CO2-free alkaline electrolysis of magnetite suspensions: impact of mineralogical source and particle size

Low-temperature alkaline electrolysis of iron oxide suspensions represents a promising carbon-free alternative to traditional blast furnace ironmaking. However, its performance depends strongly on the mineralogy and particle size of the iron oxide feed. This study investigates the electrolysis of magnetite, for which inconsistent results have been reported, using both synthetic and natural samples of different particle sizes. Synthetic hematite was examined as a benchmark for high reactivity and efficiency. Electrolysis experiments were carried out in a mechanically stirred cell with planar electrodes, using 10 wt.% iron oxide suspensions in 50 wt.% sodium hydroxide (≈ 18.3 M) at 110 °C. The process was initiated potentiostatically at 1.66 V and shortly transitioned to galvanostatic operation at 1070 A⋅m-2. Chronoamperometric and electrochemical analyses revealed that both mineral type and particle size govern the rate of iron deposition and the Faradaic efficiency (FE). Fine high-purity synthetic magnetite (d50 ≈ 1.5 µm) achieved an FE of 76%, compared with 88% for hematite of similar size and purity, due to magnetite’s enhanced catalytic activity toward the hydrogen evolution reaction (HER). Larger magnetite particles (synthetic and natural) exhibited lower FE (≈ 54%) owing to increased polarization and HER acceleration. Morphological analysis showed a transition from columnar deposits typical of hematite to nodular, non-oriented growth for magnetite. These results clarify the electroreduction mechanism of magnetite and highlight strategies to enhance the efficiency of carbon-free electrolytic ironmaking.