Method for producing magnesium hydroxide having reduced impurities and method for reducing impurities in calcium hydroxide

Abstract

To provide a method for producing magnesium hydroxide having reduced impurities by using limestone, which can easily produce magnesium hydroxide having reduced impurities.SOLUTION: The first disclosure relates to a method for producing magnesium hydroxide having reduced impurities by using limestone. The method according to the first disclosure includes a calcium hydroxide slurry formation step, a solid-liquid separation step, and a magnesium hydroxide slurry formation step. In the calcium hydroxide slurry formation step, calcium oxide derived from the limestone is mixed with water to form a slurry containing an aqueous solution of calcium hydroxide derived from the limestone and the solid impurities derived from the limestone. In the solid-liquid separation step, the slurry is solid-liquid separated into the solid impurities and the aqueous solution of calcium hydroxide. In the magnesium hydroxide slurry formation step, the aqueous solution of calcium hydroxide is reacted with a water-soluble magnesium compound to form a slurry of magnesium hydroxide.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method for producing magnesium hydroxide having reduced impurities using limestone, and a method for reducing impurities in calcium hydroxide formed from limestone. [Background technology]

[0002] Various methods for producing magnesium hydroxide have been investigated. For example, Patent Document 1 discloses a method for producing high-purity magnesium hydroxide, which comprises reacting magnesium hydroxide containing impurities with an aqueous solution containing a water-soluble calcium salt and carbon dioxide gas to produce an aqueous solution of magnesium salt and a precipitate of calcium carbonate, transferring the impurities into the precipitate and filtering off the precipitate, reacting the aqueous solution of magnesium salt with ammonia to obtain a magnesium hydroxide slurry and an aqueous solution of ammonium salt, filtering off the magnesium hydroxide slurry, and obtaining an aqueous solution containing high-purity magnesium hydroxide and the ammonium salt, excess ammonia, and unreacted magnesium salt.

[0003] The specification of Patent Document 1 discloses that "conventionally, magnesium hydroxide has been obtained by using seawater as a raw material and reacting it with milk of lime. However, such magnesium hydroxide contains relatively large amounts of boric acid, silica, alumina, iron, calcium, etc., which are mixed in from the milk of lime, etc., making it unsuitable for use in the above-mentioned luxury products in this state." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 155529 / 1983 Summary of the Invention [Problem to be solved by the invention]

[0005] In the method disclosed in Patent Document 1, after magnesium hydroxide is formed, multiple reagents are used and multiple steps are carried out to produce high-purity magnesium hydroxide. Therefore, the method disclosed in Patent Document 1 is not easy to carry out and is not necessarily preferable from the viewpoint of achieving the Sustainable Development Goals (SDGs). Therefore, an object of the present disclosure is to provide a method for producing magnesium hydroxide with reduced impurities using limestone, which can easily produce magnesium hydroxide with reduced impurities. [Means for solving the problem]

[0006] The present disclosure includes the following aspects. (First Disclosure) The present disclosure relates to a method for producing magnesium hydroxide with reduced impurities by using limestone, the method according to the present disclosure including a calcium hydroxide slurry formation step, in which calcium oxide derived from the limestone and water are mixed to form a slurry containing an aqueous solution of calcium hydroxide derived from the limestone and the solid impurities derived from the limestone.

[0007] The method according to the first disclosure includes a solid-liquid separation step, in which the slurry is separated into the solid impurities and the aqueous solution of calcium hydroxide. The method according to the first disclosure includes a magnesium hydroxide slurry formation step, in which the aqueous solution of calcium hydroxide is reacted with a water-soluble magnesium compound to form a magnesium hydroxide slurry.

[0008] (Second Disclosure) In this second disclosure, in the first disclosure, the aqueous solution of calcium hydroxide contains 50 ppm or less of Fe. (Third Disclosure) In the present third disclosure, in the first or second disclosure, the slurry in the calcium hydroxide slurry formation step contains calcium hydroxide particles, and in the present third disclosure, in the solid-liquid separation step, new water is supplied to recover at least a part of the calcium hydroxide particles as the calcium hydroxide aqueous solution.

[0009] (Fourth Disclosure) In the present fourth disclosure, in the first to third disclosures, the concentration of carbon dioxide in the aqueous calcium hydroxide solution is maintained at 1.0 ppm or less in at least one of the solid-liquid separation step and the magnesium hydroxide slurry formation step. (Fifth Disclosure) In the present fifth disclosure, in the fourth disclosure, the aqueous solution of calcium hydroxide is maintained in an inert gas atmosphere in at least one of the solid-liquid separation step and the magnesium hydroxide slurry formation step.

[0010] (Sixth Disclosure) In the present sixth disclosure, in any one of the first to fifth disclosures, at least a part of the magnesium hydroxide slurry formation step and at least a part of the solid-liquid separation step are carried out simultaneously. (Seventh Disclosure) The method according to the seventh disclosure is any one of the first to sixth disclosures, and includes a powder forming step after the magnesium hydroxide slurry forming step. In the powder forming step, the magnesium hydroxide powder is formed from the magnesium hydroxide slurry. (Eighth Disclosure) In this eighth disclosure, in the seventh disclosure, the magnesium hydroxide powder contains 50 ppm or less of Fe.

[0011] (Ninth Disclosure) In this ninth disclosure, in either the seventh or eighth disclosure, the magnesium hydroxide powder contains 0.60% by mass or less of CaCO3.

[0012] (10th Disclosure) In the present tenth disclosure, in any one of the seventh to ninth disclosures, the magnesium hydroxide powder contains 0.20 ppm or less of Cd. (Eleventh Disclosure) In this eleventh disclosure, in any one of the seventh to tenth disclosures, the magnesium hydroxide powder contains 20 ppm or less of Mn.

[0013] (12th Disclosure) In the present twelfth disclosure, in any one of the seventh to eleventh disclosures, the powder forming step further includes a hydrothermal treatment step, in which the magnesium hydroxide slurry or a dehydrate thereof is hydrothermally treated in the presence of at least one selected from the group consisting of calcium chloride, potassium chloride, and sodium chloride.

[0014] (13th Disclosure) The thirteenth disclosure relates to a method for reducing impurities in calcium hydroxide formed from limestone. The method according to the thirteenth disclosure includes a calcium hydroxide slurry forming step, in which calcium oxide derived from the limestone is mixed with water to form a slurry containing an aqueous solution of calcium hydroxide derived from the limestone and the solid impurities derived from the limestone. The method according to the thirteenth disclosure includes a solid-liquid separation step, in which the slurry is separated into the solid impurities and the aqueous solution of calcium hydroxide. Effect of the Invention

[0015] The method for producing magnesium hydroxide having reduced impurities by using limestone according to the present disclosure makes it possible to easily produce magnesium hydroxide having reduced impurities. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The method for producing magnesium hydroxide with reduced impurities using limestone according to the present disclosure, and the method for reducing impurities in calcium hydroxide formed from limestone will be described in detail below. The method for producing magnesium hydroxide with reduced impurities using limestone according to the present disclosure may be simply referred to as the "production method according to the present disclosure." The method for reducing impurities in calcium hydroxide formed from limestone according to the present disclosure may be simply referred to as the "reduction method according to the present disclosure."

[0017] The manufacturing method and reduction method according to the present disclosure include a calcium hydroxide slurry formation step, in which calcium oxide derived from the limestone is mixed with water to form a slurry containing an aqueous solution of calcium hydroxide derived from the limestone and the solid impurities derived from the limestone. Hereinafter, the slurry containing calcium hydroxide derived from the limestone and the solid impurities derived from the limestone may be referred to as a "slurry of calcium hydroxide derived from limestone."

[0018] The limestone-derived calcium hydroxide slurry can be formed, for example, as follows. Limestone is calcined at 900°C to 1300°C to obtain calcium oxide. The obtained calcium oxide is added to water so as to have a concentration of, for example, 200 g / L, and the calcium oxide and water are reacted at, for example, 90°C to obtain a reaction product. Examples of the water include ion-exchanged water, industrial water, and tap water. The reaction product is passed through, for example, a 50-mesh sieve to remove, for example, unreacted calcium oxide and / or calcium carbonate. The reaction product from which the unreacted calcium oxide and / or calcium carbonate has been removed is mixed with water to obtain the limestone-derived calcium hydroxide slurry.

[0019] The temperature at which the limestone-derived calcium hydroxide slurry is formed is preferably 1° C. or higher, and more preferably 15° C. or higher. This makes it easier to form an aqueous solution of calcium hydroxide in the slurry while preventing the slurry from freezing. The temperature at which the limestone-derived calcium hydroxide slurry is formed is preferably 80° C. or lower, more preferably 35° C. or lower, and even more preferably 35° C. or lower. This makes it possible to increase the ratio of the aqueous solution of calcium hydroxide in the slurry.

[0020] In the above limestone-derived calcium hydroxide slurry, the calcium concentration is preferably 5.0 mol / L or less, and more preferably 3.5 mol / L or less. In addition, in the above limestone-derived calcium hydroxide slurry, the calcium concentration is preferably 1.0 mol / L or more. This makes it possible to suppress an increase in the viscosity of the slurry while maintaining production efficiency.

[0021] The limestone-derived calcium hydroxide slurry generally contains 50 ppm or more of Fe, more generally 80 ppm or more, and even more generally 100 ppm or more of Fe in a powder state, although this varies depending on the limestone used as a raw material. The limestone-derived calcium hydroxide slurry generally contains 250 ppm or less of Fe, more generally 200 ppm or less, and even more generally 150 ppm or less of Fe in a powder state.

[0022] The limestone-derived calcium hydroxide slurry contains, in a powdered state, generally 0.1 ppm or more, more generally 0.2 ppm or more, and even more generally 0.3 ppm or more of Cd, although this varies depending on the limestone used as the raw material. The limestone-derived calcium hydroxide slurry contains, in a powdered state, generally 3.0 ppm or less, more generally 2.0 ppm or less, and even more generally 1.0 ppm or less of Cd.

[0023] The limestone-derived calcium hydroxide slurry contains, in a powdered state, Mn at a concentration of generally 3 ppm or more, more generally 5 ppm or more, and even more generally 10 ppm or more, although this varies depending on the limestone used as the raw material. The limestone-derived calcium hydroxide slurry contains, in a powdered state, Mn at a concentration of generally 200 ppm or less, more generally 100 ppm or less, and even more generally 50 ppm or less.

[0024] In this specification, the amount of Fe in the calcium hydroxide slurry can be measured by powdering the calcium hydroxide slurry and then using a scanning X-ray fluorescence analyzer ZSX Primus4 manufactured by Rigaku Corporation. In addition, in this specification, the amounts of Cd and Mn in the calcium hydroxide slurry can be measured by powdering the calcium hydroxide slurry and using a polarized Zeeman atomic absorption spectrophotometer ZA3000 manufactured by Hitachi High-Tech Science Corporation. In this specification, the slurry of limestone-derived calcium hydroxide can be dehydrated and dried to a constant weight to give a powdered state.

[0025] The limestone-derived calcium hydroxide slurry may further contain calcium hydroxide particles derived from limestone in addition to the limestone-derived calcium hydroxide aqueous solution and the limestone-derived solid impurities. In that case, in the solid-liquid separation step described below, new water is added to dissolve the calcium hydroxide particles in water, and the solid-liquid separation can be performed as a liquid. Examples of the water include ion-exchanged water, industrial water, and tap water.

[0026] The manufacturing method and the reduction method according to the present disclosure include a solid-liquid separation step. In the solid-liquid separation step, the slurry is separated into the solid impurities and the aqueous solution of calcium hydroxide. The solid-liquid separation step can be performed using a solid-liquid separation device. An example of the solid-liquid separation device is a Nutsche. When the solid-liquid separation device is a Nutsche, the slurry is subjected to suction filtration using the Nutsche, whereby solid-liquid separation can be performed into the solid impurities and the aqueous solution of calcium hydroxide.

[0027] When the limestone-derived calcium hydroxide slurry further contains limestone-derived calcium hydroxide particles, the calcium hydroxide particles can be recovered as an aqueous solution of calcium hydroxide in the solid-liquid separation step. For example, new water can be supplied to the limestone-derived calcium hydroxide slurry before being fed into a solid-liquid separation device, the calcium hydroxide particles can be dissolved in water to form an aqueous solution of calcium hydroxide, and the aqueous solution can be recovered as a liquid using a solid-liquid separation device. In addition, when the limestone-derived calcium hydroxide slurry is subjected to solid-liquid separation using a solid-liquid separation device, new water can be supplied to the residue containing the calcium hydroxide particles remaining in the solid-liquid separation device, and the calcium hydroxide particles can be recovered as an aqueous solution of calcium hydroxide.

[0028] The aqueous solution of calcium hydroxide separated by the solid-liquid separation step has a calcium concentration of preferably 0.0001 mol / L or more, and more preferably 0.001 mol / L or more. The aqueous solution of calcium hydroxide separated by the solid-liquid separation step has a calcium concentration of preferably 0.1 mol / L or less, and more preferably 0.01 mol / L or less. This makes it possible to recover an aqueous solution of calcium hydroxide with fewer impurities in high yield while suppressing the total amount of the aqueous solution of calcium hydroxide.

[0029] The aqueous solution of calcium hydroxide separated by the solid-liquid separation step contains Fe at a content of 50 ppm or less, more preferably 30 ppm or less, even more preferably 20 ppm or less, and even more preferably 10 ppm or less. The lower limit of the Fe content in the aqueous solution of calcium hydroxide separated by the solid-liquid separation step is not particularly limited, but is, for example, 0.1 ppm or more.

[0030] In this specification, the content of Fe in an aqueous solution of calcium hydroxide is measured as follows. (1) Add 5 mL of 6.79 mol / L nitric acid aqueous solution to 25 mL of calcium hydroxide aqueous solution, and add ion-exchanged water to make the total volume 50 mL to form a sample. (2) The amount of Fe in the sample is measured using a Hitachi High-Tech Science Corporation ICP optical emission spectrometer PS3500. (3) Multiply the measured amount of Fe by two to obtain the Fe content in the calcium hydroxide solution.

[0031] The manufacturing method and reduction method according to the present disclosure include a magnesium hydroxide slurry formation step in which the aqueous solution of calcium hydroxide is reacted with a water-soluble magnesium compound to form a magnesium hydroxide slurry. The water-soluble magnesium compounds include, for example, magnesium chloride, magnesium sulfate, bittern and / or seawater.

[0032] An example of the water-soluble magnesium compound is magnesium nitrate.

[0033] The reaction between the aqueous solution of calcium hydroxide and the water-soluble magnesium compound can be carried out, for example, by at least one of the following methods (i) to (iv). (i) Batch method-1: An aqueous solution of calcium hydroxide and a solution of a water-soluble magnesium compound are simultaneously poured into a reaction vessel to form a magnesium hydroxide slurry. (ii) Batch Method-2: A solution of a water-soluble magnesium compound is poured into an aqueous solution of calcium hydroxide to form a magnesium hydroxide slurry. (iii) Batch method-3: An aqueous solution of calcium hydroxide is poured into a solution of a water-soluble magnesium compound to form a slurry of magnesium hydroxide. (iv) Continuous process: An aqueous solution of calcium hydroxide and a solution of a water-soluble magnesium compound are continuously poured into a reaction vessel to continuously form a magnesium hydroxide slurry. In consideration of production efficiency, the continuous process (iv) is preferred.

[0034] The reaction can be carried out at a reaction temperature of preferably 1° C. or higher, and more preferably 15° C. or higher. The reaction can be carried out at a reaction temperature of preferably 80° C. or lower, and more preferably 35° C. or lower. This can improve production efficiency while suppressing freezing.

[0035] The above reaction can be carried out preferably at a pH of 9.0 or more. The above reaction can be carried out preferably at a pH of 12.0 or less. This allows the magnesium hydroxide slurry to be stably formed.

[0036] The manufacturing method according to the present disclosure and the reduction method according to the present disclosure may further include, after the magnesium hydroxide slurry formation step, a powder formation step of forming magnesium hydroxide powder from the magnesium hydroxide slurry. In the powder formation step, the magnesium hydroxide slurry can be subjected to, for example, a solid-liquid separation step, a washing step, a hydrothermal step, and / or a drying step to obtain magnesium hydroxide powder.

[0037] In the solid-liquid separation step, the magnesium hydroxide slurry can be separated into a solid component containing magnesium hydroxide particles and a liquid component by a solid-liquid separation device. Examples of the solid-liquid separation device include a Nutsche. The washing step can be carried out by adding water to the solid component, which is the residue on the solid-liquid separation device. The amount of water can be, for example, 20 times the amount of the solid component. Examples of the water include ion-exchanged water, industrial water, and tap water.

[0038] The hydrothermal step can be carried out by re-emulsifying the solid component containing magnesium hydroxide particles in ion-exchanged water to form a re-emulsion, and then subjecting the re-emulsion to hydrothermal treatment, for example, at 100 to 200°C, for example, for 0.5 to 5.0 hours.

[0039] In the hydrothermal step, for example, a chloride can be added to the re-emulsified product to promote dissolution and precipitation of magnesium hydroxide, thereby increasing the particle size of the magnesium hydroxide particles. As the particle size of the magnesium hydroxide particles increases, the D50 of the magnesium hydroxide powder formed increases, and the surface area of ​​the magnesium hydroxide powder formed by the BET method tends to decrease. The chloride includes at least one selected from the group consisting of calcium chloride, potassium chloride and sodium chloride, with calcium chloride being preferred.

[0040] In the drying step, the solid component is dried, for example, until it reaches a constant weight, thereby obtaining calcium hydroxide powder.

[0041] The magnesium hydroxide powder contains Fe at a content of preferably 50 ppm or less, more preferably 40 ppm or less, even more preferably 30 ppm or less, even more preferably 20 ppm or less, and even more preferably 10 ppm or less. In the magnesium hydroxide powder, the lower limit of the Fe content is, for example, 1.0 ppm. By setting the Fe content in the magnesium hydroxide powder within the above range, it can be used for various purposes.

[0042] The magnesium hydroxide powder contains Cd at a content of preferably 0.20 ppm or less, more preferably 0.17 ppm or less, even more preferably 0.15 ppm or less, and even more preferably 0.13 ppm or less. The lower limit of the Cd content in the magnesium hydroxide powder is, for example, 0.05 ppm. By setting the Cd content in the magnesium hydroxide powder within the above range, it can be used for various purposes.

[0043] The magnesium hydroxide powder contains Mn at a content of preferably 20 ppm or less, more preferably 15 ppm or less, even more preferably 10 ppm or less, and even more preferably 7 ppm or less. The lower limit of the Mn content in the magnesium hydroxide powder is, for example, 1 ppm. By setting the Mn content in the magnesium hydroxide powder within the above range, it can be used for various purposes.

[0044] The magnesium hydroxide powder contains Si at a content of preferably 0.020 mass% or less, and more preferably 0.015 mass% or less. The lower limit of the Si content in the magnesium hydroxide powder is, for example, 0.001 mass%. By setting the Si content in the magnesium hydroxide powder within the above range, it can be used for various purposes.

[0045] The magnesium hydroxide powder contains Al at a content of preferably 0.005% by mass or less, and more preferably 0.003% by mass or less. The lower limit of the Al content in the magnesium hydroxide powder is, for example, 0.0001% by mass. By setting the Al content in the magnesium hydroxide powder within the above range, it can be used for various purposes.

[0046] The magnesium hydroxide powder contains Zn at a content of preferably 40 ppm or less, more preferably 30 ppm or less, and further preferably 20 ppm or less. The lower limit of the Zn content in the magnesium hydroxide powder is, for example, 1 ppm. By setting the Zn content in the magnesium hydroxide powder within the above range, it can be used for various purposes.

[0047] In this specification, the contents of Si, Al, Zn, Cl, S, Na and Ni in the magnesium hydroxide powder can be measured using a scanning X-ray fluorescence analyzer ZSX Primus IV manufactured by Rigaku Corporation. In this specification, the respective contents of Cd and Mn in the magnesium hydroxide powder can be measured using a polarized Zeeman atomic absorption spectrophotometer ZA3000 manufactured by Hitachi High-Tech Science Corporation. In this specification, the Fe and CaCO3 contents in the magnesium hydroxide powder can be measured using an ICP optical emission spectrometer PS3500 manufactured by Hitachi High-Tech Science Corporation.

[0048] The magnesium hydroxide slurry or magnesium hydroxide powder produced as described above is less likely to contain impurities that are poorly soluble in water and are derived from limestone. In the present disclosure, the impurities include at least one of Fe, Mn, Cd, Al, Si, Cl, Na, Ni, Zn, S, Pb, As, P, V, and Cr.

[0049] The magnesium hydroxide slurry or magnesium hydroxide powder is suitable for use as, for example, a flame retardant for polymer materials, a resin filler, a heat storage material, a coating agent for paper, and / or a catalyst, since it contains reduced amounts of impurities derived from limestone that are poorly soluble in water, such as Fe, Cd, and Mn.

[0050] In at least one of the calcium hydroxide slurry forming step, the solid-liquid separation step, and the magnesium hydroxide slurry forming step, the contact between the aqueous calcium hydroxide solution and carbon dioxide is reduced, for example, the concentration of carbon dioxide in the aqueous calcium hydroxide solution can be maintained lower than that in the atmosphere, thereby preventing calcium hydroxide and carbon dioxide from reacting with each other in the aqueous calcium hydroxide solution to form calcium carbonate, and thus making it difficult for calcium carbonate to remain in the magnesium hydroxide powder.

[0051] From the above viewpoints, the carbon dioxide concentration in the aqueous solution of calcium hydroxide is preferably 100 ppm or less, more preferably 10 ppm or less, even more preferably 1.0 ppm or less, even more preferably 0.5 ppm or less, and even more preferably 0.1 ppm or less. The lower limit of the carbon dioxide concentration in the aqueous solution of calcium hydroxide is, for example, 0.001 ppm.

[0052] In the powder formation step, particularly in the solid-liquid separation step in the powder formation step, the contact between the magnesium hydroxide slurry and carbon dioxide is reduced, for example, the concentration of carbon dioxide in the magnesium hydroxide slurry can be maintained lower than that in the atmosphere, thereby suppressing the reaction of remaining calcium and carbon dioxide in the magnesium hydroxide slurry to form calcium carbonate, and thus making it difficult for calcium carbonate to remain in the magnesium hydroxide powder.

[0053] In the powder forming step, the carbon dioxide concentration in the magnesium hydroxide slurry is preferably 100 ppm or less, more preferably 10 ppm or less, even more preferably 1.0 ppm or less, even more preferably 0.5 ppm or less, and even more preferably 0.1 ppm or less. The lower limit of the carbon dioxide concentration in the magnesium hydroxide slurry is, for example, 0.001 ppm.

[0054] In order to achieve the above-mentioned carbon dioxide concentration, in at least one of the calcium hydroxide slurry forming step, the solid-liquid separation step, and the magnesium hydroxide slurry forming step, the aqueous solution of calcium hydroxide can be held in an inert gas atmosphere, and at least one of the calcium hydroxide slurry forming step, the solid-liquid separation step, and the magnesium hydroxide slurry forming step can also be carried out under an inert gas atmosphere.

[0055] Similarly, in the powder formation step, particularly the solid-liquid separation step in the powder formation step, the magnesium hydroxide slurry can be kept in an inert gas atmosphere and can also be carried out under an inert gas atmosphere. The inert gas may be, for example, a group consisting of nitrogen gas and rare gases, such as helium gas and argon gas.

[0056] In order to achieve the above carbon dioxide concentration, an oil layer may be formed on the surface of the aqueous calcium hydroxide solution in at least one of the calcium hydroxide slurry forming step, the solid-liquid separation step, and the magnesium hydroxide slurry forming step. Similarly, an oil layer may be formed on the surface of the magnesium hydroxide slurry in the powder forming step, particularly in the solid-liquid separation step in the powder forming step. Examples of the oil that forms the oil layer include hexane, ethyl acetate, benzene, and diethyl ether.

[0057] In order to achieve the above-mentioned carbon dioxide concentration, a vessel having a predetermined S / V ratio can be used in at least one of the solid-liquid separation step, the magnesium hydroxide slurry formation step, and the powder formation step. 2 ) means the area of ​​contact between the aqueous solution of calcium hydroxide and / or magnesium hydroxide slurry contained in the container and a gas containing carbon dioxide, for example, the outside air, and V (cm 3 ) means the volume of the aqueous calcium hydroxide solution or magnesium hydroxide slurry contained in the container. The S / V ratio is preferably 0.10 cm -1 Less than or equal to 0.07 cm, and more preferably -1 This makes it possible to reduce contact between the calcium hydroxide and / or magnesium hydroxide slurry and carbon dioxide, and therefore makes it difficult for calcium carbonate to remain in the magnesium hydroxide powder.

[0058] In order to reduce the contact between the aqueous calcium hydroxide solution and carbon dioxide, a part or all of the magnesium hydroxide slurry formation step and a part or all of the solid-liquid separation step can be carried out simultaneously. In addition, in order to reduce the contact between the aqueous calcium hydroxide solution and the magnesium hydroxide slurry and carbon dioxide, a part or all of the magnesium hydroxide slurry formation step and a part or all of the solid-liquid separation step in the powder formation step can be carried out simultaneously. In addition, in order to reduce the contact between the aqueous calcium hydroxide solution and the magnesium hydroxide slurry and carbon dioxide, a part or all of the magnesium hydroxide slurry formation step and a part or all of the solid-liquid separation step in the powder formation step can be carried out simultaneously.

[0059] The magnesium hydroxide powder formed by reducing the contact between the aqueous solution of calcium hydroxide and carbon dioxide, and the magnesium hydroxide powder formed by reducing the contact between the magnesium hydroxide slurry and carbon dioxide, preferably contain 0.60 mass% or less of CaCO3, more preferably 0.40 mass% or less, even more preferably 0.20 mass% or less, and even more preferably 0.10 mass% or less. Note that the lower limit of CaCO3 in the magnesium hydroxide powder is, for example, 0.01 mass%. EXAMPLES

[0060] The present disclosure will be described below with reference to examples, but the present disclosure is not limited to these examples. [Production Example 1] Limestone was calcined at 900°C to obtain calcium oxide. The obtained calcium oxide was added to ion-exchanged water so that the calcium oxide concentration was 200g / L, and the liquid temperature was adjusted to 90°C to obtain a suspension containing calcium hydroxide. The obtained suspension containing calcium hydroxide was passed through a 50-mesh sieve to obtain a 2.22 mol / L calcium hydroxide slurry No. 1. The calcium hydroxide slurry No. 1 was dehydrated and dried to obtain calcium hydroxide powder No. 1. The composition of calcium hydroxide powder No. 1 was measured according to the method described in this specification. The results are shown in Table 1.

[0061] A container was filled with 1200 mL of ion-exchanged water. While stirring the contents of the container at 200 rpm, 15 mL of calcium hydroxide slurry No. 1 was poured into the container, and the contents of the container were stirred for 20 minutes. The contents of the container were then suction filtered using a Nutsche funnel, and the filtrate was obtained as aqueous calcium hydroxide solution No. 1. The calcium concentration in aqueous calcium hydroxide solution No. 1 was 0.0191 mol / L. The Fe content of aqueous calcium hydroxide solution No. 1 was measured. The results are shown in Table 2.

[0062] [Comparative Manufacturing Example 1] A container was filled with 1200 mL of ion-exchanged water. While stirring the contents of the container at 200 rpm, 15 mL of calcium hydroxide slurry No. 1 was poured into the container, and stirring was continued for 20 minutes to obtain calcium hydroxide aqueous solution No. 2 containing solid impurities. The calcium hydroxide aqueous solution No. 2 was stirred, and the Fe content in the calcium hydroxide aqueous solution No. 2 was measured in a state in which the solid impurities were uniformly dispersed. The results are shown in Table 2. The calcium concentration in calcium hydroxide aqueous solution No. 2 was 0.0292 mol / L.

[0063] [Table 1]

[0064] [Table 2]

[0065] [Example 1] 375 mL of calcium hydroxide slurry No. 1 was mixed with 30 L of ion-exchanged water at room temperature, and then stirred for 18.5 hours to obtain a slurry. The slurry was suction filtered using a Nutsche filter, and the obtained filtrate was obtained as an aqueous solution of calcium hydroxide No. 3. The concentration of the aqueous solution of calcium hydroxide No. 3 was 0.0194 mol / L.

[0066] A batch reaction tank was filled with 30 L of calcium hydroxide aqueous solution No. 3. While stirring the contents of the reaction tank at 300 rpm, 402 mL of 1.61 mol / L magnesium chloride aqueous solution was poured into the batch reaction tank over 5 minutes to react calcium hydroxide and magnesium chloride, thereby obtaining a first magnesium hydroxide slurry No. 1. The temperature during the reaction was 19.5°C and the pH was 10.72.

[0067] The first magnesium hydroxide slurry No. 1 was suction filtered using a Nutsche filter to obtain a filtration residue. A part of the filtrate was added to the filtration residue so that the total amount was 600 mL, and the filtration residue was re-emulsified to obtain a re-emulsion. The re-emulsion was hydrothermally treated at 170°C for 4 hours to obtain a second magnesium hydroxide slurry No. 1.

[0068] The second magnesium hydroxide slurry No. 1 was suction filtered using a Nutsche filter, and the filtration residue was washed with ion-exchanged water in an amount 20 times the solid content. The filtration residue after washing was dried at 105°C for 12 hours to obtain magnesium hydroxide powder No. 1. The Fe, Cd, and Mn contents, D50, and specific surface area measured by the BET method of magnesium hydroxide powder No. 1 are shown in Table 3. The Fe content was measured using a scanning X-ray fluorescence analyzer ZSX Primus IV manufactured by Rigaku Corporation.

[0069] D50 was measured using a laser diffraction / scattering particle size distribution analyzer MT3300EX II manufactured by Microtrack Bell Co., Ltd. D50 means the 50% particle size on a volume basis. The specific surface area was measured by the BET method using a specific surface area and pore distribution measuring device, BELsorp-max, manufactured by Microtrack BEL Co., Ltd.

[0070] [Example 2] 375 mL of calcium hydroxide slurry No. 1 was mixed with 30 L of ion-exchanged water at room temperature, and the mixture was stirred for 15.5 hours to obtain a slurry. The slurry was suction filtered using a Nutsche filter, and the filtrate was obtained as an aqueous solution of calcium hydroxide No. 4. The concentration of the aqueous solution of calcium hydroxide No. 4 was 0.0205 mol / L.

[0071] A batch reaction tank was filled with 30 L of calcium hydroxide aqueous solution No. 4. While stirring the contents of the reaction tank at 300 rpm, 424 mL of 1.61 mol / L magnesium chloride aqueous solution was poured into the batch reaction tank over 5 minutes to react calcium hydroxide and magnesium chloride, thereby obtaining a first magnesium hydroxide slurry No. 2. The reaction temperature was 21.2°C and the pH was 10.42.

[0072] The first magnesium hydroxide slurry No. 2 was suction filtered using a Nutsche filter to obtain a filtration residue. 50 mL of a 5.2 mol / L calcium chloride aqueous solution was added to the filtration residue, and then ion-exchanged water was further added so that the total amount became 600 mL, to re-emulsify the filtration residue and obtain a re-emulsion. The re-emulsion was subjected to hydrothermal treatment at 170°C for 4 hours to obtain a second magnesium hydroxide slurry No. 2.

[0073] The second magnesium hydroxide slurry No. 2 was suction filtered using a Nutsche filter, and the filtration residue was washed with ion-exchanged water in an amount 20 times the solid content. The filtration residue after washing was dried at 105°C for 12 hours to obtain magnesium hydroxide powder No. 2. The Fe, Cd, and Mn contents, D50, and specific surface area by BET of magnesium hydroxide powder No. 2 are shown in Table 3. The Fe content was measured using a scanning X-ray fluorescence analyzer ZSX Primus IV manufactured by Rigaku Corporation.

[0074] [Comparative Example 1] Ion-exchanged water was added to calcium hydroxide slurry No. 1 to obtain a 1.05 mol / L calcium hydroxide slurry. A batch reaction tank was filled with 365 mL of a 1.79 mol / L magnesium chloride aqueous solution. While stirring the contents of the reaction tank at 200 rpm, 529 mL of a 1.05 mol / L calcium hydroxide slurry was poured into the batch reaction tank over 5 minutes to react the calcium hydroxide and magnesium chloride, thereby obtaining a first magnesium hydroxide slurry No. 3. The reaction temperature was 23.0°C and the pH was 9.10.

[0075] The first magnesium hydroxide slurry No. 3 was hydrothermally treated at 170° C. for 4 hours to obtain a second magnesium hydroxide slurry No. 3. The second magnesium hydroxide slurry No. 3 was suction filtered using a Nutsche filter, and the filtration residue was washed with ion-exchanged water in an amount 20 times the amount of solid matter. The filtration residue after washing was dried at 105°C for 12 hours to obtain magnesium hydroxide powder No. 3. The Fe, Cd, and Mn contents, D50, and specific surface area by BET of magnesium hydroxide powder No. 3 are shown in Table 3. The Fe content was measured using a scanning X-ray fluorescence analyzer ZSX Primus IV manufactured by Rigaku Corporation.

[0076] [Table 3]

[0077] It can be seen that magnesium hydroxide powders No. 1 and No. 2 in Examples 1 and 2 have lower contents of Fe, Cd and Mn than magnesium hydroxide powder No. 3 in Comparative Example 1.

[0078] [Example 3] 122 mL of calcium hydroxide slurry No. 1 produced in Production Example 1 was mixed with 10 L of ion-exchanged water at room temperature and stirred for 30 minutes to obtain calcium hydroxide slurry No. 2. At room temperature, 131 mL of magnesium chloride aqueous solution with a concentration of 1.7 mol / L was placed in a filter bottle and stirred at 800 rpm using a magnetic stirrer. A Buchner funnel was attached to the filter bottle, and calcium hydroxide slurry No. 2 was filtered using the Buchner funnel to simultaneously separate the calcium hydroxide aqueous solution into solid and liquid and react with calcium hydroxide and magnesium chloride.

[0079] After the addition was completed, the reaction mixture was stirred for 30 minutes to obtain magnesium hydroxide slurry No. 3. The S / V ratio during the reaction was 0.02 cm -1 Magnesium hydroxide slurry No. 3 was filtered under reduced pressure and then washed with ion-exchanged water in an amount 20 times the mass of the magnesium hydroxide to obtain a magnesium hydroxide cake. The magnesium hydroxide cake was dried at 105°C for 24 hours to obtain magnesium hydroxide powder No. 4. The Fe and CaCO3 contents of magnesium hydroxide powder No. 4 are shown in Table 4, and the Cl, Si, S, Al, Zn, Fe, Na and Ni contents are shown in Table 5.

[0080] [Example 4] Magnesium hydroxide powder No. 5 was obtained in the same manner as in Example 3, except for the following. -To obtain calcium hydroxide slurry No. 2, 12.2 mL of calcium hydroxide slurry No. 1 and 1 L of ion-exchanged water were used. The S / V ratio during the reaction was 0.06 cm -1 It was. The process from obtaining calcium hydroxide slurry No. 2 to obtaining magnesium hydroxide cake was carried out under a nitrogen atmosphere using a glove bag, and the carbon dioxide concentration was kept at 1.0 ppm or less. The Fe and CaCO3 contents of magnesium hydroxide powder No. 5 are shown in Table 4.

[0081] [Comparative Example 2] The calcium hydroxide slurry No. 2 was filtered under reduced pressure using a Buchner funnel and a filter bottle to obtain an aqueous calcium hydroxide solution. The concentration of the obtained aqueous calcium hydroxide solution was 0.02 mol / L. At room temperature, 129 mL of a 1.7 mol / L aqueous magnesium chloride solution was placed in a new filter bottle and stirred at 800 rpm using a magnetic stirrer. The aqueous calcium hydroxide solution was poured into the new filter bottle, and the aqueous calcium hydroxide solution and the aqueous magnesium chloride solution were reacted. After the pouring was completed, the reaction mixture was stirred for 30 minutes to obtain a magnesium hydroxide slurry No. 4. The S / V ratio during the reaction was 0.02 cm -1 In the same manner as in Example 1, magnesium hydroxide powder No. 6 was obtained from magnesium hydroxide slurry No. 4. The Fe and CaCO3 contents of magnesium hydroxide powder No. 6 are shown in Table 4, and the Cl, Si, S, Al, Zn, Fe, Na and Ni contents are shown in Table 5.

[0082] [Comparative Example 3] 410 mL of a 1.7 mol / L magnesium chloride aqueous solution was placed in a stainless steel reaction vessel, and the temperature of the aqueous solution was adjusted to 35° C. while stirring at 200 rpm using a chemical stirrer. 290 mL of a 2.22 mol / L calcium hydroxide slurry produced in Production Example 1, adjusted to 35° C., was poured into the stainless steel reaction vessel over 5 minutes to obtain magnesium hydroxide slurry No. 5. The S / V ratio during the reaction was 0.14 cm -1 Magnesium hydroxide slurry No. 5 was filtered under reduced pressure and then washed with deionized water in an amount 20 times the mass of the magnesium hydroxide to obtain a magnesium hydroxide cake. The magnesium hydroxide cake was dried at 105°C for 24 hours to obtain magnesium hydroxide powder No. 7. The Fe and CaCO3 contents of magnesium hydroxide powder No. 7 are shown in Table 4, and the Cl, Si, S, Al, Zn, Na and Ni contents are shown in Table 5.

[0083] It can be seen that the magnesium hydroxide powders No. 4 and No. 5 obtained in Examples 3 and 4 have a reduced amount of CaCO3 compared to the magnesium hydroxide powder No. 6 obtained in Comparative Example 2. This is believed to be due to the fact that in Examples 3 and 4, the aqueous calcium hydroxide solution absorbs carbon dioxide, suppressing the production of calcium carbonate.

[0084] It can be seen that the magnesium hydroxide powder No. 4 obtained in Example 3 has reduced amounts of Si, Al, Zn, and Cl, compared to the magnesium hydroxide powder No. 7 obtained in Comparative Example 3. It is believed that the Si, Al, and Zn are due to being removed as solid impurities in the solid-liquid separation process. It is believed that the Cl is due to the reaction conditions.

[0085] [Table 4]

[0086] [Table 5]

Claims

1. A method for producing magnesium hydroxide having reduced impurities using limestone, comprising the steps of: a calcium hydroxide slurry formation step of mixing the calcium oxide derived from the limestone with water to form a slurry containing an aqueous solution of calcium hydroxide derived from the limestone and the solid impurities derived from the limestone; a solid-liquid separation step of separating the slurry into the solid impurities and the aqueous solution of calcium hydroxide; a magnesium hydroxide slurry formation step of reacting the aqueous solution of calcium hydroxide with a water-soluble magnesium compound to form a magnesium hydroxide slurry; A method comprising:

2. The method according to claim 1 , wherein the aqueous solution of calcium hydroxide separated in the solid-liquid separation step contains 50 ppm or less of Fe.

3. The slurry in the calcium hydroxide slurry formation step contains calcium hydroxide particles, In the solid-liquid separation step, fresh water is supplied to recover at least a part of the calcium hydroxide particles as the aqueous solution of calcium hydroxide. The method of claim 1.

4. The method according to claim 1 , wherein the concentration of carbon dioxide in the aqueous calcium hydroxide solution is maintained at 1.0 ppm or less in at least one of the solid-liquid separation step and the magnesium hydroxide slurry formation step.

5. The method according to claim 4, wherein the aqueous solution of calcium hydroxide is maintained in an inert gas atmosphere in at least one of the solid-liquid separation step and the magnesium hydroxide slurry formation step.

6. 2. The method of claim 1, wherein at least a portion of the magnesium hydroxide slurry forming step and at least a portion of the solid-liquid separation step are carried out simultaneously.

7. 10. The method of claim 1, further comprising, after said magnesium hydroxide slurry forming step, a powder forming step of forming a magnesium hydroxide powder from said magnesium hydroxide slurry.

8. 8. The method of claim 7, wherein the magnesium hydroxide powder contains 50 ppm or less of Fe.

9. The magnesium hydroxide powder contains 0.60% by mass or less of CaCO 3 The method of claim 7, comprising:

10. 8. The method of claim 7, wherein the magnesium hydroxide powder contains 0.20 ppm or less of Cd.

11. 8. The method of claim 7, wherein the magnesium hydroxide powder contains 20 ppm or less Mn.

12. 8. The method according to claim 7, wherein the powder forming step further comprises a hydrothermal treatment step of hydrothermally treating the magnesium hydroxide slurry or a dehydrated product thereof in the presence of at least one selected from the group consisting of calcium chloride, potassium chloride and sodium chloride.

13. 1. A method for reducing impurities in calcium hydroxide formed from limestone, comprising the steps of: a calcium hydroxide slurry formation step of mixing the calcium oxide derived from the limestone with water to form a slurry containing an aqueous solution of calcium hydroxide derived from the limestone and the solid impurities derived from the limestone; a solid-liquid separation step of separating the slurry into the solid impurities and the aqueous solution of calcium hydroxide; A method comprising:

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