| Compared with other coke structure,needle coke has extremely important application value in high power,ultra high power graphite electrode,battery electrode material,nuclear protection material and so on,because of its high orderly structure,high density,low resistivity and high mechanical strength.Because China’s coal resource is much more abundant than oil resource,coking companies in the production process obtain a considerable amount of coal tar pitch(CTP).CTP has high carbon content,low prices and is easy to be graphitized,which makes CTP an important source of raw materials for the preparation of needle coke with highly ordered structure.By studying the effects of modification,co-carbonation and external force field on the formation of mesophase,improving the structure and performance of needle coke has become the breakthrough of scientific research.This study is of great significance to achieve the efficient use of coal resource and to increase the added-value of downstream products of coal.In this paper,high-temperature CTP was used as material.In order to avoid the influence of large QIs and impurities on the structure of coke.The CTP was first refined under the role of the electric field so as to obtain refined CTP.Then,using PP as additive and mechanical stirring force as the guiding force,the influence of both PP and mechanical stirring force on the structure and performance of mesophase,semi-coke and needle coke was explored.In order to further reduce the viscosity of the system,selecting the engine oil as the dispersion medium,the synergistic effects of EO and mechanical agitation on the structure and properties of semi-coke and needle coke were investigated.Finally,more anisotropic structures were formed through the secondary growth of MCMBs so as to improve the structure and performance of the coke.The structure and performance of mesophase,semi-coke and needle coke were characterized by OM,particle size analyzer,XRD,TG,SEM,BET,EQCM,resistivity meter and other analytical means.The conclusions are presented as follws:1.The addition of PP can effectively enhance the content of anisotropic mesophase structure and the most suitable amount is 10 wt.%.The mechanical stirring force has a significant guiding effect on the fused ring aromatic molecules.The rate of stirring and the mode of action have great influence on the molecular orientation.Specifically,the effect of intermittent stirring is better than continuous stirring.Stirring rate should be appropriate.If the rate is too small,the resulting force is not enough to produce a guiding role on the molecule.If the rate is too big,strong force will disrupt the arrangement of molecules.The semi-coke which was obtained under the condition of intermittent stirring rate of 100 rpm has not only the most fibrous structure but also the highest carbonization yield.The needle coke has a minimum resistivity of 676 Ω·mm~2/m,which is about 50% of the resistivity of the unsweetened sample.2.Engine oil,as dispersion medium of CTP,can be cracked and gasified to generate hydrocarbon radicals with higher activity and small molecule gases in the process of closed high temperature carbonization.Hydrocarbon radicals can react with large aromatic molecules in the mother liquor.This reaction is called alkyl radical transfer reaction.The gas flow produced by small molecules also has important impacts on the orientation of mesophase liquid crystal molecules.The main influences are presented as follows: The viscosity of the carbonized system is effectively reduced and the viscosity value is reduced from 220 mPa.s to 60 mPa.s,which improves the flow performance of the mesophase and enhances the reactivity.At the same time,the removal of small molecule gases exerted a certain shear force on the fused ring aromatic molecules.The size of shear force depends on the addition amount of engine oil.The most suitable amount is 3 wt.%.Excessive addition amount easily leads to the chaos of escaping direction of small molecules,which gives rise to the chaotic arrangement of fused macromolecules and is conducive to the formation of fiber structure.The synergistic effects between EO and stirring role with a certain rate exerted positive parts on the accumulation of mesophase building units and the full escape of small molecule gases.The optimal stirring rate is 60 rpm.The content of semi-coke fiber reached 72.63%,which was nearly 20.83% higher than that without stirring.The resistivity of needle coke has a minimum value of 539 Ω·mm~2/m.This proves that EO is better than PP in terms of improving the structure of semi-coke and enhancing the performance of the coke.3.The secondary growth of the mesocarbon microbeads can increase the size of the particles,which is beneficial to the formation of more anisotropic mesophase tissue,thereby improving the semi-coke structure and reducing the resistivity of the needle coke.Specifically,the addition of MCMBs has an important effect on secondary growth and the most suitable amount is 15 wt.%.The average size of the obtained mesocarbon microbeads was 36.62 μm,which increased by 20% compared to that of the first growth.The softening point was 168 °C,which increased by 26°C compared to the first growth.When the addition ratio exceeds a certain critical value,the formation of the mesophase is saturated and the size and number of MCMBs do not increase any more.The secondary growth of MCMBs increased the content of anisotropic tissue,laying a better material basis for the formation of ordered fiber structure.The semi-coke significantly improved.The proportion of fibrous tissue reached the maximum value of 66%,which increased by 19% compared to the first growth.The electrical performance of needle coke has been effectively improved.And the minimum value of the resistivity value is 1080 Ω·mm~2/m,which is reduced by 880 Ω·mm~2/m. |
