Subcritical Extraction of Coal Tar Slag and Analysis of Extracts and Raffinates

Abstract

Coal is an important energy source for the development of modern society. The processing and utilization of coal have brought significant economic benefits for people, but at the same time, a large amount of coal-based solid waste is produced. Coal tar slag is one of the many types of solid waste. Coal tar slag contains a large number of PAHs (Polycyclic Aromatic Hydrocarbons) that are carcinogenic to humans and are therefore categorized as hazardous waste. There is a large historical stockpile of coal tar slag, and if not properly handled, it will cause great harm to people and the environment; therefore, the disposal of coal tar slags is a hot issue for scholars to study. In this paper, the toxic PAHs in coal tar slag were removed by subcritical extraction, and the extracts and raffinates were tested using infrared spectroscopy, GC-MS (gas chromatography–mass spectrometry), and PY-GC-MS (Pyrolysis Gas Chromatography–Mass Spectrometry). The results showed that after three subcritical extractions at 60 °C, there were obvious aromatic hydrocarbon absorption peaks in the extracts, and the intensity of aromatic hydrocarbons in the extracts was significantly reduced. In the first extract, mainly aliphatic hydrocarbons and aromatic hydrocarbons dominated; the relative content of aliphatic hydrocarbons was 28.68%, and the relative content of aromatic hydrocarbons was 56.56%. In the second extract, mainly aliphatic hydrocarbons and ethers dominated; the relative content of aliphatic hydrocarbons was 37.11%, and the relative content of ethers was 18.5%. In the third extract, mainly aliphatic hydrocarbons dominated, and the relative content of aliphatic hydrocarbons was 81.78%. Only one substance, benzaldehyde, was detected in the PY-GC-MS results of the third extract, and this substance is not included in the national hazardous waste list. After subcritical extraction, the coal tar residue is transformed from coal-based hazardous waste to coal-based solid waste that can be used directly, which is convenient for people to recycle coal tar residue in the future.

1. Introduction

The world’s three major fossil energy sources are oil, natural gas, and coal, but China’s energy structure is “rich in coal, poor in oil, and poor in gas”, so coal will dominate China’s social development for a long time [1]. The development and use of coal have introduced good economic benefits for social development, but a large amount of coal-based waste has been generated in the process of mining and utilization [2,3]. Coal tar slag is a kind of viscous solid residue produced in the process of coal pyrolysis and coking, with an irritating odor, which contains heteroatomic compounds such as N, S, etc., and it is mixed with a large number of polycyclic aromatic hydrocarbons (PAHs). These PAHs, such as naphthalene, fluoranthene, pyrene, and benzo[a]pyrene, etc., have a high degree of carcinogenicity to human beings and are classified as hazardous waste by the U.S. Environmental Protection Agency (USEPA), especially benzo[a]pyrene. The gastric cancer induction rate in mice can reach 8% when 1 mg of benzo[a]pyrene is ingested orally; moreover, the carcinogenicity rate increases to 70% when the total dose is 10 mg. Benzo[a]pyrene mainly causes tumors in epithelial tissues, such as skin, lung, stomach, and digestive tract cancers [4,5]. China is a large coal utilization country; every year it produces a large amount of coal tar slag. If not disposed of properly, this coal tar slag will cause great harm to the environment and people. The clean and harmless recycling of coal tar slag has been identified as a major problem in studies [6,7].

At present, people commonly use coal tar slag as a resource in coal coking, as a fuel, and in the preparation of carbon materials [8]. Chen [9] investigated the coking mechanism of coal liquefaction residue in coal coking by mixing the coal direct liquefaction residue with coking coal. He found through experiments that adding 10% of coal direct liquefaction residue can reduce the high caking coal by 13%, and the mechanical strength of coal can be increased from 90% to 97%. The direct coal liquefaction residue can improve the quality of coke by squeezing the viscous metamorphic layer into the pores between the coal particles at the critical stage of coal pyrolysis (380–450 °C). In the composition of coal tar slag, it can be seen that the fixed carbon content in coal tar slag is high, so its calorific value is high, and it is a good secondary energy source; however, because its combustion may produce NO, SO₂, and other gases that can be harmful to the environment and people, it is necessary to pre-treat it. Xu [10] investigated the reduction in the viscosity of coal tar slag with the aid of microwave treatment by using thermogravimetric tests and tube furnace combustion to investigate the combustion phenomena of the coal tar slag before and after treatment. The experimental results show that high temperature and high liquid–solid ratio in microwave-assisted coal tar slag treatment can greatly reduce its viscosity. Moreover, after the combustion of coal tar slag, the NO emission was less than 140 mg/m³, and the SO₂ emission was less than 100 mg/m³. The combustion of coal tar slag treated in this way reduces the emission of toxic gases, and it can be used as a fuel for fluidized beds used in industrial production. Due to the high carbon content in coal tar slag, it can be used as an excellent raw material for the preparation of new carbon materials. Liang [11] utilized semi-coke to prepare Ni/C-based composite microwave absorbers. The synthesis mainly involved loading an ethanol solution of nickel nitrate onto SC followed by in situ carbothermal reduction. The results showed that thermal treatment at 700 °C and above with a fixed Ni loading of 20 wt% could exploit the properties of the SC carbon-based material by removing most of the coal tar and also inducing complete reduction.

The above-described treatments for the recovery of coal tar slag were used directly, ignoring the aromatic hydrocarbons in the coal tar slag. People used to commonly extract aromatic hydrocarbons in coal tar slag for ordinary solvent extraction and supercritical extraction. Ordinary solvent extraction is simple and easy to realize, but the extraction effect is poor, and commonly used organic solvents have a certain degree of toxicity, which can easily harm personnel. The effect of supercritical extraction is good, and the extractant is generally CO₂, non-toxic, and non-hazardous, but the pressure required for supercritical extraction is too great, and the cost of the equipment is high. In this article, we used a new extraction technology to recover aromatic hydrocarbons from the coal tar slag, which are important raw materials in chemical production. Removing these aromatic hydrocarbons allows the coal tar slag to be more easily recycled as solid waste [12]. Subcritical extraction is a novel separation technology that uses subcritical fluids as extractants. In this state, the extractant is above its boiling point but below its critical temperature and pressure. Under these conditions, molecular diffusion enhances mass transfer rates, and the permeability and solubility of weakly polar and nonpolar substances significantly improve. Butane is commonly used as the solvent. The process involves immersing the material in the extractant in a closed, oxygen-free, low-pressure vessel, allowing fat-soluble components to transfer to the liquid extractant through molecular diffusion. The extractant is then separated from the product via reduced-pressure evaporation [13,14]. Subcritical extraction is a low-temperature technology that preserves the active ingredients of the extract without destruction or oxidation. It is efficient, energy saving, and suitable for the large-scale industrial production. The extractant and target product are easily separated, improving product purity and quality. The entire process is non-toxic, environmentally friendly, and aligns with modern green chemistry trends. In summary, subcritical extraction is an efficient, environmentally friendly, energy-saving technology with broad applications. As technology advances, it will play an important role in agriculture, medicine, cosmetics, and other industries [15,16]. Ding [17] separated high-temperature coal tar samples into 10 narrow oil fractions and one solid-phase asphaltene product using supercritical fluid extraction with n-pentane at 220 °C and pressures ranging from 5 to 15 MPa. The results showed that the cumulative yield increased with pressure, especially at lower pressures, and the rate of increase gradually decreased. The total yield of the oil extract was 78.36%, while the bitumen yield was 21.64%.

Fourier Infrared Spectroscopy (FTIR) determines the chemical composition of a substance by analyzing the specific vibrational frequencies of chemical bonds between molecules, which produce characteristic absorption peaks under infrared light. GC-MS separates substances based on properties such as the boiling point or polarity, identifies the signals transmitted by the GC through the mass spectrometer, and generates mass spectra. PY-GC-MS involves thermally cracking solid substances to break large molecules into smaller ones, which are then analyzed using GC-MS [18,19]. Qi [20] pyrolyzed two Victorian lignite samples and characterized the pyrolysis products using GC-MS and PY-GC-MS. The results showed that the low-temperature pyrolysis products contained abundant triterpene polycyclic compounds and aliphatic hydrocarbons, while the yield of aromatic hydrocarbons increased significantly with a higher pyrolysis temperature. Sun [21] used GC-MS to analyze the composition of washed fractions from low-temperature and high-temperature coal tar. The experimentals revealed that petroleum ether effectively extracted light fractions, primarily consisting of aliphatic hydrocarbons, aromatic hydrocarbons, and phenolics. Four dehydrogenation reactions occurred in the rapid pyrolysis process, including alkane dehydrogenation, alkane cleavage, polymerization, and bridge bond cleavage. This method is also applicable for the quantitative analysis of oil sands and oil shale.

In this article, coal tar slag was extracted using subcritical butane at 60 °C, with three extraction cycles performed. The extracts and raffinates were collected after each cycle and analyzed using FTIR, GC-MS, and PY-GC-MS to determine their organic components. This approach was used to evaluate the extraction efficiency of aromatic hydrocarbons. The innovative application of subcritical extraction technology in the coal chemical industry is a new expansion of its application field of subcritical extraction and provides novel insights for the sustainable development of the coal chemical industry chain.

2. Materials and Methods

2.1. Experimental Materials

The coal tar slag used in this experiment was obtained from the production process of orchid charcoal at the Yulin Orchid Charcoal Factory. After sampling, the slag was crushed, sieved to a particle size below 200 mesh (0.075 mm), and dried in an oven at 40 °C for over 2 h. The dried samples were cooled to room temperature and stored in a desiccator containing color-changing silica gel for further use. The subcritical extraction equipment produced by the Henan Institute of Subcritical Extraction utilized butane as the extractant. A schematic diagram is shown in Figure 1.

2.2. Subcritical Extraction Experiments

A 1000 g sample of prepared coal tar slag was weighed in the extraction tank. The tank door was tightened, and butane was added at a mass-to-volume ratio of 1:10 (coal tar slag to butane). The temperature was set to 60 °C. After 60 minutes of extraction, the first extract and 300 g of raffinate were collected. The remaining coal tar slag was extracted for another 60 minutes, and the second extract and 300 g of raffinate were collected. This process was repeated once more to obtain the third extract and raffinate. All three extracts and raffinates were stored in a desiccator.

The whole experimental process is illustrated in Figure 2.

EXT1: subcritical 60 °C extraction of the first extract; RAF1: subcritical 60 °C first extraction raffinate; EXT2: subcritical 60 °C extraction of the second extract; RAF2: subcritical 60 °C second extraction raffinate; EXT3: subcritical 60 °C extraction of the third extract; RAF3: subcritical 60 °C third extraction raffinate; FTIR: Fourier Transform Infrared Spectroscopy (FTIR); GC-MS: gas chromatography–mass spectrometry (GC-MS); PY-GC-MS: Pyrolysis Gas Chromatography–Mass Spectrometry (PY-GC-MS).

2.3. Test Analysis

Infrared spectroscopy test: Infrared spectroscopy measurements were conducted using a Thermo Fisher Scientific Nicolet IN10&iZ10 FTIR spectrometer (Waltham, MA, USA) with KBr pellets. The scanning time was 2 min, the resolution was 4 cm⁻¹, and the scanning range was 400–4000 cm⁻¹.

GC-MS: The GC-MS-QP 2010S gas chromatography–mass spectrometry (GC-MS) instrument was manufactured by Shimadzu Corporation, Nishinokyo Kuwabara-cho, Japan; the column was 30.0 m × 250 μm × 0.25 μm, the carrier gas was high-purity He, the temperature of the ion source was 230 °C, the temperature of the inlet port was 300 °C, the injection mode was shunt injection, the ratio of shunt injection was 120:1, and the injection volume was 1 μL. The warming program was initialized. The temperature increase program was 60 °C, residence time of 5 min, 5 °C/min to 300 °C, and retained for 8 min.

PY-GC-MS test: The instrument was an Agilent 7890B Thermal Cracking Mass Spectrometer (TCS-MS), Santa Clara, CA, USA; Firstly, the sample was pyrolyzed by the thermal cracker; the temperature of the sample was set at 200 °C, the time of the thermal cracking was 10 s, the injection volume of the thermal cracking was 0.5 mg, and the sample was detected using the gas chromatography–mass spectrometer (GC-MS), and the chromatographic column was an HP-5 ms Ultra. The column was HP-5ms Ultra 30.0 m × 250 μm × 0.25 μm, the carrier gas was high-purity He, the temperature of the ion source was 230 °C, the scanning mode was Scan, the scanning mass range was 40–500 μ, the splitting ratio was 150:1, and the injection volume was 1 μL. The warming program was 5 °C/min at room temperature to 60 °C for 10 min, then 8 °C/min to 100 °C for 5 min, and then 8 °C/min for 5 min at room temperature for 5 min. The temperature increase program was 5 °C/min to 60 °C for 10 min at room temperature, then 8 °C/min to 100 °C for 5 min, followed by 10 °C/min to 200 °C for 5 min.

Compound identification in extracts and raffinates was conducted using GC-MS and PY-GC-MS. The detected compounds were identified using NIST17 (National Institute of Standards and Technology Mass Spectral Library), with compound names and species determined based on confidence levels and similarity indices. Subsequently, semi-quantitative analysis was performed using the peak area method to determine compound species and relative concentrations.

Data analysis and plotting in the article were performed using Origin2021 software from OriginLab, Inc. of Northampton, MA, USA.

Experimental apparatus and equipment details are provided in

Table 1. Experimental apparatuses and equipment.

Equipment Name	Equipment Model	Address	Manufacturer
Muffle furnace	KSL-1100X	Hefei, China	Hefei Kejing Material Technology Co., Ltd.
Electrically heated blast drying oven	101-OAB	Tianjin, China	Tianjin Taist Instrument Co., Ltd.
Elemental Analyser	Elementar Vario EL II	Hebi, China	Hebi Yingtai Electronic and Electrical Appliance Co., Ltd.
Sulfur meter	5E-S3200	Yuncheng, China	Nanfeng Chemical Group Co., Ltd.
Subcritical extraction equipment	CBE-T-1L	Henan, China	Henan Subcritical Extraction Research Institute
Infrared spectrometer	Nicolet iN10&iZ10	Waltham, MA, USA	Thermo Fisher Sciedtific, USA
GC-MS	7890A-5975C	Nishinokyo Kuwabara-cho, Japan	Shimadzu Manufacturing, Japan
PY-GC-MS	7890A-5975C	Santa Clara, CA, USA	Agilent Corporation of the United States

.

3. Results and Discussion

3.1. Coal Tar Slag Basic Property Test

The results of industrial analysis and elemental analysis of coal tar slag are as follows in

Table 2. Industrial and elemental analysis of raw coal tar slag samples.

Sample	Industrial Analysis/W%				Elemental Analysis/W%, daf
	Mad	Aad	Vdaf	FCad	C	H	N	S	O*
Coal tar slag	5.33	13.43	17.58	67.55	74.64	2.46	1.83	0.56	20.51

The industrial analysis adopts the “Industrial Analysis Methods of Coal” GBT212-2022 [22] to conduct the test. M_ad: moisture content in coal tar slag; A_ad: ash content in coal tar slag; V_daf: volatile matter content in coal tar slag; FC_ad: fixed carbon content in coal tar slag. C, H, and N indicate elemental content in elemental analyzer test, and S indicates elemental content in sulfur meter test. O* stands for O. The elemental analysis results were obtained using the difference subtraction method.

.

It can be seen in the results of industrial analysis that the fixed carbon content in the samples is high; it can be seen in the results of elemental analysis that the sample varieties also contain more O elements, which may be due to the reaction with oxygen in the air during the coking of the raw coal.

3.2. Infrared Spectroscopy and PY-GC-MS Tests on Raw Coal Tar Slag Samples

Figure 3 below shows the infrared spectra of the original sample of coal tar slag, from which it can be seen that the original sample of coal tar slag has obvious absorption peaks at 3430 cm⁻¹, 1600 cm⁻¹, 1430 cm⁻¹, 870 cm⁻¹, 807 cm⁻¹, and 745 cm⁻¹. The absorption peak appearing at 3430 cm⁻¹ is generally that of the -OH functional group. The absorption peaks appearing at 1600 cm⁻¹ are generally C=C stretching vibrations, which indicate that coal tar slag may contain aromatic hydrocarbons. The absorption peak at 1430 cm⁻¹ is generally a C-H bending vibration, which indicates that the coal tar slag may contain aliphatic hydrocarbons. The absorption peaks at 870 cm⁻¹, 807 cm⁻¹, and 745 cm⁻¹ are generally =C-H out-of-plane bending vibrations, which indicates that the coal tar slag may contain aromatic hydrocarbons. Infrared spectroscopy is an important reference in the scientific research process, but the infrared spectroscopy test is generally not able to determine the composition of the substance, and other tests must be used to further determine the composition of the components in the tar slag.

Figure 4 below shows the total ion flow chromatogram of coal tar slag samples tested using PY-GC-MS.

In the total ion flow chromatogram 3 and

Table 3. Composition of original coal tar slag samples. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font indicate the information of other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aliphatic Hydrocarbons
5	Tetradecane	C14H30	198.39	1.23
	Dodecane	C12H26	170.33
	Hentriacontane	C31H64	436.80
	Aromatic Hydrocarbons
	Bicyclic Aromatic Hydrocarbons
2	2,7-dimethyl-Naphthalene	C12H12	156.22	0.15
	2,3-dimethyl-Naphthalene
	1,8-dimethyl-Naphthalene
3	2,3,6-trimethyl-Naphthalene	C13H14	170.25	0.55
	4,6,8-Trimethylazulene
	1,6,7-trimethyl-Naphthalene
4	4,6,8-Trimethylazulene	C13H14	170.25	1.34
	1,4,5-trimethyl-Naphthalene
	1,6,7-trimethyl-Naphthalene
	Tricyclic Aromatic Hydrocarbons
6	1-methyl-Phenanthrene	C15H12	192.25	4.22
	4-methyl-Phenanthrene
	l-methyl-Anthracene
7	4-methyl-Phenanthrene	C15H12	192.25	3.86
	4-methyl-Phenanthrene
	2-methyl-Anthracene
8	1-methyl-Anthracene	C15H12	192.25	1.80
	1-methyl-Phenanthrene
	4-methyl-Phenanthrene
9	3,4-Dimethylphenanthrene	C16H14	206.28	2.57
	di-p-Tolylacetylene
	2,7-dimethyl-Phenanthrene
10	3,6-dimethyl-Phenanthrene	C16H14	206.28	1.36
	2,7-dimethyl-Phenanthrene
	l,2-dihydro-1-phenyl-Naphthalene
	Tetracyclic Aromatic Hydrocarbons
11	Fluoranthene	C16H10	202.25	2.16
	Fluoranthene
	Pyrene
12	Pyrene	C16H10	202.25	1.18
	Pyrene
	Fluoranthene
	Aldehyde
1	Benzaldehyde	C7H6O	106.12	9.47
	Benzaldehyde
	Benzaldehyde

of the compositional composition of the tarry slag, it can be seen that 12 volatilizable substances were detected in the original sample of tarry slag, including one aliphatic hydrocarbon substance with a relative content of 1.23%, 10 aromatic hydrocarbon substances with a relative content of 15.85%, and one aldehyde substance with a relative content of 9.46%. The next experiment will remove these aromatic hydrocarbons.

3.3. Infrared Spectroscopic Testing of Coal Tar Slag Raffinates and Extracts

Figure 5 and Figure 6 below show the infrared spectra of the extracts from three subcritical extractions of coal tar slag at 60 °C.

Figure 5 shows the infrared spectroscopy of three 60 °C subcritical extracts. The first extract exhibits strong absorption peaks at 700–900 cm⁻¹, suggesting abundant volatile aromatic hydrocarbons. In contrast, the second and third extracts show significantly reduced peaks in this region, indicating that most aromatic hydrocarbons were removed during the first extraction. In Figure 5, the solid red line in the region of 700–900 cm⁻¹ indicates the distribution of absorption peaks in the extract. Figure 6 shows the infrared spectra of Figure 5’s magnified red line area; it is more obvious that the first extractive aromatic hydrocarbon absorption peaks are of high intensity, while the second and third almost have no obvious absorption peaks.

Figure 7 and Figure 8 below show the infrared spectra of the raffinates from three subcritical extractions of coal tar slag at 60 °C.

Figure 7 shows the infrared spectra of three raffinates, revealing no significant absorption peak at 700–900 cm⁻¹ (red line region), indicating minimal aromatic hydrocarbon content. Figure 8 presents an enlarged view of this region, demonstrating spectral flattening with increasing extraction cycles and the near absence of absorption peaks, suggesting the effective removal of volatile and toxic PAHs. These infrared spectra results are crucial references for our study, which will further investigate the extracts and raffinates using GC-MS and PY-GC-MS analyses.

3.4. GC-MS Testing of Coal Tar Slag Extracts

The previous infrared spectroscopy measurement is an important reference to analyze the extracts and raffinates of coal tar slag, but it can only detect the functional groups and chemical bonds in the substance and cannot analyze the composition of the substance. Therefore, we continued with the extraction of coal tar slag using GC-MS. Figure 9 presents the total ion flow chromatogram for the first extraction of coal tar slag at a subcritical 60 °C.

Table 4. Compositional composition of the first extract of coal tar slag. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font are the information on other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aliphatic hydrocarbons
3	Heptadecane, 2,6,10,15-tetramethyl-	C21H44	296.60	0.51
	Heptadecane, 2,6,10,15-tetramethyl-	C21H44	296.60
	2-Bromo dodecan	C32H40BrN5O5	654.60
4	Nonane, 3,7-dimethyl-	C11H24	156.31	0.30
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	Carbonic acid, tridecyl vinyl ester	C16H30O3	270.41
6	Octacosane, 2-methyl-	C29H60	408.80	0.74
	2-Methyltetracosane	C25H52	352.70
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
14	Heneicosane	C21H44	296.60	2.10
	2-Methylhexacosane	C27H56	380.70
	2-Methyltetracosan	C25H52	352.70
22	Cyclohexane, (1-butylhexadecyl)-	C26H52	364.70	0.22
	Fumaric acid, dodecyl 2,3,4,6-tetrachlorophenyl ester	C22H28Cl4O4	498.30
	Fumaric acid, heptadecyl octyl ester	C29H54O4	466.70
24	Heneicosane	C21H44	296.60	4.45
	2-Methylhexacosane	C27H56	380.70
	1-Decanol, 2-hexyl-	C16H34O	242.44
29	Tetrapentacontane	C54H110	759.40	0.99
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	Heptacosyl trifluoroacetate	C29H55F3O2	492.70
32	2-Methylhexacosane	C27H56	380.70	0.53
	1-Decanol, 2-hexyl-	C16H34O	242.44
	5-Methyl-Z-5-docosene	C23H46	322.60
34	Cyclohexane, octadecyl-	C24H48	336.60	0.34
	17-Pentatriacontene	C35H70	490.90
	17-Pentatriacontene	C35H70	490.90
35	Dotriacontane	C32H66	450.90	2.07
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
39	Squalane	C30H62	422.80	1.24
	1-Decanol, 2-hexyl-	C16H34O	242.44
	1-Dodecanol, 2-octyl-	C20H42O	298.50
40	Tetracontane	C40H82	563.10	2.11
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	1-Decanol, 2-octyl-	C18H38O	270.50
41	Dotriacontane	C32H66	450.90	5.65
	1-Decanol, 2-hexyl-	C16H34O	242.44
	17-Pentatriacontene	C35H70	490.90
48	11-Methyltricosane	C24H50	338.70	1.50
	1-Decanol, 2-hexyl-	C16H34O	242.44
	17-Pentatriacontene	C35H70	490.90
49	2-Methylhexacosane	C27H56	380.70	0.68
	17-Pentatriacontene	C35H70	490.90
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
51	17.alpha.(H),21.beta.(H)-Homohopane	C31H54	426.80	0.31
	1-Decanol, 2-hexyl-	C16H34O	242.44
	2-Dodecen-1-yl(-)succinic anhydride	C16H26O3	266.38
52	Pentatriacontane	C35H72	492.90	1.13
	17-Pentatriacontene	C35H70	490.90
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
53	Cholestane, (5.alpha.,14.beta.)-	C27H48	372.70	0.40
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
	17-Pentatriacontene	C35H70	490.90
55	2-Methylhexacosane	C27H56	380.70	0.39
	17-Pentatriacontene	C35H70	490.90
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
56	28-Nor-17.alpha.(H)-hopane	C29H50	398.70	0.64
	2-Dodecen-1-yl(-)succinic anhydride	C16H26O3	266.38
	7-Hexadecenal, (Z)-	C16H30O	238.41
57	Tetrapentacontane	C54H110	759.40	0.66
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
	Heptacosyl trifluoroacetate	C29H55F3O2	492.70
59	Tetracontane	C40H82	563.10	0.39
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
	Heptacosyl trifluoroacetate	C29H55F3O2	492.70
60	17.alpha.(H),21.beta.(H)-Hopane	C30H52	412.70	0.93
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
	Undec-10-ynoic acid, tetradecyl ester	C25H46O2	378.60
61	17.alpha.(H),21.beta.(H)-Homohopane	C31H54	426.80	0.40
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
	cis-1-Chloro-9-octadecene	C18H35Cl	286.90
	Aromatic hydrocarbons
	Tricyclic aromatic hydrocarbons
2	9H-Fluorene, 9-methylene-	C14H10	178.23	0.89
	9,10-Ethanoanthracene-11,12-dicarbonitrile, 9,10-dihydro-, cis-	C18H12N2	256.30
	Phenanthrene	C14H10	178.23
5	Phenanthrene, 2-methyl-	C15H12	192.25	2.70
	Phenanthrene, 2-methyl-
	Phenanthrene, 4-methyl-
7	Phenanthrene, 9-ethyl-	C16H14	206.28	0.42
	Anthracene, 9-dodecyltetradecahydro-	C26H48	360.70
	Phenanthrene, 9-dodecyltetradecahydro-	C26H48	360.70
8	Phenanthrene, 2,5-dimethyl-	C16H14	206.28	2.46
	Phenanthrene, 2,5-dimethyl-
	Phenanthrene, 4,5-dimethyl-
11	Anthracene, 9-(1-methylethyl)-	C17H16	220.31	2.16
	Phenanthrene, 2,3,5-trimethyl-
	Anthracene, 9-(1-methylethyl)-
12	[14]Annulene, 1,6:7,12-bis(methano)-, anti-	C16H14	206.28	0.29
	18.alpha.-Olean-3.beta.-ol, acetate	C32H54O2	470.80
	5-Fluoro-2-trifluoromethylbenzoic acid, eicosyl ester	C28H44F4O2	488.60
15	2,3,5-trimethyl-Phenanthrene,	C17H16	220.31	0.63
	2,3,5-trimethyl-Phenanthrene,
	2,3,5-trimethyl-Phenanthrene,
17	Retene	C18H18	234.30	2.23
	Retene
	Retene
21	1-benzyl-3-methylnaphthalene	C18H16	232.30	0.50
	Fumaric acid, dodecyl 2,3,4,6-tetrachlorophenyl ester	C22H28Cl4O4	498.30
	5-Methyl-Z-5-docosene	C23H46	322.60
	Tetracyclic aromatic hydrocarbons
10	Pyrene	C16H10	202.25	9.43
	Fluoranthene
	Pyrene
16	11H-Benzo[b]fluorene	C17H12	216.28	5.50
	11H-Benzo[b]fluorene
	Pyrene, 2-methyl-
18	Fluoranthene, 2-methyl-	C17H12	216.28	2.08
	Pyrene, 2-methyl-
	Fluoranthene, 2-methyl-
19	Pyrene, 1-methyl-	C17H12	216.28	4.67
	Pyrene, 1-methyl-
	Fluoranthene, 2-methyl-
23	Pyrene, 1,3-dimethyl-	C18H14	230.30	3.82
	Pyrene, 1,3-dimethyl-	C18H14	230.30
	Succinic acid, 2,2,3,3-tetrafluoropropyl heptadecyl ester	C24H42F4O4	470.60
27	Benz[a]anthracene	C18H12	228.30	1.17
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Tetradecanoic acid, hexadecyl ester	C30H60O2	452.80
28	Triphenylene	C18H12	228.30	6.60
	2-Aminopent-4-enoic acid, N-(2-ethylhexyloxycarbonyl)-, undecyl ester	C25H47NO4	425.60
	2-Aminopent-4-enoic acid, N-octyloxycarbonyl-, undecyl ester	C25H47NO4	425.60
30	1,3,9-Trimethylpyrene	C19H16	244.33	0.95
	5-Methyl-Z-5-docosene	C23H46	322.60
	Tetradecanoic acid, hexadecyl ester	C30H60O2	452.80
37	Bicyclo[4.1.0]hepta-1,3,5-triene, 2,5-diphenyl-	C19H14	242.30	2.15
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Carbonic acid, decyl heptadecyl ester	C28H56O3	440.70
38	Benz[a]anthracene, 8-methyl-	C19H14	242.30	1.97
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Benz[a]anthracene, 8-methyl-	C19H14	242.30
	Pentacyclic aromatic hydrocarbons
43	Chrysene, 5-ethyl-	C20H16	256.30	0.57
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
	17-Pentatriacontene	C35H70	490.90
47	Perylene	C20H12	252.30	3.60
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
	Pentafluoropropionic acid, octadecyl ester	C21H37F5O2	416.51
50	Benzo[a]pyrene	C20H12	252.30	1.77
	17-Pentatriacontene	C35H70	490.90
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
	Alcohols
42	1-Decanol, 2-hexyl-	C16H34O	242.30	1.01
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
	Heptacosyl trifluoroacetate	C29H55F3O2	492.70
58	4a,7,7,10a-Tetramethyldodecahydrobenzo[f]chromen-3-ol	C17H30O2	266.40	0.14
	17-Pentatriacontene	C35H70	490.90
	2-Dodecen-1-yl(-)succinic anhydride	C16H26O3	266.38
	Esters
31	Succinic acid, 4-methylhept-3-yl nonyl ester	C21H40O4	356.50	0.28
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
	1-Decanol, 2-hexyl-	C16H34O	242.44
36	Palmitic acid vinyl ester	C18H34O2	282.50	0.27
	17-Pentatriacontene	C35H70	490.90
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
45	Palmitic acid 2-nonanol ester	C27H54O2	410.70	0.25
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
46	Carbonic acid, decyl heptadecyl ester	C28H56O3	440.70	0.69
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
54	Phytyl, 2-methylbutanoate	C25H48O2	380.60	0.39
	E-10,13,13-Trimethyl-11-tetradecen-1-ol acetate	C19H36O2	296.50
	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70
	Ketones
26	2-(2-Indanylidene)-1-indanone	C18H14O	246.30	1.46
	2-(2-Indanylidene)-1-indanone	C18H14O	246.30
	Undec-10-ynoic acid, tetradecyl ester	C25H46O2	378.60
	Phenols
13	1-Hydroxypyrene	C16H10O	218.25	1.87
	1-Hydroxypyrene	C16H10O	218.25
	Olean-12-ene	C30H50	410.70
	Ethers
33	Nonyl tetracosyl ether	C33H68O	480.90	0.48
	1-Decanol, 2-hexyl-	C16H34O	242.44
	5-Methyl-Z-5-docosene	C23H46	322.60
	Nitrogen compounds
20	Naphthalen-2-ol, 1-(4-morpholyl)(phenyl)methyl-	C21H21NO2	319.40	1.52
	2-[1-(2-[1,3]Dithian-2-yl-ethyl)-pent-4-enyloxy]-tetrahydropyran	C16H28O2S2	316.50
	Fumaric acid, dodecyl 2,3,4,6-tetrachlorophenyl ester	C22H28Cl4O4	498.30
25	Ellipticine	C17H14N2	246.31	1.70
	Fumaric acid, heptadecyl octyl ester	C29H5O2	466.70
	Tetradecanoic acid, hexadecyl ester	C30H60O2	452.80
	Sulfurous compounds
1	Sulfurous acid, 2-ethylhexyl isohexyl ester	C17H30O3S	314.50	0.27
	Tridecane, 1-iodo-	C13H27I	310.26
	Sulfurous acid, butyl nonyl ester	C13H28O3S	264.43
	Other compounds
9	Silane, trichlorooctadecyl-	C18H37Cl3Si	387.90	0.90
	Di-n-decylsulfone	C20H42O2S	346.60
	Diglycolic acid, 2-bromo-4-fluorophenyl decyl ester	C20H28BrFO5	447.30
44	Hexacosane, 1-iodo-	C26H53I	492.60	2.08
	1-Decanol, 2-hexyl-	C16H34O	242.44
	1-Decanol, 2-octyl-	C18H38O	270.50

below shows the results of the identification and analysis of the 61 volatile organic components detected in Figure 9, and the relative content was calculated based on the corresponding peak areas of the compounds.

As can be seen in the total ion flow chromatogram 9 and Table 4 of the substance composition, a total of 61 compounds were detected in the first extract, including 24 aliphatic hydrocarbons with a relative content of 28.68%, 22 aromatic hydrocarbons with a relative content of 56.56%, 2 alcohols with a relative content of 1.15%, 5 esters with a relative content of 1.88%, 1 ketone with a relative content of 1.46%; 1 phenolic substance with a relative content of 1.87%; 1 ether substance with a relative content of 0.48%; 2 nitrogen-containing compounds with a relative content of 3.22%; 1 sulfur-containing compound with a relative content of 0.27%; and 2 other compounds with a relative content of 2.98%. The main substances detected in the first extract were aliphatic hydrocarbons and aromatic hydrocarbons, among which the content of aromatic hydrocarbons was the highest, which was in line with the results of the previous infrared spectroscopy test.

Figure 10 below shows the total ion flow chromatogram of the GC-MS test on the second extract of coal tar slag at a subcritical 60 °C.

Table 5. Constituent composition of the second extract of coal tar slag. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font are the information on other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aliphatic hydrocarbons
3	Cyclohexane, tricosyl-	C29H58	406.80	2.73
	Dodecane, 5-cyclohexyl-	C18H36	252.50
	Undecane, 5-cyclohexyl-	C17H34	238.50
5	1-(1,5-dimethylhexyl) -4-(4-methylpentyl)-Cyclohexane	C20H40	280.50	2.23
	Octadecanoic acid, 2-oxo-, methyl ester	C19H36O3	312.50
	Heneicosane, 11-cyclopentyl-	C26H52	364.70
6	pentacosyl-Cyclohexane	C31H62	434.80	3.44
	Dodecane, 5-cyclohexyl-	C18H36	252.50
	Undecane, 5-cyclohexyl-	C17H34	238.50
8	2-methyl-Octacosane	C29H60	408.80	7.13
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Hexacosyl trifluoroacetate	C28H53F3O2	478.70
9	2,6,10,15-tetramethyl-Heptadecane	C21H44	296.60	6.11
	Hexacosyl trifluoroacetate	C28H53F3O2	478.70
	Sulfurous acid, octadecyl 2-propyl ester	C21H44O3S	376.60
13	2-Methyltetracosane	C25H52	352.70	8.38
	Hexacosyl trifluoroacetate	C28H53F3O2	478.70
	1-Decanol, 2-hexyl-	C16H34O	242.44
16	17.alpha.(H),21.beta.(H)-Homohopane	C31H54	426.80	7.09
	17.alpha.(H),21.beta.(H)-Hopane	C30H52	412.70
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
	Carboxylic acids
12	Tetracosyl benzoate	C31H54O2	458.80	1.96
	Hexacosyl trifluoroacetate	C28H53F3O2	478.70
	Dotriacontanal $$ n-Dotriacontanal	C32H66	450.90
	Alcohols
15	5-(7a-Isopropenyl-4,5-dimethyl-octahydroinden-4-yl)-3-methyl-pent-2-en-1-ol	C20H34O	290.50	3.55
	2,4a,8,8-Tetramethyldecahydrocyclopropa[d]naphthalene	C15H26	206.37
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
	Aldehydes
1	2-Butyn-1-al diethyl acetal	C8H14O2	142.20	1.36
	2-Propenamide	C3H5NO	71.08
	Stearic acid hydrazide	C18H38N2O	298.50
	Ethers
2	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60	6.40
	Octadecanoic acid, 2-oxo-, methyl ester	C19H36O3	312.50
	10-Methylnonadecane	C20H42	282.50
10	Fumaric acid, trans-hex-3-enyl tridecyl ester	C23H40O4	380.60	7.65
	Tricosyl pentafluoropropionate	C26H47F5O2	486.60
	Docosyl pentafluoropropionate	C25H45F5O2	472.60
11	Fumaric acid, octadecyl trans-hex-3-enyl ester	C28H50O4	450.70	4.45
	Hexacosyl trifluoroacetate	C28H53F3O2	478.70
	Docosyl pentafluoropropionate	C25H45F5O2	472.60
	Sulfurous compounds
7	Sulfurous acid, octadecyl 2-propyl ester	C21H44O3S	376.60	11.26
	Sulfurous acid, octadecyl 2-propyl ester	C21H44O3S	376.60
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	Other compounds
4	2-Bromo dodecane	C12H25Br	249.23	5.68
	1-Dodecanol, 2-octyl-	C20H42O	298.50
	1-Decanol, 2-octyl-	C18H38O	270.50
14	Phosphonofluoridic acid, ethyl-, decyl ester	C12H26FO2P	252.31	1.56
	Heptyl ethylphosphonofluoridate	C9H20FO2P	210.23
	1,1-Dimethylpropyl ethylphosphonofluoridate	C7H16FO2P	182.17

below shows the results of the identification and analysis of the 16 volatile organic components detected in Figure 10.

As can be seen in the total ion flow chromatogram 10 and Table 5 of the substance composition, a total of 16 substances were detected in the second extract, including 7 aliphatic hydrocarbons, with a relative content of 37.11%; 1 carboxylic acid, with a relative content of 1.96%; 1 alcohol, with a relative content of 3.55%; 1 aldehyde, with a relative content of 1.36%; 3 esters, with a relative content of 18.50%; 1 sulfur-containing compound with a relative content of 11.26%; and 2 other compounds with a relative content of 7.24%. In the results of the second extract, it can be preliminarily concluded that after two subcritical extractions, no aromatic hydrocarbon substances were detected in the extract, and the largest proportion of the extract was aliphatic hydrocarbon substances, which indicated that the aromatic hydrocarbon substances in the coal tar slag had been completely extracted.

Figure 11 below shows the total ion flow chromatogram of the GC-MS test on the third extract of coal tar slag at a subcritical 60 °C.

Table 6. Constituent composition of the third extract of coal tar slag. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font are the information on other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aliphatic hydrocarbons
1	Octacosane,2-methyl-	C29H60	408.80	1.19
	Heneicosane	C21H44	296.60
	Pentadecane, 8-hexyl-	C21H44	296.60
2	Heneicosane	C21H44	296.60	1.08
	Tetracosane, 1-iodo-	C24H49I	464.50
	Pentacosane	C25H52	352.70
3	Eicosane	C20H42	282.50	1.97
	2-Methylhexacosane	C27H56	380.70
	Octacosane, 2-methyl-	C29H60	408.80
5	3,8-dimethyl-Decane	C12H26	170.33	0.43
	Decane, 2,3,5-trimethyl-	C13H28	184.36
	Hexadecane, 1-iodo-	C16H33I	352.34
6	Tridecane, 5-cyclohexyl-	C19H38	266.50	0.44
	Cyclohexane, (1-butylhexadecyl)-	C26H52	364.70
	Tridecane, 4-cyclohexyl	C19H38	266.50
7	Pentacosane	C25H52	352.70	1.19
	Octacosane,2-methyl-	C29H60	408.80
	Octacosane,2-methyl-	C29H60	408.80
9	Cyclohexane, 1-(cyclohexylmethyl)-2-methyl-, cis-	C14H26	194.36	0.45
	Cyclohexane, 1-bromo-2-methyl-	C7H13Br	177.08
	Cyclohexane, 1-isopropyl-1-methyl-	C10H20	140.27
10	Eicosane	C20H42	282.50	1.46
	11-Methyltricosane	C24H50	338.70
	Carbonic acid, octadecyl vinyl ester	C21H40O3	340.50
11	2-Methylhexacosane	C27H56	380.70	2.04
	Dotriacontane	C32H66	450.90
	Heneicosane	C21H44	296.60
13	Cyclohexane, octadecyl-	C24H48	336.60	0.76
	Cyclohexane, 1,1′-(1,3-propanediyl)bis-	C15H28	208.38
	n-Heptadecylcyclohexane	C23H46	322.60
15	11-Methyltricosane	C24H50	338.70	0.29
	9-Methylheneicosane	C22H46	310.60
	9-methylheptadecane	C18H38	254.50
16	Heneicosane	C21H44	296.60	3.89
	Pentacosane	C25H52	352.70
	Tetracontane	C40H82	563.10
17	1,1-dicyclohexyl-Heptane	C19H36	264.50	0.19
	Cyclohexane, 1-(cyclohexylmethyl)-2-methyl-, cis-	C14H26	194.36
	Cyclohexane, 1-(cyclohexylmethyl)-4-methyl-	C14H26	194.36
19	Tetrapentacontane	C54H110	759.40	0.78
	Eicosane	C20H42	282.50
	Hexadecane, 2,6,10,14-tetramethyl-	C20H42	282.50
20	Pentatriacontane	C35H72	492.90	10.47
	2-Methylhexacosane	C27H56	380.70
	Octacosane, 2-methyl-	C29H60	408.80
21	7,7-Diethylheptadecane	C21H44	296.60	2.42
	Pentadecane, 6-methyl-	C16H34	226.44
	Hentriacontane	C31H64	436.80
23	Octacosane, 2-methyl-	C29H60	408.80	0.86
	Heptadecane, 2,6,10,15-tetramethyl-	C21H44	296.60
	Hexadecane, 2,6,10,14-tetramethyl-	C20H42	282.50
25	Cyclohexane, octadecyl-	C24H48	336.60	0.85
	Cyclohexane, nonadecyl-	C25H50	350.70
	2-Cyclohexylnonadecane	C25H50	350.70
27	11-Methyltricosane	C24H50	338.70	1.14
	Carbonic acid, octadecyl vinyl ester	C21H40O3	340.50
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
30	Tetracontane	C40H82	563.10	16.35
	Dotriacontane	C32H66	450.90
	Pentacosane	C25H52	352.70
32	Pentadecane, 2,6,10,14-tetramethyl-	C19H40	268.50	1.90
	11-Methylpentacosane	C26H54	366.70
	11-Methyltricosane	C24H50	338.70
33	3,3-Diethylpentadecane	C19H40	268.50	0.87
	3,3,13,13-Tetraethylpentadecane	C23H48	324.60
	Hentriacontane	C31H64	436.80
34	Dotriacontane	C32H66	450.90	4.52
	Tetracontane	C40H82	563.10
	Pentacosane	C25H52	352.70
35	Pentacosane	C25H52	352.70	1.22
	Pentacosane	C25H52	352.70
	Dotriacontane	C32H66	450.90
37	2-Methylheptacosane	C28H58	394.80	4.41
	Dotriacontane	C32H66	450.90
	Tetrapentacontane	C54H110	759.40
40	Octacosane, 2-methyl-	C29H60	408.80	1.71
	Pentatriacontane	C35H72	492.90
	Carbonic acid, octadecyl vinyl ester	C21H40O3	340.50
41	11-Methyltricosane	C24H50	338.70	5.33
	Pentatriacontane	C35H72	492.90
	Carbonic acid, octadecyl vinyl ester	C21H40O3	340.50
42	2-Methylhexacosane	C27H56	380.70	4.16
	Pentatriacontane	C35H72	492.90
	Hexacosane, 1-iodo-	C26H53I	492.60
44	17.alpha.(H),21.beta.(H)-Hopane	C30H52	412.70	0.40
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
	A’-Neogammacer-22(29)-ene	C30H50	410.70
45	Tetrapentacontane	C54H110	759.40	2.91
	Eicosane	C20H42	282.50
	Dotriacontane	C32H66	450.90
46	17.alpha.(H),21.beta.(H)-Homohopane	C30H52	412.70	0.64
	17.alpha.(H),21.beta.(H)-Homohopane	C30H52	412.70
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
49	Trispiro[4.2.4.2.4.2.]heneicosane	C21H36	288.50	0.44
	4a,7,7,10a-Tetramethyldodecahydrobenzo[f]chromen-3-ol	C17H30O2	266.40
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
53	1-(2,2,3,5,6-Pentamethylcyclohex-4-enyl)- 9-(3,3,4-trimethylcyclohex-1-enyl)- 3,6-dimethyl-6-ethenyl-dec-4-ene	C34H58	466.80	0.12
	1,1,6-trimethyl-3-methylene-2-(3,6,9,13-tetramethyl-6-ethenye-10,14-dimethylene-pentadec-4-enyl)cyclohexane	C33H56	452.80
	Cholestane, (5.alpha.,14.beta.)-	C27H48	372.70
54	28-Nor-17.alpha.(H)-hopane	C29H50	398.70	1.72
	17.alpha.(H),21.beta.(H)-Homohopane	C31H54	426.80
	17.alpha.(H),21.beta.(H)-Homohopane	C31H54	426.80
56	17.alpha.(H),21.beta.(H)-Hopane	C30H52	412.70	1.95
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
	28-Nor-17.alpha.(H)-hopane	C29H50	398.70
58	17.alpha.(H),21.beta.(H)-Homohopane	C31H54	426.80	1.23
	17.alpha.(H),21.beta.(H)-Hopane	C30H52	412.70
	17.alpha.(H),21.beta.(H)-Hopane	C30H52	412.70
	Alcohols
8	1-Decanol, 2-hexyl-	C16H34O	242.44	2.02
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	2-Methylhexacosane	C27H56	380.70
22	Octacosanol	C28H58O	410.80	0.43
	Tetracosyl trifluoroacetate	C26H49F3O2	450.70
	Tricosyl trifluoroacetate	C25H47F3O2	436.60
28	1-Dodecanol, 2-octyl-	C20H42O	298.50	0.43
	Nonyl tetracosyl ether	C33H68O	480.90
	Carbonic acid, octadecyl vinyl ester	C21H40O3	340.50
38	1-Decanol, 2-hexyl-	C16H34O	242.44	2.18
	Docosyl octyl ether	C30H62O	438.80
	Octyl tetracosyl ether	C32H66O	466.90
	Ethers
12	Docosyl octyl ether	C30H62O	438.80	0.40
	2-Methylhexacosane	C27H56	380.70
	5-Methylnonacosane	C30H62	422.80
26	Nonyl tetracosyl ether	C33H68O	480.90	0.19
	Dotriacontane	C32H66	450.90
	Eicosane	C20H42	282.50
50	Isophthalic acid, 3,7-dimethyloct-6-enyl tridecyl ester	C31H50O4	486.70	0.26
	Citronellyl palmitoleate	C26H48O2	392.70
	Citronellyl oleate	C28H52O2	420.70
	Esters
14	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60	1.04
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	Pentatriacontane	C35H72	492.90
24	Carbonic acid, octadecyl vinyl ester	C21H40O3	340.50	0.65
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Docosyl octyl ether	C30H62O	438.80
36	Carbonic acid, decyl hexadecyl ester	C27H54O3	426.70	1.46
	Heptacosyl trifluoroacetate	C29H55F3O2	492.70
	Carbonic acid, decyl heptadecyl ester	C28H56O3	440.70
39	Carbonic acid, decyl octadecyl ester	C27H54O3	426.70	0.79
	2-Methylhexacosane	C27H56	380.70
	2-Methyltetracosane	C25H52	352.70
43	Carbonic acid, decyl hexadecyl ester	C27H54O3	426.70	2.39
	Dotriacontane	C32H66	450.90
	Hentriacontane	C31H64	436.80
51	Phytyl decanoate	C30H58O2	450.80	0.12
	1-(2,2,3,5,6-Pentamethylcyclohex-4-enyl)-9-(3,3,4-trimethylcyclohex-1-enyl)-3,6-dimethyl-6-ethenyl-dec-4-ene	C34H58	466.80
	Allopregnane	C21H36	288.50
52	Valtrate	C22H30O8	422.50	0.84
	Squalane $$ Tetracosane, 2,6,10,15,19,23-hexamethyl-	C30H62	422.80
	11-Methyltricosane	C24H50	338.70
55	Isophthalic acid, 3,7-dimethyloct-6-enyl tridecyl ester	C31H50O4	486.70	0.26
	Heptacosyl trifluoroacetate	C29H55F3O2	492.70
	Hexacosyl trifluoroacetate	C28H53F3O2	478.70
	Ketones
47	21-Nor-5.alpha.-cholest-24-en-20-one	C26H42O	370.60	0.14
	2-Methyl-E-7-octadecene	C19H38	266.50
	5-Methyl-Z-5-docosene	C23H46	322.60
	Nitrogen compounds
4	Hexadecanedinitrile	C16H28N2	248.41	0.09
	1-Decanol, 2-hexyl-	C16H34O	242.44
	Carbonic acid, eicosyl vinyl ester	C23H44O3	368.60
	Sulfurous compounds
18	Sulfurous acid, cyclohexylmethyl heptyl ester	C14H28O3S	276.44	0.37
	Sulfurous acid, di(cyclohexylmethyl) ester	C14H26O3S	274.42
	Sulfurous acid, cyclohexylmethyl pentadecyl ester	C22H44O3S	388.60
31	Sulfurous acid, cyclohexylmethyl pentadecyl ester	C22H44O3S	388.60	1.06
	Sulfurous acid, di(cyclohexylmethyl) ester	C14H26O3S	274.42
	2-Pyrazoline, 5-ethyl-1,4-dimethyl-	C7H14N2	126.20
	Other compounds
29	Tetracosyl pentafluoropropionate	C27H49F5O2	500.70	0.44
	Nonyl tetracosyl ether	C33H68O	480.90
	1-Decanol, 2-hexyl-	C16H34O	242.44
48	Triacontane, 1-bromo-	C30H61Br	501.70	0.61
	Hexacosane, 1-iodo-	C26H53I	492.60
	Dotriacontane	C32H66	450.90
57	2- Bromopropionic acid, octadecyl ester	C21H41BrO2	405.50	0.53
	Heptadecane, 9-(2-cyclohexylethyl)-	C25H50	350.70
	Cyclohexane, (2-decyldodecyl)-	C28H56	392.70

below shows the results of the identification and analysis of 58 volatile organic components detected in Figure 11.

As can be seen in the total ion flow chromatogram 11 and the substance composition Table 6, a total of 58 substances were detected in the third extract, including 36 aliphatic hydrocarbons with a relative content of 81.78%, 4 alcohols with a relative content of 5.06%, 3 ethers with a relative content of 0.85%, 8 esters with a relative content of 7.55%, 1 ketone with a relative content of 0.14%, 1 nitrogen-containing compound with a relative content of 0.09%, 2 sulfur-containing compounds with a relative content of 1.43%, and 3 other compounds with a relative content of 1.58%. The percentage of aliphatic hydrocarbons in the third extract detection was very high, reaching 81.78%, and no aromatic hydrocarbon substances were detected, which was consistent with the infrared spectroscopy results and the GC-MS results of the second extract, indicating that the volatilizable aromatic hydrocarbon substances in the coal tar slag were extracted to completion. The next three extracts of the subcritical extraction will be detected, and the results of the extract detection will be used to corroborate the conclusions reached so far.

3.5. Coal Tar Slag Raffinate PY-GC/MS Test

After the previous infrared spectroscopy and GC-MS extract results, a preliminary conclusion can be drawn: after subcritical 60 °C extraction on coal tar slag, a large number of aromatic hydrocarbons were detected in the extract; in the second and third extracts, aromatic hydrocarbons were not detected, indicating that the tar slag contained volatile aromatic hydrocarbons. A PY-GC-MS test will be performed on the three raffinates to confirm this conclusion.

Figure 12 below shows the PY-GC-MS total ion flow chromatogram of the first raffinates of the subcritical extraction of tar slag.

Table 7. Composition of the first extraction raffinate of coal tar slag. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font are the information on other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aliphatic hydrocarbons
2	Cyclododecane	C12H24	168.32	0.79
	1-Tetradecanol	C14H30O	214.39
	Cyclotetradecane	C14H28	196.37
	Aldehydes
1	Benzaldehyde	C7H6O	106.12	7.60
	Benzaldehyde
	Benzaldehyde
	Carboxylic acids
3	n-Hexadecanoic acid	C16H32O2	256.42	37.56
	Butyraldehyde, semi carbazone	C3H7N3O	101.11
	n-Hexadecanoic acid	C16H32O2	256.42

below shows the results of the identification and analysis of the three volatile organic components detected in Figure 12.

As can be seen in total ion flow chromatogram 12 and Table 7 of the composition of the extract, only three volatile substances were detected in the first extract, which were aliphatic hydrocarbons, with a relative content of 0.79%; aldehydes, with a relative content of 7.60%; and carboxylic acids, with a relative content of 37.56%. These three substances are not in the 16 PAHs announced by the U.S. Environmental Protection Agency, indicating that there are no volatile toxic PAHs in the raffinates after a subcritical 60 °C extraction.

Table 8. Composition of the second extracted raffinate of coal tar slag. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font are the information on other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aldehydes
1	Benzaldehyde	C7H6O	106.12	7.70
	Benzaldehyde
	Benzaldehyde
	Carboxylic acids
2	n-Hexadecanoic acid	C16H32O2	256.42	37.55
	n-Hexadecanoic acid
	n-Hexadecanoic acid

below shows the results of the identification and analysis of the two volatile organic components detected in Figure 13.

As can be seen in total ion flow chromatogram 13 and Table 8 of the composition of the raffinates, only two volatile substances were detected in the second raffinate, which were aldehydes, with a relative content of 7.70%, and carboxylic acids, with a relative content of 37.55. Both substances were not included in the 16 PAHs published by the U.S. Environmental Protection Agency, which indicated that after the two subcritical 60 °C extractions, there were no volatile toxic PAHs in the raffinates, and the test results are consistent with the previous extracts.

Table 9. Composition of the third extracted raffinate of coal tar slag. The compounds marked in red in the first row of each substance in the table are those that were present in the coal tar slag after judgment, and those in black font are the information on other MS ion fragments detected.

No.	Compound Names	Proposed Formula	Molecular Weight	Relative Content, %
	Aldehydes
1	Benzaldehyde	C7H6O	106.12	11.84
	Benzaldehyde
	Benzaldehyde

below shows the results of the identification and analysis of one volatile organic component detected in Figure 14.

As can be seen in total ion flow chromatogram 14 and extract composition Table 9, only one volatilizable substance was detected in the third raffinate, which was an aldehyde, with a relative content of 11.84%, and this substance was not in the 16 PAHs published by the U.S. Environmental Protection Agency, which indicated that after the three subcritical 60 °C extractions, there was no volatilizable toxic PAH in the raffinate, and the preliminary conclusion was again confirmed to be credible.

4. Conclusions

This study demonstrates that subcritical extraction can extract the toxic aromatic hydrocarbons from coal tar slag. Firstly, the basic properties of coal tar slag were tested, and it was found that coal tar slag contained a large amount of volatile toxic aromatic hydrocarbons. These toxic aromatic hydrocarbons were extracted via subcritical 60 °C three times. According to the test results, it can be concluded that the first extract had a large number of aromatic hydrocarbon substances, with a relative content of 56.56%; the second extract showed that the main component was aliphatic hydrocarbon substances, with a relative content of 37.11%; the third extract showed a main component of aliphatic hydrocarbon substances, with a relative content of 81.78%. No aromatic hydrocarbons were detected in the three raffinates, indicating that after a subcritical extraction, all the toxic aromatic hydrocarbons in the coal tar slag were extracted, and the “purified” coal tar slag can be reused as a new resource. Compared with the supercritical extraction in the previous work by Ding [17], this study improves efficiency, and the extraction conditions are easier to realize, which saves on equipment costs. While subcritical extraction is well-established in biomedicine, our application in coal chemical processing represents a novel expansion of this technology, offering new perspectives for industry researchers and significantly contributing to coal chemical industry chain development.