Study On The Influence Of Oxidative Modification On The Structure And Properties Of Needle Coke Prepared From Coal Tar Pitch

Needle coke is a gray carbon product with metallic luster.It is widely used in the preparation of high/ultra-high power graphite electrode materials and other fields due to its high structural linearity,high true density,low resistivity,low thermal expansion coefficient,high mechanical strength.However,as a large coal country,my country’s coal resources are far more abundant than oil resources,and about 6.6 million tons of coal tar（CTP）is associated with it every year.It has the characteristics of high carbon content,easy graphitization,low cost and easy availability,which makes the development of coal Compared with oil-based needle coke,have natural advantages and is more in line with my country’s national conditions.Although the production and preparation technology of needle coke in my country is relatively mature,the quality of needle coke produced still has some gaps compared with developed countries.For this reason,many researchers have modified coal pitch in different ways,hoping to further improve its structure to prepare needle coke with more fiber structure and better quality.In view of this,in this study,high-temperature coal tar pitch was used as raw material,and impurities such as quinoline insolubles in the pitch were removed by electric field method.Using oxygen as the oxidant,directly oxidizing the refined coal tar pitch（RCTP）obtained after pretreatment,exploring the effect of oxidation modification time on the composition and properties of coal tar pitch.The obtained oxidatively modified coal tar pitch（MCTP）was further carbonized and calcined to obtain anisotropic mesophase structure（AMT）,semi-coke（SC）and needle coke（NC）.Also explored both the effects of modification on AMT,SC,NC and a quantitative relationship curve with good fitting liner between the aromatic hydrogen content of coal tar pitch and AMT content.In order to further improve the coke structure and improve the properties of needle coke order degree and graphitization degree,the effect of graphene addition on AMT,SC and NC was explored,which provided a feasible improvement method for improving needle coke process.The structures and properties of modified coal tar pitch,anisotropic mesophase tissue,semi-coke and needle coke were characterized by means of infrared spectrometer（FT-IR）,hydrogen nuclear magnetic resonance analysis（¹HNMR）,elemental analysis,thermogravimetric analysis（TG）,polarizing microscope,X-ray diffractometer（XRD）and scanning electron microscope（SEM）.The following are the main conclusions reached:（1）The CTP was oxidatively modified under alkaline conditions with oxygen as the oxidant.The softening point of the modified MCTP increased in different degrees,the C and H content decreased,and the O content increased.In MCTP,the content of H_ar increases,the content of H_al decreases,the ratio of H/C decreases,the structure of condensed rings increases,and the degree of polymerization increases,which is conducive to the formation,growth and fusion of mesophase microspheres,and is conducive to the formation of Semi-coke and needle coke with semi-structured crystal structures.On the whole,MCTP has more aromatic C-H stretching vibration peaks and more obvious aromatic ring skeleton vibration absorption peaks.Compared with RCTP,the aliphatic C-H stretching vibration peaks are much weaker,and The C=O stretching vibration peaks are much stronger in MCTP.This indicates that through oxidative modification,the branched chain of light alkanes in coal tar pitch is oxidized into the liquid phase to leave the system or escape from the system in gaseous form such as CO₂,CH₄,which makes MCTP contain a large amount of aromatic structures and aromatic hydrogen,It contains less alkane structure and aliphatic hydrogen,and some oxygen-containing functional groups such as carboxyl group,ester group,etc.are introduced into MCTP through oxidative modification.In addition,through oxidative modification,the thermal stability of MCTP was changed,so that the thermal stability of MCTP was significantly improved,and the carbon residue rate increased,indicating that MCTP has better thermal stability.Among them,the MCTP-5 modified for 5 h has the most significant change,the aromatic hydrogen content is68.15%,which is 26.63%higher than that of RCTP;the carbon residue rate is 53.94%,which is 27.02%higher than that of RCTP.（2）Oxidative modification has a significant effect on the structure and properties of AMT and SC formed by carbonization of coal tar and the structure and properties of NC formed after calcination.Compared with unmodified RCTP,the AMT content formed by carbonization of MCTP is higher,the structure of SC is more ordered,the crystal structure is more regular,and the NC presents a better streamlined fiber structure,the crystal structure is more orderly and uniform,and the crystal plane is more regular.The interlayer spacing is lower,the number of aromatic nucleus layers is larger,the graphitization degree is higher,and the true density is higher.Among them,AMT-5 produced by carbonization of modified coal tar pitch（MCTP-5）for 5 h has the highest content of anisotropic mesophase tissue content,which is 94.1%,which is 35.5%higher than that of unmodified AMT.The semi-coke SC-5 has the densest crystal structure,the lowest interplanar spacing,d₀₀₂ of 0.3490 nm,the largest grain size,L_c of 1.3692nm,and the densest graphite flake stacking;needle coke NC-5 after high temperature calcination The graphite flakes are more densely packed,the interlayer spacing is significantly reduced,d₀₀₂ is 0.3424 nm,the grain size is the largest,L_c is 3.468 nm,the number of streamlined fiber structures is significantly increased,and the number of aromatic nuclei is the largest,M is 11.13,and the graphitization degree G is the highest.is 18.6%,and the true densityρ_T is 2.098 g·cm^-3,which is 0.159 g·cm^-3 higher than that of unmodified NC.（3）Adding different proportions of graphene to MCTP-5 and RCTP has a great influence on the content of AMT produced by carbonization,semi-coke and NC after calcination.With the increasing amount of graphene added,the content of anisotropic mesophase tissue of AMT-G and AMT-5-G,the L_c of SC-G and SC-5-G,the orientation,the graphitization and the streamlined fiber structure of NC-G and NC-5-G have been improved in varying degrees.Among them,when the addition amount of graphene is 0.5 wt.%,AMT-G-0.5 and AMT-5-G-0.5 have the most content of anisotropic mesophase tissue,the L_c of SC-G-0.5 and SC-5-G-0.5are 1.2736 nm and 1.8488 nm respectively,NC-G-0.5 and NC-5-G-0.5 have the most fibrous tissue and the most obvious structure,the smallest interlayer spacing which are 0.3433 nm and0.3411 nm respectively,and the number of aromatic nuclei are 10.27 and 11.35,the degrees of graphitization are 8.14%and 33.7%respectively.Compared with NC-5-G-0 without adding graphene,NC-5-G-0.5 was increased by 15.1%.The true densityρ_T are 2.031 g·cm^-3 and 2.156g·cm^-3 respectively,which are increased 0.074 g·cm^-3 and 0.158 g·cm^-3 respectively.