Investigation of dissolution kinetics of spent co-mo hydrodesulfurization catalyst in sulfuric acid solutions

Hydrodesulfurization (HDS) catalysts are used to remove sulfur from refined petroleum products. HDS catalysts contain Mo, Co, Ni and V compounds on γ-Al2O3 substrate. The catalysts used in the HDS process can be reused through a series of processes called regeneration. However, after several regeneration cycles, it loses its activity and become non-functional. Such catalysts are considered spent catalysts and are classified as hazardous waste because they contain significant amounts of metals, sulfur, and volatile compounds. In recent years, recovery of metallic values such as Mo, Co, Ni etc. from spent HDS catalysts has become important. Considering both economic and environmental conditions, the use of spent catalysts as a secondary source for metal recovery has become important. In this study, the leaching kinetics of spent Co-Mo HDS catalyst in sulfuric acid (H2SO4) solutions was investigated. The effects of stirring speed, temperature, H2SO4 concentration and solid/liquid ratio on the leaching rate were determined. It was reported that the stirring speed, H2SO4 concentration and solid/liquid ratio had no significant effect on the leaching rate, and the temperature was the most important parameter affecting the leaching rate. Spent Co-Mo HDS catalyst was roasted at 773 K for 5 h prior to the leaching experiments for the conversion of sulphide compounds to corresponding oxides and the elimination of volatile petroleum residues. XRD, XRF and SEM-EDS analyses were used for the characterization of the roasted Co-Mo HDS catalyst and leach residues. In the study, solvent extraction process was also applied to separate molybdenum from other metallic compounds in the leach solution. Alamine 304-1 was used as the organic extractant and ammonia (NH3) was used as the stripping solution in the solvent extraction process. The effects of pH, extraction time and stripping time parameters on the solvent extraction of molybdenum were investigated. McCabe – Thiele diagrams were constructed by carrying out experiments at different A/O (aqueous phase/organic phase) ratios. In this way, the optimum A/O ratios and the number of stages required for extraction and stripping processes were determined. The results showed that the optimum pH value for the extraction of molybdenum was 1, and the optimum extraction and stripping times were 10 and 30 minutes, respectively. It was determined using McCabe – Thiele diagrams constructed for the extraction and stripping of molybdenum that 98,5% of molybdenum can be extracted in three stages at A/O = 2/3, while 95% of Mo in the organic phase can be stripped in two stages at A/O = 3/2. Avrami Kinetic Model was found to fit the best for experimental data obtained from leaching reactions of MoO3, CoO, and Al2O3. Kinetic parameters were calculated using this kinetic model and model kinetic equations, which give fractional conversion as a function of time, were derived. Apparent activation energies for the leaching reactions of MoO3, CoO, and Al2O3 compounds were calculated as 34.7, 17.09, and 81.84 kJ mol-1, respectively.