Coal Tar Production

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Production
The complete process of coal tar production is shown in Figure 1. The coal tar is produced by carbonization of coal. In this process, the coal is heated at 900-1100 ºC and the evolved vapors are condensed to form liquid, from which ammonia is removed to obtain a black viscous crude coal tar. The composition of tar so obtained depends upon the origin and composition of starting material used viz. bituminous (soft) coal, anthracite (hard) (Arnold 1997, Thami 2002).
Crude coal tar can be further purified using various techniques viz. solvent extraction, fractional distillation or coloum chromatography. The extraction of crude coal tar with alcohol containing 5% w/w Tween 80 produces Liquor carbonis detergens (LCD). Similarly, the distillate and residue obtained after fractional distillation of CCT are known as coal tar creosotes and coal tar pitch respectively. The creosotes consist of light oils (2-8%), middle oils (8-10%), heavy oils (8-10%) and anthracene oils (16-20%) (Gosselin et al. 1984). Pitch is mostly made up of highly condensed aromatic hydrocarbons (Kershaw 1993). Medical pix lithantracis is a mixture of pitch (66.67%) and tar oils (33.33%) (Roelofzen 2007).
Physical and chemical characteristics
Crude coal tar is a black and viscous liquid. It has obnoxious naphthalene like odor and a prickly burning taste (Gennaro1990). It is heavier than water. It is “practically insoluble” in water, whereas “all or almost all” dissolves in benzene or nitrobenzene (Budavari 1989).
According to EPA 1994 & IARC 1985, CCT contains approximately 10,000 compounds which consist of 48% hydrocarbons, 42% carbon and 10% water. The chemical composition of coal tar depends upon the temperature of distillation (IARC Monogr Eval C...

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...rmulation in mice (Bhatia 2009, 2011).
The therapeutic benefits of coal tar are the results of synergistic or additive effects of its more than thousand chemical constituents whose concentrations vary depending upon the source and distillation conditions. There is also not any compendial requirement which any coal tar sample must meet. There is not any specific analytical technique which can detect these variations in constituents. The disadvantages which are associated with currently used techniques viz. HPLC, GC aggregate the problem. Hence, the development of low cost, highly sensitive analytical method which can detect the minute alterations in the composition of coal tar will be helpful in product control, ensuring uniform safety and efficacy of drug product.
Conclusion
The safety and efficacy of coal tar in mild to moderate psoriasis is well documented.

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