48 BULLETIN 1179, U. S. DEPARTMENT OF AGRICULTURE. 
By comparing the figures given in Tables 20 and 21 with those in 
Tables 3 to 7, pages 15 and 16. it will be seen that the estimated cost per 
ton (or per unit) of P 2 5 produced by the fuel furnace method is con- 
siderably below that obtained by either the electric furnace or sul- 
phuric acid process. If the acid thus produced be used in the manu- 
facture of double acid phosphate the economy of this method 
appears even more marked. 
Whether or not efficiencies can be obtained throughout a protracted 
run commensurate with those indicated during certain short periods 
of actual operation must yet be proven, and it is the intention of the 
writers to establish this point, if possible, with the present plant by 
eliminating the mechanical difficulties already cited. But the fun- 
damental principles established and the progress which has been 
made in large-scale experiments so far conducted make it appear 
that this general method of producing phosphoric acid will be com- 
mercially applied. 
SUMMARY AND CONCLUSIONS. 
During the last decade there has been a tendency toward a higher 
specialization in the American industries, but while the American 
fertilizer industry up to the time of the European war had shown a 
rather steady growth, in some regards it had lagged considerably 
behind. It is now apparent, however, that fertilizer practice as a 
whole and phosphate production in particular are on the verge of a 
profound change. This bulletin describes certain research investiga- 
tions of the Fertilizer Division of the Bureau of Soils and was pre- 
pared with a view to showing the commercial possibilities of the 
volatilization process for producing phosphoric acid and phosphatic 
fertilizers. 
The fundamental principles involved in the pyrolytic production 
of phosphoric acid are (1) at high temperatures, silica assumes the 
properties of a relatively strong acid and can displace the phosphoric 
acid of phosphate rock forming silicates of lime and free phosphoric 
anhydride (P 2 5 ) ; (2) in the presence of a reducing agent, such as 
coke, elemental phosphorus is produced and the decomposition of 
the rock is brought about at considerably lower temperatures. 
This process has four advantages over the sulphuric acid method 
of producing soluble phosphate. (1) Low-grade phosphates (high in 
silica) unfit for sulphuric acid treatment may be used, since the pres- 
ence of silica is necessary for the smelting method. (2) Since the 
furnace process utilizes mine-run material, it is possible to dispense 
with the costly steps of washing and screening the rock, which en- 
tails the loss of so much phosphate. (3) The furnace process calls 
for no sulphuric acid, which under present conditions is hauled to the 
fertilizer plants as acid and hauled away again as gypsum in acid 
phosphate. This acid acts primarily as a reagent and actually dilutes 
the final product. (4) By the use of electric or fuel furnaces near 
the phosphate mines it is possible to produce a relatively concen- 
trated product which can stand heavy handling charges and the cost 
of long freight hauls. 
The pyrolytic process of producing phosphoric acid is based on the 
old metnod of making phosphorus, but the steps employed to-day 
are much less cumbersome and costly than in the earlier days of 
phosphorus manufacture. Numerous patents have been taken out 
