PROCESSING OF SPENT ULTRA-DEEP HYDRODESULFURIZATION CATALYSTS

This work discusses the recycling of spent NiMo/Al2O3 catalysts from ultra-deep hydrodesulfurization (HDS) units. They contain a refractory coke and fouling elements from the processed feedstocks. Samples were preoxidized (600 degrees C) for 12 h using a very slow heating rate to eliminate coke and volatiles. Experiments were performed in the presence of HF + H2O2 mixtures under stirring at 25-60 degrees C for 1-4 h. The amount of insoluble matter was the lowest (< 1 wt.% of the initial mass) after leaching for 2 h at 60 degrees C using an excess of 50 vol.% HF and 20 vol.% H2O2. Leached molybdenum and vanadium were extracted together with Alamine 304 (tri-dodecylamine) in n-heptane at pH below 2. Aluminum and iron were precipitated from the raffinate as Na3AlF6 (+ Na3FeF6) by adjusting pH at similar to 4 using 6 mol L-1 NaOH. Nickel was either recovered by solvent extraction using D2EHPA (bis-(2-ethylhexyl)phosphoric acid) in n-heptane at pH 6 or precipitated at pH 8 using 6 mol L-1 NaOH. The first method proved better as less contaminants were present. After slow evaporation of the final aqueous solution, a mixture of Na2SiF6 and NaF was recovered. Fluoride losses were very low (similar to 0.10 wt.%).