THE MANUFACTURE OF ACID PHOSPHATE. 19 
out and broken up where there is a good circulation of air. Me- 
chanical excavators have not been successfully worked in the factories 
of this country, however, and the old-style chamber or den is employed 
almost entirely. 
The den system is the only one which can be successfully employed 
where it is necessary to absorb the fumes given off in the manufacture 
of acid phosphate. Each den is equipped with a flue near the top, 
which allows the gases and vapors from the freshly made acid phos- 
phate to escape or be drawn off by means of a fan. The flue leads 
into a washer or scrubber, which consists either of a wooden tower in 
winch jets of water are constantly spraying or of a number of com- 
partments through which the gases are made to circulate while they 
are continually sprayed with water. Under such conditions the 
gaseous compound silicon tetrafluoride is decomposed with precipita- 
tion of silica and formation of hydrofluosilicic acid, as shown in the 
equation on page 8. The hydrofluosilicic acid, together with any 
hydrofluoric acid which may have escaped from the mass of acid 
phosphate, is absorbed by the water. The acid solution thus pro- 
duced is used to some extent in the manufacture of fluosilicates of 
the alkalies which are used in the production of enamel. 
Both the initial cost and running expenses of the "den" system are 
greater than those of the "open-dump" method, but a high-grade 
product in excellent mechanical condition can be obtained in a 
short time by the former method without allowing the objectionable 
fumes to escape into the atmosphere. Most factories are equipped 
with at least two dens (sometimes four) built close together, with 
the acidulator or mixer placed on the dividing wall above them. In 
this way work can be carried on with little interruption, for while 
one den is being emptied the other may be filled. The capacity of 
the dens varies from 50 to 300 tons, depending on the size of the 
mixing plant. 
THE OPEN-DUMP SYSTEM. 
The "open-dump" method is largely used in the South Atlantic 
States. The mixture of acid and rock is discharged into an auto- 
matic dump car and carried to the storage shed, where it is dumped 
on an open pile. In order that the chemical reactions may get a fair 
start before the mixture spreads out in thin layers, it is allowed to 
Leat up and thicken somewhat in the mixing pan; frequently it is 
>ermitted to remain in the dump car until it has nearly set. Many 
>perators, however, claim to obtain good results by dumping the 
Laterial almost immediately. Sometimes, in order to prevent the 
acid phosphate from spreading, a partly open bin is employed. The 
Laterial after standing in this bin for 8 or 10 days is taken up by 
levators and dumped on a storage pile. 
The acid phosphate made by the "open-dump" method naturally 
takes much longer to arrive at its maximum availability and optimum 
