Representation Of Desulfurizaion And Dephosphorization Ability Of Slags Based On Ion And Molecular Coexistence Theory

With the development of metallurgy technology, the requirement of ultra-low sulfur, low phosphorous clean steel have become increasingly demanding. To achieve this goal, metallurgical scientists have measured the desulfurization, dephosphorization data and iron oxide activity data of large number of different metallurgical slags to find the physical and chemical properties for achieving the optimal development and production process. Based on ion and molecular coexistence theory(IMCT), experiments have been designed to study the desulfurization and dephosphorization properties of metallurgical slag at different stages of oxygen potentials.The reaction ability of iron oxides in slags directly affects the desulfurization or dephosphorization effects, one consensus that low oxygen potential is conductive to desulfurization reaction and high oxygen potential is benefit for dephosphorization reaction have been achieved by metallurgical workers, thus, it can be deduced that the desulfurization or dephosphorization ability of slags cannot be accurately represented unless the oxidation effect of slags can be predicted. In this study, experiments have been made to develop IMCT-NFe,O model. Meanwhile, a large number of the activity data of iron oxides with large compositional variation from different literatures showed satisfactory agreement with the calculated results by the present model. In addition, the activity of iron oxide aFe,O is predominately controlled by NFe,O other than NFe2P3 or NFeO·Fe2O3 according to the model calculation.During the ironmaking process of blast furnace, the reaction atmosphere is in favor of desulfurization reaction due to the ultra-low oxidation potential, and CaO-MgO-SiO2-Al2O3 slag system can be regarded as one of excellent desulfurization slags. Desulfurization experiments of ShouQin blast furnace slags have been taken out to establish thermodynamic model, and the calculated sulfur distribution or sulfide capability by the present model can be reliably adopted. The mass action concentration of slag components, just like the classical physical chemistry concept of activity, can represent the actually reaction ability with more accuracy compared with that of mass percentage, and the linear relationship between mass action concentration and sulfur distribution or sulfide capability is very clear. This model can quantitatively calculate the desulfurization contribution of free CaO and free MgO at 1773 K as 97% and 3%.In the process of basic steelmaking, slag basicity generally lies in the range from 2 to 5 with high content of iron oxide which can be converted into high oxygen potential that is in general much larger than 10-11 pa. From basic dephosphorization reaction, slags with high basicity and high iron oxide are in favor of dephosphorization. Thus, a thermodynamic model has been developed for calculating the dephosphorization ability of CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al2O3-P2O5 based on experiments. Meanwhile, this model can be reliably extended to CaO-FeO-Fe2O3-P205, CaO-SiO2-FeO-Fe2OH3-P2O5, CaO-FeO-Fe2O3-Al2O3-P2O5, CaO-MgO-FeO-Fe2O3-SiO2-P2O5 and CaO-SiO2-MgO-FeO-Fe2O3-MnO-P2O5 slags at different temperatures.The linear relationship between mass action concentration and phosphorus distribution is somehow poor compared with that of sulfur distribution. This phenomenon can be explained as that dephosphorization reaction is controlled by the comprehensive effects of slag basicity and iron oxides, and any component alone is difficult to control the whole dephosphorization reaction. Considering the difference of slag-gas reaction and slag-metal reaction, the concepts of phosphate capacity CPO43- and phosphate capacity index CPO43-,index have been proposed, and the developed IMCT-CPO43-model and IMCT-CPO43-,index model can not only predict the experimental results of CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al203-P205 slags, but also predict the industrial production results. According to the modelling calculation,3CaO·P2O5 and 4CaO·P2O5 made main contribution of dephosphorization with 97% and 2.88%, and other dephosphorization molecules’contribution are relatively small which can be neglected.Sulfur atoms in liquid iron at the slag metal interface can replace free oxygen atom vectors [O] during the process of entering the slag phase which can provide the opportunities for coupling reaction of dephosphorization. Adjusting oxygen potential exerts no influence upon sulfide capacity or phosphate capacity, but increasing oxygen potential leads to lower sulfur distribution and higher phosphorus distribution. The intersection of sulfur distribution straight line and phosphorous distribution straight line can be regarded as the appropotional oxygen potential which is great of temperature dependence. To maintain the optimal relationship of sulfur distribution and phosphorous distribution, the corresponding iron oxide should be subtly increased at the elevated temperatre. The developed desulfurization and dephosphorization coupling model in this study can be applied to CaO-SiO2-MgO-Al2O3-FeO slags in the field of novel flash ironmaking technology and CaO-Al2O3-Fe,O slags reported in the literature. The oxygen potential in the above-mentioned both slags were adjusted by the mass content of iron oxide, and increasing the mass content of iron oxide in slags leaded to decreasing sulfur distribution and increasing phosphorous distribution simultaneously. Sulfur distribution ratio and the phosphorus distribution ratio tended to balance when the mole fraction of iron oxide was 0.13.