A low temperature molten salt electrolyte screening method and a solid state electrorefining aluminum method thereof

Abstract

The application discloses a low-temperature molten salt electrolyte screening method, comprising the following steps: 1, candidate chloride theoretical deposition potential screening; 2, candidate molten salt system liquidus surface projection calculation and parameter setting; 3, candidate molten salt theoretical viscosity calculation; 4, molten salt composition screening condition setting; 5, molten salt composition point determination. A low-temperature molten salt electrolyte, LiCl-KCl-CaCl2 molten salt composition is LiCl: 51-55 mol%, KCl: 40-44 mol%, CaCl2: 2-5 mol%, and in the ternary base salt, 5 mol% AlF3 is additionally added. A solid-state electrolytic refining aluminum method based on the low-temperature molten salt electrolyte, comprising the following steps: A, pretreatment of molten salt electrolyte and electrode; B, constant current electrolytic refining aluminum based on the molten salt electrolyte; C, cathode product collection and anode cleaning. In the electrolysis process, the average cell voltage is 0.42-0.64 V, the cathode product purity reaches more than 99.9%, and the current efficiency reaches more than 90%.

Description

Technical Field

[0001] This invention belongs to the field of molten salt electrolytic refining technology for waste aluminum, specifically relating to a low-temperature molten salt electrolyte screening method and its solid-state electrolytic refining method for aluminum. Background Technology

[0002] In the field of aluminum recycling, in addition to the problem of low recycling rates, there is also the issue of product degradation due to the accumulation of impurities. Existing aluminum recycling technologies suffer from complex processes and limited adaptability to raw materials.

[0003] Among them, molten salt electrolysis has the advantages of high purification efficiency, good adaptability to waste aluminum with mixed components and low pollution. Furthermore, the three-layer liquid electrolysis method has realized the industrial application of aluminum electrolytic refining. However, this method relies on the density difference between the crude aluminum anode, molten electrolyte and pure aluminum liquid cathode layer to achieve stratification, which limits the range of molten salt system and composition selection. In addition, it also has the problem of energy consumption as high as 15.7 kWh / kg, which is even higher than the energy consumption of primary aluminum production. For example, existing literature 1 (Guo B, Wang Y, Dong W, et al. Electrolytic Purification of Aluminum Scrap in NaCl-KCl-Na3AlF6 Molten Salt System[C] / / TMS Annual Meeting & Exhibition. Cham: SpringerNature Switzerland, 2025: 1267-1272.) uses a geometrically side-by-side electrolytic structure. Although this reduces the dependence on density difference, the problem still exists that the fine aluminum particles generated at the cathode cannot be completely incorporated into the molten cathode, resulting in a current efficiency of only 77.55%. In addition, the liquid electrolytic refining method in this technical solution requires temperatures above 700°C, which results in high operating temperatures and high energy consumption.

[0004] For example, another work by the research group in existing literature 1, and existing literature 2 (Huan S, Wang Y, Peng J, et al. Recovery of aluminum from waste aluminum alloy by low-temperature molten salt electrolysis[J]. Minerals Engineering, 2020, 154: 106386.), uses an AlCl3-rich low-temperature molten salt system for solid-state electrolytic purification of waste aluminum alloy, which can achieve the desired results at an electrolysis temperature of 170℃ and a flow rate of 30 mA / cm². 2This technology achieves aluminum electrolytic recovery at low current densities, significantly reducing the electrolysis operating temperature and achieving a current efficiency of over 94%. However, this technology suffers from several drawbacks. Firstly, the low current density results in low electrolysis yield and low purity of the cathode product (only 99.3%). Secondly, the highly hygroscopic nature of AlCl3-rich molten salt makes its composition susceptible to moisture, necessitating stringent requirements for raw material drying, equipment sealing, and inert atmosphere control.

[0005] As can be seen from existing literature 1 and existing literature 2, in the process of electrolytic refining of waste aluminum, the choice of molten salt electrolyte not only affects the electrolysis temperature and operational stability, but also directly affects the ion transport capacity, current efficiency and cathode product purity during the electrolysis process.

[0006] High-temperature liquid electrolytic refining systems, due to their high electrolysis temperatures (above 700°C), have advantages such as strong ion transport capabilities, high current density, and high yield. However, they also suffer from serious energy consumption problems.

[0007] Although the temperature can be significantly reduced to 170℃ using the low-temperature molten salt electrolyte system based on AlCl3, the molten salt mass transfer capacity is significantly reduced, and electrolysis cannot be carried out under high current density conditions because increasing the current density will lead to significant concentration polarization, which will seriously affect the purity of the cathode product and the stability of the cathode deposition process.

[0008] Therefore, simply lowering the electrolysis temperature cannot simultaneously meet the requirements of low energy consumption, high yield, and high purity.

[0009] To lower the operating temperature and enhance the mass transfer capacity of the molten salt, this research group used MgCl2-based molten salt with a higher melting point to increase the pure aluminum yield per unit time. For example, existing literature 3 (Guo B, Wang Y, Li R, et al. Sustainable recovery of pure aluminum from scrap aluminum alloy via solubleanode electrolysis: Cathodic deposition behavior of Al and Mg ions in MgCl2–KCl–NaCl molten salt system[J]. Journal of Cleaner Production, 2025, 521:146271.) used 48.6 wt.% MgCl2-30.9 wt.% KCl-20.5 wt.% NaCl, at an electrolysis temperature of 500℃ and a mass transfer capacity of 150 mA / cm². 2Solid-state electrolytic recycling of waste aluminum alloys was achieved at a current density of 99.84%. However, this technology could not consistently produce high-purity aluminum throughout the entire electrolysis process. The reason for this is that Mg... 2+ With Al 3+ The theoretical deposition potentials are similar; therefore, during cathode deposition, the process does not directly enter the stable Al deposition stage, but rather undergoes Mg deposition. 2+ Underpotential deposition, Mg 2+ With Al 3+ Co-deposition and Al 3+ Sedimentation occurs in three separate stages.

[0010] To avoid co-deposition of electrolyte components with aluminum, this research group selected Li chloride, which belongs to the same group as Mg, as the main component of the molten salt electrolyte. For example, existing literature 4 (Lu X, Zhang Z, Hiraki T, et al. A solid-state electrolysis process for upcycling aluminum scrap[J]. Nature, 2022, 606(7914): 511-515.) uses a 58.6 mol% LiCl-41.4 mol% KCl system to achieve solid-state electrolytic recovery of aluminum at 500℃, achieving a current efficiency of 95.6% and a cathode product purity of 99.8%.

[0011] Similar existing technologies used by this research group include: Reference 5 (Guo B, Wang Y, Huang Y, et al. Upcycling of scrap aluminum to pure aluminum through molten salt electrolysis[J]. Process Safety and Environmental Protection, 2024, 191: 94-101.) uses a 56 mol% LiCl-36 mol% KCl-8 mol% NaCl system, at an electrolysis temperature of 550℃, and 150 mA / cm². 2 At a current density, a product purity of 99.8% and a current efficiency of over 90% were achieved.

[0012] While existing literature 4 and literature 5 can both achieve electrolytic purification of waste aluminum, they suffer from high LiCl content in the molten salt system. Besides exhibiting the same hygroscopic properties as AlCl3 molten salt, LiCl also presents the problem of high raw material costs.

[0013] The analysis of the five existing technologies of this research group reveals that, in addition to the aforementioned technical problems, their work also suffers from the use of traditional trial-and-error methods to screen low-temperature molten salt electrolytes. Such traditional trial-and-error methods rely on empirical experiments, lack theoretical guidance, and suffer from high costs due to long component optimization cycles.

[0014] For the problem of designing molten salt composition using experimental trial-and-error methods, machine learning can be used to solve it. For example, existing literature 6 (Ravichandran A, Honrao S, Xie S, et al. Computational design of lowmelting eutectics of molten salts: A combined machine learning and thermodynamic modeling approach[J]. The Journal of Physical Chemistry Letters, 2023, 15(1): 121-126.) combines machine learning with thermodynamic models to predict the eutectic temperature of multi-component combinations. The model's average absolute percentage error is about 6%, and it performs high-throughput screening of a large number of molten salt combinations. By increasing the volume difference of component molecules and the configuration entropy, the eutectic temperature is reduced, thus achieving the screening and design of molten salt composition.

[0015] Similar existing technologies include existing literature 7 (Zhou X, Huang L, Zhou Y, et al. MachineLearning-Assisted KCl-CaCl2-LiCl Electrolyte Design for Low-Temperature,High-Performance Calcium-Based Liquid Metal Batteries[J]. Advanced Science,2026: e75994.), which screens the third component based on thermodynamic stability, melting point, density, and cost, and uses phase diagram analysis and random forest model to predict the composition of KCl-CaCl2-LiCl ternary molten salt, obtaining a molten salt of 13 mol% KCl-35 mol% CaCl2-52 mol% LiCl, enabling calcium-based liquid metal batteries to operate stably at 480 ℃.

[0016] As can be seen from the aforementioned existing literature 6 and 7, machine learning can accelerate material design through feature importance analysis and process condition optimization. However, when such technical solutions are applied to the field of solid-state electrolytic aluminum refining, the following technical problems exist:

[0017] 1. Since solid-state electrolytic refining of aluminum is still in its early stages, there is a lack of existing research data on solid-state electrolytic refining of aluminum, which cannot meet the basic requirements for the amount of experimental data for machine learning training, directly leading to the inability to guarantee the reliability of predictions.

[0018] 2. Similarly, since the above-mentioned research on solid-state electrolytic refining of aluminum is still in the early stage, the theoretical laws based on solid-state electrolytic refining of aluminum cannot meet the requirements of machine learning for screening molten salt electrolyte components.

[0019] To summarize the technical problems existing in the above-mentioned literature, the current technology cannot meet the requirements for establishing a low-temperature molten salt electrolyte screening method that comprehensively considers the risk of metal ion co-deposition, LiCl dosage, theoretical melting point and molten salt viscosity, and can verify electrolysis stability, current efficiency and purification effect through solid-state electrolysis. The reasons are as follows: 1. It is necessary to determine the cathodic reduction behavior of metal ions in molten salt components to reduce the risk of impurity co-deposition and ensure the purity of cathode products. 2. While reducing the amount of LiCl, it is necessary to ensure that the molten salt system has a low melting point and viscosity to meet the low-temperature operation requirements of solid-state electrolytic refining of waste aluminum; 3. It is necessary to balance the operational stability and electrolysis efficiency of the molten salt system, so that the cell voltage is maintained within a low and stable range during the electrolysis process, in order to meet the requirements of low energy and high efficiency in the electrolysis process. Summary of the Invention

[0020] To address the problems of existing technologies, the purpose of this invention is to provide a low-temperature molten salt electrolyte screening method and its solid-state electrolytic aluminum refining method. The basic principle involves calculating the candidate metal ions relative to Cl2 / Cl based on the Gibbs free energy change of the metal chloride formation reaction. - The theoretical deposition potential of the electrode, combined with liquid phase surface projection calculations, determines the low-melting-point liquid phase region, eutectic point, and near-eutectic composition of candidate molten salt systems. Low-melting-point and low-LiCl-content components are then screened. Furthermore, an ideal mixing viscosity is introduced as a screening condition to obtain a low-temperature molten salt electrolyte system suitable for solid-state electrolytic refining of aluminum from waste aluminum. Specifically, this involves… 1. Based on the thermodynamic data of metal chloride formation reactions, the Gibbs free energy change corresponding to the metal chloride formation reactions is converted into the metal ion / metal couple relative to Cl2 / Cl. - The theoretical deposition potential of the electrode is used to screen for chloride components that are stable under aluminum electrolytic refining conditions, thereby reducing the risk of impurity metal deposition and ensuring the purity of the cathode product. 2. Based on the LiCl-KCl binary low-temperature molten salt system, it is extended to a ternary molten salt system. The composition range of the low melting point liquid phase region, eutectic point and near eutectic point of each system is calculated by liquidus projection. According to the influence of molten salt composition on LiCl dosage and theoretical melting point, reducing LiCl dosage is beneficial to reducing electrolyte cost, but the molten salt composition deviates from the low melting point liquid phase region, which will lead to an increase in melting point. Therefore, by controlling the content of each chloride component, a ternary molten salt composition range with both low LiCl dosage and low melting point can be obtained. 3. Molten salt viscosity affects ion transport, anode interface mass transfer, and cell voltage stability during electrolysis to some extent. Lower molten salt viscosity is beneficial for ion migration and anode dissolution, while excessively high viscosity increases mass transfer resistance and leads to concentration polarization. Therefore, the theoretical viscosity of candidate ternary molten salt systems at electrolysis temperature was calculated using the ideal mixing method, and an ideal mixing viscosity of no more than 2.7 mPa·s at 500℃ was used as the screening condition to further screen low-temperature molten salt systems that combine low melting point, low LiCl content, and good mass transfer characteristics.

[0021] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0022] A method for screening low-temperature molten salt electrolytes includes the following steps: Step 1: Screening of theoretical deposition potentials for candidate chlorides. First, the Gibbs free energy of the formation reactions of different metal chlorides is obtained using thermodynamic calculations. Then, based on the thermodynamic relationship between Gibbs free energy and electrode potential, the deposition potentials of Al, Li, K, Ca, Na, and Sr relative to Cl2 / Cl are plotted. - Theoretical deposition potential-temperature variation curve of the electrode;

[0023] In step 1, the thermodynamic calculation parameters include the reaction equation, temperature range, and calculation step size;

[0024] The chemical equation is input using the method shown in formula (1).

[0025] The temperature range is set to be 600-1100 K;

[0026] The calculation step size is set to 100 K;

[0027] The theoretical deposition potential is calculated using the thermodynamic relationship between Gibbs free energy and electrode potential, as shown in formula (2).

[0028] In the formula, E(V) is the theoretical deposition potential, and ΔG ϴ(J) represents the Gibbs free energy change, n represents the number of electrons transferred, and F(C·mol⁻¹) represents the free energy change. -1 ) is Faraday's constant;

[0029] Step 2: Calculate the liquidus projection and set parameters for the candidate molten salt system, and calculate the liquidus projection phase diagram of the ternary system;

[0030] In step 2, the parameters for calculating the liquid phase surface projection include the setting of the molten salt system and the temperature range;

[0031] The candidate molten salt systems include LiCl-KCl-CaCl2, LiCl-KCl-NaCl, and LiCl-KCl-SrCl2;

[0032] The temperature range is set to be 300-1500 K;

[0033] Step 3: Calculate the theoretical viscosity of the candidate molten salt. Based on the liquidus projection diagram obtained in Step 2, determine the eutectic point of each system. Use the ideal mixing method to calculate the viscosity of the candidate ternary eutectic composition points at the electrolysis temperature.

[0034] In step 3, the specific calculation method for the ideal mixed viscosity is shown in formula (3). η ideal (mPa·s) is the ideal mixed viscosity, η1-η3 is the viscosity of each component, and x1-x3 is the mole fraction of each component;

[0035] Step 4: Set the screening conditions for molten salt components. The candidate molten salt system should simultaneously meet the requirements of low viscosity, reduced LiCl dosage, and reduced melting point.

[0036] In step 4, there are four filtering conditions, specifically:

[0037] Screening condition 1: The low-melting-point liquid phase region is determined by the liquidus projection phase diagram, and the eutectic point and near-eutectic composition are used as the candidate composition range.

[0038] Screening condition 2: The ideal mixing viscosity of the eutectic point composition of the candidate ternary molten salt system at 500 °C is not higher than 2.7 mPa·s;

[0039] Screening condition 3: The molar fraction of LiCl in the candidate ternary molten salt system is lower than 59.2 mol% in the LiCl-KCl binary eutectic composition.

[0040] Screening condition 4: The theoretical melting point of the candidate ternary molten salt system is lower than 353℃ of the LiCl-KCl binary eutectic composition;

[0041] Step 5: Determining the composition of the molten salt. Based on the phase diagram of each liquid phase surface projection, the molten salt composition that meets the screening criteria is determined.

[0042] A low-temperature molten salt electrolyte, wherein the LiCl-KCl-CaCl2 molten salt composition is LiCl: 51-55 mol%, KCl: 40-44 mol%, CaCl2: 2-5 mol%, and wherein an additional 5 mol% AlF3 is added to the ternary basic salt, the AlF3 mole fraction being calculated based on the total amount of molten salt after the addition of AlF3;

[0043] A solid-state electrolytic refining method for aluminum based on low-temperature molten salt electrolyte includes the following steps:

[0044] Step A, Pretreatment of molten salt electrolyte and electrodes: After the base salt of molten salt electrolyte and aluminum ion source are mixed evenly, the molten salt electrolyte is dried. At the same time, scrap aluminum alloy is used as the anode and 3N grade pure aluminum is used as the cathode. The electrodes are polished with 400-mesh sandpaper to remove the oxide film on the electrode surface and ensure that the electrode surface is flat.

[0045] In step A, the drying conditions are: drying temperature of 150-250℃ and drying time of 12-48 h.

[0046] Step B, constant current electrolytic refining of aluminum based on molten salt electrolyte, firstly, the molten salt electrolyte is preheated, then the molten salt electrolyte is completely melted, and finally, at the electrolysis temperature, the anode and cathode are suspended in parallel and immersed in the molten salt electrolyte for constant current electrolysis. Step B is carried out entirely under argon gas conditions.

[0047] In step B, the preheating temperature is 180-220℃, the preheating time is 1-5 h, and the melting temperature is 350-400℃.

[0048] In step B, the electrolysis temperature is 450-550℃, and the current density is 100-250 mA / cm². 2 The electrolysis time is 2 hours, the distance between the anode and cathode is 3-6 cm, and the argon flow rate is 5-10 L / min.

[0049] Step C, collection of cathode products and cleaning of anode: After electrolysis in step B is completed, the cathode products are collected and the anode is cleaned.

[0050] The average cell voltage during electrolysis is 0.42-0.64 V, the purity of the cathode product is over 99.9%, and the current efficiency is over 90%.

[0051] The technical effects of this invention have been tested and confirmed to be:

[0052] The cell voltage test during the electrolysis process showed that when using the selected LiCl-KCl-CaCl2 molten salt electrolyte for solid electrolytic refining of aluminum, the average cell voltage during the electrolysis process was 0.42-0.64V, and the cell voltage of the LiCl-KCl-CaCl2 system did not show a continuous upward trend during the 2-hour electrolysis process.

[0053] SEM test results show that after electrolysis of the LiCl-KCl-CaCl2 system, the anode mud on the anode surface has a loose skeleton-like structure. After cleaning, the anode surface is relatively flat overall, the aluminum matrix dissolution area is relatively uniformly distributed, and there is no obvious large-area local preferential dissolution area. The cathode products include two morphologies: granular and scaly dendrites.

[0054] ICP testing showed that the aluminum purity in the cathode product after electrolysis with LiCl-KCl-CaCl2 molten salt electrolyte reached over 99.9%; compared with scrap aluminum alloy anodes, the contents of Si, Cu, Mg, Mn, Fe, Cr, La, and Ce were all significantly reduced.

[0055] Current efficiency calculations show that when using LiCl-KCl-CaCl2 molten salt electrolyte for solid-state electrolytic refining of aluminum, the current efficiency reaches over 90%.

[0056] Compared with the prior art, the present invention has the following advantages:

[0057] 1. Calculate the theoretical reduction deposition potential of different metal ions based on the thermodynamic data of metal chloride formation reactions, and screen for metal ions with theoretical deposition potentials much negative than Al. 3+ Chlorides reduce the metal ions and Al in the molten salt components. 3+ The risk of co-deposition is reduced, which helps to ensure the purity of the cathode products;

[0058] 2. Based on the LiCl-KCl binary low-temperature molten salt system, the composition range of the low-melting-point liquid phase region, eutectic point, and near-eutectic point of the candidate ternary molten salt system is determined by liquid phase surface projection. Combined with theoretical viscosity screening, it is possible to reduce the amount of LiCl used while taking into account the theoretical melting point and mass transfer performance of the molten salt system. This provides theoretical guidance for the composition design of low-temperature molten salt electrolytes and reduces experimental screening costs and cycles.

[0059] 3. When the LiCl-KCl-CaCl2 ternary low-temperature molten salt electrolyte obtained by the screening method of this invention is applied to the solid-state electrolytic refining of waste aluminum, it can maintain a low and stable cell voltage at a low operating temperature and obtain high current efficiency and cathode product purity, thus realizing low-energy, high-efficiency and stable operation of the solid-state electrolytic refining process of waste aluminum. Attached Figure Description

[0060] Figure 1 For different metals relative to Cl2 / Cl- Theoretical deposition potential versus temperature curve of the electrode; Figure 2 The calculated value of the theoretical mixing viscosity at 500 °C is the eutectic composition point of the candidate molten salt system. Figure 3 The cell pressure variation curves with electrolysis time are shown for Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3. Figure 4 The image shows the morphology of the cathode products after electrolysis of the LiCl-KCl-CaCl2 ternary molten salt electrolyte in Example 1. Figure 5 The images show the morphology of the anode surface after electrolysis of the LiCl-KCl-CaCl2 ternary molten salt electrolyte in Example 1 and the anode substrate after cleaning. Figure 6 The image shows the morphology of the cathode products after electrolysis of the LiCl-KCl binary molten salt electrolyte in Comparative Example 1. Figure 7 The images show the morphology of the anode surface after electrolysis and the anode substrate after cleaning in Comparative Example 1 using the LiCl-KCl binary molten salt electrolyte. Figure 8 The image shows the morphology of the cathode products after electrolysis of the LiCl-KCl-NaCl ternary molten salt electrolyte in Comparative Example 2. Figure 9 The image shows the morphology of the cathode products after electrolysis of the LiCl-KCl-SrCl2 ternary molten salt electrolyte in Comparative Example 3. Detailed Implementation

[0061] The present invention will be further described in detail through embodiments and with reference to the accompanying drawings, but this is not intended to limit the scope of the invention.

[0062] Table 1 is provided to facilitate the differentiation of the various molten salt components and their electrolysis parameters involved in the embodiments and comparative examples.

[0063] Table 1. Molten salt composition and electrolysis parameters used in the examples and comparative examples.

[0064]

[0065] Example 1

[0066] A method for screening low-temperature molten salt electrolytes includes the following steps:

[0067] Step 1: Screening of theoretical deposition potentials for candidate chlorides. First, the Gibbs free energy of the formation reactions of different metal chlorides is obtained using thermodynamic calculations. Then, based on the thermodynamic relationship between Gibbs free energy and electrode potential, the deposition potentials of Al, Li, K, Ca, Na, and Sr relative to Cl2 / Cl are plotted. -The theoretical deposition potential-temperature curve of the electrode, as shown in the figure below. Figure 1 As shown;

[0068] The thermodynamic calculation parameters include the reaction equation, temperature range, and calculation step size;

[0069] The chemical equation is input using the method shown in formula (1).

[0070] The temperature range is set to be 600-1100 K;

[0071] The calculation step size is set to 100 K;

[0072] The theoretical deposition potential is calculated using the thermodynamic relationship between Gibbs free energy and electrode potential, as shown in formula (2).

[0073] In the formula, E(V) is the theoretical deposition potential, and ΔG ϴ (J) represents the Gibbs free energy change, n represents the number of electrons transferred, and F(C·mol⁻¹) represents the free energy change. -1 ) is Faraday's constant;

[0074] Screening was conducted based on the theoretical deposition potential-temperature curve, within the aluminum electrolytic refining temperature range, for Ca... 2+ / Ca、Na + / Na and Sr 2+ The theoretical deposition potential of / Sr is higher than that of Al. 3+ / Al is more negative and will not precede Al. 3+ Reduction deposition occurs at the cathode; therefore, CaCl2, NaCl, and SrCl2 were selected as chlorides for constructing candidate low-temperature molten salt systems.

[0075] Step 2: Calculate the liquidus projection and set parameters for the candidate molten salt system, and calculate the liquidus projection phase diagram of the ternary system;

[0076] The candidate molten salt systems include LiCl-KCl, LiCl-KCl-CaCl2, LiCl-KCl-NaCl, and LiCl-KCl-SrCl2;

[0077] Note that the liquidus projection phase diagram of the binary system LiCl-KCl was calculated simultaneously as a control system.

[0078] The temperature range is set to be 300-1500 K;

[0079] Step 3: Calculate the theoretical viscosity of the candidate molten salt. Based on the liquidus projection diagram obtained in Step 2, determine the eutectic point of each system. Use the ideal mixing method to calculate the viscosity of the candidate ternary eutectic composition points at the electrolysis temperature. The specific calculation method is shown in formula (3).

[0080] In the formula, η ideal (mPa·s) represents the ideal mixture viscosity, η1-η3 represents the viscosity of each component, and x1-x3 represents the mole fraction of each component.

[0081] The calculation results are as follows Figure 2 As shown, the ideal mixing viscosities of the ternary molten salt eutectic composition points of LiCl-KCl-CaCl2, LiCl-KCl-NaCl, and LiCl-KCl-SrCl2 at 500℃ are 2.68 mPa·s, 2.76 mPa·s, and 3.1 mPa·s, respectively.

[0082] It should be noted that although the ideal mixing viscosity of the LiCl-KCl binary system is 2.45 mPa·s, it is substantially different from that of the ternary molten salt electrolyte. Therefore, it is not used as a candidate molten salt system, but as a control system in the subsequent specific embodiments, i.e., a comparative example.

[0083] Step 4: Set the screening conditions for molten salt components. The candidate molten salt system should simultaneously meet the requirements of low viscosity, reduced LiCl dosage, and reduced melting point.

[0084] There are four filtering criteria, specifically:

[0085] Screening condition 1: The low-melting-point liquid phase region is determined by the liquidus projection phase diagram, and the eutectic point and near-eutectic composition are used as the candidate composition range.

[0086] Screening condition 2: The ideal mixing viscosity of the eutectic point composition of the candidate ternary molten salt system at 500 °C is not higher than 2.7 mPa·s;

[0087] Screening condition 3: The molar fraction of LiCl in the candidate ternary molten salt system is lower than 59.2 mol% in the LiCl-KCl binary eutectic composition.

[0088] Screening condition 4: The theoretical melting point of the candidate ternary molten salt system is lower than 353℃ of the LiCl-KCl binary eutectic composition;

[0089] Step 5, Determination of molten salt composition points: Based on the phase diagrams projected at each liquidus surface, the molten salt composition meeting the screening criteria is determined. Specifically,

[0090] The composition of the LiCl-KCl-CaCl2 molten salt is: LiCl: 51-55 mol%, KCl: 40-44 mol%, CaCl2: 2-5 mol%

[0091] To demonstrate the effectiveness and practicality of the low-temperature molten salt electrolyte screening method, a ternary molten salt with a composition range of 53 mol% LiCl-44 mol% KCl-3 mol% CaCl2, referred to as LiK-Ca, was selected as Example 1 for solid-state electrolytic refining experiments.

[0092] A solid-state electrolytic refining method for aluminum based on LiK-Ca ternary molten salt specifically includes the following steps:

[0093] Step A: Pretreatment of molten salt electrolyte and electrodes. LiCl, KCl, CaCl2 and AlF3 are used as molten salt electrolytes, with LiCl, KCl and CaCl2 as the base salts of the molten salt electrolyte and AlF3 as the aluminum ion source. After mixing LiCl, KCl, CaCl2 and AlF3 evenly, the molten salt electrolyte is dried. At the same time, scrap aluminum alloy is used as the anode, with the main impurity elements including Si, Cu, Mn, Fe, Cr, La and Ce. 3N grade pure aluminum is used as the cathode. The electrodes are polished with 400-mesh sandpaper to remove the oxide film on the electrode surface and ensure that the electrode surface is flat.

[0094] The molar percentages of LiCl, KCl, and CaCl2 are 53 mol%, 44 mol%, and 3 mol%, respectively. An additional 5 mol% of AlF3 is added to the basic salt, and the specific calculation method is shown in formula (4).

[0095] In the formula, c is the mole fraction of AlF3, and n is the amount of substance;

[0096] The drying conditions are: drying temperature of 150℃ and drying time of 48 h;

[0097] Step B, constant current electrolytic refining of aluminum based on molten salt electrolyte, firstly, the molten salt electrolyte is preheated at a preheating temperature of 200℃ for 1 hour, then the molten salt electrolyte is completely melted at a melting temperature of 350℃, and finally, under the electrolysis temperature conditions, the anode and cathode are suspended in parallel and immersed in the molten salt electrolyte for constant current electrolysis. Step B is carried out entirely under argon gas conditions.

[0098] The constant current electrolysis conditions are as follows: electrolysis temperature 500℃, electrolysis current 750 mA, and current density 200 mA / cm². 2The electrolysis time was 2 hours; the area of ​​the molten salt immersed portions of the anode and cathode was 3.75 cm². 2 The spacing is 6 cm; the flow rate of the argon gas is 10 L / min;

[0099] To demonstrate the cell voltage and stability of the LiK-Ca molten salt electrolyte, a cell voltage test was conducted. The test results are as follows: Figure 3 As shown in Table 2, the average cell pressure of LiK-Ca is 0.47 V, and the cell pressure does not show a continuous upward trend during electrolysis.

[0100] Table 2. Cell voltage, cathode product purity, and current efficiency during electrolysis of different molten salt systems.

[0101]

[0102] Step C, collection of cathode products and cleaning of anode: After electrolysis in step B is completed, the cathode products are collected and the anode is cleaned.

[0103] The method for collecting the cathode products is as follows: First, the cathode products are ultrasonically treated with water as the ultrasonic medium for 30 minutes. Then, the cathode products are ultrasonically cleaned with ethanol as the ultrasonic medium for 30 minutes. Finally, the cathode products are dried to constant weight at a drying temperature of 200°C.

[0104] The cleaning method for the anode is as follows: first, the anode is washed with water as the washing solution for 2 minutes; then, the anode is washed with ethanol as the washing solution for 2 minutes; finally, the anode is dried to constant weight at a drying temperature of 200°C.

[0105] To demonstrate the morphology of the cathode products and the anode surface before and after cleaning after electrolysis with LiK-Ca molten salt electrolyte, SEM tests were performed.

[0106] The test results of the cathode products are as follows: Figure 4 As shown, the cathode products contain two morphologies: granular and flaky dendrites.

[0107] Test results of the anolyte surface before cleaning are as follows: Figure 5 As shown, the anode mud on the anode surface before cleaning has a loose skeletal structure;

[0108] The test results of the cleaned anode surface are as follows: Figure 5 As shown, the anode surface is smooth after cleaning, which indicates that the aluminum matrix dissolution area is evenly distributed and there is no local preferential dissolution area.

[0109] To further demonstrate the refining effect, i.e., purity, of the cathode products, ICP testing was performed on the scrap aluminum alloy anode and cathode products. The test results are shown in Table 3. The aluminum purity in the cathode products was 99.94%, and the impurity content was 0.06 wt.%. Compared with the scrap aluminum alloy anode, the contents of impurity elements Si, Cu, Mg, Mn, Fe, Cr, La, and Ce were all significantly reduced.

[0110] Table 3. ICP test results (wt.%) of aluminum alloy anode and cathode products from waste materials.

[0111]

[0112] Furthermore, the current efficiency of the electrolysis process is calculated. The current efficiency is calculated as the ratio of the actual aluminum production to the theoretical aluminum production.

[0113] The specific calculation method for the theoretical aluminum production is shown in formula (5).

[0114] Where M(g) is the theoretical aluminum yield, K is the electrochemical equivalent of aluminum, T(s) is the electrolysis time, and I(A) is the electrolysis current.

[0115] The calculated current efficiency is shown in Table 2. The current efficiency of LiK-Ca is 91%.

[0116] To further demonstrate the effectiveness and practicality of the low-temperature molten salt electrolyte screening method, a binary molten salt was used as a control system. Specifically, a binary molten salt with a composition of 59.2 mol% LiCl-40.8 mol% KCl, referred to as LiK, was used as Comparative Example 1 for solid-state electrolytic refining experiments.

[0117] Meanwhile, the ternary molten salts that did not meet the screening criteria were used as Comparative Examples 2 and 3, with the following specific components:

[0118] Comparative Example 2 is a ternary molten salt of 55 mol% LiCl-36 mol% KCl-9 mol% NaCl, abbreviated as LiK-Na;

[0119] Comparative Example 3 is a ternary molten salt consisting of 49.9 mol% LiCl, 40.9 mol% KCl, and 9.2 mol% SrCl2, abbreviated as LiK-Sr.

[0120] Comparative Example 1

[0121] A solid-state electrolytic refining method for aluminum based on LiK binary molten salt is described. The steps are the same as those in Example 1 unless otherwise specified. The difference is that step A uses a binary basic salt composed of 59.2 mol% LiCl and 40.8 mol% KCl.

[0122] To demonstrate the cell pressure and stability of the LiK molten salt electrolyte, a cell pressure test was conducted. The test results are as follows: Figure 3 As shown in Table 2, the average cell voltage of LiK is 0.56 V, and the cell voltage gradually increases with electrolysis. The test results indicate that the average cell voltage of LiK is too high and does not meet the application requirements.

[0123] To demonstrate the morphology of the cathode products and the anode surface before and after cleaning after electrolysis with LiK molten salt electrolyte, SEM tests were performed.

[0124] The test results of the cathode products are as follows: Figure 6 As shown, the cathode product is not substantially different from that of Example 1, that is, it also contains both granular and scaly dendrite morphologies.

[0125] Test results of the anolyte surface before cleaning are as follows: Figure 7 As shown, the anode mud on the anode surface before cleaning is in a dense, agglomerated state. Compared with Example 1, it can be seen that the introduction of Ca element can change the anode mud from a dense, agglomerated state to a loose, skeletal state. The dense, agglomerated state is not conducive to the interfacial contact and mass transfer process between the anode and the molten salt, which leads to an increase in the anode interfacial resistance during electrolysis, which is directly manifested as an increase in cell pressure and a decrease in stability.

[0126] The test results of the cleaned anode surface are as follows: Figure 7 As shown, after cleaning, the anode surface has local pits and holes, as well as uneven dissolution areas. Compared with Example 1, it can be seen that the introduction of Ca element can improve the uniformity of anode dissolution and avoid local preferential dissolution.

[0127] To further demonstrate the refining effect, i.e., purity, of the cathode product in Comparative Example 1, ICP testing was performed on the scrap aluminum alloy and the cathode product. The test results are shown in Table 3. The aluminum purity in the cathode product was 99.88%, and the impurity content was 0.12 wt.%. Compared with the scrap aluminum alloy anode, the contents of impurity elements Si, Cu, Mg, Mn, Fe, Cr, La, and Ce were all significantly reduced.

[0128] Furthermore, the current efficiency of the electrolysis process was calculated using the same formula as in Example 1. The results are shown in Table 2, where the current efficiency of LiK is 79%. Compared with Example 1, it can be seen that the introduction of Ca can significantly improve the current efficiency.

[0129] Comparative Example 2

[0130] A solid-state electrolytic refining method for aluminum based on LiK-Na ternary molten salt is described. The steps are the same as those in Example 1 unless otherwise specified. The difference is that in step A, a ternary basic salt composed of 55 mol% LiCl-36 mol% KCl-9 mol% NaCl is used.

[0131] To demonstrate the cell pressure and stability of the LiK-Na molten salt electrolyte, a cell pressure test was conducted. The test results are as follows: Figure 3 As shown in Table 2, the average cell voltage of LiK-Na is 0.52 V, and the cell voltage gradually increases as electrolysis proceeds.

[0132] To demonstrate the morphology of the cathode products after electrolysis with LiK-Na molten salt electrolyte, SEM analysis was performed. The test results of the cathode products are as follows: Figure 8 As shown, the cathode product is not substantially different from that of Example 1, that is, it also contains both granular and scaly dendrite morphologies.

[0133] To further demonstrate the refining effect, i.e., purity, of the cathode products, ICP testing was performed on the scrap aluminum alloy anode and cathode products. The test results are shown in Table 3. The aluminum purity in the cathode products was 99.82%, and the impurity content was 0.18 wt.%. Compared with the scrap aluminum alloy anode, the contents of impurity elements Si, Cu, Mg, Mn, Fe, Cr, La, and Ce were significantly reduced. However, compared with Comparative Example 1, the introduction of Na element actually reduced the purity of the cathode products.

[0134] Furthermore, the current efficiency of the electrolysis process was calculated using the same formula as in Example 1. The results of the current efficiency calculation are shown in Table 2. The current efficiency of LiK-Na is 93.6%.

[0135] Comparative Example 3

[0136] A solid-state electrolytic refining method for aluminum based on LiK-Sr ternary molten salt is described. The steps are the same as those in Example 1 unless otherwise specified. The difference is that step A uses a ternary basic salt composed of 49.9 mol% LiCl-40.9 mol% KCl-9.2 mol% SrCl2.

[0137] To demonstrate the cell pressure and stability of the LiK-Sr molten salt electrolyte, a cell pressure test was conducted. The test results are as follows: Figure 3 As shown in Table 2, the average cell voltage of LiK-Sr is 0.64 V. The cell voltage gradually increases as electrolysis progresses, and significant fluctuations occur in the later stages of electrolysis.

[0138] To demonstrate the morphology of the cathode products after electrolysis with LiK-Sr molten salt electrolyte, SEM analysis was performed. The test results of the cathode products are as follows: Figure 9As shown, the cathode product is not substantially different from that of Example 1, that is, it also contains both granular and scaly dendrite morphologies.

[0139] To further demonstrate the refining effect, i.e. purity, of the cathode products, ICP testing was conducted on the waste aluminum alloy anode and cathode products. The test results are shown in Table 3. The aluminum purity in the cathode products was 99.90%, and the impurity content was 0.1 wt.%. Compared with the waste aluminum alloy anode, the contents of impurity elements Si, Cu, Mg, Mn, Fe, Cr, La and Ce were significantly reduced.

[0140] Furthermore, the current efficiency of the electrolysis process was calculated using the same formula as in Example 1. The calculated current efficiency results are shown in Table 2, with the current efficiency of LiK-Sr being 80.7%. Although Sr and Ca belong to the same main group, a comparison between Comparative Example 3 and Comparative Example 1 shows that the improvement in current efficiency due to the introduction of Sr is negligible.

[0141] As can be seen from the above Examples 1, 1, 2 and 3, the influence of introducing different elements on the cathode products and refining effect is negligible. However, different elements have a significant impact on the surface morphology of the anode during electrolysis, which in turn has a significant impact on the average cell voltage and current efficiency.

Claims

1. A method for screening low-temperature molten salt electrolytes, characterized in that... Includes the following steps: Step 1: Screening of theoretical deposition potentials for candidate chlorides. First, the Gibbs free energy of the formation reactions of different metal chlorides is obtained using thermodynamic calculations. Then, based on the thermodynamic relationship between Gibbs free energy and electrode potential, the deposition potentials of Al, Li, K, Ca, Na, and Sr relative to Cl2 / Cl are plotted. - Theoretical deposition potential-temperature variation curve of the electrode; Step 2: Calculate the liquidus projection and set parameters for the candidate molten salt system, and calculate the liquidus projection phase diagram of the ternary system; Step 3: Calculate the theoretical viscosity of the candidate molten salt. Based on the liquidus projection diagram obtained in Step 2, determine the eutectic point of each system. Use the ideal mixing method to calculate the viscosity of the candidate ternary eutectic composition points at the electrolysis temperature. Step 4: Set the screening conditions for molten salt components. The candidate molten salt system should simultaneously meet the requirements of low viscosity, reduced LiCl dosage, and reduced melting point. Step 5: Determining the composition of the molten salt. Based on the phase diagram of each liquid phase surface projection, the molten salt composition that meets the screening criteria is determined.

2. The screening method according to claim 1, characterized in that: In step 1, the thermodynamic calculation parameters include the reaction equation, temperature range, and calculation step size; The chemical equation is input using the method shown in formula (1). The temperature range is set to be 600-1100 K; The calculation step size is set to 100 K; The theoretical deposition potential is calculated using the thermodynamic relationship between Gibbs free energy and electrode potential, as shown in formula (2). In the formula, E(V) is the theoretical deposition potential, and ΔG ϴ (J) represents the Gibbs free energy change, n represents the number of electrons transferred, and F(C·mol⁻¹) represents the free energy change. -1 ) is the Faraday constant.

3. The screening method according to claim 1, characterized in that: In step 2, the parameters for calculating the liquid phase surface projection include the setting of the molten salt system and the temperature range; The candidate molten salt systems include LiCl-KCl-CaCl2, LiCl-KCl-NaCl, and LiCl-KCl-SrCl2; The temperature range is set to 300-1500 K.

4. The screening method according to claim 1, characterized in that: In step 3, the specific calculation method for the ideal mixed viscosity is shown in formula (3). η ideal (mPa·s) is the ideal mixed viscosity, η1-η3 is the viscosity of each component, and x1-x3 is the mole fraction of each component.

5. The screening method according to claim 1, characterized in that: In step 4, there are four filtering conditions, specifically: Screening condition 1: The low-melting-point liquid phase region is determined by the liquidus projection phase diagram, and the eutectic point and near-eutectic composition are used as the candidate composition range. Screening condition 2: The ideal mixing viscosity of the candidate ternary molten salt system at the eutectic point composition at 500 °C is not higher than 2.7 mPa·s; Screening condition 3: The molar fraction of LiCl in the candidate ternary molten salt system is lower than 59.2 mol% in the LiCl-KCl binary eutectic composition. Screening condition 4: The theoretical melting point of the candidate ternary molten salt system is lower than 353℃ of the LiCl-KCl binary eutectic composition.

6. A low-temperature molten salt electrolyte, characterized in that: The composition of the LiCl-KCl-CaCl2 molten salt is as follows: LiCl: 51-55 mol%, KCl: 40-44 mol%, CaCl2: 2-5 mol%. In addition, 5 mol% AlF3 is added to the ternary basic salt. The molar fraction of AlF3 is calculated based on the total amount of substance of the molten salt after the addition of AlF3.

7. A solid-state electrolytic refining method for aluminum based on low-temperature molten salt electrolyte, characterized in that... Includes the following steps: Step A, Pretreatment of molten salt electrolyte and electrodes: After the base salt of molten salt electrolyte and aluminum ion source are mixed evenly, the molten salt electrolyte is dried. At the same time, scrap aluminum alloy is used as the anode and 3N grade pure aluminum is used as the cathode. The electrodes are polished with 400-mesh sandpaper to remove the oxide film on the electrode surface and ensure that the electrode surface is flat. In step A, the drying conditions are: drying temperature of 150-250℃ and drying time of 12-48 h. Step B, constant current electrolytic refining of aluminum based on molten salt electrolyte, firstly, the molten salt electrolyte is preheated, then the molten salt electrolyte is completely melted, and finally, at the electrolysis temperature, the anode and cathode are suspended in parallel and immersed in the molten salt electrolyte for constant current electrolysis. Step B is carried out entirely under argon gas conditions. Step C, collection of cathode products and cleaning of anode: After electrolysis in step B is completed, the cathode products are collected and the anode is cleaned.

8. The solid-state electrolytic refining method for aluminum according to claim 7, characterized in that: In step B, the preheating temperature is 180-220℃, the preheating time is 1-5 h, and the melting temperature is 350-400℃. In step B, the electrolysis temperature is 450-550℃, and the current density is 100-250 mA / cm². 2 The electrolysis time was 2 hours, the distance between the anode and cathode was 3-6 cm, and the argon flow rate was 5-10 L / min.

9. The solid-state electrolytic refining method for aluminum according to claim 8, characterized in that: The average cell voltage during electrolysis is 0.42-0.64 V, the purity of the cathode product is over 99.9%, and the current efficiency is over 90%.

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