JPH0158131B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing easily sinterable high-purity fine magnesia powder. Because high-purity magnesia exhibits excellent heat resistance, electrical insulation, and translucency, it is widely used in magnesia-based new ceramic products such as high-grade magnetic products, electrical insulation elements, heat-resistant and translucent materials, and infrared-transmissive materials. There is. Magnesia-based new ceramic products generally use magnesia (MgO) powder as the main raw material and are manufactured by sintering it. In this case, it is well known that the finer the particle size of the magnesia powder raw material, the higher the bulk density of the sintered body, and a product with excellent properties can be obtained. Magnesia powder obtained by calcining magnesium hydroxide or basic magnesium carbonate is used. For example, a method for obtaining high-purity fine powder magnesia by adding magnesium chloride to an aqueous sodium carbonate solution to precipitate magnesium carbonate, aging it, and then calcining the synthesized basic magnesium carbonate was researched by the National Institute for Inorganic Materials, Science and Technology Agency. Report No. 30, 5~
12 (1982). However, since magnesia has a high melting point of 2800℃, in order to obtain a sintered body with high bulk density,
Usually, a firing temperature of 1,700°C or higher is required, and special firing methods such as hot pressing must be used. However, the special firing method described above has the disadvantage of high production costs, so LiF,
The sintering temperature can be lowered by adding sintering aids such as MgF ₂ or NaF, or by treating the surface with benzene etc. to improve the sinterability of magnesia powder.
A method is used in which a magnesia sintered body is obtained by lowering the temperature to 1400 to 1600°C. For example, a method using LiF as a sintering aid is disclosed in Japanese Patent Publication No. 16246/1983, and
A method using MgF ₂ is disclosed in JP-A-50-153798. Also, the method of using NaF as a sintering aid is described by M.Banerjee & DWBudworth:
Trans.Brit.Ceram.Soc., 71 (3), 51-53 (1972). However, the method of adding a sintering aid has the drawback that the purity of the sintered body is reduced, thereby impairing the properties of high-purity magnesia, and limiting its use. Regarding the method of treating the surface of magnesia powder with benzene to improve sinterability,
It is disclosed in Publication No. 16108. Furthermore, O. Yamaguchi, H. Tonami & K.
Shimizu: Chem.Left., 8 , 799-802 (1970) also reports that magnesia produced by calcining magnesium hydroxide synthesized by an alkoxide method has excellent sinterability. However, these methods have a disadvantage that the manufacturing cost of the resulting magnesia powder is high because the steps are extremely complicated. In addition, in all of the above methods, magnesia powder obtained by calcining magnesium salt is used, but such magnesia powder undergoes crystal growth during calcination, and as the particle size increases, it also deteriorates. Since agglomerates are formed, a step of pulverizing them is required in order to use them as raw materials for sintering. However, the magnesia powder obtained by calcining such a magnesium salt cannot be crushed below a certain limit even by mechanical crushing, and agglomerates also remain. These reduce the packing density of the magnesia powder, making it difficult to produce a sintered body with a high bulk density, and causing deterioration of its properties. Furthermore, when performing mechanical pulverization, contamination of impurities is unavoidable, and this pulverization process is undesirable because it causes a decrease in the purity of the magnesia powder. On the other hand, it is well known that magnesia powder is produced by burning magnesium in the presence of oxygen, and there have already been reports on a method for producing magnesia powder by a gas phase reaction method that utilizes this reaction. Specifically, Ciekoslovakia Patent Publication No. 139208 discloses that magnesium is heated in an inert gas atmosphere to generate magnesium vapor, and the magnesium vapor and oxygen are brought into contact with each other in a countercurrent flow to oxidize the magnesium. A method for producing high purity magnesia powder is disclosed which comprises producing magnesia. The magnesia powder obtained using this method is described as having a particle size of 1 μm or less. However, by this method, it is difficult to obtain fine particulate magnesia powder with a particle size of 0.1 μm or less, for example. The present invention provides a method for producing fine magnesia powder with extremely small particle size and high purity. Step of introducing magnesium vapor into the oxidation reaction section; (2) In the oxidation reaction section, the magnesium vapor and the oxygen-containing gas are mixed at a temperature where the oxygen partial pressure of the oxygen-containing gas is at least twice the magnesium vapor partial pressure, and the reaction temperature is A step of oxidizing magnesium to produce magnesia by bringing them into contact with each other in parallel flow at a temperature of 1200 to 2000°C; and (3) a step of collecting the produced fine magnesia powder in a fine magnesia powder collecting section. A method for producing easily sinterable high-purity magnesia fine powder characterized by comprising the following. Next, the manufacturing method of the present invention will be explained in more detail using an example of a manufacturing method using the apparatus shown in the accompanying drawings. The magnesia fine powder manufacturing equipment shown in Fig. 1 is as follows:
It consists of a magnesium evaporation section A, an oxidation reaction section B, and a collection section C for the generated fine magnesia powder. The evaporation section A and the oxidation reaction section B have a tubular structure in which the outside is made of a porcelain tube 1, and the porcelain tube 1 can be heated in an electric furnace 2.
The evaporator A is provided with a magnesium vapor outlet in substantially only one direction, and the other parts are substantially sealed, and the metal magnesium charged inside is appropriately heated to produce magnesium vapor. The magnesium evaporator is provided with a structure that can prevent oxygen-containing gas such as air from entering the evaporator by generating . As the above-mentioned magnesium evaporator, a generator/distillation apparatus commonly called a retort can be used. The retort is indicated by (3) in FIG. 1 (hereinafter, when the term "retort" is used without particular limitation, the above-mentioned structures are collectively referred to). If the magnesium evaporator has two or more openings in different directions and oxygen-containing gas mixes or flows into the evaporator through these openings, nitrides, for example, may be present on the surface of the molten magnesium. This is undesirable because a film is formed which inhibits the occurrence of magnesium evaporation, and impurities such as magnesium nitride are mixed into the produced fine magnesia powder, reducing the purity of the magnesia. The raw material magnesium metal 5 needs to be heated in the retort 3 to a temperature higher than the boiling point of magnesium (1107° C. at normal pressure). If the heating temperature is lower than the boiling point of magnesium, that is, 1107°C at normal pressure, oxygen-containing gas will enter from the magnesium steam outlet 4 of the retort because the vapor pressure of magnesium is low, and for example, magnesia will be absorbed near the steam outlet of the retort. is generated and may clog the steam outlet 4, which is undesirable. Metallic magnesium 5 is charged into the retort 3, heated in the electric furnace 2, and turned into steam, which is supplied to the oxidation reaction section B through the magnesium steam outlet 4. The magnesium vapor supplied to the oxidation reaction section B comes into contact with the oxygen in the gas supplied to the oxidation reaction section B from the oxygen-containing gas supply path 6 and is oxidized to produce magnesium oxide (MgO: magnesia).
Note that it is advantageous to use air as the oxygen-containing gas. However, since fine magnesia powder has extremely high activity and tends to adsorb carbon dioxide and water vapor, it is preferable to remove water vapor and carbon dioxide from the air used in advance. In addition to the above-mentioned air, the oxygen-containing gas may include a mixture of air and another gas, oxygen alone, and a mixture of oxygen and an inert gas. It is necessary that the magnesium vapor discharged from the magnesium vapor outlet 4 of the retort in the oxidation reaction section and the oxygen-containing gas supplied from the oxygen-containing gas supply path 6 be brought into contact with each other in parallel flow. That is, the steam outlet 4 of the retort needs to be open toward the collection section C of the fine magnesia powder. When these are brought into contact in countercurrent flow, the reaction continues under a high partial pressure of magnesium vapor, which increases the particle size of the produced magnesia powder, which is not preferable for the purpose of the present invention. In addition, in the case of countercurrent contact, the flow lines between the magnesium vapor and the oxygen-containing gas are disturbed, and the magnesia powder product adheres to the tube wall of the oxidation reaction section, resulting in a decrease in the collection rate of the produced magnesia powder. There is also. It is necessary to adjust the oxygen partial pressure in the oxidation reaction section to be at least twice the magnesium vapor partial pressure. This partial pressure can be easily adjusted by controlling the evaporation rate of magnesium and the flow rate of the oxygen-containing gas. If the oxygen partial pressure is lower than twice the partial pressure of magnesium vapor, the particle size of the produced magnesia powder becomes large, which is not preferable. The temperature of the oxidation reaction section, that is, the magnesium oxidation reaction temperature, needs to be 1200 to 2000°C. The oxidation reaction of magnesium is an exothermic reaction.
Generates 6080cal/g of heat. For this reason, the magnesium combustion temperature is usually adjusted by controlling the evaporation rate of magnesium, the flow rate of gas, and the temperature of the oxidation reaction section. If this temperature exceeds 2000℃, the particle size of the magnesia produced will increase,
This is not preferable, and if the temperature is lower than 1200°C, suboxides are generated, which is not preferable. The fine magnesia powder produced by the reaction under the above conditions is collected by the filter 7 in the collection section C along with the flow of the oxygen-containing gas. As mentioned above, fine magnesia powder has extremely high activity, and when it comes into contact with air, it tends to adsorb carbon dioxide and water vapor in the air. For this reason, it is desirable to attach a cover 8 to the collection section C to block air. Further, the above reaction is usually carried out under reduced pressure or normal pressure, but it is preferable to maintain the pressure inside the reactor constant using an exhaust pump 9 or the like. When the reaction is carried out under reduced pressure, the magnesium heating temperature in the evaporation section can of course be set lower than that under normal pressure. The fine magnesia powder obtained by the production method of the present invention consists of very fine particles with a particle size (BET diameter) of 0.1 μm or less. This fine magnesia powder is obtained as periclase crystals that maintain a cubic shape, and almost no secondary aggregates are observed. Further, the purity of the produced magnesia is comparable to the purity of the magnesium used as a raw material. The technology for obtaining high-purity magnesium (purity of 99.9% or higher) is now commonplace, and high-purity magnesium is easily available as a commercial product, so the present invention uses such high-purity magnesium as a raw material. By carrying out the manufacturing method described above, easily sinterable high purity magnesia fine powder can be easily manufactured. Moreover, because of its extremely small particle size, it can be used at relatively low temperatures (e.g. 1400 m
It is possible to form a dense sintered body (for example, a bulk density of 3.50 or more, a relative density of 98.7% or more) at a temperature of about 3.5°C or higher. Furthermore, the present invention allows oxidation in the oxidation reaction section to be carried out using air, which simplifies the structure of the production equipment, significantly lowers the production cost of high-purity fine powder magnesia, and therefore produces high-quality powdered magnesia. magnesia-based neuceramics products,
For example, high-grade magnetic products, electrically insulating elements, heat-resistant, light-transmitting materials, infrared-transmitting materials, and the like can be supplied to the market at low prices. Next, Examples and Comparative Examples of the present invention will be shown. Examples 1 to 3 Purity was determined using the apparatus shown in Figure 1 of the attached drawings.
Using 99.9% magnesium and air from which water vapor and carbon dioxide have been removed, the heating temperature of magnesium in the retort is 1200°C, and the air flow rate is adjusted so that the magnesium heating temperature in the oxidation reaction section is the temperature shown in Table 1. The oxidation reaction of magnesium was carried out by adjusting the temperature of the oxidation reaction section. The reaction was continued until generation of magnesium vapor was no longer observed. Table 1 shows the particle size (BET diameter) of the produced fine magnesia (MgO) powder. The produced magnesia fine powder was a periclase crystal and had a purity of 99.9%. Furthermore, the collection rate of fine magnesia powder in the collection section was over 90%.

[Table] Examples 4 to 6 Using the same equipment and raw materials as Examples 1 to 3,
The air flow rate was set to 30 ml/min, the oxygen partial pressure/magnesium partial pressure ratio was set to 3.0, and the heating temperature by the electric furnace was adjusted, and the heating temperature of magnesium in the retort and the combustion temperature of magnesium vapor in the oxidation reaction section are shown in Table 1. Magnesium was oxidized under the same conditions as Examples 1 to 3 except for the temperature. Table 2 shows the particle size (BET diameter) of the produced fine magnesia powder. The produced magnesia fine powder was a periclase crystal and had a purity of 99.9%. Furthermore, the collection rate of fine magnesia powder in the collection section was over 90%.

[Table] Examples 7 to 9 Using the same equipment and raw materials as Examples 1 to 3,
Example except that the air flow rate was 50 ml/min, the oxygen partial pressure/magnesium partial pressure ratio was 5.0, and the heating temperature in the electric furnace was adjusted so that the combustion temperature of magnesium vapor in the oxidation reaction section was set to the temperature shown in Table 3. Magnesium was oxidized under the same conditions as in 1 to 3. Table 3 shows the particle size (BET diameter) of the produced fine magnesia powder. The produced magnesia fine powder was a periclase crystal and had a purity of 99.9%. Furthermore, the collection rate of fine magnesia powder in the collection section was over 90%.

[Table] Examples 10 to 12 Particle diameters obtained in Examples 1, 3, and 7 (BET diameter)
High-purity fine powder magnesia of 0.10, 0.06, and 0.10 μm was first molded using a mold at a pressure of 100 Kg/cm ² and then hydrostatically molded at a pressure of 2000 Kg/cm ² .
Table 4 shows the bulk density and relative density of the magnesia sintered body obtained by firing this molded body in vacuum (10 ^-4 torr) at 1400° C. for 3 hours.

[Table] Comparative Example 1 Magnesium with a purity of 99.9% was evaporated using a combustion boat instead of the retort for magnesia evaporation in the high-purity fine powder magnesia production equipment shown in Figure 1, and water vapor and carbon dioxide were removed from the air. Air oxidation experiments of magnesium were carried out by contacting with. At this time, the heating temperature of the combustion boat for magnesium evaporation was 1200℃, and the temperature of the oxidation reaction section was 1000℃.
℃, and the air flow rate was 2 ml/min. Note that this experiment was terminated when no magnesium vapor was observed to be generated. the result,
The particle size (BTE diameter) of the magnesia powder collected in the collection section was 0.80 μm, and the collection rate was 30%. Furthermore, the obtained magnesia powder contained 1% magnesium nitride. On the other hand, magnesium nitride was formed on the surface of the magnesium in the combustion boat, and the evaporation of the magnesium was inhibited. Comparative Example 2 A counter-current magnesia powder manufacturing device was used, which was constructed by swapping the positions of the oxygen-containing gas supply path 6 and the collecting section C of the parallel-flow magnesia fine powder manufacturing device shown in Fig. 1. Using 99.9% pure magnesium and air from which water vapor and carbon dioxide have been removed, magnesium is heated to an evaporation temperature of 1200°C.
℃, the temperature of the oxidation reaction section was 1000℃, and the air flow rate was 20ml/min. When the oxidation reaction of magnesium was carried out, the particle size (BET diameter) was
Magnesia powder of 1.0 μm was obtained. In addition, most of the magnesia powder that was not collected in the collection section adhered to the wall of the oxidation reaction section, and its average particle size (BET diameter)
was 1.5 μm.

[Brief explanation of drawings]

FIG. 1 shows an example of a high-purity magnesia fine powder manufacturing apparatus that can be used to implement the present invention. A: Magnesium evaporation section, B: Oxidation reaction section,
C: Collection part for generated fine magnesia powder, 1: Porcelain tube, 2: Electric furnace, 3: Retort, 4: Magnesium vapor outlet, 5: Magnesium metal, 6: Oxygen-containing gas supply part, 7: Filter , 8: Cover, 9: Exhaust pump.

Claims (1)

[Claims] 1 (1) A step of evaporating metallic magnesium in a magnesium vapor atmosphere in a magnesium evaporation section and introducing the generated magnesium vapor into an oxidation reaction section; (2) A step of evaporating the magnesium metal in a magnesium evaporation section; Magnesium is oxidized by bringing steam and oxygen-containing gas into contact with each other in parallel flow under conditions where the oxygen partial pressure in the oxygen-containing gas is at least twice the magnesium vapor partial pressure and the reaction temperature is 1200 to 2000°C. and (3) collecting the generated magnesia fine powder in a magnesia fine powder collecting section. Law. 2. The process of evaporating metallic magnesium and introducing the generated magnesium vapor into the oxidation reaction part is performed using magnesium having a structure in which a magnesium vapor outlet is provided in only one direction and the other parts are substantially sealed. The method according to claim 1 is carried out by a method of heating magnesium to a temperature higher than the boiling point of magnesium in an evaporator and discharging the generated magnesium vapor from an outlet of the evaporator. A method for producing sinterable high-purity fine magnesia powder. 3. The method for producing easily sinterable high-purity fine magnesia powder according to claim 2, wherein the magnesium evaporator is a retort.