Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
  • C25C3/12—Anodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/06—Operating or servicing

Definitions

• the present invention relates to a method and means for extracting heat from an electrolysis cell for production of aluminium. Specifically, it relates to the cooling of the anode/stubs/yoke assembly by heat conduction upwards along the anode stem, and the enhancement and control of this cooling effect.
• the anode assembly in aluminium cells consists of the anode stem (rod), the anode yoke with stubs (studs), and the carbon anode block.
• the stem is attached at its upper end to the anode beam by means of a clamp, and its lower end is connected to the anode yoke.
• the stubs are integrated with the anode carbon block.
• the anode stem can be made of aluminium or copper, while the yoke is made of aluminium, copper or as normal made of steel.
• the stubs are made of steel.
• the electric and mechanic connection between the stem and the yoke is constituted by a bimetallic plate.
• One conventional way of fastening the stubs in holes in the carbon block is by means of cast iron.
• the anode stem plays an important role in the energy balance of the cell. Approximately 50 percent of the electrical energy input to the cell is lost as heat. Up to 50 percent of the heat loss takes place at the top of the cell, and the major part of this again is through the anode.
• each anode carbon block from the electrolyte and upwards. Some of this passes through the anode cover material on top of the anode, but most of the heat (about 5 kW per anode) is conducted through the stubs and into the yoke. About 4 kW is then dissipated from the yoke and stubs by electromagnetic radiation and convective heat transfer, while the remaining 1 kW is conducted into the anode rod. Part of the latter heat is dissipated into the gas between the top crust and the superstructure, and part of it is dissipated outside the superstructure.
• the easiest way to increase the heat losses is by increasing the number of stubs in each anode, or by increasing the diameter of the stubs. Besides increasing the heat loss, this has the inherent benefit of decreasing the electric resistance of the anode assembly.
• the increase in the heat loss through the stubs is less than proportional to the increase in the cross-sectional area, and the larger stub dimensions may give problems with anode cracking.
• Increased heat losses from the stubs/yoke will also lead to increased temperature in the raw gas. There are, at least, three reasons why this is not desired; 1 ) Increased maintenance costs related to the filter bags in the dry scrubber if the temperature increases above their designed operating temperature, 2) It is important to keep the temperature of the superstructure below certain limits due to the numerous electromechanical installations in this area, and 3) There may be increased heat stress on the operators working in the vicinity of the cell. The extra heat losses must therefore be compensated by increased air suction into the cells.
• the air flow in the exhaust ducts and the gas scrubbing system is the far largest mass flow in an aluminium plant (e.g., 80 t air/t Al), and the cost of transporting the gas is approximately proportional to the cube of the volumetric flow.
• increased suction rate may also require a scaling up of the equipment related to the dry scrubbing system.
• NO 318 164 B1 discloses a method for control of inert electrodes in an electrolysis cell for aluminium production.
• the problem to be solved is to reduce dissolution of the anode material by transporting heat away from the active anode surface and to reduce deposit formation on the active surface of the cathode by preferably keeping the temperature of this surface higher than that of the electrolyte.
• the electrolytic process based on inert electrodes can be enhanced.
• One main purpose of cooling the anode assembly as described in accordance with the present invention is to be able to raise the amperage on the cell while maintaining the side and end ledge (frozen bath) in the bath phase without reducing the ACD, without increasing the dimension of the stub and yoke and thereby without increasing the temperature of the raw gas.
• Removing heat from the anode with an active cooling will also increase the efficiency of stub, yoke and stem as a heat sink for heat leaving the interpolar distance where most of the heat is generated. The reason for this is because the specific electrical and thermal conductivity of steel will increase and thereby leading to an increased heat loss through the stub and yoke and also because less internal heat will be generated in the material (steel). Calculation on a heat balance model with active cooling of the anodes has shown possibility for a 10 % increase in the amperage maintaining the interpolar distance and keeping the side ledge constant.
• the basic idea in the present invention is to extract more heat from the interior of the cell, as well as reducing the heat dissipated into the raw gas, by increasing the amount of heat conducted from the cell along the anode stem. Enhancement of the heat removal from the cell can be achieved by improvement of the conduction along the stem or by installing a convective heat transfer circuit machined inside or fixed on the stem. The heat transfer fluid is circulated down to the yoke where it is heated up. It brings back this heat outside of the superstructure where the heat is released. Heat intake and release can be enhanced by phase transition of the refrigerant (boiling and condensation).
• Fig. 1 discloses in general an anode assembly
• Fig. 2 a-b disclose two embodiments of cross sectional views of anode stems in accordance with the invention
• Fig. 3 discloses a diagram showing temperature gradients along an anode stem, calculated for four cases as discussed in the following text.
• anode assembly for an electrolysis cell that comprises an anode stem 1 which is connected to an anode beam 2 and an anode yoke 3 from which stubs 4 provide further electric contact to a carbon anode 5.
• the anode stem is cooled by increasing the surface area of the stem above the cell's superstructure 6, or by applying a cooling medium that circulates along the stem.
• the anode cooling can be combined with the use of a thermal insulation material 7 at the anode stem below (inside) the superstructure.
• FIG. 2a and 2b there is shown two embodiments for arranging medium transport inside the anode stem 1.
• the Figures show possible technical solutions, which may also be used in combination with cooling of the anode yoke (WO 2006 088375).
• the anode stem 1 contains a longitudinal pipe 22 for the cold fluid supplied or recycled at the top, and another longitudinal pipe 23 for the hot fluid coming from the bottom of the stem or from the yoke and the bottom of the stem.
• the latter pipe is thermally insulated 24 in order to avoid heating of the cold fluid or the anode stem itself.
• the pipes can be made two in parallel as in Fig. 2a or concentric as in Fig. 2b.
• the anode stem 1 ' contains a longitudinal pipe 22' for cold fluid supplied or recycled at the top, and another longitudinal pipe 23' for hot fluid coming from the bottom of the stem or from the yoke and the bottom of the stem.
• the pipes are arranged concentric with a layer of insulation 24' between them.
• the preferred technical solution should as earlier stated be a fluid that evaporates at the lower part of the stem or within the anode yoke, and is condensed at the upper part of the stem. Since there is a relatively large surface of contact between the anode beam and the stem, the heat from the top of the stem can be extracted by cooling the anode beam. This eliminates the extra work needed during anode replacement, if the fluid supply to and from the stem or yoke must be connected and disconnected.
• the anode stem should be supplied with a relief valve, in case increasing temperature should lead to an unacceptable pressure build-up.
• Circulation of the cooling medium can be forced by a pump or a compressor. Circulation can also be simply triggered by buoyancy.
• This is the classical concept of thermosiphon.
• the heat transfer fluid is heated at the bottom (yoke). It expands and flows to the top (outside the electrolysis cell) where it is cooled. Its density increases and it falls back to the yoke.
• CO2 based thermosiphon was found particularly promising. CO2 is an inert gas reducing safety issues, and heat exchange properties are very good. Calculations showed that 0.014kg/s of CO2 at 50bars could carry 3kW between the hot side (yoke) at 300 0 C and the top of the stem maintained at 100 0 C.
• thermosiphon operates in transcritical mode. Very large density difference between the cold and hot sides, and then large flows can be achieved without phase transition which greatly reduces the risk of instabilities.
• the heat transfer fluid In order ensure a large heat extraction, the heat transfer fluid must be cooled above the superstructure. There are numerous ways of realising this cooling. The simplest way, but not the more effective, is to increase the surface area of heat transfer circuit above the superstructure with cooling fins. Those fins could for instance be sprayed by water or by a forced flow of air. The forced air flow can be provided by a fan, a lance delivering pressurized air, or by any other appropriate means.
• a more advanced solution would be to couple the top the heat transfer circuit with an external cooling module. Heat exchange between the heat transfer fluid and refrigerant could be ensured by a proper heat exchanger.
• the pipe that transport the warm gas upwards through the hanger is widened at the top of the hanger, i.e. to a small container. The container should be placed above the area where the current goes into the hanger from the anode beam.
• An option that would solve all problems related to connection and disconnection during replacement of an anode would be to dissipate the heat into the anode beam by conduction across the electrical contact surface. This may require cooling of the anode beam, which would lead to added benefits such as decreased ohmic resistance and better mechanical properties of the anode beam (increased creep resistance). Ideally the heat extracted should be utilized for power production.
• the cooling circuit would then preferably be of Rankine type with an expansion turbine driving a generator.
• Heat extracted from several anode stems can be collected and led to an energy conversion unit conveniently arranged outside the pot room.
• the invention will help stabilizing the temperature in the hanger and yoke at a lower level than today and make it possible to remove the bimetallic joint. If not removing it, it will live for a longer period.
• the proposed technical solution can also be used by regulating the effect input to the cell under normal operation instead of moving the anode up and down (power pulsing). If the cell needs more heat, less heat is removed from all or some of the anode assemblies on the cell, and if the cell needs less heat more heat could be removed from the anode assemblies than normal. In this way the need for upwards and downwards movements of the anode to increase or reduce the heat input to the cell will be less and therefore it will be possible to keep a more constant interpolar distance (ACD). By keeping the ACD more constant the fluctuation in the bath level will be reduced, and also the process control will be improved since movements of the anode normally will disturb the resistance signal to the regulator deciding the alumina addition.
• ACD interpolar distance
• a lower temperature on the yoke will also make it more easy to use other materials in the yoke than steel, by instance copper with a higher thermal conductivity and higher electrical conductivity than steel. Even an aluminium yoke could be considered.
• a simplified model of the anode stem and its surroundings was made.
• the model takes into account the thermal conduction along the anode stem and the heat dissipated from the stem.
• the heat transferred from the stem to the surroundings was calculated using a single heat transfer coefficient intended to contain both the convectional heat transfer and the electromagnetic radiation.
• the model was not intended to be very accurate, but still, the results should be regarded as much better than order-of-magnitude-estimates.
• the boundary between the lower end of the anode stem and the bimetallic plate was assumed to be constant (280 0 C).
• Case 2 No thermal insulation on stem, stem cooled to 50 0 C 1 m from the lower end.
• Case 3 Stem thermally insulated below (inside) the superstructure, and cooled to 50 0 C 1 m from the lower end.
• Case 4 Stem thermally insulated below (inside) the superstructure, but no extra cooling.
• Case 3 is comparable to Case 2, except that the stem is thermally insulated below (inside) the superstructure. In this case, the amount of heat conducted into the stem becomes lower, but on the other hand, the heat dissipated into the raw gas is eliminated. Insulating the yoke is therefore an effective means of reducing the raw gas temperature. When comparing Case 3 and Case 4, however, it is clear that insulating the stem should only be done in combination with cooling, or else there will be a considerable decrease in the heat conducted into the stem.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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• 2008
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• 2009
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