This invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte such as cryolite, provided with non-carbon, metal-based, anodes designed for such aluminium electrowinning cells.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
U.S. Pat. No. 4,999,097 (Sadoway) describes anodes for conventional aluminium electrowinning cells provided with an oxide coating containing at least one oxide of zirconium, hafnium, thorium and uranium. To prevent consumption of the anode, the bath is saturated with the materials that form the coating. However, these coatings are poorly conductive and have not been used.
U.S. Pat. No. 4,504,369 (Keller) discloses a method of producing aluminium in a conventional cell using massive metal oxide anodes having a central vertical through-opening for feeding anode constituents and alumina into the electrolyte, to slow dissolution of the anode.
U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes metal anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained during electrolysis by the addition of small amounts of a cerium compound to the molten cryolite electrolyte. This made it possible to have a protection of the surface from the electrolyte attack and to a certain extent from gaseous oxygen but not from nascent monoatomic oxygen.
Several designs for oxygen-evolving anodes for aluminium electrowinning cells were proposed in the following documents. U.S. Pat. No. 4,681,671 (Duruz) discloses vertical anode plates or vertical blades operated in low temperature aluminium electrowinning cells. U.S. Pat. No. 5,310,476 (Sekhar/de Nora) discloses oxygen-evolving anodes consisting of roof-like assembled pairs of anode plates. U.S. Pat. No. 5,362,366 (de Nora/Sekhar) describes non-consumable anode shapes, such as roof-like assembled pairs of anode plates, as well as a downwardly curved flexible sheet or wire or bundle of wires. U.S. Pat. No. 5,368,702 (de Nora) discloses vertical tubular or conical oxygen-evolving anodes for multimonopolar aluminium cells. U.S. Pat. No. 5,683,559 (de Nora) describes an aluminium electrowinning cell with oxygen-evolving bent anode plates which are aligned in a roof-like configuration facing correspondingly shaped cathodes. U.S. Pat. No. 5,725,744 (de Nora/Duruz) discloses vertical oxygen-evolving anode plates, preferably porous or reticulated, in a multimonopolar cell arrangement for aluminium electrowinning cells operating at reduced temperature.
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.
It is an object of the invention to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes.
It is also an object of the invention to provide an aluminium electrowinning cell with one or more anodes which have a large surface area and a high electrochemical activity for the evolution of oxygen and which permit fast oxygen gas release and circulation of alumina rich electrolyte between the anodes and a facing cathode.
An object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which are easy and economic to manufacture.
Another object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which are made of a long lasting anode material leading to commercially acceptable produced aluminium and which can be shaped at will.
A further object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which are made of an anode material having a low solubility in the electrolyte.
An important object of the invention is to provide an aluminium electrowinning cell with one or more metal-based non-carbon anodes whose design permits an enhanced electrolyte circulation and which can be maintained dimensionally stable and do not excessively contaminate the product aluminium.
The invention provides a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises at least one non-carbon metal-based anode having an electrically conductive metallic structure with an electrochemically active anode surface on which, during electrolysis, oxygen is anodically evolved, and which is suspended in the electrolyte substantially parallel to a facing cathode. Such metallic structure comprises a series of parallel horizontal anode members, each having an electrochemically active surface on which during electrolysis oxygen is anodically evolved, the electrochemically active surfaces being in a generally coplanar arrangement to form said active anode surface. The anode members are spaced apart to form longitudinal flow-through openings for the circulation of electrolyte driven by the fast escape of anodically evolved oxygen.
Depending on the cell configuration some or all of the flow-through openings may serve for the flow of alumina-rich electrolyte to an electrolysis zone between the anode(s) and the cathode and/or for the flow of alumina-depleted electrolyte away from the electrolysis zone. When the anode surface is horizontal or inclined these flows are ascending and descending. Part of the electrolyte circulation may also take place around the metallic anode structure.
A substantially uniform current distribution can be provided from a current feeder through conductive transverse metallic connectors to the anode members and their active surfaces.
As opposed to known oxygen-evolving anode designs for aluminium electrowinning cells, in an anode according to this invention the coplanar arrangement of the anode members provides an electrochemically active surface extending over an expanse which is much greater than the thickness of the anode members, thereby limiting the material cost of the anode.
The electrochemically active anode surface is usually substantially horizontal or inclined to the horizontal.
In special cases, the electrochemically active anode surface may be vertical or substantially vertical, the horizontal anode members being spaced apart one above the other, and arranged so the circulation of electrolyte takes place through the flow-through openings. For example, the anode members may be arranged like venetian blinds next to a vertical or substantially vertical cathode.
In one embodiment, two substantially vertical (or downwardly converging at a slight angle to the vertical) spaced apart adjacent anodes are arranged between a pair of substantially vertical cathodes, each anode and facing parallel cathode being spaced apart by an inter-electrode gap. The adjacent anodes are spaced apart by an electrolyte down-flow gap in which alumina-rich electrolyte flows downwards until it circulates via the adjacent anodes"" flow-through openings into the inter-electrode gaps. The alumina-rich electrolyte is electrolysed in the inter-electrode gaps thereby producing anodically evolved oxygen which drives alumina-depleted electrolyte up towards the surface of the electrolyte where the electrolyte is enriched with alumina, and induces the downward flow of alumina-rich electrolyte.
The anode members may be spaced-apart blades, bars, rods or wires. The bars, rods or wires may have a generally rectangular or circular cross-section, or have in cross-section an upper generally semi-circular part and a flat bottom. Alternatively, the bars, rods or wires may have a generally bell-shape or pear-shape cross-section.
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.
Each anode member may be an assembly comprising an electrically conductive first or support member supporting or carrying at least one electrochemically active second member, the surface of the second member forming the electrochemical active surface. To avoid unnecessary mechanical stress in the assembly due to a different thermal expansion between the first and second members, the first member may support a plurality of spaced apart xe2x80x9cshortxe2x80x9d second members.
The electrochemically active second member may be electrically and mechanically connected to the first support member by an intermediate connecting member such as a flange. Usually, the first member is directly or indirectly in contact with the electrochemically active second member along its whole length which minimises during cell operation the current path through the electrochemically active member. Such a design is particularly well suited for a second member made of an electrochemically active material which does not have a high electrical conductivity.
Such an anode member design is also suitable when the member is an entire body of electrochemically active material which is oxidation resistant and porous (such as bulk oxide) and which has an ionic conductivity permitting the oxidation of oxygen ions within the active material. When such an active material covers an oxidisable substrate, the substrate is possibly oxidised thereby expanding underneath the electrochemically active material subjecting it to mechanical damaging stress. By providing a support member which has a barrier to oxygen on its surface, such as chromium oxide, and which is electrically conductive but not necessarily electrochemically active, the support member is not oxidised by possible ionic oxygen reaching it. Ionic oxygen remains within the electrochemically active material and is eventually converted into monoatomic and biatomic oxygen therein.
The parallel anode members should be connected to one another for instance in a grid-like, net-like or mesh-like 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 connection can be achieved by at least one 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 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 grid-like, 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.
Usually, when the anode is not made of bulk electrochemically active material, the anode may have an oxygen-evolving coating, which may be an applied coating or a coating obtained by surface oxidation of a metallic anode substrate. Usually the coating is made of metal oxide such as iron oxide.
The anode(s) may slowly dissolve in the electrolyte. Alternatively, the operating conditions of the cell may be such as to maintain the or each anode dimensionally stable. For instance, a sufficient amount of anode constituents may be maintained in the electrolyte to keep the anode(s) substantially dimensionally stable by reducing or preventing dissolution thereof into the electrolyte.
The cell may comprise at least one aluminium-wettable cathode. The aluminium-wettable cathode may be in a drained configuration. Examples of drained cathode cells are described in U.S. Pat. No. 5,683,130 (de Nora), WO99/02764 and WO99/41429 (both in the name of de Nora/Duruz).
The cell may also comprise means to facilitate dissolution of alumina fed into the electrolyte, for instance by using electrolyte guiding members above the anode members as described in PCT/IB99/00017 (de Nora) inducing an up-flow and/or a down-flow of electrolyte through and possibly around the anode structure.
The electrolyte guide members may be secured together 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 cell may also comprise means to thermally insulate the surface of the electrolyte to prevent the formation of an electrolyte crust on the electrolyte surface, such as an insulating cover above the electrolyte, as described in co-pending application WO99/02763 (de Nora/Sekhar).
A further aspect of the invention is a method of producing aluminium in a cell as described above. The method comprises passing an electric current through the anode members of the or each anode as electronic current and therefrom through the electrolyte to the cathode as ionic current, thereby producing aluminium on the cathode and oxygen on the electrochemically active anode surfaces whose escape induces an electrolyte circulation through the anode""s flow through openings.
The invention also provides a non-carbon metal-based anode of a cell for the electrowinning of aluminium as described above. The anode has an electrically conductive metallic structure with an electrochemically active anode surface resistant to oxidation and fluoride-containing molten electrolyte, on which, during electrolysis, oxygen is anodically evolved, and which is suspended in the electrolyte substantially parallel to a facing cathode. Such metallic structure comprises a series of parallel horizontal anode members, each having an electrochemically active surface on which during electrolysis oxygen is anodically evolved. The electrochemically active surfaces are in a generally coplanar arrangement to form the active anode surface. The anode members are spaced apart to form longitudinal flow-through openings for the circulation of electrolyte driven by the fast escape of anodically evolved oxygen.
Anodes of the present invention may consist of or preferably may be coated with an iron oxide-based material possibly obtained by oxidising the surface of an anode substrate which contains iron. Suitable anode 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 (Fe2O3) 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 NiFe2O4/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 (950-970xc2x0 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 electrochemically active surface of the anode(s) is iron oxide-based, the electrolyte may comprise an amount of iron species and dissolved alumina preventing dissolution of the iron oxide-based electrochemically active surface. The amount of iron species and alumina dissolved in the electrolyte and preventing dissolution of the iron oxide-based electrochemically active surface of the or each anode should be 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 the amount of anode constituents, in particular iron species, in the electrolyte which prevents at the operating temperature the dissolution of the or each anode if the alumina feed itself does not contain enough iron, anode 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 electrochemically active surface of the anode is 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 anode constituents to a commercially acceptable level, the cell should be operated at a sufficiently low temperature so that the required concentration of dissolved alumina and anode 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 910xc2x0 C., usually from 730 to 870xc2x0 C. The electrolyte may contain NaF and AlF3 in a molar ratio NaF/AlF3 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 %.
The 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 %.
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