Combustion of carbonaceous shale: a case study of an incident at an Iron Ore Mine

Abstract

Spontaneous combustion incidents are known to occur at coal mining operations, associated with the presence of coal, coal-shales, and pyrite. A premature detonation incident occurred at an iron ore mine in 2020 in a black carbonaceous shale unit, known to be associated with the presence of pyrite. The unit was never chemically classified or modelled separately, and the risk for spontaneous combustion or the potential for ground reactivity was poorly defined. This report investigates the incident with the aim of initially classifying the composition of the carbonaceous shale unit, and secondly determining the propensity for spontaneous combustion to occur in the future. The classification of the carbonaceous shale unit was determined through X-ray fluorescence (XRF), carbon and sulphur speciation (CCS), proximate and ultimate analysis, and X-ray diffraction (XRD). Numerous studies have been done on determining a method to evaluate the risk of a spontaneous combustion, the Wits-Ehac Index classification is one such method that was used in this study. The Wits-Ehac Index results were correlated with the XRF and proximate and ultimate analysis through linear regression. A comparison of the XRD results versus the Wits-Ehac results was also done. The iron ore mine carbonaceous shale was classified as a ‘poor’ carbonaceous shale, falling on the limit of being classified as a ‘rock’, due to low carbon content, low calorific value, and high ash content. The unit is characterised by a low Fe presence, except when siderite is present, and a low but variable sulphur content. Where sulphur values are relatively higher, it may be ascribed to the presence of pyrite that increases the risk of reactive ground. The Wits-Ehac results were between medium and high propensity for spontaneous combustion, and the results of the study were used to determine a spatial risk map to define risk areas within the mine. The linear regression plots showed very poor correlations between the Wits-Ehac Index results and the XRF and proximate and ultimate results. The strongest correlation existed with Nitrogen. The most valuable relationship determined in this study is between the presence of relatively high sulphur (greater than 3%) that is related to the presence of pyrite which may increase the risk for spontaneous combustion and ground reactivity with nitrate-bearing explosive emulsion.