Utilization of Dust from Silicon Production

Resumo

Silicon production by the reduction of quartzite in furnaces is accompanied by copious dust emissions containing a high proportion of valuable silica (86%, on average). It is of great interest to capture that dust for reuse in the process. The dust captured in the gas-purification system consists of particles no larger than 120 μm, with a predominance of the 20–50 μm fraction. In order to use such dust in silicon production, it must be aggregated, so as to prevent its entrainment from the furnace by the gas fluxes. Since the batch pieces must be strong enough to withstand transportation, the binder employed is liquid glass with the addition of electrofilter dust from aluminum production, which contains tars (polyaromatic hydrocarbons). Strength tests of batch samples permit the recommendation of the following composition (by mass): 24–27% dust from silica production; 51–53% carbon reducing agent (a 1: 1 mixture of petroleum coke and charcoal); 4–5% silicon siftings; 14–15% binder (a 4:1 mixture of liquid glass and electrofilter dust from aluminum production). For such batch, the strength in drop tests is 82.5%, on average. Research shows that the batch pieces obtained by this method have a porous structure (45.5%) for the formation of a well-developed active surface and corresponding apparent density (1100 kg/cm³). That supports stable furnace operation. Trials in an HTF 17/10 high-temperature furnace yield an intermediate product containing more than 44 wt % silicon carbide. On that basis, such dust aggregates may be added to the main batch in silicon production.