Electrochemical Reduction Processes in Molten Salts

Electrochemical reduction in molten salts has emerged as a transformative route for the extraction and refinement of metals and semiconductors. By dissolving metal oxides, sulfides or other precursor materials in high-temperature ionic media, oxide or chalcogenide anions migrate to the anode while metal cations undergo cathodic polarisation, yielding pure metal or alloy phases at temperatures typically between 600 °C and 1 200 °C. This approach bypasses multi-step thermochemical routes, offers near-net-shape capabilities, and can dramatically lower CO₂ emissions in sectors such as titanium, iron and aluminium production. Key developments include the Fray-Farthing-Chen Cambridge (FFC) process for titanium, direct electro-desulfurisation of transition-metal sulfides, molten oxide electrolysis for steel decarbonisation and solid oxide membrane techniques for light metals. Ongoing challenges centre on electrode–electrolyte interactions, inert-anode performance, faradaic efficiency and management of oxyhalide or perovskite-type by-products. Collectively, these advances point towards sustainable metals manufacturing, circular-economy recycling and the potential realisation of zero-emission smelting at industrial scale.

Research from Nature Portfolio

Recent studies have introduced a high-throughput chemical–electrochemical hybrid method to strip dissolved oxygen from molten titanium by exploiting the strong affinity of rare-earth metals for oxygen. Formation of mixed oxyhalides effectively reduces dissolved oxygen to below 0.1 mass%, thus enabling direct conversion of titanium oxide feeds into structural-grade titanium without intermediate chlorination steps. This technique also supports the upcycling of high-oxygen-content scrap, offering a unified refining, melting and casting workflow.

A seminal proof-of-principle investigation demonstrated direct electrolysis of a molten semiconductor (stibnite) by introducing an immiscible secondary molten salt above the catholyte. Liquid antimony collects beneath the cathodically polarised antimony sulfide while sulphur vapour evolves cleanly at a carbon anode. This single-step process avoids CO₂ and SO₂ emissions, reduces energy consumption and establishes a platform for sustainable extraction of electronically conductive chalcogenides.

Electrochemical Reduction Processes in Molten Salts publication trend

The graph below shows the total number of articles in electrochemical reduction processes in molten salts across all publications each year (not limited to Nature Index journals).

Technical terms

Molten salt electrolyte: A high-temperature ionic medium, typically composed of halide or oxide salts, used to dissolve metal precursors and conduct ionic species during electrolysis.

Cathodic polarisation: The shift of electrode potential to more negative values to drive reduction of dissolved cations into solid or liquid metal at the cathode.

Oxyhalide: A mixed anion compound containing both oxide and halide ions, formed during chemical or electrochemical deoxygenation processes.

Perovskitization: The transformation of oxide surfaces into perovskite-type crystal structures under high-temperature conditions, which can impede ion transport.

Faradaic efficiency: The fraction of total electrical current that contributes to the desired electrochemical reaction, as opposed to side reactions or heat loss.

References

1. Direct production of low-oxygen-concentration titanium from molten titanium. Nature Communications (2024).
2. Electrolysis of iron in a molten oxide electrolyte. Journal of Applied Electrochemistry (2018).
3. Development of the Fray-Farthing-Chen Cambridge Process: Towards the Sustainable Production of Titanium and Its Alloys. JOM (2017).
4. Electrolysis of a molten semiconductor. Nature Communications (2016).
5. Interactions of molten salts with cathode products in the FFC Cambridge Process. International Journal of Minerals, Metallurgy and Materials (2020).
6. Clean Metals Production by Solid Oxide Membrane Electrolysis Process. Journal of Sustainable Metallurgy (2016).
7. Near-Net-Shape Production of Hollow Titanium Alloy Components via Electrochemical Reduction of Metal Oxide Precursors in Molten Salts. Metallurgical and Materials Transactions B (2013).
8. Direct reduction of synthetic rutile using the FFC process to produce low-cost novel titanium alloys. Journal of Materials Science (2016).
9. Electrolysis of metal oxides in MgCl 2 based molten salts with an inert graphite anode. Faraday Discussions (2016).