Straw.
The use of straw for the manufacture of paper was first brought prominently into notice about the year 1800 by Matthias Koops, who published a book printed on paper made from straw, but it was not until 1860 that this material was used in any large quantity.
Fig. 26.—A Cylindrical Digester for Boiling Fibre.
Straw is now converted into a bleached paper pulp for news and printings, and is also utilised for the manufacture of straw boards.
The production of a white paper pulp from straw is carried out in a manner similar to that used in the case of esparto fibre, viz., by digestion with caustic soda under pressure and subsequent bleaching. As the straw contains considerable quantities of siliceous matter, the chemical treatment necessary to reduce the material to paper pulp is more severe, a stronger solution of caustic soda being used, and the process of digestion being carried out at a higher temperature.
For the best quality of straw cellulose, the material is cut up into small pieces by machines which resemble an ordinary chaff-cutter, and the knots taken out by a separating machine. In most cases, however, the whole straw is simply cut up into small lengths of about one to two inches long, and placed at once in the digester. When the straw is contaminated with foreign weeds, sand, husks, and similar substances, as is usually the case, it is carefully hand-picked by girls, who remove these impurities, which tend to produce particles of unbleached matter in the finished pulp. The expense of this preliminary cleaning process is more than compensated for by the enhanced value of the bleached straw pulp.
Digesting.—The cut straw is boiled in rotary cylindrical or spherical vessels, stationary upright boilers of the vomiting type being seldom employed because the circulation of the caustic soda liquor does not take place freely with straw packed in the latter.
As the material is very bulky, some of the liquor is first put into the boiler and the steam admitted while the straw is being thrown in. By this means the straw is softened and reduced in bulk, so that a larger quantity can be added before the digester is quite full. The full amount of caustic soda is then made up by further additions of liquor, and the contents of the digester heated by high-pressure steam for four to six hours.
The conditions of treatment are shown by the following trial:—
| Amount of straw | 5,600 lbs. |
| Caustic soda, 20 per cent. | 1,120 lbs. |
The caustic soda was added in the form of a liquor, having a volume of 2,012 gallons and a specific gravity of 1·055.
| Time of boiling | 5 hours. |
| Pressure | 60 lbs. |
Washing.—The boiled straw is discharged into large tanks placed below the digester and washed with hot water, the smallest possible quantity being used consistent with complete washing in order to prevent the accumulation of large volumes of weak lye. The spent liquor and washing waters are drained off into store tanks and evaporated in a multiple effect apparatus by the same process as that used for esparto pulp. The last washings are usually run away because the percentage of soda in them is too small to pay for the cost of recovery.
The final washing of the straw pulp is completed by the use of a breaking engine or potcher. As straw pulp contains a large proportion of cellular matter which cannot be regarded as true fibres, there is always a danger of considerable loss in yield if the use of the breaking engine is extensively adopted, because the short cells escape through the meshes of the drum-washer. The washing is most economically effected in the tanks if a good yield of pulp is required.
Separating out Knots.—The broken pulp from the breaking engines is diluted with large quantities of water and pumped over sand traps in order to remove knots and weeds which have resisted the action of the caustic soda. These traps consist of long shallow trays, perhaps sixty to eighty yards long, one yard wide, and nine inches deep, containing boards which stretch from side to side, sloping at an angle, and nailed to the bottom of the trays. The dilute pulp flows through the trays, leaving the heavy particles, knots, and foreign matter behind the sloping boards, and finally passes over the strainers, which retain any large coarse pieces still remaining.
Making Sheets of Pulp.—The mixture from the strainers contains a large excess of water which has to be removed before the pulp can be bleached. For this purpose a wet press machine (see page [103]) or a presse-pâte (see page [85]) is employed, and the wet sheets of pulp are then ready for bleaching.
Bleaching.—The process by which the pulp is bleached is exactly similar to that used for treating esparto.
From 1870 to 1890 large quantities of straw were used for the manufacture of newspaper in conjunction with esparto and wood pulp, but the price of the material was gradually advanced so that it could not be used with advantage, especially as the production of wood pulp gave a material which was much cheaper, and which could be utilised at once without chemical treatment.
In the manufacture of newspaper the tendency during recent years has been to make the paper mill operations as mechanical as possible and to dispense with the preliminary operations which are essential for the manufacture of half-stuff, the chemical processes being left in the hands of the pulp manufacturers.
The manufacture of straw cellulose is now practically confined to Germany, but small quantities of the bleached straw cellulose are imported because the pulp imparts certain qualities to paper which improve it, notably in making cheap printing papers harder and more opaque.