Paper and Its Uses/Chapter 2
CHAPTER II
REDUCTION TO PULP
Whatever material may be used for making into paper it has to undergo stages of preparation which can be divided into removing all foreign matter and dirt, reduction to fibrous state, bleaching, beating, and lastly converting what is the pulp into paper. If the material has already been manufactured, as in the case of rags, ropes, sails, sacking, and other textiles, the first process is somewhat simpler than in the case of really raw materials, such as esparto, bamboo, or wood. But here again the first and second operations may have been carried out before the papermaker handles the material, for wood, esparto, and bamboo are imported as pulp boards. In the case of esparto the quantity so imported is very small, but the quantity of wood pulp is enormous. It will be advisable to take the materials in order, so that the difference as well as the similarity of treatment may be traced.
Rags are purchased already graded. There are some twenty to thirty grades of rags regularly quoted in the market reports, and the layman might fancy that the papermaker could unpack the bales and proceed to make paper from these graded rags. Unfortunately he finds a large quantity of undesirable material, such as silk, wool, buttons, elastic, and dirt, that must be removed. First the rags are sorted, and cut into pieces of uniform size, the undesirable parts mentioned being put aside as useless, and the seams cut open or thrown out. Standing at a bench, the top of which is wire netting, the sorter takes rags from a pile, and cuts them on a scythe-like knife which stands out obliquely from the bench. A large amount of dust escapes through the netting, and the rags are sorted into bins as more suitable for one class of paper than another. The rags are next taken to the willow or dusting machine, where they are subjected to violent treatment, the teeth of the machine carrying the rags against other teeth, giving them a thorough shaking and loosening the dust, which falls away. As they are cleaned, the rags are taken to the top of the building by a travelling band and dropped into the mouth of a boiler prepared to receive them. For rags a special spherical rotary boiler is employed, and when a charge has been filled in, a definite quantity of a solution of caustic soda in water is added. The lid is securely fastened, steam is passed in, and the boiler is kept rotating slowly for about eight hours. When the dirt in the rags has been thoroughly loosened the rotation is stopped, steam is shut off, the dirty water is run off, clean water is run in. The boiler is again revolved, the rags rinsed, and then the lid is removed and the boiler emptied by continuing the revolution.
Next comes the washing and breaking, both of which may be carried out in the beating engine. The beating engine, of the Hollander type, consists of a large vessel with rounded ends, with a central division running down the length of the engine. Two cylinders revolve: one, a very heavy cylinder known as the beating roll, reaches to the bottom of the engine and bears a number of knives on its surface, which knives, in conjunction with a bedplate also bearing knives, break the rags into smaller fragments and open the threads, loosening the fibres, and allowing dirt to come away. The second and smaller cylinder is employed as a washing drum. It is covered with wire gauze, through which the water passes, and as the drum revolves the dirty water passes into the interior, where a number of bucket compartments carry the water and pass it through the axis of the drum to the waste pipe. When the rags are filled into the beating engine clean water is run in, the beating roll is kept out of contact
Fig. 1.—Beating Engine, with covers partially removed to show interiors of Beating Roll and Washing Drum.
with the bottom knives, and the rags are kept in circulation. The washing cylinder is in action, and the roll being gradually lowered the dirt is eliminated. When this stage is reached the washing drum is lifted, the beating roll lowered, and the rags are gradually reduced in size until they attain a state of fibrous pulp, being known technically as "half-stuff." In most instances the next process is bleaching. There are special drawing papers, of which "O.W." and "Unbleached Arnold" are examples, which are the colour of the original rags, no bleaching having taken place. But usually a weak solution of bleaching powder (chloride of lime) is let into the engine and thoroughly mixed with the pulp. When the bleach is thoroughly incorporated the half-stuff is let down into large tanks, made of stoneware or cement, having perforated bottoms, and there the bleach completes its task, and the pulp is allowed to drain.
Next comes the beating, at which stage the blending of different fibres may take place. The object of beating is to reduce the bleached pulp to fibres, and to reduce the length of the fibre in accordance with the requirements of the paper to be made. The rags are chosen according to the class of paper desired—softer rags for soft papers, and, of course, stronger rags for strong papers. For blottings, filter papers, and lithographic papers, soft rags, sharp beater knives, quick beating are adopted. For dense, hard papers, such as ledger, type-writing, bank, imitation parchment papers, duller knives, slow beating, with a gradual lowering of the beater roll is the order. The normal time for beating the pulp for an ordinary rag paper may be taken as eight hours.
To take the next material, esparto, and to follow it in the same way. Esparto arrives in bales, fastened either with ropes of esparto or with iron bands. Esparto travels through the mill in the same way as rags, that is, from the ground floor, where it is unpacked and dusted, upwards by means of a series of claws, along a travelling band where pickers remove foreign substances. In its travel broken fibres and dirt escape, and the grass arrives at the mouth of an upright cylindrical boiler, stationary, and so arranged that the boiling liquor is vomited over the mass of esparto. The boiler is filled, and a fairly strong solution of caustic soda is run in, the manhole is fastened down, and steam under pressure introduced. After several hours boiling the siliceous and waxy substances taken up by the growing plant are dissolved, the dirty water is run out, small quantities of clean water let in to wash out as much soda as possible. Most of the soda is recovered, but that process, though of great importance to the paper-maker, need not be treated here. The washed esparto is conveyed to the breaking engine for treatment similar
Fig. 2.—Edge Runner or Kollergang.
to that given to rags, being washed, broken, and bleached. In many mills the half-stuff is carried over strainers, and by the use of the presse-pate machine (a paper-making machine with only a "wet end") made into sheets. The half-stuff in sheets is filled into trucks and stored or taken direct to the beaters. Owing to the small dimensions of esparto the reduction to the fibrous state is easily accomplished, and very little beating is necessary.
Wood, chemical or mechanical, usually finds its way to the paper mills in the form of pulp boards, and is known as chemical or mechanical wood pulp. No boiling is necessary, but the boards are fed into the breaking engine, and reduced to half-stuff, a little bleach liquor added to chemical wood, and the contents of the engine, when sufficiently reduced, are let down to the draining tanks for the bleach to expend itself. Then the pulp is ready for the beating engine, where it is reduced to the necessary degree of fineness.
Some materials are more effectively reduced to pulp in the edge runner or kollergang. This machine is similar in appearance to a mortar mill, but the arrangement is slightly different. The pan of the machine is stationary, and the stones revolve and travel round the pan. Only a small quantity of water is used with the pulp, and waste papers which require rubbing apart only, and strong wood pulps of which the fibres are drawn out, and not in any way reduced in length, are treated in this machine more economically and more effectively than in the beating engine.