Effect Of B₂O₃ On The Physical And Chemical Properties Of High Basicity CaO-SiO₂-FeO-MgO Slag System

In China,90%of the total crude steel comes from converter steelmaking.As an important slagging agent,fluorite can cause to decrease effectively the melting temperature and viscosity of the slag,accompanying alternatively equipment corrosion and environmental pollution.Therefore,the development of an alternative non-fluorinated slag agent in the converter steelmaking process becomes necessary and urgent.Based on the current research status of fluorine-free slag for the ferrous metallurgical process,the steel slag can be extracted as CaO-SiO₂-FeO-MgO system and form a new slag system by the addition of B₂O₃.The systematic studies on the slag properties of CaO-SiO₂-FeO-MgO-B₂O₃ are carried out by the thermodynamic calculation and experimental verifications.The main conclusions are given as follows:?1?The effect of B₂O₃ content on the phase evolution of CaO-SiO₂-FeO-MgO-B₂O₃ system has been investigated.The calculation results show that with the increase of B₂O₃ content,the primary crystal region containing the composition point shifts from the monoxide phase region to the dicalcium silicate phase zone and then to the calcium borosilicate phase zone.The low-temperature equilibrium phases comprise mainly monoxides and dicalcium silicate without B₂O₃ addition.As the B₂O₃ content increases,large quantities of borosilicates precipitate from molten slag,and then borates crystallize,to form the slag main phases.These calculation results are consistent well with experimental analyses.?2?The influence of B₂O₃ content on the melting characteristics of CaO-SiO₂-FeO-MgO-B₂O₃ system has been studied.The calculation results show that with the increase of B₂O₃ content,the liquidus temperature of the slag system declines,and the solidus temperature decreases first and then increases,then continues to decrease.The melting characteristic temperatures of the slag system were tested via the hemispherical point method,and it was found that the flow temperature,hemispherical temperature,and softening temperature of the slag system were all declining with the B₂O₃ content increasing.It can be seen that the melting characteristic temperature of the slag system with 5wt.%of B₂O₃ content is close to that of the fluorine-containing steel slag.?3?The effects of B₂O₃ content on the viscosity flow characteristics of CaO-SiO₂-FeO-MgO-B₂O₃ system have been investigated.It can be concluded that when the B₂O₃content increases from 5wt.%to 15wt.%,the melt viscosity,the critical viscosity of the temperature,and the viscous flow activation energy are all decreasing,indicating that the addition of B₂O₃ can significantly decrease the viscosity of the slag system.When the B₂O₃ content is 12wt.%,the critical viscosity of the temperature and melt viscosity at high temperature?1550??is close to the fluorine-containing steel slag.?4?The effects of B₂O₃ content on the slag structure of the CaO-SiO₂-FeO-MgO-B₂O₃ system was studied.It can be found that as the B₂O₃ content increases from 5wt.%to 15wt.%,the quantities of[BO₃]and[BO₄]structural units in the molten slag are both increasing.[BO₃]exists mainly as a ring form and the four O atoms in[BO₄]are mainly connecting to Si atoms.As the number of bridge oxygen in the system increases,the melt structure becomes more complicated,resulting in the degree of polymerization increases.?5?The effects of B₂O₃ content on the desulfurization and dephosphorization of the slag system have been investigated.It can be seen that with the increase of B₂O₃content,the desulfurization rate fluctuates around 60%,while the dephosphorization rate first increases and then decreases,and the peak occurs at 10wt.%of B₂O₃ content.This indicates that the slag system is of certain desulfurization and dephosphorization.In summary,the addition of B₂O₃ can significantly reduce the melting temperature and the viscosity of the slag system,and facilitate the desulfurization and dephosphorization of the slag system,implying the substitution of B₂O₃ for fluorite is feasible.