A novel reductive alkali roasting of chromite ores for carcinogen-free Cr6+-ion extraction of chromium oxide (Cr2O3) – A clean route to chromium product manufacturing!

Highlights: • The reduction of chromite ores with alkali eliminates the risk of carcinogenic Cr⁶⁺- ion production during the extraction of chroimum oxide and Fe-Cr-alloy. The alumina is also extracted as a by-product of the process. • The CO generated in the process is partly utilized for energy generation by combustion via CO+0.5O₂ = CO₂ reaction. Less than 10 % of CO₂ may be sequestrated for alumina recovery. • There is no solid and water waste in the process which contains carcinogenic Cr⁶⁺- ions. 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.