Phosphates 



stirred constantly, and the result is a precipitate of 

 bi- and tricalcic phosphates and some foreign matter. 

 The precipitate is then dried at a low temperature. 

 When it has been properly made it ought to contain 

 only a little tricalcic phosphate. 



This chemical phosphate is of a fineness im- 

 possible to obtain by grinding. It is, owing 

 to its bicalcic form (which is soluble in citrate of 

 ammonia), and also to its extreme fineness that its 

 action is so favourable. The tricalcic phosphate 

 is not taken into account in fixing its price. It 

 should only be bought on analysis, and when the 

 price of the unit is not higher than that of slag. 



Basic Slag,^ Slag, Thomas's Slag. 



This is a bye-product of manufactured steel 

 from minerals which contain phosphoric acid. Iron 

 containing phosphoric acid breaks easily under the 

 hammer when cold. Therefore before minerals con- 

 taining this acid can be utilised the acid must be 

 eliminated. Thomas and Gilchrist discovered the 

 way to extract phosphate from iron ore in 1878, and 

 at first the residue or slag was simply thrown away. 

 Small heaped-up mountains of it could be seen 

 outside steel works. After conclusive trials, how- 

 ever, it was utilised as manure. 



To extract the phosphate and obtain the slag 

 the inside of a Bessemer converter is lined with 

 limestone, and lime is also added to the molten 

 contents. Then a very strong current of air forced 

 into the mass oxidises the impurities and they are 



^ It is called " basic " because it contains 3 or more per cent. 

 of free lime, of the total lime contents of 35-55' 



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