Method for purifying 6N-grade ultra-high-purity aluminum based on floating zone melting and directional solidification

Abstract

The invention relates to the technical field of metal purification, and discloses a 6N-grade ultra-high-purity aluminum purification method based on suspension zone melting and directional solidification, which comprises the following steps: forming a solid aluminum shell in-situ crucible by using a vertical cold crucible, and carrying out directional solidification to enrich impurities by adopting a first process mode; a second process mode is adopted, and two-stage serial extraction is executed in the impurity enrichment area, specifically, firstly, rapid reciprocating movement is conducted to break a membrane to remove volatile impurities, and then slow one-way movement segregation is conducted to remove non-volatile impurities. And the process conflict of volatile and non-volatile impurity removal mechanisms is solved.

Description

TECHNICAL FIELD

[0001] The application relates to a 6N-grade ultra-high-purity aluminum purification method based on suspension zone melting and directional solidification, and belongs to the technical field of metal purification. BACKGROUND

[0002] Currently, obtaining 6N-grade ultra-high-purity aluminum is a key material prerequisite for supporting the development of semiconductor, optoelectronic and other high-tech industries. In industry, such high-purity metals are mainly obtained by melting and crystallization methods based on impurity segregation principles, such as directional solidification or suspension zone melting technology. However, the application of these mature melting and crystallization processes designed for high-melting-point materials directly to aluminum, which has low melting point, high thermal conductivity and high chemical activity, is subject to objective constraints between process methods and material nature known to those skilled in the art. To achieve 6N-grade purity, any secondary contamination of the melt by crucible materials must be eliminated at the source, which requires the use of suspension zone melting without a crucible to eliminate contamination. The low melting point and high thermal conductivity of aluminum make its suspension melting zone unstable in the electromagnetic field, making it difficult to maintain a flat and stable solid-liquid interface required for segregation purification, resulting in a narrow process window or engineering difficulties in stable implementation.

[0003] At the same time, the natural formation of a high-melting-point dense oxide film on the surface of the aluminum melt further exacerbates this objective constraint. This oxide film not only mechanically interferes with the stability of the melting zone, but also chemically blocks the melt, hindering the gas phase removal channel of trace volatile impurities and exacerbating the composition supercooling at the interface front, leading to early failure of the segregation purification process. Existing technical routes therefore face the following constraints: directional solidification with a crucible, although easy to control the crystallization interface, is difficult to economically break through the 5N purity limit due to crucible contamination; in an attempt to improve purification and optimize crystallization quality within the framework of a crucible, those skilled in the art have also proposed improved control methods, for example, a Chinese invention patent with publication number CN118835098A discloses a method and device for extracting high-purity aluminum by directional solidification and high-purity aluminum. This scheme attempts to generate a suspended crystal by applying air cooling at the top of the melt, combined with directional solidification water cooling at the bottom, in order to impact the solid-liquid interface and refine the grains. However, this method still falls within the technical scope of using a graphite crucible, and the purity limit that can be achieved (≥5N) confirms that crucible contamination is a fundamental obstacle to breaking through 6N-grade purity. The introduction of top air cooling and bottom water cooling creates a complex thermal field, increasing the difficulty of process control, and does not help remove the oxide film. Suspension zone melting without a crucible avoids contamination, but the physical properties of aluminum make it difficult to stably achieve suspension stability and crystallization interface control in engineering.

[0004] Therefore, how to provide a new process path to cooperatively achieve efficient segregation and removal of non-volatile and volatile impurities has become a technical problem to be solved by the present application. SUMMARY

[0005] The application provides a 6N ultra-high purity aluminum purification method based on suspension zone melting and directional solidification, which mainly aims to solve the problem that the prior art is difficult to simultaneously solve the objective constraints among crucible pollution, melt suspension instability and efficient segregation removal of various impurities in the process of ultra-high purity aluminum purification.

[0006] To achieve the above-mentioned purpose, the application provides a 6N ultra-high purity aluminum purification method based on suspension zone melting and directional solidification, which runs in a vertical cold crucible induction melting system, the vertical cold crucible induction melting system generates an induction heating field, the method is executed in a vacuum or inert atmosphere, and comprises the following steps: Step a, melting raw material aluminum, and forming a solid aluminum shell as a self-made crucible by using the cooling wall of the vertical cold crucible of the vertical cold crucible induction melting system to contain internal molten aluminum; Step b, starting a first process mode, moving at a uniform speed between the vertical cold crucible and the induction heating field to make the internal molten aluminum directionally solidify from bottom to top, so as to push and enrich the volatile impurities and the non-volatile impurities together to the impurity enrichment zone at the top of the melt; Step c, when the directional solidification reaches a preset threshold, stopping the uniform speed movement and starting a second process mode; Step d, in the second process mode, adjusting the induction heating field to form a narrow melt zone in the impurity enrichment zone; Step e, in the second process mode, performing the following two serial extraction stages on the impurity enrichment zone: the first extraction stage: driving the narrow melt zone to perform a preset number of fast reciprocating movements in the impurity enrichment zone, the fast reciprocating movements generate electromagnetic stirring intensity sufficient to periodically break the oxide film at the top of the melt, so as to promote the volatile impurities to be removed by volatilization from the exposed melt, thereby forming an intermediate melt with reduced volatile impurity concentration; the second extraction stage: after the first extraction stage is completed, in the intermediate melt, controlling the narrow melt zone to perform at least one slow and one-way movement starting from the bottom of the impurity enrichment zone, and the one-way movement is performed under the segregation condition of pushing and enriching the non-volatile impurities to the top end of the impurity enrichment zone; Step f, stopping heating to make the region rich in non-volatile impurities at the top end solidify, and removing the solidified region from the obtained material rod.

[0007] Preferably, between step c and step d, a transition step is further included, the transition step comprising: gradually reducing the rate of the relative uniform movement in step b to zero according to a preset non-linear decay curve; and gradually adjusting the power configuration and operating frequency of the induction heating field from a long-range heating mode suitable for directional solidification to a narrow-range heating mode suitable for narrow-range melting zone, the gradual adjustment of the power configuration and operating frequency being set as a preset coordinated control program for maintaining the dynamic balance of the solid-liquid interface during the phase transition.

[0008] Preferably, in step b, further comprising: monitoring the load impedance of the induction heating field in real time, and dynamically adjusting at least one operating parameter of the induction heating field according to the monitoring result of the load impedance, so as to maintain the thermal field morphology of the induction heating field constant during the directional solidification process.

[0009] Preferably, the operating parameter comprises the operating frequency of the induction heating field.

[0010] Preferably, the vertical cold crucible comprises a circulating cooling medium, and in step b, further comprising: monitoring the temperature difference between the inlet temperature and the outlet temperature of the circulating cooling medium in real time, and dynamically adjusting the power of the induction heating field according to the monitoring result of the temperature difference, so as to maintain the thickness of the solid aluminum shell stable.

[0011] Preferably, step f comprises: before stopping heating, performing a stress release annealing treatment on the impurity enrichment zone and the adjacent part of the solidified rod below the impurity enrichment zone by using the induction heating field; and guiding the asymmetric solidification of the impurity enrichment zone by asymmetrically cooling the vertical cold crucible.

[0012] Preferably, in step b, further comprising: continuously monitoring one or more electrical operating parameters of the induction heating field, and the preset threshold in step c is a critical characteristic threshold of the electrical operating parameters, which corresponds to the impurity concentration of the impurity enrichment zone at the top of the melt.

[0013] Preferably, the electrical operating parameter comprises a power factor, and the determination rule of the critical characteristic threshold comprises: calculating the relative change rate of the real-time monitored value of the power factor and its initial value at the beginning of step b, and comparing the relative change rate with a preset change rate threshold corresponding to the constitutional undercooling critical point, wherein the relative change rate By The calculation is The real-time monitored value of the power factor is The initial value of the power factor is The preset change rate threshold is.

[0014] Preferably, in step d, adjusting the induction heating field includes changing the power configuration or operating frequency of the induction coil to transform the induction heating field from a long-range heating field suitable for directional solidification to a narrow-range heating field suitable for zone extraction.

[0015] Preferably, the rapid reciprocating movement in the first extraction stage and the unidirectional movement in the second extraction stage in step e are both performed within the total length of the impurity-rich zone.

[0016] Compared with the prior art, the present application has the following beneficial effects: 1. By melting the raw aluminum in the vertical cold crucible and using the solid aluminum shell formed by the cooling wall as a self-made crucible to contain the internal molten aluminum, the high-purity aluminum melt obtains physical stability in the process without contacting any external crucible material to avoid secondary pollution, thereby avoiding the technical contradiction between melt suspension instability and crucible secondary pollution in the field of ultra-high-purity aluminum purification from the structure.

[0017] 2. The directional solidification and zone melting crystallization methods are time-sequentially reorganized and functionally separated in the process flow. The directional solidification method is used to efficiently preliminarily separate the main part of the raw material and push the impurities to the top of the melt. The function of the zone melting method is redefined from traditional full-length purification to only extreme extraction of the impurity-rich zone. This reconstruction of the process time sequence avoids direct conflict between the two methods in the physical field, making the entire purification process more efficient and stable in the metallurgical logic.

[0018] 3. In the step of in-situ zone extraction at the top, the electromagnetic stirring generated by the reciprocating movement of the narrow-range melting zone realizes the coordinated removal of impurities with different physical properties, which helps the non-volatile impurities to enrich at the top according to the principle of segregation. The continuous disturbance or rupture of the oxidation film at the top of the melt exposes the pure melt surface, providing a physical channel for the volatilization of volatile impurities in a vacuum environment and solving the problem of hindering volatilization removal by the oxidation film. BRIEF DESCRIPTION OF DRAWINGS

[0019] Fig. 1 The figure is a schematic diagram of the closed-loop control logic of the purification process of the present application. Fig. 2 The figure is an optimization calibration diagram of the holding time of stress relief annealing of the present application. Fig. 3 The figure is a schematic diagram of the core structure of the vertical cold crucible purification system of the present application. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below, but it should be understood that the detailed description of the present application is intended to explain the present application, not to limit the protection scope of the present application.

[0021] The present application provides a 6N ultra-pure aluminum purification method based on floating zone melting and directional solidification, which is performed in a vertical cold crucible induction melting system, which includes a vertical cold crucible, an induction heating field and its control unit, and a mechanical driving mechanism for realizing the relative movement of the cold crucible and the heating field, and the whole purification process is carried out in a vacuum or inert atmosphere such as high-purity argon environment; the core of the method is to reconstruct the purification process into two time-separated and functionally complementary process modes, namely the first process mode with directional solidification as the core, and the second process mode with top in-situ zone extraction as the core; the method executes step a, vertically placing a 5N pure aluminum rod into a vertical cold crucible (a split, water-cooled metal crucible such as a red copper crucible); starting the induction heating field to melt the raw aluminum, due to the strong cooling effect of the cold crucible wall such as circulating deionized water, the outermost layer of aluminum liquid in contact with the crucible wall will solidify to form a dynamically balanced solid aluminum shell, which acts as a self-generated crucible to physically isolate the internal pure aluminum liquid from the external crucible such as the copper wall, while realizing contamination-free containment, it also avoids the problem of suspended instability of the aluminum liquid; execute step b, start the first process mode; in this mode, the induction heating field is adjusted to a long-range heating field, for example, by using a lower working frequency and a longer induction coil configuration to establish a wide thermal gradient suitable for directional solidification in the melt; at the same time, the control system drives the relative uniform movement between the vertical cold crucible and the induction heating field, for example, by using a pulling mechanism to pull the cold crucible away from the main heating area at a very slow rate (e.g. 0.5mm / min to 5mm / min); under the condition of strong axial heat dissipation such as through the bottom of the cold crucible or a special seeding rod and stable pulling rate, the solid-liquid interface of the internal aluminum liquid stably advances from bottom to top under the condition of directional solidification; according to the principle of segregation, all impurities with low solubility in the solid phase, including volatile impurities and non-volatile impurities, are collectively and efficiently pushed to the top of the remaining melt to form an impurity-rich zone. For example, 0.5mm / min to 5mm / min) to pull the cold crucible away from the main heating area at a uniform speed; under the condition of strong axial heat dissipation such as through the bottom of the cold crucible or a special seeding rod and stable pulling rate, the solid-liquid interface of the internal aluminum liquid stably advances from bottom to top under the condition of directional solidification; according to the principle of segregation, all impurities with low solubility in the solid phase, including volatile impurities and non-volatile impurities, are collectively and efficiently pushed to the top of the remaining melt to form an impurity-rich zone.

[0022] In the execution of the first process mode (step b), to ensure the stability and efficiency of the process, the system can also perform two dynamic compensation controls in parallel; one is to maintain the stability of the solid aluminum shell thickness, to cope with the power fluctuations of the induction heating field, the system monitors the temperature difference (dT) between the inlet temperature and the outlet temperature of the circulating cooling medium flowing through the vertical cold crucible in real time; the dT value is proportional to the heat flux through the solid aluminum shell, which can indirectly represent the thickness; the control system compares the real-time monitoring result of the temperature difference with the dT reference value corresponding to the ideal shell thickness, and dynamically adjusts the total power of the induction heating field through the feedback control loop (such as PID control) to keep the dT value around the reference value, thereby ensuring the physical integrity of the self-grown crucible; the second is to maintain the thermal field shape of the induction heating field constant during the directional solidification process, to cope with the load impedance drift caused by the change of solid-liquid phase ratio, the system monitors the load impedance of the induction heating field in real time; and according to the monitoring results (drift of R(t) and X(t)) of the load impedance, at least one working parameter of the induction heating field is dynamically adjusted, preferably the working frequency (f) is dynamically adjusted to compensate for the change of skin depth, and ensure that the temperature gradient in front of the solid-liquid interface remains flat and stable; execute step c, determine the end point of the first process mode and start the second process mode; the preset threshold here is the key process node, indicating that the DS process has reached its physical limit, that is, the interface is about to form a composition undercooling; in the preferred embodiment, the threshold is not a fixed length, but a closed-loop feedback based on the physical state of the melt; specifically, in step b, the system also continuously monitors one or more electrical operating parameters of the induction heating field, which corresponds to the impurity concentration of the impurity enrichment zone at the top of the melt, because the enrichment of impurities (especially metal impurities Fe, Si) will change the total electrical conductivity of the melt; the electrical operating parameters include power factor; the preset threshold in step c is defined as the critical characteristic threshold of the power factor; the determination rule of the threshold can include: calculating the relative change rate of the real-time monitoring value of the power factor and its initial value at the beginning of step b , wherein ; and compare the with the preset change rate threshold corresponding to the composition undercooling critical point ; once the monitoring value reaches , the control system immediately determines that the DS process is over, stops the relative uniform speed movement, and starts the second process mode.

[0023] To ensure smooth transition from the first process mode (DS) to the second process mode (FZ) and avoid interface instability caused by sudden changes in thermal field and flow field, a transition step is included between steps c and d; the transition step executes a preset coordinated control program that synchronously controls two actions: reducing the relative uniform speed movement rate in step b (​ ), gradually decrease to zero following a pre-set non-linear decay curve such as exponential decay; while the rate is decaying, the power configuration of the induction heating field and the operating frequency are gradually and smoothly adjusted from a long-range heating mode suitable for directional solidification to a narrow-range heating mode suitable for zone refining extraction, this asynchronous gradual change of the pulling rate and the heating field ensures the dynamic balance of the solid-liquid interface during the morphological transition (from flat to meniscus), ensuring the continuity of impurity segregation; then, perform step d, adjust the induction heating field in the second process mode; the adjustment can include changing the power configuration of the induction coil, such as switching to a coil turn group with fewer turns and more concentration, or changing the operating frequency such as increasing the frequency to utilize the skin effect, the purpose is to convert the long-range heating field originally used for DS into a narrow-range molten zone with high energy concentration, which is applied to the top end of the melt that has been enriched in impurities, i.e. in the impurity enrichment zone; perform step e, perform two-stage serial extraction on the impurity enrichment zone such as the top 10% length range of the rod in the second process mode; perform the first extraction stage: the core goal of this stage is to remove the volatile impurities (Mg, Zn); the control system drives the narrow-range molten zone to perform a pre-set number of times such as 5-15 times of rapid reciprocating movement in the impurity enrichment zone; the rate and amplitude of this rapid reciprocation are set to be sufficient to generate a strong electromagnetic stirring (MHD) effect, the strength of the electromagnetic stirring is sufficient to periodically break or disturb the Al2O3 oxide film that objectively exists on the top end of the melt, which hinders the volatilization; under the continuous vacuum environment of the equipment, the volatile impurities can be quickly volatilized and removed from the agitated and exposed melt; after this stage is completed, an intermediate melt with reduced concentration of volatile impurities is formed.

[0024] Immediately after the first extraction stage is completed, the second extraction stage is started: the core objective of this stage is to limit the removal and enrichment of non-volatile impurities (Fe, Si); the control system switches the control mode to zone segregation mode in the intermediate melt; the narrow melt zone is controlled to perform at least one, preferably 1-3, unidirectional movement (e.g. from bottom to top) starting from the bottom (e.g. at 90% height) of the impurity enrichment zone; the rate of this unidirectional movement is controlled at a very low level to meet the maximum segregation condition required by classical zone segregation, avoiding melt backmixing, thereby pushing and enriching the non-volatile impurities to the top of the impurity enrichment zone; finally, step f is performed to stop heating after the unidirectional movement of the second extraction stage is completed; to avoid cracking of the finished product due to thermal stress, step f preferably includes a controlled final solidification process; specifically, before stopping heating, the inductive heating field is adjusted to a low-power, long-focus mode, and a stress relief annealing treatment at a temperature lower than the melting point is performed on the impurity enrichment zone, i.e. the final solidification zone at the top, and the adjacent part of the solidified rod below; while reducing the heating power, asymmetric solidification of the impurity enrichment zone is induced by asymmetrically cooling the vertical cold crucible; for example, by independently controlling the flow of cooling medium in different regions of the split cold crucible, a strong cold end is intentionally created on one side of the final solidification zone, forcing solidification to start from that side and driving the last solidification shrinkage hole and thermal stress concentration point to the edge of the waste zone, thereby protecting the high-value 6N product interface; after final solidification is completed, the rod is removed after cooling to room temperature, and the final solidification zone at the top, which is enriched in non-volatile impurities, is removed from the resulting rod by mechanical cutting or other means; the remaining main part of the rod is the 6N ultra-pure aluminum.

[0025] Example 1: This example is a specific application of the disclosed technical solution in a specific industrial scenario, which is used to process a batch of 5N-grade raw aluminum with complex composition, not only the content of non-volatile impurities (represented by Fe and Si) is relatively high, but also the concentration of volatile impurities (represented by Mg and Zn) is higher than that of the conventional batch. In this purification process, a stable solid aluminum shell self-crucible is established in the vertical cold crucible according to step a, and then the first process mode of step b is started to stably push most of the impurities including Fe, Si, Mg and Zn to the impurity enrichment zone at the top of the melt by directional solidification from bottom to top; when the relative change rate of the power factor of the inductive heating field touches the preset critical characteristic threshold , the system determines that the directional solidification has reached the limit and automatically stops the relative uniform movement of the first process mode.

[0026] ​The system switches to the second process mode, performs step d, and adjusts the induction heating field from the long-range heating mode to a narrow-melt zone that only covers the top impurity-rich zone. The key point here is that the system does not use a single zone melting method, but starts the two-stage serial extraction of step e, performs the first extraction stage, and controls the system to drive the narrow-melt zone to perform 10 fast reciprocating movements in the impurity-rich zone. The movements produce strong electromagnetic stirring that periodically breaks the oxide film on the surface of the melt, providing a volatilization channel for high-concentration Mg and Zn impurities, so that they are preferentially and quickly removed in a vacuum environment. After the completion of this stage, the control system automatically switches to the second extraction stage. At this time, since the volatile impurities in the melt have been largely removed, strong stirring to break the film is no longer needed. Instead, the system controls the narrow-melt zone to perform a slow one-way movement from the bottom of the impurity-rich zone. The movement is performed under conditions that meet the limit segregation, and pushes and enriches the non-volatile impurities Fe and Si, which have no volatilization interference, to the topmost final solidification zone.

[0027] Example 2: A 5N-grade aluminum rod of the same batch was selected, and its initial content (by weight) of key impurities, as analyzed by glow discharge mass spectrometry (GD-MS), was as follows: Fe 0.52 ppmw, Si 0.78 ppmw, Mg 0.31 ppmw, and Zn 0.40 ppmw, with a total of 2.01 ppmw. The inventive sample group and three control groups were set up on the same vertical cold-crucible induction melting system, which was equipped with a high-frequency induction heating unit, a precision downward-pulling mechanism (rate control accuracy 0.05 mm / min), and a high-vacuum system. All sample groups were melted under the same conditions, and the system was operated in the same way. The step a is performed in a vacuum environment of the Pa, and a stable solid aluminum shell self-generating crucible is formed by melting and forming a stable solid aluminum shell in a vertical cold crucible; the first process mode of the step b is performed in the sample group of the application, an induction heating field is adjusted to a long-range heating mode, the cold crucible is moved at a speed of 1.0 mm / min, the internal aluminum liquid is directionally solidified from bottom to top, and the rod is solidified by 90 %; the step c stops relative movement and starts the second process mode; the step d adjusts the induction heating field to form a narrow melting zone in the top 10 % impurity enrichment zone; the step e performs serial extraction, performs the first extraction stage, drives the narrow melting zone to perform 10 times of rapid reciprocating movement in the impurity enrichment zone at a speed of 15 mm / min, performs the second extraction stage, controls the narrow melting zone to start from the bottom of the impurity enrichment zone and perform 1 time of slow and single-directional movement to the top end at a speed of 0.5 mm / min; and finally, the step f is performed to finish solidification; the process parameters of the control group 1 are basically the same as those of the sample group of the application, and the difference lies in that the first extraction stage is omitted in the step e, that is, after the narrow melting zone is formed in the step d, the slow and single-directional movement at a speed of 0.5 mm / min is directly performed; the process parameters of the control group 2 are basically the same as those of the sample group of the application, and the difference lies in that the second extraction stage is omitted in the step e, that is, after the 10 times of rapid reciprocating movement is performed, the step f is immediately performed to finish solidification; the control group 3 only performs the first process mode of the step b, stops heating and finishes solidification after being solidified by 90 %, and does not perform any second process mode; after the purification of all the sample groups is completed, the top 10 % of the finished solidification zone is mechanically removed, the main rod is sampled from the top, the middle and the bottom, the impurity content is detected by GD-MS, and the average value is calculated, and the results are shown in Table 1.

[0028] Table 1: Comparison table of impurity content of the main rod (90 % part) after purification of each sample group The data in Table 1 shows that the total impurity of the control group 3 (only directional solidification) is 1.40 ppmw; the removal effect of Fe and Si (0.09 / 0.16 ppmw) of the control group 1 (missing the first extraction stage) is close to that of the sample group of the application (0.07 / 0.13 ppmw), but the removal effect of Mg and Zn (0.28 / 0.35 ppmw) is poor; the removal effect of Mg and Zn (0.04 / 0.03 ppmw) of the control group 2 (missing the second extraction stage) is close to that of the sample group of the application (0.02 / 0.01 ppmw), but the removal effect of Fe and Si (0.41 / 0.55 ppmw) is lower than that of the control group 3 (0.38 / 0.51 ppmw), which shows that the melt backmixing has an adverse effect on the segregation; the total impurity of the sample group of the application which performs complete serial extraction is 0.23 ppmw, and the purity is 99.999977 %, which reaches the 6N level standard.

[0029] Example 3: This example combinesFigs. 1 to 3 A method for purifying 6N ultra-high purity aluminum based on floating zone melting and directional solidification is described as shown in Fig. 1 , wherein 5N raw material aluminum is subjected to a purification physical process including melting, directional solidification and zone melting to produce 6N ultra-high purity aluminum, and the physical process is strictly monitored and adjusted by a purification control system, which is responsible for dynamic compensation and mode switching, and reads preset threshold values from process parameter configurations , and the purification physical process feeds back load impedance monitoring data, cooling medium temperature difference dT and electrical parameters such as power factor P(t) to the purification control system in real time, based on these feedback data and preset values, the purification control system sends mode switching instructions, frequency adjustment instructions and power adjustment instructions to the purification physical process, and records process data as P(t), dT and R(t).

[0030] As shown in Fig. 2 , the horizontal axis is the holding time (minutes), and the vertical axis is the relative index %. The microcrack density shows a nonlinear downward trend with the extension of holding time: at 10 minutes, its relative index is at the highest point of 65%, and in the interval of 10 to 30 minutes, it decreases to 25%, and in the interval of 30 to 60 minutes, the decrease tends to be flat, and finally reaches 15%. At the same time, the Fe back diffusion remains at a low level of 7-8% in the interval of 10 to 30 minutes, but starts to accelerate after 30 minutes, reaching 22% at 60 minutes. Therefore, the two curves form a clear process trade-off point at 30 minutes, at which the microcrack density has been greatly reduced, and the Fe back diffusion has not yet begun to increase, indicating that this time point is the preferred balance point that takes into account low crack density and low back diffusion.

[0031] As shown in Fig. 3 , the main body is a vertical cold crucible, surrounded by an induction heating coil. Inside the crucible, due to the effect of the cooling wall, a solid aluminum shell self-crucible is formed, which physically isolates the melt from the crucible. The shell contains aluminum in different states from top to bottom: the top is the impurity enrichment zone, the induction heating field is focused in this area to form a narrow melting zone indicated by the dashed line, the middle is the main body of the aluminum liquid, and the bottom is the solidified area. The figure also clearly shows two dynamic processes: one is the maintenance of thermal balance, through the cooling medium flowing into the crucible wall from bottom to top, and the cooling medium flowing out from the top, and the other is the advancement of the purification process, indicated by the directional solidification arrow at the bottom, showing that the solidified area is growing from bottom to top.

[0032] Example 4: This example provides a calibration procedure for determining key process parameters in step e (the first extraction stage) and step f. The calibration focuses on the rate, amplitude (range of movement within the total length of the impurity enrichment zone), and number of reciprocating movements in step e (the first extraction stage). The goal is to maximize the removal of volatile impurities (represented by Mg) in the shortest possible time while suppressing backmixing of non-volatile impurities (represented by Fe). Multiple identical feed rods that have completed directional solidification in step b and formed the impurity enrichment zone are used. Three sets of gradient experiments are set: Group A (rate calibration): the reciprocating amplitude is fixed at 80% of the impurity enrichment zone length, the number of reciprocating movements is fixed at 10, and the reciprocating rates are set to 5 mm / min, 10 mm / min, 15 mm / min, and 20 mm / min respectively; Group B (number of reciprocating movements calibration): the reciprocating amplitude is fixed at 80%, the rate is fixed at 15 mm / min, and the number of reciprocating movements is set to 3, 5, 10, and 15 respectively; Group C (amplitude ... reciprocating movement is fixed at 15 mm / min, and the number of reciprocating movements is set to 3, 5, 10, and 15 respectively; Group C (amplitude calibration): the reciprocating amplitude is fixed at 80%, the re (Standardization): The fixed extraction rate was 15 mm / min, and the number of cycles was 10. The reciprocating amplitude was set to 50%, 80%, and 95% respectively. After the first extraction stage, all sample groups skipped the second extraction stage, immediately solidified, and samples were taken for analysis of the Mg content (characterizing volatilization efficiency) and Fe distribution gradient in the impurity enrichment zone (characterizing backmixing degree through multi-point sampling). The experimental results showed that: when the rate was below 10 mm / min, electromagnetic stirring was insufficient to stabilize the broken oxide film, and the Mg removal rate was low; when the rate was above 20 mm / min, the melt fluctuated violently, and the Fe distribution gradient tended to flatten, indicating that backmixing was aggravated; when the number of cycles was less than 5, the volatilization time was insufficient, and the Mg residue was high; after more than 10 cycles, the Mg removal rate increased slowly, but the Fe backmixing increased with time; when the amplitude was less than 80%, the melt at the edge of the impurity enrichment zone was not sufficiently stirred. After comprehensive consideration, a set of optimal parameters was determined to be: rate 15 mm / min, amplitude 80%, and number of cycles 10.

[0033] Secondly, the temperature and time of the stress-relieving annealing treatment in step f were calibrated. The goal was to maximize the release of residual thermal stress at the interface between the feed bar, especially the final solidification zone and the main feed bar, without causing back diffusion of the enriched non-volatile impurities. Multiple identical feed bars that had completed the second extraction stage of step e and where non-volatile impurities were enriched at the very top were used. A gradient experiment was set up: Group D temperature calibration: the holding time was fixed at 30 minutes, and the annealing temperature was set to (…). -150 ), ( -100 ), ( -50 ),in Estimated melting point of impurity alloys in the final solidification zone; Group E time calibration: fixed temperature ( -100 ), respectively; after annealing and final solidification according to step f, the stress release effect is characterized by observing the microcrack density in the interface area through metallographic microscope, and the impurity back diffusion degree is characterized by high-precision cutting and GD-MS analysis of the Fe content at 0.5 mm below the interface; the test results show that when the temperature is lower than (- -150 ) or the time is shorter than 10 minutes, the stress release is insufficient and the microcrack density is high; when the temperature is higher than (- -50 ) or the time is longer than 60 minutes, although the cracks are reduced, the Fe content below the interface appears non-edge climbing, indicating unacceptable back diffusion; after comprehensive consideration, a group of preferred parameters is determined as the annealing temperature (- -100 ) and the holding time of 30 minutes.

[0034] Example 5: This embodiment provides a calibration procedure for determining the key control parameters required for the method of the present application to run on a specific vertical cold crucible induction melting system, to ensure the adaptability of the process to different equipment thermal characteristics and raw material batch fluctuations, and to calibrate the circulating cooling medium temperature difference for maintaining the solid aluminum shell thickness stable in step b, i.e. the dT reference value; load standard 5N raw material aluminum in the system and perform step a melting to form a self-made crucible; set a medium power and maintain the induction heating field constant, so that the system reaches thermal equilibrium, at which time the temperature difference (dT) between the inlet and outlet temperatures of the cooling medium is monitored in real time and stabilized; stop heating to make the rod solidify, and after being taken out, cut it axially, and physically measure the average thickness of the solidified solid aluminum shell; repeat the above process by adjusting the set medium power value by gradient until the measured average thickness of the aluminum shell reaches the engineering target value that takes into account the pollution isolation and melting efficiency, such as 8mm to 12mm, record and store the stable dT reading at this time, which is defined as the dT reference value of this equipment under this working condition, for closed-loop feedback control in subsequent production.

[0035] Secondly, calibrate the critical feature threshold value for determining the end point of the first process mode in step c, using a batch of representative 5N raw material aluminum with known initial total impurity concentration, complete steps a and b on the system with calibrated dT reference value, i.e. perform bottom-up directional solidification under power compensation to maintain the dT reference value constant; during this process, the control system continuously monitors and records the power factor of the induction heating field at a high frequency (such as 1Hz) and calculates its relative change rate in real time , where , is the real-time monitoring value, is the initial value at the beginning of step b; meanwhile, the critical impurity enrichment concentration at which the batch material will be compositionally undercooled under the current solidification condition is determined by calculation (based on the equilibrium partition coefficient of the target impurity in aluminum and the solidification rate, using the composition undercooling criterion) or experiment (by interrupting the experiment at different solidification heights and sampling the melt composition in front of the interface); finally, the time point at which the critical impurity enrichment concentration is actually reached in the experiment is time-stamped aligned with the recorded history curve, and the value corresponding to the time point is found out; the value (or multiplied by a safety factor, such as 0.95) is set as the critical characteristic threshold of the batch material for automatic switching judgment of subsequent batches.

[0036] Example 6: This example provides a calibration procedure for determining the key dynamic compensation control logic required in steps c and b of the method of the present application to cope with power fluctuations or system disturbances caused by solid-liquid phase change; the calibration is for the working parameter adjustment logic used to maintain the inductive heating field thermal field morphology constant in step b; in addition to real-time monitoring of the load impedance, the system also actually measures the temperature gradient and the interface shape at the solid-liquid interface (through a pre-set array of micro-thermocouples) during the directional solidification process of step b; by introducing controllable power frequency (f) disturbances, and correlating the recorded changes in load impedance , , with the measured and the deviation of the interface shape; based on multiple sets of experimental data, a response model or lookup table of impedance-frequency-thermal field morphology of the specific equipment system is established, which is used to dynamically adjust the working frequency in a feedforward manner according to the real-time monitored load impedance drift in actual production, in order to maintain the thermal field morphology (especially and the interface flatness) at the target value; secondly, the pre-set coordinated control program for the transition step between steps c and d is calibrated; the goal of the program is to ensure that the solid-liquid interface remains stable when transitioning from the first process mode (flat interface) to the second process mode (meniscus-shaped interface), avoiding impurity encapsulation; a set of gradient experiments are designed to test different combinations of draw rate decay curves and power / frequency gradient curves, where the draw rate decay curve can be set as , and are the time constants of draw rate and power, respectively; the interface morphology during the transition process is monitored online by high-speed photography and thermal imaging, and immediately after the transition is completed, the interface is metallographically analyzed to detect the presence or absence of cellular crystals or dendrites; by comparing different , ) the interface instability case under the combination, determine the optimal time constant combination, the interface transition process under the combination is the most smooth, and there is no instability sign, and the synergy control program is solidified in the control system.

[0037] It is apparent for those skilled in the art that the present application is not limited to the details of the foregoing exemplary embodiments, and the present application can be implemented in other concrete forms without departing from the spirit or essential characteristics of the present application.

[0038] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for purifying 6N grade ultra-high purity aluminum based on floating zone melting and directional solidification, which is operated in a vertical cold crucible induction melting system that generates an induction heating field, and is performed under vacuum or inert atmosphere, characterized in that, The method comprises the following steps: Step a, melting raw material aluminum, and forming a solid aluminum shell as a self-made crucible by using the cooling wall of the vertical cold crucible of the vertical cold crucible induction melting system to contain internal molten aluminum; Step b, starting a first process mode, and pushing and enriching volatile impurities and non-volatile impurities together to an impurity enrichment zone at the top of the melt by relatively uniform movement between the vertical cold crucible and the induction heating field to make the internal molten aluminum directionally solidify from bottom to top; Step c, stopping the relatively uniform movement when the directional solidification reaches a preset threshold, and starting a second process mode; Step d, in the second process mode, adjusting the induction heating field to form a narrow melt zone in the impurity enrichment zone; Step e, in the second process mode, performing the following two serial extraction stages on the impurity enrichment zone: a first extraction stage: driving the narrow melt zone to perform a preset number of rapid reciprocating movements in the impurity enrichment zone, the rapid reciprocating movements generating electromagnetic stirring intensity sufficient to periodically break the oxide film at the top of the melt, and promoting the volatile impurities to be removed by volatilization from the exposed melt to form an intermediate melt with reduced volatile impurity concentration; a second extraction stage: after the first extraction stage is completed, controlling the narrow melt zone to perform at least one slow unidirectional movement in the intermediate melt starting from the bottom of the impurity enrichment zone, and the unidirectional movement is performed under the segregation condition of pushing and enriching the non-volatile impurities to the top end of the impurity enrichment zone; Step f, stopping heating to make the region enriched with non-volatile impurities at the top end solidify, and removing the solidified region from the obtained rod.

2. The method for purifying 6N ultra-high purity aluminum according to claim 1, wherein, Between step c and step d, a transition step is further included, and the transition step comprises: gradually reducing the speed of the relatively uniform movement in step b to zero according to a preset nonlinear decay curve; and gradually adjusting the power configuration and working frequency of the induction heating field from a long-range heating mode suitable for directional solidification to a narrow-range heating mode suitable for the narrow melt zone while the speed is decaying, and the gradual adjustment of the power configuration and working frequency and the decay of the speed are set as a preset cooperative control program for maintaining the dynamic balance of the solid-liquid interface during the shape transition.

3. The method according to claim 1, wherein the method is characterized by, In step b, the load impedance of the induction heating field is further monitored in real time, and at least one working parameter of the induction heating field is dynamically adjusted according to the monitoring result of the load impedance to maintain the thermal field shape of the induction heating field constant during the directional solidification.

4. The method according to claim 3, wherein the method is characterized by, The working parameter comprises the working frequency of the induction heating field.

5. The method according to claim 1, wherein the method is characterized by, The vertical cold crucible comprises a circulating cooling medium, and in step b, the temperature difference between the inlet temperature and the outlet temperature of the circulating cooling medium is further monitored in real time, and the power of the induction heating field is dynamically adjusted according to the monitoring result of the temperature difference to maintain the thickness of the solid aluminum shell stable.

6. The method according to claim 1, wherein the method is characterized by, Step f comprises: before stopping heating, performing a stress relief annealing treatment on the impurity enrichment zone and the adjacent part of the solidified rod below the impurity enrichment zone by using the induction heating field; and guiding the impurity enrichment zone to asymmetrically solidify by asymmetrically cooling the vertical cold crucible.

7. The method according to claim 1, wherein the method is characterized by, In step b, further comprising: continuously monitoring one or more electrical operating parameters of the induction heating field, and the preset threshold in step c is a critical characteristic threshold of the electrical operating parameters, which corresponds to the impurity concentration of the impurity-enriched zone of the melt top end.

8. The method according to claim 7, wherein the method is characterized by, The electrical operating parameter comprises a power factor, and the determination rule of the critical characteristic threshold comprises: calculating a relative change rate of a real-time monitoring value of the power factor and an initial value thereof at the beginning of step b, and comparing the relative change rate with a preset change rate threshold corresponding to the critical point of the component supercooling, wherein the relative change rate By The calculation is obtained, The real-time monitoring value is the power factor, The initial value is the power factor, The preset change rate threshold.

9. The method according to claim 1, wherein the method is characterized by, In step d, adjusting the induction heating field comprises changing the power configuration or working frequency of the induction coil to transform the induction heating field from a long-range heating field suitable for directional solidification to a narrow-range heating field suitable for zone extraction.

10. The method according to claim 1, wherein the method is characterized by, The rapid reciprocating movement in the first extraction stage and the unidirectional movement in the second extraction stage in step e are both within the range of the total length of the impurity-enriched zone.

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