Cleaner extraction of vanadium from vanadium-chromium slag based on MnO2 roasting and manganese recycle

Abstract MnO2 roasting was proposed as a suitable roasting method for vanadium extraction from vanadium-chromium slag (V–Cr slag). Acid leaching and vanadium precipitation were carried out for the recovery of vanadium, then V2O5 was prepared. Manganese in the vanadium wastewater could be precipitated and recycled to the next roasting processes. The effects of roasting temperature and the amount of MnO2 on the phase evolution, along with elements distribution and leaching behaviors were discussed by XRD, SEM and leaching experiments. Two methods of manganese recycle were provided and evaluated. Results show that during MnO2 roasting, the stable vanadium spinel was oxidized and combined with Mn2O3, which was the decomposition product of MnO2 to form the acid-soluble manganese vanadate Mn2V2O7. Chromium, on the other hand, was combined with iron to generate the acid-insoluble (Fe0.6Cr0.4)2O3. After leaching the samples roasted at 850 °C in a molar ratio n(MnO)/n(V2O3) of 1.75 with acid at a pH value of 2.5, 89.48% of vanadium and 81.15% of manganese was leached due to the dissolution of Mn2V2O7, while only 0.15% of chromium and 0.02% of iron could be extracted simultaneously, improving the separation efficiency of vanadium with other impurities in V–Cr slag. V2O5 of 99.14% purity was produced after precipitating vanadium and ammonium vanadate roasting. As the highlight stage during MnO2 roasting, manganese could be recycled by precipitating with CO2 in an alkaline environment or precipitating directly with the addition of NH3·H2O. The products of precipitation were MnCO3 and Mn3O4 respectively, which could be both transformed into Mn2O3 before the combination of vanadium and manganese, which is consistent with the reaction during MnO2 roasting.