The Manufacture of Chemical Wood Pulp.
Most vegetable fibres are converted into pulp by alkaline processes, that is by digesting the raw material with caustic soda and similar alkaline substances. Wood may be treated in two ways, one of which is the ordinary soda process, and the other an acid treatment requiring the use of sulphurous acid.
Preparation of the Wood.—The logs of wood are cut up and barked exactly as in the case of mechanical pulp. The short two-foot pieces are then cut up into small flakes about one inch square and half an inch thick by means of a machine known as a chipper. This is similar in construction to a barker, consisting of a heavy iron disc rotating at a high speed inside a stout cover. The disc revolves in a vertical position, and three projecting knives slice up the logs into flakes. For this purpose the disc is provided with three slots which radiate from the centre towards the circumference for about 12 inches. The knives can be adjusted so that they stand up through the slots and above the surface of the disc to any required distance.
In order to ensure uniformity in the size of the chips, the practice is frequently adopted of sifting the wood leaving the chipper. The sieve is a large skeleton drum, the outer surface of which is made of a coarse wire cloth capable of passing all pieces of the size mentioned. Larger chips and pieces are retained in the drum as it revolves in a horizontal position and only fall out on reaching the extreme end of the machine.
The Digesters.—The object of boiling the wood under pressure with chemicals is to dissociate the valuable fibrous portion of the plant from the resinous and non-fibrous portion. In this process the wood loses half its weight, the yield of pulp being about 50 per cent., and the remainder is dissolved out by the chemical solution. The conditions of treatment are extremely varied in character, the quality of the pulp produced varying in proportion.
The digesters are either spherical, cylindrical, or egg-shaped, being constructed to revolve at a slow rate of speed, or fixed permanently in an upright position. Spherical boilers are usually 9 or 10 feet in diameter, the cylindrical digesters being 40 or 50 feet high and 12 or 15 feet diameter, the larger ones being capable of taking 20 tons of wood for each operation.
Fig. 33.—Wood Pulp Digester, partly in elevation, partly in section.
For the alkaline process the interior of the digester does not require any special treatment, but with the acid process the internal portion of the boiler is carefully lined with a thick layer of acid-resisting brick and cement.
The contents of the digester are heated by means of high-pressure steam, which is blown direct into the mass or passed through a coil lying at the bottom of the vessel. In the former case the steam is condensed by the liquor, the volume of which is consequently increased, while in the latter case the condensed steam is drawn off continuously from the pipes. Each system has its own particular advantages.
Different Kinds of Chemical Wood Pulp.—According to the method of treatment so the quality of the pulp varies. The chemicals used, the system of boiling, the temperature of digestion, the strength of the solutions, the duration of the cooking period, and, last but not least, the species of wood, are all determining factors in the value of the ultimate product.
Soda Pulp.—This is prepared by digesting wood with caustic soda in revolving boilers for eight or ten hours at a pressure of 60 to 80 lbs.
Sulphate Pulp.—Prepared by digesting the wood with a mixture of caustic soda, sulphide of soda, and sulphate of soda.
Sulphite Pulp.—The process most generally adopted for the manufacture of wood pulp is the treatment of the material in brick-lined digesters with bisulphite of lime for eight to nine hours at a pressure of 80 lbs.
Mitscherlich Pulp.—This is sulphite pulp prepared by digesting the wood at a much lower temperature and for a longer period than the ordinary sulphite. The steam is not blown direct into the mass of wood, and the pressure seldom exceeds 45 or 50 lbs., the time of boiling occupying 45 to 50 hours. So called from the name of the inventor.
Sulphite Wood Pulp.—This name is given to pulp prepared by digesting wood with solutions containing sulphurous acid, or salts of sulphurous acid. The acid is produced by burning sulphur or certain ores containing sulphur, such as copper or iron pyrites, in special ovens. The most modern form of oven consists of a cylindrical cast-iron drum revolving slowly in a horizontal position on suitable bearings. The sulphur is thrown at intervals, or fed automatically, into the oven, the amount of air being carefully regulated to avoid the formation of sulphuric acid in the later stages of preparation. The sulphur is also burnt in stationary ovens which consist of flat shallow closed trays.
Fig. 34.—View of ordinary Sulphur-burning Ovens.
The hot sulphurous acid gas passes through pipes and is cooled, after which it is brought into contact with water and lime for the production of the bisulphite of lime. This is accomplished by one of two methods as follows.
Tower System.—The cool gas is drawn into high towers usually built of wood, 7 or 8 feet diameter, which are filled with masses of limestone. From tanks at the top of each tower a carefully regulated quantity of water flows down upon the limestone and absorbs the ascending column of gas, this being drawn into the tower from the bottom. The limestone is simultaneously dissolved, and the liquid which flows out from the pipes at the bottom of the tower consists of lime dissolved in sulphurous acid, together with a certain proportion of free sulphurous acid. This is generally known as a solution of bisulphite of lime.
Tank System.—The somewhat costly tower system has in many cases been superseded by the use of a number of huge wooden vats, 10 to 12 feet diameter and 8 to 10 feet high. These tanks are filled with water and a known quantity of slaked lime. The gas is forced into the tanks by pressure or drawn through by suction, and the conversion of the milk of lime into bisulphite of lime proceeds automatically. In order to ensure complete absorption the gas passes through the tanks in series, so that the spent gases leaving the vats do not contain any appreciable amount of sulphurous acid.
In order to obtain pulp of uniform quality it is necessary that the liquor should be of constant composition. The formula differs in the various mills according to the conditions which are found most suitable.
Sulphite Digesters.—The almost universal form of boiler employed in cooking wood by the sulphite process is a tall cylindrical vessel of about 50 feet in height, and 14 to 15 feet internal diameter, lined with acid-resisting brick.
This form of digester is capable of holding 20 tons of wood at one charge, yielding 10 tons of finished pulp.
The chipped wood is discharged into the digesters from huge bins erected just above the openings to the digesters, so that the latter can be filled without any delay and the requisite quantity of sulphite liquor added.
The manhole or cover is at once put on, securely fastened, and steam turned on gradually until the pressure reaches 70 or 80 lbs., at which pressure the cooking is steadily maintained. The progress of the operation is watched and samples of the liquor drawn from the boiler at intervals to be tested, so that the boiling may be stopped when the results of the testing show the wood is sufficiently cooked.
There is no special difficulty in this operation, provided the necessary conditions are observed. It is important that the wood should be dry, and that the proportion of sulphite liquor per ton of dry wood should be constant. If the wood happens to be wet, due allowance must be made for the excess water and a somewhat stronger liquor used in order to compensate for this. Other precautions of a similar character are observed in order to minimise the danger of an insufficiently cooked pulp.
Washing.—When the pulp has been boiled, a process which generally occupies seven or eight hours, the steam is shut off and the contents of the boiler blown out into large vats known as blow-out tanks, the pressure of steam remaining in the digester being sufficient to empty the softened pulp in a few minutes. Much of the spent sulphite liquor, now containing the dissolved resinous and non-fibrous portions of the original wood, drains away from the mass in the tank, and then copious supplies of clean water are added in order to wash out the residual liquors which it is essential to remove.
Numerous other devices are employed to ensure the complete washing of the boiled pulp.
Screening.—The production of a high-class pulp necessitates proper screening to eliminate coarse pieces of unboiled wood and the knots, the latter not being softened completely. The methods adopted vary according to requirements.
For uniform clean pulp that can be bleached easily the material from the blow-out tanks is, after washing, mixed with large quantities of water and run through sand traps, which consist of long shallow wide boxes provided with slanting baffle-boards to retain knots and large pieces of unsoftened wood, the pulp thus partially screened being subsequently treated in the proper screening apparatus.
Sometimes the washed pulp is sent direct to the screens and the well-boiled fibres sorted out by a system of graded screens, which separate the completely isolated fibres from the bulk and retain the larger pieces, these being broken down in a suitable engine and put back on the screens.
The machinery employed for screening chemical pulp is identical with that used for the treatment of mechanical wood pulp.
Finishing.—The ordinary sulphite pulp is worked up into the form of dry sheets for the market and not sent out in a wet state as the mechanical wood. There are several practical disadvantages in preparing the latter in a dry condition which do not, however, occur with chemical pulp.
Hence the pulp after being screened is not pressed but submitted to a different process. From the screens the mixture of pulp and water, the latter being present in large quantity, is pumped into a concentrator, or slusher, as it is termed, by means of which some of the water is taken out.
The slusher consists of a wooden box divided into two compartments by a vertical partition. In the larger compartment a hollow drum covered with a fine wire cloth revolves, the construction and purpose of which are precisely the same as that of the wet press machine used for mechanical pulp.
As the drum revolves the pulp adheres to the outer surface, while the water passes through the wire cloth. The drum is not completely immersed in the mixture, so that the skin of pulp is brought out of the water by the rotation of the drum. When this takes place the contact of a wooden or felt covered roll which revolves on the top of the drum causes the pulp to be transferred from the drum to the roll. The wet pulp is continuously scraped off by an iron bar or doctor, as it is called, resting on the surface of the roll, and it finally drops into the second compartment of the slusher in a more concentrated form ready for the drying machine.
Drying.—The mass of wet pulp from the slusher is conveyed into a circular reservoir or stuff chest, which serves to supply the machine used for converting the pulp into dry sheets.
The machine is to all intents and purposes a Fourdrinier paper machine, and the process is similar to that used for the manufacture of paper. The pulp flows in a continuous stream on to a horizontal endless wire, which carries it forward as a thin layer; the water drains through the meshes of the wire, further quantities being removed by suction boxes, which draw away the water by virtue of the vacuum produced by special pumps. The wet sheet then passes between the couch rolls which compress the pulp, squeezing out more water, and then through press rolls, which finally give a firm adherent sheet of pulp containing 70 per cent. of water. The sheet is dried by passing over a number of steam heated cylinders, which cause all the moisture to evaporate from the pulp. At the end of the machine the dry pulp is cut up into sheets of any convenient size, and packed up in bales of two or four cwts.
Mitscherlich Sulphite Pulp.—This term is applied to sulphite wood prepared by submitting the chipped wood to a comparatively low pressure for a long period. The wood is placed in the stationary upright form of digester with the requisite amount of liquor, and the heating produced by the passage of steam through a leaden coil lying at the bottom of the digester, so that the steam does not condense in the liquor but in the coil, from which it is drawn off. The pressure seldom exceeds 45 lbs. but the duration of the cooking is thirty-six to forty-eight hours. The boiler is not emptied under pressure, but the pulp is discharged from the digester after the pressure has been lowered, and the manhole taken off. The contents are usually shovelled out by the workmen.
The pulp is carefully washed, screened and made up into wet sheets on the ordinary wet press machine. This pulp is never dried on the Fourdrinier like the common sulphite, as its special qualities can only be preserved by the treatment described. This pulp is particularly suitable for parchment papers, grease proofs and transparent papers.
Soda Wood Pulp.—The chipped wood is boiled in stationary or revolving digesters for eight or nine hours at a pressure of 70 or 80 lbs. A solution of caustic soda is employed, about 16 to 20 per cent. of the weight of the wood being added to the contents of the digester. Live steam is blown direct into the mass, and after the operation the spent liquor is carefully kept for subsequent treatment. The pulp is washed in such a manner that the amount of water actually used is kept down to the smallest possible volume consistent with a complete removal of soluble matters. This is done in order that the spent liquors may be treated for the recovery of the soda.
Recovery of Spent Liquors.—When wood is cooked by the soda and sulphate processes the solutions containing the dissolved organic matter from the wood can be evaporated, and the original chemical recovered. In the case of soda pulp the method of treatment is as follows: the spent liquors and the washings are evaporated by means of a multiple effect vacuum apparatus to a thick syrup. The concentrated liquor produced is then burnt in special furnaces, all the organic matter being consumed, leaving a black mass which consists mainly of carbonate of soda. The mass is washed with water to remove the carbonate which is afterwards converted into caustic soda by being boiled with lime.
Fig. 35.—Spruce Wood Pulp.
The spent liquors from the sulphite process have no value, for they cannot be recovered by this method. At present the whole of the sulphur used and the organic matter dissolved from the wood is lost. This means the loss of about 250 to 350 lbs. of sulphur and nearly 50 per cent. of the weight of wood for every ton of pulp produced.