The Manufacture of Mechanical Wood Pulp.
Wood is converted into pulp suitable for the manufacture of paper by methods which produce two distinct varieties. The first is mechanical wood pulp, so called because it is made by a purely mechanical process. The second is termed chemical wood pulp from the fact that the material is submitted to chemical treatment.
Ground Wood and Cellulose.—The two varieties of pulp are sometimes distinguished by the use of the terms ground wood and cellulose. In the former case the description implies a product consisting of pulp obtained by grinding wood into a fibrous condition, while in the second the word suggests a purified chemical product freed from the resinous and non-fibrous constituents found in wood. This is, in fact, the essential difference, for mechanical wood pulp consists of fibres which have been torn away from wood by means of a grindstone; it differs but slightly in chemical composition from the original raw material and contains most of the complex substances natural to wood. Chemical wood pulp, on the other hand, consists of fibre isolated from wood in such a manner that the complex non-fibrous substances are more or less entirely removed. The difference between these two pulps is shown in the following approximate analysis of spruce wood, and of the pulp derived from it. The composition of the mechanical pulp is practically identical with that of the wood itself.
Composition of Spruce Wood, and of Chemical Wood Pulp (Spruce).
| — | Wood (Spruce). | Chemical Wood Pulp. |
| Cellulose | 53·0 | 88·0 |
| Resin | 1·5 | 0·5 |
| Aqueous Extract | 2·5 | 0·5 |
| Water | 12·0 | 8·0 |
| Lignin | 30·5 | 2·5 |
| Ash | 0·5 | 0·5 |
| 100·0 | 100·0 | |
The use of mechanical wood pulp is generally confined to the manufacture of news, common printings and packing papers, cardboards, and boxboards. It possesses very little strength, quickly discolours when exposed to light and air, and gradually loses its fibrous character. The chemical wood pulp is a strong fibre, from which high-class papers can be manufactured, the colour and strength of which leave little to be desired.
Species of Wood.—The woods most commonly used for the manufacture of wood pulp belong to the order Coniferæ, or cone-bearing trees. In Europe the spruce and silver fir are the chief species, while in America spruce, balsam, pine, and fir are employed. The harder woods, such as hemlock, beech, larch and others, are not converted into pulp by the mechanical process.
Timber Operations.—The trees are cut down in the early part of winter by gangs of men specially trained to the work. The organisation of a lumber camp when the operations are of an extensive character is very complete and carefully arranged, every detail being attended to in order to get out the wood as cheaply and expeditiously as possible. The branches and small tops are removed from the trees when they are fallen, and the trunks cut into logs of 12, 14, or 16 feet in length, and afterwards piled up on the banks of the nearest river, or on the ice, ready for the breaking up of the winter.
As soon as the ice breaks up and the rivers become navigable the logs are floated down to their destination, in some cases hundreds of miles from the scene of operations. Where rivers are not available the timber is brought out by horses or bullocks, or by means of a light railway.
Log Cutting.—As the timber arrives at the mill it is carefully measured, both as to its diameter and length, in order that a record may be kept of the quantity used. Some of the logs are piled up in the storeyard for use in the winter, and the remainder converted into pulp day by day. The logs are first cut into short pieces about 2 feet long by means of a powerful circular saw, the arrangements for this work being devised so as to keep down the cost of labour as much as possible. All waste pieces are thrown aside to be utilised as fuel.
Barking.—The bark on the logs is removed in one or two ways. Much of it is knocked off during the transfer from the forest to the mill, but even then the wood requires to be cleaned. In Norway and Sweden the wood is treated in a tumbler or a barker, while in America and Canada the use of the tumbler is practically unknown.
The barker consists of a heavy iron disc fitted with knives, usually three in number, which project from the surface of the disc about half or three-quarters of an inch. The barker rotates in a vertical position, and the short pieces of wood are brought one by one into contact with the disc in such a manner that the bark is shaved off by the knives. The machine is provided with conveniences for pressing the wood against the disc and for turning the logs as they are barked.
Fig. 28.—A Pair of Barkers for removing Bark from Logs of Wood.
The machine is encased in a strong cast-iron cover, and all the bark shaved off is carried away by the strong current of air set up by the rapid motion of the disc, and subsequently burnt.
The tumbler system is quite different. In this case the short pieces are thrown into a large circular drum with hot water, and the bark taken off by the friction of the pieces as the drum rotates. The loss of material is of course less in this process, but the wood is not cleaned quite so effectively.
Fig. 29.—View of Horizontal Grinder (A), with Section (B).
The wood at this stage can be used either for the manufacture of mechanical or chemical pulp. As a general rule the pieces are taken indiscriminately for either process, but sometimes the wood is sorted out, the clean stuff free from knots and blemishes being reserved for high quality chemical pulp.
Grinding.—The main feature of the grinding process is the attrition of the wood when held against the surface of a rapidly revolving grindstone, the fibres as they are rubbed off being instantly carried away from the stone by a current of water. A complete description of the machines used and the modifications of the process practised by manufacturers is impossible in this book, but the following points will be sufficient.
The machine consists of a large grindstone about 54 inches in diameter, and 27 inches thick. It rotates in a vertical or in a horizontal position at a high speed. The stone revolves inside a casing which is provided with a number of pockets, so called, into which the pieces of wood are thrown at regular intervals, as fast as the wood is ground by the friction of the stone.
A continual stream of water playing upon the surface of the stone washes away the pulp into a tank or pit below the machine.
The quality of the pulp may be varied by the conditions under which it is made. By limiting the proportion of water so that the wood remains in contact with the stone for a longer time the temperature of the mass in the pockets rises. Such hot ground pulp, as it is termed, is tough and strong.
When the fibres are washed away from the stone as fast as they are produced the temperature does not rise, and cold ground pulp is made, which is not characterised by the somewhat leathery feel of the pulp made at the higher temperature.
The surface of the stone plays an important part also. If the stone is smooth the wood is rubbed away slowly, but if the surface has been roughened and grooved by means of a special tool the fibres are torn away quickly. In the first case the pulp comes from the stone in a finely-ground state and in a uniform condition, while in the second the pulp is coarse and chippy.
The output of the machine is, however, much increased by the use of sharp stones and by the application of considerable pressure to the blocks of wood.
Fig. 30.—A Vertical Grinder for making Hot Ground Mechanical Wood Pulp.
Screening.—The mixture of water and pulp leaving the grinder falls into a tank below the stone, all large chips being retained by means of a perforated plate. The finer pulp, still too coarse for use, is then pumped to the screens, which serve to remove all chippy and coarse fibres and produce a uniform material. The shaking sieve consists of a shallow tray, the bottom of which is a brass plate or series of plates perforated with small holes or slits. The pulp flows on to the tray, which is kept in a state of violent agitation, the fine pulp passing through the holes and the coarser pieces working down to the lower edge of the tray into a trough which carries them away. The flat screen is somewhat different in construction, but the principle of separation is the same. It consists of brass perforated plates forming the bottom of a shallow cast-iron tray, continually agitated by means of cams fixed to the under surface of the trays.
Fig. 31.—Centrifugal Screen for Wood Pulp.
The centrifugal screen is a cage made of finely perforated brass sheeting which revolves at a very high rate of speed inside a circular cast-iron vessel. The pulp flows into the interior of the cage, the fine fibres being forced through the screen by the centrifugal action of the machine, and the coarse material is retained.
Fig. 32.—Section of Centrifugal Screen for Wood Pulp.
Wet Pressing.—The pulp leaving the screens is mixed with such a large quantity of water that it is necessary to concentrate it. This is effected by means of the wet press machine ([Fig. 41]). The pulp and water are pumped into a wooden box in which revolves a large hollow drum, the surface of this drum consisting of a fine wire cloth of about 60 or 70 mesh. The drum is not entirely immersed in the mixture, so that as it rotates the pulp forms a skin or thin sheet on the surface, and the water passes away through the wire into the interior of the hollow drum. The drum carries the thin sheet out of the box and above the level of the mixture until it comes into contact with an endless blanket or felt, which is pressed against that part of the drum not immersed in the liquid.
By this means the thin sheet is transferred to the felt and carried between squeezing rolls to the finishing rolls. The felt, carrying on its upper surface the thin sheet of pulp, passes between two rolls, usually 16 to 20 inches in diameter, the upper being made of wood and the lower one of cast iron. The pulp adheres to the upper drum and the felt passes round the lower drum back to the box containing the mixture of pulp and water; the thin sheet is continuously wound on the upper roll until a certain thickness is reached.
When this occurs the attendant removes the thick sheet by a dexterous movement of a sharp stick across the face of the roll. The wet pulp at this stage consists of 30 per cent. air-dry pulp and 70 per cent. of water.
Hydraulic Pressing.—The sheets taken from the wet press machine are folded into a convenient shape and piled up, coarse pieces of sacking being placed between the sheets. At stated intervals the piles are submitted to pressure in hydraulic presses in order to remove further quantities of water, which slowly drains away through the sacking. In this way a mass of pulp in the form of thick folded sheets containing 50 per cent. of dry wood pulp is produced.
The pieces of sacking are taken out and the sheets put up in bales of any required weight, usually 2 cwt. or 4 cwt.