| Abstract: | The selective oxidation of elements in molten matte and metals is reviewed and discussed from thermodynamic viewpoint,Ni vs S in nickel matte,Cr,V,Nb, Mn or P vs C in iron melts being taken as examples of practical application.On grounds of thermodynamical analysis,a concept of transition temperature of oxi- dation has been introduced,and methods of calculating the transition temperature with either the two-steps- or the one-step-method are presented.On the assumption that no nucleation energy is required for the formation of new phases (gas bub- bles and the condensed oxidation products),the transition temperature of oxida- tion is independent of the forms of oxygen present,be it gaseous oxygen,dissolved O] in metal melts or FeO from the slag,and also independent of the pressure or activity of oxygen,but it depends solely upon the nature and activity (pressure) of the reactants and products taking part in the reaction.Furthermore,the transi- tion temperature of oxidation is not a fixed temperature,it varies continuously with the composition of the bath as the oxidation process goes on.Reducing the pressure of the gaseous product would help to lower the transition temperature of oxidation. With the transition temperature of oxidation theoretically calculated,it is possible to deduce the optimal conditions for allowing one element preferentially oxidized while leaving the other element practically intact in the bath.Small- scale experiments and industrial practices show that the concept of the transition temperature of oxidation does successfully serve as a guide to control the blowing (smelting) operation,with the result that the desired element could be oxidized selectively.The kinetic factors which influence the sequence of oxidation of ele- ments in the bath are also briefly discussed. For the desulphurization of nickel matte,the decarburization in the 18-9 stainless steel manufacture (including the AOD process)and the decarburization of high-carbon ferromanganese,the blowing operation should be run so that the temperature of the bath should always be higher than the transition temperature of oxidation corresponding with the composition of the bath.But for the dechro- mizing of pig iron,and the extraction of V or Nb from the pig iron,smelting operation should be carried on,so that the temperature of the bath should be kept always lower than the transition temperature of oxidation for the respective com- position of the bath.As regards the sequence of oxidation of P with respect to C,besides the effect of the transition temperature of oxidation,the basicity and the FeO-content of the slag for holding P_2O_5 as well as the pressure exposed to or sustained by the CO-bubbles should be both considered. |
