Eco-friendly two-step leaching of molybdenum and nickel from spent hydrodesulfurization catalysts using glycine and alkaline solutions

The recycling of spent hydrodesulfurization (HDS) catalysts has gained significant attention due to the valuable metals they contain, such as molybdenum and nickel. Effective recycling methods not only offer economic benefits but also reduce environmental impacts. This study proposes a two-step eco-friendly approach to extract Mo and Ni from spent HDS catalysts using an alkali solution and glycine as a green reagent. In the first step, 85.7 % of Mo was leached using 0.15 M NaOH (pH 10.5, S/L 2:100, 24h, room temperature). In the second step, 95.8 % of Ni was leached using an alkaline glycine solution. The optimal conditions for Ni leaching were determined to be pH 11, H₂O₂ concentration of 1 %, glycine concentration of 2M, solid/liquid ratio of 1:100, leaching duration of 4 h, and a temperature of 85 °C. Inductively coupled plasma optical emission spectrometer analysis confirmed the selective leaching of Mo and Ni through this two-step process. The findings demonstrate that this method is an effective strategy for recycling spent Ni-Mo/Al₂O₃ catalysts. The use of glycine in the presence of peroxide not only achieves high leaching efficiency but also minimizes the use of inorganic acids, presenting a novel approach for greener recovery of spent catalysts in the future.