MANUFACTUKE OF SOLUBLE PHOSPHATES. 77 



out of account, it gives rise to an excessive amount of gypsum which 

 brino-s numerous drawbacks in its train as will be seen in the sequel. 

 The quantity of acid absorbed by the carbonate of lime in normal 

 amount (1 part of CaCOg in tribasic phosphate of lime requires 

 1-5 parts of SO3 ^) is compensated to a great extent by the ad- 

 Tantages got in " mixing " and by the quality of the superphosphate. 

 Phosphates free from carbonate of lime do not heat but very 

 poorly in contact with acid ; the reaction is consequently slow, the 

 ■operation takes longer and the final product is difficult to dry. To 

 remedy this drawback in treating phosphates of this nature, apatites 

 for example, it is best to mix them with phosphatic chalk when 

 being ground, but the superphosphate which results is not so 

 homogeneous as that got from the chalky phosphates of Algeria. 

 The amount of acid required to decompose the carbonates is calcu- 

 lated from the following equations : — 



CaCO. + H0SO4 + HoO = CaS0;2H._,0 + CO, 

 100" + 98 + 18 ^ , —- ^ 



172 + 44 



21(3 ^ . ' 



216 

 MJ.CO, + HoSO, + 6H2O = MgSO.TH^O + CO., 

 84 " + yS + 108 246 + 44 



•- Y 



290 290 



Magnesia is also met with, but in smaller quantities, as tribasic 

 or neutral phosphate. The following reaction then occurs : — 



Mg..(P04)2 + 2H0SO, + 16H„0 = MgH4(POJ22H,0 + 2MgS047HoO 

 2MgHP04 + H0SO4 + 9H.p = MgH4(P04)2,2HoO + MgSO^THaO 



Acid phosphate of magnesia is not deliquescent and not decomposed 

 by water. 



Iodine. — Certain phosphates contain iodine, as calcium iodide 

 (CalJ, which is converted by the sulphuric acid into hydriodic acid 

 at the ordinary temperature of the " mixing " ; when the mass heats 

 the acid reacts on the hydriodic acid, and the iodine liberated 

 escapes as a violent vapour. Phosphoric acid has no action on 

 hydriodic acid. 



Calcium Fluoride. — Most phosphates contain more or less 

 calcium fluoride. This substance is likewise decomposed by sul- 

 phuric acid at a temperature above 40'' C. (104° F.). It is then 

 given off as gaseous hydrofluoric acid. But the latter acts in its 

 turn on silicic acid and forms with it silicic fluoride. These two 

 reactions may be represented thus : — 



CaF. + H,SO, + 2H.,0 = CaS0,2H.,0 + 2HF 

 SiO, + 2CaF, +" 2H,S0, + 2HoO = SiF^ + 2CaS0^2H,0. 



1 Evidently dilute acid of 120° Tw.— Tr. 



