JPH03115592A - Molten salt electrolytic cell - Google Patents

Abstract

PURPOSE:To easily and power-savingly remove the impurity ions in a molten salt bath and to purify the bath when the bath contains impurity ions of different valencies by charging the bath into the cathode chamber of an electrolytic cell provided with a ceramic diaphragm having pores and applying a current. CONSTITUTION:The molten salt bath 5 for electrolytically refining Mg consisting essentailly of MgCl2 and contg. Fe ion as the metallic impurity is charged into the anode chamber 8 and cathode chamber 9 of the electrolytic cell having an Fe cathode 4 and a graphite anode 3 and provided with a ceramic diaphragm 6 of sintered alumina having many pores <200mum in diameter or a quartz diaphragm 6 having pores <500mum in diameter, and a DC current is applied. The Fe ion in the bath in the cathode chamber 9 is reduced, deposited on the cathode 4 and removed, and the Fe ion in the bath in the anode chamber 8 is not passed through the diaphragm 6 and left. Consequently, the Fe ion is not moved between the cathode and anode by the presence of the diaphragm 6 or repeatedly reduced and oxidized, hence power is not wasted, and a high- purity bath having a low content of impurities is obtained in the cathode chamber 9 as the material for electrolytic Mg. Since the Fe is left in the bath 5 in the anode chamber 8, the bath can be purified in the cathode chamber 9.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is particularly useful when electrolytically depositing metal ions having different ionic valences contained in a molten salt, in particular to remove impurities contained in a chloride-containing bath salt. The present invention relates to a molten salt electrolytic cell used for removing metals by electrolytic deposition.

(Conventional technology) In molten salt electrolysis, for example, iron ions (Fe'').

When electrolyzing a bath temperature containing metal ions with different ionic valences, such as Fe''), the metal ions (Fe'') reduced at the cathode move through the bath temperature and have a higher ionic valence at the anode. It is oxidized to metal ions (Fe''), and these oxidized metal ions move to the cathode side again and are reduced to metal ions (Fe'') having a low ionic valence. Since such reduction and oxidation are repeated, electrical energy is not used effectively and current efficiency decreases.

To deal with this problem, in the case of an aqueous solution system, the method of preventing the movement of metal ions as described above by utilizing the permselectivity of the ion exchange membrane is widely used, but in the case of a molten salt system, this method is widely used. In the absence of such suitable materials, ceramics such as terracotta plates and asbestos-processed products have traditionally been used as diaphragm materials.

(Problem to be Solved by the Invention) However, even if the above-mentioned diaphragm material is used, it is not possible to completely suppress the movement of the above-mentioned metal ions, which often results in a significant decrease in current efficiency. there were.

An object of the present invention is to provide an electrolytic cell that prevents the movement of metal ions as described above and enables electrolysis with high current efficiency in molten salt electrolysis.

(Means for Solving the Problem) In order to achieve the above object, the present inventors have repeatedly studied the diaphragm of an electrolytic cell, and as a result, the present inventors have devised a diaphragm for metal ions by appropriately selecting the material of the diaphragm and the diameter of the pores. I learned that it is possible to prevent movement through.

The present invention is based on the above findings, and its gist is the following molten salt electrolytic cells i) and ii).

i) A molten salt electrolytic cell characterized in that a ceramic diaphragm having pores with a diameter of 200 μm or less is provided between an anode and a cathode.

ii) A molten salt electrolytic cell characterized in that a quartz diaphragm having pores with a diameter of 500 μm or less is provided between an anode and a cathode.

Alumina sintered crystal is suitable as the ceramic, but other ceramics such as mullite, cordierite, or zirconia, non-oxide ceramics such as silicon carbide, graphite, and silicon nitride, etc.
Any ceramic that can withstand attack by the molten salt to be electrolyzed and has pores with a diameter within the range of the present invention can be used.

In addition, the above-mentioned quartz is preferably made into a plate shape by sintering quartz particles, but is not limited to a sintered body as long as it is a quartz plate having pores with a pore diameter within the range of the present invention. Available for use.

FIG. 1 is a schematic cross-sectional view showing an example of a molten salt electrolytic cell according to the present invention. This is an electrolytic bath salt purification tank for removing Mn and Mn. In the figure, l is the outer frame of the electrolytic cell, 2 is the lid, 3 is the anode, 4 is the cathode, 5 is the bath whose main component is magnesium chloride, 6 is the diaphragm having pores, and 7 is the electrolytic cell lining. . Graphite is usually used for the anode 3, and iron is used for the cathode 4. Further, the part where the anode 3 is attached with the diaphragm 6 as a boundary is an anode chamber 8, and the part where the cathode 4 is attached is a cathode chamber 9. Note that the cathode 4 is usually not exposed above the bath 5, but is inserted into the bath 5 by penetrating the lining 7 from the side of the electrolytic cell.

(Function) In order to remove impurities such as Fe and Mn in the bath temperature using the electrolytic cell shown in Fig. 1 above, a bath is introduced into the electrolytic cell,
Electrolysis can be carried out by applying a predetermined voltage between the anode and cathode.
Fe and Mn are deposited on the cathode and removed from the bath.

To describe a specific example, in FIG. 1, a mixed bath salt 5 (MgCl
t: 20%, CaCj! x: 30%, NaC
Q: 50%), and a voltage of 2.0 to 2.4 V, which is lower than the decomposition voltage of magnesium chloride, is applied between the anode 3 and the cathode 4. re exists as Fe1 or Fe'' Fe ions at the bath temperature, but the Fe ions contained in the bath 5 in the cathode chamber 9 are converted to Fe3°-* p e'' by electrolysis.
''' → Fe' (metallic Fe) is reduced and deposited on the cathode 4. During this time, since Fe'' and Fe'+ cannot pass through the pores of the diaphragm 6, Pe'' enters the anode chamber 8. The Fe ions in the anode chamber 8 do not move and become oxidized, and the Fe ions in the anode chamber 8 do not flow into the cathode chamber 9. Therefore, the Fe ions in the cathode chamber 9
The e-ions are removed, the bath is purified, and the current flowing between the picture electrodes 3.4 is reduced. The purified bath is removed from the cathode chamber 9 and fed to an electrolytic cell for producing magnesium. The bath in the anode chamber 8 remains as it is, but is moved to the cathode chamber 9 if necessary and purified as described above.

The reason why Fe ions cannot pass through the pores of the diaphragm is that
This is considered to be because e ions do not exist alone, but are weakly bonded to several Cl ions <cp, ->, resulting in a large ionic radius.

Judging from the examples described later, when the diameter of the pores is 200 μm or less in the case of a ceramic diaphragm and 500 μm or less in the case of a quartz diaphragm, passage of Fe ions is blocked. Although it is not clear why the diameter (upper limit) of the pores that prevent the passage of Fe ions differs depending on the material, it is presumed that static electricity on the surface of the diaphragm caused by the application of voltage is involved.

Note that there is no particular lower limit to the diameter of the pores, but the diameter of the picture electrode 3.
When a voltage is applied between 4, a current flows, that is,
It is necessary for ions to pass through the diaphragm and move from one electrode to the other, and in the electrolysis of the above-mentioned chloride-based mixed bath salt, Cj2- is thought to play this role, and therefore, It is necessary that the pores have a size that allows at least C1- to pass through.

Regarding the number of pores, if the number of pores is extremely small, there will be a delay in the passage of ions through the diaphragm, increasing resistance and making it difficult to conduct electricity.
The number of holes must be large enough to not cause a significant increase in resistance.

In the case of a sintered body, the required number of holes is usually sufficient.

(Example 1) A quartz particle sintered plate (1
00an + , width 100+m,
A height of 150 mm) was created. A total of 1.2 kg of molten salt (MgCl
z: 20%, CaClz: 30%, NaCl: 5
0%), and further Fe as an impurity in each electrode chamber 8.9.
Cff1s and MnCl t were added, and the temperature of the bath was brought to 7 by heating with an electric heater (not shown) from the outside of the electrolytic cell.
The temperature was maintained at 00°C. In the anode chamber 8, Re concentration and Mn
The concentrations were 0.90% and 0.72%, respectively, and in the cathode chamber 9 they were 0.80% and 0.39%, respectively.

Next, a voltage of 2.5 μ, which is lower than the decomposition voltage of magnesium chloride, was applied between the anode 3 and the cathode 4, and electrolysis was continued for 3.5 hours while sampling the bath at regular intervals.

Meanwhile, metals (Fe and Mn) are precipitated at the cathode 4,
Chlorine was generated at anode 3.

The measurement results of FeJ degree and Mn concentration in the bath temperature are shown in Figure 2.From the same figure, the Fe concentration and MnlJ degree both decreased to 0.004% in the cathode chamber 9, the bath was transparent, and the electrolytic treatment It can be seen that impurities are removed. In addition, in the anode chamber 8, the Fe concentration and Mn concentration were only slightly decreased, and the bath also exhibited a reddish-brown color, which was not much different from the state before electrolysis.

(Example 2) A quartz sintered body with a pore diameter of 500 μm was used as a diaphragm, and electrolytic treatment was performed using the same equipment and bath as in Example 1 under the same conditions. Pa and Mn added as impurities
The concentrations in the anode chamber are 0.018% and 0.01%, respectively.
9%, 0.021% and 0.019% in the cathode chamber, respectively.
Met.

The measurement results of the Felfi degree and Mn concentration in the bath temperature after 3 hours of electrolysis are shown in FIG. 3. The Fe concentration and the ■ concentration decreased to o, oos%, and 0.002%, respectively, in the cathode chamber.

In addition, in the anode chamber, there was no difference in either the Fe concentration or the Mn concentration before and after electrolysis.

(Example 3) Using an alumina sintered crystal with a pore diameter of 200 pm or less as a diaphragm, and using the same equipment and bath as in Example 1,
Electrolytic treatment was performed under the same conditions. P added as an impurity
The concentrations of e and Mll are each 0.02 in the anode chamber.
1%, 0.014%, and 0.019% in the cathode chamber, respectively.
, 0.015%.

The measurement results of Fe concentration and Mnfi degree in the bath temperature after 3 hours of electrolysis are shown in FIG. 4. Fe1 degree and Mn11 degree decreased to 0.004% and 0.001%, respectively, in the cathode chamber.

In addition, although the Fefi degree decreased slightly in the anode chamber, Mnl
There was no difference in d degree before and after electrolysis.

(Comparative Example 1) A quartz plate with a pore diameter of 1 mm was used as a diaphragm, and electrolytic treatment was performed using the same equipment and bath as in Example 1 under the same conditions.

Figure 5 shows the measurement results of the Fe1 degree and Mn concentration in the bath temperature after 3 hours of electrolysis.The Fe1 degree and Mn concentration in the cathode chamber did not fall below 0.005%, and the current efficiency was as low as that of the example. The current efficiency was only 16.0% of that in No. 1.

(Comparative Example 2) A ceramic filter with a pore diameter of 1 mm was used as a diaphragm (
Electrolytic treatment was performed using the same equipment and bath as in Example 1 under the same conditions.

The measurement results of Fefi degree and MnlJ degree in the bath temperature after 4 hours of electrolysis are shown in Figure 6.
The nfi degree did not fall below 0.005% in any case, and the current efficiency was 15.5% of the current efficiency in Example 1.

(Comparative Example 3) A zirconia ceramic filter with a pore diameter of 11 m was used as a diaphragm, and electrolytic treatment was performed using the same apparatus and bath as in Example 1 under the same conditions.

Figure 7 shows the measurement results of the Fe concentration and Mn concentration in the bath temperature after 3 hours of electrolysis. It was 12.0% of the total.

(Comparative Example 4) A zirconia-mullite ceramic filter with a pore diameter of Ion was used as a diaphragm, and electrolytic treatment was performed using the same equipment and bath as in Example 1 under the same conditions.

FIG. 8 shows the measurement results of FelJ degree and Mn concentration in the bath temperature after 3 hours of electrolysis. Fe4 degree and Mnf in cathode chamber
The A degree was high at 0.015% and 0.006%, and the current efficiency was 27.0% of the current efficiency in Example 1.

(Comparative Example 5) A silicon carbide-mullite ceramic filter with a pore diameter of 500 μm was used as a diaphragm, and electrolytic treatment was performed using the same equipment and bath as in Example 1 under the same conditions.

The measurement results of the Pelli degree and Mn concentration in the bath temperature after 3 hours of electrolysis are shown in Figure 9.
The n concentration did not fall below 0.005% in any case, and the current efficiency was 30.4% of the current efficiency in Example 1.

(Comparative Example 6) A cordierite ceramic filter with a pore diameter of 500 μm was used as a diaphragm, and electrolytic treatment was performed using the same equipment and bath as in Example 1 under the same conditions.

The measurement results of Felli degree and Mn1 degree in the bath temperature after 3 hours of electrolysis are shown in Figure 10.
1 + J1 degree does not fall below 0.012%,
The current efficiency was 24.0% of the current efficiency in Example 1.

(Effect of the invention) When the molten salt electrolytic bath of the present invention is used, for example, Fe
Even if metal ions such as ions and Mn ions are contained, the movement of these metal ions through the diaphragm is hindered. Therefore, metal ions reduced at the cathode are oxidized at the anode and then reduced again at the cathode, which eliminates wasteful consumption of electrical energy, making it possible to perform electrolysis while maintaining high current efficiency. Purification and purification of baths, as well as electrowinning of metals, can be carried out efficiently.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of a molten salt electrolytic cell of the present invention. FIG. 2 to FIG. 10 are graphs showing the relationship between Fe and Mn concentrations and electrolysis time when Fe and Mn are contained as impurities in the bath temperature, and FIGS. FIG. 5 to FIG. 1O are comparative examples.

Claims (2)

[Claims] (1) A molten salt electrolytic cell characterized in that a ceramic diaphragm having pores with a diameter of 200 μm or less is provided between an anode and a cathode. (2) A molten salt electrolytic cell characterized in that a quartz diaphragm having pores with a diameter of 500 μm or less is provided between an anode and a cathode.