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CHEMICAL MANUEES. 



Sieves. — The sieves used in the chemical manure trade are of 

 two kinds, viz. sorting sieves and sieves for fine flour. The 

 sorting sieves are combined with the coarse crushers and serve 

 to sort the crushed material ; what passes through the sieve is 

 taken to the mills to be reduced to a fine powder, whilst the core is 

 returned to the crusher to be again crushed. Machines of this nature 

 are conical or cylindrical. They consist essentially of an axis, 

 which passes through their entire length, and carries iron stays, on 

 which a perforated sheet-iron sieve, more rarely a metallic wire 

 gauze sieve, is mounted, the meshes of which have a section varying 

 with the nature of the material to be ground. They are fitted with 

 a feed hopper and a discharge pipe. 



The second class of sieves are more especially used for the 

 sifting of the flour produced by the mills. They are also used to 



Fig. 19. — Sifting Machine (driven direct, enclosed in wood cover). 



sort bone' dust. The matter which passes through the sieve is 

 bagged up, whilst the core is returned to the mills to be reduced 

 to the desired fineness. As this material gives off much dust the 

 sieve is generally covered in. Discharge pipes pass through the 

 bottom of the cover. The construction of this machine varies 

 according to the nature of the material treated — with substances 

 easily sifted, the cylindrical form is used, whilst with substances 

 which pass through difficultly, a hexagonal sieve fitted with a beater 

 is used ; the frames of the sieve are interchangeable and easy of 

 access. Fig. 19 shows a sifting machine, driven direct, with 

 wooden cover, constructed by Krupp. Besides these cylindrical 

 sifters, shaking sieves are also used, comprising one or more flat 

 sieves lodged inside a cover. The cover rests on springs or is 

 suspended or driven by a crank shaft. 



