Information on substances
Creosote comes from the Greek kréas, “flesh”, and sö’zö, “save”, “preserve”. Creosote products are mixed from the medium fractions obtained in the distillation of tar, to meet specifications of such physical parameters as density, water content and distillation fractions. These are often defined in various standards, geared to the various functional requirements traditionally maintained by the users.
Creosote can be made from both wood and coal tar. Wood tar is mostly used as such, and so output of wood tar creosote is very small. This type of creosote has a low boiling point and consists mainly of phenol, cresol and guaiacol.
Wood creosote is used for pharmaceutical purposes and for incense manufacturing, which as been going on for many years in Sweden.
Coal tar is above all a by-product from the coking plants of steel mills. Tars are also formed in such processes as production of town gas and domestic coke. Their chemical composition varies because different processes are used, e.g. with very different processing temperatures. This of course makes a difference to the substances included in the different distillates. Creosote is made from the distillates which have boiling points in excess of 200ºC. The distillation is discontinued at about 360ºC. The end point ends on the specifications of viscosity and softness of the residue, the coal tar pitch. The fractions distilled below 200ºC contain, for example, benzenes, toluene, xylene and phenol and are used as chemical raw materials or fuels.
The medium distillates are known, according to distillation interval, as naphthalene oils, middle oils, creosote oils, wash oils, anthracene oils and heavy oils, for example, and above all contain the substances which have boiling points within the distillation interval. Different manufacturers extract different numbers and widths of fractions, according to type of facility and customer requirements. Unfortunately, not only is roughly the same function called by a variety of names, but one and the same name can represent different fractions extracted in different distillation intervals, depending on trademark. One thing they all have in common, though, is that in the distillation interval they all mostly contain boiling hydrocarbons of aromatic type including 1% or so of every substance. Small amounts of all the other substances in the tar can also be found in every fraction.
Naphthalene oils are distilled between 200ºC and 250ºC (the boiling point of naphthalene is 217ºC) and they can contain up to 60-65% naphthalene. This reaction can be used for extracting naphthalene. In addition to methyl naphthalenes they also contain indene and biphenyl, heterocyclic compounds like quinoline and lighter tar bases and tar acids like pyridine and phenol. About 50 different substances have been identified in this fraction.
Middle oils, creosote oils and wash oils are distilled at between 250ºC and 300ºC and contain various combinations of 2 aromatic rings and single alkyl groups, e.g. acenaphthene, dibenzophurane and methyl fluorenes.
The heavier distillates are called anthracene oil and/or heavy oil, the distillation interval is about 300-350ºC and, as the name implies, they can be used for anthracene extraction. These mostly include more or less conjugated systems of 3 and 4 aromatic rings, which, for example, in addition to anthracene, also include fluoranthene, pyrene, crysene and systems of hetero-atoms like carbazol. Conjugated 4-ring systems and heavier molecules mostly remain in the residue, pitch. The temperature at which the distillation is discontinued has a great influence on the chemical composition of the next lighter fraction.
The products known as creosote are mixed, then, from these fractions. Sometimes residual products from other processes in the plant, e.g. residual oils from the extraction of naphthalene or phenol, are also used in them. The result is an amber-coloured to black liquid, which is practically insoluble in water.
Creosotes are very widely used for impregnating wood. Another common use is to pyrolyse them to carbon black. A minor proportion is used for admixture to pitch and asphalt, to give them the right consistency for use, e.g. as road surfacing. Tar coal products also make good fuels; creosote has roughly the same boiling point as petroleum fuels like diesel and the lighter fuel oils.
No coal tar creosotes are manufactured in Sweden. Such creosote is imported for use as a wood impregnating agent.