Mobilization of Cr(VI) from chromite ore processing residue through acid treatment

Batch leaching studies on chromite ore processing residue (COPR) were performed using acids to investigate leaching of hexavalent chromium, Cr(VI), with respect to particle size, reaction time, and type of acid (HNO3 and H2SO4). Aqueous Cr(VI) is maximized at approximately 0.04 mol Cr(VI) per kg of dry COPR at pH 7.6–8.1. Cr(VI) mobilized more slowly for larger particles, and the pH increased with time and increased more rapidly for smaller particles, suggesting that rate limitations occur in the solid phase. With H2SO4, the pH stabilized at a higher value (8.8 for H2SO4 vs. 8.0 for HNO3) and more rapidly (16 h vs. 30 h), and the differences in pH for different particle sizes were smaller. The acid neutralization capacity (ANC) of COPR is very large (8 mol HNO3 per kg of dry COPR for a stable eluate pH of 7.5). Changes to the elemental and mineralogical composition and distribution in COPR particles after mixing with acid indicate that Cr(VI)-bearing solids dissolved. However, concentrations of Cr(VI) > 2800 mg kg− 1 (> 50% of the pre-treatment concentration) were still found after mixing with acid, regardless of the particle size, reaction time, or type of acid used. The residual Cr(VI) appears to be partially associated with poorly-ordered Fe and Al oxyhydroxides that precipitated in the interstitial areas of COPR particles. Remediation strategies that use HNO3 or H2SO4 to neutralize COPR or to maximize Cr(VI) in solution are likely to require extensive amounts of acid, may not mobilize all of the Cr(VI), and may require extended contact time, even under well-mixed conditions.