Modes of occurrence of tungsten in coals: a review

Abstract

Relevance. The necessity to know the conditions and forms of W concentration in coals for solving a number of scientific and engineering problems at complex development of coal deposits. Aim. Complex estimation of W modes of occurrence in coal for development of measures for rational ecologically safe use of coal. Methods. Correlation analysis, scanning electron microscopy, infrared spectroscopy, coal group analysis, inductively coupled plasma mass spectrometry, instrumental neutron activation analysis. Results and conclusions. The modes of occurrence of W in coal were studied by a complex of methods. In the majority of W-enriched coal deposits, a negative significant correlation of its content in coal and coal ash with ash yield was found, indicating its association with organic matter. Using the method of coal group composition analysis, it was found that the main carrier and concentrator of W in the studied lignite deposits is organic matter. The contribution of the mineral phase in general in W-rich coals and in coals with its normal content does not exceed 20%, usually less than 5%. These conclusions are also confirmed by infrared spectroscopy data, according to which no more than 15% of the metal in the samples studied is associated with mineral phases. The association of W with high molecular humic acids predominates. In anomalously W-enriched lignites, the humic acid phase represents 76 to 88% of the gross metal content. The role of bitumen and low-molecular-weight humic acids in the balance of W is marginal at their different levels in the coals. Mineral phases of W are not characteristic of coal. Nano-microinclusions of scheelite, wolframite, hubnerite and ferberite were recorded as isolated occurrences. The main mineral phases are associated with Fe and Mn hydroxides, in which W is presented as a trace element in the amount of 1–5% together with other elements (Ge, As, etc.). In more metamorphosed bituminous and anthracite coals, authigenic mineral formations were found, represented by tungstite, phyllotungstite, native W and complex Fe-Mn-Ca-W-O mineral phases.