178 SOILS AND FERTILIZERS 



ferent types of phosphates as hard rock, soft rock, land 

 pebble, and river pebble. The pebble phosphates are 

 found either on land or collected in cavities in the 

 water courses, and are generally spherical masses of 

 variable size. The soft rock phosphate is easily 

 crushed, while the hard rock requires pulverizing with 

 rock crushers. Phosphate rock usually contains from 

 40 to 70 per cent, of calcium phosphate, the equiva- 

 lent of from 17 to 30 per cent, phosphoric acid. The 

 remaining 30 to 60 per cent, is composed of fine sand, 

 limestone, alumina and iron compounds, with other 

 impurities, which often render a phosphate unsuitable 

 for manufacturing high-grade fertilizer. Raw phos- 

 phate rock is usually sold at the mines for $1.75 to 

 $4.50 per ton. 



217. Superphosphate. — Pulverized rock phosphate 

 known as phosphate flour, is treated with commercial 

 sulphuric acid and soluble monocalcium phosphate 

 obtained. The amount of sulphuric acid used is de- 

 termined from the composition of the rock. Impuri- 

 ties as calcium carbonate and calcium fluoride react 

 with sulphuric acid and cause a loss of acid. Ordi- 

 narily, a ton of high-grade phosphate rock requires a 

 ton of sulphuric acid. The mixing is usually done in 

 lead-lined tanks. A weighed amount of phosphate 

 flour is placed in the tank, and the sulphuric acid 

 added, through lead pipes, from the acid tower. The 

 mixing of the acid and phosphate is done with a me- 



