J. Cent. South Univ. Technol. (2008) 15: 819-823

DOI: 10.1007/s11771-008-0151-3

Electrical conductivity of (Na₃AlF6-40%K₃AlF₆)-AlF3-Al₂O₃ melts

HUANG You-guo(黄有国)^{1, 2}, LAI Yan-qing(赖延清)¹, TIAN Zhong-liang(田忠良)¹,

LI Jie(李 劼)¹, LIU Ye-xiang(刘业翔)¹, LI Qing-yu(李庆余)²

 (1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China;

2. School of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China)

Abstract: The effects of contents of AlF₃ and Al₂O₃, and temperature on electrical conductivity of (Na₃AlF₆-40%K₃AlF₆)- AlF₃-Al₂O₃ were studied by continuously varying cell constant (CVCC) technique. The results show that the conductivities of melts increase with the increase of temperature, but by different extents. Every increasing 10 ℃ results in an increase of 1.85×10^-2, 1.86×10^-2, 1.89×10^-2 and 2.20×10^-2 S/cm in conductivity for the (Na₃AlF₆-40%K₃AlF₆)-AlF₃ melts containing 0%, 20%, 24%, and 30% AlF₃, respectively. An increase of every 10 ℃ in temperature results an increase about 1.89×10^-2, 1.94×10^-2, 1.95×10^-2, 1.99×10^-2 and 2.10×10^-2 S/cm for (Na₃AlF₆-40%K₃AlF₆)-AlF₃-Al₂O₃ melts containing 0%, 1%, 2%, 3% and 4% Al₂O₃, respectively. The activation energy of conductance was calculated based on Arrhenius equation. Every increasing 1% of AlF₃ results in a decrease of 0.019 and 0.020 S/cm in conductivity for (Na₃AlF₆-40%K₃AlF₆)-AlF₃ melts at 900 and 1 000 ℃, respectively. Every increase of 1% Al₂O₃ results in a decrease of 0.07 S/cm in conductivity for (Na₃AlF₆-40%K₃AlF₆)-AlF₃-Al₂O₃ melts. The activation energy of conductance increases with the increase in content of AlF₃ and Al₂O₃.

Key words: aluminum electrolysis; electrical conductivity; activation energy; additive; superheat

1 Introduction

Al₂O₃ can be dissolved well by elpasolite^[1-4]. The problems of deposition of Al₂O₃ and cathodic crust are hopeful to be solved by elpasolite at low temperature electrolysis^[5-6]. Meanwhile, the service condition of inert anode will be improved and important in energy saving during the process of aluminum electrolysis by the application of elpasolite. Therefore, the research on physicochemical properties of elpasolite or mixture of elpasolite and cryolite has attracted much attention^[7-10].

In order to develop a new kind of aluminum electrolyte which has properties of lower crystallized temperature and higher solubility of Al₂O₃, and to improve service condition of inert anode, liquidus temperature of Na₃AlF₆-K₃AlF₆ mixture melts was studied systematically in Refs.[11-13]. However, 35%- 40% of the total energy consumption of electrolysis is in the voltage of bath during aluminum electrolysis^[14]. Therefore, studying the relationship between electrical conductivity and its influencing factors, and adjusting the compositions of bath and technological parameter are important for the energy consumption in aluminum electrolysis^[15-17]. Moreover, the study on conductance properties of bath will promote the research on structure of melts and migration character of ions under the effect of electric field^[18].

However, most literatures concerned the system containing K₃AlF₆ or K₃AlF₆-Al₂O₃, and the study on the electrical conductivity of system containing mixture of K₃AlF₆ and Na₃AlF₆ was rarely. In this work, the electrical conductivity of the mixture (Na₃AlF₆- 40%K₃AlF₆)-AlF₃-Al₂O₃ was studied by the continuously varying cell constant technique based on Refs.[19-20].

2 Experimental

Na₃AlF₆, K₃AlF₆ and Al₂O₃ were reagent grade. AlF₃ was high pure reagent. In order to remove the water, all reagents were dried at 393 K for over 24 h in a vacuum drying box.

The schematic drawing of measurement assembly used in high temperature experiment is shown in Fig.1. A pyrolytic boron nitride (BN) tube was used as conductivity cell. Alternating current (AC) impedance was determined by an EG & G Model 273A type potentiostat and 5210 lock-in amplifier. The electrical conductivity of melts was measured by the continuously varying cell constant technique. The cross-sectional area of pyrolytic BN cell tube was determined by measuring the electrical conductivity of KCl aqueous. In this work, the AC amplitude was 5 mV; the frequency was varied from 1 to 50 kHz.

Fig.1 Schematic drawing of measurement of electrical conductivity for molten salts: 1—Motion sensor; 2—Leading wire of work electrode; 3—Motion setting; 4—Thermocouple for temperature measurement; 5—Corundun chip; 6—Heating wire; 7—Pt electrode;  8—Thermal couple of controller; 9—Al₂O₃ tube; 10—Gas inlet; 11—Base; 12—Bracket; 13—Electrode bar;  14—Gas outlet;  15—Cooling water duct; 16—Cell; 17—Graphite crucible;  18—Melts; 19—Leading wire of counter electrode

3 Results and discussion

3.1 Effect of temperature on electrical conductivity of melts

Fig.2 shows the electrical conductivity of the system (Na₃AlF₆-40%K₃AlF₆)-24%AlF₃ and (Na₃AlF₆- 40%K₃AlF₆)-24%AlF₃-Al₂O₃ as a function of tempera- ture, respectively. As shown in Fig.2, the electrical conductivity (s) is almost linear function of the temperature (t) and the variation trend for the two systems is similar. As shown in Fig.2(a), every increase of 10℃ temperature results in an increase about 1.85×10^-2, 1.86×10^-2, 1.89×10^-2 and 2.20×10^-2 S/cm in the electrical conductivity of the systems (Na₃AlF₆ + K₃AlF₆)-AlF₃ containing 0, 20%, 24% and 30% AlF₃, respectively. While, from Fig.2(b), every increase of 10 ℃ temperature results in an increase about 1.89×10^-2, 1.94×10^-2, 1.95×10^-2, 1.99×10^-2 and 2.10×10^-2 S/cm in the electrical conductivity for melts with Al₂O₃ content of 0, 1%, 2%, 3% and 4%, respectively.

Fig.2 Electrical conductivity as function of bath temperature for different melts: Symbols—Experiment data in this work; Lines—Calculated values based on Arrhenius equation; (a) (Na₃AlF₆-40%K₃AlF₆)-AlF₃ melt; (b) (Na₃AlF₆-40%K₃AlF₆)- 24%AlF₃- Al₂O₃

Molten cryolitic fluoride is ionic conductor. The melts contain not only some simple ions, such as Na⁺, K⁺ and F^-, but also some large volume complexes, such as AlF₆^3-, AlF₅^2- and AlF₄^-. The increase of temperature results in the increase of velocity of ions in melts, whichresults in the increase of electrical conductivity of melts. At the same time, the increase of temperature increases the ionization degree of complex ions, such as AlF₆^3-, AlF₅^2- and AlF₄^-. These complex ions partly decompose into small volume ion. Therefore, in unit volume, the number of ions involved in conductance increases. This is one of the reasons for the increase of electrical conductivity of melts. In microscopic analysis, only those ions with energy equal to or higher than activation energy take participate in the conductance of ions. The increase of temperature results in more ions to obtain sufficient kinetic energy, which means more ions involved in conductance. Finally, the electrical conductivity increases with the increase of temperature.

Quantitatively, the relationship between electrical conductivity and temperature is determined by the Arrhenius equation:

                            (1)

where  s is the specific conductivity, S/cm; A is the pre-exponential related to chemical composition, S/cm; E is the activation energy of conductance, J/mol; T is the temperature of the melts, K; R is the universal gas constant and has the value of 8.314 J/( K·mol). By fitting the experimental electrical conductivities of different systems at different temperatures to Eqn.(1), A and E of different systems were obtained, respectively. The results are listed in Table 1 and Table 2, respectively.

Table 1 Pre-exponential (A) and activation energy (E) of conductance of (Na₃AlF₆-40%K₃AlF₆)- AlF₃ melt

Table 2 Pre-exponential (A) and activation energy (E) of conductance of (Na₃AlF₆-40%K₃AlF₆)- 24%AlF₃-Al₂O₃ melt

Based on Eqn.(2), the calculated values of electrical conductivity are shown in Fig.2 (line).

3.2 Effects of AlF₃ content on electrical conductivity

The influence of AlF₃ content on the electrical conductivity of melts (Na₃AlF₆-40%K₃AlF₆) is shown in Fig.3. As shown in Fig.3, the electrical conductivity of melts decreases with increasing content of AlF₃. Every increase of 1% AlF₃ in the AlF₃ content range of 0-30% results in a decrease about 0.02 S/cm in the electrical conductivity for the melts (Na₃AlF₆-40%K₃AlF₆)-AlF₃ at 1 273 K. While every increase of 1% AlF₃ results in a decrease about 0.019 S/cm in the electrical conductivity at 1 173 K. This effect is probably due to the reaction of AlF₃ with the free F^- under formation of large volume complexes, such as AlF₅^2- and AlF₄^-, after the addition of AlF₃ into the cryoltie mixture. The molar fractions of AlF₅^2- and AlF₄^- increase with the increase of the content of AlF₃^[21], which increases the interaction to Na⁺ and hinders the migration of Na⁺. Therefore, the increase in AlF₃ content results in the decrease of the electrical conductivity.

Fig.3 Electrical conductivities of cryolitic (Na₃AlF₆- 40%K₃AlF₆)-AlF₃ mixtures at different temperatures

Moreover, as shown in Fig.3, the electrical conductivity of system (Na₃AlF₆-40%K₃AlF₆)-AlF₃ is lower than that of system Na₃AlF₆-AlF₃. The main reasonis that though both of Na and K are alkali, the radius of K⁺ is longer than that of Na⁺. Therefore, the rate of migration of K⁺ is slower than that of Na⁺ in the electric field. Na₃AlF₆ has better conductance performance than K₃AlF₆ in the same condition (the electrical conductivity of Na₃AlF₆ is 0.55 S/cm higher than that of K₃AlF₆). Thus, the electrical conductivity decreases after the addition of the elpasolite into cryolite.

Activation energy of conductance is related to the energy-barrier through which ions transport, and the higher the activation energy of conductance, the less the electrical conductivity. Activation energies of conductance are 10.5, 12.5, 13.2, and 14.4 kJ/mol for system containing 0%, 20%, 24%, and 30% AlF₃ respectively. Activation energy of conductance increases with increasing AlF₃ content. The experimental electrical conductivity decreases with increasing AlF₃ content and activation energy of conductance.

3.3 Effect of Al₂O₃ content on electrical conductivity of melts

Fig.4 shows the electrical conductivity of system (Na₃AlF₆-40%K₃AlF₆)-24%AlF₃ as a function of Al₂O₃ content. As shown in Fig.4, the electrical conductivity of (Na₃AlF₆-40%K₃AlF₆)-24%AlF₃ decreases with increasing content of Al₂O₃ at different temperatures. Every increase of 1% Al₂O₃ results in a decrease about 0.07 S/cm in the electrical conductivity in the temperature range of 1 123-1 223 K. Considering the influence of Al₂O₃ on liquidus temperature of bath, the electrical conductivity decreases further at the same superheat after the effect of temperature is removed. The electrical conductivity of (KF-AlF₃-Al₂O₃) (cryotile ratio (CR)=1.3) melts was determined by KRYUKOVSKY et al^[8]. The results show that every increase 1% of Al₂O₃ content results in a decrease about 0.025 S/cm in the electrical conductivity. The electrical conductivity of (KF-AlF₃-Al₂O₃) (CR=1.2) melts was measured by HIVES and THONSTAD^[7]. However, the results show that every increasing 1% of Al₂O₃ content results in a decrease about 0.010 S/cm in the electrical conductivity, which is lower than that in this work.

Fig.4 Electrical conductivity as function of Al₂O₃ content for cryolite mixture

Firstly, Al₂O₃ is decomposed, which releases O^2- after adding Al₂O₃ into cryolite. Because the radius of O^2- is close to that of F^-, F^- in AlF₆^3- complex ion will be substituted by O^2-. The aluminum-oxygen-fluorine complex (such as Al₂OF₆^2- and Al₂O₂F₄^2-) comes into being. These complex ions are similar to AlF₅^2- and AlF₄^- with large volume and hinder the migration of Na⁺. Thus, the electrical conductivity of melts decreases after the addition of Al₂O₃.

The activation energies of conductance of (Na₃AlF₆- 40%K₃AlF₆)-24%AlF₃-Al₂O₃ melt are 14.1, 14.9, 16.0 and 17.7 kJ/mol for systems containing 1%, 2%, 3%, and 4% Al₂O₃, respectively. The activation energy of conductance increases with the increase of Al₂O₃ content, which means that the migration of the current of carrying species is blocked by Al₂OF₆^2- and Al₂O₂F₄^2-.

4 Conclusions

1) The electrical conductivity is almost a linear function of the temperature. The conductive capacity of melts increases with the increase of the temperature, but by different extents. Every increase of 10 K temperature results in an increase about 1.85×10^-2, 1.86×10^-2, 1.89×10^-2 and 2.2×10^-2 S/cm in the electrical conductivity for the melts with AlF₃ content of 0%, 20%, 24%, and 30%, respectively. Every increase 10 K in temperature results in an increase about 1.89×10^-2, 1.94×10^-2, 1.95×10^-2, 1.99×10^-2 and 2.10×10^-2 S/cm for the melts containing 0%, 1%, 2%, 3% and 4% Al₂O₃, respectively. Based on Arrhenius equation, the activation energies of conductance are calculated for both (Na₃AlF₆-40%K₃AlF₆)-AlF₃ melt and (Na₃AlF₆-40% K₃AlF₆)-24%AlF₃-Al₂O₃ melt.

2) For the system (Na₃AlF₆-40%K₃AlF₆)-AlF₃, the electrical conductivity decreases with the increase of AlF₃ content. Every increase of 1% AlF₃ results in a decrease about 0.019 and 0.02 S/cm in the electrical conductivity with the AlF₃ content from 0 to 30% at    1 173 and 1 273 K, respectively. The activation energy of conductance of (Na₃AlF₆-40%K₃AlF₆)-AlF₃ melt increases with the increase of AlF₃ content.

3) The electrical conductivity decreases with the increase of Al₂O₃ content for the system (Na₃AlF₆- 40%K₃AlF₆)-24%AlF₃ at different temperatures. Every increase of 1% Al₂O₃ results in a decrease about 0.07 S/cm in the electrical conductivity with the Al₂O₃ content ranging from 0 to 4% within the temperature range of   1 123-1 223 K. The activation energy of conductance of (Na₃AlF₆-40%K₃AlF₆)-24%AlF₃-Al₂O₃ melt increases with the increase of Al₂O₃ content.

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Foundation item: Project(2005CB623703) supported by the Major State Basic Research and Development Program of China; Project(2008AA030503) supported by the National High-Tech Research and Development Program of China; Project(GUIKEJI 0639032) supported by Applied Basic Research in Guangxi Province, China

Received date: 2008-05-14; Accepted date: 2008-07-25

Corresponding author: HUANG You-guo, Doctoral candidate; Tel: +86-731-8830474; E-mail: huangyg72@163.com

(Edited by YANG Hua)