THE MANUFACTURE OF PHOSPHORIC ACID. 155- 



mnddy ; injectors are the most convenient, Init it must be remem- 

 bered that they dilute the acid 1° to 2 " B. ^ 



The principal difference which exists between double superphos- 

 phate and ordinary superphosphate lies in the fact that the lattei' is a. 

 mixture of monocalcic phosphate and gypsum, whilst double super- 

 phosphate only contains as impurities a few per cents of gypsum, 

 and forms essentially a monocalcic phosphate containing free phos- 

 phoric acid in excess. That is why double superphosphate is less- 

 easily dried than ordinary superphosphate, and must therefore be 

 very carefully dried by artificial means. 



The process of manufacture just described ap]3ears very simple ; 

 it none the less requires the control of a good chemist, the more so- 

 as the raw material used is always contaminated with impurities. 

 The following example will show what field of action is here open 

 to the chemist. Wetzlar's factory, of which the daily production of 

 double superphosphate was about 12 tons, made a product in 1883 

 containing 34 per cent P^O,-, soluble in water and 13 per cent 

 P.,0- insoluble in water ; two years afterwards the Pp., soluble in 

 water had risen to 43 to 45 per cent with 3-6 insoluble in water. 

 100 kg. of phosphoric acid in the merchandise consisted therefore — 



In''l883 of 72-3 [34 x 100 ^ 47] of phosphoric acid solube in water.. 



In 1885 of 90-8 [45 x 100-^48] per cent phosphor c acid soluble 



in water. 



As at that time phosphoric acid soluble in water was valued at 

 0-65 francs the kg., whilst insoluble in acid was not generally paid 

 for at all, the above improvement represents an increase in profit 

 of £25 per day without taking into account the other advantages- 

 attached to the improvement in the working. The capital point in 

 the manufacture of phosphoric acid is to dissolve the least possible 

 amount of sesquioxides (FeAl).O.^, because it is to their presence in 

 superphosphate that its retrogradation is due. It is clear also that 

 the phosphates ought to be exhausted as completely as possible ta 

 reduce the expenditure of sulphuric acid to the minimum and to- 

 have the least impurities, lime, hydrofluoric acid, etc., in the weak 

 solutions. Up to 1884, it was believed that the longer the action 

 w^as prolonged in the mixing tank, the more complete was the ex- 

 traction of the phosphoric acid. It was further believed that a 

 temperature of 60' to 80° C. or 140" C. (176= F.j was necessary, or 

 was at least conducive to the progress of the operation. Thus the 

 acid and the water were mixed in the mixing vat, and the heat of the 

 reaction brought the temperature of the mixture to 70" C. (158° F.) ; 



1 It is difficult to see why the ordinary appliames for raising sulphuric acid 

 in manure works cannot be used, viz. the ordinary " montejus," sometnnes- 

 called the egg or air vessel. This air vessel is tilled with acid by a syphon trom 

 a tank alongside. When full, air is pumped into it by one pipe and the acid 

 rises from it up another pipe into the ten ton store tank overhead. Any hitch 

 with the pumps described by the author mean of course the stoppage of the 

 mixing when the store tank is empty. The air vessel with ordinary attentioa 

 gives no trouble. — Tp.. 



