A method of producing rare earth metals from molten salt electrolysis

Abstract

The application belongs to the technical field of rare earth metallurgy, and discloses a method for preparing rare earth metal by electrolyzing molten salt. The method comprises the following steps: taking rare earth oxide as electrolytic raw material, and performing electrolysis in molten salt electrolyte under a protective atmosphere; and collecting the deposit after electrolysis, so that the rare earth metal is obtained. Through the synergistic effect of multi-dimensional process, the application successfully constructs an efficient, green and stable rare earth metal electrolysis preparation system. The system completely eliminates carbon pollution, significantly improves the stability of the electrolysis process and the quality of the product, realizes efficient and stable preparation of high-purity rare earth metal, and can meet the stringent requirements of high-end electronics, aerospace, high-performance permanent magnet materials and other fields for ultra-high-purity rare earth products.

Description

Technical Field

[0001] This invention relates to the field of rare earth metallurgy technology, specifically to a method for preparing rare earth metals by electrolysis of molten salt. Background Technology

[0002] Rare earth elements are crucial strategic resources supporting the development of many key sectors of the national economy, including metallurgy, petrochemicals, electronics, and aerospace. The high-quality and efficient preparation of rare earth metals and alloys directly impacts the upgrading and core competitiveness of high-end manufacturing industries. Currently, the main method for large-scale industrial production of rare earth metals and alloys is molten salt electrolysis. Among these, the fluoride molten salt electrolysis system has become the mainstream technology for industrial applications due to its relatively environmentally friendly nature and the absence of chlorine gas during electrolysis. However, it still suffers from numerous technical shortcomings in actual industrial production, severely restricting the production efficiency, product purity, and production stability of rare earth metals and alloys. Consequently, it struggles to meet the stringent requirements for ultra-high purity rare earth products in fields such as high-end electronics, aerospace, and high-performance permanent magnet materials.

[0003] Existing industrial rare earth fluoride molten salt electrolysis technology has significant shortcomings in areas such as electrolytic cell structure and electrode system, electrolysis temperature and process control, electrolyte system, and impurity behavior. Specifically, regarding electrode and electrolytic cell structure design, the industry has made numerous attempts to improve these aspects, but existing solutions have failed to fundamentally solve the core problems. For example, Chinese patent CN104372382A discloses an electrolytic cell design using parallel vertical plate electrodes. While this structure optimizes current distribution to some extent and helps improve current efficiency, both the anode and the inner liner of the electrolytic cell are made of graphite. During high-temperature electrolysis, the graphite anode reacts with the electrolysis system to generate CO2 gas, causing not only raw material loss but also introducing carbon impurities into the product. This leads to a decrease in the purity of rare earth metals and alloys, failing to meet the high-purity rare earth product requirements of advanced applications. Furthermore, the wear and tear on the graphite material shortens the lifespan of the electrodes and electrolytic cell, increasing production and maintenance costs and downtime losses. Chinese patent CN106119900A proposes a structure in which a square hollow cathode is surrounded by a ring anode. This design can create a relatively uniform electric field in the electrolysis system, which is beneficial to reduce local electrolysis anomalies. However, under the high current density conditions commonly used in industrial production, the problem of excessively high current density in the anode area remains prominent, which aggravates the oxidation loss and corrosion rate of the anode and leads to a significant increase in the frequency of anode replacement. This not only increases operating costs but also affects the continuity of production due to frequent shutdowns. At the same time, impurities generated by anode corrosion may be mixed into the product, further degrading the purity and consistency of the product.

[0004] Furthermore, in terms of electrolysis temperature and process control, existing rare earth molten salt electrolysis processes generally employ high-temperature operation above the melting point of rare earth metals, allowing the reduction products to collect in liquid form at the bottom of the tank for easy collection and discharge. However, rare earth metals have a certain solubility in fluoride molten salts. Under high-temperature conditions, prolonged contact between rare earth metals and fluoride molten salts significantly increases the dissolution loss of metals. The dissolved rare earth metals may react with other impurities in the molten salt or recrystallize during subsequent cooling, leading to reduced current efficiency and decreased purity of the final product. This fails to meet the high purity requirements of rare earth metals and alloys in high-end electronics, aerospace, and other fields. At the same time, high-temperature electrolysis also increases energy consumption, raises production costs, and places higher demands on the high-temperature resistance of the electrolytic cell, exacerbating cell wear and further increasing production and maintenance costs.

[0005] In summary, existing rare earth molten salt electrolysis technologies generally face multiple problems in actual production, including severe anode loss, insufficient product purity, limited production efficiency, and high energy consumption, making it difficult to support the development needs of large-scale, green, low-carbon, and high-quality rare earth metal preparation at the industrial level. Therefore, how to reduce anode loss, improve product purity, and effectively control energy consumption and production costs while ensuring efficient and stable electrolysis has become a key technical challenge for promoting the iteration and industrial upgrading of rare earth molten salt electrolysis technology. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing rare earth metals by electrolyzing molten salt.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] This invention provides a method for preparing rare earth metals by electrolysis of molten salt, comprising the following steps:

[0009] Under a protective atmosphere, rare earth oxides are used as electrolytic raw materials and electrolysis is carried out in a molten salt electrolyte. After electrolysis, the precipitate is collected to obtain the rare earth metal.

[0010] The molten salt electrolyte comprises rare earth fluorides, lithium fluoride, and barium fluoride; the mass of the rare earth fluorides is 50%-70% of the mass of the molten salt electrolyte.

[0011] The electrolysis temperature is lower than the primary crystallization temperature of the rare earth metal;

[0012] The electrolysis adopts a single-pulse constant current mode with a duty cycle of 60%-80%, and the effective electrochemical area of ​​the anode is 6-12 times that of the effective electrochemical area of ​​the cathode.

[0013] This invention, based on the regulation mechanism of molten salt electrolysis and interfacial electrochemistry, achieves high current efficiency, high yield, and high purity preparation of rare earth metals through the synergistic design of multi-dimensional processes. Specifically, firstly, at the electrolyte system level, this invention uses a ternary fluoride molten salt system as the electrolyte, providing a stable ion conduction medium for the dissolution and dissociation of rare earth oxides. During electrolysis, rare earth oxides dissolve and dissociate into rare earth ions in the anode region, which then migrate to the cathode under the influence of an electric field and are reduced to the metallic state, achieving the precipitation of the metallic state. Secondly, at the electrode process regulation level, this invention uses an inert anode and regulates its effective electrochemical area, thereby effectively suppressing the discharge reaction intensity of molten salt anions on the anode surface, avoiding the anode effect caused by local overpotential, and slowing down the electrochemical corrosion rate of the anode material. Furthermore, at the thermodynamic and kinetic synergy level, this invention controls the electrolysis temperature below the primary crystallization temperature of the target rare earth metal, weakening the thermodynamic dissolution tendency of rare earth metals in molten salt, reducing the side reaction rate between the metal and the electrolyte, thereby suppressing the reverse oxidation loss of the product and improving the current efficiency. Furthermore, this invention introduces a single-pulse constant current mode during the deposition process. By periodically switching on and off the power, the cathode double layer structure and the mass transfer boundary layer state are controlled, which promotes the uniform reduction of metal ions, obtains a dense and flat metal deposition layer, reduces molten salt inclusions, and improves product purity.

[0014] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the rare earth metal includes at least one of lanthanum, cerium, praseodymium, and neodymium.

[0015] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the mass of the rare earth fluoride is one or a combination of 50%, 55%, 60%, 65%, and 70% of the mass of the molten salt electrolyte.

[0016] This invention controls the mass percentage of rare earth fluorides within the above-mentioned range, and combines them with lithium fluoride and barium fluoride. This ensures that the rare earth oxide raw materials have sufficient dissolution rate and mass transfer efficiency in the molten salt, while maintaining a suitable initial crystallization temperature and viscosity of the molten salt system, thus laying a good foundation for the physical properties of the subsequent electrolysis process.

[0017] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the mass ratio of lithium fluoride to barium fluoride is (1.5-2):1.

[0018] Preferably, the mass ratio of lithium fluoride to barium fluoride is one or a combination of 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, and 2:1.

[0019] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the anode and the cathode are plate-shaped structures and are arranged in parallel or around each other in space.

[0020] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the anode and the cathode are made of tungsten and / or molybdenum.

[0021] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the anode and the cathode are vertically arranged in the molten salt electrolyte, with an immersion depth of 2cm-5cm.

[0022] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the geometric configuration of the anode is any one of a bi-fold configuration, a tri-fold configuration, a quadri-fold configuration, or a regular hexagonal ring configuration.

[0023] Preferably, the anode has a hexagonal ring configuration.

[0024] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the electrolysis temperature is T. e The primary crystallization temperature of the rare earth metal is T. p , among which, T p -T e =10-50.

[0025] Preferably, the T e The value range is 770℃-1000℃; the T value is... p The value range is 780℃-1050℃.

[0026] The primary crystallization temperature of rare earth metals in this invention refers to the temperature at which rare earth metals begin to precipitate solid crystals from the liquid state during molten salt electrolysis. For pure metals, it is equivalent to their melting point.

[0027] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the average current density of the cathode is 1.2 A·cm⁻¹. -2 -4.8A·cm -2 The average current density of the anode is 0.2 A·cm. -2 -0.4A·cm -2 .

[0028] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the pulse period of the single-pulse constant current mode is 10s.

[0029] In a preferred embodiment of the method for preparing rare earth metals by electrolysis of molten salt according to the present invention, the effective electrochemical area of ​​the anode is one or both of the following: 6 times, 8 times, 10 times, and 12 times the effective electrochemical area of ​​the cathode.

[0030] In this invention, the ratio of the effective electrochemical area of ​​the anode to the effective electrochemical area of ​​the cathode specifically refers to the ratio of the effective electrochemical areas of the anode and cathode immersed below the molten salt surface. When the immersion depths of the anode and cathode are the same, this ratio of effective electrochemical areas is the ratio of the geometric areas of the anode and cathode. Compared with the prior art, the beneficial effects of this invention are: the electrolysis system of this invention is completely free of carbon components, and the anode process does not produce CO2 gas, avoiding greenhouse gas emissions and carbon impurities contaminating the product, which is in line with the development direction of green metallurgy. Secondly, this invention, through the design of a large-area inert anode, reduces the anode current density, effectively suppresses the occurrence of the anode effect, significantly slows down the electrochemical loss of the anode material, extends the electrode lifespan, and ensures the long-term stable operation of the electrolysis process. Meanwhile, this invention utilizes electrolysis at a low temperature below the primary crystallization temperature of the target rare earth metal, significantly reducing the thermodynamic dissolution loss of the rare earth metal in the molten salt. Furthermore, the rapid cooling process after electrolysis greatly shortens the contact time between the product and the molten salt, doubly suppressing reverse oxidation and secondary reactions, significantly improving current efficiency, reducing the risk of molten salt inclusions, and stably obtaining high-purity rare earth metals. In addition, this invention's method for preparing rare earth metals by electrolyzing molten salt allows for flexible control of the preparation processes of different rare earth metals and their alloys, demonstrating good technical versatility and promising prospects for industrial application. Attached Figure Description

[0031] Figure 1 This is a top view of the electrode structure used in Embodiment 1 of the present invention; ① is the anode, ② is the cathode;

[0032] Figure 2 This is a top view of the electrode structure used in Embodiment 2 of the present invention; ① is the anode, ② is the cathode;

[0033] Figure 3 This is a top view of the electrode structure used in Embodiment 5 of the present invention; ① is the anode, ② is the cathode;

[0034] Figure 4 This is a top view of the electrode structure used in Embodiment 6 of the present invention; ① is the anode, ② is the cathode;

[0035] Figure 5 This is a top view of the electrode structure used in Comparative Example 3 of the present invention; ① is the anode, and ② is the cathode. Detailed Implementation

[0036] To better illustrate the objectives, technical solutions, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0037] The following description, in conjunction with specific embodiments, illustrates the practical effects of the present invention.

[0038] Unless otherwise specified, the experimental methods used in the examples are conventional methods; the materials, reagents, equipment, etc. used are all commercially available unless otherwise specified.

[0039] In the embodiments of the present invention, the adjustment of the effective electrochemical area ratio between the anode and the cathode is mainly achieved through the following two methods:

[0040] (1) Change the anode configuration (same depth): Under the premise of keeping the cathode size and the immersion depth of the anode and cathode unchanged, the total geometric area of ​​the anode can be changed by increasing or decreasing the number of anode folds. Since the immersion depth is constant, the increase factor of the anode geometric area directly corresponds to the increase factor of its effective electrochemical area.

[0041] (2) Change the immersion depth (isomorphism): While keeping the electrode configuration (shape) unchanged, the actual contact area between the anode and the electrolyte is changed by adjusting the depth of the anode immersion below the molten salt surface, thereby adjusting the effective electrochemical area ratio.

[0042] Example 1: Preparation of metallic lanthanum (La)

[0043] (1) Preparation of electrolytes and raw materials

[0044] Weigh and mix the following mass percentages to prepare the molten salt electrolyte: 70 wt% lanthanum fluoride (LaF3), 20 wt% lithium fluoride (LiF), and 10 wt% barium fluoride (BaF2).

[0045] (2) Electrode arrangement

[0046] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 1 The tungsten anode and the central rectangular tungsten cathode are arranged in a hexagonal ring configuration. The surface area of ​​the anode is 12 times that of the cathode. The immersion depth of the anode and cathode is the same, 3.5 cm. That is, the effective electrochemical area of ​​the anode is also 12 times that of the cathode.

[0047] (3) Atmosphere protection and melting

[0048] Seal the electrolytic cell and perform a vacuuming-purification-high-purity argon (Ar) cycle at least three times to ensure that the air in the system is completely replaced. Under the protection of the argon atmosphere, raise the temperature to completely melt the mixed electrolyte and maintain the electrolysis temperature at 900℃ (the primary crystallization temperature of metallic lanthanum is about 920℃).

[0049] (4) Electrolysis process

[0050] After the temperature stabilizes, lanthanum oxide (La₂O₃) is added to the anode region as an electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10 s (including an 8 s on-time, a 2 s off-time, and a duty cycle of 80%); and an average current density at the cathode of 2.4 A·cm⁻¹. -2 The average current density at the anode is 1 / 12 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0051] (5) Product separation and cooling

[0052] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0053] (6) Product testing

[0054] The product was tested according to the method given in the national standard GB / T 14635-2020 "Chemical Analysis Methods for Rare Earth Metals and Their Compounds - Determination of Total Rare Earth Content".

[0055] The obtained metal was found to be lanthanum with a purity of 99.3%, and the current efficiency was 93.7% based on statistical analysis of the electrolysis process and product quality.

[0056] Example 2: Preparation of metallic cerium (Ce)

[0057] (1) Preparation of electrolytes and raw materials

[0058] Weigh and mix the following mass percentages to prepare the molten salt electrolyte: 70 wt% cerium fluoride (CeF3), 20 wt% lithium fluoride (LiF), and 10 wt% barium fluoride (BaF2).

[0059] (2) Electrode arrangement

[0060] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 2 The tungsten anode and the central rectangular tungsten cathode with a four-fold configuration shown have an immersion depth of 3.5 cm for both the anode and cathode. The effective electrochemical area of ​​the anode is 10 times that of the effective electrochemical area of ​​the cathode.

[0061] (3) Atmosphere protection and melting

[0062] Seal the electrolytic cell and perform a vacuuming-purification-high-purity argon (Ar) cycle at least three times to ensure that the air in the system is completely replaced. Under the protection of the argon atmosphere, raise the temperature to completely melt the mixed electrolyte and maintain the electrolysis temperature at 770°C (the primary crystallization temperature of metallic cerium is about 795°C).

[0063] (4) Electrolysis process

[0064] After the temperature stabilizes, cerium oxide (CeO2) is added to the anode region as the electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10s (including an 8s on-time, a 2s off-time, and a duty cycle of 80%); and an average current density at the cathode of 2.4 A·cm⁻¹. -2 The average current density at the anode is 1 / 10 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0065] (5) Product separation and cooling

[0066] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0067] (6) Product testing

[0068] The obtained metal was found to be cerium with a purity of 99.0% and a current efficiency of 82.4%.

[0069] Example 3: Preparation of metallic neodymium (Nd)

[0070] (1) Preparation of electrolytes and raw materials

[0071] Weigh and mix the following mass percentages to prepare the molten salt electrolyte: 70wt% neodymium fluoride (NdF3), 20wt% lithium fluoride (LiF), and 10wt% barium fluoride (BaF2).

[0072] (2) Electrode arrangement

[0073] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 1 The tungsten anode and the central rectangular tungsten cathode, which are arranged in a regular hexagonal ring configuration, are both immersed to a depth of 3.5 cm. The effective electrochemical area of ​​the anode is 12 times that of the effective electrochemical area of ​​the cathode.

[0074] (3) Atmosphere protection and melting

[0075] Seal the electrolytic cell and perform a vacuuming-purification-high-purity argon (Ar) cycle at least three times to ensure that the air in the system is completely replaced. Under the protection of the argon atmosphere, raise the temperature to completely melt the mixed electrolyte and maintain the electrolysis temperature at 1000℃ (the primary crystallization temperature of neodymium is about 1024℃).

[0076] (4) Electrolysis process

[0077] After the temperature stabilizes, neodymium oxide (Nd₂O₃) is added to the anode region as an electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10 s (including an 8 s on-time, a 2 s off-time, and a duty cycle of 80%); and an average current density of 3 A·cm⁻¹ at the cathode. -2 The average current density at the anode is 1 / 12 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0078] (5) Product separation and cooling

[0079] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0080] (6) Product testing

[0081] The obtained metal was found to be neodymium with a purity of 99.1% and a current efficiency of 90.7%.

[0082] Example 4:

[0083] The difference between the preparation method of lanthanum metal in this embodiment and that in Example 1 is only that: in step (1), the fluoride electrolyte includes the following components by mass percentage: 50wt% lanthanum fluoride (LaF3), 30wt% lithium fluoride (LiF), and 20wt% barium fluoride (BaF2).

[0084] The remaining steps are the same as in Example 1;

[0085] Process phenomena: During electrolysis, it was found that the dissolution rate of La2O3 raw material was slower than that in Example 1, and the voltage of the electrolytic cell fluctuated occasionally.

[0086] Test results: The purity of lanthanum metal was 98.8%, and the current efficiency was 93.0%.

[0087] Example 5:

[0088] The difference between the preparation method of metallic lanthanum in this embodiment and that in Example 1 is only that in step (2), the following method is used: Figure 3The diagram shows a tungsten anode with a three-fold configuration and a central rectangular tungsten cathode; both electrodes are immersed to a depth of 3.5 cm, and the effective electrochemical area of ​​the anode is 8 times that of the cathode.

[0089] The remaining steps are the same as in Example 1;

[0090] Process phenomena: The cell voltage is relatively stable during the electrolysis process, with occasional random fluctuations.

[0091] Test results: The purity of lanthanum metal was found to be 99.0%, and the current efficiency was 92.6%.

[0092] Example 6:

[0093] The difference between the preparation method of metallic lanthanum in this embodiment and that in Example 1 is only that in step (2), the following method is used: Figure 4 The diagram shows a bifold tungsten anode and a central rectangular tungsten cathode; the immersion depth of both the anode and cathode is 3.5 cm, and the effective electrochemical area of ​​the anode is 6 times that of the cathode.

[0094] The remaining steps are the same as in Example 1;

[0095] Process phenomena: The cell voltage is relatively stable during the electrolysis process, but there are occasional low-frequency fluctuations in the cell voltage.

[0096] Test results: The purity of lanthanum metal was found to be 98.6%, and the current efficiency was 92.3%.

[0097] Example 7:

[0098] The difference between the preparation method of metallic lanthanum in this embodiment and that in Example 1 is only that in step (2), the following method is used: Figure 1 The diagram shows a tungsten anode with a hexagonal surrounding configuration and a central rectangular tungsten cathode; the cathode has an immersion depth of 4.0 cm, the anode has an immersion depth of 2.0 cm, and the effective electrochemical area of ​​the anode is 6 times that of the cathode.

[0099] The remaining steps are the same as in Example 1;

[0100] Process phenomena: The cell voltage is relatively stable during the electrolysis process, but there are occasional low-frequency fluctuations in the cell voltage.

[0101] Test results: The purity of lanthanum metal was 98.8%, and the current efficiency was 92.0%.

[0102] Example 8:

[0103] The difference between the preparation method of lanthanum metal in this embodiment and that in Example 1 is only that: in step (4), the pulse period is 10s (where the power-on time is 6s, the power-off time is 4s, and the duty cycle is 60%).

[0104] The remaining steps are the same as in Example 1;

[0105] Process phenomena: The cell voltage is relatively stable during the electrolysis process.

[0106] Test results: The surface of the electrolyzed lanthanum metal was relatively dense and had good uniformity. The purity of the lanthanum metal was measured to be 99.1%, and the current efficiency was 92.1%.

[0107] Comparative Example 1: Preparation of metallic lanthanum (La)

[0108] (1) Preparation of electrolytes and raw materials

[0109] Weigh and mix the following mass percentages to prepare the molten salt electrolyte: 40 wt% lanthanum fluoride (LaF3), 40 wt% lithium fluoride (LiF), and 20 wt% barium fluoride (BaF2).

[0110] (2) Electrode arrangement

[0111] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 1 The tungsten anode and the central rectangular tungsten cathode, which are arranged in a regular hexagonal ring configuration, are both immersed to a depth of 3.5 cm. The effective electrochemical area of ​​the anode is 12 times that of the cathode.

[0112] (3) Atmosphere protection and melting

[0113] Seal the electrolytic cell and perform a vacuuming-purification-high-purity argon (Ar) cycle at least three times to ensure that the air in the system is completely replaced. Under the protection of the argon atmosphere, raise the temperature to completely melt the mixed electrolyte and maintain the electrolysis temperature at 900℃ (the primary crystallization temperature of metallic lanthanum is about 920℃).

[0114] (4) Electrolysis process

[0115] After the temperature stabilizes, lanthanum oxide (La₂O₃) is added to the anode region as an electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10 s (including an 8 s on-time, a 2 s off-time, and a duty cycle of 80%); and an average current density at the cathode of 2.4 A·cm⁻¹. -2 The average current density at the anode is 1 / 12 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0116] During electrolysis, it was found that the dissolution rate of the La2O3 raw material was slow and the voltage of the electrolytic cell fluctuated greatly.

[0117] (5) Product separation and cooling

[0118] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0119] (6) Product testing

[0120] After electrolysis, it was found that the amount of cathode deposits obtained was significantly less than that in Example 1, and the purity of lanthanum metal was 93.7% and the current efficiency was 81.7%.

[0121] Comparative Example 2: Preparation of metallic lanthanum (La)

[0122] (1) Preparation of electrolytes and raw materials

[0123] Weigh and mix the following percentages by mass to prepare the molten salt electrolyte: 85 wt% lanthanum fluoride (LaF3), 10 wt% lithium fluoride (LiF), and 5 wt% barium fluoride (BaF2).

[0124] (2) Electrode arrangement

[0125] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 1 The tungsten anode and the central rectangular tungsten cathode, which are arranged in a regular hexagonal ring configuration, are both immersed to a depth of 3.5 cm. The effective electrochemical area of ​​the anode is 12 times that of the cathode.

[0126] (3) Atmosphere protection and melting

[0127] The electrolytic cell is sealed, and a vacuum-purification-high-purity argon (Ar) cycle is performed at least three times to ensure that the air in the system is completely replaced. The temperature is raised under the protection of the argon atmosphere to completely melt the mixed electrolyte. However, the viscosity of the mixed electrolyte is too high at this time. In order to ensure that it has the necessary fluidity during the electrolysis process, the electrolysis temperature is increased to 960°C (the primary crystallization temperature of lanthanum is about 920°C).

[0128] (4) Electrolysis process

[0129] After the temperature stabilizes, lanthanum oxide (La₂O₃) is added to the anode region as an electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10 s (including an 8 s on-time, a 2 s off-time, and a duty cycle of 80%); and an average current density at the cathode of 2.4 A·cm⁻¹. -2 The average current density at the anode is 1 / 12 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0130] During electrolysis, it was found that the dissolution rate of the La2O3 raw material was slow and the voltage of the electrolytic cell fluctuated greatly.

[0131] (5) Product separation and cooling

[0132] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0133] (6) Product testing

[0134] After electrolysis, it was found that the separation between the deposited metal and the electrolyte was poor, and the deposit contained a lot of solidified salt particles. Furthermore, the purity of the lanthanum metal was 93.9%, and the current efficiency was 82.0%.

[0135] Comparative Example 3: Preparation of metallic lanthanum (La)

[0136] (1) Preparation of electrolytes and raw materials

[0137] Weigh and mix the following mass percentages to prepare the molten salt electrolyte: 70 wt% lanthanum fluoride (LaF3), 20 wt% lithium fluoride (LiF), and 10 wt% barium fluoride (BaF2).

[0138] (2) Electrode arrangement

[0139] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 5 The tungsten anode and the central rectangular tungsten cathode in the single-plate configuration shown have an immersion depth of 3.5 cm for both the anode and cathode, and the effective electrochemical area of ​​the anode is twice that of the effective electrochemical area of ​​the cathode.

[0140] (3) Atmosphere protection and melting

[0141] Seal the electrolytic cell and perform a vacuuming-purification-high-purity argon (Ar) cycle at least three times to ensure that the air in the system is completely replaced. Under the protection of the argon atmosphere, raise the temperature to completely melt the mixed electrolyte and maintain the electrolysis temperature at 900℃ (the primary crystallization temperature of metallic lanthanum is about 920℃).

[0142] (4) Electrolysis process

[0143] After the temperature stabilizes, lanthanum oxide (La₂O₃) is added to the anode region as an electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10 s (including an 8 s on-time, a 2 s off-time, and a duty cycle of 80%); and an average current density at the cathode of 2.4 A·cm⁻¹. -2 The average current density at the anode is half that at the cathode; electrolysis continues for 90 minutes.

[0144] During electrolysis, obvious signs of the anodic effect can be observed: the cell voltage exhibits high-frequency fluctuations (the fluctuation amplitude exceeds 20% of the original voltage).

[0145] (5) Product separation and cooling

[0146] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0147] (6) Product testing

[0148] Tests showed that the purity of lanthanum metal was 92.4% and the current efficiency was 78.7%.

[0149] Comparative Example 4: Preparation of metallic lanthanum (La)

[0150] (1) Preparation of electrolytes and raw materials

[0151] Weigh and mix the following mass percentages to prepare the molten salt electrolyte: 70 wt% lanthanum fluoride (LaF3), 20 wt% lithium fluoride (LiF), and 10 wt% barium fluoride (BaF2).

[0152] (2) Electrode arrangement

[0153] The homogeneously mixed electrolyte is placed in an electrolytic cell lined with tungsten, and then inserted as follows: Figure 1 The diagram shows a tungsten anode with a hexagonal surrounding configuration and a central rectangular tungsten cathode. The cathode is immersed to a depth of 3.5 cm, and the anode is immersed to a depth of 4.7 cm. The effective electrochemical area of ​​the anode is 16 times that of the effective electrochemical area of ​​the cathode.

[0154] (3) Atmosphere protection and melting

[0155] Seal the electrolytic cell and perform a vacuuming-purification-high-purity argon (Ar) cycle at least three times to ensure that the air in the system is completely replaced. Under the protection of the argon atmosphere, raise the temperature to completely melt the mixed electrolyte and maintain the electrolysis temperature at 900℃ (the primary crystallization temperature of metallic lanthanum is about 920℃).

[0156] (4) Electrolysis process

[0157] After the temperature stabilizes, lanthanum oxide (La₂O₃) is added to the anode region as an electrolytic feedstock. Electrolysis is performed using a single-pulse constant current power supply with the following parameters: pulse period of 10 s (including an 8 s on-time, a 2 s off-time, and a duty cycle of 80%); and an average current density at the cathode of 2.4 A·cm⁻¹. -2 The average current density at the anode is 1 / 16 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0158] The voltage is stable during the electrolysis process, but the anode structure inside the electrolytic cell is large, which leads to poor thermal uniformity inside the cell.

[0159] (5) Product separation and cooling

[0160] After electrolysis is completed, power is immediately stopped. Under the protection of argon atmosphere, the electrode assembly is quickly raised to 15cm above the molten salt surface, and the argon flow rate is increased to purge and cool the cathode deposit. After the electrolytic cell cools down, the metal deposited on the cathode is removed.

[0161] (6) Product testing

[0162] After electrolysis, the concentration of the cathode deposited metal decreased, and a few diffuse deposits appeared at the edges. The purity of the lanthanum metal was 94.7%, and the current efficiency was 81.8%.

[0163] Comparative Example 5: Preparation of metallic lanthanum (La)

[0164] The only difference between the preparation method of this comparative lanthanum metal and Example 1 is that in step (4), the pulse period is 10s (where the on time is 5s, the off time is 5s, and the duty cycle is 50%).

[0165] The remaining steps are the same as in Example 1;

[0166] Process phenomena: During electrolysis, the cell voltage fluctuates slightly intermittently and has poor stability.

[0167] Test results: After electrolysis, the obtained lanthanum metal deposit was relatively dense, but the surface uniformity was average. There were slight dendrites under the microscopic level. The purity of lanthanum metal was 94.6% and the current efficiency was 83.7%.

[0168] Comparative Example 6: Preparation of metallic lanthanum (La)

[0169] The only difference between the preparation method of this comparative lanthanum metal and Example 1 is that in step (4), the pulse period is 10s (where the on time is 9s, the off time is 1s, and the duty cycle is 90%).

[0170] The remaining steps are the same as in Example 1;

[0171] Process phenomena: During electrolysis, the cell voltage fluctuates slightly intermittently and has poor stability.

[0172] Test results: After electrolysis, the surface of the obtained lanthanum metal deposit was relatively smooth, but there were small pores in some areas. The purity of the lanthanum metal was 96.1% and the current efficiency was 82.9%.

[0173] Comparative Example 7: Preparation of metallic lanthanum (La)

[0174] The only difference between the preparation method of lanthanum metal in this comparative example and that in Example 1 is that in step (4), a constant current DC power supply is used for electrolysis, and the electrolysis parameters are set as follows: the average current density at the cathode is 2.4 A·cm⁻¹. -2 The average current density at the anode is 1 / 12 of the average current density at the cathode; electrolysis continues for 90 minutes.

[0175] The remaining steps are the same as in Example 1;

[0176] Process phenomena: During electrolysis, the cell voltage fluctuations increase significantly, and the stability is poor;

[0177] Test results: After electrolysis, the sediment was irregular in shape and contained molten salt. The purity of lanthanum was 98.1% and the current efficiency was 83.4%.

[0178] Comparative Example 8: Preparation of metallic lanthanum (La)

[0179] The only difference between the preparation method of this comparative lanthanum metal and Example 1 is that the electrolysis temperature in step (3) is 1050℃;

[0180] The remaining steps are the same as in Example 1;

[0181] Test results: After electrolysis, it was found that the electrode plates were severely corroded at this temperature, affecting the electrode life. The purity of the obtained lanthanum metal was 92.1%, and the current efficiency was 79.3%.

[0182] As can be seen from the test results of the above embodiments and comparative examples, this invention successfully constructs a highly efficient, green, and stable rare earth metal electrolytic preparation system through the synergistic effect of multi-dimensional processes. Specifically, in terms of the electrolyte system, this invention controls the mass percentage of rare earth fluorides within the range of 50%-70%, and combines it with lithium fluoride and barium fluoride. This ensures that the rare earth oxide raw materials have sufficient dissolution rate and mass transfer efficiency in the molten salt, while maintaining a suitable initial crystallization temperature and viscosity of the molten salt system, thus laying a good foundation for the subsequent electrolytic process. Secondly, in terms of electrode structure design, this invention uses anodes and cathodes made of inert materials, and controls the effective electrochemical area of ​​the anode and cathode through a special geometric configuration (surround type, multi-fold type). This avoids the anode effect and cell voltage fluctuation caused by high current density due to an excessively small anode area, and also prevents the anode area from being too large. The uneven thermal field distribution and sediment dispersion caused by the large volume of sediment in the tank have enabled long-term stable operation of the electrolysis process. Furthermore, in terms of electrolysis process control, this invention uses a single-pulse constant current power supply to effectively improve the microstructure of the cathode deposit, suppress dendrite growth and molten salt inclusions, thereby improving product purity while ensuring the density of the deposited layer. Moreover, this invention strictly controls the electrolysis temperature within a range that is lower than the primary crystallization temperature of the target rare earth metal and the temperature difference does not exceed 50°C. Through the combined strategy of "low-temperature electrolysis + rapid separation", the dissolution loss and secondary reactions of rare earth metals in high-temperature molten salt are suppressed from both thermodynamic and kinetic perspectives, fundamentally solving the core problems of low current efficiency and poor purity in traditional high-temperature electrolysis processes.

[0183] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing rare earth metals by electrolysis of molten salt, characterized in that, Includes the following steps: Under a protective atmosphere, rare earth oxides are used as electrolytic raw materials and electrolysis is carried out in a molten salt electrolyte. After electrolysis, the precipitate is collected to obtain the rare earth metal. The molten salt electrolyte comprises rare earth fluorides, lithium fluoride, and barium fluoride; the mass of the rare earth fluorides is 50%-70% of the mass of the molten salt electrolyte. The electrolysis temperature is lower than the primary crystallization temperature of the rare earth metal; The electrolysis adopts a single-pulse constant current mode with a duty cycle of 60%-80%, and the effective electrochemical area of ​​the anode is 6-12 times that of the effective electrochemical area of ​​the cathode.

2. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The rare earth metals include at least one of lanthanum, cerium, praseodymium, and neodymium.

3. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The mass of the rare earth fluoride is 70% of the mass of the molten salt electrolyte.

4. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The mass ratio of lithium fluoride to barium fluoride is (1.5-2):

1.

5. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 4, characterized in that, The mass ratio of lithium fluoride to barium fluoride is 2:

1.

6. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The anode and the cathode are plate-shaped structures and are arranged in parallel or around each other in space.

7. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The anode's geometric configuration can be any one of the following: bi-fold configuration, tri-fold configuration, quadrature configuration, or regular hexagonal ring configuration.

8. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The electrolysis temperature is T. e The primary crystallization temperature of the rare earth metal is T. p , among which, T p -T e =10℃-50℃.

9. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The average current density of the cathode is 1.2 A·cm. -2 -4.8A·cm -2 The average current density of the anode is 0.2 A·cm. -2 -0.4A·cm -2 .

10. The method for preparing rare earth metals by electrolysis of molten salt as described in claim 1, characterized in that, The pulse period of the single-pulse constant current mode is 10s.

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