SAVING ENERGY BY APPLICATION OF OZONE IN THE THERMOMECHANICAL PULPING PROCESS

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1 SAVING ENERGY BY APPLICATION OF OZONE IN THE THERMOMECHANICAL PULPING PROCESS M. Lecourt*, B. Struga +, T. Delagoutte + and M. Petit-Conil + * AFOCEL - InTechFibres, Wood-Process Laboratory, BP 251, Grenoble cedex 9, France. + Centre Technique du Papier InTechFibres, BP 251, Grenoble cedex 9, France. corresponding author : michel.petit-conil@webctp.com ABSTRACT To reduce energy consumption in TMP pulping, ozone was tested in the main line (directly in refiners) or in the reject line. The experiments were carried out at pilot plant scale (ozone in refiners) and at laboratory scale (reject treatment) with spruce chips. The application of ozone in the eye of the primary or secondary refiners modified the fibre separation mechanisms, reducing electrical energy consumption by 10 to 20%. It was demonstrated that ozone mainly oxidised the wood extractives and consequently modified the fibre surface chemistry and the final pulp quality. When used in refiners, ozone decomposition at high temperature and ozone reactions with fibre wall components were in conflict and reduced the ozone efficiency. Ozone treatment of TMP rejects seemed a very promising technology. An ozone charge of 1 to 2% on rejects gave an economy of 10 to 20% of energy, degradation of wood extractives and modification of lignin at the fibre surface. The reintroduction of these ozonated-refined reject fibres in the TMP accepts improved the strengths and the peroxide bleachability of the final pulp. These effects could quite easily compensate the costs of ozone generation and use. INTRODUCTION Ozone was used for the first time in a mechanical pulping process in 1964 /1/. It was mainly applied on SGW, RMP or TMP pulps after secondary refining to increase pulp strengths. The efficiency could be classified as such: TMP > RMP > SGW. It was also tested between defibering and refining stages of TMP or CTMP processes to reduce energy consumption and to enhance pulp quality /2-9/. For softwood species, the main drawn back was a decrease in brightness whereas the inverse was observed for hardwood species, revealing that the lignin structure was an important parameter in ozone reaction. When a charge of 2.5% in ozone was applied on birch primary-refined CTMP in alkaline medium as an interstage, the energy consumption was decreased by 45%, the tensile and tear indexes increased by about 90% /10/. These interesting results were associated with a brightness and pulp yield loss of 3 points. Kojima et al. /11-12/ demonstrated that the oxidation of lignin by ozone modified the fibre surface. Petit-Conil et al. /13/ studied in detail the effect of inter-stage ozone treatment. Important energy savings, up to 40% for an ozone charge of 3%, and pulp quality enhancement were achieved with softwoods and hardwoods without detrimental effect on brightness and pulp bleachability. Ozone reacted with fibre wall components at the fibre surface /14-15/. In the macromolecule of lignin, ozone reacted 1) by oxidizing the lateral chains associated with a depolymerisation of the macromolecule, 2) by opening the aromatic ring and 3) by forming water-soluble organic acids. This delignification occurred in the composite lamella modifying the fibre flexibility. With polysaccharides, an oxidation of terminal hemiacetal groups produced some aldonic acids. The oxidation of primary and secondary alcohol groups generated some carbonyl and carboxyl groups by opening the pyranosidic ring. In mechanical pulps, the cellulose was generally protected by the lignin and only the hemicelluloses seemed to be affected /16/. All these chemical reactions modified the hydrophilicity of the fibre surface, enhancing interfibre bonding potential. Furthermore, ozone could act as a chemical refining agent: it increased fibre wall hydration and facilitated microfibril separation. This resulted in a decrease in pulp freeness after ozonation /17-18/. Robert et al. /19/ analysed, by 13 C NMR, the lignin of ozonated softwood TMP. The carbon skeleton presented far fewer structural modifications than the lignin extracted from chemical pulps. There was no correlation between the strength improvement and the increase in carbonyl groups in lignin. The GPC analysis revealed that ozone did not degrade the polymeric structure of lignin in mechanical pulp. It was suggested that fibre flexibility could be the main modification and explanation of the pulp strength improvement. Recently Hsieh et al. /20/ demonstrated that ozone was an efficient bleaching agent for TMP if charges between 0.1 and 1% were used. Saharinen et al. /21/ enhanced the bulk and the internal cohesion of multilayer board by

2 ozonating mechanical pulp. A charge of ozone lower than 3% at low consistency increased the Scott bond of long fibres by 250% without decrease in bulk. Ozone has also been proven to possess other, well-known, potential applications such as effluent treatment. Ozone has been a selective agent for removing resin and fatty acids in CTMP effluents /22/. Small charges of ozone jave been sufficient for degrading these detrimental components in effluents and reducing effluent toxicity. Today, mechanical pulp producers are faced with a drastic increase in energy costs. Even if ozone production consumes electricity, its application in the process is an alternative to be considered not only to decrease specific energy but also to enhance pulp quality, pulp bleachability, runnability on the paper machine and the pulping effluent. As ozone is now widely used in chemical pulp bleaching with well-controlled technologies., it is interesting to consider it for mechanical pulp manufacture. The objectives of this work were to determine the potential energy savings of ozone use in the main line or in the rejects line of the TMP process. The ozone effects on the pulp quality and on the main fibre wall components, and more particularly on the wood extractives, were studied. MATERIALS AND METHODS TMP Production with Introduction of Ozone in the Refiners Thermomechanical pulps were produced on the CTP refiner mechanical pulp pilot plant with Norway spruce chips. Ozone (~1% on o.d. chips) was introduced in the eye of the primary or secondary refiner. As refiners could be considered excellent mixers, they can increase the efficiency of chemical reactions. Either ozone or oxygen was supplied under pressure (6 bar) directly in the eye of the primary or secondary refiner just when the chips entered the defibering or refining zone of the plates in order to keep the ozone from moving away with the back-flow steam. A mechanical pre-treatment of the chips was done with a MSD pressafiner to reduce their size and to improve the accessibility of the gas to the fibres,. The TMPs were manufactured under the following conditions: Pre-steaming for 20 seconds at 110 C Steaming at 120 C for 5 minutes under a pressure of 150 kpa Primary refining under pressure with continuous feeding of the chips at a rotor speed of 3500 rpm (plate pattern: Andritz D2A505) In line secondary refining at atmospheric pressure (plate pattern: Andritz C2976) Screening of the pulp with the Lam RCF screening system equipped with 3mm-holed and 0.3mm-slotted screens. Ozone was produced with an Ozonia OZAT generator with a flow of 1.68 m 3 /h under a pressure of 1 bar and pressurised before use. Ozonation of primary-refined TMP A primary-refined spruce TMP was treated with 1.1% ozone at 30% consistency and 60 C in a specific rotating laboratory reactor. The pulp was then refined in a 12 single disc refiner at 8% consistency equipped with refining plate pattern. Ozonation of TMP Screening Rejects Industrial Norway spruce TMP rejects were submitted to different ozone charges at 60 C and 35% consistency without a ph buffer. The experiments were carried out in a rotating flask equipped with a fritted glass gas dispersion tube. The pulp sample was preheated at 60 C and the rotating reactor was immersed in a thermostatic bath to keep constant the temperature. The control and the ozonated samples were refined at atmospheric pressure with a 12 Sprout Waldron refiner equipped with refining plate pattern at 8-10% consistency to reach different freeness levels. To evaluate the impact of ozonated rejects on the final TMP quality, the rejects refined at 100 ml CSF were reintroduced into the accepted TMP with the following ratio: 55% rejects 45% accepts. Peroxide Bleaching and Pulp Analysis The bleaching experiments were carried out with 3% H 2 O 2, 2.1% NaOH, 2% commercial Na 2 SiO 3, 0.2% DTPA, 70 C, 2h, 17% consistency in polyethylene bags. After bleaching chemical introduction and mixing, the pulp was maintained at 70 C in a thermostatic bath. Pulp properties were determined on 80 g/m² Rapid Köthen handsheets according to ISO standards. Fibre characteristics were measured with MorFi, CyberFlex, CyberBond analysers. Carboxylic acid groups in lignin were analysed with Tappi T273 om-93 method. Wood extractives were measured according to the Tappi T204 method. Extractives components were analysed by GC.

3 RESULTS AND DISCUSSION TMP Production with Introduction of Ozone in the Refiners Effect of Ozone on Energy Consumption A low charge of ozone injected in the primary refiner induced a different pulping behaviour: specific energy decreased when freeness decreased (figure 1). Oxygen had no significant effect on the specific energy. The use of a similar ozone charge in the secondary refiner gave important energy savings for freeness lower than 100 ml CSF. The most interesting option for reducing the specific energy of the TMP process was the association of the chip mechanical pre-treatment and the ozone: 15 to 20% energy savings were observed. While the use of this chip mechanical pre-treatment reduced energy consumed, the combination of the two treatments was cumulative (results not shown). It was interesting to note that a TMP pulp at 50 ml CSF could be produced with only 2100 kwh/t when the chips were compressed and defiberised with 1% of ozone. A charge in ozone of only 1% was sufficient to modify the TMP process parameters. Specific Energy, kwh/t Freeness, ml CSF Control 1.1% O2 in primary refiner 1.1% O3 in primary refiner M SD+1.15% O3 in primary refiner 1.1% O3 in secondary refiner Figure 1: Evolution of specific energy with freeness for the manufacture of spruce TMP. Impact of oxygen or ozone injected in the refiners and of the chip mechanical pre-treatment. Ozone had a limited effect on primary refiner energy consumption (figure 2), whereas the combination of ozone and chip mechanical pre-treatment facilitated the separation of the fibres because the reduction of chip size facilitated the reactions with ozone. In spite of the energy consumed for this mechanical pre-treatment (~100 kwh/t), this supplement of energy was compensated by energy savings of 20%in the primary refiner. Primary refiner energy, kwh/t Freeness, ml CSF Secondary refiner energy, kwh/t Control Freeness, ml CSF 1.1% O2 in primary refiner 1.1% O3 in primary refiner MSD+1.15% O3 in primary refiner 1.1% O3 in secondary refiner Figure 2: Evolution of primary refiner and secondary refiner energy with freeness for the manufacture of spruce TMP. Impact of oxygen or ozone injected in the refiners and of the chip mechanical pre-treatment.

4 Ozone had the most impact during pulp refining (figure 2). It had an effect on the fibre development and on the breakage of shives present in the primary-refined pulp. From these experiments, it was concluded that ozone in refiners could be an alternative for reducing energy consumption during TMP pulping and that the gas was still efficient at the temperature reached in the plate gap (~180 C). Even if decomposition reaction occurred, part of the gas was sufficient enough to modify the wood pulping behaviour. Effect of Ozone on TMP Quality If ozone in the refiners had no significant effect on the TMP bulk (variation < 5%), the tensile index was significantly improved (figure 3). When ozone was injected in the primary refiner, the tensile index increased by 15% at any freeness level. This improvement was associated with a conservation of tear strength, except when ozone was combined with chip mechanical pre-treatment (figure 3). The mechanical pre-treatment slightly degraded the fibre intrinsic strength, by acting on the earlywood fibres, with a consequence on the tensile strength, explaining why the combination of ozone and chip mechanical pre-treatment did not develop more tensile strength. Oxygen was also able to produce stronger pulp at the same specific energy. Ozone seemed to modify fibre flexibility without internal degradation, affecting some of the physical pulp properties. Slightly lower air permeability and Scott Bond values were also observed for all the TMPs produced with gas injected into the refiners. Only the combination of chip mechanical pre-treatment and ozone improved the internal cohesion of the pulp. 5,5 30,0 Tear index, mnm²/g 5,0 4,5 4,0 3,5 3,0 2,5 2, Control 1.1% O2 in primary refiner 1.1% O3 in primary refiner MSD+1.15%O3 in primary refiner 1.1% O3 in secondary refiner Tensile index, Nm/g 29,0 28,0 27,0 26,0 25,0 24,0 23,0 22,0 21,0 20, Freeness, ml CSF Freeness, ml CSF Figure 3: Evolution of tear and tensile indices with freeness for the manufacture of spruce TMP. Impact of oxygen or ozone supplied to the eye of the refiner and of the mechanical pre-treatment of the chips. Oxygen and ozone in the refiner increased the brightness of the unbleached TMP, but the efficiency was higher when the gas was injected in the primary refiner (figure 4). This effect of oxygen on pulp brightness has already been observed /23/. Ozone reacted with the fibre wall components, limiting the generation of chromophores during defibering and refining. Otherwise, ozone had no significant detrimental effect on the scattering coefficient. Brightness, % ISO Control 1.1%O2 in primary refiner 1.1%O3 in primary refiner MSD+1.15% O3 in primary refiner 1.1%O3 in secondary refiner Scattering coefficient, m²/kg 70,0 68,0 66,0 64,0 62,0 60,0 58,0 56,0 54,0 52,0 50, Freeness, ml CSF Freeness, ml CSF Figure 4: Evolution of brightness and light scattering coefficient with freeness for the manufacture of spruce TMP. Impact of oxygen or ozone injected in the refiners and of the chip mechanical pre-treatment.

5 If ozone or oxygen, injected into the refiners had no significant effect on the mean fibre length, the fibre width decreased, except when ozone was combined with a chip mechanical pre-treatment (figure 5). The fibre separation mechanism seemed to be modified. The main effect of ozone or oxygen was observed on the bonding potential of the fibres (figure 6). The fibre surface chemistry was modified so that higher hydrogen bonding occurred. Some of the fibre wall components were degraded by the gas. The external fibrillation slightly improved, due to easier fibre refining. Finally, the shives content was slightly reduced when ozone was injected in the refiners (not shown). The ozone modified the behaviour of the wood particles in the plate gap, facilitating fibre separation and increasing the defibering/refining efficiency by modifying the friction coefficient and the fibre surface chemistry. Mean fibre length, mm 0,88 0,86 0,84 0,82 0,8 0,78 0,76 0,74 0,72 0, Freeness, ml CSF Control 1.1%O2 in primary refiner 1.1%O3 in primary refiner MSD+1.15% O3 in primary refiner 1.1%O3 in secondary refiner Mean fibre width, µm Freeness, ml CSF Figure 5: Evolution of mean fibre length and width with freeness for the manufacture of spruce TMP. Impact of oxygen or ozone injected in the refiners and of the chip mechanical pre-treatment. Fibrillation index, % Freeness, ml CSF Control 1.1%O2 in primary refiner 1.1%O3 in primary refiner MSD+1.15%O3 in primary refiner 1.1%O3 in secondary refiner Relative bonded area index, % Freeness, ml CSF Figure 6: Evolution of fibrillation index and relative bonded area index (RBA) with freeness for the manufacture of spruce TMP. Impact of oxygen or ozone injected in the refiner and of the chip mechanical pre-treatment. Finally ozone had no detrimental effect on the TMP bleachability (table 1). The highest brightness was reached with ozone in the secondary refiner or in combination with chip mechanical pre-treatment. In all cases where ozone was used, peroxide savings were noticed with the corresponding decrease in bleaching costs. Trial Control O 2 in primary refiner O 3 in primary refiner MSD + O 3 in primary refiner O 3 in secondary refiner Brightness, % ISO Peroxide consumption, % Brightness gain, % ISO Bleachability, % ISO/% H 2 O 2 consumed Table 1: Alkaline peroxide bleaching of spruce TMP (100 ml CSF) produced conventionally or in presence of ozone or oxygen in the refiners.

6 Effect of Ozone on the Fibre Wall Components Examination with light microscope revealed some differences in the pulp suspension (figures 7-8). Oxygen-treated TMP fibres appeared brighter and were more damaged and cut at the ends. When ozone was injected in the primary or secondary refiner, the fibres were more flexible and many long fibrils were observed in the pulp suspension. Figure 7: Microphotographs of spruce control TMP fibres (left) and oxygen-treated TMP fibres (right) at a magnification of 20x. Figure 8: Microphotographs of spruce TMP fibres produced with ozone in the primary refiner (left) and the secondary refiner (right) at a magnification of 20x It is recognised that, when applied on refined mechanical pulp, ozone creates carboxylic acid groups on lignin and polysaccharides, explaining the increase in strength properties (table 2). Compared to the control TMP, the ozone, injected in the refiners, did not generate carboxylic acid groups. Due to the accuracy of the method, the differences measured were not significant. On the contrary, peroxide bleaching became much more efficient in creating carboxylic acid groups. The increase in tensile strength was not explained by an increase in carboxylic acid groups. Trial Control O 2 in primary refiner O 3 in primary refiner MSD + O 3 in primary refiner O 3 in secondary refiner Unbleached pulp, mmol COOH/100g of pulp Bleached pulp, mmol COOH/100g of pulp Table 2: Carboxylic acidic group content in unbleached or peroxide-bleached spruce TMP produced conventionally or in presence of ozone or oxygen in the refiners. It is hypothesised that ozone did not react with lignin when injected into the refiner. In order to validate this hypothesis, Klason lignin content was measured on the control and ozone-treated (in secondary refiner) pulps and did not show any variation:

7 o Control: 27.9% o Ozone-treated pulp (secondary refiner): 28.0% Moreover, 13 C NMR spectroscopy on lignin extracted from these pulps gave the same spectra, confirming that the lignin moiety was not structurally affected by ozone, used as such. To complete this investigation, the wood extractives, which cover an important part of the fibre surface were measured on the different pulps (table 3). Ozone reacted with the wood extractives present in the wood and at the fibre surface. The decrease in wood extractives could explain the higher tensile index and relative bonding area index. The degradation of wood extractives from the fibre surface liberated some sites for hydrogen bonding during sheet formation. Brandal and Lindheim /24/ demonstrated that resin deposited at fibre surface impeded the fibre linkages. A solvent extraction of the pulp induced an increase in pulp strengths. Soteland /8/ suggested a preferential reaction with wood extractives in the very early stages of ozonation. Trial Control O 3 in primary refiner O 3 in secondary refiner Wood extractives content, % Variation, % Table 3: DCM-wood extractives in spruce TMP produced conventionally or in presence of ozone or oxygen in the refiners. COD increased when oxygen or ozone was injected into the refiner (table 4). The increase was higher when ozone was used in the secondary refiner where the fibre wall components were more accessible. A chemical reaction was present, confirming that ozone reacted with certain of the fibre wall components, namely the wood extractives. Ozone also modified fibre chemistry by reducing the cationic demand, but after peroxide bleaching, the cationic demand was only increased for the pulp produced with ozone in the secondary refiner. This would have an incidence on paper machine wet end chemistry. Trial Control O 2 in primary refiner COD, g/l kg/t of pulp Dissolved solids, g/l O 3 in primary refiner After MSD MSD + O 3 in primary refiner O 3 in secondary refiner kg/t of pulp Cationic demand, eq/t of pulp After bleaching Table 4: Analysis of effluents from TMP (100 ml CSF) produced conventionally or with oxygen or ozone in the refiners. TMP Production with Inter-Stage Ozone Treatment In order to complete our investigation on the potential uses for ozone in the mechanical pulping process, an interstage ozone treatment was considered for spruce TMP (table 5). This treatment of this pulp did not result in refining energy savings. The only interesting advantages were the improvement of pulp physical properties: tensile (+49%) and tear (+29%) indices, Scott Bond (+85%) and a decrease in shives content (-27%). The main drawback was the decrease in brightness (-2.2 points) and in fibre length (- 38%), indicating that ozone reactions favoured the formation of some chromophores during refining and fibre cutting. It was also interesting to note that the mean shive width was divided by two, confirming the enhanced refining ability of the ozone-treated pulps. Some attention might be given to adapting refining conditions to these pulps to avoid fibre cutting. When a peroxide bleaching stage was carried out on these pulps, similar final brightness levels were attained: 74.6% ISO and 74.2% ISO for TMP and ozone-treated pulp, respectively. The difference in brightness of the unbleached pulp was compensated by a slightly better bleachability. Compared to the control, the inter-stage ozone treatment generated more COD in the effluent and revealed a drastic decrease in pulping yield. The coarseness of the defibered pulp was favourable to the chemical reactions of ozone with the fibre wall components. Some lignin and wood extractives degradation occurred.

8 TMP 1.1% O 3 Freeness, ml CSF Primary refiner energy, kwh/t Secondary refiner energy, kwh/t Specific energy, kwh/t Bulk, cm 3 /g Tensile index, N.m/g Tear index, mnm²/g Scott bond energy, J/m² Brightness, % ISO Absorption coefficient, m²/kg Scattering coefficient, m²/kg Mean fibre length, mm Mean fibre width, µm Coarseness, mg/m Mean curl index, % Shives content, % Mean shive length, mm Table 5: Comparison of spruce TMP produced conventionally or with an inter-stage ozone treatment. Ozonation of TMP Screening Rejects Effect of Ozone on Energy Consumption TMP rejects were submitted to different ozone charges at high consistency before refining. The treatment yield was significantly affected when the ozone charge was higher than 3% and corresponded to the formation of acetic acid, which decreased the ph to 3 (figure 9). Reject refining energy consumption decreased after the ozone treatment: for freeness levels of 100 ml CSF, energy savings of 20 to 30% were observed with an ozone charge of 1 to 2% (figure 10). Pulping yield, % Pulping yield ph Ozone charge, % ph 2 1 Refining energy, kwh/t Freeness, ml CSF Control 1% O3 2% O3 3% O3 5% O3 Figure 9: Evolution of pulping yield and ph with ozone charge after treatment of spruce TMP rejects. Figure 10: Evolution of refining energy with freeness for spruce TMP rejects refined conventionally or after ozone treatment. Effect of Ozone on TMP Quality As previously observed, ozone treatment decreased TMP reject bulk, explained by an increase in fibre flexibility (figure 11). As TMP rejects were essentially composed of shives and long, rigid fibres, an increase in fibre flexibility was important for the paper properties of the final pulp, when the refined rejects were added. This was confirmed by the tensile index improvement and the slight decrease in tear index, especially when the ozone charge was higher than 3% (figure 12).

9 Bulk, cm 3 /g 1,85 1,8 1,75 1,7 Control 1,65 1% O3 1,6 2% O3 1,55 3% O3 1,5 5% O3 1, Freeness, ml CSF Tensile index, Nm/g Control 70 1% O3 65 2% O3 3% O3 60 5% O Tear index, mnm²/g Figure 11: Evolution of bulk with freeness for spruce TMP rejects refined conventionally or after ozone treatment. Figure 12: Relationship of tensile index and tear index for spruce TMP rejects refined conventionally or after ozone treatment. But this ozone treatment of spruce TMP rejects had a detrimental effect on the corresponding brightness and the scattering coefficient (figure 13). The higher the ozone charge, the lower the brightness of the refined rejects. As already observed with the inter-stage treatment, the refining of ozone-treated fibres favoured the formation of chromophores. Fines content was lower due to the higher fibre flexibility reducing fibre cutting during refining. The decrease in the scattering coefficient was explained by the chemical reactions provoked by the gas on the fibre surface. The scattering coefficient of the refined rejects pulp had a limited impact on the scattering behaviour of the final pulp, which contained considerably more fines. Brightness, % ISO Control 45 1% O3 40 2% O3 3% O3 35 5% O Freeness, ml CSF Scattering coefficient, m²/kg Control 25 1% O3 20 2% O3 15 3% O3 10 5% O Freeness, ml CSF Figure 13: Evolution of brightness and scattering coefficient with freeness for spruce TMP rejects refined conventionally or after ozone treatment. Ozonation had no significant effect on the fibre characteristics of the refined reject pulp, except on the fibre width which increased, revealing that some fibre hydration and swelling had occurred. The higher the ozone charge, the higher the width increase. When reintroducing the refined rejects into the TMP accepts, it was possible to evaluate the impact of rejects ozonation on the final TMP quality. Tensile index of the final TMP was enhanced without no tear index change (figure 14). The scattering coefficient and brightness decreased slightly but if an ozone charge of 1 to 2% was applied, this loss was limited. Moreover, reject ozonation enhanced the final TMP bleachability with peroxide: for 1% ozone applied on the rejects, final TMP brightness was slightly improved with 10% less peroxide.

10 Property Nm/g, mnm²/g, % ISO or m²/kg Ozone charge, % Tensile index Tear index Brightness Scattering coefficient Property, % ISO or % Ozone charge, % Final brightness Peroxide consumption Figure 14: Evolution of physical and optical properties of final TMP with ozone charge applied on the rejects before refining and before reintroduction into the accepts. Effect of reject ozonation on final TMP bleachability. Effect of Ozone on the Fibre Wall Components Examination with light microscope revealed significant differences in fibre development during refining (figures 15-16). The refined rejects were mainly composed of individualised long fibres. After ozone treatment, the fibres appeared more flexible, less cut and better developed, explaining the increase in tensile strength. Figure 15: Microphotographs of spruce TMP reject fibres refined conventionally (left) and after application of 1% ozone (right) at a magnification of 40x. Figure 16: Microphotographs of spruce TMP reject fibres refined after application of 2% ozone (left) or 5% ozone (right) at a magnification of 40x.

11 The fibres were stained with the Herzberg reagent. The presence of lignin coloured them in yellow. The colouration became blue when some delignification had occurred. When rejects were ozone-treated before refining, some delignified zones appeared on the fibre surface, indicating certain localised lignin degradation (figure 17). Figure 17: Microphotographs of spruce TMP reject fibres refined conventionally (left) and after application of 1% ozone (right) at a magnification of 160x. This result was confirmed by chemical analysis (table 6). Ozone degraded the lignin but also the wood extractives of the rejects fibres. With only 1% ozone applied on the rejects before refining, 60% of the wood extractives and 10% of the lignin were degraded. If the ozone charge had no significant effect on the extractives content, the higher the ozone charge, the greater the delignification. The wood extractives in rejects were rapidly degraded and the lignin degradation explained the higher fibre flexibility and bonding potential. Ozone charge, % Lignin content in Rejects in Accepts, % 25.8 in final TMP, % Wood extractives in Rejects, % in Accepts, % 0.7 in final TMP, % Table 6: Klason lignin content and wood extractives content of rejects pulp, accepts and final TMP. Effect of Ozone on The Different Wood Extractives In order to determine which wood extractives component(s) the ozone degraded, model compounds of resin and fatty acids were treated with ozone and analysed. To degrade 1 mole of oleic acid, 2.6 moles of ozone were needed. The comparison of chromatograms before and after ozonation revealed the appearance of degradation products, for which the peaks appeared at lower retention time; this supposed their size could be smaller than oleic acid. To degrade 1 mole of abietic acid, 12.1 moles of ozone were needed, due to the presence of two double bonds in the molecule. The GC analysis revealed the formation of hydroxyl groups, carboxylic groups and a removal of the carbon-carbon double bonds. Products formed during the ozonation of abietic acid were certainly more soluble. To validate these results, a softwood TMP was treated with ozone at different charges and the resulting wood extractives analysed (figure 18). Lignans were highly degraded whereas lipophilic compounds were less attacked by the ozone. A charge in ozone of 2% seemed to be sufficient to induce important wood extractives degradation: o 85% of lignans o 55% of sterols o 45% of triglycerides and fatty acids o ~40% of steryl esters o 35% of resin acids The resin acids, among all the other wood extractives, were less amenable to ozone degradation. COD of TMP effluents, obtained by soaking ozone-treated pulps for 3 hours at 60 C and centrifugating them at 3000g for 10

12 minutes, increased due to the transfer of ozone-degradation products to liquid phase (figure 18). At a low ozone charge, it was supposed that ozone mainly reacted with dissolved and dispersed substances in the water surrounding the fibres. At higher charges, ozone reacted with wood extractives generating soluble products, which explains the increase in COD. 100 fatty acids lignans steryl esters resin acids sterols triglycerides 1000 % degraded extractives during ozonation Ozone charge, % on the pulp. COD, mg/l Ozone charge, % on the pulp Figure 18: Degradation of the main wood extractives compounds and evolution of COD with ozone charge after the treatment of a spruce TMP. As ozone was able to degrade wood extractives, it was also used to degrade the extractives contained into TMP effluents (figure 19). Most of the wood extractives in the aqueous phase were degraded by the ozone. It was not possible to degrade all the resin acids (85%) due to the lower reactivity of dehydroabietic acid towards ozone. All the other main wood extractives could be completely degraded by ozone. As observed for the pulp, lignans were rapidly degraded, due to their phenolic structure. For the fatty acids, only the unsaturated were degraded. Extractives degraded, % fatty acids resin acids lignans sterols steryl esters triglycerides Ozone charge, mg/l Figure 19: Degradation of the main wood extractives compounds contained in spruce TMP effluent by ozone. In summary, ozone was able to degrade the wood extractives in chips, pulp and effluents. The quantity of ozone needed to degrade 1 mg of lipophilic extractives was: o 4.3 mg when the pulp was considered and the corresponding extractives contents were lower than 1% o 20.3 mg when the pulp contained a higher degree of extractives content o 19 mg if the water was treated CONCLUSIONS The introduction of ozone in the thermomechanical pulping process was examined in consideration of different introduction points: in the main line refiners, as inter-stage and on the rejects before refining. Injection in the main line refiners or in rejects treatment seemed to be the alternatives most interesting and compatible with industrial

13 conditions. Some energy savings (20 to 30%) were obtained with enhancement of the corresponding pulp quality, associated with a decrease in wood extractives and/or lignin degradation, depending on the raw materials treated. Ozone degraded almost all the main wood extractives, except dehydroabietic acid and saturated fatty acids. If the rejects treatment alternative were to be used, a high-consistency reactor could be implemented and the impacts would be: o Reject refining energy savings, o Enhancement of the refined reject pulp and final pulp qualities o Modification in pulping effluents with less wood extractives, o Decrease in reinforcement pulp used on the paper machine, because of the improved final TMP strengths, o Fewer bleaching chemicals, especially in hydrogen peroxide, due to better final TMP bleachability. From an economical point of view, the ozone production costs were compensated by the advantages mentioned, even if the effects on the paper machine cannot be reviewed without data obtained at industrial level. If we consider the following data: o Electricity cost: 0.05 /kwh o Ozone cost (other the fence): 1.5 /kg o Hydrogen peroxide cost: 0.52 /kg o Energy consumption of a reject refiner: 1200 kwh/t o TMP unit with a capacity of 600 t/d with 55% of the production considered as screening rejects o Energy savings of 25% in the reject refiner with 2% ozone o Peroxide saved in bleaching of the final pulp: 10% of the 3% introduced. The ozone treatment cost an extra 6.7 per tonne of final TMP. As the final TMP tensile strength was improved by 20%, the ozone extra-cost could be compensated by the decrease in reinforcement pulp used. If we consider the following data: o Softwood bleached kraft pulp (market price): 730 /t o TMP production cost (50 ml CSF - industrial data): 250 /t o Fibrous composition of the paper produced: 80% TMP - 20% kraft pulp for supercalendered papers (SC) 65% TMP - 35% kraft for light weight coated papers (LWC) The ozone extra-cost would be compensated by a decrease of 1.3 and 0.9 points in the reinforcement pulp content for the manufacture of SC papers and LWC papers, respectively. These figures demonstrate that ozone treatment of rejects could be realistically considered at industrial scale. Moreover, other advantages, such as extractives degradation and better runnability (fewer paper breaks on the paper machine and stops for felt cleaning) provide additional incentives for rejects ozone treatment. REFERENCES 1. Ruffini, G., Improvement of bonding in wood pulps by the presence of acidic groups, Svensk Papperstding, 69 (3), pp. 72 (1966). 2. Allison, R.W., Effect of ozone on high temperature TMP, Appita Journal, 32 (4), pp (1979). 3. de Choudens, C., Monzie, P., Pâtes TMP. Traitement par l'ozone, Atip. 32 (9), pp (1978). 4. Eriksson, E., Sjöström, E., The influence of acid groups on the physical properties of high-yield pulps, Tappi, Journal, 51 (1), pp. 56 (1968). 5. Lindholm, C.A., Ozone treatment of mechanical pulp. Part 2 : Influence on strength properties, Paperi ja Puu, 59 (2), pp , 53-58, 60, 62. (1977). 6. Lindholm, C.A., Ozone treatment of mechanical pulp. Part 3 : Influence on optical properties, Paperi ja Puu. 59 (4a), pp , , (1977). 7. Lindholm, C.A., Gummerus, M., Comparison of alkaline sulphite and ozone treatment of SGW, PGW and TMP fibre s, Paperi ja Puu, 65 (8), pp (1983). 8. Soteland, N., The effect of ozone on mechanical pulps, Pulp Paper Canada, 78 (7), pp (1977). 9. Vasudevan, B., Panchapakesan, B., Gratzl, J.S., Holmbom, B., The effect of ozone on strength development and brightness reversion characteristics of high yield pulps, proceedings from the 1987 Tappi Pulping Conference, Washington, D.C, Proceedings, pp

14 10. Soteland, N., Interstage ozone treatment of hardwood high yield pulp, Paperi ja Puu, 64 (11), pp , 710, (1982). 11. Kojima, Y., Yoon, S.L., Kayama, T., A study of production of CTMP from hardwood. Part B : Characterization of pulp produced by CTMP-O 3 process, Japan Tappi Journal, 42 (10), pp (1988). 12. Kojima, Y., Yoon, S.L. (1991), Distribution of lignin in the cell wall of ozonized CTMP fibres, proceedings from the th International Symposium on wood and pulp chemistry, Melbourne, Australia, pp Petit-Conil, M. de Choudens, C., Espilit, T., Ozone in the production of softwood and hardwood high-yield pulps to save energy and improve quality, Nordic Pulp and Paper Research Journal, 13 (1), pp (1998). 14. Kibblewhite, R.P., Brookes, D., Allison, R.W., Effect of ozone on the fiber characteristics of thermomechanical pulps, Tappi Journal, 63 (4), pp (1980). 15. Rothenberg, S., Shaw, J., Durst, W.B., Effect of chemical modification on the properties of lignin-containing fibres, Paperi ja Puu, 63 (3), pp , (1981). 16. Magara, K., Ikeda, I., Tomimura, Y., Hosoya S., Accelerated degradation of cellulose in the presence of lignin during ozone bleaching, Journal of Pulp and Paper Science, 24 (8), pp. 264 (1998). 17. Petit-Conil, M., Principes de préparation des pâtes CTMP de bois résineux et feuillus. Application à la mise au point de procédés, Thèse de l'institut National Polytechnique de Grenoble (1995). 18. Petit-Conil, M., Robert, A., Pierrard, J.M., Fundamental principles of mechanical pulping from softwoods and hardwoods. Part 1 : theoretical aspects, Cellulose Chemistry and Technology, 31 (1/2), pp (1997). 19. Robert, D.R. Szadeczki, M., Lachenal, D., Chemical characteristics of lignins extracted from softwood TMP after O 3 and ClO 2 treatment, proceedings from the 215 th national ACS meeting, Lignin: historical, biological and materials perspectives, Dallas, Texas, USA, chapter 27, pp (1999). 20. Hsieh, J.S., Long, P.X., BaOsman, A., Kinetic study of ozone treatment on mechanical pulp, proceedings from the 2000 Tappi Pulping/Process & Product Quality Conference, Boston, MA, Paper Saharinen, E., Nurminen, I., Improving internal bonding strength and bulk for folding boxboard middle layer by ozone treatment, proceedings from the 2001 International Mechanical Pulping Conference, Helsinki, Finland, pp Roy-Arcand, L., Archibald, F., Selective removal of resin and fatty acids from mechanical pulp effluents by ozone, Water Research, 30 (5), pp (1996). 23. Petit-Conil, M., de Choudens, C., A new chemithermomechanical process : use of oxygen in the primary refining stage. Part 1: oxidative sulfonation for hardwood and softwood CTMP pulping, Das Papier, 48 (10), pp (1994). 24. Brandal, J., Lindheim, A., The influence of fibre surface of extractives in groundwood pulp on fibre bonding, Pulp and Paper Canada, 67 (10), pp. T (1966). ACKNOWLEDGEMENT The authors are grateful to the following companies, which supported this project: Ademe, Andritz, Ato Fina, Cascades La Rochette SA, International Paper SA, Matussière & Forest, M-real corporation, Ozonia International, Papeteries de Gascogne, Stora Enso Corbehem, UPM Kymmene. The authors would like to particularly acknowledge François Cottin and Christiane Balme for carrying out this work and EFPG for the chemical analyses of carboxyl groups and wood extractives.

15 Saving Energy by Application of Ozone in the Thermomechanical Pulping Process M. Lecourt, B. Struga, T. Delagoutte and M. Petit-Conil International Mechanical Pulping Conference Minneapolis, May 6-10, 2007

16 Objectives of this work To use mixing effect of refiners when O 3 is introduced in the eye of the refiner To determine treatment efficiency at different stages In the main line (primary refiner, secondary refiner, interstage) In the rejects line

17 Plan of the Presentation State of the art Results Main line implementation Reject line treatment Conclusions and perspectives

18 State of the art of ozone use in mechanical pulping Ozone is : a strong oxidizing agent used in all pulping processes: Deinking Kraft pulping Bleaching Mechanical pulping for energy savings

19 Trials carried out at pilot plant scale on spruce chips First stage 1% O 3 Chip input Screw Drainage zone Compression finger MSD + First stage 1% O 3 Second stage 1% O 3 O 3 added at different points of the process Same ratios of O 3 Comparison with oxygen treatment at primary refiner Introduction of a mechanical treatment of the chips (plug screw)

20 Ozone in the eye of the refiners Energy consumption Total SEC, kwh/t Control O 2 in I O 3 in I O 3 in II MSD O 3 in I Freeness, ml CSF MSD+ O 3 > O 3 >O 2

21 Ozone in the eye of the refiners Energy consumption Total SEC, kwh/t Control O 2 in I O 3 in I O 3 in II MSD O 3 in I 10% 20% Freeness, ml CSF MSD+ O 3 > O 3 >O 2

22 Ozone in the eye of the refiners Handsheets properties O 3 in I same tear Tear index, mnm²/g 5,5 5 4,5 4 3,5 3 2,5 Control O 2 in I O 3 in I O 3 in II MSD O 3 in I better tensile Tensile index, Nm/g Freeness, ml CSF

23 Ozone in the eye of the refiners Optical properties O 3 or O 2 treatment higher initial brightness Brightness, % ISO Control O 2 in I O 3 in I O 3 in II MSD O 3 in I 64 low impact on LSC LSC, m²/kg Freeness, ml CSF

24 Ozone in the eye of the refiners Final brightness % or % ISO ,1 71,7 Peroxide consumption Brightness Control O 2 in I O 3 in I MSD O 3 in I O 3 in II 71,2 73,6 Alkaline peroxide bleaching (3% H 2 O 2, 2.1% NaOH) 72,6 MSD+ O 3 > O 3 in II > O 3

25 Ozone in the eye of the refiners Fibre characteristics No significant effect on mean fibre length O 3 in I or II refiner mean fibre width shive content better fibre separation better efficiency of refining

26 Ozone in the eye of the refiners Fibre bonding potential With O 3 treatment higher RBA Relative bonded area index, % Control O 2 in I O 3 in I O 3 in II MSD O 3 in I less fibrillation Fibrillation index, % Freeness, ml CSF

27 Ozone in the eye of the refiners Impact on fibre wall components Extractives content (%) With O 3 less extractives 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 Control -50% extractives 03 in primary refiner MSD+O3 in primary refiner Complementary results: in COD no significant effect on cationic demand, except for the use of O 3 in the secondary refiner ( )

28 Ozone in the eye of the refiners Impact on lignin chemistry: carboxylic acid groups % COOH (mmol/100 g odp) Control O2 in I O3 in I O3 in II MSD O3 in I Control bleached by H2 O 2 MSD O3 in Ibleached by H 2 O 2 More acid groups with MSD + O 3 treatment in I

29 Ozone in the eye of the refiners Conclusion Refiner can be used as a reactor to homogenise ozone treatment of chips or pulp Promising results with chips treatments lower energy consumption better fibre separation better pulp properties higher bleaching potential lower extractive content

30 Trials carried out at pilot plant scale on spruce chips O 3 [C]

31 Ozone on TMP refining rejects Ozonation of industrial spruce TMP rejects Treatment at pulp ph and 46% consistency at 60 C Various proportions injected 1 % 2 % 3 % 5 %

32 Ozone on TMP refining rejects Yield and energy Energy savings up to 30% Refining energy, kwh/t 2500 Control % O 3 2% O % O % O Mass losses at high % O 3 Pulp Yield, % Freeness, ml CSF Ozone charge, % Effluent ph

33 Ozone on TMP refining rejects Pulp quality Lower bulk Higher tensile index Bulk, cm 3 /g Tensile index, Nm/g 1,85 1,8 1,75 1,7 1,65 1,6 1,55 1,5 1, Freeness, ml CSF Control 1% O 3 2% O 3 3% O 3 5% O 3

34 Ozone on TMP refining rejects Optical properties The more the O 3 injected the lower the brightness the lower the LSC Brightness, % ISO Scattering coefficient, m²/kg Control 1% O 3 2% O 3 3% O 3 5% O 3 Freeness, ml CSF

35 Ozone on TMP refining rejects Pulp fibre morphology Slightly higher mean fibre length with ozone Higher mean fibre width No impact on shives content nor straightness

36 Ozone on TMP refining rejects Fibre morphology: Control 2% O 3 5% O 3 The more the O 3 the more affected the fibre morphology the more the local delignification The more the blue color surface, the more the delignification (Herzberg agent)

37 Fibre wall components: Ozone on TMP refining rejects 30 1 Lignin content, % Rejects Rejects 0,8 0,6 0,4 0,2 Extractives content, % Ozone charge, % Treating rejects with O 3, decrease in lignin decrease in extractive content

38 Pulp recombination: Ozone on TMP refining rejects Final pulp quality TMP Accept Refined rejects (100ml CSF) 45% 55% Final pulp

39 Ozone on TMP refining rejects Final pulp quality Fibre wall components: Lignin content, % Final TMP Rejects Final TMP Rejects Ozone charge, % 1 0,8 0,6 0,4 0,2 0 Extractives content, % O 3 in reject plays a major role in the decrease in lignin and extractive content in final pulp

40 Handsheets physical properties: Ozone on TMP refining rejects Final pulp quality Tensile index, Brightness, Scattering coefficient Tensile index, Nm/g Brightness, % ISO L Scattering coefficient, m²/kg Tear index, mnm²/g Ozone charge, % mnm²/g Tendencies observed on reject pulp observed on final pulp

41 Ozone on TMP refining rejects Final pulp quality Bleached pulp optical properties: 100 Q: 0.4% DTPA, 25 min, 60 C, 5% consistency P: 3% H 2 O 2, 1.9% NaOH, 2.1% silicate, 2h, 70 C, 28% consistency % ISO or % consommed Final brightness Peroxide consumption The more the O 3, Ozone charge, % the higher the final brightness the more the H 2 O 2 consumption

42 Ozone on TMP refining rejects Potential new process Normal process Chips Washing Presteaming/ steaming Primary refiner Secondary refiner Bleaching Thickening Screening Latency chest Reject refiner Reject Screening

43 Modified process with ozone Ozone on TMP refining rejects Potential new process Chips Washing Presteaming/ steaming Primary refiner Secondary refiner -10% H 2 O 2 Bleaching Thickening Screening Latency chest Ozone Reactor (2%) Reject refiner -25% energy pulp quality Enhancement of pulp quality: in reinforcement pulp in paper sheet breaks Reject Screening paper machine stops for cleaning (less pitch)

44 Conclusions O 3 in mechanical pulping energy pulp quality through wood extractives O 3 can be added at different points in a TMP line: Defibering Refining Reject refining

45 Economy for a reject treatment Mill producing 600 t/d of mechanical pulp Rejects representing 55% of the production Refining rejects energy: 1200 kwh/t 2% O 3 allows: 25% energy savings 10% peroxide saving Extra cost = 6.7 /t At least 1,3% reinforcement pulp Savings = 7 /t Using Ozone can save money!!! 0.05 /kwh O 3 : 1.5 /kg H 2 O 2 : 0.52 /kg Softwood bleached kraft pulp: 730 /t TMP cost: 250 /t SC paper: 80% TMP / 20% NBSK

46 Conclusions and perspectives More savings could be expected with: a better machine runnability a decrease in pitch problems (fewer cleaning stops) and in web breaks a reduction in effluent toxicity Similar results obtained with treatment of SGW rejects. Still to be performed Validation at industrial scale Understanding of the mechanisms