Extraction and value-added utilization of alumina from coal fly ash via one-step hydrothermal process followed by carbonation

Coal fly ash (CFA), containing 25–50 wt% of Al2O3, is a typical and massive secondary aluminum resource. However, alumina recovery from the CFA by traditional processes is very difficult due to 40–60 wt% SiO2 content and low Al2O3/SiO2 mass ratio. In this study, a “one-step” hydrothermal process was proposed to extract Al2O3 from CFA; a mixture of lime and NaOH was used to enhance the dissolution and separation of Al/Si components. The results showed that 60.14% Al2O3 leaching could be obtained for treatment at 210 °C temperature for 3 h in the presence of 30 g/L NaOH and CaO/SiO2 molar ratio of 1.0. During the hydrothermal process, the Si- and Al-bearing components of CFA were sufficiently decomposed in lime and NaOH mixture; the dissolved silicate ions were then combined with calcium ions to form tobermorite, while the aluminate ions were concentrated in the liquor, thereby achieving selective leaching of alumina from CFA. Owing to the limited solubility, part of the aluminate ions was also converted into aluminiferous zeolite, hence lowering the alumina leaching ratio. The lixivium was of low caustic ratio that was favorable for the preparation of high-value pseudo-boehmite via carbonation process at 80–90 °C with the ultimate pH of 9.5–10.0.