Hydrometallurgical recovery of high-purity molybdenum from spent iron-molybdate catalysts via ammoniacal leaching and anti-solvent crystallization.

Abstract

Molybdenum is a critical raw material for advanced alloys and the chemical industry; however, its supply is increasingly constrained by geopolitical and resource limitations. An efficient recycling of molybdenum from secondary resources is therefore essential to mitigate the supply risk and promote maximum utilization of resources. In this study, a low-energy hydrometallurgical process is proposed for the recovery of molybdenum from spent iron-molybdate catalysts, integrating ammoniacal leaching with anti-solvent crystallization. Conventional leaching using ammonia solution alone achieved less than 90% efficiency and required a high ammonia concentration of 5.0 mol/L. This consumption was significantly reduced to only 2.0 mol/L by employing an ammonia-ammonium solution. The required NH4OH-(NH4)2SO4 ratio was optimized to achieve selective molybdenum dissolution that resulted in 92% leaching efficiency while limiting iron co-dissolution to below 5%. The residual iron, governed by the quasi-equilibrium of iron pentaamine complexation, was effectively supressed through controlled settling of the leachate. Subsequently, ethanol-assisted anti-solvent crystallization, performed at an aqueous-to-organic volumetric ratio of 1:2, enabled the direct recovery of high-purity (NH4)6Mo7O24(H2O)4 with 95% yield under ambient conditions. This work demonstrates a scalable and resource-efficient strategy for molybdenum recovery through integrating the selective leaching approach with low-energy crystallization technique.

Keywords: Ammonia-ammonium leaching; Ammonium heptamolybdate; Metal recycling; Quasi-equilibrium of iron-ammine; Spent catalysts.