Mechanism of sodium tetraborate on melting characteristics and mineral evolution behavior of Zhundong coal ash

The fusibility of coal ash directly affects the safe and stable operation of gasifiers. However, due to the significant differences in the chemical composition of coal ash from different mining areas, the effects of the same flux on the melting behavior still required further research. In this study, the influence of sodium tetraborate on the ash melting characteristics of Wucaiwan coal (WCW) and Jiangjunmiao coal (JJM) was systematically investigated. And the melting process was observed via the heating stage microscope. Mineral transformations during melting process were also analyzed by XRD and FactSage thermodynamic software. The results indicated that sodium tetraborate significantly reduced the ash melting temperatures, but the effects on two samples were quite different. For JJM ash, as the addition ratio increased from 0% to 25%, the flow temperature decreased steadily from 1220 °C to 913 °C. While for WCW ash, the flow temperature first decreased and then increased with the increase of the addition ratio, reaching a minimum of 862 °C at 20% addition ratio, which was 333 °C lower than that of the raw ash. Heating stage microscope observations indicated that at 20% addition ratio, WCW ash showed obvious melting at 900 °C, while the JJM coal ash started to melt at 1000 °C. XRD and FactSage results revealed that sodium tetraborate reacted with Fe, Mg, and Ca bearing minerals in ashes at high temperatures to form low-melting borates, such as ludwigite and calcium borosilicate. It also facilitated the formation of low-melting aluminosilicates (albite and nepheline), further reducing the ash melting temperature.