Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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trolled and maintained. The economics currently favor only relatively large installations but development of lower cost approaches is likely. Sulfuric acid anodize solution and phosphoric acid bright dip bath can both be regenerated using DD or acid retardation, which is a sorption process using ion exchange resins. The cost and complexity of such recovery operations require economic evaluation on a case by case basis. Chromic acid anodizing solutions can be regenerated by the use of cation exchange or ME. Both technologies can be used to remove the accumulating alu- minum together with other metal impurities such as copper and zinc. The life expectancy of the resin is shorter than on normal waste treatment applications, but the method is still practical and economical. The use of ME has shown effective purification and maintenance capabilities of these baths. Pickling is commonly used in steel mills for the surface finishing of steel prod- ucts or as a pretreatment operation for a galvanizing process. Large volumes of spent acid containing metal contaminants are generated. Among the various methods available for acid purification and recovery, DD is very useful for the recovery of free acid from spent pickling baths. Both sulfuric and hydrochloric acids are commonly used for cleaning steel. Sulfuric acid can be regenerated by crystallization of ferrous sulfate. Hydrochloric acid can be recovered by distilling off the acid and leaving behind the iron oxide. These technologies have been used for many years in large installations and by tolling reclaimers but are not likely to ever be economical for small metal-finishing or galvanizing plants where the production cannot justify the capital investment. Iron and Steel Concentrated chromic acid solutions are used to etch plastic surfaces prior to plat- ing. These operations consume very high quantities of chemicals and generate large quantities of sludge. Standard practice today is to reclaim essentially all of the chromium from such an operation through a combination of evaporation and electrochemical oxidation of the trivalent chromium. Today, a combination of evaporation and ME can be used to extend the oper- ating time of a chromic acid etch indefinitely. Plastic Etching Alkaline cleaners are probably the most widely used process baths in all of met- al finishing. Treatment significance will increase as water recycling becomes a more prevalent practice. Most cleaner formulations are antagonistic to good treatment of a metal-finishing effluent because they are chemically formulated to keep dirt and oil in suspension. If their concentration is high enough in an effluent this same effect prevents efficient removal of the precipitated metals. Dumps of alkaline cleaners, passing through a treatment system, are a noto- rious source of upsets and a high contributor to the TDS in a metal-finishing effluent. In addition there are certain cases where large finishing operations on small sewer systems, or small receiving streams, may have a problem meeting requirements for the organic content due to wetting agents and detergents. The cleaning of parts in surface-finishing operations generates a lot of impu- rities in the cleaner bath. These impurities, such as oils, dirt, and soil, wear out the cleaner baths and have to be removed to extend the life of the cleaner. Free or tramp oil is usually removed with a skimmer. Emulsified oil will usually build up in the bath, with some of it splitting into a floating layer where it will be removed Alkaline Cleaners 558

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