Before being cauaticized, it is usual to reduce the strength of the liquor to 20° or 22° Tw. Occasionally the reduction is carried down to 14°, but a liquor of 20° causticizes as readily as one at 14°, and the extra amount of water simply represents an after extra expenditure of fuel. Steam is blown in until a temperature of 100° is attained, and the mass of liquor begins to boil. A quantity of quicklime contained in a convenient cage, which keeps back all stones and big lumps, is then lowered into the vessel, and the steaming and agitation arc continued until a sample of the liquor, filtered, gives no effervescence with dilute hydrochloric or sulphuric acid. A simple view of the reaction in the caueticizer ie the following :— NI:4C% CaO = Na,O Besides this, the sulphide of sodium is converted into sulphate, and the alumina and silica are carried down with the calcium carbonate. A usual charge of well-burned lime consiate of 14 cwt. per ton of 60 per cent. caustic soda, but the process is, as a rule, carried out in only a rough fashion, an excess of lime being put into the cage. About an hour and a half is required for the oaueticizing of a batch of liquors.
After completion of the operation, the contents of the vessel are allowed to settle for half an hour or so, during which time the insoluble portions rapidly subside. The clear caustic liquor is then drawn off, and a fresh lot of diluted tank liquor run in upon the lime mud, and the causticizing operation repeated. The mud is not removed after every operation, because a certain amount of undecomposed lime is always present, and serves to causticize the next charge to some extent. After a second operation, fresh water, is run in upon the mud and the whole well agitated. The washings are run off to dilute the tank liquors, and the mud placed upon the "filter." This filter is usually a half boiler, cut nally. The bottom is paved with bricks in somewhat similar fashion to that already described when explaining the con struction of the lixiviating vats, a channel being left down the centre, and the bricks only loosely put in. The actual filter is formed by layers of coke to a depth of 9 inches or so, the bottom layer composed of good-sized lumps, the top of email pieces, and a covering of coarse sand or cinders.
Over the filter are laid perforated iron plates or grids, upon which the mud is placed. When a batch is spread over the grids, it ie allowed some little time to drain, and then tho roughly washed with water. The drainings and washings are utilized in diluting tank liquor, and the finally hard, close mud is shovelled out of the filter and wheeled away to the ball furnaces or mixing depOt. An ingenious mechanical contrivance ie often used to amid the draining and washing of the mud. A 2-in. iron pipe is bolted upon the bottom of the boiler, below the filter, and communicates with a small air-tight tank placed upon a higher level, and connected in its turn with a vacuum pump. Upon the top of this tank is an air-cock, and set into the bottom a pipe to convey away the collected water and liquor. When a batch of mud
is spread over the filter, the vacuum pump is set away, and draws up, first the drainings and then the washings. These collect in the tank and are run off to their destination. The completely washed mud should not contain above 1.5 per cent. of caustic soda. About 40 per cent. of it is calcium carbonate, 4 per cent. calcium hydrate, and 50 per cent. water. The remaining constituents x 2 are alumina, oxide of iron, and magnesia, with traces of chloride, sulphate, and carbonate of soda.
To return to the caustic liquors. These are run from the causticizers into settlers, and, after clearing, transferred to a wrought or cast-iron concentrating pan. The best form is the " boat " pan already described. Two of these, or a boat pan and a boiler, may be conveniently built at the end of a ball furnace and worked with waste heat. Concentration in a boiler or wrought-iron pan is not advisable beyond 30° Tw., as the liquor eats the iron away rapidly. In any case, the liquor in these first " weak " pans should not be allowed to concentrate beyond 35° Tw. It is then transferred to cast-iron pans—usually called " strong," where evaporation is carried on till a density of 70° Tw. is attained. Heat in the " strong " pans is supplied by two fireplaces built in front of the two side flues running along the pan. A convenient arrangement is to place a " weak " at the end of a " strong " pan, and take the fire from the one under the other. Evaporation in both pans must be carefully conducted so that no boiling over may occur. If 70 per cent. caustic be required, concen tration is continued up to 82° Tw. During this operation, occasionally in the " weak," but rapidly in the " strong " pans, the carbonate, sulphate, and chloride of sodium separate out, together with a little caustic soda. These salts are fished out and the liquor replenished from time to time. When the desired strength is attained the pans are allowed to cool down and the contents to settle. The clear liquor is then siphoned off into a series of settlers. The remaining salts and residue in the " strong " pans is shovelled out, drained, and worked up in the balls. The composition of these fished salts is about as follows :— Sodium sulphate .. Sodium carbonate .. .. „ sulphide .. 0.10 „ hydrate 9.00 „ sulphite and hyposulphite 3.00 Insoluble chloride .. Water .. .. 30 . 00Settling before running into the pots is often dispensed with, the liquors being allowed a further time to clear in the "strong " pans, but a better quality of caustic is made when separate settlers are used. Instead of judging by the density of the strong liquors, they are sometimes run off when a temperature of 138° (280° F.) is attained. A few pounds of nitrate of soda are usually added before running off. When previous and careful oxidation however has been performed, this second oxidation of the salts is not necessary, and involves some loss.