ALKALI WASTE: BLAST-FURNACE SLAG. 



351 



The slags from iron furnaces consist essentially of lime (CaO), silica 

 (Si0 2 ), and alumina (A1 2 3 ), though small percentages of iron oxide 

 (FeO), magnesia (MgO), and sulphur (S) are commonly present. 

 Slag may therefore be regarded as a very impure limestone or a very 

 calcareous clay from which the carbon dioxide has been driven off. 



Two plants are at present engaged in the United States in the 

 manufacture of true Portland cement from slag, and there seems to be 

 no reason why this cheap and satisfactory raw material should not 

 become an important factor in the cement industry of the country. 



Slags in general. Slags are the fusible silicates formed, during the 

 smelting or refining of metals, by the combination of the fluxing materials 

 with the gangue of the ore. The composition of the slag, therefore, will 

 be determined by the composition and relative proportions of the fluxes 

 and the gangue. In general, the slag will contain only those elements 

 which are present in either the gangue or the flux, though it may carry 

 also a percentage, usually small, of the metal which is being smelted or 

 treated. In some processes also the composition of the slag may be 

 slightly modified through the action of the fuel, from which certain 

 impurities may be taken up. 



While many elements may occur in slags, those which are of uni- 

 versal or even common occurrence are comparatively few. The slags 

 commonly found in iron metallurgy consist essentially of silica, alu- 

 mina, iron oxide, and lime, with or without magnesia. Alkalies, sul- 

 phur, and phosphorus are also almost invariably present, but in small 

 percentages. 



The following analyses of slags from various furnaces will serve 

 to give some idea of the range in composition of these products. 



TABLE 159. 

 ANALYSES OF IRON-FURNACE SLAGS. 



