Vanadium Extraction by Acid Leaching from Vanadium Slag Produced by Microwave-Assisted Calcification Roasting: Leaching Behavior and Optimization

Vanadium slag is an important secondary vanadium resource. Although microwave-assisted calcification roasting improves the leachability of vanadium-bearing phases, further extraction can still be limited during acid leaching, making optimization of the leaching process essential for efficient vanadium recovery. Using this slag, leaching was evaluated at different temperatures, times, liquid-to-solid ratios (L/S), sulfuric acid concentrations, and agitation speeds. A Box–Behnken design (BBD) was used to optimize leaching parameters within the selected ranges. Residue phase composition and microstructure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). Leaching efficiency increased with temperature, L/S, and acid concentration, but plateaued above 60 °C, 6 mL·g−1, and 14 wt.%, respectively; increases beyond 50 min or 200 rpm gave marginal improvements. Analysis of variance (ANOVA) of the BBD model ranked the statistical effects of the four linear terms within the investigated BBD range as sulfuric acid concentration > L/S > leaching time > temperature. Within the selected BBD parameter ranges, optimization yielded 64.95 °C, 55.21 min, 6.56 mL·g−1, and 15.17 wt.% sulfuric acid, with agitation fixed at 200 rpm. Validation gave an average leaching efficiency of 92.92%, with a relative error of 0.205% compared with the model prediction. After leaching, Mn2V2O7 was undetected. The residue mainly contained irregular particles, 10–30 μm acicular or plate-like CaSO4·2H2O crystals, and minor residual vanadium-bearing CrVO3 and CaVH2Si4O12 phases. Surface CaSO4·2H2O deposition and refractory-phase encapsulation of vanadium-bearing constituents increased mass-transfer resistance and limited further leaching. This study clarified the relative effects of the investigated leaching conditions on vanadium leaching efficiency within the design range and the interactions among these conditions, and provided microstructural evidence related to the factors limiting further vanadium leaching, thereby providing theoretical guidance for the efficient extraction of vanadium from vanadium slag.