ELECTROLYTIC CELL WITH IMPROVED ALUMINA SUPPLY

Abstract: An electrolytic cell for the electrowinning of aluminium comprises an electrochemically active foraminate metallic anode structure (15) for the evolution of oxygen and the escape of evolved oxygen therethrough which is spaced by an inter-electrode gap above a facing cathode (20). The cell further comprises means for promoting dissolution of powder alumina (32) fed to the surface of the electrolyte (30) and for supplying alumina-rich electrolyte to the inter-electrode gap by inducing an electrolyte circulation up from and down to the inter-electrode gap driven by the escape of anodically evolved oxygen through the foraminate anode structure. These means comprise electrolyte guide members (5) such as baffles or funnels having at least one inclined surface (6) immersed in the molten electrolyte (30) above the foraminate structure (15).

Description: ELECTROLYTIC CELL WITH IMPROVED ALUMINA SUPPLY Field of the Invention The present invention concerns a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte such as cryolite, having means to promote dissolution of alumina into the electrolyte and supply of alumina-rich electrolyte to the inter-electrode gap, as well as a metallic anode of special design for such a cell provided with these means and a method to produce aluminium utilising this cell. Background of the Invention The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950°C is more than one hundred years old. This process conceived almost simultaneously by Hall and Heroult, has not evolved as many other electrochemical processes. A major drawback of conventional cells is due to the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD) , also called inter-electrode gap (IEG), must be kept at a safe minimum value of approximately 5 cm to avoid short circuiting between the aluminium cathode and the anode or re-oxidation of the metal by contact with the C0 2 gas formed at the anode surface. Another drawback of the conventional cells is the anode effect which occurs when the electrolyte in a cell contains insufficient dissolved and/or non-uniform distribution of alumina-rich electrolyte below the entire active surface of the anodes and therefore allows the electrolysis of the fluoride-based electrolyte which produces fluorine and fluoride-based gas . The fluoride-based gas accumulates under the anodes and greatly inhibits the current transport between the anodes and the cathodes . Hence, the anode effect manifests itself by a sudden increase of the cell voltage. The voltage increase can vary from 7-8 volts up to 30 volts in commercial cells. US Patent 4,602,990 (Boxall/Gamson/Green/Traugott) describes a drained cathode cell having a bubble generated bath circulation, however this design could not achieve the expected constant voltage. The ACD reduction was coupled with an undesired reduction of the electrical conductivity of the bath caused by the increase of gas bubbles concentration in the reduced electrolyte between the drained cathodes and the anodes . European Patent Application No. 0 393 816 (Stedman) describes another design for a drained cathode cell improving the bubble evacuation. However, such drained cathode configuration cannot ensure optimal distribution of the dissolved alumina. Most of the alumina is electrolysed on the parts of the cathodes close to the dissolution point, whereas remote areas of the cathodes are poorly fed with alumina. This is due to the gradual depletion of the alumina concentration in the electrolyte while the electrolyte is moving between the electrodes where its electrolysis takes place. This insufficient distribution of dissolved alumina can cause the exposure of the cell to the anode effect, an uneven consumption of the electrodes and a non-optimal utilisation of the cathode surfaces leading to a decrease of the current efficiency and the cell performance. US Patent 4,504,369 (Keller) discloses an anode comprising a massive oxide-based anode having a central vertical through-opening for feeding anode constituents and alumina into the electrolyte. However, this cell design does not address the problem of dissolution and distribution of dissolved alumina between anodes and facing cathodes. US Patent 4,681,671 (Duruz) discloses a low temperature aluminium electrowinning cell with a series of vertical anode plates or vertical blades located above a horizontal perforated cathode plate and an electrolyte circulation generated by means of a pump or electromotive forces . US Patent 5,310,476 (Sekhar/de Nora) describes cells for the electrowinning of aluminium, having wedge-shaped cathode blocks and oxygen-evolving anodes made of anode plates fitting like roofs over the wedges. The anode plates are joined together and have openings adjacent the top of their inclined faces for the escape of anodically generated oxygen . US Patent 5,368,702 (de Nora) discloses designs including tubular or conical vertical oxygen-evolving anodes located inside and facing correspondingly shaped cathodes. The tubes and the conical surfaces forming the anodes have lateral openings guiding the escape of anodically released oxygen to generate an electrolyte flow between the anodes and the facing cathodes. In the cell described in US Patent 5,683,559 (de Nora) bent oxygen-evolving anode plates face a series of juxtaposed V-shaped cathode surfaces. The inclination of the anodes assists in releasing the anodically formed gases through a central opening. It is suggested to enhance gas release by providing ridges on the anodes or making the anodes foraminate . US Patent 5,725,744 (de Nora/Duruz) describes a multimonopolar aluminium electrowinning cell operating at reduced temperature with vertical or inclined anode and cathode plates, electrolyte being circulated between the anode and cathode plates by anodically-produced oxygen lift. US Patent 5,938,914 (Dawless/LaCamera/Troup/Ray/ Hosier), issued on August 17, 1999 describes an aluminium electrowinning cell having vertical inert anodes interleaved with vertical cathodes. The anodes are covered with an angled roof which diverts anodically evolved oxygen bubbles to agitate the cell's molten electrolyte to improve dissolution of alumina. While the foregoing references indicate continued efforts to improve the operation of aluminium electrowinning cell operations by using oxygen-evolving anodes none of them has found any commercial acceptance yet. Objects of the Invention It is an object of the invention to provide a cell for the electrowinning of aluminium, with metallic anodes operating either with a stabilised pool of aluminium or in a drained configuration, having means to promote dissolution of alumina added to the electrolyte and for supplying alumina-rich electrolyte to the inter-electrode gap where electrolysis takes place. Another object of the invention is to provide an anode of a cell for the electrowinning of aluminium whose design promotes dissolution of alumina and the supply of alumina-rich electrolyte between the electrochemically active surfaces of the anode and of a facing cathode. An important object of the invention is to provide means for the dissolution of alumina fed to a thermally insulated cell by feeding and spreading powder alumina on top of the electrolyte which forms no crust A further object of the invention is to provide a cell for the electrowinning of aluminium having improved means for guiding the escape of anodically produced gas, in particular oxygen, to generate an electrolyte circulation between the inter-electrode gap and the electrolyte surface of the cell, thereby permitting an increase of alumina dissolution. Summary of the Invention The invention relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a thermally insulated fluoride-containing crustless molten electrolyte. The cell comprises an electrochemically active foraminate metallic anode structure for the evolution of oxygen and the escape of oxygen therethrough, which anode is spaced by an inter-electrode gap above a facing cathode on which during operation aluminium is produced. The cell further comprises means for promoting dissolution of powder alumina fed to the surface of the electrolyte and for supplying alumina-rich electrolyte to the inter-electrode gap by inducing an electrolyte circulation up from and down to the inter-electrode gap driven by the escape of anodically evolved oxygen through the foraminate anode structure. These means comprise electrolyte guide members having at least one inclined surface immersed in the molten electrolyte above the foraminate anode structure. The electrolyte guide members may comprise downwardly converging inclined surfaces guiding a downward flow of alumina-rich electrolyte to the inter-electrode gap and/or upwardly converging surfaces guiding an upward flow of alumina-depleted electrolyte from the inter-electrode gap, driven by anodically-evolved oxygen. Preferably, to prevent the formation of an electrolyte crust on the surface of the molten electrolyte, the cell comprises means to thermally insulate the surface of the electrolyte, such as an insulating cover above the electrolyte as described in co-pending application WO99/02763 (de Nora/Sekhar) . Usually, the foraminate anode structure and the facing cathode are horizontal or at a corresponding slope, typically at an angle below 60°. The electrolyte guide members may be adapted to retrofitted cells, in particular Hall Heroult cells provided with suitable foraminate metallic anodes. The electrolyte guide members may be utilised in cells operating with a deep, a shallow or a stabilised pool of aluminium, or in a drained configuration, as for example described in US Patent 5,683,130 (de Nora), WO99/02764 and W099/41429 (both in the name of de Nora/Duruz) . An important feature of retrofitted deep pool cells is that the means for promoting dissolution of alumina lead to cell operation which has many of the advantages associated with drained cathode configuration. The electrolyte guide members may comprise vertical parallel sections extending from the bottom of the inclined surface to the foraminate anode structure and/or from the top of the inclined surfaces up to close to the surface of the electrolyte. The bottom end of each electrolyte guide member may extend up from the foraminate anode structure. If required, the bottom ends of the electrolyte guide members may be spaced apart above the or each anode to allow alumina-rich electrolyte flowing down from the bottom ends of the electrolyte guide members to be horizontally dispersed by the anodically-evolved upward flowing oxygen. In this case, part or all of the electrolyte may enter the inter-electrode gap by passing around the electrode structure. The electrolyte guide members may be situated relative to the surface of the electrolyte so that the upwardly flowing anodically-evolved oxygen generates turbulence above the electrolyte guide members to enhance the dissolution of alumina. The uppermost end of each electrolyte guide member may be immersed in the electrolyte by no more than 5 cm below the surface of the electrolyte. In one embodiment, the electrolyte guide members consist of a series of baffles parallel to the surface of the electrolyte. The baffles are arranged in a spaced-apart parallel configuration and laterally inclined, to form alternate pairs of upward converging surfaces and of downward converging surfaces . Alternatively, the electrolyte guide members may form a plurality of funnels which may be in the shape of truncated cones or truncated pyramids . The foraminate metallic anode structure may comprises a series of parallel spaced-apart coplanar electrochemically active anode members, for instance spaced-apart blades, bars, rods or wires. Each blade, bar, rod or wire may be generally rectilinear, or alternatively, in a generally concentric arrangement, each blade, bar, rod or wire forming a loop to minimise edge effects of the current during use. For instance, each blade, bar, rod or wire can be generally circular, oval or polygonal, in particular rectangular or square, preferably with rounded corners. The parallel anode members should be connected to one another, for instance in a grid-like, net-like or meshlike configuration of the anode members. To avoid edge effects of the current, the extremities of the anode members can be connected together, for example they can be arranged extending across a generally rectangular peripheral anode frame from one side to an opposite side of the frame. Alternatively, the anode members can be transversally connected by at least one transverse connecting member. Possibly the anode members are connected by a plurality of transverse connecting members which are in turn connected together by one or more cross members . For concentric looped configurations, the transverse connecting members may be radial . In this case the radial connecting members extend radially from the middle of the parallel anode member arrangement and optionally are secured to or integral with an outer ring at the periphery of this arrangement . Advantageously, the transverse connecting members are of variable section to ensure a substantially equal current density in the connecting members before and after each connection to an anode member. This also applies to the cross member when present. Usually, each metallic anode comprises at least one vertical current feeder arranged to be connected to a positive bus bar. Such a current feeder is mechanically and electrically connected to one or more transverse connecting members or to one or more cross members connecting a plurality of transverse connecting members, so that the current feeder carries electric current to the anode members through the transverse connecting member (s) and where present through the cross member (s). Where no transverse connecting member is present the vertical current feeder is directly connected to the anode members which are in a gridlike, net-like or mesh-like configuration. The vertical current feeder, anode members, transverse connecting members and where present the cross members may be secured together for example by being cast as a unit. Assembly by welding or other mechanical connection means is also possible. Likewise, the electrolyte guide members may be secured together for example by being cast as a unit, welding or using other mechanical connecting means to form an assembly. This assembly can be connected to the vertical current feeder or secured to or placed on the foraminate anode structure . The cathodes of the cell are advantageously aluminium-wettable, in particular they may be in drained configuration for instance having a sloping surface, as stated above. The invention also relates to an oxygen evolving anode of an electrolytic cell as described above. The anode comprises an electrochemically active foraminate metallic structure for the evolution of oxygen which during operation is immersed in the an electrolyte and spaced by an inter- electrode gap above a facing cathode on which aluminium is produced. The anode further comprises means arranged to promote dissolution of powder alumina fed to the surface of the electrolyte and supply alumina-rich electrolyte to the inter-electrode gap during operation, as described above. A further aspect of the invention is a method of producing aluminium in a cell as described above. The method comprises dissolving alumina in the electrolyte by feeding the alumina in the form of powder into the crustless molten electrolyte from above the electrolyte guide members, and passing an ionic current between the active foraminate anode structure and the facing cathode thereby carrying out electrolysis in the inter-electrode gap to produce aluminium on the cathode and oxygen on the foraminate anode structure. The means for promoting dissolution of powder alumina and for supplying alumina-rich electrolyte to the inter- electrode gap is arranged to induce an electrolyte circulation up from and down to the inter-electrode gap driven the escape of anodically evolved oxygen through the foraminate anode structure. Another aspect of the invention relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a thermally insulated fluoride- containing crustless molten electrolyte. The cell comprises an electrochemically active foraminate metallic anode structure for the evolution of oxygen and which is spaced by an inter-electrode gap above a facing cathode on which during operation aluminium is produced. The cell further comprises means for promoting dissolution of powder alumina fed to the surface of the electrolyte and for uniformly distributing and feeding alumina-rich electrolyte through the foraminate structure to the inter-electrode gap. These means comprise electrolyte guide members which are located in the electrolyte above the foraminate anode structure. The electrolyte guide members comprise downwardly converging surfaces immersed in the electrolyte which are arranged to: promote dissolution of alumina fed above their downward converging surfaces; and feed alumina-rich electrolyte down through their downward converging surfaces and through the foraminate structure to the inter-electrode gap. Yet another aspect of the invention relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a thermally insulated fluoride- containing crustless molten electrolyte. The cell comprises an electrochemically active foraminate metallic anode structure for the evolution of oxygen and which is spaced apart by an inter-electrode gap above a facing cathode on which during operation aluminium is produced. The cell further comprises means for promoting dissolution of powder alumina fed to the surface of the electrolyte and for uniformly distributing and feeding alumina-rich electrolyte through and/or around the foraminate structure to the inter- electrode gap. These means comprise electrolyte guide members which are located in the electrolyte above the foraminate anode structure. The electrolyte guide members comprise upwardly converging surfaces immersed in the electrolyte which are arranged to: guide an upward flow of alumina-depleted electrolyte driven by anodically evolved oxygen escaped through the foraminate anode structure to promote dissolution of alumina fed above their upwardly converging surfaces; and feed alumina-rich electrolyte down through and/or around the foraminate anode structure to the inter-electrode gap. Materials and Operation The foraminate metallic anode structures and/or the electrolyte guide members of the present invention may consist of or preferably are coated with an iron oxide-based material possibly obtained by oxidising the surface of the substrate of a foraminate anode structures and/or an electrolyte guide members which contains iron. Examples of suitable materials are described in greater detail in co- pending application PCT/IB99/01360 (Duruz/de Nora/Crottaz) , PCT/IB99/00015 (de Nora/Duruz), PCT/IB99/01361 (Duruz/de Nora/Crottaz), PCT/IB99/01362 (Crottaz/Duruz) , PCT/IB99/01977 (de Nora/Duruz) and PCT/IB99/01976 (Duruz/de Nora) . In known processes, even the least soluble anode material releases excessive amounts of constituents into the bath, which leads to an excessive contamination of the product aluminium. For example, the concentration of nickel (a frequent component of proposed metal-based anodes) found in aluminium produced in small scale tests at conventional cell operating temperatures is typically comprised between 800 and 2000 ppm, i.e. 4 to 10 times the maximum acceptable level which is 200 ppm. Iron oxides and in particular hematite (Fe 2 U 3 ) have a higher solubility than nickel in molten electrolyte. However, in industrial production the contamination tolerance of the product aluminium by iron oxides is also much higher (up to 2000 ppm) than for other metal impurities . Solubility is an intrinsic property of anode materials and cannot be changed otherwise than by modifying the electrolyte composition and/or the operative temperature of a cell. Small scale tests utilising a NiFe 2 U 4 /Cu cermet anode and operating under steady conditions were carried out to establish the concentration of iron in molten electrolyte and in the product aluminium under different operating conditions . In the case of iron oxide, it has been found that lowering the temperature of the electrolyte decreases considerably the solubility of iron species. This effect can surprisingly be exploited to produce a major impact on cell operation by limiting the contamination of the product aluminium by iron. Thus, it has been found that when the operating temperature of the cell is reduced below the temperature of conventional cells (950-970°C) an anode covered with an outer layer of iron oxide can be made dimensionally stable by maintaining a concentration of iron species and alumina in the molten electrolyte sufficient to reduce or suppress the dissolution of the iron-oxide layer, the concentration of iron species being low enough not to exceed the commercial acceptable level of iron in the product aluminium. The presence of dissolved alumina in the electrolyte at the anode surface has a limiting effect on the dissolution of iron from the anode into the electrolyte, which reduces the concentration of iron species necessary to substantially stop dissolution of iron from the anode. When the surface of the foraminate metallic anode structures/electrolyte guide members is iron oxide-based, the electrolyte may comprise an amount of iron species and dissolved alumina that prevents dissolution of the iron oxide-based surface. The amount of iron species and alumina dissolved in the electrolyte should be sufficient to prevent dissolution of the iron oxide-based surface but such that the product aluminium is contaminated by no more than 2000 ppm iron, preferably by no more than 1000 ppm iron, and even more preferably by no more than 500 ppm iron. To maintain in the electrolyte an amount of constituents of the foraminate anode structures/electrolyte guide members, in particular iron species, which prevents at the operating temperature the dissolution of the foraminate anode structures/electrolyte guide members if the alumina feed itself does not contain enough iron, the constituents may be fed into the electrolyte intermittently, for instance periodically together with alumina, or continuously, for example by means of a sacrificial electrode. When the foraminate anode structures/electrolyte guide members are iron oxide-based, iron species may be fed into the electrolyte in the form of iron metal and/or an iron compound such as iron oxide, iron fluoride, iron oxyfluoride and/or an iron-aluminium alloy. To limit contamination of the product aluminium by cathodically-reduced constituents of the foraminate anode structures/electrolyte guide members to a commercially acceptable level, the cell should be operated at a sufficiently low temperature so that the required concentration of constituents, in particular iron species, in the electrolyte is limited by the reduced solubility of iron species in the electrolyte at the operating temperature . The cell may be operated with an operative temperature of the electrolyte below 910°C, usually from 730 to 870°C. The electrolyte may contain NaF and AlF 3 in a molar ratio NaF/AlF 3 required for the operating temperature of the cell comprised between 1.2 and 2.4. The amount of dissolved alumina contained in the electrolyte is usually below 8 weight%, preferably between 2 weight% and 6 weight% . Since the electrolyte guide members need not be electrochemically active or conductive, their surface may also be made of non-conductive electrolyte resistant materials . The electrolyte guide members may be made of any ceramic or oxide which is resistant to the electrolyte, such as silicon nitride, aluminium nitride, boron nitride, magnesium ferrite, magnesium aluminate, magnesium chromite, zinc oxide, nickel oxide and alumina. However, the guide members may be made of the same materials as the anodes . The surfaces of the guide members, or of inactive parts of anodes which during cell operation are exposed to molten electrolyte, in particular those parts near the surface of the electrolyte, may be protected with a zinc- based coating, in particular containing zinc oxide with or without alumina, or zinc aluminate. During cell operation, to substantially inhibit dissolution of such a surface, the concentration in the electrolyte of dissolved alumina should be maintained at or above 3 to 4 weight% . Brief Description of the Drawings The invention will now be described with reference to the schematic drawings, wherein: - Figure 1 shows an aluminium electrowinning cell operating with anodes fitted with electrolyte guide members according to the invention; - Figures 2, 3 and 4 show enlarged parts of variations of the electrolyte guide members shown in Figure 1 during cell operation; - Figure 5 is a cross section of another anode with electrolyte guide members according to the invention only one of which is shown; - Figure 6 shows a plan view of half of an assembly of several electrolyte guide members like the one shown in Figure 5 ; - Figure 7 is a plan view of the anode shown Figure 5 with half of an assembly of electrolyte guide members as shown in Figure 6 ; and - Figure 8 is a plan view of a variation of the anode of Figure 7 shown without electrolyte guide members. Detailed Description Figure 1 shows an aluminium electrowinning cell according to the invention provided with a series of foraminate metallic anodes 10 having a generally horizontal anode structure 12,13,15 below a series of electrolyte guide members 5 according to the invention immersed in a crustless molten electrolyte 30. The cell comprises insulating means such as an insulating cover (not shown) covering the electrolyte to prevent the formation of an electrolyte crust on the surface of the electrolyte 30. Such a cover may be provided as described in WO99/02763 (de Nora/Sekhar) . The anodes 10 face a horizontal cathode cell bottom 20 connected to a negative busbar by current conductor bars 21. The cathode cell bottom 20 is made of conductive material such as graphite or other carbonaceous material coated with an aluminium-wettable refractory cathodic coating 22 on which aluminium 35 is produced and from which it drains or on which it forms a shallow pool, a deep pool or a stabilised pool. The molten aluminium 35 produced is spaced apart from the facing anodes 10 by an inter-electrode gap. Pairs of anodes 10 are connected to a positive bus bar through a primary vertical current feeder 11 ' and a horizontal current distributor 11" connected at both of its ends to a foraminate anode 10 through a secondary vertical current distributor 11'". The secondary vertical current distributors 11'" are mounted on the anode structure 12,13,15, on a cross member 12 which is in turn connected to two or more transverse connecting members 13 for connecting a series of anode members 15. The current feeders 11 ' , 11 ", 11 '", the cross member 12 , the transverse connecting members 13 and the anode members 15 are mechanically secured together by welding, rivets or other means. The anode members 15 have an electrochemically active lower surface 16 where oxygen is anodically evolved during cell operation. The anode members 15 are in the form of parallel rectilinear rods in a foraminate coplanar arrangement, laterally spaced apart from one another by inter-member gaps 17. The inter-member gaps 17 constitute flow-through openings for the circulation of electrolyte and the escape of anodically-evolved gas released at the electrochemically active surfaces 16. The cross member 12 and the transverse connecting members 13 provide a substantially even current distribution through the anode members 15 to their electrochemically active surfaces 16. The current feeder 11, the cross member 12 and the transverse connecting members 13 do not need to be electrochemically active and their surface may passivate when exposed to electrolyte. However they should be electrically well conductive to avoid unnecessary voltage drops and should not substantially dissolve in electrolyte. Variations may be made to the anode structure 12,13,15, for instance as disclosed in PCT/IB99/00018 (de Nora) . As described above, the electrochemically active surface 16 of the anode members 15 can be iron-oxide based, in particular hematite-based. Suitable anode materials are described in PCT/IB99/01360 (Duruz/de Nora/Crottaz), PCT/IB99/00015 (de Nora/Duruz) , PCT/IB99/01361 (Duruz/de Nora/Crottaz), PCT/IB99/01362 (Crottaz/Duruz) , PCT/IB99/01977 (de Nora/Duruz) and PCT/IB99/01976 (Duruz/de Nora) . The iron oxide surface may extend over all immersed parts 11 '", 12 , 13 , 15 of the anode 10, in particular over the immersed part of the secondary vertical current distributor 11'" which is preferably covered with iron oxide at least up to 10 cm above the surface of the electrolyte 30. The immersed but inactive parts of the anode 10 may be further coated with zinc oxide. However, when parts of the anode 10 are covered with zinc oxide, the concentration of dissolved alumina in the electrolyte 30 should be maintained above 3 wt% to prevent excessive dissolution of zinc oxide in the electrolyte 30. The core of all anode components 11 ', 11 ", 11 '", 12,13,15 is preferably highly conductive and may be made of copper protected with successive layers of nickel; chromium; nickel; copper and, optionally, a further layer of nickel. The anodes 10 are further fitted with a series of electrolyte guide members forming means for promoting the dissolution of powder alumina fed into the crustless molten electrolyte 30 in the form of parallel spaced-apart inclined baffles 5 located above and adjacent to the foraminate anode structure 12,13,15. The baffles 5 provide upper downwardly converging surfaces 6 and lower upwardly converging surfaces 7 that deflect gaseous oxygen which is anodically produced below the electrochemically active surface 16 of the anode members 15 and which escapes between the inter-member gaps 17 through the foraminate anode structure 12,13,15. The oxygen released above the baffles 5 promotes dissolution of alumina fed into the electrolyte 30 above the downwardly converging surfaces 6. A similar anode design was proposed in US Patent 4,263,107 (Pellegri) for improving electrolyte circulation in aqueous brine electrolysis. The anode was made of conventional anode materials for brine electrolysis, such as titanium coated with a platinum group metal oxide, having a foraminate active anode structure. Although, this anode design is well adapted for electrolyte circulation and gas release in brine electrolysis, it has never been proposed or suggested for use in aluminium electrowinning cells, which differ substantially to chlor-alkali cells, and in particular to improve the dissolution of fed alumina. The aluminium-wettable cathodic coating 22 of the cell shown in Figure 1 can advantageously be a slurry- applied refractory hard metal coating as disclosed in US Patent 5,651,874 (de Nora/Sekhar) . Preferably, the aluminium-wettable cathodic coating 22 consists of a thick coating of refractory hard metal boride such as TiB 2 , as disclosed in W098/17842 (Sekhar/Duruz/Liu) , which is particularly well suited to protect the cathode bottom of a drained cell as shown in Figure 1. The cell also comprises sidewalls 25 of carbonaceous or other material. The sidewalls 25 are coated/impregnated above the surface of the electrolyte 30 with a boron or a phosphate protective coating/impregnation 26 as described in US Patent 5,486,278 (Manganiello/Duruz/Bellό) and in US Patent 5,534,130 (Sekhar) . Below the surface of the electrolyte 30 the sidewalls 25 are coated with an aluminium-wettable coating 23, so that molten aluminium 35 driven by capillarity and magneto-hydrodynamic forces covers and protects the sidewalls 25 from the electrolyte 35. The aluminium-wettable coating 23 extends from the aluminium-wettable cathodic coating 22 over the surface of connecting corner prisms 28 up the sidewalls 25 at least to the surface of the electrolyte 30. The aluminium-wettable side coating 23 may be advantageously made of an applied and dried and/or heat treated slurry of particulate TiB 2 in colloidal silica which is highly aluminium-wettable. Alternatively, above and below the surface of the electrolyte 30, the sidewalls 25 may be covered with a zinc- based coating, such as a zinc-oxide coating optionally with alumina or a zinc aluminate coating. When a zinc-based coating is used to coat sidewalls 25 or anodes 10 as described above, the concentration of dissolved alumina in the molten electrolyte 30 should be maintained above 4 weight% to substantially prevent dissolution of such a coating. During cell operation, alumina is fed to the electrolyte 30 all over the baffles 5 and the metallic anode structure 12,13,15. The fed alumina is dissolved and distributed from the bottom end of the converging surfaces 6 into the inter-electrode gap through the inter-member gaps 17 and around edges of the metallic anode structure 12,13,15, i.e. between neighbouring pairs of anodes 10 or between peripheral anodes 10 and sidewalls 25. By passing an electric current between anodes 10 and facing cathode cell bottom 20 oxygen is evolved on the electrochemically active anode surfaces 16 and aluminium is produced which is incorporated into the cathodic molten aluminium 35. The oxygen evolved from the active surfaces 16 escapes through the inter-member gaps 17 and is intercepted by the upwardly converging surfaces 7 of baffles 5. The oxygen escapes from the uppermost ends of the upwardly converging surfaces 7 enhancing dissolution of the alumina fed over the downwardly converging surfaces 6. The aluminium electrowinning cells partly shown in Figures 2, 3 and 4 are similar to the one shown in Figure 1. In Figure 2 the guide members are inclined baffles 5 as shown in Figure 1, In this example the uppermost end of each baffle 5 is located just above mid-height between the surface of the electrolyte 30 and the transverse connecting members 13. Also shown in Fig. 2, an electrolyte circulation 31 is generated by the escape of gas released from the active surfaces 16 of the anode members 15 between the inter-member gaps 17 which is deflected by the upward converging surfaces 7 of the baffles 5 confining the gas and the electrolyte flow between their uppermost edges . From the uppermost edges of the baffles 5, the anodically evolved gas escapes towards the surface of the electrolyte 30, whereas the electrolyte circulation 31 flows down through the downward converging surfaces 6, through the inter-member gaps 17 and around edges of the metallic anode structure 12,13,15 to compensate the depression created by the anodically released gas below the active surfaces 17 of the anode members 15. The electrolyte circulation 31 draws down into the inter- electrode gap dissolving alumina powder 32 fed into the crustless molten electrolyte from above the downward converging surfaces 6 to be uniformly distributed to the inter-electrode gap. Figure 3 shows part of an aluminium electrowinning cell with baffles 5 operating as electrolyte guide members like those shown in cell of Figure 2 but whose surfaces are only partly converging. The lower sections 4 of the baffles 5 are vertical and parallel to one another, whereas their upper sections have upward and downward converging surfaces 6,7. The uppermost end of the baffles 5 are located below but close to the surface of the electrolyte 30 to increase the turbulence at the electrolyte surface caused by the release of anodically evolved gas. Figure 4 shows a variation of the baffles shown in Figure 3, wherein parallel vertical sections 4 are located above the converging surfaces 6,7. By guiding and confining anodically-evolved oxygen towards the surface of the electrolyte 30 with baffles or other confinement means, in particular as shown in Figures 3 and 4, oxygen is released so close to the surface as to create turbulence above the downwardly converging surfaces 6, promoting dissolution of alumina fed thereabove . It is understood that the electrolyte confinement members 5 shown in Figures 1,2,3 and 4 can either be elongated baffles, or instead consist of a series of vertical chimneys of funnels of circular or polygonal cross- section, for instance as described below. Figures 5 and 7 illustrate an anode 10 ' having a circular bottom, the anode 10 ' being shown in cross-section in Figure 5 and from above in Figure 7. On the right hand side of Figures 5 and 7 the anode 10 ' is shown with electrolyte guide members 5' according to the invention. The electrolyte guide members 5 ' represented in Figure 7 are shown separately in Figure 6. The anode 10' shown in Figures 5 and 7 have several, for example four, concentric circular anode members 15. The anode members 15 are laterally spaced apart from one another by inter-member gaps 17 and connected together by radial connecting members in the form of flanges 13 which join an outer ring 13 ' . The outer ring 13 ' extends vertically from the outermost anode members 15, as shown in Figure 5, to form with the radial flanges 13 a wheel-like structure 13,13', shown in Figure 7, which secures the anode members 15 to a central anode current feeder 11. As shown in Figure 5, the innermost circular anode member 15 partly merges with the current feeder 11, with ducts 18 extending between the innermost circular anode member 15 and the current feeder 11 to permit the escape of oxygen produced underneath the central current feeder 11. Each electrolyte guide member 5 ' is in the general shape of a funnel having a wide bottom opening 9 for receiving anodically produced oxygen and a narrow top opening 8 where the oxygen is released to promote dissolution of alumina fed above the electrolyte guide member 5 ' . The inner surface 7 of the electrolyte guide member 5' is arranged to canalise and promote an upward electrolyte flow driven by anodically produced oxygen. The outer surface 6 of the electrolyte guide member 5 ' is arranged to promote dissolution of alumina fed thereabove and guide alumina-rich electrolyte down to the inter- electrode gap, the electrolyte flowing mainly around the foraminate structure. As shown in Figures 6 and 7 , the electrolyte guide members 5' are in a circular arrangement, only half of the arrangement being shown. The electrolyte guide members 5' ■ are laterally secured to one another by attachments 3 and so arranged to be held above the anode members 15, the attachments 3 being for example placed on the connecting members 13 as shown in Figure 7 or secured as required. Each electrolyte guide member 5 ' is positioned in a circular sector defined by two neighbouring radial flanges 13 and an arc of the outer ring 13 ' as shown in Figure 7. The arrangement of the electrolyte guide members 5 ' and the anode 10' can be moulded as units. This offers the advantage of avoiding mechanical joints and the risk of altering the properties of the materials of the electrolyte guide members 5' or the anode 10' by welding. The anodes 10 ' and electrolyte guide members 5 ' can be made of any suitable material resisting oxidation and the fluoride-containing molten electrolyte, for example as disclosed in PCT/IB99/01360 (Duruz/de Nora/Crottaz) , PCT/IB99/00015 (de Nora/Duruz), PCT/IB99/01361 (Duruz/de Nora/Crottaz), PCT/IB99/01362 (Crottaz/Duruz) , PCT/IB99/01977 (de Nora/Duruz) and PCT/IB99/01976 (Duruz/de Nora) . Figure 8 illustrates a square anode 10 ' as a variation of the round anode 10 ' of Figures 5 and 7 but shown without its electrolyte guide members. The anode 10' of Figure 8 has generally rectangular concentric parallel anode members 15 with rounded corners. Electrolyte guide members similar to those of Figures 5 to 7 but in a corresponding rectangular arrangement cover the anode 10 ' .

Claims: CLAIMS 1. An electrolytic cell for the electrowinning of aluminium from alumina dissolved in a thermally insulated fluoride-containing crustless molten electrolyte, comprising an electrochemically active foraminate metallic anode structure for the evolution of oxygen and the escape of evolved oxygen therethrough and which is spaced by an inter- electrode gap above a facing cathode on which during operation aluminium is produced, the cell further comprising means for promoting dissolution of powder alumina fed to the surface of the electrolyte and for supplying alumina-rich electrolyte to the inter-electrode gap by inducing an electrolyte circulation up from and down to the inter- electrode gap driven by the escape of anodically evolved oxygen through the foraminate anode structure, said means comprising electrolyte guide members having at least one inclined surface immersed in the molten electrolyte above the foraminate anode structure. 2. The cell of claim 1, wherein the electrolyte guide members comprise downwardly converging inclined surfaces guiding a downward flow of alumina-rich electrolyte to the inter-electrode gap. 3. The cell of claim 1, wherein the electrolyte guide members comprise upwardly converging surfaces guiding an upward flow of alumina-depleted electrolyte from the inter- electrode gap, driven by anodically-evolved oxygen. 4. The cell of claim 1, wherein the electrolyte guide members comprise upwardly and downwardly converging surfaces guiding an upward flow and a downward flow of electrolyte. 5. The cell of any preceding claim, wherein the bottom end of each electrolyte guide member extends up from the foraminate anode structure. 6. The cell of claim 2, wherein bottom ends of the electrolyte guide members are spaced apart above the or each anode to allow alumina-rich electrolyte flowing down from the bottom ends of the electrolyte guide members to be dispersed by anodically-evolved upward flowing oxygen. 7. The cell of claim 3, wherein the electrolyte guide members are situated below the surface of the electrolyte so that the anodically-evolved upward flowing oxygen generates turbulence in the electrolyte above the electrolyte guide members to enhance the dissolution of alumina. 8. The cell of claim 7, wherein the uppermost end of each electrolyte guide member is immersed in the electrolyte by no more than 5 cm below the surface of the electrolyte. 9. The cell of claim 4, wherein the electrolyte guide members consist of a generally horizontal series of baffles arranged in a spaced-apart parallel configuration and laterally inclined, to form alternate pairs of upward converging surfaces and pairs of downward converging surfaces . 10. The cell of any one of claims 1 to 8 , wherein the electrolyte guide members form a plurality of funnels. 11. The cell of any one of claims 1 to 8 , wherein the electrolyte guide members are in the shape of truncated cones or truncated pyramids . 12. The cell of any preceding claim, wherein the electrolyte guide members have surfaces of a ceramic or an oxide resistant to the electrolyte. 13. The cell of claim 12, wherein the surfaces of the electrolyte guide members are iron oxide-based. 14. The cell of any preceding claim, wherein the electrolyte guide members are secured together as a unit. 15. The cell of any preceding claim, wherein the foraminate anode structure and the facing cathode are horizontal or at a corresponding slope. 16. The cell of any preceding claim, wherein the foraminate anode structure comprises a series of parallel spaced-apart coplanar electrochemically active anode members. 17. The cell of any preceding claim 1, comprising at least one aluminium-wettable drained cathode. 18. The cell of claim 17, wherein the aluminium-wettable drained cathode has a sloping drained cathode surface. 19. An oxygen evolving anode of an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising an electrochemically active foraminate metallic structure for the evolution of oxygen and the escape of evolved oxygen therethrough and which during operation in an electrolyte in a cell is spaced by an inter-electrode gap above a facing cathode on which aluminium is produced, the anode further comprising means arranged to promote dissolution of powder alumina fed to the surface of the electrolyte and supply alumina-rich electrolyte to the inter-electrode gap during operation by inducing an electrolyte circulation up from and down to the inter-electrode gap driven by the escape of anodically evolved oxygen through the foraminate anode structure, said means comprising electrolyte guide members having at least one inclined surface which in operation is immersed in the electrolyte above the foraminate anode structure. 20. The anode of claim 19, wherein the foraminate structure comprises a series of parallel spaced-apart coplanar electrochemically active anode members. 21. The anode of claim 20, wherein the anode members are spaced-apart blades, bars, rods or wires. 22. The anode of claim 21, wherein each blade, bar, rod or wire is generally rectilinear. 23. The anode of claim 21, wherein the spaced-apart blades, bars, rods or wires are in a generally concentric arrangement, each blade, bar, rod or wire forming a loop. 24. The anode of claim 23, wherein each blade, bar, rod or wire is generally circular, oval or polygonal. 25. The anode of claim 20, 21 or 22, wherein the anode members are in a grid-like, net-like or mesh-like configuration . 26. The anode of any one of claims 20 to 24, wherein the anode members are connected by one or more transverse connecting members to carry electric current to the anode members . 27. The anode of claim 26, wherein the anode members are connected by a plurality of transverse connecting members which are in turn connected together by one or more cross members to carry electric current to the anode members through the transverse connecting member (s). 28. The anode of claim 27, comprising at least one vertical current feeder arranged to be connected to a positive bus bar which is mechanically and electrically connected to one or more transverse connecting members or to one or more cross members connecting a plurality of transverse connecting members, for carrying electric current to the anode members through the transverse connecting member (s) and, where present, through the cross member(s). 29. The anode of claim 28, wherein the vertical current feeder, anode members, transverse connecting member (s) and, where present, cross member (s) are secured together as a unit. 30. The anode of claim 28 or 29, wherein the electrolyte guide members are secured together and secured to the vertical current feeder. 31. The anode of any one of claims 19 to 29, wherein the electrolyte guide members are secured to or placed on the foraminate anode structure. 32. The anode of any one of claims 19 to 31, wherein the foraminate anode structure has an iron oxide-based electrochemically active surface. 33. A method of producing aluminium in a cell as defined in any one of claims 1 to 18, comprising dissolving alumina in the electrolyte by feeding the alumina in the form of powder into the crustless molten electrolyte from above said electrolyte guide members, and passing an ionic current between the active foraminate anode structure and the facing cathode thereby carrying out electrolysis in the inter- electrode gap to produce aluminium on the cathode and oxygen on the foraminate anode structure, and inducing by said means for promoting dissolution of powder alumina and for supplying alumina-rich electrolyte to the inter-electrode gap, an electrolyte circulation up from and down to the inter-electrode gap driven the escape of anodically evolved oxygen through the foraminate anode structure. 34. An electrolytic cell for the electrowinning of aluminium from alumina dissolved in a thermally insulated fluoride-containing crustless molten electrolyte, comprising an electrochemically active foraminate metallic anode structure for the evolution of oxygen and which is spaced by an inter-electrode gap above a facing cathode on which during operation aluminium is produced, the cell further comprising means for promoting dissolution of powder alumina fed to the surface of the electrolyte and for uniformly distributing and feeding alumina-rich electrolyte through the foraminate structure to the inter-electrode gap, said means comprising electrolyte guide members which are located in the electrolyte above the foraminate anode structure, the electrolyte guide members comprising downwardly converging surfaces immersed in the electrolyte and being arranged to: - promote dissolution of alumina fed above their downward converging surfaces; and - feed alumina-rich electrolyte down through their downward converging surfaces and through the foraminate structure to the inter-electrode gap. 35. An electrolytic cell for the electrowinning of aluminium from alumina dissolved in a thermally insulated fluoride-containing crustless molten electrolyte, comprising an electrochemically active foraminate metallic anode structure for the evolution of oxygen and which is spaced by an inter-electrode gap above a facing cathode on which during operation aluminium is produced, the cell further comprising means for promoting dissolution of powder alumina fed to the surface of the electrolyte and for uniformly distributing and feeding alumina-rich electrolyte through and/or around the foraminate structure to the inter- electrode gap, said means comprising electrolyte guide members which are located in the electrolyte above the foraminate anode structure, the electrolyte guide members comprising upwardly converging surfaces immersed in the electrolyte and being arranged to: - guide an upward flow of alumina-depleted electrolyte driven by anodically evolved oxygen escaped through the foraminate anode structure to promote dissolution of alumina fed above their upwardly converging surfaces; and - feed alumina-rich electrolyte down through and/or around the foraminate anode structure to the inter- electrode gap.

Inventors: DE NORA VITTORIO, DE NORA, VITTORIO
Applicants: MOLTECH INVENT SA, NORA VITTORIO DE, DE NORA, VITTORIO, MOLTECH INVENT S.A.
Application year: 2000
Main classification: C25C3
IPC codes: C25C3