IRON, COBALT, AND NICKEL 323 



impurities do not separate of themselves, or do not fuse. The difficulty 

 of the industry lies in forming an easily-fusible slag, into which the 

 whole of the foreign matter of the- ore would pass and flow down to the 

 bottom of the furnace above the heavier cast iron. This is effected by 

 mixing certain fluxes with the ore and charcoal, A flux is a substance 

 which, when mixed with the foreign matter of the ore, forms a fusible 

 vitreous mass or slag. The flux used for silica is limestone with clay ; 

 for limestone a definite quantity of silica is used, the best procedure 

 having been arrived at by experiment and by long practice in iron 

 smelting and other metallurgical processes* 7 



Thus the following materials have to be introduced into the furnace 

 where the smelting of the iron ore is carried on : (1) the iron ore, 

 composed of oxide of iron and foreign matter ; (2) the flux required to 

 form a fusible slag with the foreign matter ; (3) the carbon which is 

 necessary (a) for reducing, (6) for combining with the reduced iron 

 to form cast iron, (c) principally for the purpose of combustion and 

 the heat generated thereby, necessary not only for reducing the iron 

 and transforming it into cast iron, but also for melting the slag, as well 

 as the cast iron and (4) the air necessary for the combustion of the 

 charcoal. The air is introduced after a preparatory heating in order to 

 economise fuel and to obtain the highest temperature. The air is 

 forced in under pressure by means of a special blast arrangement. 

 This permits of an exact regulation of the heat and rate of smelting. 

 All these component parts necessary for the smelting of iron must be 

 contained in a vertical, that is; shaft furnace, which at the base must 

 have a receptacle for the accumulation of the slag and cast iron formed, 

 in order that the operation may proceed without interruption. The 

 walls of such a furnace ought to be built of fireproof materials if it be 



7 The composition of slag suitable for iron smelting most often approaches the 

 following : 50 to 60 p.c. SiO 2 , 5 to 20 A1 8 O 3 , the rest of the mass consisting of MgO, 

 CaO, MnO, FeO. Thus the most fusible slag (according to the observations of 

 Bodeman) contains the alloy Al2O 5 ,4CaO,7SiO 2 . On altering the quantity of magnesia 

 and lime, and especially of the alkalis (which increases the fusibility) and of 1 silica 

 (which decreases it), the temperature of fusion changes with the relation between the total 

 quantity of oxygen and that in the silica. Slags of the composition) RO,SiO 2 are easily 

 fusible, have a vitreous appearance, and are very common. Basic slags approach the 

 composition 2RO,Si0 2 . Hence, knowing the composition and quantity of the foreign 

 matter in the ore, it is at once easy to find the quantity and quality of the flux which 

 must be added to form a suitable slag. The smelting of iron is rendered more complex 

 by the fact that the silica, SiO 2 , which enters into the slag and fluxes is capable of form- 

 ing a slag with the iron oxides. In order that the least quantity of iron may pass into 

 the slag, it is necessary for it to be reduced before the temperature is attained at which 

 the slags are formed (about 1000), which is effected by reducing the iron, not with char- 

 coal itself, but with carbonic oxide. From this it will be understood how the progress of 

 the whole treatment may be judged by the properties of the slags. Details of this 

 complicated and well-studied subject will be found in works on metallurgy. 



