Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/22—Collecting emitted gases

Definitions

• the present invention relates to a system and a method for recovering energy released by an igneous electrolysis cell during the manufacture of aluminum.
• an electrolytic cell having a heat exchanger in its walls is difficult to manufacture, and it is very difficult to ensure the maintenance of the exchanger, and its possible replacement.
• the present invention is intended to provide a simpler energy recovery system and some embodiments allow greater energy recovery.
• the subject of the invention is a system for recovering energy released by at least one igneous electrolysis cell generating fumes and gases during the manufacture of aluminum, the recovery system comprising a first loop of heat exchange traversed by a coolant, the first loop comprising: a first heat exchanger traversed by the heat transfer fluid; and a recovery unit capable of recovering the heat of the coolant having passed through the first heat exchanger; characterized in that the recovery system further comprises: a primary circuit capable of collecting at least a portion of the fumes and gases generated by the electrolytic cell, the first heat exchanger being disposed outside and at a distance from the electrolytic tank; the first heat exchanger being traversed by the fumes and gases collected by the primary circuit to heat the coolant.
• the subject of the invention is also a method of recovering energy released by an igneous electrolysis cell during the manufacture of aluminum, characterized in that the method comprises the following steps:
• the method further comprises the following steps:
• FIG. 1 is a diagrammatic sectional view of an electrolytic cell; and an energy recovery system according to a first embodiment of the invention
• FIG. 2 is a diagram illustrating the steps of the energy recovery method according to the first embodiment of the invention.
• FIG. 3 is a diagrammatic sectional view of an electrolysis cell and of a system for recovering energy according to a second embodiment of the invention
• FIG. 4 is a diagram illustrating the steps of the energy recovery method according to the second embodiment of the invention
• Figure 5 is a schematic sectional view of an electrolytic cell and an energy recovery system according to a third embodiment of the invention.
• FIG. 6 is a diagram illustrating the steps of the energy recovery method according to the third embodiment of the invention.
• the same or similar elements of the first, second and third embodiments of the energy recovery system are hereinafter referred to by the same references and are described only once.
• an igneous electrolysis cell 2 comprises a parallelepipedal box 4 open at its upper base and whose bottom carries carbonaceous blocks constituting the cathode 6.
• This box 4 contains an electrolyte bath 8 consisting of alumina dissolved in cryolite, brought to a temperature between 950 ° and 1000 ° C.
• anodes 10 are immersed.
• the alumina is decomposed into aluminum 12 forming a metal bath which covers the cathode 6, and oxygen which reacts with each anode 10 and causes the progressive combustion.
• the aluminum 12 is regularly removed from the electrolysis cell 2.
• the upper part of the electrolyte bath 8 is solidified, thus constituting a crust 14 which covers the bath 8 and thermally insulates it.
• the reaction at each anode 10 gives rise to an emission of fumes and gases 18, 29 which migrate to the top of the tank comprising pollutants such as carbon dioxide and carbon monoxide, sulfur dioxide, gaseous hydrogen fluoride (HF), carbon and alumina particles, dusts and fluorinated compounds.
• pollutants such as carbon dioxide and carbon monoxide, sulfur dioxide, gaseous hydrogen fluoride (HF), carbon and alumina particles, dusts and fluorinated compounds.
• a movable tubular steel rod installed between two anodes 10, pierces the crust 14 and Alumina is injected into the electrolyte bath 8.
• This rod hereinafter referred to as a metering injector 16 is operable in a vertical movement by means of a jack, preferably a pneumatic cylinder, to pierce the crust 14.
• the energy recovery system 22 comprises a primary circuit 24 for collecting the fumes and gases produced by the electrolytic cell 2, and a heat exchange loop 26, ci after called the first heat exchange loop 26.
• the primary circuit 24 includes a recovery device 28 for fumes and gases
• the cover 20 is not hermetic and outside air is sucked under the hood 20 by the leaks due to the depression. This air mixes in large quantities with the fumes and gases coming from the tank. In the rest of the text we will no longer distinguish this air smoke and gas and we will call by the generic terms smoke and gas.
• the fumes and gases 29 are sucked by the recovery device 28 with a specific flow rate of about 75,000 to 100,000 Nm / ton Aluminum (which corresponds to 8400 to 11000 NmVh for a tank of 350 kA).
• the fumes and gases 29 collected by the primary circuit 24 have a temperature of about 110 - 160 ° C. They contain 200 - 800 mg / Nm 1 of dust, 150 - 400 mg / Nm 3 of gaseous hydrogen fluoride and 100 - 400 mg / Nm 3 of sulfur dioxide
• the first treatment unit 30 is able to filter the dust contained in the fumes and gases 29 collected by the recovery device 28, and to eliminate most of the hydrogen fluoride gas by adsorption of hydrogen fluoride gas from these fumes. and gas on alumina which is then separated from the fumes and gases 29 by filtering.
• the first processing unit 30 is adapted to treat the fumes and gases 29 according to a dry process known under the name "DRY-SCRUBBER DS". This process allows a dust reduction of greater than 98% by filtration and a reduction of hydrogen fluoride gas of about 99.8% by adsorption and filtration.
• the amount of hydrogen fluoride gas contained in the fumes and gases exiting the first treatment unit 30 is less than 0.5 mg / Nm.
• the amount of dust contained in the fumes and gases coming out of the first unit of treatment is less than 5 mg / Nm
• the fluorinated alumina generated by the treatment of fumes and gases 29 by the first treatment unit 30 is introduced into the electrolytic cell 2 by means of the metering piercer 16.
• the primary circuit 24 further comprises a scrubber 32 with seawater or with a basic solution capable of eliminating, by absorption and chemical reaction, the sulfur dioxide (SO 2 ) contained in the fumes and gases leaving the first treatment unit 30, and a chimney 34 capable of evacuating the remaining fumes and purified gases.
• a scrubber 32 with seawater or with a basic solution capable of eliminating, by absorption and chemical reaction, the sulfur dioxide (SO 2 ) contained in the fumes and gases leaving the first treatment unit 30, and a chimney 34 capable of evacuating the remaining fumes and purified gases.
• the flue gas 29 comprises less than 60 mg / Nm 1 of sulfur dioxide.
• the first heat exchange loop 26 comprises a heat exchanger 36, hereinafter called the first heat exchanger 36, and a heat recovery unit 38 of a heat transfer fluid.
• the loop 26 is traversed, on the one hand, by the coolant, and on the other hand, at the level of the first heat exchanger 36, by the fumes and gases 29 leaving the recovery device 28 before they pass through the first treatment unit 30.
• the heat transfer fluid heated by the first heat exchanger 36 is used, for example, to produce electricity by an ORC cycle generator, that is to say a generator.
• Organic Rankine Cycle is used, for example, to produce electricity by an ORC cycle generator, that is to say a generator.
• Organic Rankine Cycle is for example constituted by water, oil or an inert gas.
• the first heat exchanger 36 is disposed outside the electrolysis cell 2. For example, it is disposed at a predefined distance greater than or equal to one meter from the electrolytic cell 2 to avoid any risk of contact between the heat transfer fluid and the electrolysis cell 2.
• the first heat exchanger 36 is able to cool the fumes and gases 29 of the primary circuit 24 by a temperature of 110-160 ° C. at a temperature of 70-100 ° C.
• a bypass line 39 is installed on the primary circuit 24 at each end of the first heat exchanger 36 to allow the fumes and gases 29 to bypass the first heat exchanger 36, when it is dirty and must be cleaned or replaced or renovated.
• the energy recovery method begins with a step 100 for collecting the fumes and gases 29 by the recovery device 28.
• the fumes and gases 29 collected by the primary circuit 24 are transported outside and away from the electrolysis cell 2.
• the heat transfer fluid of the first heat exchange loop 26 is warmed by passing fumes and gases 29 through the first heat exchanger 36.
• the recovery unit 38 recovers heat from the heat transfer fluid that has passed through the first heat exchanger 36.
• the processing unit 30 processes the fumes and gases 29 collected by the primary circuit 24 by filtering the dust and removing most of the hydrogen fluoride gas. Then, the fluorinated alumina generated by the first treatment unit 30 is introduced into the electrolyte bath 8 by the metering injector 16. During a step 116, part of the sulfur dioxide contained in the fumes and gases 29 collected by the primary circuit 24 is removed by absorption and chemical reaction, by the washer 32.
• the primary circuit 24 does not include a scrubber 32.
• the step 116 is not performed.
• the fumes and gases 29 leaving the first treatment unit 30 are directly discharged through the chimney 34.
• the recovery unit 38 is a system for using the heat of the coolant to produce cold or heat.
• the recovery system 40 according to the second embodiment of the invention, illustrated in FIG. 3, comprises a primary circuit 24 similar to the primary circuit 24 of the recovery system 22 according to the first embodiment of the invention. except that this circuit does not have a washer 32.
• the fumes and gases 29 collected by the recovery device 28 of the primary circuit 24 are sucked at a rate of 70000 to 100000 Nm 3 / ton of aluminum product. They comprise 100-800 mg / Nm 1 of dust, 30-100 mg / Nm 1 of hydrogen fluoride gas, 20-100 mg / Nm 3 of sulfur dioxide, 2-4 g / NmMe carbon dioxide and 0, 1 - 0.3 g / NmMe carbon monoxide. These fumes and gases collected by the primary circuit are referenced hereinafter by the reference 29.
• These fumes and gases 29 collected by the primary circuit have a temperature of about 110-160 ° C. before passing through the first heat exchanger 36, and a temperature of about 70-100 ° C. at the outlet of the first heat exchanger. 36.
• the fumes and gases 29 comprise less than 5 mg / Nm3 dust and less than 0.5 mg / Nm 3 of gaseous hydrogen fluoride.
• the recovery system 40 according to the second embodiment comprises a first heat exchange loop 26 similar to the heat exchange loop 26 of the recovery system 22 according to the first embodiment of the invention, with the exception of that this first heat exchange loop 26 further comprises a second heat exchanger 42.
• the second heat exchanger 42 is disposed outside and at a distance of at least one meter from the electrolytic cell.
• the second heat exchanger 42 is disposed downstream of the first heat exchanger 36, that is to say that the coolant passes first through the first heat exchanger 36 and then the second heat exchanger 42 before join the recovery unit 38.
• the recovery system 40 further includes a secondary circuit 44 for collecting a portion of the fumes and gases 18 produced by the electrolysis cell 2.
• the fumes and gases collected by the secondary circuit 44 are referenced hereinafter by the reference 18.
• This secondary circuit 44 comprises a hood, hereinafter called “local hood” 46 directly embedded in the crust 14 which collects the fumes and gases 18, also called anodic gas, escaping through the hole drilled by the metering piercer 16.
• Local hood 46 houses dosing device 16. Local hood 46 is connected to a flue gas collection tube 48.
• An opening 50 made in the local hood 46 can suck fumes and gases 29 collected by the primary circuit 24 and located under the hood 20 to lower the temperature of the fumes and gases 18 collected by the secondary circuit 44 and sucked by the hood Local 46.
• the opening 50 in the local hood can be set to change the ratio of smoke and gas collected by the primary circuit / fumes and gases collected by the secondary circuit to act on the anodic gas capture efficiency and the resulting temperature of the mixture.
• 4 to 6 local hoods 46 can be installed in an electrolysis cell 2 from 300 to 400 kA to ensure a good distribution of the suction and capture the maximum of fumes and gases 18.
• the pipes between the second heat exchanger 42 and each local hood 46 may be insulated to avoid energy losses that would be important given the small diameter of the pipes.
• the fumes and anodic gas 18 collected by the secondary circuit 44 contain 1.2 - 8 g / Nm 1 of fluorine gas, 1-8 g / Nm 1 of sulfur dioxide, 110-280 g / Nm 1 of carbon dioxide and 10-26 g / Nm 3 of carbon monoxide.
• They have a temperature of about 200-350 ° C. before passing through the second heat exchanger 42, and a temperature of about 70-100 ° C. after passing through the second heat exchanger 42.
• the secondary circuit 44 further comprises a processing unit 52, hereinafter called a second processing unit 52, a washer 53 and a capture unit 54 connected to the chimney 34.
• the second processing unit 52 is similar to the first processing unit 30 situated in the primary circuit 24. It is capable of removing most of the hydrogen fluoride gas from the fumes and gases 18 collected by the secondary circuit 44 by adsorption and filtering. According to this second embodiment, the second processing unit 52 uses partially fluorinated alumina obtained by the treatment of the fumes and gases 29 collected by the primary circuit 24 by the first treatment unit 30. Then, the fluorinated alumina generated by the second processing unit 52 is introduced into the electrolytic cell 2 by the metering injector 16.
• the fumes and gases 18 collected by the secondary circuit 44 comprise a value of the order of 1 mg / Nrrf of gaseous hydrogen fluoride.
• the scrubber 53 is similar to the scrubber 32 described in the first embodiment of the invention. At the outlet of the scrubber 53, the fumes and gases 18 collected by the secondary circuit 44 have a value of less than 30 mg / Nm 3 of sulfur dioxide. They have a temperature of about 30-40 ° C.
• the capture unit 54 is intended to capture the carbon dioxide by absorption with a solution of ammonia or with amines or other equivalent techniques.
• the energy recovery method starts with the same steps 100 and 102 as the recovery method according to the first embodiment of the invention.
• the fumes and gases 18 located between the crust 14 and the electrolyte bath 8 are collected by the secondary circuit 44, via the local hoods 46.
• steps 112 and 114 which are identical to steps 112 and 114 of the method illustrated in FIG.
• the hydrogen fluoride gas of the fumes and gases 18 collected by the secondary circuit 44 is treated by the second processing unit 52.
• This treatment is similar to the treatment carried out by the first treatment unit 30. the exception that the partially fluorinated alumina generated by the treatment of fumes and gases 29 collected by the primary circuit 24 is used to adsorb the hydrogen fluoride gas from the fumes and gases 18 collected by the secondary circuit 44.
• the fluorinated alumina is introduced into the electrolyte bath 8 by the metering piercer 16.
• the fresh alumina is used to adsorb the gaseous hydrogen fluoride of the fumes and gases containing the least pollutants, that is to say the fumes and gases 29 collected by the primary circuit 24, and is then reused to adsorb the gaseous hydrogen fluoride gas and gas 18 containing a higher concentration of pollutants, that is to say the fumes and gases 18 collected by the secondary circuit 44.
• a mixture, possibly in varying proportions, of fresh alumina and partially fluorinated alumina may be used in second processing unit 52.
• a step 117 the majority of the sulfur dioxide contained in the fumes and gases 18 collected by the secondary circuit 44 is removed by a washer 53.
• the capture unit 54 eliminates by absorption or by other techniques (adsorption, membrane filtration, etc.) a portion of the carbon dioxide from the fumes and gases from the scrubber 53.
• the fumes and gases 29 treated by the treatment unit 30 and the fumes and gases treated by the capture unit 54 are discharged through the stack 34.
• the secondary circuit 44 does not include a capture unit 54.
• the fumes and gases leaving the scrubber 53 are directly directed towards the chimney 34.
• a washer is disposed on the primary circuit 24 between the treatment unit 30 and the chimney 34.
• a bypass line 39 is mounted on either side of one of the first 36 and the second 42 heat exchangers to allow short-circuiting, for example, during the cleaning of these heat exchangers. .
• the recovery system 56 according to a third embodiment of the invention, shown in FIG. 5, comprises a primary circuit 24 and a secondary circuit 44 identical to the primary 24 and secondary 44 circuits of the recovery system 40 according to the second embodiment of FIG. embodiment of the invention.
• the recovery system 56 further comprises a first 26 and a second 62 heat exchange loops.
• the first heat exchange loop 26 of the third embodiment is similar to the first heat exchange loop 26 of the recovery system 40 of the second embodiment of the invention with the exception of the existence of a third heat exchanger 60, also outside the tank 2.
• the third heat exchanger 60 is disposed downstream of the second heat exchanger 42, that is to say that the heat transfer fluid first passes through, the first heat exchanger 36 then, the second heat exchanger 42 and, finally, the third heat exchanger 60 before joining the recovery unit 38.
• the temperature of the coolant of the first loop 26 is about 80-100 ° C. at the inlet of the first heat exchanger 36, of about 100 ° -120 ° C. at the inlet of the second heat exchanger 42, approximately 150 ° - 250 0 C at the inlet of the third heat exchanger 60, and finally about 200 ° - 400 0 C at the outlet thereof.
• the second heat exchange loop 62 comprises a pipe traversed by an intermediate heat transfer fluid.
• the pipe passes through at least one lateral wall 64 of the electrolytic cell 2 and then the third heat exchanger 60.
• the intermediate heat transfer fluid comprises, for example, helium, air or other gas inert vis-a-vis the liquid aluminum.
• the intermediate heat transfer fluid recovers the heat from the walls 64 of the tank, and delivers it to the third exchanger 60. Before entering the third exchanger 60, the temperature of the intermediate heat transfer fluid is between 250-600 0 C.
• the energy recovery method according to the third embodiment of the invention is identical to the recovery method according to the second embodiment, with the exception that it comprises between the steps 106 and 112, a step 108 and a step 110.
• step 108 the intermediate heat transfer fluid of the second heat exchange loop 62 passes through the wall or walls 64 of the electrolytic cell and is thus heated.
• step 110 the intermediate heat transfer fluid passes through the third heat exchanger 60 and thus heats the coolant of the first heat exchange loop 26 already preheated in the exchangers 36 and 42.
• the recovery unit 38 generates electricity from the heat recovered by the coolant having passed through the first 36, the second 42, and the third 60 heat exchangers.
• branch lines 39 are mounted on either side of the second heat exchanger 42 to allow short-circuiting, for example, during the cleaning of this heat exchanger.
• a scrubber 32 is further installed between the processing unit 30 and the chimney 34 in the second and third embodiments of the invention.
• the recovery system 40, 56 of the second and third embodiments of the invention comprises two different smoke and gas treatment circuits having different pollutant percentages and different temperatures.
• Each circuit 24, 44 for treating fumes and gases is adapted to the pollutant levels therein.
• the recovery systems 40, 56 of the second and third embodiments make it possible to increase the efficiency of the first heat exchange loop 26 by passing fumes and gases at medium temperature in a first heat exchanger 36, then the passage fumes and gases at a higher temperature in a second heat exchanger 42, and optionally in a third heat exchanger 60 at an even higher temperature.
• the recovery system 56 according to the third embodiment of the invention comprises two heat exchange loops 26, 62 able to recover the energy, one of the walls 64 of the electrolysis cell, the other, the energy of the two circuits 24, 44 for treating fumes and gases.

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• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
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• 2009
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• 2010
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