Various industries are carried on in Leblanc alkali works, as follows:—
- Manufacture of sodium sulphate.
- Manufacture of hydrochloric acid.
- Preparation of chlorine.
- Employment of chlorine for the manufacture of bleaching-powder and of chlorates.
- Manufacture of ordinary alkali from sulphate of soda.
- Manufacture of caustic soda.
- Manufacture of soda crystals.
- Recovery of sulphur from alkali waste.
1. Manufacture of Sodium Sulphate.—This is commercially known as salt-cake, and is made by decomposing common salt with sulphuric acid of about 80%, the reaction being 2NaCl + H2SO4＝Na2SO4 + 2HCl. This reaction proceeds in two stages. At first principally acid sodium sulphate, NaHSO4, is formed together with some normal sulphate; later, when the temperature has risen, the NaHSO4 acts with more NaCl so that nearly all of it is converted into Na2SO4. The gaseous hydrochloric acid evolved during all this time must be absorbed in water, unless it is directly converted into chlorine (see below, 2 and 3).
The process is carried out either in hand-wrought furnaces, or mechanical furnaces, both called “decomposing” or “salt-cake furnaces.” In the former case, the first reaction is produced in cast- iron pans or “pots,” very heavy castings of circular section, fired from below, either directly or by the waste heat from the muffle-furnace. The reaction is completed in a “roasting-furnace.” The latter was formerly often constructed as a reverberatory furnace, which is easy to build and to work, but the hydrochloric acid given off here, being mixed with the products of the combustion of fuel, cannot be condensed to strong acid and is partly, if not entirely, wasted. It is, therefore, decidedly preferable to employ “muffle-furnaces” in which the heating is performed from without, the fire-gases passing first over the arch and then under the bottom of the muffle. This requires more time and fuel than the work in “open” furnaces, but in the muffles the gaseous hydrochloric acid is separated from the fire-gases, just like that evolved in the pot, and can therefore be condensed into strong hydrochloric acid, like the pot-acid. This roaster-acid is, however, of less value than the pot-acid, as it contains more impurities.
It is not easy to keep the muffles permanently tight, and as soon as any leakages occur, either hydrochloric acid must escape into the fire-flue, or some fire-gases must enter into the muffle. The former is decidedly more objectionable than the latter, as it means that uncondensed hydrochloric acid is sent into the air. This drawback has been overcome by the construction of “plus-pressure” furnaces (figs. 1 and 2), where the fire-grate is placed 11 ft. below the top of the muffle. In consequence the fire-gases, when arriving there by the chimney shaft (a), have already a good upward draught, and when circulating round the muffle are at a lower pressure than the gases within the muffle, so that in case of any cracks being formed, no hydrochloric acid escapes into the fire-flues, but vice versa.
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| Figs. 1. and 2.—Salt-cake Furnace. (Sectional Elevation and Plan.) Figs. 1–9 from Lunge’s Handbuch der Soda-Industrie, by permission of Friedr. Vieweg u. Sohn. |
Since the work with ordinary hand-wrought salt-cake furnaces is disagreeable and costly, many attempts have been made to construct mechanical salt-cake furnaces. Of these J. Mactear’s furnaces (fig. 3) have met with the greatest success. They consist of a horizontal pan, 17 ft. wide, which is made up of a central pan (e), and a series of concentric compartments (c1), (c2), (c3), and which is supported on a frame (d d), revolving round a perpendicular axis on the wheels (n n). It is with an arch and heated on the top from one side (l), either by an ordinary coal-grate or by a gas-producer. A set of stirring blades carried in the frame (b b), and driven by gearing, passes through a gap in the arch in such a manner that the gases cannot escape outwards. The salt is conveyed to the furnace by a chain of buckets running on the pulley (g), and passing into the hopper (h), and through the pipe (i ) is mixed with the proper amount of acid supplied by the pipe (f ). The mixture is fed in continuously to the central pan (e), whence it overflows into the compartments (c1), (c2), (c3) successively until it reaches the circumference, where it is discharged continuously by o and p into the collecting-box (q), being now converted into salt-cake. This furnace acts very well, and has been widely introduced both in Great Britain and in other countries, but it has one great drawback, apart from its high cost, viz. that all the hydrochloric acid gas gets mixed with fire-gases, and consequently is condensed in a weaker and less pure form than from ordinary pots and muffles. This has led some factories which had introduced such furnaces to revert to hand-wrought muffle-furnaces.
Much was expected at one time from the “direct salt-cake process” of Hargreaves and Robinson, in which common salt is subjected
