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 deg 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 deg 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 deg 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 ~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 (~0.10 wt.%). (author)