Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2002-03-30
2003-10-28
Bell, Bruce F. (Department: 1746)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S380000, C205S384000, C205S386000, C205S387000, C205S379000, C205S372000, C204S243100, C204S247300, C204S247400
Reexamination Certificate
active
06638412
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to inhibiting dissolution of an oxygen-evolving anode of a cell for the production of aluminium from alumina dissolved in an sodium ion-containing molten electrolyte.
BACKGROUND ART
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 Héroult, 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 CO
2
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. 4,614,569 (Duruz/Derivaz/Debely/Adorian), U.S. Pat. No. 4,680,094 (Duruz), U.S. Pat. No. 4,683,037 (Duruz) and U.S. Pat. No. 4,966,674 (Bannochie/Sherriff) describe non-carbon 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 by the addition of a cerium compound to the molten cryolite electrolyte. This made it possible to have a protection of the surface from the electrolyte attack.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer. Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve.
WO00/06802 (Duruz/de Nora/Crottaz) discloses a method of keeping an anode with a transition metal oxide layer dimensionally stable during operation in an aluminium electrowinning cell by maintaining in the electrolyte a sufficient concentration of transition metal species and dissolved alumina.
U.S. Pat. No. 6,248,227 (de Nora/Duruz) discloses an aluminium electrowinning anode having a metallic anode body which can be made of various alloys. During use, the surface of the anode body is oxidised by anodically evolved oxygen to form an integral electrochemically active oxide-based surface layer, the oxidation rate of the anode body being equal to the rate of dissolution of the surface layer into the electrolyte. This oxidation rate is controlled by the thickness and permeability of the surface layer which limits the diffusion of anodically evolved oxygen therethrough to the anode body.
WO00/06803 (Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/42534 (de Nora/Duruz), WO01/42536 (Duruz/Nguyen/de Nora) disclose further developments of metal-based aluminium production anodes.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry for commercial aluminium production because their lifetime is limited.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method of increasing the lifetime of transition metal-containing alloy anodes during operation in an aluminium electrowinning cell, in particular anodes made of a homogeneous metal alloy, such as a cast alloy or possibly an electroformed alloy.
The invention relates to a method of inhibiting dissolution of an oxygen-evolving anode of a cell for the production of aluminium from alumina dissolved in an sodium ion-containing molten electrolyte comprising a cathodic material that is predominately active for the reduction of sodium ions rather than aluminium ions. The oxygen-evolving anode comprises a transition metal-containing alloy having an integral oxide layer containing predominantly one or more transition metal oxides which slowly dissolve in the electrolyte and are compensated by oxidation of the alloy at the alloy/oxide layer interface.
According to the invention, the method comprises providing a sodium-inert layer on the sodium-active cathodic material and electrolysing the dissolved alumina whereby oxygen is anodically evolved and aluminium ions rather than sodium ions are cathodically reduced on the sodium-inert layer to inhibit the presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for chemically reducing the transition metal oxides and evolved oxygen, in particular molecular oxygen. The sodium-inert layer is used as a dissolution inhibitor of the anode by its effect in inhibiting reduction of the transition metal oxides by sodium metal and in maintaining the evolved oxygen at the anode at a concentration such as to produce at the alloy/oxide layer interface stable and coherent transition metal oxides having a high level of oxidation.
The present invention is based on two different observations about the operation of a cell utilising transition metal-alloy anodes.
The first observation relates to the quality of the anode's integral oxide layer which slowly dissolves in the electrolyte and is compensated by oxidation of the alloy at the alloy/oxide layer interface.
A high concentration of oxygen, in particular molecular oxygen, at the anode surface permits the formation of transition metal oxides having a high level of oxidation. It has been observed that such metal oxides have a greater stability in the electrolyte and thus a lower dissolution rate than metal oxides of lower oxidation level. In addition, metal oxides having a high level of oxidation have a greater coherence and form integral anode oxide layers with a greater imperviousness to electrolyte and oxygen diffusion which also reduces the oxidation rate of the alloy and inhibits corrosion.
Thus a high concentration of oxygen, in particular molecular oxygen, at the surface of a transition metal-alloy anode with an integral oxide layer surprisingly maintains the anode whereas a low concentration of oxygen leads to faster oxidation and corrosion of the anode.
The second observation relates to the wear-rate of a transition metal alloy-based anode operated in an aluminium production cell which has surprisingly been found to be significantly higher when the cell is operated with a cathodically polarised carbon material which is directly exposed to the molten electrolyte than when the carbon material is shielded from the electrolyte by a sodium-inert layer, such as molten aluminium, a boride coating or a fused alumina layer.
As opposed to sodium-inert materials, a sodium-active material leads to the reduction of sodium ions rather than aluminium ions. Usually such sodium-active materials, e.g. carbon, chemically combine with sodium during cathodic reduction which lowers the required sodium reduction energy in comparison to the energy of sodium reduction on an inert or neutral surface, such as molten aluminium, to an extent that sodium ions rather than aluminium ions are cathodically reduced.
Furthermore, sodium metal produced by cathodic reduction of sodium ions is very soluble in the molten electrolyte and thus can easily migrate to the anode.
It follows that sodium metal near the anode will chemically reduce the oxygen evolved on the anode leading to depletion of oxygen at the anode. As mentioned above, a lower concentration of oxygen at the anode leads to faster oxidation and corrosion of the anode.
Furthermore, sodium metal dissolved in the electrolyte at the anode may chemicall
De Nora Vittorio
Duruz Jean-Jacques
Bell Bruce F.
Deshmukh Jayadeep R.
Moltech Invent S.A.
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