Tar is a dark, viscous liquid or semi-solid substance composed of a complex mixture of hydrocarbons and free carbon. It is the initial liquid byproduct created through the high-temperature breakdown of organic matter, a process known as destructive distillation or carbonization.
The production of tar stems from two distinct source materials: fossil fuels, primarily coal or crude petroleum, and organic plant matter such as resinous wood. In both cases, the raw material is heated in an environment with little to no oxygen, which causes it to decompose chemically without catching fire.
Defining Tar, Pitch, and Bitumen
Tar is the liquid product of the destructive distillation of coal or wood, containing a wide range of volatile organic compounds. It serves as the raw material that is further refined.
Pitch is the solid or semi-solid residue that remains after crude tar has been heated and distilled to remove its lighter, more volatile components. This process concentrates the heavier hydrocarbons, making pitch a refined form of tar with a much higher melting point. Coal tar pitch, for example, is a common industrial product used as a binder.
Bitumen (often called asphalt) is chemically distinct, primarily derived from crude petroleum refining or found naturally. While visually similar and used for similar purposes like road paving, bitumen is a petroleum product, not a byproduct of destructive distillation. True tars contain more aromatic hydrocarbons.
The Industrial Process: Creating Coal Tar
Modern industrial tar production centers on coal tar, largely a byproduct of the coking process used in the steel industry. This method involves heating bituminous coal in specialized coke ovens to high temperatures, typically 1,000 to 1,100 degrees Celsius, in the absence of air to prevent combustion.
As the coal decomposes, it releases volatile compounds, referred to as coke oven gas. This hot gas mixture is cooled, causing the vaporized hydrocarbons to condense into raw coal tar. This crude tar is a complex, black, viscous fluid containing hundreds of chemical compounds, including polycyclic aromatic hydrocarbons.
To create commercially viable products, crude coal tar undergoes fractional distillation to separate components based on their boiling points. Lighter fractions, such as naphtha, naphthalene, and creosote oils, distill off at lower temperatures. The heavy, non-volatile residue remaining after the lighter components are removed is the coal tar pitch.
Traditional Methods: Producing Wood Tar
The traditional method of producing wood tar, often called dry distillation, involves the pyrolysis of wood, most commonly resin-rich pine. This technique is carried out in earthen pits, mounds, or brick kilns with a severely restricted oxygen supply. The temperatures used are significantly lower than those in industrial coal coking.
The wood, often consisting of resinous heartwood or stumps, is slowly heated, causing the organic material to decompose. This heating “sweats” the resinous compounds out as a vapor, which then cools and condenses, dripping down for collection as wood tar.
The process is controlled to ensure the wood does not ignite completely, producing charcoal as the main solid residue. Wood tar is a valuable liquid byproduct whose quality depends heavily on the resin content of the original wood. Historically, this method was a significant source of waterproofing and preservative materials.
Primary Applications of Tar
Coal tar pitch is widely used as a binder in the production of carbon electrodes for the aluminum and steel industries. It is also used in roofing materials, where it provides a durable, waterproof membrane.
Wood tar, particularly pine tar, is valued for its natural preservative properties and is traditionally used to treat and waterproof wood. It is a common coating for the hulls of wooden boats and the exterior of log buildings, offering protection against moisture and decay. It is also found in cosmetic and pharmaceutical preparations, such as medicated shampoos and soaps.
Both coal tar and its pitch derivatives are utilized in road construction, although petroleum-derived bitumen is the predominant material. Coal tar-based sealants are applied to pavement surfaces to protect the underlying asphalt. The light oil fractions separated during coal tar distillation serve as chemical feedstocks for the synthesis of dyes, plastics, and pharmaceuticals.