A very important part of this operation is the condensation of the hydrochloric acid gas, winch is disengaged in large volumes during the decomposition of the salt. As already stated, it was formerly allowed to escape into the atmosphere. The acid flues convey it to the condensing towers, which are generally filled with pieces of burnt coke, through which a supply of water is kept running. The gas enters at the bottom of the first tower, passes upward, and descends the second, and is gradually absorbed by the water, form ing strong liquid acid, run out by openings at the bottom of the condenser. So perfect is the system of condensation now in use at some works, that of the acid pro duced by 100 tons of pure chloride of sodium which should yield 62 tons, as much trs 58i1- tons have actually been collected; and it has been instanced, as a curious illustration of tins in another way, that Mr. Muspratt's great works, which were at one time forced out of Liverpool as a nuisance, have been established there again without causing any protest. Nevertheless such works being still eousidcrcd obnogious to their neighbor hoods, an "alkali act" was passed by parliament in July, 1863, " For the more effectual condensation of muriatic (hydrochloric) acid gas in alkali works." It compels every manufacturer of alkali to secure the condensation of not less than 95 per cent of the muriatic gas evolved in his works, under a penalty not exceeding 1'50. This act was amended in 1874 by which copper and sulplfuric acid works were placed under similar restrictions. The hydrochloric acid obtained in this process is mostly used in the manufacture of bleaching-powder.
Second Operation—Me Conversion of the Sulphate of Soda into Black ash, caged also Ball-svda.--This is effected by healing a mixture of sulphate of soda, carbonate of lime, and coal, in a reverberatory furnace. The proportions now used are the same as those first recommended by Leblanc—viz., sulphate of soda, 100 parts; carbonate of lime, 100 parts; carbon (charcoal), 55 parts. But as coal is employed in England instead of char coal, the quantity used is generally 75 to 100 of each of the other two ingredients. The " balling furnace" used in this operation has two beds, the one being raised a few inches above the other, The waste heat from the fire-place is usually employed in boiling down the soda-lye. The charge is thrown into the bed of the balling furnace away from the fire-place after it has been raised to a bright red heat, and remains till it becomes suffi ciently heated throughout the whole mast. It is then transferred to the jinxing bed, which is next the fire, and exposed to a higher heat, when it shortly begins to soften and flux into a mass like dough. In about half an hour the charge is withdrawn in a red-hot state by the working door, and received into iron barrows, where it solidifies into blocks of crude soda termed lull-soda, or hlack-ash. A new kind of balling furnace, first introduced at the ;farrow Chemical works, South Shields, is now becoming extensively used. It differs from the one described in having that portion of it for receiving the mixture of sulphate of soda, coal, and lime in the form of a brick-lined iron cylinder. large enough in some
instances to decompose 50 tons of the sulphate in 24 hours. This cylinder lies in a hori zontal position, and is made to revolve slowly by engine-power. Tile materials are intro duced by means of a hopper; the fire-place, which does not rotate, is placed at one end of the cylinder; and the arrangement for evaporating the lye is somewhat similar to that employed in the furnace described. This rotary furnace admits of the work being much better done than by the older reverberatory furnace, as it not only saves labor, but pre vents loss of soda by volatilization.
In this process there is first a reduction of sulphate of soda to sulphide of sodium with evolution of carbonic acid; then a conversion of the sulphide of sodium and carbonate of lime into carbonate of soda and sulphide of calcium; and finally the excess of car bonate of lime is reduced by the carbon to caustic lime and carbonic oxide.
Third Operation—The Preparation of Carbonate of Soda from the Black-ash. by Li.rivia tion and Emnoration.—For some purposes the crude soda, or black-ash, is used without further purification; for example, in the making. of soap. Its lixiviation is effected by the use of a series of iron tanks, or vats, into which it is placed with water. Several tanks, each of the capacity of 600 gallons, rise above one another in successive stages, so that the liquor of the highest can be run into the next lower, and so on. The black ash is introduced fresh into the lowest vat; it then passes from vat to vat, and is taken away exhausted at the highest one. The water, on the contrary, comes in fresh at the top, and in passing downward encounters less exhausted ash in each succeeding vat, and finally passes away from the lowest a fully saturated solution. In most soda works, the vats are now arranged differently, although the ash may be said to be ex hausted in the same way. In the new arrangement, the vats are placed horizoutally, and advantage is taken of the fact that solutions in becoming richer become also heavier, so that, although the tanks are all on a level, the water runs through them with what is virtually a downward flow. We have not space to describe minutely this very elegant and economical plan; it will he enough to say that it completely obviates the necessity of lifting the ash from vat to vat, because any two contiguous ones can be made at pleasure the highest and lowest points, and, therefore, those of ingress and egress for the lixiviating fluid. " Each vat, in due rotation, is emptied and refilled; and thus each in torn successively occupies the highest, lowest, and all intermediate points of the de clivity." The next stage is the evaporation of the soda-lye, which is conducted in a variety of ways. A common method consists in using the waste heat of the balling furnace, the flame from which passes over the surface of the liquor. 'With proper manipulation the soda falls to the bottom, and is raked out at intervals through a side-door, and drained upon a sloping surface.