Redistribution and transformation mechanisms of gallium and germanium during coal combustion

Abstract

Gallium (Ga) and germanium (Ge) are strategically critical rare elements (CREs) which are of growing significance for the rising demand of emerging energy-efficient technologies and high-tech applications. With the increasing demand and exhaustion of conventional ores of these CREs, extraction of CREs from metalliferous coal may be a viable approach to providing stable alternative sources. Understanding of the transformation behavior and associations of CREs during combustion processes are of significance for the design of recovery applications. The Ga/Ge coal from Mangniuhai Coalfield was collected to determine the redistribution and transformation behavior of CREs during combustion at 1000 °C. The effect of ash components on the redistributions of Ga and Ge during combustion was determined via both experimental laboratory simulations and thermodynamic equilibrium calculations. The experiments were performed in a fluidized bed reactor in combustion temperature range of 500–1000 °C. Sequential chemical extraction was conducted to determine the transformation characterizations of associations of Ga and Ge during combustion. The concentrations of Ga and Ge in combustion residues are reached to the cut-off grades suggest that these CREs deserve further utilization. Ga and Ge are mainly enriched in fly ash (>60%) when compared to that of bottom ash. The redistribution characterizations of Ga and Ge during combustion with ash components are determined by combustion temperature, ash component type and adding amount. Ga and Ge can be redistributed to ash phases more effectively by adding Si, Al, Na and K -based additives during the combustion. Ga and Ge are retained in ash phases as a result of the formation of non-volatilized steady species arising from interactions with the ash components. Si, Al, Na and K based compounds are regarded as the appreciable additives for the excellent capture potential of gaseous CREs during high temperature combustion. The results may provide useful information for the development of CREs enrichment, extraction and separation technologies from coal.