SYSTEM AND PROCESS FOR COLLECTING EFFLUENTS FROM AN ELECTROLYTIC CELL

Abstract

The invention provides a system and a process for collecting effluents produced by an electrolytic cell intended for the production of aluminium and for drawing said effluents away from the cell in a flow of gas. The system comprising a hooding to confine the effluents, at least one outlet channel to collect said flow of gas and suction means to draw said flow of gas away from the cell. The hooding includes removable hoods and, optionally, at least one door to get access to the inside of the hooding. The system further comprises at least one pipe for blowing pressurized air within the outlet channel so as to increase the rate of said flow of gas. Pressurized air supply is activated at a specified pressure Po so as to obtain a specified flow rate Ro.

Description

FIELD OF THE INVENTION

The invention relates to the production of aluminium by igneous electrolysis. It more particularly relates to the extraction and treatment of airborne effluents produced by electrolytic cells designed for the production of aluminium.

BACKGROUND ART

Aluminium metal is produced industrially by igneous electrolysis, i.e. by electrolysis of alumina in solution in a molten cryolite bath using the well-known Hall-Héroult process. A plant for the production of aluminium comprises a plurality of electrolytic cells, typically several hundreds, which are arranged in rows and connected in series. U.S. Pat. No. 6,409,894 in the name of Aluminium Pechiney describes possible arrangements of plants intended for the production of aluminium using electrolytic cells.

Electrolytic reactions, secondary reactions and high operating temperatures lead to the production of airborne effluents that, in particular, contain carbon dioxide, fluorinated products and dust (alumina, electrolyte bath, etc.).

Release of these effluents into the atmosphere is severely controlled and regulated, not only concerning the ambient atmosphere in the electrolysis room, for the safety of personnel operating close to the electrolytic cells, but also for atmospheric pollution. Pollution regulations in many countries impose limits on effluent quantities released into the atmosphere.

In order to avoid releasing effluents in the ambient atmosphere, it is known to provide an electrolytic cell with an effluent extraction system that typically includes a hooding for confining the effluents and a fan for sucking up the effluents. The hooding is linked through a duct network to a chemical treatment installation common to a series of cells.

The electrolytic cells need to be tended during operation. For example, worn anodes need to be changed for new ones and the liquid aluminium produced by the cells needs to be regularly tapped. For that purpose, the hooding includes means, such as hoods or doors, for getting access to the inner part of the cells for tending operations. However, the removal of hoods or the opening of access doors decreases the collection efficiency of the extraction system and lets some effluents escape into the surrounding atmosphere.

U.S. Pat. No. 4,668,352 in the name of Aluminium Pechiney discloses a device and a process wherein the suction means automatically go into an increased suction mode when the opening of the hooding is detected. More precisely, the temperature of the gases in the extraction ducts of each cell is continuously measured and the system switches into the increased suction mode when an abrupt temperature drop caused by the opening of the hooding is detected in a duct. The increased suction mode is obtained by actuating a movable shutter or flap.

International Patent Application No. WO 01/36716 in the name of Norsk Hydro discloses a double suction system that, for each cell, comprises a second collection channel, a complementary fan and optionally a three-way valve. This system is complicated and includes mechanical means subject to the harsh conditions caused by the effluents. Moreover, this solution significantly increases the investment cost because it requires separate duct networks.

The applicant addressed the problem of finding industrially acceptable alternative means for efficiently increasing the extraction rate of an electrolytic cell.

DESCRIPTION OF THE INVENTION

An object of the invention is a system for collecting effluents produced by an electrolytic cell intended for the production of aluminium and for drawing said effluents away from the cell in a flow of gas, said system comprising a hooding to confine the effluents, at least one outlet channel to collect said flow of gas and suction means to draw said flow of gas away from the cell through said at least one outlet channel, said hooding including removable hoods and, optionally, at least one door, to get access to the inside of the hooding, wherein said system further comprises at least one pipe comprising:

• • a first end that is directly or indirectly connected to a pressurized air supply and
  • a second end that is located inside said at least one outlet channel, includes at least one aperture and is oriented so that pressurized air can be projected through said aperture in a manner that increases the rate of said flow of gas within said at least one outlet channel.

Another object of the invention is a process for collecting effluents produced by an electrolytic cell intended for the production of aluminium and for drawing said effluents away from the cell in a flow of gas circulating in at least one outlet channel, wherein said process comprises:

• • providing the cell with a system for collecting effluents according to the invention,
  • connecting said at least one pipe to a pressurized air supply,
  • activating said suction means so as to create a flow rate in said at least one outlet channel,
  • supplying pressurized air in said at least one pipe at a specified flow rate so as to increase the rate of said flow of gas within said at least one outlet channel.

Pressurized air is typically supplied in said pipe(s) when at least one hood is removed from the cell or when said door is opened.

Advantageously, the pressure and flow rate of the pressurized air in said pipe(s) is adjusted according to the actual suction needs. This embodiment of the invention enables tighter control on the needs for pressurized air supply.

The invention makes it possible to efficiently vary the rate of the flow of gas in the outlet channel(s) without requiring excessively large pressure or flow rates for the pressurized air. The invention avoids using mechanical parts within the flow of effluents coming out of the cell.

The applicant estimates that the gas flow rate R in the outlet channel(s), i.e. the rate of flow of the effluent-carrying gas coming out of a cell, can be increased by a factor between 1.5 and 3 by using a specified pressurized air flow rate Ro, i.e. the rate of flow of the pressurized air blown by the aperture of a pipe in the outlet channel of the cell, that is between 5 and 15% of the normal gas flow rate in the outlet channel(s) and a pressure Po of pressurized air smaller than about 5 bars.

Said pressure Po may be higher than 5 bars, for example when one uses a compressed air system available in a plant to directly supply said pipe with pressurized air without reducing the pressure of the compressed air supply. Such a variation of the invention simplifies said system and is suited for procedures in which few pipes are being supplied simultaneously or in which a mobile or removable arrangement is used when needed on specified electrolytic cells, for example when an electrolytic cell is being started up after refurbishment of its pot.

In order to reduce energy consumption by reducing compressed or pressurized air consumption, said pressure Po of pressurized air is preferably between 30 and 300 kPa (i.e., 0.3 to 3 bars),; and more preferably between 70 and 120 kPa (i.e., 0.7 to 1.2 bar). Such a variation of the invention is particularly advantageous for procedures in which the gas flow rate of several electrolytic cells may be boosted simultaneously. Such a variation is especially suited for fixed systems.

The invention is described in more detail below by reference to preferred embodiments and the appended figures.

FIG. 1 illustrates a cross section view of a typical electrolytic cell intended for the production of aluminium.

FIG. 2 illustrates the upper part of an electrolytic cell equipped with a system for collecting effluents.

FIG. 3 schematically illustrates an arrangement of electrolytic cells that includes a system for collecting effluents and common suction means.

FIGS. 4 and 5 schematically illustrate embodiments of an electrolytic cell equipped with a system for collecting effluents according to the invention.

FIG. 6 illustrates a possible embodiment of a system according to the invention.

FIG. 7 illustrates possible variations of a system according to the invention.

An electrolytic cell (1) designed for the production of aluminium is generally rectangular, with long sides that are typically 10 to 20 meters long and short sides that are typically 3 to 5 meters long and often referred to as ends.

As illustrated in FIG. 1, an electrolytic cell (1) comprises a pot (2) that is usually located below a floor (100) common to several cells and comprises a steel shell (3) lined with refractory material (4, 4′). The pot (2) typically includes carbonaceous cathode blocs (5) that are connected to external electrical conductors (7) using a cathode bar (6) made of an electrically conducting material such as steel. In use, the pot (2) contains a pad of liquid aluminium (8) and an electrolytic bath (9).

As illustrated in FIG. 1, an electrolytic cell (1) also typically includes a plurality of anodes (10, 10′), which are typically made of a carbonaceous material. The anodes (10, 10′) are connected to external electrical conductors (7′) using anode stems (11, 11′) sealed in the anodes and secured to common conductors (12, 12′) called anode beams using removable connectors. The anodes (10, 10′) are partially immersed in the electrolytic bath (9) and are protected from oxidation by a protecting layer (13), called a bath crust, that is mostly comprised of alumina and crushed bath.

An electrolytic cell (1) typically further includes one or more alumina feeders that usually include a hopper (14) for feeding alumina (15) at specified locations within the cell. In modern cells, the feeders are continuously supplied by an alumina conveyor (16) that runs along the cell.

An electrolytic cell (1) further includes a hooding (20) capable of confining effluents produced by the cell (1). As illustrated in FIGS. 1 and 2, the hooding (20) includes a plurality of removable hoods (21, 21′), which are also called covers, on the long sides of the cell to get access to the inside of the hooding from either of the long sides. An electrolytic cell (1) typically includes between 10 and 30 hoods (21, 21′) on each long side, which are usually arranged side by side. The hoods (21, 21′) usually comprise a handle (22, 22′) to facilitate their handling. Hoods (21, 21′) are usually removed for tending the inside of the cell. Typically, a few hoods (21′) are removed from one side of the cell when a worn anode (10′) is to be changed for a new one and put back on the cell when the anode changing operation is completed.

In several technologies, the hooding (20) also includes a door or doors (23) at one end of the cell to get access to the inside of the hooding from that end. The doors (23) are typically shutter doors. The doors (23) are often referred to as tapping doors because they are often used for tapping liquid aluminium out of the cell. This operation is done on a regular basis to remove some of the liquid aluminium (8) produced by the cell.

The hooding (20) typically further includes longitudinal channels (24, 24′) that run along the top of the cell. The flow of effluents circulates within these channels.

As illustrated in FIGS. 2 and 3, the hooding (20) is connected to at least one outlet channel (25) that is coupled to suction means (30, 31). The outlet channel (25) is typically a duct or a conduit. For safety reasons, an intermediate insulating channel (26) is usually interposed between the outlet channel(s) (25) and the suction means (30, 31). The suction means (30, 31) produce a flow of gas that sucks the effluents out of the cell. The flow of gas flows at a rate R. The suction means (30, 31) typically include at least one conduit (30) and at least one fan (31). The suction conduit(s) (30) and the fan(s) (31) may be common to several cells.

As illustrated in FIG. 3, rows of cells are usually connected to common suctions means (30, 31). In this figures, the cells are seen from above.

The normal gas flow rate of a cell depends on the type of cell. For example, the normal gas flow rate typically used for an AP18 type cell of

Aluminium Pechiney, when operated with a current intensity of about 180 000 Amperes, is about 1.4 Nm³/s, while the normal gas flow rate typically used for an AP30 type cell of Aluminium Pechiney, when operated with a current density of about 300 000 Amperes, is about 2.1 5 Nm³/s.

In modern plants, the flow of gas that carries the effluents goes through an installation (40) for the treatment of said effluents.

The effluents comprise a gaseous part (especially containing air, carbon dioxide and fluorinated products, such as hydrogen fluoride) and a solid or “dust” part (containing alumina, electrolytic bath, etc). The effluents are confined by the hooding (20), captured by suction and treated in the treatment installation(s) (40) of the plant. The treatment processes usually remove the solid particles contained in the effluents, typically using separation means such as filters or electrostatic precipitators, extract the fluorine contained in the effluents and leave a residual gas fraction containing a negligible amount of solid particles and fluorinated products. The residual gas fraction mainly contains air and carbon dioxide. Treated air is exhausted through a chimney (32).

Well-known processes for removing the fluorine from the effluents are the so-called wet scrubbing and dry scrubbing processes.

According to the wet scrubbing processes, the flow of gas is usually made to react with compounds, typically sodium carbonate, dissolved in water to form a liquor contained in a wet scrubber. The reacted fluorine comes out of the process in the form of solid compounds, typically CaF2 after reacting the liquor with lime.

According to the dry scrubbing processes, the flow of gas is made to react with powder alumina in a reactor so as produce fluorinated alumina that is partly or completely re-used to feed electrolytic cells.

Treatment installations (40) typically comprise a bank of treatment units in parallel, each unit usually comprising a reactor and separation means.

A system for collecting effluents produced by an electrolytic cell (1) comprises a hooding (20) to confine the effluents, at least one outlet channel (25) to collect and draw the effluents in a flow of gas and suction means (30, 31) to draw said flow of gas away from the cell.

According to the invention, the system further comprises at least one pipe (50) for blowing pressurized air into the outlet channel (25) so as to increase the rate of the flow of gas within the outlet channel (25). Said pipe (50) comprises a first end (51), or “inlet end”, that is directly or indirectly connected to a pressurized air supply (53) and a second end (52), or “outlet end”, that is located inside said outlet channel or one of the outlet channels (25). The pressurized air supply (53) can supply pressurized air at a specified pressure Po and a specified flow rate Ro.

The second end (52) of the pipe (50) includes at least one aperture (54) and is oriented so that pressurized air can be projected through said aperture (54) in a manner that increases the rate of said flow of gas. Typically, said second end (52) is oriented so that pressurized air is projected substantially along the direction of said flow of gas, as exemplified in FIGS. 4 to 6. The projected air forms a jet that boosts the gas flow when needed. The dimension of said aperture (54) is typically between 5 mm² and 1300 mm², and more typically between 300 mm² and 1000 mm². The aperture (54) typically has a circular section with a diameter that is typically between 3 and 40 mm, and more typically between 10 and 35 mm. The total surface area of all said apertures (54) in a given outlet channel (25) is preferably comprised between 300 and 1300 mm² so as to provide enough flow boosting capacity.

The second end (52) of the pipe(s) (50) may optionally be fitted with a nozzle that forms said aperture (54) so as to simplify maintenance and changes of pressurized air flow patterns.

The rate of flow of pressurized air that is ejected through said aperture (54) depends on the air pressure Po inside the pipe or pipes (50) and the size and shape of the aperture (54). In use, the flow rate is preferably adjusted by varying the air pressure Po.

The effluents collecting system according to the invention may include more than one pipe (50) for blowing pressurized air into the outlet channel(s) (25). In other words, the system may include several pipes (50) penetrating in an outlet channel (25) so that their second end (52) with an aperture (54) is located inside the outlet channel (25).

The outlet channel(s) (25) may be substantially straight, as illustrated in FIG. 4. The outlet channel(s) (25) may optionally include a length of duct (27) with an internal cross section that varies along said length and said second end (52) may be located within said length of duct. Said length of duct (27) has an inlet (271) and an outlet (272). In an advantageous embodiment of the invention, said length of duct (27) includes a constriction (28) between said inlet (271) and outlet (272). The inner cross section of the constriction (28) is smaller than the inner cross section of the inlet (271) and the inner cross section of the outlet (272). The length of duct (27) may include a part having the shape of a Venturi duct. The inner cross section of the length of duct (27) may vary smoothly between the inlet (271) and the outlet (272).

FIG. 5 illustrates a variation of this embodiment wherein the outlet channel (25) comprises a first straight section (273) with a first inner cross section, a second straight section (274) with a second inner cross section and a third straight section (275) with a third inner cross section, and wherein said second cross section is smaller than said first and third cross sections so as to form said constriction (28). In that variation, said length of duct (27) includes a first section (276) having a truncated-cone shape located between said first (273) and second (274) straight sections and a second section (277) having a truncated-cone shape located between said second (274) and third (275) straight sections.

The second end (52) of the pipe (50) is preferably located in the vicinity of said constriction (28), typically upstream of a plane (29) where the section of said constriction (28) is narrowest as illustrated in FIG. 5.

In FIGS. 4 and 5, the cells (1) are seen from the side.

In another variation of the invention the system may comprise one or more primary outlet channels (25′, 25′″) merging into a single, main outlet channel (25′″). FIG. 6 illustrates possible embodiments of such a variation wherein the system includes two primary channels (25′, 25″). The cells are seen from above. In the embodiment illustrated in FIG. 6(A), the second end (52) of the pipe (50) is located inside said main outlet channel (25′″). In the embodiment illustrated in FIG. 6(B), the system comprises a first pipe (50′) and a second pipe (50″), a first end (51′, 51″) of each pipe being connected to a pressurized air supply (53), a second end (52′) of the first pipe (50′) being located inside one of said primary outlet channels (25′), a second end (52″) of the second pipe (50″) being located inside the other one of said primary outlet channels (25″). The pressurized air supply (53) is typically common to both pipes (50′, 50″) and optionally to a plurality of cells.

In a possible variation of the invention the system further includes at least one adjustable or removable flow balancing means (60, 60′, 61) located in said at least one outlet channel (25) or downstream thereof. Said flow balancing means makes it possible to balance the normal throughput of each cell of a series of cells in a plant. Said flow balancing means is typically located downstream of said at least one aperture (54) of said at least one pipe (50). When a system for collecting effluents includes one or more intermediate insulating channels (26) said flow balancing means (60, 60′, 61) may be located either downstream or upstream of each said intermediate insulating channel (26). Said flow balancing means is typically selected from diaphragms, shutters and flaps, and may typically be activated by an actuator such as a jack. FIG. 7 illustrates possible embodiments of such variations. In the example illustrated in FIG. 7(A), the system includes a shutter (60) located in a part (251) of the outlet channel that is located downstream of an intermediate insulating channel (26). Said shutter (60) may be vertical, as illustrated, or horizontal or oriented in any other direction. In the example illustrated in FIG. 7(B), the system includes a flap (60′) located upstream of an intermediate insulating channel (26). Said flap (60′) is typically secured to a shaft (61) so as to enable its swivelling. Said flap (60′) typically comprises an orifice so as to allow some flow of air when closed.

The pipe or pipes (50, 50′, 50″) are advantageously connected to the pressurized air supply (53) through a valve (55, 55′, 55″). The valve (55, 55′, 55″) enables a specific activation and control of the specified pressure and flow rate in the pipe or pipes (50, 50′, 50″). The valve (55, 55′, 55″) may be coupled to a regulation system so as to enable automatic control of the specified pressure and flow rate in the pipe or pipes (50, 50′, 50″). A valve (55, 55′, 55″) may be common to more than one pipe (50, 50′, 50″).

According to a possible embodiment of the invention, a pressure reducer may be inserted between said at least one pipe (50, 50′, 50″) and said pressurized air supply (53) so as to make it possible to reduce the pressure to a specified value that is typically between 30 and 300 kPa. Such an embodiment is particularly suited for use with a compressed-air supply.

According to another advantageous possible embodiment of the invention, said pressurized air supply (53) includes a blower that provides pressurized air directly at a specified pressure, which is typically between 30 and 300 kPa. Said blower may be common to more than one electrolytic cell. Such an embodiment saves energy by avoiding compressing air to a value much higher than said specified pressure.

A process for collecting effluents advantageously includes connecting the pipe or pipes (50) of an effluents collecting system according to the invention to a pressurized air supply (53), activating the suction means (30, 31) and supplying pressurized air in said pipe or pipes (50, 50′, 50″) at a specified flow rate Ro.

The supply of pressurized air in said pipe or pipes (50) may be activated manually and/or automatically. The latter embodiment may be implemented using temperature and/or pressure sensors. For example, the temperature and/or the pressure of the gas flowing in the outlet channel(s) (25) may be measured continuously and the supply of pressurized air in said pipe(s) (50) may be activated manually or automatically when a rapid drop in temperature or pressure is detected. For that purpose, a cell (1) may be equipped with a probe or sensor for measuring the pressure and/or the temperature of the flow of gas coming out of the cell and the probe or sensor may be connected to monitoring device that displays alerts signals and/or activates the supply of pressurized when temperature or pressure limits are exceeded. The supply of pressurized air is advantageously activated by a control valve (55, 55′, 55″) or the like, such as an electrically controlled valves or pneumatically controlled valves. Electrically controlled valves can advantageously be connected to a regulation system that can automatically control and activate them.

When said system further includes said at least one adjustable or removable flow balancing means (60, 60′, 61), the process for collecting effluents typically includes opening or removing said flow balancing means so as to ease the flow of gas and thereby further increase the throughput in said at least one outlet channel (25, 25′, 25″, 25′″). Said opening or removal of said flow balancing means (60, 60′, 61) may be done manually or automatically and by activating an actuator.

Typically, the suction means (30, 31) are continuously activated during the electrolysis process and the pressurized air supply (53) is activated when needed and according to needs. Pressurized air is typically supplied in said pipe or pipes (50) when at least one hood (21) is removed from the cell or when a door, usually a tapping door, (23) is opened.

In an advantageous application of the invention the pressurized air may be supplied in said at least one pipe (50, 50′, 50″) when an electrolytic cell (1) is being started up. This may occur when a new cell is being started or when a refurbished cell is being re-started, typically after changing the refractory lining (4, 4′) and cathode blocs (5) of its shell (3).

The specified pressure Po and flow rate Ro may be selected according to needs, in particular according to the suction needs of the system, which may depend on the size of the orifice created by the removal of hoods or the opening of a door. Hence, in an advantageous embodiment of the invention, pressurized air is supplied in said pipe or pipes (50) at a first specified flow rate Ro1, typically by providing a first specified pressure Po1, when at least one hood (21) is removed from the cell and at a second specified flow rate Ro2, typically by providing a second specified pressure Po2, when a door (23) is opened. The first specified pressure Po1 and flow rate Ro1 are typically higher than the second specified pressure Po2 and flow rate Ro2, respectively, so as to increase the gas flow rate for hoods removal more than for door opening since the removal of hoods usually requires a more important air draft than the opening of a door.

Hence, the gas flow rate of a cell has a normal value when the pressurized air supply is not activated and at least a first modified value when the pressurized air supply is activated. Optionally, the gas flow rate of the cell may have a second or more modified values when the pressurized air supply is activated. The modified values are higher than the normal value, thus amounting in an increased flow rate. The normal value for the gas flow rate typically corresponds to the situation when all hoods (21) are in place, the first gas flow rate typically corresponds to the situation when one or more hoods (21) are removed for changing an anode and the second gas flow rate typically corresponds to the situation when a tapping door is opened to remove liquid aluminium from the cell, and the first modified value is higher than the second modified value, e.g. 2 to 3 times the normal gas flow rate when several hoods are removed for changing an anode and 1.5 to 2 times the normal gas flow rate when a door is opened for tapping liquid aluminium or when an electrolytic cell is being started up with all hoods in place.

The ratio Po/P between the pressure Po inside said pipe or pipes 20 (50, 50′, 50″) and the pressure P inside the outlet channel or channels (25, 25′, 25″, 25′″) where the second end (52, 52′, 52″) of the pipe or pipes (50, 50′, 50″) is located is preferably selected so as to avoid shock waves and ensure optimal efficiency in regard to sonic conditions. Said specified flow rate Ro is typically between 5 and 15% of said gas flow rate R. The pressure Po inside the pipe(s) is typically smaller than 5 bars although it may in some embodiments be greater than 5 bars.

The suction means typically include at least one fan (31). This fan (31) provides a normal flow rate in the outlet channel(s) (25, 25′, 25″, 25′″). The outlet channel(s) (25, 25′, 25″, 25′″) is (are) typically connected to the fan (31) by a suction conduit (30). Advantageously, the suction means include a conduit (30) that is common to at least two electrolytic cells (typically a plurality of electrolytic cells) and is connected to at least one common fan (31). The fan (31) is usually located in an installation (40) for the treatment of said effluents or downstream thereof.

LIST OF NUMERIC REFERENCES

• 1 Electrolytic cell
• 2 Pot
• 3 Shell
• 4, 4′ Refractory lining material
• 5 Carbonaceous cathode blocs
• 6 Cathode bar
• 7, 7′ External electrical conductors
• 8 Pad of liquid aluminium
• 9 Electrolytic bath
• 10, 10′ Anodes
• 11, 11 Anode stems
• 12, 12′ Anode beams
• 13 Protecting layer
• 14 Alumina feed hopper
• 15 Alumina
• 16 Alumina conveyor
• 20 Hooding
• 21, 21′ Hoods or covers
• 22, 22′ Handles
• 23 Door
• 24, 24′ Longitudinal channels
• 25 Outlet channel
• 25, 25′ Primary outlet channels
• 25′″ Main outlet channel
• 251 Part of outlet channel
• 26 Intermediate insulating channel
• 27 Length of duct
• 271 Inlet of length of duct
• 272 Outlet of length of duct
• 273 First straight section
• 274 Second straight section
• 275 Third straight section
• 276 First section with a truncated-cone shape
• 277 Second section with a truncated-cone shape
• 28 Constriction
• 29 Plane
• 30 Suction conduit
• 31 Fan
• 40 Installation for the treatment of effluents
• 50, 50′, 50″ Pipe
• 51, 51′, 51″ First end of pipe
• 52, 52′, 52″ Second end of pipe
• 53 Pressurized air supply
• 54 Aperture
• 55, 55′, 55″ Valve
• 60 Shutter
• 60′ Flap
• 61 Shaft
• 100 Floor

Claims

1. System for collecting effluents produced by an electrolytic cell intended for the production of aluminium and for drawing said effluents away from the cell in a flow of gas, said system comprising a hooding to confine the effluents, at least one outlet channel to collect said flow of gas and suction means to draw said flow of gas away from the cell through said at least one outlet channel, said hooding including removable hoods, to get access to the inside of the hooding, wherein said system further comprises at least one pipe comprising:

a first end that is directly or indirectly connected to a pressurized air supply; and

a second end that is located inside said at least one outlet channel includes at least one aperture and is oriented so that pressurized air can be projected through said aperture in a manner that increases the rate of said flow of gas within said at least one outlet channel.

2. System according to claim 1, wherein said second end is oriented so that pressurized air can be projected substantially along the direction of said flow of gas.

3. System according to claim 1, wherein said second end is fitted with a nozzle that forms said aperture.

4. System according to claim 1, wherein the dimension of said at least one said aperture is between 5 mm2 and 1300 m2.

5. System according to claim 1, wherein the dimension of said at least one aperture is between 300 mm2 and 1000 mm2.

6. System according to claim 1, wherein said at least one pipe is connected to said pressurized air supply through a valve.

7. System according to claim 6, wherein said valve is selected from electrically controlled valves and pneumatically controlled valves.

8. System according to claim 6, wherein said valve is coupled to a regulation system.

9. System according to claim 1, wherein said pressurized air supply can supply pressurized air at a specified pressure and a specified flow rate.

10. System according to claim 9, wherein the specified flow rate is between 5 and 15% of said gas flow rate.

11. System according to claim 9, wherein the specified pressure is smaller than 5 bars.

12. System according to claim 9, wherein the specified pressure is comprised between 30 and 300 kPa.

13. System according to claim 9, wherein the specified pressure is comprised between 70 and 120 kPa.

14. System according to claim 1, wherein a pressure reducer is inserted between said at least one pipe and said pressurized air supply so as to make it possible to reduce the pressure to a specified value.

15. System according to claim 1, wherein said pressurized air supply includes a blower that provides pressurized air directly at a specified pressure value.

16. System according to claim 1, wherein said at least one outlet channel includes a length of duct with an internal cross section that varies along said length and wherein said second end is located within said length of duct.

17. System according to claim 16, wherein said length of duct has an inlet and an outlet and includes a constriction between said inlet and outlet.

18. System according to claim 17, wherein said second end is located in the vicinity of said constrictions.

19. System according to claim 17, wherein said second end is located upstream of a plane where the section of said constriction is narrowest.

20. System according to claim 1, wherein the suction means include at least one fan.

21. System according to claim 1, wherein said suction means include a conduit that is common to at least two electrolytic cells and is connected to at least one common fan.

22. System according to claim 21, wherein said fan is located in an installation for the treatment of said effluents or downstream thereof.

23. System according to claim 1, wherein said system further includes at least one adjustable or removable flow balancing means located in said at least one outlet channel or downstream thereof.

24. System according to claim 23, wherein said flow balancing means is selected from the group comprising diaphragms, shutters and flaps.

25. Process for collecting effluents produced by an electrolytic cell intended for the production of aluminium and for drawing said effluents away from the cell in a flow of gas circulating in at least one outlet channel, wherein said process comprises:

providing the cell with a system according to claim 1,

connecting said at least one pipe to a pressurized air supply,

activating said suction means so as to create a flow rate in said at least one outlet channel,

supplying pressurized air in said at least one pipe at a specified flow rate so as to increase the rate of said flow of gas within said at least one outlet channel.

26. Process according to claim 25, wherein the supply of pressurized air in said at least one pipe is activated manually or automatically, or a combination thereof.

27. Process according to claim 25, wherein pressurized air is supplied in said at least one pipe when at least one hood is removed from the cell.

28. Process according to claim 25, wherein the hooding is further provided with at least one door to get access to the inside of the hooding, and wherein pressurized air is supplied in said at least one pipe when said doors is opened.

29. Process according to claim 25, wherein the hooding is further provided with at least one door to get access to the inside of the hooding, and wherein pressurized air is supplied in said at least one pipe at a first specified flow rate when at least one hood is removed from the cell and at a second specified flow rate when said doors is opened.

30. Process according to claim 25, wherein pressurized air is supplied in said at least one pipe when said cell is being started up.

31. Process according to claim 25, wherein the ratio Po/P between the pressure Po inside said at least one pipe and the pressure P inside said at least one outlet channel where the second end of said at least one pipe is located is selected so as to avoid shock waves.

32. Process according to claim 25, wherein the specified flow rate is between 5 and 15% of said gas flow rate.

33. Process according to claim 25, wherein, said system is further provided with at least one adjustable or removable flow balancing means located in said at least one outlet channel or downstream thereof, and said process includes opening or removing said flow balancing means so as to ease the flow of gas and thereby further increase the throughput in said at least one outlet channel.