JPS6227009B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for immobilizing strontium in an aqueous solution. High-level radioactive waste liquid contains strontium, which is a dangerous nuclide, and if left untreated, it poses a risk of pollution. Conventional methods for separating and solidifying strontium from high-level radioactive waste liquid include solidifying it as a component of borosilicate glass together with other nuclides, separating strontium by ion exchange with zeolite, and heating it at 1000 to 1200℃. Two methods are known: heating to convert strontium aluminosilicate into a strontium aluminosilicate compound and fixing it, and solidifying strontium by dissolving strontium in potassium sites by firing with potassium titanate at high temperatures. However, since the borosilicate vitrification method uses nitrates and the like during solidification, it requires a high melting temperature during melting, which can lead to erosion of the crucible material. In addition, the solidified material has poor durability as phase separation and crystallization occur due to aging and accumulation of decay heat, and the leaching rate of strontium from the solidified material is on the order of 10 ^-7 g/cm ² day, which is a drawback. There is. The method using zeolite has the advantage of a low strontium leaching rate of 2 to 3 × 10 ^-9 g/cm ² ·day, but it is chemically inert under hydrothermal conditions. It has the disadvantage of being unstable. In addition, the method using potassium titanate is said to have a lower leaching rate than borosilicate glass, but because the amount of solid solution is small, a large amount of potassium titanate is required, which has the disadvantage of increasing the overall amount of waste. With this method, it is not possible to dissolve only strontium in solid solution. The present invention aims to improve the drawbacks of conventional methods and provides a method for selectively separating strontium and converting it into a chemically stable solid. 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., the fibrous titanium hydrate TiO ₂ Succeeded in producing mH ₂ O (m = 0 to 3) (Patent Application No. 1983-676856
−93460). Further research on the properties of the obtained fibrous titanium hydrate revealed that the fibrous titanium hydrate adsorbs and exchanges strontium ions in an aqueous solution, forming a strontium adsorbent Sr _x O nTiO ₂
mH ₂ O (x = 0.5 to 1, n = 2 to 8, m
= 2 to 8). In addition, when the strontium adsorbent is heated at a temperature of around 1000°C, it becomes a mixture of strontium titanate and titanium dioxide with a perovskite structure, and strontium is fixed at the center of the cube made of eight TiO ₆ octahedrons, making it difficult to leach out. To become a. To obtain a stable solidified body with a low leaching rate despite a large amount of immobilization. Furthermore, the leaching rate of strontium is reduced in the mixed phase of 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 leaching rate of strontium 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 containing strontium. Although it can be obtained by adsorption or ion exchange, fibrous materials are preferable because they have a large adsorption amount and are easy to handle, and fibrous materials having a crystalline layered structure are particularly preferred. Adsorption and ion exchange of strontium in an aqueous solution may be carried out by immersion in the aqueous solution or by passing the strontium aqueous solution through a column filled with an adsorbent. Strontium in an aqueous solution is absorbed by the strontium adsorbent Sr _x O・nTiO ₂・mH ₂ O (where x, n,
m is the same as above). The amount of adsorption and the amount of ion exchange vary depending on the concentration of strontium, hydrogen ion concentration, reaction time, temperature, etc. Further, the values of x, n, and m can be determined by heat-treating the adsorbent at a temperature of around 1000° C. to crystallize it, and identifying the synthesized phase using X-ray powder diffraction. To immobilize strontium, the strontium adsorbent is heat-treated at a temperature of 900 to 1300°C to form a mixture of strontium titanate and titanium dioxide (rutile phase). Next, the mixture is pressure-molded 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, which reduces the volume and reduces the leaching rate of strontium, increasing durability. The result will be a large one. 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, the strontium adsorbent may be directly hot pressed. The strontium adsorption and ion exchange material used in the present invention is a titanate,
TiO immobilizes strontium in a _6- octahedral linkage pattern, thus improving the efficiency of traditional silicate zeolites.
The immobilization is superior compared to that in the SiO ₄ tetrahedral connection mode. In addition, although the radiation decay process generates considerably high temperatures, it is stable even at high temperatures, and is particularly stable under hydrothermal conditions (700℃, 1000 atm hot water), making it an excellent material for stably fixing strontium. have an effect. Example 1 (1) Production of fibrous potassium titanate (i) Melting method 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 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 was a mixed phase fiber of K ₂ Ti ₄ O ₉ and K ₂ Ti ₂ O ₅ . (ii) Flux method TiO ₂ and K ₂ CO ₃ powders in a molar ratio of 1:0.33
The composition {(K ₂ O) _1/3 ·TiO ₂ } was mixed with K ₂ MoO ₄ powder in a molar ratio of 30:70. Approximately 80 g of the mixture was filled into a approximately 100 ml platinum crucible, heated and melted at 1130°C for 4 hours, and then slowly cooled to 950°C at a rate of 8°C/h.
After heating and melting again at 1130℃ for 4 hours, 8℃/
Fibers were grown by slow cooling to 950°C at a rate of 1 h and repeated growth reactions. The obtained fibrous crystalline material was taken out from the crucible by dissolving the flux with water. The fibers were aggregates of bundle-like single crystals with an average length of 2 mm and a diameter of 0.05 to 0.1 mm, and were composed of a single K ₂ Ti ₄ O ₉ phase. In addition, by using Na ₂ CO ₃ instead of the fiber composition {(K ₂ O) _1/3・TiO ₂ }, the fiber composition {(Na ₂ O) _1/3・TiO ₂ }
However, the actual product was K ₂ Ti ₄ O ₉ fibers, and relatively long fibers were obtained without any particular adverse effects. Further, when a large amount of Na ₂ CO ₃ component is added, a mixed phase of potassium hexatitanate (K ₂ Ti ₆ O ₁₃ ), rutile (TiO ₂ ), etc. is formed. (2) Production of fibrous titania hydrate (i) Melt-processed fiber The fiber obtained by the method (1)-(i) above is
Approximately 1 at a ratio of 10g to 100ml of HCl aqueous solution
immersed in the aqueous solution for an hour to extract the _K2O component;
The titanium hydrate was obtained by washing with water and air drying. The X-ray powder diffraction pattern of the titanium hydrate using a copper anticathode showed that it was a crystalline fiber showing broad peaks around 2θ=10°, 25.6°, and 48.6°. (ii) Flux method fiber The fiber obtained by the method (1)-(ii) above is
Approximately 10g at a ratio of 10g to 100ml of HCl aqueous solution
immersed in the aqueous solution for an hour to extract the _K2O component;
The titanium hydrate was obtained by washing with water and air drying. The X-ray powder diffraction pattern of the titania hydrate is 2θ=
The strongest peak was at 9.8°, 17.8°, 24.3
It shows relatively weak but sharp diffraction peaks at angles such as ゜, 27.3゜, 30.0゜, 33.7゜, and 37.5゜. (3) Adsorption and ion exchange of strontium in aqueous solution (i) 2 g per 1 saturated aqueous solution of 0.04MSrOH
The titania hydrate fibers produced by the method (2)-(i) above were soaked for 6 days without stirring, and then filtered and air-dried. As a result of chemical analysis of this adsorbent,
_{The composition of Sr0.9Ti4.0O8.9}・_{6.7H2O is} shown _. The exchange capacity for strontium of this composition is
It is 4.5meq/g. This powder X-ray diffraction diagram shows 2θ=25.2°,
Broad peaks were observed around 25.6°, 36.4°, and 44.0°. The product that was heat-treated at 1000℃ for 30 minutes was identified from the powder X-ray diffraction pattern as strontium titanate SrTiO ₃ and rutile.
It became a mixed phase of TiO ₂ . This follows the reaction: SrTi ₄ O ₉ →SrTiO ₃ +3TiO ₂ (ii) The titanium hydrate fiber produced by the method (2)-(ii) above is soaked in the same manner as in (3)-(i) above,
Air-dried. Results of chemical analysis of this adsorbent
_{The composition is Sr 1.0 Ti 6.0 O 13.0 ・}6.7H ₂ O. The exchange capacity for strontium of this composition is
It is 3.8meq/g. This powder X-ray diffraction diagram shows a sharp strongest line peak at 2θ = 8.1°, as well as 15°, 16.1°, 23.9°, 26.6°,
Broad peaks were observed around 30°, 34°, and 41°. When the adsorbent was heat-treated at 1000° C. for 30 minutes, a mixed phase of strontium titanate SrTiO ₃ and rutile TiO ₂ was identified from powder X-ray diffraction. This follows the reaction: SrTi ₆ O ₁₃ →SrTiO ₃ +5TiO ₂ (4) Molding and immobilization of strontium titanate Two types of strontium adsorbents obtained in (3) (SrTi ₄ O ₉・6.7H ₂ O, SrTi ₆ O ₁₃・6.7H ₂ O) were calcined at 1000°C for 1 hour to crystallize them into strontium titanate and titanium dioxide. 0.25g of these two-phase mixture under a pressure of 500Kg/ ^cm2 ,
After molding into a belet shape with a diameter of 1.3 cm and a thickness of 0.1 cm, it was fired at 1000°C for 15 hours. Its specific surface area was measured by nitrogen gas adsorption method and was found to be 11.8×
It was 10 ⁴ cm ² /g. (5) Leaching of strontium in pure water The solidified material obtained by the method (4) above was immersed in 50 ml of distilled water, and while stirring, changes in the amount of strontium leached over time were examined. The amount of strontium leached out after immersion for 24 hours was measured by atomic absorption spectrometry and the results are shown in Table 1 below.

[Table] 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 strontium (g), S: Surface area of sample (cm ² ),
t: Leaching time (h) The detection limit of strontium is (4×
0 ^-7 mol/l, which was below this detection limit value. Example 2 Leaching of strontium from a non-sintered body Mixed phase powder of strontium titanate and titanium dioxide obtained by firing each of the two types of strontium adsorbents obtained in (3) of Example 1 at 1000°C for 1 hour. The raw leaching rate was measured. SrTi ₄ O ₉・6.7H ₂ O composition, 0.162g, SrTi ₆ O ₁₃・
Using 0.270g of 6.7H ₂ O composition, Example 1
The amount of strontium leached into pure water was determined by immersion in pure water for 24 hours using method (5). The results were as shown in Table 2 below.

[Table] As a result, the leaching rate is higher than that of the molded sintered immobilized body (Table 1). Therefore, it is better to use a shaped sintered body for immobilization. Note that the specific surface area of the sample was calculated as 11.8×10 ⁴ cm ² /g.

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 = 2 to 4) (where m = 0 to 3)
By adsorbing and ion-exchanging strontium in an aqueous solution, a strontium adsorbent Sr _x O.
nTiO ₂ mH ₂ O (x=0.5~1, n=2~
8, m=2 to 8), and heating the strontium adsorbent to 900 to 1300°C to obtain a mixture of strontium titanate SrTiO ₃ and titanium dioxide. 2 Claim 1, in which the mixture of strontium titanate and titanium dioxide is pressure-molded and sintered.
Strontium immobilization method described in section.