MXPA97003458A - Method to separate substances from a gaseous medium by adsorption in s - Google Patents

Abstract

The present invention relates to a method for separating fluorine-containing substances from a gas emitted from a process for the production of aluminum by means of adsorption on solid, particulate aluminum oxide in a dry adsorption process, characterized in that it comprises : treating the gas in a first dry adsorption process partially used particulate aluminum oxide to absorb fluorine-containing substances, passing aluminum oxide countercurrent to the gas, separating particulate aluminum oxide with fluorine-containing substances adsorbed from the gas under the gas first stage of adsorption, before the gas is transferred to a second stage of dry adsorption, remove part of particulate aluminum oxide separated with fluorine-containing substances adsorbed from the adsorption process and recycle the part of particulate aluminum oxide separated with substances that contain fluoride to the process for the production of alum The remaining separated aluminum oxide being recirculated in the first adsorption stage, and after removal of the aluminum oxide, the gas being supplied to a second dry adsorption stage and treating the essentially unused reactive particulate aluminum oxide gas, passing unused aluminum oxide countercurrent to the gas, after which particulate aluminum oxide is separated from the gas flowing under the second stage of dry adsorption, before the gas is discharged into the surrounding atmosphere and at least parts of the oxide Separated aluminum stream downstream of the second adsorption stage is transferred to the first adsorption stage

Description

METHOD FOR SEPARATING SUBSTANCES FROM A GASEOUS MEDIUM BY DRY ADSORPTION FIELD OF THE INVENTION This invention relates to a method for separating, by dry adsorption and for recovery purposes, impurities, such as fluorine-containing gases and fluorine-containing powder, from a gas that is emitted from a process for the production of aluminum. The gas emitted from the process is put in contact with an adsorbent in the form of particulate aluminum oxide, which can be recycled as raw material for the process. To be more specific, the invention relates to a multi-stage countercurrent process which combines the effective cleaning of the gas with a high degree of concentration of fluorine-containing substances on the adsorbent. In an environmentally advantageous embodiment of the invention, the sulfur dioxide is simultaneously removed from the gas.

DESCRIPTION OF THE PREVIOUS TECHNIQUE In a process for the electrolytic production of aluminum, such as the Hall-Héroult process where aluminum is produced by reducing aluminum oxide in a REF: 24570 molten electrolyte in the form of a mineral containing fluorine to which aluminum oxide is supplied, the process gases are charged with fluorine-containing substances, such as hydrogen fluoride and fluorine-containing powder. Being extremely harmful to the environment, these substances have to be separated before the process gases are discharged into the surrounding atmosphere, while at the same time the melt containing fluorine is essential for the electrolytic process. The recovery of the fluorine-containing compounds from the gases generated in the production of aluminum suffers from the drawback that the process gas is usually also charged with other substances, such as sulfur dioxide, which originate mainly from the oxidation of the electrodes but to some degree also of impurities found in the raw material. If they are recycled to the process or together with the adsorbent, those substances will be emitted to the process gases and thus will be concentrated in a cycle that arises in the electrolytic process and gas treatment. If they concentrate on the process, those substances often have an adverse effect on the performance of the process or interfere with the process in some other way, thereby adversely affecting the economy of the process. Consequently, these substances must be removed from the adsorbent before it is recycled into the process. For environmental reasons, the amount of sulfur dioxide discharged from the process should be reduced. It has been known for the use of the dry adsorption process to clean the gases generated in the production of aluminum, in which case the aluminum oxide can be used as an adsorbent. Aluminum oxide (AI2O3), which is the raw material supplied to the process for the production of aluminum, has a greater capacity to adsorb (more specifically, to chemically adsorb) hydrogen fluoride. Aluminum oxide powder of commercial qualities and a particle size in the range of 0.03-0.16 mm has a porous structure and an active surface area of 40-80 m2 / g, so that large amounts of hydrogen fluoride can be adsorbed. before the aluminum oxide saturates. However, it is true that the adsorption capacity decreases when the active surface is completely covered by molecules of adsorbed hydrogen fluoride, that is when the aluminum oxide is saturated with hydrogen fluoride. Usually, the particulate aluminum oxide is mixed efficiently and turbulently with the gases of the aluminum production process in a fluidized bed or some other contact reactor, the hydrogen fluoride is then adsorbed onto the aluminum oxide. The aluminum oxide, which now contains adsorbed fluorides, is separated downstream from the contact reactor with the aid of one or more filters. The aluminum oxide is then supplied to the aluminum production process, and the fluorides are recovered. However, sulfur dioxide is also adsorbed to a certain degree (as a rule 10-30%) in these processes, and sulfur dioxide thus accompanies the aluminum oxide back to the aluminum production process, where it is released to the process gases in the furnace. In actual practice, sulfur dioxide is thus not removed from the gases, but instead is recycled and undesirably concentrated in a system that includes the aluminum production furnace and the gas cleaning equipment, and an increase in the sulfur dioxide content in the air is also obtained in the premises. If it is desired to reduce the environmentally hazardous discharge of sulfur dioxide from aluminum production, the sulfur dioxide has to be separated from the combustion gases with the help of wet separators arranged downstream of the dry adsorption processes of the technique previous. However, such wet separators used to separate the sulfur dioxide from the gases represent a very expensive solution, since the amounts of gas involved are considerable and the concentration of sulfur dioxide in them is low, for example in comparison with the the combustion gases of a plant that runs on fossil fuel. For this reason, most aluminum plants in the world continue to discharge all of the sulfur dioxide into the surrounding atmosphere. An object of this invention is to provide a method for, with the aid of dry adsorption on aluminum oxide, to essentially remove all fluorine-containing substances for recovery purposes, as well as to efficiently remove sulfur dioxide for environmental reasons, from a Gas emitted from a process for the production of aluminum. Another object of the invention is to provide a process by which sulfur dioxide and, to a certain degree, also other undesirable impurities on the adsorbent can be removed therefrom before the adsorbent is recycled into the aluminum production process, thus avoiding the recirculation and accumulation of those substances in the system. Yet another object of the invention is to provide a process which, in comparison with the dry air cleaning processes of the prior art, results in the separation and sustained or improved recovery of the fluorine-containing substances, which maintain or improve at the same time the environmentally friendly nature of the process (low emissions), compared to the processes of the prior art mentioned above.

DESCRIPTION OF THE INVENTION According to the invention, these objects are achieved by means of an adsorption process which comprises at least two stages of dry adsorption, in which a gas, which is generated in a process for the production of aluminum, is charged to the less with fluorine-containing substances which may be gaseous or particulate, it is mixed with and contacted with particulate aluminum oxide, to thereby separate at least the fluorine-containing substances from the gas. The adsorption stages are arranged in the form of one or more contact reactors, in which the gas is treated by being mixed and contacted with particulate aluminum oxides. In the adsorption process according to the invention, the gas is treated in a first stage of dry adsorption with at least partially used particulate aluminum oxide, so that a substantial part of the gaseous fluorides found in the gas are adsorbed. on the adsorbent, the aluminum oxide with the adsorbed fluorine-containing substances is separated from the gas downstream of the first adsorption stage, after which part of the aluminum oxide separated with the adsorbed fluorine-containing compounds is removed from the adsorption process , and the rest of the aluminum oxide is recirculated in the first adsorption stage, while at the same time the gas is transferred to a second stage of dry adsorption arranged downstream of the first adsorption stage, the gas which now has a Substantially reduced content of fluorine-containing substances is then, in the second stage of dry adsorption, treated with aluminum oxide r ectivo essentially not used in particulate form, to adsorb therefore any fluorine-containing substances remaining in the gas after the first adsorption stage and to adsorb other gases, such as sulfur dioxide, and the particulate aluminum oxide is then removed from the gas downstream of the second adsorption stage, before the gas is discharged into the surrounding atmosphere or undergoes further downstream treatment, and at least part of the aluminum oxide separated from the gas downstream of a contact reactor included in the second adsorption stage it is transferred to a contact reactor included in the first adsorption stage. As is evident from the foregoing, the particulate aluminum oxide passes through the steps of the adsorption process in countercurrent to the gas. The aluminum oxide used is first supplied to a contact reactor which is included in the second stage of dry adsorption and wherein the aluminum oxide is mixed with and put in contact with the gas. From the contact reactor included in the second adsorption stage, at least some of the partially used aluminum oxide is now transferred to a contact reactor included in the first adsorption stage. When supplied to a contact reactor included in the first adsorption stage, the aluminum oxide from the second adsorption stage is mixed and brought into contact with the gas in this first adsorption step. After passing through a contact reactor included in the first adsorption stage, part of the dried particulate aluminum oxide is separated, the aluminum oxide is now essentially saturated with at least fluorine-containing substances and is removed from the process, by therefore to recycle the fluorine-containing substances to the process for the production of the aluminum, the rest of the aluminum oxide is recirculated in the first adsorption stage.

This recirculation is motivated by two reasons. First, it is desired to control and optimize the adsorption of the process gas in the first adsorption stage. Second, it is desired to obtain mainly the desorption of substances such as sulfur dioxide, which has been adsorbed on the aluminum oxide in the second stage of adsorption, to prevent therefore any substantial recycling of those substances to the electrolytic process. Sulfur (sulfur dioxide) or phosphorus (phosphorus pentoxide) could be recycled to the electrolytic process, this can have an adverse effect on the performance of this process. Since aluminum oxide has a much higher affinity with hydrogen fluoride than with gases such as sulfur dioxide, it is possible to partially recycle the adsorbent in at least the first adsorption stage, to verify which substances are recycled to the process electrolytic together with the adsorbent transferred to the electrolysis furnace, thus having undesirable substances, such as sulfur dioxide and phosphorus pentoxide, recirculated and concentrated in the system, including the electrolysis furnace and the gas treatment equipment . All these gases are adsorbed and molecularly bound to the active surface of the oxide particle in a dry adsorption process. Since, however, hydrogen fluoride has a higher affinity with the oxide than with sulfur dioxide, easily adsorbed sulfur dioxide will be desorbed, while hydrogen fluoride takes the place of sulfur dioxide on the active surface. . Under excellent contact conditions between the process gas and the adsorbent, the adsorption process is routed to an equilibrium state with a very high proportion of hydrogen fluoride adsorbed on the oxide surface, where adsorbed sulfur dioxide only occurs if there is an excess of active adsorbent surface in relation to the amount of hydrogen fluoride present in the process. Due to the fact that the adsorbent is recirculated in the first adsorption stage, the process approaches this state of equilibrium. As a result, the adsorption of undesirable substances can be verified and minimized, so that only a minimum of those substances are recycled to the electrolysis furnace together with the adsorbent. In an embodiment of the invention which is intended to be used when it is desired to prevent undesirable substances, such as sulfur dioxide, which have been adsorbed on the adsorbent being recycled to the electrolysis furnace, but nevertheless these substances can be allowed to be discharged to the surrounding atmosphere, the adsorbent (aluminum oxide) is transferred from the second adsorption stage directly to the first adsorption stage, where it is recirculated while the sulfur dioxide is desorbed. Desorption is guided towards a state of equilibrium. Sulfur dioxide is emitted from the electrolysis process and accompanies the process gas to the first adsorption stage. However, the adsorption of the sulfur dioxide in this first stage is controlled and minimized through the recirculation of the adsorbent in this step. As a result, the sulfur dioxide will be concentrated in a cycle between the first and second adsorption stages, whereby essentially the sulfur dioxide will not be recirculated between the electrolysis furnace and the first adsorption stage. In the steady state, a state of equilibrium is finally established by itself, in which the amount of sulfur dioxide discharged into the surrounding atmosphere is equal to the amount of sulfur dioxide emitted into the process gases in the furnace. electrolysis DRAWING For purposes of exemplification, the invention will now be described in more detail with the aid of a preferred embodiment, reference is made to the accompanying drawing.

DESCRIPTION OF THE DRAWING In the Hall-Héroult process, aluminum is produced by reducing aluminum oxide, which dissolves in a molten mass of fluorine-containing minerals, with the help of electrolysis in an electric reduction furnace 1. Electrolysis takes place at a temperature of approximately 960 ° C. The melt partially disintegrates during the process, and the volatile components are released in the gaseous state. As a result, the gases emitted from the process contain fluorine compounds, such as hydrogen fluoride (HF) and fluorine-containing powder. Being extremely harmful to the environment, those substances have to be separated from the process gases before they are discharged into the surrounding atmosphere. At the same time, however, those substances containing fluorine represent a considerable loss of value. Apart from the fluorine-containing compounds, certain combustion products, such as sulfur dioxide, are present in the carbon anodes, which burn during the process. Sulfur dioxide should be removed from the adsorbent not only to prevent it from being recycled to the process, but also because it is desirable, for environmental reasons, to reduce sulfur dioxide discharges from the process without having to install large and expensive plants for the process. treatment of large quantities of gases that have a low content of sulfur dioxide. When the invention is used to treat a gas 2 emitted from a process 1 for the production of aluminum, the fluorine-containing substances are separated from the gas in a countercurrent adsorption process comprising at least two stages of dry adsorption 3, 4 The gas charged with fluorine-containing substances is treated in a first adsorption stage 3, which in the Figure is shown as a contact reactor 3. In this contact reactor 3, the gas is mixed and brought into contact with a Partially used particulate adsorbent in the form of aluminum oxide, which is transported with the gas stream in the contact reactor 3, the content of fluorine-containing substances in the process gas is reduced. The adsorption of sulfur dioxide during the treatment in the first adsorption stage 3, when the content of fluorine-containing substances in the gas is at its maximum, is suppressed since substances such as hydrogen fluoride have a much higher affinity with the aluminum oxide that with the sulfur dioxide. In this first adsorption step, the sulfur dioxide is thus only adsorbed on an excess surface on the aluminum oxide, which is not covered by, for example, hydrogen fluoride. If the aluminum oxide on which the sulfur dioxide has been adsorbed comes into sufficiently intensive contact with the gas containing hydrogen fluoride, the sulfur dioxide will be released and replaced by hydrogen fluoride. After the treatment in the first adsorption stage 3, the particulate aluminum oxide is separated from the gas before the latter, which now has a very low content of hydrogen fluoride, is transferred to a second stage of dry adsorption 4 to be treated there . The particulate aluminum oxide, which has a high content of adsorbed fluorine-containing substances, such as hydrogen fluoride, is separated from the gas together with the major part of particulate fluorine-containing compounds downstream of the first adsorption step 3 with the aid of the mechanical separation devices of the prior art 31, such as cyclones. Some aluminum oxide 33, which corresponds to the amount of spent aluminum oxide supplied to the second adsorption stage 4 of the adsorption process and which is charged with adsorbed fluorine-containing substances, is recycled (in 33) to process 1, while the rest of the aluminum oxide is recirculated (in 32) within the first adsorption stage 3. Through a sufficient recirculation and due to the affinity difference of the aluminum oxide with the hydrogen fluoride and sulfur dioxide respectively, it must be ensured that the main part of the fluorine-containing substances in the gas is adsorbed even in the first adsorption stage 3, although there is no essential adsorption of sulfur dioxide. Instead, a substantial amount of the sulfur dioxide 1 adsorbed on the aluminum oxide is desorbed. As a result, essentially all of the sulfur dioxide will accompany the gas, so that the fluorine-containing substances vital to process 1 can be recycled with a good yield of 33, while avoiding the recirculation and concentration of sulfur dioxide in the process. Also the second adsorption stage 4 is arranged in the form of one or more contact reactors 4 co Located downstream of the first adsorption stage 3. From the first adsorption stage 3 and the next separator 31, the gas is transferred to the adsorber. to contact reactors 4, where it is mixed with and contacted with reactive and essentially unused, fresh aluminum oxide. In contact reactor 4, any remaining gaseous fluorine, as well as sulfur dioxide, are adsorbed in an amount that depends on the degree to which the adsorption capacity of the fresh adsorbent (aluminum oxide) allows low gas adsorption. affinity. After the treatment in the second adsorption stage 4, the adsorbent is separated from the gas with the aid of a filter 41, such as a bag filter, whereby the gas, which has been efficiently cleaned of all substances containing fluorine, can be discharged into the surrounding atmosphere by 5, while aluminum oxide loaded with a substantial amount of sulfur dioxide adsorbed in the second adsorption stage 4 is, according to the invention, transferred to the first adsorption stage 3. Through the suitable recirculation of aluminum oxide 32 in the adsorption step 3, a substantial amount of the sulfur dioxide adsorbed on the aluminum oxide, when brought into contact with the process gas having a high content of Hydrogen fluoride will be desorbed in the adsorption step 3. The sulfur dioxide released is then conducted together with the process gas to the second adsorption stage 4. Due to the desorption of the sulfur dioxide, there is an increase in the active surface over the aluminum oxide that is available for the adsorption of hydrogen fluoride, resulting in highly efficient adsorption of hydrogen fluoride,. so that a very high degree of gaseous fluorine adsorption is obtained in the first adsorption stage.3. Through the aluminum oxide 33 which is transferred to the reduction process 1 from the first adsorption stage 3, substantially all the fluorine-containing substances emitted from the reduction process 1 to the process gas 2 are recycled to the reduction process 1 However, essentially the sulfur dioxide is not recycled to the reduction process 1 together with the aluminum oxide 33 transferred from the first adsorption stage 33 to the reduction process 1. Due to the fact that the sulfur dioxide is desorbed in the first adsorption stage, the partially clean process gas 30 transferred to the second adsorption stage 4 will have an increased sulfur dioxide content, which to some degree is reduced in the second adsorption stage 4. In the steady state, a state of equilibrium is established with respect to the recirculation of the concentrated sulfur dioxide between the two adsorption stages 3 and 4, the amount of dioxide of sulfur discharged together with the clean process gas 5 is equal to the amount of sulfur dioxide supplied together with the process gas not yet cleaned. In an embodiment of the invention, also the sulfur dioxide discharged to the surrounding atmosphere together with the clean process gas 5 is reduced by treating the aluminum oxide charged with sulfur dioxide from the second adsorption stage 7 in a desorption step 8. In this desorption step 8, substantially all of the adsorbed sulfur dioxide is desorbed by being heated and mixed with a carrier gas flowing therethrough 81. The carrier gas 82 leaving the desorption stage 8 will then have a high concentration of sulfur dioxide, essentially all the Sulfur dioxide has been emitted from aluminum oxide through desorption. Due to the low affinity of sulfur dioxide with aluminum oxide, aluminum oxide has a very restricted ability to adsorb sulfur dioxide. In this way, even at such low gaseous fluorine content in the process gas 30 transferred to the second adsorption stage 4 and has an essentially negligible effect on the desorption of sulfur dioxide during this adsorption step 4, a poor separation of the sulfur dioxide from the process gas, in the quality of the adsorbent is low and / or if the sulfur dioxide content of the process gas supplied to this second adsorption stage 4 is high. In one embodiment of the invention, the ability to remove sulfur dioxide is mainly directed to the level of recycling (at 83) part of the aluminum oxide treated in the desorption step 8 to the second adsorption stage 4, where it contributes to increasing the amount of active adsorbent. The amount of aluminum oxide treated in the desorption step 8 in this way will be increased proportionally to the amount of aluminum oxide recycled at 83 from the desorption stage 8 to the second adsorption stage 4. In the desorption step 8, the sulfur dioxide is desorbed due to the effect of the heating and the carrier gas that flows through it, 81 taking the sulfur dioxide with it out of the system. If the desorption treatment in step 8 is carried out correctly, only a small amount of carrier gas 81 is required, while at the same time a high sulfur dioxide concentration is obtained in the carrier gas 82 leaving the desorption stage. Sulfur dioxide in the carrier gas can, at a reasonable cost, be washed or converted to commercial products, such as liquid sulfur dioxide, sulfuric acid or sulfur, using well-known processes, since there is only a small amount of carrier gas 82 involved, which is why the treatment team may be small in size. The slight heating of the aluminum oxide required to desorb the sulfur dioxide in the desorption step 8 does not result in the desorption of the small amount of hydrogen fluoride that has been adsorbed in the second adsorption stage 4. After the step of desorption 8, the aluminum oxide is conducted to the first adsorption stage 3, as described above. After having passed in this way the adsorption process in two stages in dry, 3, 4 countercurrent to the gas and be substantially adsorbed all the hydrogen fluoride and other fluorine-containing substances of the gas, aluminum oxide is supplied to the process 1 for the production of aluminum. The sulfur dioxide content of aluminum oxide is very low and is essentially limited to the amount that has been adsorbed and remains during the treatment in the first adsorption stage 3. Other certain substances, such as phosphorus, which have been carried with the gas from the aluminum production process 1 and that reduce the real yield in the electrolytic process, they have an adverse effect on the process and therefore they must be removed. The phosphorus in the form of particulate phosphorus pentoxide is removed from the process gas in a final filtration step 41 and can thus be concentrated in the system, including the electrolysis furnace and the gas treatment equipment. It has been found that the treatment for the removal of sulfur dioxide 8 also removes a certain amount of phosphorus, thus reducing the accumulation thereof in the system. Since, according to the method of the invention, the particulate adsorbent (aluminum oxide) passes the two stages 3, 4 of the countercurrent adsorption process to the gas, where the gas and the adsorbent are transported together with the current in the adsorption stages 3, 4, the adsorbent is efficiently exhausted and essentially all of the hydrogen fluoride separated in the first adsorption stage 3 and recycled together with the adsorbent to the aluminum production process 1, while the sulfur dioxide is separated in the second adsorption stage 4 and is removed from the adsorbent in the desorption step 8. The separation of the sulfur dioxide can be adjusted efficiently through the recirculation in 83 of the adsorbent of the desorption step 8 to the second stage of adsorption 4. This two-stage process results in high-yield recycling of the fluorine-containing substances that are desired to be recirculated from the process, while that the sulfur dioxide can be separated into itself and be either neutralized in an alkaline scrubbing tower or converted to a form of commercially available products. Since the process according to the invention in its simplest form reduces the recirculation and accumulation of sulfur dioxide and in its more elaborate form it also reduces the recirculation and accumulation of such a contaminant as phosphorus in the aluminum production process, achieves an improved efficiency in the electrolytic process for the production of aluminum, since this process in other circumstances could be adversely affected by increasing the content of these substances. Since the sulfur can be separated in one embodiment of the invention, the environmental friendliness of the aluminum production can be fully improved.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (5)

1. A method for separating, when treating a gas emitted from a process for the production of aluminum, fluorine-containing substances of the gas by means of adsorption on particulate aluminum oxide, solid in a dry adsorption process, characterized in that the gas is treated with particulate aluminum oxide in at least two stages, the aluminum oxide passes the stages of the adsorption process in countercurrent to the gas; the gas is treated in a first adsorption stage with the aluminum oxide that has been partially used; the particulate aluminum oxide with adsorbed fluorine-containing substances is separated from the gas downstream of the first adsorption stage, before the gas is transferred to the second stage of dry adsorption; Part of the particulate aluminum oxide separated with adsorbed fluorine-containing substances is removed from the adsorption process with a view to recycling the fluorine-containing substances to the process for the production of aluminum, and the remainder of the separated aluminum oxide is recirculated in the first adsorption stage; and the gas is, after separation of the aluminum oxide, supplied to the second stage of dry adsorption and there treated with essentially unused reactive particulate aluminum oxide, after which the particulate aluminum oxide is separated from the normal gas downstream of the second dry adsorption stage, before the gas is discharged into the surrounding atmosphere, and at least part of the aluminum oxide separated downstream of the second adsorption stage is transferred to the first adsorption stage.

2. The method according to claim 1, characterized in that the partially used aluminum oxide is recirculated in the first adsorption stage, and the amount of recirculated aluminum oxide is verified and controlled in this way to optimize the adsorption of the substances that contain fluoride in the desorption of sulfur dioxide in the first adsorption stage.

3. The method according to claim 1 or 2, for treating a gaseous medium containing at least hydrogen fluoride and sulfur dioxide, characterized in that the aluminum oxide, which was separated downstream of the second adsorption stage and charged with adsorbed sulfur dioxide, it is treated in a desorption stage, where the aluminum oxide is heated and a carrier gas flows through it, thereby desorbing a substantial amount of the sulfur dioxide adsorbed on the aluminum oxide .

The method according to claim 3, characterized in that the aluminum oxide treated in the desorption step is recycled to the second desorption stage to increase the adsorption capacity in this step.

5. The method according to claim 3 or 4, characterized in that water vapor, gaseous nitrogen or other non-oxidizing gas flows through the aluminum oxide in the desorption stage. SUMMARY OF THE INVENTION The treatment, through a dry adsorption process of a gas of a hot electrolytic process for the production of aluminum comprises at least two stages (3, 4). The particulate aluminum oxide (the adsorbent) passes through stages (3, 4) of the countercurrent adsorption process to the gas. In this way, the gas is treated with a partially used adsorbent in a first adsorption step, after which the particulate adsorbent is separated from the downstream gas of the first adsorption stage. Part of the separated particulate adsorbent is removed (33) from the adsorption process with a view to recycling the adsorbed fluorine-containing substances to the electrolytic process. The rest of the separated adsorbent is recirculated in the first adsorption stage to optimize the adsorption of the fluorine-containing substances and the desorption of the sulfur dioxide in this step. Simultaneously, the gas is transferred to a second adsorption stage. In this second stage, the gas is treated with reactive particulate aluminum oxide, essentially unused, so that any gaseous fluorine remaining in the gas is very efficiently adsorbed, although at the same time a substantial part of the sulfur dioxide in the gas is also adsorbed. . Finally, this particulate aluminum oxide is separated from the gas downstream of the second adsorption stage, before the gas is discharged into the surrounding atmosphere. The separated aluminum oxide is transferred to the first adsorption stage, optionally after passing to the desorption stage (8) for the removal of the adsorbed sulfur dioxide to reduce the sulfur dioxide discharges from the aluminum production. The separation of the sulfur dioxide in the second adsorption stage is improved by recycling, to the second adsorption stage, the aluminum oxide which has undergone the desorption treatment.