Preparation And Microstructure Control Of Coal-Based Needle Coke

Needle coke is mainly used as the raw materials for high power and ultra-high power graphite electrode in steel making industry as well as anode of lithium ion battery due to its low coefficient of thermal expansion,high mechanical strength and high electrical conductivity,as a result of its high graphitization degree.However,the quality of the needle coke in the delayed coker is not uniform.The quality of the needle coke from the bottom part of the coker is generally poor,which seriously endangers the development level and international competitiveness of the needle coke industry of China.How to improve the quality of the needle coke is still a big challenge.In the present thesis,high temperature coal tar was used as raw material for the preparation of purified pitch with low quinoline insoluble content（0.05 wt.%）by distillation,extraction and sedimentation.The needle coke was prepared by co-pyrolysis of purified pitch with coking heavy oil or hydrogenated oil.The effect of the raw material composition and carbonization conditions on the formation and microstructure of the needle coke was investigated by the means of GC/MS,FT-IR,¹H-NMR,elemental analysis,polarizing microscope,scanning electron microscope,XRD,Raman,and etc.Based on the mesophase theory,the formation mechanism of the uneven quality of the needle coke was explored and the key factors affecting the microstructure of the needle coke were revealed.The detailed research results are as follows:（1）The formation and microstructure of needle coke are strongly influenced by the raw material composition and liquid phase carbonization conditions.The carbonization temperature and pressure are the main factors that affect the microstructure and formation rate of mesophase pitch.In addition,the content of quinoline insoluble in the raw material,the proportion of circulating oil and the gas flow rate have great effects on the viscosity and molecules orientation of the mesophase pitch at the solidification stage.The experimental results show that,using the mixture of purified pitch and coking heavy oil（mass ratio of 1:1）as raw material,the high-quality needle coke with excellent flow texture and high mesophase content（95 vol.%）could be produced at 495 ^oC for 6 h,under pressure of 1 MPa,with gas flow rate of 0.1 L min^-1.The carbon layer spacing of the green coke is 0.3453 nm.（2）The formation of the mosaic coke in the bottom part of the coker appeared to be caused by the highly reactive components in the coal tar pitch.Using partially saturated hydrogenated anthracene oil as hydrogen-donor solvent can inhibit the rapid formation of the high viscosity mesophase spheres from the highly reactive components in coal tar pitch by increasing the hydrogen free radical concentration,which can thus effectively improve the quality of the coke.The results show that the micromosaic coke in the bottom part of the coker completely disappeared by adding 20%of the hydrogenated anthracene oil under the carbonization condition of 495 ^oC,1 MPa and 6 h.The obtained green coke shows the excellent flow texture and high mesophase content（100 vol.%）.The scanning electron microscopy images showed the highly ordered needle-like texture microstructure of the needle coke,which possesses a carbon layer spacing of 0.3442 nm.