Transformation Characteristics Of Pyrite During Coal Accumulation,Flotation And Pyrolysis

Pyrite is a common mineral found in coal seams in China,which can be transformed and have significant impacts on coal development and the environment during the utilization process.Understanding pyrite’s distribution and transformation characteristics is crucial for efficient desulfurization and clean resource utilization.Pyrite’s autothermal oxidation properties can also contribute to spontaneous combustion during coal accumulation.Furthermore,pyrite undergoes mineral transformations during coal utilization,but research on its mechanism and characteristics is still developing.This dissertation focuses on analyzing the geological conditions of coal seams and pyrite deposits in two coalfields,the Upper Permian Longtan Formation coal in Xingyi Coalfield,Guizhou Province and the Carboniferous Permian Taiyuan Formation coal in Datong Coalfield,Shanxi Province,which have varying coal-forming environments and coal quality characteristics.Based on the distribution characteristics of the coal mining and utilization process,a series of experiments were conducted in sequence to analyze the geological conditions of the coal seams,including flotation experiments of pyrite in coal,pyrite oxidation experiments under different conditions,and high-temperature combustion experiments of coal bodies.Modern analysis and testing methods,such as microscopic observations（optical microscope,scanning electron microscope,electron probe X-ray microanalysis）and mineral element content analysis（X-ray diffraction analysis,X-ray fluorescence analysis,inductively coupled plasma mass spectrometry,and X-ray photoelectron spectroscopy）,were used to comprehensively study the transformation characteristics and mechanism of pyrite in different processes.The results of this research are shown:（1）Identified the spatial distribution of sulfur and the enrichment characteristics of associated elements in coal from Datong coalfield of Shanxi provinceThe abundance of sulfur in coal is largely affected by regression,magmatic intrusion,topography and fault structure.Most of the areas with high sulfur content in coal in the study area are adjacent to magmatic activity and fault distribution.From bottom to top of No.8 to No.4 coal,the higher content of S_t,d,S_o,d,S_p,d and S_s,d in coal shifts from southeast to northwest as a whole.The form of sulfur in coal is mainly pyrite sulfur.The more complete the crystal form and the larger the grain size of pyrite is,the higher the sulfur content is,and the proportion of sulfur form of pyrite in coal is gradually increasing.Elements such as Cr,Co,Ni,Cu,Mo,Cs,Zn,V,Tl,U,As,Pb,Cd,Cr,Sb,Se in coal are closely related to pyrite in coal,and harmful elements such as As,Pb,Cd,Cr,Sb,Se are relatively rich Set in fine-grained pyrite with defective crystals.（2）Discuss the occurrence causes of pyrite in coal and its influence on flotation desulfurization effect.Pyrite in Guizhou and Shanxi coals generally has a poor crystal form and is relatively sulfur-poor.Mostly of them distributed in the vitrinite,especially in the matrix vitrinite（including frambo（?）dal,euhedral crystalline,and fine-grained agglomerated pyrite）,and a small number of them are filled in the cavities of other components or the gaps between components（such as frambo（?）dal,fissure filling shape）.The sulfate reduction is mainly affected by the change of sea level,which limited the growth of pyrite particles.However,in the middle stage of coal mineralization,the supply of pyrite-forming materials is abundant,the ore-forming environment is continuous and stable,and the ore-forming temperature is relatively high,which can formed the Fine-grained pyrite with good crystal morphology.Pyrite with a particle size small than 15μm in pulverized coal can form a complex symbiotic relationship with organic matter,which is the critical scale for crushing and improving the effective flotation efficiency of pyrite.（3）Clarified the oxidation and dissolution process of pyrite during coal accumulation and its influencing factors.Under normal conditions,the oxidation reaction of pyrite samples is slow,whereas the addition of bacteria and Fe3+ions can significantly increase the oxidation rate of pyrite.Additionally,the oxidation degree and oxidation rate of coal-pyrite are slightly higher than those of mineral-pyrite samples under the same environment.This may be due to the relatively high impurity content in coal-pyrite samples,including associated iron-containing minerals such as siderite,iron dolomite stone,and hematite;impurity elements existing in the crystal lattice or crystal gap;and residual humic acid;the poorer integrity of the crystal structure,and the relatively larger specific surface area of the mineral crystal,all of which help to increase the rate of oxidation reactions.Furthermore,the organic matter in coal is closely related to coal-pyrite and thus affects the oxidation reaction process of coal-pyrite.（4）Revealed the formation and transformation mechanism of pyrite during pyrolysisA combustion transformation model of pyrite has been proposed to summarize the transformation process of pyrite under combustion conditions:（1）direct oxidation to form hematite;（2）pyrolysis to form pyrrhotite,and continued oxidation to form hematite;（3）first pyrrhotizes,then oxidizes to magnetite,and then oxidizes to form hematite;（4）first pyrolyzes to form magnetite,and then oxidizes to form hematite.The conversion process is closely related to the particle size and occurrence form of pyrite.Generally,the more stable the structure,the larger the particle size,and Sulfur is relatively hard to lose in pyrite.In an air atmosphere,when the combustion temperature is less than 500°C,pyrite is easy to oxidize,with a higher crystal integrity and higher thiophene sulfur content,and relatively easy to form pyrrhotite and hematite.At 500°C≤combustion temperature<700°C,pyrite is relatively easy to oxidize to form magnetite and hematite,with lower crystal integrity and higher thiophene sulfur content.When the combustion temperature is≥700°C,pyrite is relatively easy to oxidize to hematite.The atmosphere,combustion temperature,sample particle size,thiophene content,density,and crystal integrity in the sample organic sulfur will all affect the conversion degree of pyrite and the content distribution of staged products.There are 70 figures,22 tables,204 references in this dissertation.