Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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quate operator and maintenance training. There is constant activity in the mar- ketplace with new developments and promising breakthroughs in technology. Marketing claims can often make the situation bewildering, but it is appropriate to bear in mind that the laws of chemistry, physics, and economics will prevail. The fundamental law of ecology teaches that there is no free lunch. Mother Nature is a tough task mistress. She has made it much easier and less costly to mix things together than to take them apart. SOURCES OF WASTE There are three categories of waste that must be considered when formulating a waste minimization program. Bath Drag-Out to Rinses This is the carryover of concentrated process baths on the workpieces, which Most of the process baths used in metal finishing are expendable and must be peri- odically discarded when their chemical activity is below a level acceptable for pro- duction purposes. is removed by stagnant and flowing water rinses. Bath Dumps This is a catch-all category including both accidental and purposeful incidental waste sources such as tank overflows, drips from workpieces, leaking tanks or pipes, spills of chemicals, salt encrustations, equipment and floor wash-down water, oil drips, or spills from gear boxes, etc. Historically, most of the emphasis on recovery technologies has focused on rin- sewater since it constitutes the majority of the flow leaving an operation and neces- sitates expensive waste treatment. Bath dumps are usually infrequent and are low in volume. Often, dumped baths can be hauled to a distant location by a waste service provider for final treatment and disposal. A subsequent section of this arti- cle will discuss the possibility of regeneration for certain of these baths to elim- inate the need for periodic dumping. Floor spills are nearly impossible to manage by the application of recovery tech- nologies due to their unpredictable and intermittent nature and to the fact that they are so heterogeneous in composition. The primary attack on floor spills is tight operating and process control, adequate operator and safety training, pro- grams to eliminate accidents, and, of course, good housekeeping. The following sections will deal with the techniques applied to rinsewater. These can be divided into those that return a concentrated solution back to the origi- nating process and those that aim to recover metals or chemicals for use elsewhere. Floor Spills CONCENTRATE RECOVERY METHODS There are a number of important factors that should be considered in regard to returning concentrate to the originating process. First, the majority of metal-fin- ishing process baths is ultimately expendable. They have a finite life and are peri- odically discarded. Recycling of drag-out simply accelerates this process and will give no net gain unless some regeneration scheme is employed on the process bath itself. Thus, recovery of drag-out is most often considered only for the baths that operate in a reasonably balanced condition, primarily the process baths. A general recovery schematic for return methods is pictured in Figure 1. In the case of those electroplating baths where return of drag-out seems prac- 541

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