Stepwise extraction and utilization of silica and alumina from coal fly ash by mild hydrothermal process

Abstract

Coal fly ash (CFA) is a potential secondary resource. In this study, silica and alumina from CFA were leached and utilized through a gentle alkali hydrothermal process. The results of CFA desilication with a single NaOH solution showed that amorphous silica was dissolved at a relatively low temperatures. Under optimized conditions, the leaching rate of SiO2 reached 33.81 %, while that of Al2O3 only was 1.37 %. Amorphous honeycomb calcium silicate hydrate was then well prepared by using silicon-rich liquid with CaO. The new aluminosilicate structures (sodalite) were detected in desilicated coal fly ash (DSCFA), which hinder the silica dissolution. After the optimal hydrothermal leaching in Ca(OH)2+NaOH system, the leaching rate of Al2O3 in DSCFA reached 39.84 %. The dealuminated CFA were mainly composed of friendly nano-flake tobermorite. With the increase of temperature, time, alkali concentration and Ca/Si, the tobermorite phase gradually changed to hibschite. The obtained alumina lixivium was well transformed into valuable porous boehmite via a carbonation process. Finally, the pilot tests excellent results, leaching about 41 % of SiO2 and 42 % of Al2O3, further demonstrating the feasibility of the whole hydrothermal process of desilication with a single alkali followed by aluminum extraction with mixed alkali.