Modes of arsenic occurrence in coal from the Santanghu Coalfield, Xinjiang, Northwest China: evidence from coal petrography, coal facies, mineralogy and geochemistry

The Santanghu Coalfield is an important coal reserve in China. It has been found that the arsenic content is at a very high level at some drilling locations in the Hanshuiquan ore district of the Santanghu Coalfield, with a maximum content as high as 190 mu g/g. In order to determine the states of the occurrence of As, 22 coal samples from the Xishanyao Formation of the Hanshuiquan ore district were analysed using sequential chemical extraction experiments and atomic fluorescence spectrometry. The results show that the concentrations of As are ranked as follows, from high to low: 'inorganic soluble' state, 'inorganic insoluble' state (residual form state), and 'organic' state. Although arsenopyrites (FeAsS) and As-bearing pyrites were detected by electron probe microanalysis, they are not common in the samples. By combining the results of low-temperature ashing and X-ray diffraction (XRD) experiments, it was found that the modes of As occurrences are not mainly in arsenopyrites or As-bearing pyrites. To confirm the modes of As occurrence in coal, we used an improved experimental method: reductive difference. As a result, we found that most inorganic soluble As occurs in the form of arsenate minerals, followed by pyrite via isomorphism.