The percentage of cellulose in esparto is given on p. [57]. The following numbers obtained by the authors are somewhat higher. They are calculated on the absolutely dry fibre; those mentioned are on the air-dry samples:—
| Cellulose per cent. | |
|---|---|
| Spanish | 58·0 |
| Tripoli | 46·3 |
| Arzew | 52·0 |
| Oran | 49·6 |
There is no doubt that considerable differences occur even between different specimens of the same kind of grass.
It is the practice in some mills to wash the grass in a series of tanks connected together in the same way as the lixiviating tanks of an alkali works. They are so arranged that pure water flows in at one end, passes through fresh lots of grass in succession, and issues at the further end highly charged with the soluble products of the grass. By such an arrangement the grass can be washed without any loss of fibre, and with a minimum quantity of water. This latter feature is of great importance in mills where it is necessary to evaporate the whole of the waste liquors from the esparto, as they are then obtained in a very concentrated form. Even with such an arrangement it is advisable to give the grass a short final treatment in the washing engine.
The washing having been completed, and the esparto having been broken up into “half-stuff,” it is now ready to be bleached (see Chap. VII.).
The presse-pâte system, originally adopted for the treatment of straw, has of late years been extensively applied to esparto.
The presse-pâte consists of the wet end of a paper machine, and is furnished with sand-tables and strainers. The pulp is allowed to flow on to the wire cloth, so as to form a thick web of pulp. The bulk of the water passes away through the wire cloth; a further quantity is removed by the {97} vacuum-boxes and couch-rolls. The pulp, containing from 50 to 60 per cent. of water, is wound round an iron rod until a sufficiently large roll is formed.
The advantages of the presse-pâte system are the possibility of the removal of dirt and unboiled portions by means of the sand-tables and strainers, and the very complete washing and removal of the products of the action of bleaching powder.
It also enables manufacturers to dispense with the somewhat costly methods of dry and wet picking.
The presse-pâte system can also be applied to the unbleached pulp.