MANUFACTURE OF PHOSPHORIC ACID. 45 
DISCUSSION OF EXPERIMENTAL RESULTS. 
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 nearly 
completely from briquetted charges of phosphate rock, sand, and coke 
in large-scale operations by means of burning fuel. 
It has also been shown that by the use of high-grade refractories 
and proper water-cooling devices a furnace can be constructed which 
will withstand the combined effects of extremely siliceous slags and 
the high temperatures (1,500 : to 1 650 : C attained in this process. 
Other points brought out by these experiments are the feasibility 
and advantages of having auxiliary equipment for heat regeneration 
and to make sure that only oxidized products arc transmitted to the 
electrical precipitator where the phosphoric acid is collected. 
In the present furnace, however, which has a relatively short shaft 
or charge chamber, it is necessary to localize or concentrate the 
highest temperature upon the furnace hearth. During the last pro- 
tracted test fof 70 hours; just described, the impurities present in 
the oil frequently made it necessary to operate the burners at a rather 
high pressure in order to obtain proper atomization of the fuel. This 
high pressure caused a greater fuel consumption than was either 
essential or desirable, since it resulted in a long flame which burned 
well up in the shaft, causing the entire charge to soften and settle 
down upon the hearth in a semifluid condition. The lower levels of 
this viscous slag, therefore, were not exposed to the maximum tem- 
perature for a sufficient time to drive off the phosphoric acid either 
rapidly or completely. In a furnace of larger dimensions with a higher 
shaft, and where much greater air pressures are employed (such as 
in the ordinary blast furnace;, trouble of this kind would hardly be 
encountered, since the charge is kept in a more or less floating con- 
dition, 50 and therefore while descending through the shaft is exposed 
to the necessary high temperatures for a considerably longer period 
than is possible in a plant of the present size. 
The concentration of the heat in the crucible of this small furnace, 
however, may be accomplished by a more thorough atomization of 
the oil at the lower pressures, which will result in shorter flames and 
less fuel consumption. These conditions were apparently fulfilled in 
some of the other experimental runs where rapid evolution and ex- 
cellent yields of P 2 5 were obtained. 
For the efficient working of this furnace it is also essential to elimi- 
nate the rather frequent shutdowns, such as were necessitated by the 
failure of the blowing equipment. The ratio of surface area to the 
volume of this furnace is such that radiation losses are far more 
serious than in a plant of large capacity. Even fluctuations in the 
amount of air delivered are apt to cause the formation of scaffolds 
in the shaft which tether upset the relation between the oil and the 
air required for its combustion. Because of the small diameter of 
the furnace shaft, a scaffold often means a complete bridging over 
of the charge, which puts an added load on the blower to force the 
air through this partially fused mass. While scaffolds also form at 
times in large blast furnaces, the complete bridging over of the charge 
^ Johnson, J . E., jr. Principles, Operation, and Products of the Blast Furnace .p. 115-122 (1918). 
