Abstract

High-alumina coal fly ash (CFA) is considered as an important and cheap resource of aluminum. On account of high valence aluminum often trapped in aluminosilicate basic glassy phase and mullite (Al₆Si₂O₁₃), efficiently separating aluminum and silicon is an essential precondition for maximizing the recycling of alumina from CFA, which is conducive to reducing the environment danger. In this study, a novel process combining alkali calcination and hydrochemical treatment was proposed to efficiently separate aluminum and silicon in CFA. The reactivity of CFA was greatly improved after the calcination treatment. The results show that alumina-rich phase, mullite, was transformed into nepheline (NaAlSiO₄) by calcination with NaOH at a mass ratio 0.8:1 (w_NaOH:w_CFA) and calcination temperatures of 400 °C, which improved the dissolution of aluminum during the hydrochemical process and enriched the silicon in the residue. Then, the influence of the parameters of the hydrochemical process on alumina extraction was investigated. The results indicated that the two-stage method was more efficient than the direct hydrochemical process. The optimum conditions of the hydrochemical process were 40% NaOH solution and heating at 260 °C for 60 min. Approximately 94% of the alumina was easily recovered, whereas most of the silicon was converted to NaCaHSiO₄. The combined treatment method highlights the potential for aluminum production from CFA.

Graphical abstract