FR3112842A1 - DEVICE FOR COOLING GAS EFFLUENTS FROM AN INSTALLATION FOR THE PRODUCTION OF ALUMINUM BY IGNITE ELECTROLYSIS - Google Patents

Abstract

This device for cooling gaseous effluents from an installation for the production of aluminum by igneous electrolysis, is intended to be positioned between a collector of said effluents, whether it is an individual collector emanating from each of the tanks electrolysis (1) of said installation, or a general collector (6) at which lead said individual collectors, and a treatment center for said effluents. This device consists of a plurality (8) of hollow tubes (9) mounted parallel to each other, with a diameter smaller than the diameter of said individual collectors or of said general collector (6), and within which said gaseous effluents circulate , and constituting an air/air exchanger: - one of the ends of said tubes (9) being in communication with an upstream plenum (10), at which said general collector (6) or said individual collector (5) ends, - the opposite end of said tubes (9) being in communication with a downstream plenum (11), itself in communication with a pipe (13) leading to the gas treatment center. Figure for abstract: Fig 5

Description

DEVICE FOR COOLING GAS EFFLUENTS FROM AN INSTALLATION FOR THE PRODUCTION OF ALUMINUM BY IGNITE ELECTROLYSIS

Field of invention

The invention belongs to the field of the production of aluminum by igneous electrolysis, and more particularly relates to the treatment of the gaseous effluents resulting from the implementation of this process.

Prior state of the art

The production of aluminum by the so-called igneous electrolysis process is currently widespread and mastered. The electrolysis of alumina in the presence of molten cryolite generates the production of gaseous effluents, which contain in particular carbon dioxide, fluorinated products, and in particular hydrofluoric acid HF, and dust. In view of the increase in the severity of anti-pollution standards, the discharge of these effluents cannot occur as such, and are in fact treated at the level of an effluent treatment center, with the precise aim of satisfying the standards. environmental.

Devices have therefore been developed capable of, on the one hand, capturing the gaseous effluents leaving the electrolysis tanks or cells, then treating them in such a way as to meet these environmental standards. Such effluent treatment centers traditionally include filtration means, most often in the form of sleeves made of polymer, and typically of polyester, capable of capturing dust, and, on the other hand, treatment means chemicals intended to neutralize fluorinated gases via an adsorption process on alumina.

If such treatment of gaseous effluents by means of these treatment centers is generally satisfactory, they are however insufficient in terms of quality, since, as is the tendency of the operators of such aluminum production plants, we want to increase the production capacity. Indeed, such an increase results in an increase in the intensity of the electrolysis current and, as a corollary, typically in an increase in the volume of the gaseous effluents, in addition to an increase in their temperature.

In order to combat this rise in temperature, which is likely to impact the integrity of the filtration means and reduce the efficiency of the treatment of fluorinated gases, various technical solutions have been proposed.

These include the principle of diluting the effluent gases with ambient air, typically by means of dilution traps located upstream of the effluent treatment centre(s). In addition to the fact that the principle implemented requires a significant flow of added air to achieve the desired temperature reduction, it also generates a greater volume of gas to be treated and therefore a treatment device that is oversized as a result, effectively affecting the overall economics of the aluminum production facility. This is also the reason why this principle is generally only implemented as a back-up, in exceptional situations and when the outside environment is itself very hot.

Another technical solution has also been proposed, consisting of injecting water droplets into the effluent gases. The evaporation of said water droplets induces the cooling of said gases (see for example EP 1 172 326). This relatively inexpensive process, however, is little implemented or in a limited way to ensure a lowering of the temperature of the effluent gases below the maximum allowable temperature typically by the filtration sleeves, in this case 135 ° C .

It has also been proposed to implement air/water exchangers, such as for example described in document EP 2 431 498. Typically, these exchangers consist of tubes inside which the effluent gases from the electrolytic cells flow. , and outside of which circulates a cooling fluid, in particular water, advantageously in the opposite direction, the water acting as a heat transfer fluid intended to cool the tubes and consequently the gases.

If, from a thermal point of view, these exchangers prove to be effective and in particular make it possible to lower the desired temperature of the gaseous effluents emanating from the tanks, they nevertheless generate hot water resulting from the heat exchange at the level tubes, which environmental standards prohibit discharging at such a temperature, and which therefore require the implementation of batteries of air coolers in a closed circuit. Doing so significantly increases installation costs. Moreover, in a number of countries where water resources are low, this type of device may prove to be inappropriate, even if the water circulates in a closed circuit.

In summary, to date there is no device for reducing sufficiently and effectively the temperature of the gaseous effluents from the electrolytic cells in a concomitant manner with the ease of implementation, the reduction of the size, the reduced production costs and easy maintenance.

This is the object of the present invention.

The invention thus proposes a device for cooling gaseous effluents coming from an installation for the production of aluminum by igneous electrolysis, intended to be positioned between a collector of said effluents, whether it is an individual collector emanating of each of the electrolytic cells of said installation, or of a general collector at the level of which lead said individual collectors, and a treatment center for said effluents.

According to the invention, this device consists of a plurality of hollow tubes mounted parallel to each other, with a diameter smaller than the diameter of the individual collector or the general collector, tubes within which said gaseous effluents circulate:
– one of the ends of said tubes being in communication with an upstream plenum, at which the said individual collector or the general collector ends,
– the other of the ends of said tubes being in communication with a downstream plenum, itself in communication with a pipe leading directly or indirectly to the gas treatment center.

In other words, unlike the devices of the prior art, the general principle adopted by the device of the invention is based on an air/air exchanger, the heat exchange taking place by convection between the outside air transiting at the periphery and in contact with the tubes and the gaseous effluents transiting inside the tubes. Obviously, the number of these tubes and their diameter are determined in such a way as to optimize this heat exchange according to the quantity of effluent gases and their temperature to be treated, but also in order to reduce the risks of deposit of material within the tubes, the risk of abrasion, the risk of scaling, in addition to minimizing pressure drops.

Thus, according to an advantageous characteristic of the invention, in order to optimize this heat exchange, all or part of the tubes constituting the exchanger is provided with external radial fins emanating from the periphery of the tubes, and able to optimize convection.

Advantageously, these fins have a thickness comprised between 0.5 and 3 millimeters, and extend from the outer peripheral wall of the tubes by a distance typically comprised between 20 and 60 millimeters. These dimensions are determined in order to optimize the heat exchange.

In addition, these fins result in fact from one or more spirals, defining between each turn a pitch of between 10 and 100 millimeters, again, this pitch being determined to optimize heat exchange. These coils are typically rolled tight around the tube to ensure the best contact between the tube and the fin, to maximize heat transfer. Typically, the spiral or spirals constituting the fins are only welded to the tube at their two ends.

According to another advantageous characteristic of the invention, and still with a view to improving and optimizing heat transfer, these tubes, and where appropriate the fins, are made of a material chosen from the group comprising aluminum , black steel, stainless steel and galvanized steel.

Typically, the tubes have a length of between 6 and 12 meters, and a diameter of between 50 and 300 millimeters. These values are purely indicative.

Advantageously, when the tube is provided with fins, the spiral or spirals are mounted on tubes of standard dimensions.

According to another advantageous characteristic of the invention, the tubes can be staggered relative to each other, again in order to optimize the circulation of the outside air in contact with the tubes, and therefore to optimize the surface of exchange between the outside and the peripheral surface of said tubes.

Typically, the inter-tube distance, i.e. between two generatrices of two contiguous tubes in the same exchanger, is between 5 and 100 millimeters if the tubes have no fins. On the other hand, if the tubes are equipped with fins, it is necessary to take into account the size they generate, this distance being able to reach several hundreds of millimetres.

With the aim of optimizing the installation of such an exchanger, the tubes are mounted on a modular frame (skid), typically in groups of 100 to 200, it being thus specified that depending on the quantity of gas in effluent to be treated, several of these modular frames can be mounted side by side and connected to the same general collector.

Furthermore, in order to reduce the pressure drops inherent in the turbulence generated by the flow at the inlet of the tubes, and more specifically the phenomena of separation of the gas stream within said tubes, the upstream plenum is equipped with means configured to promote the introduction of gases into the tubes. These means are typically provided at the level of one of the constituent walls of said plenum, said wall being pierced with through holes of diameter corresponding to the diameter of said tubes positioned plumb with the inlet of the tubes, and being provided with deflectors or equivalent systems.

Alternatively, said wall is folded, so as to intrinsically define deflectors or pipes capable of reducing the turbulence of the effluents.

According to another characteristic of the invention, the downstream plenum is also provided with means, substantially of the same design as those implemented at the level of the upstream plenum, and whose function is to reduce turbulence and overspeed zones at the level of the outlet pipe leading to the effluent treatment center.

According to another characteristic of the invention, an additional source of air is added to the exchanger, that is to say other than the ambient air surrounding the exchanger, this source typically consisting of a fan or equivalent, capable of increasing the flow of air intended to come into contact with said tubes, again with the aim of optimizing the heat exchange.

According to yet another characteristic of the invention, one or more fans or equivalent devices are positioned within the outlet pipe leading to the effluent treatment center, in order to compensate for the aforementioned pressure drops.

According to another advantageous characteristic of the invention, thermoelectric modules are mounted on the fins associated with the tubes, so as to transform the heat resulting from the heat exchange into electrical energy, and thus recover a fraction of the heat thus dissipated.

Brief description of figures

The way in which the invention can be implemented and the resulting advantages will emerge better from the examples of embodiment which follow, given by way of indication and not limitation, in support of the appended figures.

Figure 1 is a schematic representation of an electrolytic cell and its connection to a general collector.

FIG. 2 is a schematic representation of the flow of the gaseous effluents according to a first embodiment of the invention.

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

There is a schematic representation analogous to the , of another embodiment of the invention, implementing a fan mounted on the outlet pipe leading to the effluent treatment center.

There is a schematic representation in perspective of an embodiment of the device of the invention, connected to the general collector.

There is a schematic representation in perspective of an embodiment of a tube implemented in the device of the invention.

There is a schematic representation in perspective of a first embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic representation in perspective of a second embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic representation in perspective of a third embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic sectional and top view of the embodiment of the means of the .

There is a schematic sectional and top view of an alternative embodiment of the means of the

There is a schematic representation in perspective of a fourth embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic sectional and top view of the embodiment of the means of the .

There is a schematic representation in perspective of a fifth embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic sectional and top view of the embodiment of the means of the .

There is a schematic sectional view of a sixth embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic sectional view of a seventh embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There is a schematic sectional view of an eighth embodiment of the means implemented within the upstream plenum in order to reduce the pressure drop.

There represents a schematic representation of a fin associated with a tube of the device of the invention, equipped with thermoelectric modules.

There is a schematic representation in cross section illustrating a first mode of arrangement of the tubes of the device of the invention.

There is a view analogous to , of another mode of arrangement, in this case staggered, of the tubes of the device of the invention.

Detailed description of the invention

We were therefore represented within the a schematic view of an electrolytic cell. Such a tank (1) conventionally consists of a plurality of anodes (2) fixed by anode rods (3) on an electrically conductive frame, and partially immersed in a bath of alumina and molten cryolite. Removable covers (4) allow the anodes (3) to be changed. The tank (1) is connected by an individual pipe (5) to a general collector (6), in order to collect, then to conduct the gaseous effluents generated within said tank during the electrolysis operation.

Such architecture is well known, so there is no need to describe it here in more detail.

According to the particular embodiment of the invention described, the general collector or collectors (6) lead to the device for cooling the gaseous effluents thus collected, illustrated schematically in FIGS. 2 to 5. However, the invention also extends to such a device for cooling said gaseous effluents, not mounted on the general collector or collectors (6), but on the individual collector or collectors (5), the principle remaining however identical.

More specifically, and in connection with the , the gaseous effluents emanating from the general collectors (6) are conveyed by a pipe (7) to a set (8) of hollow tubes (9), mounted parallel to each other.

The connection between the pipe (7) and the inlet of the tubes (9) is made at the level of an upstream plenum (10), more detailed below. The gaseous effluents pass through said tubes (9), then are collected at the level of a downstream plenum (11), in communication with another pipe (12), connected in turn to a collector (13) intended to convey said gaseous effluents after passing through the tubes (9), and therefore after cooling at an effluent treatment center (not shown in this ).

This set (8) of tubes (9) is positioned outside the civil engineering structures housing the series or series of electrolytic cells (1), and in particular in the open air, in order to allow air environment to circulate in contact with the outside of said tubes, and to allow, by convection, to cool the gaseous effluents circulating within the tubes. It is however specified that in the configuration according to which the assembly (8) is mounted, not on a general collector (6), but on an individual collector, said assembly is then positioned inside the engineering structure civil.

If within the , only one set (8) of tubes has been illustrated, it may be envisaged to have several of them, mounted in series or in parallel, in order to achieve the desired reduction in temperature of the gaseous effluents before they end at the effluent treatment center, for the reasons mentioned in the preamble.

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

The set (8) can typically comprise between 100 and 200 of such tubes, mounted in the form of skids, thus giving the device a modular character, and furthermore favoring all the associated logistics.

Within the same skid, all the tubes (8) are identical and positioned parallel to each other. They can advantageously be mounted aligned with each other ( ), or staggered ( ), in order to optimize the displacement of the ambient air (represented by the arrows) in contact with said tubes, and therefore as a corollary, improve the heat exchange by convection.

Furthermore, and in order to further increase the heat exchange, each tube (9) is provided with radial fins (16), extending from the outer wall (15) of said tubes (see ). These fins can in fact consist of one or more metal plates, advantageously of the same metal as that constituting the tube, with a thickness of between 0.5 and 3 millimeters, and fixed to said outer wall by welding at each end. uniquely. This or these helical spirals, with the same axis as the axis of revolution of the tube in question, define at each rotation a fin, separated from the contiguous fins by the same pitch, typically between 10 and 100 millimeters.

There are shown in Figures 2 to 4, three possible configurations of the invention.

Within the , is illustrated the basic configuration, in which the assembly (8) of tubes (9) is subjected only to the action of the ambient air. Depending on the number of assemblies and/or tubes per assembly, in addition to the volume of gaseous effluents to be treated or the temperature of said effluents leaving the tanks (1), such a configuration may prove to be sufficient.

Downstream of the exchanger (8), the cooled gases are mixed in one or more reactors (21) with so-called “fresh” metallurgical alumina, previously stored in a silo (19). In these reactors, the gaseous HF will be adsorbed on the alumina; this so-called “fluorinated” alumina is then separated from the purified gas of HF in one or more filters (20). One or more draft fans (23) ensure the depression of the whole and the evacuation of the clean gases in one or more chimneys. The fluorinated alumina is stored in a silo (22) before being used as raw material for supplying the tanks (1).

In the event that the configuration thus described is insufficient in terms of lowering the temperature of the gaseous effluents, another configuration, as illustrated in provides for the implementation of forced air, typically by means of one or more fans (26) or equivalent devices. Such fans are in this case dimensioned in such a way as to achieve the desired reduction in the temperature of the gaseous effluents.

Furthermore, in order to reduce as far as possible the pressure drops resulting from the entry of the gaseous effluents into the tubes (8), a means described further on is positioned at the level of the upstream or entry plenum (10). detail in relation to figures 7 to 18.

Firstly, the inlet (10) and outlet (11) plenums of the exchanger typically have a beveled shape, as can be clearly seen on the . The widest part or base of the bevel communicates directly with the pipe (7), and is contiguous with the first tubes (9) constituting the assembly (8), that is to say at the level of the zone where the velocity of the gaseous effluents is the most important. Then the beveled shape of the plenum in the direction of the most distant tubes (9) makes it possible to maintain the speed of said gaseous effluents in the plenum within an acceptable range as the gases feed the tubes (9) (typically between 14m/s and 20m/s) while ensuring a homogeneous distribution of said gases between the tubes (9), making it possible to optimize heat dissipation.

The beveled shape of the downstream plenum (11) contributes to a similar result.

Furthermore, the means illustrated in FIGS. 7 to 15 contribute to achieving this homogeneity.

The general principle underlying these different variants of said means is based on the implementation of a profile or deflector of appropriate shape, and in particular curved, in order to minimize the separation of the gas stream at the inlet of the tubes (9), by with regard to the upstream plenum.

Thus, in , is illustrated a first embodiment of such a means, on which has been materialized by the reference (27), the sheet of the plenum (10) being positioned at the inlet of the exchanger (8). This sheet (27) is pierced with through holes (28), of identical diameter to that of the tubes (9), and positioned opposite each of said tubes. The sheet (27) is provided with deflectors (29), typically in sagittal section in the shape of a shark's fin, fixed to said sheet, for example between spacers (30), at the edge of each of the through orifices, and downstream in relation to the direction of circulation of the gaseous effluents materialized by the arrow. Due to the curved profile of said deflectors, in addition to their positioning with respect to the direction of the gas flow, the pressure drop is significantly reduced.

There illustrates a variant of the , in which instead of deflectors (29), there is welded at each of the through-holes (28) of the sheet (27), a hollow elbow (31), oriented in the direction of the gas flow, and capable of ensuring a change in direction of approximately 45° of said gas flow, but again according to a curved profile.

Figures 9 and 10 illustrate another variant of this means, respectively in perspective view and in section and in top view. In this variant, a half-round (32) is welded to the sheet (27), upstream of each of the rows of through orifices (28) with respect to the direction of arrival of the gas flow.

There illustrates a variant of Figures 9 and 10, respectively in section and in top view. Compared to this embodiment, half-round portions (33) extending between the half-rounds (32) are added.

Figures 12 and 13 illustrate another embodiment of these means. In this case, said means consist of circular half-rounds (34), each welded to the periphery of each through hole (28) of the sheet (27).

Figures 14 and 15 illustrate yet another embodiment of these means. The latter are again made up of partial circular sections of half-rounds (35), welded to the sheet (27) upstream of each of the rows of through orifices (28) with respect to the direction of arrival of the gas flow.

Still with the aim of reducing the pressure drop, the upstream plenum can have a configuration of the type illustrated in figures 16 to 18.

Thus, in the embodiment of the , the sheet (27), intended to be positioned upstream of the tubes, is bent, defining a broken line, the faces of which positioned in the vicinity of the tubes are pierced with through holes, at the level of which are welded hollow profile elbows curve (36), the other end of which is welded to the tubes (9).

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

In the same way, and always with the aim of minimizing the pressure drop of the assembly, and more particularly of homogenizing the speeds in the downstream plenum or outlet plenum (11) by minimizing the turbulence and the overspeed zones there. , is positioned at said downstream plenum (11) the same type of means as those previously described.

Still with the aim of overcoming the problem arising from the loss of pressure, inherent in the passage of gaseous effluents within the exchanger (8), it can be envisaged (see ), to position one or more fan(s) (24) or equivalent devices downstream of the downstream plenum (11). Such fans are designed so as to operate under the specific conditions characteristic of the gaseous effluents emanating from the tanks (1). Thus, the constituent material of the blades of the fan (24) is chosen so as to resist the dusty gases liable to generate abrasion and scaling phenomena. Advantageously, a specific steel grade is used, such as S690QL according to standard EN10025-6.

Furthermore, axial type fans are typically used, which meet specific needs characterized by a high flow rate (several tens of m ³ /s) and a relatively low pressure differential (typically <2000Pa).

Several of these fans (24) can be placed in parallel, so as to provide redundancy.

In an advantageous embodiment of the invention, advantage is taken of the very large exchange surface offered by the fins (16) with which the tubes (9) are provided (several thousand m ² per exchanger), and the fact that due to the environment in which the exchanger is immersed, there is a surface temperature of said fins higher by an amount close to 40°C above the ambient temperature, to recover the thermal energy thus generated and transform it into electrical energy. To this end, thermoelectric cells (25) are positioned on said fins (16) for this purpose, as illustrated in the .

In a typical application, several thousand kW of thermal energy have to be dissipated for each processing center. Considering an energy efficiency of the order of a few percent (<10%) for these thermoelectric modules, it may be possible to recover a few tens of kW.

The implementation of thermoelectric cells offers the advantage of modularity; in a more restricted way, only a small fraction of the fins could thus be equipped, so as to supply a certain number of electrical equipment located near the exchanger (measuring instruments, lighting, etc.).

Claims (14)

Device for cooling gaseous effluents coming from an installation for the production of aluminum by igneous electrolysis, intended to be positioned between a collector of said effluents, whether it is an individual collector (5) emanating from each of the electrolytic cells (1) of the said installation, or of a general collector (6) at the level of which the said individual collectors (5) lead, and a treatment center for the said effluents, characterized in that this device consists of a plurality (8) of hollow tubes (9) mounted parallel to each other, with a diameter smaller than the diameter of said individual collectors (5) or of said general collector (6), and within which said gaseous effluents circulate, and constituting a air/air exchanger:
– one of the ends of said tubes (9) being in communication with an upstream plenum (10), at the level of which ends said general collector (6) or said individual collector (5),
– the opposite end of said tubes (9) being in communication with a downstream plenum (11), itself in communication with a pipe (13) leading to the gas treatment center. Device for cooling gaseous effluents according to claim 1, in which all or part of the tubes (9) constituting the exchanger (8) is provided with external radial fins (16), emanating from the periphery (15) of the tubes (9), and capable of optimizing convection. Gaseous effluent cooling device according to Claim 2, in which the fins (16) have a thickness of between 0.5 and 3 millimeters, and extend from the outer peripheral wall (15) of the tubes (9) of a distance between 20 and 60 millimeters. Device for cooling gaseous effluents according to one of Claims 2 and 3, in which the fins (15) consist of one or more helical spirals, the axis of rotation of which coincides with the axis of revolution of the tubes (9), the pitch of the propeller(s) thus formed being between 10 and 100 millimeters. Device for cooling gaseous effluents according to one of Claims 1 to 4, in which the tubes (9) are made of a material chosen from the group comprising aluminium, black steel, stainless steel and galvanized steel. Device for cooling gaseous effluents according to one of Claims 1 to 5, in which the tubes (9) are staggered with respect to each other. Device for cooling gaseous effluents according to one of Claims 1 to 6, in which the inter-tube distance within an exchanger (8) is between 5 and several hundreds of millimetres. Gaseous effluent cooling device according to one of Claims 1 to 7, in which the tubes (9) are mounted on a modular frame in groups of 100 to 200. Device for cooling gaseous effluents according to one of Claims 1 to 8, in which the exchanger (8) further comprises at least one additional air source (26), that is to say other than the ambient air surrounding the exchanger, this source typically consisting of a fan or equivalent, capable of increasing the flow of air intended to come into contact with said tubes (9). Device for cooling gaseous effluents according to one of Claims 1 to 9, in which the upstream (10) and downstream (11) plenums of the exchanger (8) have a wedge shape. Device for cooling gaseous effluents according to one of Claims 1 to 10, in which the upstream plenum (10) is provided with means (29, 31, 32, 33, 34, 35) attached to one of the plates ( 27) constituting said plenum, and capable of reducing separations of the gas stream at the inlet of the tubes (9) and thus of minimizing pressure drops. Device for cooling gaseous effluents according to one of Claims 1 to 11, in which the downstream plenum (11) is provided with means capable of reducing the turbulence and the overspeed zones in this same plenum and thus of minimizing the losses of charge. Device for cooling gaseous effluents according to one of Claims 1 to 12, in which one or more fan(s) (24) or equivalent devices is (are) installed downstream of the exchanger (8), so as to overcoming the pressure drop inherent in the passage of the gaseous effluents within said exchanger. Device for cooling gaseous effluents according to one of Claims 2 to 13, in which the fins (16) of the tubes (9) are provided with thermoelectric modules (25), intended to transform the heat dissipated by the said fins into electrical energy. .