The waste treatment from electroplating industry mainly consist of cyanide treatment and chromium treatment.

Cyanide treatment: (alkaline chlorination):

• In alkaline chlorination for cyanide treatment, chlorine is added to a waste containing free cyanide and sufficient alkali is added to raise the pH, the free cyanide is oxidized to cyanate with cyanogens chloride as an intermediate product.

• This reaction is normally instantaneous or takes not more than 10 minutes. With excess chlorine, however cyanide of the first stage is further slowly oxidized to carbon dioxide and nitrogen.

• This second stage of reaction takes from one half to one hour. The chlorination is always done with thorough agitation to provide proper mixing of cyanide with chlorine to avoid formation of solid cyanide precipitates which resist action of chlorine in solution.

• It is important to carry the reaction all the way to carbon dioxide and nitrogen, as otherwise there is a chance of cyanates being reduced back to cyanides.

• In general chlorination is accomplished by two methods:

  • A batch type

  • Continuous type

Chromium treatment:

• Chromates, dichromates and chromic acid occur in the waste waters generated from chromium plating, anodizing and other metal finishing operations.

• The Hexavalent chromium ions in these wastes are highly toxic even in very low concentrations and need to be almost completely removed from the waste water before they are discharged into a stream, sewer or on land.

• The most effective and economical way of treatment involves reduction to trivalent state and subsequent precipitation with an alkali.

• Reduction to trivalent state takes place most effectively in acid solution. Ferrous ion is used to reduce chromate quantitatively in acid solution. Ferrous sulphate along with sulphuric acid is commonly used for this purpose.

• This reduction takes about one hour. Other reducing agents are sulphur dioxide and sodiumbisulphate. Maximum reduction occurs in the pH range 2 to 2.5. Fresh mineral acids or stored spent pickle acids are used for this purpose.

• The pH is lowered by adding 5% sulphuric acid.