A smelting process for silver ingot

Abstract

The present invention belongs to the technical field of high-purity metal smelting, and discloses a smelting process of silver ingots. The process comprises the following steps: placing a mold and a crucible containing a silver ingot in a heating furnace, wherein the mold is located at the casting port side of the crucible; heating the crucible in a vacuum atmosphere, and before the heating temperature reaches the melting point of the silver ingot, starting to pass a protective gas, maintaining a negative pressure in the furnace, and continuing to heat until the silver ingot is completely melted, and then cooling to a casting temperature of 980-1050°C; maintaining the casting temperature and the negative pressure in the furnace, and casting the casting material in the crucible into the mold in three stages until the casting of the material is completed, and stopping heating; after the ingot is cooled, it is taken out of the furnace to obtain a silver ingot. The process flow of the present invention is simple, the obtained ingot is of high quality, and the silver burn loss is small during the entire smelting process, and the cost is low.

Description

Technical Field

[0001] The invention belongs to the technical field of high-purity metal smelting, and more specifically relates to a smelting process for silver ingots. Background Art

[0002] High-purity silver targets are widely used in the production of thin layers of electrodes or reflective layers due to their excellent properties such as low resistance and high reflectivity, such as in optical recording media, solar cells, low-emissivity glass, liquid crystal displays and other products.

[0003] The preparation of silver targets usually requires plastic deformation and heat treatment of the melted silver ingots. If the ingots have many defects, scraps will be produced or the prepared silver targets will be unqualified, resulting in high costs.

[0004] The main methods of silver ingot melting are vacuum induction melting or atmospheric melting. However, if the melting temperature, mold, casting speed and other conditions are not paid attention to during the melting process, the impurities and gas content of the ingots produced are relatively difficult to control, and more burnout is produced during the melting process, resulting in excessively high costs. Summary of the invention

[0005] In view of the above problems existing in the prior art, the object of the present invention is to provide a smelting process for high-purity silver ingots, which can improve the quality of the ingots and save costs.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is: A smelting process for silver ingots comprises the following steps: (1) placing a mold and a crucible containing a silver ingot in a heating furnace, with the mold located on the pouring port side of the crucible and a riser disposed above the mold opening; (2) Heat the crucible in a vacuum atmosphere. Before the heating temperature reaches the melting point of the silver ingot, start passing protective gas to a furnace pressure of 300-500 mbar. Continue heating until the silver ingot is completely melted, then cool down to the casting temperature of 980-1050°C. (3) Maintaining the casting temperature and the gas pressure in the furnace, the casting material in the crucible is cast into the mold in three stages: the first stage: casting 40-50% of the casting material at a casting speed of 6-8 kg / s; the second stage: casting 20-40% of the casting material at a casting speed of 4-5 kg / s; the third stage: casting the remaining casting material at a casting speed of 5-9 kg / s, until the material casting is completed and the heating is stopped; (4) After the ingot is cooled, it is taken out of the furnace to obtain a silver ingot.

[0007] Furthermore, in step (1), the silver ingot is pretreated before use: the surface of the silver ingot is wiped with isopropyl alcohol or alcohol and a dust-free cloth, and then dried.

[0008] Furthermore, in step (1), the purity of the silver ingot is greater than or equal to 99.9%.

[0009] Furthermore, in step (1), the mold is a cast iron mold.

[0010] Furthermore, in step (1), the inner wall of the mold is sprayed with a boron nitride release agent.

[0011] Furthermore, in step (1), the wall thickness of the mold is 80-100 mm. Using this thickness range, the heat conduction effect after casting is relatively good, the ingot grains are relatively fine, and it is easy to demould.

[0012] Furthermore, the riser is a trapezoidal structure with a smaller opening at the top and a larger opening at the bottom.

[0013] Furthermore, the outer circumference of the riser is wrapped with insulation cotton. During the casting process, the riser is placed directly above the mold, and the outer circumference of the riser is wrapped with insulation cotton to ensure that the metal liquid can perform a good shrinkage compensation effect during the solidification process, thereby improving the ingot quality.

[0014] Furthermore, step (2) is specifically as follows: close the furnace cover, start heating and vacuuming in the furnace until the vacuum degree in the furnace is 10 -1 to 10 -2 Pa, the temperature in the furnace is 700-900°C, protective gas is started, and heating is continued to 1050-1200°C until the material is completely melted, heating is stopped, and the temperature is lowered to the casting temperature. Further preferably, protective gas is started to be passed until the gas pressure in the furnace is not less than 300mbar. Further preferably, protective gas is started to be passed until the gas pressure in the furnace is 300-500mbar.

[0015] Furthermore, in step (2), the heating rate in the furnace is controlled to be 10-15°C / second.

[0016] Furthermore, in step (2), the protective gas is an inert gas.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The process flow of the invention is simple, defects of the obtained ingot are concentrated in the riser, the quality of the ingot can be improved, and the burnout of silver is controlled within one thousandth during the entire smelting process, so that a silver ingot with fewer defects and a low burnout rate is obtained, which greatly reduces the cost.

[0018] The present invention provides a riser structure. During the casting process, the solidified liquid volume decreases. When the molten metal fills the mold, the liquid solidifies into a solid and forms shrinkage cavities. The molten metal in the riser is replenished in time, so that the shrinkage cavities only exist in the riser, thereby improving the quality of the ingot.

[0019] In the process of melting the silver ingot, the protective gas is introduced when a certain vacuum degree and temperature are reached. At this time, the gas in the raw material has been volatilized and drawn away, and the volatilization speed of silver is relatively slow, thereby greatly reducing the loss of silver. DETAILED DESCRIPTION

[0020] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and carefully in combination with preferred embodiments below, but the protection scope of the present invention is not limited to the following specific embodiments.

[0021] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0022] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0023] Example 1 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 7kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 6kg / s; in the third stage, the remaining 20% ​​is restored to 7kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0024] The shrinkage holes of the silver ingot obtained in this embodiment are about 10 mm and all concentrated on the riser; and the loss of silver in the whole smelting process is within one thousandth.

[0025] Example 2 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 12°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 7 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 4 kg / s; in the third stage, the remaining 20% ​​is restored to 7 kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0026] The shrinkage holes of the silver ingot obtained in this embodiment are about 10 mm and all concentrated on the riser; and the loss of silver in the whole smelting process is within one thousandth.

[0027] Example 3 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 7 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 4 kg / s; in the third stage, the remaining 20% ​​is restored to 7 kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0028] The shrinkage holes of the silver ingot obtained in this embodiment are about 10 mm and all concentrated in the riser; and the loss of silver in the whole smelting process is within one thousandth.

[0029] Example 4 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 40% of the casting material is 6 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 5 kg / s; in the third stage, the remaining 30% is restored to 6 kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0030] The shrinkage holes of the silver ingot obtained in this embodiment are about 15 mm and all concentrated in the riser; and the loss of silver in the whole smelting process is within one thousandth.

[0031] Example 5 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 45% of the casting material is 6 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 5 kg / s; in the third stage, the remaining 25% of the casting material is restored to 6 kg / s until the casting of the material is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0032] The shrinkage holes of the silver ingot obtained in this embodiment are about 15 mm and all concentrated in the riser; and the loss of silver in the whole smelting process is within one thousandth.

[0033] Example 6 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 45% of the casting material is 6 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 5 kg / s; in the third stage, the remaining 25% is restored to 6 kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0034] The shrinkage holes of the silver ingot obtained in this embodiment are about 15 mm and all concentrated in the riser; and the loss of silver in the whole smelting process is within one thousandth.

[0035] Comparative Example 1 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 15°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa, the metallic silver has partially melted, and the temperature under vacuum reaches 1000°C, argon gas is introduced at 300 mbar; (7) Under the argon protection environment, the temperature is reduced to the casting temperature of 980°C, the casting temperature and protective atmosphere pressure are maintained, and preparation for casting is started; (8) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 2 kg / s; in the second stage, 50% of the casting material is cast at a speed of 5 kg / s until the casting of the material is completed; (9) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0036] The silver ingot obtained in this comparative example has a part of shrinkage cavities at the riser position, but there are some gas defects at the bottom of the ingot; the loss of silver in the whole smelting process is within one thousandth.

[0037] The gas defect at the bottom of this comparative example is formed because the casting speed is slow, resulting in the silver liquid absorbing gas and falling on the inner wall of the mold during the early casting process. Because the mold temperature is low, the liquid solidifies to form pores.

[0038] Comparative Example 2 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 15°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa, the metallic silver has partially melted, and the temperature under vacuum reaches 1000°C, argon gas is introduced at a pressure of 200 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature is reduced to the casting temperature of 980°C, the casting temperature and protective atmosphere pressure are maintained, and preparation for casting is started; (8) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 7 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 4 kg / s; in the third stage, the remaining 20% ​​is restored to 7 kg / s until the material casting is completed; (9) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0039] The shrinkage holes of the silver ingot obtained in this comparative example are about 10 mm and all concentrated on the riser; the loss of silver in the whole smelting process is 0.2%.

[0040] During the melting process of this comparative example, silver volatilizes greatly because it is in a vacuum state, but the amount of argon gas is small, resulting in a portion of silver vapor being plated on the inner wall of the furnace, causing greater burn loss.

[0041] Comparative Example 3 This comparative example does not use a riser structure, which is as follows: (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold and place it in an oven at 150°C for drying for 2 hours. The wall thickness of the cast iron mold is 80 mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that casting can enter, place the silver ingot in the graphite crucible, and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 7 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 4 kg / s; in the third stage, the remaining 20% ​​is restored to 7 kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0042] The shrinkage holes of the silver ingot obtained in this comparative example are 100-180 mm at the upper end of the ingot. In order not to affect the plastic deformation at the rear end, the shrinkage holes must be completely removed, resulting in a low material utilization rate; the loss of silver in the entire smelting process is within one thousandth.

[0043] In the casting process of this comparative example, the temperature of the liquid near the edge of the mold is low and solidifies quickly. However, due to the fast casting speed, a large amount of liquid is replenished in time during the solidification process of the center, resulting in the last solidification in the center, forming a larger shrinkage cavity.

[0044] Comparative Example 4 This comparative example does not use a riser structure, which is as follows: (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold and place it in an oven at 150°C for drying for 2 hours. The wall thickness of the cast iron mold is 80 mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that casting can enter, place the silver ingot in the graphite crucible, and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, the crucible is quickly turned over so that the molten metal is quickly poured into the mold. The casting speed is 10 kg / s to complete the casting; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0045] The shrinkage cavities of the silver ingot obtained in this comparative example are concentrated in the upper end 50-180 mm of the ingot. In order not to affect the plastic deformation of the rear end, the shrinkage cavities need to be completely removed, resulting in a low material utilization rate; the loss of silver in the entire smelting process is within 0.2%.

[0046] In the casting process of this comparative example, the temperature of the liquid near the edge of the mold is low and solidifies quickly. However, due to the fast casting speed, a large amount of liquid is replenished in time during the solidification process of the center, resulting in the last solidification in the center, forming a larger shrinkage cavity.

[0047] Comparative Example 5 This comparative example does not use a riser structure, which is as follows: (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride release agent on the inner wall of the cast iron mold and place it in an oven at 150°C for drying for 2 hours. The wall thickness of the cast iron mold is 80 mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that casting can enter, place the silver ingot in the graphite crucible, and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, continue heating until the silver is completely melted, and then start filling with argon gas at 300 mbar; (7) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (8) During the casting process, the average speed is maintained at 6 kg / s to complete the casting; (9) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0048] The shrinkage cavities of the silver ingot obtained in this comparative example are concentrated in the upper 50-100 mm of the ingot. In order not to affect the plastic deformation of the rear end, the shrinkage cavities need to be completely removed, resulting in a low material utilization rate; the loss of silver in the entire smelting process is within 0.2%.

[0049] In this comparative example, the casting is performed at a uniform speed. During the casting process, the liquid temperature near the edge of the mold is low and solidifies quickly. However, due to the fast casting speed, a large amount of liquid is replenished in time during the solidification process of the center, resulting in the last solidification in the center. The formed shrinkage cavity is deeper and there is no riser, resulting in more material needing to be sawed off from the top and bottom of the ingot.

[0050] The loss in this comparative example is large because the pressure is not applied until the silver is completely melted under vacuum, resulting in more silver vapor volatilization during the vacuuming process.

[0051] Comparative Example 6 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 50% of the casting material is 3kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 8kg / s; in the third stage, the remaining 20% ​​is restored to 3kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0052] The shrinkage holes of the silver ingot obtained in this comparative example are about 20 mm and all concentrated in the riser. However, there are solidification pits at the bottom of the ingot after peeling. The main reason is that after the molten metal is cast, the early casting speed is too slow, resulting in the new molten metal not having time to compensate for the shrinkage, so solidification pits appear at the bottom edge of the ingot. Such solidification pits will affect the plastic deformation effect and cause defects inside the ingot. In addition, the loss of silver in the entire smelting process is within one thousandth.

[0053] Comparative Example 7 (1) Fix the graphite crucible with the furnace charge in the coil, turn on the heating, and start the furnace drying to dry the moisture in the furnace charge; (2) Take a 99.99% high-purity silver ingot, wipe the surface clean, and place it in a vacuum drying oven at 100°C for 2 hours to dry; (3) Spray boron nitride mold release agent on the inner wall of the cast iron mold, spray boron nitride and insulation cotton on the surface of the riser and riser pressure iron, and place them in an oven for drying at 150°C for 2 hours. The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom. The wall thickness of the cast iron mold is 80mm; (4) After drying, place the cast iron mold on the side of the graphite crucible casting mouth plate to ensure that it can be cast into the mold, place the riser on the upper end of the mold mouth, add insulation cotton around the riser, and then press it with a pressure iron to ensure that the riser floats after the molten metal is cast. Finally, place the silver ingot in the graphite crucible and close the furnace door; (5) First, turn on the vacuum pump to evacuate the material, increase the power, and heat slowly at a heating rate of 10°C / s. During the heating process, as the gas in the raw material is discharged, the vacuum degree will first decrease and then tend to a stable state. (6) When the vacuum degree in the furnace reaches a stable level of 0.1 Pa and the metallic silver has not yet melted, and the temperature under vacuum reaches about 700°C, argon gas is filled at 300 mbar to prevent excessive volatilization of silver; (7) Under the argon protection environment, the temperature continues to rise and the silver ingot begins to melt slowly; (8) When the temperature reaches 1100°C, the silver ingot is completely melted, and then the temperature is reduced to the casting temperature of 980°C, and the casting temperature and protective atmosphere pressure are maintained, and the casting is ready to start; (9) During the casting process, in the first stage, the casting speed of the first 40% of the casting material is 9 kg / s; in the second stage, the casting speed of 30% of the casting material is slowed down and cast at a speed of 5 kg / s; in the third stage, the remaining 30% is restored to 9 kg / s until the material casting is completed; (10) Turn off the furnace power and allow it to cool naturally. After 12 hours, when the temperature is around 70°C, take out the silver ingot.

[0054] The shrinkage holes of the silver ingot obtained in this embodiment are about 50 mm and concentrated in the riser; the loss of silver in the whole smelting process is within one thousandth.

[0055] In this comparative example, the casting speed in the first stage is too fast, and the molten metal splashes after hitting the mold. The surface roughness of the obtained ingot is poor, which is easy to cause defects in the later plastic deformation. Meanwhile, the shrinkage hole of the silver ingot obtained in the present invention is about 50 mm, compared with the shrinkage hole of 10 mm of the silver ingot obtained in the previous embodiment, the utilization rate of the material in this comparative example is relatively low.

[0056] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of the present invention.

Claims

1. A smelting process for silver ingots, characterized in that: The following steps are involved: (1) placing a mold and a crucible containing a silver ingot in a heating furnace, with the mold located on the pouring port side of the crucible and a riser disposed above the mold opening; (2) Heat the crucible in a vacuum atmosphere. Before the heating temperature reaches the melting point of the silver ingot, start passing protective gas to maintain negative pressure in the furnace. Continue heating until the silver ingot is completely melted, and then cool down to the casting temperature of 980~1050℃; (3) Maintaining the casting temperature and negative pressure in the furnace, pouring the casting material in the crucible into the mold in three stages: the first stage: pouring 40-50% of the casting material at a casting speed of 6-8 kg / s; The second stage: pouring 20-40% of the casting material at a pouring speed of 4-5 kg / s; the third stage: pouring the remaining casting material at a pouring speed of 5-9 kg / s until the casting of the material is completed and the heating is stopped; (4) After the ingot is cooled, it is taken out of the furnace to obtain a silver ingot.

2. The smelting process of silver ingot according to claim 1, characterized in that: In step (1), the silver ingot is pretreated before use: the surface of the silver ingot is wiped with isopropyl alcohol or alcohol and a dust-free cloth, and then dried.

3. The smelting process of silver ingot according to claim 1, characterized in that: In step (1), the purity of the silver ingot is greater than or equal to 99.9%.

4. The smelting process of silver ingot according to claim 1, characterized in that: In step (1), the mold is a cast iron mold; the inner wall of the mold is sprayed with a boron nitride release agent; and the wall thickness of the mold is 80-100 mm.

5. The smelting process of silver ingot according to claim 1, characterized in that: In step (1), the crucible is a graphite crucible.

6. The smelting process of silver ingot according to claim 1, characterized in that: The riser is a trapezoidal structure with a small opening at the top and a large opening at the bottom.

7. The smelting process of silver ingot according to claim 1, characterized in that: The outer wall of the riser is wound with thermal insulation cotton.

8. The smelting process of silver ingot according to claim 1, characterized in that: Step (2) is as follows: close the furnace cover, turn on the heating and vacuum pumping in the furnace until the vacuum degree in the furnace reaches 10 -1 ~10 -2 Pa, the temperature in the furnace is 700~900℃, protective gas is started to be passed until the pressure in the furnace is not less than 300mbar, and heating is continued to 1050~1200℃ until the material is completely melted, then heating is stopped and the temperature is lowered to the casting temperature.

9. The smelting process of silver ingot according to claim 1, characterized in that: In step (2), the heating rate in the furnace is controlled to be 10-15°C / second.

10. The smelting process of silver ingot according to claim 1, characterized in that: In step (2), the protective gas is an inert gas.