Two-Stage Sulfuric Acid Leaching of Vanadium-Bearing Carbonaceous Siliceous Ore: Oxidative Pressure Leaching, Kinetics, and Mechanism

Efficient vanadium recovery from carbonaceous siliceous ores is challenging because of the complex mineral occurrence of vanadium and the high chemical resistance of the quartz-rich matrix. In this study, a two-stage processing scheme comprising atmospheric decarbonization–neutralization followed by oxidative autoclave leaching was developed for vanadium-bearing quartzite from the Balasausqandiq deposit (Kyzylorda Region, Kazakhstan). Under the optimized autoclave conditions (140 °C, 120 min, 150 kg t−1 H2SO4, 5 kg t−1 HNO3, 3 atm O2, and a solid-to-liquid ratio of 1:1), 88.4% of vanadium was extracted. XRD and SEM–EDS analyses revealed preferential dissolution of reactive V- and Fe-bearing minor phases, while the quartz-rich matrix remained largely preserved, producing a residue containing 72.5 wt.% SiO2 and <0.01 wt.% carbon. Among the shrinking-core models tested, the product-layer diffusion model provided the closest fit to the available kinetic data (R2 = 0.9787–0.9865). A three-temperature Arrhenius analysis over 120–140 °C yielded a preliminary apparent activation energy of 54.55 kJ mol−1. Thermodynamic calculations supported the oxidative pathway, showing that V(IV) oxidation by O2 is favorable at 140 °C (ΔrG° = −75.1 kJ mol−1). The combined results indicate selective acid–oxidative dissolution of accessible vanadium-bearing phases followed by increasingly restricted extraction from resistant mineral domains within the silica-rich residual matrix. The proposed process integrates efficient vanadium recovery with the generation of a silica-rich residue with potential for use as a secondary metallurgical feedstock, subject to further evaluation.