CC BY
21Chemical Journal of Kazakhstan
ISSN 1813-1107, elSSN 2710-1185 https://doi.org/10.51580/2021-1/2710-1185.24
Volume 2, Number 74 (2021), 14 - 20
UDC 662.749.38
OBTAINING OF MESOPHASE PITCHES FROM COAL TAR
A.M. Imangazy1'2
1 JSC «A.B. Bekturov Institute of Chemical Sciences», Almaty, Kazakhstan 2 JSC «Al-Farabi Kazakh National University», Almaty, Kazakhstan E-mail: kazpetrochem@gmail.com
Abstract: This article presents the results of research on mesophase pitch production from coal tar. The preparation of mesophase pitch was carried out by heat treatment in an argon atmosphere at temperatures of 300, 350, and 400 °C. The resulting carbon pitches were analyzed by scanning electron microscopy, Raman spectroscopy, and energy-dispersive analysis. An increase in the degree of surface degradation and the number of mesophase centers per unit area was observed with an increase in the treatment temperature to 300 °C. At 350 °C, a transition from an isotropic to an anisotropic structure was observed, where the mesophase centers were about 2 ^m in size. A similar aniso-tropic structure was observed for a sample of coal tar obtained at 400 °C, and in some areas, a layered structure was observed, which could be associated with an increase in the graphitization degree of the samples. The particle size of the mesophase increases to 3.55 microns. The results of energy dispersive analysis showed that an increase in temperature leads to a decrease in the sulfur content. At 400 °C, sulfur is completely removed from the coal tar pitch composition. A correlation between the heat treatment temperature and the structure of the obtained pitch was established.
Keywords: coal tar, processing, heat treatment, mesophase pitch. 1. Introduction
Coal is widely used as a heating agent and in power generation [1], and is also an initial material in the production of valuable products. Coal coke is a product of coking coal processing, which has found its application in industry, in the production of steel and cast iron, in the chemical industry, etc. In the process of coal coking, coke oven gas, coal tar, and a mixture of aromatics are released [2].
Coal tar is a complex mixture of aromatic hydrocarbons, heterocyclic sulfur-, oxygen- and nitrogen-containing compounds. Tars, according to their chemical properties, are divided into three groups: neutral, acidic, and basic [3]. According to [4], the world coke production in 2019 amounted to about 700 million tons, at which about 35 million tons of coal tar were produced and about 50% of which was further processed. By further processing, such valuable products as benzene, toluene, xylenes, etc, as well as impregnating oil, plastics, electrode pitches, carbon fibers, binder pitch, etc. could be obtained.
Citation: Imangazy A.M. Obtaining of mesophase pitches from coal tar. Chem. J. Kaz., 2021, 2(74), 14-20. DOI: https://doi.org/10.51580/2021-1/2710-1185.24
One of the possible ways to process coal tar is to produce pitch. There are two main types of pitch: isotropic (non-mesophase) and anisotropic (mesophase). Mesophase pitches are usually obtained by the heat treatment, as a result of which chemical reactions occur with a change in structural characteristics - the formation of mesophases. The heat treatment was carried out in a wide temperature range in an inert atmosphere of nitrogen, argon, or helium [5]. The formation of liquid crystal structures (mesophase) occurred in the temperature range 300-500 °C [6]. Mesophase crystallites were composed of condensed high molecular weight aromatic compounds with an interplanar spacing of 0.34 nm. The course of mesophase transformations depends on the physicochemical characteristics of the feedstock and the temperature regime of processing. An important characteristic of pitch is the content of sulfur and insoluble residues, which determine the quality of the pitch [7]. The preparation of mesophase pitches with a high degree of aromaticity is described in [8]. In this work, the results of studies on the production of polyaromatic resins by extraction of low-temperature soot are presented. The quality of mesophase pitches and their fiber-forming properties were determined by the degree of aromaticity of the original resin, the presence of highly condensed structures in it [9]. The transition of carbon pitch into the mesophase structure occurs through the stage of an intermediate isotropic-mesophase structure formation, under the temperature influence. The transition is accompanied by the removal of gaseous products and a change in the H/C ratio [10].
In this work, coal tar obtained during the processing of coal from the Shubrakol deposit (Kazakhstan) was used. The reserves of the Shubrakol deposit are about 1.5 billion tons. The consumers of special coke produced from Shubarkol coal are the largest enterprises of Kazakhstan: ferroalloy plants of «KazChrome» JSC, «Kazphosphate» JSC, «KazZinc» JSC, Russia and Ukraine. Some scientists [12] were conducting research on the extraction of hydrocarbon products from coal tar, as well as the production of boiler coke fuel as an alternative to fuel oil.
2. Experimental part
Coal tar is a thick black liquid with a specific odor and a viscosity of 1.35 g/cm3. For the experiments, the mesophase pitch preparation was carried out by heat treatment in an argon atmosphere at temperatures of 300, 350, and 400 °C. The argon flow rate was 90 cm3/min. Heat treatment was carried out in a tubular furnace with a quartz reactor with a diameter of 3 cm. A pre-dried and weighed porcelain boat was filled with initial coal tar, after which the boat was placed in a quartz reactor and purged with argon to remove air from the reactor and exclude contact with oxygen. Argon was passed for 5 minutes, after which the reactor was heated to temperatures from 300 to 400 °C. The heating rate was 13 °C/min. The time of temperature treatment was 2 hours; after the end of the heat treatment process, the heating of the reactor was stopped, the sample was cooled to room temperature without being removed from the reactor in an argon
atmosphere. After removing it from the reactor, the boat with the final product was weighed to establish the weight loss.
After heat treatment, the initial coal tar from a viscous-flowing state goes into a solid, with an increase in volume. The resulting coal tar pitch has a porous structure, which occurs due to the removal of low-boiling fractions in the form of vapors, which lead to the formation of loose and spongy material.
The resulting product was investigated by scanning electron microscopy (SEM), EDAX analysis, and Raman spectroscopy.
3. Results and discussion
The samples of coal tar before and after the heat treatment at various temperatures were weighed to determine the mass loss (Table 1).
Table 1 - Coal tar masses before and after the heat treatment
Heat treatment temperature, °С Mass of coal tar before the heat treatment, g. Mass of coal tar after the heat treatment, g. Mass loss value, %
300 3.5 1.9 46
350 2.4 1.2 50
400 2.3 0.9 61
Based on the data in Table 1, it can be seen that the highest weight loss was observed at a temperature of 400 °C and is 61% of the initial weight. The heat treatment removed volatiles from the coal tar. After the heat treatment at different temperatures, coal tar was examined to Quanta 200i 3D scanning electron microscope (Figure 1a).
Figure 1a shows the scanning electron microscopy images. Analysis of the images shows that at 300 °C, an increase in the degree of surface degradation is observed. At a processing temperature of 350 °C, a transition from an isotropic to anisotropic structure is also observed. For this sample, all volatile fractions were removed. The sample surface was uniform, the size of the mesophase centers was about 1.5-2 ^m. For a coal tar pitch sample obtained at 400 °C, a similar anisotropic structure was observed; in some areas, a layered structure is observed which is associated with an increase in the degree of graphitization of the sample. The sizes of mesophase particles increased to 3.5-4.5 ^m.
To establish the effect of the heat treatment on the composition of the initial coal tar, elemental analysis was carried out for coal tar pitch obtained at different temperatures (Figure 1b). For samples of coal tar pitch obtained at temperatures equal to 300 and 350 °C, the sulfur content was observed from 0.24 to 0.26 wt.%. An increase in the processing temperature to 400 °C leads to the complete removal of sulfur from the composition of coal tar pitch. Sulfur is contained in coal tar in the form of sulfur-containing heterocyclic aromatic compounds.
Figure 1c shows the Raman spectra of coal tar pitch obtained by processing the initial coal tar at different temperatures. Raman spectroscopy analysis was carried out under excitation by unpolarized radiation of a semiconductor diode laser at a wavelength of ^exc = 473 nm. The interpretation of the Raman spectra
Figure 1 - Images of scanning electron microscopy (a), EDAX analysis (b), Raman spectra (c) of coal tar after the heat treatment at different temperatures: 300 °C, 350 °C, and 400 °C.
was carried out on the basis of the analysis of the review article [13]. Figure 1 shows a graph of the combined spectra.
Analysis of Raman spectra allows one to evaluate the effect of the heat treatment on the degree of graphitization of the initial coal tar. For pure graphite, two main first-order peaks were observed at a wavelength of 1356 cm-1 (D-peak, Defective Raman zone) and 1575-1582 cm-1 (G-peak due to the presence of carbon atoms in the sp2 state and located in planes of graphite grids), spectral lines of the second order in the region of ~ 2710 cm-1. While interpreting Raman spectra, the following indicators are important: AD, AG - the values of wavelengths for D and G peaks, respectively, in cm-1; ID, IG - the intensity of D and G peaks in relative units; R - the ratio of intensities of D and G peaks (ID/IG). While graphite nanocrystallites appear in the sample, the G peak shifts from 1575-1582 cm-1 to higher values of ~1600 cm-1. In the sample treated at 300 °C, a shift of the G peak to the region of higher frequencies ~ 1405-1428 cm-1 was observed, this could be explained by the fact that the samples contain clusters with a small number of aromatic rings. The generalized data on the wavelengths of the G and D peaks, their intensities, and the R index are presented in Table 2. For the sample treated to 350 °C, a significant change in the intensities and positions of the G and D peaks was observed, characterized by the removal of all volatile fractions and the beginning of the transition from a disordered structure to
a more ordered structure with the formation of mesophase centers. For the samples treated to 400 °C, a shift of the D peak to the range of 1600-1610 cm-1 was observed, which is explained by the formation of nanocrystalline mesophase centers.
Table 2 - Characteristics of G and D peaks for coal tar samples treated at different temperatures
Sample XG, cm-1 Ш, cm-1 Ig Id R(Id/Ig)
300 °С 1406 1605 792 1042 -
350 °С 1356 1600 1103 1606 0.6867
400 °С 1362 1610 395 638 0.7215
Based on the data in Table 1, an estimated calculation of the degree of graphitization can be made using the equation [14]:
g(%) = [l -£]• 100 (1)
where g is the degree of graphitization, %; R is the ID/IG ratio; n is the maximum value of R obtained during the study (in this case, 0.7215).
Thus, the calculation showed that for the 350 °C sample the degree of graphitization was about 5%. For the 400 °C sample, this indicator could not be determined, since the R value was taken as the maximum value (n).
4. Conclusion
Based on the conducted research, a correlation between the heat treatment temperature and the structure of the resulting coal tar pitch was established. The results of scanning electron microscopy showed that with an increase in the heat treatment temperature, the number of mesophase centers per unit volume of the final pitch increases. The diameters of mesophase particles range from 3 to 15 ^m. The highest number of mesophase particles was observed for pitch obtained at a temperature of 400 °C. The results of the energy-dispersive analysis showed that heat treatment at a temperature of 400 °C leads to the complete removal of sulfur. On the basis of Raman spectroscopy data, the calculated graphitization values of coal tar pitch obtained at 350 °C were about 5%.
Acknowledgments: I would like to express my gratitude to Smagulova Gaukhar and Kaidar Bayan (al-Farabi Kazakh National University, Almaty, Kazakhstan) for their help in providing this research and discussing its results. Also, I would like to thank Renata Nemkayeva (National Nanotechnology Laboratory, Almaty, Kazakhstan) for the provided microscopic and spectroscopic analysis of the samples obtained in this research.
Information about author:
Imangazy A.M. - Scientific Researcher, e-mail: kazpetrochem@gmail.com, ORCID ID: https://orcid.org/0000-0001-7834-1022
References
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Тушндеме
К6М1Р ШАЙЫРЫНАН МЕЗОФАЗАЛЫ ШАЙЫР АЛУ А.М. Имангазы1'2
1 «Э.Б. Бектуров атындагы Химия гылымдары институты» АЦ, Алматы, Цазахстан
2 «Эл-Фараби атындагы Цазащ упттыщуниверситетi» АЦ, Алматы, Цазахстан E-mail: kazpetrochem@gmail.com
Б^л макалада KeMip шайырынан мезофазалы шайыр алу жешндеп зерттеу-лердщ нэтижелерi келпршген. Мезофаза кадамын дайындау аргон атмосферасында 300, 350 жэне 400 °C температураларда термиялык ендеу аркылы жузеге асырылды. Термиялык ендеу температурасы мен алынган шайырдьщ к¥рылымы арасында езара байланыс орнатылды. Алынган кeмiртегi кабаттарын электронды сканерлеу микроскопиясы, Раман спектроскопиясы жэне энергия дисперсиялык анализi аркы-
лы зерттелд^ вндеу температурасыныц 300 °C дейiн жогарылауымен беттщ деградация дэрежесiнiн жэне аудан бiрлiгiне шаккандагы мезофаза ортальщтарыньщ санынын артуы байкалды. 350 °C температурасында изотроптыдан анизотропты к¥рылымга эту байкалады, м^нда мезофаза орталыктарыныц мeлшерi шамамен 2 мкм к^райды. ¥ксас анизотропты к¥рылым 400 °С температурада алынган кeмiр шайырыныц Yлгiсi Yшiн байкалды, ал кейбiр аудандарда кабаттар к¥рылымы байкалды, б^л Yлгiлердi графиттеу дэрежесiнiн жогарылауымен байланысты болуы мYмкiн. Мезофазанын бэлшектер мeлшерi 3.5-5 микронга дейiн артады. Энергети-калык дисперсиялык талдаудын нэтижелерi температуранын жогарылауы куюрт-тiн тeмендеуiне экелетiнiн кэрсетп. 400 °C температурада кYкiрт кeмiр шайыры шайырыныц к¥рамынан толыгымен алынады.
ТYЙiн свздер: кeмiр шайыры, eндеу, термиялык eндеу, мезофазалы шайыр.
Резюме
ПОЛУЧЕНИЕ МЕЗОФАЗНЫХ ПЕКОВ ИЗ КАМЕННОУГОЛЬНОЙ СМОЛЫ А.М. Имангазы1'2
1 АО «Институт химических наук им. А.Б. Бектурова», Алматы, Казахстан
2 АО «Казахский национальный университет им. аль-Фараби», Алматы, Казахстан E-mail: kazpetrochem@gmail.com
В данной статье представлены результаты исследований по получению мезо-фазного пека из каменноугольной смолы. Получение мезофазного пека проводили путем термической обработки в атмосфере аргона при температурах 300, 350 и 400 °С. Была установлена корреляция между температурой термообработки и структурой полученного пека. Полученные углеродные пекы были исследованы методами сканирующей электронной микроскопии, Раман-спектроскопии, и энергодисперсионного анализа. Увеличение степени деградации поверхности и количества мезофазных центров на единицу площади наблюдалось при повышении температуры обработки до 300 °C. При 350 °C уже наблюдается переход от изотропной структуре к анизотропной, где размер мезофазных центров составляет около 2 мкм. Аналогичная анизотропная структура наблюдалась для образца каменноугольной смолы, полученного при 400 °C, а на некоторых участках наблюдалась слоистая структура, что могло быть связано с увеличением степени графитизации образцов. Размер частиц мезофазы увеличивается до 3,5-5 мкм. Результаты энергодисперсионного анализа показали, что повышение температуры приводит к снижению содержания серы. При температуре 400 °С сера полностью удаляется из состава каменноугольного пека.
Ключевые слова: каменноугольная смола, переработка, термообработка, мезофазный пек.
