SODA SALTS.
11 7
So far we have dealt with Leblanc’s process in its originality; the plan ofM. Schaffner for regenerating the sulphur from the soda-waste we will touchupon presently. Meanwhile it will be interesting and instructive to considerthe theory of the process as far as we have presented it to our reader. Forthe elucidation of the theory we are indebted to the researches of Mr. Gossageand M. Scheurer-Kestner. Chemists long held that the calcination of amixture of sulphate of soda, carbonate of lime, and carbon resulted in thecarbon yielding carbonic oxide, and converting the sulphate of soda intosulphuret of sodium, this in turn becoming decomposed by the carbonate oflime, yielding ultimately carbonate of soda, oxysulphuret of calcium, andcarbonic acid according to the following formulae :—
(1) . Na 2 S0 4 +4C = Na 2 S+ 4 C 0 .
(2) . 2Na 2 S + 3CaC0 3 = 2Na 2 C0 3 + Ca0,2CaS + C 0 2 .
But Unger has proved beyond doubt that only carbonic acid, and not thelower oxide of carbon, is formed, the carbonic oxide resulting merely fromexcess of carbon and its action upon the carbonate of lime. The reduction ofthe carbonate of lime by the carbon is effected at a much higher temperaturethan that at which the sulphate is reduced, so that carbonate of soda is formedbefore carbonic oxide. Unger therefore distinguished three stages in theprocess, viz.:—
1. During [the redudtion of the sulphate, the evolution of carbonic acid,according to the formula—
Na 2 S 0 4 -f 2 C = Na 2 S+ 2 C 0 2 .
2. The sulphuret of sodium and carbonate of lime are decomposed—
Na 2 S-t-CaC 0 3 = Na 2 C 0 3 + CaS.
3. The excess of carbonate of lime is reduced by the carbon—
2CaC0 3 + 2C = 2CaO+4CO.
Theory gives 100 parts of sulphate as requiring only 20 parts of carbon, butin pradtice as much as 35 to 75 per cent of carbon is employed.
We now turn to the utilisation of the soda-waste by the process carriedout by M. Schaffner. Of the importance of the improvement upon Leblanc’sprocess, it may be stated that the accumulated waste from the alkali works atDieuze contains, according to M. Oppenheim, ,£150,000 worth of sulphur.M. Schaffner’s method includes three processes:—
x. The preparation of the waste liquor, or liquor containing the sulphur.
2. The decomposition of the sulphur.
3. The refining or preparing of the sulphur for the market.
1. The soda-waste is permitted to oxidise in heaps in the open air, the resultbeing the formation of oxysulphurets and polysulphurets, and subsequentlyhyposulphites. The lumps of waste are then lixiviated, and a concentratedliquor obtained, the waste being allowed to remain in the tanks, while heatedgases from a chimney are forced through perforations in the bottom of thetanks ; after eight to ten hours the liquor is colledted in a reservoir.
2. The lixiviation liquor is next decomposed by hydrochloric acid in closedstone or iron vessels, sulphurous acid gas being evolved and sulphur precipitated.
CaS 2 0 3 + 2HCl = CaCl 2 + S 0 2 + S + H 2 0 .
3- The sulphur is very simply prepared, being mixed with water to a paste,and subjected to the heat of steam at a pressure of 1} atmospheres. Themolten sulphur settling to the bottom of the vessel is run off into moulds.