Multistage Refining of Silicon Using CaO-SiO2-Al2O3 and CaO-SiO2-MgO Slags

Multistage slag treatment was carried out for silicon refining with a focus of removal of boron. Boron is one of the difficult elements to remove from silicon. Depending on the initial concentration, boron can be removed from silicon into slag to a certain level. Previous studies have attained final 50-100 ppmw (parts per million weight) boron in silicon using slag treatment. The term 'multistage' refers to a process, where the resulting silicon from the initial slag treatment is further refined multiple times using a new slag. In this study, slags with initial composition of 45CaO-45SiO2-10Al2O3 and-10MgO (wt %) were utilized. Based on the thermodynamics, boron level in the silicon is expected to be lowered sequentially because the new slag will always have the same capacity to absorb boron to reach the equilibrium level. The amount of boron at equilibrium can be represented by a distribution coefficient (LB = (ppmwB)slag/(ppmwB)Si). The current study was carried out through combined thermodynamic calculations and high-temperature experimentations. The thermodynamic calculations were carried out for reactions at 1500, 1550, and 1600 degrees C for four different scenarios: (1) Single-stage refining with varied slag-to-silicon ratio, (2) Multistage refining with slag-to-silicon ratio of 1:1, (3) 2:1, and (4) 4:1. The multistage experimental work was carried out at 1550 degrees C under an inert gas atmosphere each time for 2 h, with slag-to-silicon ratio of 1:1 and 2:1. The thermodynamic simulation results indicated that the boron distribution coefficient essentially stays constant at around 1.8 to 2, with varying slag-to-silicon ratio and temperature. The simulation also showed that boron can be removed by approximately 78% in each stage. The experimental results measured LB in a similar range, between 1.4 and 2.9 and confirmed that, in total, the boron concentration in the silicon was reduced by an average of 98% for slag-to-silicon ratio of 2:1, and 95-97% for slag-to-silicon ratio of 1:1 after four stages of treatment.