PHOSPHORUS FERTILIZERS 147 



with acid, the solution of phosphate is very slow, and the 

 product is difficult to dry. Where such a material is being 

 used, it is better to mix it with one of the poor grade rock 

 phosphates containing higher proportions of carbonate of 

 lime. The reactions of calcium carbonate with sulphuric 

 acid and magnesium carbonate are given in the equations : 



CaC0 3 + H 2 S0 4 + H 2 = CaSO 4 . 2H 2 O + CO 2 

 MgC0 3 + H 2 S0 4 + 6H 2 = MgS0 4 . 7H 2 + CO 2 



Magnesium sulphate can also crystallize with 6 molecules 

 of water. Occasionally, but very rarely, magnesia is met 

 with as a phosphate. The acid phosphate of magnesia is 

 not deliquescent, like the corresponding calcium compound, 

 nor is it at all readily decomposed by water. Up to 5 % of 

 calcium carbonate may be regarded as a beneficial amount ; 

 greater quantities merely absorb sulphuric acid to little 

 purpose, add to the weight of the resulting material, and 

 cost money in freight and cartage to no ultimate advantage. 

 Fluorine. Many of the phosphates contain fluorine, 

 either as calcium fluoride or as apatite. In the latter case, 

 reaction takes place as described on p. 145 in equations 

 (2), (5) and (8). Should any calcium fluoride be present, 

 reaction takes place according to the equation : 



CaF 2 + H 2 SO 4 + 2H 2 O = CaSO 4 . 2H 2 O + 2HF 

 The ordinary rock phosphates almost invariably contain a 

 certain amount of earthy material, so that there is a fair 

 amount of silica present. In the absence of water, hydro- 

 fluoric acid acts upon silica according to the following 

 equation : 



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



but the silicon tetrafluoride so produced reacts with water, 

 according to the following equation : 



3 SiF 4 + 4 H 2 = 2H,SiF 6 + Si(OH) 4 



Hydrofluoric acid is not merely capable of acting upon silica, 

 but also upon complex silicates, and will attack the brick- 

 work of the den if it has not already obtained sufficient 

 silica from the superphosphate. As the mass by its heat 



