PHOSPHORUS FERTILIZERS 153 



floor space can be afforded, this method of drying is quite 

 efficient, but much labour is spent in spreading. Special 

 machines with a rotary movement are also constructed for 

 drying superphosphate in a continuous manner. The removal 

 of water modifies the relationship between the four forms 

 of phosphorus in the superphosphate, viz. between the tri- 

 calcium, di-calcium and mono-calcium phosphates and the free 

 phosphoric acid. In the case of well-made superphosphate, 

 the amount of free phosphoric acid is reduced by drying, 

 since, owing to the removal of water, free phosphoric acid re- 

 acts with di-calcium phosphate with the formation of mono- 

 calcium phosphate. By careful drying, the percentage of 

 water soluble phosphate is raised, and the mechanical condition 

 is improved. In the case of badly made superphosphate which 

 contains large quantities of free phosphoric acid and in which 

 the physical condition is bad, the removal of water during 

 drying is very slow, and sometimes only superficial ; owing 

 to the relative absence of di-calcium phosphate, the reduction 

 of free phosphoric acid by drying is very slight, so that 

 actually, after the removal of water, the free phosphoric 

 acid may become slightly higher, instead of slightly lower, 

 due to the general concentration of all the constituents. 

 Consequently, badly made superphosphates do not benefit 

 much by artificial systems of drying. 



Some artificial drying of superphosphates may be carried 

 out by admixture with other materials. The other materials 

 used may be small amounts of rock phosphates, which have 

 been dusted in the way described on p. 150, or gypsum may 

 be added, merely as a means of absorbing liquor rather than 

 as an agent of desiccation. Drying superphosphates by 

 admixture is more commonly carried out in the general process 

 of making compound manures, described on p. 179. Where 

 superphosphates are overheated in the effort to dry them, 

 the phosphoric acid may be dehydrated. If phosphoric 

 acid be heated at 105 C. (221 F.), for three hours, as 

 much as 10% may be converted into pyro-phosphoric 

 acid, whilst if heated to 200 C. (392 F.), most of the 

 ortho-phosphoric acid will have been dried and changed 



