Long-term treatment issues with chromite ore processing residue (COPR): Cr(6+) reduction and heave.

A pugmill treatability study was conducted to remediate chromite ore processing residue (COPR) using ferrous sulfate heptahydrate (FeSO(4) x 7H(2)O) as a reductant. Two different types of COPR, with respect to particle size and mineralogy, were tested in this study. Two different stoichiometric ratios of FeSO(4) x 7H(2)O to Cr(6+) (5x and 8x) were applied to reduce Cr(6+) to Cr(3+). The effectiveness of FeSO(4).7H(2)O treatment was assessed using the toxicity characteristic leaching procedure (TCLP) tests and X-ray absorption near edge structure (XANES) analyses. TCLP results obtained from the pugmill treatability study showed that TCLP Cr concentrations were less than the TCLP regulatory limit of 5mg/L upon 8x FeSO(4) x 7H(2)O treatment for up to 420 days but may fail to meet this regulatory limit in the long-term. XANES results obtained from samples cured for 300 days showed that all of the treated samples failed the New Jersey Department of Environmental Protection (NJDEP) clean up level for Cr(6+) of 240 mg/kg. However, the Cr(6+) concentration from the sample with the smaller particle size approached 240 mg/kg (338 mg/kg), suggesting that particle size reduction prior to the addition of reductant may improve the effectiveness of the treatment. COPR heaving was investigated with unconfined swell tests upon 5x and 8x FeSO(4) x 7H(2)O treatment. The formation of ettringite, an expansive material, was investigated following the swell tests using X-ray powder diffraction (XRPD). Significant heaving (>50 vol%) was observed at curing times of 138 days for the 5x treatment and the ettringite formation was identified by XRPD analyses.