Mechanism of Mechanical Activation-Assisted Selective Decalcification of Vanadium-Bearing Steel Slag

Selective decalcification of vanadium-bearing steel slag via hydrochloric acid pretreatment was investigated in this work. The influences of hydrochloric acid concentration, temperature, reaction time, and liquid-to-solid ratio on Ca and V leaching from mechanically activated and non-activated slag are evaluated. The results show that mechanical activation significantly enhances decalcification selectivity. At an HCl concentration of 1.5 mol·L−1, 40 °C, 8 min, and a liquid-to-solid ratio of 7 mL·g−1, the Ca extraction reached 77.10%, while the V loss rate remains at only 6.08%, resulting in a separation index (SI) of 12.7, significantly higher than that of the non-activated slag (SI = 3.9). XRD and SEM-EDS analyses show the disappearance of Ca2SiO4, CaO, and Fe3O4 after decalcification, accompanied by the formation of a loose and porous surface structure. Ca2SiO4 preferentially dissolves due to its porous structure, whereas Ca2Fe2O5 remains because of its dense layered structure. Furthermore, the lower bond energy of Ca-O than V-O facilitates selective decalcification. ICP analysis shows that the CaO content decreases from 40.83% to 19.87%, while the V2O5 content increases from 2.19% to 2.49%, resulting in vanadium enrichment. Kinetic analysis suggests a shrinking-core model for the decalcification process, with the reaction being predominantly governed by surface chemical control. The apparent reaction order and activation energy are 0.73 and 20.28 kJ·mol−1 respectively. These findings provide theoretical support for selective calcium removal and vanadium enrichment from vanadium-bearing steel slag.