8 BULLETIN 144, U. S. DEPARTMENT OP AGRICULTURE. 



CALCIUM FLUORIDE. 



Fluorides are present in almost all phosphate rock. Some samples 

 contain as high as 8 per cent of calcium fluoride (CaF 2 ). The amor- 

 phous phosphates as a rule contain smaller quantities of this com- 

 pound than apatite. 



Calcium fluoride reacts with sulphuric acid, giving gaseous hydro- 

 fluoric acid (HF) and calcium sulphate, thus: 



Calcium fluoride. Sulphuric acid. Hydrofluoric acid. Calcium sulphate. 

 CaF 2 + H 2 S0 4 = 2HF + CaS0 4 



But hydrofluoric acid (HF) acts upon the silica or silicates present 

 in the mass, producing gaseous silicon, tetrafluoride (SiF 4 ), and water 

 or steam, thus : 



Hydrofluoric acid. Silica. Silicon tetrafluoride. Water. 



4HF + Si0 2 = SiF 4 + 2H 2 



Silicon tetrafluoride in turn is decomposed by water with the 

 formation of hydrofluosihcic acid (H 2 SiF 6 ) and precipitation of pure 

 silica (Si0 2 ), thus: 



Silicon tetrafluoride. Water or steam. Hydro fluosilicic acid. Silica. 

 3SiF 4 + 2H 2 = 2H 2 SiF Q + Si0 2 



Before this last reaction takes place, however, much of the silicon 

 tetrafluoride escapes from the mass and can be detected by its pene- 

 trating odor and smarting effect on the eyes and nose. 



Veiy high grade acid phosphate can be made from rock containing 

 large amounts of fluorine, because, as pointed out above, many of the 

 products formed during the process escape as gases or vapors, leaving 

 the mass correspondingly richer in phosphoric acid. These gases 

 also, in forcing their way out of the acid phosphate, tend to render it 

 porous and more readily dried. The product, therefore, can be easily 

 broken up and mixed with other ingredients to make a complete 

 fertilizer. 



The main objections to using phosphates high in fluorides are, first, 

 the increased quantity of sulphuric acid necessary to decompose these 

 compounds, and, second, the noxious and even poisonous nature of 

 the gases evolved during their decomposition. 



COMPOUNDS OF IRON AND ALUMINUM. 



Iron and aluminum oxides, either in the free state or combined as 

 phosphates, are the most objectionable of the impurities found in 

 phosphate rock. These substances even when present in very small 

 quantities cause a certain amount of " reversion" in the superphos- 

 phate, and when present in large quantities are likely to produce a 

 sticky acid phosphate unfit for commercial purposes. 



The phosphate of iron in natural occurrences may conveniently be 

 represented by the formula FeP0 4 , although actually it is probably 



