Leaching Mechanism of Cr(VI) from Chromite Ore Processing Residue with Nitric Acid

The leaching behavior of hexavalent chromium (Cr(VI)) and major metal elements relevant to Cr(VI) leaching from chromite ore processing residue (COPR) to HNO3 are investigated by batch leaching tests. According to the test results, aqueous Cr(VI) accounted for 70-80% of the content of total Cr. Cr(VI) concentration was controlled by the dissolution degree of Cr(VI)-bearing minerals (such as hydroandradite) as the pH value ranged from 7.58 to 10.47, and by the adsorption degree of Cr(VI) on metal amorphous precipitates as pH value ranged from 6.18 to 7.58. It takes time for the penetration of acid into COPR and the release of alkalinity from the interior zone of particles. So, it could be speculated that the fine-sized particles would react with acid more effectively. 14mol HNO3 per 1kg COPR is needed to maintain the pH value to achieve 7.5 for 48 hours. This indicates the acid neutralization capacity of the test sample is relatively large. It suggests that the application of acid treatment method for COPR disposal is feasible but costly.