Recovery of precious metals from spent Mo–Co–Ni/Al2O3 catalyst in organic acid medium: Process optimization and kinetic studies

In present study, the leaching kinetics of the spent Mo–Co–Ni/Al₂O₃ catalyst was investigated in the presence of formic acid as an organic leaching agent. Firstly, the spent catalyst was roasted in different roasting temperature (200–700?°C) and time (15–240?min), the maximum metal extraction was achieved that at 500?°C with 90?min. Then, the leaching experiments were carried out to determine the influences of process parameters following; particle size, liquid/solid ratio, formic acid concentration, leaching temperature, leaching time and stirring speed. According to the experimental results, the highest dissolution rates of molybdenum (Mo, 75.82%), cobalt (Co, 96.81%), nickel (Ni, 93.44%) and aluminum (Al, 19.46%) were reached under optimum experimental conditions; particle size +75???30?µm; liquid/solid ratio 10?ml/g; formic acid concentration 0.6?M; leaching temperature 80?°C; leaching time 90?min and stirring speed 300 r/min. Moreover, the leaching kinetics clearly reveal that the leaching reaction is controlled by liquid film diffusion and that the activation energy values (E_a) of Co, Ni, Mo and Al were to be 24.49, 25.98, 32.36 and 33.47?kJ/mol, respectively. In conclusion, the leaching process can be conducted in the presence of formic acid for the various industrial wastes in similar structure and composition to Mo–Co–Ni/Al₂O₃ spent catalyst.