Abstract
This report investigates the wetting behaviour and reactions involving liquid silicon (Si) and SiO2 – CaO – Al2O3 slag, silicon carbide (SiC), and carbon (C) substrates under carbon monoxide (CO) atmosphere at 1550 and 1650 °C. The interaction between these is important for understanding reaction mechanisms in industrial Si production. β-SiC is often found within the slag in industrial furnaces. Prior studies have found that β-SiC forms from molten Si in contact with CO gas. This study investigates whether this formed SiC layer incorporates into the slag phase over time.
Wetting experiments showed similar wetting behaviour for liquid Si and SiO2 – CaO – Al2O3 slag on SiC substrates. For the slag-SiC system at 1650 °C, stable wetting was not achieved due to reactions between the slag and the SiC. Reference experiments with slag on carbon substrates at 1550 °C and 1650 °C with 5 minutes holding time demonstrated that no measurable SiC formation occurred in the absence of Si, indicating that direct contact between slag and carbon was insufficient to produce SiC under the investigated conditions.
Experiments involving both Si and slag on carbon substrates revealed rapid SiC formation. SiC was observed already at 1650 °C without holding time, confirming that SiC formation proceeds rapidly once liquid Si is present. However, the amount of SiC formed was considerably lower than that observed in experiments involving only Si and carbon under a CO atmosphere [1], [2], [3]. This indicates that the presence of slag influences the extent of SiC formation, likely through limiting the Si surface available for reaction and some consumption of SiC. At 1550 °C, only limited reaction between the slag and the SiC phase was observed. SiC was formed on the Si droplet and within the carbon substrate, while no slag penetration into the substrate was detected. In contrast, experiments conducted at 1650 °C showed more extensive interaction between the phases. SiC particles were found both at the Si-slag interface and within the slag phase, indicating transport of SiC from the interface into the slag. In addition, mixed regions containing both SiC and slag were observed within the carbon substrate.
Analysis of the slag composition showed little or no change in SiO2 concentration after experiments at 1550 °C. At 1650 °C, SiO2 consumption was observed after only 5 minutes of holding time, and the overall extent of SiO2 consumption generally increased with holding time. However, the degree of consumption was also influenced by the amount of Si and slag, and the Si-to-slag ratio. Only minor concentration gradients were observed within the slag, suggesting that convective flow contributed to homogenization of the slag composition during the experiments.
The results demonstrate that temperature influences the interaction between Si, slag, and carbon. The presence of slag reduces the extent of SiC formation compared with Si-carbon systems without slag. At 1650 °C, the SiC that forms can participate in further reactions with the slag, leading to both SiO2 consumption and incorporation of SiC into the slag phase.