Effect of SiO2 on the reduction of chromite sinter for ferrochrome production

Abstract

Ferro-chrome is presently produced from lump chromite ores in the submerged arc furnace. Majority of the lump ore is very lean (~30% Cr) in nature. Production of a better quality ferro-chrome requires using high-grade chromite ores; but these are fragile and tend to form fines during handling. In order to utilize these fines direct sintering of chromite fines has been carried out to produce fluxed sinter which contains 36% Cr2O3, 20% FeO, 8% SiO2, 9% MgO, 11% Al2O3, 6% CaO etc. In the present study the performance of developed chromite sinter to produce ferrochrome in graphite-lined 50 kVA electric arc furnace has been conducted and the effect of SiO2 on smelting property has been studied. The thermodynamic calculations for the chromite sinter smelting (Cr2O3-Fe2O3-SiO2-CaO-MgO-Al2O3) were done with the Equilib module of FactSage 6.4 to study the effect of silica in lowering the slag melting temperature. It was found that with increase in SiO2, the melting point of slag decreases. It has also been found from the calculations that required SiO2 concentration would be 33% to obtain a slag of melting point around 1660 0C. Thus calculation suggests external SiO2 addition of 656 g is necessary for 10 kg sinter. In actual experiment equivalent amount (752 g) of quartzite was added as source of SiO2. Smelting experiments were conducted in presence of coke as reductant and several parameters viz. tapping temperature, specific power consumption, flux (silica) requirement, metallic recovery has been studied. The slag tapping temperature measured 1789°C (100 °C super heat above the slag melting temperature) and was appeared to be sufficiently fluid, having viscosity 1.1 poise. Thus, quartzite (SiO2) addition decreases the liquidus temperature of slag, as has been found from the thermodynamic estimation. The obtained Fe-Cr alloy has metallic yield of 81%, chromium recovery of 88% and specific Power consumption of 13 kWh/kg of Fe-Cr alloy. Produced ferrochrome has composition 55%Cr, 36% Fe, 7.8%C, 1% Si and the final slag Cr and Fe was very low (0.76% Cr2O3, 0.98% FeO). Thus addition of SiO2 provides an encouraging result in smelting of developed sinter to produce ferrochrome.