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.