146 SUPEEPHOSPHATES 



A1 2 (SO 4 ) 3 + CaH 4 P 2 8 = 2A1P0 4 + CaS0 4 + 2H. > S0 4 

 Fe 2 (S0 4 ) 3 + CaH 4 P 2 O 8 = 2FeP0 4 + CaS0 4 + 2H 2 S0 4 . 



The free sulphuric acid thus formed would be used in acting upon 

 some of the unchanged Ca 3 P 2 O s as before. 



In either case, a quantity of phosphoric acid formerly in the soluble 

 condition as CaH 4 P 2 8 would pass into an insoluble form. Such 

 phosphoric acid and phosphates are often spoken of as "retrograde/' 

 " reduced," or " reverted " phosphate. They generally possess a higher 

 manurial value than tricalciurn phosphate. 



In the process of manufacturing superphosphate, the ground phos- 

 phate is mixed with the suitable quantity of sulphuric acid (chamber 

 acid of specific gravity 1'55) in "mixers" made of wood lined with 

 lead, or of iron lined with fire-brick, and provided with stirrers rotated 

 by gearing. The carbon dioxide, hydrofluoric or hydrochloric acid, 

 and steam evolved by the action are led away into a "scrubber" or 

 "condenser," in which the steam condenses and absorbs the acids. 

 The prevention of their escape into the atmosphere is enforced by law. 

 When the lining of the "mixer" is fire-clay (highly silicious), phos- 

 phates containing calcium fluoride cause the production of gaseous 

 silicon fluoride : 



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



Si6 2 + 4HF = SiF 4 + 2H 2 O. 



The silicon fluoride escapes with the other gases and, on contact with 

 the condensed steam, produces a gelatinous presipitate of silica and a 

 solution of hydrofluosilicic acid, H 2 SiF (; : 



3SiF 4 + 4H 2 = SiO 4 H 4 + 2H 2 SiF . 



The presence of fluorine in phosphates is thus attended with the 

 production of the very corrosive hydrofluoric acid, the consequent wear 

 and corrosion of the fire- clay lining, and the accumulation of the very 

 bulky, gelatinous silicic acid in the condenser, tending to choke it. 

 The bye-product, hydrofluosilicic acid, possesses antiseptic properties 

 and can be used as a preservative of farm -yard manure. When the 

 admixture of the acid and phosphate is complete, the contents of the 

 "mixer" are transferred to pits or "dens" made of brick- work or 

 masonry. The mixture at this stage is usually semi-fluid and runs 

 easily. In the " dens " the union of the calcium sulphate with water 

 takes place, resulting in the formation of crystals of the same composi- 

 tion as the mineral gypsum, CaS0 4 .2H 2 O. This causes the product to 

 " set " or " dry," and after this it is ground in suitable mills and screened 

 and is ready for the market. 



" Double superphosphate " is sometimes made by preparing phos- 

 phoric acid by the action of excess of acid upon one portion of the 

 phosphate, removing the greater part of the calcium sulphate, and 

 adding the phosphoric acid to another portion of the original phosphate 

 thus converting the tricalcium into monocalcium phosphate : 



Ca 3 P 2 O s + 4H 3 PQ 4 = 3CaH 4 P L ,O s . 



Such a product is very concentrated and may contain as much as 40 

 per cent of phosphoric acid, nearly all in the soluble form. 



