124 CHEMICAL FERTILIZERS 



The fineness is measured by the percentage which will 

 pass a sieve containing TOO meshes to the inch. Occasional^ 

 the samples may show a degree of fineness much lower than 

 the figures given above, but those lower than the lowest in 

 the above table should be considered of doubtful value. The 

 reducing power is determined by dissolving the slag in dilute 

 sulphuric acid, and titrating with permanganate. This 

 figure has often been alluded to as representing the ferrous 

 oxide in the slag, but nothing is more certain than that this 

 figure represents no such thing. It is given in the above 

 table because it often appears in tables of analyses. Probably 

 what is represented more than anything else is the amount 

 of metallic iron present. If a slag contains ferric iron and 

 metallic iron, the latter would reduce the ferric salts in solution 

 to ferrous salts. In the equation Fe 2 (SO 4 ) 3 + Fe = 3FeSO 4 , 

 it will be noticed that one part of metallic iron can reduce two 

 parts of iron in the ferric condition, and return in the analysis 

 three parts of ferrous iron. The opinion not infrequently 

 given, that slags containing much ferrous iron are not easy 

 to grind, is readily understood, when one remembers that it 

 is mainly the quantity of metallic iron that is represented by 

 the figures which state the percentage of ferrous iron. 



Owing to the changes in the methods of manufacturing 

 slag which have been introduced by using the basic open- 

 hearth process, the composition of slag has undergone much 

 change during recent years. Until comparatively recently, 

 practically the whole of the phosphatic basic slag, which had 

 been placed upon the market, contained phosphoric acid 

 with a solubility of 70-95 % by Wagner's citric acid 

 test. During the past few years, steel manufacturers, at 

 least those using the basic open-hearth process, have been 

 introducing fluorspar into the furnace, which has enabled 

 greater quantities of lime to be added without making the 

 slag too thick to flow conveniently. A special feature has 

 been given to this new type of slag, since the fluorspar 

 reduces the solubility of the phosphate in citric acid to only 

 20-50 ..%. Robertson has shown that although the first 

 extraction with citric acid according to Wagner's method 



