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Chromium is usually found in plating shop and paint shop rinse water in the form of the dichromate anion Cr2O7=. It must be reduced to Cr+3 before it can precipitate. The reduction process is usually done at a pH of 3 with sodium bisulfite as the reducing agent, although other reducing agents such as ferrous ion can also be used. Low concentrations of dichromate can be reduced at a pH as high as 6.
Chrome from pickling and etch operations are usually found as chrome 2 or chrome 3 and do not need to be reduced before precipitation.
Once the chrome is reduced, it can be precipitated as the hydroxide at a pH of 8 to 9, or as the phosphate or sulfide at a higher pH. The chrome 3 carbonate is fairly soluble and should not be used for precipitation. Again when using phosphate or sulfide, a two-step process is recommended to remove the complexed chrome sulfide or chrome phosphate. Ferrous ion is recommended when treating the sulfide and calcium ion is recommended when treating the phosphate.
Chrome can be removed by both cation and anion resin. The chromate or dichromate ion can be removed by the anion resin. Although in large concentrations, it will oxidize the resin shortening the resins life. As chrome 3 or chrome 2, it can be removed effectively in a weak base cation resin in the sodium form.
Chrome cannot be removed by reduction to the metal since it has a high oxidation potential.
The same equipment used for treating the above metals can be used for chrome removal provided a reduction step is used to reduce the chrome to +3 or + 2.
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