Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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remaining metal content can be precipitated with the help of specialized flocculants. Acid Residues Acid solutions quickly dissolve metals from the parts being processed and, like the alkaline chemicals, respond well to simple neutralization techniques to precip- itate the metal content. Acid and alkaline rinsewaters are typically mixed together for treatment and help to neutralize each other. Cyanide Residues The rinses following the brass plating bath will contain cyanide, copper, and zinc. This rinsewater is typically subjected to a cyanide destruct process, which oxi- dizes and decomposes the cyanide to harmless chemicals and also precipitates the copper and zinc content. The metallic sludge is then collected on filters and dis- posed of as hazardous solid waste. Solid Waste The waste treatment methods above generate hazardous solid waste in the form of metal-bearing precipitate, which is commonly collected on a particle filter cartridge or plate filter element. This solid waste can be sent out to a licensed waste treater for proper stabilization and landfilling. Dragout Rinses These are often used as preliminary rinses following a heated process tank such as a heated cleaning tank or plating tank. Dragout rinses are perhaps the single most effective and least costly way to minimize chemicals in the drain. They are typ- ically followed by a treated rinse, which is fed to ion exchange or other treatment. For process solutions carrying only a moderate level of metals, a single dragout rinse is sufficient. A brass plating tank, on the other hand, will contain fairly high concentrations of cyanide, which is costly to treat. Consequently, it is common to see two or three dragout rinses used to minimize the level of cyanide sent to waste treatment. Copper and Selenium-Bearing Effluent Room-temperature oxidizers are perhaps the simplest to operate because they respond so well to treatment by ion exchange techniques. Some lines are set up with the rinsewaters going in two different directions, so to speak; the rinsewaters from the alkaline clean and acid tarnish removers tend to neutralize each other in the drain and are sent to a pH adjustment to complete the precipitation process; meanwhile, the rinsewaters following the copper/selenium-based oxidizers can be treated by ion exchange to purify the water and reuse it with none of this water enter- ing the drain. Another option, in many cases, is to treat all the rinsewater in the line with ion exchange. Since all the rinses can contain metals none can be considered sewerable. But, since the total dissolved solids content of these rinses is usually quite low ion exchange is able to purify all the rinsewaters, in many lines, and return them to the rinse tanks to be reused over and over again. In general ion exchange works well when the total dissolved solids content of the water is 1,000 ppm or less. For higher concentrations pH adjustment and neu- tralization techniques are more efficient. Most ion exchange systems are equipped with a conductivity light, which signals the operator that the resin tanks are satu- rated and ready for regeneration. The regeneration can be performed on site or the resin tanks can be shipped to a licensed waste treater for regeneration. Responsible chemical suppliers offer advice on proper waste treatment tech- niques for their products. There is a great deal of additional information avail- 392

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