Formation of Light-Weight Ferroalloys in the Fe2O3-Al2O3-C System at 1550 °C: Influence of Silica Impurities

Light-weight ferro-aluminium alloys are finding increasing application in the transport sector to reduce overall weights, energy costs and CO2 emissions. As the primary production processes of both Fe and Al are among the most energy-intensive industrial processes in the world, there is an urgent need to develop alternate routes for producing Fe-Al alloys. Our group has successfully produced these alloys in the Fe2O3-Al2O3-C system by producing molten iron in situ, followed by the reduction of alumina at 1550 °C and pick-up of Al by Fe. In this article, we report on the influence of silica, a typical impurity present in iron oxide and reductant carbon, on the reduction reactions in this system, and on the formation of ferroalloys. In-depth investigations were carried out on the Fe2O3-SiO2-C and Fe2O3-Al2O3-SiO2-C systems at 1550 °C for times of up to 60 min. Detailed HRSEM/EDS and XRD analysis was carried on the quenched reaction products recovered after various heat treatments. A complete reduction of silica and alumina was observed in the Fe2O3-SiO2-C system, along with the formation of FeSi and SiC. The reduction reactions were relatively slow in the Fe2O3-Al2O3-SiO2-C system. While the formation of SiC, FeSi and mullite (Al6Si2O13) was observed, even small amounts of Fe-Al alloys could not be detected. The presence of silica impurities reduced the formation of Fe-Al to negligible levels by depleting molten iron from the reaction zone, a key ingredient for the low-temperature carbothermic reduction of alumina. This study shows that some impurities can be highly detrimental to the reaction kinetics and the formation of ferroalloys, and great care needs to be exercised during the choice of reaction constituents