MANUFACTURE OF PHOSPHORIC ACID. 51 
deposits of rim-of-mine phosphate in Florida and Tennessee are of 
such composition that they can be used in the furnace process either 
by the addition of sand to raise their silica content or by reinforcing 
with somewhat high-grade phosphates to give the proper silica-lime 
ratio in the charge. A series of laboratory tests were conducted with 
a view to determining the best proportions of silica and lime in the 
briquet ted charge for the most rapid evolution of P 2 5 . The nearly 
complete volatilization of phosphoric acid from briquets having a 
silica-lime ratio of 69 to 31 was attained in from 30 to 35 minutes 
and the slag was sufficiently molten to flow freely at 1,600° C. 
Another point brought out by these experiments was the possibility 
of using bituminous coal as a reducing agent in the briquets in lieu 
of coke. 
The large scale operations on smelting briquet ted charges of phos- 
phate rock, sand, and coke were practically all conducted in a furnace 
holding 700 pounds of briquetted charge and having certain features 
of both the open-hearth and blast-furnace types. Near the top of 
the furnace shaft a flue leads into a dust chamber and from thence 
into a series of stoves for burning any combustible gases evolved 
from the furnace and regenerating the heat in the effluent gases. 
The auxiliary equipment of this earlier furnace was not entirely 
satisfactory, so it was later torn down and replaced by more suitable 
equipment. Oil burners set into water-cooled steel jackets furnished 
the fuel required for the heating of the furnace and in the earlier 
work three small positive-pressure blowers delivered the air required 
for the combustion of this oil. The phosphate used in these experi- 
ments was mine-run material obtained from the hard-rock fields of 
Florida. Two tests conducted with this equipment are recorded, one 
of these lasting for a period of only 10 hours and the other for a 
period of 20 hours. In the first experiment an average evolution of 
52 per cent of P 2 5 was obtained from the charge, and while the 
slag finally congealed to the point where it was not possible to tap 
the furnace, the fuel consumption toward the latter part of the run 
indicated that the rate of evolution was 88.5 pounds per hour or 
8^ pounds of P 2 5 per gallon of fuel oil consumed. 
The next test was continued for a period of 20 hours, during which 
time 3,500 pounds of briquetted charge were smelted and run out of 
the furnace as a molten slag. While numerous mechanical difficul- 
ties were encountered in the earlier stages of this run, during the last 
3^ hours the furnace operated quite efficiently, temperatures as high 
as 1,600° C. being attained during this period and an average voia- 
ization of 97 per cent P 2 5 obtained. The rate of evolution during 
the latter part of this test was 46.3 pounds of P 2 5 per hour with a 
fuel-oil consumption of 8.3 gallons per hour. On this basis a volatil- 
liization of 5^ pounds of P 2 5 per gallon of fuel oil was obtained. 
While the electrical precipitator was operated for only a portion of 
the time, a considerable quantity of phosphoric acid varying in 
strength from 21 ,5 to 64 per cent'H 3 P0 4 was collected, the stronger 
acid being obtained toward the last of the test when the temperature 
of the system had risen to the point where very much less moisture 
was condensed. 
The refractory lining of the furnace after this test, however, was 
found to be in such bad condition that the furnace proper was rebuilt 
and carborundum brick used in those portions where the highest 
