Electrical Conductivities of High Aluminum Blast Furnace Slags

Abstract

Electrical conductivity is one of the most important thermophysical properties of oxide melts. In the present study, the electrical conductivities of CaO–SiO₂–MgO–Al₂O₃ type high aluminum blast furnace slags were measured by using four electrode method. It can be concluded that the electrical conductivity decreases as increasing the content of Al₂O₃, because Al³⁺ has a strong preference to form AlO₄⁵⁺ tetrahedron and incorporates into the network of SiO₄⁴⁺ which enhances the degree of polymerization of melts. As adding MgO to the slags, the electrical conductivity also decreases due to the stronger polarization ability and thus weaker diffusion ability of small Mg²⁺ ion relative to Ca²⁺ ion. However, the electrical conductivity increases with gradually increasing the CaO/SiO₂ ratio, owing to the increase of concentration of Ca²⁺ ion which acts as the charge carrier and the decreases of degree of polymerization.

1. Introduction

At present, it is urgent to deal with the metallurgical slag produced in the process of blast furnace ironmaking efficiently and environmentally. Most of the slags are used as building and roadbed materials after being cooled by water or air, such as concrete.¹⁾ However, it is not an effective way to recycle abundant sensible heats. In order to make better use of the wasted heat energy, some researchers tried to use blast furnace slags to produce slag wool or microcrystalline glass with high added values. In order to fulfill the compositional requirements, it is necessary to adjust composition by adding some additives into the liquid blast furnace slags.²⁾ In this process, the additives are often melted by using the heating elements of molybdenum electrodes. Hence, as a prominent parameter, the electrical conductivity of slag should be clearly known for carrying out more effective electric heat. In addition, electrical conductivity is one of the most important thermophysical properties of oxide melts. It plays an important role in the design and optimization of electric smelting furnaces. Furthermore, it could provide fundamental information to understand the structures of molten slag. However, compared to viscosity, much less attention has been paid to electrical conductivity, because of the difficulty of experimental measurements. Limited data couldn’t provide enough fundamental supports to the practical production. For example, the minimum values of electrical conductivity were found in both CaO–Al₂O₃–SiO₂³⁾ and CaO–FeO_x–Al₂O₃–SiO₂ melts with the change of CaO/Al₂O₃ molar ratio.⁴⁾ It was also found that when replacing K₂O by Na₂O while keeping the concentrations of other components constant in the CaO–SiO₂–MgO–Al₂O₃–K₂O system, the electrical conductivity first decreases and then increases, or the mixed-alkali effect occurs.⁵⁾ Therefore, more data of electrical conductivity are urgently needed. In this study, the influences of Al₂O₃ and MgO, as well as the CaO/SiO₂ ratio on the electrical conductivity of CaO–SiO₂–MgO–Al₂O₃ type blast furnace slags will be investigated.

2. Materials and Experiments

The compositions of the samples are shown in Table 1. These compositions are classified into four different groups: In groups A, B, and C, the mass ratio of CaO/SiO₂ keeps constant, respectively, but the content of Al₂O₃ gradually increases; in group D, the mass ratio of CaO/SiO₂, and the content of Al₂O₃ keep constant, while the content of MgO increases from D1 to D3. Slag samples were prepared using reagent grade SiO₂, Al₂O₃, MgO powders. SiO₂, Al₂O₃, and MgO were calcined at 1273 K for 10 hours in a muffle furnace. Then the prepared CaO by roasting CaCO₃ at 1273 K for 10 hours and other reagents were precisely weighted, according to the compositions shown in Table 1, and thoroughly mixed in an agate mortar for the following experiments.

Table 1. Compositions for electrical conductivity measurements (mass fraction).

Compositions	CaO	MgO	Al2O3	SiO2	CaO/SiO2
A1	35.56	10	10	44.44	0.8
A2	33.33	10	15	41.67	0.8
A3	31.11	10	20	38.89	0.8
A4	28.89	10	25	36.11	0.8
B1	40	10	10	40	1
B2	37.5	10	15	37.5	1
B3	35	10	20	35	1
B4	32.5	10	25	32.5	1
C1	43.64	10	10	36.36	1.2
C2	40.91	10	15	34.09	1.2
C3	38.18	10	20	31.82	1.2
C4	35.45	10	25	29.54	1.2
D1	37.5	5	20	37.5	1
D2	35	10	20	35	1
D3	32.5	15	20	32.5	1

A four-terminal method was used to measure the electrical conductivity in this study. The resistance of the wire and electrode should be removed from the total resistance in order to obtain an accurate conductivity value. The contribution of the interfacial resistance can not be ignored when the current electrode is used in common for the potential one. The application of the four-electrode technique can avoid those difficulties. The electrical conductivity of the melts can be obtained by the following equation:   

$$\sigma =C/R_x$$(1)

where C is the cell constant and R_x is the slag melts resistance.

The schematic diagram of the experimental apparatus is shown in Fig. 1. The inner diameter of alumina working tube was 40 mm. Pt–Rh (30 wt-%) was used to make the electrodes, which consisted of a tip (25 mm in length and 0.8 mm in diameter) and a same material wire, but 0.3 mm in diameter and 800 mm in length. The four electrodes were inserted into two double-hole alumina tubes, fixed with short alumina tubes of the same diameter, and then the central electrode was separated by 6 mm through the two supporting tubes of the plug. Two double-hole alumina tubes could move up and down along the two supporting tubes. The immersion depth of the electrode was measured with a vernier caliper.

Fig. 1.

Schematic diagram of the experimental apparatus.

Before measuring resistance of slags, the standard aqueous KCl (1.0 mol/L) solution was used for cell calibration at temperature from 293 K to 295 K. After obtaining the cell constant, the prepared slag sample was packed into a platinum crucible with the diameter and height of 26 mm and 25 mm, respectively. The crucible was placed in the constant temperature zone of the furnace with the heating elements of MoSi₂. After the temperature measured by a type B (Pt-6 pct Rh/Pt-30 pct Rh) thermocouple reached 1873 K and held for 2 h (which is enough for the decomposition of carbonate and escape of CO₂ as well as the uniformity of melts based on our preliminary experiments), the electrodes were lowered slowly until touching the surface of the melt. The depth of the melt is about 12 mm. During this process, the resistance was continuously monitored. When the tips of the electrodes contacted with the surface of melt, the resistance significantly decreased. From this critical point, the electrodes were lowered 3 mm which is the same as that during the cell calibration. The resistance measurement was carried out at every 50 K interval from 1873 K to 1673 K. At each temperature before measurement, the melt was kept for 30 min first to ensure the equilibrium. All the measurements were carried out at the frequency of 20 kHz.

3. Results

In order to assess the accuracy of the obtained experimental data, the data from the literature are used for comparisons. Electrical conductivity from the work of Martin et al.,⁶⁾ Nesterenko et al.⁷⁾ and Sarkar et al.⁸⁾ are compared with the conductivities of compositions A1, C2, and C3, respectively, as shown in Table 2. From Table 2, it can be seen that there are not so large differences between the currently measured data and these from the literatures.

Table 2. Comparison with data from literatures for the similar compositions.

Source	Martin	Present study (A1)	Nesterenko	Present study (C2)	Sarkar	Present study (C3)
1823 K	0.175	0.199	0.359	0.321	0.280	0.240
1873 K	0.248	0.246	0.42	0.404	0.344	0.320

All the measured data of different compositions at different temperature are shown in Table 3. The logarithms of electrical conductivity for groups A through C as functions of the reciprocal of temperature are displayed in Figs. 2, 3, and 4. From these figures, it can be concluded that there is always a linear relationship between the logarithm of conductivity and the reciprocal of temperature. In other words, the temperature dependence of conductivity obeys the Arrhenius law described as follows,   

$$ln\sigma =ln{\sigma }_0-E/RT$$(2)

where σ is the electrical conductivity, Ω⁻¹·cm⁻¹; σ₀ is the pre-exponent factor; E is the activation energy of electrical conductivity, J/mol; R is the gas constant, 8.314 J/(mol·K); T is the absolute temperature, K.

Table 3. Electrical conductivity for different compositions, Ω⁻¹·cm⁻¹.

Compositions	1873 K	1823 K	1773 K	1723 K	1673 K
A1	0.246	0.199	0.158	0.123	0.094
A2	0.209	0.167	0.131	0.102	0.077
A3	0.172	0.141	0.112	0.089	0.070
A4	0.147	0.122	0.097	0.077	0.060
B1	0.323	0.265	0.210	0.153	0.107
B2	0.215	0.175	0.136	0.113	0.085
B3	0.193	0.148	0.121	0.096	0.074
B4	0.160	0.128	0.105	0.083	0.066
C1	0.867	0.649	0.475	0.357	0.251
C2	0.404	0.321	0.246	0.187	0.150
C3	0.320	0.240	0.180	0.128	0.096
C4	0.244	0.180	0.137	0.101	0.077
D1	0.233	0.172	0.133	0.108	0.079
D2	0.192	0.147	0.120	0.095	0.073
D3	0.142	0.113	0.088	0.073	0.052

Fig. 2.

Change of electrical conductivity with temperature for different compositions in group A.

Fig. 3.

Change of electrical conductivity with temperature for different compositions in group B.

Fig. 4.

Change of electrical conductivity with temperature for different compositions in group C.

From Figs. 2, 3, and 4, as the Al₂O₃ content is gradually increased from A1 to A4, B1 to B4 and C1 to C4, respectively, the conductivity monotonously increases. From Fig. 5, it can be concluded that as gradually increasing MgO content, while keeping the CaO/SiO₂ mass ratio and content of Al₂O₃ constant, there is a decrease on the electrical conductivity of CaO–SiO₂–MgO–Al₂O₃ melts. The influence of CaO/SiO₂ ratio at a fixed content of Al₂O₃ on electrical conductivity is shown in Figs. 6, 7, 8. From those figures, it can be seen that the electrical conductivity increases with increasing the ratio of CaO/SiO₂.

Fig. 5.

Change of electrical conductivity with temperature for different compositions in group D.

Fig. 6.

Change of electrical conductivity with CaO/SiO₂ ratio, at x(Al₂O₃)=10%.

Fig. 7.

Change of electrical conductivity with CaO/SiO₂ ratio, at x(Al₂O₃)=15%.

Fig. 8.

Change of electrical conductivity with CaO/SiO₂ ratio, at x(Al₂O₃)=20%.

4. Discussion

4.1. Influence of Al₂O₃ on Electrical Conductivity of Groups A, B and C

The conductivity of the slag includes electronic conductance and ion conductance. Electronic conductance is often caused by transition metal oxides such as iron oxides in the slag. Oxides other than transition metal oxides usually contribute only to ion conductance. There is no transition metal oxide in the current compositions, so charge transfer is mainly due to ions. Normally, because of the small ionic radii and large valences of Al³⁺ and Si⁴⁺ ions, which lead to large interactions with the nearby ions, they contribute little to charge conductance.⁹⁾ There are three types of oxygen ions in oxide melts based on the classification of Fincham and Richardson: bridging oxygen (O⁰), bonded with two cations from acidic oxides, e.g. Si⁴⁺ in [SiO₄], or Al³⁺ in [AlO₄] tetrahedron after being charge balanced; non-bridging oxygen (O⁻), bonded with one cation from acidic oxide and one cation from basic oxide; free oxygen (O²⁻), bonded with two cations from basic oxides.¹⁰⁾ The mobility of bridged or non-bridged oxygen is very weak, because they combine with Si⁴⁺ or Al³⁺ ions to form strong covalent bonds.¹¹⁾ However, the diffusion of free oxygen ion is much easier, because of its ionic bond dominated chemical bond with the metal cation from the basic oxide. However, because of the low concentration of free oxygen owing to the low basicity of composition in this study, the contribution of oxygen ion can be neglected. Therefore, Ca²⁺ and Mg²⁺ ions mainly contribute to conductance in the present study.

It is widely believed that the ionic conductance of oxide melt is determined by the concentration of mobile ion and the degree of polymerization. The larger the concentration of the mobile ion or the lower the degree of polymerization, the larger the conductivity will be. It can be seen from Figs. 2, 3 and 4 that the electrical conductivity of groups A, B and C monotonously decreases as increasing Al₂O₃ content. As mentioned above, Al³⁺ could bond with bridging or non-bridging oxygen to form the covalent bond, so Al³⁺ have strong preferences to form AlO₄⁵⁺ tetrahedrons and incorporate into the network of SiO₄⁴⁺ when there are enough metal cation to participate into the charge compensation (as shown in Table 1), which leads to the decrease on conductivity, as schematically expressed in Fig. 9.

Fig. 9.

Schematic diagrams of charge compensation of Ca²⁺ ions.

4.2. Influence of MgO on Electrical Conductivity

From Fig. 5, it can be seen that the addition of MgO to blast furnace type slags can lead to a decrease in electrical conductivity. As mentioned above, Ca²⁺ and Mg²⁺ ions mainly contribute to conductance. Generally, when there are several basic oxides in a melt containing Al₂O₃, there is a strict order for which cation carrys out the charge compensation of Al³⁺ ions.^12,13,14) The field strength of cation plays an important role in the priority of charge compensation of Al³⁺. Cation with a lowest columbic force (I) between the cation and oxygen anion $I=2Q/(r_{M^{z+}}+r_{O^{2-}})$, (where Q is the valence of M ion and $r_{M^{z+}}$ and $r_{O^{2-}}$ are the radii of M^z+ and oxygen ions, respectively) takes precedence over the charge compensation. It will be much easier for the cation with a small I value to contribute their O²⁻ ion to Al³⁺ ion to form [AlO₄].¹⁵⁾ Due to the difference in the radii of the two ions, r (Ca²⁺) > r (Mg²⁺), priority for charge compensation is in the order of Ca²⁺ > Mg²⁺. Therefore, in Group D, since the ratio of CaO/Al₂O₃>1, the Ca²⁺ cation is enough for charge compensation. After the charge compensation, the remaining Ca²⁺ cations act as the network modifier. Therefore, there will be less and less Ca²⁺ cation acting as network modifier to break the network as the content of MgO increases. In addition, the polarization ability of small radii Mg²⁺ cation is greater than that of Ca²⁺ cation, and they have a strong interaction with surrounding anions. Thus, the moving ability of Mg²⁺ is weaker than Ca²⁺. Consequently, all of the above factors lead to a decrease in electrical conductivity as adding MgO.

4.3. Influence of Ratio of CaO/SiO₂ on Electrical Conductivity

The influence of CaO/SiO₂ ratio at a fixed content of Al₂O₃ on electrical conductivity is shown in Figs. 6, 7, 8. It can be seen from those figures that, as gradually increasing the CaO/SiO₂ ratio, the electrical conductivity of molten slag increases. From the Table 1, all the compositions fulfill CaO/Al₂O₃ >1. In this case, there are already enough Ca²⁺ ions participating into the charge compensation of Al³⁺ ions, and the extra Ca²⁺ ion with the increase of CaO/SiO₂ will be mainly used for breaking the network and increasing ion conduction. Both the increase of Ca²⁺ ion concentration and the decrease of the degree of polymerization as increase CaO/SiO₂ ratio are beneficial for the increase of conductivity.

4.4. Comparisons between Measured and Calculated Electrical Conductivities

In the previous study,¹⁶⁾ a model was proposed to calculate the electrical conductivity of CaO–MgO–Al₂O₃–SiO₂ system based on the optical basicity:   

$$ln\sigma =lnA-B/(RT)$$(3)

where A is a pre-exponent factor, Ω⁻¹·cm⁻¹; B is the activation energy, J/mol; R is the gas constant, 8.314 J/(mol·K), and T is the absolute temperature, K.

The parameter B in the above equation is the linear function of Λ^corr:   

$$B=m\cdot {\mathrm{\Lambda }}^{corr}+n$$(4)

where m and n are constants, J/mol; Λ^corr is the optical basicity.

In the present study, the optical basicity of CaO–SiO₂–MgO–Al₂O₃ slag system is calculated as follows:   

$${\mathrm{\Lambda }}^{corr}=\frac{x_{CaO}+0.78\times x_{MgO}+0.8\times x_{A{l}_2{O}_3}+0.96\times x_{Si{O}_2}}{x_{CaO}+x_{MgO}+2\times x_{A{l}_2{O}_3}+2\times x_{Si{O}_2}}$$(5)

The parameters in Eqs. (3) and (4) are A = 5054 Ω⁻¹cm⁻¹, m = −276838 J/mol, n = 323789 J/mol. According to the model, the electrical conductivity of CaO–SiO₂–MgO–Al₂O₃ system could be calculated. The comparisons of measured electrical conductivity and calculated values are shown in Fig. 10, with a mean deviation Δ of 18.62%, which was calculated in terms of the following equation:   

$$\mathrm{\Delta }\text{=}\frac{\text{1}}{N}\times \sum _{i=1}^N\frac{|k_{i,mea}-k_{i,cal}|}{k_{i,mea}}\times 100\%$$(6)

where k_i,mea and k_i,cal are the measured and estimated electrical conductivity, respectively, and N represents the number of the samples.

Fig. 10.

Comparisons between measured and calculated electrical conductivity.

5. Conclusions

The electrical conductivity of CaO–SiO₂–MgO–Al₂O₃ melts was measured in the present study. From experimental results, the following conclusions could be drawn.

(1) The electrical conductivity increases with increasing the temperature, and the temperature dependence of electrical conductivity obeys the Arrhenius law.

(2) The addition of Al₂O₃ or MgO could lead to the decrease of the electrical conductivity of CaO–SiO₂–MgO–Al₂O₃ type high aluminum blast furnace slags.

(3) As the CaO/SiO₂ ratio increases, the concentration of free Ca²⁺ ion increases while the degree of polymerization of the melt decreases, both of which result in the increase of conductivity.

Acknowledgement

Thanks are given to the financial supports from the National Natural Science Foundation of China (51734002).

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