Selective sulfidation of metal compounds

There is urgent, unprecedented demand for critical by-product and co-product metallic elements for the infrastructure (magnets, batteries, catalysts and electronics) needed to power society with renewable electricity(1-3). However, the extraction of d-block and f-block metals from mineral and recycled streams is thermodynamically difficult, typically requiring complete dissolution of the materials, followed by liquid-liquid separation using metal-ion complexing or chelating behaviour(4,5). The similar electronic structure of these metals results in poor separation factors, necessitating immense energy, water and chemicals consumption(6-8). Here a metal-processing approach based on selective anion exchange is proposed. Several simple process levers (gas partial pressure, gas flowrate and carbon addition) are demonstrated to selectively sulfidize a target metal from a mixed metal-oxide feed. The physical and chemical differences between the sulfide and oxide compounds (for example, density, magnetic susceptibility and surface chemistry) can then be exploited for vastly improved separation compared with liquid-liquid methods(9). The process conditions of sulfidation are provided for 56 elements and demonstrated for 15 of them. An assessment of the environmental and economic impacts suggests a path towards 60-90% reductions in greenhouse gas emissions while offering substantial capital cost savings compared with liquid-liquid hydrometallurgy.

Automated summary

This study proposes a metal-processing approach based on selective anion exchange to achieve efficient separation of metallic elements. By manipulating process control parameters, the target metal can be selectively sulfidized from a mixed metal-oxide feed, and the differences between sulfide and oxide compounds can be exploited for improved separation. The results of this study indicate significant reductions in energy and chemical consumption, as well as positive environmental and economic impacts.