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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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