This study deals with the feasibility of the oxidative leaching of molybdenum and cobalt sulphides contained in a spent hydrodesulphurisation catalyst using H2O2 in acidic medium. The study was first carried out with MoS2 in order to define the important parameters of the leaching procedure and then applied to an unroasted CoMo/Al2O3 spent catalyst containing 11 wt % of Mo and 2.7 wt % of Co. In both cases, a 23 factorial design was used. The results obtained with MoS2 highlighted the necessity to limit the increase in temperature of the medium to minimize the decomposition of H2O2 and the necessity to keep pH constant to avoid an important acidification of the medium which could cause dissolution of alumina matrix when leaching will be applied to spent catalysts. The oxidative leaching of the unroasted CoMo/Al2O3 spent catalyst was performed without previous grinding. In a single-step, at pH = 1.3 with a stoichiometric factor equal to 2.4, an L/S ratio equal to 7.5, and an H2O2 concentration of 3.75 mol·L−1, it was possible to recover molybdenum and cobalt simultaneously with leaching yields of 90% and 83%, respectively, without dissolving more than 8% Al. In these conditions, the maximal temperature recorded during the experiment was about 60 °C.

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This paper describes the recovery of molybdenum from a spent CoMo/Al2O3 by soda leaching after a roasting step. First, the leaching was performed on a catalyst industrially roasted at 800 °C during 20 min. In order to reach a molybdenum leaching yield higher than 85%, the process was optimized using a 23 design of experiments with parameters such as NaOH amount, leaching time, and temperature. Unfortunately, in the best leaching conditions defined by the factorial design, the leaching yield of molybdenum never exceeded 60% because of an incomplete roasting of the catalyst proved by the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis of the residue. In a second part, the roasting was investigated on an unroasted catalyst containing 23.9% Al, 11% Mo, 2.9% Co, 7.9% S, and 6.3% C using a central composite design. By changing the thermal treatment conditions (700 °C, 20 min) and performing the best soda leaching conditions (an amount of sodium hydroxide equal to the twice of the required stoichiometry and a leaching time of 4 h at 60 °C), it was possible to extract more than 90% of molybdenum initially present in the catalyst.