The occurrence modes and geologic origins of arsenic in coal from Santanghu Coalfield, Xinjiang

Santanghu Coalfield, located in the northeast of Xinjiang, is an important coal base in China. In the geological prospecting stage, we found that the contents of arsenic were high at some drilling cores in Hanshuiquan district, Santanghu Coalfield, with the maximum content reaching 190 μg/g. Therefore, the research of enrichment processes and origins of arsenic is imminent. In this work, twenty-two coal samples from Xishanyao Formation were collected and tested with X-ray Diffraction (XRD) analysis, coal petrography and quality analysis, and sporopollen analysis. Furthermore, the quantitative research on occurrence modes of arsenic was operated by sequential chemical extraction experiment (SCEE) on seven samples with relatively high arsenic content from No. 9 coal seam in the middle part of Xishanyao Formation. According to the result, most of the samples from No. 9 coal seam possess high arsenic content. The contents of arsenic in all occurrence modes were ranked from high to low as follows: sulfide bound arsenic, residual form arsenic, organic bound arsenic, adsorption arsenic. For the isomorphism or solid solution form, arsenic mostly exist in crack filling pyrite of coal, contrarily, only a little occurs in clay minerals and organic matters. The further analysis show that there are three geologic origins that lead to the arsenic enrichment of coal, including the tectonic fracture-hydrothermal solution activities, the regional distribution of chemical elements and source of terrigenous detrital material, and the type of coal-forming plants. Among them, tectonic fracture-hydrothermal solution activities is the main source of arsenic in study area, which results in the sulfide bound arsenic occupying the largest proportion of total arsenic. There is a close relationship between the extraction methods and washing methods of arsenic in different occurrence modes. Therefore, this research plays a positive guiding role in the washing and utilization of high‑arsenic coal. We will further study the washing properties of high arsenic coals to improve the utility value of high arsenic coal in future investigation.