American continental phosphate rock consists essentially of submicrocrystalline fluorapatite, C:al~F2(PO4)6, which contains some excess fluorine and a small amount of sodium. The compounds Calo(OH)z (PO,),, Cas(H2O)Z-(P04)6, and CaloCOs(P04)e.H20 form extensive series of solid solutions with fluorapatite, some members of which occur in geologically recent rocks. The phosphate rock of Curacao Island is essentially a hydrate of tricalcium phosphate, Cas(H20)n(PO&. Hydroxyfluorapatite, Calo(F,0H)~(P04)e, in which only about half of OH-is replaced by F-, occurs on Christmas, Nauru, and Ocean Islands.HE phosphate rock deposits of the world have usually been described as consisting of various amorphous T and microcrystalline apatite-like minerals. Lacroix (25) considered certain French phosphorites to consist essentially of colloidal tricalcium phosphate, collophane, crystalline staffelite (francolite), and crystalline dahllite (podolite). Schaller (58) concluded that phosphorites consist of the following compounds in solid solutions: Dahllite 9Cs0~3P~Oa~CaCOs~HzO Francolite 9Ca0.3PzOrCa(F1, COa).H?O Collophane 9Ca0.3PzOsCaCOs~HzO + xHzO Fluorapatite SCa0.3PzOs~CaFz Hydroxyapatite 9CaO.3PzOvCa(OH)rRogers (35) writes: "The principal constituents of phosphorites or so-called phosphate rocks and also of fossil bones is an amorphous substance with properties sufficiently characteristic to be entitled to recognition as a distinct mineral. I t may be called 'collophane.' " He gives as the approximate formula of collophane:3Ca3( PO4)~1-2Ca( C03,Fz,S04,0).xHz0
The phosphorus is oxidized either within or outside the furnace proper and is finally recovered as orthophosphoric acid.18 From the standpoint of commercial operation of the process, one of the primary functions of the silica is to combine with the lime to form a liquid slag which can be tapped from the furnace. The investigations of Berthier,1 Hempel,10Nielsen,16 and Ross, Mehring, and Jones19 show, however, that silica also plays a definite part in accelerating the reaction and reducing the temperature at which reduction begins. The last-named investigators have further shown that it is not necessary to form a liquid slag in order to obtain practically complete volatilization of phosphorus from small mixtures of phosphate rock, silica, and coke.Definite information on the factors affecting the reduction by carbon, in the absence of silica, of tricalcium phosphate, which for most purposes may be considered as the primary constituent of phosphate rock, is essential in order to be able to draw reliable conclusions regarding the effect of silica on the reaction. Berthier1 was unable to reduce phosphate of lime with carbon alone, and according to Thorpe22 tricalcium phosphate is not reduced by carbon in the absence of silica except at excessively high temperatures, and even 1
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