Capture of a concentrated pollutant component from an electrolysis cell for the production of aluminum

Abstract

The invention relates to an assembly for capturing at least one polluting component present in a gaseous flow to be treated which comes at least from an electrolysis cell (101) for the industrial production of aluminum by igneous electrolysis, said assembly comprising two treatment systems (110, 120) connected in series so that at least part of the gaseous flow to be treated is capable of circulating from an inlet (E1) to an outlet (S1) of a first treatment system (110) connected to the electrolysis cell (101), then from an inlet (E2) to an outlet (S2) of a second treatment system (120). Figure for abstract: Figure 1

Description

Title of the invention: Capture of a concentrated polluting component originating from an electrolysis cell for the production of aluminum

[0001] The present invention relates to an assembly for capturing at least one polluting component present in a gas flow to be treated which originates at least from a cell for the industrial production of aluminum by igneous electrolysis.

[0002] The invention also relates to a gas treatment center comprising such an assembly.

[0003] The invention also relates to a method for capturing at least one polluting component present in a gas flow to be treated which comes at least from a tank for the industrial production of aluminum by igneous electrolysis.

[0004] Aluminum is produced in an industrial installation which includes, among other things, electrolysis tanks arranged in a building. The aluminum is then produced according to the well-known Hall-Héroult process, in each electrolysis tank being contained an electrolyte bath.

[0005] Conventionally, each tank extends in a transverse direction, and the tanks are arranged next to each other in a longitudinal direction.

[0006] Each tank comprises a support structure called a “superstructure” and a plurality of anodes, which are conventionally made of carbon.

[0007] Electrolysis tanks generally comprise a steel box, usually parallelepipedal and open at its upper part, an internal lining which is generally formed by blocks of refractory materials, and a cathode assembly, located at the bottom of the box, which comprises blocks of carbonaceous material, called "cathode blocks", and metal connecting bars to which the electrical conductors used to carry the electrolysis current are fixed.

[0008] The anodes are removably attached to a movable metal frame, called the "anode frame", by a removable connector. The anode frame is carried by the superstructure and attached to electrical conductors, called "positive risers", used to carry the electrolysis current.

[0009] Generally, more than a hundred tanks are arranged next to each other, in rows or lines, in the longitudinal direction. The tanks in a row are electrically connected in series using connecting conductors. The tanks are arranged so as to clear a circulation aisle along the installation and an access route between the tanks.

[0010] Furthermore, the electrolyte bath contained in each of the tanks is made up of: exclusively by alumina dissolved in fluorine salts such as complex fluorides like cryolite, the electrolyte bath being brought to a temperature between 950°C and 1000°C. In this bath, one or more anodes are immersed. When an electric current is applied between the anodes and the cathode assembly, an electrolysis reaction is activated and the alumina decomposes into aluminum forming a metallic bath which covers the cathode blocks, and into oxygen which reacts with each anode and causes its progressive consumption.

[0011] Aluminum is regularly removed from electrolysis tanks.

[0012] The upper part of the anodes is covered with a protective layer containing a powdered mixture of crushed electrolyte bath and alumina. Under the action of heat, this mixture partly solidifies, thus forming a crust that covers the anodes in order to thermally insulate them and protect them from oxidation in air.

[0013] Conventionally, the electrolysis reactions, especially at each anode, and the secondary reactions that occur in the electrolyte baths, as well as the high operating temperatures, result in the production of fumes and gases, which contain pollutant components, in the form of gases and / or dusts, such as carbon dioxide and monoxide, sulfur dioxide, hydrogen fluoride, carbon and alumina particles, dusts or fluorinated products. These fumes and gases migrate towards the top of the cells, that is to say in the direction of the upper part of the cells.

[0014] Note that the decomposition of alumina, during the electrolysis reaction, leads to a reduction in its content in the electrolyte bath. When this content falls below a limit value, a movable steel rod generally installed between two anodes, pierces the crust and another device injects alumina into the electrolyte bath by gravity fall through the hole thus created. The rod, commonly referred to as a piercer, can be maneuvered in a vertical movement using a jack, preferably pneumatic, to pierce the crust.

[0015] The majority of the fumes and gases trapped under the crust then escape through the holes drilled by the drills. These fumes and gases are lodged, after migration towards the upper part of the tanks, under a set of removable hoods which cover the open part of the electrolysis tanks. It should be noted, however, that a part of the fumes and gases containing the polluting components also escapes through cracks and openings which exist in the crust to in turn also migrate towards the upper part of the tanks.

[0016] It should however be noted that the discharge of these polluting components into the atmosphere is strictly controlled and regulated, not only with regard to the ambient atmosphere of the electrolysis hall, for reasons of working conditions of the personnel operating near the electrolysis tanks, but also with regard to the Air pollution. Pollution regulations in several states impose limits on the amounts of polluting compounds released into the atmosphere.

[0017] It is then of both economic and ecological interest to capture at least part of these polluting components.

[0018] This is why there are now installations which make it possible to generate, from these gases and fumes, at least one gas flow and to treat it in order to separate polluting components before releasing the treated gas flow into the atmosphere.

[0019] With this in mind, the gas flow is conventionally collected, via collection means, then sent to at least one capture assembly contained in a state-of-the-art Gas Treatment Center (commonly designated by the acronym "CTG"), in which polluting components are captured, then filtered in gas / particle separation means.

[0020] These collection means are arranged at the upper parts of the electrolysis tanks and configured to extract all or selectively the fumes and gases after they have migrated towards the upper part of the tanks to normally be housed under the set of removable hoods. These collection means contain at least one suction device, and one or more conduits which run along the electrolysis tanks and which join to connect to at least one extraction duct connected to the CTG. The suction device is then configured to cause a depression under the hoods in order to contain and suck up the gas flow to be treated. For example, the suction device may be a fan.

[0021] It should be noted that since the covers' joints are not airtight, outside air is drawn through them due to the depression. This outside air mixes with the fumes and gases coming from the electrolysis tanks and thus ends up in the gas stream to be treated. The resulting mixture can then contain between 80% and 99% by volume of the effectively drawn outside air.

[0022] Conventionally, the extracted gas stream to be treated comprises: - between 200 and 500 mg / Nm3 of dust, - between 200 and 500 mg / Nm3 of gaseous hydrogen fluoride (HF), and - between 200 and 400 mg / Nm3 of sulfur dioxide (SO2).

[0023] It should be noted that Nm3 commonly designates the unit of measurement "normal cubic meter".

[0024] It should be noted that the suction device is conventionally configured so that the gas flow to be treated, thus generated, preferably circulates in the extraction sheath with a flow rate of 1.4 to 5 Nm3 / s per electrolysis tank depending on their size. The gas flow to be treated generally circulates at a temperature between 100 and 200°C, which represents, in view of the suction flow rate, 30 to 35% of the heat losses per tank of electrolysis.

[0025] The gas flow to be treated then circulates in the CTG, which is conventionally constituted by a contacting zone configured to bring a capture agent, for example an adsorption and / or chemical neutralization agent of the powdery material type, into contact with the gas flow. The contacting zone is supplied with capture agent via supply means. The capture agent is capable of capturing polluting components. In the contacting zone, the capture agent is charged with polluting component. At the outlet of the contacting zone, a gas flow is obtained which comprises capture agent charged with polluting component.

[0026] For example, as a capture agent, mention may be made of alumina (A12O3), preferably in the form of a powdered material and having a high specific surface area, i.e. greater than 65 m2 / g.

[0027] After this contact, the gas flow which comprises the capture agent loaded with polluting component is introduced into a gas / particle separation means at the outlet of which there is a treated gas flow which is separated from the capture agent loaded with polluting component.

[0028] The contacting zone may be contained in a reactor, generally a vertical sheath in which the gas flow is generally ascending, or directly in the gas / particle separation means.

[0029] By reactor, as used herein, is meant any enclosure in which a reaction takes place by bringing a capture agent, which may be powdered material, into contact with a gaseous flow containing polluting components. It may thus be a fluidized bed reactor, of a type known per se, in which the gaseous flow to be treated circulates directly through a fluidized bed of powdered materials.

[0030] Furthermore, it should be noted that the gas / particle separation means is conventionally a cyclone or a filter with filter media, for example a bag filter or filter pocket filter.

[0031] With alumina as a capture agent, the CTGs currently used have hydrofluoric acid capture efficiencies close to 99.8% so as to obtain, at the CTG outlet, treated gas flows defined by mass concentrations of hydrofluoric acid of less than 0.5 mg / Nm3.

[0032] However, the search for energy savings, whether through the reduction of thermal leaks from the electrolysis tanks, or through a limitation of the suction flow rate generated by the suction device, leads to the collection, on the electrolysis tanks, of a gas flow to be treated that is both hotter and more concentrated in polluting agents than those that are currently to be treated.

[0033] However, for such heat and such concentrations, current CTGs do not allow for optimized decontamination which would make it possible to obtain flows treated gases defined by a mass concentration of hydrofluoric acid less than or equal to 0.5 mg / Nm3, knowing that the gas flow to be treated can then be defined by very high mass concentrations of hydrofluoric acid, dust and / or SO2, i.e. between 600 and 2500 mg / Nm3 for each of these polluting components.

[0034] There is therefore a need to provide a solution which makes it possible to overcome at least some of the aforementioned drawbacks.

[0035] A first object of the invention is to propose a capture assembly which makes it possible to capture polluting components contained in gas flows having such concentrations of polluting components, with a capture efficiency greater than that of the assemblies of the prior art.

[0036] A second object of the invention is to propose a capture assembly which makes it possible to significantly reduce the investment and operating costs linked to capture.

[0037] A third object of the invention is to propose a capture assembly configured to optimize the use of the gas / particle separation means conventionally included in the capture assemblies of the prior art.

[0038] Thus, according to a first aspect, the invention proposes an assembly for capturing at least one polluting component present in a gas flow to be treated which comes at least from an electrolysis cell for the industrial production of aluminum by igneous electrolysis, the assembly comprising two treatment systems connected in series so that at least part of the gas flow to be treated is capable of circulating from an inlet to an outlet of a first treatment system connected to the electrolysis cell, then from an inlet to an outlet of a second treatment system, each of the two treatment systems comprising, from its inlet to its outlet in the direction of circulation of the gas flow to be treated: - a means for supplying a capture agent of the powdery material type, capable of capturing the at least one pollutant component present in the gas stream, the supply means being configured to bring the capture agent into contact with the gas stream and obtain a gas mixture comprising the capture agent loaded with the pollutant component, and - a gas / particle separation means configured to obtain a gas stream separated from the capture agent loaded with the pollutant component,

[0039] the assembly being characterized in that the supply means of the second treatment system is connected to a source containing the capture agent not loaded with the pollutant component, and

[0040] in that the supply means of the first treatment system is connected to a source containing the capture agent loaded with the pollutant component.

[0041] Various additional features may be provided alone or in combination: - the source containing the capture agent loaded with polluting component is the gas / particle separation means of said second treatment system when it is further configured to obtain a flow of capture agent loaded with polluting component, - the source containing the capture agent loaded with polluting component is an ancillary source configured to store and / or generate the capture agent loaded with polluting component, - the gas / particle separation means of the first treatment system and / or of the second treatment system comprise a filter media filter, - the gas / particle separation means of the first treatment system is a cyclone, - the supply means of the second treatment system is further connected to a source of capture agent loaded with polluting component, - the source of capture agent loaded with polluting component is connected to the gas / particle separation means of the second treatment system so that the second treatment system is partly supplied with capture agent not loaded with polluting component and partly with capture agent loaded with polluting component from the gas / particle separation means of the second treatment system, - the supply means of the first treatment system is further connected to the gas / particle separation means of the first treatment system so that the first treatment system is partly supplied with capture agent loaded with pollutant component from the gas / particle separation means of the first treatment system, - the means of supplying the first treatment system is further connected to a source of capture agent not loaded with polluting component, - the assembly further comprises a means for cooling at least a portion of the capture agent loaded and / or not loaded with polluting component to a temperature greater than or equal to 5°C before supplying the first treatment system and / or the second treatment system, - the assembly further comprises a means for cooling the gas flow to be treated upstream of the first cooling system, the cooling means being located between the at least one electrolysis tank and the reactor of the first treatment system, - the assembly further comprises a means for cooling the gas flow leaving the first treatment system, the cooling means being

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[0049] located between the two treatment systems, - the capture agent is alumina, - at least one polluting component is hydrofluoric acid or sulfur dioxide. According to a second aspect, the invention proposes a gas treatment center comprising an assembly as described above. According to a third aspect, the invention proposes a method for capturing at least one polluting component present in a gas flow to be treated which comes at least from an electrolysis cell for the industrial production of aluminum by igneous electrolysis, the method being capable of implementing an assembly as described above and comprising the following steps: a) bringing the gas flow to be treated into contact with a capture agent loaded with polluting component in a first treatment system to obtain a first gas mixture comprising capture agent loaded with polluting component, b) separation of the first gas mixture to obtain a first gas flow, called the first treated gas flow, separated from the capture agent loaded with polluting component, c) contacting the first treated gas stream with a capture agent not loaded with polluting component in a second treatment system to obtain a second gas mixture, d) separation of the second gas mixture to obtain a second gas flow, called the second treated gas flow, separated from the capture agent loaded with polluting component. Various additional features may be provided alone or in combination: - the gas flow to be treated is also brought into contact with a capture agent not loaded with polluting component, - during step c), the first treated gas flow is further brought into contact with a capture agent loaded with polluting component, - the method further comprises a step of cooling the capture agent loaded and / or not loaded with polluting component to a temperature greater than or equal to 5°C before bringing it into contact with the gas flow to be treated and / or the first gas flow treated, - the method further comprises, before step a) of contacting, a step of cooling the gas flow to be treated, - the method further comprises, before step c) of contacting, a step of cooling the first treated gas flow, Other features and advantages of the invention will become apparent during the reading the detailed description which follows in which are presented examples which are solely illustrative and in no way limitative of the scope of the invention and from the following illustrations in which

[0050] [Fig.l] [Fig.l] is a schematic representation of a capture assembly according to an exemplary embodiment of the invention.

[0051] [Fig.2] [Fig.2] is a schematic representation of an alternative embodiment according to an exemplary embodiment of the invention.

[0052] [Fig.3] [Fig.3] is a schematic representation of an alternative embodiment according to an exemplary embodiment of the invention.

[0053] [Fig.4] [Fig.4] is a schematic representation of an alternative embodiment which combines the exemplary embodiments of the invention illustrated in Figures 2 and 3.

[0054] In the following description, identical, similar or analogous elements will be designated by the same alphanumeric references.

[0055] At least one capture assembly according to the invention is connected to collection means arranged in the upper part of the electrolysis tanks. This configuration is conventionally implemented with the collection means currently used and makes it possible to collect the fumes and gases which migrate towards the top of the tanks, that is to say towards the upper part of the tanks.

[0056] These collection means contain at least one suction device, and one or more conduits which run along the electrolysis tanks and which join to connect to at least one extraction sheath connected to the CTG. The suction device is then configured to cause a depression under the hoods and suck up the gas flow to be treated which is then sent to the capture assembly according to the invention, which is a CTG.

[0057] It should be noted that the suction device can be included in the capture assembly or can be located upstream or downstream of the latter in the direction of circulation of the gas flow to be treated.

[0058] Advantageously, the suction device is configured so that the gas flow to be treated thus generated preferably circulates with a flow rate of less than 2 NmVs per electrolysis tank, preferably between 0.2 and 2 NmVs per electrolysis tank.

[0059] Note that the suction device, which can contain any known suction means suitable for use in electrolysis installations, also makes it possible to overcome the pressure loss experienced by the gas flow, particularly during its passage through the elements constituting the capture assembly.

[0060] The capture assembly is advantageously adapted for treating gas flows which contain at least one polluting component present at a mass concentration of between 600 and 2500 mg / Nm3.

[0061] For example, the polluting component may be hydrofluoric acid (HF) under gaseous form, or sulfur dioxide. The gas stream to be treated may also include a mixture of pollutant components containing both hydrofluoric acid and sulfur dioxide. Preferably, the pollutant component to be captured in the capture assembly is hydrofluoric acid in gaseous form.

[0062] For example, the gas flow to be treated which enters each of the capture assemblies according to the exemplary embodiments illustrated in Figures 1 to 4 described below can then comprise:

[0063] - between 600 and 2500 mg / Nm3 of dust, - between 600 and 2500 mg / Nm3 of gaseous hydrofluoric acid (HF), and - between 600 and 2500 mg / Nm3 of sulfur dioxide (SO2).

[0064] The gas flow to be treated which enters the capture assemblies according to the embodiments described below can circulate at a temperature above 200°C, preferably between 200 and 300°C.

[0065] Preferably, the capture assembly according to the invention is then configured to capture hydrofluoric acid present in the gas flow to be treated coming from the electrolysis cells. This capture is made possible by bringing the hydrofluoric acid into contact with at least one suitable capture agent which is an adsorption and / or chemical neutralization agent.

[0066] By capturing at least hydrofluoric acid, within the meaning of the present invention, is meant capturing hydrofluoric acid and fixing it on the capture agent by physicochemical adsorption or by chemical reaction and thus drastically reducing its concentration in the circulating gas flow.

[0067] In particular, the capture agent may be selected from the capture agents currently used and known to those skilled in the art. Advantageously, when the polluting component is hydrofluoric acid or a mixture comprising it, the capture agent is alumina which has properties enabling it to effectively capture hydrofluoric acid.

[0068] It should be noted that alumina is also capable of capturing sulfur dioxide. In the embodiments described below, the focus is solely on the capture of hydrofluoric acid by alumina.

[0069] The alumina is of the powdery material type. By powdery material, as used herein, particles are meant. In what follows, we then speak of alumina particles.

[0070] With reference to Figures 1 to 4, sets of capture according to embodiments of the invention will now be described below.

[0071] It should be noted that in these figures, only one electrolysis cell 101 is illustrated to facilitate the schematic representation, even though, in reality, industrial aluminum production plants following the Hall - Héroult process contain obviously several electrolysis cells emitting gases and fumes that need to be treated. Each of these electrolysis cells is then connected to at least one capture set according to the invention.

[0072] The capture assembly according to each of the exemplary embodiments illustrated in FIGS. 1 to 4 comprises at least two processing systems 110, 120 connected in series, that is to say that the output SI of the first processing system 110 is connected to the input E2 of the second processing system 120.

[0073] As illustrated in the figures, the first processing system 110 is directly connected to the second processing system 120. By first processing system 110 directly connected to the second processing system 120, within the meaning of the present invention, it is meant that the output SI of the first processing system 110 is fluidically connected to the input E2 of the second processing system 120 without any other element being present between them.

[0074] However, in a variant not illustrated in the figures, the first processing system 110 may be indirectly connected to the second processing system 120. By first processing system 110 indirectly connected to the second processing system 120, within the meaning of the present invention, it is meant that the output SI of the first processing system 110 is fluidically connected to at least one other element itself fluidically connected to the inlet E2 of the second processing system 120. However, several other elements may be arranged between the output SI of the first processing system 110 and the inlet E2 of the second processing system 120, some of these other elements then being fluidically connected in series.

[0075] For example, as other elements, there may be means for heating or cooling the gas flow, means for chemical treatment of the gas flow, a common sheath (commonly designated by the term plenum) to which are connected both the outlet SI of the first treatment system, and the inlet E2 of the second treatment system 120.

[0076] According to the exemplary embodiments of the invention illustrated in Figures 1 to 4, the gas flow to be treated first circulates in a first circulation pipe C1 connected to the electrolysis tank 101, then from an inlet E1 to the outlet S1 of the first treatment system 110. The gas flow to be treated then circulates from the inlet E2 to an outlet S2 of the second treatment system 120, possibly also in the common sheath.

[0077] Advantageously, the capture assembly of these exemplary embodiments may further comprise a means for cooling the gas flow to be treated arranged upstream of the first treatment system 110 (variant not illustrated in the figures). In particular, the cooling means is for example located between the electrolysis tank 101 and the first treatment system 110 (not illustrated in the figures). Such a cooling means makes it possible to cool the gas flow coming from the electrolysis tank 101 and facilitate its treatment in the first treatment system 110 in particular.

[0078] The gas flow to be treated is then treated successively in the first treatment system 110 then in the second treatment system 120. In particular, the gas flow, after passing through the first treatment system 110, circulates in a second circulation pipe C2 before being introduced into the second treatment system 120.

[0079] Then, the treated gas flow which has passed into the first treatment system 110 then into the second treatment system 120 circulates in a third circulation pipe C3 before being sent into the atmosphere after, for example, having passed through a chimney, or being directed towards another treatment system to capture other polluting components not captured by the two treatment systems 110 and 120.

[0080] In particular, the first treatment system 110 comprises, from its inlet E1 to its outlet S1 in the direction of circulation of the gas flow to be treated, first a contacting zone in which the gas flow is likely to be brought into contact with alumina particles loaded with hydrofluoric acid, then a gas / particle separation means 113. It should be noted, however, that, in an alternative embodiment not shown in the figures, the contacting zone is located between the electrolysis tank 101 and the suction device.

[0081] Alumina loaded with hydrofluoric acid, or fluorinated alumina, is alumina partially loaded with hydrofluoric acid. For example, partially loaded alumina particles can be obtained after bringing fresh alumina particles into contact with hydrofluoric acid. The loaded alumina is, in the context of the invention, alumina still capable of capturing hydrofluoric acid.

[0082] Preferably, the alumina particles loaded with hydrofluoric acid contain fluorine at a mass concentration of between 0.1 and 1.5% inclusive relative to the total weight of the particles.

[0083] Bringing the gas flow into contact with alumina particles loaded with hydrofluoric acid allows the latter to capture more hydrofluoric acid and to approach its optimal capture rate.

[0084] The contact zone of the first treatment system 110 is fluidically connected to a means 114 for supplying alumina particles loaded with hydrofluoric acid. This supply means 114 may be any type of known particle injection device.

[0085] To this supply means 114 is then connected a source of alumina particles loaded with hydrofluoric acid.

[0086] This source of alumina particles loaded with hydrofluoric acid may, for example, be an element contained in the capture assembly and configured to transmit alumina particles loaded with hydrofluoric acid, or an additional source configured to store and / or generate alumina particles loaded with hydrofluoric acid.

[0087] For example, as an additional source, mention may be made of any type of gas / particle separation means known to those skilled in the art and configured to generate a flow of alumina particles loaded with hydrofluoric acid, a silo containing alumina particles loaded with hydrofluoric acid, or any means for distributing alumina particles loaded with hydrofluoric acid and connected to several gas / particle separation means. This list is in no way limiting.

[0088] Advantageously, as can be seen in Figures 1 to 4, the source of alumina particles loaded with hydrofluoric acid connected to the supply means 114 of the first treatment system 110 is the gas / particle separation means 123 of the second treatment system 120. The supply means 114 is then fluidically connected, in particular by a first recirculation pipe 140, to the gas / particle separation means 123 of the second treatment system 120 so as to extend the residence time of the loaded alumina particles from this second treatment system 120. Here, the contacting zone of the first treatment system 110 is therefore supplied with alumina particles that have already been brought into contact with hydrofluoric acid during its passage in the gas flow circulating in the second treatment system 120 and introduced via the inlet E2.

[0089] The feed means 114 of the first treatment system 110 is fed with alumina particles loaded with hydrofluoric acid in proportions of between 0.5 and 5 t / h.

[0090] Preferably, as illustrated in the figures, the contacting zone of the first treatment system 110 is contained in a reactor 111 which is supplied with alumina particles loaded with hydrofluoric acid via the supply means 114 of the first treatment system 110.

[0091] In an alternative embodiment not illustrated in the figures, the contact zone is contained directly in the gas / particle separation means 113 of the first treatment system 110 without it being necessary to use a dissociated reactor. For example, the contact zone can then be located close to the inlet of the gas / particle separation means 113.

[0092] At the end of such contact, a first gaseous mixture is then obtained comprising, among other things, hydrofluoric acid not captured by the alumina particles, and alumina particles charged with hydrofluoric acid defined by a capture rate increased compared to that of the charged alumina particles which were introduced into the contacting zone by the feed means 114.

[0093] The reactor 111 of the first treatment system 110 is fluidically connected, by a transfer pipe 112, to the gas / particle separation means 113 of the first treatment system 110.

[0094] The gas / particle separation means 113 of the first treatment system 110 is configured to obtain a first treated gas flow which is discharged via the outlet S1, outlet S1 being located for example in the upper part of the gas / particle separation means. By first treated gas flow is meant a first gas flow separated from the alumina particles loaded with hydrofluoric acid. The gas / particle separation means 113 of the first treatment system 110 is further configured to obtain a flow of alumina particles loaded with hydrofluoric acid which can be discharged via another outlet which is located for example in the lower part of the gas / particle separation means.

[0095] In particular, the gas / particle separation means 113 of the first treatment system 110 is further configured to filter the dust contained in the gas flow so as to separate it from the first gas mixture.

[0096] For example, the gas / particle separation means 113 of the first treatment system 110 may be a dust removal means. It may further comprise a known filter which is capable of separating, from the gas flow, alumina particles loaded with hydrofluoric acid.

[0097] Advantageously, the gas / particle separation means 113 of the first treatment system 110 is a device for mechanically separating alumina particles loaded with hydrofluoric acid. In particular, it is a filtration device containing a filter media filter such as a bag filter or a filter bag filter capable of maintaining alumina particles fully loaded with hydrofluoric acid. It should be noted that the bag filter is conventionally composed of a plurality of filtration modules through which the gas flow passes.Thus, almost all of the hydrofluoric acid - fully charged alumina particles and most of the dust are separated from the incoming gas stream in the gas / particle separator 113 and, as will be seen further below, at least a part of these hydrofluoric acid - fully charged alumina particles and of these dusts can be directed towards the electrolysis cell 101 via a first recovery conduit RI to be introduced therein, for example, using the lance - doser. In addition, the particles that deposit on the surfaces of the sleeves form a cake composed mainly of hydrofluoric acid - fully charged alumina particles. The hydrofluoric acid - fully charged alumina particles directed towards the electrolysis cell 101 are preferably alumina particles containing fluorine at a mass concentration of more than 1.5% relative to the... total weight of the particles. It should be noted that the alumina particles loaded with hydrofluoric acid can be treated before being introduced into the electrolysis tank 101. Thus, the recovery pipe RI can, for example, comprise any filtering means or treatment device known to those skilled in the art (not illustrated in the figures).

[0098] In an alternative embodiment, the gas / particle separation means 113 of the first treatment system 110 may be an aeraulic selection device such as a cyclone. It should be noted that the aeraulic selection device operates a separation between the alumina particles loaded with hydrofluoric acid and the dust particles. The aeraulic selection here defines a selection operated on the basis in particular of the size of the particles, using a carrier gas such as that constituting the gas flow to be treated. It may however be influenced by other granulometric characteristics of the particles, combined with the size, for example the density, shape or porosity of the particles.

[0099] The use of a cyclone also makes it possible to have a first treatment system 110 which is compact, less bulky and less energy-intensive than a first treatment system comprising the reactor 111 / mechanical separation device combination.

[0100] According to the exemplary embodiments illustrated in Figures 1 to 4, the first treated gas flow leaving the first treatment system 110 is then directed towards the second filtration system 120 via the second circulation pipe C2. This first treated gas flow contains a portion of gas flow to be treated, a small portion of dust and a small portion, or residual portion, of alumina particles loaded with hydrofluoric acid which would not have been separated from the gas flow in the first treatment system.

[0101] It should be noted that on leaving the first treatment system 110, the first treated gas flow comprises hydrofluoric acid at a concentration of between 10 and 500 mg / Nm3.

[0102] According to the exemplary embodiments illustrated in Figures 1 to 4, the second treatment system 120 comprises, from its inlet E2 to its outlet S2 in the direction of circulation of the gas flow, a contacting zone in which the first treated gas flow is likely to be brought into contact with fresh alumina particles which will, upon contact with the first gas flow, become charged with hydrofluoric acid to obtain alumina particles charged with hydrofluoric acid. The second treatment system 120 further comprises a gas / particle separation means 123 configured to obtain a second gas flow, called the second treated gas flow (which is more generally designated as the treated gas flow) separated from the alumina particles charged with hydrofluoric acid.

[0103] The contacting zone of the second treatment system 120 is preferably contained in a reactor 121. The reactor 121 is supplied with fresh alumina particles by a supply means 124 connected to a first supply pipe Al, itself connected to a source 125 (or storage means) of fresh alumina particles. This source 125 of fresh alumina particles may, for example, be a fresh alumina silo or any means of distributing fresh alumina particles.

[0104] For the purposes of the present invention, fresh alumina means alumina not loaded with hydrofluoric acid, i.e. alumina free of hydrofluoric acid or alumina which does not contain fluorine.

[0105] In an example not illustrated in the figures, the assembly may also further comprise a means for cooling the fresh alumina particles connected to the supply means of the second treatment system 120 so as to lower the temperature of the fresh alumina particles to a temperature greater than or equal to 5°C before introducing them into the reactor 121 of the second treatment system 120.

[0106] Thus, according to the invention, the reactor 121 of the second treatment system 120 is therefore configured to receive, via a first inlet, fresh alumina particles, and via a second inlet, the first treated gas flow so as to obtain, at the outlet of the reactor 121, a second gas mixture.

[0107] Preferably, the reactor 121 of the second treatment system 120 is supplied with fresh alumina particles in proportions of between 0.5 and 5 t / h.

[0108] After passing through the reactor 121 of the second treatment system 120, the gas flow then consisting of the second gas mixture, circulates in a transfer pipe 122 before being introduced into the gas / particle separation means 123 of the second treatment system 120.

[0109] The gas / particle separation means 123 of the second treatment system 120 is particularly advantageous insofar as it is configured to obtain, on the one hand, the second treated gas flow which is discharged via the outlet S2, i.e. the second gas flow which is separated from almost all of the hydrofluoric acid contained in the gas flow to be treated entering the capture assembly according to the exemplary embodiments, and on the other hand, a second flow of alumina particles loaded with hydrofluoric acid which can be discharged via another outlet and be injected into the contacting zone of the first treatment system 110 by its supply means 114.

[0110] The gas / particle separation means 123 of the second treatment system 120 may be a dust removal means. It may comprise a filter capable of filtering the alumina particles loaded with hydrofluoric acid.

[0111] Advantageously, the filter is a filter with filter media, for example with filter fabric such as a bag filter.

[0112] The gas / particle separation means 123 of the second treatment system 120 may be a mechanical separation device such as that described above for the gas / particle separation means 113 of the second treatment system 110.

[0113] The second treated gas flow then leaves the gas / particle separation means 123 of the second treatment system at a temperature between 100 and 130°C.

[0114] The treated gas stream, i.e. the second treated gas stream exiting the second treatment system via outlet S2, is capable of circulating in the third circulation line C3. Any element known to those skilled in the art may be connected to the third circulation line C3 to further treat the treated gas stream before its discharge into the atmosphere.

[0115] For example, the treated gas flow leaving S2 through the third circulation pipe C3 can then be released into the atmosphere through a chimney, possibly using a fan.

[0116] By implementing the capture assembly described above, the quantity of hydrofluoric acid contained in the second treated gas flow leaving the second treatment system 120 is less than or equal to 0.5 mg / Nm3. The quantity of dust contained in the second treated gas flow leaving the second treatment system 120 is, for its part, less than or equal to 5 mg / Nm3.

[0117] According to an exemplary embodiment not illustrated in the figures, the assembly may further comprise a means for cooling the second flow of alumina particles loaded with hydrofluoric acid from the gas / particle separation means 123 of the second treatment system 120 to a temperature greater than or equal to 5°C. This cooling means is fluidically connected to the gas / particle separation means 123 of the second treatment system 120 and to the supply means 114 of the contacting zone of the first treatment system 110.

[0118] In an alternative embodiment not illustrated in the figures, the supply means 114 of the first treatment system 110 may, in addition to being connected to the gas / particle separation means 123 of the second treatment system 120 as a source of alumina particles loaded with hydrofluoric acid, also be connected to a source of fresh alumina particles. Preferably, this source of fresh alumina particles may be dissociated from the source of alumina particles loaded with hydrofluoric acid so as to have two separate sources connected to the supply means 114 by equally separate fluid circuits. However, in another exemplary embodiment, the supply means 114 may be at least connected to a storage means itself connected, on the one hand, to the gas / particle separation means 123 of the second treatment system 120 as a source of alumina particles loaded with hydrofluoric acid. particles 123, and on the other hand, to a source of fresh alumina particles, so that this storage means is capable of transmitting, to the supply means 114, a mixture of particles comprising both fresh alumina particles and also charged alumina particles. It should be noted that this source of fresh alumina particles may be the one which is connected to the supply means 124 of the reactor 121 of the second treatment system 120. The fact of supplying the contacting zone of the first treatment system 110 with fresh alumina particles and alumina particles charged with hydrofluoric acid makes it possible to increase the efficiency of capture of the hydrofluoric acid by the alumina. Preferably, in this configuration, the assembly may also further comprise a means for cooling the fresh alumina particles to a temperature greater than or equal to 5°C.This cooling means is fluidically connected at least to the source adapted to send fresh alumina particles to the supply means 114 of the contacting zone, preferably of the reactor 111, of the first treatment system 110.

[0119] In a non-illustrated exemplary embodiment, the reactor 121 of the second treatment system 120 is further supplied, via its supply means 124, with alumina particles loaded with hydrofluoric acid. The mixture of fresh alumina particles and loaded alumina particles, in variable proportions, can be stored in a single source connected to the supply means 124. This makes it possible to guarantee a homogeneous mixture of the alumina before it is brought into contact with the gas flow.

[0120] According to an exemplary embodiment not illustrated in the figures, the assemblies illustrated in figures 1 to 4 may further comprise a means for cooling the first treated gas flow leaving the first treatment system 110, the cooling means being fluidically connected to the outlet SI of the first treatment system 110 and to the inlet E2 of the second treatment system 120. Such a configuration has the advantage of protecting the possible filtering medium from the gas / particle separation means 123 of the second treatment system 120. Such a configuration also makes it possible to increase the capacity for capturing hydrofluoric acid by the alumina.

[0121] According to the embodiment variant illustrated in [Fig.2], the supply means 124 of the reactor 121 of the second treatment system 120 is further connected to the gas / particle separation means 123 of the second treatment system 120 by a second recirculation pipe 142. Thus, the reactor 121 of the second treatment system 120 is then supplied, via its supply means 124, both with fresh alumina particles, and also with alumina particles loaded with hydrofluoric acid which have been separated from the second gas flow passing through the gas / particle separation means 123. These alumina particles loaded with hydrofluoric acid hydrofluoric acid are contained in the second flow of alumina particles loaded with hydrofluoric acid from the gas / particle separation means 123 of the second treatment system 120. Thus, the supply means 124 is then further fluidically connected to this gas / particle separation means. In this variant, the second gas / particle separation means 123 of the second treatment system 120 is then configured to transmit alumina particles loaded with hydrofluoric acid both to the contacting zone of the first treatment system, preferably therefore to the reactor 111 of the first treatment system 110, and also to the reactor 121 of the second treatment system 120. For example, the reactor 121 of the second treatment system 120 can be supplied in proportions of between 0.5 and 5 t / h with the second flow of alumina particles loaded with hydrofluoric acid from the gas / particle separation means 123.

[0122] According to the variant illustrated in [Fig.3], the supply means 114 of the first treatment system 110 is further connected to the gas / particle separation means 113 of the first treatment system 110, by a third recirculation pipe 144, so that the first treatment system 110 is partly supplied by the first flow of alumina particles loaded with hydrofluoric acid coming from the gas / particle separation means 113 of the first treatment system 110 and separated from the gas flow which passes through it.

[0123] It should be noted that the means for cooling the gas flows mentioned above may be heat exchangers, means for injecting water in fine drops then implementing the principle of cooling by evaporation, or even pins. For example, a pin is understood to mean a sheath emerging from the electrolysis tank 101 extended with a large heat exchange surface.

[0124] It should be noted that a multitude of exemplary embodiments combining at least two of the embodiment variants described above can be implemented. For example, [Fig.4] illustrates an embodiment variant which combines those illustrated in Figures 2 and 3.

[0125] According to the embodiments and its variants described above, the capture assembly according to the invention allows the recirculation of alumina particles loaded with hydrofluoric acid after it has been brought into contact with the gas flow to be treated so as to increase its residence time in order to achieve the optimal capture rate of the alumina particles. In this way, the capture assembly according to the invention makes it possible to increase the capture efficiency of the hydrofluoric acid and at the same time to significantly reduce the investment and operating costs linked to the capture, while maximizing the contact between the gas flow and the alumina particles. Thus, optimal use of the alumina particles and less landfilling of the alumina particles loaded with hydrofluoric acid are guaranteed. still capable of capturing hydrofluoric acid. Using the assemblies described above, a first rough treatment is then carried out in the first treatment system 110 then a second so-called finishing treatment in the second treatment system 120.

[0126] In particular, according to an advantageous use of the capture assemblies according to the exemplary embodiments of the invention described above, the quantity of fresh alumina particles introduced into the reactor 121 of the second treatment system 120 is equal to the quantity of alumina particles loaded with hydrofluoric acid introduced directly into the electrolysis tank 101 via the first recovery pipe RI. By proceeding in this way, almost total capture of the hydrofluoric acid present in the gas flow to be treated is guaranteed.

[0127] Exemplary embodiments and variants have been described above in which the capture assemblies comprise two treatment systems 110 and 120 connected in series. However, the invention is not limited to these two treatment systems and could include more. For example, the capture assemblies could comprise three treatment systems connected in series and numbered in the direction of circulation of the gas flow: a first treatment system, a second treatment system and a third treatment system. The separation means of the third treatment system can then be connected to the reactor of the first treatment system, or to the reactor of the second treatment system to allow the recirculation of the alumina particles loaded with hydrofluoric acid.

[0128] Furthermore, several first treatment systems, as described above, connected in parallel can be configured to recover the gases and fumes coming from the several electrolysis tanks actually contained in the industrial aluminum production installations. These several first treatment systems can be fluidically connected to a common sheath (commonly designated by the term plenum) which, itself, can be connected to one or more second treatment systems, as described above, arranged in parallel. In this way, the first treated gas streams leaving the first treatment systems are mixed in the common sheath before the resulting mixture is introduced into the second treatment systems. In this case, at least one of the second treatment systems is fluidically connected to at least one of the first treatment systems.

[0129] It should be noted that several capture assemblies according to the invention can be used in an industrial aluminum production installation. In this case, each capture assembly is connected to a single electrolysis tank, or to a group of electrolysis tanks.

[0130] It should also be noted that one can also use a single set of capture according to the invention in an industrial aluminum production installation which would then be connected to all of the electrolysis tanks.

[0131] In the following, a method for capturing hydrofluoric acid present in the gas stream to be treated is described. This method uses at least one of the capture assemblies described above.

[0132] In a first optional step, the method may firstly comprise a preliminary step of cooling the gas flow to be treated via the cooling means.

[0133] Then, a step a) consisting of bringing the gas flow to be treated into contact with alumina particles loaded with hydrofluoric acid is carried out in the contacting zone of the first treatment system 110, for example in the reactor 111. This makes it possible to obtain the first gas mixture.

[0134] Advantageously, this step a) can consist of bringing the gas flow to be treated into contact with, in addition, fresh alumina particles.

[0135] Then, a step b) of separation of the first gas mixture is implemented to obtain, on the one hand, the first treated gas flow separated from the alumina particles loaded with hydrofluoric acid, and on the other hand, the first flow of alumina particles loaded with hydrofluoric acid.

[0136] Optionally, the method can then comprise a step of cooling the first treated gas flow before introducing it into the reactor 121 of the second treatment system 120.

[0137] Then, a step c) of bringing the first treated gas stream into contact with fresh alumina particles is implemented in the reactor 121 of the second treatment system 120 to obtain a second gas mixture.

[0138] Advantageously, during step c), the first treated gas flow is further brought into contact with alumina particles loaded with hydrofluoric acid. This alumina loaded with hydrofluoric acid can for example either come from the gas / particle separation means 123 of the second treatment system 120 of the same capture assembly, or come from a gas / particle separation means of a second treatment system of another capture assembly contained in the gas treatment center.

[0139] Finally, a step d) of separation of the second gas mixture is carried out to obtain, on the one hand, the second treated gas flow separated from the alumina particles loaded with hydrofluoric acid, and on the other hand, the second flow of alumina particles loaded with hydrofluoric acid which may optionally be reinjected into the contacting zone of the first treatment system 110 and / or into the contacting zone of the second treatment system 120.

[0140] Preferably, the method further comprises a step of cooling the alumina particles loaded with hydrofluoric acid and / or fresh up to a temperature greater than or equal to 5°C before being brought into contact with one of the aforementioned gas flows, in particular the gas flow to be treated coming from the electrolysis tank 101 and / or the first treated gas flow leaving the gas / particle separation means 123 of the second treatment system 120.

[0141] Alternatively, the supply means 114 and 124 of the reactors 111 and 121 of the first and / or second treatment systems 110 and 120 may be configured to introduce, into the contact zones, a mixture comprising several capture agents, in particular alumina and at least one other capture agent capable of capturing sulfur dioxide.

[0142] Naturally, other modes of implementing the invention could have been envisaged by those skilled in the art without departing from the scope of the invention defined by the claims below.

Claims

Claims

1. Assembly for capturing at least one polluting component present in a gas flow to be treated which comes at least from an electrolysis cell (101) for the industrial production of aluminum by igneous electrolysis, said assembly comprising two treatment systems (110, 120) connected in series so that at least part of the gas flow to be treated is capable of circulating from an inlet (El) to an outlet (SI) of a first treatment system (110) connected to the electrolysis cell (101), then from an inlet (E2) to an outlet (S2) of a second treatment system (120), each of said two treatment systems (110, 120) comprising, from its inlet (El, E2) to its outlet (SI, S2) in the direction of circulation of the gas flow to be treated: - a means (114, 124) for supplying a capture agent, of the powdery material type, capable of capturing the at least one polluting component present in said gas flow, the supply means being configured to bring the capture agent into contact with the gas flow and obtain a gas mixture comprising capture agent loaded with polluting component, and - a gas / particle separation means (113, 123) configured to obtain a gas flow separated from the capture agent loaded with polluting component and a flow of capture agent loaded with polluting component, said assembly being characterized in that the supply means (124) of said second treatment system (120) is connected to a source containing capture agent not loaded with polluting component (125), and in that the supply means (114) of said first treatment system (110) is connected to a source containing capture agent loaded with polluting component.

2. An assembly according to claim 1, wherein the source containing the capture agent loaded with polluting component is the gas / particle separation means (123) of said second treatment system. (120) when further configured to obtain a capture agent flow loaded with polluting component.

3. An assembly according to claim 1, wherein the source containing the capture agent loaded with polluting component is an ancillary source configured to store and / or generate the capture agent loaded with polluting component.

4. Assembly according to one of claims 1 to 3, in which the gas / particle separation means (113, 123) of the first treatment system (110) and / or of the second treatment system (120) comprise a filter media filter.

5. Assembly according to one of claims 1 to 4, in which the gas / particle separation means (113) of the first treatment system (110) is a cyclone.

6. Assembly according to one of claims 1 to 5, in which the supply means (124) of the second treatment system (120) is further connected to a source of capture agent loaded with polluting component.

7. An assembly according to claim 6, wherein the source of capture agent loaded with polluting component is connected to the gas / particle separation means (123) of said second treatment system (120) so that the second treatment system (120) is partly supplied with capture agent not loaded with polluting component and partly with capture agent loaded with polluting component coming from the gas / particle separation means (123) of the second treatment system (120).

8. Assembly according to one of claims 1 to 7, in which the supply means (111) of the first treatment system (110) is further connected to the gas / particle separation means (113) of the first treatment system (110) so that the first treatment system (110) is partly supplied with capture agent loaded with polluting component from the gas / particle separation means (113) of the first treatment system.

9. Assembly according to one of claims 1 to 8, in which the supply means (114) of the first treatment system (110) is further connected to a source of capture agent not loaded with polluting component.

10. An assembly according to one of claims 1 or 9, further comprising means for cooling at least a portion of the capture agent. loaded and / or not loaded with polluting component up to a temperature greater than or equal to 5°C before feeding the first treatment system and / or the second treatment system.

11. Assembly according to one of claims 1 to 10, further comprising a means for cooling the gas flow to be treated upstream of the first cooling system (110), the cooling means being located between the at least one electrolysis tank (101) and the reactor (111) of the first treatment system (110).

12. Assembly according to one of claims 1 to 11, further comprising a means for cooling the gas flow leaving the first treatment system (110), the cooling means being located between the two treatment systems (110, 120).

13. An assembly according to one of claims 1 to 12, in which the capture agent is alumina.

14. An assembly according to one of claims 1 to 13, wherein the at least one polluting component is hydrofluoric acid or sulfur dioxide.

15. Gas treatment center comprising an assembly according to one of claims 1 to 14.

16. Method for capturing at least one polluting component present in a gas stream to be treated which comes at least from an electrolysis cell (101) for the industrial production of aluminum by igneous electrolysis, the method being capable of implementing an assembly according to one of claims 1 to 14 and comprising the following steps: a) bringing the gas stream to be treated into contact with a capture agent loaded with polluting component in a first treatment system (110) to obtain a first gas mixture comprising the capture agent loaded with polluting component, b) separating the first gas mixture to obtain a first treated gas stream separated from the capture agent loaded with polluting component, c) bringing the first treated gas stream into contact with a capture agent not loaded with polluting component in a second treatment system (120) to obtain a second gas mixture,d) separation of the second gas mixture to obtain a second treated gas flow separated from the capture agent loaded with polluting component.,

17. A method according to claim 16, wherein in step a), the flow The gas to be treated is also brought into contact with a capture agent not loaded with polluting components.

18. Method according to one of claims 16 or 17, according to which during step c), the first treated gas flow is further brought into contact with a capture agent loaded with polluting component.

19. Method according to one of claims 16 to 18, further comprising a step of cooling at least part of the capture agent loaded and / or not loaded with polluting component to a temperature greater than or equal to 5°C before bringing it into contact with the gas flow to be treated and / or the first gas flow treated.

20. Method according to one of claims 16 to 19, further comprising, before step a) of contacting, a step of cooling the gas flow to be treated.

21. Method according to one of claims 16 to 20, further comprising, before step c) of contacting, a step of cooling the first treated gas flow.

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