Revenue from Waste & Zero Discharge from Anodizing Plants

Revenue From Waste & Zero Discharge From Anodizing Plants

By R.C. Furneaux, S.A. Finlayson, N.A. Darby, T. Miyashita, B.R. Ellard, and G.C. Holywell

Plants engaged in batch anodizing Very frequently, acid and alkaline fluorides, while boric acid may be aluminum use a large number of waste solutions are collected sepa- present in the electrocoloring solution. chemicals in different process rately and then brought together for solutions. Sludge disposal is an neutralization. Following the addition Control Technology issue, and residual ions in the of a flocculant, the solution is The Etching Process discharge may interfere with clarified. The supernatant is dis- Dealing with etching can be difficult compliance. Technology in use and charged to a river or sewer, and the because the finish it produces is under development that addresses sludge is consolidated using a filter critical to the market acceptance of these problems includes etch press before transporting to landfill. the product. recovery or elimination, ion retar- The discharge is essentially a dilute In western Europe, the market dation to regenerate acids, potential solution of sodium sulfate, and the expects a very matte finish. This is co-products, closed-loop systems for sludge is an amorphous aluminum achieved using a long-life etch with a electrocoloring, and the minimiza- hydroxide with 15–30 percent solids, high level of dissolved aluminum held tion and reuse of rinsewater. These containing trace heavy-metal hydrox- in solution by a suitable sequestrant. methods are discussed in this edited ides. One German plant uses expen- Typically, the solution contains 100 version of a paper presented at the sive high-pressure tube filters, and is g/L free-sodium hydroxide and 130 16th AESF/EPA Pollution Preven- able to achieve almost 50 percent g/L aluminum, and may dissolve tion & Control Conference, held solids. more than 80 g/m2 to give a 60° February 13–15, 1995, at Orlando, specular gloss value of 20. Using a FL. The complete presentation and Origin of Potential Pollutants long-life etch means the solution additional illustrations appear in Anodizing sludge is mainly composed needs only to be dumped infrequently. the Proceedings of the conference, of aluminum hydroxide, and its main The solutions are, however, very and can be ordered through AESF source is the etching process. Figure 1 viscous, which leads to high dragout Publications Sales. shows how the amount of sludge to the rinse (typically 800 mL/m2). At produced per m2 of metal processed steady-state operation, all the dis- nodized aluminum has depends on the metal weight loss in solved aluminum becomes diluted in environmental advantages over the etch and sludge solids content. A rinsewater and goes to the effluent Aalternative materials. Organic typical plant may dissolve 90 g/m2 in treatment plant for neutralization. solvents are not used, avoiding VOC the etch. In contrast, anodizing to In Japan, the normal product is emissions. Polymeric materials are produce a 20 µm film will only relatively bright (60° specular gloss not involved, which allows for easy dissolve about 10 g/m2. value of 90) and the market will recycling of products after use. The A range of pollutants may be tolerate the appearance of die lines. aqueous solutions, however, result in present in the clarified discharge and Relatively dilute etch solutions are wastes that have to be disposed of in subject to environmental controls. used, typically 65 g/L free sodium an acceptable manner. Sulfate arises from the anodizing and hydroxide and 30 g/L aluminum electrocoloring solutions. Some without additives. Unlike long-life The Anodizing Line localities limit sulfate discharge to etch, these solutions can be recircu- Batches of extruded sections pass sewers, because of the potential lated through a crystallizer, which through a number of tanks containing degradation of sewer concrete.1 Other precipitates aluminum hydroxide, aqueous solutions, each constituting a potential pollutants in the discharge purifying the sodium hydroxide for unit operation, and many followed by are suspended solids caused by reuse.2 In contrast to anodizing rinsing tanks. Typical operations inadequate clarification or filtration, sludge, this precipitate is relatively include: Degrease; etch; desmut/ and trace heavy metals, which were crystalline, has higher purity, can neutralize; anodize; electrocolor; and insufficiently precipitated at the easily be consolidated to give 85–90 seal. Primary chemicals used include: neutralization stage. Heavy metals percent solids, and is salable as a raw

Various alkalis; NaOH; HNO3; originate from the electrocoloring material to the manufacturers of water 3 H2SO4; Sn, Ni or Co salts. Intervening process, and from nickel salt solutions treatment chemicals. rinses are important to prevent used for fixing organic dyes, hydro- Recovery etch systems, as used in unacceptable contamination of thermal sealing or cold impregnation. Japan, are now also used in other subsequent operations. The latter may also involve the use of parts of the world. Difficulties have arisen, however, because there seems

88 PLATING & SURFACE FINISHING Fig. 1—Dependence of sludge production on etch weight loss for Fig. 2—Gloss vs. weight loss for different etch systems. different sludge solids contents.

to be an international trend toward used by a plant in Spain. Sodium yielding further purified sodium more matte finishes, which certain aluminate solution is the waste for hydroxide and an aluminum hydrox- plants are only achieving with very disposal. This can be used for ide co-product. high metal removal (~120 g/m2). This municipal water treatment,8 and has Reaction of spent etch solution with is higher than necessary with a long- been considered in Germany to lime allows caustic recovery, and life etch (Fig. 2). remove phosphate from municipal generates an aluminate that may be Alternative technologies are water to be discharged to rivers.9 used as a cement additive.5 becoming available. Mechanical etch A plant in the Netherlands is minimizes the use of alkaline solu- operating a process where the etch The Anodizing Process tions. The use of wet blasting with solution is regenerated by adding a Anodizing solutions must be dumped alumina has been found to be unsuit- silicate that precipitates zeolite A. regularly when the dissolved alumi- able, because of rapid degradation of Such technology has also been num reaches an unacceptable level the alumina. Dry blasting with steel investigated in Japan for etch recov- (20 g/L, although some plants may shot is available now.4 This produces ery,10 and is being used in the U.S. to adopt a much lower maximum). a very matte, although slightly rough, regenerate extrusion die cleaning Diffusion dialysis is used in Japan finish, obscures die lines well, and is solution. Zeolite A has application as to control the level of aluminum in claimed to generate very little waste. a builder for phosphate-free deter- the anodizing solution.3 Sulfate ions It is recommended that dry blasting be gents and as a desiccant. pass across the membrane, producing followed by a brief “etch” in sodium A method used in Switzerland regenerated acid. The waste is a dilute hydroxide, which only removes a few involves the precipitation of crystal- aluminum sulfate that, although of g/m2 aluminum. A French plant has line aluminum hydroxide in the etch little value, may be used in the installed this process. tank.11 The solution is recirculated production of alum for water treat- Other processes have been devel- through a filter to remove the precipi- ment. There have been problems with oped for the recovery of etch solu- tate. It is claimed that a chemical this technology, because the fragile tions.5 Methods that require etch equilibrium is established between the membranes have tended to foul, and reformulation may be less attractive solute species and the solid material. manual cleaning is expensive. because of the risk that they may not A membrane process, diffusion Many plants are using ion retarda- be able to reproduce the finish dialysis, has been proposed to remove tion.6 It also produces a dilute required by the market. In other cases, sodium hydroxide from the solution aluminum sulfate waste. The process it is not clear whether etch additives for return to the etch tank.12 The etch involves an ion exchange resin that are recovered with the sodium solution is passed across one side of physically retards the transfer of acid hydroxide or lost into the co-product. an ion exchange membrane, while through the column, allowing salts to Ion retardation has been used water flows across the other side. be transmitted relatively unimpeded. extensively for the recovery of acids Sodium ions pass through the mem- Subsequently, the column may be used in pickling solutions.6 This brane to produce the regenerated regenerated using water to produce process has been extended to include stream, leaving a sodium aluminate acid of similar concentration to the alkaline etch solutions,7 and is being solution that goes to a crystallizer, free acid in the original solution.

May 1995 89 material for the inorganic chemicals Table 1 industry. Performance of Ion Retardation & Diffusion Dialysis For Acid Recovery Sealing Solutions Different standards have influenced Stream Ion Retardation Diffusion Dialysis the development of sealing technol- Spent acid 184 g/L H SO 170 g/L H SO 2 4 2 4 ogy in different parts of the world. 12.2 g/L Al 20 g/L Al Japan makes extensive use of elec-

Recovered Acid 175 g/L H2SO4 120 g/L H2SO4 trodeposited lacquers to seal anodic 4.2 g/L Al 0.5 g/L Al films, possibly because of tight Waste 13 g/L H SO 50 g/L H SO standards on the alkaline resistance of 2 4 2 4 2 12 g/L Al 19.5 g/L Al anodized finishes. European standards have stressed the importance of admittance tests. Consequently, boiling-water sealing for long times is Table 1 gives examples, comparing Technology exists for closed-loop favored. Different accelerated tests the performance of diffusion dialysis systems that effectively eliminate have been emphasized by North and ion retardation, indicating that ion certain heavy metal salts of electro- American standards, making the retardation has higher acid recovery. coloring baths from the effluent adoption of cold impregnation Both processes are effective at treatment system. In Japan, rinsewater techniques easier.14 controlling the aluminum concentra- from counter-flowed rinse tanks The effectiveness of cold impregnation in the anodizing bath. following the electrocoloring bath is tion techniques depends on maintain- concentrated by reverse osmosis and ing bath contamination below certain Electrocoloring Solutions the concentrate is returned to electro- levels (Table 2),14 which determines Tin, nickel and cobalt salt solutions coloring solution (Fig. 3).13 Impurities the solution lifetime. The presence of are used worldwide to produce bronze that accumulate in this solution, fluorides and heavy metals in the and black finishes. Because of their sulfate and sodium, are removed by formulations may cause environmen- light adsorption characteristics, ion exchange. Because of the chemi- tal difficulties on disposal. Such however, deposits of each metal cal similarities between nickel and considerations have to be offset produce bronzes of slightly different cobalt, it is anticipated that such a against the energy savings associated hue. This difference is generally system would be equally effective for with these processes in comparison sufficient for customers not to want cobalt-based solutions. with boiling-water sealing. mixed products. Consequently, Alternatively, nickel or cobalt can Boiling water seals need to be regional and national differences have be removed from rinsewater using ion dumped regularly, because of the arisen concerning electrocoloring exchange, and then returned to the buildup of colloidal aluminum solutions. Nickel is used almost electrocoloring solution.3 hydroxide that sticks to, and adversely exclusively in Japan, while tin Closed-loop tin recovery can be affects, the appearance of the work, predominates in Europe. The use of more difficult. Because tin electro- and the accumulation of sulfate, cobalt is declining in favor of tin coloring solutions are quite acidic, which will affect sealing quality. The because of the cost of cobalt salts and reverse osmosis membranes that have maximum concentration for sulfate is their environmental impact in effluents. limited acid resistance may be often taken to be 250 mg/L. The unsuitable. What is more important, contamination level, however, is tin electrocoloring solutions tend to sufficiently low and environmental precipitate stannic hydroxide during problems are unlikely. use, which cannot easily be regenerated to the stannous ion needed for Water Consumption electrodeposition. Recirculating The main use for water is rinsing. systems are under development in There are opportunities to eliminate which the hydroxide can be removed rinses, and to minimize water con- in a form amenable to consolidation sumption. into a high-solids sludge. This Degreaser formulations are avail- material has potential value as a raw able that are compatible with the etch so that no intervening rinse is necessary. Table 2 Similarly, if sulfuric Limits on Contaminants acid (preferably spent In Cold Impregnation Solutions anodizing solution) can be used in place of Contaminant Limit (mg/L) nitric acid for the Phosphate 5 desmut, then there is no Aluminum 250 need for a rinse Sodium or Potassium 300 following the operation. Ammonium 1500 Also, if a nickel-free Fig. 3—Schematic diagram for nickel sulfate recovery. hydrothermal seal is

90 PLATING & SURFACE FINISHING while etch rinsewater with 20 g/L Table 3 total sodium hydroxide be processed Technologies for Desalinated Water by reverse osmosis. Other factors, however, have to be considered. The Regime Of ability of the technique to produce a Lowest Operating concentrated stream must be evalu- Technology Cost (g/L) Water Purity ated. For example, there are currently technical limitations with reverse Evaporation >100 Very high osmosis membranes for either highly Reverse osmosis 10–100 High acidic or highly basic streams because Electrodialysis 0.5–10 Low of membrane stability. This probably Ion exchange <0.5 Medium excludes them from concentrating etch and anodizer rinsewaters to used, it has a low level of contamina- rinses, which enables reductions in process concentrations. Cation tion and may be further employed as volumetric flow rate. Also, feeding exchange electrodialysis membranes rinsewater, particularly after being the static rinse back to the process are not well developed, making them passed through a heat exchanger solution partly counteracts losses unsuitable for concentrating etch system. caused by dragout. rinsewater. Electrodialysis is probably Counterflow rinses, static rinses, For the reuse of rinsewater, it is the most appropriate technology for spray rinse systems and adequate necessary to concentrate the solutions concentrating anodizing rinsewater, drainage practices are frequently used so they may be returned to the process while evaporation is preferred for etch to minimize rinsewater consumption tanks. This assumes that the process rinsewater. and carry-over. Typically, horizontal solutions have recovery systems that Eliminating rinses or reducing their lines consume water at about 40 L/m2, enable aluminum to be removed. If water consumption requires an while the value may be reduced to the water generated by the concentra- understanding of the effect of carry- half in a vertical line. tion process is sufficiently pure, it over on the product quality produced Generally, spray rinse systems are may be reused for further rinsing. by subsequent operations. Mass installed above a process tank to wash Technologies potentially appropriate balance analyses show how contami- loads and flight bar systems on for concentration are evaporation, nants may build up to a steady-state withdrawal from the tank and reduce electrodialysis, reverse osmosis and level. For instance, modifying rinsing carry-over, or over a conventional ion exchange. after anodizing is subject to certain rinse tank to reduce water consump- Experience with desalination requirements. The first rinse should tion associated with the rinsing systems determines which methods be sufficiently acid (

Rinsewater flow (L/m2) Rinsewater flow rate (L/m2)

Fig. 4—Advantage of counterflow rinsing over parallel rinsing. Fig. 5—Rinse tank concentration vs. rinsewater flow rate (counterflow).

May 1995 91 will progressively acidify the coloring Such analyses, however, provide no and the results were used in compari- bath and affect its operation. Acidic information about the time to achieve son with a fixed drag-out of 450 mL/ tin electrocoloring baths are more steady-state concentrations. Figure 6 m2. This shows that the concentration tolerant of acid carry-over than shows how the concentrations in the in the final rinse may be too high by a neutral cobalt baths. Steady-state rinse tanks increase with time. With a factor in excess of three, if it is mass balance analyses show the rinse flow rate of 1000 L/hr, steady- assumed that drag-out does not vary. advantage of counterflow rinsing over state concentrations are approached parallel rinsing, where each rinse tank after about 150 hr operating time. The Revenue From Waste has its own feed and effluent going to rinsewater flow is assumed to be End-of-pipe waste treatment can be a waste. Consider the following constant over that time. This cost for which there is no return on parameters: analysis indicates that, assuming no the investment. Regeneration systems perturbations, a number of weeks are preferred because they involve a • Rinse time per tank: 3 min; may elapse before steady-state saving in the cost of chemicals. In • Loads per hour: 2; conditions are achieved. It is also order to get an even better return on • Tank volume: 40 m3; interesting to compare Figs. 5 and 6. investment, the waste should be a • Drag-in per load: 80 L; For a rinsewater flow rate of 2.8 L/m2 source of revenue. This can be • Area per load: 175 m2; (1000 L/hr), Figure 5 indicates that achieved by producing potentially • Process solution: 275 g/L. the first rinse will have a steady-state salable co-products (Table 4). The concentration of about 41 g/L, while ability to sell particular co-products, The improvement in rinsing, based Figure 6 shows a higher value of however, depends on local opportuni- on the concentration in the last rinse, about 52 g/L. It is believed that ties and the presence of impurities. increases with rinsewater flow rate relying solely on steady-state mass With unstable, low value or and more dramatically as the number balance analyses may underestimate hazardous co-products, transportation of rinses increases (Fig. 4). For the rinsing requirements. distance and the related costs, it is counterflow rinse system with three Drag-out is the most important best to have a local customer. Sodium tanks, the steady-state concentration factor governing rinsing requirements. aluminate, for example, may be depends on the rinsewater flow rate With optimum draining practices, this insufficiently stable to allow long (Fig. 5). In this case, effective depends on solution viscosity, which transportation distances. High rinsing is achieved with a flow rate in turn is controlled by solution transportation costs may be associated of about 1.2 L/m2 load area. In temperature and concentration. In a with waste solutions and anodizing practice, a number of process tanks rinse system, the solution concentra- sludge with its high water content. would be feeding loads into the tion, and most likely the temperature, Impurities may also be critical to same rinse system, which would will vary in the rinse tanks during the marketability. This is particularly true increase the rinsing requirements. If time before steady-state is achieved. for the manufacturers of water a line contains three etches and This means that drag-out will vary. treatment chemicals used for potable three anodizing tanks, this analysis Figure 7 indicates the effect of water. Co-products from etch recov- suggests that it should be feasible to assuming that drag-out is constant ery systems have the highest purity operate with a rinsewater flow of with time. A linear relationship with because they are contaminated with less than 10 L/m2. solution concentration was used to only alloying constituents. Neverthe- determine the instantaneous drag-out, less, these may be at a higher level

Time (h) Rinswater flow rate (L/m2) Fig. 6—Time history of concentrations in a three-tank Fig. 7—Effect of drag-out assumption on final rinse tank countercurrent rinse. concentration.

92 PLATING & SURFACE FINISHING than in the raw materials convention- ally used. This does not necessarily Table 4 preclude anodizing line co-products Applications of Potential Co-products from Anodizing Lines from all water treatment applications. It is important to determine whether Co-product Application the application really requires a high- Anodizing sludge Cement purity product, or whether the purity Water treatment chemicals level is an inherent feature of the Absorbents conventional raw material. For Etch recovery sludge Water treatment chemicals example, alumina has a high purity Firebricks because this is a characteristic of the Ceramics Bayer process. Fire retardants Certain cements are sensitive to the presence of sodium or sulfate. Sodium aluminate Water treatment Industrial ceramics and refractories Zeolite A Washing powders are expected to be white; the presence Desiccants of heavy metals may impart color. Spent anodizing acid Waste neutralization Many anodizers are successfully Dilute Al (SO ) Water treatment chemicals selling their wastes as co-products. 2 4 3 One Italian anodizer has an alum 8% alum Water treatment plant next door. Co-products require Tin hydroxide sludge Fire retardants the same considerations as main-line products in quality and consistency. achieved using an ion exchange unit. manner without building-in redun- Implementation Similarly, the accumulation of acid in dancy. Developments for each plant Because a waste treatment plant can neutral pH electrocoloring baths will depend on its local circumstances represent a significant cost, planning should be investigated. regarding environmental legislation for incremental installation is impor- and opportunities to sell co-products. tant. Actions should stage develop- Conclusions 3. National standards and markets ments through the generation of 1. Environmental technology is have led to diverse products internation- revenue from waste toward “zero available for anodizing plants to ally, which in some cases inhibit the discharge” operation. Zero discharge produce salable co-products from implementation of the best environmen- operation may never be needed, waste and institute zero discharge tal technology. Examples relate to although at least one Japanese operation. This includes recovery etching, electrocoloring and sealing. anodizer already has to meet that systems for etch, anodizing and 4. An important element of any requirement. Technology planning electrocoloring solutions. technology plan is the selection of the toward that goal ensures that no 2. A technology plan is important to most appropriate etching technology equipment is installed that later stage implementation in a logical suffers unplanned redundancy. Each plant has to develop its own plans based on its own circumstances. This depends on the facilities being used, and particularly on local opportunities to sell potential co-products. Initial actions should be aimed at reducing rinsewater flow rates. Subsequently, recovery systems may be introduced to eliminate heavy metal-containing effluent. The installation of etch recovery is cost- effective, but requires careful consid- eration of product requirements and means to dispose of waste acid. It spreads capital expenditure if an acid recovery system can be introduced later. The final stages to achieve zero discharge involve reuse of rinsewater. Here, mass balance analyses are important to understand how impurities may build up, and avoid unacceptable contamination of process solutions. It is particularly important to control sodium contamination of electrocoloring baths that may be Free Details: Circle 174 on postpaid reader service card.

May 1995 93 to satisfy market needs and long-term NLM, whose comments and experi- 5. D.A. Cornwell, Proc. 37th Purdue Industrial environmental requirements. ence have been invaluable. Waste Conference, 119-126 (1983). 6. W. Götzelmann, L. Hartinger, and Gülbas, 5. Mass balance analyses are important Metalloberfläche, 41, 208-212 & 315-322 tools in developing a technology plan, References (1987). particularly in determining how impurities 1. G. Schuster, and K.-H. Kästle, Industrie- 7. J. Amigó, 2nd International Congress on will accumulate in a complex recycling Anzeiger, 16, 22-23 (1991). Aluminum, Interall Publications, 2, 27-50 system. 2. J. Patrie, Revue de l’Aluminum, 77-88, (1993). February 1976. 8. H. Potter, Aluminum Finishing, 13, No. 4/ 3. W.S. Anderson, Annual Technical 5, 27-30 (1993). Acknowledgments Conference, Inst. Metal Finishing, 9. D. Brodalla, EURAS Member’s Assembly, The authors thank the management of September 1992. EURAS, 1992. Alcan International, Ltd. for permis- 4. E. Strazzi, and R. Finessi, Aluminum 10. Y. Umehara, Japan Kokai, 51, 43 (1975). Finishing, 14, No. 3, 17-20 (1994). 11. P. Fischer, EURAS Members’ Assembly, sion to publish this work, and also EURAS, 1994. acknowledge colleagues in Alcan and 12. T. Davis, U.S. Patent 5049233. 13. Japanese Patent 1413949 14. M.R. Kalantary, D.R. Gabe and D.H. Ross, Aluminum Finishing, 13, No. 4/5, 33-37 (1993). 15. P.M. Bungay, H.K. Lonsdale, and de Pinho, M. N., Synthetic Membranes: Science, Engineering and Applications, 217, D. Reidel Publishing Co. (1986). About the Authors Dr. Robin C. Furneaux is a chartered chemist with Alcan Interna- tional, Ltd., Banbury, Oxon, England 0X167SP, where he has most recently been working with environmental and aluminum recycling technologies, particularly the identification and development of integrated systems to minimize the environmental impact of effluents from aluminum processing operations. Dr. Furneaux holds degrees from the University of Sheffield and UMIST. He is a member of the Royal Society of Chemistry and the Institute of Metal Finishing. He has more than 25 publications, including five patents. N.A. Darby is a registered environmental auditor and a chartered engineer, currently working with recycling aluminum-containing packaging materials at Alcan International, Ltd. He is a graduate in chemical engineering from the University of Leeds. B.R. Ellard, left, is a scientist at Alcan International, Ltd., Banbury, England. He has worked 25 years at the research center in analytical, lubrication, paint/pretreatment, and specializing in anodizing for the last 15 years. Teruo Miyashita is research director of surface finishing and corrosion at Nippon Light Metal Co., Ltd., Research and Development Center, Shizuoka, Japan. He specializes in color anodizing, surface finishing, and corrosion and cathodic protection of aluminum. He holds degrees in metallurgical engineering from the Tokyo Institute of Technology, and previously served with Alcan International, Ltd. in England and Canada. Free Details: Circle 175 on postpaid reader service card.

94 PLATING & SURFACE FINISHING