JP7256061B2 - Method for treating hard-to-filter substances - Google Patents

Description

The present invention relates to a treatment method for improving the filterability of difficult-to-filter substances, and in particular, difficult-to-filter substances such as iron oxide in the oxidative decomposition solution of used ion-exchange resins and filter sludge generated in the process performed at nuclear power plants. The present invention relates to a treatment method for improving filterability of a filterable substance.

As a treatment method for filter sludge and used ion exchange resin used for wastewater containing radionuclides and solid content recovery treatment at nuclear power plants, a method of wet oxidative decomposition by the action of hydrogen peroxide has been proposed. This wet decomposition method is a method of oxidatively decomposing organic waste such as used ion exchange resin with hydrogen peroxide using iron or the like as a catalyst under a temperature condition of about 100 ° C. and a pressure condition of atmospheric pressure. OH radicals generated from hydrogen peroxide cut the chain of bonds in the organic waste, which is finally decomposed into carbon dioxide and water. For example, sulfuric acid is produced when the cation exchange resin is decomposed, and ammonia is produced when the anion exchange resin is decomposed.

Low-level radioactive waste generated at nuclear power plants, not limited to waste generated during decommissioning, is categorized by law as follows and disposed of according to each standard.
Level I [L1]: Low-level radioactive waste with a relatively high concentration of radioactive materials. It is buried in an artificial structure at a depth of 50m to 100m (burial at a marginal depth (managed for 300 years)).
Level II [L2]: Low-level radioactive waste with a relatively low concentration of radioactive material. Buried in a man-made structure about 10m underground (buried in a concrete pit (managed for 300 years))
Level III [L3]: Low-level radioactive waste with an extremely low concentration of radioactive materials, buried directly underground (uncut trench burial (managed for 30 to 50 years)).

Substances remaining after the wet decomposition are crud derived from radioactive corrosion products accumulated in the reactor primary cooling system (mainly iron oxide: α-Fe ₂ O ₃ ) and Fe catalyst (Fe(OH) ₃ ). It is a decomposition liquid consisting of solid iron impurities containing such as and a slurry containing sodium sulfate as a main component. Since solid iron impurities have a high radiation dose, it is necessary to bury them as L1 waste in the above category. If the two can be separated, the amount of highly radioactive radioactive waste can be reduced.

In addition, since radioactive substances contained in solid iron impurities may generate hydrogen gas due to radiolysis when water coexists, it is desirable to reduce the water content in the solid iron impurities to be finally disposed. In addition, sodium sulfate accompanying the solid content causes leaks, corrosion, etc., and increases the weight of the waste, so it is desired to reduce it.

Therefore, if only the solid iron impurities are efficiently filtered and dried from the decomposed slurry, the disposal amount of L1 waste can be reduced and the effects of hydrogen generation and liquid leakage can be reduced. The derived solid iron impurities such as Fe(OH) ₃ have the problem that they themselves are difficult to filter and are difficult to separate by filtration.

In general, the freeze-thaw method is also known as a method for improving the filtration performance of difficult-to-filter substances. there were. In addition, when the salt concentration is high, especially when NaCl coexists at a high concentration (several percent or more), there is a problem that it does not freeze even when cooled to about -15°C due to the depression of the freezing point. When NaCl coexists, it is necessary to lower the temperature to -22°C or lower, which places a heavy burden on refrigerators and the like.

In addition, as a method of improving the dehydration performance of difficult-to-filter substances, a method of lowering the pH using an ion-exchange resin before freezing and thawing (Patent Document 1: JP-A-55-157393); A method (Patent Document 2: JP-A-55-157394) is also known. Since an additional ion-exchange resin is used, slurry containing radioactive substances is not efficient because it requires a process of removing radioactive impurities by new adsorption. In addition, the above patent document describes that the addition of ferric chloride as a flocculating agent is not preferable.

JP-A-55-157393 JP-A-55-157394

Therefore, a method for improving the filtration performance of suspensions of difficult-to-filter substances has been desired. Also, since water may react with radiation to generate hydrogen, it is desirable that the water content be as small as possible.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a suspension containing hard-to-filter substances such as solid iron impurities can be frozen and re-frozen by appropriately adding a soluble metal salt to the suspension. We have found that the filtration performance can be improved by melting, and have completed the process.

The configuration of the present invention is as follows.
[1] A soluble metal salt is added to a suspension of a difficult-to-filter substance containing a radioactive substance, and then the pH is adjusted to 6 or more to produce a metal hydroxide solid, and the suspension containing the metal hydroxide solid is obtained. A method for treating difficult-to-filter substances by freezing and re-thawing the liquid.
[2] The method for treating hard-to-filter substances of [1], wherein the soluble metal salt is at least one of iron salt, nickel salt, cobalt salt and manganese salt.
[3] The processing method of [2], wherein the soluble metal salt is iron sulfate.
[4] The treatment method of [1] to [3], wherein the hard-to-filter substance contains iron oxide (α-Fe ₂ O ₃ ) and iron hydroxide (Fe(OH) ₃ ) as main components.
[5] The weight of the soluble iron salt converted to Fe(OH) ₃ , where the content (weight) of the iron element in the difficult-to-filter substance is converted to α- _Fe2O3 is ₁ . The treatment method of [4], wherein the addition is made in an amount that is 0.1 times or more of the ratio.
[6] Decomposition of solid iron impurities and sodium sulfate as main components, wherein the suspension or solution of the difficult-to-filter substance is obtained by wet decomposition of spent ion-exchange resins and filter sludge from nuclear power plants. The processing method of [1], which is a liquid.
[7] The treatment method of [1] to [6], wherein the suspension after freezing and rethawing is filtered.

According to the present invention, it is possible to improve the filterability of the wet decomposition slurry containing iron impurities, which are substances that are difficult to filter, thereby facilitating the removal of sodium sulfate and water. The washing efficiency of the separated solid substances is also high, and the flow and ventilation of the filter cake are facilitated, so that the washing efficiency is improved and drying is facilitated.

Therefore, it is possible to reduce the generation of hydrogen gas due to decomposition of water due to high radiolysis in burial disposal.

Since the high-dose resin has a relatively high radioactivity concentration, the wet decomposition slurry is planned to be disposed of as L1 waste. Salt carry-over can be reduced. In addition, since sodium sulfate solution with low radiation dose can be separated from clad with high radiation dose, sodium sulfate solution can be treated as the current L2 waste, making it possible to reduce L1 waste. Become.

According to the present invention, by combining with the wet decomposition process, the high-dose resin treatment process can be made compact and the generation of hydrogen gas due to radioactivity can be suppressed, so it is expected to become an effective technology for L1 waste disposal. Be expected.

It is also possible to dry the solids collected from the high-dose resin treatment decomposition solution and dispose of them as solids. can be used.

Furthermore, according to the present invention, it is possible to reduce mainly salts such as sodium sulfate from L1 waste. And there is a possibility of application to nuclear power related sites.

2 shows the results of filtration evaluation in Example 1. FIG. The results of filtration evaluation in Example 2 are shown. 4 shows the results of filtration evaluation in Example 3. FIG. The results of filtration evaluation in Comparative Example 1 are shown.

Embodiments of the present invention will be described in detail below.
Recalcitrant materials containing radioactive materials to be treated by the method of the present invention contain mainly solid iron impurities. Solid iron impurities contain iron oxide (α-Fe ₂ O ₃ ) and iron hydroxide (Fe(OH) ₃ ) as major components. For example, such solid iron impurities include crud (mainly composed of iron oxide: α-Fe ₂ O ₃ ) derived from corrosion products that accumulate in the reactor primary cooling system, and iron catalysts used in wet cracking. derived from The main component of the clad depends on the constituent materials of the cooling system, water quality conditions, and operating conditions, but the main constituent material is derived from stainless steel. As a difficult-to-filter substance, or when chromium, nickel and their oxides are contained, the same treatment can be carried out.

The hard-to-filter suspension consists mainly of solid iron impurities obtained by wet decomposition of used ion-exchange resins from nuclear power plants and filter sludge, which is collected by filtration, and sodium sulfate. A liquid is preferably used. In the following, wet decomposition will be described as an example.

In the wet decomposition method for treating radioactive organic waste, for example, as described in JP-A-2000-56986 and JP-B-61-9599, radioactive organic waste is peroxidized in the presence of iron ions in an aqueous medium. It is oxidatively decomposed by the action of hydrogen. Radioactive organic wastes include used granular or powdered ion-exchange resins specifically generated at nuclear power plants, as well as filter sludge. 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 can be applied to treatment of decontamination waste liquids containing citric acid, oxalic acid, and EDTA.

Both ferrous ions Fe ²⁺ and ferric ions Fe ³⁺ can be used as iron ions, and iron sulfate, iron nitrate, iron chloride and the like are used as iron sources. Wet oxidation with hydrogen peroxide proceeds well when the aqueous medium is acidic, so an acid such as sulfuric acid is added at the start of the reaction. In addition, since ammonia and amines in the waste are retained in the liquid during the reaction and do not volatilize, they are released after the reaction is completed by adding an alkali such as caustic soda to neutralize the liquid. Through the above-described treatment, a decomposition solution containing crud (iron oxide) and sodium sulfate as main components is produced. In the present invention, such a decomposition liquid is used as a processing liquid.

The concentrations of solid iron impurities and sodium sulfate contained in the treatment liquid used in the present invention are not particularly limited, but usually the amount of solid iron impurities is 1 to 100 g/liter and the amount of sodium sulfate is 2 to 200 g/liter. be

Freezing and re-thawing In the present invention, after adding a soluble metal salt to the treatment liquid, the pH is adjusted to 6 or more to generate a metal hydroxide solid, and the suspension containing the metal hydroxide solid is frozen and re-frozen. Melt.
As the soluble metal salt, the pH is not particularly limited as long as it produces a hydroxide solid, but in the present invention, it is preferably at least one of iron salt, nickel salt, cobalt salt, and manganese salt. Iron salts are more preferred.

The soluble iron salt has a weight ratio of 0.0 in terms of Fe(OH) ₃ when the content (weight) of the iron element in the difficult-to-filter substance in terms of α-Fe ₂ O ₃ is 1. It is added in an amount of 1 time or more, preferably 0.1 to 2 times, more preferably 0.15 to 0.4 times. Similarly, when the hard-to-filter substance is an element other than iron, it is added so that the weight ratio falls within the above range.
When a soluble metal salt other than iron is used, it is added so that the weight ratio in terms of metal hydroxide falls within the above range.

The soluble iron salt is not particularly limited, but it is necessary to freeze the treatment solution, so a salt having a solubility that does not significantly lower the freezing point is preferable, and has a solubility of 50 to 150 g / liter in water at 20 ° C. Salt is preferred. Specifically, the soluble iron salt is preferably iron (II) or (III) sulfate. When iron sulfate is used, sodium sulfate is generated when the pH is adjusted to prepare a solid, and the sodium sulfate salt becomes less soluble when the temperature is lowered. The salt concentration does not become high, and freezing point depression does not easily occur. This facilitates freezing even at a relatively high temperature of about -15°C. When iron chloride or the like is added as a soluble iron salt, the solid content does not precipitate, and due to the depression of the freezing point due to the salt such as sodium chloride generated, it does not freeze when cooled to about -15 ° C. Japanese Patent Application Laid-Open No. 55-157393 and Japanese Patent Application Laid-Open No. 55-157394 disclose that the addition of salts such as iron chloride is not preferable for filtration of difficult-to-filter substances. Sulfates such as manganese sulfate, cobalt sulfate, and nickel sulfate can also be used.

After adding the soluble metal salt in the present invention, the pH of the treatment liquid is adjusted to 6 or more. The pH adjustment is performed by a known method, for example, by adding an alkali such as caustic soda to adjust the pH of the liquid to 6 or higher. The pH is not particularly limited as long as it can produce a metal hydroxide solid, but is preferably adjusted within the range of 8-9. The clad itself is difficult to filter, but when the metal hydroxide solid is produced, the clad is incorporated into the solid, making it easier to filter by freezing and thawing. Even if the metal hydroxide solid is added in advance and the freeze-thaw treatment is performed, the effect of the present invention is not exhibited.

Before the freeze-thaw treatment, it is possible to reduce the volume of the treatment liquid by evaporating water in order to reduce the volume. good.
The treatment liquid containing metal hydroxide solids has a solids concentration of 40 to 120 g/l and a salt concentration of 80 to 240 g/l. Those within this range can increase the filtration efficiency by freezing and rethawing.

In the present invention, the treatment liquid is frozen, the frozen material is thawed, and the thawed liquid is supplied to a solid-liquid separator to separate the residue and the filtrate. Hard-to-filter substances including radioactive substances are recovered as residue together with metal hydroxide solids.

By freezing, the water grows as ice crystals, and the solid content moves between the ice cubes, is compressed, and is condensed while being combined and coalesced into large blocks to improve filtration characteristics. Furthermore, when the whole is frozen, the expansion force of ice formation applies a strong compressive force to the block-shaped solid matter, so even if it forms a gel-like structure, it is destroyed and included inside. Water will flow out and the dehydration effect will be higher. Therefore, the filterability can be improved even for solid iron impurities, which are difficult to filter.

Next, when this frozen material is heated and thawed, since the shape of the block-shaped coarse particles is maintained even in the thawed state, solid matter with high density coarse particles settles out, and moisture (including soluble salts) is separated as supernatant water.
Freezing is not particularly limited as long as it is a temperature at which water in the treatment liquid freezes.

Thawing can be done by removing the frozen state and warming until liquefied, but even if it is natural thawing, heating means such as a heater or a hot water bath may be used.

The solid-liquid separator can be exemplified by known means such as centrifugal separation in addition to vacuum filtration, press filtration and the like.
Although the filtrate contains soluble salts such as sodium sulfate, since the hard-to-filter substances including radioactive substances are separated as solids, it can be disposed of as wastewater with low radiation dose. Moreover, it is also possible to recover soluble salts in the washed aqueous phase by further washing the residue with water during the solid-liquid separation.

If necessary, the solid-liquid separated residue may be dried to remove moisture. Residue with improved filterability is easy to remove moisture, so it is easy to wash and dry.
Since the sodium sulfate solution recovered in the wastewater can be used as L2 waste, it is possible to reduce L1 waste.

As an outline of a device , for example, a processing facility, a processing liquid storage tank for temporarily storing the processing liquid of the object to be processed, a concentration tank connected to this storage tank by a pipe to concentrate the processing liquid, and most of the concentrated processing liquid are supplied. It is composed of a freeze-thaw device for freezing and thawing and solid-liquid separation, and a solid-liquid separation device for solid-liquid separation of the treatment liquid after thawing. A storage tank or a concentrated layer may be provided with a stirring means.

As the refrigerator, any known refrigerator can be used without particular limitation, and for example, an ammonia absorption refrigerator can be used. This is because the use of ammonia as a refrigerant is convenient because it functions as a refrigerant that is inexpensive and has excellent thermodynamic properties without using chlorofluorocarbons, which are harmful to the environment. If waste heat energy from sludge incineration, cogeneration, or the like is supplied as a drive source for operating the ammonia absorption refrigerator, the running cost can be further reduced, which is convenient.

The thawed treatment liquid is solid-liquid separated by a dehydrator such as a vacuum dehydrator, the residue is washed and dried, and the filtrate is concentrated as necessary and then treated according to the classification of radioactive waste. .

EXAMPLES The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples.

Example 1
As a simulated treatment of filter sludge from a nuclear power plant, Powdex resin was wet-decomposed in a reactor with a decomposition reaction volume of 2 L to prepare a treatment solution with a crud concentration of 4% by mass (in terms of α-Fe ₂ O ₃ ). bottom. The pH of the treatment liquid was 12. Fe ₂ (SO ₄ ) ₃ ·nH ₂ O (n≈7) was added as a soluble iron salt to 200 ml of the resulting treatment liquid in terms of iron hydroxide (Fe(OH) ₃ ), and a predetermined pair of α-Fe The pH decreased to 3 or less when _{the 2} O ₃ weight ratio was 0.85 times. A NaOH solution was added to this to change the pH, and Fe(OH) ₃ was precipitated at each pH. Then, after freezing to -15°C in the freezer of a household refrigerator, returning to room temperature and thawing, filtering using a suction filter with a filtration area: 3.2 cm ² (filtration diameter: 41 mmφ). Velocity was measured.
The results are shown in Figure 1. FIG. 1(a) shows changes in filtration rate depending on pH, and FIG. 1(b) shows photographs of observed states of the filtrate.

At a pH of 7 to 10, the filterability was good, so the filtration rate was high, and the filtrate was colorless and transparent without turbidity. On the other hand, when the pH was 2.5, iron hydroxide that could not be filtered was mixed in the filtrate, resulting in reddish turbidity. In addition, under treatment conditions with a high pH, the filtrate was colored, which is considered to be due to the dissolution of iron hydroxide.

Example 2
Iron sulfate was added to the treatment liquid prepared in the same manner as in Example 1 so that Fe(OH) ₃ was 0.1 to 2 times as much as α-Fe ₂ O ₃ . A NaOH solution was added to the decomposed solution, which had become weakly acidic due to the addition of iron sulfate, so as to adjust the pH to 9. In Example 2, it was then frozen at -15°C, thawed, and the filtration rate was measured. The results are shown in FIG. Fig. 2(a) shows the change in filtration rate depending on the addition ratio of iron sulfate, and Fig. 2(b) shows a photograph of the condition of the filtrate. , and the addition ratio of 0.2 to 1.0 greatly improved the filtration rate. The filtrate became transparent when the addition ratio was 0.1 or more.

Example 3
Iron sulfate was added to the treatment liquid (pH=12) prepared in the same manner as in Example 1. The amount _{of iron sulfate added was such that Fe(OH)3 was} 0.25 times the amount of α- _Fe2O3 . A NaOH solution was added to this to change the pH, and Fe(OH) ₃ was precipitated at each pH. Thereafter, freeze-thaw treatment was performed in the same manner, and the filtration rate was measured.
Results are shown in FIG. FIG. 3(a) shows a change in filtration rate depending on pH, and FIG. 3(b) shows a photograph of the state of the filtrate.
As a result, an improvement in the filtration rate was observed at a pH of 6 to 12. In addition, the filtration rate was improved at an addition ratio of 0.2 to 1.0. The filtrate had a pH of 6 to 9 and was clear and colorless. At a pH of 2.5, iron hydroxide that could not be filtered was mixed into the filtrate, making it red and turbid.

Comparative example 1
As an iron salt to be added, iron chloride was added instead of iron sulfate, and the amount added was 0.25, 0.5, 0.75 and 1.0 times, and NaOH solution was added to make it alkaline. , freeze-thaw treatment was performed and the filtration rate was measured. The results are shown in FIG.
At the freezing temperature of -15°C, it did not freeze completely, and the filtration rate did not improve regardless of the presence or absence of freezing.

Claims (7)

After adding a soluble metal salt to a suspension of a hard-to-filter substance containing a radioactive substance, the pH is adjusted to 6 or more to produce a metal hydroxide solid, and the suspension containing the metal hydroxide solid is frozen. A method for treating a difficult-to-filter substance, characterized by performing re-melting treatment. 2. The method for treating hard-to-filter substances according to claim 1, wherein the soluble metal salt is at least one of iron salt, nickel salt, cobalt salt and manganese salt. 3. The treatment method according to claim 2, wherein said soluble metal salt is iron sulfate. 4. The method according to any one of claims 1 to 3, wherein the hard-to-filter substance contains iron oxide (α-Fe ₂ O ₃ ) and iron hydroxide (Fe(OH) ₃ ) as main components. How to handle. The weight ratio of the soluble iron salt converted to Fe(OH _{) 3} when the content (weight) when the iron element of the difficult-to-filter substance is converted to α-Fe ₂ O 3 is ₁ , 5. The treatment method according to claim 4, wherein the addition amount is 0.1 times or more. The suspension or solution of the hard-to-filter substance is a decomposed solution containing solid iron impurities and sodium sulfate as main components, obtained by wet decomposition of spent ion exchange resins and filter sludge from nuclear power plants. The processing method according to claim 1, characterized by: The processing method according to any one of claims 1 to 6, characterized in that the suspension after freezing and rethawing is filtered.

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