JP2012149301A - Molten salt electrolytic cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably improve the current efficiency by reducing current leakage caused by the flow of a molten salt climbing over electrodes, which causes a problem in a multipolar type electrolytic cell, without being affected by the variation in current amount.SOLUTION: A multipolar type molten salt electrolytic cell has two or more multipoles 8 between a positive electrode 6 and a negative electrode 7, and a molten salt flows from the positive electrode side to the negative electrode side while climbing over the multipoles 8 during electrolytic operation. In the multipolar type molten salt electrolytic cell, the thickness of the multipole 8 provided on the outermost side and adjacent to the negative electrode 7 (negative electrode side multipole thickness Tc) is made larger than the thickness of the multipole 8 provided on the innermost side and adjacent to the positive electrode 6 (positive electrode side multipole thickness Ta). Preferably, when the negative electrode side multipole thickness Tc/positive electrode side multipole thickness Ta is defined as multipole thickness coefficient K, K satisfies 1.05≤K≤4.

Description

  The present invention relates to a molten salt electrolytic cell used for production of metal Mg, and more particularly to a multipolar molten salt electrolytic cell in which a plurality of bipolar electrodes are arranged between an anode and a cathode.

When industrially producing metal Mg, a molten salt electrolysis method is frequently used in which a molten salt containing MgCl ₂ is electrolyzed at a temperature equal to or higher than the melting point of Mg. Further, as an electrolytic cell used here, a highly efficient multipolar electrolytic cell is attracting attention. A method for producing metal Mg by a molten salt electrolysis method using a multipolar electrolytic cell will be described with reference to FIGS. FIG. 1 is a longitudinal side view of a multipolar electrolytic cell, and FIG. 2 is a view taken along the line AA of FIG. FIG. 3 is an enlarged front view of the main part of the electrolysis chamber during operation.

As shown in FIGS. 1 and 2, the electrolytic cell 1 containing molten salt 2 containing MgCl ₂ therein. The inside of the electrolytic cell 1 is separated into an electrolytic chamber 4 and an Mg recovery chamber 5 by a partition wall 3. In the electrolysis chamber 4, flat plate-like anodes 6 and cathodes 7 made of carbon are alternately arranged on the roast brick 9 in the tank width direction, and between the adjacent anodes 6 and cathodes 7, carbon A flat plate-shaped bipolar electrode 8 is arranged for improving current efficiency. The upper part of the anode 6 projects upward through the cover 10 of the electrolysis chamber 4 for energization.

  Here, the upper surface level of the bipolar electrode 8 is higher than the upper surface level of the cathode 7, and gradually increases as the anode 6 is approached. As will be described in detail later, this is for smooth bath convection of the molten salt 2.

  On the other hand, the Mg recovery chamber 5 communicates with the electrolysis chamber 4 through two upper and lower openings 11, 11 provided in the partition 3. The Mg recovery chamber 5 is provided with a bath surface level adjusting device 12 composed of a bottom open container soaked in the molten salt 2. Further, a heat exchanger is provided as a temperature adjuster 13 for the molten salt 2 so as to surround the bath surface level adjuster 12. Further, a thermometer 14 and a bath surface level measuring device 15 are provided.

In the electrolysis operation, a direct current flows between the anode 6 and the cathode 7 in the electrolysis chamber 4. Thus, MgCl ₂ in the molten salt 2 is electrolyzed, metal Mg are generated. Further, chlorine gas is generated between the electrodes along with this electrolysis. The chlorine gas generated between the electrodes rises, and the molten salt 2 rises between the electrodes along with this gas lift. In order to smoothly discharge the molten salt 2 that has risen between the electrodes toward the cathode 7, the upper surface level of the bipolar electrode 8 is lowered stepwise from the anode 6 to the cathode 7. Is set even lower. As a result, during the electrolytic operation, as shown in FIG. 3, the molten salt 2 rising between the electrodes flows over the bipolar electrode 8 from the anode 6 side to the cathode 7 side. More specifically, every time the molten salt 2 gets over the bipolar electrode 8, it merges with the molten salt 2 that has risen between the next electrodes, thereby gradually increasing the amount of overcoming, while moving from the anode 6 side to the cathode 7 side. Flows over the double pole 8 and flows.

  Metal Mg produced in the electrolysis chamber 4 is carried to the Mg recovery chamber 5 by circulating convection of the molten salt 2 and floats on the molten salt 2 in the Mg recovery chamber 5 to form the Mg layer 16.

  The bath surface level when the molten salt 2 is not energized in the electrolysis chamber 4 substantially matches the upper surface level of the cathode 7 as shown in FIG.

  One of the problems in the molten salt electrolysis method using such a multipolar electrolytic cell is that the molten salt 2 flows over the bipolar electrode 8 and the cathode 7 in the electrolytic chamber 4 and flows through the molten salt. It is a leak. This problem will be described in detail with reference to FIG. The broken line in FIG. 3 represents the flow of the molten salt 2, and the solid line represents the flow of electricity.

  The current in the multipolar electrolytic cell flows from the anode 6 to the cathode 7 through the bipolar electrode 8 and the molten salt 2 between the electrodes, and contributes to the electrolysis of the molten salt 2. When a flow over the molten salt 2 is generated, a current flows directly from the anode 6 to the cathode 7 through the flow over the molten salt 2. Since this current does not pass through the double electrode 8 and the molten salt 2 between the electrodes, it becomes a leakage current that does not contribute to the electrolysis of the molten salt 2 and causes a reduction in current efficiency. For this reason, various devices have been devised to reduce the leakage current passing over the overflow flow of the molten salt 2, one of which is the bath surface level operation of the molten salt 2 described in Patent Document 1.

  This measure focuses on the fact that the heights H1 and H2 of the overcoming flow change according to the energization amount for electrolysis. By reducing the bath level and increasing the bath surface level of the molten salt 2 when energizing a small current, the heights H1 and H2 of the overpass flow are reduced. The leakage current passing through the flow over the molten salt 2 is reduced. By the way, when the bath surface level of the molten salt 2 is lowered uniformly, the height H1 and H2 of the overcoming flow are properly controlled when the energization amount is large, but the overcoming flow is generated when the energization amount is small. Even if it occurs, Mg stays between the electrodes because the heights H1 and H2 are small. If this happens, leakage due to Mg occurs and the current efficiency is significantly reduced.

  This bath level operation according to the energization amount is effective when the energization amount is greatly changed between the normal time zone and midnight as described in Patent Document 1. When the energization amount in the normal time zone is 90 kA and midnight is 150 kA, the average daily energization amount is 120 kA, and the displacement from this average energization amount is as large as 25% in both the normal time zone and midnight. In such a case, the countermeasure described in Patent Document 1 is effective.

  However, when the change in the energization amount is small, the effect of the countermeasure described in Patent Document 1 is small. For example, in the case where electrolysis is performed within 90% or more of the time within a range of ± 10% or less of the average daily energization amount, the effect of the countermeasure described in Patent Document 1 is small. In other words, the countermeasure described in Patent Document 1 is a technique for minimizing a decrease in current efficiency due to a change in energization amount, rather than a technique for increasing current efficiency.

JP 2002-317293 A

  The object of the present invention is to reduce current leakage caused by the flow of molten salt over the electrode, which is a problem in a multipolar electrolytic cell, without being affected by changes in the amount of current, and to stably improve current efficiency. An object of the present invention is to provide a molten salt electrolyzer that can be used.

  In order to achieve the above object, the present inventors examined the cause of the problem of the countermeasure described in the cited document 1. As a result, it was concluded that one of the causes of the problem described in the cited document 1 involves the operation of raising and lowering the bath surface level of the molten salt. We planned to solve the problem and repeated further studies. The knowledge obtained as a result will be described with reference to FIG.

  In FIG. 3, two double poles 8 are arranged between the anode 6 and the cathode 7, and the upper surface level of each pole is gradually lowered from the anode 6 toward the cathode 7. During the electrolytic operation, the molten salt 2 rises between the anode 6 and the anode-side bipolar 8 due to bubble lift. The molten salt 2 that has risen travels over the anode 8 on the anode side, and further travels over the cathode 8 on the cathode 7 and heads onto the cathode 7. At the same time, the molten salt 2 rises between the anode-side bipolar 8 and the cathode-side bipolar 8. The molten salt 2 travels over the cathode 8 on the cathode 8 and toward the cathode 7. Furthermore, the molten salt 2 also rises between the cathode 8 and the cathode 7. This molten salt 2 goes directly onto the cathode 7. As a result, the flow rate of the molten salt 2 over the bipolar electrode 8 increases in the order on the bipolar electrode 8 on the anode side and on the bipolar electrode 8 on the cathode side.

  Strictly speaking, the current leak is from the current leak L1 from the anode 6 not passing through the anode-side bipolar 8 to the cathode-side bipolar 8 and from the anode-side bipolar 8 not passing through the cathode-side bipolar 8. It is roughly divided into a current leak L2 to the cathode 7. The former current leakage amount affects the height H1 of the overflow on the anode-side bipolar 8 and the latter influences the cathode-side bipolar 8 on the cathode-side bipolar 8. Since the latter overpass flow, which has a flow height of H2, has a lower resistance than the lower overpass flow, the amount of current leak is large, and this affects the overall current leak. The degree also increases.

  Therefore, the present inventor has determined the amount of current leak L2 from the anode side bipolar 8 having a large current leak amount and a large influence on the entire current leak to the cathode 7 via the cathode side bipolar 8. We thought that reducing it would be effective in reducing the overall current leak, and we examined its specific measures from various angles. As a result, it has been found effective to increase the thickness of the cathode-side bipolar 8.

  That is, when the thickness of the cathode-side bipolar 8 is increased, the flow length of the molten salt 2 that crosses over the cathode-side bipolar 8 increases in the flow direction, and the electrical resistance of the overflow increases, thereby increasing the current leak L2. Decrease. The energization is performed from above the anode 6, but if the thickness of the cathode-side bipolar 8 is large, the amount of current in the vertical direction at the bipolar 8 increases. As a result, the current density between the cathode-side bipolar 8 and the cathode 7 is leveled, the electrical resistance therebetween decreases, and the current leak L2 via the overpass flow on the cathode-side bipolar 8 is relative. Decrease.

  The molten salt electrolytic cell of the present invention has been completed on the basis of such knowledge, and has two or more bipolar electrodes between the anode and the cathode, and these anodes are overcome during the electrolytic operation. A molten salt electrolytic cell in which molten salt flows from the cathode side to the cathode side, wherein the thickness of the outermost bipolar electrode adjacent to the cathode is the cathode-side bipolar electrode thickness Tc, and the thickness of the innermost bipolar electrode adjacent to the anode is the anode When the side bipolar thickness is Ta and the cathode side bipolar thickness Tc / the anode side bipolar thickness Ta = the bipolar thickness factor K, the bipolar thickness factor K> 1 is satisfied.

  In the molten salt electrolytic cell of the present invention, the bipolar electrode thickness coefficient K> 1 is satisfied, and the cathode-side bipolar electrode thickness Tc is larger than the anode-side bipolar electrode thickness Ta. The electric resistance in the flowing direction of the molten salt on the outermost bipolar electrode adjacent to the cathode having the largest H increases. In addition, when the amount of current in the vertical direction in the outermost bipolar electrode increases, the current density is leveled with the cathode, and the electrical resistance with the cathode is reduced. As a result, the current leak that passes over the outermost double pole adjacent to the cathode that has the greatest influence on the overall current leak is reduced, and the current efficiency is efficiently increased.

  This technique is more effective for operations with a larger energization amount in which the flow rate over the electrode is larger and the flow heights H1 and H2 are larger, specifically, the current per unit width in the outermost bipolar electrode adjacent to the cathode. When the quantity x the number of multiple poles = the conduction coefficient A, it is particularly effective for the operation of A ≧ 40 kA / m or more, and the effectiveness is even higher in the operation of A ≧ 60 kA / m or more, and A ≧ 80 kA / m or more. Higher than in operation. As shown in FIG. 6, the width direction of the double pole is a horizontal direction perpendicular to the direction of current flow, and per unit width means per unit length in the width direction.

  The bipolar pole thickness coefficient K reduces the electric resistance in the flow direction of the molten salt on the outermost bipolar pole adjacent to the cathode where the heights H1 and H2 of the overflow flow on the bipolar pole are the largest, A larger value is desirable from the viewpoint of increasing the amount of current in the vertical direction, but the distance between the anode and the cathode is specified, and if it is too large, the thickness of the bipolar electrode other than the outermost bipolar electrode will be small. This is a problem in terms of strength in operation. For this reason, the bipolar thickness coefficient K is preferably 1.05 or more and 4 or less, more preferably 1.1 or more and 3 or less, and particularly preferably 1.3 or more and 2.5 or less.

  As for the bipolar thickness other than the outermost bipolar electrode adjacent to the cathode, as described above, since the interelectrode distance between the anode and the cathode is specified, the total thickness of the bipolar electrodes is also specified. Therefore, it is necessary to reduce the thickness of the other bipolar electrode by increasing the thickness of the outermost bipolar electrode. When the number of bipolar poles is 2, there is no room for selection. However, when the number of bipolar poles is 3 or more, the number of bipolar poles other than the outermost bipolar pole is 2 or more, and it is necessary to consider their thickness. Usually, the thickness is the same, but the thickness may be increased stepwise from the anode side to the cathode side depending on the height of the overflow. In any case, it is important to set the outermost double pole thickness Tc to the maximum or one of the maximum thickness groups and the innermost double pole thickness Ta to the minimum or one of the minimum thickness groups.

  The number of bipolar electrodes may be two or more, and the number is not particularly limited. However, if the number is too large, the electrode structure in the electrolytic cell becomes complicated.

  In the molten salt electrolytic cell of the present invention, the thickness of the outermost bipolar electrode adjacent to the cathode (cathode-side bipolar thickness Tc) is greater than the thickness of the innermost bipolar electrode adjacent to the anode (anode-side bipolar thickness Ta). By making it large, current leakage due to the outermost bipolar crossover flow that has a large effect on the overall current leakage due to the flow over the electrode is effectively reduced, so that the current efficiency can be increased efficiently. In addition, it is easy to implement, can avoid an increase in size and complexity of the electrode structure in the electrolytic cell, and is excellent in economic efficiency.

It is a vertical side view of a multipolar electrolytic cell. It is an AA line arrow figure of FIG. 1, and is a front view of an electrolysis chamber. It is explanatory drawing of the current leak resulting from the rise of the molten salt on a double pole, and is a front view which shows the detail of the electrode structure in an electrolysis chamber. It is a detailed explanatory view of the electrode structure corresponding to FIG. 3 for explaining the structure of the main part in the molten salt electrolyzer of the present invention. FIG. 4 is a detailed explanatory view of an electrode structure corresponding to FIG. 3 for explaining another structure of a main part in the molten salt electrolytic cell of the present invention. It is a schematic diagram which shows an electrode width direction.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG.

  The basic structure of the molten salt electrolytic cell of this embodiment is substantially the same as that of the molten salt electrolytic cell shown in FIGS. What is different from the molten salt electrolytic cell shown in FIGS. 1 and 2 is an electrode structure in an electrolytic chamber of the electrolytic cell. The basic structure of the electrode is substantially the same as that of the molten salt electrolytic cell shown in FIGS. 1 and 2, and as shown in FIG. 4, the anode 6 and the cathode 7 are alternately arranged in the lateral width direction in the electrolytic chamber of the electrolytic cell. In addition to the arrangement, it is a multipolar type in which two or more, here three, bipolar electrodes 8 are respectively arranged between the anode and the cathode 7.

  The anode 6 projects through the cover of the electrolysis chamber for power supply (see FIG. 2). The upper surface level of the cathode 7 is the lowest, and the upper surface levels of the three bipolar electrodes 8 are gradually lowered toward the cathode 7. This is because, as described above, the molten salt 2 that rises between the electrodes by the gas lift during operation smoothly flows and discharges to the side. The difference between the molten salt electrolytic cell shown in FIGS. 1 and 2 is that the thickness T of two or more bipolar electrodes 8 is the same in the molten salt electrolytic cell shown in FIGS. On the other hand (see FIG. 3), in the molten salt electrolytic cell of this embodiment, the thickness of the outermost bipolar electrode 8 adjacent to the cathode 7 (cathode-side bipolar thickness Tc) out of the three bipolar electrodes 8 is the same. The thickness is larger than the thickness of the other two bipolar electrodes 8.

  More specifically, the total thickness of the three bipolar electrodes 8 is the same as that of the molten salt electrolytic cell having the same basic structure, and the thickness of the outermost bipolar electrode 8 (the cathode-side bipolar electrode). As the thickness Tc) is increased, the total thickness of the remaining two bipolar poles 8 is reduced, and the total thickness is equally divided between the remaining two bipolar poles 8. As a result, the thickness of the outermost bipolar electrode 8 (cathode-side dual electrode thickness Tc) is maintained without changing the distance between the anode 6 and the cathode 7 and without changing the distance between the electrodes. As a result, the thickness of the outermost bipolar electrode 8 (cathode-side bipolar thickness Tc) becomes larger than the thickness of the innermost bipolar electrode 8 adjacent to the anode 6 (cathode-side bipolar). It is larger than the thickness Ta).

  That is, when the cathode-side bipolar thickness Tc / the anode-side bipolar thickness Ta = the bipolar thickness coefficient K, the bipolar thickness coefficient K> 1. As described above, the multipole thickness coefficient K is preferably 1.05 or more and 4 or less, more preferably 1.1 or more and 3 or less, and particularly preferably 1.3 or more and 2.5 or less.

  In the molten salt electrolytic cell of the present embodiment, the thickness of the two bipolar electrodes 8 excluding the outermost bipolar electrode 8 adjacent to the cathode 7 is the same, but as shown in FIG. The thickness of the adjacent innermost bipolar electrode 8 (cathode-side bipolar thickness Ta) is made thinner than the thickness of the intermediate cathode 8, and the outermost electrode adjacent to the cathode 7 from the innermost bipolar electrode 8 adjacent to the anode 6. The thickness may be reduced stepwise over the double pole 8. In short, the thickness of the innermost bipolar electrode 8 adjacent to the anode 6 (cathode-side bipolar thickness Ta) is the smallest or one of the minimum thickness groups, and the thickness of the outermost bipolar electrode 8 adjacent to the cathode 7. (Cathode side double pole thickness Tc) is the maximum or one of the maximum thickness groups.

  Also in this case, the bipolar pole thickness coefficient K is preferably 1.05 or more and 4 or less, more preferably 1.1 or more and 3 or less, and particularly preferably 1.3 or more and 2.5 or less. .

During the electrolytic operation, an electrolytic current flows from the molten salt 6 to the cathode 7 through the three bipolar electrodes 8, and the molten salt (MgCl ₂ ) is electrolyzed between the electrodes. Along with the electrolysis, gas (chlorine gas) is generated and rises between the electrodes, and the molten salt 2 rises between the electrodes due to this bubble lift. The molten salt 2 rising between the anode 6 and the innermost bipolar electrode 8 adjacent to the anode 6 is on the innermost bipolar electrode 8, on the intermediate bipolar electrode 8, and on the outermost bipolar electrode adjacent to the cathode 7. Over 8 and over to the cathode 7. The molten salt 2 rising between the outermost bipolar electrode 8 and the intermediate bipolar electrode 8 passes over the intermediate bipolar electrode 8 and the outermost bipolar electrode 8 adjacent to the cathode 7 to reach the cathode 7. . The molten salt 2 that has risen between the middle bipolar electrode 8 and the outermost bipolar electrode 8 gets over the outermost bipolar electrode 8 and reaches the cathode 7. The molten salt 2 rising between the outermost bipolar electrode 8 and the cathode 7 reaches the cathode 7 directly.

  As a result, the flow rate of the molten salt 2 over the bipolar electrode 8 increases in order on the innermost bipolar electrode 8, on the intermediate bipolar electrode 8, and on the outermost cathode 8.

  The most significant influence on the overall current leak is that the current flow through the molten salt on the outermost cathode 8 where the flow-over flow rate is large and the height is increased, and as a result, the electric resistance is minimized. However, in the molten salt electrolytic cell of the present embodiment, the thickness of the outermost bipolar electrode 8 (cathode side bipolar electrode thickness Tc) is made larger than the thickness of the other bipolar electrode 8, so The length in the flow direction of the molten salt 2 over the outermost bipolar electrode 8 is increased, and the electric resistance of the flow over is increased. Secondly, energization is performed from above the anode 6, but if the thickness of the outermost bipolar 8 (cathode-side bipolar thickness Tc) is large, the amount of current in the vertical direction at this bipolar 8 increases. The current density between the outermost bipolar electrode 8 and the cathode 7 opposed to the outermost bipolar electrode 8 is leveled, and the electric resistance therebetween is reduced. As a result, the current leak via the crossover flow on the outermost bipolar 8 that has the greatest influence on the overall current leak is reduced, and the current efficiency is increased by reducing the overall current leak.

Metal Mg was produced from MgCl ₂ using a molten salt electrolytic cell having the electrode structure shown in FIG. At that time, the thickness of the three bipolar electrodes 8 arranged between the molten salt 6 and the cathode 7 was changed. Specifically, the case where the thicknesses of the three bipolar electrodes 8 are all 70 mm (total thickness 210 mm) is a conventional example, and the outermost bipolar electrode 8 in contact with the cathode 7 is not changed without changing the total thickness from the conventional example. The thickness (cathode-side bipolar thickness Tc) was increased, and the thickness of the intermediate bipolar electrode 8 and the thickness of the innermost bipolar electrode 8 in contact with the anode 6 (anode-side bipolar thickness Ta) were reduced accordingly. . The thickness of the intermediate bipolar 8 and the thickness of the innermost bipolar 8 in contact with the anode 6 (anode-side bipolar thickness Ta) were the same.

  Table 1 shows the electrode structure (bipolar thickness and multipolar thickness coefficient K) and current efficiency in each electrolytic operation. The current efficiency was expressed as a ratio when the current efficiency in the conventional example was 100.

  As can be seen from Table 1, the thickness of the outermost bipolar electrode 8 in contact with the cathode 7 (cathode-side bipolar thickness Tc) is larger than the thickness of the other bipolar electrode 8, and the thickness of the innermost bipolar electrode 8 in contact with it. The current efficiency is increased by setting the bipolar thickness coefficient K, which is a ratio to the thickness (anode-side bipolar thickness Ta), to more than 1. Although the current efficiency tends to increase as the double pole thickness coefficient K increases, as described above, if the double pole thickness coefficient K is excessively increased, a problem arises in terms of mechanical strength and is restricted. As described above, the preferable thickness condition of the bipolar electrode 8 is 1.05 or more and 4 or less, more preferably 1.1 or more and 3 or less, and particularly preferably 1. 3 or more and 2.5 or less.

  In the above-described embodiment, the thickness of the intermediate bipolar electrode 8 and the thickness of the innermost bipolar electrode 8 in contact with the anode 6 (anode-side bipolar thickness Ta) are the same. However, as shown in FIG. The thickness of the intermediate bipolar electrode 8 is increased so that the thickness of the bipolar electrode 8 is gradually reduced from the side of the cathode 6 to the side of the cathode 7, and the thickness of the innermost bipolar electrode 8 in contact with the anode 6 (the anode side) The double pole thickness Ta) may be reduced.

  Further, the multipolar electrode structure in the molten salt electrolytic cell is a flat electrode plate arranged in parallel in the above-described embodiment, but a cylindrical bipolar electrode and a cathode around a cylindrical or cylindrical anode. The electrode assemblies arranged in a concentric manner may be arranged side by side in the electrolytic chamber.

Claims (3)

  A molten salt electrolytic cell that has two or more bipolar electrodes between an anode and a cathode, and over which these molten electrodes flow during electrolytic operation, the molten salt flows from the anode side to the cathode side, adjacent to the cathode The thickness of the outermost bipolar electrode is the cathode-side bipolar thickness Tc, the thickness of the innermost bipolar electrode adjacent to the anode is the anode-side bipolar thickness Ta, and the cathode-side bipolar thickness Tc / the anode-side bipolar thickness Ta = A molten salt electrolytic cell having a bipolar electrode thickness coefficient K of more than 1 when the bipolar electrode thickness coefficient K is used.   The molten salt electrolyzer according to claim 1, wherein the bipolar electrode thickness coefficient K is 1.05 or more and 4 or less.   3. The molten salt electrolysis cell according to claim 1 or 2, wherein A ≧ 40 kA / m or more of electrolytic operation when the amount of current per unit width in the outermost bipolar electrode adjacent to the cathode × the number of bipolar electrodes = conduction coefficient A Molten salt electrolyzer used for

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