JPH0213031B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention is a method for producing high-purity lithium in which the concentration of alkali metals and alkaline earth metals such as sodium and calcium is suppressed. Conventional technology Conventionally, lithium was produced by using carbon or graphite for the anode and iron or steel for the cathode, electrolyzing lithium chloride alone or a mixed salt of lithium chloride and potassium chloride in a molten state, and depositing lithium on the cathode. A molten salt electrolytic method is known in which molten salt is collected using molten salt. Problems to be Solved by the Invention In order to obtain high-purity lithium with sodium and calcium content of 200 ppm or less, it is necessary to produce lithium using only the conventional molten salt electrolyte process described above, which requires sodium chloride as an impurity as the raw material lithium chloride. but
It is essential to have a high purity of 83ppm or less, and calcium chloride is 90ppm or less, which makes it extremely expensive.
In addition, in the case of electrolyzing lithium chloride that contains many impurities, the crude lithium produced by the electrolysis must be distilled, but not only is it inevitable to increase costs due to large capital investments, but mass production is not possible because it is a batch method. . Means for Solving the Problems The present invention was made to solve the above problems, and consists of 34-64% by weight of lithium chloride and 66-36% by weight of potassium chloride, or a mixture of both components. A mixed molten salt made by adding 1 to 20% by weight of sodium chloride to the liquid, using solid aluminum as the cathode, at a cathode current density of 0.005 to 1 A/cm ² so that no metal floats on the bath surface around the cathode. A method for producing high-purity lithium, which is characterized in that a solid aluminum-lithium alloy produced by electrolysis in a range is used as an anode, lithium is eluted from the anode, and is deposited and collected around the cathode where it does not react with or alloy with lithium. It is. The present invention will be explained in detail below. The present inventor used solid Al as the cathode in the electrolysis of a mixed molten salt of LiCl and KCl, and the cathode current density was
If electrolysis is carried out at 0.005 to 1 A/ ^cm2 , the precipitated Li will not float to the surface of the electrolytic bath, and Na and
High-purity Al cathode without Ca precipitation
It was discovered that an Al-Li alloy could be formed. At that time, the current efficiency reached almost 100%. The reason why a high-purity Al-Li alloy is formed in this way is that the Li deposited on the cathode surface by electrolysis is solid.
While the decomposition voltage of LiCl decreases due to the depolarization effect of diffusing into Al to generate Li-Al compounds and reducing the polarization of the cathode due to this generated compound,
Since Na does not have such a depolarization effect, the decomposition voltage of NaCl does not change, and the decomposition voltage of CaCl ₂ decreases due to the depolarization effect of Ca due to alloying, but the diffusion of Ca in the alloy is less than that of Li. As a result, the decomposition voltage does not change. As a result, only Li precipitates,
This is thought to be due to the fact that Na and Ca do not mix into the cathode material. The present invention is based on the above findings and considerations, and applies these findings to the production of high purity lithium. In the present invention, the electrolytic bath components consist of LiCl: 34 to 64% by weight and KCl: 66 to 36% by weight, and the desired effect can be obtained in both component ranges. It can be added in an amount of 1 to 20% by weight. Addition of NaCl lowers the melting point of the LiCl-KCl mixed salt and lowers the electrical resistance of the electrolytic bath, which is advantageous in reducing power consumption in the electrolytic process. Within the above range, Na precipitation does not occur even if the NaCl concentration in the electrolytic bath becomes high. but,
When the amount of NaCl added exceeds 20% by weight, the electrical resistance of the bath increases. Moreover, when it is less than 1% by weight, the melting point does not decrease significantly. In the present invention, the cathode current density is 0.005 to 1A/cm ²
shall be. When the cathode current density is increased beyond 1A/ ^cm2 , the amount of precipitated Li floating on the bath surface near the cathode becomes larger than the amount that diffuses into the Al of the cathode.
The alloying yield of Li to cathode Al becomes low. On the other hand,
When the cathode current density is less than 0.005 A/cm ² , the amount of Li precipitated is small, and as a result, the amount of Al-Li alloy produced is reduced, resulting in a decrease in the productivity of the target product. In the present invention, there are two methods for using a solid aluminum-lithium alloy produced by electrolysis as an anode. To explain this with the help of drawings, Fig. 1 explains a method for producing an aluminum-lithium alloy by electrolysis, in which a crucible 2 is provided in an outer cylinder 1 of an electrolytic furnace, and an electrolytic bath 3 is poured into it. A graphite anode 4 and a solid aluminum cathode 7 are placed opposite each other.
The graphite anode 4 is housed in a chlorine gas collection/discharge pipe 5, and an anode lead rod 6 is connected thereto. A cathode lead rod 8 is connected to the solid aluminum cathode 7. In this way, electrolysis is performed under specific conditions using the specific electrolytic bath 3 as the electrolytic bath 3 to convert the cathode into a high-purity aluminum-lithium alloy. Next, as shown in FIG. 2, the aluminum-lithium alloy is transferred to the elution tank, and the anode lead rod 6 is connected to the aluminum-lithium alloy 9 in the crucible 2, and the cathode lead rod 8 is connected to the iron cathode 11. Connect and oppose. Surrounding the iron cathode 11 with a lithium collection tube 13,
By electrolyzing, lithium is eluted from the anode, and the lithium 12 is deposited and collected in the lithium collection tube 13. Figure 3 shows another method.
The aluminum-lithium alloy 9 is placed in the middle of the crucible 2 as a partition wall, and the electrolytic bath is separated into two chambers to enable bipolarization. One anode chamber 1
4 contains an electrolytic bath 3 as defined in claim 1, and as in the case of FIG. Further, an iron cathode 11 is placed in the cathode chamber 15, which is filled with an electrolytic bath 10 defined in claim 2, surrounded by a lithium collection tube 13, and connected to a cathode lead rod 8. When electricity is applied to the graphite anode 4 and the iron cathode 11, the aluminum-lithium alloy 9 in the middle becomes a bipolar electrode with the anode chamber side becoming the cathode and the cathode chamber side becoming the anode, and is electrolyzed to deposit lithium 12 around the iron cathode 11. , collect this. In addition, sodium chloride or calcium chloride in the electrolytic bath during anode elution becomes NaCl + Li = LiCl + Na, CaCl ₂ + 2Li = 2LiCl + respectively due to a chemical reaction with the eluted lithium.
Since a Ca substitution reaction occurs and contaminates the eluted metal, it is required that the concentrations of sodium chloride and calcium chloride in the electrolytic bath be sufficiently small in order to obtain high purity lithium. On the other hand, when the concentration is relatively high, the above-mentioned substitution reaction progresses, and the purity of the initially eluted metal is low, but as a result, the concentration of sodium chloride and calcium chloride in the bath decreases, and when the concentration reaches a certain critical value, After that, the eluted lithium is not contaminated and can maintain high purity. When we set the concentration of sodium and calcium in the lithium grade target as Xppm, the above critical values for sodium chloride and calcium chloride are 10
It was found that it was +0.3Xppm. These baths use expensive lithium chloride or potassium chloride with fairly high purity depending on the target quality, but this bath is not consumed electrolytically and only a small amount of loss is observed due to evaporation or adhesion to the lithium. Since it is only a small part of the manufacturing process, its proportion in the manufacturing cost can be ignored. Furthermore, as mentioned earlier, even if an electrolytic bath containing sodium chloride or calcium chloride in an amount exceeding this critical value is used, the concentration will attenuate as lithium is eluted, and after a certain amount of precipitation reaches a critical value, Lithium of target quality can be obtained. Examples The present invention will be described in detail with reference to examples. Example 1 An aluminum-20wt% lithium alloy was produced under the following conditions. Note that the mother alloy composition in the table indicates the average lithium concentration in the entire cathode aluminum including the unalloyed portion in the cathode aluminum.

[Table] Next, using the above alloy as an anode and an iron cathode, electrolysis was carried out in an electrolytic bath at a temperature of 450°C with a composition of 50wtLiCl + 50wt% KCl. Impurities in LiCl are
NaCl was 3000ppm and CaCl ₂ was 150ppm. The electrolytic conditions were a cathode current density of 0.4 A/cm ² . The initial purity of precipitated lithium is
Na4300ppm and Ca440ppm, but after the purity stabilized when the impurities in the electrolytic bath reached the critical value.
Na was 10ppm and Ca was 33ppm. Comparative example 1 20wt% aluminum of the anode in Example 1
When graphite was used instead of Li alloy, the precipitated lithium became Na7200ppm and Ca330ppm after the purity stabilized.
It was hot. Example 2 Aluminum obtained in Example 1
Electrolysis was performed using a 20wt% Li alloy as a bipolar electrode as follows. The electrolytic bath is 50wt% LiCl+ in both the anode and cathode chambers.
The composition was 50wtKCl and the temperature was 450℃. Impurities include NaCl2000ppm in the anode chamber,
CaCl ₂ 80ppm, cathode chamber NaCl 13ppm,
CaCl ₂ 20ppm. The electrolytic conditions were anode current density of 0.4A/cm ² , cathode current density of 0.4A/cm ² , and double-electrode anode surface current density of 0.4A/cm 2 .
It was 0.05A/ ^cm2 . Lithium precipitated from the early stage of precipitation contained 10 ppm Na,
It was of high purity with Ca35ppm. Example 3 In Example 2, the impurities in the anode chamber were
The same procedure as in Example 2 was carried out except that NaCl was 300 ppm, CaCl ₂ was 600 ppm, and the impurities in the cathode chamber were NaCl 200 ppm and CaCl ₂ 200 ppm. After the purity of the precipitated lithium stabilized, Na15ppm,
It was of high purity with Ca25ppm. Effects of the Invention According to the present invention, extremely high purity lithium metal can be produced only by an electrolytic process, regardless of the amount of sodium chloride and calcium chloride in the raw material lithium chloride. Since the applied voltage for eluting lithium from the anode is small, the power cost does not increase significantly. In addition, even when a high-purity electrolytic bath is used for anode elution, consumption is extremely small, and low-grade lithium chloride, which has many impurities, can be used as the raw material consumed in electrolysis, making it extremely inexpensive and expensive. Pure lithium can be produced. The high-purity lithium obtained by the present invention is useful as a long-life lithium battery anode active material, alloy additive material, fusion reactor blanket material, etc.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the aluminum-lithium alloy manufacturing process in an example of the present invention, FIG. 2 is an explanatory diagram of the same lithium precipitation process, and FIG. 3 is a modified example of the lithium precipitation process. 1... Electrolytic furnace outer cylinder, 2... Crucible, 3... Electrolytic bath, 4
...graphite anode, 5...chlorine gas collection/discharge pipe, 6
...Anode lead rod, 7...Solid aluminum cathode, 8
... Cathode lead rod, 9 ... Aluminum-lithium alloy, 10 ... Electrolytic bath, 11 ... Iron cathode, 12 ... Lithium, 13 ... Lithium collection tube.

Claims (1)

[Claims] 1. 34 to 64% by weight of lithium chloride and 66% by weight of potassium chloride
A mixed molten salt consisting of ~36% by weight or with addition of 1 to 20% by weight of sodium chloride to the mixture of both components, using solid aluminum as the cathode, at a cathode of 0.005 to 1 A/cm ² A solid aluminum-lithium alloy produced by electrolyzing at a current density within a range where metal does not float on the bath surface around the cathode is used as an anode, and lithium is eluted from the anode and deposited around the cathode where it does not react with or alloy with lithium. A method for producing high-purity lithium, which comprises collecting the lithium. 2 Anode elution is performed using 34 to 64% by weight of lithium chloride and potassium chloride, where the sodium chloride concentration and calcium chloride concentration in the mixed molten salt are 10+0.3Xppm or less, respectively, when the sodium and calcium concentrations in the lithium grade target are Xppm. A method for producing high-purity lithium according to claim 1, which is carried out using a mixed molten salt comprising 66 to 36% by weight. 3 For anode elution, set the alloy anode current density to 0.005
2. The method for producing high-purity lithium according to claim 1 or 2, wherein the method is carried out at a temperature of 1 A/cm ² and within a period during which the anode surface does not transfer to aluminum. 4. The method for producing high-purity lithium according to claim 1, wherein the anode is a partition wall or a diaphragm serving as a bipolar electrode that separates an anode chamber and a cathode chamber. 5. The electrolytic bath according to claim 4, in which the electrolytic bath is the mixed molten salt described in claim 1 in the anode chamber, and the mixed molten salt described in claim 2 in the cathode chamber. Method for producing pure lithium.