EP0047227A2 - Device for the regulation of the heat flow of an aluminium fusion electrolysis cell, and method of operating this cell - Google Patents

Abstract

The comparatively high supply of electrical energy to an aluminium fusion electrolysis cell should overcome without damage an enforced interruption, for example due to a defect or a reduction during peak loadings due to private households. The cell insulation (13) between steel bath (12) and carbon lining (14) is reinforced by up to 50% and up to double the amount of alumina (32) is poured onto the electrolyte crust (30). End faces of heat exchanger tubes (34) built into the interior of the cell as dead-end projects through peripheral parts of the cell (10) and terminate a heat exchanger (38, 44) disposed outside the cell. For 70-80% of the normal value of the current level, the cell is in thermal equilibrium without heat being removed or supplied. At higher current levels, in particular at the normal value, heat is continuously removed. At lower values, the interpolar distance is increased or heat is supplied from another energy source. <IMAGE>

Description

The invention relates to a device for regulating the heat flow of a melt flow electrolysis cell for the production of aluminum and a method for maintaining the thermal balance of this cell at any current strengths between 50 and 125% of the normal value of the cell current.

For the production of aluminum by electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which largely consists of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon bottom of the cell, the surface of the liquid aluminum forming the cathode. Anodes which consist of amorphous carbon in conventional processes are immersed in the melt. At the carbon anodes, the electrolytic decomposition of the aluminum oxide produces oxygen, which combines with the carbon of the anodes to form CO ₂ and CO. The electrolysis takes place in a temperature range of approximately 940 - 970 ° C.

In the course of electrolysis, the melt flow of aluminum oxide becomes poor. With a lower concentration of 1-2% by weight of aluminum oxide in the electrolyte, there is an anode effect, which results in a voltage increase of, for example, 4-4.5 V to 30 V and above. At the latest then the crust must be hammered in and the aluminum oxide concentration increased by adding new aluminum oxide (alumina).

Under normal production conditions, the electrolysis cell is in thermal equilibrium, ie the ohmic produced in the cell with the direct electrolysis current Heat is continuously dissipated to the environment to the extent that the cell remains at a constant temperature. If the strength of the direct electrical current is increased or decreased, the temperature of the electrolyte increases or decreases until a new thermal equilibrium has been established.

At a constant current, the temperature of the equilibrium state of the electrolytic cell can be influenced by adding heat or changing the amount of heat to be removed.

In the field of aluminum electrolysis, externally controlled cooling by means of closed heat convection circuits is known. According to SU-PS 605 865 and 663 760, heat is extracted from an electrolysis cell laterally and downwards with such a circulation system via valves and pipes.

SU-PS 633 937 discloses a combination of heat extraction and supply, the circulation system being formed not only in the cathodic part but also on the anode of the Soederberg cell.

SU-PS 600 214 shows cooling tubes made of silicon carbide, which are arranged in the deposited metal. These tubes, through which a cooling medium flows, regulate the temperature of the electrolytic cell to a certain value, so the regulation is carried out from the outside, as in the preceding Russian patents.

• - Die Transportkapazität ist nicht genügend, das System ist träge
• - die Systeme sind nicht selbstregulierend
• - die Systeme sind konstruktiv aufwendig und wenig flexibel, d.h. es kann nur an bestimmten Stellen
• - im allgemeinen an den Aussenflächen - gekühlt bzw. erwärmt werden.

The known circulation systems in which heat is supplied or removed by convection have the following disadvantages:

• - The transport capacity is not sufficient, the system is sluggish
• - the systems are not self-regulating
• - The systems are structurally complex and not very flexible, ie it can only be used at certain points
• - generally on the outer surfaces - are cooled or heated.

The inventors have therefore set themselves the task of providing a device for regulating the heat flow of a melt flow electrolysis cell for the production of aluminum and a method for maintaining the thermal balance of this cell with a heat regulating device which transport a large amount of heat in a self-regulating manner and thus temporarily or permanently generate current intensities of Can withstand 50 - 125% of the normal value of the cell direct current without damage.

• - eine verstärkt ausgestaltete Zellenisolation, gebildet durch eine bis zu 50 % dickere Isolationsschicht zwischen Stahlwanne und Kohleauskleidung und bis zur doppelten Menge Tonerde auf der Kruste aus erstarrtem Elektrolytmaterial, und
• - mit einer Stirnseite blind im Zelleninnern eingebaute Wärmerohre, welche ein reversibel verdampf- und kondensierbares Wärmeträgermedium enthalten, periphere Teile der Zelle durchgreifen und im ausserhalb der Zelle angeordneten Wärmeaustauscher enden.

With regard to the device, the object is achieved according to the invention by

• - A reinforced cell insulation, formed by an up to 50% thicker insulation layer between the steel tub and coal lining and up to twice the amount of clay on the crust of solidified electrolyte material, and
• - With one end face blindly installed in the interior of the cell, which contain a reversibly evaporable and condensable heat transfer medium, reach through peripheral parts of the cell and end in the heat exchanger arranged outside the cell.

The heat pipes used according to the invention are known per se, for example from the magazine Chem.-Ing.-Tech. 50 (1978) No. 11, pages A654 ff. The containers, which are sealed in a vacuum-tight manner, have a capillary structure on the inside, which, for example, is sintered from textile or wire mesh, grooves Structures etc. can be formed. After evacuation, the heat pipes are filled with a small amount of liquid as the heat transport medium until the capillary structure is just saturated. This liquid is in equilibrium with its vapor in the remaining interior of the heat pipe.

If one end of the tube is heated and the other end is cooled, the liquid on the warm side evaporates, absorbing the heat of vaporization. The steam flows to the other, cold side of the heat pipe and condenses there, giving off the heat of condensation to the cooling medium. The condensate flows back to the warm side under the action of the capillary force.

A heat pipe essentially consists of three zones: an evaporation zone, an isolated adiabatic zone and a condensation zone.

In the conventional production of aluminum by means of melt flow electrolysis, approximately 60% of the electrical energy supplied to the cell is lost as heat losses. About 60% of it escape upwards, 10% through the bottom and 30% through the side walls (including cathode bar connections). In the case of oversized cell isolation, each cell must first of all be better isolated upwards. This is done by pouring up to an additional ton of aluminum oxide onto the solidified electrolyte crust, which can double the insulating clay layer, for example. The side and bottom areas are improved by arranging, for example, a thicker insulation layer in the cell structure. The cathode bar ends can also be narrowed so that their heat radiation to the outside is reduced.

• - Im Kohleboden der Zelle werden Wärmerohre etwa vertikal angeordnet, wobei die obere Stirnseite vorzugsweise auf gleicher Höhe wie die obere Seitenfläche der Kathodenbarren endet. Beim Kühlen der Zelle liegt die Verdampfungszone des Wärmerohres höher als dessen Kondensationszone, es muss deshalb darauf geachtet werden, dass das Wärmerohr nur so lang ist, dass die Kapillarkraft grösser als die Schwerkraft bleibt (hydrostatische Druckdifferenz). Beim Erwärmen der Zelle liegt die Verdampfungszone unter der Kondensationszone, Schwerkraft und Kapillarkraft wirken in der gleichen Richtung.
• - Auch in den Kathodenbarren, also horizontal, können Wärmerohre angeordnet werden. Damit der elektrische Widerstand der Zelle nicht erhöht wird, muss der Barrenquerschnitt entsprechend angepasst werden.
• - Weiter können Wärmerohre in die seitliche Kohleauskleidung eingeführt werden, wobei sie vorzugsweise ungefähr horizontal angeordnet sind.
• - Schliesslich können die Wärmerohre in den Anodenstangen zu den Anodenkörpern führen. Diese Variante kann insbesondere beim Einsatz von unverbrauchbaren Anoden zweckmässig sein. Hier wirken Schwer- und Kapillarkraft bei der Wärmezufuhr in entgegengesetzter Richtung.

The installation of heat pipes in aluminum electrolysis cells is particularly useful at the following locations:

• - In the carbon bottom of the cell, heat pipes are arranged approximately vertically, the upper end preferably ending at the same height as the upper side of the cathode bars. When the cell is cooled, the evaporation zone of the heat pipe is higher than its condensation zone, so care must be taken that the heat pipe is only so long that the capillary force remains greater than gravity (hydrostatic pressure difference). When the cell is heated, the evaporation zone lies below the condensation zone, gravity and capillary force act in the same direction.
• - Heat pipes can also be arranged in the cathode bars, i.e. horizontally. So that the electrical resistance of the cell is not increased, the bar cross section must be adjusted accordingly.
• - Furthermore, heat pipes can be inserted into the side coal lining, wherein they are preferably arranged approximately horizontally.
• - Finally, the heat pipes in the anode rods can lead to the anode bodies. This variant can be expedient in particular when using unusable anodes. Here gravity and capillary force act in the opposite direction when heat is supplied.

The introduction of heat pipes into the cell from above is difficult because generally most of the top of the cell is covered by the carbon anodes, which are continuously replenished and after a few weeks need to be replaced. When operating the cells, the crust has to be at least partially hammered in, which makes it difficult to arrange heat pipes next to the anodes. In addition, the arrangement of a cooling medium above the cell is much more difficult than next to or below.

The ends of the heat pipes protruding from the cell advantageously have a surface serving as a heat exchanger, e.g. in a lamella form of a known type, which in turn is arranged in a metallic plate or in a channel through which a cooling or warming medium flows. When the cell is cooled, it is quite possible that the medium emerging from the cooling channel and heated by heat exchangers is used, for example for heating purposes, either directly or via a store.

A lamellar heat exchanger is expediently attachable over the ends of a heat pipe. With this attachment, the effect of a heat pipe of simple shape is significantly increased because the surface serving for heat exchange is multiplied.

Because of the high temperatures in aluminum melt flow electrolysis cells, heat pipes with an alkali metal are preferably used as the heat transfer medium, sodium being used in particular for practical and economic reasons. In the event of a defect in the heat pipe, it would not matter if the relatively small amount of sodium in the heat pipe were to get into the electrolysis cell. In contrast, an outflow of sodium into a cooling channel located outside the cell would be extremely dangerous because this metal reacts violently when it comes into contact with water.

When using heat pipes with sodium as the heat transport medium or heat carrier for cooling aluminum electrolysis cells, therefore, a primary heat exchanger, which protrudes from the electrolysis cell and is expediently provided with a lamellar attachment, is preferably created, which in turn is in engagement with a secondary heat exchanger. The primary heat exchanger, in particular a cooling circuit, is filled with a liquid organic coolant for higher temperatures which is compatible with sodium as well as with water and air, for example with DOWTHERM from the well-known chemical company DOW Inc .. The primary heat exchanger can also consist of a metal block or a metal plate with good thermal conductivity. Water or air can be passed through the secondary heat exchanger, in particular a cooling channel. When using a primary and secondary heat exchanger, a defect in a heat pipe is not harmful because the sodium outside the cell only comes into contact with the organic coolant, but not with water or air.

If the economy in terms of installation and operating costs permits, the melt flow electrolysis cell for the production of aluminum can be provided with heat pipes which pump heat from a heating medium into the cell. In the event of a prolonged power failure, the cell can be heated from the outside to such an extent that the temperature of the electrolyte remains above a critical value which prevents the complete solidification.

In relation to the method, the object of maintaining the thermal equilibrium of a melt flow electrolysis cell for the production of aluminum at any current strengths between 50 and 125% of the normal value of the cell direct current is achieved according to the invention in that the current strength exceeds 70-80% of the normal value Heat withdrawn to a corresponding extent, whereas with an amperage between 50 and 70 - 80% of the normal value, the interpolar distance is increased or heat is supplied from another energy source.

Because of the increased cell insulation, heat is continuously dissipated at the normal value of the cell direct current. Depending on the design of the oversized cell insulation, the electrolysis cell is in thermal equilibrium with a current intensity reduced to 70-80% without heat dissipation. Compared to a normally insulated cell, the cathodic current density can be increased according to the invention in such a way that the same current intensity flows through the cell overall during normal operation.

If the electrical energy supplied to the cell is reduced over a shorter or longer period, less heat is drawn off from the electrolysis cell, and the heat pipes act as variable thermal insulation. This allows the thermal equilibrium of the melt flow electrolysis cell to be restored in a self-regulating manner after a relatively short time. The production of aluminum is normal at a level which is reduced in accordance with the reduced energy supply.

• - Die Marktsituation macht eine Produktionsdrosselung notwendig
• - Der Spitzenbedarf der Privathaushalte bewirkt kurzzeitige Netzüberlastungen
• - Pannen am Gleichrichter, die einen teilweisen Ausfall des elektrischen Normalstromes zur Folge haben.

The energy supplied to the electrolytic cell can be varied, in particular reduced, for various reasons, whether intentionally or unintentionally, for a short or long period, for example:

• - The market situation makes it necessary to cut production
• - The peak demand of private households causes short-term network overloads
• - Breakdowns on the rectifier, which result in a partial failure of the normal electrical current.

The supply of electrical energy, which fluctuates greatly during the day, is of particular importance: shortly before and over noon and in the early evening, private households consume large amounts of electrical energy, while practically nothing is consumed after midnight until the early hours of the morning. It would therefore be of great economic interest for aluminum electrolysis, which is also energy-intensive, if it could work to complement the electricity consumption of private households.

The invention allows the power supply to the electrolysis furnaces to be controlled in such a way that the power supply is reduced during the peak consumption of private households, but is increased during the night hours.

Although the current reduction is preferably at most 30% in order not to exceed the self-regulating area of the heat pipes, an electrolysis cell designed according to the invention can tolerate a current reduction of up to 50% if the interpolar distance is increased accordingly and / or heat is supplied from another energy source.

The change in temperature which occurs as a result of a decrease or increase in the current intensity in order to achieve the thermal equilibrium of the cell may only fluctuate within relatively narrow limits, for example ^± 10 ° C., because any change in temperature causes the side board formed from solidified electrolyte material to increase or decrease.

• - _Fig. 1 einen Querschnitt durch eine Aluminiumschmelzflusselektrolysezelle mit seitlich angeordneten Wärmerohren, und
• - Fig. 2 einen Ausschnitt eines Querschnittes im unteren Bereich einer Aluminiumschmelzflusselektrolysezelle, mit einem senkrecht angeordneten Wärmerohr.

The invention is explained in more detail with the aid of schematic vertical sections. Show it:

• - _F ig. 1 shows a cross section through an aluminum melt flow electrolysis cell with laterally arranged heat pipes, and
• 2 shows a section of a cross section in the lower region of an aluminum melt flow electrolysis cell, with a heat pipe arranged vertically.

According to FIG. 1, an aluminum melt flow electrolysis cell 10, which essentially consists of a steel tub 12, an insulating layer (not shown), a cathodic carbon block 14 with cathode bars 16 embedded therein, and anode blocks 18 with spades 20 and anode rods 22, is arranged on a base plate 24. On the bottom of the trough-shaped coal block 14 is the liquid aluminum 26, which is deposited during the electrolysis process. The anodes 18 are immersed in the molten electrolyte 28 from above. This electrolyte material has solidified into a solid crust 30 in the lateral and upper region. A layer of alumina 32 is heaped onto this crust and thus forms excellent thermal insulation.

In the lateral longitudinal area, the steel trough 12, the insulation layer and the outer area of the carbon block 14 are penetrated by heat pipes 34. A lamellar heat exchanger with a large surface area is arranged in the outer region of the heat pipes 34. The outer end of the heat pipe 34, in the present case with a heat exchanger 36, is arranged in a channel 38. This channel can be replaced by a metal block of good thermal conductivity, then no heat exchanger 36 is necessary.

_M are cooled uss the electrolytic cell 10, the channel or the metal block 38 is initially cooled and the heat pipes 34 operate in the direction of arrow 40. must, however, the cell from a reason mentioned above, heat can be supplied, so the metal block 38 about the working temperature the electrolytic cell 10 is heated, the heat pipes then work in the opposite direction of the arrow 40.

Fig. 2 shows a heat pipe 34 which passes through the steel trough 12, the insulation 13 and partially the carbon bottom 14 of an electrolysis cell. The liquid aluminum 26 lies on the bottom of the coal block 14.

A lamellar radiator attachment 42 is attached to the lower end of the heat pipe 34. The lower end with the attachment 42 projects into a primary cooling channel 44 which is filled with an organic coolant 46. This coolant is inert to alkali metals, especially sodium, even at high temperatures. The lower area of the primary cooling channel 44 has a downwardly directed lamellar bulge 50, through which the organic coolant circulates in the direction of the arrow 48, around an extension 52 of the cap 42 designed as a partition.

The lamella-like lower part 50 of the primary cooling tube 44 in turn protrudes into a secondary cooling channel 54. A conventional cooling medium 56, in particular air or water, flows through this secondary cooling channel 54.

With this embodiment, which allows the removal of heat in the area of the bottom of the aluminum electrolysis cell, the use of alkali metal heat pipes makes it completely harmless. Should a breakdown occur, the alkali metal only flows into the inert organic coolant, but does not come into contact with water or moist air.

Claims (10)

1. Device for regulating the heat flow of a melt flow electrolysis cell for the production of aluminum,
marked by - A reinforced cell insulation, formed by an up to 50% thicker insulation layer (13) between the steel trough (12) and the coal lining (14) and up to twice the amount of clay (32) on the crust (30) made of solidified electrolyte material, and - With one end face blindly installed in the interior of the cell (34), which contain a reversibly evaporable and condensable heat transfer medium, reach through peripheral parts of the cell and end in the heat exchanger (38, 44) arranged outside the cell. 2. Device according to claim 1, characterized in that an end face of approximately vertically arranged heat pipes (34) ends in the carbon bottom of the cell (10), preferably at the same height as the upper side surface of the cathode bars (16). 3. Device according to claim 1 or 2, characterized in that heat pipes (34) in cathode bars (16) or anode rods (22), the cross-sectional area of which is correspondingly enlarged, are arranged. 4. The device according to at least one of claims 1-3, characterized in that an end of approximately horizontally arranged heat pipes (34) ends in the side coal lining. 5. The device according to at least one of claims 1-4, characterized in that the ends of the heat pipes (34) projecting into the heat exchanger (38, 44) arranged outside the cell are provided with a lamella attachment (42) functioning as a heat exchanger. 6. The device according to at least one of claims 1-5, characterized in that the outer heat pipe ends protrude into a primary heat exchanger (44), which in turn is in engagement with a secondary heat exchanger (54). 7. The device according to claim 6, characterized in that the primary, designed as a cooling circuit heat exchanger (44) is filled with a liquid organic coolant suitable for higher temperatures, which is compatible with alkali metals as well as air and water, and the secondary, heat exchanger (54) designed as a cooling circuit is filled with air or water. 8. The device according to claim 6, characterized in that the primary heat exchanger (44) is a metal block or a metal plate of high thermal conductivity. 9. A method for maintaining the thermal equilibrium of a melt flow electrolysis cell for the production of aluminum with a device according to one of claims 1-8, at any current strengths between 50 and 125% of the normal value of the cell direct current, characterized in that
that at a current strength exceeding 70-80% of the normal value, heat is withdrawn to a corresponding extent, at a current strength between 50 and 70-80% of the In contrast, the interpolar distance is increased or heat is supplied from another energy source. 10. The method according to claim 9, characterized in that heat is continuously removed at the normal value of the cell direct current.

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