T. Howard Butler, Ph.D., M.Sc, F.I.C.

Managing Director Of William Butler & CO. (Bristol), Ltd., Tar, Rosin, And Petroleum Distillers.

Chapter XXXV. Introduction

Early History

Few industries are more dependent on distillation than that of coal tar. It was in consequence of a discovery by Bethel in 1838 that it was realised that valuable products'could be obtained from coal tar by submitting it to distillation. Since that date the industry has thrived, and every year brings out discoveries of further ingredients in tar, and further uses for the bodies that have for a long time been known to exist in it.

Bethel's discovery was that an oil suitable for the effective preserving of timber could be obtained by the distillation of tar. The first distillation of tar actually dates back much further than this, although the value of the process was not then realised.

Henry Haskins took out a patent in 1746, but probably used wood tar, as gas tar was very little known so early as this. Again, Long-staffe and Daleton aver that they erected the first distillery in Leith in 1822, for the purpose of producing an oil that was used as a water-proofing medium. It is also asserted that a plant was erected in the neighbourhood of Manchester in 1834 for manufacturing an oil that would dissolve the residual pitch to make a black varnish.

Nature Of Coal Tar

Coal tar is essentially the thick viscous liquid which is obtained by the carbonisation of coal, and is a by-product in the ordinary manufacture of coal gas. It should be differentiated from all the other forms of tar.

When coal is submitted to dry distillation in horizontal or vertical retorts, the products of carbonisation are gas, ammoniacal liquor, and tar. The tar comes off in the form of a fog suspended in the gas, from which it is condensed or scrubbed out by suitable means, when the resulting products contain varying proportions of ammoniacal liquor and tar.

Tar is a thick black liquid, and is a very complex mixture of many widely different substances. Its composition varies enormously according to the mode of its formation in the gas or coke works. Some of the principal factors governing its composition are the following:1. The Nature of the Coal.

2. The Temperature of Carbonisation. - This is probably the most important factor of all, as tar obtained from low temperature carbonisation differs widely from that from high. The former is by far the most valuable to the tar distiller, as it contains more naphthas and phenol, and less pitch and free carbon. But the production of gas is much lower, and as the tar is a by-product it is not a commercial proposition to utilise this method.

3. The Kind of Retort. - A horizontal retort gives an entirely different tar from that afforded by a vertical retort. The tar from the former contains larger quantities of aromatic hydrocarbons, more phenol, and more naphthalene, but less phenol homologues. The vertical retort tends to produce more paraffins, less pitch, and less free carbon.

4. The Shape of the Retort. - The length of time the hot gases evolved from the coal remain in contact with the walls of the retort has a great influence on the nature of the tar produced and depends largely upon the size and shape of the retort.

5. The Depth of the Coal Charge in the Retort. - The depth of the charge in a vertical retort is naturally much greater than in a horizontal one. Again, similar horizontal retorts worked under identical conditions, except that the depth of the coal charge is varied, will produce tar of quite different composition.

There are many other details in gas or coke manufacture which may affect the composition of tar, but they need not be enumerated here; and it should always be borne in mind that the coal is carbonised for the manufacture of gas or coke, and consequently the quality of the tar has too frequently to be left to chance.

Different Kinds of Tar

The field of the tar distiller is a very wide one, and although generally confined to coal tar and coke oven tar, he frequently has to deal with other forms of tar. They may be very briefly mentioned :1. Coke Oven Tar. - It was not until the discovery of Knab, Hauport and Carves in the late 'fifties that it was realised what enormous waste was going on in the making of metallurgical coke for the iron industry in the old beehive oven. In this form of oven all products went to waste, as the coal was carbonised purely for the production of coke. Even after many of the new ovens had been erected on the Continent there was much prejudice against them in this country, as it was thought that an inferior coke was obtained if the plant was enclosed so as to collect the tar and ammonia.

In 1887 further progress was made by Franz Brunck of Dortmund, who introduced the recovery of benzole from coke oven products, a step of extreme importance to all industries connected with coke ovens and benzole.

It is not necessary to go further into this question of by-product coke ovens beyond mentioning the names of some of the most important ones in use at the present time, such as The Coppee Oven. The Otto-Hoffmann Oven. The Otto-Hilgenstock Oven. The Simon-Carves Oven. The Hussener Oven. The Semet-Solvav Oven.

The quality of coke oven tar is, like that of coal tar, very variable indeed, but is similar enough to enable the distiller to mix the tars and work them up together. Generally speaking, however, coke oven tar is less viscous, has a lower specific gravity, contains less naphtha and phenols, although often as much or even more total tar acids, and it leaves less pitch.

2. Blast Furnace Tar. - The combustible material used in blast furnaces for the manufacture of pig iron is either coke or coal. The most modern plants use coal, but the earlier forms only coke, which naturally does not produce much volatile material such as ammonia and tar. On the other hand, the amount of these produced when coal is used is very large indeed, and in some cases the quantity is 12 to 13 times as great as that obtained from the same kind of coal when heated in ordinary gas retorts. The tar comes off similarly in a fog suspended in the gas, but is in a physically different form, which makes it even more difficult to separate from the gas. Many plants and processes have been devised and patented to separate the tar, but it is hardly necessary to mention them here. The gas is thus purified and is used for raising steam, distilling the tar, etc., and a well-equipped works need use hardly any coal outside that fed into the furnaces.

The composition of blast furnace tar is very different from that of gas tar. It contains very small quantities of the lower aromatic hydrocarbons, large quantities of bases and phenols, some solid paraffins, and a small quantity of pitch. The phenols are the higher homologues, and in this fact lies the value of the tar, for blast furnace creosote is used largely for obtaining disinfectants having high Bideal-Walker coefficients, whereas gas tar creosote is unsuitable for this purpose, as it is not so rich in the higher homologues of phenol.

Blast furnace tar is distilled and treated in a similar way to coal tar, but, owing to the difference in its composition, cannot be efficiently worked up with it.

3. Oarburetted Water-gas Tar. - Water gas is obtained by passing steam through white-hot coke or anthracite, and is formed according to the following reactions :1. 2H20 +C = 2H2 + C02

2. C02 + C = 2CO.

The mixture of carbon monoxide and hydrogen thus formed is useful for heating, but not for lighting, as it burns with an almost non-luminous blue flame, hence the term "Blue Water Gas." It is usual to "carburett" the water gas with crude petroleum hydrocarbons by passing the two together through red-hot retorts. This carburetted water gas gives up on scrubbing a certain amount of tar, which unfortunately forms an emulsion with water, which is very difficult to separate. Long standing has very little effect, and the most efficacious remedy is centrifuging.

This tar is of little value, as it contains only small quantities of naphthas, no phenols, but chiefly high-boiling hydrocarbons, and it leaves little pitch. In fact, it is not greatly changed from its original composition as it entered the retorts to serve as the carburettor.

It can be mixed with coal tar and distilled with it, provided the proportion is not large.

4. Producer Gas Tar. - This is very viscous and contains a large quantity of water which is exceedingly difficult to separate. When distilled, it produces no light oils, but paraffins, high-boiling tar acids, and a large percentage of pitch.

5. Oil-gas Tar. - Reference need only be made to this, as very little is made. Oil gas is formed by passing aliphatic hydrocarbons, generally crude petroleum, through red-hot tubes. Part of the oil escapes decomposition and part is decomposed, leaving a residue of carbon, the result being the oil-gas tar.

6. Wood Tars. - These tars are produced in almost every country of the world which contains big forests, and where charcoal burning is practised. They are entirely different from coal tar and are rarely distilled.

Composition of Coal Tar

The composition of coal tar, as has already been stated, varies so enormously, according to its mode of production, that it is useless to attempt to give any definite analysis. It can be said, however, that it generally consists ofWater. - Properly ammoniacal liquor, the quantity of which is generally about 5 per cent, although it may reach even 40 per cent. A tar with this amount of water is an emulsion, is separated with great difficulty, but has the appearance of a good tar free from water.