Abstract
The carbothermic reduction of iron ores in blast furnaces emits approximately 3–4 Gt of CO₂ annually, corresponding to 6–10% of global emissions. Low-temperature alkaline electrolysis (LTAE), developed under the ArcelorMittal Volteron (formerly SIDERWIN) program, offers a carbon-free alternative by directly producing metallic iron from aqueous electrolysis of iron oxide suspensions in concentrated caustic electrolytes at ~110 °C. The direct cathodic conversion of iron oxide suspended particles to metallic iron is a complex electrowinning process with several questions as to the underlying mechanism and the effects of key mineralogical and process parameters governing performance in terms of current deposition, Faradaic efficiency, metal yield and purity. The highlights from a laboratory scale investigation examining various parameters with emphasis on the effects of iron oxide particle size and mineral type will be presented. Electrolysis experiments were conducted using 10 wt.% suspensions of hematite, magnetite and goethite minerals (both synthetic minerals and natural ore concentrates) in 50 wt.% NaOH at 110 °C. Feedstocks were comprehensively characterized in terms of particle size distribution and specific surface area, as well as phase and elemental composition. Among the tested high-purity iron oxides, hematite exhibited the highest Faradaic efficiency (up to ~88%), followed by goethite (~82%) and magnetite (up to ~77%), while maintaining industrially relevant current densities (~700-1000 A.m-2) at cell voltages near 1.6 V. The lower efficiency observed for magnetite is attributed to enhanced hydrogen evolution, likely promoted by its catalytic properties. Particle size was identified as a critical parameter controlling effective cathode–particle interactions and, consequently, overall electrolysis performance. Furthermore, process optimization through adjustment of electrolysis duration and current density improved both metallic iron purity and Faradaic efficiency. Iron ore feedstock modification via grinding (particle size distribution control) and magnetic separation (mineralogical adjustment) further enhanced the electrolytic processing of complex iron ore concentrates. These results provide insight into the interplay between mineralogy, particle-scale phenomena and electrochemical conditions in LTAE, and establish pathways for improving the efficiency of carbon-free ironmaking and controlling metal deposition characteristics.