Understanding the atomic-scaled evolution mechanism of V and Cr in V slag with high Cr content during magnesiation roasting toward tailing toxicity minimization

V slag with high Cr content (VS-HCC) serves as the primary raw material for extracting V, and the evolution mechanism of V and Cr in VS-HCC decides the eco-friendliness of V extraction process. To minimize the Cr(VI) and residual V(V) content in tailings produced after V extraction, this work introduces the "atomic atmosphere" method to investigate the evolution mechanism of V and Cr in VS-HCC during magnesiation roasting at the atomic scale. In the early stage of roasting at 900℃, the FeV₂O₄ and FeCr₂O₄ spinel disappear and are converted into Fe₂O₃•V₂O₃ and (Fe_0.6Cr_0.4)₂O₃, with the chemical states of V and Cr unchanged; Fe₂TiO₄ spinels are oxidized to generate FeTi₂O₅. After roasting for 10 min, Fe₂SiO₄ structure is completely destroyed. Both the conversion of spinels and the complete decomposition of Fe₂SiO₄ have created favorable conditions for the oxidation of V-bearing phases. After roasting for 50–90 min, V atoms transitioned from the mixed valence states (V^4 +/V⁵⁺) to V⁵⁺. This transformation involved the combination of Mg²⁺, O^2- V⁵⁺and Mn²⁺ ions, resulting in the formation of Mg₂V₂O₇ and Mn₂V₂O₇ crystals. Meanwhile, Cr atoms still exist as nontoxic Cr³⁺ in the form of (Fe_0.6Cr_0.4)₂O₃, ensuring the eco-friendliness of the magnesiation roasting of VS-HCC.