20 Nature of Mineral Phosphates 



The solubility of Florida pebble phosphate in citric acid is noticeably 

 less than that of Makatea Island phosphate. The average of twenty 

 citric extracts gives a ratio P2O5 : CaO of 1 : 148. Extracts made with 

 hydrochloric acid instead of citric acid give exactly the same ratios. 

 The curves (Fig. 2) for the portion passing " 100" sieve and the portion 

 refusing "30" sieve are quite normal, and clearly demonstrate that 

 whatever its nature, only one phosphate is being extracted. 



A calculation based on the figures obtained for the citric acid 

 extracts or hydrochloric extracts gives the formula 4Ca3P20g . 3CaO 

 (P2O5 : CaO :: 1 : 1-48) as representing the phosphate present in Florida 

 pebble phosphate. 



Calcined Florida Pebble Phosphate. 



Five grams of the phosphate in a platinum dish were calcined for 

 4| hours in a Fletcher muf&e furnace. A sugar extract was made to 

 remove any free lime, and this was followed by five half-hour extractions 

 with 500 c.c. of 2 per cent, citric acid. 



That a change in the composition of the phosphates has been 

 produced by calcining is obvious, and a glance at the curve on Fig. 2 

 shows that at least two phosphates are being dissolved by the citric 

 acid. More phosphoric acid is soluble in the second citric extract than 

 in the first. There is a big increase in the ratio PgOg : CaO in the first 

 citric extract, and as any free lime would be removed by the sugar 

 extract it would appear that the phosphate dissolving in the first 

 extract has a much higher lime content than the original phosphate. 

 Sihca derived from the sand in the sample of Florida pebble phosphate 

 has also been brought into combination. The ratio P0O5 : CaO of 

 1 : 1-38 obtained in the 3rd, 4th and 5th extracts suggests at once the 

 presence of the phosphate 2Ca3P208 . CaO of which Makatea Island 

 phosphate is composed. The formation of this phosphate would account 



