WO1991016965A1

WO1991016965A1 - Method and apparatus for increasing the degree of utilisation for a dry sorbent

Info

Publication number: WO1991016965A1
Application number: PCT/SE1991/000287
Authority: WO
Inventors: Bjarne Hebnes
Original assignee: ABB Fläkt AB
Priority date: 1990-04-27
Filing date: 1991-04-23
Publication date: 1991-11-14
Also published as: AU7788791A; SE9001529D0; SE9001529L; SE467952B

Abstract

In a method and an apparatus for increasing the degree of utilisation for a dry sorbent for sorption of gaseous pollutants, such as hydrogen fluoride, from a polluted process gas, such as exhaust gases from electrolytic furnaces, the sorbent is separated after its reaction with the gaseous pollutants in a fabric filter (1), whereupon a proportion of the separated sorbent is recycled to the polluted process gas by internal circulation inside the fabric filter. Fresh sorbent is supplied to the fabric filter (1) via at least one sorbent inlet (11), while circulated sorbent is removed from the fabric filter via at least one sorbent outlet (12) separated from the sorbent inlet. The polluted process gas is supplied to the fabric filter (1) through the gas inlet (3) thereof in such manner that only a partial flow of the process gas is introduced at the sorbent inlet (11) of the fabric filter (1) and is supplied with fresh sorbent.

Description

METHOD AND APPARATUS FOR INCREASING THE DEGREE OF

UTILISATION FOR A DRY SORBENT

Field of the Invention The present invention relates to a method for increasing the degree of utilisation for a dry absorbent and/or adsorbent, hereinafter referred to as sorbent, for absorption and adsorption, respectively, of gaseous pol¬ lutants, such as hydrogen fluoride, from a polluted pro- cess gas, such as exhaust gases from electrolytic fur¬ naces (such as aluminium smelters), the sorbent being separated in a fabric filter after reacting with the gaseous pollutants, said fabric filter having a gas inlet for supplying the polluted process gas, a gas outlet for removing the cleaned process gas, and dust separating means, whereupon a proportion of the separated sorbent is recycled to the polluted process gas by internal circula¬ tion inside the fabric filter, fresh sorbent being sup¬ plied to said fabric filter via at least one sorbent inlet, while circulated sorbent is removed from the fabric filter via at least one sorbent outlet separated from the sorbent inlet. The invention also relates to an apparatus for carrying out the method. Description of the Prior Art In the manufacture of aluminium in electrolytic fur¬ naces, exhaust gases are formed which contain both gaseous pollutants, such as hydrogen fluoride, and particulate pollutants, such as fluorine-containing dust. Since hydro¬ gen fluoride is injurious to the environment at the same time as fluoride is needed in the reduction of the alumi¬ nium oxide in the electrolytic furnaces, it is desirable to separate the fluoride from the exhaust gases before emitting them into the atmosphere, thereby making it pos¬ sible to recycle the fluoride to the electrolytic fur- naces. This is usually achieved by supplying the aluminium oxide which is to be reduced in the electrolytic furnaces, to the exhaust gases in a contact reactor or transport conduit. The aluminium oxide acts as a sorbent for the gaseous hydrogen fluoride and converts, in a chemical sorption process, this gaseous pollutant into a particu- late pollutant containing aluminium oxide particles which are more or less saturated with hydrogen fluoride. This particulate pollutant is then separated together with the fluorine-containing dust in a subsequent dust separator. To increase the degree of utilisation for the aluminium oxide, the material separated in the dust separator is usually recycled to the exhaust gases. The number of times the aluminium oxide is recycled before being supplied to the electrolytic furnaces is determined by the concentra¬ tion of hydrogen fluoride of the exhaust gases, the desir¬ ed degree of separation, the quality of the aluminium oxide and the design of the contact reactor.

Since aluminium oxide is a highly abrasive dust, increased recirculation of the separated material and, thus, an increased dust concentration make the wear of transport conduits and dust separators increase. This wear also results in the aluminium oxide being polluted by the worn-off material, e.g. iron from the transport con- duits, and the particle size of the aluminium oxide decreasing. Smaller aluminium oxide particles imply, inter alia, that fine aluminium oxide particles are spread in the process hall surrounding the electrolytic furnaces, which is undesirable from working environment aspects. An increased concentration of dust in the dust separator also results in an increase of the pressure drop and, conse¬ quently, the energy consumption.

If aluminium oxide particles collide at high speed with metallic surfaces, aluminium oxide can be deposited on the surfaces and form coatings which subsequently can come off the surfaces in the form of hard and sharp alumi¬ nium oxide scales. As the above-mentioned dust separator usually is a fabric filter in the form of a bag filter whose bags would quickly be torn by these scales, the dust separator is frequently provided with a preseparator which separates the scales from the exhaust gases before these are caused to pass through the filter bags. However, in the preseparator the coarse aluminium oxide particles, too, are separated, which means that the coarse sorbent particles having the largest surface and, thus, the greatest reaction potential are not deposited in the dust which is collected on the outside of the bags and in which the remaining, gaseous pollutants are brought into close contact with the aluminium oxide and converted into parti- culate pollutants.

Since the aluminium oxide stays in the reactor just for a few seconds but remains on the bags until these are cleaned, which takes place about 1-10 times an hour, the residence time of the aluminium oxide on the filter bags is a few hundred times longer than in the reactor. This means that the fine aluminium oxide particles, which need a short time to react with the gaseous pollutants, have a long total residence time, while the coarse aluminium oxide particles, which need a long time to react with the gaseous pollutants, have a short total residence time. To increase the residence time of the coarse particles and, consequently, the degree of utilisation for the aluminium oxide, the aluminium oxide must thus be recycled a great number of times, which causes various drawbacks in the plant for cleaning exhaust gases.

US-4,501,599 suggests for example that fresh and recycled aluminium oxide be added to the exhaust gases from the electrolytic furnaces in a special mixing chamber, before they are introduced into a dust separator for separating reacted and unreacted aluminium oxide and dust entrained from the electrolytic furnaces. The purpose of the mixing chamber is to provide intimate mixing of the gaseous pollutants of the exhaust gases and the aluminium oxide, thereby contacting the major part of these pollu- tants with the aluminium oxide and converting it into par- ticulate pollutants which are separable in the dust sepa¬ rator.

In this case, the dust separator is a fabric filter provided with a preseparator for separating the coarse particles. As a result, the coarse particles are contacted but with the gaseous pollutants in the mixing chamber. Since the residence time in the mixing chamber is short, only part of these particles has time to react with these pollutants, despite the intimate mixing in the chamber, before they are separated in the preseparator. In spite of the complicated and expensive mixing chamber, the material separated in the fabric filter must also in this case be circulated a number of times between the mixing chamber and the fabric filter in order to obtain an acceptable utilisation of the sorption plant.

DE-38 01 913 suggests that the fresh sorbent be sup¬ plied to the exhaust gases immediately prior to the intro¬ duction thereof into a bag filter functioning both as a contact reactor and as a dust separator. The material separated in the bag filter thus need only be recirculated internally inside the filter to be contacted once more with uncleaned exhaust gases. Further, no special separa¬ tion of the coarse particles of the separated material takes place, but both fine and coarse particles are circu¬ lated together in the filter. The coarse dust thus has a residence time in the filter, which is of the same length as that of the fine dust. This implies that the exhaust gas cleaning plant need not be provided either with a spe- cial preseparator, contact reactor or with special con¬ duits for conveying the separated material from the bag filter to the contact reactor. This results in a plant which is both simple and inexpensive, at the same time as the surfaces that can be subjected to wear are reduced. Since the internal recirculation of the separated material occurs by means of part of the partly cleaned exhaust gases, a recycling of the exhaust gases is also obtained here. This means that the rate of flow and thus the velocity of the exhaust gases inside the filter increase, which results in a higher degree of wear of the filter bags and a reduced concentration of the gaseous pollutants. Such a reduction implies, in turn, a lower degree of efficiency of the sorption process. Because the exhaust gases are caused to flow at high speed through a collecting pipe inside the filter, before they are brought into contact with the filter bags, it is besides a great risk that scales are formed inside the filter, see p 2, last paragraph.

Just before the separated material is contacted with the uncleaned exhaust gases, it is caused to pass over apertured dust guide plates through whose apertures part of the separated material is conducted to the lower part of the filter housing to be conveyed to the electrolytic furnaces. The particles of the separated material which are not to be recycled back to the exhaust gases are thus selected absolutely randomly, which is also the case in the exhaust gas cleaning plant according to the above- mentioned US patent. This means that fresh, reactive sor¬ bent particles can be conducted away after being circu¬ lated once inside the filter, whereas such sorbent par¬ ticles as have already been recycled several times and have a high degree of saturation are recycled to the exhaust gases. In other words, it is not possible to control how many times the fresh sorbent is circulated before being conducted to the electrolytic furnaces, which results in a low degree of utilisation for the sorbent.

Summary of the Invention Technical Problem

It thus constitutes a technical problem to provide a high degree of utilisation for the sorbent, at the same time as it should produce as little wear as possible of the components of the exhaust gas cleaning plant. Object of the Invention

The object of the present invention therefore is to solve the above technical problem of providing a high degree of utilisation for the sorbent, at the same time as the wear is minimised. Summary of the Invention

The above-mentioned object is achieved in that the polluted process gas is supplied via the gas inlet into the fabric filter in such manner that only a partial flow of the process gas is introduced at the sorbent inlet of the fabric filter and is supplied with fresh sorbent, whereby the fresh sorbent, for the difference in sorbent concentration occurring between the partial flow and the remaining flow of the process gas to be levelled out, is caused to move in the direction of the sorbent outlet where the sorbent is removed and the sorbent concentration is at its lowest, at the same time as the flow of the pro¬ cess gas and the action of gravity make the sorbent circu¬ late at least twice inside the fabric filter between the dust separating means and the gas inlet, without simulta¬ neous recycling of polluted process gas.

The gas inlet of the fabric filter is designed rela¬ tive to the sorbent inlet so that only part of the gas inlet communicates with the sorbent inlet, while the sor- bent inlet and the sorbent outlet are arranged at the cir¬ cumference of the fabric filter in a predetermined spaced- apart relationship. General Description of the Invention

According to the present invention, the above-men- tioned problem of providing a high degree of utilisation for the sorbent is solved, at the same time as the wear is minimised, by attaining such control of the movement of the fresh sorbent inside the fabric filter that no fresh sorbent particles leave the fabric filter before having circulated at least twice therein. The wear is minimised in that the circulation occurs internally inside the fabric filter, without sorbent particles being caused at high speed to collide with any metallic surfaces, and in that no simultaneous recycling of polluted process gas occurs, see p 4, second paragraph, and the paragraph bridging pp 4 and 5. The sorbent is circulated preferably 2-100, especial ly 2-20, times inside the fabric filter.

A larger proportion of the process gas is introduced preferably at the sorbent inlet of the fabric filter than at the sorbent outlet thereof by a control means extendin across the gas inlet.

In order to attain the above-mentioned control of th movement of the fresh sorbent inside the fabric filter, the gas inlet is designed in relation to the sorbent inlet so that only part of the gas inlet communicates with the sorbent inlet, while the sorbent inlet and the sorbent outlet are arranged at the circumference of the fabric filter in a predetermined spaced-apart relationship.

Since only the partial flow of the process gas which flows through the above-mentioned part of the gas inlet is supplied with fresh sorbent, there is a difference in sorbent concentration between this partial flow and the remaining flow of the process gas. This difference in sor¬ bent concentration results in the sorbent moving in the direction of the sorbent outlet where the sorbent is removed and the sorbent concentration is at its lowest, at the same time as it is caused, by the flow of the process gas towards the dust separating means and under the action of gravity, to move between the dust separating means and the gas inlet. As a result, the sorbent is caused to make a helical movement between the sorbent inlet and the sorbent outlet.

The sorbent inlet and the sorbent outlet are arranged in such a predetermined spaced-apart relationship that the sorbent has time to be added to and separated from the process gas at least twice, before it is removed through the sorbent inlet. This guarantees a high utilisation of the sorbent. The fabric filter is preferably a bag filter. A control means can be mounted across the gas inlet, thereby controlling the introduction of the process gas into the fabric filter so that it flows through the filter in such a manner that, in coaction with the above-men¬ tioned difference in sorbent concentration, it causes the sorbent to move in the direction of the sorbent outlet. Thus, the pitch of the helical movement of the sorbent can be affected by the control means. The surface of the control means preferably increases in a direction away from the sorbent inlet of the fabric filter, towards the sorbent outlet thereof. Consequently, a larger proportion of the process gas is supplied at the point in the fabric filter where the fresh sorbent is added, as compared to the point where the recycled sorbent is removed. This means that the pitch of the helical move¬ ment of the sorbent increases.

The control means is preferably designed as a ser¬ rated plate. This makes the sorbent particles spread over a larger part of the cross-sectional surface of the gas inlet than if the control means had been designed as a straight plate. The teeth of the plate also produce whirls in the process gas, i.e. the process gas is given a turbu¬ lent flow configuration. These effects result in a more uniform distribution of the sorbent particles in the pro¬ cess gas.

The length of the teeth of the plate decreases pre¬ ferably in a direction away from the sorbent inlet towards the sorbent outlet. One or two sorbent inlets are preferably arranged in one or two sides of the fabric filter, while one or two sorbent outlets are preferably arranged in another side or in two other sides of the fabric filter.

Preferably only one sorbent inlet is arranged in one side of the fabric filter, while only one sorbent outlet is arranged in another side of the fabric filter. The sorbent outlet is preferably arranged in a side opposite said one side of the fabric filter.

The control means extends preferably from one side of the gas inlet, which adjoins one side of the fabric fil- ter, to the opposite side thereof which adjoins the oppo¬ site side of the fabric filter. Description of a Preferred Embodiment

The invention will now be described in more detail with reference to the accompanying drawings. Fig. 1 is a schematic side view of an apparatus according to the present invention;

Fig. 2 is a front view of a different embodiment of the apparatus in Fig. 1; and

Fig. 3 is a top plan view of the apparatus in Fig. 2. The apparatus in Fig. 1 is a bag filter 1 which com¬ prises a filter housing 2 having a gas inlet 3 for supply¬ ing the polluted process gas to be cleaned, such as exhaust gases from electrolytic furnaces for reduction of aluminium oxide, and a gas outlet 4 for removing the cleaned process gas. The interior of the filter housing is divided by means of a plate 5 into a lower filter chamber 6 for the polluted gas and an upper filter chamber 7 for the cleaned gas, which communicate with the gas inlet 3 and the gas outlet 4, respectively. The bag filter also comprises dust separating means in the form of filter bags 8, usually in an amount of 100-800, especially 540, which are arranged in longitu¬ dinal and transverse rows. At their open upper end, the filter bags are detachably mounted on the plate 5. Their length is 3-8 m, usually 6 m, and their diameter is 100-300 mm, usually 130 mm. The distance between two adjoining filter bags is in the longitudinal direction 130-500 mm, usually 160 mm, and in the transverse direc¬ tion 130-500 mm, usually 220 mm. The lower filter chamber 6 comprises a bottom portion 9 and a filter portion 10 in which the filter bags 8 are mounted. The bottom portion 9 has a vertical front wall 9a, two vertical opposite side walls 9b, 9c, and an inclined rear wall 9d serving as the bottom of the filter housing 2. This configuration gives the vertical section of the bottom portion the shape of a right-angled tri- angle, see Fig. 1, whereas its horizontal section is given the shape of a rectangle, see Fig. 3. The filter portion 10 is provided with a vertical front wall 10a, two verti¬ cal opposite side walls 10b, 10c, and a vertical rear wall lOd, which implies that both its vertical section and its horizontal section are rectangular.

As shown in Fig. 2, a sorbent inlet 11 for supplying fresh sorbent particles, such as aluminium oxide par¬ ticles, is arranged in one of the side walls 9b of the bottom portion, while a sorbent outlet 12 for removing circulated sorbent particles is arranged in the opposite side wall 9c at a predetermined distance from the sorbent inlet. A guide plate 13 for conducting recycled sorbent particles to the sorbent outlet 12 is pivotably connected to the upper end of the sorbent outlet. The guide plate extends in the vertical direction up to the lower end of the filter bags, see Fig. 2, and in the horizontal direc¬ tion to the rear wall 9d of the bottom portion, see Fig. 3. Fig. 3 also shows that the angular deviation of the guide plate 13 relative to the side wall 9c of the bottom portion 9 is indicated by the angle β .

The gas inlet 3 is positioned at the lower ends of the walls of the bottom portion 9 and comprises a vertical front wall 3a, two vertical opposite side walls 3b, 3c, and a vertical rear wall 3d. This means that both its ver- tical section and its horizontal section have the shape of a rectangle, see Figs. 1 and 3. In order to prevent the polluted gas from flowing past the sorbent outlet 12, where the occurrence of sorbent particles is small, the side wall 3c of the gas inlet is positioned a distance inside the side wall 9c of the bottom portion. These side walls are interconnected by a partition 14 having a ver¬ tical part 14a connected to the side wall 3c of the gas inlet, and an inclined part 14b connected to the side wal 9c of the bottom portion. The side wall 3b, however, is arranged directly below the bottom portion side wall 9b into which the sorbent outlet 11 opens, see above. As seen in Fig. 1, not the entire filter portion 10 is provided with filter bags, but the first row of filter bags is arranged about 1 m from the front wall 10a of the filter portion. As a result, a flow duct 15 for the pol¬ luted gas is formed inside the lower filter chamber 6 directly above the gas inlet 3.

Nozzle tubes 16 provided with nozzles 17 are arrange in the upper filter chamber 7 in such manner that a nozzl 17 is positioned above the open upper end of each filter bag 8. During cleaning of the filter bags, a short and powerful compressed-air pulse is supplied to the filter bags from a pressure tank (not shown) via the nozzle tubes 16 and the nozzles 17 according to a well-defined cleaning programme.

The function of the bag filter will now be described in more detail. The process gas polluted with gaseous pol¬ lutants, such as hydrogen fluoride, and particulate pollu¬ tants, such as a fluorine-containing dust, is supplied via the gas inlet 3 to the bottom portion 9 of the lower fil¬ ter chamber 6. In this context the process gas is con- tacted with the sorbent particles via the lower end of the inclined rear wall 9d. The sorbent particles are thus mixed with the process gas and the gaseous pollutants thereof which to a certain extent are sorbed by the sor¬ bent particles and converted into particulate pollutants. Subsequently the process gas flows up through the flow duct 15 and into the filter bags 8 at a speed of about 0.025-0.4 m/s, whereupon it flows out to the upper filter chamber 7 via the upper open ends of the filter bags. From there, the process* gas is conducted via the gas outlet 4 to a chimney (not shown) to be emitted to the atmosphere. When the process gas passes through the filtering material of the filter bags, which is e.g. polyester, its particulate pollutants are deposited on the outsides of the filter bags and form layers of dust thereon. When passing the dust layers of the filter bags, the remaining gaseous pollutants of the process gas are sorbed by unsa- turated sorbent particles therein and separated from the process gas. The dust layers produce an increase of the pressure drop across the filter bags, the increase being proportional to the thickness of the dust layers.

When cleaning the filter bags, the particles included in the dust layers then drop down on the rear wall 9d of the bottom portion 9 under the action of gravity and slide along this wall down to the gas inlet 3 where they are recycled to the process gas. Since the process gas mainly flows in between the filter bags from the flow duct 15, the dropping motion of the particles is not essentially counteracted by process gas flowing upwards. This means that the particles are not recycled to the filter bags, without having passed the lower end of the rear wall 9d. Since the gas inlet 3 is designed so that only part thereof communicates with the sorbent inlet 11, the fresh sorbent is only supplied to the partial flow of the pro¬ cess gas, which passes through the above-mentioned part of the gas inlet. In order to balance the difference in sor¬ bent concentration, which consequently arises between this partial flow and the remaining flow of the process gas, the fresh sorbent particles move in the direction of the sorbent outlet where they are removed from the filter and the sorbent concentration thus is at its lowest, at the same time as they are caused to move between the filter bags and the gas inlet owing to the upward flow of the process gas towards the filter bags 8 and under the action of gravity. These two motions make the sorbent particles perform a helical movement while they move from the sor¬ bent inlet to the sorbent outlet. This smooth movement ensures that the sorbent particles are not crushed during their recirculation inside the lower filter chamber 6. When the sorbent concentration is at its lowest at the sorbent outlet, the thickness of the dust layers and the pressure drop across the filter bags consequently are at a minimum at the sorbent outlet, which also contributes to the process gas and, thus, the sorbent being caused to move in the direction of the sorbent outlet.

In order to be able to control how many times the fresh sorbent particles are circulated inside the lower filter chamber 6, before they are removed via the sorbent outlet 12, a control means in the form of an inclined, serrated plate 18, as shown in Figs. 2 and 3, can be posi¬ tioned across the upper end of the gas inlet 3. Further, this plate is at its rear edge attached to the lower end of the rear wall 9d of the bottom portion 9. The plate is designed such that the points of its teeth 19 are posi¬ tioned at the same distance from the front wall 3a of the inlet, whereas the distance between the roots of the teeth and the front wall 3a decreases in a direction away from the side wall 3b to the side wall 3c, i.e. the length of the teeth 19 decreases in this direction. The surface of the plate, however, increases in this direction, see Fig. 3, and thus covers a larger part of the cross-sectional surface of the gas inlet at the side wall 3c than at the side wall 3b. Fig. 3 also shows that the roots of the teeth are positioned along a straight line making an angle a with the rear edge of the plate.

In the embodiment according to Figs. 2 and 3, the plate 18 thus brings the process gas into contact with the sorbent particles which via the teeth 19 of the plate 18 are spread over a larger part of the cross-sectional sur¬ face of the gas inlet and are mixed with the incoming pro¬ cess gas. This mixing operation is facilitated by the whirls that are produced in the process gas by the teeth of the plate. In this embodiment, a more satisfactory mix¬ ing of the process gas and the sorbent particles than in the embodiment illustrated in Fig. 1 thus is obtained. Since the plate 18 covers a larger part of the cross- sectional surface of the gas inlet at the side wall 3c, where the recycled sorbent particles are removed via the sorbent outlet 12, than at the side wall 3b, where the fresh sorbent particles are supplied via the sorbent inlet 11, a larger part of the process gas is introduced at the side wall 9b of the bottom portion 9 than at the opposite side wall 9c. Since the process gas strives to be distri¬ buted uniformly over all the filter bags 8, the uneven distribution of the process gas over the cross-sectional surface of the gas inlet causes the gas, which is supplied at the side wall 9b, to flow in the direction of the oppo¬ site side wall 9c and, in coaction with the above- mentioned difference in sorbent concentration, thus causes the sorbent particles to move in the direction of the sor¬ bent outlet.

How many times the fresh sorbent particles are circu¬ lated inside the lower filter chamber is determined by the size of the angles and. β and the distance between the sorbent inlet and the sorbent outlet. An increase of the angle a results in the plate 18 covering a larger part of the cross-sectional surface of the gas inlet at the side wall 3c, whereby the pitch of the helical movement increases. On the other hand, an increase of the angle β results in a larger proportion of the circulated particles being removed. An increase of the distance between the sorbent inlet and the sorbent outlet results in the sor¬ bent particles being moved a longer distance inside the filter, before they are removed via the sorbent outlet. The number of circulations having the values a, β as shown in Fig. 3 and the above-mentioned distance is about 10, being about 4°, β being about 8° and the distance being about 5 m.

Since the rate of flow of the process gas at the side wall 9c of the bottom portion is lower in the embodiment according to Figs. 2 and 3 than in the embodiment accord¬ ing to Fig. 1, a higher sorbent concentration is obtained at the sorbent outlet 12, where the sorbent particles have a high degree of saturation and thus are of poor quality, according to the embodiment in Figs. 2 and 3. This causes an improvement of the separation of the gaseous pollutants of the process gas in the bottom portion 9 and the filter portion 10 adjacent the side walls 9c and 10c in this embodiment.

The invention is of course not limited to the embodi¬ ment described above, but can be modified in various ways within the scope of the appended claims.

For example, the bottom portion 9 can be provided with two sorbent inlets and one sorbent outlet instead of one sorbent inlet 11 and one sorbent outlet 12. The sor¬ bent inlets can be arranged in the side walls 9b and 9c of the bottom portion, while the sorbent outlet can be arranged midway between the side walls at the rear wall 9d of the bottom portion. It is also possible to provide the bottom portion 9 with two sorbent outlets and one sorbent inlet, and in this case the sorbent outlets can be arrang- ed at the side walls 9b and 9c of the bottom portion, while the sorbent inlet can be arranged midway between the side walls in the rear wall 9d.

For example, the gas inlet 3 and the filter chambers 6 and 7 can be of circular cross-section instead of rec- tangular.

For example, the control means can be formed as guide vanes arranged in the gas inlet or a wedge-shaped plate instead of a serrated plate 18.

Claims

1. A method for increasing the degree of utilisation for a dry absorbent and/or adsorbent, hereinafter referred to as sorbent, for absorption and adsorption, respective¬ ly, of gaseous pollutants, such as hydrogen fluoride, from a polluted process gas, such as exhaust gases from elec¬ trolytic furnaces, said sorbent being separated in a fabric filter (1) after reacting with the gaseous pollutants, said fabric filter having a gas inlet (3) for supplying the polluted process gas, a gas outlet (4) for removing the cleaned process gas, and dust separating means (8), whereupon a proportion of the separated sorbent is recycled to the polluted process gas by internal circulation inside said fabric filter, fresh sorbent being supplied to said fabric filter (1) via at least one sorbent inlet (11), while circulated sorbent is removed from said fabric filter via at least one sorbent outlet (12) separated from the sorbent inlet, c h a r a c ¬ t e r i s e d in that the polluted process gas is supplied via the gas inlet (3) to said fabric filter (1) in such manner that only a partial flow of the process gas is introduced at the sorbent inlet (11) of said fabric filter (1) and is supplied with fresh sorbent, whereby the fresh sorbent, for the difference in sorbent concentration occurring between the partial flow and the remaining flow of the process gas to be levelled out, is caused to move in the direction of the sorbent outlet (12) where the sorbent is removed and the sorbent concentration is at its lowest, at the same time as it is caused, by the flow of the process gas and under the action of gravity, to circulate at least twice inside said fabric filter (1) between the dust separating means (8) and the gas inlet (3), without simultaneous recycling of polluted process gas.

2. The method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the sorbent is circulated 2-100, preferably 2-20, times inside said fabric filter (1).

3. The method as claimed in claim 1 or 2, c h a - r a c t e r i s e d in that a larger proportion of the process gas is introduced at the sorbent inlet (11) of said fabric filter (1) than at the sorbent outlet (12) thereof by a control means (18) arranged in said gas inle (3).

4. An apparatus for carrying out the method accordin to any one of the preceding claims, for increasing the degree of utilisation for a dry absorbent and/or adsor¬ bent, hereinafter referred to as sorbent, for absorption and adsorption, respectively, of gaseous pollutants, such as hydrogen fluoride, from a polluted process gas, such as exhaust gases from electrolytic furnaces, said apparatus comprising a fabric filter (1) having a gas inlet (3) for supplying the polluted process gas, a gas outlet (4) for removing the cleaned process gas, dust separating means (8), at least one sorbent inlet (11) for supplying fresh sorbent, and at least one sorbent outlet (12) separated from said sorbent inlet and adapted to remove circulated sorbent, c h a r a c t e r i s e d in that said gas inlet (3) is designed relative to the sorbent inlet (11) so that only part of the gas inlet communicates with the sorbent inlet, and that the sorbent inlet (11) and the sorbent outlet (12) are arranged at the circumference of said fabric filter in a predetermined spaced-apart rela¬ tionship.

5. The apparatus as claimed in claim 4, c h a ¬ r a c t e r i s e d in that the fabric filter is a bag filter (1).

6. The apparatus as claimed in claim 4 or 5, c h a ¬ r a c t e r i s e d by a control means (18) extending across said gas inlet (3) and adapted to control the introduction of the polluted process gas into said fabric filter (1) and the introduction of the sorbent into the polluted process gas.

7. The apparatus as claimed in claim 6, c h a ¬ r a c t e r i s e d in that the surface of said control means (18) increases in a direction away from the sorbent inlet (11) of said fabric filter, towards the sorbent out¬ let (12) thereof.

8. The apparatus as claimed in claim 6 or 7, c h a ¬ r a c t e r i s e d in that said control means is in the shape of a serrated plate (18).

9. The apparatus as claimed in claim 8, c h a ¬ r a c t e r i s e d in that the length of the teeth (19) of said plate (18) decreases in a direction away from the sorbent inlet (11) of said fabric fil er (1), towards the sorbent outlet (12) thereof.

10. The apparatus as claimed in any one of claims 4-9, c h a r a c t e r i s e d in that one or two sor¬ bent inlets (11) are arranged at one (9b; 9d) or two (9b, 9c) of the sides of said fabric filter, and that one or two sorbent outlets (12) are arranged at another side (9c; 9d) or two other sides (9b, 9c) of said fabric filter.

11. The apparatus as claimed in claim 10, c h a ¬ r a c t e r i s e d in that only one sorbent inlet (11) is arranged at one side (9b) of said fabric filter, and that only one sorbent outlet (12) is arranged at another side (9a, 9c, 9d) of said fabric filter.

12. The apparatus as claimed in claim 11, c h a ¬ r a c t e r i s e d in that said sorbent outlet (12) is arranged at a side (9c) opposing said one side (9b).

13. The apparatus as claimed in claim 12, c h a ¬ r a c t e r i s e d in that said control means (18) extends from one gas inlet side (3b) which adjoins one fabric filter side (9b), to the opposite gas inlet side (3c) adjoining the opposite side (9c) of said fabric filter.