Tar is a dark, viscous, organic liquid, but it is not a single substance. Instead, it encompasses several materials with similar appearances and properties, each derived from different organic sources. These materials have been utilized across centuries for various purposes, ranging from waterproofing to road construction. The distinct methods of production for these different types of tar contribute to their unique characteristics and applications.
Understanding Different Tars
The primary types of tar encountered are coal tar, wood tar, and bitumen. Each originates from a distinct raw material, giving them unique compositions and uses. Coal tar is a byproduct of processing coal, a fossil fuel formed from ancient plant matter. Wood tar, on the other hand, is derived directly from plant material, specifically wood. Bitumen, often known as asphalt in North America, primarily comes from crude petroleum, another fossil fuel.
Producing Coal Tar
Coal tar is generated as a byproduct during the destructive distillation of coal, a process carried out to produce coke for the steel industry. This method involves heating coal to high temperatures, typically between 900 and 1,200 degrees Celsius, in the absence of oxygen within specialized coke ovens. The heat causes the complex organic compounds within the coal to break down into simpler molecules.
During this thermal decomposition, volatile gases are released from the coal. These gaseous products are then cooled and condensed, separating into a crude coal tar, along with other byproducts like coke oven gas and ammoniacal liquor. Crude coal tar is a dense, black liquid, recognized as a complex mixture of aromatic hydrocarbons, phenols, and other chemical compounds. Different fractions can be recovered through further distillation processes, yielding substances used in various chemical and industrial applications.
Producing Wood Tar
Wood tar is produced through the destructive distillation of wood, a process also known as pyrolysis, often occurring with charcoal production. This method involves heating wood in an environment with limited or no oxygen. Historically, this was achieved using tar kilns, where logs, often from resin-rich trees like pine or birch, were stacked in a pit or kiln and covered with earth or peat to restrict airflow. Heat was applied, causing the wood to decompose without fully burning.
The volatile components released during this heating process are collected and condensed to form wood tar. Traditional methods allowed the liquid tar to seep out into a collection vessel at the bottom of the kiln. Modern processes utilize distillation equipment to control the temperature and collect the volatile compounds more efficiently. The composition of wood tar can vary depending on the type of wood used and the specific pyrolysis conditions.
Producing Bitumen
Bitumen, widely used in road construction, is primarily derived from crude petroleum through a refining process. Crude oil is a complex mixture of hydrocarbons with varying boiling points. The production of bitumen begins with heating crude oil in an atmospheric distillation tower, typically to temperatures ranging from 300 to 350 degrees Celsius.
During this initial distillation, lighter fractions like gasoline and diesel evaporate at lower temperatures and are separated. The heavier, non-volatile components, known as atmospheric residuum, remain at the bottom of the tower. This residuum is then transferred to a vacuum distillation unit, where it is further heated under reduced pressure. Operating under vacuum allows the remaining heavier oils to vaporize at lower temperatures, leaving bitumen as the heaviest residue.