74 MANUFACTURE OF FERTILIZING MATERIALS 



have been made to use hot sulphuric acid and in- 

 ject hot air into the chamber, so as to render the 

 phosphate perfectly soluble and to start the drying 

 of the superphosphate, so as to simplify the final 

 operations. But the results obtained were not 

 satisfactory. The opinion of certain specialists 

 may be endorsed. Such experiments will never 

 be successful, because it is irrational to exceed a 

 temperature of 100 degrees C. in the "den" except 

 in the case of phosphate of a very good quality of 

 which there is no need to fear retrogradation. It 

 is better to leave the substance to itself during its 

 chemical transformation and let it be settled by 

 insensible gradations. Experiments show, more- 

 over, that the injection of hot air into the mass 

 gives it the consistency of mastic, which the manure 

 manufacturer always tries to avoid, knowing full 

 well that the porosity of the superphosphate is the 

 best condition to realize for subsequent operations. 

 The construction of the decomposition house is 

 not very complicated. The walls are two-brick 

 thick; they are covered inside with a coating which 

 resists acid. To consolidate them and prevent 

 them yielding under the pressure of the mixing 

 they are fortified by iron T pieces, fixed to the 

 base by masonry, and joined to the roof by cramp- 

 ing irons. The roof consists of iron T pieces three 

 feet apart, laid on the walls and connected together 

 by iron rods or arches of masonry, the whole being 

 covered by a coat of cement. All the ironwork is 

 covered by paint to resist acid fumes. The house 



