JP5069865B2 - High purity magnesium hydroxide powder and method for producing the same - Google Patents

Description

  The present invention is used, for example, as a deposition source when a protective film for a plasma display panel (hereinafter also referred to as “PDP”) is manufactured using a vacuum deposition method such as an electron beam deposition method or an ion plating method. The present invention relates to a high-purity magnesium hydroxide powder for a magnesium oxide sintered body and a method for producing the same.

  Development of a PDP using the discharge light emission phenomenon is being promoted as a flat display that is easily increased in size. In the AC type (AC type) PDP having a structure in which the transparent electrode is covered with a glass dielectric, in order to prevent the surface of the dielectric layer from being altered due to ion bombardment sputtering and increasing the discharge voltage, generally, a dielectric is used. A protective film is formed on the body. The protective film is required to have a low discharge voltage and excellent sputtering resistance.

  Conventionally, a magnesium oxide (hereinafter sometimes referred to as “MgO”) film has been used as a protective film that satisfies this requirement. Since the MgO film is an insulator having excellent sputtering resistance and a large secondary electron emission coefficient, the discharge start voltage can be lowered, which contributes to extending the life of the PDP. At present, the MgO protective film is generally formed on a dielectric by an electron beam evaporation method using MgO as a target material.

  Conventionally, MgO single crystal pulverized products have been mainly used as MgO vapor deposition materials for PDP. However, as the performance of PDP increases, the technical level required by PDP manufacturers increases, and further improvement of the PDP vapor deposition material is desired. For this reason, high-purity polycrystalline MgO particles that allow easy adjustment of the amount of additive elements can be obtained. It is shifting to a sintered body to be sintered.

  A sintered body that sinters high-purity polycrystalline MgO particles is based on increasing the purity of magnesium hydroxide as a raw material. The manufacturing method of magnesium hydroxide powder has been developed for a long time, and many patents have been filed. New uses of magnesium hydroxide for PDP have been pioneered, and a number of improvements have been made to the production method for obtaining magnesium hydroxide having suitable physical properties. Conventionally, however, magnesium oxide derived from magnesium hydroxide has a certain degree of purity, but high purity magnesium hydroxide in which all individual metal impurities are 10 ppm or less has not been obtained. . For this reason, magnesium hydroxide as a raw material for an MgO sintered body that satisfies the characteristics required with the performance enhancement of PDP has not always been satisfactory in terms of purity. Therefore, in order to improve the performance of the PDP, in particular, to increase the secondary electron emission coefficient and reduce the power consumption, it is necessary to reduce the impurity elements that affect the secondary electron emission coefficient. When magnesium oxide is produced from high-purity magnesium hydroxide in which all impurities are 10 ppm or less, the impurity content can be extremely reduced, so that a uniform protective film can be produced stably. Can be expected.

  Magnesium ions are generally separated as hydroxides, phosphates and the like. The method of separating as a hydroxide has been put into practical use for a long time, and is roughly classified according to the type of treatment agent. In the method using sodium hydroxide as a treating agent, when sodium hydroxide is added to a solution containing magnesium ions, precipitation of magnesium hydroxide occurs as shown in the following formula.

Mg ²⁺ + 2OH ⁻ = Mg (OH) ₂

Solubility product Mg (OH) ₂ is represented by [Mg] [OH] ^2, at room temperature is approximately 1.2 × 10 ^-11 mol / l. When an equivalent amount of sodium hydroxide solution is added to magnesium ion at once, it reacts instantaneously, resulting in fine precipitation that is very poor in settling and difficult to filter. Many efforts have been made.

  In order to obtain a highly pure magnesium compound, conventionally, a method of precipitating magnesium carbonate by adding sodium carbonate to magnesium chloride obtained from bittern in Shioda has been adopted. However, since this method consumes a large amount of expensive sodium carbonate, the resulting magnesium compound is expensive. Moreover, in Japan, there is a shortage of bittern, so this method has become increasingly expensive.

  Therefore, magnesium hydroxide is produced by adding dry lime obtained by hydration of slaked lime or calcined dolomite to seawater, thereby producing a method for producing magnesium oxide and magnesium carbonate industrially. In such a method, the magnesium compound can be mass-produced at low cost, but impurities are mixed from the raw material, and the purity of the obtained magnesium compound is not sufficient. Even if high-grade limestone or magnesium hydroxide produced using dolomite is fired, the purity of the obtained magnesium oxide is only 96 to 98.5% by mass.

  As a method for increasing the purity of magnesium hydroxide, Patent Document 1 (Japanese Patent Laid-Open No. 58-120514) discloses that high-purity magnesium chloride is separated from a magnesium chloride aqueous solution containing Fe, Al, and Ca as impurities, Disclosed is a method in which the obtained magnesium chloride is dissolved in water, and then ammonium is added thereto to precipitate and separate magnesium hydroxide. As a result, high purity magnesium hydroxide of 99.95% by mass or more is obtained as MgO. Yes. Further, Patent Document 2 (Japanese Patent Laid-Open No. 2001-302232) discloses that MgO fine particles composed of high-purity single crystals in which fine primary particles are independent are hydrated, whereby the MgO equivalent purity is 99.98% by mass or more. High purity magnesium hydroxide is disclosed. Furthermore, in Patent Document 3 (Japanese Patent Laid-Open No. 2002-255544), an MgO fired product in which the content of a specific inorganic compound is adjusted is hydrated under specific conditions in the presence of a water-soluble magnesium salt. Thus, it is disclosed that a purity of magnesium hydroxide of 99.5% by mass or more can be obtained.

  However, the high-purity magnesium hydroxide disclosed in these prior arts has a purity of less than 99.99% by mass, which is at least fourine or more that satisfies the advanced characteristics required as the performance of PDP increases. The high purity level has not been reached.

On the other hand, as high-purity magnesium oxide, for example, Patent Document 4 (Japanese Unexamined Patent Application Publication No. 2004-84017) discloses a raw material for producing a magnesium oxide vapor deposition material used when forming a protective film for a dielectric layer of an AC plasma display panel. Magnesium oxide powder characterized in that the MgO purity is higher than 99.98% by mass, the specific surface area is in the range of 5 to 10 m ² / g, and the shape of the primary particles is cubic. It is disclosed.

  However, this magnesium oxide powder is produced by a gas phase oxidation method in which metallic magnesium is directly oxidized. The method for producing high-purity magnesium oxide by the vapor phase method requires a large amount of production equipment and requires a sophisticated reaction operation. For this reason, it is difficult to obtain a purity of 99.99% by mass or more, and the production cost is considered to be high, and the actual situation is that a satisfactory product has not yet been obtained.

  On the other hand, Patent Document 5 (Japanese Patent Laid-Open No. 61-209911) describes a method for producing high-purity magnesium oxide. Here, a crude raw material containing magnesium is dissolved with a mineral acid to obtain a crude solution of a magnesium mineral salt, and an alkali is added to the crude solution to precipitate and remove impurities in the crude raw material. A mineral salt is obtained, and an alkali is added to the purified solution until the pH becomes 10 or more, and the alkali-added solution is hydrothermally treated at a temperature of 120 ° C. or more, and consists of a double salt of magnesium hydroxide and sulfate. A method for producing magnesium oxide is described, which comprises producing a magnesium compound, washing the magnesium compound with water, dehydrating, and heating the dehydrated product at a temperature of 1000 ° C. or higher.

  Conventionally, magnesium oxide is mainly produced by drying and baking a precipitate of a magnesium compound. However, at that time, it was sufficient to have a certain degree of purity for fine ceramics. However, as today, PDP applications have been newly developed, and more highly purified MgO has been required. There is a need for improvement.

  Thus, currently commercially available magnesium hydroxide has a large impurity content and is insufficient as a raw material for a magnesium oxide sintered body for PDP. Moreover, even if it is a high purity material, there is a problem that it is difficult to obtain a sintered MgO having a relative density of 98% or more if the aggregated grains are large and the BET specific surface area is small.

JP 58-120514 A JP 2001-302232 A JP 2002-255544 A JP 2004-84017 A JP-A 61-209911

  The present invention is a commercially available magnesium hydroxide as a raw material used for manufacturing a MgO sintered body used as a target material for forming an MgO film on a substrate using a method such as an electron beam evaporation method. An object of the present invention is to provide a magnesium hydroxide powder that solves the problem that the impurity content is large and insufficient and a method for producing the same.

  Common impurities in magnesium oxide include Si, Al, Ca, and Fe, which are contained in a relatively large amount compared to other impurity elements. Since these impurity amounts depend on the raw material source of the magnesium compound to be used, selection of the raw material source is important. In order to solve the above problems, for example, the present inventors conducted a synthesis experiment of magnesium hydroxide using various magnesium ion-containing materials when synthesizing a starting material for an MgO sintered body, and It came to be completed.

That is, the present invention is a magnesium hydroxide powder in which each of Si, Al, Ca, Fe, V, Cr, Mn, Ni, Zr, B, and Zn is 10 ppm or less and the purity is 99.99 mass% or more. is there. The magnesium hydroxide powder of the present invention preferably has a 50% particle size of 1.0 × 10 ⁻⁶ m or less and a BET specific surface area of 8 to 30 m ² / g. The present invention further provides a magnesium oxide powder obtained by firing this magnesium hydroxide powder, and a vapor deposition material for a PDP protective film using this magnesium oxide powder.

  In the present invention, in particular, the impurities contained in the magnesium salt are separated very effectively by purifying all the raw materials of magnesium hydroxide to be used, and then performing a hydrothermal treatment in addition to the precipitation operation by adding an alkali. Thus, it is possible to provide high-purity magnesium hydroxide intended for a raw material for a PDP vapor deposition material.

  Therefore, according to the present invention, Si, Al, Fe, V, Cr, Mn, Ni, Zr, B, and Zn are each 10 ppm or less, and Ca is 30 ppm or less. An aqueous solution obtained by mixing pure water purified to 1 μS / cm or less is reacted with an alkaline aqueous solution having an alkali content of 20 to 50% by mass to produce magnesium hydroxide, and then the produced magnesium hydroxide is added to the autoclave. This is a method for producing high-purity magnesium hydroxide, which is subjected to hydrothermal treatment with, followed by filtration, washing with water and drying.

  By firing, the magnesium hydroxide powder of the present invention is 10 ppm or less in each of Si, Al, Ca, Fe, V, Cr, Mn, Ni, Zr, B, and Zn, and the purity is 99.99 mass%. The above magnesium oxide powder can be provided. In particular, since a high-purity magnesium oxide can be produced by a combination of relatively simple solution reaction steps that cannot be obtained by a conventional wet method, the industrial effect is great.

  The present invention provides a magnesium hydroxide powder in which each of Si, Al, Ca, Fe, V, Cr, Mn, Ni, Zr, B, and Zn is 10 ppm or less and the purity is 99.99 mass% or more. In addition, after purifying the raw materials used in advance, magnesium hydroxide is precipitated and separated.

  The raw material of the magnesium hydroxide powder is preferably magnesium chloride, more preferably anhydrous magnesium chloride, as a magnesium compound that becomes magnesium ions in an aqueous solution. As for the purity of magnesium chloride, Si, Al, Fe, V, Cr, Mn, Ni, Zr, B, and Zn each need to be 10 ppm or less and Ca must be 30 ppm or less. Further, not limited to magnesium chloride, after being dissolved in water with a magnesium compound, each of Si, Al, Fe, V, Cr, Mn, Ni, Zr, B, and Zn is 10 ppm or less, and Ca is 30 ppm or less. Any compound can be used. In addition, in the method of hydrating magnesium oxide produced by oxidizing metal magnesium by a vapor phase method or the method of hydrating magnesium chloride derived from seawater, it is difficult to purify and remove impurities contained in the raw material, and once high purity After manufacturing the magnesium clinker, it is necessary to rehydrate and purify it to produce high-purity magnesium hydroxide of three nine levels.

  Water is mixed with the above magnesium chloride raw material to obtain a magnesium chloride aqueous solution. The amount of water added is preferably 2 to 5 times that of anhydrous magnesium chloride. At this time, ion-exchanged ultrapure water is used as the water. In particular, since the amount of Si may be significantly contained in water, it is necessary to use ultrapure water that has been refined to an electric conductivity of 0.1 μS / cm or less through an ion exchange resin.

Next, in order to purify and remove impurities such as Si contained in the magnesium chloride aqueous solution, before performing this reaction using sodium hydroxide, an alkali is used so that the reaction rate is 20 mol% with respect to magnesium ions. The source is charged and a purified (primary) reaction yields a highly purified MgCl ₂ solution. By the above reaction, 20 mol% of the total magnesium ions contained in the magnesium chloride aqueous solution and impurities that precipitate as hydroxide are precipitated and removed, and the purity of the remaining purified magnesium chloride solution is increased. Sodium hydroxide is preferable in terms of low impurity contamination, but ammonia water can also be used. Sodium hydroxide is preferably an aqueous solution in which an alkali having an alkali content of 20 to 50% by mass is dissolved. In addition, in order to purify and remove impurities such as Si mixed in NaOH itself, before using sodium hydroxide in this reaction, the reaction rate is 10 mol% with respect to hydroxide ions. A magnesium chloride solution is added to perform a preliminary reaction to increase the purity. By the above reaction, 10 mol% of hydroxide ions in the aqueous sodium hydroxide solution react with magnesium ions and precipitate as magnesium hydroxide, and impurities are precipitated and removed. To reduce.

Next, the obtained purified MgCl ₂ solution is subjected to a secondary reaction with an alkali source such as NaOH to obtain a magnesium hydroxide slurry. Subsequently, the obtained magnesium hydroxide slurry was hydrothermally treated in an autoclave (temperature: 100 to 150 ° C., time) so that the particles became crystals having a 50% particle diameter of 1 μm or less and a specific surface area of 8 to 30 m ² / g. : 0 to 60 minutes), the magnesium hydroxide of the present invention is obtained. By performing autoclaving, it is considered that crystal grains can be prepared and impurities incorporated in the crystals can be leached into the solution, and as a result, the amount of impurities can be reduced.

  The magnesium hydroxide of the present invention obtained by such a production method has high purity and excellent dispersibility, so that abnormal grain growth does not occur during firing, and the subsequent molding and the fired product with respect to the theoretical density of magnesium oxide. A sintered body having a relative density of 98% or more can be obtained. Moreover, since the magnesium hydroxide of the present invention obtained by such a production method has high purity and excellent dispersibility, it can also be used for flame retardant applications and the like.

  On the other hand, magnesium hydroxide obtained by a normal liquid phase reaction is in a state in which fine particles are aggregated and has poor filterability and water washing efficiency, and thus has a problem of reduced productivity and high impurity content. Magnesium hydroxide with large agglomerated grains and a small BET specific surface area can easily form coarse-grained magnesium oxide with abnormal grain growth after firing, and the relative density of the sintered and sintered magnesium oxide sintered body is 90% or less. Sometimes. The present invention has solved these abnormal grain growth problems.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
Magnesium chloride (MgCl ₂ ) (analytical values of impurity elements: Si, Al, Fe, V, Cr, Mn, Ni, Zr, B, Zn are each 10 ppm or less and Ca is 30 ppm or less) 1.0 kg Was dissolved in ion-exchanged water (electric conductivity is 0.1 μS / cm or less), and the solution was made up to 3 liters (MgCl ₂ = about 3.5 mol / l). As the MgCl ₂ in the reaction rate of 20 mol%, MgCl ₂ solution and NaOH solution (alkalinity 20-50% by weight) each fed to the reactor at a metering pump and the roller pump, was continuously reacted. The reaction slurry was allowed to overflow from the reactor in a residence time of 30 minutes, and a flocculant was added to the produced magnesium hydroxide in an amount of 500 ppm to cause precipitation, and the supernatant (purified magnesium chloride solution) was recovered.

While stirring the recovered purified magnesium chloride solution, an NaOH solution (alkaline content: 20 to 50% by mass) was added so that the reaction rate of MgCl ₂ was 90 mol% and stirred for 30 minutes. The produced magnesium hydroxide slurry was subjected to hydrothermal treatment at 130 ° C. for 1 hour in an autoclave. The hydrothermally treated magnesium hydroxide slurry was filtered, washed with water and dried to obtain magnesium hydroxide powder. Images taken by SEM (scanning electron microscope) of this magnesium hydroxide are shown in FIGS. 1 to 3, and the results of measuring chemical components and physical properties are shown in Table 1.

  Further, the obtained magnesium hydroxide was baked in an electric furnace at 1300 ° C. for 60 minutes to obtain magnesium oxide. Table 2 shows the measurement results of chemical components and physical properties of this magnesium oxide.

  The obtained magnesium oxide powder was pulverized with a wet pot mill in an alcohol solvent for 24 hours with a nylon ball with an iron core (about φ15 mm). After natural drying, it is ignited with a hot air dryer at 120 ° C., and then a power kneader (trade name: PK type, manufactured by Dalton Co., Ltd.) is used for 5 minutes at a rotational speed of 250 rpm, and a binder (trade name: Metrolose 90SH- 400, manufactured by Shin-Etsu Chemical Co., Ltd.) was granulated while adding 6% by mass.

  Next, the granulated powder was molded with a press machine (trade name: SR100-1P-9H, manufactured by Ebara Seiki Co., Ltd.) at a molding pressure of 200 MPa, and then fired for four at 1650 ° C. in the atmosphere using a gas furnace. Thus, a vapor deposition material pellet having a length of 3 mm, a width of 5 mm, and a thickness of 2 mm was obtained. The relative density and secondary electron emission coefficient of the pellet were measured. The results are shown in Table 3.

(Physical property measurement)
The physical property values of magnesium hydroxide (Mg (OH) ₂ ) powder and magnesium oxide (MgO) powder were measured by the following methods.
(1) Measurement of the amount of impurities in Mg (OH) ₂ and MgO The measured trace impurities include Si, Al, Ca, Fe, V, Cr, Mn, Ni, Zr, B, and Zn in total 11 metal elements These contents were measured after the sample was acid-dissolved using an ICP emission spectroscopic analyzer (trade name: SPS-1700VR, manufactured by Seiko Instruments Inc.). The purity of Mg (OH) ₂ and MgO was calculated as a value obtained by subtracting the total value of the 11 kinds of impurities measured from 100.00% by mass.
(2) Average particle diameter The average particle diameter of the powder was determined by measuring the 50% particle diameter with a laser diffraction particle size measuring device (trade name: HIRA, manufactured by Nikkiso Co., Ltd.).
(3) Specific surface area The specific surface area of the powder was measured by the BET method.
(4) Relative density The bulk density of the vapor deposition material was determined by the Archimedes method. The relative density of the sintered body was calculated by setting the density of the MgO single crystal to 3.58.
(5) Characteristic evaluation of PDP protective film Using the obtained MgO vapor deposition material as a target material, a protective film measurement sample for PDP is produced by forming a film on a stainless steel substrate to a thickness of 100 nm using an electron beam vapor deposition apparatus. did. The obtained measurement sample was set at the target position of the secondary electron measuring device, and then the activation treatment was performed in a high vacuum, and then the secondary electron emission coefficient was measured. The sample temperature at the time of measuring the secondary electron emission coefficient was 300 ° C., and the ion acceleration voltage was 300V.

[Comparative Example 1]
Commercially available magnesium oxide (gas phase method high-purity magnesium oxide fine powder) 1.0 kg was added to 30 liters of warm water maintained at 60 ° C. and stirred for 4 hours to hydrate the magnesium oxide fine powder, and magnesium hydroxide A slurry was obtained. Next, the obtained magnesium hydroxide slurry was filtered, dehydrated and dried to obtain a magnesium hydroxide solid, and then pulverized with a pulverizer to produce a magnesium hydroxide powder. The results of measuring the chemical components and physical properties of this magnesium hydroxide are shown in Table 1.

  Next, the obtained magnesium hydroxide was baked in an electric furnace at 1300 ° C. for 60 minutes to obtain magnesium oxide. Table 2 shows the measurement results of chemical components and physical properties of this magnesium oxide. Thereafter, in the same manner as in Example 1, vapor deposition material pellets having a length of 3 mm, a width of 5 mm, and a thickness of 2 mm were obtained. The relative density and secondary electron emission coefficient of the pellet were measured. The results are shown in Table 3.

[Comparative Example 2]
Commercially available magnesia clinker (seawater-based high-purity magnesium clinker) was pulverized, and 1.0 kg of the obtained magnesium oxide granules were put into 10 liters of a 0.25 mol / l magnesium acetate aqueous solution and stirred into an autoclave. After reacting at 130 ° C. for 2 hours, the product was washed with water, filtered and dried to produce magnesium hydroxide powder. The results of measuring the chemical components and physical properties of this magnesium hydroxide are shown in Table 1.

  Next, the obtained magnesium hydroxide was baked in an electric furnace at 1300 ° C. for 60 minutes to obtain magnesium oxide. Table 2 shows the measurement results of chemical components and physical properties of this magnesium oxide. Thereafter, in the same manner as in Example 1, vapor deposition material pellets having a length of 3 mm, a width of 5 mm, and a thickness of 2 mm were obtained. The relative density and secondary electron emission coefficient of the pellet were measured. The results are shown in Table 3.

[Comparative Example 3]
Commercially available magnesium oxide (gas phase method high purity magnesium oxide fine powder) was used. FIG. 4 shows an image taken with an SEM of magnesium oxide, and Table 2 shows the results of measurement of chemical components and physical properties. Thereafter, in the same manner as in Example 1, vapor deposition material pellets having a length of 3 mm, a width of 5 mm, and a thickness of 2 mm were obtained. The relative density and secondary electron emission coefficient of the pellet were measured. The results are shown in Table 3.

It is the image which image | photographed the magnesium hydroxide of this invention Example 1 with SEM (scanning electron microscope). It is the image which image | photographed the magnesium hydroxide of this invention Example 1 with SEM. It is the image which image | photographed the magnesium oxide of this invention Example 1 with SEM. It is the image which image | photographed the magnesium oxide of the comparative example 3 with SEM.

Claims (1)

Each of Si, Al, Ca, Fe, V, Cr, Mn, Ni, Zr, B, and Zn is 10 ppm or less, the purity is 99.99 mass% or more, and the 50% particle size is 1.0 × 10. ^-6 m or less and a BET specific surface area of 8 to 30 m ² / g, a method for producing high purity magnesium hydroxide,
Magnesium chloride in which each of Si, Al, Fe, V, Cr, Mn, Ni, Zr, B, and Zn is 10 ppm or less and Ca is 30 ppm or less and the electrical conductivity were refined to 0.1 μS / cm or less. An aqueous solution mixed with pure water is reacted with an alkaline aqueous solution having an alkali content of 20 to 50% by mass to produce magnesium hydroxide, and then the produced magnesium hydroxide is hydrothermally treated in an autoclave and filtered. , Washing with water, and drying, a method for producing high-purity magnesium hydroxide.