112 



CHEMICAL MANUEES. 



has been well made it soon crumbles when exposed to the air ; if 

 badly made it forms lumps which do not crumble in the air, and 

 the outside of which is damp from free phosphoric acid, whilst the 

 inside forms any nucleus of neutral phosphate of lime : this occurs 

 when too strong acid has been used. The core, sifted from the 

 material, is dried in an oven if it cannot be crushed in a Carr's 

 disintegrator. 



In a general manner the reaction which superphosphate under- 

 goes during the process of being rendered soluble, and which consists 

 in the precipitation of sulphate of lime and its conversion into 

 crystallized gypsum, is accomplished rapidly. Certain phosphates, 

 however, form exceptions to this rule : crystallization is stimulated 



Fig. 24.— A. Superphosphate Den. B. Discharge Apertures, 

 veyor. D. Elevator Eeceiver. E. Elevator. 



C. Belt Con- 



by stirring them with the shovel and the process finished by 



drying. 



Neiu Methods for the Mechanical Extraction of SujJerjjhosjjhate 

 from the ''Den'\ — The manipulation of superphosphate being 

 dangerous, on account of the toxic gases which are disengaged, 

 efforts have been made to empty the houses mechanically. But 

 the appliances used are costly and defective, especially owing to the 

 false position at the door of the excavation plant. Benker and 

 Hartmann's mechanical extractor suppresses these drawbacks, in so 

 far that the excavating organ is arranged so as to support its shaft 

 at its two ends. Figs. 25-28 show the arrangement which forms the 

 subject of their invention. Fig. 25 is a longitudinal section. Fig. 

 26 is an end view. Fig. 27 is a plan. Fig. 28 is a detail. 



