Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Electroplating generates copious amounts of wastewater to be treated, normally for removal and/or destruction of materials such as chromium,cyanide, nickel, cadmium, copper, lead, and zinc. The wastewater from such systems is usually at the extremes of the pH scale de- pending upon which stage is being treated, thus requiring neutral- ization. Processes such as alu- minum anodizing will also pro- duce significant amounts of pol- lutants, similar in some cases to electroplating systems, with high amounts of hexavalent chromium and other metals to be removed and highly acidic and alkaline wastewater to be neutralized. WASTE TREATMENT SYSTEMS Fig. 1. Clarifiers Waste treatment systems are put into place to remove the various pollutants entering the waste stream from plant operations. These systems have grown in so- phistication over time from sim- ple settling ponds to complex os- motic filtration units. The typi- cal waste treatment system con- sists of a series of tanks in which wastewater can be collected and chemically treated as necessary to remove con- taminants. Depending on the rate of water flow, the system may be continuous or may involve batch treatment. (As a rule, systems in which spray washer rinse run-off and "dumped" washer stages are the prevalent material in the waste stream can typically be treated on a batch basis; systems consuming large amounts of water on a continuous basis, such as electroplating or electrocleaning lines, are often treated in a continuous system.) In the case of materials such as hexavalent chromium a dedicated tank might be necessary for segregation/treatment of a particular pollutant. The material can then be treated, adjusted, and, quite pos- sibly, removed from the water, which then moves on in the treatment system. Oth- er tanks may simply be used for pH adjustments, such as those to neutralize highly acidic or alkaline materials. Once the wastewater is adjusted to the desirable state, it moves into the area of solids removal. This may be done through phys- ical filtration,such as a sand filter system, or through gravity separation, such as would be done in a settling pond. Commonly, however, the particles/pollutants remaining in the water at this point either are not heavy enough to rapidly set- tle in a simple still pond or are not in a form to ever settle under normal means. To facilitate this process, inorganic materials such as lime or alum can be added to help flocculate the solid pollutants, bringing them together in a mass. Organic polymers can also be used to coagulate the smaller particles, as can combination products made up of polymers and inorganic salts. To further facilitate the set- tling of these pollutants, a piece of equipment known as a clarifier is used. The 253

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