Esparto.
FIG. 23.
The first treatment that the esparto undergoes is that of “picking.” The bands of the bales in which esparto is packed, generally by hydraulic pressure, are cut, and the grass is spread out on tables by women, who carefully remove such impurities as weeds, root-ends, &c., which from their nature are with great difficulty boiled and bleached, and which if not removed would be liable to appear in the finished paper as dark-coloured specks, technically known as “sheave.” This treatment is called “dry-picking,” in contradistinction to a subsequent process, known as “wet-picking.” A portion of the table is covered with coarse wire gauze, through which when the grass is spread over it loose particles of sand, dirt, &c., escape. The cleaning of esparto can be much better accomplished by means of machinery. Fig. 23 is an elevation of a machine manufactured for the purpose by Messrs. Masson, Scott and Bertram. The grass, which may be in the form of sheaves, as taken from the bales, is put in by the hopper A. It passes thence to a conical {91} drum made of steel bars placed very close together, driven rapidly by the shaft B. It is provided with five rows of teeth. Fixed to the sides of the willow there is also a row of stationary teeth. The grass is thoroughly broken up and dusted by the action of the teeth, and is transferred to the wide end of the willow at C, where it is carried forward on the travelling tables D. During its passage along the tables it can be examined and picked by women standing on the platforms E. The dust and dirt passing through the steel bars of the drum are drawn away by a fan through the pipe F.
This machine is also adapted so as to carry the cleaned grass to the boiler-house. It is taken forward by means of travelling rakes over the tops of a series of boilers, any one of which can be furnished by simply opening a door corresponding in position with the lid of the boiler.
Boiling.—The ordinary form of boiler is shown in Fig. 24. It is known as a vomiting boiler. The grass is put in by the door E, which is hinged, and is counterbalanced by the weights L. It is securely fastened down by the screws F. Steam enters by the inner pipe A, which dips a little below the perforated false bottom B. Surrounding the steam-pipe is a wider pipe C, open at the top, which is made slightly trumpet-shaped; also open at the bottom, where it ends in a kind of shoulder, on which the false bottom rests. The lower part has two or more openings G cut away, through which the liquor can freely pass.
In order to get as large a charge into the boiler as possible, steam is turned on while furnishing, and a quantity of caustic soda lye is also run in, which has the effect of softening the grass and making it more compact. This is continued until the boiler is well filled.
The action of the boiler is as follows:—The steam passing through the pipe A heats the liquor that has drained from the grass through the perforations in the false bottom, and forcing it up the wide pipe C, causes it to strike against the dome or bonnet D, and distribute itself again over the grass. {92} This is technically known as “vomiting.” In this way a constant circulation of liquor is kept up.
FIG. 24.
The boiler is emptied by the door H and the liquor is run off by the cock I. The boilers are usually supplied with a safety valve E, and also with a pipe for letting off the steam when the boiling is finished. The boiling generally takes from four to six hours. The quantity of soda necessary depends upon the nature of the grass, Spanish requiring less than African. {93}
Routledge, to whom the introduction of esparto as a paper-making material is due, gives 10 per cent. as the proper quantity.
The pressures employed vary from 5 lb. to 45 lb. There has been a growing tendency of late to employ the higher pressures for reasons already stated.
FIG. 25.
Of late years great improvements in the form of the boiler have been introduced. Fig. 25 is an illustration of Roeckner’s Patent Boiler. The vomit pipe is outside the boiler. Steam entering by the cock D, forces the liquor up the vertical vomit pipe and distributes it over the grass. The pipe K is used for heating the liquor at the commencement of the operation by means of waste steam. F F are gauges for indicating the height of the liquor. The grass is put in by the opening G. The pipes A B and C are for the supply of {94} steam, strong lye and water. The boiled grass is discharged by the circular door E.
The boiler holds three tons of grass and the boiling is completed in about 2 1⁄2 hours, the pressure being about 35–40 lb. per square inch. It is claimed that by its use a saving both of time and soda is effected.
FIG. 26.
Sinclair’s Patent Boiler is shown in Fig. 26. The vomit pipes, of which there are two, are made of thin steel plates {95} riveted to opposite sides of the boiler. The liquor drains through the perforated false bottom, and is then forced up the vomit pipes above the perforated plates through which it is distributed over the grass in a number of fine jets. The boiler is charged by the door e and emptied by j. The small cock a is used as a blow-through cock; the opening b is used for blowing-off steam when emptying the boiler; c for letting in caustic soda lye; d for water. Steam enters by the small branch of the T pipe at the bottom of the boiler, the other, g being used for running off the waste liquor. A boiler holds from two to three tons of grass, and a boiling is completed in about two hours at a pressure of 40–50 lb. per square inch.
The two forms of boiler above described possess the obvious advantage of having the whole or nearly the whole of the interior available for holding grass; the boilers can therefore be made to hold more, and the boiling is moreover much more evenly accomplished.
Washing.—The boiling being completed, the steam is allowed to escape and the liquor to collect at the bottom of the boiler where it is run away by the pipes placed at the bottom for the purpose. Water is then run in and steam turned on for a short time; this is also run off, and the grass drained as completely as possible. The boiled grass is then emptied into trucks and taken away to the washing engines. These resemble those already described under rags.
During the process of washing, a certain amount of the shorter fibres find their way through the meshes of the wire-cloth. In addition to this a large proportion of the cellular tissue surrounding the fibro-vascular bundles (see Fig. 12, p. [57]) is carried away. If the wash-water be examined under the microscope, it will be seen to consist largely of this cellular matter. Though this entails a certain loss of cellulose, its removal is in other respects advantageous, as it is possessed of hardly any “felting” properties, and it is moreover exceedingly difficult to bleach.
The amount of fibre actually obtained in practice is but little below that contained in the grass. A certain loss is {96} inevitable, but this probably does not exceed 1 or 2 per cent.
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.
It may be interesting at this point to say something about the substances which are removed from the esparto by the caustic soda. On referring to p. [57] it will be seen that the original grass contains nearly half its weight of extractive matter, the removal of which has to be effected. Only a small proportion of this is extractive matter in the ordinary sense of the term, i.e. that can be extracted by the usual solvents; the remainder is intimately combined with the cellulose. The action of the caustic soda is to resolve these bodies, the cellulose remaining behind, and the other constituents being dissolved as resinous bodies by the soda. A certain proportion of the mineral constituents, notably the silica, dissolves in the lye, the latter forming silicate of soda.
On neutralizing the liquor with an acid, the bulk of the dissolved constituents is thrown down as a dark brown resinous mass. If this crude product be purified, it is found to consist of a definite body having the formula C21H24O8. By the action of chlorine on this resin a bright yellow chlorine compound is formed, resembling the compound from jute described on p. [18]. If the resin be treated with concentrated nitric acid, a bright yellow body is formed, which forms definite compounds with bases, and which has the property of dyeing animal fibres a bright orange colour. In addition to this body, a yellowish-white wax can be isolated. It sometimes happens that this wax is but imperfectly dissolved in the caustic lye; if the waste liquor be allowed {98} to stand, the wax is occasionally found to collect in small quantities on the surface.
Various attempts have been made from time to time to remove from the waste liquors the soluble matter derived from the esparto by the addition of lime. This throws it down as a very voluminous precipitate, exceeding difficult of filtration. The filtrate contains only caustic soda. The difficulties of removal of the precipitate, however, are almost insurmountable. The usual method of disposing of the liquor is to evaporate it to dryness and ignite it, as will be subsequently described under the head of “Soda Recovery,” Chapter XII.