Fig. 62.
Marshall's Perfecting Engine.—This engine, a longitudinal section of which is shown in Fig. 62, has been introduced into this country by Messrs. Bentley and Jackson, and is described in Industries[31] as follows:—"The machine, which is the invention of Mr. F. Marshall, of Turner's Falls, Mass., U.S.A., is used in one of the processes of paper manufacture, and has for its purpose the more effectual drawing of the pulp fibre, the clearance of knots from the pulp previous to its delivery on to the paper-making machine, and the saving of time in the treatment of the material. As will be seen in the illustration (Fig. 62), the machine consists essentially of a cast-iron conical casing, bored, and fitted with about two hundred elbowed steel knives, G, placed in sections. At the large end of this conical casing is placed a movable disc, also fitted with about two hundred and ten steel knives, F, and capable of adjustment by means of a screw, worm, worm-wheel, and hand-wheel, E. The revolving cone and disc are of cast iron, fitted with straight steel knives firmly keyed upon a hammered iron shaft, and carefully balanced to prevent vibration. The knives of the revolving cone and disc are brought into contact with the stationary knives by means of the hand-wheel, E, and the disc-knives can be independently adjusted by means of the hand-wheel C, which actuates a screw on the conical casing by means of the worm and worm-wheel shown. The machine is driven by means of a pulley A, and the whole machine is mounted on a cast-iron base-plate. The pulp material enters the engine in the direction indicated by the arrow, B, at the small end of the cone, and is by the rotary and centrifugal action of the revolving cone, propelled to its large end, and during its passage is reduced to a fine pulp by the action of the knives. It then passes through the knives, F, of the stationary and rotating discs, by which the fibres are further crushed or split up, all knots or strings rubbed out, and the pulp effectually cleared previous to its exit through the passage D." We are informed that the machine is capable of treating from 900 lbs. to 1,200 lbs. of pulp per hour. The power required to drive it is estimated at from 40 i.h.p. to 50 i.h.p. when making 300 revolutions per minute. This, however, is dependent on the amount of friction caused between the surfaces of the fixed and revolving knives. The flow space occupied is 12ft. 6in. in length, and 4ft. in width. The perfecting machine, in its complete form, is shown in Fig. 63.
Fig. 63.
[CHAPTER XVIII.]
RECOVERY OF SODA FROM SPENT LIQUORS.
Recovery of Soda.—Evaporating Apparatus.—Roeckner's Evaporator.—Porion's Evaporator—American System of Soda Recovery.—Yaryan Evaporator.
Recovery of Soda.—Probably one of the most important improvements in modern paper-making, at least from an economical point of view, is the process of recovering one of the most costly, and at the same time most extensively used, materials employed in the manufacture—soda. While not a great many years since (and in some mills is still the case even now), it was customary to allow the spent soda liquors resulting from the boiling of various fibres to run into the nearest rivers, thus not only wasting a valuable product, but also polluting the streams into which they were allowed to flow, means are now adopted by which a considerable proportion of the soda is recovered and rendered available for further use. The means by which this is effected are various, but all have for their object the expulsion of the water and the destruction of the organic matters dissolved out of the fibrous substances in the process of boiling with caustic soda solutions. One of the main objects of the various methods of recovering the soda from spent liquors is to utilise, as far as practicable, all the heat that is generated from the fuel used, whereby the process of evaporation may be effected in the most economical way possible. The principle upon which the most successful methods are based is that the flame and heat pass over and under a series of evaporating pans, and through side flues, by which time the heat has become thoroughly utilised and exhausted. When all the water has been expelled, the resulting dry mass is ignited and allowed to burn out, when the black ash that remains, which is carbonate of soda, is afterwards dissolved out, and the alkaline liquor causticised with lime in the usual manner. According to Dunbar, 8 cwt. of recovered ash and 4½ cwt. of good lime will produce 900 gallons of caustic ley at 11° Tw. The liquor is then pumped into settling tanks, from which it is delivered to the boilers when required.