Research On The Graphitization Process Of Coal And The Migration Rule Of Coal-derived Minerals

Graphite is an important strategic resource.Despite natural graphite is an abundant resource,most of them are of low purity and thus cost great for preparation.Consequently,recent years have witnessed the growing importance of artificial graphite research.Petroleum coke is the main raw material for preparing artificial graphite at present.However,with the supply shortage and the fluctuating cost of petroleum,it is urgent to seek a substitute raw material from both resource and economic perspective.Anthracite could be employed as a kind of raw material for preparing graphite because of its high carbon content,but the ubiquitous ash-forming minerals,which are impurities in this process,would cause serious problems,e.g.adhere to graphite at high temperature.Research on transforming the three-dimensional-structured coal macromolecule into the layer-structured graphite could provide theoretical guidance for developing graphite with a specific structure,e.g.large area lamellar graphite and microcrystalline graphite with high intensity.Research on the change and migration laws of minerals in the graphitization process could lay a theoretical foundation for removing impurity substances from anthracite and produce high purity graphite.Research on the synergistic relationship between mineral migration and carbon laminarization in the graphitization process is beneficial to facilitate the graphitization process and improve the graphitization speed.Taixi anthracite was employed as the raw material.Scanning Electron Microscope（SEM）,combined with Transmission Electron Microscope（TEM）,were used for characterizing the change law of microscopic morphology and lattice structure during the graphitization process.Then the process of transforming from the three-dimensional organic structure to a planar ordered structure could be investigated.The results show that multiple mechanisms co-existed in the graphitization process.（1）The internal main structure of the anthracite could produce fractures along with the interfaces of different minerals as the graphitization temperature increases.Meanwhile,local carbon atoms gradually turned well ordered,and thus produced the isotropic polycrystalline graphite.（2）Under the temperature of 2000℃,several minute carbon spheres started to appear on the surfaces and fractures of the anthracite.And as temperature increases,the spheres gradually grew larger,merged with other spheres,and finally ruptured when their diameter reached 100 mm.When the temperature rose to 3000℃,anisotropic lamellar graphite crystal with flexible,wrinkled structure was formed.（3）The intermediate silicon carbide,which is formed by the reaction of primary or secondary quartz with carbon in anthracite at 1800°C,was decomposed to form a highly well ordered graphite sheet at 2600°C.Therefore,all the amorphous carbon in different chemical environments in raw coal could be transferred into graphite,but the crystal structures of the graphite were quite distinct.The graphitization process is complicated and could not be generalized to a single mechanism.Raman and X-ray diffraction（XRD）spectra were employed to characterize the arrangement order of the carbon atoms,the micro-crystal size and the trend of graphitization degree in the graphitization process of Taixi anthracite.Furthermore,with the help of thermodynamics and kinetics,the dominating factors in different temperature ranges during the graphitization process were analyzed.The experimental results demonstrate that the graphitization process of Taixi anthracite could be divided into three stages from both thermodynamics and kinetics perspective.（1）298-1773 K:it is an endothermic process,mainly including the pyrolysis and polycondensation reaction of anthracite macromolecules.（2）1773-2473 K:the chaotic layer structure gradually turned to the ordered structure,and the carbon layer spacing was reduced to release the latent heat in the system.Meanwhile,this stage was accompanied by the formation and decomposition of carbides,indicating that the system was endothermic.That is,the exothermic reaction co-existed with the endothermic reaction in this temperature range.（3）2473-3273 K:graphite associated with lattice defects absorbed heat and recrystallized,resulting in the elimination of the lattice defects and furtherly the ordered arrangement of the carbon atoms.However,under the elevated temperature,the ordered structural transformation in the graphitization process always companied by the locally disordered thermal motion of atoms.Hence,when the graphite with a certain degree of perfection was converting to a higher stage,its entropy value decreased,making it difficult to produce defect-free graphite.Plasma low-temperature asher and XRD were employed to analyze the type,grade and occurrence mode of minerals in the anthracite.Subsequently,high-temperature heat treatment under pure argon was applied to pure minerals,mixed minerals and minerals in the carbon environment to compare the thermal environment in the graphitization process.Finally,the crystal structure,functional group and melting point of the minerals treated with different temperatures were characterized by XRD,infrared spectroscopy and ash melting point analyzer.The transformation process and the melting characteristics of minerals in the anthracite,as well as the synergistic effect of coal and minerals in the graphitization process,were investigated.The results show that Taixi anthracite mainly contained six kinds of minerals,such as quartz,illite,kaolin,pyrite,calcite and calcined gypsum,among which quartz and illite accounted for 86%.Mixed minerals had a lower flow temperature than the pure minerals because of the formation of low-temperature eutectics at elevated temperatures.In the graphitization process,most of the minerals migrated from the interior of the anthracite to the fractures of the anthracite in the form of gas or liquid as the temperature increased to 1500°C,and then escaped from the furnace cavity with the argon gas flow.Above 2700°C,only a small amount of silicon presented in the form of silicon carbide,and most of the silicon escaped the furnace in the form of aluminosilicate with alkaline metal elements（sodium,potassium,magnesium,etc.）in the raw coal.To explore the universality of the above discussion,coal samples with varying degrees of metamorphism were selected as the raw materials.Different pretreatment methods following by the identical heat treatment conditions were carried out to graphitize the raw materials.The effects of raw coal structure and its ash and volatiles content on coal-based graphite products were investigated.It is interesting to note that the coal-based graphite materials with different microstructures（polycrystalline graphite,monocrystalline graphite,spherical graphite,honeycomb graphite,and rod-like graphite）were obtained.The results demonstrate that the graphitization effect of high-rank coal was superior to that of low-rank coal under identical graphitization conditions.Different minerals have different effects on graphitization of coal.A proper amount of metallic minerals was beneficial to the growth of the graphite lattice,but excess clay minerals would result in the increase of impurities,which would furtherly decrease the purity of the product and finally affect its graphitization degree.Pretreating the raw materials with acid pickling could facilitate the graphitization degree of products while slowdown the graphitization speed.Finally,synthetic graphite materials with hierarchical pores and large specific surface areas were successfully prepared by one-step impregnation with lignite as the carbon source,sulfuric acid（H₂SO₄）as the oxidant,and phosphoric acid（H₃PO₄）as the activator.Moreover,its adsorption performance for dye wastewater was investigated.The results show that this very kind of graphite possessed outstanding adsorption performance for methyl orange solution（99.9%adsorbed within 60 min）.The obtained graphite could be used repeatedly,and its adsorption performance maintained above 84.6%after five cycles.There are 65 figures,25 tables and 192 references in this paper.