Patent Publication Number: US-2021164119-A1

Title: Apparatus and method for controlled alumina supply

Description:
TECHNOLOGY FIELD 
     The present disclosure relates to an apparatus and a method for removing pollutants from process effluent gas produced by electrolytic cells used in an aluminum production plant. More particularly, the present disclosure relates to an apparatus and a method for controlling alumina supply to an electrolytic cell and to a singularly dedicated dry scrubber. 
     BACKGROUND OF THE DISCLOSURE 
     In the process for electrolytic production of aluminum, such as by the Hall-Héroult process, aluminum is produced by reducing aluminum oxide in an electrolytic smelting pot filled with melted electrolyte in the form of a fluoride-containing mineral. During the process, effluent gas is produced comprising fluoride-containing substances such as hydrogen fluoride (HF) and fluorine containing dust. As these substances are extremely damaging to the environment, the substances must be separated before the process effluent gas may be discharged into the surrounding atmosphere. At the same time, the fluorine-containing melt is essential to the electrolytic process, and thus recovery of the fluoride-containing substances is desirable for recirculation to the electrolysis process. This recirculation may take place by adsorption of the fluorine-containing substances on a particulate adsorbent. 
     As noted, the electrolytic reaction occurring in the electrolytic smelting pots produces process effluent gas in the form of hot, particle-laden effluent gas, typically cleaned in a gas cleaning unit before being discharged to the atmosphere. An example of a gas cleaning unit for cleaning the effluent gas generated in electrolytic smelting pots is disclosed in U.S. Pat. No. 5,885,539. The gas cleaning unit disclosed in U.S. Pat. No. 5,885,539 comprises a first contact reactor and a second contact reactor. The effluent gas from the electrolytic smelting pots is first forwarded to the first contact reactor and is, in the first contact reactor, brought into contact with recycled alumina. The partly cleaned effluent gas is then forwarded to the second contact reactor and is, in the second contact reactor, brought into contact with fresh alumina. The partly used alumina is recycled from the second contact reactor to the first contact reactor. A dust removal device removes the alumina from the effluent gas, which is then discharged to the atmosphere. 
     The system for recovery of fluoride compounds comprises a filter system, which is included in a closed system. Stable transport of effluent gas from the aluminum production process to the filter system is important. Stable transport is accomplished using gas ducts through which the effluent gas, by means of large fans, is conveyed. The gas ducts comprise main ducts and branch ducts fluidly connected to the filter system. For each aluminum production electrolytic cell, a branch duct is brought into or connects with the main duct. The cross section of the main duct increases gradually, by means of diffusers as the transported effluent gas quantity increases. It is very important for the environment, but also for the electrolytic process, that effluent gas distribution is as even as possible. Traditionally, even effluent gas distribution is achieved by an increasingly stronger throttling of the effluent gas transported within the branch duct, the closer the proximity of the particular branch duct to the suction fans. Throttling represents sheer energy loss through a pressure drop. 
     Gas cleaning units for cleaning of process effluent gas produced during electrolytic processing of aluminum include both centralized systems and decentralized systems. Centralized systems often connect to one or several halls comprising electrolytic cells whereby each hall may comprise from 70 to 200 electrolytic cells, with cleaning equipment arranged centrally between the halls or outside. The centralized system connects with each of the electrolytic cells by means of comprehensive and costly ductwork. Aluminum oxide used as an adsorbent agent during the effluent gas cleaning process is stored in separate silos, i.e., a silo for aluminum oxide storage before use and a silo for aluminum oxide storage after use in the effluent gas cleaning process. Aluminum oxide stored after use is later transported back to each cell by means of transportation vehicles, cranes or other transportation system for aluminum, such as a system for transportation of aluminum in a compact phase. 
     Decentralized systems are used to clean process effluent gas from 5 to 40 electrolytic cells, more preferably from 10 to 20 electrolytic cells. As such, less ductwork is required, and transportation needs for movement of aluminum oxide are greatly reduced. Large flexibility is achieved as to operation start up, and distance between aluminum oxide storage and the electrolytic cells may be minimized. Additional benefits achieved by decentralized systems are described in U.S. Pat. No. 6,406,524. 
     Although systems for cleaning process effluent gas produced during electrolytic processing of aluminum are known, improved systems that reduce operation costs, reduce equipment footprint, reduce capital costs, and/or increase adaptability to meet specific system requirements for larger production facilities, are still needed in the aluminum production industry. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure relates to an apparatus and a method for removing pollutants from process effluent gas produced by electrolytic cells used in an aluminum production plant. More particularly, the present disclosure relates to an apparatus and a method for controlling a supply of alumina to an electrolytic cell and to a singularly dedicated dry scrubber contact reactor. According to an embodiment of the present disclosure, apparatus is provided wherein each single aluminum electrolytic cell is arranged at a level below that of a singularly dedicated dry scrubber contact reactor for removal of gaseous pollutants, such as hydrogen fluoride, from effluent gas produced by the aluminum production process. The single aluminum electrolytic cell comprises a number of anode electrodes, typically six to thirty anode electrodes, typically arranged in two parallel rows extending along a length of the electrolytic cell and extending into molten contents of a bath. The electrolytic cell also comprises one or more cathode electrodes. The process occurring in the electrolytic cell may be the well-known Hall-Héroult process in which aluminum oxide, also referred to herein interchangeably as “alumina”, is dissolved in a melt of fluorine containing minerals and electrolyzed to form aluminum. Hence, the electrolytic cell functions as an electrolysis cell. Powdered aluminum oxide is supplied to the electrolytic cell from an alumina hopper via gravity, fluidization, mechanical transport and/or similar means. Powdered aluminium oxide is supplied to the bath of the electrolytic cell by means of feeders. Each feeder may be provided with a feeding pipe, a feed port and a crust breaker operative for forming an opening in a crust that often forms on a surface of contents within the bath. An example of a crust breaker is described in U.S. Pat. No. 5,045,168. 
     The electrolysis process occurring in the electrolytic cell generates significant amounts of heat, dust particles, and effluent gas including but not limited to hydrogen fluoride, sulphur dioxide, carbon dioxide, and perfluorinated chemicals (PFCs), i.e., pollutants. The electrolytic cell is arranged within an enclosed housing that defines an interior area. The interior area of the housing includes an outlet. A fan draws effluent gas from the housing via the outlet into an effluent gas treatment system. The fan is preferably located downstream of the effluent gas treatment system to generate a negative pressure within the effluent gas treatment system. However, other arrangements may be utilized for the transport of effluent gas. Due to the negative pressure generated by the fan, some volume of ambient air is drawn into the housing interior area mainly via gaps or openings between side wall doors of the housing. The effluent gas drawn from the housing interior area thereby comprises effluent gas, dust particles generated in the aluminum production process, and a volume of ambient air. 
     In the singularly dedicated effluent gas treatment system arranged at a level vertically above that of the electrolytic cell, effluent gas flows upwardly through a dry scrubber contact reactor in which an adsorbent agent, typically aluminum oxide, is dispersed and thereafter utilized in the aluminum production process. The dispersed aluminum oxide mixes with the effluent gas and interacts with some components of the effluent gas, particularly hydrogen fluoride, HF, and sulphur dioxide, SO 2 , to produce contacted gas. The particulate adsorption products formed by the interaction of aluminum oxide with hydrogen fluoride and sulphur dioxide are entrained in the contacted gas flowing vertically or upwardly from the dry scrubber contact reactor through the effluent gas treatment system to a fabric filter. The particulate adsorption products are removed from the contacted gas via the fabric filter to produce treated gas. In addition to removing hydrogen fluoride and sulphur dioxide from the effluent gas, the effluent gas treatment system via the fabric filter also separates at least a portion of the dust particles entrained within the contacted gas from the housing interior area. 
     The subject dry scrubber contact reactor is arranged downstream of the alumina hopper, which according to one embodiment extends horizontally across a partly porous bottom surface of the effluent gas treatment system housing. Arranged a distance vertically below the partly porous bottom surface is a solid base wall. The effluent gas treatment system housing comprises a top, a partly porous bottom surface with a solid base wall just below, and two opposed side walls defining an open interior. The dry scrubber contact reactor is supplied alumina via the alumina hopper. As such, the alumina flows across the partly porous bottom surface of the effluent gas treatment system from a flow control device to the dry scrubber contact reactor via gravity, fluidization, mechanical transport and/or similar means. The dry scrubber contact reactor is equipped with an effluent gas inlet for a flow of effluent gas therethrough with alumina dispersal into and mixture with the effluent gas within the dry scrubber contact reactor. The effluent gas inlet is arranged between a portion of the side wall of the housing and a retaining wall that abuts free ends of the porous bottom surface and solid base wall, and extends vertically upwardly from the free ends to a free overflow edge. The retaining wall is distanced from the side wall to allow a flow of effluent gas therebetween into the dry scrubber contact reactor. Similarly, according to one embodiment, the dry scrubber contact reactor is arranged between a movable scrubber free wall that extends generally parallel to the side wall from a free base end to an opposed free top end. The scrubber free wall may be electronically and/or manually movable through adjustment of an arm equipped with hinges connected thereto. The arm, connected to the side wall, may have a hinge at or near the side wall. The arm, also connected to the scrubber free wall, may have a hinge at or near the scrubber free wall. Further, the arm may also have a hinge arranged between those of the side wall and the scrubber free wall. The arm equipped with hinges, or other movement mechanism, enables electronic and/or manual movement of scrubber free wall vertically, i.e., closer to or farther from the porous bottom surface, and horizontally, i.e., closer to or farther from the side wall. Through movement and positioning of the scrubber free wall, the supply of alumina to the dry scrubber contact reactor may be controlled or adjusted. Hence, if the scrubber free wall is arranged relatively closer to the porous bottom surface, the supply of alumina is decreased. If the scrubber free wall is arranged relatively farther from the porous bottom surface, the supply of alumina is increased. If the scrubber free wall is arranged relatively closer to the side wall, the supply of alumina is increased. If the scrubber free wall is arranged relatively farther from the side wall, the supply of alumina is decreased. The flow control device controls the flow of alumina to the alumina hopper. The flow control device comprises a first portion formed of an elongated tubular or similarly shaped hollow configuration. The first portion is arranged vertically with a top inlet end fluidly connected to a primary alumina supply. An opposed bottom outlet end of the first portion is connected to or unitarily formed with a vertically arranged second portion. The second portion defines an open interior area extending from a free base edge of sides that taper inwardly and upwardly to an opposed connection end. The connection end of the second portion connects to the bottom outlet end of the first portion. As such, primary alumina flows through the flow control device from the top inlet end of the first portion to a base opening defined by the free base edge of the second portion. The flow control device may be movable. The flow control device may be electronically and/or manually movable through adjustment of an arm equipped with hinges connected thereto. The arm is connected to the side wall and may have a hinge at or near the side wall. The arm is also connected to the flow control device and may have a hinge at or near the flow control device. Further, the arm may have a hinge arranged between those at the side wall and at the flow control device. The arm equipped with hinges, or other such movement mechanism, enables electronic and/or manual movement of the flow control device vertically, i.e., closer to or farther from the porous bottom surface, and horizontally, i.e., closer to or farther from the side wall. Through movement and positioning of the flow control device, the supply of alumina to the alumina hopper, and hence, the electrolytic cell and the dry scrubber contact reactor may be controlled or adjusted. 
     Since the electrolytic cell is supplied alumina from the alumina hopper, which also supplies alumina to the dry scrubber contact reactor, the rate of alumina demand by the electrolytic cell determines or controls the rate of alumina supply via the flow control device to the singularly dedicated dry scrubber contact reactor. Accordingly, alumina is transported from a primary alumina supply, to the flow control device arranged vertically within the effluent gas treatment system housing for a gravity fed flow of alumina therethrough. The free base edge of the flow control device is arranged a predetermined distance based on system requirements from the partly porous bottom surface of the effluent gas treatment system housing to be within, or below a top surface of the alumina within the alumina hopper. According to the embodiment, an air supply is fluidly connected to the housing to supply air between the solid base wall and the partly porous bottom surface. The air supply may be a fan, a blower, or similar such device. Air supplied between the solid base wall and the partly porous bottom surface flows upwardly through openings arranged in a portion or portions of the porous bottom surface thereby fluidizing a portion of the alumina supported on the porous bottom surface. As such, a certain static amount of primary alumina intentionally builds under the second portion of the flow control device adjacent the side wall of the effluent gas treatment system housing. As the static amount of primary alumina builds within the open interior area of the second portion, gravity flow of alumina through the first portion becomes slowed or blocked. As a certain amount of alumina flows from beneath the second portion via fluidization and/or gravity to the alumina hopper supplying alumina to the electrolytic cell via a feeding pipe, a portion of the static amount of the primary alumina becomes free and shifts away, again allowing a flow of primary alumina from the first portion. Such flow of alumina continues unless or until the flow is again slowed or blocked by a build-up of a static amount of primary alumina beneath the second portion of the flow control device. Through this ebb and flow of primary alumina from the flow control device, alumina supplied to the electrolytic cell, as well as to the dry scrubber contact reactor downstream of the feeding pipe, is controlled. For a further, possibly as an “as needed” intermittent boost, the air supply may be connected to one or more air booster devices. According to an embodiment, an air booster device may be arranged at the side wall below the flow control device. The air booster device arranged below the flow control device may be used intermittently to locally increase air supply to alter or boost fluidization conditions of the alumina under the second portion of the flow control device to intermittently increase alumina supply to the alumina hopper. Also, according to the embodiment, an air booster device may be arranged above the porous bottom surface at the retainer wall. The air booster device arranged at the retainer wall may be used intermittently to locally increase air supply to alter or increase fluidization conditions to intermittently increase alumina supply to the dry scrubber contact reactor. 
     The subject fabric filter is arranged in an upper portion of the effluent gas treatment system housing at a level vertically above the flow control device, alumina hopper, and dry scrubber contact reactor. The subject fabric filter comprises a support wall arranged to extend across a portion of the effluent gas treatment system housing to create a barrier separating an “after-filter” area on one side of the support wall from a “before-filter” area on the opposite side of the support wall. A plurality of openings extends through a thickness of the support wall, with each opening equipped with a replaceable fabric filter bag that extends from the opening into the before-filter area. Arranged in the after-filter area is an outlet through which treated gas flows outwardly from the after-filter area of the effluent gas treatment system housing, to further treatment equipment or to the atmosphere. 
     A method of using the subject effluent gas treatment system comprises arranging the subject effluent gas treatment system at a level vertically above that of an electrolytic cell operable to produce aluminum, fluidly connecting the subject effluent gas treatment system to the electrolytic cell via a feeding pipe and an effluent gas outlet, wherein the feeding pipe is connected to the effluent gas treatment system alumina hopper and the effluent gas outlet is connected to the dry scrubber contact reactor, supplying alumina to the effluent gas treatment system via a flow control device, wherein the flow control device supplies alumina to the electrolytic cell and to the dry scrubber contact reactor at a rate based on the electrolytic cell alumina demand, interacting dispersed alumina with effluent gas in the dry scrubber contact reactor for pollutant removal from the effluent gas to produce contacted gas entrained with particulate adsorption products, and removing the particulate adsorption products from the contacted gas in a fabric filter to produce clean gas. 
     A method of using the subject flow control device for alumina supply comprises providing a vertically arranged flow control device within an effluent gas treatment system housing, the flow control device comprising a vertically arranged elongated first portion, and a vertically arranged second portion, arranged a predetermined distance above a partly porous bottom surface of the effluent gas treatment system housing, supplying alumina to the flow control device from an alumina supply for a gravity feed of alumina through the flow control device to an alumina hopper for alumina supply to an electrolytic cell and to a dry scrubber contact reactor, and controlling a rate of supply of alumina to the dry scrubber contact reactor based on alumina demand by the electrolytic cell. 
     In summary, the subject effluent gas treatment system comprises a singular, singularly dedicated, effluent gas treatment system arranged at a level vertically above that of a singular aluminum electrolytic cell, a housing defining an interior area of the singular effluent gas treatment system, a flow control device arranged vertically within the interior area comprising an elongated hollow first portion and a tapered second portion arranged a predetermined distance from a partly porous bottom surface of the housing, an adsorbent hopper extending across the partly porous bottom surface of the housing between the flow control device and a dry scrubber contact reactor, a feeding pipe fluidly connected between the adsorbent hopper and the singular aluminum electrolytic cell for supplying adsorbent to the singular aluminum electrolytic cell, and an effluent gas outlet in a cell housing for the singular electrolytic cell, fluidly connected to the dry scrubber contact reactor for interaction of the effluent gas with adsorbent supplied from the adsorbent hopper to produce contacted gas. The effluent gas treatment system further comprises air booster devices to alter or boost adsorbent fluidization within the system. The subject effluent gas treatment system further comprises a fabric filter operable to remove particulate adsorption products and dust from the contacted gas. The fabric filter comprises a plurality of removable fabric filter bags arranged within the housing at a level vertically above that of the flow control device, adsorbent hopper, and the dry scrubber contact reactor. The flow control device and/or a portion of the dry scrubber contact reactor may be movable to affect adsorbent fluidization, and the flow control device of the effluent gas treatment system controls a rate of supply of adsorbent to the dry scrubber contact reactor based on a rate of adsorbent demand by the electrolytic cell. 
     In summary the subject flow control device comprises a vertically arranged elongated hollow first portion, a vertically arranged second portion defining an open interior area, the second portion comprising a tapered wall extending between a base edge and a connection top, wherein the connection top is fluidly connected to a bottom open end of the first portion, an adsorbent supply fluidly connected to an open top end of the first portion, and a portion of an adsorbent hopper arranged a predetermined distance vertically below the flow control device, wherein the flow control device is mechanically operable to control a rate of supply of adsorbent to a dry scrubber contact reactor based on a rate of adsorbent demand by an aluminum electrolytic cell. The base edge of the second portion of the subject flow control device is arranged vertically below a top surface of adsorbent in the adsorbent hopper. Further, the first portion of the subject flow control device is tubular or similar hollow configuration. The base edge of the second portion of the subject flow control device is of a larger dimension than that of the connection top of the second portion. 
     In summary a method of using the subject flow control device comprises arranging the flow control device comprising a vertically arranged elongated hollow first portion, a vertically arranged second portion defining an open interior area, the second portion comprising a tapered wall extending between an open free base edge and a connection top, wherein the connection top is fluidly connected to a bottom outlet end of the first portion, an adsorbent supply fluidly connected to an open top inlet end of the first portion, and a portion of an adsorbent hopper arranged a predetermined distance vertically below the flow control device, and operating the flow control device to control a rate of supply of adsorbent to a dry scrubber contact reactor based on a rate of adsorbent demand by an aluminum electrolytic cell. According to the subject method, adsorbent flows via gravity through the flow control device from the open top inlet end of the first portion to the open free base edge of the second portion. The adsorbent is alumina. The subject method further comprises increasing the rate of supply of adsorbent to the dry scrubber contact reactor via an air booster device. 
     The subject method further comprises reducing the rate of supply of adsorbent to the dry scrubber contact reactor through static adsorbent build-up in an open interior area defined by the second portion. The method further comprises reducing the rate of supply of adsorbent flow through the flow control device to the dry scrubber contact reactor through static adsorbent build-up in an open interior area defined by the second portion and static adsorbent build-up below the second portion. According to the subject method, the open free base edge of the second portion is of a larger dimension than that of the connection top of the second portion. Also, according to the subject method, the connection top of the second portion is of like dimension and configuration as that of the bottom outlet end of the first portion. 
     A benefit of the subject system is that control of the alumina and fluoride balance may be electrolytic cell specific. Hence, if one electrolytic cell for some reason is generating more hydrogen fluoride gas, the feeder may be actuated to supply more alumina to the electrolytic cell to adsorb more fluoride to reduce the amount of fluoride lost from the electrolytic cell. Further, when the feeder is actuated, the crust breaker is first operated inside the bath to open a hole through which the alumina is supplied into the bath contents. This operation generates a significant amount of hydrogen fluoride gas. As such, during the operation the feeder supplies more alumina to the electrolytic cell. The additional alumina supplied to the electrolytic cell adsorbs more fluoride to reduce the amount of fluoride lost from the electrolytic cell during the operation. 
     Preferably, a hydrogen fluoride sensor, sulphur dioxide sensor and/or perfluorinated chemicals sensor is installed on or relatively near the treated gas outlet for additional feeder control via a controller, and emissions control. Further objects and features of the present disclosure will be apparent from the following detailed description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described in more detail below with reference to the appended drawings in which: 
         FIG. 1  is a schematic side cross sectional view of an aluminium production plant equipped with an embodiment of the subject effluent gas treatment system; and 
         FIG. 2  is a schematic side cross sectional view of an aluminium production plant equipped with another embodiment of the subject effluent gas treatment system. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  are each schematic representations of an aluminum production plant  10 . The main components of aluminum production plant  10  include an aluminum production electrolytic cell room  12  in which a plurality of aluminum production electrolytic cells  14  may be arranged. In each of  FIGS. 1 and 2 , only one aluminum production electrolytic cell  14  is depicted for purposes of clarity and simplicity, but it will be appreciated that electrolytic cell room  12  may typically comprise 50 to 200 electrolytic cells  14 . Each aluminum production electrolytic cell  14  comprises a number of anode electrodes  16 , typically six to thirty anode electrodes  16 , typically arranged in two parallel rows extending along the length of electrolytic cell  14  and extending into contents  18  of bath  20 . The electrolytic cell  14  also comprises one or more cathode electrodes  22 . The process occurring in the electrolytic cell  14  may be the well-known Hall-Héroult process in which aluminum oxide or alumina A, language used interchangeably herein, is dissolved in a melt of fluorine containing minerals and electrolyzed to produce aluminum. Hence, electrolytic cell  14  functions as an electrolysis cell. Powdered aluminum oxide A is supplied to electrolytic cell  14  from an alumina hopper  24  integrated in an effluent gas treatment system  26  singularly dedicated to a single electrolytic cell  14 . Powdered aluminum oxide A is supplied to the bath  20  by means of feeders  28  controlled by a controller  126 . Each feeder  28  is provided with a feeding pipe  30 , a feed port  32 , and a crust breaker  34  operative for forming an opening in a crust that often forms on a surface  18 A of contents  18 . An example of a crust breaker  34  is described in U.S. Pat. No. 5,045,168. Each feeder  28  is electronically connected to the controller  126 . Controller  126  may also be electronically connected to a hydrogen fluoride sensor  33  arranged in treated gas outlet  102 . An example of a hydrogen fluoride sensor  33  is disclosed in EP 2181753. Other sensors  33  may likewise be arranged in the treated gas outlet  102 , such as a sulphur dioxide sensor, a perfluorinated chemicals sensor, a carbon dioxide sensor, and/or a similar pollutant sensor. Controller  126  may also be electronically connected to air supply  120  and booster devices  124 . Controller  126  is discussed in more detail below. 
     The electrolysis process occurring in electrolytic cell  14  generates large amounts of heat H, dust particles DP, and effluent gas EG including but not limited to hydrogen fluoride, sulphur dioxide and carbon dioxide, i.e., pollutants. A cell housing  36  defines an interior area  36 A in which bath  20  is arranged. An effluent gas EG inlet  66  is fluidly connected to interior area  36 A. A fan  40  draws effluent gas EG from interior area  36 A and through the effluent gas treatment system  26 . Fan  40  is preferably located downstream of effluent gas treatment system  26  to generate a negative pressure in the effluent gas treatment system  26 . However, fan  40  could also, as an alternative, be arranged elsewhere depending on plant  10  requirements. Fan  40  creates via fluidly connected effluent gas EG inlet  66  a suction in interior area  36 A of cell housing  36 . As a result of the negative pressure in cell housing  36 , a volume of ambient air AA is drawn into interior area  36 A mainly via gaps or openings  42  at side wall doors  44 . The effluent gas EG drawn from interior area  36 A via effluent gas EG inlet  66  comprises effluent gas EG, dust particles DP generated in the aluminum production process, and a volume of ambient air AA. 
     In effluent gas treatment system  26 , effluent gas EG is mixed in dry scrubber contact reactor  46 , with an adsorbent, which is typically aluminum oxide A that is thereafter utilized in the aluminum production process. Aluminum oxide A interacts with some components of the effluent gas EG, particularly hydrogen fluoride, HF, and sulphur dioxide, SO 2 . The particulate adsorption products PP formed by the reaction of aluminum oxide A with hydrogen fluoride and sulphur dioxide are separated from the contacted gas CG by a fabric filter  48 . In addition to removing hydrogen fluoride and sulphur dioxide from the effluent gas EG, effluent gas treatment system  26  via fabric filter  48  also separates at least a portion of the dust particles DP entrained in the effluent gas EG from interior area  36 A. 
     Optionally, treated gas TG flowing out of effluent gas treatment system  26  via treated gas outlet  102  is further treated in a sulphur dioxide removal device  50 . Sulphur dioxide removal device  50  removes most of the sulphur dioxide remaining in the treated gas TG after treatment in effluent gas treatment system  26 . Sulphur dioxide removal device  50  may for example be a seawater scrubber, such as that disclosed in U.S. Pat. No. 5,484,535, a limestone wet scrubber, such as that disclosed in EP 0 162 536, or another such device that utilizes an alkaline absorption substance for removing sulphur dioxide from production gas. 
     Optionally, treated gas TG flowing from effluent gas treatment system  26 , or from the sulphur dioxide removal device  50 , is further treated in a carbon dioxide removal device  52 , operable to remove at least some of the carbon dioxide from the treated gas TG. Carbon dioxide removal device  52  may be of any type suitable for removing carbon dioxide gas from production gas. An example of a suitable carbon dioxide removal device  52  is that which is equipped for a chilled ammonia process. In a chilled ammonia process, treated gas TG is contacted with, for example, ammonium carbonate and/or ammonium bicarbonate solution or slurry at a low temperature, such as 0° to 10° C., in an absorber  54 . The solution or slurry selectively absorbs carbon dioxide gas from the treated gas TG. Hence, treated gas TG, containing mainly nitrogen gas and oxygen gas, flow from absorber  54  for release to the atmosphere. The spent ammonium carbonate and/or ammonium bicarbonate solution or slurry is transported from absorber  54  to a regenerator  56  in which the ammonium carbonate and/or ammonium bicarbonate solution or slurry is heated to a temperature of, for example, 50° to 150° C. causing a release of the carbon dioxide in concentrated gas form. The regenerated ammonium carbonate and/or ammonium bicarbonate solution or slurry is then returned to the absorber  54 . The concentrated carbon dioxide gas flows from regenerator  56  to a gas processing unit  58  in which the concentrated carbon dioxide gas is compressed. The compressed concentrated carbon dioxide CC may be disposed of, for example by being pumped into an old mine, or the like. An example of a carbon dioxide removal device  52  of the type described is disclosed in US 2008/0072762. It will be appreciated that other carbon dioxide removal devices  52  may also be utilized. 
     Although the subject effluent gas treatment system  26  is described herein as singularly dedicated to an electrolytic cell  14 , the scope of the subject disclosure encompasses applications wherein use of the subject effluent gas treatment system  26  may be dedicated to more than one electrolytic cell  14 . In the singularly dedicated effluent gas treatment system  26  arranged at a level vertically above that of the electrolytic cell  14 , effluent gas EG flows upwardly through the dry scrubber contact reactor  46 . The subject dry scrubber contact reactor  46  is arranged downstream of the alumina hopper  24 , which extends horizontally across a porous bottom surface  60 A of the effluent gas treatment system  26  housing  60 . Porous bottom surface  60 A, as used herein, collectively refers to either a wholly porous surface or a partially porous surface depending upon needs of the effluent gas treatment system  26 . Arranged a distance vertically below porous bottom surface  60 A is solid base wall  60 D. The effluent gas treatment system  26  housing  60  comprises a top  60 B, a porous bottom surface  60 A with a solid base wall  60 D therebelow, and two opposed side walls  60 C defining an open interior  62 . The dry scrubber contact reactor  46  is supplied a fluidized and/or gravity flow of alumina A from the alumina hopper  24 . As such, the alumina A flows across the porous bottom surface  60 A of the effluent gas treatment system  26  from a flow control device  64  to the dry scrubber contact reactor  46  equipped with the effluent gas EG inlet  66 . Within the dry scrubber contact reactor  46 , alumina A is dispersed into and mixed with the effluent gas EG flowing into the effluent gas treatment system  26  housing  60  via effluent gas EG inlet  66 . Effluent gas EG inlet  66  is arranged between a portion of side wall  60 C and a retaining wall  66 A with a base end  66 B abutting free ends  61  of porous bottom surface  60 A and solid base wall  60 D and extending vertically upwardly from the base end  66 B to a free overflow edge  66 C. Retaining wall  66 A is distanced from side wall  60 C to allow a flow of effluent gas EG therebetween into dry scrubber contact reactor  46 . Similarly, dry scrubber contact reactor  46  is arranged between a movable scrubber free wall  46 A that extends generally parallel to side wall  60 C from a free base end  46 B vertically to an opposed free top end  46 C. Scrubber free wall  46 A may be electronically movable via controller  126  and/or manually movable through adjustment of an arm  49  equipped with hinges  47  connected thereto. Arm  49 , connected to side wall  60 C may have a hinge  47  arranged at or near side wall  60 C. Arm  49 , connected to scrubber free wall  46 A may have a hinge  47  arranged at or near scrubber free wall  46 A. Further, arm  49  may have a hinge  47  arranged between those of side wall  60 C and scrubber free wall  46 A. Arm  49  equipped with hinges  47 , or a similar movable mechanical device, enables electronic movement through controller  126 , or manual movement, of scrubber free wall  46 A vertically, i.e., closer to or farther from porous bottom surface  60 A, and horizontally, i.e., closer to or farther from side wall  60 C. Hence, upon arrangement of scrubber free wall  46 A relatively closer to porous bottom surface  60 A, supply of alumina A to the dry scrubber contact reactor  46  is decreased. Upon arrangement of scrubber free wall  46 A relatively farther from porous bottom surface  60 A, supply of alumina A to the dry scrubber contact reactor  46  is increased. Upon arrangement of scrubber free wall  46 A relatively closer to adjacent side wall  60 C, supply of alumina A to the dry scrubber contact reactor  46  is decreased. Upon arrangement of scrubber free wall  46  A relatively farther from adjacent side wall  60 C, supply of alumina A to the dry scrubber contact reactor  46  is increased. Through movement and positioning of scrubber free wall  46 A, supply of alumina A to the dry scrubber contact reactor  46  may be controlled or adjusted. The flow control device  64  comprises a first portion  68  formed of an elongated tubular or other similar hollow configuration. The first portion  68  is arranged vertically with a top inlet end  70  fluidly connected to a primary alumina supply  72  via an adjustable duct  74 . An opposed bottom outlet end  76  of the first portion  68  is connected to or unitarily formed with a vertically arranged second portion  78 . The second portion  78  defines an open interior area  80  extending from a free base edge  82  of sides  84  that extend tapering inwardly and upwardly to a connection end  86 . The connection end  86  of the second portion  78  fluidly connects to the bottom outlet end  76  of the first portion  68 . As such, alumina A flows via gravity through the flow control device  64  from the top inlet end  70  of the first portion  68  to a base opening  88  defined by the free base edge  82  of the second portion  78 . Although the subject flow control device  64  is described herein as having a tubular first portion  68  and a tapered second portion  78  for controlled gravity flow, the scope of the subject disclosure encompasses other shapes and/or configurations functionable as the subject flow control device  64  disclosed herein. The flow control device  64  may be movable. Flow control device  64  may be electronically movable via controller  126  and/or manually movable through adjustment of an arm  69  with hinges  67  connected thereto. Arm  69 , connected to side wall  60 C may be equipped with a hinge  67  at or near side wall  60 C. Arm  69 , connected to flow control device  64  may be equipped with a hinge  67  at or near flow control device  64 . Further, arm  69  may be equipped with a hinge  67  between those of side wall  60 C and flow control device  64 . Arm  69  equipped with hinges  67  enables electronic movement via controller  126 , or manual movement, of flow control device  64  vertically, i.e., closer to or farther from porous bottom surface  60 A, and horizontally, i.e., closer to or farther from side wall  60 C. Hence, upon arrangement of flow control device  64  relatively closer to porous bottom surface  60 A, supply of alumina A to the alumina hopper is decreased. Upon arrangement of flow control device  64  relatively farther from porous bottom surface  60 A, supply of alumina A to the alumina hopper is increased. Upon arrangement of flow control device  64  relatively closer to adjacent side wall  60 C, supply of alumina A to the alumina hopper is decreased. Upon arrangement of flow control device  64  relatively farther from adjacent side wall  60 C, supply of alumina A to the alumina hopper is increased. Through movement and positioning of flow control device  64 , supply of alumina A to the electrolytic cell  14  and the dry scrubber contact reactor  46  may be controlled or adjusted. 
     Since the electrolytic cell  14  is supplied alumina A from the alumina hopper  24  that also supplies alumina A to the dry scrubber contact reactor  46 , alumina A demand by the electrolytic cell  14  determines or controls the rate of alumina supply to the singularly dedicated dry scrubber contact reactor  46 . Accordingly, alumina A is transported from a primary alumina supply  72 , to the flow control device  64  arranged vertically within the effluent gas treatment system  26  housing  60  for a gravity fed flow of alumina A therethrough. The free base edge  82  of the flow control device  64  is arranged a predetermined distance D from the porous bottom surface  60 A of the effluent gas treatment system  26  housing  60  within the flow of alumina A within the alumina hopper  24 . According to one embodiment, an air supply  120  is fluidly connected to housing  60  to supply air G between solid base wall  60 D and porous bottom surface  60 A at ends  63 , opposite free ends  61 . Air supply  120  may be a fan, a blower, or similar such device. Air G supplied between solid base wall  60 D and porous bottom surface  60 A flows upwardly through openings  122  arranged across all, a portion or portions of porous bottom surface  60 A thereby fluidizing a portion of alumina A supported by porous bottom surface  60 A. As such, a certain static amount of alumina A intentionally builds under the second portion  78  of the flow control device  64  and adjacent side wall  60 C of the effluent gas treatment system  26  housing  60 . As the static amount of alumina A builds within the open interior area  80  of the second portion  78 , gravity flow of alumina A through the first portion  68  is slowed or blocked. As a certain fluidized amount of alumina A flows from the second portion  78  to the alumina hopper  24  supplying alumina A to the electrolytic cell  14  via a feeding pipe  30 , a portion of the static amount of the alumina A shifts via gravity to again allow a flow of alumina A from the first portion  68  unless or until the flow is again slowed or blocked by the static amount of alumina A built up under the second portion  78  of the flow control device  64 . Through this ebb and flow of alumina A from the flow control device  64 , alumina A is supplied to the electrolytic cell  14  and flow of alumina A to the dry scrubber contact reactor  46  downstream of the feeding pipe  30  to the electrolytic cell  14 , is controlled. For further, possibly as an “as needed” intermittent boost, air supply  120  may be connected to one or more air booster devices  124 . According to one embodiment, an air booster device  124  may be arranged at side wall  60 C below flow control device  64 . Air booster device  124  below flow control device  64  may be used intermittently to locally increase air G supply to intermittently alter or increase fluidization conditions under the second portion  78  of the flow control device  64 . According to another embodiment, an air booster device  124  may be arranged above porous bottom surface  60 A at retainer wall  66 A. Air booster device  124  may be used intermittently at retainer wall  66 A to locally increase air G supply to intermittently alter or increase alumina A supply to the dry scrubber contact reactor  46 . 
     The subject fabric filter  48  is arranged in an upper portion  92  of the effluent gas treatment system  26  housing  60  at a level vertically above that of the flow control device  64 , alumina hopper  24 , and dry scrubber contact reactor  46 . The subject fabric filter  48  comprises a support wall  90  arranged within the upper portion  92  of the effluent gas treatment system  26  housing  60  fluidly separating an “after-filter” area  94  on one side of the support wall  90  from a “before-filter” area  96  on an opposite side of the support wall  90 . A plurality of openings  98  extends through a thickness T of the support wall  90 , with each opening  98  equipped with a replaceable fabric filter bag  100  that extends from the opening  98  into the before-filter area  96 . Arranged within the after-filter area  94  is a treated gas outlet  102  through which treated gas TG flows outwardly from the after-filter area  94  of the effluent gas treatment system  26  housing  60 , to optional further treatment equipment  50 ,  52  or to the atmosphere. 
     In  FIG. 1 , side  84  of lower portion  78  of flow control device  64  is arranged adjacent to housing  60  side wall  60 C, and the fabric filter  48  is arranged vertically, i.e., the fabric filter bags  100  extending vertically. Another embodiment, schematically illustrated in  FIG. 2 , side  84  of lower portion  78  of flow control device  64  is of an abridged configuration to enable a relatively closer abutting arrangement with housing  60  side wall  60 C, and the fabric filter  48  is arranged horizontally, i.e., the fabric filter bags  100  extending horizontally. Further, embodiments with the flow control device  64  arranged adjacent to housing  60  side wall  60 C (as illustrated in  FIG. 1 ) and the fabric filter  48  arranged horizontally (as illustrated in  FIG. 2 ), or the flow control device  64  in an abridged configuration (as illustrated in  FIG. 2 ) and the fabric filter  48  arranged vertically (as illustrated in  FIG. 1 ), are also considered within the scope of this disclosure. 
     A method of using the subject effluent gas treatment system  26  comprises arranging the subject effluent gas treatment system  26  at a level vertically above that of an electrolytic cell  14  producing aluminum, fluidly connecting the subject effluent gas treatment system  26  to the electrolytic cell  14  via a feeding pipe  30  and an effluent gas outlet  104 . As such, the feeding pipe  30  is connected to the alumina hopper  24 , and the effluent gas outlet  104  is connected to the effluent gas EG inlet  66  of dry scrubber contact reactor  46 . Alumina A is supplied to the effluent gas treatment system  26  via the flow control device  64 , wherein the flow control device  64  supplies alumina A to the electrolytic cell  14  and the dry scrubber contact reactor  46  at a rate based on electrolytic cell  14  demand. The alumina A supplied to the dry scrubber contact reactor  46  interacts with the effluent gas EG in the dry scrubber contact reactor  26  for pollutant removal from the effluent gas EG to produce contacted gas CG entrained with particulate adsorption products PP. The method further comprises removing the particulate adsorption products PP from the contacted gas CG in an associated fabric filter  48  to produce treated gas TG. 
     A method of using the subject flow control device  64  for alumina A supply comprises providing the vertically arranged flow control device  64  within an effluent gas treatment system  26  housing  60 , the flow control device  64  comprising a vertically arranged elongated first portion  68 , and a vertically arranged second portion  78 , arranged a predetermined distance D above a horizontal porous bottom surface  60 A of the effluent gas treatment system  26  housing  60 , supplying alumina A to the flow control device  64  from a primary alumina supply  72  for a gravity feed of alumina A through the flow control device  64  to an alumina hopper  24  for alumina A supply to an electrolytic cell  14  and to a dry scrubber contact reactor  46 , and controlling a rate of supply of alumina A to the dry scrubber contact reactor  46  based on alumina A demand by the electrolytic cell  14 . 
     A benefit of the subject plant  10  is that control of the alumina A and fluoride balance is electrolytic cell  14  specific. Hence, if an electrolytic cell  14  for some reason is generating more hydrogen fluoride gas, the hydrogen fluoride sensor  33  electronically transmits the hydrogen fluoride measurement to the controller  126 , and via electronic control via the controller  126 , the feeder  28  will supply more alumina A to the electrolytic cell  14  to adsorb more fluoride to reduce the amount of fluoride lost from the electrolytic cell  14 . Further, when the feeder  28  is actuated, the crust breaker  34  is first operated inside the bath  20  to open a hole through which the alumina A is supplied into the bath  20  contents  18 . This operation generates a significant amount of hydrogen fluoride gas. As such, during the operation the feeder  28  will supply more alumina A to the electrolytic cell  14 . The additional alumina A supplied to the electrolytic cell  14  adsorbs more fluoride to reduce the amount of fluoride lost from the electrolytic cell  14  during the operation. Preferably, the hydrogen fluoride sensor  33  is arranged on or relatively near the effluent gas EG outlet  102  for controller  126  control of feeder  28  supply of alumina A to electrolytic cell  14 . Additionally, one or more hydrogen fluoride sensors  33  may be arranged in various other locations within aluminium production plant  10 , such as but not limited to adjacent feeder  28  and/or adjacent effluent gas outlet  104 . Like the hydrogen fluoride sensors  33 , other sensors  33  may also be arranged in plant  10 , such as sulphur dioxide sensors, carbon dioxide sensors, perfluorinated chemicals sensors, and/or similar pollutant sensors, for emissions control. 
     In summary, the subject effluent gas treatment system  26  comprises a singular, singularly dedicated effluent gas treatment system  26  arranged at a level vertically above that of a singular aluminum electrolytic cell  14 , a housing  60  defining an interior area  62  of the singular effluent gas treatment system  26 , a flow control device  64  arranged vertically within the interior area  62  comprising an elongated hollow first portion  68  and a tapered second portion  78  arranged a predetermined distance D from a horizontal porous bottom surface  60 A of the housing  60 , an adsorbent hopper  24  extending across the horizontal porous bottom surface  60 A of the housing  60  between the flow control device  64  and a dry scrubber contact reactor  46 , a feeding pipe  30  fluidly connected between the adsorbent hopper  24  and the singular aluminum electrolytic cell  14  for supplying adsorbent to the singular aluminum electrolytic cell  14 , and an effluent gas outlet  104  in a cell housing  36  for the singular electrolytic cell  14 , fluidly connected to the effluent gas EG inlet  66  of dry scrubber contact reactor  46  for reaction of the effluent gas EG with adsorbent supplied from the adsorbent hopper  24  to produce contacted gas CG. The subject effluent gas treatment system  26  further comprises air booster devices  124  to alter or boost adsorbent fluidization within the system  26 . The subject effluent gas treatment system  26  further comprises a fabric filter  48  operable to remove particulate adsorption products PP and dust DP from the contacted gas CG. The fabric filter  48  comprises a plurality of removable fabric filter bags  100  arranged within the housing  60  at a level vertically above that of the flow control device  64 , adsorbent hopper  24 , and the dry scrubber contact reactor  46 . The flow control device and/or a portion of the dry scrubber contact reactor is movable to affect adsorbent fluidization and/or flow, and the flow control device  64  of the effluent gas treatment system  26  controls a rate of supply of adsorbent to the dry scrubber contact reactor  46  based on a rate of adsorbent demand by the electrolytic cell  14 . 
     In summary, the subject flow control device  64  comprises a vertically arranged elongated hollow first portion  68 , a vertically arranged second portion  78  defining an open interior area  80 , the second portion  78  comprising a tapering wall  84  extending between a base edge  82  and a connection top  86 , wherein the connection top  86  is fluidly connected to a bottom open end  76  of the first portion  68 , an adsorbent supply  72  fluidly connected to an open top end  70  of the first portion  68 , and a portion of an adsorbent hopper  24  arranged a predetermined distance D vertically below the flow control device  64 , wherein the flow control device  64  is mechanically operable to control a rate of supply of adsorbent to a dry scrubber contact reactor  46  based on a rate of adsorbent demand by an aluminum electrolytic cell  14 . The base edge  82  of the second portion  78  of the subject flow control device  64  is arranged vertically below a top surface S of adsorbent in the adsorbent hopper  24 . Further, the first portion  68  of the subject flow control device  64  is tubular or of a similar hollow configuration. The base edge  82  of the second portion  78  of the subject flow control device  64  is of a larger dimension than that of the connection top  86  of the second portion  78 . 
     In summary, a method of using the subject flow control device  64  comprises arranging the flow control device  64  comprising a vertically arranged elongated hollow first portion  68 , a vertically arranged second portion  78  defining an open interior area  80 , the second portion  78  comprising a tapering wall  84  extending between an open free base edge  82  and a connection top  86 , wherein the connection top  86  is fluidly connected to a bottom outlet end  76  of the first portion  68 , an adsorbent supply  72  fluidly connected to an open top inlet end  70  of the first portion  68 , and a portion of an adsorbent hopper  24  arranged a predetermined distance D vertically below the flow control device  64 , and operating the flow control device  64  to control a rate of supply of adsorbent to a dry scrubber contact reactor  46  based on a rate of adsorbent demand by an aluminum electrolytic cell  14 . According to the subject method, adsorbent flows via gravity through the flow control device  64  from the open top inlet end  70  of the first portion  68  to the open free base edge  82  of the second portion  78 . The adsorbent is alumina A. The subject method further comprises increasing the rate of supply of adsorbent to the dry scrubber contact reactor  46  via an air booster device  124 . The subject method further comprises reducing the rate of supply of adsorbent to the dry scrubber contact reactor  46  through static adsorbent build-up in an open interior area  80  defined by the second portion  78 . The method further comprises reducing the rate of supply of adsorbent flow through the flow control device  64  to the dry scrubber contact reactor  46  through static adsorbent build-up in an open interior area  80  defined by the second portion  78  and static adsorbent build-up below the second portion  78 . According to the subject method, the open free base edge  82  of the second portion  78  is of a larger dimension than that of the connection top  86  of the second portion  78 . Also, according to the subject method, the connection top  86  of the second portion  78  is of like dimension and configuration as that of the bottom outlet end  76  of the first portion  68 . 
     While the present disclosure has been described with reference to a number of embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements without departing from the scope thereof. In addition, many modifications may be made to adapt a particular situation or material to the teachings of this disclosure without departing from the essential scope thereof. Therefore, it is intended that this disclosure not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc., do not denote any order or importance, but rather the terms first, second, etc., are used to distinguish one element from another.