Process for the electrolytic production of pure aluminum
SUBSTANCE: The process involves the use of anodes containing biphasic metallic copper-based and iron-base alloys that also contain small amounts of nickel, consisting of an iron-rich reactive phase and a copper-rich solid inert phase containing 30 to 77 wt% copper, 23 to 65% by weight of iron and 15% by weight of nickel, wherein the content of the reactive phase in the Cu-Fe-Ni two-phase alloy is 24-83% and the inert phase is in the space between the dendrites of the reactive phase.
EFFECT: Possibility to achieve a significant reduction in the corrosion rate of anodes in fluoride melts containing aluminum oxide under conditions of anodic polarization and to obtain aluminum with a low metal content - components of the anode are guaranteed.
The invention relates to the field of non-ferrous metallurgy and can be used for the production of metals by electrolysis of molten electrolytes with inert anodes, in particular for the electrolytic production of aluminum in cryolite-alumina melts.
In the last few decades there has been intensive work on the manufacture of marashdeh anodes (“refractory” or “inert”) to replace the consumable carbon anodes in the electrolytic production of aluminum. The replacement is expected to reduce costs in the production of aluminum, the most compact construction of the technological system (electrolyzer) with lower heat losses, increase the ecological safety of production. The focus is on metal alloys, as these are technologically more sophisticated materials [1, 2] compared to ceramic and Karett materials. The first work in this direction concentrated on alloys with a high nickel content [3-5]. These materials should be used in melts that are traditionally used in the industrial production of aluminum by electrolysis (cryolite ratio KO = 2,2-3,0, T = 950-1000 ° C). The following is the cryolite ratio, KO = [NaF] / [AlF3], the ratio of the molar concentrations of sodium fluoride and aluminum fluoride in the melt (conventionally, such melts CA that have a high temperature). It was also shown that by lowering the temperature of the electrolyte (while reducing the KO's), a significant reduction in the corrosion rate of some metals (typical components of the alloys in the melt during anodic polarization [2]) can be achieved. At the same time, nickel-containing alloys show a significant deterioration in stability with a simultaneous reduction in the TO melt due to the preferential formation of nickel fluoride layers on the surface of the anode lohaprasadaya [6]. Therefore, the study of alloys of copper with a low nickel content began [2, 7-14]. Reduction of CO and the operating temperature leads to a shift in the balance between on the surface of the anode the solid products of oxidation and dissolved metal complexes in the melt, which is accompanied by the formation of some conditions lohaprasadaya layers on the surface of the anode and increase the rate of Corrosion. Thus, when the temperature of the electrolysis and the corresponding change in the composition of the electrolyte requires the definition of the composition of the metal alloys, the surface of which are formed by non-conductive phase during anodic polarization.
For the first time, copper / iron / nickel based alloys have been proposed as the material for Malaysiaboleh anodes in melts with a high content of aluminum fluoride (low QRS and t is melting temperatures) [7]. Since the optimum material was offered a high porosity (density of 60-70% of theoretical) of the anode alloy containing 25 to 70 wt% Cu, 15 to 60 wt% Ni and 1 to 30 wt% Fe , When the anode is made by powder metallurgy and used in the melt containing 42-48 mol% AlF3. Further work in this direction has been actively developed [8-14].
The prototype of the present invention is a patent [14] in which the best results have been obtained by the decomposition resistance of such metal alloys. In this patent it is proposed to use as a material for Malaysiaboleh anode alloys containing 10 to 70% by weight Cu, 15 to 60% by weight Ni and the balance of the iron. Also indicated in [14] is the interval of the composition: from 20 to 50 wt% Cu, from 20 to 40 wt% Ni and 20 to 40 wt% Fe. Since all of these alloys are biphasic, since their crystallization occurs from the iron-rich melt phase in the form of dendrites, between which the second copper-rich phase crystallizes to ensure the best possible resistance to decomposition in the proposed prototype, which subjects the casting to a special heat treatment to obtain metastable monophasic state. The electrolysis is to be proposed at a temperature of not more than 900 ° C in cryolite-glinozemnykh melts with the liquidus temperature of 715-860 ° C by passing a direct current between the cathodes and the anodes.
Studies on the degradation behavior of the alloys of the copper-iron-nickel composite in melts of different composition showed that the formulations proposed in [14] are not optimal: there is a significant amount of nickel, which in many cases forms a barrier layer of non-conductive Nickel fluoride and leads to the rapid destruction of the anode. In addition, the alloys are subjected to a special heat treatment to obtain a metastable single phase state which is less stable in electrochemical polarization compared to biphasic alloys of the same elemental composition.
A major disadvantage of the prototype is a significant corrosion rate of the anode material which prevents the use of such compositions in the industry due to the high level of contamination of the aluminum components of the anode. The concentration of nickel, copper and iron in the resulting cathode aluminum is controlled by GOST 11069-2001. It was expressly stated that the content of copper and nickel should not exceed 0,05 or 0,03% and iron 0,35% for the technical purity of aluminum.
The present invention aims to increase the corrosion resistance of the inert anode-based alloying system Cu-Fe-Ni as compared to alloys whose composition is proposed in patent [14].
The solution to this problem is provided by the process of electrolytically producing aluminum from aluminum-containing fluoride melt in the electrolytic cell at a temperature of less than 950 ° C by passing a constant current between the cathodes and the anodes according to the claimed invention using two-phase alloy anodes Cu-Fe-Ni, which consists of an enriched, in the form of dendrites occurring iron-reactive phase and is enriched in copper-solid phase and 30 to 77 wt .-% copper, 23 to 65 wt .-% iron and up to 15 wt .-% contains nickel.
The method can supplement these essential features.
The method can be used with anodes in which the iron content in the two-phase alloy Cu-Fe-Ni exceeds the nickel content by not less than twice.
The method can be used for anodes in which the content of the reactive phase in the two-phase alloy Cu-Fe-Ni is 24-83% and the inert phase is in the space between the dendrites of the reactive phase.
Therefore, the solution to this problem is achieved primarily by reducing the total nickel content in the alloy to values of not more than 15% by weight when the claims indicate the content of copper and iron. To reduce the risk of formation of oxides and fluorides with nickel-iron content in the alloy, the nickel content should be at least twice that.
It has also been demonstrated that the biphasic alloys have a higher stability in electrochemical polarization than the single-phase alloys of the same elemental composition. When a phase is rich in iron, as part of a biphasic alloy, the second phase dissolves and oxidizes much faster and is therefore called a reactive phase. Accordingly, the second phase, which is enriched with copper, is called the inert phase. The presence of a reactive phase and the continuity of the inert solid phase have a significant impact on the mechanism and corrosion rate of the anode.
Only in the presence of a reactive phase and continuity is the inert solid phase provided by uniform oxidation of the alloy and limited by its mechanical destruction by oxidation and dissolution of the reactive phase in the surface layer of the anode. The content of both phases in the system Cu-Fe-Ni at constant Ni content up to 15 wt .-% can be varied within wide limits.
The number of phases in the alloy is clearly linked to its elemental composition and can easily be determined using the corresponding ternary phase diagram. Optimal elemental composition of the anodes used: from 30 to 77% by weight Cu, up to 15% by weight Ni and from 23 to 65% by weight Fe - primarily identifies the correlation of the phases clearly. The content of the reactive phase in the two-phase alloy Cu-Fe-Ni can be 24-83%, and the inert phase is in the space between the dendrites of the reactive phase.
Thus the problem is solved, with a simultaneous optimization of the structure and the main parameters of the microstructure of the material of the anode - reactive phase and continuity inert solid phase.
Achieved by the use of the invention, the technical result is achieved by increasing the corrosion resistance of the anode, which is used in the electrolysis of aluminum oxide-containing fluoride melts at a temperature of less than 950 ° C, which ensures the reduction of contamination of the resulting aluminum components of the anode ,
For the experimental verification of the proposed materials, samples of anodes with different compositions (see table) and the results of their testing under the conditions of anodic polarization in cryolite-aluminum oxide melts of different composition were used. Samples of metal anodes Cu-Fe with the addition of Ni and without it were prepared by melting the source powders of pure metals in a resistance furnace in an inert atmosphere. The melt was held at a temperature of 10-30 ° C for 1600-1650 minutes to calculate the composition and then poured into the mold. Receive a cylindrical anode with a diameter of 8 to 15 mm and the height is from 30 to 150 mm were arc-welded to the electrical power supply. The electrolysis took place at an anode current density of about 0,3-0,7 A / cm2 in a graphite crucible with 400 grams of melt. The tests were carried out at temperatures of 760 and 920 ° C, melting at 1,3 and 1,86 and the alumina content of 2%. A melt was made from a mixture of reagents Na3AlF6, AlF3, Al2O3qualification not lower than “h”. Graphite was used as the cathode. During the electrolysis was periodic exposure to the molten alumina with an interval of 30 minutes, the test duration was not less than 2 hours. The immersion depth of the electrodes in the melt was usually 10-15 mm (active area of the anode approx. 3-4 cm2).
For a quantitative comparison of the corrosion rate of biphasic alloys which during electrolysis shows the formation of a long porous layer by selective oxidation and dissolution of the reactive phase, we have used the value of the integral corrosion rate which characterizes the percentage of power (%). which is used for the oxidation of the metallic base of the anode during the electrolysis. The integral corrosion rate was calculated on the basis of electron microscopic data obtained from the cross sections of the samples after laboratory investigations. The calculation was not only based on the geometric changes of RA is the size of the anode, but taking into account the volume of pores formed in the surface layer of the alloy. Thus, the integral indicator of corrosion rate of the anodes characterizes the average differential current corrosion for a given total current density during electrolysis. Since all experiments were performed under identical conditions, the calculated integral corrosion rate can be used to directly compare the observed corrosion rate of materials with different microstructures and the length of the porous layers.
The table shows that the prototype prototype anode (# 1) has a high corrosion rate. At the same time, the transition from the single-phase alloy to the two-phase alloy and the reduction of the nickel content in the alloy leads to a rapid decrease in the total oxidation rate of the material, which is less likely to produce nickel fluoride. However, a small amount of nickel in the alloy, which leads to the formation of the oxide layer of nickel ferrite, has a positive effect on the decomposition resistance of the material. Thus, the minimum corrosion rate shows an alloy with a nickel content of about 8 wt .-%. High stability also shows two-component alloys Cu-Fe, in which the content of the reactive phase near 50-60%. The best oxidation resistance is shown by Plav # 6 and # 11. For such materials, a minimum flow in the melt (and thus in the aluminum components) of the anode is achieved.
As a result of the laboratory investigations, the proposed material-optimized compositions and microstructures exhibit high stability in the aluminum oxide-containing fluoride melts under conditions of anodic polarization. Therefore, the anodes of these materials have a low corrosion rate and allow you to obtain the aluminum with a low content of alloy components.
1. A method for the electrolytic production of aluminum from aluminum-containing fluoride melt in the electrolytic cell at a temperature of less than 950 ° C by passing a direct current between the cathode and the anode, characterized in that the use of anodes of a two-phase alloy Cu-Fe-Ni, consisting of enriched iron-reactive phase and enriched with copper-inert solid phase containing 30 to 77 wt% copper, 23 to 65 wt% iron and up to 15 wt% nickel.
2. A method according to claim 1, characterized in that the use of anodes in which the iron content in the two-phase alloy Cu-Fe-Ni exceeds the nickel content by not less than twice.
3. Method according to claim 1, characterized in that the use of anodes consists in that the content of the reactive phase in the two-phase alloy Cu-Fe-Ni is 24-83%, since the inert phase is in the space between the reactive phase of the dendrites ,
Translation of the Russian patent by the Institute for Rare Earths and Metals. We apologize for the German language used in this article, ultimately it's about the content.
