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.
Contact John Strandberg
email: rwaterguy@rwaterguy.com