PHOSPHATE FERTILIZERS 17 1 



The phosphoric acid is separated from the gypsum be- 

 fore acting upon the phosphate flour. In this way, 

 superphosphate containing' from 35 to 45 per cent, 

 of phosphoric acid is produced. When fertilizers are 

 to be transported long distances this concentrated 

 product is preferable. The terms ( acid ' and ( super- 

 phosphate ' have been generally used to designate 

 both the first product produced by the action of sul- 

 phuric acid and that produced by phosphoric acid, but 

 of late there is a tendency to restrict the term ( acid 

 phosphate ' to the product formed by the action of 

 sulphuric acid, and the term ' super-phosphate ' to the 

 concentrated product formed by the action of phos- 

 phoric acid. 



218. Commercial Value of Phosphoric Acid. The 

 commercial value of phosphoric acid in fertilizers is 

 determined by the value of the crude phosphate rock, 

 cost of grinding and treating with sulphuric acid, and 

 cost of transportation. The price of phosphoric acid 

 in superphosphates usually ranges from 5 to 6 cents 

 per pound. The field value, that is the increased 

 yields obtained from the use of superphosphates, may 

 not be in accord with the commercial value because so 

 many conditions govern their use. The phosphoric 

 acid obtained from feed-stuffs is usually considered 

 worth about a cent a pound less than that from super- 

 phosphates. Water-soluble phosphoric acid is general- 

 ly rated a half cent per pound higher than citrate-sol- 

 uble phosphoric acid. 



219. Phosphate Slag. In the refining of iron ores 



