Effect of B₂O₃ Addition on Electrical Conductivity and Structural Roles of CaO-SiO₂-B₂O₃ Slag

Abstract

The effect of B₂O₃ addition on electrical conductivity of CaO-SiO₂-B₂O₃ slag was investigated using the four-electrode method. The slag structure was determined from Raman and nuclear magnetic resonance spectra. The experimental results show that the electrical conductivity, as the B₂O₃ content in the slag increased from 0 to 10 pct, monotonously increases from 0.0769 to 0.2642 Ω⁻¹ cm⁻¹ at temperatures ranging from 1783 to 1873 K (1510 °C to 1600 °C). The effects of temperature on electrical conductivity obeyed the Arrhenius law, and the electrical conductivity increased as the temperature increased. The Raman spectra results indicated that addition of B₂O₃ led to an increase of Q³(Si) at the cost of Q²(Si), which will increase the degree of polymerization of the slag. ¹¹B nuclear magnetic resonance spectra showed that both BO₃ trigonal and BO₄ tetrahedral structures increased with the increasing B₂O₃ content. There was a competitive effect between the presence of these structures and an enhanced degree of polymerization of slags in the networks, which resulted in the increased electrical conductivity of the slag. A structure–electrical conductivity model for CaO-SiO₂-B₂O₃ slag was established.

Introduction

The conductivity of slag has an important role in the molten oxide electrolysis process and can directly affect product quality and energy consumption. A large number of metals, including aluminum, magnesium, and titanium, have been produced by the electrolysis of molten slags and salts.[1,2] In recent years, the electrochemical nature of metallurgical reactions is receiving more attention; closely related to this is the electrical conductivity of the slag. Study of the electrical conductivity of the slag can help in understanding the transfer behaviors of oxygen ions and electrons in the slag and the electrochemical nature of reactions.

Electrical conductivity is also an important thermophysical property of a molten slag. This can provide not only a foundation for establishing a model, but also information regarding the structures of a slag. For instance, the earliest evidence for the ionic structure of oxide melts resulted from electrical conductance measurements.[3,4] The transport properties of silicate slags, such as conductivity and viscosity, are predominantly affected by their structure. Knowledge of the structure of silicate melts is an effective way to understand the conductivity properties of molten slags.

There have been numerous attempts in the fields of metallurgy and glass science to measure these macroscopic thermophysical properties as a function of the composition of silicates based on a microscopic view of the structure.[5,6,7,8,9,10,11,12,13,14,15,16,17] Most attempts have studied the relationship between the structure and viscosity of a slag. Research on electrical conductivity relies on the relationship between composition and conductivity. In this study, the relationship between the structure and electrical conductivity of CaO-SiO₂-B₂O₃ slag was investigated.

CaO-SiO₂-B₂O₃ slag has important significance for the smelting and refining processes of silicon metallurgy, so the study of its structure and macroscopic properties is useful. The macroscopic properties of these glasses and melts are primarily determined by their microscopic structures. In this study, a macroscopic property (electrical conductivity) of CaO-SiO₂-B₂O₃ slag with different B₂O₃ contents was measured, and Raman and nuclear magnetic resonance (NMR) spectroscopy were used to study their microscopic structures. This enabled better evaluation of relationships between the structure and thermophysical properties, such as conductivity, of these melts. A structure–electrical conductivity model for CaO-SiO₂-B₂O₃ slag was also established.

Experimental

Conductivity Measurement

A four-electrode technique was used to measure the electrical conductivity of CaO-SiO₂-B₂O₃ slag. To obtain accurate values, the resistances of lead wires and electrodes should be excluded from the total measured resistance, and the four-electrode method avoids the influence of interface polarization.

A series of CaO-SiO₂-B₂O₃ ternary samples were designed with a fixed CaO/SiO₂ mass ratio of 0.67, but with the gradually increasing B₂O₃ content. The sample compositions are shown in Table I. Slag samples were prepared using reagent grade SiO₂, CaCO₃, and B₂O₃ powders (analytically pure, Sinopharm Chemical Reagent Co., Ltd, China), all of which were calcined at 1273 K (1000 °C) for 10 hours in a muffle furnace before use to decompose any contained carbonate and hydroxide present before being used. Each sample mixture was accurately weighed according to the compositions shown in Table I and thoroughly mixed in an agate mortar before use.

Table I Compositions for Electrical Conductivity Measurements (Mass Fraction)

The experimental setup is schematically shown in Figure 1. A detailed description of the device was introduced in our previous study.[18,19] Before measuring the resistance of the slag, the cell was calibrated at temperatures from 293 K to 298 K (20 °C to 25 °C). A standard solution of KCl (1 mol L⁻¹) was used to determine the cell constant. For the conductivity measurements of the slags, each prepared sample was packed into a platinum crucible (Φ26 mm × 26 mm × 0.3 mm), which was first placed inside a corundum crucible (Φ48 mm × 126 mm × 3 mm) and then in the constant-temperature zone of the furnace. Once the target temperature of 1873 K (1600 °C) was reached, it was held there for 2 hours to ensure that a homogeneous slag composition is achieved. The electrodes were slowly lowered until they made contact with the surface of the melt. When the tips of the electrodes contacted the surface of melt, the resistance measured by the electrochemical station rapidly and significantly decreased. The electrodes were then lowered to a depth of 3 mm below the surface. Resistance measurements were carried out at 30 K intervals while cooling from 1873 K (1600 °C). To ensure the equilibrium of the melt, it was held there for 30 minutes at each temperature before the measurement was made. All measurements were carried out at a frequency of 20 kHz.

Fig. 1

Schematic diagram of experimental setup

Structure Determinations

Fifty grams of each sample composition was loaded into a graphite crucible (Φ45 mm × 100 mm × 4 mm) and placed in a muffle furnace. The temperature of the furnace was raised to 1873 K (1600 °C) at a rate of 10 K min⁻¹ and then held for 2 hours under Ar atmosphere to ensure homogeneity the slag composition. The premelting liquid samples were then quickly poured into cold water. The water-quenched samples were dried in an oven at 353 K (80 °C) for 10 hours then crushed and ground to 200 mesh. X-ray diffraction (XRD; SmartLab, Rigaku, Japan) measurements were performed to ensure that the water-quenched samples remained in the glass state. The results, shown in Figure 2, indicated that none of the samples contained precipitated crystals.

Fig. 2

X-ray diffraction results of the premelted sample

The glass samples were characterized using Raman and NMR spectroscopy. A laser confocal Raman spectrometer (LabRAM HR800, Horiba Jobin Yvon Company) was used for this purpose. The power we used was 50 pct and the number of scans is three. The scanning time is 10 seconds. Solid-state ¹¹B MAS-NMR (AVANCE III HD 500 MHz,Avance III 500M, Bruker with a Speed up to 35 KHz high-speed probe) measures were used to identify the structure role of ¹¹B in the glass. The samples were analyzed using an NMR spectrometer employing the ¹¹B one pulse MAS technique equipped with a Bruker AVANCE II 500 operating at ¹¹B frequency of 160.5 MHz. Approximately 50 to 100 mg of samples was placed into a zirconium oxide rotor with a diameter of 4 mm and kel-F caps. For all the measurements, a spinning speed of 7 KHz was applied, and the recycle delays were set to 1 second. About 128 scans were acquired. The power level for pulse was 200 w, and the high–power pulse was 7.5 μs.

Results and Discussion

Conductivity Measurements

The measured electrical conductivity data for different slag compositions at temperatures ranging from 1783 K to 1873 K (1510 °C to 1600 °C) are shown in Figure 3. The experimental results show that as the B₂O₃ content in the CaO-SiO₂-B₂O₃ slag increased from 0 to 10 pct, the electrical conductivity monotonously increased from 0.0769 to 0.2642 Ω⁻¹ cm⁻¹ in this temperature range. The logarithms of electrical conductivity as a function of reciprocal temperature are shown in Figure 4. There was a linear relationship between these two parameters, and the electrical conductivity increased with the increasing temperature. It was concluded that the temperature dependence of electrical conductivity complied with the Arrhenius equation[20]:

Fig. 3

Electrical conductivity as a function of temperature and slag composition

Fig. 4

Change in electrical conductivity as a function of temperature for different contents of B₂O₃

$$ \ln \sigma = \ln A - \frac{E}{RT}, $$

(1)

where σ is the electrical conductivity, Ω⁻¹ cm⁻¹; A is the pre-exponential factor, Ω⁻¹ cm⁻¹; E is the activation energy of electrical conductivity, J mol⁻¹; R is the gas constant, 8.314 J mol⁻¹ K⁻¹; and T is the absolute temperature, K. The activation energies were calculated from the Arrhenius equation, as shown in Table II. The values were relatively close to those of CaO-SiO₂-FeO_x[21] and Fe_xO-SiO₂-CaO-Al₂O_3.[19]

Table II Activation Energy of Electrical Conductivity for Each Sample

There are two reasons for the increase of electrical conductivity with the increasing temperature. On the one hand, the ionic distance increases with the increasing temperature, the viscous resistance of molten slags decreases, and so the electrical conductivity increases; on the other hand, the spacing between cations and anions increases, and the interionic attraction between cations and anions decreases. Therefore, the binding energy decreases, and the O²⁻ ions can be more easily plundered by the Si⁴⁺ ions to break the network, which will decrease the degree of polymerization (DOP) and increase the electrical conductivity of molten slag. Figure 4 shows that conductivity increased with the increasing B₂O₃ content from 0 to 10 pct.

Structure Determination

Raman spectra for the various slags are shown in Figure 5. According to the literature.[22,23,24,25] the Raman spectra can comprise three bands attributed to Si-related structures: these are assigned to the O-Si-O bending vibration, Si-O symmetric stretching vibration, and stretching vibration of the SiO₄ tetrahedral at 300 to 580, 580 to 770, and 770 to 1150 cm⁻¹, respectively. The B₂O₃ content had a strong influence on the network structure of the slags. Figure 5 shows that, as the SiO₂ content decreased, the relative intensity of the bands associated with O-Si-O bending (300 to 580 cm⁻¹) and Si-O symmetric stretching (580 to 770 cm⁻¹) became less pronounced, and, the peaks at ~ 1080 cm⁻¹, which were assigned to Q³,[26,27,28,29] became more pronounced as the B₂O₃ content increased. The peaks at about 1350 and 1450 cm⁻¹ were attributed to BO₄ tetrahedral and BO₃ trigonal structures, respectively.[30,31] These peaks became more pronounced as the B₂O₃ content increased.

Fig. 5

Raman spectra of the CaO-SiO₂-B₂O₃ melts

The stretching vibration of the SiO₄ tetrahedral (770 to 1150 cm⁻¹) can reflect the DOP of a slag, so essential for understanding the structure of a silicate melt. According to the literature,[32,33,34,35,36,37,38] the bands at about 860, 910, 960, 1080, and 1200 cm⁻¹ can be assigned to Q⁰, Q¹, Q², Q³, and Q⁴ respectively. To better understand the structure of the slag, it is necessary to quantitatively calculate the proportions of Q⁰–Q⁴. Deconvolution of the Raman spectra was performed in the range of 800 to 1200 cm⁻¹. The results are shown in Figure 6. To reflect changes in the content of Q⁰–Q⁴ more intuitively, Eq. [2][39,40,41] was used.

Fig. 6

Deconvolution of the Raman spectra: (a) CSB0, (b) CSB1, (c) CSB2, and (d) CSB3

$$ X_{i} = {{\left( {{{A_{i} } \mathord{\left/ {\vphantom {{A_{i} } {S_{i} }}} \right. \kern-0pt} {S_{i} }}} \right)} \mathord{\left/ {\vphantom {{\left( {{{A_{i} } \mathord{\left/ {\vphantom {{A_{i} } {S_{i} }}} \right. \kern-0pt} {S_{i} }}} \right)} {\left( {\sum\limits_{i = 0}^{3} {{{A_{i} } \mathord{\left/ {\vphantom {{A_{i} } {S_{i} }}} \right. \kern-0pt} {S_{i} }}} } \right)}}} \right. \kern-0pt} {\left( {\sum\limits_{i = 0}^{3} {{{A_{i} } \mathord{\left/ {\vphantom {{A_{i} } {S_{i} }}} \right. \kern-0pt} {S_{i} }}} } \right)}}, $$

(2)

where A_i is the area of the ith peak and S_i is the respective Raman scattering coefficient. The values of S₀ to S₃ are 1, 0.514, 0.242, and 0.009, respectively. The results are shown in Figure 7, from which it can be seen that as the B₂O₃ content increased, the Q³ content increased significantly, but the Q² content decreased significantly. In other words, the increase of Q³ occurred at the cost of Q².

Fig. 7

Mole fractions of Qⁿ (Si) units in silicate networks

The nonbridging oxygens also can reflect the DOP of a slag. Equation [3][42] was used to reflect the change in the contents of nonbridging oxygens (NBO/T).

$$ {\raise0.7ex\hbox{${\text{NBO}}$} \!\mathord{\left/ {\vphantom {{\text{NBO}} T}}\right.\kern-0pt} \!\lower0.7ex\hbox{$T$}} = 4Q^{0} + 3Q^{1} + 2Q^{2} + Q^{3} $$

(3)

The corresponding results are shown in Figure 8. The NBO/T content decreased with the increasing B₂O₃ content, indicating that the addition of B₂O₃ increased the DOP of the slag network. In other words, B₂O₃ acted as a network former in these slags. In the pure B₂O₃ slag state (B-O-B), all oxygens are assumed to originally exist as bridging oxygens. In a silicate slag, B₂O₃ will capture the oxygens of Q² (Si) once it is introduced into the networks; therefore, more Q³ (Si) is formed at the cost of Q²(Si). The reactions can be described in the form of Eqs. [4] and [5]. The network will therefore contain Si-O-Si and Si-O-B bridging oxygens. From Figure 6, it was concluded that the main reaction is the conversion from Q² to Q³, as shown in Figure 9.

Fig. 8

Nonbridged oxygen content (NBO/T) as a function of slag composition

Fig. 9

Schematic reaction for the conversion of Q² to Q³

$$ 2{\text{Si{-}O}}^{\text{-}} + {\text{B{-}O{-}B}} \to {\text{Si{-}O{-}Si}} + 2{\text{B{-}O}}^{\text{-}} $$

(4)

$$ {\text{Si{-}O}}^{\text{-}} + {\text{B{-}O{-}B}} \to {\text{Si{-}O{-}B}} + {\text{B{-}O}}^{\text{-}} $$

(5)

To further study changes of the slag structure, ¹¹B magic-angle spinning (MAS)-NMR spectra results were analyzed, as shown in Figure 10. According to the Raman studies, it was concluded that the regions from − 3.7 to 18 and − 15 to − 3.7 ppm are attributed to the BO₃ trigonal and BO₄ tetrahedral structures, respectively.[43,44,45] The BO₃ trigonal structure was also identified by ring and non-ring BO₃ units that are located at ~ 5.5 and ~ 1.5 ppm, respectively. The BO₄ tetrahedral was also identified by the BO₄ (1B3Si) and BO₄ (0B4Si) units located at about − 5.5 and − 10 ppm, respectively.[41,42,43] The ¹¹B MAS-NMR spectra results showed that the intensities of both the BO₃ trigonal and BO₄ tetrahedral peaks increased with the increasing B₂O₃ content.

Fig. 10

¹¹B magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra of CaO-SiO₂-B₂O₃ melts

Structure–Electrical Conductivity Model for CaO-SiO₂-B₂O₃ slag

The CaO-SiO₂-B₂O₃ slag can be considered to contain ionic conductors, where the Ca²⁺ cation is assumed to be the main charge carrier when subjected to an electric field. Equation [6] can be used to describe the electrical conductivity:

$$ \sigma = czF\mu, $$

(6)

where σ is the electrical conductivity(S m⁻¹); c is the concentration of Ca²⁺(mol m⁻³); z is the charge number of Ca²⁺; F is Faraday’s constant (C mol⁻¹); and µ is mobility (m² V⁻¹ s⁻¹). For this slag, the mobility (µ) of the charge carrier is the only unknown factor. Based on the Einstein relation, mobility can be related to a species diffusion coefficient, as shown in Eq. [7]:

$$ \mu = \frac{zF}{RT}D, $$

(7)

where D is the species diffusion coefficient (m² s⁻¹). Thus, Eq. [6] can be presented as follows:

$$ \sigma = \frac{{z^{2} F^{2} c}}{RT}D. $$

(8)

An electrical conductivity model was developed based on the Nernst–Einstein equation. The species diffusion coefficient D can be represented by the effective diffusivities of the cations,[46] which are described as a function of slag structure. In this slag, the simple oxygen-bonding structures of slag mainly included nonbridging oxygens (bridged, broken, and free oxygen) and B-O bonding (BO₃ trigonal and BO₄ tetrahedral units). Therefore, the effective diffusivity D is described using Eq. [9]:

$$ \begin{aligned} D &= D_{0} e^{{\frac{ - E}{RT}}} \\ D &= D_{0} e^{{ - \frac{{({\text{B}} + {\text{C}}_{\text{NBO}} x_{\text{NBO}} + {\text{C}}_{{{\text{BO}}_{3} }} x_{{{\text{BO}}_{3} }} + {\text{C}}_{{{\text{BO}}_{4} }} {\text{X}}_{{{\text{BO}}_{4} }} )}}{RT}}}, \end{aligned} $$

(9)

where D₀ is the pre-exponential factor; E is the activation energy; B is the activation energy when the diffusing species is not being hindered by the medium through which it is diffusing; C is the increase in activation energy due to the increase in polymerization of the slag expressed by the pair fractions of NBO, BO₃, and BO₄. Equation [8] then can be expressed as follows:

$$ \sigma = \frac{{z^{2} F^{2} c}}{RT}D_{0} e^{{ - \frac{{({\text{B}} + {\text{C}}_{\text{NBO}} x_{\text{NBO}} + C_{{{\text{BO}}_{3} }} x_{{{\text{BO}}_{3} }} + C_{{{\text{BO}}_{4} }} X_{{{\text{BO}}_{4} }} )}}{RT}}} $$

(10)

The conductivity of a slag is determined by ionic and electronic conductivities. In silicate melts used in metallurgy, conductance is primarily determined by cation conduction; this was proved by the study of Martin and Derge,[47] which concluded that conductivity in a CaO-SiO₂-Al₂O₃ slag occurs via cation. According to theory, there are simple metal cations and complex polymeric anions in a slag. Since the 1950s, a lot of research has been undertaken on the structure of slags. Most metal oxides release oxygen ions to form metal cations, while some expensive cerium oxides receive oxygen ions from suboxides to form complex anions. Complex anions have a large influence on the electrical conductivity. The electronic conductivity is an important parameter used to measure the transmission speed of electrons in the slag. Under certain conditions, although the electronic conductivity and the number of electron migrations in an ionic slag are small, they will have a significant impact on the electrochemical reaction. Many studies have been undertaken on the electrical conductivity of slags. It has been found that slags that contain FeO, MnO, and TiO exhibit electronic conductivity.[48,49,50,51]

From previous studies, we know the slag that was used in the current study exhibits anion conductivity. The electrical conductivity of the slag is usually determined in two ways: from the ion concentration and from the structure of the slag. For this slag, charge transport is completed by Ca²⁺ ions. Si⁴⁺ and B³⁺ have large valencies and small ionic radii, and therefore, they have large interactions with nearby ions and form network units, which leads to small contributions of B³⁺ and Si⁴⁺ ions to the charge conductance.[52] Oxygen ions will connect with B³⁺ or Si⁴⁺, forming bridging or nonbridging oxygens, the mobility of which will be weak.[53] Hence, it can be concluded that the main ion that contributes to charge conductance is Ca²⁺.

For this CaO-SiO₂-B₂O₃ slag, the conduction ion is Ca²⁺, and the values of z, F, R, and T are constant. The differences between the slags are the concentrations of Ca²⁺ and the activation energy which is determined by slag structure. The pair fractions of NBO, BO₃, and BO₄ were calculated from the Raman and NMR data. A structure–electrical conductivity model for the CaO-SiO₂-B₂O₃ slag was established, as follows:

$$ \sigma = \frac{{z^{2} F^{2} c}}{RT} \times 0.0874 \times e^{{\frac{{(342613 - 92411 \times x_{\text{NBO}} - 1172114 \times x_{{{\text{BO}}_{3} }} + 1129822 \times X_{{{\text{BO}}_{4} }} )}}{RT}}} $$

(11)

For a specified CaO-SiO₂-B₂O₃ slag, the pair fractions of NBO, BO₃, and BO₄ can be calculated from the results of Raman and NMR; then, the Eq. [11] can be used to estimate the electrical conductivity of CaO-SiO₂-B₂O₃ slag.

It is known that microscopic structures determine the macroscopic properties of slag melts; therefore, the variation of the electrical conductivity for these slags will be a comprehensive consequence of their microscopic structures. On adding B₂O₃ to CaO-SiO₂-B₂O₃ slag, the DOP of the melt and the intensities of both the BO₃ trigonal and BO₄ tetrahedral peaks increased, which affected the diffusivity of the cation: first, an increase in DOP of the slag led to a decrease of electrical conductivity; second, the BO₃ trigonal and BO₄ tetrahedral units appeared after adding B₂O₃ to the slag. The BO₃ trigonal, as one of the dominant B-related units, is a two-dimensional structure and will greatly reduce the symmetry and the strength of the networks. Furthermore, due to the influence of BO₄ group avoidance, the BO₄ tetrahedral is preferentially connected to nearby Si atoms and introduced into the silicate networks. This reduces the uniformity of the network structure, and as a result, the strength of the slag structure will decrease. These deformations facilitate Ca²⁺ to jump from one site to another. In contrast to the effect of the increasing DOP, the increases in BO₃ trigonal and BO₄ tetrahedral structures could cause the resistance of the charge conductance to decrease observably, resulting in an increase of electrical conductivity. In other words, for these slags, the effect of the increasing electrical conductivity caused by the presence of the BO₃ trigonal and BO₄ tetrahedral units is stronger than the effect of decreasing electrical conductivity caused by the DOP. This is the reason for the increase of electrical conductivity with the increasing B₂O₃ content.

Conclusion

A four-electrode technique was used to investigate electrical conductivity changes of CaO-SiO₂-B₂O₃ slag with the addition of B₂O₃. It was concluded that conductivity and temperature comply with the Arrhenius equation. As the B₂O₃ content increased, the electrical conductivity increased. The reason may be that BO₃ trigonal and BO₄ tetrahedral units were produced with the addition of B₂O₃. These units decreased the symmetry and uniformity of the structure, and thereby reduced its strength and increased the electrical conductivity of the slag. A structure–electrical conductivity model for CaO-SiO₂-B₂O₃ slag was established