Chromium is usually found in industrial plating shop rinse water in the form of chromic acid. The chromium typically exists in either a +6 or +3 oxidation state. Using traditional treatment methods, Cr+3 can be treated by precipitating the hydroxide salt at a pH of 9. Traditional treatment techniques are much more complex when treating waste with Cr+6 or “hex chrome.” It must be reduced to Cr+3 before it can precipitate. The reduction process is usually done at a pH of 3 with sodium bisulfite as the reducing agent, although other reducing agents such as ferrous ion can also be used. Once the chrome is reduced, it can be precipitated as the hydroxide at a pH of 8 to 9.
Wastech has developed a process that eliminates the need to reduce hex chrome to the trivalent state for precipitation using MetFloc chemistry. The reduction process is typically based on ORP, and is dependent on the concentration of chromium in the waste. Eliminating this step reduces the amount of treatment equipment, instrumentation, chemistry, and real estate required to implement the treatment process. The MetFloc process is based solely on pH, making it easy to control, without overdosing chemistry. The end result is a smaller, lower cost system, that generates less sludge while using less chemical.
Wastech designs small and large batch type precipitation systems for treatment of concentrated chromic acid baths. They range from compact, stand alone units, to large, multiple skid systems. Most of Wastech’s batch systems feature PLC control and are fully automated.
For higher flows of contaminated waste, a continuous treatment system is needed. Wastech’s continuous heavy metal removal systems are optimized for each and every customer’s application. We test waste samples in our laboratory prior to designing a system to guarantee the treatment results.
Trace amounts of chrome can also be removed by ion exchange. Wastech ion exchange systems are designed to treat plating rinse water with trace amounts of metals so that the water can be re-used as rinse water again. The water is sent through cation and anion resin beds, along with activated carbon and/or media filtration to produce deionized water that can be returned to the process. The difference in a Wastech ion exchange system is that the resin is regenerated on-site, eliminating the fees for bottle haul off. Since the regenerate waste will contain any chrome and other metals removed during treatment, a vacuum distillation system can be used to concentrate the regenerate even further to reduce the amount of liquid waste hauled away. The purified water from the vacuum distillation system can also be re-used in the process. The customer saves money by having less city water coming in, by eliminating the need to polish that city water for use as deionized rinse water, and by eliminating the waste stream going to the sewer.