Method for separating radium and coal ash matrix from coal ash

Abstract

To provide a method capable of stably obtaining Ra-226 and recycling a base material after removing the Ra-226.SOLUTION: A separation method comprising: mixing coal ash containing radium with a solution containing nitric acid, hydrochloric acid, or a combination thereof; eluting the radium into the solution; maintaining a matrix of the coal ash as a solid; and separating the radium from the matrix of the coal ash. The method of claim 1, further comprising: SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for separating radium and coal ash matrix from coal ash. [Background technology]

[0002] Pharmaceuticals using alpha-ray emitting nuclides have made great progress, and Ac-225 is considered to be a useful nuclide in the future. Ra-226 would be ideal as a raw material for producing Ac-225 using accelerators, but there is a global shortage of Ra-226, resulting in global competition (Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Robertson AKH, Ramogida CF, Schaffer P, Radchenko V. Development of 225Ac Radiopharmaceuticals: TRIUMF Perspectives and Experiences. Curr Radiopharm. 2018;11(3):156-172 Summary of the Invention [Problem to be solved by the invention]

[0004] Since Ra-226 is a uranium-series nuclide, it would be ideal to extract it from uranium ore that contains a large amount of U-238, but such uranium deposits are unevenly distributed geographically, and because it is a strategic material, it is extremely difficult to obtain outside of Japan. Therefore, a method for obtaining Ra-226 in a stable manner is needed. [Means for solving the problem]

[0005] The gist of the present invention is as follows. (1) A separation method comprising mixing coal ash containing radium with a solution containing nitric acid, hydrochloric acid, or a combination thereof, dissolving the radium into the solution while maintaining the coal ash matrix as a solid, and separating the radium from the coal ash matrix. (2) The separation method according to (1) above, wherein the concentration of the solution is 2 to 6 mol / L. (3) The separation method according to (1) or (2) above, which comprises recovering the separated coal ash matrix by filtration. (4) The separation method according to (3) above, further comprising recycling the recovered coal ash matrix through a coal ash recycling process. (5) The separation method according to any one of (1) to (4) above, which comprises recovering the separated radium by coprecipitating it with manganese dioxide. [Effects of the Invention]

[0006] According to the present invention, Ra-226 can be obtained stably, and the base material can be recycled after the Ra-226 is removed. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a graph showing the separation rates of Ba separated in Examples 1 to 8. [Figure 2] FIG. 2 shows XRD profiles of coal ash before separation treatment in Examples 1 and 5, and XRD profiles of the coal ash matrix after separation treatment. [Figure 3] Figure 3 shows elemental mapping of Ba and Al by synchrotron X-ray fluorescence (XRF) of coal ash produced in Japan. [Figure 4] FIG. 4 shows photographs of the exterior of the prepared coal ash in Example 1, which were taken after the coal ash was separated in a nitric acid solution into Ra, Ba, which behaves similarly to Ra, and a coal ash matrix, and the filtered and dried matrix was placed on filter paper. [Figure 5]FIG. 5 shows photographs of the exterior of the prepared coal ash in Example 5, which were taken after the coal ash was separated in a nitric acid solution into Ra, Ba, which behaves similarly to Ra, and a coal ash matrix, and the filtered and dried matrix was placed on filter paper. DETAILED DESCRIPTION OF THE INVENTION

[0008] The present disclosure is directed to a separation method that separates radium (Ra-226) from the coal ash matrix by mixing coal ash containing radium with a solution containing nitric acid, hydrochloric acid, or a combination thereof, to dissolve the radium into the solution while maintaining the coal ash matrix as a solid.

[0009] According to this method, radium (Ra-226) (hereinafter simply referred to as Ra), which is useful in nuclear medicine and medical treatment, can be easily separated (extracted) from coal ash by dissolving it in a solution containing nitric acid, hydrochloric acid, or a combination thereof (hereinafter also referred to as a separation solution). According to the present invention, the matrix (base material) (hereinafter also referred to as a matrix) of coal ash from which Ra has been separated can be maintained as a solid. Because the matrix can be separated from Ra while remaining solid without dissolving in the separation solution, it can be recycled in the same way as conventional methods.

[0010] Coal is produced in a reducing environment, so uranium (U) transforms from U(VI) to the unnecessary U(IV), which then becomes concentrated. Ra is a uranium-series nuclide, so its concentration in coal is high. Ra is an alkaline earth metal and is refractory, so when coal is burned, it becomes even more concentrated in the coal ash, which is the residue left behind. Coal contains approximately 13-30 Bq / kg of Ra, and the coal ash produced by burning this coal can contain approximately 10 times that amount, or approximately 130-300 Bq / kg of Ra.

[0011] For example, the amount of alpha rays obtained from one target (e.g., 10 mg of Ra) used in cyclotron cancer therapy is 225As a raw material for producing radium (enough for 25 patients with Ac), 370MBq of Ra is required. For example, if coal ash contains approximately 300Bq / kg of Ra and the Ra recovery rate is 50%, it is calculated that approximately 2,500 tons of coal ash would be sufficient for one target. The amount of coal ash produced in Japan in 2021 is 12 million tons / year, which is equivalent to approximately 48g of Ra, so there is a sufficient amount of coal ash available. In this way, the amount of Ra that can be contained in coal ash is extremely large compared to the 5Bq / kg of Ra contained in radium hot spring water. Furthermore, since targets can be used repeatedly in accelerators, etc., repeated irradiation would actually enable irradiation of more patients. 225 Ac can be provided.

[0012] This method for separating Ra and matrix from coal ash also has the following advantages. Coal ash is a waste product of coal-fired power plants and can be readily obtained in large quantities for recycling, making it possible to continue using it as a Ra resource for a long period of time. Coal ash is available as a powder, making it easy to separate and process Ra. Because primary production of Ra is possible, a steady supply of Ra can be secured without being affected by diplomatic relations with other countries. Currently, 1 mg of Ra is said to be worth 100 million to 1 billion yen, making it possible to add extremely high added value to coal ash, which is a waste product. Waste recycling can promote pharmaceutical development, thereby making it possible to save lives.

[0013] Furthermore, this method achieves a high separation rate (extraction rate) of Ra from coal ash, and the matrix after Ra separation can be maintained as a solid without undergoing structural changes, making it possible to carry out conventional coal ash recycling processes.

[0014] According to this method, it is possible to separate Ra from coal ash at a rate of preferably 20% or more, more preferably 30%, even more preferably 40% or more, even more preferably 50% or more, and even more preferably 60% or more.

[0015] The quantitative determination and separation rate (extraction rate) of Ra can be performed using conventional alpha ray counting methods or methods for quantifying daughter nuclides produced from Ra. However, alpha ray counting is difficult, and quantification of daughter nuclides takes a relatively long time until radioactive equilibrium is reached. Ra can also be quantitatively analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Alternatively, because the amounts of Ba and Ra in coal ash behave essentially the same, the separation rate can be measured using Ba, which is an alkaline earth element with similar chemical properties, as a substitute for Ra. The separation rates (extraction rates) of Ba and Ra obtained by quantitative analysis using X-ray fluorescence analysis (XRF), inductively coupled plasma mass spectrometry (ICP-MS), etc., are essentially the same for the two elements.

[0016] In this method, the separation solution used to dissolve Ra from the coal ash and separate Ra from the matrix contains nitric acid, hydrochloric acid, or a combination thereof, and preferably contains nitric acid.

[0017] The separation solution does not contain strong acids such as hydrofluoric acid or aqua regia. As a result, after Ra is eluted from the coal ash and the Ra and matrix (base material) are separated, the matrix can maintain its crystalline structure before the separation process, and the matrix is ​​not substantially decomposed. This means that Ra is eluted relatively selectively from the coal ash, and the separation solution into which Ra has been eluted contains few impurities other than Ra. Since the separation solution contains few impurities other than Ra, it is also possible to reduce the amount of waste liquid after Ra recovery.

[0018] In this method, the separation solution is a relatively easy-to-handle inorganic acid and does not contain strong acids such as hydrofluoric acid or aqua regia. Therefore, the container for the separation solution can be a tank or glass container with an inner lining made of a common material such as polypropylene. This eliminates the need for special containers or operating equipment, reducing the burden on the work and the environment. The separation solution also does not need to contain hydrogen peroxide or perchloric acid. If coal ash contains organic matter derived from incompletely burned coal, the hydrogen peroxide or perchloric acid reacts with the organic matter, generating gas or causing a violent reaction that could lead to an explosion. However, the absence of hydrogen peroxide or perchloric acid in the separation solution reduces the burden on the equipment and work.

[0019] The concentration of the separation solution is not particularly limited as long as it is a concentration that can elute Ra from the coal ash and separate Ra from the matrix, but it can be a relatively low concentration, preferably 1 to 10 mol / L, more preferably 2 to 9 mol / L, even more preferably 3 to 8 mol / L, and even more preferably 4 to 7 mol / L, for example, 3 to 7 mol / L or 2 to 6 mol / L. By using a separation solution with the above-mentioned preferred concentration, the burden on the work and the environment can be further reduced.

[0020] The temperature of the separation solution used to separate Ra from the matrix by eluting Ra from the coal ash (hereinafter also referred to as separation treatment) is not particularly limited, and the elution may be carried out at room temperature for, for example, 1 to 9 hours, 2 to 8 hours, or 3 to 7 hours. To increase the elution rate of Ra from the coal ash, the separation solution may be heated to approximately 30 to 100°C, 40 to 90°C, or 50 to 80°C.

[0021] The amount of the separation solution may be at least an amount that can elute Ra, and may be, for example, 200 L or more per 100 kg of coal ash.

[0022] The separation process can be carried out by a batch method or a column method. The batch method involves placing coal ash in a separation solution in a container and eluting Ra into the separation solution, and the coal ash may be added to the separation solution in the container. The column method is a method in which the separation solution is poured into coal ash packed in a column. The batch method is simpler than the column method, but the column method allows the filtration operation to be omitted.

[0023] In this method, Ra and the matrix are separated from coal ash because Ra is selectively eluted from the coal ash into the separation solution. The selective elution of Ra from coal ash is thought to be due to the presence of Ra in a phase separate from the matrix.

[0024] Figure 3 shows elemental mapping of Ba and Al by synchrotron X-ray fluorescence (XRF) of coal ash, a waste product generated at thermal power plants. Ra is a radioactive element with an extremely low molar concentration, making mapping by X-ray fluorescence measurement difficult. Therefore, the distribution of Ba, which is also an alkaline earth metal and has similar chemical behavior, was measured instead. The area enclosed by a square indicates a high concentration of Ba. The distribution of Al in coal ash matches the distribution of elements that make up the main components of coal ash, such as Si and O, but the distribution of Ba does not match the distribution of Al. Because Ba and Ra exist in a phase separate from the main component phase of the coal ash matrix, they are thought to be selectively eluted from coal ash even in a separation solution with a relatively low concentration.

[0025] Conventionally, coal ash has had to undergo some kind of final treatment, such as recycling. According to this method, the crystalline structure of the coal ash matrix is ​​maintained even after separation, so that coal ash can be used as is after Ra separation in conventional coal ash recycling. Coal ash is recycled, for example, in road construction work, and because the crystalline structure of the matrix before separation is maintained even after separation, and there is no substantial change in the physical properties of the coal ash, it can be recycled as is.

[0026] The maintenance of the crystalline structure of the coal ash matrix before and after the separation treatment can be analyzed by powder X-ray diffraction (XRD). If there is no change in the main quartz (Qz) peak and the mullite (Mu) peak in the powder X-ray diffraction pattern of the coal ash before and after the separation treatment, it can be determined that the crystalline structure is maintained. "No change in the peaks" means that there is substantially no change in the peak position and peak half-width. "No substantial change in the peak position and peak half-width" preferably means that the difference in peak position is within 0.1° (2θ) and the difference in half-width is within 0.1° (2θ). The wollastonite (WO) peak at 29.2° (2θ) disappears after the separation treatment, but the WO peak is derived from calcium, which is an alkaline earth element like Ra and is not included in the matrix components.

[0027] The matrix after the separation treatment (residue after Ra separation) can be filtered and dried, crushed as needed, and used directly in a conventional recycling process. The recycling process may include, for example, storage in a coal ash silo, material mixing, curing, pressure vibration molding, heated and humidified curing, or a combination thereof. The matrix after the separation treatment may be optionally subjected to the removal of residual nitrate ions (NO3 - ), hydrochloride (Cl - ), or a combination thereof, may be subjected to a neutralization treatment.

[0028] The composition of the coal ash used in this method (hereinafter simply referred to as coal ash) is not particularly limited as long as it contains Ra, and may contain, for example, Si, Ti, Al, and Fe in addition to Ra. Si, Ti, Al, and Fe may be contained in the coal ash as silicon oxide, titanium oxide, aluminum oxide, and iron oxide. The coal ash may also contain Ca, Mg, Mn, Na, K, and P. Ca, Mg, Mn, Na, K, and P may be contained in the coal ash as calcium oxide, magnesium oxide, manganese oxide, sodium oxide, potassium oxide, and phosphorus oxide. The coal ash may also contain elements such as Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Y, Sr, Zr, Nb, Ba, Pb, La, Ce, and Th.

[0029] It is preferable that coal ash contains a large amount of Ra, but this method can be applied to any coal ash that contains Ra. The coal ash preferably contains 200 Bq / kg or more, more preferably 300 Bq / kg or more of Ra.

[0030] Si may be contained in the coal ash in an SiO2 equivalent amount of, for example, 30 to 80 mass% or 40 to 70 mass%. Ti may be contained in the coal ash in an TiO2 equivalent amount of, for example, 0.1 to 5 mass% or 0.5 to 2 mass%. Al may be contained in the coal ash in an Al2O3 equivalent amount of, for example, 10 to 50 mass% or 20 to 35 mass%. Fe may be contained in the coal ash in an Fe2O3 equivalent amount of, for example, 0.5 to 20 mass% or 1 to 15 mass%.

[0031] Ca may be contained in the coal ash in an amount of, for example, 0.1 to 20 mass% or 0.5 to 10 mass% converted into CaO. Mg may be contained in the coal ash in an amount of, for example, 0.1 to 5 mass% or 0.2 to 5 mass% converted into MgO. Mn may be contained in the coal ash in an amount of, for example, 0.001 to 1 mass% or 0.01 to 0.1 mass% converted into MnO. Na may be contained in the coal ash in an amount of, for example, 0.001 to 5 mass% or 0.002 to 3 mass% converted into NaO. K may be contained in the coal ash in an amount of, for example, 0.1 to 5 mass% or 0.3 to 2 mass% converted into KO.

[0032] Ra dissolved in the separation solution during the separation process can be recovered using the Mn coprecipitation method. The solid matrix can be recovered by filtration. Ra can be coprecipitated by mixing manganese dioxide with the separation solution in which Ra is dissolved in a container. Ra can also be coprecipitated by passing the separation solution in which Ra is dissolved through manganese dioxide, such as manganese fiber, in a column. The Mn coprecipitation method allows Ra to be coprecipitated by mixing manganese dioxide with the separation solution in which Ra has been dissolved, from which the matrix has been recovered by filtration, and enables large-scale concentration of Ra and a significant scale-down of the amount of liquid handled after concentration.

[0033] Manganese dioxide is primarily MnO2(IV).

[0034] The amount of manganese dioxide added to co-precipitate Ra need only be an amount that will co-precipitate Ra. For example, if 100 kg of coal ash with a Ra content of 200 Bq / kg is separated using 200 L of separation solution, 1 g or more of MnO2 may be used.

[0035] In the Mn coprecipitation method, Ba, which may be contained in coal ash, co-precipitates with Ra, but Ra and Ba can be separated using a resin such as Sr-resin (registered trademark), an ion exchange resin that uses crown ether.

[0036] Manganese dioxide coprecipitated with Ra can be dissolved in a mixed solution of hydrogen peroxide and nitric acid, and then Ra can be recovered using an ion exchange resin. The mixed solution of hydrogen peroxide and nitric acid can be any concentration that can dissolve manganese dioxide, for example, a mixed solution of 20 mL of 1 mol / L nitric acid containing 4% hydrogen peroxide.

[0037] The process of recovering Ra using ion exchange resins is, for example, to pass a mixed solution of Ra and co-precipitated manganese dioxide through a column packed with DGA resin (manufactured by Eichrom) to separate the rare earth elements (REEs). 3+ Ra can be recovered by removing the alkali metals and the like through a column packed with DOWEX 50x8 (manufactured by Bio-Rad, 200-400 mesh), removing Ba through a column packed with Sr-resin (registered trademark) (manufactured by Eichrom). [Example]

[0038] Example 1 50 mL of a 7 mol / L nitric acid solution was prepared as a separation solution. 100 mg of coal ash, a waste material generated at a thermal power plant with a Ra content of 170 Bq / kg and the composition (including Ba) shown in Tables 1 to 3, was prepared. The coal ash was added to the prepared 7 mol / L nitric acid solution, mixed, and left to stand for 60 minutes to separate the nitric acid solution from the matrix (residue). The Ba content in the nitric acid solution and the crystalline structure of the matrix were analyzed. Figure 4 shows photographs of the prepared coal ash and the matrix (residue) after treatment with the nitric acid solution, separated by filtration, and then the dried matrix placed on filter paper.

[0039] The composition of the coal ash was measured using XRF (Rigaku ZSX Primus II) with a glass bead method. Table 1 shows the mass of oxides for high-concentration components, and the elemental amounts (ppm) for trace components. In this specification, the unit "ppm" means "mg / kg." The total is slightly less than 100% by mass due to the volatilization of volatile substances such as sulfur (sulfate group) and carbon (carbonate group) during the high-temperature bead preparation, resulting in loss on ignition. The amount of barium in nitric acid solution, which behaves similarly to Ra, was measured using an ICP (Agilent 7700). The barium separation rate (extraction rate) was calculated from the ratio of the amount of barium in the nitric acid solution after separation to the amount of barium in the coal ash before separation. The crystal structure of the coal ash and the matrix after separation was measured using an XRD (Rigaku RINT-ULTIMA-2100). The XRD profile of the matrix after the separation treatment was measured for the matrix recovered from the nitric acid solution by filtration and drying.

[0040] Example 2 The Ra and the matrix were separated in the same manner as in Example 1, except that a 2 mol / L nitric acid solution was used as the separation solution, and the Ba content in the separation solution and the crystalline structure of the matrix were analyzed.

[0041] Example 3 The Ra and the matrix were separated in the same manner as in Example 1, except that a 7 mol / L hydrochloric acid solution was used as the separation solution, and the Ba content in the separation solution and the crystalline structure of the matrix were analyzed.

[0042] Example 4 The Ra and the matrix were separated in the same manner as in Example 1, except that a 2 mol / L hydrochloric acid solution was used as the separation solution, and the Ba content in the separation solution and the crystalline structure of the matrix were analyzed.

[0043] Example 5 The solution containing Ra and Ba and the matrix were separated, and the Ba content and the crystalline structure of the matrix were analyzed in the same manner as in Example 1, except that JCFA-1, which has a Ra content of 138 Bq / kg and the composition shown in Tables 1 to 3, was used instead of ash. Figure 5 shows photographs of the appearance of the prepared coal ash and the matrix (residue) that was filtered and dried after the coal ash was separated into Ra and Ba, which behaves similarly to Ra, in a nitric acid solution, and then placed on filter paper.

[0044] (Examples 6 to 8) Ra and Ba, which behaves similarly to Ra, were separated from the matrix in the same manner as in Examples 2 to 4, except that JCFA-1 (soil and sediment standard sample, coal fly ash, manufactured by the National Institute of Advanced Industrial Science and Technology, National Research and Development Agency) having the composition shown in Tables 1 to 3 was used instead of Ash. The Ba content in the separated solution and the crystalline structure of the matrix were then analyzed.

[0045] [Table 1] [Table 2] [Table 3]

[0046] Table 4 shows the Ba separation rates in Examples 1 to 8. Figure 1 shows a graph of the Ba separation rates in Examples 1 to 8. [Table 4]

[0047] 2 shows the XRD profiles of the coal ash before separation treatment and the powder X-ray diffraction patterns of the matrix after separation treatment in Examples 1 and 5. In both the powder X-ray diffraction patterns of Ash used in Example 1 and JCFA-1 used in Example 5, the main peak of quartz (Qz) at 26.7° (2θ) and the peak positions and peak half-widths of mullite (Mu) at 15.4° (2θ) and 35.2° (2θ) were essentially unchanged before and after separation treatment.

Claims

1. A separation method comprising mixing coal ash containing radium with a solution containing nitric acid, hydrochloric acid, or a combination thereof, dissolving the radium into the solution while maintaining the coal ash matrix as a solid, and separating the radium from the coal ash matrix.

2. 2. The separation method according to claim 1, wherein the concentration of the solution is 2 to 6 mol / L.

3. 10. The method of claim 1, further comprising recovering the separated coal ash matrix by filtration.

4. 4. The method of claim 3, further comprising recycling the recovered coal ash matrix through a coal ash recycling process.

5. 5. The method according to claim 1, further comprising recovering the separated radium by coprecipitation with manganese dioxide.

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