A post-weld heat treatment method for an aluminum electrolysis cell shell

Abstract

The application provides a post-weld heat treatment method of an aluminum electrolysis cell shell, 1) installing a graphite anode and a graphite cathode on a heat treatment pool; 2) then loading resistance material and the shell into the heat treatment pool, and controlling the resistance material to completely bury, surround and cover the shell inside and outside and up and down and left and right; 3) connecting direct current, the direct current output by the graphite anode and the graphite cathode is transmitted to the resistance material, the resistance material generates heat when the direct current passes through the resistance material, the heat generated by the resistance material heats the shell to 500-700 DEG C and performs tempering treatment with heat preservation, and then naturally cools down after the tempering treatment is finished; 4) then moving part of the resistance material and the shell out; the welding residual stress is eliminated, the shape and size of the shell are stabilized, the distortion is reduced, the performance of base material and a welding joint is improved, the plasticity of the welding seam metal is improved, the hardness of a heat affected zone is reduced, the fracture toughness is improved, the fatigue strength is improved, the yield strength is recovered or improved, and harmful gas in the welding seam metal is released.

Description

A method for post-weld heat treatment of aluminum electrolytic cell shell Technical Field

[0001] This invention belongs to the technical field of aluminum electrolysis equipment, and specifically relates to a post-weld heat treatment method for the shell of an aluminum electrolysis cell. Background Technology

[0002] Modern aluminum production employs the cryolite-alumina molten salt electrolysis method. Molten cryolite serves as the solvent, alumina as the solute, carbonaceous material as the anode, and molten aluminum as the cathode. A strong direct current is applied, and an electrochemical reaction occurs at the electrodes within the electrolytic cell at 950℃-970℃, producing aluminum. The cathode product is molten aluminum, which is extracted from the cell using a vacuum ladle and sent to the foundry. After purification and clarification in a holding furnace, it is cast into aluminum ingots or directly processed into wire rods, profiles, etc. The anode products are primarily carbon dioxide and carbon monoxide gases, containing a certain amount of harmful gases such as hydrogen fluoride and solid dust. Currently, prebaked anode cells are mainly used.

[0003] The commonly used aluminum electrolytic cell structure mainly consists of a steel shell, side furnace walls built inside the steel shell, cathode carbon block steel rod assemblies, and a bottom refractory insulation layer. The cell shell is a load-bearing device, primarily constructed by welding steel side panels, a bottom plate, horizontal edge plates, external support columns, and bottom I-beams. This forms an aluminum electrolytic cell shell structure with an external concave cradle-like support frame and an internal steel plate shell structure. For example, a 500KA aluminum electrolytic cell shell is cradle-shaped, weighing approximately 65 tons, with an external length of 20.5 meters, an external width of 5.22 meters, and an external height of 1.42 meters.

[0004] The aluminum electrolysis cell shell is made by welding, containing numerous welds of considerable length and variety. Theoretically, performing a post-weld heat treatment after welding assembly would significantly improve performance. However, the cell shell is characterized by its large size, heavy weight, numerous reinforcing ribs, many irregular parts, large welding deformation, and excessive weight and width. Currently, there is no heat treatment furnace large enough to accommodate this cell shell, making post-weld heat treatment impossible.

[0005] Therefore, how to perform post-weld heat treatment on the shell of an aluminum electrolysis cell that is huge in size, huge in weight, and extremely wide is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a post-weld heat treatment method for the shell of an aluminum electrolytic cell.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A method for post-weld heat treatment of the shell of an aluminum electrolytic cell, comprising the following steps performed sequentially:

[0009] 1) First, build a rectangular heat treatment pool using refractory bricks and refractory mortar, and open electrode holes on the end walls at both ends of the length of the heat treatment pool.

[0010] Then, the graphite anode is inserted from the outside of the tank into the electrode hole at one end of the heat treatment tank until the inner end of the graphite anode extends into the cavity inside the tank. The graphite cathode is inserted from the outside of the tank into the electrode hole at the other end of the heat treatment tank until the inner end of the graphite cathode extends into the cavity inside the tank.

[0011] 2) Then lay a layer of resistance material at the bottom of the heat treatment tank, and then hang the welded and assembled tank shell on the resistance material at the bottom of the heat treatment tank, controlling the opening of the tank shell to face upwards.

[0012] Then, resistance material is filled into the inner cavity of the tank, ensuring the tank is filled to the brim.

[0013] Then, resistive material is filled into the gap between the outer side wall of the tank shell and the inner side wall of the heat treatment tank, so that the outer side wall of the tank shell is covered by the resistive material, and the resistive material in the gap covers the graphite anode and graphite cathode to form an electrical circuit.

[0014] Then continue to fill the heat treatment tank with resistance material, so that the resistance material completely buries, surrounds and covers the tank shell;

[0015] Then, a layer of insulation material is laid on the resistive material for insulation.

[0016] 3) Use a rectifier transformer to supply DC power to the graphite anode and graphite cathode respectively. The DC power output from the graphite anode and graphite cathode is transferred to the resistive material. When the DC power passes through the resistive material, the resistive material heats up. The heat generated by the resistive material heats the tank shell, heats the tank shell to the tempering temperature and holds it at that temperature for a period of time for tempering treatment. After the tempering and holding is completed, it is naturally cooled until the temperature of the tank shell drops below 100℃.

[0017] 4) After the tempering process is completed, remove the resistance material from the tank shell and the gaps, and then lift the tempered tank shell out of the heat treatment tank, leaving the resistance material at the bottom of the tank shell in preparation for lifting into the next tank shell to be heat treated after welding.

[0018] Preferably, the inner dimensions of the heat treatment tank are 22-24m in length, 6-8m in width, and 1.5-2.5m in height.

[0019] Preferably, the thickness of the resistive material covering any part of the inner and outer walls of the control tank is 30-60cm.

[0020] Preferably, in step 3), the tempering temperature is 500℃-700℃, and the holding time for tempering is 3-6h.

[0021] This application has achieved the following beneficial technical effects:

[0022] This application constructs a heat treatment tank larger than the tank shell, loads the tank shell and resistive material into the heat treatment tank, and controls the resistive material to completely bury, surround and cover the inside, outside, top, bottom, left and right sides of the tank shell. Then, direct current is connected, and the direct current output from the graphite anode and graphite cathode is transferred to the resistive material. When the direct current passes through the resistive material, the resistive material heats up. The heat generated by the resistive material heats the tank shell to 500℃-700℃ and holds it at that temperature for tempering treatment. After the tempering treatment, the tank shell is allowed to cool naturally. This successfully achieves post-weld heat treatment of aluminum electrolysis tank shells that are huge in size, huge in weight, and huge in width, solving the problem that post-weld heat treatment was impossible due to the huge size, huge weight, and huge width of the aluminum electrolysis tank shell.

[0023] This post-weld heat treatment eliminates residual welding stress in the tank shell, stabilizes the shape and size of the tank shell, reduces distortion, improves the performance of the base material and weld joint, enhances the plasticity of the weld metal, reduces the hardness of the heat-affected zone, improves fracture toughness, improves fatigue strength, restores or enhances the yield strength reduced during cold forming, enhances resistance to stress corrosion, and further releases harmful gases in the weld metal, especially hydrogen, preventing the occurrence of delayed cracks. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention, and not all of them. Therefore, all other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] This application provides a post-weld heat treatment method for the shell of an aluminum electrolytic cell, comprising the following steps performed sequentially:

[0026] 1) First, build a rectangular heat treatment pool using refractory bricks and refractory mortar, and open electrode holes on the end walls at both ends of the length of the heat treatment pool.

[0027] Then, the graphite anode is inserted from the outside of the tank into the electrode hole at one end of the heat treatment tank until the inner end of the graphite anode extends into the cavity inside the tank. The graphite cathode is inserted from the outside of the tank into the electrode hole at the other end of the heat treatment tank until the inner end of the graphite cathode extends into the cavity inside the tank.

[0028] 2) Then lay a layer of resistance material at the bottom of the heat treatment tank, and then hang the welded and assembled tank shell on the resistance material at the bottom of the heat treatment tank, controlling the opening of the tank shell to face upwards.

[0029] Then, resistance material is filled into the inner cavity of the tank, ensuring the tank is filled to the brim.

[0030] Then, resistive material is filled into the gap between the outer side wall of the tank shell and the inner side wall of the heat treatment tank, so that the outer side wall of the tank shell is covered by the resistive material, and the resistive material in the gap covers the graphite anode and graphite cathode to form an electrical circuit.

[0031] Then continue to fill the heat treatment tank with resistance material, so that the resistance material completely buries, surrounds and covers the tank shell;

[0032] Then, a layer of insulation material is laid on the resistive material for insulation.

[0033] 3) Use a rectifier transformer to supply DC power to the graphite anode and graphite cathode respectively. The DC power output from the graphite anode and graphite cathode is transferred to the resistive material. When the DC power passes through the resistive material, the resistive material heats up. The heat generated by the resistive material heats the tank shell, heats the tank shell to the tempering temperature and holds it at that temperature for a period of time for tempering treatment. After the tempering and holding is completed, it is naturally cooled until the temperature of the tank shell drops below 100℃.

[0034] 4) After the tempering process is completed, remove the resistance material from the tank shell and the gaps, and then lift the tempered tank shell out of the heat treatment tank, leaving the resistance material at the bottom of the tank shell in preparation for lifting into the next tank shell to be heat treated after welding.

[0035] In one embodiment of this application, the inner cavity of the heat treatment tank has a length of 22-24m, a width of 6-8m, and a height of 1.5-2.5m.

[0036] In one embodiment of this application, the thickness of the resistive material covering any part of the inner and outer wall surfaces of the control tank is 30-60 cm.

[0037] In one embodiment of this application, in step 3), the tempering temperature is 500℃-700℃, and the holding time for tempering is 3-6h.

[0038] In this application, the purpose of post-weld heat treatment is to eliminate welding stress and improve weld microstructure and overall performance.

[0039] During welding, due to uneven heating and cooling, as well as the inherent or external constraints of the component, welding stress will always be generated in the component after welding. The presence of welding stress in the component will reduce the actual load-bearing capacity of the weld joint area, cause plastic deformation, and in severe cases, lead to component failure. The most common method to eliminate residual stress is high-temperature tempering, which involves placing the weldment in a heat treatment tank and heating it to a certain temperature (below Ac1) and holding it at that temperature for a certain time. By utilizing the reduction in the yield strength of the material at high temperatures, plastic flow will occur in areas with high internal stress, elastic deformation will gradually decrease, and plastic deformation will gradually increase, thereby reducing stress. High-temperature tempering is chosen in most cases.

[0040] After welding, the weld joint will develop a hardened structure, which deteriorates the mechanical properties of the material. In addition, this hardened structure may lead to joint failure under welding stress and hydrogen. After heat treatment, the metallographic structure of the weld joint is improved, which enhances the plasticity and toughness of the weld joint, thereby improving the overall mechanical properties of the weld joint.

[0041] In this application, the cavity inside the heat treatment tank is filled with a resistance material for heating by electricity. After being energized, the high temperature of 500℃-700℃ is mainly generated by the heating of the resistance material, which indirectly heats the tank shell.

[0042] According to the heating method, it belongs to the indirect heating electric furnace, which means that the resistance material itself is a conductor, and the tank shell is heated by resistance, so that the heat treatment after welding is completed.

[0043] The heat treatment tank is powered by DC and uses low voltage and high current for heating, which greatly reduces energy consumption;

[0044] The resistance in the heat treatment tank is mainly provided by the resistance material. During the entire post-weld heat treatment process, the heat is mainly transferred to the tank shell by the resistance material. The current ratio entering the tank shell is very small, resulting in a small amount of heat generated by the tank shell itself. About 95% of the heat is generated by the resistance material, and only 5% of the heat is generated by the tank shell itself.

[0045] Resistance material refers to the material used for resistive heating when electricity is applied. Generally, granular metallurgical coke or graphitized metallurgical coke is used as resistance material, preferably calcined petroleum coke. The particle size of the resistance material is 5-25mm. The current passes through the furnace core resistor composed of the aluminum electrolysis cell shell and the resistance material to generate heat, heating the cell shell to the temperature required for post-weld heat treatment.

[0046] In this application, the graphite anode and graphite cathode are long cylindrical in shape, with an outer diameter of 350mm-650mm and a length of 1.5m-2m.

[0047] The methods and apparatus not described in detail in this invention are all prior art and will not be elaborated further.

[0048] To better understand the present invention, the following embodiments are provided for further detailed description of the present invention, but they should not be construed as limiting the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the protection scope of the present invention.

[0049] Example 1

[0050] A method for post-weld heat treatment of the shell of an aluminum electrolytic cell, comprising the following steps performed sequentially:

[0051] 1) First, build a rectangular heat treatment pool using refractory bricks and refractory mortar, and open electrode holes on the end walls at both ends of the length of the heat treatment pool.

[0052] The internal dimensions of the heat treatment tank are 23m long, 7m wide, and 2.0m high;

[0053] Then, the graphite anode is inserted from the outside of the tank into the electrode hole at one end of the heat treatment tank until the inner end of the graphite anode extends into the cavity inside the tank. The graphite cathode is inserted from the outside of the tank into the electrode hole at the other end of the heat treatment tank until the inner end of the graphite cathode extends into the cavity inside the tank.

[0054] 2) Then lay a layer of resistance material at the bottom of the heat treatment tank, and then hang the welded and assembled tank shell on the resistance material at the bottom of the heat treatment tank, controlling the opening of the tank shell to face upwards.

[0055] Then, resistance material is filled into the inner cavity of the tank, ensuring the tank is filled to the brim.

[0056] Then, resistive material is filled into the gap between the outer side wall of the tank shell and the inner side wall of the heat treatment tank, so that the outer side wall of the tank shell is covered by the resistive material, and the resistive material in the gap covers the graphite anode and graphite cathode to form an electrical circuit.

[0057] Then continue to fill the heat treatment tank with resistance material, so that the resistance material completely buries, surrounds and covers the tank shell;

[0058] Then, a layer of insulation material is laid on the resistive material for insulation.

[0059] The thickness of the resistive material covering any part of the inner and outer walls of the control tank is 40-60cm.

[0060] 3) Use a rectifier transformer to supply DC power to the graphite anode and graphite cathode respectively. The DC power output from the graphite anode and graphite cathode is transferred to the resistive material. When the DC power passes through the resistive material, the resistive material heats up. The heat generated by the resistive material heats the tank shell, heats the tank shell to the tempering temperature and holds it at that temperature for a period of time for tempering treatment. After the tempering and holding is completed, it is naturally cooled until the temperature of the tank shell drops below 100℃.

[0061] In step 3), the tempering temperature is 550℃-650℃, and the holding time for tempering is 5 hours.

[0062] 4) After the tempering process is completed, remove the resistance material from the tank shell and the gaps, and then lift the tempered tank shell out of the heat treatment tank, leaving the resistance material at the bottom of the tank shell in preparation for lifting into the next tank shell to be heat treated after welding.

[0063] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for post-weld heat treatment of the shell of an aluminum electrolytic cell, characterized in that, The process includes the following steps performed sequentially: 1) First, construct a rectangular heat treatment tank using refractory bricks and refractory mortar, and open electrode holes on the end walls at both ends along the length of the heat treatment tank; then, insert the graphite anode from the outside into the electrode hole at one end of the heat treatment tank until the inner end of the graphite anode extends into the cavity inside the tank, and insert the graphite cathode from the outside into the electrode hole at the other end of the heat treatment tank until the inner end of the graphite cathode extends into the cavity inside the tank; 2) Then, heat treatment... A layer of resistive material is laid at the bottom of the tank. Then, the welded and assembled tank shell is suspended on the resistive material at the bottom of the heat treatment tank, with the opening of the tank shell facing upwards. Next, resistive material is filled into the inner cavity of the tank shell, ensuring it is completely filled. Then, resistive material is filled into the gap between the outer side wall of the tank shell and the inner side wall of the heat treatment tank, ensuring the outer side wall of the tank shell is covered by resistive material, and that the resistive material in the gap covers the graphite anode and graphite cathode to form an electrical path. Then, continue filling the heat treatment tank with resistive material until it completely covers the tank shell; then, lay a layer of insulating material on top of the resistive material for insulation; 3) Use a rectifier transformer to supply DC power to the graphite anode and graphite cathode respectively. The DC power output from the graphite anode and graphite cathode is transferred to the resistive material. When the DC power passes through the resistive material, the resistive material heats up. The heat generated by the resistive material heats the tank shell to the tempering temperature and holds it for a period of time for tempering treatment. After the tempering and holding are completed, allow it to cool naturally until the temperature of the tank shell drops below 100℃; 4) After the tempering treatment is completed, remove the resistive material from the tank shell and the gaps. Then, lift the tempered tank shell out of the heat treatment tank, retaining the resistive material at the bottom of the tank shell for lifting into the next tank shell to be heat treated after welding; The inner dimensions of the heat treatment tank are 22-24m long, 6-8m wide, and 1.5-2.5m high; control the thickness of the resistive material covering the inner and outer walls of the tank shell to be 30-60cm.

2. The post-weld heat treatment method for the shell of an aluminum electrolytic cell according to claim 1, characterized in that, In step 3), the tempering temperature is 500℃-700℃, and the holding time for tempering is 3-6 hours.