Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-01-29
2003-02-18
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S372000, C205S384000, C205S385000, C205S389000, C205S392000, C205S394000, C204S243100, C204S245000, C204S247300
Reexamination Certificate
active
06521116
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte provided with dimensionally stable oxygen-evolving anodes, and to methods for the fabrication and reconditioning of such anodes, as well as to the operation of such cells to maintain the anodes dimensionally stable.
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.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. As mentioned hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry.
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 barrier layer. However, full protection of the alloy substrate was difficult to achieve.
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 found commercial acceptance.
U.S. Pat. No. 4,504,369 (Keller) discloses a method for producing aluminium in a conventional cell using anodes whose dissolution into the electrolytic bath is reduced by adding anode constituent materials into the electrolyte, allowing slow dissolution of the anode. However, this method is impractical because it would lead to a contamination of the product aluminium by the anode constituent materials which is considerably above the acceptable level in industrial production. To limit contamination of the product aluminium, it was suggested to reduce the reduction rate of the dissolved constituent materials at the cathode, by limiting the cathode surface area or by reducing mass transfer rates by other means. However, the feasibility of these proposals has never been demonstrated, nor was it contemplated that the amount of the anode constituent materials dissolved in the electrolyte should be reduced.
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 by the addition of small amounts of cerium to the molten cryolite electrolyte so as to protect the surface of the anode from the electrolyte attack. All other attempts to reduce the anode wear by slowing dissolution of the anode with an adequate concentration of its constituents in the molten electrolyte, for example as described in U.S. Pat. Nos. 4,999,097 (Sadoway) and 4,504,369 (Keller), have failed.
In known processes, even the least soluble anode material releases excessive amounts constituents into the bath, which leads to an excessive contamination of the product aluminium. For example, the concentration of nickel (a frequent component of stable anodes) found in aluminium produced in laboratory tests at conventional cell operating temperatures is typically comprised between 800 and 2000 ppm, i.e. 4 to 10 times the acceptable level which is 200 ppm.
The extensive research which was carried out to develop suitable metal anodes having limited dissolution did not find any commercial acceptance because of the excessive contamination of the product aluminium by the anode materials.
OBJECTS OF THE INVENTION
A major object of the invention is to provide an anode for aluminium electrowinning which has no carbon so as to eliminate carbon-generated pollution and increase the anode life.
A further object of the invention is to provide an aluminium electrowinning anode material with a surface having a high electrochemical activity for the oxidation of oxygen ions for the formation of bimolecular gaseous oxygen and a low solubility in the electrolyte.
An important object of the invention is to reduce the solubility of the surface layer of an aluminium electrowinning anode, thereby maintaining the anode dimensionally stable without excessively contaminating the product aluminium.
Another object of the invention is to provide operating conditions for an aluminium electrowinning cell under which conditions the contamination of the product aluminium is limited.
A subsidiary object of the invention is to provide a cell for the electrowinning of aluminium whose side walls are resistant to electrolyte, thereby allowing operation of the cell without formation of a frozen electrolyte layer on the side walls and with reduced thermal loss.
SUMMARY OF THE INVENTION
The invention relates to dimensional stabilisation of oxygen-evolving anodes of cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte. It has been found that dissolution of anodes comprising a transition metal-based oxide surface, in particular an electrochemically active outside layer of iron oxide, cobalt oxide, nickel oxide or combination thereof, can be kept dimensionally stable during electrolysis by maintaining in the electrolyte a sufficient concentration of dissolved alumina and transition metal species which are present as one or more corresponding transition metal oxides in the anode surface, and operating the cell at a sufficiently low temperature so that the required concentration of the transition metal species in the electrolyte is limited by the reduced solubility thereof in the electrolyte at the operating temperature, which consequently limits to an acceptable level the contamination of the product aluminium by the transition metals which are present as one or more corresponding transition metal oxides in said outside layer.
The invention is particularly but not exclusively concerned with iron oxide-containing electrochemically active anode surfaces and will be further described and illustrated with particular reference thereto.
It has been observed that iron oxides and in particular hematite (Fe
2
O
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 operating temperature of a cell.
Laboratory scale cell tests utilising a NiFe
2
O
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 t
Crottaz Olivier
de Nora Vittorio
Duruz Jean-Jacques
Bell Bruce F.
Deshmukh Jayadeep R.
Moltech Invent S.A.
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