18 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
raise the temperature to 120° to 250° C. Carbon dioxide, steam, and 
gaseous compounds of fluorine work their way out of the mass and 
escape through the flue at the top of the chamber, leaving the acid 
phosphate in a dry, porous condition. 
After standing in the den for about 24 hours the reactions are 
practically complete and the material is ready to be removed. The 
heavy wooden doors at opposite sides of the den are then opened and 
the acid phosphate is removed. Frequently the floors of the den 
are so constructed that they can be opened and the acid phos- 
phate discharged into a hopper or upon an acid-proof belt beneath, 
whence it is taken up by elevators and dumped on the storage pile. 
The emptying of the den is not only a disagreeable operation, but is 
attended with considerable danger. The temperature of the acid 
phosphate contained therein, even after standing from 24 to 36 
hours, is still very high (from 130° to 150° C.), and the fumes given 
off by this hot material are quite poisonous. Great care must be 
exercised in digging out the phosphate to prevent large masses of the 
loose material from falling upon the laborers.. 
In order to do away with these dangers efforts have been made to 
empty the dens mechanically. A number of processes have been 
devised 1 in most of which the excavator or cutter is introduced into 
the chamber after the acid phosphate is cured. Special forms of 
chambers are required in some of the processes, while in others the 
excavating device is adaptable to almost any form of den after the 
latter has undergone some slight alterations. The more general 
scheme of emptying the dens mechanically is as follows : 
A device consisting of an endless chain, which either rotates on a 
shaft or can be moved laterally in the den, is fitted with knives or 
teeth which cut or break up the acid phosphate and at the same time 
convey it to a chute. This device is either introduced horizontally 
at the top of the den or vertically at the side. In the former case 
the cutter is so arranged that it is mechanically lowered or sinks 
automatically after completing the circuit of the chamber. If the 
cutter is introduced vertically at one end of the chamber it cuts 
away the pile of acid phosphate from the side. It is claimed that the 
latter method is less likely to cause the material to pack. The 
removal of acid phosphate mechanically seems to be ordinarily a 
rather slow process, since the cutters or scrapers, if run at any 
great speed, cause the material to become heated and gummy. 
One process of emptying the dens more rapidly consists of a combined 
cutter and fan. The latter helps keep the material cool while exca- 
vation is going on. Another method of emptying the den consists 
of having the floors mounted on rollers so that one side of the chamber 
may be swung open and the whole mass of acid phosphate wheeled 
i U. S. Patents 892,593, 899,042, 940,583, 949,055, 956,792, 1,013,334, 1,037,464, 1,033,854, 1,070,296. 
