Status Quo of the Metal Finishing Sector in South Africa

Transcription

1 Status Quo of the Metal Finishing Sector in South Africa Report supplementing WRC Report TT 645/15: NATSURV 2 Water and Wastewater Management in the Metal Finishing Industry by HA Ally, W Kamish and T van der Spuy

2 ACKNOWLEDGEMENTS The research in this report emanates from a project that was undertaken by the Water Research Commission, entitled: NATSURV 2: Water and Wastewater Management in the Metal Finishing Industry (Ed 2) The Steering Committee is thanked for contributing their knowledge and insights to the project and the content of this guide. The project team would like to extend their thanks to the following people: All the metal finishing companies who gave of their time to complete the survey and participate in the site visits and interviews The regulators who provided input into the development of the guide ii

3 TABLE OF CONTENTS Page No 1. BACKGROUND INTRODUCTION Industry overview THE OVERALL OBJECTIVES OF THE PROJECT DESCRIPTION OF THE RESEARCH PRODUCTS A CURRENT OVERVIEW OF THE METAL FINISHING INDUSTRY IN SOUTH AFRICA THE GENERIC PROCESS DESCRIPTION THE METAL FINISHING INDUSTRY A GENERAL PROCESS DESCRIPTION OF ELECTROPLATING PRE-TREATMENT PROCESSES, CONTAMINANTS AND THEIR SOURCES Mechanical pre-treatment Electrolytic and chemical polishing Solvent degreasing Pickling Electrolytically assisted pickling, activation and degreasing Metal stripping Drag-out and rinsing THE SPECIFIC SURFACE TREATMENTS REGIMES AND GENERAL WASTES GENERATED Copper and copper alloy plating Nickel electroplating Chromium plating Zinc and zinc alloy plating Cadmium plating Tin and alloy plating Precious metal plating Autocatalytic plating Immersion or displacement coating Colour anodising on aluminium Sealing following anodising Phosphating coatings Chromium conversion coatings Metal colouring Chemical blacking oxide coatings Brightening Etching alkaline etching of aluminium Chemical milling POST TREATMENT ACTIVITIES ANODISING OF ALUMINIUM Sealing following anodising POWDER COATING Examples of Pre-Processing on various substrates prior to powder coating WASTES GENERATED DURING THE METAL FINISHING PROCESS AND TREATMENT FURTHER CLEANER PRODUCTION INITIATIVES FOR THE INDUSTRY iii

4 4.1.1 Management and housekeeping THE HOT DIP GALVANIZING PROCESS DEGREASING THE ACID PICKLING STAGE RINSING STAGES THE FLUX PROCESS DRYING THE HOT DIP GALVANIZING PROCESS QUENCHING AND THE PASSIVATION STAGE CHANGES IN THE PLATING INDUSTRY SINCE THE 1980 S AND PROJECTED CHANGES A DEFINITION FOR CLEANER PRODUCTION EFFECTIVE WATER USE AND MANAGEMENT DRAGOUT MINIMISATION AND MANAGEMENT THE CONTINUOUS RUNNING RINSES RINSEWATER FLOW CONTROL SPRAY RINSING ION-EXCHANGE TECHNOLOGY REVERSE OSMOSIS ULTRAFILTRATION EFFLUENT TREATMENT REGIMES Neutralisation and precipitation of effluent The oxidation and reduction processes Chrome reduction Improving settling using flocculants Cross-flow membrane technology Compressing and filtering the wastewater sludge CHECKLIST OF CURRENT INTERNATIONAL AND LOCAL CLEANER PRODUCTION TECHNIQUES Jigging Pre-treatment measures Plating Post plating Wastewater treatment and sludge disposal Good housekeeping techniques Inspection Personnel CHANGES IN THE HOT DIP GALVANIZING INDUSTRY THE COMMON POLLUTANTS AND THEIR EFFECTS CHECKLIST OF CURRENT INTERNATIONAL AND LOCAL CLEANER PRODUCTION TECHNIQUES Jigging Degreasing iv

5 7.2.3 Acid pickling Rinse tanks Fluxing stage Drying The hot dip galvanizing Quenching and passivation General plant cleaner production options METAL FINISHING CONCEPTS FOR THE FUTURE ELECTROPLATERS RECTIFIERS AND PULSE PLATING NANOTECHNOLOGY Nanocrystalline Cobalt-Phosphorus Coatings as a Hard Chrome Alternative PLATING WITH IONIC LIQUIDS DEVELOPMENTS IN POWDER COATING REFERENCES LIST OF TABLES Table 1 Typical copper plating and wastes generated Table 2 Types of nickel solutions used Table 3 Types of zinc electrolyte solutions Table 4 Electrolytic solutions used in cadmium plating Table 5 Precious metal coatings Table 6 Types of autocatalytic plating Table 7 Chemicals used in chemical milling of metals Table 8 Control parameters for chemical milling of aluminium and alloys Table 9 Typical treatment regimens for effluent treatment Table 10 Possible treatment(s) specific to each constituent of concern Table 11 Typical components of an Environmental Management Plan (EMS) Table 12 Pollutants and their sources during hot dip galvanizing LIST OF FIGURES Figure 1 A generic process flow diagram for the metal finishing industry... 4 Figure 2 A typical hot dip galvanizing process (Zalcon, 2000) Figure 3 Counterflow rinsing (Hyder Consulting and Hemsley 1999) Figure 4 A typical airlift pump (SAMFA, 2013) Figure 5 Effluent treatment in the metal finishing industry (SAMFA, 2013) Figure 6 Flowchart of the inputs and outputs of the hot dip galvanizing process Figure 7 Spray rinsing with immersion rinse and drain boards (CPMFI, 2003) Figure 8 A typical flux purification system (CPMFI, 2003) v

6 GLOSSARY Anode Anodising Barrel plating Bonderising Brightener Cathode Chelating agent Chromating Complexing agent Desmut Drag-out E-Coating Electroless plating Electrolyte Electroplating Etch Galvanizing Jig Nitriding the positive electrode of an electrolytic cell, usually it is comprised of the metal to be deposited the process in which a hard protective oxide layer is formed on aluminium in an electrolytic cell composed of sulphuric acid. an electroplating process that takes place in a rotating perforated container. a phosphating process to protect against corrosion. a reagent added to the electroplating process to produce bright deposits. the negative electrode of an electrolytic cell on which the metallic deposit is plated. a type of complexing agent (e.g. EDTA) which combines with the metal ions to form a stable compound. a passivation process by which a protective chromate coating is applied to zinc or cadmium-coated steel. a compound which combines with metals to form a stable solution. (e.g. cyanides and ammonium compounds). a nitric acid bath used in the anodising process between the etching and anodising stages to remove smut that may have developed as a result of the etching process the water or solution that adheres to work when it is removed from a tank. This liquid is then carried over into the subsequent tank. more correctly known as electrophoresis. The electrolytic deposition of paint onto an article from a suitable aqueous electrolyte. a process which employs solutions from which a metal is deposited by chemical reduction instead of electro-deposition. It is also known as autocatalytic plating. a solution that conducts an electric current by means of ions contained in the solution. the deposition of a metallic coating onto the cathode in an electrolytic cell. The cell contains an electrolyte composed of a solution of salts of the metal being deposited. a caustic bath used in the anodising process to remove the existing oxide layer from work to be anodised. Acidic etching solutions are used in the plating of plastics to provide a keyed surface for subsequent metal deposits. a process in which a zinc coating is applied to steel to provide protection against corrosion. The two types are (a) Electrogalvanizing galvanizing by electroplating and (b) Hot-dip galvanizing galvanizing by immersion of the base metal in molten zinc. A device for securing components in a suitable manner and orientation for electroplating or anodising. The jig is immersed in the electrolyte and a direct current is passed through the jig. a process in which a hard surface is provided on special types of steel by heating in gaseous ammonia. Also known as nitrogen case-hardening. vi

7 Phosphating Pickling Sealing Soils Specific water intake Specific pollution load Strike Wetting agent an immersion process in which a zinc, iron or manganese phosphate conversion coating is produced on the surface of components, primarily to promote paint adhesion and/or guard against under paint corrosion. the removal of oxides or other compounds from a metal surface by means of an acid, usually hydrochloric or sulphuric. an operation performed on anodised aluminium by immersion in a tank of hot water ( C) to close microscopic pores in the surface and prevent staining. De-ionised water is required for optimum results. dust and other dirt deposits which must be removed from work which is to undergo a metal finishing operation. the volume of water used in a metal finishing process to produce one square metre of finished work. The figure is based on the "effective surface area" of the article being treated and is expressed in m 3 /m 2. the quantity of a specified contaminant that is discharged in the final effluent in the processing of one "effective" square metre of work usually expressed in g/rn 3. A plating solution usually used to prevent immersion deposition onto a work piece through depositing a thin initial film of metal prior to a heavier deposit from a subsequent plating solution. The film itself is also referred to as a strike e.g. a copper strike from a cyanide solution on a zinc object prior to a heavier deposit from an acid copper bath. An additive which reduces the surface tension of a liquid allowing it to spread more easily over a solid surface. Wetting agents also help prevent pitting during some plating processes, and facilitate improved drip off of process solution on removal from the tank vii

8 ACRONYMS AFSA AGM COD CPMFI DANIDA DANIDA DTI DTI DWA EDTA JHB KZN MCEP MFA-SA RO SAMFA SPL SS SWI TDS US WRC WUE Aluminium Association of South Africa Annual general meeting Chemical oxygen demand Cleaner Production in the Metal Finishing Industry Danish Aid organisation Danish International Development Agency Danish Technological Institute Department of Trade and Industry Department of Water Affairs Ethylenediaminetetraacetic acid Johannesburg KwaZulu-Natal Manufacturing Competitiveness Enhancement Programme Metal Finishing Association-South Africa reverse osmosis South African Metal Finishing Association ( specific pollution load suspended solids specific water intake total dissolved solids University of Stellenbosch Water Research Commission Water use efficiency viii

9 1. BACKGROUND 1.1 INTRODUCTION The last National Survey relating to the management of resources in the Metal Finishing Industry was completed in October 1987, 25 years ago. Since then there has been concerted effort to develop coating systems that rely on more environmentally friendly chemicals along with scientific evaluation of process lines that has resulted in significant improvements to layout and handling methods. Since the last NATSURV, a portion of the South African metal finishers have benefitted from financial and technical assistance from DANIDA (the Danish Foreign Aid agency) with the implementation of our own Resource Conservation program. This initiative, the CPMFI project [Cleaner Production in the Metal Finishing Industry] ran for a period of three and a half years, officially ending in 2004 and culminating in the establishment of SAMFA, the South African Metal Finishing Association. Ultimately eighteen companies rebuilt their plants with partial subsidies from DANIDA totalling 1.2 million rand. Together they achieved combined savings of around 3.8 million rand in year one and on average recouped their investments through savings in an average period of 18 months. Scores of others made smaller but measurable improvements using their own resources. To quantify the extent to which the metal finishing industry in South Africa has changed since the last NATSURV conducted in 1987, it was again necessary to collate up-to-date information such that meaningful conclusions could be drawn. The major outcomes expected from this study are as follows: A review of current state of the metal finishing sector which would provide the platform for further initiatives aimed at positively transforming the sector. The development of a product that will allow more cost effective planning for the sector, resulting in better run factories with the potential to become more profitable. An assessment of the extent of compliance with ISO and Chemicals Management Action Plans (CMAP) to provide an indication of the protective measures currently provided to workers in this sector. To provide an indication of the varying degrees of training required for general worker safety awareness, the use of personal protective gear, correct operational procedures, improving the state of poor housekeeping and to rectify poor administration Industry overview It is often necessary to use metals or metal alloys which have certain surface properties, e.g. resistance to corrosion, hardiness, high temperature tolerance, etc. Generally, no one metal will possess all the necessary properties. For example, steel has many good properties. It is inexpensive, abundant and a strong material that can easily be worked into all manner of shapes and forms. However, its major flaw is poor resistance to corrosion, causing it to rust severely in damp atmospheres. It needs to be coated with another material to provide corrosion resistance. The term metal finishing refers to a range of techniques that treat metal surfaces for its intended purpose. Metal Finishing enables the engineer to combine the good mechanical properties of one metal with the desirable surface properties of another. 1

10 Electroplating thus offers the engineer a wide choice in combining various metals in determining a specific objective. Currently, metal finishing operations take place in two broadly defined divisions: 1. The jobbing shop sector (or commercial finishers): This sector includes businesses that offer metal finishing services to companies that elect not to install an in-house finishing capability to deal with their finishing requirements. 2. In-house finishing operations: It is possible that this sector has experienced growth since the last NATSURV, particularly amongst OEM (original equipment manufacturers) and second tier automotive component manufacturers (SAMFA, 2013). Some of the reasons for this growth are: Inventory control is less complicated as parts are not transported to off-site locations; The manufacturer is able to concentrate his resources on dealing with specific component configurations and finishes; and The manufacturer is better able to manage finishing quality control, with no third party involvement Even with a national survey, an estimate of the number of metal finishing installations in South Africa is difficult to quantify as a significant percentage of the work is performed in-house as an essential part of an overall manufacturing activity. The remainder of the metal finishing work is undertaken on a contract basis by specialist companies that include some large and dominant companies along with a greater number of smaller operators. Not all of the latter necessarily adhere to desirable industry standards, local and national legislation. 1.2 THE OVERALL OBJECTIVES OF THE PROJECT The project objectives as formulated in the Agreement with the Water Research Commission were to: 1. Provide a general overview of the metal finishing industry in South Africa, its changes since 1980 and its projected change. 2. Evaluate and document the generic industry processes 3. Determine the water consumption and specific water intake 4. Determine the wastewater generation and typical pollutant loads 5. Determine local electricity, water and effluent prices and by-laws within which these industries function. 6. Critically evaluate the water (inclusive of wastewater) management processes adopted and provide recommendations 7. Evaluate the industry adoption of the following concepts: cleaner production, water pinch, energy pinch, life cycle assessments, water footprints, and ISO to name a few. 8. Provide recommendations for best practice The major findings obtained in meeting the abovementioned objectives are described in this report in the traditional format of previous NATSURV documents. Other information which could support this document, but cannot necessarily be included within the structure of the 2

11 NATSURV document have been completed as supporting documents and will be discussed further in the ensuing sections. 1.3 DESCRIPTION OF THE RESEARCH PRODUCTS In the original proposal one major research product and two supporting documents were envisaged for this Project: NATSURV 2 : Water and Wastewater Management in the Metal Finishing Industry (Ed 2) An overview of the status quo of the metal finishing sector in South Africa An overview of source reduction methods which are or can be employed in the metal finishing sector. The project yielded three deliverables in the form of reports: Ally, S.H., Kamish, W. and van der Spuy, A. (2015). NATSURV 2: Water and Wastewater Management in the Metal Finishing Industry (Ed 2). WRC Report No. K5/2224, Water Research Commission, Pretoria. Ally, S.H., Kamish, W. and van der Spuy, A. (2015). Status Quo of the Metal Finishing Sector in South Africa. WRC Report No. K5/2224, Water Research Commission, Pretoria. (This report) Ally, S.H., Kamish, W. and van der Spuy, A. (2015). Source Reduction Measures Employed in the Metal Finishing Sector. WRC Report No. K5/2224, Water Research Commission, Pretoria. The major findings from the survey on industries in the metal finishing sector are described in this report. The updated NATSURV 2: Water and Wastewater Management in the Metal Finishing Industry (Ed 2) can provide support to professionals in the plating industry, officials of the DWS as well as practitioners who supply support services in the field of cleaner production in the metal finishing sector. The supporting documents provide a more detailed account of the status quo and the extent of implementation of cleaner production (CP) technologies in the metal finishing sector. Once again, the documents can be used by the sector as well as those providing support services to the sector as a point of departure for what is practically possible in term of CP implementation. 3

12 2. A CURRENT OVERVIEW OF THE METAL FINISHING INDUSTRY IN SOUTH AFRICA It is often necessary to use metals or metal alloys which have certain surface properties, e.g. resistance to corrosion, hardiness, high temperature tolerance, etc. Generally, no one metal will possess all the necessary properties. For example, steel has many good properties. It is inexpensive, abundant and a strong material that can easily be worked into all manner of shapes and forms. However, its major flaw is poor resistance to corrosion, causing it to rust severely in damp atmospheres. It needs to be coated with another material to provide corrosion resistance. The term metal finishing refers to a range of techniques that treat metal surfaces for its intended purpose. Metal Finishing enables the engineer to combine the good mechanical properties of one metal with the desirable surface properties of another. Electroplating thus offers the engineer a wide choice in combining various metals in determining a specific objective. An estimate of the number of metal finishing installations in South Africa is difficult to quantify as a significant percentage of the work is performed in-house as an essential part of an overall manufacturing activity. The remainder of the metal finishing work is undertaken on a contract basis by specialist companies that include some large and dominant companies along with a greater number of smaller operators. Not all of the latter necessarily adhere to desirable industry standards, local and national legislation. 2.1 THE GENERIC PROCESS DESCRIPTION The term metal finishing incorporates a wide variety of different activities and these are compartmentalised into three main operations and shown in Figure 1. Pre-treatment Surface treatment Post treatment Figure 1 A generic process flow diagram for the metal finishing industry 1. The pre-treatment process This procedure is essential and prepares the metal to be plated. It usually involves the cleaning of the surface which ensures a good surface finish. This process could be of two types, (a) chemical surface treatment or (b) mechanical surface treatment of the object or both. a. Chemical surface treatment includes processes such as degreasing, pickling and phosphating b. Mechanical surface treatment includes descaling, brushing, scouring and polishing. 2. The surface treatment of the product is performed via anodising, electroplating, e-coating, chemical surface treatments, powder coating, or hot dip galvanizing. 3. The post treatment process usually involves passivation, lacquering, painting and/or rinsing of the object. Of these treatment processes mentioned above, only those which were found to use substantial quantities of water or generate substantial quantities of effluent have been studied. Some finishing operations, such as nitriding (and other similar processes) require very little water (Binnie, 1987) and were not covered in NATSURV 2. The electroplating, anodising and chemical surface treatment prior 4

13 to powder coating have been found to be the most water-use-intensive of this group, and this guide deals exclusively with these three processes. The annual water consumption by the metal finishing industry during 1987 was determined to be nearly 9 x 10 6 m 3, accounting for 0.7% of the total water intake of the industrial sector in South Africa with approximately 80% of this water being discharged as effluent (Binnie, 1987). 5

14 3. THE METAL FINISHING INDUSTRY Today there are many metal finishing techniques, including electroplating, anodising, hot dip galvanizing, powder coating and even painting. For the purposes of this study, each of the aforementioned techniques will be dealt with independently. Most of the information for this section was obtained from SAMFA training manual, 3.1 A GENERAL PROCESS DESCRIPTION OF ELECTROPLATING Electroplating is the process of depositing a metallic coating onto a metal or other conducting surface in an electrolytic cell where the electrolyte is a solution of a dissolved metallic compound that provides ions. The component for plating is the cathode (negative) and a direct current is passed through the cell from an anode (positive) to the cathode. The product to be plated is first thoroughly cleaned of grease and any other dirt by immersion in acid and alkaline cleaning solutions. It is then processed in a solution containing salts of the metal (the electrolyte) with which it is to be coated. Some common metals electro-deposited are: Zinc and zinc alloys for anticorrosion properties Chromium and nickel for decorative, corrosion, wear and temperature resistance Copper and gold for decorative and industrial properties Silver for decorative and electrical properties Tin for food containers and electrical contacts Some of the properties imparted to articles by electroplated finishes include hardness, lubricity, solderability, conductivity, improved torque tension characteristics, enhanced appearance, reflectivity and anti-galling characteristics. In the electrolytic cell, the metal that will be plated to an object is solution in the form of positively charged ions. The product to be plated is connected to the negative terminal (cathode) of a source of direct current (DC), while the positive electric terminal (anode) is connected to another conductor which is also placed into the solution. The product and the conductor are the electrodes through which the electric current enters and leaves the solution. The positively charged metal ions are attracted to the cathode and the coating is deposited on the product's metal surface. In ideal circumstances, the metal in the solution and the metal of the positive electrode will be the same, and the electricity will cause metal from the electrode to enter the solution and replace metal taken from the solution to coat the product. The thickness of the layer deposited on the product depends on the strength of the electric current, the concentration of metallic ions in the solution, the distance between the product and the electrodes, the geometry of the electrodes and the duration of time the product has been in the solution. In some instances, inert anodes are used such as carbon. In order that a satisfactory electro-deposit can be applied, the article must first be cleaned from all traces of grease, rust, oxides and other contaminants and the surface must be activated to ensure maximum deposit adherence onto the object. This is accomplished by passing the articles through a series of tanks containing the necessary cleaning and activating solutions. A basic plating sequence is as follows: hot alkali soak, dilute acid dip, electroplate and dry. Each process is followed by one or more running rinse tanks in which traces of electrolyte and other solutions remaining on the article are removed and prevented from entering and contaminating the subsequent process solutions. As indicated earlier, components may receive a series of electrodeposits such as copper, nickel and a final deposit of chromium. Articles to be plated may either be mounted on jigs (racks) or else continuously tumbled through the various solutions in perforated 6

15 barrels. Barrel plating is particularly suitable for small components of robust construction that are not likely to be damaged by affecting each other and, where individual attachment to electrical contacts would be very time consuming. 3.2 PRE-TREATMENT PROCESSES, CONTAMINANTS AND THEIR SOURCES Articles/workpieces that are to be treated must be cleaned from dust, grease and, in the case of castings, moulding flash. There should also be no corrosion on the surface of the article so that a uniform application and permanent adhesion of the treated surface can be ensured. Many articles are oiled in transit or from a previous operation (such as pressing) to prevent corrosion. Pre-treatment steps therefore include the removal of greases, oil and oxides to provide chemically active surfaces for the subsequent treatment Mechanical pre-treatment Mechanical pre-treatment processes consist of linishing (removal of roughness with sanders) and polishing, abrasive blasting and deburring and/or tumbling. Mechanical polishing produces a flowed amorphous surface on the article/workpiece by subjecting the article to high pressures and high local temperatures. Individual components may be linished using abrasive belts, and then polished with an abrasive paste applied with fabric mops which remove fine marks and give the article/workpiece a highly polished finish. Mechanical pre-treatments may be viewed as a cleaner production initiative when compared to other process that uses chemicals to polish/clean the surface of the articles/workpieces, which affect the discharge in water effluents. Abrasive blasting Abrasive blasting involves the use of shot or grit to clean the surfaces of the article/workpiece, but softer, finer abrasives such as ground nutshells along with a host of other options are also used. Abrasive blasting generates solid wastes and with non-ferrous metals, the wastes may be hazardous. Deburring and/or tumbling When using deburring and/or tumbling methods articles/workpieces are mixed with pre-formed abrasive media and tumbled or vibrated for up to several hours. It can also be used in aqueous compounds that include chemical additives to clean, deburr and pickle the parts. Wastes generated may be contaminated with oils, surfactants and abrasive particles. Where an aqueous compound is used, the effluent may require specific treatment to eliminate metals in solution and reduce the chemical oxygen demand (COD). In some cases, it may be possible to recycle the effluent after centrifugation by simple filtration or ultrafiltration (SAMFA, 2013). The solid residues from the pretreatment may be treated in a suitable waste treatment plant. The residues may also be hazardous. Although mechanical pre-treatments may be viewed as a cleaner production initiative when compared to other process, this is not the case when an aqueous compound containing chemical additives is used Electrolytic and chemical polishing Processes in electrolytic and chemical polishing include electropolishing, plasma-electrolytic polishing, electrolytic polishing and chemical polishing. These processes are applied to aluminium mainly, but stainless steel is often electropolished as well, e.g. for bathroom and kitchen racks and trays. The potential advantages of electrolytic and chemical polishing are: 7

16 The ability to use a single production line because they are similar in operation to anodising and electroplating processes, the suitability for bulk treatment, so that labour costs are appreciably lower, the clean surface improves subsequent deposit adhesion and also provides a high corrosion resistance and, pre-treatment offers superior reflectivity and colour (SAMFA, 2011). Some of the chemicals used in electropolishing are toxic and spent electrolytes and acids may have a low ph and high concentration of dissolved metals which will need to be treated, before disposal as effluent or managed as hazardous waste. Rinsing waters may therefore also require treatment. Electropolishing Electropolishing is an electrochemical method used for smoothing, polishing, deburring and cleaning various metals. Electropolishing removes a fine surface layer electrolytically, and is often used for smooth and bright finishes. The article (anode) is immersed in electrolyte and DC electric current is connected between the article and cathode. The article becomes polarised and metal ions start to diffuse to the cathode, and metal is removed preferentially from the high spots so that the hills and the valleys even out resulting in a polished finish. The effluent generated will contain spent electrolyte and may be treated in chemical wastewater treatment plants before discharging. Plasma-electrolytic polishing Plasma-electrolytic polishing is an alternative method for some applications. It differs from conventional electropolishing because of the electrolytes and process parameters used. Instead of using mixed acids, a proprietary electrolyte solution is used, which is safer for employees and the environment. In this process, the electric potential between anode and cathode is 200 V to 400 V DC depending on the solution composition and temperature, which can be in the range of 40 o C to 95 o C. The effluent contains spent salt solutions and plasma-electrolytic polishing is a cleaner production initiative as the ph of the effluent is not acidic and less acidic fumes are produced during the high temperature polishing stage. Electrolytic and chemical polishing processes for aluminium These processes are designed to replace or reduce mechanical polishing or be employed after mechanical polishing. Hot, highly concentrated acid mixtures such as especially phosphoric acid, sulphuric acid and sometimes nitric acid are used. The temperature is usually higher than 80 o C. The effluent generated contains spent electrolyte solutions and must be treated in a chemical wastewater treatment plants Solvent degreasing Solvent degreasing is usually carried out by using chlorinated hydrocarbons, alcohols, terpenes, ketones, mineral spirits or hydrocarbons. The most common solvents are chlorinated hydrocarbons (CHCs). CHCs are used due to its high cleaning efficiency, universal applicability, quick drying and incombustibility, but its use is restricted by environmental and health legislation. Two types of solvent degreasing processes may occur, namely cold cleaning and vapour phase degreasing. In cold cleaning the articles are immersed in the solvent or cleaned in a stream of solvent. In vapour phase degreasing the solvent is vaporised and the cold component suspended in the vapour. The vapour condenses on the component dissolving grease and drained off with the dirt and grease, leaving the component clean and dry. The main components of the aqueous cleaning system are alkalis or acids, silicates, phosphates and complexing and wetting agents. Rinse-waters may require simple ph treatment in wastewater 8

17 treatment plants. Cleaning solutions may need to be separated from other process effluents to avoid interference with the wastewater treatment plant by excess surfactants. Process tanks operate at 50 o C to 90 o C and may require fume extraction. The cleaner production initiative for hazardous degreasing solvents (such as trichloroethylene) is often the replacement with biodegradable alkaline degreasers. In aqueous cleaning, the articles are placed in an aqueous process solution for several minutes, or placed in a spray bath. The solutions are usually alkaline, but may be neutral or acidic and usually operate at increased temperatures of 40 o C to 90 o C to enhance the cleaning effect. The installation of recovery equipment is not therefore required as a consequence of changing to newer degreasers Pickling Pickling is applied to degreased/clean metallic surfaces. During the pickling processes disturbed or adhering layers such as scale, oxide films and other corrosion products of the metal are stripped from the surface prior to other surface treatment processes. The strong oxide layers are removed by a chemical reaction with an acid-based pickling agent. The pickling agent may include hydrochloric acid, sulphuric acid, hydrofluoric acid, nitric acid or phosphoric acid. Hydrochloric acid or sulphuric acids are normally used while the other acids mentioned are used for alloy steels containing more than 6% alloys. Mixtures of the aforementioned acids are also used. The typical pickling reaction is the metal oxide reacting with the pickling solution to form the corresponding metal ion and water. The removal of the oxides is dependent on specified acid concentrations, temperature and pickling times. The pickling time is reduced with increasing acid concentration and operational temperatures. Some guidelines that are followed are usually that: The maximum pickling effect is reached with a sulphuric acid concentration of 25% and an optimal temperature of 60 o C, when using hydrochloric acid a concentration of 18 to 22% is used when temperatures are increased from 30 o C to 35 o C. However, when using hydrochloric acid as a pickling agent, air emissions are formed, Hydrofluoric acid is used for the pickling of cast iron at a concentration of 20 to 25% where temperatures of 35 o C to 40 o C are generally preferred and, for aluminium the process is called desmutting and nitric acid is used at concentrations below 150 g/l. Descaling and pickling are not normally applied to aluminium processing because the natural oxide layer on aluminium is very thin. Etching is rather carried out on aluminium and its alloys, by placing aluminium in sodium hydroxide solutions. Spent pickling solutions require either treatment and disposal, through an effluent treatment system, or disposal as liquid wastes. The iron that is removed in significant amounts has adverse effects on the wastewater treatment systems such as increased sludge production. Generally, effluents from pickling processes can be easily treated in typical wastewater plants. The life of the pickling acid can be extended by using proprietary chemical additives that tie up the metals allowing them to be filtered out easily using standard pump-filter units Electrolytically assisted pickling, activation and degreasing Pickling can be enhanced by making the article anodic. Non-electrolytic pickling is sometimes followed by electrolytic activation to remove oil and dirt. The oil and dirt are removed by the electrolysis of hydrogen at the surface of the cathode, and oxygen gas at the surface of the anode. Wetting agents are omitted to prevent foaming, however cyanides or other complex agents may be added to improve the activation of steel items. If cyanides (which are hazardous) are used, then it will 9

18 be present in the wastewater effluent. Rinse effluents and used solutions can be treated as alkaline or cyanide solutions in wastewater treatment plants prior to discharge. Cyanides in cyanide solutions are usually treated by oxidation before being released as effluent Metal stripping Metals are stripped when it is necessary to process defectively electroplated components without losing the properties of the base material. The metal is then re-plated to the correct specifications. There can be increased waste production in sludge from treatment and used acids as well as wasted water Drag-out and rinsing Drag-out is defined as the liquid which adheres to the article surface after any process. As a result, rinsing is carried out as a common activity after nearly all process steps. Rinsing is necessary to prevent cross-contamination of process solutions and to ensure there is no deterioration of the workpiece and/or article surface by residual chemicals. The rinse-water may vary in quality depending on the process requirements and may be required to treat prior to discharge. Metals can only be treated or moved to another waste stream but cannot be destroyed. Cyanides are usually treated by oxidation, but complex agents may need to be treated separately to enable metals to be successfully treated afterward. Past SAMFA experience has shown that surfactants, brighteners, and other additives may interfere in wastewater treatment or have their own environmental impacts (SAMFA, 2011). A reduction of drag-out is a primary measure for minimising losses of chemicals, operating costs and environmental problems in rinse-waters of the plating industry. Many rinsing techniques have also been developed to reduce water consumption to a minimum, e.g. spray rinsing as opposed to immersion rinsing. Spray rinsing is a method where water is sprayed onto the items as a water mist as opposed to immersion in a rinse tank (Figure 7). Other methods include line rinsing where the rinsing process can be extended by two or more rinsing tanks each with running water in a series. Cascade rinsing (or more correctly, counterflow rinsing) is a multistage rinsing process, where the same rinse water flows through all tanks. Water from rinsing stages can also be recycled by using water from one rinsing stage for another rinsing stage where the chemical or physical characteristics acquired in the first stage can be exploited in the second stage without requiring any additional treatment. In so doing, water consumption is reduced and the chemicals needed to modify or balance the ph is reduced. Using multiple rinse stages achieves a high rinsing rate with a small amount of rinsing water, thus reducing the consumption of water. The rinsing methods can be listed in order of decrease in raw water consumption: re-circulation 2-stage chemical rinse, 3-stage cascade rinsing, 3-stage line-rinsing, 2-stage cascade rinsing, optimum spray rinsing, 2- stage line-rinsing and 1-stage rinsing with running water. 3.3 THE SPECIFIC SURFACE TREATMENTS REGIMES AND GENERAL WASTES GENERATED The following is a description of some of the popular surface treatment processes as well as the wastes generated Copper and copper alloy plating Copper plating is common for items in daily use such as coins, buttons or zip fasteners. These types of workpieces can be plated in jigs or barrels. Acid copper plating has become the standard choice for copper plating, whereas pyrophosphate copper electrolytes no longer play an important role. 10

19 However, copper cannot be deposited directly onto mild steel due to the displacement effect (electrochemical series). The displacement effect is used to predict whether a given metal will displace another, from its salt solution: The metal having low standard reduction potential will displace the metal from its salt's solution which has higher value of standard reduction potential. A cyanide copper underlay, therefore, remains the popular choice. Copper can also not be deposited onto zinc die castings directly from an acid electrolyte. The different copper plating processes are listed in Table 1. Table 1 Typical copper plating and wastes generated Type of copper plating Cyanide copper Acid copper Electrolyte solution Copper cyanide, sodium cyanide Copper cyanide Potassium cyanide, potassium hydroxide Copper cyanide, sodium cyanide, potassium sodium tartrate Copper sulphate Sulphuric acid (60 to 90 g/l) Metal content 15 to 20 g/l (copper) Layer thickness Uses 2 to 3 µm For strike plating on steel and zinc 25 to 50 g/l 6 to 8 µm Produces thicker layers in barrel plating for high performance electrolytes 40 to 60 g/l No data given Also high performance providing higher current densities, enhanced brightness and reduction in tendency to anode passivation 50 to 60 g/l No data given Makes polishing and buffing redundant Wastewater effluent and treatment Sodium-based solutions can be regenerated by batch or continuous precipitations of sodium carbonate Potassium-based electrolytes have to be discarded as soon as the content of potassium carbonate exceeds 90 g/l Potassium-based electrolytes have to be discarded as soon as the content of potassium carbonate exceeds 90 g/l (is formed from the oxidation of cyanide) None given Copper sulphate Sulphuric acid (180 to 200 g/l) 20 g/l No data given Through-hole, panel and pattern plating of printed circuit boards None given Pyrophosphate copper Brass Copper pyrophosphate (110 g/l) Potassium pyrophosphate (400 g/l) Citric acid (10 g/l) Ammonia (3 g/l) Copper cyanide Zinc cyanide Sodium cyanide (70 to 90 g/l) Bronze Stannate Copper cyanide Zinc cyanide Potassium cyanide (6 to 10 g/l) Compiled from CPMFI, 2003 and SAMFA, 2013 No data given No data given Shielding on heat treated parts, drawing aid for wires, forms thick bright layers that need little or no polishing, through-hole and panel plating 8 to 15 g/l 5 to 30 g/l 4 to 10 g/l 4 to 20 g/l 1 to 4 g/l 11 Due to the ammonia content, separate treatment from other effluents containing metals is required No data given For decorative purposes ph 10 effluent No data given For decorative plating, as a substitute for nickel in jewellery to avoid skin allergy effects Potassium carbonate solutions have to be discarded after it exceeds 40 g/l

20 The effluent generated in acid copper processes can be treated in a wastewater plant for low ph as well as to remove the copper. Pyrophosphate copper effluents have to be treated with lime (CaCO 3 ) because sodium or potassium hydroxides (NaOH and KOH) do not precipitate the copper from the pyrophosphate. Bronze copper effluents can be treated for ph, cyanide and metals in a wastewater treatment plant which contains a cyanide oxidation step. Zinc cyanide can be substituted with acid zinc or alkali cyanide-free zinc for reduced environmental emissions. Copper cyanide can be substituted with acid- or pyrophosphate copper except for strike plating processes on steel, zinc die casts, aluminium and aluminium alloys. The frequency of dumping bath solutions can be reduced by preventing the contamination of the baths. Methods include using anodes and chemicals of the highest purity, providing sufficient rinsing and operating with frequent analytical control of the bath solution (Binnie, 1987). In spite of the fact that there is a greater emphasis on clean technology, and where possible the replacement of cyanide based solutions, the advantages of plating from cyanide based copper solutions are such that these systems continue to be used extensively. Modern day approaches to handling cyanide are informed and the methods of cyanide destruction are well researched and established. It is, therefore, possible to work with cyanide safely and ensure safe disposal. The main advantages of cyanide copper plating are: Cyanide copper solutions are not as corrosive as acid copper and do not tend to attack articles. An undercoat of cyanide copper prevents attack and facilitates subsequent coverage by acid copper. Cyanide copper solutions do not produce immersion coatings that prevent adhesion on steel, zinc and aluminium castings, as does acid copper, therefore its use as an undercoat or strike is invaluable. Cyanide copper has better throwing power and less thickness variation over the surface of the work-piece. It is possible to achieve good coverage over complexly shaped parts. Some of the disadvantages are: Cyanide copper solutions are not generally capable of good levelling, i.e. it is capable of filling imperfections in the metal to render a smoother finish. However research has shown that with periodic reverse and pulse plating with proprietary additives, significant levelling can be achieved. Additional effort and expense has to go into handling and disposal safety programs Nickel electroplating Nickel electroplating has the primary function to improve the resistance of articles to corrosion, wear and abrasion, although nickel also provides a smooth, highly reflective and corrosion-resistant coating below a range of other coatings for decorative finishes. The most important application of nickel is in nickel/chromium electroplated coatings, commonly called chrome plating. It consists of a very thin chromium topcoat (1%) over an undercoat of nickel (99%). Brass, gold and silver topcoat systems are used as alternatives to chromium. Nickel can also be used on its own without any topcoat. This is generally only for engineering purposes, e.g. refurbishment of worn components. Nickel matrixes can be formed in which inert, non-metallic particles, such as silicon carbide, diamond or polytetrafluoroethylene (PTFE) are incorporated by co-deposition to improve engineering properties such as hardness, abrasion resistance and coefficient of friction. Electrodeposited nickel alloys of commercial significance include zinc-nickel, nickel-cobalt and nickel-iron. The types of nickel solutions are listed in Table 2. 12

21 Table 2 Types of nickel solutions used Solution type Solution content Temp. ph Wetting agents Watts-type nickel Nickel sulphamate-based (most frequently used in electroforming applications where the low internal stress of the deposits they produce is absolutely vital) Nickel chloride-based (very limited used due to high internal stresses of deposits) Nickel sulphate 240 to 375 g/l Nickel chloride 35 to 60 g/l Boric acid 30 to 45 g/l Nickel sulphamate 350 to 600 g/l (allows higher current) Boric acid 35 to 45 g/l Nickel chloride 1 to 15 g/l (sometimes) Woods nickel strike solution: Nickel chloride hexahydrate 240 g/l Range of o C o C (more common) o C o C (more common) 70 o C (high deposition rates) o C No data given Added to reduce pitting (wetting agents used do not usually interfere with typical wastewater treatments) Added to reduce pitting No data given Organic compounds Added to modify metallurgical structure of nickel to produce either lustrous/bright appearance or semibright/satin nickel deposits Saccharin and naphthalene trisulphonic acid added increase deposit hardness or to control deposit internal stress No data given Hydrochloric acid 125 ml/l (used to provide an initial adherent nickel layer on the surface of materials where it is difficult to achieve adhesion) Nickel sulphate-based Nickel strike solution: Nickel sulphate 70 g/l No data given No data given No data given No data given Sulphuric acid 100 g/l Nickel-cobalt alloy (used in electroforming because they are harder than pure nickel and nickeliron alloys) Solutions based on standard Watts-type Based on Watts-type Based on Watts-type Based on Watts-type Based on Watts-type Compiled from CPMFI, 2003 and SAMFA, 2013 Soon after 2008, nickel sulphate, nickel carbonate and other soluble nickel slats were classified as carcinogens when inhaled as an aerosol. An exposure limit has been set of 0.1 mg/m 3 as nickel on an eight hour time weighted average (SAMFA, 2013). If nickel-plated finishes cannot be substituted with something else, then methods will have to be found to lessen the nickel mists that emanate from the plating baths. The hydrogen and oxygen liberated by electrolysis also escapes as a mist and this will have to be dealt with through properly designed extraction units. Another health effect is the sensitivity of between 4 and 20% of the population to contact with nickel, resulting in a rash known as contact dermatitis and commonly referred to as nickel itch. Platers are aware that an operator working on a nickel-plating line will sometimes develop an allergy to nickel after extended exposure. It is generally accepted in plating circles that once this happens, that 13

22 particular individual cannot work with nickel again. Normally, however, as far as the public is concerned, this condition is predominantly caused by close skin contact with jewellery items such as wristwatches, bangles, earrings and posts and sometimes clothing fasteners that have been coated with nickel. This is a problem that is of great concern to the plater as it has become necessary to stop user exposure to consumer products, especially jewellery. Alternatives to nickel that have been experimented with are: White Bronze, an alloy deposit like brass from a cyanide based solution. A thin flash of Palladium over bright acid copper as the underlay coat. A tin-nickel alloy that has been specially developed which is non-allergenic, according to its promoters. The wastewaters generated can be treated in a typical wastewater treatment plant. However, wastewaters containing nickel must be separated from cyanide-bearing wastewaters because nickel forms stable complexes with cyanide which are difficult to treat. Furthermore, contact with other complexing agents should also be avoided. Solid residues will require management as hazardous waste. The frequency of dumping bath solutions can be reduced by preventing the contamination of the baths. Methods include using anodes and chemicals of the highest purity, providing sufficient rinsing and operating with frequent analytical control of the bath solution (Binnie, 1987). Exhaust extraction is regularly used and mist eliminators are also often employed Chromium plating Chromium plating has found wide usage both as a functional coating and as a decorative surface finish because of its typical hardness and wear resistance properties. It is also widely used in packaging applications. Bright chrome is usually applied as a thin layer to prevent corrosion for decorative purposes and can be plated from either hexavalent or trivalent chromium electrolytes. The deposit thickness is generally in the range of 0.1µm to 0.4 µm with a treatment time of between 2 and 13 min. Hexavalent chromium plating contains 80 to 400 g/l chromic acid and 0.8 to 5 g/l sulphate. Trivalent chromium plating, on the other hand, is based on chromium III compounds, and contains only about 20 g/l of the trivalent chromium. Black chromium plating is for decorating black pieces. They are based on chromic acid electrolytes with 350 to 520 g/l chromic acid. Deposit layers are porous and less than 1 µm thick. Hard chrome plating consists of heavy deposits applied on particular components to give high resistance to mechanical and wear damage. These can be plated from hexavalent chromium electrolytes. The physical and mechanical properties of the plated coating are influenced by both the type of catalyst chosen and operating temperatures. Hexavalent chromium (where contained in the effluent) is genotoxic (causes mutations) and carcinogenic. Effluents may be treated in a typical wastewater treatment plant, where chromium VI is reduced to chromium III, followed by flocculation and precipitation. Chromium III solutions do not require separation and reduction prior to treatment. Furthermore, the lower electrolyte concentrations in trivalent chromium electrolytes have a lower viscosity than the hexavalent electrolyte. This result in a better draining of the plated parts, and subsequently less drag-out occurs, the loss of electrolyte is reduced, less effluent treatment is required and less chromium-containing waste is produced. Sludge disposal from wastewater treatment plants can therefore be reduced by up to 90% if trivalent chromium electrolytes are used. For black chromium plating, it may also be a necessary to treat nitrates and/or fluorides in any wastewaters. The use of trivalent chromium eliminates the carcinogenic properties and other hazards associated with hexavalent chromium. Furthermore, the lower chromium content in trivalent chromium plating 14

23 lowers the viscosity of the electrolyte solution which permits easier draining of the articles/workpieces, with less drag out and up to an 80% reduction in sludge production (SAMFA Metal Finishing Handbook, 2008). However, trivalent chromium plating can only be used for decorative purposes, and not for hard chromium plating. In hard chromium plating processes, emissions may therefore be reduced by covering the plating solution during plating and using air extraction with condensation to recover the hexavalent chromium. The hexavalent chromium solutions can also be operated on a closed loop basis. The frequency of dumping bath solutions can be reduced by preventing the contamination of the baths. Methods include using anodes and chemicals of the highest purity, providing sufficient rinsing and operating with frequent analytical control of the bath solution (Binnie, 1987). The following sections are a précised version of the health and environmental implications of chrome plating taken from CPMFI of 2003 and SAMFA, Chromic acid, CrO 3, the hexavalent compound used to make up chrome plating solutions is very toxic, a suspected carcinogen and hazardous. When preparing these solutions the use of safety clothing and apparatus, including goggles, apron, boots and respirator, is necessary. Chromic acid is a strong oxidant and should be stored away from flammable materials. It can spontaneously cause fires when in contact with other products in the stores, including activated carbon, paper bags, rags, etc. The mists are particularly aggressive. In earlier times before awareness of safety had reached today s standards, some chrome plating workers had the entire septum of the nose corroded away by these fumes, so that there was no separation between the nostrils. Skin contact with chrome compounds and solutions should also be avoided, as chronic ulceration, so-called chrome sores can result. Special barrier creams and ointments are available to both prevent and treat this condition. Hexavalent chromium salts are classified as hazardous substances, are carcinogenic and have very serious effects on biological processes at sewerage processing plants. Stringent limits apply to discharge of chrome effluents. In Denmark, the tolerated level of CrO 3 in the effluent sample is zero. Trivalent Chromium Plating Over the years, there have been numerous attempts to develop a chromium plating process without the need for chromic acid (Chrome Vl) and its associated problems. The British Non-Ferrous Metals Research Association carried out preliminary work on trivalent (Chrome III) electrolytes in the late 60's and early electrolyte prototypes were undergoing commercial trials in the UK in the early 70's. Further development work in the USA improved the process, and due to the environmental pressure in the USA this electrolyte improvement maintained its momentum, and forced the progressive development of technically, and commercially viable processes. By the late 70 / early 80 s, full-scale production plants were in operation in the USA (CPMFI, 2003 and SAMFA, 2013). Commercially viable chloride and sulphate based trivalent processes (available from several suppliers) are now in operation in several companies in the UK and in other countries. The major reasons for the business owner to switch to trivalent chrome plating are: a) Meets the legal requirements for substitution with a less hazardous alternative where available. b) Eliminates the carcinogenic and other hazards associated with hexavalent chromium from the workplace. c) Significantly reduces drag-out losses, associated effluent treatment and sludge disposal requirements. d) Potential for reduction in reject rate and increased productivity. e) Much improved energy efficiency. 15

24 f) Trivalent plating systems are now a proven and established technology in many applications. When changing to a trivalent process one-off costs will be incurred for disposing of the hexavalent chromium solution, re-lining the process tank with physical vapour deposition (PVD) (after removing and disposing the contaminated lead), and replacing the lead/antimony anodes with carbon, plus an ion exchange system. Ion exchange resin technology has improved; costs are much lower, the equipment is easy to use in controlling the metallic contamination, without the need for production to be interrupted. In a British case study, regeneration of the ion exchange columns is necessary periodically and they have established that the resins will last about 3 years before they need to be replaced (SAMFA, 2013). These new fourth generation products are available in South Africa. They can replace conventional hexavalent chromium processes in many instances and offer superior colour and finish compared to the first generation systems. The process has excellent covering power and a good cathode efficiency.. Trials have been done in South Africa, but at the time of producing this report, there were no tri-valent chrome plating installations locally Zinc and zinc alloy plating Zinc and zinc alloy plating are the most widely used electrolytic surface treatments, providing corrosion resistance and/or cheap decorative coating to a wide variety of iron and steel items for the automotive, construction and other industries. The most common electrolyte systems are described in Table 3. Table 3 Types of zinc electrolyte solutions Type of electrolyte system Content Metal concentration Uses Alkaline cyanide zinc (Current efficiency of 50% to 75%) Alkaline cyanide-free zinc (Current efficiency of 65% to 70%) Acid zinc (Cathode efficiency of 93% to 96%) Zinc oxide Sodium hydroxide (80 to 120 g/l) Sodium cyanide (5 to 100 g/l) Zinc oxide Sodium hydroxide or potassium hydroxide (100 to 150 g/l) Zinc chloride Potassium chloride and/or sodium chloride (130 to 180 g/l) Boric acid (10 to 40 g/l) Wetting agent 10 to 30 g/l For technical corrosion-resistant layers. The cyanide-based electrolyte has good throwing power into holes and blind spaces. 5 to 15 g/l Applied for technical corrosionresistant layers. 30 to 55 g/l For decorative purposes, e.g. on furniture frames, shopping trolleys and baskets. Zinc alloy Zinc-iron from alkaline cyanide-free electrolytes <1% Fe For automotive applications Zinc-cobalt from acid or alkaline cyanide-free electrolytes <3% Co Zinc-nickel from acid (ammonium chloride-based) or alkaline non cyanide electrolytes Compiled from CPMFI, 2003 and SAMFA, 2013 <15% Ni Cyanide in alkaline cyanide zinc rinse-waters can be readily oxidised by several methods in the wastewater treatment plants. Zinc can readily be removed in a typical wastewater treatment. Zinc from alkaline cyanide-free zinc rinses can readily be removed in a wastewater treatment plant. Effluents from acid zinc processes may be treated in a typical wastewater treatment system. Treatment of effluents from ammonium chloride-based electrolytes has to be treated separately. Almost all the drag-out can be recovered. Special treatment is not required to remove the nickel from 16

25 the effluent for zinc-nickel plating processes because of the high dilution factor that occurs in rinsewaters. Additional chlorination treatments will be necessary to destroy nickel amine complexes before the nickel can be precipitated from the effluent stream. Cyanide based electrolyte solutions can be substituted with alkaline cyanide-free zinc and acid zinc for zinc electroplating. This reduces waste in the wastewater effluent and reduces organo-halogens (AOX). Due to an increasing environmental consciousness that began stirring in the 1960 s, research continued on the development of acid and non-cyanide zinc plating systems on a parallel basis. A lot of effort went into acid systems based on zinc sulphate, with some degree of success but it was not until Schloetter Laboratories in Germany patented their ammonium chloride bright acid zinc process that any serious contender to replace cyanide zinc plating emerged. The development of this brightening system and basic formulation became the cornerstone on which all subsequent work in this field was based and which remains the industry standard (SAMFA, 2013). The advantages of acid chloride zinc plating are: Reduced effluent treatment requirement. No cyanide oxidation step. Simple precipitation of zinc metal by elevating the ph. Cathode current efficiencies of 95-98% within normal working parameters Brightness levels much higher than cyanide zinc plating Good levelling characteristics not found in cyanide or alkaline non-cyanide zinc plating. The ability to plate on cast iron and similar materials not plateable in cyanide zinc. Solution is more conductive requiring less power than cyanide zinc to cover the same area, potentially increasing productivity by 25-30% over cyanide installations. Reduced hydrogen embrittlement safer on high tensile components and spring steel. Anodes do not dissolve in the solution and can be left in place during idle periods. Solutions are more free-rinsing than cyanides, reducing drag out losses. New generation brighteners can tolerate temperatures of up to 50 o C without losing efficiency. Where the main disadvantages of acid chloride zinc plating is: Severe leaching problems due to the thinner solution which gets into entrapment areas easily. The corrosiveness of the acid chloride solution results in worse attack on the surrounding areas of plate than does cyanide solution. Some components can therefore simply not be plated in acid zinc without redesigning to promote free rinsing. There is no third-stage electro-cleaning effect in acid based solutions. Therefore precleaning is as critical as would be for nickel plating and may involve additional expense and equipment. Precipitates of ferrous hydroxide that are generated in the plating process have to be removed by continuous filtration, particularly in rack plating applications. Although less of a problem in barrel plating operations it remains necessary to perform batch filtration treatments from time to time. The process cannot take place in ordinary mild steel tanks and needs specialised equipment like titanium hooks or baskets for anodes. Metal distribution on the article is not good. Work on the alkaline non-cyanide zinc bath continued apace since the need for environmentally acceptable alternatives first started. Unlike the acid chloride bath, successes did not come early in these experiments and it was not until the early 1990 s that it was demonstrated that problems had been overcome and that viable non-cyanide alternatives could be considered by the average plating shop. Now that the majority of difficulties have been overcome, non-cyanide alkaline systems are 17

26 poised to displace both cyanide and acid systems. These baths are also referred to as zincate baths since they rely solely on sodium zincate, Na 2 ZnO 2, for their metal ions. The advantages of alkaline non-cyanide plating: The basic solution is less expensive than other systems but currently relies heavily on proprietary additives to make it work properly, virtually negating the cost advantage. The solution may be operated in steel tanks. No change of equipment is necessary if converting from a cyanide plating system. In terms of toxicity, the zincate bath is the least toxic of all available systems. Zincate baths have excellent throwing power and metal distribution, making them especially suited to plating into deep recesses such as tubes, boxes, cages, etc. They feature faster plating cycles with higher rack loading. There is evidence of operators doubling their production when switching from cyanide to zincate baths. They produce brilliant and ductile deposits. Suppliers of modern formulations claim tolerance of temperatures of around 50 o C. The deposits readily accept a chromate conversion coating from standard formulations as used for cyanide zinc plating. Results may differ slightly from that achieved on the deposit from a cyanide solution. Low metal content reduces effluent treatment costs. No cyanide oxidation step. The disadvantages of alkaline non-cyanide plating: Cleaning is vitally important, perhaps even more so than with acid zinc plating. Zincate baths do not possess any of the levelling ability of their acid zinc counterparts. Control of the zinc content is more critical. Dissolution of the anodes has to be more closely monitored. Often an additional reservoir of zincate solution has to be installed, the only purpose of which is to act as a dissolving cell to provide strong fresh solution to balance up the zinc content in the system Cadmium plating Cadmium is similar to zinc both as a metal and as regards plating. There is not a great deal of difference between the two in protecting steel against corrosion. Cadmium plating is mainly used to protect parts made of steel, aluminium or titanium alloys. It is more corrosion resistant than zinc, however the toxicity of cadmium has limited its use to vital technical uses only. Examples include metals in aviation and aerospace, military equipment, mining, nuclear industries and some safety critical electrical contacts. Acidic and alkaline cyanide baths can be used for cadmium plating. The electrolytes used are listed in Table 4. Table 4 Electrolytic solutions used in cadmium plating Types of electrolytic solutions Based on cyanide Constituents Sodium hydroxide Constituent concentration 20 g/l Operating temperature 20 o C to 35 o C Sodium cyanide 120 g/l Based on fluoroborate Cadmium Cadmium fluoroborate 20 to 30 g/l 250 g/l 20 o C to 35 o C Ammonium fluoroborate 60 g/l 18

27 Types of electrolytic solutions Constituents Boric acid Constituent concentration 25 g/l Operating temperature Based on sulphate Cadmium sulphate 52 to 85 g/l 18 o C to 30 o C Based on chloride Sulphuric acid Cadmium chloride 50 to 120 g/l 114 g/l No data given Ammonium chloride 112 g/l Complexing agent Compiled from CPMFI, 2003 and SAMFA, g/l Effluent may be treated by chemical-physical treatment. It can be very difficult to obtain low emission values for cadmium by precipitation therefore additional treatment may be required such as separate treatment at the point source prior to mixing with other effluents, e.g. electrolysis, mobile ion exchangers and evaporation. It is possible to plate cadmium in closed loop systems where rinse water from the first rinse is returned to the process solution. Closing the loop can be achieved by cascade rinsing, ion exchange, membrane techniques or evaporation. Cadmium and its salts are toxic. When its melting point of 321 o C is reached, it produces poisonous fumes of cadmium oxide. It does bio-accumulate in the body as it is excreted very slowly. Cadmium enters the body through inhalation or ingestion. The most likely source for inhalation by a plater is from mists above the plating bath. Ingestion would usually be accidental as a result of not observing safety rules, consuming food in the plating shop or without washing hands properly. Exposure to cadmium compounds over extended periods can lead to damage to the kidneys and lungs. Cadmium and cadmium compounds are now known human carcinogens. [Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta] One of the silent dangers is when cadmium gets into the food chain through effluent discharge and dumping (SAMFA, 2013). As described in the chapter on zinc plating, a lot of effort has gone into finding suitable alternatives as a safe replacement for cadmium. At the moment the two most promising systems seem to be zincnickel and tin-zinc alloys (SAMF, 2013) Tin and alloy plating Tin and tin alloy plating deposits are ductile, resistant to corrosion, easy to coat, and have high throwing and good distribution properties. With the exception of the tin-lead alloy, the deposits are non-toxic. The main application of tin plating is in the coating of steel coil for packaging of food, beverages and aerosols. It is also widely used in printed circuit boards, electronic components, appliance chassis and also kitchen utensils. Several electrolytes are used and are available, such as acid stannous sulphate, acid tin fluoroborate, alkaline sodium or potassium stannate (tin compound). Tin lead plating is the most commonly used tin plate alloy. It is used as solder coat in different alloy ratios. As the lead component is toxic, a range of lead-free alternatives has been developed. Non-fluoroboric tin lead electrolytes are available and based on the organic methane sulphonate acid. The wastewaters will contain sulphonic acid. Effluents may be treated in typical wastewater plant. Fluoroborate bath effluents should be pre-treated separately to a typical wastewater treatment plant Precious metal plating Thin layers of less than 1 µm of precious metals can be plated onto metals to make a wide range of items appear valuable. They also provide stain and corrosion resistance. The conductivity, hardness and wear resistance have led to a widespread application in the electric and electronic industries as 19

28 well. Coatings can be made of silver, gold, palladium and alloys, rhodium and platinum. The details are listed in Table 5. Table 5 Precious metal coatings Precious metal plating Constituents Constituent concentration Uses Silver (Ag) Potassium-silver cyanide Potassium cyanide Ag: 30 to 65 g/l 100 to 160 g/l For jewellery, ornaments, trophies and giftware. Also on components of electrical and electronic devices. Potassium carbonate 15 to 20 g/l Gold (Au) Potassium cyanoaurate ph buffering agents Au: 2 to 6 g/l For connectors, printed circuit boards, integrated circuits, semiconductor manufacture, bathroom fittings, giftware, tableware, buttons, watches, pens, jewellery, spectacle frames Sometimes alloying metals: cobalt, nickel, iron, indium No data given Hardens, increases the wear resistance and brightens the deposit. Strike solution: gold potassium cyanide 1 to 2 g/l To deposit the initial layer of gold to promote adhesion Alkaline solutions containing free cyanide No data given Jewellery plating Non-cyanide gold electrolytes: gold sulphite complexes Au: 8 to 15 g/l Same as for cyanide electrolytes Palladium (Pd) and alloys Complex tetraamine palladium (II) dichloride Pd: 4 to 20 g/l Coating spectacle frames and writing implements Ammonia Palladium-nickel alloy Pd: 75 to 80% Ni: 20 to 25% Coating spectacle frames and writing implements Rhodium (Rh) Rhodium (III) sulphate or phosphate (Layer thickness: 0.05 to 0.5 µm for decorative purposes, and 0.5 to >8 µm for technical purposes) Rh: 2.5 to 20 g/l Deposited on silver to prevent staining. Hardness and wear resistance are suitable for technical applications on reed contacts and heavy duty connectors. Also applied on reflectors for optical equipment. Platinum (Pt) Acid electrolytes based on chloride, sulphate, nitrate and nitrite complexes of platinum Alkaline electrolytes based on phosphate, ammonia and sodium hydroxide complexes Compiled from CPMFI, 2003 and SAMFA, 2013 Pt: 6 to 40 g/l Thin layers applied for decorative purposes, thicker layers applied on electrical devices and equipment for the chemical industry Potassium carbonate concentrations over 200 g/l need to be discarded. If cyanides (which are hazardous) are used, then it will be present in wastewater effluent. Cyanide can be readily oxidised in typical effluent treatment in wastewater treatment plants. Effluent treatment is similar to other 20

29 electroplating rinse streams in a typical wastewater treatment plant. Electrolytic and ion exchange recovery of gold may be practised Autocatalytic plating Autocatalytic plating is also known as electroless plating which is a catalytic chemically reduced coating. The fundamental reaction requires the presence of a catalytic metal (which is the metal being deposited) that allows the reaction to proceed. The advantages of using electroless plating are: All surfaces are in contact with the fresh solution, the deposit will be uniform over the entire surface, regardless of the surface shape, Plated deposits are less porous than electroplating of the same metal, racking or fixing is greatly simplified and, non-conducting material may be coated. Autocatalytic plating may include nickel and copper electrolytes on both metals and plastics. Details are summarised in Table 6. Table 6 Types of autocatalytic plating Type autocatalytic plating Constituents Constituent concentration Uses Nickel on metals Electrolytes: Nickel sulphate, nickel chloride Reducing agent: Sodium hypophosphite Chelating agents: organic carboxylic acids Buffers: Sodium hydroxide, sodium carbonate Ni: 2 to 10 g/l 10 to 50 g/l 10 to 50 g/l No data given Uniform thickness is achieved irrespective of shape, and resistance is high against wear, abrasion and corrosion with good adhesion on base metal. Can therefore be applied in data storage devices, components for chemical, oil and gas industry, automotive, machine tool and electronics industries, plastics moulding tools Brighteners: Cadmium or Lead 1 to 5 mg/l 3 mg/l Nickel-phosphorus alloys P: 2 to 15% Nickel on plastics Nickel sulphate, nickel chloride Ni: 2 to 5 g/l - Reducing agents: Sodium hypophosphite Dimethylaminoborane 5 to 20 g/l >10 g/l Chelating agents: organic acids No data given Weakly sulphuric acid/sodium hydroxide, ammonia hydroxide Acid ph: 3 to 6 / Alkaline ph: 8 to 10 Copper on metals and plastics Copper electrolyte Sodium hydroxide Chelating agents (EDTA) or tartrates Cu: 2 to 5 g/l 15 to 20 g/l 10 to 15 g/l 5 to 10 g/l Applied on small items such as buttons, fashion jewellery. Also plastic housings for electric shielding and printed circuit boards. Reducing agents: formaldehyde 3 to 5 g/l 21

30 Compiled from CPMFI, 2003 and SAMFA, 2013 Autocatalytic coatings generate more waste than other plating techniques. Effluents may have to be pre-treated before being discharged to the wastewater treatment plant. The strong chelating agent may require separate wastewater treatment. Trace amounts of cadmium and lead may be present in the effluent. Typical effluent treatment will remove metals except where effluents contain strong chelating agents. Effluent containing the solvent EDTA requires separate treatment because EDTA prevents the precipitation of metals in effluent treatment (SAMFA, 2013). The frequency of dumping bath solutions can be reduced by preventing drag in of contaminants by providing effective pre-cleaning and rinsing and operating with frequent analytical control of the bath solution However, electroless plating baths have a finite life and have to be discarded after a given number of cycles. A bath cycle is completed when the same amount of metal has been added to the bath as was originally present at make-up Immersion or displacement coating Immersion or displacement coatings are non-catalytic chemically reduced coatings. They are formed when the metal to be deposited is precipitated upon its reduction in solution. This can occur chemically from solution or if the metallic article is more active than the ions in the solution. Deposits are often non-adherent and of poor physical quality, although careful attention to solution composition and operating conditions can produce deposits acceptable for certain purposes such as flash gold deposits on fashion trim Colour anodising on aluminium Aluminium can be coloured in many shades and colours in conjunction with or after sulphuric acid anodising. There are four general methods, namely immersion-, electrolytic-, interference- and integral colouring. Immersion colouring is the most widely used colouring method. The anodised workpieces or articles are immersed in a water-based organic or inorganic dye solution before sealing. In electrolytic colouring, the anodised aluminium is placed in an acid solution containing metal salts and an alternating current is applied. The film obtains a colour characteristic of the metal salts used. Electrolytic and immersion colouring can be combined to form new shades. In interference colouring, the appearance is produced by interference effects between two light-scattering layers, i.e. between the electrochemically deposited metal layer at the bottom of pores and the aluminium oxide-aluminium interface beneath. With integral colouring, the aluminium oxide layer is coloured itself during the anodising process. Colouring occurs either by anodising in a solution of special organic acids or by normal anodising in sulphuric acid of special aluminium alloys with substances that are not oxidised such as Al-Si or Al-Fe-Mn. This technique has almost entirely been replaced by electrolytic colouring. Metals used in electrolytic colouring may require treatment in a wastewater treatment plant prior to discharge. Some organic dyes may require additional wastewater treatment Sealing following anodising Sulphuric acid anodising is normally followed by a sealing process. Sealing improves the corrosion and stain resistance of the oxide layers. It also prevents organic dyes from leaching out and improves the light fastness. Sealing may be carried out in hot or cold processes. With hot sealing, the process is carried out by immersing the anodised parts in hot or boiling deionised water (95 to 96 o C) for three minutes per µm thickness. Sealing with steam achieves the same effect. Cold sealing methods have been developed for operating temperatures of 25 to 35 o C, and 60 o C. 22

31 No constituents of concern are present in wastewater effluent and this water may be recycled and used in sealing (AFSA, 2013) Phosphating coatings Phosphate coatings are the most widely used conversion coatings and probably the most widely used surface treatment. They are used to treat steel, aluminium and zinc for cold forming, coil coating, rust proofing, bearing surface lubrication, paint base and electrical insulation. Phosphating is an important step in the pre-treatment of parts that have to be powder coated. Phosphate coatings are micro-porous in nature. They make an excellent undercoat for the powder coating or wet paint that is to follow. The phosphating solution is acidic being based on phosphoric acid. There are different types of phosphate coatings. The most important to powder coaters are ironphosphate and zinc phosphate coatings, but manganese phosphate is also used in certain cases. Apart from a phosphoric acid base, the solutions will contain ingredients known as accelerators or oxidisers that influence the speed and the structure of the deposit and possibly surfactants. Iron Phosphating Phosphating is what it known as a conversion process. In the case of iron-phosphate, the simpler of the processes, the external steel (iron) surface of the job is converted to an iron-phosphate coating. The phosphoric acid etches the surface of the steel (iron) and the iron that is removed from the part in the etching process is released into the solution as iron ions. Unlike zinc phosphate solutions that contain dissolved zinc to start with, the iron phosphate solution gets its iron by dissolving it off the job. The chemistry involved causes the steel (iron) part being coated to develop a positive charge in the solution. The iron ions in the bath that were etched off the work-piece undergo a reaction in the phosphoric solution to become iron phosphate ions which happen to be negatively charged. During this process, the ph at the surface of the part in contact with the solution (the interface between part and solution) rises. This rise in ph causes the iron phosphate ions in the vicinity of the metal to precipitate onto the metal as an amorphous coating of iron phosphate. The word amorphous refers to the fact that the phosphate deposit lacks an ordered structure; the bits that make it up vary in shape and size throughout its mass. There are several options for applying iron-phosphate coatings. Apart from immersion coating in the process tank there is also the hand-held spray wand option which can produce good results, especially on awkwardly shaped parts, short runs or one-off jobs. Finally there is the multi-stage spray system that is suited to many types of work and is one of the most widely used methods of moving parts quickly and efficiently through the production line. Depending on the requirement, these machines may have as few as two stages or as many as eight. The more stages that are included the more thorough the process and the better the quality that one can expect to achieve. Iron phosphate traditionally works best in the region of ph 5.0 (SAMSA, 2013). Zinc Phosphating Zinc phosphate coatings are superior to iron phosphate coatings and they are generally specified where superior corrosion resistance is necessary and in outdoor environments. Zinc phosphate solutions cannot double as degreasers and cleaners as can iron-phosphate baths. And where the iron phosphate gets its iron off of the work piece, the zinc phosphate bath has to contain dissolved zinc. The solution therefore contains the following: Phosphoric acid Accelerators 23

32 Zinc salts Fluorides (in some instances) Where iron phosphate deposits are amorphous in nature, the zinc phosphate deposit is crystalline. The size of the crystals is controlled by the addition of chemicals such as slightly alkaline suspensions of titanium salts that are sometimes included as part of the formula in the cleaning tank or alternatively the conditioning chemical salts may be included in a dedicated rinse station directly prior to zinc phosphating. The process results in the formation of iron sludge and a collection and disposal system for the sludge has to be included in the design of this processing station. Zinc phosphate solutions need more careful management than iron phosphate solutions. Ideally, the maintenance additions of accelerator and zinc phosphate should be made automatically using chemical dosing pumps. Today it is possible to source pre-treatment processes free of heavy metals that generate little or no sludge. The performance is claimed to be similar to iron-phosphate and almost as good as zincphosphate. Formulated to have less environmental impact this category of product has very low phosphorous content. The absence of heavy metals ensures simplified waste treatment. Another variety of product is a trivalent chrome based conversion treatment that can be used on iron and steel as well as aluminium and its alloys (CPMFI, 2003) Chromium conversion coatings Chromium conversion coatings are used to enhance corrosion protection on various metal surfaces, including electroplated zinc and cadmium, zinc die-castings, tin, aluminium, magnesium and magnesium alloys, copper, brass and bronze, nickel, silver and stainless steel. Chromium conversion coatings are often referred to as chromating because the process originally used only hexavalent chromium. It is an essential step in post treatment of zinc plating. Phospho-chromating exist with both hexavalent chromium and trivalent chromium versions and is used in the treatment of aluminium prior to painting. Typical composition of solutions is chromic acid, dichromate, chloride, fluorides, sulphates, borates, nitrates and acetates. These are used in different combinations and concentrations to produce different colours and layer characteristics. Conversion of hexavalent chromium to trivalent chromium in wastewater treatment is necessary. Hexavalent chromium can be replaced with trivalent chromium for a more environmentally acceptable alternative. No specific health and safety consideration exists for trivalent chromium and the reduction of Cr(VI) to Cr(III) will no longer be required in wastewater treatment. That said, Cr(III) processes usually contain ten times the amount of chromium when compared with Cr(VI) processes in chromium conversion coatings. Trivalent chromium conversion coatings processes can produce the appearance traditionally associated with hexavalent processes over acid, cyanide or alkaline non-cyanide zinc plating systems. They also usually produce a clear or blue coating, but there are formulations that produce a yellow iridescent colouration not as deep as associated with traditional Chrome(VI) based films. Today s formulations provide even better corrosion protection than the old hexavalent formulations, especially when used with seals and topcoats (CPMFI, 2003) Metal colouring Metal colouring is used to obtain a wide range of shades and colours over different metals by heat treatment, chemical immersion or electrolytic treatment. Metal colouring processes are used on brass, copper and steel parts. The most commonly used system is chemical immersion. Originally, sulphide and polysulphide solutions were used in alkaline media containing sodium, ammonium or barium salts while being operated at room- to high temperatures. It is currently more usual to use solutions containing metallic ions (e.g. copper, selenium, molybdenum) in an acid medium and at room temperature. 24

33 Depending on the precise formulations used, effluents may need to be pre-treated separately prior to typical wastewater treatment Chemical blacking oxide coatings Immersion-type chemical oxidation coatings are used mainly for changing the appearance of the metal, as a paint base or for their oil-retention characteristics. Steel, stainless steel, copper, brass and aluminium may be blackened. The original formulations for steel were based on a mixture of sodium hydroxide and sodium nitrate/chlorite ranging from 120 g/l to 840 g/l in concentration and operated at high temperature. Rinse water from this process is likely to require ph adjustment. Sophisticated proprietary cold blacking products are now available for various metals (CPMFI, 2003) Brightening In the brightening of steel, concentrated nitric acid is used to form a very clean surface. Copper and brass are brightened by oxidising a surface layer. Aluminium and some of its alloys can be brightened by chemical or electrochemical processes. Spent brightening solutions may require treatment for low ph and possibly metal content prior to disposal as effluent Etching alkaline etching of aluminium The most frequently used method for etching aluminium is aqueous solutions of caustic soda, which may or may not contain additives. It can be used for general cleaning purposes, to produce a stain or matt finish for decorative purposes, for deep engraving or chemical milling. Wastewater contains caustic soda which may be treated in typical wastewater treatment plants Chemical milling Chemical milling is a process used to remove metal on workpieces by dissolution in a caustic or acid bath without an external source of energy. Chemical milling is mostly used on aluminium alloys, but chemical milling can also be used on titanium alloys, stainless steel and some special alloys with a nickel, cobalt or magnesium base. The chemicals used in chemical milling for aluminium, titanium and stainless are listed in Table 7 and the main control parameters for aluminium are displayed in Table 8. All these parameters affect the speed of processing, the thickness regularity and roughness of the finish. Table 7 Chemicals used in chemical milling of metals Metal used in chemical milling Aluminium Titanium Chemicals used in milling Caustic soda with additives sodium, gluconate and sodium sulphide Hydrofluoric acid (20 g/l to 50 g/l) and nitric acid (50 g/l to 70 g/l) Stainless steel Compiled from CPMFI, 2003 and SAMFA, 2013 Hydrochloric acid with nitric and phosphoric acid 25

34 Table 8 Control parameters for chemical milling of aluminium and alloys Main control parameters in chemical milling for aluminium and aluminium alloys Concentration of the bath 100 g/l to 150 g/l sodium hydroxide Temperature 80 o C for older processes, up to 110 o C for high speed processes Concentration of dissolved metals 70 g/l to 90 g/l aluminium Composition of the alloy to be chemically milled Compiled from CPMFI, 2003 and SAMFA, 2013 The main problem experienced for the treatment of effluents in chemical milling is the large quantity of sludge that is produced due to the dissolved metals. Aluminium concentrations below 70 g/l are usually maintained in the milling bath. Furthermore, normal wastewater treatment can be used. 3.4 POST TREATMENT ACTIVITIES Post treatment activities include the drying of workpieces or articles. This can be done by using hot water, hot air or air knives. Furthermore, heat treatment can be carried out on pieces to avoid the hydrogen embrittlement formed in pickling, cathodic cleaning, and the electro-deposition of metal. 3.5 ANODISING OF ALUMINIUM The anodising of metals is an electrolytic surface oxidation process which enhances the natural aptitude for the metal to oxidise. Aluminium is the most important material that is anodised, with alumina (Al 2 O 3 ) formed at the surface. In the majority (90%) of cases of aluminium anodising, sulphuric acid (H 2 SO 4 ) is used as electrolyte. Other compounds include phosphoric acid, sulphuric/oxalic acids, sulphuric/salicylic acids and chromic acids. Magnesium, titanium, tantalum and niobium are also anodised in smaller quantities (AFSA, 2013). The workpiece or article to be treated is anodic. During the anodising process, the negatively charged anion migrates to the anode where it is discharged with a loss of one or more electrons. The metal reacts with the oxygen of the anion and a layer of oxide forms on the surface. In sulphuric acid anodising of aluminium, a colourless, transparent aluminium oxide is formed on most aluminium alloys, however alloys containing high quantities of elements such as iron, manganese and silicon, tend to display layers in colours of grey or brown. The anodising voltage is in the range of V, while the temperature of the electrolyte is generally 20 o C ± 5 o C. The sulphuric based electrolyte generally contains 190 g/l ± 40 g/l sulphuric acid (H 2 SO 4 ). Chromic acid anodising also forms an aluminium oxide film on most aluminium alloys ranging from a light to dark grey colour. This process is mainly used for aerospace and military applications. The voltage cycle depends on the alloy treated and must be closely followed to avoid etching, while the temperature is kept within the range of o C. The electrolyte solution contains 30 g/l to 100 g/l chromic acid (AFSA, 2013). Anodising of magnesium gives the best protection for abrasive wear. The coatings are relatively porous and contain crystalline phases like magnesium di-hydroxide (Mg(OH) 2 ) and magnesium oxide (MgO). Phosphoric acid anodising produces a porous, hydration-resistant oxide and increases fracture toughness. The anodic coatings resulting from phosphoric acid anodising have a high porosity compared to those produced with other processes. This type of anodising is used mainly in the aerospace sector as a pre-treatment for adhesive bond priming. The anodic film produced by this method is typically between 0.1µm to 5.0µm thick and appears as an opaque iridescent film on the aluminium surface. It is therefore not suited as a decorative finish. The anodic film is usually left in an 26

35 unsealed state prior to adhesive bond priming which is typically required to be carried out within 72 hours after anodising ( 2013). Effluent may be treated in typical wastewater treatment plants. The same cleaner production initiatives apply for the chemicals used in anodising which are used in other electroplating techniques; however, closed loop rinsing with ion exchange cannot be implemented. This is because the chemicals removed are of similar environmental impact and quantity to the chemicals required for regeneration Sealing following anodising Sulphuric acid anodising is normally followed by a sealing process. Sealing improves the corrosion and stain resistance of the oxide layers. It also prevents organic dyes from leaching out and improves the light fastness. Sealing may be carried out in hot or cold processes. With hot sealing, the process is carried out by immersing the anodised parts in hot or boiling deionised water (95 to 96 o C) for three minutes per µm thickness. Sealing with steam achieves the same effect. Cold sealing methods have been developed for operating temperatures of 25 to 35 o C, and 60 o C. No constituents of concern are present in wastewater effluent and this water may be recycled and used in sealing (AFSA, 2013). 3.6 POWDER COATING Powder coating is an advanced method of applying a decorative and protective finish to mild steel, aluminium, and other metals, creating a wide range of products that are used by both industries and consumers. The process results in a uniform, high quality and attractive finish. The powder coating industry continues as the fastest growing finishing technology owing to cost and environmental requirements. Powder costs less to apply than liquid paint that provides equivalent performance. Negligible amounts of volatile organic compounds (VOC's) are released into the atmosphere. In addition, unused or over-sprayed powder can be recovered and reused so any waste is minimal and can be disposed of easily and safely. Phosphating is an important step in the pre-treatment of parts that have to be powder coated. Phosphate coatings are micro-porous in nature. They make an excellent undercoat for the powder coating that is to follow. The phosphating solution is acidic being based on phosphoric acid. There are different types of phosphate coatings that we will consider. The most important to powder coaters are iron-phosphate and zinc phosphate coatings, but manganese phosphate is also used in certain cases Examples of Pre-Processing on various substrates prior to powder coating Seven stage phosphating process for mild steel 1) Alkaline Degreaser (Hot Soak) 60 C 2) Rinse 3) Acid Pickle 4) Rinse 5) Iron Phosphate 6) Rinse 7) Passivation (Preferably chrome free) 27

36 Environmentally friendly alternatives to conventional phosphating Today it is possible to source pre-treatment processes free of heavy metals that generate little or no sludge. The performance is claimed to be similar to iron-phosphate and almost as good as zincphosphate. Formulated to have less environmental impact this category of product has very low phosphorous content. The absence of heavy metals ensures simplified waste treatment. Another variety of product is a trivalent chrome based conversion treatment that can be used on iron and steel as well as aluminium and its alloys. Aluminium pre-treament conventional 1) Alkaline Degreaser (Hot Soak) 2) Caustic Etch 3) Rinse 4) Nitric Acid de-smut 5) Rinse 6) Chromate Conversion 7) Rinse 8) De-ionised Rinse 9) Drying Station Eco-friendly options for aluminium pre-treatment The Chrome 6 based conversion process listed in the conventional aluminium pre-treatment sequence above has been around for many decades. However, the hazards to human health and to the environment have been well documented. Experimentation with Chrome 6 free alternatives started many years ago and in the 1990's the forerunners of today s modern generation products had already made their appearance in the market place. With almost two decades of operating experience notched up and hundreds of users worldwide, the performance of these products has been proven to be equivalent to the traditional chrome 6 based products. Users of chrome-free technology include the world s biggest names in the aluminium finishing industry. Hot dip galvanized metal The following advice is provided on the website Powder Coating Online*: Hot dip galvanize and do not water or chromate quench. Remove all drainage spikes and surface defects. Powder coat within 12 hours of galvanizing. Do not get surfaces wet. Do not leave outside. Keep the surface clean. Do not transport uncovered loads. Diesel fumes will contaminate surface. If surface contamination has occurred or is suspected, clean surface with proprietary solvent / detergent designed for pre-cleaning prior to powder coating. Use zinc phosphate pre-treatment if highest adhesion is required. Surface must be perfectly clean. Zinc phosphate has no detergent action and will not remove oil or soil. 28

37 Use iron phosphate if standard performance is required. Iron phosphate has a slight detergent action and will remove small amounts of surface contamination. Best used for pregalvanized products. Pre-heat work prior to powder application. Use degassing' grade polyester powder only. Check for correct curing by solvent testing. Adjust pre-heat and line speed to ensure full cure. * In some high-end applications, the part is electrocoated following the pre-treatment process and prior to the powder coating application. This has been particularly useful in automotive and other applications requiring performance characteristics. Another method of preparing the surface prior to powder coating is known as sandblasting and shot blasting. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing for products made of wood, plastic, or glass. The most important properties to consider are chemical composition and density; particle shape and size; and impact resistance. Silicon carbide grit blast medium is brittle, sharp, and suitable for grinding metals and lowtensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives that are sensitive to articles such as aluminum, but still suitable for de-coating and surface finishing. Sand blast medium uses high-purity crystals that have low-metal content. Glass bead blast medium contains glass beads of various sizes. Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. This method of preparation is highly efficient on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces. The wastewaters from a powder coater need to be treated in a typical wastewater treatment plant before discharging to the MWTP. Pre-treatment may include separation of the liquid wastes to remove solvents and settling or precipitation of solid materials. The typical contaminants to the wastewater stream are generally, oil and grease, metals (zinc from the article, or the zinc phosphate bath), and ph are the concerns from an environmental reporting standpoint. However, sludge or contaminant buildup in the cleaner and phosphate stages may force regular cleanout of these stages. The waste treatment process is usually a simple ph adjustment and flocculation (AFSA, 2013). 29

38 4. WASTES GENERATED DURING THE METAL FINISHING PROCESS AND TREATMENT Constituents of concern which are potentially released during metal finishing processes include metals, organic material, cyanides, hypochlorite, chlorine, absorbable organohalogens (AOX), surfactants, complexing agents, acids/alkalis, ions, solvents, dusts and general wastes. The emission of pollutants in wastewaters and the production of waste are considered more significant than emissions to air for this study. Since most surface treatments by metal deposition are water-based, with significant quantities of wastewater is generated. The most effective method for preventing pollutants entering the water environment is in the minimisation of the loss of materials, where these materials are lost by drag-out into rinse waters. Furthermore, wastewater treatment serves to remove the constituents of concern before discharge into the surrounding water bodies. These treatments include filtration, absorption techniques, crystallisation, atmospheric evaporation, vacuum evaporation, electrolysis, ion exchange, electrode-ionisation, acid sorption, membrane filtration, reverse osmosis, diffusion dialysis, membrane electrolysis and electro-dialysis. These techniques are listed in Table 9. Table 9 Typical treatment regimens for effluent treatment Treatment type Filtration Absorption techniques Crystallisation Atmospheric evaporation Vacuum evaporation Electrolysis plating out Description of treatment Sand filters are used for cleaning raw water or polishing effluents Belt filters or filter presses are used with higher solids applications such as wastewater sludges in conjunction with coagulants The filter medium with the filtrate is usually disposed of as a waste Activated carbon is used to adsorb unwanted organic substances formed from breakdown products in a solution Activated carbon will also remove a portion of the useful organic chemical additives (e.g. brighteners), which will need replacing The absorbent material along with the retentate and filter medium is usually disposed of as a waste, although precious metals may be recovered Various evaporation and cooling systems are used to bring solutions to a super-saturation point where solid crystals form and can be separated from solution Atmospheric evaporation occurs when solution are heated. It reduces the volume of process solutions and allows drag-out to be returned of fresh chemicals to be added to the process solution Evaporators may be used with a condenser to recover distilled water Reduced pressure and elevated temperature combine to separate constituents with relatively high volatility from constituents with lower volatility Transition metals can be removed from wastewater streams by plating out on high surface area electrodes in metal recovery cells 30

39 Treatment type Electrolysis oxidation Ion exchange resin Electro-deionisation Acid (resin) sorption Ion exchange liquid/liquid Membrane filtration Description of treatment It is possible to oxidise both unwanted organic by-products and metals in solutions, such as trivalent chromium to hexavalent chromium Ions in solution are selectively removed by exchanging positions with resin-functional groups. The direct ion exchange treatment of wastewater provides a means of concentrating multivalent cations for subsequent treatment on column regeneration or by plating out. Water from ion exchange can be recycled. Ions are removed using conventional ion exchange resins Acid sorption is configured similarly to ion exchange. Resins are designed to selectively adsorb mineral acids while excluding metal salts (adsorption phase) Purified acid is recovered for re-use when the resin is regenerated with water (desorption phase) Ionic contaminants are removed from process solutions into immiscible primary liquid extraction solutions. Secondary liquid extraction solutions are sued to remove the contaminants and to regenerate the primary extraction solution. Water from ion exchange can be recycled. Membrane filtration can be used for the purification and recirculation of oily water Various types of membrane filtration exist that are dependent on the pore size Microfiltration is a membrane filtration technology that uses low pressures with pore sizes in the range of 0.02 to 10 microns, to separate relatively large particles in the macromolecular and micro particle size range Ultrafiltration passes ions and rejects macromolecules of to 0.1 microns and removes organics from process solutions Nanofiltration is used for larger size rejection reverse osmosis (rejects molecules larger than to μm) for partial desalination of rinse water, removal of aluminium from pickling baths and concentration of chromating chemical in rinse water Purified water can be re-used for the degreasing bath or rinsing 31

40 Treatment type Reverse osmosis Diffusion dialysis Membrane electrolysis Electro-dialysis Description of treatment Reverse osmosis is effectively a filtration of ions through a semi-permeable membrane at high pressure which desalinates chemically treated wastewater It provides an alternative means of concentrating metal impurities for subsequent removal This approach can be capital intensive and any solids or organics have to be removed prior to treatment The chemicals from zinc, chrome, copper and nickel plating baths can be re-circulated Diffusion dialysis is a membrane separation process that typically uses an anionic exchange membrane to transport acid anions and protons from waste acid solutions into deionised water streams The anions and protons are treated in wastewater treatments plants and the acid is recovered Membrane electrolysis used one or more ion-selective membranes to separate electrolyte solutions within an electrolytic cell The membranes are ion-permeable and selective Anions and cations are removed from solutions with an applied electric field in cells with alternation anion- and cationpermeable membranes Various acids from electrolyte solutions can be recycled Compiled from CPMFI, 2003 and SAMFA, 2013 The possible treatment(s), specific to each constituent of concern is listed in Table 10. Table 10 Possible treatment(s) specific to each constituent of concern Constituent of concern Immiscible organics: Non-halogenated (oils, greases, solvents) and halogenated (oils, degreasing solvents, paint solvents) Soluble organics: Wetting agents, brighteners, organic ions and ligands Acids and alkalis Particulate material: Metal hydroxides, carbonates, powders, dusts, film residues, Possible treatment Reduced to solubility limit by physical separation (e.g. flotation) or by liquid/liquid phase separation Followed by either air stripping (activated carbon) or oxidation to carbon dioxide (using UV irradiation and hydrogen peroxide addition) Soluble organics increase the difficulty in removing metals by flocculation Concentration may be reduced by oxidation (by UV irradiation and hydrogen peroxide addition Dissolved organics increase COD; biological treatment may be necessary ph adjustment is usually required ph may be partially neutralised by mixing with other streams May be removed by settling or filtration Filtration uses a filter or belt press to produce a cake manageable as a solid 32

41 Constituent of concern metallic particles Metals Complexing agents: Sequestering and chelating agents Ammonia, Nitrites Cyanides Sulphide Fluorides Phosphate compounds Other salts Compiled from CPMFI, 2003 and SAMFA, 2013 Possible treatment For soluble anions the capture of precious metals for re-use, e.g. platinum, gold, silver, rhodium and ruthenium may be achieved by electrochemical recovery or ion exchange In some cases it may be necessary to reduce the oxidation state of the metal ion as the higher oxidation state may not be readily flocculated and precipitated by ph change Multivalent ions are most conveniently removed by precipitation and ph adjustment May be removed by precipitation with the predecessor of activated carbon process Microbiological oxidation is also a possibility for removal of complexing agents Ammonia can be removed by steam stripping or by oxidation to nitrogen with sodium hypochlorite Nitrites can also be oxidised with sodium hypochlorite Note that AOX may be formed when using hypochlorite solutions Cyanide from degreasing may be oxidised by using hypochlorite or chlorine gas at high ph When in excess sulphide can be precipitated out as elemental sulphur on oxidation with hydrogen peroxide or iron (III) salts. Is readily precipitated out as calcium fluoride at a ph above 7 Precipitated out as calcium hydroxide phosphate Other ions such as Cl -, SO 2-4, K +, Na + and Ca + Sulphate can be readily precipitated as calcium sulphate Ion exchange, reverse osmosis or evaporation to remove other ions Sludge waste is produced during metal finishing processes, which are the solid particles suspended in the water effluent. These solids may be removed from the main effluent (called sludge dewatering) by precipitation or filtering through a filter press, belt press or centrifuge to produce a cake manageable as a solid. When operating at pressures above 15 bar, the final cake may have 15 to 35% solids. The filter can then be dried further to lower the water content. Some waste solutions can be disposed of as liquid or hazardous wastes, or may be recovered or recycled. Examples of wastes that are recovered or recycled include autocatalytic plating, spent etchants and the sludge from anodising. Further steps to abate potential releases to the environment include cleaner production initiatives, which aim to minimise the use of water and material discharged from the processes. It is important to note, however, that the minimisation of water usage can increase the concentration of dissolved salt and various metals, which increases the solubility of metals in the wastewater. Further, 33

42 it can become difficult to maintain a stable ph in the narrow margins required to minimise the solubility of individual metals when dealing with a mixture. This suggests that when dealing with a mixture of metals it will become increasingly difficult to optimise every parameter. 4.1 FURTHER CLEANER PRODUCTION INITIATIVES FOR THE INDUSTRY Management and housekeeping The most practical way and cost effective method of improving plant operation to reduce the wastewater generated is by improving management and housekeeping. The best environmental performance is usually achieved by the installation of the best technology and its operation in the most effective and efficient manner. An Environmental Management System (EMS) is a tool that operators can use to address design, construction, maintenance, operation and decommissioning issues (Lochner, 2005). Environmental management systems typically ensure the continuous improvement of the environmental performance of the installation. Components of the EMS shown in Table 11 should typically include: A definition of the environmental policy Planning and establishing objectives and targets Implementation and operation of procedures Checking and corrective action Management review Preparation of a regular environmental statement Validation by certification body or external EMS verifier Design considerations for end-of-life plant decommissioning Development of cleaner technologies and Benchmarking Furthermore, if workpieces or articles are treated incorrectly, in terms of incorrect specification or application of the correct specification, significant amounts metal stripping or scrapping of the workpiece/article is to be expected. Implementation of a Quality Management System (QMS) can reduce the reworking and scrap. Avoiding rework minimises losses in raw material, energy and water inputs, as well as minimising wastewater treatment and the generation of sludge and liquid acid wastes. Table 11 Typical components of an Environmental Management Plan (EMS) Components of EMS Definition of environmental policy Planning Description Top management are responsible for defining an environmental policy that includes a commitment pollution prevention and control and to comply with all relevant applicable environmental legislation The policy must provide a framework for setting and reviewing environmental objectives and targets, must be documented and available to all interested parties Procedures to identify the environmental aspects of the installation and the legal and other requirements to which the organisation subscribes Establishing and reviewing documented environmental objectives and targets, and establishing and regularly updating an environmental management programme 34

43 Components of EMS Implementation and operation of procedures Checking and corrective action Management review Preparation of a regular environmental statement Validation by certification body or external EMS verifier Design considerations for end-of-life plant decommissioning Development of cleaner technologies Benchmarking Compiled from Lochner, 2005 Description Effective environmental management must ensure that procedures are known in areas of Structure and responsibility Training, awareness and competence Communication Employee involvement Documentation Efficient process control Maintenance programmes and Emergency preparedness and response Establishing and maintaining documented procedures to monitor and measure key characteristics of operation and activities Establishing and maintaining procedures for defining responsibility and authority for handling and investigating non-conformance with permit conditions Establishing records, EMS audits and periodic evaluation of legal compliance Reviewing the EMS at regular intervals to ensure its suitability, adequacy and effectiveness Ensure that necessary information is collected to carry out evaluation and documentation of the review This statement compares the results achieved by the installation to the environmental objectives and targets Having the EMS validated by an accredited certification body or external EMS verifier to enhance the credibility of the system Giving consideration the environmental impact from the eventual decommissioning of a unit Giving consideration to the development of cleaner technologies that incorporate techniques at the earliest possible design stage to be more effective and cheaper Carrying out systematic and regular comparisons with the sector on a national or regional scale Other management techniques include a reduction in reworking by process specification and quality control which minimises wastewater treatment and the generation of sludge and liquid acid wastes. A theoretical optimisation of the process line improves the consumption of water, energy and conservation of raw materials, as well as minimising emissions to water. Real time process control systems that collect data and react to maintain predetermined process values improve plant efficiency and product quality as well as lowering the emissions. Recording all water inputs enables corrective action to be taken when optimum water usage levels are exceeded as well as allowing for quick troubleshooting the process. 35

44 5. THE HOT DIP GALVANIZING PROCESS Hot dip galvanized (HDG) coatings are produced by a metallurgical reaction between iron and the molten zinc. A series of hard abrasion resistant iron/zinc alloys are formed and these are over-coated with relatively pure zinc as the product is withdrawn from the galvanizing kettle. The various layers all play a significant role in the provision of corrosion protection. For the protective coating of zinc to be formed, the steel surface is required to be free from all contaminants such as mill scale, rust, grease and oil. The hot dip galvanizing process entails immersion into a series of cleaning and pre-treatment chemicals prior to immersion into the molten zinc. The principle of hot dip galvanizing is shown in the flow charts in Figure 2. Hot dip galvanizing by the dry method is most commonly used in South Africa and the most common pollutants are shown in Table 12. Figure 2 A typical hot dip galvanizing process (Zalcon, 2000) Table 12 Pollutants and their sources during hot dip galvanizing Common pollutants Polluting sources 1. Alkalis, NaOH, Ca(OH) 2 Degreasing, neutralisation at WWTP 2. Acids, H 2 SO 4, HCl, ZnCl 2 /NH 4 Cl Acid pickling, flux 3. Organics, oil, grease solvents, complexing Degreasing agents 4. Heavy metal containing waste, Fe, Zn Acid pickling, rinsing water, flux, WWTP 5. Chromium (VI) waste Passivation 6. ZnCl 2, ZnO, NH 4 Cl, NH 3, HCl, H 2 SO 4, oil Hot dip galvanizing, acid pickling, flux 7. Suspended solids, ph, heavy metals, sludge Wastewater treatment plant The main processes in hot dip galvanizing include degreasing, rinse(s), pickling, rinse(s), flux, hot dip galvanizing, zinc kettle and quenching. These generic processes are described in the following sections. 36

45 5.1 DEGREASING An alkaline degreasing treatment is necessary to remove oil and grease residues, such as cutting oils from earlier manufacturing processes. This is effectively carried out by means of an emulsifying heated caustic soda solution (50-70 o C) followed by water rinsing to ensure no carryover into acid. Some plants make use of acid based degreasers either added to the acid pickling solution or in a separate bath. This method is normally less effective in the removal of heavy oil and grease as well as other contaminants such as paint. A further disadvantage is that contaminants are more likely to be carried over into the acid pickling solution and even into the subsequent flux solution, thus detrimentally affecting the effective action of these chemicals. Floating oil is particularly detrimental. Solvent degreasing is not recommended as it redistributes the contaminant as a thin continuous grease film on the product. This is environmentally damaging, and demands strict safety equipment and procedures to maintain operator safety. The main environmental, health and safety issues are: Chemical exposure. Alkaline solution with silicates, phosphates, surfactants and additives. High consumption of degreaser due to poor maintenance. Heat loss due to poor insulation of tank. Poor degreasing will influence the pickling and cause the need for longer pickling time resulting in over consumption of acid and generation of more spent acid. Poor degreasing causes more reworks. Loss of degreaser through drag out. Disposal of sludge contaminated with heavy metals, oils and grease. 5.2 THE ACID PICKLING STAGE Hydrochloric acid (HCl) and sulphuric acid (H 2 SO 4 ) solutions are almost exclusively used. Sometimes, if castings are being processed, hydrofluoric acid (HF) solutions may be used, but this is particularly hazardous and expert advice should be sought. The aim of the pickling process is to remove the surface scale without attacking the underlying steel surface. This demands the use of inhibitors in the pickling acid. This reduces the rate at which the iron content builds up in the pickling acid bath. An inhibitor contributes to the reduction of zinc pick up and gives a better surface finish. The most commonly used pickling acid is hydrochloric acid (HCl). Its main advantage is that heating is normally not required other than in extremely cold climates. The most efficient cleaning is achieved at a solution temperature of about 25 o C. This is easily achieved by exothermic characteristics of the pickling reaction between HCl and steel. The disadvantage of using HCl is that the transport costs over long distances are high. HCl (hydrogen chloride) is a gas dissolved in water at a maximum concentration of between about 33% depending on altitude above sea level. It follows that some 67% of the solution for which transport costs are paid is water. HCl tends to remove surface scale and rust without the steel article being excessively attacked, particularly if an inhibitor is used. H 2 SO 4 tends to remove similar surface deposits by reacting with the underlying steel article. Sulphuric acid has the advantage that it can be transported in concentrated form (98%) while it is used at a concentration of 10%. Energy cost are however, required (heating to a temperature of o C). The use of waste heat from the galvanizing kettle can be used but this is normally inadequate and additional heat is required. Galvanizing kettle waste heat is better utilised for drying purposes after the fluxing process. In South Africa, the initial cost of H 2 SO 4 is substantially lower than the cost of HCl but this is largely offset by the energy costs applicable when H 2 SO 4 is used (Zalcon, 2000). 37

46 H 2 SO 4 is not compatible with the flux. Any H 2 SO 4 carry over into the flux bath causes its deterioration. When using HCl, any free acid carry over into the flux bath can be of benefit in producing either zinc chloride or ammonium chloride. Outside the main process there is a stripping acid. Normally it is partly spent pickling acid, which is used to dissolve zinc from suspension devices, tools and faulty products (rejects). The main environmental, health and safety issues are: Chemical exposure. HCl fumes from the pickling bath. Corrosive nature of the fumes. High consumption of acid due to poor maintenance. Poor pickling may cause reworks. Loss of acid through drag out. Disposal of sludge and spent acid. 5.3 RINSING STAGES Water rinsing is used after both the degreaser and pickling baths to stop the chemical reaction and to reduce drag-in of chemicals and impurities into the following process bath. Separate tanks should be used for degreaser and acid rinses. Usually the water is from municipal supply although some galvanizers use borehole water. Rinsing can take place in static rinse tanks where rinse water is partly used for topping up the process bath. In some galvanizing plants, a running rinse is used. Where an efficient wastewater treatment plant is in place, recycled water can be used for rinsing purposes. The main environmental, health and safety issues are: Use of excessive water for rinsing. Treatment of the effluent from the rinsing process. Disposal of wastewater. Disposal of sludge from the wastewater treatment plant. 5.4 THE FLUX PROCESS The application of a flux film prior to immersion in the molten zinc is extremely important. The action of the flux is to remove remaining impurities on the product surface and to maintain cleanliness until the product is dipped into the galvanizing kettle. The flux solution consists of ammonium chloride and zinc chloride. Good control of flux quality provides good quality coatings and reduces cost in a number of ways. Ideally, flux should be heated to a temperature of about 70 o C. This assists in the drying of the flux film thus preventing flash rust of steel surfaces after pickling and prior to hot dip galvanizing. The addition of aluminium to the flux provides several benefits. It reduces the zinc surface tension upon withdrawal of work from the zinc thus ensuring a bright and smooth finish. It also reduces surface ash (ZnO) formation due to a compact aluminium oxide (Al 2 O 3 ) film that forms on the zinc surface. A further benefit is that, even at this low concentration, aluminium reduces the metallurgical reaction between zinc and iron, thus diminishing the incidence of excessively thick coatings associated with the galvanizing of reactive steels. If the flux is weak, the optimum quantity of aluminium cannot be added to the zinc since this will result in discontinuities in the coating. At the same time, similar defects will occur if any excessive quantity of aluminium is added even if the flux density is at the recommended level. Acid carry over can be converted into either ammonium chloride (ammonium hydroxide additions) or zinc chloride (zinc oxide additions). 38

47 The main environmental, health and safety issues are: Chemical exposure. High consumption of flux due to poor maintenance. Lack of purification of the flux to reduce the accumulation of iron. Dissolved iron in flux generates dross in the galvanizing kettle and results in the loss of zinc. Heat loss due to poor insulation of flux bath. Losses of flux solution due to high drag out. 5.5 DRYING Apart from the drying effect achieved by heating the flux, many galvanizers also make use of a drying oven or other drying facilities in order to dry out the steel surface and flux film completely. This reduces to a minimum the possibility of moisture being dragged into the galvanizing kettle, resulting in zinc splashings, which can cause burns to personnel. The main environmental, health and safety issues are: Heat exposure. Flux decomposition by high drying temperatures. High energy consumption due to poor design of drying facilities. 5.6 THE HOT DIP GALVANIZING PROCESS Adequate degreasing, pickling and fluxing allows the molten zinc to react chemically with the steel surface of an immersed component, producing Zn/Fe layers of varying composition and thickness at the interface. The quality of the coating for a particular composition of steel relies on: the quality of the pre-treatment process; the quality of the zinc; the temperature of the galvanizing kettle; the amount of aluminium in the galvanizing kettle time of immersion; rate of withdrawal Dross is a waste product largely made up of Fe/Zn alloy, which forms during the hot dip galvanizing process. Dross consists of approximately 96% zinc and 4% iron. It precipitates to the galvanizing kettle floor from where it must be removed (drossing) regularly. Excess dross remaining in the kettle may cause severe damage to the walls due to the development of local hot spots, which can result in perforation of the 50-mm thick galvanizing kettle plate. A 50-mm lead layer is normally used to cover the bottom of the galvanizing kettle to facilitate the removal of dross from the bottom and to protect the kettle floor from attack by molten zinc. Excess dross accumulation will also reduce heating efficiency and kettle capacity. While dross, which is stirred up during galvanizing adversely affects coating quality. Ash is produced by the disturbance of the surface of the zinc in contact with air, which entraps zinc in the oxidised zinc film. Ash contains about 80% zinc of the total weight. Incorrect removal of ash can lead to significant amounts of lost zinc. The ash on the surface can be skimmed using a wooden blade and removed using a perforated ladle. The ash and dross should be sold for reprocessing. In South Africa they are mainly sold to ZinChem and MR Zinc The main environmental, health and safety issues are: Fumes from the galvanizing kettle. The air emissions from the kettle are approximately 95% particles and 5% gas. The particles contain NH 4 Cl (50-75%), ZnO (5-15%), ZnCl 2 (3-5%), Zn (0-5%), and C (0-3%). The gases contain traces of NH 3, H 2 O, HCl and oil High temperature exposure. 39

48 Loss of zinc through ash and dross. High consumption of zinc due to thicker coatings. Surface oxidation of the molten zinc. High consumption of aluminium. Risk of accidents and burns, e.g. zinc splashings. High energy consumption of the process. Heat loss due to poor insulation. Rejects due to poor pre-treatment process 5.7 QUENCHING AND THE PASSIVATION STAGE After withdrawal from molten zinc, galvanized products are quenched in water to solidify the coating and cool down the product. Air-cooling is used if there is a possibility that thermal shock will cause a structure to warp. A passivating chemical (usually a dichromate solution) is normally included in the quench water to prevent the formation of white rust (wet storage staining) on freshly galvanized surfaces. Freshly exposed zinc is reactive and in the presence of moisture, unstable zinc oxide and zinc hydroxide (white rust) form to a lesser or greater degree. With time, a stable surface film of basic zinc carbonate is naturally formed. It is this protective film that provides long-term durability to the coating. Passivation is provided in order to provide temporal protection until such time that the durable zinc carbonate film has naturally developed. This is demonstrated by the dull grey appearance that the coating assumes with time. The formation of white rust on freshly galvanized surfaces is more likely to occur in South Africa than in Europe where passivation is frequently considered unnecessary (Zalcor, 2000). The problem in South Africa is largely due to the extremes in maximum and minimum temperatures experienced during a 24-hour period, which results in condensation of moisture from the atmosphere when the dew point is reached. The most commonly used passivating chemical is sodium dichromate. Its use is however, discouraged and even forbidden in some industrialised countries. This is because it contains hexavalent chrome, which is environmentally unfriendly. The main environmental, health and safety issues are: Chemical exposure. Use of toxic dichromate solution for passivation. Disposal of sludge. 40

49 6. CHANGES IN THE PLATING INDUSTRY SINCE THE 1980 S AND PROJECTED CHANGES The need for guidelines to reduce water intake and waste-water disposal by industry is of national concern in view of South Africa's water scarcity. To establish norms for water intake and waste-water disposal, the Water Research Commission (WRC) in collaboration with the Department of Water Affairs (DWA) contracted Binnie and Partners, a firm of consulting engineers, in the 1980 s to undertake a National Industrial Water and Waste-water Survey (NATSURV) of all classes of industry. The results obtained in the survey of the metal finishing industry form the basis of the guide on Water and Waste-water Management in the Metal Finishing Industry. Since then many changes have occurred in the industry and it is the purpose of this survey to document the changes since the 1980 s with respect to the pollution to water, cleaner production and novel new international and national processes that limit pollutants to water and the environment and that are inherently safer. The following sections will generically address some of the changes within the industry that have been implemented in various facilities as well as indicating opportunities for cleaner production within the sector. These ensuing sections are taken from SAMF 2010 and Plating lines consist of similar basic components, irrespective of the process being practised. The line will always have a cleaning stage followed by a plating stage and finally the post-plating processes that may be as elementary as final rinses, but could include passivation processes and/or lacquering. Since 1987, many positive changes have been realised in the major plating companies based on the recommendations of the NATSURV 2 study as well as the best practise in the industry. This shift towards cleaner production is not entirely because of economics but rather a combination of good business practices as well as economic issues and ever-growing environmental pressure on the industry. The 1987 report recommends the following practises for the plating industry: Prevention of bath contamination and subsequent dumping of solutions; Prevention of shock loads; and Effluent treatment before discharge to the municipal sewer system. 6.1 A DEFINITION FOR CLEANER PRODUCTION It is generally understood that cleaner production is a preventive, environmental protection initiative. It is also intended to minimise waste and emissions and maximize product output. Improvements of organisation and technology help to reduce or suggest better choices in use of materials and energy, and to avoid general waste, wastewater generation, gaseous emissions, and also waste heat and noise. The concept was developed as a programme of UNEP (United Nations Environmental Programme) and UNIDO (United Nations Industrial Development Organization). The programme was meant to reduce the environmental impact of industry and is loosely based on pollution prevention pays. It has found more international support than all other comparable programmes. Starting from the simple idea to produce with less waste, cleaner production was developed into a concept to increase the resource efficiency of production in general. In the US, the term pollution prevention is more commonly used for cleaner production. Some examples for cleaner production options are: Documentation of consumption and motoring any significant changes as process related (to identify losses from poor process operation, poor education and training, mistakes) Substitution of raw materials and auxiliary materials (especially environmentally friendly, renewable materials and energy) Increase of useful life of materials and process liquids (by avoiding drag in, drag out, contamination, spills, etc.) Improved process control and automation Recycle or reuse of waste (internal or external) 41

50 New, low waste processes and technologies It is clear that cleaner production is not end-of-pipe treatments or effluent treatments for the plating industry but rather a fresh look at the entire process and applying cleaner production techniques. Thus, it is imperative that cleaner production be applied first, then the end-of-pipe effluent treatment regimens if it is required. 6.2 EFFECTIVE WATER USE AND MANAGEMENT Water is a vital component in the electroplating process. It is used to make up nearly every process tank, except for chlorinated and other solvent degreaser tanks. It is needed to rinse off the residue of the preceding chemical step before proceeding to the next step. For plating to be successful, the work pieces have to meet acceptable standards of cleanliness between the various processes in the sequence. Work pieces also have to be absolutely clean before being finally dried off and packed, or else residual chemical from the process will dry on the surfaces and spoil the finish. It may even leach out from holes or narrow gaps where parts fit together. The modern plater is faced with two major dilemmas: Water is becoming a scarce and expensive commodity in a water scarce southern Africa and there is pressure on the industry to reduce its usage of water. Water is needed to rinse off plating chemicals between stages and post-plating to ensure good quality finished products. This rinse water carries the chemicals to sewers connected to municipal sewerage systems where increasingly strict limits are imposed in respect of contaminants and heavy metals. Heavy penalties may be imposed on transgressors. In response to these problems as well as to prevent punitive action, the plater has to: Reduce the amount of fresh water that is metered into the operation. Plate as effectively as possible (financially and to standards) to prevent contamination of tanks and avoid reworking work pieces. Prevent the escape of heavy metals and other contaminants into the effluent. 6.3 DRAGOUT MINIMISATION AND MANAGEMENT Every time a load of work is removed from the plating tank, a film of the plating solution referred to as dragout clings to the work-pieces. This film thickness is affected by the density and temperature of the solution and whether it is acid or alkaline based. Alkaline solutions tend to cling to surfaces more than acids thereby increasing dragout. The first tool assisting in the reduction of water and the recovery of raw materials is the tank known as the dragout tank. At least one and preferably two of these tanks should follow every electroplating process tank. In processes that place an extremely heavy load on the environment, such as chromeplating, three dragout tanks are even better. The dragout tank starts out as a static tank of clean water. Work from the plating tank is immersed into this tank first. In plating nickel for example, when a rack of work is removed from the plating tank and plunged into the dragout tank, most of the solution clinging to the work pieces on the rack starts to become diluted by the water in the dragout tank and to mix with the water in the dragout tank. This dilution and mixing can be accelerated and made more efficient by lifting and lowering the jig in and out of the water and shaking it around. If there was a perforated airline on the base of the dragout tank jetting air bubbles through the water would also accelerate the process of dislodging the clinging film of plating solution from the work pieces and facilitate the dissolution of the chemicals throughout the dragout tank ( June 2013). A large percentage (around 95%) of the chemicals once clinging to the jig would be removed and a small quantity would stay behind. It is 42

51 possible to measure the precise concentration of chemicals present in the dragout tank. At the end of a shift, this would amount to a significant amount of the actual plating solution that had been removed from the bath and transferred to the dragout. By then, the amount removed could be measured in grams per litre, and a value could be attached to it relative to the cost of the make-up cost of the solution. Remember that the dragout contains everything, including often, expensive brighteners and other additives. If this was simply discharged, the facility would have lost revenue as a result of disposing a usable chemical solution as well as discharging measurable amounts of heavy metals and high COD chemicals into the sewer system and would become liable for a fine. If they had placed the dragout solution in a suitable tank, dosed it with appropriate chemicals to precipitate the metals, and sent the clear supernatant to the sewer, they would have been performing the so-called end-of-pipe response commonly referred to as effluent treatment. Theoretically, they would have done an acceptable thing, but they would have added two outcomes that are more undesirable: Spending unrecoverable money on the chemicals to precipitate the chemicals from the plating solution that we had already bought and paid for. creating sludge of heavy metal that now has to be disposed of in a high hazardous landfill at additional cost. The effluent problem was simply shifted to another location at additional expense. A solution which is implemented by various facilities at present (automated and manual operations) is to reduce the amount of plating solution entering the dragout tank by withdrawing the jigs slowly from the solution and allowing an extended drip-off time above the actual plating tank. Waiting for a minimum of 10 seconds and up to 20 seconds if circumstances permit ensures that a large percentage of the plating solution will run back into the plating bath where it belongs, and where it can perform its original intended function. This drip off action can be accelerated by tilting jigs and by careful orientation of parts to facilitate drainage back to the tank. Vibration and bumping of jigs also can assist in dislodging the plating solution. It is important to return the solution from the dragout tank to top up the solution originally lost from the plating tank through dragging out and evaporation as a result of the elevated temperature. The tank is more properly described as a static recovery rinse. The water is static and they are able to return the solution to its source, effectively recovering it without any additional expense. The part to be plated should exit the last acid treatment with a clean and active surface. It may be so active that it oxidises slightly during the water rinsing immediately before plating. To counter this, the part is often put through a pre-dip to remove this oxide, and then goes without rinsing into the plating tank. Some dragout from the pre-dip will be carried over into the plating solution. Therefore, the composition of the pre-dip should be such that any carry-over will not be harmful. In cyanide based plating solutions such as zinc, copper and brass a dilute solution of sodium cyanide is used. This removes light oxide and can be carried over without damage to the plating solution. For acid type plating solutions such as nickel or acid copper dilute sulphuric acid is used and termed a sour rinse. For other plating processes it is usually easy to select a suitable pre-dip. Thus, dragout tanks are used after an actual plating process, but they could even be used to save solutions like alkaline cleaners. A single dragout tank will take an enormous load off the rinses following, but two dragout tanks will have an even more dramatic effect in reducing the load on the running rinses. In this case, the process tank is topped up from the first dragout tank which in turn is replenished by solution from the second less concentrated dragout tank. 43

52 It is only possible to feed as much solution back to the tank as was lost through the original dragout and evaporation. In the case of tanks operating at ambient temperature where evaporative losses are minimal it is more difficult to make use of dragout. This is especially true when operators are taking extra care to allow work to drip off over the tank ensuring that minimum volumes of solution are actually dragged out of the process solution. In addition, water is being introduced into the process tank via pre-dips. Dragout use is most successful when tanks are operating at higher temperatures. It is usually in the operator s interest to operate the tank at the upper limits of recommended temperature to create space for recovered dragout. The costs of energy to maintain the tank at the upper end will usually outweigh the costs of lost raw material and additional treatment and disposal costs. Here of course there is a good argument for having properly insulated tanks to reduce heat losses through the tank walls and save on energy costs. It is possible to concentrate the solution in the dragout tanks into a lesser volume by installing purpose built evaporators. These units are coupled to the process tank and are designed to accelerate evaporation through various mechanisms. An alternative is to use an off-line evaporator tank in which excess dragout is contained and reduced through evaporation to a usable concentrate. Dragout contamination occurs when the process tank is allowed to become contaminated through gradual build-up of organic contaminants in the process solution. This can arise from the breakdown of additives such as brighteners, levellers and wetting agents, or the carryover of contaminants from previous process. Some contaminants may have been air-borne, or delivered in the air used for agitating the solution. Generally, the counter to this is to have effective filtration in the process bath, and it is can also be useful to run carbon filtration on the dragout tank. As far as metallic contamination is concerned, this should be prevented by appropriate pre-plating procedures and to guard against components dropping off jigs and into the tank where they slowly dissolve. In any event, metallic contaminants can usually be plated out, either in the process tank or in a stand-by tank created for this purpose. Ion-exchange membranes can also be utilised to remove such contaminants. 6.4 THE CONTINUOUS RUNNING RINSES It is not possible to practice electroplating with only static rinses or dragouts. Static rinses would simply become more and more concentrated until they could no longer perform as rinses. Clean water has to be introduced into the system. For this, the running rinse system is used. It is a tank that has a water inlet at a point, preferably near the base, and an outlet at a point furthest from the inlet and situated near the top (i.e. an overflow system). When the tank is filled with water up to the level of the outlet port and new water is introduced through the inlet an equivalent amount of water will be displaced and run out through the outlet. With a continuous flow of water via the inlet, there will be a continuous discharge via the outlet. In a single tank system, this outlet would normally discharge straight into the effluent stream. If the tank following the plating process tank were a single running rinse then each time we introduced a jig of work into this rinse the solution dragged in would be discharged directly into the effluent stream. This is a wasteful way of plating with respect to raw materials and fresh water consumption. All the dragout introduced into the running rinse tank is immediately lost as effluent. This explains why the running rinse must be preceded by a dragout (static recovery) system. In a single tank rinse system, a high volume of water has to pass through the tank in order to keep the water clean enough to maintain a satisfactory cleaning performance. This is overcome by using a system of several tanks connected in series known as counterflow rinsing. In counterflow rinsing the water flows between the rinsing stages in a direction counter to that of the object flow, which can be either continuous or discontinuous. In electroplating the article or workpiece can therefore be immersed in the first tank (Rinse A) and then the second and so forth until it exits the final rinse tank (Rinse D), while fresh water flows from the final rinse tank and is let out of the first rinse 44

53 tank (see Figure 3). The overflow from Rinse A has the highest metal concentration and can either be sent to the wastewater treatment plant or be recycled to the process tank if more rinse stages are required. Figure 3 Counterflow rinsing (Hyder Consulting and Hemsley 1999) The water required to effectively rinse articles or workpieces can be improved by extending the rinsing process with more rinsing tanks/stages. The freshwater consumption in counterflow rinsing is calculated from the following formula (Binnie, 1987): Q C n n = D 0 C n = D n F Equation 1 Where, n is the number of rinsing stages; Q n is the consumption of fresh rinsing water (volume/time) when using counter-current rinsing with n stages; D is the drag-out from the cleaning vat (volume/time); C 0 is the concentration of the constituent in the plating bath (mass/volume); C n is the concentration of the constituent in the n th rinsing tank (mass/volume); and F is the dilution factor (dimensionless). It is seen from Equation 1 that the amount of freshwater consumption (Q n ) will decrease for a decrease of drag-out (D), and increase in the number of rinsing stages (n), and a decrease in the dilution factor (F). The dilution factor is dependent on the metal concentration that can be tolerated in a rinse without having adverse effects on the rinsing quality (SAMFA, 2013) while the drag-out is dependent on the workload, workpiece configuration and dripping time of workpieces over the process tank. Since the dilution factor has a minimum limit, the freshwater consumption can only be minimized by employing methods which reduce drag-out and by increasing the number of rinsing stages. The use of a five stage counterflow rinse has been reported as an ideal situation design if a closed loop system is contemplated and showed a saving of 99.6% of fresh water used. (SAMFA, 2013) Normally a maximum of three rinses is considered adequate (99% saving) due to plant costs and space limitations.. Because the required flow is so little through a five tank counterflow system that it 45