The operation is finished when all the sodium sulphide has been converted into normal sodium carbonate, partly also into acid sodium carbonate (bicarbonate) NaHCO 3; at the same time a precipitate is formed, consisting of ferrous sulphide, alumina and silica, which is removed by another settling tank, and the clear liquor is now ready either for boiling down in a " fishing-pan " for the manufacture of white soda-ash, or for the process of causticizing.
The greater portion of all the soda-ash of commerce is now made by Solvay's apparatus, which alone we shall describe in this place, although it should be borne in mind that the principles laid down by Dyar and Hemming have been and are still successfully carried out in a number of factories by an entirely different kind of apparatus.
Although the consumption of salt-cake for the manufacture of alkali is now much less than formerly, since the Leblanc alkali process has been greatly restricted, yet it is largely made and will continue to be made for the use of glassmakers, who use it for the ordinary description of glass in the place of soda-ash.
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.
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.