MANUFACTURE OF PHOSPHORIC ACID. 
17 
obtained by the same process using high-grade rock, and where the 
phosphoric acid obtained by smelting such low-grade materials is 
utilized in making double superphosphate, the final cost of the unit 
of P 2 5 comparesVery favorably with that produced by the sulphuric 
acid" method. At this point, however, the double superphosphate 
has a distinct advantage over the ordinary acid phosphate, in that it 
is so concentrated that it can be handled, shipped, and distributed 
at a minimum cost. 
THEORETICAL HEAT BALANCES. 
The data obtained in these experiments and given in Tables Nos. 3 
to 7, inclusive, indicate that where phosphoric acid is produced in 
the electric furnace the main cost item is that of electric power. 
Where this electric energy is available at a price as low as $25 per 
horsepower year, the actual cost of power is over 70 per cent of the 
total charge against the ton of P 2 5 in the form of H 3 P0 4 . With a 
properly designed furnace, however, and efficient auxiliary equipment 
for conserving the heat wasted in the ordinary crucible type of fur- 
nace, it should be possible to reduce the power cost per unit of phos- 
phoric acid very materially. In Tables 8 and 9, modified from two 
prepared by George T. Southgate. formerly employed in this bureau, 
the heat balance of two electric furnaces for the production of phos- 
phoric acid, one an open crucible and the other of the shaft type, are 
given. Certain assumptions were made in making up these tables, 
but the figures are considered conservative and comparable in so far 
as the relative heat economies are concerned. In both instances 
sufficient carbon is assumed to be present to reduce completely the 
phosphoric acid to elementary phosphorus and enough silica added 
to give a slag having the composition CaSi0 3 . 
Table 8. — Theoretical heat balance of electric furnace of simple crucible type. 
[Capacity 10 tons P 2 5 per day of 24 hours.] 
Actions affecting temperature. 
Heat generated 
(plus). 
Heat consumed 
(minus). 
Item 
No. 
Thousands 
of 
kilogram- 
calories 
per ton 1 
of P2O5. 
Per cent 
of 
total. 
Thousands 
of 
kilogram- 
calories 
per ton l 
of P 2 5 . 
Per cent 
of 
total. 
1 
Absorbed by burden before fusion 2 
\ ! 
2 
3 
Absorbed by burden in fusing ' 
Absorbed by endothermic reactions in ore 
1 
4,100 
36. 1 
4 
Evolved by exothermic reactions in ore 
420 
1, 140 
2,660 
2,850 
3.7 
10.0 
23.4 
25.0 

Evolved by oxidation of C to CO 
6 
Evolved bv oxidation of this CO to CO » 
7 
Evolved by oxidation of P to PoOs 
8 
Removal of heat bv cooling masonry 
1,619 
1,707 
2, 350 
1,600 
14. 2 
9 
Removal of heat by slag 3 
15. 
10 
Removal of heat by evolved gases (at 650° C) 
20.fi 
11 
Removal of heat bv unburned CO* 
14 1 
12 
Heat supplied by electric energy 
Total 
5 4,306 
37.9 
11,376 
100.0 
11,376 
100.0 
Econo 
In 
In 
In 
In 
my: 6 
pounds PjOi per kilowatt hour 
0.44 
2,000 pounds P 2 Oi per kilowatt year 
1 90 
2,000 pounds P 2 O s per horsepower vear 
1.43 
over-all thermal efficiency , per cent 
36. 10 
1 Metric ton. 
2 Included in item 9. 
•Including small amount removed by ferrophosphorus. 
* Assuming one-third of CO unburned in furnace. 
* 5,010 kilowatt hours. 
•Production based on 90 per cent recovery of P 2 Oj in furnace charge, 
52670—23 3 
