Method for treating difficult-to-filter substances

Abstract

To provide a method for treating hard-to-filter substances that can improve filtration performance after freeze-re-thaw treatment and improve waste volume reduction by reducing a solid content during solid-liquid separation, even a solution of hard-to-filter substances containing radioactive substances and ions of specific metals.SOLUTION: A method for treating hard-to-filter substances includes: an insolubilizing step of applying oxidizing treatment to a solution of hard-to-filter substances containing radioactive substances and ions of a specific metal of at least one of Ni, Pb, Cu, Zn, Co, Mn, and Cr, oxidizing the ions of the specific metal, and generating an insoluble compound; and a freeze-re-thaw step of adding a soluble metal salt to the suspension of the hard-to-filter substances containing the insoluble compound to form a metal hydroxide solid and performing freeze-re-thaw treating of the suspension containing the metal hydroxide solid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for processing poorly filterable materials containing radioactive materials and ions of specific metals. [Background technology]

[0002] A method of wet oxidative decomposition using hydrogen peroxide has been proposed as a treatment method for filter sludge and spent ion exchange resins used in the recovery and treatment of wastewater and solids containing radionuclides at nuclear power plants. This wet decomposition method involves the oxidative decomposition of organic waste such as spent ion exchange resins using hydrogen peroxide, with iron or other catalysts, under temperature conditions of approximately 100°C and atmospheric pressure. The OH radicals generated from hydrogen peroxide break the bond chains of the organic waste and oxidize it, ultimately decomposing it into carbon dioxide and water. For example, sulfuric acid is produced when cation exchange resins are decomposed, and ammonia is produced when anion exchange resins are decomposed.

[0003] Low-level radioactive waste generated at nuclear power plants, including not only waste generated during decommissioning, is classified according to the law as follows and disposed of in accordance with the respective standards. Level I [L1]: Low-level radioactive waste with a relatively high concentration of radioactive material. It is buried in artificial structures at a depth of approximately 50m to 100m underground (burial at a safe depth (managed for 300 years)). Level II [L2]: Low-level radioactive waste with relatively low concentrations of radioactive materials. It is disposed of by burying it in an artificial structure about 10 meters underground (concrete pit burial (managed for 300 years)). Level III [L3]: Low-level radioactive waste with extremely low concentrations of radioactive material, which is disposed of by direct underground burial (burial in an open trench (managed for 30-50 years)).

[0004] The material remaining after the wet decomposition is a decomposition liquid consisting of solid iron impurities, including cladding (mainly composed of iron oxide: α-Fe2O3) and iron catalysts (Fe(OH)3) derived from radioactive corrosion products accumulating in the reactor's primary cooling system, and a slurry mainly composed of sodium sulfate. Because the solid iron impurities have a high radiation level, they must be disposed of as L1 waste in the aforementioned classification and buried. However, the sodium sulfate aqueous solution itself has a low concentration of radioactive material and can be treated as L2 waste. If the two can be separated, the amount of radioactive waste with high radiation levels can be reduced.

[0005] Furthermore, because the radioactive materials contained in solid iron impurities have high radiation doses, and hydrogen gas can be generated through radiolysis when water is present, it is desirable to reduce the water content in solid iron impurities that are to be final disposed of. In addition, sodium sulfate, which is accompanied by the solid components, is considered a type of waste that requires consideration for safety and is thought to potentially affect handling facilities and human barriers. For this reason, it is desirable to reduce the amount of water and sulfates.

[0006] Therefore, efficiently filtering and separating only solid iron impurities from the slurry decomposition liquid and drying them would reduce the amount of L1 waste to be disposed of, as well as minimize the effects of hydrogen generation and leakage. However, solid iron impurities such as Fe(OH)3 derived from the cladding and iron catalyst are themselves difficult to filter and separate, posing a challenge.

[0007] One known method for improving the filtration performance of such poorly filterable substances is the freeze-re-thaw method (see, for example, Patent Document 1). In this freeze-re-thaw method, a soluble metal salt is added to a suspension of poorly filterable substances to produce a metal hydroxide solid. By freezing and re-thawing the suspension containing this metal hydroxide solid, only solid iron impurities can be efficiently filtered and separated from the slurry decomposition liquid, thereby reducing the amount of L1 waste to be disposed of. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2020-173130 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] When a solution containing a difficult-to-filter substance contains a high concentration of ions of a specific metal, such as Ni ions, for example, 500 mg / L or more, the improvement of filtration performance after freeze-re-thaw treatment is easily hindered. To address this problem, the treatment method for difficult-to-filter substances disclosed in Patent Document 1 aims to improve filtration performance by increasing the amount of soluble metal salt added as an auxiliary agent. However, in this case, increasing the amount of soluble metal salt leads to an increase in the amount of metal hydroxide generated, which increases the cake (solid content) volume after filtration and hinders volume reduction.

[0010] This invention has been made in consideration of these circumstances, and aims to provide a method for treating poorly filterable materials that can improve the filtration performance after freeze-re-thaw treatment and reduce the amount of solids during solid-liquid separation, even in solutions of poorly filterable materials containing radioactive materials and ions of at least one specific metal from Ni, Pb, Cu, Zn, Co, Fe, Mn, and Cr, thereby improving the volume reduction of waste. [Means for solving the problem]

[0011] As a result of diligent research to solve the above problems, the inventors have discovered that by adding a divalent iron salt to a solution of a poorly filterable substance containing a high concentration of metal ions to generate a metal hydroxide, and then performing air bubbling, the metal ions dissolved in the solution can be ferrite-converted into an insoluble compound, thereby improving the filtration performance by freeze-re-thaw cycles even with a reduced amount of soluble metal salts used, and have now completed the invention.

[0012] In other words, to solve the above problems, this invention proposes the following means. The method for treating hardly filterable substances of the present invention comprises an insolubilization step of subjecting a solution of hardly filterable substances containing radioactive substances and ions of at least one specific metal selected from Ni, Pb, Cu, Zn, Co, Mn, and Cr to an oxidation treatment to oxidize the ions of the specific metal to form an insoluble compound, and a freeze-thaw step of adding a soluble metal salt to a suspension of hardly filterable substances containing the insoluble compound to produce a metal hydroxide solid and subjecting the suspension containing the metal hydroxide solid to freeze-thawing.

[0013] According to the present invention, according to the method for treating hardly filterable substances of the present embodiment, even in a solution of hardly filterable substances containing a high concentration of ions of a specific metal such as Ni ions that hinder modification by freeze-thawing in the freeze-thaw step, in the insolubilization step, ions of a specific metal such as Ni ions and a divalent iron salt can be made into insoluble ferrite by performing an oxidation treatment such as air bubbling. Thereby, in the freeze-thaw step, a sufficient modification effect can be obtained by adding a small amount of a freezing aid, and the filtration performance can be improved. Therefore, even in a solution (waste liquid) containing a high concentration of ions of a specific metal that affects freeze-thawing, such as represented by Ni ions, both improvement in filtration performance and volume reduction of the solid content after solid-liquid separation can be achieved, and it becomes possible to reduce L1 waste.

[0014] Further, in the present invention, the insolubilization step may be a step of adding a divalent iron salt and then neutralizing with an alkali hydroxide to generate an intermediate solution containing a hydroxide of the specific metal, and subjecting this intermediate solution to an oxidation treatment by air bubbling to generate the ferrite-converted insoluble compound.

[0015] Further, in the present invention, the air bubbling in the insolubilization step may be performed in the range of a liquid temperature of 40 to 100°C and a pH of 10 to 11.5.

[0016] Further, in the present invention, the air flow rate in the air bubbling in the insolubilization step may be in the range of 0.5 to 5 NL / min per 1 L of the intermediate solution.

[0017] In the present invention, the specific metal is Ni, and the concentration of Ni ions contained in the solution of the hardly filterable substance may be in the range of 500 to 50,000 mg / L.

[0018] In the present invention, the hardly filterable substance may contain iron oxide (α-Fe2O3) and iron hydroxide (Fe(OH)3).

[0019] In the present invention, the radioactive substance may be a component of a decomposition solution obtained by wet decomposition of used ion exchange resins in a nuclear power plant and filter sludge, or a component of a waste solution obtained by eluting used ion exchange resins.

[0020] In the present invention, it may further have a solid-liquid separation step, which is a subsequent step of the freeze-thawing step and filters the suspension after freeze-thawing.

Effects of the Invention

[0021] According to the present invention, even for a solution of a hardly filterable substance containing radioactive substances and ions of specific metals, it is possible to improve the filtration performance after freeze-thawing treatment and reduce the solid content during solid-liquid separation, thereby improving the volume reduction of waste. A treatment method for hardly filterable substances can be provided.

Brief Description of the Drawings

[0022] [Figure 1] It is a flowchart showing step by step the treatment method of hardly filterable substances according to an embodiment of the present invention. [Figure 2] It is a graph showing the results of Verification Example 1. [Figure 3] It is a graph showing the results of Verification Example 2. [Figure 4] It is a graph showing the results of Verification Example 3. [Figure 5] It is a graph showing the results of Verification Example 4.

Modes for Carrying Out the Invention

[0023] A method for treating poorly filterable substances according to one embodiment of the present invention will be described below with reference to the drawings. The embodiments described below are provided specifically to better illustrate the spirit of the invention and do not limit the present invention unless otherwise specified.

[0024] Figure 1 is a flowchart illustrating a step-by-step method for processing a difficult-to-filter substance according to one embodiment of the present invention. The method for processing poorly filterable materials according to this embodiment includes: an insolubilization step S1 in which an intermediate solution containing a metal hydroxide is generated from a solution of poorly filterable materials containing radioactive materials and ions of a specific metal, and air is bubbling into this intermediate solution to generate an insoluble compound in which the specific metal has been ferriteized; a freeze-re-thaw step S2 in which a soluble metal salt is added to the suspension of poorly filterable materials obtained in the insolubilization step S1, and the suspension containing the metal hydroxide solid is frozen and re-thawed; and a solid-liquid separation step S3 in which the suspension obtained in the freeze-re-thaw step S2 is filtered.

[0025] The poorly filterable material treated by the treatment method of this embodiment contains radioactive material and ions of a specific metal, and the specific metal ion contains at least one metal ion from among Ni, Pb, Cu, Zn, Co, Fe, Mn, and Cr. In this embodiment, the Ni ion is given as an example of the specific metal ion.

[0026] Furthermore, poorly filterable substances mainly consist of iron oxide (α-Fe2O3) and iron hydroxide (Fe(OH)3). For example, such solid iron impurities originate from cladding (mainly composed of iron oxide: α-Fe2O3) that accumulates in the reactor's primary cooling system and from iron catalysts used in wet decomposition. The main components of the cladding depend on the materials used in the cooling system, water quality conditions, and operating conditions, but the main component is stainless steel. Ni ions also originate from the constituent materials.

[0027] A solution or suspension of poorly filterable materials containing Ni ions is preferably prepared using a decomposition solution that mainly consists of solid iron impurities obtained by wet decomposition of spent ion exchange resins and filter sludge (filter collectibles) from nuclear power plants, and sodium sulfate, and further contains a high concentration (e.g., 500 to 50,000 mg / L) of Ni ions. Wet decomposition will be described below as an example.

[0028] (Pre-process: wet decomposition process) In methods for treating radioactive organic waste, as described in Japanese Patent Publication No. 2000-56986 and Japanese Patent Publication No. 61-9599, for example, the radioactive organic waste is oxidatively decomposed by reacting it with hydrogen peroxide in an aqueous medium in the presence of iron ions. Specifically, radioactive organic waste includes not only spent granular or powdered ion exchange resins generated at nuclear power plants, but also filter sludge, which is subject to similar treatment.

[0029] The filter sludge can be treated with either cellulose-based or acrylic fiber-based filter aids. Waste solvents generated from nuclear fuel reprocessing plants can also be treated, and the process can be applied to the treatment of decontamination waste liquids containing citric acid, oxalic acid, and EDTA.

[0030] Iron ions are ferrous ions (Fe) 2+ ferric ion Fe 3+ All of these can be used, with iron sulfate, iron nitrate, and iron chloride being used as iron sources. Furthermore, since wet oxidation with hydrogen peroxide proceeds well when the aqueous medium is acidic, an acid such as sulfuric acid is added to initiate the reaction.

[0031] Furthermore, since ammonia and amines contained in the waste are retained in the liquid during the reaction and do not volatilize, they are released after the reaction is complete by adding an alkali such as caustic soda to neutralize the liquid. Through the above treatment, a decomposition liquid mainly composed of cladding (iron oxide) and sodium sulfate is produced. In this invention, such a decomposition liquid is used as the treatment liquid.

[0032] The concentrations of solid iron impurities and sodium sulfate contained in the treatment liquid used in this embodiment are not particularly limited, but usually, the solid iron impurities are contained in the range of 1 to 100 g / L, and sodium sulfate is contained in the range of 2 to 200 g / L. Also, the concentration of Ni ions contained in the treatment liquid is, for example, contained in the range of 500 to 50,000 mg / L.

[0033] (Insolubilization step S1) First, using the solution (acidic solution) of the difficult-to-filter substance containing the radioactive substance and Ni ions (ions of a specific metal) obtained in the wet decomposition step as described above, Fe 2+ salt as the source is dissolved. Although the iron salt is not particularly limited, in order to generate Fe 2+ in the solution of the difficult-to-filter substance, it is preferable to use a water-soluble inorganic iron salt. For example, a salt having a solubility of 50 to 150 g / liter in water at 20 °C is preferable. Specifically, it is preferable that the divalent iron salt is iron(II) sulfate.

[0034] Next, the solution of the difficult-to-filter substance in which this iron salt is dissolved is neutralized to about pH 7 using an alkali hydroxide compound, for example, sodium hydroxide (neutralization process S11). As a result, Ni 2+ and Fe 2+ present in the solution of the difficult-to-filter substance bind to the hydroxyl group, and an intermediate solution in which nickel-iron hydroxide is generated is obtained (refer to formula (1)). Ni 2+ +2Fe 2+ +6OH - →NiFe2(OH)6···(1)

[0035] Then, air is blown into this intermediate solution to perform air bubbling (oxidation process S12). As a result, the nickel-iron hydroxide binds to oxygen in the air, and insoluble nickel ferrite (NiFe2O4) precipitates (refer to formula (2)). NiFe2(OH)6 + 1 / 2O2 → NiFe2O4 + 3H2O···(2)

[0036] The conditions for air bubbling at this time are preferably such that the intermediate solution is pH 10 to 11.5 using an alkali hydroxide compound, such as sodium hydroxide. The temperature of the intermediate solution is preferably in the range of 40 to 100°C, preferably 60 to 80°C. The air flow rate during air bubbling is preferably in the range of 0.5 to 5 NL / min per liter of intermediate solution.

[0037] Furthermore, for bubbling to produce nickel ferrite, any gas capable of oxidizing nickel-iron hydroxide can be used, including not only air, but also oxygen or any gas of any composition containing oxygen.

[0038] Furthermore, when ammonia nitrogen (NH4-N) is dissolved in a solution of a difficult-to-filter substance, the insolubilization process S1 allows the treated solution to be adjusted to a pH of 10 or higher and to be sufficiently exposed to air through air bubbling. This causes the dissolved ammonia nitrogen to move from the liquid phase to the gas phase as free ammonia, thereby reducing the concentration of ammonia nitrogen.

[0039] The insolubilization process S1 described above allows for the precipitation of Ni ions as insoluble nickel ferrite (NiFe2O4) in a solution of a radioactive material and a poorly filterable material containing Ni ions, thereby enhancing the effect of the reforming aid in the freeze-re-thaw process S2 described later.

[0040] In this embodiment, Ni is given as a specific metal, but the insolubilization step S1 described above can be carried out in exactly the same way for other metals such as Pb, Cu, Zn, Co, Mn, and Cr. When Pb, Cu, Zn, Co, Mn, and Cr are represented as M, air is blown into the intermediate solution containing M to perform air bubbling (oxidation step S12), and M·iron hydroxide combines with oxygen in the air to precipitate insoluble M ferrite (MFe2O4) (see equation (3)). MFe2(OH)6+1 / 2O2→MFe2O4+3H2O···(3) However, M = Pb, Cu, Zn, Co, Mn, Cr

[0041] For example, if M is Pb, lead ferrite (PbFe2O4) precipitates; if M is Co, cobalt ferrite (CoFe2O4) precipitates; and if M is Mn, manganese ferrite (MnFe2O4) precipitates. All of these ferrites are insoluble and can be processed in the subsequent freeze-re-thaw step S2 in exactly the same way as nickel ferrite.

[0042] (Freeze-re-thaw step S2) Next, a soluble metal salt is added as a freezing aid to the suspension of the poorly filterable material, which has been converted into insoluble nickel ferrite (NiFe2O4) by the insolubilization step S1. Then, the pH is raised to 6 or higher to generate a metal hydroxide solid, and the suspension (treatment solution) containing the metal hydroxide solid is frozen and thawed again. The soluble metal salt (freezing aid) is not particularly limited as long as it produces a solid hydroxide, but it is preferably at least one of the following: iron salt, cobalt salt, or manganese salt, with iron salt being particularly preferred.

[0043] The soluble iron salt is added in an amount such that, when the iron content (by weight) of the poorly filterable substance is converted to α-Fe2O3, the weight ratio converted to Fe(OH)3 is 0.1 times or more, preferably 0.1 to 2 times, and more preferably 0.15 to 0.4 times. Similarly, if the poorly filterable substance is an element other than iron, it is added in such a way that the weight ratio falls within the above range. Furthermore, when using soluble metal salts other than iron, they are added in such a way that their weight ratio, converted to metal hydroxide, falls within the range described above.

[0044] While there are no particular restrictions on the soluble iron salt, a salt with sufficient solubility to avoid significant freezing point depression is preferred, given the need to freeze the processing solution. A salt with a solubility of 50 to 150 g / liter in water at 20°C is preferred. Specifically, iron(III) sulfate is preferred. When iron sulfate is used, sodium sulfate is produced when the pH is adjusted to prepare the solid. Since the solubility of sodium sulfate decreases as the temperature drops, it precipitates as a solid once it reaches a certain concentration. As a result, the salt concentration does not become too high, and freezing point depression is less likely to occur. This makes freezing easier even at relatively high temperatures of around -15°C.

[0045] Furthermore, when soluble iron salts such as iron chloride are added, no solid matter precipitates, and the freezing point depression caused by the resulting sodium chloride and other salts prevents freezing at cooling temperatures of around -15°C. Additionally, Japanese Patent Publication No. 55-157393 and No. 55-157394 disclose that the addition of salts such as iron chloride is undesirable for filtering poorly filterable substances. Sulfates such as manganese sulfate and cobalt sulfate can also be used.

[0046] In this embodiment, the above-mentioned soluble metal salt is added to the suspension of the poorly filterable substance, and then the pH of the treatment solution is adjusted to 6 or higher. pH adjustment is performed by known methods; for example, by adding an alkali such as caustic soda to raise the pH of the solution to 6 or higher. The pH is not particularly limited as long as it is within the range in which metal hydroxide solids can be formed, but it is preferably adjusted to the range of 8 to 9. Although the cladding itself is poorly filterable, when metal hydroxide solids are formed, the cladding is incorporated into the solids, making them easier to filter by freeze-thaw treatment. Note that even if metal hydroxide solids are formed in advance and then freeze-thawed, the effect of the freeze-re-thaw step S2 will not be achieved.

[0047] Furthermore, before the freeze-thaw treatment, it is possible to reduce the volume of the treatment solution by evaporating water, or by reducing the pressure of the treatment solution or heating it to evaporate the water. The solid concentration of the treatment solution containing metal hydroxide solids is in the range of 40-120 g / l, and the salt concentration is in the range of 80-240 g / l. Solutions within this range can achieve high filtration efficiency through freeze-re-thaw cycles.

[0048] In the freeze-re-thaw process S2, the water crystals grow as ice crystals due to freezing, and the solids move between the ice crystals and are compressed, consolidating and combining into large block-like structures as they become concentrated. This improves the filtration characteristics in the subsequent solid-liquid separation process S3.

[0049] Furthermore, when the entire material freezes, the expansion force of ice formation applies a strong compressive force to the block-shaped solid material. This breaks down any gel-like structures that may have formed, causing the water contained within to leak out and resulting in a higher dehydration effect. Therefore, even solid iron impurities, which are difficult to filter, can have their filtration improved.

[0050] Next, when this frozen material is heated and thawed, the block-like shape of the coarse particles is maintained even in the thawed state. As a result, the dense, coarse-particle solid settles, and the water (containing soluble salts) is separated as supernatant water. There are no particular restrictions on the freezing temperature, as long as it is the temperature at which the water in the treatment liquid freezes.

[0051] Thawing can be achieved by releasing the frozen state and heating it until it becomes liquefied, but natural thawing or the use of heating methods such as a heater or hot water bath are also acceptable.

[0052] (Solid-liquid separation process S3) Next, the thawed liquid obtained by thawing the frozen material in the freeze-re-thaw process S2 described above is supplied to a solid-liquid separator to separate it into residue (solid phase) and filtrate (liquid phase). Difficult-to-filter materials, including radioactive substances, are recovered as residue along with metal hydroxide solids. Examples of solid-liquid separation devices used in the solid-liquid separation step S3 include known means such as vacuum filtration, press filtration, and centrifugal separation.

[0053] Although the filtrate contains soluble salts such as sodium sulfate, poorly filterable substances including radioactive materials are separated as solids, so it can be disposed of as wastewater with a low radiation dose. It is possible to further wash the residue with water during solid-liquid separation and recover soluble salts in the washing aqueous phase.

[0054] The solid-liquid separated residue may be dried to remove moisture as needed. The residue with improved filterability is more easily washed and dried because moisture is removed more easily.

[0055] As described above, according to the method for processing poorly filterable substances of this embodiment, even if the solution of a poorly filterable substance contains a high concentration of ions of specific metals such as Ni ions, which hinders modification by freezing and thawing in the freeze-re-thawing step S2, in the immobilization step S1, the ions of specific metals such as Ni ions and iron ions can be converted into insoluble ferrite by performing an oxidation treatment such as air bubbling.

[0056] As a result, in the freeze-re-thaw process S2, sufficient modification effect can be obtained with the addition of a small amount of freezing aid, thereby improving filtration performance. Therefore, even with solutions (waste liquids) containing metal ions that affect freeze-re-thaw, such as Ni ions, it is possible to achieve both improved filtration performance and volume reduction of solids after solid-liquid separation, thereby reducing L1 waste. Since the unit cost of burying L1 waste is higher than the current unit cost of L2 waste, reducing L1 waste can lead to a reduction in the disposal costs of waste containing radioactive materials.

[0057] Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be carried out in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. This embodiment and its variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Examples]

[0058] The effects of the present invention were verified. (Verification Example 1) As samples, solutions containing radioactive material and poorly filterable material including Ni ions were prepared. Three types of treatment solutions were created: one without a freeze-re-thaw process (Comparative Example 1), one without an immobilization process but with the addition of 2 w / v% of a freeze aid (Fe(OH)3) and the subsequent freeze-re-thaw process (Comparative Example 2), and three with both an immobilization and freeze-re-thaw processes (Examples 1 and 2). In Example 1, the theoretical amount of ferrite formation in the immobilization process was set to 1x, while in Example 2, the theoretical amount of ferrite formation was set to 2x. Solid-liquid separation (filtration) was then performed on each treatment solution, and the average filtration resistivity was measured. The results are shown as a graph in Figure 2.

[0059] As shown in Figure 2, in Comparative Example 1, where the solution containing the difficult-to-filter substance was filtered as is, the average filtration resistivity was 1.9 × 10⁻⁶. 12 The value was m / kg. In Comparative Example 2, where only the freeze-re-thaw process was performed without insolubilizing Ni ions, the average filtration resistivity was 7.5 × 10⁻⁶. 9 The value remained at m / kg. In contrast, in Example 1 of the present invention, in which Ni ions were insolubilized, the average filtration resistivity was 5.2 × 10⁻⁶. 8 In m / kg, in Example 2, the average filtration resistivity was 1.2 × 10⁻⁶. 9 The resistance could be reduced to m / kg. Therefore, as in the present invention, a remarkable improvement in the average filtration resistivity was confirmed by making specific metal ions insoluble by ferriteization or other means.

[0060] (Verification Example 2) For Examples 1 and 2 and Comparative Examples 1 and 2 described above, the relationship between the amount of freezing aid added in the freeze-re-thaw process and the average filtration resistivity was investigated (Comparative Example 1 did not have any freezing aid added). The results are shown as a graph in Figure 3.

[0061] According to the results shown in Figure 3, in Comparative Example 2, the average filtration resistivity was 1 × 10⁻⁶. 9To obtain m / kg, it was necessary to add 6 w / v% or more of the freezing aid. On the other hand, in Examples 1 and 2, where ions of a specific metal were ferrite-formed, it was confirmed that similar filtration performance could be obtained even with an added amount of 2 w / v% of the freezing aid.

[0062] (Verification Example 3) For Examples 1 and 2 and Comparative Examples 1 and 2 described above, the relationship between the concentration of the freezing aid in the freeze-re-thaw process and the volume of the cake (solid content) after filtration was investigated (Comparative Example 1 did not have any freezing aid added). The results are shown as a graph in Figure 4.

[0063] According to the results shown in Figure 3, the average filtration resistivity is 1 × 10⁻⁶. 9 To obtain m / kg, a freezing aid of 6 w / v% or more is required (see Verification Example 2). The cake volume at this concentration is 100 cm³. 3 That concludes the report. On the other hand, in Example 1, where ferriteization was performed, the average filtration resistivity was 1 × 10⁻⁶. 9 Since adding 2 w / v% of the freezing agent is sufficient to obtain m / kg, the cake volume at this concentration is approximately 40 cm³. 3 It was confirmed that the cake volume can be significantly reduced by insolubilizing specific metal ions, such as by ferriteizing them.

[0064] (Verification Example 4) In Example 2 described above, the treatment solution was prepared by setting the temperature conditions for the immobilization process (ferrite formation) to 30°C, 40°C, 60°C, 75°C, and 90°C, respectively, and the average filtration resistivity was measured over 5 hours, taking into account a realistic ammonia stripping time. These results are shown as a graph in Figure 5.

[0065] According to the results shown in Figure 5, in the liquid temperature range of 40°C to 90°C, 2 × 10 9 m / kg ~ 3 × 10 9 The result was approximately m / kg, and it was confirmed that there was no significant variation due to the processing temperature in the immobilization process. On the other hand, at a liquid temperature of 30°C, the result was 9 × 10 9The result was m / kg, and it was found that the average filtration resistivity was higher than in the liquid temperature range of 40°C to 90°C. Therefore, it is more preferable to perform air bubbling in the immobilization process within the liquid temperature range of 40°C to 100°C (below the boiling point).

Claims

1. An insolubilization step is performed by oxidizing a solution of a poorly filterable material containing a radioactive material and ions of at least one specific metal from Ni, Pb, Cu, Zn, Co, Mn, and Cr, thereby oxidizing the ions of the specific metal and generating an insoluble compound. The process includes a freeze-re-thaw step in which a soluble metal salt is added to a suspension of a poorly filterable substance containing the aforementioned insoluble compound to produce a metal hydroxide solid, and the suspension containing the metal hydroxide solid is frozen and then thawed. The method for treating a difficult-to-filter substance is characterized in that the insolubilization step involves adding a divalent iron salt, neutralizing it with alkali hydroxide to produce an intermediate solution containing the hydroxide of the specified metal, and then performing an oxidation treatment on this intermediate solution by air bubbling to produce the ferrite-formed insoluble compound.

2. The method for treating a difficult-to-filter substance according to claim 1, characterized in that the air bubbling in the insolubilization step is performed at a liquid temperature of 40 to 100°C and a pH of 10 to 11.

5.

3. The method for treating a difficult-to-filter substance according to claim 1, characterized in that the air flow rate in the air bubbling in the insolubilization step is in the range of 0.5 to 5 NL / min per liter of the intermediate solution.

4. The method for treating a poorly filterable substance according to any one of claims 1 to 3, characterized in that the specified metal is Ni, and the concentration of Ni ions contained in the solution of the poorly filterable substance is in the range of 500 to 50,000 mg / L.

5. The aforementioned poorly filterable substance is iron oxide (α-Fe ２ O ３ ) and iron hydroxide (Fe(OH) ３ A method for treating a difficult-to-filter substance according to any one of claims 1 to 3, characterized by including ).

6. The method for treating a difficult-to-filter substance according to any one of claims 1 to 3, characterized in that the radioactive material is a component of a decomposition solution obtained by wet decomposition of spent ion exchange resin and filter sludge from a nuclear power plant, or a component of waste liquid obtained by eluting spent ion exchange resin.

7. A method for treating a difficult-to-filter substance according to any one of claims 1 to 3, further comprising a solid-liquid separation step for filtering the suspension after the freeze-re-thaw process, as a post-freeze-re-thaw step.

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