Effect of CaO-MgO-FeO-SiO 2 - x Na 2 O Slag System on Converter Dephosphorization

: Na 2 O is an alkaline oxide, which can signiﬁcantly improve the dephosphorization ability of converter slag. The effect of Na 2 O on the dephosphorization of converter slag was analyzed with a high-temperature dephosphorization experiment in a MoSi 2 resistance furnace. We found that the dephosphorization rate increased with the increase of (Na 2 O) in the dephosphorization slag. The elements of Ca, Si, O, and P in the dephosphorization slag are distributed in the same area, mainly in the form of phosphate minerals, such as Ca 2 SiO 4 · 0.05Ca 3 (PO 4 ) 2 and 6Ca 2 SiO 4 · Ca 3 (PO 4 ) 2 . After adding Na 2 O, part of the Na will replace the Ca in the phosphorus-containing phase to form Ca 2 SiO 4 · Ca 2 Na 2 (PO 4 ) 2 . The industrial test showed that the average dephosphorization rate in the early stage of the test heats with the CaO-MgO-FeO-SiO 2 -0.5%Na 2 O slag system could reach 62.39%, which was 19.62% higher than that of the conventional heats. The average basicity of the ﬁnal slag was 0.19% lower than that of the conventional heats, while w (P 2 O 5 ) increased by 0.36%, and T.Fe decreased by 0.69%. The average consumption of the slagging materials was 35.93 kg/t, which was 7.24 kg/t less than that of the conventional heats. Through thermodynamic calculation, we found that with the increase of (Na 2 O), the phosphorus distribution ratio between slag and steel increased signiﬁcantly, the area of the liquid phase zone of the slag system increased continuously, and the viscosity decreased continuously.


Introduction
For most steel, phosphorus is a harmful element, and its segregation in steel will cause uneven structure, reduce the plasticity and toughness of steel, cause 'cold brittleness', and seriously affect its service performance [1].Therefore, in order to meet the market demand for low-phosphorus steel, various converter dephosphorization processes have been developed by major steel mills, for example, the MURC (Multi-Refining Converter) of Nippon Steel [2], the SGRS (Slag Generation Reduced Steelmaking) of Shougang [3,4], and the BRP (BOF Refining Process) of Baosteel [5].These processes mainly rely on the converter double slag method or the duplex method to achieve dephosphorization.Converter slag is mainly produced by the oxidation of silicon, phosphorus, iron, manganese, and other elements in molten iron and the addition of lime, dolomite, and other slagging materials.In addition, there are some other ways of formation, such as the blast furnace slag brought into the converter and the eroded converter lining [6].In 2021, Chinese crude steel production reached 1.03 billion tons, and the byproduct converter slag in the smelting process also increases year by year.At present, about 1 billion tons of slag are stored in China, occupying a large amount of land and polluting the environment.
Before the high temperature dephosphorization experiment, the initial molten iron was prepared by the pre-melting of pig iron and ferrophosphorus; the compositions are shown in Table 1.Then, the high-temperature dephosphorization experiment was carried out in the MoSi 2 resistance furnace; the experimental device is shown in Figure 1.The 400 g molten iron sample was put into the magnesium oxide crucible (ϕ60 mm × 72 mm) of the outer-sleeve graphite crucible (ϕ70 mm × 100 mm) and put into the furnace.The sample was melted at 1653 K in an argon atmosphere, then 60 g of dephosphorization slag containing different amounts of Na 2 CO 3 (the compositions are shown in Table 2) was added, followed by appropriate stirring to promote melting.After 30 min of reaction, the dephosphorization slag sample was taken with a molybdenum rod, and the molten iron sample was extracted with a quartz tube.Before the high temperature dephosphorization experiment, the initial molten iron was prepared by the pre-melting of pig iron and ferrophosphorus; the compositions are shown in Table 1.Then, the high-temperature dephosphorization experiment was carried out in the MoSi2 resistance furnace; the experimental device is shown in Figure 1.The 400 g molten iron sample was put into the magnesium oxide crucible (φ60 mm × 72 mm) of the outer-sleeve graphite crucible (φ70 mm × 100 mm) and put into the furnace.The sample was melted at 1653 K in an argon atmosphere, then 60 g of dephosphorization slag containing different amounts of Na2CO3 (the compositions are shown in Table 2) was added, followed by appropriate stirring to promote melting.After 30 min of reaction, the dephosphorization slag sample was taken with a molybdenum rod, and the molten iron sample was extracted with a quartz tube.In order to explore the effect of Na 2 O on the melting point of the slag system, the melting point of the CaO-MgO-FeO-SiO 2 -xNa 2 O slag system was tested with a slag melting characteristic tester.The hemispherical point temperature of the sample was defined as its melting temperature.Before the test, the samples were uniformly ground in agate mortar and mixed with anhydrous ethanol to make a 3 mm × 3 mm cylindrical sample.Each sample was heated at the same rate and tested 3 times to obtain the average.

Viscosity Calculation Method
Using FactSage8.1 to calculate the viscosity of the slag, we first opened the 'Viscosity' module.Then, we selected the component and entered its mass fraction (g) and input temperature (K), selected the viscosity unit (Pa•s), selected the 'Melts' melt database, and clicked the 'Calculate' button to calculate the relevant values.

Industrial Program
In order to improve the dephosphorization effect, the production process of a 130 t converter in a steel plant was improved, and the industrial test was carried out using CaO-MgO-FeO-SiO 2 -xNa 2 O slag.The single-slag smelting process mode was adopted.After the blowing was stable, lime, magnesium oxide ball, and sodium-containing slag were added with the appropriate slagging method.Among them, lime was added in 6~7 batches, the magnesium oxide ball was added in 2 batches, and the sodium-containing slagging material was added in 2~3 batches.Due to the erosion effect of excessive alkaline oxide on the furnace lining, the w(Na 2 O) in the slag was controlled at about 0.5% in the early stage of the test heats to improve the dephosphorization effect.In the later stage of smelting, the basicity of the final slag should be controlled at 2.0~2.5.In the production process, the molten iron samples were taken by sublance at the 6 min blowing and the blowing end point, respectively, to compare the dephosphorization effect of each stage.The slag samples at the end of blowing were analyzed and compared.

Analysis Method of Molten Iron Sample
The molten iron sample was polished with 600 mesh sandpaper to remove the surface oxide, and then the debris sample was obtained with a multifunctional drilling machine.Next, the mass fractions of carbon and sulfur were detected with an infrared carbon and sulfur analyzer (instrument model LECO-CS230), and the mass fractions of silicon, manganese, and phosphorus were detected with an inductively coupled plasma emission spectrometer (ICP, instrument model PE-Avio500).

Analysis Method of Slag Sample
The dephosphorization slag was ground into powder with a crusher and then screened with 200-mesh sieves to obtain a slag sample with a particle size meeting the detection standard.The composition of the slag sample was detected with an X-ray fluorescence spectrometer (XRF, ZSXPrimus II), and the Na 2 O in the slag sample was detected by ICP.An X-ray diffractometer (XRD, Uitima IV) was used to detect and analyze the phase of the slag sample.The diffractometer used a copper target with a scanning range of 15-80 • and a scanning speed of 10 • /min.The morphology of the dephosphorization slag was observed with a tungsten filament scanning electron microscope (SEM, ZEISS EVO18), and the corresponding regions and locations were selected for energy dispersive spectroscopy (EDS) component surface analysis and point analysis.

Laboratory Dephosphorization Experimental Results
The test results of the dephosphorization experiment are shown in Table 3, and the test results of the dephosphorization slag compositions are shown in Table 4.With the increasing of the w(Na 2 O) in the slag, the dephosphorization rate and phosphorus distribution ratio increased.After adding Na 2 O with a mass fraction of about 1% to the slag, the dephosphorization rate reached 76.30%, and the phosphorus distribution ratio reached 5.24, which are 4.45% and 0.19% higher, respectively, than the heat without adding Na 2 O, indicating that adding a small amount of Na 2 O can improve the dephosphorization ability of the slag.Figure 2 shows the XRD patterns of the dephosphorization slag samples.Figure 2a indicates that the phosphorus element mainly exists in the form of phosphate phases, such as Ca 2 SiO 4 •0.05Ca 3 (PO 4 ) 2 and 6Ca 2 SiO 4 •Ca 3 (PO 4 ) 2 , in the slag without Na 2 O.When Na 2 O is added to the dephosphorization slag, some Na will replace the Ca in the phosphoruscontaining phase, thus forming a Ca 2 SiO 4 •Ca 2 Na 2 (PO 4 ) 2 phase and some Na 3 PO 4 phase.Chuanming Du [27] and Kan Yu [28] also found that alkaline oxides, such as Na 2 O, are conducive to the formation of a phosphorus-rich solid solution in dephosphorization slag.In addition, Figure 2 shows that the addition of Na 2 O will also form a low-meltingpoint phase, such as Na 2 Si 2 O 5 , which can achieve rapid slag melting in the early stage of converter smelting and promote dephosphorization reactions.Figure 3 shows the SEM observation results of the experiment's dephosphorization slag samples.The figure indicates that the dephosphorization slag mainly has a white area and a gray area, which is consistent with the research results of previous researchers [29][30][31]; however, they all found that the gray area can be divided into a gray phosphoruscontaining area and a gray phosphorus-free area.In this regard, the energy spectrum analyzer was used to quantitatively analyze the components in different positions of the white and gray areas; the results are shown in Table 5.They indicate that there are more elements of Fe, Mg, Mn, and O in the white area 1, which is an iron-rich phase; the gray Figure 3 shows the SEM observation results of the experiment's dephosphorization slag samples.The figure indicates that the dephosphorization slag mainly has a white area and a gray area, which is consistent with the research results of previous researchers [29][30][31]; however, they all found that the gray area can be divided into a gray phosphorus-containing Metals 2023, 13, 844 6 of 15 area and a gray phosphorus-free area.In this regard, the energy spectrum analyzer was used to quantitatively analyze the components in different positions of the white and gray areas; the results are shown in Table 5.They indicate that there are more elements of Fe, Mg, Mn, and O in the white area 1, which is an iron-rich phase; the gray area 2 is rich in phosphorus, and its P element is greater than the other areas; and the gray area 3 is the matrix phase, with mainly the elements Ca, Si, and O. Figure 3 shows the SEM observation results of the experiment's dephosphorization slag samples.The figure indicates that the dephosphorization slag mainly has a white area and a gray area, which is consistent with the research results of previous researchers [29][30][31]; however, they all found that the gray area can be divided into a gray phosphoruscontaining area and a gray phosphorus-free area.In this regard, the energy spectrum analyzer was used to quantitatively analyze the components in different positions of the white and gray areas; the results are shown in Table 5.They indicate that there are more elements of Fe, Mg, Mn, and O in the white area 1, which is an iron-rich phase; the gray area 2 is rich in phosphorus, and its P element is greater than the other areas; and the gray area 3 is the matrix phase, with mainly the elements Ca, Si, and O.   4 indicates that Ca, Si, O, and P are distributed in the gray area 2, which is consistent with the energy spectrum analysis results in Table 5.Combined with the XRD phase analysis results in Figure 2, these elements exist in the form of a phosphate phase.The distribution map of each element in Figure 5 indicates that after adding Na 2 O to the dephosphorization slag, the Na element mainly exists in the gray area containing phosphorus.Combined with the XRD analysis in Figure 2, it mainly forms a Ca 2 SiO 4 •Ca 2 Na 2 (PO 4 ) 2 phase.and the No. 3 slag sample, respectively.The distribution of each element in Figure 4 indicates that Ca, Si, O, and P are distributed in the gray area 2, which is consistent with the energy spectrum analysis results in Table 5.Combined with the XRD phase analysis results in Figure 2, these elements exist in the form of a phosphate phase.The distribution map of each element in Figure 5 indicates that after adding Na2O to the dephosphorization slag, the Na element mainly exists in the gray area containing phosphorus.Combined with the XRD analysis in Figure 2, it mainly forms a Ca2SiO4•Ca2Na2(PO4)2 phase.

Industrial Results
The phosphorus mass fraction and dephosphorization rate of the conventional heats and test heats after blowing 6 min and the end point are shown in Table 6.The composition of the molten iron used in the two production processes is the same.Table 6 shows that the average dephosphorization rate of the molten iron in the conventional heats blowing for 6 min is only 42.77%, while the test heats can reach 62.39%, and the dephosphorization rate in the early stage of the converter increased by 19.62%.In addition, the average phosphorus mass fraction of the molten iron in the test heats is 0.041%, which is 0.019% lower than that in the conventional heats.
Moreover, the average dephosphorization rate at the end of the conventional heats is 62.14%, and the average dephosphorization rate at the end of the test heats can reach 72.03%, which is a 9.89% increase.When w(Na2O) reached 0.79% in the dephosphorization slag designed by Zhiqiang Zhou [20], the dephosphorization rate increased about 10%.In addition, the average mass fraction of the phosphorus at the end of the test heats is 0.03%, which is 0.01% lower than that of the conventional heats.The average value of the final slag compositions of the conventional and test heats is

Industrial Results
The phosphorus mass fraction and dephosphorization rate of the conventional heats and test heats after blowing 6 min and the end point are shown in Table 6.The composition of the molten iron used in the two production processes is the same.Table 6 shows that the average dephosphorization rate of the molten iron in the conventional heats blowing for 6 min is only 42.77%, while the test heats can reach 62.39%, and the dephosphorization rate in the early stage of the converter increased by 19.62%.In addition, the average phosphorus mass fraction of the molten iron in the test heats is 0.041%, which is 0.019% lower than that in the conventional heats.Moreover, the average dephosphorization rate at the end of the conventional heats is 62.14%, and the average dephosphorization rate at the end of the test heats can reach 72.03%, which is a 9.89% increase.When w(Na 2 O) reached 0.79% in the dephosphorization slag designed by Zhiqiang Zhou [20], the dephosphorization rate increased about 10%.In addition, the average mass fraction of the phosphorus at the end of the test heats is 0.03%, which is 0.01% lower than that of the conventional heats.
The average value of the final slag compositions of the conventional and test heats is shown in Table 7.The average basicity of the final slag of the test heats is 2.31, which is 0.19% lower than that of the conventional heats.The average w(P 2 O 5 ) in the final slag of the test heats is 2.83%, which is 0.36% higher than the conventional heats.The dephosphorization effect is enhanced, while reducing the consumption of materials such as lime.In addition, the average value of T.Fe in the final slag of the test heats is 14.93%, which is 0.69% lower than 15.62% of the conventional heats.The average value of the slagging materials consumption of the conventional and test heats is shown in Table 8.The table indicates that the average lime consumption of the test heats is 26.15 kg/t, which is 6.16 kg/t lower than that of the conventional heats.The change of the magnesium oxide ball consumption is not obvious; the average consumption of dolomite decreased by 2.67 kg/t; and the consumption of sodium-containing slagging material increased by 1.54 kg/t.The average consumption of slagging materials is 35.93 kg/t, which is 7.24 kg/t lower than that of the conventional heats.In addition, the average consumption of the iron and steel material in the test heats is 1052.23 kg/t, which is 1.37 kg/t lower than that in the conventional heats.

Effect of Na 2 O on Dephosphorization Reaction
In the conventional production process of the converter, the oxidative dephosphorization reaction occurs at the steel-slag interface.The relationship between the standard Gibbs free energy and the temperature of the dephosphorization reaction equations between CaO/Na 2 O and molten iron [32][33][34][35] are shown in Figure 6.The figure shows that the Gibbs free energy of Equation ( 3) is lower than the other two reactions at the same temperature, which indicates that the dephosphorization reaction between Na 2 O and molten iron occurs more easily.

Effect of Na 2 O on Phosphorus Distribution Ratio 4.2.1. Establishment of Phosphorus Distribution Ratio Model
The phosphorus distribution ratio (L P ) is one of the important parameters reflecting the dephosphorization ability of slag.As the phase diagram can accurately reflect the structure and thermodynamic properties of metallurgical melts, many scholars [36][37][38][39][40][41] used Ion and Molecule Coexistence Theory (IMCT) to characterize the physical and chemical properties of slag, and then obtained an IMCT-L P prediction model with high accuracy.Based on IMCT, a calculation model of the phosphorus distribution ratio of slag containing Na 2 O was established in this article, and the influence of Na 2 O on the phosphorus distribution ratio between slag and molten iron was discussed.
In the conventional production process of the converter, the oxidative dephosphorization reaction occurs at the steel-slag interface.The relationship between the standard Gibbs free energy and the temperature of the dephosphorization reaction equations between CaO/Na2O and molten iron [32][33][34][35] are shown in Figure 6.The figure shows that the Gibbs free energy of Equation ( 3) is lower than the other two reactions at the same temperature, which indicates that the dephosphorization reaction between Na2O and molten iron occurs more easily.Combined with the phase diagram, we determined that the slag has the following structural units, in which the chemical reaction formula, standard Gibbs free energy, and equilibrium constant of complex molecules can be referred to [32][33][34][35].
(1) Ions: Ca 2+ , Fe 2+ , O 2-, Mn 2+ , Mg 2+ , Na + ; (2) Simple molecules: SiO 2 , Al 2 O 3 , P 2 O 5 ; (3) Complex molecule: The total equilibrium mole number of all the structural units in 100 g slag can be calculated by the following formula: where n 1 ~n6 represents the molar number of simple components under equilibrium conditions; n c1 ~nc36 represents the molar number in the equilibrium of 36 composite molecules; and ∑n i represents the sum of the molar numbers of each structural unit in the slag during equilibrium, mol.
N i is defined as the mass action concentration of the structural unit i, which is equal to the ratio of the equilibrium molar number of structural unit i to the total molar number of all structural units in the equilibrium system.The expression is as follows: We assume that the mole fractions of the initial oxides before the equilibrium of 100 g slag are b According to the law of the conservation of mass, the molar number of each oxide is constant before and after the reaction, and the conservation formulas can be established.Taking CaO as an example: temperature in the early stage increased the w(Na 2 O) in slag to 0.5%, thereby ensuring the high phosphorus distribution ratio between the molten iron and slag and achieving the purpose of rapid dephosphorization in the early stage.

Effect of Na2O on Phosphorus Distribution Ratio
The effects of w(Na2O) on LP at 1573 K, 1623 K, and 1673 K are shown in Figure 8.The figure indicates that a small amount of Na2O can significantly improve the phosphorus distribution ratio.Free oxygen ions can promote the formation of phosphate, according to the IMCT, and the free oxygen ions in the slag are generated by the dissociation of alkaline oxides [33].As an alkali metal element, sodium is more alkaline than CaO, and it is easier to dissociate free oxygen ions, thus it shows better dephosphorization ability.In this industrial test, using the thermodynamic conditions of dephosphorization at lower temperature in the early stage increased the w(Na2O) in slag to 0.5%, thereby ensuring the high phosphorus distribution ratio between the molten iron and slag and achieving the purpose of rapid dephosphorization in the early stage.
According to the literature [44], Na2CO3 can be used as an additive in lime-based dephosphorization flux.Na2CO3 is decomposed into Na2O and CO2 by heating, and Na2O can improve the phosphorus distribution ratio.However, Na2O is highly active in the alkaline slag, which is easy to volatilize from the slag, and volatile Na2O will cause serious corrosion to the furnace lining; therefore, the mass fraction of Na2O in slag should not be too high.Thus, the w(Na2O) in the slag was controlled less than 1.0% in these industrial trial heats.

Effect of Na2O on Phosphorus Distribution Ratio
The effects of w(Na2O) on LP at 1573 K, 1623 K, and 1673 K are shown in Figure 8.The figure indicates that a small amount of Na2O can significantly improve the phosphorus distribution ratio.Free oxygen ions can promote the formation of phosphate, according to the IMCT, and the free oxygen ions in the slag are generated by the dissociation of alkaline oxides [33].As an alkali metal element, sodium is more alkaline than CaO, and it is easier to dissociate free oxygen ions, thus it shows better dephosphorization ability.In this industrial test, using the thermodynamic conditions of dephosphorization at lower temperature in the early stage increased the w(Na2O) in slag to 0.5%, thereby ensuring the high phosphorus distribution ratio between the molten iron and slag and achieving the purpose of rapid dephosphorization in the early stage.
According to the literature [44], Na2CO3 can be used as an additive in lime-based dephosphorization flux.Na2CO3 is decomposed into Na2O and CO2 by heating, and Na2O can improve the phosphorus distribution ratio.However, Na2O is highly active in the alkaline slag, which is easy to volatilize from the slag, and volatile Na2O will cause serious corrosion to the furnace lining; therefore, the mass fraction of Na2O in slag should not be too high.Thus, the w(Na2O) in the slag was controlled less than 1.0% in these industrial trial heats.According to the literature [44], Na 2 CO 3 can be used as an additive in lime-based dephosphorization flux.Na 2 CO 3 is decomposed into Na 2 O and CO 2 by heating, and Na 2 O can improve the phosphorus distribution ratio.However, Na 2 O is highly active in the alkaline slag, which is easy to volatilize from the slag, and volatile Na 2 O will cause serious corrosion to the furnace lining; therefore, the mass fraction of Na 2 O in slag should not be too high.Thus, the w(Na 2 O) in the slag was controlled less than 1.0% in these industrial trial heats.Figure 9 shows the liquid region of the CaO-FeO-SiO2 slag system at 1573 K after adding the Na2O calculated by FactSage8.1.It indicates that with the increasing of the w(Na2O) in the slag, the area of the liquid phase increases continuously, and the liquid phase region moves to the low w(FeO) region.Na2O has a great influence on the liquidus in the region of low binary basicity and has little influence in the region of high binary basicity.Figure 10 shows the experiment results of different fractions of Na2O on the melting point of the tested slag.It indicates that with the increasing of the w(Na2O) in slag, the melting point decreases continuously.When the w(Na2O) increases from 0 to 0.5%, the melting point of the tested slag decreases from 1645 K to 1639 K, which is consistent with the law measured by Jiang Diao [11].The addition of Na2O can generate low-meltingpoint compounds, such as Na2Si2O5 and Na2Ca2Si3O9, in the slag, which helps to reduce the melting point of the slag [45,46], achieve rapid slag melting in the early stage of converter smelting, and promote the dephosphorization reactions.Figure 9 shows the liquid region of the CaO-FeO-SiO2 slag system at 1573 K after adding the Na2O calculated by FactSage8.1.It indicates that with the increasing of the w(Na2O) in the slag, the area of the liquid phase increases continuously, and the liquid phase region moves to the low w(FeO) region.Na2O has a great influence on the liquidus in the region of low binary basicity and has little influence in the region of high binary basicity.Figure 10 shows the experiment results of different fractions of Na2O on the melting point of the tested slag.It indicates that with the increasing of the w(Na2O) in slag, the melting point decreases continuously.When the w(Na2O) increases from 0 to 0.5%, the melting point of the tested slag decreases from 1645 K to 1639 K, which is consistent with the law measured by Jiang Diao [11].The addition of Na2O can generate low-meltingpoint compounds, such as Na2Si2O5 and Na2Ca2Si3O9, in the slag, which helps to reduce the melting point of the slag [45,46], achieve rapid slag melting in the early stage of converter smelting, and promote the dephosphorization reactions.).After the viscosity of the slag is reduced, the fluidity is enhanced, the mass transfer capacity between the molten iron and slag is enhanced, and the dephosphorization reaction is promoted [47][48][49].

Effect of Na2O on Viscosity of Slag
Figure 11 shows the viscosity change of the CaO-FeO-SiO2 slag after adding the Na2O calculated by FactSage8.1.It indicates that the viscosity of the slag decreases with the increasing of w(Na2O).After the viscosity of the slag is reduced, the fluidity is enhanced, the mass transfer capacity between the molten iron and slag is enhanced, and the dephosphorization reaction is promoted [47][48][49].

Conclusions
(1) With the increase of w(Na2O), the dephosphorization rate increases, and the Ca, Si, O, and P elements in the dephosphorization slag are distributed in the same area, mainly in the form of phosphate minerals, such as Ca2SiO4•0.05Ca3(PO4)2and 6Ca2SiO4•Ca3(PO4)2.After adding Na2O, part of the Na will replace the Ca in the phosphorus-containing phase, forming a Ca2SiO4•Ca2Na2(PO4)2 phase.(2) After adding sodium-containing slagging material, the average dephosphorization rate of blowing for 6 min and at the end point can reach 62.39% and 72.03%, which are 19.62% and 9.89% higher, respectively, than the corresponding values of the conventional heats.The average final slag basicity of the test heats is 0.19% lower than that of the conventional heats, while the average w(P2O5) of the final slag increases by 0.36%, and the average T.Fe decreases by 0.69%.The average slagging materials consumption of the test heats is 35.93 kg/t, which is 7.24 kg/t lower than that of the conventional heats.(3) Through thermodynamic calculation, we found that with the increase of w(Na2O), the phosphorus distribution ratio between the slag and the molten iron increases significantly, the area of the liquid phase zone of the slag system increases continuously, and the viscosity decreases continuously.

Figure 6 .
Figure 6.Relationship between standard Gibbs free energy and temperature of reaction equations.Figure 6. Relationship between standard Gibbs free energy and temperature of reaction equations.

Figure 6 .
Figure 6.Relationship between standard Gibbs free energy and temperature of reaction equations.Figure 6. Relationship between standard Gibbs free energy and temperature of reaction equations.

Figure 8 .
Figure 8.Effect of different mass fraction of Na2O on lgLP.

Figure 8 .
Figure 8.Effect of different mass fraction of Na2O on lgLP.Figure 8. Effect of different mass fraction of Na 2 O on lgL P .

Figure 8 .
Figure 8.Effect of different mass fraction of Na2O on lgLP.Figure 8. Effect of different mass fraction of Na 2 O on lgL P .

4. 3 .
Figure 9 shows the liquid region of the CaO-FeO-SiO 2 slag system at 1573 K after adding the Na 2 O calculated by FactSage8.1.It indicates that with the increasing of the w(Na 2 O) in the slag, the area of the liquid phase increases continuously, and the liquid phase region moves to the low w(FeO) region.Na 2 O has a great influence on the liquidus in the region of low binary basicity and has little influence in the region of high binary

4. 3 .
Effect of Na2O on Physicochemical and Chemical Properties of Slag System 4.3.1.Effect of Na2O on Melting Point of Slag

Figure 10 .
Figure 10.Effect of different mass fractions of Na2O on melting point of slag system.

4. 3 .
Effect of Na2O on Physicochemical and Chemical Properties of Slag System 4.3.1.Effect of Na2O on Melting Point of Slag

Figure 10 .
Figure 10.Effect of different mass fractions of Na2O on melting point of slag system.Figure 10.Effect of different mass fractions of Na 2 O on melting point of slag system.

Figure 10 .
Figure 10.Effect of different mass fractions of Na2O on melting point of slag system.Figure 10.Effect of different mass fractions of Na 2 O on melting point of slag system.

4.3. 2 .
Figure 11 shows the viscosity change of the CaO-FeO-SiO 2 slag after adding the Na 2 O calculated by FactSage8.1.It indicates that the viscosity of the slag decreases with the increasing of w(Na 2 O).After the viscosity of the slag is reduced, the fluidity is enhanced, the mass transfer capacity between the molten iron and slag is enhanced, and the dephosphorization reaction is promoted[47][48][49].

Table 6 .
Average value of phosphorus mass fraction and dephosphorization rate.

Table 6 .
Average value of phosphorus mass fraction and dephosphorization rate.