Synergistic extraction of rare earth elements and alumina from coal fly ash by potassium pyrosulfate

Abstract

Although coal fly ash (CFA) contains a high content of rare earth elements (REEs), the related extraction methods have limitations because of their low efficiencies, high levels of energy consumption, and other drawbacks. To address these problems, in this study, we examined the coextraction of REEs and Al2O3 from two types of Al2O3-rich CFA, pulverized CFA (PCFA) and circulating fluidized bed fly ash (CFBFA) using low-temperature calcination in the presence of K2S2O7. The total REEs, heavy REEs (HREEs), and light REEs (LREEs) extraction efficiencies were determined using different K2S2O7/Al2O3 molar ratios and calcination temperatures and correlated with the Al2O3 extraction efficiency using Pearson correlation coefficient analysis. The REEs are concentrated within CFA particles encapsulated in an aluminosilicate glass phase, and the REEs extraction efficiency is related to the form of Al in CFA. The extraction efficiencies of Al2O3 and REEs increase as the K2S2O7/Al2O3 molar ratio and calcination temperature increase, and the extraction selectivity of the more industrially valuable HREEs from CFBFA is higher. At high K2S2O7/Al2O3 molar ratios, the extraction of REEs from PCFA is more efficient than that from CFBFA with the regeneration of the highly active Al–O–Si bonds in CFBFA. The Al2O3 extraction efficiencies of PCFA as well as CFBFA correlate strongly with the total REEs, HREEs, and LREEs extraction efficiencies. The developed extraction technology has the potential to promote CFA valorization and expand REEs resources, thus mitigating the bottlenecks of REEs procurement.