The highest quality of all is white 70 per cent. This resembles white 60 per cent. in appearance, but is much purer, and more expensive.
A very pure form of caustic soda has within the last few years been introduced by the Greenbank Alkali Company. It consists of small fragments, more or less crystalline, of almost pure sodium hydrate. It can be packed in wooden casks, and it dissolves very readily in water. It contains 76 per cent. of alkali. It is, however, much too costly for boiling purposes; it might, however, be used for the preparation of rosin size.
The following analyses of different forms of caustic soda and their relative prices will possibly be of interest:—
| (Davis.) White (70 Per Cent. Na2O). | (Davis.) White (60 Per Cent. Na2O). | (Morrison.) Cream (60 Per Cent. Na2O). | |
|---|---|---|---|
| Sodium hydrate | 89·60 | 75·25 | 70·00 |
| Sodium carbonate | 2·48 | 2·53 | 5·00 |
| Sodium chloride | 3·92 | 17·40 | 7·00 |
| Sodium sulphate | 3·42 | 4·40 | 2·00 |
| Sodium sulphide | 0·02 | 0·03 | — |
| Sodium silicate | 0·30 | 0·30 | — |
| Sodium aluminate | trace | trace | — |
| Water | none | none | 15·80 |
| Insoluble matter | none | none | 0·20 |
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Their relative prices per ton f.o.b Liverpool, in December 1886, were:—70 per cent. white, 8l. 5s.; 60 per cent. white, 7l. 5s.; 60 per cent. cream, 7l.
All the above-mentioned forms of caustic soda occur in the form of more or less hard masses, and are contained in thin wrought-iron drums, which are filled with the caustic in a fused state. The soda is removed by breaking the drum with a chisel and hammer. The lumps of caustic may be put direct into the boiler, though it is better to dissolve them previously in water and allow any insoluble impurities to settle to the bottom of the solution.
Instead of buying caustic soda direct, it is the custom of some paper-makers to prepare it for themselves from carbonate of soda, which can now be obtained in various forms, some of great purity.
It occurs as caustic ash, containing about 40 per cent. of alkali as carbonate, and about 10 per cent. as caustic soda; soda-ash containing about 52 per cent. total alkali, nearly all as carbonate; and refined soda-ash containing 57–58 per cent. of alkali as carbonate. Some forms of carbonate of soda, such, for example, as those made by the ammonia process, are of even greater purity. The method of converting sodium carbonate into caustic soda will be described subsequently.
Unless the arrangements for washing and draining the lime-mud are very perfect, it is probably cheaper for the paper-maker to buy his caustic direct from the maker.