Selective recovery of molybdenum from spent iron-molybdate catalysts through NH4OH(NH4)2SO4 leaching and solvent extraction

An integrated hydrometallurgical process was developed for the selective recovery of molybdenum from spent iron-molybdate catalysts generated during formaldehyde production. Ammoniacal buffer leaching was statistically optimized using response surface methodology based on central composite design to evaluate the effects of NH 4 OH:(NH 4 ) 2 SO 4 molar ratio, total ammonia concentration, pulp density, temperature, and time on molybdenum dissolution. The developed quadratic model demonstrated strong predictive capability and identified the ammonia-ammonium ratio as the most influential parameter. The kinetic analysis revealed that leaching proceeds through a multi-step mechanism involving NH 3 -NH 4 + coupled solution equilibria, surface reactions, and interfacial transport. Under optimized conditions, 98.3% molybdenum leaching was experimentally achieved with only 2.1% iron co-dissolution in ammoniacal solution. The pregnant leach solution was directly treated by solvent extraction using protonated tri- n -octylamine modified with tributyl phosphate in kerosene. Selective extraction of 96% molybdenum was achieved at pH 5.4 without measurable iron co-extraction. Two-stage counter-current extraction enabled an overall extraction efficiency of 99.96%, demonstrating strong potential for continuous and scalable operation. Quantitative stripping with NaOH solution produced a molybdenum-rich solution (~103 g·L −1 Mo), from which high-purity sodium molybdate (Na 2 MoO 4 .2H 2 O) crystals were recovered by evaporative crystallization. The integrated process combines NH 4 OH (NH 4 ) 2 SO 4 leaching, liquid-liquid separation, and crystallization techniques into an efficient hydrometallurgical route for recovering molybdenum from spent Fe Mo catalysts and provides a promising strategy for the efficient utilization of secondary molybdenum resources.