Apparent versus true density for the volume-to-weight transformation in coal blends

This work reviews the procedures involved in the conversion of the petrographic characterization data of blended coals into weight percentages of the single coals in the blends, the selection of the most suitable parameter for describing the density of the coals being critical. First, the fundamentals of the volume-to-weight transformation of the petrographic data were developed by means of mathematical formulae. This part of the procedure highlighted the need to use a density value adapted to the resolving power of the optical microscope. It was found that mercury penetration at 5 MPa, as determined by mercury porosimetry, was best suited for this purpose. A literature survey of published coal density data revealed that the use of correlations between coal density and rank was not an accurate enough procedure to deal with the relatively low corrections involved in the transformation of petrographic data. In the end, three binary blends were prepared from six carefully selected coals and polished particulate blocks of these blends were petrographically analysed to determine their compositions on a volume per cent basis. The best fit of these volume percentages with the weight percentages used in the formulation of the blends was found for a density correction based on mercury penetration at 5 MPa, in agreement with the empirical analysis.