Experimental study of rate of silica formation from active oxidation of liquid silicon

In the silicon and ferrosilicon industry there are several environmental challenges, one of them being diffuse emissions of fume. Fume emissions in the plant are often poorly captured, and thus contribute to making the indoor environment harmful for the workers. On-going research into solutions to reduce the emissions is hence vital. In earlier studies, the main source of fugitive emissions - amorphous silica fume formation - was established; oxidation of liquid silicon during tapping and refining. The limiting factor for the fuming rate was found to be the oxygen availability at the liquid silicon surface, and a silica flux was estimated from measurements. In order to better understand the mechanisms behind industrially observed fuming rates, experiments performed in a 75 kW induction furnace were carried out, and the rate of fume formation as a function of gas composition and flow velocity was measured. The particle size of fume particles, formed due to active oxidation of a moving liquid silicon surface, was also determined as a function of gas flow rate, and gas composition. The fume was collected in an industrial type filter, and all equipment was weighted before and after each experiment in order to monitor the weight loss/gain. The results of the experimental study showed that the flux of silica increased with increased gas velocity above the liquid surface, and was found to correlate well with mass transfer rates calculated from impinging jet theory. The size of the particles was also found to be dependent on the gas flow rate. The findings are discussed in light of recent measurements and modeling of Si oxidation rate in industrial and small scale.