THE MANUFACTURE OF PHOSPHORIC ACH). 159 



2 centrifugal pumps. 



3 membrane pumps. 

 Cooling and dilution plant. 



Sulphuric acid store tank, 10 cubic metres capacity. 



^^ per cent sulphuric acid store tank, 20 cubic metres capacity. 



2 sulphuric acid measuring tanks. 



2 wash water tanks, 15 cubic metres capacity. 



Dilute phosphoric acid tank, 40 cubic metres capacity. 



Concentrated phosphoric acid tank, 10 cubic metres capacity. 



Membrane pump for concentrated phosphoric acid. 



Oven for drying the phosphoric acid. 



Lead piping. 



1 steam engine 25 H.P. with shafting. 



As mentioned at the outset, the phosphoric acid industr}" has 

 lost much of its importance. Of twelve large factories at work 

 twenty years ago, there are only four or live still at work, which 

 also make various kinds of phosphates, of which a few words will 

 now be said. 



Manufacture of Superpliosphate of Potassium and Ammonium. 

 — -The oldest and the most important of the phosphoric acid deriva- 

 tives is the neutral phosphate of sodium, Na.,HPOj, which occurs in 

 the form of beautiful monoclinic crystals consisting of 20 per cent 

 Na.,0, 20 per cent P.,0,- and 60 per cent of water, and can therefore 

 be prepared easily in the pure state. The phosphate of sodium is 

 not employed as manure, but rather the phosphate of potash and of 

 ammonia about to be described. The m.anufacture of phosphates of 

 potassium and ammonium presents no difficulties, especially that of 

 potassium, owing to its weak crystallizing power. From that point 

 of view, the mono-ammonium phosphate AmH.,PO^, which is the 

 most stable of all the phosphates of ammonia, behaves much better. 

 It crystallizes without water of crystallization in beautiful tetragonal 

 prisms with a pointed apex. The acid phosphates are generally 

 very difficult to filter when they are neutralized with soda. Now, as 

 the pure non-ammoniacal phosphate is much less used as a manure 

 than the impure phosphate of ammonia, the manufacture of the 

 latter furnishes a pure ammoniacal salt as a secondary product. 

 The most simple process to manufacture phosphate of ammonia as 

 well as sulphate of ammonia, is to use gas liquor, working with the 

 well-known apparatus of Feldmann. The salt is not separated from 

 the muddy precipitate obtained, which is likewise used as manure, 

 but there forms at the surface fine crystalline efflorescences of 

 AmH^POj, which it is easy to collect apart and to purify by fresh 

 crystallization. In order to prepare in the same manner a crude 

 phospho-potassic manure, potash must be employed as the raw 



