Research On The Petrographic Structure And Melting Properties Of Fruorine-free Steelmaking Slag Containing Boron Mud

It is widely accepted that as an important reaction medium during pyrometallurgical process, the mineral compositions, structure and properties of the molten slag play a vital role in the converter steelmaking process. During the conventional steelmaking process, Ca F2 is employed as fluxing agents, which can improve the fluidity of the slag. However, the fluoride will erode the lining and the equipment, and the volatilization and acidification of fluorine will pollute the environment and endanger human’s health. Therefore, the development of new fluorine-free flux has gained more and more attentions in modern society. It has been confirmed that B2O3 has good fluxing effects on the CaO-based slag, but it costs too high to employ B2O3 as fluxing agent merely. Considering that boron mud is waste and it includes effective fluxing components such as B2O3 and Fe2O3, it is advisable to employ B2O3 and boron mud as compound fluxes.In this paper, the structure and melting properties of the slag with boron mud were studied. Firstly, the effects of fluxing agent and basicity on the structure of molten slag were investigated by NBO/T and structure parameter Q. And the mineral compositions and microstructure of the slag system containing boron mud were also studied through XRD, Raman spectrum and mine microscope methods. In addition, the melting temperature and viscosity of slag system were determined by the hemisphere point and rotating cylinder method respectively. The main conclusions include:(1) The value of structural parameter Q of CaO-SiO2-B2O3(R=3.0) slag system increases with B2O3 content increasing, the value of Q increases from 1.65 to 2.51 sharply with B2O3 content changing from 5% to 15%,which indicates that B2O3 plays the role of network formers in the slag system. For CaO-SiO2-boron mud(R=3.0) slag system, the value of Q increases from 0.88 to 1.55 slowly as the boron mud content changes from 5% to 40%. In addition, the Q parameter decreases with the basicity of the molten slag increasing.(2) XRD results show that the main mineral phases of CaO-SiO2-B2O3-boron mud(R=3.0) include calcium and magnesium silicate, calcium silicate and a small amount of CaO?B2O3 also precipitates. Raman spectrum analysis results show that with the increase of boron mud content and basicity of the slag system, the complex silicate network is gradually disintegrating, and the structure of the slag system tends to become simpler. Petrographic analysis shows that in the CaO-SiO2-B2O3-boron mud system, C3 S phases distribute with a tabular morphology, and lath-shaped calcium and magnesium silicate C3MS2 and circular C2 S and CaO ? B2O3 phases also exist. In addition, there is still not melted f-CaO in the slag. In addition, the mineral phases of the slag with boron mud distribute evenly, indicating that molten slag samples melted sufficiently.(3) The melting temperature of CaO-SiO2-B2O3 slag decreases with the increase of B2O3 content, and the drop speed rises with B2O3 increasing. For CaO-SiO2-B2O3-boron mud(CaF2) slag, the melting temperature declines from 1480℃ to 1341℃ as the boron mud increases from 1% to 11%, while the variation range of the melting point of the slag containing CaF2 is 1465~1289℃.(4) With the increase of fluxes boron mud and CaF2 content, the viscosity of CaO-SiO2-B2O3-boron mud(CaF2) slags decrease; while for CaO-SiO2-Fe2O3-B2O3-boron mud slag system, its viscosity decreases firstly and then increases with boron mud content increasing, while the viscosity of the slag containing CaF2 declines gradually with the increase of CaF2 content.In summary, when boron mud was employed as fluxing agent, the calcium and magnesium silicate and calcium borate phases with low melting point precipate, which is conductive to melt rapidly. In addition, with the boron mud content increasing, the structure of the slag system tends to become simpler. And the fluxing effects of boron mud(5~9%) and CaF2 on the steelmaking slag are close. Therefore, boron mud can be considered to substitute for CaF2 as fluxing agent in the steelmaking process.