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hair, the skin, and most animal substances, sulphur,and some metallic sulphides, silica, alumina, andsome metallic oxides which are quite insoluble inwater.
The oldest method of making caustic soda is dueto Scheele ; it consists in the decomposition ofsodium chloride by litharge. This method was em-ployed in England for producing, besides causticsoda, the fine pigment known as Patent yellow. Theoperation consisted in intimately mixing and rubbingtogether 1 part of salt, and from 4 to 6 parts of lith-arge with a little water.
About twenty-four hours are required to completethe decomposition, during which time the mixturemust be constantly agitated. It is then allowed tostand a few hours to settle, and the caustic sodasolution is then decanted, and the residue wellwashed with water. The decanted solution on eva-poration yields very pure sodium hydrate, and the leadcompound, after being dried and fused, becomes ofa beautiful yellow colour.
The history of this very important branch ofalkali manufacturing is a very short one, as faras the production of solid caustic is concerned,though this is now practised on an enormous scalein Lancashire , and to some extent also on the Tyne.It seems to have occurred to Gossage before anybodyelse (in 1853) to adopt an operation opposite tothat of carbonating the caustic in tank liquors, viz. —to separate the carbonate (along with other sodiumsalts) from the liquors by concentration and fishingout, and to leave the hydrate in solution as redliquor, from which by further concentration solidcaustic could be produced. The first article thusproduced was of a very inferior colour, owing to thepresence of iron sulphide and oxide in considerablequantities; but very soon improvements upon thiswere patented by Stott (1855) and Gossage (1857),whose object was the removal of the sulphides. In1857 Bakewell introduced the packing of causticin iron drums, which is now the usual way.
About this time manufacturers, finding the de-mand for caustic to exceed the supply, also beganto causticize tank liquor direct by lime; and as thegreat dilution of the liquors caused the expenseof concentration to be very high, Dale (in 1859)patented the process of concentrating the weakcaustic liquors in steam boilers, and using the steamfor various purposes. The next real improvementwas made in 1860 by Ralston, who prescribed, inlieu of keeping the heat as low as it had beenformerly usual, to work at a greater heat, and tocontinue the evaporation till the iron separates asferric oxide, and is precipitated at the bottom of thevessel; the clear caustic floating over it to beseparated from the bottoms. This process for thefirst time allowed of the manufacture of white caustic,before this only cream caustic could be produced;and at the same time a stronger article (70 percent.) could be produced in the place of 60 per cent,caustic, which was up to that time the only prac-ticable strength. This was, however, only possible byemploying much stronger melting pots than were used
up to that time. For several years not much whitecaustic was made ; but in 1872 Pauli published theprocess which is now the only one in practical use,and which will be described hereafter. Its mainfeature was to heat commercial caustic to fusion andkeep it at that temperature through the night, when,besides the oxide of iron, the alumina is separatedas a crystalline silicate. About the same timeDeacon patented a process for producing crystals ofcaustic soda, by evaporating until the boiling tem-perature is 177° C., cooling down to 70°, and runningoff into cast-iron cones for crystallization ; a hydrateof the following composition crystallizes out.
Sodium oxide,. 50‘5
Sodium hyposulphite,. 0’5
Sodium chloride,. 1 '8
Water,. 47'2
The cones must be kept warm, and when their con-tents have cooled down to 50°, the mothers are runoff through a plug hole in the bottom; the conesare then heated by steam outside, which melts thecrystals inside so that they can be ladled off intothe drums. This process has, however, never beena commercial success. The demand for causticbecoming stronger, some works, principally on theTyne, actually began to make it by redissolvingordinary soda ash, causticizing it by lime, and eva-porating the lyes; and this extremely costly andirrational process has been in use for many years,and may be so yet here and there. The great bulkof the caustic soda found in commerce is made atWidnes , where one firm turns out equivalent to250 tons of 60 per cent, caustic per week, andseveral others from 60 to 80 tons each, and at St.Helens, where there are four or five works of thesame size. Most of this (not all, as stated byDavies, to whose paper on white caustic soda thewriter is indebted for most of the above historicaldata, as well as for a great portion of the followingdescription) is made by causticizing vat liquors bylime, whilst a small portion of white and most ofthe cream caustic is made from the red liquors.Wagner’s statement in his “Technology” (English edition of 1873), that “ the use of lime for the trans-formation of sodium carbonate into caustic soda hasbeen abandoned long since,” is therefore quiteerroneous.
Where caustic is made, the mixture for the ballsis now regularly partly compounded with the limemud from the causticizers, and with the fished saltsseparating at various stages of the concentrationof the causticized liquors. The following are ballmixtures given by Davies from actual experience:—