Metal Finishing Guide Book

2013

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chemical processes are essentially steady-state processes and lend themselves to tight statistical control. In comparison, finishing processes are more readily categorized as unsteady-state processes that are relatively chaotic from a process standpoint and, as a consequence, are more difficult to monitor and control. This characteristic has nourished the relatively straightforward "lime-and-settle" method of treating toxic wastes and has hindered the acceptance and application of what are now a well-documented set of chemical process techniques for reducing the high level of waste generated by surface-finishing processes. In an ideal finishing process, there would be no bath drag-out. Chemical losses would be restricted only to those chemicals that are consumed in cleaning and preconditioning surfaces and to those portions of the plating baths, which produce the desired surface coating or condition. In the real world, bath drag-out is, of course, unavoidable. Drag-out can be reduced to some extent by instituting such mechanisms as increasing dwell time over baths, decreasing bath surface tension, forward pumped spray rinses, air knives, etc. Despite such efforts, substantial quantities of bath can still be lost to the rinse system. The net result is that bath drag-out continues to be the primary contributor to the extraordinary quantity of chemical waste generated by the surface-finishing industry. This article reviews a number of well-demonstrated and proven chemical recovery methods, collectively known as separation technologies, for reducing or in some cases reversing bath drag-out. When properly selected and applied, one or more of these technologies in combination can be confidently used to separate and recover dragged-out bath or specific chemical components or values of certain baths or solutions and to separate and condition rinsewaters for recycle and reuse in the plating process. Each technology separates the constituents of a solution differently. For example, evaporation separates the solvent (water) from the rest of the bath constituents. All other techniques affect separation on either a molecular or an ionic level. The choice of technology, or combination of technologies, is determined by both bath chemistry (what the chemistry lets you do) and by the underlying operating economics. ECONOMICS OF RECOVERY VERSUS TREATMENT There are essentially four approaches that can be taken to evaluate point-source recovery potential in given metal-finishing operations. Operating Savings Plating facilities with existing and adequate waste treatment systems can readily assess operating savings for a candidate recovery technology. A given recovery technology is evaluated on the basis of savings on purchased process chemicals and associated waste treatment chemicals plus any resultant savings in sludge handling and disposal cost. If the payback on invested capital is attractive, the recovery system should be installed. Avoidance of Waste Treatment Capital Cost Operating cost is the primary consideration for a new plant or for existing plants with an inadequate treatment system. In this case the economic evaluation incorporates an added factor; the avoidance of additional capital investment for waste treatment capacity. 596

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