MANUFACTUEE OF SUPEEPHOSPHATE. 103 



Filter Press. — The filter press is capable of interesting applica- 

 tions, not only in the manufacture of double superphosphates and 

 precipitated phosphates, but also in the treatment of raw phosphates, 

 in view of their enrichment each time that they have to be washed 

 or mixed. 



After mixing, the materials have a semi-fluid, non-pasty con- 

 sistency. It is, therefore, necessary to separate the solids from 

 the liquid mass in which they are suspended. The old decantation 

 process gave good results. But it is easy to understand that the 

 enormous quantity treated daily adapted itself badly to such an 

 extremely slow process requiring numerous vessels and consider- 

 able space. In well-organized factories at the present day, a rapid 

 decanter answers an urgent want. This rapid decanter is 

 nothing more or less than the filter press, the yield of which is 

 twenty times greater than that of the old process. 



Under a small compass the filter press is, therefore, an apparatus 

 presenting such a filtering surface that the solids and liquids are 

 automatically and instantaneously separated. This apparatus con- 

 sists essentially of a certain number of wood or metal plates 

 separated by hollow frames, in which the solid matter aggregates 

 as more or less compact cakes, whilst the liquid is pressed outside, 

 passing through appropriate filter cloths. 



The filter press is fed by a pump which propels the mud into 

 the filtering chambers. The chambers are made tight by the pres- 

 sure system of the filter and by the packing which the filter cloths 

 give between the different plates when the latter are pressed in 

 the centre. As regards the filtration of phosphatic material, 

 manures, etc., the filter presses ought to exhibit such peculiarities 

 of construction as to ensure the flow from the material without the 

 formation of channels during the course of the operation. On the 

 other hand, when the cakes are formed in the frames of the filter 

 presses they still contain a certain amount of mother liquor. To 

 obtain products as pure as possible these cakes must be washed 

 thoroughly to purge them from the liquids which they contain. 

 The filters should be so arranged that this operation of washing is 

 thoroughly and absolutely effective. The " Etablissement des Filtre 

 Philippe " of Paris, with whom the author has had business relations 

 for more than twenty years, has specialized for many j^ears in the 

 construction of special filter presses for manures. Its make 

 exhibits all guarantees for good working according to the nature 

 and consistency of the products to be treated.^ 



Mixing of the Phosphate icith Sulphuric Acid.— As already 

 mentioned, the phosphate was formerly rendered soluble m pits, 

 where the acid and phosphate were hand-mixed with suitable tools. 



1 The best duck (cotton) fabric will not stand superphosphate liquor a day. 

 The writer found a woollen blanket to stand well. — Tr. 



