THE MANUFACTURE OF ACID PHOSPHATE. 25 
DOUBLE ACID PHOSPHATE. 
Tho double acid phosphate now marketed contains from two to 
three times as much soluble phosphoric acid as ordinary superphos- 
phate, and is therefore very valuable in the manufacture of concen- 
trated fertilizers. Before the discovery of extensive deposits of 
high-grade phosphate rock, both in this country and abroad, the 
making of double superphosphate was widely practiced, since it 
afforded a ready means of utilizing low-grade phosphates. Now, 
however, most of the commercial rock is so high in phosphoric acid 
that it is unnecessary to resort to schemes for enriching the soluble 
product obtained therefrom, but in Germany, France, and several 
other foreign countries, as well as in the State of South Carolina, 
where a comparatively low grade of phosphate is mined, this process 
is still used with considerable success. 
The two main chemical reactions involved in the manufacture of 
double acid phosphate are: First, sufficient dilute sulphuric acid is 
added to phosphate rock to convert the hypothetical tricalcium 
phosphate into phosphoric acid and gypsum; and second, the phos- 
phoric acid thus obtained is used to convert the tricalcium phosphate 
of a fresh supply of rock into monocalcium phosphate. The reactions 
in their simplest form may be represented thus: 
(1 ) Ca 3 (P0 4 ) 2 + 3H 2 S0 4 = 2H 3 P0 4 + 3CaS0 4 
(2) 4H 3 P0 4 + Ca 3 (P0 4 ) 2 = 3CaH 4 (P0 4 ) 2 
The process, however, is by no means as simple as it at first appears, 
for there are several distinct operations which not only require the 
watchfulness of a competent superintendent but the control of a 
skillful chemist. 
The phosphate rock and dilute sulphuric acid (16° B.) are run into a 
vat simultaneously and stirred thoroughly for 15 or 20 minutes. It 
is inadvisable to use warm acid to decompose the rock or to stir for a 
protracted period, since under such conditions the compounds of 
iron and aluminum are dissolved only to be precipitated again later 
on, causing the reversion of a part of the phosphoric acid. The 
quantity of sulphuric acid required to bring about the desired reac- 
tions should be carefully ascertained from analyses of the raw material, 
since either an excess or an insufficient quantity will cause trouble 
in the subsequent operations. 
The muddy solution is run into a tank from which it is pumped to a 
filter press where the sediment and gypsum is separated, the clear 
phosphoric-acid solution being run into evaporating pans. The 
residue in the filter press is then washed with water till the washings 
have a concentration of 0.25° B. or less. These washings are used to 
dilute the sulphuric acid employed in the process. 
