Simultaneous oxidation of Cr(III) and extraction of Cr(VI) from chromite ore processing residue by silicate-assisted hydrothermal treatment

The great challenge of remediation of chromite ore processing residue (COPR) lies in how to effectively eliminate or extract the highly toxic and mobile Cr(VI) incorporated in the solid sludge. This work developed a facile method to detoxify industrial COPR by using hydrothermal treatment assisted with sodium silicate (HTNa2SiO3). The results reveal that Na2SiO3 greatly promotes the extraction of Cr(VI) in COPR. After the treatment, the Cr(VI) concentration in the leachate by toxicity characteristic leaching procedure (TCLP) markedly decreased to 1.1 mg L-1, well below the regulatory limit of 1.5 mg L-1 (GB 16889-2008, China EPA). X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) mapping confirm that neither Cr(VI) nor Cr(III) were detected on the surface of COPR after treatment. Further study reveals that in the hydrothermal conditions, SiO32- ions not only act as the anion exchange agent to delaminate layered phases in COPR like hydrotalcites to release the encapsulated Cr(VI), but also help to oxidize Cr(III) in COPR to soluble Cr(VI), both of which distinctively increased the leaching of chromium from COPR. This study provides a novel strategy for both COPR detoxification and recovery of Cr(VI) for resource recycling.