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 sodium bisulphate.
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.