148 CHEMICAL FERTILIZERS 



is always giving off steam, the latter and silicon tetra- 

 fluoride will often meet where the gases escape, and it is 

 therefore very common to see in the flues, in the doors and 

 any odd corners of the den, large masses of sponge-like silica, 

 which have been produced by these chemical changes. 

 Different observers discover different proportions between 

 the fluorine that escapes and that which remains. It is only 

 to be expected that there should be this great difference, as 

 the water present and the heat evolved will influence these 

 proportions considerably. vSilicates of lime and alumina 

 occur in some of the rock phosphates, the former are 

 decomposed by either sulphuric or phosphoric acid, but only 

 somewhat slowly. Part of the reversion alluded to on p. 154 

 is possibly due to some such slow reaction taking place. 



Other Halogens. Along with true apatite, there are 

 present in the rock phosphates some bodies of analogous 

 composition, in which chlorine, iodine or hydroxyl takes the 

 place of fluorine. Chlorine, if present, is evolved as hydro- 

 chloric acid, and passes off with the remaining gases into the 

 washer. Iodine may also be present in traces, when it is given 

 off as hydriodic acid. As the latter is partly oxidized by 

 atmospheric oxygen, small quantities of free iodine are 

 occasionally liberated, and give a slight violet tinge to the 

 effluent gas. 



Iron and Alumina. The presence of oxides of iron and 

 alumina in the raw phosphates produces many objectionable 

 results, so much so that in certain cases raw phosphates 

 become quite unsuitable for superphosphate manufacture. 

 Very often the iron is present as ferric phosphate, although 

 occasionally pyrites may be found, as in the Tennessee phos- 

 phates. Decomposition between the sulphuric acid and the 

 phosphates may take place according to the following 

 equations : 



(1) 3 FeP0 4 + 3H 2 S0 4 - FeH 6 P 2 O 8 + Fe 2 (SO 4 ), 



(2) 2FeP0 4 + 3H 2 S0 4 = 2H 3 PO 4 + Fe 2 (SO 4 ) 3 



If the iron present is equal to less than 2 % of oxide of iron 

 in the raw phosphate, little injury is done, because to that 

 degree phosphate of iron will remain in solution. The ferric 



