Recovery of molybdates from an alkaline leachate of spent hydrodesulphurisation catalyst – proposal of a nearly-closed process

In this work, we developed a simple, effective, low-cost and more environmentally friendly process (with low energy requirements and minimized effluent emissions) to recover molybdenum from an alkaline leaching solution of a spent hydrodesulphurisation catalyst, as a final product with high purity and in high yield. Alkaline leaching of spent Ni–Mo hydrodesulphurisation catalysts results in a solution that mainly contains molybdenum and aluminum. Before recovering molybdenum, previous separation of aluminum from the solution, as a precipitate of aluminum hydroxide, was performed by lowering the pH to 8, at room temperature, with a yield of 99.2%. Recovery of molybdenum was studied by precipitation in the form of a salt of strontium or lead molybdate. The first process allowed a recovery of 96% of Mo at 25 °C, as tetragonal SrMoO4, with high purity (99%). Precipitation with lead at 25 °C resulted in a maximum recovery of 99.8% of the molybdenum, as tetragonal PbMoO4, with a purity of 99.5%. Finally, the destination of the final solution is discussed and a flow sheet considering the production of strontium/lead molybdate is proposed. ► Nearly closed cycle for recycling Mo from an alkaline leaching solution of an HDS spent catalyst. ► Precipitation of aluminum as Al(OH)3, with high yield. ► Effective recovery of molybdenum as SrMoO4 or PbMoO4, with high purity. ► Advantages of the process: (i) limited generation of residues; (ii) use of simple and inexpensive separation technologies.