# Subcritical Extraction of Coal Tar Slag and Analysis of Extracts and Raffinates
## 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]. [PERSON] [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 \({}^{\circ}\)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, SO\({}_{2}\), and other gases that can be harmful to the environment and people, it is necessary to pre-treat it. [PERSON] [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/m\({}^{3}\), and the SO\({}_{2}\) emission was less than 100 mg/m\({}^{3}\). 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. [PERSON] [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 \({}^{\circ}\)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.