2O4 SOILS AND FERTILIZERS 



either on land or collected in cavities in water courses, 

 and are generally spherical masses of variable size. 

 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 equivalent of from 17 to 30 per cent 

 phosphoric acid. The remaining 30 to 60 per cent is 

 fine sand, limestone, alumina, and iron compounds, with 

 other impurities, which often render a phosphate un- 

 suitable for manufacture into high-grade fertilizer. 



232. Superphosphate. - - Pulverized rock phosphate, 

 known as phosphate flour, is treated with commercial 

 sulphuric acid to obtain soluble monocalcium phosphate. 

 The amount of sulphuric acid used is determined by the 

 composition of the rock. Impurities as calcium carbon- 

 ate and calcium fluoride react with sulphuric acid and 

 cause a loss of the acid. Ordinarily, a ton of high-grade 

 phosphate rock requires a ton of sulphuric acid. The 

 mixing is done in lead-lined tanks. A weighed amount 

 of phosphate flouris 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 mechani- 

 cal mixer, driven by machinery. From the mixing tank 

 the material is passed into other large tanks, where two 

 or three days are allowed for the completion of the 

 reaction. The mass is placed in piles to solidify and is 

 then ground and sold as superphosphate. In the manu- 

 facture of superphosphate, gypsum (CaSO 4 .2H 2 O) is 



