Determination of pyritic sulphur and organic matter contents in Spanish subbituminous coals by X-ray power diffraction

Abstract

Two analytical methods for the indirect determination of pyritic sulphur and ash contents in Spanish subbituminous coals by means of X-ray diffraction (XRD) are described. An indirect analytical method for pyritic sulphur is used to determine this type of sulphur in high-sulphur coals from the Teruel and Mequinenza mining areas (northeastern Spain). The analytical method is based on the quantification by m eans of XRD of the hematite content resulting from the oxidation of iron sulphides after coal ashing at 750°C. It has been possible to use this analytical method for the particular coals studied because they do not contain iron carbonates or organic-fixed iron in appreciable amounts. It can thus be assumed that the hematite in the ash proceeds exclusively from the oxidation of iron sulphide.

The method of determining the ash content in coal by means of XRD was devised in order to analyze small amounts of samples from density separations. The small amount of sample available (some fractions lower than 0.5g) and the need to analyze various parameters (sulphur, mineral and trace element contents) in the sample did not allow us to determine the inorganic content by ashing. The method permits the total recovery of the sample for subsequent analyses and the determination of the different mineral phases. The method devised is based on the quantification of scattering due to the organic matrix in XRD. The organic matrix, as an amorphous matrix, increases the X-ray diffraction background intensity (scattering curve) by amorphous scatterring, to an extent proportional to the organic matter content. The main advantages of this method is its speedy determination (4 min) and the possibility of sample recovery for subsequent analyses.