Modifying Coal Tar Pitch Through Chemical Cross-linking Method To Reduce Toxic Pahs Content

With the rapid developments of the National Highway Network and urban road construction, road asphalts with the higher quantity and quality are demanded. Coal tar pitch(CTP) is rich in resources and inexpensive in prices in China, but then due to the content of toxic polycyclic aromatic hydrocarbons(PAHs) in CTP exceeds the relevant environmental standards and its application in road construction is limited. To solve the problems,such as excess capacity of CTP, expensive prices and insufficient production of petroleum pitch, using CTP as asphalt for road construction is a good way and modifing CTP to suppress its toxicity before using is necessary.In this paper, CTP was modified with the cross-linking agents p-phthalaldehyde /1,4-benzenedimethanol over the catalysis of p-toluenesulfo--nic acid in cyclohexane solvent to reduce the toxic PAHs content. The modified conditions were optimized and some basic properties of CTP before and after modification were measured and compared. Finally, the modified mechanism was preliminarily exploried through the reactions between model compounds Anthracene, Fluoranthene, Pyrene, Benzo[g,h,i]perylene and modifiers. The main conclusions are as follows:(1) Under the effect of the solvent cyclohexane, p-toluenesulfonic acid as catalyst, using cross-linking agents p-phthalaldehyde /1,4-benzenedimethanol to chemically modify CTP for detoxification and the toxic PAHs contents were significantly reduced. Since the relatively large dissolved amount of CTP and relatively good dissolved selectivity of toxic PAHs in cyclohexane solvent, coupled with the solvent role in making the toxic PAHs, modifiers and catalyst forming a relatively homogeneous reaction system, two main issues of high modification temperature and low utilization rate of modifiers in conventional modified methods are well solved.(2) The optimum conditions of using p-phthalaldehyde to modify CTP were as fllows: 8 %(relative to the amount of CTP) additive amount of p-phthalaldehyde, 5 % additive amount of p-toluenesulfonic acid, the reaction temperature was 60 ℃ and the reaction time was 4 h. Under that conditions the total content of 16 kinds of toxic PAHs decreased by 82.75 %. After modification, the CTP softening point reduced from 101.5 ℃ to 77.1 ℃ and the CTP type has been transformed from high temperature CTP into medium temperature CTP. The CTP toluene insolubles content increased from 26.36 % to 44.91 % and quinoline insolubles content reduced from 9.30 % to 1.01 %. The CTP coking values remained unchanged. 8 kinds of toxic PAHs were detected in the CTP fume, the average reduction rate of which was 8.68 %. The CTP water solubles included 6 kinds of toxic PAHs and the total content increased by 1.52 time.(3) The optimum conditions of using 1,4-benzenedimethanol to modify CTP were as fllows: 6 % additive amount of 1,4-benzenedimethanol, 12 % additive amount of p-toluenesulfonic acid, the reaction temperature was 50 ℃ and the reaction time was 2 h. Under that conditions the total content of 16 kinds of toxic PAHs decreased by 74.14 %. After modification, the CTP quinoline insolubles content slightly reduced from 9.30 % to 7.96 %. The CTP softening point increased to 118.4 ℃ and the CTP toluene insolubles content increased to 40.81 %. The CTP coking values also remained unchanged. The average reduction rate of 8 kinds of toxic PAHs in the CTP fume was 17.67 % and the total content of 6 kinds of toxic PAHs included in the CTP water solubles increased by 1.91 time.(4) Through the research and analysis by GC, FT-IR and TG-DTG on the reaction between CTP/model compounds and modifiers, it can be concluded that the electrophilic substitution cross-linking reaction took place between PAHs and p-phthalaldehyde/1,4-benzenedimethanol. Besides as to the reaction, the reactivity of PAHs increased with the number of its aromatic rings.