Analysis.—The value of soda ash, carbonate of soda, and recovered ash depends on the amount of available alkali (Na2O) present.
A weighed quantity (15·5 grammes conveniently) is dissolved in a measured volume of distilled water (500 c.c.), and titrated with standard normal hydrochloric acid, methyl orange indicator being used.
Caustic Soda.—Raw vegetable fibres may be reduced to the condition of paper pulp by treatment with caustic soda. In practice this process is largely resorted to for the manufacture of pulp from esparto, straw, and wood, the spent caustic soda being recovered and used again.
The paper-maker prepares the caustic required for digesting the raw material from recovered ash and carbonate of soda.
A convenient volume of clear liquor obtained by lixiviating the recovered ash is boiled with lime in suitable causticising pans, the reaction being represented as follows:—
| Na2CO3 | + | CaO | + | H2O | = | 2 NaOH | + | CaCO3. |
| Soda ash | + | Lime | + | Water | = | Caustic soda | + | Chalk. |
According to this equation, 100 lbs. of soda ash require 53 lbs. of quicklime, but a slight excess is generally added, 58 or 60 lbs. being the usual amount actually employed. Several precautions should be observed in the process of causticising.
(1) The liquor from the recovered soda should be bright and clear, indicating complete incineration of the ash.
(2) The liquor is best causticised at a density between 1·050 and 1·100 (10-20, Twaddell). With stronger solutions the reaction is complicated and the yield of caustic soda reduced. Lunge has shown that if the density of the solution is 1·025 the proportion of soda causticised is 99·5 per cent., whereas at a density of 1·150 it is only 94·5 per cent. In the latter case the caustic soda formed acts upon the chalk produced and is reconverted into carbonate.