AU2015359022B2

AU2015359022B2 - Electrolysis plant and method for treating cell gases

Info

Publication number: AU2015359022B2
Application number: AU2015359022A
Authority: AU
Inventors: Guillaume Girault; Stéphane Petit
Original assignee: Rio Tinto Alcan International Ltd
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 2014-12-08
Filing date: 2015-12-02
Publication date: 2020-06-25
Anticipated expiration: 2035-12-02
Also published as: WO2016092357A1; EP3230496A4; FR3029537A1; EP3230496B1; CN107208290B; CA2969558C; RU2017123812A; RU2709660C2; RU2017123812A3; CA2969558A1; AU2015359022A1; DK201770366A1; CN107208290A; EP3230496A1; BR112017011044B1; FR3029537B1; BR112017011044A2; DK180039B1

Abstract

The invention relates to a plant (1000) comprising a series of cells (2) and a circuit for treating gases produced in the cells (2), which treatment circuit has a first treatment stage (100) for treating hydrogen fluoride, a second treatment stage (102) for treating sulphur dioxide, and an alumina supply circuit for supplying the first treatment stage (100) and the second treatment stage (102) with alumina. The plant (1000) further comprises a heat exchanger (142) positioned between the first treatment stage and the second treatment stage.

Description

ELECTROLYSIS PLANT AND METHOD FOR TREATING CELL GASES

The present invention relates to an aluminum reduction plant for the production of aluminum by electrolysis and a process for treating the pot gases generated during the electrolytic reaction in this aluminum reduction plant.

The production of aluminum by electrolysis enables aluminum metal to be manufactured from alumina. The electrolysis reaction is conventionally performed in an electrolytic cell. An aluminum reduction plant conventionally comprises one or more series of electrolytic cells. Each series comprises several hundred electrolytic cells electrically connected to each other in series. The electrolytic cells comprise a pot shell, a cathode placed at the 10 bottom of the pot shell, an electrolytic bath in which alumina is dissolved and an anode plunging inside the electrolytic bath.

According to the principle of electrolysis, a continuous electrical current, called an electrolysis current, flows between the anode and the cathode and the aluminum produced forms a layer of liquid aluminum.

Gases, called pot gases, are generated during the electrolysis process. These gases contain hydrogen fluoride (HF), sulfur dioxide (SO2), carbon dioxide (CO₂) and dust or solid particles including fluorine.

Pot gases are usually collected in the suction hoods positioned in the top part of the electrolytic cells. These hoods are interconnected by a duct system leading to a gas 20 treatment center (GTC).

Suction fans draw the pot gases from these hoods and lead them through the ductwork to the gas treatment center. These fans are used to maintain all the electrolytic cells at negative pressure thereby capturing the gases generated during the electrolysis reaction. This suction is also involved in cooling the internal components of the electrolytic cells.

Given the inadequate sealing of conventional hooded electrolytic cells, the suction flow of pot gases is relatively high and conventional aluminum reduction plants include one or more gas treatment centers per series of electrolytic cells, in order to treat large volumes of gas.

However, this poses problems of costs and space.

In addition, gas treatment centers in existing aluminum reduction plants are typically adapted to treat only hydrogen fluoride, without treating sulfur dioxide or carbon dioxide also present in the pot gases that are released into the environment.

There are gas treatment centers designed to treat hydrogen fluoride and sulfur dioxide.

This is known, for example, from patent document WO96/15846 and patent document

WO2013/093268.

In patent document WO96/15846, the gas treatment center has two treatment stages: a first treatment stage comprising a single reactor for treating hydrogen fluoride and a second treatment stage comprising a single reactor for treating sulfur dioxide.

However, in document WO96/15846, the reactor forming the first treatment stage is insufficient to treat pot gases containing a high concentration of hydrogen fluoride and sulfur dioxide. Furthermore, this reactor is dimensioned to treat the pot gases of a whole series of electrolytic cells. This reactor is therefore cumbersome. This means that this reactor has to be kept away from the electrolytic cells. Pot gases therefore travel a significant distance between leaving the electrolytic cells and this reactor, during which these pot gases lose their heat The alumina, which is has become charged with fluorine in the reactor, is introduced into the electrolytic cells and for this purpose travels a substantial distance from leaving the reactor to being introduced into the electrolytic cells. Transportation of alumina from the reactor to the electrolytic cells therefore takes a considerable time, of the order of several hours to several days, including an intermediate storage phase in a silo, and is achieved by means of ducts swept by air at room temperature through which the alumina is transported. Also, the alumina has returned to a relatively low temperature close to ambient temperature before being introduced into the electrolytic cells, despite the transfer of heat produced in the reactor from the tank gases to the alumina.

Patent document WO2013/093268 discloses a plurality of reactors for treating hydrogen fluoride and sulfur dioxide contained in the pot gases. These reactors are arranged in series relative to each other. This helps reduce the cost and clutter and spatial requirements.

However, this serial arrangement has a limit in the treatment capacity of pot gases containing a high concentration of hydrogen fluoride and sulfur dioxide. In addition, the serial arrangement means that these reactors have to be positioned next to each other; these reactors therefore form an indivisible whole, and because of this, are necessarily located relatively far away from the electrolytic cells.

It will also be noted that some aluminum reduction plants of prior art include gas treatment centers equipped with a wet scrubber for treating the sulfur dioxide

However, these wet scrubbers are expensive.

2015359022 09 Jun 2017

These disadvantages of aluminum reduction plants of prior art are accentuated by the fact that the electrolytic cells tend to have emitted pot gas characteristics that differ substantially from what existed before. The electrolytic cells being developed by the applicant tend to improve the overall sealing of the electrolytic cells giving a significant reduction in pot gas suction flow rates. These flows tend to drop from a value typically between 70,000 and 120,000 Nm /tAl (Nm per ton of aluminum produced) for existing pot technologies, to a value between 15,000 and 50,000 Nm³/tAl.

This significant drop in the suction flow rate results in a significant increase in concentrations of hydrogen fluoride and sulfur dioxide inside the electrolytic cells. For hydrogen fluoride, these concentrations move from around [200-400] mg/Nm to around [1000-2000] mg/Nm³.

The significant decrease of the suction flow is also accompanied by a significant rise in temperature of the pot gases. The temperature of the pot gases tends to rise to about 130°C for pre-existing electrolytic cells to over 200°C, 300°C or even 400°C for the new generation of electrolytic cells.

These significant increases in temperature and concentrations of hydrogen fluoride and sulfur dioxide affect the performance of gas treatment centers in existing aluminum reduction plants and imply a need for improved performance.

At the same time, environmental regulations are becoming stricter, implying that greater efficiency than today will be required, given the anticipated increase in concentrations of hydrogen fluoride, sulfur dioxide and carbon dioxide in the pot gases.

Finally, from an economic perspective of efficiency and with a view to preserving environmental resources, it is common to try to reduce the overall energy consumption of aluminum reduction plants, especially electrolytic cells and gas treatment centers.

In this context, the present invention aims to overcome one or more of the aforementioned drawbacks. The proposed aluminum reduction plant provides one or more of the following: more effective treatment of pot gases, lower (1309121 l_2):GMM:dah

3a

2015359022 09 Jun 2017 spatial requirements and improved energy efficiency, and a pot gas treatment process that allows more effective treatment of pot gases and improved energy efficiency.

According to an aspect of the present disclosure, there is provided an aluminum reduction plant for production of aluminum by electrolysis, the aluminum reduction plant comprising: a series of electrolytic cells; and a gas treatment circuit for treating pot gases generated by the series of electrolytic cells during an electrolysis reaction, the gas treatment circuit comprising: a first treatment stage configured to treat hydrogen fluoride in the pot gases, a second treatment stage configured to treat sulfur dioxide in the pot gases, an alumina supply circuit configured to supply alumina to the first treatment stage and the second treatment stage, and a heat exchanger placed between the first treatment stage and the second treatment stage to cool the pot gases after treatment of the pot gases in the first treatment stage and before treatment of the pot gases in the second treatment stage.

According to another aspect of the present disclosure, there is provided a treatment method for pot gases emitted by the series of electrolytic cells in the aluminum reduction plant as described above, the method comprising the steps of: treating hydrogen fluoride in the pot gases in the first treatment stage by adsorption onto the alumina; treating sulfur dioxide in the pot gases in the second treatment stage by adsorption onto the alumina; and cooling the pot gases with the heat exchanger after treatment of the pot gases in the first treatment stage and before treatment of the pot gases in the second treatment stage.

To this end, the present invention relates to an aluminum reduction plant for the production of aluminum by electrolysis, the aluminum reduction plant comprising a series of electrolytic cells and a gas treatment circuit for treating pot gases generated by the series of electrolytic cells during the electrolysis reaction, (1309121 l_2):GMM:dah the gas treatment circuit comprising a first treatment stage configured to treat the hydrogen fluoride in the pot gases, a second treatment stage configured to treat the sulfur dioxide in the pot gases and an alumina supply circuit configured to supply alumina to the first treatment stage and the second treatment stage, the aluminum reduction plant being characterized in that the aluminum reduction plant includes a heat exchanger placed between the first treatment stage and the second treatment stage.

In other words, the heat exchanger is placed downstream of the first treatment stage and upstream of the second treatment stage, upstream and downstream being understood as in relation to the direction of flow of the pot gases in the gas treatment circuit. In this way, the pot gases leaving the first treatment stage are cooled before being treated in the second treatment stage.

An advantage of using a heat exchanger before the second treatment stage is that the temperature of the pot gases can be lowered to a level compatible with the adsorption of sulfur dioxide on the alumina in the second treatment stage, for example to a temperature less than or equal to 100°C or less than or equal to 70°C, thereby improving the treatment efficiency of the sulfur dioxide.

The volume of gas to be treated in the second treatment stage is also minimized as is the cost associated with this treatment, which is very closely linked to the volume of gas to be treated.

The heat exchangers associated with the pot gases are conventionally arranged upstream of the gas treatment centers to treat the hydrogen fluoride. Such conventional positioning results from the desire, in the heat exchangers, to make the most of the heat contained in the pot gases before bringing these gases into contact with the alumina, and the need to lower the temperature of the pot gases so as to be able to use conventional filtration system filters that are not efficient when the gas temperature is high.

The applicant found that, on the contrary, positioning a heat exchanger after the first treatment stage and before the second stage treatment was particularly advantageous.

The passage of high-temperature gases, in particular above 150°C, in the first gas treatment stage, makes for good selectivity of the adsorption of fluorine on alumina as compared to the adsorption of sulfur on alumina. In this way, the sulfur is not adsorbed on the alumina in the first gas treatment stage and is not returned to the electrolytic cell with the fluorided alumina in the first gas treatment stage.

Also, heat transfer to preheat the alumina in the first gas treatment stage may be all the more effective when the gas temperature is high and lead to a direct enhancement of the energy derived from the gas heat, unlike the energy obtained through a heat exchanger, so that the solution of the present invention is particularly advantageous In addition, if necessary, when the reactor of the first treatment stage is configured to separate the pot gases and solid particles carried by the pot gases, for example when the reactor is a cyclone reactor, dust has already been removed from the gas flow passing through the heat exchanger, so that the heat exchanger does not become clogged as is the case with the conventional positioning.

In addition, the applicant observed that lowering the temperature of the gases before they move into the second gas treatment stage, in particular below 100°C and preferably below 70°C, facilitates the step involving desorption of the sulfur adsorbed on the alumina. Lowering the temperature of the sulfur-containing gases promotes adsorption by physical adsorption of sulfur dioxide on the alumina at the expense of the adsorption by chemical adsorption of the sulfur dioxide on alumina. However, desorption of physically adsorbed sulfur dioxide on alumina requires less energy than desorption of chemically adsorbed sulfur dioxide on alumina. Such lowering of the temperature, in addition to being enhanced by the energy resulting from the heat exchange, advantageously helps towards a reduction in the overall energy required to operate the gas treatment center and the aluminum reduction plant

Also, lowering the temperature of the gases before they pass into the second gas treatment stage ensures that the residual hydrogen fluoride remaining in the pot gases after treatment in the first treatment stage is completely adsorbed, the adsorption of hydrogen fluoride itself being more efficient when the temperature is lowered

In sum, it follows from the above that placing a heat exchanger downstream of the first treatment stage and upstream of the second treatment stage significantly improves the energy efficiency of the pot gas treatment and the performance of the pot gas treatment, so this feature makes the aluminum reduction plant suitable for the treatment of pot gases with high concentrations of hydrogen fluoride and sulfur dioxide.

The aluminum reduction plant gas treatment circuit is therefore particularly suited to recent type of electrolytic cells, i.e. those in which the temperature of the pot gases reaches high levels (greater than 200°C and more particularly between 300-400°C instead of 100-150°C for older generations of cells), with a low pot gas suction flow (between 15,000 and 50,000 Nm³/tAI instead of being typically between 70,000 and 120,000 Nm³/tAI for older generations of cells), and/or a high concentration of hydrogen fluoride, sulfur dioxide and dust inside these cells (for hydrogen fluoride equal to or greater than

1000-2000 mg/Nm³ instead of concentrations of approximately 200-400 mg/Nm³ for older generations of cells).

The heat exchanger is advantageously designed to reduce the temperature of the pot 5 gases from a temperature of 400°C at the inlet of the heat exchanger to a temperature of less than or equal to 100°C and preferably less than or equal to 70°C at the outlet of the heat exchanger.

In this way, the aluminum reduction plant according to the invention is effective for treating the pot gases generated by the latest electrolytic cells in which the gas temperature can 10 reach 200°C and even 300°C or even 400°C.

Advantageously, the second treatment stage includes a reactor configured to fix sulfur dioxide by adsorption onto alumina coming from the alumina feed circuit.

It will be noted that the second treatment stage may include a reactor configured to separate the sulfurized alumina, from the adsorption of sulfur dioxide on the fresh 15 alumina, from the pot gases from which sulfur dioxide has been removed. To this end, the reactor of the second treatment stage may include a bag filter.

In this way, as they leave the second treatment stage, the pot gases are scrubbed free of sulfur dioxide (and hydrogen fluoride, with the first treatment stage preceding the second treatment stage).

Advantageously, the first treatment stage may include a reactor configured so as to fix the hydrogen fluoride by adsorption onto the alumina coming from this alumina feed circuit.

According to a preferred embodiment, the first treatment stage comprises a plurality of reactors, each reactor being associated with a group of electrolytic cells, each group of electrolytic cells comprising one or more electrolytic cells from the series of electrolytic 25 cells, and each reactor comprises:

- a first inlet connected to the related group of electrolytic cells in order to collect the pot gases generated by said group of electrolytic cells,

- a second inlet connected to the alumina supply circuit to supply each reactor with alumina to fix the hydrogen fluoride in the pot gases by adsorption onto said alumina, 30 and

- a first outlet to route the pot gases treated in the first treatment stage to the second treatment stage.

Ί

In this way, the aluminum reduction plant according to the invention offers the advantage of treating the hydrogen fluoride in a plurality of parallel reactors, each reactor being assigned to the treatment of pot gases from a group of electrolytic cells .

This results in more effective treatment than in prior art, which makes it possible to treat 5 pot gases having high concentrations of hydrogen fluoride and sulfur dioxide.

This also results in a reduction in the size of these reactors, allowing significant space saving in the aluminum reduction plant. In the case of state of the art electrolytic cells, it also makes it possible to place these reactors near these electrolytic cells to take advantage of the high temperature of the pot gases.

According to a preferred embodiment, each reactor is configured to preheat the alumina that is to supply the group of electrolytic cells to which this reactor is associated, by heat transfer between the pot gases flowing in said reactor and said alumina and each reactor comprises a second outlet to supply the group of electrolytic cells with the alumina preheated within the reactor by the pot gases.

This feature has the advantage of significantly reducing the energy consumption of the electrolytic cells in each group of electrolytic cells because the energy provided by preheating the alumina that supplies the electrolytic cells is not taken from the electrolytic cells themselves, unlike in prior art, but from the pot gases flowing in the pot gas treatment circuit. For recent electrolytic cells, this allows a significant reduction in the 20 energy consumption of the electrolytic cell: about 50-100 kWh/tAI.

Preheating also makes it possible to dry the alumina, i.e. to eliminate part of the water contained in the alumina, thereby making it possible to reduce emissions of hydrogen fluoride to the pot, the water contained in the alumina being a significant source of the hydrogen fluoride emissions in contact with the electrolytic bath. It therefore creates a 25 virtuous cycle.

This feature is particularly advantageous when the electrolytic cells are recent electrolytic cells for which the temperature of the pot gases is particularly high (greater than 200°C and more particularly of the order of 300°C to 400°C). The highest temperature of the pot gases in the most recent electrolytic cells opens the way, in an aluminum reduction plant 30 according to the invention, towards a more economic usage of the significant amount of energy contained in these fumes (of the order of about 350kW/pot for the most recent electrolytic cells), whereas the energy of the pot gases is dissipated in prior art.

As the alumina is preheated in the reactors, it means that compact reactors can be used. More generally, it results in an aluminum reduction plant with lower spatial requirements.

According to a preferred embodiment, each reactor comprises an enclosure defining a volume in which both the pot gases from the related group of electrolytic cells and the alumina for supplying this group of electrolytic cells flow, the enclosure being configured to allow heat transfer by direct contact between the pot gases and the alumina for supplying the related group of electrolytic cells.

This has the advantage of treating the hydrogen fluoride contained in the pot gases by means of the alumina which is designed to feed the electrolytic cells, at the same time as performing the heat transfer. Also, the fluorine released by this group of cells as gaseous hydrogen fluoride is returned to the same group of cells, as fluorinated alumina, which 10 gives better control the amount of fluoride in the electrolytic bath.

The alumina supplying each group of electrolytic cells is therefore also the fluorinated alumina resulting from the adsorption of hydrogen fluoride on the alumina within the reactor associated with this group of electrolytic cells. This means that the alumina used for the gas treatment circuit is also the alumina used for the electrolytic reaction.

The alumina for supplying the electrolytic cells previously passes through the reactors of the first treatment stage: this alumina enters via the second reactor inlet, adsorbs the fluorine in the reactors and exits the reactor via the second outlet towards the electrolytic cells.

This results in lower operating costs because the aluminum reduction plant is in this way 20 more economical in terms of raw materials.

This also results in a compact aluminum reduction plant because the alumina feed circuit for the electrolytic cells is also that of the reactors of the first treatment stage.

According to a preferred embodiment, each reactor of the first treatment stage is configured to separate the pot gases, the solid particles transported by the pot gases, and 25 the fluorided alumina resulting from the adsorption of fluorine on the alumina injected into the reactor via the second inlet.

This feature is particularly advantageous when used in combination with the feature according to which each reactor is configured to preheat the alumina for supplying the group of electrolytic cells to which this reactor is associated, by heat transfer, in particular 30 by direct contact between the pot gases flowing through the reactor and the alumina.

This feature makes it possible to have reactors providing several functions, and thereby benefit from compact equipment. This gives significant space savings in the aluminum reduction plant according to the invention.

According to a preferred embodiment, the reactors for the first treatment stage are cyclonic reactors.

This feature offers the advantage of compact reactors that can therefore be placed as close as possible to the electrolytic cells making it possible to separate the pot gases, the 5 solid particles carried by these pot gases, and the fluorinated alumina from the fluorine adsorption on the alumina injected into the reactor via the second inlet.

According to a preferred embodiment, each reactor is arranged at least 40 meters, especially less than 20 meters, and preferably less than 10 meters from each electrolytic cell of the group of electrolytic cells with which this reactor is associated.

This reduces heat loss: the closer the reactor within which the heat transfer takes place is to the electrolytic cells with which this reactor is associated, the less time the pot gases and preheated alumina have to cool, and therefore the higher the heat exchange efficiency.

According to a preferred embodiment, each group of electrolytic cells includes a maximum 15 of four, preferably a maximum of three, electrolytic cells from the series of electrolytic cells.

One advantage of this feature is an optimized compromise between a minimum number of reactors in order to reduce costs, and effective treatment and a location close to the electrolytic cells of each reactor to reduce the consumption of the electrolytic cells.

According to a preferred embodiment, each group of electrolytic cells comprises several electrolytic cells and the electrolytic cells from each group of electrolytic cells are adjacent electrolytic cells from the series of electrolytic cells.

This feature ensures that each reactor is located as close as possible to the nearest electrolytic cells with which this reactor is associated, which makes it possible to preserve 25 as much as possible, and take advantage of, the heat of the pot gases circulating in this reactor to preheat alumina to feed the electrolytic cells.

According to one preferred embodiment, each group of electrolytic cells comprises a single electrolytic cell.

One advantage individualizing the association between electrolytic cells and reactors is 30 individual recycling of the fluorine emitted by each electrolytic cell, which allows optimized operation of the electrolytic cell.

The fluorine leaving the electrolytic cell in the form of hydrogen fluoride returned directly to the electrolytic cell as fluorinated alumina. The amount of fluorine lost individually by each electrolytic cell is substantially equal to the amount of fluorine returning to each of these electrolytic cells, ensuring optimal balancing of the fluorine flows and simplifying the control methods for feeding the electrolytic cells.

According to a preferred embodiment, the gas treatment circuit comprises an alumina feed circuit configured to supply alumina to the second treatment stage, and the second treatment stage comprises a reactor configured to fix sulfur dioxide by adsorption onto alumina from this alumina feed circuit.

This feature makes it possible to treat the sulfur dioxide present in the pot gases without resorting to the use of a wet scrubber as in prior art, thereby substantially reducing costs. The first treatment stage allowed the concentration of hydrogen fluoride in the gases to be sufficiently lowered for the adsorption of sulfur dioxide on the alumina in the second treatment stage to allow effective treatment of the sulfur dioxide The preferential adsorption of hydrogen fluoride on the alumina in relation to the adsorption of sulfur dioxide on the alumina renders the treatment of sulfur dioxide inefficient if the hydrogen fluoride concentration is too high.

This feature also improves the efficiency of the treatment of the pot gases, as the second treatment stage, by bringing the pot gases into contact with fresh alumina, in this way makes it possible firstly to fix the residual hydrogen fluoride remaining in the pot gases after treatment in the first stage treatment reactors, and secondly to fix a significant part of the sulfur dioxide. Hydrogen fluoride is treated in two successive stages for greater efficiency.

According to a preferred embodiment, the aluminum reduction plant comprises a desorption device configured to allow the desorption of sulfur dioxide fixed onto the alumina, the desorption device comprising an inlet connected to the second treatment stage so as to collect the sulfurized alumina resulting from treatment of the sulfur dioxide in the second treatment stage by adsorption of the sulfur dioxide on the alumina, and an outlet connected to the second inlet of the first treatment stage reactors, so that the alumina feeding the first treatment stage reactors is the desulfurized alumina coming from the desorption device.

An advantage of this feature is that the alumina used for treating the gases, both sulfur dioxide and hydrogen fluoride, opens into the electrolytic cells in order to feed the electrolysis reaction within the electrolytic cells. This gives savings in raw materials and limits spatial requirements. This desorption treatment may advantageously be carried out by means of a thermal method involving exposure of the sulfurized alumina to temperatures that allow desorption, using the heat recovered from the pot gases.

Moreover, by returning to the first treatment stage, through which the pot gases are passing, still at high temperature as they directly leave the electrolytic cell(s), the alumina is dried after desorption, which lowers hydrogen fluoride emissions as part of a virtuous cycle, as previously described.

This is particularly advantageous when the water vapor is used as a desorption fluid in the desorption device.

It will be noted that the desorption device is configured so that only the sulfur dioxide is desorbed, but not the residual fluorine adsorbed in the second treatment stage.

Advantageously, the energy recovered in the heat exchanger is used to heat a desorption 10 fluid designed to be injected into the desorption device.

According to a preferred embodiment, the heat exchanger comprises a heat transfer fluid circulation circuit configured to reduce the temperature of the pot gases leaving the reactors of the first treatment stage by heating the heat transfer fluid circulating in this heat transfer fluid circulation circuit, and the heat transfer fluid circulation circuit is 15 configured to transfer heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device.

This feature will also enhance the still high temperature of the pot gases leaving the first treatment stage. This increases the overall energy efficiency.

The heat transfer fluid is, for example, water.

Preferably, the desorption fluid is water vapor.

This allows efficient desorption at lower cost. In addition, a by-product of desorption is sulfuric acid, which is readily recoverable.

Alternatively, the desorption fluid to be injected into the desorption device may be a hot gas or a mixture of hot gas, such as nitrogen. It will be noted that hot gas is taken to mean 25 gas whose temperature is at least equal to 350°, and preferably at least 400°. The temperature of the hot gas is in fact designed to cause the desorption of sulfur dioxide; this temperature is for example in the range [400-700]°C or even [400-1000] C.

Alternatively, the heat exchanger includes a heat transfer fluid circulation circuit configured to reduce the temperature of the pot gases leaving the reactors of the first 30 treatment stage by heating the heat transfer fluid flowing in this heat transfer fluid circulation circuit, and the heat transfer fluid is used directly as desorption fluid to be injected into the desorption device.

This heat transfer and desorption fluid is preferably liquid water and/or steam.

According to a preferred embodiment, the aluminum reduction plant comprises a treatment unit configured to treat the desorption fluid used to desorb the sulfur dioxide present in the sulfurized alumina.

This limits the environmental impact of desorption.

Desorbing in this case means separating the sulfur dioxide from the alumina on which the sulfur dioxide was originally adsorbed.

According to a preferred embodiment, the gas treatment circuit includes a moistening device upstream of the second treatment stage.

This feature offers the advantage of improving the treatment efficiency of the sulfur dioxide in the second treatment stage.

According to a preferred embodiment, the gas treatment circuit comprises a third treatment stage configured to treat the carbon dioxide present in the pot gases.

An advantage of this feature is to reduce the impact of the production of aluminum by electrolysis on the environment.

The invention also relates to a treatment method for pot gases emitted by the electrolytic cells of a series of electrolytic cells in an aluminum reduction plant, especially an aluminum reduction plant having the above characteristics, the process comprising the steps of:

- treating hydrogen fluoride in the pot gases in the first treatment stage,

- treating sulfur dioxide in the pot gases in the second treatment stage, the method being characterized in that it comprises a step of: cooling the pot gases with a heat exchanger after treatment of the pot gases in the first treatment stage and before treatment of the pot gases in the second treatment stage.

This method gives improved treatment performance and energy efficiency.

An advantage is to reduce the temperature of the pot gases to a level compatible with the adsorption of sulfur dioxide on the alumina in the second treatment stage, for example to a temperature less than or equal to 100°C, thereby improving the treatment efficiency of the sulfur dioxide. This also offers the advantage of minimizing the volume of gas to be treated in the second treatment stage, the cost of which is very closely linked to the volume of gases to be treated.

The treatment of hydrogen fluoride in the first treatment stage may include the injection of alumina into this first treatment stage to fix the hydrogen fluoride by adsorption onto the injected alumina.

The treatment of sulfur dioxide in the second treatment stage may include the injection of alumina into this second treatment stage to fix the sulfur dioxide by adsorption onto the injected alumina.

According to a preferred embodiment, the step involving treating the hydrogen fluoride in the pot gases in the first treatment stage includes treatment of the hydrogen fluoride in parallel in a plurality of reactors, each reactor being associated with a group of electrolytic cells from the series of electrolytic cells, each group of electrolytic cells comprising one or more electrolytic cells from the series of electrolytic cells.

This method provides a more effective treatment of hydrogen fluoride, because of the treatment of hydrogen fluoride in parallel and by group of electrolytic cells. This method therefore allows pot gases having a high concentration of hydrogen fluoride to be treated.

According to a preferred embodiment, the method comprises the step of: preheating the alumina to feed the electrolytic cells of each group of electrolytic cells by heat transfer between said alumina and the pot gases collected by the reactor associated with the corresponding group of electrolytic cells.

This preheating reduces the specific energy consumption of the electrolytic cells, and also dries the alumina, thereby making it possible to reduce emissions of hydrogen fluoride to the cell, as explained above.

Preheating can be carried out within each of the reactors of the first treatment stage. Advantageously, the preheated alumina is introduced directly into the electrolytic cell without first passaging through a hopper or other storage equipment between the reactor of the first stage and the electrolytic cell.

According to a preferred embodiment, said heat transfer is achieved by direct contact between said alumina and said pot gases.

In other words, the alumina to feed the electrolytic cells is injected into the reactors of the first treatment stage.

This improves the efficiency of the heat transfer. Direct contact also permits the adsorption of fluorine on the alumina to feed the electrolytic reaction, i.e. using the same alumina both to treat the hydrogen fluoride and to feed the electrolysis reaction.

An advantage of this feature is therefore also to consume less in terms of quantity and type of raw materials, because the alumina used for treating the hydrogen fluoride is used to feed the electrolytic reaction in the electrolytic cells.

According to a preferred embodiment, the method comprises, in each reactor of the first treatment stage, the step of: separating the pot gases, the solid particles transported by the pot gases, and the fluorided alumina after adsorption of the fluorine on the alumina injected into each reactor.

This feature makes it possible to have reactors providing several functions, and thereby benefit from compact equipment. This gives significant space savings in the aluminum 5 reduction plant according to the invention.

According to a preferred embodiment, the step of treating the hydrogen fluoride in the pot gases in a first treatment stage comprises treating the hydrogen fluoride via one reactor per electrolytic cell.

The process therefore includes individualized treatment of electrolytic cells. An advantage 10 of this individualization is individual recycling of fluorine emitted by each electrolytic cell, which allows simplified control of the fluorine content in the cells and optimized operation of the electrolytic cell.

According to a preferred embodiment, the method comprises the step of: supplying the electrolytic cells exclusively with fluorided alumina resulting from the adsorption of 15 hydrogen fluoride on the alumina injected into the reactors of the first treatment stage.

One advantage of this feature is that the fluorine released by a group of cells as gaseous hydrogen fluoride is returned to the same group of cells, as fluorinated alumina, which gives better control the amount of fluoride in the electrolytic bath.

According to a preferred embodiment, the method comprises:

- fixing sulfur dioxide onto the alumina by adsorption in a reactor of the second treatment stage, so as, firstly, to obtain pot gases that are free of sulfur dioxide and, secondly, sulfurized alumina,

- separating the sulfur dioxide present in the sulfided alumina by desorption in a desorption device, so as to obtain desulfurized alumina,

- injecting the alumina desulfurized in the reactors of the first treatment stage so as to adsorb the fluorine present in the pot gases and obtain fluorinated alumina,

- route the fluorinated alumina from the reactors of the first treatment stage to the electrolytic cells in order to supply the electrolysis reaction with alumina.

One advantage is a saving in raw materials in the treatment of pot gases because the 30 alumina used to treat the pot gases is also the alumina used to feed the electrolytic cells.

The desulphurized alumina from the second treatment stage contains the residual fluorine from the gases which was not adsorbed in the first treatment stage, plus the fluorine adsorbed in the first treatment stage into which the desulfurized alumina is introduced.

According to a preferred embodiment, separation by desorption is performed by direct contact between said sulfurized alumina and a desorption fluid, said desorption fluid being preferably water vapor.

According to a preferred embodiment, the pot gas cooling step is performed by heat transfer between the pot gases and a heat transfer fluid, in order to heat the heat transfer fluid, the method further comprising a step of transferring heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device to separate the sulfur present in the sulfurized alumina by desorption.

In this way, the energy required for desorption is provided by the heat exchanger, which improves energy efficiency.

Preferably, the desorption fluid is water vapor.

According to another preferred embodiment, the pot gas cooling step is performed by heat transfer directly between the pot gases and a desorption fluid, in order to heat the desorption fluid to be injected into the desorption device to separate the sulfur present in the sulfurized alumina by desorption.

According to a preferred embodiment, the method comprises a step of: moistening the pot gases before the step of treating the sulfur dioxide present in the pot gases in the second treatment stage.

This feature offers the advantage of improving the treatment efficiency of the sulfur dioxide.

According to a preferred embodiment, the method comprises the step of: capturing the carbon dioxide present in the pot gases.

An advantage of this feature is to reduce the impact of the production of aluminum by electrolysis on the environment, and especially on greenhouse gas emissions.

The invention also relates to an aluminum reduction plant for the production of aluminum by electrolysis, this aluminum reduction plant comprising a series of electrolytic cells and a gas treatment circuit for treating pot gases generated by the series of electrolytic cells during the electrolysis reaction, the gas treatment circuit comprising a first treatment stage configured to treat the hydrogen fluoride in the pot gases, a second treatment stage configured to treat the sulfur dioxide in the pot gases and an alumina supply circuit configured to supply alumina to the first treatment stage, characterized in that the first treatment stage comprises a plurality of reactors, each reactor being associated with a group of electrolytic cells, each group of electrolytic cells comprising one or more electrolytic cells from the series of electrolytic cells, and each reactor comprising:

- a second inlet connected to the alumina supply circuit to supply each reactor with alumina to fix the hydrogen fluoride in the pot gases by adsorption onto said alumina, and

In this way, the aluminum reduction plant offers the advantage of treating the hydrogen fluoride in a plurality of parallel reactors, each reactor being assigned to the treatment of pot gases from a group of electrolytic cells .

This results in more effective treatment than in prior art, which makes it possible to treat pot gases having high concentrations of hydrogen fluoride and sulfur dioxide.

The second treatment stage includes a reactor configured to fix sulfur dioxide by adsorption onto alumina coming from the alumina feed circuit.

It will be noted that the second treatment stage may include a reactor configured to separate the sulfurized alumina, from the adsorption of sulfur dioxide on the fresh alumina, from the pot gases from which sulfur dioxide has been removed. To this end, the reactor of the second treatment stage may include a bag filter.

The aluminum reduction plant may also include some or all of the associated features and benefits that have been previously mentioned in connection with the aluminum reduction plant described above. In this way, the aluminum reduction plant may include the following advantageous features.

According to a preferred embodiment, each reactor of the first treatment stage is 20 configured to separate the pot gases, the solid particles transported by the pot gases, and the fluorided alumina resulting from the adsorption of fluorine on the alumina injected into the reactor via the second inlet.

According to a preferred embodiment, each group of electrolytic cells includes a maximum of four, preferably a maximum of three, electrolytic cells from the series of electrolytic 30 cells.

According to a preferred embodiment, the aluminum reduction plant comprises a desorption device designed to allow the desorption of sulfur dioxide fixed onto the alumina, the desorption device comprising an inlet connected to the second treatment stage so as to collect the sulfurized alumina resulting from treatment of the sulfur dioxide in the second treatment stage by adsorption of the sulfur dioxide on the alumina, and an outlet connected to the second inlet of the first treatment stage reactors, so that the alumina feeding the first treatment stage reactors is the desulfurized alumina coming from the desorption device.

According to a preferred embodiment, the gas treatment circuit includes a heat exchanger interposed between the first treatment stage and the second treatment stage.

An advantage of this feature is to reduce the temperature of the pot gases to a level compatible with the adsorption of sulfur dioxide on the alumina in the second treatment stage, for example to a temperature less than or equal to 100°C or less than or equal to 70°C, thereby improving the treatment efficiency of the sulfur dioxide.

This feature also offers the advantage of minimizing the volume of gas to be treated in the second treatment stage, the cost of which is very closely linked to the volume of gases to be treated.

The passage of high-temperature gases, in particular above 150°C, in the first gas treatment stage, makes for good selectivity of the adsorption of fluorine on alumina as compared to the adsorption of sulfur dioxide on alumina. In this way, sulfur dioxide is not adsorbed on the alumina in the first gas treatment stage and is not returned to the electrolytic cell with the fluorided alumina in the first gas treatment stage.

Also, the heat transfer discussed above for preheating the alumina in the first gas treatment stage is all the more effective when the gas temperature is high and leads to a direct enhancement of the energy derived from the gas heat, unlike the energy obtained through a heat exchanger, so that this solution is particularly advantageous

In addition, if necessary, when the reactor of the first treatment stage is configured to separate the pot gases and solid particles carried by the pot gases, for example when the reactor is a cyclone reactor, dust has already been removed from the gas flow passing through the heat exchanger, so that the heat exchanger does not become clogged as is the case with the conventional positioning.

In addition, the applicant observed that lowering the temperature of the gases before they move into the second gas treatment stage, in particular below 100°C and preferably below 70°C, facilitates the step involving desorption of the sulfur dioxide adsorbed on the alumina. Lowering the temperature of the sulfur-containing gases promotes adsorption by physical adsorption of sulfur dioxide on the alumina at the expense of the adsorption by chemical adsorption of the sulfur dioxide on alumina. However, desorption of physically adsorbed sulfur dioxide on alumina requires less energy than desorption of chemically adsorbed sulfur dioxide on alumina. Such lowering of the temperature, in addition to being enhanced by the energy resulting from the heat exchange, advantageously helps towards a reduction in the overall energy required to operate the gas treatment center and the aluminum reduction plant

The aluminum reduction plant gas treatment circuit is therefore particularly suited to recent type of electrolytic cells, i.e. those in which the temperature of the pot gases reaches high levels (greater than 200°C and more particularly between 300-400°C instead of 100-150°C for older generations of cells), with a low pot gas suction flow (between 15,000 and 50,000 Nm³/tAI instead of being typically between 70,000 and 120,000

Nm³/tAI for existing technologies used in older generations of cells), and/or a high concentration of hydrogen fluoride, sulfur dioxide and dust inside these cells (for hydrogen fluoride equal to or greater than 1000-2000 mg/Nm³ instead of concentrations of approximately 200-400 mg/Nm³ for older generations of cells).

In this way, the aluminum reduction plant is effective for treating the pot gases generated by the latest electrolytic cells in which the gas temperature can reach 200°C and even 10 300°C or even 400°C.

The heat exchanger is advantageously designed to reduce the temperature of the pot gases from a temperature of 400°C at the inlet of the heat exchanger to a temperature of less than or equal to 100°C and preferably less than or equal to 70°C at the outlet of the heat exchanger.

Advantageously, the energy recovered in the heat exchanger is used to heat a desorption fluid designed to be injected into the desorption device.

According to a preferred embodiment, the heat exchanger comprises a heat transfer fluid circulation circuit configured to reduce the temperature of the pot gases leaving the reactors of the first treatment stage by heating the heat transfer fluid circulating in this 20 heat transfer fluid circulation circuit, and the heat transfer fluid circulation circuit is configured to transfer heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device.

The heat transfer fluid is, for example, water.

Preferably, the desorption fluid is water vapor.

Alternatively, the desorption fluid to be injected into the desorption device may be a hot gas or a mixture of hot gas, such as nitrogen. It will be noted that hot gas is taken to mean gas whose temperature is at least equal to 350°, and preferably at least 400°. The temperature of the hot gas is in fact designed to cause the desorption of sulfur dioxide; this temperature is for example in the range [400-700]°C or even [400-1000] C.

Alternatively, the heat exchanger includes a heat transfer fluid circulation circuit configured to reduce the temperature of the pot gases leaving the reactors of the first treatment stage by heating the heat transfer fluid flowing in this heat transfer fluid circulation circuit, and the heat transfer fluid is used directly as desorption fluid to be injected into the desorption device.

The invention also relates to a treatment method for pot gases emitted by the electrolytic 10 cells of a series of electrolytic cells in an aluminum reduction plant, the process comprising the steps of:

- treating hydrogen fluoride in the pot gases in the first treatment stage,

- treating sulfur dioxide in the pot gases in the second treatment stage, the method being characterized in that it includes a step involving treating the hydrogen 15 fluoride in the pot gases in the first treatment stage comprises treating the hydrogen fluoride in parallel in a plurality of reactors, each reactor being associated with a group of electrolytic cells from the series of electrolytic cells, each group of electrolytic cells comprising one or more electrolytic cells from the series of electrolytic cells.

This method provides a more effective treatment of hydrogen fluoride, because of the 20 treatment of hydrogen fluoride in parallel and by group of electrolytic cells. This method therefore allows pot gases having a high concentration of hydrogen fluoride to be treated.

This method may also include some or all of the associated features and benefits that have been previously described in connection with the method described above. In this 30 way, this method may include the following advantageous features.

According to a preferred embodiment, the method comprises, in each reactor of the first treatment stage, the step of: separating the pot gases, the solid particles transported by the pot gases, and the fluorided alumina after adsorption of the fluorine on the alumina 10 injected into each reactor.

According to a preferred embodiment, the method comprises the step of: supplying the 15 electrolytic cells exclusively with fluorided alumina resulting from the adsorption of hydrogen fluoride on the alumina injected into the reactors of the first treatment stage.

According to a preferred embodiment, the method comprises:

- fixing sulfur dioxide onto the alumina by adsorption in a reactor of the second treatment stage, so as, firstly, to obtain pot gases that are free of sulfur dioxide and, 20 secondly, sulfurized alumina,

According to a preferred embodiment, the method comprises a step of: cooling the pot gases before the pot gases enter the second treatment stage.

An advantage of this feature is to reduce the temperature of the pot gases to a level 30 compatible with the adsorption of sulfur dioxide on the alumina in the second treatment stage, for example to a temperature less than or equal to 100°C, thereby improving the treatment efficiency of the sulfur dioxide. This feature also offers the advantage of minimizing the volume of gas to be treated in the second treatment stage, the cost of which is very closely linked to the volume of gases to be treated.

According to a preferred embodiment, the pot gas cooling step is performed by heat transfer between the pot gases and a heat transfer fluid, in order to heat the heat transfer 5 fluid, the method further comprising a step of transferring heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device to separate the sulfur present in the sulfurized alumina by desorption.

Preferably, the desorption fluid is water vapor.

According to another preferred embodiment, the pot gas cooling step is performed by heat 10 transfer directly between the pot gases and a desorption fluid, in order to heat the desorption fluid to be injected into the desorption device to separate the sulfur present in the sulfurized alumina by desorption.

According to a preferred embodiment, the method comprises a step of: moistening the pot gases before the step of treating the sulfur dioxide present in the pot gases in the second 15 treatment stage.

Other features and advantages of this invention will be clearly apparent from the following detailed description of an embodiment provided by way of a non-limiting example with 20 reference to the appended drawings, in which:

- figure 1 is a schematic view of an embodiment of an aluminum reduction plant according to the invention,

- figure 2 is a schematic sectional view of an electrolytic cell of prior art,

- figure 3 is a schematic view of a reactor of the first or second treatment stage of one embodiment of an aluminum reduction plant according to the invention.

Figure 1 shows a known aluminum reduction plant 1. Aluminum reduction plant 1 is designed for the production of aluminum by electrolysis and is more particularly an aluminum works.

Aluminum reduction plant 1 comprises a series of electrolytic cells 2, for the production of 30 aluminum by electrolysis. Series of electrolytic cells is taken to mean comprises a set of electrolytic cells electrically connected to each other in series. This series of electrolytic cells 2 is designed to be traversed by an electrolysis current of up to several hundred thousand amperes.

Figure 2 shows an example of an electrolytic cell 2, for the production of aluminum by electrolysis. As shown in figure 2, the electrolytic cells 2 conventionally comprise a pot shell 4, cathode blocks 6 arranged at the bottom of pot shell 4, the cathode blocks 6 being traversed by conductive bars designed to collect the electrolysis current to route it to a 5 subsequent electrolytic cell in the series, and anode blocks 8 partially immersed in an electrolytic bath 10, above the cathode blocks 6. A layer 12 of liquid aluminum, covering the cathode blocks, is formed as the reaction proceeds. Electrolytic cells 2 may further comprise a feed hopper 14 through which electrolytic cells 2 are supplied with alumina. A set of hoods 16 closes the pot shell 4 in order to limit heat losses from and pot gas 10 leakage generated during the electrolysis reaction.

Electrolytic cells 2 may be electrolytic cells of an earlier generation.

Preferably, electrolytic cells 2 are a recent generation of electrolytic cells. Aluminum reduction plant 1 is particularly suitable for electrolytic cells 2 of the latest generation. Electrolytic cells 2 of a recent type is understood to mean electrolytic cells having, in 15 normal operation, a high pot gas temperature (greater than 200°C, and more particularly between 300-400°C), low pot gas suction flow ([15000-50000] Nm³/tAI), and/or a high concentration of hydrogen fluoride, sulfur dioxide and dust inside these cells (equal to or greater than 1000-2000 mg/Nm³). The recent type of electrolytic cells may have high sealing performance, particularly in terms of their set of hoods, which explains the low 20 suction flow and its consequences, namely high temperature and concentration of pot gases. These recent electrolytic cells are for example those developed by the applicant.

As illustrated in Figure 1, the aluminum reduction plant 1 further comprises a gas treatment circuit, the gas treatment circuit being designed to treat pot gases generated by the series of electrolytic cells 2 during the electrolysis reaction.

In the following description, upstream and downstream are to be understood in relation to the direction of movement of the gases in the gas treatment circuit.

The gas treatment circuit comprises a first treatment stage 100 configured to treat the hydrogen fluoride in the pot gases, and a second treatment stage 102 configured to treat the sulfur dioxide in the pot gases.

The gas treatment circuit comprises an alumina feed circuit configured to feed alumina to the first treatment stage 100. In this way, treatment of the hydrogen fluoride in the pot gases is performed by adsorption of the fluorine on this alumina.

The first treatment stage 100 includes a plurality of reactors 104 in parallel. Each reactor 104 is associated with a group of electrolytic cells 2, i.e. with at least one electrolytic cell 2 35 of the series

Each reactor 104 is associated with a distinct group of electrolytic cells 2. In other words, each group of electrolytic cells 2 works in connection with a distinct reactor 104 from reactors 104.

As illustrated in figure 1 or figure 3, a reactor 104 from the first treatment stage 100 comprises:

- a first inlet 106 connected to the related group of electrolytic cells 2 in order to collect the pot gases generated by said group of electrolytic cells 2, during the electrolysis reaction,

- a second inlet 108 connected to the alumina supply circuit to supply each reactor 104 with alumina to fix the hydrogen fluoride in the pot gases by adsorption onto said alumina, and

- a first outlet 110 to route the pot gases treated in the first treatment stage 100 to the second treatment stage 102 as shown by the arrow 20.

Reactors 104 are arranged in series relative to each other. Therefore each reactor 104 treats only a portion of the pot gases generated by the series of electrolytic cells 2. More particularly, each reactor 104 treats only the pot gases from the group of electrolytic cells 2 with which this reactor 104 is associated, and each reactor 104 can therefore be arranged close to the associated group of electrolytic cells 2.

The use of a first treatment stage 100 comprising a plurality of reactors 104 in parallel associated with distinct groups of electrolytic cells 2 followed by a second treatment stage 102 makes it possible to obtain concentrations of hydrogen fluoride of the order of about [0.5-1.0] mg/Nm³ and of sulfur dioxide of less than 400mg/Nm³ at the outlet of the gas treatment circuit.

The gas treatment circuit may include an exhaust stack 105 at the outlet.

Each reactor 104 is advantageously configured to preheat the alumina to feed the group of electrolytic cells 2 with which each reactor 104 is associated.

This preheating is carried out in each reactor 104 by heat transfer between the pot gases circulating in each reactor 104 and the alumina for feeding the group of electrolytic cells 2 with which each reactor 104 is associated.

Each reactor 104 further comprises a second outlet 112 to feed the group of electrolytic cells 2 with alumina preheated within the reactor 104 by the pot gases, as shown by arrow 22 in figure 1.

Reactors 104 can therefore be used to preheat the alumina up to the temperature of the pot gases, i.e. in particular up to 200°c or even 300-400°C when the electrolytic cells used are electrolytic cells of recent type.

This significantly reduces the energy consumption of the electrolytic cells 2 because the energy provided by preheating the alumina that feeds the electrolytic cells 2 is not taken from the electrolytic cells 2 themselves, unlike in prior art, but from the pot gases flowing in the pot gas treatment circuit. For example, the specific consumption of electrolytic cell 2 can therefore be reduced by about 100kWh/tAI (100 kWh per ton of aluminum produced). Preheating the alumina by means of the pot gases enables maximum enhancement of the energy normally dissipated in prior art.

Preheating also makes it possible to dry the alumina, i.e. to eliminate part of the water contained in the alumina, thereby making it possible to reduce emissions of hydrogen fluoride to the pot, the water contained in the alumina being a significant source of the hydrogen fluoride emissions in contact with the electrolytic bath.

More particularly, each reactor 104 comprises an enclosure 114, shown in figure 3, defining a volume in which both the pot gases from the group of cells 2 associated with each reactor 104 and the alumina for feeding this group of electrolytic cells 2 circulate. Enclosure 114 is configured to permit heat transfer through direct contact between the pot gases and the alumina designed to feed the corresponding group of electrolytic cells 2.

In this way, the hydrogen fluoride contained in the pot gases is treated by means of the alumina which is designed to feed the electrolytic cells, at the same time as performing the heat transfer. The alumina supplying each group of electrolytic cells is therefore also the fluorinated alumina resulting from the adsorption of hydrogen fluoride on the alumina within the reactor associated with this group of electrolytic cells.

Fluoride released as gaseous hydrogen fluoride by each group of electrolytic cells 2 is returned to the same group of electrolytic cells 2 as fluorinated alumina, which allows better control the amount of fluorine in the electrolytic bath.

To optimize the performance of the preheating, reactors 104 are positioned close to the group of electrolytic cells 2 with which each reactor 104 is associated. Specifically, reactors 104 are positioned at the outlet of the electrolytic cell(s) 2 of the group of electrolytic cells 2 with which each reactor 104 is associated.

Each reactor 104 is arranged at least 40 meters, especially less than 20 meters, and preferably less than 10 meters from each electrolytic cell 2 of the group of electrolytic cells 2 with which this reactor 104 is associated.

The closer reactor 104 within which the heat transfer takes place is to the electrolytic cells with which reactor 104 is associated, the less time the pot gases and the preheated alumina have to cool between cells 2 and reactor 104 (for the pot gases) and between reactor 104 and the electrolytic cells 2 (for the fluorinated alumina). The efficiency is thereby improved.

Advantageously, each reactor 104 is configured to separate the pot gases, the solid particles transported by the pot gases, and the fluorided alumina resulting from the adsorption of fluorine on the alumina injected into the reactor via the second inlet 108.

In particular, reactor 104 is adapted to separate pot gases scrubbed free of hydrogen 10 fluoride, alumina and dust with high efficiency, typically greater than 90%, to prevent the risk of deposits.

Reactors 104 may include for this purpose bag filters to filter the dust.

Reactors 104 may be standard Venturi reactors.

Preferably, reactors 104 are cyclonic reactors, as illustrated in figure 3. This provides 15 efficient filtration, while giving compact reactors 104, which makes it possible firstly to place the reactors 104 closer to the electrolytic cells 2 to improve preheating, and secondly to reduce the space required in the aluminum reduction plant 1.

Arrow 24 shows the intake of alumina (preferably desulfurized as will be described below) inside reactor 104 via the second inlet 108 The alumina is dispersed by a plurality of 20 nozzles 116 so as to form a shower of particles.

Arrow 26 shows the intake, through the first inlet 106, of pot gases generated by the group of electrolytic cells 2 associated with each reactor 104.

Arrow 20 shows the removal of the pot gases scrubbed clean of hydrogen fluoride, via the first outlet 110, and routed toward the second treatment stage 102.

The first inlet 108 is here placed at a height lower than the first outlet 110. In this way, within the enclosure 114 of reactors 104, pot gases flow upwardly, against the flow of the alumina falling by gravity to the second outlet 112 from the array of nozzles 116 placed at the top of reactors 104.

Enclosure 114 is shaped to generate a cyclone separating dust from pot gas. Enclosure 30 114 includes a funnel-shaped wall 118 leading to the second outlet 112.

At the second outlet 112, arrow 28 shows the outlet fluorinated alumina and dust. Fluorided alumina is injected into the associated electrolytic cells 2 to feed the electrolysis reaction.

It will be noted that each group of electrolytic cells 2 includes a maximum of four, preferably a maximum of three, electrolytic cells 2 from the series of electrolytic cells. The first treatment stage 100 therefore comprises a reactor 104 for up to four or three electrolytic cells 2.

This provides an advantageous compromise between a minimum number of reactors 104 in order to reduce costs, and effective treatment and a location close to the electrolytic cells of each reactor to reduce the consumption of the electrolytic cells.

In the case where each group of electrolytic cells 2 comprises a plurality of electrolytic cells 2, and to allow reactors 104 to be positioned as close as possible to the electrolytic cells 2 with which these reactors 104 are associated, the electrolytic cells 2 from each group of electrolytic cells 2 are adjacent electrolytic cells from the series of electrolytic cells 2.

According to one preferred embodiment, each group of electrolytic cells 2 comprises a single electrolytic cell 2. In other words, aluminum reduction plant 1 comprises as many reactors 104 as electrolytic cells 2, and each reactor 104 is associated with a separate electrolytic cell 2

This allows individual recycling of the fluorine emitted by each electrolytic cell 2, which allows optimized operation of each electrolytic cell 2.

As illustrated in figure 1, the gas treatment circuit comprises an alumina feed circuit configured to feed alumina to the second treatment stage 102. In this way, the hydrogen fluoride in the pot gases is treated by adsorption of the sulfur dioxide on this alumina.

To this end, the second treatment stage 102 includes a reactor 120 configured to fix sulfur dioxide by adsorption onto alumina coming from the alumina feed circuit.

The second treatment stage 102, and more exactly the reactor 120, includes a first inlet 122 connected to the first treatment stage 100 to let the pot gases that have previously been treated in the first treatment stage 100 into the second treatment stage 102, a second inlet 124 connected to the alumina feed circuit to feed the second treatment stage 102 with alumina to fix sulfur dioxide by adsorption onto said alumina, a first outlet 126 to discharge the pot gases treated in the second treatment stage 102, i.e. the pot gases scrubbed free of hydrogen fluoride and sulfur dioxide, and a second outlet 128 discharge the sulfurized alumina from the adsorption of sulfur dioxide on the alumina fed into the second treatment stage 102.

The second treatment stage 102 is subsequent to the first treatment stage 100, i.e., placed downstream of the first treatment stage 100. Pot gases necessarily pass through the reactor 104 of the first treatment stage 100 before they reach the second treatment stage 102.

The first treatment stage 100, because of its plurality of reactors 104 in parallel, allowed the concentration of hydrogen fluoride in the gases to be sufficiently lowered for the 5 adsorption of sulfur dioxide on the alumina to allow effective treatment of the sulfur dioxide in the second treatment stage 102. The preferential adsorption of hydrogen fluoride on the alumina in relation to the adsorption of sulfur dioxide on the alumina renders the treatment of sulfur dioxide inefficient if the hydrogen fluoride concentration is too high.

Reactor 120 may advantageously be configured to separate the sulfurized alumina, from 10 the adsorption of sulfur dioxide on the fresh alumina, from the pot gases from which sulfur dioxide has been removed. To this end, the reactor of the second treatment stage 102 may be a cyclone reactor, like reactor 104 in figure 3

Preferably, the second treatment stage 102 includes a single reactor 120, or a plurality of reactors 120 arranged side by side, providing an advantage in terms of compactness.

Arrow 24 shows the intake of alumina inside reactor 120 via the second inlet 124 The alumina is dispersed by a plurality of nozzles 130 so as to form a shower of particles.

Arrow 26 shows the intake via the first inlet 122, of pot gases having previously been treated by the first treatment stage 100.

Arrow 20 shows removal of pot gases scrubbed free of sulfur dioxide and hydrogen 20 fluoride, via the first outlet 126

The first inlet 122 is placed at a height lower than the first outlet 126. In this way, within an enclosure 132 of reactor 120, pot gases flow upwardly, against the flow of the alumina falling by gravity to the second outlet 128 from the array of nozzles 130 placed at the top of reactor 120.

Enclosure 132 is shaped to generate a cyclone separating sulfurized alumina from pot gas. Enclosure 132 includes a funnel-shaped wall 134 leading to the second outlet 128.

At the second outlet 128, arrow 28 shows the outlet of the sulfurized alumina resulting from the adsorption of sulfur dioxide on the alumina fed into the second treatment stage 102.

The aluminum reduction plant 1, according to a preferred embodiment comprises a desorption device 136. The desorption device 136 is configured to allow the desorption of sulfur fixed on the sulfurized alumina leaving the second treatment stage 102.

The desorption device 136 includes an inlet 138 connected to the second treatment stage

102 so as to collect the sulfurized alumina resulting from treatment of the sulfur dioxide in the second stage 102 of the adsorption treatment of sulfur dioxide on the alumina, and an outlet 140 connected to the second inlet 108 of reactors 104 of the first treatment stage

100.

In this way, the alumina feeding the reactor 104 of the first treatment stage is desulfurized alumina from the desorption device 136.

The desorption device 136 is adapted to desorb at least 80% of the sulfur dioxide adsorbed by the alumina in the second treatment stage 102.

The desorption device 136 is configured so that only the sulfur dioxide is desorbed, but not the residual fluorine that may have been adsorbed in the second treatment stage 102.

Preferably when the aluminum reduction plant 1 includes electrolytic cells 2 of a recent type and therefore the temperature inside reactors 104 is high (greater than 200°C and more particularly between 300-400°C), the gas treatment circuit may further comprise a 15 humidification device 121, arranged between the desorption device 136 and the reactors 104 to humidify the desulfurized alumina coming from the desorption device 136 before this alumina enters reactors 104 of the first treatment stage 100.

The alumina circulation circuit of the aluminum reduction plant 1 serves both to treat the pot gases and to feed electrolytic cells 2 with alumina. This alumina feed circuit 20 comprises:

- an optional means of storage for fresh alumina, such as a hopper 139 leading to the second inlet 124 of the second treatment stage 102,

- the second treatment stage 102,

- the desorption device 136,

- possibly a humidification device 121 to moisten the desulfurized alumina leaving the desorption device 136

- the reactors 104 of the first treatment stage,

- the electrolytic cells 2.

As illustrated in figure 1, fresh alumina is first injected into the second treatment stage 30 1 02, where it is used for the adsorption of sulfur dioxide. The alumina sulphide leaving the second treatment stage 102 is then routed to the desorption device 136. The desulfurized alumina leaving the desorption device 136 can be humidified, and is then distributed into the reactor 104 of the first treatment stage 100, where this alumina adsorbs the fluorine of the pot gases while warming up substantially in contact with the pot gases. Fluoridated and preheated alumina leaving the reactors 104 is finally routed to the group of electrolytic cell(s) 2 with which each reactor 104 is associated, in order to feed the electrolysis reaction.

According to a preferred embodiment, the gas treatment circuit 142 includes a heat exchanger interposed between the first treatment stage 100 and the second treatment stage 102.

The heat exchanger 142 is preferably designed to reduce the temperature of the pot gases from a temperature of 400°C at the inlet of the heat exchanger to a temperature of 10 less than or equal to 100°C and preferably less than or equal to 70°C at the outlet of the heat exchanger.

Preferably, the aluminum reduction plant 1 comprises a single heat exchanger 142, to improve compactness.

The heat exchanger 142 may advantageously include a heat transfer fluid circulation 15 circuit. This heat transfer fluid circulation circuit is configured to decrease the temperature of the pot gases leaving reactor 104 of the first treatment stage 100 by heating the heat transfer fluid circulating in the heat transfer fluid circulation circuit. The heat from the pot gases is transferred to the heat transfer fluid. In addition, the heat transfer fluid circulation circuit is configured to transfer the heat from the heat transfer fluid previously heated by 20 the pot gases to a desorption fluid, this desorption fluid being designed to be injected into the desorption device 136, via a second inlet 143 of the desorption device 136 to perform desorption.

This improves desorption efficiency while enhancing the thermal energy of the pot gases, therefore which also improves energy efficiency.

The heat transfer fluid is, for example, water.

Separation by desorption may preferably be carried out by direct contact between the alumina and the sulfurized desorption fluid.

Preferably, the desorption fluid is water vapor. In this way, the desorption step is carried out by exposing the sulfurized alumina to superheated steam, i.e. water vapor at a 30 temperature greater than or equal to 120°C.

Arrow 30 in figure 1 represents the intake of cold desorption fluid into the heat exchanger 142. Arrow 32 shows the desorption fluid, for example superheated steam, having been heated by the heat transfer fluid which has itself been heated by the pot gases flowing in the heat exchanger 142. As shown by arrow 32, the desorption fluid flows to the desorption device 136.

Alternatively, the desorption fluid to be injected into the desorption device 136 may be a hot gas or a mixture of hot gas, for example nitrogen. It will be noted that hot gas is taken 5 to mean gas whose temperature is at least equal to 350°, and preferably at least 400°.

The temperature of the hot gas is in fact adapted to cause the desorption of sulfur dioxide. This temperature is higher than the desorption temperature of the sulfur dioxide, i.e. higher than 200°C; this temperature is for example in the range [400-700]°C or even [4001000] C.

The aluminum reduction plant 1 comprises a treatment unit 144 configured to treat the desorption fluid used to desorb the sulfur dioxide present in the sulfurized alumina. To this end, the desorption device 136 includes an outlet 146 routing the desorption byproduct(s) to the treatment unit 144. For example, in the case of water vapor, the treatment unit 144 is advantageously a sulfuric acid collecting device, to collect the sulfuric acid resulting from the desorption by means of water vapor, in order to enhance this sulfuric acid

The gas treatment circuit advantageously comprises a humidification device 148 upstream of the second treatment stage 102, in order to improve the efficiency of the sulfur dioxide treatment in the second treatment stage 102.

As illustrated in figure 1, the gas treatment circuit preferably comprises a third treatment 20 stage 150. The third treatment stage 150 is configured to process the carbon dioxide present in the pot gases.

The third treatment stage 150 includes an inlet 152 connected to the first outlet 126 of the second treatment stage 102 to receive the pot gases that have been pretreated by the second treatment stage 102, and an outlet 154 to discharge the pot gases scrubbed free 25 of carbon dioxide, sulfur dioxide and hydrogen fluoride to the exhaust stack 105.

The third treatment stage 150 comprises for example an absorption tower.

A treatment method for pot gases emitted by the electrolytic cells 2 of a series of electrolytic cells in an aluminum reduction plant 1, especially the aluminum reduction plant 1 having some or all of the features described above, is described below. Identical 30 elements are therefore designated hereafter by the same reference numbers.

The method comprises the steps of:

- treating hydrogen fluoride in the pot gases in the first treatment stage 100, and

- treating sulfur dioxide in the pot gases in the second treatment stage 102.

In addition, the step involving treating the hydrogen fluoride in the pot gases in the first treatment stage 100 comprises treating the hydrogen fluoride in parallel in a plurality of reactors 104, each reactor being associated with a group of electrolytic cells 2 from the series of electrolytic cells, each group of electrolytic cells 2 comprising one or more electrolytic cells 2 from the series of electrolytic cells.

Each reactor 104 is associated with a group of electrolytic cells 2, i.e. with at least one electrolytic cell 2 of the series

Each reactor 104 is associated with a distinct group of electrolytic cells 2. In other words, each group of electrolytic cells 2 works in connection with a distinct reactor 104 from io reactors 104.

The method may additionally comprise the step of: preheating the alumina to feed the electrolytic cells 2 of each group of electrolytic cells 2 by heat transfer between this alumina and the pot gases collected by the reactor 104 associated with the corresponding group of electrolytic cells 2.

Preheating can be carried out within each of the reactors 104 of the first treatment stage 100.

It will be noted that the heat transfer is preferably accomplished by direct contact between the alumina for feeding the electrolytic cells 2 and the pot gases flowing within the reactor 104.

In this way, the alumina for supplying the electrolytic cells 2 is injected into the reactor 104 of the first treatment stage 100, where this alumina adsorbs the fluorine in the pot gases.

The method advantageously comprises, in each reactor 104 of the first treatment stage, the step of: separating the pot gases, the solid particles transported by the pot gases, and the fluorided alumina after adsorption of the fluorine on the alumina injected into each reactor 104.

According to a preferred embodiment, the step of treating the hydrogen fluoride in the pot 30 gases in a first treatment stage 100 includes treating the hydrogen fluoride via one reactor 104 per electrolytic cell 2.

The process therefore includes individualized treatment of electrolytic cells 2. In other words, each group of electrolytic cells 2 includes a single electrolytic cell 2; there is one reactor 104 of the first treatment stage 100 per electrolytic cell 2.

The method may include the step of: feeding the electrolytic cells 2 exclusively with fluorided alumina resulting from the adsorption of hydrogen fluoride on alumina.

According to a preferred embodiment, the method comprises:

- fixing sulfur dioxide onto the alumina by adsorption in a reactor 120 of the second 5 treatment stage 102, so as, firstly, to obtain pot gases that are free of sulfur dioxide and, secondly, sulfurized alumina,

- separating the sulfur dioxide present in the sulfided alumina by desorption in a desorption device 136, so as to obtain desulfurized alumina,

- injecting the alumina desulfurized in the reactors 104 of the first treatment stage 100 10 so as to adsorb the fluorine present in the pot gases and obtain fluorinated alumina,

- route the fluorinated alumina from the reactors 104 of the first treatment stage 100 to the electrolytic cells 2 in order to supply the electrolysis reaction with alumina.

The method advantageously comprises the step of: cooling the pot gases before the pot gases enter the second treatment stage 102 between the first treatment stage 100 and the second treatment stage 102, via a heat exchanger 142.

This pot gas cooling step may be performed by heat transfer between the pot gases and a heat transfer fluid, in order to heat the heat transfer fluid, the method further comprising a 20 step of transferring heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device 136 to separate the sulfur present in the sulfurized alumina by desorption.

In this way, the energy required for desorption is provided by the heat exchanger 142, which improves energy efficiency.

Preferably, the desorption fluid is water vapor.

The method may include the step of: wetting the pot gases before the stage of treating the sulfur dioxide in the pot gases in the second treatment stage 102, particularly via a humidification device 148 positioned between the first treatment stage 100 and the second treatment stage 102 of treatment, possibly downstream of the heat exchanger 30 142.

The method may include the step of: wetting the desulfurized alumina designed to feed the reactors 104 of the first treatment stage 100. This wetting step may be performed by means of a humidifying device 121 positioned between the desorption device 136, downstream of the latter, and the reactors 104 of the first treatment stage 100.

Advantageously, the method comprises the step of: capturing the carbon dioxide present in the pot gases, especially in a third treatment stage 150 which may include an absorption tower.

The method may further include any other step mentioned in the detailed description above of the aluminum reduction plant 1.

The invention relates to a second aluminum reduction plant 1000, particularly an aluminum works, for the production of aluminum by electrolysis. Similar elements between the aluminum reduction plant 1 described above and the second aluminum reduction plant 1000 described below are designated by the same reference numbers.

Like aluminum reduction plant 1, the second aluminum reduction plant 1000 comprises a series of electrolytic cells 2, for the production of aluminum by electrolysis. This series of electrolytic cells 2 is designed to be traversed by an electrolysis current of up to several hundred thousand amperes.

Figure 2 shows an example of an electrolytic cell 2, for the production of aluminum by electrolysis, as described above.

Electrolytic cells 2 may be electrolytic cells of an earlier generation.

Preferably, electrolytic cells 2 are a recent generation of electrolytic cells, as previously described in connection with the aluminum reduction plant 1. These recent electrolytic cells are for example those developed by the applicant. Aluminum reduction plant 1000 is particularly suitable for electrolytic cells 2 of the latest generation.

Like aluminum reduction plant 1 described above, and as illustrated in figure 1, aluminum reduction plant 1000 further comprises a gas treatment circuit, the gas treatment circuit being designed to treat pot gases generated by the series of electrolytic cells 2 during the electrolysis reaction.

The feed circuit is further configured to supply the second treatment stage 102 with alumina and this second aluminum reduction plant 1000 comprises a heat exchanger 142 arranged between the first treatment stage 100 and the second treatment stage 102.

As described above in greater detail, placing a heat exchanger downstream of the first treatment stage 100 and upstream of the second treatment stage 102 significantly improves the performance of the pot gas treatment and the energy efficiency of this treatment.

The second aluminum reduction plant 1000 may also include some or all of the features and advantages of aluminum reduction plant 1 described above.

The first treatment stage 100 may include a plurality of reactors 104 in parallel. Each reactor 104 is associated with a group of electrolytic cells 2, i.e. one or more electrolytic cells 2 of the series

- a first inlet 106 connected to the related group of electrolytic cells 2 in order to collect the pot gases generated by said group of electrolytic cells 2,

The gas treatment circuit may include an exhaust stack 105 at the outlet.

This preheating is carried out in each reactor 104 by heat transfer between the pot gases 5 circulating in each reactor 104 and the alumina for feeding the group of electrolytic cells 2 with which each reactor 104 is associated.

Reactors 104 can therefore be used to preheat the alumina up to the temperature of the pot gases, i.e. in particular up to 200°c or even 300 - 400°C when the electrolytic cells used are electrolytic cells of recent type.

This significantly reduces the energy consumption of the electrolytic cells 2 because the energy provided by preheating the alumina that feeds the electrolytic cells 2 is not taken 15 from the electrolytic cells 2 themselves, unlike in prior art, but from the pot gases flowing in the pot gas treatment circuit. For example, the specific consumption of electrolytic cell 2 can therefore be reduced by about 100kWh/tAI (100 kWh per ton of aluminum produced). Preheating the alumina by means of the pot gases enables maximum enhancement of the energy normally dissipated in prior art.

More particularly, each reactor 104 comprises an enclosure 114, shown in figure 3, 25 defining a volume in which both the pot gases from the group of cells 2 associated with each reactor 104 and the alumina for feeding this group of electrolytic cells 2 circulate. Enclosure 114 is configured to permit heat transfer through direct contact between the pot gases and the alumina designed to feed the corresponding group of electrolytic cells 2.

In this way, the hydrogen fluoride contained in the pot gases is treated by means of the 30 alumina which is designed to feed the electrolytic cells, at the same time as performing the heat transfer. The alumina supplying each group of electrolytic cells is therefore also the fluorinated alumina resulting from the adsorption of hydrogen fluoride on the alumina within the reactor associated with this group of electrolytic cells.

To optimize the performance of the preheating, reactors 104 are positioned close to the 5 group of electrolytic cells 2 with which each reactor 104 is associated. Specifically, reactors 104 are positioned at the outlet of the electrolytic cell(s) 2 of the group of electrolytic cells 2 with which each reactor 104 is associated

Each reactor 104 is arranged at least 40 meters, especially less than 20 meters, and preferably less than 10 meters from each electrolytic cell 2 of the group of electrolytic cells 10 2 with which this reactor 104 is associated.

The closer reactor 104 within which the heat transfer takes place is to the electrolytic cells 2 with which reactor 104 is associated, the less time the pot gases and the preheated alumina have to cool between cells 2 and reactor 104 (for the pot gases) and between reactor 104 and the electrolytic cells 2 (for the fluorinated alumina). The efficiency is 15 thereby improved.

Advantageously, the ducts for transporting the alumina from the reactor to the electrolytic cells are heat lagged, particularly by surrounding the pipes with thermal insulation These . ducts do not advantageously include any hopper or other storage equipment.

Advantageously, each reactor 104 is configured to separate the pot gases, the solid 20 particles transported by the pot gases, and the fluorided alumina resulting from the adsorption of fluorine on the alumina injected into the reactor via the second inlet 108.

In particular, reactor 104 is adapted to separate pot gases scrubbed free of hydrogen fluoride, alumina and dust with high efficiency, typically greater than 90%, to prevent the risk of deposits.

Reactors 104 may include for this purpose bag filters to filter the dust.

Reactors 104 may be standard Venturi reactors.

Preferably, reactors 104 are cyclonic reactors, as illustrated in figure 3. This provides efficient filtration, while giving compact reactors 104, which makes it possible firstly to place the reactors 104 closer to the electrolytic cells 2 to improve preheating, and 30 secondly to reduce the space required in the second aluminum reduction plant 1000.

Arrow 24 shows the intake of alumina (preferably desulfurized as will be described below) inside reactor 104 via the second inlet 108 The alumina is dispersed by a plurality of nozzles 116 so as to form a shower of particles.

Enclosure 114 is shaped to generate a cyclone separating dust from pot gas. Enclosure 10 114 includes a funnel-shaped wall 118 leading to the second outlet 112.

At the second outlet 112, arrow 28 shows the outlet fluorinated alumina and dust. Fluorided alumina is then injected into the associated electrolytic cells 2 to feed the electrolysis reaction.

It will be noted that each group of electrolytic cells 2 includes a maximum of four, 15 preferably a maximum of three, electrolytic cells 2 from the series of electrolytic cells. The first treatment stage 100 therefore comprises a reactor 104 for up to four or three electrolytic cells 2.

This provides an advantageous compromise between a minimum number of reactors 104 in order to reduce costs, and effective treatment and a location close to the electrolytic 20 cells of each reactor to reduce the consumption of the electrolytic cells.

In the case where each group of electrolytic cells 2 comprises a plurality of electrolytic cells 2, and to allow reactors 104 to be positioned as close as possible to the electrolytic cells 2 with which these reactors 104 are associated, the electrolytic cells 2 from each group of electrolytic cells 2 are adjacent electrolytic cells from the series of electrolytic 25 cells 2.

According to one preferred embodiment, each group of electrolytic cells 2 comprises a single electrolytic cell 2. In other words, the second aluminum reduction plant 1000 comprises as many reactors 104 as electrolytic cells 2, and each reactor 104 is associated with a separate electrolytic cell 2

The second treatment stage 102, and more exactly the reactor 120, includes a first inlet 122 connected to the first treatment stage 100 to let the pot gases that have previously 5 been treated in the first treatment stage 100 into the second treatment stage 102, a second inlet 124 connected to the alumina feed circuit to feed the second treatment stage 102 with alumina to fix sulfur dioxide by adsorption onto said alumina, a first outlet 126 to discharge the pot gases treated in the second treatment stage 102, i.e. the pot gases scrubbed free of hydrogen fluoride and sulfur dioxide, and a second outlet 128 discharge 10 the sulfurized alumina from the adsorption of sulfur dioxide on the alumina fed into the second treatment stage 102.

The second treatment stage 102 is subsequent to the first treatment stage 100, i.e., placed downstream of the first treatment stage 100. Pot gases necessarily pass through the reactor 104 of the first treatment stage 100 before they reach the second treatment 15 stage 102.

The first treatment stage 100, because of its plurality of reactors 104 in parallel, allowed the concentration of hydrogen fluoride in the gases to be sufficiently lowered for the adsorption of sulfur dioxide on the alumina to allow effective treatment of the sulfur dioxide in the second treatment stage 102. The preferential adsorption of hydrogen fluoride on the 20 alumina in relation to the adsorption of sulfur dioxide on the alumina renders the treatment of sulfur dioxide inefficient if the hydrogen fluoride concentration is too high.

Reactor 120 may advantageously be configured to separate the sulfurized alumina, from the adsorption of sulfur dioxide on the fresh alumina, from the pot gases from which sulfur dioxide has been removed. To this end, the reactor of the second treatment stage 102 25 may be a cyclone reactor, like reactor 104 in figure 3

Arrow 20 shows removal of pot gases scrubbed free of sulfur dioxide and hydrogen fluoride, via the first outlet 126

The second aluminum reduction plant 1000, according to a preferred embodiment comprises a desorption device 136. The desorption device 136 is configured to allow the desorption of sulfur fixed on the sulfurized alumina leaving the second treatment stage 102.

The desorption device 136 includes an inlet 138 connected to the second treatment stage 15 102 so as to collect the sulfurized alumina resulting from treatment of the sulfur dioxide in the second stage 102 of the adsorption treatment of sulfur dioxide on the alumina, and an outlet 140 connected to the second inlet 108 of reactors 104 of the first treatment stage 100.

In this way, the alumina feeding the reactor 104 of the first treatment stage is desulfurized 20 alumina from the desorption device 136.

The desorption device 136 is configured so that only the sulfur dioxide is desorbed, but not the residual fluorine adsorbed in the second treatment stage 102.

Preferably when the second aluminum reduction plant 1000 includes electrolytic cells 2 of a recent type and therefore the temperature inside reactors 104 is high, the gas treatment circuit may further comprise a humidification device 121, arranged between the desorption device 136 and the reactors 104 to humidify the desulfurized alumina coming from the desorption device 136 before this alumina enters reactors 104 of the first treatment stage 30 100

The alumina circulation circuit of the second aluminum reduction plant 1000 serves both to treat the pot gases and to feed electrolytic cells 2 with alumina. This alumina feed circuit comprises:

- the second treatment stage 102,

- the desorption device 136,

- the reactors 104 of the first treatment stage,

- the electrolytic cells 2.

As illustrated in figure 1, fresh alumina is first injected into the second treatment stage 10 102, where it is used for the adsorption of sulfur dioxide. The alumina sulphide leaving the second treatment stage 102 is then routed to the desorption device 136. The desulfurized alumina leaving the desorption device 136 can be humidified, and is then distributed into the reactor 104 of the first treatment stage 100, where this alumina adsorbs the fluorine of the pot gases while warming up substantially in contact with the pot gases. Fluoridated 15 and preheated alumina leaving the reactors 104 is finally routed to the group of electrolytic cell(s) 2 with which each reactor 104 is associated, in order to feed the electrolysis reaction.

The heat exchanger 142 is preferably designed to reduce the temperature of the pot gases from a temperature of 400°C at the inlet of the heat exchanger to a temperature of 20 less than or equal to 100°C and preferably less than or equal to 70°C at the outlet of the heat exchanger.

Preferably, the second aluminum reduction plant 1000 comprises a single heat exchanger 142, to improve compactness.

The heat exchanger 142 may advantageously include a heat transfer fluid circulation 25 circuit. This heat transfer fluid circulation circuit is configured to decrease the temperature of the pot gases leaving reactor 104 of the first treatment stage 100 by heating the heat transfer fluid circulating in the heat transfer fluid circulation circuit. The heat from the pot gases is transferred to the heat transfer fluid. In addition, the heat transfer fluid circulation circuit is configured to transfer the heat from the heat transfer fluid previously heated by 30 the pot gases to a desorption fluid, this desorption fluid being designed to be injected into the desorption device 136, via a second inlet 143 of the desorption device 136 to perform desorption.

The heat transfer fluid is, for example, water.

Preferably, the desorption fluid is water vapor. In this way, the desorption step is carried out by exposing the sulfurized alumina to superheated steam, i.e. water vapor at a temperature greater than or equal to 120°C.

Alternatively, the desorption fluid to be injected into the desorption device 136 may be a hot gas or a mixture of hot gas, for example nitrogen. It will be noted that hot gas is taken to mean gas whose temperature is at least equal to 350°, and preferably at least 400°. The temperature of the hot gas is in fact adapted to cause the desorption of sulfur dioxide. This temperature is higher than the desorption temperature of the sulfur dioxide, i.e. higher than 200°C; this temperature is for example in the range [400-700]°C or even [4001000] C.

The second aluminum reduction plant 1000 comprises a treatment unit 144 configured to treat the desorption fluid used to desorb the sulfur dioxide present in the sulfurized alumina. To this end, the desorption device 136 includes an outlet 146 routing the desorption byproduct(s) to the treatment unit 144. For example, in the case of water vapor, the treatment unit 144 is advantageously a sulfuric acid collecting device, to collect the sulfuric acid resulting from the desorption by means of water vapor, in order to enhance this sulfuric acid

As illustrated in figure 1, the gas treatment circuit preferably comprises a third treatment stage 150. The third treatment stage 150 is configured to process the carbon dioxide present in the pot gases.

The third treatment stage 150 includes an inlet 152 connected to the first outlet 126 of the second treatment stage 102 to receive the pot gases that have been pretreated by the second treatment stage 102, and an outlet 154 to discharge the pot gases scrubbed free of carbon dioxide, sulfur dioxide and hydrogen fluoride to the exhaust stack 105.

The third treatment stage 150 comprises for example an absorption tower.

The invention also relates to a second treatment method for pot gases emitted by the 5 electrolytic cells 2 of a series of electrolytic cells in an aluminum reduction plant 1000, especially the second aluminum reduction plant 1000 having the above characteristics. Identical elements are therefore designated hereafter by the same reference numbers.

Like the method described above, this second method comprises the steps of:

- treating sulfur dioxide in the pot gases in the second treatment stage 102.

The second method is characterized in that the second method comprises the step of: cooling the pot gases with a heat exchanger after treatment of the pot gases in the first treatment stage and before treatment of the pot gases in the second treatment stage.

This cooling step can be performed by means of the heat exchanger 142.

This second method may further include some or all of the features and benefits of the method described above.

The step involving treating the hydrogen fluoride in the pot gases in the first treatment stage 100 may advantageously comprise treating the hydrogen fluoride in parallel in a plurality of reactors 104, each reactor being associated with a group of electrolytic cells 2 20 from the series of electrolytic cells, each group of electrolytic cells 2 comprising one or more electrolytic cells 2 from the series of electrolytic cells.

Each reactor 104 is associated with a distinct group of electrolytic cells 2. In other words, 25 each group of electrolytic cells 2 works in connection with a distinct reactor 104 from reactors 104.

The second method may additionally comprise the step of: preheating the alumina to feed the electrolytic cells 2 of each group of electrolytic cells 2 by heat transfer between this alumina and the pot gases collected by the reactor 104 associated with the corresponding 30 group of electrolytic cells 2.

Preheating can be carried out within each of the reactors 104 of the first treatment stage

100.

It will be noted that the heat transfer is preferably accomplished by direct contact between the alumina for feeding the electrolytic cells 2 and the pot gases flowing within the reactor 5 104.

The second method advantageously comprises, in each reactor 104 of the first treatment stage, the step of: separating the pot gases, the solid particles transported by the pot 10 gases, and the fluorided alumina resulting from adsorption of the fluorine on the alumina injected into each reactor 104.

According to a preferred embodiment, the step of treating the hydrogen fluoride in the pot gases in a first treatment stage 100 includes treating the hydrogen fluoride via one reactor 104 per electrolytic cell 2.

The second process therefore includes individualized treatment of electrolytic cells 2. In other words, each group of electrolytic cells 2 includes a single electrolytic cell 2; there is one reactor 104 of the first treatment stage 100 per electrolytic cell 2.

The second method may include the step of: feeding the electrolytic cells 2 exclusively with fluorided alumina resulting from the adsorption of hydrogen fluoride on alumina.

According to a preferred embodiment, the second method comprises:

- fixing sulfur dioxide onto the alumina by adsorption in a reactor 120 of the second treatment stage 102, so as, firstly, to obtain pot gases that are free of sulfur dioxide and, secondly, sulfurized alumina,

- separating the sulfur dioxide present in the sulfided alumina by desorption in a 25 desorption device 136, so as to obtain desulfurized alumina,

- injecting the alumina desulfurized in the reactors 104 of the first treatment stage 100 so as to adsorb the fluorine present in the pot gases and obtain fluorinated alumina,

The pot gas cooling step may be performed by heat transfer between the pot gases and a heat transfer fluid, in order to heat the heat transfer fluid, the method further comprising a step of transferring heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device 136 to separate the sulfur present in the sulfurized alumina by desorption.

Preferably, the desorption fluid is water vapor.

The second method may include the step of: wetting the pot gases before the stage of treating the sulfur dioxide in the pot gases in the second treatment stage 102, particularly 10 via a humidification device 148 positioned between the first treatment stage 100 and the second treatment stage 102 of treatment, downstream of the heat exchanger 142.

The second method may include the step of: wetting the desulfurized alumina designed to feed the reactors 104 of the first treatment stage 100. This wetting step may be performed by means of a humidifying device 121 positioned between the desorption device 136, 15 downstream of the latter, and the reactors 104 of the first treatment stage 100.

Advantageously, the second method comprises the step of: capturing the carbon dioxide present in the pot gases, especially in a third treatment stage 150 which may include an absorption tower.

The second method may further include any other step mentioned in the detailed 20 description above of the aluminum reduction plant 1000 according to the invention.

Of course the invention is not in any way limited to the embodiment described above, this embodiment only being provided by way of example. Modifications are possible, in particular from the point of view of the constitution of the various components, or through replacement by technical equivalents, without thereby going beyond the scope of 25 protection of the invention.

Claims

CLAIMS:

1. An aluminum reduction plant for production of aluminum by electrolysis, the aluminum reduction plant comprising:

a series of electrolytic cells; and a gas treatment circuit for treating pot gases generated by the series of electrolytic cells during an electrolysis reaction, the gas treatment circuit comprising:

a first treatment stage configured to treat hydrogen fluoride in the pot gases, a second treatment stage configured to treat sulfur dioxide in the pot gases, an alumina supply circuit configured to supply alumina to the first treatment stage and the second treatment stage, and a heat exchanger placed between the first treatment stage and the second treatment stage to cool the pot gases after treatment of the pot gases in the first treatment stage and before treatment of the pot gases in the second treatment stage.
2. The aluminum reduction plant according to claim 1, wherein the second treatment stage includes a second reactor configured to fix the sulfur dioxide by adsorption onto alumina coming from the alumina supply circuit.
3. The aluminum reduction plant according to either claim 1 or 2, wherein the first treatment stage includes a first reactor configured to fix the hydrogen fluoride by adsorption onto alumina coming from the alumina supply circuit.
4. The aluminum reduction plant according to any one of claims 1 to 3, wherein the first treatment stage comprises a plurality of the first reactors, each of the first reactors being associated with a group of the series of electrolytic cells, the group of electrolytic cells comprising one or more electrolytic cells from the series of electrolytic cells, and each of the first reactors comprising:

a first inlet connected to the related group of electrolytic cells in order to collect the pot gases generated by said group of electrolytic cells, a second inlet connected to the alumina supply circuit to receive alumina from the alumina supply circuit; and (1309121 l_2):GMM:dah

2015359022 09 Jun 2017 a first outlet to route the pot gases treated in the first treatment stage to the second treatment stage, wherein the first treatment stage fix the hydrogen fluoride in the pot gases by adsorption onto said alumina.
5. The aluminum reduction plant according to claim 4, wherein each of the first reactors is configured to preheat the alumina by heat transfer between the pot gases flowing in said first reactor and said alumina, and each of the first reactors comprises a second outlet to supply the group of electrolytic cells with the alumina preheated within the first reactor by the pot gases.
6. The aluminum reduction plant according to claim 5, wherein each of the first reactors comprises an enclosure defining a volume in which both the pot gases from the related group of electrolytic cells and the alumina supplying the group of electrolytic cells flow, the enclosure being configured to allow heat transfer by direct contact between the pot gases and the alumina supplying the related group of electrolytic cells.
7. The aluminum reduction plant according to any one of claims 4 to 6, wherein each of the first reactors of the first treatment stage is configured to separate the pot gases, solid particles transported by the pot gases, and the fluorided alumina resulting from the adsorption of the hydrogen fluoride on the alumina injected into the first reactor via the second inlet.
8. The aluminum reduction plant according to any one of claims 4 to 7, wherein the first reactors of the first treatment stage are cyclonic reactors.
9. The aluminum reduction plant according to any one of claims 4 to 8, wherein each of the first reactors is arranged less than 40 meters, from each electrolytic cell of the group of electrolytic cells with which the first reactor is associated.
10. The aluminum reduction plant according to any one of claims 4 to 9, wherein each of the groups of electrolytic cells includes a maximum of four electrolytic cells from the series of electrolytic cells.
11. The aluminum reduction plant according to any one of claims 4 to 10, wherein each group of electrolytic cells comprises several electrolytic cells and the electrolytic cells from each group of electrolytic cells are adjacent electrolytic cells from the series of electrolytic cells.

(1309121 l_2):GMM:dah

2015359022 09 Jun 2017
12. The aluminum reduction plant according to any one of claims 4 to 10, wherein each group of electrolytic cells includes a single electrolytic cell.
13. The aluminum reduction plant according to any one of claims 4 to 12, further comprising a desorption device configured to allow the desorption of sulfur dioxide fixed onto the alumina, the desorption device comprising an inlet connected to the second treatment stage so as to collect the sulfurized alumina resulting from treatment of the sulfur dioxide in the second treatment stage by adsorption of the sulfur dioxide on the alumina, and an outlet connected to the second inlet of the first reactors , so that the alumina feeding the first reactors is the desulfurized alumina coming from the desorption device.
14. The aluminum reduction plant according to claim 13, wherein the heat exchanger comprises a heat transfer fluid circulation circuit configured to reduce the temperature of the pot gases leaving the first reactors by heating the heat transfer fluid circulating in the heat transfer fluid circulation circuit, and the heat transfer fluid circulation circuit is configured to transfer heat from the heated heat transfer fluid to a desorption fluid to be injected into the desorption device.
15. The aluminum reduction plant according to any one of claims 1 to 14, wherein the gas treatment circuit includes a humidification device upstream of the second treatment stage.
16. A treatment method for pot gases emitted by the series of electrolytic cells in the aluminum reduction plant according to any one of claims 1 to 15, the method comprising the steps of:

treating hydrogen fluoride in the pot gases in the first treatment stage by adsorption onto the alumina;

treating sulfur dioxide in the pot gases in the second treatment stage by adsorption onto the alumina; and cooling the pot gases with the heat exchanger after treatment of the pot gases in the first treatment stage and before treatment of the pot gases in the second treatment stage.
17. The treatment method according to claim 16, wherein the step of treating the hydrogen fluoride in the pot gases in the first treatment stage comprises treating the hydrogen fluoride in parallel in the plurality of first reactors, each of the first reactors being associated with the (1309121 l_2):GMM:dah

2015359022 09 Jun 2017 group of electrolytic cells from the series of electrolytic cells, each group of electrolytic cells comprising one or more electrolytic cells from the series of electrolytic cells.
18. The treatment method according to claim 17, the method further comprising the step of: preheating the alumina to feed the electrolytic cells of each group of electrolytic cells by heat transfer between said alumina and the pot gases collected by the first reactors associated with the corresponding group of electrolytic cells.
19. The treatment method according to claim 18, wherein said heat transfer is achieved by direct contact between said alumina and said pot gases.
20. The treatment method according to any one of claims 17 to 19, the method further comprising the steps of:

fixing sulfur dioxide onto the alumina by adsorption in the second reactor of the second treatment stage, so as, firstly, to obtain pot gases that are free of sulfur dioxide and, secondly, sulfurized alumina, separating the sulfur dioxide present in the sulfided alumina by desorption in a desorption device, so as to obtain desulfurized alumina, injecting the alumina desulfurized in the first reactors of the first treatment stage so as to adsorb the fluorine present in the pot gases and obtain fluorinated alumina, route the fluorinated alumina from the first reactors of the first treatment stage to the electrolytic cells in order to supply the electrolysis reaction with alumina.