Natural Graphitization Process Of The Coal-based Natural Graphite

With the new energy revolution,graphite has attracted more and more attention as an important anode material for lithium-ion batteries.Presently,the graphite materials that are put into use in large quantities include natural graphite and synthetic graphite.Among them,synthetic graphite consumes a lot of energy in graphitization and also generates a lot of carbon emissions,which runs counter to China’s goal of "carbon peaking by 2030 and carbon neutrality by 2060".Therefore,more and more attention is paid to natural graphite.Natural graphite can be classified into flake graphite,lump graphite,and coal-based natural graphite.Among them,flake graphite has large resource reserves,since it is easy to flotation,easy to purification,and has a high degree of graphitization,it is widely used in all walks of life,but it also faces problems such as resource exhaustion and high price.While for the coal-based natural graphite,as its ore body is concentrated and easy to develop and utilize,it is treated as a potential substitute mineral.However,in comparison with flake graphite,coal-based graphite is often used in the low-industrial value-added fields due to its small grain size,which is difficult to purify,coexists with coal,and has a low and unstable degree of graphitization,resulting in a serious waste of resources.To solve the existing problems in utilization,promote the transformation of coal-based graphite into high economic value-added industries.It is necessary to have a thorough understanding of the mineral formation,structural evolution,and mineralization of coal-based graphite.However,the current research on coal-based natural graphite often focuses on a single mining area.The formation of graphite minerals in a specific area is affected by the special local geological conditions,so it is difficult to draw the general laws of graphite mineralization,and the obtained samples are not enough to cover each transition phase throughout the coal-to-graphite process,resulting in a lack of dynamic understanding of the entire mineral formation process.We selected samples from major coal-based natural graphite mining areas in China,including Panshi.Jilin province,Lutang,and Xinhua.Hunan province,Jarud.Inner Mongolia,and Fengxian.Shaanxi province,where magma intrusion leads to coal natural graphitization.Firstly,the metamorphic degree and the average structure of the whole coal were characterized by conventional analyses,XRD,and Raman.Method for determining the degree of graphite metamorphism by parameters and the average structure of the whole coal.Using the scanning electron microscope,polarized light microscope,and microRaman spectroscopy were tested.Based on the morphological and structural observation,reflectance,and micro-Raman spectroscopy of the microscopic components.There are great differences in the internal microscopic components of coals and coal-based natural graphites with different metamorphic degrees.In anthracite,there are vitrinite and inertinite.The vitrinite is dominated by desmocollinite and telocollinite;the inertinite is commonly pyrofusinite;In meta-anthracite,there were thermoplastic structure,mosaic structure,crystalline tar,pyrolytic carbon,and other newly formed microscopic microstructures components present;In semi-graphite,these new formed microscopic components were further graphitized;In graphite,there are three types of graphite microscopic components,namely matrix graphite,flake graphite,and crystalline aggregates.Among them,the content of matrix graphite is the highest,the degree of metamorphism of crystalline aggregates is the highest,and the flake graphite is the largest.The three types of graphite are mixed and embedded,and some of them are combined into a graphite shell.According to the optical characteristics and Raman parameters of microscopic components,it can be inferred that different microscopic components follow three graphitization paths.(1)In-situ solid-phase graphitization: The stable component(vitrine or inertinite)is heated and melted to form a thermoplastic structure,and then subjected to magma baking to form a mosaic structure,which is further graphitized to form matrix graphite.(2)Liquid-phase graphitization: the exinite were heated thus a certain degree of graphitization occurs.And the graphitized structural units migrate to the weak pores and cracks under the action of stress and precipitate as liquid tar,and then further graphitize to form dense crystalline graphite.(3)Gas-phase graphitization: The graphitization occurs in which the side chain groups of the small molecules of the exinite or vitrinite are decomposed by thermal decomposition and recrystallized on the surface of the stable component in the form of pyrolytic carbon.After discussing the differential graphitization of coal,we further discuss the macromolecular structure changes during the formation of coal-based natural graphite and quantify the process.First,select representative samples from many places,including anthracite(PSBMS),anthracite(ZLT),meta-anthracite(FX),semi-graphite(BC5),low structural order graphite(SXL3),and high structural order degree graphite(PSZK),observe the evolution law of the coal microcrystalline layer group and the existing structural defects.With the help of the HRTEM image analysis method,the length,orientation,curvature,and stacking of a series of coal-based graphite samples were analyzed respectively.The lattice fringes in the anthracite stage are scattered and spliced to form small microcrystalline domains.Different small microcrystalline domains are connected by various structural defects to form a long and curved graphite layer.With the healing of structural defects,the curved graphite layer gradually becomes.Straight,threedimensionally ordered graphitic structures are formed.The result table of quantitative analysis shows that the lattice fringes of coal evolve gradually from anthracite stage to metamorphic anthracite stage,and there is no sudden change;the orientation increases suddenly from metamorphic anthracite to semi-graphite stage,and after entering the semigraphite stage,the orientation evolution tends to be more stable.The tortuosity evolution also increased sharply from the metamorphic anthracite to the semi-graphite stage,which showed that the stripe aspect ratio was relatively stable and the stripe deflection angle and the number of stripe deflection segments increased,indicating that large-scale splicing of graphite molecules occurred in this stage;The stacking degree shows a sudden change in the stage of graphite with low structural order degree to high structural order degree graphite,which reflects the stacking of small graphite micro-domains to form large-scale micro-domains,and there is a large number of structural defects healing,and the graphite structure further tends to dense.