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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 electricity1,2,3. 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 behaviour4,5. The similar electronic structure of these metals results in poor separation factors, necessitating immense energy, water and chemicals consumption6,7,8. 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 methods9. 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.

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Fig. 1: The S2/SO2 ratio for selective sulfidation of metals and the corresponding gaseous space time, space velocity and CDSR levers.
Fig. 2: Application of selective sulfidation for LIB recycling, rare-earth magnet recycling and rare-earth mineral processing.
Fig. 3: Capital costs and environmental impact estimates for selective sulfidation coupled with physical separation, compared with conventional hydrometallurgical processing.

Data availability

All data relevant to the results presented are included herein, within the Supplementary Information or available from the corresponding author upon request.

Code availability

Spreadsheets and code supporting technoeconomic analysis and life cycle assessment are available on the Harvard Dataverse repository at


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We thank the US Department of Energy and the US National Science Foundation for their financial support; H. Higuchi and Sumitomo Metal Mining for providing samples of scandium oxide; and K. Daehn, A. Culbertson, T. Close, L. Rush, A. Caldwell and M. E. Wagner for their insight.

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C.S. and A.A. designed the project, wrote the manuscript and prepared the figures. C.S. carried out the experiments, modelling and analysis.

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Correspondence to Antoine Allanore.

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Extended data figures and tables

Extended Data Fig. 1 Gas species partial pressures as thermodynamic levers to control compound stability.

a, Sc-O-S Kellogg diagram (T = 1,000 °C, 1 atm), illustrating the role of \({P}_{{{\rm{S}}}_{2}}\), \({P}_{{{\rm{O}}}_{2}}\) and \({P}_{{{\rm{SO}}}_{2}}\) on scandium compound stability. b, Fe–O–S–C predominance diagram, illustrating the role of PCO, \({P}_{{{\rm{CO}}}_{2}}\), \({P}_{{{\rm{S}}}_{2}}\) and \({P}_{{{\rm{SO}}}_{2}}\) on iron compound stability (\({P}_{{{\rm{S}}}_{2}}\) fixed at 0.05 atm). The sulfide becomes the only stable compound of Fe in the presence of carbon at increasing \({P}_{{{\rm{S}}}_{2}}\) values, as for many critical metals including Co, In, Mn, Ni, Sn, W and Zn. In a, b, solid lines correspond to phase domains, dotted lines correspond to equilibrium gas compositions at 1 atm total pressure and dashed lines correspond to \({P}_{{{\rm{SO}}}_{2}}\).

Extended Data Fig. 2 Reaction and solution contributions to sulfidation selectivity.

a, b, (See Supplementary equations (1), (2), (3), (4), (5), (6) and (7) in Supplementary Information). When \({\psi }_{rxn}\gg {\psi }_{sol}\), the sulfidation thermodynamics are reaction-dominated, solution effects are minimal and the sulfidation thermodynamics are well described by those of the pure compound. When \({\psi }_{sol}\gg {\psi }_{rxn}\), the sulfidation thermodynamics are solution-dominated and reaction effects are minimal, the sulfidation thermodynamics are not well described by those of the pure compound, and knowledge of the solution behaviour is essential to determine sulfidation spontaneity. c, d, Equilibrium S2/SO2 ratio as a function of sulfur partial pressure and temperature at 1 atm for a gas consisting exclusively of sulfur–oxygen-containing species at equilibrium. As derived in the Supplementary Information, the equilibrium \({P}_{{{\rm{S}}}_{2}}\) / \({P}_{{{\rm{SO}}}_{2}}\) ratio that satisfies \(\psi \) corresponds to (\({P}_{{{\rm{S}}}_{2}}\)/\({P}_{{{\rm{SO}}}_{2}}\))crit.

Extended Data Fig. 3 Experimental devices used for sulfidation and recovery.

a, A packed-bed, flow-through reactor (12), is placed in a furnace (1) equipped with an alumina tube (2) to conduct selective sulfidation using elemental sulfur evaporated from the bottom of the tube (11). b, A Hallimond cell is used for the recovery by flotation of the sulfide phases formed in a.

Extended Data Fig. 4 Lanthanum oxide sulfidation reaction kinetics as measured in a graphite packed bed reactor.

a, Conversion versus time as a function of temperature shows reaction rate increasing with temperature, consistent with the notion of thermal activation of the reaction. b, Sulfur partial pressure corresponding to conversion rate in a. c, Modified Sherwood number \((Sh{\prime} )\) for lanthanum oxide sulfidation kinetics experiments. Following Sohn’s criteria65, for \(Sh{\prime} > 30\) external mass transfer limitations to the observed reaction rate are negligible. d, Fluid–solid reaction modulus \((\hat{\sigma })\) for lanthanum oxide sulfidation kinetic experiments. For \({\hat{\sigma }}^{2} < 0.01\), intergrain diffusion limitations to the observed rate of reaction are negligible65. Intragrain diffusion limitations are addressed in the Supplementary Information and Supplementary Table 3. e, Comparison of the kinetically limited (Supplementary Information) rate of oxygen liberation for sulfidation versus sulfur gas concentration. The slope of natural log of the oxygen liberation rate vs the natural log of the sulfur concentration is the reaction order, observed to be approximately first order with respect to sulfur. f, Arrhenius plot of the natural log of the reaction rate constant vs inverse temperature. The activation energy is found to be 114 kJ mol−1, with a pre-exponential factor of 1.08 × 104 s−1.

Extended Data Fig. 5 Selective sulfidation of LiNi1/3Mn1/3Co1/3O2 (abbr. NMC111 or NMC333) separated into Ni-rich sulfide (1), Co-rich sulfide (2) and Mn oxysulfide (3) phases.

a, Optical dark field image showing Ni, Co and Mn-rich phases coalesced to approximately 100–500 μm in size that support physical separation following sulfidation at 1,000 °C. b, SEM–EDS analysis reveals distinct Ni-rich sulfide (1, Ni0.75Co0.25S), Co-rich sulfide (2, Ni0.33Co0.67S) and Mn oxysulfide (3, MnO0.2S0.8) phases. c, SEM–EDS maps illustrate minimal Mn inclusion in Ni–Co phases and vice versa post-sulfidation.

Extended Data Fig. 6 Selective sulfidation of calcined rare-earth, iron, boron ((Nd,Pr,Dy)–Fe–B) magnet separated into an iron-rich sulfide phase (1) with neodymium-rich oxide (2) inclusions.

a, Upon sulfidation, calcined (Nd,Pr,Dy)–Fe–B particles 90–212 μm in size sintered to approximately 1–2 mm in size, with Nd-rich oxide (1) regions approximately 20–100 μm in size that are large enough to support physical separation from the bulk Fe-rich sulfide (2) phases (SEM/BEC image). b, SEM–EDS analysis reveals minimal inclusion of Fe,Dy into the Nd,Pr-rich inclusions and vice versa post-sulfidation.

Extended Data Fig. 7 Sulfidative sintering and selective sulfidation of synthetic defluorinated, dethoriated, light rare-earth element bastnaesite (Ln2O3) separates into neodymium-rich and lanthanum-rich phases.

a, Ln2O3 particles (25–45 μm) sulfidized with carbon in an alumina flow-through packed-bed reactor at 1,400 °C sintered to approximately 100–300 μm in size (darkfield optical image). b, Ln10OS14 (1) and Ln2O2S (2) approximately 20–100 μm in size are observed to form upon sulfidation, large enough to support physical separation (polarized optical image, 90°). c, EPMA/WDS elemental analysis reveals sulfidation is selective, with Nd enriched in the oxygen-rich Ln2O2S phase (2) and La enriched in the sulfur-rich Ln10OS14 phase (1).

Extended Data Fig. 8 Flowsheet of a generic selective sulfidation process.

The process consists of selective sulfidation in a multihearth fluidized bed reactor, product comminution and physical separation via froth flotation, and downstream gas handling and treatment via a cyclone separator for solid particle removal and acid plant for SO2 recovery, and assumes an equimolar, mixed, binary oxide feed. The system boundary for life cycle assessment is depicted, over the impact categories of global warming potential (GWP), terrestrial acidification (TA) and water resource depletion (WRD), for a functional unit of 1 kg of selective sulfidation feed. The impacts of flows originating within the system boundary are evaluated from the cradle to usage in the process, while the impacts of flows originating outside the system boundary are evaluated from the system gate to usage in the process. The impacts of flows exiting within the system boundary are evaluated from outlet of the process to the grave, while flows exiting outside the system boundary are evaluated from production in the process to the system gate.

Extended Data Fig. 9 Capital cost (CAPEX) and operating cost (OPEX) distributions for the generic selective sulfidation process (Extended Data Fig. 8, Supplementary Figs. 13).

Distributions for selective sulfidation with and without feed pretreatments and with and without CDSR are determined via Monte Carlo simulation, with probability distributions for CAPEX, OPEX and operating condition parameters described in Supplementary Tables 14, 15. Pretreatment steps for feed drying, sintering and roasting/calcination each marginally increase the CAPEX and OPEX of selective sulfidation by 10% to 20%. CDSR generally decreases CAPEX at the expense of increases in OPEX and environmental impacts (Extended Data Fig. 10).

Extended Data Fig. 10 Global warming potential (GWP), water resource depletion (WRD) and terrestrial acidification (TA) distributions for the generic selective sulfidation process (Extended Data Fig. 8, Supplementary Figs. 13), with and without feed pretreatments, with and without CDSR, for a functional unit of 1 kg of selective sulfidation feed.

Distributions are determined via Monte Carlo simulation, with probability distribution for operating condition parameters described in Supplementary Tables 14, 15. The inclusion of feed pretreatments increases GWP by about 50%, WRD by 30% and TA by double, while CDSR increases GWP by a factor of 3–5 times and WRD by a factor of 3–4 times. The bimodal nature of the GWP distribution is due to differences in oxygen content of the three model chemistries considered in the Monte Carlo simulation, highlighting the role of system chemistry in determining environmental impact.

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Stinn, C., Allanore, A. Selective sulfidation of metal compounds. Nature 602, 78–83 (2022).

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