Iron-based anode for obtaining aluminum by the electrolysis of melts

Abstract

The invention concerns non-ferrous metallurgy, particularly an anode for electrolytically obtaining aluminum by the electrolysis of fluoride melts. The anode for obtaining aluminum by means of the electrolysis of melts at a temperature of less than 930 C. consists of a base executed of an alloy containing 65-96%.sub.wt of iron, less than 35%.sub.wt of copper, less than 20%.sub.wt of nickel, and one or several additives from molybdenum, manganese, titanium, tantalum, tungsten, vanadium, zirconium, niobium, chromium, aluminum (less than 1%.sub.wt) cobalt, cerium, yttrium, silicon, and carbon totaling less than 5%, and a protective oxide layer comprising iron oxides and complex oxides of iron, copper, and nickel. The protective oxide layer on the anode surface is obtained by preliminary oxidation in air at a temperature of 850-1050 C. or subsequently in the electrolysis process by oxidation with oxygen evolving at the anode.

Claims

1. An anode for obtaining aluminum by means of the electrolysis of melts at a temperature of less than 930 C., the anode comprising: a base comprised of an alloy containing iron, copper, nickel, and one or several additives selected from molybdenum, manganese, titanium, tantalum, tungsten, vanadium, zirconium, niobium, chromium, aluminum, cobalt, cerium, yttrium, silicon, and carbon, and a protective oxide layer, wherein the base comprises components in the following quantitative proportions, in % by weight: Fe65-96, Culess than 35, Niless than 20, and additives of an admixture of molybdenum, manganese, titanium, tantalum, tungsten, vanadium, zirconium, niobium, chromium, aluminum (less than 1), cobalt, cerium, yttrium, silicon, and carbon, totaling less than 5, and wherein the protective oxide layer comprises iron oxides and complex oxides of iron copper, and nickel.

2. The anode according to claim 1, wherein the base is made by casting in a metal or sand mold.

3. The anode according to claim 1, wherein the protective oxide layer on the anode surface is obtained by preliminary oxidation in air at a temperature of 850-1050 C. or subsequently in the electrolysis process by oxidation with oxygen evolving at the anode.

4. The anode according to claim 1, wherein the protective oxide layer has a thickness of 0.1-3.0 mm.

5. The anode according to claim 1, wherein the base does not comprise aluminum.

6. The anode according to claim 1, wherein the protective oxide layer comprises more than 50% by weight iron oxides.

7. The anode according to claim 1, wherein the protective oxide layer does not contain nickel oxide.

Description

(1) As an example, a thin-section is shown in FIG. 1 in cross-section from sample 80Fe20Ni (No. 4 in the table) after testing for 10 hr. In FIG. 1, it is shown that a thin porous layer (2) is formed on a metal base (1) after testing, at the surface, whereof a protective oxide layer (3) is found with a thickness of up to 1 mm. It must be noted that contamination of the aluminum with iron in all the anodes exceeds the limits specified in GOST 11069-2001s. However, it must be considered that in the initial period of anode operation, the rate of anode component solution in the melt is at a maximum. Since oxidation is the principal mechanism of anode corrosion for all the alloys, the anode corrosion rate decreases after the formation of a protective oxide layer of sufficient thickness and aluminum contamination is accordingly reduced. The anode based on the prototype (No. 8) has a high rate of corrosion, but contamination of the aluminum in iron therewith is sufficiently low. This is explained by the selective solution of aluminum from the alloy of the anode base, wherein aluminum fluoride forms and accumulates beneath the oxide layer, forming a fluoride interlayer, which contributes to the development of accelerated fluoride degradation of the alloy (FIG. 2). A porous layer is therefore completely lacking in the alloy. In FIG. 2, it is seen that a layer (4) of fluorides with a considerable thickness is found between a thin external oxide layer (3) and the metal base (1). Over a certain operating time for such an anode, a large part of the alloy will be destroyed and collapse of the degradation products accumulated beneath the oxide layer into the melt will occur; a rapid increase in aluminum contamination in iron will follow.

(2) TABLE-US-00001 TABLE Anode current Decrease Thickness Contamination of Serial density, in diameter, of porous aluminum, % wt nos. Anode composition, % wt A/cm.sup.2 m zone, m Fe Ni Cu 1 44Fe44Cu12Ni 0.5 870 0 0.67 0.06 0.28 2 65Fe25Cu10Ni 0.5 490 0 1.17 0.07 0 3 65Fe35Cu 0.5 5 813 0.5 0 0 4 80Fe20Ni 0.5 60 340 0.49 0.01 0 5 80Fe10Cu10Ni1(Mn + Si) 0.75 0 480 0.57 0 0 6 96Fe1.5Cu1.5Ni1(Mn + Si) 0.75 150 355 1.29 0 0 7 99Fe1(Mn + Si) 0.75 1600 145 3.21 0.02 0 8 90Fe10Al (prototype) 0.5 1390 0 0.75 0 0

(3) Thus, the experimental data confirm that the invention allows for a reduction in the corrosion rate of inert anodes made of iron-based alloys when obtaining aluminum by means of the electrolysis of alumina dissolved in fluoride melts at a temperature below 930 C. and, consequently, a reduction in contamination with iron of the aluminum being obtained. The task set for the invention is thereby resolved.