Modes of occurrence of scandium in coals and peats (A review)

The modes of occurrence of scandium in peat and coal from different deposits in Siberia, Russian Far East, Kazakhstan, and Mongolia was studied with the use of a set of methods. The fractions of coal with different densities and the group components of peat and brown coal were considered. The selective leaching of Sc from brown coal and black coal with ammonium acetate and concentrated acids was performed, and a correlation between the Sc content and the ash content was studied. It was established that, at the early stages of a coal-formation process, the bulk of scandium in low- and medium-ash peats and brown coal was bound to organic matter. Humic substances (fulvic acids + humic acids) are the main carriers and concentrators of scandium. In all cases, the scandium content of humic acids was higher than the average concentration in peat and brown coal. A decrease in the yield of humic acid fractions in the coalification process decreases their fraction in the total balance of scandium; however, the fraction of insoluble humic substances increases in this case. The modes of occurrence of scandium changes in the course of coalification. In low-rank black coal (grade D), a considerable fraction of scandium occurs in the organic matter. The mineral forms of scandium predominate in mature black coal. Scandium is a constituent of silicates, aluminosilicates, phosphates, and aluminophosphates; it likely forms its own mineral species.