Negligible emissions and highly efficient recovery of Mo, V, and Al from spent hydroprocessing catalysts through alkaline leaching and solvent extraction

Spent petroleum hydroprocessing catalysts (HPCs) are considered hazardous waste due to their high content of heavy metals and organic pollutants, posing a risk of environmental pollution and significant loss of valuable metals if not properly disposed. This study aimed to characterize the spent HPCs using optical microscopy, SEM-EDS, XRF, C & S analyzer, XRD, and XPS. Additionally, the effectiveness of six organic solvents in removing the black oil covering the surface of spent HPCs was investigated. Furthermore, the recovery of Mo, V, and Al from the spent HPCs was examined through stepwise leaching using NaOH + NaClO2 and NaOH, followed by the separation of Mo, V, and Al via selective precipitation of Al and solvent extraction using Aliquat 336. The results indicated that toluene and tetrahydrofuran exhibited the highest efficiency in removing the black oil. Under the optimized oxidizing alkaline leaching conditions (0.09 g/mL NaOH + 0.05 g/mL NaClO2, 90 °C, 1 h), Mo, V, and Al achieved leaching efficiencies of 99.9 %, 99.9 %, and 20.1 %, respectively. Subsequently, Al in the residue was recovered through leaching with NaOH, resulting in a total leaching efficiency of 95 % under the optimized alkaline leaching conditions (0.12 g/mL NaOH, 180 °C, 3 h). The Al in both leaching solutions was precipitated by adjusting the pH to 8.60. For the subsequent Mo/V separation, the best conditions involved either four stages of counter-current extraction or two stages of cross-current extraction using a solution of 21 wt% Aliquat 336 dissolved in a mixture of kerosene and isooctanol (volume ratio 9:1) at a pH of 8.60. Finally, a hydrometallurgical flowchart for the recovery of Mo, V, and Al from spent HPCs was proposed.