JPH0130766B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical field to which the invention pertains] The present invention relates to lithium aluminate (LiAlO ₂ ). In other words, the above electrolyte tile contains alkali carbonate (e.g. Li ₂ CO ₃ −
The present invention particularly relates to _a method for producing LiAlO ₂ powder, which is _the raw material for this matrix. [Prior art and its disadvantages] LiAlO ₂ has three types of crystal forms: α, β, and γ, but γ-LiAlO ₂ is considered to be the stable phase at high temperatures, and both α and β-LiAlO ₂ are It is known that it irreversibly transforms to γ-LiAlO ₂ at high temperatures. In addition, the crystal form and primary particle size of LiAlO ₂ generated depending on the type of starting materials, heating conditions, etc.
Products with different shapes etc. can be obtained. Particle shapes of LiAlO ₂ are known to be lump-like, plate-like, rod-like, and others. On the other hand, LiAlO ₂ as a matrix used in molten carbonate fuel cells is required to have the following properties. (b) Excellent ability to retain electrolyte (fine and high specific surface area). (b) Excellent alkali resistance (low solubility in molten carbonate). (c) Excellent heat resistance (no change in crystal morphology or grain growth for long periods even in molten carbonate). (d) An electrolyte tile with excellent thermal cycle performance can be obtained. Generally, the electrolyte retention performance of the matrix and the thermal cycle performance of the electrolyte tile are said to be greatly influenced by the particle shape of the LiAlO ₂ powder, which is the raw material for the matrix.
LiAlO ₂ is said to be preferred. This is Tokukai Akira
As stated in Publication No. 53-140300, it has a felt-like structure in which rod-shaped crystal particles are randomly intertwined, resulting in high retention performance for molten electrolyte and excellent stability against thermal cycles. It is said that Therefore, in order to meet the above-mentioned requirements, it is desirable to use γ-LiAlO ₂ that has a rod-like particle shape and has a high specific surface area. β-
Although LiAlO ₂ is known, there has been no report that γ-LiAlO ₂ having a rod-like particle shape was obtained. Moreover, the conventional method for producing rod-shaped β-LiAlO ₂ also has problems in terms of its properties or production technology, as will be described later. Various methods for producing LiAlO ₂ have been developed, but
The main methods include the following. (a) Carbonate mixing method: Li ₂ CO ₃ −K ₂ CO ₃ −Al ₂ O _3-3
The final composition of the components after synthesis is
A method of synthesis by heating after mixing to achieve a Li ₂ CO ₃ −K ₂ CO ₃ −LiAlO ₂ ratio (b) Method using lithium hydroxide (LiOH):
LiOH−Al ₂ O ₃ (or add KOH to this)
Method of synthesis from a system mixture (c) Chloride method: A method in which LiOH and Al ₂ O ₃ (or aluminum hydroxide) are reacted in a molten alkali chloride Among these methods, (a) This method has the advantage of requiring fewer manufacturing steps because there is no need to mix an electrolyte afterwards, but so far, rod-shaped LiAlO ₂ has been obtained using carbonate as the lithium source. There are no reports. In the method (b), a slurry of LiOH-KOH-Al ₂ O ₃ mixture with water is evaporated to dryness, and then this is heated to 450°C in the air or the sample that has been evaporated to dryness is carbonated. By keeping it at room temperature for 6 hours in a gas atmosphere and then heating it to 600℃,
Rod-shaped β-LiAlO ₂ with a specific surface area of 12 m ² /gr
is said to be obtained. However, the specific surface area of LiAlO ₂ obtained by this method is 10
m ² /gr, which is not sufficient in terms of electrolyte retention performance, and furthermore, the need to create a CO ₂ atmosphere during the manufacturing process poses problems in terms of manufacturing costs, mass productivity, etc. On the other hand, method (c) is a method developed by General Electric Company of the United States. In this method, reactant: flux = 45:55 (weight ratio), LiOH/Al ₂ O ₃ 3H ₂ O =
1.02 (equivalent ratio) and NaCl:KCl=1:1 (molar ratio) at 650 to 700℃, LiAlO ₂ mainly consisting of β,
It is also said that γ-LiAlO ₂ is generated at temperatures above 800°C, but the particle shape of β-LiAlO ₂ is an aggregate.
Furthermore, there is no description of the particle shape of γ-LiAlO ₂ . In addition, the present inventors have developed a method of using a LiCl-KCl mixture, which melts at a lower temperature than the NaCl-KCl system, as a flux as an improved method of using chloride as a fluxing agent. minute details
Patent pending for manufacturing method of LiAlO ₂ (Japanese Patent Laid-Open No. 1983
-45113), but even in this case, it was not possible to obtain LiAlO ₂ having a rod-like particle shape when the synthesis temperature was in the range of 360 to 900°C. On the other hand, as another method to obtain rod-shaped β-LiAlO ₂ ,
The following method is proposed in Japanese Patent Publication No. 53-140300. First, from an aqueous solution containing lithium ions and aluminum ions, Li ₂ O−Al ₂ O ₃ −
Lithium hydroxoaluminate represented by nH ₂ O is precipitated, washed and dried, and then heat treated at 200 to 800°C to obtain plate-like or microcrystalline β-LiAlO ₂ .
This is then heat-treated in an alkali carbonate melt at 500 to 800°C to obtain rod-shaped β-LiAlO ₂ . However, the method of the above patent includes a step of forming a precipitate, and when forming a precipitate, 50~
Although it is necessary to maintain a predetermined temperature between 100℃, this method depends on the properties of LiAlO ₂ obtained depending on the stirring conditions and temperature when forming the precipitate, especially the porosity, Li/Al ratio, and particle size. There is a drawback that it is difficult to obtain a powder with highly reproducible and homogeneous properties because of slight changes in the properties. Furthermore, in the patent, plate-like or microcrystalline β-
It is said that rod-shaped β-LiAlO ₂ can be obtained by heat-treating LiAlO ₂ in molten alkali carbonate at 500-800°C, but plate-shaped or microcrystalline β-
When LiAlO ₂ is heated in molten alkali carbonate,
It is said that fine particles of about 0.2μ to 0.6μ change into lumpy particles of several μ in a few tens of hours, and at the same time a significant decrease in specific surface area occurs, and the above method has a high specific surface area. Obtaining fine rod-like LiAlO ₂ is quite difficult. [Object of the Invention] The object of the present invention is to eliminate the drawbacks of the conventional methods as described above, and to provide a new manufacturing method that is easy to manufacture and can obtain LiAlO ₂ having a rod-like particle shape and a high specific surface area. It is about providing. [Key Points of the Invention] High specific surface area and rod-like particle shape
As a result of various studies on the improved chloride method using the LiCl-KCl-based flux shown in JP-A No. 58-45113 due to the presence of LiAlO ₂ , the present inventors found that by the method described below, It has been found that LiAlO ₂ having a rod-like particle shape can be obtained not only of the conventionally known B type but also of the γ type, which has not been reported so far. The method for obtaining rod-shaped LiAlO ₂ consists of the following three steps. In the first step, β-LiAlO ₂ in which plate-shaped particles are aggregated is heated by heating a flux consisting of a LiOH-Al ₂ O _3- based reactant and an alkali metal chloride above the melting temperature of the flux. Generate. In the next second step, the fluxing agent and excess LiOH are removed. In the third step, the obtained β-LiAlO ₂ is heated at 550℃
Heat treatment is performed at the above temperature to obtain rod-shaped LiAlO ₂ . Here, the synthesis temperature in the first step needs to be within a temperature range in which β-LiAlO ₂ is produced, as shown in the examples below. When the main product in the first step is γ- _LiAlO2 , the heat treatment in the third step is
LiAlO ₂ having a rod-like particle shape cannot be obtained even if carried out in the temperature range of ~1000°C. As a result of various studies conducted by the present inventors using a flux made of an alkali metal chloride, the lower limit of the reaction temperature at which γ-LiAlO ₂ is substantially observed in the presence of a flux was 650°C. Therefore, the synthesis temperature in the first step needs to be 650°C or lower. Also, from the above, the flux used in the first step is
Any mixture of alkali metal chlorides with a melting point of 650℃ or less is sufficient, and LiCl− with a melting point of 350℃ is sufficient.
Fluxing agents such as KCl-based or NaCl-KCl-based with a melting point of 650°C can be used, but in order to obtain LiAlO ₂ with a high specific surface area, it is necessary to use a flux that melts at the lowest possible temperature and react at a low temperature. It is better to perform this, and from this point of view, LiCl-KCl system is more preferable. On the other hand, the temperature of the heat treatment in the third step is 550 to 1000℃.
is preferred. At temperatures below 500°C, the rate at which the particle shape changes to rod-like particles is extremely slow, and at temperatures above 1000°C, the specific surface area decreases significantly, which is undesirable. Here, the temperature of the heat treatment in the third step is 550 to 700.
In the temperature range of 0.degree. C., the crystal morphology of LiAlO ₂ remains B-type, and only the particle shape changes to a rod-like shape. Moreover, when the temperature of the heat treatment is 750° C. or higher, LiAlO ₂ having a γ-type and rod-like particle shape can be obtained. As described above, according to the method of the present invention, not only rod-shaped β-LiAlO ₂ can be obtained by simply heat-treating β-LiAlO ₂ consisting of aggregates of plate-shaped particles, but also
Rod-shaped γ-
It has also become possible to newly manufacture LiAlO ₂ . The present inventors have also found that β-LiAlO ₂ obtained by a method other than the method using a flux as described above has a rod-like particle shape by heat treatment.
We investigated whether LiAlO ₂ could be obtained. The results are shown in the comparative example below. In the comparative example, LiOH−
This study investigated a method for obtaining β-LiAlO ₂ from an Al ₂ O ₃ mixture. In this method, fine particles of β-
LiAlO ₂ is obtained, but even if it is heat-treated in the range of 500 to 1000℃, it has a rod-like particle shape.
LiAlO ₂ could not be obtained. In this way, β-LiAlO ₂ synthesized from a LiOH-Al ₂ O ₃ mixture without a flux does not yield rod-shaped LiAlO ₂ even if it is heat-treated; Although the reason why rod-shaped LiAlO 2 is obtained by heat treatment in the case of β-LiAlO ₂ synthesized using 3-LiAlO ₂ is not yet clear, the present inventors estimate that the change to rod-shaped particles due to heat treatment is due to This is complicated due to factors such as the crystal morphology, particle shape, and particle size of LiAlO ₂ before heat treatment, and the fact that trace amounts of chloride, lithium hydroxide, etc. that remain even after cleaning act as flux at the particle interface. It is thought that this is affecting the [Examples of the Invention] The contents of the present invention will be explained in more detail below based on Examples. Example ₁ Lithium hydroxide monohydrate (LiOH・H ₂ O) and aluminum oxide (γ-
Al ₂ O ₃ ), and lithium chloride (LiCl) and potassium chloride (KCl) were prepared as fluxing agents.
These raw materials were weighed so as to have the following blending ratio. Reactant: Fluxing agent = 45:55 (weight ratio) LiOH・H ₂ O/γ-Al ₂ O ₃ = 1.02 (equivalent ratio) LiCl: KCl = 58:42 (molar ratio) Ethyl alcohol was added to the mixture. The mixture was made into a slurry and mixed in a ball mill for 17 hours.
Next, after removing the ethyl alcohol in the slurry using a vacuum dryer, the slurry was placed in a high-purity alumina crucible and heated to 360-800°C at a heating rate of 100°C/H in an electric furnace.
The temperature was raised to .degree. C., kept heated for 1 hour, and then allowed to cool to room temperature. The sample obtained by the above operation was
It is a mixture of LiAlO ₂ , alkali chloride and Li ₂ O which is the residue of LiOH-H ₂ O added in slight excess. Therefore, next, the sample was washed in order to separate and collect only LiAlO ₂ . That is, the above mixture was dispersed in excess ion-exchanged water to dissolve water-soluble components (alkali chloride, Li ₂ O). After stirring for a certain period of time, the mixture was allowed to stand, and the supernatant was thoroughly washed by the so-called decanting method, followed by filtration. In order to remove residual ions such as Li ⁺ , K ⁺ , and Cl ^-, the product was further thoroughly washed with water on paper, and then, after separating LiAlO ₂ , it was dried in a dryer at 150° C. for 10 hours. Table 1 shows the results of investigating the specific surface area, crystal morphology, and particle shape of this dried product. As you can see from Figure 1, the synthesis temperature is between 360 and 600.
In the temperature range, β-LiAlO ₂ consisting of aggregates of fine plate-like particles of 0.5 μm or less was obtained and had a specific surface area of 40 to 80 m ² /gr. Also, at temperatures above 700℃,
γ- consists of aggregates of fine particles of 0.5μ or less
It was _LiAlO2 . Next, among the LiAlO ₂ obtained in this way, three types of synthesis temperatures of 400°C, 550°C, and 700°C were heated to 500 to 1000°C at a heating rate of 100°C/H, and kept at the specified temperature for 24 hours. After being maintained, it was allowed to cool to room temperature. FIG. 2 shows the results of investigating the specific surface area, crystal morphology, and particle shape of LiAlO ₂ obtained by this operation. As shown in Figure 2, β-LiAlO ₂ is composed of aggregates of plate-like particles obtained at a synthesis temperature of 600°C or lower.
It can be seen that when heat-treated at a temperature range of 550 to 1000°C, it changes to LiAlO ₂ having a rod-like particle shape with a diameter of about 0.2 to 0.4 μ and a length of about 1 to 3 μ.
Moreover, the crystal morphology of the obtained rod-shaped LiAlO ₂ was B type when the heat treatment temperature was 700°C or lower, but it was γ type when the heat treatment temperature was 750°C or higher. Figure 3 shows an electron micrograph of β-LiAlO ₂ composed of aggregates of plate-like particles obtained at a synthesis temperature of 550°C.
In addition, Fig. 4 shows an electron micrograph of rod-shaped β-LiAlO ₂ obtained by heating the β-LiAlO 2 shown in Fig. 3 at 700°C for 24 hours, and Fig. 5 shows an electron micrograph of the β-LiAlO ₂ shown in Fig. 3.
An electron micrograph of rod-shaped γ-LiAlO ₂ obtained by heat-treating LiAlO ₂ at 800° C. for 24 hours is shown. Example 2 LiOH・H ₂ O− consisting of the blending ratio shown below
An Al ₂ O ₃ .3H ₂ O (gypsite)-NaCl-KCl mixture was prepared, and two types of LiAlO ₂ having different synthesis temperatures were prepared by a method similar to Example 1. Reactant: Fluxing agent = 45:55 (weight ratio) LiOH・H ₂ O / Al ₂ O ₃・3H ₂ O = 1.02 (equivalent ratio) NaCL:KCl = 1:1 (molar ratio) Synthesis conditions: 650℃× 1 ^H , 800°C x 1 ^H After removing the fluxing agent in the same manner as in Example 1.
The properties of LiAlO ₂ are shown in Table 1.

[Table] As shown in Table 1, even when a NaCl-KCl-based flux is used, β-LiAlO ₂ consisting of aggregates of plate-like particles similar to Example 1 can be obtained at a synthesis temperature of 650°C. It can be seen that the specific surface area is smaller than that when LiCl-KCl-based flux is used. Next, these two samples were heat-treated in the same manner as in Example 1, and the properties of LiAlO ₂ obtained are shown in Table 2.

〔Effect of the invention〕

As described above, according to the method of the present invention, LiAlO ₂ having a rod-like particle shape and a high specific surface area can be obtained without using a precipitation method with complicated operating conditions. Furthermore, in addition to the conventionally known rod-shaped β-LiAlO ₂ , the previously unknown rod-shaped γ-
LiAlO ₂ could also be obtained, making it possible to produce electrolyte tiles for molten carbonate fuel cells that have better thermal cycle performance and electrolyte retention performance than conventional ones. In addition to the electrolyte matrix for molten carbonate fuel cells, the method for producing LiAlO ₂ powder of the present invention can also be applied to
LiAlO ₂ sintered bodies are used in parts that require excellent performance such as thermal cycle performance and mechanical strength, and can also be applied to the ceramic field, such as crucibles for heat treating molten salt, and corrosion-resistant bricks. It is possible.

[Brief explanation of drawings]

Figure 1 shows LiAlO ₂ in the presence of LiCl-KCl flux.
Figure 2 is a diagram showing the relationship between _{the synthesis temperature when synthesizing β-LiAlO 2 and} the properties of the LiAlO ₂ produced. Figure 3 is an electron micrograph of β-LiAlO ₂ in the form of an aggregate consisting of fine plate-like particles.
FIG. 4 is an electron micrograph of β-LiAlO ₂ having a rod-like particle shape, and FIG. 5 is an electron micrograph of γ-LiAlO ₂ having a rod-like particle shape.

Claims (1)

[Claims] 1. A method for producing lithium aluminate (LiAlO ₂ ) powder having a rod-like particle shape, which comprises the following steps. (b) Aluminum oxide (Al ₂ O ₃ ) or aluminum oxide hydrate (Al ₂ O ₃ −nH ₂ O) and lithium hydroxide (LiOH) as raw materials for the synthesis of LiAlO ₂
In addition, a mixture of at least two or more alkali metal chlorides having a melting point of 650°C or lower is used as a fluxing agent for the reaction, and these are mixed at a temperature higher than the melting point of the fluxing agent.
The first step is to generate β-LiAlO ₂ by heating to a temperature of 650°C or lower. (b) A second step of washing and removing chloride, which is a fluxing agent. (c) A third step of heat treating β-LiAlO ₂ powder at a temperature of 550 to 1000°C. 2. In the method described in claim 1,
A method for producing lithium aluminate powder, characterized in that a lithium chloride (LiCl)-potassium chloride (KCl) mixed chloride is used as a flux in the first step.