Microwave-Assisted Sulfuric Acid Leaching of Vanadium from Pre-Decalcified Vanadium-Bearing Steel Slag

Vanadium-bearing steel slag has attracted considerable attention as a secondary source of vanadium. Pre-decalcified vanadium-bearing steel slag served as the feedstock to evaluate vanadium leaching under microwave irradiation and conventional water-bath heating by varying sulfuric acid concentration, temperature, leaching time, and liquid-to-solid ratio. The leaching performance was further evaluated through countercurrent leaching experiments, and kinetic analysis was conducted to investigate the rate-controlling behavior. The results demonstrated that the optimum leaching conditions under microwave irradiation were a liquid-to-solid ratio of 8 mL·g^-1, a leaching time of 60 min, a sulfuric acid concentration of 15%, and a temperature of 70 °C, yielding a vanadium leaching rate of 74.43%. Multistage leachate reuse increased the normalized V enrichment index from 74.43% after the initial single-stage leaching to 89.93% after two reuse stages, with only a limited further increase thereafter. Kinetic analysis indicated a constant-particle-size shrinking-core mechanism for vanadium leaching. Sulfuric acid concentration exhibited an apparent reaction order of 1.80, and the apparent activation energy was 57.20 kJ·mol^-1. Internal diffusion was identified as the predominant rate-controlling step, while the contribution of the interfacial chemical reaction could not be neglected. Microwave irradiation enhanced vanadium extraction through volumetric and selective heating effects, potentially promoting structural disruption of the pre-decalcified slag and facilitating leaching agent diffusion into the particle interior. These findings provide an effective hydrometallurgical strategy for sustainable vanadium recovery from industrial solid waste.