Solvent extraction separation of molybdenum as thio-molybdate complex from alkaline tungsten leach liquor of spent HDS catalyst - A pilot study

Spent nickel-tungsten catalyst generated from hydrotreating petroleum industry always contains molybdenum about 0.1-2% as an impurity. Separation of Mo from spent Ni-W catalyst leach solutions is a critical step to produce high pure tungsten products like ammonium paratungstate (APT) and tungsten trioxide (WO3) for catalyst and tools applications. Though Mo and W have chemically similar properties but it is possible to exploit separation of Mo from W by modifying the aqueous phase species. In the present paper, alkaline leach liquor with a composition of 0.72 g/L Mo and 18 g/L W was taken. Selective separation of Mo as its thiocomplex from W was examined by Aliquat 336, a quaternary ammonium compound. Experimental parameters such as extraction pH, concentration of extractant and metal for extraction and selection of reagent and concentration for stripping were optimized to obtain maximum separation of Mo. Variation of extractant concentration indicated extracted Mo species as (R3CH3N)2MoS4. The McCabe-Thiele plots indicated that 2 and 3-counter-current stages are necessary at O:A ratio of 1:1 and 3:1 for quantitative extraction and stripping of molybdenum, respectively. A pilot run for 330 h was operated with 3-stages of extraction, 1-stage of scrubbing, 3-stages of stripping and one stage of washing based on the equilibrium data to verify the performance of Mo separation from W from commercial point of view.