Multistage slag treatment was carried out to investigate boron removal from metallurgical grade
silicon. Boron is known to be one of the most difficult elements to remove from silicon. Depending
on boron initial concentration, boron can be removed from silicon into certain level; most previous
studies have attained final 50–100 ppmw (parts per million weight) boron in the 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, an initial slag composition
of 45CaO-45SiO2-10Al2O3 (wt per cent) was utilised. Using this technique, boron level in the silicon
was expected to be gradually lowered because the new slag will always have more capacity to reach
the equilibrium distribution coefficient (LB = (ppmwB)slag/(ppmwB)Si). The current study was carried
out through combined thermodynamic calculations (using FactSage™ ver 8.2, by Thermfact Ltd,
thermochemical package) and high-temperature experimentation. The thermodynamic calculations
were carried out for reactions at 1500, 1550, and 1600°C for three different scenarios:
1. Single stage refining with varied slag to Si ratio.
2. Multistage refining with slag to Si ratio of 1:1.
3. Multistage refining with slag to Si ratio of 2:1.
The multistage experimental work was carried out at 1550°C under inert gas atmosphere each time
for 2 hrs, with slag to Si ratio of 2:1. The simulation results indicate that the boron distribution
coefficient stay essentially constant with varying slag to Si ratio and temperature, around 1.8 to 2,
and the experimental results measured LB in the same range, between 1.5 and 2.5. By harnessing
this feature, the simulation showed that boron can be removed by approximately 78 per cent in each
stage. The experimental result confirmed that the boron concentration in the silicon was reduced by
a total of 97.8 per cent from its initial value using four stages of slag treatment, from initial 270 ppmw
down to 6 ppmw.