Metal Finishing Guide Book

2013

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operator and maintenance training. There is constant activity in the marketplace 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 is removed by stagnant and flowing water rinses. Bath Dumps Most of the process baths used in metal finishing are expendable and must be periodically discarded when their chemical activity is below a level acceptable for production purposes. Floor Spills 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 rinsewater since it constitutes the majority of the flow leaving an operation and necessitates 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 article will discuss the possibility of regeneration for certain of these baths to eliminate the need for periodic dumping. Floor spills are nearly impossible to manage by the application of recovery technologies 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, programs 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 originating process and those that aim to recover metals or chemicals for use elsewhere. 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 metalfinishing process baths is ultimately expendable. They have a finite life and are periodically 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. 599

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