STOEIXG OF SUPERPHOSPHATE. 



135 



in no way influence the accuracy of the method. If the exact tem- 

 perature and pressure be found, they remain the same in all cases 

 so long as the raw material is not changed. This method is far 

 from solving the question of retrogradation, but it is interesting, 

 because an idea can be gained from it of the retrogradation of a 

 given phosphate. Dr. Grueber gives the results of experiments 

 which he has made with Schucht's apparatus, of which the following 

 is a brief summary : — 



1. A Florida phosphate, 35 per cent P.,0., and 2-52 of sesqui- 

 oxides, when rendered soluble gave 18' 7 per cent PoO- soluble 

 in water, and 1-21 of insoluble PoO.. Treated after Schucht's 

 method, under a pressure of four atmospheres and at a tempera- 

 ture of 40" C. (10i° F.) (the apparatus being home-made could not 

 do better), it contained 18"2 per cent of PoO^ soluble in water, and 

 1-76 of insoluble PoO-. A second Florida phosphate, with 35-96 

 per cent PoO^ and 2'38 per cent sesquioxides, gave immediately 



Fig. 35. — Retrogradation Indicator (Schucht). 



after being rendered soluble, 18-8 per cent PoO_:^, and 0'81 insoluble. 

 After Schucht's process, 16-67 per cent soluble PoO^, and 2-62 per 

 cent insoluble, which shows the efficiency of Schucht's apparatus, 

 that is to say, that the particles of phosphate, intimately mixed 

 and pressed hot, show an accentuated tendency to retrograde. 



2. A comparison was made with a superphosphate stored hot in 

 a big heap. A Florida phosphate with 35-8 per cent P0O5, and 

 2-09 per cent of sesquioxide, was in use. This phosphate, tested 

 immediately after being "dissolved," gave 18'1:0 per cent P0O5 and 

 1-1 per cent insoluble. With Schucht's method it gave 17-25 per 

 cent soluble P..O5, and 2-06 per cent insoluble. The stored heap 

 showed at the end of a year 17*28 per cent PoO.,, and 1*96 per cent 

 insoluble. Consequently the stored phosphate confirms the accuracy 

 of Schucht's method. In the middle (height) of the heap a- retro- 

 gradation of only 0-21 per cent occurred and no retrogradation in 

 the top. 



