Selective Mo Recovery from Spent Hydrodesulfurization (HDS) Mo-Co Catalysts: Comparison of Acidic Leaching and Oxidative Roasting

Two alternative first-stage routes for selective molybdenum recovery from a pretreated spent Co-Mo/Al2O3 hydrodesulfurization catalyst were investigated and compared using the same feedstock: direct sulfuric-acid leaching and oxidative volatilization. The study combined single-factor process tests with XRD, SEM-EDS, XRF, ICP-OES, and particle-size analyses to relate extraction behavior to the distribution of Mo and Co within the catalyst. Direct leaching achieved 98.1% Mo extraction in 2 mol/L H2SO4 at 90 °C for 2 h, while approximately 95% extraction was obtained at only 0.1 mol/L H2SO4 and 40 °C. Co co-extraction remained at approximately 12%, and Al extraction was generally 0.6–0.9%, demonstrating a selective first-stage separation. Water leaching provided an extraction of 51.3% Mo because readily accessible MoO3 forms hydrated aqueous Mo(VI) species. Oxidative roasting removed 92.2% Mo from the coarsely ground material at 1400 °C for 1 h, while fine grinding increased Mo volatilization to 97.8%, whereas no measurable Co volatilization was observed. The two routes therefore offer different advantages: mild leaching lowers thermal demand and produces a Mo-bearing solution, whereas roasting avoids liquid reagents and produces a Mo-bearing vapor that can be collected by controlled condensation. The residual Mo fraction was associated with MoO2 and Mo-bearing regions enclosed by the stable CoAl2O4-Al2O3 matrix. The results establish a comparative basis for selecting an appropriate first-stage Mo-removal route before separate recovery of Co and possible valorization of the alumina-rich residue.