Co-melting properties and mineral transformation behavior of mixtures by MSWI fly ash and coal ash

Abstract

The disposal of municipal solid waste incineration fly ash (MSWI FA), as one of hazardous wastes, is restricted by the energy consumption in the melting process. Coal ashes are suitable additives to lower the melting temperature thanks to the enrichment of Si, Al and the low costs. Herein, MSWI FA was added with three representative coal ashes at varying ratios to uncover the co-melting mechanism. The three types of coal ashes contain high Ca (SMca), high Si (CPca) and medium Ca, Si (SEHca), respectively. Ash fusibility temperature (AFT) test, heating and water quenching, X-ray diffraction, scanning electron microscopy and FactSage analysis were conducted. Results show that both CPca and SEHca can decrease the flow temperature of FA. After the blending with 40% CPca or SEHca, the flow temperature of FA dropped from 1460 to 1240 and 1280 °C respectively. However, only CPca-40% led to the formation of uniform vitreous bodies at 1300 °C. SMca did not considerably impact the melting temperature. As the Si/Ca ratio increased from 0.19 to 0.62, main minerals in the mixtures were transformed at 1300 °C as follows: Ca2SiO4+Ca3Al2O6+Ca12Al14O33→Ca12Al14O33+ Ca2SiO4→Ca2Al2SiO7 (glass phase). FactSage calculation revealed that when nepheline was formed, the coal ash content was just enough for reducing FA melting temperature below 1300 °C. To control the melting temperature below 1300 °C, we suggest the composition in coal ashes to be: 43.75%–87.50% SiO2, 12.50%–25.00% Al2O3, and as little CaO as possible.