CHAPTER IV

ESPARTO. STRAW. NOTES ON BEATING

Esparto Sorting, cleaning and boiling-Bleaching, beating, loading, sizing-Manufacture of rosin size-Colouring-Making paper on the Fourdrinier machine-Straw -Preparation of straw pulp-Washing, bleaching, &c. -Beating

The Preparation of Esparto Pulp.

The

Cleaning.-Esparto grass reaches the paper-mill in the form of huge bales containing the grass bundles tightly packed together by hydraulic pressure. These bales are opened and the bundles of esparto loosened by women. material is then carefully sorted and thoroughly cleaned either by hand or machinery. Usually the loosened grass is passed through a "willow," or duster, similar to that employed for rags, which beats out all dust, dirt, small stones and sand, and discharges the grass upon a travelling band, so that the coarse rootends, weeds, and extraneous matter can be readily picked out if necessary.

The cleaned grass is carried from the willowing machine to any desired height by means of travelling bands, and thus furnished to the digesters.

Boiling. In a well-arranged paper-mill the boilers are erected in convenient positions so that they can be quickly filled with the grass. The type of digester now almost exclusively used for esparto grass is known as Sinclair's Vomiting Boiler. It is a stationary cylindrical digester capable of holding two to three tons of grass, provided with a man-hole at the top, through which the fibre is furnished from the floor above, and a second man-hole fitted to one side of the boiler near the bottom, through which the boiled grass may be emptied out.

Steam is admitted through a pipe at the bottom of the boiler, and this causes "vomit " a continuous circulation of the caustic soda liquor upwards through pipes, fitted to the opposite sides of the apparatus, and over a perforated plate

fixed in the upper part of the boiler, the liquor then falling back upon the grass. The digester is also provided with suitable inlet pipes for caustic soda liquor, and for clean water, and outlet pipes for blowing off the steam and for the discharge of the spent liquor.

The boiler is first supplied with the proper quantity of caustic soda liquor, and the steam turned on in order that the grass may be quickly saturated, for by this means a much larger quantity of grass can be filled in. The upper man-hole is bolted down, and the full working pressure of steam admitted. The period occupied in boiling depends on the strength of the liquor and the pressure of steam. A boiling usually occupies four to five hours, the steam pressure being 40 to 50 lbs. per square inch, the proportion of caustic soda 14 to 16 per cent. on the weight of grass.

When the boiling is completed the caustic liquid, now perfectly black from the presence of the non-cellulose constituents of the esparto removed by the operation, is discharged into storetanks. The boiled pulp is carefully washed under a system which ensures a thorough cleansing of the fibre with the least possible quantity of water, because the washings are not thrown away as in the case of rags, but are run down into the "black liquor store-tanks to be subsequently dealt with.

Soda Recovery. These black liquors contain all the caustic soda originally used, together with organic matter removed from the esparto grass, the latter amounting to about 50 per cent. of the weight of grass treated. The liquors are boiled down and concentrated to a small bulk by suitable appliances, generally some form of vacuum evaporating machinery, and this gives a thick syrupy liquid, which readily burns when run on to a hearth and brought into contact with a fire. By this means the organic matter is burnt off, and the mass left behind consists mainly of impure carbonate of soda. This residue is known as "recovered ash," and when boiled in suitable tanks with lime is reconverted into caustic soda, which can be utilised over again. About 75 to 85 per cent. of the soda used in the treatment of esparto can be recovered.

Fig. 26. Plan and Section of Sinclair
Vomiting Esparto Boiler.

Washing. The fibre, washed by a inore or less elaborate system for the

removal of the black liquor, is nearly always submitted to a final washing in the breaking-engine to remove the last traces of dirty water, the operation reducing the grass at the same time to the condition of pulp.

Bleaching. The bleaching of esparto pulp is accomplished by methods which have been found suitable for rags, such as the system of steeping in tanks, or circulating the pulp and bleach liquor in the breaker. Of recent years, esparto has been bleached in tall cylindrical vessels, the mixture of pulp and bleach

being kept in constant circulation by a centrifugal pump which discharges the stuff from the bottom of the vessel through a pipe into the open top, where the pulp falls on a disc of wood and spreads over the surface of the mixture, coming freely into contact with the air, a method said to be economical in the use of bleach, but which is probably wasteful in the matter of power. (See Fig. 54.)

Presse-pâte. The bleached pulp is then thoroughly washed to remove every trace of bleach residues, and worked up into moist sheets of pulp on a presse-pâte. The presse-pâte closely resembles the wet end of a paper-machine, and it is used for converting bleached pulp into sheets for convenience in handling and transport. The pulp mixed with a large quantity of water flows through a series of shallow open boxes, or sand-traps, provided with wooden baffle-plates which serve

to retain dirt and coarse root-ends incompletely digested, and then on to strainers. The purer fibre passes on to a moving endless wire-cloth, about thirty feet long and sixty or seventy inches wide. The wire-cloth is supported in a horizontal position on brass rolls, and the continuous rotation of the rolls causes the wire to travel between the sand-traps and the couch-roll, carrying on its upper surface the mixture of wet pulp.. As the pulp flows on the wire and is carried forward, the water falls through the meshes of the cloth, leaving a coherent mass of pulp on the wire. Further quantities of water are drawn from the pulp by means of vacuum-boxes placed immediately under the surface of the wire, and finally, the pulp passes under the heavy couch-roll, which squeezes more water out. The sheet

of wet pulp is led carefully on to an iron spindle, and a continuous roll of pulp is produced which consists of esparto half-stuff.

In some cases the presse-pâte is dispensed with, and the bleached pulp after washing is run down into draining-tanks, to be afterwards dug out with shovels into trucks as required.

Beating. The beating of esparto pulp for the production of pure esparto papers is quite different to the treatment necessary in the manufacture of strong rag papers. The individual fibres possess peculiarities of structure compelling the paper-maker to adopt a method of beating which will bring out qualities looked for in esparto paper. The differences in the appearance and feel of esparto papers cannot be varied to such an extent as with fibres like rag and wood, but on the one hand it is possible to make a soft bulky antique, and on the other a fairly strong supercalendered writing out of esparto pulp. The main factor in producing this difference is the method of beating.

Esparto half-stuff can be beaten in the "Hollander," a machine which has been described in chap. iii. It is, however, usual to employ the Umpherston beater, or some of the more modern types of beaters, in which the circulation of the pulp is not maintained by the beater-roll, but by some independent contrivance.

The Umpherston beater is similar in principle to the old "Hollander," in that the engine is divided into two channels, but the second channel is underneath the first, so that the pulp is discharged by the beater-roll over the backfall into the second channel, and circulates underneath the backfall, returning to the front of the beater roll ready for continued treatment.

In the production of soft bulky esparto papers the stuff is put into the beater

and beaten for a short period, varying from an hour to an hour and a half, the object being to reduce the pulp to a suitable length and condition without allowing the water to assimilate with the fibres, since prolonged beating causes the pulp to work "wet," as fully explained in chap. iii., which is undesirable in pulps intended for bulky light papers.

In the production of the stronger, more closely made writing papers, the esparto is beaten for two to four hours according to requirements, the result being a harder sheet.

Loading. During the actual process of beating, certain more strictly chemical operations are carried out. If a mineral filling is required, it is added to the pulp soon after the beating has commenced, to ensure thorough incorporation. China-clay is the mineral most frequently added, and this is used either