The invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a crustless fluoride-containing molten electrolyte at a temperature below 910xc2x0 C., as well as the production of aluminium in such cell.
The production of aluminium today utilises cells for the electrolysis of alumina dissolved in cryolite with an excess of approximately 10 weight % aluminium fluoride, operating at a temperature of approximately 950xc2x0 C., utilising carbon anodes.
Several patents have been filed and many granted concerning anode and cathode materials, shape, cell designs, operating conditions etc., and many solutions to specific problems have been proposed. However, no overall arrangement has heretofore been proposed which meets up to all the practical requirements for the industrial production of aluminium with low contamination.
The metal anodes suggested until now are highly soluble in the electrolyte utilised contaminating the aluminium produced, and have other drawbacks such as low electrical conductivity, short life and high cost.
All or some of these drawbacks can be eliminated by operating the cells at lower temperature which would require a high circulation of the electrolyte to maintain a sufficiently high concentration of alumina in the inter-electrode gap.
U.S. Pat. No. 4,681,671 (Duruz) proposed the production of aluminium by the electrolysis of alumina in a crustless fluoride-containing molten electrolyte at a temperature below 900xc2x0 C. by effecting steady state electrolysis using an oxygen evolving anode but at a low anode current density. This led to the development of multimonopolar cell designs, described in U.S. Pat. No. 5,725,744 (de Nora/Duruz). Such designs are however not compatible with the use of cathodes made from carbon blocks protected with an aluminium-wettable slurry-applied coating of titanium diboride as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar).
Efforts have been made to achieve the advantages of low temperature electrolysis in cells with drained cathodes made of carbon blocks coated with an aluminium-wettable coating, but so far have not led to an accepted design meeting up to all requirements. WO 99/02764 (de Nora) and WO 99/02763 (de Nora/Sekhar) disclosed drained cells with oxygen evolving anodes, operating with a crustless electrolyte maintained by a thermal insulating cover. Electrolyte circulation was provided by sloping anodes and cathodes.
U.S. Pat. No. 5,983,914 (Dawless/LaCamera/Troup/Ray/Hosler) proposes to improve the dissolution of alumina in an electrolyte at 700xc2x0 to 940xc2x0 C. by using a sloping roof covering an array of vertical anodes and cathodes, the sloping roof intercepting and guiding anodically evolved oxygen.
One object of the invention is to provide an aluminium electrowinning cell incorporating nickel-iron alloy based anodes that can be operated without excessive contamination of the produced aluminium.
Another object of the invention is to provide an aluminium electrowinning cell operating with a crustless electrolyte, that can achieve high productivity, low contamination of the product aluminium, and whose components resist corrosion and wear.
Yet another object of the invention is to provide an aluminium electrowinning cell including nickel-iron alloy based anodes which remain substantially insoluble at the cell operating temperature.
An overall object of the invention is to provide a cell for the electrowinning of aluminium from alumina dissolved in a crustless fluoride-containing molten electrolyte, in particular at low temperatures, which overcomes the various drawbacks of the previous proposals.
The invention proposes a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell uses nickel-iron alloy based anodes for producing aluminium of low contamination and of commercial high grade quality. Each anode has an oxygen-evolving electrochemically active surface. The cell comprises a cathode having a drained cathode surface and operating at reduced temperature without formation of a crust or ledge of solidified electrolyte. The molten electrolyte is substantially saturated with alumina, particularly on the electrochemically active anode surface, and with species of at least one major metal present at the surface of the nickel-iron alloy based anodes.
A xe2x80x9cmajor metalxe2x80x9d refers to a metal which is present at the surface of the nickel-iron alloy based anode in an atomic and/or ionic form, in particular in one or more oxide compounds, in an amount of at least 25% of the total amount of metal atoms and/or ions present at the surface of the nickel-iron alloy based anode. Typically, such a metal can be iron, nickel or another major alloying metal of the nickel-iron alloy based anode, if such is present at the surface of the anode.
Usually, the operating temperature of an NaFxe2x80x94AlF3 molten electrolyte is from 730xc2x0 to 910xc2x0 C. or from 780xc2x0 to 880xc2x0 C., in particular from 820xc2x0 to 860xc2x0 C., and preferably below 850xc2x0 C. The concentration of alumina dissolved in the electrolyte is at most about 8 weight %, usually between 2 weight % and 6 weight %. The molten electrolyte may also contain MgF2 and/or LiF in an amount of up to 5 weight % each. Further low temperature electrolytes are disclosed in U.S. Pat. No. 4,681,671 (Duruz).
For instance, a molten electrolyte containing about 3 weight % Al2O3 as well as NaF and AlF3 in a weight ratio NaF/AlF3 from about 0.71 to 0.81 is typically operated in the range of 780xc2x0 and 860xc2x0 C. at about 10xc2x0 C. above its solidification temperature.
As described in patent application PCT/IB99/01976 (Duruz/de Nora), AlF3 may be present in such a high concentration in the electrolyte that fluorine-containing ions rather than oxygen ions are oxidised on the electrochemically active surface, however only oxygen is evolved, the evolved oxygen being derived from the dissolved alumina present near the electrochemically active anode surfaces.
The drained cathode is preferably aluminium-wettable and may be associated with an aluminium collection channel along the cell for collecting produced molten aluminium draining from the drained cathode surfaces and leading into a central aluminium collection reservoir across the cell from where the produced molten aluminium can be evacuated from the cell. The drained cathode may comprise two inclined drained cathode surfaces arranged generally in a V-shape extending along the cell formed by upper surfaces of cathode blocks that extend across the cell, the cell being divided by the aluminium collection channel along the cell and by the central aluminium collection reservoir across the cell, the reservoir being formed by recessed spacer blocks spacing the cathode blocks.
Unlike in conventional cells where undissolved alumina collects as sludge on the cell bottom which prevents electrolysis from taking place, this configuration offers the advantage that any undissolved alumina can deposit on and flow together with the aluminium produced from the drained cathode surfaces into the collection recess from where it can be recovered, for instance when the product aluminium is tapped, without interfering with the normal course of electrolysis. A cell bottom design incorporating this feature is described in patent application PCT/IB99/00698 (de Nora), filed Apr. 16, 1999.
The cell has side walls contacted by the molten electrolyte and made of material resistant to the molten electrolyte including fused alumina, carbides and/or nitrides, such as silicon carbide, silicon nitride and boron nitride.
Preferably, the drained cathode surface on which aluminium is produced and from which the produced aluminium is drained comprises, or is associated with, inclined drained surfaces adjacent to the side walls. These inclined drained surfaces are inclined down towards the centre of the cell to keep the produced aluminium out of contact with the side walls.
Ledgeless and crustless cell operation may be achieved by means of a thermal insulation of the cell, including a sidewall insulation and an insulating cover above the molten electrolyte surface, sufficient to prevent the formation of any crust of solidified electrolyte or ledge of solidified electrolyte on the cell side walls. For example, the inside of the insulating cover can be held at a temperature differential as little as 10xc2x0 C. below the temperature at the surface of the molten electrolyte. To allow for servicing of the anodes, the cover may be arranged to permit the removal and insertion of the anodes from/into the molten electrolyte. For this, it can include individually removable sections permitting removal of individual anodes or groups of anodes without adversely affecting the thermal balance, as disclosed in WO 99/02763 (de Nora/Sekhar).
The insulating cover may be of composite structure, having an inner surface layer of material resistant to fumes from the molten electrolyte, an insulating core and an outer support structure providing mechanical strength.
Optionally, the cell may comprise means for supplying heat, e.g. burners, between the insulating cover and the surface of the molten electrolyte to prevent cooling leading to the formation of an electrolyte crust when the insulating cover is removed.
The cell may comprise means for supplying powdered alumina between the thermal insulating cover and the molten electrolyte surface. The alumina supplying means may comprise a device for distributing preheated alumina by spraying or blowing it over the molten electrolyte surface.
Unlike the conventional point feeder devices used for cells with a frozen crust, these alumina supply means are arranged to distribute the supplied powdered alumina preferably over all of the molten electrolyte surface from where the alumina dissolves as it enters the electrolyte to maintain an even concentration of dissolved alumina in the circulating electrolyte. However, the supplied alumina may be distributed over selected areas of the molten electrolyte surface, usually making up a substantial part of the total surface. Such alumina distribution means, as described in patent application PCT/IB99/00968 (de Nora/Berclaz), filed Apr. 16, 1999, includes a device for spraying or blowing the alumina which is advantageously preheated.
The alumina to be sprayed or blown may be stored in a reservoir located above the cell and preheated. The heat evacuated from the cell with the gas produced during electrolysis and/or the heat conducted by stems feeding current to the active anode structures is optionally used to pre-heat the stored alumina. The alumina may alternatively or additionally be preheated while it is introduced into the cell above the molten electrolyte by blowing it with hot gas or a flame.
Means are provided for inducing electrolyte circulation generated by upward lift of oxygen released from the anodes, whereby the electrolyte circulates towards the molten electrolyte surface and down to the inter-electrode gap. These means can include sloped surfaces of the anodes facing sloping cathodes, or can include baffles, funnels or other electrolyte guide members with converging surfaces, arranged above a foraminate anode of open structure comprising a series of vertical through openings for the fast release of anodically produced oxygen and for the down flow of alumina-rich electrolyte into the anode-cathode gap for electrolysis, as described in patent application WO 00/40781 (de Nora), filed Jan. 8, 1999.
The means for inducing electrolyte circulation may comprise electrolyte guide members with converging surfaces. The guide members may be arranged above a foraminate anode of open structure comprising a series of vertical through openings for the rapid escape of anodically produced oxygen and for the down flow of alumina-rich electrolyte into the anode-cathode gap for electrolysis.
These means for inducing electrolyte circulation, together with the previously-described means for distributing alumina, result in enrichment of the electrolyte with dissolved alumina at a concentration which is close saturation even in the inter-electrode gap. The saturation of the electrolyte with alumina and its strong circulation limit the depletion of alumina and maintain a near-saturation concentration of dissolved alumina in the depleted electrolyte. As explained below, the presence in the electrolyte of alumina at a saturation concentration or close to saturation, together with dissolved metal species at or nearly at their saturation concentration which is reduced by the presence of alumina, inhibits dissolution of the nickel-iron alloy based anodes.
Usually, each electrochemically active anode surface comprises iron and nickel as metals and/or oxides. For example, the electrochemically active anode surface may comprise nickel ferrite. The electrochemically active anode surface may be an integral oxide based outer layer which can be obtained by oxidising the surface of a nickel-iron alloy body or layer, for example as disclosed in WO 00/06803 (Duruz/de Nora/Crottaz) and WO 00/06804 (Crottaz/Duruz). The electrolyte may contain dissolved iron and/or nickel species in an amount sufficient to inhibit dissolution of such an electrochemically active iron oxide and nickel oxide anode surface as described in WO 00/06802 and WO 00/06803 (Duruz/de Nora/Crottaz).
In one embodiment, the nickel-iron alloy anodes are surface oxidised in an oxidising atmosphere before use to produce an openly porous nickel metal rich outer portion which consists predominantly of nickel metal, as disclosed in PCT/IB99/01976 (Duruz/de Nora) and whose surface constitutes an electrochemically active anode surface of high surface area which in use is active for the oxidation of ions.
The open porosity can be produced before use by heat treatment in an oxidising atmosphere, e.g. at 1000xc2x0-1200xc2x0 C. for 0.5-5 hours in air or another oxygen-containing atmosphere, which removes iron from the nickel-iron alloy by diffusion and oxidises the removed iron. Such a porosity contains cavities which are partly or completely filled before use with nickel and/or iron oxides and during use with fluorides of at least one metal selected from iron, nickel and aluminium. A similar porosity can be formed by electrolytic dissolution of part of the iron of the alloy""s outer portion, which can be carried out by passing a current though the anode at low current density on the anode""s surface, typically 1 to 100 mA/cm2, in a fluoride-based electrolyte, for instance an electrolyte at a temperature below 870xc2x0 C. and consisting essentially of cryolite with an excess of AlF3 in an amount of about 25 to 35 weight % of the electrolyte, before use in an aluminium production cell or in-situ at start-up of the anode. Furthermore, these two methods of producing the porosity may be combined, e.g. partial conditioning of the anode by oxidation treatment can be completed by electrolytic dissolution.
An anode""s electrochemically inactive surface which is exposed to molten electrolyte can be made of the same materials used for the electrochemically active anode surface or of other materials which are resistant to molten electrolyte.
The cell usually comprises means to adjust the positioning of the anodes over the drained cathode surface. These means may form part of an anode superstructure under which the anodes are suspended, the superstructure for example including one or more motors for small linear and/or angular displacements of the anodes and for fine adjustments of the inter-electrode distance. For instance, each anode is associated with an individual motor for linear displacements of the anode so the inter-electrode distance is adjustable for each anode separately in order to achieve a substantially uniform and equal current distribution between the cathode bottom and each anode and to prevent formation of local current peaks.
Alternatively, the anodes are positioned above the cathode bottom using electrically non-conductive spacer elements to ensure a constant inter-electrode distance. These spacer elements are made of a material resistant to the product aluminium, the molten electrolyte and the anodically produced oxygen, such as fused alumina, silicon carbide, silicon nitride or boron nitride, and may be embedded in the cathode bottom or mechanically secured to the anodes.
Each active anode structure can be made of a series of spaced apart parallel anode rods which are mechanically and electrically connected, usually with at least one connecting cross-member arranged transversally over the anode rods. This connecting member is preferably of variable section, i.e. decreasing from the middle of the active anode structure, where current is centrally fed from an anode stem, towards the extremities of the active anode structure, in order to feed current at a substantially uniform current density over the entire active anode structure.
Optionally, each anode is associated with means to oscillate it, for instance around at least one axis, to enhance distribution of dissolved alumina in the inter-electrode gap. At least one axis of oscillation can be substantially vertical to the drained cathode surface.
The product aluminium collected in the aforementioned central recess is of an acceptable purity due to the fact that the molten electrolyte contains dissolved metal species, corresponding to metal(s) of the nickel-iron alloy based anodes, in particular iron, at or nearly at a saturation concentration but which is reduced by the presence of dissolved alumina maintained in the circulating molten electrolyte and by the low temperature of the electrolyte. These combined effects inhibit dissolution of the nickel-iron alloy based anodes and lead to a concentration, in the produced molten aluminium, of the metals and/or metal species which are present as one or more corresponding metals and/or oxides at the electrochemically-active surface of the anodes, within commercially acceptable limits as explained in greater detail in patent applications WO 00/06802 and WO 00/06802 (both in the name of Duruz/de Nora/Crottaz).
In summary, the product aluminium has an acceptably low contamination due to the combined effect of operating with a low temperature molten electrolyte with improved electrolyte circulation and alumina distribution using nickel-iron alloy based anodes that are substantially insoluble in the electrolyte at the low operating temperature, and wherein the aluminium collection is separated from the side walls facilitating ledgeless operation.
A preferred embodiment of the invention combines several aspects of the cell described hereabove, as set out in claim 35.
Such a cell combines low temperature operation with crustless molten electrolyte with electrolyte circulation. The cell has an aluminium-wettable drained cathode and uses nickel-iron alloy based anodes which have low solubility. The cell has a single central aluminium collection channel and a central reservoir for collection of the produced molten aluminium which, thanks to the cell features and operating conditions, is of low contamination.
In contrast to the low-temperature cell disclosed in U.S. Pat. No. 4,681,671 (Duruz), the cell according to the invention can make use of a unipolar cathode made of an assembly of carbon cathode blocks protected with an aluminium-wettable protective coating. Moreover, whereas this US patent preferred an external circulation for enrichment of the molten electrolyte with alumina, the cell according to the invention achieves an internal circulation by means not suggested by the patent.
Compared to the drained cells with oxygen evolving anodes of WO 99/02764 (de Nora), the invention provides improved distribution of alumina and electrolyte circulation, in addition to lower contamination of the product aluminium and better protection of cell components, notably the side walls. Moreover, the invention is not limited to making use of inclined or vertical anode/cathode surfaces to produce the electrolyte circulation, neither is it limited to an inclined roof covering vertical anode and cathode packs as disclosed in U.S. Pat. No. 5,983,914 (Dawless/LaCamera/Troup/Ray/Hosler).
The invention thus provides an overall combination which has heretofore not been suggested and which leads to significant advantages.
In summary, the cell according to the invention combines a plurality or preferably most or all of the following features:
1) a molten electrolyte at reduced temperature, typically between 780xc2x0 and 880xc2x0 C., preferably between 820xc2x0 and 860xc2x0, and in particular below 850xc2x0 or 830xc2x0 C.;
2) cathodes of drained configuration;
3) cathodes wetted by molten aluminium;
4) an electrolyte integrally in a molten state;
5) no formation of any ledge or crust of frozen electrolyte on the sidewalls, at the surface of the molten electrolyte or on the bottom of the cell;
6) nickel-iron based alloy containing anodes with an electrochemically active surface;
7) nickel-iron alloy based anodes having an electrochemically active surface comprising in particular iron and/or nickel species including oxides;
8) an electrolyte saturated or substantially saturated with the main element(s), in particular iron and/or nickel species, of the electrochemically active anode surfaces;
9) an insulating cover fitted over the cell and preventing the molten electrolyte from freezing;
10) active anode structures suspended with anode stems for feeding current, which stems are electrically highly conductive below the insulating cover;
11) a powder alumina dispersion system for uniform or substantially uniform alumina feeding over the molten electrolyte;
12) an alumina reservoir on top of the cell containing powdered alumina which is preheated using the heat generated by the cell;
13) gas burners below the insulating cell cover above the molten electrolyte, used to prevent electrolyte from freezing when the insulating cover or a section thereof is removed to insert or extract an anode or for another maintenance operation;
14) an electrolyte circulation induced by oxygen gas lift which is preferably controlled by deflectors arranged above the anode active structure;
15) each anode-cathode distance being individually settable to achieve a substantially uniform and equal current density and current distribution between the cathode bottom and each facing anode;
16) anode structures designed to feed electrical current at a substantially uniform current density to the active anode surface;
17) anode active surfaces prevented from contacting product aluminium during cell operation;
18) molten electrolyte substantially saturated with dissolved alumina, especially in the vicinity of the active anode surfaces;
19) active anode surfaces operating at a substantially uniform current density with no local current peaks;
20) molten electrolyte substantially saturated at the operating temperature with the main element of the electrochemically active anode surfaces and with dissolved alumina;
21) electrochemically inactive and active immersed surfaces of the anodes being all made of the same material; and
22) active anode surfaces sloped to permit rapid upward escape of anodically evolved gas facilitating electrolyte circulation.
Another aspect of the invention concerns a method of electrowinning aluminium in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based molten electrolyte as described above. The method comprises supplying alumina to the molten electrolyte where it is dissolved and electrolysing the dissolved alumina in the inter-electrode gap, to produce oxygen gas on the nickel-iron alloy based anodes and aluminium on the drained cathodes. Oxygen can be produced by oxidising oxygen-containing ions directly on the active surfaces or by firstly oxidising fluorine-containing ions that subsequently react with oxygen-containing ions, as described in PCT/IB99/01976 (Duruz/de Nora).
For instance, the electrolyte may contain AlF3 in such a high concentration that fluorine ions rather than oxygen ions are oxidised on the electrochemically active anodes surfaces that are catalytically active for the oxidation of fluorine-containing ions rather than oxygen ions, however, only oxygen is evolved. The evolved oxygen is derived from the dissolved alumina present near the electrochemically active anode surfaces.
The oxidation of fluorine-containing ions rather than oxygen ions on the anode surface inhibits oxidation of the anode by oxidised oxygen ions, in particular monoatomic nascent oxygen, formed on the anode surface. Thus, oxygen is formed at a distance of the anode surface either by reaction of oxygen ions with oxidised fluorine containing ions or by decomposition of transient oxidised oxyfluoride ions.
The mechanism of oxidation of fluorine-containing ions rather than oxygen ions can be achieved by operating the cell with a nickel-iron anode having a openly porous nickel metal rich outer portion as electrochemically active surface as described above.
As nickel and cobalt behave very similarly under the above described cell conditions, in modifications of the above aspects of the invention, the nickel of the anodes is wholly or predominantly substituted by cobalt. For example, the anode is made from a nickel-cobalt-iron alloy or a cobalt-iron alloy.