A method for recovering thorium and rare earth elements from rare earth waste.

Abstract

This invention discloses a method for recovering thorium and rare earth elements from rare earth waste residue. The method includes: (S1) roasting the rare earth waste residue for the first time, mixing it with an inorganic acid for a second roasting, and then leaching it to obtain a feed solution containing thorium and rare earth elements; (S2) extracting the feed solution containing thorium and rare earth elements using an organic phase containing an extractant; (S3) washing the organic phase obtained after extraction in step (S2) with a washing liquid to allow rare earth elements to enter the aqueous phase while thorium remains in the organic phase; (S4) back-extracting the thorium-containing organic phase obtained in step (S3) with a back-extraction liquid to recover thorium; wherein, in step (S3), the washing liquid is NH4HCO3. This invention, through 10-stage cascade extraction, achieves a thorium content of less than 0.5 mg / L in the aqueous phase outlet; the final rare earth recovery rate is over 95%, and the cerium loss is at most 3%.

Description

Technical Field

[0001] This invention belongs to the field of rare earth extraction and separation technology, specifically, it relates to a method for recovering thorium and rare earth elements from rare earth waste residue. Background Technology

[0002] The research group of Professor Wu Wenyuan at Northeastern University studied the valence state changes of Ce during calcination at different temperatures. Using methods such as TG and DTA, they found that at certain temperatures, some Ce(III) is converted to Ce(IV) during calcination. Industrially, oxidative calcination or the addition of an oxidant after calcination can also be used to convert Ce(III) to Ce(IV) as completely as possible, facilitating Ce(IV) separation. However, some cerium is lost in the waste residue generated in industrial processes, and the selective extraction of Th(IV) and Ce(IV) by common extractants is not significant. Therefore, Th(IV) and Ce(IV) are usually completely leached out and then separated, with selective back-extraction performed on Th(IV) and Ce(IV). For example, a reducing back-extraction method is used, employing P507 to extract and separate cerium, thorium, and fluorine, followed by back-extraction of Ce(IV) with hydrogen peroxide and hydrochloric acid; after recovering cerium oxide, Th(IV) is back-extracted with sulfuric acid to finally obtain ThO2. Cerium and thorium can be separated by using different back-extraction agents. The Beijing Fangzheng Rare Earth Technology Research Institute proposed a method to separate Ce(IV) by oxidizing and roasting bastnaesite, then mixing it with a coordination precipitant for solid-liquid separation. The precipitate is then calcined to obtain cerium oxide, while thorium remains in the filtrate. This method is easy to operate, effective, and low-cost. Currently, there are two main extraction separation methods: first, utilizing the different extraction capacities of organic phases for Ce(IV) and Ce(III), using a reducing agent to reduce Ce(IV) to Ce(III) and back-extract it from the organic phase; second, utilizing the fact that CeO2 is less soluble in dilute acids than other rare earth elements for separation. However, the separation efficiency of thorium and rare earth elements in existing technologies remains low. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a method for recovering thorium and rare earth elements from rare earth waste. The method involves calcining the rare earth waste with calcium hydroxide, followed by roasting and leaching with an inorganic acid (e.g., ammonium sulfate), and then using an ionic liquid for thorium removal via a cascade process to separate thorium and rare earth elements. This method eliminates the need for acid back-extraction, reducing acid and alkali consumption during thorium separation. Furthermore, the use of the detergent NH4HCO3 efficiently separates thorium and cerium, reducing the separation process, increasing the thorium back-extraction rate, and minimizing cerium loss.

[0004] The technical solution of the present invention is as follows:

[0005] A method for recovering thorium and rare earth elements from rare earth waste, the method comprising the following steps:

[0006] (S1) After the rare earth waste residue is roasted for the first time, it is mixed with inorganic acid for a second roasting, and then heated and leached to obtain a raw material liquid containing thorium and rare earth elements.

[0007] (S2) Extraction is performed using an organic feed solution containing extractant and containing thorium and rare earth elements;

[0008] (S3) The organic phase obtained after extraction in step (S2) is washed with a washing solution so that rare earth elements enter the aqueous phase and thorium remains in the organic phase.

[0009] (S4) The thorium-containing organic phase obtained in step (S3) is back-extracted using a back-extraction solution to recover thorium;

[0010] In step (S3), the washing solution is NH4HCO3.

[0011] In this invention, the organic phase containing the extractant refers to kerosene containing the extractant.

[0012] In this invention, the raw material liquid containing thorium and rare earth elements is the rare earth waste residue leachate.

[0013] According to the present invention, in step (S1), the rare earth element is selected from at least one of La, Ce, Pr, Nd, Pm, Sm, Eu, Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0014] According to the present invention, in step (S1), the raw material liquid further includes at least one of Al, Ca, Mg and Fe.

[0015] According to the present invention, in step (S1), the rare earth waste residue can be mixed with calcium hydroxide during the first roasting to stabilize the fluorine in the rare earth waste residue. Preferably, the mass of calcium hydroxide is 15-30 wt% of the rare earth waste residue, for example, 25 wt%.

[0016] According to the present invention, in step (S1), the temperature of the first roasting is 800-1000°C, for example 900°C; the time of the first roasting is 3-6 hours, for example 4 hours.

[0017] According to the present invention, in step (S1), the inorganic acid is selected from at least one of hydrochloric acid, ammonium chloride, and ammonium sulfate; preferably, the concentration of the inorganic acid is 2-7 mol / L, exemplary is 6 mol / L, and is preferably ammonium sulfate.

[0018] According to the present invention, in step (S1), the temperature of the second calcination is 350-600°C, for example 500°C; the time of the second calcination is 3-7 hours, for example 5 hours.

[0019] According to the present invention, in step (S1), water may be added during the heating leaching process; the water content is not particularly limited, as long as it is sufficient to fully dissolve the roasted product.

[0020] According to the present invention, in step (S1), the temperature of the heating leaching is 70-110°C, exemplarily 90°C; the heating leaching time is 1-4 hours, for example 2 hours.

[0021] This invention uses ammonium sulfate as a leaching additive to efficiently combine with rare earth waste residue to prepare a raw material solution containing thorium and rare earth elements, which can reduce the radioactivity of rare earth waste residue.

[0022] According to the present invention, in step (S1), the thorium leaching rate in the raw material solution containing thorium and rare earth elements can reach 50%.

[0023] According to the present invention, in step (S1), the mass ratio of rare earth waste residue to inorganic acid is 1:(1-10), preferably 1:(1-6).

[0024] According to the present invention, in step (S2), the volume ratio of the feed liquid containing thorium and rare earth elements to the organic phase is 1:(2-5), for example 1:3.

[0025] According to the present invention, in step (S2), the extractant is an ionic liquid [N 1888 [DEAO], its structural formula is shown below;

[0026]

[0027] According to the present invention, in step (S2), the concentration of the extractant is 0.01-0.1 mol / L, preferably 0.02-0.05 mol / L, and exemplaryly 0.02-0.042 mol / L.

[0028] According to the present invention, in step (S2), the extraction temperature is 20-40°C, preferably 25-30°C; the extraction time is 5-30 min, preferably 10-20 min.

[0029] According to the present invention, in step (S3), after mixing the washing liquid with the organic phase obtained after extraction in step (S2), the pH of the system is 2-3. At this pH, cerium carbonate is a precipitate, and thorium carbonate is a soluble compound.

[0030] According to the present invention, in step (S3), the concentration of NH4HCO3 is 0.01mol / L-1.5mol / L, preferably 0.015mol / L-1mol / L.

[0031] According to the present invention, in step (S3), the volume ratio of the washing liquid to the organic phase in step (S2) is 1:(1-3), exemplarily 1:1.5.

[0032] According to the present invention, in step (S4), the back-extraction solution is, for example, at least one of Na2C2O4 solution and NaOH solution, preferably a mixture of Na2C2O4 solution and NaOH solution; preferably, the concentration of Na2C2O4 solution is 0.1-0.3 mol / L, and the concentration of NaOH solution is 3 mol / L-6 mol / L; preferably, the concentration of the back-extraction solution is 0.27 mol / L Na2C2O4 and 5 mol / L NaOH solution.

[0033] According to the present invention, in step (S4), the volume ratio of the back-extraction solution to the organic phase in step (S2) is (1-3):1, exemplarily 1 mL of the organic phase and 0.5 mL of the back-extraction solution consisting of 0.5 mL of saturated Na2C2O4 and 0.5 mL of NaOH solution.

[0034] According to the present invention, in step (S4), the back-extraction temperature is 20-40°C, preferably 25-30°C; the back-extraction time is 5-30 min, preferably 15-25 min.

[0035] According to an embodiment of the present invention, the above-mentioned extraction and separation process includes n-stage extraction, m-stage washing and p-stage back-extraction, wherein n is 2-6, m is 1-5 and p is 1-4; preferably n is 2-4, m is 2-4 and p is 1-3.

[0036] For example, the organic phase is added from stage 1, the feed solution containing thorium and rare earth elements is added from stage n, the washing solution is added from stage n+m, and the back-extraction solution is added from stage n+m+p.

[0037] According to the present invention, the method further includes step (S5), adding the alkaline liquid generated in the method to the rare earth waste leachate to increase the pH of the leachate. For example, the alkaline liquid generated includes NaOH solution, ammonium bicarbonate, Na2C2O4 solution, etc.

[0038] The beneficial effects of this invention are:

[0039] (1) The present invention uses 10 rows of cascade extraction to make the thorium content in the aqueous phase outlet less than 0.5 mg / L; the final rare earth yield is more than 95%, and the cerium loss is at most 3%.

[0040] (2) In the prior art, acidic detergents are often used, while the present invention uses ammonium bicarbonate (NH4HCO3) as the washing liquid. Ammonium bicarbonate can bind rare earth element Ce, and the washing effect is stronger. During the washing process, the difficult-to-extract components are mainly washed from the organic phase to the aqueous phase, so that the easily extractable components in the organic phase do not continue to increase, while the difficult-to-extract components are further reduced.

[0041] (3) In this invention, rare earth waste residue is roasted and acid-leached to obtain a leachate with a high rare earth content, at 0.042 mol / L [N 1888 Separation of rare earth elements and Th under [DEAO] conditions. [N] 1888 [DEAO] can bind with thorium, and thorium and [N] 1888 The ratio of [DEAO] to action is approximately 1:4; [N] 1888 The maximum capacity of ][DEAO] combined thorium can reach 1.02 g / L, which can meet the requirements for thorium removal in rare earth slag leaching solution.

[0042] (4) This invention uses saturated Na₂C₂O₄ solution and NaOH solution as the back-extraction solution. After passing through an ionic liquid extractant [N...] 1888 After 10 rows of cascade extraction using [DEAO], a rare earth solution with a thorium content of less than 0.5 mg / L was obtained. This invention's method does not rely on acid, reducing the use of alkali; only an NH4HCO3 washing solution is used for washing. The back-extraction process uses saturated Na2C2O4 and NaOH solutions. The alkaline liquids generated during the cascade process (including wastewater and washing solution) can be added to the leachate to adjust the system pH, ensuring no excess wastewater or waste salt is generated during the cascade process. This reduces the environmental and storage pressure on rare earth waste residue, allows for the effective utilization of rare earth elements in the acid-dissolved residue, and enables the collection of the radioactive element thorium. This invention helps improve enterprise efficiency and reduces wastewater discharge and waste salt generation. Attached Figure Description

[0043] Figure 1 This is the cascade extraction process of the present invention.

[0044] Figure 2 This is a graph showing the compositional changes of the aqueous phase containing rare earth elements in step (S3) of test example 2.

[0045] Figure 3 The graph shows the variation of thorium back-extraction rate with different concentrations of washing solution in Examples 2-6. Detailed Implementation

[0046] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0047] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0048] Example 1

[0049] In the following examples, the cascade process flow ratio is as follows: 3 mL organic phase, 1 mL feed solution containing thorium and rare earth elements, 2 mL washing solution NH4HCO3 (NH4HCO3 concentration is 0.015 mol / L), 3 mL back-extraction solution (including 1.5 mL Na2C2O4 solution and 1.5 mL NaOH solution), and the extractant concentration is 0.042 mol / L. Four-stage washing and three-stage extraction are adopted.

[0050] according to Figure 1 Cascade extraction process in Figure 1 In the middle, the top layer is numbered 1, 3, 5, and 7 from left to right; the bottom layer has three middle layers numbered 2, 4, and 6 from left to right. The specific process for recovering thorium and rare earth elements from rare earth waste using an extractant is as follows:

[0051] (1) First, 100g of rare earth waste residue was roasted with calcium hydroxide at 900℃ for 4 hours, and finally 106g of solid was obtained. The amount of calcium hydroxide doped was 25% of the mass of rare earth waste residue.

[0052] (2) Weigh 50g of the roasting product in step (1), add 300g of ammonium sulfate and stir evenly, roast at 500℃ for 5h, add 400mL of water, heat at 90℃ for 2h, filter to obtain residue, and collect the filtrate to obtain raw material liquid containing thorium and rare earth elements.

[0053] In the feed solution containing thorium and rare earth elements, the thorium leaching rate is 50%.

[0054] (3) Extraction was performed using an organic feed solution containing thorium and rare earth elements, with the extractant ionic liquid [N1888][DEAO] at room temperature and for 20 min.

[0055] The organic phase consists of 3 mL (extractant concentration of 0.042 mol / L), and the feed solution containing thorium and rare earth elements consists of 1 mL.

[0056] (4) Wash the organic phase obtained after extraction in step (3) with 2 mL of washing solution NH4HCO3 (NH4HCO3 concentration is 0.015mol / L) to allow rare earth elements to enter the aqueous phase and thorium to remain in the organic phase;

[0057] (5) The thorium-containing organic phase obtained in step (4) is back-extracted using 3 mL of back-extraction solution (including 1.5 mL of 0.27 mol / L Na2C2O4 solution and 1.5 mL of 5 mol / L NaOH solution) to recover thorium. The back-extraction temperature is room temperature and the back-extraction time is 15 min.

[0058] The Th(IV) content in the aqueous phase outlet was reduced to 0.5 mg / L by 10-row cascade extraction, while the thorium content in the organic phase outlet was 11 mg / L and the cerium content was 30 mg / L; the thorium yield was 95% and the cerium yield was 95%.

[0059] (6) By reusing the alkaline liquid (such as ammonium bicarbonate, sodium hydroxide, etc.) in the cascade process into the feed liquid containing thorium and rare earth elements, the pH is adjusted to maintain the pH of the system at 2.2, and finally the thorium yield reaches 95% and the cerium yield reaches 97%.

[0060] Examples 2-5

[0061] The difference between Examples 2-6 and Example 1 is that the concentration of [N1888][DEAO] is 0.021 mol / L, the pH of the raw material solution containing thorium and rare earth elements is 2.2, and the concentrations of the washing solution are 0.015 mol / L, 0.1 mol / L, 0.5 mol / L, 1 mol / L, and 1.5 mol / L, respectively.

[0062] Test Example 1

[0063] Study on the separation effect of different concentrations of NH4HCO3 washing solution on rare earth elements and thorium

[0064] This invention employs a small-volume, multiple-stage washing method (i.e., four-stage washing) to study the washing of organic phases loaded with Ce(IV) and Th(IV). By controlling the concentration of NH4HCO3, it helps to reduce the diffusion of Th(IV) from the organic phase into the aqueous phase.

[0065] like Figure 3As shown, the separation of Ce(IV) and Th(IV) in organic phases loaded with Ce(IV) and Th(IV) was studied using different concentrations (0.015 mol / L, 0.1 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L) of NH4HCO3 washing solution in Examples 2-6. The concentration of NH4HCO3 in the washing solution was varied in Example 1 for testing; that is, the concentrations of NH4HCO3 in the washing solution were 0.1 mol / L, 0.5 mol / L, 1 mol / L, and 1.5 mol / L, respectively. The organic phase obtained after extraction in step (S2) was washed with these concentrations. Figure 3 It can be seen that the thorium back-extraction rate gradually increases with the increase of NH4HCO3 concentration. When the NH4HCO3 concentration reaches 0.5 mol / L, the Th(IV) back-extraction rate can reach 88%. When the NH4HCO3 concentration is increased to 1.5 mol / L, the Th(IV) back-extraction rate reaches 100%. To avoid precipitation during the extraction process, the concentration of the washing solution should be controlled below 0.1 mol / L.

[0066] This invention demonstrates that by gradually increasing the concentration of the washing solution (from 0.015 mol / L to 1.5 mol / L), the concentration of the washing solution has a significant impact on thorium extraction. Controlling the concentration of NH4HCO3 helps reduce the diffusion of Th(IV) from the organic phase to the aqueous phase. Considering the difference between Ce(IV) and Th(IV), the amount of Th(IV) separated from the organic phase should be minimized while the amount of Ce(IV) separated should be increased. As the foregoing analysis indicates, multiple washes at appropriate concentrations are necessary.

[0067] Test Example 2

[0068] In this invention, both Ce(IV) and Th(IV) are easily extractable components that enter the organic phase during the cascade process. When the extractant and feed solution come into contact, a large amount of Th(IV) is extracted into the organic phase, while only a small amount of Ce(IV) is extracted into the organic phase. This extraction ensures that Th(IV) is extracted into the organic phase. In the fourth-stage washing, the action of a low-concentration NH4HCO3 solution (e.g., below 0.015 mol / L) with Ce(IV) achieves the washing of most of the Ce(IV), leaving only a small amount of Ce(IV) remaining in the organic phase to be back-extracted along with Th(IV). Figure 2 This is a composition change diagram of the aqueous phase containing rare earth elements in step (S3) of Example 1, where 1 is the aqueous phase outlet and 7 is the organic phase outlet.

[0069] The embodiments of the present invention have been described above by way of example. However, the scope of protection of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for recovering thorium and rare earth elements from rare earth waste, characterized in that, The method includes the following steps: (S1) After the rare earth waste residue is roasted for the first time, it is mixed with ammonium sulfate for a second roasting, and then heated and leached to obtain a raw material liquid containing thorium and rare earth elements. In step (S1), the rare earth waste residue is mixed with calcium hydroxide for roasting during the first roasting; the temperature of the first roasting is 800-1000℃; the temperature of the second roasting is 350-600℃. (S2) Extraction is performed using an organic feed solution containing thorium and rare earth elements; the extractant is an ionic liquid [N 1888 [DEAO], its structure is as follows: ； (S3) The organic phase obtained after extraction in step (S2) is washed with a washing solution to allow rare earth elements to enter the aqueous phase while thorium remains in the organic phase; (S4) The thorium-containing organic phase obtained in step (S3) is back-extracted using a back-extraction solution to recover thorium; In step (S3), the washing solution is NH4HCO3; the concentration of NH4HCO3 is 0.01 mol / L-0.1 mol / L. In step (S4), the back-extraction solution is at least one of Na2C2O4 solution and NaOH solution.

2. The method according to claim 1, characterized in that, In step (S1), the rare earth element is selected from at least one of La, Ce, Pr, Nd, Pm, Sm, Eu, Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

3. The method according to claim 1, characterized in that, In step (S1), the raw material liquid also includes at least one of Al, Ca, Mg and Fe.

4. The method according to claim 1, characterized in that, In step (S1), the mass of calcium hydroxide is 15-30 wt% of the rare earth waste residue.

5. The method according to claim 1, characterized in that, In step (S1), the first roasting time is 3-6 hours.

6. The method according to claim 1, characterized in that, The concentration of ammonium sulfate is 2-7 mol / L.

7. The method according to claim 1, characterized in that, In step (S1), the second roasting time is 3-7 hours.

8. The method according to claim 1, characterized in that, In step (S1), water needs to be added for heating during leaching.

9. The method according to claim 1, characterized in that, In step (S1), the temperature for heating and leaching is 70-110℃; the heating and leaching time is 1-4h.

10. The method according to claim 1, characterized in that, In step (S1), the mass ratio of rare earth waste residue to ammonium sulfate is 1:(1-10).

11. The method according to claim 1, characterized in that, In step (S2), the volume ratio of the feed liquid containing thorium and rare earth elements to the organic phase is 1:(2-5).

12. The method according to claim 1, characterized in that, In step (S2), the concentration of the extractant is 0.01-0.1 mol / L.

13. The method according to claim 1, characterized in that, In step (S3), the volume ratio of the washing liquid to the organic phase in step (S2) is 1:(1-3).

14. The method according to claim 1, characterized in that, In step (S4), the volume ratio of the back-extraction solution to the organic phase in step (S2) is (1-3):

1.

15. The method according to claim 1, characterized in that, The method further includes step (S5), which involves adding the alkaline liquid generated in the method to the rare earth waste leaching solution.

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