An energy-saving and cleaner method for recycling coal gangue and aluminium chips: Preparing in-situ nitride whiskers reinforced ceramics for thermal storage

An energy-saving and cleaner method was provided for recycling coal gangue (CG) and aluminium (Al) chips to prepare in-situ nitride (Si3N4, SiAlON) whiskers reinforced ceramics for solar thermal storage by aluminothermic nitridation. The in-situ synthesis mechanism of nitride whiskers and the optimized processing parameters of preparing the ceramics from the binary CG-Al system were studied systematically. Results indicated that Al chips as reducing agent could reduce CG to produce silicon at 1000–1100 °C, and their relics provided pores for in-situ synthesizing nitride whiskers by the gaseous reaction between SiO, Al2O and N2 via Vapor-Solid mechanism. The in-situ nitride whiskers increased the fracture toughness effectively. A conversion order of mullite–X1-SiAlON–β-SiAlON for the CG-Al system under N2 atmosphere was confirmed by XRD. A certain size range of under 300-mesh for Al chips was the limit to realize the conversion of X1-SiAlON to β-SiAlON. Decreasing the size of Al chips or increasing the sintering temperature enhanced the production and evaporation of silicon, decreasing the thermal conductivity and fracture toughness greatly. The in-situ aluminothermic nitridation method can save 29–33 GJ/t energy in recycling CG and Al chips and in separately producing nitride whiskers, and also avoid the emission of hazardous gas and the inhaled toxicity of whiskers. The ceramic products show good competency on thermal storage applications in clean energy.