Novel extraction of valuable metals from circulating fluidized bed-derived high-alumina fly ash by acid–alkali–based alternate method

Abstract

The recycling of valuable metals from circulating fluidized bed (CFB)-derived high-alumina fly ash (HAFA) paves the way for ash utilization with high economic benefits. However, obtaining an ideal extraction ratio for metals using a single acid treatment under moderate conditions is difficult. A moderate acid–alkali–based alternate method was used in this study to extract aluminum (Al) from CFB-derived HAFA. The dissolution behavior of lithium (Li), gallium (Ga), and rare earth elements (REY) was investigated. The characteristics of leaching residues were studied by XRD, 27Al and 29Si MAS NMR, BET, and SEM-EDS. Results show that Al in the units with low degree of polymerization distributed on the particle surface was preferentially dissolved in the HCl solution. HCl could not penetrate the particle interior and thus greatly limited the improvement of the Al extraction ratio. NaOH solution effectively removed SiO2 that accumulated on the particle surface and broke the Si–O–Al units in the residues. The exposed and liberated Al was further extracted with the HCl solution. Finally, 78% of Al2O3, 80% of Li, 72% of Ga, and 55% of REY in the ash were extracted with the HCl solution. Moreover, 63% of SiO2 in the ash was dissolved in the NaOH solution. The acid leaching residue presented abundant mesopores and a high specific surface area of 205 m2/g. The findings imply that the proposed method that combines the extraction of valuable metals and the preparation of mesoporous materials provides a new way to utilize CFB-derived HAFA.