Manufacturers frequently sort the bones in such a way that materials of different quality are obtained. Thick, compact bones are utilized for the manufacture of animal charcoal, a comparatively small percentage of bone-meal resulting in crushing such bones.
Incompact, porous bones, on the other hand, yield not only crummy animal charcoal of less value, but in stamping also a larger percentage of bone-meal than compact bones. Hence they are as a rule directly worked for fat, glue and steamed bone-meal, no attempt being made to convert them into such granular pieces as are suitable for the production of charcoal.
For this purpose, the bones are first broken by a crusher or mill into coarse pieces, and the fat extracted by a special process or together with the glue in one operation. The latter method would seem to be the most suitable, time and labor being thereby saved, but it must be borne in mind that fat extracted by itself brings a much better price than that obtained by steaming, and besides the yield of glue is larger from steamed bones which have been previously degreased.
The crushed bones—whether degreased or not—are subjected to the action of high-pressure steam. The apparatus, Fig. 47, used for this purpose consists of a cylinder of thick boiler-plate, 10 to 13 feet high and 3 to 4 feet in diameter. E and A are manholes, which can be closed steam-tight. The pipe D leads to the steam-boiler and opposite to D is a short pipe, H. The cylinder is further fitted with the perforated false bottom, S, and the bent pipe, L.
As a rule, four to six, and in larger plants even more, of such cylinders are combined to a battery. In this case the discharge pipe, L, terminates in a common collecting vessel, and the steam-pipes, D, branch off from a main steampipe. The battery may be enclosed by brickwork, but is preferably placed upon a suitable foundation and surrounded by woodwork, the intermediate space between woodwork and cylinders being filled with sawdust. This plan offers the best means of keeping the heat together, and the further advantage that, in case one of the cylinders becomes defective, it can be readily taken out and replaced by a new one.
Fig. 47.
In order to be able to fill the cylinders rapidly and with the least expenditure of power, it is advisable to place the bone-crusher at such a height that the crushed bones fall directly into carriages which are run upon a small railway over the charging holes of the cylinders, and emptied. In front of the manholes, A, for discharging the bones is also a railway, so that the crushed bones can be directly emptied into carriages and conveyed to the stamping mill.
The cylinder having been filled with bones, is closed steam-tight. The cock, H, is then opened and steam admitted by opening the cock D. The steam passing in at first, is cooled off by coming in contact with the bones and condensed to water. However, the temperature in the cylinder soon becomes so high that the steam is no longer condensed and, having first expelled the air in the cylinder through the pipe H, it escapes through the latter in the form of a powerful jet. When this is the case H is closed and high-pressure steam allowed to act upon the bones.
The fat contained in the bones melts and trickles down. On the bottom of the cylinder collects a fluid which contains glue, is of a milky turbidity due to admixed drops of fat, and with a quite thick layer of fat upon its surface. From time to time—about every hour—the cock L is slightly opened. By the pressure of the steam the glue-liquor is expelled with great force through the pipe L, the latter being closed when by the peculiar noise it is noticed that only steam escapes.