JPS6156017B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for immobilizing cesium in an aqueous solution. High-level radioactive waste liquid contains cesium, and if left untreated, it can cause pollution and be dangerous. Conventionally, methods for separating and solidifying cesium from high-level radioactive waste liquid include solidifying it with borosilicate glass, separating cesium by ion exchange with zeolite, and heating it to 1000 to 1200°C to form the polesite mineral phase. A method of converting and fixing is known. However, since the borosilicate vitrification method uses nitrates and the like during solidification, the crucible material is eroded during melting, the melting temperature is high, and cesium volatilizes. In addition, the solidified material has poor durability due to phase separation and crystallization due to aging and accumulation of decay heat, and the cesium leaching rate of the solidified material is 10 ^-7 g/
The drawback is that leaching is large on the order of cm ² day.
The method using zeolite has the advantage of a low cesium leaching rate of 2 to 3 × 10 ^-9 g/cm ² ·day, but the ion exchange capacity of zeolite is small, and Since the heat treatment temperature for fixing cesium is relatively high at 1000 to 2000°C, there is a drawback that cesium evaporates during heat treatment. The present invention aims to improve the drawbacks of the conventional method, and has high adsorption and ion exchange properties for cesium, no volatilization of cesium during heat treatment to solidify cesium, and leaching of cesium from the solidified product. The purpose of the present invention is to provide a method for immobilizing cesium with low yield. The present inventor first formed a fibrous material from a melt of TiO ₂ and K ₂ O to produce fibrous potassium titanate.
By eluting _the K _{2 O} component from this fibrous potassium titanate with an acid aqueous solution etc., fibrous titanium hydrate TiO ₂ Successfully produced mH ₂ O (m=0 to 3). (Special Application No. 53-676856,
54-93460) Further, as a result of continuing research on the properties of the obtained fibrous titania hydrate, it was found that the fibrous titania hydrate adsorbs and ion-exchanges cesium in an aqueous solution.
It was found that cesium titanate Cs _x O.nTiO 2.mH ₂ O (x=0.5-2, n=1-8, _m =0-7). In addition, when cesium is adsorbed and ion-exchanged, a specific metal oxide or a metal salt that becomes an oxide when heated is mixed, and an excessive amount of titanium dioxide is added or heat-treated without adding titanium dioxide. The connection of TiO ₆ octahedrons, which are unique to alkali metals, results in a hollandite-type compound with a tunnel structure, which fixes cesium in the tunnel structure and makes it difficult to leach out. Furthermore, the leaching rate of cesium is reduced in the mixed phase of hollandite-type compounds and titanium dioxide. In addition, it becomes a highly durable and stable mineral phase. It has also been found that when this is pressure-molded and sintered, the cesium leaching rate is further reduced and the durability is excellent. The present invention was completed based on this knowledge. The titanium hydrate used in the present invention may be in any form such as an amorphous gel, an amorphous or crystalline powder or granule, or an amorphous or crystalline fibrous material, and may contain cesium. Although it can be obtained by adsorption and ion exchange, fibrous materials are preferable because they have a large amount of adsorption and are easy to handle. In particular, fibrous materials having a crystalline layered structure are preferred. Adsorption and ion exchange of cesium in an aqueous solution may be carried out by immersion in the aqueous solution or by passing the cesium aqueous solution through a column filled with an adsorbent. Cesium in aqueous solution is cesium titanate
Cs _x O·nTiO ₂ ·mH ₂ O (x, n, m are the same as above). The amount of adsorption and ion exchange varies depending on the concentration of cesium, reaction time, temperature, etc. Moreover, the values of x, n, and m can be determined by crystallizing by heat treatment at a temperature of 500 to 1000° C. and determining the synthesized phase by X-ray powder diffraction method. In order to immobilize cesium, Ti
It is known to be effective as a replacement component for
Oxides or heating of divalent metals such as Mg, Co, Ni, and Cu (hereinafter collectively referred to as M〓) and trivalent metals such as A, Fe, Cr, Mn, Co, and Ga (hereinafter collectively referred to as M〓) One or two of these metal salts, such as carbonates, bicarbonates, nitrates, and hydroxides, are converted into oxides by
If necessary, an excessive amount of titanium dioxide, for example, up to about 10 moles per mole of hollandite-type cesium titanate, is mixed and polished. The obtained polished frame is heated at 500 to 1300°C to form a crystallized hollandite structure. Its composition is as follows. (1)Cs _x M〓 _y/2 Ti _8-y/2 O ₁₆ (x=0.5~2.0, y=0.5~2), (2)
Cs _x M〓 _y Ti _8-y O ₁₆ (where x and y are the same as above), and an excessive amount of titanium is mixed as an anatase phase or a rutile phase. If the heating temperature is less than 500°C, it will take a long time, and if it exceeds 1300°C, cesium will volatilize. Next, if these are pressure-formed at a pressure of 5Kg/cm ² to 500Kg/cm ² and then sintered at a temperature of 100℃ or higher and lower than the melting temperature, the volume will be reduced and the cesium leaching rate will be low, resulting in poor durability. It becomes something big. Instead of this two-step method of pressure forming and sintering, pressure forming and sintering may be performed simultaneously using a hot press method.
Alternatively, when adding a metal oxide to cesium titanate to convert it into hollandite-type cesium titanate, this composition mixture may be directly hot-pressed. The cesium adsorption and ion exchange material of the present invention is made of titanic acid and fixes cesium in the TiO ₆ octahedral connection mode, unlike the conventional silicate zeolite SiO ₄ tetrahedron connection mode. Immobilization is superior compared to those that are immobilized inside. Furthermore, since the fixing process can be carried out at a temperature lower than 1300° C., there is no risk of cesium volatilizing during sintering, and since the fixation has a hollandite structure, there is little leaching of cesium. In addition, for example, the radioactive element of cesium transfers to barium through decay, but even if the alkali component in the alkali metal titanate of the hollandite structure in the present invention is exchanged with barium, it remains stable and durable due to this decay process. Although the temperature does not decrease and the temperature rises to a considerably high temperature (700 to 1000°C) due to the accumulation of decay heat, it is stable even at high temperatures, and has an excellent effect of stably fixing cesium. Example 1 (1) Production of fibrous potassium titanate TiO ₂ and K ₂ CO ₃ powders were mixed at a molar ratio of 2:1. Approximately 45 g of the mixture was filled into a 100 ml platinum crucible and melted by heating at 1000° C. for 30 minutes. The melt was discharged into another metal container (the bottom of which was water-cooled from the outside), where it was rapidly cooled and crystallized into fibers. The resulting fibrous crystalline mass was defibrated by immersing it in water for about 2 hours. The defibrated fibers were bundles with a diameter of 0.1 to 0.5 mm and an average length of 5 mm. Since this fiber has poor crystallinity, it was heated at 900°C for 30 minutes. this is
The fibers were a mixed phase of K ₂ Ti ₄ O ₉ and K ₂ Ti ₂ O ₅ . (2) Production of fibrous titania hydrate The fibers obtained in (1) above were dissolved in 1N-HC aqueous solution.
The K ₂ O component was extracted by immersing it in the aqueous solution for about 1 hour at a ratio of 10 g to 100 ml, followed by washing with water and air drying to obtain titania hydrate. The value of m for TiO ₂ ·mH ₂ O was 0.5 to 1.5. The X-ray powder diffraction pattern of the titanium hydrate with copper anticathode shows 2θ=10°,
It was a crystalline fiber showing broad peaks around 25.6° and 48.6°. (3) Adsorption and ion exchange of cesium in aqueous solution
The titanium hydrate fibers were immersed in a proportion of 0.1 g per 100 ml of 0.5NCsOH aqueous solution for 24 to 48 hours without stirring, and then dried with air. This powder X-ray diffraction diagram showed an extremely broad peak around 2θ=30°. this
The diffraction pattern of the cesium hexatitanate CS ₂ Ti 6 O 13 phase obtained by heat treatment at 900°C for 30 minutes matched that of the cesium hexatitanate CS 2 Ti ₆ O ₁₃ phase.
Air-dried items have a moisture content of approximately 12.5%.
It was cesium hexatitanate in the aqueous phase of CS ₂ Ti ₆ O _{13.5.3H 2} O. The adsorbed ion exchange amount of Cs ions was about 31%. (4) Conversion of cesium hexatitanate to hollandite type cesium titanate Dehydrated product of cesium hexatitanate (CS ₂ Ti ₆ O ₁₃ )
Added at a ratio of 0.067g of A ₂ O ₃ to 0.5g,
After mixing the powder, it was heated at 1000°C for 24 hours or more. What was obtained was hollandite type cesium titanate. (5) Molding and immobilization of hollandite-type cesium titanate 0.431 g of hollandite-type cesium titanate (Cs ₂ A ₂ Ti ₆ O ₁₆ ) obtained in (4) was molded to a diameter of 1.3 cm under a pressure of 500 Kg/cm ² After forming into pellets with a thickness of 0.1 cm, they were fired again at 1000°C for 15 hours. The surface area was measured by nitrogen gas adsorption method.
It was 5.8×10 ⁴ cm ² /g. (6) Leaching of cesium in pure water The solidified material was immersed in 100 ml of distilled water, and the behavior of pH change over time was investigated while stirring. The increase in pH value associated with the leaching of cesium ions reached a peak in about 30 minutes, and then showed a tendency to gradually decrease. This decrease in pH value is thought to be because leaching of cesium was extremely small and the effect of dissolving carbon dioxide in the air was greater. When the 24-hour immersion was repeated three times (72 hours), the amount of cesium leached was measured by atomic absorption spectrometry, and the result was 1.33×10 ^-7 g/cm ² ·day. The leaching rate was calculated using the following formula. L=A/S・1/t L: Leaching rate (g/cm ²・24h), A: Leaching amount of cesium (g), S: Surface area of sample (cm ² ), t:
Leaching time (h) Example 2 While changing the x and y values of the composition of hollandite type cesium titanate Cs _x M _y Ti _8-y O ₁₆ in (4) of Example 1, the excess TiO ₂ change the amount of mixture,
Heat treatment was performed at 1000°C for 24 hours. The leaching rate of cesium in pure water was as follows.

[Table] Note that when anatase is used as a raw material for TiO ₂ and heat treatment and sintering are performed at a temperature of 1000°C or less, it becomes an anatase phase instead of a rutile phase. As is clear from this result, adding excess TiO ₂ reduces the leaching rate. The cesium leaching rate in pure water of the mixture of 12 moles of TiO ₂ to 1 mole of Cs ₂ A ₂ Ti ₆ O ₁₆ produced by the above method and the mixture obtained by pressure molding and sintering is as follows: It was as follows.

[Table] G As is clear from the results, it can be seen that the leaching rate of cesium decreases when pressure molding and sintering are performed. In addition, in the examples, the case of A ₂ O ₃ was given, but Mg, Co〓, Ni, Cu, Fe, Cr, Mn,
Co〓 and Ga were also obtained by replacing Ti in the same manner, and almost the same results were obtained.

Claims (1)

[Claims] 1 Potassium titanate K ₂ O・nTiO ₂ (however, n
Titanium hydrate TiO ₂ mH ₂ O obtained by extracting the K ₂ O component from = 1 to 6) (where m = 0 to 3)
Cesium titanate Cs _x O・nTiO ₂・mH ₂ O
(However, x=0.5~2, n=1~8, m=0~
7) Mg, Co〓,
Oxides of metals selected from Ni, Zn, Cu, A, Fe, Cr, Mn, Co〓, Ga or their metal salts which become oxides by firing are mixed, and this mixture is heated to
A method for fixing cesium characterized by heating to 1300°C to form a hollandite-type cesium titanate compound or a mixture of the cesium titanate compound and titanium dioxide, and sintering this.