Quantification of the Modes of Occurrence of 42 Elements in Coal Samples from the U.S., Canada, Brazil, Great Britain, Australia, Wyoming, and North Dakota - 2018

Coal and Related Carbon Resources / Lignite, Graphite, Graphene, & Carbon Uses

Finkelman, et al., reported in 2018 that a series of 20 coal samples from Pennsylvania, West Virginia, Kentucky, Alabama, Maryland, Canada, Brazil, Great Britain, Australia, Wyoming, and North Dakota were subjected to sequential leaching with ammonium acetate, hydrochloric acid, hydrofluoric acid, and nitric acid to quantify the modes of occurrence of about 42 elements. The results indicate that within a coal rank the leaching behavior of many elements are broadly similar. However, the behavior of many elements varies between bituminous and subbituminous coals. While the leaching behavior of most elements is consistent with previous mode of occurrence interpretations there were a number of unanticipated results. About 75% of the arsenic in bituminous coals appears to be associated with sulfides, probably pyrite, however only 40% of the arsenic in the low rank coals appears to be associated with pyrite.

In the bituminous coals, we interpret that about 60% of the beryllium is associated with silicates, but in the low-rank coals only 30% is silicate-associated. In the bituminous coals 25% of the magnesium is associated with carbonates, but in the low rank coals, 75% of the magnesium is in the carbonates. Fifty-five percent of the molybdenum was associated with the sulfides in the bituminous coals and in the low rank coals 65% was in the silicates. In the bituminous coals the rare earth elements (REE) are predominantly in the phosphate minerals (70% for the light REE and 50% for the heavy REE) but in the low rank coals the REE (50–60%) are associated with the clays.

Sodium is predominantly (80%) associated with silicates in the bituminous coals and with the organics (65%) in the low rank coals. Titanium appears to occur more in the clays (65% bituminous, 70% low rank) than in oxides (35% bituminous, 15% low rank). Tungsten is in the sulfides (50%) in the bituminous coals but in the clays (60%) in the low rank coals. Uranium is associated with the silicates (40%) and insoluble phases such as zircon (30%) in the bituminous coals, but is associated with the organics (55%) in the low rank coals. Many of these rank differences reflect a higher detrital input and greater sulfide mineralization of the bituminous coals.

The authors concluded that selective leaching results from 20 coal samples have allowed the authors to make an initial attempt at quantifying the modes of occurrence of 42 elements. The results indicate that within a coal rank, the leaching behavior of many elements are broadly similar. However, the behavior of many elements varies between bituminous and subbituminous coals. The authors recognize the limits of the selective leaching procedure, especially in trying to identify phases that are insoluble or only partially soluble. Nevertheless, the authors conclude that the quantification of the mode of occurrence presented in this paper are reasonable estimates for these elements in most coals.

The authors trust that this effort will encourage other researchers to further refine these estimates and to resolve some of the unusual leaching behaviors that the authors observed in their results.