Preparation And Characterization Of Coal-based Mesophase Pitch

The mesophase is a kind of discotic nematic liquid crystal formed by parallel arrangement of planar aromatic molecules.It is the intermediate liquid crystal state that the asphalt organic material transforms into to solid semi-coke.It exhibits optical anisotropy under polarizing microscope.The mesophase pitch is a mixture composed of various discoid fused aromatic hydrocarbons.The mesophase is a good precursor of various advanced carbon material,such as mesophase pitch-based carbon fiber,mesophase pitch binder and mesocarbon microspheres,because it can be made from various raw materials and has excellent performances and hight carbon yield.These advanced carbon materials have played an irreplaceable role in aircraft manufacturing,steel smelting and advanced technology.Petroleum and coal-based pitch are the main crude materials of intermediate phase asphalt bitumen.In China,oil resources are scarce,coal resources is relatively abundant,moreover,a large number of high-temperature coal tar produced from coking industry is badly in need of treatment.Therefore,the development of the technology for producing mesophase pitch from coal-based asphalt is very important to make full use of our advantages of resources.It is also vital to the sustainable utilization of resource.In this study,high temperature coal tar as raw material was used to prepared coal-based pitch with suitable softening point and quinolone insoluble(QI)matter by vacuum distillation and heat treatment.The mixed solvent-static method was utilized to purify the coal-based pitch in order to obtain purified coal-based pitch with QI less than 0.1%and high yield.The refined asphalt was prepared by catalytic hydrogenation of purified coal-based pitch.Finally,coal-based mesophase pitch was prepared by thermal polycondensation treatment of refined asphalt.Furthermore,the process parameters of each preparation step were investigated in detail and optimized.The main results and conclusions are summarized as following:(1)The high temperature coal tar was treated by～vacuum distillation to obtain coal tar pitch with a soting point at about 155~165 ℃,appropriate QI content and higher yield.The effects of vacuum heat treatment temperature,nitrogen flow rate and decompression time on the softening point,QI content and yield of coal tar pitch were investigated.It was found that the softening point and QI content of coal tar pitch increased while the yield of coal tar pitch decreased with the increase of the decompression time,heat treatment temperature and N2 flow rate.This is ascribed to the fact that volatilization of small molecular components and dealkylation reaction of aromatic side chain mainly occurred during the decompression process.The higher degree of decompression led to the higher softening point,higher QI,lower yield of coal tar pitch.When the coal tar pitch was prepared by the heat treatment at 320 ℃with N2 flow rate of 0.25 cmL.g-1.min-1 for 160 min.The softening point of coal tar pitch is 160 ℃,the QI is 9.73%，the yield is 45.3%,which were accord with the requirements.(2)The QI matter in the coal tar pitch was removed by mixed solvent-static method to obtain purified coal-based pitch with QI less than 0.1%.The influences of the ratio of diesel oil/wash oil,solvent ratio,sedimentation separation temperature and settling time on the QI and yield of purified coal-based pitch were studied.The optimum process parameters were determined.The purified asphalt with QI less than 0.1%and 43.4%yield was obtained,when the coal tar pitch was treated at settling temperature 110 ℃ for 3 h with diesel oil/wash oil = 40%/60%and the solvent ratio(mixed solvent/coal tar pitch)= 2/1.(3)The purified asphalt was treated by catalytic hydrogenation to gain refined asphalt.The effects of the temperature and time of hydrogenation,filling the initial pressure of H2 and other factors on the qualities of refined asphalt were investigated.The optimum hydrogenation parameters were determined.The refined asphalt with the QI less than 0.1%,6.85%of the QS-TI component and 65.82%of the TS-HI component was obtained,when the purified asphalt was hydrogenated at 380 ℃ with 9 MPa of the initial hydrogen pressure for 3 h.(4)The refined asphalt was treated by thermal polycondensation to prepare coal-based mesophase pitch.The effects of heat polymerization temperature and constant temperature time on the characters of mesophase pitch were studied.These results showed that the softening point,anisotropy content,TI and QI of mesophase pitch increased,while the yield of mesophase pitch decreased with the increase of heat polymerization temperature and constant temperature time.It was due to the fact that the dehydrogenation and polymerization of the asphalt molecules happened mainly during the thermal polycondensation process.We can achieve the same effect at high temperature for a short time or at low temperature for a long time.However,if the temperature is too high,the thermal polymerization reaction is intense and the dehydrogenation and polymerization of the asphalt molecules are generated,which casuing the poor mesophase.At the optimized temperature,the asphalt molecules gradually polymerized to form the fused aromatic hydrocarbons with larger molecular weight,while aromatic macromolecules grew up and self-assembled to form the mesophase pitch with nematic phase,as a result,the microcrystalline structure of mesophase would be more perfect.The optimum process parameters were also determined.The mesophase pitch with softening point at 252 ℃,85%of anisotropic content,67.73%of TI,52.18%of QI was prepared at 420 ℃ for 10 h.