Migration Rule And Enrichment Mechanism Of Toxic Elements During Rock-coal Alteration In Shitai Coal Mine,Huaibei

The thermal metamorphism of magma intrusion into coal seam?referred to as alteration effect between coal and intrusion?is abnormally developed in Huaibei region of China.These Yanshanian invader will not only destroy the stability and exploitability of coal seams,but also seriously endanger the production of coal fields and have a great impact on the utilization of coal resources.It will also affect and drive the redistribution of elements in coal seam,resulting in the abnormal accumulation of toxic elements in coal coke,and even harms the environment and human health.In order to investigate the migration and anomalous enrichment of toxic elements during the alteration process between coal and intrusion,samples of intrusive rocks,alteration zones,fire clip,natural coke and unaffected coal from No.3 coal bed in the Shitai coal mine in northern part of Anhui Province have been collected for studying the coal quality characteristics,mineral characteristics and element characteristics by applying methods of proximate analysis,ultimate analysis,optical microscopic observation,XRD and ICP-MS analysis.The results show that:?1?The coal metamorphic degree increases as it approaches the rock mass.From coal?natural coke?fire clip?alteration zone,the content of ash,water and C in coal increases,the content of volatilization and H decreases while is no obvious trend of change in the content of N because of the intrusion of magma.?2?As it approaches the rock mass,slurry source material increases.Near the invader,the mineral species in the fire clip and alteration belt increases,the alteration intensity increases,the mosaic structure and porosity?bubble?are more developed,the vitrinite decreases to disappear.Away from the intrusive body,the unaffected coal seam is weakened and the mineral decreases.?3?Near the rock mass,some toxic elements are enriched.Among them,V,Cr,Mn,Co,Ni,Zn,Ga,Rb,Zr and Cd are enriched in the intrusive rock,Th is enriched in the altered zone,Pb is enriched in the fire clip,Cu,Sr,Mo and Ba are enriched in the natural coke,As and Sb are relatively stable in the coal seam.The analysis shows that the metamorphic belt between coal and intrusion in Shitai coal mine have the following migration and enrichment characteristics:?1?The enrichment of Mn is closely related to magma intrusion.Mn is extremely high in intrusive rocks and its enrichment is directly related to magmatic hydrothermal fluids?the magma itself is rich in Mn?.At the same time,due to the magmatic intrusion,the volatile content of coal decreases,and some Mn volatiles to gas or liquid phase products with the loss of volatile content.?2?As and Sb elements are mainly derived from coal seams.The content of As and Sb in the whole coal seam is at a relatively stable level,mainly from the coal-forming soil and sediment,and the magma has little effect on it.In addition,As and Sb will volatilize with the pyrolysis of coal after thermal decomposition of coal.?3?The migration and enrichment of most toxic elements such as V,Cr,Zr,Co,Ni,Zn,Cd,Ga,Rb,Ba,Sr,Mo,Pb,Cu,and Th are the comprehensive results of magma invasion of coal.The content of these elements in intrusive rocks and unaffected coal is high,partly from the sedimentary environment,partly from the minerals?clay minerals,sulfide,siderite,etc.?brought by magmatic hydrothermal fluids,is the result of the interaction between the sedimentary environment and magmatic hydrothermal fluids.In addition,due to magma intrusion coal seam thermal metamorphism,V and Mo condensed with carbon into a polymer compound remained in the semi-coke.Zr and Rb volatilize with loss of volatilization,Sr and Ba increase with the increase of ash content,Co,Ni,Cu and Pb volatilize with the decomposition of pyrite.Zn evaporates into the atmosphere,Cd enters the sphalerite during hydrothermal metasomatism,Cr is mainly distributed in coking solid products,Ga may volatilize GaH and Ga₂O,GaO or their organic derivatives,Th due to the increase of moisture,coal hole fissure increase and rapid precipitation.The main innovations and findings of this study are as follows:?1?The coal quality characteristics,mineral composition,and toxic element content characteristics of different degrees of metamorphic coal in the metamorphic belt between coal and intrusion are analyzed,and the migration mechanism and enrichment of toxic elements are revealed in the alteration process between coal and intrusion.?2?It was found that magmatic intrusion not only brought toxic elements such as V,Cr,Mn,Co,Ni,Cu,Ga,Rb,Sr,Zr,Mo,Ba,Pb,and Th,but also driven the migration and enrichment of these elements between magma and coal.