Furnace drawing and baking method for nickel melting intermediate frequency furnace

Abstract

The invention discloses a furnace ramming and baking method for a nickel melting intermediate frequency furnace, which comprises the following steps of: taking coarse and fine fused magnesite dry materials with the magnesium oxide content of about 85 percent as main ramming materials, boric acid and water glass as sintering agents and adhesives, asbestos cloth as a heat insulation layer and a nickel skin crucible as a middle filling crucible of the intermediate frequency furnace, so that the purity of nickel in the production process of atomized nickel powder is ensured; and the used materials are low in price and have the cost advantage, and the furnace drawing operation requirement is simple and convenient. According to the furnace baking method disclosed by the invention, the finally built furnace body is baked according to low-power and high-power furnace baking requirements after furnace ramming is completed, so that the sintering layer, the semi-sintering layer and the heat insulation and insulation layer are sequentially formed from inside to outside, the final furnace life is ensured to be more than 80 furnaces, and the production cost of atomization powder making is effectively reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of powder metallurgy, and in particular relates to a method for firing and baking a nickel-melting medium-frequency furnace. Background Art

[0002] Atomization powder making is one of the methods for making metal and alloy powders. The atomization method has the advantages of short process flow, high metal recovery rate, no pollution, low energy consumption, and meets the requirements of developing a circular economy and national industrial policies. Domestic large-scale powder making manufacturers and research institutes are constantly developing this technology. High-pressure water atomization, protective gas atomization, rotary electrode atomization, plasma atomization and other technologies have been applied in industrial powder making. With the further adjustment of the country's processing industry, the unique advantages of atomization powder making are becoming more and more obvious.

[0003] The most important first process of atomization powder making is to use medium frequency furnace to melt metal or metal alloy. Improving the furnace age of the melting process is of great significance to reducing the production cost of atomization powder making. If the average furnace age is calculated as 40 furnaces, the processing cost of the production process can be reduced by 1.5% for every increase in furnace age. Therefore, increasing the furnace age is one of the main ways to reduce the production cost of atomization powder making.

[0004] Medium frequency furnace charge is also called medium frequency furnace lining material, medium frequency furnace refractory material, medium frequency furnace dry vibration material, medium frequency furnace ramming material. It is divided into acidic, neutral and alkaline charge. Acidic charge is made of high purity quartz and fused quartz as the main raw materials, with suitable additives as sintering agent; neutral medium frequency charge is made of alumina and high aluminum materials as the main raw materials, with composite additives as sintering agent; alkaline charge is made of high purity fused corundum, high purity fused magnesia and high purity spinel as the main raw materials, with suitable additives as sintering agent.

[0005] Acidic, neutral and alkaline medium frequency furnace charges are widely used in coreless medium frequency furnaces and cored induction furnaces as lining materials for medium frequency furnaces. Different furnace charges can be selected according to the materials of the smelted metal or alloy. However, the cost price, furnace charging method and service life (furnace age) of different furnace charges are also different, so it is very important to choose economical, affordable and reliable furnace charges.

[0006] At the same time, factors that affect the furnace life also include furnace-building technology and furnace-baking process control skills. Therefore, after choosing the appropriate medium frequency furnace charge and sintering agent, it is also necessary to master the necessary furnace-building and furnace-baking skills to ultimately ensure that the built furnace body meets the metal smelting requirements. Summary of the invention

[0007] The purpose of the present invention is to solve the technical problems of furnace beating, furnace baking and improving furnace life involved in furnace body construction in the process of atomization powder making by using a small medium frequency induction furnace, and in particular to provide a medium frequency furnace beating and baking method with low manufacturing cost, simple operation and long furnace life for nickel smelting in the process of atomized nickel powder preparation. The method uses coarse and fine fused magnesia sand dry materials as main furnace beating materials, boric acid and water glass as added sintering agents and adhesives, asbestos cloth as heat insulation layer, nickel skin crucible as the middle filling crucible of the medium frequency furnace, and after the furnace beating is completed, the finally built furnace body is baked according to the requirements of low power and high power furnace baking to form a sintering layer, a semi-sintering layer and a heat insulation and insulating layer, thereby ensuring that the final furnace life is more than 80 furnaces.

[0008] To this end, the present invention adopts the following technical solutions: Step 1: Fix the coil: Fix the intermediate frequency furnace coil, clean the metal materials on the coil, keep the turns parallel and without short circuit, tighten the vertical rod screws, and apply refractory material on the coil; Step 2: Material preparation: prepare 300kg of dry magnesia, 10-15kg of boric acid and water glass, 1.5-2m of asbestos cloth, and 1 nickel crucible; Step 3: Mixing: Pour the magnesia sand onto the iron plate, add the boric acid and stir with a plastic shovel to mix evenly. Avoid mixing metal impurities during the mixing process. Gradually add water and stir. The final amount of water is enough to pinch it into shape by hand. Step 4: Cover the coil: Lay the asbestos cloth on the inner wall of the coil to ensure that the inner wall is completely covered; Step 5: Make the furnace bottom: The total material used for the furnace bottom is 80~100kg, and the height of the furnace bottom is compacted to 10~20cm; Step 6: Install the nickel crucible: fix the nickel crucible in the center of the furnace bottom and pad it firmly; Step 7: Make the furnace wall: The total material of the furnace wall is 80~100Kg. During the process of making the furnace wall, the thickness of the furnace wall should be uniform, and it should be U-shaped as much as possible and parallel to the furnace coil; Step 8: Make the furnace collar: Add water glass to the remaining magnesia sand and stir evenly with a plastic shovel. The furnace collar should be 10-20cm wide and 10-20cm high, and a V-shaped feeding port should be reserved above the furnace mouth. Step 9: Low-power furnace baking: Add 3 to 5 nickel blocks to the furnace chamber in front of the furnace to form the bottom, start from 3 to 8kw, increase 4 to 6kw every 6 hours, and bake the furnace at low power for 24 hours; Step 10, high-power furnace baking: high-power furnace baking for 12 hours, increase 4~6kw every 2 hours, reach 45~55kw insulation standby, the whole furnace baking process is completed.

[0009] As a further improvement of the technical solution of the present invention, in step 1, the coil needs to be fixed vertically, concentric with the furnace shell, and the coil turns are evenly spaced and cannot touch each other to avoid short circuit. The bolts of the external poles must be tightened to ensure that the coil is securely installed. At the same time, refractory coating or clay needs to be applied to the coil to prevent the melt from leaking and scalding the coil.

[0010] Further, in step 2, the magnesia is fused magnesia, and the coarseness and fineness of the magnesia dry material need to be matched for use, wherein 0-1 mesh is 100 kg, 1-3 mesh is 75 kg, 3-5 mesh is 75 kg, and -250 mesh is 50 kg. The boric acid is a chemical binder for magnesia, and mainly plays a bonding role in the medium frequency furnace. The amount of boric acid added during use should be controlled, otherwise it will affect the service life of the furnace lining, and it may also wear the furnace. It should be introduced reasonably, and the amount used is 1% to 1.5% of the total weight of the fused magnesia. The main component of the water glass is sodium silicate, commonly known as sodium silicate, which is an inorganic substance with the chemical formula Na2O·nSiO2. Its aqueous solution is commonly known as water glass, which is a mineral adhesive. It is mainly used in the process of furnace construction. The amount used depends on the actual situation to ensure that the furnace surface is moist and can be bonded. According to many years of practical experience, the optimal amount is 10-15 kg of 100 kg of magnesia. The purpose of using water glass is to ensure that the furnace is firmly bonded and more durable. If it is not used in furnace wall construction, on the one hand, water glass is not easy to evaporate, which will cause difficulties in furnace baking, and on the other hand, it is to prevent the furnace wall from being too hard to bond and difficult to dismantle the furnace. The asbestos cloth mainly plays a role in heat insulation. The nickel skin crucible is a homemade crucible filled in the middle of the medium frequency furnace. The size is determined by the diameter of the medium frequency furnace coil and the thickness of the furnace wall to be built. It is necessary to open a number of holes in the nickel skin in order to allow water vapor to escape during the furnace baking process.

[0011] Furthermore, in step three, the mixing must ensure that the coarse and fine magnesia sands are mixed evenly, and the amount of water added should not be too much, and finally it can be formed by hand.

[0012] Furthermore, in step 4, when covering the coil, ensure that the asbestos cloth completely covers the bottom of the medium frequency furnace and the coil, and there must be no gaps when overlapping each other. At the same time, ensure that 15 to 20 cm is reserved and laid flat on the outside of the coil, and use an object of a certain weight to press and fix it to prevent the asbestos cloth from falling during the furnace operation.

[0013] Furthermore, in step 5, the furnace bottom is beaten by spreading magnesia sand on the bottom of the coil 3 to 4 times, and each time it is compacted with a hammer, and the force is applied evenly during the compaction process to ensure that the furnace bottom is flat and the thickness is consistent. At the same time, the compacted surface needs to be scraped before adding magnesia sand each time, in order to prevent the compacted surface from being too smooth and causing unstable bonding and delamination.

[0014] Furthermore, in step six, the nickel crucible needs to be installed after the furnace bottom is paved, and the nickel crucible needs to be placed in the center. The nickel crucible must be padded. The nickel crucible is installed so that the crucible is directly attached to the magnesia after melting, to prevent the magnesia from melting into the nickel liquid, causing the impurity content of the nickel liquid to be too high, affecting the quality of the final product. At the same time, in order to prevent the nickel crucible from loosening and deforming during the furnace, a nickel plate of a certain weight can be placed in the crucible to press and fix it.

[0015] Furthermore, in step seven, the furnace wall is beaten in 15 to 25 times. Each time 3 to 5 kg of magnesia is added, the wall is compacted with a hammer. Before adding magnesia each time, the compacted surface needs to be scraped to prevent stratification, and at the same time, ensure that the furnace bottom and the furnace wall are compacted without gaps.

[0016] Furthermore, in step eight, when beating the furnace collar, it is necessary to add water glass to the remaining magnesia sand and stir it evenly before use to ensure that there is no gap at the junction of the furnace collar and the nickel crucible, and there should also be no gap between the furnace collar and the furnace wall. After the furnace is beating, it is necessary to clean up the debris inside the nickel crucible and the surface of the furnace body.

[0017] Furthermore, in step nine, the low-power drying furnace needs to strictly adjust the power as required, in order to gradually evaporate the water and allow the water vapor to gradually dissipate, thereby preventing the magnesia from being hydrated and causing the magnesia to decompose and fail, thereby increasing the porosity and possibly causing the furnace body to crack and leak.

[0018] Furthermore, in step ten, the high-power baking furnace needs to strictly adjust the power as required, in order to make the medium-frequency furnace sinter better, and to form the sintering layer, semi-sintering layer, insulation and insulating layer from the inside to the outside respectively, and the baking process must be continuous and uninterrupted.

[0019] The beneficial effects of the present invention are: 1. The method for firing and baking a nickel molten medium frequency furnace of the present invention uses coarse and fine fused magnesia sand dry material with a magnesium oxide content of about 85% as the main firing material, boric acid and water glass as sintering agents and adhesives, asbestos cloth as a heat insulation layer, and a nickel crucible as a filling crucible in the middle of the medium frequency furnace. It not only ensures the purity of nickel in the production process of atomized nickel powder, but also has cost advantages due to the low price of the materials used, and the furnace firing operation is simple.

[0020] 2. Through the furnace baking method described in the present invention, after the furnace is built, the finally built furnace body is baked according to the requirements of low-power and high-power furnace baking, so that a sintering layer, a semi-sintering layer, and an insulating and insulating layer are formed from the inside to the outside, ensuring that the final furnace life is more than 80 furnaces, effectively reducing the production cost of atomization powder making. DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with specific embodiments: Step 1: Check whether the coil of the medium frequency furnace is fixed intact, clean the nickel material that has leaked out of the coil, make sure that the coils are fixed and parallel, check whether the screws of the vertical poles are tight, and use refractory red glue to repair the two damaged places of the coil glue.

[0022] Step 2: Prepare 4 bags of 0-1 mesh fused magnesia (25kg / bag), 3 bags of 1-3 mesh fused magnesia (25kg / bag), 3 bags of 3-5 mesh fused magnesia (25kg / bag), 2 bags of -250 mesh fused magnesia (25kg / bag), 3.2kg of boric acid, 1 bucket of water glass (10kg / bucket), cut 1.8m of asbestos cloth, and weld 1 nickel crucible (350mm high, 280mm top, 260mm base).

[0023] Step 3: Prepare a clean iron plate, pour all the fused magnesia and boric acid on it, stir them back and forth with a plastic shovel, and mix them evenly. Then one person uses a watering can to water, and another person continues to stir them back and forth. After each stir, pinch them with your hands to test, and stop adding water and stirring after pinching them with your hands until they are formed. The mixed wet materials are set aside.

[0024] Step 4. Lay the prepared asbestos cloth on the inner wall of the coil, stagger it up and down at the joints to prevent the coil from leaking out. At the same time, use scissors to cut the excess asbestos cloth at the furnace mouth into roughly the same size. The cutting depth cannot exceed the furnace mouth coil. Lay the cut openings up and down in an staggered manner, and use a weight to press the asbestos cloth head to prevent it from falling.

[0025] Step 5. Use a plastic bucket to hold about 30kg of mixed wet magnesia sand, pour it all on the asbestos cloth at the bottom of the furnace, use a furnace hammer to compact it a little bit, then use a tool to scrape the smooth surface, then add another bucket of mixed wet magnesia sand, repeat the above operation to compact and scrape 3 times to complete the furnace bottom operation.

[0026] Step 6. Place the nickel crucible on the bottom of the furnace. Find the center with a ruler, put the used motor in the nickel crucible to prevent it from shifting, and use a small amount of magnesia sand at the bottom of the furnace to pad it.

[0027] Step seven, use tools to scrape the surface of the magnesia sand on the bottom of the furnace outside the nickel crucible, use a dustpan to add a small amount of mixed magnesia sand wet material, use the furnace hammer to compact it a little bit, and then use tools to scrape the smooth surface after compacting it, and then add a small amount of mixed magnesia sand wet material, repeat the above operation to compact and scrape 20 times to complete the furnace bottom operation.

[0028] Step 8. Add the prepared water glass to the remaining magnesia sand, stir it evenly with a plastic shovel, remove the weight and the waste motor, and pile the mixed magnesia sand on the furnace panel and coil little by little, with a stacking width of about 15cm and a height of about 12cm. Make a V-shaped feeding port by hand at the center of the furnace mouth. Then clean up the spilled magnesia sand and tools.

[0029] Step 9. After cleaning, add 4 nickel blocks to the furnace to lay the bottom, and start to increase the medium frequency power of the medium frequency furnace to 5kw, increase it to 11kw after 6 hours, increase it to 16kw after 12 hours, increase it to 20kw after 18 hours, and increase it to 25kw after 24 hours.

[0030] Step 10: Increase to 30kw after 26 hours, increase to 36kw after 28 hours, increase to 40kw after 30 hours, increase to 45kw after 32 hours, increase to 49kw after 34 hours, and increase to 54kw for insulation after 36 hours. The whole furnace-building and baking process is now completed.

[0031] Embodiment 2: The present embodiment provides a method for starting and baking a nickel melting medium frequency furnace, comprising the following steps: Step 1: Check whether the coil of the medium frequency furnace is fixed well, clean the nickel material that has leaked out of the coil, make sure that the coils are fixed and parallel, check whether the screws of the vertical poles are tight, and the refractory putty is intact and no repair is required.

[0032] Step 2: Prepare 4 bags of 0-1 mesh fused magnesia (25kg / bag), 3 bags of 1-3 mesh fused magnesia (25kg / bag), 3 bags of 3-5 mesh fused magnesia (25kg / bag), 2 bags of -250 mesh fused magnesia (25kg / bag), 3.5kg of boric acid, 1 and a half barrels of water glass (10kg / barrel), cut 2m of asbestos cloth, and weld 1 nickel crucible (350mm high, 280mm top, 260mm base).

[0033] Step 3: Prepare a clean iron plate, pour all the fused magnesia and boric acid on it, stir them back and forth with a plastic shovel, and mix them evenly. Then one person uses a watering can to water, and another person continues to stir them back and forth. After each stir, pinch them with your hands to test, and stop adding water and stirring after pinching them with your hands until they are formed. The mixed wet materials are set aside.

[0034] Step 4. Lay the prepared asbestos cloth on the inner wall of the coil, stagger it up and down at the joints to prevent the coil from leaking out. At the same time, use scissors to cut the excess asbestos cloth at the furnace mouth into roughly the same size. The cutting depth cannot exceed the furnace mouth coil. Lay the cut openings up and down in an staggered manner, and use a weight to press the asbestos cloth head to prevent it from falling.

[0035] Step 5. Use a plastic bucket to hold about 30kg of mixed wet magnesia sand, pour it all on the asbestos cloth at the bottom of the furnace, use a furnace hammer to compact it little by little, then use a tool to scrape the smooth surface, then add another bucket of mixed wet magnesia sand, repeat the above operation to compact and scrape 4 times to complete the furnace bottom operation.

[0036] Step 6. Place the nickel crucible on the bottom of the furnace. Use a tape measure to find the center. Place the used motor in the nickel crucible to prevent it from shifting. Use a small amount of magnesia sand at the bottom of the furnace to pad it.

[0037] Step seven, use tools to scrape the surface of the magnesia sand on the bottom of the furnace outside the nickel crucible, use a dustpan to add a small amount of mixed magnesia sand wet material, use the furnace hammer to compact it a little bit, and then use tools to scrape the smooth surface after compacting it, and then add a small amount of mixed magnesia sand wet material, repeat the above operation to compact and scrape 18 times to complete the furnace bottom operation.

[0038] Step 8. Add the prepared water glass to the remaining magnesia sand, stir it evenly with a plastic shovel, remove the weight and the waste motor, and pile the mixed magnesia sand on the furnace panel and coil little by little, with a stacking width of about 13cm and a height of about 15cm. Make a V-shaped feeding port in the center of the furnace mouth by hand. Then clean up the spilled magnesia sand and tools.

[0039] Step 9. After cleaning, add 5 nickel blocks into the furnace to lay the bottom, and start to increase the medium frequency power of the medium frequency furnace to 5kw, increase it to 10kw after 6 hours, increase it to 16kw after 12 hours, increase it to 21kw after 18 hours, and increase it to 26kw after 24 hours.

[0040] Step 10: Increase to 31kw after 26 hours, increase to 36kw after 28 hours, increase to 41kw after 30 hours, increase to 45kw after 32 hours, increase to 50kw after 34 hours, and increase to 55kw for insulation after 36 hours. The whole furnace-building and baking process is now completed.

[0041] Embodiment 3: The present embodiment provides a method for starting and baking a nickel melting medium frequency furnace, comprising the following steps: Step 1: Check whether the coil of the medium frequency furnace is fixed intact, clean the nickel material that has leaked out of the coil, make sure that the coils are fixed and parallel, check whether the screws of the vertical poles are tight, and use refractory red glue to repair one damaged part of the coil glue.

[0042] Step 2: Prepare 4 bags of 0-1 mesh fused magnesia (25kg / bag), 3 bags of 1-3 mesh fused magnesia (25kg / bag), 3 bags of 3-5 mesh fused magnesia (25kg / bag), 2 bags of -250 mesh fused magnesia (25kg / bag), 3kg of boric acid, 1 bucket of water glass (10kg / bucket), cut 1.6m of asbestos cloth, and weld 1 nickel crucible (350mm high, 280mm top, 260mm base).

[0043] Step 3: Prepare a clean iron plate, pour all the fused magnesia and boric acid on it, stir them back and forth with a plastic shovel, and mix them evenly. Then one person uses a watering can to water, and another person continues to stir them back and forth. After each stir, pinch them with your hands to test, and stop adding water and stirring after pinching them with your hands until they are formed. The mixed wet materials are set aside.

[0044] Step 4. Lay the prepared asbestos cloth on the inner wall of the coil, stagger it up and down at the joints to prevent the coil from leaking out. At the same time, use scissors to cut the excess asbestos cloth at the furnace mouth into roughly the same size. The cutting depth cannot exceed the furnace mouth coil. Lay the cut openings up and down in an staggered manner, and use a weight to press the asbestos cloth head to prevent it from falling.

[0045] Step 5. Use a plastic bucket to hold about 30kg of mixed wet magnesia sand, pour it all on the asbestos cloth at the bottom of the furnace, use a furnace hammer to compact it a little bit, then use a tool to scrape the smooth surface, then add another bucket of mixed wet magnesia sand, repeat the above operation to compact and scrape 3 times to complete the furnace bottom operation.

[0046] Step 6. Place the nickel crucible on the bottom of the furnace. Find the center with a ruler, put the used motor in the nickel crucible to prevent it from shifting, and use a small amount of magnesia sand at the bottom of the furnace to pad it.

[0047] Step seven, use tools to scrape the surface of the magnesia sand on the bottom of the furnace outside the nickel crucible, add a small amount of mixed magnesia sand wet material with a dustpan, use the furnace hammer to compact it a little bit, and then use tools to scrape the smooth surface after compacting it, and then add a small amount of mixed magnesia sand wet material, repeat the above operation of compacting and scraping 22 times to complete the furnace bottom operation.

[0048] Step 8. Add the prepared water glass to the remaining magnesia sand, stir it evenly with a plastic shovel, remove the weight and the waste motor, and pile the mixed magnesia sand on the furnace panel and coil little by little, with a stacking width of about 16cm and a height of about 14cm. Make a V-shaped feeding port in the center of the furnace mouth by hand. Then clean up the spilled magnesia sand and tools.

[0049] Step 9. After cleaning, add 3 nickel blocks into the furnace to lay the bottom, and start to increase the medium frequency power of the medium frequency furnace to 6kw, increase it to 10kw after 6 hours, increase it to 14kw after 12 hours, increase it to 19kw after 18 hours, and increase it to 24kw after 24 hours.

[0050] Step 10: Increase to 28kw after 26 hours, increase to 34kw after 28 hours, increase to 39kw after 30 hours, increase to 44kw after 32 hours, increase to 50kw after 34 hours, and increase to 54kw for insulation after 36 hours. The whole furnace-building and baking process is now completed.

Claims

1. A method for firing and baking a nickel melting medium frequency furnace, characterized in that: The following steps are involved: Step 1: Fix the coil: Fix the coil of the medium frequency furnace, clean the metal materials bonded to the coil, keep the turns parallel and without short circuit, tighten the vertical rod screws, and apply refractory materials on the coil; Step 2: Material preparation: prepare dry magnesia, boric acid, water glass, asbestos cloth, and nickel crucible; The magnesia is fused magnesia, and the weight ratio of 0-1 mesh, 1-3 mesh, 3-5 mesh, and -250 mesh in the magnesia is 100: (70-80): (70-80): (45-55); the amount of boric acid is 1%-1.5% of the total weight of the magnesia; the amount of water glass used for 100kg magnesia is 10-15kg; Step 3: Mixing: Pour the magnesia sand on the workbench, add boric acid and stir to mix evenly. Avoid mixing metal impurities during the mixing process. Add water while stirring until it is tightly kneaded into shape by hand. Step 4: Cover the coil: Lay the asbestos cloth on the inner wall of the coil to ensure that the inner wall is completely covered; Step 5: Make the furnace bottom: The total material used for the furnace bottom is 80~100kg, and the height of the furnace bottom is compacted to 10~20cm; Step 6: Install the nickel crucible: fix the nickel crucible in the center of the furnace bottom and pad it firmly; Step 7: Make the furnace wall: The total material of the furnace wall is 80~100Kg, and the wall thickness is controlled to be uniform and U-shaped, and the wall thickness is parallel to the furnace coil; Step 8: Make the furnace collar: Add water glass to the magnesia dry material in step 2 and stir evenly. The furnace collar should be 10-20cm wide and 10-20cm high, and a V-shaped feeding port should be reserved above the furnace mouth. Step 9, low power oven: add 3~5 nickel blocks to the furnace chamber before the oven is baked to form the bottom; start from 3~8kw, increase 4~6kw every 5.5~6.5 hours, and bake the oven at low power for 23~25 hours; Step 10, high-power furnace baking: high-power furnace baking for 11 to 13 hours, increase 4 to 6kw every 1.5 to 2.5 hours, reach 45 to 55kw insulation standby, then the whole furnace baking process is completed.

2. The method for firing and baking a nickel-melting medium-frequency furnace according to claim 1, characterized in that: In step one, the coil is installed vertically and concentrically with the furnace shell; the coil turns are evenly spaced and cannot touch each other to avoid short circuits; the external pole bolts are tightened to ensure that the coil is securely installed.

3. The method for firing and baking a nickel-melting medium-frequency furnace according to claim 1, characterized in that: In step 4, the asbestos cloth covers the bottom of the medium frequency furnace and the coil completely, overlapping each other without any gaps; 15 to 20 cm is reserved on the top of the asbestos board to extend flat to the outside of the coil to prevent the asbestos cloth from falling during the furnace operation.

4. The method for firing and baking a nickel-melting medium-frequency furnace according to claim 1, characterized in that: In step 5, spread the magnesia sand on the bottom of the coil in 3 to 4 times, gradually solidifying it to ensure that the furnace bottom is flat and of uniform thickness; Scrape the compacted surface before adding magnesia each time.