26 BULLETIN 144, U. S. DEPARTMENT OP 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 
