# Subcritical Extraction of Coal Tar Slag and Analysis of Extracts and Raffinates
## 1 Introduction
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 CO\({}_{2}\), 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]. [PERSON] [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 \({}^{\circ}\)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]. [PERSON] [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. [PERSON] [21] used GC-MS to analyze the composition of washed fractions from low-temperature and high-temperature coal tar. The experiments revealed that petroleum ether effectively extracted light fractions, primarily consisting of aliphatic hydrocarbons, aromatic hydrocarbons, and phenolics.