JPS6212173B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-purity magnesia powder, and in particular to a method for producing high-purity magnesia powder, in which a magnesia sintered body having a bulk density close to the theoretical density can be obtained without additives at a low temperature of 1400°C. This invention relates to a method for producing magnesia powder.

Magnesia has excellent base resistance and electrical insulation at high temperatures, and has a large coefficient of thermal expansion and thermal conductivity.
Moreover, it has the characteristics of good transmittance to infrared rays and visible light, and low cost. Therefore, it is widely used as a high-temperature heat-resistant material, a high-temperature insulating material, a high-temperature lighting material, etc., and has recently attracted attention as a fine ceramic material. The magnesia sintered body for high-temperature heat resistance is dense and has high hot strength. Magnesia sintered bodies for high-temperature insulation have high resistance to hydration. Magnesia sintered bodies for high-temperature lighting are required to be highly dense and have excellent transparency.

Conventional industrial magnesite (95-98% by weight
MgO) or magnesium hydroxide (97-99 wt% MgO) obtained from seawater, a sintered body with a bulk density of only 95% can be obtained by firing at 1600°C. In addition, (1) magnesia obtained by thermally decomposing magnesium hydroxide by hydrolysis of magnesium alcoholate ^can be densified to 98-99% of the theoretical density by firing at 1400℃, but it is expensive because it uses alcohol. There are also major drawbacks, such as the need for special care in operation. (2) Highly pure magnesia can be obtained using the ion exchange method or solvent extraction method, but this also has the drawback of being extremely expensive, and in order to densify the bulk density to about 95%, it is necessary to High temperature firing is required.

Conventionally, hot pressing, additives, etc. have been used to obtain a highly dense sintered body with a density close to the theoretical density. In the hot pressing method, it is not only difficult to manufacture large-sized or complicated-shaped products, but also requires heat treatment after sintering, which has the drawback of low productivity and high cost.

In addition, with the additive method, the purity of the sintered body decreases,
It has drawbacks such as loss of properties such as hot strength and high temperature insulation.

An object of the present invention is to provide a method for producing high-purity magnesia powder for high-density sintered bodies used in high-temperature heat-resistant materials, high-temperature insulating materials, high-temperature lighting materials, fine ceramic materials, and the like. The second purpose is to provide a method for inexpensively manufacturing high-purity magnesia powder that can produce magnesia sintered bodies with a bulk density close to the theoretical density at a temperature of about 1400°C without additives. be.

As a result of intensive research to achieve the above-mentioned object, the inventors of the present invention have prepared magnesium salt and alkali carbonate at low temperature.
A metastable precipitate is created through a low-concentration aqueous reaction, and this is aged to change into a stable precipitate.
It was discovered that by hydrolyzing this to basic magnesium carbonate and then calcining it, an excellent, highly pure magnesium powder could be obtained. The present invention was completed based on this finding.

The gist of the present invention is that in the reaction of synthesizing basic magnesium carbonate from magnesium salt and alkali carbonate, the initial concentration of both aqueous solutions is 0.3 to 1.0 mol/
Set the reaction temperature to 20-40℃, and the pH of the mother salt to 9.6-40℃.
A metastable precipitate is produced by reacting under the conditions of 10.3, and the precipitate is aged at 30 to 45°C until the pH of the mother salt becomes 8.3 or less to transform it into a stable precipitate. A method for producing high-purity magnesia powder, which is characterized by hydrolyzing it to basic magnesium carbonate until the pH becomes 10.6 to 10.8, and then calcining it at 700 to 1100°C in air or an oxygen atmosphere.

Magnesium salts used in the present invention include magnesium chloride, magnesium sulfate, magnesium nitrate, magnesium fluoride, and the like, with magnesium chloride and magnesium sulfate being preferred. Examples of alkali carbonates include ammonium carbonate,
A basic precipitant such as alkali bicarbonate cannot be used because a stable precipitate is formed from the beginning, and it is necessary to use an alkali carbonate such as sodium carbonate or potassium carbonate, or a mixture thereof.

The initial concentration of both the magnesium salt and the aqueous alkali carbonate solution is 0.3 to 1.0 mol/concentration, preferably 0.4 to 1.0 mol/concentration.
It is necessary that the amount is in the range of 0.5 mol/. 1.0
If the concentration exceeds mol%, the properties of the metastable precipitate will become non-uniform and residual impurities (especially Na) will increase.If the concentration is less than 0.3 mol/%, stable amorphous precipitates will coexist from the beginning. I can't use it because it does.

The initial concentrations of both solutions should be the same. If this is reversed, there will be a disadvantage that residual impurities will increase.

Also, the mixing ratio of both solutions is that magnesium salt is
50% by volume is sufficient.

The magnesium salt and the aqueous alkali carbonate solution may be added by adding the alkali carbonate aqueous solution to the aqueous magnesium salt solution, or vice versa. With the former addition method, a sintered body with a small grain size and microstructure can be obtained.
The latter addition method yields a sintered body with a large grain size and microstructure. The mother liquor temperature when adding this is 20 to 40
℃, preferably 25-35℃, and the maximum pH value of the mother liquor coexisting with the metastable precipitate is 9.6-10.3. When the mother liquor temperature exceeds 40℃, stable precipitates coexist from the beginning. Further, if it is lower than 20°C, there will be a disadvantage that the transient property will be significantly lowered and residual impurities will be increased. If the pH of the mother liquor falls outside the range of 9.6 to 10.3, different compounds will be mixed in.

The metastable precipitate thus obtained is
40-10 hours at 45℃, preferably 24-10 hours at 33-37℃
Hold and mature for 18 hours. If the ripening temperature is lower than 30°C, ripening will not proceed sufficiently, and not only will the maturation be poor, but also a large amount of impurities (particularly Si and Na) will be contained.
When the temperature exceeds 45°C, impurities increase and sinterability deteriorates. The relationship between aging temperature and aging time is approximately expressed by the following equation.

log t(min)=2.353-T(°C)/34 When washing the precipitate stabilized by aging, the precipitate is hydrolyzed to basic magnesium carbonate until the pH of the liquid becomes 10.6 to 10.8. The pH of the liquid is 10.6-10.8
Otherwise, different compounds will remain and the precipitate will be non-uniform.

Next, this basic magnesium carbonate is heat treated at 700 to 1100°C in air or oxygen atmosphere.
However, if the water of crystallization is completely evaporated at 300 to 400℃, followed by thermal decomposition (decarboxylation) and further sintering at 900℃, a powder with uniform properties and good sinterability can be obtained. More preferred.

Pressure molding of the obtained magnesia powder using a conventional mold is sufficient, but preferably it is further hydrostatic pressed. The molding pressure of the mold may be 100 kg/cm ² or more so that the molded product is not damaged.

The magnesia powder obtained by the method of the present invention has excellent properties obtained by sintering a magnesia sintered body having a bulk density close to the theoretical density at a low temperature of 1400°C without additives. This makes it possible to select different materials for the sintering furnace, simplify the structure, and reduce thermal energy.

In addition, its production also depends on the precipitation conditions of the raw material mother salt, aging,
Not only is it easy to control the hydrolysis conditions, but also the precipitate has good permeability and purity.
It can be obtained up to 99.99% by weight, and has the effect of easily obtaining a product with high adsorption properties for organic solvents at a low cost.

Example 1 0.4 mol of magnesium chloride/1000 ml of an aqueous solution was added dropwise to 0.4 mol of sodium carbonate/1000 ml of an aqueous solution and reacted at 25°C for 30 minutes to form a metastable precipitate (PH
10.6) was obtained. This precipitate was aged at 35°C for 24 hours to obtain a stable precipitate (PH8.3). This precipitate was washed and filtered 6 times with distilled water in step 1.2.
Basic magnesium carbonate was obtained through hydrolysis (PH10.5). This was dried at 110℃ for a day and night, and then dried at 400℃.
After dehydrating in dry air at ℃ until the dew point becomes constant, it is thermally decomposed at a constant temperature increase of 10℃/min to 900℃.
The powder was calcined for 20 hours to obtain magnesia powder. Its purity was 99.99% by weight (SiO ₂ 35ppm, CaO 30ppm).

0.3 g of the obtained magnesia powder was molded with a pressure of 150 kg/cm ² to make a molded body of 8φ x 3 mm.
Further hydrostatic pressing was carried out at a pressure of 2 tons/cm ² . The obtained molded body was heated in air at a constant rate of temperature increase of 10° C./min and baked at 1400° C. for 2 hours.
The bulk density of the sintered body is 3.549g/cm ³ (99.02%), the particle size is 9μ, and the impurities are 35ppm of SiO ₂ and 35ppm of CaO.
It was 30ppm.

In addition, in the above method, when basic magnesium carbonate was produced by changing the aging temperature and magnesia powder obtained under the same conditions was sintered, the bulk density and pH of the sintered body were as shown in Figure 1. It was as shown in. 1 is the bulk density curve, and 2 is the PH curve. This shows that a product with good bulk density can be obtained when the aging temperature is 25 to 45°C.

Comparative example 1 Both concentrations of magnesium salt and alkali carbonate were
A sintered body was produced in the same manner as in Example 1 except that the amounts were changed to 0.05 mol/ and 1.4 mol/. The bulk density of the sintered body is 3.412 g/cm ³ (95.17
%) 1.4 mol/ is 3.450 g/cm ³ (96.23%)
It was hot.

Comparative Example 2 A sintered body was produced in the same manner as in Example 1 except that the mixing ratio of magnesium salt was 35% by volume and 70% by volume. The bulk density of the sintered body is 35% by volume.
3.349g/cm ³ , (93.42%), 3.432 for 70% by volume
g/cm ³ (95.73%).

Example 2 When magnesium sulfate was used in place of magnesium chloride in the method of Example 1, the resulting sintered body had a uniform particle size and a bulk density of 3.514 g/cm ³
(98.02%).

Example 3 When potassium carbonate was used in place of sodium carbonate in the method of Example 1, the resulting sintered body had a uniform particle size and a bulk density of 3.506 g/cm ³ (97.80 g/cm 3 ).
%).

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the bulk density and PH of a calcined product of magnesia powder obtained when a metastable precipitate is aged at varying temperatures. 1: Bulk density curve, 2: PH curve.

Claims (1)

[Claims] 1. In the reaction of synthesizing basic magnesium carbonate from magnesium salt and alkali carbonate, the initial concentrations of both aqueous solutions are 0.3 to 1.0 mol/2, the reaction temperature is 20 to 40°C, and the pH of the mother salt is 9.6 to 10.3. to form a metastable precipitate, and the precipitate is
Aged at 30-45℃ until the pH of the mother salt becomes 8.3 or less, turning it into a stable precipitate.
A method for producing high-purity magnesia powder, which comprises hydrolyzing the powder to basic magnesium carbonate to a concentration of 10.6 to 10.8, followed by calcining at 700 to 1100°C in air or an oxygen atmosphere.