RU2452797C2 - Method of metal production using ceramic anode - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: electrolysis is carried out using anode with modifying admixtures able to form solid solutions of substitution with trivalent cations A³⁺ and quinquevalent cations B⁵⁺ in rutile structure. Note here that total amount of said admixtures does not exceed 30 wt % while electrolysis is performed at temperature lower than 950°C. Said modifying admixtures to introduce trivalent cations A³⁺ contain compounds of Al, Mn, Cr, In, while for introduction a quinquevalent cations B⁵⁺ in they contain compounds of Sb, Nb, Ta. Anode comprises also metallic component not interacting with oxide material at temperature of synthesis and operation in amount of not over 40 wt %. Cu, Ni, noble metals Ag, Au, Pt, Pd and alloys thereof are used as said metallic component.

EFFECT: lower rate of anode corrosion and higher purity of metal.

3 cl, 1 tbl

Description

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

In recent decades, intensive work is underway to create low-consumable (“non-combustible”, or “inert”) anodes to replace consumable carbon anodes in the electrolytic production of aluminum. As a result of the replacement, it is expected that aluminum production costs will be reduced, the design of the technological apparatus (electrolyzer) will be more compact with less heat loss, and cleaner production. A lot of oxide materials and oxide-based materials have been described in connection with the possibility of making low-consumable anodes from them [1]. Tin dioxide has a rutile structure and has very low solubility in cryolite-alumina melts in the absence of reducing agents; therefore, it was one of the first to be proposed as inert anodes for producing aluminum by electrolysis [2, 3].

Pure tin dioxide has low electrical conductivity and poor sintering, which does not allow to obtain ceramic material with high electrical conductivity and low porosity. Therefore, it was proposed to introduce various modifying additives into the composition of ceramics, improving either the sintering ability or electrical conductivity of the material. Oxides of Fe, Sb, Cu, Mn, Nb, Zn, Cr, Co, W, Cd, Zr, Ta, In, Ni, were considered as such additives with a content of up to 20 wt.% (Usually 1-3 wt.%). Ca, Ba, V, Bi, Ti, Hf, Mg, Sr, Al, Ga, Si, Ge, As [3-6]. As a rule, the choice of the optimal ceramic composition was carried out according to two key features: high density / low porosity of the resulting ceramic (good sinterability) and high conductivity of the material. According to these characteristics, ceramics containing 1-3 wt.% CuO and Sb ₂ O _{3 are} considered optimal as additives that increase the sintering and conductivity of the material, respectively. Therefore, it was precisely such formulations that were studied in detail and proposed for introduction into production [1, 4, 6, 8, 9]. Ceramics of this composition are analogues of the materials of the present invention.

Since tin is practically insoluble in metal aluminum, and its segregation at grain boundaries during solidification of the metal leads to a sharp deterioration in the mechanical properties of the metal, the technological restrictions on the permissible level of tin content are very stringent. In primary aluminum grades, the maximum concentration of tin impurity is not specifically indicated in GOST 11069-2001, i.e. it is determined based on the restriction on the content of the “remaining elements” (not more than 0.02-0.03% for technical grade aluminum).

To reduce the corrosion rate of tin dioxide-based anodes, it was proposed to use an anode with a shielded three-phase boundary [6], as well as an anode with a protective poorly conductive coating that improves current distribution [9]. One of the promising ways to increase the corrosion resistance of tin dioxide-based inert anode materials is the synthesis of complex compounds with a rutile structure. In [10], it was proposed to use complex oxides with the general formula A _x ³⁺ B _x ⁵⁺ Sn _2-2x O ₄ (A = Cr, Fe, Al, B = Sb, Nb, V) as such compounds, which have an increased stability in cryolite-alumina melt. However, none of the proposed solutions allows to obtain aluminum with a tin content of less than 0.02-0.03% in melts and at temperatures traditionally used in the industrial production of aluminum by electrolysis (cryolite ratio KO = 2.2-3.0, t = 950-1000 ° FROM). Here, the cryolite ratio KO = [NaF] / [AlF ₃ ] is the ratio of molar concentrations of sodium fluoride and aluminum fluoride in the melt. Conventionally, such melts are called high-temperature.

In recent years, the possibility of a significant reduction in the electrolysis temperature through the use of medium- and low-temperature fluoride melts with a low KO and a melting point has been actively investigated, which ensures the electrolysis process at temperatures below 950 ° C [1, 11]. This should make it possible to significantly reduce the corrosion rate of the anode material as a result of a decrease in the solubility of tin dioxide with a decrease in the CO of the melt and the electrolysis temperature [12]. However, with a decrease in KO and working temperature of the melt, a sharp acceleration of corrosion of the anode was observed [12, 13].

A prototype of the present invention is a patent [3], which describes a method for the electrolytic production of aluminum from an alumina-containing fluoride melt, under conditions when the part of the anode in contact with the melt consists of an oxide conductive ceramic material chemically resistant to fluoride melt. The oxide conductive ceramic material according to the prototype contains at least 80% SnO ₂ , one or more oxides of Fe ₂ O ₃ , ZnO, Cr ₂ O ₃ , Sb ₂ O ₃ , Bi ₂ O ₃ and one or more oxides of Ta ₂ O ₅ , Nb ₂ O ₅ and WO ₃ . One suitable composition of ceramic materials is indicated to include 98% SnO ₂ , 1.5% Sb ₂ O ₃ , 0.3% Fe ₂ O _3, and 0.2% ZnO. For the manufacture of the anode, ceramic materials of these compositions are sintered at temperatures of 1000-1450 ° C.

The main disadvantage of the prototype is although low, but significantly exceeding the maximum permissible level of aluminum contamination with tin, which is due to the significant solubility of tin dioxide in high-temperature fluoride melts. On the other hand, an experimental verification showed that on the anodes whose compositions correspond to analogues and prototype, when the melt temperature decreases, there is a sharp acceleration of degradation processes (selective dissolution of modifying additives, melt impregnation, SnO ₂ recrystallization and reprecipitation, porosity increase), accompanied by a sharp increase voltage on the cell, the appearance of the limiting current and mechanical destruction of the ceramic material. The latter limits the possibility of reducing the corrosion rate of inert anodes based on tin dioxide.

The present invention is to reduce the corrosion rate of inert anodes based on tin dioxide and contamination of the resulting metal by the components of the anode.

The solution of this problem is achieved by the fact that a trivalent A ³⁺ cation (or several trivalent cations) capable of forming substitutional solid solutions in the structure of rutile is introduced into the composition of a ceramic material based on tin dioxide, a structural type of rutile. To stabilize the resulting substitutional solid solution and increase the solubility in the lattice of the A ³⁺ cation and the general electrical conductivity of the ceramic anode, a pentavalent B ⁵⁺ cation is simultaneously introduced into its composition, also capable of forming solid substitution solutions in the structure of rutile, for which compound A is added, respectively and compound B, the total amount of which does not exceed 30% wt. The process of electrolytic production of metals with anodes from such a ceramic material is carried out in alumina-containing medium and low temperature fluoride melts at electrolysis temperatures less than 950 ° C.

An embodiment of the invention is supplemented by particular features that contribute to the solution of the task.

For the purpose of introducing A ³⁺ cation into the composition of the ceramic material in the manufacture of anodes, tin dioxide is supplemented with modifying additives - oxides or other compounds A, for example, Fe, Al, Mn, Cr, In compounds, and with the aim of introducing B ⁵⁺ cation - compounds B for example compounds of Sb, Nb, Ta.

In order to increase the electrical conductivity of the ceramic material, a metal component can be added to the anode that does not interact with the oxide material at synthesis and operating temperatures in an amount of not more than 40 wt%. The metal components used are Cu, Ni, noble metals Ag, Au, Pt, Pd and their alloys.

The technical result achieved by using the invention is ensured by the increased resistance to degradation of these anodes during the electrolysis of alumina-containing medium and low temperature fluoride melts at a temperature of less than 950 ° C, which reduces the corrosion rate of the inert anode and contamination of the aluminum obtained by the anode components.

For the experimental verification of the claimed materials, anode samples of various compositions were prepared (see table) and their degradation stability was tested under conditions of anode polarization in cryolite-alumina melts of various compositions. Samples of ceramic materials based on tin dioxide of various compositions were prepared by solid-phase ceramic synthesis, including joint grinding of components in a planetary mill, pressing using a temporary technological binder (polyvinyl acetate) and sintering at 1280-1300 ° C for 10 hours. For conducting electrochemical degradation tests, ceramic anodes were made in the form of bars 15 × 15 × 100 mm. As cathodes, bars of 15 × 15 × 100 mm of a composite material based on titanium diboride or carbon bars coated with titanium diboride were used. The ceramic anode was fixed on a copper pin, which plays the role of a current supply, and the contact area was isolated from the action of molten vapor by corundum cement. The depth of immersion of the electrodes in the melt, as a rule, was 60-70 mm (the working area of the anode is about 40 cm ² ). The tests were carried out at an operating current of 20 A (anode current density of 0.5 A / cm ² ). The electrolysis was carried out in a graphite crucible containing 2.1 kg of melt saturated with alumina. The melt was prepared from a mixture of reagents Na ₃ AlF ₆ , AlF ₃ , Al ₂ O ₃ qualifications not lower than "h". During electrolysis, periodic loading of alumina into the melt was carried out with an interval of 5-30 minutes. The test duration was at least 10 hours. The tin content in aluminum obtained during electrolysis was determined using the atomic absorption method of analysis after the dissolution of the metal sample in hydrochloric acid. The electrical resistivity of ceramics at various temperatures was measured by the four-point method using a platinum sensor.

The table contains the test results in cryolite-alumina melts of various synthesized materials, both analogues (No. 1-3) and prototype (No. 4), and new materials (No. 5-12). All materials were tested under the same conditions and had slightly different porosity, which makes their comparison correct based on, in particular, the content of tin impurity in aluminum metal obtained during electrolysis.

From the data of the table it follows that the analogues and prototype of the proposed material undergo complete destruction with a decrease in the temperature of electrolysis, which is accompanied by a significant increase in aluminum contamination with tin.

The simultaneous introduction of tin and pentavalent cations A ³⁺ and B ⁵⁺ into the composition of tin dioxide can significantly reduce the level of contamination of the product by tin at a temperature of 920 ° C: from 0.26% (No. 2) to 0.089-0.17% (No. 5, No. 8, No. 9). However, the most significant effect is observed during electrolysis in a low-temperature melt (750 ° C), in which there is a very high stability of the proposed compositions of inert anodes, and the tin content in aluminum does not exceed 110-130 ppm (No. 6, No. 7). The introduction of trivalent cations in the composition of ceramics leads to a regular decrease in the conductivity of the material. With a small concentration of the corresponding additive (MnO ₂ in examples No. 5-7), the conductivity decreases by 2-4 times, which, however, allows the use of such materials in electrolysis. A further increase in the content of trivalent cations by increasing the concentration of additives (Fe ₂ About ₃ and Al ₂ About ₃ in examples No. 8-10) makes it impossible to conduct electrolysis at 750 ° C due to the high resistance of the ceramic anode. Uneven current distribution, significantly increasing with increasing resistance of the anode material, leads to an increase in the corrosion rate of the anode (cf. No. 5 and No. 8-10). A decrease in the resistance of such materials can be achieved by introducing a metal component into the composition of the anode (No. 11), while achieving both a decrease in the cell voltage during electrolysis and the level of aluminum contamination. Similar results can be achieved with the introduction of the metal phase in the composition of ceramics with a low content of trivalent cation (No. 12).

As the results of laboratory testing show, the proposed oxide materials are highly stable in alumina-containing medium and low temperature fluoride melts under conditions of anodic polarization. Therefore, the anodes of these materials have a low corrosion rate and make it possible to obtain aluminum with a low content of anode components.

Information sources

1. Galasiu, R. Galasiu, J. Thonstad, Inert Anodes for Aluminum Electrolysis, 1 st Edition, Aluminum-Verlag, Germany, 2007.

2. Belyaev A.I., Studentsov Y.V. Electrolysis of alumina with non-combustible anodes from oxides // Light Metals. 1937. No3. S.17-21.

3. H.-J. Klein, Process for the electrolytic production of aluminum, US Patent 3718550, 02.27.1973.

4. H. Alder, Process for the electrolysis of a molten charge using inconsumable bi-polar electrodes, US Patent 3930967, 6.01.1976.

5. H. Alder, Process for the electrolysis of a molten charge using inconsumable anodes, US Patent 3974046, 08/10/1976.

6. H. Alder, Inconsumable electrodes, US Patent 4057480, 8/8/1977.

7. H. Alder, Anode of dimensionally stable oxide-ceramic individual elements, US Patent 4357226, 11.2.1982.

8. D.R.Secrist, J. M. Clark, Corrosion-Resistant ceramic electrode for electrolytic processes, US Patent 4,484,997, 11/27/1984.

9. J. M. Clark, D. R. Secrist, Monolithic composite electrode for molten salt electrolysis, US Patent 4491510, January 1, 1985.

Claims (3)

1. The method of electrolytic production of metals from molten electrolytes containing oxides of these metals, comprising electrolysis using an anode of an oxide conductive ceramic material based on tin dioxide having a rutile type structure, with modifying additives to form substitutional solid solutions with trivalent metal cations A ³⁺ and pentavalent metal cations B ⁵⁺ in said structure, characterized in that the electrolysis is carried out using an anode in which the total amount of citing additives does not exceed 30 wt.%, at a temperature of less than 950 ° C. 2. The method according to claim 1, characterized in that they use an anode containing compounds of Fe, Al, Mn, Cr, In as modifying additives to obtain cations of trivalent metal A ³⁺ and compounds Sb, Nb, Ta as modifying additives for for the preparation of pentavalent cations B ⁵⁺ . 3. The method according to claim 1, characterized in that they use an anode that additionally contains a metal component that does not interact with tin dioxide at an electrolysis temperature selected from Cu, Ni, as well as noble metals Ag, Au, Pt, Pd and their alloys, in an amount of not more than 40 wt.%.