Fig. 24.
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.