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 

 which 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 

 heat up and thicken somewhat in the mixing pan; frequently it is 

 permitted to remain in the dump car until it has nearly set. Many 

 operators, however, claim to obtain good results by dumping the 

 material almost immediately. Sometimes, in order to prevent the 

 acid phosphate from spreading, a partly open bin is employed. The 

 material after standing in this bin for 8 or 10 days is taken up by 

 elevators 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 





