Comprehensive Analysis of Thermochemical Phase Evolution and Strategic Recovery of Valuable Elements from High-Alumina Coal Fly Ash at Different Thermal Conditions

Abstract

The evaluation of the application potential of high-alumina coal fly ash (HACFA) resources relies on the assessment of the phase structure and leaching behavior of valuable elements. Herein, we systematically investigated the particle size, specific surface area, and morphology of HACFA produced at various temperatures and quantitatively analyzed phase transformations using the Rietveld method. The results revealed that the HACFA particle size gradually increased, specific surface area gradually decreased, and pores became lesser with the increasing calcination temperature of high-alumina coal (HAC) from 400°C to 1300°C. Furthermore, HACFA phases sequentially changed across various temperatures, its main phases at a calcination temperature of 400°C included 29.8% kaolinite, 6.9% boehmite, and 5.4% calcite, but the crystalline phases were the most abundant and turned into 41.2% mullite, 13.7% corundum, and 11.6% plagioclase when the calcination temperature reached 1,300°C. Additionally, the leaching behavior of 28 major and trace valuable elements of HACFA under acidic and alkaline conditions was investigated. Most metal elements easily leached from HACFA under acidic conditions at calcination temperatures of 600°C–800°C: the optimal leaching efficiencies of Al, Li, K, Rb, Mg, Ca, Sr, and Ba were 67.64%, 92.71%, 76.67%, 73.46%, 86.74%, 98.87%, 98.42%, and 61.19%, respectively. Si, a nonmetallic element, achieved a high leaching efficiency of 48.27% under alkaline conditions at a calcination temperature of 1,300°C. Interestingly, only Mo exhibited a high leaching efficiency of approximately 98% under acidic and alkaline conditions.

Keywords: high-alumina coal, calcination, crystalline phase, Rietveld method, leaching behavior