Arbuzov, and 5 co-authors (2019) report that 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.
The modes of occurrence of chemical elements in coal can be important indicators of the conditions of their formation and the nature of geochemical anomalies. They are also important factors in determining the technology for the extraction of valuable elements from coal and coal by-products. Determining the modes of occurrence of elements in coal can be difficult due to the many different possible modes of occurrence and also due to the difficulty of quantifying the modes of occurrence. Determining the modes of occurrence is especially important for the rare earth elements (REE) because extracting them from coal and coal ash for use in a wide range of technological devices is receiving global attention.
There has been significant progress in the study of the modes of REE occurrence in coals for three decades after a publication of Reference [5]. Until recently there has been the notion in the literature that the bulk of the REE in coal was concentrated in inorganic compounds, mainly adsorbed on clay minerals, as isomorphous components in various minerals (especially clays) and in the rare earth phosphates and aluminophosphates. Calculations performed by Finkelman [5] for the bituminous Waynesburg coal (West Virginia, USA) based on electron-microscopy data showed that micro-inclusions (1-2 microns) of rare earth phosphates (monazite, xenotime) and aluminophosphates (crandallite group minerals, which Campbell [1968, pp.12-13 and well logs] reported above the top of the phosphate zone in the Georgina Basin, Queensland) were able to provide 100% of the total content of REE in the coal. [This suggests that phosphate deposits are also likely sources of REE, see Campbell (1969, p. 26 (Figure 1). Notice the lanthanum and yttrium values are near 1,000 ppm].
Using a suite of bench samples from a Texas lignite strip mine, Finkelman estimated that no more than 10% of the total REE in the lignite had an organic association; the remaining 90% of the REE were associated with REE-bearing minerals. In a recent attempt to quantify the modes of occurrence of elements in coal. Finkelman et al. [16] concluded that in bituminous coals 70% of the light REE (La, Ce, Nd, and Sm) and Y were in phosphate minerals, 20% in clays, and 10% in carbonates and organics. Whereas, 50% of the heavy REE (Yb and Lu) were in phosphates, 20% in clays and 30% in organics and carbonates. In a limited number (6) of low-rank coals they found that 60% of the light REE and Y were associated with clays, 20% in phosphates, and 20% in carbonates and organics. Of the heavy REE 50% were associated with clays, 25% in organics and carbonates, and 25% in phosphates.
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