Tuesday, 23 June 2015

Coal tar and Coal tar dyes

Today coal tar is seen as a menacing cancer-giving substance. This of course wasn't always the case and careers in the gas industry were built on knowledge of tars and how they could be used.  The following article describes its application in the development of dyes.  It was written in the 1920s-or 1930s and comes from Copartnership Herald - which was the house journal of the east London based Commercial Gas Company.
It is worth noting - what the article doesn't say - that most of the research described here took place in east London. largely in Hackney Wick.  Perkin himself, very much the hero of tar based dye stuffs - was brought up in Stepney and did much of his lonely research as a lad in a room above his father's shop.
We shouldn't put down the achievements of these researchers because of our subsequent knowledge. Arguably we wouldn't know of the dangers had people like them not looked at new substances and analysed them - and the development of these coal tar dyes led to other research, as well as leading to major industries and consumer goods, which we still buy.
It is of historical interest that the first English patent, referring to the destructive distillation of coal, was granted in the year 1681 to .J .J. Becher and Henry Serle for "a new way of making pitch and tarre out of pit- coale never before found out or used by any other." At first the industry was of very small proportions, and. not until the beginning of the nineteenth century was the distillation of coal carried out extensively, and then 110t for the purpose of producing tar and pitch, but for the making of coal gas. The tar thus formed forced itself upon the notice of the gas manufacturer since it could ·not be thrown away without causing a nuisance. As it was impossible to get rid of it by running it into the streams and rivers, the problem was solved to some extent by burning the tar as a fuel. About the year 1830, tar was used at the Glasgow Gas Works for heating the retorts by pouring it over the coke. Some relief was also brought about by its use as a paint for wood and metal work, and for this purpose the more volatile portions of the tar were removed by distillation, the spirit so obtained being used either as a substitute or turpentine or as a solvent for rubber in the manufacture of a waterproof material, which is still known by the name of the original Glasgow manufacturer, Mackintosh. Some of the residue from the distillation was burned for the production of lampblack, and this was used in the making of blacking and printers' ink.
The demand for tar still lagged behind the supply until the year 1838, when .John Bethel introduced a process for preserving or •• pickling" timber, thus starting an industry which has now attained enormous proportions, and still forms at the present day one of the most important outlets for coal tar oils.
During the year 1845 there was founded ill London the Royal College of Chemistry, where A. VV. Hofmann and his students interested themselves in the nature and composition of coal tar. One of the 'earliest results to be obtained was the isolation from it of the hydrocarbon benzene. These investigations not only led to the isolation of some of the main constitucnts of coal tar, but were the beginning from which the vast modern industry of coal tar dyes, drugs, and explosives has grown. As early as in the year 1831 it was known that when benzene is treated with concentrated nitric acid it is converted into an oily liquid nitro-benzene, which was manufactured in small quantities, 'and used under the name of essence of mirbane for scenting soap. Nitro-benzene in its turn, as was found by Bechamp in 1854, could be converted into aniline by the action of a mixture of acetic acid and finely divided iron upon the nitro-benzene. It was with this compound, aniline, that W H. Perkin made his important discovery of the first coal tar dye. Whilst engaged in an attempt to produce quinine from simpler substances, he treated a solution of aniline in dilute sulphuric acid with potassium dichromate. As a result there separated out dark-coloured resinous mass, and from this material Pcrkin obtained the first known aniline dye, which was manufactured and sold under the name of aniline purple or mauve, the name given to it by the French dyers. The success which attended the introduction of mauve, the vogue of which among the women became so common that Punch referred to it as " The mauve measles," naturally led chemists to try the action of other substances on aniline, and although they did not succeed in making mauve, their efforts led to the discovery of a new dye, aniline red, magenta or fuchsine. The formation of this dye had been observed as early as 1856, but the success of its manufacture was not achieved until the year l860, when two English chemists, Medlock and Nicholson prepared it by the action of arsenic acid on commercial aniline. Just as aniline formed the basis of manufacture for mauve and of magenta, so magenta in its turn became the starting-point for the preparation of a series of new dyes, the number of which began rapidly to increase. In 186l aniline blue was prepared by heating magenta with aniline in the presence of benzoic acid, and by treating this dye with concentrated sulphuric acid Nicholson produced the more valuable blue which possessed the advantage of being soluble in water. This dye was more suitable for dyeing wool than the dyes previously prepared.
Although the preparation of new dyes and the perfecting of their industrial production was carried on with much vigour along the lines opened up by W. H. Pcrkin, chemists were not unmindful or the need of more theoretical investigations for the purpose of determining the composition and constitution of these new substances. In this work Hofmann took a leading part, and in 1862 confirmed what had already been discovered by Nicholson, that magenta could not be obtained from the pure aniline but only from the commercial aniline which contained the two substances, ortho- and para-toluidine, as impurities. Owing to these developments, a demand was created for the more volatile portions of the coal tar which had to be rejected by the timber "pickling" industry.
Although, in the past, crude coal tar was employed not only as a fuel but also for the manufacture of roofing felt, the tarring of roads and other purposes, the water or ammoniacal liquor present in the tar was found to be detrimental, so that now only a small amount is used in the crude static
Tar is a complex mixture of substances, and these vary considerably, their relative amounts depending upon the kind of coal distilled, and the conditions under which the distillation is carried out. The temperature, shape of retort, and the time during which the volatile products remain in contact with the red hot walls of the retort, influence to a wide extent its chemical and physical properties. "Then the distillation is carried out at a low temperature the tar contains a large percentage of hydrocarbons of the paraffin and olefine series, and a small' amount of naphthalene and “free carbon." If the distillation is carried out at a high temperature, as in the case when coal is distilled for illuminating gas or hard coke, the tar contains only traces of paraffinoid hydrocarbons, the predominating hydrocarbons being those of the benzene, naphthalene, and anthracene series (aromatic hydro- carbons). The "free carbon" content of this tar is generally high. In practice the tar obtained from heavily charged retorts is of a superior quality to that produced from those lightly charged, and contains less" free carbon" and a higher percentage of light oils.
Of the total quantity of tar produced in produced in gas works, about 60 per cent. is deposited in the hydraulic main, the remainder being carried forward with this gas to the condensers and tar extractors.

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