From waste to wealth: Separation of alumina from coal fly ash and synergistic preparation of soil amendments

Large-scale comprehensive utilization of coal fly ash (CFA) has become an inevitable issue. In this study, one-step mixed alkali (KOH + CaO) hydrothermal extraction of alumina and synergistic preparation of soil amendment from CFA was developed. The Al 2 O 3 and SiO 2 in CFA were dissolved rapidly under the action of KOH, and the dissolved silicate was transformed into tobermorite under the action of CaO. This approach realized efficient separation of aluminum and silicon with simultaneous regeneration of KOH. The optimum leaching ratio of Al 2 O 3 was 56.44 %, while the leaching ratio of SiO 2 was 3.38 %. The dealuminated CFA (DACFA) was simultaneously transformed into porous aggregates intertwined with tobermorite fibers, which are rich in Si, Ca and K 2 O nutrients due to the excellent cation exchange of tobermorite or formation of tobermorite and leucite minerals. The effective Si, Ca, and K 2 O in F90 were 16.19 %, 20.45 % and 5.31 %, respectively, which met the primary standard of fertilizer. The pot experiment showed that DACFA effectively improved soil quality and immobilized soil heavy metals, thereby accelerating the growth of pakchoi. Through this technology, the high-value transformation from CFA to alumina and soil amendments is expected to facilitate large-scale utilization of CFA and enhance agricultural productivity.