Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2002-04-27
2004-08-31
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S380000, C205S392000, C205S396000, C204S244000, C204S245000, C204S247000, C204S293000, C204S247400
Reexamination Certificate
active
06783656
ABSTRACT:
FIELD OF THE INVENTION
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 910° C., as well as the production of aluminium in such cell.
BACKGROUND OF THE INVENTION
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 950° 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 900° 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 700° to 940° C. by using a sloping roof covering an array of vertical anodes and cathodes, the sloping roof intercepting and guiding anodically evolved oxygen.
OBJECTS OF THE INVENTION
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.
SUMMARY OF THE INVENTION
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 “major metal” 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 NaF—AlF
3
molten electrolyte is from 730° to 910° C. or from 780° to 880° C., in particular from 820° to 860° C., and preferably below 850° 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 MgF
2
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 % Al
2
O
3
as well as NaF and AlF
3
in a weight ratio NaF/AlF
3
from about 0.71 to 0.81 is typically operated in the range of 780° and 860° C. at about 10° C. above its solidification temperature.
As described in patent application PCT/IB99/01976 (Duruz/de Nora), AlF
3
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 m
De Nora Vittorio
Duruz Jean-Jacques
Deshmukh Jayadeep R.
MoltechInvent S.A.
Valentine Donald R.
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