Multi-stage integrated molten salt electrolysis metal refining system and method

Abstract

The invention relates to a multi-stage integrated molten salt electrolysis metal refining system and method. The multi-stage integrated molten salt electrolysis metal refining system comprises an integrated electrolytic bath, a collaborative flow control system, a centralized power supply control system, a metal extraction device and an inert atmosphere and temperature monitoring system. A plurality of crucibles are arranged in the same electrolytic cell in parallel and are sequentially filled with molten salt electrolyte with progressively increased purity, and step-by-step reduction of metal ions and impurity separation are realized by setting a potential gradient; liquid metal is conveyed to the rear stage from the front stage in the heating and heat preservation pipeline, and finally high-purity products are collected at the final stage. The method is suitable for high-purity extraction of metals such as gallium, indium, lithium and tin. The system is compact in structure and stable in operation, has good refining selectivity and process expansibility, and is suitable for high-end metal material preparation scenes.

Description

Technical Field

[0001] This invention relates to the fields of metallurgical engineering and electrochemical purification technology, specifically to a molten salt electrolytic refining system and its refining method, and more specifically, to a multi-stage integrated low-temperature molten salt electrolytic refining system and method suitable for the graded purification of high-purity metals (such as gallium, indium, lithium, tin, bismuth, etc.), belonging to the category of precision purification of functional metals and green metallurgical process technology. Background Technology

[0002] In the fields of modern metallurgy and materials processing, especially in high-precision industries such as semiconductors, new energy, and aerospace, the purity of metallic materials directly determines their performance and product reliability. To obtain high-purity functional metals, molten salt electrolysis technology has been widely used in metal purification processes due to its advantages in thermal efficiency, energy consumption control, and adaptability to handling metals in multiple valence states.

[0003] Traditional molten salt electrolytic refining systems typically employ a single-stage, single-cell structure, using a constant cathode potential to electroreduc and deposit metal ions containing impurities. While this technology offers a simple process flow, it suffers from numerous limitations when handling raw material systems with complex impurity types and similar deposition potentials. Particularly in purification scenarios where the potential difference between coexisting metal ions is only tens of millivolts, traditional single-cell electrolysis often fails to achieve effective separation, easily leading to impurity co-deposition and substandard product purity.

[0004] To overcome the above problems, some studies have proposed connecting multi-stage electrolysis processes in series, that is, using multiple independent electrolytic cells to purify the electrolyte step by step according to a potential progression. However, such multi-cell systems still have the following prominent problems in practical engineering implementation: 1. Low system integration and large footprint: Multi-slot distributed layout requires a large number of external connection pipes and individual control modules, which occupy a lot of space and are complicated to install and maintain; 2. Severe heat loss and low energy efficiency: The tanks are connected by external metal or heat-resistant pipes, resulting in long heat transfer paths and difficulty in uniform temperature control, leading to severe heat loss. 3. Inconsistent reaction conditions make control difficult: Differences in temperature field, electric field, and atmosphere in each tank cause fluctuations in reaction parameters, affecting the consistency and selectivity of metal precipitation; 4. Backmixing of impurities between stages is difficult to avoid: Liquid metal is prone to backmixing during the flow between tanks, which affects the refining effect of subsequent stages and reduces the purity of the target metal; 5. Inability to achieve intelligent collaborative control: Traditional multi-slot structures are mostly distributed control systems, which are difficult to manage and execute refined linkage strategies in a unified manner, resulting in a low degree of automation.

[0005] Furthermore, in low-temperature molten salt systems, due to the low melting point and high reactivity of the metals, if high-precision thermal control and flow regulation cannot be achieved, problems such as metal solidification and blockage, pipeline corrosion failure, and electrode potential drift are likely to occur, further exacerbating the instability of system operation.

[0006] Therefore, in order to address the core challenges in high-purity metal electrolytic purification, such as inaccurate potential control, inconsistent thermal fields, dispersed structures, high risk of backmixing, and poor automation capabilities, there is an urgent need to develop a new electrolytic refining system and method that integrates structure, modularizes functions, coordinates processes, and enables intelligent control, so as to achieve efficient separation of complex metal impurities and high-purity extraction of target metals. Summary of the Invention

[0007] To overcome the problems of inefficient impurity separation, low potential control precision, and poor system integration in existing molten salt electrolysis processes, this invention proposes a compact, hierarchically coordinated, integrated multi-stage molten salt electrolysis system and its supporting refining method. This system achieves selective reduction of metal ions, pre-deposition of impurities, and final enrichment of target metals by arranging multiple independent electrolysis units in parallel within the same electrolytic cell, combined with progressively set cathode potential gradients and temperature-controlled metal flow paths. This results in high-purity refining. The core technical solution of this invention is described in detail below, focusing on the system structure, key functional modules, and control methods.

[0008] In one embodiment of the present invention, a multi-stage integrated low-temperature molten salt electrolytic refining system is provided, comprising: An integrated electrolytic cell has at least two independent crucibles arranged side by side inside, and each crucible forms a multi-stage refining sequence. Each crucible is equipped with an anode and a cathode and contains an independent low-temperature composite molten salt electrolyte, wherein the molten salt electrolyte is of the same type but the purity increases progressively from the previous stage to the next stage. A collaborative flow control system includes an interstage delivery pipeline connecting each stage of crucibles, a regulating valve, a solenoid valve, an interstage circulation pump, and a flow meter, for controllably delivering liquid metal obtained from cathode reduction in the previous stage crucible to the next stage crucible. The interstage delivery pipeline has an active heating structure to prevent the liquid metal from solidifying, so as to ensure the continuous and stable flow of the liquid target metal during the interstage transfer process. A centralized power control system includes a DC regulated power supply, a control host and power supply lines. The control host is configured to independently regulate the cathode potential of each crucible to form a progressively decreasing cathode potential gradient along the multi-stage crucible sequence, and to execute a potential-flow linkage control strategy to achieve dynamic matching between the cathode reduction rate and the interstage feeding rate. A metal extraction and collection system, located in the final stage crucible, is used to collect high-purity target metals; a global inert atmosphere protection system is used to provide a uniform inert atmosphere environment for the integrated electrolytic cell; A centralized temperature monitoring system is used to monitor and regulate the real-time temperature of the integrated electrolytic cell and interstage delivery pipelines.

[0009] Furthermore, the interstage delivery pipeline has a jacketed structure, including a corrosion-resistant metal inner tube, an external insulation layer, and an armored heating wire disposed in the jacket gap. The heating wire is connected to an independent temperature control power supply, and the control host performs temperature setting and closed-loop feedback adjustment to keep the temperature of the liquid target metal in the delivery pipeline stably maintained within the range of 10°C to 30°C above its melting point.

[0010] Furthermore, the centralized power control system further includes a potential compensation module, which dynamically compensates the preset cathode potential based on the real-time temperature data fed back by the centralized temperature monitoring system, so that the cathode potential fluctuation of each crucible is controlled within ±5 mV.

[0011] Preferably, the cooperative flow control system maintains the interstage transport process in a low Reynolds number laminar flow state (Re < 2000) by adjusting the speed of the interstage circulating pump and the opening of the regulating valve, and can switch to a pulse transport mode as needed to suppress interstage backmixing and maintain the independence of electrolyte components.

[0012] Preferably, the centralized power control system is configured to execute a potential-flow linkage control strategy based on the real-time collected flow meter signal and cathode deposition current signal, so as to keep the metal feeding rate and deposition rate matched by dynamically adjusting the circulating pump speed and cathode potential, thereby improving deposition efficiency and system stability.

[0013] Optionally, the metal extraction and collection system includes a cone-shaped collector, a heating wire, and a liquid metal collection tank, and can be configured as a scraping collection device, a liquid metal confluence device, or a vacuum suction collection device according to the physical properties of the target metal.

[0014] Optionally, the low-temperature composite molten salt electrolyte is selected from one or more of LiCl-KCl, NaCl-KCl, ZnCl2-NaCl-KCl, and AlCl3-NaCl-KCl, with an operating temperature range of 100℃ to 400℃; the system is suitable for refining low-melting-point or reactive metals such as gallium, indium, tin, bismuth, and lithium.

[0015] In one embodiment of the present invention, a metal electrolytic refining method using the above-described system is provided, comprising the following steps: S1, under the protection of an inert atmosphere, low-temperature composite molten salt electrolytes of different purities are added to crucibles of each stage and heated to the set working temperature. At the same time, the interstage conveying pipeline is preheated to a range of 10°C to 30°C above the melting point of the target metal. S2, Add the target metal raw material containing impurities to the first-stage crucible; S3, set a cathode potential gradient that decreases step by step along the crucible sequence, so that impurity metal ions with more positive precipitation potential are preferentially deposited in the previous stage; S4, activate the interstage flow control system to controllably transport the liquid metal deposited in the previous stage crucible to the next stage crucible; S5, During the flow of the target metal, different metal ions are selectively deposited at the cathode at the corresponding potential according to their deposition potential differences; S6, high-purity target metal is obtained in the final crucible and extracted through a collection system.

[0016] Furthermore, in step S3, the cathode potentials at each stage are compensated and adjusted based on real-time temperature, deposition current, and interstage flow data to stabilize the cathode potentials at each stage within the range of ±5 mV of the set value.

[0017] Preferably, the interstage conveying process in step S4 adopts a low Reynolds number laminar flow or pulse conveying mode, and the liquid metal is kept from solidifying during the conveying process by actively heating the conveying pipeline, thereby achieving continuous and stable multi-stage electrolytic refining.

[0018] This method is applicable to the refining of rare earth metals, low melting point metals or active metals, and the purity of the target metal obtained can reach 99.99% or higher.

[0019] Based on the above technical solutions, the multi-stage integrated molten salt electrolysis metal refining system and method proposed in this invention achieves a key process breakthrough by integrating multiple independent refining units within a single integrated electrolytic cell, and combining a low-temperature composite molten salt system, a step-by-step cathode potential control strategy, and a thermally coupled metal flow channel. This effectively overcomes the technical bottlenecks of existing multi-cell series molten salt electrolysis systems, such as low integration, large heat loss, difficulty in coordinated control, and difficulty in finely separating impurities. Specifically: Firstly, by constructing a "dual gradient synergistic separation mechanism" of electrolyte purity gradient and cathode potential gradient in the same tank, active impurities are selectively deposited in advance using a high-impurity electrolyte and a more positive potential in the front stage, and the target metal is efficiently deposited in the back stage using a high-purity electrolyte and a more negative potential. This enables multi-metal systems with only tens of millivolts difference in deposition potential to achieve high selective purification, breaking through the limitation of traditional single-potential systems that cannot balance purity and efficiency. Secondly, the integrated structure places crucibles of all levels in the same closed thermal field and inert atmosphere, which greatly reduces heat loss, potential disturbance and impurity back mixing caused by pipeline transportation, and significantly improves system integration, thermal utilization and operational stability. Third, the collaborative flow control system adopts a low Reynolds number steady-state or pulsed flow mode, combined with active heating and insulation pipelines and dynamic flow regulation, to achieve safe directional transport of liquid metal, avoid solidification and blockage, maintain the independence of interstage components, and ensure continuous operation. Fourth, the supporting precision electrical control strategy uses "potential-flow linkage" and "temperature compensation algorithm" to adjust the cathode potential and pump speed at each stage based on the flow meter, thermocouple and current signal closed loop, so that the deposition rate and metal supply are precisely matched, ensuring that the metal ion concentration is stable and the grain morphology is consistent, and obtaining high-purity and high-consistency metal products. Fifth, the system adopts a modular design, which can flexibly configure the electrolyte system, potential range and collection device according to the melting point, potential and chemical activity of the metal. It is suitable for the purification of a variety of metals such as gallium, indium, tin, bismuth and lithium, and has excellent versatility and scalability. Sixth, the low-temperature composite molten salt system keeps the operating temperature below 400℃, resulting in low energy consumption, minimal corrosion, and avoids the volatilization and harmful gas emissions of traditional high-temperature chloride processes. The fully enclosed inert atmosphere further enhances the protection of active metals, making it safe, environmentally friendly, and industrially feasible.

[0020] Therefore, this invention significantly improves the separation efficiency, product purity, and operational reliability of the multi-stage molten salt electrolytic refining process by integrating structural integration, heat flow integration, precise control, and process flexibility. It can meet the stringent requirements for raw material quality in the fields of high-purity metals, functional materials, and new energy, and has broad industrialization value.

[0021] Furthermore, the core technical effect of this invention stems from the dual coupling effect of the cathode potential gradient and the molten salt electrolyte purity gradient: First, by utilizing the minute potential difference in the deposition of different metal ions, selective reduction is achieved under progressively controlled cathode potentials, allowing the target metal and impurity metals to separate sequentially according to their electrical driving forces in each electrolysis unit. Second, by constructing a molten salt purity gradient that progresses from high impurity concentration to high purity, the initial stage is more conducive to the preferential deposition of active impurities, while the subsequent stage maintains the highest activity of the target metal ions, thereby further enhancing the deposition selectivity of the final stage. Third, low Reynolds number directional transport between stages ensures that the molten salt and metal are transferred in a laminar flow mode, avoiding cross-stage backmixing. Once the flow state turns into turbulence, electrolyte mixing will severely damage the separation effect. In summary, this invention achieves the unexpected effect of efficient staged separation in a system with a deposition potential difference of only tens of millivolts, a capability that cannot be achieved by traditional single-cell electrolysis or conventional series devices. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of the multi-stage integrated molten salt electrolytic refining system described in this invention.

[0023] Marked in the image: 101a, 101b, 101c - Metal raw material and molten salt electrolyte storage tanks; 102a, 102b, 102c - Electrode interfaces; 103a, 103b, 103c - Electrode fixtures; 104a, 104b, 104c - Metal wires; 105a, 105b, 105c - Anodes; 106a, 106b, 106c - Cathodes; 107a, 107b, 107c - Molten salt electrolytes; 108a, 108b, 10 8c - Crucible (multi-stage independent setup); 109 - Integrated electrolytic cell; 201a, 201b - Regulating valves; 202a, 202b - Interstage conveying pipelines; 203a, 203b - Circulating pumps; 204a, 204b - Flow meters; 205a, 205b - Solenoid valves; 206 - Power supply line; 207 - Control host; 208 - Power cord; 209 - DC regulated power supply; 210 - Control wires; 301 - Conical metal collector; 302 - Heating wire; 303 - Liquid metal collection tank; 401 - Temperature acquisition module; 402 - Monitoring computer; 403 - Thermocouple; 501 - Argon cylinder; 502 - Inert gas pipeline.

[0024] Figure 2 This is a functional flow diagram of the multi-stage integrated molten salt electrolytic refining system described in this invention, illustrating the process logic and module collaboration relationships of the entire process from raw material input to target metal extraction.

[0025] The process shown in the diagram includes the following functional modules and operational stages: 1. Raw material input and power configuration stage: This includes a device for feeding metal raw materials containing impurities and a raw material storage tank; A DC regulated power supply and multi-level cathode potential gradients are set up for graded control of each electrolytic crucible.

[0026] 2. System initialization and preparation phase: Activate the inert gas protection system; Fill each crucible with a low-temperature composite molten salt electrolyte; Start the temperature monitoring system and heat the system to the operating range of 100–400℃.

[0027] 3. Core refining process: Staged electrolysis: The first-stage crucible is set with a correction potential to achieve preferential deposition of active impurities; The second-stage crucible is set at an intermediate potential to achieve the deposition of impurities at an intermediate potential and further purification of the target metal. The third-stage crucible is set with a target potential to achieve selective deposition and final separation of high-purity metal ions.

[0028] 4. Target Metal Cooperative Flow Control System: Metal flows at each stage are directed through actively heated pipelines to prevent back mixing; The conveying process is linked to the electrolysis process for control.

[0029] 5. Potential-flow linkage control strategy: The control unit dynamically adjusts the cathode potential and circulation pump speed based on information from the flow meter, temperature monitoring, and electrode feedback. Achieving a step-by-step purification process from initial refining to final high-purity extraction.

[0030] 6. Metal collection system: Includes a cone-shaped collector, a heating device, and a liquid metal storage tank; To achieve continuous output and collection of high-purity metals, ensuring constant temperature and stable product quality.

[0031] The figure clearly illustrates the innovative process concept and collaborative mechanism of the present invention system, which is based on "structural integration + electronic control linkage + graded purification + high-purity collection". Detailed Implementation

[0032] To ensure the stable operation of the multi-stage molten salt electrolysis system of this invention under engineering conditions, this invention has developed a set of stable and reproducible "engineering parameter windows" through extensive experimental verification, including key operating parameters such as cathode potential gradient range, molten salt electrolyte purity gradient range, and temperature window range.

[0033] 1. Cathode potential gradient window Testing revealed that the optimal range of the multi-stage cathode potential for target metals such as gallium, indium, and tin is as follows: Level 1: Target metal standard potential +50 mV to +150 mV Level 2: Target metal standard potential +10 mV to +50 mV Level 3: Target metal standard potential ±0 mV to -20 mV When the interstage potential difference is less than 30 mV, impurity ions begin to deposit across stages; When the interstage potential difference is greater than 100 mV, the deposition current efficiency of the target metal decreases by 15–25%.

[0034] Therefore, the above window is the optimal range that balances "selective separation" and "deposition efficiency".

[0035] 2. Molten salt purity gradient window The typical impurity content ranges for the three-stage molten salt are as follows: Level 1: 1000–5000 ppm Level 2: 100–500 ppm Level 3: <50 ppm When the level 1 impurities exceed 5000 ppm, it will cause severe fluctuations in cathode polarization and affect the stability of the potential compensation algorithm.

[0036] 3. Temperature window The temperature of the multi-stage delivery pipeline is set to the target metal melting point +10~30℃.

[0037] For example, gallium (melting point 29.8℃) is prone to local solidification when the temperature of the delivery pipeline is below 35℃; There is no solidification when the pipeline temperature is maintained at 45℃±2℃.

[0038] 4. Cathode current density window To ensure a stable deposition rate of metal ions and the effectiveness of the cathode potential control strategy, this invention preferably sets the cathode current density in the range of 50–400 mA / cm². This range can be appropriately adjusted according to the metal system, electrode geometry, and electrolyte composition.

[0039] Specifically, below 50 mA / cm², the deposition rate is slow and coarse grains are prone to occur; above 400 mA / cm², the deposition rate is... 2 At this time, the increased polarization and potential compensation burden can easily lead to the co-deposition of impurities or uneven deposition morphology.

[0040] Therefore, the current density window selected in this invention helps to achieve a comprehensive balance between deposition efficiency, deposition selectivity and electrode stability.

[0041] The establishment of the above window parameters ensures the long-term stability of the system under continuous operation conditions, providing an operable basis for the implementation of the present invention.

[0042] like Figure 1 As shown, the multi-stage integrated low-temperature molten salt electrolytic refining system of this invention comprises multiple cooperative functional sub-units. The system's technical framework revolves around "integrated electrolytic cell + multi-stage electrolysis unit + cathode potential control + metal flow path," and through the coordinated operation of multiple modules, it supports the step-by-step purification and high-purity deposition of target metals under low-temperature conditions. This section will begin with the overall structure, providing a detailed analysis of the functional positioning, structural configuration, and integrated operational logic of each major unit.

[0043] I. System Overview and Overall Configuration In one possible embodiment of the present invention, such as Figure 1 As shown, a multi-stage integrated low-temperature molten salt electrolytic refining system includes: an integrated electrolytic cell (109), multiple crucibles arranged in parallel (108a, 108b, 108c), a collaborative flow control system, a centralized power control system, a metal collection device, and an inert atmosphere and temperature monitoring system.

[0044] The integrated electrolytic cell (109) is a fully enclosed structure made of corrosion-resistant alloy material or ceramic composite material. Inside the cell are at least three independent, heat-insulated crucible units. Each crucible has an anode (105a~105c) and a cathode (106a~106c), and is filled with an electrolyte molten salt (107a~107c). This electrolyte can be LiCl-KCl, NaCl-KCl, or other low-melting-point halide mixtures. The purity of the electrolyte in each crucible can be set in a stepped increase according to process requirements to meet the needs of progressive metal purification.

[0045] The collaborative flow control system includes heated and insulated metal pipelines (202a, 202b) installed between each crucible, matching solenoid valves (205a, 205b), regulating valves (201a, 201b), circulating pumps (203a, 203b), and flow monitoring devices (204a, 204b). This system, under the command of the control host, quantitatively delivers the liquid metal deposited from the cathode in the previous stage crucible to the next stage crucible, achieving continuous feeding during the staged electrolysis process. To prevent solidification of the liquid metal during delivery, the metal pipelines are equipped with armored heating wires, with a temperature control range of 10°C to 30°C above the metal's melting point, and closed-loop regulation is performed by the central control unit.

[0046] During operation, the system is first heated to the electrolyte's operating temperature range (e.g., 200℃~400℃) under an inert atmosphere. Then, different cathode potentials are sequentially set for each crucible, precisely controlled by a centralized power supply control system. This ensures that impurity metal ions are preferentially deposited in the upstream crucibles, while target metal ions are deposited in the downstream crucibles. Metal transfer between crucibles is achieved through low Reynolds number laminar flow, effectively suppressing backmixing.

[0047] It should be noted that the above structures and parameters can be adjusted according to the physicochemical properties of different metals (such as gallium, indium, lithium, etc.). For example, when processing gallium, a metal with a low melting point, the pipeline insulation temperature can be set to 45°C, while for lithium, it is recommended to operate in an inert environment above 200°C. The above embodiments do not constitute a limitation on the scope of protection of this invention.

[0048] II. System Initialization and Operation Process To achieve the high-purity metal purification goal of this invention, a precise start-up and operation control process must be executed sequentially after the system is built. For example... Figure 2 As shown, this section provides a detailed description of the entire process of the system from startup preparation to electrolysis process control, including key steps such as atmosphere protection, temperature management, potential setting, metal transfer and deposition recovery, aiming to illustrate the collaborative operation mechanism and parameter control logic of this system.

[0049] The operation process of the multi-stage integrated low-temperature molten salt electrolysis refining system of the present invention mainly includes the following steps: system preparation, molten salt filling and preheating, potential gradient setting, staged electrolysis process control, liquid metal transportation, and high-purity metal extraction. Each functional module works together through the integrated control host (207) to achieve precise control of the whole process.

[0050] 1. System Initialization Phase Before the system starts operating, the global inert atmosphere protection system is first activated. High-purity inert gas is introduced into the integrated electrolytic cell (109) through argon cylinder (501) and its pipeline (502) to form a stable oxygen-free working environment and prevent the active metals from undergoing oxidation reactions during electrolysis or transfer. Subsequently, the low-temperature molten salt electrolyte in each crucible is heated to the target operating temperature (generally between 100℃ and 400℃, depending on the selected molten salt system) by a heating device.

[0051] At the same time, the heating system of the interstage conveying pipelines (202a, 202b) is activated simultaneously. The heating power of the outer armored heating wire is set by the control host to keep its temperature in a constant range of 10~30℃ above the melting point of the target metal, so as to avoid solidification and blockage of the liquid metal during the transmission process.

[0052] 2. Cathode potential gradient setting After all crucibles reach the preset temperature, the centralized power supply control system starts operating. The control host (207) sets the cathode potential for each stage of the crucible, forming a gradually changing potential gradient. For example, the cathode potential of the first-stage crucible is set to a relatively positive potential to preferentially reduce and deposit active impurity ions; the second-stage crucible is set to an intermediate potential to further remove neutral impurity metal ions with similar potentials; the third-stage crucible is set to the most negative potential to selectively reduce target metal ions, thereby achieving high-purity deposition in the final stage.

[0053] 3. Control of liquid metal transport During electrolysis, the liquid target metal formed by reduction in the previous stage crucible is directionally transported to the next stage crucible via circulating pumps (203a, 203b) and conveying pipelines (202a, 202b) under the command of the control unit, completing the staged feeding. To ensure flow stability and refining effect, the control unit collects feedback signals from the flow meters (204a, 204b) in real time and, combined with the changes in electrolysis current at each stage, executes a potential-flow linkage control algorithm to dynamically adjust the cathode potential and pump speed of the next stage, achieving a match between the feeding rate and the reduction deposition rate.

[0054] To prevent back-mixing between stages, the system can be switched to an intermittent pulse flow mode. By controlling the periodic opening and closing of solenoid valves (205a, 205b), the liquid metal enters the subsequent crucible in a wave-like flow state, further maintaining the independence of the electrolyte composition and the reducing environment.

[0055] 4. High-purity metal deposition and collection In the final stage crucible, the target metal ions, after being purified through stages, are selectively reduced and deposited on the cathode surface to form high-purity metal. Depending on the physical state of the target metal, the collection system can be selected for scraping collection (solid), heated liquid storage tank collection (liquid), or vacuum suction device (active metal) to extract the product. The storage tank (303) is equipped with an external heating wire (302) to maintain a stable temperature of the collected metal and prevent secondary solidification or oxidation.

[0056] Through the above operating process, the system of the present invention can realize automated start-up, closed-loop precision control and continuous and stable output, and is suitable for green and efficient refining of a variety of high-purity metals.

[0057] III. Implementation Examples (Reference) Figure 1 and Figure 2 ) To further illustrate the technical effects and applications of the present invention, several specific embodiments are given below, taking into account typical metal purification targets. These embodiments are configured based on the system structure and control strategy of the present invention, and the adaptability, stability, and high-purity extraction capability of the present invention are verified through staged electrolysis processes of different metals. It should be understood that these embodiments are not intended to limit the present invention, and those skilled in the art can adjust the material selection, potential parameters, number of electrolysis stages, etc., according to different process requirements, which still fall within the protection scope of the present invention.

[0058] Example 1: A three-electrode electrolysis configuration suitable for high-purity indium (In) refining This embodiment provides a method for graded purification of indium metal based on the multi-stage integrated low-temperature molten salt electrolytic refining system described in this invention. It is suitable for processing crude indium raw materials containing impurities such as iron and aluminum, and finally obtaining 5N (99.999%) grade high-purity indium products.

[0059] In this implementation, the system configuration is as follows: Figure 1 As shown, the electrolytic cell (109) contains three independent crucibles (108a, 108b, and 108c), which are the first, second, and third refining units, respectively. Each crucible is filled with a low-temperature molten salt eutectic system composed of LiCl-KCl-InCl3, wherein: The electrolyte in the first-stage crucible (108a) is of industrial grade purity, with a high InCl3 content and a high concentration of impurity metal ions. The electrolyte in the second-stage crucible (108b) was prepared using laboratory-grade reagents. The electrolyte in the third-stage crucible (108c) is prepared with high-purity anhydrous electrolyte, and the InCl3 molar ratio is strictly controlled within the system's stable range.

[0060] During operation, the control host (207) sets three cathode potentials, namely: First stage: -0.25 V (vs. Ag / AgCl), mainly used for preferential reduction of active impurities such as Fe³⁺ and Al³⁺ with positive reduction potentials than indium; Second stage: -0.45 V, used for further separation of neutral impurities with similar potentials; The third stage: -0.60 V, corresponding to the reduction potential of In³⁺, enables highly selective reduction and deposition.

[0061] The raw material is added to the first-stage crucible through the raw material tank (101a), and the molten salt temperature is controlled at approximately 170°C. Under the protection of an inert atmosphere (high-purity argon), the system operates continuously. The liquid reduced indium in the first-stage crucible is transported to the next stage through actively heated delivery pipes (202a, 202b) to achieve multi-stage separation.

[0062] The operating speed of the circulating pumps (203a, 203b) is dynamically adjusted by the main unit. In conjunction with the signal and current feedback from the flow meters (204a, 204b), the system executes a potential-flow linkage strategy in real time. The cathode potential fluctuation of the entire process is stably controlled within ±5 mV, effectively ensuring the selectivity and consistency of the precipitation process.

[0063] The system ran continuously for 72 hours without interruption. The first two stages successfully removed >99% of impurities such as Fe and Al, and the final crucible deposition yielded metallic indium with a purity greater than 99.999%. The product has uniform grains, a smooth surface, no inclusions, and its conductivity test results are consistent with those of the high-purity indium standard sample.

[0064] This embodiment verifies the operational stability and purification efficiency of the system of the present invention in the scenario of graded refining of indium metal, and provides effective process support for the preparation of high-purity indium materials.

[0065] Example 2: Low-Temperature Operation Scheme Applicable to Liquid Gallium (Ga) Purification This embodiment provides a crude gallium purification method based on the multi-stage integrated low-temperature molten salt electrolytic refining system described in this invention. It is suitable for removing impurity ions such as lead (Pb), zinc (Zn), and tin (Sn) to obtain high-purity liquid gallium metal.

[0066] like Figure 1 and Figure 2 As shown, the system configuration is basically the same as in Example 1. The electrolytic cell (109) is equipped with three crucibles connected in series (108a, 108b, 108c), and the electrodes of each crucible are numbered as anode (105a–105c) and cathode (106a–106c). To adapt to the low melting point characteristics (29.8℃) of gallium, this example uses an AlCl3–NaCl–KCl eutectic low-temperature molten salt system, and the overall operating temperature is controlled between 45℃ and 60℃.

[0067] The purity of the molten salt in each crucible increases progressively from front to back: The first stage is an industrial-grade molten salt solution with a high doping content; The second level is the analytical purity level; The third stage uses high-purity reagents that have undergone drying and purification, with impurity concentrations controlled at the ppm level.

[0068] The cathode potential is set as follows: The first-stage crucible is set to a cathode potential of -0.20 V (vs. Ag / AgCl) to preferentially reduce lead and zinc ions with deposition potentials higher than gallium. The second stage is set to -0.35 V to further remove impurities such as tin (Sn); The third stage is set to -0.52 V, corresponding to the standard reduction potential of Ga³⁺ ions, to achieve selective deposition of the target metal.

[0069] To ensure stable transfer of liquid gallium between different refining stages, the interstage transport pipelines (202a, 202b) employ a heat tracing jacket structure with temperature control, and the heating temperature is set at 45℃ ±2℃ to ensure the liquid remains fluid throughout the process. During system operation, the circulating pumps (203a, 203b) are adjusted synchronously with the electrode potential under the control of the main unit (207) to avoid the accumulation of metal residues in the preceding stages.

[0070] During electrolysis, a potential-flow linkage control strategy is used to achieve dynamic matching between the metal supply and deposition in the preceding and following stages. The system also incorporates a pulse delivery program (solenoid valves 205a / 205b) to reduce the risk of backmixing in the early stages of the process and further improve refining selectivity.

[0071] After 48 hours of continuous operation, the first two stages effectively remove impurity ions such as Pb²⁺, Zn²⁺, and Sn²⁺, and the final cathode deposition yields liquid gallium metal with a purity of not less than 99.995%. The product is output through the cone-shaped collector (301) and the constant-temperature storage tank (303) in the collection system. The resulting product exhibits good flowability and consistency, making it suitable for subsequent crystal material preparation processes.

[0072] This embodiment verifies the stable control capability and process adaptability of the system of the present invention in the selective reduction and purification of liquid gallium, and has good potential for promotion.

[0073] Example 3: Design of a stable separation process for highly reactive metallic lithium (Li) This embodiment provides a method for purifying metallic lithium based on the multi-stage integrated molten salt electrolysis system described in this invention, which is suitable for extracting high-purity metallic lithium from crude lithium raw materials containing coexisting active metal impurities such as sodium (Na) and potassium (K).

[0074] like Figure 1 As shown, the system structure is consistent with the aforementioned embodiment, consisting of an integrated electrolytic cell (109) and three sets of independent crucibles (108a, 108b, 108c) forming a graded refining sequence. Each crucible is equipped with a dedicated anode (105a–105c) and cathode (106a–106c). The electrodes are connected to the control host (207) and the regulated power supply (209) via power supply lines (206) and power lines (208), respectively. The electrolyte adopts a LiCl-KCl eutectic mixed salt system, which has good lithium-ion conductivity and thermal stability, with a melting point of approximately 350℃, making it suitable for lithium electrolysis.

[0075] To meet the extremely high reaction sensitivity of lithium metal to oxygen and water, the entire system operation is carried out under the protection of high-purity argon gas (purity ≥99.999%). The inert atmosphere system consists of an argon gas cylinder (501) and a closed circulation pipeline (502) to ensure that the inside of the electrolytic cell is kept in a low oxygen and low water state for a long time.

[0076] The operating potential settings for each crucible are as follows: First-stage crucible: The cathode potential is set to -2.30 V (vs. Ag / AgCl), mainly used to preferentially reduce deposited metal impurities with corrected potentials, such as sodium (Na⁺) and potassium (K⁺); Second-stage crucible: The potential is set to -2.60 V to further remove ions with similar potentials but not the target ions; The third-stage crucible has a cathode potential of -2.90 V, corresponding to the standard reduction potential of Li⁺ ions, enabling selective deposition of metallic lithium.

[0077] Since lithium has a melting point of 180.5℃ and reacts readily with air during transport, to ensure system safety and continuous operation, the interstage metal transport pipelines (202a, 202b) are externally equipped with a high-temperature armored heating structure. The heating temperature is set at 200℃ ±5℃ to ensure that the lithium metal remains in a liquid state throughout the process. Combined with the real-time monitoring signals from the thermocouple (403) and temperature acquisition module (401) of the control host, the system can dynamically adjust the local temperature of the pipelines and collection chambers to avoid the risks of solidification and blockage.

[0078] Liquid lithium is transferred between stages by circulating pumps (203a, 203b). An intermittent pulse delivery strategy is adopted during the process, and the ripple flow is controlled by solenoid valves (205a, 205b), which significantly reduces backmixing and reflux and ensures the independence of the components in the crucible.

[0079] The final deposited liquid high-purity lithium metal was extracted from the bottom of the final stage crucible using a vacuum suction collection device and stored in a temperature-controlled collection tank (303). After the system ran continuously for 36 hours, the test results showed that the purity of the lithium metal reached over 99.997%, the concentrations of impurities sodium and potassium decreased significantly, and the cathode deposition layer was uniform and free of oxidation spots.

[0080] This embodiment illustrates that the system of the present invention can perform precise electrolytic purification of highly reactive metals in a highly stable, low-oxygen environment. It has adaptability and scalability to light, reactive metals such as lithium, sodium, and calcium, and is particularly suitable for the production needs of high-purity lithium materials in the field of new energy materials.

[0081] The system of this invention underwent continuous operation verification under typical operating conditions for gallium, indium, and lithium. Results showed that, under consistent heating and inert atmosphere conditions, the three cathode potentials remained stable at ±3–5 mV, the temperature difference between the three crucibles was controlled within ±2℃, and the temperature fluctuation in the interstage piping did not exceed ±1.5℃. The system could operate continuously and stably for 72–120 hours without blockage, backmixing, or abnormal shutdowns. Taking the indium system as an example, compared to traditional single-tank systems, the deposition efficiency was improved by 18–25%, impurities were reduced by 90–95%, and recovery rate was increased by 12–20%. These results fully verify the engineering feasibility and long-term stability of the structure of this invention.

[0082] IV. Adaptability and Expansion Description In addition to standard refining processes for single metals, the system of this invention also possesses excellent engineering scalability and process compatibility. Through modular structure, multi-channel potential control, and intelligent data acquisition design, this system can flexibly adapt to multi-level structural extensions, multi-metal refining tasks, and remote industrial deployment needs. This section further explains the scalability strategies and technical advantages of this system in terms of stage expansion, control integration, multi-metal synergistic purification, and industrial deployment.

[0083] 1. System architecture scalability Without altering the core integrated design framework, the system can be expanded as needed to four, five, or even more levels of crucible electrolysis units. By adding additional independent crucibles, matching electrodes, and interstage flow lines, more refined potential distributions and multi-stage separation processes can be constructed. This type of expansion is particularly suitable for ultra-high purity metal preparation processes with complex impurity compositions, extremely small potential differences, or extremely high purity requirements, such as semiconductor-grade aluminum, tantalum, and beryllium.

[0084] 2. Control and data system compatibility The system control host (207) has remote interface and signal acquisition capabilities, and supports connection with standard industrial communication protocols such as industrial Ethernet, RS485 / Modbus, and CAN bus. Optional remote control platform and data acquisition and recording module are available. Through linkage with the host computer system, users can remotely set operating parameters, issue early warnings, trace historical data, and analyze operating trends, facilitating process optimization and quality traceability management.

[0085] 3. Processing capacity for polymetallic mixed raw materials The system supports gradient separation and selective purification of crude materials containing multiple metal ions. By adjusting the electrolyte composition, potential gradient settings, and flow strategies, sequential fractional refining can be performed on metals with different precipitation potential differences. For example, in processing blends of zinc, indium, and tin, a five-stage refining process can achieve the individual purification and enrichment of the target components, thereby improving the overall recovery rate and the purity of individual metals.

[0086] 4. Modular and engineering implementation capabilities All functional units of this system (electrolytic cell, pump circuit, electrical control, electrolyte unit, heating system, etc.) adopt standardized and modular design, possessing good industrial adaptability and engineering layout flexibility. Multiple systems can be arranged in parallel to achieve a doubling of production capacity; modules can be configured or replaced as needed, facilitating rapid deployment and upgrades for different metal categories or production capacity targets.

[0087] This invention is not only applicable to the high-purity refining of single metals, but also has good process variability, intelligent control and industrial adaptability. It is one of the key supporting equipment for the next generation of green metallurgical processes and high-end material preparation.

[0088] V. Controlled Experiment To verify the necessity of the "dual gradient structure + synergistic flow control" of this invention, multiple sets of control experiments were conducted. The results showed that when the potential gradient was removed and the three crucibles were uniformly set to -0.55 V, impurities such as Fe³⁺ and Al³⁺ were significantly co-deposited in the second and third stages, and the purity of the target metal decreased from 5N (99.999%) to 3N (99.9%). Black spots and inclusions appeared in the deposition layer, proving that the absence of a potential gradient could not prevent impurities from migrating across stages. Under the condition of removing the molten salt purity gradient and using high-purity molten salt throughout the process, the current efficiency of the front stage decreased by 20-40% due to the rapid accumulation of impurities. Potential compensation was frequently triggered due to polarization surges, and the system could only run continuously for 8-12 hours before the electrolyte needed to be replaced, indicating that the molten salt gradient is crucial for staged purification and load balancing. When the active heating and delivery were removed and ordinary metal pipes were used instead, liquid gallium locally solidified within 18-25 minutes, causing pipe blockage, which in turn led to material shortage and current decay in the subsequent stages, forcing the system to shut down. The comprehensive comparison results fully demonstrate that the structure of the present invention has irreplaceable necessity and system stability in continuous refining.

[0089] To facilitate understanding of the key technical concepts in this invention, the following terms are hereby uniformly defined: 1. Cathode potential gradient: The cathode potential difference set sequentially between multiple electrolysis units, gradually becoming positive from the raw material end to the product end, used to achieve selective reduction deposition of metal ions.

[0090] 2. Low Reynolds number laminar flow: Liquid metal in the pipe flows stably under the condition of Re < 2000, which is conducive to component separation and suppresses backmixing.

[0091] 3. Armored heating wire: A heating element encased in a metal sheath, used to maintain the temperature inside a pipeline or device and prevent the metal from solidifying.

[0092] 4. Potential-flow linkage control: Based on the feedback of deposition current and metal conveying flow rate, the cathode potential and flow rate are dynamically adjusted in real time to achieve matching of reduction and feeding rates.

[0093] To achieve coordinated matching of reaction efficiencies at all levels.

[0094] The potential values ​​in this specification are based on the Ag / AgCl reference electrode. In industrial applications, an equivalent reference electrode can be selected according to the specific system. The potential conversion relationship is a conventional technique in this field.

[0095] Furthermore, the system of this invention exhibits excellent industry compatibility in engineering deployment. The system's modular structure is highly standardized, and its key interfaces are highly adaptable to existing molten salt electrolysis process equipment and electrical control systems, allowing for seamless integration into existing refining production lines or pilot-scale platforms. It supports interface with industrial DCS systems, remote monitoring terminals, and digital platforms, facilitating rapid system deployment, operation and maintenance integration, and intelligent upgrades, demonstrating promising prospects for widespread adoption and significant engineering transformation value.

[0096] In summary, this invention integrates multiple electrolysis units within a single electrolytic cell, combining progressively varying cathode potentials and electrolyte purity gradients with coordinated flow control and centralized power supply control strategies. This results in a compact, precisely controlled, and highly adaptable multi-stage integrated molten salt electrolytic refining system and method. This system is suitable for the electrolytic purification of various high-purity metals, possesses excellent process scalability and engineering application value, and is particularly suitable for the high-purity refining needs of semiconductor materials, new energy metals, and other functional materials.

[0097] It should be noted that the embodiments described in this specification are merely preferred embodiments of the present invention, and the technical features involved can be combined and used as needed in different embodiments. The described technical content does not constitute a limitation on the scope of protection of the present invention. All equivalent substitutions or modifications made in accordance with the concept and technical solution of the present invention without departing from the spirit of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A multi-stage integrated low temperature molten salt electrolysis refining system, characterized by, The application relates to a multi-stage integrated electrolytic cell for the production of high-purity liquid metal, which comprises: an integrated electrolytic cell (109) with at least two independent crucibles (108a, 108b, 108c) arranged side by side inside the cell, each of the crucibles constituting a multi-stage refining sequence; each of the crucibles is provided with an anode (105a, 105b, 105c) and a cathode (106a, 106b, 106c) and contains an independent low-temperature composite molten salt electrolyte (107a, 107b, 107c), wherein the molten salt electrolytes are of the same system but have gradually increasing purity from the front stage to the rear stage; a coordinated flow control system, which comprises inter-stage conveying pipelines (202a, 202b) connecting the crucibles, regulating valves (201a, 201b), electromagnetic valves (205a, 205b), inter-stage circulating pumps (203a, 203b) and flow meters (204a, 204b) and is used for controllable conveying of liquid metal reduced by the cathode in the front-stage crucible to the rear-stage crucible, wherein the inter-stage conveying pipelines are provided with an active heating structure for maintaining the non-solidification of the liquid metal, so as to ensure the continuous and stable flow of the liquid target metal during the inter-stage transfer; a centralized power supply control system, which comprises a direct-current stabilized power supply (209), a control host (207) and power supply lines (206, 208), the control host (207) is configured to independently regulate and control the cathode potential of each stage of the crucible, so as to form a cathode potential gradient gradually decreasing along the multi-stage crucible sequence, and to perform a potential-flow linkage control strategy, so that the cathode reduction rate and the inter-stage feeding rate are dynamically matched; a metal extraction and collection system arranged in the last-stage crucible and used for collecting high-purity target metal; a global inert atmosphere protection system used for providing a unified inert atmosphere environment for the integrated electrolytic cell (109); a centralized temperature monitoring system used for monitoring and regulating the real-time temperature of the integrated electrolytic cell and the inter-stage conveying pipelines.

2. The system of claim 1, wherein, The inter-stage conveying pipelines (202a, 202b) are of a jacketed structure, which comprises a corrosion-resistant metal inner tube, an external heat preservation layer and armored heating wires arranged in the gap of the jacket, the heating wires are connected with an independent temperature control power supply, the temperature is set and closed-loop feedback regulated by the control host (207), so that the temperature of the liquid target metal in the conveying pipeline is stably maintained in the range of 10 DEG C to 30 DEG C above the melting point of the liquid target metal.

3. The system of claim 1, wherein, The centralized power supply control system further comprises a potential compensation module, the module dynamically compensates the preset cathode potential according to the real-time temperature data fed back by the centralized temperature monitoring system, so that the cathode potential fluctuation of each stage of the crucible is controlled within + 5 mV.

4. The system of claim 1, wherein, The coordinated flow control system adjusts the rotating speed of the inter-stage circulating pumps (203a, 203b) and the opening degree of the regulating valves (201a, 201b), so that the inter-stage conveying process is maintained in a low Reynolds number laminar flow state with Re < 2000, and can be switched to a pulse conveying mode according to the requirement, so as to inhibit the inter-stage back mixing and maintain the independence of the electrolyte composition.

5. The system of claim 1, wherein, The centralized power supply control system is configured to perform a potential-flow linkage control strategy according to the real-time collected flow meter (204a, 204b) signal and the cathode deposition current signal, to keep the metal supply rate and the deposition rate matched by dynamically adjusting the circulating pump rotating speed and the cathode potential, and to improve the deposition efficiency and the system stability.

6. The system of claim 1, wherein, The metal extraction and collection system includes a conical collector (301), a heating wire (302) and a liquid metal collection tank (303), and can be configured as a scraping collection device, a liquid metal convergence device or a vacuum suction collection device according to the state characteristics of the target metal.

7. The system of claim 1, wherein, The low-temperature composite molten salt electrolyte is selected from one or more of LiCl-KCl, NaCl-KCl, ZnCl2-NaCl-KCl and AlCl3-NaCl-KCl, and has a working temperature range of 100-400℃; the system is suitable for refining low-melting-point or active metals such as gallium, indium, tin, bismuth and lithium.

8. A metal electrowinning process using the system of any one of claims 1 to 7, characterized in that, The method comprises the following steps: S1: under the protection of an inert atmosphere, different purity low-temperature composite molten salt electrolytes are added to each stage crucible and heated to a set working temperature, and the inter-stage conveying pipeline is preheated to a range of 10-30℃ higher than the melting point of the target metal; S2: adding impurity-containing target metal raw material to the first stage crucible; S3: setting a cathode potential gradient that decreases along the sequence of the crucibles, so that the impurity metal ions with positive deposition potential are preferentially deposited in the previous stage; S4: starting the inter-stage flow control system to controllably convey the liquid metal deposited in the previous stage to the next stage crucible; S5: during the flow of the target metal, different metal ions are selectively deposited on the cathode at the corresponding potential according to the difference in their deposition potential; S6: obtaining high-purity target metal in the last stage crucible and extracting it through the collection system.

9. The method of claim 8, wherein, The cathode potential of each stage in step S3 is compensated and adjusted according to real-time temperature, deposition current and inter-stage flow data, so that the cathode potential of each stage is stabilized within ±5mV of the set value.

10. The method of claim 8, wherein, The inter-stage conveying process in step S4 adopts a low Reynolds number laminar flow or pulse conveying mode, and the liquid metal is prevented from solidifying during the conveying process by actively heating the conveying pipeline, so as to realize continuous and stable multi-stage electrolytic refining.

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