154 CHEMICAL MANUEES. 



cent phosphoric acid, vaporizing 270 tons of water with an expense- 

 of 349 tons of coal, 100 kg. (220 lb.) of coal vaporized, 770 litres 

 (1694 lb.) of water (say 1 lb. of coal furnished nearly 8 lb. of 

 steam), and yielded 160 kg. (352 lb.) of concentrated phosphoric 

 acid. The evaporation was at the rate of 1626 litres (say 32^ cwt.) 

 per hour. 



The ovens installed at the Wetzlar factory were not so big. When 

 the acid was brought to the desired density it sometimes happened, 

 owing to want of supervision, that in the parts of the pan most 

 exposed to the heat the phosphoric acid was evaporated to dryness, 

 and vitreous lumps of a greyish-blue colour, partially diaphanous, 

 formed. The analysis of this vitreous mass was very difficult, be- 

 cause it was impossible to dissolve it by ordinary methods. Prof.. 

 Krantt, of Hanover, succeeded in dissolving it, by heating it with 

 borax ; the analysis gave 70 per cent of P^.O^, besides alumina, iron, 

 lime, alkali, and hydrofluoric acid ; silica, supposed, to be present 

 in large amount, looking to the vitreous nature of the mass, was 

 only found in traces combined with fluorine. The substance thus 

 consisted chiefly of metaphosphate. In the evaporation of the 

 phosphoric acid, the steam entrains a great part of the dissolvedi 

 HF and H.SiFg. 



Manufacture of Double Superphosjjhate. — The greater part of 

 the phosphoric acid so produced was used in the manufacture of 

 double superphosphates. This manufacture is distinguished from 

 ordinary superphosphate manufacture by the fact that phosphoric 

 acid is substituted for the sulphuric acid. The phosphate is then 

 rendered soluble according to the following equation : — 



Ca-.(PO,)., + 4H..P0, = 3CaH,(P04)., 



310 8U2 ■ . -^ 



. ^ 702 



702 



It is clear that in actual practice the method of conversion is 

 not so simple, because phosjohates containing all manner of im- 

 purities are used, and the nature of the phosphoric acid, as well as 

 that of the superphosphate, plays a preponderant role. The phos- 

 phates which are the most easily dissolved by phosphoric acid, are^ 

 sandy phosphates with a high percentage of carbonate of lime, such 

 as Somme phosphate and Malogne phosphate. This phosphate is 

 mixed in the proportion of 1 ton to 4 tons of 54 per cent phosphoric- 

 acid. This mixing is done in the same manner, and with the plant, 

 as that in which the phosphate is dissolved by sulphuric acid. 

 To elevate the concentrated phosphoric acid and bring it to the^ 

 mixing apparatus, either a membrane pump, a centrifugal pum.p, or 

 an^ injector is used. The lirst of these pumps is sure in its working 

 and can elevate the acid to any desired height ; the second, which 

 lends itself more particularly to the elevation of dilute phosphoric- 

 acid, is liable to stop work ng when the acid is concentrated or- 



