Method for removing radioactive element thorium in rare earth mineral

Abstract

The present invention relates to a method for removing radioactive element thorium in a rare earth mineral, comprising: mixing the rare earth mineral with selenium dioxide in water, reacting radioactive element thorium with selenium dioxide by hydrothermal method, cooling to form a crystal, and separating the crystal to remove the radioactive element thorium. In the invention, tetravalent element thorium is selectively bound to inorganic ligand selenium dioxide in a hydrothermal environment to form a crystal, thereby achieving removal of radioactive element thorium. The method has high crystallization rate and high decontamination efficiency, and removes thorium from trivalent lanthanide element by crystallization solidification under a uniform reaction condition. Compared to a conventional industrial method for thorium separation, the method has low energy consumption and high separation ratio, enables one-step solidification separation, and effectively avoids the disadvantages of redundant separation operations and a large amount of organic and radioactive liquid wastes.

Claims

1. A method for removing radioactive element thorium in monazite, comprising steps of: mixing the monazite with selenium dioxide in water, reacting the radioactive element thorium in the monazite with selenium dioxide by a hydrothermal method, cooling a resulting solution to form a crystal, and separating the crystal to remove the radioactive element thorium.

2. The method as claimed in claim 1, wherein the hydrothermal method includes performing the reaction at 200-230 C. for 1-3 days.

3. The method as claimed in claim 1, wherein the resulting solution is cooled to room temperature after the reaction is completed.

4. The method as claimed in claim 2, wherein the cooling rate is 4-10 C./h.

5. The method as claimed in claim 1, further comprising a step of washing the crystal after separating the crystal.

6. The method as claimed in claim 5, wherein the crystal is washed with water, alcohol and dilute nitric acid.

7. The method as claimed in claim 6, wherein the dilute nitric acid has a mass concentration of 5-10%.

8. The method as claimed in claim 1, wherein the crystal is a needle-like and flocculent crystal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of crystals generated in the method of the present invention;

(2) FIG. 2 is a powder diffraction pattern of the resulting crystals according to embodiment 1 of the present invention;

(3) FIG. 3 is a powder diffraction pattern of the resulting crystals according to embodiment 2 of the present invention;

(4) FIG. 4 is a powder diffraction pattern of the resulting crystals according to embodiment 3 of the present invention;

(5) FIG. 5 is a powder diffraction pattern of the resulting crystals according to embodiment 4 of the present invention;

(6) FIG. 6 is a powder diffraction pattern of the resulting crystals according to embodiment 5 of the present invention;

(7) FIG. 7 is a powder diffraction pattern of the resulting crystals according to embodiment 6 of the present invention;

(8) FIG. 8 is a powder diffraction pattern of the resulting crystals according to embodiment 7 of the present invention; and

(9) FIG. 9 is a schematic diagram of the separation principle of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) The invention will be further illustrated in more detail with reference to the accompanying drawings and embodiments. It is noted that, the following embodiments only are intended for purposes of illustration, but are not intended to limit the scope of the present invention.

Embodiment 1

(11) Separation of Binary Lanthanide and Actinide Elements

(12) In this example, crystals of rare earth elements (Th(NO.sub.3).sub.4.6H.sub.2O and La(NO.sub.3).sub.3.6H.sub.2O) were substituted for a rare earth mineral, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(13) 0.1 mmol Th(NO.sub.3).sub.4.6H.sub.2O solid, 0.1 mmol La(NO.sub.3).sub.3.6H.sub.2O solid, and 0.2 mmol SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C. and heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature (20-30 C.) to give a crystal product. The crystal product was washed with deionized water and the washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(14) Following the forgoing method, the amount of SeO.sub.2 was adjusted to 0.4 mmol, 0.6 mmol, 0.8 mmol and 1.0 mmol respectively, other conditions were kept unchanged, and the same reaction was performed to give crystal products.

(15) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 2. In the FIG. 2, a curve (6) is a spectrum of a powder structure simulated by software for a single tested crystal structure (in the FIG. 2, Ln(SeO.sub.3).sub.2 simulated represents the simulation results of Ce(SeO.sub.3).sub.2 crystal, and Structure type2 indicates one of the most important characteristic peaks of each curve. FIGS. 3-8 are similar to this figure). Curves 1-5 are powder diffraction spectra measured for crystals actually obtained after the reaction with different concentrations of SeO.sub.2. In the figures, their main characteristic peaks are the same, namely, peaks of each curve are the same as those of the structure type of Ce(SeO.sub.3).sub.2 crystal, and this indicates that they have the crystal type of the same structure.

(16) The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 1.

(17) TABLE-US-00001 TABLE 1 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th/La R.sub.Th/La factor (Th) Sample 1 78.2 0.017 4656.9 98.95% Sample 2 17.4 8.7E5 200837.0 ~100% Sample 3 8.91 8.7E5 101993.2 ~100% Sample 4 6.67 9.7E5 69168.5 ~100% Sample 5 10.2 8.2E5 123933.7 ~100%

(18) In table 1, samples 1-5 represent the products for SeO.sub.2 in an amount of 0.2 mmol, 0.4 mmol, 0.6 mmol, 0.8 mmol and 1.0 mmol respectively. The recovery rate is a recovery rate of Th.

Embodiment 2

(19) Separation of Binary Lanthanide and Actinide Elements

(20) In this example, crystals of rare earth elements (Th(NO.sub.3).sub.4.6H.sub.2O and Eu(NO.sub.3).sub.3.6H.sub.2O) were substituted for a rare earth mineral, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(21) 0.1 mmol Th(NO.sub.3).sub.4.6H.sub.2O solid, 0.1 mmol Eu(NO.sub.3).sub.3.6H.sub.2O solid, and 0.2 mmol SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C. and heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature (20-30 C.) to give a crystal product. The crystal product was washed with deionized water and a washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(22) Following the forgoing method, the amount of SeO.sub.2 was adjusted to 0.4 mmol, 0.6 mmol, 0.8 mmol and 1.0 mmol respectively, other conditions were kept unchanged, and the same reaction was performed, to give crystal products.

(23) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 3. It is shown that the doping reaction generates only one structure type of crystals. The crystals have the same structure as that of Ce(SeO.sub.3).sub.2 crystal. The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 2.

(24) TABLE-US-00002 TABLE 2 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th/Eu R.sub.Th/Eu factor (Th) Sample 1 70.1 0.074 949.4 95.53% Sample 2 6.04 3.4E4 17559.6 99.98% Sample 3 4.68 3.5E4 13530.3 99.98% Sample 4 4.18 3.2E4 13112.2 99.99% Sample 5 2.69 3.8E4 7028.5 99.99%

(25) In table 2, samples 1-5 represent the products for SeO.sub.2 in an amount of 0.2 mmol, 0.4 mmol, 0.6 mmol, 0.8 mmol and 1.0 mmol respectively.

Embodiment 3

(26) Separation of Binary Lanthanide and Actinide Elements

(27) In this example, crystals of rare earth elements (Th(NO.sub.3).sub.4.6H.sub.2O and Yb(NO.sub.3).sub.3.6H.sub.2O) were substituted for a rare earth mineral, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(28) 0.1 mmol Th(NO.sub.3).sub.4.6H.sub.2O solid, 0.1 mmol Yb(NO.sub.3).sub.3.6H.sub.2O solid, and 0.2 mmol SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C. and heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature (20-30 C.) to give a crystal product. The crystal product was washed with deionized water and a washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(29) Following the forgoing method, the amount of SeO.sub.2 was adjusted to 0.4 mmol, 0.6 mmol, 0.8 mmol and 1.0 mmol respectively, other conditions were kept unchanged, and the same reaction was performed, to give crystal products.

(30) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 4. It is shown from FIG. 4 that the doping reaction generates only one structure type of crystals. The crystals have the same structure as that of Ce(SeO.sub.3).sub.2 crystal. The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 3.

(31) TABLE-US-00003 TABLE 3 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th/Yb R.sub.Th/Yb factor (Th) Sample 1 76.1 0.025 3038.9 98.13% Sample 2 9.08 9.6E5 94135.3 99.99% Sample 3 4.69 1.9E4 24568.0 99.99% Sample 4 2.70 8.9E4 3044.2 99.96% Sample 5 2.27 0.001 1822.5 99.96%

(32) In table 3, samples 1-5 represent the products for SeO.sub.2 in an amount of 0.2 mmol, 0.4 mmol, 0.6 mmol, 0.8 mmol and 1.0 mmol respectively.

(33) It can be seen from the results of embodiments 1-3 that binary lanthanide-actinide and thorium can be separated by the fractional crystallization method, and the separation ratio in a first separation can reach about 200,000, 170,000 and 90,000 respectively.

Embodiment 4

(34) Separation of Ternary Lanthanide and Actinide Elements

(35) In this example, crystals of rare earth elements (Th(NO.sub.3).sub.4.6H.sub.2O, Ce(NO.sub.3).sub.3.6H.sub.2O, and La(NO.sub.3).sub.3.6H.sub.2O) were substituted for a rare earth mineral, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(36) 0.05 mmol Th(NO.sub.3).sub.4.6H.sub.2O solid, 0.05 mmol Ce(NO.sub.3).sub.3.6H.sub.2O solid, 0.1 mmol La(NO.sub.3).sub.3.6H.sub.2O and SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel in a molar ratio of 1:1:2:4, 1:1:2:8, 1:1:2:12, 1:1:2:16 and 1:1:2:20 respectively, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C., heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature (20-30 C.) to give a crystal product. The crystal product was washed with deionized water and a washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(37) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 5. It is shown from FIG. 5 that the doping reaction generates only one structure type of crystals. The crystals have the same structure as that of Ce(SeO.sub.3).sub.2 crystal. The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 4.

(38) TABLE-US-00004 TABLE 4 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th(Ce)/La R.sub.Th(Ce)/La factor (Th) Sample 1 34.1 0.292 173.4 99.87% Sample 2 29.0 0.066 592.4 99.99% Sample 3 21.5 0.028 1034.6 99.99% Sample 4 16.1 0.021 1007.6 99.97% Sample 5 15.9 0.006 3163.1 99.94%

(39) In table 4, samples 1-5 represent the products after the reaction of Th(NO.sub.3).sub.4.6H.sub.2O solid, Ce(NO.sub.3).sub.3.6H.sub.2O solid, La(NO.sub.3).sub.3.6H.sub.2O, and SeO.sub.2 in a molar ratio of 1:1:2:4, 1:1:2:8, 1:1:2:12, 1:1:2:16 and 1:1:2:20 respectively.

Embodiment 5

(40) Separation of Ternary Lanthanide and Actinide Elements

(41) In this example, crystals of rare earth elements (Th(NO.sub.3).sub.4.6H.sub.2O, Ce(NO.sub.3).sub.3.6H.sub.2O, and Eu(NO.sub.3).sub.3.6H.sub.2O) were substituted for a rare earth mineral, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(42) 0.05 mmol Th(NO.sub.3).sub.4.6H.sub.2O solid, 0.05 mmol Ce(NO.sub.3).sub.3.6H.sub.2O solid, 0.1 mmol Eu(NO.sub.3).sub.3.6H.sub.2O, and SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel in a molar ratio of 1:1:2:4, 1:1:2:8, 1:1:2:12, 1:1:2:16 and 1:1:2:20 respectively, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C., heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature (20-30 C.) to give a crystal product. The crystal product was washed with deionized water and a washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(43) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 6. It is shown from FIG. 6 that the doping reaction generates only one structure type of crystals. The crystals have the same structure as that of Ce(SeO.sub.3).sub.2 crystal. The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 5.

(44) TABLE-US-00005 TABLE 5 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th(Ce)/Eu R.sub.Th(Ce)/Eu factor (Th) Sample 1 20.8 0.430 74.7 99.81% Sample 2 16.7 0.175 133.0 99.91% Sample 3 12.9 0.091 192.2 99.91% Sample 4 12.5 0.032 485.0 99.92% Sample 5 11.4 0.007 2154.4 99.95%

(45) In table 5, samples 1-5 represent the products after the reaction of Th(NO.sub.3).sub.4.6H.sub.2O solid, Ce(NO.sub.3).sub.3.6H.sub.2O solid, Eu(NO.sub.3).sub.3.6H.sub.2O, and SeO.sub.2 in a molar ratio of 1:1:2:4, 1:1:2:8, 1:1:2:12, 1:1:2:16 and 1:1:2:20 respectively.

Embodiment 6

(46) Separation of Ternary Lanthanide and Actinide Elements

(47) In this example, crystals of rare earth elements (Th(NO.sub.3).sub.4.6H.sub.2O, Ce(NO.sub.3).sub.3.6H.sub.2O, and Yb(NO.sub.3).sub.3.6H.sub.2O) were substituted for a rare earth mineral, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(48) 0.05 mmol Th(NO.sub.3).sub.4.6H.sub.2O solid, 0.05 mmol Ce(NO.sub.3).sub.3.6H.sub.2O solid, 0.1 mmol Yb(NO.sub.3).sub.3.6H.sub.2O, and SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel in a molar ratio of 1:1:2:4, 1:1:2:8, 1:1:2:12, 1:1:2:16 and 1:1:2:20 respectively, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C. and heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature (20-30 C.) to give a crystal product. The crystal product was washed with deionized water and a washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(49) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 7. It is shown from FIG. 7 that the doping reaction generates only one structure type of crystals. The crystals have the same structure as that of Ce(SeO.sub.3).sub.2 crystal. The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 6.

(50) TABLE-US-00006 TABLE 6 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th(Ce)/Yb R.sub.Th(Ce)/Yb factor (Th) Sample 1 35.3 0.392 138.6 ~100% Sample 2 23.8 0.157 209.1 ~100% Sample 3 16.3 0.077 285.2 99.99% Sample 4 13.4 0.026 670.6 99.91% Sample 5 13.2 0.006 2779.63 99.97%

(51) In table 6, samples 1-5 represent the products after the reaction of Th(NO.sub.3).sub.4.6H.sub.2O solid, Ce(NO.sub.3).sub.3.6H.sub.2O solid, Yb(NO.sub.3).sub.3.6H.sub.2O and SeO.sub.2 in a molar ratio of 1:1:2:4, 1:1:2:8, 1:1:2:12, 1:1:2:16 and 1:1:2:20 respectively.

Example 7

(52) Separation of Multiple Lanthanide and Actinide Elements

(53) In this example, a rare earth ore, monazite, was simulated with crystals of multiple rare earth elements, to verify the effect of a method for removing radioactive element thorium in a rare earth mineral of the present invention.

(54) 0.2 mmol of a mixture of Ln(NO.sub.3).sub.3.6H.sub.2O (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Y) and Th(NO.sub.3).sub.4.6H.sub.2O (in the mixture: about 20% La element, 43% Ce element, 4.5% Pr element, 16% Nd element, 3% Sm element, 0.1% Eu element, 1.5% Gd element, 0.6% Dy element, 0.2% Er element, 0.1% Yb element, 2.5% Y element, and 810% Th element) and SeO.sub.2 were placed in a 10 mL polytetrafluoroethylene reaction vessel in a ratio of 1:1, 1:2, 1:3, 1:4 and 1:5 respectively, and 2 mL deionized water was added. The resulting mixture was sealed, heated up to a temperature of 230 C. and heated for 3 days, and then cooled at a rate of 8.3 C./h to room temperature. The crystal product was washed with deionized water and a washing solution was collected in a 10 mL centrifuge tube and diluted to volume. The resulting crystal product was washed with ethanol and 5-10% dilute nitric acid, and then allowed to dry at room temperature.

(55) The generated crystals were characterized by a powder diffractometer, as shown in FIG. 8. It is shown from FIG. 8 that the doping reaction generates only one structure type of crystals. The crystals have the same structure as that of Ce(SeO.sub.3).sub.2 crystal. The crystal was dissolved in concentrated nitric acid and then diluted to a low acid concentration. The concentration of each element was determined by ICP-Ms and ICP-OES. The concentration of each element in the washing solution was similarly determined by ICP-Ms and ICP-OES, and then a separation factor and a yield were calculated. The results are shown in Table 7.

(56) TABLE-US-00007 TABLE 7 Separation factor and yield Molar Crystal Washing solution Separation Recovery rate ratio R.sub.Th(Ce)/Ln R.sub.Th(Ce)/Ln factor (Th) Sample 1 13.8 0.420 32.9 99.93% Sample 2 11.1 0.133 83.5 99.98% Sample 3 7.24 0.016 462.7 99.99% Sample 4 7.19 0.005 1541.3 99.98% Sample 5 6.34 0.003 1996.5 99.99%

(57) In table 7, samples 1-5 represents the products after the reaction of the mixture and SeO.sub.2 in a molar ratio of 1:1, 1:2, 1:3, 1:4 and 1:5 respectively.

(58) The results of embodiments 4-7 indicate that the separation ratio of trivalent elements and tetravalent elements can reach about 1800 in the separation system of ternary and multi-component lanthanide and actinide elements.

(59) The above description is only preferred embodiments of the present invention and not intended to limit the present invention, it should be noted that those of ordinary skill in the art can further make various modifications and variations without departing from the technical principles of the present invention, and these modifications and variations also should be considered to be within the scope of protection of the present invention.