Sorting.
Paper is sold in sheets of definite sizes, and is made up into reams containing from 480 to 516 sheets. These sizes {176} correspond to different trade names—such, for example, as crown, demy, royal, imperial, &c. The weight per ream is usually expressed in addition to the name, thus: 14-lb. demy, 18-lb. double crown, &c. In this way the consumer is enabled to make a rough comparison of the thickness of the different kinds of paper.
In making paper it is the duty of the machineman to examine and weigh from time to time a sheet of the desired size, in order to ensure uniformity. Special lever balances can be obtained, showing at a glance the weight per ream of different numbers of sheets.
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CHAPTER XII. CAUSTIC SODA, RECOVERED SODA, ETC.
As we have already pointed out, caustic soda and lime are the only alkalis employed by the paper-maker for boiling fibres; the special cases in which carbonate of soda is used have been mentioned in their proper place.
The preparation of milk of lime requires little or no description, as the operation is a simple and tolerably familiar one. Before using, it should be passed through a fine wire sieve, to keep back sand, coal, and similar impurities which the lime invariably contains.
Caustic soda is met with in commerce in four forms, differing from each other in alkaline strength and colour, and of course in price. The lowest quality is what is known as caustic “bottoms”: it consists of that portion remaining at the bottom of the caustic-pot after the clear fused caustic soda has been ladled out, and it contains a considerable quantity of ferric oxide and other insoluble impurities. It is of a dark reddish brown colour, and contains 50–60 per cent. of alkali (Na2O). Its use in paper-making cannot be recommended, except for the preparation of the very lowest grade of pulp. The solution should be allowed to remain at rest, in order that the insoluble matter may subside.
The next in quality is known as cream caustic, so called from its slightly brown colour. It is usually sold containing 60 per cent. of alkali, in the form of a tolerably friable mass, having a crystalline structure. It is a very suitable form of alkali for the paper-maker.
The next in order is what is called 60 per cent. white. It {178} consists of a hard white mass, requiring considerable force to break it. Though whiter in appearance than cream caustic, it is in reality less pure, as it contains a considerable quantity of salt. It is made by continuing the evaporation of the caustic liquor to a further point than is the case with cream caustic, the result being that the whole of the water is driven off. A small quantity of nitre is then added to oxidise the sulphides and other compounds which impart the colour to cream caustic. Common salt is then added to reduce its strength to 60 per cent., in order to satisfy the whim of the consumer, who insists upon an article of a given definite strength. On this account it is somewhat inferior to cream caustic, and is, moreover, more expensive.
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.