Hydrochloric acid is, in fact, a by-product in the manufacture of carbonate of soda, and is generated during the first stage of the operation, known as the salt-cake process, which consists in the decomposition of salt by sulphuric acid, and is accomplished in a furnace called the salt-cake furnace.
The hydrochloric acid gas which is given off escapes from the furnace through a flue with the products of combustion into high brick towers filled with coke or stones, over which a stream of water trickles down, the whole of the acid vapours are thus condensed, the smoke passing off by a chimney connected with the towers. The diluted acid solution thus formed is concentrated by the aid of the apparatus shown in section in figs. 1, 2, and 3.
This apparatus consists of several cast-iron cylinders, 57 feet long by 27 feet in diameter, closed in the same manner as gas retorts, by lids luted with clay. One of the lids has an opening o, into which is fitted the stoneware or leaden pipe a, conveying the hydrochloric acid to the condensing apparatus. The other, or posterior lid, is also provided with an opening d, through which is passed the tube of a leaden funnel, so that after the retort is filled with salt sulphuric acid may be poured in. The construction of the furnace in which two retorts are usually placed, permits the flame of the fire at O to play round the cylinders before reaching the flue leading to the chimney F. B is an arch over the furnace. The first stage of the operation consists in filling each cylinder with 330 lbs. of salt. The lids or covers are then luted on, and the fire is kindled. The requisite quantity of strong sulphuric acid is next poured into the retort, and the funnel having been withdrawn from D, the hole is covered by a clay plug.
Fig. 1.
As soon as the reaction is over, the 396 lbs. of sulphate of soda produced are removed, and the operation repeated.
The condensation apparatus 1 and 3 is composed of rows of Woulfe’s bottles, partly filled with water, care being taken to place the first pairs of these bottles in a tank of cold water.
The condensation of the last portions of hydrochloric acid gas is effected either by the aid of the coke columns, or in leaden chambers, into which fine jets of cold water are injected on all sides.
“A saturated solution of hydrochloric acid in water has the specific gravity of 1·21; and when heated in a retort, loses at first hydrochloric acid gas, but after a time an aqueous acid distils over, at the ordinary atmospheric pressure, containing 20·22 per cent. of hydrochloric acid, and boiling constantly at 110° C. If the distillation be conducted under diminished pressure, the liquid boils at a lower temperature, and attains a composition which is different for each boiling point; hence the dilute acids thus obtained by boiling the solution of hydrochloric acid gas in water, cannot be considered as definite compounds of hydrochloric acid and water.”[348]
[348] Roscoe and Dittmar.