MANUFACTURE OF PHOSPHORIC ACID. 
27 
too rapidly. This chamber was supported by arches of carborundum 
brick above a hollow hearth intended to receive the molten silicate. 
The whole was surrounded by an outer chamber into the opposite walls 
of which were first set water-cooled tuyeres to deliver preheated air 
for the combustion of the coke as the charge descended upon the 
hollow hearth. These tuyeres, ho vever. were later replaced by two 
oil burners so arranged that their flames played upon and around the 
lover part of the inner chamber, heating the charge by radiation 
through the 4^ -inch walls. Any fumes which were evolved from the 
fusing mass were to be drawn down through the charge chamber into 
the outer chamber and passed, together with the gases of combustion, 
into the Cottreli precipitator previously described. 
This indirect method of heating the charge, however, was so 
inefficient that after several trials, lasting from 18 to 24 hours each, 
it was abandoned as impractical. Little or no slag was obtained and 
only a relatively small 
quantity of phosphoric 
acid was driven off 
from the sintered prod- 
uct. 
These experiments 
showed clearly that in 
order to make the proc- 
ess economically prac- 
tical the calorific power 
of the fuel must be 
more completely util- 
ized, which could only 
be done by the direct 
heating of the mass. 
This conclusion, how- 
ever, complicated the 
problem of how to 
maintain the reducing 
conditions necessary 
for the volatilization of 
the phosphoric acid 
when the maximum 
efficiency of the crude- 
oil flame or that of any 
oxidizing conditions 
.o-^te^i 
V^7 
Fig. 10.— Vertical section of oil-burning furnace in which the first 
large scale experiments on the volatilization of phosphoric acid 
were conducted. 
other fuel can only be attained under 
The most promising type of furnace for this 
purpose seemed to be a modification of the blast furnace, wherein 
sufficient coke is used to maintain a reducing atmosphere both in the 
crucible and in the shaft. The run-of-mine phosphates with which 
the experiments were being conducted, however, are of such a 
character that it is impossible to handle them in a plant of the blast- 
furnace type, because of the large percentage of finely divided mate- 
rial present in the mass, through which it is impossible to force a draft. 
THE B3IQUETTING OF MINERAL PHOSPHATES. 
Accordingly experiments were undertaken with a view to briquet- 
ting the mixtures of finely ground pebble phosphate, sand, and coke, 
using various binders, such as magnesium chloride, calcium chloride, 
sodium fluoride, sodium silicate, phosphoric acid, acid sludge from 
