WO2022018192A1

WO2022018192A1 - Device for cooling flue gas originating from a plant for the production of aluminum by fused-salt electrolysis and plant implementing such a device

Info

Publication number: WO2022018192A1
Application number: PCT/EP2021/070492
Authority: WO
Inventors: Guillaume Emmanuel GIRAULT
Original assignee: Reel Alesa
Priority date: 2020-07-24
Filing date: 2021-07-22
Publication date: 2022-01-27
Also published as: FR3112842A1; EP4185739A1; CA3184214A1

Abstract

This device for cooling flue gas originating from a plant of aluminum production by fused-salt electrolysis is intended to be positioned between a collector of said flue gas, be it an individual collector originating from each of the electrolysis pots (1) of said plant, or a general collector (6) having said individual collectors emerging at the level thereof, and a center for processing said flue gas. This device is formed of a plurality (8) of hollow tubes (9) assembled parallel to one another, having a diameter smaller than the diameter of said individual collectors or of said general collector (6) and having said flue gas flowing therethrough, and forming an air-to-air heat exchanger:  one of the ends of said tubes (9) being in communication with an upstream plenum (10), having said general collector (6) or said individual collector (5) emerging at the level thereof,  the opposite end of said tubes (9) being in communication with a downstream plenum (11), itself in communication with a pipe (13) reaching the gas processing center.

Description

DEVICE FOR COOLING FLUE GAS ORIGINATING FROM A PLANT FOR THE

PRODUCTION OF ALUMINUM BY FUSED-SALT ELECTROLYSIS AND PLANT

IMPLEMENTING SUCH A DEVICE DOMAIN OF THE INVENTION

The invention pertains to the field of fused-salt electrolysis aluminum production, and more particularly aims at the treatment of the flue gas resulting from the implementation of this method.

BACKGROUND

The production of aluminum by the so-called fused-salt electrolysis method is now widely known and controlled. The electrolysis of aluminum in the presence of fused cryolite generates the production of flue gas, which particularly contains carbon dioxide, fluorinated products, and particularly hydrofluoric acid HF, and dust. Due to the increased severity of anti-pollution standards, the discharge of this flue gas cannot occur as such, and is thus processed in a flue gas processing center, precisely to comply with environmental standards. Devices capable, on the one hand, of capturing the flue gas at the outlet of the electrolysis cells or pots, and then of processing it to comply with these environmental standards have thus been developed. Such flue gas processing centers conventionally comprise filtering means, most often in the form of sleeves made of polymer, and typically of polyester, capable of capturing dust, and, on the other hand, chemical treatment means intended to neutralize the fluorinated gases via a method of adsorption on alumina.

Although such a treatment of the flue gas by means of these processing centers is generally satisfactory, it is however insufficient in terms of quality, from the moment that, as is the trend for operators of such aluminum production plants, the production capacity is desired to be increased. Indeed, such an increase results in an increase in the intensity of the electrolysis current and, accordingly, typically in the increase of the flue gas volume, in addition to their temperature increase. To fight this temperature increase, likely, in particular, to impact the integrity of the filtering means and to decrease the efficiency of the fluorinated gas treatment, various technical solutions have been provided.

They comprise the principle of diluting the flue gas with ambient air, typically by means of dilution doors located upstream of the flue gas processing center(s). Apart from the fact that the implemented principle requires a significant flow of added air to achieve the desired temperature decrease, it also generates a larger volume of gas to be treated and thus an accordingly oversized processing device, thereby affecting the general economy of the aluminum production plant. This is actually the reason why this principle is generally only implemented as a backing, in exceptional situations and when the outside environment is itself very warm.

Another technical solution comprising injecting water droplets into the flue gas has also been provided. The evaporation of said water droplets induces the cooling of said gases (see for example EP 1 172 326). This relatively inexpensive method is however little implemented or only in a limited way to ascertain a decrease in the flue gas temperature below the maximum temperature typically admissible by the filtering sleeves, in the case in point 135°C.

It has also be provided to implement air-to-water heat exchangers, such as for example described in document EP 2431 498. Typically, these exchangers are formed of tubes having the flue gas originating from the electrolysis pots flowing therethrough, and having a coolant fluid, particularly water, flowing outside thereof, advantageously in the opposite direction, the water playing the role of a heat-carrying fluid intended to cool the tubes and accordingly the gases.

Although, thermal speaking, these exchangers are efficient and particularly enable to achieve the desired temperature decrease of the flue gas originating from the pots, they however generate hot water resulting from the heat exchange at the pipe level, which is forbidden to be rejected at such a temperature by environmental standards, and which thereby requires the implementation of batteries of cooling towers in closed circuit. Thereby, the cost of the plant is significantly increased. Besides, in a number of countries where the water resource is low, this type of device may turn out being inappropriate, even though the water flows in closed circuit. As a summary, there is currently no device enabling to sufficiently and efficiently decrease the temperature of the flue gas originating from the electrolysis pots together with an easy implementation, a bulk decrease, decreased implementation costs, and an easy maintenance.

Such is one of the objects of the present invention.

Further, plants for the collection of such flue gas conventionally comprise collection tubes or collectors connected to each electrolysis pot, called individual collectors, and a general collector at the level of which said individual collectors are connected, and indented to collect the gases collected by the tubes. To obtain a substantially constant flow rate of the flue gas at the level of the series of electrolysis pots all along the length of the general collector, the individual pipes or collectors are equipped with a differential pressure device, of diaphragm or butterfly valve type, intended to create a head loss. However, the head losses thus caused require oversizing the flue gas suction element(s).

The aim targeted by the implementation of these differential pressure devices is to balance the suction flow rates on the pots, the action of these differential pressure devices being different for each pot: the pots most distant from the inlet of the general collector thus having a lower additional head loss. However, such an additional head loss generates a higher power consumption.

To overcome this difficulty, it has been provided to implement flue gas bypass loops, connected between the general collector and the individual collectors, said bypass loop typically extending in ambient air, obviously below the flue gas temperature. However, this technology, although it induces little or no additional head loss, however generates a particularly limited decrease in the temperature of said flue gas, in any case insufficient for a number of applications.

The invention thus aims, according to a second aspect, at a plant of the type in question, enabling to sufficiently cool the flue gas originating from each of the electrolysis pots or cells, without generating a head loss, or by decreasing the latter with respect to plants of the prior state of the art. SUMMARY

According to a first aspect, the invention thus provides a device for cooling flue gas originating from a plant of aluminum production by fused-salt electrolysis, intended to be positioned between a collector of said flue gas, be it an individual collector originating from each of the electrolysis pots of said plant, or a general collector having said individual collectors emerging at the level thereof, and a center for processing said flue gas.

According to the invention, this device is formed of a plurality of hollow tubes assembled parallel to one another, having a diameter smaller than the diameter of said individual collectors or of said general collector, which tubes have said flue gas flowing therethrough, said tubes being in contact with the ambient air to form an air-to-air heat exchanger:

• one of the ends of said tubes being in communication with an upstream plenum, having said individual collector or said general collector emerging at the level thereof, the other end of said tubes being in communication with a downstream plenum, itself in communication with a pipe directly or indirectly reaching the gas processing center.

In other words, conversely to prior art devices, the general principle retained by the device of the invention relies on an air-to-air heat exchanger, the heat exchange occurring by convection between the outer air transiting at the periphery of and in contact with the tubes and the flue gas transiting within the tubes. Of course, the number of these tubes and their diameter are determined to optimize this heat exchange according to the quantity of flue gases and to their temperature to be processed, but also to decrease risks of deposition of material in the tubes, risks of abrasion, risks of scale formation, in addition to minimize head losses.

Thus, according to an advantageous feature of the invention, to optimize this heat exchange, all or part of the tubes forming the exchanger is provided with external radial fins originating from the periphery of the tubes, and capable of optimizing the convection.

Advantageously, these fins have a thickness in the range from 0.5 to 3 millimeters, and extend from the external peripheral wall of the tubes over a distance typically in the range from 20 to 60 millimeters. These dimensions are determined to optimize the heat exchange. Further, these fins typically result from one or a plurality of spirals, defining between each turn a pitch in the range from 10 to 100 millimeters, this pitch being here again determined to optimize the heat exchange. These spirals are typically rolled tight around the tube to ensure the best contact between the tube and the fin, to maximize the heat transfer. Typically, the spiral(s) forming the fins are only welded on the tube at their two ends.

According to another advantageous feature of the invention, and still to improve and optimize the heat transfer, these tubes, and if present, the fins, are made of a material selected from the group comprising aluminum, black steel, stainless steel, and electroplated steel.

Typically, the tubes have a length in the range from 6 to 12 meters, and a diameter in the range from 50 to 300 millimeters. These values are purely indicative.

Advantageously, when the tube is finned, the spiral(s) are assembled on tubes of standard dimensions.

According to the invention, the tubes are aligned with one another. However, and according to an alternative feature of the invention, the tubes may be assembled in quincunx with respect to one another. Whatever the assembly of the tubes relative to one another, the desired aim is to optimize the flowing of outer air in contact with the tubes, and thus to optimize the exchange surface area between the outside and the peripheral surface of said tubes.

Typically, the distance between tubes, that is, between two generating lines of two contiguous tubes in a same exchanger, is in the range from 5 to 100 millimeters if the tubes are finless. However, if the tubes are provided with fins, the bulk that they generate has to be taken into account, and this distance may reach several hundreds of millimeters.

To optimize the installation of such an exchanger, the tubes are assembled on a skid, typically by group of from 30 to 200, it being specified that according to the quantity of flue gas to be treated, a plurality of these skids may be assembled side by side and coupled to a same general collector. Further, to decrease head losses inherent to the turbulences generated by the flow at the inlet of the tubes, and more precisely phenomena of separation of the gas stream within said tubes, the upstream plenum is provided with means configured to favor the introduction of the gases into the tubes. These means are typically formed at the level of one of the walls forming said plenum, said wall being pierced with through openings having a diameter corresponding to the diameter of said tubes positioned vertically in line with the inlet of the tubes, and being provided with deflectors or with equivalent systems.

As a variant, said wall is folded, to intrinsically define deflectors or pipes capable of decreasing the turbulences of the flue gas.

According to another feature of the invention, the downstream plenum is also provided with means, substantially of same design as those implemented at the level of the downstream plenum, and having the function of decreasing turbulences and overspeed areas at the level of the outlet pipe reaching the flue gas processing center.

According to another feature of the invention, the exchanger is added a source of additional air, that is, other than the ambient air surrounding the exchanger, this source being typically formed of a fan or the like, capable of increasing the air flow intended to come into contact with said tubes to optimize the heat exchange.

According to still another feature of the invention, one or a plurality of fans or equivalent devices are positioned within the outlet pipe reaching the flue gas processing center, to compensate for the previously-mentioned head losses.

According to another advantageous feature of the invention, thermoelectric modules are assembled on the fins associated with the tubes, to transform the heat resulting from the heat exchange into electric energy, and thus value a fraction of the heat thus dissipated.

According to a second aspect, the invention also aims at a plant for the collection of the flue gas originating from electrolysis pots for the production of aluminum by fused-salt electrolysis. This plant comprises, for each pot, an individual collector of said flue gas, each connected to a general collector, said general collector conveying the flue gas thus collected to a flue gas processing center. According to the invention, for at least part of said pots, a flue gas cooling device of the previously-described type is interposed between the individual collector of the considered pot and the general collector.

According to a first variant of the plant of the invention, the latter comprises at least one series of n consecutive pots ( .. m, m_+i, . n_z), having N (Ni. . Ni, Ni_+i,. . N_z) individual flue gas cooling devices of the type in question associated therewith, the individual collector of pot ni is connected to the upstream plenum of device Ni, having its downstream plenum coupled to the general collector in the vicinity of the area occupied by pot m_+i, and accordingly the individual collector of pot m_+i is connected to the upstream plenum of cooling device Ni_+i, having its downstream plenum coupled to the general collector in the vicinity of the area occupied by said pot ₊₂. In this configuration, said individual flue gas cooling devices are positioned in quincunx, to minimize the general bulk resulting from these individual cooling devices.

According to another variant of the plant of the invention, the individual flue gas cooling devices of two consecutive pots extend adjacently and parallel to each other. In this configuration, the individual collector of pot is connected to the upstream plenum of individual cooling device Ni, having its downstream plenum connected to the general collector in the vicinity of the area occupied by pot m_+i, and accordingly, the individual collector of pot m_+i is connected to the upstream plenum of individual cooling device Ni_+i, having its downstream plenum connected to the general collector vertically in line with pot , typically upstream of the place of connection of the downstream plenum of the individual cooling device Ni to the general collector. In this configuration, the flue gas originating from individual cooling device Ni_+i ends up in the general collector upstream of the flue gas originating from cooling device Ni. Further, the access to the plenums, respectively upstream and downstream of each of said cooling devices, and accordingly the maintenance operations likely to occur at the level of these devices, are facilitated. Further, the actual plant is simplified, by suppressing the support structures of said devices, typically for one pot out of two.

In still another variant of the plant of the invention, the individual flue gas cooling devices are also oriented parallel and two by two for two consecutive pots, these two devices being "coupled" to each other by a double plenum, each of said plenums being provided with a valve capable of closing, and thus of thus differentiating when needed, an upstream plenum and a downstream plenum for each of said devices, and where here again, the flue gas originating from individual cooling device Ni+i end up in the general collector upstream of the flue gas originating from cooling device Ni.

The notions of upstream and downstream are to be understood with respect to the flue gas flow direction within the general collector.

BRIEF DESCRIPTION OF THE DRAWINGS

The way in which the invention may be implemented and the resulting advantages will better appear from the following non-limiting embodiments, in relation with the accompanying drawings.

Figure l is a simplified representation of an electrolysis pot and of its coupling to a general collector.

Figure 2 is a simplified representation of the travel of the flue gas according to a first embodiment of the invention.

Figure 3 is a simplified representation similar to Figure 2 of a second embodiment of the invention, implementing the principle of forced convection.

Figure 4 is a simplified representation similar to Figure 2, of another embodiment of the invention, implementing a fan assembled on the outlet pipe reaching the flue gas processing center.

Figure 5 is a simplified perspective representation of an embodiment of the device of the invention, connected to the general collector.

Figure 6 is a simplified perspective representation of an embodiment of a tube implemented in the device of the invention.

Figure 7 is a simplified perspective representation of a first embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 8 is a simplified perspective representation of a second embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 9 is a simplified perspective representation of a third embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 10 is a simplified cross-section and top view of the embodiment of the means of Figure 9.

Figure 11 is a simplified cross-section and top view of a variant of the embodiment of ht means of Figure 9. Figure 12 is a simplified perspective representation of a fourth embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 13 is a simplified cross-section and top view of the embodiment of the means of Figure 12.

Figure 14 is a simplified perspective representation of a fifth embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 15 is a simplified cross-section and top view of the embodiment of the means of Figure 14.

Figure 16 is a simplified cross-section view of a sixth embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 17 is a simplified cross-section view of a seventh embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 18 is a simplified cross-section view of an eighth embodiment of the means implemented within the upstream plenum to decrease the head loss.

Figure 19 shows a simplified representation of a fin associated with a tube of the device of the invention, equipped with thermoelectric modules.

Figure 20 is a simplified representation in transverse cross-section illustrating a first mode of arrangement of the tubes of the device of the invention.

Figure 21 is a view similar to Figure 20, of another mode of arrangement, in the case in point in quincunx, of the tubes of the device of the invention.

Figure 22 is a simplified perspective representation of a portion of the flue gas collection plant according to the invention.

Figure 23 is a view similar to Figure 22 of another embodiment of the invention.

Figure 24 is a view similar to Figure 23 of an alternative embodiment of the invention.

Figure 25 is a simplified top view of the plant of Figure 24,

Figure 26 is a lateral view of the plant of Figure 24.

Figure 27 is a profile view of the plant of Figure 24.

Figure 28 is a simplified view of the operation of still another embodiment. DETAILED DESCRIPTION

A simplified view of an electrolysis pot has thus been shown in Figure 1. Such a pot (1) is conventionally formed of a plurality of anodes (2) fastened by anode rods (3) on an electrically conductive frame, said anodes being partially immersed in a fused cryolite and alumina melt. Removable covers (4) enable to change the anodes (3). The pot (1) is coupled by an individual pipe or collector (5) to a general collector (6), to collect and then conduct the flue gas generated within said pot during the electrolysis operation at the level of a flue gas processing center (not shown).

Such an architecture is perfectly well known, so that there is no need to describe it in further detail herein.

According to the described specific embodiment of the invention, the general collector(s) (6) emerge into the device for cooling the flue gas thus collected, schematically illustrated in Figures 2 to 5. However, the invention also concerns such a device for cooling said flue gas, which is not assembled on the general collector(s) (6), but on the individual collector(s) (5), the principle however remaining identical.

More precisely, and in connection with Figure 5, the flue gas originating from the general collectors (6) is conveyed by a pipe (7) to an assembly (8) of hollow tubes (9) assembled parallel to one another.

The connection between the pipe (7) and the inlet of the tubes (9) is formed at the level of an upstream plenum (10), further detailed hereafter. The flue gas crosses said tubes (9) and is then collected at the level of a downstream plenum (11), in communication with another pipe (12), coupled in turn to a collector (13) intended to convey said flue gas after its passage through the tubes (9) and thus after the cooling at the level of a flue gas processing center (not shown in this Figure 5).

This assembly (8) of tubes (9) is positioned outside of the civil engineering structures housing the series of electrolysis pots (1) and particularly in free air, to enable ambient air to flow in contact with the outside of said tubes, and to enable, by convection, the cool the flue gas flowing within the tubes. It is however specified that in the configuration according to which the assembly (8) is assembled, rather than on a general collector (6), on an individual collector, said assembly is then positioned inside of the civil engineering structure. Whatever the configuration, an air-to-air heat exchanger is thus formed, the cooling of the flue gas transiting within the tubes (9) resulting from the convection with outer air with respect to said tubes.

Although, in Figure 5, only one assembly (8) of tubes has been illustrated, it can be envisaged to have a plurality thereof, assembled in series or in parallel, to obtain the desired decrease in the flue gas temperature before the latter end up in the flue gas processing center, for the reasons discussed as a preamble.

These tubes, having a typical length in the range from 6 to 12 meters, and a diameter typically in the range from 50 to 300 millimeters, are advantageously made of aluminum, due to the good thermal properties of this metal, in addition to its low density.

The assembly (8) may typically comprise between 100 and 200 of such tubes, assembled in the form of skids, thus giving the device a modular character, and further favoring all the associated logistics.

Within a same skid, all the tubes (8) are identical and positioned parallel to one another. They may advantageously be assembled in alignment with one another (Figure 20) or in quincunx (Figure 21), to optimize the displacement of ambient air (represented by the arrows) in contact with said tubes, and thus accordingly improve the heat exchange by convection.

Additionally, and to further increase the heat exchange, each tube (9) is provided with radial fins (16) extending from the external wall (15) of said tubes (see Figure 6). These fins may in facts be made of one or a plurality of metal plates, advantageously made of the same metal as that forming the tube, having a thickness in the range from 0.5 to 3 millimeters, and fastened to said external wall by welding to each end only. This or these helical spirals, of same axis as the axis of revolution of the considered tube, define at each rotation a fin, separated from the contiguous fins by a same pitch typically in the range from 10 to 100 millimeters.

Figures 2 to 4 show three possible configurations of the invention. Figure 2 illustrates the basic configuration, where the assembly (8) of the tubes (9) is only submitted to the action of ambient air. According to the number of assemblies and/or of tubes per assembly, in addition to the volume of flue gas to be treated or to the temperature of said flue gas at the outlet of the pots (1), such a configuration may turn out being sufficient.

Downstream of the exchanger (8), the cooled gases are mixed in one or a plurality of reactors (21) with so-called "fresh" metallurgical-grade alumina, previously stored in a silo (19). In these reactors, the gaseous HF will adsorb on the alumina; this so-called "fluorinated" alumina is then separated from the HF-purified gas in one or a plurality of filters (20). One or a plurality of exhaust fans (23) ensure the depressurizing of the assembly and the discharge of the clean gases into one or a plurality of chimneys. The fluorinated alumina is stored in a silo (22) before being used as a raw material for the feeding of the pots (1).

Under the assumption where the configuration thus described is insufficient in terms of flue gas temperature decrease, another configured, such as illustrated in Figure 3 provides the implementation of pulsed air, typically by means of one or a plurality of fans (26) or of equivalent devices. Such fans are in this case sized to achieve the desired flue gas temperature decrease.

Further, to decrease as much as possible the head loss resulting from the entering of the flue gas into the tubes (8), one positions at the level of the upstream or inlet plenum (10) means described in further detail in relation with Figures 7 to 18.

Further, the inlet (10) and outlet (11) plenums of the exchanger typically have a tapered shape, as can be well observed in Figure 5. The widest portion or base of the taper directly communicates with the pipe (7) and is contiguous to the first tubes (9) forming the assembly (8), that is, at the level of the area where the flue gas speed is the greatest. Then, the tapered shape of the plenum towards the most distant tubes (9) enables to maintain the speed of said flue gas in the plenum within an acceptable range as the gas feeds the tubes (9) (typically between 14 m/s and 20 m/s) while ensuring a homogeneous distribution of said gas between the tubes (9), enabling to optimize the heat dissipation.

The tapered shape of the downstream plenum (11) contributes to a similar result. Further, the means illustrated in Figures 7 to 15 contribute to reaching this homogeneity.

The general principle underlying these different variants of said means relies on the implementation of a profile or deflector of appropriate shape, and particularly curved, to minimize the separation of the gas stream at the inlet of the tubes (9), as concerns the downstream plenum.

Thus, Figure 7 illustrates a first embodiment of such means, on which reference (27) materializes the plate of the plenum (10) which positions at the level of the inlet of the exchanger (8). This plate (27) is pierced with through openings (28), having a diameter identical to that of the tubes (9) and positioned opposite each of said tubes. The plate (27) is provided with deflectors (29), typically in sagittal cross-section in the shape of a shark fin, fastened to said plate, for example, between spacers (30), at the border of each of the through openings, and downstream with respect to the flue gas flow direction materialized by the arrow. Due to the curved profile of said deflectors, in addition to their positioning with respect to the gas flow direction, the head loss is significantly decreased.

Figure 8 illustrates a variant of Figure 7, where instead of the deflectors (29), one welds at the level of each of the through openings (28) of the plate (27) a hollow elbow (31) oriented towards the gas flow, and capable of ensuring a direction change of approximately 45° of said gas flow, but here again according to a curved profile.

Figures 9 and 10 illustrate another variant of these means, respectively in perspective and in cross-section view and in top view. In this variant, a half round piece (32) is welded on the plate (27) upstream of each of the lines of through openings (28) with respect to the gas flow incoming direction.

Figure 11 illustrates a variant of Figures 9 and 10, respectively in cross-section and in top view. Half-round portions (33) extending between the half-round pieces (32) are added with respect to this embodiment.

Figures 12 and 13 illustrate another embodiment of these means. In the case in point, said means are formed of circular half-round pieces (34), each welded to the periphery of each through opening (28) of the plate (27). Figures 14 and 15 illustrate still another embodiment of these means. The latter are here again formed of partial circular cross-sections of half-round pieces (35) welded on the plate (27) upstream of each of the lines of through openings (28) with respect to the gas flow incoming direction.

Still to decrease the head loss, the upstream plenum may have a configuration of the type of that illustrated in Figures 16 to 18.

Thus, in the embodiment of Figure 16, the plate (27), intended to be positioned upstream of the tubes, is folded, defining a broken line, having its surfaces positioned in the vicinity of the tubes pierced with through openings, at the level of which hollow elbows with a curved profile (36), having their other end welded to the tubes (9), are welded.

Figures 17 and 18 illustrate a principle similar to that of Figure 16.

In the same way, and still to minimize the head loss of the assembly, and more particularly to homogenize the speeds in the downstream plenum or outlet plenum (11) by minimizing turbulences and overspeed areas therein, the same type of means as those previously described are positioned at the level of said downstream plenum (11).

Still to overcome the issue due to the head loss, inherent to the passage of the flue gas within the exchanger (8), it may be envisaged (see Figure 4) to position one or a plurality of fan(s) (24) or equivalent devices downstream of the downstream plenum (11). Such fans are known to operate in the specific conditions characteristic of the flue gas originating from the pots (1). Thus, the material forming the blades of the fan (24) is selected to resist dusty gases likely to generate abrasion or scale formation phenomena. A specific steel grade such as the S690QL according to standard EN10025-6 is advantageously used.

Further, axial-type fans are typically used, which meet the specific needs characterized by a high flow rate (several tens of m3/s) and a relatively low pressure differential (typically < 2,000 Pa).

A plurality of these fans (24) may be arranged in parallel, to ensure a redundancy. In an advantageous embodiment of the invention, advantage is taken of the very large exchange surface area provided by the fins (16) equipping the tubes (9) (several thousands of m2 per exchanger), and of the fact that due to the environment into which the exchanger is plunged, a surface temperature of said fins greater by a quantity close to 40°C above the room temperature is available to collect the thermal power thus generated and transform it into electric power. For this purpose, thermoelectric cells (25) are positioned on said fins (16) to achieve this, as illustrated in Figure 19.

In the context of a typical application, several thousands of kW of thermal power are to be dissipated for each processing center. Considering a power efficiency in the order of a few percents (<10%) for these thermoelectric modules, it may be envisaged to recover a few tens of kW.

The implementation of thermoelectric cells provides the advantage of modularity; more restrictedly, only a fraction of the fins might thus be equipped, to power a number of electric appliances located close to the exchanger (measurement instruments, lightings, ...).

Different variants of the positioning of individual flue gas cooling devices in relation with Figures 22 to 28 will be described hereafter. In the drawings, the flow direction of said flue gas in the general collector (6), enabling to define the notion of upstream and downstream, has been materialized by an arrow.

A simplified view of a first embodiment of the plant for the collection of the flue gas originating from a series of electrolysis pots has been shown in relation with Figure 22.

In this drawing, the pots as such have not been shown. However, it shows the individual collectors (5) originating from each of the pots. According to the invention, these individual collectors (5) are thus not directly coupled to the general collector (6), but emerge into an individual flue gas cooling device (8) of the type of those described in relation with Figures 1 to 21. It can thus be observed, in this Figure 22, that these devices (8) are oriented along a same direction parallel to the general collector (6) but however offset with respect to one another and typically positioned in quincunx. As a corollary, it can be observed that the length of each of these individual devices, provided with their respective upstream and downstream plenums, typically 6 meters, substantially corresponds to the distance between pots. In this embodiment, the upstream plenum of each of these individual cooling devices (8i, 8i_+i) is coupled to the individual collector (5i, 5i_+i) of each of the pots, and accordingly, the downstream plenum of said devices is coupled at the level of the general collector (6) via a downstream pipe (14i, 14i_+i). However, and due to this quincunx positioning, it can be observed that the downstream plenum of the individual cooling device (8i) of the pot (i) emerges at the level of the general collector (6) by a downstream pipe (14i) substantially vertically in line with the area occupied by the consecutive pot (i+1). Accordingly, concerning said consecutive pot (i + 1), for which the individual flue gas cooling device (8i_+i) is offset, and in the case in point a little more distant from the considered pot, and for bulk reasons, the upstream plenum of said individual cooling device (8i_+i) of said pot (i + 1) is located vertically in line with the concerned pot, and the downstream plenum is also connected to the general collector (6) by a downstream pipe (14i_+i) substantially vertically in line with the consecutive pot (i + 2).

Concerning the embodiment of the invention shown in Figures 23 and 24, the individual flue gas cooling devices (8) are always oriented along a same direction parallel or substantially parallel to the direction of the general collector (6). However, they are positioned adjacently, two by two for an assembly of two consecutive pots. The differences between these two variants essentially lie in the positioning of the downstream pipes (14) originating from the individual devices (5).

Thus, for a first assembly of two consecutive pots (i, i + 1), thus having two parallel individual flue gas cooling devices (8i, 8i _{+ i}) parallel and adjacent to each other, the flue gas which has been cooled, that is, after having transited through the individual device of the pot (i + 1) systematically emerge into the general collector (6) upstream of this same cooled flue gas of the first one (i) of said two pots. Thereby, the plant is simplified due to the implementation of fewer support structures. Accordingly, the accessibility to the plenums, respectively upstream and downstream of each of the individual cooling devices, and thus the maintenance of these devices, are optimized.

According to still another embodiment of the invention very schematically shown in Figure 28, where here again for two consecutive pots, the individual flue gas cooling devices (8i, 8i₊ 1) are positioned parallel to each other and adjacently to each other, said pair of devices thus implemented is coupled upstream and downstream by a respective intermediate plenum (37, 38). These intermediate plenums are likely to form, respectively, the upstream plenum of one of the individual devices (5) and the downstream plenum of the adjacent individual device, and vice versa. For this purpose, these intermediate plenums are provided with a valve or an equivalent device (39, 40), likely to differentiate, at the level of each of said intermediate plenums, the downstream plenum and the upstream plenum of each of said intermediate cooling devices. Thereby, when the valves (39, 40) in question are closed, the configuration is that described in relation with the embodiment described in Figures 23 to 27.

However, if the flow rate on pot n is desired to increase, to maximize the efficiency of the collection of the flue gas emitted during the maintenance operations carried out on said pot n, when the corresponding cover (4) is removed, it becomes possible to bypass the considered individual device (5i) by opening the valve (39), whereby the flue gas originating from said pot n are then not cooled and are directly conveyed to the level of the general collector (6) via the downstream pipe (14i_+i). The same operation can be envisaged with pot n + 1, by opening the valve (40).

Accordingly, said downstream pipes (14i, 14i_+i) are also likely to receive a valve-type member (41, 42) to reach their total or partial closing to optimize this bypass operation.

In this configuration, the maintenance operations on the individual flue gas cooling devices (8) may be carried out efficiently without altering the general operation of the plant.

Further, and as discussed hereabove, the plant of the invention does not require equipping each of the pots with such an individual flue gas cooling device. Thereby, by selecting the number of these devices and their implantation, the generated head loss is relatively small and in any case minimizes the impact on the total head loss of the flue gas collection and processing circuit of the plant.

Claims

1. Device for cooling flue gas originating from a plant of aluminum production by fused- salt electrolysis, intended to be positioned between a collector of said flue gas, be it an individual collector (5) originating from each of the electrolysis pots (1) of said plant, or a general collector (6) having said individual collectors (5) emerging at the level thereof, and a center for processing said flue gas, characterized in that this device is formed of a plurality (8) of hollow tubes (9) assembled parallel to one another, having a diameter smaller than the diameter of said individual collectors (5) or of said general collector (6) and having said flue gas flowing therethrough, said tubes being in contact with the ambient air to form an air-to-air heat exchanger:

• one of the ends of said tubes (9) being in communication with an upstream plenum

(10), having said general collector (6) or said individual collector (5) emerging at the level thereof, · the opposite end of said tubes (9) being in communication with a downstream plenum

(11), itself in communication with a pipe (13) reaching the gas processing center.

2. Flue gas cooling device according to claim 1, wherein all or part of the tubes (9) forming the exchanger (8) is provided with external radial fins (16), originating from the periphery (15) of the tubes (9) and capable of optimizing the convection.

3. Flue gas cooling device according to claim 2, wherein the fins (16) have a thickness in the range from 0.5 to 3 millimeters, and extend from the external peripheral wall (15) of the tubes (9) over a distance in the range from 20 to 60 millimeters.

4. Flue gas cooling device according to any of claims 2 and 3, wherein the fins (15) are formed of one or a plurality of helical spirals, having their rotation axis confounded with the axis of revolution of the tubes (9), the pitch of the helix or of the helices thus formed being in the range from 10 to 100 millimeters.

5. Flue gas cooling device according to any of claims 1 to 4, wherein the tubes (9) are made of a material selected from the group comprising aluminum, black steel, stainless steel, and electroplated steel.

6. Flue gas cooling device according to any of claims 1 to 5, wherein the tubes (9) are assembled to be aligned with respect to one another.

7. Flue gas cooling device according to any of claims 1 to 5, wherein the tubes (9) are assembled in quincunx with respect to one another.

8. Flue gas cooling device according to any of claims 1 to 7, wherein the distance between tubes within an exchanger (8) is in the range from 5 to several hundreds of millimeters.

9. Flue gas cooling device according to any of claims 1 to 8, wherein the tubes (9) are assembled on a skid by group of from 30 to 200.

10. Flue gas cooling device according to any of claims 1 to 9, wherein the exchanger (8) further comprises at least one source of additional air (26), that is, other than the ambient air surrounding the exchanger, this source being typically formed of a fan or the like, capable of increasing the air flow intended to come into contact with said tubes (9).

11. Flue gas cooling device according to any of claims 1 to 10, wherein the upstream (10) and downstream (11) plenums of the exchanger (8) are tapered.

12. Flue gas cooling device according to any of claims 1 to 11, wherein the upstream plenum (10) is provided with means (29, 31, 32, 33, 34, 35) placed on one of the plates (27) forming said plenum, and capable of decreasing the separations of the gas stream at the inlet of the tubes (9) and thus to minimize head losses.

13. Flue gas cooling device according to any of claims 1 to 12, wherein the downstream plenum (11) is provided with means capable of decreasing turbulences and overspeed areas in this same plenum and thus of minimizing head losses. 14. Flue gas cooling device according to any of claims 1 to 13, wherein one or a plurality of fan(s) (24) or equivalent devices is (are) installed downstream of the exchanger (8) to check the head loss inherent to the passage of the flue gas within said exchanger. Flue gas cooling device according to any of claims 2 to 14, wherein the fins (16) of the tubes (9) are provided with thermoelectric modules (25), intended to transform the heat dissipated by said fins into electric energy.

Plant for the collection of the flue gas originating from electrolysis pots for the production of aluminum by fused-salt electrolysis, comprising for each of said pots an individual collector (5) of said flue gas each connected to a general collector (6), said general collector conveying the flue gas thus collected to a flue gas processing center, wherein part at least of said pots has, interposed between the individual collector (5) of the considered pot and the general collector (6), a device for cooling said flue gas (8) according to any of claims 1 to 15.

Plant for the collection of the flue gas originating from electrolysis pots for the production of aluminum by fused-salt electrolysis according to claim 16, said plant comprising at least one series of n consecutive pots ( ..., m, m_+i, . n_z), having N (Ni. . ., Ni, Ni_+i,. . .,

N_z) individual flue gas cooling devices (8) associated therewith, in which plant the individual collector (5i) of pot n, is connected to the upstream plenum of the individual flue gas cooling device (Ni, 8i) having its downstream plenum coupled to the general collector (6) in the vicinity of the area occupied by pot _+i, and accordingly the individual collector (5i_+i) of pot m_+i is connected to the upstream plenum of the cooling device (Ni_+i, 8i_+i) having its downstream plenum coupled to the general collector (6) in the vicinity of the area occupied by said pot ₊2, said individual flue gas cooling devices being positioned in quincunx.

N_z) individual flue gas cooling devices (8) associated therewith, in which plant said individual flue gas cooling devices of two consecutive pots extend adjacently and parallel to each other.

19. Plant for the collection of the flue gas originating from electrolysis pots for the production of aluminum by fused-salt electrolysis according to claim 18, wherein the individual collector (5i) of pot n, is connected to the upstream plenum of the individual flue gas cooling device (Ni, 8i) having its downstream plenum coupled to the general collector (6) in the vicinity of the area occupied by pot _+i, and accordingly the individual collector

(5i_+i) of pot n,-i is connected to the upstream plenum of the individual cooling device (Ni_+i, 8i_+i) having its downstream plenum coupled to the general collector (6) vertically in line with the area occupied by pot n,, the flue gas originating from the individual cooling device (Ni_+i, 8i_+i) emerging into the general collector (6) upstream of the flue gas originating from the cooling device (Ni, 8i).

20. Plant for the collection of the flue gas originating from electrolysis pots for the production of aluminum by fused-salt electrolysis according to claim 18, wherein the individual flue gas cooling devices of two consecutive pots are coupled to each other by an intermediate plenum (37, 38), each of said plenums being provided with a valve (39,

40) capable of closing, and thus of thus differentiating when needed, an upstream plenum and a downstream plenum for each of said devices.

21. Plant for the collection of the flue gas originating from electrolysis pots for the production of aluminum by fused-salt electrolysis according to claim 20, wherein the downstream pipes originating from the individual flue gas cooling devices are each provided with a valve (41, 42) capable of generating the total or partial closing of said downstream pipes.