Experimental and Thermodynamic Investigation of Si–Ca–Al Alloy Slag Refining: Effects of Slag SiO2 Content and Slag-to-Alloy Ratio

Silicon is an essential material for metallurgical and energy applications, yet its conventional carbothermic production is associated with high CO2 emissions. The SisAl process offers a low-emission alternative based on the reduction of SiO2 using aluminium as the reductant, followed by slag refining of the resulting Si–Ca–Al alloy. This study investigates the refining stage using CaO–SiO2-based slags with varying SiO2 contents and slag-to-alloy ratios. Equilibrium simulations and laboratory-scale experiments in an induction furnace were conducted to evaluate the influence of these parameters on the normalized Si concentration in the alloy. Increasing the initial SiO2 content of the slag and the slag-to-alloy ratio generally resulted in higher normalized Si concentrations in the recovered alloy. At 60 wt.% SiO2 and a slag-to-alloy ratio of 5:1, an average normalized silicon content of 97.72 wt.% was achieved, with residual Ca and Al contents of 0.78 and 0.17 wt.%, respectively, while individual experiments exceeded 98 wt.% Si. Phase separation was identified as an additional factor affecting refining performance. The results demonstrate the feasibility of the investigated slag-refining approach and provide suitable process parameters for further development of the SisAl process.