Process Phenomena in Furnaces Producing Ferrosilicon Alloys
Abstract
The production of ferrosilicon alloys is carried out in electric arc furnaces through carbothermic reduction. The reduction process involves the use of quartz, iron containing raw materials, and carbon reductants which are heated to high temperatures by the electric arc. Recent excavations of Si and FeSi furnaces reveal varying archaeologies related to various process phenomena such as cavity formation, slag accumulation, SiC production, and condensates formation.
The aim of this study is to better understand the process phenomena present in industrial ferrosilicon furnaces by verifying a laboratory-scale experimental method that simulates the conditions present in these furnaces in a controlled environment. In this work, the following variables have been investigated: Si content of the alloy (52, 75 or 96 wt%), holding temperature (1850 or 1900 °C), holding time (30 or 60 min), nature of carbon reductant (coke, coal, or charcoal), nature of iron containing raw materials (iron ore pellets, mill scale, or metallic iron) and the presence of SiO producing layer. These investigations yielded details regarding the cavity formation mechanism, SiO condensates formation, reduction of iron oxides, softening/melting of quartz, transformation of carbon materials into SiC, slag formation, and finally alloy production.
Cavity formation has been found to be dependent on the formation of SiO condensates, which will glue the charge together and prevent further downward movement of the charge. A mechanism for cavity formation has been suggested, involving both upward and downward growth directions. Fe oxide pellets and mill scale appear to be mainly reduced through solid-gas reactions involving CO/H2. The introduction of Fe to the system enables the production of Si at temperatures below 1700 °C, where SiO2 will react with the dissolved carbon in the molten iron. Carbon reductants play an important role in the production of ferrosilicon alloys through the formation of the intermediate SiC phase. The transformation of carbon reductants into SiC was observed in the high temperature area of the crucible, typically located below the cavity. Charcoal and pyrolyzed woodchips have the highest reactivity with SiO and extent of transformation, followed by coal char and lastly coke. SiC has also been observed above the cavity wall, both within the white condensate layer and as SiC whiskers. In quartz particles, larger cracks have been observed with increasing temperatures. Softening of quartz has been observed at temperatures above 1600 °C. Disintegration of quartz has not been observed in this work, mainly attributed to the gradual heating of the quartz particles. Various oxide species have been identified in this work and can be divided into two main categories: oxides in which the atomic ratio of Fe to Si was larger than 1 were denoted as fayalitic oxides, whereas oxides in which the atomic ratio of Fe to Si was smaller than 1 were denoted as silicate oxides. Both type of oxides has been observed both over and under the cavity.
The in-depth study of the temperature profiles and process gas analysis in selected experiments reveals that, in the presence of iron oxide, the FeSi production process is divided into three main thermal zones:
1. Zone 1: At T <600 °C, the coking/devolatilization reactions of carbon materials and the evaporation of water will dominate.
2. Zone 2: At T between 600-1500 °C, the FeOx reduction reactions will dominate, coupled with the Boudouard reaction.
3. Zone 3: At T >1500 °C, the SiO2 reduction reactions will dominate.
The gaseous species present in zone 1, described above, are mainly H2 and CH4 with small amounts of CO, whereas zone 2 is dominated by CO2. In zone 3, the main gaseous species analyzed was CO, but SiO will also be present.
Various metallic phases were produced in the experimental work, including Fe2Si (β), FeSi (ε), FeSi2 (ζα), Fe2Si5 (ζβ) and Si. The presence of these alloys indicates successful FeSi and Si alloys production, validating the suitability of this experimental method in simulating the operational conditions of ferrosilicon furnaces.
Lastly the two measurement campaigns carried out in both Si and FeSi furnaces indicates that the temperatures recorded at the surface of the charge are higher than temperatures within the charge, with temperatures as low as 400-600 °C being recorded. Analysis of the process gas indicates that it mainly consists of CO, and small amounts of H2 and CH4. The process gas produced in the FeSi furnace contained significant amount of CO2 (approximately 5%). SiO will also be present but was not analyzed.