In larger plants, the above described mode of extracting the glue-stock with water has been superseded by the use of steam in a cylindrical wrought-iron boiler, twice as high as wide, and capable of withstanding a pressure of three atmospheres. The boiler is furnished with a perforated false bottom underneath which terminates a steam pipe. It is filled from above with previously softened glue-stock and the charging hole hermetically closed. Steam is then gradually admitted and exerts at once a dissolving influence upon the stock. A portion of the steam condenses and forms with the dissolved glue-stock a concentrated jelly which collects between the true and false bottoms.
For the escape of air a cock is provided which is closed as soon as steam commences to escape from it.
Fig. 12.
The advantages of this process are obvious. A larger quantity of glue-stock can be extracted than in the boiler previously described, and there is no danger of injury by scorching and consequent damage to the color of the glue. More highly concentrated solutions are obtained in a shorter time, and the spoiling of the glue solution by too long continued cooking is prevented by drawing off the solution as quickly as formed. The escaping hot vapors may be utilized for drying the glue, softening the raw material, etc., the entire quantity of heat being thus utilized. A further great advantage of this method is that there is less annoyance from badly-smelling vapors than when boiling is done over an open fire. A number of such boilers can be arranged in one room and supplied from a common steam boiler.
Fig. 12 represents a boiler for extracting glue-stock with the use of steam. It is provided with a lid, D, which is removed for charging the boiler. The aperture, E, in front, serves for the removal of the residue. Above the true bottom there is another false bottom, perforated and movable, which can be covered with straw for preliminary filtration. The steam reaches the glue-stock through a pipe which passes through the actual and false bottoms, and is perforated above the latter. The resulting jelly collects between the true and false bottoms, where it is less exposed to the action of hot steam. The escaping steam passes through the pipe, F, which is provided with a stock-cock. The pressure in the boiler is indicated by the manometer, K. After throwing the materials into the boiler they can be covered with warm water, or, after the lid is closed, warm water is introduced from a reservoir through a special pipe and distributed over the material through a rose.
The boiler stands upon a frame sufficiently high to allow of conveniently placing a vessel under the pipe G, through which the jelly is discharged. The vessel, when full, is conveyed to the settling vat, or the arrangement may be such that the jelly is directly run into the settling vat.
In many large plants open jacketed pans heated by steam are still used for treating the material. Fig. 13 shows an arrangement with two of such pans; of course one, or a larger number may be used, according to requirement. In the illustration the pan I on the left is shown in front view, and the pan II on the right, in section. K1 is the actual pan enclosed by the jacket K. Steam circulates in the space between pan and jacket, whereby the stock in the pan is heated. K1, in addition, is furnished with a steam coil S, which may, however, be omitted.
The steam enters through the pipe D, the space between pan and jacket, passes into the coil S, and escapes at b. The water formed by the condensation of steam in the space between pan and jacket, as well as that which runs off at b from the coil S, is carried away by the pipe A.
The pipe L serves for conveying hot water to the pans, and the pipe F for the discharge of the finished glue liquor. The stirrer R, is furnished with two paddles, and is set in motion by a transmission on the ceiling of the room. It serves for keeping the stock in the pans constantly agitated, solution being thus very much promoted.