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

• a device for collection of hot gas from an electrolysis process and a method for gas collection with said device
• the present invention relates to a method and a device for collection off gases in an electrolysis cell, in particular a cell for aluminium production.
• the superstructure above the cell has several individual point feeders connected to the cell superstructure.
• the gas collection system has several suction points distributed along the process gas duct, located in the top of the superstructure, but as a separate system adjacent to the alumina feeding system. Since at least one anode normally has to be replaced by a new anode every day, modern prebake cells has a superstructure with many lids covering the area between the cathode and the gas skirt located just below the anode beam to prevent the flue gases from entering the potroom.
• the air entering the inside the superstructure also provides air cooling of the upper part of the cell with its installed equipment (pneumatical-, electrical- and electronical- equipment).
• the air entering the inside the superstructure also provides air cooling of the upper part of the cell with its installed equipment (pneumatical-, electrical- and electronical- equipment).
• PTS Pot Tending Suction
• efficient flue gas collection can be obtained during this operation by increasing the suction volume significantly by setting the cell into Pot Tending Suction (PTS) mode for instance via a separate suction string.
• PTS Pot Tending Suction
• the gas suction can change from normal to PTS, and the increased suction volume enables handling the anode replacements with several lids removed from the cell without any flue gases entering the potroom, i.e maintaining the negative pressure inside the cell super structure.
• Feeding alumina to an electrolysis cell was performed more than a century ago by manually breaking the top crust of alumina and feeding alumina powder to the cell.
• the crust breakage was later done by a crust breaking wheel, than a crust breaker beam and finally an electronically controlled point crust breaker, which is being installed at basically all new smelters being built.
• point feeding is therefore considered state of the art.
• the production of aluminium also gives effluents, mainly CO 2 with traces of CO, but also significant amounts of HF and SO 2 .
• effluents leave the electrolytic process through one layer of solidified crust above the electrolyte, through feeding holes but also through the crust itself.
• the present invention generally relates to gas collection, preferably with an alumina feeder integrated.
• the invention relates to a method of collection of concentrated process gas for further treatment.
• this device enables collection of process gas with enough elevated temperatures suitable for heat-recovery, such as flue gas that has a temperature of more than 10O 0 C, preferably more than 150°C.
• Cooling the process gas will contribute to reduced gas flow rate and pressure drop, with reduced fan power as a consequence.
• the largest reduction in pressure drop is achieved by cooling the process gas as close to the aluminium cells as possible.
• the energy content of the process gas can be recovered in a heat exchanger (heat recovery systems) in which the process gas gives off heat (is cooled) to another fluid suitable for the application in question.
• the heat recovery system can be located:
• Cooling of the raw gas upstream the fans in combination with heat recovery is a solution that will reduce both the process gas volume flow rate and the pressure drop in channel system and gas cleaning plant.
• the suction can thereby be increased without the need of changing the dimensions of channels and gas cleaning plant.
• the heat recovered from the process gas is available as process heat for various heating and processing purposes, like CO 2 sequestration.
• the present suction device for gas collection is able to obtain an efficient collection of the flue gases produced in the cell without alumina or anode cover material (ACM) entering the suction device.
• ACM anode cover material
• US Patent 4, 770,752 from 1988 describes a system where the gas collection cap is placed in contact with the crust in correspondence of a hole provided in the crust.
• the purpose of this invention is to collect the flue gases from the cell for purification of fluoride components by alumina and thereafter return the alumina and the fluorides to the cell again by a separate alumina feeder. CO 2 scrubbing and heat recovery are not mentioned except from preheating the alumina.
• This invention has a limitation with respect to maintenance and possible damages occurring during anode replacements since the cap is situated so close to the anodes and the crust. There is no indication of any plant which utilised this invention, supporting the said drawbacks.
• JP Patent 57174483 from 1981 describes a method and device for continuous measurement of current efficiency of an aluminium electrolysis cell.
• the purpose is to measure current efficiency quickly and continuously and to control supplying of raw materials by collecting the gases produced from the cell continuously, measuring the concentrations of CO 2 and CO successively, converting these to electrical signals and inputting the signals to a controller.
• the collection device is not fully described but seems to be situated in contact with the crust with the drawbacks just described.
• US Patent 4, 770, 752 from 1988 describes a system where a cap is placed in contact with the crust in correspondence of a hole provided in the crust.
• the purpose of this invention is to collect the flue gases from the cell for purification of fluorine components by alumina situated close to the cell and thereafter feed the alumina and said components directly back into the same cell from which they have been emitted.
• US Patent 5,968,334 describes removal of at least one of the gases CF 4 and C 2 F 6 from the flue gases from an electrolysis cell using a membrane.
• the present invention relates further to the principles of Distributed Pot Suction (DPS) where one can combine feeding the raw material alumina to the cell and at the same time extract a more CO 2 -concentrated flue gas from a hole in the top crust in the cell than what is standard procedure in the aluminium industry today.
• DPS Distributed Pot Suction
• the suction can also be arranged at other places above the crust in the cell, if appropriate.
• Fig. 1 discloses one embodiment of a distributed pot suction (DPS) device in accordance with the invention
• Fig. 2 discloses a fluid dynamic model of the flue gas collection from a suction device comprising a cap with a single wall design
• Fig. 3 discloses a fluid dynamic model of the flue gas collection from a suction device comprising a cap with a double wall design
• Fig. 4 shows (in part) a picture of a double wall collection cap seen from underneath
• Fig. 5a discloses in a cross sectional view, a second embodiment of a DPS
• Fig. 5b discloses in a side view the DPS shown in Fig. 5a, rotated 90 degrees about its length axis
• Fig. 5c discloses in an enlarged view, a distributor plate for the DPS shown in Fig.
• Fig. 6 discloses a diagram showing the CO 2 concentration in a cell with traditional flue gas collection from inside the cell superstructure
• Fig. 7 discloses a diagram that shows the CO 2 concentration under varying conditions from "normal” to the left, to “pure DPS collection "to the right,
• Fig. 8 discloses a schematic a gas flow pattern in the superstructure of a cell operated with five DPS units, seen from above,
• Fig. 9 discloses a diagram that shows the pressure distribution / gas flow in an electrolysis cell with suction out of top of the cells superstructure
• Fig. 10 discloses a diagram that shows the pressure distribution / gas flow in an electrolysis cell with suction in accordance with the DPS present invention and with no suction in the top of the superstructure.
• DPS distributed pot suction device
• One of the prototypes designed during the development of the invention had a single wall collection cap 4' (see the CFD modelling results of the collection efficiency in Figure 2).
• Another version of the suction cap 4 had double walls (see Figure 3) where the suction velocity between the double walls is significantly higher than in the centre. Thicker lines indicate higher suction rates.
• FIG. 1 a pneumatic cylinder of a crust breaker is indicated at reference sign 1, the breaker is attached to the DPS main parts.ln the Figure there is shown collection cap 4, alumina feeding tube 3, gas suction duct 5 with a valve 6. At the other side of the valve there is shown a duct 2.
• the suction for the DPS that is introduced through the dedicated duct 2 may preferably be connected retrofit to an existing feeder, or alternatively it could also be part of a new assembly replacing an existing feeder.
• the alumina may be feed from a fluidised feeder but also mechanical feeders.
• the gas When the gas is drawn through the duct 2, it will be collected into a main duct/manifold on the pot superstructure conveying gas from all feed points (not shown).
• the gas is from this transition points transported to the fume treatment systems (i.e. Fluoride recovery, and SO 2 removal) and introduced from there to any commercial CO 2 scrubbing system able to handle the actual concentrations of CO 2 or as input to combustion systems such as gas turbines, coal power plants or biomass combustion plant.
• the main collection duct for DPS points on the superstructure can be closed, and the main ducts in the pot superstructure is activated to support pot tending suction (PTS) from the pot (i.e. increasing the pot suction volume 2-4 times higher than normal.
• PTS pot tending suction
• the up-concentrated process gas is hotter than normal which makes it suitable for heat recovery.
• the warmer gas may damage the superstructure and electronics placed there.
• One way to solve this new challenge is to thermally insulate the components of the gas-collection systems within the superstructure and to the place where the heat recovery can take place outside the cell.
• Another alternative can be to arrange the gas collection caps and its corresponding ducting with some space with regard to other installations inside the superstructure of the cell.
• Process gases from several cells can be connected to the same heat recovery unit.
• the process gas is then sent for classical fume treatment, removing dust, HF and SO 2 .
• the flue gas might has to be purified sufficiently not to damage these process steps.
• the main features of one embodiment of the present invention consist in the integration of the point suction system with the alumina point feeder having a crust breaker.
• the step forward caused by the DPS is changed composition and increased temperature of the collected process gas.
• the gas collected by the DPS will contain much less "false air” and consequently have higher concentration of hazardous gases (Fluoride, SO x , and CO 2 ). This will ease the fluoride recovery and SO x removal.
• the aim is to increase the concentration of CO 2 to such a level that commercial available CO 2 scrubbing technologies can be utilised to remove it. Also, because of the smaller amount of air and installation straight above the feeding points the collected off-gas has increased temperature compared to the regular process gas, which increases the potential for heat exchange.
• process gas collection cap could be customised for any type of point feeder, and also be arranged in the vicinity of such feeder without being an integrated part of it.
• the suction in duct 2 in figure 1 can also be split into two independent suction flows that can be regulated, where the suction from the space 11 between the inner- and outer walls 14, 13, and the suction at the inside 12 of the cap 4 can be independently regulated. See also Fig. 4.
• the inner wall 14 of the suction cap 4 can be both solid and perforated, i.e. provided with holes or not (not shown). Furthermore, the walls of the suction cap 4 can be angled outwards in such a way that the suction velocity vector can be aimed in any angle between 0-180 degrees downwards towards the crust.
• Fig. 5a it is disclosed, in a cross sectional view, a second embodiment of a DPS, integrated with a point feeder (PF).
• PF point feeder
• an inner wall 28 shaped as a rectangular sleeve
• an outer wall 26 also shaped as a rectangular sleeve.
• the space between the inner and outer walls defines a suction space between these two walls, with opening 15.
• the inner wall extends closer towards the crust than outer wall, and has suction opening 16.
• Fig. 5b discloses in a side view the DPS as shown in Fig. 5a, rotated 90 degrees about its length axis.
• the outer wall 26, outlet 22 and 22' and a manifold plate 30 In this view there is shown the outer wall 26, outlet 22 and 22' and a manifold plate 30.
• the manifold plate is shown in more detail in an enlarged view, in Fig. 5c.
• the purpose of the manifold plate is to distribute the suction through outlet 22, 22' evenly into the space between the outer and inner walls. This is achieved by the arrangement of appropriate openings, O, O', O", O'" or slots through the plate.
• the plate has further openings for the alumina feed tube 23' and one stem of the point feeder PF.
• the lower part of the outer wall 26 may be provided with a diverging deflector (not shown).
• the deflector can be represented by a plate shaped part at all sides of the wall, and preferably having an angle ⁇ with regard to the horizontal plane. The purpose of the deflector is to assist the guiding of the flow of gases that is sucked into the gas cap.
• the angle ⁇ may preferably be of magnitude 30 - 60 °.
• a dust trap 29 in the inner part of the cap to avoid alumina and other particular constituents to follow the sucked off gas further into the gas evacuating system.
• the dust trap in such an embodiment can be represented by one or more slots 29 in the inner wall, i.e. the wall dividing the space between the double walls from the inner space of the suction cap.
• the slot is arranged near the top wall of the inner space in the cap, and in such manner that when suction is applied to the annular space, there will be a suction of gas through said slot.
• the effective gas flow opening of the slots can be designed in a manner where a suction in the space between the outer- and inner walls also will generate an appropriate suction inside the space defined by the inner wall, thus defining a relationship between the suction rate of inlet 15 versus inlet 16.
• the cross sectional area between the inner wall and the outer wall is increasing downstream a flow from the second inlet 15, thus reducing gas velocity.
• the suction cap is preferably placed at a distance from the crust allowing the anodes to pass beneath it during anode change.
• the cap is placed at a minimum distance to the crust depending on the suction rate. Preferably the distance is in order 10 to 1000 mm.
• the distance has to take into account the pickup velocity for alumina/anode cover material (ACM) which is in order of 7 metres per second, hence the said distance between the cap and the top of the crust should ensure that this level of velocities at the surface of the crust is not reached.
• ACM alumina/anode cover material
• This embodiment of DPS is designed to separate by physical measures the hot gas to be sucked off and the technical parts of the crust breaker as much as possible, to induce as little thermal stress as possible to vital parts of the crust breaker.
• Fig. 6 there is disclosed a diagram that shows the CO 2 concentration in a cell with traditional flue gas collection from inside the cell superstructure.
• Fig. 7 there is disclosed a diagram that shows the CO 2 concentration under varying conditions from "normal” to the left, to “pure DPS collection “to the right.
• Fig. 8 is a diagram that shows a schematic flow pattern in a cell, based upon five DPS units in the cell, seen from above. The arrows indicate the gas flow pattern above the crust, which is clearly directed towards the individual suction points.
• FIG. 9 there is shown the pressure distribution / gas flow in an electrolysis cell of commonly known type with evacuation "E" of process gas in the top of the cells' superstructure.
• evacuation "E” evacuation of process gas
• the cell is in a normal operation modus of a cell with closed superstructure, and all lids closed.
• Fig. 10 there is shown a pressure distribution in an electrolysis cell with evacuation "E" of process gas in accordance with the present invention by means of five DPS units and with no suction in the top of the superstructure.
• the cell is in a normal operation modus of a cell with closed superstructure.
• the CO2 capture and storage in accordance with the present invention can in one embodiment be performed in the following steps: I) CeII CO2 production

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Treating Waste Gases (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Sampling And Sample Adjustment (AREA)

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• 2008
  • 2008-09-19 NO NO20084014A patent/NO332375B1/no unknown
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