Efficient reduction of Cr(VI) and recovery of Fe from chromite ore processing residue by waste biomass

Abstract

The traditional chromite ore processing residue (COPR) detoxification process is carried out in strong acid or anoxic high-temperature condition, which not only has a high input cost but leading difficulties for resource utilization of treated COPR. This study first reported the efficient reduction of Cr(VI) and recovery of Fe from COPR by waste molasses-assisted hydrothermal treatment. Under optimal conditions (i.e., the Na2CO3 dosing of 0.5 mol/L, the molasses/COPR mass ratio of 10%, hydrothermal treatment at 140 °C for 150 min), the Cr(VI) reduction efficiency reached 99.96 ± 0.01%, and the leaching toxicity of total Cr met the USEPA regulatory limit of 5 mg/L. Moreover, Fe was recycled in the form of magnetic magnesioferrite with Fe2O3 grade of 52.58 ± 0.90%, which can be used as raw material in steel industry. The Cr(VI) reduction reaction conforms to a pseudo-first-order kinetic model, and the reaction rate constants increase with increasing temperature (120 ∼160 °C). As a potential mechanism of the reaction, the Mg-Al hydrotalcite (LDHs) structure collapses in the presence of Na2CO3 during the hydrothermal treatment, and the embedded Cr(VI) is released, which is then reduced to Cr(OH)3 by reducing substances such as fructose, sucrose, and glucose in waste molasses. From the standpoint of a circular economy, this study may provide a potential strategy for minimizing resource input and waste/emission production by harmless disposal and resource utilization of COPR and waste molasses.