A method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag

Abstract

The present invention provides a method for stepwise recovering lithium and rare earths from rare earth molten salt electrolysis slag, belonging to the technical field of metallurgical waste slag treatment and secondary resource recycling. The method comprises the following steps: (1) reacting the rare earth molten salt electrolysis slag with sulfuric acid for leaching to obtain a lithium-containing solution and a leaching residue; (2) mixing the leaching residue with an additive for low-temperature roasting transformation to obtain a roasting product; (3) performing water leaching on the roasting product to obtain a water leaching residue; (4) performing acid leaching on the water leaching residue and adding a reducing agent to obtain a rare earth leaching solution and a leaching residue; wherein, the additive is an alkali and activated carbon, and the reducing agent is formaldehyde or hydrogen peroxide. The method of the present invention realizes the mineral phase transformation and efficient extraction of lithium and rare earths in the rare earth molten salt electrolysis slag, realizes the separation of lithium and rare earths, and at the same time, fluorine can be recovered, having the advantages of high extraction rates of lithium and rare earths and being green.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgical waste residue treatment and secondary resource recycling, and particularly relates to a method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag. Background Art

[0002] China is the world's largest rare earth producer, accounting for more than 70% of the world's output. At present, the main methods for preparing rare earth metals are metallothermic reduction and molten salt electrolysis methods, and the fluoride system electrolysis method is the most common molten salt electrolysis method. It is estimated that the rare earth molten salt electrolysis slag generated during the molten salt electrolysis process is about 5% of the metal amount. The rare earth molten salt electrolysis slag contains lithium and rare earths, and their contents are respectively 1-6% and 40-80% (varying). Because their contents are much higher than the grade in the original ore, the rare earth molten salt electrolysis slag has very high recycling value; however, its composition is complex, and the main existing phases are fluorides, resulting in difficult and efficient extraction of rare earths. At present, the recycling methods mainly focus on acid and alkali methods for extracting rare earths. The leaching rate of some rare earths is low, only part of the rare earths can be extracted, and less attention is paid to lithium. Therefore, how to achieve the efficient comprehensive recovery of rare earths and lithium in rare earth molten salt electrolysis slag is an urgent problem to be solved. Summary of the Invention

[0003] To solve the above technical problems, the present invention proposes a method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag.

[0004] The present invention proposes a method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag, comprising the following steps:

[0005] (1) Leaching the rare earth molten salt electrolysis slag with sulfuric acid to obtain a lithium-containing solution and a leaching residue. The lithium-containing solution is used for lithium recovery and can be used as a raw material for preparing lithium carbonate. The reaction equation involved in this step is:

[0006] 2LiF + H2SO4 = Li2SO4 + 2HF

[0007] Li2SO4 + CO3 2- = Li2CO3 + SO4 2-

[0008] (2) Mixing the leaching residue with an additive and performing low-temperature transformation roasting to obtain a roasting product. The additive is an alkali and activated carbon, and the alkali is sodium hydroxide or potassium hydroxide. The reaction equations involved in this step are as follows, where RE is a rare earth element:

[0009] REF3 + 3OH - = RE(OH)3 + 3F -

[0010] REF4 + 4OH- = RE(OH)4 + 4F -

[0011] 4RE(OH)4 + C = 4RE(OH)3 + 2H2O↑ + CO2↑

[0012] (3) Crush the calcined product obtained in step (2) and pass it through a 100-mesh sieve, then leach it with water to obtain a water-leached residue and a fluoride-containing solution; (4) Acid-leach the water-leached residue with hydrochloric acid or sulfuric acid, and add a reducing agent during the acid-leaching process to obtain a rare earth leaching solution and a leaching residue, realizing the recovery of rare earth. The reducing agent is formaldehyde and / or hydrogen peroxide. The reaction equations involved in this step are:

[0013] 2RE(OH)4 + H2O2 = 4RE(OH)3 + H2O + O2↑

[0014] 4RE(OH)4 + HCHO = 4RE(OH)3 + 3H2O + CO2↑

[0015] RE(OH)3 + 3H + = RE 3+ + H2O

[0016] Furthermore, the present invention does not limit the source of rare earth molten salt electrolysis slag, as long as it is the rare earth molten salt electrolysis slag produced by molten salt electrolysis of a single rare earth metal or alloy using a fluoride salt system. However, the effect is more obvious for rare earth molten salt electrolysis slag containing variable-valence rare earth elements such as terbium, cerium, praseodymium, etc. The total content of rare earth elements in the rare earth molten salt electrolysis slag is 20 - 80%, and the total content of lithium is 1% - 6%. Both the rare earth elements and lithium mainly exist in the form of fluorides.

[0017] Furthermore, in step (1), the mass fraction of the sulfuric acid is 70% - 98%, and the liquid-solid ratio of the sulfuric acid to the rare earth molten salt electrolysis slag is 3:1 - 10:1.

[0018] Furthermore, in step (1), the reaction temperature of the leaching is 30°C - 95°C, and the reaction time is 1h - 30h.

[0019] Furthermore, in step (2), the dosage of the base in the additive is 1 - 1.5 times the mass of the leaching residue, and the dosage of carbon is 0 - 8% of the mass of the leaching residue (dry basis).

[0020] Furthermore, in step (2), the reaction temperature of the low-temperature transformation roasting is 100°C - 700°C, and the reaction time is 0.5h - 6h.

[0021] Furthermore, in step (3), the liquid-solid ratio of the calcined product to water is 3:1 - 10:1.

[0022] Further, in step (3), the reaction temperature for water leaching is 30°C - 95°C, and the reaction time is 0.5 h - 6 h.

[0023] Further, in step (4), the mass ratio of the reducing agent to the acid leaching residue is 0.01 - 0.15:1. The water leaching residue is acid leached with hydrochloric acid or sulfuric acid, and the end point acidity is controlled to be 0.5 mol / L during the acid leaching process.

[0024] Further, in step (4), the reaction temperature for the acid leaching is 30°C - 95°C, and the reaction time is 0.5 h - 6 h.

[0025] Compared with the prior art, the present invention has the following advantages and technical effects:

[0026] The method of the present invention realizes the mineral phase transformation (i.e., lithium fluoride is transformed into lithium sulfate) and efficient extraction of lithium and rare earths in the rare earth molten salt electrolysis slag. By preferentially acid leaching to extract lithium, and then performing low-temperature carbothermal reduction roasting for transformation, the rare earth fluoride is transformed into rare earth hydroxide, and fluorine is transformed into soluble fluoride. By using the water leaching method, fluorine dissolves while rare earths remain in the slag, realizing the efficient separation of rare earth elements and fluorine. At the same time, fluorine can be recovered, and it has the advantages of high extraction rates of lithium and rare earths and being green. Specific Embodiments

[0027] The various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0028] It should be understood that the terms used in the present invention are only for describing specific embodiments and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0029] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0030] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the description of the present invention, which will be apparent to those skilled in the art. Other embodiments obtained from the description of the present invention will be apparent to those skilled in the art. The description and examples of the present invention are merely exemplary.

[0031] Regarding the terms "comprising", "including", "having", "containing", etc. used herein, they are all open-ended terms, meaning including but not limited to.

[0032] The present invention provides a method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag, comprising the following steps:

[0033] (1) Leaching the rare earth molten salt electrolysis slag with sulfuric acid to obtain a lithium-containing solution and a leaching residue. The lithium-containing solution is used for lithium recovery and can be used as a raw material for preparing lithium carbonate. The reaction equation involved in this step is:

[0034] 2LiF + H2SO4 = Li2SO4 + 2HF

[0035] Li2SO4 + CO3 2- = Li2CO3 + SO4 2-

[0036] (2) Mixing the leaching residue with an additive for low-temperature transformation roasting to obtain a roasting product. The additive is an alkali and activated carbon, and the alkali is sodium hydroxide or potassium hydroxide. The reaction equations involved in this step are as follows, where RE is a rare earth element:

[0037] REF3 + 3OH- = RE(OH)3 + 3F-

[0038] REF4 + 4OH- = RE(OH)4 + 4F-

[0039] 4RE(OH)4 + C = 4RE(OH)3 + 2H2O↑ + CO2↑

[0040] (3) Crushing the roasting product obtained in step (2) and passing it through a 100-mesh sieve, then leaching with water to obtain a water leaching residue and a fluorine-containing solution;

[0041] (4) Acid-leaching the water leaching residue with hydrochloric acid or sulfuric acid, and adding a reducing agent during the acid-leaching process to obtain a rare earth leaching solution and a leaching residue, and realizing the recovery of rare earths from the leaching solution. The recovery process is a conventional technical means in the art. The reducing agent is formaldehyde and / or hydrogen peroxide. The reaction equation involved in this step is:

[0042] 2RE(OH)4 + H2O2 = 4RE(OH)3 + H2O + O2↑

[0043] 4RE(OH)4 + HCHO = 4RE(OH)3 + 3H2O + CO2↑

[0044] RE(OH)3 + 3H + = RE 3+ + H2O

[0045] The source of the rare earth molten salt electrolysis slag is not limited in the present invention. As long as it is the rare earth molten salt electrolysis slag produced by molten salt electrolysis using a fluoride salt system to prepare single rare earth metals or alloys, it is acceptable. However, the effect is more obvious for those containing variable valence rare earth elements such as terbium, cerium, praseodymium, etc. In the examples of the present invention, the used rare earth molten salt electrolysis slag is collected from a rare earth factory of molten salt electrolysis slag. After measurement, the total content of the used rare earth elements is 20 - 80%, and the total content of lithium is 1% - 6%. Both the rare earth elements and lithium mainly exist in the form of fluorides.

[0046] The rare earth molten salt electrolysis slag is leached with sulfuric acid. An appropriate sulfur concentration can promote the leaching rate of lithium in the rare earth molten salt electrolysis slag. In order to improve the recovery rate of lithium elements, in step (1) of the examples of the present invention, the mass fraction of the sulfuric acid is 70% - 98%, and the liquid - solid ratio of the sulfuric acid to the rare earth molten salt electrolysis slag is 3:1 - 10:1.

[0047] During the process of leaching the rare earth molten salt electrolysis slag with sulfuric acid, the leaching temperature is related to the lithium leaching rate. In order to obtain a higher leaching rate, in step (1) of the examples of the present invention, the reaction temperature of the leaching is 30°C - 95°C, and the reaction time is 1h - 30h.

[0048] The function of the additive is to reduce the +4 - valent rare earth to +3 - valent for variable valence rare earths. Therefore, in step (2) of the examples of the present invention, an appropriate amount of additive is added. Among them, the dosage of the base in the additive is 1 - 1.5 times the mass of the leaching residue, and the dosage of carbon is 0 - 8% of the mass of the leaching residue.

[0049] The function of low - temperature transformation roasting is to transform rare earth fluoride into rare earth hydroxide. Too high a temperature will cause serious agglomeration, which is not conducive to subsequent rare earth leaching, and too low a temperature will not cause the transformation of rare earth. Based on this, in step (2) of the examples of the present invention, the reaction temperature of the low - temperature transformation roasting is 100°C - 700°C, and the reaction time is 0.5h - 6h.

[0050] In step (3) of the examples of the present invention, the liquid - solid ratio of the roasting product to water is 3:1 - 10:1.

[0051] The process of leaching the roasting product with water is to remove fluorine and dissolve the generated sodium fluoride. Controlling both the leaching reaction temperature and time is beneficial to the dissolution of sodium fluoride. Therefore, in step (3) of the examples of the present invention, the reaction temperature of leaching with water is 30°C - 95°C, and the reaction time is 0.5h - 6h.

[0052] The function of the reducing agent is to reduce the rare earth with variable valence, reducing the +4-valent rare earth to +3-valent. In step (4) of the embodiment of the present invention, the mass ratio of the reducing agent to the acid leaching residue is 0.01 - 0.15:1. The water leaching residue is acid leached with hydrochloric acid or sulfuric acid, and the end-point acidity is controlled at 0.5 mol / L during the acid leaching process, aiming to remove impurities while preventing the precipitation of rare earths.

[0053] The purpose of acid leaching the water leaching residue again is to leach rare earth hydroxide into the solution to achieve the purpose of recovering rare earths. In step (4) of the embodiment of the present invention, the reaction temperature of the acid leaching is 30°C - 95°C, and the reaction time is 0.5 h - 6 h.

[0054] Each raw material used in the embodiment of the present invention is commercially available.

[0055] The technical solution of the present invention is further described below through examples.

[0056] Example 1

[0057] A method for stepwise recovering lithium and rare earths from rare earth molten salt electrolysis slag, comprising the following steps:

[0058] (1) Mix the rare earth molten salt electrolysis slag with sulfuric acid with a mass fraction of 90% according to a liquid-solid ratio of 6:1, and carry out leaching reaction at 80°C for 10 h to obtain a lithium-containing solution and a leaching residue, realizing the recovery of lithium. The lithium-containing solution can be used as a raw material for preparing lithium carbonate, and the lithium leaching rate (i.e., the lithium content in the lithium-containing solution) reaches 75.98%;

[0059] (2) Mix the leaching residue with additives (sodium hydroxide and activated carbon). The dosage of sodium hydroxide is 1 time the mass of the leaching residue, and the dosage of carbon is 4% of the mass of the leaching residue (dry basis). Carry out low-temperature transformation roasting at 400°C for 2 h to obtain a roasting product containing rare earth hydroxide and soluble fluoride salt;

[0060] (3) Crush the roasting product and pass it through a 100-mesh sieve, then add water to remove fluorine according to a liquid-solid ratio of 5:1, and carry out leaching reaction at 60°C for 2 h to obtain a water leaching residue (mainly rare earth hydroxide) and a fluorine-containing solution;

[0061] (4) Mix the water leaching residue with hydrochloric acid, carry out acid leaching reaction at 75°C for 3 h, add a reducing agent (formaldehyde) during the acid leaching process, and control the end-point acidity at 0.5 mol / L during the acid leaching process to obtain a rare earth leaching solution and a leaching residue, realizing the recovery of rare earths from the leaching solution. The rare earth leaching rate reaches over 90.55%.

[0062] Example 2

[0063] A method for stepwise recovering lithium and rare earths from rare earth molten salt electrolysis slag, comprising the following steps:

[0064] (1) Mix the rare earth molten salt electrolysis slag with sulfuric acid with a mass fraction of 98% at a liquid-solid ratio of 8:1, and carry out leaching reaction at 90 °C for 20 h to obtain a lithium-containing solution and leaching residue, realizing the recovery of lithium. The lithium-containing solution can be used as a raw material for preparing lithium carbonate, and the lithium leaching rate reaches 98.20%;

[0065] (2) Mix the leaching residue with additives (sodium hydroxide and activated carbon). The dosage of sodium hydroxide is 1.4 times the mass of the leaching residue, and the dosage of carbon is 6% of the mass of the leaching residue (dry basis). Carry out low-temperature transformation roasting at 500 °C for 4 h to obtain a roasting product containing rare earth hydroxide and soluble fluoride salt;

[0066] (3) Crush the roasting product and pass it through a 100-mesh sieve, then add water according to a liquid-solid ratio of 10:1, and carry out leaching reaction at 90 °C for 4 h to obtain a water leaching residue (mainly rare earth hydroxide) and a fluoride-containing solution;

[0067] (4) Mix the water leaching residue with hydrochloric acid, carry out acid leaching reaction at 90 °C for 4 h, add a reducing agent (hydrogen peroxide) during the acid leaching process, and control the final acidity to 0.5 mol / L to obtain a rare earth leaching solution and leaching residue, and recover rare earth elements from the rare earth leaching solution. The rare earth leaching rate reaches more than 98.68%.

[0068] Example 3

[0069] A method for cascade recovery of lithium and rare earth from rare earth molten salt electrolysis slag, comprising the following steps:

[0070] (1) Mix the rare earth molten salt electrolysis slag with sulfuric acid with a mass fraction of 98% at a liquid-solid ratio of 6:1, and carry out leaching reaction at 70 °C for 15 h to obtain a lithium-containing solution and leaching residue, realizing the recovery of lithium. The lithium-containing solution can be used as a raw material for preparing lithium carbonate, and the lithium leaching rate reaches 96.52%;

[0071] (2) Mix the leaching residue with additives (potassium hydroxide and activated carbon). The dosage of potassium hydroxide is 1.2 times the mass of the leaching residue, and the dosage of carbon is 5% of the mass of the leaching residue (dry basis). Carry out low-temperature transformation roasting at 600 °C for 3 h to obtain a roasting product containing rare earth hydroxide and soluble fluoride salt;

[0072] (3) Crush the roasting product and pass it through a 100-mesh sieve, then add water according to a liquid-solid ratio of 10:1, and carry out leaching reaction at 90 °C for 4 h to obtain a water leaching residue (mainly rare earth hydroxide) and a fluoride-containing solution;

[0073] (4) Mix the water leaching residue with sulfuric acid, carry out acid leaching reaction at 80 °C for 3 h, add a reducing agent (formaldehyde) during the acid leaching process, and control the final acidity to 0.5 mol / L to obtain a rare earth leaching solution and leaching residue, realizing the recovery of rare earth. The rare earth leaching rate reaches more than 95.88%.

[0074] Example 4

[0075] A method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag, comprising the following steps:

[0076] (1) Mix the rare earth molten salt electrolysis slag with sulfuric acid having a mass fraction of 70% at a liquid-solid ratio of 3:1, and carry out leaching reaction at 95 °C for 1 h to obtain a lithium-containing solution and a leaching residue, realizing the recovery of lithium. The lithium-containing solution can be used as a raw material for preparing lithium carbonate, and the lithium leaching rate reaches 55.24%;

[0077] (2) Mix the leaching residue with additives (sodium hydroxide and activated carbon). The dosage of sodium hydroxide is 1 time the mass of the leaching residue, and the dosage of carbon is 8% of the mass of the leaching residue. Carry out low-temperature transformation roasting at 100 °C for 6 h to obtain a roasting product containing rare earth hydroxide and soluble fluoride salt;

[0078] (3) Crush the roasting product and pass it through a 100-mesh sieve, then add water at a liquid-solid ratio of 10:1, and carry out leaching reaction at 30 °C for 0.5 h to obtain a water-leached residue and a fluoride-containing solution;

[0079] (4) Mix the water-leached residue with hydrochloric acid, carry out acid leaching reaction at 30 °C for 6 h, add a reducing agent (hydrogen peroxide) during the acid leaching process, and control the final acidity to 0.5 mol / L to obtain a rare earth leaching solution and a leaching residue, realizing the recovery of rare earths. The rare earth leaching rate reaches more than 35.36%.

[0080] Example 5

[0081] A method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag, comprising the following steps:

[0082] (1) Mix the rare earth molten salt electrolysis slag with sulfuric acid having a mass fraction of 70% at a liquid-solid ratio of 10:1, and carry out leaching reaction at 30 °C for 30 h to obtain a lithium-containing solution and a leaching residue, realizing the recovery of lithium. The lithium-containing solution can be used as a raw material for preparing lithium carbonate, and the lithium leaching rate reaches 50.65%;

[0083] (2) Mix the leaching residue with additives (potassium hydroxide and activated carbon). The dosage of potassium hydroxide is 1.5 times the mass of the leaching residue, and the dosage of carbon is 1% of the mass of the leaching residue. Carry out low-temperature transformation roasting at 700 °C for 0.5 h to obtain a roasting product containing rare earth hydroxide and soluble fluoride salt;

[0084] (3) Crush the roasting product and pass it through a 100-mesh sieve, then add water at a liquid-solid ratio of 3:1, and carry out leaching reaction at 95 °C for 0.5 h to obtain a water-leached residue and a fluoride-containing solution;

[0085] (4) Mix the water leaching residue with sulfuric acid, carry out acid leaching reaction at 95 °C for 0.5 h, add a reducing agent (formaldehyde) during the acid leaching process, control the final acidity to 0.5 mol / L, obtain rare earth leaching solution and leaching residue, realize the recovery of rare earth, and the rare earth leaching rate reaches more than 91.67%.

[0086] The above is only the preferred specific implementation mode of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag, characterized in that, It includes the following steps: (1) Leaching rare earth molten salt electrolysis slag with sulfuric acid to obtain a lithium-containing solution and leaching residue, and the lithium-containing solution is used for lithium recovery; (2) Mixing the leaching residue with an additive for low-temperature transformation roasting to obtain a roasted product, and the additive is an alkali and activated carbon; (3) Leaching the roasted product with water to obtain a water leaching residue and a fluorine-containing solution; (4) Acid leaching the water leaching residue and adding a reducing agent during the acid leaching process to obtain a rare earth leaching solution and a leaching residue, realizing the recovery of rare earth, and the reducing agent is formaldehyde and / or hydrogen peroxide; The rare earth molten salt electrolysis slag is a rare earth molten salt electrolysis slag produced by molten salt electrolysis of a fluoride system to prepare a single rare earth metal or alloy. The total content of rare earth elements in the rare earth molten salt electrolysis slag is 20%-80%, and the total content of lithium is 1%-6%; In step (2), the reaction temperature of the low-temperature transformation roasting is 100°C - 700°C, and the reaction time is 0.5h - 6h; In step (1), the mass fraction of the sulfuric acid is 70% - 98%, and the liquid-solid ratio of the sulfuric acid to the rare earth molten salt electrolysis slag is 3:1 - 10:1; In step (2), the dosage of the alkali in the additive is 1.0 - 1.5 times the mass of the leaching residue, and the dosage of the activated carbon is 0 - 8% of the mass of the leaching residue; In step (3), the liquid-solid ratio of the roasted product to water is 3:1 - 10:1; In step (4), the mass ratio of the reducing agent to the water leaching residue is 0.01 - 0.15:1, and the end-point acidity of the water leaching residue during acid leaching is controlled at 0.5 mol / L.

2. The method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag according to claim 1, characterized in that, In step (1), the reaction temperature of the leaching is 30°C - 95°C, and the reaction time is 1h - 30h.

3. The method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag according to claim 1, characterized in that, In step (3), the reaction temperature of water leaching is 30°C - 95°C, and the reaction time is 0.5h - 6h.

4. The method for cascaded recovery of lithium and rare earths from rare earth molten salt electrolysis slag according to claim 1, wherein In step (4), the reaction temperature of the acid leaching is 30°C - 95°C, and the reaction time is 0.5h - 6h.