Cleaner and efficient vanadium extraction from vanadium slag: optimal adaptation mechanism between calcification roasting and reductive leaching processes

Efficient recovery of critical metal vanadium from vanadium slag, while reducing the use of limestone and lowering the roasting temperature is a pivotal challenge. Here, we report the effect of calcification roasting conditions of vanadium slag on the leaching behaviour of vanadium in a novel H₂SO₄ - (NH₄)₂SO₃·H₂O reductive leaching system. Furthermore, we elucidate for the first time the dissolution behaviour of various phase-pure vanadate compounds formed during roasting, thereby uncovering the adaptation mechanism between calcification roasting and reductive leaching process. The results show that the optimum roasting process adaptation for the reductive leaching system was n(CaCO₃/V₂O₃) = 1, with a roasting temperature of 850 °C for 120 min and a heating rate of 5 °C/min, the vanadium leaching efficiency reached 97.43 %. Compared to the conventional calcification roasting - H₂SO₄ leaching process, vanadium leaching efficiency increased by 8.64 %, whilst CaCO₃ consumption was reduced by 50 %. Even with a 50 °C reduction in roasting temperature, vanadium leaching efficiency remained at 93.75 %. By fully utilizing the inherent Mn and Mg in vanadium slag for vanadate conversion, increased the contribution of Mn and Mg to vanadium extraction by 9.24 % and 2.28 %, respectively, thereby reliance on limestone is greatly reduced, meanwhile the reduction leaching system is perfectly adapted to the vanadate phase transformation. Synergistic action of H₂SO₄ and (NH₄)₂SO₃·H₂O promotes vanadate decomposition and inhibits vanadium hydrolysis, achieving the 100 % dissolution of Ca₂V₂O₇, Mn₂V₂O₇, and Mg₂V₂O₇. This new low calcium roasting - reduction leaching process reduces roasting additive costs and CO₂ emissions by 50 %, and decreases the tailings gypsum production by 50 %, this offer a new path for cleaner utilization of vanadium slag.