Fig. 52.
A lead steam pipe D furnished with several narrow flat outlet pipes placed in the direction in which the stirrer revolves, lies about 4 inches above the bottom of the tank. W is the pipe for the admission of water, S the lead pipe connected with the sulphuric acid reservoir, and A an outlet of boards for carrying off the vapor evolved from the mass in admitting the sulphuric acid. R is a wooden hopper, which serves for the introduction of the bone-ash, and is removed when the tank has been filled, the aperture being closed with a well-fitting wooden lid. The bottom of the tank is furnished with a lead discharge-cock.
Water is run into the tank, the bone-ash being simultaneously introduced through the hopper R, and the stirrer allowed to revolve slowly to effect an intimate mixture. Sulphuric acid and steam are then at the same time admitted. The steam heats the fluid very quickly to the boiling-point, and assists the action of the stirrer, it passing out from the outlet pipes in the same direction in which the stirrer revolves.
When the required quantity of sulphuric acid has been introduced, the admission of steam is interrupted, the stirrer, however, being constantly kept in motion. To keep the mass hot, steam is for a few minutes admitted every hour. When the sulphuric acid has acted for 24 hours, decomposition is complete, and the liquor is discharged through the cock on the bottom of the tank.
For the evaporation of the liquor leaden pans are used, and this operation is continued until the fluid has attained a specific gravity of 1.45. The pans rest upon cast-iron plates covered with a layer of clay or sand, to prevent them from being injured by the fire gases. For heating the pans, the fire gases escaping from the distilling furnace or the bone kiln are utilized.
The fluid having been evaporated to the above-mentioned specific gravity is now mixed with charcoal powder, or rather granulated charcoal of the size of small peas, in the proportion of 20 to 25 parts of charcoal to 100 of liquor. The mixture is then quickly dried in shallow cast-iron pans heated by a direct fire. Much sulphurous acid is evolved during this operation, and provision must be made for carrying off the vapors arising from the pans.
When the mass has been dehydrated so far that it balls together, it is taken from the pan by means of shovels and brought into a copper cylinder which is furnished with a sheet-iron sieve-like bottom through which it is forced into another pan. In this second pan the mass is moderately heated until a sample of it still evolves slight vapors and when pressed with the hand, after cooling somewhat, appears still moist without, however, being sticky. The material is now ready for distilling and 100 parts of concentrated solution of 1.45 specific gravity and 20 to 25 parts of charcoal yield about 77 parts by weight of so-called distilling mass.
It is best to bring the hot mass as taken from the pan at once into the retorts, as it is very hygroscopic and, if allowed to lie exposed to the air, would absorb moisture and require to be again dried. If it cannot be immediately subjected to distillation, it is advisable to keep it in sheet-metal boxes tightly closed.
The liquor obtained in treating bones for the manufacture of glue with hydrochloric acid may, as mentioned in Chapter V, be advantageously utilized in the manufacture of phosphorus. In order to obtain in crystallized form the acid calcium phosphate contained in it, the liquor has to be concentrated by evaporation and as during this operation hydrochloric acid vapors constantly escape, provision must be made for their removal from the workroom. The operation is conducted as follows: The flue of the furnace for distilling the phosphorus, through which the fire gases usually escape to the chimney, is made to communicate with a long low chamber, which can be tightly closed and connects at the other end with a high chimney. The flue is fitted with a slide and by opening the latter, the fire gases are forced to pass through the chamber before reaching the chimney.