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 ºC) 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 25-60oC for 1-4 h. The amount of insoluble matter was the lowest (< 1 wt.% of the initial mass) after leaching for 2 h at 60oC 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.%).

This work details the chemical characterization of four NiMo/Al2O3 hydroprocessing generation catalysts (1979-1986, 1993-1998, 2003-2008 and 2008-2011) employed to perform hydrodesulfurization (HDS) of diesel fractions in the same hydrotreater of a Brazilian refinery. The basic differences are the quality of the feedstocks and the HDS experimental conditions. Samples were characterized by X-ray fluorescence, X-ray diffraction, 13C CP-MAS NMR, elemental analysis, loss of volatiles, specific surface area and average pore volume. The amount and variety of foreign elements deposited on the catalyst increased in the most recent generations due to the higher amounts of impurities in the processed feedstocks. Coke became more aromatic and loss of textural properties was more prominent in these samples as expected from the more drastic HDS conditions and the quality of the crude diesel. On the other hand, loss of volatiles was lower due to the low carbon content, partial oxidation of sulfur to sulfate species and oxidation of some elements deposited on the catalyst. Coke tended to ignite as it became more aromatic. For this reason, leaching of decoked samples from the most recent generations by sulfuric acid presented very low yields due to the formation of refractory oxides that are insoluble in the leachant.