MANUFACTURE OF SUPERPHOSPHATES. 155 



The free sulphuric acid thus formed would be used in acting 

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



In either case a quantity of phosphoric acid formerly in the 

 soluble condition as CaH 4 P 2 K 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 mammal value than trical- 

 cium phosphate. 



In the process of manufacturing superphosphate the ground 

 phosphate is mixed with the suitable quantity of sulphuric acid 

 (chamber acid of sp. gr. 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, hydro- 

 fluoric or hydrochloric acid, and sieam 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 -fH 2 S0 4 = 



Si0 2 +4HF = Si 



The silicon fluoride escapes with the other gases and on 

 contact with the condensed steam produces a gelatinous 

 precipitate of silica and a solution of hydrofluosilicic acid, 

 H 2 SiF (i 



3SiF 4 + 4H 2 = Si0 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, hydrofluo- 

 silicic 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-w r ork 

 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 



