Page images
PDF
EPUB

Thursday, 31st October.

P. A. PETERSON, Vice-President, in the Chair.

Student's Paper No. 3.

WOOD FIBRE FOR PAPER-MAKING.

By F. A. BOWMAN, M.A., B.E., Stud. Can. Soc. C.E.

After a brief sketch of the origin of paper, and of the various raw materials which have, at times, been employed in its manufacture, the author proceeds as follows:

The ordinary merchantable varieties of wood-fibre are divided into Mechanical or Ground Wood Pulp, Soda or Chemical Wood Fibre, and Sulphate or Acid Wood Fibre. This is the order in which they have come into the market, and technically they should be arranged as follows: mechanical wood pulp, being wood ground to a pulp on a grindstone, with the fibres only partially separated, and all the incrustating materials still in it; sulphite fibre, being wood partially boiled in an acid solution until the fibres are completely separated, but some little silica still remains; soda fibre or wood, completely boiled in an alkaline solution, until it is a pure vegetable fibre or cellulose.

Mechanical or ground wood pulp cannot be called a fibre at all, as the wood is merely separated into particles, regardless of the natural fibre, and containing all the resins and silicious matters.

In discussing wood pulp and fibre, the question has often been put to the writer: "Cannot sawdust be used to make pulp?" It cannot be used successfully, as the saw has cut the fibres too short. Many experiments have been tried, and a kind of pulp produced from it, but it is useless for any purpose, except perhaps as a filling material to other fibres, or, in other words, as an adulterant.

The process of manufacture of ground wood pulp is simple, and consists in rasping off the particles parallel to the fibres by forcing the wood against a grindstone. The Voelter process is the best known and most used on this side of the Atlantic.

The wood is completely barked, cut into one-foot lengths, and the knots bored out; if large, the stick is split. The principal part of the apparatus is the grindstone, which is about 4 feet diameter, and 15 inches face. Its axis is set horizontally, and it makes 150 revolutions per

minute. The wood is pressed against the stone by shoes at the end of screwed rods, which slowly advance as the wood is ground off. A stream of water plays continually on the stone, and washes off the pulp. The pulp, much diluted with water, passes over a series of revolving drums, covered with wire cloth increasing in fineness. The drums have hollow trunnions, and the pulp which passes through the wire runs out through the trunnions to the next finer drum. The product is 50 per cent, of the rough wood employed. A Voelter machine uses 50 horse-power (French), and produces 500 kilos. of saleable pulp a day.

In the Grellinger process the wood is held against the side of the grindstone instead of the edge, and the patentees claim that the pulp produced is longer and better than that from the Voelter machine. The principal advantage is that the pulp is never reground, while on the Voelter machine part of the wood ground on the top of the stone runs down and gets reground at the lower shoes. In the Grellinger process, centrifugal sorters are used instead of the drums of Voelter.

The woods principally used in making both ground pulp and true fibre are spruce, poplar, and a little hemlock. The dense hardwoods are practically useless as fibre producers, either when ground or boiled. The fibres are too small and weak to be of any use.

10

Vegetable fibre, or cellulose, forms the principal part of every vegetable. The chemical formula for it is C Ho Os, and its composition varies very slightly, as is shewn by the following table from Wagner's Chemical Technology:

[blocks in formation]

The properties which make a vegetable fibre suitable or otherwise for paper-making are entirely physical. All fibres adapted to spinning into textile fabrics are well suited to the manufacture of paper, and some that are too short for the former purpose are still very suitable for the latter. In this latter class are found all kinds of wood fibres. Cotton fibres have the peculiarity that when dried they twist spirally like a corkscrew, which, of course, much increases their felting power.

Flax fibres have sharp points arranged along them, which serve the same purpose. Wood fibres, when freed from the incrustating materials, are in the form of a flattened ribbon with thickened edges, and composed of thin transparent membranes. Mr. Aimé Girard has made a careful microscopical examination of the various fibres used in paper-making, and in a paper read before the French Academy, gives the following as the result of his investigations: The fibres after pulping, that is, when ready for the paper machine, are one-sixteenth inch long, and in very fine pulps one-fiftieth or less. He finds that fineness is essential, and considers that the fibre should have a diameter of one-fiftieth of its length, and that strength is of not so much consequence as, when paper is torn, the fibres are not broken but merely slit apart. The fibres should be elastic. He divides them into five classes: 1st. Round fibres freely flexible and elastic, as flax and hemp. 2nd, Similar fibres of a smoother and less vigorous kind, as jute, esparto, hop, dwarf palm, sugar cane, etc. 3rd. Fibre cellular matter, derived from straw, and containing both fibres and cellular matter. 4th. Flat fibres, such as cotton, and the wood fibres produced by chemical processes. 5th. Imperfect material, such as ground wood; this is not flexible enough,

The method known as the soda or alkali process, by which chemical or soda fibre is produced, was invented about 1870. The first works erected were at Manayunk, Pennsylvania. In England and other countries, where wood is scarce and high priced, few manufactories have been started, but there are a large number in the United States and on the continent of Europe. The wood employed is generally pine or spruce, sometimes aspen or poplar. It is cleaned of bark and knots, and cut into chips of a uniform size and thickness, by a machine with revolving knives. These chips are packed into a boiler about 40 feet long and 4 feet diameter, and a strong solution of caustic soda is pumped in. The boilers are heated by steam, and the wood cooked under a pressure of about 150 lbs. to the square inch for from 5 to 6 hours. The boiling and washing processes are similar to those employed for rags, esparto, etc., except that the solution is stronger. When the wood comes out of the boiler, it is still in the form of chips, but if rubbed between the finger and thumb, it separates into fibres, and is now ready to be treated by washing, bleaching, beating, etc., like any other paper-making material. The pulp is of a dark brown colour, and always requires bleaching, except for the common brown papers. The process of regaining the soda from the "black liquor" of the boiler, essential in boiling rags, etc., on account of the saving of expense and the prevention of the pollution of rivers, becomes absolutely necessary in boiling wood, on account of the much larger quantity of soda used.

The regaining is accomplished in the following manner: The spent liquor from the boiler is evaporated to dryness in pans, and the residue burnt, the result being the removal of all organic matter and the production of soda ash; the latter is causticized by dissolving it in water and agitating it in a tank with lime. The soda ash is crude carbonate of soda, and the agitating of the solution with lime produces carbonate of lime and a solution of caustic soda. For every ton of soda ash causticized, 9 to 15 cwt. of lime are required. In the best plants, 75 to 80 per cent. of the original charge of soda in the boiler is regained, The necessity for this regaining plant severely handicaps this process.

The next process to be considered is the sulphite wood fibre or acid process. This is the last to be developed, although prior in invention to the soda process. Benjamin C. Tilghmann, the inventor of the sand blast, and a resident of Philadelphia, took out a patent in 1867, for the treatment of wood with bisulphite of lime, in order to remove the incrustating materials. He never developed the process, and consequently the patent lapsed and became public property.

Dr. Mitzcherlich, the celebrated German chemist, was, at the time of his death, carrying on a series of experiments with bisulphite of lime as a boiling solution for wood pulp. His nephew, a paper-maker, and present owner of the Mitzcherlich patents, appropriated the results of these experiments, and took out a set of patents on the process and on the machines and boilers connected with it. His patent on the process was cancelled some years later, as it was identical with Tilghmann's. He cuts his wood into discs and prepares his solution in towers, but comparatively little is known of the process, as all employees and licensees are under oaths of secrecy. The other patent sulphite processes are the Francke, the Ekman, the Graham, the Olive and Partington, the Flodquist, and the Ritter-Kellner. In all these, except the Ekman, the chemical employed is bisulphite of lime, and the patents are on the boilers, the wood-preparing machinery and the method of preparing the solution. Mr. Ekman employs bisulphite of magnesia. Of these processes the Francke and the Ritter-Kellner are the most important and practical; the Ekman, the Graham, and the Olive and Partington are employed in a few mills.

The following descriptions of the sulphite process and the figures of cost refer to the Francke system, but are generally applicable to the other practical methods. The wood comes to the mill in six-foot lengths with six strips of bark peeled off to facilitate the complete barking. These sticks are cut by a circular saw into two-foot lengths, which are passed to the borers, who bore out all knots with machine augurs. They are then barked by a machine which is really a revolving plane. It is an abso

lute necessity to have all bark removed, as nothing in the process and touches it; it comes out from boiling and bleaching unscathed, appears as a brown speck in the paper. The clean wood is fed into a simple vertical barking plane that cuts it into small chips, which pass slowly down an inclined shoot, along each side of which women are stationed to pick out knots that may have escaped the borers. It is now ready for the boiler.

The solution employed to boil the wood in is bisulphite of lime, this is a combination of 2 parts of sulphurous acid with 1 part of lime. It is a colourless pungent liquid, which emits sulphurous acid gas, and in time by exposure to the air becomes oxidized into sulphate of lime, the sulphurous acid also forming sulphuric acid. When boiled with wood at a pressure of 60 to 65 lbs. to the square inch, the solution is forced into the pores. The sulphurous acid is set free, and the lime combines with the resin of the wood to form a soluble resinate. Small quantities of sulphate of lime are produced during the boiling, and care has to be exercised to prevent its being mixed with the fibre. One style of boiler is especially designed to this end.

The preparation of the solution is simple. Sulphurous acid is generated from pyrites in ovens similar to those used in the manufacture of sulphuric acid. The fumes of acid are led to the bottom of a tower 40 to 90 feet high. This tower is divided into several shafts by solid partitions, and each shaft is provided with gratings at certain points. On these gratings are piled blocks of limestone. A large tank at the top of the tower supplies a steady rain of water that trickles down over the limestone. The fumes of sulphurous acid rise through the shafts and combine with the limestone to form bisulphate of lime, which is absorbed in the water, and runs into tanks at the bottom of the tower, whence it is conveyed to the boilers.

The boilers used by Mr. Francke are 40 ft. long and 7 ft. diameter, of steel, lined with inch of lead. They are set horizontally on friction rollers, and revolved by spur gear. Steam is admitted by a pipe with a trunnion joint situated in the axis of the boiler. The lead lining in these boilers is the greatest source of trouble connected with their management. Everything that the solution comes in contact with must be protected with lead. The coefficient of expansion of lead is more than twice that of iron, and in addition, lead is so inelastic that on cooling it does not return to its original size. When expanded by the heat of the steam, the lead plates confined by the iron shell must buckle. This repeated buckling and straining of the plates rapidly crystallizes them, and then they crack, letting the solution through to the iron.

« PreviousContinue »