Revenue from Waste & Zero Discharge from Anodizing Plants
Total Page：16
File Type：pdf, Size：1020Kb
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.