10 BULLETIN 312, U. S. DEPARTMENT OF AGRICULTURE. 
distances. In order to produce a more concentrated phosphatic 
fertilizer, the following method is employed: 
The ground phosphate rock is mixed with dilute sulphuric acid, and 
the phosphoric acid thus produced is separated from the gypsum and 
impurities both by decantation and filtration. The acid is then con- 
centrated by evaporation and sold as such or used to treat another 
batch of phosphate rock in the production of double acid phosphate, 
which contains as high as 40 per cent of phosphoric acid (P,O,). The 
equations showing these reactions may be represented thus: 
Ca,(PO,)+3H,S0,+6H,0=2H,P0,+3(CaS0,.2H,0): 
4H, PO,+Ca,(PO,).=3CaH,(PO,)o. 
A list of the patents on this subject, arranged in chronological order, 
is given in Table III, Appendix. 
Several of the processes cited in Table III, if they accomplish what 
is claimed for them, should make it possible to produce soluble 
phosphate more cheaply than by the methods now generally used, or 
to produce a more concentrated fertilizer, which will admit of shipping 
it long distances. The following processes have features of interest 
from either a scientific or an economic standpoint. 
The process of Designolle ' consists in treating phosphate rock sus- 
pended in water with sulphur dioxide under pressure, producing 
thereby a solution of monocalcium phosphate and sulphite of lime— 
ee Ca,(PO,)2+280,+2H,0=CaH,(PO,).+2CaSO3. 
The suspended matter is allowed to settle in some suitable vessel 
and the solution is boiled with steam to drive off the excess of sulphur 
dioxide and to precipitate the calcium sulphite. The solution of 
monocalcium phosphate is then poured off, evaporated to a sirupy 
consistency, and treated with plaster of Paris to take up the excess 
of water. 
Bergmann ? claims that dicalcium phosphate free from calcium sul- 
phite is obtained by first mixing phosphate rock with sulphurous acid 
in the cold, then adding monocalcium phosphate to the solution, and 
finally boiling the solution to precipitate the dicalcium phosphate and 
drive off sulphur dioxide. 
In the process of Machalske,? phosphate rock is subjected to the 
action of sulphur dioxide in a small quantity of water. The resulting 
mass is leached with a dilute solution of sulphur dioxide to extract 
the soluble phosphates, and the residue, which is said to contain a 
large percentage of calcium sulphite, is calcined to recover the sulphur 
dioxide, which can be used again. 
1 United States Patent No. 196881 (1877). 
2 United States Patent No. 852371 (1907). 
3 United States Patent No. 902425 (1908). 
