PART XVI.
Other products obtainable from Coal, namely:—Coal Tar—Pitch—Coal Oil—Ammoniacal Liquor, and conversion of the latter into Carbonate, and Muriate of Ammonia.
Coal Tar.
The coal tar is so called from its resembling common tar in its appearance, and most of its qualities.
This substance is deposited in the purification of the coal gas, in a separate vessel destined to receive it. See fig. 3, [plate I.]
In the year 1665, Becher, a German chemist, brought to England his discovery for extracting tar from coal, this distillation he performed in close vessels. It is not mentioned in the records of the time, whether Becher obtained, or rather collected, any other articles than the tar.
Several works have been, at different times, erected both in England and on the continent, to procure from coal a substitute for tar; but they have turned out unprofitable speculations.
In 1781, the Earl of Dundonald invented a mode of distilling coal in the large way, which enabled him not only to form the coke, but, at the same time, to save and collect the tar. Even this process, however, for which a patent was taken out, gained very little ground. Its object was too limited; for though some of the proximate constituent parts of coal were procured, they were obtained at an expence that nearly balanced the profits; and no attention whatever was paid to the coal gas, which constitutes by far the most valuable part obtainable from pit coal.
Coal tar is now used with advantage largely in the Royal Navy, and also for painting and securing wood that is exposed to the action of air. The wood being warmed, the tar is applied cold, and penetrating into the pores, gives the timber an uncommon degree of hardness and durability.
The quantity of tar obtainable from a given quantity of coal, varies according to the manner in which the decomposition of the coal is affected. See [page 122].
The tar obtained from Newcastle coal is specifically heavier than that produced from cannel coal; hence it sinks in water, whereas the latter swims on the surface of that fluid.
To render coal tar fit for use, it requires to be evaporated to give it a sufficient consistence. If this process be performed in close vessels, a portion of an essential oil is obtained, which is known by the name of
Coal Oil.
To obtain this oil, a common still is charged with coal tar, and, being properly luted, the fire is kindled and kept up very moderate, for the tar is very apt to boil up in the early part of the process. The first product that distils over is principally a brown ammoniacal fluid, mixed with a good deal of oil. As the process advances, and the heat is increased, the quantity of ammoniacal liquor lessens, and that of oil increases, and towards the end of the distillation the product is chiefly oil.
The oil and ammoniacal water which distil over do not mix, so that they may be easily separated by decantation. The oil is a yellowish inferior kind of naptha, which is very useful in painting ships, and for making common varnishes. It has lately been employed as a substitute for whale oil, to be burnt in out door lamps.
The contrivance by means of which this oil is burnt in lamps[61] consists of a fountain reservoir to supply and preserve a constant level. The burner with its wick is placed in the axis of the lamp, and supplied with the oil from the fountain reservoir, placed on the outside of the lamp. The air is admitted by an aperture at the bottom of the lamp. The current of air in passing through the lamp envelopes the burner and urges the flame, which is extremely bright; but it is essential that the flame should be small. The draught tube proceeding from the centre of the reflector above the flame carries away the smoke.
[61] All the lamps on Waterloo Bridge, and the streets adjoining the bridge are lighted by means of tar oil.
1430 pounds of coal tar, produce 360 pounds of essential oil. The residue left after the distillation is
Pitch.
If the coal tar is wanted to be converted into pitch, without obtaining the oil which it is capable of furnishing, the evaporation of it may be performed in a common boiler; but as it is extremely liable to boil over, the greatest precaution is necessary in conducting the evaporation. A spout or rim is added to the common boiler into which the tar spreads itself as it rises, and by this means becomes cooled, and the boiling over is checked.
1430 pounds of coal tar produce 9 cwt. of pitch. A subsequent evaporation with a gentle heat, converts the coal pitch into a substance greatly resembling asphaltum.
Manufacture of Carbonate of Ammonia from the Ammoniacal Liquor of Pit Coal.
The ammoniacal liquor obtained in the gas light manufacture, is employed for the production of carbonate of ammonia. The average quantity of this liquor, obtainable from a chaldron, (27 cwt.) of Newcastle, or Sunderland coal, amounts to from 180 to 220 pounds. It is chiefly composed of carbonate and sulphate of ammonia. The quantity of ammonia contained in it, varies considerably. The strongest liquor is obtained from coal that readily cake, ([page 45]); a gallon (or eight and a half pounds weight) of ammoniacal liquor usually requires for saturation, from fifteen to sixteen ounces of sulphuric acid of a specific gravity 1,84. The weakest ammoniacal liquor is obtained from those species of coal which do not cake, and which by a single combustion are reduced to light ashes. It requires only from eight to ten ounces of sulphuric acid, of the before mentioned specific gravity for its saturation.
The following process is employed in the large way, for obtaining carbonate of ammonia from the ammoniacal liquor. To 108 gallons[62] of the liquor contained in a cask, are added 125 pounds[63] of finely ground sulphate of lime, which has been previously deprived of moisture by heat. The cask is bunged up, and the mixture after being stirred together for a few minutes, is left undisturbed for three or four hours. Sixteen ounces of sulphuric acid are then added, the mixture is again agitated, and is again suffered to stand undisturbed for four or six hours. If the liquor be now examined, it will turn blue litmus paper, red.
[62] One gallon of the strongest ammoniacal liquor, weighs eight and a half pounds.
[63] This quantity is evidently too large, but the workmen assert, that an excess of sulphate of lime causes the carbonate of lime which is formed, to subside more readily, and the excess of sulphate of lime can do no injury.
In this operation a double decomposition takes place, the sulphate of lime yields part of its sulphuric acid, to the carbonate of ammonia of the liquor, to form sulphate of ammonia, and the carbonic acid of the ammonia, combines with the lime of the sulphate of lime, to form carbonate of lime, which falls to the bottom, the supernatant fluid contains in solution, sulphate of ammonia.
When the liquor has become clear, it is pumped out of the barrel into shallow cast iron boilers, where it is evaporated slowly. During this process, a portion of sulphate of lime is deposited which is removed, and as the liquor becomes more concentrated, part of the sulphate of ammonia begins to crystallize and falls to the bottom. It is shovelled out from time to time into wicker baskets, placed slanting over the rim of the boiler, that the liquor which drains off from the crystals may not be lost, and lastly the whole fluid is evaporated to dryness.
108 gallons of ammoniacal liquor from Newcastle coal, produce upon an average, one and a half cwt. of dry sulphate of ammonia. To decompose it, one cwt. is mixt with one quarter of a cwt. of finely ground chalk, previously deprived of moisture by heat. The mixture is introduced (as expeditiously as possible) into cast iron retorts,[64] heated nearly to a dull redness, and when the lid of the retorts have been rendered air tight, the fire is raised gradually till the retorts are of a strong red heat. The carbonate of ammonia developed from the contents of the retorts, is made to sublime into a leaden barrel-shaped receiver, connected with the retorts, by means of a pipe four inches in diameter, proceeding from the upper extremity of each retort, and opposite to the mouth piece. The leaden receiver is furnished with a leaden cover, fitting into a groove, where it is made air tight by lute. The receiver which is supported upon a stand is provided at its base, with a small pipe, furnished with a stopper. This pipe is left open till the liquid products are got rid of during the sublimatory process. In the centre of the cover, or at any other convenient part of the apparatus, is made a small hole, slightly stopped with a wooden peg, to give vent to the elastic fluid that becomes evolved during the process.
[64] Of the usual form and dimensions, described [page 58].
The time requisite for the operation depends on the mode in which the retorts are set, the temperature kept up and other practical circumstances. A charge of 120 pounds of the mixture of sulphate of ammonia and chalk in one retort, is usually decomposed in twenty-four hours. When the operation is at an end, and the receiver having become cold, the cover is taken off, and the sublimed carbonate of ammonia adhering to the sides of the receiver is detached by a chissel and mallet, and after being freed from any casual impurities, is packed up in stone jars for sale.
One cwt. of dry sulphate of ammonia, produces from sixty pounds, to sixty-five pounds, of pure carbonate of ammonia. In some establishments, the carbonate of ammonia is subjected to a second sublimation by means of a gentle heat; but this is quite unnecessary if the process has been conducted carefully.
Manufacture of Muriate of Ammonia from the Ammoniacal Liquor of Coal.
It must be obvious that the ammoniacal liquor may be employed with great advantage for the production of muriate of ammonia. For if the solution of sulphate of ammonia obtained from the ammoniacal liquor by means of sulphate of lime, as before stated, be mixed with common salt, (or any other muriate) another decomposition takes place. The muriatic acid of the common salt, unites to the ammonia of the sulphate of ammonia, and produces muriate of ammonia, and the sulphuric acid of the sulphate of ammonia, combines with the soda of the common salt, and produces sulphate of soda, or glauber salt.
The liquor containing these two salts being evaporated, the glauber salt begins to crystallize, and is removed from time to time. The evaporation is continued till as much as possible of the glauber salt has been separated, and the muriate of ammonia begins to crystallize on the surface of the fluid in the form of a feathered star. The remaining fluid is then run off into coolers, and deposits little else than muriate of ammonia, till it gets below the temperature of 76° Fahr. at which time the crystals are to be removed, lest they should be mixed with glauber’s salt which now begins to be again deposited. After the muriate of ammonia has been suffered to drain in baskets, it is heated in shallow pans to drive off as much water of crystallization as possible. It is then removed whilst still hot, into earthenware jars, glazed within, and fitted with a cover, (having a hole of about half an inch in diameter in its centre,) luted on with clay. The jars are put in a cast iron pot over a strong fire, in a furnace capable of containing from six to eighteen jars, surrounded with sand up to the edge of the pot, and also having about two and a half inches of sand on the cover, confined by an iron ring about three inches deep, and two inches less in diameter than the cover, in order that if the luting should give way in any part, it may be repaired without suffering the covers (which should be kept during the sublimation at about 320° Fahr.) to be cooled by the removal of a large portion of the sand.
These earthen jars may be filled to within two inches of the top, with the dried salt gently pressed in, but not rammed close; and the fire which has been lighted some time before, is now to be raised gradually till the iron pots are of a pretty strong red heat all round, being so placed by mean of flues in the furnace that the upper part may be first heated, the bottom resting on solid brick work.
During the first impression of the heat, a portion of the salt carrying with it a quantity of watery vapour not separated during the drying of the salt, will escape through the hole in the cover, which must be left open till all the aqueous part is exhaled: this is known by bringing a piece of cold smooth iron plate near the hole, in order to condense the sublimate, which becoming more and more dry, at length attaches itself firmly to the plate, in the form of a dry semi-transparent crust.
At this time the hole is to be stopped with lute, more sand is to be put on the cover, and the heat continued till it is judged that nearly the whole of the muriate of ammonia is sublimed. The time requisite for this purpose depends on the construction of the furnace, the size of the pots, the briskness of the fire, and other circumstances only to be learnt by experience.
The process should be stopped before the sublimation has entirely ceased, as the heat in some parts of the jar may be too great when it is nearly empty, and either by volatilizing a part of the salt itself, or elevating a portion of foreign matter from which it can never be kept wholly free, and thus giving the cake a yellow tinge, and a scorched, opake, crackled appearance.
The same defects are likely to happen, when any part of the luting having given way, is obliged to be repaired by wet lute, when the sublimation is pretty far advanced: consequently glass vessels are preferable, except on account of the expence, as they must always be broken to pieces in order to get out the cake: the earthenware jars on the contrary will serve for several sublimations, even the covers, if well glazed, will last two operations. The sublimation being finished and the apparatus having become sufficiently cool, the tops of the jars are to be taken off, and the cakes of sal-ammoniac that are found adhering to them are to be separated, and placed for a day or two in a damp atmosphere, which softens their surface a little, and thus facilitates the removal of any superficial impurities. Lastly, the cakes are packed up in casks for sale.
The excise laws have hitherto operated strongly against the establishment of manufactories of muriate of ammonia in England. Hence an immense quantity of sulphate of ammonia obtained from the gas light ammoniacal liquor, is exported from this country to the continent, solely from the extreme rigour of the excise relating to the use of common salt, and it is only this that has hitherto prevented the establishment of manufactories of sal-ammoniac from the ammoniacal liquor of the gas light process upon a large scale.
Chemical manufactories, of all others, will least bear excise, because many of them are worked according to secret processes, which, if made public, must pass into other countries; and the greatest part of the profit ceases together with the export. The vexatious introduction of excise officers into manufacturing laboratories, it is evident, puts an end to all secrecy of operation. There are several chemical processes which interruption will extremely injure, and others which it totally destroys, and as on the whole they in general are of a nature in which interference of others is most peculiarly vexatious, in all probability, if the excise be extended to manufactures of this nature, it will eventually put a stop to most of them, and greatly injure the revenue by causing thereby to cease the duties which at present arise from the exports and imports to a large amount, now depending on the chemical trade of Great Britain.
We have now gone through all the improvements by which the gas light manufacture has been distinguished during the interval which has elapsed since the publication of our former work[65] on this subject; and perhaps the reader may be inclined to think, from the extraordinary height to which improvement has been carried in this art, that little or nothing more remains to be desired with regard to it. Let it be remembered, however, that the whole art is only in its infancy. There is yet a wide field for improvement in the construction of the apparatus. Ingenious men may speculate from what has been done, to what remains to be effected, which no doubt will lead to objects of the greatest utility, and most extended national importance.
[65] A practical treatise on gas light.