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State characteristics of electroplating industry waste. Also explain method for reduction of hexavalent chromium to trivalent chromium.
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CHARACTERISTICS OF WASTEWATER

1)More than 90% of the wastewater discharged from a plating shop comes from rinsing of workpieces with the occasional discharges of process solution. spillage or cleaning of process devices.

2)The characteristics of electroplating wastewater are a function of the types of plating processes, rinsing methods and plating practices.

3)It is found that the characteristics of the plating wastewater varied from one factory to another even though both might be engaged in the same plating processes.

4)The combined plating wastewater was generally clear, colourless and with a pH ranging from acidic to slightly basic (pH 1-7-8-2).

5) Cyanide was detected in the combined waste water discharged from copper and zinc plating factories with concentrations ranged from 1-6 mg/l. As the baths of these two plating processes contained cyanide, the sources of cyanide detected in the waste water should be derived from the washing of the drag-out solution still attached on the surface of the plated items.

6)Similarly, chromium was detected in substantial amount (1-40 mg/l) in the wastewater discharged from factories engaged in chromium plating and zinc plating. The sources of the chromium were from the drag-out of the chromium plating bath and the passivating bath of the zinc plating operation. The chromium in the wastewater existed mostly in the hexavalent form.

7)Depending upon the type of plating processes. heavy metals such as copper, nickel, zinc, aluminium and silver were also detected in the wastewater and the concentrations of which were generally less than 400 mg/l.

8)In summary, the combined plating wastewater generally contained cyanide, hexavalent chromium and heavy metals. As these chemicals possess a cer tain degree of toxicity to the human health and aquatic life, it is therefore necessary to provide treat ment to reduce the level of these contaminants before the waste-water is allowed to discharge into receiving water-bodies.

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