JPH024554B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a method for producing a porous sintered body of lithium aluminate. (Technical background of the invention and its problems) Conventional ceramic porous bodies include unglazed pottery, foamable glass ceramics, and ceramic foam, although the properties of pores are different. However, when these are used as porous plates for electrolyte tiles for fuel cells, they lack the hardness and density required of ceramics. Furthermore, when the pores are filled with various inorganic substances or electrolytes, there are many cases where the amount of filling is small and the ion permeability and electron conductivity are insufficient. In addition, even if ion permeability and electron conductivity are satisfied, there are drawbacks such as the inorganic substances and electrolytes filled in the pores being washed away from the pores, and the fact that the ceramic skeleton alone does not have sufficient strength. I had a lot of things. When making ceramic sheets, there are injection molding methods, extrusion molding methods, isostatic pressing methods, etc., which do not use papermaking methods. However, when using these methods to make ceramic sheets with a large area of 30 cm square or more, the surface cracks or becomes distorted, making it impossible to obtain a uniform sheet. In addition, in the conventional method of making a porous sintered body, α-alumina is added as a sinterable material powder and 51 to 70% by weight of wood pulp is added as a fibrous material, and after wet kneading and agglomeration, There is a method of obtaining a thin and dense porous body by firing a sheet-like material obtained by paper making. However, in this method, the content of wood pulp is high in order to obtain high porosity.
Furthermore, since α-alumina was used, it was necessary to sinter at a high temperature of 1,500 to 1,600°C to obtain a sintered material. Furthermore, α-alumina and lithium salts such as lithium hydroxide, lithium carbonate, and lithium sulfate are added to
R-lithium aluminate is produced by heating to 1000°C and reacting, then wood pulp and resin are added to this, papermaking is made, the temperature is raised to about 1000 to 1300°C, and porous sintering is made by firing. I found a way to form a body. In this case, firing had to be carried out twice, once when producing the r-lithium aluminate and once when producing the porous sintered body, which had the disadvantage that a considerable amount of thermal energy was required. (Objective of the Invention) The present invention provides a material that is dense, strong, has a high porosity, and has a maze of pores, which is sufficient to eliminate or compensate for the above-mentioned drawbacks. The present invention provides a method for producing a porous sintered body of ceramics. (Summary of the invention) That is, the present invention provides that, for 1 mole of alumina,
0.75 to 2.0 mol of a lithium compound consisting of lithium hydroxide or lithium salt, and 0.25 to 1.0 mol of an alkali inorganic compound consisting of an alkali hydroxide or alkali salt other than the lithium compound, in powder form with this mixing ratio, were added to the water together with the fibers. Stir and mix to make an aqueous slurry, adjust the pH value of the liquid to 6 to 10, add a flocculant to adsorb and agglomerate the powder to the fibers, and make paper.By doing so, the total dry weight before firing is A sheet-like or plate-shaped paper molded product with a fiber content of 3 to 15% by weight is obtained, and after drying, the organic fibers in the molded product are burned and vaporized, and then subjected to normal firing. By sintering the powder to obtain a sintered product, and placing the sintered product in water to elute the alkaline aluminate content, the porosity can be reduced to 40-40.
80% of the porous sintered body of lithium aluminate. (Details of the invention) As the porous sintered body, alumina is dense and
It has high heat resistance and excellent electrical insulation properties. However, when used as an electrolyte tile for fuel cells, the pores of sintered alumina are filled with electrolytes of lithium carbonate and potassium carbonate. Lithium reacts with alumina and turns into lithium aluminate. In the present invention, γ-lithium aluminate, which has the most stable structure among lithium aluminates, is produced by reacting and sintering alumina with a mixture of lithium hydroxide, alkali hydroxide, lithium salt, and alkali salt. After that, the alkali aluminate is eluted leaving the r-lithium aluminate portion, thereby producing a porous sintered body. More specifically, in the method for producing a porous sintered body of lithium aluminate, lithium hydroxide, alkali hydroxide, lithium salt, alkali salt, and alumina are reacted to produce a sintered body of alkali aluminate. make. The lithium salt used here is one or more selected from lithium chloride (LiCl), lithium carbonate (Li ₂ CO ₃ ), lithium nitrate (LiNO ₃ ), and lithium sulfate (Li ₂ SO ₄ ). Used with lithium oxide (LiOH) or each alone. In short, lithium salt or lithium hydroxide is used as a source of lithium.
It is used in an amount of 0.75 to 2.0 mol per mol of alumina. On the other hand, as the alkali component other than lithium, an alkali hydroxide or an alkali salt is used. The alkali hydroxides mentioned here include potassium hydroxide (KOH), sodium hydroxide (NaOH), magnesium hydroxide (Mg(OH) ₂ ),
At least one selected from calcium hydroxide (Ca(OH) ₂ ) is used, and at least one selected from potassium salt, sodium salt, magnesium salt, and calcium salt is used as the alkali salt. These alkali hydroxides or alkali salts are mixed in a proportion of 0.25 to 1.0 mol per 1 mol of alumina, and after becoming a sintered product, they are eluted as alkali aluminates and play a role in forming pores in the porous body. Is Umono. These inorganic compounds and fine alumina, preferably having an average particle size of 1 micron or less, are stirred and mixed in powder form together with fibers in water to form an aqueous slurry. The fibers may include organic fibers such as wood pulp and synthetic resin fibers, as well as inorganic fibers such as asbestos and ceramic fibers. However, since organic fibers are burned out and vaporized during firing, they contribute to increasing the porosity of the porous body and are useful. Further, if inorganic fibers are used, there is an advantage that they not only serve as a carrier for the powder together with the organic fibers during the papermaking stage, but also remain in the sintered product after firing and serve as reinforcement. The aqueous slurry is used to obtain a sheet or plate-shaped paper product containing a large amount of powder by a papermaking method. This aqueous slurry is prepared by using water in an amount of about 4 to 20 times the solid content consisting of the fibers and powder mentioned above. Regarding the pH of this slurry liquid, a pH of about 3 to 11 is generally suitable for coagulation and paper making. However, in the present invention, strong alkaline to neutral materials are used as raw materials for the inorganic sintered body. For this reason, it is not a good idea to adjust the pH toward the acidic side, especially to a pH of 3 to 4, when performing aggregation. Consider flocculants and surfactants and adjust the pH range to 6.
It was made to aggregate at ~10. If the pH is greater than 10, the viscosity of the slurry liquid increases, making it difficult to aggregate, and the filtration rate becomes slow. As the flocculant, it is preferable to use a polyacrylamide-based polymer flocculant, and if an amphoteric surfactant is used in combination with this, the capture rate of powder into the fibers can be increased. By making paper,
The aim is to obtain a sheet-like or plate-like molded product for firing with a fibrous content of 3 to 15% by weight based on the total dry weight. During firing, the organic fiber content in the paper molded product is reduced to 500%.
It burns out and vaporizes at around ~600℃, and is then fired at a firing temperature of 1000~1200℃. At this time, alumina and alkali salts react as follows.

[Table] As a result, the sintered body consists of lithium aluminate (LiAlO ₂ ) and alkali aluminate (NaAlO ₂ ,
KAlO ₂ , MgAl ₂ O ₄ , CaAl ₂ O ₄ ).
In order to leave only lithium aluminate in this mixture, the pH value is set to 6 to 10, preferably 50.
Place the sintered material in hot water at a temperature above ℃. Lithium aluminate is insoluble in the alkaline aqueous solution, so it remains, whereas alkali aluminate (excluding lithium aluminate) dissolves, so it is leached out until the elution is finished. Once the alkali aluminate has leached out, it is washed with water and dried to form a porous sintered body of lithium aluminate. The molar ratio of lithium hydroxide, alkali hydroxide, lithium salts, alkali salts, and alumina is adjusted so that the porosity is 40 to 80% during alkali aluminate elution. That is, if the amount of alkali salt or alkali hydroxide is increased from the set range in the molar ratio of the powder described above, the amount of alkali aluminate will increase, and the porosity will increase after elution. Therefore, the skeletal strength of lithium aluminate decreases, making it susceptible to cracking. Furthermore, if the amount of alkali salt or alkali hydroxide is reduced below the set range, the porosity necessary for the porous plate will be insufficient. Although the explanation has been mixed, when preparing the aqueous slurry liquid for papermaking, the pH of strong alkaline to weak alkaline should be 6 to 6.
To adjust to 10, hydrochloric acid, sulfuric acid, nitric acid, etc. are used. However, when these acids are used, more highly soluble Li salts and alkali salts are produced, increasing the viscosity of the liquid, resulting in agglomeration and poor paper-making efficiency. Therefore, in the present invention, for adjusting the pH of the slurry liquid,
Preferably, this is carried out by blowing _CO2 . This effect is
Li salts and alkali salts turn into carbonates when _CO2 is injected, and because the solubility of the crystals is low, crystals precipitate in the liquid, and the viscosity of the liquid does not increase, resulting in agglomeration with fibers and inorganic sintering raw materials. It also has the effect of good paper-making properties. Furthermore, it is practical to circulate the white water that remains after papermaking, that is, the white water that is dehydrated during papermaking to make sheets. Normally, this white water is treated as wastewater and discharged, but in the present invention, Li is added to the white water.
It contains a considerable amount of water-soluble substances such as salts, alkali salts, and flocculants, so it is necessary to send it to wastewater treatment as is.
Wastewater treatment costs are high, and raw materials are wasted, making it uneconomical. The firing temperature is usually the temperature required for Li salt and alkali salt to react with alumina to form alkali aluminate and lithium aluminate. And the necessary firing conditions for lithium aluminate are 1100
It takes about 1 hour at ~1300℃. However, in the present invention, since alumina, lithium salt, alkali salt, lithium aluminate, and alkali aluminate are mixed, the melting temperature is lowered by the eutectic effect, which naturally has the effect of lowering the sintering temperature. The sintering temperature is 1,000 to 1,200°C, and the process is completed in one hour. When eluting the alkali aluminate to remove lithium aluminate from the sintered product, lithium aluminate is insoluble in water and gradually reacts with the acid and decomposes in an acidic aqueous solution. Alkali aluminate dissolves in neutral, acidic, and alkaline aqueous solutions. In order to produce a porous body of lithium aluminate, an alkali aluminate must be eluted from the sintered product. In the present invention, the alkali aluminate is leached out with an aqueous solution having a pH of 6 to 10 in order to leave the skeleton of the lithium aluminate porous body and obtain a porosity of 40 to 80%. As a result, a porous lithium aluminate sintered body with fine pore diameter and arbitrarily adjusted porosity of 40 to 80% is obtained. As described above, by agglomerating lithium salts, alkali salts, alumina, and fibers, forming paper, firing, washing, and drying, the porosity is as large as 40 to 80%, and the pores become maze-like. It becomes possible to manufacture dense and strong electrolyte tiles for fuel cells. Specific examples of the present invention will be described below. In addition, all compositions are weight ratios. <Example 1> (A) Sample preparation Lithium hydroxide monohydrate 38 parts (manufactured by Kanto Kagaku Co., Ltd., reagent special grade) Sodium hydroxide 32 parts (manufactured by Kanto Kagaku Co., Ltd., reagent special grade) α-Alumina 102 (Manufactured by Sumitomo Aluminum Refining Co., Ltd., average particle size 0.3μ) Wood pulp 10 parts White Water 1000 parts Water 100 parts (B) Coagulant and surfactant Amphoteric surfactant 20% aqueous solution 5 parts (Lion Co., Ltd.) Polyacrylamide-based polymer flocculant (anionic type) 0.1% aqueous solution 60 parts (manufactured by Sanyo Chemical Industries, Ltd., product name "Lipomin LA")
AH-200P") Polyacrylamide polymer flocculant (cationic) 0.1% aqueous solution 60 parts (manufactured by Sanyo Chemical Industries, Ltd., product name: "Sunfloc C")
-009P") First, add 38 parts of lithium hydroxide, 32 parts of sodium hydroxide, and α-alumina to a 300ml ball mill.
Add 102 parts and grind for 48 hours. As a result, the particle size of lithium hydroxide, sodium hydroxide, and α-alumina becomes 0.3μ or less. Next, add 100 parts of water and 1000 parts of white water to a 2-sized container.
and 10 parts of wood pulp, stir for about 20 minutes,
The pulp is thoroughly dispersed and beaten in an aqueous solution, and a mixture of lithium hydroxide, sodium hydroxide and α-alumina that has been crushed in advance is added thereto.
Stir for about a minute to make an aqueous slurry. This aqueous slurry becomes strongly alkaline as some of the lithium hydroxide and sodium hydroxide are dissolved in water. Therefore, the liquefied carbon dioxide gas cylinder containing 40.2 kg was subjected to primary and secondary depressurization to adjust the pressure to 0.2 kg/cm ² g. A blowout nozzle for blowing out fine air bubbles used in an air washing bottle is attached to the carbon dioxide gas outlet, and carbon dioxide gas is blown into the aqueous solution in the two containers containing the aqueous slurry. While stirring the slurry,
Blow in carbon dioxide until the pH is 8 to 9, and the pH is 8.
Check with a PH test paper that it has become ~9. next,
This was also made in advance with 5 parts of an amphoteric surfactant (Lipomin LA 20% aqueous solution), 60 parts of a polyacrylamide polymer flocculant (anionic type: 0.1% aqueous solution of Sunfloc AH-200P), and a polyacrylamide polymer. Add 60 parts of flocculant (cationic: 0.1% aqueous solution of Sunfloc C-009P),
Stir for about 30 seconds to coagulate. The sample agglomerated as described above is made into paper using a paper making machine to form a sheet of 300 mm square and 1.5 to 2.2 mm thick.
This is dried in a dryer at 120°C for 1 to 2 hours and then placed in an electric furnace. The electric furnace is heated in an oxidizing atmosphere or, if necessary, with air flowing, at a rate of 50°C/hour from room temperature, and held for 2 hours at 600°C, the temperature at which the wood pulp is burnt and vaporized. After that, lithium carbonate, sodium carbonate and α produced by reaction with _CO2
-200℃/up to 1100℃ where alumina reacts and lithium aluminate and sodium aluminate sinter.
When the temperature inside the electric furnace reaches 1100℃,
This temperature is maintained for 1 hour to mix and sinter lithium aluminate and sodium aluminate. After sintering, let it cool in the furnace, let it cool down to room temperature, and put the cooled sintered product into a vat of about 600 mm long x 500 mm wide x 40 mm high filled with water. Place it in water and rinse with a small amount of water. At this time, sodium aluminate other than lithium aluminate and excess sodium carbonate that does not participate in the formation of sintered products dissolve in the water and become alkaline. If the washing water becomes alkaline, dissolution and leaching of lithium carbonate and sodium carbonate by sodium aluminate will become worse, so if necessary, use a diluted acidic aqueous solution (PH2
~3) is added to maintain the pH of the washing water at 6-8. PH
Rinse with water until the pH of the test paper reaches 7. When the pH reaches 7, remove it from the vat and dry it in a dryer at 120°C for about 2 hours to remove water. Lithium aluminate (r-lithium aluminate) ) porous plates are manufactured. <Example 2> (A) Sample preparation Lithium chloride 76 parts (manufactured by Kanto Kagaku Co., Ltd., special grade reagent) Calcium sulfate 34 parts (manufactured by Kanto Kagaku Co., Ltd., special grade reagent) γ-Alumina 102 parts (manufactured by Nippon Aerosil ( Co., Ltd., average particle size 0.1 μ or less) Alumina fiber 24 parts (Toshiba Monoflex Co., Ltd., fiber bulk) White 1000 parts Water 200 parts (B) Coagulant and surfactant Amphoteric surfactant 20% aqueous solution 5 Part (manufactured by Lion Corporation, trade name: "Lipomin LA") Polyacrylamide-based polymer flocculant (anionic type) 0.1% aqueous solution 60 parts (manufactured by Sanyo Chemical Industries, Ltd., trade name: "Sunfloc")
AH-200P'') Polyacrylamide polymer flocculant (cationic) 0.1% aqueous solution 60 parts (manufactured by Sanyo Chemical Industries, Ltd., product name: ``Sunfloc C'')
-009P'') Using the above compositions (A) and (B), a porous sintered product of γ-lithium aluminate was obtained in the same manner as in Example 1. <Example 3> (A) Sample preparation Lithium carbonate 74 parts (manufactured by Kanto Kagaku Co., Ltd., reagent special grade) Magnesium nitrate hexahydrate 77 parts (manufactured by Kanto Kagaku Co., Ltd., reagent special grade) α-alumina 102 parts (manufactured by Nikkei Kako Co., Ltd., average particle size 0.4μ) 20 parts of asbestos fiber (manufactured by Carry Corporation (Canada),
AY) White Water 1000 parts Water 100 parts (B) Flocculant and surfactant Amphoteric surfactant 20% aqueous solution 5 parts (manufactured by Lion Corporation, trade name "Lipomin LA") Polyacrylamide polymer flocculant (anion 60 parts of 0.1% aqueous solution (manufactured by Sanyo Chemical Industries, Ltd., product name: "Sunfrost")
AH-200P'') Polyacrylamide polymer flocculant (caotine type) 0.1% aqueous solution 60 parts (manufactured by Sanyo Chemical Industries, Ltd., product name: ``Sunfloc C'')
-009P'') Using the above compositions (A) and (B), a porous body of γ-lithium aluminate was obtained in the same manner as in Example 1. (Effects of the Invention) The ceramic porous body thus obtained has excellent performance as an electrolyte tile used in fuel cells and is also excellent in durability.
Furthermore, since the pores are fine and uniformly distributed, with a diameter of 1 μm or less, it can be applied to catalyst carriers for combustion exhaust gas and molecular sieves. According to the present invention, a ceramic porous body is produced that is thin, dense, strong, has high porosity, and has continuous pores like a labyrinth. The present invention incorporates the advantages of both the conventional method of producing a thin, dense metal oxide without pores and the method of producing a foamable porous material, and the porosity can also be adjusted by adjusting the amount of fiber added. It can be freely changed by controlling the amount of sodium aluminate produced. Furthermore, the present invention simplifies the manufacturing process because the complicated process after foaming with polyurethane, which is a problem in ceramic foam manufacturing, is eliminated. In addition, expansion due to heat, which could not be used conventionally,
Places that undergo repeated shrinkage or are subject to stress, especially thermal stress, such as electrolyte tiles for fuel cells.
It can be used in places where electrical stress and mechanical stress are applied repeatedly. Furthermore, since paper-forming technology is used to form sheet-like objects for firing, it is possible to continuously manufacture sheet-like objects of uniform thickness, and the thickness can also be varied within a variety of ranges. be.

Claims (1)

[Claims] 1. 0.75 to 1 mol of lithium hydroxide or a lithium compound consisting of lithium salt per mol of alumina
2.0 mol and 0.25 to 1.0 mol of an alkali inorganic compound consisting of an alkali hydroxide or an alkali salt excluding the lithium compound in powder form at this blending ratio,
Before firing, the powder is stirred and mixed with the fibers in water to form an aqueous slurry, the pH value of the slurry is adjusted to 6 to 10, and a coagulant is added to adsorb and aggregate the powder to the fibers. A sheet-like or plate-shaped paper molded product is obtained with a fiber content of 3 to 15% by weight based on the total dry weight of the product, and after drying, the organic fibers in the molded product are burned and vaporized. The powder is then sintered by normal firing to obtain a sintered product.
The sintered product is placed in hot water with a pH value of 6 to 10 and a temperature of 50°C or higher to elute the alkali content of aluminate excluding lithium aluminate, thereby increasing the porosity to 40 to 40.
80% of the porous sintered body of lithium aluminate. 2. The method for producing a porous sintered body according to claim 1, wherein the fibrous material is at least one selected from wood pulp, synthetic resin fiber, asbestos, and ceramic fiber. 3. The method for producing a porous sintered body according to claim 1, wherein the lithium salt is at least one selected from lithium chloride, lithium carbonate, lithium nitrate, and lithium sulfate. 4 Alkali hydroxides include KOH, NaOH, Mg
The method for producing a porous sintered body according to claim 1, wherein the porous sintered body is at least one selected from (OH) ₂ and Ca(OH) ₂ . 5 Alkaline salts include potassium salt, sodium salt,
The method for producing a porous sintered body according to claim 1, wherein the porous sintered body is at least one selected from magnesium salts and calcium salts. 6. The method for producing a porous sintered body according to claim 1, wherein the alumina has an average particle size of 1 micron or less. 7. The method for producing a porous sintered body according to claim 1, wherein carbon dioxide gas is blown into the aqueous slurry as a means for adjusting the pH value of the aqueous slurry to 6 to 10. 8. The method for producing a porous sintered body according to claim 1, wherein the firing temperature is 1000 to 1200°C.