On this account the testing of wood pulp for the amount of moisture is an important part of the work of the paper-mill chemist.

The Manufacture of Chemical Wood Pulp.

Pulp prepared by treating wood with various chemical reagents under proper conditions is termed chemical wood pulp in order to distinguish it from pulp manufactured by the mechanical

process already described. There are several methods employed for the production of paper-making fibre from wood, but the sulphite process is the best known and most universally adopted.

Logs. The logs of wood used for the manufacture of both chemical and mechanical pulps are subjected to precisely the same preliminary treatment. They are sawn up and barked, as shown above, and used indifferently in either process. As a rule, the short two-foot pieces are roughly. sorted, so that the clean logs, free from decaying knots, may be preferably used for chemical pulp. In some cases the knots are bored out, but this is an expensive operation and only applied under special circumstances.

Chipping the Logs.-The short logs are next chipped up into small pieces or flakes about one inch square and a quarter of an inch thick by means of a powerful machine called a chipper. This is a massive iron disc rotating rapidly in a vertical position, being partially encased by an iron cover. From the surface of the disc project three steel knives, similar to those used in the barker, except that the knives are placed much closer to the centre of the disc to minimise the strain produced on the machine during use. The log is thrown into a short iron tube, which forms part of the casing and which is inclined at an angle of forty-five degrees to the surface of the disc, so that the wood is chipped into flakes at that angle. The wood is continuously discharged from the chipper and transferred by travelling belts or any convenient form of conveyer to the digester-house, where it is usually stored in huge wooden bins erected over the mouth of each digester.

Fig. 48.--Schopper's Oven for testing Moisture in Wood Pulp.

Chemical Process.-The next operation involves the digestion of the wood with a chemical solution at a high temperature. The details of carrying out this digestion process vary considerably, the differences relating chiefly to the time of boiling, the temperature, and the strength of the chemical solution. As a

general rule, with the stronger liquors and higher temperatures the operation is more quickly carried out, but the product is not invariably of the highest quality. Two systems of digestion are in vogue, viz., the Mitscherlich or "slow-cooking process, the second being known as the "quick-cook" or high pressure method. High Pressure System.-The digester, which is a tall cylindrical steel vessel, averaging 14 to 15 feet internal diameter, with a height of 50 feet, lined with special acid-resisting brick, is almost completely filled with the chips of

wood, and the liquor run in as quickly as possible. After the man-hole has been closed down, high pressure steam is blown direct into the digester and the contents boiled for a period of eight to ten hours. By this means the wood is reduced from its original hard condition to a soft pulpy mass.

The progress of the "cook" is carefully watched, especially towards the end of the operation, small quantities of the solution being withdrawn and tested at intervals. When the pulp is digested to the required extent, the contents of the digester are blown out under pressure into huge washing-tanks. The spent liquor drains away from the pulp and the latter is thoroughly washed with clean water.

It is interesting to note that about 50 per cent. of the actual weight of wood is dissolved by the chemical liquor. At present there is no systematic method

for the treatment of the waste liquors with a view of recovering any by-products. Attempts have been made from time to time in this direction and certain substances have been obtained which find a limited use, but so far no general and practical method has been found, and the importance of the question may be. judged from the fact that the loss entailed includes not only one-half the weight of wood put into the digester, but also the loss of about 300 lbs. of sulphur for every ton of pulp produced.

Screening. The pulp having been thoroughly washed is carefully screened in order to remove knots and other portions of incompletely boiled wood. Many systems are in use for efficient screening, and one of the simplest methods employed

is to pass the whole of the pulp over a series of screens, some of which are very fine and only allow the best quality of fibre to pass through, while others are coarser and give a pulp of somewhat inferior character, but suitable for common papers. The "Success" screen consists of brass plates, in which are cut a number of fine slits, and these plates are fitted into an iron frame so constructed as to form a shallow box. The bottom of the box, fitted with the brass plates, is kept in a state of violent agitation, so that the fine pulp finds its way through the slits while the coarse pulp is prevented from passing away.

Pressing. As the proportion of water to pulp at this stage is very large, the whole of the diluted pulp is next run through a wet press machine in order to separate out the pulp.

The pulp as it is taken from the wet press machine contains about 70 per cent. of water, and part of this is further removed by means of the hydraulic

press, and the moist sheets containing 50 per cent. of water are then packed up into bales ready for export.

Most of the sulphite pulp, however, is shipped from the pulp-mill in a dry In this case the process described is modified as follows:

state.

Making Dry Pulp.-The washed pulp taken from the digesters is carefully screened and then run off into large tanks, or chests, and converted into sheets on a machine which closely resembles an ordinary paper-machine, the diluted mixture of pulp flowing on to an endless wire-cloth to which is imparted a shaking motion. The water drains away, leaving the pulp on the upper surface of the wire in the form of a sheet. Further quantities of water are abstracted by means of a vacuum box, placed under the wire, and the moist sheet so formed

is pressed between rollers and led over a number of large steam-heated cylinders. The sheet of pulp is thus completely dried, and as it reaches the end of the machine it is automatically cut into sheets of the required size.

Sulphite Liquor.-The liquor used for digesting the pulp is known as bisulphite of lime. It is prepared by bringing the vapour of burning sulphur into contact with lime and water. In the "Tower" system, ordinary flowers of sulphur are burned in a specially constructed oven, and the sulphurous acid gas obtained is led away into tall towers filled with huge lumps of limestone, which are kept in a moist condition by a stream of water flowing from tanks at the top of each tower. As the gas passes upwards through the limestone it enters into combination with the water and lime, the whole of the gas being absorbed so that the liquid flowing out at the bottom of the tower is a strong solution of bisulphite of lime of constant strength. This is pumped into store-tanks and run into the digesters as required.

In the "Tank" system the gas is simply pumped into large circular vats in

which ordinary powdered lime mixed with water is kept in a state of violent agitation. The gas enters into combination with the milk of lime, giving a solution of bisulphite of lime.

The amount of sulphur required to produce a ton of air-dry pulp varies between 350 and 300 lbs.

"Mitscherlich" Process.-A wood pulp particularly suitable for certain special papers is prepared by this method due to Mitscherlich. The conditions of digesting are. radically different from those employed in the manufacture of the ordinary sulphite pulp, the pressure being seldom above 45 lbs., and the time

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occupied by the "cook" averaging about thirty-six to forty-eight hours. over, the steam does not come into direct contact with the contents of the digester, but passes through a lead coil, while the condensed water produced flows away.

The pulp is not blown out of the boiler under pressure, but is removed by shovels and rakes through a large man-hole near the bottom of the boiler after the pressure has been reduced. The pulp is partly washed before it is taken out of the digester, and more thoroughly washed afterwards in very long shallow "Knot" catchers, which retain hard unboiled chips and knots, and allow the properly digested pulp to flow out.

Soda Pulp. The ordinary commercial soda pulp is a soft brown-coloured fibre of excellent quality which is useful for the production of bulky papers. The word "soda" is a somewhat comprehensive term applied in a general way

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