Stepwise extraction and utilization of silica and alumina from coal fly ash by mild hydrothermal process
Lang Xing, Xinran Li, Pengxu Cao, Jun Luo, Hao Jiang
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 SiO 2 reached 33.81 %, while that of Al 2 O 3 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 Al 2 O 3 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 SiO 2 and 42 % of Al 2 O 3, further demonstrating the feasibility of the whole hydrothermal process of desilication with a single alkali followed by aluminum extraction with mixed alkali.