Mechanochemical versus stirred alkaline leaching for sustainable recovery of vanadium and molybdenum from spent Hydrodesulfurization catalysts

This study compares stirred and mechanochemical alkaline leaching methods for recovering vanadium (V) and molybdenum (Mo) from spent hydrodesulfurization (HDS) catalysts. Stirred leaching was optimized using response surface methodology by evaluating the following variables: NaOH concentration (1–3 M), solid content (5–15%), stirring rate (100–300 rpm), and leaching time (1−2 h). Optimal conditions (2.5 M NaOH, 5% solids, 300 rpm, 1 h) achieved 77.7% V and 89.0% Mo dissolution, increasing to 85.1% V and 94.1% Mo after 2 h. Increasing the temperature to 60 °C enhanced recovery by approximately 4%, but significantly increased aluminum co-dissolution, thereby reducing selectivity; particle size reduction had a negligible influence. Mechanochemical leaching, optimized at 2.0 M NaOH, 5% solids, and 60 min milling, outperformed stirred leaching with recoveries of 97.4% V and 94.1% Mo, while lowering NaOH consumption by about 20%. Economic analysis indicated that the mechanochemical leaching process requires a lower capital investment (approximately 93,500 USD for a plant capacity of 5000 tons/year), while also reducing operating costs by about 67 USD/ton and increasing overall revenues by around 485 USD/ton of processed catalyst, despite its higher energy demand. Overall, mechanochemical leaching demonstrates superior technical performance and economic viability. It reduces chemical consumption, minimizes secondary waste, and mitigates environmental impact, representing a sustainable and cost-effective strategy for large-scale recovery of valuable metals from spent HDS catalysts, thereby supporting circular economy objectives in the refining sector.