52 BULLETIN 1179, U. S. DEPARTMENT OF AGRICULTURE. 
temperature is attained. A new blower was installed, better aux- 
iliary equipment for conserving the heat of the evolved gases built, 
and a recording pressure gauge and pyrometers also installed. The 
furnace proper of this plant was operated a number of times, but it 
was finally decided to water-cool the furnace in order to conduct 
tests over more protracted periods. Accordingly a new furnace shaft 
consisting of a steel shell lined with carborundum and fire brick was 
constructed, over which a water curtain was caused to flow. Later 
a gas-tight steel shell (water-cooled) was also built around the furnace 
crucible and cast-iron cinder notches (water-cooled) inserted in the 
slag holes. One of the tests conducted with the new equipment was 
continued efficiently for five and a half hours after the furnace was 
first charged, and during this period 1,175 pounds of briquets were 
smelted and tapped off as a molten slag. The phosphoric acid content 
of this slag was 0.72 per cent, equivalent to a volatilization of 96.7 
)er cent of P 2 5 , but since the fuel-oil consumption was relatively 
high (17 gallons per hour) the yield of P 2 5 per gallon of fuel oil was 
only 3.2 pounds. Just after the furnace was tapped an explosion 
blew off the top of one of the stoves and rendered a continuation of 
this test impracticable. 
The next test was continued for a period of 70 hours, during 
which time over 4 tons of charge were smelted and tapped off as a 
molten slag. The data obtained during this 3-day run was not 
altogether satisfactory, owing to a number of mechanical difficulties 
and the poor quality of the fuel oil used. A volatilization of from 
46 to 94.5 per cent of P 2 5 was obtained, but the sediment in the 
crude oil necessitated a very much higher fuel consumption than 
was desirable for the efficient working of the furnace. Therefore the 
relation between the P 2 0- evolved and the fuel consumed was very 
unfavorable. This last furnace equipment, however, stood up admi- 
rably under the high temperatures attained and is in a stand-by 
condition for further experimentation. 
The results of the tests so far conducted with the semicommercial 
equipment at Arlington Experimental Farm, Va., have demonstrated 
conclusively that it is feasible to drive off phosphoric acid practically 
completely from run-of-mine phosphates in large-scale operations by 
means of burning fuel. They have also shown that a furnace can be 
constructed which will withstand for a protracted period the combined 
effects of extremely siliceous slags and the high temperatures attained 
in this process, but before data can be obtained which will show con- 
clusively the commercial feasibility of producing phosphoric acid by 
this process a number of the mechanical difficulties encountered 
must be remedied. 
During the tests so far conducted, however, with the semicommer- 
cial equipment, there were periods of from three to five hours when 
the furnace appeared to operate quite efficiently, and if the data 
taken at such times are considered indicative of normal working 
conditions, the volatilization method of producing phosphoric acid 
appears economically superior to the ordinary sulphuric acid method 
or to the electric-furnace process. Based on the data gathered 
between certain periods of actual operation the estimated cost of 
production by the fuel-furnace process is $36.31 per ton (or 36 cents 
per unit) of P 2 5 in the form of 75 per cent H 3 P0 4 and $30.98 per 
