Production method for magnesium oxide

Abstract

The present invention provides a new production method with which magnesium oxide having a higher purity can be obtained.　The present disclosure pertains to a production method for magnesium oxide. A production method for magnesium oxide according to the present disclosure comprises: a carbonation step; a separation step; a crystallization step; and a firing step.　The carbonation step is for blowing carbon dioxide gas into a suspension liquid of magnesium hydroxide and calcium hydroxide to obtain an aqueous magnesium hydrogencarbonate solution.　The separation step is for subjecting the aqueous magnesium hydrogencarbonate solution to solid-liquid separation.　The crystallization step is for crystallizing magnesium carbonate from the aqueous solution obtained in the separation step.　The firing step is for firing the magnesium carbonate to obtain magnesium oxide.　In addition, the present disclosure is characterized in that the pH of the suspension liquid is adjusted in the carbonation step.

Description

Magnesium oxide manufacturing method

[0001] The present invention relates to a novel method for producing magnesium oxide.

[0002] Magnesium oxide is used in various industrial fields as an industrial raw material and a pharmaceutical raw material. For example, magnesium oxide is known to exhibit excellent effects in various applications such as medicines, foods, vulcanization accelerators, pigments, chemical heat storage materials, battery materials, ceramic materials, adsorbents, abrasives, and catalysts.

[0003] Magnesium oxide is industrially produced by various methods using seawater or ores as natural raw materials. Among these methods, dolime (CaO.MgO) or dolomide (CaMg(CO)) is a mineral resource. ３ ) ２ ) to obtain magnesium carbonate as an intermediate product, and finally magnesium oxide.

[0004] For example, Patent Document 1 proposes a method for extracting magnesium oxide from semi-burned dolomite, including a slaking / carbonation step, a separation step, and a recovery / calcination step. The slaking / carbonation step involves adding semi-burned dolomite containing 11% by mass or less of free calcium oxide and 10% by mass or more of free magnesium oxide to a solution into which carbon dioxide-containing gas is blown and stirred, and carbonating the semi-burned dolomite simultaneously. The separation step involves filtering the elution residue, primarily composed of calcium carbonate, to obtain a filtrate containing the magnesium component of the semi-burned dolomite. The recovery / calcination step involves adding an additive that precipitates magnesium carbonate salt to the filtrate to recover the magnesium component as magnesium carbonate salt, and calcining the recovered magnesium carbonate salt at a temperature above its decomposition temperature to obtain magnesium oxide with a purity of 99% by mass or more. The magnesium oxide extraction method of Patent Document 1 is said to be capable of obtaining high-purity magnesium oxide with a purity of 99% by mass or more.

[0005] Furthermore, Patent Document 2 proposes a method for removing impurities in a method for producing magnesium carbonate by reacting magnesium hydroxide slurry with carbon dioxide gas. Specifically, the proposed impurity removal method involves maintaining the pH, which decreases during the carbonation process, at a pH value of 7.6 to 8.0 at which magnesium carbonate does not dissolve 100%, and filtering the reaction liquid after carbonation using undissolved magnesium produced at the end of carbonation as a precoat agent.

[0006] JP 2014-80347 A JP 63-40722 A

[0007] Depending on the application, magnesium oxide may be required to have an extremely high purity, with impurities removed as much as possible. However, even when the magnesium oxide extraction method of Patent Document 1 is used, impurities may still remain, and further improvements in the removal of impurities from magnesium oxide are desired. Furthermore, even when the method of Patent Document 2 is diverted as part of a magnesium oxide production method, it remains unclear whether impurities from magnesium oxide can be sufficiently removed.

[0008] Therefore, an object of the present invention is to provide a novel production method that can obtain magnesium oxide with higher purity.

[0009] The present disclosure includes the following aspects.

[0010] (First Disclosure) The first disclosure is a method for producing magnesium oxide. The method for producing magnesium oxide of the first disclosure includes the following carbonation step, separation step, crystallization step, and calcination step. The carbonation step is a step of blowing carbon dioxide gas into a suspension of magnesium hydroxide and calcium hydroxide to obtain an aqueous magnesium bicarbonate solution. The separation step is a step of subjecting the aqueous magnesium bicarbonate solution to solid-liquid separation. The crystallization step is a step of crystallizing magnesium carbonate from the aqueous solution obtained in the separation step. The calcination step is a step of calcining the magnesium carbonate to obtain magnesium oxide. The first disclosure is characterized in that the pH of the suspension is adjusted in the carbonation step.

[0011] (Second Disclosure) A second disclosure is the production method according to the first disclosure, characterized in that in the carbonation step, the pH of the suspension is adjusted to a range of 6.0 or more and less than 8.0.

[0012] (Third Disclosure) The third disclosure is the manufacturing method according to the first disclosure or the second disclosure, characterized in that in the carbonation step, the suspension is heated to a temperature of 0°C or higher and 50°C or lower, and carbon dioxide gas is blown in.

[0013] (Fourth Disclosure) A fourth disclosure is the manufacturing method according to any one of the first to third disclosures, wherein the suspension is pressurized in the carbonation step.

[0014] (Fifth Disclosure) A fifth disclosure is the manufacturing method according to any one of the first to fourth disclosures, characterized in that in the carbonation step, the suspension contains at least one material selected from dolim and dolomide as a raw material.

[0015] (Sixth Disclosure) A sixth disclosure is the manufacturing method according to any one of the first to fifth disclosures, characterized in that in the carbonation step, the concentration of the suspension is 1 to 30 g / L.

[0016] (Seventh Disclosure) The seventh disclosure is the manufacturing method according to any one of the first to sixth disclosures, wherein the crystallization step further includes a heating step in which the crystallized magnesium carbonate is heated.

[0017] (Eighth Disclosure) An eighth disclosure is the manufacturing method according to the seventh disclosure, characterized in that in the heating step, the magnesium carbonate is heated at a temperature of 50°C or higher and 250°C or lower.

[0018] (Ninth Disclosure) A ninth disclosure is the manufacturing method according to the seventh or eighth disclosure, characterized in that in the heating step, the magnesium carbonate is heated for a heating time of 1 hour or more and 72 hours or less.

[0019] According to the present invention, a novel production method can be provided that can obtain magnesium oxide with higher purity.

[0020] A preferred embodiment of the method for producing magnesium oxide of the present invention will be described in detail below.

[0021] [Method for producing magnesium oxide] One embodiment of the present invention is a method for producing magnesium oxide, comprising the following carbonation step, separation step, crystallization step, and calcination step. In the carbonation step, carbon dioxide gas is blown into a suspension of magnesium hydroxide and calcium hydroxide to obtain an aqueous magnesium hydrogen carbonate solution. In the separation step, the aqueous magnesium hydrogen carbonate solution is subjected to solid-liquid separation. In the crystallization step, magnesium carbonate is crystallized from the aqueous solution obtained in the separation step. In the calcination step, magnesium carbonate is calcined to obtain magnesium oxide.

[0022] In the manufacturing method of this embodiment, the pH of the suspension is adjusted in the carbonation step.

[0023] By adjusting the pH of the suspension in the carbonation step, it is possible to precipitate impurities contained in the suspension as solids while leaving the magnesium bicarbonate, which is a magnesium component, dissolved. In other words, the degree of dissolution of impurities can be determined by the pH of the suspension. Examples of impurities include Fe, Al, Si, As, and Pb. The pH of the suspension can be adjusted depending on the types of impurities contained in the suspension or the types of impurities that are to be preferentially removed, so that impurity components with a relatively low ionization tendency precipitate as solids.

[0024] The impurities precipitated as a solid by adjusting the pH of the suspension can be removed by solid-liquid separation in the subsequent separation step. That is, the impurities contained in the magnesium hydrogen carbonate after the carbonation step can be sufficiently reduced. By sufficiently removing the impurities in this separation step, the magnesium oxide obtained through the subsequent crystallization step and calcination step will have an extremely low impurity content, resulting in magnesium oxide with a higher purity.

[0025] In the separation step, it is believed that the impurity components can be precipitated as a solid by adjusting the pH depending on the type of impurity to be removed. In this embodiment, the carbonation step and the separation step are considered to be one set, and these steps may be repeated multiple times. In this case, the pH of the magnesium hydroxide suspension may be adjusted to be different for each set. Alternatively, the pH of the magnesium hydroxide suspension may be adjusted to be the same for each set.

[0026] As described above, the manufacturing method of this embodiment has an extremely low impurity content and produces magnesium oxide with higher purity, which has the advantage of contributing to the achievement of the SDGs (Sustainable Development Goals) adopted at the United Nations Summit.

[0027] The suspension of magnesium hydroxide and calcium hydroxide that can be used in the production method of this embodiment is not particularly limited. For example, the suspension of magnesium hydroxide and calcium hydroxide can be made from at least one material selected from dolim and dolomide. That is, the production method of this embodiment may include the following step of producing a suspension of magnesium hydroxide and calcium hydroxide before the carbonation step.

[0028] For example, the step of producing a suspension of magnesium hydroxide and calcium hydroxide may be a step of adding water to at least one selected from dolim and dolomide to form a suspension. The water used in this step may be, for example, ion-exchanged water.

[0029] The suspension of magnesium hydroxide and calcium hydroxide obtained by the above-mentioned production process is subjected to the next carbonation process.

[0030] Hereinafter, each step that may be included in the method for producing magnesium oxide of this embodiment will be described in detail.

[0031] <Carbonation Step> The carbonation step is a step in which carbon dioxide gas is blown into a suspension of magnesium hydroxide and calcium hydroxide to obtain an aqueous magnesium hydrogencarbonate solution. The carbon dioxide gas that can be used in the carbonation step may contain other gases as long as the carbonation of magnesium hydroxide is not inhibited. Furthermore, when blowing carbon dioxide gas into the suspension, it is preferable to blow the carbon dioxide gas while stirring the suspension.

[0032] In this carbonation step, the pH of the suspension is adjusted as described above. The pH of the suspension may be adjusted appropriately depending on the type and content of impurities contained in the suspension. For example, the pH of the suspension in the carbonation step may be adjusted to less than 8.0, less than 7.6, less than 7.5, or less than 7.4. For example, the pH of the suspension in the carbonation step may be adjusted to 6.0 or more, 6.5 or more, 7.0 or more, or 7.1 or more.

[0033] In the carbonation step, it is preferable to adjust the pH of the suspension to a range of 6.0 or more and less than 8.0. By adjusting the pH of the suspension to within this range, it is possible to more reliably precipitate multiple types of impurities contained in the suspension. As a result, magnesium oxide with even higher purity can be obtained.

[0034] If the pH of the suspension is less than 6.0, both Mg and Ca will dissolve, and there is a risk that calcium carbonate will be incorporated into the magnesium carbonate intermediate product described below. That is, there is a risk that impurities contained in calcium carbonate will also be incorporated into the magnesium carbonate. Furthermore, if the pH of the suspension is 8.0 or higher, the amount of dissolved magnesium component will be reduced, which may result in a deterioration in yield. A more preferable upper limit of the pH of the suspension is pH 7.5. A more preferable lower limit of the pH of the suspension is pH 6.5.

[0035] In the carbonation step, the pH of the suspension can be adjusted by blowing carbon dioxide gas in. Therefore, in the carbonation step, it is preferable to blow carbon dioxide gas in while continuously or intermittently measuring the pH of the suspension.

[0036] In the carbonation step, the supply rate of carbon dioxide gas when blowing carbon dioxide gas into the suspension is not particularly limited. For example, the supply rate of carbon dioxide gas is 5.0 L / min or less. For example, the supply rate of carbon dioxide gas is 0.1 L / min or more. A preferred upper limit of the supply rate of carbon dioxide gas is 3.0 L / min. A preferred lower limit of the supply rate of carbon dioxide gas is 0.2 L / min.

[0037] If the supply rate of carbon dioxide gas is more than 5.0 L / min per 1 L of the magnesium hydroxide suspension, the supplied carbon dioxide gas will not dissolve completely in the magnesium hydroxide suspension. If the supply rate of carbon dioxide gas is less than 0.1 L / min per 1 L of the magnesium hydroxide suspension, productivity will decrease.

[0038] In the carbonation step, the concentration of the suspension is not particularly limited. For example, the concentration of the suspension is 30 g / L or less. For example, the concentration of the suspension is 1 g / L or more. The concentration of the suspension is preferably 28 g / L or less. Furthermore, the concentration of the suspension is preferably 3 g / L or more. The concentration of the suspension can be calculated using the following formula: Concentration of suspension (g / L) = Weight of magnesium hydroxide (g) / Volume of suspension (L)

[0039] If the concentration of the magnesium hydroxide suspension is higher than 30 g / L, the magnesium hydroxide will not dissolve completely, resulting in a decrease in yield, whereas if the concentration of the magnesium hydroxide suspension is lower than 1 g / L, productivity will decrease.

[0040] In the carbonation step, the temperature of the suspension when blowing in carbon dioxide gas is not particularly limited, but it is preferable to blow in carbon dioxide gas at a temperature of 0° C. or higher and 50° C. or lower in the carbonation step. When the temperature of the suspension is within this range, multiple types of impurities contained in the suspension can be more reliably precipitated. As a result, magnesium oxide with even higher purity can be obtained.

[0041] A more preferable upper limit of the temperature of the suspension in the carbonation step is 40°C. A still more preferable upper limit of the temperature of the suspension in the carbonation step is 30°C. A more preferable lower limit of the temperature of the suspension in the carbonation step is 20°C.

[0042] In the carbonation step, when carbon dioxide gas is blown into the suspension, it is preferable to pressurize the suspension. Pressurizing the suspension makes it easier for Mg to dissolve, and makes it easier to separate MgO and CaO. The pressure of the carbon dioxide gas when pressurizing the suspension can be, for example, 0.01 MPa or more, 0.05 MPa or more, or 0.1 MPa or more. The pressure of the carbon dioxide gas when pressurizing the suspension can be, for example, 10 MPa or less, 5 MPa or less, or 1 MPa or less.

[0043] In the carbonation step, it is preferable that the aqueous magnesium hydrogen carbonate solution after the carbon dioxide gas is blown into contains undissolved magnesium carbonate. If the aqueous magnesium hydrogen carbonate solution after the carbon dioxide gas is blown into contains such undissolved magnesium carbonate, solid-liquid separation after the carbonation step becomes easy. As a result, magnesium oxide of higher purity can be obtained.

[0044] Furthermore, in the carbonation step, carbon dioxide gas may be blown into the suspension, followed by air. By first blowing carbon dioxide gas into the suspension, the magnesium component can be dissolved while Fe, an impurity, can be precipitated. Next, air can be blown into the suspension to precipitate the Fe that is still dissolved as FeO. This makes it possible to obtain magnesium oxide of even higher purity, with Fe as an impurity further reduced.

[0045] The impurities precipitated as a solid in the carbonation step as described above can be removed by solid-liquid separation in the subsequent separation step. That is, the aqueous magnesium hydrogen carbonate solution obtained in the carbonation step is subjected to the subsequent separation step.

[0046] <Separation step> The separation step is a step of subjecting the aqueous magnesium hydrogen carbonate solution after the carbonation step to solid-liquid separation. More specifically, the separation step is a step of separating the aqueous magnesium hydrogen carbonate solution, which is a liquid component, from impurities, which are solid components. This step removes the impurities from the aqueous magnesium hydrogen carbonate solution, thereby obtaining a highly pure aqueous magnesium hydrogen carbonate solution.

[0047] In this specification, "separation" includes not only the case where the solid content and the liquid content are completely separated, but also the case where a small amount of water is unavoidably contained in the solid content.

[0048] The separation means that can be used in the separation step is not particularly limited, and examples thereof include filtration means, membrane separation means, centrifugation means, solid-liquid separation means, and natural settling means.

[0049] In the separation step, the aqueous magnesium bicarbonate solution obtained after the carbonation step may be used as is, but is not limited to this. For example, the separation step may be performed in advance by adding water to the aqueous magnesium bicarbonate solution to adjust the concentration. Examples of the water used in this step include ion-exchanged water. The separation step may be performed once, or may be performed in two or more separate steps.

[0050] The aqueous magnesium hydrogen carbonate solution from which impurities have been removed by the above separation steps is subjected to the next crystallization step.

[0051] <Crystallization Step> The crystallization step is a step of crystallizing magnesium carbonate from the aqueous solution obtained in the separation step. The means for crystallizing magnesium carbonate from the aqueous solution after the separation step is not particularly limited, and examples thereof include heating means. That is, in the crystallization step, magnesium carbonate can be crystallized by heating the aqueous solution after the separation step.

[0052] The heating temperature when crystallizing magnesium carbonate is not particularly limited, and examples thereof include temperatures of 30°C or higher. A preferred lower limit of the heating temperature is 50°C. A preferred upper limit of the heating temperature when crystallizing magnesium carbonate is not particularly limited, and is, for example, 100°C. A preferred upper limit of the heating temperature is 95°C. In this specification, the "heating temperature when crystallizing magnesium carbonate" may be referred to as the "crystallization temperature" to distinguish it from the heating temperature in the heating step described below. By setting the crystallization temperature to a temperature within the above preferred range, the precipitated needle-like crystals of magnesium carbonate become plate-like crystals. This reduces the bulk of magnesium carbonate, improving the handleability of magnesium carbonate.

[0053] The shape of magnesium carbonate crystals is inherited by the shape of magnesium oxide crystals obtained by calcining the magnesium carbonate. In other words, by improving the handling properties of magnesium carbonate, the handling properties of the magnesium oxide finally obtained can be improved.

[0054] The heating time when crystallizing magnesium carbonate is not particularly limited, and can be, for example, 1 minute or more. A preferred lower limit of the heating time when crystallizing magnesium carbonate is 15 minutes. The preferred upper limit of the heating time when crystallizing magnesium carbonate is not particularly limited, but from the viewpoint of productivity, it is, for example, 600 minutes, and more preferably 180 minutes. In this specification, the "heating time when crystallizing magnesium carbonate" may be referred to as the "crystallization time" to distinguish it from the heating time in the heating step described below. This crystallization time refers to the time from when the aqueous solution after the separation step is heated to the crystallization temperature and the temperature is maintained. By setting the crystallization time within the above preferred range, the precipitated needle-like crystals of magnesium carbonate become plate-like crystals. This reduces the bulk of magnesium carbonate, improving the handleability of magnesium carbonate.

[0055] (Heating Step) In the present embodiment, the crystallization step may further include a heating step of heating the crystallized magnesium carbonate. By heating the crystallized magnesium carbonate, the needle-like crystals of magnesium carbonate become plate-like crystals. This reduces the bulk of the magnesium carbonate, improving the handleability of the magnesium carbonate.

[0056] (Heating temperature) In the heating step, the heating temperature of magnesium carbonate is not particularly limited, and examples thereof include a temperature of 50° C. or higher and 250° C. or lower. From the viewpoint of the handleability of magnesium carbonate, the lower limit of the heating temperature is preferably 60° C. Furthermore, from the viewpoint of the handleability or productivity of magnesium carbonate, the upper limit of the heating temperature is preferably 150° C.

[0057] (Heating Time) In the heating step, the heating time of magnesium carbonate is not particularly limited, but may be, for example, 1 hour or more and 72 hours or less. From the viewpoint of the handling property of magnesium carbonate, the lower limit of the heating time is preferably 3 hours. Furthermore, from the viewpoint of the handling property or productivity of magnesium carbonate, the upper limit of the heating time is preferably 48 hours.

[0058] Since the magnesium carbonate crystallized by the above-described crystallization step is obtained as a solid content in an aqueous solution, the magnesium carbonate can be obtained as a solid by subjecting the aqueous solution containing the crystallized magnesium carbonate to solid-liquid separation. That is, the crystallization step may further include a step of subjecting the aqueous solution containing the crystallized magnesium carbonate to solid-liquid separation to obtain magnesium carbonate as a solid.

[0059] The means for performing solid-liquid separation of the aqueous solution containing the crystallized magnesium carbonate is not particularly limited, and for example, the same separation means as in the above-mentioned separation step can be used.

[0060] In the crystallization step, a washing step of the crystallized magnesium carbonate may be carried out as necessary.

[0061] The magnesium carbonate obtained by the above crystallization step is subjected to the next calcination step.

[0062] <Caking Step> The baking step is a step in which magnesium oxide is obtained by baking the magnesium carbonate obtained in the crystallization step.

[0063] The calcination means for calcining magnesium carbonate is not particularly limited as long as it can produce magnesium oxide. Examples of such calcination means include a calcination furnace or a microwave.

[0064] The calcination temperature when calcining magnesium carbonate is not particularly limited as long as it is a temperature at which magnesium oxide can be produced. Examples of such a calcination temperature include temperatures of 500°C or higher. The upper limit of the calcination temperature is not particularly limited, but is, for example, 1500°C from the viewpoint of the quality or productivity of magnesium oxide. The calcination temperature is preferably 700°C or higher. Furthermore, the calcination temperature is preferably 1200°C or lower.

[0065] The calcination time when calcining magnesium carbonate is not particularly limited as long as it is a time that can produce magnesium oxide. Examples of such a calcination time include a time of 1 minute or more. The upper limit of the calcination time is not particularly limited, but from the viewpoint of productivity, it is, for example, 24 hours. The calcination time is preferably 30 minutes or more. Furthermore, the calcination time is preferably 6 hours or less.

[0066] The magnesium oxide obtained by the above firing step may be subjected to additional treatment steps as necessary, such as a surface treatment step in which the surface of magnesium oxide particles is treated with various surface treatment agents, a crushing step in which the magnesium oxide is crushed into powder, a classification step in which the magnesium oxide is classified by particle size, or a molding step in which the magnesium oxide is molded into a predetermined shape.

[0067] According to the manufacturing method of this embodiment as described above, magnesium oxide with an extremely low impurity content and higher purity can be obtained.

[0068] The manufacturing method of the present invention is not limited to the above-described embodiments or the examples described below, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0069] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0070] Example 1 A pressure vessel with a volume of 0.98 L was prepared and a suspension of magnesium hydroxide and calcium hydroxide was placed in it. Carbon dioxide gas was blown into the pressure vessel at a partial pressure of 0.20 MPa for 70 minutes while stirring the suspension of magnesium hydroxide and calcium hydroxide, to obtain an aqueous magnesium hydrogen carbonate solution.

[0071] The obtained aqueous magnesium hydrogen carbonate solution was subjected to solid-liquid separation using a Nutsche separator to remove solid impurities. The solution was then heated to 90°C and maintained at that temperature for 60 minutes to precipitate magnesium carbonate. The solution was then filtered to obtain a solid magnesium carbonate product. The obtained solid was heated at 105°C for 24 hours to obtain magnesium carbonate powder.

[0072] A crucible for firing was prepared and magnesium carbonate was placed in it. This crucible was then placed in a firing furnace that had been preheated to 900°C. After being placed in the crucible, firing was carried out at 900°C for 2 hours under atmospheric pressure to obtain a fired magnesium oxide product. The resulting fired product was sieved through a 150-micron filter to obtain the magnesium oxide powder of Example 1.

[0073] (Quantitative Analysis) The raw materials magnesium hydroxide and calcium hydroxide (Mg(OH)) used in the production of magnesium oxide in Example 1 were ２ Ca(OH) ２ ) was quantitatively analyzed using a scanning X-ray fluorescence analyzer "ZSX Primus IV" manufactured by Rigaku Corporation. The results are shown in Table 1 below.

[0074]

[0075] Magnesium carbonate (MgCO), the intermediate product of Example 1 ３ ) was quantitatively analyzed using a scanning X-ray fluorescence analyzer "ZSX Primus IV" manufactured by Rigaku Corporation. The results are shown in Table 2 below.

[0076]

[0077] As can be seen from the amounts of impurities in the magnesium carbonate above, it was found that the method for producing magnesium oxide in Example 1 can produce magnesium oxide with few impurities.

[0078] The method for producing magnesium oxide of the present invention can be suitably used for producing magnesium oxide that can be used for various applications such as pharmaceuticals, foods, vulcanization accelerators, pigments, chemical heat storage materials, battery materials, ceramic materials, adsorbents, abrasives, and catalysts.

Claims

1. a carbonation step of blowing carbon dioxide into a suspension of magnesium hydroxide and calcium hydroxide to obtain an aqueous magnesium hydrogen carbonate solution; A separation step of subjecting the aqueous magnesium hydrogen carbonate solution to solid-liquid separation; a crystallization step of crystallizing magnesium carbonate from the aqueous solution obtained in the separation step; A calcination step of calcining the magnesium carbonate to obtain magnesium oxide, A method for producing magnesium oxide, wherein in the carbonation step, a pH of the suspension is adjusted so that the concentration of the suspension is 1 to 30 g / L.

2. The method according to claim 1 , wherein the pH of the suspension is adjusted to within a range of 6.0 or more and less than 8.0 in the carbonation step.

3. The method according to claim 1, wherein in the carbonation step, the suspension is heated to a temperature of 0° C. or more and 50° C. or less, and the carbon dioxide gas is blown into the suspension.

4. The method according to claim 1 , wherein the suspension is pressurized in the carbonation step.

5. The method according to claim 1, wherein in the carbonation step, the suspension is made of at least one material selected from the group consisting of dolimes and dolomides.

6. The method according to claim 1 , wherein the crystallization step further comprises a heating step of heating the crystallized magnesium carbonate.

7. The method according to claim 6, wherein the magnesium carbonate is heated at a temperature of 50° C. or more and 250° C. or less in the heating step.

8. The method according to claim 6 or 7, wherein in the heating step, the magnesium carbonate is heated for a heating time of 1 hour or more and 72 hours or less.