Steaming and the occasional discharge of melted fat are continued until on testing a sample of the liquor running off, it is noticed to be free from fat. The liquor in the cylinder is then expelled by the pressure of steam, the steam-cock D closed, the manhole A opened, and steam again admitted through D. By the steam-pressure the greater portion of the bones in the cylinder is expelled through the manhole A. The bones coming from the cylinder are pliable and soft, and, after drying, are readily converted by grinding into bone-meal.
For the manufacture of animal charcoal it is of the utmost importance that steaming should be interrupted at the time when the bones are completely degreased. If, however, only fat, glue and bone-meal are to be produced, steaming may advantageously be continued for a longer time.
The longer the bones are subjected to the action of high-pressure steam, the more complete the conversion of glue-yielding substance into glue will be. To be sure, the bone-meal obtained from such bones will contain somewhat less nitrogen than the product from bones not steamed quite so long. However, the content of phosphates will in both cases be the same, and on this depends, in the main, the fertilizing value of bone-meal.
The fluid discharged from the cylinder consists of a mixture of glue-liquor and drops of fat. It is run into a large vat, in which it is kept warm for a few hours, when the fat rises and collects in a coherent mass on the surface. The fat is then drawn off through cocks in the upper portion of the vat, while the glue-liquor is discharged from the bottom of the vat, running first upon a very fine meshed sieve, which retains the coarser bodies held in suspension, and then directly into the evaporator. In the latter the liquor is evaporated to the desired strength, when it is run into the clarifying vats, and finally into the cooling vessels.
With the above-described process, the simultaneous utilization of the bones for animal charcoal is only possible if the crushed steamed bones are passed through a sieve for the purpose of sorting out the granular pieces of suitable size. However, in the process above described, incompact bones are, as a rule, used which give but a small percentage of granulated pieces, and the latter yield an inferior quality of animal charcoal. It is therefore best to use the steamed bones from which the fat and glue have been extracted for the production of bone-meal.
For the manufacture of animal charcoal, the bones have to be carefully sorted, fresh bones rich in organic substance being best for the purpose, and the hardest and thickest pieces should be selected. Previous to carbonization, the bones are degreased by extraction with benzine or carbon disulphide, and then crushed.
Carbonization was formerly effected in iron pots having a capacity of about 25 quarts each. However, by this process a uniform product of good quality cannot be obtained, and, besides, the total quantity of organic substance of the bones is lost. At present carbonization is effected in retorts, whereby large quantities of animal charcoal are in a comparatively short time obtained, and, besides, the products of destructive distillation can be completely utilized. An essential product of distillation is a large quantity of inflammable gases, which can be used for heating the retort-furnace or for illuminating the entire plant, it being, however, best to arrange the conduits so that the gases can be used for either purpose.
A detailed discussion of the methods for gaining and further working of the products of distillation is not within the scope of this work, and only a brief description of a plant for the manufacture of animal charcoal will here be given.
Fig. 48.
