High-efficiency recycling of Mo and Ni from spent HDS catalysts: Enhanced oxidation with O2-rich roasting and selective separation with organic acid leaching- complexation extraction

Spent petroleum refining catalyst is regarded as the important secondary resource for valuable metals. However, common recycling strategies, including soda roasting, acid and alkaline solutions leaching and chemically precipitation, produced large quantities of high salinity wastewater. This study proposed an efficient method to recovery of Mo and Ni from the spent hydrodesulfurization (HDS) catalyst via O₂-rich roasting and organic acid leaching with the advantage of less salinity wastewater production. The transformation of Mo(IV) into soluble Mo(VI) was enhanced by O₂-rich atmosphere roasting, and 98.64% of Mo(IV) was oxidized at 650 ℃ for 2 h in atmosphere containing 30% of O₂. The oxidation process of Mo(IV) was agreed with the shrinkage pore model, and regulated by surface reaction and internal diffusion. 97.97% of Mo(VI) was leached from roasted product by oxalic acid, separated with complexation extraction agent of Ala-TBP and recovered as (NH₄)₈Mo₁₀O₃₄ and (NH₄)₂Mo₃O₁₀ by evaporative crystallization. Ni was leached out from spent catalyst with 1 mol/L acetic acid, and precipitated as NiC₂O₄ with oxalic acid. 95.92% of Mo and 96.77% of Ni were recovered from spent HDS catalyst with this recycling route. This study provided a high-efficient and eco-friendly method to recovery of valuable metals from spent catalyst.