CN113432439B

CN113432439B - Cooling method for aluminum electrolysis cell after stopping operation

Info

Publication number: CN113432439B
Application number: CN202110863317.8A
Authority: CN
Inventors: 穆罕默杜·阿齐兹·迪奥普; 王兆文; 石忠宁; 刘爱民; 杨酉坚; 林明
Original assignee: Northeastern University China
Current assignee: Northeastern University China
Priority date: 2021-07-29
Filing date: 2021-07-29
Publication date: 2022-09-06
Anticipated expiration: 2041-07-29
Also published as: CN113432439A

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

Cooling method for aluminum electrolysis cell after stopping operation

Technical Field

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.

Background

In an aluminium smelter, raw aluminium is extracted from ore (alumina) by passing direct current through electrolytic cells in series. The basic process of operation of these cells is named Hall-Herroult process by the name of the inventor. A typical Hall-heroult cell consists of a cell body, an anode assembly and a superstructure to limit heat loss at the top of the cell. The cell body comprises a steel casing lined with refractory insulating material and a cathode block. During operation, molten salt cryolite (Na) is filled in the electrolytic bath ₃ AlF ₆ ) To which alumina (Al) was added ₂ O ₃ ) Powdered and dissolved to form a solution. Passing a current even exceeding 350kA from the anode through the electrolyte comprising alumina to the cathode lining produces an alumina reduction process and maintains the temperature of the electrolyte at 960 ℃. The aluminium metal produced during electrolysis sinks to the bottom of the reduction tank and the gases produced are released through the cell hood. The aluminium is tapped from the bottom and then conveyedTo a furnace in a casting chamber where it is alloyed and cast into various forms of primary aluminum products.

It is known that the Hall-Heroult process is a continuous process, which cannot be easily stopped and restarted by an aluminium smelter. However, various conditions including cell failure, wear and high cathode fall can cause cell operation to be interrupted. For example, if the cell operation is interrupted for more than 4 hours during which time the metal in the bath of the cell body solidifies, a remanufacturing process is typically required to leave the expensive cell housing for subsequent use. Prior to reconditioning, 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 (e.g., outdoors) because the use of rapid cooling may result in failure of the tank inner shell material. This natural cooling has proven to be a long process, requiring 5 to 9 days, depending on the ambient temperature. Since a subsequent reconditioning of the cell after the cell has been closed requires the previous cell shell and the reconditioning process cannot be started unless the cell has cooled to a temperature at which all lining material can be safely stripped from the cell, the delay caused by the cooling affects the capacity of the smelter. Furthermore, environmental regulations do not allow cooling of the cells outdoors, since cooling outdoors may release harmful gases. Cooling the electrolysis cell in the enclosed space further increases the cooling time, which prolongs the production time of the smelter and reduces the production capacity.

Providing a reserve tank has long been a solution to this problem. However, sometimes smelters need to shut down a large number of cells for maintenance in a short time, and replacement of spare cells is also costly. The method for effectively reducing the cooling time of the stopped electrolytic cell, which is suitable for the cooling process of the conventional electrolytic cell, does not involve high cost, and is a feasible method for solving the problem.

In order to increase the heat extracted from the cell, the us patent (application serial No.4,073,714) describes the use of a plurality of aluminium extruded cooling bars partially immersed in the cell anode mix, the non-immersed portions of the bars being cooled by natural convection at the anode surface, and this design is not suitable for cooling after the cell has been taken out of service, since the cell anodes are typically extracted before the cell is shut down for cooling. Furthermore, natural convection has proven to be slow in cooling a closed cell, and therefore improvements to the process require the incorporation of expensive technical measures. Us patent (application serial No.6,251,237) proposes the use of localized, dispersed jets on the cell shell to control the heat flux of the cell, designed to provide a plurality of air jets that maintain cell heat balance parameters during cell repair. This method is not suitable for cooling off-line cells which are out of service because the lining material between the cell shell and the bath limits the heat in this direction. This method does not provide a significant reduction in cooling time compared to its cost. Furthermore, since this method only effectively cools the shell, it is difficult to affect the lining material, and undesirable temperature gradients can occur, while thermal stresses can be induced on the shell because the expansion of the inner lining material is in the opposite direction to the contraction of the shell.

Disclosure of Invention

In view of the real solution of the problem of the prior art that the cooling time of the aluminum electrolysis cell which stops running is too long to delay production, the invention provides a cooling method suitable for the aluminum electrolysis cell which stops running, and the smelting plant can be ensured to avoid unnecessary yield reduction.

The invention provides a cooling method for an aluminum electrolysis cell used in the process of producing aluminum by Hall-Elut electrolysis after the operation of the aluminum electrolysis cell is stopped off line. The method is provided with a thermal radiation absorption system, a forced convection system and an impinging jet system which are combined together to cool the aluminum electrolytic cell after the operation is stopped.

The heat radiation absorption system comprises a metal pipe (preferably an aluminum 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; a part of each metal pipe is arranged above the aluminum liquid and the ledge, is parallel to each other and extends from one end of the electrolytic bath to the other end of the electrolytic bath, and the part of the metal pipe arranged above the aluminum liquid and the ledge is called as a heat collector; preferably, the metal tubes of the heat collector are equally spaced from each other and welded to each other by means of metal plates, or a plurality of tubes are welded together in groups by means of metal plates, so that the heat collector is formed in a plate-shaped structure having a larger area to increase the area for collecting heat. The metal plate and the metal pipe have high heat radiation absorption capacity, and the surface of the metal plate and the metal pipe can be coated with a high-emissivity coating to prevent overheating.

The heat collector absorbs heat energy in the groove through heat radiation, particularly heat energy at the aluminum liquid and the ledge, the cooling liquid circulating pump pumps the cooling liquid into the metal pipe, the cooling liquid flows through the heat collector above the aluminum liquid and the ledge through the metal pipe, namely flows from one end of the electrolytic bath to the other end, heat absorbed by the heat collector is extracted through radiation heat transfer of the surface, then the cooling liquid flows into the cold source, the heat absorbed by the heat collector is brought into the cold source, the cooling liquid returns to the cooling liquid circulating pump after being cooled in the cold source and enters the metal pipe again, and recycling of the cooling liquid is achieved.

The heat collector can be arranged in a Z shape besides being arranged in parallel with the upper surface of the electrolytic bath, or the metal pipes arranged as the heat collector are respectively arranged at two ends of the electrolytic bath and form a certain angle with the upper surface of the electrolytic bath, and then are connected in the middle of the electrolytic bath to form an inverted V shape when viewed from the side. This can enlarge the area of the heat collector and improve 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 through the adjustment of the metal pipe pipelines.

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

The forced convection system comprises an air pipe and a blower, the air pipe is arranged on the outer side of the bottom end of the side tank shell of the aluminum electrolytic tank, and the blower blows cold air into the air pipe to form forced convection cooling with the bottom end of the side tank shell of the aluminum electrolytic tank. The air pipe can also be arranged at the bottom of the cell shell of the aluminum electrolytic cell and penetrates through the transverse length (width) of the electrolytic cell to form forced convection cooling, the bottom of the cell shell of the electrolytic cell is generally provided with an I-shaped support, and the bottom air pipe can be arranged in the gap of the supports. The air flow velocity in the air duct in the forced convection system is 5-20 m/s.

The impact jet system comprises an air compressor (also can be a blower) and a pipeline network communicated with the air compressor (or the blower), 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 (the upper part of the aluminum liquid and the ledge of 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, the aluminum liquid and the ledge; compressed air is sent into the pipeline network by the air compressor, flows at high speed in the pipeline network, is sprayed out from the nozzle, forms air impact jet flow with the hot surfaces of key positions such as the side cell shell, the top plate of the electrolytic cell, the aluminum liquid, the cell wall and the like, and cools the parts. In order to prevent the waste gas in the electrolytic bath from polluting the environment, reduce the retention time of the air after impacting jet flow in the area near the hot surface and improve the cooling efficiency, a ventilation system can be arranged above the electrolytic bath, and the waste gas generated when the electrolytic bath is cooled is pumped out of the electrolytic bath, thereby forming an air flow loop of an air compressor, a pipeline network, a nozzle, the hot surface and the ventilation system and being beneficial to the centralized treatment of the waste gas.

Since a heat collector in a thermal radiation absorption system is also arranged above the network above the electrolysis cell and it is preferable to keep the jet area above the electrolysis cell closed in order to prevent the exhaust gases from flowing out, a cover plate is arranged above the heat collector, which cover plate communicates with the ventilation duct of the ventilation system, through which the air after impinging on the jet is drawn from around the electrolysis cell. Namely, a pipeline network of the impinging jet system above the electrolytic bath, a heat collector of the thermal radiation absorption system, a cover plate and a ventilation pipeline of the ventilation system are sequentially arranged above the electrolytic bath from bottom to top.

Through simulation calculation of an electrolytic bath cooling model, the optimal values of various parameters of an impinging jet system above the electrolytic bath are set as follows: the gas flow velocity of a nozzle of the impinging jet system is 5 m/s-12 m/s, preferably 5m/s, the diameter of a pipeline in a pipeline network of the impinging jet system is 0.16-0.28 m, preferably 0.16m, a cooling water pipeline can be arranged beside the pipeline to prevent overheating above the electrolytic cell, namely above the aluminum liquid and the ledge, the arrangement mode of the nozzles above the electrolytic cell is uniform distribution and linear arrangement, the nozzles are preferably circular nozzles, the diameter of the nozzle outlet is 0.05 m-0.06 m, preferably 0.053m, the nozzles are uniformly spaced and uniformly distributed in the same plane, the spacing is 0.5 m-1 m, preferably 0.62m, and the distance between the nozzles and the surface of the aluminum liquid is 50 cm. A combination of the respective preferred values allows an optimum cooling effect.

Also set up pipe network and the nozzle of striking efflux system on the side cell shell of electrolysis trough and roof, the pipeline diameter, nozzle exit diameter, go out the nozzle gas velocity of flow and also can refer to the pipeline network of electrolysis trough top, the distance on nozzle and side cell shell, roof surface can take 25 ~ 50cm, the relative electrolysis trough top of these parts cools off more easily, consequently in a side of cell shell, or a side of roof, it can realize the cooling to set up 3 minimum nozzles, can adjust the nozzle number according to actual conditions.

The 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.

Drawings

FIG. 1: cross section structure and material schematic diagram of DX aluminum electrolysis cell, wherein:

1-side cell shell, 2-vermiculite, 3-refractory brick, 4-carbon electrode, 5-ledge, 6-side carbon block, 7-refractory brick, 8-silicon carbide, 9-tamping paste, 10-top plate, 11-aluminum liquid, 12-cell shell bottom and 13-cathode steel bar.

FIG. 2: the side section of the width of the electrolytic cell and a material engineering drawing, wherein: 201-calcium silicate, 202-vermiculite, 203-casting material, 204-steel, 205-refractory brick, 206-side carbon block, 207-cathode carbon block, 208-side carbon block, 209-casting material, 210-side carbon block, 211-tamping paste and 212-tamping carbon paste.

FIG. 3: the side section material engineering drawing of the length of the electrolytic cell.

FIG. 4 is a schematic view of: piping network of impinging jet system and associated ventilation system schematic wherein: 401-piping network above the electrolyzer, 402-nozzles, 403-cover plate, 404-ventilation pipe.

FIG. 5: axial view of the cooling system from the inside of the cell, wherein: 501-heat collector, 502-metal tube, 503-piping network of impinging jet system at side cell shell and ceiling, 504-air duct.

FIG. 6: a length-side cross-sectional view of the cooling system, wherein: 505-cold source, 506-blower.

FIG. 7 is a schematic view of: a width-side cross-sectional view of the cooling system.

FIG. 8: and (3) an axial view of the cooling system under the external view of the electrolytic cell.

FIG. 9: the conditions that the convective heat transfer coefficients of the side cell shell center, the top plate, the cell shell bottom and the like under the free convection and the forced convection change with the temperature are as follows, wherein: 901-side shell center, free convection, 902-ceiling, free convection, 903-shell bottom, free convection, 904-side shell center, forced convection, 905-ceiling, forced convection, 906-shell bottom, forced convection.

FIG. 10: cooling curve in the center of the side cell shell.

FIG. 11: heat flux in the center of the side cell casing.

FIG. 12: cooling profile of the top plate of the cell shell.

FIG. 13: heat flux of the top plate of the cell shell.

FIG. 14: cooling curve of the bottom of the tank.

FIG. 15 is a schematic view of: heat flux at the bottom of the slot.

FIG. 16: schematic diagram of the forced convection cooling scheme of the tank shell.

FIG. 17: schematic view of a heat collector of a thermal radiation absorption system (axial view).

FIG. 18: schematic representation (top view) of a heat collector of a thermal radiation absorption system.

FIG. 19 is a schematic view of: schematic view of a heat collector of a thermal radiation absorption system (width side cross-sectional view).

FIG. 20: oblique type heat collector schematic (axial view).

FIG. 21: oblique-type heat collector schematic view (width side sectional view).

FIG. 22: zigzag heat collector (axial view).

FIG. 23: zigzag heat collector (width side sectional view).

FIG. 24: schematic view of a heat collector unit panel.

FIG. 25: the parameters of the nozzles arranged above the electrolysis cell optimize the curve.

FIG. 26: basic nozzle unit of the pipe network.

Detailed Description

The aluminum electrolysis cell used in the present embodiment is a DX technology aluminum electrolysis cell, and the cross-sectional structure and material thereof are shown in fig. 1. The cell consists of an outer shell made of steel grade S275J, with multiple layers of lining material inside. The support/cell shell consists of a relatively loose shell supported by a solid structure, the vertical sides of the cell have reinforcing i-beams and the bottom of the cell has i-shaped supports for support to counteract outward and upward bowing on both sides of the cathode.

The lining material of the bottom of the electrolytic cell is heat insulating bricks, which prevent a large loss of heat through the bottom of the electrolytic cell, and the special materials used in this part of the electrolytic cell include vermiculite plates, refractory bricks and calcium silicate plates. These materials have low thermal conductivity and are stiff enough to support the other structures above. Above the bottom, the lining material is cathode carbon blocks which act as the cathode of the cell. The cathode carbon block is generally more suitable for use in a lining than a rammed paste because the cathode carbon block can provide higher strength, higher density and lower electrical resistivity. The cathode carbon blocks are uniformly distributed over the entire length of the electrolytic cell, and a tamping paste is arranged in the gap between the cathode carbon blocks and the electrolytic cell to form a liquid-tight container for the reduction reaction. The degree and rate of penetration of the electrolyte into the bottom of the cell largely determine the cell life and energy efficiency, and thus the sealing effect of the ramming paste is an important factor in determining the cell life and energy efficiency. The lining material is also used to separate the electrolyte/aluminium located inside the cell from the cell shell side walls, the material used for the cell side lining needs to be able to maintain the heat balance required for the cell, the materials used for the side lining in DX cells are carbon bricks and silicon carbide bricks, which are set on the basis of their thermal properties: carbon and silicon carbide bricks allow heat to flow through the sides of the trough, thereby forming a ledge.

To investigate the situation of free convection cooling of aluminium electrolysis cells, temperature readings of various parts of the aluminium electrolysis cell that were out of service were collected during the free convection cooling. To this end, thermocouples were placed on the top plate, side cell shells, cell shell bottom and center of the electrolyte/aluminum liquid after shutdown. Each reading is collected at two adjacent positions of the cell to ensure complete confidence. The temperature test was started immediately after the cell was taken out of service and was continued for approximately 7 days during each stage of the cell movement from the cell building to the service room. The temperature of the electrolyte before and after tapping of the aluminium was tested, and the temperature of the top plate of the cell, the bottom of the cell and the metal remaining in the cell was tested over time, all in the middle part of the cell.

In the side cell shell, thermocouples are arranged at the upstream end, the downstream end and the aluminum outlet end of the side cell shell of the electrolytic cell. The top plate of the electrolytic cell is provided with three thermocouples which are respectively arranged at an upstream end, a downstream end and an aluminum outlet end. Two thermocouples were placed at the bottom of the cell shell, 800 mm from the upstream and downstream sides, respectively. Thermocouples could not be placed 800 mm from the upstream and downstream sides in the cell house, and therefore measurements were taken at 100 mm from both edges. The specific instrument used was as follows: a Marshal Tip thermocouple assembly for detecting the temperature of the electrolyte/aluminum liquid, a Fluke and Aristo thermometer for detecting the surface temperature of the cell shell (including the top plate, the side cell shell and the bottom of the cell shell). From these set thermocouples, cell temperature data were obtained.

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

The CFD model was modeled and simulated by ANSYS and Star-CCM +.

The 2D engineering drawing of the cell is drawn using AutoCAD, mapping the material actually used in the cell into the 2D engineering drawing, as shown in fig. 2 and 3, showing the width side section and length side section of the cell, respectively.

The cooling system for a DX electrolytic cell in the present embodiment comprises: (1) forced convection system: the aluminum electrolysis cell comprises an air pipe and a blower, wherein the air pipe is arranged on the outer side of the bottom end of a cell shell on the side surface of an aluminum electrolysis cell and also can be arranged at the bottom of the cell shell of the aluminum electrolysis cell simultaneously, cold air is blown into the air pipe by the blower, the cold air can be introduced into the bottom of the aluminum electrolysis cell by the air pipe to form forced convection, and the introduction speed can be 5-20 m/s. (2) 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 surface cell shell of the electrolytic cell and above the electrolytic cell, and the pipeline network is provided with nozzles at one side facing the side surface cell shell of the electrolytic cell, the top plate of the electrolytic cell, the aluminum liquid and the ledge; compressed air is sent into the pipeline network by the air compressor, flows at high speed 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. (3) 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; a part of the metal tubes are arranged above the aluminum liquid and the ledge and are parallel to each other and extend from one end of the electrolytic bath to the other end of the electrolytic bath, the part of the metal tubes are called as heat collectors, and the metal tubes can be mutually welded together through metal plates or a group of the metal tubes are welded together through the metal plates, so that the heat collectors form a plate-shaped structure with a larger area to increase the heat collecting area. The cooling liquid circulating pump pumps the cooling liquid into the metal pipe, the cooling liquid flows through the aluminum liquid and the upper part of the ledge through the metal pipe, then flows into the cold source, and returns to the cooling liquid circulating pump for circulation after being cooled by the cold source.

Fig. 5-8 show visualizations of the system, in which only a quarter of an electrolytic cell is drawn for illustration. The piping network of the impinging jet device above the aluminum liquid and ledge is not shown in the figures for clarity, nor is the ventilation system shown. And the heat radiation absorption system only shows a part of metal pipes and a cold source, but it is not difficult to understand that after the cooling liquid in the metal pipes flows into the cold source for cooling, the cooling liquid is pumped into the metal pipes again through a cooling liquid circulating pump for recycling.

Regarding the piping network of the impinging jet system disposed above the molten aluminum and the ledge and the ventilation system associated therewith, as shown in fig. 4, in the impinging jet cooling technique of the present embodiment, for the nozzles in the piping network disposed above the molten aluminum and the ledge, an in-line array of circular nozzles is employed, which is the most effective impinging jet configuration for a given gas flow rate. In the embodiment, a 50cm gap is reserved between the nozzle and the aluminum liquid according to the size of the electrolytic cell, and the nozzle on the outer edge is also 50cm away from the vertical plane where the inner side wall of the electrolytic cell is located. The velocity of the gas stream exiting the nozzle is believed to vary from 5m/s to 20 m/s. A preferred impinging jet array occurs when the space between adjacent nozzles is open to the ambient environment, allowing air to continue to flow upward and directly expel the heated exhaust air. In order to prevent the environment from being polluted and simultaneously to enable air to form a complete loop, a ventilation system, an impact jet system pipeline network arranged above the aluminum liquid and the ledge are adopted during design, as shown in figure 4, a cover plate is arranged above the pipeline networks, the cover plate is communicated with a ventilation pipeline of the ventilation system, and waste gas flowing upwards after impact jet is pumped out of the electrolytic bath through the ventilation system. It should be noted that fig. 4 is only a schematic view of the piping network of the impinging jet system and the associated ventilation system arranged above the individual aluminium liquid and ledge, and although not shown in fig. 4, there is actually a heat collector of the thermal radiation absorption system described above between the piping network and the cover plate.

The pipe network and the nozzles of the impinging jet system are arranged above the aluminum liquid/electrolyte and the ledge (also called above the electrolytic cell), above the top plate of the electrolytic cell and at the side cell shell of the electrolytic cell, and the heat of the part above the aluminum liquid/electrolyte and the ledge can be effectively removed from the electrolytic cell through impinging jets due to the better positions. Since the air duct in the designed forced convection system cannot effectively cool the upper part of the lateral cell shell due to the blockage of the cathode steel bar in the electrolytic cell, it is proposed to also locally cool this part by impinging jets, so that compressed air is introduced into a specific location of the lateral cell shell through the network of pipes and nozzles, enhancing the cooling effect by increasing the air velocity at the surface of the cell shell and introducing turbulence. It is also possible to use impinging jets to cool the roof, since the roof is also an effective area for heat dissipation from the cell.

The aluminum liquid/electrolyte and ledge are the hottest areas of the cell and are the most efficient cooling locations. However, cooling this part entirely by means of impinging jets produces a lot of harmful fumes, which, even with ventilation systems, can still be a major environmental hazard. Therefore, the cooling of the aluminum liquid/electrolyte and ledge also employs a thermal radiation absorption system. The heat collector of the thermal radiation absorption system itself is made of a material with high radiation absorptivity, such as aluminum (metal tube and metal plate welded between the metal tubes), and the surface of the heat collector may be coated with a high emissivity material to prevent overheating. After the operation of the electrolytic cell is stopped and the anode of the electrolytic cell is removed, the heat collector is moved to the position above the aluminum liquid and the ledge, so that the cooling liquid flows through the heat collector to collect heat from the high-temperature surface, flows into the cold source to be cooled and then is introduced into the circulation again.

The configuration of the nozzle array (nozzle diameter and spacing) is optimized by parametric studies to fit the gap between the aluminium liquid in the cell and the nozzle array, so as to operate with the maximum possible efficiency. In an optimal configuration, the heat transfer relationship can be deduced 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 studied. After optimizing the processes, the cooling systems were combined and examined for the effect on the resulting cooling curves.

1. CFD model of air duct cooling (forced convection)

A Computational Fluid Dynamics (CFD) model based on a three-dimensional finite element is established for a shell (a tank shell) of an electrolytic tank of an aluminum smelting plant, and an air inlet pipeline with the size of 0.02m is arranged below a cathode steel bar. The model simulates the air flow forced at a velocity of 5m/s over the cathode steel bar, the lateral cell shell outside and the cell shell bottom. The temperature-dependent heat transfer coefficient is estimated by modeling the exact shape of the isothermal surfaces and the cell shell in the appropriate ambient air domain, where inlet velocity boundary conditions are introduced at specific locations on the ground. Due to the strong dependence of the domain flow on temperature, a coupling-based solver is used. Induced turbulence was modeled using the RNG k-epsilon reynolds average model. The obtained convective heat transfer coefficients at the center of the side cell casing, the ceiling, and the bottom of the casing, etc. positions as a function of temperature are shown in fig. 9, and are compared with the heat transfer coefficients at these positions in the case of free convection in fig. 9.

By introducing forced convection the heat transfer coefficient increases three times at the centre of the side cell shell and two times at the bottom of the cell shell, but the heat transfer coefficient in the region of the top plate is hardly affected since the top plate is far from the cooling channel. Similar curves were established for the other parts of the cell shell by this model, and these curves were used for the boundary conditions of the FHT (fast Hartley transform) model. The previously obtained steady state solution is used as the initial condition for the model. The residual aluminum liquid and electrolyte in the model are respectively 4.65cm and 1cm in height. The model simulates the process of removing the aluminum liquid/electrolyte within 1 hour by breaking the interface between the aluminum liquid/electrolyte and the residual metal on the upper part. This ensures that the metal heat is removed from the model after the steady state solution is obtained. The solidification of the metal at a temperature of 660 ℃ is modeled by inputting a command to extract the latent heat. The transfer of the electrolytic plant to the service room is thermally modeled by the change in ambient temperature, ignoring the effects of wind during this movement. The model introduces surface-to-surface radiation to estimate the performance of the thermal radiation absorption system. A surface-to-surface radiation model was simulated using a one-dimensional equation and errors of up to 15% were recorded. It is assumed that all objects are grey 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: cooling the side surface independently;

case 2: cooling the sides and the bottom;

case 3: cooling the top part separately;

case 4: the top, sides and bottom are cooled simultaneously.

The temperature and the historical heat flux were estimated in each case for each part of the cell shell. Since the modeling was with a quarter cell (14U/S), the heat flux estimate was one-quarter of that of a DX cell, and the total heat flux can be multiplied by 4. Fig. 10 and 11 show the temperature recording (cooling curve) and the heat flux at the center of the side cell shell for these four cases. In cases 1 through 4, the cooling rate at the side cell shells increased. This represents an overall increase in cooling performance of the shell from the sides of cases 1-4. The heat flux history of figure 11 also shows that the rate of heat absorption in this part of the cell is improved from cases 1-4.

Fig. 12 and 13 show the recording of the change in temperature and heat flux of the top plate of the cell housing. In cases 1-4, the cooling rate of the top plate of the cell shell was also increased. However, during cooling in cases 1-4, the heat flux in this portion is less than in a free convection environment. Since no cooling is applied to the top plate portion of the trough, it is indicated that the other cooled portions may extract heat that naturally flows through the top plate.

In these cases, the cooling curve and heat flux history recorded at the bottom of the cell demonstrate this assumption (fig. 14 and 15). Fig. 15 shows that applying forced convection cooling to the bottom of the cell shell results in faster cooling and heat extraction. However, when forced cooling is applied only to the side cell casing, the cooling rate at the bottom of the cell increases, but the heat flux decreases.

Case 1 and case 2 did not cause significant changes in the cooling curve and heat flux. However, careful observation shows that the cooling rate of the aluminum liquid is slightly increased in case 1 and case 2, but the heat dissipation rate of the aluminum liquid is slightly reduced compared with the value in free convection. This indicates that the heat flow initially removed from the top by free convection is in both cases discharged from the side. Thus, the overall increase in the cooling rate of the aluminum liquid in

cases

1 and 2 is relatively minimal. However, in case 3, when cooling is performed only from the top, the cooling rate and the heat dissipation rate of the aluminum liquid are increased. While in case 4, omnidirectional cooling further increases the cooling rate and heat dissipation rate.

These results indicate that the best approach to heat sink cooling is to use the concept of case 4. Since the insulation on the sides and bottom of the cell limits heat dissipation, the transfer of heat from one side to the other depends on the magnitude of the heat transfer coefficient, rather than being removed from the heat source. On the other hand, removing heat from only the top reduces the heat flux at the sides and bottom, i.e., the thermal resistance in those directions increases. In addition, uneven cooling rates can cause thermal stresses that can damage the steel cell casing.

2. Forced convection cooling system scheme for tank shell

When air is forced to contact the surface, air particles that contact the surface will become clogged and eventually stop. This results in adjacent fluid layers becoming slower due to viscous stresses (friction) between adjacent fluid elements moving at different speeds. Subsequent fluid layers are also occluded, but not to the same extent as the first layer. This trend continues until a region where stagnant fluid has a negligible effect on flow velocity (fig. 16). The region where the velocity gradient is created is called the velocity boundary layer.

Also, a temperature gradient is created when the temperature of the fluid is different from the temperature of the tank. The region where this temperature gradient exists is called the thermal boundary layer. The conditions in this boundary layer determine the nature of the 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 of electrolytic cells. In this forced convection cooling technique, convective heat transfer from the shell is achieved by increasing the velocity of the air at its surface, which is a more economically viable option, although it is also possible to reduce the temperature of the ambient air to achieve this. In a smelter, this cooling solution can be implemented by placing the ductwork on the floor at the bottom end of the side shell of the electrolytic cell, as shown in fig. 8 and 16, and blowing cold air into the ductwork by means of a blower.

Unlike the upward facing hot plates, the downward facing hot plates (i.e. the bottom of the cell shell for aluminum reduction cells) are 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 cold air is at the bottom and the less dense hot air is very close to the hot surface, and the system is at equilibrium. The larger area of the bottom of the cell housing means that there will be a considerable equilibrium area for such air stagnation. To eliminate this stagnation, it is proposed to provide an air duct also at the bottom, into which cool air is blown. This process will increase heat transfer from the bottom of the cell by reducing the ambient temperature at the bottom of the cell housing, increasing the air flow velocity and introducing turbulence. In addition, it is also desirable to be able to produce the effect of reducing the overall ambient temperature of the service room. The bottom of the cell is typically provided with i-shaped supports and the air ducts at the bottom of the cell can be arranged between these support structures across the transverse length of the cell.

3. Scheme of heat radiation absorption system

Thermal radiation absorption systems remove heat by radiation. It is well known that radiation systems are most effective at high temperatures, so the aluminum liquid and ledge are the target locations for the heat removal system. This solution involves placing an efficient radiation absorber (heat collector) above the open cell to enhance the heat transfer from the aluminum liquid/ledge during cooling. The heat collected by the heat collector is conducted away from the system by the coolant flowing through the heat collector. A schematic of the heat collector of the thermal radiation absorption system is shown in fig. 17-19. During operation, the heat collector is simply lowered towards the open cell in the service room. The cooling liquid is transferred from one longitudinal side (length side) of the trough to the other side through the metal tubes in the heat collector. As shown in fig. 18, the cooling liquid in the two adjacent metal tubes of the heat collector can be set to flow in opposite directions, so as to ensure uniform and stable surface temperature of the heat collector. The coolant flowing out of the heat collector is cooled by passing through a cold source before being reintroduced into the circuit by means of a coolant circulation pump.

Unlike a completely planar structure of the heat collector, exposing a portion of the surface area of the metal tube to the hot surface also increases the surface area of the collector by about 40%. Theoretically, this improves the radiation heat transfer capability of the heat collector, and welding metal plates between metal tubes is a more efficient method of improving the radiation heat transfer capability. To further enhance the heat dissipation effect, the orientation of the metal tubes of the heat collector may be modified to maximize the surface area of the heat collector above the aluminum level. For example, the heat collectors in which the metal pipe is inclined at an angle (for example 45 °) at the two ends of the cell, as shown in fig. 20 and 21, since the modelling is on a quarter of the cell, fig. 20 and 21 show only one half of the metal pipe, it is conceivable that the other half of the metal pipe is symmetrical to fig. 20, and the metal pipe and the upper surface of the cell, viewed from the lateral sides, form a triangle or an inverted V. A zigzag heat collector may also be used as shown in fig. 22 and 23.

4. Impinging jet system scheme

In the scheme of the cooling system, the impinging jet array of impinging aluminum liquid is used for improving the heat transfer coefficient of the surface of the aluminum liquid. Linear arrays of circular nozzles, staggered arrays of circular nozzles, and slit nozzles are three options that can be used for air impingement jet system design. In selecting the array configuration, it is preferable to allow the air after impinging the jets to continue flowing upward and directly discharge the exhaust gas when the space between adjacent nozzles is open to the ambient environment. Since this method may pollute the environment of the smelter, a suitable ventilation system is used in its design. Furthermore, to avoid overheating, cooling water pipes may be arranged on both sides of the pipe network.

5. Performance evaluation and parameter optimization of cooling solutions

The performance of the cell cooling scheme in this example was evaluated with the aid of a mathematical model that had been developed. The adopted technology is forced convection at the cell shell (mainly the bottom end and the bottom of the side cell shell), heat radiation absorption above aluminum liquid and impact air jet flow arranged above the electrolytic cell (mainly above the aluminum liquid and the cell wall), the electrolytic cell top plate and the side cell shell. For the arrangement of forced convection ductwork, the cooling scheme is based on cooling the electrolysis cell by forcing air over the cell shell, which requires high accuracy of modeling the airflow over the cell shell structure (translating into millions of discrete nodes). This method was employed in evaluating the cooling scheme and a special model was developed to estimate the effect of air flow rate on the heat transferred through the cell casing. The heat transfer coefficient related to the flow rate calculated by the model is used in the model of the electrolytic cell and then used to evaluate the cooling performance of the electrolytic cell. Two other solutions, involving radiation and impinging jets on a flat aluminum liquid surface, were also evaluated for their performance, but for which no fluid domains were set that would introduce impractical calculated durations. It is important to note that the active cooling technique described above is only enabled in the model after the cells have been moved to the cell service room (14 hours after shut down) because cooling cannot be performed on the potline in the smelter.

For the shell forced convection cooling scheme, a 3D CFD model is developed for the shell of a DX electrolytic cell, and a 17cm x 40cm air pipe is arranged below the cathode steel bar. This dimension is chosen so that the ductwork is adapted to fit between the i-shaped support structures of the cell support, becoming part of the cell shell, with minimal heat transfer resistance. The temperature-dependent heat transfer coefficient is calculated by modeling the exact shape of the isothermal layer and the cell shell in a suitable ambient air domain, which model is similar to the free convection model. However, in this model, a duct was introduced as an inlet velocity boundary condition, forcing the gas flow to flow at a velocity of 5m/s to 20m/s over the cathode steel bar, the side cell casing, the bottom of the cell casing. Due to the strong dependence of the domain flow on temperature, a coupling-based solver was used, modeling induced turbulence using the RNGk-s raynatz mean model, and also including gravitational effects. The computational domain was discretized using a grid with a maximum size of 0.002 m. Empirically, the boundary conditions outside the cell casing are modeled as ambient temperature walls, rather than pressure vents, as the latter can cause back pressure. The momentum and energy equations are respectively 10 ^-3 And 10 ^-6 Convergence criterion of residual value. The wall temperature and air velocity were varied and a temperature dependent heat transfer coefficient was calculated for each of the rack surface sections. Through simulation calculation, when air flow rates of 5m/s, 10m/s and 20m/s are adopted, the air flow rates can be reduced by about 22, 24 and 2 relative to natural convection cooling respectivelyCooling time of 6 hours.

The radiation cooling scheme was evaluated by placing the collector at a distance above the surface of the aluminum liquid and placing an air field between them. The net radiant flux on each surface is a function of the surface properties, and thermal boundary conditions are imposed on the surface, calculated to balance the radiation. By considering each surface and the way it exchanges radiation with all other surfaces, radiation balance can be achieved on all surfaces of the whole closed system. Thus, the first step of the model is to calculate each surface-to-surface interaction. After the surface is broken down into patches and the angular coefficients of the patch pairs are calculated, the radiant flux on each patch can be obtained independently by applying a spectral radiation balance across all surfaces of the entire closed system. An innovative approach is taken to obtain the emitted power, reflectance and transmittance values of the patches by surface averaging the unit surface values of all the boundary surfaces that make up each patch. Studies were conducted to determine the effect of varying the heat collector temperature, clearance from the surface of the aluminum liquid, and heat collector area. And determining the distance between the proper heat collector and the top of the electrolytic cell to be 1-1.2 m. The temperature range considered is from 10 to 25 deg.c and the zigzag heat collector is used to increase the area of the heat collector. FIG. 24 is a design drawing of a unit plate of a heat collector, wherein the metal pipe and the metal plate are made of aluminum, the inner diameter of the metal pipe is 10cm, the thickness of the metal pipe is 1cm, 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), 14 such unit plates are required for the electrolytic cell. If a sloped heat collector in fig. 20 or a zigzag heat collector in fig. 22 is used, different cell plate sizes and numbers may be required depending on the slope angle or zigzag direction.

In an impinging jet system, the optimum nozzle arrangement is achieved for the nozzles in the piping network above the electrolyzer by selecting the nozzle parameters that produce the greatest heat transfer at a particular gas flow rate. At a given total gas flow rate and a fixed nozzle to cooling surface clearance (H), optimum values for nozzle diameter (D) and nozzle spacing(s) were found.

FIG. 25 is a parameter optimization curve of nozzles disposed above an electrolytic cell when the nozzles of the impinging jet system are circular, the array mode is in-line distribution, the gas flow velocity of the outlet nozzles is 5m/s, and the distance from the surface of the molten aluminum is 50 cm. The result shows that under the condition, the optimized nozzle outlet diameter is 0.05-0.06 m, and the optimal value is 0.053 m. As can be seen from FIG. 25, the diameter of the pipes in the piping network of the impinging jet system is preferably 0.16m to 0.28m, most preferably 0.16m, and the nozzles disposed above the electrolytic cell are preferably spaced apart from each other by 0.5m to 1m, most preferably 0.62m, and are uniformly distributed in the same plane.

With this structure, a total of 132 nozzles uniformly distributed above the electrolytic cell used in the present embodiment are required. The basic nozzle units of the piping network of fig. 26 were designed, the piping material being aluminum and the nozzles being brass or stainless steel, with 3 nozzles above each basic nozzle unit, and a total of 44 such nozzle units being required to cool the entire cell above for optimum results.

The system model represents air impingement jets, plenum ducts, and cover plates by combining velocity inlet, pressure outlet, and wall boundary conditions. Thus, when air at the same temperature as the ambient environment impinges the jet in contact with the surface of the molten aluminum, the waste air is directed through the outlet at ambient pressure. The process is continued, and the high-speed air is continuously used for replacing the waste air to remove the heat of the aluminum liquid.

The system can reduce the cooling time of the electrolytic cell by about 60 hours by combining a forced convection system, an impinging jet system and a thermal radiation absorption system.

Table 1 shows the materials and specifications of the various portions of the cooling system actually used according to the size of the electrolytic cell in the present embodiment. It will be appreciated by those skilled in the art that in practice in electrolytic cells of different sizes, parameters such as the actual height of the heat collector, the total number of nozzles, the number of metal tubes in the heat collector, etc. may be adjusted to the specific cell size, in addition to the preferred values defined in the claims, and therefore the values in this table are given by way of example only and are not intended as limitations on the parameters of the various parts of the cooling system.

TABLE 1

Claims

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.