Particle size and pH effects on remediation of chromite ore processing residue using calcium polysulfide (CaS5)

A long-term bench scale treatability study was performed to assess the ability to remediate chromite ore processing residue (COPR) using calcium polysulfide (CaS(5)). COPR materials were characterized with respect to particle size, pH, curing period and mineralogy. A stoichiometric ratio of sulfide species to hexavalent chromium (Cr(6+)) of 2 was used for the long-term treatment of COPR. The effectiveness of CaS(5) treatment was assessed using the toxicity characteristic leaching procedure (TCLP), alkaline digestion, and X-ray absorption near edge structure (XANES) analyses. The formation of ettringite, known as a heaving agent, was investigated following the treatment of CaS(5), using X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM) along with an energy dispersive X-ray spectroscopy (EDX). Overall, after a curing period of 18 months, the TCLP total chromium (Cr) and alkaline digestion (Cr(6+)) results obtained from the treatability study showed that the concentrations were lower than 5 mg L(-1) and 9 mg kg(-1), respectively. However, XANES results obtained from samples cured for 18 months showed that all of the treated samples had higher Cr(6+) concentrations than shown using alkaline digestion. The lowest XANES Cr(6+) concentration of 610.2 mg kg(-1) was obtained from the sample with a particle size less than 0.075 mm and a pH value of 9. Particle size reduction prior to the addition of the reductant, along with pH reduction was found to be strongly associated with the treatment performance. Ettringite formation, due to pH increase over time in the samples, where the initial pH was adjusted to 9, was verified by XRPD and SEM-EDX analyses, indicating that a pH less than 9 should be maintained to avoid ettringite formation.