Chemistry and Leaching Behavior of Chromite Ore Processing Residue from the Soda Ash Process

This study presents chemical, mineralogical, and leaching characterization of chromite ore processing residue (COPR) derived from the soda ash process, which is currently replacing the lime-based process in China, the world's largest chromite processor. Results indicated that, while soda ash COPR had 8,500mg/kg Cr(VI), 90% of that was readily available in deionized water and thus the result of incomplete dissolution during the leaching process. The remaining 850mg/kg of Cr(VI) could only be leached by decreasing the pH from 12.5 to 8, and geochemical modeling indicated that hydrotalcite (Mg4Al2(OH)(12)CO(3)2H(2)O) is a potential solubility controlling phase that can substitute chromate in its structure. Buffering capacity of soda ash COPR is substantially lower compared with lime-based COPR, with 1mol(c)/kg of H+ required to decrease its pH to 8. Mineralogical makeup of soda ash COPR includes chromite, hematite, and calcite, as identified by X-ray diffraction, and potentially also magnesioferrite and tobermorite, as predicted by geochemical modeling. Overall, results of this study provide the first geochemistry-based evidence that soda ash COPR is deemed to be a more environmentally friendly waste compared with lime-based COPR. Widespread conversion of chromite processing factories to this process in China and elsewhere will have important implications for environmental protection.