A plating shop typically generates a waste stream that includes nickel, chromium, copper, zinc, cyanide, and other ions. Current technology for treating a waste stream from a plating shop generally involves separating the waste stream into alkaline, acid, chromate and cyanide waste streams. The cyanide stream is made strongly alkaline and is treated with a hypochlorite (bleach) to totally destroy the cyanide. The chromate containing stream is treated with metabisulfite or sulfur dioxide in order to reduce the hexavalent chromium to the trivalent state. Then all streams are combined and made strongly alkaline. This forces all the heavy metals to precipitate as oxides and hydroxides. Usually a flocculant is added to encourage full precipitation.
At this point the precipitates can be filtered out of the combined waste stream. Then the stream is neutralized with an acid. The treatment system is complex, including many tanks; feed loops, under automatic control, for introducing reagents to these tanks; impellers or equivalent devices for adequate mixing of reagents with the waste streams; and at least one filter press for separating the water from the precipitated metals. Not infrequently, the whole system is under computer control. Plating wastes treated in this manner typically meet requirements for discharge into water courses and sanitary sewers. The sludges generated are generally trucked to a sanitary landfill.
It can be readily appreciated that the system described above is large, costly, and requires massive quantities of chemicals for its operation. Further it is not readily amenable to treatment of acid mine waste and clean up of hazardous waste sites. The types of systems suitable for treating acid mine wastes, for recovering metals, and for removing heavy metals from the soil at hazardous waste sites vary considerably from the typical plating waste treatment system described above. Such systems are typically designed for each specific application and so no generalized design can be given.
Development of a treatment system which can remove heavy metals and other harmful contaminants from plating waste streams, acid mine wastes and hazardous waste sites or recover valuable metals in a simple and economical manner represents a great improvement in the field of waste treatment and satisfies a long felt need of the waste treatment industry.