Lunar steelmaking via direct oxygen refining of Fe-Si alloys

Thermal processing methods are currently being explored by space agencies and private companies alike that can extract oxygen and metals from lunar soil using molten regolith electrolysis and carbothermal reduction techniques. These processes produce Fe-Si alloys with over 10 wt% Si, which makes them brittle. This work provides a means to reduce Si content in ferrosilicon alloy by direct oxygen injection into molten Fe-Si alloys. The paper first uses thermodynamic modelling to optimize the refining conditions such as the oxygen quantity, refining temperature and flux requirement in Fe-Si alloy refining. The modelling predicts the refined metal composition and the corresponding slag chemistry. Equilibrium calculations predicted that the addition of 15 g of oxygen is required to facilitate easy separation of slag and metal during the refining of 100 g of Fe91Si9 alloy. The addition of CaO facilitates the early slag formation during the Fe-Si refining process. Refining at 1600 °C with 10 g O2 and 9 g CaO is predicted to recover 100 % of Fe from 100 g Fe91Si9 alloy. The Si content in the liquid Fe at this O2 level is about 0.6 wt%, making it suitable for lunar steel production. The thermodynamic modelling was validated through laboratory refining experiments conducted on lab-synthesised Fe91Si9 alloy. Oxygen blowing at a flow rate of 500 ml/min reduced the silicon content to 2 to 3 wt% within 5 min, with a further decrease to below 0.5 wt% after 10 min, resulting in nearly pure iron. After 30 min of oxygen blowing, no recoverable bulk metallic phase was observed, and the recovered product was predominantly oxidised material. Experiments at 1600 °C with 500 ml/min oxygen flowrate successfully reduced Si content from 9 to 1 wt%. This method supports Lunar Steel production and Earth's electrical steel recycling technologies.