Fixation of Metals in Slags

Historically, slags have been an important by-product of the smelting of ores to produce metals. They serve the primary purpose of dissolving all of the extraneous rock into one homogeneous liquid from which the heavier metal reduced to its elemental state can separate by simple gravity segregation. They can also serve to absorb certain metal oxides while allowing another to be reduced to the elemental state. In some 

Achieving low fusion temperatures was largely adventitious in the earliest times but over the many years empirical additions were discovered. Chief among these were iron oxide, silica and lime added individually and under the right circumstances. Although we know now, soda and potash from the fuel ashes mixing with the ore also played a significant role. 

partly by the accumulation of empirical knowledge and partly by the interpretation of phase diagrams, we can deal with the fusion of oxide mixtures by deliberate planning. However, science is a long way from managing such designs by purely intellectual methods. 

We can start, as did the ancient craftsmen, with the fusion of the iron oxide, FeO, with silica, SiO . The phase diagram for those binary mixtures show that whereas Si02 fuses at about 1713 C and FeO at 13 9 C, mixtures containing between 20 and 40 weight percent FeO fuse below 1250 C. Complexing with additions of another iron oxide, Fe203, in amounts of up to 10%, can lower the fusion temperature to about 1150 C. 

From this point we can proliferate other additions in limited amounts which further depress the fusion temperature. Notable of these are lime, alumina, soda and potash. The limitation in amounts is shown by phase diagrams to the facts that, above a limit, fusion temperatures climb rapidly. For example, an addition of 20% lime to the sio2-70% FeO mixture will lower the fusion temperature to 1150 C but a further addition up to 40% will bring the fusion temperature to over 1400 C. 

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