THE ROLE OF DIFFERENT ALKALI SOURCES IN DEINKING AND BLEACHING PROCESSES

Transcription

1 THE ROLE OF DIFFERENT ALKALI SOURCES IN DEINKING AND BLEACHING PROCESSES Madhu S. Mahagaonkar, M.Sc.. (By Research - University of Bombay, India) Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy. ok k vcicoo Chemistry Department, University of Tasmania, November, 1995.

2 DECLARATION This thesis contains no material which has been accepted for the award of any other higher degree or graduate diploma in any other tertiary institution, and to the best of my knowledge, this thesis contains no material previously published or written by another person, except where due reference is made in the text of the thesis. 4014, cai-v M.S. Mahagaonkar November, AUTHORITY OF ACCESS This thesis may be mad.e available for loan and limited copying in accordance with the Coo.yright Act _M. S. MAHAGAONKAR

3 ACKNOWLEDGMENT I would like to acknowledge with gratitude the guidance provided by supervisors, Dr. Karen Stack, Mr. Paul Banham and Dr. Lawrie Dunn, over the last three and a half years. My thanks go to Australian Newsprint Mills, Australian Government (Department of Industry, Technology and Regional Development), and Solvay Interox Pty. Ltd., for funding this project. My appreciation is extended to Australian Newsprint Mills for the use of their excellent library, laboratory equipments and special thanks go to Mr. Paul Banham for providing several technical resources. I would like to thank Prof. Bob Johnston, Director, Australian Pulp and Paper Institute (APPI), Mr. Jeffrey Peters, Member of Technical Editorial Committee (TAPPI), Mr. Jim Knewstubb, Technical Editor (APPITA) and Dr. Noel Roberts for their valuable suggestions associated with my research work. I am also grateful to Prof. Paul Haddad, Prof. Allan Canty and Dr. Michael Hitchman for their support during the course of the project. My thanks also go to the staff and the members of the Paper Group in the Chemistry Department at the University of Tasmania. I am grateful to Mr. Yos Ginting for providing help in operating different computer programs. I would like to thank Mr. Bob Head, Marketing and Technical Manager of DuPont (Australia) Ltd. for his support during thesis writing. Finally, I would like to thank my wife, Yash and my two little ones, Sejal and Raj for their constant love, care and support throughout the entire course of this degree.

4 DEDICATION This thesis is dedicated to my wife Yash and my two little ones, Sejal and Raj, for their constant love, support and care throughout the entire course of this degree. Without them this work would not have been possible. i v

5 ABSTRACT This thesis is divided into two parts. The first and major part deals with deinking and the second part deals with peroxide bleaching. DEINKING The use of deinked fibre in the paper industry has increased dramatically in recent times. Enormous progress has been made in recent years in improving the deinking process. However, deinking chemistry remains one area which is not well understood and not completely optimized. There is widely held belief that the inclusion of approximately 10% ash in the feed is desirable for effective removal of ink from newsprint. However, some recently reported studies have found that the inclusion of magazines does not enhance deinking of newsprint. Many factors can have an influence on the efficiency of flotation deinking of newsprint and magazines. In order to have a better understanding of this complex process, the influence of some important process variables (flotation time and ph, different magazine type and quality, influence of sodium silicate and water hardness) during flotation deinking of newsprint and magazines have been examined in this thesis. No evidence could be found to support the idea that ash components from magazines facilitate the removal of ink from newsprint. There has been significant recent interest in attempting to improve the strength and optical properties of recycled fibre. However, no study has been reported which investigates comprehensively the effect of dein king on the properties of recycled paper after the deinking stages of pulping and flotation. This work also addresses the effects of different stages of deinking on the physical and optical properties of recycled paper. Furnishes of mixed magazine (coated) and newsprint were used to investigate changes in properties of recycled paper during deinking. In most cases, optical and physical properties showed reverse trends after pulping and flotation stages. These differences

6 can be explained by the change in proportions of mechanical and chemical fibre, filler, fines and ink during flotation. After the pulping stage, trends in strength properties were influenced by detrimental effects of filler from magazines. However, after the flotation stage, due to loss of most of the filler and fines, an enhancement in all strength properties occurred. Brightness and light absorption coefficient appeared to be influenced more by ink content, when large amounts of filler are present. The light scattering coefficient was dominated by the fibre type. Extended periods of flotation (greater than ten minutes) had no significant effect on properties of the recycled paper investigated. Studies have also been carried out to examine the effects of the various alkali sources on flotation deinking of newsprint and coated magazines. At the same time, the effects of each alkali used in the deinking process with regard to the strength, optical and surface properties of recycled paper have been investigated. This study also addresses the effect of sodium silicate on the deinking efficiency of each alkali. Significant differences in the strength and optical properties of recycled paper occurred with the use of different alkalis. These observed differences can be explained by the hydrolysing effect of the alkalis. BLEACHING Recently there has been interest in the use of other sources of alkali in peroxide bleaching, particularly magnesium oxide and calcium oxide. These bases may offer the advantage of decreased pollution problems associated with salinity, which arise when discharging conventional effluents into a fresh water system. Magnesia as an alternative alkali source may be particularly attractive in the Australian context as there are very large deposits of high quality mineral available at Kunwarara in Northern Queensland. In this study, sodium hydroxide and magnesium oxide have been compared as alkali sources in single-stage and two-stage bleaching of Eucalyptus v i

7 regnans cold caustic soda (CCS) and Pinus radiata thermomechanical pulp (TMP). The effects of peroxide stabilizers have also been studied in the presence of individual alkali. Several studies have been reported on recycling spent liquor in the bleaching process for mechanical pulps using sodium hydroxide as an alkali source. However, no work has been reported in the literature using magnesium oxide as an alkali source employed in multi-stage bleaching of mechanical or chemical pulps. In this work, the possibilities of internal recycling of residual liquor during sequential two-stage peroxide bleaching processes using either sodium hydroxide or magnesium oxide as an alkali have been investigated. This study includes the investigation of various recirculation strategies for bleaching of Pinus radiata TMP and Eucalyptus regnans CCS pulp, without altering the pulp concentration in the first and second stage of the bleaching processes. In general, there was a small to no improvement in brightness response in going from one-stage to a two-stage process, although some exceptions show interesting results. Under certain conditions, recycling residual peroxide produces detrimental effects on the bleaching response of Pinus radiata TMP. vii

8 Definition of Terms GLOSSARY Alkali hydrolysis: Saponification of esters and lactones on the fibre surface to form carboxylic acid. Brightness: A measure of the amount of light reflected from the sheet at a specific wavelength of light usually 457nm which is in the blue region. Bulk: The ratio of thickness to basis weight of a sheet of paper and it is inversely proportional to density. Burst index: A measure of the hydrostatic pressure required to rupture paper when deformed in an approximate sphere 30.5 mm in diameter at a controlled rate of loading. Caliper: CCS: Chelating agent: Thickness of paper measured under specified conditions. Cold caustic soda pulping using Eucalypts. A chemical which removes metal ions from the pulp by binding them. Chemical pulp: Pulp formed by dissolution of lignin from the wood by chemical means. Chromophores: The moieties which give pulp its colour, mainly present in the lignin. C.O.D.: DTPA: Chemical oxygen demand. An organic chelating agent, diethylene-triamine-penta- aceticacid. Filler: Inorganic materials which are incorporated into the fibre sheet to improve optical properties like brightness, opacity and printability of the paper. Fines: Fibre fraction which passes through a 76tim screen. viii

9 Flotation: floated Formation: A process in which pulped stock of recycled paper is in water to remove ink. The uniformity with which fibres are distributed in the paper. Freeness: Canadian standard freeness, an arbitrary measure of the drainage properties of paper stock under specified conditions. Furnish: Grammage: Light scattering coefficient: Pulp mixture used in the papermaking process. Weight of paper is expressed as grams per square meter. A measure of the degree light can penetrate the sheet and be reflected from the surface between or within fibres and eventually be returned from the surface to the observer. OMG: ONP: Old magazines used in paper recycling. Old newsprint, especially newsprint which is being recycled. Opacity: Percentage of light which fails to travel through a sheet of paper. Oven dried (o.d.) pulp: Pulp which has had all water removed from it by drying in an oven. This enables the total mass of wood fibre in a pulp slurry to be determined and is the usual basis from which chemical concentrations are expressed. Pigments: Coloured, water insoluble substances which do not dye the fibres but are held in the sheet to improve colour, brightness and opacity of the sheet. The term is also used in ink formulations as the solids that give colour to ink.

10 Porosity: The resistance of a paper, of given dimensions, to the passage of air under standardized conditions of pressure, temperature and relative humidity. Pulping: Runnability: Paper is repulped during recycling process. How well a given furnish performs on the paper machine, particularly with respect to paper breaks. Stabilizer: A chemical which minimises hydrogen peroxide decomposition. Stretch: A measure of the amount of distortion that paper undergoes under tensile stress. Tear index: A measure of the amount of work done in tearing the paper through a fixed distance after the tear has been started. Tensile index: A measure of the force required to pull a standard sheet of paper apart. TMP: Thermomechanical pulp.

11 TITLE DECLARATION ACKNOWLEDGMENT DEDICATION ABSTRACT GLOSSARY INTRODUCTION TABLE OF CONTENTS ii iii iv viii xvii CHAPTER 1 DEINKING - GENERAL OVERVIEW 1.1 Utilization of Recycled Fibre History of Deinking Printing Processes and Inks Deinkability Mechanism of Detaching Ink Pulping Fibre Swelling and the Breaking of Interfibre Bonding Saponification Wetting Emulsification Solvation Dispersion Anti-redeposition Chemicals Used in Deinking Formulations Ink Removal Process Screening Cleaning Washing Flotation Flotation Deinking-Description of Process Mechanism of Flotation-Air bubble and Ink Particle Attachment Role of Calcium Ions in Flotation Deinking Deinking of Newsprint and Coated Magazines 15 REFERENCES 17 x i

12 CHAPTER 2 EFFECTS OF PROCESS VARIABLES DURING FLOTATION DEINKING OF NEWSPRINT AND MAGAZINES 2.1 Introduction Effects of Process Variables During Flotation Deinking of Newsprint and Magazines Effects of Feedstock Compositions, Flotation Time and ph Glossy Magazines (OMG) With -30% Ash Content Lower Quality Magazine (OMG) With -9% Ash Content Effect of Sodium Silicate Effect of Sodium Silicate on Newsprint (100% ONP) and Different Furnishes of Magazines (100% OMG) Effect of Alkali Darkening on Newsprint (100% ONP) Effect of Calcium Ions in Flotation Deinking of Newsprint and Coated Magazines Conclusions 35 REFERENCES 36 CHAPTER 3 EFFECTS OF FILLERS AND DIFFERENT TYPES OF PULPS ON THE PROPERTIES OF THE PAPER. 3.1 Literature Review Role of Fillers in Papermaking Process Introduction to Paper Testing Optical Properties of Paper Kubelka-Munk Theory Kubelka-Munk Equation Effects of Fillers and Pulps on the Optical Properties Opacity Light Scattering Coefficient Brightness Effects of Fillers and Pulps on Strength Properties Effect of Fillers on Strength Properties Effect of Different Types of Pulps on Strength Properties Effects of Fillers on Porosity, Bulk and Surface Smoothness Porosity 49 x i i

13 Bulk Surface Smoothness Effects of Recycling on the Properties of Secondary Fibre Effect on Recycling on Strength Properties Effect of Deinking on the Properties of Secondary Fibre 51 REFERENCES 53 CHAPTER 4 EFFECTS OF DEINKING ON OPTICAL AND PHYSICAL PROPERTIES OF SECONDARY FIBRE AFTER PULPING AND FLOTATION 4.1 Introduction Effect of Flotation Time on the Properties of Recycled Paper Effect of Deinking on Ash Content Effect of Deinking on Optical Properties Brightness Light Absorption Coefficient Light Scattering Coefficient Opacity Ink Speck Measurements Effect of Deinking on Strength Properties Tear Index, Burst Index, Tensile Index Stretch Effect of Deinking on Other Properties Freeness Porosity Roughness Bulk Conclusions 84 REFERENCES 85 CHAPTER 5 THE ROLE OF DIFFERENT ALKALIS IN FLOTATION DEINKING OF NEWSPRINT AND MAGAZINES 5.1 Literature Review Improvement of Strength Properties of Recycled Fibre 89

14 5.1.2 A Mechanism for the Alkali Strengthening of Different Pulps Effect of Swelling and Carboxylic Acid Groups on Strength Properties Alkali Hydrolysing Effect on Strength Properties Effects of Different Deinking Processes on the Properties of Recycled Fibre Effect of DeinIcing on the Environment The Type of Effluents From Deinking Processes Introduction Effect of Deinking on Optical Properties Using Different Alkalis Brightness Image Analysis, L*a*b* and Light Scattering Coefficient Effect of Deinking on Strength Properties Using Different Alkalis Tear Index, Tensile Index, Burst Index and Stretch Effect of Deinking on Other Properties Using Different Alkalis Freeness Porosity Effect of Deinking on Environment Using Different Alkalis Effluent Study Conclusions 119 REFERENCES 121 CHAPTER 6 BLEACHING - GENERAL OVERVIEW 6.1 The Structure of Wood The Sources of Colour in Wood and its Measurement Use of Hydrogen Peroxide in the Pulp and Paper Industry Decomposition of Hydrogen Peroxide Stabilization of Hydrogen Peroxide Reagent Recycling in Hydrogen Peroxide Bleaching Recycling in Single-Stage Bleaching Process Recycling in Two-Stage Bleaching Processes Alkaline Peroxide Bleaching With Alternative Alkali Sources 139 REFERENCES 142 x i v

15 CHAPTER 7 PEROXIDE BLEACHING OF Pinus radiata TMP AND Eucalyptus regnans COLD CAUSTIC SODA PULP WITH SODIUM HYDROXIDE AND MAGNESIUM OXIDE IN SINGLE-STAGE AND TWO-STAGE BLEACHING PROCESSES 7.1 Introduction Single-Stage Bleaching of Eucalyptus regnans CCS Pulp A Comparison Between Sodium Hydroxide and Magnesium Oxide in Single-Stage Bleaching of Eucalyptus regnans CCS Pulp Effect of Stabilizers Using Sodium Hydroxide Effect of Stabilizers Using Magnesium Oxide Effect on Bleaching Response Using Sodium Hydroxide and Magnesium Oxide Effect on Peroxide Consumption Using Sodium Hydroxide and Magnesium Oxide Effect of Extended Bleaching time in Single-Stage Bleaching of Eucalyptus regnans CCS Pulp Using Sodium Hydroxide and Magnesium Oxide Two-Stage Peroxide Bleaching of Eucalyptus regnans CCS Pulp Experimental Design for Two-Stage Bleaching Process A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Without Using Stabilizers (Additives) A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Using Sodium Silicate A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Using Magnesium Sulfate A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Using DTPA Two-stage Peroxide Bleaching of Pinus radiata TMP A Comparison Between Sodium Hydroxide and X V

16 Magnesium Oxide in Two-Stage Bleaching of Pinus radiata TMP Pulp Conclusions 172 REFERENCES 175 CHAPTER 8 SUMMARY AND CONCLUSIONS 8.1 Deinking Bleaching 180 CHAPTER 9 EXPERIMENTAL 9.1 DeinIcing Stock Preparation and Reagents Pulping and Flotation Experimental Error and Confidence Limits Handsheet Formation, Physical Testing and Optical Measurements X-ray Microanalysis Electron Scanning Microscopy Effluent Analysis Fibre length Measurement and Water Retention Values Bleaching Reagents Pulping and Bleaching Procedure Determination of Residual Peroxide Measurement of Pulp Brightness 189 REFERENCES 191 x v i

17 INTRODUCTION This thesis is divided into two sections. The first and main section is about deinking while the second part deals with peroxide bleaching. A general overview of deinking is presented in Chapter 1. In this Chapter the proposed mechanisms by which ink is removed are described. In Chapter 2, results of an investigation into the effects of some of the process variables on flotation deinking of newsprint and magazines are presented. A review of the role of fillers and different pulps in the papermaking process is presented in Chapter 3. In particular, the effects of filler addition on optical and strength properties, as well as some other paper and pulp properties are summarised. In Chapter 4, the effect of deinking on the optical, strength and other pulp properties after pulping and flotation stage are studied while in Chapter 5 the effect of different alkali sources on the pulp properties and effluent are investigated. Chapter 6 presents a general review of peroxide bleaching and reagent recycling in the peroxide bleaching processes. In Chapter 7, the results of the effect of peroxide bleaching of Pinus radiata Thermomechanical pulp (TMP) and Eucalyptus regnans Cold Caustic Soda (CCS) pulp with sodium hydroxide and magnesium oxide in a singlestage and a two-stage process are presented. A summary of the experimental results for both the deinking and bleaching sections are presented in Chapter 8. In this Chapter the conclusions drawn from the experimental work are given. Details of the experimental procedures are given in Chapter 9.

18 CHAPTER 1 DEDIKING - GENERAL OVERVIEW 1.1 Utilization of Recycled Fibre Recycled fibre has been an important source of papermaking fibre for the last 100 years, particularly for packaging grades. The main driving force for its use has been economics. However, in the past 10 to 15 years there has been a rapid expansion of recycled fibre use into other paper grades'. This continuing rapid growth is highlighted in Figure 1, which shows a predicted increase in the total use of deinked fibre from 11 millions tonnes in 1988 to 31 million tonnes by Actual Estimated 2001 Millio n Tonnes (/) 14 c.) ca ce) 'et Figure 1: World deinked pulp usage (after Barassi and Welsford I) A check of Jaako Poyry's data banks shows 92 deinking plant projects, worldwide, which are currently under active consideration or in the process of construction. These have a combined capacity of 6.7 million tonnes'. This rapid expansion has occurred for two main reasons: 1

19 * an increase in environmental awareness and problems of waste disposal, causing communities and governments to pressure the paper industry into increasing its recycling rates 1,2. * significant and continuing improvements in deinking technology, making ink removal more cost effective and thus allowing the use of recycled fibre in the higher quality white grades 1, History of Deinking The first successful deinking experiments were carried out and patented by Matthias Koops in England in Commercial manufacture was not achieved at that stage as the mill went bankrupt'. The first recorded deinking of printed waste paper was around 1900 and was confined to chemical rag and wood based papers for quality reasons'. Deinking of mechanical printed fibre did not begin until 1922 in the USA. The early development of deinking was based on hot caustic chemistry and extensive countercurrent washing. Sodium hydroxide and related alkalis were used to saponify the resins in the ink and to facilitate the release of the ink bond. Additional soap was added to increase the carrying capacity for the released ink pigments and this was assisted by the use of solvents. Around 1960 a "new" process using flotation cells was borrowed from the mining industry and used to float the ink from the fibre'. Today flotation deinking is included in almost all deinking systems because of its good ink removal efficiency, reduced fibre loss and reduced water use'. It may have taken 1400 years from the invention of ink to the development of deinking technology, but the technology has rapidly advanced in the last years as has the use of deinked pulp'. Increasing waste paper utilization rates, development of new printing inks, varnishes and adhesives and tighter environmental constraints will continue to provide new challenges for all involved in deinking 2

20 1.3 Printing Processes and Inks The deinkability of a wastepaper furnish is directly related to the process by which it was printed, the types of ink used and the method used to dry the ink. With conventional impact or contact printing, the image is put onto an impression cylinder or "plate", usually by photomechanical means. Paste or liquid ink is applied to the impression cylinder and the image is transferred to the paper directly or indirectly (offset) via a blanket cylinder. Such types of printing include letterpress, flexographic, lithographic and gravure 3. In contrast, non-impact printing methods use the image which is stored in digital form in a computer memory. Transfer of the image to the paper is accomplished via electrostatic or electrical charge. The ink used with electrostatic printing is a dry powder of coloured resin binders 3. Inks are formulated to provide specific properties in terms of printability, drying, colour and end use 3. Although the composition of ink may vary greatly depending on its use, the three common components are: colorant, modifier and vehicle. Colorants are typically finely divided small particle size solids called pigments. Some common examples are carbon black and titanium dioxide. Small amounts of ingredients called modifiers are incorporated into the ink formulation to impart a specific property to the ink3. For example, waxes may be added to improve scuff resistance and sheet sticking. In order to improve ink transfer, some modifiers like lubricants/greases are also added 3. A key component of the ink formulation is the vehicle or "binder". It acts as a carrier for the colorant or pigment, to transport it to the paper being printed3. It also binds the pigment to the printed surface. Typically, the vehicle will contain solvent, and synthetic resins to provide bonding of the pigment to the paper. The type of vehicle used in ink formulation will depend on both the printing process and the drying method3. Drying is required because the ink on the printed surface must be transformed from a fluid film to a more rigid and permanent film. Often, more than one drying method is used with a 3

21 given printing process which includes oxidation, evaporation, precipitation, crosslinking or polymerisation of the vehicle Deinkability How easily and effectively the dried ink film can be removed from a printed waste furnish (i.e. its "deinkability") is primarily determined by the type of vehicle and drying method used 3. Typically, the colorants and modifiers do not pose deinking problems. However, many vehicles form tough films resistant to both mechanical and chemical treatment upon drying 3. In general, chemically crosslinked ink films are more difficult to break up and remove from the fibre surface 3. Many of the ink films formed by oxidation are not only strong but flexible as well. Upon alkali swelling of the fibres, these films will expand elastically rather than break up. The different drying methods, vehicles used and relative deinkability have been well discussed in the literature Mechanism of Detaching Ink In simple terms, conventional deinking can be broken down into two steps 3 * Detachment or removal of inks and coatings from the fibre surface * Separation of ink and contaminants from the pulp slurry In the first step, the ink must not only be detached from the fibre surface but, more importantly, be broken up into particles of appropriate size and surface chemistry 3. Each of these steps has different requirements in terms of optimum ink particle size and surface chemistry5-10. Subsequent separation steps may include screening, cleaning, washing, flotation 3,5 4

22 1.6 Pulping Inks are detached from the fibre surface and rendered innocuous in the pulper without further mechanical treatment. This is accomplished by adjusting the pulping conditions and chemistry to achieve maximum wetting, emulsification, solvation and dispersion of the ink particles. Since there is no further mechanical treatment for ink removal, it is very important that the ink be sufficiently broken up and dispersed in the pulper 3. In the deinking of newsprint and magazines, the conditions used in the pulping are typical. The parameters like high alkaline ph ( ), temperature (50-65 C), pulping time (20-45 minutes) and stock consistency (5-10%) are commonly practiced in the recycling industry 3. However, the selection of these conditions essentially depends upon the type of waste paper furnish used in recycling 3. Chemicals are added to enhance the thermal and mechanical action of the pulper. The addition of chemicals causes a number of complex mechanisms to occur simultaneously Fibre Swelling and the Breaking of Interfibre Bonding: This process begins as soon as the wastepaper is immersed in water. The water molecules form hydrogen bonds with the cellulose molecules 3. The effect is enhanced with the addition of sodium hydroxide or sodium carbonate and elevation of temperature4. The fibre swelling process liberates ink particles which are not strongly held to the fibre surface 3. It also helps to loosen more strongly held coatings and brittle ink films, which allows them to be reduced in particle size by the mechanical action of the pulper Saponification: A chemical reaction that proceeds under alkaline conditions to convert an ester to its component alcohol and acid is known as saponification 3. Some of the binders (metallic 5

23 resinates) used in gravure inks and dried by evaporation are esters which can be saponified at high alkaline ph and temperature Wetting: This is a surface or interfacial phenomenon that plays a key role in pulping liquor penetration into the fibre network. Effective penetration facilitates fibre swelling, saponification and mechanical break up of the inks and coatings 3. Since a printed surface tends to be water repellent, a chemical must be added to the pulper to affect interfacial tension and promote wetting 3. These chemicals are called surfactants or surface active materials. "Surfactants" is a catch-all term that covers chemicals like dispersants, collectors wetting agents, displectors, anti-redeposition aids and the like 7. Displectors which are the surfactants of choice for many years, are ideally suitable for combination washing-flotation system. They cause sufficient adhesion between ink particles and air bubbles for flotation to be effective. However, they also cause ink particles to retain sufficient hydrophilicity to remain dispersed so that they can be removed during washing 6. Surfactants are molecules that consist of a "head" which is water soluble or hydrophilic and a "tail" which is oil soluble or hydrophobic. When surfactant is introduced into the pulper or just prior to flotation, the hydrophobic end gets associated with the ink, oil and dirt while the hydrophilic end remains in the water7. The size and strength of the hydrophilic group in relation to the hydrophobic group dictates the function of the surfactant3,14. The ratio between the two groups is expressed as a numerical value called the hydrophilic-lipophilic balance or HLB 3. In deinking, the types of surfactants used for wetting are ethoxylated alkyl phenols, ethoxylated fatty acids or ethoxylated linear alcohols Emulsification: Similar to wetting, emulsification is a surface phenomenon that requires the addition of surfactants to alter the interfacial tension between the phases 3. In deinking, emulsification breaks up inks. When surfactants are added in the pulper, the oily 6

24 vehicle is emulsified into the pulping liquor. As a result, pigment particles are liberated Solvation: This is the dissolving of a solid into a liquid 3. Certain deinking formulations may contain solvent3. The solvent will probably soften or weaken the binder making it more susceptible to mechanical break up or chemical dispersion Dispersion: This is the act of suspending solid particles in a liquid. The process is a complex one, which involves both chemical and mechanical considerations. Dispersion is important in washing deinking where the ink must be small, finely divided and well suspended in the water phase to completely separate the ink from fibre 3. Suspended ink particles in a liquid, have a tendency to agglomerate when they come in contact with each other, because of attractive forces between them 3. To keep particles dispersed and prevent agglomeration, some dispersants are added. They form an electrical barrier around the particle. Dispersants include higher molar mass anionic or non-ionic surfactants, polymers (polyacrylates) or silicates Anti-redeposition: This is the prevention of suspended particles from precipitating onto the substrate from which they were removed 3. Anti-redeposition aids prevent the dispersed ink particles being settled onto the fibres. Otherwise, subsequent separation steps such as washing will not be effective 3. Anti-redeposition aids are believed to be adsorbed on both the suspended particles and the fibre, thereby increasing the repulsive forces between the two 3. Sodium carboxymethyl cellulose(cmc) is a commonly used as an antiredeposition aid. 7

25 1.6.8 Chemicals Used in Deinking Formulations The types of chemicals used in the pulping stage are selected relative to the furnish type3. The chemicals employed in the deinking formulations are listed in Table 1. The function of each chemical is also highlighted in the Table 1. Table 1: Chemical addition for deinking formulation (after Olson and Letscher 3 ) Chemical Dosage (%) Function Sodium hydroxide 1-2 High alkalinity, fibre swelling Sodium silicate 2-4 ph buffer, stabilizer for peroxide Surfactant Wetting Fatty acid Collector for ink particles Calcium salts Precipitates fatty acid to form soap necessary for ink removal Chelant Binds metal ions which can cause peroxide degradation Hydrogen peroxide 1-3 Reverses alkali darkening 1.7 Ink Removal Processes A number of mechanisms can separate foreign particles from a pulp slurry based on different principles of operation which may include screening, cleaning, washing, and flotation, or a combination of these3,5,6,8, Screening: Separates by difference in size and stiffness3,5.

26 1.7.2 Cleaning: Centrifugal cleaning separates mainly by the difference in specific gravity. It is extremely effective 5 at removing particles larger than m. Light cleaners remove particles lighter than water (specific gravity <1), while heavier cleaners can remove particles heavier than water Washing: Washing, which is basically water extraction, separates particles that are small enough to move with the water stream. Generally, these particles are less than 20[1.m in diameter 3,5,11. Washing is not effective for highly coated wastepaper since the ink cannot be dispersed into small particles 3,5,11. To save water and increase fibre yield, the counter current washing process is very often used 3, Flotation: Flotation is the selective separation of ink particles from pulp slurry using their natural or induced hydrophobic properties. The particles attach to air bubbles and are carried to the surface of the slurry in a froth. The froth is then skimmed or suctioned from the surface of the slurry and the particles are collected 3,5,11. Similar to washing, flotation begins with ink and fibre separation in the pulping stage3,5, 1 1,14. However, for flotation it is important that the pulping conditions be carefully controlled 3. The optimum particle size for ink flotation is 30 to 601im. Figure 2 shows the response of the removal mechanisms as a function of particle size 5. According to this model (Figure 2), washing is ineffective5 on specks and is only able to remove very small particles less than 10gm. Flotation is most efficient in the gm range 3,5. 9

27 PA RTI CL E CO UN T PARTICLE SIZE. pm i.. t dispersion Size (Washers) face - active properties, FSur (Flotation) 1 Specific gravity (Centrifugal cleaners) f. Size. stiffness (Screens) PARTICLE SIZE. pm Figure 2: Particle size distribution and unit operation removal efficiency (after McCool and Silveri 5 ) Flotation Deinking Description of Process The flotation process has become the dominant deinking method due to greater ink removal efficiency A flotation deinking system consists of a cell, a high speed agitator, an overflow for froth removal and a mechanical paddle or suction for removing the froth. This is shown schematically in Figure 3. 10

28 Overflow for ink sludge Mechanical paddle for froth/sludge removal Recycled fibre at 1% consistency Figure 3: A schematic diagram of a flotation deinking cell The high speed agitator induces a partial vacuum which in turn causes air to enter the system and combine with the stock and flotation agents to form small air bubbles3,5,11,14. A absorption agent, or collector binds the released ink particles to the hydrophobic surface of air bubbles rendering them more hydrophobic 3,5,11,14. The particles attached to air bubbles are carried to the surface of the slurry in a froth. The froth is then skimmed by paddles or suction. Flotation is carried out at consistencies of approximately I% with a retention time of 5-10 minutes. Efficient flotation of ink is dependent on three factors: (I) Collision between ink particle and air bubble (2) Attachment of ink particle to air bubble (3) Flotation of ink/bubble complex to the surface 11

29 Mechanism of Flotation Air Bubble and Ink Particle Attachment Flotation is an effective and efficient process to remove ink particles from an ink-laden fibre slurry 3,5,6,7,10. As air is bubbled through the fibre suspension, ink particles collect at the bubble-water interface and move upwards to form a froth. The froth is removed and skimmed off, taking with it the concentrated ink particles 3,5,10. Figure 4 shows a schematic representation of the flotation of an ink particle. In order for the particle to be floated, the total upward pull of the particle must balance its apparent we igh t 10,12. Theory shows that 10,12 the vertical force applied to the particles is given by yaw p cose = v (d p - d w ) g ( 1 ) where l aw = Surface tension at the air-water interface = Perimeter of the particle = Volume of the particle = Density of particle = Density of water phase = Gravitational constant = Contact angle between the particle and the bubble 12

30 Water W/A 1 S/A Air Ink Particle Slw Figure 4: Contact angle between ink particle and air bubble in aqueous media (after West and Daubitz 10). The relationship between three phases, air,water and solid is given by Young's equation I 7 S/A = 7 S/W + 7 W/A cos() (2) where 7 S/A' 7S/W' and y W/A are the surface energies at the solid-air, solid-water and water-air interfaces respectively. The force required to break the particle-bubble interface is called the "work of adhesion", WS/Aand is equal to the work required to separate the solid-air interface and produce separate air-water and solid-water interfaces i.e. W S/A = 7 W/A ± 7 s/w - 7s/A (3) Combining equation 2 and 3 gives w = 7 S/A W/A (1 - cos $0) (4 ) Equations 3 and 4 reveal that as the contact angle increases so does the W between S/A the bubble and the particle and the more resilient the particle-bubble complex is to disruptive forces

31 Role of Calcium Ions in Flotation Deinking Removal of the ink from the pulp slurry proceeds in the flotation cel1 3,5,6.7,8. When fatty acid collectors are used Ca2+ ions are added to the slurry as CaC12 or Ca(OH)2 before it is sent to the ce11 3. The Ca2+ forms the insoluble soap of the fatty acid which precipitates onto ink particles. A mechanism for flotation deinking has been proposed by Larsson, Stenius and Odberg 13. They theorize that when the calcium soap is precipitated, it forms a layer of small particles around the ink particles. This gives the ink particles the surface properties of the calcium soap (Figure 5). In the presence of excess calcium the particles become hydrophobic and acquire a low zeta potential. They can be then easily attached to air bubbles 3,13. PRECIPITATE CALCIUM 30AP ARTICLE 3 Figure 5: A role of calcium ions in flotation deinking (after Larsson et a1 13) 14

32 1.8 Deinking of Newsprint and Coated Magazines Generally, analysis of deinking begins with a review and a study of ink types and printing processes used in the printing industry. In the deinking of newsprint, the inclusion of a level of ash (8-10% of furnish weight) is considered necessary for good flotation results 14. This is most effectively carried out by the inclusion of coated magazines rather than other ash sources. The original incorporation of the ash of the magazines into a coating matrix bound by a binder seems important, perhaps because ink-absorbing flotation-susceptible coating flakes are formed". However, some of the commonly accepted assumptions on this issue have been disputed 15. Figures 6 and 7 show ink attachment in newsprint (uncoated paper) and coated magazines respectively. In the case of newsprint, ink particles are in contact with the fibres (Figure 6). In contrast to this, Figure 7 shows that in coated magazines ink remains on the surface of the coating material and never penetrating to the fibre 5. Figure 6: Ink attachment in newsprint (after McCool and Silveri 5) 15

33 Figure 7: Ink attachment in coated magazines (after McCool and Silveri 5 ) 16

34 REFERENCES (1). Barassi, J. and Welsford, J., Appita, 45(5), 308, (1992). (2). Higgins, H. G., Tappi, 75(3), 99, (1992). (3). Olson, C. R., Letscher, M. K., Appita, 45(2), 125, (1992). (4). Sjostrom, E., in "Wood Chemistry Fundamentals and Applications", Academic Press, New York, Chapter 9, (1981). (5). McCool, M. A. and Silveri, L., Tappi, 70(11), 75, (1987). (6). Shrinath, A., Szewczak, J. T. and Bowen. J. I., Tappi, 74(7), 85, (1991). (7). Ferguson, L. D., Tappi, 75(7), 75, (1992). (8). Carr, W. F., Tappi, 74(2), 127, (1991). (9). Paraskevas, S., Tappi Seminar Notes, 41, (1989). (10). West, Y. and Daubitz, F., Tappi Pulping Conference, 1195, (1992). (11). Pfalzer, L., Tappi, 63(9), 114, (1980). (12). Davies, J. T. and Rideal, E. K., in "Interfacial Phenomena", 2nd Edition, Academic Press, New York, 421, (1963). 17

35 (13). Larsson, A., Stenius, P. and Odberg, L., Svensk Papperstidning, 18, R158, (1984). (14). Blain, T. J., Tappi Seminar Notes, 113, (1992). (15). Letscher, M. K. and Sutman, F. J., Pulp and Paper Science, 18(6), J225, (1992). 18

36 CHAPTER 2 EFFECTS OF PROCESS VARIABLES DURING FLOTATION DEINKING OF NEWSPRINT AND MAGAZINES. 2.1 Introduction There is much current interest in the recycling of paper in Australia 1,2 and an improved understanding of deinking chemistry will make a very important contribution in the future development of these processes 1,2,12. The use of deinked fibres in the paper industry has increased dramatically in recent years, and with the current environmental awareness and legislation, this trend is expected to continue 1,2,12. The brightness requirements for deinked pulp depend upon the waste paper furnish and the end use of the deinked product. Whatever the final brightness requirement, maximum ink removal during the deinking process is of paramount importance 12. A poorly deinked pulp cannot be efficiently bleached if the ink is smeared and the brightness loss cannot be compensated for by post bleaching 12. Enormous progress has been made in recent years in improving the deinking process hardware 12. Although deinking chemistry has been optimized, additional information is required for further developments in deinking technology. A number of different stages are used in deinking, including screening, cleaning, washing and flotation, with each being most effective over a defined range of particle size 3-7. Flotation deinking has gained wide acceptance in Europe and Japan, while washing is widely practiced in North America8,9. The new deinking operation at Australian Newsprint Mills Albury facility uses a flotation stage, after repulping of a mixture of newspapers and magazines. Many factors can have an influence on the efficiency of deinking processes, and there has been significant recent interest in attempting to correlate process variables with properties of the deinked paper It is common practice to include about 30% coated magazines (OMG) with newsprint (ONP) furnish in commercial operations 917,18. In addition to providing a source of chemical pulp fibres which can 19

37 offset losses in strength properties in recycling 19, there is a widely held belief that the inclusion of approximately 10% ash in the feed is desirable for effective removal of ink from newsprint 917,18. It has been suggested that as the major proportion of ink is localised on the coating in magazine paper, removal of the clay leads to high ink removal efficiencies and consequently improved brightness. However, some recently reported studies have found that the inclusion of magazines does not enhance deinking of newsprini20. The exact role of coated magazines in the deinking of newsprint has not been thoroughly investigated. The role of sodium silicate in the deinking of magazine-newsprint has been addressed in the literature. However, the literature is somewhat divided on the effect of sodium silicate on the flotation deinking of 70:30 ONP/OMG mixtures. Ferguson 21 has reported that silicate addition has beneficial effects on the brightness of deinked pulp, both in the absence and presence of DTPA, while Mathur 23 has found that silicate addition has a little effect on the final brightness of pulp after flotation. Mathur 23 suggested that this difference may be due to the fact that other studies were not conducted at constant alkalinity, while in his work, adjustments were made to allow for the p1-1 effects of the sodium silicate. In this work, the role of sodium silicate has been investigated on different types of furnishes. In flotation deinking, the effect of calcium ions with the use of fatty acid collectors has been discussed in the literature 25,26. However, very little has been reported correlating the effect of acidic flotation ph and calcium ions (generated from the furnish itself) on flotation deinking of newsprint and magazines. In this study, the effect of calcium ions (generated from feedstock) together with a range of ph has been discussed. In order to have a better understanding of the technique involved in deinking of newsprint, an attempt has been made to investigate the various process parameters. In this chapter, the effects of (1) varying the ratio of magazines to newsprint in the furnish, (2) magazine type and quality, (3) flotation time and ph, (4) calcium water hardness and (5) the influence of varying sodium silicate dose introduced in the repulping stage have been studied. 20

38 2.2 Effects of Process Variables During Flotation Deinking of Newsprint and Magazines Effects of Feedstock Compositions, Flotation Time and ph Glossy Magazines (OMG) With -30% Ash Content Figure 1 shows the effect of feedstock composition on the pulp brightness after pulping. As the proportion of magazines (OMG) in the furnish is increased, there is a significant drop in pulp brightness. After the pulping stage, this observed decrease in the brightness can be attributed to the magazine component carrying a higher ink load than ONP. This effect has been reported previously in a recent study by Letscher and Sutman 20. These workers also found that the addition of OMG to the furnish does not increase the efficiency of deinking of ONP Brightness ( %ISO) % OMG Figure 1: The effect of feedstock composition on brightness after pulping. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. 21

39 Figure 2 illustrates the influence of flotation time on the final brightness of the pulps obtained. For all feedstocks tested, the most rapid increase in brightness occurs within the first 5-10 minutes, after which there is a much more gradual increase in brightness over extended times. The relative ordering of the sequence of the final brightnesses is established after 5 minutes and the same pattern is maintained thereafter. O 0% OMG 10% OMG O 30% OMG a 50% OMG 70% OMG A 80% OMG 100% OMG Flotation Time (minutes) Figure 2: Effect of flotation time on final handsheet brightness for feedstock with various ratios of ONP/OMG. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 10.5; hardness (CaCO3) 200 ppm. It should be noted, however, that as the pulps with highest OMG content in the furnish are initially the darkest, the brightness-time profiles for the complete series must intersect during the first five minutes of flotation. At very short flotation times, the ordering of pulp brightnesses for the samples 22

40 will not correspond to that observed after 5 minutes. This observation is important in that it shows the potential difficulties which could be encountered in trying to establish feedstock-response correlations at some arbitrarily chosen short flotation time. The results reported in Figure 2 were obtained without any adjustment of ph in the flotation stage, so that flotation occurred at ph It has been observed that improved deinking efficiency can be obtained by using less alkaline conditions during flotation 21. This result is confirmed in Figure 3 which shows that for deinking of a 70:30 ONP/OMG mixture, adjustment of the flotation ph to 8.5 by acid addition gives improved final handsheet brightnesses. This ph (8.5) was also found to be optimum by other researchers when identical experimental conditions were used CID e a CA al o ph 10.5 ph Flotation Time (minutes) Figure 3: The influence of flotation ph on final brightness, using 30% OMG. Pulping conditions: 1% H202, 1% Na0H, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C; hardness (CaCO3) 200 ppm. 23

41 It was, however, still found that the influence of magazine addition on newsprint deinking exhibits a very similar pattern of behaviour to that at ph 10.5, and this is illustrated in Figure Brightness ( %ISO) a 8 0 O 0% OMG 30% OMG O 80% OMG 100% OMG Flotation Time (minutes) Figure 4: Variations of final brightness with flotation time for different feedstock compositions at flotation ph 8.5. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 8.5; hardness (CaCO3) 200 PPm. Figures 2 and 4 show that higher final pulp brightnesses (after flotation) are obtained with increasing OMG/ONP ratios in this study. This is presumably due to the combined effect of several factors: (1) Ash (filler) retention in deinked pulp (due to white fillers like titanium dioxide and calcium carbonate originating from magazine coating) (2) Ink removal at the flotation stage (3) Chromophore removal due to bleaching effect occurring in the pulping stage (4) Retention of brighter chemical pulp (originating from magazine) 24

42 In order to differentiate between increased brightness due to addition of higher brightness materials from the magazines (i.e. fibres or coating components) and effects on the effectiveness of ink removal from the ONP, a comparison such as shown in Figure 5 can be used. This figure shows the results for deinking of ONP/OMG together with results obtained after preparing handsheets derived from blending deinked pulps from ONP and OMG feedstock using similar experimental conditions. Also shown are calculated results for the blended samples, obtained by taking a weighted average of the brightnesses of pulps from independently deinked ONP and OMG O Actual (ONP/OMG pulped & deinked together) Mixed Pulp (ONP/OMG pulped & deinked individually) O Theoretical (calculated values from weighted average) % OMG Figure 5: Final pulp brightness as a function of furnish composition plotted together with corresponding brightness for pulp blends prepared from individually deinked OMG and ONP. Calculated values of brightness for blended pulps also shown. Figure 5 shows that there is a close correspondence between the actual brightnesses of the blended samples and the calculated values. However, the observed brightnesses of sheets prepared after deinking ONP and OMG together are significantly lower than the corresponding values from blended samples. This confirms the results of previous studies which report that there is no positive effect of OMG on deinking of 0NP 20, and in fact, a negative influence for this particular system is observed. 25

43 Lower Quality Magazines (OMG) With -9% Ash Content When a magazine furnish (TV Week) of lower quality (lower brightness, less fillers) was used for flotation deinlcing studies, very different brightness trends were obtained when compared with those found using glossy coated magazines, as in Figures 2 and 4. The final brightness obtained with increased OMG(lower quality magazine) content was lower than the brightness found using ONP alone (Figure 6) % OMG 80% OMG El 60% OMG M 30% OMG A 0% OMG Flotation Time (minutes) 60 Figure 6: Variations of final brightness with flotation time using different feedstock combinations of OMG (lower quality magazine, ash content -9%) and ONP. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 8.5; hardness (CaCO3) 200 PPm- 26

44 This trend was observed despite the fact that the OMG (lower quality magazine) furnish possessed an appreciable ash content of approximately 9%. This may suggest that the initial quality and brightness of the magazine furnish has a large bearing on the final brightness obtained when using OMG/ONP mixtures. The brightness of the magazine base stock is generally higher than the newsprint and so the reduced brightness observed is possibly due to the amount of ink and ink removal efficiency of the magazine furnish. These results may also suggest that the presence of ash during flotation deinking doesn't always result in an overall brightness gain. 2.3 Effect of Sodium Silicate Effect of Sodium Silicate on Newsprint (100% ONP) and Different Furnishes of Magazines (100% OMG) The application of sodium silicate in deinking processes is well established, particularly when the furnish contains significant proportions of groundwood grades. Compared to the use of sodium hydroxide alone, silicate provides better ink removal with less fibre damage 3. It has been suggested that the presence of silicate may contribute to wetting, peptization, ink dispersion, alkalinity, buffering and peroxide stabilization 8,10,21,22. Figure 7 shows the effect of increased silicate addition on final pulp brightness for different types of magazines furnishes (100% OMG) and newsprint (100% ONP) and 70:30 mixture of ONP and OMG (coated, with ash content -30%). Also shown are results obtained for a sample of uncoated magazines (100% OMG, with ash content -9%). It is clear that addition of silicate has a significant effect on increasing final brightness for the 100% ONP and uncoated magazine samples (ash content -9%), which both contain high proportions of mechanical pulp. For coated magazines (ash contents -30%), addition of sodium silicate has a slight negative influence on brightness, while for the 70:30 ONP/OMG (coated, with -30% ash content) mixture silicate level has a little effect. At the pulping stage, for all mixtures of magazines and newsprint, no significant change was observed in ph's with the addition of sodium silicate. 27

45 O 100% OMG (30% ash) O 100% OMG (9% ash) O 100% ONP 70% ONP + 30% OMG (30% ash) Sodium silicate (%) Figure 7: The effect of sodium silicate addition on final brightness of different furnishes. Pulping conditions: 1% H202, 1% NaOH, Na2SiO3 (0-3%), 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 8.5; hardness (CaCO3) 200 PPm. Figure 8 shows corresponding results using image analysis to provide ink speck data from handsheets prepared by deinking the three feedstocks. For 100% ONP, increased sodium silicate dose produces a marked reduction in speck count as shown in figure 8a, which correlates with the increased brightness observed in Figure 7. In contrast, for 100% OMG (coated, with ash content 30%), increased silicate dose produces a higher ink speck count (Figure 8b) together with reduced handsheet brightness (Figure 7). 28

46 150-0 Light Specks Dark Specks C J L Sodium silicate (%) (b) El Light Specks Dark Specks Sodium silicate %) 0 Light Specks Dark Specks Sodium silicate (%) Figure 8: Effect of sodium silicate addition on ink speck count as observed by image analysis (a) 100% ONP (b) 100% OMG (coated, ash content -30%) (c) 70:30 ONP/OMG (coated, ash content -30%) mixture. 29

47 Figure 8c shows that the ink speck count decreases with silicate dose for the 70:30 ONP/OMG (coated, with ash content 30% ) mixture, although there is little change in brightness. Figure 8 also shows that the ink speck count is significantly higher for deinked samples from OMG compared to ONP, although the brightness shows the reverse trend. Figure 8 shows that calculated values (derived from weighted average) for ink specks lie below the actual values which are obtained with the mixtures of magazine and newsprint when pulped and deinked together, while the observed brightnesses are below the calculated values. This suggests that the efficiency of deinking of newsprint is not enhanced by the addition of magazines for this particular system Effect of Alkali Darkening on Newsprint (100% ONP) Figure 9 shows the influence on handsheet brightness of 100% ONP with increasing sodium hydroxide concentration applied with and without the introduction of sodium silicate (1%). 0 With 1% sodium silicate Without sodium silicate NaOH (%) Figure 9: Effect of addition of sodium hydroxide (1-3%) and sodium silicate (0-1%) on final handsheet brightness for deinking of 100% ONP. Pulping conditions: 1% H202, NaOH (1-3%), Na2SiO3 (0-1%), 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%; Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 8.5; hardness (CaCO3) 200 PPm. 30

48 In the absence of sodium silicate, brightness reaches a maximum value corresponding to 2% applied sodium hydroxide. At this alkali level, the corresponding profile for speck count in Figure 10b shows that ink removal reaches a limiting value. The decrease in brightness observed in figure 9, in the absence of sodium silicate, is likely due to alkali darkening (a) 40-0 Light Specks Dark Specks ch' 30 cip NaOH (%) (b) Light Specks Dark Specks FL., NaOH (%) Figure 10: Effect of addition of sodium hydroxide (1-3%) and sodium silicate (0-1%) on ink speck count by image analysis for deinking of 100% ONP (a) With 1% sodium silicate (b) Without sodium silicate. 31

49 Increased values of b* (Figure 11), which is an indication of yellowness, can be correlated to the decrease in brightness observed in Figure 9. A negligible change was observed in b* values when sodium silicate was present in the system. 80 I=1 Brightness (%ISO) b* NaOH (%) Figure 11: Effect of sodium hydroxide addition on final brightness and b* values for 100% ONP, in absence of sodium silicate This suggests that the pulp yellowing effect was prominent at high alkali concentrations. With sodium silicate present (Figure 9), the final brightness increases with the sodium hydroxide dose, while the ink speck count decreases over the range of alkali addition studied (Figure 10a). In summary, in the absence of sodium silicate, brightness decreases at sodium hydroxide addition above 2% concentration due to alkali darkening (pulp yellowing). In the presence of sodium silicate, an enhancement in brightness and ink removal occurs at increasing sodium hydroxide addition. It is believed that sodium silicate may be enhancing the stability of hydrogen peroxide which offsets the alkali darkening effect associated with high alkalinity. 32

50 2.4 Effect of Calcium Ions in Flotation Deinking of Newsprint and Coated Magazines Turvey25 and Larsson et a126 reported that, when fatty acid collectors are used, Ca 2+ ions are added during the flotation. The Ca2+ form the insoluble soap of the fatty acid which precipitates onto the ink particles. This gives the ink particles the hydrophobic properties (low zeta potential). As a result, efficient ink removal results25,26. Figure 12a shows that calcium ions, initially present as CaCO3 originate from the surface coating of magazines. However, under acidic flotation conditions calcium carbonate is much more soluble. ph was found to make little difference to the calcium concentration obtained from ONP alone. This is not surprising considering the low ash content of newsprint (approximately < 0.5%). Figure 12b shows corresponding results with brightness. Under acidic flotation conditions, 100% OMG (coated, with -30% ash content) shows a significant enhancement in brightness, while 70:30 ONP/OMG (coated, with -30% ash content) mixture shows some improvement. Expectedly, the brightness of 100% ONP (0% OMG) is found unaffected, as there is no coating material (fillers) present in the furnish. At high acidic flotation ph, significant foaming (vigorous bubble formation) was observed with 100% OMG stock. This could be due to the evolution of carbon dioxide arising from calcium carbonate (coating) and acid interaction. The observed enhancement in brightness at high acidic flotation ph and high calcium carbonate concentration, could be due to vigorous bubbling action resulting from carbon dioxide evolution (efficient attachment of ink particle and air bubble). It is unlikely to have brightness enhancement with increased concentration of CaCO3 (>200 ppm), as most studies reported that excessive CaCO3 concentration (>200 ppm) has detrimental effect on ink removal caused by ink redeposition effect

51 Calcium carbonate (ppm) I) O 0% OMG O 30% OMG 100% OMG Brightness ( % ISO) (b) O 0% OMG O 30% OMG 100To OMG ph 7.5 0_ Figure 12: Effect of ph on (a) Concentration of calcium ions (b) Brightness, using various ratios of ONP/OMG. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C. 34

52 2.5 Conclusions Our studies show that during flotation deinking of newsprint and coated magazines, increasing the proportion of glossy magazines (of ash content -30%) in the furnish results in a deinked pulp with higher brightness. However, brightness after pulping showed the reverse trend. Consequently, the sequence of brightness at very short flotation times may not reflect the final sequence after extended flotation times. The addition of lower quality magazines (of lower ash content -9%) to newsprint in the deinking furnish resulted in a lower final brightness, suggesting that the magazine type and quality has a large influence on the overall brightness gain during deinking. No evidence could be found to support the idea that ash components from the magazines facilitate the removal of ink from newsprint. Addition of sodium silicate leads to higher brightness and reduced ink speck count for deinking of newsprint. The reverse trends were observed for magazines (coated with -30% ash content). 35

53 REFERENCES 1. Higgins, H.G., Tappi, 75(3), 99, (1992). 2. Barassi, J., and Welsford, J., Appita, 45(5), 308, (1992). 3. Ferguson, L. D., Tappi, 75(8), 49, (1992). 4. McCool, M. A. and Silveri, L., Tappi, 70(11), 75, (1987). 5. Shrinath, A., Szewczak, J. T. and Bowen, J. I., Tappi, 74(7), 85, (1991). 6. Carr, W. F., Tappi, 74(2), 127, (1991). 7. Paraskevas, S., Tappi Seminar Notes, 41, (1989). 8. Woodward, T. W., Tappi Short Course, Chemical Processing Aids, 85, (1991). 9. Schriver, K. E., Bingham, S. J. and Fraizer, M. W., Tappi Pulping Conference, 133, (1990). 10. Liphard, M., Schreck, B. and Hornfeck, K., Pulp and Paper Canada, 94(8), 27, (1983). 11. Borchardt, J. K., Tappi, 76(11), 147, (1993). 12. Mathur, I., Pulp and Paper Canada, 94(10), T310, (1993). 13. Maughan, S., Appita Conference, 345, (1994). 36

54 14. Marchildon, L., Daneault, C., Lapointe, M. and Pelletier, C., Pulp and Paper Canada, 94(5), T145, (1993). 15. Ferguson, L. D., Pulp and Paper Canada, 94(4), T86, (1993). 16. Renders, A., Tappi, 76(11), 155, (1993). 17. Read, B. R., Tappi Pulping Conference, 851, (1991). 18. Blain, T. J., Tappi Seminar Notes, 113, (1992). 19. Chatterjee, A., Roy, D. N. and Whiting, P., CPPA Technical Section, Annual Meeting, A277, (1992). 20. Letscher, M. K. and Sutman, F. J., Pulp and Paper Science, 18(6), J225, (1992). 21. Ferguson, L. D., Tappi, 75(7), 75, (1992). 22. Forester, W. K., Tappi, 70(5), 127, (1987). 23. Mathur, I., Tappi Pulping Conference, 1015, (1991). 24. Allison, R. W. and Graham, K. L., Pulp and Paper Science, 61(1), J28, (1990). 25. Turvey, R. W., CPPA Recycling Forum, 123, (1991). 26. Larsson, A., Stenius, P. and Odberg, L., Svensk Papperstidning, 18, R158, (1984). 27. Ariadi, B. and Abbot, J., Appita Conference, 339, (1994). 28. Weigl, J., Scheidt, W., Phan-Tri, D. and Gramann, H., PTS Deinking Symposium, 733, (1987). 37

55 CHAPTER 3 EFFECTS OF FILLERS AND DIFFERENT TYPES OF PULPS ON THE PROPERTIES OF THE PAPER 3.1 Literature Review Role of Fillers in Papermaking Process The process of adding mineral matter to paper stock prior to the formation of the sheet has been practised since the ancient days of papermaking. The addition of fillers such as clay, calcium carbonate and titanium dioxide is regarded as an integral part of the papermaking process'. In fact, some paper qualities cannot be obtained without fillers, or they would be more expensive to achieve. Fillers are highly desirable in printing papers because they increase the opacity, raise the brightness and generally improve the printing properties'. The clay particles are more readily wetted by ink than the fibres and the clay will also produce more and finer capillaries in the sheet'. The application of fillers is especially important when opacity is needed at a low basis weight and they are invaluable in packaging grades where low permeability should be combined with opacity for light protection of the foodstuff 1 Other characteristics imparted to paper through filling are improved softness, improved dimensional stability and more rapid absorption of water and other liquids'. The principal fillers used are clay, calcium carbonate, talc, titanium dioxide, zinc sulfide, calcium sulfate, diatomaceous silica and blanc fixel. Generally, clay, calcium carbonate and titanium dioxide are the most widely used fillers. Ideally, fillers and pigments should have a high degree of whiteness, a high index of refraction, small particle size, low solubility in water and low specific gravity'. Fillers may be used in almost all paper grades, but especially in those grades where the optical properties and 38

56 the printability are more important than the strength'. The use of a high percentage of filler will also result in some undesirable effects, principally a decrease in strength'. A normal filler addition may be from 10 to 15% but up to 30% filler may be used provided the filler is cheap' Introduction to Paper Testing Although all papers are made from essentially the same raw material, there is considerable variation in the properties of commercial papers. Some of these variations are intentionally introduced by the papermaker2. Some properties are desirable in some papers and very undesirable in others. The papermaker has many ways of changing the properties of paper. However, the various properties are interrelated. The good papermaker tries to choose the combination of various properties which gives the best paper for the intended use2. The characteristics of paper can be classified as physical, optical, chemical, electrical or microscopical. The physical characteristics include the common strength tests of tensile, burst, tear and fold as well as such tests as stiffness, hardness, softness, density, weight and thickness. Optical properties include light transmittance, light absorption and light reflection, which are measured as opacity, brightness, gloss and colour. Some fundamental properties of paper are density, dielectric constant, strength, colour, hardness, light absorption, tensile strength and thickness Optical Properties of Paper The appearance of paper depends upon its optical properties and much paper is sold on the basis of its appearance. The most important optical properties are colour, brightness, opacity and gloss. Important factors 2 in determining the optical properties of the finished paper include (1) type of pulp used, (2) amount of bleaching, (3) presence of fillers or surface coatings, (4) presence of dyes or coloured pigments, 39

57 (5) method of stock preparation and sheet formation, (6) presence of minor ingredients such as rosin and starch (7) finishing operations that alter the surface of the sheet Kubelka - Munk Theory Although brightness is a useful parameter for specifying the end use properties of paper, it is not a fundamental quantity but rather a complicated function of two more basic phenomena, light scattering and light absorption. Light scattering occurs when incident light strikes an internal or external fibre surface in paper while light absorption arises from the absorption of light by chromophoric substances. In 1931, Kubelka and Munk derived a cumbersome and incomplete set of equations to describe the relative contributions of light scattering and light absorption to brightness measurements. Kubelka6,7 (1948) was later able to provide simplified and more practical forms of the so-called Kubelka-Munk equations, allowing light absorption and light scattering effects to be calculated from brightness measurements. Derivation of the Kubelka-Munk equations is based on the scattering and absorption of light in successive thin layers of homogeneous materiai 3,6,7,8 (Figure 1). The material is assumed to have a mass per unit area (grammage) of W kg/m 2 and each thin layer has a grammage of dw kg/m2. Two quantities, S and K, representing light scattering and light absorption phenomena respectively are defined so that if the surface layer of material is exposed to light of intensity I, a fraction of light I.S.dW is scattered while the amount I.K.dW is absorbed in the layer. The so-called light scattering (S) and light absorption (K) coefficients are both expressed in units of (grammage) -1 i.e. m2/kg. The remaining light not absorbed or scattered in the surface layer continues through to the next layer where the absorption and scattering processes are repeated. 40

58 Incident light of intensity, I Scattering, I.S.dW Absorption, I.K.dW Transmitted light, I-I.(K+S).dW Figure 1: Diagram showing the physical basis for derivation of the Kubelka-Munk equations. K and S represent light absorption and scattering coefficients respectively (units m2/kg) and W represent the mass/unit area of the surface (units kg/m2) Kubelka-Munk Equations The complete derivation of the Kubelka-Munk equations from first principles is complex and results in many forms of equations. However, only a few have practical applications. An excellent summary of the Kubelka-Munk equations as applied to the optical properties of paper has been presented by Robinson 9. The most practical forms of the equations are presented in equations 1-3. SW = R. 2. inf In (I OR inf 1. R2 inf 1 --D (I) K 1 ( 1 l + =_ S 2 R. inf +R. =a inf (2) 41

59 1 1 R 2 R. i nf inf =Va2 1 ( 3 ) where Rinf = Ro = C2. W = K = reflectance of an infinitely thick pile of sheets reflectance of a single sheet on a background opacity = Ro/Rinf grammage = mass/unit area (kg/m2) absorption coefficient (m 2/kg) S = scattering coefficient (m 2/kg) When measuring the light absorption coefficients of papersheets using the Kubelka- Munk equations, care must be taken to ensure that the sheets conform to three 'rules' arising from theoretical considerations during derivation of the equations 6,7,8. The three rules are: (a) (b) (c) that the optical density of the sheet must not to be too low that light must not be substantially absorbed before scattering that light scattering must not dominate over light absorption Effects of Fillers and Pulps on the Optical Properties Opacity The optical properties of paper are governed by the same rules of physics as other materials. The basis of Kubelka-Munk theory is the assumption that two parameters, the specific absorption coefficient (K) and the specific scattering coefficient (S), can be used to explain both brightness and opacityl. According to Kubelka-Munk, the opacity will depend upon the number of individual particles within the sheet, that is, basis 42

60 weight (grammage) and on the refractive index of these particles and the surrounding mediuml. Some of the factors such as apparent density, bonding, formation, beating, calendering, amount and type of filler, amount of lignin, presence of coloured pigments, starch can have major influences on the opacity. It also depends on the geometry of the measuring instrument and wavelength of light being used to measure opacity 2. The increase in opacity that results from filling is due to an increase in the scattering coefficient of the paper. In filled paper, pigment and fibre scatter light independently of each other30. There are three interfaces involved, namely, fibre-air, pigment-fibre and pigment-air2. Reflection and scattering are greatest at the pigment-air interface. Less scattering occurs at the pigment-fibre interface. In pigment filled papers, most of the surface area of the pigment is not in optical contact with the fibres and hence most of the scattering occurs at pigment-air interfaces 2. Anything that increases the number of pigment-fibre interfaces at the expense of pigment-air interfaces will lower the opacity 2. For a given pulp, the opacity is a direct function of the fineness of the fibres or fibrediameter2. Fibre length does not affect the number of transverse fibre-air interfaces because nearly all fibres lie in the plane of the sheet and light passes through them transversely 2. However, the effect of fibre size on the specific surface and opacity are found to be interrelated 31. Because of inherent light-absorption and light-scattering powers, each major grade of pulp has an individual effect on the opacity 2. The opacity obtainable with any pulp depends on the degree of cooking, the amount of beating and pressing and other papermaking factors that affect the degree of bonding in the sheet2. In general, it may be stated that the lower the reflectivity of the pulp from which the paper is made, the higher the opacity of the paper2. 43

61 Light Scattering Coefficient Figure 2 shows that the relationship between scattering coefficient and percent filler in the sheet is not rectilinear'. The filler seems to become less efficient as the amount in the sheet is increased'. However, according to Staff and Youngl, this trend is not universally true. Hemstockl I has pointed out that scattering coefficient of the filler in the papersheet is dependent upon the combination of two factors, essentially pigment and the type of pulp. In practice, it is difficult to assign a definite scattering coefficient to a given filler'. It will depend on the processing conditions under which the filler is applied'. Different pigments and different pulps have different scattering coefficient values (Table 1). m 21 k g 100 Ti 02 Anatase 80 Sca tte r ing coe f fic ien t 60,' Synthetic Ca-silicate. ".. Coating clay Fitter day 40 // V /. '/ Filler in the sheet by weight Figure 2: Effect of different fillers on light scattering coefficient (after Barnet") 44

62 Table 1: Specific scattering coefficient for pulp and pigments (after BOhmerl.) Type of filler / pulp Scattering coefficient (m 2/kg) Chemical pulp Mechanical pulp Precipitated CaCO Ground CaCO Coating clay Filler clay Anatase TiO Rutile TiO Brightness White pigments such as calcium carbonate, titanium dioxide and zinc sulfide can be used to increase the brightness of paper. The increase in sheet brightness obtained by filling depends partly on the original brightness of the pigment and partly on the particle size and "hiding power" of the pigment'. However, the addition of filler will not always increase the brightness'. The effect obtained depends upon the extent to which the pulp has been beaten'. Pigments improve the ageing characteristics of paper. This may be important when mechanical pulp is used because of the colour instability of the pulp. The pigment will, however, be more efficient if applied in a coating layer'. Figure 3 shows the effect of different fillers on the brightness of printing paper made from low-brightness mechanical pulp'. 45

63 , 65 TiO2 Anata se Synthetic At- hydrate Coating "--- Clay....../...,.../..--./.../ Filler ' :: Ctay./ /...c.: " *4 Filler in sheet Figure 3: Effect of different fillers on the brightness of printing paper made from lowbrightness mechanical pulp (after Bohmerl) Effect of Fillers and Pulps on Strength Properties Effect of Fillers on Strength Properties The price paid for the improvement of the optical properties of the paper through the addition of filler is a significant loss in the strength'. There are two reasons for this loss: (1) Paper with and without fillers are compared at the same basis weight. Consequently, a sheet containing filler will contain less fibre and will accordingly be weaker I. (2) The strength of paper is due mainly to fibre-fibre bonds. The pigment particles occupy space between the fibres and, thus, interfere with the fibre bonding. This leads 46

64 to an increased number of fibre-air and pigment-air interfaces, which is the main reason why the opacity is increasedl. Since the strength reduction is related to the reduction of bonds between the fibres, different strength properties may not be reduced to the same extent'. It is well known that tensile and bursting strength follow the same trend 12. It is assumed that the addition of fillers will have approximately the same effect on the tensile strength, the bursting strength and folding endurance 13. The tear strength may, however, be affected differentlyl. It has been frequently observed that tensile and tear are inversely related'. Davidson 14 has reported that small amount of the pigment known as whiting gave an increase in the strength. Aarefjord 13 found, however, that several fillers give a small but consistent reduction of the tear, as shown in figure 4. r4e Te ar in dex m N % Filler in sheet Figure 4: Effect of fillers on tear index. Different symbols represent different fillers (after Aarefjord 13) 47

65 The addition of filler has a much greater effect on tensile, bursting strength and folding endurance'. Essentially, the effect of fillers on strength properties depends on several variables used in the papermaking process". If high percentages of fillers are used, the strength may be seriously impaired. There are two ways to improve this situation. The furnish may be changed by incorporating more high-strength fibre into the sheet, or the filler content may be reduced by use of a more efficient filler'. Reduced filler content means less strength loss because the filler occupies less volume in the sheet and causes less interference with the internal bonding of the sheet' Effect of Different Types of Pulps on Strength Properties Mechanical pulp is widely used in printing papers due to its good printability and low raw material costs 15. In order to enhance the strength properties and runnability of the wet and dry paper web, chemical softwood pulp is added to the furnish 15. The amount added varies from 0-15% in newsprint. In order to keep the raw material cost acceptable, the addition of expensive chemical reinforcing pulp should be kept to the minimum level that gives tolerable strength properties 15. Retulainen 15 reported that blending two pulps rarely gives a linear change in paper strength properties with the blend ratio. This is especially true with mechanical and chemical pulps 15. The same study concluded that the effect of different pulp blends on strength properties is not a straightforward relationship. It is due to the complexity of 48

66 the mechanism involved in the process 15. The predictability of the resulting paper properties of the blend depends upon the component properties and the processing conditions such as pulping, drying, calendering 15, Effects of Fillers on Porosity, Bulk and Surface Smoothness Porosity An indication of porosity can be obtained by measuring the resistance of a paper sample of given dimensions to the passage of air under standardized conditions of pressure, temperature and relative humidity2. The results are expressed in arbitrary units as the time taken for the passage of a given volume of air or as the amount of air passed in a given period of time 2. This measurement is frequently termed as "porosity", but it is reported as air resistance 2. Air resistance measurements are commonly used as a control test for paper manufacture because of the indirect correlation between porosity, formation and strength of the paper2. Normally, strength is directly related to porosity, although there are some exceptions reported in the literature 32. Generally, air resistance decreases with increasing amounts of filler. The bulky fillers such as calcium carbonate cause the greatest decrease in air resistance 2. A study 33 found that low percentages of fillers reduce the air resistance by reducing fibre bonding but that high percentages of fillers increase the air resistance by plugging the sheet Bulk Due to higher specific gravity of filler (compared with fibre), an addition of a filler would give a paper of lower bulk'. However, the literature on this subject appears divided34,35. A small addition of filler may actually increase bulk' (figure 5). This 49

67 effect is noticeable in particular with short-fibred pulpl. An opposite effect has also been observed i. e. a reduction of bulk at high filler levels mil/kg 1.5 Birch Sulfate 1.3 Spruce Sulfite % Clay Figure 5: Bulk as a function of percent clay in the sheet (after Barnett) Surface Smoothness It is well known that the addition of filler improves the printing properties of the papersheet I. This improvement can be attributed to different phenomena that result from the filler addition'. Most pigments have a greater affinity for printing inks than the fibre surface'. The presence of pigments creates a finer pore structure that increases the rate of oil absorption'. Different pigments show different degrees of oil absorption 36. Fillers generally seem to improve the smoothness of the paper after calendering This is probably because the finer particles tend to reorient under the influence of the calendering rolls and to fill the voids between the fibresl Effects of Recycling on the Properties of Secondary Fibre Effect of Recycling on Strength Properties During the last thirty years, the effect of recycling on pulp properties has been investigated in many different institutions and countries. With the increasing use of 50

68 secondary fibre, it has become more important to know how the fibre affects the properties of the product. Such information is essential for planning the use of a limited quantity of fibre to obtain the maximum benefit at the least cost. The effects of recycling on strength properties of different pulps are well documented in the literature 1622 One impediment to the wider use of recycled fibre is the loss of strength that occurs when these materials are repulped 19. These losses in strength were related to the fibre morphology of the species and the number of times the fibre was recycled24. It was found that different pulp types showed different recycling effects 16. After recycling, mechanical pulp fibres became flatter and more flexible giving a denser, stronger sheet 16, While beaten chemical pulp fibres "hornified" (loss of swelling) resulting in a bulkier, weaker sheet 16. The same study concluded that the effects of recycling occur at different rates in different pulps 16. For a mechanical/chemical blend, overall effects were dictated by the net result of these different rates 16. There are several studies, which examined the effects of different techniques for enhancing the strength of secondary fibre 19. Repulping under alkaline conditions and refining are the most commonly used methods to improve the strength of secondary fibres 19. The quality of secondary fibres for papermaking depends on the treatment these fibres have received during previous papermaking processes, in addition to the recycling process itself23, Effect of Deinking on the Properties of Secondary Fibre The use of recycled deinked fibre in the paper industry has increased significantly in recent years 27,28. Unlike the studies on recycling of different pulps, there are few publications which discuss the effect of deinking on the properties of secondary fibre17,18,25,26. It is common practice to include about 30% coated magazines in the deinking of newsprint. The inclusion of magazines provides a source of chemical pulp fibres which can offset losses in strength properties in recycling 21. There has been a 51

69 significant recent interest in attempting to improve the quality of recycled fibre with regard to strength, optical and physical properties 17,18,25,26. Some studies reported that the strength properties of deinked pulps depend on the type of deinking process used In the deinking of different stocks of magazines, it was observed that washing and flotation showed a significant difference in the strength properties (Figure 6). This difference was attributed to the different degrees of extraction of ash and fines 18, SO 14A.GA2 NES C PO 25 I. ASH n. ASH MAGA2 NES C PO MAGAZ NES CPO 25% ASH P. ASH ZS ASH 21. ASH Figure 6: A comparison of strength properties of deinked magazine stock using flotation and washing processes (after Pfalzer 25 ) 52

70 REFERENCES 1 BOhmer, E., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 2 Brandon, C. E., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 21, Properties of Paper, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 3 Robertson, G. J., in "Fundamentals Of Paper Performance", Chapter 9, 'General and Theoretical Concepts of the Optical Properties of Paper', Appita Technical Association, (1985). 4. Lundqvist, M., Svensk Papperstidning, 82(1), 16, (1979). 5. Axegard, P., Moldenius, S. and Olm, L., Svensk Papperstidning, 82(5), 131, (1979). 6. Kubelka, P., J. Opt. Soc. Amer., 38(5), 448, (1948). 7. Kubelka, P., J. Opt. Soc. Amer., 38(12), 1067, (1948). 8. Teder, A. and Tormund, D., Trans. Tech. Assoc. CPPA, 3(2), TR 41, (1977). 9. Robinson, J. V., Tappi, 58(10), 152, (1975). 53

71 10. Staff, R. E. and Young, R. H., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 11. Hemstock, G. A., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 12. Beazley, K. M., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 13. Aarefjord, T., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 14. Davidson, R. R., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 15. Retulainen, E., Paperi ja puu, 74(5), 419, (1992). 16. Howard, R. C. and Bichard, W., Pulp and Paper Science, 18(4), J151, (1992). 17. Claydon, P., Newsprint Conference, 121, (1989). 18. Helmling, 0., Siiss, H. U. and Eul, W., Tappi Pulping Conference, 407, (1986). 54

72 19. Bhat, G. R., Heitmann, J. A. and Joyce, T. W., Tappi, 74(9), 151,(1991). 20. Koran, Z., Tappi, 77(6), 167, (1994). 21. Chatterjee, A., Roy, D. N. and Whiting, P., CPPA Technical Section, Annual Meeting, A277, (1992). 22. Howard, R. C. and Bichard, W., Recycling Forum, 81, (1991). 23. Gratton, M. F., Recycling Forum, 65, (1991). 24. Bobalek, J. F. and Chaturvedi, M., Tappi, 72(6), 123, (1989). 25. Pfalzer, L., Tappi, 63(9), 113, (1980). 26. Wang, H. and Jan, I.Y., Appita Conference, 121, (1991). 27. Higgins, H. G., Tappi, 75(3), 99, (1992). 28. Barassi, J. and Welsford, J., Appita, 45(5), 308, (1992). 29. Ferguson, L. D., Paper Technology, 33(10), 14, (1992). 30. Doughty, R. H., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 21, Properties of Paper, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 55

73 31. Parsons, S. R., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 21, Properties of Paper, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 32. Harpham, J. A., Reid, A. R. and Turner, H. W., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 21, Properties of Paper, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 33. Lane, W. H., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 21, Properties of Paper, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 34. Beazley, K. M. and Petereit, H., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 35. Riddell, M. C., Jenkins, B., Rivers, A. and Waring, I., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience,'New York, (1981). 36. Shaw, M. B. and Simmons, R. H., in "Pulp and Paper - Chemistry and Chemical Technology", Chapter 15, Filling and Loading, Casey, J. P. (Ed.), Volume 3, 3rd Edition, Wiley-Interscience, New York, (1981). 56

74 CHAPTER 4 EFFECTS OF DEENKING ON OPTICAL AND PHYSICAL PROPERTIES OF SECONDARY FIBRE AFTER PULPING AND FLOTATION 4.1 Introduction There has been significant recent interest in attempting to improve secondary fibre quality with respect to its strength and optical properties. It is believed that paper looses its strength when it is recycled. However, this traditional view is being challenged by modern research. Effects of recycling on different types of pulp fibres are well documented in the literature 1-3. Unfortunately, the results of these studies have often been conflicting. Ferguson 3 reported that mechanical pulp showed improved strength properties with recycling because the fibres become progressively flatter and more flexible in the alkaline environment of recycling. On the other hand, Howard and Bichard 2 reported that different pulp types showed different recycling effects depending upon the type of recycling system used. Some commercial recycling (deinking) processes use 30% coated magazine in flotation deinking of newsprint 4,5. The addition of magazine has been found to improve the optical and strength properties of recycled paper, because of the introduction of fillers and chemical fibre. However, the literature appears to be divided on the role of magazines in deinking of newsprint 6-8. Effects of deinking on the properties of recycled fibre have been reported in the literature8-15. However, most studies focus on comparison of virgin fibre and deinked fibre. The influence of deinking on paper properties arises principally 57

75 from ash (filler) loss, fractionation of fibre (type of pulps) and the ink. These factors can have different influences on the properties of recycled paper during different stages of deinking. Due to the presence of several such components in this system, different interactions may be taking place at the same time thus adding to the complexity of the whole system. Some studies reported that the strength properties of deinked pulps depend on the type of deinking process used 9, I 3. In deinking of different stocks of magazines it was observed that washing and flotation showed a significant difference in the strength properties9,13. This difference was attributed to the different degrees of extraction of ash (fillers) and fines 9,13. However, these studies have not investigated the effects taking place at the pulping stage. Most of these studies have not addressed the role of different types of pulps, the ink, fillers and fines in the properties of recycled paper during different stages of deinking. In order to have a better understanding of the many components (fillers, fines, the ink and the type of pulps) involved in the deinking system, this study investigates the.role each component has in regards to the properties of recycled paper at different stages of deinking. This study also investigates the dominating effect exerted by these components at different stages of deinking, and their resultant effect on each property of recycled paper. To our knowledge, no study has been reported which discusses comprehensively the effects of deinking on optical and physical properties of recycled paper after the various stages of deinking such as pulping and flotation. 58

76 4.2 Effect of Flotation Time on the Properties of Recycled Paper The results discussed in this study were obtained using ten minute flotation time. This study found that extended periods of flotation (greater than ten minutes) have no significant effect on the properties investigated. This effect is illustrated in Figure 1. However, a study by Mahagaonkar and Abbot shows a further improvement in brightness after extended periods of flotation 6 (chapter 2, Figure 2). 59

77 STRETCH ( %) FREENESS ( C.S.F.) E 500 (a) (c) tai a FLOTATION TIME (min) FLOTATION TIME (min) FLOTATION TIME (min) it -01 E 480 z E-1 z 440 E-4 E-4 eg u 400 cr.a u (d) FLOTATION TIME (min) Figure 1: The effect of extended flotation time on (a) Freeness (b) Bulk (c) Stretch (d) Light scattering coefficient. 0, 0% OMG;, 30% OMG;0, 80% OMG;411, 100% OMG. 60

78 4.3 Effect of Deinking on Ash Content Table 1 presents results of an elemental analysis of magazine (100% OMG) and newsprint (100% ONP) after pulping and flotation stages. The results of the elemental analysis indicate that the deinked pulp contains filler from several sources including aluminium silicates (clay), titanium dioxide and calcium carbonate. A major change in the composition of the pulp was observed to occur during the flotation process. Large amounts of fillers and fines were lost from the pulp during flotation. Changes in the composition of the ash were also observed with a decrease in the level of % CaO, % Si02 and % Al203 (Table 1 on next page). 61

79 Table 1: Elemental analysis of magazine and newsprint after pulping and flotation Deinking stage % OMG % Ash %% _ S03 Si02 % Al203 % MgO % CaO % Na20 % FeO % TiO2 Pulping Flotation Pulping Flotation _

80 Figure 2 shows that the total amount of filler retained in handsheets prepared after pulping and flotation (deinking) was proportional to the amount initially present. The initial ash contents of the furnishes are also shown individually for comparison. The linear relationship observed between the percentage of OMG and the ash content of the pulp is due to the addition of fillers and coating originating from the magazines. The lowest ash level was obtained with 0% OMG (100% ONP) indicating a negligible percentage of fillers present in the newsprint. A 10% reduction in the ash content of the 100% OMG furnish was observed to occur after the pulping stage (Figure 2). 30 After pulping After flotation a Furnish % OMG Figure 2: The effect of deinking on Ash content. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 8.5; hardness (CaCO3) 200 ppm. This result is likely to have occurred from a loss of fillers and fines during the sheetmaldng process. It is anticipated that some loss of calcium carbonate would occur at acidic papermaking conditions (handsheets were prepared at ph 4.5). 63

81 A significant foaming was observed on repulped stock of 100% OMG. This was presumably due to the evolution of carbon dioxide resulting from carbonate-acid interaction. Though these conditions (acidic ph) were less than ideal for samples containing calcium carbonate, this ph was chosen because it reflects the typical industrial papermaldng conditions. Figure 3 shows the correlation between brightness gain in pulping and flotation stages and the amount of ash lost between these two stages. An increase in brightness was observed to occur with increasing ash (% OMG). However, it does not suggest that removal of the clay (magazine coating) leads to high ink removal efficiencies. BRIGHTNESS GAIN ( %ISO) ASH LOSS (%) Figure 3: The effect of deinking on Ash loss and Brightness. Pulping conditions: 1% H202, 1% NaOH, 1% Na2SiO3, 0.4% surfactant, 0.2% DTPA; pulping time 20 minutes, temperature 50 C, pulp consistency 8%. Flotation conditions: Pulp consistency 1%, temperature 50 C, ph 8.5; hardness (CaCO3) 200 ppm. 64

82 Mahagaonkar and Abbot showed that in flotation deinking of newsprint, increasing the proportion of magazine in the furnish results in a deinked pulp with higher brightness6 (chapter 2). The same study 6 (chapter 2) reported that no evidence could be found to support the idea that ash component from magazine facilitates the removal of ink from newsprint. In figure 3, the observed increase in brightness with increasing ash (To OMG) could be due to the combined effect of several factors: (1) Despite the ash losses, significant ash (filler) retention in deinked pulp adds to the brightness of the deinked pulp 8 (due to white fillers like titanium dioxide and calcium carbonate) (2) Ink removal at the flotation stage (3) Chromophore removal due to bleaching effect occurring in the pulping stage 16,17 (4) Retention of brighter chemical pulp originating from magazine Effect of Deinking on Optical Properties Brightness Figure 4 shows the effect of deinking on brightness. After the pulping stage, as the proportion of magazine (% OMG) in the furnish is increased, there is a significant drop in brightness. With the high proportion of filler (originating from OMG) in the furnish, one would expect repulped stock to have higher brightness. However, 65

83 80 After pulping o After flotation BRIGHTNESS % OMG Figure 4: The effect of deinking on Brightness. the observed decrease in brightness can be attributed to the increased ink content (originating from magazine). It also suggests that the effect exerted by the ink predominates over that of the filler. A study by Mahagaonkar and Abbot 6 has shown this effect (in chapter 2, Figure 1). After flotation, the observed increase in brightness can be attributed either to removal of ink, filler retention 8 (white filler like titanium dioxide and calcium carbonate), and chromophore removal, i.e. bleaching effect occurring at the pulping stagei 6,17 and/or retention of brighter chemical pulp 8 (originating from OMG) in the deinked pulp. 100% ONP (0% OMG) showed very little change in brightness suggesting that either (1) little or no ink removal or (2) the alkaline darkening process occurring at high ph in mechanical pulp may be offsetting the brightness gains that occur with ink removal and any bleaching that may have occurred. 66

84 4.4.2 Light Absorption Coefficient Figure 5 shows the effect of deinking on light absorption coefficient. After the pulping stage, the observed increase in the light absorption coefficient with the addition of OMG can be attributed to the magazine component carrying a higher ink load than ONP. Any bleaching, which occurs during this stage, serves to remove chromophores and therefore should decrease the light absorption coefficient. Generally, white fillers like calcium carbonate and titanium dioxide are used to increase the brightness of paper 18,19, as a result light absorption coefficient decreases. However, despite the presence of such fillers in the furnish (Table 1), the effect of higher ink content appears to predominate over the filler effect, resulting in increased light absorption coefficient as the paper became darker. '; E-( After pulping o After flotation C.) W4 0 0 c-) 40 - E* a., cf) % OMG Figure 5: The effect of deinking on Light absorption coefficient. 67

85 After flotation, because of the removal of the ink, reductions were observed in light absorption coefficient. The furnish with 0% OMG (100% ONP) showed very little change in light absorption coefficient as it showed in brightness (Figure 4). The trend showing decreasing brightness with increasing light absorption coefficient was found to be unaffected in both stages of deinking (Figure 6). Though the slopes of the relationship are different for the two stages. 100 BRIGHTNESS ( %ISO) ABSORPTION COEFFICIENT (cm2 /g) Figure 6: The relationship between Brightness and Light absorption coefficient Light Scattering Coefficient Figure 7 shows the effect of deinking on light scattering coefficient. After the pulping stage, despite the large amounts of clay in the OMG, the light scattering coefficient of the pulped OMG was less than that of ONP. This is likely due to the fibre in the OMG being predominantly chemical pulp in origin and having a lower light scattering coefficient than mechanical type pulp present in ONP. 68

86 ria 500 cj E-4 ;: U (-7 c/) After pulping o After flotation % OMG Figure 7: The effect of deinking on Light scattering coefficient. 0 A greater decrease in scattering coefficient was observed after the flotation stage with the addition of OMG. This is due to the change in the proportions of chemical fibre and filler present in the system. Loss of filler that occurs in the flotation stage enhances the effect of chemical pulp originating from OMG Opacity Figure 8 shows the effect of deinking on opacity. The observed increase in the opacity with the addition of OMG after pulping can be attributed to increasing amount of fillers (originating from OMG). In the paper making process, it is common practice to add fillers for the improvement of the optical properties of the paper 18,19. This is believed to occur because the pigment particles occupy voids between the fibres which leads to an increased number of fibre-air and pigment-air interfaces. As a result, opacity is increased 18,19. 69

87 OPACITY ( %) After pulping o After flotation % OMG Figure 8: The effect of deinking on Opacity. After flotation, reductions were observed in opacity due to the loss of filler and retention of lower opacity chemical pulp (originating from OMG) in the deinked pulp. According to the Kubelka-Munk theory 18,19, opacity is determined by both the light scattering coefficient and the light absorption coefficient and as either increases, opacity will increase. This effect was observed after the flotation stage. However, after the pulping stage, despite the decrease in light scattering coefficient (Figure 7) with increasing percentage of OMG, the increase in light absorption coefficient (Figure 5) contributed to the increase in opacity (Figure 8) Ink Speck Count Measurements Figures 9(a) and 9(b) show ink speck counts after the pulping and flotation stages respectively. After the pulping stage, the ink speck count showed increasing values 70

88 (a) (21 After pulping (3676) DARK SPECK CO UNT (34) (358) (2579) 0 30 % OMG (b) 0 After flotation E (42) (45) c4 30 ^ r:4 15 (18) (5 ) % OMG Figure 9: The image analysis: Ink speck counts (a) after pulping (b)after flotation. with increasing percentage of OMG added as more ink was added into the system. After flotation, despite the highest value of brightness, 100% OMG still showed the highest ink speck count. This suggests that the values of brightness do not 71

89 necessarily represent the true extent of ink removal 6,16,20,21. It appears that the higher the ink content in the deinking system, the greater the final ink speck count. In summary, increased OMG content leads to increased brightness, increased ink speck counts and reduced opacity and light scattering ability after flotation stage. The lowest percentage of ink removal (85%) was observed with 0% OMG (100% ONP) highlighting the poor ink removal from the newsprint. However, 99% of ink removal was achievable with 100% OMG. An addition of 30% OMG in newsprint was found optimum. Interestingly, further addition of OMG could yield only 3% high ink removal efficiency. This could be one of the reasons for selecting 70:30 ratio (newsprint to magazine) in commercial deinking processes. 4.5 Effect of Deinking on Strength Properties Retulainen 22 reported that blending two pulps, particularly a mechanical and chemical pulp, rarely gave a linear change in properties with the blend ratio. The same study 22 showed that the system was complex with several mechanisms functioning simultaneously. Some of them have a positive effect while others have a negative effect on strength properties of the blend, making it difficult to draw a straightforward conclusion 22. The predictability of resulting paper properties depends on the component properties and nature of the system. The properties of paper made from a blend do not usually follow the weighted mean of the components 22. In this chapter the system chosen for deinking is not only composed of two different pulps but it is further complicated by the additional major factors (a) fillers and coating material present in the magazines, (b) ink and (c) chemicals (alkalis) used in the system. Some studies showed that the strength properties like tensile 72

90 index can be affected in different proportions with the use of different alkalis in the bleaching and deinking processes 16,23. A study by Mahagaonkar et a/ 16 concluded that the change in strength properties was due to the hydrolysing effect of individual alkalis used in deinking process which can affect the bridge forming property of the fibre. This will be discussed in more detail in chapter Tear index, Burst index, Tensile index Figures 10(a), 10(b) and 10(c) show the effect of the deinking process upon tear index, burst index and tensile index respectively. After the pulping stage, each strength property decreased with increased percentage of OMG. This effect can be attributed to the presence of filler and coating materials (originating from OMG) in the repulped stock. After the flotation stage, however, an enhancement in all strength properties occurred. It is well known that the addition of chemical pulp to mechanical pulp leads to increase in strength 22 (as chemical pulp is inherently stronger than mechanical pulp). This is mainly due to an increased bonding and an increase in load bearing capacity of the fibrous component22. Addition of filler, on the other hand, reduces strength 18,19 The strength of paper is due mainly to fibre-fibre bonds. Pigments (filler) particles occupy space between fibres and interfere with fibre bonding hence a significant reduction in strength occurs 18,19. Crawford24 and Riddell et a/25found that 10% filler gave a 20 to 25% reduction of tensile and burst. This strength reduction is related to the reduction in fibre bonding potentia1 26,

91 (a ) After pulping o After flotation % OMG a.. t.t v cn 1.4 cc) 0 (b) 0 0 After pulping o After flotation % OMG a G After pulping o After flotation 20 E % OMG Figure 10: The effect of deinking on strength properties (a) Tear index (b) Burst index (c) Tensile index. 74

92 At the pulping stage, despite the addition of chemical pulp from magazine, there was a continuous reduction in all strength properties. This suggests that the filler component was exerting an overriding negative influence and suppressing the effect exerted by chemical pulp. After the pulping stage, the 100% OMG furnish showed the lowest values of burst, tear and tensile because of the high percentage of ash (fillers) while 0% OMG (100% ONP) showed the highest values suggesting that a negligible amount of filler was present in the newsprint. After flotation, due to the loss of a major portion of the filler in the flotation stage, the beneficial effect of the chemical fibre in OMG predominated over the detrimental effect of the filler component. The observed enhancement in all strength properties occurring after flotation, can be explained in terms of the change in the proportions of chemical fibre, mechanical fibre, filler and fines in the two stages. The improvement in tear, tensile and burst indices occurring after flotation may be mainly due to loss of fillers and fines. However, it can also depend upon the combined effect exerted by mechanical fibre, chemical fibre, fillers and fines retained in deinked pulp. The effect of loss of fillers on the overall improvement of strength properties was difficult to quantify. As anticipated, 0% OMG (100% ONP) did not show a significant change in tear and tensile. However, it shows some increase in burst index after flotation. 100% OMG showed the highest tear, tensile and burst, which is as expected, as chemical pulps usually have higher tear, tensile and burst indices when compared with mechanical pulps2,19. This is one of the reasons why magazines are included with newspapers in the deinking process 75

93 4.5.2 Stretch Figure 11(a) shows the effect of deinldng on stretch. Generally, stretch can be expected to follow trends in the tensile index 19 and this was observed after flotation (Figure 11b). After the pulping stage, however, the reverse trend to tensile index was observed. i.e. increasing stretch with increasing OMG. This behaviour was not anticipated and is difficult to explain 2.8 F % OMG TENS ILE INDEX ( N. m/g) STRETCH (%) Figure 11: The effect of deinking on strength properties (a) Stretch (b) The relationship between Stretch and Tensile index. 76

94 4.6 Effect of Deinking on Other Properties Freeness Monitoring and controlling freeness is important in the paper mills because freeness directly affects basis weight, caliper, moisture and strength 28. It is often stated that freeness provides a measure of the speed at which a paper machine can be run 28. To make a consistent, high quality product, freeness must be monitored and controlled before and after refining28. Figure 12 shows the effect of deinking on freeness. In the pulping and flotation stages, freeness increased with increasing percentage of OMG, indicating the presence of higher freeness chemical pulp 2. After flotation, freeness was slightly lower than at the pulping stage for a given percentage of OMG. This drop in freeness cannot be attributed to the loss of fines or filler. 400 Ca) After pulping o After flotation % OMG Figure 12: The effect of deinking on Freeness. If loss of fillers was a key factor in the freeness difference between pulped and floated pulp, (for a given percentage of OMG) then one might expect to see a significant difference in freeness (between pulped and floated pulp) as percentage 77

95 of OMG increases. However, Figure 12 indicates an almost constant drop in freeness at a given percentage of OMG. It suggests that freeness is not influenced by the fillers. Howard and Bichard 2 showed that the pulps recycled with fines retained, showed a drop in freeness (generally pulps with high fines have low freeness). The loss of fines would increase freeness and hence floated stock would be higher than the pulped stock. However, Figure 12 shows an opposite effect (i.e. floated stock have lower freeness than pulped stock). It suggests that, freeness is influenced by the factors other than fines and fillers, such as possible effects of chemicals used in the deinking process. The concentration of chemicals is reduced in the flotation stage due to dilution (by factor 8) compared to pulping stage. Some recent studies showed that despite the presence of fines and filler in the deinked pulp, freeness was found to be influenced by the effects of chemicals such as different alkalis or enzymes employed in deinking process 16, Porosity Figure 13 shows the effect of deinking on porosity. Porosity increased with increasing percentage of OMG at the pulping and flotation stages. This is due to incorporation of chemical pulp into the system. In Figure 13, at a given OMG percentage, the observed drop in porosity between pulped and floated samples cannot be attributed to the loss of filler or fines. 78

96 PO ROSITY (ml/min) After pulping 13 After flotation % OMG Figure 13: The effect of deinking on Porosity. If a loss of filler or fines was a key factor in the porosity drop between pulped and floated pulp, then one might expect to see the increase in porosity drop (between pulping and flotation stages at a given percentage of OMG) with increase of OMG percentage. However, Figure 13 indicates that, porosity drop at a given OMG percentage does not show an increasing trend. This suggests that porosity has been influenced by other factors such as effects exerted by chemicals used in the deinking process rather than loss of filler or fines. A recent study by Mahagaonkar etall 6 showed that porosity of deinkecl pulp can be influenced more by the effects exerted by chemicals (used in deinking process) rather than filler or fines. This will be discussed in more detail in chapter 5). Air resistance measurements are commonly used as a control test for paper manufacture because of the indirect correlation between porosity, formation and strength of the paper 18,19. Normally, strength is directly related to porosityi 8,19. However, there are few exceptions reported in the literature 30. In our study a trend showing high porosity and low strength was predominant in the pulping stage. However, the same trend was not observed after flotation. This effect is illustrated in Figure

97 POROSITY (ml/min) (-1 'RI 2.6 E al x X * CID g I I I POROSITY (ml/min) 800 TENSILE INDEX (N. m/g ) After pulping 13 After flotation POROSITY (ml/min) Figure 14: The relationship between Porosity and Strength properties using different mixtures of ONP/OMG (0%, 30%, 80% and 100% OMG) (a) Tear index (b) Burst index (c) Tensile index. 80

98 4.6.3 Roughness Figure 15 shows the effect of deinking on surface roughness of the handsheets. In the pulping stage as the percentage of OMG increased, roughness decreased linearly as chemical pulps (originating from OMG) generally have a smoother surface than mechanical pulps 18,19. It is well known that the addition of fillers improves the printing properties of the sheeti 8,19,31. Most pigments have a greater affinity for printing inks than the fibre surface 18,19. Filler generally improves the smoothness of paper after calendering 18,19,31. Shaw and Simmons 31 found that incorporation of clay and calcium carbonate into the sheet resulted in a distinct reduction of roughness. The observed reduction in roughness with increasing quantity of OMG at the pulping stage can also be attributed to the fillers originating from OMG. 5.2 ROUGHNESS 5.0 -; After pulping 0 After flotation % OMG Figure 15: The effect of deinking on Roughness. 0 Roughness values for a given OMG content were higher after flotation compared to after pulping. This increase could be due to a loss of fillers in the flotation stage. 81

99 4.6.4 Bulk Figure 16 shows the effect of deinking on bulk. The pulped and floated stocks show a decrease in bulk with increasing % OMG. This is consistent with the introduction of lower bulk chemical pulp and fillers through the OMG furnish. For any given furnish an increase in bulk is generally observed after flotation and this increase is attributed to loss of filler in the flotation stage. However, increased use of chemical pulp resulted in decreased bulk (trend observed in the flotation stage). Scanning electron image of floated sample of 0% OMG (100% ONP, Figure I7a) appears bulkier than floated sample of 100% OMG (Figure 17b). 0 After pulping 0 After flotation % OMG Figure 16: The effect of deinking on Bulk. 82

100 (b) Figure 17: Scanning electron micrographs of floated samples (a) 100% ONP (0% OMG) and (b) 100% OMG. 83

101 4.7 Conclusions Our studies show that, during the process of deinking, the trends observed in strength properties like burst, tear and tensile can be explained viewing the system as being composed of three main components: mechanical fibre, fibre derived from chemical pulp and filler. After the pulping stage, the trends obtained in the strength properties were influenced by the detrimental effects exerted by filler. However, after the flotation stage, an enhancement in all strength properties occurred. This can be attributed to the change in the proportions of chemical fibre, mechanical fibre, filler, and fines in the pulping and flotation stages. Loss of a major portion of the filler enabled the beneficial effect of the chemical fibre to predominate over the detrimental effect of the filler component. Because of the complexity of the system, the effect of loss of fillers and fines on the overall improvement of strength properties was difficult to quantify. Stretch typically increases with tensile in flotation stage but it decreased in the pulping stage. Despite a significant amount of filler in the system, optical properties like brightness and light absorption coefficient were influenced more by the ink content. The light scattering coefficient was found to be dominated by the fibre type. Despite having the highest brightness, 100% OMG had the highest ink speck count. Porosity and freeness were influenced more by the effects exerted by chemicals used in the system rather than fines and fillers. After ten minutes -of flotation, there was no advantage in continuing the deinking process for extended periods, as there was no significant effect on properties of recycled fibre. 84

102 REFERENCES (1). Chatterjee, A., Roy, D. N. and Whiting, P., CPPA Technical Section, Annual Meeting, A277 (1992). (2). Howard, R. C. and Bichard, W., Pulp and Paper Science, 18(4), 151, (1992). (3). Ferguson, L. D., Paper Technology, 33(10), 14, (1992). (4). Schriver, K. E., Bingham, S. J. and Fraizer, M. W., Tappi Pulping Conference, 133, (1990). (5). Read, B. R., Tappi Pulping Conference, 851, (1991). (6). Mahagaonkar, M. S. and Abbot, J., CPPA Conference, Jasper, Alberta, Canada (1994). (7). Letscher, M. K. and Sutman, F. J., Pulp and Paper Science, 18(6), J225, (1992). (8). Rao, R.N., Kuys, K. and Abbot, J., Appita Conference, 357, (1994). (9). Suss, 0., Helmling, H. U. and Eul, W., Tappi Pulping Conference, 407 (1986). (10). Ferguson, L. D., CPPA Recycling Forum, 57, (1987). 85

103 (11). Claydon, P., Newsprint Conference, 121 (1989). (12). Wang, H. and Jan, I. Y., Appita Conference, 121 (1991). (13). Pfalzer, L., Toppi, 63(9), 113, (1980). (14). Michaud, M. G. and Harvey, E. H., Newsprint Conference, 191, (1989). (15). Phipps, J., Paper Technology, 35(6), 34, (1994). (16). Mahagaonkar, M. S., Stack, K. and Dunn, L., Tappi Papermaker Conference, 113, (1995). (17). Renders, A., Tappi Pulping Conference, 233, (1992). (18). "Pulp and Paper Chemistry and Chemical Technology", Chapter 15, 'Filling and Loading', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (19). "Pulp and Paper Chemistry and Chemical Technology", Chapter 21, 'Properties of Paper', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (20). McCool, M. A. and Taylor, C. J., Tappi, 66(8), 69, (1983). (21). Mckinney R. W. J. - Tappi Pulping Conference, 29, (1987). (22). Retulainen, E., Paperi ja puu, 74(5), 419 (1992). 86

104 (23). Nystrom, M., Pykalainen, J. and Lehto, J., Paperi ja putt, 75(6), 419 (1993). (24). Crawford, J. J., in "Pulp and Paper Chemistry and Chemical Technology", Chapter 15, 'Filling and Loading', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (25). Riddell, M.C., Jenkins, B., Rivers, A. and Waring, I., in "Pulp and Paper Chemistry and Chemical Technology", Chapter 15, 'Filling and Loading', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (26). Beazley, K. M. and Petereit, H., in "Pulp and Paper Chemistry and Chemical Technology", Chapter 15, 'Filling and Loading', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (27). Davidson, R. R.,,in "Pulp and Paper Chemistry and Chemical Technology", Chapter 15, 'Filling and Loading', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (28). Coenen, T. J., "Recycled Paper Technology - An Anthology Of Published Papers", 272, Tappi Press, USA, (1994). (29). Prasad, D. Y., Heitman, J. A. and Joyce, T. W., "Recycled Paper Technology - An Anthology of Published Papers", Tappi Press, 134, (1994). 87

105 (30). Harpham, J. A., Reid. A. R. and Turner. H. W., in "Pulp and Paper Chemistry and Chemical Technology", Chapter 21, 'Properties of Paper', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). (31). Shaw, M. B. and Simmons, R. H., in "Pulp and Paper Chemistry and Chemical Technology", Chapter 15, 'Filling and Loading', Third edition, Volume III, Casey, J. P. (Ed), A Willey-Interscience Publication, USA, (1981). 88

106 CHAPTER 5 THE ROLE OF DIFFERENT ALKALIS IN FLOTATION DEINKING OF NEWSPRINT AND MAGAZINES 5.1 Literature Review Improvement of Strength Properties of Recycled Fibre It is believed that significant losses in strength occur when paper is recycled. The traditional view that paper turns into "mush" after four to seven recycles is being challenged by modern researchl. The losses occurring in strength are related to the fibre morphology of the species, type of recycling process and the number of times the fibre is recycled2,3. Recycled paper loses strength mainly because of fibre stiffening and hornification that occur when virgin fibres are dried during their initial papermaking cycle5. This phenomenon is difficult to reverse 5. Essentially, the strength properties of deinked pulp depend on the type of deinking process used 2,3,9. There are several techniques used to improve the strength of secondary fibre. Generally, strength loss can be regained by refining4. Unfortunately, this usually reduces drainage and production capacity 5. Increased refining also limits the amount of strength that can be regained by refining in future cycles 5 The use of chemical additives, which improve the strength properties without changing the repulping requirement, can provide an alternative method to refining 5,6. Two resins often used are an anionic polymer, which is capable of facilitating hydrogen bonding and a cationic polymer, which is capable of forming strong electrostatic bonds between fibres and fines5,63. These resins improve the dry strength of paper by increasing both the strength and the area of the interfibre bonds5,6,7. 89

107 Treatment of wastepaper with sodium hydroxide increases the freeness and the strength properties of recycled fibre 5,13. Sodium hydroxide treatment promotes fibre swelling, thereby increasing fibre flexibility and surface conformability 5. Both alkaline treatment and delignification can improve the papermaking potential of recycled fibres 5. The delignification treatment was found to improve bonding and strength characteristics probably because of softening, swelling and lignin removal 5. Some studies show that a combination of alkali and high-shear-field (HSF) treatment may be a better alternative to obtain high product quality from secondary fibre 5. The strength properties of the recycled paper obtained by this treatment are higher than those obtained by refining 5. A enzymatic deinking method showed a significant improvement in strength properties and drainage characteristics compared to conventional deinking 9, A Mechanism for the Alkali Strengthening of Different Pulps Methods for strengthening and brightening mechanical pulps have been the subject of research for many years". One route towards upgrading the strength properties is to treat the pulp with alka1i Richter 15 suggested that the essential feature of this process is the removal of noncellulosic materials from the cell wall. Neale, Pearson and Sommerville 16 and also others 14 have suggested that the alkaline treatment swells the fibres and that the accompanying plasticisation enhances the ability of the fibres to conform and bond to one another during sheetmaking Effect of Swelling and Carboxylic Acid Groups on Strength Properties An important function of the peroxide in papermaking process, besides the elimination of chromophores, is to improve the mechanical properties of mechanical pulp 17. It is known that one of the important factors in upgrading strength properties during oxidative treatment with peroxide or oxygen is the introduction of carboxylic acid 90

108 groups into the pull:0 1,17. The swelling of fibres (fibre saturation point) caused by alkali treatment is believed to generate carboxylic acid groups by saponification of pulp esters or lactones 11,17 (Figure 1). The results showed that the pulp strength increases as carboxylic acid groups are introduced 11,17 (Figure 2). The increase in strength is coupled both to the swelling of the pulp and to an increased concentration of carboxylic groups on the fibre surfaces 11, o < O Spruce O Aspen I Untreated pulps ACIDIC GROUPS. mmol/kg Figure 1: Changes in the swelling of mechanical pulps with their acidic group contents (after Katz, Liebergott and Scallan 11 ) Tens ile in de x,. knm / kg Carboxylic groups, gmol/g Figure 2: The tensile index versus the content of carboxylic acid groups (after Engstrand and Sjogren 17 ) 91

109 Several studies have been published describing the effects of acidic groups and chemical conditions on cellulosic pulp properties, i.e. strength, beatability The acidic groups in the fibres consist of carboxylic acid groups originally present in the wood or created during oxidative treatment, such as hydrogen peroxide bleaching 17. Ionization of the acidic group results in increased swelling of the fibres due partly to the electrostatic repulsion between the negatively charged carboxylate anionsi Pulp and paper properties are known to be related to swelling of cellulosic fibres 17. There is a linear correlation between the tensile strength of handsheets made from chemical pulp fibres and the degree of swelling of the fibres in different ionic formsi 7,20. The ionic form also seems to affect the beating and refining operations in the manufacturing of chemical and mechanical pulps 17,21. The basic factors controlling swelling have been reported to be the type of cation bound to the acidic groups, the degree of dissociation and the ionic strength of the solution surrounding the ge1 17,22. The swelling effect of fibre was found to be related to the lignin content in the pulp. In the case of mechanical pulp, the swelling of the fibres is very limited compared with that of chemical pulp fibres where the lignin has been removed by chemical treatment Alkali Hydrolysing Effect on Strength Properties A study by Nystrom et a124 found that different alkalis give different effects on strength properties of the fibre. Essentially, these effects are dependent on the degree of hydrolysing effect of individual alkali24. The same study concluded that decrease in tensile index caused by reduction in cross-linking between fibres can be correlated to an increase in freeness24 (Figure 3). 92

110 40 7:1 Tensile Index N m/g tr) 25.0 N CSF (ml) Figure 3: Alkali hydrolysing effect of different alkalis on strength properties of mechanical pulp (after Nystrom et a124) 5.2 Effects of Different Deinking Processes on the Properties of Recycled Fibre The selection of the deinking process has always been dependent on the quality of recycled fibre set by the end user. The properties of recycled fibre were significantly changed with the use of different deinking processes 3. Pfalzer3 found that with the use of flotation and washing processes of different magazine stock, a significant difference in the strength properties was observed. This change was attributed to the degrees of extraction of ash and fines, depending upon the type of process selected 3. Some recent studies show that enzymatic deinking processes yield superior results compared to conventional alkaline deinking method 9. 10,25. These studies concluded that a significant energy savings can be possible. The recycled fibre produced from 93

111 these processes was found to have improved optical and strength properties 9,10,25. These processes do not need an expensive effluent treatment as required by conventional alkaline deinking process 9,10, Effect of Deinking on the Environment The pressure to conserve natural resources and reduce waste from the community has led to an increased trend toward recycling of waste paper Studies on paper recycling showed that paper recycling can be cost effective and can help the environment26,27,30. However, paper recycling is not costless 26,27,30. The selectivity of each recycling process depends on several factors such as economics of the process, quality of recycled fibre and the effect on environment26,27. Contrary to some popular opinion, paper recycling is not without environmental cost 26,27. While the technology to remove ink from printed paper has been developed over several years, less is known about the components released in the deinking process, the route where by they leave and, where primary plus secondary treatment is employed to treat the effluent 29. Environmental concerns and increasing costs have also driven technical developments to reduce and treat the solid and liquid emissions from the deinking plant The Type of Effluents From Deinking Processes The effluent streams from the deinking plant are mainly the filtrates produced from reject thickening and some bleed-off from the process 27. Trace concentration of dioxins and furans have been found in some deinking plant effluents. The main source of these is believed to be from chlorine bleached paper in the waste paper supply 27. Heavy metals from the printing inks have also been a source of some concern although the majority of them are retained in the paper 26,27. There is presently a trend towards using non-metal containing inks, resulting in decreasing metal discharges from the deinking plants27. Deinking effluent is often high in BOD from dissolved starch. The presence 94

112 of starch and other polymers may make it difficult to remove suspended solids by conventional sedimentation techniques 26. Deinking plants today typically have yields around 80-90%, so that significant quantities of solid waste must be disposed of. Rejects from pulping, screening and cleaning consist mainly of plastic, metal, glass, sand and other inert material. They are then disposed of to landfill 27. The largest reject streams consist of fibre, ink and fillers removed in the flotation sludge 27. Sludge disposal is commonly effected through landfill, composting with other wastes such as bark or burning but deinking sludge may have a high ash content and consequently a poor fuel value26. The landfill option is becoming increasingly unpopular because of shortage of landfill sites and rising costs 26,27. Other aspects of the environmental side of deinking include the beneficial effect of relieving pressure on landfill sites, which has been a major incentive for recycling in the United States 26. Liquid effluents from wastepaper are similar to those from mechanical pulping, the following loads being typical26: biochemical oxygen demand: kg/t ; chemical oxygen demand: kg/t; non filtrable residue : kg/t. 95

113 5.4 Introduction The two most common techniques to remove ink particles from old newsprint are washing and flotation deinking. The chemistry involved in the two processes is based on different principles 25. Washing systems attempt to reduce the size of the ink particles, thus facilitating removal of the ink particles with the filtrate 25. On the other hand, a flotation system aims to keep the size of the ink particles large so that effective surface collection can be achieved using specific chemical systems 25. These processes use conventional alkaline deinking agents such as sodium hydroxide, sodium silicate and hydrogen peroxide25. These deinking agents have been proved to be potentially environmentally damaging chemicals 25. In the alkaline deinking processes, dislodging of the ink from the fibres takes place by swelling the fibre Structure. Pulping under high alkalinity assists this process 28. Sodium hydroxide is the most common source of alkalinity added during deinking. With the use of sodium hydroxide, at some paper mills in Australia, there is a problem with saline discharge into the river. Some recent bleaching studies showed that sodium hydroxide can be substituted by divalent bases such as magnesium oxide or calcium oxide. Some researchers found that fibre properties can be influenced by the use of different alkalis in bleaching process 24. However, no attempt has been made to investigate the role of alternative alkalis in deinking processes. The aim of this study was to explore the possibility of substituting conventional sodium hydroxide by divalent bases such as magnesium oxide and calcium oxide to provide the alternative alkali source in the deinking process. Other alkalis like calcium hydroxide, magnesium hydroxide and ammonium hydroxide have also been investigated. At the same time, the effects of each alkali used in the deinking process with regard to the optical and other physical properties of recycled fibre have also been investigated. 96

114 This study also examines the effect of each alkali on the environment (effluent). To our knowledge, no investigation has been reported which discusses comprehensively the effects of different alkalis on the properties of deinked pulp. This study has been carried out to examine the effects of the various alkali sources on flotation deinking of offset printed newsprint (ONP) and coated magazines (OMG) with ash content of approximately 30%. A furnish of 70% ONP and 30% OMG was used throughout all experiments. The literature appears to be divided on the involvement of sodium silicate in deinking process. The role of sodium silicate in deinking chemistry remains one area which is not well understood and not completely optimized. This study also addresses the effect of sodium silicate on the deinking efficiency of each alkali. 5.5 Effect of Deinking on Optical Properties Using Different Alkalis Brightness Figures la - If show the effect of the addition of sodium silicate and various alkalis on final brightness (after flotation). Experiments were carried out using 0.2% DTPA (optimized amount) to avoid excessive use of sodium silicate31. Standard conditions of pulping and flotation were followed (see experimental section, chapter 9). Some researchers have reported enhancement in the brightness of deinked pulp with increasing sodium silicate concentrations without optimizing the total alkalinity 2,32. Mathur33 showed that total alkalinity optimization was necessary in silicate deinking. In this work, all alkalis incorporated in the pulping stage were varied from 1% to 4% with and without 1% sodium silicate. For all alkalis, over the concentration range investigated, 1% concentration of each individual alkali was found to give the maximum brightness and maximum ink removal. 97

115 Most of the alkalis show poor solubility and low ph values compared to sodium hydroxide 34. For alkalis like calcium oxide and magnesium oxide, only 2-3% of the added alkali was found soluble 24,34,53. In order to generate more alkalinity at the pulping stage, higher dosages (up to 4%) of each alkali have been employed in this study. Higher dosages of low solubility alkalis can produce a buffering effect (ph stability)24,52. The solubilities 59 of different alkalis used in this work are listed in the Table I. Table 1: Solubilities of the alkalis 59. _. Alkali _ Solubility (gms/100 cc) in hot water. Ca(OH) CaO Mg(OH)22 MgO _. _ NaOH E 347 The effect of sodium silicate was found to be different for each alkali. Silicate addition was found to have its greatest effect when magnesium oxide was used. At optimized conditions, a net increase of 11 points in brightness was observed (Figure I a). Calcium hydroxide and magnesium hydroxide showed 3 and 4 points increase in brightness respectively with silicate addition (Figure lb and lc). The beneficial effect obtained with magnesium alkalis and calcium hydroxide can be attributed to the formation of insoluble magnesium and calcium silicate complexes. These complexes agglomerate small ink particles to form a suitable particle size required for efficient ink removal 35 Very little effect was observed with the addition of sodium silicate with sodium hydroxide and ammonium hydroxide (Figure ld and le respectively). This suggests that, for both these alkalis, silicate free deinking is possible. Detrimental effects were observed when silicate was added with calcium oxide (Figure If). A decrease of 3 points in brightness was observed. 98

116 Ideally, calcium oxide should show the same effect as calcium hydroxide because in aqueous medium calcium oxide forms calcium hydroxide. Hence a beneficial effect should have occurred with sodium silicate addition (as observed with calcium hydroxide). The detrimental effect of silicate addition suggests that some other complex reaction mechanism is occurring at the pulping and flotation stages. Very little change occurs in ph's (at the pulping stage), as the amount of MgO (Figure I a) and Mg(OH)2 (Figure 1c) increases. This indicates that the limit of solubility of these alkalis has been reached. The decrease in brightness observed with increased alkali concentration (Figures la and lc) cannot be attributed to alkali darkening, as no more [01-1] - concentration is being generated. The observed decrease in brightness could be due to redeposition of ink (poor ink removal) resulting from the interaction between ink particles and undissolved alkali particles. It can be seen that ph's (at the pulping stage) with MgO (Figure la) are higher than the Mg(OH)2 (Figure lc). Ideally MgO should have the same ph range as Mg(OH) 2, as MgO readily hydrates to Mg(OH) 2. The observed difference in ph's for these alkalis can be attributed to their molar concentration difference. The concentrations of alkalis employed in this study are based on a weight percentage. However, on molar basis the concentration of Mg(OH) 2 present in the solution would be less than MgO (hence lower ph). The observed increase in ph's (at the pulping stage) with increased amount of Ca(OH)2 (Figure lb) and CaO (figure if) can be attributed to the complex reaction mechanism occurring at the pulping stage (e.g. interaction of alkali with different functional groups present on the surface of fibre, filler or ink). Since both alkali charges (1-4%) are well above their solubilities, it is unlikely to have additional [OM- concentration being generated from these alkalis: The complexity of the system occurring at the pulping stage has been discussed in subsequent investigation. 99

117 Brightn ess ( %ISO) 65 - (a) :1 With 1% Silicate Without Silicate 45 PH Mg0 (%) El With 1% Silicate 65 - (b) 12 Without Silicate Brightnes s ( % ISO) A 45 ph Ca(OH) 2 (%) Brightness ( %ISO) (c) El With 1% Silicate Without Silicate 45 ph Mg(OH) 2 (%) Figure 1: Effect of alkali addition on Brightness (%ISO) of deinked pulp, with and without sodium silicate (1%); (a) MgO (b) Ca(OH)2 (c) Mg(OH)2 100

118 Brightness ( %ISO) 65 (4) O With 1% Silicate [0] Without Silicate ph NaOH (%) Brightness ( %ISO) 65 - (e) O With 1% Silicate Without Silicate 45 ph NH 4 OH (%) Brightness ( To ISO) 65 (f) O With 1% Silicate [21 Without Silicate 45 ph CaO (%) Figure 1: Effect of alkali addition on Brightness (%ISO) of deinked pulp, with and without sodium silicate (1%); (d) NaOH (e) NH4OH (f) CaO In order to obtain the optimized sodium silicate concentration, the same study was conducted with increasing silicate dose (up to 4%). However, no additional effects 101

119 were found with greater than 1% sodium silicate (Figure 2). In summary, 1% sodium silicate was found optimum for all alkalis except calcium oxide. 65 Brightne s s ( % ISO) A Mg(OH) 2 A Ca(OH) 2 O MgO NH OH 4 O CaO NaOH Sodium silicate (%) Figure 2: Effect of sodium silicate addition (0-4%) on Brightness (%ISO) of deinked pulp using various alkalis (1%). Increased concentration (more than 1%) of each alkali, at the pulping stage, did not result in an increase in the final brightness (after flotation). It was also observed that the addition of sodium silicate did not influence significantly the ph of the pulp at the pulping stage. A similar conclusion was made by Mathur 33. In the pulper, alkalinity is necessary for swelling the fibres and thus efficient detachment of ink particles from the fibres results. Very high alkalinity levels cause alkali darkening 36,37. This can be avoided by the use of hydrogen peroxide in the pulper. Generally, the pulping conditions are not ideal for hydrogen peroxide bleaching 35,39,

120 Unlike bleaching, the system chosen for deinking is very complex. The complexity of this system is governed by several effects exerted by different components of the system. The factors involved in this complex system could be as follows: (a) Different type of pulps (mechanical and chemical) (b) Fillers and coating material present in the magazines (c) Ink (d) Type of alkali used in the system (e) The effect of papermaking conditions (previous treatment in the papermaking process) on chemical reactions (bleaching) occurring at the pulping stage 39,54 The bleaching effect occurring at the pulping stage cannot be correlated to bleaching chemistry directly35,39,54. Galland et a/39,54 reported that no simple correlation was found between brightness and peroxide consumption at the pulping stage. It can be attributed to the effect of papermaking conditions on peroxide bleaching 39. A study of the effect of papermaking conditions on peroxide bleaching efficiency showed that resin sizing and alum addition increase peroxide consumption and reduce brightness 39,54. The same study concluded that drying, calendering, coating, starch and filler addition also have an effect on peroxide bleaching efficiency 39. Due to the complexity of the system, the results obtained at the pulping stage were difficult to explain with respect to the bleaching chemistry 35,39,54,

121 Though the pulping conditions are not ideal for bleaching, a small amount of brightness gain was obtainable due to bleaching at the pulping stage 35. Figure 3 shows the effect of various alkalis on brightness after pulping and flotation, at appropriate deinking conditions (1% alkali and optimized amount of sodium silicate). Significant variations in the brightness response for each alkali occur both after the pulping and flotation stages. Since no ink removal occurs at the pulping stage, the change in brightness observed was essentially due to the bleaching effect35,39, [2] After pulping O After flotation 60 Br ightness ( %ISO) ph 10.4 Alkali NaOH MgO NH 4 OH Mg(OH)2 Ca(OH) 2 CaO Figure 3: Comparison of the bleaching response of each alkali after the pulping stage and brightness variations after the flotation stage (at 1% alkali and optimized amount of sodium silicate). A trend of increasing final brightness (after flotation) with increasing ph (at pulping stage) was observed after flotation. However, this trend is difficult to correlate with ph's observed at the pulping stage. The brightness obtained after the flotation could be due to the combined effect of filler retention (white filler like calcium carbonate and 104

122 titanium dioxide), ink removal, some bleaching in the pulping stage and retention of brighter chemical pulp (originating from magazines) 33,40, Image Analysis, L* a* b* and Light Scattering Coefficient Commonly used techniques for evaluating deinking performance include brightness improvement and speck count. Brightness measurements alone have limited value in secondary fibre recycling. It is basically a reflectance measurement which is influenced by the presence of mineral fillers, coating materials, various chemicals as well as the ink that is present42,43. Figure 4 shows the ink speck counts and brightness of each alkali using appropriate deinking conditions (I% alkali and optimized amount of sodium silicate). It can be seen that calcium oxide, calcium hydroxide and magnesium hydroxide yield lower brightness and poor ink removal compared to sodium hydroxide, magnesium oxide and ammonium hydroxide. 105

123 150 El Dark Speck Count Brightness(%ISO) 100-4: CaO Ca(OH) 2 Mg(OH) 2 NH- 4 OH MgO NaOH Figure 4: Effect of various alkalis on the Ink speck counts and Brightness (%ISO) of deinked pulp (at 1% alkali and optimized amount of sodium silicate). These latter three alkalis (sodium hydroxide, magnesium oxide and ammonium hydroxide) showed the same ink speck count and hence the same extent of ink removal. However, the brightness response was different, with magnesium oxide and ammonium hydroxide showing a marginally lower brightness compared to sodium hydroxide. Many researchers44 have reported that colour measurements made with L* a* b* colour scale are preferable to brightness measurements made at 457 nm as a means to assess ink removal. Colour measurements were made using L* a* b* scale. The L* number, which is a measure of the greyness (and therefore ink removal) 33 and bleaching, showed the same trends as did the brightness measurements at 457 nrn (Figure 5). In this study a* values, which are an indication of the green-red tint of paper, did not show a significant change during the process of deinking. 106

124 Brightness (%ISO) Figure 5: The relationship between Brightness (%ISO) and L* using different alkalis (at 1% alkali and optimized amount of sodium silicate). Values of b*, which are an indication of yellowness of the paper, were observed to be 2-3 points lower before flotation (after pulping) than after flotation (Figure 6) (at I% alkali and optimized amount of sodium silicate). This implies that alkali yellowing reactions predominated over perhydroxyl bleaching reactions. A similar conclusion was made by Mathur33. Despite high alkalinity (ph-11), alkali yellowing effect was not prominent at the pulping stage. This was presumably due to hydrogen peroxide which offset the alkali darkening effect associated with high ph 55. However, after flotation, residual peroxide concentration gets reduced by 8 times (due to stock dilution from 8% to 1%) though alkalinity in flotation cell remains high. As a result, alkali yellowing reactions predominate. 107

125 b*(after pulping) b*(after flotation) CaO Ca(OH) 2 Mg(OH) 2 NH 4 0H MgO NaOH Figure 6: Effect of various alkalis on the b* values of deinked pulp before and after flotation (at 1% alkali and optimized amount of sodium silicate). - The effect of alkali darkening (yellowing) in deinking of ONP has been reported earlier45 (in chapter 2). It was also noted that during alkali-silicate optimization (Figures la - If), b* values did not change with varying both the amounts of sodium silicate and alkali added. - This suggests that the decrease in brightness observed in Figures la - if, for the various alkalis with increasing alkali addition, was due to the extent of ink removal and not a pulp yellowing effect. 108

126 Figure 7 shows the effect of deinking on the light scattering coefficient using appropriate deinking conditions (i.e. 1% alkali and optimized amount of sodium silicate). It was observed that the changes in light scattering coefficient, using different alkali sources, were substantial. However, a negligible difference in opacity was observed. Similar results have been obtained in the enzymatic deinking process NaOH CaO NH 4 OH MgO Ca(OH) 2 Mg(OH) 2 Figure 7: Effect of various alkalis on Light scattering coefficient of deinked pulp (at 1% alkali and optimized amount of sodium silicate). These differences in the light scattering coefficient can be attributed to changes in structural parameters such as specific surface area or crystallinity index 25,

127 5.6 Effect of Deinking on Strength Properties Using Different Alkalis A study by Mahagaonkar and Banham 47 (in chapter 4) reported that, in the deinking of ONP/OMG mixtures, after the pulping stage, pulp strength was influenced by detrimental effects exerted by filler originating from 0MG 47. However, after the flotation stage, an enhancement in all strength properties was observed. This was attributed to loss of a major portion of filler from the system and the beneficial effect of the chemical fibre predominating over the filler effects47. This effect was found to be dependent upon the ash content present in the hand sheets 47. In this study, negligible changes in the ash contents of the hand sheets were found (using appropriate deinking conditions for each alkali i.e. at 1% alkali and optimized amount of sodium silicate). This suggests that the amount of filler retained in all hand sheets was the same. Therefore it was reasonable to assume that the effect exerted by the filler was the same for the different alkali sources. The deinked pulp produced from various alkali sources, using appropriate deinking conditions (1% alkali and optimized amount of sodium silicate) was also found to have the same average fibre length and the same water retention values Tear Index, Tensile Index, Burst Index and Stretch Figures 8,9,10 show the effect of deinking upon tear, tensile and burst indices respectively. Experiments were performed using appropriate deinking conditions (at 1% alkali and optimized amount of sodium silicate). Despite the same amount of filler retention (ash content) in the hand sheets and the same average fibre length, pulps deinked with different alkalis show a distinct change in strength properties. 110

128 NaOH CaO NH OH MgO Ca(OH) Mg(OH) Figure 8: Effect of various alkalis on Tear index of deinked pulp (at 1% alkali and optimized amount of sodium silicate) ) VA 32 NaOH CaO NH 4 OH /.4" F ,. MgO Ca(01-1) Mg(OH) 2 Figure 9: Effect of various alkalis on Tensile index of deinked pulp (at 1% alkali and optimized amount of sodium silicate). 2.4 e 2.2 ts cl) -cz N aoh 7-77 ' CaO NH OH MgO Ca(OH) Mg(OH) Figure 10: Effect of various alkalis on Burst index of deinked pulp (at 1% alkali and optimized amount of sodium silicate). 111

129 Figure 11 shows the relationship between freeness and tensile index using appropriate deinking conditions (1% alkali and optimized amount of sodium silicate). A trend of low freeness with high tensile can be attributed to the hydrolysis effect exerted by each alkali used in the deinking process. Since the hydrolysing ability of the various alkalis will affect the hydrogen bonding potential of the fibres. In enzyme deinked pulp, enzymatic hydrolysis led to a similar trend in strength properties 25. This effect was attributed to the changes in the hemi cellulose content and the breakdown of ligninhemicellulose linkages, which facilitates the release of lignin Freeness (CSF) Figure 11: The relationship between Tensile index and Freeness using different alkalis (at 1% alkali and optimized amount of sodium silicate ). A study by Phipps 55 reported that strength properties of recycled paper are essentially dependent upon the conditions of repulping. Most repulping is carried out at high alkalinity to induce fibre swelling (alkali hydrolysis). The swelling of fibres caused by alkali treatment is due to the generation of carboxylic acid groups resulted from saponification of esters and lactones present in the pulp 11,17. The increase in strength is related to the swelling of the pulp and an increased concentration of carboxylic 112

130 groups on the fibre surface. The generation of carboxylate group depends on the type of system used54,55. In this chapter, alkali hydrolysing effect of each alkali will depend upon two major factors; (a) solubility of alkali (b) complex chemical interaction occurring between different components (chemical additives used in papermaking, the ink, pulp types, type of magazine coating) of the system. Due to complexity of the system, the effect exerted by each component on alkali hydrolysing effect of each alkali is difficult to identify. A study by Nystrom et a124 showed that changes occurring in strength properties of mechanical pulp were essentially dependent upon the hydrolysis effect of the type of alkali used in the bleaching process 24. The decrease in tensile with increase of freeness was correlated to a reduction in crosslinking between fibres 24. The hydrolysing effect of the alkalis was giving the fibres more bridge-forming properties 24. The same study concluded that divalent ions bind to the bridge-forming sites in the fibres thus preventing both bridging and hydrolysis and consequently low strength results 24. In this chapter, divalent alkalis like magnesium oxide and calcium oxide show lower values of tensile, tear and burst compared to sodium hydroxide (Figures 8-10). The high strength properties observed when sodium hydroxide was used as the alkali demonstrate the beneficial effect of sodium hydroxide on the bridge forming property of the pulp. Sodium hydroxide was also found to have the highest hydrolysis constant pkh49. Figure 12 shows the effect of deinking on stretch. Generally, stretch can be expected to follow the same trend as tensile index 58. However, the reverse trend was observed, namely increasing stretch with decreasing tensile (Figure 13). In the deinking of ONP/OMG mixtures47, this type of unusual trend has been reported in chapter

131 2.35 NaOH CaO NH OH 4 MgO Ca(OH) Mg(OH) 2 2 Figure 12: Effect of various alkalis on Stretch of deinked pulp (at 1% alkali and optimized amount of sodium silicate). Tensile index (N. m/g ) Stretch (%) Figure 13: The relationship between Stretch and Tensile index using different alkalis (at 1% alkali and optimized amount of sodium silicate). 114

132 5.7 Effect of Deinking on Other Properties Using Different Alkalis Freeness Figure 14 shows the effect of deinking on freeness using different alkalis at appropriate deinking conditions (1% alkali and optimized amount of sodium silicate). Despite the same average fibre length and the same water retention values, all the deinked pulps using different alkali sources show substantial changes in freeness. These changes can also be attributed to hydrolysis effects. Prasad et a125 and Lee et a146 showed that change in pulp freeness was dependent upon the extent of hydrolysis (1) v) c NaOH CaO NH-OH MgO Ca(OH) 2 Mg(OH) ' 4 2 Figure 14: Effect of various alkalis on Freeness of deinked pulp (at 1% alkali and optimized amount of sodium silicate). 115

133 5.7.2 Porosity Figure 15 shows the effect of deinking on porosity using different alkali sources at appropriate deinking conditions (1% alkali and optimized amount of sodium silicate). Despite the same ash content (filler retention) in the sheets, pulps deinked with different alkalis show a significant change in porosity. It suggests that porosity is influenced by effects exerted by chemicals used in the process rather than filler or fines present in the system Poros ity (m l/m in ) NaOH CaO NH--OH MgO Ca(OH) 2 Mg(OH) 2 4 Figure 15: Effect of various alkalis on Porosity of deinked pulp (at 1% alkali and optimized amount of sodium silicate). Normally strength is directly related to porosity 58. However, in this study, a trend of high porosity and low strength (tensile) was observed (Figure 16). The same effect 47 was also observed at the pulping stage of deinking of ONP/OMG mixtures in chapter 4. Other properties, such as bulk and roughness, did not show any significant change. 116

134 Poros ity (ml/min) Tensile index (N. m/g) Figure 16: The relationship between Porosity and Tensile index using different alkalis (at 1% alkali and optimized amount of sodium silicate). 5.8 Effect of Deinking on Environment Using Different Alkalis Effluent Study An effluent study has been made on filtered sludge samples of each alkali, using appropriate deinking conditions (1% alkali and optimized amount of sodium silicate). Figures 17 and 18 show chemical oxygen demand and dissolved solids, respectively. The study on bleaching effluents, using different alkalis showed that alkalis like sodium hydroxide and sodium carbonate, which are capable of giving high alkaline ph, show high C. 0. D. values 24. Due to high alkaline ph the long chain molecules in the pulp are hydrolysed to smaller ones. As a- result, increase in C. 0. D. was observed24,56. Our results showed the same effect. Higher C. 0. D. values obtained for the alkalis like sodium hydroxide, magnesium oxide and ammonium hydroxide can be attributed to 117

135 high alkaline ph generated by each alkali at the pulping stage. Alkali like sodium hydroxide24,56 dissolves wood components like ligninwhich increases the C.O.D c-) NaOH PH MgO NH OH Mg(OH) Ca(OH) CaO Figure 17: Effect of various alkalis on Chemical Oxygen Demand (at 1% alkali and optimized amount of sodium silicate) WW11171Z/72 NaOH MgO NHOH CaO Ca(OH)2 Mg(OH) 2 PH Figure 18: Effect of various alkalis on Dissolved solids (at 1% alkali and optimized amount of sodium silicate). 118

136 For ammonium hydroxide, being a weak base (hence less hydrolysing effect), one would expect lower value of C.O.D. compared with other alkalis. However, it shows high value of C.O.D. and it could be due to the complex effect occurring at the pulping stage. Sodium hydroxide shows the highest dissolved solids in the effluent. By and large, high alkaline ph producing alkalis show high dissolved solids (Figure 18). The alkalis other than sodium hydroxide show lower values of dissolved solids. This suggests that they are more friendly to the environment, because of less dissolution of the pulp in the effluent occurred. 5.9 Conclusions Our study showed that magnesium oxide and ammonium hydroxide can be used as alternative alkalis in flotation deinking. Despite the same ink removal, both the alkalis produced slightly lower brightness pulp compared to sodium hydroxide. Alkalis like calcium oxide, calcium hydroxide and magnesium hydroxide show poor ink removal. Each alkali showed a different bleaching response at the pulping stage. However, due to the complexity of the system, these results were difficult to explain with respect to bleaching chemistry. The effect of addition of sodium silicate was found to vary with the different alkalis. A beneficial effect was observed with magnesium oxide, calcium hydroxide and magnesium hydroxide, while a detrimental effect was observed with calcium oxide. A negligible effect was found using sodium hydroxide and ammonium hydroxide. This suggests that for these latter two alkalis, silicate free cleinking can be possible. 119

137 Substantial differences in the effect of various alkalis on light scattering coefficient were observed. However, a negligible difference in opacity occurred. This can be attributed to the changes in structural parameters such as specific surface area or crystallinity index of the fibre. Significant changes in strength properties of the pulps deinked with different alkalis occurred despite no change in ash content (filler retention in handsheets) and average fibre length of pulps being observed. These properties are also influenced by alkali hydrolysing effect. The variations in the hydrolysing effect influence the hydrogen bonding potential of the fibres. Deinked pulp using sodium hydroxide showed the highest tear, tensile and burst indices and the lowest freeness, porosity and stretch. Divalent alkalis like magnesium oxide and calcium oxide showed lower values of tear, tensile and burst indices compared to sodium hydroxide. The divalent ions appear to bind to the bridge forming sites on the fibres thus preventing bridging and hydrogen bonding (hence reduction in strength). Stretch showed the reverse trend with tensile. Other properties like bulk and roughness were unaffected by varying the alkali. High values of C.O.D. and dissolved solids were obtained for alkalis like sodium hydroxide, magnesium oxide and ammonium hydroxide. This can be attributed to the high alkaline ph generated by these alkalis at the pulping stage. 120

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142 47. Mahagaonkar, M. S. and Banham, P., Paper accepted by Appita and will be published in November issue, (1995). 48. Retulainen, E., Paperi ja puu, 74(5), 419, (1992). 49. "Inorganic Chemistry-Principles Of Structure And Reactivity", Ed. J. E. Huheey, 214, Harper and Row Publishers, UK, (1975). 50. Letscher, M. K. and Sutman, F. J., Pulp and Paper Science, 18(6), J225, (1992). 51. Dionne, P. Y., Seccombe, R., Vromen, M. R. and Crowe, R. W., Paper Technology, 36(3), 29, (1995). 52. Abadie-Maumert, F. A. and Soteland, N., International Pulp Bleaching Conference, 99, (1985). 53. Vromen, M. R., Appita Conference, 167, (1993). 54. Galland, G., Bernard, E. and Verac, Y., Paper Technology, 30(12), 28, (1989). 55. Phipps, J., Paper Technology, 35(6), 34, (1994) 56. Bunce, P. G., Paper Technology, 34(2), 28, (1993). 57. Heimburger, S. A. and Meng, T. Y., Pulp and Paper, 65(13), 116, (1991). 125

143 58. "Pulp and Paper - Chemistry and Chemical Technology", Chapter 21, 'Properties of paper', 3rd edition, Volume 3, Casey, J. P. (Ed), A Wileyinterscience publication, USA, (1981). 59. "CRC Hand Book of Chemistry and Physics", 54th Edition, Weast, R. C. (Ed), CRC Press, USA, (1973). 126

144 CHAPTER 6 BLEACHING - GENERAL OVERVIEW 6.1 The Structure of Wood Wood, in simple terms, can be regarded as a matrix of fibres composed of cellulose, hemicellulose, lignin and low molecular weight compounds called extractivesl. Cellulose is the major wood component, making up approximately one half of both softwoods and hardwoods. It can be characterised as a linear high molecular weight polymer built up exclusively of 13 - D - glucose. Due to its chemical, physical properties and supramolecular structure it can fulfil its function as the main structural component of the plant cell walls 2. Hemicelluloses are a group of polysaccharides which, like cellulose, provide physical support for the plant. Generally, hemicellulose comprises between 20 and 30% of the dry weight of wood. Lignin is another of the major components of wood saturated with a highly aromatic polymer. It acts as a glue which binds the cellulose fibres together to give wood its inherent rigidity and strength 3. Softwoods generally contain between 26-32% lignin while hardwoods have a slightly lower lignin content of 20-28% 4. The molecules of lignin are built up quite differently from those of the polysaccharides, as they consist of an aromatic system composed of phenylpropane units. Extractives are the least abundant component of wood and may be divided into three sub-groups comprising aliphatic compounds (fats and waxes), terpenes and terpenoids and phenolic compounds4. The content and composition of extractives varies considerably among wood species and also within different parts of the same tree 4. Extractives are characterised by their ready extractibility in organic solvents such as 127

145 ethanol, acetone and dichloromethane, although a smaller proportion of extractives (e.g. tannins) may be extracted with water. 6.2 The Sources of Colour in Wood and its Measurement All wood possesses colour, however the degree of colouration differs significantly between trees of different species. Pinus radiata is a relatively bright species in comparison with hardwood species such as Eucalyptus regnans. In early studies on the sources of colour in some softwoods, a significant correlation was noted between the colour and lignin content of both spruce (picea abies) 5 and Douglas fir6. Further studies on the same area confirmed that lignin was the major source of colour in softwoods and as a result, lignin is regarded as the component primarily responsible for imparting colour to softwood species 6,7. The extractive components of some tree species (e. g. Eucalypt) are known to be highly coloured due to the presence of light absorbing polyphenolic compounds. These extractives are removed to a large extent during the grinding of the wood, which is carried out in alkaline solution 8. The bleaching of lignin rich wood pulp is essentially the result of removal of light absorbing substances originating from the lignin 5. The colour of unbleached pulp is generally yellow or brown which results from absorption of visible light in the complementary part of the visible spectrum, the blue region. The colour of pulp is usually quantified by the so called "brightness" measurement. Brightness refers to the diffuse reflectance of a thick pile of paper handsheets made from the same pulp source and measured at a fixed wavelength. The brightness measurement is expressed as the percentage of light reflected from a sample relative to a standard white plate which is assigned a value of 100% reflectance. The variables (handsheet preparation, the optical geometry of apparatus and the type of white standard used) have been standardized by the formation of the ISO brightness scale. Most modem diffuse spectrometers (such as Elrepho 2000 used in the present work) are manufactured to meet ISO specification and 128

146 report brightness measurements in % ISO units when the correct white standard is used. 6.3 Use of Hydrogen Peroxide in the Pulp and Paper Industry Two general processes are commonly employed for the production of wood pulps, mechanical pulping and chemical pulping. Mechanical pulping involves the use of physical force (grinding and refining) to separate the wood fibres. This process has the advantage of producing high yield pulps, as essentially all of the wood components are retained9. As a result, the lignin which contains the major colour forming components present in the wood is not removed during pulping, the resulting bleached pulp is of a relatively low brightness'. In contrast, chemical pulping results in the removal of the majority of the lignin giving higher bleached pulp brightness, but low yields 9. Hydrogen peroxide has been shown to be an effective reagent for the bleaching of both mechanical and chemical pulps. The bleaching of mechanical pulps is most commonly carried out by reduction of chromophores using sodium dithionite (Na2S204) or by oxidation of chromophores with hydrogen peroxide (H202). Although chromophores are effectively eliminated using dithionite, the reduced leucochromophores are reoxidised to chromophores in the presence of oxygen and light to give a yellow coloured paper4. Dithionite solutions are corrosive and result in frequent wear and tear of paper machine wires and other equipments. They also suffer the disadvantage of reacting readily with oxygen. Consequently, loss of bleaching agent occurs 5. Hydrogen peroxide efficiently degrades chromophores in lignin and produces a brighter more stable pulp than dithionite bleaching. The corrosion problem associated with dithionite is also greatly reduced in peroxide bleaching. Hydrogen peroxide is also regarded as a "clean" or "environmentally friendly" bleaching reagent, as its decomposition products are oxygen and water. For these reasons, hydrogen peroxide 129

147 has become increasingly popular as a lignin retaining bleaching agent for mechanical pulps. The effectiveness of hydrogen peroxide as a bleaching agent for mechanical pulps is attributed to the action of the perhydroxyl anion (H02 -) 11 which exists in equilibrium with undissociated hydrogen peroxide under alkaline conditions, as shown in equation (1). H202 + OH 1=-7 H02 + H20 ( 1 ) In conventional peroxide bleaching, sodium hydroxide is added to hydrogen peroxide solution to promote the formation of perhydroxyl anion, however excessive addition of alkali catalyses the decomposition of perhydroxyl anion to oxygen according to the overall reaction in equation (2). 2H H0 (2) As a result of base catalysed decomposition at high ph, industrial peroxide bleaching is usually performed at an initial ph value of about 11 which falls during bleaching to a final value of about 9 as carboxylic acids are formed by oxidation reactions. To gain a better understanding of peroxide bleaching in terms of the active bleaching agent (H02-) the equilibrium constant for the equilibrium between hydrogen peroxide and the perhydroxyl anion in equation (1) must be accurately known. Teder and Tormund 12 have shown that the temperature dependence of the base dissociation constant for hydrogen peroxide, Kb, can be described by the equation : 1330 pkb = V[Na + ] ( 3 ) 130

148 where T = absolute temperature (K) and Kb [011][11202 i [1110;- ] (4) Decomposition of Hydrogen Peroxide From equation (3) and (4) it is evident that formation of the perhydroxyl anion is favoured by increasing temperature. However, industrial peroxide bleaching is generally carried out at 50 C - 60 C since peroxide decomposition is accelerated at higher temperatures, leading to wastage of reagent. The overall reaction for base catalysed decomposition of peroxide (equation 2) has been shown to occur through a variety of mechanisms involving hydroxyl and superoxide anion radicals 135 (Figure 1). The effects of these short lived radical intermediates on pulp brightness are still largely unknown. H202 + H02 02* -- + HO H *- + H HO 02+ H02- + HO FIGURE 1: Example of base catalysed peroxide decomposition mechanisms involving generation of radical species (after Gierer 15, Agnemo and Gellerstedt 14 and Roberts etal 13). 131

149 The presence of transition metal ions such as iron, copper and manganese is also known to catalyse the decomposition of alkaline hydrogen peroxide. Several peroxide decomposition mechanisms involving transition metals are shown in Figure 2. These metals have been found to originate in the native wood, in the grinding water, in impurities in the bleaching reagents or through corrosion of equipment during pulp production 5. Copper and iron show moderate catalytic activity towards decomposition, however manganese has the most serious effect on peroxide stability 16. H202 + M HO. + HO + M + M+ + H02 + HO. > M H20 M+ + 02' M + 02 HO HO FIGURE 2 : Example of peroxide decomposition mechanisms involving transition metal ions (after Barel et al 17, Walling 18, Gierer 15 and Agnemo and Gellerstedt 14). M = Fe2+, Mn 3+, Cut Stabilization of Hydrogen Peroxide To minimize peroxide decomposition during bleaching, two types of additives are normally employed during peroxide bleaching. The first type stabilizes peroxide bleaching liquors by providing a buffering action in the normal bleaching ph range of Sodium silicate is the most widely used additive of this type and has been found to give considerable improvements in the bleaching response of mechanical pulp 19. In addition, sodium silicate appears to deactivate metal ions towards decomposition reactions 16. However, the exact mechanism of deactivation is poorly understood. Magnesium ions, added as magnesium sulphate, have a similar deactivation effect on metal ions and produce a synergistic effect when added with sodium silicate, although the mechanism for this is also largely unknown

150 The second type of additive reduces peroxide decomposition by removing trace levels of transition metals ions from pulp in a pre-bleaching treatment. Pre-treatment typically involves the application of a chelating agent such as ethylenediamine tetra acetic acid (EDTA), pentasodium diethylenetriamine penta acetate (DTPA) or diethylenetriamine pentamethylene phosphoric acid (DTPMPA). Manganese and copper are easily removed by pre-treatment with chelating agents, but a proportion of the iron remains firmly complexed to natural chelants in the lignin and cannot be completely removed 5. The peroxide bleaching of mechanical pulp is usually carried out in a downflow tower, however processes such as refiner peroxide bleaching are also practised. In tower bleaching, thickened pulp of a consistency between 10% and 20% is fed into a bleaching tower. The bleaching chemicals are metered individually by pump and are mixed together just prior to their addition to the bleaching tower. The pulp is mixed thoroughly during the bleaching process which generally lasts between 1.5 to 3 hours5. The bleached pulp is diluted with water to about 4 % consistency before being discharged from the bottom of the tower where it must be adjusted to a slightly acidic ph to prevent alkali darkening reactions. 6.4 Reagent Recycling in Hydrogen Peroxide Beaching Recent studies into fundamental aspects of the chemistry and kinetics phenomenon of peroxide bleaching have led to an increased understanding of the factors affecting the consumption of peroxide and the brightness gain of the pulp20,2 1. By applying this fundamental knowledge it is possible to arrive at a new set of conditions specifically designed to minimise the decomposition of hydrogen peroxide whilst maintaining adequate bleaching rate. Conventional bleaching has been conducted under conditions in which hydrogen peroxide is inherently unstable, due mainly to the ph, but at which bleaching occurs relatively rapidly. At a lower ph the stability of peroxide is enhanced, but the bleaching rate is diminished. This may be offset by increasing the hydrogen 133

151 peroxide in the bleaching mixture. As a result, appreciable quantity of hydrogen peroxide remains in residual liquor. To operate bleaching economically using higher concentrations of peroxide, recycling of residual peroxide is necessary. Cost of peroxide and restrictions on discharge of inorganic salts are the two main drawbacks of current peroxide bleaching processes. In order to overcome these problems several strategies of recycling residual liquor have been investigated 30,33,35. Although alkaline hydrogen peroxide has been used commercially as a bleaching reagent for mechanical pulps for many years, there is continuing interest in maximising the efficiency of the process 22,23. In current commercial practice, 1 to 3% hydrogen peroxide (on oven dried pulp) is applied under highly alkaline conditions. Process variables such as temperature, time and stock consistency can be varied along with the application of stabilizing agents such as sodium silicate and complexing agents 23,24. This process has been optimized on the premise that the bleaching chemicals are used once and then discarded (single-stage bleaching). The discharge (discarded liquor) can include significant amounts of the residual peroxide (typically 10% to 40% of initial charge), together with alkali and stabilizers. Interest in bleaching mechanical pulps to high brightness, requiring relatively large charges of hydrogen peroxide, has led to the development of new bleaching processes in which the residual peroxide is used25. The idea of recycling the bleaching reagent in peroxide bleaching is not new. Ever since the late sixties when the price of hydrogen peroxide dropped due to a more efficient manufacturing process, the use of moderate concentrations of hydrogen peroxide to obtain a high brightness product has made recycling a key factor in offsetting costs associated with higher peroxide concentrations. In 1969 mill trials of recycling began in Sweden26, where a pulp brightness of 80% was being sought. To bleach to this brightness required higher concentrations of hydrogen peroxide than conventionally used, and thus also led to higher concentrations of residual chemical on the completion of bleaching. This residual was recycled to the pulping stage, where it 134

152 was used as shower water in stone-grinding operations26. Since then several other methods for the use of residual bleaching chemicals have been investigated, including use in refining operations27, in single stage recycling operations28,29,30, two-stage bleaching processes 31,32,33. and in Recycling in Single-Stage Bleaching Processes In a single-stage recycling system, residual bleaching chemicals are pressed from the bleached pulp, and added back to the bleach reactor, together with fresh unbleached pulp and fresh chemicals. One Swedish mill currently uses this system in the production of tissue and fluff grades. There are few reported studies on the viability of liquor recycle in a single-stage peroxide bleaching process 28,34. Solinas28 showed that liquor recycle under conventional peroxide bleaching conditions is viable, provided the pulp is treated with a chelating agent and washed prior to bleaching. Strand et a136 reported that, in single-stage bleaching process recycle of peroxide filtrate is valuable especially when the peroxide charge is high and therefore the peroxide residuals high. They concluded that, when the brightness target is lower, % ISO, a single stage process may be sufficient. Figure 3 illustrates the single-stage recycle model recommended by Strand et al 36. One Stage Peroxide System FIGURE 3: A single-stage peroxide bleaching process with recycle (after Strand et al 36). 135

153 The study by Abbot, et al 35 explored the feasibility of a bleaching system using low pulp concentration conditions (4%) to facilitate internal liquor recycling in a single-stage process. These conditions require that high applications of peroxide (as charge on o.d. pulp) are used, to maintain peroxide Concentration in the liquor. However, residual peroxide in the pulp after pressing can also be recovered by washing, and the dilute peroxide solution obtained can potentially be used for other purposes such as deinldng (Figure 4). The system illustrated in figure 4 also has the potential to allow adjustments in the alkali / peroxide balance to reduce the amount of applied alkali. This may be of significance in situations where salt discharge in the effluent into fresh water supplies is restricted35. Alkali Pulp Peroxide PRIMARY BLEACH Pubo Liquor WAS H Wash Liquor Pulp or Recycled firre Alkali SECONDARY BLEACH Or DE INKING Pulp and Effluent FIGURE 4: A possible design for coupled peroxide bleaching-deinking system (after Abbot, et al 35) Hook et a/ 25 have concluded that single and two-stage bleaching are identical in terms of hydrogen peroxide consumption, but state that single-stage bleaching can only compete with two-stages if the pulp is well washed prior to bleaching and the bleach consistency is high. 136

154 Meyerant et a129 found that a two-stage process gave no significant benefit over a single-stage recycling process. In this study the residual peroxide was pressed from the pulp and added back to the bleach with a full charge of fresh chemicals. Only around 35% of the residual chemicals were able to be recycled, leading to a steady state in terms of brightness gain and peroxide consumption after 4 or 5 cycles. To test whether the accumulation of extraneous matter adversely affected the recycled liquor, a series of 12 cycles was carried out in which 15% of the fresh peroxide was saved by the addition of recycled peroxide. The brightness gain and peroxide consumption for each cycle was found to be approximately constant, showing that recycled liquor is as efficient as fresh chemicals 29. The current literature dealing with single-stage recycling seems to suggest that the benefits in this way are negligible. However, this appears to be a function of the way in which the recycling has been carried out. A single-stage recycling process may offer better efficiency than both a conventional bleach and a two-stage bleaching system if the conditions are appropriate. Chelation and careful washing to remove soluble organic matter and complexed transition metal ions, fixing the consistency and maintaining a constant level of peroxide rather than adding a full charge each cycle will give the best results for this type of process Recycling in Two-Stage Bleaching Processes There has been recent interest in utilizing residual peroxide through two-stage PP (peroxide-peroxide bleaching process) bleaching sequences 33,36. Such processes may save on chemical costs but also require higher capital investment for a two-tower bleaching configuration. In high-yield pulp bleaching, a brightness above 80% ISO is essential. Otherwise, the pulp is not likely to be accepted in the market for producing printing and writing grades or for sanitary paper products. Bleaching of these high quality mechanical pulps to high brightness, over 80% ISO, is very costly due to the 137

155 high price of hydrogen peroxide. The bleach plant must be carefully designed to utilize the charged peroxide efficiently by recycling residual peroxide in a strategic fashion. Peroxide bleaching systems including single and two-stages with various recirculation strategies for bleaching of mechanical pulps have been optimized 33,36. Strand et al 36 reported that, when bleaching spruce to a high brightness of 80% ISO, savings of $ / ODMT pulp can be made by installing a two-stage process compared to a single-stage process. All peroxide should be charged in the second stage and peroxide filtrate recycled to the first stage. Medium consistency in the first tower and higher consistency in the second tower is recommended 36. This process has been illustrated in Figure 5. Two Stage Peroxide System 12% consistency Bleach chemicals Bleach tower Bleach chemicals Bleach tower - Press 45% 6% 1 1Fresh water Recycled pressate FIGURES : Two-stage bleaching process (after Strand et al 36). Although the process has now been adopted industrially, several researchers still question whether there really is an improvement in efficiency with this process 33,34. Lachenal et al 33 reported that the recycling of residual hydrogen peroxide in the PP (two-tower bleaching) process is not as efficient as the PE (peroxide alkali activation) process. They further, concluded that the PP process needs a huge capital investment. Consequently, the economics of the process would be impaired to.a very large extent

156 6.5 Alkaline Peroxide Bleaching With Alternative Alkali Sources Mechanical pulp is the main fibre component in many of today's printing and writing paper grades. As the brightness of mechanical pulp is much lower than that of chemical pulp, there is an urgent need to develop improved bleaching processes for mechanical pulp so that it can replace the chemical pulp component in different types of paper, which would be more economical. Additionally, the greater demands being placed on effluent treatment are forcing the pulp and paper industry to find bleaching chemicals which pollute the environment as little as possible 39. The effects of using sodium hydroxide as a base in peroxide bleaching are well known". Peroxide bleaching together with sodium hydroxide and sodium silicate is one of the most widely used methods for the bleaching of mechanical pulp. However, there are a number of potential drawbacks associated with the use of the sodium hydroxide and silicate combination. Sodium silicate leads to rapid wear and tear of paper machines37 and has detrimental effects to the flocculating agents used in the effluent treatment38. High saline discharge resulted from sodium salts is a potential problem with the use of this combination. At Albury, the Australian Newsprint Mill, beside the Murray River, on the Victoria - NSW border, there is a problem with saline discharge into the river water since some of the by-products from bleaching processes are sodium salts. The Murray River discharges more than one million tonnes of salt annually into the ocean, while upstream near Albury it carries about tonnes of salt a year. In recent years, due to the introduction of new deinking plant and new peroxide bleaching processes, the output of dissolved salts has been increased significantly. There has been recent interests in the possibility of substituting divalent bases such as magnesium oxide or calcium oxide to provide the alkali source39,40. There are both 139

157 economic and environmental incentives to consider alternative bases 39,40. The bases like magnesium oxide and calcium oxide may offer the advantage of decreased pollution problems associated with salinity, which arises when discharging conventional effluents into fresh water system 39. Nystrom et at 39 reported that sodium hydroxide, which is used together with sodium silicate in conventional bleaching of mechanical pulp, can be replaced by other alkalis (magnesium oxide, calcium oxide, magnesium hydroxide, calcium hydroxide). The alkalis containing divalent ions gave the required brightness results without the use of silicate. Most of the alkalis caused less dissolution of the pulp into the residual liquor, which means that they are more friendly to the environment. Surprisingly, the economy of the process did not suffer from the use of these new alkalis 39. The only drawback was that the new alkalis always caused a decrease in mechanical strength of bleached pulp 39. Magnesium, unlike sodium, is comparatively harmless to plants and vegetation and so would be suitable as an alternative alkali source for an environmental point of view. Economically, sodium hydroxide is a relatively expensive reagent. It is produced by electrolysis, with chlorine as by-product. Due to growing environmental awareness the demand on chlorine is decreasing. However, the electrolysis process does not allow for higher ratios of sodium hydroxide and lower quantities of chlorine. In contrast, magnesia (magnesium oxide), being derived from naturally occurring minerals, is relatively easy to obtain 41. In Australia there is a world scale magnesite deposit located in Kunwarara, north west of Rockhampton, Queensland 41. Magnesia is prepared commercially by thermally decomposing the mineral magnesite. The map in Figure 6 shows the locality of the deposit

158 FIGURE 6: Kunwarara magnesite deposit locality". 141

159 REFERENCES 1. Barker, G. J. and Cullinan, H. T., Search, 20(6), 195 (1989). 2. Fengel, D. and Wegener, G., in "Wood: Chemistry, Ultrastructure, Reactions", Chapter 3, 'Chemical Composition and Analysis of Wood', 1st Edition Walter De Gruyter, Berlin, (1984). 3. Yan, J. F. and Johnson, D. C., "Delignification and Degelation: Analogy in Chemical Kinetics", J. Applied Polymer Sci., 26, 1623 (1981). 4. Sjostrom, E., in "Wood Chemistry", 1st Edition, Academic Press, New York, (1984). 5. Lords, V., in "Pulp and Paper - Chemistry and Chemical Technology", Volume 1, Chapter 5, 'Bleaching', J. P. Casey. (Ed.), 3rd Edition, Wiley-Interscience, New York, (1980). 6. Wilcox, M. D., Svensk Papperstidning, 78(1), 22, (1975). 7. Norrstrom, H., Svensk Papperstidning, 72(2), 25, (1969). 8. Hillis, W.E. and Swain, T., in "Wood Extractives", Chapter 12, 'Extractives in Groundwood and Newsprint', W. E. Hillis (Ed.), 1st Edition, Academic Press, London, (1962). 9. Smook, G. A., J. Chem. Tech. Biotechnol, 45, 15, (1989). 10. Hart, J. R., Pulp Paper, 55(6), 13.8, (1981). 142

160 11. Andrews, D. H. and Singh, R. P., in "The Bleaching of Pulp", Chapter 8, 'Peroxide Bleaching', R. P. Singh (Ed.), Tappi Press, Atlanta, (1979). 12. Teder, A. and Tormund, D., Svensk Papperstidning, 83(4), 106, (1980). 13. Roberts, J. L., Morrison, M. M. and Sawyer, D. T., J. Am. Chem. Soc., 100(1), 329, (1978). 14. Agnemo, R. and Gellerstedt, G., Acta. Chem. Scand., B33(5), 337, (1979). 15. Gierer, J., 4th Int. Symp. Wood and Pulp. Chem., 279, (1987). 16. Colodette, J., Fairbank, M. G. and Whiting, P., Tech. Sect. CPPA, 75th Annual Meeting, B45, (1989). 17. Baral, S., Lume-Pereira, C., Janata, E. and Henglein, A., J. Phys. Chem., 89, 5779, (1985). 18. Walling, C., Acc. Chem. Res., 8, 125, (1975). 19. Ali, T., McArthur, D., Scott, D., Fairbank, M. and Whiting, P., Tech. Sect. CPPA, 72nd Annual Meeting, B15, (1986). Lundquist, M., Svensk Papperstidning, 82(1), 16, (1979). 21. Wright, P. and Abbot, J., J. Wood Chem. Tech. 11(3), 349, (1991). 22. Bergman, E. K. and Edwards, L. L., Pulp Paper, 87(6), 97, (1986). 143

161 23. Burton, J. T., Pulp Paper, 95(4), 114, (1986). 24. Kuczynski, K., Nijs, H. and May, B. H., Tappi, 71(8), 142, (1988). 25. Hook, J., Wallin, S. and Akerlund, G., Tappi Pulping Conference, 267, (1989). 26. Lindahl, A. and Norberg, P. H., Pulp Paper Mag. Can., 81(6), T138, (1980). 27. Sharpe, P. E. and Rothern, S., Tappi, 71(5), 109, (1988). 28. Solinas, M., Pulp Paper Mag. Can. 77(3), 59, (1976). 29. Meyerant, P., Presley, J. R. and Kouk, R. S., International Mechanical Pulping Conference, 81, (1987). 30. Abbot, J., Vanderaa, S., Wright, P., Ritz, A. and Ault, K., Appita, 45(6), 393, (1992). 31. Lachenal, D., Dechoudens, C. and Bourson, L., Tappi, 70(3), 119, (1987). 32. Siiss, H.U. and Eul, W., Das Papier, 42(10A), V23, (1988). 33. Lachenal, D., Dubreuil, M. and Bourson, L., Tappi, 73(10), 195, (1990). 34. Whiting, P., Pulp Paper, 91(6), 60, (1990). 144

162 35. Abbot, J., Mahagaonkar, M. S., Rao, R., Ault, K., Vanderaa, S., Tech. Sect. CPPA, Spring Conference, (1992). 36. Strand, E., Koponen, R., Edwards, L. L., Moldenius, S., Viljakainen, E., Tappi Pulping Conference, 497, (1987). 37. Ellis, M.E. and Bast, I., Tappi Pulping Conference, 103, (1990). 38. Linhart, F., Auhorn, N. J., Degen, H. G. and Lorz, R., Tappi, 70(10), 179, (1987). 39. Nystrom, M., Pykalainen, J., Lehto, J., Paperi ja puu, 75(6), 419, (1993). 40. Soteland, N., Abadie, H., Maumert, F. A. and Arnevik, T. A., Tappi. Int. Pulp. Bleach. Conference, 231, (1988). 41. Koenig, R. L., Chemistry In Australia, 59(5), 225, (1992). 145

163 CHAPTER 7 PEROXIDE BLEACHING OF Pinus radiata TMP AND Eucalyptus regnans COLD CAUSTIC SODA PULP WITH SODIUM HYDROXIDE AND MAGNESIUM OXIDE IN SINGLE AND TWO-STAGE BLEACHING PROCESSES 7.1 Introduction Mechanical pulps have been bleached with alkaline peroxide for many years'. Traditionally, the pulps are bleached in a single-stage process. In addition to hydrogen peroxide and alkali, other reagents such as sodium silicate, magnesium salts, and complexing reagents such as DTPA are often added 1,2. These reagents can either buffer the system or act as stabilizers to suppress peroxide decomposition through side reactions. Much work has been done to optimize the application of the various reagents 3-7. Reduction in peroxide decomposition leads to higher final pulp brightness, and also higher residual peroxide levels. Residual peroxide is traditionally decomposed at the end of the bleaching process during neutralisation with sulfur dioxide'. This residual peroxide may represent a significant proportion of the initial charge with the efficient use of stabilizing reagents. There has been recent interest in utilizing residual peroxide, for example through two-stage PP (two-tower bleaching system) bleaching sequences Such processes may save on chemical costs, but they require higher capital investment for a two-tower bleaching configuration 12. The efficient use of hydrogen peroxide during bleaching has motivated a number of studies investigating the effect of recycling residual peroxide in single-stage and two- stage processes Unfortunately, the results of these studies have often been 146

164 conflicting. An additional problem with studies of this type is that they fail to consider the alkalis, other than sodium hydroxide. To our knowledge, there have been no studies reported on magnesium oxide used as alternative alkali source in two-stage bleaching processes. There have been a few studies on the effects of a two-stage process using residual peroxide8,9,10. However, the majority of these processes are of limited scope. Most of these studies have failed to address the role of different alkalis when used in conjunction with and without stabilizers, in the peroxide bleaching of mechanical pulps, in particular, Eucalyptus regnans CCS (cold caustic soda) pulp. There has been no systematic study on recycling of residual peroxide, in the bleaching of Eucalyptus regnans CCS pulp and Thermomechanical pulp (TMP). The saline discharge associated with the use of sodium hydroxide as an alkali has caused some environmental concerns recently in Australia 15. The use of magnesia (magnesium oxide), as an alternative alkali source can be useful. Unlike sodium hydroxide, it may offer the advantage of decreased pollution problems such as salinity, high COD's and dissolved solids 14,16,18. Magnesium alkalis are found suitable from an environmental point of view, as they have low solubil ity 16,18. As a result, alkalinity in the effluent does not rise too high 16,18. Magnesia, as an alternative alkali source, may be particularly attractive in the Australian context as there are very large deposits of high quality mineral available at Kunwarara in Northern Queensland 20. The present investigation was conducted in an attempt to achieve three major objectives: (1) To examine the use of magnesium oxide as an alternative alkali source in peroxide bleaching of mechanical pulps in both single-stage and two-stage bleaching processes, in conjunction with and without conventional stabilizers, such as sodium silicate, DTPA and magnesium sulfate. 147

165 (2) To study a comparison between single-stage and two-stage processes using residual peroxide in the spent liquor in the bleaching of Pinus radiata TMP and Eucalyptus regnans CCS pulps (3) To investigate various recirculation strategies of residual peroxide of the spent liquor. 148

166 7.2 Single-Stage Bleaching of Eucalyptus regnans CCS Pulp A Comparison Between Sodium Hydroxide and Magnesium Oxide in Single-Stage Bleaching of Eucalyptus regnans CCS Pulp The perhydroxyl anion [(00H) -] is the agent generally considered to be responsible for bleaching in hydrogen peroxide systems'. Below ph 9, the concentration of the perhydroxyl anions is low'. A ph range is usually required to produce sufficient anion for hydrogen peroxide bleaching 1,14. Magnesium oxide readily hydrates to magnesium hydroxide which in turn dissociates in the following way 14 : MgO + H20 6 Mg(OH)2 fl Mg W The above equation indicates that magnesium hydroxide is a dibasic alkali 14. This means a relatively lower amount of magnesium oxide can substitute for sodium hydroxide 14. Magnesium hydroxide is classified as a mild base, although its saturated aqueous slurry has a ph of This is adequate to activate hydrogen peroxide 14. The use of magnesium oxide as an alternative alkali source during peroxide bleaching of Pinus radiata TMP has been reported previously 14,15,18. These studies concluded that magnesia is generally less effective as an alkali source, producing lower brightness gains after a constant bleaching time, with other factors such as consistency and temperature held constant. Studies aimed at optimizing the bleaching response of Eucalyptus regnans CCS pulps using hydrogen peroxide and sodium hydroxide have been reported 21,,26, With regard to Eucalyptus regnans CCS pulp, the only studies reported in the literature aim at optimizing the bleaching response using hydrogen peroxide and sodium hydroxide 21,22,26,

167 7.2.2 Effect of Additives (Stabilizers) Using Sodium Hydroxide Figure la shows the results for brightness response over a range of alkali addition for sodium hydroxide, after four hours at 60 C using 10% consistency with an initial peroxide charge of 2.0% on CCS pulp. Addition of either DTPA (0.1% on pulp), magnesium sulfate (2% on pulp) or sodium silicate (5% on pulp) progressively improves the brightness gain, so that a brightness of 74% ISO could be obtained under optimized conditions using sodium silicate, compared with 64% ISO with no additives present Effect of Additives (Stabilizers) Using Magnesium Oxide Some studies reported thati 5 with magnesium oxide, addition of sodium silicate alone (without magnesium sulfate) produced the best response in brightness gain and peroxide stabilization. Hence, in this study sodium silicate and magnesium sulfate are added separately (without using mixtures of sodium silicate and magnesium sulfate). Figure lb shows results from similar experiments substituting magnesium oxide for sodium hydroxide. A brightness of 70% ISO can be achieved with this pulp using magnesium oxide as the alkali source in the absence of other additives. However, introduction of either DTPA, sodium silicate or magnesium sulfate as an additive does not improve the brightness response. 150

168 75 (a) Brightness (%ISO) c_a X DTPA A Sodium silicate 0 Magnesium sulfate Without Additives NaOH (% on pulp) Figure la: Single-stage bleaching of Eucalyptus regnatzs CCS pulp with NaOH as base. Conditions: 10% pulp consistency, 4 hours bleaching time, 2% hydrogen peroxide (on o.d. pulp); Initial brightness 50.3% (ISO). Brightnes s (%ISO) 75 (b) X DTPA A Sodium silicate 0 Magnesium sulfate Without Additives MgO (% on pulp) Figure lb: Single-stage bleaching of Eucalyptus regnans CCS pulp with MgO as base. Conditions: 10% pulp consistency, 4 hours bleaching time, 2% hydrogen peroxide (on o.d. pulp); Initial brightness 50.3% (ISO). 151

169 7.2.4 Effect on Bleaching Response Using Sodium Hydroxide and Magnesium Oxide A comparison of results in figures la and lb show that, in contrast to bleaching systems using sodium hydroxide as the base, the addition of additives in conjunction with magnesia produces a reduction in brightness gain for peroxide bleaching of Eucalyptus regnans CCS pulp. It can be seen that magnesium oxide alone (without additives) produces higher brightness response than sodium hydroxide alone (without additives). In peroxide peroxide bleaching of Pinus radiata TMPI 5, magnesium oxide and sodium hydroxide were compared without using additives. It was observed that magnesium oxide produced lower brightness gain compared with sodium hydroxidei 5. This was attributed to the efficiency of producing active bleaching species, which was correlated to the higher alkalinity of sodium hydroxide 15. However, in Figures la and lb, the higher bleaching response of magnesium oxide alone (without additives) over sodium hydroxide alone (without additives), suggests that the alkalinity is not the only deciding factor, which can affect the bleaching response. It is possible that the bleaching response also depends upon the type of pulp being studied. The literature appears to be divided on the effect of stabilizers using magnesium oxide as the alkali. It was observed that interaction of stabilizers with magnesium oxide was essentially dependent upon the type of pulp. In the peroxide bleaching of Pinus radiata TMP 15, stabilizers such as sodium silicate and magnesium sulfate showed significant brightness gain. Abadie-Maumert and Soteland23 found that, for peroxide bleaching of Mg-bisulphite pulps, the highest brightness obtained using a sodium hydroxide and sodium silicate combination was also achievable by substituting magnesium oxide, without using additives. Their study further concluded that, over a range of addition of either sodium silicate or DTPA no significant positive effect was found in the bleaching response when Magnesium oxide was used as the alkali source. The same study

170 concluded that the interaction of additives like DTPA, sodium silicate with magnesium oxide depend upon the lignin content of the pulp studied. For peroxide bleaching of SGW pulps, Nystrom et a1 16 reported that, using sodium hydroxide, sodium bicarbonate or calcium hydroxide as an alkali source, improved results are obtained with sodium silicate present. However, for magnesium oxide and magnesium hydroxide better brightness results are obtained in the absence of sodium silicate, as observed in our study of peroxide bleaching of Eucalyptus regnans CCS pulps. In summary, it can be concluded that in mechanical pulp bleaching, alkalinity is not the only factor which can influence the bleaching response. It may also depend upon the interaction between additives and alkalis and the type of pulp studied Effect on Peroxide Consumption Using Sodium Hydroxide and Magnesium Oxide Figures 2a and 2b show peroxide consumption using either sodium hydroxide and magnesium oxide with different additives. Sodium silicate gives the greatest degree of peroxide stabilization for both alkalis, and therefore providing more residual peroxide in bleach liquor as compared with other additives. Both alkalis show maximum decomposition of peroxide when no additives are used. This effect of peroxide decomposition can be correlated to the lowest brightness response observed in the sodium hydroxide case. However, it is difficult to explain an opposite effect observed in the magnesia case (maximum brightness response with maximum decomposition of peroxide). When magnesia was used as the alkali, with approximately the same range of peroxide consumption, DTPA shows better brightness results over magnesium sulfate. 153

171 Brightness ( % IS O) 80 (a) 70 - X DTPA A Magnesium sulfate 0 Sodium silicate Without additives Peroxide consumed (% on pulp) Figure 2a: Peroxide consumption with NaOH (1-2.5% on o.d. pulp) as base. Conditions: 10% pulp consistency, 4 hours bleaching time, 2% hydrogen peroxide (on o.d. pulp). Brightness ( % ISO) (b) x DTPA A Magnesium sulfate. 0 Sodium silicate Without additives Peroxide consumed (% on pulp) Figure 2b: Peroxide consumption with MgO (1-2.5% on o.d. pulp) as base. Conditions: 10% pulp consistency, 4 hours bleaching time, 2% hydrogen peroxide (on o.d. pulp) Effect of Extended Bleaching Time in Single-Stage Bleaching of Eucalyptus regnans CCS Pulp Using Sodium hydroxide and Magnesium Oxide Figure 3 shows the effect of extended periods of bleaching time on the brightness response for both alkali sources. The optimized conditions were selected for both 154

172 bleaching processes. The peroxide charge (2% on o.d. pulp), consistency (10%) and temperature (60 C) were kept constant. Sodium hydroxide (1.8% on o.d. pulp) was used in combination with sodium silicate (5 % on o.d. pulp). Magnesium oxide (1.8 % on o.d. pulp) was used without any additives. Initially, magnesium oxide produced an increasing brightness trend, although after four hours, a gradual decrease was observed. In contrast, sodium hydroxide showed continuous increase in brightness throughout the extended periods of bleaching time as shown in figure NaOH 0 MgO Brightness ( % ISO) Time (hours) Figure 3: The effect of extended bleaching time. Abadie-Maumert and Soteland23 found that, for peroxide bleaching of Mg-bisulphite pulps, magnesium oxide and sodium hydroxide showed a continuous gradual enhancement in brightness with extended periods of bleaching. Their study further concluded that both alkalis did not show a great difference in brightness with extended periods of bleaching. In the peroxide bleaching of SGW pulps, Nystrom et a/ 16 found that the magnesium alkalis such as magnesium oxide and magnesium hydroxide showed a significant enhancement in brightness with longer bleaching time. The same study showed, in contrast to the observations of Abadie-Maumert and Soteland 23, the 155

173 effect of extended bleaching time on sodium hydroxide-sodium silicate combination showed inferior results when compared against magnesium alkalis 16 In our work on bleaching of Eucalyptus regnans CCS pulps, however, sodium hydroxide-sodium silicate combination shows a distinct superiority over magnesium oxide. Magnesium oxide also produced a significant drop in brightness over extended bleaching periods. This suggests that magnesium oxide decreases the bleaching response at longer times. However, an opposite effect was observed for peroxide bleaching of SGW pulps 16. Unlike the peroxide bleaching of SGW 16 and Mg-bisulphite pulps 23, magnesium oxide shows inferior results compared to sodium hydroxide in Eucalyptus regnans CCS bleaching. Studies of peroxide bleached pine bisulphite pulp and SGW pulps concluded that, the better brightness stability was produced by the buffering effect of residual magnesium oxide during extended bleaching periods 16,23W Our study did not show this positive effect of magnesium oxide during bleaching of Eucalyptus regnans CCS pulps, and it can be concluded that the effect of magnesium oxide may depend upon other factors such as the nature of pulp studied. It is reported that 31 magnesium oxide bleaches needed longer retention time (extended bleaching) to achieve maximum brightness gains than sodium hydroxide. However, in our work no such effect occurs. It suggests that magnesium oxide as a substitute alkali source is not quite as effective in terms of brightness gain as sodium hydroxide. 7.3 Two-Stage Peroxide Bleaching of Eucalyptus regnans CCS Pulp In the literature, there are several publications which discuss the effect of two-stage bleaching 8,9,10,11,24,25. The majority of work has focussed on CTMP and CMP bleaching. In contrast, there are very few publications which discuss the effect of two-stage bleaching on Eucalypt based pulps 25,26. Lachenal et a/ 10,24 reported that the 156

174 merits of two-stage bleaching were essentially dependent upon the utilization of residual hydrogen peroxide in the spent liquor. In order to reactivate peroxide in the spent liquor, extra alkali is introduced at the right time and in the appropriate quantity. This can lead to significant brightness gains 10. Strand et a128 also reported the same results. These studies recommended that, all peroxide should be charged in the second stage and peroxide filtrate recycled to the first stagei 0,24, Experimental Design for Two-Stage Bleaching Process In our two-stage bleaching experiments, fresh bleaching chemicals were added to pulp in the second stage. The bleached pulp was then diluted and pressed. The residual chemicals recovered in the press liquor were then added to the first bleaching stage to pretreat pulp in the first stage. A simplified flow sheet for the process is shown in Figure 4. Unbleached pulp and makeup bleaching chemicals (if used) Fresh bleaching chemicals Bleaching Stage 1 Bleached pulp NO" Bleaching Stage. 2 Bleached pulp Residual bleaching chemicals. Figure 4: A simplified flow diagram for a two-stage peroxide bleaching process. In this work optimized conditions were used for single-stage bleaching and subsequently the same conditions were used in second stage. The range of total 157

175 bleaching time employed in the two-stage process varied from 9-12 hours. In high pulp consistency experiments, ph measurements have been reported on filtered liquor. Magnesium oxide probably dissolved out some lignin or other coloured component from the pulp as the liquor became yellowish, which was not the case with sodium hydroxide 16. Figures 5-8 show the results for two-stage bleaching processes with Eucalyptus regnans CCS pulp, using either sodium hydroxide or magnesium oxide as the alkali source, in conjunction with, and without, several additives (DTPA, sodium silicate, magnesium sulfate). Pulps were pretreated in the first stage using the filtrate from bleaching the same pulp type under second stage conditions. In view of wasteful addition of peroxide in recycled liquor, no fresh peroxide charge was added in the first stage of the bleaching process 25. Both first and second-stage bleaching processes were carried out at 60 C and 10% consistency. Results for single-stage bleaching using second-stage conditions are also shown for comparison for both alkali sources A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Without Using Stabilizers (Additives) Figures 5a and 5b show the results for a two-stage process for both alkali sources without using any additives. During the first stage, for both alkalis, a significant decrease in alkalinity of the residual liquors (low ph - 9) and significant consumption of peroxide was observed. The low concentration of residual peroxide in the liquor can be attributed to the absence of stabilizers (more decomposition of peroxide). In order to reactivate peroxide in the residual liquor, extra alkali was addec1 10. The effect of varying the amount of each alkali in the first stage was studied. 158

176 For both alkalis, duration was also varied in the first stage (Figures 5a and 5b). In the second stage the initial peroxide charge (2% on o.d. pulp), alkali charge (NaOH 2.5% and MgO 1.8% on o.d. pulp), temperature (60 C), pulp consistency (10%) and bleaching time (4.0 hours) were held constant. Brightness ( %ISO) 80 (a) Time (Hours) % NaOH Figure 5a: Two-stage bleaching of Eucalyptus regnans CCS pulp with NaOH as base, with no additives. Continuous line shows level for brightness corresponding single-stage bleaching. 80 Brightness ( % ISO) r vvrr At 7 A,V V AV A r dr A yr rer r r 60 Time (Hours) % Mg Figure 5b: Two-stage bleaching of Eucalyptus regnans CCS pulp with MgO as base, with no additives. Continuous line shows level for brightness corresponding single-stage bleaching. 159

177 Results show that sodium hydroxide produces a small brightness gain of 0.5% ISO and magnesium oxide shows a gain of 1.5% ISO under optimized conditions as compared to single-stage bleaching. The final brightness values attained using NaOH ( % ISO) are consistently 7 points below those attained with MgO ( % ISO). It can be seen that, with sodium hydroxide (Figure 5a), increased addition of alkali caused a decrease in brightness and duration showed very little effect. While with magnesium oxide (Figure 5b), increased addition of alkali showed a small increase in brightness. However, duration had no effect. Both the alkalis showed the same ph range at the end of two-stage bleaching ( ). This makes it difficult to correlate bleaching response of each alkali with ph alone A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Using Sodium Silicate Figures 6a and 6b show the results for a two-stage process for both alkali sources using sodium silicate as an additive. During the first stage, the duration (Figures 6a) was varied. However, no extra sodium hydroxide was added, because residual liquor showed a significant alkalinity (ph 11). Extra addition of sodium hydroxide could have resulted in alkali darkening of the pulp". With magnesium oxide (Figure 6b), both duration and alkali charge were varied in the first stage. In the second stage, the initial charge of peroxide (2% on o.d. pulp), alkali (NaOH and MgO 1.8% on o.d. pulp) and sodium silicate (5% on o.d. pulp) were kept constant. Pulp consistency (10%), temperature (60 C) and bleaching time (4.0 hours) were also kept constant. 160

178 Brightness ( %ISO) (a) 60 Time (Hours) Figure 6a: Two-stage bleaching of Eucalyptus regnans CCS pulp with NaOH as base, with sodium silicate. Continuous line shows level for brightness corresponding single-stage bleaching. e - 80 (b ) CID Y bi v 60 Time (Hours) % Mg Figure 6b: Two-stage bleaching of Eucalyptus regnans CCS pulp with MgO as base, with sodium silicate. Continuous line shows level for brightness corresponding single-stage bleaching. The combinations of sodium hydroxide-sodium silicate (Figure 6a) and magnesium oxide-sodium silicate (Figure 6b) show the brightness enhancement using the two-stage process produces gains of 1.51% ISO and 0.4% ISO respectively under optimized 161

179 conditions as compared to single-stage process. Results also show that, the final brightness values attained using MgO ( % ISO) are consistently 7-8 points below those attained with NaOH ( % ISO). As observed earlier, for both the alkalis, duration did not show any effect on bleaching response. The high residual peroxide obtained in liquor (at first stage), is presumably due to the stabilizing effect of sodium silicate. However, despite high residual peroxide, both the alkalis did not show significant improvement in brightness compared to single-stage bleaching A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Using Magnesium Sulfate Figures 7a and 7b show the results for a two-stage process for both alkali sources using magnesium sulfate as an additive. During the first stage, the duration and magnesium sulfate charge (Figures 7a and 7b) were varied with alkali charge constant (NaOH 1.5% and MgO 1.6 % on o.d. pulp). In the second stage, initial peroxide charge (2% on o.d. pulp), magnesium sulfate (2% on o.d. pulp) and alkali charge (NaOH 1.5 % and MgO 1.6 % on o.d. pulp) were held constant. Temperature (60 C), pulp consistency (10%), and bleaching time (4.0 hours) were also kept unchanged. The combinations of Na0H-MgSO4 (Figure 7a) and Mg0-MgSO4 (Figure 7b) show that using a two-stage processes a brightness gain of 4.1 % ISO and 1.1 % ISO respectively is achieved under optimized conditions as compared to single-stage bleaching. Results also show that the final brightness values attained using NaOH ( % ISO) are 5-8 points higher than those attained with MgO ( To ISO). It can be observed that single-stage brightness (73.3% ISO) obtained by the Na0H-Na2S io3 combination (Figure 6a), can be achieved with a Na0H-MgSO4 combination (Figure 7a). This suggests that under certain conditions, sodium silicate 162

180 can be replaced by magnesium sulfate as an additive in Eucalyptus regnans CCS pulp bleaching. For both the alkalis, duration had no effect. With increased amount of magnesium sulfate, both the alkalis did not show a change in the ph range. However, sodium hydroxide showed a significant enhancement in bleaching response while magnesium oxide showed little effect. This suggests that bleaching response cannot to be correlated with only alkalinity or ph of bleaching medium. Brightness ( %IS O) (a) 60 Time (Hours) % MgSO Figure 7a: Two-stage bleaching of Eucalyptus regnans CCS pulp with NaOH as base, with magnesium sulfate. Continuous line shows level for brightness corresponding single-stage bleaching. Brightnes s ( %ISO) Time (Hours) % MgS (b) A_ IIV r I A A WA er A r ro, Figure 7b: Two-stage bleaching of Eucalyptus regnans CCS pulp with MgO as base, with magnesium sulfate. Continuous line shows level for brightness corresponding single-stage bleaching. 163

181 7.3.5 A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Eucalyptus regnans CCS Pulp Using DTPA Figure 8a and 8b show the results for a two-stage process for both alkali sources using DTPA as an additive. For both the alkalis, DTPA charge (0.1% o.d. pulp) was kept constant during the first stage. However, duration and alkali charge (Figures 8a and 8b) were varied. In the second stage, initial peroxide charge (2% on o.d. pulp), alkali (NaOH 1.4% and MgO 1.8% on o.d. pulp), DTPA (0.1% on o.d. pulp), temperature (60 C), pulp consistency (10%) and bleaching time (4.0 hours) were kept constant. The combinations of Na0H-DTPA (Figure 8a) and MgO-DTPA (Figure 8b) when used in a two-stage processes produce a brightness gain of the same magnitude. The brightness enhancement for both alkali sources was limited to 1.6 % ISO under optimized conditions. Results obtained show that, final brightness values attained using MgO ( % ISO) are consistently one point higher those attained with NaOH ( % ISO). No change in ph occurred despite the increased addition of magnesium oxide with increased bleaching response (Figure 8b). For both the alkalis duration had no much effect. 164

182 80 (a) CID 1 ts:,> 75 a) 70 1:1: Time (Hours) % NaOH Figure 8a: Two-stage bleaching of Eucalyptus regnans CCS pulp with NaOH as base, with DTPA. Continuous line shows level for brightness corresponding single-stage bleaching. Time (Hours) % MgO Figure 8b: Two-stage bleaching of Eucalyptus regnans CCS pulp with MgO as base, with DTPA. Continuous line shows level for brightness corresponding single-stage bleaching. 165

183 Ui 8-6 airi!,4 m pasputuwns am dind The results obtained from a two-stage b leachi ng process of Eucalyptus regnans CCS Bleaching efficiency (Brightness gain/peroxide consumed) Maximum brightness (%ISO) Brightness gain (%ISO) tr.) Ui (1) NaOH t") NaOH Cr' NaOH MgO MgO MgO Na0H/Na SiO 23 Na0H/Na SiO 23 Na0H/Na SiO 23 MgO/Na SiO, 2 3 MgO/Na SiO 2 3 MgO/Na SiO 23 ro NaOHIM gs0 4 ed c.) Na0H/MgS0-4 MgO/MgS0 4 MgO/MgSO4 Na0H/DTPA Na0H/DTPA MgO/MgSO4 Na0H/MgS0 4 Na0H/DTPA \N MgO/DTPA MgO/DTPA MgO/DTPA

184 By and large, these results show slightly higher bleaching response for a two-stage process over that of a single-stage process (Figure 9a). The combination of Na0H- MgSO4 shows a maximum gain 4.1 % ISO over single-stage bleaching (Figure 9a). Figure 7a shows the effect of using magnesium sulfate as an additive instead of sodium silicate for two-stage bleaching of Eucalyptus regnans CCS pulp under optimized conditions, the maximum attainable brightness lies between that for the MgO case (Figure 5b) and the Na0H-silicate combination (Figure 6a). This is illustrated in Figure 9b. The combinations of Na0H-Na2SiO3 and Na0H-without additives, show the highest and the lowest bleaching efficiency respectively (Figure 9c). There are very few publications which discuss multi-stage bleaching of Eucalypt based pulp25,26. However, none of them has addressed the role of magnesium oxide as an alkali source used in multi-stage bleaching of any kind of pulps. The comparative study of Mulcahy et a125 showed a slightly higher brightness response for the "steep" process over that of single and two-stage tower processes in the absence of recycled spent peroxide liquor. However, when liquor was recycled, the brightness of twostage tower bleached pulp matched that of "steep" bleached pulp. The same study showed that no further improvement was observed even after doubling the peroxide charge in recycled liquor. Garland and Nelson 26 tried different multi-stage bleaching sequences for Eucalyptus regnans CCS pulp but such processes did not appreciably increase the brightness level. In our work, different combinations were tried with sodium hydroxide and magnesium oxide in conjunction with several additives and we found a slightly higher bleaching response for a two-stage process with a few exceptions. 167

185 7.4 Two-Stage Peroxide Bleaching of Pinus radiata TMP A Comparison Between Sodium Hydroxide and Magnesium Oxide in Two-Stage Bleaching of Pinus radiata TMP Pulp Figures 10a and 10b show results of two-stage processes for Pinus radiata TMP pulps using both alkali sources. During first stage, keeping DTPA charge (0.1% on o.d. pulp) constant, the duration and alkali charge (Figures 10a and 10b) were varied. In the second stage (Figure 10a), alkali charge (NaOH 1.5% on o.d. pulp), peroxide charge (3% on o.d. pulp), DTPA (0.1% o.d. pulp), temperature (60 C), consistency (10%) and bleaching time (2.5 hours) were kept constant. In case of magnesium oxide when used as an alkali, during second stage (Figure 10b), DTPA charge (0.1% on o.d. pulp) was maintained along with peroxide charge (2% on o.d. pulp) and alkali charge (MgO, 0.5% on o.d. pulp). Bleaching time (2.0 hours), consistency (10%) and temperature (60 C) were kept constant. Results for bleaching of Pinus radiata TMP using sodium hydroxide as an alkali show brightness enhancement using two-stage process is limited to 1.5% ISO as compared to single-stage (figure 10a). However, using magnesium oxide as an alkali source (Figure 10b) produces a gain of 4.5% ISO under optimized conditions producing a superior final brightness to that attained using sodium hydroxide in the two-stage process. 168

186 Brightness (%ISO) V I Time (Hrs) rm.ir. 0 50" 9" aolf '74 r. ir AA AA PAl PAl PAlA PAl PA % NaOH Figure 10a: Two-stage bleaching of Pinus radiata TMP with NaOH. Conditions: 10% pulp consistency, temperature 60 C. Continuous line shows level for brightness corresponding single-stage bleaching. Brightness ( To ISO) 67 Time (Hrs) % Mg Figure 10b: Two-stage bleaching of Pinus radiata TMP with MgO. Conditions: 10% pulp consistency, temperature 60 C. Continuous line shows level for brightness corresponding single-stage bleaching. Several studies have been reported on recycling spent liquor in bleaching process for mechanical pulps using sodium hydroxide as an alkali source8, I 0,12,13,29. No work has been reported in the literature using magnesium oxide as an alkali source employed in multi-stage bleaching of mechanical or chemical pulps. In our study, two-stage bleaching processes show a very slight increase in brightness over single-stage processes using sodium hydroxide as an alkali. In the case of magnesium oxide, a 169

187 significant brightness gain was observed. The bleaching response gradually increased with increase of sodium hydroxide charge in first stage bleaching (Figure 10a). It suggests that, hydrogen peroxide present in spent liquor was reactivated by adding extra alkali 10. The addition of magnesium oxide in the first stage also shows an increasing trend in bleaching response only up to 1% magnesium addition (Figure 10b). However, further addition decreases brightness despite the fact that no change occurred in ph's. It suggests that the bleaching response cannot be correlated to ph of the bleaching medium. Duration showed a little effect with magnesium oxide while sodium hydroxide had no effect. Figure 11 shows results of two-stage processes for Pinus radiata TMP carried out at 4% consistency and 50 C temperature using sodium hydroxide as an alkali. The pulps were prechelated with DTPA and washed prior to bleaching. During the first stage of bleaching, duration and alkali charge were varied. In second stage, alkali charge (NaOH 1.0% on o. d. pulp), peroxide charge (3% on o.d. pulp) and bleaching time 150 minutes were kept constant along with constant consistency 4% and temperature 50 C. Results for single-stage bleaching using second stage conditions are shown for comparison. Results obtained from two-stage bleaching show a detrimental effect of recycling residual liquor on brightness ( % ISO) compared with single-stage bleaching brightness (70.1% ISO). 170

188 77 Brightnes s ( %ISO) 75 " Time (Minutes) % NaOH Figure 11: Two-stage bleaching of Pinus radiata TMP with NaOH. Conditions: 4% pulp consistency, temperature 50 C. Continuous line shows level for brightness corresponding single-stage bleaching. This study shows that a comparison between single-stage and two-stage processes is not a straightforward relationship. Several studies have reported that the success of the individual process is governed by many factors such as consistency and target brightness. Meyrant et a129 reported that no difference was found in the performance of recycled bleach liquor and fresh bleach liquor with regard to brightness development, making it difficult to justify a sophisticated bleaching process (two-stage) over a simpler process (single-stage) based on brightness gain alone. Strand et al8 reported that when bleaching spruce to a high brightness of 80% ISO, a significant amount of savings could be possible by installing a two-stage process compared to one-stage process. Their study was conducted at medium consistency in the first tower and high consistency in the second tower, although all peroxide was charged in the second stage and peroxide filtrate was recycled to the first stage8. 171

189 In this work, various recirculation strategies have been investigated for bleaching of Pinus radiata TMP and Eucalyptus regnans CCS pulp, without altering the consistency at the first and second stage of bleaching processes. The results obtained in our study suggest that the bleaching response at the end of a two-stage process cannot be correlated to the alkalinity or ph of the bleaching medium alone. In our study, no straightforward correlation was found between brightness gain, peroxide consumption and the alkalinity of bleaching medium. The resultant effect observed at the end of twostage bleaching could be due to different effects exerted by several factors: ( I) Effects occurring at the single-stage bleaching which subsequently carried forwarded to a two-stage process (2) Solubility of the alkali (3) The interaction between alkali and stabilizers (4) The type of pulp (effect on lignin) In view of these complex effects the interpretation of the observed results is not straightforward. However, this work will provide a information about the effect of a two-stage bleaching process and will help to carry out further investigations in this area. 7.5 Conclusions In contrast to bleaching systems using sodium hydroxide as the base, the addition of additives (DTPA, sodium silicate, magnesium sulfate) in conjunction with magnesium oxide produces a reduction in brightness gain. However, magnesium oxide alone (without stabilizers) produces higher brightness response than sodium hydroxide alone (without stabilizers) indicating that the alkalinity of the bleaching medium is not the only deciding factor in bleaching. 172

190 In single-stage bleaching of Eucalyptus regnans CCS pulps, during extended period of bleaching time, no buffering effect of residual magnesium oxide is observed. Maximum brightness gain obtained with sodium hydroxide could not be achieved with magnesium oxide even using extended bleaching time. This suggested that magnesium oxide as a substitute alkali source is not quite as effective in terms of brightness gain as sodium hydroxide. In two-stage bleaching of Eucalyptus regnans CCS pulps, in presence of DTPA, both alkalis show similar brightness gains at optimized conditions. Magnesium oxide shows maximum brightness when used with no additives. In the presence of magnesium sulfate, sodium hydroxide shows a significant brightness gain compared with magnesium oxide. Under optimized conditions, sodium silicate can be replaced by magnesium sulfate as an additive used in Eucalyptus regnans CCS pulp bleaching. For both alkalis, with the use of sodium silicate, a high level of residual peroxide was obtained in the liquor. However, despite high residual peroxide, both alkalis did not show significant brightness gain over single-stage bleaching process. No simple correlation was observed between brightness, ph of the bleaching medium and peroxide consumption. For two-stage bleaching of Pinus radiata TMP, magnesium oxide shows superior final brightness to that attained using sodium hydroxide. Under certain conditions, recycling residual peroxide produces detrimental effect on bleaching response of Pinus radiata TMP. By and large, there was small to no improvement in brightness response in going from a one-stage to a two-stage process, although some exceptions show interesting results for both pulps (Eucalyptus regnans CCS and Pinus radiata TMP). In view of marginal gains (compared to single-stage process) obtained in the brightness of two-stage 173

191 bleached Eucalyptus regnans CCS pulp, it appears that the idea of recycling the bleach liquor in two-stage bleaching processes may not have significant industrial advantages. Differences observed in bleaching response with the various alkalis and stabilizers are not solely due to ph and alkalinity effects, but other factors such as the interaction between the additives, alkalis and pulp type. The lignin content, wood species and pulp pretreatment may be playing an important part in explaining many of the variations in bleaching responses observed both in the literature and our current work. Further work is needed in this area to investigate the effect of these factors. 174

192 REFERENCES 1. Andrews, D. H. and Singh, R. P. in "The Bleaching of Pulp", R. P. Singh (Ed.),3rd Edition, C 2. Allison, R. A., Appita, 36(5), 362, (1983). 3. Colodette, J., Fairbank, M. G. and Whiting, P., Pulp and Paper Science, 16(2), J53, (1990). 4. Burton, J. T., Campbell, L.L. and Donnini, G. P., Pulp and Paper Canada, 88(6), T224, (1987). 5. Colodette, J., Rothenberg, S. and Dence, C. W., Pulp and Paper Science, 15(2), J45, (1989). 6. Ali, T., Fairbank, M., Mcarthur, D., Evans, T. and Whiting, P., Pulp and Paper Science, 14(2), J23, (1988). 7. Burton, J. T., Pulp and Paper Science, 12(4), J95, (1986). 8. Strand, E., Koponen, R., Edwards, L. L., Moldenius, S. and Viljakainen, E., Tappi Pulping Conference, 497, (1987). 9. Lachenal, D., Bourson, L.and Lachapelle, R., CPPA Annual Meeting, B193, (1989). 10. Lachenal, D., Dubreuil, M. and Bourson, L., Tappi, 73(10), 195 (1990). 175

193 11. Hook, J. and Wallin, S., Tappi Pulping Conference, 267, (1989). 12. Abbot, J., Mahagaonkar, M., Rao, R., Ault, K.and Vanderaa, S., CPPA - Spring Conference, (1992). 13. Abbot, J., Vanderaa, S., Wright, P. and Ault, K., Appita, 45(6), 393, (1992). 14. Vromen, M. R., Appita Conference, 167, (1993). 15. Griffiths, P. and Abbot, J., Appita, 47(1), 50, (1994). 16. Nystrom, M., Pykalainen, J. and Lehto, J., Paperi ja puu, 75(6), 419, (1993). 17. Lords, V., in "Pulp and Paper - Chemistry and Chemical Technology", Volume 1, Chapter 5, "Bleaching", J. P. Casey, (Ed.), 3rd Edition, Wiley-Interscience, New York, (1980). 18. Maughan, S. E., Appita Conference, 123, (1992). 19. Soteland, N., Abadie-Maumert, F. A. and Arnevik, T. A., International Pulp Bleaching Conference, 231, (1988). 20. Koenig, R. L., Chemistry In Australia, 59(5), 225, (1992). 21. Michell, A. J., Nelson, P. J. and Chin, C. W., Appita, 42(6), 443, (1986). 22. Ariadi, B. and Abbot, J., Appita, 45(3), 178, (1992). 176

194 23. Abadie-Maumert, F. A. and Soteland, N., International Pulp Bleaching Conference, 99, (1985). 24. Lachenal, D., Dechoudens, C. and Bourson, L., Tappi, 70(3), 119, (1987). 25. Mulcahy, J., Neilson, M. and Maddern, K. N., Appita, 42(6), 424, (1989). 26. Garland, C. and Nelson, P., Appita, 42(5), 354, (1989). 27. Maughan, S. E., Appita Conference, 195, (1992). 28. Strand, E., Moldenius, S., Koponen, R., Viljakainen, E. and Edwards, L., Tappi, 71(7), 130, (1988). 29. Meyrant, P., Kouk, R. S. and Presley, J. R., Bleaching Pulping Conference, 81, (1987). 30. Allison, R. W. and Graham, K. L., Pulp and Paper Science, 16(1), J28, (1990). 31. Dionne, P. Y., Seccombe, R., Vromen, M. R. and Crowe, R. W., Paper Technology, 36(3), 29, (1995). 177

195 CHAFFER 8 SUMMARY AND CONCLUSIONS 8.1 Deinking The present study has investigated various aspects of flotation deinking of newsprint and magazines. Particular emphasis has been placed on the elucidation of the mechanisms involved during flotation deinking of newsprint and magazines. In order to have a better understanding of this complex process, studies have been made on the effect of process variables such as flotation time, feedstock compositions and ph. No evidence could be found to support the idea that ash components from the magazines facilitates the removal of ink from newsprint. The effects of different stages of deinking on properties of recycled paper have been investigated. Furnishes of mixed magazine and newsprint showed distinct differences in the properties after pulping and flotation. These differences can be explained by the change in proportions of mechanical and chemical fibres, filler, fines and ink during flotation. After the pulping stage, the trends in strength properties were influenced by detrimental effects of filler from magazines. However, after the flotation stage, due to loss of a major portion of filler and fines, an enhancement in all strength properties occurred. Because of complexity of the system, the effect of loss of fillers and fines on the overall improvement of strength properties was difficult to, quantify. The investigations also revealed that, despite the presence of filler in the system, optical properties like brightness and light absorption coefficient appeared to be influenced more by ink content. The light scattering coefficient was dominated by the fibre type. After flotation, despite the highest brightness value, the all-magazine furnish (100% OMG) had the highest ink speck count. Porosity and freeness were influenced more by 178

196 the effects exerted by chemicals used in the system rather than fines and fillers. Extended periods of flotation had no significant effect on properties of the recycled paper. The effect of deinking using various alkali sources was also investigated. Significant differences in the strength and optical properties of the deinked pulp occurred with the use of different alkalis. This study showed that magnesium oxide and ammonium hydroxide can be used as alternative alkalis in flotation deinking of newsprint and magazines. Despite the same ink removal, both the alkalis produced slightly lower brightness pulp compared to sodium hydroxide. Alkalis like calcium oxide, calcium hydroxide and magnesium hydroxide show poor ink removal. This could be due to reduced solubility of the alkalis and complex reaction mechanism occurring in the pulping stage. Each alkali showed a different bleaching response at the pulping stage. However, due to complexity of the system, these results were difficult to explain with respect to bleaching chemistry. Significant changes in strength properties of the pulps deinked with different alkalis occurred despite no change in ash content (of the handsheets) and average fibre length of pulps being observed. These properties were influenced by alkali hydrolysing effect. Variations in the hydrolysing effect of the various alkalis influences the hydrogen bonding potential of the fibres. Deinked pulp using sodium hydroxide showed the highest tear, tensile and burst indices and the lowest freeness, porosity and stretch. Divalent alkalis like magnesium oxide and calcium oxide showed lower values of tear, tensile and burst indices compared to sodium hydroxide. These divalent ions bind to the bridge-forming sites in the fibres and thus prevent both bridging and hydrolysis and consequently reduce pulp strength. 179

197 Other properties like bulk and roughness were unaffected by varying the alkali. High values of C.O.D. and dissolved solids were obtained for alkalis like sodium hydroxide, magnesium oxide and ammonium hydroxide. These alkalis were found to generate high alkaline ph in the pulping stage. These high alkaline conditions can lead to dissolution of wood components like lignins which increases C.O.D. The same effect was also observed with dissolved solids present in the effluent. In this work the role of sodium silicate was investigated. The effect of addition of sodium silicate was found to vary with the different alkalis. A beneficial effect was observed with magnesium oxide, calcium hydroxide and magnesium hydroxide. This effect was attributed to the formation of insoluble magnesium and calcium silicate complexes which agglomerate small ink particles to form a suitable particle size required for efficient ink removal. A detrimental effect of sodium silicate observed with calcium oxide was attributed to the complex reaction occurring in the pulping stage. A negligible effect of sodium silicate addition with sodium hydroxide and ammonium hydroxide suggested that silicate free deinkin2 was possible with these alkalis. 8.2 Bleaching In this study, sodium hydroxide and magnesium oxide have been compared as alkali sources in single-stage and two-stage bleaching of Eucalyptus regnans CCS and Pinu.s radiata TMP. In contrast to using sodium hydroxide, addition of DTPA, sodium silicate or magnesium sulfate to magnesia produces a reduction in pulp brightness gain. However, magnesium oxide alone (without stabilizers) produced higher brightness response than sodium hydroxide alone (without stabilizers) indicating that the alkalinity of the bleaching medium was not the only deciding factor in the bleaching of Eucalyptus regnans CCS pulp. 180

198 Possibilities of internal recycling of liquor during two-stage peroxide bleaching processes using either sodium hydroxide or magnesium oxide as an alkali, have also been investigated. Utilization of residual peroxide in a sequential two-stage process was generally ineffective during alkaline peroxide bleaching of Pinus radiata TMP and Eucalyptus regnans CCS pulps, although some exceptions showed encouraging results. Under certain conditions, recycling residual peroxide produced detrimental effects on the bleaching response of Pinus radiata TMP. Despite high residual peroxide, both alkalis did not show significant brightness gain over single-stage bleaching process. For both alkalis, no simple correlation was observed between brightness, ph of the bleaching medium and peroxide consumption. Differences observed in bleaching response with the various alkalis and stabilizers were not solely due to ph and alkalinity effects, but other factors such as the interaction between the additives, alkalis and pulp type. The lignin content, wood species and pulp pretreatment may be contributing factors in explaining the variations in bleathing responses observed in this work. Further work is needed in this area to investigate the effects of these factors. 181

199 CHAPTER 9 EXPERIMENTAL 9.1 Deinking - (Chapters 2,4,5) Stock Preparation and Reagents Old newsprint (ONP) was obtained in batches of recently printed offset newspaper (3 months old) from The Mercury Press in Hobart, while a range of magazines (OMG) were obtained from Angus and Robertson book store in Hobart. A selection of highly coated magazines (e.g. Cosmopolitan and Cleo ) was taken as representative of coated magazines with high ash content (30%). Magazines with lower ash content (9%), designated OMG (low) was represented by samples of TV Week. As with the newspapers used, the age of these magazines was approximately 3 months. Magazines and newsprint were individually cut into mm. squares. All staples and glues were removed prior to pulping, and samples were stored in opaque plastic bags. In chapter 4, four samples ranging from 0 to 100% OMG were used in the deinking process. In chapter 5, a combination of 70% (ONP) and 30% (OMG) was used as a feedstock. The sources of alkalinity used in this work were sodium hydroxide (98%), magnesium oxide (99%), magnesium hydroxide (99%), calcium oxide (98%), calcium hydroxide (98%), and ammonium hydroxide (30%). All were obtained from BD1-1 Chemicals. Hydrogen peroxide (30%) and DTPA (99%) were obtained from BDH Chemicals and Aldrich respectively. Sodium silicate (30%) was from Aluminates (Tas) Pty. Ltd., Burnie, Tasmania. The quantities of the chemicals charged in the deinking formulations were calculated as a percentage of the oven 182

200 dry weight of paper fed to the pulper. The charges of chemicals added were corrected to 100% purity of each chemical. The deinking surfactant employed was from Buckman laboratories (Type BRD-2307) and was found by GC/MS analysis to consist predominantly of stearic, oleic and linoleic fatty acids Pulping and Flotation A Lamort Deinkit was used for pulping and flotation. In the pulping stage, a Helico pulper was installed into the unit. For flotation, the Lamort Helico rotor was replaced by the Lamort Hyperflotation kit incorporating an aeration screen and air suction column on the rotor and an overflow weir for ink sludge collection (See photographs in Figures 1 a and lb). In the pulping stage, optimum conditions were used. Pulping was carried out using 750 g o.d. weight (as such OMG and ONP) at 8% pulp concentration (p.c.). Chemicals (1% hydrogen peroxide, 0.4% surfactant, 0.2% DTPA, 1% sodium hydroxide, 1% sodium silicate on o.d. weight) were added to hot water at 50 C prior to addition of the waste paper. Once a good rolling/mixing action was achieved, the pulping stage was continued for 20 minutes. In the flotation stage, optimum conditions were used and 450 g o.d. of the repulped stock was diluted to 1% p.c.; a constant ph 8.5 was maintained. A ten minute of flotation time was used. Surfactant employed in this study was fatty acid based; an optimum level of water hardness (as CaCO3) was maintained in the flotation cell (200 ppm) 7,8, Experimental Error and Confidence Limits The extent of variations obtained in optical and physical properties measurement was very low, when Lamort Deinkit was used. The percentage error was less than 0.5% for all optical, strength and other physical properties measurements. Every reported value is an average of six different measurements. 183

201 Figure la: A Lamort deinkit operating in the pulping mode. Figure lb: A Lamort deinkit operating in the flotation mode. 184