4 BULLETIN 1179, U. S. DEPARTMENT OF AGRICULTURE. 
lime ratio in the charge, the general course of the reaction is usually 
represented by the following simple equation : 
Ca 3 (P0 4 ) 2 + 3Si0 2 + 5C = 3CaSi0 3 + P 2 + 5CO. 
The phosphorus evolved is oxidized by air or carbon dioxide to 
phosphorus pentoxide either within or outside of the furnace proper 
according to the following equation : 
(1) 2P 2 + 50 2 = 2P 2 5 
(2) P 2 + 5C0 2 = P 2 5 + 5CO. 
Actual experience has proven, however, that in oxidizing phos- 
phorus either with air or carbon dioxide there is ordinarily sufficient 
moisture present to form orthophosphoric acid and even where the 
Cottrell method of precipitation is employed the final product is a 
liquid. This reaction is represented thus: 
P 2 5 + 3H 2 = 2H 3 P0 4 . 
ADVANTAGES OF THE VOLATILIZATION PROCESS. 
The pyrolytic process of producing phosphoric acid for fertilizer 
purposes appears to offer four distinct advantages over the sulphuric 
acid process even where the actual cost of the power or fuel required 
per unit of soluble P 2 5 is somewhat greater than the cost of the 
sulphuric acid necessary to effect the same result. 
(1) This process makes it possible to utilize low or medium grade 
phosphate deposits which are unfit for treatment with sulphuric 
acid, either because of their low content of P 2 5 or their relatively 
high content of such impurities as oxides of iron and aluminum. 
Where the presence of 6 to 8 per cent of these impurities will cause 
a rock to be rejected as unsuitable for acid phosphate manufacture, 
this amount of alumina tends to aid rather than interfere with the 
reactions sought in the furnace process. Iron likewise, while very 
objectionable in the sulphuric acid process, causes no trouble in the 
furnace method other than its tendency to " fix " some of the phos- 
phoric acid in the form of ferrophosphorus. 9 The formation of a 
certain amount of this compound, however, does not necessarily 
mean a loss, since it finds a ready market in the steel industry. The 
chief impurity in phosphate rock is usually silica, which is often 
present in such large amounts as seriously to dilute the acid phos- 
phate made therefrom. Yet such phosphates may require further 
additions of sand in order to obtain the proper silica-lime ratio for 
successful treatment by the furnace process. 
(2) The furnace method of treating phosphate rock makes it possible 
to dispense with the elaborate washing and screening processes now 
so extensively employed in Florida and Tennessee to separate the 
phosphate rock from the gangue or matrix in which it is embedded. 
As has been previously pointed out, 10 this mechanical separation 
involves the loss of a great deal of finely divided phosphate which 
would be practically all saved if the " run-of-mine " material were, 
treated directly by the furnace scheme. 
(3) The adoption of this method would largely eliminate the use of 
sulphuric acid which under present conditions is hauled to the fertilizer 
9 An alloy of iron and phosphorus containing as a rule from 10 to 20 per cent of the latter element. 
10 Phosphorus in Fertilizer, Yearbook, U. S. Department of Agr., 1920, p. 217. 
