Ultrahigh-purity oxygen-free copper and preparation method thereof

Abstract

The invention relates to ultra-pure oxygen-free copper and a preparation method thereof.The preparation method comprises the steps that firstly, cathode copper is added into a graphite crucible of an electric vacuum high-temperature purification furnace to be melted into molten copper; secondly, after the molten copper is subjected to standing and heat preservation, a heating cover of the electric vacuum high-temperature purification furnace is slowly lifted upwards, and the molten copper in the graphite crucible is slowly solidified from the bottom till the molten copper is completely solidified; and then, the solidified cast ingot is taken out, the two ends of the cast ingot are cut off, the remaining cast ingot obtained after cutting is conducted repeatedly according to the first two steps, and the ultra-pure oxygen-free copper cast ingot is obtained after circulation is conducted for 3-5 times. The purity of the prepared ultra-pure oxygen-free copper is 5N-6N, the oxygen content is less than 2ppm, and the hydrogen content is less than 1ppm. The method is simple and reliable to operate, cast ingots with different copper contents can be prepared according to different application directions, and meanwhile, the oxygen content is also controlled at an extremely low level due to the fact that the cast ingots are extremely low in gas content and good in compactness.

Description

Technical Field

[0001] The invention belongs to the field of nonferrous metal smelting and processing, and specifically relates to ultra-high purity oxygen-free copper and a preparation method thereof. Background Art

[0002] Oxygen-free copper refers to pure copper that does not contain oxygen or any deoxidizer residue. But in fact, it still contains very trace amounts of oxygen and some impurities. According to the standard, oxygen content is no more than 0.003%, total impurity content is no more than 0.05%, and copper purity is greater than 99.95% to be oxygen-free copper.

[0003] Ultra-high purity oxygen-free copper is increasingly used in the current high-tech field, and people's requirements for the purity of ultra-high purity oxygen-free copper are also getting higher and higher. With the development of large-scale integrated circuits and large-size displays, the copper interconnects required for chips and displays are mostly plated on the circuit by magnetron sputtering, which has high requirements for copper purity, density and oxygen content. The purity of copper targets used for display leads must be 5N or above, and the purity of copper used for chips must be 6N or above; the oxygen content must be below 5ppm, and there must be no defects such as looseness and pores. However, the purity of conventional cathode copper is currently between 99.90-99.9935%, and the trace impurities in it will affect the properties of copper.

[0004] At present, conventional methods for purifying copper mainly include electron beam melting, suspension melting, zone melting and other methods. These methods have problems such as high production costs and difficulty in operation, which limit the development and application of large-scale integrated circuits and large-size displays. Summary of the invention

[0005] The purpose of the present invention is to provide an ultra-high purity oxygen-free copper and a preparation method thereof. The copper liquid is repeatedly purified by utilizing the fact that the contents of trace impurity elements in the solid phase precipitated and the remaining liquid phase are different during solidification under the condition close to equilibrium solidification in a vacuum environment, so as to achieve the purpose of preparing ultra-high purity oxygen-free copper.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a method for preparing ultra-high purity oxygen-free copper, comprising the following steps: S1, adding cathode copper into a graphite crucible of an electric vacuum high-temperature purification furnace, and melting the cathode copper into copper liquid in a vacuum and high-temperature environment of the electric vacuum high-temperature purification furnace; S2, after the copper liquid is left to stand and keep warm, the heating cover of the electric vacuum high-temperature purification furnace is slowly lifted upward, and the heating cover leaves the graphite crucible, so that the copper liquid in the graphite crucible slowly solidifies from the bottom until the solution is completely solidified; S3, taking out the solidified ingot, cutting off the two ends of the ingot, and repeating steps S1 and S2 for the remaining ingot after cutting, and obtaining an ultra-high purity oxygen-free copper ingot after 3-5 cycles.

[0007] In step S1, the purity of the cathode copper is 3N~5N.

[0008] In step S1, the vacuum degree of the electric vacuum high temperature purification furnace is less than 5Pa.

[0009] In step S2, the insulation temperature of the copper liquid is 1160~1300℃.

[0010] In step S2, the holding time of the copper liquid is 0.5-1.5h.

[0011] In step S2, the heating hood rises at a speed of 10-50 mm / h.

[0012] In step S3, the weight of the parts cut off at both ends of the ingot each time accounts for 1-25% of the weight of the ingot before cutting.

[0013] In step S3, the total length of the ingot cut off is 6-12 mm.

[0014] During the 3rd to 5th cycle, the lifting speed of the heating hood is gradually reduced, and the lifting speed of the heating hood in the last cycle is reduced by 7% to 14% compared with the lifting speed of the heating hood in the previous cycle.

[0015] The present invention further provides an ultra-high purity oxygen-free copper, which is prepared by the ultra-high purity oxygen-free copper preparation method, has a purity of 5N-6N, an oxygen content of less than 2ppm, and a hydrogen content of less than 1ppm.

[0016] The process principle of the present invention is: according to the solidification theory, the trace impurity elements (solutes) in the copper liquid will be redistributed in the process close to the equilibrium solidification, and the precipitated solid phase and the remaining liquid phase are not distributed equally. The solid phase solidified first contains impurities (solutes) with a higher concentration than the remaining liquid phase; when the solidification is almost over, the remaining liquid phase contains impurities (solutes) with a higher concentration than the solidified solid phase. The closer to the equilibrium solidification, the more obvious this difference is, and the lifting speed of the heating hood can be slowed down. The slower the lifting speed, the closer to the equilibrium solidification, so that the solute distribution difference is more obvious, which is more conducive to purification and obtaining high-purity oxygen-free copper. In the process of multiple cycles, the later the product purity is, the more difficult it is to further purify, so it is necessary to reduce the lifting speed of the heating hood one by one so as to be closer to the equilibrium solidification and achieve further purification and impurity removal.

[0017] According to the principle of solute distribution during the solidification process, if the solid phase in the initial solidification process and the part formed by the solidification of the remaining liquid phase when the solidification is almost completed can be removed separately during the solidification process of liquid pure copper, the impurities (solutes) of different properties in the copper liquid can be removed respectively in the initial solidification stage and the final solidification stage. Repeatedly, the purity of the copper can be continuously improved.

[0018] While the process of purification and impurity removal is close to equilibrium solidification, trace oxygen and carbon in the graphite crucible are reduced to generate CO under vacuum conditions, which escapes from the melt together with a very small amount of hydrogen and is removed, thereby obtaining ultra-high purity, high density, low-oxygen and oxygen-free copper.

[0019] The beneficial effects of the present invention are as follows: in the preparation process of the present invention, when the copper liquid is close to equilibrium solidification and crystallization under vacuum conditions, the metal impurity elements are enriched in the early stage of solidification and the final stage of final solidification due to directional solidification, and are removed in the process of cutting off the two ends of the ingot; at the same time, trace oxygen and carbon in graphite are reduced to CO, which escapes from the melt together with a very small amount of hydrogen and is removed; the process is continuously repeated, so as to achieve the purpose of preparing ultra-high purity and high-density low-oxygen oxygen-free copper, and the prepared oxygen-free copper has a purity of 99.999-99.9999% (5N-6N), an oxygen content of less than 2ppm, and a hydrogen content of less than 1ppm.

[0020] The preparation method is simple and reliable to operate. It can control the preparation of ingots with different copper contents according to different application directions. At the same time, due to the extremely low gas content of the ingots and good density, the oxygen content is also controlled at an extremely low level. The entire preparation process is carried out under conditions close to equilibrium solidification, and there will be no defects such as ingot inclusions that are fatal to the material. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with the embodiments, but they are not intended to limit the invention in any way.

[0022] Embodiment 1: A method for preparing ultra-high purity oxygen-free copper, comprising the following steps: S1. Place 500 kg of dry cathode copper with no oil or water on the surface, cut edges and a purity of 99.995% (4N5) neatly into a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and cover it with a heating cover to completely cover the graphite crucible; use a vacuum pump to evacuate the vacuum degree in the graphite crucible to below 3 Pa, and energize the resistance wire of the heating cover to heat it, and gradually increase the temperature in the furnace to 1160~1200℃, so that the cathode copper gradually melts into liquid in a vacuum and high-temperature environment; S2: Keep the temperature of the copper liquid at 1160-1200°C and keep it still for 0.5h; after the insulation process, slowly lift the heating cover at a moving speed of 20 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, thereby obtaining a copper ingot; S3: After taking out the copper ingot, cut off 4mm on both sides of the head and tail to remove the part with higher content of impurity elements; put the remaining copper ingot back into the electric vacuum high-temperature purification furnace, repeat the above S1 and S2 operations to continuously improve the purity of oxygen-free copper; after 3 cycles, ultra-high purity oxygen-free copper is finally obtained, with a purity of 99.9999% (6N), an oxygen content of 0.6ppm, and a hydrogen content of 0.5ppm.

[0023] It should be noted that during the three cycles of purification, the lifting speed of the heating hood was gradually reduced, and for the sake of operation, the lifting speed of the heating hood in each cycle was an integer value; specifically, the lifting speed of the heating hood in the second cycle of purification was 18 mm / h, which was 10% lower than the lifting speed of the heating hood in the first cycle of purification, which was 20 mm / h; in the third cycle of purification, the lifting speed of the heating hood was 16 mm / h, which was 11% lower than the second. In each cycle, except for the lifting speed of the heating hood, other process parameters remained unchanged.

[0024] Embodiment 2: A method for preparing ultra-high purity oxygen-free copper, comprising the following steps: S1. Place 450 kg of dry cathode copper with no oil or water on the surface, cut edges and a purity of 99.998% (4N8) neatly into a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and cover it with a heating cover to completely cover the graphite crucible; use a vacuum pump to evacuate the vacuum degree in the graphite crucible to below 3 Pa, and energize the resistance wire of the heating cover to heat it, and gradually increase the temperature in the furnace to 1200~1240℃, so that the cathode copper gradually melts into liquid in a vacuum and high-temperature environment; S2: Keep the temperature of the copper liquid at 1200-1240℃ and keep it still for 1h; after the insulation process, slowly lift the heating cover at a moving speed of 10 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, thereby obtaining a copper ingot; S3: After taking out the copper ingot, cut off 5mm on both sides to remove the part with higher impurity element content; put the remaining copper ingot back into the electric vacuum high-temperature purification furnace, repeat the above S1 and S2 operations to continuously improve the purity of oxygen-free copper; after 5 cycles, ultra-high purity oxygen-free copper is finally obtained, with a purity of 99.99992% (6N2), oxygen content <0.5ppm, and hydrogen content <0.5ppm.

[0025] It should be noted that during the five cycles of purification, the lifting speed of the heating hood was gradually reduced, and for the sake of operation, the lifting speed of the heating hood in each cycle was an integer value; specifically, the lifting speed of the heating hood in the second cycle of purification was 9 mm / h, which was 10% lower than the lifting speed of the heating hood in the first cycle of purification, which was 10 mm / h; in the third cycle of purification, the lifting speed of the heating hood was 8 mm / h, which was 11% lower than the second; in the fourth cycle of purification, the lifting speed of the heating hood was 7 mm / h, which was 13% lower than the third; in the fifth cycle of purification, the lifting speed of the heating hood was 6 mm / h, which was 14% lower than the fourth. In each cycle, except for the lifting speed of the heating hood, other process parameters remained unchanged.

[0026] Embodiment 3: A method for preparing ultra-high purity oxygen-free copper, comprising the following steps: S1. Place 580 kg of dry cathode copper with no oil or water on the surface, cut edges and a purity of 99.95% (3N5) ​​neatly into a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and cover it with a heating cover to completely cover the graphite crucible; use a vacuum pump to evacuate the vacuum degree in the graphite crucible to below 5 Pa, and energize the resistance wire of the heating cover to heat it, and gradually increase the temperature in the furnace to 1250~1300℃, so that the cathode copper gradually melts into liquid in a vacuum and high-temperature environment; S2: Keep the temperature of the copper liquid at 1250-1300℃ and keep it still for 1.5h; after the insulation process, slowly lift the heating cover at a moving speed of 30 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, thereby obtaining a copper ingot; S3: After taking out the copper ingot, cut off 6mm on both sides of the head and tail to remove the part with higher content of impurity elements; put the remaining copper ingot back into the electric vacuum high-temperature purification furnace, repeat the above S1 and S2 operations to continuously improve the purity of oxygen-free copper; after 4 cycles, ultra-high purity oxygen-free copper is finally obtained, with a purity of 99.9995% (5N5), an oxygen content of 0.8ppm, and a hydrogen content of 0.6ppm.

[0027] It should be noted that during the four cycles of purification, the lifting speed of the heating hood was gradually reduced, and for the sake of operation, the lifting speed of the heating hood in each cycle was an integer value; specifically, the lifting speed of the heating hood in the second cycle of purification was 27 mm / h, which was 10% lower than the lifting speed of the heating hood in the first cycle of purification, 30 mm / h; in the third cycle of purification, the lifting speed of the heating hood was 25 mm / h, which was 7% lower than the second; in the fourth cycle of purification, the lifting speed of the heating hood was 23 mm / h, which was 8% lower than the third. In each cycle, except for the lifting speed of the heating hood, other process parameters remained unchanged.

[0028] Embodiment 4: A method for preparing ultra-high purity oxygen-free copper, comprising the following steps: S1. Place 500 kg of dry cathode copper with no oil or water on the surface, cut edges and a purity of 99.95% (3N5) ​​neatly into a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and cover it with a heating cover to completely cover the graphite crucible; use a vacuum pump to evacuate the vacuum degree in the graphite crucible to below 4 Pa, and energize the resistance wire of the heating cover to heat it, and gradually increase the temperature in the furnace to 1180~1220℃, so that the cathode copper gradually melts into liquid in a vacuum and high-temperature environment; S2: Keep the temperature of the copper liquid at 1180-1220°C and keep it still for 0.6h; after the insulation process, slowly lift the heating cover at a moving speed of 50 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, thereby obtaining a copper ingot; S3: After taking out the copper ingot, cut off 5mm on both sides to remove the part with higher impurity element content; put the remaining copper ingot back into the electric vacuum high-temperature purification furnace, repeat the above S1 and S2 operations to continuously improve the purity of oxygen-free copper; after 3 cycles, ultra-high purity oxygen-free copper is finally obtained, with a purity of 99.999% (5N), an oxygen content of 1.9ppm, and a hydrogen content of 0.9ppm.

[0029] It should be noted that during the three cycles of purification, the lifting speed of the heating hood was gradually reduced, and for the sake of operation, the lifting speed of the heating hood in each cycle was an integer value; specifically, the lifting speed of the heating hood in the second cycle of purification was 45 mm / h, which was 10% lower than the lifting speed of the heating hood in the first cycle of purification, which was 50 mm / h; in the third cycle of purification, the lifting speed of the heating hood was 40 mm / h, which was 11% lower than the second. In each cycle, except for the lifting speed of the heating hood, other process parameters remained unchanged.

[0030] Embodiment 5: A method for preparing ultra-high purity oxygen-free copper, comprising the following steps: S1. Place 600 kg of dry cathode copper with no oil or water on the surface, cut edges and purity of 99.996% (4N6) neatly into a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and cover it with a heating cover to completely cover the graphite crucible; use a vacuum pump to evacuate the vacuum degree in the graphite crucible to below 3 Pa, and energize the resistance wire of the heating cover to heat it, and gradually increase the temperature in the furnace to 1220~1250℃, so that the cathode copper gradually melts into liquid in a vacuum and high-temperature environment; S2: Keep the temperature of the copper liquid at 1220-1250℃ and keep it still for 1 hour; after the insulation process, slowly lift the heating cover at a moving speed of 30 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, thereby obtaining a copper ingot; S3: After taking out the copper ingot, cut off 3mm on both sides to remove the part with higher content of impurity elements; put the remaining copper ingot back into the electric vacuum high-temperature purification furnace, repeat the above S1 and S2 operations to continuously improve the purity of oxygen-free copper; after 3 cycles, ultra-high purity oxygen-free copper is finally obtained, with a purity of 99.9996% (5N6), an oxygen content of 1.8ppm, and a hydrogen content of 0.7ppm.

[0031] It should be noted that during the three cycles of purification, the lifting speed of the heating hood was gradually reduced, and for the sake of operation, the lifting speed of the heating hood in each cycle was an integer value; specifically, the lifting speed of the heating hood in the second cycle of purification was 27 mm / h, which was 10% lower than the lifting speed of the heating hood in the first cycle of purification, which was 30 mm / h; in the third cycle of purification, the lifting speed of the heating hood was 25 mm / h, which was 7% lower than the second. In each cycle, except for the lifting speed of the heating hood, other process parameters remained unchanged.

[0032] The following comparative examples are designed to contrast with the above embodiments to illustrate the beneficial effects of the technical solutions of the present invention.

[0033] Comparative Example 1: Based on Example 1, in each cycle of purification, the lifting speed of the heating hood remains unchanged at 20 mm / h, and other process parameters remain consistent with Example 1. The purity of the ultra-high purity oxygen-free copper finally obtained is 99.998%, that is, 4N8, the oxygen content is 0.8 ppm, and the hydrogen content is 1.7 ppm.

[0034] Compared with Example 1, the purity of oxygen-free copper is reduced and is lower than 5N, and the oxygen content and hydrogen content are increased, and the increase in hydrogen content is large, which is higher than the 1ppm required by the present invention. This is because in the process of step-by-step purification, the speed at which trace solutes migrate in the melt will become slower and slower, and a slower solidification process is required for purification. If the same moving speed is used, trace impurity solutes and hydrogen and oxygen contents will be difficult to completely remove from the melt at the same speed, and it will take longer time to achieve a better removal effect.

[0035] Comparative Example 2: Based on Example 2, the number of cycles of purification is 6 times, and the purity of the obtained oxygen-free copper is 99.99993%, that is, 6N3, the oxygen content is less than 0.5ppm, and the hydrogen content is less than 0.5ppm.

[0036] Compared with Example 2, the number of purification cycles in this comparative example increases, and is more than 3-5 times required by the present invention, and the purity of the copper will eventually increase, and the lower the purity of the copper, the greater the increase in purity after purification, and the higher the purity, the less the increase in purity after purification. When the purity of copper approaches or reaches 6N, the purity of the copper after purification increases very little. Therefore, although the increase in the number of cycles can improve the purity of the product, too many cycles will increase the preparation cost of the product, and with the increase in the number of purification cycles, it will become increasingly difficult to improve the purity of the product in the later stage, so the number of cycles is not the more the better.

[0037] Comparative Example 3: Based on Example 2, the number of cycles of purification was 2, and the purity of the obtained oxygen-free copper was 99.991%, i.e. 4N1, the oxygen content was 2.3 ppm, and the hydrogen content was 1.9 ppm.

[0038] Compared with Example 2, after the cyclic purification in this comparative example, the purity of oxygen-free copper is only 4N1, which is lower than the 5N~6N required by the present invention, and the oxygen content and hydrogen content exceed the requirements of the present invention. This is because as the number of purification cycles increases, the impurities in the oxygen-free copper will be gradually removed, the purity will gradually increase, and the oxygen content and hydrogen content will also decrease accordingly, so a certain number of cycles is required.

[0039] By comparing Examples 2 and 3, it is illustrated that in the preparation of oxygen-free copper, the importance of the number of purification cycles is demonstrated. Too low a number affects the purity of the product, while too high a number increases the overall preparation cost. Therefore, the number of cycles proposed in this application is 3-5 times, which can take into account both the quality and preparation cost of the final product.

[0040] Comparative Example 4: On the basis of Example 3, the insulation temperature of the copper liquid is 1320-1350°C, and the other process parameters are the same as those in Example 3. The purity of the obtained oxygen-free copper is 99.999%, i.e. 5N, the oxygen content is 2.0 ppm, and the hydrogen content is 1.1 ppm.

[0041] Compared with Example 3, in this comparative example, since the insulation temperature exceeds the 1160-1300° C. required by the present invention, the oxygen content and hydrogen content in the prepared oxygen-free copper are slightly higher than the range required by the present invention (oxygen content <2 ppm, hydrogen content <1 ppm). The analysis is as follows: The variation pattern of hydrogen and oxygen content is similar to that of other impurities, and is related to the initial state of copper. However, under normal circumstances, as the number of purification cycles increases, the hydrogen and oxygen content of the melt has been basically completely removed after the purification times reach more than 3 times, and is lower than the solubility of hydrogen and oxygen in solid copper, meeting the use requirements. However, when the melt temperature increases, the solubility of hydrogen and oxygen in the melt will increase accordingly, the difficulty of removal will increase, and the residual amount in the copper will also increase accordingly.

[0042] In addition, if a lower holding temperature, such as 1130°C, is used on the basis of Example 3, the melt fluidity is poor and the migration speed of impurities in the melt is slow. In this way, under the same number of purification cycles, the purity of the obtained oxygen-free copper is lower than that of Example 3, and the hydrogen and oxygen content is relatively high. Therefore, the present invention sets the holding temperature of the copper liquid to 1160-1300°C.

[0043] Comparative Example 5: Based on Example 4, the vacuum degree of the electric vacuum high temperature purification furnace is 6-8Pa, and the other process parameters are the same as those in Example 4. The purity of the obtained oxygen-free copper is 99.998%, i.e. 4N8, the oxygen content is 2.5ppm, and the hydrogen content is 1.8ppm.

[0044] Compared with Example 4, the purity of oxygen-free copper is lower, lower than the 5N~6N required by the present invention, and the oxygen content and hydrogen content both exceed the requirements of the present invention (oxygen content <2ppm, hydrogen content <1ppm). The analysis is as follows: Compared with Example 4, the gas content in the melt, especially hydrogen, is removed by negative pressure under vacuum conditions. When the vacuum degree of the furnace is reduced, the sealing conditions of the furnace are not enough, and the melt may have a certain contact with the atmosphere. At this time, the melt and the hydrogen and oxygen in the air have a certain reaction, so that there is a certain balance between the air absorption and degassing of the melt. When the equilibrium point is reached, the gas in the melt is difficult to remove. Therefore, when the vacuum degree is lower than the range of less than 5Pa required by the present invention, it is manifested as not only removing the gas in the melt, but also ultimately causing the hydrogen and oxygen content in the oxygen-free copper product to be higher than that in Example 4. In addition, the increase in the hydrogen and oxygen content will also cause an increase in the viscosity combined with copper, resulting in the purification effect being affected.

[0045] It should be noted that when the hydrogen and oxygen content in oxygen-free copper is lower than 0.5ppm, the detection instruments will report it as <0.5ppm.

[0046] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Those skilled in the art should understand that the specific implementation modes of the present invention may be modified or replaced by equivalents with reference to the above embodiments. Any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the scope of protection of the pending claims.

Claims

1. A method for preparing ultra-high purity oxygen-free copper, characterized in that: The steps include: S1, adding cathode copper into a graphite crucible of an electric vacuum high-temperature purification furnace, and melting the cathode copper into copper liquid in a vacuum and high-temperature environment of the electric vacuum high-temperature purification furnace; S2, after the copper liquid is left to stand and keep warm, the heating cover of the electric vacuum high-temperature purification furnace is slowly lifted upward, and the heating cover leaves the graphite crucible, so that the copper liquid in the graphite crucible slowly solidifies from the bottom until the solution is completely solidified; S3, taking out the solidified ingot, cutting off the two ends of the ingot, and repeating steps S1 and S2 for the remaining ingot after cutting, and obtaining an ultra-high purity oxygen-free copper ingot after 3-5 cycles.

2. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S1, the purity of the cathode copper is 3N~5N.

3. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S1, the vacuum degree of the electric vacuum high temperature purification furnace is less than 5Pa.

4. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S2, the insulation temperature of the copper liquid is 1160~1300℃.

5. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S2, the holding time of the copper liquid is 0.5-1.5h.

6. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S2, the heating hood rises at a speed of 10-50 mm / h.

7. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S3, the weight of the parts cut off at both ends of the ingot each time accounts for 1-25% of the weight of the ingot before cutting.

8. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: In step S3, the total length of the ingot cut off is 6-12 mm.

9. The method for preparing ultra-high purity oxygen-free copper according to claim 1, characterized in that: During the 3rd to 5th cycle, the lifting speed of the heating hood is gradually reduced, and the lifting speed of the heating hood in the last cycle is reduced by 7% to 14% compared with the lifting speed of the heating hood in the previous cycle.

10. An ultra-high purity oxygen-free copper, characterized in that: The ultra-high purity oxygen-free copper is prepared by the preparation method of any one of claims 1 to 9, with a purity of 5N~6N, an oxygen content of less than 2ppm, and a hydrogen content of less than 1ppm.