50 BULLETIN 1179, U. S. DEPARTMENT OF AGRICULTURE. 
method. The main cost in producing phosphoric acid in the electric- 
furnace is that of electric power, hut with a properly designed furnace 
and efficient auxiliary equipment it should be possible to reduce the 
power cost per unit of phosphoric acid very materially. Tables are 
given containing the thermal heat balance of two types of electric fur- 
naces, one in which practically all of the heat in the effluent gases is lost 
and the other of the shaft type wherein much of the heat is regener- 
ated. It is pointed out that in a furnace charge containing sufficient 
carbon for the reduction of the phosphoric acid to elemental phos- 
phorus the over-all furnace reactions are exothermic. If, therefore, 
no heat were lost in the evolved gases from radiation and in the slag 
which is tapped off the reactions when started should go to an end 
without power consumption . 
While it is entirely possible that future developments of our 
water-power resources may make electric energy available at a price 
considerably below the present prevailing rates, under existing 
conditions the thermal unit can be obtained very much more cheaply 
from burning fuel than it can by electric energy. Therefore, if the 
necessary temperature can be attained and the conditions fulfilled 
for volatilizing phosphoric acid by means of burning fuel in lieu of the 
electric arc a great saving in the cost of production should be effected. 
Charts and a table comparing the cost of the heat unit produced by 
electric power at present rates with that obtained from standard 
fuels at prevailing prices show very conclusively that the cost of the 
thermal unit obtained by fuel is far below that obtained by electric 
energy. 
The writers therefore undertook an investigation to determine if 
fuel could not be substituted for the electric arc. In the preliminary 
laboratory experiments where pure samples of tricalcium phosphate, 
quartz flour, and coke were employed it was found possible to 
completely eliminate phosphoric acid from such mixtures providing 
reducing conditions were maintained within the mass and the 
materials so proportioned as to give a rather highly siliceous but 
fusible slag. The ratio of silica to lime adopted was approximately 
61 to 39, which is very close to the proportions of these ingredients 
in calcium trisilicate (Ca 2 Si 3 8 ) . The work was then continued on a 
larger scale. In these tests mine-run phosphates were employed and 
an oil-burning furnace used of such a design that the charge was heated 
indirectly. This type of furnace was abandoned as impractical since 
it was evident that in order to make the process economically prac- 
tical the full calorific power of the fuel should be utilized, which can 
only be done by the direct heating of the mass. The most practical 
type of furnace to employ seemed to be a modified form of the blast 
furnace, but in order to maintain reducing conditions and handle a 
charge of mine-run phosphate containing much finely divided mate- 
rial it was necessary to briquet or nodulize the mixture. Accordingly, 
experiments were undertaken with a view to briquetting the mixtures 
of finely ground phosphate, sand, and coke. Various binders were 
employed, but none of them were satisfactory either from the stand- 
point of price or their poor binding qualities. It was found, however, 
that in many typical phosphate deposits the percentage of very finely 
divided material which is classed as clay was ample to give the 
mixture sufficient plasticity to make excellent briquets when the 
mass was moistened with about 7 to 10 per cent water. Many 
