Patent Application
20080271996
This invention relates to the technical domain of electrolytic cells, and particularly cells used in the electrolysis process used for industrial production of aluminium.
More particularly, this invention relates to equipment of the side walls of an electrolytic cell pot, designed to recover heat energy lost through the sides of the pot by heat exchange with a heat transfer fluid, while protecting and preserving the sides of the pot and improving operating conditions of this pot.
International patent application WO 2004/083489 A1 describes an electrolytic cell for aluminium production in which the pot is provided with a kerb slab equipped for recovery of heat energy, by circulation of a gas or liquid heat transfer fluid. The heat transfer fluid is circulated through internal channels, arranged in a “coil” type path within the body of the kerb slab, gas tight cavities being obtained by various means at the time of production and assembly of side panels, with internal profiles obtained by casting and carbonaceous resin elements that are eliminated during sintering. The heat transfer fluid is added and extracted through ceramic pipes or connectors, glued to the material of the side panels.
Leak tightness of the heat transfer fluid circuit thus made up is complex and difficult to achieve using the means described, either for cavities formed in the bulk of the material making up the panels, or at the junction between the slab supply pipes, particularly because sintering of the parts is accompanied by large shrinkage of the material, and the residual porosity of the material close to the cavities can induce leaks.
Document WO 87/00211 that also relates to electrolytic cells for aluminium production is based on the control of the electrolysis bath temperature by a complex system of gas circuits provided in cooling chambers arranged not only in the side walls of the pot, but also in the anode and in the refractory material at the bottom of the pot, the heat transfer gas preferably being helium. The controlled cooling system is designed to minimise the thickness of insulating refractory materials and to recover heat energy. This system appears very expensive, both in implementation and in operation.
U.S. Pat. No. 4,222,841 describes a tubular heat exchanger above the electrolysis bath, due to the use of thermal insulation by refractory or carbonaceous panels separating the bath and the heat exchanger to prevent the formation of crusts. No information is given about the nature of the materials from which the heat exchanger is made, which is particularly subject to fluoride corrosion at high temperature. This document also describes a tubular heat exchanger at the sidewalls and the bottom of the pot containing the bath, once again without giving any further information about the structure and materials from which the exchanger is made.
Finally, document WO 01/94667 describes placement of cooling side panels that are insulated from the outside shell by a thick refractory material, inside an electrolytic pot for the production of aluminium, as a substitute for conventional kerb slabs. Each side panel is cooled by evaporation of a metal or alloy such as zinc, sodium or a sodium—lithium alloy in the liquid state. The panel evaporation chamber is covered by a condensation chamber cooled by circulation of a heat transfer gas. Such a device is complex and introduces obvious safety problems. Furthermore, the refractory material may be attacked by metals or alloys in the liquid or vapour phase with which it is in contact.
Faced with this state of the art, the purpose of this invention is to provide a new and advantageous technique for the side wall of an electrolytic pot, with the function, in addition to the function of surrounding the kerb of the crucible, of making a heat exchanger in order to recover heat energy lost through the sides of the pot, with a large exchange surface area, and to control the thickness of the frozen bath ridge that protects the side lining materials of the pot from chemical attack by the liquid aluminium and the molten salts bath, the proposed solution being intended to be simple and therefore economic, while remaining safe, and having a high heat energy recovery efficiency, and possibly with modulation of this energy recovery.
To achieve this, the purpose of the invention is an electrolytic pot that can be used for the production of aluminium, comprising sidewalls provided with a heat exchanger through which a heat transfer gas can pass, characterised in that the sidewalls of the pot are at least partly made of a porous material over at least a fraction of their height and their thickness, to enable circulation of the heat transfer gas, the part or each part made of a porous material being connected to heat transfer gas inlet and outlet means.
Thus according to the invention, the sidewalls of the electrolytic pot include porous parts in which the required heat exchange takes place. These porous parts, with open pores, have a high porosity such that the number of pores and the size distribution of these pores can enable circulation of the heat transfer gas without any excessive head loss, between an entry point and an exit or extraction point, gas circulation preferably being controlled by suction from the extraction side.
In one simple embodiment, the porous material may have homogenous characteristics over the entire height and width of the porous part. However according to an advantageous variant, the porous material has variable characteristics, and particularly porosity, thickness and/or thermal conductivity characteristics, over the height of the porous part, so as to obtain successive zones in the direction of the height with different heat exchange characteristics. The porous part can thus be optimised either by a progressively varying gradient, or by a sub-division in the direction of the height into successive zones with distinct characteristics (such as porosity and/or thickness) chosen to achieve the required power for a corresponding heat transfer gas flow. The sidewalls of the pot may also comprise at least one pipe through which the heat transfer gas can be circulated along a preferred path, particularly towards the bottom of the porous part and/or from the top of the porous part.
The porous part may be formed from one or several porous slabs, each slab having a monolithic structure made of a porous material.
According to one embodiment of the invention, the sidewalls of the electrolytic pot comprise at least a first part made of a dense material located on the inner side of these walls, and at least a second part made at least partially of a porous material, located between the first part and the outside shell of the pot, in other words the outer side of these walls facing the outside shell of the pot.
The first part made of a dense material is typically in contact with a kerb with an inclined edge which, together with the cathode blocks, form the cathode crucible. A contact material may be inserted between said first part and the kerb so as to reduce the thermal resistance at this interface. During operation, said first part may possibly be in contact with the upper part of the ridge of the frozen electrolytic bath. The heat exchange in this case is done in the part made of a porous material, on the outer side of the sidewalls.
The part made of a dense material may also be formed by one or several slabs with a monolithic structure. According to one variant, the part made of a dense material may be joined to the part made of a porous material by assembly. The two parts of these walls, namely the inner part made of a dense material and the outer part made of a porous material, can be assembled using a refractory material such as concrete, typically in the form of a refractory grout, or by a standard glue for the materials concerned, or by a special glue for this application.
Instead of deciding to assemble two distinct materials, one after the other through the thickness of the sidewalls of the pot, in one variant of the invention, the sidewalls of the electrolytic pot include a structure formed from monolithic slabs made from a material with variable porosity in the direction of the thickness of said sidewalls.
The slabs may form individual heat exchanger devices.
A porous part may be obtained by a process comprising the production of a porous polymer body comprising one or several polymer foams with open porosity, preparation of a suspension of a ceramic precursor, impregnation of the porous body with this suspension, drying of the impregnated suspension, baking of the porous body in order to burn organic components and sintering of the porous body. American patent U.S. Pat. No. 5,039,340 describes such a process. A material with variable porosity can be obtained by forming an initial body comprising a foam with variable porosity or superposition of two or more distinct porous foams with different porosities. A slab comprising a part made of a dense material and a part made of a porous material may be obtained by forming the dense part by casting, vibrocompaction and pressing of a ceramic precursor, and by combining the dense body thus obtained with one or several porous bodies as described above, typically after impregnating them with a ceramic precursor, baking and sintering preferably being done after bringing the dense body and the porous body together.
The porous material used may be formed from a metal or a metal alloy, a heat conducting ceramic or a mix or combination of these materials, and is typically in the form of a foam. Preferably, the metal or metal alloy has a melting point exceeding 800° C. and is resistant to oxidation at temperatures of above 250° C., like nickel-based alloys (in other words containing more than 50% by weight of nickel). Said metal or metallic alloy preferably has a coefficient of thermal expansion less than 25×10−6 K−1, such as Inconel® 686 containing nickel, chromium, molybdenum and tungsten. The intrinsic thermal conductivity of ceramic is preferably greater than 5 W/m.K, and preferably even greater than 20 W/m.K. In order to increase the compatibility of thermal expansion between the dense material and the porous material, the porous material is preferably made from a foam containing a majority of silicon carbide (SiC), silicon nitride and/or aluminium nitride, that are heat exchanging materials according to the meaning of this invention (a majority meaning more than 50% by weight). In this case, the porous material preferably contains at least 70% by weight of heat conducting ceramic, and preferably 85% by weight of heat conducting ceramic, for example the remainder may be oxide type mineral binders such as silicates or oxinitrides.
The porous material has a porosity of more than 70%, preferably more than 80% (the porosity being defined in this case as being the void ratio).
Considering more particularly the case of sidewalls comprising a first part made of a dense material and a second part made of a porous material, it is preferable for the part made of a dense material to have a porosity of less than 20%. The dense material is preferably a ceramic material containing at least 70% by weight of silicon carbide (SiC) and more preferably 85% by weight of silicon carbide (SiC).
In combination with the distribution in the direction of the thickness of the sidewalls of the pot, the part made of a porous material that is the source of the heat exchange may be present around all or part of the height of these walls.
According to a first possibility, the part made of a porous material extends substantially over the entire height of the sidewalls of the pot, so that a controlled quantity of heat energy produced by electrolysis can be drawn out over an extended surface area.
According to another possibility, the part made of a porous material extends over a limited portion of the total height of the sidewalls of the pot, particularly over a fraction of the order of one third to one half of the height of this pot, so as to concentrate the heat exchange and therefore the drawn off thermal flux, facing limited and judiciously selected areas, for example at the interface between the liquid aluminium layer and the molten salts bath, known as being a critical area with regard to stability of the ridge.
The heat transfer gas inlet and outlet means may in particular be located at the top and at the bottom of the (each) part made of a porous material, therefore near the top part and bottom part of the sidewalls of the pot, particularly in the case of a porous area extending substantially over the entire height of the sidewalls.
However, particularly in the case of a part made of a porous material divided in the direction of the height into successive zones having different heat exchange characteristics, it is advantageously planned to use at least one additional inlet or one additional outlet of the heat transfer gas located at an intermediate height, particularly at the transition between two successive zones, as a function of heat exchange needs in these different zones.
The heat transfer gas inlet and outlet means may also be distributed along the horizontal dimension of a part of the sidewalls of the pot made of a porous material, particularly by arranging the inlet and outlet at the two horizontally opposite ends of a part made of a porous material. This is particularly applicable to the case of a long part made of a porous material in order to achieve the most uniform possible heat exchange over the entire length of said part made of a porous material.
In that respect, it can be noted that a part made of a porous material may extend over a zone very much longer than a slab (for example when the porous part is formed by the assembly of two or more slabs made of a porous material). In these cases, the junction between slabs is made so as to enable the heat transfer gas to flow between the porous zones in each adjacent slab. The junction cement may be concrete, refractory grout or appropriate glue. The number of heat transfer gas inlets and outlets can thus be limited.
Preferably, the heat transfer gas inlet(s) have orifices located at a level higher than the liquid level in the pot, in other words the inlet orifices are near the top of the sides of the shell or are around the periphery of the top of the shell, or if the actual inlets are located near the bottom of the pot for technical reasons, these inlets are extended by tubes oriented upwards and with their orifices at a level higher than the liquid level in the pot, in order to avoid a flow of hot liquids outside the outside shell of the pot, particularly when the porous zone extends substantially over the entire height of the sidewalls of the pot, and therefore the shell, in the case of a local failure of the different heat exchanger devices.
For the heat transfer gas outlet, and more particularly for extraction of this gas by suction, at least one side manifold is advantageously provided connected to several heat transfer gas outlets. Preferably, each side of the electrolytic pot is equipped with at least one manifold, all manifolds being connected to a common suction unit. The two long sides of the pot may each be equipped with two parallel manifolds.
Regardless of the position of the heat transfer gas inlet and outlet orifices, the through cross-section of these orifices or of some of these orifices is advantageously made adjustable using flaps. These flaps may be pre-set when cold, before the electrolytic pot is put into operation, as a function of local specificities and the pot design. The design and manufacture of the manifold(s) through which the heat transfer gas is extracted, is preferably such that before the above mentioned flaps are adjusted, an equivalent head loss at the suction is obtained in all individual heat exchanger devices in the pot connected to these flaps, so as to obtain constant heat flows per unit volume.
The heat transfer gas may be air or an inert gas, typically nitrogen, helium or argon, or a mix of air and inert gas.
If air is used as the heat transfer gas, air inlets may be open to the surrounding atmosphere, more particularly into the space located between adjacent pots, and in this case only the air outlets are connected to suction manifolds. Air inlets are thus composed of single orifices with an appropriate shape and size, that operate by depression to supply a heat exchanger device, in other words a part made of a porous material.
However, in another preferred embodiment in which air is used as the heat transfer gas, this air is recycled to increase its inlet temperature into the porous zone, and consequently its outlet temperature from this zone so as to increase the usage efficiency of the recovered energy, for example through an external heat exchanger. In this case, a distribution network is provided to bring air drawn off at the outlet orifices to the inlet orifices. The design of this distribution network assures an identical head loss at all air inlet orifices to the pot, in order to obtain uniform operation.
Similarly, if the heat transfer gas contains an inert gas, it is advantageous to recycle it due to its value.
Finally, if a distribution network is provided, the embodiment without recycling the heat transfer gas, and therefore with air intake in the space located between the pots, can be combined with the production mode with air recycling using direct air inlet valves into the distribution network, these valves possibly being located at different points in the distribution network, preferably in combination with isolating valves that can isolate the different portions of the distribution network from each other. This “combined” mode has the advantage that it enables work to be done on the distribution network, or can temporarily overcome a failure in the “upstream” part of the gas recycling system, or compensate for air losses in the circuit.
Advantageously, a thermal insulation is also provided between the part made of a porous material and the shell of the pot, and more precisely between firstly the outer face of the part made of a porous material, and secondly the inner face of the pot shell. The layer of insulating material limits thermal losses, which makes it possible to improve energy recovery. The thermal insulation is advantageously made of a fibrous material that can act as a deformable buffer to protect kerb slabs, by absorbing any thermal expansions of the pot, particularly during its temperature rise when the cell is brought into service. The insulation typically forms a substantially vertical layer with a thickness of between 10 and 100 mm, and preferably between 15 and 50 mm.
The heat transfer gas preferably travels by suction and therefore by depression, through the heat exchanger device so that if the device fails, it will not blow heat transfer gas into the structure of the pot, kerb blocks, the bottom of the pot or liquid phases. This variant of the invention may be used by connecting the electrolytic cell to a suction system that can circulate a heat transfer gas by depression in the part or each part made of a porous material.
Finally, another purpose of the invention is an industrial aluminium production plant comprising a plurality of electrolytic pots like those mentioned above, that are connected through manifolds to a heat transfer gas circuit directed towards energy recovery means, comprising at least one external heat exchanger and/or at least one electricity generator.
The heat exchanger system as a whole proposed by the invention has the following advantages:
The invention will be better understood with the help of the following description made with reference to the appended diagram showing various embodiments of this electrolytic cell with a pot with sidewalls provided with a heat exchanger, for use as examples:
As can be seen on
An electrolytic cell 23 is formed by the assembly of a pot 22 and one or several carbonaceous anodes 6 in the upper part above the crucible, and connected to a current inlet through anode multipodes 7.
In service, the crucible 40 contains a layer of liquid aluminium 9 above which there is a molten electrolyte bath 10 based on cryolite into which each anode 6 dips. The assembly formed by the liquid aluminium layer 9 and by the electrolyte bath 10 is surrounded by a solidified bath zone called the “ridge” 11 close to the sidewalls 8 and in contact with kerb blocks 4′ of the crucible 40.
Seen from above, the electrolytic pot has a generally rectangular shape, with two long sides and two short sides.
An electrolytic cell is usually associated with other similar cells arranged in line with free spacings (therefore filled by air) between the pots of these cells.
As illustrated in the right part of
In this case, a second part closer to the outer side and therefore facing the inner face of the shell 2, is composed of a porous material 13 over almost its entire height. The porous material, that is typically a foam (preferably a silicon carbide foam) has an appropriate porosity, typically between 10 and 90 ppi (namely between about 4 and 36 pores/cm) and preferably between 20 and 70 ppi (namely approximately between 8 and 28 pores/cm) so as to offer a low head loss while maintaining a high heat exchange capacity.
The thickness of the dense part 12 may be between 10 and 100 mm and preferably between 30 and 50 mm, while the thickness of the part made of a porous material 13 is between 5 and 50 mm, and preferably between 10 and 25 mm, and typically between 15 and 25 mm. These two parts 12 and 13 can be assembled at their interface 14 using special glue in the form of a suspension or a paste, containing a mix of a mineral filler with an average size grading of less than 250 μm, and a silicone resin. In the case of a suspension, the mix may possibly contain a solvent to solubilise the resin and form a fluid suspension. International patent applications WO 03/033435 and WO 03/033436 describe possible glues.
A heat insulating fibrous material (not shown) may be inserted between the shell and the porous part so as to reduce heat losses to the shell, for recovery of this heat. This insulation may also be put into compression between the outer wall of the part 13 made of a porous material, and the inner face of the shell 2.
The part made of a porous material 13 is designed to allow air to enter through an air inlet orifice 15 near the bottom of the sidewall 8 and then pass through the part from bottom to top in
Air, that is not recycled in the case illustrated in
Air passing through the heat exchangers thus formed recovers heat energy released within the pot and transfers this energy to outside the pot. The range of heat flux thus evacuated through the walls 8 of the pot typically varies between 1 and 35 kW/m2.
The air outlet orifice 16 is advantageously provided with a flap 19 for varying the air outlet cross section.
According to an inverse arrangement illustrated in
Conversely, it is also possible to keep a single air inlet orifice 15 and to multiply air outlets as illustrated in
Other combinations of air inlets and outlets distributed over the height of the sidewalls 8 of the pot are also possible, the embodiments described in detail above only being examples.
As can be seen in
The height of the part made of a porous material 13 may be further reduced and for example only represent about one third of the total height of the sidewalls 8 of the pot, as illustrated in
In all embodiments described above with reference to
In another type of embodiment illustrated in
In one variant not illustrated related to the previous embodiment, the parts made of a porous material are joined together to form a continuous strip with alternating inlet and outlet orifices around the perimeter of the electrolytic pot.
In all cases, in order to force circulation of air or another heat transfer gas into the parts 13 made of a porous material, the side or end surfaces of these parts made of a porous material can be made leak tight by impregnation or sealing, thus preventing any leaks or losses of air or gas.
The modules comprise at least a first porous section 13a and a second porous section 13b and internal conduits 51, 52, 53 through which the heat transfer gas can be circulated along preferred paths. The inlet points 15 and outlet points 16 are preferably in a part of the modules 30 that will be in the upper part of the heat exchanger. The said first and second porous sections 13a, 13b may be distinct slabs placed adjacent to each other or parts of the same porous slab.
In the example illustrated, the modules 30 are provided with a down conduit 51 through which the heat transfer gas flows towards the bottom part of the first porous section 13a, a first horizontal conduit 52 through which the heat transfer gas flow can be distributed along the first porous section 13a and a second horizontal conduit 53 to collect the heat transfer gas from the second porous section 13b. The walls 43, 44, 45, 46 of said conduits 51, 52, 53 may be formed using metallic or ceramic elements such as tubes, by moulding and/or sealing of porous surfaces using glues or refractory cements.
The first porous section 13a has a first porosity, and particularly a first number of pores per unit length. The second porous section 13b has a second porosity, and particularly a second number of pores per unit length. The number of pores per unit length is typically expressed in ppi or pores per cm.
These composite modules have the advantage that they enable a uniform and substantially vertical circulation velocity of the heat transfer gas, to avoid the formation of a thermal gradient along the walls of a pot that could be harmful to the shape of the solidified bath ridge.
These composite modules 30 advantageously comprise a support slab 12 made of a heat conducting material, typically a dense ceramic material like that defined above, that will be located on the inner side of the pot, typically in contact with the kerb 4′. These composite modules 30 may also comprise a layer 29 of a heat insulating material, typically a fibre, designed to be located on the outer side, typically in contact with the inside face of the shell 2.
These composite modules may be placed adjacent to each other so that an inlet 15, usually colder, can be placed close to an outlet 16, usually warmer, which is conducive to better uniformity of the temperature by mutual compensation, particularly by the possible use of a heat conducting support slab 12.
The first porous section 13a is advantageously located at the average height of the interface between liquid aluminium 9 and the liquid electrolyte bath 10 in the pot during operation, so as to enable better heat exchange due to higher porosity and due to the conduit to the first porous section 13a of all or part of the heat transfer gas originating directly from the inlet 15.
In the case in which the level of the interface between the liquid aluminium 9 and the liquid electrolyte bath 10 is at the first porous section 13a, the first porosity is preferably greater than the second porosity. For example, in this case the first number of pores per unit length is typically between 50 and 70 ppi and the second number of pores per unit length is typically between 30 and 50 ppi.
The composite modules 30 illustrated in
Said gluing and sealing operations may be done using refractory glue like that described above.
Finally,
Tests have been carried out with a board made of ceramic porous material so as to evaluate heat exchange capacities possible with the materials according to the invention. A heat exchanger was made by gluing a 25 mm thick board of porous ceramic foam made of silicon carbide with a porosity of 20 ppi (8 pores/cm) with a porous volume of 88%, onto a 40 mm thick board of dense material based on silicon carbide bonded with silicon nitride. The effective thermal conductivity of the porous board was between 0.50 and 1 W.m−1.K−1. The glue was a refractory grout. The heat exchanger was placed at the entrance to a furnace to replace the door by using a metal frame. The heat exchanger device was isolated using fibrous materials around the frame. Thermocouples located at different locations, particularly at the inlet and outlet of the heat transfer fluid, were used to quantify heat exchange in the porous zone. The exposed surface area facing the furnace was 400 cm2.
The furnace was heated to a fixed temperature and was then kept at this temperature during the series of tests, the air flow circulating in the exchanger being controlled using a flow meter.
Table I shows the heated air outlet temperature and the heat flux recovered by the exchanger, as a function of the flow of heat transfer fluid, namely air. These tests show that when the cooling airflow increases, the air temperature at the outlet reduces and the recovered heat flow increases.
The inventors reckoned that, for a comparable density, the increase in the number of pores does not significantly increase the head loss in the porous medium up to about 60 ppi, but it does increase the heat exchange surface area with the heat transfer gas.
Obviously, the invention is not limited to the embodiments of this electrolytic pot described above as examples; on the contrary, it includes all variants that respect the same principle. Thus, in particular, changing the nature of materials, in particular the porous materials from which the heat exchanger is made can be changed without departing from the scope of the invention, and these materials may also be metallic or hybrid (such as a combination of silicon carbide and metal). Similarly, the number and positions of air or heat transfer gas inlet and outlet orifices related to porous parts or zones can be varied without departing from the scope of the invention. Finally, there is a wide variety of methods of using and/or converting the recovered thermal energy.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0511503 | Nov 2005 | FR | national |
| PCT/FR06/02465 | Nov 2006 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/002465 | 11/6/2006 | WO | 00 | 5/13/2008 |
