14 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 



QUANTITY, STRENGTH, AND TEMPERATURE OF SULPHURIC ACID. 



The quantity and strength of sulphuric acid which should be used 

 in treating phosphate rock is a perplexing problem, yet many manu- 

 facturers give it little consideration, proceeding in a "rule of thumb" 

 manner without regard to the composition of the rock. The reason 

 why many of these latter obtain such good results may be explained 

 by the fact that they have been using one grade of rock for years 

 and have thus learned by actual experience the proper proportions 

 of acid and rock to use. A sudden change in the composition and 

 grade of the phosphates often results in a loss of both acid and rock 

 in attempts to find the quantity of acid required for this new mate- 

 rial. While actual trial mixings should be made when testing out a 

 new grade of phosphate, these tests should be intelligently con- 

 ducted with due regard to the composition of this rock. Take, for 

 example, a sample of high-grade Florida hard rock phosphate having 

 the following composition : 



Per cent. 



Moisture 0.5 



Calcium fluoride (CaF 2 ) 4. 5 



Tricalcium phosphate (Ca 3 (POJ 2 ) 80. 



Calcium carbonate (CaC0 3 ) 3. 5 



Aluminum phospnate (A1P0 4 ) 6. 



Silica (Si0 2 ) 5. 5 



Total 100. 



The problem is to convert the phosphoric acid of this rock into a 

 soluble form and yet obtain a dry product which can be uniformly 

 spread on the soil or readily mixed with other ingredients to make 

 up a complete fertilizer. The reactions which are desired may be 

 represented in their simplest form, thus: 



(1) Conversion of tricalcium phosphate to monocalcium phos- 

 phate and calcium sulphate: 



1 molecule 2 molecules 1 molecule 2 molecules 

 Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = CaH 4 (P0 4 ) 2 + 2CaS0 4 



Wt. 310 Wt. 196 Wt. 234 Wt. 272 



(2) Conversion of calcium fluoride to hydrofluoric acid and calcium 



sulphate : 



1 molecule 1 molecule 2 molecules 1 molecule 



CaF 2 + H 2 S0 4 = 2HF + CaS0 4 



Wt. 78 Wt. 98 Wt. 40 Wt. 136 



(3) Conversion of iron phosphate to iron sulphate and phosphoric 



acid: 



2 molecules 3 molecules 1 molecule 2 molecules 

 2FeP0 4 + 3H 2 S0 4 = Fe 2 (S0 4 ) 3 + 2H 3 (P0 4 ) 

 Wt. 302 Wt. 294 Wt. 400 Wt. 196 



