Processing method

Abstract

The present invention provides a treatment method that can reduce the concentration of radioactive contaminants such as Cs in the ash to be treated. [Solution] The processing method comprises a first water washing step S10 in which the ash to be treated containing the recovered radioactive material is washed with water, an acid washing step S20 in which the ash to be treated washed in the first water washing step S10 is washed with acid, and a second water washing step S30 in which the ash to be treated washed in the acid washing step S20 is washed with water.

Description

Technical Field

[0001] The present invention relates to a treatment method for treating ash to be treated containing radioactive substances.

Background Art

[0002] Conventionally, attempts have been made to appropriately treat contaminants containing radioactive cesium generated by the accident at the Fukushima Daiichi Nuclear Power Plant. For example, in Patent Document 1, a thermal volume reduction method is shown in which a mixture of radioactive cesium contaminants mixed with lime or calcium chloride is heated in a melting furnace or a rotary kiln to volatilize radioactive cesium and recover fly ash (cesium concentrate). Then, it is disclosed that the recovered radioactive cesium concentrate is dissolved by washing with water, or that the radioactive cesium contaminants are placed under hot water (subcritical water) conditions and the radioactive cesium is replaced with a polyvalent cation (chemical treatment) to transfer it to an aqueous solution.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Melting fly ash, for example, the melting fly ash generated in the melting furnace of the waste treatment business No. 1 in the Futaba-machi volume reduction facility (intermediate storage facility), has a radioactive Cs concentration of 200,000 to 600,000 Bq / kg. The melting fly ash is composed of a water-soluble salt and an insoluble component, and most of the radioactive Cs is contained in the water-soluble salt. Similar to the "washing with water" shown in Patent Document 1, by washing the fly ash with pure water and dehydrating it using a filter press, most of the radioactive Cs transfers to the dehydration filtrate. However, since a part of the radioactive Cs is contained in the insoluble component of the fly ash, radioactive Cs remains in the insoluble component of the dehydration cake (water content of about 40%). The Cs concentration of the dehydration cake is about 10,000 to 30,000 Bq / kg.

[0005] Dehydrated cake exceeding the 8000 Bq / kg standard set by the Act on Special Measures for Dealing with Radioactive Material Contamination (hereinafter referred to as the "Special Measures Act") cannot be disposed of as industrial waste as is, according to the Special Measures Act. It would need to be disposed of outside the designated area, such as outside Fukushima Prefecture, which raises concerns about increased volume and costs.

[0006] The present invention provides a treatment method that can reduce the concentration of radioactive contaminants such as Cs in the ash to be treated. [Means for solving the problem]

[0007] [Concept 1] The processing method according to the present invention is The first water washing process involves washing the treated ash containing the recovered radioactive material with water, The first water washing step involves washing the ash to be treated with acid, and It is equipped with.

[0008] [Concept 2] The processing method according to Concept 1 is: The process may also include a second water washing step in which the ash to be treated, which has been washed in the acid washing step, is washed with water.

[0009] [Concept 3] In a processing method based on concept 1 or 2, The acid may be hydrochloric acid.

[0010] [Concept 4] In a processing method based on any one of concepts 1 to 3, The molar equivalent ratio of acid-soluble components such as Ca and Fe contained in the treated ash washed in the first water washing step to the acid may be 0.8 to 1.2.

[0011] [Concept 5] In a processing method based on any one of concepts 1 to 4, The liquid-solid ratio of the ash to be treated washed in the first water washing step and the acid may be 4 to 15 [kg / kg].

[0012] [Concept 6] In the treatment method according to any one of Concepts 1 to 5, The concentration of the acid may be 1 to 8% by weight.

[0013] [Concept 7] The treatment method according to any one of Concepts 1 to 6 includes a first dehydration step of dehydrating the ash to be treated that has undergone the first water washing step, and a second dehydration step of dehydrating the ash to be treated that has undergone the acid washing step, and The ash to be treated that has undergone the first dehydration step may be washed with acid in the acid washing step.

[0014] [Concept 8] In the treatment method according to any one of Concepts 1 to 7, The radioactive substance may contain radioactive cesium. [Advantages of the Invention]

[0015] According to the present invention, the concentration of radioactive contaminants such as Cs in the ash to be treated can be reduced. [Brief Description of the Drawings]

[0016] [Figure 1] A flow chart showing an example of the treatment method according to an embodiment of the present invention. [Figure 2] A schematic diagram showing an example of the treatment apparatus according to an embodiment of the present invention. [Figure 3] A schematic diagram showing another example of the treatment apparatus according to an embodiment of the present invention. [Figure 4] A diagram for explaining an example of the present invention. [Figure 5] A diagram showing the next step of FIG. 4, and a diagram for explaining an example of the present invention. [Figure 6]This figure shows the next step after Figure 5 and is a diagram illustrating an embodiment of the present invention. [Figure 7] A flowchart showing the adsorption and stabilization process and the wastewater treatment process, which are processing steps after the washing and dewatering process in an embodiment of the present invention. [Figure 8] Graphs showing the relationship between the molar equivalent ratio of hydrochloric acid and the Cs removal rate, and graphs showing the relationship between the molar equivalent ratio of hydrochloric acid and the dissolution ratio. [Figure 9] Graphs showing the relationship between the liquid-to-solid ratio and the Cs removal rate, and graphs showing the relationship between the liquid-to-solid ratio and the dissolution ratio. [Figure 10] A diagram illustrating the process of water washing and acid washing according to embodiments of the present invention. [Modes for carrying out the invention]

[0017] Embodiment The processing method of this embodiment relates to the washing and dewatering process S100 (see Figure 7). The ash to be processed is typically molten fly ash generated in a melting furnace.

[0018] As shown in Figure 1, the processing method of this embodiment includes a first water washing step S10 in which the ash to be treated containing the recovered radioactive material is washed with water, an acid washing step S20 in which the ash to be treated washed in the first water washing step S10 is washed with acid, and a second water washing step S30 in which the ash to be treated washed in the acid washing step S20 is washed with water. The radioactive material may contain radioactive cesium (Cs). The inventors have confirmed that the first water washing step S10 dissolves water-soluble salts, but by performing the acid washing step S20 thereafter, the radioactivity concentration in the ash to be treated can be reduced to an extremely low level (see Figure 10). As a result, the radioactivity concentration of the dewatered cake obtained from the ash to be treated can be reduced to 8000 Bq / kg or less. In addition, by performing the acid washing step S20, impurities contained in the ash to be treated can be dissolved, and the total amount of ash to be treated can be reduced. Reducing the total amount of ash to be treated in this way is beneficial in that it can lower the processing cost of the dewatered cake. Furthermore, since the acid washing solution contains radioactive materials and it is necessary to wash the acid washing solution off the ash to be treated, it is preferable to perform the second water washing step S30 after the acid washing step S20.

[0019] The ash to be treated may be received from the outside into the plant's storage section 300 (see Figure 2) (see S1 in Figure 1). After taking an appropriate amount of the received ash from the storage section 300 (see S2 in Figure 1), the above treatment may be carried out.

[0020] The processing method and the plant implementing this method may be used to appropriately process ash to be processed, which originates from contaminated materials generated by the Fukushima Daiichi Nuclear Power Plant accident.

[0021] A first dewatering step S12 may be performed after the first water washing step S10. A second dewatering step 22 may be performed after the acid washing step S20. A third dewatering step 32 may be performed after the second water washing step S30. Performing the first dewatering step S12 prevents the acid solution in the subsequent acid washing step S20 from being diluted, allowing the ash to be treated to be acid washed at an appropriate concentration. Performing such dewatering steps results in the generation of a dewatered cake. A neutralization step to neutralize the acid may be performed between the acid washing step S20 and the second dewatering step 22.

[0022] As shown in Figure 2, the apparatus of this embodiment may include a processing tank 230, a water supply unit 210 for supplying water, and an acid supply unit 220 for supplying an acidic aqueous solution. When performing acid washing treatment in the processing tank 230, a dilute acidic aqueous solution may be supplied directly from the acid supply unit 220 to the processing tank 230, or an appropriate amount of water supplied from the water supply unit 210 and a concentrated acidic aqueous solution supplied from the acid supply unit 220 may be mixed and supplied to the processing tank 230. A stirring unit 240 for stirring the stored liquid may be provided inside the processing tank 230. The ash to be treated may be transported from the storage unit 300 and placed in the processing tank 230 before water is supplied, or the ash to be treated may be mixed into the water after a certain amount of water has been placed in the processing tank 230.

[0023] It is preferable that hydrochloric acid be used in the acid washing process S20. The inventors have confirmed that, compared to other acids such as sulfuric acid and citric acid, hydrochloric acid is particularly advantageous because it can be implemented at a low cost and does not cause other adverse effects. For example, when sulfuric acid was used as the acid, gypsum was formed. Also, when citric acid was used as the acid, the COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand) in the aqueous solution increased.

[0024] The concentration of the acid solution may be 1 to 8% by weight, with a lower limit of preferably 1% by weight and more preferably 2% by weight. The upper limit of the acid solution concentration is preferably 5% by weight and more preferably 4% by weight. The inventors have confirmed that if the concentration of the acid solution is low, the effect of separating radioactive materials from the ash to be treated cannot be expected, and it is preferable that the concentration be 1% by weight or more, and more preferably 2% by weight or more. Even if the concentration of the acid solution exceeds 4% by weight, the efficiency of separating radioactive materials from the ash to be treated does not change significantly, and in the case of exceeding 5% by weight, the difference in effect did not change much. On the other hand, if the concentration of the acid solution is high, adverse effects such as increased corrosion of tanks, pipes, pumps, etc. that constitute the system may occur in the plant. For this reason, it is preferable that the concentration of the acid solution is 8% by weight or less.

[0025] The molar equivalent ratio of acid-soluble components such as Ca and Fe contained in the treated ash washed in the first water washing step to the acid is preferably 0.8 to 1.2. By using such a ratio, acid-soluble components such as Ca and Fe can be dissolved more reliably.

[0026] Insoluble particles are mainly composed of compounds such as Ca, and by dissolving them with acid, it is possible to elute Cs from the particles. However, there is an optimal range for the amount of acid to be added. Here, the equivalent amount of acid is defined as the amount of acid that exactly dissolves the dissolved components in the particles. If the ratio of this acid equivalent (hereinafter referred to as the acid equivalent ratio) is less than 1.0, the insoluble particles will not dissolve sufficiently, increasing the amount of residual Cs and decreasing the Cs removal rate. On the other hand, even if the acid equivalent ratio is greater than 1.0, the dissolution of insoluble particles remains almost unchanged and the Cs removal rate plateaus (see Figure 8). However, as the amount of acid added increases, the amount of residual acid also increases, leading to increased corrosion of the equipment and making it uneconomical. Considering the variability in the properties of the target material, the appropriate acid equivalent ratio for practical operation is preferably between 0.8 and 1.2.

[0027] The liquid-to-solid ratio of the ash to be treated to the acid is preferably 4 to 15 kg / kg. This ratio is beneficial because it facilitates the treatment of the ash with acid.

[0028] There is an optimal range for the liquid-to-solid ratio relative to insoluble particles. If the liquid-to-solid ratio is too small, the concentration of insoluble particles increases, resulting in a highly viscous slurry. As a result, contact between the insoluble particles and the acid becomes insufficient, leading to increased dissolution ratio and Cs elution rate. Furthermore, the dissolved solution contains eluted Cs, and if the liquid-to-solid ratio is small, the Cs concentration increases, resulting in high-concentration adhesion to the surface of the remaining particles. Therefore, if this is not sufficiently removed, the Cs removal rate will be low. On the other hand, if the liquid-to-solid ratio is too large, the dissolution ratio and Cs removal rate plateau (see Figure 9). However, the equipment becomes larger and the amount of wastewater increases, making it uneconomical. Considering these factors, and taking into account variations in the properties of the target material, an appropriate liquid-to-solid ratio for practical operation is preferably between 4 and 15.

[0029] The ash to be treated after the first dewatering step S12 may be washed with an acidic aqueous solution in the acid washing step S20. The ash to be treated after the second dewatering step S22 may be washed with water in the second water washing step S30.

[0030] As shown in Figure 2, the processing apparatus may have a dewatering section 250. The dewatering section 250 is provided with a filter 255, and in the dewatering process, dewatering may be performed by filtering through the filter 255. Typically, a filter press is performed in the dewatering section 250. The water or acidic solution that has passed through the filter 255 is recovered in the recovery section 260 and sent to a subsequent process for processing the water or acidic solution. Also, by removing the water or acidic solution through the filter 255, a dewatered cake is generated.

[0031] Post-processing steps for water or acidic aqueous solution include the adsorption and stabilization step S200 and the wastewater treatment step S300 (see Figure 7). In the adsorption and stabilization step S200, radioactive materials in the water or acidic aqueous solution that have undergone dewatering in the dewatering section 250 are adsorbed and stabilized, such as by suppressing elution, to produce a stabilized body. The stabilized body is, for example, solidified with cement to suppress the elution of radioactive materials. The radioactivity concentration of this stabilized body is high. Because radioactive materials are concentrated in the stabilized body, the radioactivity concentration in the adsorption-treated water after the adsorption and stabilization step S200 is reduced, and it is then appropriately treated in the wastewater treatment step S300.

[0032] The treatment tanks, etc., may be separated for each treatment process in the first water washing process S10, the acid washing process S20, and the second water washing process S30. For example, as shown in Figure 3, a first treatment tank 230a, a first stirring unit 240a, a first dewatering unit 250a, a first filter 255a, and a first recovery unit 260a may be provided to carry out the process related to the first water washing process S10. Similarly, a second treatment tank 230b, a second stirring unit 240b, a second dewatering unit 250b, a second filter 255b, and a second recovery unit 260b may be provided to carry out the process related to the acid washing process S20. A third treatment tank 230c, a third stirring unit 240c, a third dewatering unit 250c, a third filter 255c, and a third recovery unit 260c may be provided to carry out the process related to the second water washing process S30.

[0033] In the first water washing step S10, it is desirable that the washing treatment be carried out with water in an amount (by weight) of 5 to 15 times the amount of ash to be treated. In the acid washing step S20, the washing treatment may be carried out with an acid aqueous solution that is 5 to 20 times (by weight) the amount of the ash to be treated (dewatered cake) after dewatering in the first dewatering step S12, or more specifically, 7 to 15 times (by weight). In the second water purification process, the ash to be treated (dewatered cake) after dewatering in the second dewatering process 22 may be washed with 5 to 15 times its weight in water.

[0034] (Examples) The radioactive Cs concentration of the rewashed fly ash used in the example was 3360 (=16800 × 0.2) Bq. Acid washing was performed using 2000 g of hydrochloric acid (liquid-to-solid ratio 10) (reference numeral 110) on 200 g of rewashed fly ash (reference numeral 100 in Figure 4) (acid washing step S20). In the acid washing, the rewashed fly ash was mixed in an aqueous hydrochloric acid solution. The rewashed fly ash used in this example is ash obtained by replacing the pore water containing radioactive Cs in the dehydrated cake produced through the first water washing step S10 and the first dehydration step S12 with sufficient fresh water, and then drying it.

[0035] In Figure 5, reference numeral 120 indicates the state of ash insoluble matter 125 suspended in pore water 128 after being recovered by the filter (after the second dehydration step S22), with a weight of approximately 516g. On the other hand, reference numeral 130 indicates the dehydrated filtrate after passing through filter 255, with a weight of approximately 1683g. Reference numeral 121 in Figure 5 shows the separation of pore water 128 and ash insoluble matter 125 due to the precipitation of the ash insoluble matter. It was confirmed that the higher the hydrochloric acid concentration, the higher the radioactive Cs concentration in the dehydrated filtrate and the higher the density of the dehydrated filtrate. When acid washing was performed with hydrochloric acid at a concentration of 7.2 wt%, the total amount of radioactive Cs in the dehydrated filtrate 130 + pore water 128 was 3348 (=1580 × 2.119) Bq, confirming that 99.6 wt% of the total amount of radioactive Cs was eluted.

[0036] Figure 6 shows the process after adding water (reference numeral 140 in Figure 6) to the ash insoluble matter 125 and performing the second water washing step S30 (see reference numeral 145 in Figure 6), followed by the third dewatering step S32. The "ash insoluble matter + interstitial water" in the filter was washed thoroughly with water, dried, and its weight and radioactive Cs concentration were measured. The ash insoluble matter 125 weighed approximately 67 g, confirming that its weight had decreased to 34% of its weight before washing. The radioactive Cs concentration of the ash insoluble matter 125 was 500 Bq / kg.

[0037] The effects obtained by the embodiment of this product can be summarized as follows: In this embodiment, as an example, after washing the fly ash with water, the dewatered cake is washed again using an acid such as hydrochloric acid. Washing with an acid such as hydrochloric acid dissolves the insoluble ash containing radioactive Cs, and more than 90% by weight of the radioactive Cs dissolves in the dewatered filtrate. Furthermore, by re-dehydrating the slurry after washing with an acid such as hydrochloric acid, and then adding a rinse wash with water, the radioactivity concentration of the dehydrated cake can be reduced to 8000 Bq / kg or less.

[0038] The above-described embodiments and the disclosure of the drawings are merely examples for explaining the invention described in the claims, and the invention described in the claims is not limited by the above-described embodiments or the disclosure of the drawings. Furthermore, the description of the claims at the time of filing is merely an example, and the description of the claims can be appropriately modified based on the description in the specification, drawings, etc. [Explanation of Symbols]

[0039] S10 First water washing process S12 First dehydration process S20 Acid cleaning process S22 Second dehydration process S30 Second water washing process S32 Third dehydration process

Claims

1. The first water washing process involves washing the treated ash containing the recovered radioactive material with water, The first water washing step involves washing the ash to be treated with acid, and A second water washing step in which the ash to be treated, which has been washed in the acid washing step, is washed with water, A method for treating ash, comprising the following components.

2. A first water washing step of washing the treated ash containing the recovered radioactive material with water, A first dewatering step is performed on the ash to be treated after the first water washing step, The first dewatering step involves an acid washing step in which the ash to be treated, which has undergone dewatering treatment in the first dewatering step, is washed with acid. A second dewatering step involves dewatering the ash to be treated after the acid washing step, Equipped with, A treatment method comprising washing the ash to be treated, which has undergone the first dewatering step, with acid in the acid washing step.

3. The first water washing process involves washing the treated ash containing the recovered radioactive material with water, The first water washing step involves washing the ash to be treated with acid, and A neutralization step for neutralizing the aforementioned acid, A dewatering step is performed on the ash to be treated after the neutralization step, A method for treating ash, comprising the following components.

4. The method for treating ash according to claim 2, further comprising a second water washing step of washing the ash to be treated, which has undergone dewatering in the second dewatering step, with water.

5. The treatment method according to any one of claims 1 to 3, wherein the acid is hydrochloric acid.

6. The treatment method according to any one of claims 1 to 3, wherein the concentration of the acid is 1 to 8% by weight.

7. The processing method according to any one of claims 1 to 3, wherein the radioactive material contains radioactive cesium.

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