MANUFACTURE OF PHOSPHORIC ACID. 85 
The data given in the above table show that, on the whole, an 
increase in the silica content of these briquetted mixtures resulted 
in a more rapid evolution of phosphoric acid, but usually at the 
sacrifice of the fluidity of the slag. 
The ratios of silica to lime in the first and third batches of bri- 
quetes — namely, 59 : 41 and 63 : 37 — do not correspond to any definite 
calcium silicates but the proportions of these two ingredients (61 : 39 
and 68 : 32) in the second and fourth batches coincide closely to the 
ratios of silica to lime in calcium trisilicate (Ca 2 Si 3 8 ) and calcium 
di-meta silicate (CaSi 2 5 ), respectively. 
Where the proportion of silica to lime was 59 :41 (first batch of 
briquets) it required from 40 to 50 minutes to drive off the phos- 
phoric acid from the charge at a temperature of 1,600° C. In every 
other instance, however (where the silica content was higher), the 
nearly complete volatilization of phosphoric acid was accomplished 
in from 30 to 35 minutes. Yet the slight increase in the rate of 
evolution of P 2 5 where the proportion of silica to lime was raised 
above that in the second batch of briquets (namely, 61 : 39) was 
insufficient to offset the disadvantages of having a more viscous slag 
to tap and a greater weight of charge to smelt. Therefore a charge 
having the proportion of 61 silica to 39 lime was adopted as the most 
practical for the subsequent large-scale experiments. 46 
Another important point brought out by these experiments is the 
possibility of using bituminous coal as a reducing agent in the bri- 
quets in lieu of coke. Not only was it found that the evolution of 
the volatile hydrocarbons which are given off in the preliminary 
heating of such mixtures did not disintegrate or split the briquets 
but that there was an actual deposition of carbon (from the decom- 
position of hydrocarbons) throughout the mass which seemed to aid 
in the subsequent reduction of the phosphate mineral. These facts 
open up an interesting line of research on the feasibility of sub- 
stituting coal for at least part of the coke in the furnace charge. It 
seems probable, however, that such a procedure would necessitate 
altering the design of the furnace so that the extra amount of 
combustible gases evolved could be taken care of before the phosphoric 
acid was passed into the collector. 
The several types of briquets used in this experimental work are 
shown in Plate IV, Figure 1, and the effect of the preliminary 
heating on these briquets (before fusion takes place) is shown in 
Plate IV, Figure 2. 
The average temperature obtained in the experiments recorded 
above was approximately 1,600° C, although at times individual 
readings showed as high as 1,700° C. Either of these temperatures, 
however, is below the maximum obtained in blast-furnace practice, 
and where water-cooling devices are employed no undue erosion of 
the furnace lining is anticipated. It is true that the acid character 
of the slag produced in smelting the phosphate charge has a tendency 
to attack more vigorously a fire-brick lining than the basic or neutral 
slag obtained in the smelting of iron, but a balance would soon be 
46 In the larger scale experiments subsequently described it was found, however, that the slags 
produced from charges of this composition congealed so readily that the tapping of the furnace was 
often rendered somewhat difficult. Further experimental work is being conducted with a view to 
determining a charge composition which will give a slag of greater fluidity at lower temperatures than 
1,600° C. without materially reducing the evolution of PjOs. 
