Process Improvement Of Refining Anthracene And Carbazole From Crude Anthracene By Solvent Crystallization Method

Anthracene and carbazole are crucial chemical raw materials, which can be widely applied in the dyes, pigments, pharmaceuticals, pesticides, photoelectric materials and other industries. Therefore, in recent years, the demand for anthracene and carbazole shows a rising tendency. The isolation of anthracene and carbazole is rather difficult and the solvent crystallization method is frequently applied in the purification of anthracene and carbazole from the crude anthracene of the coal tar. The content of purified anthracene and carbazole gotten from the traditional process are only about 90% and 93-95% respectively, which cannot meet the standard for high purity (>97%) products. Consequently, it is of great practical significance to seek for a suitable solvent system, in which the technology of getting the highly purified anthracene and carbazole from crude anthracene could be improved and promoted through optimizing the process parameters.This research tested the solubility of anthracene and carbazole in dimethylbenzene, chlorobenzene, perchloroethylene and dimethylbenzene-perchloroethylene mixed solvent by liquid-solid balance method, which is the base for the separation of anthracene and carbazole. The results showed that the solubility of anthracene and carbazole increased with the rising temperature; and at a certain temperature, the order of the solubility of anthracene and carbazole was chlorobenzene> dimethylbenzene> dimethylbenzene-perchloroethylene> perchloroethylene in these four solvent systems. Moreover, the solubility of anthracene and carbazole increased gradually with the rising proportion of dimethylbenzene in the dimethylbenzene-perchloroethylene mixed solvent. However, there was no significant difference in the solubility selectivity of anthracene and carbazole with the change of temperature in the four solvent systems. At a certain temperature, the order of the solubility selectivity of anthracene and carbazole was perchloroethylene> chlorobenzene> dimethylbenzene. As for the dimethylbenzene-perchloroethylene mixed solvent, the solubility selectivity was between that of perchloroethylene and dimethylbenzene, and decreases with the increasing proportion of dimethylbenzene in the solvent.There are three stages in the improved process of preparing the highly purified anthracene and carbazole from crude anthracene through the solvent crystallization method:firstly, two steps to take off phenanthrene by dimethylbenzene; furthermore, efficient separation of anthracene and carbazole using DMF or DMF-isopropanolamine mixed solvent; and finally the refinement of crude carbazole for four times with chlorobenzene. The results displayed that the content of phenanthrene decreases from 25.52% to 2.41% and its removal rate reaches 90% or above in the first stage, which alleviated the impact of the impurities such as phenanthrene on the following separation of anthracene and carbazole. During the second stage, The DMF-isopropanolamine mixed solvent (the best ratio of 7:3) was used to separate anthracene and carbazole efficiently, and the content of anthracene in the filter cake and carbazole in the residue was 92.83% and 64.80%, and their single step yield was 81.52% and 70.82% respectively. Using DMF to refine anthracene, the recovery rate was more than 70% in one pass and the content was 98.93%. In the final stage, utilizing chlorobenzene to refining crude carbazole for four times, the content of carbazole could reach 98.97%; without circulation of the mother liquor, the recovery rate of carbazole was more than 40% in one pass. Compared with the traditional process, the purity of anthracene and carbazole is enhanced by the improved process, and the wash times of carbazole has been reduced to 3-4 times from 7-8 times.The result of the material balance in the one pass flow manifested that the solid material loss rates in the process of taking off phenanthrene, isolating anthracene and carbazole and the secondary refining anthracene were less than 4% respectively. However, the solid material loss rate in the process of refining crude carbazole added up from 2.70% to 9% with the increase of refining times. The total loss rate of the solvent during the whole process was 38.45%.Among them, the process to take off phenanthrene accounted for 90.10%, which mainly concentrated in the first wash time; and the separation of anthracene and carbazole and the purification of crude carbazole accounted for about 10% of the total loss rate.To improve the yield of the target product, the mother liquor will be cycled in the refining process of crude anthracene. In this study, cyclic experiments were carried out to investigate the dimethylbenzene from the first step and the filter residue respectively. The results indicated that the content of carbazole can stay above 98% and the yield of carbazole increases by 5% when the filter residue was recycled. While the dimethylbenzene from the first step was recycled, with the increase of cyclic times, the naphthalene of raw materials accumulated and its highest concentration could be up to 0.345 mg/ml in the reclaimed dimethylbenzene. Besides, the content and yield of carbazole in the filter cake decreases of 2.77% and 7.05% respectively.