Fundamental Research On The Structure And Physicochemical Properties Of Aluminosilicate Based Blast Furnace Slag

With the consumption of high-grade iron ore resources,the use of low-grade and difficult-to-handle iron ore will become an alternative way to alleviate the depletion of iron ore resources.High alumina iron ore will become an important resource for future steel production.However,the utilization of high alumina iron ore in the blast furnace will inevitably increase the Al₂O₃ content in the slag,and will result in the slag system translation from silicate based to aluminate based.The high temperature physicochemical properties of conventional silicate slag will not be suitable for the utilization of high alumina iorn ore.In this case,the CaO-MgO-Al₂O₃-SiO₂-TiO₂ slag system with high alumina content was studied in this work.The effects of Al₂O₃ on the slag structure and high temperature physicochemical properties?viscosity,density,surface tension,electrical conductivity,sulfide capacity?were studied experimentally.The relationship between composition,structure and properties of this slag system was investigated.A viscosity model based on slag structure was established.The feasibility of aluminosilicate slag in blast furnace was discussed.The technical characteristics of the utilization of high alumina iron ore in the blast furnace were clarified.Firstly,the structural information of high alumina slag was investigated by molecular dynamics simulation,and the results shown that the Si-O bond is the most stable bond,and then is the Al-O bond,the Ca-O bond is weakest with the longest bond length.The combination of Si⁴⁺,Al³⁺with O^2-forms a tetrahedral structure,but the AlO₄^5-is not as stable as the SiO₄^4-tetrahedron and that some highly coordinated Al³⁺units exist in the slags.The obvious preference for NBO is coordinated with Si⁴⁺,whereas Al³⁺tends to be localized in more polymerized environments as network intermediate phases.The AlO₄^5-tetrahedron is surrounded by AlO₄^5-tetrahedron instead of SiO₄^4-tetrahedron.Ca²⁺is more energetically active than Mg²⁺as the charge compensation ion.Combining with the Raman spectroscopy analysis,it is can be found that with increasing of Al₂O₃ content,the"red shift"phenomenon can be observed obviously,and the slag structure tends to be complicated.The concetration of structural unit of Al-O^-,Al-O-Si and Si?Q⁰+Q¹?are gradually decreased,while the content of Al-O⁰ structural unit increased.The concetration of Si?Q²+Q³?is slightly decreased firstly and then increased.When substitution SiO₂ by Al₂O₃,the concetration of Al-O⁰and Si?Q²+Q³?structural units decrease firstly and then increase,while the concetration of Al-O-Si and Si?Q⁰+Q¹?structural units increase then decrease.The concetration of Al-O^-structure the unit is gradually increased,and the slag structure has the minimum degree of polymerization when the?wt.%Al₂O₃?/?wt.%SiO₂?is 0.92.Secondly,the effects of Al₂O₃ content and?wt.%Al₂O₃?/?wt.%SiO₂?on the high temperature physicochemical properties of aluminosilicate slag were investigated.The relationship between slag structure and properties was discussed.The results show that:?1?The viscosity increases with increasing the Al₂O₃ content at the range of 16 to 24wt.%and the?wt.%Al₂O₃?/?wt.%SiO₂?up 0.92,but it decreases when Al₂O₃ content is more than 24 wt.%and the?wt.%Al₂O₃?/?wt.%SiO₂?below 0.92.The free running temperature increases with increasing the Al₂O₃ content and appears to show a peak at an?wt.%Al₂O₃?/?wt.%SiO₂?of 0.92.Temperature dependence of the viscosity is checked,from which the apparent activation energy is calculated.The change of the apparent activation energy is in accordance with the change of viscosity.?2?The density decreases with increasing the Al₂O₃ content up to nearly 24 wt.%at a fixed basicity of1.2,subsequently increases.Increasing the?wt.%Al₂O₃?/?wt.%SiO₂?from 0.47 to 0.92caused an increase in the density at a fixed CaO content,and the density decreases slightly when the?wt.%Al₂O₃?/?wt.%SiO₂?is greater than 0.92.?3?The surface tension increases upon increasing the Al₂O₃ content and?wt.%Al₂O₃?/?wt.%SiO₂?.The change in the surface tension can be explained by the decrease in the surface-active component?SiO₂?and the increase in the number of charge compensators at the melt surface.?4?Electrical conductivity and viscosity have a negative correlation.?5?The sulfide capacity decreases with increasing the Al₂O₃ content at a fixed basicity of 1.20.Meanwhile,increasing the?wt.%Al₂O₃?/?wt.%SiO₂?from 0.47 to 0.79 with a fixed CaO content causes a significant increase in the sulfide capacity of the slags and a slight increase when the?wt.%Al₂O₃?/?wt.%SiO₂?is more than 0.79.?6?According to the slag structure,it can be found that the critical point of viscosity,density and conductivity is near the composition of the slag translation from silicate to aluminate.That is,?autom Al?/?autom Si?is about 1.0,indicating that the effect of Al₂O₃ content and?wt.%Al₂O₃?/?wt.%SiO₂?has different effects of the physicochemical properties on the silicate based and aluminate based slag.A structure-based viscosity model has been developed for the CaO-MgO-FeO-SiO₂-Al₂O₃ system and its subsystems.This model can be used to predict the viscous behavior of CaO-MgO-FeO-SiO₂-Al₂O₃ slag system and its subsystems in the range of Newtonian fluid.In the model,the relationship between viscosity,structure and temperature is clarified by kinetic theory.The charge compensation effect of AlO₄^5-tetrahedron and the effects of different metal cations on the viscosity are considered.The oxygen ions bonded with non-compensated Al³⁺ions were defined as excess bridge oxygens?O*?.The concentration of different types of oxygen ions are calculated and used to express the activation energy.A critical analysis of the experimental data for these slag systems has been conducted and used in the optimization of a model.The present model is capable of predicting the viscosities in the CaO-MgO-FeO-SiO₂-Al₂O₃system and its subsystems over wide ranges of composition and temperature above the liquidus within experimental uncertainties,and the average of relative errors for this model was found to be less than 20%.The physicochemical properties of slag in a wide range of composition range were predicted.According to the blast furnace operation,the feasibility of the alumina slag-based slag system was discussed from slag-iron separation and quality.An aluminate based slag range??38⁴2wt.%?CaO-?26³0wt.%?Al₂O₃-?20²5 wt.%?SiO₂-9wt.%MgO?was reported that the physicochemical properties meet the requirements of blast furnace operation.In summary,the structure and properties of high alumina content blast furnace slag were inverstigated systematically in this study.An aluminate-based slag system was proposed innovatively,and this work will provide scientific basis and theoretical guidance for the large-scale utilization of high-alumina iron ore in the blast furnace process.