STORING OF SUPERPHOSPHATE. 121 



be obstructed, the machine must be stopped and the sieve cleaned. 

 It is cleaned by beating and scraping with a broom, using sand if 

 need be. A mechanical beater is often installed on the sieve, but 

 it can only be used for cylindrical sieves. The latter again can 

 only be used for very dry superphosphates, because in spite of the 

 mechanical beating they are rapidly obstructed and difficult to 

 clean. 



If the superphosphate is in a pulverulent condition, it is simply 

 sifted without crushing ; only the core from the sieve then passes 

 through the crusher at a lower level, from which it falls into the 

 receiver of the elevator, which spreads it again on the sieve. The 

 elevator, crusher, and sieve act simultaneously. However, it is 

 better to drive the crusher independently, or by an electric motor, 

 for in many cases the core on the sieves is not important, and the 

 action of the crusher only intervenes usefully at rare intervals. 

 It is hardly necessary to say that these machines should be easy 

 of access. Fig. 31 represents a unit consisting of elevator, crusher, 

 and sieve, mounted on iron frames. This unit, movable on rollers, 

 may easily be conveyed from one depot to another as required. 

 Manipulations and cost of conveyance, often heavy, are thus avoided. 

 An installation of this nature only requires three workmen. The 

 first feeds the cups, the second looks after the machine and bags 

 up, the third conveys the superphosphate to the depot. In case of 

 a break-down the three mutually assist each other. It is to be 

 noted that damp weather is bad for grinding and sifting ; the 

 superphosphate is very deliquescent, and adheres to the machine. 



Artificial Drying of Superjjliosphate. — If it be hardly possible 

 to effect economy in crushing and dissolving with the machinery 

 described above, it is quite otherwise with the operations relating 

 to the finishing of the superphosphate, that is to say, those that 

 intervene between " dissolving " and dispatch of the finished pro- 

 duct. There are factories where these operations cost double what 

 is required in a rational installation. In the present economical 

 conditions of the industry, it is necessary to supplant ineffective 

 hand labour, always costly, as much as possible by mechanical 

 methods ; a machine can be stopped when desired, whilst hand 

 labour must be kept on continually, otherwise it goes. It must, 

 therefore, be reduced to a minimum, the superphosphate finished 

 with the least possible delay, without having to store it for a 

 longer or shorter time, to render it saleable. In this chain of 

 reasoning, the attention of manufacturers is drawn to the machinery 

 and processes described further on. 



Theoretical Review of Drying. — The moisture of a superphos- 

 phate is not wholly due to a high water content per cent, but also 

 to the presence of an important proportion of free phosphoric acid 

 in supersaturated .superphosphates. Thus, when a superphosphate 



