EP1948849A2

EP1948849A2 - Electrolytic cell with a heat exchanger

Info

Publication number: EP1948849A2
Application number: EP06831067A
Authority: EP
Inventors: Airy-Pierre Lamaze; Richard Laucournet; Christian Barthelemy
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 2005-11-14
Filing date: 2006-11-06
Publication date: 2008-07-30
Also published as: CA2627956A1; NO20082380L; CN101305114A; WO2007057534A2; RU2008123600A; US20080271996A1; FR2893329B1; FR2893329A1; AU2006314430A1; BRPI0618543A2; ZA200804542B; WO2007057534A3

Abstract

The invention relates to electrolytic cells used for producing aluminum. The lateral walls (8) of the cell, which surround its crucible (4, 4') are comprised, on at least a fraction of their height and/or their thickness, of a porous material part (13) capable of permitting the circulation of a heat transfer gas, the porous material part (13) being connected to inlet (15) and outlet means (16) of the heat transfer gas whereby constituting a heat exchanger that recovers the heat energy lost on the sides of the cell.

Description

Electrolysis tank with heat exchanger

The present invention relates to the technical field of electrolysis cells, in particular the cells used in the electrolysis process used for the industrial production of aluminum.

More particularly, this invention is concerned with equipping the side walls of an electrolytic cell tank, intended to recover, by heat exchange with a coolant, the thermal energy lost on the sides of the tank, while ensuring the protection and preservation of the sides of the tank and improving the operating conditions of this tank.

The international patent application WO. 2004/083489 A1 discloses an electrolysis cell for the production of aluminum, the vessel is provided with a border slab equipped for the recovery of thermal energy, by circulation of a coolant, gaseous or liquid. The circulation of the coolant is through internal channels, arranged in a "serpentine" path in the mass of the edge slab, gastight cavities being obtained by various means at the time of development and assembly side panels, with molded internal profiles and carbon resin elements which are removed during sintering. The introduction and extraction of the coolant is provided by ceramic pipes or fittings glued to the material of the side panels.

The sealing of the heat transfer fluid circuit, thus constituted, is complex and difficult to obtain by the means described, either at the cavities formed in the material of the panels, or at the junction between the feed tubes. of the slab, this in particular because the sintering of the parts is accompanied by a significant shrinkage of the material, and that the residual porosity of the material near the cavities can induce leaks. WO 87/00211, which also relates to electrolysis cells for the production of aluminum, is focused on controlling the temperature of the electrolysis bath by a complex system of gas circuits, provided in chambers cooling means arranged not only in the side walls of the tank, but also in the anode and in the bottom refractory material, the heat transfer gas being preferably helium. The controlled cooling system is supposed to minimize the thickness of insulating refractory materials, and recovering thermal energy. This system appears eminently expensive, in its implementation and its operation.

US Pat. No. 4,222,841 provides a tubular heat exchanger above the electrolytic bath, by means of thermal insulation by refractory or carbonaceous panels, separating the bath from the exchanger to prevent the formation of crusts. No details are given as to the nature of the constituent materials of the exchanger, which is particularly stressed in high-temperature fluorinated corrosion. This document also provides a tubular heat exchanger at the side walls and the bottom of the tank containing the bath, again without further specification of the structure and the constituent materials of the exchanger.

Finally, the document WO 01/94667 describes, in an electrolysis cell for the production of aluminum, in substitution of conventional border slabs, the establishment of lateral cooling panels insulated from the outer box by a thick refractory material. Each side panel is cooled by evaporation of a metal or alloy in the liquid state, such as zinc, sodium or a sodium-lithium alloy. The evaporation chamber of the panel is surmounted by a condensation chamber, cooled by circulating a heat-transfer gas. Such a device is complex and poses obvious safety problems. In addition, the refractory material can be attacked by the metals or alloys in liquid or vapor phase, in contact with which it is located.

In view of this state of the art, the object of the present invention is to provide a new and advantageous design of the side wall of an electrolytic cell, whose function, other than that of surrounding the edge of the crucible, of producing a heat exchanger in order to recover the thermal energy lost by the sides of the tank, with a large exchange surface, and to control the thickness of the frozen bath slope which protects the side materials of the tank of the chemical attack by the liquid aluminum and molten salt bath, the solution proposed to be simple, therefore economical, while being safe, and a high efficiency with respect to the recovery of thermal energy, with a possible modulation of this energy recovery. For this purpose, the subject of the invention is an electrolytic cell, which can be used for the production of aluminum, comprising side walls provided with a heat exchanger, adapted to be traversed by a heat-transfer gas, characterized in that the side walls of the tank comprise, over at least a fraction of their height and their thickness, at least a portion of porous material capable of permitting a circulation of the heat transfer gas, the or each portion of porous material being connected to input and output means of the heat transfer gas.

Thus, according to the invention, the side walls of the electrolytic cell include porous parts in which the desired heat exchange takes place. These porous, open-pored parts have a high porosity, such that the number of pores and the size distribution of these pores can provide a transport of heat transfer gas without excessive pressure drop, between an entry point and a point of entry. outlet or extraction, the circulation of the gas is preferably provided by suction on the extraction side. The porous material may, in a simple embodiment, have uniform characteristics over the entire height and width of the porous portion. However, according to an advantageous variant, the porous material has variable characteristics, in particular of porosity, thickness and / or thermal conductivity, over the height of the porous part, in order to obtain, in the direction of the height, successive zones possessing different characteristics of heat exchange. The porous part can thus be optimized either by a progression by gradient, or by a subdivision in the direction of the height, in successive zones of characteristics (such as porosity and / or thickness) distinct and chosen to ensure the desired power, for a corresponding heat transfer gas flow. The side walls of the tank may further comprise at least one pipe capable of circulating the heat transfer gas along a preferential path, in particular towards the bottom of the porous part and / or from the top of the porous part.

The porous portion may be formed of one or more porous slabs, each slab having a monolithic structure of porous material.

According to one embodiment of the invention, the side walls of the electrolytic cell comprise at least a first portion of dense material located on the inner side of these walls, and at least a second portion made at least partially of porous material, located between the first part and the outer chamber of the vessel, that is to say the outer side of these walls, facing the outer chamber of the vessel. The first portion of dense material is typically in contact with an inclined edge edge which together with the cathode blocks form the cathode crucible. A contact material may be interposed between said first portion and the edge to decrease the thermal resistance at that interface. In operation, said first portion may possibly be in contact with the upper portion of the fixed electrolytic bath slope. The heat exchange is here carried out in the portion of porous material, located on the outside of the side walls.

The portion of dense material may also be formed by one or more slabs of monolithic structure. Alternatively, the dense material portion may be joined to the portion of porous material by assembly. The assembly of the two parts of these walls, namely the inner part of dense material and the outer part of porous material, is achievable by a refractory material such as concrete, typically in the form of refractory slurry, or by a usual adhesive for the materials concerned, or by a specific glue for this application.

Instead of providing the assembly of two different materials, succeeding in the direction of the thickness of the side walls of the tank, a variant of the invention provides that the side walls of the electrolytic cell comprise a structure formed by from monolithic slabs, made of a material with variable porosity in the direction of the thickness of said side walls.

The slabs can form individual heat exchange devices. A porous part may be obtained by a process comprising producing a porous polymer body comprising one or more open porosity polymer foams, preparing a suspension of a ceramic precursor, impregnating the porous body with this suspension, the drying of the impregnated suspension, the baking of the porous body in order to burn the organic components and the sintering of the porous body. US Pat. No. 5,039,340 describes such a method. A variable porosity material can be obtained by forming an initial body having a variable porosity foam or a superposition of two or more distinct porous foams having different porosities. A slab having a portion of dense material and a portion of porous material can be obtained by forming the dense part by casting, vibrating and pressing a precursor of ceramic, and by joining the dense body thus obtained to one or more porous bodies as described above, typically after impregnating them with a ceramic precursor, the firing and the sintering being preferably carried out after having united the dense body and the porous body. The porous material used may be formed of a metal or metal alloy, a thermally conductive ceramic or a mixture or combination thereof, and is typically in the form of a foam. Preferably, the metal or metal alloy has a melting point greater than 800 ^° C. and is resistant to oxidation at temperatures greater than 250 ^° C., such as nickel base alloys (that is to say containing more than 50% by weight of nickel). Said metal or metal alloy preferably has a thermal expansion coefficient of less than 25 × 10 ^-6 K ^-1 , such as Nnconel® 686 containing nickel, chromium, molybdenum and tungsten.The intrinsic thermal conductivity of the ceramic is preferably greater than 5 W / mK, and more preferably greater than 20 W / mK. In order to increase the thermal expansion compatibility between the dense material and the porous material, the latter is advantageously a foam mainly containing silicon carbide (SiC ), silicon nitride and / or aluminum nitride, which are thermally conductive materials within the meaning of the present invention (mainly meaning more than 50% by weight) In the latter case, the porous material preferably contains less than 70% by weight of thermally conductive ceramic, and more preferably 85% by weight of thermally conductive ceramic, the remainder being for example, inorganic oxide binders, such as silicates or oxynitrides.

The porous material has a porosity greater than 70%, preferably greater than 80% (porosity being defined here as the void ratio).

Considering more particularly the case of side walls having a first portion of dense material and a second portion of porous material, it is preferred that the dense material portion has 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 side walls of the tank, the portion of porous material which is the seat of the heat exchange may be present on all or part of the height of these walls.

According to a first possibility, the portion of porous material extends over substantially the entire height of the side walls of the tank, which allows to extract on a large surface a controlled amount of heat energy produced by electrolysis.

According to another possibility, the portion of porous material extends over a limited portion of the total height of the side walls of the tank, in particular on a fraction of the order of one third to one half of the height of this tank, so as to concentrate the heat exchange, and therefore the heat flow withdrawn, in front of limited and judiciously chosen zones, for example at the interface between the liquid aluminum layer and the molten salt bath, known to be a critical area with respect to slope stability.

The input and output means of the heat transfer gas may be located in particular at the top and bottom of the or each portion of porous material, so in the upper part and in the lower part of the side walls of the tank, this in particular in the case of a porous zone extending over substantially the entire height of the side walls.

However, particularly in the case of a portion of porous material divided in the direction of the height in successive zones having different characteristics of heat exchange, advantageously at least one additional inlet or outlet of the heat transfer gas located at intermediate height is advantageously provided. , in particular at the transition between two successive zones, according to the heat exchange needs in these different zones.

The inlet and outlet means for the heat-transfer gas may also be distributed over the horizontal dimension of a part made of porous material of the side walls of the tank, in particular by arranging the inlet and the outlet respectively at the two horizontally opposite ends of the tank. Part of a porous material. This applies in particular to the case of a portion of porous material of great length, in order to ensure a heat exchange as much as possible homogeneous over the entire length of said portion of porous material.

In this regard, it should be noted that a portion of porous material may extend over a zone of length much greater than that of a slab (for example, when the porous part is formed by assembling two or several slabs of porous material). In these cases, the junction of the slabs is performed so as to allow the flow of heat transfer gas between the respective porous zones of contiguous slabs. The joining cement may be a concrete, a refractory slurry or a suitable glue. The number of heat transfer gas inlets and outlets can thus be limited.

Preferably, in order to avoid, in the event of local failure of the heat-exchange devices, a flow of hot liquids outside the external chamber of the vessel, in particular in the case where the porous zone extends over substantially the entire height of the side walls of the tank, therefore of the box, the heat transfer gas inlet (s) have orifices situated at a level higher than the level of the liquid in the tank, that is to say that the inlet orifices are located partly high sides of the box or on the periphery of the top of the box, or that, if the inputs themselves are located in the lower part of the tank for technical reasons, these entries are extended by pipes directed upwards and having their orifices located at a level higher than the level of liquid in the tank.

For the exit of the heat transfer gas, and more particularly for the suction extraction of this gas, advantageously at least one side collector is provided, connected to a plurality of heat transfer gas outlets. Preferably, each side of the electrolytic cell is equipped with at least one collector, all collectors can be connected to a common central vacuum. The two long sides of the tank can each be equipped with two parallel collectors. Whatever the position of the inlets and outlets of the heat transfer gas, the passage section of these or some of them is advantageously made adjustable by means of registers. These registers can be preset cold, before starting the operation of the electrolysis cell, depending on the local specificities of the design of the tank. The design and construction of the collector (s), through which the extraction of the heat-transfer gas is carried out, is preferably such that, before adjustment of the above-mentioned registers, a loss of suction charge which is equivalent in all individual heat exchange devices of the tank connected to them, so as to obtain constant heat flow volumes. The heat-transfer gas may be air, or an inert gas, typically nitrogen, helium or argon, or a mixture of air and inert gas.

If air is used as heat transfer gas, the air inlets can be opened to the surrounding atmosphere, more particularly in the space between adjacent tanks, and only the air outlets are in this case connected to suction manifolds. The air inlets are thus constituted by simple orifices, of suitable shape and size, which operate by depression to feed an exchanger device, in other words a portion of porous material. However, in another preferred embodiment of the use of air as a heat-transfer gas, this air is recycled to increase its inlet temperature in the porous zone, and consequently its outlet temperature of this zone, in order to increase the recovery efficiency of the recovered energy, for example through an external heat exchanger. In this case, a distribution network is provided, to return to the inlet ports the air taken at the outlets. The design of this distribution network ensures a pressure drop that is identical to all the air inlet openings of the tank, in order to obtain a homogeneity of operation.

Similarly, in the case where the heat transfer gas contains an inert gas, it is advantageous to recycle it because of its value.

Finally, if a distribution network is provided, it is possible to combine the embodiment without recycle of the heat transfer gas, thus with suction of the air in the space between the tanks, and the embodiment with recycling. air, by means of direct air intake valves in the distribution network, these valves can be located at different points of the distribution network, preferably in combination with isolation valves able to isolate some other different portions of the distribution network. This "combined" mode has the advantage of allowing interventions on the distribution network, or of temporarily mitigating a failure in the "upstream" part of the gas recycling system, or of compensating for air losses in the system. circuit.

Advantageously, a thermal insulator is furthermore disposed between the part made of porous material and the chamber of the vessel, and more precisely between the outer face of the part made of porous material, on the one hand, and the internal face of the chamber of the vessel. , on the other hand. The layer of insulating material limits thermal losses, which increases the energy recovery. The thermal insulation is advantageously a fibrous material so that it can act as a deformable pad to protect the edge slabs by absorbing any thermal expansion of the tank, especially during its rise in temperature at the start of operation. the cell. The insulation typically forms a substantially vertical layer whose thickness is between 10 and 100 mm, and preferably 15 and 50 mm.

The heat transfer gas preferably circulates by suction, and thus by depression, through the heat exchange device, so that, in the event of failure of the device, the device does not blow heat transfer gas into the structure of the tank, the blocks border, the bottom of the tank or the liquid phases. This variant of the invention can be implemented by connecting the electrolysis cell to a suction system adapted to circulate a heat transfer gas by depression in the or each portion of porous material.

Finally, the subject of the invention is also an industrial plant for producing aluminum, comprising a plurality of electrolysis cells as previously defined, which are connected by collectors to a heat-transfer gas circuit directed towards recovery means. energy, comprising at least one external heat exchanger and / or at least one electricity generator. Overall, the heat exchanger system proposed by the invention has the following advantages:

- This heat exchanger system is of great simplicity of realization, which makes it economical and reliable;

- Being located closer to the source of thermal energy, the system allows optimal recovery of this energy; the valorization of this energy is optimal because it is done at high temperature;

The implementation of the invention does not require a questioning of the structure of the electrolytic cell, which contributes to the simplicity and allows a retrofitting of existing tanks; - The energy recovery is easily adjustable, depending on the heat transfer gas inlet temperature and its flow rate, which allows to participate in the regulation of the intensity of service of the electrolysis tanks according to the needs of production or availability of electricity. The heat exchanger system is used to evacuate all or part of the excess energy with respect to a determined operating point; The invention makes it possible to increase the precision of the control of the thickness of the electrolytic bath of frozen bath which protects the materials of the side walls of the tanks against the chemical attack by the liquid aluminum and the bath of molten salts. This makes it possible, in association with the preceding points, to develop new electrolysis cells with a markedly increased specific power, with identical or improved energy balance, with the possibility of modulating the intensity of the tank without disturbing the thermal equilibrium of the tank. tank.

The invention will be better understood with the aid of the description which follows, with reference to the appended schematic drawing showing, by way of example, various embodiments of this electrolytic cell with side-walled tank provided with an exchanger thermal:

Figure 1 is a partial vertical sectional view of an electrolysis cell according to the present invention; Figure 2 is a partial sectional view, corresponding to the right part of Figure 1, showing a first variant of the cell;

Figure 3 is a view similar to Figure 2, illustrating a second variant;

Figures 4 to 9 are views similar to the preceding, illustrating further variants of the cell object of the invention;

Figure 10 is a vertical sectional view, along X-X of Figure 11, of another embodiment of an electrolysis cell according to the invention;

Fig. 11 is a top plan view, partially, of the cell of Fig. 10; Figure 12 shows a modular assembly according to an advantageous variant of the invention.

FIG. 13 is a diagram illustrating a mode of recovery of the thermal energy of cells according to the invention.

As shown in FIG. 1, an electrolytic cell 22, used in the production of aluminum from alumina, generally comprises:

an outer casing 2 made of steel,

a floor lining 3 made of refractory material,

a crucible 40, intended to be cathodically polarized, formed wholly or partly of cathode blocks 4 and of a border 4 ', which is typically formed of edge blocks made of carbonaceous materials, a current arrival at the cathode, by horizontal steel bars, which pass through the caisson 2 and are sealed to the cathode blocks 4,

- Side walls of tank 8, detailed below. An electrolysis cell 23 is formed by assembling a tank 22 and one or more carbon anodes 6, in the upper part, above the crucible, and connected to a current supply by anodic multipodes 7.

In use, the crucible 40 contains a layer of liquid aluminum 9, surmounted by a molten electrolyte bath 10 based on cryolite in which each anode 6 is plunged. The assembly formed by the liquid aluminum layer 9 and by the electrolyte bath 10 is surrounded by a solidified bath zone, called "slope" 11, close to the side walls 8 and in contact with the edge blocks 4 'of the crucible 40. Plan view from above, the tank Electrolysis has a generally rectangular shape, with two long sides and two short sides.

An electrolysis cell is usually associated with other similar cells, arranged in line, with free spaces (thus filled with air) between the cells of these cells. As illustrated in the right-hand part of FIG. 1, the side walls 8 of the electrolysis cell are, according to the present invention, divided in the direction of their thickness into at least two adjacent parts. A first portion 12, located on the inner side and thus in contact with the edge blocks 4 '(and possibly the slope 11) is made of a dense refractory material, as much as possible gas-tight, especially in the form of a a plurality of slabs of silicon carbide (SiC).

A second part, located on the outer side, thus facing the inner face of the box 2, is constituted here, over virtually its entire height, by a porous material 13. The porous material, which is typically a foam (preferably a foam silicon carbide), has a suitable porosity, typically between 10 and 90 ppi (or between about 4 and 36 pores / cm), and preferably between 20 and 70 ppi (or between about 8 and 28 pores / cm) , in order to offer a low pressure drop while maintaining a high heat exchange capacity. The thickness of the dense portion 12 may be between 10 and

100 mm, and preferably between 30 and 50 mm, while the thickness of the porous material portion 13 is between 5 and 50 mm, preferably between 10 and 25 mm, and typically between 15 and 25 mm. The assembly of these two parts 12 and 13, at their interface 14, is achievable by means of a specific adhesive, in the form of a suspension or paste, containing a mixture of a mineral filler with an average particle size of less than 250. μm, and a silicone resin. In the case of a suspension, the mixture may optionally contain a solvent to solubilize the resin and form a fluid suspension. International applications WO 03/033435 and WO 03/033436 describe possible adhesives. A fibrous material, thermal insulation (not shown), can be introduced between the box and the porous portion to reduce heat losses to the box, for their recovery. This insulator can also be placed in compression between the outer wall of the porous material part 13, and the inner face of the box 2. The porous material part 13 is intended to be traversed, from bottom to top in FIG. air that is admitted through an air inlet 15, located in the lower part of the side wall 8, and which is extracted by an air outlet port 16, located in the upper part of the side wall 8.

The air, which is not recycled in the case illustrated in Figure 1, is taken outside the tank, and in particular in a free space between tanks, and it is admitted by depression following the arrows F1 in FIG. 15. This air, extracted by the air outlet orifice 16, travels along the arrow F2 a tubing 17 connected to this orifice 16 and reaches a lateral collector 18, which extends horizontally along one side of the tank and is itself connected to a central vacuum (not shown). The same manifold 18 can thus group the air flows extracted from several similar heat exchangers, which follow one another along a side wall 8 of the tank.

The air flowing through the heat exchangers, thus constituted, recovers thermal energy released in the tank, and transfers this energy to the outside of the tank. The heat flux range thus evacuated by the walls 8 of the vessel typically ranges from 1 to 35 kw / m ² .

The air outlet orifice 16 is advantageously provided with a damper 19 for adjusting the air outlet section. In an inverted arrangement, illustrated in FIG. 2 (on which the corresponding elements are designated by the same reference marks), the orifice 15 is located at the top while the air outlet 16 is located at the bottom of the side wall 8.

Figure 3 shows an embodiment approaching that of Figure 1, but wherein the portion of porous material 13 is subdivided, in the direction of the height, into two successive porous partial areas 13a and 13b. More particularly, the lower sub-zone 13a has a greater porosity, and the upper sub-zone 13b has a smaller porosity, conducive to a more intense heat exchange, this partial zone 13b being preferably substantially at the height of the liquid aluminum 9 and the electrolyte bath 10. The air here successively traverses the lower sub-zone 13a and then the upper sub-zone 13b. In a variant not shown, the air inlet is located in the upper part of the side wall, the air flowing in this case first the upper portion 13b, then the lower portion 13a. FIG. 4 represents another variant, in which the portion of porous material 13 is subdivided, in the direction of height, into three successive porous partial zones, ie a lower partial zone 13c, an intermediate partial zone 13d and an upper partial zone. 13th.

FIG. 5 represents yet another variant, approaching the embodiment according to FIG. 3, but including an additional air intake orifice 20 situated at the level of the transition between the lower porous partial zone 13 a and the porous partial zone. upper 13b. As represented by the arrow F3, an additional air flow is admitted by depression into the additional inlet port 20, and this flow is added, within the upper partial zone 13b, to the flow rate. air admitted through the lower inlet 15.

Conversely, it is also possible to keep a single air inlet 15, but to multiply the air outlets as shown in FIG. 6; in this case, the different air outlet orifices 16a, 16b and 16c, located at different heights on the side wall 8, each have their register 19a, 19b or 19c, and are all connected to the same lateral collector 18, - even connected to the central vacuum.

Other combinations of air inlets and outlets, distributed over the height of the side walls 8 of the tank, are still conceivable, the previously detailed embodiments being only examples. FIG. 7, in which the elements corresponding to those previously described are still designated by the same reference numerals, shows another embodiment, in which the portion of porous material 13, constituting the heat exchanger, extends only over a fraction of the total height of the side walls 8 of the electrolytic cell, for example about half the height of these walls. In particular, the portion of porous material 3 is here present in the upper half of this height of walls, so as to be also at the height of the liquid aluminum layer 9 and the electrolyte bath 10. The orifice of The air inlet 15 is thus about halfway up the side wall 8, while the air outlet port 16 is at the top. Of course, like the previous examples, a reversal of the position of the air inlet and the air outlet is possible here.

As shown in FIG. 8, also in the case of a portion of porous material 13 extending only over a fraction of the height of the side walls 8, this portion of porous material remains divisible into two or more partial zones, in this example an upper partial zone 13a and a lower partial zone 13b of different porosities. An additional inlet or air outlet could be provided at the junction of the two porous partial areas 13a and 13b. The height of the portion of porous material 13 can be further reduced, and represent for example only about one third of the total height of the side walls 8 of the tank, as shown in Figure 9.

In all the embodiments described above, with reference to FIGS. 1 to 9, the inlet and outlet orifices, whatever their number, are distributed over the height of the side walls 8 of the tank.

In another embodiment, as illustrated in FIGS. 10 and 11, the inlet and outlet orifices 16 are located at the same height, at the two horizontally opposite ends of the successive porous material parts 13, having a horizontal extension. . As shown more particularly in FIG. 11, each portion of porous material 13 extends horizontally along several adjacent slabs 12 constituting the side walls 8. Each portion of porous material 13 thus has a length much greater than that of slabs 12 and 12. spaces between cradles 21 of the box 2. As previously, the outlet ports 16 are connected to collectors 18 which extend along the sides of the electrolysis tank. In a variant not illustrated, resulting from the previous embodiment, the porous material parts meet to form, on the perimeter of the electrolysis cell, a continuous strip with alternating inlet and outlet ports. In all cases, in order to force the circulation of air or other heat-transfer gas in the parts made of porous material 13, it is possible to seal the lateral or terminal surfaces of these parts made of porous material, by impregnation or clogging, thus avoiding any leaks and losses of air or gas. Figure 12 illustrates a modular assembly of porous sections according to a particularly advantageous embodiment of the invention. The drawing shows two adjacent modules seen at (A) according to section AA of FIG. 12 (B), at (B) along sections BB and B'-B 'of FIG. 12 (A), and at ( C) according to section CC of Fig. 12 (A). In this embodiment, the modules 30 are composite and have a first side 31 intended to be in the bottom of the heat exchanger and a second side 32 intended to be in the top of the heat exchanger.

The modules comprise at least a first 13a and a second 13b porous sections and internal pipes 51, 52, 53 able to circulate the heat transfer gas along preferential paths. The inlet and outlet points 16 are preferably in a part of the modules 30 intended to be in the upper part of the heat exchanger. Said first and second porous sections 13a, 13b may be separate juxtaposed slabs or parts of the same porous slab. In the illustrated example, the modules 30 have a downcomer 51 capable of directing the flow of heat transfer gas towards the lower part of the first porous section 13a, a first horizontal pipe 52 able to distribute the flow of heat transfer gas along the first porous section 13a and a second horizontal pipe 53 capable of collecting the heat transfer gas from the second porous section 13b. The walls 43, 44, 45, 46 of said channels 51, 52, 53 may be formed using metallic or ceramic elements, such as tubes, by molding and / or by sealing the porous surfaces with the aid of glues or refractory cements.

The first porous section 13a has a first porosity, and in particular a first number of pores per unit length. The second porous section 13b has a second porosity, and in particular a second number of pores per unit length. The number of pores per unit length is typically expressed in ppi or in pores per cm.

These composite modules have the advantage of promoting a flow velocity of the substantially vertical heat transfer gas and uniform, which avoids the formation of a thermal gradient along the walls of a tank that could be detrimental to the shape of the embankment solidified.

These composite modules 30 advantageously comprise a support slab 12 of thermally conductive material, typically a dense ceramic material as defined above, intended to be located on the inside of the tank, typically in contact with the edge 4 ". 30 may further comprise a layer of thermally insulating material 29, typically a fiber, intended to be located on the outside of the tank, typically in contact with the inner face of the box 2.

These composite modules can be juxtaposed so as to place an inlet 15, which is generally cooler, close to an outlet 16, which is generally warmer, which promotes a greater uniformity of the temperature by mutual compensation, in particular via a possible support slab 12 thermally conductive.

The first porous section 13a is advantageously located at the average height of the interface between the liquid aluminum 9 and the liquid electrolyte bath 10 in the tank in operation, so as to ensure greater heat exchange by means of a greater porosity and through the pipe to the first porous section 13a of all or part of the heat transfer gas directly from the inlet 15. In cases where the level of the interface between the liquid aluminum 9 and the bath of Liquid electrolyte 10 is located at the first porous section 13a, the first porosity is preferably greater than the second porosity. For example, the first number of pores per unit length is, in these cases, 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 FIG. 12 can be obtained by a method comprising:

the provision of a support slab 12 made of thermally conductive ceramic; providing a first porous slab 13a and a second porous slab 13b or providing a porous slab having a first porous portion 13a and a second porous portion 13b;

optionally, the supply of a thermally insulating material 29;

bonding said one or more porous slabs to the support slab 12, so as to form to seal the porosities and to form a sealed surface 41 and an effective thermal contact at their interface 14, that is to say between a determined surface of the support slab and a determined surface of said one or more porous slabs;

- The possible bonding of the insulating material 29 on an opposite surface of said one or more porous slabs, so as to seal the pores and form a sealed surface 42;

- The clogging of the lateral sides 44, 47 of said one or more porous slabs to make them wholly or partially sealed;

- The formation of the pipes 51, 52, 53.

Said collages and blockages can be made using a refractory adhesive such as that described above.

Finally, FIG. 13 illustrates, very schematically and by way of example, a method for recovering the thermal energy recovered in several electrolysis cells, as previously described. The air or the heat-transfer gas, coming from the different cells 23 of an industrial aluminum production plant, and in particular recovered from the lateral collectors 18, is directed towards the central vacuum and then into a feed circuit 24 to an external heat exchanger 25 for heating, or electricity generation applications that can be directly used in the plant tanks, which by nature are power consumers. This arrangement can also be used to reduce the ambient temperature of the electrolysis rooms by evacuation outside the electrolysis room of the heat produced by the tanks.

Tests were carried out with a ceramic porous material plate in order to evaluate the heat exchange capacities accessible with the materials according to the invention. A heat exchanger was manufactured by gluing on a plate of dense silicon carbide material, bonded with silicon nitride, with a thickness of 40 mm, a 25 mm plate of porous silicon carbide ceramic foam which the porosity is 20 ppi (8 pores / cm) and represents a pore volume of 88%. The effective thermal conductivity of the porous plate was between 0.50 and 1 W.nrr ¹ .K " ^1. The glue was a refractory slurry.The heat exchanger was placed at the entrance of an oven instead. of the door by means of a metal frame The heat exchange device was isolated by means of fibrous material around the frame Thermocouples placed at different locations, especially at the inlet and the outlet of the coolant, allowed to quantify heat exchange in the porous zone The exposed surface facing the furnace represented 400 cm ² .

The oven was heated to a set temperature, and maintained at this temperature during the test series, the flow of air flowing through the exchanger being controlled by means of a flow meter.

Table I shows, depending on the flow rate of the coolant, namely air, the exit temperature of the heated air and the heat flow recovered by the exchanger. These tests show that as the cooling air flow increases, the temperature of the outlet air decreases and the heat flow recovered increases.

Table I

The inventors believe that the increase in the number of pores, at comparable density, does not substantially increase the pressure drop in the porous medium up to about 60 ppi, while increasing the heat exchange surface with the heat transfer gas.

It goes without saying that the invention is not limited to the embodiments of this electrolysis cell which have been described above, for example; it embraces, on the contrary, all variants respecting the same principle. It is thus, in particular, that one would not depart from the scope of the invention by a modification of the nature of the materials, particularly with regard to the porous materials constituting the heat exchanger, which can also be metallic, or mixed (such as a combination of silicon carbide and metal). In the same vein, the number and the positions of the air inlet or outlet or heat transfer orifices, relative to the porous parts or zones, are widely variable without moving away from the frame. of the invention. Finally, the modes of use and / or conversion of the recovered thermal energy are widely variable.

Claims

1. Electrolytic cell (22), usable for the production of aluminum, comprising side walls (8) provided with a heat exchanger adapted to be traversed by a heat-transfer gas, characterized in that the side walls (8) of the vessel have, on at least a fraction of their height and their thickness, at least one portion of porous material (13) adapted to allow circulation of the heat-transfer gas, the or each porous part (13) being connected to inlet (15) and outlet (16) means for the heat transfer gas.

2. Electrolytic cell according to claim 1, characterized in that the porous material (13) is in the form of foam.

3. Electrolytic cell according to any one of claims 1 or 2, characterized in that the porous material (13) is formed of a material selected from metals, metal alloys and ceramics whose intrinsic thermal conductivity is greater than 5 W / mK and a mixture or combination of these materials.

4. Electrolytic cell according to claim 3, characterized in that said ceramic mainly contains silicon carbide, silicon nitride and / or aluminum nitride.

5. Electrolytic cell according to any one of claims 3 or 4, characterized in that the porous material (13) is a ceramic foam containing at least 70% by weight of thermally conductive ceramic.

6. electrolysis tank according to claim 3, characterized in that said metal or metal alloy has a coefficient of thermal expansion less than 25.10 ^"6 K- ^1.

7. Electrolytic cell according to any one of claims 1 to 6, characterized in that the portion of porous material (13) has a porosity greater than 70%.

8. Electrolytic cell according to any one of claims 1 to 7, characterized in that the portion of porous material (13) is formed by one or more porous slabs.

9. Electrolytic cell according to any one of claims 1 to 8, characterized in that the thickness of the porous material portion (13) is between 5 and 50 mm.

10. Electrolysis cell according to any one of claims 1 to 9, characterized in that the side walls (8) of the tank comprise at least a first portion of dense material (12) located on the inner side of these walls ( 8), and at least a second portion made at least partially of porous material (13) between the first portion (12) and the outer chamber (2) of the vessel.

11. Electrolytic cell according to claim 10, characterized in that the dense material is a ceramic material containing at least 70% by weight of silicon carbide.

12. Electrolytic cell according to any one of claims 10 to 11, characterized in that the dense material portion (12) has a porosity of less than 20%.

13. Electrolytic cell according to any one of claims 10 to 12, characterized in that the first and second parts of the side walls (8), namely the inner part of dense material (12) and the outer part made of material porous (13), are joined by a refractory material (14).

14. Electrolytic cell according to claim 13, characterized in that the first and second parts of the side walls are joined by an adhesive, in the form of a suspension, containing a mixture of inorganic filler with a mean particle size of less than 250 μm, a silicone resin and an organic solvent ensuring the solubilization of the resin and the rheology of the suspension.

15. Electrolytic cell according to any one of claims 10 to 14, characterized in that the dense material portion (12) is formed by one or more slabs of monolithic structure.

16. Electrolytic cell according to any one of the claims

10 to 14, characterized in that the side walls (8) of the vessel comprise a structure formed from monolithic slabs, made of a material with variable porosity in the direction of the thickness of said side walls (8).

17. Electrolytic cell according to any one of claims 10 to 16, characterized in that the thickness of the dense portion (12) is between 10 and 100 mm.

18. Electrolytic cell according to any one of claims 1 to 17, characterized in that said portion of porous material (13) extends over substantially the entire height of the side walls (8) of the tank.

19. Electrolytic cell according to any one of claims 1 to 18, characterized in that said portion of porous material (13) extends over a limited portion of the total height of the side walls (8) of the tank.

20. Electrolytic cell according to claim 19, characterized in that said portion of porous material (13) extends over a fraction of the order of one third to one half of the height of the side walls (8) of tank.

21. Electrolytic cell according to claim 19 or 20, characterized in that said portion of porous material (13) overlaps the interface between the liquid aluminum layer (9) and the molten salt bath (10).

22. Electrolytic cell according to any one of claims 1 to 21, characterized in that the porous material (13) has variable characteristics, in particular porosity, thickness and / or thermal conductivity, over the height of the porous part. (13), in order to obtain, in the direction of the height, successive zones (13a, 13b; 13c, 13d, 13e) having different characteristics of heat exchange.

23. Electrolysis cell according to any one of claims 1 to 22, characterized in that the input means (15) and outlet (16) of the heat transfer gas are located in the upper part and in the lower part of the or each part made of porous material (13).

24. Electrolytic cell according to claim 23, characterized in that the inlet means (15) and outlet (16) of the coolant gas are located in the upper part and in the lower part of the side walls (8) of the tank .

Electrolytic cell according to Claims 22 and 23, characterized in that at least one inlet (20) and / or one outlet is provided.

(16a, 16b) additional heat transfer gas located at intermediate height.

Electrolytic cell according to Claim 25, characterized in that the additional inlet (20) and / or outlet (16a, 16b) of the heat-transfer gas is situated at the transition between two successive zones (13a, 13b). 13c, 13d, 13e).

27. Electrolytic cell according to any one of claims 1 to 26, characterized in that the inlet and outlet means of the heat transfer gas are distributed over the horizontal dimension of a portion of porous material (13) of the walls side (8) of the tank.

28. Electrolytic cell according to claim 27, characterized in that the inlet (15) and the outlet (16) are respectively arranged at the two horizontally opposite ends of a portion of porous material (13).

29. Electrolytic cell according to any one of claims 1 to 28, characterized in that the inlet (s) (15) of heat transfer gas have orifices located above the liquid level in the tank, that is to say say in the upper part of the box (2) or on the periphery of the box (2).

30. Electrolytic cell according to any one of claims 1 to 29, characterized in that there is provided at least one side collector (18), connected to a plurality of outlets (16) of the heat transfer gas.

31. Electrolytic cell according to claim 30, characterized in that each side of the tank is equipped with at least one manifold (18), all the collectors can be connected to a common central vacuum.

32. Electrolytic cell according to any one of claims 1 to 31, characterized in that the passage section of the inlet and outlet ports (15, 16) of the heat transfer gas, or some of them, is adjustable by means of registers (19).

Electrolytic cell according to one of Claims 1 to 32, characterized in that the heat-transfer gas used is air, and in that the air inlets (16) are open to the surrounding atmosphere. .

34. Electrolysis cell according to any one of the claims

1 to 32, characterized in that the heat-transfer gas used is air, an inert gas or a mixture of air and inert gas, and in that this heat-transfer gas is recycled by a distribution network leading to the orifices inlet (15) the air or gas withdrawn at the outlet ports (16).

35. Electrolytic cell according to claim 34, characterized in that it combines the embodiments without recycling and with recycling of air, by means of direct air intake valves located at different points of the network. of distribution.

36. Electrolytic cell according to claim 35, characterized in that it further comprises isolation valves adapted to isolate from each other different portions of the distribution network.

37. Electrolytic cell according to any one of claims 1 to 36, characterized in that a heat insulator (29) is disposed between the porous material portion (13) and the chamber (2) of the tank.

38. Electrolytic cell according to claim 37, characterized in that the thermal insulator is a fibrous material.

39. Electrolytic cell according to any one of claims 37 or 38, characterized in that the insulator forms a substantially vertical layer whose thickness is between 10 and 100 mm.

40. Electrolytic cell according to any one of claims 1 to 39, characterized in that it is connected to a suction system adapted to circulate a heat transfer gas by depression in the or each portion of porous material (13). ).

41. Electrolytic cell according to any one of claims 1 to 40, characterized in that the side walls (8) of the vessel further comprise at least one pipe (51, 52, 53) adapted to circulate the gas coolant along a preferential path towards the bottom of the porous portion and / or from the top of the porous portion, so as to promote a flow velocity of the substantially vertical heat transfer gas and uniform.

42. Electrolytic cell according to any one of the claims

1 to 41, characterized in that the one or more porous parts (13) are assembled as a module (30).

43. An electrolysis cell according to claim 42, characterized in that said module (30) comprises at least a first porous section (13a) having a first porosity, a second porous section (13b) having a second porosity and being disposed of so as to be located above the first porous section in said side walls (8) tank, the first porosity being greater than the second porosity, a first horizontal pipe (52) capable of distributing the flow of heat transfer gas along the first porous section (13a) and a second horizontal pipe (53) capable of collecting the heat transfer gas from the second porous section (13b).

44. An electrolysis cell according to claim 43, characterized in that said module comprises an inlet (15) for the admission of the heat-transfer gas and an outlet (16) for the extraction of the heat-transfer gas, and in that the inlet (15) and the outlet (16) are located in a part of the module (30) intended to be in the upper part of said heat exchanger.

45. An electrolysis cell comprising a vessel according to any one of claims 1 to 44.

46. An industrial aluminum production plant, comprising a plurality of electrolytic cells (22) according to any one of claims 1 to 45, connected by collectors (18) and by a central vacuum unit to a circuit of heat transfer gas (24) directed towards energy recovery means, comprising at least one heat exchanger (25) and / or at least one electricity generator.