Microstructural analyses of Cr(VI) speciation in chromite ore processing residue from the soda ash process

The microstructure of Chromite Ore Processing Residue (COPR) derived from the soda ash roasting process was investigated prior to and after removal of water exchangeable chromate using a host of microscopy and spectroscopy techniques. Soda ash COPR consists mostly of a magnesioferrite (MgFe2O4) matrix that has substantial substitution of trivalent chromium (Cr) for iron. The chromite particles are generally larger than the overall particle size distribution of COPR, containing most of the Cr mass in areas that are greater than 20 mu m in diameter; chromite particles are also associated with most of the non-exchangeable hexavalent Cr (Cr(VI)), even though the binding mechanism is not well understood. The remaining non-exchangeable Cr(VI) was found in association with the surrounding Si- and Al-matrix, with spectroscopic evidence of the presence of Cr(VI)-hydrotalcite.