Efficient immobilization and utilization of chromite ore processing residue via hydrothermally constructing spinel phase Fe2+(Cr3+X, Fe3+2-x)O4 and its magnetic separation.

Chromite ore processing residue (COPR) has been a severe environmental contaminant which is worthy of attention. In this study, we developed an eco-friendly and practical technology for effectively stabilizing and recovering Cr(VI) in COPR via combining FeSO4 reducing agent and the hydrothermal treatment. A stable spinel phase product was formed during detoxification. In addition, the ferrochrome resources in the treated COPR can be obtained by magnetic separation. As we studied, the hydrothermal environment promoted the release of unstable Na2CrO4 from COPR into the solution, and the released CrO42- was reduced to Cr(III) by FeSO4. Subsequently, Cr(III), Fe(II) and Fe(III) were hydrothermally mineralized to form the magnetic spinel phase Fe2+(Cr3+X, Fe3+2-x)O4 (FeCr spinel substance), which was conducive to the magnetic separation of ferrochrome resources. Under the optimal hydrothermal conditions (0.15 g FeSO4/2 g COPR, treatment at 180 °C for 8 h), the total Cr leaching concentration of treated COPR (COPR-HT) was decreased from 120.51 mg L-1 to 0.23 mg L-1, well below the regulatory limit of 1.5 mg L-1 (HJ/T 301-2007, China EPA). After 300 days aging under atmospheric conditions, the total Cr leaching concentration of COPR-HT was still below 1.5 mg L-1. Besides, the COPR-HT after magnetic separation contained 11.52 wt% Cr2O3 and 53.44 wt% Fe2O3, which can be used as the raw material for steel industry. The underlying mechanism of COPR stabilization was explained by XRD, XPS and SEM-EDS analysis. This work converted the toxic and unstable Cr(VI) in COPR into the long-term stable FeCr spinel substance that is easy to magnetically separate. It has important reference for the harmless disposal and resource utilization of other chromium-containing hazardous wastes including chromium slag and electroplating sludge.