STOEING OF SUPEEPHOSPHATE. 127 



converted into a fine powder. The superphosphate is transferred 

 by a conveyor into a hopper which feeds it into a drum armed with 

 knives. This drum consists of a posterior disc and an anterior 

 ring. Between the disc and the ring there are arranged tangentially 

 at fixed distances steel blades or knives, covering the openings for 

 the exit of the finished superphosphate. Sixteen blades of this 

 sort pass in front of the hopper aperture, at a speed of 300 revolu- 

 tions a minute, consequently 4800 blows per minute. The slices 

 so produced are very fine and do not set on accumulating. The 

 drum is driven by shaft and pulley. On the hopper side and 

 beneath it are w^rought-iron plates, on which the finished super- 

 phosphate, issuing from the drum, falls; from these plates it falls 

 into trucks. The working of the installation is as follows : The 

 drum is driven by a pulley and turns at great speed. This speed, 

 which is thus transmitted to the knives, is calculated in such a 

 w^ay as to exceed the rapidity of the fall of the superphosphate into 

 the^ feed hopper. In that way, all the superphosphate is forced to 

 pass over the knives, and the uncut lumps are prevented from pass- 

 ing through the apertures and falling into the truck. The spaces 

 between the knives are regulated according to the speed of the 

 periphery of the drum and reciprocally. It follows that the knives 

 succeed each other in their action, penetrate into tha material and 

 scrape it through successive layers. Very thin slices are thus 

 obtained, which fall to powder. This division is still further 

 favoured by the speed of rotation of the drum, which produces 

 a strong current of air acting on the substance, and producing 

 a cyclonic motion in the parts detached by the knives. The 

 pulverized material falls from the plates into a truck. The current 

 of air produced by the rotation of the drum imparts a certain speed 

 to the phosphate powder detached by scraping. The wrought-iron 

 plates also serve to turn this current of air in its tangential course, 

 so that the powder only falls into the trucks at the speed it would 

 fall by its own weight. The too abrupt fall of the material is thus 

 avoided. Finally, the current of air produced in the drum frees the 

 superphosphate from any gases with bad odour, and expels them 



outside. 



Dr. Lutjens afterwards modified his processes, first, in combining 

 drying with the scraping machine, afterwards in cooling the dried 

 superphosphate. The process thus modified realizes to the letter 

 the problem enunciated above, to produce a superphosphate ready 

 for dispatch in twenty-four hours. For this purpose the superphos- 

 Xohate cut in very fine slices, falls into an upper drying trough, 

 where it is exposed on all sides to the action of hot air ; this 

 completely traverses the mass, heats it and renders it friable. 

 From the upper trough the superphosphate falls into a second 

 trough placed underneath, where drying is com^Dleted by a current 



