JP7592308B2 - Thorium adsorbent, its manufacturing method, method for extracting thorium from a test aqueous solution using the same, and method for recovering thorium from thorium-containing ore - Google Patents

Description

The present invention relates to a thorium adsorbent using porous silica, a method for producing the same, a method for extracting thorium from a test aqueous solution using the same, and a method for recovering thorium from thorium-containing ore.

Thorium, an actinide element, is radioactive and stable in solution or solids as a tetravalent ion, and its compounds are considered to have low toxicity. However, thorium ions can migrate into drinking water or groundwater during the mining process of geological ores, nuclear fuel waste, etc., and can worsen environmental problems. In addition, exposure to large amounts of thorium precipitates in the liver, spleen, etc., raising concerns about its effects on the human body. As a result, a method for selectively extracting thorium is required.

On the other hand, technologies have been developed to adsorb strontium ions and cesium ions using mesoporous silica (see, for example, Patent Documents 1 and 2). However, these mesoporous silicas cannot selectively adsorb thorium.

JP 2013-17919 A JP 2013-40852 A

The objective of the present invention is to provide a thorium adsorbent using porous silica that selectively adsorbs thorium, a method for producing the same, a method for extracting thorium from a test aqueous solution using the same, and a method for recovering thorium from thorium-containing ore.

The thorium adsorbent according to the present invention comprises a sponge-like agglomerate made of porous silica, the agglomerate having a hierarchical structure with micropores, mesopores and macropores, the agglomerate having a BET specific surface area in the range of 150 ^m2 /g to 200 ^m2 /g and a pore volume in the range of 0.18 ^cm3 /g to 0.3 ^cm3 /g, the agglomerate being particles with a particle size in the range of 10 μm to 70 μm, the agglomerate having elongated grooves arranged in the longitudinal direction, selectively adsorbing or releasing thorium, thereby solving the above-mentioned problem.
The surface and pores of the porous silica may be modified with at least a chelate compound represented by any one of formulas 1 to 3.

Here, L is a divalent group, and * represents a bonding site with the porous silica.
The mass ratio of the chelate compound to the porous silica may be in the range of 0.1 or more and 0.5 or less.
The mass ratio of the chelate compound to the porous silica may be in the range of 0.15 or more and 0.25 or less.
The porous silica contains at least silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and optionally nitrogen (N) and an M element (a Group 1 element or a Group 2 element), and when the total is 100% by mass, the mass percent concentration of each element is:
40≦Si≦45
47≦O≦52
1≦C≦5,
0.5≦P≦2,
0≦N≦2, and
0≦M≦5
may be satisfied.
The surface and pores of the porous silica may be further modified with a chelate compound represented by any one of formulas 4 to 6.

Here, L is a divalent group, and ... represents a hydrogen bond with the porous silica or a bond by van der Waals forces.
The L may be an alkylene group having 1 to 20 carbon atoms, which may or may not have a substituent.
The method for producing the thorium adsorbent according to the present invention includes adding a plant-derived silica source obtained from wheat straw to an alkaline solution to prepare a plant-derived silica solution, adding cetyltrimethylammonium bromide (CTAB) to the plant-derived silica solution to obtain a raw material mixed solution, hydrothermally synthesizing the raw material mixed solution to precipitate a reaction product consisting of silica from the plant-derived silica source and the cetyltrimethylammonium bromide, and calcining the reaction product to remove the cetyltrimethylammonium bromide, thereby solving the above problem.
The step of preparing the plant-derived silica solution may include refluxing the alkaline solution to which the plant-derived silica source has been added at a temperature in the range of 55°C or more and less than 65°C.
The raw material mixed solution may be obtained by adding cetyltrimethylammonium bromide so that the mass ratio of cetyltrimethylammonium bromide to the plant-derived silica in the plant-derived silica solution is in the range of 0.1 to 0.4.
The hydrothermal synthesis may include adding an acid to the reaction product, washing, and drying.
Firing the reaction product may involve firing the reaction product at a temperature in the range of 200° C. to 800° C. for a period of 2 hours to 24 hours.
The method may further include mixing the calcined product obtained by calcining the reaction product with the chelate compound represented by formula 7 or 8.

Here, L is a divalent group.
The mixing may involve mixing the chelate compound such that a mass ratio of the chelate compound to the baked product is 0.1 or more and 0.5 or less.
Following said mixing, the method may further comprise washing and drying the mixed product.
The method may further include preparing the plant-derived silica source prior to preparing the plant-derived silica solution, and the preparing step may further include washing and drying the straw, treating the dried straw with an acid having a concentration in the range of 0.25M to 0.5M and refluxing it at a temperature range of 65°C to less than 75°C for 1 hour to 10 hours, washing the acid and drying it at a temperature range of 80°C to 110°C for 5 hours to 10 hours, and calcining the dried straw at a temperature range of 600°C to 800°C for 5 hours to 12 hours to decarbonate it.
The method for extracting thorium from a test aqueous solution according to the present invention comprises contacting the above-mentioned thorium adsorbent with the test aqueous solution to adsorb thorium in the test aqueous solution, thereby solving the above-mentioned problem.
The method for recovering thorium from a thorium-containing ore according to the present invention includes adding the thorium-containing ore to an acid solution selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid to prepare the thorium-containing solution, contacting the thorium adsorbent with the thorium-containing solution to allow the thorium adsorbent to adsorb thorium in the thorium-containing solution, discharging treated water in which thorium has been extracted from the thorium-containing solution, and contacting the thorium adsorbent with thorium adsorbed thereon with an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid to liberate the adsorbed thorium, thereby solving the above-mentioned problem.
The pH of the thorium-containing solution may be in the range of 4 to 6 inclusive.
The adsorption may be performed by contacting the thorium adsorbent with the thorium-containing solution for a period of time of 30 minutes or more and 100 minutes or less.

The thorium adsorbent of the present invention comprises an aggregate made of porous silica. The porous silica has a hierarchical structure with micropores, mesopores and macropores, has a BET specific surface area in the range of 150 m ² /g to 200 m ² /g, and has a pore volume in the range of 0.18 cm ³ /g to 0.3 cm ³ /g. Furthermore, the aggregate has a particle size in the range of 10 μm to 70 μm, and has elongated grooves arranged in the longitudinal direction inside. As a result, the thorium adsorbent of the present invention can selectively adsorb or release thorium, which is advantageous for maintaining human health and purifying the environment. The thorium adsorbent of the present invention can be used as a column packing material.

Schematic diagram showing the thorium adsorbent of the present invention. FIG. 1 is a schematic diagram showing another thorium adsorbent of the present invention. Schematic diagram showing the adsorption mechanism using the thorium adsorbent of the present invention modified with a chelating compound. A flowchart showing the process for producing the thorium adsorbent of the present invention. A flow chart showing the process for producing another thorium adsorbent of the present invention. FIG. 1 is a schematic diagram showing a thorium adsorption column using the thorium adsorbent of the present invention. A flow chart showing the process of adsorbing thorium from a test aqueous solution using the thorium adsorbent of the present invention. A flow chart showing the process of extracting and recovering thorium from thorium-containing ore using the thorium adsorbent of the present invention. Graph showing the dependence of thorium leaching from ECR on various conditions 1 shows an SEM image and an HR-TEM image of a sample of Example 1. FIG. 1 shows XRD patterns of samples of Examples 1 and 2. FIG. 1 shows nitrogen adsorption/desorption isotherms (A) and TG-DTA curves (B) of the samples of Examples 1 and 2. FIG. 1 shows FTIR spectra of samples from Examples 1 and 2. FIG. 1 shows the XRD pattern of the sample of Example 2 after a batch thorium adsorption test using a thorium standard solution. FTIR spectrum of the sample of Example 2 after batch thorium adsorption testing with thorium standard solution FIG. 1 shows the results of a batch-type thorium adsorption test using a thorium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a batch thorium adsorption test using a thorium standard solution for the samples of Example 1 and Example 2. FIG. 1 shows the results of a batch thorium adsorption test using thorium standard solutions containing various ions for the samples of Examples 1 and 2. FIG. 1 shows further results of batch thorium adsorption tests using thorium standard solutions containing various ions for the samples of Examples 1 and 2. FIG. 1 shows the results of a release (desorption) test using samples of Example 1 and Example 2 in which thorium was adsorbed by the batch method. FIG. 1 shows the results of a batch-type thorium adsorption/desorption repeat test using a thorium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a column-type thorium adsorption test using a thorium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a release (desorption) test using the samples of Example 1 and Example 2 in which thorium was adsorbed by the column method. FIG. 1 shows the results of a column-type thorium adsorption/desorption cycle test using a thorium standard solution for the samples of Examples 1 and 2. FIG. 1 shows the results of a batch thorium adsorption test using a thorium-containing solution by ECR-containing ore for the sample of Example 2. FIG. 1 shows the results of batch and column thorium adsorption and desorption tests using a thorium-containing solution with ECR-containing ore for the samples of Example 1 and Example 2. FIG. 1 shows the results of a batch-type thorium adsorption test using a thorium-containing solution by ECR-containing ore for the samples of Examples 1 to 7.

The following describes an embodiment of the present invention with reference to the drawings. Note that similar elements are given similar reference numerals and their description will be omitted.

Figure 1 is a schematic diagram showing the thorium adsorbent of the present invention.

The thorium adsorbent of the present invention comprises a sponge-like aggregate 100 made of porous silica 110. The aggregate 100 has a hierarchical structure with micropores (pore diameter: 0.5 nm or more and less than 2 nm), mesopores (pore diameter: 2 nm or more and less than 50 nm), and macropores (pore diameter: 50 nm or more and 10 μm or less). By having such pores of various sizes, a hierarchical structure is formed, which can promote the uptake, diffusion, and adsorption of thorium. The present inventors have found that the aggregate 100 has a BET specific surface area in the range of 150 m ² /g to 200 m ² /g, a pore volume in the range of 0.18 cm ³ /g to 0.3 cm ³ /g, and particles having a particle diameter in the range of 10 μm to 70 μm, and selectively adsorbs or liberates thorium and functions as a thorium adsorbent. In particular, the aggregate 100 includes elongated grooves aligned along its length, allowing thorium to be adsorbed into the mesopores or channels via the grooves.

The thorium (Th) of interest in this invention may be either naturally occurring or produced by nuclear reactions in a reactor. The naturally occurring thorium isotope is mainly 232Th, but there are also 230Th, 234Th (uranium series intermediate decay products), 227Th, 231Th (actinium series intermediate decay products), and 228Th (thorium series intermediate decay products). Thorium has a long half-life and is radioactive. When 232thorium absorbs a neutron, it becomes 233thorium, which undergoes beta decay to become 233platactinium and then 233uranium.

In addition, in the subject to be treated (also referred to as the test liquid, for example), thorium species have three types of oxidation states, namely, Th(II), Th(III), and Th(IV), with Th4 ⁺ being stable, and there are compounds such as thorium dioxide, thorium fluoride, thorium hydroxide, thorium nitrate, and thorium carbonate. Hereinafter, for the sake of simplicity, the term thorium will be used to refer to all isotopes and ions in various oxidation states.

The presence of micropores (diameter 0.5 nm or more and less than 2 nm), mesopores (diameter 2 nm or more and less than 50 nm), and macropores (diameter 50 nm or more and less than 10 μm) in the aggregate 100 can be determined by the NLDFT method (Non-Local Density Functional Theory) based on the nitrogen adsorption and desorption isotherm. In simple terms, it is sufficient if the adsorption and desorption isotherm is a mixture of IUPAC type I, type II or III, and type IV or V. In this specification, an aggregate having micropores, mesopores, and macropores is said to have a hierarchical structure.

In the aggregate 100, the mesopores having a size of 2 nm or more and less than 50 nm mainly function as spaces for adsorbing and retaining thorium, and preferably have a pore volume in the range of 0.2 ^cm3 /g or more and 0.3 ^cm3 /g or less. The mesopores become active sites for thorium, can promote the capture of thorium, and can efficiently adsorb thorium.

The aggregate 100 is a particle having a particle size in the range of 10 μm to 70 μm, preferably 50 μm to 70 μm. The elongated grooves arranged in the longitudinal direction preferably have a length of 250 nm to 2 μm. The arranged grooves can be combined with mesopores or channels to promote the adsorption of thorium. The elongated grooves more preferably have a length of 1 μm to 2 μm. The particle size and groove length are values measured using Image J (ver. 1.51n; open source, public domain image processing software) based on electron microscope images.

FIG. 2 is a schematic diagram showing another thorium adsorbent of the present invention.
FIG. 3 is a schematic diagram showing the adsorption mechanism using the thorium adsorbent of the present invention modified with a chelating compound.

The thorium adsorbent of the present invention may preferably comprise an aggregate of particles in which the surface and pores of porous silica are modified with a chelate compound 210 represented by formulas 1 to 3. The chelate compound 210 represented by formulas 1 to 3 is chemically bonded to the porous silica.

Here, L is a divalent group, and * represents a bonding site with the porous silica.
L is not particularly limited as a divalent group, and examples of the divalent hydrocarbon group which may have a heteroatom include, for example, an alkylene group (preferably having 1 to 20 carbon atoms), a cycloalkylene group (preferably having 3 to 20 carbon atoms), an alkenylene group (preferably having 2 to 20 carbon atoms), an alkynylene group (preferably having 2 to 20 carbon atoms), and combinations thereof, as well as combinations of the above with -C(O)-, -C(O)O-, -OC(O)-, -O-, -S-, and -NR- (R represents a hydrogen atom or a monovalent organic group).

Among these, L is preferably an alkylene group having 1 to 15 carbon atoms. This promotes the modification of the porous silica with the chelate compound 210. Even more preferably, L is an alkylene group having 1 to 10 carbon atoms. This allows more chelate compound 210 to be modified on the porous silica, allowing more thorium to be adsorbed. Even more preferably, L is an alkylene group having 1 to 2 carbon atoms.

The thorium adsorbent of the present invention may also comprise an aggregate of particles in which the surface and pores of porous silica are modified with a chelating compound 210 represented by formulas 4 to 6. That is, the chelating compound 210 may be bonded to the mesoporous silica 110 by hydrogen bonds or van der Waals forces in addition to the above-mentioned chemical bonds.

L is a divalent group, and the "..." next to OH indicates that it is hydrogen bonded to the porous silica or bonded by van der Waals forces. Since L is as described above, the explanation is omitted.

The chelate compound 210 may be modified to the porous silica by chemical bonds, hydrogen bonds, or van der Waals forces, or any combination thereof.

As shown in FIG. 2, the chelate compound 210 may be applied to the outer surface of the mesoporous silica 110, to the internal pores, or both.

Figure 3 shows a schematic diagram of chelating compound 210 adsorbing thorium, with the phosphoryl or carboxyl groups of chelating compound 210 being able to selectively adsorb and release thorium. As a result, the thorium adsorbent shown in Figure 2 can adsorb more thorium than the thorium adsorbent shown in Figure 1.

Examples of such chelating compounds 210 include the following. These can be easily modified into porous silica and can selectively adsorb thorium.

When the chelating compound 210 is modified, the aggregate 200 preferably has a BET specific surface area in the range of 150 ^m2 /g to 170 ^m2 /g and a pore volume in the range of 0.2 ^cm3 /g to 0.25 ^cm3 /g. Although the specific surface area and pore volume are slightly decreased by modifying the chelating compound 210, the adsorption efficiency of thorium can be improved due to the ability of the chelating compound 210 itself to adsorb thorium.

The thorium adsorbent of the present invention preferably has a mass ratio of the chelate compound to the porous silica in the range of 0.1 to 0.5. This allows thorium to be efficiently adsorbed while maintaining the specific surface area and pore volume.

More preferably, the thorium adsorbent of the present invention has a mass ratio of the chelate compound to the porous silica in the range of 0.15 to 0.25. This allows thorium to be adsorbed more efficiently while maintaining the specific surface area and pore volume.

In the thorium adsorbent of the present invention modified with the chelate compound 210, the porous silica preferably contains at least silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and optionally nitrogen (N) and an M element (a Group 1 element or a Group 2 element), and when the total is taken as 100 mass%, the mass percent concentration of each element is:
40≦Si≦45
47≦O≦52
1≦C≦5,
0.5≦P≦2,
0≦N≦2, and
0≦M≦5
This allows thorium to be adsorbed more efficiently.

In the thorium adsorbent of the present invention modified with the chelating compound 210, more preferably, the mass percent concentration of each element is:
42≦Si≦44
48≦O≦51
1.5≦C≦3,
0.5≦P≦2, and
0≦M≦5
This allows thorium to be adsorbed more efficiently.

In the thorium adsorbent of the present invention modified with the chelating compound 210, the porous silica may further contain a Group 1 element or Group 2 element in addition to the above elements. In this case, the content, when the total is taken as 100 mass%, is preferably 2.5 mass% or more and 5 mass% or less. Among the Group 1 elements or Group 2 elements, sodium and potassium are preferred. These elements do not reduce the adsorption efficiency of thorium.

Next, a method for producing the thorium adsorbent of the present invention shown in FIG. 1 will be described.
FIG. 4 is a flow chart showing the process for producing the thorium adsorbent of the present invention.

The manufacturing method of the present invention uses a sol-gel method. Each step will be described below.
Step S410: Add the plant-derived silica source obtained from wheat straw to an alkaline solution to prepare a plant-derived silica solution. The plant-derived silica solution may be called a sol-gel solution. The plant-derived silica source is hydrolyzed to form silanol. In the present invention, the plant-derived silica source obtained from wheat straw is used to produce a thorium adsorbent having a unique structure, properties and characteristics, which is low-cost and environmentally friendly.

The alkaline solution may be a strong alkali, such as a hydroxide of an alkali metal (lithium, sodium, potassium, rubidium, cesium, etc.), a hydroxide of tetraalkylammonium, or a hydroxide of an alkaline earth metal (calcium, strontium, barium, europium, etc.). The solvent may be water or an alcohol such as ethanol or methanol.

Prior to the step of preparing the plant-derived silica solution in step S410, a plant-derived silica source may be prepared from wheat straw. In particular, the wheat straw is washed and dried (e.g., at 90°C). The dried wheat straw is treated with acid (e.g., hydrochloric acid greater than 0.25 M and less than 0.5 M) and refluxed at a temperature range of 65°C to less than 75°C for 1 hour to 10 hours. After cooling, the straw is washed to remove the acid and dried (e.g., at 80°C to 110°C) for 5 hours to 10 hours. The straw is then calcined (e.g., at 600°C to 800°C for 5 hours to 12 hours) to decarbonate the plant-derived silica source.

In step S410, preparing the plant-derived silica solution may involve refluxing the alkaline solution to which the plant-derived silica source has been added at a temperature range of 55°C or higher and lower than 65°C. This promotes hydrolysis.

Step S420: Cetyltrimethylammonium bromide (CTAB) is added to the plant-derived silica solution to obtain a raw material mixture solution. The raw material mixture solution has a sufficiently high viscosity compared to the plant-derived silica solution and may be called a wet sol. As a result, the formed silanols are polymerized to form siloxane bonds. Furthermore, the siloxane bonds are reconstructed into sponge-like aggregates with an ordered hierarchical structure by the surfactant CTAB.

In step S420, the mass ratio of CTAB to plant-derived silica in the plant-derived silica solution is preferably in the range of 0.1 to 0.4. This promotes reconstruction into a sponge-like aggregate having a hierarchical structure.

Step S430: The raw material mixed solution is subjected to hydrothermal synthesis. This causes the precipitation of a reaction product consisting of silica from the plant-derived silica source and CTAB. Hydrothermal synthesis is a reaction in which the raw material mixed solution is reacted in a sealed container such as an autoclave in the presence of hot water at high temperature and pressure. For example, hydrothermal synthesis is performed at a temperature range of 100°C to 150°C for a period of 1 hour to 24 hours.

In step S430, an acid may be added to the reaction solution after hydrothermal synthesis to neutralize it. The acid may be hydrochloric acid, sulfuric acid, nitric acid, or the like. The resulting reaction product may be washed and dried.

Step S440: The reaction product obtained in step S430 is calcined to remove CTAB. As a result, CTAB is removed and porous silica, which is a sponge-like aggregate that maintains an orderly hierarchical structure, is obtained. There are no particular restrictions on the calcination conditions as long as the CTAB is removed, but it is preferably calcined at a temperature range of 200°C to 800°C for 2 hours to 24 hours. This removes the CTAB. More preferably, the calcination is performed at a temperature range of 450°C to 650°C for 6 hours to 10 hours. In this way, a thorium adsorbent is obtained that is made of porous silica shown in Figure 1 and is made of a sponge-like aggregate having a hierarchical structure with micropores, mesopores, and macropores.

Next, a method for producing another thorium adsorbent of the present invention shown in FIG. 2 will be described.
FIG. 5 is a flow chart showing a process for producing another thorium adsorbent of the present invention.

The separate thorium sorbent is performed following step S440 of FIG.
Step S510: The fired product obtained in step S440 is mixed with the chelate compound represented by formula 7 or 8. This causes the surface and pores of the porous silica to be modified with the chelate compound.

L is a divalent group, and as described above, the explanation is omitted.

Examples of such chelating compounds include the following. These can be easily modified into porous silica and can selectively adsorb thorium.

In step S510, the mass ratio of the chelate compound to the fired product (i.e., the porous silica shown in FIG. 1) is preferably 0.1 or more and 0.5 or less. This allows the chelate compound to modify the surface and pores of the fired product while maintaining the specific surface area and pore volume of the fired product.

In step S510, the chelating compound may be added directly, or may be mixed with an aprotic donor solvent such as toluene or hexane, and dissolved. The mixture may be stirred at a temperature range of room temperature (20-30°C) to 50°C for 1 hour to 24 hours.

Step S520: The product after mixing is washed and dried. Washing may be performed multiple times using an alcohol such as ethanol and water. This allows the unreacted chelate compound to be removed. Drying may be performed at a temperature range of 50°C to 100°C for 1 hour to 24 hours. In this way, a thorium adsorbent made of porous silica with modified surfaces and pores as shown in Figure 2 is obtained.

Next, applications of the thorium adsorbent of the present invention will be described. The thorium adsorbent of the present invention can be used in either a batch system or a column system. Here, we will explain the case where it is used in a column system.

Figure 6 is a schematic diagram showing a thorium adsorption column using the thorium adsorbent of the present invention.

The thorium adsorption column 600 of the present invention has an adsorbent 610. The adsorbent 610 is the thorium adsorbent 100 of the present invention described with reference to Figures 1 and 2. The adsorbent 610 is packed in a container (not shown).

As shown in FIG. 6, the thorium adsorption column 600 may further include a top filter 620, a bottom filter 630, and column connections 640 and 650. The thorium adsorption column 600 has an opening on the top filter 620 side and an opening on the bottom filter 630 side, and a solution containing thorium (thorium-containing solution) is passed from one opening to the other opening, whereby the thorium-containing solution comes into contact with the thorium adsorbent 610. This causes the thorium in the thorium-containing solution to be adsorbed. The adsorption column may also be called an adsorption tower.

The top filter 620 prevents the thorium adsorbent 610 from scattering in the liquid when it is passed through. The bottom filter 630 prevents the thorium adsorbent 610 from leaking out of the container (not shown). The column connectors 640 and 650 function to connect to piping for supplying or discharging the liquid passed through the thorium adsorption column 600, such as piping for a purification system.

Next, a method for extracting thorium from a test aqueous solution using the thorium adsorption column 600 shown in FIG. 6 will be described.
FIG. 7 is a flow chart showing the process for adsorbing thorium from a test aqueous solution using the thorium adsorbent of the present invention.

Step S710: The thorium adsorbent is brought into contact with the test aqueous solution. For example, in the thorium adsorption column 600 of FIG. 6, the test aqueous solution is passed through the column connections 640, 650 and the top filter 620. If the test aqueous solution contains thorium, the thorium in the test aqueous solution is selectively adsorbed by the thorium adsorbent.

Preferably, the pH of the test aqueous solution is adjusted to 4 or more and 6 or less. A known acid or base may be used to adjust the pH. The adsorption of thorium is promoted by adjusting the pH. More preferably, the pH of the test aqueous solution is adjusted to 4 or more and 5 or less. This suppresses the electrostatic repulsion between Th(IV) and H ₃ O ⁺ at the active site of the thorium adsorbent, and also suppresses the generation of hydroxides such as [Th(OH) ₂ ] ²⁺ and [Th ₂ (OH) ₂ ] ⁶⁺ , thereby maintaining the adsorption ability.

Preferably, the contact time in step S710 is 30 minutes or more. This promotes the adsorption of thorium. There is no particular upper limit, but it may be 100 minutes. If the contact time is longer than this, no change in the amount of adsorption is observed. It is also advisable to contact the mixture while stirring. From the viewpoint of efficiency, the contact time should be 50 minutes or more and 70 minutes or less.

The contact temperature in step S710 may be room temperature, but heating to a temperature range of 60°C or higher and 80°C or lower can enhance adsorption efficiency.

Although not shown, following step S710, the treated water from which thorium has been extracted from the test aqueous solution is discharged. For example, in the thorium adsorption column 600 of FIG. 6, the treated water may be discharged through the bottom filter 630.

The thorium adsorbent with adsorbed thorium can be released by contacting it with an acid such as sulfuric acid, hydrochloric acid, or nitric acid. This makes the thorium adsorbent reusable. From the standpoint of release efficiency, hydrochloric acid is preferred.

Preferably, the thorium adsorbent having thorium adsorbed thereon is brought into contact with an acid such as hydrochloric acid having a concentration of 0.05M to 0.5M for 40 minutes to 120 minutes. This allows the adsorbed thorium to be liberated. Even more preferably, the thorium adsorbent having thorium adsorbed thereon is brought into contact with an acid such as hydrochloric acid having a concentration of 0.1M to 0.4M for 40 minutes to 80 minutes. This allows the adsorbed thorium to be efficiently liberated.

Figure 8 is a flow chart showing the process of extracting and recovering thorium from thorium-containing ore using the thorium adsorbent of the present invention.

Step S810: The thorium-containing ore is added to an acid solution selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid to prepare a thorium-containing solution. This allows the thorium in the thorium-containing ore to be leached out, even if the ore has a low thorium content. The acid concentration (M) in the acid solution is preferably in the range of 1.5 M to 3 M per 1 g of thorium-containing ore. Within this range, thorium can be effectively leached out.

In preparing the thorium-containing solution, the thorium-containing ore and the acid are mixed and stirred, preferably for 60 to 120 minutes. This increases the leaching efficiency. More preferably, heating during stirring increases the leaching efficiency, and the heating temperature is preferably in the range of 60°C to 80°C.

In preparing the thorium-containing solution, the amount of acid (mL) per 1 g of thorium-containing ore is preferably in the range of 10 mL to 40 mL. This range promotes the leaching of thorium.

Step S820: The thorium adsorbent is brought into contact with the thorium-containing solution. For example, in the thorium adsorption column 600 of FIG. 6, the thorium-containing solution is passed through the column connections 640, 650 and the top filter 620.

Preferably, the pH of the thorium-containing solution is adjusted to 4 or more and 6 or less. A known acid or base may be used to adjust the pH. The adsorption of thorium is promoted by adjusting the pH. More preferably, the pH of the thorium-containing solution is adjusted to 4 or more and 5 or less. This suppresses the electrostatic repulsion between Th(IV) and H ₃ O ⁺ at the active site of the thorium adsorbent, and also suppresses the generation of hydroxides such as [Th(OH) ₂ ] ²⁺ and [Th ₂ (OH) ₂ ] ⁶⁺ , thereby maintaining the adsorption ability.

Preferably, the contact time in step S820 is 50 minutes or more. This promotes the adsorption of thorium. There is no particular upper limit, but it may be 100 minutes. Even if the contact time is longer than this, no change in the amount of adsorption is observed. It is also recommended to contact the material while stirring. In this way, the thorium adsorbent of the present invention can be used to adsorb thorium extracted from thorium-containing ore.

Step S830: Discharge the treated water from which thorium has been extracted from the thorium-containing solution. For example, in the thorium adsorption column 600 of FIG. 6, the treated water may be discharged through the bottom filter 630.

Step S840: The thorium adsorbent with adsorbed thorium is contacted with an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid to release the adsorbed thorium. This makes the thorium adsorbent reusable. From the viewpoint of release efficiency, hydrochloric acid is preferred.

Preferably, the thorium adsorbent having thorium adsorbed thereon is brought into contact with an acid such as hydrochloric acid having a concentration of 0.05M to 0.5M for 40 minutes to 120 minutes. This allows the adsorbed thorium to be liberated. Even more preferably, the thorium adsorbent having thorium adsorbed thereon is brought into contact with an acid such as hydrochloric acid having a concentration of 0.1M to 0.4M for 40 minutes to 80 minutes. This allows the adsorbed thorium to be efficiently liberated.

Next, the present invention will be described in detail using specific examples, but please note that the present invention is not limited to these examples.

[Preparation of plant-derived silica source]
Wheat straw (collected in the north of Egypt) was washed with tap water to remove impurities, and then dried at 90° C. for 24 hours. The dried straw was then refluxed in 0.5 M hydrochloric acid (HCl) at 70° C. for 4 hours. After cooling, the hydrochloric acid-treated straw was filtered, and the residue was washed with Milli-Q water to remove the acid. The residue was then dried at 100° C. for 12 hours. The dried body was calcined at 700° C. for 7 hours to obtain a white plant-derived silica source.

[Equipment used in evaluation]
The morphology of the sample was photographed with a field emission scanning electron microscope (FE-SEM, JSM-6500F, manufactured by JEOL Ltd.) and a high-resolution transmission electron microscope (HRTEM). The accelerating voltage during SEM observation was 20 kV. The accelerating voltage during TEM observation was 200 kV. Wide-angle X-ray diffraction (D8 Advance, manufactured by Bruker) was measured using an 18-kW diffractometer to identify the sample. The nitrogen adsorption/desorption isotherm (77 K) of the sample was measured using a fully automatic gas adsorption apparatus (BELSORP36, manufactured by Microtrack-Bell Co., Ltd.). The pore size, pore volume, and specific surface area by the BET method were obtained from the isotherm. Thermogravimetric differential thermal analysis (TG-DTA) was performed using a differential thermal/thermogravimetric simultaneous measurement apparatus (TG-60, manufactured by Shimadzu Corporation). The absorption spectrum in the infrared region was examined using a Fourier transform infrared spectrophotometer (FTIR, manufactured by Shimadzu Corporation, FTIR Prestage-21).

The amount of thorium was measured before and after thorium adsorption using an inductively coupled plasma mass spectrometer (ICP-MS, Perkin Elmer, Elan-6000) equipped with a Perkin Elmer lambda 3b double beam UV-visible programmable spectrophotometer. Direct gamma counting was performed using a high purity germanium (HPGe) EG&G Alltec model instrument (GMX60P4).

[Thorium adsorption/desorption test]
Thorium adsorption and desorption tests were carried out in both column and batch modes.
The glasses used were cleaned with 5% nitric acid and Milli-Q water. All tests were performed in triplicate.

The column method was as follows: A plastic net and polyethylene wool were placed in a fixed vertical glass column (length 12.5 cm, diameter 1.0 cm), and a thorium adsorbent was placed on top of the net. Using the column, the effects of the flow rate of the test liquid, the dosage of the thorium adsorbent, and the thorium concentration were investigated. The amount of thorium before and after adsorption was measured by ICP-MS.

The adsorption amount (q _e , mg/g) and the adsorption efficiency (Ads.%) were calculated from the following formulas.
q _e =(C ₀ -C _e )(V/w) (1)
Ads. %={(C ₀ - C _e )/C ₀ }×100 (2)
where C ₀ and C _e are the initial and equilibrium concentrations [Th(IV), mg/L], V is the volume of the test solution (L), and w is the mass of the thorium adsorbent (g).

Next, the acid was passed through a column packed with the thorium adsorbent to desorb (release) thorium. The release efficiency (E.%) was calculated using the formula E.% = ( _{C.E./C.A .} ) x 100, where _C.E. and _C.A. are the released thorium concentration and the adsorbed thorium concentration.

The batch method was carried out as follows. 20 mL of the test liquid (a thorium standard solution described below) was mixed with 20 mg of thorium adsorbent while stirring. The thorium concentration and pH in the thorium standard solution, the mixing time, and the mixing temperature were changed to investigate the amount of thorium adsorbed and the adsorption efficiency. As with the column method, sulfuric acid was passed through the thorium adsorbent to which thorium had been adsorbed, and the thorium was desorbed. The calculations of the amount of thorium adsorbed, the adsorption efficiency, and the release efficiency were as described above, so a description thereof will be omitted.

[Test solution]
Thorium standard solutions with various concentrations were used as test solutions for the thorium adsorption/desorption test.

Furthermore, a thorium-containing solution in which thorium was leached from a thorium-containing ore was prepared as a test liquid for use in a thorium adsorption/desorption test in the following manner.
Egyptian cataclasite (ECR) was collected from the Abu Rushied region in the southeastern desert of Egypt and crushed to 200 mesh (approximately 0.075 mm). Elemental analysis of the ECR was performed to measure the amount of oxides. The results are shown in Table 1. In Table 1, "REE" stands for rare earth elements. Furthermore, the ECR contained 619.3 Bq/Kg of 232Th isotope as measured using a HPGe spectrometer.

1 g of crushed ECR was added to and stirred in various volumes of hydrochloric acid solutions (0.1M to 4M) at various temperatures for various times to allow thorium to leach out. The leaching efficiency (L%) was calculated using the following formula. These results are shown in Figure 9.
L%=( _CL / _CS )×100 (3)
where C _L (Bq/Kg) and C _S (Bq/Kg) are the thorium concentrations in the acid solution at equilibrium and initial conditions.

Figure 9 shows the dependence of thorium leaching from ECR on various conditions.

Figure 9 (A) shows the dependence of leaching efficiency on hydrochloric acid concentration. All results are from adding 1 g of ECR to hydrochloric acid of various concentrations (0.1 M to 0.4 M, 10 mL) and stirring for 60 minutes at room temperature (25°C ± 2). Figure 9 (A) shows that it is effective to use hydrochloric acid of 1.5 M or more and 3 M or less for 1 g of thorium-containing ore.

Figure 9 (B) shows the time dependence of leaching efficiency. In all cases, 1 g of ECR was added to 2 M hydrochloric acid (10 mL) and stirred at room temperature (25°C ± 2) for 5 to 180 minutes. Figure 9 (B) shows that a high leaching efficiency of nearly 60% was observed for leaching times of 60 minutes or more. The upper limit of the leaching time should be 120 minutes.

Figure 9 (C) shows the dependence of leaching efficiency on the amount of hydrochloric acid solution. All results are from adding 1 g of ECR to various amounts of hydrochloric acid (2 M, 2 mL to 50 mL) and stirring at room temperature (25°C ± 2) for 60 minutes. According to Figure 9 (C), high leaching efficiency was observed when 10 mL or more of hydrochloric acid was used. For this reason, it is preferable to use an acid solution of 10 mL to 40 mL per 1 g of ECR.

Figure 9 (D) shows the temperature dependence of leaching efficiency. All results are from adding 1 g of ECR to hydrochloric acid (2 M, 10 mL) and stirring for 60 minutes at various temperatures (25°C to 80°C). Figure 9 (D) shows that a high leaching efficiency of over 80% can be achieved if the temperature is in the range of 60°C to 80°C.

Based on the above results, a thorium-containing solution using EGR-containing ore, described below, was prepared by adding 1 g of ECR to 10 mL of hydrochloric acid (2 M), heating to 80°C ± 2°C, and stirring for 60 minutes. At this time, 99.5% of the thorium was extracted from the thorium-containing ore.

[Example 1]
In Example 1, a thorium adsorbent made of porous silica was produced under the conditions shown in Table 2 using a plant-derived silica source and cetyltrimethylammonium bromide (CTAB, manufactured by Sigma-Aldrich) as a surfactant.

The plant-derived silica source (2.5 g) was added as an alkaline solution to 2.5 M sodium hydroxide (20 mL), and the solution was refluxed at 60° C. for 5 to 7 hours to dissolve the source, preparing a plant-derived silica solution (step S410 in FIG. 4). The plant-derived silica solution was filtered, washed with boiled Milli-Q water (10 mL), and cooled. Cetyltrimethylammonium bromide (CTAB, 0.5 g) was added thereto, and the mixture was stirred for 1 hour to obtain a raw material mixture solution (step S420 in FIG. 4). The raw material mixture solution was a sol with a higher viscosity than the plant-derived silica solution.

The raw material mixture solution was transferred to a Teflon (registered trademark) autoclave and hydrothermally synthesized to precipitate a reaction product consisting of silica and CTAB (step S430 in FIG. 4). The conditions for hydrothermal synthesis were to raise the temperature to 130°C at a heating rate of 10°C/min and hold the temperature for 10 hours. 50% sulfuric acid was added dropwise to the resulting reaction solution while stirring, and the pH of the reaction solution was adjusted to 8. The resulting reaction product was washed with boiled Milli-Q water to remove the alkali, and then dried at 70°C for 12 hours.

The dried reactant was calcined at 550°C for 8 hours to remove the CTAB (step S440 in Figure 4). In this way, porous silica was obtained. The sample thus obtained is called the sample of Example 1 or HOMS.

The results of the SEM image, HRTEM image, elemental analysis, XRD diffraction, TG-DTA curve, FTIR spectrum, BET specific surface area, and pore volume of the obtained sample of Example 1 are shown in Figures 10 to 13 and Tables 4 and 5.

The thorium adsorption and release efficiencies were measured for the samples obtained in Example 1 using the batch and column methods as appropriate. The results are shown in Figures 16 to 27.

[Example 2]
In Example 2, the surface and pores of the porous silica obtained in Example 1 were modified with a chelate compound shown in Table 3 (3-phosphonopropionic acid (PPA, manufactured by Sigma-Aldrich)).

Specifically, PPA (0.2 g) was dissolved in dry toluene (20 mL), to which the sample of Example 1 (1 g) was added and stirred at room temperature (25° C.) for 5 hours (step S510 in FIG. 5). The mixture was then filtered and washed three times with absolute ethanol and Milli-Q water to remove unreacted PPA. The product was dried in an oven at 70° C. for 12 hours and dehydrated (step S520 in FIG. 5). The sample thus obtained is referred to as the sample of Example 2 or HOMS-L.

As with the sample of Example 1, the results of SEM images, HRTEM images, elemental analysis, XRD diffraction, TG-DTA curves, FTIR spectra, BET specific surface area, and pore volume of the sample of Example 2 are shown in Figures 11 to 13 and Tables 4 and 5.

As with the sample of Example 1, the adsorption efficiency and release efficiency of thorium were measured for the sample of Example 2 by appropriately adopting the batch method and column method. In addition, the XRD pattern and FTIR spectrum of the sample of Example 2 after thorium adsorption were measured. These results are shown in Figures 14 to 27.

[Examples 3 to 7]
In Examples 3 to 7, the surface and pores of the porous silica obtained in Example 1 were modified with the chelate compounds shown in Table 3 in the same manner as in Example 2, and the adsorption and release of thorium was confirmed. The samples obtained in this manner are referred to as the samples of Examples 3 to 7 or HOMS-L1 to HOMS-L5.

For simplicity, the synthesis conditions for the samples in Examples 1 to 7 above are shown in Tables 2 and 3, and the results are summarized and explained.

Figure 10 shows SEM and HR-TEM images of the sample in Example 1.

Figure 10(A) is an SEM image of the sample of Example 1, and Figures 10(B) and 10(C) are HR-TEM images of the sample of Example 1. The sample of Example 1 was a sponge-like aggregate consisting of particles with a particle size in the range of 10 μm to 70 μm. As shown in Figure 10(A) b and c, the sample of Example 1 had elongated grooves aligned in the longitudinal direction and had a length of 250 nm to 2 μm.

According to FIG. 10(B), the outer surface of the aggregate had many holes with diameters of 200 nm to 250 nm. According to FIG. 10(C), the inner surface of the aggregate also had interconnected holes with diameters of 200 nm to 250 nm. This also demonstrated that the sample of Example 1 was a sponge-like aggregate. The samples of Examples 2 to 7 also exhibited a similar appearance.

Table 4 shows the results of elemental analysis of the samples of Examples 1 and 2.

According to Table 4, the sample of Example 2 modified with a chelate compound contains silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and has the following mass percent concentrations:
42≦Si≦44
48≦O≦51
1.5≦C≦3, and
0.5≦P≦2
It was found that the above condition was satisfied. The samples of Examples 3 to 7 also had the same composition.

Figure 11 shows the XRD patterns of the samples of Examples 1 and 2.

The samples of Example 1 and Example 2 had broad peaks in the range of 2<2θ<2.5 and had d ₁₀₀ spacings of 4.1 nm and 4.019 nm, respectively. This suggests that the samples of Example 1 and Example 2 have voids in the mesoscale region. The decrease in the lattice spacing suggests that the surface of the sample of Example 2 is modified with a chelate compound. The broad peaks at 2θ=30° and 43° in the wide-angle XRD pattern are due to amorphous silica. The XRD patterns of the samples of Example 3 to Example 7 were similar to that of Example 2.

Figure 12 shows the nitrogen adsorption/desorption isotherms (A) and TG-DTA curves (B) of the samples of Examples 1 and 2.

According to FIG. 12(A), the nitrogen adsorption/desorption isotherm shows a mixture of IUPAC types I, II, and IV, with micropores, mesopores, and macropores, and it was found that the samples of Examples 1 and 2 have a hierarchical structure. As shown in the inset, the pore size distribution was calculated using nonlocal density functional theory (NLDFT) and had a peak at 4.78 nm. It was also found that the pores in the samples of Examples 1 and 2 have similarity and regularity. Although not shown, the samples of Examples 3 to 7 also showed similar nitrogen adsorption/desorption isotherms.

Table 5 shows the BET specific surface area and pore volume of the samples of Examples 1 and 2.

As shown in Table 5, both the BET specific surface area and the pore volume were reduced by modifying the surface with a chelating compound. It was confirmed that the samples of Examples 2 to 7 also had a BET specific surface area in the range of 250 m ² /g or more and 170 m ² /g or less, and a pore volume in the range of 0.2 cm ³ /g or more and 0.25 cm ³ /g or less.

According to FIG. 12(B), the sample of Example 1 showed a mass loss of 2.61% by mass between 90°C and 420°C. This is due to the evaporation of residual solvent, water molecules, and CTAB surfactant. On the other hand, the sample of Example 2 showed a total mass loss of 5.6% by mass over two stages between 27°C and 600°C. The first mass loss of 2.28% by mass between 27°C and 200°C is due to the evaporation of physically adsorbed water, ethanol, etc. The second mass loss of 3.22% by mass between 200°C and 600°C is due to the decomposition of organic groups such as CTAB, chelating compounds, and silanols.

The small mass loss of the samples in Examples 1 and 2 indicates that the samples in Examples 1 and 2 have excellent durability in harsh high-temperature environments.

Figure 13 shows the FTIR spectra of the samples of Examples 1 and 2.

Both spectra showed the following characteristics: the broad bands at 3550 cm ^-1 and 1646 cm ^-1 were due to the stretching, antisymmetric stretching, and bending of OH groups caused by silanol groups (Si-OH) and water molecules (H-O-H) physically adsorbed on the surface; the peaks at 1078 cm ^-1 , 797 cm ^-1 , and 456 cm ^-1 were due to the stretching motions of Si-O-Si and O-Si-O.

On the other hand, the FTIR spectrum of the sample of Example 2, unlike that of the sample of Example 1, had small bands at 1653 cm ^-1 and 958 cm ^-1 . These are due to the bending HO-P=O and stretching P-O due to the chelating compound (PPA in this case) modified on the surface and in the pores, respectively. The broad band at 3460 cm ^-1 indicates the OH group of the chelating compound, and the small peak at 1880 cm ^-1 indicates the C=O group of the chelating compound. Although not shown, the samples of Examples 3 to 7 also showed similar FTIR spectra.

From the above results, it was shown that the samples of Examples 1 to 7 had sponge-like aggregates made of porous silica, had BET specific surface areas in the range of 150 m ² /g to 200 m ² /g, and had pore volumes in the range of 0.18 cm ³ /g to 0.3 cm ³ /g. Furthermore, it was shown that the aggregates were particles with particle sizes in the range of 10 μm to 70 μm, and had a hierarchical structure with micropores, mesopores, and macropores. The aggregates had elongated channels aligned in the longitudinal direction. It was shown that the surfaces and pores of the samples of Examples 2 to 7 were modified with a specific chelating compound.

Figure 14 shows the XRD pattern of the sample of Example 2 after a batch thorium adsorption test using a thorium standard solution.

Figure 14 also shows the XRD patterns of the samples of Example 1 and Example 2 before the adsorption test. According to Figure 14, the sample of Example 2 after the adsorption test also showed an XRD pattern similar to that of the sample before the adsorption test, with a broad peak in the range of 2<2θ<2.5 and a lattice spacing d ₁₀₀ of 4.018 nm.

Figure 15 shows the FTIR spectrum of the sample of Example 2 after a batch thorium adsorption test using a thorium standard solution.

According to Fig. 15, the FTIR spectrum of the sample of Example 2 after the adsorption test showed the bands of O=Th=O and Th=O stretching bonds at 802 cm ^-1 and 623 cm ^-1 , and the band of Th-O bond at 463 cm ^-1 , in addition to the bands described with reference to Fig. 13. Although not shown, the sample of Example 1 after the adsorption test also showed a similar spectrum.

The above results show that the samples of Examples 1 and 2 function as thorium adsorbents and are chemically and physically stable even after adsorption.

FIG. 16 shows the results of a batch-type thorium adsorption test using a thorium standard solution for the samples of Examples 1 and 2.
FIG. 17 shows the results of a batch-type thorium adsorption test using a thorium standard solution for the samples of Example 1 and Example 2.

Figure 16 (A) shows the pH dependence of the amount of thorium adsorbed (adsorption efficiency) when 20 mL of thorium standard solution (100 mg/L) and 20 mg of the samples of Examples 1 and 2 were mixed at 25 ± 2°C for 30 minutes. HCl and NaOH were used to adjust the pH.

Figure 16 (A) shows that the samples of Examples 1 and 2 showed high adsorption amounts exceeding 80% in the pH range of 4 to 6, with a pH range of 4 to 5 being particularly preferable.

Figure 16(B) shows the mixing time dependence of the amount of thorium adsorption when 20 mL (100 mg/L) of a pH 4 thorium standard solution was mixed with 20 mg of samples from Examples 1 and 2 at 25±2°C. According to Figure 16(B), when the contact time was 30 minutes or more, all samples showed a high amount of adsorption exceeding 90%.

Figure 16 (C) shows the dose dependence of the amount of thorium adsorption when 20 mL (100 mg/L) of a standard thorium solution at pH 4 was mixed with 5 mg to 100 mg of samples from Examples 1 and 2 at 25 ± 2°C for 60 minutes.

16(C), the amount of thorium adsorption increased with increasing mass of the sample, but even when more than 20 mg of sample was used, the amount of thorium adsorption did not change any more. This is thought to be because the active sites in the sample were interfered with by insoluble and stable hydroxides such as [Th(OH) ₂ ] ²⁺ and [ _Th2 (OH) ₂ ] ⁶⁺ .

Figure 17(A) shows the temperature dependence of the amount of thorium adsorption when 20 mL (100 mg/L) of a pH 4 thorium standard solution was mixed with 20 mg of samples from Examples 1 and 2 at various temperatures for 60 minutes. According to Figure 17(A), the sample from Example 1 showed an increase in the amount of thorium adsorption with increasing temperature, but all samples showed high thorium adsorption amounts exceeding 90% at room temperature. In particular, when not modified with a chelating compound, it is recommended to heat to a temperature range of 60°C to 80°C.

Fig. 17(B) is a diagram showing the relationship between the amount of adsorption _qe and the thorium concentration _C0 in the thorium-containing solution. According to Fig. 17(B), it was found that when the thorium concentration in the thorium-containing solution exceeds a predetermined concentration, the amount of adsorption is saturated.

Furthermore, according to Figures 16 and 17, the sample of Example 2, whose surface and pores were modified with a chelating compound, showed a higher adsorption amount and higher adsorption efficiency than the sample of Example 1, whose surface was not modified, and was found to be an excellent thorium adsorption material.

Figure 18 shows the results of a batch thorium adsorption test using thorium standard solutions containing various ions for the samples of Examples 1 and 2.

20 mL (100 mg/L) of a ^thorium standard solution at pH 4 further contained Cr ³⁺ , Cu ²⁺ , Ni ²⁺ , Zn ²⁺ , Zr ⁴⁺ , Ba ²⁺ , Sr ²⁺ , V ³⁺ , Pb ²⁺ , and rare earth ions (REE) at the same concentration as Th 4+ ions. The amount of adsorption of each ion was investigated when such a thorium standard solution was mixed with 20 mg of the samples of Examples 1 and 2 at 25±2°C for 60 minutes.

According to Figure 18, it was found that the samples of Examples 1 and 2 selectively adsorb thorium ions (Th 4+ ) although they also adsorb small amounts of Cr ³⁺ , Cu ²⁺ , Ni ²⁺ , Zn ²⁺ , Zr ⁴⁺ , Ba ²⁺ , Sr ²⁺ , V ³⁺ , Pb ²⁺ , and rare earth ions ( ^REE ).

Figure 19 shows another result of a batch thorium adsorption test using thorium standard solutions containing various ions for the samples of Examples 1 and 2.

A thorium standard solution (pH 4, 20 ^mL , 100 mg/L) was prepared containing Cr ³⁺ , Cu ²⁺ , Ni ²⁺ , Zn ²⁺ , Zr ⁴⁺ , Ba ²⁺ , Sr ²⁺ , V ³⁺ , Pb ²⁺ , and rare earth ions (REE) at the same concentration as Th 4+ ions. Figure 19(A) shows the amount of thorium ions adsorbed when each thorium standard solution was mixed with 20 mg of samples from Examples 1 and 2 at 25±2°C for 60 minutes. According to Figure 19(A), all samples selectively adsorbed 90% or more of thorium.

Thorium standard solutions (pH 4, 20 ml, 100 mg/L) of types G1 to G7 were prepared, with the same content of each ion.
G1 (Th ⁴⁺ , Cr ³⁺ , Ca ²⁺ )
G2 (Th ⁴⁺ , Cu ²⁺ , Mg ²⁺ )
G3 (Th ⁴⁺ , Ni ²⁺ , Zn ²⁺ )
G4 (Th ⁴⁺ , Zr ⁴⁺ , Ba ²⁺ ) G5 (Th ⁴⁺ , Pb ²⁺ , Sr ²⁺ , V ³⁺ ), G6 (Th ⁴⁺ , Cu ²⁺ , Ni ²⁺ , rare earth ions)
G7 (Th ⁴⁺ , Cr ³⁺ , Ba ²⁺ , Zr ⁴⁺ , Pb ²⁺ )

Figure 19 (B) shows the amount of thorium ions adsorbed when the thorium standard solutions G1 to G7 were mixed with 20 mg of the samples of Examples 1 and 2 at 25 ± 2°C for 60 minutes. According to Figure 19 (B), all samples selectively adsorbed 90% or more of thorium.

Figure 20 shows the results of a release (desorption) test using samples from Examples 1 and 2 in which thorium was adsorbed by the batch method.

Figure 20 (A) shows the results of adding 0.1 g of the samples of Examples 1 and 2 after the thorium adsorption test to 10 mL of hydrochloric acid of various concentrations and contacting them for 30 minutes at 25 ± 2°C. Figure 20 (A) shows that thorium can be efficiently liberated by using hydrochloric acid with a concentration of 0.1 M or more and 0.4 M or less.

Figure 20(B) shows the results when 0.1 g of the samples of Examples 1 and 2 after the thorium adsorption test was added to 10 mL of 0.1 M hydrochloric acid and allowed to come into contact for 5 to 150 minutes at 25±2°C. Figure 20(B) shows that thorium can be efficiently liberated by using a time of 40 to 80 minutes.

Figure 21 shows the results of a batch-type repeated thorium adsorption/desorption test using a thorium standard solution for the samples of Examples 1 and 2.

The adsorption conditions were: 10 mL of a pH 4 thorium standard solution (100 mg/L) and 20 mg of the samples from Examples 1 and 2 were mixed at 25±2°C for 60 minutes. The desorption conditions were: 10 mL of 0.1 M hydrochloric acid and 0.1 g of the samples from Examples 1 and 2 were mixed at 25±2°C for 45 minutes.

According to Figure 21, the thorium adsorption efficiency and thorium release efficiency of the samples of Examples 1 and 2 decreased slightly as the number of repetitions increased, but still showed high efficiency even after more than 10 repetitions.

These results indicate that the samples of Examples 1 and 2 are reusable thorium adsorbents.

Figure 22 shows the results of a column-type thorium adsorption test using a thorium standard solution for the samples of Examples 1 and 2.

22A and 22B show the dependence of C _eff /C ₀ on the amount of thorium standard solution (pH 4, 100 mg/L) passed through the samples (2 g) of Examples 1 and 2, respectively, at various rates. C ₀ is the initial thorium concentration in the thorium standard solution, and C _eff is the thorium concentration removed from the thorium standard solution.

Figures 22(C) and (D) show the flow rate dependence of C _eff /C ₀ when thorium standard solution (pH 4, 100 mg/L) was passed through the samples of Examples 1 and 2 at various doses at 4 mL/min. Figures 22(E) and (D) show the flow rate dependence of C _eff /C ₀ when thorium standard solutions (pH 4) of various concentrations were passed through the samples (2 g) of Examples 1 and 2 at 4 mL/min.

Figure 22 shows that if the samples of Examples 1 and 2 are used in a column of thorium adsorbent, all of the thorium in the test liquid can be adsorbed by adjusting the amount of test liquid passed through, the flow rate of the test liquid, the thorium concentration in the test liquid, and the dose amount filled into the column.

Figure 23 shows the results of a release (desorption) test using samples from Examples 1 and 2 in which thorium was adsorbed using the column method.

Figure 23(A) shows the dependence of the thorium release concentration on the amount of 0.2 M of various acids at a flow rate of 0.5 mL/min on the samples of Examples 1 and 2 after the thorium adsorption test. Figure 23(A) shows that thorium can be released using any acid, but that thorium can be released particularly efficiently using hydrochloric acid.

Figure 23(B) shows the dependence of the thorium release concentration on the amount of hydrochloric acid passed through 2 g of samples from Examples 1 and 2 after the thorium adsorption test when various concentrations of hydrochloric acid were passed through at a rate of 0.5 mL/min. Figure 23(B) shows that thorium can be released efficiently by using hydrochloric acid with a concentration of 0.1 M or more and 0.4 M or less.

Figure 24 shows the results of repeated column-type thorium adsorption/desorption tests using a thorium standard solution for the samples of Examples 1 and 2.

The adsorption conditions were as follows: 1,500 mL of thorium standard solution (pH 4, 100 mg/L) was passed through the samples (2 g) of Examples 1 and 2 at a flow rate of 4 mL/min. The desorption conditions were as follows: 40 mL of 0.2 M hydrochloric acid was passed through the samples (1 g) of Examples 1 and 2 at a flow rate of 0.5 mL/min.

According to Figure 24, the thorium adsorption efficiency and thorium release efficiency of the samples of Examples 1 and 2 decreased slightly as the number of repetitions increased, but still showed high efficiency even after more than 10 repetitions.

From the above, it was shown that the samples of Examples 1 and 2 function as reusable thorium adsorbents that selectively adsorb thorium. In particular, it was shown that modification with a chelating compound results in a superior thorium adsorbent. It was shown that the thorium adsorbent of the present invention can be applied to both the batch method and the column method.

Figure 25 shows the results of a batch thorium adsorption test using a thorium-containing solution with ECR-containing ore for the sample of Example 2.

Figure 25 shows the gamma ray spectrum of the sample of Example 2 after the adsorption test and the release test. From Figure 25, it can be seen that the sample of Example 2 adsorbed the 232Th isotope species and other radioactive substances leached from the ECR-containing ore. In addition, the count number was a value comparable to that used in the field of thorium adsorption and removal.

Figure 26 shows the results of batch and column thorium adsorption/desorption tests using a thorium-containing solution from ECR-containing ore for the samples of Examples 1 and 2.

The adsorption conditions for the batch method were: 10 mL of thorium-containing solution (pH 4) and 20 mg of the samples of Examples 1 and 2 were mixed at 25±2°C for 60 minutes. The desorption conditions for the batch method were: 10 mL of 0.1 M hydrochloric acid and 0.1 g of the samples of Examples 1 and 2 were mixed at 25±2°C for 45 minutes. The adsorption conditions for the column method were: 1000 mL of thorium-containing solution (pH 4) was passed through the samples of Examples 1 and 2 (2 g) at a flow rate of 4 mL/min. The desorption conditions for the column method were: 40 mL of 0.2 M hydrochloric acid was passed through the samples of Examples 1 and 2 (1 g) at a flow rate of 0.5 mL/min. The samples of Examples 1 and 2 after the adsorption test were analyzed by ICP-MS.

As shown in Figure 26, it was shown that the thorium adsorbent of the present invention can adsorb the 232Th isotope leached from actual ore with a high adsorption efficiency of over 70%, whether using the column method or the batch method.

Figure 27 shows the results of a batch-type thorium adsorption test using a thorium-containing solution with ECR-containing ore for the samples of Examples 1 to 7.

The batch-type adsorption conditions were: 10 mL of thorium-containing solution (pH 4) and 20 mg of samples from Examples 1 to 7 were mixed for 60 minutes at 25±2°C. Figure 27 also shows the results for the samples from Examples 1 and 2 shown in Figure 26.

As shown in Figure 27, all samples function as thorium adsorbents that adsorb thorium. In particular, it was shown that modification with a chelating compound results in a better thorium adsorbent. It was shown that the chelating compounds modified with the above-mentioned formulas 7 and 8 are effective.

The thorium adsorbent of the present invention can selectively adsorb and release thorium, making it applicable to various devices and methods for removing and recovering thorium.

100 Aggregate 110 Porous silica 210 Chelate compound 600 Thorium adsorption column 610 Adsorbent 620 Top filter 630 Bottom filter 640, 650 Column connection part

Claims (20)

The present invention comprises a sponge-like aggregate of porous silica,
The aggregates have a hierarchical structure with micropores, mesopores and macropores;
The aggregates have a BET specific surface area in the range of 150 m ² /g or more and 200 m ² /g or less, and a pore volume in the range of 0.18 cm ³ /g or more and 0.3 cm ³ /g or less,
The aggregates are particles having a particle size in the range of 10 μm to 70 μm,
the aggregate has longitudinally aligned elongated grooves;
A thorium adsorbent that selectively adsorbs or liberates thorium. The thorium adsorbent according to claim 1, wherein the surface and pores of the porous silica are modified with at least a chelate compound represented by any one of formulas 1 to 3.
Here, L is a divalent group, and * represents a bonding site with the porous silica. The thorium adsorbent according to claim 2, wherein the mass ratio of the chelate compound to the porous silica is in the range of 0.1 to 0.5. The thorium adsorbent according to claim 3, wherein the mass ratio of the chelate compound to the porous silica is in the range of 0.15 to 0.25. The porous silica contains at least silicon (Si), oxygen (O), carbon (C) and phosphorus (P), and optionally nitrogen (N) and an M element (a Group 1 element or a Group 2 element), and when the total is 100% by mass, the mass percent concentration of each element is:
40≦Si≦45
47≦O≦52
1≦C≦5,
0.5≦P≦2,
0≦N≦2, and
0≦M≦5
The thorium adsorbent according to any one of claims 2 to 4, which satisfies the above. The thorium adsorbent according to any one of claims 2 to 5, wherein the surface and pores of the porous silica are further modified with a chelate compound represented by any one of formulas 4 to 6.
Here, L is a divalent group, and ... represents a hydrogen bond with the porous silica or a bond by van der Waals forces. The thorium adsorbent according to claim 2 or 6, wherein L is an alkylene group having 1 to 20 carbon atoms, with or without a substituent. Adding a plant-derived silica source obtained from wheat straw to an alkaline solution to prepare a plant-derived silica solution;
Adding cetyltrimethylammonium bromide (CTAB) to the plant-derived silica solution to obtain a raw material mixed solution;
subjecting the raw material mixed solution to hydrothermal synthesis, thereby precipitating a reaction product consisting of silica from the plant-derived silica source and the cetyltrimethylammonium bromide;
and calcining the reactant to remove the cetyltrimethylammonium bromide. The method according to claim 8, wherein preparing the plant-derived silica solution includes refluxing the alkaline solution to which the plant-derived silica source has been added at a temperature range of 55°C or more and less than 65°C. The method according to claim 8 or 9, wherein the raw material mixed solution is obtained by adding cetyltrimethylammonium bromide so that the mass ratio of cetyltrimethylammonium bromide to the plant-derived silica source in the plant-derived silica solution is in the range of 0.1 to 0.4. The method according to any one of claims 8 to 10, wherein the hydrothermal synthesis includes adding an acid to the reactants, washing them, and drying them. The method according to any one of claims 8 to 11, wherein the calcination of the reactant comprises calcining the reactant at a temperature in the range of 200°C to 800°C for a period of 2 hours to 24 hours. The method according to any one of claims 8 to 12, further comprising mixing the calcined product obtained by calcining the reactant with a chelate compound represented by formula 7 or formula 8.
Here, L is a divalent group. The method according to claim 13, wherein the mixing step mixes the chelate compound so that the mass ratio of the chelate compound to the baked product is 0.1 or more and 0.5 or less. The method of claim 13 or 14, further comprising washing and drying the mixed product following the mixing. The method further includes preparing the plant-derived silica source prior to preparing the plant-derived silica solution, the preparing comprising:
washing and drying the wheat straw;
Treating the dried wheat straw with an acid having a concentration ranging from 0.25M to 0.5M and refluxing it at a temperature ranging from 65°C to 75°C for a period of 1 hour to 10 hours;
washing the acid and drying the acid at a temperature in the range of 80° C. to 110° C. for a period of 5 hours to 10 hours;
The method according to any one of claims 8 to 15, further comprising calcining the dried wheat straw at a temperature range of 600 ° C. to 800 ° C. for a period of 5 hours to 12 hours to decarbonate the straw. 1. A method for extracting thorium from a test aqueous solution, comprising the steps of:
A method comprising: contacting the thorium adsorbent according to any one of claims 1 to 7 with the test aqueous solution to adsorb thorium in the test aqueous solution. 1. A method for recovering thorium from thorium-containing ore, comprising the steps of:
adding the thorium-containing ore to an acid solution selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid to prepare a thorium- containing solution;
A method for producing a thorium-containing solution comprising contacting the thorium adsorbent according to any one of claims 1 to 7 with the thorium-containing solution and allowing the thorium adsorbent to adsorb thorium in the thorium-containing solution;
Discharging treated water from which thorium has been extracted from the thorium-containing solution;
contacting the thorium adsorbent having thorium adsorbed thereon with an acid selected from the group consisting of sulfuric acid, hydrochloric acid and nitric acid to liberate the adsorbed thorium. The method of claim 18, wherein the pH of the thorium-containing solution is in the range of 4 to 6. The method according to claim 18 or 19, wherein the adsorption comprises contacting the thorium adsorbent with the thorium-containing solution for a period of 30 minutes or more and 100 minutes or less.