Municipal solid waste incineration fly ash (MSWI FA) is an important waste product that holds considerable potential for valourisation. In addition to major phases such as CaSO4, CaCO3, NaCl, KCl, and silicates, these ashes contain significant amounts of valuable elements like copper (Cu), zinc (Zn), lead (Pb), and others, where the specific composition depends on the source of the waste and the incineration process used. This study aims to investigate the melting behaviour of municipal MSWI FA samples from various incineration technologies, including rotary kiln, grate furnace, and circular fluidised bed, as a background for pyro/hydrometallurgical metal extraction. The experimental study was designed to research the effect of salt composition on the melting temperature and phase formations of different ash types, as well as metal migration between phases, using a sessile drop furnace. As a complimentary approach to the experimental study, thermodynamic modelling (FactSage™ ver 8.3, by Thermfact/CRCT and GTT-Technologies) was used to predict the phase formations of different fly ashes using Scheil-Gulliver cooling of molten ash. The observed melting point of the samples varied between 1000–1400°C depending on the ash type, without any trend of salt composition effect on the melting point. Upon solidification, there were three distinct phases observed in the samples: a metallic phase, a crystalline, non-metallic phase with inhomogeneous shape pattern, and an amorphous matrix phase. The findings indicate that the predominant component in the matrix phase was Ca-O-Si, implying the formation of calcium silicate slag. Elemental mapping showed metallic droplets consisting of primarily Fe-P phases, while the crystalline non-metallic phase is concentrated in Ca and S. The furnace atmosphere (Ar versus CO) had no significant impact on the phase formations. Thermodynamic modelling results were in good agreement with the experimental study, except for P-rich metallic phases, showing the formation of non-metallic and complex silicate slag phase formations.
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Soylu et al. (2024) studied this question.