MANUFACTURE OF PHOSPHORIC ACID. 6 
The fundamental principle involved is that at high temperatures 
(1,600° to 1,800° C.) silica assumes the properties of a relatively 
strong acid in so far as its ability to combine with bases is concerned, 
and therefore it can displace the phosphoric acid of phosphate rock 
forming silicates of lime and free phosphoric anhydride (P 2 5 ). The 
latter compound being highly volatile at elevated temperatures is 
driven off as a fume and may be collected either by absorption in 
water or by means of the Cottrell electrical precipitator, which is 
described later in this bulletin. When carbon or coke is added to 
the mixture to be smelted elemental phosphorus is produced, and if 
reducing conditions are maintained throughout the operation the 
decomposition of the rock and expulsion of its phosphorus content 
maybe brought about at considerably lower temperatures (1,300° 
to 1.500° C). 
According to Nielsen, 4 while tricalcium phosphate and silica begin 
to react at a temperature of 1,150° C. and continue to form various 
compounds up to 1.650° C. unless a reducing agent is present these 
chemical changes merely consist in certain combinations of the two 
substances and no phosphoric acid is evolved. This same author 
states that while CO will not reduce tricalcium phosphate this latter 
compound is completely reducible by carbon, the reduction beginning 
at 1,400° C. He claims, however, that there always remains some 
phosphorus in the residue because the CaO formed unites with the 
undecomposed calcium phosphate to form more basic compounds of 
phosphoric acid which are not reduced by carbon. Peacock 6 takes 
issue with Nielsen on this point, stating that if the evolved gases 
are removed as last as they are formed complete dispersion of phos- 
phorus is obtained by heating mixtures of phosphate rock and carbon. 
Carnot, 8 Stead, 7 and Kroll, 8 however, have recognized the exist- 
ence of certain nonvolatile compounds of phosphoric acid, silica, 
and lime in basic slag where an excess of lime is always present, and 
it seems logical to the writers that such compounds would form in 
simple mixtures of phosphate rock and carbon after sufficient P 2 5 
were evolved to render the residue basic. Nielson is wrong, however, 
in his conclusion that P 2 5 is not evolved upon heating mixtures of 
phosphate rock and silica in the absence of a reducing agent, for 
later experimentation has shown that where a relatively high per- 
centage of silica is added phosphoric acid is copiously evolved when 
the temperature of the mass approaches 1,800° C. 
The presence of a reducing agent is essential, however, in order to 
bring about the volatilization of phosphoric acid (or phosphorus) at 
the lower temperatures and the writers have found that by briquet- 
ting finely ground mixtures of phosphate rock, silica, and coke (thus 
insuring reducing conditions within the mass and a close contact 
between the reacting materials) the volatilization of P 2 5 begins con- 
siderably below 1,300° C. and by prolonged heating at this tempera- 
ture the bulk of this acid may be driven off. 
While various silicates of lime are no doubt formed in smelting 
mixtures of phosphate rock, sand, and coke, depending on the silica- 
* Ferrum, vol. 10. p. 97-111 (1913). Translated in American Fertilizer, vol. 39, No. 6, p. 63 (1913). 
5 American Fertilizer, vol. 39, No. 6, p. 67 (1913). 
« Compt. I- end. 97, p. 136 (18831. 
1 Trans. Chem. Soc. 51, p. 601 (1887). 
• Jour. Iron and Steel Inst. 2, p. 126 (1911). 
