A novel reductive alkali roasting of chromite ores for carcinogen-free Cr⁶⁺-ion extraction of chromium oxide (Cr<inf>2</inf>O<inf>3</inf>) ¿ A clean route to chromium product manufacturing!

© 2020 Elsevier B.V.A novel reduction reaction for extracting Cr2O3 from chromite ores is demonstrated by excluding the formation of carcinogenic chromate (Cr6+) intermediates. We have investigated in detail the underpinning high-temperature reduction reaction: FeCr2O4+Na2CO3+2[C]=[Fe]+Na2Cr2O4+3CO(g), which defines the process chemistry for the formation of sodium chromite (Na2CrO2) as an intermediate product for Cr2O3 extraction. After high-temperature reduction, the magnetic separation, aqueous and acid leaching of reaction products yielded 81 wt% and 70 wt% pure Cr2O3 from low (~4 wt%) and high (>8 wt%) silica-containing chromite ores, respectively. The process diagram explains the extraction of Cr2O3, Fe-Cr alloy, Al2O3, and MgO-Al2O3-silicate, reuse of CO2 for Na2CO3 recovery, and energy generation from CO combustion for demonstrating Cr6+-free extraction of metallic and mineral values from chromite ores. The process chemistry demonstrates the extraction of 75¿80 % pure Cr2O3 from NaCrO2 by leaching with 0.05¿0.5 M dilute H2SO4 in controlled pH conditions. The detailed chemical analysis of leachates after Cr2O3 extraction shows that the acid leachates with residual concentrations of ~150 ppm Cr3+-ions can be recycled in situ for reusing water, for eliminating the risk of Cr6+-ion formation from atmospheric oxidation. The novel extraction route may be able to displace the current oxidative process for chromite ore processing by retrofitting.