CN106222703A - Multistep selective electrolysis reclaims the method for metal in hard alloy scraps - Google Patents

Abstract

The invention provides a method for multi-step selective electrolytic recovery of metals in waste hard alloys, which includes the following operations: the first step of electrolysis: using waste hard alloys as an anode, inserting an anode and a cathode into a molten salt medium, and Electrolysis in the molten salt dielectric; the second step of electrolysis: take out the cathode used in the first step, insert the second cathode into the molten salt medium, and then perform the second step of electrolysis; or, take out the anode and clean it before using a new one The molten salt medium is subjected to the second step of electrolysis, and the electrolysis method is constant voltage electrolysis or constant current electrolysis. In the method proposed by the present invention, in the electrolysis process, the method of multi-cathode and stepwise electrolysis is adopted, and by controlling the electrolysis parameters, the types of ions entering the molten salt medium when the anode material-waste cemented carbide is dissolved are controlled. After diffusion and mass transfer, the ions dropped from the anode migrate to the cathode, and are deposited as simple metal by discharge, and different elements are deposited on different cathodes, so as to realize the purpose of selectively recovering the metal elements in the cemented carbide.

Description

Method for recovering metal from waste cemented carbide by multi-step selective electrolysis

technical field

The invention belongs to the field of metallurgy, in particular to a method for recovering metal from waste cemented carbide.

Background technique

Tungsten is a rare metal with high melting point, high specific gravity and high hardness, and it is an irreplaceable strategic resource. Its carbide and bonding metal can be made into high-composite cemented carbide by powder metallurgy, which has high hardness, high wear resistance, high elastic modulus, high compressive strength, and good chemical stability (acid, alkali, high temperature, etc.) Oxidation), low impact toughness, low expansion coefficient, thermal conductivity, electrical conductivity and other characteristics. It has been widely used in machinery manufacturing, mining, transportation, energy exploration, architectural decoration and other fields. According to statistics, after entering the 21st century, the world's annual output of cemented carbide has reached 38,000 tons, of which China's cemented carbide output is about 15,000 tons, more than one-third of the world's cemented carbide output ^[1] . Therefore, from the perspective of production scale and production volume, China is truly the world's largest producer. However, with the rapid development of the national economy in recent years, the consumption of tungsten ore resources has increased rapidly, the phenomenon of excessive mining and abuse of tungsten ore is serious, and the utilization efficiency of resources is very low, which has rapidly reduced the reserves of tungsten ore resources. On the other hand, the rapid development of the tungsten products industry has inevitably resulted in the accumulation of a large amount of cemented carbide waste, and the discarded cemented carbide due to various reasons reaches more than half every year ^[2,3] . It not only pollutes the environment, but also causes waste of metal resources such as tungsten and cobalt ^[4,5] .

At present, there are more than ten main methods for recycling waste cemented carbide and tungsten-based alloys in various countries in the world, mainly including high-temperature treatment, mechanical crushing, oxidation, zinc melting, electrochemical methods, and in these methods The extension of the new technology on ^[6] . Among these methods, the high temperature treatment method, mechanical crushing method and oxidation method cannot achieve the separation of tungsten and cobalt, and only the alloy mixture with the same composition as the cemented carbide scrap can be obtained ^[7] . Zinc melting process is relatively simple, short process, small investment, low cost, especially suitable for processing waste cemented carbide with cobalt content less than 10%, suitable for small enterprises to use waste cemented carbide to remanufacture cemented carbide, but this A kind of method environmental pollution is bigger, and energy utilization rate is not high. Electrochemical methods are mainly aqueous solution electrochemical method ^[8] , ionic liquid electrochemical method ^[9-11] and molten salt electrochemical method ^[12,13] . Among them, the main research and development in the field of waste cemented carbide recovery is the recovery of cobalt by electrochemical method in aqueous solution. This method recovers the cobalt metal in the cemented carbide through electrification and electrolysis. Although the aqueous solution electrolysis method can shorten the cobalt recovery process, simplify the operation steps, increase the cobalt recovery rate, and improve the working environment, the electrolysis process is prone to anode passivation and current efficiency reduction, which has certain limitations. At the same time, due to the electrochemical window of the aqueous solution, only the cobalt metal in the cemented carbide can be recovered, but the recovery of tungsten ions with a relatively negative deposition potential cannot be realized.

The molten salt electrolysis method uses molten salt as the electrolyte and waste cemented carbide as the anode to directly recover metal elements in one step, and the recovered products are collected at the cathode. The whole process is simple, there is no exhaust gas or waste liquid discharge, and the recovery efficiency is high. It is in line with the general trend of short process, low cost and environmental friendliness in the metallurgical industry. Chinese patent CN104018190A proposes a method for recovering tungsten-cobalt metal by molten salt electrolysis of waste cemented carbide, but the separation of the product is not realized. However, although some other hydrometallurgical methods and redox methods have achieved the separation of metal elements, they all have limitations. Therefore, it is necessary to find an environmentally friendly, efficient and simple method for selectively recovering metal elements in cemented carbide.

references

[1] Xu Chuanhua. Foreign Cobalt Production [J]. Journal of Beijing Research Institute of Mining and Metallurgy, 1980, (3): 31.

[2] Xu Tao. Analysis on the development trend of cemented carbide high-end products and new materials [J]. Cemented Carbide, 2011,28(6):395-402.

[3]J.C.Lee,E.Y.Kim,J.H.Kim.Recycling of WC Co hardmetal sludge by a new hydrometallurgical route[J].International Journal of Refractory Metals and Hard Materials,2011,29(3):365-371.

[4] M. Watanabe, C. Pornthep, S. Kuroda, J. Kawakita, J. Kitamura, K. Sato, Development of WC-Co coatings by warm spray deposition for resource savings of tungsten, Journal of the Japan Institute of Metals, 2007,32(71):853-859.

[5] Chen Hao, Li Jianbo, Yang Jiangao, Research progress and development trend of cemented carbide recycling, Nonferrous Metal Science and Engineering, 2012, 9(12): 18-22.

[6] Zhai Xin. Current status of cemented carbide recycling technology [J]. Nonferrous Metal Recycling and Utilization, 2004,35(1):14-16.

[7] Fang Xingjian. Crushing process of waste cemented carbide [P]: China, 102049521AEP.2011.

[8] Liang Huqi, Chen Shiguan, Electrochemical treatment of waste cemented carbide to recover tungsten and cobalt, Journal of Shanghai University (Natural Science Edition), 1995, 17(65): 105-112.

[9]Y.T.Hsieh,M.C.Lai,H.L.Huang,I.W.Sun,Specification of cobalt-chloride-based ionic liquids and electrodeposition of Co wires,Electrochimica Acta,2014,36(117):217-223.

[10] A.Unemoto, H.Ogawa, Y.Gambe, I.Honma, Development of lithium-sulfurbatteries using room temperature ionic liquid-based quasi-solid-state electrolytes, Electrochimica Acta, 2011, 28(125): 386-394 .

[11]Y.L.Zhu,Y.Katayama,T.Miura,Effects of coumarin and saccharin onelectrodeposition of Ni from a hydrophobic ionic liquid,Electrochimica Acta,2014,17(123):303-308.

[12]E.Juzeliunas,A.Cox,D.J.Fray,Electro-deoxidation of thin silicalayer in molten salt-Globular structures with effective light absorption,Electrochimica Acta,2012,37(68):123-127.

[13] K. Yasuda, S. Kobayashi, T. Nohira, R. Hagiwara, Electrochemical formation of Nd Ni alloys in molten NaCl KCl NdCl3, Electrochimica Acta, 2013, 8(92): 349-355.

Contents of the invention

Aiming at the deficiencies in the technical field, the purpose of the present invention is to propose a method for recovering metal in waste cemented carbide.

Realize above-mentioned object technical scheme of the present invention is:

A method for selectively recycling metals in waste cemented carbide with multiple cathodes, comprising the following operations:

The first step of electrolysis: use waste cemented carbide as the anode, insert the anode and the first cathode in the molten salt medium during the first step of electrolysis, and electrolyze in the molten molten salt dielectric, the electrolysis temperature is 600 ~ 1000 ℃, and the electrolysis method is Constant voltage electrolysis or constant current electrolysis, gas protection during electrolysis;

The second step of electrolysis: take out the first cathode, insert the second cathode into the molten salt medium electrolyzed in the first step, and then perform the second step of electrolysis; or, take out the anode and insert it into a new molten salt medium with the second cathode , carry out the second step of electrolysis; the electrolysis temperature is 600-1000°C, and the electrolysis method is constant voltage electrolysis or constant current electrolysis.

In the second step, only the cathode is replaced, or a new molten salt system is replaced, and the obtained cathode products are different. The mechanism may be due to the low concentration of carbon atoms in the new molten salt system, which is not enough to react with the tungsten deposited on the cathode.

The gas protection is protection with a non-oxidizing gas, and the non-oxidizing gas may be composed of one or more of nitrogen, argon, helium, or other gases that do not contain oxygen.

Wherein, the molten salt medium in the first step and the new molten salt medium in the second step are both NaCl-KCl molten salt systems with a molar ratio of 1:0.2-2.0. Other molten salt systems can also be used for electrolysis, and the NaCl-KCl system has the advantages of being cheap and easy to separate.

The waste cemented carbide is one of tungsten-cobalt cemented carbide, tungsten-titanium-cobalt cemented carbide, tungsten-titanium-tantalum, tungsten-titanium-niobium cemented carbide, specifically YG3, YG6, YG8, YG10, One of YG16, YG20, YT15, YG11.

Wherein, the materials of the first cathode and the second cathode are independently one of molybdenum, titanium, stainless steel, carbon, and graphite.

Wherein, the voltage of the constant voltage electrolysis is 0.1V-1.6Vvs.Ag/AgCl, and the current density of the constant current electrolysis is 0.01-1A/cm ² .

Further, the first step of electrolysis is constant voltage electrolysis, the voltage is 0.1V-0.5Vvs.Ag/AgCl, and the electrolysis time is 6-12 hours.

As one of the preferred technical solutions of the present invention, the second step of electrolysis is constant voltage electrolysis, and the electrolysis voltage is 0.6-1.6Vvs.Ag/AgCl.

Or, the second step of electrolysis is constant current electrolysis, and the cell voltage of electrolysis is kept below 3.2V by controlling the current density.

In the second step of electrolysis, the first cathode is taken out, and the second cathode is inserted into the molten salt medium of the first step of electrolysis, and then the second step of electrolysis is performed, and the electrolysis time is 1 to 48 hours; or, after the anode is taken out and cleaned, and the second step of electrolysis The two cathodes are inserted into a new molten salt medium, and the second step of electrolysis is performed, and the electrolysis time is 1 to 4 hours.

About the time of the second step of electrolysis: If the second step is to change the molten salt, the purpose is to obtain pure tungsten, then the electrolysis time is 1 to 4 hours, and the purity of the product will decrease after 4 hours; if the second step does not change the molten salt, the purpose is Obtaining WC depends only on the amount of anode waste alloy, and it can be electrolyzed for a long time.

Among them, after the first cathode is taken out, it is placed in water for ultrasonic treatment together with the molten salt medium near the first cathode, and then solid-liquid separation is carried out to obtain the metal; after the second step of electrolysis, the second cathode is taken out, and the The molten salt medium is placed in water for ultrasonic treatment, and then solid-liquid separation is performed to obtain the second-step electrolysis product.

Further, the solid-liquid separation method includes one or more of centrifugal separation, filtration, and drying, wherein the centrifugal speed of the centrifugal separation is 2000-6000 rpm, and the drying temperature is 30-100°C.

The method of the present invention can adopt a kind of multi-cathode electrolysis device for reclaiming metal elements in waste cemented carbide, which comprises a power supply, an electrolytic cell, an anode, a first cathode, and a second cathode, and the electrolytic cell is a crucible, so Both the first cathode and the second cathode are connected to the power supply through wires. The electrolytic cell is placed in a closed container, and the container is provided with a temperature control device and a gas protection device. The crucible may be an alumina crucible.

The beneficial effects of the present invention are:

The method proposed by the present invention adopts multi-cathode and step-by-step electrolysis in the electrolysis process, and controls the types of ions that enter the molten salt medium when the anode material-waste hard alloy dissolves, by controlling the electrolysis parameters. After diffusion and mass transfer, the ions dropped from the anode migrate to the cathode, and are deposited as simple metal by discharge, and different elements are deposited on different cathodes, so as to realize the purpose of selectively recovering the metal elements in the cemented carbide. This method has a short technological process, and can recover various metal elements in the hard alloy; it is environmentally friendly, and there is no waste gas and liquid discharge; the equipment is simple, and only needs an electrolytic cell that can control the temperature.

The tungsten carbide, tungsten and cobalt materials separated and obtained by the method have high purity and can be applied to fields such as military industry, aerospace, mechanical processing, mining tools, electronic communication, and construction.

Description of drawings

Fig. 1 is the structural representation of multi-electrode electrolyzer of the present invention;

Fig. 2 is the schematic diagram of the structure of the electrolyzer during the first step of electrolysis in embodiment 3.

In the figure, 1: power supply, 2: first cathode, 3: second cathode, 4: waste cemented carbide anode, 5: molten salt, 6: crucible. 7 electrochemical workstations, 8 reference electrodes.

Fig. 3 is the cathode product XRD diagram of the selective recovery of tungsten-cobalt metal by electrolysis of YG6 type cemented carbide by multi-cathode method in embodiment 1.

Fig. 4 is the cathode product XRD figure of embodiment 2 adopting multi-cathode method to electrolyze YG6 type cemented carbide to selectively recover tungsten-cobalt metal.

Fig. 5 is the cathode product XRD diagram of the selective recovery of tungsten-cobalt metal by electrolysis of YG6 type cemented carbide in embodiment 3.

Fig. 6 is the cathode product XRD diagram of the selective recovery of tungsten-cobalt metal by electrolysis of YG6 type cemented carbide in embodiment 4.

Figure 7 is the XRD pattern of the cathode product of the selective recovery of tungsten-cobalt metal by electrolysis of YG11 type cemented carbide in Example 5 by the step-by-step electrolysis method.

detailed description

The present invention is illustrated below through the preferred embodiments. It should be understood by those skilled in the art that the examples are only used to illustrate the present invention and not to limit the scope of the present invention.

In the examples, unless otherwise specified, the means used are conventional means in the art.

Test example: comparison of electrolysis time in the first step

DC power supply is used to electrolyze YG6 hard alloy, the molten salt system used for electrolysis is an equimolar NaCl-KCl molten salt system, the protective gas is argon, the anode is YG6 hard alloy block, the cathode is molybdenum sheet, and the reference electrode is Ag/AgCl, control the electrolysis temperature to 750°C, 0.4V constant voltage electrolysis, the electrolysis time is 1h, 2h, ... to 12h, a total of 12 tests, and then change to the second cathode for the second step of constant voltage electrolysis Electrolysis, voltage 0.8V, time 2h.

After the second step of electrolysis, take the powder separated from the second cathode for XRD, and compare it with the standard spectrum PDF#15-0805. It is found that the samples obtained on the second cathode have the characteristics of Co during the first step of electrolysis for 1h to 8h There is no Co peak in the samples after 8h. Considering the loss of electric energy comprehensively, it is advisable to choose the electrolysis time of the first step as 8h.

Example 1:

In the present embodiment, use the device shown in Figure 1:, comprise power source 1, electrolyzer, waste cemented carbide anode 4, first cathode 2, second cathode 3, described electrolyzer is crucible 6 (aluminum oxide crucible) , molten salt 5 is placed in the crucible, and both the first cathode and the second cathode are connected to the power supply 1 through wires. The electrolytic cell is placed in a closed container, and the container is provided with a temperature control device and a gas protection device.

Selective recovery of tungsten-cobalt metal by electrolysis of YG6 hard alloy by multi-cathode method: the molten salt system used in electrolysis is an equimolar NaCl-KCl molten salt system, the protective gas is argon, the anode is YG6 hard alloy block, and the cathode is For molybdenum sheets, the temperature of the electrolysis is controlled at 750°C, and the electrolysis method of the first step is constant voltage electrolysis. First, apply a voltage of 0.4V relative to the reference electrode between the anode and the first cathode, and electrolyze for 8h;

The electrolysis method of the second step is constant voltage electrolysis. Pull out the first cathode, insert the second cathode, apply a voltage of 0.8V relative to the Ag/AgCl reference electrode between the anode and the second cathode, and perform electrolysis for 2h.

The cathode after electrolysis and the molten salt near the cathode are placed in water, washed with ultrasonic water, and the solid powder is taken, and then centrifuged, filtered, and dried to remove impurities. Wherein the centrifugal speed is 5000rpm, and the drying temperature is 100°C.

The XRD pattern of the cathode product obtained after electrolysis is shown in Figure 3, the product obtained on the first cathode is pure cobalt, and the product on the second cathode is WC. The product sample of the first cathode is detected by EDS (energy dispersive spectrometer), and there are only peaks of cobalt and carbon on the spectrum, and the average purity of Co obtained is 92%.

Example 2

The multi-cathode method is used to electrolyze the YG6 type hard alloy to selectively recover tungsten-cobalt metal, and the electrolysis device is the same as that in Example 1. The molten salt system used in electrolysis is an equimolar NaCl-KCl molten salt system, the protective gas used in the experiment is argon, the anode is a YG6 type hard alloy block, the cathode is a graphite rod, the electrolysis temperature is 750°C, and the electrolysis method is the first step constant Piezoelectrolysis and second step constant current electrolysis.

First, apply a voltage of 0.4V relative to the reference electrode between the anode and the first cathode, and electrolyze for 8 hours; then pull out the first cathode, insert the second cathode, and apply a constant current of 72mA/ ^cm2 between the anode and the second cathode, Electrolysis 2h. The cell voltage is always lower than 3.2V during the electrolysis process.

The powder obtained from the cathode is ultrasonically washed, centrifuged, filtered, and dried to remove impurities. The centrifugal speed is 4000rpm, and the drying temperature is 80°C.

Figure 4 shows the XRD patterns of the products collected from the first cathode and the second cathode, wherein the product of the first cathode is pure cobalt, and the product of the second cathode is WC. The product sample of the first cathode is detected by EDS (energy dispersive spectrometer), and there are only peaks of cobalt and carbon on the spectrum, and the average purity of Co obtained is 94%.

Example 3

The step-by-step electrolysis method is used to electrolyze YG6 type hard alloy to selectively recover tungsten-cobalt metal. The difference between the electrolysis device and Example 1 is that the constant voltage electrolysis adopts a three-electrode system, and the reference electrode 8 is also inserted into the molten salt 5. The first cathode 2. Both the reference electrode 8 and the waste cemented carbide anode 4 are connected to the electrochemical workstation 7 ( FIG. 2 ), and other settings are the same as in Embodiment 1.

The molten salt system used in electrolysis is an equimolar NaCl-KCl molten salt system, the protective gas used in the experiment is argon, the anode is YG6 type hard alloy block, the first cathode is molybdenum sheet, the reference electrode is Ag/AgCl, the electrolysis temperature 750°C, the electrolysis method is the first step of constant voltage electrolysis and the second step of constant voltage electrolysis.

In the first step of electrolysis, apply a voltage of 0.4V relative to the reference electrode between the anode and the cathode for 8 hours of electrolysis; after electrolysis, cool down, take out the remaining anode block, clean it and use it as the anode for the second step of electrolysis. The second step is constant voltage electrolysis, replaced by a new NaCl-KCl molten salt system, the second cathode is a new molybdenum sheet, the applied voltage is 0.8V, and the electrolysis time is 2h. The powder obtained from the cathode is ultrasonically washed, centrifuged, filtered, and dried to remove impurities. The centrifugal speed is 4000rpm, and the drying temperature is 70°C.

The product obtained in the first step of electrolyzing the cathode is pure cobalt, and the product obtained in the second step of electrolyzing the cathode is pure tungsten. Fig. 5 is the XRD pattern of the product obtained in example 3. The product sample of the first cathode is detected by EDS (energy dispersive spectrometer), and there are only peaks of cobalt and carbon on the spectrum, and the average purity of Co obtained is 93%.

Example 4

Selective recovery of tungsten-cobalt metal by electrolysis of YG6 type cemented carbide by step-by-step electrolysis: the electrolysis device is the same as in Example 3.

The molten salt system used in the experiment is an equimolar NaCl-KCl molten salt system, the protective gas used in the experiment is argon, the anode is a YG6 hard alloy block, the cathode is a graphite rod, the reference electrode is Ag/AgCl, and the electrolysis temperature is 750°C , The electrolysis method is constant voltage electrolysis and constant current electrolysis. In the first step of electrolysis, apply a voltage of 0.4V relative to the reference electrode between the anode and the cathode for 8 hours of electrolysis; after electrolysis, cool down, take out the remaining anode block, clean it and use it as the anode for the second step of electrolysis.

In the second step, a new NaCl-KCl molten salt system is replaced by electrolysis. The second cathode is a molybdenum sheet. A constant current of 72mA/ ^cm2 is applied between the cathode and anode, and the electrolysis time is 2h. The cell voltage is always lower than 3.2V during the electrolysis process. The powder obtained from the cathode is ultrasonically washed, centrifuged, filtered, and dried to remove impurities. The centrifugal speed is 2000-6000rpm, and the drying temperature is 30-100°C.

The product obtained in the first step of electrolyzing the cathode is pure cobalt, and the product obtained in the second step of electrolyzing the cathode is pure tungsten. Fig. 6 is the XRD spectrum of the product obtained in Example 4. The product sample of the first cathode is detected by EDS (energy dispersive spectrometer), and there are only peaks of cobalt and carbon on the spectrum, and the average purity of Co obtained is 92%.

Example 5

Selective recovery of tungsten-cobalt metal by electrolysis of YG11 type cemented carbide by step-by-step electrolysis: the electrolysis device is the same as in Example 3.

The molten salt system used in the experiment is an equimolar NaCl-KCl molten salt system. The protective gas used in the experiment is argon. Constant current electrolysis. In the first step of electrolysis, apply a voltage of 0.4V relative to the reference electrode between the anode and the cathode, and electrolyze for 9 hours; The first step is to apply a constant current of 72mA/cm ² between the cathode and the anode, and the electrolysis time is 2h. The powder obtained from the cathode is ultrasonically washed, centrifuged, filtered, and dried to remove impurities. The centrifugal speed is 2000-6000rpm, and the drying temperature is 30-100°C.

It can be known from XRD detection (FIG. 7) that the product obtained from the cathode electrolysis in the first step is pure cobalt, and the product obtained from the cathode electrolysis in the second step is pure tungsten.

The above embodiments are only descriptions of preferred implementations of the present invention, and are not intended to limit the scope of the present invention. On the premise of not departing from the design spirit of the present invention, various technical solutions of the present invention can be made by ordinary engineers and technicians in the field. Variations and improvements should fall within the scope of protection defined by the claims of the present invention.

Claims (10)

1. A method for multi-step selective electrolysis recovery of metal in waste cemented carbide, characterized in that, comprising the following operations: The first step of electrolysis: use waste cemented carbide as the anode, insert the anode and the first cathode in the molten salt medium during the first step of electrolysis, and electrolyze in the molten molten salt dielectric, the electrolysis temperature is 600 ~ 1000 ℃, and the electrolysis method is Constant voltage electrolysis or constant current electrolysis, gas protection during electrolysis; The second step of electrolysis: take out the first cathode, insert the second cathode into the molten salt medium electrolyzed in the first step, and then perform the second step of electrolysis; or, take out the anode and insert it into a new molten salt medium with the second cathode , carry out the second step of electrolysis; the electrolysis temperature is 600-1000°C, and the electrolysis method is constant voltage electrolysis or constant current electrolysis. 2. The method according to claim 1, characterized in that the molten salt medium in the first step and the new molten salt medium in the second step are both NaCl-KCl molten salt systems with a molar ratio of 1:0.2 to 2.0; The waste cemented carbide is one of tungsten-cobalt cemented carbide, tungsten-titanium-cobalt cemented carbide, tungsten-titanium-tantalum and tungsten-titanium-niobium cemented carbide. 3. The method according to claim 1, wherein the material of the first cathode and the second cathode is independently one of molybdenum, titanium, stainless steel, carbon, and graphite. 4. The method according to claim 1, characterized in that, the voltage of the constant voltage electrolysis is 0.1V-1.6Vvs.Ag/AgCl, and the current density of the constant current electrolysis is 0.01-1A/cm ² . 5. The method according to any one of claims 1 to 4, characterized in that the first step of electrolysis is constant voltage electrolysis, the voltage is 0.1V-0.5V vs.Ag/AgCl, and the electrolysis time is 6-6. 12 hours. 6. The method according to any one of claims 1-4, characterized in that the second step of electrolysis is constant voltage electrolysis, and the electrolysis voltage is 0.6-1.6 Vvs.Ag/AgCl. 7. The method according to any one of claims 1-4, characterized in that the second step of electrolysis is constant current electrolysis, and the cell voltage of electrolysis is kept below 3.2V by controlling the current density. 8. The method according to any one of claims 1 to 4, characterized in that, in the second step of electrolysis, the first cathode is taken out, and the second cathode is inserted into the molten salt medium of the first step of electrolysis to carry out the second step. Step electrolysis, the electrolysis time is 1-48h; or, after the anode is taken out and cleaned, and the second cathode is inserted into a new molten salt medium, the second step electrolysis is performed, the electrolysis time is 1-4h. 9. The method according to any one of claims 1 to 4, characterized in that, after the first cathode is taken out, the molten salt medium near the first cathode is placed in water for ultrasonic treatment, and then solid-liquid separation is performed to obtain Metal: After the second step of electrolysis, the second cathode is taken out, and the molten salt medium near the second cathode is placed in water for ultrasonic treatment, and then solid-liquid separation is performed to obtain the second step electrolysis product. 10. The method according to claim 8, wherein the solid-liquid separation method includes one or more of centrifugal separation, filtration, and drying, wherein the centrifugal speed of the centrifugal separation is 2000-6000rpm , The drying temperature is 30-100°C.