Formation of Carbon Nanotubes and Microsilica during the Production of Crystalline Silicon in Three-Phase Ore-Thermal Furnaces

The annually generated waste of silicon production in Irkutsk oblast equals 20 000 t/year, and the volume of waste accumulated in the three sludge fields of JSC Silicon exceeds 3 million m 3 . The dust from the gas cleaning systems of the ore-thermal furnaces is the main waste type of the crystalline silicon production. In that regard, this work presents a study of the chemical composition of the dust and the possibilities of using its valuable components: amorphous silica and carbon nanotubes (CNTs). The possibility of separating this product by flotation into three components—the sand fraction, the flotation tailings enriched in SiO 2 , and the froth product enriched in CNT-type carbon—is shown. We have studied the CNT structure and determined the following physicomechanical properties: the elasticity modulus (2000 GPa), the ultimate strength (75 GPa), and the thermal conductivity (4000 W/(m K)). The heat amount required to obtain 1 kg of CNTs in ore-thermal furnaces is calculated. On the base of a material balance of technical silicon electrosmelting, we found that, during the endothermic process, 153 kg of CNTs are formed per 1 t of crystalline silicon, as well as 336 kg of the flotation tailings containing 75% of the amorphous microsilica (AMS) particles. From calculations of the heat effect and the Gibbs energy of the AMS formation reactions, we reveal that all processes are exothermic, and the oxidation process of the silicon carbide particles by air oxygen (2SiC + 3O 2 → 2SiO 2 + 2CO) has the highest thermodynamic probability. We calculate the economic efficiency of the amorphous silica use for the foundry silumin production and demonstrate the results: fast payback period (6 months) and high profitability level ($819 672).