The Effect of Substituting FeO with CaO on the Rheological and Surface Properties of Silicate Melts

Abstract

A comprehensive understanding of the structural impact of composition is crucial in designing converter slag to optimize its rheological and surface properties during the smelting process. In this study, glassy CaO-SiO₂-Fe_xO samples with varying CaO/Fe_xO ratios were prepared to simulate the slag in the initial stage of converter melting. The viscosity and surface tension of the slag at 1300–1600 °C were measured, and the microscopic essence of physical properties was further analyzed using Raman spectroscopy technology. The findings reveal that as CaO replaces FeO, [SiO₄]-tetrahedra gradually depolymerize from Q⁴(Si) to Q⁰(Si), while [FeO₆]-octahedra gradually transform into [FeO₄]-tetrahedra, resulting in a decrease in the degree of polymerization of the slag. The slag with a lower degree of polymerization exhibits reduced activation energy of viscous flow and increased surface tension. Therefore, it is of great significance to appropriately control the CaO/Fe_xO ratio in the early stage of smelting to improve the rheological and surface properties of the slag.

1. Introduction

Converter steelmaking is a pivotal step in the contemporary steel material preparation process, playing a crucial role in subsequent refining, solidification, and final quality control of liquid steel. Steelmaking is essentially a slagging process wherein the slag assumes an important metallurgical function by enhancing the steel quality and smelting efficiency [1,2]. As converter steelmaking progresses towards high-quality, high-efficiency, environmentally friendly, and safe practices, there is an increasing demand for the metallurgical function of slag that closely correlates with its physical properties. Therefore, it becomes imperative to establish a comprehensive understanding of the relationship between its physical properties, metallurgical function, and chemical composition to effectively enhance its metallurgical performance.

With the advancement of research, there has been a growing recognition of the essence of the slag microstructure, leading to extensive exploration of its correlation with macroscopic physical properties and metallurgical functionalities. In investigations on slag viscosity, Gi et al. [3] observed that the substitution with larger alkali metal cations, e.g., Li⁺, Na⁺, and K⁺, increased the viscosity of the melts and enhanced the degree of polymerization of the Q³(Si) structural units. Wen et al. [4] found that the addition of SiO₂ leads to the increase of the slag viscosity and the activation energy increases. According to deconvolution results of XPS, as SiO₂ content in glassy slag increases, the number of bridging oxygens increases, indicating a more polymerized structure and a larger viscosity.

In the investigation of surface tension, Gao et al. [5] discovered a significant increase in the surface tension of CaO-SiO₂-Na₂O-CaF₂ slag with an increasing CaO/SiO₂ ratio, which could be attributed to the depolymerization of the slag. Sukenaga et al. [6] investigated the effect of CaO/SiO₂ on the surface tension of CaO-SiO₂-Al₂O₃-MgO slag within a temperature ranging from 1723 to 1823 K and found that with the increase in the CaO/SiO₂ ratio from 1.1 to 1.7, a large amount of unsaturated non-bridging oxygen was formed on the melt surface, resulting in an increase of surface tension. The surface tension is closely related to its thermodynamic properties. Soledade et al. [7] described about the significance of the Butler equation for the examination of the surface tension of the liquid mixture. At present, surface tension models are mostly established through this model.

Current research mainly focuses on investigating the impact of changes in the CaO/SiO₂ ratio on the physical properties of slag. However, in the converter smelting process, continuous dissolution of limestone leads to ongoing substitution of FeO by CaO in the slag [8]. CaO/Fe_xO is also an important factor affecting the physical properties. Therefore, it is essential to systematically analyze the structural behavior of ions to explore the transformation of physical properties. The present work investigated the viscosity, surface tension, and structure of a simplified CaO-SiO₂-Fe_xO slag to reveal the transformation of physical properties from a microscopic perspective. The research findings contribute to a deeper comprehension of the micro-level transformation mechanism governing macroscopic physical properties and metallurgical behavior of converter slag during the smelting process, thereby providing guidance for the refined design of slag composition and precise regulation of the smelting process.

2. Materials and Methods

2.1. Preparation of Slag Samples

Reagent-grade CaO, SiO₂, and FeC₂O₄∙2H₂O powders were utilized to synthesize the samples. When the temperature exceeds 850 °C, FeC₂O₄·2H₂O can decompose into FeO. To investigate the effects of replacing FeO with CaO on slag properties and microstructure, six experimental slags were designed with varying CaO content (5%, 15%, 25%, 35%, 40%, and 50%). The preparation process of quenched slag has been described in our previous research [9]. The quenched slag was subjected to X-ray fluorescence spectroscopy (XRF) and X-ray diffraction (XRD) analysis to determine whether a glassy sample with satisfactory chemical composition was prepared.

Table 1. Composition of CaO-SiO₂-FeO-Fe₂O₃ slags (mass fraction, %).

No.	CaO	SiO2	FeO	Fe2O3	Fe2+/T.Fe
CSF1	6.47	38.41	33.66	21.46	0.63
CSF2	14.34	40.66	30.03	14.97	0.69
CSF3	24.37	41.55	23.09	10.99	0.70
CSF4	36.41	39.66	14.57	9.36	0.63
CSF5	41.56	40.83	11.99	5.62	0.70
CSF6	52.14	37.65	5.63	4.58	0.58

presents the chemical composition of the slag, wherein CaO and SiO₂ contents are consistent with the design composition. Some FeO is oxidized to Fe₂O₃, and the Fe²⁺/TFe ratio is maintained at around 0.7. Figure 1 shows a typical XRD pattern of the slag. All the XRD profiles only showed a broad peak around the diffraction angle 2θ of 30°, which belongs to the typical glass phase characteristics.

2.2. Performance Testing

The viscosity and surface tension of CaO-SiO₂-Fe_xO slag were measured using the rotating cylinder method and the pulling tube method, respectively. Figure 2 shows the experimental equipment, wherein temperature control is achieved through a microcomputer program with a precision of ±0.5 °C. Surface tension tests were conducted to obtain the average value from six measurements of the surface tension at that temperature. Meanwhile, three viscosity tests were performed, and their average value represented slag viscosity.

2.3. Microstructure Detection

The microstructure of quenched slag is analyzed using a LabRAM HR800 Raman spectrometer equipped with a CCD detector. Raman spectra were recorded using an Ar⁺ laser with an excitation wavelength of 488 nm as the light source. The scanning range of the spectrum is 200–2000 cm⁻¹, and the spectral resolution is 0.65 cm⁻¹. The testing curve was imported into Origin 8.5 software, and the smoothing and baseline correction were performed to remove noise and fluorescence effects, respectively. Referring to the relevant literature, an appropriate frequency range was selected to deconvolute by Gaussian function.

3. Results

3.1. Viscosity

The viscosity curves of CaO-SiO₂-Fe_xO slag with different temperatures and CaO/Fe_xO ratios are presented in Figure 3. In Figure 3a, the slag viscosity generally decreases with increasing temperature and substitution of CaO for FeO. Notably, the addition of CaO causes a significant decrease in the viscosities of CSF1-3 slags, indicating that CaO exerts a stronger influence on slag viscosity compared to FeO.

The relationship between viscosity and temperature is commonly described using the Arrhenius formula, where lnη and T⁻¹ are linearly correlated. The linear regression analysis is shown in Figure 3b. The fitting coefficients of each slag are greater than 0.95, demonstrating a strong linear association between lnη and T⁻¹. From this, the activation energies of the viscous flow of CSF1-6 slags can be determined as 160, 139, 133, 115, 110, and 109 kJ·mol⁻¹, respectively.

The activation energy of viscous flow in metallurgical melts can be defined as the total energy required for particles to form holes and migrate between holes. As CaO replaces FeO, the decreasing activation energy at 1300–1600 °C indicates an increase in the number of holes in the melt and an enhancement of migration ability, thereby causing a decrease in slag viscosity. However, when the addition of CaO exceeds 35%, there is only a relatively small reduction in the activation energy observed, suggesting that it reaches a lower state.

3.2. Surface Tension

Figure 4 shows the surface tension curves of the slags under different CaO/Fe_xO conditions as a function of temperature. It can be seen that the surface tension of the slag gradually increases with the substitution of CaO for FeO at the same temperature; as the temperature increases, the surface tension of each slag shows a linear decrease, the slope of which represents the temperature coefficient of surface tension. The temperature coefficients are sequentially obtained as −3.16, −3.14, −2.9, −2.44, and −1.8, respectively. As CaO content increases, the absolute value of the temperature coefficient shows a decreasing trend, and the decrease amplitude gradually increases. This indicates that CaO weakens the dependence of surface tension on temperature, and the higher content of CaO results in even weaker reliance on changes in temperature.

3.3. Melt Structure

Figure 5 shows the Raman spectra of CaO-SiO₂-Fe_xO slag in the frequency range of 400–1200 cm⁻¹. The vibration peaks in the low-frequency region below 500 cm⁻¹ are believed to be associated with the deformation and vibration of tetrahedron [10,11], and no further analysis will be conducted. The intermediate frequency range consists of four vibration peaks near 535, 580, 670, and 710 cm⁻¹. The vibration peaks near 535 and 710 cm⁻¹ correspond to the bending vibrations of Si-O and Si-O-Si bonds [12,13], respectively, while the vibration peaks near 580 and 670 cm⁻¹ represent [FeO₆]-octahedra and [FeO₄]-tetrahedra [14,15], respectively. As CaO replaces FeO, there is a gradual decrease in relative intensity observed for these vibration peaks at around 580 and 670 cm⁻¹, which is consistent with the decreasing trend of FeO content from 55 wt.% to 10 wt.%.

The high-frequency region consists of six characteristic peaks, of which the peaks near 855, 927, 960, 1060, and 1130 cm⁻¹ represent the [SiO₄]-tetrahedra with bridging oxygen numbers of 0, 1, 2, 3, and 4 [11,16,17], respectively, and the peak near 1000 cm⁻¹ is related to the vibration of Si-O-Fe bonds [18]. The Raman spectra are deconvoluted according to the above assignments, and the results are shown in Figure 6.

The relative area fraction of structural units is calculated to clearly analyze the changes in slag structure after CaO replacing FeO, as illustrated in Figure 7. With the increase of CaO, Q⁰(Si) content increases while Q³(Si) and Q⁴(Si) content decreases. Meanwhile, Q¹(Si) and Q²(Si) exhibit fluctuations but show an overall decreasing trend. This can be attributed to the dissociation of O²⁻ from both CaO and FeO, resulting in the gradual depolymerization of [SiO₄]-tetrahedra by cutting off Si-O-Si bonds. The depolymerization process can be represented by Equation (1), which is consistent with the depolymerization mechanism proposed by Mysen et al. [19].

(1)

In addition, it was noted that [FeO₄]-tetrahedra gradually increased while [FeO₆]-octahedra decreased. This phenomenon is particularly pronounced at higher levels of CaO content. The dissociated O²⁻ from CaO preferentially combines with Si⁴⁺ to form the Si-O bonds due to the stronger energy compared to Fe-O bonds, resulting in the depolymerization of the aforementioned [SiO₄]-tetrahedra. As O²⁻ further increases, excessive O²⁻ combines with Fe³⁺ to form [FeO₄]-tetrahedra, causing the increase of [FeO₄]/[FeO₆]. Similar results were reported by Rüssel and Wiedenroth [14], where an increase in basicity is beneficial for the stability of [FeO₄]-tetrahedra. Despite enhancing the networking role of Fe³⁺, the slag still exhibits the depolymerization of [SiO₄]-tetrahedra and the degree of polymerization of the slag is reduced.

4. Discussion

4.1. Effect of CaO-FeO Substitution on Rheological Property

To elucidate the relationship between slag viscosity and its structure, a linear fit between the activation energy of viscous flow and the structural parameters is plotted in Figure 8. The activation energy is inversely proportional to the molar fractions of simple [SiO₄]-tetrahedra, while it is directly proportional to the molar fractions of complex [SiO₄]-tetrahedra. This indicates that the activation energy of viscous flow is smaller in the melts with lower polymerization degrees, which is consistent with the study by Yang et al. [20].

The viscosity variation of CaO-SiO₂-Fe_xO slag can be microscopically analyzed, as shown in Figure 9. When the CaO addition is less than 35%, the substitution of FeO by CaO leads to preferential destruction of Si-O-Si bonds by dissociated O²⁻ due to its strong affinity with Si⁴⁺ [21]. Consequently, significant depolymerization is observed in highly polymerized Q⁴(Si) and Q³(Si); complete depolymerization of Q⁴(Si) occurs when CaO content reaches 25%. Since Fe³⁺ has limited O²⁻ around it, some Fe³⁺ ions act as network modifiers. Therefore, the significant decrease in viscosity with increasing CaO at this stage can be attributed to the depolymerization of highly polymerized [SiO₄]-tetrahedra in the slag.

When CaO content exceeds 35%, O²⁻ destroys the Si-O-Si bonds in Q³(Si) and Q²(Si), depolymerizing into simpler Q¹(Si) and Q⁰(Si). Furthermore, an increase in O²⁻ concentration promotes the formation of [FeO₄]-tetrahedra by Fe³⁺, resulting in a reduction in free Fe³⁺ content [14]. However, the slag remains in a state of dissociation overall. Therefore, the slag viscosity gradually decreases, but the reduction amplitude of viscosity decreases due to the increase of [FeO₄]/[FeO₆].

4.2. Effect of CaO-FeO Substitution on Surface Property

For oxide melts, ions are primarily bound together by electrostatic force, and strong electrostatic forces can lead to significant surface tension. Generally, the electrostatic force is inversely proportional to the distance between ions [22]. Based on this relationship, the reason for the decrease in surface tension of slag with increasing temperature can be explained.

On the one hand, the irregular thermal motion of ions increases with temperature, resulting in a larger distance between ions, which weakens the interaction force between ions. On the other hand, as the temperature increases, simple [SiO₄]-tetrahedra increases while complex [SiO₄]-tetrahedra decreases, resulting in a reduction in the average ion radius of the slag. According to Einstein’s equation, both elevated temperature and decreased ion radius can increase the average displacement. This implies a decrease in activation energy required for ion motion and intensifies the irregular thermal motion of ions, causing larger interionic distances and reduced slag surface tension.

The Butler equation describes the relationship between surface tension, temperature, and activity, as follows in Equation (2) [23].

$$\sigma ={\sigma }_i+\frac{\mathrm{R}T}{A_i}\ln \frac{a_i^S}{a_i^B}$$

(2)

The surface tension of slag is influenced by its microstructure, so it is assumed that the Butler equation is a function of the concentration of structural units. The activity terms a_i^S and a_i^P in the equation can be replaced by the mole fraction of structural units, and a linear fitting between the surface tension of slag and the logarithm of structural parameters is established, as shown in Figure 10.

The fitting coefficients of surface tension with the linear equations of lnQ⁰(Si)+Q¹(Si), lnQ²(Si)+Q³(Si), lnNBO/Si, and lnNBO/T are 0.756, 0.715, 0.969, and 0.974, respectively. The comparison shows that the linear correlation between surface tension and lnQ⁰(Si)+Q¹(Si), lnQ²(Si)+Q³(Si) is weak; however, the linear relationship between surface tension and lnNBO/Si and lnNBO/T is strong. This indicates that surface tension has a stronger dependence on the overall microstructure of the slag; that is, lowly polymerized slag results in high surface tensions.

Figure 11 shows the microstructure analysis of the variation of surface tension. When the CaO content is 5%, many complex [SiO₄]-tetrahedra in the slag are observed with a larger average ion radius and lower interaction force between ions, resulting in a lower surface tension at this stage. As CaO replaces FeO, an increase in O²⁻ breaks the bridging oxygen bond, causing a gradual depolymerization of complex [SiO₄]-tetrahedrons into the simple structural units. The depolymerization of [SiO₄]-tetrahedra results in a decrease in the average ion radius of the slag and an increase in the interaction force between ions, thereby causing an increase in surface tension. Furthermore, the increase of O²⁻ leads to an increase in the number of ions in the slag, resulting in a decrease in the distance between ions and an increase in the interaction force. Therefore, there is a significant increase in slag surface tension with increasing CaO content.

5. Conclusions

The viscosity, surface tension, and Raman spectra of CaO-SiO₂-Fe_xO slag were investigated to identify the structural evolution and property transformation of the converter slag during the smelting process. The typical conclusions are summarized as follows:

(1) The change in slag structure can be divided into two stages with the substitution of CaO for FeO. When the CaO content in the slag is less than 35%, the complex [SiO₄]-tetrahedra depolymerizes into simple structural units. When the CaO content exceeds 35%, most of the complex [SiO₄]-tetrahedra are depolymerized, and only a portion of [SiO₄]-tetrahedra continue to depolymerize. Meanwhile, Fe³⁺ ions combine with O²⁻ ions to form more [FeO₄]-tetrahedra, resulting in an increase in the [FeO₄]/[FeO₆] ratio. Nevertheless, the slag is still in a state of depolymerization as a whole.

(2) When the addition of CaO is less than 35%, as CaO replaces FeO, the dissociated O²⁻ in the melt preferentially destroys the Si-O-Si bonds due to its strong affinity with Si⁴⁺, resulting in a significant depolymerization of highly polymerized [SiO₄]-tetrahedra. The depolymerized slag reduces its activation energy of viscous flow and increases the ion interaction force, resulting in a decrease in slag viscosity and an increase in surface tension.

(3) When CaO content exceeds 35%, O²⁻ not only destroys the Si-O-Si bonds in Q³(Si) and Q²(Si) units but also combines with Fe³⁺ to form more [FeO₄]-tetrahedrons. Consequently, as CaO replaces FeO, the viscosity and surface tension of the slag still maintain the original trend, but the reduction amplitude of viscosity and the increase amplitude of surface tension decrease.