Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 558 of 707

has to pay freight costs plus a fee to the processor for removal and treatment. A typ- ical example would be a sludge containing 5 to 10% copper or nickel, which can be used as a feedstock for a pyrometallurgical operation (a smelter). Such metal-fin- ishing sludge is a richer source of feedstock than the typical ore mined from the ground. On the other hand metal-finishing sludge is typically highly variable in com- position and can contain a significant amount of inorganic salt in the entrained water. Halides can be particularly troublesome in a smelting operation. From the standpoint of long-term liability, the metal finisher needs to consider that 90 to 95% of such sludge will not be turned into product at the smelter but will wind up in the smelter's residues. Although such recycling may appear advantageous under today's regulations, the long-term environmental significance of smelter residue needs to be factored into the decision. A more promising situation exists if a metal finisher generates a segregated sludge that consists essentially of a single metal. Single metal sludges containing only tin, nickel, cadmium, copper, or zinc have excellent potential for being used as feedstock for reclaiming operations, which can operate in an environ- mentally "clean" manner, producing little or no residue. Furthermore, the met- al content of such segregated sludge may be a candidate for in-house recovery by the metal finisher by redissolving the sludge and applying EMR. Segregated sludge is the natural by-product of the closed-loop or integrated rinse treatment method, which has been successfully practiced for decades in both the U.S. and Europe. REGENERATION OF BATHS Historically, most of the effort on recovery was focused on drag-out; however, most of the chemical load from a metal-finishing operation will usually be found in the dumps of expendable process baths and the losses from purification of plating solutions or sludge removal of the process tank. Operations, such as cleaning, pickling, bright dipping, etching, and chemical milling, are worth being inves- tigated for recovery potential. Some of these applications are discussed in the fol- lowing. EMR as described earlier is highly effective on many copper pickling and milling solutions including sulfuric acid, cupric chloride, and ammonium chloride solutions. Solutions based on hydrogen peroxide are generally best regenerated by crystallization and removal of copper sulfate with the crystals being sold as a byproduct or redissolved for EMR. Copper and Its Alloys Bright dipping in highly concentrated nitric/sulfuric acid is a difficult challenge for regeneration because the solution volumes involved are usually quite small (5-25 gal) and the drag-out losses are very high. Regeneration is theoretically pos- sible by distillation of the nitric acid and removal of copper sulfate but the eco- nomics are not likely to be attractive for most metal finishers. This approach does have potential for larger plating plants or for large-scale, centralized recovery facil- ities, which serve a number of plants. Aluminum and Its Alloys The caustic etch used in many aluminum finishing lines and the chemical milling solution used for aircraft components can be regenerated by crystallization and removal of aluminum trihydrate; however, the process must be carefully con- 557

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