JP7487025B2 - Manufacturing method and manufacturing device for geopolymer molded body - Google Patents

Description

An embodiment of the present invention relates to a manufacturing technology for geopolymer molded bodies.

Low-level radioactive waste generated from nuclear power plants and other sources is generally solidified with cement or other solidification materials before disposal. In this cement-based solidification method, in order to mix the solidification materials homogeneously, water must be added until the material becomes paste-like, and then the mixture must be kneaded while adjusting the viscosity until it is lower than a certain value. This cement-based solidification method has the problem that the volume reduction of radioactive waste is reduced by adding a large amount of mixing water. Furthermore, the paste-like mixture is prone to problems such as adhering to the agitator during kneading and to piping during transport, which requires frequent equipment maintenance.

A technology is being considered that reduces the amount of water used by mixing geopolymer solidification material in powder form with a small amount of water and compressing and solidifying it. This type of solidification process using compression molding does not require the mixing of large amounts of water, and is expected to avoid the aforementioned decrease in the volume reducibility of radioactive waste and other problems, as well as simplify the mixing process. Note that with geopolymers, a method is generally used in which a large amount of water is mixed to create a paste before solidifying, but this has the same issues as the aforementioned solidification method using cement.

The geopolymer is composed of an amorphous inorganic solidification material called aluminosilicate, whose main components are aluminum and silicon, and unlike cement, does not contain water of hydration in its molecular structure. In addition, solidified bodies molded from a paste state inevitably contain a considerable amount of water, but solidified bodies made by compression molding can avoid this situation.

The water contained in the solidified body is decomposed by the radioactivity of the mixed radioactive waste, causing hydrogen to be generated. If the hydrogen gas concentration increases during storage of the waste body, there is a concern that a hydrogen explosion may occur. On the other hand, compressed solidified geopolymer bodies have a low water content, and most of the water can be removed by drying, which has the advantage of reducing the amount of hydrogen generated by radioactivity.

JP 2018-65731 A

The compressed and molded geopolymer described above is mixed with a small amount of water so that it does not turn into a paste, and the solidification material is mixed. However, the compressed and solidified geopolymer molded in this way has the problem that it has lower mechanical strength and is inferior in durability compared to the general processing method in which the geopolymer is first turned into a paste and then solidified. This is thought to be because the contact between the alkali irritant, which is necessary to start the reaction, and the water in the solidification material is insufficient, so the solidification reaction does not proceed uniformly. In addition, the hydroxide or silicate used as this alkali irritant is highly hygroscopic in powder form and easily deliquesces, which makes it difficult to store and manage.

The embodiment of the present invention was made taking these circumstances into consideration, and aims to provide a manufacturing technology for geopolymer molded bodies that have a low moisture content and high mechanical strength.

A method for manufacturing a geopolymer molded body according to an embodiment includes a step of supplying a powder material mainly composed of aluminum and silicon, a step of supplying an aqueous solution of an alkaline irritant that polymerizes the powder material, a step of supplying radioactive waste consisting of a pulverizable solid, a step of mixing the powder material, the radioactive waste , and the aqueous solution to generate a moist powder, and a step of compressing the moist powder to form a molded body. The aqueous solution of the alkaline irritant is a solution of a silicate in which a hydroxide is dissolved, and the silicate is at least one of lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate. The hydroxide is at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide .

Embodiments of the present invention provide a manufacturing technique for geopolymer molded bodies that have low moisture content and high mechanical strength.

1 is a schematic diagram showing a manufacturing apparatus for a geopolymer molded body according to a first embodiment of the present invention. Schematic diagram showing a part of a geopolymer molded body manufacturing apparatus according to the second embodiment. FIG. 1 is a process diagram illustrating an embodiment of a method for producing a geopolymer molded body according to the present invention. 11 is a table showing conditions for examples in which the effects of each embodiment were confirmed.

First Embodiment
Hereinafter, an embodiment of the present invention will be described based on the accompanying drawings. Figure 1 is a schematic diagram showing a manufacturing apparatus for a geopolymer molded body according to the first embodiment of the present invention (hereinafter, simply referred to as "manufacturing apparatus 10A"). Thus, the manufacturing apparatus 10A (10) includes a first supply section 11 for supplying a powder material 21 mainly composed of aluminum and silicon, a second supply section 12 for supplying an aqueous solution 22 of an alkaline stimulant that polymerizes the powder material 21, a mixing section 15 for mixing the powder material 21 and the aqueous solution 22 to generate a moistened wet powder 25, and a compression molding section 16 for compressing the moist powder 25 to form a molded body 26.

Geopolymer is a general term for amorphous inorganic polymers produced by a condensation polymerization reaction between alumina-silica and an alkaline irritant, which will be described later. The powder material 21 is an amorphous powder whose main component is a compound (alumina-silica) containing aluminum (Al) and silicon (Si). Specifically, the powder material 21 may be metakaolin, blast furnace slag, incineration ash, fly ash, zeolite, silica fume, amorphous silicon dioxide, aluminum oxide, aluminum hydroxide, etc.

The alkaline stimulant causes the powder material 21 to undergo a polymerization reaction, and at least one of hydroxides and silicates can be used. Specific examples of hydroxides that can be used include lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide. Examples of silicates that can be used include lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate. Note that silicates come in a variety of chemical forms, such as ortho and meta, and any chemical form of silicate can be used as the alkaline stimulant without being limited to a specific chemical form.

The aqueous solution 22 of the alkaline stimulant is prepared by dissolving the alkaline stimulant in the aqueous solvent at a concentration equal to or lower than the saturation solubility. The concentration of the alkaline stimulant in the aqueous solution 22 is set so that the desired polymer composition is obtained stoichiometrically without excess or deficiency relative to the amount of aqueous solvent that uniformly provides the desired moistened state to the wet powder 25.

The first supply unit 11 supplies a predetermined amount of powder material 21 to the mixing unit 15. The second supply unit 12 supplies a predetermined amount of an aqueous solution 22 of an alkaline irritant to the mixing unit 15. The mixing unit 15 mixes the supplied powder material 21 and aqueous solution 22 to form a moist powder 25.

Here, "humid" refers to a state in which the moist powder 25 contains moisture to the extent that it has not yet turned into a slurry. Specifically, this refers to a state in which the aqueous solution 22 of the alkaline irritant is evenly distributed over the surfaces of the particles that form the powder material 21, and the aqueous solution 22 is not separated from the moist powder 25 when compressed into a molded body 26.

The aqueous solution 22 mixed with the powder material 21 can be prevented from forming a slurry by setting the upper limit of the proportion of the aqueous solution 22 in the entire wet powder 25 at 40 wt%. The upper limit is preferably set at 35 wt%, and more preferably at 25 wt%. On the other hand, the lower limit of the aqueous solution 22 mixed with the powder material 21 is set to a value that contains the amount of alkaline irritant required for the polymerization reaction, assuming that the aqueous solution 22 is a saturated aqueous solution.

The molded body 26 is formed by compressing the wet powder 25 placed in the mold 18 using the compression molding section 16. The pressure for compressing the wet powder 25 is preferably 1.0 MPa or more. This allows the shape stability of the molded body 26 to be maintained. There is no particular upper limit to the compression pressure of the wet powder 25, but it is set within a range that can be achieved by the compression molding section 16. The mold 18 is preferably a collapsible type to make it easier to demold the molded body 26.

The manufacturing apparatus 10 further includes a curing section 17 that cures the molded body 26 in a temperature- and humidity-controlled environment. The curing section 17 promotes a polymerization reaction between the alumina silica in the powder material 21 and the alkaline irritant in the aqueous solution 22 in the molded body 26 that has been cut out of the mold form 18. The curing section 17 can also dry the molded body 26 and reduce the moisture content by adjusting the temperature and humidity. Although the polymerization reaction naturally proceeds inside the molded body 26 without using the curing section 17, the polymerization reaction can be promoted by adjusting the temperature and humidity of the atmosphere of the molded body 26 using the curing section 17. This improves the mechanical strength of the molded body 26.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to Fig. 2. Fig. 2 is a schematic diagram showing a part of a geopolymer molded body manufacturing apparatus (hereinafter, simply referred to as "manufacturing apparatus 10B") according to the second embodiment. In Fig. 2, parts having the same configuration or function as Fig. 1 are indicated by the same reference numerals, and duplicated explanations are omitted.

In this way, the manufacturing apparatus 10B (10) of the second embodiment includes the configuration of the manufacturing apparatus 10A of the first embodiment, as well as a third supply section 13 that supplies radioactive waste 23. The radioactive waste 23 supplied from the third supply section 13 to the mixing section 15 is mixed with the powder material 21 and the aqueous solution 22 to produce a moist powder 25. In this way, in the manufacturing apparatus 10B of the second embodiment, the moist powder 25 also contains radioactive waste.

The radioactive waste contained in the wet powder 25 may be, for example, a radionuclide adsorbent used in water purification. In addition, various radioactive wastes made of pulverizable solids may also be contained in the wet powder 25.

Third Embodiment
An embodiment of the manufacturing method of the geopolymer molded body according to the present invention will be described with reference to the process diagram of Figure 3 (see Figure 1 or Figure 2 as appropriate). A powder material 21 mainly composed of aluminum and silicon is supplied (S11). In this case, the supply of the powder material 21 can be performed by weighing a specified amount from a storage tank (not shown) or by directly taking it out from a distribution bag, regardless of the supply means. In addition, when the powder material 21 is a mixture of multiple materials, these materials may be mixed in advance and supplied, or each may be separated and supplied separately.

Next, an aqueous solution 22 of an alkaline stimulant that causes a polymerization reaction of the powder material 21 is supplied (S12). This aqueous solution 22 is prepared in advance by dissolving the alkaline stimulant in a water solvent and setting the concentration to a predetermined value. The supply amount of this aqueous solution 22 is set according to the supply amount of the powder material 21.

Next, the powder material 21 and the aqueous solution 22 are mixed to produce moist powder 25 (S13). This mixing operation is continued until the aqueous solution 22 of the alkaline irritant is evenly distributed throughout the powder material 21.

Next, the wet powder 25 is compressed to form a molded body 26 (S14). The wet powder 25, which has fluidity due to this compression, becomes a solidified molded body 26. Note that this compression does not cause the aqueous solution 22 to separate from the wet powder 25.

Next, the molded body 26 is cured in a temperature and humidity controlled environment (S15). This promotes the polymerization reaction between the alumina silica that constitutes the molded body 26 and the alkaline stimulant, and promotes the improvement of the mechanical strength of the molded body 26.

Next, examples in which the effects of this embodiment were confirmed will be described. Figure 4 is a table showing the conditions of examples in which the effects of each embodiment were confirmed. In this table, a mixture of metakaryon, silica fume, or blast furnace slag corresponds to powder material 21. An aqueous solution of potassium metasilicate in which potassium hydroxide is dissolved corresponds to an aqueous solution of an alkaline irritant 22.

(Comparative Example 1)
Comparative Example 1 shows a case where the alkaline irritant (sodium metasilicate anhydrous, potassium hydroxide) is mixed in powder form (not in the form of an aqueous solution 22) with a powder material (metakaolin) together with a small amount of water. Specifically, the mixture was prepared with the mixing ratio of each compound in the whole being 58 wt% metakaolin, 28 wt% sodium silicate anhydrous, 11 wt% potassium hydroxide (all in powder form), and 3 wt% water.

The resulting mixture was placed in a 30 mm diameter mold and compressed for 10 minutes at a molding pressure of 56 MPa. The mixture was then removed from the mold to obtain a compression molded body. This was then cured for 15 days at an air temperature of 25°C and a relative humidity of 90%. The uniaxial compressive strength of the cured geopolymer molded body was measured and found to be 6.7 MPa.

(Comparative Example 2)
Comparative Example 2 shows a case where an alkaline irritant (potassium metasilicate, potassium hydroxide) is mixed with a powder material (metakaolin) together with a large amount of water solvent to form a paste. Specifically, the mixture was prepared with the mixing ratio of each compound as a whole being 31 wt% metakaolin, 16 wt% silica fume, 12 wt% of 30% potassium metasilicate aqueous solution, 18 wt% powdered potassium hydroxide, and 23 wt% water.

The resulting paste-like mixture was placed in a 50 mm diameter mold and cured for 14 days under conditions of 25°C temperature and 97% relative humidity. The uniaxial compressive strength of the cured geopolymer molded body was measured and found to be 29 MPa. In this way, by turning the mixture into a paste and then forming it into a geopolymer molded body, it is possible to improve the mechanical strength compared to Comparative Example 1. However, since the solidified body molded from the paste state inevitably contains a considerable amount of water, an increase in the amount of hydrogen generated due to radioactivity is unavoidable.

Example 1
In Example 1, an aqueous solution 22 of an alkaline irritant (potassium metasilicate, potassium hydroxide) is mixed with a powder material 21 (metakaolin, silica fume) to form a wet powder 25. Specifically, the wet powder 25 was prepared by mixing the overall mixing ratio of each compound as follows: metakaolin 45 wt%, silica fume 24 wt%, potassium metasilicate aqueous solution with a concentration of 30% 17 wt%, and powdered potassium hydroxide 14 wt%.

The powdered potassium hydroxide was dissolved in advance in the prepared potassium metasilicate aqueous solution 22. The obtained wet powder 25 was treated under the same compression and curing conditions as Comparative Example 2. The uniaxial compressive strength of the cured geopolymer molded body was measured and found to be 48 MPa. Thus, the geopolymer molded body molded by compressing the wet powder 25 exhibited improved mechanical strength compared to those molded in Comparative Examples 1 and 2.

Example 2
In Example 2, an aqueous solution 22 of an alkaline irritant (potassium metasilicate, potassium hydroxide) is mixed with powder materials 21 (metakaolin, silica fume, blast furnace slag) to form a wet powder 25. Specifically, the wet powder 25 was prepared by mixing the overall mixing ratios of each compound as follows: metakaolin 31 wt%, silica fume 13 wt%, blast furnace slag 31 wt%, 30% concentration potassium metasilicate aqueous solution 19 wt%, and powdered potassium hydroxide 6.0 wt%.

The powdered potassium hydroxide was dissolved in advance in the prepared potassium metasilicate aqueous solution 22. The obtained wet powder 25 was treated under the same compression and curing conditions as in Comparative Example 2. The uniaxial compressive strength of the cured geopolymer molded body was measured and found to be 28 MPa. Thus, the geopolymer molded body molded by compressing the wet powder 25 exhibited improved mechanical strength compared to that molded in Comparative Example 1.

Furthermore, in Examples 1 and 2, in comparison with Comparative Example 2, the powder material 21 and the like are mixed without being turned into a paste, which improves the adhesion of the powder material 21 and the like to the mixing section 15, reduces the frequency of equipment maintenance, and increases manufacturing efficiency. Furthermore, in Examples 1 and 2, in comparison with Comparative Example 2, the solidified body molded from the moist powder 25 contains a small amount of water, which makes it possible to suppress the amount of hydrogen generated by radioactivity. Furthermore, in Examples 1 and 2, in comparison with Comparative Examples 1 and 2, the alkaline irritant, which is highly hygroscopic and easily deliquesces, can be stored in the aqueous solution 22, which makes it easier to handle and reduces the burden on safety management.

According to at least one embodiment of the manufacturing method for a geopolymer molded body described above, by compressing a wet powder made by mixing an aqueous solution of an alkaline irritant with a powder material, it is possible to obtain a geopolymer molded body with a low moisture content and high mechanical strength.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as set forth in the claims, as well as the scope and spirit of the invention.

10 (10A, 10B)... Manufacturing apparatus (manufacturing apparatus) for geopolymer molded bodies, 11... First supply section, 12... Second supply section, 13... Third supply section, 15... Mixing section, 16... Compression molding section, 17... Curing section, 18... Formwork, 21... Powder material, 22... Aqueous solution of alkaline irritant, 23... Radioactive waste, 25... Wet powder, 26... Molded body.

Claims (4)

providing a powder material having a base of aluminum and silicon;
supplying an aqueous solution of an alkaline stimulant that causes a polymerization reaction of the powder material;
providing radioactive waste comprising a pulverizable solid;
mixing the powder material, the radioactive waste and the aqueous solution to produce a moist powder;
and compressing the wet powder to form a molded body .
The aqueous solution of the alkaline irritant is obtained by dissolving a hydroxide in an aqueous solution of a silicate,
The silicate is at least one of lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate;
The method for producing a geopolymer molded body , wherein the hydroxide is at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide . In the method for producing a geopolymer molded body according to claim 1 ,
A method for producing a geopolymer molding, wherein the aqueous solution mixed with the powder material has a maximum ratio of 40 wt % of the total wet powder. In the method for producing a geopolymer molded body according to claim 1 or 2 ,
The method for producing a geopolymer molded body further comprises a step of curing the molded body in a temperature and humidity controlled environment. A first supply unit that supplies a powder material containing aluminum and silicon as main components;
A second supply unit that supplies an aqueous solution of an alkaline stimulant that causes a polymerization reaction of the powder material;
a third supply for supplying radioactive waste comprising a pulverizable solid;
a mixing section for mixing the powder material, the radioactive waste, and the aqueous solution to generate a moist powder;
A compression molding unit that compresses the wet powder to form a molded body ,
The aqueous solution of the alkaline irritant is obtained by dissolving a hydroxide in an aqueous solution of a silicate,
The silicate is at least one of lithium silicate, sodium silicate, potassium silicate, rubidium silicate, and cesium silicate;
The hydroxide is at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide .

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