The Occurrence Modes And Geologic Origins Of Arsenic In Coal From Hanshuiquan Ore District,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?g/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 of SCEE,the sum of the arsenic content in each occurrence mode extracted by SCEE is generally equal to the total arsenic content,with a high coincidence rate of 96.67%–98.77%.These results enlighten that the experimental means of SCEE is feasible to quantitatively study the occurrence mode of arsenic in coal.Most of arsenic occurs in the inorganic bound mode including sulfide bound arsenic and residual form arsenic,occupying up to 90.6% of the total arsenic content.Organic bound arsenic combined with heteroatoms such as oxygen and sulfur in macromolecules as well as carbon atoms in coal through chemical bonds,contributing a lower content in samples from 4.41% to 22.11%,with an average of 9.4%.And the low content of organic bound arsenic is closely related to the properties of coal-forming plants,the preservation conditions of vegetation and the groundwater activities.Inorganic bonded arsenic can be further extracted as Adsorption arsenic(water soluble / exchangeable arsenic),inorganic soluble arsenic(sulfide bonded arsenic and arsenate bound arsenic)and inorganic insoluble arsenic(residual arsenic).Adsorption arsenic affects a little on the overall arsenic distribution because of its insignificant content.The proportions of residual arsenic in the samples are large and unstable,ranging from 1.32% to 35%,with an average of 9.23%.There is a positive correlation between the residual arsenic content and the ash yield of coal,which indicates that arsenic can be stored in clay minerals,forming the residual arsenic.Inorganic soluble arsenic accounts for the heaviest proportion of total arsenic(42.59%-84.15),with a mean value of 73.09%,which is the most important inorganic binding arsenic.The inorganic soluble arsenic can be subdivided into sulfide combined arsenic and arsenate bound arsenic.Among them,sulfide bound arsenic accounted for a relatively low proportion of inorganic soluble arsenic(17.70%-51.05),with an average of 29.13%.Arsenate combined arsenic accounts for 48.95%-82.30% of inorganic soluble arsenic,averaging 70.87%,which is the most important component of inorganic soluble state.Sulfide bound arsenic and arsenate bound arsenic are related in origin.The geologic origins of high arsenic coal are diverse.Before the coal accumulation period,a large number of low-temperature hydrothermal activities related to fracture development provide sufficient arsenic supply for the area,and the high arsenic background in Santanghu Coalfield provides conditions for the accumulation of arsenic in the coal seam.The coal forming plants in the study area are all advanced plants,and the ability of combining arsenic is weak,resulting in the low organic bound arsenic content in the study area,which is not the main source of high arsenic in the region.Tectonic fracture-hydrothermal activity and groundwater activity are the main factors affecting the accumulation of arsenic in coal,and also the most important geological source of inorganic soluble arsenic in high arsenic coal from Santanghu coalfield.The washing characteristics of arsenic in coal are closely related to the occurrence state of arsenic.This paper has a positive guiding role in the washing and utilization of high arsenic coal.It can reduce the environmental pollution and improve the utilization rate of coal.