Modelling of ferrosilicon smelting in submerged arc furnaces

Ferrosilicon alloys are commonly manufactured in submerged electric arc furnaces with little slag. In the presence of iron, silica will be reduced by carbon to give a maximum of similar to22 wt% silicon in the liquid alloy. The rest of the carbon will be consumed to transform silica to silicon carbide at similar to 1810 K. Higher grades of ferrosilicon alloy may be produced owing to the reactions occurring between silicon carbide and silica at temperatures above 1810 K. Thermodynamic data on the standard free energy of formation of species are used in the study to calculate the required smelting temperatures at various silicon contents of the alloy. The sum of partial pressures of carbon monoxide and silicon monoxide must equal the applied pressure of 1 atm at the smelting temperature. It is important to know the activity coefficient of silicon in the alloy as a function of temperature, and the silicon content of the alloy using literature data. Mass and enthalpy balances are used to determine the carbon and electricity requirements of the process. The recycling of silicon monoxide is promoted by maintaining a bed of a certain height so that evolved gases are cooled owing to heat exchange between the gas and solid phases. It might result in a saving of more than 3000 kWh/t of Fe-80Si alloy. The reduction of silica is found to account for just 47.6% of the total energy that is added via the calorific value of carbon and the electricity in producing the alloy. Further improvement in the performance is visualised by reducing electrical losses and recovering as much as possible the calorific value of outgoing carbon monoxide.