CN113432439B - Cooling method for aluminum electrolysis cell after stopping operation - Google Patents

Abstract

The invention belongs to the technical field of the production of aluminium by electrolysis using the Hall-Heroult (Hall-Heroult) method. More particularly, the invention relates to a cooling method after the stop of the operation of the aluminum electrolysis cell, and a cooling device for reducing the cooling time of the electrolysis cell after the shut-down and saving the cooling time is arranged. The invention is provided with a thermal radiation absorption system, a forced convection system and an impact jet system which are combined to cool the aluminum electrolysis cell after the operation is stopped, and provides specific parameters of each system. The invention can reduce the cooling time of the electrolytic cell by about 60 hours by combining the forced convection system, the impinging jet system and the thermal radiation absorption system.

Description

A kind of cooling method after aluminum electrolytic cell stops working

technical field

The present invention belongs to the technical field of the production of aluminium by electrolysis using the Hall-Héroult method. More specifically, the present invention relates to a cooling method for an aluminum electrolytic cell after shutdown, providing a cooling device for reducing the cooling time of the electrolytic cell after closing and for saving the cooling time.

Background technique

In an aluminum smelter, primary aluminum is extracted from ore (alumina) by passing direct current electricity through series-connected electrolyzers. The basic process by which these cells work is named the Hall-Héroult process after its inventor. A typical Hall-Héroult cell consists of a cell body, an anode assembly and a superstructure that limits heat loss at the top of the cell. The cell body consists of a steel shell lined with refractory insulating material and cathode blocks. During operation, the electrolytic cell is filled with molten salt cryolite (Na ₃ AlF ₆ ), to which alumina (Al ₂ O ₃ ) powder is added and dissolved to form a solution. A current of even more than 350 kA is passed from the anode through the alumina-containing electrolyte to the cathode liner, resulting in an alumina reduction process and maintaining the temperature of the electrolyte at 960°C. The aluminium metal produced by the electrolysis process sinks to the bottom of the reduction tank, while the gases produced are released through the cell hood. The aluminium is drawn from the bottom and conveyed to the furnace of the casting chamber, where it is alloyed and cast into various forms of primary aluminium products.

As we all know, the Hall-Héroult process is a continuous process and an aluminium smelter cannot easily be stopped and restarted. However, various conditions including cell failure, wear and high cathode voltage drop can cause interruption of cell operation. For example, if the operation of the electrolyzer is interrupted for more than 4 hours, during which time the metal in the bath of the tank solidifies, a rebuilding process is usually required, which preserves the expensive electrolyzer shell for subsequent use. Before regenerating, the cell must be shut down, the anodes removed, and the entire cell allowed to cool. Typically, this cooling process is performed in a free convection environment (eg outdoors) because rapid cooling may result in failure of the tank shell material. This free cooling has proven to be a lengthy process, taking 5 to 9 days, depending on the ambient temperature. Cooling-induced delays as subsequent rebuilding of an electrolyser requires the previous electrolyser cell shell after the electrolyser has been shut down, and the reconditioning process cannot begin unless the electrolyser has cooled to a temperature at which all lining material can be safely stripped from the cell. Affects the production capacity of the smelter. In addition, environmental regulations do not permit the cooling of electrolyzers outside due to the release of harmful gases. Cooling the electrolyser in an enclosed space further increases the cooling time, extending the smelter's production time and reducing production capacity.

Providing spare slots has long been the solution to this problem. However, there are times when a smelter needs to shut down a large number of electrolytic cells for maintenance in a short period of time, and replacement of spare cells is also costly. Applicable to the cooling process of conventional electrolyzers, the method of effectively reducing the cooling time of out-of-service electrolyzers does not involve high costs and is a feasible solution to this problem.

In order to increase the heat extraction from the electrolysis cell, US Patent (Application Serial No. 4,073,714) describes the use of a plurality of aluminum extruded cooling rods partially immersed in the anode mixture of the electrolysis cell, the unimmersed portions of the rods passing through the natural Convective cooling, which is not suitable for cooling after an aluminum cell is out of service, as the cell anode is usually drawn out before the cell is closed for cooling. In addition, natural convection has been shown to be slow in cooling closed electrolyzers, so improvements to this method require the incorporation of some expensive technical means. US Patent (Application Serial No. 6,251,237) proposes the use of localized, dispersed jets over the cell shell to control the heat flux of the cell, designed to provide multiple air cells that maintain cell heat balance parameters during cell repair nozzle. This method is not suitable for cooling off-line electrolyzers that are out of service because the lining material between the tank shell and the molten pool limits the heat in this direction. The cooling time reduction is not significant enough for this approach compared to its cost. In addition, since this method only effectively cools the tank shell, it is difficult to affect the lining material, which can create undesired temperature gradients and cause heat on the tank shell because the expansion of the inner lining material is in the opposite direction to the contraction of the tank shell. stress.

SUMMARY OF THE INVENTION

Considering the lack of a real solution to the problem that the cooling time of the out-of-service aluminum electrolytic cell is too long to delay production in the prior art, the present invention proposes a cooling method suitable for the out-of-service aluminum electrolytic cell to ensure that the smelter Unnecessary yield drops can be avoided.

The invention proposes a cooling method for an aluminum electrolytic cell used in the process of producing aluminum by Hall-Eilut electrolysis after the off-line operation is stopped. In the method, a heat radiation absorption system, a forced convection system, and an impinging jet system are arranged, and the three are combined to cool the aluminum electrolytic cell after the operation is stopped.

The heat radiation absorption system includes a metal pipe (preferably an aluminum pipe), a cooling liquid circulating pump, and a cold source, and the metal pipe, the cooling liquid circulating pump, and the cold source are interconnected; a part of each metal pipe is arranged above the aluminum liquid and the groove, Parallel to each other, extending from one end of the electrolytic cell to the other end of the electrolytic cell, this part of the metal tubes arranged above the molten aluminum and the tank side is called the heat collector; The metal plates are welded together, or several tubes are welded together through the metal plates in a group, so that the heat collector forms a plate-shaped structure with a larger area, so as to increase the area for collecting heat. The metal plate and metal tube itself has a high thermal radiation absorption capacity, and the surface can be coated with a high emissivity coating to prevent overheating.

The heat collector absorbs the heat energy in the tank through thermal radiation, especially the heat energy at the aluminum liquid and the tank side. At the collector, that is, it flows from one end of the electrolytic cell to the other end, and the heat absorbed by the heat collector is extracted through the surface-to-surface radiative heat transfer, and then the cooling liquid flows into the cold source, bringing the heat absorbed by the heat collector into the cold source. In the middle, the cooling liquid is cooled in the cold source and then returned to the cooling liquid circulation pump, and then enters the metal pipe again to realize the recycling and utilization of the cooling liquid.

In addition to being arranged in parallel with the upper surface of the electrolytic cell, the heat collector can also be arranged in a zigzag shape, or the metal pipes of the heat collector can be arranged in a certain shape with the upper surface of the electrolytic cell at the two ends of the electrolytic cell. The angle is set and then connected in the middle of the electrolyzer to form an inverted V shape when viewed from the side. This expands the area of the heat collector and improves the ability to absorb heat.

In addition, in order to maintain the overall temperature coordination of the heat collector, the cooling liquid in the two adjacent metal pipes at the heat collector can flow in opposite directions by adjusting the metal pipe pipeline.

Preferably, the height of the heat collector is set to be 1m-1.2m, preferably 1.2m, from the top of the aluminum electrolytic cell, and the initial temperature of the cooling liquid when entering the heat collector is 10°C-25°C.

The forced convection system includes an air duct and a blower. The air duct is arranged outside the bottom end of the tank shell on the side of the aluminum electrolytic cell. The blower blows cold air into the air duct to form forced convection cooling with the bottom end of the tank shell on the side of the electrolytic cell. The air duct can also be set at the bottom of the tank shell of the aluminum electrolytic cell, and pass through the horizontal length (width) of the electrolytic cell to form forced convection cooling. the gaps between these brackets. The air velocity in the air duct in the forced convection system is 5 to 20 m/s.

The impinging jet system includes an air compressor (which can also be a blower) and a pipeline network communicated with the air compressor (or blower), and the pipeline network is arranged at the side tank shell of the electrolytic cell, the top plate of the electrolytic cell and above the electrolytic cell ( Refers to the aluminum liquid and the top of the tank side of the electrolytic cell), the pipeline network is equipped with nozzles on the side facing the side tank shell of the electrolytic tank, the top plate of the electrolytic tank, the aluminum liquid and the tank side; the air compressor sends the compressed air into the pipeline network, compressing The air flows at a high speed in the pipe network and is ejected from the nozzle, forming an air impact jet with the hot surface of the side tank shell, the top plate of the electrolytic tank, the molten aluminum and the tank side, etc., to cool these parts. In order to prevent the exhaust gas in the electrolytic cell from polluting the environment, and to reduce the residence time of the air after hitting the jet in the area near the hot surface and improve the cooling efficiency, a ventilation system can be installed above the electrolytic cell to cool the electrolytic cell. The exhaust gas is extracted from the electrolyzer, which not only forms the air flow loop of the air compressor-pipe network-nozzle-hot surface-ventilation system, but also facilitates the centralized treatment of the exhaust gas.

Because the heat collector in the heat radiation absorption system is also set on the pipe network above the electrolytic cell, and in order to prevent the exhaust gas from flowing out, it is better to keep the jet area above the electrolytic cell closed, so a cover plate is set above the heat collector. The plates are in communication with the ventilation ducts of the ventilation system through which the air after impinging the jets is drawn from around the electrolytic cell. That is, above the electrolytic cell, from bottom to top are the piping network of the impinging jet system above the electrolytic cell, the heat collector of the heat radiation absorption system, the cover plate and the ventilation pipes of the ventilation system.

After the simulation calculation of the cooling model of the electrolytic cell, the optimal values of the parameters of the impinging jet system above the electrolytic cell are set as follows: the gas flow rate of the exit nozzle of the impinging jet system is 5m/s～12m/s, preferably 5m/s, Above the electrolytic cell, that is, above the aluminum liquid and the tank side, the diameter of the pipes in the pipeline network that hits the jet system is 0.16-0.28m, preferably 0.16m. A cooling water pipe can be set next to the pipe to prevent overheating. Above the electrolytic cell, the nozzle The arrangement between them is uniform distribution and linear arrangement. The nozzles are preferably circular nozzles. The diameter of the nozzle outlet is 0.05m to 0.06m, preferably 0.053m. The intervals between the nozzles are consistent and evenly distributed in the same plane. 1m, preferably 0.62m, and the nozzle is 50cm away from the surface of the molten aluminum. Combinations of the individual preferred values allow for optimum cooling.

The pipe network and nozzles of the impinging jet system are also arranged on the side tank shell and top plate of the electrolytic cell. The pipe diameter, nozzle outlet diameter, and gas flow rate of the nozzle can also refer to the pipe network above the electrolytic tank. The surface of the nozzle and the side tank shell and top plate can also be referred to. The distance can be 25-50cm. These parts are easier to cool than the top of the electrolytic cell. Therefore, on one side of the tank shell or one side of the top plate, a minimum of 3 nozzles can be set to achieve cooling, and the number of nozzles can be adjusted according to the actual situation. .

The present invention can reduce the cooling time of the electrolytic cell by about 60 hours through the combination of the forced convection system, the impinging jet system and the heat radiation absorption system.

Description of drawings

Figure 1: Schematic diagram of the cross-sectional structure and material of the DX aluminum electrolytic cell, in which:

1-Side tank shell, 2-Vermiculite, 3-Refractory brick, 4-Carbon electrode, 5-Slot side, 6-Side carbon block, 7-Refractory brick, 8-Silicon carbide, 9-Tamping paste, 10-top plate, 11-aluminum liquid, 12-bottom of tank shell, 13-cathode steel rod.

Figure 2: Side section and material engineering drawing of the width of the electrolytic cell, in which: 201-calcium silicate, 202-vermiculite, 203-castable, 204-steel, 205-firebrick, 206-side carbon block, 207-cathode Carbon block, 208-side carbon block, 209-castable, 210-side carbon block, 211-ramming paste, 212-ramming carbon paste.

Figure 3: Material Engineering Drawing of Electrolyzer Length Side Section.

Figure 4: Schematic diagram of the piping network of the impinging jet system and its matching ventilation system, in which: 401-pipeline network above the electrolyzer, 402-nozzle, 403-cover plate, 404-ventilation pipe.

Figure 5: Axial view of the cooling system from the inside of the electrolyzer, where: 501 - Heat collector, 502 - Metal pipes, 503 - Pipe network at side tank shell and top plate of impinging jet system, 504 - Air duct.

Figure 6: Length side sectional view of the cooling system, wherein: 505 - cold source, 506 - blower.

Figure 7: Width side sectional view of the cooling system.

Figure 8: Axial view of the cooling system from the outside of the cell.

Figure 9: Convective heat transfer coefficients as a function of temperature at the center of the side shell, the top plate and the bottom of the shell under free convection and forced convection, where: 901-side shell center, free convection, 902-top plate, free Convection, 903 - Bottom of tank, free convection, 904 - Center of side tank, forced convection, 905 - Top plate, forced convection, 906 - Bottom of tank, forced convection.

Figure 10: Cooling curve at the center of the side tank shell.

Figure 11: Heat flux in the center of the side tank shell.

Figure 12: Cooling curve for the top plate of the tank shell.

Figure 13: Heat flux in the top plate of the tank shell.

Figure 14: Cooling curve at the bottom of the tank.

Figure 15: Heat flux at the bottom of the tank.

Figure 16: Schematic diagram of the forced convection cooling scheme of the tank shell.

Figure 17: Schematic (axial view) of the heat collector of the thermal radiation absorption system.

Figure 18: Schematic diagram (top view) of the heat collector of the thermal radiation absorption system.

Figure 19: Schematic (width side cross-sectional view) of the heat collector of the thermal radiation absorption system.

Figure 20: Schematic (axial view) of a tilted heat collector.

Figure 21: Schematic diagram of a sloped heat collector (width side cross-sectional view).

Figure 22: Z-shaped heat collector (axial view).

Figure 23: Z-shaped heat collector (width side sectional view).

Figure 24: Schematic of the heat collector cell plate.

Figure 25: Parameter optimization curve for the nozzle set above the electrolytic cell.

Figure 26: Basic nozzle unit for a pipe network.

Detailed ways

The aluminum electrolytic cell used in this embodiment is a DX technology aluminum electrolytic cell, and its cross-sectional structure and materials are shown in FIG. 1 . The cell consists of an outer shell made of S275J grade steel with multiple layers of lining material inside. The bracket/cell shell consists of a relatively loose shell supported by a strong structure, with reinforced I-beams on the vertical sides of the cell and I-shaped brackets on the bottom of the cell for support to counteract outward and upward bowing on both sides of the cathode .

The lining material at the bottom of the cell is insulating bricks, which prevent a large amount of heat from dissipating through the bottom of the cell. Special materials used in this part of the cell include vermiculite slabs, refractory bricks and calcium silicate slabs. These materials have low thermal conductivity and are stiff enough to carry other structures above. Above the bottom, the lining material is the cathode carbon block that acts as the cathode of the cell. Cathode carbon blocks are generally more suitable for lining than tamping pastes because they provide higher strength, higher density and lower resistivity. The cathode carbon blocks are evenly distributed over the entire length of the electrolytic cell, and there is a tamping paste in the gap between the cathode carbon blocks and the electrolytic cell, forming a liquid-tight container for the reduction reaction. The penetration degree and penetration rate of the electrolyte at the bottom of the electrolytic cell largely determine the service life and energy utilization rate of the electrolytic cell. Therefore, the sealing effect of the tamping paste is an important factor determining the service life and energy utilization rate of the electrolytic cell. The lining material is also used to separate the electrolyte/aluminium located in the cell from the side walls of the cell shell, the material used for the side lining of the cell needs to be able to maintain the heat balance required by the cell, the material used for the side lining in the DX cell is Carbon and silicon carbide bricks, which are set based on their thermal properties: Carbon and silicon carbide bricks allow heat to flow through the sides of the grooves, creating the sides of the grooves.

To study free convection cooling of aluminum cells, temperature readings were collected from various parts of the aluminum cell that were out of service during free convection cooling. To do this, place thermocouples on the top plate, side tank shells, bottom of the tank shell, and in the center of the electrolyte/aluminum after shutdown. Each reading was collected at two adjacent locations in the slot to ensure complete confidence. The temperature test started immediately after the electrolyzer was taken out of operation and continued for nearly 7 days during the various stages of the electrolyzer's movement from the electrolysis plant to the maintenance room. The temperature of the electrolyte before and after aluminum production was tested, and the temperature of the top plate, the bottom of the cell and the remaining metal in the cell were tested as a function of time. These tests were all carried out in the middle part of the cell.

In the side tank shell, thermocouples are arranged at the upstream end, the downstream end and the aluminum outlet end of the side tank shell of the electrolytic cell. Three thermocouples are arranged on the top plate of the electrolytic cell, respectively at the upstream end, the downstream end and the aluminum outlet end. Two thermocouples were placed at the bottom of the electrolytic cell shell, 800 mm from the upstream and downstream sides, respectively. In the electrolysis plant, it is not possible to install thermocouples at a distance of 800 mm from the upstream and downstream sides, so the measurement is performed at a distance of 100 mm from both edges. The specific instruments used are as follows: used to detect the surface temperature of the tank shell (including the top plate, side tank shell and bottom of the tank shell) for detecting the temperature of the electrolyte/aluminum liquid, Marshal Tip thermocouple assembly for detecting the temperature of the electrolyte/aluminum liquid, Fluke and Aristo thermometers. Obtain cell temperature data from these set-up thermocouples.

In the electrolysis plant, the first set of measurements was made 15 minutes before and 15 minutes after the outage, and then every hour for 19 hours. Outside the electrolysis plant, the first set of measurements was completed 3 hr. 20 min after the cell was taken offline and transferred, and the last set of measurements was completed 6 days and 23 hours after the power outage.

The CFD model is modeled and analyzed by ANSYS and Star-CCM+.

The 2D engineering drawing of the electrolytic cell is drawn with AutoCAD, which maps the actual materials used in the electrolytic cell into the 2D engineering drawing, as shown in Figures 2 and 3, showing the width side section and the length side section of the electrolytic cell, respectively.

The cooling system for the DX electrolytic cell in this embodiment includes: (1) Forced convection system: including an air duct and a blower, and the air duct is located outside the bottom end of the side tank shell of the aluminum electrolytic cell, and can also be installed in the electrolytic cell tank shell at the same time. At the bottom, the blower blows the cold air into the air duct, and the air duct can introduce the cold air into the bottom of the electrolytic cell to form forced convection, and the introduction speed can be 5m/s-20m/s. (2) The impinging jet system includes an air compressor and a pipeline network communicated with the air compressor. The pipeline network is arranged at the side tank shell of the electrolytic cell and above the electrolytic tank. A nozzle is set on one side of the molten aluminum and the tank side; the air compressor sends the compressed air into the pipeline network, the compressed air flows in the pipeline network at a high speed, and is ejected from the nozzle, which is connected with the side tank shell, the top plate of the electrolytic tank, the molten aluminum and the tank. Helps create air impingement jets on hot surfaces. (3) The heat radiation absorption system includes a metal pipe, a cooling liquid circulating pump, a cold source, and the metal pipe, the cooling liquid circulating pump, and the cold source are connected to each other; a part of the metal pipe is arranged above the aluminum liquid and the groove, parallel to each other, from the One end of the electrolytic cell is extended to the other end of the electrolytic cell. This part of the metal tube is called the heat collector. These metal tubes can also be welded to each other through metal plates, or several tubes are welded together through metal plates in a group to make the heat collector. Form a larger area plate structure to increase the area for collecting heat. The cooling liquid circulation pump pumps the cooling liquid into the metal pipe, the cooling liquid flows through the metal pipe through the aluminum liquid and the top of the groove, and then flows into the cold source, and then returns to the cooling liquid circulation pump for circulation after being cooled by the cold source.

Figures 5-8 show visualizations of the system, in which only a quarter of the electrolyzer is drawn for illustration. In the figure, for the sake of clarity, the piping network of the impinging jet device above the molten aluminum and the trough is not drawn, and the ventilation system is not drawn. The heat radiation absorption system only shows part of the metal tubes and the cold source, but it is not difficult to understand that after the cooling liquid in the metal tube flows into the cold source for cooling, it is pumped into the metal tube again through the cooling liquid circulating pump for recycling.

Regarding the pipeline network of the impinging jet system set above the molten aluminum and the tank top and the matching ventilation system, as shown in FIG. 4 , in the impinging jet cooling technology of this embodiment, The nozzles in the pipe network, using an in-line array of circular nozzles, are the most efficient impinging jet configuration for a given gas flow rate. In this embodiment, according to the size of the electrolytic cell, a gap of 50 cm is left between the nozzle and the molten aluminum, and there is also a gap of 50 cm between the nozzle at the outer edge and the vertical plane where the inner side wall of the electrolytic cell is located. The airflow velocity out of the nozzle is considered to vary from 5 m/s to 20 m/s. A preferred array of impinging jets occurs when the space between adjacent nozzles is opened to the surrounding environment, allowing continuous upward flow of air and direct discharge of heated waste air. In order to prevent pollution of the environment and to make the air form a complete loop, a ventilation system is used in the design. The ventilation system and the impact jet system pipeline network set above the aluminum liquid and the tank are shown in Figure 4. In these pipeline networks A cover plate is arranged above the plate, and the cover plate is connected to the ventilation duct of the ventilation system, and the exhaust gas flowing upward after hitting the jet is drawn out of the electrolytic cell through the ventilation system. It should be noted that Figure 4 is only a schematic diagram of the separate aluminum liquid and the pipe network of the impinging jet system set above the trough and the matching ventilation system. Although it is not drawn in Figure 4, it is actually in the pipe network. And between the cover plate, there is also the heat collector of the thermal radiation absorption system described above.

The piping network and nozzles of the impingement jet system are arranged above the aluminum liquid/electrolyte and the tank side (also called above the electrolytic tank), above the top plate of the electrolytic tank and at the side tank shell of the electrolytic tank, above the aluminum liquid/electrolyte and the tank side Due to the better location, this part of the heat can be effectively removed from the cell by impinging jets. Because the air duct in the designed forced convection system cannot effectively cool the upper part of the side tank shell due to the blockage of the cathode steel rod in the electrolytic cell, it is proposed to also use the impinging jet to locally cool this part, so that the compressed air passes through the pipe network and The nozzles are introduced into specific locations on the side troughs to enhance cooling by increasing air velocity over the trough surfaces and introducing turbulence. In the same way, the impinging jet can also be used to cool the top plate, because the top plate is also an effective area for heat dissipation of the electrolytic cell.

The molten aluminium/electrolyte and tank sides are the hottest areas of the electrolysis cell and the most efficient cooling locations. However, if the impact jet is completely relied on to cool this part, a lot of harmful fumes will be generated, and even if there is a ventilation system, there will still be a greater environmental hazard. Therefore, the cooling of the molten aluminum/electrolyte and the tank side also employs a thermal radiation absorption system. The heat collector itself of the thermal radiation absorption system is made of materials with high radiation absorption rate, such as aluminum (metal tube and metal plate welded between the metal pipes), and the surface of the heat collector can also be coated with a coating of high emissivity material. layer to prevent overheating. After the electrolytic cell is stopped and the anode of the electrolytic cell is removed, the heat collector is moved over the molten aluminum and the tank side, and the cooling liquid flows through the heat collector, collects heat from the high temperature surface, flows into the cold source for cooling and reintroduces the circulation.

The configuration of the nozzle array (nozzle diameter and spacing) was optimized by parametric studies to fit the gap between the molten aluminum and the nozzle array in the electrolytic cell to operate with the maximum possible efficiency. In the optimal configuration, the heat transfer relationship can be derived by varying the air injection velocity and the surface temperature of the cooling body.

Each of these cooling systems described above will initially be considered separately, and their effect on the overall cooling curve will be investigated. After optimizing these processes, these cooling systems were combined and the effect on the resulting cooling curves was examined.

1. CFD model of duct cooling (forced convection)

A 3D finite element-based computational fluid dynamics (CFD) model was established for the shell (tank shell) of an electrolytic cell in an aluminum smelter, and a 0.02m-sized intake duct was set below the cathode steel rod. The model simulates a forced flow of air flow at a velocity of 5 m/s over the cathode steel rod, the outside of the side tank shell and the bottom of the tank shell. The temperature-dependent heat transfer coefficients were estimated by modeling the exact shape of the isothermal surface and the trough shell in the appropriate ambient air domain, with inlet velocity boundary conditions introduced at specific locations on the ground. Due to the strong dependence of domain flow on temperature, a coupling-based solver is used. The induced turbulence was modeled using the RNG k-ε Reynolds averaging model. The obtained convective heat transfer coefficients at the center of the side shell, the top plate and the bottom of the shell as a function of temperature are shown in Fig. 9 and compared with the heat transfer coefficients under free convection at these positions in Fig. 9 .

By introducing forced convection, the heat transfer coefficient is increased threefold at the center of the side shell and twofold at the bottom of the shell, but the heat transfer coefficient in the top plate region is hardly affected because the top plate is far away from the cooling channels. Similar curves were established for the rest of the tank shell through this model, and these curves were used for the boundary conditions of the FHT (Fast Hartley Transform) model. The previously obtained steady-state solution was used as the initial condition for this model. The heights of residual aluminum liquid and electrolyte in the model are 4.65 cm and 1 cm, respectively. The model simulates the removal of molten aluminum/electrolyte within 1 hour by breaking the interface between the upper molten aluminum/electrolyte and the residual metal. This ensures that metal heat is removed from the model after the steady state solution is obtained. Metal solidification at a temperature of 660°C is modeled by entering commands to extract latent heat. The transfer of the electrolysis plant to the maintenance room is thermally modeled by changes in ambient temperature, ignoring the effects of wind during this movement. The model introduces surface-to-surface radiation to estimate the performance of thermal radiation absorbing systems. The surface-to-surface radiation model was simulated using a one-dimensional equation and a maximum error of 15% was recorded. Assume that all objects are gray and their radiance is constant. The emissivity of the heat collector is 0.9 and the emissivity of the electrolyte is 0.45. Four models were built to evaluate performance as follows:

Case 1: Side cooling alone;

Case 2: Cool the sides and bottom;

Case 3: The top is cooled separately;

Case 4: The top, sides and bottom are cooled at the same time.

The temperature and historical heat flux in each case were estimated for each section of the tank shell. Because the modeling was done with a quarter cell (14U/S), the heat flux estimate is one quarter that of the DX cell and can be multiplied by 4 to get the total heat flux. Figures 10 and 11 show the temperature records (cooling curves) and heat fluxes at the center of the side tank shell for these four cases. In case 1 to case 4, the cooling rate at the side tank shell is increased. This represents an overall increase in cooling performance of the tank shell from the case 1-4 side. The heat flux history in Figure 11 also shows that from Cases 1-4, the heat absorption rate for this part of the cell has increased.

Figures 12 and 13 show the temperature and heat flux change records of the tank top plate. In cases 1-4, the cooling rate of the tank top plate was also increased. However, in the cooling process of cases 1-4, the heat flux of this part is smaller than that of the free convection environment. Since no cooling was applied to the top plate portion of the trough, it is suggested that other cooled portions may have extracted heat that naturally flows through the top plate.

In these cases, the cooling curves and heat flux history recorded at the bottom of the tank support this hypothesis (Figures 14 and 15). Figure 15 shows that when forced convection cooling is applied to the bottom of the tank shell, faster cooling and heat extraction results. However, when only the side tank shells are forcedly cooled, the cooling rate at the bottom of the tank increases, but the heat flux decreases.

Cases 1 and 2 do not cause significant changes in cooling curves and heat fluxes. However, careful observation shows that cases 1 and 2 will slightly increase the cooling rate of the molten aluminum, but compared with the values under free convection, the heat dissipation rate of the molten aluminum decreases slightly. This indicates that the heat flow initially removed from the top by free convection is removed from the side in both cases. In this way, the overall increase in the cooling rate of the molten aluminum in cases 1 and 2 is relatively minimal. However, in case 3, the cooling rate and heat dissipation rate of the molten aluminum are increased when cooling is performed only from the top. While in case 4, omnidirectional cooling further increases the cooling rate and heat dissipation rate.

These results show that the best cooling method for heat dissipation is to use the ideas in Case 4. Since heat dissipation is limited by thermal insulation on the sides and bottom of the cell, the transfer of heat from side to side depends on the magnitude of the heat transfer coefficient rather than removal from the heat source. On the other hand, removing heat only from the top reduces the heat flux to the sides and bottom, ie the thermal resistance increases in those directions. In addition, uneven cooling rates can cause thermal stresses that can damage the steel tank shell.

2. Forced convection cooling system scheme of tank shell

When air is forced to contact the surface, the air particles that touch the surface will be blocked and eventually stop. This causes adjacent fluid layers to slow down due to viscous stresses (frictional forces) between adjacent fluid elements moving at different speeds. Subsequent layers of fluid were also blocked, but not to the same extent as the initial layers. This trend continued to the region where the stagnant fluid had a negligible effect on the flow rate (Figure 16). The region where the velocity gradient occurs is called the velocity boundary layer.

Likewise, when the temperature of the fluid differs from the temperature of the tank, a temperature gradient is created. The region where this temperature gradient exists is called the thermal boundary layer. Conditions in this boundary layer determine the nature of heat transfer from the fluid to the tank. The nature of heat transfer from flat surfaces is now well understood and there are corresponding experimental statistics, but there are no good experimental statistics for such relatively complex surfaces as electrolyzers. In this forced convection cooling technique, the convective heat transfer of the tank shell is achieved by increasing the air flow rate on its surface. Although this can also be achieved by reducing the temperature of the ambient air, increasing the air velocity is a more economical way to viable option. In the smelter, this cooling scheme can be realized by placing the air duct on the ground at the bottom end of the tank shell on the side of the electrolytic cell, as shown in Figure 8 and Figure 16, and blowing cold air into the air duct through a blower.

Unlike the hot plate facing upwards, the hot plate facing downwards (the bottom of the tank shell in the case of aluminum electrolysis cells) is not suitable for buoyancy-driven free convection. This is due to the negative temperature gradient of the air from the plate to the ground. The denser, cooler air is at the bottom, and the less dense, warmer air is located very close to the hot surface, and the system is in equilibrium. The larger area of the bottom of the tank means that there will be a considerable balance area of this air stagnation. In order to eliminate this stagnation, it is proposed to also provide an air duct for blowing cold air at the bottom. This process will increase heat transfer from the bottom of the cell by reducing the ambient temperature at the bottom of the cell shell, increasing air flow velocity and introducing turbulence. In addition, it is also expected that the effect of reducing the overall ambient temperature of the maintenance room can be produced. The bottom of the electrolytic cell is generally provided with an I-shaped support, and the air duct at the bottom of the electrolytic cell can be arranged between these support structures and pass through the lateral length of the electrolytic cell.

3. Thermal radiation absorption system scheme

Thermal radiation absorption systems drive heat away by radiation. It is well known that radiant systems are most effective at high temperatures, so the molten aluminum and sump are the target locations for this heat extraction system. The solution involves placing an efficient radiation absorber (heat collector) above the open electrolysis cell to enhance the heat transfer from the molten aluminium/bath during cooling. The heat collected by the heat collector is conducted away from the system by the cooling fluid flowing through the heat collector. The schematic diagram of the heat collector of the thermal radiation absorption system is shown in Figure 17-19. During operation, the heat collector is simply lowered towards the open cell in the maintenance compartment. The cooling liquid is passed from one longitudinal side (length side) of the tank to the other through metal pipes in the heat collector. As shown in FIG. 18 , the cooling liquids in the two adjacent metal tubes of the heat collector can be set to flow in opposite directions to ensure uniform and stable surface temperature of the heat collector. Coolant flowing from the heat collector flows through a cooling source for cooling before being reintroduced into the circulation via the coolant circulation pump.

Unlike a heat collector with a completely planar structure, exposing part of the surface area of the metal tube to the hot surface also increases the surface area of the collector by about 40%. In theory, this improves the radiative heat transfer capacity of the heat collector, so welding metal plates between metal tubes is a more effective way to increase the radiative heat transfer capacity. To further enhance heat dissipation, the metal tube orientation of the heat collector can be modified to maximize the surface area of the heat collector on the aluminum liquid surface. For example, a heat collector with metal tubes inclined at an angle (such as 45°) at the two ends of the electrolytic cell, as shown in Figures 20 and 21, since the modeling is on a quarter of the electrolytic cell, Figures 20 and 21 21 shows only half of the metal tube. It is conceivable that the other half of the metal tube is symmetrical with Figure 20. The metal tube and the upper surface of the electrolytic cell form a triangle or an inverted V shape when viewed from the lateral side. A zigzag heat collector can also be used, as shown in Figures 22 and 23.

4. Impact jet system scheme

In this cooling system solution, an array of impinging jets impinging on the molten aluminum is used to increase the heat transfer coefficient on the surface of the molten aluminum. Linear arrays of circular nozzles, staggered arrays of circular nozzles, and slit nozzles are three options available for air impingement jet system designs. When choosing an array configuration, it is preferred that the air after impinging jets be allowed to flow continuously upwards and discharge the exhaust gas directly when the space between adjacent nozzles is open to the surrounding environment. Since this method may contaminate the environment of the smelter, a suitable ventilation system has been incorporated into its design. In addition, to avoid overheating, cooling water pipes can be arranged on both sides of the pipe network.

5. Performance evaluation and parameter optimization of cooling scheme

With the help of the developed mathematical model, the performance of the electrolytic cell cooling scheme in this example was evaluated. The technologies used are forced convection at the tank shell (mainly the bottom end of the side tank shell and the bottom of the tank shell) through the air duct, heat radiation absorption above the molten aluminum, and above the electrolytic cell (mainly the molten aluminum and the tank top). above), impinging air jets set at the top plate and side tank shells of the electrolysis cell. For the forced convection duct arrangement, the cooling scheme is based on cooling the electrolyzer by forcing air over the tank shell, which requires high accuracy (translated into millions of discrete nodes) for modeling the airflow over the tank shell structure. This approach was used in evaluating this cooling scheme, and a special model was developed to estimate the effect of air velocity on heat transfer through the tank shell. The flow rate-dependent heat transfer coefficients calculated with the model were used in the model of the electrolytic cell, which was then used to evaluate the cooling performance of the electrolytic cell. The performance of the other two schemes, involving radiating and impinging jets on a flat molten aluminum surface, was also evaluated, but for which no fluid domains were set that would introduce unrealistic computational durations. It is very important to note that since cooling cannot be done on the electrolyser train in the smelter, the above active cooling techniques are only enabled in the model after the electrolyser has been moved to the electrolyser maintenance room (14 hours after outage) .

For the tank shell forced convection cooling scheme, a 3D CFD model was developed for the tank shell of the DX electrolyzer with a 17cm x 40cm air duct below the cathode steel rods. This size is chosen so that the duct fits between the I-shaped support structures of the cell support, forming part of the cell shell with minimal resistance to heat transfer. The temperature-dependent heat transfer coefficients are calculated by modeling the exact shape of the isothermal layer and trough shell in a suitable ambient air domain, which is similar to the free convection model. However, in this model, the pipeline is introduced as the inlet velocity boundary condition, forcing the gas flow at the velocity of 5m/s to 20m/s in the cathode steel rod, the side tank shell, and the bottom of the tank shell. Due to the strong dependence of the domain flow on temperature, a coupling-based solver was used to model the induced turbulence with the RNGk-ε Reynolds averaging model, and gravitational effects were also included. The computational domain is discretized using a grid with a maximum size of 0.002m. As a rule of thumb, the boundary conditions outside the tank shell are modeled as ambient temperature walls, rather than pressure outlets, as the latter causes back pressure. The momentum and energy equations use convergence criteria for residual values of 10 ^-3 and 10 ^-6 , respectively. Varying the wall temperature and air velocity, and calculating the temperature-dependent heat transfer coefficient for each bracket surface section. According to the simulation calculation, when the air velocity of 5m/s, 10m/s and 20m/s is adopted, the cooling time can be reduced by about 22, 24 and 26 hours respectively compared with natural convection cooling.

Radiative cooling schemes were evaluated by placing the collector at a distance above the molten aluminum surface with an air domain in between. The net radiative flux at each surface is a function of the properties of the surface, and thermal boundary conditions are applied to the surface to equilibrate the radiation by calculation. Radiation balance can be achieved across all surfaces of a closed system by considering each surface and the way it exchanges radiation with all other surfaces. Therefore, the first step in the model is to calculate each surface-to-surface interaction. After decomposing the surface into patches and calculating the angular coefficients of patch pairs, the radiant flux on each patch can be obtained independently by applying a spectral radiance balance across all surfaces of the entire closed system. An innovative method was used to obtain the transmit power, reflectance and transmittance values of the patches by surface averaging the element surface values of all boundary surfaces that make up each patch. Studies were conducted to determine the effect of changing the heat collector temperature, the gap with the molten aluminum surface, and the heat collector area. Determine the distance between the suitable heat collector and the top of the electrolytic cell to be 1-1.2m. Considering the temperature range from 10°C to 25°C, the zigzag heat collector is used to increase the area of the heat collector. Figure 24 is the design drawing of the unit plate of the heat collector, the metal tube and the metal plate are made of aluminum, the inner diameter of the metal tube is 10 cm, the thickness is 1 cm, and the thickness of the metal plate is 2 cm. When these unit plates are arranged parallel to the surface of the molten aluminum (or the upper surface of the electrolytic cell), the electrolytic cell requires 14 such unit plates. If the inclined heat collector of Figure 20 or the zigzag heat collector of Figure 22 is used, different cell plate sizes and numbers may be required depending on the angle of inclination or the direction of the zigzag zigzag.

In the impinging jet system, for the nozzles in the pipe network above the electrolyzer, an optimal nozzle arrangement is achieved by choosing the nozzle parameters that produce the greatest heat transfer at a specific gas flow rate. Optimal values for nozzle diameter (D) and nozzle spacing (s) were found for a given total gas flow rate and a fixed nozzle-to-cooling surface gap (H).

Figure 25 shows the parameter optimization curve of the nozzle set above the electrolytic tank when the nozzle of the impinging jet system is circular, the array is arranged in-line, the gas flow rate of the outlet nozzle is 5m/s, and the distance from the surface of the molten aluminum is 50cm. The results show that under this condition, the optimized nozzle outlet diameter is 0.05-0.06m, and the optimal value is 0.053m. According to Figure 25, it can be seen that the diameter of the pipes in the pipe network of the impinging jet system is preferably 0.16m to 0.28m, and the optimal value is 0.16m. , distributed uniformly in the same plane.

With this structure, a total of 132 uniformly distributed nozzles are required above the electrolytic cell used in this embodiment. The basic nozzle unit of the pipe network as shown in Figure 26 is designed. The pipe material is aluminum, and the nozzle material is brass or stainless steel. There are 3 nozzles on each basic nozzle unit, and a total of 44 such nozzle units are needed to cool the entire electrolysis. above the groove for best results.

This system model represents air impinging on jets, ventilation system ducts, and covers by combining velocity inlet, pressure outlet, and wall boundary conditions. Thus, when the impinging jet of air at the same temperature as the surrounding environment comes into contact with the surface of the molten aluminum, the waste air is directed through the outlet at ambient pressure. The process continues, continuously replacing waste air with high-velocity air, removing heat from the molten aluminum.

This system can reduce the cooling time of the electrolyzer by about 60 hours by combining the forced convection system, the impinging jet system and the heat radiation absorption system.

Table 1 shows the materials and specifications of each part of the cooling system actually used according to the size of the electrolytic cell in this embodiment. Those skilled in the art should know that when actually applied to electrolytic cells of different sizes, in addition to the preferred values defined in the claims, parameters such as the actual height of the heat collector, the total number of nozzles, and the number of metal tubes in the heat collector are It can be adjusted according to the specific electrolytic cell size, so the values in this table are only examples, not as limitations on the parameters of each part of the cooling system.

Table 1

Claims (8)

1. A cooling method after the stop of the operation of an aluminum electrolytic cell is characterized in that a thermal radiation absorption system, a forced convection system and an impact jet system are arranged to cool the aluminum electrolytic cell after the stop of the operation;

the heat radiation absorption system comprises a metal pipe, a cooling liquid circulating pump and a cold source, wherein the metal pipe, the cooling liquid circulating pump and the cold source are communicated with each other; one part of the metal pipe is arranged above the aluminum liquid and the ledge, is parallel to each other and extends from one end of the electrolytic cell to the other end of the electrolytic cell, and the part of the metal pipe is a heat collector; the cooling liquid circulating pump pumps the cooling liquid into the metal pipe, the cooling liquid flows through the metal pipe above the aluminum liquid and the ledge, then flows into the cold source, is cooled by the cold source and then returns to the cooling liquid circulating pump for circulation; the metal pipes at the heat collectors of the thermal radiation absorption system are equal in distance, the metal pipes at the heat collectors are welded together through metal plates, and the surfaces of the metal plates and the metal pipes are coated with high-emissivity coatings;

the forced convection system comprises an air pipe and a blower, the air pipe is arranged outside the bottom end of the shell on the side surface of the aluminum cell, and the blower blows cold air into the air pipe; the forced convection system also comprises an air pipe arranged at the bottom of the electrolytic bath shell;

the impact jet system comprises an air compressor and a pipeline network communicated with the air compressor, the pipeline network is arranged at the side shell of the electrolytic cell, the top plate of the electrolytic cell and above the electrolytic cell, and the pipeline network is provided with a nozzle at one side facing the side shell of the electrolytic cell, the top plate of the electrolytic cell, aluminum liquid and ledge; compressed air is sent into the pipeline network by the air compressor, flows in the pipeline network, is sprayed out from the nozzle and forms air impact jet flow with the side cell shell, the top plate of the electrolytic cell, the aluminum liquid and the hot surface of the cell wall; the heat collector of the thermal radiation absorption system is arranged above the network of pipes above the electrolysis cell in the impinging jet system.

2. The method of claim 1, wherein a cover plate and a ventilation system are disposed above the heat collector, the cover plate is connected to the ventilation system, and the air in the aluminum electrolysis cell is discharged into the ventilation system.

3. The method for cooling aluminum reduction cells after shutdown in claim 1, wherein the flow direction of the coolant in two adjacent metal tubes of the heat collector is opposite.

4. A method for cooling an aluminum electrolysis cell after shutdown according to claim 1, wherein the metal tube of the heat collector is disposed parallel to the upper surface of the electrolysis cell, or in a zigzag pattern, or at both ends of the electrolysis cell at an angle to the upper surface of the electrolysis cell.

5. The method for cooling an aluminum reduction cell after the aluminum reduction cell is stopped, wherein the flow rate of air in the air duct in the forced convection system is 5-20 m/s.

6. A method for cooling an aluminum reduction cell after shutdown as claimed in claim 1, wherein the height of the heat collector is: the distance between the top of the aluminum electrolytic cell and the aluminum electrolytic cell is 1 m-1.2 m, and the initial temperature of the cooling liquid when entering the heat collector is 10-25 ℃.

7. The method for cooling the aluminum reduction cell after stopping the operation of the aluminum reduction cell according to claim 1, wherein the gas flow velocity of the nozzle outlet of the impinging jet system is 5m/s to 12m/s, the diameter of the pipeline in the pipeline network above the electrolysis cell of the impinging jet system is 0.16m to 0.28m, the cooling water pipeline is arranged beside the impinging jet system, the arrangement of the nozzles in the pipeline network above the electrolysis cell is uniform distribution and linear arrangement, the nozzles are circular nozzles, the nozzle outlet diameter is 0.05m to 0.06m, the nozzles are spaced 0.5m to 1m apart, and the nozzle is 50cm away from the surface of the aluminum liquid.

8. The method for cooling aluminum reduction cells after the aluminum reduction cells stop operating according to claim 7, wherein the gas flow rate of the outlet nozzle of the impinging jet system is 5m/s, the diameter of the pipeline in the pipeline network above the electrolysis cells of the impinging jet system is 0.16m, the diameter of the outlet nozzle is 0.053m, and the nozzles are spaced from each other by 0.62 m.

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