RU2106431C1 - Charge for manufacturing inert anodes - Google Patents

Abstract

FIELD: nonferrous metallurgy. SUBSTANCE: invention relates to charge for manufacturing inert anodes designed for producing metals via electrolysis of melts, in particular, for electrolytic aluminum production in cryolite-silica melts. Charge contains 73-83% NiO•NiFe₂O₄, 10-20% CuO, 2-12% copper powder, and 1-2% polymer solution (calculated for carbon residue), CuO to carbon residue weight ratio being (8-12):1 and percentage of introduced and formed (as calculated) metal phase in material being 15-20%. EFFECT: improved process quality of charge. 1 tbl

Description

 The invention relates to the field of non-ferrous metallurgy and can be used for the manufacture of inert anodes for producing metals by electrolysis of melts, in particular for the electrolytic production of aluminum in cryolite-alumina melts.

 Recently, in developed countries, intensive work is underway to create inert anodes for replacing carbon materials in the electrolytic production of aluminum, since inert anodes have several advantages over carbon ones: the ability to work with short interelectrode distances; compact design with less heat loss; environmentally friendly production.

Known patent [1] for sintered electrodes with a matrix of yttrium oxide Y ₂ O ₃ and other electrically conductive compounds and electro-catalytic material on the working surface. The sintered body of Y ₂ O _{3 is} impregnated with a solution of a metal salt, dried, heated in an oxygen atmosphere to convert salts to the desired oxide.

 The disadvantages of this anode are: the inefficiency of using scarce and expensive yttrium oxide, as well as a noticeable dissolution of the anode in a cryolite-alumina melt, followed by aluminum contamination with yttrium.

More complex inert anodes are known for the analysis of aluminum with spinel, perovskite, pyrochlore, rutile oxide structures and their mixtures [2]. The anodes obtained by depositing a NiO • Ta ₂ O ₅ + NiO • NiB ₂ O ₅ layer of La ₂ Zr ₂ O ₇ on the sintered block had the best characteristics.

 The use of scarce and expensive materials, as well as the insufficiently high electrical conductivity of these anodes, leads to the conclusion that their use on an industrial scale for the production of aluminum is unacceptable.

The closest set of essential features to the claimed invention is the composition of an inert anode for the electrolytic production of aluminum in cryolite-alumina melts [3]. The anode material is cermet sintered from powders of the oxides NiO and NiFe ₂ O ₄ , to which powders of metallic nickel 0.1–10% and copper 10–30% are added. The powders are mixed, pressed into a plate in the shape of an anode and heated to form a monolithic sample. The electrical conductivity of the resulting material is approximately 100 Ohm ^-1 cm ^-1 .

 The disadvantage of this method is the low electrical conductivity and, as a consequence, the high cost of electricity during the electrolysis of aluminum, as well as insufficient corrosion resistance, which leads to the transition of the cathode components into the final product, i.e., to aluminum pollution.

The present invention is to increase the electrical conductivity of the inert anode, as well as improving its corrosion resistance in molten electrolyte. Currently, the conditions of aluminum electrolysis in an aggressive molten electrolyte based on fluoride salts at high temperatures of 900 - 1000 ^o C lead to the transition of the components of the inert anode to the electrolyte, and then to the final product - aluminum. Thus, the solution of the problem of the present invention is reduced to the search for such a composition and the ratio of the components of the mixture, in which inert anodes are obtained with a minimum corrosion rate, which leads to pure aluminum. At the same time, the task of obtaining a high-electrode material is being solved, which makes it possible to increase the current efficiency — a very important technical indicator in the electrolysis of molten salts.

The technical result achieved by using the invention arises from the fact that the mixture for the manufacture of inert anodes containing nickel, iron oxides and copper powder additionally contains copper oxide and an organic polymer solution in the following ratio of components, wt.%
NiO • NiFe ₂ O ₄ - 73 - 83
Copper Powder - 2 - 12
Copper (II) oxide - 10 - 20
The polymer solution is 1 - 2 (by carbon residue)
moreover, the ratio of CuO: C introduced into the charge of copper oxide and polymer solution (in terms of carbon residue) is 8 - 12: 1.

The claimed charge differs from the closest analogue in that it additionally contains copper oxide and an organic polymer solution with a ratio of components, wt.%:
NiO • NiFe ₂ O ₄ - 73 - 83
Copper Powder - 2 - 12
Copper (II) oxide - 10 - 20
The polymer solution is 1 - 2 (by carbon residue)
moreover, the ratio of copper oxides and polymer solution introduced into the charge (in terms of the carbon residue) of CuO: C is 8 - 12: 1.

When mixing the mixture in the mixer, the particles of solid components are coated with a layer of organic polymer. When the mixture is heated, thermolysis of the polymer occurs with the formation of carbon and volatile components. Volatile thermolysis products are removed, and finely dispersed carbon reduces copper oxide and partially nickel and iron oxides to elemental metals. The polymer solution simultaneously plays the role of a binder in the formation of the product. Evenly distributed in the body of the anode, metal particles of copper, nickel and iron at elevated temperatures (> 950 ^o C) form a continuous framework, which determines the high electrical conductivity of the whole product as a whole, which leads to an increase in current efficiency.

 The ratio of the components of the mixture is selected in such a way as to achieve minimum corrosion rates. The corrosion ability of the material is characterized by the density of the residual current I (corrosion current), which is determined from the polarization curve, and, with increasing density of the residual current I, the corrosion resistance decreases, and a large amount of impurities passes into the cathode aluminum. The ratio of copper oxide and the polymer solution in the mixture is proposed based on the total consumption of resin carbon for the reduction of CuO. With a CuO: carbon residue ratio greater than 12: 1, not all copper oxide is reduced to metal, and with ratios less than 8: 1, reduction in noticeable amounts of iron and nickel oxides begins during electrolysis.

 For experimental verification of the claimed composition of the mixture, several mixtures of ingredients were prepared (see table). A 50% solution of phenol-formaldehyde resin in acetone was used as a carbon-containing reducing agent and a binder. With a higher resin content, the solution is too viscous and does not provide high-quality wetting of the powders.

For the preparation of the mixture used copper powder grade PMS-A, oxides of copper, nickel and iron qualification "CHDA". The mixture was obtained by mixing the components in a porcelain mortar. Samples of 5 • 5 • 40 mm were pressed from the resulting mixture and sintered in an argon atmosphere; they were slowly (16 hours) heated to 1075 ^° C with a 2-hour exposure at this temperature. Then the samples for 2 hours were heated to 1300 ^o C, kept for eight hours and cooled with the oven. At lower temperatures and sintering times, the samples were not obtained with stable properties and had significant porosity. The homogeneous samples obtained were used to measure the electrical conductivity κ and the residual current density I (corrosion current) at the thermodynamic potential of oxygen evolution, which is a characteristic of the corrosion resistance of the material.

 The table shows the values of κ and I samples with different ratios of ingredients and known composition.

 The scatter in the electrical conductivity of samples of the same composition did not exceed, as a rule, 20%. The table shows the average values from 3 to 4 experiments.

 From the data of the table it follows that the mixture of the proposed compositions (3, 5, 11) provides inert anodes with a higher electrical conductivity, which accordingly provides energy savings in the electrolysis of aluminum from cryolite-alumina melts, increases the current efficiency and energy.

 The experiments showed that the amount of introduced and formed (calculated) metal phase in the material should be 15 - 20%. At less than 15% contents of the metal phase, the metal conductivity of the material is not ensured, and at higher than 20% metal contents, the density of the residual current increases significantly, which leads to contamination of the cathode aluminum by the components of the anode.

An analysis of impurities in cathode aluminum showed that with a corrosion current of 15 mA / cm ² and lower, a high technical result is achieved: the content of the main substance in cathode aluminum is 99.0% and higher. Primary aluminum with a basic substance content of 99.0% is used in industry without further purification.

Literature
1. V. De Nora, FMSpaziante, F. Nidola (Diamond Shamrock Texhnogies SA) USPat, 4.098.669 (1978) /
2. K. Billehaug, HADye. Inert anodes for fluminium electrolysis in Hall-heroult cells-Aluminum, 1981, N 3, pp 228 - 231.

3. Cermet anode compositipns with high contnt allou phase: Pat. USA 4.871.438, MKI ⁴ C 25 B 11/04. Marsohman Steveo, Davis Normanc. Batelle Mtmorial Instr. - N 116474, declared. 11/03/87, publ. 10/03/89, NKI 204/291 (RZ "Metallurgy", 1991, N 5, ref. 5G 263P).

Claims (1)

The mixture for the manufacture of inert anodes, including NiO • NiFe ₂ O ₄ , copper powder, characterized in that it additionally contains CuO and a solution of an organic polymer in the following ratio, wt.%:
NiO • NiFe ₂ O ₄ - 73 - 83
CuO - 10 - 20
Copper Powder - 2 - 12
The polymer solution is 1 - 2 (by carbon residue)
moreover, the ratio of CuO: carbon residue is 8-12: 1, and the amount of introduced and formed by calculation of the metal phase in the material is 15-20%.

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