BEATING OR REFINING.
Beating.—Mr. Dunbar's Observations on Beating.—Mr. Arnot on Beating Engines.—Mr. Wyatt on American Refining-Engines.—The Beating Engine.—Forbes' Beating-Engine.—Umpherston's Beating Engine.—Operation of Beating.—Test for Chlorine.—Blending.
Beating.—One of the most important operations in the manufacture of first-class paper is that of beating, by which the half-stuff becomes reduced to a fine state of division, and the fibres which, in the condition of half-stuff, are more or less loosely held together in a clotted state, become separated, and are thus put into a condition in which they will intertwine with each other, or felt, as it is termed, when submitted to the vibratory motion of the wire-cloth of the paper machine. The beating-engine, or beater, as it is commonly called, much resembles in construction the washing- and breaking-engine, but since it is required to still further reduce the pulp to a condition suitable for paper-making, the knives of this engine are more numerous and are made to revolve more rapidly. In this engine the half-stuff is cleansed from bleach, hydrochloric or sulphuric acid—whichever acid may have been used in the bleaching—chloride of calcium, and the various products resulting from the decomposition of the chloride of lime. In this engine, also, the loading, sizing, and colouring materials are worked up with the pulp, and the stuff fully prepared for its final transfer direct to the paper-machine. Before describing the various forms of beating-engines which have been from time to time introduced, including some of the most recent types, to which special attention will be drawn, we purpose quoting some observations of well-known experts in paper manufacture which will be read with interest, since they fully explain the importance that attaches to the proper manipulation of the beating-engine for the production of paper of high quality.
Mr. Dunbar's Observations on Beating.—There is no operation of the paper-mill that requires more careful attention and experienced judgment than that of beating, or refining, to bring the pulp to the finest possible condition for paper-making; in this department, Mr. Dunbar urges, "none but thoroughly efficient men should be employed, for it is here that the paper is really made—that is, the quality of the paper produced at the paper-machine will be in proportion to the treatment the material has received; and if the half-stuff sent to the beating-engines is not subjected to judicious manipulation and careful preparation for the special paper to be made, all future doctoring will prove unsatisfactory."
Mr. Arnot on Beating Engines.—On this subject Mr. Arnot says:—"Upon the management of the beating-engine the character of the paper produced largely depends. What is wanted is not a mincing or grinding of the fibre, but a drawing out or separation of the fibres one from another; in fact, the name of the machine indicates pretty accurately the nature of the action required—beating. Long, fine fibres can only be produced [obtained] by keeping the roll slightly up off the bed-plate, and giving it time to do the work. Sharp action between the roll and the bed-plate will, no doubt, make speedy work of the fibre, but the result will be short particles of fibre only, which will not interlace to make a strong felt. Indeed, the action I refer to will reduce the long, strong fibre of linen to little better than that of wood or straw. Practice and careful observation can alone make a good beater-man, and for the finer classes of paper none but careful, experienced men should be entrusted with the management of the beating-engine. Sometimes the operation is conducted in two successive engines, the first being called the intermediate beater, but I have hitherto failed to see wherein the advantage of this system lies. The time usually occupied in beating esparto for printing-paper is about four hours, while for rags the time may vary from four to twelve hours, or even more." This, however, depends upon the nature of the rags themselves, and the purposes to which they are to be applied.
Mr. Wyatt on American Refining-Engines.—Referring to the engines adopted in America, Mr. Wyatt says:—"There are various modifications of the original Jordan, the principal ones being the Marshall, Jeffers, and improved Jordan; but I gathered that experience proves the Jordan type to be the most practical and efficient in the end, and is one of the most generally used. One Jordan is required for each machine, refining all the stuff supplied to it. The roll, or plug, runs from 350 to 400 revolutions per minute, the horse-power consumed varying from 25 to 40 horse-power according to the work done, and an engine will do up to 1,000 lbs. of pulp per hour. The time saved in the beating-engine by the use of the Jordan is just about one-third of what would otherwise be necessary, that is to say, pulp requiring otherwise six hours beating only takes four hours if finished in the Jordan. The half-beaten pulp is emptied into a stuff-chest, and the Jordan is furnished with a small stuff-pump and service-box, just as at the paper-machine what the Jordan does not take flows back again into the chest: the pulp from the Jordan is run into the ordinary machine stuff-chests. The finished pulp can be taken from the Jordan at three different levels from the circumference of the roll, or plug. If the pulp is wanted 'free,' it is drawn from the bottom of the engine; if wanted 'wet,' or well greased, it is drawn from the top; and if medium from the centre."
The Beating-Engine.—The ordinary form of beater consists of a cast-iron trough 13 feet 6 inches long × 6 feet 6 inches wide, and the bottom is dish-shaped, so as to prevent the pulp from lodging, which would inevitably be the case if the bottom were flat, as the pulp would be apt to lodge in the angles formed by the junction of the bottom with the vertical walls of the trough. The iron trough is fitted with a cast-iron roll, 3 feet 6 inches × 3 feet 6 inches, which is provided with 69 "roll-bars," or knives, arranged in 23 groups of 3 bars each; this roll is suspended upon a malleable iron shaft 5 inches in diameter, resting upon side levers; suitable gearing is attached by which the roll can be lifted or lowered at will, the action being uniformly equal on both sides, by which the knives of the roll are kept uniform with those of the bed-plate beneath. The bed-plate, furnished with 20 steel knives, of the same length as the roll, is placed immediately beneath the roll. When the knives of the bed-plate are straight they are fitted into the plate-box at an angle, but in some cases they are bent at a slight angle, when they are termed elbow plates. There have been, however, many improvements in the beating-engine introduced of late years, some of which are of considerable importance, and to some of these we will now direct attention. Although our own manufacturers have introduced improvements in beaters which have been fully recognised by the trade, the American engineers have not been behindhand in devising modifications which appear to have some important advantages. The Jordan beater, which has been extensively adopted in the States, consists of a roll in the form of a truncated cone, furnished with knives in the usual way; this revolves in a box of a similar form, fitted with knives in the direction of its length, but at slightly different angles. In this engine the stuff enters at the narrow end through a box having an arrangement which regulates its flow, and the pulp is discharged by several openings in the cover at the wider end. In an engine invented by Mr. Kingsland there is a circular chamber furnished with knives covering its sides; between this is a circular plate, also fitted with knives, which revolves. The stuff enters through a pipe in the centre of one of the sides of the chamber, and flows out through an opening in the opposite side.
Forbes' Beating Engine.—This engine, an illustration of which is given in Fig. 21, is manufactured by Bertrams, Limited, of St. Katherine's Works, Edinburgh. The engine has three chambers, two rolls, and a mixing wheel; the rolls, only one of which is uncovered in the engraving, are fixed in the outer channels, and the mixing wheel is placed in the middle channel. By this arrangement the pulp flows alternately into the two outer channels, and after passing through the rolls again it enters the centre channel at the opposite end.
Umpherston's Beating Engine.—This engine, for which a patent was granted in 1880, has been successfully adopted at the Daily Chronicle and other mills, and presents several important advantages, one of the chief being that it occupies much less ground space than ordinary beating-engines. Indeed, we have heard it remarked of this engine that it will do double the amount of work in the same ground space as the ordinary engine, and this, in some mills, would be a decided advantage. The construction of this beater, a drawing of which is shown in Fig. 22, is thus described by the patentee:—"In the common and almost universal form of engines used for preparing pulp for paper-making, the pulp travels horizontally in a trough with semi-circular ends, and straight sides, partly divided longitudinally by a partition called the midfeather, around which the pulp flows from the back of the roll to its front, where it passes under the roll and over the bottom working-plate, and is again delivered over the back fall to pass again round the midfeather to the front of the roll. In the course of these repeated revolutions part of the pulp near the circumference of the tub has much farther to travel than the part near the midfeather, and consequently is not so often operated upon, and the pulp is thus unequally treated. As an improvement upon this form of tub, I make it so that the pulp passes from the back of the roll to its front through a longitudinal passage under the back fall, the pulp thus moving as through an inverted syphon, the superincumbent weight of the semi-fluid pulp, as delivered over the back fall of the roll, pressing it along this passage and upwards, to enter again in front of the roll. The roll A, bottom plate B, and the form of the back fall C, are similar to those of ordinary engines, but the trough is formed with the passage D under the bottom plate B, so that the semi-fluid contents of the engine, in travelling from the back fall C to the front of the roll A, pass by means of the passage D under the bottom plate B in the direction indicated by the arrows, the superincumbent weight of the semi-fluid pulp, as it is delivered over the back fall C at the back of the roll A, pressing it along the under passage D and upwards to the front of the roll A. The position of a drum-washer is shown at E, and at F is seen a section of the cross shaft for raising or lowering both ends of the roll A simultaneously; G is the roll cover, which may be of any usual form. By this invention the semi-fluid pulp is acted upon in a more effective manner, and its particles are also more equally treated than has hitherto been the case."
The beating-engines are usually driven from a separate engine, but Messrs. Bertrams have introduced a system of direct driving for these engines by which, it is said, there is a considerable saving in power. The accompanying engravings, Figs. 23 and 24, show a series of eight beaters, each carrying 300 lbs. of pulp, driven by one of their compound direct-driving steam-engines, and now being worked at the Forth Paper Mills.
Operation of Beating.—Having referred to some of the more important improvements connected with the beating-engines, we will proceed to explain the operation of beating as briefly as possible. The bleached half-stuff is removed from the tray of the press in caked masses, and in this condition is conveyed in trucks or boxes to the beating-engine. The first thing to be attended to is the removal of the last traces of chlorine from the pulp, which, if not effectually done, would cause injury to the size, and also corrode the strainer plates and wire-gauze of the paper-machine. It is possible to wash out the chlorine by an abundant application of pure water, but this method of removing the chlorine is very tedious and occupies a long time, while it also involves the use of enormous quantities of water—a serious consideration in some mills; to this may be added the still more important fact that by the method of washing out the chlorine a considerable loss of fibre takes place. The plan most usually adopted is to neutralise the chlorine left in the pulp by the application of suitable chemical agents, whereby the chlorine is rendered inert. These agents, technically termed "antichlors," are sometimes objected to, however, although they are in themselves practically harmless so far as their action upon cellulose is concerned. Mr. Arnot, who has considered this subject very thoroughly, says:—"I do not think there is much in this objection, as those agents that are soluble pass through the wire of the machine almost completely, while those that are insoluble are in the finest possible state of division and pearly white. The chemical agent most largely used is hyposulphite of soda, but hyposulphite of lime is also employed, and those agents, known by the name of 'antichlor,' are put into the engine in such a quantity as will ensure the neutralisation of the whole of the chlorine. The products of the reaction, when the soda salts are used, are chloride of sodium (common salt) and sulphate of soda (Glauber's salt), and, when the lime salt is used, chloride of calcium and sulphate of lime, the latter identical with the pearl hardening so well known as a loading agent." From this it will be seen that little or no harm can possibly occur either to the fibre or the metal work of the machine by the employment of the neutralising agents named, and when it is borne in mind that the simple washing of the pulp would occupy the beating-engine for a lengthened period and exhaust a considerable quantity of water—which, as we have said, would in some mills be a serious matter—the adoption of the neutralising method would undoubtedly have the preference.
The engine, being partly filled with water, is set in motion, and the bleached half-stuff introduced in small quantities at a time, each portion being allowed to become thoroughly mixed with the water before the next batch is added. The charging of the beater with half-stuff is kept up until the mass becomes so thick that it will only just move in the trough under the action of the revolving roll. If the beater is of the older type, portions of the pulp are liable to lodge in corners, to remove which the "beater-man" uses a wooden paddle, with which tool he also pushes the slowly moving pulp in the direction of the roll, especially when the stiff mass appears to move too slowly. At this stage the neutralisation of the chlorine in the pulp is effected, which is done by adding a solution of hyposulphite of soda, a little at a time, until the liquor ceases to redden blue litmus paper, strips of which should be dipped into the pulp every few minutes until the paper persistently retains its blue colour. This operation should be conducted with great care, so as to exactly neutralise the traces of chlorine without adding an excess of the hyposulphite of soda. Besides this salt, other substances are used as "antichlors," as, for example, hyposulphite of lime, which is prepared by boiling milk of lime (slacked lime made into a thin mixture with water) and flour of sulphur in an iron vessel until the latter is dissolved, when, after cooling and settling, the resulting solution, which is of an orange-yellow colour, is ready for use. One great objection to the use of hyposulphite of lime, however, is that when decomposed by the chloride of lime remaining in the pulp sulphur is set free, which, mingling with pulp, will impart to it a yellow tint; besides this, in passing over the drying cylinders of the machine the sulphur present in the paper may attract oxygen from the air, converting it into sulphuric acid, which must inevitably prove injurious to the manufactured paper. Sulphite of soda has also been used as an antichlor, and is said to be preferable to hyposulphite of soda,[21] inasmuch as the latter salt is liable to decompose with the liberation of free acid, which is not the case with the sulphite of soda.
Test for Chlorine.—Instead of relying solely upon the litmus paper test when applying the antichlor, the following test for chlorine may also be used with advantage:—Take 2 drachms (120 grains) of white starch, and make it into a paste with a little cold water; then pour over it about half a pint of boiling water, stirring briskly; to this add 1 drachm of iodide of potassium, and stir until dissolved and well incorporated with the starch solution. The mixture is then to be allowed to cool, when it is ready for use. A few drops of this mixture dropped upon a small sample of the pulp will indicate if any chlorine be present by the spot assuming a blue colour; if such be not the case, the pulp may be considered free from chlorine.
During the beating, the roll, which should make not less than 220 revolutions per minute, is lowered, a little at a time, so that the cutting edges of the bars and plate may be brought together gradually and equally until the pulp is reduced to the desired condition. The pulp is made long or short according to the quality of paper to be produced; news papers, which require strength, are made of long-fibred pulp, while writing paper, or paper of fine texture, is made of shorter pulp. The stuff should be what is called "mellowed" in the engine, which is effected by a judicious working of the roll, not lowering it suddenly but gradually, and not much at one time, on the plate, until the pulp attains the fineness required. This is generally arrived at in about three and a half to four hours, though sometimes the beating of pulp from rags is continued for more than double that time. It should be added that if the cutting edges of the roll and plate are brought together suddenly and too closely, the fibre will be cut, and as a consequence the paper produced will be tender.
Esparto, which, in the process of boiling becomes reduced to such a soft condition that the fibres may be readily separated by the fingers, does not require such excessive beating as rags; indeed, the perfect disintegration of the fibres of esparto is practically accomplished in about half the time occupied by rags, and often much less, but this of course depends upon the nature of the esparto itself and upon the thoroughness of the boiling. Wood pulps also require but moderate beating, since the process of disintegration is generally pretty effectually accomplished by the processes to which the raw material is subjected in the course of manufacture into half-stuff, which is the condition in which this paper material is furnished to the manufacturer.
Blending.—To produce papers of the different qualities required by the trade, a system of blending is adopted, which may be effected—(1) by mixing the materials in the raw state, or the rags, previous to boiling; and (2) blending the half-stuff in the beating-engine. The latter method, however, is generally preferred. Sometimes, also, pulps of different character are beaten separately and then mixed in the stuff-chests, where they are mixed as thoroughly as possible before passing on to the machine, but this method would be less likely to ensure a perfect mixture of the respective pulps than would be effected with proper care in the beater. The proportions of the several materials to be blended is also a matter of important consideration. In blending esparto with rag stuff, if the former be in excess it becomes reduced to the proper condition before the latter is sufficiently fine, which causes the rag fibre to appear in "knots and threads" in the manufactured paper. But if the rag stuff be allowed to predominate, the beating is conducted as though no esparto were present, by which, while the rag stuff becomes reduced to the proper length of fibre, the esparto, which is still further reduced, in mingling with the longer fibre of the rags forms what is called a "close" paper. Mr. Dunbar, in his useful little work, "The Practical Paper-maker," furnishes a series of receipts for blending for high-class papers, as also the proportions of colouring matter to be used, which the reader will do well to consult. For news papers, esparto and straw pulps are generally used, in varying proportions according to the nature and quality of the esparto; these proportions have to be regulated according to the judgment of the paper-maker, and vary greatly at different mills. A large quantity of sulphite and other wood pulps are also used, those coming from Scandinavia and Germany being especially suited to the requirements of the English manufacturer. Mechanical wood pulp is also used in a moderate degree—sometimes up to 15 per cent., in some English mills, but it is said that in Germany this paper stock is sometimes used to the extent of 90 per cent.
