Coal, and What We Get from It

by Raphael Meldola

Fig. 5.—Mansfield’s still. R the heating burner, A the body of the still with stopcock, i, for running out the contents. B the still-head kept in a cistern, C, of hot water or other liquid. The vapour generated by the boiling of the liquid in A, partly condenses in B, from whence the higher boiling-point portion flows back into the still. The uncondensed vapour passes into the condensing-worm, D, which is kept cool by a stream of water, and from thence flows into the receiver S. By opening m in the side-pipe any higher boiling-point oil condensing in the delivery-pipe can be run back into the still.

The operation of tar-distilling is about as unromantic a process as can be imagined, but it must be briefly described before the subsequent developments of the industry can be appreciated properly. It has already been explained that the tar is a complex mixture of many different substances. These various compounds boil at certain definite temperatures, the boiling-point of a chemical compound being an inherent property. If a mixture of substances boiling at different temperatures is heated in a suitable vessel the compounds distil over, broadly speaking, in the order of their boiling-points. The separation by this process is not absolute, because compounds boiling at a certain temperature have a tendency to bring over with them the vapours of other compounds which boil at a higher temperature. But for practical purposes it will be sufficient to consider that the general tendency is for the compounds of low boiling-point to come over first, then the compounds of higher boiling-point, and finally those of the highest boiling-point. This is the principle made use of by the tar-distiller. The tar-still is a large iron pot provided with a still-head from which the vapours boil out into a coil of iron pipe kept cool in a vessel of water (see [Fig. 6]). The still is heated by a fire beneath it, and the different portions which condense in the iron coil are received in vessels which are changed as the different fractions of the tar come over. The process is what chemists would call a rough fractional distillation. The first fractions are liquid at ordinary temperatures, and the water in the condenser is kept cold; then, as the boiling-point rises, the fraction contains a hydrocarbon which solidifies on cooling, and the water in the condenser is made hot to prevent the choking up of the coil. Every one of these fractions of coal-tar, from the beginning to the end of the process, has its story to tell—all the chief constituents of the tar separated by this means have by chemical science been converted into useful products.

Fig. 6.—Sectional diagram of tar-still with arched bottom. The fireplace is at i; the hot gases pass over the bridge k and through g into the flues h, h. The pipe at c is to supply the still with tar; a is the exit pipe connected with the condenser, and b a man-hole for cleaning out the still. The condenser and bottom pipe for drawing off pitch have been omitted to avoid complication.

It is customary at the present time to collect four distinct fractions from the period when the tar begins to boil quietly, i.e. from the point when the small quantity of watery liquor which is unavoidably entangled with the tar has distilled over, by which time the temperature in the still is about 110° C. The small fraction that comes over up to this temperature constitutes what the tar-distiller calls “first runnings.” From 110° C. to 210° C. there comes over a limpid inflammable liquid known as “light oil,” and this is succeeded by a fraction which shows a tendency to solidify on cooling, owing to the separation of a solid crystalline hydrocarbon known as naphthalene. This last fraction, boiling between 210° C. and 240° C., is known as “carbolic oil,” because it contains, in addition to the naphthalene, the chief portion of the carbolic acid present in the tar. From 240° C. to 270° C. there comes over another fraction which shows but little tendency to solidify in the condensing coil, and which is known as “heavy oil,” or “creosote oil.” From 270° C. up to the end of the distillation there distils a fraction which is viscid in consistency, and has a tendency to solidify on cooling owing to the separation of another crystalline hydrocarbon known as anthracene, and which gives the name of “anthracene oil” to this last fraction. When the latter has been collected there remains in the still the black viscid substance known as pitch, which is obtained of any desired consistency by leaving more or less of the anthracene oil mixed with it, or by afterwards mixing it with the heavy oil from previous distillations. The process carried out in the tar-still thus separates the tar into—(1) First runnings, up to 110° C. (2) Light oil, from 110° to 210° C. (3) Carbolic oil, from 210° to 240° C. (4) Creosote oil, from 240° to 270° C. (5) Anthracene oil, from 270° to pitch. (6) Pitch, left in still.

It has already been said that coal-tar is a complex mixture of various distinct chemical compounds. Included among the gases, ammoniacal liquor, and tar, the compounds which are known to be formed by the destructive distillation of coal already reach to nearly one hundred and fifty in number. Of the substances present in the tar, about a dozen are utilized as raw materials by the manufacturer, and these are contained in the fractions described above. The first runnings and light oil contain a series of important hydrocarbons, of which the three first members are known to chemists as benzene, toluene, and xylene, the latter being present in three different modifications. The carbolic oil furnishes carbolic acid and naphthalene, and the anthracene oil the hydrocarbon which gives its name to that fraction. Here we have only half a dozen distinct chemical compounds to deal with, and if we confine our attention to these for the present, we shall be enabled to gain a good general idea of what chemistry has done with these raw materials. The products separated during the processes which have to be resorted to for the isolation of these raw materials have also their uses, which will be pointed out incidentally.

Beginning with the first runnings and the light oil, from which the hydrocarbons of the benzene series are separated, we have to make ourselves acquainted with the treatment to which these fractions are submitted by the tar-distiller. The light oil is first distilled from an iron still, similar to a tar-still, and the first portions which come over are added to the oily fraction brought over by the water of the first runnings. The separation of the oil from the water in this last fraction is a simple matter, because the hydrocarbons float as a distinct layer on the water, and do not mix with it. We have at this stage, therefore, four products to consider, viz. 1st, the oil from the first runnings; 2nd, the first portions of the light oil; 3rd, the later portions of the light oil; and 4th, the residue in the still. The first and second are mixed together, and the third is washed alternately with alkali and acid to remove acid and basic impurities, and can then be mixed with the first and second products. The total product is then ready for the next operation. The last portion of the light oil which remains in the still is useless as a source of benzene hydrocarbons, and goes into the heavy oil of the later tar fractions.

The process of purification is thus far one of fractional distillation combined with chemical washing. In fact, all the processes of purification to which these oils are submitted are essentially of the same character. The principle of fractional distillation has already been explained sufficiently for our present purpose. The process of washing a liquid may appear mysterious to the uninitiated, but in principle it is extremely simple. If we pour some water into a bottle, and then add some liquid which does not mix with the water—say paraffin oil—the two liquids form distinct layers, the one floating on the other. On shaking the bottle so as to mix the contents, the two liquids form a homogeneous mixture at first, but on standing for a short time separation into two layers again takes place. Now if there was present in the paraffin oil some substance soluble in the oil, but more soluble in water, such as alcohol, we should by the operation described wash the alcohol out of the oil, and when the liquids separated into layers after agitation the watery layer would contain the alcohol. By drawing off the oil or the water the former would then be obtained free from alcohol—it would have been “washed.” This operation is precisely what the manufacturer does on a large scale with the coal-tar oils. These oils contain certain impurities of which some are of an acid character and dissolve in alkalies, while others are basic and dissolve in acid. The oil is therefore agitated in a suitable vessel provided with mechanical stirring gear with an aqueous solution of caustic soda, and after separation into layers the alkaline solution retaining the acid impurities is drawn off. Then the oil may be washed with water in the same way to remove the lingering traces of alkali, and then with acid—sulphuric acid or oil of vitriol—which dissolves out basic impurities and certain hydrocarbons not belonging to the benzene series which it is desirable to get rid of. A final washing with water removes any acid that may be retained by the oil.

The total product containing the benzene hydrocarbons is put through such a series of washing operations as above described, and is then ready for separation into its constituents by another and more perfect process of fractional distillation. This final separation is effected in a piece of apparatus somewhat complicated in structure, but simple in principle. It is a development on a large scale of the apparatus used by Mansfield in his early experiments. The details of construction are not essential to the present treatment of the subject, but it will suffice to say that the vapours of the boiling hydrocarbons ascend through upright columns, in which the compounds of high boiling-point first condense and run back into the still, while the lower boiling-point compounds do not condense in the columns, but pass on into a separate condenser, where they liquefy and are collected. But even with this rectification we do not get a perfect separation—the hydrocarbons are not perfectly pure from a chemical point of view, although they are pure enough for manufacturing purposes. Thus the first fraction consists of benzene containing a small percentage of toluene, then comes over a mixture containing a larger proportion of toluene, then comes a purer toluene mixed with a small percentage of xylene. The boiling-points of the three hydrocarbons are 81° C., 111° C., and 140° C. respectively; but owing to the nature of fractional distillation, a compound of a certain boiling-point always brings over with it a certain quantity of the compound of higher boiling-point, and that is why the rectifying column effects only a partial separation.