Study on modes of occurrence and enhanced leaching of critical metals (lithium, niobium, and rare earth elements) in coal gangue

This paper first studied the occurrence modes of critical metals (lithium: Li, niobium: Nb, and rare earth elements: REY) in coal gangue (CG) by sequential chemical extraction, and the distribution and enrichment degree of critical metals in different density and size fractions were also studied, and then CG was pretreated with intercalation and calcination to strengthen the extraction of critical metals. The results of sequential chemical extraction showed that the residual fraction was the dominant occurrence mode of critical metals. The enrichment ratios of different density fractions were 0.57-1.23 for Li, 0.03-1.34 for Nb, 0.69-1.36 for REY, and different size fractions were 0.89-1.23 for Li, 0.46-2.21 for Nb, 0.63-1.17 for REY. Density separation and particle size classification can be used to enrich critical metals. Direct leaching of 3 M hydrochloric acid resulted in 5.76% Li and 0.74% Nb recovery. Further, the leaching efficiency of Li was improved to 17.83% using pretreatment of intercalation, and the crystal structure of kaolinite was retained in the leaching residue. In comparison, calcination pretreatment led to 92.74% Li recovery, while the leaching efficiencies of Al and Fe were correspondingly improved. This paper can provide a theoretical foundation and technological support for the extraction of critical metals from CG. • Occurrence modes of critical metals in coal gangue were studied via sequential chemical extraction. • The technological process of the enrichment of critical metals from coal gangue by density separation and particle size classification was provided. • 17.83% of lithium was extracted by acid leaching after pretreatment of intercalation, and the crystal structure of kaolinite in the residue was retained.