MANUFACTUKE OF BONE SUPERPHOSPHATE. 177 



horns, glass and scrap iron are separated by hand on the shaking 

 table, or on a revolving table placed at the end of the shaking 

 table, or the sifting machine. Generally, they rest content with 

 receiving the bones on an inclined endless band, along which the 

 female sorters are arranged. The band moves slowly, so as to give 

 time for sorting, and its slope enables the bones to be fed into the 

 hopper of a bone crusher. Before falling into the crusher, the 

 bones issuing from the chain fall on to a narrow iron plane inclined 

 at 45°, divided into two parts and soldered by a copper band. Each 

 of these parts corresponds to one of the poles of a strong electro- 

 magnet situated underneath. It follows that if the iron falls on the 

 inclined plane it is retained, and eliminated by the sorter who had 

 allowed it to pass on to the chain. In this method of sorting, the 

 earth necessarily follows the bones into the crusher ; nevertheless, a 

 certain part of the earth is not fed on to the chain when the man 

 feeding uses as a shovel a fork with close teeth in shovelling the 

 bones thrown on to a perforated sheet of iron over a pit where the 

 earth collects. Although this earth may be removed in subsequent 

 operations, it is always more advantageous to remove it before 

 crushing, so as not to have pulverulent matter in the fat extraction 

 and in the glue autoclaves. There is little useful matter in this 

 earth, from which the small particles of bones are removed by finer 

 sifting. They are mixed with bone meal for manure. 



Bone Crushing. — Two sorts of tools are used in bone crushing ; 

 one working at a slow speed, which is the toothed crusher, and the 

 centrifugal crusher at great speed, such as that constructed by 

 Weidknecht. It is well to have several successive crushers with 

 duplicate exchange pieces, so that they may be replaced rapidly. 

 The first crusher is the coarse crusher, and the second the finishing 



mill. 



(1) Sloio-speed Crusher. — It consists of two large rolls formed 

 by thick circular toothed bosses and with teeth a little obtuse, 

 alternately with non-toothed bosses of the same thickness, but of a 

 less diameter by the height of two teeth ; the aggregate of the bosses 

 is mounted on a hexagonal steel axis. The rolls are fitted up in 

 such a manner that the teeth of the one correspond to the circular part 

 of the smaller diameter of the other. They are separated by the 

 space desired, to crush coarser or finer. For that purpose, they are 

 driven by steel cog-wheels with deep teeth, allowing a certain dis- 

 placement. One of the rolls is on movable bearings, capable of 

 sliding if the resistance to be overcome is too gi^eat, being brought 

 back to the original position by a system of springs. A good fly- 

 wheel is necessary to overcome passing obstructions. The crushers 

 built by Krupp attain the same end, and are made of very hard 

 special steel. There are two crushers, the one after the other, fed 

 by cup chain elevators in the case of the second crusher, so as to 



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