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CHAPTER X.

SODA PROCESSES OTHER THAN LEBLANC'S, INCLUDING THE AMMONIA PROCESS."

UNDER this heading it is our intention to briefly indicate, and in some measure describe, processes which have been proposed from time to time, either as supplementary to, or substitutive of, the one in general use. Most of them are improved modifications of old processes, and have been tried on a limited scale, while others are actually in use to a small extent.

Some years ago Mr. Hunt proposed a process, founded on some experiments of Mr. Gossage, and which, indeed, was an old friend in a new garb. It was proposed to obtain crude sulphide of sodium by fluxing salt-cake with coke or coal; the product was to be broken into lumps and placed in iron cisterns provided with false bottoms, under which carbonic anhydride was forced, the gases passing out at the top. Each vessel was to be heated by steam, and the carbonate of sodium so obtained to be purified by recrystallisation. As for the sulphuretted hydrogen evolved in the reaction, this was mixed with air and burnt under a stratum of oxide of iron kept red-hot by the burning gases, and in this way sulphurous anhydride was produced for use in the chambers. The chief difficulties encountered in the working of such a process are those which Mr. Weldon hopes to avoid,

and as we have already described his process and its advantages in our last chapter, we may pass from its consideration.

Mr. Arrot, Mr. Hunt, and others have also proposed at various times to manufacture caustic soda from the sulphide made as above, by the use of oxides, such, for instance, as those of iron, manganese, zinc, and copper. Where iron is used, a double compound of sulphide of iron and sodium results, which is difficult to manipulate. But in any case a greater difficulty so far, consists in the manufacture of the sodic sulphide. This has been attempted by causing a fused stream of sulphate of sodium to flow through red-hot coke or coal, but with partial success. If by Weldon's plan, good results should be obtained, then these various other processes would merit reconsideration.

The processes we shall next describe, refer to the extraction of soda contained in felspar, cryolite, and other minerals.

Ward proposed to frit any natural silicate, as pumicestone, with calcium fluoride and chalk, in a reverberatory furnace, by which operation, supplemented by lixiviation of the product, a solution of caustic soda is obtained; the residuum he employed as a hydraulic cement.

Newton patented the use of the following proportions:-100 parts felspar, 50 parts phosphate of lime, and 300 to 400 parts of lime, all in powder. This mixture was calcined at a low red heat for two hours, by which means there are formed silicate of lime and phosphate of sodium; the silicate of lime combines with the silicate of alumina, and on lixiviation, the lime decomposes the phosphate of sodium, reproducing phosphate of lime and

giving caustic soda, which remains in solution. The residue obtained in these operations was recommended

as a manure.

The attempts to obtain soda from cryolite, which is a double fluoride of sodium and aluminium, originated with Spilsbury, and were revived by Moigno. But it is by a process devised by Julius Thomsen in 1850, that such an amount of soda is actually manufactured at the present day, as to consume annually 12,000 to 15,000 tons of cryolite. Cryolite occurs almost exclusively in South Greenland, where it forms a solid mass of about 600 feet in length and 200 feet in width, and of unknown depth. This mine is the property of the Danish Government.

The following description of the manufacture of soda from this mineral is taken from a paper by J. Lawrence Smith. The decomposing agent employed is lime, and the process can be performed either in the wet or dry.

The Dry Process.-The cryolite, crushed to powder, is mixed with slaked lime or powdered chalk in such proportions that for each equivalent of pure cryolite there shall be rather more than six equivalents of lime. In the next process of calcination, the great difficulty of the process is encountered. The heat must only be great enough to effect the decomposition, for if the temperature exceeds this, the materials fuse, and produce a product it is impossible to lixiviate. About two hours are required to complete the decomposition of a charge of 1,000 lb., and the product, when properly made, is a granular loose mass of an ash colour. After cooling, it is lixiviated by water, and yields a solution of soda and sodium aluminiate, which marks on an average 26° to 28° Baumé. The

liquors are placed in large horizontal cylinders, provided with agitators, where the solution is treated with carbonic acid, produced by the combustion of coke and washed with water, by being made to traverse stacks of faggots over which water trickles. The carbonic acid converts the caustic soda and aluminate into carbonate, setting free the alumina, thus

2 (Na3A103)+3CO2-3Na2CO3+Al2O3.

The liquors now mark about 31° Baumé, and furnish soda, while the alumina is allowed to settle, and in this state of granular powder constitutes a valuable source of alums. The carbonate of soda obtained is tolerably pure, containing only 1 or 2 per cent. of sulphate of sodium, derived from the sulphides associated with the cryolite.

The Wet Process differs from the dry, according as the required product be carbonate, or caustic soda. If carbonate be desired, the proportions above stated are used; but when caustic is the desideratum, fifteen equivalents of lime are employed to two of cryolite, the resulting products being caustic soda, aluminate of lime, and fluoride of calcium, which last-named body is formed in either case. The operation is conducted in a large vertical cylinder heated by steam and provided with an agitator, and takes from two to three hours' boiling. The liquors are filtered and settled, and are treated with carbonic acid as above (with the same result) if carbonate of soda be desired. On the other hand, when caustic soda is required, the liquors are simply evaporated in the usual way, yielding caustic soda of 75 per cent.

The wet process has many advantages over the dry method, but inasmuch as lyes of only 10° Baumé are so

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produced, there is an enormous consumption of fuel necessitated by evaporation.1

There are four establishments in Germany consuming about 2,000 tons of cryolite annually, and in 1867 one was started in Pennsylvania for working 6,000 tons. At this works, in Natrona, caustic soda is made as above on a large scale. Bauxite (a mineral containing 80 per cent. of alumina), is sometimes mixed with the cryolite to increase the yield of alumina.

The following equations, although they may not represent exactly what occurs in the two forms of the process described, nevertheless do so fairly:

1st. Dry Process_

2(3NaF,AlF3)+6CaO=2Na,A1O3+6CaF2.

Any caustic soda present in the product would seem to result only from the presence of water in the lime used, thus

2 (Na,AlO3)+3CaH2O2=6NaHO+3CaO+Al2O3.

2nd. Wet Process

2(3NaF,AIF3)+6CaH2O2=6NaHO+6CaF2+Al2O33H2O.

The various products manufactured from cryolite have a market value of more than $1,500,000 in gold.

Allied to the processes just described are others in which salt is decomposed by silica and steam. Blanc and Blazille (1840) used a mixture of 280 parts of salt and 200 of sand, which was enclosed in red-hot iron cylinders, and exposed to a current of steam. In this

It is not easy to see why four or five such cylinders should not be worked in a series, like as vats are worked in Lancashire. In this way the liquor from the first operation would be used in the second, and so on, and thus stronger liquors would be obtained.