older pans, described above. In this apparatus only the first of the pans is heated directly, usually by means of ordinary boiler-steam circulating round a number of pipes, containing the liquid to be concentrated. The steam rising from the latter is passed into a similar pan, in which it circulates round another set of pipes, but as it could not bring the liquid in the latter to boil under ordinary conditions, the second pan is connected with a vacuum-pump so that the boiling-point of the liquid in this pan is lowered. This pan may be followed by a third pan, in which a stronger vacuum is maintained, and so forth. By this means the latent heat of the steam, issuing from all pans but the last, is utilized for evaporating purposes, and from half to three-fourths of the fuel is saved.
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| Fig. 9.—Caustic Soda “Finishing-pot.” (Sectional Elevation.) |
After being concentrated up to a certain point, and after the separation of nearly all the salts, the caustic liquor is transferred to cast-iron “finishing-pots” (fig. 9), holding from ten to twenty tons. Here it is further boiled down until the greater part or nearly all of the water has been removed, and until the salts on cooling would set to a solid mass. This requires ultimately a good red heat. Before the mass has reached that point the sulphides still present have been destroyed, either by the addition of solid nitrate of soda or by blowing air through the red-hot melt. Before finishing, the molten mass must be kept at a quiet heat for some hours in order to settle out the ferric oxide which it always contains, and which becomes insoluble (through the destruction of the sodium ferrite) only at high temperatures. When it has completely cleared, the liquid caustic is ladled or pumped out into sheet-iron drums, holding about 6 cwt. each, where it solidifies and forms the caustic soda known to commerce.
The best caustic soda tests from 75 to 76 degrees of “available soda”; this is only a few per cent removed from the composition of pure NaOH, which would be＝77·5 degrees Na2O. Most of the caustic soda is sold at a strength of 70 degrees, sometimes as low as 60 degrees.
Caustic soda is used in very large quantities in the manufacture of soap, paper, textile fabrics, alizarin and other colouring matters, and for many other purposes.
7. Soda-Crystals.—Another product made in alkali works is soda-crystals. Their formula in Na2CO3, 10H2O, corresponding to 37% of dry sodium carbonate. They are made by dissolving ordinary soda-ash in hot water, adding a small quantity of chloride of lime for the destruction of colouring matter and the oxidation of any ferrous salts present, carefully settling the solution, without allowing its temperature to fall below the point of maximum solubility (34° C.), and running the clarified liquid into cast-iron crystallizers or “cones,” where, on cooling down, most of the sodium carbonate is separated in large crystals of the decahydrated form. This process lasts about a week in winter, and up to a fortnight in summer. In France the crystallization of soda is performed not in large tanks but in sheet-iron dishes holding only about 14 cwt., and requires only from 27 to 48 hours in the cool season; it is not carried on at all in warmer climates during the summer months. The mother-liquor, drained from the soda-crystals, on boiling down to dryness yields a very white, but low-strength soda-ash, as the soluble impurities of the original soda-ash are nearly all collected here; it is called “mother-alkali.”
Although the soda-crystals contain the alkali combined with such a large quantity of water, they are made in large quantities, because their form, together with their complete freedom from caustic soda, makes them very suitable for domestic purposes. Hence they are best known as “washing-soda.” Sometimes they are made, not from soda-ash, but from Leblanc soda-liquor before “finishing” the ash, or from the crude bicarbonate of the ammonia-soda process by prolonged boiling, until nearly half of the carbonic acid has been expelled.
Formerly bicarbonate of soda was made from Leblanc soda-crystals by the action of carbonic acid, but this article is now almost exclusively made in the ammonia-soda process.
8. The Recovery of Sulphur from Alkali-waste.—For many years all the sulphur used in the Leblanc process in the shape of sodium sulphate, and originally imported into the manufacture in the shape of brimstone or pyrites, was wasted in the crude calcium sulphide remaining from the lixiviation of black-ash. This “alkali-waste,” also called tank-waste or vat-waste, was thrown into heaps where the calcium sulphide was gradually acted upon by the moisture and the oxygen of the air. The sulphur was by these converted partly into gaseous sulphuretted hydrogen, partly into soluble polysulphides, thiosulphates and other soluble compounds, and in all shapes caused a nuisance which became more and more intolerable as the number and size of alkali works increased. Both the air and the water in their neighbourhood were contaminated thereby.
Both this nuisance and the loss of the sulphur (whose cost sometimes amounted to more than half of the total cost of the soda-ash) led to many attempts at extracting the sulphur from the alkali-waste. This was first done with a certain amount of success by the processes of M. Schaffner (1861) and L. Mond (1862), but as these required the use of hydrochloric acid, and as they only recovered about half of the sulphur, they were superseded by another—a process which had been originally proposed by W. Gossage in 1837, but has been made practicable only by the inventions of C. F. Claus, in 1883, and from 1887 onward by the technical skill of Messrs Chance Brothers, of Oldbury. The Claus-Chance process, as it is called, comprises the following operations. The wet alkali-waste as it comes from the lixiviating vats, is transferred into upright iron cylinders in which it is systematically treated with lime-kiln gases until the whole of the calcium sulphide has been converted into calcium carbonate, the carbon dioxide of the lime-kiln gases being entirely exhausted. The sulphur issues as sulphuretted hydrogen, mixed with the nitrogen of the air. It is mixed with fresh air containing sufficient oxygen for the combustion of the hydrogen, and the mixture is passed through red-hot iron oxide (burnt pyrites) which by its catalytic action causes the reaction H2S+O＝H2O+S to take place. By cooling the vapours the sulphur is condensed in a very pure form, and about 85% of the whole of it is recovered, the remaining 15% escaping in the shape of sulphur dioxide (SO2) and H2S. Unfortunately it has been hitherto found impossible to deal with these gases in any profitable way.
It should be noted that this “recovered sulphur,” which is equal in purity to the “refined brimstone” of commerce, has a far higher value than the sulphur contained in the originally employed pyrites, so that the recovery is a paying process, in spite of the somewhat considerable cost of the plant and of the working operations. It has been introduced at most large Leblanc alkali works, and has, so to say, given them a new lease of life.
