Effects of reducing environment and fusible components on carbothermal reduction–nitridation reaction of coal gangue at high temperature under N2 atmosphere

Abstract

The synthesis of SiAlON materials by the carbothermal reduction–nitridation (CRN) method paves the way for utilization of coal gangue with significant economic benefits. Excessive carbon (C), which provides a high residual C content in the product, is added to the raw materials to ensure a high yield of SiAlON. The industrial solid waste of coal-coke powder is used as a reductant. In this study, the effects of the reducing environment and C content on the CRN reaction of coal gangue using corundum and graphite crucibles are investigated and a method to reduce the amount of C added to the raw materials is proposed. The process is carried out under a N2 flow in the range of 1300–1500 °C with a holding time of 3 h. The effects of fusible components including CaO, Fe2O3, and MgO on the CRN reaction and micromorphology are discussed. The graphite crucible considerably promotes the conversion of mullite to β-SiAlON and is effective for the reduction in residual C content in the product. When the graphite crucible is used, the mullite wholly transforms to β-SiAlON and corundum, and β-SiAlON and AlN at 1400 °C when the C contents in the raw materials are 11% and 18%, respectively. The content of residual C in the product is approximately 1% (or lower). The β-SiAlON content decreases with the additions of CaO and MgO in the raw materials owing to sintering and melting. The addition of Fe2O3 promotes the formation of β-SiAlON whiskers according to the vapor–liquid–solid mechanism. These results would be useful in parameter optimization for a cleaner production of β-SiAlON powder using the CRN method.