Optimized two-step sulfuric acid leaching for sustainable recovery of valuable metals from hazardous sodium carbonate roasting–leaching residues of spent hydrodesulfurization catalysts†

Sodium carbonate (Na₂CO₃) roasting–leaching residues from spent hydrodesulfurization (HDS) catalysts remain hazardous and are often neglected in existing research. This study introduces an optimized two-stage sulfuric acid (H₂SO₄) leaching strategy, combined with nitric acid (HNO₃) as an oxidant, to efficiently extract valuable metals and isolate insoluble α-Al₂O₃ (accounting for about 20% of the total Al recovered). The acidic leachate from the first stage was directly reused in a second leaching step, maximizing acid reuse and resource efficiency. Under optimized conditions, Mo leaching reached 97.5% and Co leaching reached 96.6%, while approximately 80% of Al was selectively recovered as potassium alum (KAl(SO₄)₂·12H₂O). This approach not only addresses the treatment of Na₂CO₃ roasting–leaching residues but also transforms hazardous waste into valuable resources, advancing environmental sustainability.