Formation mechanism of an undesirable by-product in the mild hydro-chemical process for the extraction of alumina from fly ash and its mitigation

Abstract

The mild hydro-chemical process has been commercially demonstrated as an effective method to extract alumina from high alumina fly ash. This study investigated the formation mechanism of an undesirable aluminum-containing by-product, hydrated sodium calcium aluminosilicate (HSCA, 1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O), produced during the mild hydro-chemical process and its mitigation strategy. The effects of reaction temperature, residence time and mole ratio of calcium to silicon on the alumina extraction and phase transformation of the leaching residues have been investigated, confirming the formation of HSCA phase being responsible for the decrease of alumina extraction efficiency. At the low Ca(OH)2 addition, the HSCA phase could be easily transformed from Na8Al6Si6O24(OH)2(H2O)2 combining with Ca(OH)2. On the other hand, when the addition of Ca(OH)2 was abundant, the existence of the NaCaHSiO4 product layer would insulate most of Ca(OH)2 from the Na8Al6Si6O24(OH)2(H2O)2 causing formation of HSCA. Based on the reaction mechanism and coating effect analysis, the mitigation strategy was also proposed. By increasing stirring speed to break the NaCaHSiO4 coating layer, HSCA phase could be completely avoided and a qualified alumina extraction efficiency of 90.84% could be achieved.