MANUFACTURE OF PHOSPHORIC ACID. 49 
on methods of producing phosphorus, phosphoric acid, and phosphate 
compounds either in an electric or fuel furnace. In some of these 
processes a reducing agent is employed and in some the carbon is 
left out of the charge. Combinations of the fuel and electric furnaces 
have also been proposed, but until recently it has been the general 
belief that the complete volatilization of phosphoric acid from mix- 
tures of phosphate rock. sand, and coke could only be attained in an 
electric furnace. Accordingly, the first work conducted in this bu- 
reau was carried on in a simple electric furnace using high-grade 
phosphate rock and fairly pure samples of sand and coke. In this 
investigation the Cottrell method of electrical precipitation was em- 
ployed for the collection of phosphoric acid fumes. This method 
nas proven very satisfactory and has the advantage of recovering the 
Eroduct in a relatively pure and concentrated form which can be 
andled and shipped at a minimum cost. A description of this pre- 
cipitator equipment is given in detail. 
The investigation was then carried out on a larger scale with a 
view to determining the commercial possibilities of the process. A 
cooperative arrangement was entered into with an outside concern 
and protracted tests made over a period of several days and weeks. 
Assuming power could be obtained at $25 per horsepower year the 
estimated cost per pound of P 2 5 in the form of phosphoric acid was 
3.37 cents, exclusive of interest, maintenance, and depreciation 
chanres. No means of heat regeneration were employed, but even 
on the basis of these figures the cost of the unit of P 2 5 was not very 
much in excess of that in acid phosphate during the war period, when 
the price of sulphuric acid was exceptionally high. Furthermore, by 
using the volatilized acid collected by the Cottrell precipitator for 
treating high-grade phosphate rock for the production of double 
superphosphate, the final cost of the unit of soluble P 2 5 can be ma- 
terially reduced. Since these experiments were conducted electric 
furnaces of commercial size for the simultaneous production of ferro- 
phosphorus and phosphoric acid have been erected and economically 
operated. Most of the phosphoric acid produced, however, is sold 
for purposes other than fertilizers, as for these purposes it commands 
a higher price. 
There is another factor, however, which has a direct bearing on the 
cost of manufacturing P 2 5 by the furnace process. This is the great 
saving which can be effected by employing low-grade and run-of-mine 
phosphates unfit for treatment with sulphuric acid. A number of 
experiments were undertaken in the electric furnace at Aj-lington 
Experimental Farm. Va., to determine if these run-of-mine phos- 
phates could be successfully employed in the volatilization process. 
The results of this investigation are given in tabulated form, showing 
that complete volatilization of phosphoric acid may be readily ob- 
tained by the use of such charges. There are given also the estimated 
costs of phosphoric acid per ton and per unit" by the sulphuric acid 
process and by the electric furnace process, using in some cases high- 
grade washed rock, and in others run-of-mine phosphates, and then 
treating phosphate rock with the phosphoric acid thus obtained. 
These estimates show that by the use of run-of-mine phosphates it is 
possible to reduce the cost per unit of P,0 5 to a point where it com- 
pares more favorably with that produced by the sulphuric acid 
