Effective Chromium Extraction from Chromium-containing Vanadium Slag by Sodium Roasting and Water Leaching

Abstract

Chromium (Cr) was concentrated as chromium spinel in the vanadium slag produced by pre-oxidizing vanadium-containing hot metal in Pan-steel in China. Due to the chromium-rich Hongge ores used, chromium concentration in this slag was much higher than that in common vanadium slags. To propose a process for effective Cr extraction from the slag, mechanisms of Cr oxidation in Pan-steel vanadium slag under sodium roasting conditions were studied experimentally and with thermodynamic simulations. Chromium slags without vanadium were synthesized in the laboratory to mimick mineralogical characteristics of Pan-steel vanadium slag and characterized by XRD, SEM/EDS and TG-DSC techniques. Conditions for Cr extraction from the chromium slag after sodium roasting by leaching with water were studied and optimized. Results showed that Cr spinel was encapsulated in the silicate phase. During sodium roasting, Cr spinel was oxidized and decomposed after oxidation and decomposition of the olivine phase. Sodium carbonate began to react with chromium oxide, which was produced by decomposition of Cr spinel, at 537°C and large amount of sodium chromate appeared after 800°C. However, multi-component liquid phase (mixture of Na₂Cr₂O₄, Na₂CrO₄ and Na₂CO₃) was produced after 1000°C, which suppressed the conversion of Cr(III) to Cr(VI) by inhibiting oxygen supply. A conversion about 90% was obtained when the chromium slag was roasted at 1000°C for 2 h with Na₂CO₃ addition in stoichiometric ratio to total amount of Cr₂O₃ and SiO₂. 96% of Cr in the leaching liquid was recovered as chromium sesquioxide in fine powder form and high purity by reducing and precipitating.