JP7776126B2 - Method for producing Zn-Al layered double oxide - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. (1) The 86th Annual Meeting of the Society of Chemical Engineers, Abstracts, March 8, 2021. (2) The 86th Annual Meeting of the Society of Chemical Engineers, held online using a web conferencing system, http://www3.scej.org/meeting/86a/, March 21, 2021, 10:20-11:20.

The present invention relates to a method for producing a Zn-Al-based layered double oxide.

In recent years, efforts have been made to recycle and reuse waste that would otherwise be disposed of, with a view to environmental conservation and the effective use of resources.

Wastewater containing metals discharged from plating factories, metal refineries, and other facilities is generally treated within the plant, but this wastewater treatment process generates large amounts of metal sludge. This metal sludge is formed when chemicals are added to the wastewater to turn the metals into an insoluble solid, and is usually metal hydroxides or sulfides.

For sludge containing expensive metals such as precious metals and nickel, methods have been developed to increase the purity of these metals and recycle them (see, for example, Patent Document 1). On the other hand, most sludge containing inexpensive metals such as zinc is landfilled without being recycled.

Patent No. 6616845

However, in recent years, disposal costs have risen sharply due to a shortage of landfill sites for sludge. Furthermore, landfilling metal sludge could lead to metal contamination of the soil, making it undesirable to continue landfilling from the perspective of protecting the global environment.

From this perspective, there is a need for a method to convert the zinc contained in wastewater discharged from factories into useful substances and recycle or reuse it.

The objective of the present invention is to provide a method for producing Zn-Al layered double oxides that can be used as anion adsorbents, using zinc contained in wastewater as a raw material.

The gist of the present invention is as follows.
[1] A method for producing a Zn-Al-based layered double hydroxide (LDO), comprising: a precipitation step of adding an aluminum source and a carbonate ion source to zinc wastewater containing zinc ions to precipitate a Zn-Al-based layered double hydroxide (LDH); and a calcination step of calcining the Zn-Al-based layered double hydroxide (LDH).
[2] The manufacturing method described in [1], characterized in that the step of adding the aluminum source and the carbonate ion source to the zinc wastewater comprises a first addition step of adding the aluminum source to the zinc wastewater, and a second addition step of adding the carbonate ion source to the zinc wastewater to which the aluminum source has been added.
[3] The method according to [1] or [2], wherein the carbonate ion source is an alkali metal carbonate and/or carbon dioxide gas.
[4] The method according to any one of [1] to [3], wherein the aluminum source is one or more compounds selected from the group consisting of aluminum nitrate, aluminum chloride, polyaluminum chloride, and aluminum sulfate.
[5] The manufacturing method according to any one of [1] to [4], characterized in that the molar ratio of carbonate ions to sulfate ions in the zinc wastewater to which the aluminum source and the carbonate ion source have been added is 20 or more.
[6] The method according to any one of [1] to [5], wherein the molar ratio of zinc contained in the Zn—Al-based layered double oxide (LDO) to aluminum contained in the Zn—Al-based layered double oxide (LDO) is 1.9 or more and 2.5 or less.
[7] The method according to any one of [1] to [6], wherein the content of sulfate ions in the Zn—Al-based layered double oxide (LDO) is 2 mass% or less.
[8] The method according to any one of [1] to [7], wherein the Zn—Al-based layered double oxide (LDO) is an anion adsorbent.

The present invention provides a method for producing a Zn-Al layered double oxide that can be used as an anion adsorbent, using zinc contained in wastewater as a raw material.

2 is a flowchart of a manufacturing method according to the present embodiment. 10 is a flowchart of a manufacturing method according to the present embodiment (modification); FIG. 1 is a diagram showing an XRD pattern of the Zn—Al-based layered double oxide obtained in Example 1. FIG. 1 shows XRD patterns of the Zn—Al-based layered double hydroxide and the Zn—Al-based layered double oxide obtained in Example 5.

One embodiment of the present invention is described below.

This embodiment relates to a method for producing a Zn-Al-based layered double oxide (LDO). As shown in Figure 1, the method for producing a Zn-Al-based layered double oxide of this embodiment includes a precipitation step S101 and a calcination step S102.

The precipitation step S101 is a step in which an aluminum source and a carbonate ion source are added to zinc wastewater containing zinc ions to precipitate Zn-Al layered double hydroxides (LDH).

In the precipitation step S101, the zinc wastewater to which the aluminum source and carbonate ion source are added is wastewater containing at least zinc ions, and may be, for example, zinc wastewater discharged from a plating factory, a metal refinery, or a zinc mine. The source of the zinc ions contained in the zinc wastewater is not particularly limited, but examples include zinc ions generated by dissolving water-soluble zinc salts such as ZnCl ₂ , ZnSO ₄ , and Zn(NO ₃ ) ₂ in water (wastewater).

The zinc wastewater used in the precipitation step S101 may contain zinc ions as well as other substances. The substances other than zinc ions vary depending on the source of the wastewater, but examples include metal ions other than zinc ions (hereinafter also referred to as "other metal ions"), non-metal ions, and organic substances. Examples of other metal ions include iron ions, manganese ions, sodium ions, potassium ions, chromate ions, magnesium ions, and calcium ions. Examples of non-metal ions include ammonium ions, cyanide ions, and sulfate ions. Examples of organic substances include citric acid, succinic acid, aldehydes, benzalacetone, and PEG.

When zinc wastewater contains other metal ions, the molar amount of zinc ions is preferably 90% or more of the total molar amount of metal ions (including zinc ions) excluding alkali metal ions. When the molar amount of zinc ions is 90% or more of the total molar amount of metal ions excluding alkali metal ions, Zn-Al layered double hydroxide (LDH) is more likely to precipitate than when the molar amount is less than 90%.

Substances other than zinc ions that may be contained in zinc wastewater may be dissolved in the zinc wastewater, or may exist as solids (insoluble matter) in the zinc wastewater. To facilitate precipitation of Zn-Al layered double hydroxide (LDH), it is preferable to remove the solids by solid-liquid separation before adding the aluminum source and carbonate ion source. Furthermore, if the solids that may be contained in zinc wastewater are zinc salts that can be dissolved in inorganic acids, such as zinc hydroxide or basic zinc carbonate, inorganic acid may be added to the zinc wastewater to dissolve the zinc salt, and then the aluminum source and carbonate ion source may be added.

The zinc ion concentration in zinc wastewater is not particularly limited and can be, for example, 500 gm/L or more, or 1,000 mg/L or more. The upper limit of the zinc ion concentration in zinc wastewater is also not particularly limited and can be, for example, 10,000 mg/L or less. The zinc ion concentration in zinc wastewater can be determined using an ICP optical emission spectrometer (for example, Shimadzu Corporation's Shimadzu Sequential Type High-Frequency Plasma Optical Emission Spectrometer ICPS-7510).

The pH of the zinc wastewater to which the aluminum source and carbonate ion source are added (zinc wastewater before addition) is not particularly limited, but from the perspective of facilitating precipitation of Zn-Al layered double hydroxides (LDHs) is preferably less than 7, and more preferably less than 4. When the pH of the zinc wastewater to which the aluminum source and carbonate ion source are added is 7 or higher, it may be used as is, but it is preferable to adjust the pH to less than 7 using an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid. However, when the pH of the zinc wastewater is highly alkaline, at 12 or higher, and the zinc in the solids in the zinc wastewater is less than 5% (in terms of molar amount) of the zinc ions dissolved in the wastewater, using it as is or adjusting the pH to less than 7 will not significantly affect the ease of precipitation of Zn-Al layered double hydroxides (LDHs).

The aluminum source added to the zinc wastewater is a substance that can cause aluminum ions to be contained in the zinc wastewater after addition, and examples thereof include water-soluble aluminum salts such as aluminum nitrate, aluminum chloride, polyaluminum chloride, and aluminum sulfate (aluminum sulfate hydrate). From the viewpoint of facilitating the precipitation of Zn-Al layered double hydroxide (LDH), the aluminum source is preferably polyaluminum chloride. Polyaluminum chloride is a substance expressed as [Al ₂ (OH) _n Cl _6-n ] _m (where 1≦n≦5, m≦10).

The amount of aluminum source added to zinc wastewater is not particularly limited, but can be, for example, an amount such that the molar ratio of zinc ions to aluminum ions (hereinafter also referred to as "Zn ²⁺ /Al ³⁺ ") in zinc wastewater to which the aluminum source and carbonate ion source have been added (hereinafter also referred to as "zinc wastewater after addition") is 1.5 to 3.5. From the viewpoint of improving the anion adsorption performance of the Zn-Al-based layered double oxide (LDO) obtained by the production method of this embodiment, Zn ²⁺ /Al ³⁺ is preferably 1.8 to 2.5, and more preferably 1.9 to 2.2. The concentration of aluminum ions in zinc wastewater may be determined using an ICP optical emission spectrometer (for example, Shimadzu Corporation's Shimadzu sequential high-frequency plasma optical emission spectrometer ICPS-7510). However, if the zinc wastewater does not contain aluminum ions before addition, or if the amount of aluminum ions in the zinc wastewater before addition is such that it does not affect the Zn ²⁺ /Al ³⁺ ratio, the concentration may be determined by calculation from the amount of aluminum in the aluminum source, taking into account the solubility of the aluminum source added to the zinc wastewater.

The aluminum source added to the zinc wastewater may be one that has not yet generated aluminum ions, generating aluminum ions in the zinc wastewater, or one that has been dissolved in a solvent (e.g., water) to generate aluminum ions beforehand, and then added to the zinc wastewater with aluminum ions still present.

The carbonate ion source to be added to the zinc wastewater is a substance that can cause the zinc wastewater to contain carbonate ions (CO ₃ ²⁻ ) after addition, and examples thereof include alkali metal carbonates, carbon dioxide gas, ammonium carbonate, and ammonium hydrogen carbonate. From the viewpoint of making it easier to adjust the carbonate ion concentration contained in the zinc wastewater, it is preferable to use alkali metal carbonates and/or carbon dioxide gas among these carbonate ion sources.

The amount of carbonate ion source added to zinc wastewater is not particularly limited, but from the viewpoint of improving the anion adsorption performance of the Zn—Al-based layered double oxide (LDO) obtained by the production method of this embodiment, the amount added is preferably such that the molar ratio of carbonate ions to zinc ions in the zinc wastewater after addition (hereinafter also referred to as "CO ₃ ²⁻ /Zn ²⁺ ") is 2.8 or more, more preferably 2.8 to 9, and even more preferably 4.5 to 6.5. The concentration of carbonate ions in zinc wastewater may be determined by titration in accordance with the Warder method, but if the zinc wastewater before addition does not contain carbonate ions or if the amount of carbonate ions in the zinc wastewater before addition does not affect CO ₃ ²⁻ /Zn ²⁺ , the concentration may be determined by calculation from the amount of carbonate in the carbonate ion source, taking into account the solubility of the carbonate ion source added to the zinc wastewater.

The carbonate ion source to be added to the zinc wastewater may be one that has not yet generated carbonate ions, generating carbonate ions in the zinc wastewater, or one that has been dissolved in a solvent (e.g., water) to generate carbonate ions beforehand, and then added to the zinc wastewater with carbonate ions still present.

In the precipitation step S101, the order in which the aluminum source and carbonate ion source are added to the zinc wastewater is not particularly limited; the aluminum source and carbonate ion source may be added in any order, or the aluminum source and carbonate ion source may be added simultaneously. From the perspective of facilitating the precipitation of Zn-Al-based layered double hydroxides (LDHs), it is preferable to add the aluminum source to the zinc wastewater and then the carbonate ion source to the zinc wastewater. In other words, the step of adding the aluminum source and carbonate ion source to the zinc wastewater preferably comprises a first addition step in which the aluminum source is added to the zinc wastewater, and a second addition step in which the carbonate ion source is added to the zinc wastewater to which the aluminum source has been added.

The pH of the zinc wastewater to which the aluminum source and carbonate ion source have been added (hereinafter also referred to as "zinc wastewater after addition") is not particularly limited and can be, for example, 6.5 or higher and lower than 10.5, or 7.0 or higher and lower than 8.7. From the perspective of improving the anion adsorption performance of the Zn-Al-based layered double oxide (LDO) obtained by the manufacturing method of this embodiment, the pH of the zinc wastewater after addition is preferably 7.0 or higher and lower than 8.0, and more preferably 7.0 or higher and lower than 7.3.

The pH of the zinc wastewater after addition can be adjusted by changing the type and amount of carbonate ion source added to the zinc wastewater. For example, increasing the amount of carbon dioxide gas (carbonate ion source) added can lower the pH, while increasing the amount of sodium carbonate (carbonate ion source) added can raise the pH. The pH of the zinc wastewater after addition can be adjusted by changing the type and amount of carbonate ion source added to the zinc wastewater, or by separately adding a pH adjuster to the zinc wastewater. Examples of pH adjusters that can be added to zinc wastewater include inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and hydroxides such as sodium hydroxide and potassium hydroxide. The order in which the pH adjuster is added to the zinc wastewater is not particularly limited. For example, the pH adjuster may be added between the first and second addition steps, or after the first and second addition steps.

In the precipitation step S101, an aluminum source and a carbonate ion source are added to the zinc wastewater, causing aluminum ions and zinc ions to co-precipitate in the presence of carbonate ions, resulting in the precipitation of Zn-Al layered double hydroxide (LDH).

Coprecipitation of aluminum ions and zinc ions proceeds by adding an aluminum source and a carbonate ion source to zinc wastewater. To facilitate the coprecipitation, the zinc wastewater may be stirred after the addition. The coprecipitation time (stirring time) is not particularly limited, but can be, for example, 30 minutes or more. From the perspective of improving the anion adsorption performance of the Zn-Al-based layered double oxide (LDO) obtained by the manufacturing method of this embodiment, the stirring time of the zinc wastewater after the addition is preferably 10 hours or more, more preferably 20 hours or more, and particularly preferably 24 hours or more.

The temperature of the zinc wastewater during co-precipitation of aluminum and zinc is not particularly limited and can be, for example, room temperature (1 to 30°C).

Here, we will explain the Zn-Al-based layered double hydroxide precipitated in the precipitation step S101.

Layered double hydroxides (LDHs) are crystalline (or microcrystalline) substances represented by the following general formula (1) (crystalline substances having the following general formula (1) as a repeating unit), and are known to be composed of a host layer of octahedral layers that are positively charged by substituting some of the M ²⁺ in the metal hydroxide M(OH) ² with M ³⁺ , and a guest layer that consists of anions that compensate for the positive charge of the host layer and interlayer water.
[M ²⁺ _1-x M ³⁺ _x (OH) ₂ ] [(A ^n- ) _x/n・yH ₂ O]...(1)
(In the above chemical formula (1), M ²⁺ is a divalent metal ion, M ³⁺ is a trivalent metal ion, A ^n- is an n-valent anion, x is a value greater than 0 and less than 1, n is 1 to 3, and y is a value greater than 0. It is preferable that x is 0.22 to 0.3.)

In contrast, the Zn—Al-based layered double hydroxide precipitated in the precipitation step S101 of this embodiment is a substance that contains zinc ions (Zn ²⁺ ) as M ²⁺ in the above chemical formula (1) and aluminum ions (Al ³⁺ ) as M ³⁺ in the above chemical formula (1).

The Zn—Al-based layered double hydroxide precipitated in the precipitation step S101 of this embodiment is preferably a carbonate-type Zn—Al-based layered double hydroxide represented by the following general formula (2), more preferably a substance represented by Zn ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O.
[Zn ²⁺ _1-x Al ³⁺ _x (OH) ₂ ] [CO ₃ ^2- _x/2・yH ₂ O]...(2)
(In the above formula (2), x is a value greater than 0 and less than 1, and y is a value greater than 0. It is preferable that x is 0.22 to 0.3.)

The fact that the precipitate deposited in the precipitation step S101 is a Zn-Al-based layered double hydroxide can be identified by comparing the XRD pattern of the precipitate deposited in the zinc wastewater (hereinafter also referred to as the "XRD pattern of the precipitate") with the XRD pattern of a known Zn-Al-based layered double hydroxide (hereinafter also referred to as the "XRD pattern of a known LDH"). For example, when comparing the XRD pattern of the precipitate with the XRD pattern of a known LDH, if the FOM (Figure of Merit) is 10 or less, the precipitate can be determined to be the Zn-Al-based layered double hydroxide. Furthermore, for example, the three peaks with the highest intensities can be selected from the XRD pattern of the precipitate (selecting the three peaks in descending order of intensity), and if the peak top positions (diffraction angles) and intensities are similar to the three peaks in the XRD pattern of a known LDH, and if the FOM when comparing the XRD pattern of the known LDH with the XRD pattern of the precipitate is 10 or less, the precipitate can be determined to be the Zn-Al-based layered double hydroxide. The FOM is a value that quantitatively indicates the difference between the compared XRD patterns; the smaller the FOM, the greater the match.

The conditions for measuring the XRD pattern of the precipitate may be, for example, the following: As the XRD pattern of a known LDH with which the XRD pattern of the precipitate can be compared, for example, that of Zn ₆ Al ₂ (OH) ₁₆ CO 3.4H ₂ O disclosed as DB card number 00-038-0486 of the ICDD ( _{International} Centre for Diffraction Data) can be used.
[XRD measurement conditions]
Radiation source: CuKα ray Voltage: 40 kV
Current value: 30mA
Measurement range: 2θ = 5° to 90°

Next, the calcination step S102 will be described. Calcination step S102 is a step in which the Zn-Al-based layered double hydroxide (LDH) is calcined.

In the calcination step S102 of this embodiment, the conditions for calcining the Zn-Al-based layered double hydroxide can be any known calcination conditions used to obtain a layered double oxide (LDO) from a layered double hydroxide (LDH), and are not particularly limited.

For example, the temperature at which the Zn-Al-based layered double hydroxide is calcined can be 300°C or higher and lower than 600°C. Preferably, the calcination temperature is 380°C or higher and lower than 520°C, and more preferably, the calcination temperature is 420°C or higher and lower than 510°C.

Furthermore, for example, the atmosphere in which Zn-Al-based layered double hydroxide (LDH) is calcined can be an air atmosphere or a nitrogen atmosphere. A nitrogen atmosphere is preferred.

Furthermore, for example, the calcination time for Zn-Al-based layered double hydroxide (LDH) can be 1 hour or more. Preferably, the calcination time is 2 hours or more, and more preferably, 3 hours or more. The upper limit of the calcination time is not particularly limited, but can be, for example, 8 hours or less.

In the calcination step S102 of this embodiment, the Zn—Al-based layered double hydroxide (LDH) is calcined to remove the n-valent anion represented by A ^n- in the above formula (1), thereby producing the Zn—Al-based layered double oxide (LDO).

Here, we will explain the Zn-Al-based layered double oxide produced by the manufacturing method of this embodiment.

Layered double oxides (LDOs) are crystalline (or microcrystalline) substances represented by the following chemical formula (3), and are known to have a structure in which the guest layer in layered double hydroxides (LDHs) has been removed.
M ²⁺ _2(1-x)/(2+x) M ³⁺ _2x/(2+x) O...(3)
(In the above chemical formula (3), M ²⁺ is a divalent metal ion, M ³⁺ is a trivalent metal ion, and x is a value greater than 0 and less than 1. Note that x is preferably 0.2 to 0.35.)

The Zn—Al-based layered double oxide produced by the production method of this embodiment is a substance that contains zinc ions (Zn ²⁺ ) as M ²⁺ in the above chemical formula (3) and aluminum ions (Al ³⁺ ) as M ³⁺ in the above chemical formula (3).

The Zn—Al-based layered double oxide produced by the production method of this embodiment is preferably a compound represented by the following chemical formula (4): Specific examples of the Zn—Al-based layered double oxide represented by the following chemical formula (4) include Zn ₆ Al ₂ O ₉ .
Zn ²⁺ _2(1-x)/(2+x) Al ³⁺ _2x/(2+x) O...(4)
(In the above formula (4), x is a value greater than 0 and less than 1. It is preferable that x is 0.2 to 0.35.)

The substance produced by the production method of this embodiment can be identified as a Zn-Al-based layered double oxide by comparing the XRD pattern of the produced substance (hereinafter also referred to as the "XRD pattern of the product") with the XRD pattern of a known Zn-Al-based layered double oxide (hereinafter also referred to as the "XRD pattern of a known LDO"). Specifically, for example, when the XRD pattern of the product is compared with the XRD pattern of a known LDO, if the FOM is 10 or less, the product can be determined to be the Zn-Al-based layered double oxide. Furthermore, for example, when three peaks with the highest intensities are selected from the XRD pattern of the product (selecting the three peaks in descending order of intensity), and the peak top positions (diffraction angles) and intensities are similar to the three peaks in the XRD pattern of a known LDO, and further, when the FOM is 10 or less when the XRD pattern of the known LDO is compared with the XRD pattern of the product, the product can be determined to be the Zn-Al-based layered double oxide.

The conditions for measuring the XRD pattern of the product are the same as those for measuring the XRD pattern of the precipitate described above, and therefore the explanation will be omitted. As the XRD pattern of a known LDO to be compared with the XRD pattern of the product, for example, Zn ₆ Al ₂ O ₉ disclosed as DB card number 00-051-0037 of the ICDD (International Centre for Diffraction Data) can be used.

One form of Zn-Al-based layered double oxide (LDO) includes a divalent metal oxide containing at least zinc oxide (ZnO) in which a trivalent metal (metal capable of forming a trivalent metal ion) containing at least aluminum is dissolved (hereinafter also referred to as a "solid solution form"). When this solid solution form is present, peaks due to aluminum may not appear in the XRD pattern of the product. In such cases, it is possible to determine whether a product is a Zn-Al-based layered double oxide (LDO) by utilizing the property that Zn-Al-based layered double oxide (LDO) changes (reconstructs) into a Zn-Al-based layered double hydroxide (LDH) in an aqueous solution containing anions, such as a borate solution (borate ion), a fluoride solution (fluoride ion), or a phosphate solution (phosphate ion). Specifically, when the XRD pattern of the product and the XRD pattern of a known divalent metal oxide containing at least zinc oxide (ZnO) (hereinafter referred to as the "XRD pattern of the known divalent metal oxide") show a similar agreement when compared with the XRD pattern of the known LDH described above, and when the XRD pattern of a substance obtained by immersing the product in an aqueous solution containing anions shows a similar agreement when compared with the XRD pattern of the known LDH described above, the product can be determined to be a Zn-Al-based layered double oxide (specifically, a Zn-Al-based layered double oxide in solid solution form).

As an example of the XRD pattern of a known divalent metal oxide to compare with the XRD pattern of the product, ZnO, disclosed as DB card number 00-005-0664 in the ICDD (International Centre for Diffraction Data, PDF-2 Release 2020 RDB), can be used. The XRD patterns of known LDHs have been described above, so a detailed explanation is omitted here.

As can be seen from the above formulas (3) and (4), layered double oxides (LDOs) are positively charged and typically contain counter anions that compensate for this positive charge. In the Zn-Al-based layered double oxides produced by the production method of this embodiment, the counter anions that compensate for the positive charge are not particularly limited and include, for example, sulfate ions, chloride ions, and phosphate ions. However, when sulfate ions are included as counter anions, the sulfate ion content is preferably 2% by mass or less relative to 100% by mass of the Zn-Al-based layered double oxide. Because sulfate ions are divalent anions, they have a high charge density and are strongly bound between layers. Therefore, when the Zn-Al-based layered double oxide produced by the production method of this embodiment is used as an anion adsorbent, they are difficult to exchange with other anions, which can result in reduced anion adsorption capacity. For this reason, the sulfate ion content in the Zn-Al-based layered double oxide is preferably 2% by mass or less. The sulfate ion content in the Zn-Al-based layered double oxide can be determined using an ICP optical emission spectrometer (for example, Shimadzu Corporation's Shimadzu Sequential High-Frequency Plasma Optical Emission Spectrometer ICPS-7510). Note that, in this specification, a sulfate ion content of 0% by mass means that no sulfate ions are contained.

The sulfate ion content in the Zn—Al-based layered double oxide varies depending on the type and content of anions (excluding metal ions) contained in the zinc wastewater after addition. It can be adjusted, for example, by changing the molar ratio of carbonate ions to sulfate ions in the zinc wastewater after addition (hereinafter also referred to as "CO ₃ ²⁻ /SO ₄ ²⁻ "). While CO ₃ ²⁻ /SO ₄ ²⁻ is not particularly limited, from the viewpoint of further reducing the sulfate ion content in the Zn—Al-based layered double oxide, it is preferably 20 or more, more preferably 22 or more, and particularly preferably 22 to 50. The sulfate ion content in the zinc wastewater after addition can be determined using an ICP optical emission spectrometer (e.g., Shimadzu Corporation, Shimadzu Sequential High-Frequency Plasma Optical Emission Spectrometer ICPS-7510). Note that the zinc wastewater and the aluminum source may not contain sulfate ions. In such cases, the total amount of sulfate ions in the zinc wastewater and the aluminum source will be zero.

The sulfate ion content in the Zn—Al-based layered double oxide can also be adjusted by changing the CO ₃ ²⁻ /Zn ²⁺ (molar ratio of carbonate ions to zinc ions in the zinc wastewater after addition) in the precipitation step S101. There are no particular limitations on the _{CO 3} ²⁻ /Zn ²⁺ ratio, but from the perspective of further reducing the sulfate ion content in the Zn—Al-based layered double oxide, it is preferably 2.8 or higher.

In the Zn-Al-based layered double oxide produced by the production method of this embodiment, the molar ratio of zinc contained in the Zn-Al-based layered double oxide to aluminum contained in the Zn-Al-based layered double oxide (hereinafter also referred to as "Zn/Al") is not particularly limited and can be, for example, 1.9 or more and 3.5 or less. From the perspective of improving the anion adsorption performance of the Zn-Al-based layered double oxide, Zn/Al is preferably 1.9 or more and 2.5 or less, and more preferably 1.9 or more and 2.2 or less. The aluminum and zinc contents in the Zn-Al-based layered double oxide can be determined using an ICP optical emission spectrometer (for example, Shimadzu Corporation's Shimadzu Sequential High-Frequency Plasma Optical Emission Spectrometer ICPS-7510).

The manufacturing method of this embodiment, which includes steps S101 and S102 described above, allows for the production of a Zn-Al-based layered double oxide using zinc contained in wastewater as a raw material. Furthermore, the Zn-Al-based layered double oxide produced by this manufacturing method of this embodiment can be used as an anion adsorbent that adsorbs anions.

The method for adsorbing anions by the Zn-Al-based layered double oxide according to this embodiment is not particularly limited, but for example, anions can be adsorbed by bringing a fluid containing anions into contact with the Zn-Al-based layered double oxide. The fluid containing anions may be a liquid (e.g., water) or a gas (e.g., air). Examples of anions adsorbed by the Zn-Al-based layered double oxide include borate ions, phosphate ions, nitrate ions, and sulfate ions.

Note that the manufacturing method of this embodiment described above is a manufacturing method consisting only of the precipitation step S101 and calcination step S102, but the manufacturing method of this embodiment may include other steps in addition to the precipitation step S101 and calcination step S102. For example, as shown in FIG. 2, the manufacturing method of this embodiment may include a filtration step S103 and a drying step S104 between the precipitation step S101 and the calcination step S102.

The filtration step S103 is a step in which the Zn-Al-based layered double hydroxide (LDH) precipitated in the precipitation step S101 is filtered from the zinc wastewater. The filtration process in the filtration step S103 can be performed using a conventionally known method and is not particularly limited, but can be performed using a membrane filter, for example.

The drying step S104 is a step in which the Zn-Al-based layered double hydroxide precipitated in the precipitation step S101 (or the Zn-Al-based layered double hydroxide filtered in the filtration step S103) is dried. The drying conditions in the drying step S104 can be conventionally known drying conditions and are not particularly limited. For example, the drying conditions can be in an air atmosphere at 1 to 60°C for 1 to 24 hours.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these.

Calcined bodies were obtained using the manufacturing methods described in Examples 1 to 9, and evaluations 1 to 4, described below, were performed. In the manufacturing methods described in Examples 1 to 9, actual wastewater 1 was wastewater from zinc plating using an ammonium chloride bath. It was confirmed that the zinc concentration (zinc ion concentration) was 1,200 mg/L, the pH was 2.2, the ammonium chloride concentration was approximately 500 mg/L as nitrogen, the sulfate ion concentration was 250 mg/L, and the heavy metals iron and manganese contained less than 10 mg/L each. Actual wastewater 2 was wastewater from zinc plating using an ammonium chloride bath. It was confirmed that the zinc concentration (zinc ion concentration) was 1,000 mg/L, the pH was 4.4, the sulfate ion concentration was 110 mg/L, and the heavy metals iron and manganese contained less than 10 mg/L each. Actual wastewater 1 and actual wastewater 2 were confirmed to contain almost no aluminum ions or carbonate ions.

The content of each component in Actual Wastewater 1 and Actual Wastewater 2 was measured using an ICP optical emission spectrometer (Shimadzu Corporation, Shimadzu Sequential High-Frequency Plasma Optical Emission Spectrometer ICPS-7510) and a total nitrogen analyzer (Analytik Jena AG TOC/TN Analyzer Multi N/C 3100).

[Example 1]
200 mL of actual wastewater 1 was placed in a beaker, and 0.312 g of polyaluminum chloride (Kishida Chemical Co., Ltd., aluminum oxide 30.0% or more, solubility: 71 g/100 mL) was added. Then, 11 mL of sodium carbonate aqueous solution (100 g/L, Kanto Chemical Co., Ltd., special grade, solubility: 17.4 g/100 mL) was added dropwise to adjust the pH to 7.1. After stirring for 24 hours at room temperature (23 ° C) using a magnetic stirrer, the white precipitate obtained by suction filtration was dried in a drying oven at 60 ° C. The mixture was then calcined for 4 hours at 450 ° C. under a nitrogen atmosphere (flow rate: 1 L/min) using an electric furnace to obtain the calcined body of Example 1.

[Example 2]
200 mL of actual wastewater 2 was placed in a beaker, and 0.266 g of polyaluminum chloride (Kishida Chemical Co., Ltd., aluminum oxide 30.0% or more, solubility: 71 g/100 mL) was added. Then, hydrochloric acid (Kanto Chemical Special Grade) was added to adjust the pH to 1.9. 11 mL of aqueous sodium carbonate solution (100 g/L, Kanto Chemical Co., Ltd., Special Grade, solubility: 17.4 g/100 mL) was added dropwise to this solution to adjust the pH to 7.1. After stirring for 24 hours at room temperature (23 ° C) using a magnetic stirrer, the white precipitate obtained by suction filtration was dried in a drying oven at 60 ° C. The mixture was then calcined for 4 hours at 450 ° C in a nitrogen atmosphere (flow rate: 1 L/min) using an electric furnace to obtain the calcined body of Example 2.

[Example 3]
The calcined body of Example 3 was obtained under the same conditions as in Example 1, except that an aqueous sodium hydroxide solution (200 g/L, Kanto Chemical Co., Inc., special grade) was further added to the actual wastewater, the pH of which had been adjusted to 7.1 in Example 1, to adjust the pH to 8.0.

[Example 4]
The calcined body of Example 4 was obtained under the same conditions as in Example 1, except that an aqueous sodium hydroxide solution (200 g/L, Kanto Chemical Co., Inc., special grade) was further added to the actual wastewater, the pH of which had been adjusted to 7.1 in Example 1, to adjust the pH to 9.0.

[Example 5]
The calcined body of Example 5 was obtained under the same conditions as in Example 1, except that an aqueous sodium hydroxide solution (200 g/L, Kanto Chemical Co., Inc., special grade) was further added to the actual wastewater, the pH of which had been adjusted to 7.1 in Example 1, to adjust the pH to 10.0.

[Example 6]
200 mL of actual wastewater 1 was placed in a beaker, and 0.312 g of polyaluminum chloride (Kishida Chemical Co., Ltd., aluminum oxide 30.0% or more, solubility: 71 g/100 mL) was added. Then, hydrochloric acid (Kanto Chemical Special Grade) was added to adjust the pH to 1.4. 20 mL of aqueous sodium carbonate solution (100 g/L, Kanto Chemical Co., Ltd., Special Grade, solubility: 17.4 g/100 mL) was added dropwise to this solution to adjust the pH to 7.1. After stirring for 24 hours at room temperature (23 ° C) using a magnetic stirrer, the white precipitate obtained by suction filtration was dried in a drying oven at 60 ° C. The mixture was then calcined for 4 hours at 450 ° C in a nitrogen atmosphere (flow rate: 1 L/min) using an electric furnace to obtain the calcined body of Example 6.

[Example 7]
The calcined body of Example 7 was obtained under the same conditions as in Example 1, except that the amount of aqueous sodium carbonate solution (100 g/L) added to actual wastewater 1 was changed to 12 mL and the stirring time for actual wastewater 1 to which polyaluminum chloride and sodium carbonate had been added was changed to 30 minutes.

[Example 8]
200 mL of actual wastewater 1 was placed in a beaker, and 0.312 g of polyaluminum chloride (Kishida Chemical Co., Ltd., aluminum oxide 30.0% or more, solubility: 71 g/100 mL) was added. Then, 5 mL of sodium carbonate aqueous solution (100 g/L, Kanto Chemical special grade, solubility: 17.4 g/100 mL) was added dropwise, and then sodium hydroxide aqueous solution (200 g/L, Kanto Chemical Co., Ltd., special grade) was added dropwise to adjust the pH to 7.1. After stirring for 24 hours at room temperature (23 ° C) using a magnetic stirrer, the white precipitate obtained by suction filtration was dried in a drying oven at 60 ° C. Then, the calcined body of Example 8 was obtained by calcining for 4 hours at 450 ° C. under a nitrogen atmosphere (flow rate: 1 L/min) using an electric furnace.

[Example 9]
200 mL of actual wastewater 2 was placed in a beaker, and 0.266 g of polyaluminum chloride (Kishida Chemical Co., Ltd., aluminum oxide 30.0% or more, solubility: 71 g/100 mL) was added. Then, 5 mL of aqueous sodium carbonate solution (100 g/L, Kanto Chemical Special Grade, solubility: 17.4 g/100 mL) was added dropwise to adjust the pH to 7.0. After stirring for 24 hours at room temperature (23 ° C) using a magnetic stirrer, the white precipitate obtained by suction filtration was dried in a drying oven at 60 ° C. The calcined body of Example 9 was then obtained by calcining for 4 hours at 450 ° C. under a nitrogen atmosphere (flow rate: 1 L/min) using an electric furnace.

Table 1 below shows the physical properties of the zinc wastewater used in Examples 1 to 9. Note that the zinc wastewater after addition shown in Table 1 below refers to the zinc wastewater after the addition of an aluminum source (polyaluminum chloride), a carbonate ion source (sodium carbonate), and, if necessary, a pH adjuster (hydrochloric acid or sodium hydroxide).

The calcined bodies obtained in Examples 1 to 9 were used to carry out the following evaluations 1 to 4.

<Evaluation 1: Identification of crystalline phase>
Using a powder X-ray diffractometer (Smart Lab, manufactured by Rigaku Corporation), XRD patterns of the white precipitates (precipitates) and calcined bodies of Examples 1 to 9 were obtained under the following measurement conditions.
[XRD measurement conditions]
Radiation source: CuKα ray Voltage: 40 kV
Current value: 30mA
Measurement range: 2θ = 5° to 90°

When the XRD patterns of the obtained white precipitates (precipitates) were compared with the XRD patterns of Zn ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O disclosed as DB card number 00-038-0486 of the ICDD (International Centre for Diffraction Data), the FOMs were all found to be 2 or less. From the agreement of the XRD patterns, it was understood that the white precipitates (precipitates) obtained in Examples 1 to 9 were Zn—Al-based layered double hydroxides (more specifically, Zn ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O).

Furthermore, when the XRD patterns of the obtained calcined bodies were compared with the XRD patterns of ZnO disclosed as DB card number 00-005-0664 in ICDD (PDF-2 Release 2020 RDB), the FOMs were all found to be 2 or less.

50 mL of a boric acid aqueous solution (boron concentration: 43.1 mg/L) prepared by diluting borate pH standard solution (type 2) pH 9.18 (Kanto Chemical Co., Inc.) 10 times was taken, and 100 mg of each of the calcined bodies obtained in Examples 1 to 9 was added thereto and shaken (200 times/min) at room temperature for 16 hours. After shaking, the mixture was filtered through a membrane filter with a pore size of 0.45 μm to obtain a residue (hereinafter also referred to as "calcined body after treatment with boric acid aqueous solution"). After drying these calcined bodies after treatment with boric acid aqueous solution in Examples 1 to 9, XRD patterns of the calcined bodies were obtained under the measurement conditions described above.

When the XRD patterns of the calcined bodies obtained after the boric acid aqueous solution treatment were compared with the XRD patterns of Zn ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O disclosed in ICDD DB Card No. 00-038-0486, the FOMs were all 10 or less. Note that the boric acid aqueous solutions in which the calcined bodies of Examples 1 to 9 were immersed did not contain carbonate ions, and therefore immersion of the calcined bodies of Examples 1 to 9 in boric acid aqueous solutions did not result in the formation of carbonate-type Zn-Al-based layered double hydroxides (LDHs) such as Zn ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O. However, since the carbonate in the Zn-Al-based layered double hydroxides (LDHs) does not appear in the XRD patterns, the XRD patterns of Zn ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O were used for comparison in this evaluation.

The consistency of the XRD patterns described above indicates that the calcined bodies obtained in Examples 1 to 9 are Zn-Al layered double oxides (more specifically, hexagonal cation-deficient solid solution forms in which Al is dissolved in ZnO).

Figure 3 shows the XRD pattern of the calcined body (LDO) obtained in Example 1. In Figure 3, representative (characteristic) intensities contained in the XRD pattern of ZnO are shown as a bar graph. Figure 4 shows the XRD patterns of the white precipitate (LDH) and calcined body (LDO) obtained in Example 5. In addition to the XRD patterns of the white precipitate (LDH) and calcined body (LDO), Figure 4 also shows the XRD pattern of the calcined body (LDO) after treatment with an aqueous boric acid solution.

<Evaluation 2: Zn/Al and sulfate ion contents of calcined body>
Ten mg of each of the Zn—Al-based layered double oxides (calcined bodies) obtained in Examples 1 to 9 was placed in a dilute aqueous nitric acid solution (Kanto Chemical Co., Inc., for measuring hazardous metals), and the total volume was adjusted to 50 mL with ultrapure water. This solution was appropriately diluted and introduced into a high-frequency plasma emission spectrometer (Shimadzu Sequential ICPS-7510, Shimadzu Corporation), and the concentrations of zinc, aluminum, and sulfur were measured. From the measurement results, the Zn/Al and sulfate ion contents in the Zn—Al-based layered double oxides of Examples 1 to 9 were calculated. The results are shown in Table 2 below.

<Evaluation 3: Evaluation of anion adsorption performance>
A boric acid solution (boron concentration: 43.1 mg/L) prepared by diluting a borate pH standard solution (type 2) pH 9.18 (manufactured by Kanto Chemical Co., Inc.) 10 times was taken in 50 mL, to which 100 mg of each of the Zn—Al-based layered double oxides (calcined bodies) obtained in Examples 1 to 9 was added, followed by shaking (200 rpm) at room temperature for 16 hours. After shaking, the Zn—Al-based layered double oxide (calcined body) was removed by filtration through a membrane filter with a pore size of 0.45 μm, and the boron and zinc concentrations in the resulting filtrate were measured using a high-frequency plasma emission spectrometer (Shimadzu sequential type ICPS-7510, manufactured by Shimadzu Corporation).

The measurement results for boron concentration were compared with the boron concentration in the solution before the Zn-Al layered double oxide (calcined body) was added to determine the amount of boron adsorbed per gram of boron (hereinafter also referred to as "B adsorption amount"). Furthermore, the measurement results for zinc concentration were used as the amount of zinc eluted (hereinafter also referred to as "Zn elution amount"). The results are shown in Table 2 below. Note that ND in Table 2 stands for not detected.

<Evaluation 4: Measurement of pH change before and after adsorption treatment>
Before and after the adsorption treatment performed in Evaluation 3, the pH of the boric acid solution was measured using a glass electrode hydrogen ion concentration indicator (MM-43X, manufactured by DKK-TOA Corporation) and a pH combination electrode (GST-5841C, manufactured by DKK-TOA Corporation). From the measurement results, the change in pH of the boric acid solution before and after the adsorption treatment was determined. The results are shown in Table 2 below. In Table 2, (+) indicates that the pH after the adsorption treatment was higher than that before the adsorption treatment, and (-) indicates that the pH after the adsorption treatment was lower than that before the adsorption treatment.

As is clear from the results of Evaluation 1, the manufacturing methods of Examples 1 to 9 were able to produce Zn-Al layered double oxides using zinc contained in wastewater as a raw material. Furthermore, as is clear from the B adsorption amount in Evaluation 3, the Zn-Al layered double oxides produced by the manufacturing methods of Examples 1 to 9 were able to adsorb borate ions in a boric acid solution.

Furthermore, even when the Zn—Al-based layered double oxides produced by the production methods of Examples 1 to 9 were subjected to adsorption treatment, zinc did not elute and the change in pH was limited to −1 to +1. From these results, it was understood that the Zn—Al-based layered double oxides produced by the production methods of Examples 1 to 9 are useful substances that are unlikely to cause environmental pollution even when used as adsorbents.

Claims (6)

A method for producing a Zn-Al-based layered double oxide (LDO) that is an anion adsorbent, comprising the steps of:
a precipitation step of adding an aluminum source and a carbonate ion source to zinc wastewater containing zinc ions to precipitate a Zn—Al-based layered double hydroxide (LDH);
and a calcination step of calcining the Zn—Al-based layered double hydroxide (LDH) ,
The zinc wastewater to which the aluminum source and the carbonate ion source have been added has a CO ₃ ²⁻ /Zn ²⁺ ratio of 2.8 to 9.1 and a CO ₃ ²⁻ /SO ₄ ²⁻ ratio of 22 to 50 . The manufacturing method described in claim 1, characterized in that the process of adding the aluminum source and the carbonate ion source to the zinc wastewater comprises a first adding process of adding the aluminum source to the zinc wastewater, and a second adding process of adding the carbonate ion source to the zinc wastewater to which the aluminum source has been added. The manufacturing method described in claim 1 or 2, characterized in that the carbonate ion source is an alkali metal carbonate and/or carbon dioxide gas. 4. The method according to claim 1 , wherein the aluminum source is polyaluminum chloride. 5. The method according to claim 1, wherein a molar ratio of zinc contained in the Zn—Al-based layered double oxide (LDO) to aluminum contained in the Zn—Al-based layered double oxide (LDO) is 1.9 or more and 2.5 or less. 6. The method according to claim 1 , wherein the Zn-Al-based layered double oxide (LDO) has a sulfate ion content of 2 mass % or less.