Remediation of chromite ore processing residue by hydrothermal process with starch

Hydrothermal remediation (HR) of chromite ore processing residue (COPR) was studied using starch as a reducing agent. X-ray powder diffraction (XRPD) analysis and the Toxicity Characteristic Leaching Procedure (TCLP) tests were used to evaluate the mineralogical changes of the untreated and treated COPR and the remediation effectiveness of COPR, respectively. The influencing factors for the reduction rate of Cr(VI) in COPR were investigated. The findings indicate that reaction temperature (T), reaction time (t), stoichiometric ratio of starch to Cr(VI) (X) and sulfuric acid dosage (mol-H+/kg-COPR) are the main factors, whereas liquid–solid mass ratio (L/S) is the minor one. The results obtained from HR by starch (X = 0.375) with H+ dosage of 10.80 mol/kg-COPR at 453.15 K for 120 min prove effective in meeting the TCLP regulatory limits of China Environmental Protection Agency (CEPA), and all the Cr(VI)-bearing minerals in COPR are completely destabilized. Compared with the room-temperature hydro-remediation (RTHR), reduction of Cr(VI) in COPR is complete by HR process, and the consumed dosage of starch is much less. Results also suggest that brownmillerite is the last completely destabilized Cr(VI)-bearing mineral and its complete destabilization is the key to the full success of the remediation of COPR.

Research highlights▶ Hydrothermal remediation of COPR was presented for the first time. ▶ Hydrothermal reduction of Cr(VI) in COPR using starch is extremely effective. ▶ The destabilization of brownmillerite is the key to the full remediation of COPR. ▶ The reducing agent of starch is abundant, cheap and renewable.