KR100757200B1 - Method for Preparing Immobilization Product of Waste Chloride Salts Using Zeolite Only - Google Patents

Abstract

The present invention relates to a method for preparing immobilized product of waste chloride salt using zeolite only, more specifically cesium (Cs), strontium (Sr) and barium ( Immobilized intermediates by mixing zeolites with chloride wastes containing alkali or alkaline earth metal groups such as Ba), or rare earth-based radionuclides such as cerium (Ce), lanthanum (La), neodynium (Nd), and yttrium (Y) It relates to a method for producing an immobilized product of the chloride waste using only zeolite comprising the step of preparing and transferring the immobilized intermediate to Na-based sodalite.

The immobilized product of the present invention is a Na-based sodalite, which is a more stable form, and not only has excellent leaching characteristics, but also does not add a transition accelerator in the process of transferring to sodalite, thereby achieving a final solid amount (waste) corresponding thereto. Reducing the amount of form) allows for more effective waste disposal.

LiCl salt waste, zeolite NaA, sodalite, immobilized product, cesium, strontium

Description

Method for Preparing Immobilization Product of Waste Chloride Salts Using Zeolite Only}             

1 is a result of measuring the crystal structure of zeolite 4A type by X-ray diffracto-meter (XRD),

2 is a result of measuring the crystal structure of the immobilized intermediate (top) and the crystal structure of the immobilized product (bottom) produced when the mixing ratio (LiCl / zeolite) is 0.25 by X-ray diffraction spectroscopy,

3 is a result of measuring the crystal structure of the immobilized intermediate (top) and the crystal structure of the immobilized product (bottom) when the mixing ratio (LiCl / zeolite) is 0.1 by X-ray diffraction spectroscopy,

4 is a result of measuring the crystal structure of the immobilized product by X-ray diffraction spectroscopy when the mixing ratio (LiCl / zeolite) is 15, 10, 4.5, 1.0, 0.5, 0.4, respectively,

<Description of the symbols for the main parts of the drawings>

▲: Li-A crystal

■: Li ₈ Cl ₂ -Sod. (Li ₈ Cl ₂ -Sodalite)

▣: Na ₈ Cl ₂ -Sod. (Na ₈ Cl ₂ -Sodalite)

▼: NaCl

The present invention relates to a method for producing a solid waste of chloride waste using only zeolite, and more particularly, alkalis such as cesium (Cs), strontium (Sr), and barium (Ba) generated during the reprocessing of spent nuclear fuel. Immobilization product of chloride waste using zeolite only, comprising the steps of preparing an immobilized intermediate by mixing zeolite with chloride waste comprising alkaline earth metal group or rare earth based radionuclide and transferring the immobilized intermediate to Na-based sodalite It relates to a manufacturing method of.

As the industry develops, the seriousness of environmental pollution is increasing day by day. In particular, as nuclear energy is widely used in various economic aspects such as facility investment cost as an energy source required for various industries, the efficiency of radioactive waste treatment generated from nuclear power plants and nuclear-related research institutes has emerged as an important problem. Research is actively underway.

It is important to recover these fissable materials, since there is a significant amount of fissable material, such as uranium and plutonium, left behind from nuclear fuel. The recovery of fissable material from spent fuel can be carried out by an electrorefining method using an electrochemical cell. Electrolytic refining uses an electrorefining cell, which is composed of an electrolyte and an electrode of a molten eutectic salt mixture such as LiCl / KCl, and uranium or plutonium is deposited on the electrode.

However, when the spent nuclear fuel is reprocessed by electrorefining, the eutectic salt mixture is contaminated with radionuclides such as cesium, strontium and barium and other harmful metals, so that they can no longer be used as batteries for electrorefining. The contaminated eutectic salt mixture should be discarded.

A method of treating these contaminated eutectic salt mixtures, which theoretically hinders the operation of electrorefining agents such as radionuclides and other metals, such as sodium, which generate heat such as cesium or strontium in the contaminated eutectic salt mixtures. It may be possible to recycle the eutectic salt mixture by removing possible substances.

However, cesium, strontium, or barium are present as chlorides in eutectic salt mixtures, and these radionuclide chlorides are not easily separated from the eutectic salt mixtures, and are stored in a concentrated form because they generate significant amounts of heat. Before dilution and / or cooling in other media is required. Thus, in practice, radionuclides such as cesium, strontium or barium, and other harmful metal chlorides and iodine are generally treated with a eutectic salt mixture.

In general, radioactive waste can directly damage humans and land animals and plants, and secondly, polluting the groundwater flowing through the soil can adversely affect the entire environment, such as marine and atmospheric environments. In particular, salt wastes containing radionuclides such as cesium, strontium, or barium are classified as High Level Wastes (HLW), and therefore, unlike low-level radioactive wastes, they are stored in geologic repository by themselves. Must be stored. However, since salt wastes dissolve well in water, the form of salt wastes is required to be in a leach-resistant form to prevent radionuclides and other harmful chemicals from flowing into the groundwater. There is a need for treatment methods that can capture and fix these water-soluble salts.

As a treatment to solve this problem, a method of fixing waste in a glass matrix has been proposed. However, the type of waste suitable for fixation to the glass-forming system by this method is oxide, because glass containing chloride ions is less leach resistant. Chlorine wastes are generated during electrorefining, so this method cannot be applied directly to the treatment of eutectic salts, and there is a problem that the chloride wastes can only be applied after conversion to oxides or other suitable forms.

Another fixed medium that has been proposed is mortar. Special mortars can fix lithium, potassium, cesium and strontium chloride wastes in its structure. However, when water in mortar is exposed to radiation, there is a problem of generating a large amount of hydrogen gas. Therefore, a new fixed medium is required for the treatment of chloride waste, and a method of immobilizing the chloride waste inside the zeolite using zeolite has been developed as a method of immobilizing the salt waste.

The basic unit of zeolite is tetrahedron of SiO ₄ and AlO ₄ in which aluminum or silicon are bonded to four surrounding oxygen, and they share a vertex to form a secondary bonding unit. These three-dimensional bonds form porous aluminosilicate crystals. For example, zeolite A has a truncated octahedral, a model obtained by cutting a vertex of an octahedron, as a secondary bonding unit, and four bonding units are formed by forming a double 4-ring. will be. In this case, the secondary binding unit is called sodalite.

 Representative characteristics of the zeolite is that it has excellent selective cation exchange capacity and adsorption capacity. The cation exchange property of zeolite means that the cations in the cavity easily undergo ion exchange even with the treatment of simply washing with a solution of other cations. In addition, since they selectively exchange cations, they have ion exchange properties that are not found in other amorphous ion exchange materials (various ion exchange resins). Under normal conditions, large amounts of water are present around the cations in the cavities of the zeolite structure, but are easily dehydrated when heated to 350-400 ° C. The dehydrated zeolite selectively adsorbs inorganic and organic molecules in the appropriate sized form, resulting in excellent molecular properties that can separate different molecules.

The selective adsorption characteristics of these zeolites are different depending on the size and shape of the cavities of the zeolite, and these aspects are different depending on the type of zeolite. Examples thereof include zeolite A, X, and Y. They have Si / Al ratios of 1, 1 to 1.5, and 2 to 5, respectively, which constitute the basic unit, and the pore sizes in the crystals are 4.2, 7.4, and 7.4 Å, respectively. In addition, the size of the so-called effective pore size may vary depending on the nature of the cations in the cavity of the same zeolite. That is, in the case of zeolite type A, the substitution with potassium has pores of 3 Å size, whereas the substitution with sodium and calcium shows 4 Å and 5 Å, respectively.

Except for special cases, such as when the zeolite is heated and vented, the structure breaks down or sticks to the pore surface, most of the material in the pore is reversibly desorbed. Desorption occurs and adsorption occurs on empty pores. Since the substance to be adsorbed to the zeolite must pass through the pore inlet, molecules larger than the pore inlet cannot enter the pores, and if the molecules produced in the pore are larger than the pore inlet, they cannot escape out of the pore. This results in a phenomenon in which zeolites determine the possibility of adsorption-desorption depending on the molecular size.

U.S. Patent No. H1227 (1993) describes a method of occlusion and fixation of salt wastes using the properties of such zeolites. Zeolites in the form of sodium, calcium or lithium (eg zeolite A, or zeolites in the form of a mixture of cha-bazite and erionite, or a mixture of both) and LiCl- The radioactive nucleus in the salt wastes was reacted with KCl salt wastes at a mixing temperature of about 400 ± 25 ° C. (because the zeolite structure could be destroyed at higher mixing temperatures). The species cesium, strontium and other harmful substances were ion exchanged and occluded with sodium, calcium or lithium in the zeolite to about 25 wt% of the salt waste.

The patent invention has developed a method for fixing salt wastes using a new medium to improve the existing treatment method, but because the resulting zeolite is not a monolithic solid (monolithic solid), storage of high-level waste for a long time in terms of stability The problem is that it is not suitable for storage in.

U.S. Patent No. 5613240 (1997) describes a method for transferring a zeolite immobilized with LiCl-KCl based salt waste containing radionuclides and other harmful substances to sodalite. It relates to a more stable method of treating radionuclides and other harmful substances, characterized by increasing the leaching resistance and density of the final solids through transition to sodalite. The present invention is characterized by providing a sodalite that is excellent in leaching properties of radionuclides and other harmful substances such as cesium, strontium and stable in storage form by this transition. The invention requires a reaction condition of 927 ° C. (about 1000 K), 20 MPa in order to transfer the zeolite to sodalite, and a glass precursor (or glass-producing material) that promotes the transition to sodalite in the initial stage of the reaction. glass frit) and the like are added.

In addition to the above patents, conventional methods of preparing immobilized products or solids containing sodalite crystals are not the single salt of LiCl, but the eutectic salts in which LiCl and KCl are mixed together. The temperature is about 500 ° C. or less. Therefore, the immobilized intermediate thus produced retains the zeolite crystal structure as it is, and in order to transfer them to sodalite, a transition accelerator such as the glass precursor is required.

Without the addition of these precursors, the main product after the reaction would be nepheline rather than sodalite. Nephelline's crystal structure is not useful for storing radionuclides because of its low leaching resistance. On the other hand, when the glass precursor is added, sodalite can be obtained as the main product, but the amount of the final waste solidified body increases by the amount of the glass precursor added.

The present inventors have a stable structure as sodalite, and as a result of making efforts to make an immobilized product without adding an additive such as a glass precursor material, the present invention was completed by developing a method using only zeolite.

The present invention is to overcome the above problems, alkali, alkaline earth metal groups such as cesium, strontium and barium generated in the process of reprocessing spent nuclear fuel or rare earth radionuclides such as cerium, lanthanum, neodymium, yttrium, etc. Provided is a method for preparing an intermediate by mixing the contained chloride waste with zeolite and heat treating the immobilized intermediate produced by this method, thereby providing a method for producing a more stable and superior leaching product of the chloride waste. There is a purpose.

Hereinafter, the present invention will be described in detail.

The present invention is to prepare an immobilized intermediate by mixing a zeolite and chloride waste containing alkali, alkaline earth metal group or rare earth radionuclide (first step) and transferring the immobilized intermediate to Na-based sodalite (second step) It provides a method for producing the immobilized product of the chloride waste using only zeolite comprising the step).

In the step (first step) of preparing an immobilized intermediate according to the present invention, the chloride waste may be a chloride waste comprising alkali, alkaline earth metal group or rare earth radionuclides such as cesium, strontium or barium.

The chloride waste may be LiCl-based chloride waste. In the case of foreign patent invention, LiCl / KCl-based eutectic salt wastes generated from nuclear fuel processing process are targeted to Korea, but in Korea, chloride waste generated by using different processing process technology is different from LiCl-based single salt waste. Therefore, the present invention pays attention to the advantages.

In addition, the present invention is expected to be sufficiently applicable to LiCl / KCl-based chloride wastes because of more severe temperature conditions (prior art; 500 ° C., the present invention; 650 ° C.) when compared to foreign prior art.

In the step of preparing the immobilized intermediate according to the present invention (first step), the zeolite is of zeolite A type, particularly zeolite 4A type. For other types of zeolites (e.g., zeolites X, Y, etc.), the amount of radionuclides, such as cesium, which can be contained in the zeolite pores by ion exchange and adsorption in the step of making the immobilization intermediate in the first step, is determined by This is because it is quite small.

Zeolite Type A includes zeolite Li-A, zeolite NaA, zeolite KA, zeolite CaA, and the like depending on the type of the substituted cation. In the present invention, NaA type is mainly used. The pore size of NaA type is about 4.3 mm3. In the step of preparing the immobilized product according to the present invention (first step), the zeolite NaA is a pore of the zeolite by ion exchange, adsorption and inclusion of alkali, alkaline earth metal group or rare earth radionuclides contained in LiCl-based chloride waste. It plays a role of holding in.

Also in the case of CaA type, the same effect of the invention can be obtained. In practice, however, the CaA type was prepared by ion-exchanging Na ions with Ca ions in an aqueous solution, which was made of NaA type, and only about 60% of the ion was exchanged and the remaining 40% of the Na ions were excluded.

In the step (first step) of preparing an immobilized intermediate according to the present invention, the mixing ratio of LiCl and zeolite (LiCl / zeolite; r) of the chloride waste is a weight ratio of 0.001 to less than 0.4, preferably 0.1 or more It can be 0.3 or less.

In the step of preparing the immobilized intermediate according to the present invention (first step), the temperature of the mixing reaction is preferably 613 to 690 ℃.

Immobilized intermediate is prepared by mixing the mixture in a suitable mixer (for example, blending mixer or V-mixer, etc.) at a temperature of at least 613 ° C., at which the LiCl salt is melted, by stirring or rotating to prepare immobilized intermediate. It is desirable to maintain 650 ℃.

In the step of preparing the immobilized intermediate according to the present invention (first step), the mixing reaction time is preferably 30 minutes or more and 30 hours or less, preferably 3 hours or more.

In the step (second step) of transferring the immobilized intermediate according to the present invention to Na-based sodalite, the transition reaction temperature is preferably 750 ° C. or more and 950 ° C. or less, and preferably maintained at 850 ° C. or more.

In the step (second step) of transferring the immobilized intermediate according to the present invention to Na-based sodalite, the transition reaction time is preferably 2 or more and 10 hours or less, preferably about 4 hours.

The present invention constituted as described above is a method for preparing an immobilized intermediate of chloride waste using only appropriate heat treatment and zeolite, and transferring it to Na-based sodalite, which is different from the conventionally developed method. Without heat treatment alone, excellent immobilization products can be obtained in terms of leaching properties and stabilization.

The immobilized product with Na-based sodalite crystal structure is smaller than the size of the radionuclides contained in the immobilized product (about 3.7)) by the size of the pores therein so that the internal structure of the crystal is not destroyed. It does not escape to the outside, making it suitable for storing radionuclides at disposal sites.

It is also useful for reducing the amount of immobilized product since it does not require the addition of glass precursors for the transition to sodalite.

As a result of the immobilization intermediate prepared in the immobilized product manufacturing step according to the present invention, the immobilization intermediate having a mixing ratio of 0.4 or more and 15 or less showed that the crystal structure of Li-A was the main product after the heat treatment, but the mixing ratio was 0.3. Under the following conditions, almost all of the immobilized products had excellent leaching properties and were confirmed to be transferred to Na-based sodalite in a more stable form.

Hereinafter, the present invention will be described in detail by Examples and Comparative Examples.

However, the following examples are merely to illustrate the invention, but the content of the present invention is not limited by the following examples.

Example 1 Preparation of Immobilized Intermediates and Immobilized Products of LiCl-Based Salt Wastes Containing Cesium and Strontium (Mixed Ratio of 0.25)

1. Preparation of Immobilized Intermediates (Step 1)

Simulate LiCl-based salt waste mixed with 25 g of LiCl, 1.53 g of CsCl, and 0.83 g of SrCl ₂ and zeolite NaA 100 g are placed in a blending mixer or V-type mixer and the temperature of the electric furnace surrounding the mixer is 613 ° C, the melting temperature of LiCl. It operated above. Thereafter, the mixer was rotated in an electric furnace maintained at about 650 ° C. and mixed for 5 hours or more so that the molten LiCl salt waste was mixed well with the zeolite. The crystal structure of the immobilized intermediate thus prepared was measured with an X-ray diffractometer, and the pattern of the immobilized intermediate was found to be a mixture of Li-A crystals and Li-based sodalite (Li ₈ Cl ₂ -Sod.) (FIG. 2). At the top).

2. Preparation of Immobilized Product (Second Step)

When the immobilized intermediate is heat-treated for 4 hours or more in an electric furnace maintained at 900 ° C., an immobilized product having Na-based sodalite (Na ₈ Cl ₂ -Sod.) Crystals as a main component is prepared. As a result of measuring the crystal structure of the immobilized product thus prepared by X-ray diffractometer, it was confirmed that the immobilized intermediate of the first step was transferred to Na-based sodalite having better leaching properties and more stable form (Fig. Bottom of 2).

Example 2 Preparation of Immobilized Intermediate and Immobilized Product of LiCl-Based Salt Waste Containing Cesium and Strontium (Mixed Ratio 0.1)

It was prepared in the same manner as in <Example 1> except for the mixing ratio (LiCl / zeolite) conditions. 3 shows the results of X-ray diffraction analysis for the immobilized intermediate and the immobilized product.

Table 1 below shows the immobilization intermediate and the immobilization product according to Examples 1 and 2.

Comparative Example Comparison of Crystal Structures of Immobilized Intermediates and Immobilized Products at Different Mixture Ratios (LiCl / zeolites)

The immobilization intermediate and the immobilization product were prepared in the same manner as in the above examples under the conditions of the mixing ratio of 15, 10, 4.5, 1.0, 0.5, 0.4 in the range of 0.4 or more and 15 or less. Table 1 below shows the crystal structures of the immobilized intermediate and the immobilized product according to the comparative example. The main product in the mixing ratio range was found to be Li-based sodalite (Li ₈ Cl ₂ -Sod.) Or Li-A crystal structure.

The present invention has excellent leaching properties by transferring immobilized intermediates of chloride wastes, including radionuclides and other harmful chemical species, generated in the course of reprocessing spent fuel to Na-based sodalite crystal structure, and storing them at the disposal site. It is useful for the preparation of suitable immobilization products. In addition, by preparing the immobilized product without the addition of any transition accelerator, such as glass precursors used in the conventional method as a function of promoting the transition in the transition step, to reduce the amount of the final solid to be sent to the disposal site or salt By increasing the throughput of flame waste, it is possible to treat chloride waste more effectively than conventional methods.

Claims (10)

In the method for producing the immobilized product of the chloride waste, Preparing an immobilized intermediate by mixing zeolite A with a chloride waste comprising an alkali, alkaline earth metal group or rare earth radionuclide (first step), and A method for preparing immobilized product of chloride waste using zeolite only, comprising the step of transferring the immobilized intermediate to Na-based sodalite (second step). The method of claim 1, wherein the radionuclide is cesium, strontium or barium. 2. The method of claim 1, wherein the zeolite A is zeolite NaA type. The method of claim 1, wherein the chloride waste is LiCl-based chloride waste, characterized in that the production method of the immobilized chloride waste using only zeolite. 5. The method of claim 4, wherein the mixing ratio of the LiCl-based chloride waste and the zeolite A is in a weight ratio of 0.001 or more and less than 0.4. 6. 5. The method of claim 4, wherein the mixing ratio is 0.1 to 0.3 in weight ratio. 5. The method of claim 4, wherein the mixing reaction temperature of the first step is 613 to 690 ° C. 6. The method of claim 4, wherein the mixing reaction time of the first step is 30 minutes to 30 hours. The method of claim 4, wherein the reaction temperature of the second step is from 750 to 950 ° C. 5. The method of claim 4, wherein the reaction time of the second step is 2 to 10 hours. 6.

Images