Читать книгу The Art of Paper-Making - Alexander Watt - Страница 7

Table of Contents

TREATMENT OF RAGS (continued).

Boiling Rags.—Bertrams' Rag Boiler.—Donkin's Rag Boiler.—Washing and Breaking.—Bertrams' Rag Engine.—Bentley and Jackson's Rag Engine.—Draining.—Torrance's Drainer.

Boiling Rags.—To remove greasy matters, and also to dissolve out the cementing substances from the stems of flax and shell of the cotton, the rags are next boiled in a solution of caustic soda, caustic lime, or a mixture of carbonate of soda and lime. The boiling has also the effect of loosening the dirt contained in the rags, whereby the colour of the material is greatly improved, while at the same time it is rendered more susceptible to the action of the bleaching agent. Strong linen rags will sometimes lose from one-third to one-fifth of their weight by the process of boiling. The vessels for boiling rags are of various construction, and have been the subject of numerous ingenious patents. These boilers are either cylindrical or spherical, and are also stationary or rotary—the latter form being devised for the purpose of keeping the caustic alkali solution freely diffused throughout the mass of fibre during the boiling.

Fig. 9.

Bertrams' Rag Boiler.—An illustration of a spherical boiler, as manufactured by Bertrams, Limited, of Edinburgh, is given in Fig. 9. The shell of this boiler is made from malleable iron, is 8 feet in diameter and 9 feet deep. The boiler is constructed on what is termed the "vomiting" principle, by which a free circulation of the alkaline liquor is constantly maintained. These boilers are made to withstand any pressure of steam, but the size given is usually worked at from 35 to 45 lbs. pressure, and carries about 30 cwt. of dry esparto.

Fig. 10.

Donkin's Rag Boiler.—The spherical boiler of Messrs. Bryan Donkin and Co. is shown in Fig. 10. Being of a spherical form, it is twice as strong as a cylindrical boiler of the same diameter and thickness. The plates used are, notwithstanding, of the usual substance, thus rendering it perfectly safe, durable, and suitable for high-pressure steam. The spherical shape also allows the rags to fall out by themselves when the boiler is revolving with the cover off. Within the boiler are strainers to carry off the dirt, and lifters to agitate the rags during the process of either boiling or washing. To avoid cement, or even lead joints, the gudgeons and the boiler are turned true in the lathe to fit each other, the joints being simply made with red lead. These boilers are usually about 8 feet in diameter, and are capable of boiling from 20 cwt. to 25 cwt. of rags. The idea of giving motion to the boiler, so as to insure a perfect mixture of the rags and the caustic liquor, is of American origin, and was first introduced into this country by Messrs. Bryan Donkin and Co. It is usual to fix the boiler so that it can be fed with rags through a trap in the floor above, while the boiler is in a vertical position and the lid removed. The trunnions are hollow, to admit the introduction of steam, alkaline ley, or water, and its rotary motion, which is about three times in two minutes, is given by the gearing on the left of the illustration.

The alkalies used for boiling rags are either caustic soda, soda ash, slaked lime, made into a cream and sifted, or a mixture of slaked lime and carbonate of soda. A description of the preparation of caustic soda ley will be found in another chapter. It has been customary at most of the larger paper-mills to purchase their caustic soda direct from the alkali manufacturers, who supply it in a solid form enclosed in iron drums, hermetically closed, which are broken and the contents removed and dissolved when required for use. As to the strength of caustic soda liquor to be used for boiling rags, this is regulated according to the nature and condition of the material, and the quality of the paper it is intended for (see p. 34). For the finest papers the caustic soda should be perfectly pure, and as there are various grades of this chemical substance sold by the alkali makers, only the purer qualities are used for the better kinds of paper. The proportion of caustic soda per cwt. of rags varies to the extent of from 5 to 10 per cent. of the former to each cwt. of the latter, the coarser materials, of course, requiring more alkali than those of finer quality. In cases where rags are boiled in an open boiler—as was formerly the case—a much larger proportion of caustic soda would be required than when the boiling is conducted under high pressures, as is now very generally the custom. In boiling the finer qualities of rags, less pressure of steam is required than for the coarser qualities, and the heat being proportionately lower, there is less destruction of the fibre. Some paper-makers prefer to boil the rags with caustic lime only, in which case the lime, after being slaked in the usual way, is mixed with water until it attains a milky consistence, when it is passed through a sieve to separate any solid particles which may be present. About the same percentage of lime may be used as in the former case.

When a mixture of lime and carbonate of soda is used, a method much adopted on the Continent, the lime should be well screened from lumps before being mixed with the soda. The usual method of preparing this mixture is as follows:—A wooden tank, 15 feet long, 5 feet wide, and 4 feet deep is divided into three compartments, each of which has a false bottom perforated with ½-inch holes to keep back lumps, stones, pieces of coal, etc., which frequently abound in the lime. The fresh lime is put into the first compartment, where it is slaked with water in the usual way; the resulting powder is then put into the next compartment together with sufficient water, where it is agitated until converted into what is technically termed "milk of lime." In the partition which separates the second from the third division is a movable sluice, through which the milk of lime flows into the third compartment; in this is fitted a revolving drum, similar to the drum-washer of the breaking-engine, through which the milk of lime which flows from the sluice becomes strained, and is lifted in the same way as water is lifted by the drum-washer of the breaking-engine, and is thence discharged through a pipe into the rag boilers; an additional straining can be effected by placing a fine wire strainer over the mouth of this pipe leading to the boiler, which will prevent objectionable particles from entering the boiler. Each compartment is provided with a large waste pipe, through which, by the aid of a sufficient supply of water, all impurities which have been rejected by the drum are carried away. The soda solution is prepared by dissolving the required proportion in water, and the resulting liquor, after careful straining, is introduced into the boiler to which the charge of rags has been given; the head of the boiler is then fixed in its position and steam turned on, until a pressure of about 20 to 30 lbs. to the square inch is attained, and the boiling kept up for two to six hours, according to the quality of the rags. By the Continental system of boiling rags, for No. 1 stuffs, 216 lbs. of lime and 114 lbs., of 48 per cent., soda ash are used for every 4,000 lbs. of rags; for Nos. 3 and 5 stuffs, 324 lbs. of lime and 152 lbs. of soda ash are used; and for No. 4 stuff 378 lbs. of lime and 190 lbs. of soda ash, and the boiling in each case is kept up for twelve hours, under a pressure of 30 lbs., the operation being conducted in boilers which revolve horizontally.

In boiling the finest qualities of rags, it is considered preferable to boil with lime alone, which is believed to be less injurious to delicate fibres than caustic soda. Dunbar[14] gives the following proportions of 70 per cent. caustic soda per cwt. of rags:—

S. P. F. F. F. is boiled with lime alone, then washed in the boiler, and again boiled with 2 per cent. of soda ash.

S. P. F. F. is	boiled	with	12	lbs. of	(70 per cent.)	caustic soda	per cwt.
S. P. F	"	"	14	"	"	"	"
Fines	"	"	7	"	"	"	"
Seconds	"	"	6	"	"	"	"
L. F. X.	"	"	20	"	"	"	"
C. L. F. X.	"	"	27	"	"	"	"
C. C. L. F. X.	"	"	30	"	"	"	"
F. F.	"	"	15	"	"	"	"

These are all boiled at a pressure of from 20 to 25 lbs. for 10 hours, in stationary boilers without vomit, and also in boilers revolving horizontally. In some mills, where the best qualities of paper are made, iron boilers are objected to, as small particles of oxide of iron are apt to become dislodged from the interior of the boiler, and produce discolouration of the paper. In such cases wooden vats, with mechanical stirrers, are employed; sometimes a jacketed boiler is used.

Figs. 11 and 12.

Washing and Breaking.—The removal of the dirty water resulting from the boiling is effected in the washing and breaking engine, or "rag engine," as it is commonly called, which is constructed on the same principle as the beating engine, but is provided with an extra drum, called the drum-washer, which, being covered with wire gauze, allows the washing waters to escape without permitting the fibrous stuff to pass through. The rag engine, having been invented by a Dutchman, acquired, and still retains, the name of the Hollander, and although it has been considerably improved upon, its principle is still retained in the modern engines, of which there are many different forms. The ordinary rag engine, Figs. 11 and 12, consists of a cast-iron trough A, about 10 feet long, 4½ feet wide, and 2½ feet deep, and rounded at the ends, and is firmly bolted to a wooden foundation. It is provided with a partition termed the midfeather B, of such a length as to have the trough of uniform width round it. A cylinder, or roll, C, furnished with a series of steel knives, rotates in one of the divisions formed by the midfeather, and the floor of the trough in this division is inclined in such a manner as to cause the pulp, as it travels, to pass under the roll. Beneath the roll is the bed-plate, which is fitted with a series of steel knives c c similar to those on the exterior of the roll. The distance between the knives of the roll and the bed-plate is regulated by levelling screws, which are so adjusted that both ends of the roll are raised at the same time, which is a great improvement upon the older types of breaking engines in which only one end of the roll was raised, whereby the knives became unequally worn. By the present method of regulating the distance between the respective sets of knives, any required degree of fineness can be given to the fibrous substances treated. The roll is generally caused to rotate at a speed of about 230 revolutions per minute, causing the water and rags to circulate in the engine and to be constantly under the action of the knives. In the other division F F of the trough is the drum-washer H, which, being covered with fine gauze wire, allows the water to enter, but keeps back the fibrous material. The ends of the drum are formed of two discs of wood, generally mahogany, upon which the coarse gauze is fastened as a backing, and this is covered with the fine wire gauze. The interior of the drum is sometimes furnished with a series of buckets, which conduct the water to a trough in the axis of the drum, by which it is led away. This is also accomplished by dividing the interior of the cylinder into compartments by means of a partition. The drum-washer is so arranged that it can be wholly raised out of the trough, which is necessary in certain parts of the operation, when the removal of the liquid is not required; or it can be partially raised, or otherwise, according to requirement. The floor of the compartment containing the roll C is inclined at D, so as to cause the pulp to pass directly under the roll, and at D′ is the backfall, over which the pulp travels to the opposite side of the midfeather.

In working the rag engine, it is first partly filled with water, and then set in motion; the boiled stuff is then gradually put in, and a constant supply of clean water is run in from a cistern provided with means of preventing sand or other impurities from finding their way into the engine. It is of the utmost importance that the water should be abundant and of good quality, more especially as the material (rags) is mostly required for making the finer qualities of paper. In this respect the county of Kent and a few other localities on the chalk formation are considered specially suitable for this particular manufacture.

With respect to the driving of the engines, this was formerly effected by what is called toothed gearing, but cog-wheels were afterwards replaced by iron spur-wheel gearing, which enabled manufacturers to drive four or more engines from one source of power, by continuing the line of shafting and spur-wheels; but even with small rolls the wear and tear on this system was considerable, while it was quite inadequate to the driving of a number of large rolls of 30 inches in diameter, such as are now used. The introduction of belt-gearing, by Messrs. G. and W. Bertram, proved to be a great improvement on the older system, and it is found that the rags are broken not only more uniformly, but in less time, as the rolls work more steadily on the plates than with any system of wheel-gearing, while the various working parts of the engine last longer than when subjected to the vibrating action of wheel-gearing.

Fig. 13.

Bertrams' Rag Engine.—This engine, of which a drawing is shown in Fig. 13, may be used either as a washing and breaking engine, potcher, or beater. It is provided with double lifting gear, and has "all sweeps, curves, and angles" of the most improved design to save lodgments and ensure steady and thorough travelling of the pulp. The drum-washer is shown lifted by rack and pinion and worm gear, and empties down the midfeather direct to mouthpiece. The emptying can be done by spout and pipe, or by a chamber cast on the engine, down back or front side, as well as through the midfeather; but it is not advisable that it should be emptied down the midfeather if the rag engine is to be used as a beater.

Fig. 14.

Bentley and Jackson's Rag Engine.—This form of engine is shown in Fig. 14. The trough is of cast-iron, and made whole, and the engine can be obtained of any required dimensions. The trough is provided with a sand-well, cast-iron grate, and cock in front of the roll, and a sand-well, cast-iron grate, and brass valve on the back of the midfeather, a brass let-off valve and a brass waste-water valve. The bottom of the trough is "dished," to prevent the stuff from lodging. There are two movable bridge trees, fitted with pedestals and brass steps, and wrought-iron lifting links and screws, worm-wheels, worms, cross-shaft and hand-wheel for simultaneously lifting the roll on both sides. The roll is covered by a polished pitch-pine cover. The drum-washer may have either iron or wooden ends, has strong copper brackets, and is covered with brass backing and covering wires, mounted on a wrought-iron shaft, and carried by cast-iron stands, fitted with improved lifting gear, driving-wheels, and pulley.

When the engine is set in motion by the revolving shaft or spindle, the combined action of the knives of the roll and bed-plate causes the rags, which circulate in the water, to be gradually cut into small fragments, and the operation is kept up until the rags are converted into what is technically termed half-stuff. While this process is going on, fresh water is constantly supplied by a pipe at the end of the washing-engine; and when it is found that nothing but clear water escapes from the drum-washer, this is raised, and the spindle bearing the roll is lowered, so as to bring the respective knives closer together, to enable them to cut the reduced material still finer.

Draining.—When the material is sufficiently broken, as it is termed, the engine is then emptied by means of its valves, and the contents run into large vats or drainers, furnished with perforated zinc floors, in which it is allowed to drain thoroughly; and in order to remove the water more effectually, the pulp is afterwards pressed, either by an extractor or a centrifugal drainer, which dries it sufficiently for gas-bleaching, or for treatment in the potcher or poacher. This is a larger engine than the washer, and instead of the cylinder and bars, has a hollow drum which carries on its periphery a number of cast-iron paddles, which thoroughly agitate the pulp, and thus render it more susceptible of being freely and uniformly acted upon by the bleaching agent. The drum-washer of this engine should have a finer wire than is used for the breaker.

Torrance's Drainer.—This machine, which has been extensively used, is manufactured by Messrs. J. Bertram and Son, of Edinburgh. It consists of a perforated cylindrical box, enclosed in a fixed case, which revolves at about two hundred and fifty revolutions per minute. The machine is capable of treating about 4 cwt. of pulp per hour.