158 CHEMICAL MANURES. 



-cannot stand heavy freight charges, for its selHng price is almost 

 entirely absorbed by drying expenses and loading. It is easy to 

 understand that to a factory which produces 100 tons of phosphatic 

 gypsum a day, the sale of this waste is of no great importance in 

 itself, whilst the shifting of it and its storage form a big item in 

 the general expenses account. Thus the Wetzlar factory was 

 forced to purchase a piece of land for 30,000 marks (£1500) for 

 this purpose and to instal on it an aerial conveyor at a cost of 

 16,000 marks (£800). It has already been seen that double super- 

 phosphate must be dried artificially. The principle of this opera- 

 tion consists in passing over the sul^stance a great amount of air at 

 a uniform temperature, the degree of which depends on the super- 

 phosphate. The dryers installed at first fulfilled none of these 

 conditions ; the product obtained therefore was very unequal, one 

 part was superheated and its phosphoric acid had partially retro- 

 graded. To improve the drying laboratory, experiments were 

 undertaken to find the limit of temperature at which the phosphoric 

 acid ceased to retrograde. These researches were made on Lahn 

 •double superphosphate and gave quite unexpected results. Up to 

 120^ C. (248" F.) the content of phosphoric acid soluble in water 

 underwent no modification, it even appreciably increased between 

 120^^ C. (248° F.) and 170' C. (338° F.), and it is only above 170° C. 

 that the much-dreaded retrogradation occurred. At first sight one 

 would be inclined to explain this fact in the sense that the H^PO^ 

 and G3j.,H.,(P0^)., are at a low temperature in a state of equilibrium 

 in superphosphate, and react the one on the other at a high 

 temperature to form CaH^(POj)o. It was then a question of realiz- 

 ing this temperature of 170° C. in actual practice. But this problem 

 is a difficult one and does not appear to have ever been perfectly 

 solved. It will be seen in the sequel that double superphosphate 

 made from Liege phosphate behaves in quite a ditf'erent manner ; 

 not only is there no increase in the phosphoric acid soluble in water, 

 JDut retrogradation commences as soon as the drying temperature 

 exceeds 120° C. The true cause of that difference in the behaviour 

 of the two superphosphates has not been ascertained, but it may 

 be concluded that each phosphate has a nature of its own, and 

 requires to be treated in accordance therewith. It is the phos- 

 phates poor in sesquioxides which work best, such as Florida 

 phosphate which is used in a Swedish factory. The crushing of 

 the dried superphosphate is done in a Carr's disintegratoi', like 

 an ordinary superphosphate, and presents no difficulty. The plant 

 required for making 7^ tons of 55 per cent phosphoric acid is the 

 following : — 



4 to 5 filter presses. 



1 large and 3 small mixing vats. 



