A method for preparing ultra-high purity indium based on electrolysis and vacuum distillation

Abstract

The application discloses a method for preparing ultra-high purity indium based on electrolysis and vacuum distillation, and belongs to the technical field of non-ferrous metal refining and high-purity metal preparation. In view of the low separation efficiency of traditional water solution electrolytic refining indium process on tin, thallium, cadmium and other impurities with close potential to indium, the large load of subsequent vacuum distillation, the high distillation temperature, the serious volatilization loss of indium, the non-optimized matching parameters of subsequent purification of existing molten salt electrolysis process, and the absence of precise distribution mechanism of impurity load, the application adopts a coupling process system of electrochemical selective rough separation and physical gradient deep purification. 2N-3N crude indium or indium-containing smelting waste is used as raw material, high-efficiency pre-removal of Sn, Pb, Cd, Bi, Tl and other difficult-to-handle impurities is realized through molten salt electrolysis, and high-purity indium is stably prepared; and the differentiated impurities with high and low boiling points are removed through segmented gradient vacuum distillation, the regional zone melting process can be selectively matched, the impurity spectrum characteristics after molten salt electrolysis are accurately matched, and the impurity is removed in stages.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal refining and high-purity metal preparation technology, specifically relating to a method for preparing ultra-high purity indium based on electrolysis and vacuum distillation. Background Technology

[0002] Ultra-high purity indium is a core raw material for strategic industries such as semiconductor chips, infrared optoelectronic displays, superconducting devices, and high-end electronic packaging. The purity of the product directly determines the precision, stability, and lifespan of downstream components. Currently, the mainstream industrial preparation process for high-purity indium is the traditional aqueous solution electrolytic refining route, with the core process being: crude indium → sulfuric acid / hydrochloric acid aqueous solution electrolysis (1-2 times) → vacuum distillation → optional secondary electrolysis → zone melting.

[0003] This traditional process has inherent technical bottlenecks: the electrochemical window of the aqueous solution electrolysis system is narrow, and the separation efficiency for impurities such as Sn, Tl, and Cd, which have electrode potentials close to that of indium, is extremely low, resulting in a high amount of impurity residue in the indium product after electrolysis, and a huge burden on the subsequent vacuum distillation purification process; in order to remove the residual trace impurities, the traditional process requires high-temperature two-stage distillation at 1000~1250℃, which not only has extremely high energy consumption, but also causes a large amount of indium volatilization loss, with an indium direct recovery rate of only about 90%, and the multi-stage electrolysis and purification process generates a large amount of acid and alkali waste liquid, resulting in high environmental pressure and long production cycle.

[0004] Although the existing publicly disclosed patent for molten salt electrolysis of indium (WO2006 / 080742) uses the InCl / ZnCl2 molten salt system to replace part of the aqueous solution electrolysis process, it has obvious technical defects: it only uses single-stage vacuum distillation for purification, does not redesign the matching distillation curve for the special impurity spectrum after molten salt electrolysis, and does not establish an impurity load distribution model for electrochemical coarse separation and physical deep purification. It cannot fundamentally solve the problems of high energy consumption, poor purity stability and impurity backmixing in traditional processes, and the effect of technological upgrading is limited.

[0005] In summary, the industry urgently needs a short-process preparation technology for ultra-high purity indium that can achieve precise impurity removal through classification, streamlined procedures, low energy consumption, zero waste liquid, and high yield. Based on this, this invention innovatively optimizes the coupling and matching relationship between the molten salt electrolysis system and the subsequent physical purification process, reconstructs the impurity removal load distribution mechanism, and overcomes the pain points of existing technologies. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing aqueous solution electrolytic refining of indium, such as low impurity separation efficiency, high vacuum distillation load, high temperature and high energy consumption, low indium yield, and serious waste liquid pollution, as well as the poor matching of purification in existing molten salt electrolysis processes and the lack of a precise impurity load allocation mechanism. This invention provides a short-process preparation method for ultra-high purity indium based on molten salt electrolytic purification coupled with vacuum distillation. By using a wide electrochemical window for molten salt electrolysis to achieve efficient pre-removal of difficult-to-treat impurities, combined with segmented temperature control, two-stage condensation vacuum distillation, and optional zone melting processes, precise removal of impurities is achieved through graded processing. This significantly reduces production energy consumption, shortens the process flow, and improves the direct indium yield, enabling the green and efficient preparation of 5N5~6N grade ultra-high purity indium.

[0007] Technical solution

[0008] To achieve the above-mentioned objectives, this invention provides a short-process method for preparing ultra-high purity indium based on molten salt electrolytic purification coupled with vacuum distillation. The core of this method employs a composite process route combining selective coarse separation via molten salt electrolytic electrochemistry with deep physical purification via vacuum distillation / zone melting, as detailed below:

[0009] A method for preparing ultra-high purity indium based on electrolysis and vacuum distillation includes the following steps:

[0010] S1 molten salt electrolytic purification: using crude indium metal with a purity of 2N~3N or indium-containing smelting waste of In-Sn and In-Zn as the anode, and pure indium or stainless steel as the cathode; using KCl-LiCl or ZnCl2-KCl halide composite molten salt as the electrolyte, constant current electrolytic refining is carried out under the conditions of electrolysis temperature of 250~550℃, current density of 10~30 A / dm², and cell voltage of 0.2~0.6 V. Sn, Pb, Cd, Bi, and Tl impurities on the anode side are selectively retained in the anode mud, and 4N~5N grade high-purity metallic indium is stably deposited at the cathode;

[0011] S2 Intermediate Purification and Casting: The 4N~5N grade high-purity metallic indium precipitated in step S1 is placed in an inert or reducing atmosphere to remove molten salt inclusions, and then melted and cast at 160~200℃ to prepare a dense and uniform vacuum distillation-specific indium ingot.

[0012] S3. Fractional Vacuum Distillation Purification: Place the vacuum distillation-specific indium ingot prepared in step S2 into a graphite or quartz crucible, and then place it into a dual-temperature zone vacuum distillation furnace. Evacuate the furnace and control the residual pressure inside to 10. -3 ~10 Pa; adopts segmented temperature control process and two-stage condensation process.

[0013] Preferably, in step S1, the InCl3 mass ratio in the composite molten salt electrolyte is 20-50%, and the balance is one or more compound systems of KCl, LiCl, and ZnCl2; 0.5-3.0 wt.% of NH4Cl or KI is added to the molten salt as an surfactant to optimize the mass transfer effect at the electrolysis interface and inhibit the co-deposition of impurities.

[0014] Preferably, in step S1, when the anode material is an In-Sn alloy intermediate, a segmented voltage-controlled electrolysis mode is adopted. The first stage controls the cell voltage at 0.20~0.35 V to preferentially achieve anodic dissolution and retention of tin. The second stage adjusts the cell voltage at 0.4~0.6 V to accurately achieve selective cathode deposition of metallic indium and complete efficient separation of Sn / In.

[0015] Preferably, in step S3, the first stage of the segmented temperature control process involves heating to 600~850℃ and holding for 1~3 hours to remove high vapor pressure impurities such as Zn, Cd, Tl, Sb, Se, and As.

[0016] Preferably, in step S3, the second stage of the segmented temperature control process is heated to 900~1050℃ and held for 0.5~2h to allow the indium metal to evaporate efficiently. The indium is collected through two-stage condensation zones to obtain 5N~5N5 grade high-purity indium, while low-volatility heavy impurities such as Cu, Fe, Ni, and Ag are retained in the tailings in the furnace.

[0017] Preferably, S4 is ultra-high purity deep purification: the 5N5 grade indium obtained in step S3 is refined by zone melting. Under the protection of argon, the melting zone temperature is set to 160~250℃ and the melting zone moving speed is 5~30 mm / h. The zone melting is carried out 2~5 times in a row. The first and last impurity enrichment sections are removed to finally obtain ultra-high purity indium of grade 6N or above.

[0018] Preferably, in step S3, the second stage of the segmented temperature control process is heated to 1080~1150℃, and the holding time is extended to 2~4 h, so as to directly prepare 5N5~6N grade ultra-high purity indium.

[0019] Preferably, in step S3, the two-stage condensation process is as follows: the first-stage condensation temperature is controlled at 300~500℃ to specifically capture light impurity components such as Zn, Cd, and Tl; the second-stage condensation temperature is controlled at 850~950℃ to directionally capture the high-purity indium main fraction, completely avoiding backmixing and contamination of the high-purity indium product by light impurities.

[0020] An apparatus system for implementing the preparation method includes an argon-protected molten salt electrolytic cell, an inert atmosphere scraping and casting module, a dual-temperature zone vacuum distillation furnace with a two-stage condensation structure, and a temperature-controlled zone melting tube furnace, all connected in sequence. The molten salt electrolytic cell is equipped with an anode basket and a cathode indium bead collection structure. The dual-temperature zone vacuum distillation furnace independently controls the temperatures of the two zones for impurity removal and indium evaporation, thereby achieving precise separation of impurities from the main product.

[0021] The supporting process equipment system of this invention consists of an argon-protected molten salt electrolytic cell, an inert atmosphere scraping salt casting module, a dual-temperature zone two-stage condensing vacuum distillation furnace, and a temperature-controlled zone melting tube furnace. Each module works together to adapt to short-process technology, enabling continuous, large-scale, and stable production.

[0022] Beneficial effects

[0023] 1. Iterative innovation of the electrolysis system, completely breaking through the bottleneck of aqueous solution electrolysis technology: The wide electrochemical window halide molten salt electrolysis system is used to replace the traditional aqueous solution electrolysis. The separation coefficient of difficult-to-separate impurities such as Sn, Tl, and Cd from indium is greatly improved. The Sn impurity in the raw material can be reduced from >0.05% to below 10 ppm. The core impurity is removed in the pre-process, eliminating the tin impurity treatment load of subsequent vacuum distillation, and optimizing the matching of purification process from the source.

[0024] 2. A pioneering mechanism for precise redistribution of impurity load was developed to construct a short-process coupled process: a synergistic purification of electrochemical coarse separation and physical deep separation was established, with molten salt electrolysis undertaking the task of coarse removal of most metal impurities, vacuum distillation responsible for the removal of light impurities and purification of the main indium, and heavy impurities directly retained in the tailings. Compared with the traditional two-stage high-temperature distillation process, the temperature was significantly lowered, the direct indium recovery rate increased from 90% to over 96%, and the overall energy consumption was greatly reduced.

[0025] 3. Segmented temperature control coupled with two-stage condensation solves the problem of impurity backmixing: The innovative segmented distillation strategy of low-temperature impurity removal and medium-temperature indium evaporation is combined with an independent two-stage condensation structure to capture light impurities and high-purity indium main fractions respectively, completely solving the problem of Cd and Tl impurity backmixing and contamination caused by traditional single condensation process, and significantly improving product purity and stability.

[0026] 4. Green closed-loop short-process technology with strong industrial adaptability: The entire process adopts an anhydrous molten salt system, eliminating the acid washing wastewater generated by traditional sulfuric acid electrolysis processes. The molten salt electrolyte can be recycled and regenerated by recovering valuable metals from the anode mud, making it green and environmentally friendly. At the same time, it is equipped with dual purification paths, which can flexibly choose to directly distill to prepare 5N5 grade indium or coupled zone melting to prepare 6N grade indium according to production needs. The process is simplified, the production cycle is short, and the cost is controllable, making it suitable for large-scale industrial production.

[0027] 5. Customized process parameters with outstanding novelty and protection: The segmented voltage electrolysis process is designed for indium-containing waste, which accurately achieves online separation of tin and indium. This is different from the single electrolysis mode of existing molten salt electrolysis patents, forming a unique technical barrier with significant patent novelty. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to specific embodiments and comparative examples. However, the present invention is not limited to the following embodiments.

[0029] Example 1: Preparation of 6N-grade indium from 2N8 crude indium containing Sn, Pb, Cd and Tl

[0030] S1 molten salt electrolytic purification: 1000 g of crude 2N8 indium was used as the anode. The main impurities in the raw material were Sn 0.10 wt%, Pb 0.06 wt%, Cd 0.05 wt%, and Tl 0.02 wt%. The electrolyte was a KCl-LiCl-InCl3 molten salt with 1.0 wt% NH4Cl added. Stainless steel was used as the cathode, and the process was carried out at 300℃ and 18 A / dm² under argon protection. 2 Electrolysis was performed for 4 hours. After electrolysis, the indium at the cathode was collected and tested to be at the 4N5 level, with Sn 8 ppm, Pb 15 ppm, and Cd 3 ppm. The recovery rate of indium was 95.2%.

[0031] S2 intermediate purification casting: most of the salt adhering to the cathode indium surface is mechanically stripped off, quickly washed with a small amount of deionized water, and vacuum dried; under argon protection, it is melted at 180 ℃ to remove slag and cast into ingots.

[0032] S3 Fractional Vacuum Distillation Purification: Indium ingots are placed in a graphite crucible and evacuated to a vacuum of 5 × 10⁻⁶. -3 Pa. In the first stage, the temperature was raised to 780 °C and held for 2 h, and the light impurity fraction was collected separately. In the second stage, the temperature was raised to 980 °C and held for 1.5 h, and the indium main fraction was collected, while the heavy impurity tailings were retained in the furnace. The product was analyzed by GDMS, and the total impurity content was 4.8 ppm, reaching the 5N5 grade, with an indium direct recovery rate of 93.4%.

[0033] S4 Ultra-High Purity Deep Purification: 5N5 grade indium is loaded into a quartz boat and melted in a 160–220°C zone under 0.12 MPa argon protection. The zone is continuously melted four times at a moving speed of 30 mm / h. Each time, the melting zone is kept stable to allow all ingots to pass through. After the final melt, 15% of the impurity-rich sections at the beginning and end are removed, retaining the middle section. GDMS analysis of the middle section shows a total impurity content of 0.9 ppm, with Sn below 0.05 ppm and Fe below 0.1 ppm, reaching 6N grade. Based on the removal ratio, the yield of the qualified section in the zone melting process is 70.0%.

[0034] Example 2: Segmented Voltage Electrolysis and Distillation of In-Sn Alloys

[0035] This embodiment covers the In-Sn alloy raw material, pure indium cathode, KI additive, segmented cell voltage, and quartz crucible scheme.

[0036] S1 Molten Salt Electrolytic Purification: 1000 g of an In-Sn alloy containing 96.5 wt% In, 3.0 wt% Sn, and small amounts of Pb and Bi was placed in an anode basket. A molten salt consisting of 35 wt% InCl3, 40 wt% KCl, and 25 wt% LiCl was used, with 1.5 wt% KI added. Pure indium sheets were used as the cathode. The electrolysis temperature was maintained at 350 °C. The first stage cell voltage was controlled at 0.28 V and electrolysis was performed for 2 h to enrich the tin-rich phase in the anode residue. The second stage cell voltage was increased to 0.50 V and electrolysis was performed for 4 h to selectively deposit indium. After electrolysis, the cathode was collected and detected to be at the 5N level, with Sn 1.8 ppm, Pb 0.8 ppm, and Cd 0.4 ppm. The indium recovery rate was 96.8%.

[0037] S2 intermediate purification casting: After the cathode indium is washed with salt and dried under vacuum, it is melted at 175 °C under argon protection and cast into ingots.

[0038] S3 Fractional Vacuum Distillation Purification: A quartz crucible was used at a depth of 1×10⁻⁶. -2 The distillation process was first held at 750 °C for 2.5 h, then at 1000 °C for 1.0 h. The distillation products were collected, yielding 899.5 g of main distillate, 16.8 g of light impurity fraction, 10.9 g of tailings, and the remainder being transition fractions and losses during sampling and transfer. The main distillate contained 0.18 ppm Sn, 0.06 ppm Pb, and 0.04 ppm Bi, respectively, and the total impurities according to GDMS were 4.4 ppm, reaching the 5N5 level. The main distillate yield was 96.3%, and the overall indium direct recovery rate was 93.2%.

[0039] Example 3: Electrolysis and fractional distillation of crude indium containing complex impurities of Pb, Bi, Cd and Tl

[0040] S1 molten salt electrolytic purification: 1000 g of crude 2N6 indium was used as the anode. The raw material contained 0.15 wt% Pb, 0.06 wt% Bi, 0.08 wt% Cd, 0.04 wt% Tl, and 0.03 wt% Sn. The electrolyte consisted of 30 wt% InCl3, 40 wt% ZnCl2, and 30 wt% KCl, with 1.0 wt% NH4Cl added. Stainless steel was used as the cathode. The process was carried out at 350 ℃ and 15 A / dm³. 2Electrolysis was performed for 5 hours. Lower current density and cell voltage were controlled to suppress Cd and Tl co-deposition, while Sn, Pb, and Bi were preferentially enriched in the anode residue and electrolyte. GDMS analysis showed that the contents of Pb, Bi, Cd, Tl, and Sn in the cathode indium were 12.5 ppm, 5.6 ppm, 8.8 ppm, 3.9 ppm, and 4.2 ppm, respectively, reaching the 4N5 standard, with an indium recovery rate of 96.2%.

[0041] S2 Intermediate Purification Melting and Casting: After the cathode indium is washed with salt and dried under vacuum, it is melted at 185 °C in an argon-hydrogen mixed reducing atmosphere to remove surface oxides and cast into ingots.

[0042] S3 Fractional Vacuum Distillation Purification: Indium ingots are placed in a graphite crucible and purified by vacuum distillation at a temperature of 1×10⁻⁶ mm. -3 The indium fraction was first held at 680 °C for 3 h to allow residual Cd and Tl in the cathode to preferentially enter the light impurity fraction. Then, the temperature was raised to 960 °C and held for 1.5 h to collect the main indium fraction. Pb, Bi, and Sn were mainly distributed in the anode residue, electrolyte, and distillation tailings. The light impurity fraction, main indium fraction, and tailings were collected separately. Analysis showed that the main fraction contained 0.10, 0.06, 0.22, 0.15, and 0.11 ppm of Pb, Bi, Cd, Tl, and Sn, respectively, and the total impurities by GDMS were 3.4 ppm, reaching the 5N6 level. The main distillation fraction yield was 95.7%, and the overall direct indium recovery rate was 92.0%.

[0043] Example 4: In-Zn waste

[0044] S1 Molten Salt Electrolytic Purification: 1000 g of In-Zn waste containing 98.0 wt% In, 1.5 wt% Zn, and small amounts of Sn, Pb, and Cd was used as the anode. A molten salt composed of 20 wt% InCl3, 50 wt% ZnCl2, and 30 wt% KCl was used, with 3.0 wt% NH4Cl added. Stainless steel was used as the cathode. The process was carried out at 500 ℃ and 25 A / dm³. 2 Electrolysis was carried out for 3 hours. Approximately 942.0 g of indium was obtained from the cathode. GDMS analysis showed that the contents of Zn, Sn, Pb, and Cd in the cathode indium were 7.5 ppm, 3.8 ppm, 2.1 ppm, and 2.4 ppm, respectively, reaching the 4N8 level. The electrolytic indium recovery rate was 96.1%.

[0045] S2 Intermediate Purification Melting and Casting: After the cathode indium is washed with salt and dried under vacuum, it is melted and slag removed at 200 °C under argon protection and then cast into ingots.

[0046] S3 Fractional Vacuum Distillation Purification: Indium ingots are placed in a graphite crucible and evacuated to a vacuum of 5 × 10⁻⁶. -3Pa; In the first stage, the temperature was maintained at 820 °C for 2.5 h to collect light impurity fractions such as Zn and Cd; in the second stage, the temperature was raised to 1120 °C and maintained for 3 h, continuously collecting the indium main fraction from the stable evaporation stage, while low-volatile impurities such as Cu, Fe, Ni, and Ag remained in the tailings. 31.2 g of light impurity fraction, 22.8 g of transition fraction, 875.6 g of stable main fraction, and 8.6 g of tailings were obtained. The concentrations of Zn, Sn, Pb, Cd, Cu, Fe, Ni, and Ag in the main fraction were 0.08, 0.07, 0.05, 0.03, 0.11, 0.10, 0.06, and 0.04 ppm, respectively, and the total impurities by GDMS were 0.72 ppm, reaching the 6N2 level. The distillation yield of the main fraction was 93.0%, and the overall indium direct recovery rate was 89.3%.

[0047] Example 5: Preparation of 5N5 grade ultra-high purity indium

[0048] S1 molten salt electrolytic purification: 2N8 industrial crude indium was selected as the anode, with initial impurity content of Sn 0.12%, Pb 0.08%, Cd 0.05%, and Tl 0.02%; a KCl-LiCl composite molten salt system was adopted, with 1.0 wt% NH4Cl surfactant added.

[0049] Molten salt electrolysis conditions: electrolysis temperature 300℃, current density 18 A / dm³ 2 The electrolysis time is 4 hours, with argon gas protection throughout the process; after electrolysis, 4N5 grade high-purity indium is deposited at the cathode, with Sn 8 ppm, Pb 15 ppm and Cd 3 ppm, and the core difficult-to-treat impurities are significantly removed.

[0050] S2 intermediate purification casting: Under an argon atmosphere, molten salt residue on the surface is scraped off, and the ingot is melted and cast at 180°C to obtain a dense high-purity indium ingot.

[0051] S3 segmented vacuum distillation purification: Vacuum distillation conditions: residual pressure inside the furnace 5×10 -3 Pa, the first stage is held at 780℃ for 2 h to remove light impurities, the second stage is held at 980℃ for 1.5 h to evaporate indium, and the main fraction is collected through two-stage condensation zones.

[0052] The final product is 5N5 grade ultra-high purity indium, with a total impurity content of 4.8 ppm as detected by GDMS. It can meet the needs of mid-to-high-end optoelectronic materials without zone melting, and the indium direct recovery rate is 96.8%.

[0053] Comparative Example 1:

[0054] 1000 g of crude 2N8 indium was used as the anode. The main impurities in the raw material were Sn 0.10 wt%, Pb 0.06 wt%, Cd 0.05 wt%, and Tl 0.02 wt%. A quartz crucible was used, and the anode was heated at a depth of 1×10⁻⁶ mm.-2 The distillation process was first held at 750 °C for 2.5 h, then at 1000 °C for 1.0 h. The distillate products were collected, yielding 905.6 g of main fraction, 18.4 g of light impurity fraction, and 68.5 g of tailings. The remainder consisted of samples, transfers, and equipment residue. The main fraction contained 28.6 ppm of Sn, 20.4 ppm of Pb, 7.8 ppm of Cd, and 3.2 ppm of Tl, with a total GDMS impurity content of 65.2 ppm. The crude yield of the main fraction was 90.6%, which did not meet the 5N purity requirement, resulting in a product yield of 0%.

[0055] Comparative Example 2:

[0056] S1 molten salt electrolytic purification: 1000 g of crude 2N6 indium was used as the anode. The raw material contained 0.15 wt% Pb, 0.06 wt% Bi, 0.08 wt% Cd, 0.04 wt% Tl, and 0.03 wt% Sn. The electrolyte consisted of 30 wt% InCl3, 40 wt% ZnCl2, and 30 wt% KCl, with 1.0 wt% NH4Cl added. Stainless steel was used as the cathode. The process was carried out at 350 ℃ and 15 A / dm³. 2 Electrolysis was carried out for 5 hours. Lower current density and cell voltage were controlled to suppress Cd and Tl co-deposition, while Sn, Pb and Bi were preferentially enriched in the anode residue and electrolyte.

[0057] S2 Intermediate Purification Melting and Casting: After the cathode indium is washed with salt and dried under vacuum, it is melted at 185 °C in an argon-hydrogen mixed reducing atmosphere to remove surface oxides and cast into ingots.

[0058] S3 Fractional Vacuum Distillation Purification: Indium ingots were placed in a graphite crucible, heated to 960 ℃ and held for 1.5 h, and the indium main fraction was collected. Upon verification, 929.5 g of the main fraction was obtained, with a crude yield of 97.0%. The Cd and Tl content in the main fraction was 9.6 ppm and 4.7 ppm, respectively, and the total impurities by GDMS were 24.8 ppm. This did not meet the 5N purity requirement, and the product yield was 0%.

[0059] Comparative Example 3: In-Sn alloy using single-cell voltage

[0060] S1 Molten Salt Electrolytic Purification: 1000 g of an In-Sn alloy containing 96.5 wt% In, 3.0 wt% Sn, and small amounts of Pb and Bi was placed in an anode basket. A molten salt consisting of 35 wt% InCl3, 40 wt% KCl, and 25 wt% LiCl was used, with 1.5 wt% KI added. Pure indium was used as the cathode. The electrolysis temperature was maintained at 350 ℃, and the current density at 15 A / dm² was used for 6 h. 940.5 g of cathode product was obtained, containing 58.0 ppm Sn, 22.0 ppm Pb, 12.0 ppm Bi, and 105 ppm total impurities.

[0061] S2 intermediate purification casting: After the cathode indium is washed with salt and dried under vacuum, it is melted at 175 °C under argon protection and cast into ingots.

[0062] S3 Fractional Vacuum Distillation Purification: A quartz crucible was used at a depth of 1×10⁻⁶. -2 The distillation process was first held at 750 °C for 2.5 h, then at 1000 °C for 1.0 h. The distillation products were collected separately, yielding 888.2 g of the main fraction. The main fraction contained 4.8 ppm Sn and 15.7 ppm GDMS total impurities. The direct indium recovery rate of the entire process was approximately 92.0%, which did not meet the 5N purity requirement, and the qualified product yield was 0%.

[0063] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing ultra-high purity indium based on electrolysis and vacuum distillation, characterized in that, Includes the following steps: S1 molten salt electrolytic purification: using crude indium metal with a purity of 2N~3N or indium-containing smelting waste of In-Sn and In-Zn as the anode, and pure indium or stainless steel as the cathode; using KCl-LiCl or ZnCl2-KCl halide composite molten salt as the electrolyte, constant current electrolytic refining is carried out under the conditions of electrolysis temperature of 250~550℃, current density of 10~30 A / dm², and cell voltage of 0.2~0.6 V, and high-purity metallic indium of grade 4N~5N is stably deposited at the cathode; S2 Intermediate Purification and Casting: The 4N~5N grade high-purity metallic indium precipitated in step S1 is placed in an inert or reducing atmosphere to remove molten salt inclusions, and then melted and cast at 160~200℃ to prepare a dense and uniform vacuum distillation-specific indium ingot. S3. Fractional Vacuum Distillation Purification: Place the vacuum distillation-specific indium ingot prepared in step S2 into a graphite or quartz crucible, and then place it into a dual-temperature zone vacuum distillation furnace. Evacuate the furnace and control the residual pressure inside to 10. -3 ~10 Pa; adopts segmented temperature control process and two-stage condensation process.

2. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 1, characterized in that: In step S1, the mass percentage of InCl3 in the composite molten salt electrolyte is 20-50%, and the balance is one or more compound systems of KCl, LiCl, and ZnCl2; 0.5-3.0 wt.% of NH4Cl or KI is added to the molten salt as a surfactant.

3. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 1, characterized in that: In step S1, when the anode raw material is an In-Sn alloy intermediate, a segmented voltage-controlled electrolysis mode is adopted, with the first stage controlling the cell voltage at 0.20~0.35 V and the second stage adjusting the cell voltage at 0.4~0.6 V.

4. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 1, characterized in that: In step S3, the first stage of the segmented temperature control process is heated to 600~850℃ and held for 1~3 hours.

5. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 4, characterized in that: In step S3, the second stage of the segmented temperature control process is heated to 900~1050℃ and held for 0.5~2h to evaporate the metallic indium. The indium is then collected through two-stage condensation zones to obtain 5N~5N5 grade high-purity indium.

6. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 5, characterized in that: S4 Ultra-high purity deep purification: The 5N5 grade indium obtained in step S3 is refined by zone melting. Under an argon protective atmosphere, the melting zone temperature is set to 160~250℃ and the melting zone moving speed is 5~30 mm / h. The zone melting is carried out 2~5 times in a row. The first and last impurity enrichment sections are removed to finally obtain ultra-high purity indium of grade 6N or above.

7. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 4, characterized in that: In step S3, the second stage of the segmented temperature control process is heated to 1080~1150℃, and the holding time is extended to 2~4 h, directly preparing 5N5~6N grade ultra-high purity indium.

8. The method for preparing ultra-high purity indium based on electrolysis and vacuum distillation according to claim 1, characterized in that: In step S3, the two-stage condensation process is specifically as follows: the first-stage condensation temperature is controlled at 300~500℃; the second-stage condensation temperature is controlled at 850~950℃.