CHAPTER VIII. 



SUPERPHOSPHATES AND EFFECT OF PHOSPHORIC ACID. 



The phosphates mentioned in the previous chapter, with the 

 exception of basic slag, are not always used in the raw condi- 

 tion for fertilizing purposes, but are treated with sulphuric acid 

 in the manufacture of commercial or artificial fertilizers to make 

 the phosphoric acid available; that is, to convert the phosphoric 

 acid into forms that may readily be used by the plant as food. 



Manufacture of Super or Acid Phosphate. — The manufacturing 

 of artificial fertilizers began some time after 1840 in which year 

 Liebig, a German scientist, discovered that by adding sulphuric 

 acid (oil of vitriol) to bones the phosphoric acid was made 

 soluble. This discovery paved the way for the manufacture of 

 commercial fertilizers which are sold in such large quantities 

 to-day. 



Manufacturing Sulphuric Acid. — The manufacture of super- 

 phosphate is rather tecTmical but a knowledge of this important 

 industry may prove of interest. To begin with, the manufac- 

 turer purchases pyrites or brimstone and phosphate rock. Py- 

 rites is a compound of sulphur and iron and is obtained from 

 Spain and mines in this country. The pyrites or brimstone 

 are burned in special burners and the sulphurous gases are mixed 

 with nitrous gases obtained from nitrate of soda. These mixed 

 sulphurous and nitrous gases are introduced into large high lead 

 towers and then into lead chambers which are also large and 

 high. Steam is introduced into the lead chambers, mixed with 

 the gases and sulphuric acid is formed which falls to the bot- 

 tom as a liquid. These lead towers and lead chambers are very 

 costly. 



Making Superphosphate. — The manufacturer purchases phos- 

 phates that contain sufficient tricalcuim phosphate to warrant 

 profitable treatment. Phosphates that contain considerable im- 

 purities as iron and alumina are avoided. The phosphate rock 

 is broken into small pieces and then pulverized. Certain amounts, 

 say 1,000 pounds, of phosphate powder and dilute sulphuric acid 



