Abstract
Silicon (Si) is one of the most useful elements on earth and is popular due to its semiconductor characteristics and its abundance in form of quartz (SiO2). Both metallurgical Si and ferrosilicon (FeSi) are produced industrially by carbothermic reduction of quartz (SiO2) in a submerged arc furnace. When this work started in 2017, the knowledge about slag in industrial Si and FeSi furnaces was limited. The understanding about the slag and how it behaves and reacts in the furnace are important to avoid accumulated slag in the furnaces and to optimize a good flow of materials in the furnaces. The amount of slag is one of the main differences between different furnaces and is believed to be an important factor in the productivity of the furnace. Additionally, knowledge about the slag present in the furnaces is an important step towards identifying the different zones and materials in the furnaces. This study is divided into three parts. Part 1 focuses on slag from industrial Si and FeSi furnaces, part 2 investigates the impurities in the quartz, who later contributes to the slag formation in the furnace, and part 3 studies the main action used today for removing accumulated slag from the industrial furnace: dissolving calcium oxide (CaO) in the slag to reduce its viscosity. Slag from different zones in six different Si and FeSi furnaces collected during excavations, tapped slag from three different furnaces and slag from the charge surface of two FeSi furnaces during operation are the basis for the industrial part of this work. Accumulated slag is typically found along the furnace walls, sometimes extending all the way up to the charge top, and in a thick layer at the furnace bottom, which the metal must pass to exit the tap-hole. Both the accumulated slag and the tapped slag mainly contain SiO2, CaO and Al2O3. In the accumulated slag samples, it is found that the slag towards the furnace wall in the higher parts generally has a higher SiO2 content compared to the slag accumulated at the furnace bottom. The possible explanation suggested in this study for both the existence of slag in this area, and the increased SiO2 content in the slag, is a high crater pressure that pushes the slag towards the furnace walls and upwards. Furthermore, for the slag next to the tapping channel, the slag above the tapping channel generally has a higher SiO2 content compared to the slag below it. This is believed to be due to variations in the densities, where slags with higher SiO2 content have lower densities. No significant differences were found in the composition of the slag between different tap-holes within the same furnace. Visually, the zones around the tapping channel appear similar. Next to the Si flow is a bright green layer measuring 5-15 cm in thickness. This layer mainly consists of SiO2-CaO-Al2O3 slag and smaller silicon carbide (SiC) particles. Following the green layer is a dark grey layer that is mainly SiO2-CaO-Al2O3 slag and larger SiC particles. The green and grey color seem to be dependent on the size of the SiC particles, and not the composition of the slag. The tapped slags were found to be liquid during tapping at a temperature of 1800 °C, except for the solid SiC particles present in the slag. The main difference between the normal tapped slag and the slag reported as high-viscosity slag is the increased amount of SiO2 in the slag, and the presence of SiO2 and condensates in the samples. These SiO2 areas are former quartz which has melted, but not fully dissolved in the slag. The amount of slag, the amount of solid SiC in the liquid slag and the viscosity of the slag are three of the main factors that influence the flow of slag through the tap-hole. Impurities in quartz will affect both the SiO2 properties during heating to elevated temperatures and it affects the composition of the slag in the furnaces. The impurities and the properties in the quartz of six different quartz types suited for Si and FeSi production were studied. It is found that an increased amount of impurities lowers the softening temperature and the melting time. It is also found that SiO2 dissolves in the impurities as the temperature increases. The crack formation in quartz during heating were found to mainly happen at two temperature intervals, ~300-600 °C and ~1300-1600 °C. Cracks formed from 300 °C are from an uneven SiO2 surface, from activities in form of volume change or color change in the impurity areas, or from fluid inclusion cavities, while cracks occurring from 1300 °C is believed to be due to the volume increase and the phase transformations from quartz to cristobalite. The degree of cracking is also found to be different between the different quartz types. No correlation could be found between amounts of cracks and fines formation, nor between the crack formation and the impurity composition. CaO in form of lime (CaCO3) is commonly added as a flux in Si and FeSi production to lower the viscosity of the slag, which is beneficial to ensure a good flow of materials through the furnace. The dissolution of CaO in three different compositions of SiO2-CaO-Al2O3 slag were investigated at temperatures between 1500-1600 °C. It is found that CaO dissolution into the slag is fast. The initial effect of increasing the CaO content in the slag from 15-21% to 25-30% gives a significant reduction in the viscosity. During the dissolution process, a boundary layer containing 35-42% CaO formed between the CaO particle and the slag, which corresponded to the phases CaO·Al2O3·2SiO2 or 2CaO·Al2O3·SiO2 in this study. Two models were investigated to determine the dissolution rate of the three slags. In the first model, the CaO particle is assumed to be a smooth shrinking sphere, and the rate controlled by the chemical reaction rate. The second model assumes that the rate is controlled by mass transport and depends on the diffusion rate of CaO through a boundary layer on the surface of the CaO. Both models gave similar results, and a proportional relationship between the rate constants and the viscosities was obtained. The diffusion coefficients were found to be within the range of 10-6 cm2/s.