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

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.