Method for manufacturing magnesium metal

Abstract

To provide a method for manufacturing magnesium metal from a chemical wastewater including magnesium chloride.SOLUTION: A method for manufacturing magnesium metal includes: depositing magnesium hydroxide by treating a chemical wastewater including magnesium salt with alkali metal hydroxide or alkali metal oxide, etc.; and heat-treating calcium hydroxide that becomes an auxiliary material in aluminum reduction or a mixture of calcium oxide and magnesium hydroxide in coexistence of aluminum.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing metallic magnesium from chemical wastewater containing magnesium salts.

Background Art

[0002] Two typical methods for producing metallic magnesium are known. One is a method (thermal reduction method) in which dolomite ore is calcined to form magnesium oxide, which is then mixed with ferrosilicon (reducing agent) to produce briquettes, and the briquettes are heated and reduced under high temperature in a vacuum. This method has been put into practical use (Non-Patent Document 1, Non-Patent Document 2), and a typical example is the Pidgeon method. In addition, as a reducing agent used in the thermal reduction method, a method for producing metallic magnesium by heating and reducing a mixture of magnesium oxide and aluminum at high temperature has been disclosed (Non-Patent Document 3, Non-Patent Document 4, Non-Patent Document 5). Since these production methods use dolomite as a raw material, carbon dioxide is emitted during the calcination stage. When ferrosilicon is used as the reducing agent, ferrosilicon is produced by reducing silica and sand with coke in the presence of iron, so a large amount of carbon dioxide is emitted, and the environmental load is large, which is a problem.

[0003] The other typical method is a method for producing metallic magnesium by electrolytic reduction. This method is a method for producing metallic magnesium by converting magnesium chloride collected from seawater into anhydrous magnesium chloride and electrolyzing the obtained anhydrous magnesium chloride (Patent Document 1, Non-Patent Document 2, Non-Patent Document 6, Non-Patent Document 7). This electrolytic reduction method using seawater requires a large amount of electric power for the concentration preparation and reduction of anhydrous magnesium chloride, so the environmental load is still high, which is a problem. As a method for producing magnesium oxide, which is a raw material for producing metallic magnesium, by heating, a method is disclosed in which a sulfuric acid solution containing magnesium and calcium such as drainage is concentrated or the like, and is easily and efficiently produced (Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention aims to provide a method for producing high-purity metallic magnesium with few impurities by efficiently thermally reducing magnesium salts contained in chemical wastewater containing magnesium salts such as Grignard reagents without generating carbon dioxide during the reaction process. Specifically, it aims to provide a production method for simply obtaining magnesium hydroxide, which is a raw material for metallic magnesium, alone or simultaneously with calcium hydroxide from chemical wastewater containing magnesium salts, and adding aluminum as a reducing agent to a mixture of magnesium hydroxide and calcium hydroxide or calcium oxide, followed by heat treatment to produce high-purity metallic magnesium.

Means for Solving the Problems

[0007] The present inventors diligently studied to solve the above problems and found that by treating chemical wastewater containing magnesium salts with an alkali metal hydroxide or the like to precipitate magnesium hydroxide, and subjecting a mixture of magnesium hydroxide and calcium hydroxide or calcium oxide, which serves as an auxiliary agent in aluminum reduction, to heat treatment in the presence of aluminum, it is possible to efficiently thermally reduce without generating carbon dioxide during the reaction process to obtain high-purity metallic magnesium with few impurities, thus completing the present invention. That is, the present inventors found that the above problems can be solved by the following configuration. The present invention relates to a method for producing metallic magnesium, which comprises adding an alkali metal hydroxide or an alkali metal oxide to chemical wastewater containing magnesium salts (excluding magnesium hydroxide) to precipitate magnesium hydroxide, separating the magnesium hydroxide to obtain it in a clay-like or powdery form, and heating a mixture of the magnesium hydroxide, aluminum, and calcium hydroxide or calcium oxide under reduced pressure. Further, an alkali metal hydroxide or an alkali metal oxide is added to chemical wastewater containing a magnesium salt (excluding magnesium hydroxide), magnesium hydroxide is precipitated, and the magnesium hydroxide is separated to obtain it in a clay-like or powdery form. By heating a mixture obtained by adding calcium hydroxide or calcium oxide to the magnesium hydroxide, an oxide mixture of magnesium oxide and calcium oxide is obtained, and the present invention relates to a method for producing metallic magnesium, characterized in that a mixture obtained by adding aluminum to the obtained oxide mixture is heated under reduced pressure. Furthermore, calcium hydroxide or calcium oxide is added to chemical wastewater containing a magnesium salt (excluding magnesium hydroxide), and it is precipitated as a mixture of magnesium hydroxide and calcium hydroxide. The mixture is separated to obtain it in a clay-like or powdery form, and the present invention relates to a method for producing metallic magnesium, characterized in that a mixture obtained by adding aluminum to the mixture is heated under reduced pressure.

Advantages of the Invention

[0008] According to the present invention, metallic magnesium can be provided from chemical wastewater containing a magnesium salt in which a magnesium-containing reagent such as a Grignard reagent is used. The method for producing magnesium of the present invention does not generate carbon dioxide during the reaction process, has a small carbon dioxide emission amount during production, and has a small environmental load. According to the present invention, low-energy recycling of magnesium becomes possible.

Embodiments for Carrying Out the Invention

[0009] The method for producing metallic magnesium from chemical wastewater of the present invention will be described in detail.

[0010] ·First production method The first production method of the present invention is a method for obtaining metallic magnesium by heating a mixture obtained by adding aluminum, and calcium hydroxide or calcium oxide to clay-like or powdery magnesium hydroxide under reduced pressure. The process for obtaining clay-like or powdery magnesium hydroxide from chemical wastewater containing magnesium salts (hereinafter excluding magnesium hydroxide) is described below.

[0011] The chemical wastewater containing magnesium salts used in this process is chemical wastewater treated with hydrochloric acid, sulfuric acid, nitric acid, etc. after a chemical reaction using a magnesium-containing reagent such as a Grignard reagent. Examples of magnesium salts include one or more of magnesium chloride, magnesium sulfate, magnesium nitrate, etc., and it is preferable to contain magnesium chloride in terms of high production yield of metallic magnesium. The concentration of magnesium chloride in all magnesium salts is preferably 50 mol% or more, more preferably 60 mol% or more, still more preferably 70 mol% or more, even more preferably 80 mol% or more, even more preferably 90 mol% or more, and even more preferably 95 mol% or more. The concentration of the magnesium salt in the chemical wastewater in this process is not a problem as long as it is a concentration soluble in the chemical wastewater. Furthermore, since the chemical wastewater used in this process is chemical wastewater used in a chemical reaction such as a Grignard reagent, the chemical wastewater may contain organic compounds, organometallic compounds, or inorganic compounds generated or by-produced by the reaction, or even mixtures thereof.

[0012] In the process for obtaining clay-like or powdery magnesium hydroxide, it is essential to react an alkali metal hydroxide or an alkali metal oxide. Examples of the alkali metal include sodium, potassium, lithium, etc. or mixtures thereof. In order to simply produce magnesium hydroxide, it is preferable to use sodium hydroxide or potassium hydroxide which are alkali metal hydroxides. Also, when the magnesium salt contains magnesium chloride, it is preferable to use sodium hydroxide or potassium hydroxide which are alkali metal hydroxides because water-soluble chlorides are generated as by-products and chloride ions can be easily removed by washing with water. It has been reported that the reduction rate to metallic magnesium decreases when chloride ions are present (Research Report Collection of the Salt Science Research Foundation, Vol. 1, Engineering Sciences, 2019, p51-63). Regarding the above-mentioned alkali metal hydroxide or alkali metal oxide, it can also be added to chemical wastewater as an aqueous solution or suspension. The molar percentage of the alkali metal hydroxide or alkali metal oxide used depends on the acidic state of the chemical wastewater used. However, by using 10 to 2000 mol% with respect to the magnesium ions contained in the chemical wastewater, clay-like or powdery magnesium hydroxide can be obtained. In order to obtain clay-like or powdery magnesium hydroxide with good yield, it is preferable to use 50 to 500 mol% of the alkali metal hydroxide or alkali metal oxide with respect to the magnesium ions.

[0013] Also, in this step, after adding the alkali metal hydroxide or alkali metal oxide to the chemical wastewater, by further adding a polymer flocculant, clay-like or powdery magnesium hydroxide can be efficiently obtained. As the polymer flocculant to be used, general polymer flocculants such as anionic polymer flocculants, cationic polymer flocculants, and nonionic polymer flocculants may be used. By using 0.01 to 10 wt% based on the weight ratio with respect to the obtained magnesium hydroxide, magnesium hydroxide can be efficiently obtained.

[0014] Regardless of the use of the flocculant, after adding the alkali metal hydroxide or alkali metal oxide to the chemical wastewater, magnesium hydroxide precipitates. By filtering the precipitated magnesium hydroxide, clay-like or powdery magnesium hydroxide can be obtained. When the chemical wastewater contains organic compounds, organometallic compounds, or inorganic compounds, or mixtures thereof, produced or by-produced in a chemical reaction, not only washing the filtered magnesium hydroxide with water but also washing it with an organic solvent can increase the purity of the magnesium hydroxide. For example, when an organic compound is contained, when magnesium hydroxide is obtained in a clay-like or powdery form, it is possible to remove the organic compound by washing it with an organic solvent.

[0015] The magnesium hydroxide washed after filtration contains moisture, but by performing vacuum filtration during filtration, it can be obtained as clay-like magnesium hydroxide with a high water content. On the other hand, by sufficiently performing heat drying after filtration, powdery magnesium hydroxide with a low water content can be obtained. Since it is heated in the next step, there is no limit to the water content of magnesium hydroxide, but it is preferable to prepare clay-like or powdery magnesium hydroxide with a solid content of 10% or more in order to avoid the complexity of the operation in the next step.

[0016] Next, the process of manufacturing metallic magnesium from the clay-like or powdery magnesium hydroxide obtained from chemical wastewater will be described below. In this step, aluminum is essential for reducing the clay-like or powdery magnesium hydroxide obtained from chemical wastewater. The molar percentage of aluminum to be added can reduce magnesium hydroxide with aluminum in the range of 60 to 100 mol% to obtain metallic magnesium, but in order to obtain metallic magnesium with a good yield, it is preferably in the range of 60 to 80 mol%. The aluminum to be used may be commercially available aluminum powder or aluminum slices such as reagents, but for example, shavings or crushed materials such as aluminum cans obtained as waste may also be used. The particle size or size of the aluminum (powder, slice, shaving, crushed) to be reacted is not a problem as long as it does not harm the reduction reaction, but in order to efficiently proceed the reduction reaction to metallic magnesium, it is preferably in the range of 0.01 to 5 mm. In this specification, the particle size means the average value of the maximum diameters of 30 or more particles measured with a microscope or the like.

[0017] Also, in this step, in order to obtain metallic magnesium with a good yield, it is necessary to add calcium hydroxide or calcium oxide. The molar percentage of calcium hydroxide or calcium oxide is preferably in the range of 60 to 100 mol% with respect to the clay-like or powdery magnesium hydroxide obtained from chemical wastewater, and more preferably 60 to 80 mol% in order to obtain metallic magnesium with a better yield.

[0018] In this project, it is necessary to prepare a mixed sample by mixing magnesium hydroxide, aluminum, and calcium hydroxide or calcium oxide in clay or powder form obtained from chemical wastewater. Since each raw material is solid, arbitrarily measured magnesium hydroxide, aluminum, calcium hydroxide or calcium oxide are mixed. Also, in the preparation of the mixed sample, arbitrarily measured magnesium hydroxide, aluminum, calcium hydroxide or calcium oxide can be added to water for wet mixing and then dried to prepare the mixed sample. The obtained mixed sample can be used to produce metallic magnesium by heating it under reduced pressure with a high-temperature heating device such as a muffle furnace. In this process, it is essential to heat in the temperature range of 600 to 1500 °C under reduced pressure in the range of 0.001 to 0.01 MPa.

[0019] ·Second manufacturing method The second manufacturing method of the present invention is to heat a mixture obtained by adding calcium hydroxide or calcium oxide to the clay-like or powdery magnesium hydroxide obtained in the same manner as the first manufacturing method, to obtain a mixed oxide of magnesium oxide and calcium oxide. This is a method of obtaining metallic magnesium by heating a mixture obtained by adding aluminum to this mixed oxide under reduced pressure.

[0020] The second manufacturing method is to make a mixed oxide by heating a mixture of magnesium hydroxide and calcium hydroxide obtained by the above method in a high-temperature heating device such as a muffle furnace under reduced pressure. A predetermined amount of aluminum is mixed with the mixed oxide obtained by this method in the same manner as above, and then heated in a high-temperature heating device such as a muffle furnace under reduced pressure to produce metallic magnesium.

[0021] ·Third manufacturing method Next, a method for producing magnesium metal by preparing clay-like or powdery magnesium hydroxide containing calcium hydroxide from chemical wastewater containing a magnesium salt (excluding magnesium hydroxide), which is the third production method of the present invention, will be described below. In the third production method, since the clay-like or powdery magnesium hydroxide contains calcium hydroxide, the step of mixing calcium hydroxide or calcium oxide before the reduction step can be omitted, and a good reduction rate can be exhibited.

[0022] In this step, it is essential to react calcium hydroxide or calcium oxide with the chemical wastewater. In this reaction, it can also be reacted with the chemical wastewater as an aqueous solution of calcium hydroxide. The equivalent amount of calcium hydroxide or calcium oxide used is in the range of 10 to 2000 mol% with respect to the magnesium ions contained in the chemical wastewater, so that a mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide can be obtained. In order to obtain a mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide with good yield, it is preferable to use calcium hydroxide or calcium oxide in the range of 50 to 500 mol% with respect to the magnesium ions. Also, in this step, it is preferable to use calcium hydroxide because water-soluble calcium chloride is generated as a by-product and chloride ions can be easily removed by washing with water. In order to make the ion ratio (molar ratio) of magnesium hydroxide and calcium hydroxide described later 100:60 to 100:80, the amount of water used for washing is preferably adjusted by calculating from the solubility of 0.17 wt% of calcium hydroxide in water. Note that after washing with water, calcium hydroxide may be added in order to make the ion ratio (molar ratio) of magnesium hydroxide and calcium hydroxide 100:60 to 100:80.

[0023] In this project, after adding calcium hydroxide or calcium oxide to the chemical wastewater, and then further adding a polymer flocculant, a clay-like or powdery mixture of magnesium hydroxide and calcium hydroxide can be efficiently obtained. As the polymer flocculant to be used, common polymer flocculants such as anionic polymer flocculants, cationic polymer flocculants, and nonionic polymer flocculants may be used. By using 0.01 - 10 wt% based on the weight ratio of the magnesium hydroxide content contained in the chemical wastewater, a mixture of magnesium hydroxide and calcium hydroxide can be efficiently obtained.

[0024] Regardless of whether a flocculant is used or not, after adding calcium hydroxide or calcium oxide to the chemical wastewater, a mixture of magnesium hydroxide and calcium hydroxide will precipitate. By filtering out the precipitated mixture of magnesium hydroxide and calcium hydroxide, a clay-like or powdery mixture of magnesium hydroxide and calcium hydroxide can be obtained. When the chemical wastewater contains organic compounds, organometallic compounds, or inorganic compounds generated or by-produced in a chemical reaction, or even a mixture thereof, not only washing the filtered mixture of magnesium hydroxide and calcium hydroxide with water, but also washing it with an organic solvent can increase the purity of the mixture of magnesium hydroxide and calcium hydroxide.

[0025] The mixture of magnesium hydroxide and calcium hydroxide washed after filtration contains moisture, but by performing vacuum filtration during filtration, a clay-like mixture of magnesium hydroxide and calcium hydroxide with a high moisture content can be obtained. On the other hand, by sufficiently performing heat drying after filtration, a powdery mixture of magnesium hydroxide and calcium hydroxide with a low moisture content can be obtained. Since heating will be carried out in the next step, there is no limit to the moisture content of the mixture of magnesium hydroxide and calcium hydroxide, but it is preferable to prepare a clay-like or powdery mixture of magnesium hydroxide and calcium hydroxide with a solid content of 10% or more in order to avoid the complexity of the operation in the next step.

[0026] Next, a method for producing metallic magnesium from a mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide obtained from chemical wastewater will be described below. In this step, aluminum is essential for reducing the magnesium hydroxide present in the mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide obtained from chemical wastewater. The molar percentage of aluminum to be added can be reduced with 60 - 100 mol% of aluminum with respect to magnesium hydroxide to obtain metallic magnesium, but in order to obtain metallic magnesium with a good yield, it is preferably 60 - 80 mol%. It is preferable that the metal ion ratio (molar ratio) of magnesium hydroxide, calcium hydroxide, and aluminum is 100:60:60 - 100:80:80 in order to obtain metallic magnesium with a good yield. The aluminum to be used may be commercially available aluminum powder or aluminum slices such as reagents, etc., but for example, it may be in the form of shavings or crushed materials such as aluminum cans obtained as waste. The particle size and size of the aluminum (powder, slice, shaving, crushed) to be reacted are not a problem as long as they do not harm the reduction reaction, but in order to efficiently proceed the reduction reaction to metallic magnesium, it is preferably in the range of 0.01 - 5 mm.

[0027] In this step, it is necessary to prepare a mixed sample by mixing a mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide obtained from chemical wastewater and aluminum. Since both raw materials are solids, an arbitrarily measured mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide and aluminum is mixed. Also, in the preparation of the mixed sample, a mixed sample can be prepared by adding an arbitrarily measured mixture of clay-like or powdery magnesium hydroxide and calcium hydroxide and aluminum to water, performing wet mixing, and then drying. The obtained mixed sample can be used to produce metallic magnesium by heating it under reduced pressure in a high-temperature heating device such as a muffle furnace. In this step, it is essential to heat in the temperature range of 600 - 1500 °C under reduced pressure in the range of 0.001 - 0.01 MPa.

Example

[0028] Next, the present invention will be specifically described with reference to examples, but the scope of the present invention is not limited thereto. Table 1 shows the results of the composition analysis of the chemical wastewater used by ion chromatography.

Table 1

[0029] 〔Experiment 1〕Production of magnesium hydroxide To 137 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 23.8 wt% magnesium chloride, 370 g of 10 wt% aqueous NaOH solution was added. The resulting precipitate was washed with water and dried to obtain 19.5 g of a powdery solid. Analysis of this solid showed that the crystal structure was Mg(OH) 2 (Brucite). The Mg content by chelate titration was 37.3 wt%. (Theoretical value Mg / Mg(OH) 2 = 41.7 wt%) When the solid was heated at 800 °C and analyzed by fluorescent X-ray, it was 99% as MgO. The reason for the lower Mg content of the dried solid than the theoretical value was presumed to be the influence of residual moisture.

[0030] 〔Experiment 2〕Production of magnesium hydroxide To 137 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 23.8 wt% magnesium chloride, 370 g of 10 wt% aqueous NaOH solution was added, and a polymer flocculant was further added to form flocs to improve the solid-liquid separation operation. The resulting precipitate was washed with water and dried to obtain 19.7 g of a powdery solid. Analysis of this solid showed that the crystal structure was Mg(OH) 2 (Brucite). The Mg content by chelate titration was 38.9 wt%. (Theoretical value Mg / Mg(OH) 2 = 41.7 wt%) When the solid was heated at 800 °C and analyzed by fluorescent X-ray, it was 99% as MgO. The reason for the lower Mg content of the dried solid than the theoretical value was presumed to be the influence of residual moisture.

[0031] Experiment 3: Generation of a mixture of magnesium hydroxide and calcium hydroxide 48.0 g (0.648 mol) of calcium hydroxide was added to 120 g of water to prepare a calcium hydroxide suspension. 125 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 22.9 wt% magnesium chloride was added to the suspension, and the mixture was stirred well. The obtained precipitate was washed with water to obtain clay-like magnesium hydroxide. The total amount of all water used for washing was set to 2900 g in order to make the molar ratio of Mg:Ca in the solid content 3:2. Magnesium chloride was converted to insoluble magnesium hydroxide, calcium hydroxide that reacted with magnesium chloride and hydrochloric acid was converted to water-soluble calcium chloride, and chloride ions flowed out into the washing liquid. In addition, since calcium hydroxide has a solubility of 0.17 wt% in water, 4.9 g of it dissolved and flowed out into the washing liquid with respect to the 2900 g of water used. It was confirmed by ion chromatography that Mg was not detected in the washing liquid. When the obtained solid content was analyzed by fluorescent X-ray, it was confirmed that the molar ratio of Mg:Ca was 3:2, which is a suitable composition for Mg thermal reduction smelting.

[0032] Experiment 4: Generation of a mixture of magnesium hydroxide and calcium hydroxide 48.0 g (0.648 mol) of calcium hydroxide was added to 120 g of water to prepare a calcium suspension. 125 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 22.9 wt% magnesium chloride was added to the suspension, and the mixture was stirred well. A polymer flocculant was added, and the obtained precipitate was washed with water to obtain clay-like magnesium hydroxide. The total amount of all water used for washing was set to 2900 g in order to make the molar ratio of Mg:Ca in the solid content 3:2. Magnesium chloride was converted to insoluble magnesium hydroxide, calcium hydroxide that reacted with magnesium chloride and hydrochloric acid was converted to water-soluble calcium chloride, and chloride ions flowed out into the washing liquid. In addition, since calcium hydroxide has a solubility of 0.17 wt% in water, 4.9 g of it dissolved and flowed out into the washing liquid with respect to the 2900 g of water used. It was confirmed that Mg was not detected from the washing liquid by ion chromatography. When the obtained solid content was analyzed by X-ray fluorescence, it was confirmed that the molar ratio of Mg:Ca was 3:2, which is a suitable composition for Mg thermal reduction smelting.

[0033] 〔Example 1〕 To 137 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 23.8 wt% magnesium chloride, 370 g of 10 wt% aqueous NaOH solution was added, and a polymer flocculant was further added to form flocs to improve the solid-liquid separation operation. The obtained precipitate was washed with water and dried to obtain 19.7 g of solid content of powdery magnesium hydroxide. The Mg content by chelate titration was 37.3 wt%. To 0.78 g (0.012 mol) of the obtained magnesium hydroxide, 0.89 g (0.012 mol) of calcium hydroxide and 0.32 g (0.012 mol) of aluminum powder were added and mixed in a mortar. This mixture was subjected to thermal reduction treatment at 1150 °C under vacuum for 3 hours in a muffle furnace to recover the evaporated metallic magnesium. The recovery rate as metallic magnesium was 57% (0.17 g).

[0034] 〔Example 2〕 To 137 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 23.8 wt% magnesium chloride, 370 g of 10 wt% aqueous NaOH solution was added, and a polymer flocculant was further added to form flocs to improve the solid-liquid separation operation. The obtained precipitate was washed with water and dried to obtain 19.7 g of solid content of powdery magnesium hydroxide. The Mg content by chelate titration was 37.3 wt%. To 0.98 g (0.015 mol) of the obtained magnesium hydroxide, 0.74 g (0.010 mol) of calcium hydroxide and 0.27 g (0.010 mol) of aluminum powder were added and mixed in a mortar. This mixture was subjected to thermal reduction treatment at 1150 °C under vacuum for 3 hours in a muffle furnace to recover the evaporated metallic magnesium. The recovery rate as metallic magnesium was 80% (0.29 g).

[0035] 〔Example 3〕 48.0 g (0.648 mol) of calcium hydroxide was added to 120 g of water to prepare a calcium suspension. 125 g of chemical wastewater containing 4.8 wt% hydrochloric acid and 22.9 wt% magnesium chloride was added to the suspension, and the mixture was stirred thoroughly. A polymer flocculant was added to form flocs, aiming to improve the solid-liquid separation operation. The obtained precipitate was washed with water, and the total amount of all the water used was set to 2900 g. Magnesium chloride was converted to insoluble magnesium hydroxide, and the calcium hydroxide that reacted with magnesium chloride and hydrochloric acid was converted to water-soluble calcium chloride, which flowed out into the washing liquid. Also, since calcium hydroxide has a solubility of 0.17 wt% in water, 4.9 g of it dissolved and flowed out into the washing liquid with respect to the 2900 g of water used. By this method, 32.3 g of a mixture of magnesium hydroxide and calcium hydroxide with a molar ratio of Mg metal ions to Ca metal ions of 3:2 was obtained. This mixture was calcined at 800 °C for 3 hours in the air to obtain a mixture of magnesium oxide and calcium oxide. This was mixed so that the molar ratio of magnesium oxide to aluminum became 3:2 to prepare a sample. 2.0 g of this sample was subjected to a thermal reduction treatment at 1150 °C for 3 hours under vacuum, and the evaporated metallic magnesium was recovered. The recovery rate as metallic magnesium was 80% (0.31 g).

[0036] The composition of the metallic magnesium produced in Example 2 above was measured by the ICP-AES method. The results other than Mg are shown in Table 2. The purity of the obtained metallic magnesium was 99.98%.

Table 2

[0037] 「Comparative Example 1」 The magnesium hydroxide taken out from the chemical wastewater in Experiment 1 was calcined at 800 °C for 3 hours in the air to obtain magnesium oxide. This was mixed so that the molar ratio of magnesium oxide to aluminum became 3:2 to prepare a sample. A thermal reduction treatment was carried out at 1150 °C for 3 hours under vacuum, and the evaporated metallic magnesium was recovered. The recovery rate as metallic magnesium decreased to 47%.

Industrial Applicability

[0038] According to the present invention, after treating chemical wastewater containing a magnesium salt with an alkali metal hydroxide or the like to obtain magnesium hydroxide, aluminum and calcium hydroxide are mixed, and metallic magnesium can be provided by heating the mixture.

Claims

1. To chemical wastewater containing a magnesium salt (excluding magnesium hydroxide), an alkali metal hydroxide or an alkali metal oxide is added to precipitate magnesium hydroxide, the precipitated magnesium hydroxide is separated and obtained in a clay-like or powdery form, and a mixture obtained by adding aluminum, calcium hydroxide or calcium oxide to the magnesium hydroxide is heated under reduced pressure. A method for producing metallic magnesium, characterized by the above.

2. The method for producing metallic magnesium according to claim 1, wherein the amounts of aluminum, calcium hydroxide or calcium oxide added to the clay-like or powdery magnesium hydroxide are each 60 to 100 mol% based on the magnesium hydroxide.

3. To chemical wastewater containing a magnesium salt (excluding magnesium hydroxide), an alkali metal hydroxide or an alkali metal oxide is added to precipitate magnesium hydroxide, the precipitated magnesium hydroxide is separated and obtained in a clay-like or powdery form, and the product obtained by adding calcium hydroxide or calcium oxide to the magnesium hydroxide is heated to obtain an oxide mixture of magnesium oxide and calcium oxide, and a mixture obtained by adding aluminum to the obtained oxide mixture is heated under reduced pressure. A method for producing metallic magnesium, characterized by the above.

4. When producing the oxide mixture, the amount of calcium hydroxide or calcium oxide added is 60 to 100 mol% based on the magnesium hydroxide, and the amount of aluminum added to the oxide mixture is 60 to 100 mol% based on the magnesium hydroxide. The method for producing metallic magnesium according to claim 3, characterized by the above.

5. To chemical wastewater containing a magnesium salt (excluding magnesium hydroxide), calcium hydroxide or calcium oxide is added to precipitate a mixture of magnesium hydroxide and calcium hydroxide, the mixture is separated and obtained in a clay-like or powdery form, and a mixture obtained by adding aluminum to the mixture is heated under reduced pressure. A method for producing metallic magnesium, characterized by the above.

6. A method for producing metallic magnesium according to claim 5, wherein the amount of calcium hydroxide or calcium oxide added to the chemical wastewater is 60 to 100 mol% based on magnesium hydroxide, the amount of aluminum added to the mixture is 60 to 100 mol% based on magnesium hydroxide, and when separating the mixture, it is washed with water to adjust the molar ratio of Mg to Ca in the solid content to 100:60 to 100:80.

Images