Influence Mechanism of Ce₂O₃ on Dephosphorization Process using CaO–Al₂O₃–SiO₂–MnO Based Slag

Abstract

To improve the dephosphorization capacity of CaO–Al₂O₃–SiO₂–MnO based ferromanganese slag, this work attempts to introduce Ce₂O₃ as a strong phosphorus fixative into the slag, and the influence mechanism of Ce₂O₃ on the dephosphorization process is investigated by thermodynamic analysis, structural analysis, melting temperature, and viscosity tests. The results show that after adding 20 mass% Ce₂O₃ into the slag, Ce₂O₃ can release the O²⁻ and promote the P-O⁰ bond to P-O⁻ bond transformation, thus improving the phosphorus fixation capacity of melted slag. Also, the phosphorus enrichment phases are nCa₂SiO₃–Ca₃P₂O₈ solid solution and CePO₄ phase in Ce₂O₃-containing slow cooling slag. Furthermore, the melting temperature of slag with 0–20 mass% Ce₂O₃ addition is always about 1623 K, but increases obviously when the addition of Ce₂O₃ exceeds 20 mass%. Also, adding appropriate Ce₂O₃ content into the CaO–Al₂O₃–SiO₂–MnO based slag can decrease the viscosity of slag, and the viscosity can remain relatively low with adding 15 mass%–25 mass% Ce₂O₃. In conclusion, adding appropriate Ce₂O₃ content into the CaO–Al₂O₃–SiO₂–MnO based slag can improve the phosphorus fixation capacity of slag and ameliorate the fluidity of slag, and consequently, it is conducive to remove phosphorus from ferromanganese. Finally, the high-temperature ferromanganese dephosphorization experiments were carried out, and the results show that after adding 20 mass% Ce₂O₃ into the slag, the lowest [P] content decreases from 0.31% to 0.25%, and the dephosphorization rate increases from 31.1% to 45.6%. The above results not only verify that the feasibility of Ce₂O₃ as a strong phosphorus fixative used in the ferromanganese dephosphorization slag, but also provide the theoretical guidance for optimizing the composition of Ce₂O₃-containing dephosphorization slag.

1. Introduction

[P] is harmful to the mechanical properties (e.g., strength, ductility, and toughness) of iron/steel products, and removing [P] from iron/steel has been emphasized over several decades in the steelmaking process.^1,2) Moreover, the demand of removing [P] from ferromanganese is continuously increased because the advanced high strength steels containing manganese, such as TRIP and TWIP steels, were recently developed.^3,4) Hence, the dephosphorization of ferromanganese is also an important issue. However, the removal of [P] from ferromanganese is more difficult than that from iron/steel due to the stronger binding force of [Mn]-[P] than that of [Fe]-[P], and the preferential oxidation of [Mn].⁵⁾ Therefore, to remove [P] and retain [Mn] effectively, it is necessary to employ a MnO-bearing slag with strong phosphorus fixation capacity and good fluidity in the ferromanganese dephosphorization process, in which MnO plays roles in appropriately improving the oxidation capacity of slag and retaining [Mn].

Since the pyrometallurgical process produces slags mostly comprising oxides such as Al₂O₃ and SiO₂, and CaO is the most economical and common phosphorus fixative, CaO–SiO₂–Al₂O₃ based slag is widely used in the iron/steel dephosphorization process. However, abundant researches show that CaO–SiO₂–Al₂O₃ based slag cannot effectively remove phosphorus from ferromanganese due to the insufficient phosphorus fixation capacity of CaO. Therefore, some compounds with high alkalinity, such as Na₂O and BaO, were tentatively introduced into the CaO based ferromanganese slag^6,7) to improve the phosphorus fixation capacity of slag. However, the introduction of Na₂O/BaO increases the cost of refining ferromanganese ineluctably, and Na₂O is unstable at high temperature and harmful to furnace lining. Also, [Si] is a common component in the ferromanganese, but the dephosphorization capacity of BaO-based slag is strongly affected by [Si] content in the ferromanganese, and the extremely poor dephosphorization effect is found when the [Si] content exceeds 0.2% by Ma et al.’s reports.⁸⁾ So far, no suitable slag can be widely applied in the ferromanganese dephosphorization process. Thus, it is of great significance to find another high basicity oxide that may avoid the above troubles for optimizing the composition of ferromanganese dephosphorization slag. The optical basicity values of Ce₂O₃ (1.42), which is estimated by Zhao et al.⁹⁾ according to the calculated average electronic polarizability, is greater than that of CaO (1.00), BaO (1.08), and Na₂O (1.10), and there are stable rare earth phosphate minerals such as Monazite, which contains Ce, La, Nd, Th[PO₄], in the natural environment. Furthermore, rare earth (RE) resources are abundant in some countries, such as China, but unreasonable utilization of the RE resources leads to the production of RE-bearing waste slags with massive reserves and low utilization rates.¹⁰⁾ If Ce₂O₃ can improve the phosphorus fixation capacity of CaO based slag, the above waste slags or low-grade RE ores are expected to be introduced into ferromanganese dephosphorization slag, which will significantly reduce the cost of the ferromanganese refining process. However, there are few reports on the dephosphorization experiments using Ce₂O₃-bearing slag up to now. Therefore, the effect mechanism of Ce₂O₃ on the phosphorus fixation capacity of slag is unclear, and whether CePO₄ can be formed in the dephosphorization slag has not been investigated yet.

Furthermore, the slag with good fluidity is also essential to remove [P] from ferromanganese effectively, and viscosity and melting temperature are two important indexes to evaluate the fluidity of slag. However, the effect of Ce₂O₃ on the melting temperature or viscosity of slag varies with the composition of slag. Cai¹¹⁾ et al. pointed out that CeO₂ increases the melting temperature and decreases the viscosity of CaF₂-bearing and B₂O₃-containing mold fluxes. Qi¹²⁾ et al. investigated the Ce₂O₃ on viscosity and structure of CaO–Al₂O₃–Li₂O based slag, and found that adding Ce₂O₃ could decrease the viscosity of the slag due to its effects on decreasing the polymerization of the slag. Wang¹³⁾ et al. considered that the RE oxide (Ce, La) could remarkably increase the viscosity of CaO–SiO₂ based mold fluxes, especially when the mixture content was over 10%. Up to now, the effects of the Ce₂O₃ on the melting temperature and viscosity of CaO–Al₂O₃–SiO₂–MnO based ferromanganese slag have been rarely explored. Therefore, in this work, the effects of Ce₂O₃ on phosphorus fixation capacity and fluidity of CaO–Al₂O₃–SiO₂–MnO based slag were investigated through thermodynamic analysis, structural analysis, melting temperature test and viscosity test in order to explore the effect of Ce₂O₃ on the dephosphorization mechanism of slag. Furthermore, the high-temperature ferromanganese dephosphorization experiments were also carried out to verify the feasibility of Ce₂O₃ as a strong phosphorus fixative used in the ferromanganese dephosphorization slag. The above results have a great guiding significance for the composition optimization of Ce₂O₃-containing ferromanganese dephosphorization slag.

2. Effect of Ce₂O₃ on the Phosphorus Fixation Capacity of Slag

2.1. Standard Gibbs Free Energy

It is widely believed that the P₂O₅ activity coefficient is an important and indispensable parameter to investigate the phosphorus fixation capacity of slag. To clarify the effect of Ce₂O₃ on the P₂O₅ activity coefficient in the slag, it is essential to understand the Gibbs free energy for the formation of cerium phosphate. Thiret et al.¹⁴⁾ studied the heat capacity of CePO₄ and the thermodynamic formation function of CePO₄ at 298.15 K, as shown in Eq. (1) and Table 1. The standard Gibbs free energy for the reaction of Ce₂O₃ and P₂O₅ to produce CePO₄ (Eq. (3)) can be deduced according to Eq. (2) and relevant thermodynamic data of Ce₂O₃, P₂O₅ and CePO₄. Figure 1 shows the standard Gibbs free energy that different basic components (CaO, MnO, Ce₂O₃) in the sample reacts with P₂O₅ to form phosphate.¹⁵⁾ Assuming the activities of basic components and phosphates are both 1, the order of P₂O₅ activity coefficient is calculated as follows: Ce₂O₃<CaO<MnO. Therefore, it is deduced that the Ce₂O₃-containing slag has a strong phosphorus fixation capacity.   

$$\begin{array}{l}{C}_{\text{P}}\text{(J}\cdot {\text{K}}^{-\text{1}}\cdot {\text{mol}}^{-\text{1}}\text{)}=\text{106}\text{.488}+\text{7}\text{.231}\times {\text{10}}^{-\text{2}}\text{(}T\text{/K)}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}-\text{1}\text{.876}\times {10}^{-5}{\text{(}T\text{/K)}}^2\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}-1.773\times {10}^6{\text{(}T\text{/K)}}^{-2}\end{array}$$(1)

  

$$G_T^{\theta }=H_{298.15}^{\theta }+{\int }_{298.15}^T{C}_{\text{P}}\text{d}T-T\left(S_{298.15}^{\mathrm{\theta }}+{\int }_{298.15}^T\frac{C_{\text{P}}}{T}\text{d}T\right)$$(2)

  

$${\text{(Ce}}_{\text{2}}{\text{O}}_{\text{3}}\text{)}+{\text{(P}}_{\text{2}}{\text{O}}_{\text{5}}\text{)}={\text{2(CePO}}_{\text{4}}\text{)}\hspace{1em}\mathrm{\Delta }{G}_1{}^{\theta }=-654\mathrm{\hspace{0.17em} }568-107T$$(3)

Table 1. The thermodynamic function of CePO₄ formation at 298.15 K.¹⁴⁾

Thermodynamic functions
ΔfHθ (CePO4, 298.15K)	−1969.54±3.0	KJ·mol−1
ΔfSθ (CePO4, 298.15K)	−401.51	J·K−1·mol−1
ΔfGθ (CePO4, 298.15K)	−1849.8	KJ·K−1·mol−1

Fig. 1.

Relationship between temperature and standard Gibbs free energy. (Online version in color.)

However, the above analyses may be incorrect because the activities of basic components and phosphates cannot be 1 in the actual slag. Unfortunately, the P₂O₅ activity coefficient in Ce₂O₃-bearing actual slag cannot be directly estimated using the existing methods, such as Factsage software, Basu et al.’s model,¹⁶⁾ Suito et al.’s model,¹⁷⁾ Mori et al.’s model,¹⁸⁾ and so on, due to the lack of thermodynamic datas relating to some Ce-bearing compounds. Therefore, it is necessary to find other reliable methods to explore the effect of Ce₂O₃ on the phosphorus fixation capacity of slag. The phosphorus fixation capacity of slag increases with the improvement of the stability of P⁵⁺ in the slag, and the stability of P⁵⁺ fundamentally depends on the structure of slag. Therefore, an attempt to investigate the effect of Ce₂O₃ on the phosphorus fixation capacity was carried out from the structural view in the subsequent work.

2.2. Sample Preparation

Previous researches have shown that introducing 20%–30% MnO into ferromanganese dephosphorization slag can appropriately improve the oxidation capacity, and reduce the manganese oxidation loss,¹⁹⁾ so the MnO content in the sample is fixed at 25%. The mass ratio of CaO: Al₂O₃: SiO₂ in the sample is determined according to the P₂O₅ iso-activity curves calculated by Factsage 8.1 with “FToxide” database, as shown by the blue asterisk in Fig. 2. Subsequently, the two samples with no Ce₂O₃ and 20 mass% Ce₂O₃ were designed according to the above analyses and were abbreviated as No. 1 and No. 2, respectively. Furthermore, to investigate Ce₂O₃ on the stability of P⁵⁺ in the slag, 5% P₂O₅ is also added to the two samples for structure test, and the specific sample preparation steps for structure test are as follows. First, reagent grade powders of CaO, SiO₂, Al₂O₃, MnO, P₂O₅ and CeO₂ used as raw materials were calcined in a muffle furnace for 10 h at high temperature to remove impurities and moisture, and all the powders were mixed thoroughly according to the composition of designed samples. Then, the mixed sample was put into a graphite crucible lined with a Mo sheet, and a Mo lid was set on the top of the crucible to reduce the volatilization loss. Thereafter, the two crucibles were hung in a high-temperature tube furnace with Mo wire and heated to approximately 1623 K for 5 h in the Ar atmosphere. Finally, the melted sample was quickly quenched in ice water by loosening Mo wire, and the quenched sample was crushed to less than 100 mesh for the structure test.

Fig. 2.

P₂O₅ iso-activity curves in CaO-Al₂O₃-SiO₂-25%MnO-5%P₂O₅ slag at 1623 K. (Online version in color.)

Furthermore, to investigate the stable valence state of Mn/Ce (variable valence elements) in the prepared sample under the present experimental atmosphere, the X-ray photoelectron spectroscopy (XPS) spectra of quenched samples were analyzed as follow. The computer program XPSPEAK Version 4.0 was used to fit the narrow-scan spectra after Shirley-type background subtraction. All spectra were calibrated using the adventitious C 1 s peak with a fixed value of 284.5 eV.²⁰⁾ Figure 3(a) shows the Mn 2p spectrum of quenched sample with Ce₂O₃, and the stable valence of Mn in the sample is mainly Mn²⁺ from the observation of Mn²⁺ peaks (641.7 eV and 653.6 eV) and MnO satellite feature.²¹⁾ Figure 3(b) shows that the Ce 3d spectrum of quenched sample with Ce₂O₃, and Ce in the sample is stabilized as Ce³⁺ because the intensity of Ce³⁺ peak (886 eV and 904.3 eV)²²⁾ is majorly detected and the intensity of Ce⁴⁺ peak is not observed. Therefore, Mn and Ce in the sample mainly exist as the bivalent and trivalent forms under the present experimental atmosphere, respectively.

Fig. 3.

The XPS spectra of Mn 2p (a) and Ce 3d (b) for quenched samples. (Online version in color.)

2.3. Stability of P⁵⁺ in the Slag

P forms the covalent bond with O in the slag, and consequently, the stability of P⁵⁺ mainly depends on the strength of P–O bond. Figure 4(a) shows the P 2p spectra of two quenched samples measured by the XPS method, and the binding energy of P 2p peak decreases with the addition of Ce₂O₃, which indicates the increase of the electron cloud density (the orbital overlap) between P and O atoms.²³⁾ Moreover, when the P 2p spectrum of the sample is carefully observed, it is possible to decompose the P 2p peak into two components. The two decomposed peaks are compared with those obtained peaks of orthophosphate (132.9 eV) and pyrophosphate (134.0 eV),²⁴⁾ and the two types of phosphates can be abbreviated as ${\text{Q}}_{\text{P}}^{\text{0}}$ and ${\text{Q}}_{\text{P}}^{\text{1}}$, respectively (${\text{Q}}_{\text{P}}^n$, n is the number of bridging oxygen in one [PO₄]-tetrahedron). The peak of the low binding energy component has the same position as the P 2p peak in the orthophosphate, whereas the peak of the high binding energy component corresponds to the P 2p peak in the pyrophosphate. According to the integral area of each peak, the percentage of different phosphate species (viz. ${\text{Q}}_{\text{P}}^{\text{0}}$ and ${\text{Q}}_{\text{P}}^{\text{1}}$) are also calculated, as shown in Fig. 4(b). The results show that P⁵⁺ mainly exists as ${\text{Q}}_{\text{P}}^{\text{0}}$ and ${\text{Q}}_{\text{P}}^{\text{1}}$ units in the samples, and the ${\text{Q}}_{\text{P}}^{\text{1}}$ unit transfers to ${\text{Q}}_{\text{P}}^{\text{0}}$ unit with the addition of Ce₂O₃.

Fig. 4.

Effect of Ce₂O₃ on the P 2p spectra of quenched samples. (Online version in color.)

Furthermore, the Raman spectra are also measured to investigate the structure of quenched slag, and the [PO₄]-tetrahedron can be observed in the range of 700–1200 cm⁻¹ Raman bands. Figures 5(a) and 5(b) show the deconvoluted results of Raman curves of quenched slags with different Ce₂O₃ content,^11,25) and the percentage of different phosphate species (viz. ${\text{Q}}_{\text{P}}^{\text{0}}$ and ${\text{Q}}_{\text{P}}^{\text{1}}$) are plotted as Fig. 5(c) according to the integral area of each peak. Similarly, the ${\text{Q}}_{\text{P}}^{\text{1}}$ unit transfer to the ${\text{Q}}_{\text{P}}^{\text{0}}$ unit after adding Ce₂O₃ into the slag from Raman spectra. It is widely believed that there are more P–O⁻ bonds and fewer P–O⁰ bonds in the ${\text{Q}}_{\text{P}}^{\text{0}}$ unit than that in the ${\text{Q}}_{\text{P}}^{\text{1}}$ unit, thus increasing the P–O⁻ bond and decreasing the P–O⁰ bond after the ${\text{Q}}_{\text{P}}^{\text{1}}$ unit to the ${\text{Q}}_{\text{P}}^{\text{0}}$ unit transformation. Moreover, P⁵⁺ ions partly replaced with Si⁴⁺, forming ${\text{Q}}_{\text{Si+P}}^{\text{2}}$ unit from Raman spectra, and the percentage of ${\text{Q}}_{\text{Si+P}}^{\text{2}}$ decreases with the addition of Ce₂O₃ in the slag, which indicates that the percentage of P–O⁰(-Si) bond decreases. Suzuya et al. proposed that the radius of P–O⁰ bond (1.54 Å) is larger than that of P–O⁻ bond (1.42 Å).²⁶⁾ Therefore, the average radius of P–O bond decreases after adding Ce₂O₃ into the slag. In conclusion, after adding 20 mass% Ce₂O₃ into the slag, the electron cloud density between P and O atoms increases, and the average radius of P–O bonds decreases, which enhance the stability of P⁵⁺ in the sample.

Fig. 5.

Effect of Ce₂O₃ on the Raman spectra of quenched samples. (Online version in color.)

2.4. Optical Basicity of Slag

Furthermore, abundant studies show that there is a negative correlation between the P₂O₅ activity coefficient and the optical basicity of slag.^27,28) However, the optical basicity values of oxides vary in different researches, so the optical basicity of multicomponent slag calculated by frequent formulas²⁹⁾ has low accuracy. The optical basicity proposed by Duffy and Ingram³⁰⁾ is expressed by the electron donating ability of oxygen ions. Therefore, the optical basicity is directly related to the electron distribution around the O atom. Recently, the O 1s chemical shift in XPS spectrum was intensively used for the search of adequate relation between the peak position and the optical basicity of some oxides and oxide glasses,^31,32) and the results show that the binding energy of O 1s peak decreases with theoretical optical basicity increasing. Therefore, the O 1s spectra of two samples were also measured to investigate the effect of Ce₂O₃ on the optical basicity of slag. Figure 6(a) shows the binding energy corresponding to O 1s peak decreases after adding Ce₂O₃ into the slag, which indicates the electron donating ability of oxygen ions increases in the slag, thereby increasing the optical basicity of slag. Moreover, when the O 1s spectrum of the sample is carefully observed, it is possible to decompose the O1s peak into three components. The binding energies near 532.2 eV, 531.2 eV, and 530 eV correspond to the bridging oxygen (O⁰), the non-bridging oxygen (O⁻), and the free oxygen (O²⁻), respectively, which agree well with the results from previously published reports.³³⁾ According to the integral area of each peak, the percentage of different oxygen ions species (viz. O⁰, O⁻, O²⁻) are also calculated, as shown in Fig. 6(b). As a basic oxide, Ce₂O₃ can release O²⁻, and promote the equilibrium reaction among the three oxygen anions (Eq. (4)) proceeds spontaneously in the forward direction, thus increasing the percentage of O⁻/O²⁻ and decreasing the percentage of O⁰ in the slag. Similarly, the P–O⁰ bond to P–O⁻ bond transformation should be attributed to the increase of O²⁻ released by Ce₂O₃, and Ce₂O₃ play the role of the network modifier in the slag. To further confirm the above inference, the effect of Ce₂O₃ on phosphorus enrichment phase in the slow cooling slag is also explored as follows.   

$${\text{O}}^{\text{0}}+{\text{O}}^{\text{2}-}=2{\text{O}}^{-}$$(4)

Fig. 6.

Effect of Ce₂O₃ on the O 1s spectra of quenched samples. (Online version in color.)

2.5. Phosphorus Enrichment Phases in the Slow Cooling Slag

The morphology and the phase composition of slow cooling sample were studied by the scan electron microscope and energy-dispersive spectrometer (SEM-EDS), as shown in Fig. 7. It can be seen from Fig. 7(a) that the slag with no Ce₂O₃ consists of three mineralogical phases: white phase, grey phase and dark grey phase. The grey and phosphorus enrichment phase consists of Ca, Si, O and P elements, which is inferred as nCa₂SiO₃–Ca₃P₂O₈ (nC₂S–C₃P) solid solution according to previous researches.³⁴⁾ The slag with 20 mass% Ce₂O₃ consists of two phosphorus enrichment phases: the composition of one phase is similar to that of the phosphorus enrichment phase in Fig. 7(a), and the other phase mainly composes of Ca, Ce, Si, O and P elements, as shown in Fig. 7(b). To clarify the structure of phosphorus enrichment phases, the selected area electron diffraction (SAED) patterns and energy-dispersive spectrometer (EDS) spectrum were acquired in the transmission electron microscope (TEM), as shown in Figs. 8 and 9. In the TEM mode (Figs. 8(a) and 9(a)), the SEAD patterns (Figs. 8(b) and 9(b)) are consistent with the simulated electron diffraction patterns of nC₂S–C₃P solid solution and CePO₄ (Figs. 8(c) and 9(c)). Also, in the scanning transmission electron microscopy (STEM) mode (Figs. 12(d) and 13(d)), these phosphorus-rich phases have the same elements with nC₂S–C₃P solid solution and CePO₄ (Figs. 12(e) and 13(e)). Based on the SEAD pattern and EDS spectrum from the same phase, the phosphorus enrichment phases are confirmed as nC₂S–C₃P solid solution and CePO₄. In conclusion, adding 20 mass% Ce₂O₃ into the slag, Ce₂O₃ can release the O²⁻ and promote the P–O⁰ bond to P–O⁻ bond (connected to Ce³⁺) transformation, thus improving the phosphorus fixation capacity of slag.

Fig. 7.

The morphologies and phase composition of two slow cooling samples. (Online version in color.)

Fig. 8.

The structure and composition of phosphorus enrichment phases (nC₂S–C₃P solid solution). (Online version in color.)

Fig. 9.

The structure and composition of phosphorus enrichment phase (CePO₄). (Online version in color.)

Fig. 12.

Variations of ferromanganese element concentrations with treatment time. (Online version in color.)

Fig. 13.

Time dependency of dephosphorization rates. (Online version in color.)

3. Effect of Ce₂O₃ on the Slag Fluidity

Strong phosphorus fixation capacity of slag can promote the dephosphorization reaction, and good slag fluidity can accelerate the reaction. Therefore, to investigate the effect of Ce₂O₃ on the slag fluidity, the melting temperature and the viscosity of slag were measured as follows.

3.1. Melting Temperature of Slag

A hemisphere method was used to measure the melting temperature of the sample with different Ce₂O₃ addition. First, the quenched samples with different Ce₂O₃ addition were crushed and mixed thoroughly in an evaporating dish with a little alcohol, and made into the cylindrical samples with the size of Φ 3 mm*3 mm. Then, the cylindrical sample dried naturally was placed on the center of the platform and heated at a rate of 5–10°C/min during the test, and the melting temperature recorded automatically by a computer when the height of the cylindrical sample became half of the original height. The results of the hemispherical temperature and the flowing temperature of samples are presented in Fig. 10, and the hemispherical temperature is usually considered as the melting temperature of slag. When the Ce₂O₃ content is from 0 mass% to 20 mass%, the melting temperature is always about 1623 K, and the melting temperature increases obviously with exceeded 20 mass% Ce₂O₃ addition. The above results indicate that the melting property of the ferromanganese dephosphorization slag can remain relatively stable after absorbing 0–20 mass% Ce₂O₃.

Fig. 10.

Effect of Ce₂O₃ on the melting temperature of ferromanganese slag. (Online version in color.)

3.2. Viscosity of Slag

A rotating cylinder viscometer was employed to measure the viscosity. First, about 150 g quenched sample was put into a graphite crucible (inside diameter: 40 mm; height: 80 mm) lined with Mo sheet and heated to make the samples completely melted. Then, after 30 minutes of heat preservation, the measurements of viscosity were carried out with a Mo spindle (diameter: 10 mm; height: 25 mm). The measurement was under an argon atmosphere (>99% purity) and the data were automatically collected by a computer. For viscosity measurement, the Mo spindle rotated with a speed of 200 rpm, meanwhile, the slag was cooled with a rate of −3 K/min, and the measurement was ended with a viscosity of 6 Pa·s. Figure 11(a) shows the viscosity-temperature curves of samples with different addition of Ce₂O₃. With the increase of Ce₂O₃ content, the viscosity first decreased and then increased at 1623 K, and that was always around 0.1 Pa·s when the Ce₂O₃ addition is 15 mass%–25 mass%, as shown in Fig. 11(b). Qi et al.¹²⁾ reported that adding moderate Ce₂O₃ content could decrease the viscosity of the slag due to its effect on reducing the polymerization of the slag. But excessive Ce₂O₃ addition will inevitably lead to the precipitation of high melting point phases such as Ce₂O₃,³⁵⁾ thus increasing the viscosity of slag. Therefore, to obtain the slag with low viscosity, the suitable Ce₂O₃ addition is 15 mass%–25 mass%. Considering the viscosity and the melting temperature of samples, the suitable Ce₂O₃ addition is 15 mass%–20 mass% to obtain good slag fluidity.

Fig. 11.

Effect of Ce₂O₃ on the viscosity of ferromanganese slag. (Online version in color.)

4. Ferromanganese Dephosphorization Experiments

4.1. Experimental Process

The above analyses show that adding an appropriate Ce₂O₃ content to the CaO–Al₂O₃–SiO₂–MnO based slag can improve the phosphorus fixation capacity of slag and ameliorate the slag fluidity, which is theoretically conducive to remove phosphorus from ferromanganese. To further verify the above theoretical analyses, the ferromanganese dephosphorization experiments using the No. 1 (no Ce₂O₃) and No. 2 sample (20 mass% Ce₂O₃) were carried out in a silicon molybdenum furnace. First, the alumina crucible (Φ 5 cm) loaded with the ferromanganese (200 g) was heated under the purified argon atmosphere to 1623 K. After reaching the specified temperature and keeping for a while, the quenched slag (20 g) prepared with reagent grade powders were introduced to the ferromanganese (0 minute). Use quartz tubes (Φ 4 mm) to collect ferromanganese samples every 15 minutes, and use iron spoons to collect slag samples at 45 minutes. Finally, the elements in ferromanganese were determined by silicon molybdenum blue colorimetry (Si), phosphorus molybdenum blue colorimetry (P), carbon and sulfur analyzer (C), atomic absorption spectrometry (Mn). The slag composition was analyzed by fluorescence spectrometry.

4.2. Results and Discussion

Figure 12 shows the composition variations of ferromanganese with reaction time during the experimental process. Table 2 shows the chemical composition of slag before and after ferromanganese dephosphorization experiments. When the quenched samples were introduced into the ferromanganese at 1623 K: the [Si] content first decreased rapidly to about 0.1%, and then had no obvious change; the [P] content first decreased gradually with time, and reached the lowest value at 45 minutes; The [Mn] and [C] content varied little with time. The dephosphorization process of ferromanganese is accompanied by partial desiliconization reaction, and the removal reactions of [Si] and [P] can be described by two coupling steps. First, [Si] and [P] in the ferromanganese are oxidized by oxidants (e.g. MnO) to form SiO₂ and P₂O₅ into the slag. Second, the SiO₂ and P₂O₅ are stabilized by basicity oxides (e.g. CaO or Ce₂O₃) to form stable silicate and phosphate. Moreover, [Si] is usually removed prior to [P] due to the strong reducibility of [Si]. Therefore, [Si] content first decreases rapidly after the slag is introduced into ferromanganese. However, when [Si] content decreases to about 0.1%, the desilication reaction can not continue because of the low activity of [Si] and the high activity of SiO₂, so [Si] content then has no obvious change.

Table 2. Chemical composition of slag before and after dephosphorization experiments.

Number		chemical composition of slag/mass%
		CaO	Al2O3	SiO2	MnO	Ce2O3	P2O5
No. 1	before	34.99	26.04	13.97	25.00
	after	34.30	27.56	15.98	20.56		1.38
No. 2	before	29.16	21.70	11.64	20.83	16.67
	after	27.51	23.01	13.56	18.98	15.73	1.92

Figure 13 shows the time dependency of dephosphorization rates (η_P). The effects of Ce₂O₃ on the dephosphorization rates (η_P), the phosphorus distribution ratios (L_P), the calculated optical basicity, and the binding energy of O 1s peak are shown in Fig. 14. The optical basicity of slag is calculated by Eqs. (5) and (6),²⁹⁾ where Λ_i is the optical basicity of component i, i represents the component in the slag; $X_i^{{\text{O}}^{2-}}$ is the molar fraction of oxygen ion of component i in the slag; O_i is the molar number of oxygen ion in component i; X_i is the molar fraction of component i in the slag. Also, the binding energy of O 1s peak decreases with the optical basicity increasing according to previous studies.^31,32) The results show that the addition of Ce₂O₃ can increase the optical basicity of slag, decrease the binding energy of O 1s peak, and improve the dephosphorization effects (the lowest [P] content changes from 0.31% to 0.25%, the dephosphorization ratio changes from 27% to 45%, and the phosphorus distribution ratio changes from 4.46 to 7.76). The above results demonstrate that Ce₂O₃ is a strong alkaline oxide and Ce₂O₃ can be used as a strong phosphorus fixative in the ferromanganese dephosphorization slag.   

$$\mathrm{\Lambda }=\sum _{i=1}^{\text{n}}{\mathrm{\Lambda }}_i\cdot X_i^{{\text{O}}^{2-}}$$(5)

  

$$X_i^{{\text{O}}^{2-}}=\frac{O_i\cdot X_i}{\sum _{i=1}^{\text{n}}{O}_i\cdot X_i}$$(6)

Fig. 14.

Effects of Ce₂O₃ on dephosphorization rates, phosphorus distribution ratios, calculated optical basicity and binding energy of O 1s peak. (Online version in color.)

5. Conclusions

In this work, to improve the dephosphorization capacity of CaO–Al₂O₃–SiO₂–MnO based ferromanganese slag, a strong phosphorus fixative-Ce₂O₃ is proposed and verified by thermodynamic analysis, structural analysis, melting temperature, viscosity tests and high-temperature ferromanganese dephosphorization experiments. The results are as follows:

(1) According to the comparison of standard Gibbs free energy, it is inferred that the affinity between Ce₂O₃ and P₂O₅ is stronger than that between CaO/MnO and P₂O₅ at dephosphorization temperature. However, the effect of Ce₂O₃ on the actual phosphorus fixation capacity of slag cannot be determined from a thermodynamic view due to the lack of thermodynamic data relating to some Ce-bearing compounds.

(2) When adding 20 mass% Ce₂O₃ to the ferromanganese slag: the binding energy corresponding to P 2p spectrum decreases and the P–O⁰ bond transfers to P–O⁻ bond, which indicates that the stability of P⁵⁺ in the melted slag increases; the binding energy corresponding to O 1s spectrum decreases, which indicates that the optical basicity of melted slag increases; furthermore, the phosphorus enrichment phases in the slow cooling slag are determined as the nC₂S–C₃P solid solution and CePO₄ phase by the SEM and TEM methods. In conclusion, after adding 20 mass% Ce₂O₃ into the slag, Ce₂O₃ can release the O²⁻ and promote the P–O⁰ bond to P–O⁻ bond (connected to Ce³⁺) transformation, thus improving the phosphorus fixation capacity of slag.

(3) When adding 0–20 mass% Ce₂O₃ into the slag, the melting temperature is always about 1623 K, but the melting temperature increases obviously when the addition of Ce₂O₃ content exceeds 20 mass%. Furthermore, adding appropriate Ce₂O₃ content into the slag can decrease the viscosity of slag, and the viscosity at 1623 K can remain relatively low when the addition of Ce₂O₃ is 15 mass%–25 mass%. In conclusion, the appropriate addition of Ce₂O₃ in the slag is 15 mass%–20 mass% to keep good fluidity of slag.

(4) Finally, the high-temperature ferromanganese dephosphorization experiments were carried out, and the results show that after adding 20 mass% Ce₂O₃ into the slag, the lowest [P] content decreases from 0.31% to 0.25%, and the dephosphorization rate increases from 31.1% to 45.6%. The above results verify that the feasibility of Ce₂O₃ as a strong phosphorus fixative used in the ferromanganese dephosphorization process.

Acknowledgments

Financial support to this project is provided by the National Natural Science Foundation of China (No. 51874082) and NSFC-Liaoning Joint Fund (No. U1908224).

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