WO2025164535A1 - Magnesium oxide powder and method for producing same - Google Patents

Abstract

This magnesium oxide powder has a zeta potential of -50 mV to -5 mV, as measured by adding 0.2 g of the magnesium oxide powder to 50 mL of a borate buffer solution (pH=9.18), dispersing with an ultrasonic homogenizer for 3 minutes, then adding the powder into a measurement apparatus within one minute of the dispersion, and performing zeta potential measurement under the conditions of a set temperature of 25 °C and an equilibrium time of 2 minutes. The magnesium oxide powder preferably has a circularity of 0.6 to 0.8. The magnesium oxide powder also preferably has a particle diameter D₅₀ at a cumulative frequency of 50% on a volume basis as determined by a laser diffraction scattering method of 1 μm to 200 μm.

Description

Magnesium oxide powder and its manufacturing method

The present invention relates to magnesium oxide powder and a method for producing the same.

Magnesium oxide has the property of easily reacting with water to form magnesium hydroxide. In particular, crushed magnesium oxide has a high surface activity, making it even more reactive with water. As such, magnesium oxide has issues with water resistance, and there is a demand for improving its water resistance.
In order to improve the water resistance of magnesium oxide, Patent Document 1 proposes treating the surface of magnesium oxide with a fatty acid or various coupling agents.
Patent Document 2 proposes polishing the surface of magnesium oxide to remove the grain boundary phase present on the surface, which has low water resistance.

JP 2015-160781 A US Patent Application Publication No. 2020/246864

Although the techniques described in Patent Documents 1 and 2 can improve the water resistance of magnesium oxide, there is a problem in that the use of a surface treatment agent increases the production cost. Furthermore, polishing the surface of magnesium oxide reduces the yield due to the peeling and removal of the surface, and there is also a problem in that surface polishing is not effective for magnesium oxide, which does not have a grain boundary phase with low water resistance developed on its surface.
Therefore, an object of the present invention is to provide magnesium oxide having high water resistance that can be obtained by a simple procedure, and a method for producing the same.

The present invention provides a magnesium oxide powder having a zeta potential of −50 mV or more and −5 mV or less, measured by the following measurement method.
<Measurement method>
0.2 g of the magnesium oxide powder is placed in 50 mL of borate buffer solution (pH = 9.18) and dispersed for 3 minutes using an ultrasonic homogenizer. Then, within 1 minute after the dispersion treatment, the powder is placed in a measuring device and the zeta potential is measured at a set temperature of 25°C and an equilibration time of 2 minutes.

Furthermore, the present invention provides a suitable method for producing the magnesium oxide powder, which comprises a first firing step of firing a magnesium compound to obtain a magnesium oxide sintered body;
a crushing step of crushing the magnesium oxide sintered body obtained in the first firing step to obtain a crushed magnesium oxide product;
a second firing step of firing the magnesium oxide pulverized product obtained in the pulverization step to obtain magnesium oxide powder,
The present invention provides a method for producing magnesium oxide powder, in which the firing temperature in the second firing step is set lower than the firing temperature in the first firing step.

The present invention will be described below based on preferred embodiments. First, a method for producing magnesium oxide powder of the present invention will be described. The method for producing magnesium oxide powder of the present invention can be broadly divided into the following steps (1) to (3).
(1) A step of calcining a magnesium compound to obtain a magnesium oxide sintered body (first calcination step).
(2) A step of pulverizing the magnesium oxide obtained in the first firing step to obtain a pulverized magnesium oxide product (pulverization step).
(3) A step of calcining the magnesium oxide pulverized product obtained in the pulverization step to obtain magnesium oxide powder (second calcination step).
These steps will be explained in order below.

(1) First Firing Step In this step, a magnesium compound is fired and thermally decomposed to obtain a magnesium oxide sintered body. As the magnesium compound, magnesium hydroxide and magnesium salts such as magnesium carbonate (magnesite), basic magnesium carbonate, magnesium chloride, magnesium nitrate, and magnesium sulfate can be used, with magnesium hydroxide being particularly preferred.

The preferred purity of the magnesium oxide sintered body is the same as the preferred purity of the magnesium oxide powder described below. The purity of the magnesium oxide sintered body is measured using the same method as the purity of the magnesium oxide powder described below.

There are no particular limitations on the method for calcining the magnesium compound, and any calcination furnace such as a rotary kiln, shaft kiln, tunnel kiln, pusher furnace, box-type electric furnace, or airflow type calcination furnace can be used.
The calcination may be carried out with the magnesium compound left to stand, or may be carried out while stirring, flowing, or floating the magnesium compound. Alternatively, the first calcination step may be divided into a step of calcining the magnesium compound to obtain magnesium oxide, and a step of granulating the magnesium oxide and then calcining it at a higher temperature to obtain a magnesium oxide sintered body, and each step may be carried out in a separate calcination furnace.
In the first firing step, a magnesium oxide sintered body is obtained from the magnesium compound, and therefore the firing temperature of the magnesium compound is preferably 1400°C or higher, more preferably 1600°C or higher, and even more preferably 1800°C or higher.
From the viewpoint of reducing production costs, the calcination temperature of the magnesium compound is preferably 2600°C or lower, more preferably 2400°C or lower, and even more preferably 2200°C or lower.
The calcination temperature is the temperature of the magnesium compound in the first calcination step.
The magnesium oxide sintered body obtained in the first firing step may be electrically melted in an arc furnace to adjust the properties before being subjected to the pulverization step, thereby obtaining electro-fused magnesium oxide with a larger crystal size.

The calcination time for the magnesium compound varies depending on the type of calcination furnace, but can be, for example, from 1 second to 24 hours. There are no particular restrictions on the calcination atmosphere, and any of an inert atmosphere, an oxidizing atmosphere, and a reducing atmosphere can be used.

(2) Pulverization Step After the magnesium oxide sintered body is obtained, it is then pulverized to obtain a pulverized magnesium oxide product having a desired particle size. The pulverization device can be appropriately selected depending on the properties of the magnesium oxide sintered body to be pulverized and the properties desired for the resulting magnesium oxide powder. For example, pulverized magnesium oxide products can be obtained by using crushing devices such as roll crushers and jaw crushers, and pulverizing devices such as tumbling ball mills, vibrating ball mills, roller mills, hammer mills, pin mills, and jet mills, either alone or in combination of two or more. In addition, classification may be performed during or after the pulverization step, or a pulverizer with a built-in classification mechanism may be used.
From the viewpoint of increasing the yield of magnesium oxide powder, it is preferable that 90 mass % or more of the pulverized magnesium oxide product obtained in the pulverization step pass through a sieve with an opening of 200 μm.

There are no particular restrictions on the temperature during grinding, and grinding can be carried out at room temperature, for example.

(3) Second Firing Step The second firing step is a step in which the pulverized magnesium oxide product is fired to obtain magnesium oxide powder. As a result of investigations by the present inventors, it was found that the pulverized magnesium oxide product obtained by the pulverization step has high surface activity and low water resistance. It was also found that firing the pulverized magnesium oxide product in this step can reduce the surface activity of the resulting magnesium oxide powder and improve its water resistance.

The second firing step is preferably carried out so as to prevent excessive grain growth or sintering of the magnesium oxide particles. By carrying out the second firing step in this manner, an increase in the particle size of the magnesium oxide powder caused by excessive grain growth or sintering is suppressed. As a result, magnesium oxide powder having the desired particle size can be obtained in a higher yield. From this perspective, the firing temperature T2 in the second firing step is set lower than the firing temperature T1 in the first firing step.

The firing temperature T2 in the second firing step is set lower than the firing temperature T1 in the first firing step in order to prevent the magnesium oxide particles that have been pulverized and particle size adjusted from becoming larger due to fusion. Specifically, firing is preferably performed at 1200°C or lower, more preferably at 1100°C or lower, and even more preferably at 1000°C or lower.
Furthermore, from the viewpoint of sufficiently increasing the water resistance of the magnesium oxide powder, the firing temperature T2 in the second firing step is preferably 500°C or higher, more preferably 700°C or higher, and even more preferably 800°C or higher.
The firing temperature T2 is the temperature of the pulverized magnesium oxide product in the second firing step.

There is no particular limitation on the method for firing the pulverized magnesium oxide product, and the same firing furnace as that used in the first firing step can be used. The firing furnace used in the second firing step may be the same as or different from that used in the first firing step.
The calcination time of the pulverized magnesium oxide product is preferably 1 second or more and 100 seconds or less when the calcination furnace is an airflow type calcination furnace, and in other calcination furnaces, it is preferably 0.1 hours or more and 10 hours or less, more preferably 0.5 hours or more and 5 hours or less, and even more preferably 1 hour or more and 2 hours or less.
The firing atmosphere is not particularly limited, and any of an inert atmosphere, an oxidizing atmosphere, and a reducing atmosphere can be used.

Prior to firing the pulverized magnesium oxide product, the pulverized magnesium oxide product may be mixed with water and/or a binder and granulated (granulation step), and the resulting mixture (granules) may be subjected to a second firing step. The granulation step improves the handling properties of the pulverized magnesium oxide product, such as reducing dust generation, and also adjusts the particle size of the resulting magnesium oxide powder. Granulation of the pulverized magnesium oxide product can be carried out using a device such as an extrusion granulator, a tumbling granulator, an agitation granulator, a fluidized bed granulator, a briquetting machine, a roller compactor, or a spray dryer.
As the binder, for example, organic solvents such as lower alcohols, and water-soluble organic compounds such as polyvinyl alcohol, poly-N-vinyl-2-pyrrolidone, sodium polyacrylate, and polyethylene glycol can be used.
The total amount of water and/or binder mixed per 100 parts by mass of the pulverized magnesium oxide can be, for example, 0.1 to 10.0 parts by mass, particularly 0.5 to 5.0 parts by mass.
The ground magnesium oxide product and water and/or binder can be mixed using any mixer, such as a ribbon blender, a cone-schalf blender, a V-type mixer, a tumbler mixer, or a twin-screw kneader.

If the magnesium oxide powder obtained in the second firing step does not have the desired particle size (for example, _D90 and _D50 described below), the magnesium oxide powder may be classified as necessary to obtain magnesium oxide powder having the desired particle size (for example, magnesium oxide powder having _D90 and _D50 within the numerical ranges described below). For classification, a vibrating sieve, an air classifier, a cyclone classifier, or the like may be used alone or in combination of two or more thereof.

As described above, in this production method, the firing temperature T2 in the second firing step is appropriately controlled, which prevents sintering of magnesium oxide particles in the second firing step, thereby preventing an increase in _D90 of the magnesium oxide powder obtained in the second firing step.
Specifically, after classification, particles having a particle size of 150 μm or less are obtained in a high yield of preferably 70% or more, more preferably 75% or more, even more preferably 80% or more, and particularly preferably 85% or more.

As mentioned above, when magnesium oxide is crushed, the surface activity of the resulting magnesium oxide powder increases, which can reduce the water resistance of the magnesium oxide powder. Therefore, from the perspective of preventing a decrease in water resistance, it is preferable that the magnesium oxide powder obtained by the second firing step not be crushed any further. In other words, it is preferable that the magnesium oxide powder obtained by the second firing step be classified as described above before use, lightly crushed before use, or used as is.

Next, the magnesium oxide powder of the present invention will be described. The magnesium oxide powder of the present invention has a zeta potential of -50 mV or more and -5 mV or less, measured by the method described below. Zeta potential indicates the surface condition of the particles that make up the magnesium oxide. As a result of the inventor's studies, it was found that when the zeta potential of the magnesium oxide powder is -50 mV or more and -5 mV or less, the water resistance of the surfaces of the particles that make up the magnesium oxide is increased, and as a result, the water resistance of the magnesium oxide powder as a whole is also increased. From this perspective, the zeta potential of the magnesium oxide powder is preferably -48 mV or more and -7 mV or less, and more preferably -47 mV or more and -10 mV or less.

The zeta potential of magnesium oxide powder is measured using the following method. Specifically, 0.2 g of magnesium oxide powder is placed in 50 mL of borate buffer solution (pH = 9.18) and dispersed for 3 minutes using an ultrasonic homogenizer (Nippon Seiki Seisakusho, US150T, rated output 150 W). The powder is then placed in the measuring device within 1 minute of dispersion, and the zeta potential is measured at a set temperature of 25°C with an equilibration time of 2 minutes. A Malvern Zetasizer Nano ZS ZEN3600 is used to measure the zeta potential.

In order to keep the zeta potential within the above-mentioned range, it is preferable, for example, to perform a second firing step in the above-mentioned manufacturing method and not to perform a grinding step after the second firing step.

The magnesium oxide powder of the present invention may contain unavoidable impurities or components such as boron, iron, calcium, aluminum, and silicon added to adjust the properties of magnesium oxide. The purity of the magnesium oxide powder is preferably 88% by mass or more, more preferably 90% by mass or more, and even more preferably 92% by mass or more. Furthermore, from the viewpoint of improving the water resistance of the magnesium oxide powder produced, the purity of the magnesium oxide powder is preferably 99% by mass or less, more preferably 98.5% by mass or less, and even more preferably 97% by mass or less.
The purity of the magnesium oxide powder is determined in accordance with JIS R2212-4 by quantifying the contents of CaO, SiO ₂ , Fe ₂ O ₃ , Al ₂ O ₃ and B ₂ O ₃ by ICP atomic emission spectrometry, and then subtracting the contents of the five components (CaO, SiO ₂ , Fe ₂ O ₃ , Al ₂ O ₃ and B ₂ O ₃ ) from the total by the difference method.

The purity of magnesium oxide powder is roughly the same as that of the magnesium oxide sintered compact, which is its raw material. Therefore, the preferred purity of magnesium oxide powder can be the same as the preferred purity of the magnesium oxide sintered compact described above.

The magnesium oxide powder of the present invention preferably has a circularity of 0.6 or more and 0.8 or less. Magnesium oxide powder with such a circularity can be obtained, for example, by calcining a magnesium compound (after the first calcination step in the above-described production method) and then performing a pulverization step. The pulverization step reduces the particle size, and the circularity decreases due to the effect of fracture surfaces generated by the pulverization on the particle shape. According to the above-described production method, a pulverized magnesium oxide product with a circularity of 0.60 or more and 0.80 or less is obtained by the pulverization step, and then the pulverized magnesium oxide product is subjected to the above-described second calcination step, thereby obtaining a magnesium oxide powder with improved water resistance. That is, according to the above-described production method, a magnesium oxide powder with a circularity of 0.60 or more and 0.80 or less, high water resistance, and a desired particle size can be efficiently produced.
Furthermore, when the circularity of the magnesium oxide powder is set to 0.8 or less, the contact area between the particles constituting the magnesium oxide powder increases, forming more heat conduction paths, thereby improving the thermal conductivity of the magnesium oxide powder.
From the viewpoint of making the above-mentioned advantages more pronounced, the circularity of magnesium oxide is more preferably 0.63 or more and 0.79 or less, and even more preferably 0.65 or more and 0.79 or less.

The circularity is calculated based on a projected image of the magnesium oxide powder of the present invention. Specifically, an SEM image is taken of the magnesium oxide powder dispersed and fixed on carbon tape or the like, and for particles whose individual particle shapes can be distinguished, the circularity coefficient calculated from the particle shape using image analysis software (Mac-view ver. 4: manufactured by Mounttec Co., Ltd.) is used as the circularity of the particles. Measurements are performed on 100 or more magnesium oxide particles, and the arithmetic average value is used as the circularity of the powder.

The magnesium oxide powder of the present invention preferably has an aspect ratio of 1.30 or more and 1.60 or less. Magnesium oxide powder with such an aspect ratio can be obtained, for example, by calcining a magnesium compound (after the first calcination step in the above-mentioned manufacturing method) and then performing a pulverization step. The aspect ratio is calculated based on a projected image of the magnesium oxide powder of the present invention. Specifically, an SEM image is taken of magnesium oxide powder dispersed and fixed on carbon tape or the like. For particles whose individual particle shapes can be identified, image analysis software (Mac-view ver. 4: manufactured by Mounttec Co., Ltd.) is used to calculate the short and long sides of the particles from their particle shapes. The long side/short side ratio is then used as the aspect ratio of the particles. Measurements are performed on 100 or more magnesium oxide particles, and the arithmetic average value is taken as the aspect ratio of the magnesium oxide powder.

The particle size _D90 at a cumulative frequency of 90% on a volume basis as determined by a laser diffraction scattering method of the magnesium oxide powder of the present invention is preferably 200 μm or less, more preferably 180 μm or less, and even more preferably 150 μm or less, from the viewpoints of smoothness and beautiful appearance after kneading with a resin or the like.
Furthermore, the _D90 of the magnesium oxide powder is preferably 5 μm or more, and more preferably 10 μm or more.
The particle size _D50 (median diameter) of the magnesium oxide powder at a volume-based cumulative frequency of 50% as measured by a laser diffraction scattering method is preferably 1 μm or more and 200 μm or less, more preferably 2 μm or more and 150 μm or less, even more preferably 3 μm or more and 150 μm or less, and even more preferably 5 μm or more and 100 μm or less.

The degree of water resistance of the magnesium oxide powder of the present invention is such that the mass increase rate after storing the magnesium oxide powder in an environment of 85°C and 85% relative humidity (RH) for 48 hours is preferably 2.0% or less, more preferably 1.8% or less, and even more preferably 1.6% or less.

The magnesium oxide powder of the present invention is suitable for use as a thermally conductive filler. This thermally conductive filler can be mixed with various resins to form a resin composition. Such a resin composition can be suitably used in various articles, particularly articles requiring high thermal conductivity and water resistance. Examples of such articles include lamp sockets and various electrical components in the automotive field. Examples of such articles in the electronics field include heat sinks, die pads, printed wiring boards, semiconductor package components, cooling fan components, pickup components, connectors, switches, bearings, case housings, thermal interface materials (sheets, greases), and gap fillers.
The magnesium oxide powder produced by the above method can also be used as a raw material for various ceramics.

The magnesium oxide powder and the method for producing the same of the present invention have been described above based on the preferred embodiments, but the scope of the present invention is not limited to these embodiments.
For example, in the manufacturing method of the present invention, additional firing may be performed once or multiple times in the second firing step. In this case, if necessary, the magnesium oxide powder obtained in the second firing step may be pulverized and then additional firing may be performed. Regardless of how many times firing is performed, it is preferable to use the magnesium oxide powder after the final firing without pulverization.

The present invention will be explained in more detail below using examples. However, the scope of the present invention is not limited to these examples. Unless otherwise specified, "%" means "% by mass."

Example 1
Magnesium hydroxide was produced by the seawater method using seawater and milk of lime as raw materials. The Si content of this magnesium hydroxide was adjusted so that the MgO purity of the magnesium oxide sintered body described below would be the desired value. Next, the magnesium hydroxide with the adjusted Si content was introduced into a rotary kiln and fired at 1800°C for 8 hours in an oxidizing atmosphere to obtain a magnesium oxide sintered body (first firing step). The MgO purity of the obtained magnesium oxide sintered body was 95.80% by mass as determined by ICP atomic emission spectroscopy. The composition of the magnesium oxide sintered body determined by ICP atomic emission spectroscopy is shown in Table 1.
The obtained magnesium oxide sintered body was pulverized in a ball mill to obtain a pulverized magnesium oxide product (pulverization step).
The ground magnesium oxide was introduced into a rotary kiln and fired at 888°C for 2 hours in an oxidizing atmosphere (second firing step).
The magnesium oxide powder obtained in the second firing step was classified using a vibrating sieve with a mesh size of 150 μm (classification step), and the magnesium oxide powder of Example 1 was obtained from which particles larger than 150 μm had been removed.

Examples 2 and 3
Magnesium oxide powders of Examples 2 and 3 were obtained in the same manner as in Example 1, except that the firing temperature in the second firing step was changed to the temperature shown in Table 2.

Examples 4 and 5
In carrying out the second firing step, the magnesium oxide powders of Examples 4 and 5 were obtained in the same manner as in Example 1, except that the crushed magnesium oxide was introduced into a box-type electric furnace instead of a rotary kiln, and the firing temperature in the second firing step was changed to the temperature listed in Table 2.

Example 6
Magnesium hydroxide was produced by the seawater method using seawater and milk of lime as raw materials. The Si content of this magnesium hydroxide was adjusted so that the MgO purity of the magnesium oxide sintered body described below would be the desired value. Next, the magnesium hydroxide with the adjusted Si content was introduced into a rotary kiln and fired at 1800°C for 8 hours in an oxidizing atmosphere to obtain a magnesium oxide sintered body (first firing step). The MgO purity of the obtained magnesium oxide sintered body was 97.47% by mass as determined by ICP optical emission spectroscopy. The composition of the magnesium oxide sintered body determined by ICP optical emission spectroscopy is shown in Table 1.
The obtained magnesium oxide sintered body was pulverized in a ball mill to obtain a pulverized magnesium oxide product (pulverization step).
5 wt % of water was added to the pulverized magnesium oxide product, and the mixture was granulated in a twin-axis kneader (granulation step), and then introduced into a rotary kiln and fired in an oxidizing atmosphere at 924°C for 2 hours (second firing step).
The pulverized magnesium oxide product after firing was classified using a vibrating sieve with a mesh size of 150 μm (classification step), and magnesium oxide powder of Example 6 was obtained from which particles larger than 150 μm had been removed.

Examples 7 to 10
The composition of the magnesium hydroxide was adjusted so that the magnesium oxide after firing would have the composition shown in Table 1, and the firing temperature in the second firing step was changed to the temperature shown in Table 2. Except for these points, the magnesium oxide powders of Examples 7 to 10 were obtained in the same manner as in Example 1.

Comparative Example 1
A pulverized magnesium oxide product of Comparative Example 1 was obtained in the same manner as in Example 1, except that the second firing step and the classification step were not carried out.
Comparative Example 2
A pulverized magnesium oxide product of Comparative Example 2 was obtained in the same manner as in Example 6, except that the granulation step, the second firing step, and the classification step were not carried out.

Comparative Example 3
A pulverized magnesium oxide product of Comparative Example 3 was obtained in the same manner as in Example 9, except that the second firing step was not carried out.

〔evaluation〕
The magnesium oxide powders and pulverized products obtained in the Examples and Comparative Examples were measured for median diameter ( _D50 ), _D90 , yield in the classification process, mass increase rate after the water resistance test, and zeta potential by the following methods. The average circularity was also measured by the same method. The measured physical properties of the magnesium oxide powders are shown in Table 2.

[Median diameter (D ₅₀ ) and D ₉₀ ]
A laser diffraction scattering particle size distribution analyzer (MICROTRAC MT3300EXII, manufactured by Microtrac Bell Co., Ltd.) was used. Ion-exchanged water was used as the solvent, and magnesium oxide powder was added through the sample inlet until the sample amount indicated by the instrument was appropriate. The circulation state was maintained until the peak shape of the detected particle size distribution stabilized. After the peak shape stabilized, the particle size at which the volume-based cumulative frequency was 50% (D ₅₀ : median diameter) and the particle size at which the volume-based cumulative frequency was 90% (D _{90 )} were determined. <Apparatus conditions> Light source: Semiconductor laser 780 nm 3 mW Class 1 laser Refractive index: 1.74 (MgO) - 1.333 (water) Number of measurements: Avg/3 Measurement time: 30 seconds

[Yield of classification process]
The yield of the classification step was calculated based on the following formula: The results are shown in Table 2.
Yield (%) = (mass of magnesium powder after classification) / (mass of magnesium powder before classification) × 100

[Mass increase rate after water resistance test]
The weighing bottle used was dried at 105°C for 1 hour, then allowed to cool to room temperature in a desiccator containing a quicklime-based desiccant, and the mass was measured. This process was repeated until a constant mass was reached. Approximately 10 g of magnesium oxide powder was weighed into the same weighing bottle and dried at 105°C for 1 hour. Similarly, the bottle was allowed to cool to room temperature in the desiccator and the mass was measured. This process was repeated until a constant mass was reached, and the pre-humidification mass was then measured. Next, the weighing bottle containing the magnesium oxide powder was left with the lid open in a constant temperature and humidity chamber at 85°C and 85% relative humidity for 48 hours, then dried at 105°C for 1 hour to remove adhering moisture, and then allowed to cool to room temperature in a desiccator containing a quicklime-based desiccant, and the post-humidification mass was measured. From these values, the mass increase rate after the water resistance test was calculated using the following formula. The results are shown in Table 2.
Mass increase rate (%) = (mass after humidification - mass before humidification) / (mass before humidification - mass of container) x 100

[Zeta potential]
The zeta potential was measured by the above-mentioned method using a Zetasizer Nano ZS ZEN3600 manufactured by Malvern.

As is clear from Table 2, the magnesium oxide powders of the examples that underwent the second firing step had a smaller mass increase rate and better water resistance than the magnesium oxide powders of the comparative examples that did not undergo the second firing step. Furthermore, the magnesium oxide powders of the examples had a zeta potential of -50 mV or more and -5 mV or less.
When focusing on the yield of the classification process (i.e., the proportion of magnesium oxide particles that can pass through a sieve with a mesh size of 150 μm), it can be seen that sintering of the magnesium oxide powder is suppressed by setting the firing temperature in the second firing process low.

According to the present invention, a magnesium oxide powder having high water resistance is provided. Furthermore, according to the present invention, a magnesium oxide powder having high water resistance can be produced by a simple procedure of performing at least two firing steps. The magnesium oxide powder of the present invention thus obtained is particularly suitable for use as a filler for resin compositions, a ceramic raw material, and the like.

Claims (10)

A magnesium oxide powder having a zeta potential of -50 mV or more and -5 mV or less as measured by the following measurement method.
<Measurement method>
0.2 g of the magnesium oxide powder is placed in 50 mL of borate buffer solution (pH = 9.18) and dispersed for 3 minutes using an ultrasonic homogenizer. Then, within 1 minute after the dispersion treatment, the powder is placed in a measuring device and the zeta potential is measured at a set temperature of 25°C and an equilibration time of 2 minutes. The magnesium oxide powder according to claim 1, having a circularity of 0.6 or more and 0.8 or less. 3. The magnesium oxide powder according to claim 1, wherein the particle size _D50 at a cumulative frequency of 50% on a volume basis as determined by a laser diffraction scattering method is 1 μm or more and 200 μm or less. 3. The magnesium oxide powder according to claim 1, wherein the particle size _D90 at a volume-based cumulative frequency of 90% as determined by a laser diffraction scattering method is 200 μm or less. _3. The magnesium oxide powder according to claim 1, _{wherein the purity of the magnesium oxide powder is 88% by mass or more and 99% by mass or less, as determined by quantifying the contents of CaO, SiO 2 , Fe 2 O 3 , Al 2 O 3 and} B 2 O 3 by ICP atomic emission spectrometry in accordance with JIS R2212-4, and then subtracting the contents of the five components from the total by the difference method. The magnesium oxide powder according to claim 1 or 2, which has a mass increase rate of 2.0% by mass or less after being left standing in an environment of 85°C and 85% RH for 48 hours. a first firing step of firing a magnesium compound to obtain a magnesium oxide sintered body;
a crushing step of crushing the magnesium oxide sintered body obtained in the first firing step to obtain a crushed magnesium oxide product;
a second firing step of firing the magnesium oxide pulverized product obtained in the pulverization step to obtain magnesium oxide powder,
A method for producing magnesium oxide powder, wherein the firing temperature in the second firing step is set lower than the firing temperature in the first firing step. The manufacturing method described in claim 7, wherein the firing temperature in the second firing step is set to 1200°C or less, provided that it is lower than the firing temperature in the first firing step. The manufacturing method described in claim 7 or 8, wherein the magnesium oxide pulverized product obtained in the pulverization step is mixed with water and/or a binder, and the resulting mixture is subjected to a second firing step. 9. The method according to claim 7 or 8, wherein the magnesium oxide powder obtained in the second firing step is classified to obtain magnesium oxide powder having a particle size _D90 of 200 μm or less at a cumulative frequency of 90% on a volume basis as determined by a laser diffraction scattering method.