Method for preparing 4N5 fine indium by multi-stage vacuum rectification of crude indium

Abstract

The application discloses a method for preparing 4N5 refined indium by multi-section vacuum rectification of crude indium, and belongs to the technical field of rare metal vacuum metallurgy. The method adopts a "4+1" equipment configuration composed of four continuous vacuum distillation furnaces and one intermittent vacuum distillation furnace, forms a "high-high-low-high" four-section gradient distillation framework according to a function sequence of "removing high-boiling-point impurities-removing high-boiling-point impurities again-removing low-boiling-point impurities-removing high-boiling-point impurities finally", and realizes gradient removal of high-boiling-point impurities and concentrated removal of low-boiling-point impurities respectively. The high-boiling-point and low-boiling-point enriched materials are subjected to impurity open circuit by the intermittent vacuum distillation furnace, and the indium-containing materials in each section are all returned to the first section for cyclic treatment. The application can remove various impurities in the crude indium systematically, reduce the total amount of impurities to below 20 ppm, has high indium direct recovery rate, is stable, environment-friendly and suitable for industrialized preparation of 4N5 refined indium.

Description

Technical Field

[0001] This invention relates to the field of rare metal vacuum metallurgy technology, specifically a method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium. Background Technology

[0002] Indium is an important rare metal that is widely used in ITO sputtering targets, electronic semiconductors, photovoltaic cells and alloy materials. 4N5 indium ingots, which are indium ingots with a purity of 99.995%, have become the basic raw material for the production of ITO sputtering targets.

[0003] Currently, the main publicly disclosed methods for indium purification are electrolytic refining and vacuum distillation. Electrolytic refining can effectively remove some impurities, but it suffers from problems such as wastewater and waste gas treatment, chemical reagent consumption, and a long production cycle. Vacuum distillation utilizes the difference in saturated vapor pressure between In and impurity metals in crude indium to achieve separation, offering advantages such as a short process flow, high direct indium recovery rate, no consumption of chemical reagents / auxiliaries, no generation of waste, and low production costs. Existing vacuum distillation technology has made some progress; for example, patent CN201710160302.9 describes melting crude indium and then performing n-stage vacuum distillation and x-stage condensation to obtain high-purity indium. However, existing technologies mainly focus on single-furnace multi-stage condensation schemes, lacking a systematic multi-stage synergistic treatment of the complex impurity system in crude indium, especially lacking targeted process combination design for the gradient removal of impurities with different boiling points. Existing vacuum distillation technologies mostly employ single-furnace, multi-stage condensation schemes, which have limited ability to systematically remove more than ten impurities from crude indium. They struggle to achieve efficient, differentiated, and gradient removal of high-boiling-point and low-boiling-point impurities. Furthermore, high-boiling-point impurities accumulate in the residue without effective open-circuit separation methods, and low-boiling-point impurities, after accumulation in the evaporate, also fail to separate effectively. This leads to the accumulation of impurities within the system, affecting batch-to-batch stability of product purity. Electrolysis, besides its high environmental impact and processing costs, also suffers from long production cycles, significant indium accumulation on the production line, and high operational capital requirements. Therefore, it is necessary to develop a method for preparing 4N5 refined indium that can systematically remove multiple impurities from crude indium, achieve high direct metal recovery rates, and provide a stable and controllable process. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies, such as insufficient systematic impurity removal in vacuum distillation processes, difficulty in gradient separation of high and low boiling point impurities, poor batch-to-batch purity stability due to easy accumulation of impurities, and high environmental load, long production cycle, and large capital occupation of metals in electrolytic refining methods. This invention provides a method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium. By constructing a multi-stage gradient distillation architecture of "high-high-low-high," it achieves efficient classification and gradient removal of impurities. An independent, intermittent impurity open-circuit unit and a full-process material circulation system solve the problem of impurity accumulation. While ensuring stable purity of 4N5 refined indium and high direct metal recovery, it achieves clean production with zero emissions of waste gas, wastewater, and solid waste. Furthermore, parameters can be adjusted to adapt to raw materials with different impurity contents and to prepare higher purity indium, improving process applicability and equipment utilization.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium, employing a "4+1" equipment configuration consisting of four continuous vacuum distillation furnaces and one intermittent vacuum distillation furnace. The crude indium raw material is sequentially processed through four stages of continuous vacuum distillation. The four stages of continuous vacuum distillation are arranged in a functional sequence of "removal of high-boiling-point impurities - further removal of high-boiling-point impurities - removal of low-boiling-point impurities - final removal of high-boiling-point impurities," forming a multi-stage distillation architecture of "high-high-low-high." Specifically, the first and second stages of distillation remove high-boiling-point impurities, the third stage removes low-boiling-point impurities, and the fourth stage produces 4N5 refined indium. The residue containing high-boiling-point impurities from the first stage distillation and the evaporate containing low-boiling-point impurities from the third stage distillation are fed into the intermittent vacuum distillation furnace, where impurities are separated from indium through high-boiling-point open-circuit mode and low-boiling-point open-circuit mode, respectively. All indium-containing residues from each stage of distillation and indium recovered from the intermittent vacuum distillation furnace are returned to the first stage of continuous vacuum distillation furnace for recycling. Based on the differences in saturated vapor pressure of different metals, this invention uses a multi-stage gradient distillation process to independently control the removal of high-boiling-point impurities and low-boiling-point impurities in stages. It is also equipped with an independent impurity open-circuit unit and a full-process material circulation system, which ensures the deep removal of impurities while achieving efficient recovery of indium.

[0006] The four-stage continuous distillation employs a "high-high-low-high" functional arrangement. Essentially, it utilizes the principle that high-boiling-point impurities gradually accumulate in the residual phase during multiple evaporations. Gradual depth removal is achieved through three high-boiling-point distillations in the first two stages and the final stage. Simultaneously, the preferential evaporation of low-boiling-point impurities at lower temperatures is utilized, with concentrated removal of these impurities through low-temperature distillation in the intermediate stages, avoiding interference between the removal processes of different impurity types. The intermittent vacuum distillation furnace, as an independent impurity open-circuit unit, employs different temperature regimes for high-boiling-point and low-boiling-point enriched materials. Targeted separation is achieved using the vapor pressure difference between indium and the two types of impurities, ensuring complete removal of impurities from the system and resolving product purity fluctuations caused by impurity accumulation. The entire process material recycling system returns all intermediate indium-containing materials to the first-stage distillation for reprocessing, minimizing indium loss and improving the overall direct metal recovery rate.

[0007] Furthermore, the first stage of continuous vacuum distillation is carried out at a temperature of 1000–1050℃ with a vacuum degree ≤10Pa. By adjusting the heating power to control the evaporation residue ratio, the total amount of high-boiling-point impurities is reduced to below 200ppm. The evaporated product is indium #1, and the residue is residue #1 containing high-boiling-point impurities. This stage, as the first step in the purification of crude indium, achieves the initial retention of most high-boiling-point impurities at a relatively high temperature, significantly reducing the amount of high-boiling-point impurities in the evaporated phase and reducing the load on subsequent deep purification.

[0008] Furthermore, the second stage of continuous vacuum distillation processes indium #1 at a temperature of 1000–1050°C. By reducing the residual ratio and enhancing the separation effect, the total amount of high-boiling-point impurities is reduced to below 50 ppm. The evaporated product is indium #2, and the residue is returned to the first stage of continuous vacuum distillation. This stage further enhances the enrichment and separation of high-boiling-point impurities through a smaller residual ratio, achieving deep removal of high-boiling-point impurities. At the same time, the residual material enriched with impurities is returned to the first stage of recycling, avoiding the loss of indium.

[0009] Furthermore, the third stage involves continuous vacuum distillation to process indium #2 at a temperature of 850–950℃ and a vacuum degree ≤10Pa. This process preferentially evaporates low-boiling-point impurities such as As, Cd, Zn, Tl, Bi, and Pb, leaving indium #3 as the residual product. The evaporated product is indium #3 containing low-boiling-point impurities. This stage, by lowering the distillation temperature, increases the vapor pressure difference between indium and low-boiling-point impurities, allowing the low-boiling-point impurities to preferentially enter the gas phase for concentrated removal. This avoids separation interference caused by the simultaneous evaporation of low-boiling-point impurities during the high-boiling-point distillation process.

[0010] Furthermore, the fourth stage of continuous vacuum distillation processes the No. 3 indium at a temperature of 1000–1050°C. The heating power is adjusted to control the residual ratio, ensuring that the total amount of high-boiling-point impurities and low-boiling-point impurities in the refined indium is less than 20 ppm. The evaporated product is 4N5 refined indium ingots, and the residue is returned to the first stage of continuous vacuum distillation. This stage serves as the final purification process, precisely controlling process parameters to remove trace amounts of residual high-boiling-point impurities, ensuring stable and compliant product purity. Simultaneously, the residual material is returned to the first stage for recycling.

[0011] Furthermore, in the high-boiling-point open-circuit mode of the intermittent vacuum distillation furnace, the distillation temperature is controlled at 1180–1220℃, and the vacuum degree is ≤10Pa. Indium evaporates to the condenser for recovery, while high-boiling-point impurities remain in the evaporation crucible, forming impurity lumps. In the low-boiling-point open-circuit mode, the distillation temperature is controlled at 980–1020℃, and the vacuum degree is ≤10Pa. Low-boiling-point impurities evaporate to the condenser for removal, while indium remains in the evaporation crucible for recovery. The two operating modes are customized with temperature regimes based on the impurity characteristics of the two types of enriched materials, utilizing the difference in boiling points between indium and impurities to achieve directional separation. This ensures that impurities are completely discharged from the system while maximizing the recovery of indium metal. All recovered intermittent furnace indium is returned to the first-stage continuous vacuum distillation furnace for main process recycling.

[0012] Furthermore, the crude indium raw material is a mixture of self-produced crude indium and purchased crude indium, which flows between the distillation furnaces in the form of metal ingots. During the production process, the material flows between the distillation furnaces in the form of metal ingots, and impurities are gradually enriched.

[0013] Furthermore, by adjusting the distillation temperature and evaporation residue ratio of each stage, it can be adapted to crude indium raw materials with different impurity contents, and can also be used to prepare high-purity indium with higher purity. The equipment has high utilization rate and low operating cost.

[0014] Furthermore, the vacuum level is stably controlled within ≤10Pa throughout the entire process, enabling continuous quantitative feeding and ensuring process stability.

[0015] Furthermore, the prepared 4N5 refined indium has a purity of ≥99.995% and can be applied to ITO targets, solar cell electrodes, semiconductor device interconnect materials, or lead-free solders.

[0016] Compared with the prior art, the present invention provides a method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium, which has the following beneficial effects: This invention achieves a systematic approach to impurity removal. Through a four-furnace, three-stage gradient distillation architecture, high-boiling-point impurities are removed by three-stage gradient distillation, while low-boiling-point impurities are removed in a concentrated manner. This enables the systematic and efficient removal of more than ten impurities from crude indium, solving the problem of insufficient systematic impurity removal in existing processes.

[0017] Secondly, the impurity opening is more thorough. With the 4+1 equipment configuration, high-boiling-point impurities and low-boiling-point impurities are separately gathered to the intermittent vacuum distillation furnace for independent opening, so that the impurities are completely separated from indium, avoiding the accumulation of impurities in the system and ensuring the batch stability of product purity. The total amount of impurities can be reduced from thousands or even tens of thousands of ppm at the raw material level to below 20 ppm.

[0018] Furthermore, the metal recovery rate is high. Through the whole-process material recycling system, all residues from each stage are returned to the first furnace for reprocessing, and the indium recovered from the intermittent furnace is returned to the main production line, so that the metal indium can be fully utilized.

[0019] Meanwhile, the entire process consumes no chemical reagents or auxiliary materials, and generates no wastewater or waste gas, making it a clean and green metallurgical process.

[0020] In addition, the process is flexible and controllable. By adjusting the distillation temperature and evaporation residue ratio of each stage, it can adapt to raw materials from different sources. The process has good stability, and by adjusting the parameters, the production line can be compatible with the preparation of high-purity indium, thereby improving equipment utilization and reducing operating costs.

[0021] The prepared 4N5 refined indium has a purity of ≥99.995%, which can be used as a basic raw material for the production of ITO targets, as well as in the fields of solar cell electrode materials, semiconductor device connection materials and lead-free solders. It can also be used as an intermediate raw material for the preparation of higher purity indium. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of a method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to the present invention. Detailed Implementation

[0023] The technical solution of the present invention will be described in detail below with reference to the complete process. Those skilled in the art can realize the industrial application of the process based on the content disclosed in this specification without departing from the core concept of the present invention.

[0024] Please see Figure 1This invention provides a technical solution: a method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium. Based on the fundamental principle that different metal elements have significant differences in saturated vapor pressure under vacuum, the method employs a process system consisting of four continuous vacuum distillation furnaces and one intermittent vacuum distillation furnace. The system operates according to the overall approach of gradient distillation, classification and impurity removal, open-circuit impurity removal, and recycling, ultimately producing a 4N5 refined indium product with a purity of not less than 99.995%. The continuous vacuum distillation furnace used in this method is equipped with a melting and feeding unit, a multi-stage evaporation unit, a condensation and collection unit, a vacuum pumping unit, an electric heating and temperature control unit, and a discharge unit. The melting and feeding unit heats and melts solid metal ingots into a liquid state, and achieves continuous and stable material conveying through a screw or piston-type quantitative feeding mechanism. The feeding rate can be steplessly adjusted within the design range. The multi-stage evaporation unit adopts a multi-layer graphite evaporation plate stacked structure, with overflow ports at the edge of each evaporation plate. The liquid material flows downwards layer by layer under gravity, continuously heated and evaporated during the flow, ensuring full contact and mass transfer between the gas and liquid phases, thus enhancing the separation effect. The condensation unit… The collection unit features a multi-stage gradient condensation surface, with the temperature gradually decreasing from near the evaporation zone to the discharge zone. This allows metal components with different saturated vapor pressures to condense separately at different temperature ranges, achieving initial component classification. The vacuum pumping unit employs a two-stage pumping system combining a mechanical vacuum pump and a diffusion vacuum pump, rapidly reducing the vacuum level inside the furnace to below 10 Pa. Real-time, precise, and stable pressure control within the furnace is achieved through an electric vacuum regulating valve and a pressure sensor. The electric heating and temperature control unit uses high-purity graphite as the heating element, coupled with a PID intelligent temperature control system, maintaining the distillation temperature control accuracy within ±5℃, ensuring long-term stability of process parameters. The intermittent vacuum distillation furnace is equipped with a graphite evaporation crucible, a liftable water-cooled condensation plate, a vacuum system, a segmented heating system, and upper and lower discharge mechanisms. It can switch operating modes according to different materials being processed, enabling open-circuit operation for both high-boiling-point and low-boiling-point impurities. The evaporation crucible is made of isostatic graphite, capable of withstanding temperatures above 1300℃, and does not chemically react with indium or its impurity elements, thus preventing the introduction of new impurities into the material.

[0025] The first step involves a first-stage continuous vacuum distillation to initially separate high-boiling-point impurities from the crude indium. The crude indium raw material to be processed is added in the form of metal ingots to the melting feed unit of the No. 1 continuous vacuum distillation furnace. The melting feed unit is preheated to 160–200°C, above the melting point of indium, to completely melt the solid indium ingots into a liquid metal melt. The melting process can be carried out under a micro-vacuum environment to prevent indium from oxidizing upon contact with air, thus avoiding the formation of indium oxide inclusions that would affect the quality of subsequent products. The molten liquid indium is continuously and uniformly fed into the top of the multi-stage evaporation unit of the distillation furnace through a quantitative feeding mechanism. The feeding rate can be adjusted according to the processing scale and process requirements to ensure that the material forms a uniform and stable liquid film thickness on each evaporation plate. The liquid film thickness is typically controlled within the range of 5–20 mm, ensuring sufficient heating and evaporation area while avoiding an excessively thick liquid film that would hinder the complete evaporation of the lower layers of material. The distillation temperature of the No. 1 continuous vacuum distillation furnace is controlled at 1000–1050℃. Within this temperature range, indium exhibits a high saturated vapor pressure, ensuring high evaporation efficiency and production capacity. Simultaneously, the saturated vapor pressures of high-boiling-point impurities such as Cu, Fe, and Sn are much lower than that of indium, making them difficult to volatilize with indium vapor at the same temperature. Therefore, they accumulate in large quantities in the liquid residue, achieving initial separation from indium. The vacuum level inside the furnace is controlled to ≤10Pa. A vacuum environment effectively reduces the evaporation temperature of the metal, enhances the evaporation mass transfer process, and prevents the metal from reacting with oxygen, nitrogen, and other gases in the air to form impurity compounds, thus ensuring product purity. The residual ratio of evaporation is controlled by adjusting the matching relationship between heating power and feed rate. The residual ratio refers to the ratio of the mass of the residual liquid material to the total mass of the feed. During adjustment, the residual ratio can be reduced by increasing the heating power to increase the evaporation rate or by decreasing the feed rate to extend the material residence time. Conversely, the residual ratio will increase if the heating power is increased. By controlling a suitable residual ratio, most of the indium evaporates into the gas phase, while high-boiling-point impurities are continuously enriched with the residual liquid phase, ultimately reducing the total amount of high-boiling-point impurities in the evaporated product to below 200 ppm. The indium vapor generated by evaporation enters the condensation and collection unit, where it is cooled and condensed into liquid on a suitable condensation surface. Finally, it is collected through the discharge mechanism to obtain #1 indium, which is used as the raw material for the next stage of distillation. The residue generated by distillation is #1 residue enriched with high-boiling-point impurities. It is periodically discharged from the slag discharge port at the bottom of the evaporation unit and sent to an intermittent vacuum distillation furnace for open-circuit treatment of high-boiling-point impurities to recover the residual indium metal. In this distillation process, temperature control is the core parameter affecting the separation effect. Too low a temperature will cause the indium evaporation rate to decrease and the equipment capacity to decrease; too high a temperature will cause the saturated vapor pressure of some high-boiling-point impurities to increase with the temperature, and they will evaporate into the gas phase along with indium, reducing the separation effect. Therefore, controlling the temperature in the range of 1000 to 1050℃ can achieve effective initial interception of high-boiling-point impurities while ensuring production efficiency.Maintaining a stable vacuum level is equally crucial, as fluctuations in vacuum level directly affect the saturated vapor pressure of the metal, leading to changes in the evaporation rate and separation effect. Therefore, by using a vacuum regulation system to monitor the furnace pressure in real time and dynamically adjust the opening of the extraction valve, the vacuum level is stably controlled within the set range, ensuring the stability of the process operation and the consistency of product quality.

[0026] The second step involves a second stage of continuous vacuum distillation to further remove high-boiling-point impurities. The indium obtained from the first stage distillation (No. 1) is added in metal ingot form to the melting feed unit of the second stage continuous vacuum distillation furnace. After melting, it is fed into the multi-stage evaporation unit via a quantitative feeding mechanism. The distillation temperature of the second stage continuous vacuum distillation furnace is controlled at 1000–1050℃, and the vacuum degree is ≤10Pa. Compared to the first stage distillation, the separation effect of high-boiling-point impurities is further enhanced by reducing the evaporation residue ratio. This can be achieved by appropriately increasing the heating power, reducing the feed rate, or increasing the number of evaporation plate layers, further reducing the proportion of liquid phase residue and allowing more high-boiling-point impurities to accumulate in the residual phase, thereby reducing the total amount of high-boiling-point impurities in the evaporated product to below 50 ppm. The evaporate produced in this stage of distillation is indium #2, which is collected by condensation and sent to the next stage of distillation. The resulting residue #2 is also enriched with high-boiling-point impurities, but still contains a certain amount of indium metal. Therefore, it is returned to the feed end of the #1 continuous vacuum distillation furnace via a conveyor to be mixed with the crude indium raw material and then re-distilled to avoid loss of indium metal and improve metal recovery rate. The second stage of distillation serves as a deep removal process for high-boiling-point impurities. Building upon the preliminary separation in the first stage, it further reduces the content of high-boiling-point impurities in the evaporate. Through two-stage continuous high-boiling-point distillation, a gradient removal of high-boiling-point impurities is achieved. Compared with single-stage distillation, this significantly improves the separation depth while avoiding the problem of a sharp decrease in indium direct recovery rate caused by the excessive pursuit of high removal rate in single-stage distillation. It balances separation effect and metal recovery rate.

[0027] The third step involves a third-stage continuous vacuum distillation to centrally remove low-boiling-point impurities. The indium obtained from the second-stage distillation (No. 2) is added in metal ingot form to the melting feed unit of the third-stage continuous vacuum distillation furnace. After melting, it is fed into the multi-stage evaporation unit. The distillation temperature of the third-stage continuous vacuum distillation furnace is controlled at 850–950℃, with a vacuum degree ≤10Pa. This temperature range is significantly lower than the temperature of the high-boiling-point distillation stage. Utilizing the characteristic that the saturated vapor pressure of low-boiling-point impurities such as As, Cd, Zn, Tl, Bi, and Pb is more sensitive to temperature changes, at this temperature, low-boiling-point impurities still have a relatively high saturated vapor pressure, allowing them to evaporate in large quantities into the gas phase. Meanwhile, the saturated vapor pressure of indium decreases significantly with decreasing temperature, resulting in a substantial reduction in evaporation. This achieves efficient separation of low-boiling-point impurities from indium, allowing them to be preferentially removed from the material. During distillation, low-boiling-point impurities enter the condensation and collection unit along with the evaporated product, condensing on the condensation surface to form evaporated product #3, which is enriched with low-boiling-point impurities. This evaporated product is collected periodically and sent to an intermittent vacuum distillation furnace for open-circuit treatment of low-boiling-point impurities. The remaining liquid phase is indium #3, in which most of the low-boiling-point impurities have been removed. This liquid phase is collected and sent to the next stage of distillation. This stage of distillation uses a relatively low temperature specifically for the concentrated removal of low-boiling-point impurities, avoiding the problem of simultaneous evaporation and indium separation during high-temperature distillation. By using classified temperature control, the separation efficiency of different types of impurities is improved, while reducing indium evaporation loss during the removal of low-boiling-point impurities and increasing the direct metal recovery rate.

[0028] The fourth step involves a fourth-stage continuous vacuum distillation, followed by final purification to produce 4N5 refined indium. The 3# indium obtained from the third-stage distillation is added in metal ingot form to the melting feed unit of the 4# continuous vacuum distillation furnace. After melting, it is sent to a multi-stage evaporation unit for final purification. The distillation temperature of the 4# continuous vacuum distillation furnace is controlled at 1000–1050℃, and the vacuum degree is ≤10Pa. By precisely adjusting the heating power and feed rate, the evaporation residue ratio is controlled within a suitable range. This process ultimately removes trace amounts of high-boiling-point impurities from the material and further reduces the content of potentially remaining low-boiling-point impurities. The final refined indium contains less than 20 ppm of both high-boiling-point and low-boiling-point impurities, meeting the purity requirements for 4N5 refined indium. The evaporated product from distillation is qualified 4N5 refined indium, which is collected by condensation and sent to the ingot casting mechanism. Under a protective atmosphere, it is cast into standard-sized indium ingots, which are then packaged and stored after passing composition inspection. The No. 4 residue from distillation still contains a certain amount of indium and enriched trace impurities. This residue is returned to the feed end of the No. 1 continuous vacuum distillation furnace, mixed with the raw materials, and reprocessed to recover the indium metal. The fourth stage of distillation, as the final purification step in the entire process, requires even higher precision in controlling process parameters. A high-precision temperature control system and feed control system ensure stable product purity, avoiding batch-to-batch quality fluctuations. Simultaneously, residue recycling reduces indium loss in the final stage, ensuring the overall economic efficiency of the process.

[0029] The fifth step involves intermittent vacuum distillation with open-circuit impurities and indium recovery. The intermittent vacuum distillation furnace processes the No. 1 residue from the first stage and the No. 3 evaporate from the third stage, respectively. Depending on the material being processed, it operates in two modes: a high-boiling-point open-circuit mode and a low-boiling-point open-circuit mode. The two modes are operated separately, and the furnace is cleaned before processing different materials to avoid cross-contamination of different impurities. In the high-boiling-point open-circuit mode, the collected No. 1 residue is loaded into the evaporation crucible of the intermittent vacuum distillation furnace. After sealing the furnace, the vacuum system is started to evacuate the furnace to ≤10Pa. Then, the temperature is gradually increased in a stepped manner. First, the temperature is raised to 200-300℃ and held to remove any residual moisture and volatile substances in the material. Then, the temperature is raised to 1180-1220℃. At this temperature, the indium in the residue has a sufficiently high saturated vapor pressure, which can evaporate in large quantities and rise to the condensing plate above. After cooling, it condenses into liquid indium, which flows down the condensing plate and is collected to obtain intermittent furnace indium. Meanwhile, high-boiling-point impurities such as Cu, Fe, and Sn have extremely low saturated vapor pressure at this temperature and are difficult to evaporate. They remain at the bottom of the evaporation crucible and eventually form solid impurity blocks, which are discharged from the system after distillation, thus achieving a complete open circuit for high-boiling-point impurities. In the low-boiling-point open-circuit mode, the collected evaporated material (No. 3) is loaded into an evaporation crucible, evacuated to ≤10 Pa, and then gradually heated to 980–1020 °C. At this temperature, low-boiling-point impurities such as As, Cd, Zn, Tl, Bi, and Pb preferentially evaporate in large quantities, rising to the condensing plate and condensing for removal. Indium, however, has a low saturated vapor pressure at this temperature, resulting in minimal evaporation, with most remaining in the evaporation crucible. After distillation, the indium in the crucible is collected to obtain intermittent furnace indium. The low-boiling-point impurities evaporated to the condensing plate are periodically cleaned and discharged from the system, achieving a complete open circuit for low-boiling-point impurities. The intermittent furnace indium recovered in both modes is returned to the feed end of the No. 1 continuous vacuum distillation furnace, mixed with crude indium raw material, and then reintroduced into the main process for maximum indium metal recovery. Intermittent vacuum distillation furnace, as an independent impurity open-circuit unit, centrally processes the enriched impurity material, ensuring that the impurities are completely discharged from the entire process system. This avoids the problem of product purity decline caused by the continuous accumulation of impurities in the main process. At the same time, by setting targeted temperatures to process the two types of impurities separately, it ensures thorough impurity removal while improving the recovery rate of indium.

[0030] The sixth step involves full-process material recycling and process control. Throughout the process, all residues from stage 2 and stage 4, as well as the indium recovered from the intermittent furnace, are returned to the feed end of the first-stage continuous vacuum distillation furnace. They are then mixed with the crude indium raw material and re-distilled, forming a closed-loop recycling system. All intermediate indium-containing materials are fully recovered and utilized, effectively improving the overall direct recovery rate of metallic indium. The proportion of recycled materials can be adjusted according to the raw material composition and product quality requirements. Online component detection or periodic sampling analysis ensures the stability of the mixture entering the first-stage distillation, preventing significant fluctuations in material composition from affecting the separation effect in subsequent stages. During process operation, online temperature, pressure, and flow rate monitoring devices are used to monitor the process parameters of each stage in real time. Combined with the material composition detection results, the temperature, vacuum degree, and residue ratio of each distillation stage are dynamically adjusted to form a closed-loop control system, ensuring long-term stability of product purity. When processing crude indium raw materials from different sources with varying impurity contents, the process parameters of each stage can be adjusted to suit the raw material characteristics. For example, when the raw material contains a high content of high-boiling-point impurities, the distillation temperatures of the first and second stages can be appropriately increased, or the residual ratio of the two stages can be reduced to enhance the removal effect of high-boiling-point impurities. When the raw material contains a high content of low-boiling-point impurities, the distillation temperature and material residence time of the third stage can be appropriately adjusted to enhance the removal effect of low-boiling-point impurities. By adjusting the process parameters, this process system can not only stably prepare 4N5 refined indium, but also further optimize the parameters of each stage and increase the number of distillation stages to prepare higher purity high-purity indium, enabling the production of multiple products on a single production line, improving equipment utilization, and reducing operating costs. Throughout the entire process, the vacuum level is consistently controlled within ≤10Pa, and with the continuous quantitative feeding mechanism, the stable and continuous operation of each distillation stage is ensured, avoiding fluctuations in process parameters and changes in product quality caused by pressure fluctuations and uneven feeding. The material flows between the different distillation furnaces in the form of metal ingots, which facilitates storage and transportation, while reducing the contact time and area between the material and air, reducing oxidation loss, and ensuring the purity of the material.

[0031] The specific implementation of this solution will be described in detail below through examples.

[0032] Example 1 Using crude indium raw materials (30.00 t / a of self-produced crude indium and 52.39 t / a of purchased crude indium, with a total crude indium processing capacity of 82.39 t / a) as the processing object, 4N5 indium ingots are prepared by the four-stage continuous distillation + intermittent distillation combined process of the present invention.

[0033] Raw material composition: Self-produced crude indium has an In content of 99.9873%, and the main impurities are: Pb 0.0097%, Fe 0.0011%, and Cu 0.0008%; Purchased crude indium has an In content of 98.7448%, and the main impurities are: Pb 0.1510%, Sn 0.4950%, Cu 0.1470%, and Bi 0.4320%.

[0034] Process parameters: Furnace #1: Distillation temperature 1020℃, vacuum degree 8Pa, graphite heating element power 55kW; Furnace #2: Distillation temperature 1020℃, vacuum degree 8Pa, graphite heating element power 58kW; Furnace #3: Distillation temperature 880℃, vacuum degree 8Pa, graphite heating element power 52kW; Furnace #4: Distillation temperature 1020℃, vacuum degree 8Pa, graphite heating element power 56kW; Intermittent furnace: 1200℃ for high-boiling-point distillation mode, 1000℃ for low-boiling-point distillation mode.

[0035] Operational results: 82.39 tons of crude indium were processed annually, producing 80 tons of 4N5 indium ingots. The composition of the 4N5 indium ingots is: In 99.9986%, Zn 0.0001%, Pb 0.0001%, Cu 0.0001%, Cd 0.0001%, Sn 0.0001%, Tl 0.0001%, Al 0.0001%, Fe 0.0005%, As 0.0001%. The total content of all impurities is less than 20 ppm, meeting the requirements for refined 4N5 indium -50.

[0036] Example 2 Using crude indium raw materials (a mixture of 25.00 t / a of self-produced crude indium and 56.18 t / a of purchased crude indium) as the processing object, the distillation temperature parameters of each stage were adjusted for experimental verification.

[0037] Raw material composition: Self-produced crude indium In99.9873%, purchased crude indium In98.40% (main impurities: Sn0.45%, Pb0.35%, Bi0.30%, Cu0.20%).

[0038] Process parameters: Furnace #1: Distillation temperature 1010℃, vacuum degree 6Pa, graphite heating element power 56kW; Furnace #2: Distillation temperature 1030℃, vacuum degree 6Pa, graphite heating element power 60kW; Furnace #3: Distillation temperature 900℃, vacuum degree 6Pa, graphite heating element power 54kW; Furnace #4: Distillation temperature 1030℃, vacuum degree 6Pa, graphite heating element power 58kW; Intermittent furnace: Operating parameters are the same as in Example 1.

[0039] Operational results: 81.18 tons of crude indium are processed annually, producing approximately 80 tons of 4N5 indium ingots. Analysis of the produced 4N5 indium ingots shows: In 99.9983%, total impurities approximately 9.2 ppm (Zn 0.6 ppm, Pb 0.8 ppm, Cu 0.5 ppm, Cd 0.5 ppm, Sn 0.6 ppm, Tl 0.5 ppm, Al 0.5 ppm, Fe 2.0 ppm, As 0.5 ppm).

[0040] Example 3 Using crude indium raw material (73.18 t / a of purchased crude indium) as the treatment object, the residual ratio parameters of each stage were optimized to adapt to the high impurity content of the purchased crude indium.

[0041] Raw material composition: In 98.7448%, Pb 0.1510%, Sn 0.4950%, Cu 0.1470%, Bi 0.4320%, Zn 0.0198%.

[0042] Process parameters: Furnace #1: Distillation temperature 1050℃, vacuum degree 9Pa, graphite heating element power 60kW; Furnace #2: Distillation temperature 1050℃, vacuum degree 9Pa, graphite heating element power 62kW; Furnace #3: Distillation temperature 930℃, vacuum degree 9Pa, graphite heating element power 55kW; Furnace #4: Distillation temperature 1050℃, vacuum degree 9Pa, graphite heating element power 60kW.

[0043] Results: 4N5 indium ingot composition: In 99.9985%, Zn 0.0001%, Pb 0.0001%, Cu 0.0001%, Cd 0.0001%, Sn 0.0001%, Tl 0.0001%, Al 0.0001%, Fe 0.0005%, As 0.0001%. Indium direct recovery rate is approximately 97.8%.

Claims

1. A method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium, characterized in that, The equipment configuration consisting of 4 continuous vacuum distillation furnaces and 1 intermittent vacuum distillation furnace shall be implemented according to the following steps: The crude indium raw material is sequentially processed by four stages of continuous vacuum distillation. The four stages of continuous vacuum distillation are arranged in the functional sequence of "removing high-boiling point impurities - removing high-boiling point impurities again - removing low-boiling point impurities - finally removing high-boiling point impurities", forming a multi-stage distillation structure of "high-high-low-high". The first and second distillation stages remove high-boiling-point impurities, the third distillation stage removes low-boiling-point impurities, and the fourth distillation stage produces 4N5 refined indium. The residue containing high-boiling-point impurities from the first stage of distillation and the evaporate containing low-boiling-point impurities from the third stage of distillation are fed into an intermittent vacuum distillation furnace, where the impurities are separated from indium through high-boiling-point open-circuit mode and low-boiling-point open-circuit mode, respectively. All indium-containing residual materials generated from each distillation stage, as well as indium recovered from the intermittent vacuum distillation furnace, are returned to the first stage continuous vacuum distillation furnace for recycling.

2. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 1, characterized in that, The first stage of continuous vacuum distillation is carried out at a temperature of 1000-1050℃ and a vacuum degree of ≤10Pa. The evaporation residue ratio is controlled by adjusting the heating power to reduce the total amount of high-boiling-point impurities to below 200ppm. The evaporated product is indium 1#, and the residue is residue 1# containing high-boiling-point impurities.

3. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 2, characterized in that, The second stage of continuous vacuum distillation processes indium #1 at a temperature of 1000–1050°C. By reducing the residual ratio and enhancing the separation effect, the total amount of high-boiling-point impurities is reduced to below 50 ppm. The evaporated product is indium #2, and the residue is returned to the first stage of continuous vacuum distillation furnace.

4. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 3, characterized in that, The third stage of continuous vacuum distillation processes indium #2 at a temperature of 850–950°C and a vacuum degree of ≤10Pa, which causes low-boiling-point impurities to evaporate preferentially. The residual product is indium #3, and the evaporated product is indium #3 containing low-boiling-point impurities.

5. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 4, characterized in that, The fourth stage of continuous vacuum distillation processes indium #3 at a temperature of 1000–1050°C. The heating power is adjusted to control the residual ratio, ensuring that the total amount of high-boiling-point impurities and low-boiling-point impurities in the refined indium is less than 20 ppm. The evaporated product is 4N5 refined indium ingot, and the residue is returned to the first stage of continuous vacuum distillation furnace.

6. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 1, characterized in that, In the high-boiling-point open-circuit mode of the intermittent vacuum distillation furnace, the distillation temperature is controlled at 1180–1220℃ and the vacuum degree is ≤10Pa. Indium is evaporated to the condenser plate for recovery, and high-boiling-point impurities are retained in the evaporation crucible to form impurity blocks. In the low-boiling-point open-circuit mode, the distillation temperature is controlled at 980–1020℃ and the vacuum degree is ≤10Pa. Low-boiling-point impurities are evaporated to the condenser plate for removal, and indium is retained in the evaporation crucible for recovery.

7. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 1, characterized in that, The crude indium raw material is a mixture of self-produced crude indium and purchased crude indium, which is transferred between the various distillation furnaces in the form of metal ingots.

8. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 1, characterized in that, By adjusting the distillation temperature and evaporation residue ratio of each stage, crude indium raw materials with different impurity contents can be adapted, and high-purity indium with higher purity can be prepared.

9. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 1, characterized in that, The vacuum level is kept stable within ≤10Pa throughout the process to achieve continuous quantitative feeding and ensure the stability of process operation.

10. The method for preparing 4N5 refined indium by multi-stage vacuum distillation of crude indium according to claim 1, characterized in that, The prepared 4N5 refined indium has a purity of ≥99.995% and can be used in the fields of ITO targets, solar cell electrodes, semiconductor device interconnect materials, or lead-free solders.

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