The indices of a coal’s vitrinites give an indication of its grade and are used in the coal classification system. Vitrinites are made up of black, ribbon-like bands of aromatic material that are buried deep in the coal bed matrix. The propensity for this material to form separate bands is determined by the relative molecular mass (RMM) of the constituents and by the degree of maturity of the coal. The higher the RMM, the higher the indices.
The range of vitrinite indices for a specific coal is one of the important parameters used to determine its grade. For example, low vitrinite indices are typical of brown coal and high vitrinite indices of higher rank coals, such as anthracite. The vitrinite indices provide an indication of the coal’s ash and sulfur content, which are viewed as indicative of its quality.
The anthracite vitrinite indices are used in the classification of hard coals. They are considered important in terms of their rank and ash, both of which contribute to determining the quality of the coal being mined. The vitrinite indices of anthracite coals, although moderately higher than bituminous coals, are still much lower than those of the more indurated coals.
The vitrinite indices measure the amount of vitrinites present in a coal. These elemental components determine the quality of the coal, as does the grade and amount of vitrinite. The indices are determined by measuring the amount of vitrinites present in a coal per unit area of the unit area of a coal. Vitrinites are classified according to density, specific surface area and type of material.
Vitrinites are divided into three main groups; vitrinites, macerals and minerals. Vitrinites collectively refer to the aromatic material that gives coal its dark color. Macerals are the macerated particles that form a solid mass in coal beds. These are divided into the vitrinite family and the liptinite family. Minerals are the insoluble inorganic components that are responsible for the color, hardness, and odor of coal.
The vitrinites index is the ratio of the amount of vitrinites in a coal to the total amount of particles present within the coal. It is used to characterise the maceral and mineral content of the coal. The range of vitrinites indices will depend on the type of coal being analysed. Bituminous coals will have a much lower vitrinite index than anthracite coals.
The indices of a coal’s vitrinites are used to provide an indication of the coal’s grade and composition. They measure the amount of vitrinites present and provide an indication of the coal’s ash and sulfur content, which are viewed as indicative of its quality. The range of vitrinites indices will depend on the type of coal being analysed, with bituminous coals having a much lower index than anthracite coals. Understanding the vitrinites indices of a coal is critical to its proper classification and identification, and further contributes to a better understanding of its mechanical and chemical properties.
