What Is Coal Tar?
Coal tar is a liquid substance obtained from the dry distillation of coal by cooling a hot gaseous substance.
There are two types of coal tar: low-temperature coal tar produced by low-temperature distillation and high-temperature coal tar produced by high-temperature distillation. Low-temperature coal tar is generally used.
It is basically a mixture of various aromatic hydrocarbons, and its CAS registration number is 8007-45-2.
Coal Tar used to be obtained as a byproduct of coal dry distillation in the production of city gas. However, since the raw materials for city gas have shifted to natural gas, coal tar is now produced as a byproduct of coke production for steelmaking at steel mills.
Uses of Coal Tar
The main uses of coal tar are as a raw material for tar distillation products, anti-corrosion paint, fishing net dye, oil smoke, and fuel.
In the past, it was widely used as a preservative for wood such as sleepers and wooden poles, and as a coating for tin roofs, but its use has declined in recent years due to changes in building materials and the widespread use of concrete.
It was also used as a support material for carbon electrodes.
Coal tar preparations also used to be a substance used in medicine because of its keratolytic, plasmolytic and antipruritic properties. In particular, psoriasis, a type of skin disease, was once treated with the Geckelman therapy, in which coal tar was applied externally. However, since its carcinogenicity, which will be discussed later, it is no longer used.
Tar Distillation Products
Coal tar is used as a raw material for distillation to produce tar distillate products. Tar light oil, a relatively light fraction of the fractional components, is used for BTX (benzene, toluene, and xylene) and styrene monomer.
Carbol oil and naphthalene oil are used for phenol and cresol.
Anthracene oil is used for carbon black and pitch is used for pitch coke.
Properties of Coal Tar
The main components of Coal Tar are polycyclic aromatic hydrocarbons.
The main compounds and their content are as follows:
- Naphthalene: 5%-15
- Benzene: 0.3%-1
- Phenol: 0.5%-1.5
- Benzo[a]pyrene: 1%-3
- Phenanthrene: 3%-8
It is brown or black in color, viscous liquid. It has a peculiar naphthalene-like odor. Its specific gravity is 1.18-1.23. It is slightly soluble in water and mostly soluble in benzene and nitrobenzene.
It is also partially soluble in some alcohols and caustic soda solutions.
Types of Coal Tar
Coal tar is sold as a corrosion inhibitor for various steel products, as a wood preservative, and as a reagent product for research and development.
As an anticorrosion and corrosion inhibitor, the product is available in 16 kg, 2.5 kg, and other capacities. As R&D products, they are available in capacities of 100 mL, 500 mL, etc.
Coal tar is also fractionated by distillation and sold as tar fractionated products. The main products and their applications are as follows:
- Pitch: binder for electrodes
- Creosote oil: preservative, paint
- Naphthalene: dye, pigment
- Tartaric acid: resin raw material, pigment
Other Information on Coal Tar
1. Stability and Reactivity of Coal Tar
Coal tar is stable under normal handling conditions, but will react with oxidizing substances. Contact with oxidizing substances should be avoided in storage.
Combustion of coal tar will produce black smoke, carbon monoxide, carbon dioxide, and other toxic substances.
2. Coal Tar Safety
Coal Tar is a substance that has been shown to be harmful to the human body. Specifically, it is carcinogenic, toxic by oral ingestion, mildly irritating to the skin, may cause allergic skin reactions, may cause serious eye damage, may cause genetic disorders, may damage the nervous system, and may irritate the respiratory tract.