Aluminum manufacturing method

Abstract

The present invention provides a method for producing aluminum by using fractional crystallization to enrich peritectic elements. [Solution] This is a method for producing aluminum by refining using fractional crystallization, and temperature control is performed so that the rate of increase in the solid fraction when the solid fraction is less than 10% is smaller than the rate of increase in the solid fraction when the solid fraction is 10% or more.

Description

[Technical Field]

[0001] The present invention relates to a method for producing aluminum, and more particularly to a method for producing aluminum in which the purity of aluminum is improved by fractional crystallization. [Background technology]

[0002] Refining methods for improving the purity of aluminum can be broadly divided into the "three-layer electrolysis method," which uses electrolysis, and the "segregation method," which uses the phenomenon in which solute elements move between the solid and liquid phases as the metal solidifies.

[0003] Furthermore, the segregation methods widely known are "unidirectional solidification," "rotary refining," and "fractional crystallization," and among these, "fractional crystallization" is known to have high purification efficiency and high mass production efficiency.

[0004] When aluminum with a purity of 99.70 to 99.90 wt% is used as the raw material and refined to a higher purity using fractional crystallization, the raw material generally contains several tens of wtppm of titanium, vanadium, chromium, and zirconium, which are elements that exhibit a peritectic reaction with aluminum in the phase diagram (hereinafter referred to as "peritectic elements").

[0005] Furthermore, since peritectic elements have the characteristic of concentrating in the solid phase during solidification, they end up being incorporated into the target ultra-high purity ingot when aluminum is refined by fractional crystallization.

[0006] Therefore, when an aluminum raw material containing titanium, vanadium, chromium, and zirconium is refined by fractional crystallization, boron or an aluminum master alloy containing boron, or titanium or an aluminum master alloy containing titanium is added to a melting furnace before the refinement (particularly before or after melting the raw material) (see, for example, Patent Document 1).

[0007] Here, boron or the like is added, completely dissolved, and then stirred and allowed to stand, thereby accelerating the reaction between boron and titanium, vanadium, chromium, or zirconium, and allowing high-melting-point intermetallic compounds such as titanium diboride and vanadium diboride to grow coarsely in the molten aluminum (hereinafter, this type of treatment is referred to as "boron treatment").

[0008] Intermetallic compounds formed by the boron treatment are removed by allowing them to settle in the melting furnace, or by filtering when transferring from the melting furnace to the refining furnace, or by applying both of these methods, and the titanium, vanadium, chromium, and zirconium concentrations in the refining furnace are reduced to about 1.0 wtppm. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-173718 Summary of the Invention [Problem to be solved by the invention]

[0010] However, even if the content of titanium, vanadium, chromium, and zirconium is reduced to about 1.0 wtppm by boron treatment, these elements are peritectic elements, and therefore when refined by fractional crystallization, they will be widely distributed, centered on the portion that solidified in the early stage of the refinement process.

[0011] As a result, in order to make the content (concentration) of peritectic elements in the ultra-high purity ingot obtained by fractional crystallization equal to that of the aluminum being refined, it was necessary to remove much of the part that became solid early in the refining process (the lower region), and the yield per refining process could not be said to be good.

[0012] The present invention has been made in view of the above points, and has as its object to provide a method for manufacturing aluminum that can improve yield. [Means for solving the problem]

[0013] As a result of various studies by the inventors of the present application, the following findings have been reached.

[0014] That is, in a refining facility using a fractional crystallization method such as that disclosed in JP-A-2002-534603, the increase in the solid phase ratio is controlled by changing the heater output according to the height of the crystal (solid phase) in the crucible. Then, when the amount of aluminum crystal (solid) produced exceeds a predetermined control value (crystal height at any given time), the heater output is increased.

[0015] Specifically, as shown in Figure 1(a), the height H of the solidified aluminum crystal 2 in the crucible 1 is monitored, and if the actual measured value exceeds the control value, the heater output is increased to reduce the crystal height by the excess amount (to re-melt the solid phase).

[0016] The amount of aluminum crystals produced depends on the cooling by the piston (corresponding to the "rod" in JP 2002-534603 A), does not fluctuate greatly, and is an almost constant amount. Therefore, as the slope of the control value (rate of increase in solid fraction) becomes smaller, the number of melting events due to an increase in heater output increases.

[0017] Incidentally, Figure 2 shows the relationship between "crystal height (curve)" and "control value (straight line)." As is clear from Figure 2, the increase in crystal height is not affected by the slope of the control value (rate of increase in solid fraction). That is, even when comparing "when the slope of the control value is small (see symbol A in Figure 2)" with "when the slope of the control value is large (see symbol B in Figure 2)," the increase in crystal height is the same, and it can be seen that crystals are generated at the same frequency and in the same amount regardless of the slope of the control value (rate of increase in solid fraction).

[0018] Therefore, the smaller the slope of the control value (rate of increase in solid fraction), the greater the number of solidifications (regions where the curve rises) and meltings (regions where the curve falls) until a certain solid fraction (for example, 20% solid fraction) is reached.

[0019] Here, "solidification" progresses through the formation of a nucleus, which is the starting point, and the growth of crystals around it. During the crystal growth process, peritectic elements are incorporated into the solid phase of aluminum (the peritectic elements are concentrated), and the concentration of peritectic elements in the molten metal around the crystal decreases.

[0020] As the crystal growth progresses in this way, a phenomenon occurs in which the concentration of peritectic elements is high in the center of the aluminum solid phase, where the crystal growth originated, and decreases toward the outside.

[0021] Furthermore, the solid phase obtained here (a solid phase crystallized on the piston, in which peritectic elements are concentrated) falls to the bottom of the crucible and undergoes repeated melting and solidification.

[0022] That is, in "melting," the surface of the solid phase, which is enriched with peritectic elements, melts, and the concentration of the surrounding liquid phase increases, but the effect is negligible because the concentration of peritectic elements on the surface of the solid phase is relatively low.

[0023] The liquid phase produced by melting will mix with the bulk liquid phase, but the change in concentration of the bulk liquid phase is slight and negligible (even if the liquid phase produced by melting does not mix sufficiently with the bulk liquid phase and remains around the solid phase, the peritectic elements will be incorporated during the "solidification" process described below).

[0024] In addition, during "solidification," the solid phase absorbs peritectic elements from the liquid phase and becomes concentrated. At this time, the concentration of the liquid phase around the solid phase decreases, and this liquid phase with decreased concentration (dilute liquid phase) mixes with the bulk liquid phase, lowering the concentration of the bulk liquid phase.

[0025] In this way, by repeating "solidification" and "melting", the enrichment of peritectic elements in the solid phase progresses. As the number of repetitions increases, the enrichment of peritectic elements progresses further.

[0026] [1] Based on such findings, in order to achieve the above object, the present invention provides a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined and crystallizing aluminum crystals on the surface of the pressing means, and a pressing step of pressing the aluminum crystals deposited at the bottom of the crucible with the pressing means. It is a method for producing aluminum that refines the aluminum to be refined using the fractional crystallization method. When the ratio of the aluminum crystals in the region within the crucible, above the lowest point reached by the pressing means, is defined as the solid phase ratio, the rate of increase of the solid phase ratio when the solid phase ratio is less than a predetermined threshold X (where 0 < X ≤ 13.5%) is made smaller than the rate of increase of the solid phase ratio when the solid phase ratio is greater than or equal to the threshold X.

[0027] Here, by making the "rate of increase of the solid phase ratio when the solid phase ratio is less than the threshold X (where 0 < X ≤ 13.5%)" smaller than the "rate of increase of the solid phase ratio when the solid phase ratio is greater than or equal to the threshold X", that is, by making the "slope of the control value when the solid phase ratio is less than the threshold X" smaller than the "slope of the control value when the solid phase ratio is greater than or equal to the threshold X", the enrichment of peritectic elements will progress further until the solid phase ratio reaches the threshold X (for example, 10%).

[0028] Also, the upper limit of the threshold X is set to 13.5% because by reducing the rate of increase of the solid phase ratio until the solid phase ratio reaches 13.5% and repeating solidification and melting, most of the peritectic elements in the aluminum to be refined are incorporated into the solid phase.

[0029] In addition, even if the rate of increase in the solid fraction is small (the slope of the control value is small) throughout the entire refining process, the enrichment of the peritectic elements can be realized. However, it takes an extremely long time to enrich the peritectic elements (as shown in Figure 2, when the slope of the control value is small, it takes a long time to reach a certain solid fraction), resulting in poor productivity.

[0030] Therefore, in the aluminum production method of the present invention, the rate of increase of the solid fraction is varied, such that it is small when the "solid fraction is less than threshold value X" and large when the "solid fraction is equal to or greater than threshold value X," thereby realizing the concentration of peritectic elements and avoiding the refining process from taking an excessively long time.

[0031] [2] Furthermore, in the aluminum production method of the present invention, the “number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction” when the solid fraction is less than the threshold value X can be made larger than the “number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction” when the solid fraction is equal to or greater than the threshold value X.

[0032] Furthermore, by increasing the "number of repetitions per unit increase in solid fraction," the number of repetitions required until the solid fraction reaches a threshold value X (e.g., 10%) can be increased, resulting in further enrichment of peritectic elements.

[0033] In addition, enrichment of peritectic elements can be achieved even when the number of repetitions per unit increase in solid fraction is large throughout the entire refining process. However, a large number of repetitions per unit increase in solid fraction means that the slope of the control value is small (see Figure 2), which requires an extremely long time and reduces productivity.

[0034] [3] Further, in order to achieve the above object, the present invention includes a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined and crystallizing aluminum crystals on the surface of the pressing means, and a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means. This is a method for producing aluminum in which the aluminum to be refined is refined using the fractional crystallization method. When the ratio of the aluminum crystals in the region within the crucible, above the lowest point reached by the pressing means, is defined as the solid phase ratio, when the solid phase ratio is less than a predetermined threshold X (where 0 < X ≤ 13.5%), the number of repetitions of solidification and melting of the aluminum to be refined per unit increase in the solid phase ratio is made larger than the number of repetitions per unit increase in the solid phase ratio when the solid phase ratio is greater than or equal to the threshold X.

[0035] Here, by making the "number of repetitions of solidification and melting of the aluminum to be refined per unit increase in the solid phase ratio when the solid phase ratio is less than the threshold X" larger than the "number of repetitions of solidification and melting of the aluminum to be refined per unit increase in the solid phase ratio when the solid phase ratio is greater than or equal to the threshold X", the number of repetitions until the solid phase ratio reaches the threshold X (for example, 10%) can be increased, and the enrichment of peritectic elements will progress more.

[0036] Also, the upper limit of the threshold X is set to 13.5% because by increasing the number of repetitions of solidification and melting per unit increase in the solid phase ratio up to 13.5% of the solid phase ratio, most of the peritectic elements in the aluminum to be refined are incorporated into the solid phase.

[0037] In addition, in all steps of the purification, when it is said that "the number of repetitions per unit increase in the solid phase ratio is large", as described in [2] above, it will require an extremely long time and the productivity will be poor. Therefore, in the method for producing aluminum of the present invention, by varying the number of repetitions per unit increase in the solid phase ratio such that it is large when the "solid phase ratio is less than the threshold X" and small when the "solid phase ratio is greater than or equal to the threshold X", enrichment of peritectic elements is achieved while avoiding the purification process from becoming excessively long.

[0038] [4] In addition, in the aluminum manufacturing method of the present invention, when the solid fraction is less than the threshold value X, the rate of increase of the solid fraction vrs (% / min) is 0 <vrs≦2.25×10 -2 may be satisfied.

[0039] Here, if vrs<0, it means that the solid fraction decreases over time, and the part with a high concentration of peritectic elements (the central part of the solid phase) also melts, so concentration does not progress and no effect can be expected. Furthermore, when vrs=0, the same amount of aluminum crystals as the crystallized aluminum crystals melts (the solid fraction does not increase), and the refining process never ends. Furthermore, vrs>2.25×10 -2 In the case of (1), the rate of increase of the solid fraction is too high, the number of times of repetition of solidification and melting is insufficient, and the concentration of the peritectic elements does not progress sufficiently.

[0040] On the other hand, 0 <vrs≦2.25×10 -2 When the condition (x) is satisfied, the enrichment of the peritectic elements can proceed sufficiently until the solid fraction reaches the threshold value X.

[0041] [5] Furthermore, in order to achieve the above object, the present invention provides a method for producing aluminum by using a fractional crystallization method, comprising: a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined and crystallizing aluminum crystals on the surface of the pressing means; and a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means, wherein the solid fraction is defined as the proportion of aluminum crystals in a region within the crucible above the lowest point reached by the pressing means, and the average rate of increase in the solid fraction up to a solid fraction of 13.5% is made smaller than the average rate of increase in the solid fraction after the solid fraction is 13.5%.

[0042] Here, by making the "average rate of increase in the solid fraction up to a solid fraction of 13.5%" smaller than the "average rate of increase in the solid fraction after a solid fraction of 13.5%", i.e., by making the "average value of the slope of the control value up to a solid fraction of 13.5%" smaller than the "average value of the slope of the control value after a solid fraction of 13.5%", the enrichment of the peritectic elements will progress further until the solid fraction reaches 13.5%.

[0043] The reason for using a "solid fraction of 13.5%" as the standard is that the average rate of increase in the solid fraction up to 13.5% is slowed down, and by repeating solidification and melting, most of the peritectic elements in the aluminum to be refined are incorporated into the solid phase.

[0044] [6] Furthermore, in the aluminum production method of the present invention, the average number of repetitions of solidification and melting of the aluminum to be refined per unit increase in solid fraction up to a solid fraction of 13.5% can be made larger than the average number of repetitions per unit increase in solid fraction after a solid fraction of 13.5%.

[0045] Furthermore, since the "average number of repetitions per unit increase in solid fraction" is large, the number of repetitions required to reach a solid fraction of 13.5% can be increased, resulting in further enrichment of peritectic elements.

[0046] [7] Furthermore, in order to achieve the above object, the present invention provides a method for producing aluminum by using a fractional crystallization method, comprising: a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined and crystallizing aluminum crystals on the surface of the pressing means; and a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means, wherein, if the solid fraction is defined as the proportion of aluminum crystals in a region within the crucible that is above the lowest point reached by the pressing means, the average number of repetitions of solidification and melting of the aluminum to be refined per unit increase in solid fraction up to a solid fraction of 13.5% is made greater than the average number of repetitions per unit increase in solid fraction above 13.5%.

[0047] Here, by making the "average number of times that the aluminum to be refined is solidified and melted per unit increase in solid fraction up to a solid fraction of 13.5%" larger than the "average number of times that the aluminum to be refined is solidified and melted per unit increase in solid fraction after a solid fraction of 13.5%", the number of times that the aluminum to be refined is solidified and melted can be increased until the solid fraction reaches 13.5%, and the concentration of peritectic elements will progress further.

[0048] The reason why a "solid fraction of 13.5%" is used as the standard is that by increasing the average number of times the aluminum undergoes solidification and melting per unit increase in solid fraction up to a solid fraction of 13.5%, most of the peritectic elements in the aluminum to be refined are incorporated into the solid phase.

[0049] [8] In addition, in the aluminum production method of the present invention, the purity of the aluminum to be refined can be 99.70 wt% or more and 99.9997 wt% or less, and the content of each peritectic element, which is an element that exhibits a peritectic reaction in the phase diagram with aluminum, can be less than 1.0 wtppm.

[0050] Even if the aluminum to be refined has a purity of 99.70 wt% or more but less than 99.9997 wt% and the content of each peritectic element is less than 1.0 wtppm, it can be refined with a high yield by the aluminum production method of the present invention.

[0051] [9] In addition, in the aluminum production method of the present invention, the content of peritectic elements can be reduced to less than 1.0 wtppm by adding boron to the molten aluminum to generate intermetallic compounds in the molten aluminum, and then removing the intermetallic compounds.

[0052] According to the method for producing aluminum of the present invention, even boron-treated aluminum can be refined with a high yield. [Effects of the Invention]

[0053] In the present invention, in a method for producing aluminum by refining using fractional crystallization, it is possible to concentrate peritectic elements and achieve a high yield. [Brief explanation of the drawings]

[0054] [Figure 1] FIG. 2 is a schematic diagram for explaining control of the solid phase ratio in the fractional crystallization method. [Figure 2] FIG. 10 is a schematic diagram for explaining the relationship between the crystal height and the control value. [Figure 3] 1 is a schematic process diagram illustrating an example of a method for producing aluminum to which the present invention is applied. [Figure 4] FIG. 1 is a schematic diagram illustrating a refining step in an example of a method for producing aluminum to which the present invention is applied. [Figure 5] FIG. 2 is a schematic diagram for explaining the rate of increase in the solid phase fraction in an example of a method for producing aluminum to which the present invention is applied. [Figure 6] FIG. 10 is a schematic diagram for explaining a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0055] Hereinafter, a mode for carrying out the invention (hereinafter referred to as "embodiment") will be described.

[0056] In this embodiment, first, aluminum raw materials (aluminum ingots, scrap aluminum alloy products, etc.) are melted in a melting furnace (see reference symbol S11 in FIG. 3).

[0057] Next, the molten metal is poured into a refining furnace, and a predetermined metal (for example, an Al-Cu based master alloy) is added to adjust the composition (see S12 in FIG. 3). If the concentration of each peritectic element contained in the aluminum to be refined is 1.0 wtppm or more, boron treatment may be carried out.

[0058] In this embodiment, the purity of the aluminum poured into the refining furnace (aluminum to be refined) is 99.9995 wt % or more, and contains 0.10 ppm of titanium, 0.03 ppm of vanadium, and 0.02 ppm of chromium.

[0059] The temperature of the refining furnace is controlled at the melting point of aluminum, and by adding and releasing heat equivalent to the latent heat of solidification, the melting and solidification process is repeated, gradually increasing the proportion of solids.

[0060] Specifically, as shown in Figure 4(a), a graphite piston 23 (here, the piston 23 is an example of a pressing means and also functions as a cooling body for the aluminum 21 to be refined) is immersed in a graphite crucible 22 into which the aluminum 21 to be refined has been poured, thereby causing aluminum crystals 24 to crystallize on the surface of the piston 23.

[0061] Next, as shown in FIG. 4( b ), the piston 23 is moved up and down, and the graphite ring 25 surrounding the piston 23 scrapes the crystallized mass of aluminum crystals 24 (aluminum crystal mass) down to the bottom of the crucible 22 .

[0062] Next, as shown in Figure 4(c), the aluminum crystal block deposited at the bottom of the crucible 22 is compacted (pressed) from above with the piston 23 (more specifically, the bottom of the tip of the piston 23), and the impurity-containing layer near the surface of the aluminum crystal block is melted by heating with a heater, and the molten metal containing the impurities is discharged.

[0063] By repeating this series of actions, such as "crystallization of aluminum crystals (Figure 4(a))," "scraping off aluminum crystal lumps (Figure 4(b))," and "tamping down aluminum crystal lumps (Figure 4(c))," the proportion of solids is gradually increased.

[0064] In this embodiment, first, the rate of increase of the solid fraction vrs1 is 2.22 × 10 -2 (% / min) and purification is performed (see symbol S13 in FIG. 3). The rate of increase of the solid fraction vrs1 is 2.22 × 10 -2 (% / min) until the solid fraction reaches 10% (see symbol a in FIG. 5).

[0065] Here, the "solid fraction" means the ratio of the height of the aluminum crystal excluding the region from the "bottom of the crucible 22" to the "lowest point reached by the piston 23." That is, when the lowest point reached by the piston 23 is used as a reference, the molten metal surface height during pouring is "hmax (mm)" and the aluminum crystal height at any time is "ht (mm)", then: This will be represented as JPEG0007732125000002.jpg19108 (see Figure 4).

[0066] Here, graphs 1 to 3 show the analysis results of peritectic elements when refining was performed without changing the rate of increase in the solid fraction (graph 1 is Cr, graph 2 is Ti, and graph 3 is V).

[0067] [Graph 1] JPEG0007732125000003.jpg76127

[0068] [Graph 2] JPEG0007732125000004.jpg76127

[0069] [Graph 3] JPEG0007732125000005.jpg76127

[0070] Graph 1 shows that Cr is concentrated in the region where the solid phase fraction is 13.5% or less, graph 2 shows that Ti is concentrated in the region where the solid phase fraction is 8.1% or less, and graph 3 shows that V is concentrated in the region where the solid phase fraction is 13.5% or less. That is, it can be seen from graphs 1 to 3 that most of the peritectic elements (Cr, Ti, V) are concentrated in the region where the solid fraction is 13.5% or less.

[0071] Next, when the solid fraction reaches 10%, the rate of increase of the solid fraction vrs2 becomes 4.77 × 10-2 (% / min), and purification is performed (see symbol S14 in FIG. 3). The rate of increase of the solid fraction vrs2 is 4.77 × 10 -2 The refinement at (% / min) means that the slope of the control value is steeper compared to the refinement before the solid fraction reaches 10% (see symbol b in FIG. 5).

[0072] Here, not all of the molten metal in the refining furnace is solidified, but a certain percentage of the molten metal is left and the remaining molten metal (residual metal) is discharged (see symbol S15 in FIG. 3). The reason why the residual molten metal is discharged and separated is that as the solidification of the molten metal progresses, the eutectic elements move from the solid phase to the liquid phase, and the residual molten metal contains eutectic elements at high concentrations.

[0073] Next, by cutting the region that solidified in the early stage of the purification process (lower region) and the region that solidified in the later stage of the purification process (upper region) (see symbol S16 in Figure 3), an ultra-high purity block can be obtained (see symbol S17 in Figure 3).

[0074] The reason for cutting the "region that solidified in the early stage of the refining process (lower region)" is that as the solidification of the molten metal progresses, peritectic elements move from the liquid phase to the solid phase, and the peritectic elements segregate in the lower region. Similarly, the reason for cutting the "region that solidified in the later stage of the refining process (upper region)" is that as the solidification of the molten metal progresses, the eutectic elements move from the solid phase to the liquid phase, and the eutectic elements segregate in the upper region.

[0075] [effect] In this embodiment, the rate of increase of the solid fraction when the solid fraction is less than 10% (vrs1 = 2.22 × 10 -2 ) is the rate of increase of the solid fraction when the solid fraction is 10% or more (vrs2=4.77×10 -2 ) to control the crystal height, and the solidification and melting of aluminum is repeated many times until the solid fraction reaches less than 10%, allowing the concentration of peritectic elements to progress sufficiently.

[0076] Therefore, even if the cutting allowance of the "region (lower region) solidified in the early stage of the refining process" is minimized (for example, about 12.5%), the content of peritectic elements can be reduced.

[0077] More specifically, when trying to reduce the peritectic element content using conventional technology (temperature control that does not change the rate of increase in the solid fraction), it becomes necessary to cut the lower region extensively, which reduces the yield. Furthermore, if the cutting allowance of the upper region is reduced in order to prevent the yield reduction, the concentration of the eutectic element increases.

[0078] In contrast to this, in the present embodiment, the peritectic elements can be concentrated in a narrower range in the lower region, so that it is possible to lower the concentrations of both the peritectic elements and the eutectic elements while ensuring a high yield.

[0079] The purity of the ultra-high purity ingot obtained in this embodiment is 99.9995 wt%, and it contains 0.07 ppm of titanium, 0.03 ppm of vanadium, and 0.02 ppm of chromium, thereby realizing a reduction in the content of peritectic elements.

[0080] [Variations] In the above embodiment, the control value is set to increase the rate of increase of the solid fraction when the solid fraction reaches 10%, but the control value does not necessarily have to be set to 10%. For example, as shown in FIG. 6, the rate of increase of the solid fraction may be increased when the solid fraction reaches 13%.

[0081] [Example] Examples will be described below. In this example, refining is performed using test pieces (1) to (8) shown in Tables 1-1 and 1-2. In this example, evaluations are performed for "the case where the threshold value X is fixed" and "the case where the rate of increase in the solid fraction vrs1 is fixed."

[0082] [Table 1-1] JPEG0007732125000006.jpg49162

[0083] [Table 1-2] JPEG0007732125000007.jpg49162

[0084] (Evaluation when threshold X is fixed) For the above-mentioned test pieces (1), (2), (5), (6), and (7), the threshold value X was fixed at 2.7%, and the "rate of increase in the solid fraction until the threshold value X (2.7%) was reached, vrs1 (% / min)" and the "rate of increase in the solid fraction after the threshold value X (2.7%) was reached, vrs2 (% / min)" were controlled as shown in Table 2, and refinement was performed.

[0085] [Table 2] JPEG0007732125000008.jpg32162

[0086] (Evaluation when the rate of increase of the solid fraction vrs1 is fixed) For the above-mentioned test pieces (1), (2), (3), (4), and (8), the threshold value X is set to the value shown in Table 3, and the rate of increase of the solid fraction vrs1 until the threshold value X is reached is set to 7.51 × 10 -3 The "rate of increase in the solid fraction after reaching the threshold value X vrs2 (% / min)" was controlled as shown in Table 3, and purification was performed.

[0087] [Table 3] JPEG0007732125000009.jpg32162

[0088] For the test pieces (1) to (8) that were refined under the control shown in Tables 2 and 3 above, the contents (wtppm) of peritectic elements before and after refinement are shown in Tables 4-1 and 4-2 (the analytical values ​​of the contents are rounded to two decimal places).

[0089] Incidentally, when the impurity concentration of aluminum before purification (i.e., the impurity concentration of the aluminum to be purified) is C0 (wtppm) and the impurity concentration of aluminum after purification (i.e., the impurity concentration of the ultra-high purity ingot) is Cf (wtppm), the purification efficiency can be obtained as follows. Tables 4-1 and 4-2 also show the purification efficiency of Ti (the purification efficiency is rounded to the second decimal place). JPEG0007732125000010.jpg14108

[0090] [Table 4-1] JPEG0007732125000011.jpg36170

[0091] [Table 4-2] JPEG0007732125000012.jpg37170

[0092] From Table 4-1, in the case of test piece (1) (when the temperature control does not change the rate of increase of the solid fraction), the concentration of Ti after purification is 2.16 times or more that of the aluminum to be purified, and the purification efficiency was -116.7%. In contrast, in the case of test pieces (2) to (8) (when vrs1 < vrs2), it can be seen that the concentration of Ti after purification is reduced (in the case of test pieces (2) and (4)) or the enrichment is suppressed (in the case of test pieces (3), (5), (6), (7), and (8)) compared to the aluminum to be purified.

[0093] Also, for test pieces (1), (2), (5), (6), and (7), the relationship between "vrs1" and "the purification efficiency of Ti" is shown in Graph 4.

[0094] [Graph 4] JPEG0007732125000013.jpg97127

[0095] From Graph 4, "vrs1 ≦ 3.00×10 -2When the condition "(% / min)" is satisfied, the Ti purification efficiency is "-75.0% or more," which is a significant improvement compared to the Ti purification efficiency of "-116.7%" in the case of test piece (1) (temperature control in which the rate of increase in the solid fraction is not changed).

[0096] Furthermore, graph 5 shows the relationship between the "threshold X" and the "Ti purification efficiency" for test pieces (1), (2), (3), (4), and (8).

[0097] [Graph 5] JPEG0007732125000014.jpg85127

[0098] From Graph 5, vrs1 is 7.51 x 10 -3 Under the condition of (% / min), when "threshold X≧1.4(%)" is satisfied, the Ti purification efficiency is "-20% or more," which is found to be a significant improvement compared to the Ti purification efficiency of "-116.7%" in the case of test piece (1) (temperature control in which the rate of increase in the solid fraction is not changed). Graph 2 also shows that the effect of improving refining efficiency reaches a plateau when "threshold X≧2.7(%)".

[0099] (Considerations on elements other than Ti) The equilibrium distribution coefficient k of Ti to aluminum in the region where the aluminum concentration is 99.9 wt% or more (i.e., the region where the solute concentration is less than 0.1 wt%) Ti " "The equilibrium distribution coefficient k of V to aluminum V " "The equilibrium distribution coefficient k of Cr to aluminum Cr " was experimentally determined, and k Ti =7.0, k V =1.5, k Cr = 1.5.

[0100] In addition, when not limited to the sparse region, k Ti =6.97, k V =5.97, k Cr= 1.50, but since this example is performed in the dilute region, here, k Ti =7.0, k V =1.5, k Cr =1.5 is adopted.

[0101] Here, among the test pieces (2) to (8), for the test piece (2) which had the smallest vrs1 and was closest to the equilibrium state, the "amounts of Ti, V, and Cr contained in the aluminum to be refined" and the "amounts of Ti, V, and Cr contained in the region solidified at the beginning of the refining process (lower region)" are as shown in Table 5.

[0102] [Table 5] JPEG0007732125000015.jpg22170

[0103] Table 5 shows that there is a positive correlation between the "enrichment ratio" and the "equilibrium distribution coefficient k." In other words, the larger the equilibrium distribution coefficient of an element, the more it is concentrated in the lower region. Therefore, it is believed that the effects of the present invention are effective not only for Ti but also for other peritectic elements. [Explanation of symbols]

[0104] 1 crucible 2. Aluminum crystals 21 Refined aluminum 22 Crucible 23 Piston 24 Aluminum Crystal 25 Graphite Ring

Claims

1. a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined, and crystallizing aluminum crystals on the surface of the pressing means; a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means; The aluminum to be refined is refined by a fractional crystallization method, When the ratio of the aluminum crystals in the region in the crucible that is above the lowest point reached by the pressing means is defined as a solid phase ratio, The rate of increase of the solid fraction when the solid fraction is less than a predetermined threshold value X (where 0<X≦13.5%) is set to be smaller than the rate of increase of the solid fraction when the solid fraction is equal to or greater than the threshold value X. A method for producing aluminum.

2. The number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction when the solid fraction is less than the threshold value X is made larger than the number of times the solid fraction is solidified and melted per unit increase in solid fraction when the solid fraction is equal to or greater than the threshold value X. The method for producing aluminum according to claim 1.

3. a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined, and crystallizing aluminum crystals on the surface of the pressing means; a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means; The aluminum to be refined is refined by a fractional crystallization method, When the ratio of the aluminum crystals in the region in the crucible that is above the lowest point reached by the pressing means is defined as a solid phase ratio, The number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction when the solid fraction is less than a predetermined threshold value X (where 0<X≦13.5%) is greater than the number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction when the solid fraction is equal to or greater than the threshold value X. A method for producing aluminum.

4. The rate of increase vrs (% / min) of the solid fraction when the solid fraction is less than the threshold value X is 0<vrs≦3.00×10 －２ fulfill The method for producing aluminum according to claim 1 or 2.

5. a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined, and crystallizing aluminum crystals on the surface of the pressing means; a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means; The aluminum to be refined is refined by a fractional crystallization method, When the ratio of the aluminum crystals in the region in the crucible that is above the lowest point reached by the pressing means is defined as a solid phase ratio, The average rate of increase of the solid fraction up to the solid fraction of 13.5% is made smaller than the average rate of increase of the solid fraction after the solid fraction of 13.5% A method for producing aluminum.

6. The average number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction up to the solid fraction of 13.5% is made larger than the average number of times the aluminum is solidified and melted per unit increase in solid fraction after the solid fraction of 13.5%. The method for producing aluminum according to claim 5.

7. a crystallization step of immersing a predetermined pressing means in a crucible containing liquid aluminum to be refined, and crystallizing aluminum crystals on the surface of the pressing means; a pressing step of pressing the aluminum crystals deposited on the bottom of the crucible with the pressing means; The aluminum to be refined is refined by a fractional crystallization method, When the ratio of the aluminum crystals in the region in the crucible that is above the lowest point reached by the pressing means is defined as a solid phase ratio, The average number of times the aluminum to be refined is solidified and melted per unit increase in solid fraction up to the solid fraction of 13.5% is made larger than the average number of times the aluminum is solidified and melted per unit increase in solid fraction after the solid fraction of 13.5%. A method for producing aluminum.

8. The aluminum to be refined is Its purity is 99.70 wt% or more and 99.9997 wt% or less, The content of each peritectic element, which is an element that exhibits a peritectic reaction in the phase diagram with aluminum, is less than 1.0 wtppm. The method for producing aluminum according to claim 1, claim 2, claim 3, claim 5, claim 6 or claim 7.

9. The aluminum to be refined is Boron is added to the molten metal to form an intermetallic compound in the molten metal, and then the intermetallic compound is removed, thereby making the content of the peritectic element less than 1.0 wtppm. The method for producing aluminum according to claim 8.

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