Leaching and Preparation of Co-Mo Sulfide/Oxide Nanoparticles from Spent Catalyst

Acid leaching of spent molybdenum (Mo) catalyst followed by a simple pH-selective precipitation route for recovery of Co-Mo sulfide/oxide complexes in high yield has been investigated. The effect of leaching parameters such as temperature, pulp density, particle size, time, and acid concentration on the dissolution efficiency of aforesaid metal ion was studied, revealing that the Mo and Co dissolution increased with increase in the acid concentration, temperature, and contact time but increase in the particle size led to the opposite effect on the metal ion dissolution. The optimized leaching parameters, which resulted in dissolution of 95% Mo along with 53% Co under the size fraction of 90 + 75 µ m, were 20% pulp density, 3% H 2 SO 4 concentration (V/V), 80°C, and contact time of 3 h. Precipitation experiments were carried out on mixed Co-Mo sulfide/oxide nanoparticles at different pH values, adjusted using Na 2 S or NaOH. The Co-Mo sulfide nanoflakes obtained at the optimum pH were annealed and their physicochemical properties characterized. The feasibility of precipitation of mixed Co-Mo oxide material by pH adjustment was also shown. At low pH, chemically bound Co-Mo oxide with crystalline nature was observed, whereas at high pH, physically entrapped Mo phase on Co(OH) 2 was detected. Scanning electron microscopy (SEM) of the precipitate obtained at pH 5 revealed nanofiber crystalline grains, whereas the precipitate obtained at pH 7 showed gel-like nature with aggregate morphology, and at pH 12, the precipitate was formed of denser amorphous aggregates with larger particles. A flow sheet based on the optimal results for preparation of Co-Mo sulfides or oxides is presented and discussed.