Prevention of dissolution of metal-based aluminium production anodes

Inactive Publication Date: 2005-12-08
DE NORA VITTORIO +1
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Benefits of technology

[0096] The abovementioned carbon dust, carbon monoxide, sodium, lithium or potassium metal in the electrolyte at the anode may chemically reduce oxides of the anode's surface which causes corrosion of the anode. Sodium, lithium or potassium metal may also be oxidised in the electrolyte by the anodic current which reduces the cell's current efficiency. The a

Problems solved by technology

However, full protection of the alloy substrate was difficult to achieve.
Many attempts were made to use metallic anodes for aluminium pro

Method used

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  • Prevention of dissolution of metal-based aluminium production anodes
  • Prevention of dissolution of metal-based aluminium production anodes
  • Prevention of dissolution of metal-based aluminium production anodes

Examples

Experimental program
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example 1

Comparative

[0145] One of the above identical nickel-iron alloy anodes 40 was used in a cell, as shown in FIG. 1, having cathodically polarised carbon sidewalls 15 exposed to the molten electrolyte 30.

[0146] The electrolytic bath 30 consisted of 16 weight % AlF3, 4 weight % caF2 and 6 to 6.5 weight % dissolved Al2O3, the balance being cryolite (Na3AlF6), and was at a temperature of 930° C. The aluminium layer 20 had a thickness of about 3 cm.

[0147] Electrolysis was performed at constant current corresponding to an anodic current density of 0.8 A / cm2 whereby oxygen was anodically evolved and aluminium 20 cathodically produced by electrolysis of the dissolved alumina.

[0148] The composition of the bath 30 was analysed every 12 hours by x-ray fluorescence (XRF). The Al2O3 content in the bath was maintained substantially constant by adding every 15 min an amount of Al2O3 adjusted according to the analysed composition of the bath 30.

[0149] During the first 24 hours the cell voltage wa...

example 2

[0155] Another of the above identical nickel-iron alloy anodes was used in a cell, as shown in FIG. 2, having cathodically non-polarised upper parts 17 of carbon sidewalls 15 exposed to the molten electrolyte 30, the cathodically polarised sidewall bottom parts 16 being shielded from the electrolyte by fused alumina sleeve 50.

[0156] The electrolysis was carried out under the same operating conditions as in Example 1.

[0157] Like in the previous Example, during the first 24 hours the cell voltage was stable at 3.6 volts and the Al2O3 consumption corresponded to about 60% of the theoretical value.

[0158] After this initial period the cell voltage continued to remain substantially stable. However, the Al2O3 consumption decreased. After 50 hours the Al2O3 consumption had stabilised at 50% of the theoretical value.

[0159] After 100 hours the anode 40 was removed from the bath 30 and examined.

[0160] The external dimensions of the anode 40 had not significantly changed. The wear of the a...

example 3

[0165] The last anode of the above identical nickel-iron alloy anode was used in a cell, as shown in FIG. 3, in which no carbon is exposed to the electrolyte 30.

[0166] The electrolysis was carried out under the same operating conditions as in Examples 1 and 2.

[0167] The cell voltage was stable at 3.6 volts, and the Al2O3 consumption corresponded to about 60% of the theoretical value throughout the test.

[0168] After 100 hours the anode was removed for examination. The external dimensions of the anode were substantially unchanged.

[0169] The external dimensions of the anode 40 had not significantly changed. The wear of the anode 40 led to a reduction of the average diameter of the metallic core by 0.3 mm from 20 to 19.7 mm, which is even better than in Example 2. The anode was covered by a dense and coherent oxide scale of about 200 microns thick. No noticeable anode corrosion was observed.

[0170] The improvement of the anode wear rate between Examples 2 and 3 is believed to be due...

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Abstract

A method of inhibiting dissolution of a transition metal alloy anode (40) of an aluminium electrowinning cell comprises providing a barrier layer (11,20,50,50′) on a non-anodic structural cell material (15), such as carbon, and electrolysing alumina dissolved in a molten electrolyte (30). The non-anodic structural material is able to supply an oxidisable by-product to the electrolyte and/or is active for reducing electrolyte species exposed to the structural material into an oxidisable by-product, such as sodium metal or carbon dust. However, the barrier layer inhibits the presence in the molten electrolyte (30) of the oxidisable by-product that constitutes an agent for chemically reducing the anode's transition metal oxides and anodically evolved oxygen. This inhibits reduction of the anode's transition metal oxides by the oxidisable by-product and maintains the anodically evolved oxygen 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 barrier layer may comprise molten aluminium (20) and/or a layer of refractory hard material (11,50,50′).

Description

FIELD OF THE INVENTION [0001] 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 [0002] 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. [0003] Industrial 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. [0004] Using m...

Claims

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Application Information

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IPC IPC(8): C25C3/06C25C3/08C25C7/02
CPCC25C3/08C25C3/06
Inventor DE NORA, VITTORIODURUZ, JEAN-JACQUES
Owner DE NORA VITTORIO
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