24 BULLETIN 1179, U. S. DEPARTMENT OF AGRICULTURE. 
furnace heated by fuel, where excess air must be introduced for com- 
bustion. By proper regenerative apparatus and means of utilizing 
the sensible heat in the effluent gases, however, this latter loss of 
valuable heat may be materially reduced. 
To further offset the probable greater efficiency of the thermal 
unit developed by electric energy it must be borne in mind that the 
utilization of mine-run phosphates depends upon the location of a 
furnace plant close to the source of the raw material (the phosphate 
mines), and while certain of the phosphate deposits of Tennessee are 
sufficiently close to admit of the transmission of power to the mines 
from a development at Muscle Shoals, the Florida deposits are far 
removed from any possible source of cheap hydroelectric power. 
Low-priced fuel, however, is readily available in practically all of 
the phosphate fields of this country. 
With these facts in mind the writers undertook an investigation 
to determine if complete elimination of phosphoric acid from proper 
mixtures of phosphate rock, sand, and coke could not be brought 
about at the temperatures attained and under the conditions exist- 
ing in various types of fuel-fired furnaces, and the results have shown 
that this can be done by pioperly proportioning the ingredients in 
the charge and maintaining reducing conditions within the phos- 
phate mass. 
PRELIMINARY LABORATORY EXPERIMENTS. 
The work was first undertaken on a laboratory scale using rela- 
tively pure samples of tricalcium phosphate, quartz flour, and 
carbon. These ingredients were thoroughly mixed in several pro- 
portions, placed in graphite or clay crucibles, and heated in an 
injection furnace for various periods of time with the crucibles both 
open and closed. City gas and a cold-air blast were employed in 
heating these mixtures. 
Table 11 shows in part, the results obtained by heating such mix- 
tures with and without the addition of small amounts of aluminum 
oxide. 
