JP7538638B2 - Method for producing layered double hydroxide - Google Patents

Description

The present invention relates to a method for producing layered double hydroxides.

Layered double hydroxides are known to have an anion exchange function. If this anion exchange function can be used to immobilize arsenic, fluorine, boron, selenium, hexavalent chromium, nitrite ions, and other anionic hazardous substances, it is expected that this will contribute to improving the quality of contaminated water, preventing the elution of hazardous substances, improving soil, and promoting the stabilization of hazardous substances at waste disposal sites in waste safety improvement technology and harmless environmental improvement technology. Several proposals have been made regarding methods for producing layered double hydroxides (for example, Patent Document 1).

International Publication No. 2009/072488

However, there is still room for improvement in the production of layered double hydroxides, and there is an issue with the granular layered double hydroxides crumbling. There is also an issue with turning liquids from which harmful substances have been removed using layered double hydroxides into drinking water.

Therefore, an object of the first invention is to provide a method for producing a granular layered double hydroxide that is not easily crumbled.
Another object of the second invention is to provide a method for producing a layered double hydroxide which can turn the liquid from which harmful substances have been removed into drinking water.

The method for producing a layered double hydroxide according to the first invention is a method for producing a layered double hydroxide by mixing an acidic solution and an alkaline solution, and includes a washing step of washing the layered double hydroxide with a liquid having a pressure of 0.2 MPa to 1.5 MPa prior to a compressing step of the layered double hydroxide using a compressed gas, and a drying step of drying the layered double hydroxide after the washing step so that the moisture content of the layered double hydroxide is 5% to 30%.

According to the first invention, a method for producing a granular layered double hydroxide that is resistant to crumbling can be realized.
According to the second invention, it is possible to provide a method for producing a layered double hydroxide which can turn a liquid from which harmful substances have been removed into drinking water.

1 is a schematic diagram showing a manufacturing line 1 according to a first embodiment of the present invention; 4 is a flowchart of processing by a processing unit 50 according to the first embodiment. FIG. 2 shows particle size distributions of a granular layered double hydroxide having a water content of 6.4% before and after passing water through it, (a) showing the particle size distribution before passing water through it, and (b) showing the particle size distribution after passing water through it. FIG. 2 shows particle size distributions of a granular layered double hydroxide having a water content of 11% before and after passing water through it, (a) showing the particle size distribution before passing water through it, and (b) showing the particle size distribution after passing water through it. FIG. 2 shows particle size distributions of a granular layered double hydroxide having a water content of 26.4% before and after passing water, (a) showing the particle size distribution before passing water, and (b) showing the particle size distribution after passing water. 1 is a schematic diagram showing a contaminated water purification system 100 according to a first embodiment of the present invention. FIG. 13 is a diagram showing details of a sample used in an experiment according to the second embodiment. FIG. 13 is a diagram showing experimental conditions of the second embodiment. 9(a) to 9(c) are diagrams showing the results of Experiments 1-1 and 1-2. 10(a) to 10(c) are diagrams showing the results of Experiments 2-1 and 2-2. 11(a) to 11(c) are diagrams showing the results of Experiments 3-1 and 3-2. 12(a) to 12(c) are diagrams showing the results of Experiments 4-1 and 4-2. 13(a) to 13(c) are diagrams showing the results of Experiments 5-1 and 5-2. FIG. 13 is a diagram summarizing experimental results of the second embodiment. 13 is a graph showing experimental results of the third embodiment.

First Embodiment
A first embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the embodiment described below.

The manufacturing line 1 for manufacturing the layered double hydroxide of this embodiment has a generation section 10 for generating the layered double hydroxide, a liquid supply section 30 for supplying a liquid for washing the generated layered double hydroxide, and a processing section 50 for carrying out various treatments on the generated layered double hydroxide.

The production line 1 produces granular layered double hydroxide. The granular layered double hydroxide produced in the production line 1 is filled into a container. Contaminated water is supplied to this container, and harmful substances in the contaminated water are removed by the granular layered double hydroxide, and the water is used as industrial water or drinking water. The applicant of the present application disclosed in International Application No. PCT/JP2017/046943 (International Publication No. 2018/124190) that harmful substances such as arsenic, iron, and manganese can be removed to drinking standards by layered double hydroxides. In this embodiment, in addition to removing these harmful substances, salt is also removed to a concentration suitable for drinking. Although details will be described later, it is preferable to set the electrical conductivity to 5.7 mS/cm or less. In other words, it is preferable to set the chloride ion concentration to 200 mg/L or less.

The applicant has also confirmed the phenomenon that the granular layered double hydroxide filled in the container crumbles into powder when contaminated water is passed through it, clogging the water passage in the container. Although details will be described later, the applicant has found that by setting the moisture content of the granular layered double hydroxide at the end of the drying process to 5% to 30%, preferably by setting the moisture content of the granular layered double hydroxide at the end of the drying process to 8% to 20%, and more preferably by setting the moisture content of the granular layered double hydroxide at the end of the drying process to 10% to 20%, the collapse of the granular layered double hydroxide due to the passage of water can be suppressed to a level that does not cause problems in use.

(Generation section)
The production section 10 of this embodiment is for producing a layered double hydroxide by mixing an acidic solution containing divalent metal ions and trivalent metal ions with an alkaline solution, and is mainly composed of a first supply section 11 that supplies the acidic solution, a second supply section 12 that supplies the alkaline solution, a rotary stirring section 15 having a rotating shaft 13 and blades 14 and stirring the acidic solution and alkaline solution to produce a mixed solution, and a pH adjustment section 16 below the rotary stirring section 15 that adjusts the hydrogen ion exponent of the mixed solution to stop aging of the produced layered double hydroxide.

Here, layered double hydroxide means a compound represented by the general formula M2 ⁺ _1- ^xM3+ _x (OH) ₂ ( ^An- ) _{x/ n.mH2O} (where M2 ⁺ is a divalent metal, ^{M3 +} is a trivalent metal, An- is an n-valent anion, 0<x<1, m>0). Layered double hydroxides are also called hydrotalcite-like compounds.
Examples of divalent metals include Mg ²⁺ , Fe ²⁺ , Zn ²⁺ , Ca ²⁺ , Li ²⁺ , Ni ²⁺ , Co ²⁺ , and Cu ²⁺ .
Examples of trivalent metals include Al ³⁺ , Fe ³⁺ , and Mn ³⁺ .
Examples of anions include HCO ₃ ⁻ , PO ₄ ³⁻ , SO ₄ ²⁻ , Cl ⁻ , NO ₂ ⁻ , and NO ₃ ⁻ .

The acid contained in the acidic solution may be any acid that makes the aqueous solution acidic, and examples of the acid that can be used include nitric acid and hydrochloric acid.
The alkali contained in the alkaline solution may be any alkali that makes the aqueous solution alkaline, and may be, for example, sodium hydroxide, calcium hydroxide, etc. Sodium carbonate, potassium carbonate, ammonium carbonate, aqueous ammonia, sodium borate, potassium borate, etc. These alkalis may be used alone or in combination of two or more.

Specifically, examples of such layered double hydroxides include those having the general formula Mg2 ⁺ _1- ^xAl3+ _x (OH) ₂ ( ^An- ) _{x/ n.mH2O} and those having the general formula Zn2 ⁺ _1- ^xAl3+ _x (OH) ₂ ( ^An- ) _{x/ n.mH2O} ( ^An- is an n-valent anion, m>0).

The first supply unit 11 supplies an acidic solution containing trivalent metal ions and divalent metal ions that have been separately prepared. For example, the acidic solution may be prepared and mixed with divalent metal ions and trivalent metal ions from materials required in an acidic solution preparation tank, and then supplied to the rotary stirring unit 15 via the first supply unit 11.

The second supply unit 12 is for supplying the alkaline solution to the rotary stirring unit 15. The alkaline solution may be prepared, for example, from materials required in the alkaline solution preparation tank.

The first supply unit 11 and the second supply unit 122 may be provided with a flow rate regulator that appropriately regulates the flow rate of the acidic solution and the alkaline solution. As the flow rate regulator, a conventionally known general device may be used, for example, a flow rate regulator valve.

The rotary stirring unit 15 has multiple blades 14 below the rotating shaft 13 and rotates at a predetermined speed to quickly stir the acidic and alkaline solutions to form a mixed solution. The layered double hydroxides produced tend to accumulate on the rotating shaft 13 of the rotary stirring unit 15. In order to prevent the layered double hydroxides from accumulating on the rotating shaft 13, a liquid such as water can be supplied from above the rotating shaft 13 and allowed to flow, covering the surface of the rotating shaft 13 with the liquid. This makes it possible to prevent the layered double hydroxides from accumulating on the surface of the rotating shaft 13.

For example, when producing a layered double hydroxide having the general formula Mg2 ⁺ _1- xAl3 ⁺ _x (OH) ₂ ( ^An- ) _{x/ n.mH2O} ( ^An- is an n-valent anion, m>0), first, an acidic solution containing aluminum ions and magnesium ions is prepared.

The aluminum source for the aluminum ions is not limited to any particular substance, as long as it generates aluminum ions in water. For example, alumina, sodium aluminate, aluminum hydroxide, aluminum chloride, aluminum nitrate, bauxite, alumina production residue from bauxite, aluminum sludge, etc. can be used. Furthermore, any of these aluminum sources may be used alone, or two or more types may be used in combination.

The magnesium source of magnesium ions is not limited to any specific substance, as long as it generates magnesium ions in water. For example, brucite, magnesium hydroxide, magnesite, baked magnesite, etc. can be used. These magnesium sources can be used alone or in combination of two or more types.

The aluminum compound as the aluminum source and the magnesium compound as the magnesium source do not need to be completely dissolved as long as aluminum ions and magnesium ions are present in the acidic solution. Therefore, even if the acidic solution contains aluminum compounds or magnesium compounds that are not dissolved, layered double hydroxides can be produced without any problems.

It is also known that the highly crystalline layered double hydroxide represented by Mg2 ⁺ _1- ^xAl3+ _x (OH) ₂ ( ^An- ) _{x/n.mH2O has} a molar ratio of aluminum ions to magnesium ions of 1:3 (x=0.25). Therefore, the molar ratio of aluminum ions to magnesium ions in the acidic solution is preferably in the range of 1:5 to 1:2. By setting the ratio in this range, the layered double hydroxide can be produced advantageously in terms of material balance without wasting the aluminum source and the magnesium source. It is preferable to use nitric acid or hydrochloric acid to adjust the acidic solution to an acidic state.

The alkaline solution is preferably prepared to have a pH of 8 to 14. The alkali contained in the alkaline solution is not limited to a specific substance as long as it can make the aqueous solution alkaline. For example, sodium hydroxide, calcium hydroxide, etc. can be used. In addition, sodium carbonate, potassium carbonate, ammonium carbonate, aqueous ammonia, sodium borate, potassium borate, etc. can also be used. Any of these alkalis may be used alone, or two or more types may be used in combination.
By mixing the acidic solution and the alkaline solution in a predetermined ratio, a layered double hydroxide is produced.

The pH adjustment unit 16 is disposed below the rotary stirring unit 15, and is for adjusting the hydrogen ion exponent of the mixed liquid to stop the aging of the produced layered double hydroxide. The shorter the aging time after production, the smaller the crystallite size (size of the crystallites) of the produced layered double hydroxide can be. It is preferable to stop the aging so that the crystallite size becomes 20 nm or less, preferably 10 nm or less. To prevent aging, after mixing of the acidic solution and the alkaline solution is completed, the pH of the mixed liquid may be lowered to a value at which the crystal growth of the layered double hydroxide stops.

For example, the aging of a layered double hydroxide represented by the general formula Mg2 ⁺ _1- ^xAl3+ _x (OH) ₂ ( ^An- ) _{x/n.mH2O can} be stopped by adjusting the pH to 9 or less. Also, the aging of a layered double hydroxide represented by the general formula Zn2 ⁺ _1- ^xAl3+ _x (OH) ₂ ( ^An- ) _{x/ n.mH2O} can be stopped by adjusting the pH to 5 or less.

The pH adjustment unit 16 may be provided with, for example, a pH adjustment liquid supply flow path that supplies a liquid capable of adjusting the hydrogen ion exponent (pH adjustment liquid) to the mixed liquid containing the produced layered double hydroxide. The pH adjustment liquid may be any liquid having a pH below the level required to stop aging, such as water or an acidic solution. It is also possible to use a gas (pH adjustment gas) or a solid (pH adjustment solid) that can lower the pH of the mixed liquid. A quantity adjustment device that adjusts the flow rate of the pH adjustment liquid may also be provided as appropriate.

The stirring tank 17 contains blades 14 therein, and mixes an acidic solution with an alkaline solution to produce a layered double hydroxide.
The pH adjustment tank 18 is connected to a pH adjustment unit 16 at its lower part and also to a discharge flow path 19, and stops the aging by lowering the pH of the produced layered double hydroxide. By quickly stopping the aging, layered double hydroxides with small crystallite sizes can be efficiently mass-produced.

The layered double hydroxide in slurry form produced by overflowing the mixed solution of the acidic solution and the alkaline solution flows into the slurry tank 20 from the discharge flow path 19 together with the pH adjusting liquid. The layered double hydroxide in slurry form that flows into the slurry tank 20 is transported to the processing unit 50 by the pump 25. Before explaining the processing by the processing unit 50, the liquid supply unit 30 will be explained in more detail.

(Liquid supply unit)
The liquid supply unit 30 supplies a liquid for washing the layered double hydroxide to the treatment unit 50, and in this embodiment, water is used as the liquid. The liquid supply unit 30 has a tank 31 that stores water, and a pump 32 that transports the water stored in the tank 31 to the treatment unit 50, and supplies 20 to 100 liters of water per minute, preferably 30 to 80 liters per minute, and more preferably 40 to 60 liters per minute to the treatment unit 50. This is because if the flow rate of the water supplied by the liquid supply unit 30 is below the above-mentioned lower limit, it takes a long time to wash the layered double hydroxide, and if the flow rate of the water supplied by the liquid supply unit 30 exceeds the above-mentioned upper limit, the cost of washing the layered double hydroxide increases.

The pressure at which the liquid supply unit 30 supplies water to the processing unit 50 is 0.2 MPa to 1.5 MPa, preferably 0.4 MPa to 1.0 MPa, and more preferably 0.5 MPa to 0.8 MPa. If the water pressure is below the lower limit value, the aggregated layered double hydroxide does not become granular, and if the water pressure exceeds the upper limit value, the water does not easily pass through the filter described below, and the layered double hydroxide cannot be washed. The water supplied by the liquid supply unit 30 may be tap water or purified water. Since the electrical conductivity of natural water is 100 μS/cm, it is preferable to use water with an electrical conductivity of 200 μS/cm or less, preferably 100 μS/cm or less, and more preferably 50 μS/cm or less. The lower limit of the electrical conductivity of water may be 0.01 μS/cm, which is the electrical conductivity of pure water.

(Processing section)
The treatment section 50 subjects the layered double hydroxide to various treatments such as filtration, washing, squeezing, drying, etc. The treatment section 50 has a switching valve 51 for switching the flow path, a treatment body 52, a filter cloth 53, an opening 54, a discharge section 55, a compressed gas supply section 56, etc.

In this embodiment, the switching valve 51 is a three-way switching valve, which switches between the layered double hydroxide from the processing unit 50 and the water from the liquid supply unit 30 and supplies them to the processing unit 50 .
The treatment body 52 houses a filter cloth 53 inside and has an opening 54 at the top for supplying the layered double hydroxide and water to the filter cloth 53. It also has a discharge section 55 for discharging the pH adjusting liquid and water supplied together with the layered double hydroxide, and a compressed gas supply section 56 for supplying compressed gas for the squeezing treatment.

The filter cloth 53 may be made of chemical fibers such as nylon, polyester, polypropylene, etc., and may have an air permeability of 0.1 to 1 cm ³ /cm ² ·sec.
The opening 54 is connected to a switching valve 51 and selectively supplies the layered double hydroxide and water to the filter cloth 53 .
The discharge section 55 is provided below the treatment body 52, and discharges the liquid that has passed through the filter cloth 53 to the outside of the treatment body 52. Although two discharge sections 55 are illustrated in Fig. 1, the number of discharge sections may be one, or three or more.
The compressed gas supply unit 56 is formed above the processing body 52 and is connected to a compressed gas supply source (not shown). In this embodiment, the compressed gas supply unit 56 supplies air, but other gases may be used. In addition, although two compressed gas supply units 56 are illustrated in FIG. 1, the number of compressed gas supply units 56 may be one, or three or more.
The processing of the processing section 50 of the manufacturing line 1 of this embodiment configured as above will be explained further.

(Flowchart explanation)
FIG. 2 is a flowchart of the processing of the processing unit 50 of this embodiment, and this flowchart is executed under the control of a control unit (not shown) of the processing unit 50.
The control unit switches the switching valve 51 so as to supply the layered double hydroxide in a slurry state stored in the slurry tank 20 to the filter cloth 53 (step S1). The switching operation of the switching valve 51 may be performed manually.

The filter cloth 53 allows liquids such as the pH adjusting liquid to pass through, and the liquid that has passed through the filter cloth 53 is discharged from the discharge section 55. In this manner, filtration is performed by a filter press using the filter cloth 53 (step S2). It is preferable to use a filter cloth 53 having an air permeability of 0.1 to 1 ^cm3 / ^cm2 ·sec so that the layered double hydroxide does not flow out of the filter cloth 53.

Following the filtration process, the control unit switches the switching valve 51 so that water from the liquid supply unit 30 is supplied to the filter cloth 53, and the layered double hydroxide is washed (washed) (step S3). The switching operation of the switching valve 51 may be performed manually. By mixing the acidic solution and the alkaline solution, the layered double hydroxide is produced and NaCl is also produced. In the washing (washing) process of step S3, NaCl is removed. At this time, NaCl attached to the layered double hydroxide is also removed, improving the anion exchange activity of the layered double hydroxide and improving the adsorption performance for adsorbing arsenic, fluorine, boron, selenium, hexavalent chromium, nitrite ions, and other anion-based harmful substances.

In addition, if the liquid from which harmful substances have been removed by the granular layered double hydroxide produced in this embodiment is to be used as drinking water, the NaCl concentration of this liquid must be 0.3% or less. Therefore, the NaCl concentration of the liquid used to wash the granular layered double hydroxide produced in this embodiment must be at least 0.3% or less. Since NaCl may also be contained in contaminated water, it is preferable to set the NaCl concentration of the liquid used to wash the granular layered double hydroxide to 0.1% or less. When the NaCl concentration of the liquid used to wash the granular layered double hydroxide is expressed in terms of electrical conductivity, a NaCl concentration of 0.3% approximately corresponds to an electrical conductivity of 5.7 mS/cm, so the electrical conductivity of the liquid used to wash the granular layered double hydroxide must be at least 5.7 mS/cm or less, and an electrical conductivity of 100 μS/cm or less, which is approximately the same as that of natural water, is preferable, and an electrical conductivity of 50 μS/cm or less is even more preferable.

When the NaCl concentration of the liquid used to wash the granular layered double hydroxide is expressed as chloride ion concentration, a NaCl concentration of 0.3% roughly corresponds to a chloride ion concentration of 200 mg/L, so the chloride ion concentration of the liquid used to wash the granular layered double hydroxide needs to be 200 mg/L or less, preferably 50 mg/L to 100 mg/L or less, which is roughly the same as natural water, and more preferably 0.1 mg/L or less.

The washing step in step S3 is preferably performed after the filtering step in step S2 in order to reduce the amount of water, but water from the liquid supply unit 30 may be supplied to the filter cloth 53 during the filtering step. By performing the washing step in step S3 before the squeezing step, it is possible to efficiently remove NaCl and improve the adsorption performance of the layered double hydroxide. If the dry weight of the layered double hydroxide supplied to the filter cloth 53 is 100 kg, 20 to 100 liters of water are supplied per minute for several hours. For this reason, the weight of the water supplied in the washing step is greater than the dry weight of the layered double hydroxide, and in this embodiment, the weight of the water is preferably 10 times or more and 100 times or less than the dry weight of the layered double hydroxide.

An electrical conductivity meter that measures the ease with which electricity passes is provided in the discharge section 55 to measure the electrical conductivity of the water discharged in the cleaning process, and the cleaning process is continued until this measured value falls within the range of 50 μS/cm to 5.7 mS/cm. In this embodiment, the cleaning process is terminated when the electrical conductivity falls to a first predetermined value of 200 μS/cm or less, preferably 150 μS/cm or less, and more preferably 100 μS/cm or less, which is the electrical conductivity of natural water.

When the measured value of the electrical conductivity meter becomes equal to or less than 400 μS/cm, preferably equal to or less than 300 μS/cm, more preferably equal to or less than 200 μS/cm, which is a second predetermined value, a switching valve (not shown) may be switched to circulate the water discharged from the discharge part 55 to the tank 31. In this case, a filter or water purifier may be provided between the switching valve (not shown) and the tank 31. By circulating the water discharged from the discharge part 55 to the tank 31, the water used for cleaning can be effectively utilized. In this embodiment, the second predetermined value is set to be greater than the first predetermined value.

By using the flow rate and water pressure described above in the washing step, it is possible to provide granular layered double hydroxide even if the washing step of this embodiment is added.

After switching the switching valve 51 so that the supply of the slurry layered double hydroxide and water from the liquid supply unit 30 is stopped, the control unit introduces compressed air from a compressed gas supply source (not shown) through the compressed gas supply unit 56. The layered double hydroxide contained in the filter cloth 53 is compressed by supplying compressed air from the compressed gas supply unit 56 to the filter cloth 53 (step S4). Note that in this embodiment, the squeezing step is performed within the processing body 52, but the layered double hydroxide may be contained in a container separate from the processing body 52 and the squeezing step may be performed.

The control unit performs a drying step of drying the layered double hydroxide following the squeezing step (step S5). The drying step may be performed inside the processing body 52 or outside the processing body 52, but in this embodiment, the description will continue assuming that the drying step is performed inside the processing body 52.
The layered double hydroxide can be vacuum-dried by vacuum adsorbing the steam in the filter cloth 53 using a vacuum adsorption mechanism (not shown) in the treatment body 52 and providing a flow path through which a liquid at 80°C to 100°C circulates in the treatment body 52. By lowering the air pressure inside the treatment body 52 using the vacuum adsorption mechanism (not shown), the moisture becomes steam at a low temperature, allowing efficient drying. When the vacuum adsorption mechanism (not shown) is operating, it is preferable to cover the opening 54, etc. with a lid or the like to prevent communication with the outside of the treatment body 52.
The layered double hydroxide may be dried by blowing hot air at 80° C. to 150° C., preferably 100° C. to 120° C., onto the layered double hydroxide, or by using electromagnetic waves.

In this embodiment, following the drying process, it is also possible to appropriately add processes such as a crushing process for crushing the layered double hydroxide and a sieving process for sieving the layered double hydroxide.

As mentioned above, the applicant has found that by increasing the moisture content of the granular layered double hydroxide at the end of the drying process to between 5% and 30%, it is possible to suppress the collapse of the granular layered double hydroxide due to the passage of water to a level that does not pose a problem in use. The explanation of this will be continued below.

3 is a graph showing the particle size distribution of a granular layered double hydroxide having a water content of 6.4% before and after passing water, (a) showing the particle size distribution before passing water, and (b) showing the particle size distribution after passing water, where the vertical axis shows the relative particle amount (%) and the horizontal axis shows the particle size (μm).
The granular layered double hydroxide having a water content of 6.4% was sieved in a sieving process.
As shown in Fig. 3(a), the particle size distribution before passing water is about 370 μm to 1380 μm, and the median (D50) is about 680 μm. 750 g of this granular layered double hydroxide with a water content of 6.4% was filled into a container (not shown), and a water passing experiment was performed at a water passing rate of 3.0 L/hr and a total water passing amount of 5.25 ^m3 (water passing ratio 7000 times). In this embodiment, the water passing experiment was performed using tap water, but this is not limited to this.

As shown in FIG. 3(b), the particle size distribution after passing water is mainly about 25 μm to about 1080 μm, and the median (D50) is about 180 μm. Although the particle size distribution after passing water is smaller than that before passing water, powdery layered double hydroxides of several μm to several tens of μm were not observed very much, and did not clog the water in the container (not shown). For this reason, the applicant of the present application found that if the lower limit of the moisture content is a granular layered double hydroxide of up to 5%, there is no problem in use in terms of the adsorption performance (adsorption time) of the granular layered double hydroxide. It is expected that the adsorption performance of the granular layered double hydroxide will improve in the future, and the adsorption time of harmful substances will be extended. For this reason, it is more preferable to set the lower limit of the moisture content to 8% or 10%.
The particle size distribution before and after the water flow can be measured by a particle size distribution measuring device, which may be of the laser diffraction type.

Next, the particle size distributions of the granular layered double hydroxides having moisture contents of 11% and 26.4% before and after passing water will be described, but explanations that overlap or substantially overlap with those of the granular layered double hydroxide having a moisture content of 6.4% will be omitted or simplified.
FIG. 4 is a diagram showing the particle size distribution of a granular layered double hydroxide having a water content of 11% before and after passing water through it. As shown in FIG. 4(a), the particle size distribution before passing water through it is about 370 μm to 1,380 μm, and the median (D50) is about 680 μm.
As shown in Fig. 4(b), the particle size distribution after passing water was mainly from about 35 µm to about 1300 µm, with a median (D50) of about 260 µm. Although the particle size distribution after passing water was smaller than before passing water, powder-like layered double hydroxides of several µm to several tens of µm were hardly observed, and did not clog the water in a container (not shown).

As shown in Fig. 5(a), the particle size distribution before passing water is about 370 μm to 1380 μm, with a median (D50) of about 680 μm. A container (not shown) was filled with 700 g of this granular layered double hydroxide with a water content of 26.4%, and a water passing experiment was carried out with a water passing rate of 2.8 L/hr and a total water passing volume of 4.90 m3 (water passing magnification 7000 times).
As shown in Fig. 5(b), the particle size distribution after passing water is mainly from about 55 µm to about 1300 µm, with a median (D50) of about 480 µm. Although the particle size distribution after passing water is smaller than that before passing water, powdery layered double hydroxides of several µm to several tens of µm were hardly observed, and did not clog the water in a container (not shown). For this reason, the applicant of the present application found that if the granular layered double hydroxide has an upper limit of moisture content of 30%, there is no problem in use in terms of the adsorption performance (adsorption time) of the granular layered double hydroxide.

If the moisture content exceeds 30%, the granular layered double hydroxide is not dried sufficiently and becomes sticky, which is undesirable. Considering the stability during mass production, the upper limit of the moisture content is more preferably 20%.
As described above, if the median value of the particle size distribution after water passage is about 180 μm to 480 μm, the granular layered double hydroxide does not become powdery and prevent water passage. However, in consideration of stability during mass production, it is more preferable that the median value of the particle size distribution after water passage is about 250 μm to 400 μm.

In addition, in the drying process, by having a table showing the relationship between the vacuum drying time and the moisture content of the granular layered double hydroxide, it is possible to change the moisture content of the granular layered double hydroxide from 5% to 30%. As an example of the table, 5 hours of drying will result in a moisture content of 30%, 8 hours of drying will result in a moisture content of 10%, and 10 hours of drying will result in a moisture content of 8%. This table can be created by measuring the moisture content of the granular layered double hydroxide after vacuum drying for a predetermined time using, for example, an infrared moisture meter. This table can be stored in a memory (not shown).

Figure 6 is a schematic diagram showing the contaminated water purification system 100 of the first embodiment. Granular layered double hydroxide produced on production line 1 is used in this contaminated water purification system 100 (described in detail later). Below, the contaminated water purification of this embodiment will be further explained using Figure 6. The contaminated water purification system 100 has a contaminated water tank 101, a pump 102, a pre-treatment unit 103, a contaminant removal unit 104, and a post-treatment unit 105.

The contaminated water tank 101 is a tank in which contaminated water is stored. The pump 102 is a liquid pump that discharges the contaminated water stored in the contaminated water tank 101 toward the pretreatment unit 103. The pump 102 may also be configured to pump up contaminated groundwater instead of from the contaminated water tank 101.
Some contaminated water may contain sludge, which is a precipitate of iron, manganese, etc. If this sludge accumulates in the contaminant removal section 104, there is a problem that the function of removing contaminants in the contaminant removal section 104 is reduced. Therefore, in this embodiment, a filter is provided in the pretreatment section 103 to prevent sludge, which is a precipitate of iron, manganese, etc., from flowing into the contaminant removal section 104. Examples of the filter include inorganic filters such as ceramic filters, polypropylene filters, and paper filters. In this case, the filter structure may be a depth filter (deep layer filtration type) such as a thread-wound filter or a resin-molded filter, or a surface filter (surface filtration type) such as a pleated filter or a membrane filter. Activated carbon may be provided in the pretreatment section 103 to deodorize the contaminated water, adsorb sludge, and purify the water.

The contaminant removal section 104 has a container filled with granular layered double hydroxide produced in the production line 1. This container has an inlet for receiving contaminated water that has passed through the pretreatment section 103, and an outlet for discharging the liquid from which harmful substances have been removed by the granular layered double hydroxide toward the posttreatment section 105. The granular layered double hydroxide adsorbs anion-based harmful substances such as arsenic, fluorine, boron, selenium, hexavalent chromium, and nitrite ions. In addition, NaCl generated during the production process of the granular layered double hydroxide has been removed to a drinking water level. This makes it possible to use the liquid from which harmful substances have been removed by the contaminant removal section 104 as a drink.

The post-treatment section 105 has a filter that removes suspended solids smaller in size than the filter used in the pre-treatment section 103. Note that the post-treatment section 105 may be provided with activated carbon so that the activated carbon can deodorize the liquid or purify the water.
In addition, if the contaminated water contains NaCl, the liquid that has passed through the contaminant removal unit 104 may not be suitable as drinking water. In such a case, a desalination treatment unit may be provided in the post-treatment unit 105. As the desalination treatment unit, electrodialysis, a nanofiltration membrane (NF membrane), a reverse osmosis membrane (RO membrane), or the like may be used, but is not limited to these. In this way, the liquid that has passed through the contaminated water purification system 100 of this embodiment can be used as drinking water.
If the contaminated water is in a good contaminated state, the post-treatment unit 105 may be omitted.

The above-described embodiment is merely an example for explaining the present invention, and various modifications can be made without departing from the scope of the present invention. For example, in the above-described embodiment, direct washing is performed by supplying water from the tank 31 to the filter cloth 53 by switching the switching valve 51, but indirect washing may be performed by supplying water to the filter cloth 53 from a separate flow path provided above the treatment body 52.
In the above embodiment, the electrical conductivity of the water discharged from the discharge portion 55 was measured, but the chloride ion concentration may be detected to determine whether or not the water can be circulated to the tank 31. Alternatively, the NaCl concentration of the liquid used to wash the granular layered double hydroxide may be managed according to the washing time by having a table showing the relationship between the washing time and the electrical conductivity (or the chloride ion concentration).

Second Embodiment
3 to 5 of the first embodiment, an experimental example will be described to demonstrate that by increasing the moisture content of the granular layered double hydroxide at the end of the drying step from 5% to 30%, collapse of the granular layered double hydroxide due to water flow can be suppressed to a level that does not pose a problem in use.

In this experiment, five types of samples (samples 1 to 5) were used, as shown in Figure 7. As shown in Figure 7, sample 1 had a drying time (vacuum drying time at 93 KPa and 90°C) of 10 hours, a moisture content of 8.17%, and a median particle size (median diameter: D50) of 670.34 μm before passing water. Sample 2 had a drying time of 6 hours, a moisture content of 9.66%, and a median particle size (D50) of 613.09 μm before passing water. Sample 3 had a drying time of 5 hours, a moisture content of 11.73%, and a median particle size (D50) of 632.37 μm before passing water. Sample 4 had a drying time of 4 hours, a moisture content of 17.81%, and a median particle size (D50) of 666.97 μm before passing water. For sample 5, the drying time was 3 hours, the moisture content was 24.22%, and the median particle size (D50) before passing water was 1264.37 μm.

These samples (layered double hydroxides) were placed in a filtration container, test water was passed through from above, and treated water was discharged from below. Ten experimental conditions are shown in Figure 8. For example, in experiment 1-1, ion-exchanged water (water that does not contain ions) was used as the test water, and 15 g of the above sample 1 was placed in the container as an adsorbent. The test water was passed through for 36 hours at a flow rate of 0.18/L and a space velocity of 12SV. Experiments 2-1, 3-1, 4-1, and 5-1 were conducted under the same conditions as experiment 1-1, except for the different samples. Meanwhile, experiment 1-2 was conducted under the same conditions as experiment 1-1, except for the test water being an aqueous solution containing 200 mg/L of sulfate ions. Experiments 2-2, 3-2, 4-2, and 5-2 were conducted under the same conditions as experiment 1-2, except for the different samples.

(1) Experiments 1-1 and 1-2 Figure 9(a) is a diagram showing the particle size distribution of granular layered double hydroxide (sample 1) with a water content of 8.17% before passing water through it, the left diagram is the particle size distribution of sample 1 in the upper part of the container, and the right diagram is the particle size distribution of sample 1 in the lower part of the container. Figure 9(b) is a diagram showing the particle size distribution of sample 1 after experiment 1-1 was performed, and Figure 9(c) is a diagram showing the particle size distribution of sample 1 after experiment 1-2 was performed. Here, the vertical axis is the relative particle amount (%), and the horizontal axis is the particle size (μm).

As shown in Figure 14, the particle size distribution in the upper part of the container before passing water was 670.34 μm (Figure 9(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 577.11 μm, and the median particle size (D50) in the lower part of the container after passing water was 575.55 μm (Figure 9(b)). On the other hand, in experiment 1-2, the particle size distribution in the upper part of the container before passing water was 670.34 μm (Figure 9(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 186.12 μm, and the median particle size (D50) in the lower part of the container after passing water was 598.97 μm (Figure 9(c)). The particle size distribution before and after passing water can be measured by a particle size distribution measuring device. A laser diffraction type can be used as this particle size distribution measuring device.

From these experiments 1-1 and 1-2, it was found that when ion-exchanged water was passed through the container, the layered double hydroxide of sample 1 did not crumble in either the upper or lower part of the container, but when an aqueous sulfate ion solution was passed through the container, the layered double hydroxide crumbled in the upper part of the container. On the other hand, when the moisture content was 8.17%, although the particle size distribution after passing water was smaller than before passing water, powder-like layered double hydroxide of several μm to several tens of μm was not observed very much, and it was found that it did not clog the water passing through the container.

(2) Experiments 2-1 and 2-2 Figure 10(a) is a diagram showing the particle size distribution of granular layered double hydroxide (sample 2) with a water content of 9.66% before passing water through it, the left diagram is the particle size distribution of sample 2 in the upper part of the container, and the right diagram is the particle size distribution of sample 2 in the lower part of the container. Figure 10(b) is a diagram showing the particle size distribution of sample 2 after experiment 2-1 was performed, and Figure 10(c) is a diagram showing the particle size distribution of sample 2 after experiment 2-2 was performed.

As shown in Figure 14, in experiment 2-1, the particle size distribution in the upper part of the container before passing water was 613.09 μm (Figure 10(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 572.15 μm, and the median particle size (D50) in the lower part of the container after passing water was 546.62 μm (Figure 10(b)). On the other hand, in experiment 2-2, the particle size distribution in the upper part of the container before passing water was 613.09 μm (Figure 10(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 130.75 μm, and the median particle size (D50) in the lower part of the container after passing water was 537.18 μm (Figure 10(c)).

From Experiments 2-1 and 2-2, it was found that when an aqueous sulfate ion solution was passed through the container, the layered double hydroxide collapsed at the top of the container. Furthermore, when the moisture content was 9.66%, the particle size distribution after the water was passed through was smaller than before the water was passed through, but powder-like layered double hydroxide with sizes ranging from several μm to several tens of μm was not observed very often, and it was found that the water passing through the container did not clog.

(3) Experiments 3-1 and 3-2 Figure 11(a) is a diagram showing the particle size distribution of granular layered double hydroxide (sample 3) with a water content of 11.73% before passing water through it, the left diagram is the particle size distribution of sample 3 in the upper part of the container, and the right diagram is the particle size distribution of sample 3 in the lower part of the container. Figure 11(b) is a diagram showing the particle size distribution of sample 3 after experiment 3-1 was performed, and Figure 11(c) is a diagram showing the particle size distribution of sample 3 after experiment 3-2 was performed.

As shown in Figure 14, in experiment 3-1, the particle size distribution in the upper part of the container before passing water was 632.37 μm (Figure 11(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 668.21 μm, and the median particle size (D50) in the lower part of the container after passing water was 679.27 μm (Figure 11(b)). On the other hand, in experiment 3-2, the particle size distribution in the upper part of the container before passing water was 632.37 μm (Figure 11(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 185.53 μm, and the median particle size (D50) in the lower part of the container after passing water was 649.16 μm (Figure 11(c)).

From Experiments 3-1 and 3-2, it was found that when an aqueous sulfate ion solution was passed through the container, the layered double hydroxide collapsed at the top. Furthermore, when the moisture content was 11.73%, the particle size distribution after passing the water was smaller than before passing the water, but powder-like layered double hydroxide with sizes ranging from several μm to several tens of μm was not observed very often, and it was found that the water passing through the container did not clog.

(4) Experiments 4-1 and 4-2 Figure 12(a) is a diagram showing the particle size distribution of granular layered double hydroxide with a water content of 17.81% (sample 4) before passing water through it, the left diagram is the particle size distribution of sample 4 in the upper part of the container, and the right diagram is the particle size distribution of sample 4 in the lower part of the container. Figure 12(b) is a diagram showing the particle size distribution of sample 4 after experiment 4-1 was performed, and Figure 12(c) is a diagram showing the particle size distribution of sample 4 after experiment 4-2 was performed.

As shown in Figure 14, in experiment 4-1, the particle size distribution in the upper part of the container before passing water was 666.97 μm (Figure 12 (a)), whereas the median particle size (D50) in the upper part of the container after passing water was 697.08 μm, and the median particle size (D50) in the lower part of the container after passing water was 664.84 μm (Figure 12 (b)). On the other hand, in experiment 4-2, the particle size distribution in the upper part of the container before passing water was 666.97 μm (Figure 12 (a)), whereas the median particle size (D50) in the upper part of the container after passing water was 507.01 μm, and the median particle size (D50) in the lower part of the container after passing water was 665.42 μm (Figure 12 (c)).

From Experiments 4-1 and 4-2, it was found that when an aqueous sulfate ion solution was passed through the container, the layered double hydroxide collapsed at the top. Furthermore, when the moisture content was 17.81%, the particle size distribution after passing the water was smaller than before passing the water, but powder-like layered double hydroxide with sizes ranging from several μm to several tens of μm was not observed very often, and it was found that it did not clog the water passing through the container.

(5) Experiments 5-1 and 5-2 Figure 13(a) is a diagram showing the particle size distribution of granular layered double hydroxide (sample 5) with a water content of 24.22% before passing water through it, the left diagram is the particle size distribution of sample 5 in the upper part of the container, and the right diagram is the particle size distribution of sample 5 in the lower part of the container. Figure 13(b) is a diagram showing the particle size distribution of sample 5 after experiment 5-1 was performed, and Figure 13(c) is a diagram showing the particle size distribution of sample 5 after experiment 5-2 was performed.

As shown in Figure 14, in experiment 5-1, the particle size distribution in the upper part of the container before passing water was 1264.37 μm (Figure 13(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 1426.40 μm, and the median particle size (D50) in the lower part of the container after passing water was 1367.44 μm (Figure 13(b)). On the other hand, in experiment 5-2, the particle size distribution in the upper part of the container before passing water was 1264.37 μm (Figure 13(a)), whereas the median particle size (D50) in the upper part of the container after passing water was 557.40 μm, and the median particle size (D50) in the lower part of the container after passing water was 1330.38 μm (Figure 13(c)).

From Experiments 5-1 and 5-2, it was found that when an aqueous solution of sulfate ions was passed through the container, the layered double hydroxide collapsed at the top. Furthermore, when the moisture content was 24.22%, the particle size distribution after passing the water was smaller than before passing the water, but powder-like layered double hydroxide with sizes ranging from several μm to several tens of μm was not observed very often, and it was found that it did not clog the water passing through the container.

From the above experiments 1-1 to 5-2, it was found that the collapse of layered double hydroxides is caused by ion exchange reactions, and that the higher the water content, the less likely they are to collapse even when an ion exchange reaction occurs.

From these results, the applicant has found that a granular layered double hydroxide with a moisture content of 5% to 30%, preferably 8% to 25%, is acceptable in terms of the adsorption performance (adsorption time) of the granular layered double hydroxide. If the moisture content exceeds 30%, as explained in the first embodiment above, the granular layered double hydroxide is not dried sufficiently and becomes sticky, which is undesirable. Considering stability during mass production, it is more preferable to set the upper limit of the moisture content at 20%.

Third Embodiment
As explained in the first embodiment, the liquid supply unit 30 supplies the liquid for washing the layered double hydroxide to the treatment unit 50, and according to an experiment conducted by the applicant of the present application, it was found that the washed layered double hydroxide adsorbs a larger amount of arsenic contained in water than the unwashed layered double hydroxide. This experiment will be described below.

In this experiment, tap water with arsenic trioxide dissolved therein (arsenic standard solution 1000 mg/L: Fujifilm Wako Pure Chemical Industries, Ltd.) was used as raw water so that the arsenic concentration was 100 μg/L. As a comparative example, 75 g of unwashed layered double hydroxide granules were placed in a filtration vessel, and raw water was passed through at 0.3 L/hr and a space velocity of 4 SV. On the other hand, as an example, 50 g of water-washed layered double hydroxide granules were placed in a filtration vessel, and raw water was passed through at 0.2 L/hr and a space velocity of 4 SV. Note that the amounts of layered double hydroxide used in the comparative example and the example were different, so the amount of raw water passed through was adjusted accordingly. The results of this experiment are shown in FIG. 15.

As shown in Figure 15, in the case of the comparative example (unwashed layered double hydroxide), the arsenic concentration in the treated water exceeded the drinking water standard value at a water flow rate of 7,324 L/Kg. On the other hand, in the case of the example (washed layered double hydroxide), the arsenic concentration in the treated water exceeded the drinking water standard value at a water flow rate of 13,901 L/Kg. Note that the water flow rate is a converted value.

In this way, it was found that washing the layered double hydroxide as in the example resulted in a greater amount of raw water that could be treated (amount of arsenic that could be adsorbed).

The above-described embodiment is a preferred example of the present invention. However, the present invention is not limited to this embodiment, and various modifications are possible without departing from the spirit of the present invention.

Reference Signs List 1 Production line 10 Generation section 30 Liquid supply section 50 Processing section 52 Processing body 53 Filter cloth 100 Contaminated water purification system 104 Contaminant removal section

Claims (6)

A method for producing a layered double hydroxide by mixing an acidic solution and an alkaline solution,
a washing step of washing the layered double hydroxide with a liquid having a pressure of 0.2 MPa to 1.5 MPa prior to the compressing step of the layered double hydroxide with a compressed gas,
a drying step, after the washing step, of drying the layered double hydroxide so that the moisture content of the layered double hydroxide is 5% to 30%. The method for producing a layered double hydroxide according to claim 1, wherein the drying step dries the layered double hydroxide so that the moisture content of the layered double hydroxide is 8% to 20%. The method for producing a layered double hydroxide according to claim 1 or 2, wherein the drying step is carried out after the squeezing step. The method for producing a layered double hydroxide according to any one of claims 1 to 3, wherein the drying step is carried out by vacuum drying. The method for producing the layered double hydroxide according to any one of claims 1 to 4, further comprising a step of detecting the electrical conductivity of the liquid used in the washing step or the chloride ion concentration when chloride ions are contained as anions in the layered double hydroxide. The method for producing a layered double hydroxide according to any one of claims 1 to 5, wherein the washing step is carried out using 20 to 100 liters of the liquid per minute.

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