Experimental Studies on Coupled Treatment of Chromite Ore Processing Residue

A study was performed to investigate the combined treatment of hexavalent chromium (Cr(VI)) and heaving in Chromite Ore Processing Residue (COPR). Treatment of heaving focused on the intentional exhaustion of the mineral brownmillerite, the hydration of which is considered responsible for volume expansion in COPR deposition sites in New Jersey. The addition of sufficient amounts of sulfate under different conditions of temperature, acidity, and particle size aimed in the quick transformation of brownmillerite to ettringite, a phenomenon observed during a COPR pilot scale study. Simultaneously, the reductive treatment of hexavalent chromium (Cr(VI)) in COPR was pursued using calcium polysulfide (CPS) and ferrous sulfate (FS). X-ray Absorption Near Edge Spectroscopy (XANES) analyses showed that neither reductant was able to reduce Cr(VI) below 1,000 mg/kg after three months (FS) and six months (CPS) of treatment. However, competitive sulfide oxidation by oxygen was much slower compared to ferrous iron oxidation, as evidenced by chemical analyses. Quantitative X-ray diffraction analyses showed that brownmillerite dissolved to form ettringite only in a sample treated with FS at 50°C; neither acidity or temperature alone were able to trigger brownmillerite dissolution and ettringite formation. Particle size reduction to <150 µm had also no apparent effect in the rate of brownmillerite dissolution. Thus, CPS is deemed a more appropriate reductant for Cr(VI)treatment, while the exhaustion of brownmillerite appears to be a challenging engineering task.