THE SPONTANEOUS COMBUSTION OF COAL AND ITS BY-PRODUCTS IN THE WITBANK AND SASOLBURG COALFIELDS OF SOUTH AFRICA

Abstract

Spontaneous combustion of coal seams in the Witbank and Sasolburg coalfields, South Africa, was studied in order to determine if toxic chemical elements and compounds are being mobilised into the environment. Samples of the minerals forming on the surface of coal seams, and gases escaping from vents, were analysed to verify the presence of these elements and compounds.Gas temperature measurements at coal-fire vents range from 34 °C to 630 °C. The coal-fire gas minerals (CFGM) identified included sulphur compounds and salammoniac. X-ray diffraction (XRD) studies of CFGM by-products confirmed the presence of mascagnite ((NH4)2SO4), illite ((Al,Si)4O10[(OH)2,H2O]) letovicite ((NH4)3H(SO4)2), phlogopite (KMg3(AlSi3)O10(F,OH)2), titanium dioxide (TiO2), barite (BaSO4), iron sulphate (FeSO4), gypsum (CaSO4·2H2O) and silicate. An unknown and unclassified sulphur–nitrogen–chlorine CFGM was also identified. The minerals are interpreted to have formed by condensation or sublimation; several may be alteration products. Other heavy elements found in the CFGM's are mercury, arsenic, lead, zinc, and copper. Arsenic and mercury were the major elements of potential environmental significance found accumulating around coal-fire vents.Relatively high concentrations of toluene, benzene and xylene were found in the gas collected at both Witbank and Sasolburg coalfields.