16 BULLETIN 144, U. S. DEPARTMENT OF AGEICULTURE. 



It has been found, however, that the quantity of sulphuric acid 

 theoretically required to convert phosphate rock into acid phosphate 

 does not always produce the best results. The physical condition of 

 the acid phosphate is just as important as its chemical composition, 

 and sometimes the proper mechanical condition can not be obtained 

 except by sacrificing some of the water-soluble phosphate. In factory 

 practice it is often well, therefore, to add a little less sulphuric acid 

 than is necessary to satisfy the equations outlined above. 



The strength of sulphuric acid used is another detail of great impor- 

 tance in the production of acid phosphate. The quantity of strong 

 acid sufficient to bring about the desired chemical reactions is of such 

 small bulk and has such a viscosity that it is difficult to obtain an 

 intimate mixture with the ground phosphate; moreover, the calcium 

 sulphate produced, being much less soluble in strong than hi weak 

 sulphuric acid, 1 forms a relatively insoluble coating over the phos- 

 phate, preventing further action by the acid. 



On the other hand, if very dilute acid is employed, the amount 

 required to bring about the necessary chemical reactions is so great 

 and so much water is contained therein that it is almost impossible to 

 obtain a product in good mechanical condition. The strength of acid 

 with which the best results are ordinarily obtained ranges from 50° 

 to 55° B., though phosphates very high in iron and aluminum com- 

 pounds sometimes yield better to slightly stronger acid. 



There is considerable difference of opinion concerning the tempera- 

 ture at which sulphuric acid should be added to phosphate rock. 

 Some manufacturers, however, give little heed to this important 

 point, mixing their acid and rock in the same proportions winter and 

 summer at whatever temperature the air happens to be. Others 

 believe in heating the acid to 50° to 55° C. before using, while many 

 others claim it is bad practice to use acid at a temperature below 25° 

 or above 30° C. 



It is obvious that this matter should not be disregarded entirely, 

 for in mixing acid and rock either very low or excessively high tem- 

 perature may seriously affect the product. No definite rule, however, 

 can be prescribed, for here again the composition of the phosphate 

 used is the controlling factor. 



Phosphates containing large quantities of carbonates heat up 

 rapidly when mixed with sulphuric acid. If the mixture becomes 

 very hot violent frothing occurs and the mass is apt to overflow from 

 the pan. Rocks of this type should not be treated with hot acid. 

 When dealing with phosphates high in compounds of iron and alumi- 

 num, however, it often saves time to use acid heated to a temperature 

 of 50° to 55° C. The reactions then begin promptly in the pan and 



i Bui. 33, Bureau of Soiis, U. S. Dept. of Agr., pp. 41-42 (1906). 



