System and process for collecting effluents from an electrolytic cell

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.

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.

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:

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