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



Before the phosphoric acid produced can be used for making 

 double acid phosphate it must be concentrated. This is usually done 

 by evaporating in iron pans lined with stone or some acid-resisting 

 material. After concentrating to about 56° or 58° B., it is run into 

 lead-lined tanks from which it is drawn or pumped as required. 

 The mixing of this phosphoric acid with phosphate rock and all sub- 

 sequent operations are practically the same as those employed in" 

 making ordinary acid phosphate, but the final product often has to 

 be artificially dried since it contains but a small percentage of gypsum. 

 Ordinary acid phosphate, as we have seen, is largely a mixture of 

 soluble lime phosphate and gypsum, the latter having been formed 

 from calcium sulphate by extracting the excess of water from the 

 mass. Double acid phosphate, however, consists chiefly of soluble 

 lime phosphate with but little calcium sulphate to act as a dehy- 

 drating agent, and therefore requires artificial heating to drive off 

 the excess of water. 



SUMMARY. 



The general procedure followed in making acid phosphate involves 

 numerous details of great economic importance which are not 

 thoroughly understood. 



The raw phosphatic materials which have been used in the acid- 

 phosphate industry are bone, guano, apatite, and phosphate rock. 

 Of these substances the last named has practically displaced the 

 others as a source of phosphoric acid. 



The process of making acid phosphate was devised in .order to 

 produce phosphoric acid in a soluble, or so-called "available," con- 

 dition; this done by the action of sulphuric acid on tribasic phos- 

 phates whereby less basic and more soluble phosphates are produced. 



A knowledge of the composition of the raw materials is of the 

 greatest importance in the manufacture of acid phosphate, since 

 not only the phosphate of lime but all the impurities contained in the 

 rock are acted upon by sulphuric acid and influence the composition 

 and physical condition of the finished product. 



Much phosphate rock contains organic matter which consumes a 

 certain amount of sulphuric acid, but owing to the various forms in 

 which this material occurs it is almost impossible to determine 

 except by actual trial the quantity of acid required to decompose it. 



Silica is acted upon only indirectly by sulphuric acid. 



Calcium fluoride, which is present in nearly all phosphate rock, 

 is acted upon by sulphuric acid, resulting in the formation of gaseous 

 hydrofluoric acid. This gas in turn acts upon silica and silicates, 

 producing silicon tetrafluoride. The silicon tetrafluoride is decom- 

 posed by water, forming hydrofluosilicic acid and silica. The 



