MANUFACTURE OF PHOSPHORIC ACID. 43 
The furnace was again tapped at 4.30 p. m. and this slag (fourth 
tap) showed 12.78 per cent of P 2 5 or a volatilization of 34.9 per cent. 
Since the temperature of the hearth on tapping was 1,570° C. and 
the working conditions apparently rather good during the period, 
the high P 2 O s content of this slag is somewhat difficult to explain. 
It was probably due, however, to rapid charging immediately after 
tapping the previous slag when operating conditions had been so 
adverse, and also to the fact that the oil consumption was so high 
that the material melted out of the furnace shaft at a more rapid 
rate than was desirable for the efficient elimination of P 2 5 . 
While this fourth tap was in progress 500 pounds of briquetted 
charge were added and at 6 p. m. the cinder notches were again 
closed. Between 5 and 8.30 p. m. 1.350 pounds of briquets were 
charged, but some trouble was experienced during this period, owing 
to scaffolding in the shaft from time to time. At 9.20 p. m. one side 
of the furnace was tapped (fifth tap), a portion of the slag showing 
5.9 per cent P,0 5 or a volatilization of 70.1. but another sample of 
somewhat better appearing slag showed a content of only 1.18 per 
cent P 2 5 , equivalent to a volatilization of 94.4 per cent. The slag in 
the north end of the crucible, however, was so viscous that it did 
not tap readily, a condition which was probably caused by a channel 
which opened up in the shaft toward this end of the furnace some- 
time previously, which allowed the combustion of the oil to take 
place well up in the shaft rather than upon the hearth, where the 
maximum temperature is desired. One hundred pounds of charge 
were added between 10.20 and 11.20 p. m., largely to close the chan- 
nel mentioned above, but the north end of the crucible was not 
entirely tapped free of slag until 5.45 the following morning. The 
average phosphoric acid content of this slag (sixth tap) was 7.56 per 
cent, equivalent to a volatilization of 61.9 per cent P 2 5 . During all 
of this period the precipitator was cut off for fear the unburned oil 
or carbonaceous matter which distilled over from time to time would 
cause deposition of carbon in the precipitator pipes and on the cen- 
tral wires and result in the arcing across of the high-tension current. 
When the furnace was freed from the slag of the previous charge 
and the temperature of the hearth apparently sufficiently high for 
ready volatilization of P 2 5 it was decided to put in a charge and 
allow it to run through and maintain it in a molten condition on the 
hearth until the evolution of P 2 5 was no longer apparent. Accord- 
ingly between the hours of 7 and 9 a. m. 1,100 pounds of briquets 
were charged to the furnace. This was melted and ran entirely 
through the shaft at 10.30 a. m., the top of the furnace being left 
open during this period in order to observe the fumes. This proce- 
dure, of course, resulted in enormous losses of heat energy, not only 
due to the fact that the gases of combustion were discharged directly 
into the atmosphere, and therefore no regenerative effect gained as 
is the case when the shaft is full of charge, but also because the stoves 
could not be operated under these conditions, and therefore practi- 
cally no preheated air was employed. During this period from 8 a. m. 
to 12 noon the oil pressure was maintained at about 75 pounds per 
square inch, with an average oil consumption of 20 gallons per hour. 
Fumes of P 2 5 could no longer be readily detected at noon and at 
12.45 p. m. the furnace was tapped. The south cinder notch was 
opened and a molten slag ran out readily for 15 or 20 minutes (sev- 
