Modes of Occurrence of Rare-Earth Elements (La, Ce, Sm, Eu, Tb, Yb, Lu) in Coals of Northern Asia (Review)

The modes of occurrence of six rare-earth elements (La, Ce, Sm, Eu, Yb, and Lu) in low-rank coals (lignites and sub-bituminous coals; also referred to as brown coals) from different deposits of Siberia, the Russian Far East, Kazakhstan, and Mongolia were studied using two selective leaching methods. Selective extraction of organic compounds included the extraction of organic substances with minimal effects on the mineral matter was only used for low-rank coals. The second method of the selective leaching of rare-earth elements (REE) from mineral matter in brown and bituminous coals used ammonium acetate, conc. HCl, conc. HF and 10% HNO3. The content of the six rare-earth elements in all phases (bitumen, humic acids, and residues) was determined by INAA. It was found that in the early stages of coal formation the bulk of the REE was associated with the organic matter primarily humic substances. Release of REE from humic substances during coalification results in the formation of authigenic minerals. The proportion of the REE in organic forms is gradually reduced in the coalification process until there is a complete REE transition to mineral phases in anthracite and graphite. These investigations showed that the alteration of modes of occurrence of La, Ce, Sm, Eu, Yb, and Lu from primary organic compounds to primary minerals occurred simultaneously with the organic matter alteration over the coalification process from peat to anthracite.