Method for recovering rare earth elements from rare earth molten salt electrolytic slag

Abstract

The invention belongs to the technical field of mineral substance recovery, and relates to a method for recovering rare earth elements from electrolytic residues. The method comprises the following steps: adding aluminum particles and aluminum chloride particles into the electrolytic slag, and performing high-temperature closed calcination; carrying out crushing treatment, mixing with an inorganic strong acid solution, controlling the mixing temperature, carrying out a stirring reaction, and filtering to obtain primary acid leaching residues and a primary rare earth acid leaching solution; the primary acid leaching residues are added into an inorganic strong acid solution for secondary mixing, the mixing temperature is controlled, stirring reaction is conducted, filtering is conducted, and secondary acid leaching residues and a secondary rare earth leaching solution are obtained. According to the method, the leaching rate of the rare earth elements can reach 98%, efficient recovery of the rare earth elements is achieved, the leaching rate is high, roasted products are prone to breakage, fluorine is fixed in a solid phase, environmental pollution is reduced, the cost is reduced, and the comprehensive utilization rate of resources is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of mineral recovery and relates to a method for recovering rare earth elements from rare earth molten salt electrolytic slag. Background Art

[0002] Rare earth elements have many unique physical and chemical properties and are widely used in various high-tech fields such as permanent magnets, catalysis and special materials. The production of rare earth metals by molten salt electrolysis has the advantages of low cost and high yield, so it has become the mainstream process for actual rare earth production. There are many rare earth elements in the waste slag produced by this process. Due to the increasing demand for rare earths in magnetic materials in my country, the output of materials such as neodymium iron boron has also increased year by year, and the amount of molten salt electrolytic slag produced has also increased greatly. Considering the current tight supply of rare earth mining and its continued strong price, the recovery of rare earth elements from rare earth solid waste has received more and more attention in recent years. The rare earth elements in rare earth molten salt electrolytic slag generally exist in the form of rare earth fluoride, rare earth oxyfluoride, etc., and the rare earth content is about 20%-80%. In addition, there are some other complex non-rare earth impurities (silicon dioxide, iron oxide, etc.), but these components are basically stable substances, and they cannot be separated from rare earth elements by simple acid leaching and filtration. Therefore, the researchers have tried a variety of strategies to solve the problem that rare earth fluorides (oxides) are insoluble in acid, making it difficult to dissolve rare earths into the liquid phase. The most important and effective idea is to chemically react rare earth fluorides (oxides) with other substances to generate substances that are easily soluble in aqueous solutions. For example, the concentrated sulfuric acid roasting method can react rare earth fluorides (oxides) with concentrated sulfuric acid to convert them into rare earth sulfate salts, and then the rare earth elements can be dissolved in the aqueous solution. This process will produce toxic gases such as hydrogen fluoride during the roasting process, which will cause serious corrosion to production equipment and cause certain harm to the environment. There is also an alkali roasting method, which allows alkaline substances such as sodium hydroxide, sodium carbonate, and borax to react with rare earth fluorides (oxides) under high temperature conditions to generate rare earth oxides that are soluble in acid. At the same time, fluorine will become substances such as sodium fluoride. After entering the aqueous solution, it will form fluorine-containing wastewater, which will pollute the environment. In addition, some scholars have tried to mix Ca(OH)2, CaO, calcium chloride, etc. with rare earth molten salt slag and then roast it, which can not only convert rare earth into oxides, but also keep fluorine in the solid phase in the form of calcium fluoride, turning it into solid waste, reducing the harmful impact on the environment. However, these roasting processes often require higher temperatures (generally greater than 700°C) and longer times, making the roasting process energy-intensive. Moreover, the products after most of the above roasting processes are severely agglomerated, dense and hard, and not easy to break, so they need to be repeatedly crushed, which increases energy consumption. In summary, in view of the problems existing in the current rare earth electrolytic slag recovery process, it is urgent to develop an economical, efficient and environmentally friendly recovery and extraction process. Summary of the invention

[0003] The present invention provides a technology for efficiently converting rare earth molten salt electrolytic slag into soluble rare earth compounds, and the roasted product is easy to break and can stably fix fluorine in the solid phase. The main component of the solid waste formed is aluminum fluoride, which can be used as a flux for aluminum metal smelting after impurity removal and purification. This technology solves the problems of poor rare earth leaching effect, large reagent input, and pollution of fluorine-containing waste gas and wastewater in the existing process.

[0004] In order to achieve the above object, the present invention is implemented by adopting the following technical solutions: A method for recovering rare earth elements from rare earth molten salt electrolytic slag, comprising the following steps: (1) adding aluminum particles and aluminum chloride particles to the electrolytic slag and mixing them uniformly to obtain a solid compound; (2) calcining the solid compound in a closed environment at high temperature; (3) crushing the calcined solid to obtain substance A; (4) mixing the substance A with an inorganic strong acid solution, controlling the mixing temperature and stirring the mixture to react, and filtering the mixture to obtain a primary acid leaching residue and a primary rare earth acid leaching solution; (5) Add the primary acid leaching residue to the inorganic strong acid solution for secondary mixing, control the mixing temperature and stir the reaction, filter, and obtain secondary acid leaching residue and secondary rare earth leaching solution.

[0005] Preferably, in step (1), the mass ratio of electrolytic slag: aluminum particles: aluminum chloride particles is (2-8): 1: (2-4), and the size of the aluminum particles is 80-200 mesh.

[0006] Preferably, in step (2), the calcination temperature is 500-700° C., the calcination time is 1.5-3 h, and the calcination atmosphere is air containing a trace amount of hydrogen, wherein the volume fraction of hydrogen in the total calcination atmosphere is 0.05-0.2%.

[0007] Preferably, the inorganic strong acid solution in step (4) and step (5) is a hydrochloric acid solution with a concentration of 2.5-4M, and the stirring reaction temperature is 85-95°C.

[0008] Preferably, the secondary rare earth leaching solution obtained in step (5) is recovered and reused.

[0009] Principle: Aluminum can replace rare earth elements in electrolytic slag and generate aluminum fluoride which is insoluble in water, thus stabilizing the fluorine element in the solid phase.

[0010] Aluminum chloride can not only react with rare earth fluoride to generate water-soluble rare earth chloride, but also easily sublimate at high temperature, which can play a role in loosening the slag during the roasting process. However, if there are no aluminum particles, the sublimation effect of aluminum chloride is too obvious, and a lot of excess is needed to achieve the conversion of rare earth salts. Therefore, the cost of raw material input will be relatively high. In addition, experiments have shown that adding a small amount of hydrogen during the calcination process can not only increase the rare earth recovery rate, but also make the calcined solids looser, which is conducive to further crushing and processing in the later stage.

[0011] Compared with the prior art, the advantages and positive effects of the present invention are: The method provided by the present invention can make the leaching rate of rare earth elements as high as 98%, realizing the efficient recovery of rare earth elements. Compared with the traditional rare earth electrolytic slag rare earth recovery process, this method has a higher rare earth leaching rate, the roasting product is easy to break, and fluorine is fixed into a solid phase, which reduces environmental pollution, reduces the reagent input cost, and improves the comprehensive utilization rate of resources. DETAILED DESCRIPTION

[0012] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0013] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0014] Example 1 Take 10g of rare earth molten salt electrolytic slag (source: Zhongxi Tianma New Materials Technology Co., Ltd.), 15g of aluminum chloride hexahydrate and 5g of aluminum particles (average particle size 100 mesh), mix them evenly, put them into a tube furnace, heat them up in a closed manner, and the heating rate is 10℃ / min. When the temperature rises to above 400℃, according to the volume of the tube furnace, 0.1% of the total volume of hydrogen is introduced, and the temperature continues to rise to a high temperature of 700℃, and the constant temperature is roasted for 1.8h. After the roasting is completed, it is naturally cooled to room temperature, and then the solid material is taken out. Grind and crush through a 100-mesh sieve, then add fresh or recycled 740mL of 3M hydrochloric acid solution, heat in a water bath at 90℃, and continue to stir and react for 2h, with a stirring rate of 200-800rpm. After the reaction is completed, filter and obtain the primary acid leaching solution and the primary acid leaching residue, then add the primary acid leaching residue to a fresh 370mL of 3M hydrochloric acid solution, and repeat the secondary reaction under the same conditions. That is, the mixture is heated in a water bath at 90°C and stirred continuously for 2 hours at a stirring rate of 200-800rpm. After the secondary reaction is completed, the mixture is naturally cooled to room temperature and filtered to obtain secondary acid leaching residue and secondary acid leaching liquid, which can be added to the acid solution used for the primary treatment of the roasted product. The secondary acid leaching residue is naturally air-dried for more than 10 hours to obtain the final waste residue.

[0015] The rare earth molten salt electrolytic slag used in this embodiment was analyzed by XRF (X-ray fluorescence spectrometry), and the main element contents (%) were as follows: F element 56.438, Nd element 28.551, Pr element 7.711, Fe element 4.654.

[0016] The final waste slag was analyzed by XRF, and the main elements were (%): F element 62.182, Al element 30.031, Nd element 0.323, Pr element 0.021, Fe element 2.357.

[0017] Example 2 Take 10g of rare earth molten salt electrolytic slag (source: Zhongxi Tianma New Materials Technology Co., Ltd.), 15g of aluminum chloride hexahydrate and 5g of aluminum particles (average particle size 100 mesh), mix them evenly, put them in a tube furnace, heat them up in a closed manner, and the heating rate is 10℃ / min. When the temperature rises to above 300℃, according to the volume of the tube furnace, 0.2% of the total volume of hydrogen is introduced, and the temperature is continued to rise to 500℃. Constant temperature roasting is performed for 2h. After roasting, it is naturally cooled to room temperature, and then the solid material is taken out. Grind and crush through a 200-mesh sieve, then add fresh or recycled 740mL of 3M hydrochloric acid solution, heat in a water bath at 85℃, and continue to stir and react for 2.2h at a stirring rate of 300rpm. After the reaction is completed, filter and obtain the primary acid leaching solution and the primary acid leaching residue, then add the primary acid leaching residue to a fresh 370mL of 3M hydrochloric acid solution, and repeat the secondary reaction under the same conditions. That is, the mixture is heated in a water bath at 85°C, stirred continuously for 2.2 hours, and stirred at a rate of 300 rpm. After the secondary reaction is completed, the mixture is naturally cooled to room temperature, filtered, and secondary acid leaching residue and secondary acid leaching liquid are obtained. The secondary acid leaching liquid can be added to the acid solution used for the primary treatment of the roasted product. The secondary acid leaching residue is dried at 50°C for 2 hours to obtain the final waste residue.

[0018] The composition of the rare earth molten salt electrolytic slag used in this embodiment is consistent with that in Example 1. The final waste slag is analyzed by XRF, and the main elements are (%): F element 61.976, Al element 30.248, Nd element 0.365, Pr element 0.019, Fe element 2.541.

[0019] Example 3 Take 10g of rare earth molten salt electrolytic slag (source: Zhongxi Tianma New Materials Technology Co., Ltd.), 15g of aluminum chloride hexahydrate and 5g of aluminum particles (average particle size 100 mesh), mix them evenly, put them in a tube furnace, heat them up in a closed manner, and the heating rate is 10℃ / min. When the temperature rises to above 300℃, according to the volume of the tube furnace, 0.15% of the total volume of hydrogen is introduced, and the temperature is continued to rise to 600℃. Constant temperature roasting is performed for 2.5h. After roasting, it is naturally cooled to room temperature, and then the solid material is taken out. Grind and crush through a 150-mesh sieve, then add fresh or recycled 740mL of 3M hydrochloric acid solution, heat in a water bath at 90℃, and continue to stir and react for 2h at a stirring rate of 200rpm. After the reaction is completed, filter and obtain the primary acid leaching solution and the primary acid leaching residue, then add the primary acid leaching residue to a fresh 370mL of 3M hydrochloric acid solution, and repeat the secondary reaction under the same conditions. That is, the mixture is heated in a water bath at 90°C, stirred continuously for 2 hours, and stirred at a rate of 200 rpm. After the secondary reaction is completed, the mixture is naturally cooled to room temperature, filtered, and secondary acid leaching residue and secondary acid leaching liquid are obtained. The secondary acid leaching liquid can be added to the acid solution used for the primary treatment of the roasted product. The secondary acid leaching residue is dried at 50°C for 2 hours to obtain the final waste residue.

[0020] The composition of the rare earth molten salt electrolytic slag used in this embodiment is consistent with that in embodiment 1. The final waste slag is analyzed by XRF, and the main elements are (%): F element 62.087, Al element 30.915, Nd element 0.417, Pr element 0.020, Fe element 2.493.

[0021] Comparative Example 1 The difference between this comparative example and Example 1 is that the calcination atmosphere is air and no hydrogen is added. The other conditions and operation process are consistent with Example 1. It is found that the solid obtained after the calcination is agglomerated, some solids have high hardness, and are difficult to break later, and the overall recovery rate of rare earth elements decreases.

[0022] Comparative Example 2 The difference between this comparative example and Example 1 is that the amount of aluminum chloride hexahydrate added is 25g. As a result, the calcined slag is powdery after calcination, and the particles can reach the 100 mesh requirement after simple vibration screening. However, the leaching rate of the product obtained later is reduced, and the reagent cost is increased. After XRF detection, the main element content (%) is obtained as follows: F element 59.182, Al element 28.031, Nd element 1.024, Pr element 0.186, Fe element 2.357.

[0023] Comparative Example 3 The difference between this comparative example and Example 1 is that the mass of the added aluminum particles is 15 g, and the rest of the preparation process and process conditions are consistent with Example 1. Results The leaching rate of the final product is slightly improved, but the hardness of the solid obtained by roasting is significantly increased, and it is difficult to crush.

[0024] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for recovering rare earth elements from rare earth molten salt electrolytic slag, characterized in that: Here are the steps: (1) adding aluminum particles and aluminum chloride particles to the electrolytic slag and mixing them uniformly to obtain a solid compound; (2) calcining the solid compound in a closed environment at high temperature; (3) crushing the calcined solid to obtain substance A; (4) mixing the substance A with an inorganic strong acid solution, controlling the mixing temperature and stirring the mixture to react, and filtering the mixture to obtain a primary acid leaching residue and a primary rare earth acid leaching solution; (5) Add the primary acid leaching residue to the inorganic strong acid solution for secondary mixing, control the mixing temperature and stir the reaction, filter, and obtain the secondary acid leaching residue and the secondary rare earth leaching solution.

2. The method for recovering rare earth elements from rare earth molten salt electrolytic slag according to claim 1, characterized in that: In step (1), the mass ratio of electrolytic slag: aluminum particles: aluminum chloride particles is (2-8):1:(2-4), and the size of the aluminum particles is 80-200 mesh.

3. The method for recovering rare earth elements from rare earth molten salt electrolytic slag according to claim 1, characterized in that: In step (2), the calcination temperature is 500-700° C., the calcination time is 1.5-3 h, and the calcination atmosphere is air containing a trace amount of hydrogen, wherein the hydrogen gas volume fraction is 0.05-0.2%.

4. The method for recovering rare earth elements from rare earth molten salt electrolytic slag according to claim 1, characterized in that: In step (4) and step (5), the inorganic strong acid solution is a hydrochloric acid solution with a concentration of 2.5-4M, and the stirring reaction temperature is 85-95°C.

5. The method for recovering rare earth elements from rare earth molten salt electrolytic slag according to claim 1, characterized in that: The secondary rare earth leaching solution obtained in step (5) is recovered and reused.