CH651857A5 - METAL COMPOSITION FOR INERT ELECTRODE AND ELECTROLYTIC PROCESS USING THE SAME. - Google Patents

Description

This invention relates to the electrolytic production of metals such as aluminum, lead, magnesium, zinc, zirconium, titanium, silicon, etc., and more particularly relates to an inert type electrode used in the production of these metals.

When aluminum, for example, is produced by the electrolysis of alumina dissolved in a molten salt using carbon electrodes, carbon dioxide is produced at the anode as a result of the release of oxygen by decomposition of the alumina. That is, the released oxygen reacts and consumes the carbon anode. Thus, approximately 0.33 kg of carbon should be used for each kilogram of aluminum produced. Carbon like that obtained from petroleum coke is normally used for such electrodes. However, due to the increasing cost of these cokes, it has become necessary to find a new material for the electrodes. A new material indicated would be one which would not be consumed and which would be resistant to attack by the molten bath. In addition, the new material must be able to provide high current efficiency, must not affect the purity of the metal and must be reasonable with regard to the price of the raw material and with regard to its manufacturing price.

Many efforts have been made to provide an inert electrode of the type mentioned above but apparently without the necessary degree of success to be economically feasible. That is, the inert electrodes known in the art seem to be reactive to such a degree that it causes contamination of the metal produced as well as the consumption of the electrode. For example, US Patent No. 4,039,401 indicates that significant research has been done to find non-consumable electrodes for the electrolysis of aluminum oxide in a molten salt bath and that 35 spinel structure oxides or oxides of perovskite structure have excellent electronic conductivity at a temperature of 900 to 1000 ° C, have a catalytic action for the formation of oxygen and have chemical resistance. Also, in US Pat. No. 3,960,678, there is described a method of operating a cell for the electrolysis of aluminum oxide, with one or more anodes the working surface of which is made of a material which is a ceramic oxide. However, according to the patent, the process requires a current density greater than a minimum value to be maintained over the entire surface of the anode which comes into contact with the molten electrolyte so as to minimize corrosion of the anode. It can thus be seen that there is a great need for an electrode which is substantially inert or which is resistant to attack by molten salts or molten metal to avoid contamination and the problems associated therewith.

n / a The present invention provides an electrode which is very resistant to attack by materials in an electrolysis cell and which is relatively inexpensive to manufacture.

According to the objects of the invention, there is provided an electrode material which can be used in the production of a metal such as alumi-55 nium, lead, magnesium and zinc, etc., using 'electricity. Metal is produced from a metal compound such as its oxide or a salt, used in a molten salt. The material of the electrode is made from at least two metals or compounds of metals combined to give a metallic compound in combination with its at least one of the compounds chosen from the group comprising oxides, fluorides, nitrides, sulfides, carbides and borides, the metallic compound in combination being defined by the formula:

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1 = 1

Ver- r2-

VcMj) F, Mj y ^ \

j = i iron

X F r r = l

WHERE K = Xî / F, Mj + / F'Mi = 1

Z is a number between 1.0 and 2.2; K is a number between 2.2 and 4.4; M, is at least one metal having a valence of 1,2, 3, 4 or 5 and is the same metal or the same metals wherever M is used in the composition; Mj is a metal having a valence of 2, 3.4 or 5; Xr is at least one of the elements of the group comprising O, F, N, S, C and B; m, p and n denote the number of components which can constitute Mj, Mj and Xr; FM., F'Mj, F '", or FXf are the molar fractions of M„ Mj and Xt and 0 <S F'm. <1, except when M, is Sn, Ti or Zr or when m = 1, or when Xr is oxygen and K is 3, in which case 0 <S F'M. <1.

When the metal compound is a metal oxide comprising at least two metals, the composition can be defined by the formula M (M'yM, _y) 2XK where y is a number less than 1 and greater than 0 and M is a metal having a valence of 1, 2, 3, 4 or 5 and M 'is a metal having a valence of 2, 3,4 or 5, z is the number 2, 3 or 4, X is at least one of O, F , N, S, C or B, and K is a number between 2 and 4.4, the composition being very conductive and being inert with respect to the molten salt.

Also provided is a metal compound as described herein, where at least one metal powder is dispersed in the metal compound in combination for the purpose of increasing its conductivity, the metal powder comprising Ni, Co, Fe, Cu, Pt , Rh, In, Ir and / or their alloys.

In the accompanying drawings:

Fig. 1 is a graph illustrating the change in the lattice parameter as a function of the percentage of metal oxide in excess of the stoichiometric amount.

Fig. 2 is a schematic representation of an electrolysis cell representing the inert electrode of the invention under test.

Fig. 3 is a photomicrograph showing an electrode composition according to the invention.

Fig. 4 is another photomicrograph showing copper powder dispersed in the electrode composition according to the invention.

An inert electrode suitable for use in the production of aluminum, for example, must meet certain criteria. For example, the electrode must have a high conductivity level. In addition, it must be resistant to attack by the bath. In addition, it must have a high resistance to oxidation. Other considerations include price and ease of manufacture. That is, the price must be such that the electrode can be prepared economically. All of these areas are important. For example, if the electrode is not resistant to attack, the metal, for example the aluminum produced, will then be contaminated. Or, if the conductivity is too high, the price in terms of energy then becomes too high. It can therefore be seen that these factors are very important for obtaining a completely satisfactory electrode.

As a result, it has been discovered that when the electrode is manufactured from oxides, nitrides, borides, sulfides, carbides or halides of metals or their combinations, it will meet these requirements only if the oxides or the like materials are carefully chosen and combined so as to give a combination having a specific composition. It has thus been found that, without the careful choice of the components and their combinations in determined quantities, the electrode will not have the satisfactory resistance to attack by the bath.

Thus, according to the present invention, an electrode composition is prepared from at least two metals or compounds of metals combined so as to give a metal compound in combination containing at least one of the following compounds oxide, fluoride, nitride , sulfide, carbide or boride, the metallic compound in combination being defined by the formula:

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Z is a number between 1.0 and 2.2; K is a number between 2.0 and 4.4; Mi is at least one metal having a valence of 1,2, 3, 4 or 5 and is the same metal or the same metals whenever M, is used in the composition; Mj is a metal having a valence of 2, 3, 4 or 5; X, is at least one of the following: O, F, N, S, C and B; m, p and n denote the number of components which can constitute Mi, M, and Xr; FMj, F'Mj, F'M. where FXr are the molar fractions of Mh Mj and Xr and

0 ^ S F'm. <1, except when Mi is Sn, Ti or Zr or when m = 1, or when Xr is oxygen and K is 3, in which case 0 <E F'Mi <1.

When M, is nickel or cobalt, Mj is iron and Xr is oxygen, a typical compound would be (Ni0,5COo, 5) (Fe „.6Ni„ .2Coo.2) 204. If M, also contains zirconium in addition to the above, a standard compound would then be CNÌMCooaZrM) (FeMNi (UCooj) 204. Or else if tin were substituted for zirconium, a standard compound would be (Nio, 4Coo , 2Sno, 4) (Feo, 6Nio, 2Coo.2) 204 As indicated above, it is also within the scope of the invention to use elements in place of or in addition to oxygen, for example if M, and Mj are nickel and iron respectively, fluorine can then be added in addition to oxygen, for example, to give a metal oxyfluoride like Ni (Fe0,5Ni0,4) 2O3F. It will also be noted that d other metals can be used and other elements can be used to give oxysulfides, oxynitrides, oxycarbides and metal oxyborides, etc., which are all within the scope of the present invention. The following list is typical of compounds in combination according to the invention, the compounds being metals of which at least two of them must be used in such compounds in combination:

Ni (Fe0.6Nio.4) 204; Ni (Feo.6Nio, 4) 03F; NiLiF4;

V (Mn0.8V0.2) O4; Ni (Nioto5Coo, 9s) 204; (Coot9Feo, i) (Fe2) 04; (Sno, 8Vo, 2) Co204; Co (Coo, osFeo, 95) 204; (Coo, 9Feo, i) Fe204; (Ni0.5Co „.4Fe0, l) Fe2O4; (Nio, 6Nbo, 4) (Feo.6Ni „, 4) 204;

(Ni0.8Nb0.2) (Fe0i6Co0.4) 2O4; (NÌQ6Ta04) (FefJÛCoo4) 204î (Nio-6Coo, 2Zroj) (Fe0,8Cool) 204;

(NiQ.6Hfo, 4) (Feo, 6Ni0.4) 204;

(Ni0.4Coo ^ Hfo.4) (F e0.6Co0.4) 2O4;

(Ni0.4Co0.2Zr0.4XF e0.6Co0.4) 2O4;

(Nio, 6Coo, iSno, 3) (Feo, 7Coo, 3) 204;

(Ni0.6Li0, lZr „.3) (Fe„, 7Ni0.3) 2O4; NiLi2F4;

(Nio, 7Coo, 3) Li2F4; (GeoùNio4) (Fe06Nio, 4) 204;

(Ge, :: Coo,) (Feo, 6Coo, 4) 204; (Nio, 9Cuo, i) (Feo.6Nio.4) 204;

(Ni „, 6Zr„, Nbo.2) (Fe „, 7Ni„ .3) 204, and (Co0.6Zr0.4) (F e0,7Zn0-j) 2O4.

It will be noted that some of the compounds may have more inertia than others with respect to molten metal salts and are therefore preferred. In addition, it is understood that only the metal compounds in combination having at least a reasonable degree of inertia with respect to the molten salts are advantageous with regard to their use as inert electrodes. That is to say that the compounds which do not clearly have a sufficient level of inertia with respect to the molten salts are not considered to be part of the field of the invention.

In another aspect of the present invention, at least two metals or metal compounds, such as metal oxides, can be combined to provide or contain a metal oxide in combination having the formula M (M'yM, _y) 2Ok. That is to say that after the choice of components comprising metals or

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metal oxides, they are combined in proportions which will give a composition having this formula. For the purposes of the present invention, y must be a number less than one and greater than zero. It is an important aspect of this invention that these limits be strictly followed. That is, it is important that there be less than one. It has been discovered that the metal oxide composition obtained when y is equal to one gives an electrode composition which, although having a certain resistance to attack by a bath of molten salt such as that which is used to prepare the aluminum, generally has an unacceptable resistance level. The compositions which are obtained when y is equal to one are attacked by the bath, for example the cryolite in which the alumina is dissolved, which in turn gives an unacceptable level of contamination of the metal which is produced and the need to purify it, and this also makes it necessary to replace the electrode frequently. For example, US Patent No. 3,960,678 describes that the anodes formed of Fe203 and SnO2, or NiO, or ZnO, give high levels of impurities, for example 0.80% Sn, 1.27% Fe, 0 , 45% Ni, 1.20% Fe, 2.01% Zn, 2.01% Fe, and so these materials are considered unsuitable as anodes due to the problem of impurities and the fact that the anodes are consumed. It can therefore be seen that these or similar compositions should be avoided. In the formula in question, when y is equal to zero, it will also be seen that an appropriate electrode composition is not obtained. Thus, in a preferred aspect of the invention, the value of y should be controlled so that it is a number between about 0.1 and 0.9, an appropriate range being from about 0.3 to 0.7 , in particular when the valence of M is chosen from the group comprising 1, 2, 4 and 5 and when M 'is 3. If M is formed from only two metals, it must also contain two metals throughout the formula. It is understood that M can comprise three or more metals; however, in such cases, M does not have to include all of these metals in the formula.

The value of Z must be a number between 1.0 and 2.2. The value of K must also be a number between 2 and 4.4, a typical value being between 3 and 4.1. That is, for the purposes of the present invention, M and M 'are used in the electrode composition in non-stoichiometric amounts, according to the principles of the invention.

For the purposes of the present invention, M is a metal having a valence of 1, 2, 3, 4 or 5 and M 'is a metal having a valence of 2, 3, 4 or 5. Normally, in the present invention, M and M 'are different metals, combinations of which are described below by way of illustration.

Although in the electrode composition defined by the formula M (M'yM, _y) zOK, reference is made mainly to the oxides of such compounds, the oxygen component can be replaced or substituted or partially substituted by fluorine, nitrogen , sulfur, carbon or boron. Consequently, for reasons of convenience, the composition can be defined by the formula M (M'yMi_y) zXK where X can be at least one of the components, including oxygen, which has just been mentioned.

It is part of the field of the invention to obtain the electrode composition from metals as well as from metal oxides. That is, metals are contemplated as the source of material which will provide the composition of the present invention. For example, M and M'may be metals suitable for transformation into an alloy, the proportions of which, when subjected to oxidation, would at least give the surface a layer containing or comprising a composition defined by the formula M (M'yM, _y) 20K, for example. It is understood that additional alloying elements can be used in the alloy in order to modify the characteristics of the resulting oxide. Additional elements can be added in order to modify the electrical conductivity or the resistance of the resulting oxide to attack by the bath, for example molten salt.

Fig. 1 illustrates the effect that can be obtained each time two metal oxides are combined to give an electrode composition according to the present invention. That is to say that, in order to obtain the compositions suitable for the electrodes of the invention, it is necessary when one uses two metal oxides that one of the oxides is in excess of the stoichiometric amount. On the contrary, when using two metal oxides like ZnO and Fe203, the normal stoichiometric equation is as follows:

Fe203 + ZnO - »ZnFe204 and the resulting compound is considered to be stoichiometrically balanced. In such an equation, the compound formed has a formula which is designated as being a spinel and which, although having a certain resistance to attack by the bath, for example by molten salts, does not have a satisfactory inertia, as it can be seen from US Patent No. 3960678. Consequently, the dissolution and corrosion of an electrode made from such a material leads to contamination of the metal produced and frequent replacement of the electrode, which which is economically unsatisfactory as mentioned above. Because of the problems with stoichiometric spinels containing two metal oxides, we can see that it is better to avoid them. In the present invention, the compositions having the formula M (M'yM, _y) 20K have demonstrated a higher inertia with respect to the molten salts compared to the inertia of such spinels. As indicated above, a composition according to the invention can be obtained, in the case of metal oxides, by using one of the excess oxides as shown in FIG. 1. In the case of a NiO and Fe203 system, NiO or Fe203 can be kept in excess. In a preferred embodiment, the components are mixed according to the formula so as to obtain a composition in which one of the components is in excess up to the maximum solubility limit in solid solution, which is represented by the points D or E in fig. 1.

Although the inventor does not wish to be bound by any theory, it is believed that the effect of maintaining one of the excess metal oxides is that the excess metal atoms displace the other metal atoms of the structure. network. If the excess metal atoms are smaller than the other metal atoms, the result is a decrease in the distance between the atoms in the structure and therefore a decrease in the lattice parameter, as shown on the right A-E in fig. 1. It is understood that, in different systems, the effect may be to increase the lattice parameter by using an excess of one of the oxides. This effect will be obtained if the dimension of the excess metal atom is greater than that of the other atom. An increase in the network distance is illustrated by the line A-D in FIG. 1. It is understood that point A in fig. 1 shows where there are stoichiometrically balanced compositions, for example structures of the spinel or perovskite type.

In addition to the above, it is believed that only a certain amount of substitution from one atom for another can take place to give a composition according to the invention. This point is indicated in fig. 1 at points D or E, depending on whether the metal or metal oxide is used in excess of the stoichiometric amount. The dotted line from D or E to B or C indicates the change in the network distance, if the substitution continues without interruption. When another substitution does not take place, there is then practically no change in the network distance, as represented by the lines D-B 'or E -C'.

We can see from fig. 1 that the lines A-D or A-E represent a composition according to the invention. Note that the lines D-B 'or E-C represent an additional material, such as a metal oxide, which may be present in the composition. Thus, another aspect of the invention contemplates a composition having a first portion or phase of formula M (M'yM, _y) zOK as defined above and a second portion or phase being a material essentially consisting of a metal oxide, for example as shown in fig. 3. Preferably, in this aspect of the invention, the components are mixed according to the formula to give a composition of which one of the components is in excess of the solubility limit in maximum solid solution. With reference to fig. 1, we

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will see that this limit is represented by point D or E. In addition, FIG. 3 shows a composition according to the formula where one of the components has been used in excess of the maximum solid solubility limit. When metal oxides are used to form the electrode material and the amount of metal oxide used is in excess of that required for substitution or in excess of the maximum solid solubility limit, the composition can be represented by the formula M (M'yMi_y) zOK + MO, where the letters of the formula are as defined above and MO represents the second phase. When the electrode composition is made from two metal oxides, it is preferred that the second phase comprises at least the excess metal oxide.

Fig. 3 is a 400 x micrograph of an electrode composition according to the invention. From the examination of fig. 3, we will see that different phases are present. A phase called the first phase has a composition according to the formula of the invention. That is to say that on microphotography, the first phase, noted or represented as the zones which are essentially gray, has a composition defined by the formula M (M'yM, _y) 20K. The second phase, represented by dark gray areas, represents the material in excess of that where the substitution can be accepted by the network structure. That is to say that the dark areas of the second phase are represented by the line D-B 'or E-C in FIG. 1. The darkest areas of the micrograph represent gaps in the composition. The composition shown in fig. 3 was prepared from NiO and Fe203 where 51.7% of NiO by weight was mixed with 48.3% by weight of Fe203 to obtain a composition consisting essentially of Ni (Fe0.7Ni0.3) 2O4, NiO being present in an amount representing about 20% in excess of the stoichiometric amount.

The compositions mentioned are important embodiments of the invention. That is, the compositions mentioned are important in that, if a second phase is present, it must then be chosen carefully so as not to harm the properties of the composition. It is important that the first phase constitutes the main part of the composition and that the second phase constitutes a minor part. According to fig. 1, we can see that the excess, in percentage, of material, that is to say metallic,

can determine the amount of the second portion.

When the electrode composition is formed of a first and a second phase, as explained above, it is important that the metal oxide used to constitute the minor portion is chosen carefully. We have found that better results can be obtained when the second phase has a network structure compatible with the first phase.

With regard to compositions having the formulas mentioned above, Mi must be at least one of Ni, Sn, Zr, Zn, Co, Mn, Ti, Nb, Ta, Li, Fe or Hf. M can also be a metal on this list. When Mi includes N and a tetravalent metal like Sn, Ti or Zr, then m is> 3. Mj must be at least one of Fe, V, Cr, Mn, Al, Nb, Ta, Zr, Sn, Zn, Co, Ni, Hf or Y, and M'can also be a metal on this list. Preferably, the composition is prepared from at least two metal oxides of these metals. A preferred composition is prepared from NiO and Fe203. A typical composition using NiO and Fe203 is Ni (Fey_0,7Ni / = „, 3) 2O4 or Ni, .6Fe, .404. In the NiO and Fe 2 O 3 system, y can be between 0.2 and 0.95 and y 'between 0.05 and 0.80. Other compositions which can be prepared according to the present invention include Co (Fey = 0i6Coy = 0.4) 204 where the starting components are Co304 and Fe203. In the Co304 and Fe203 system, y can also be between 0.4 and 0.95 and y 'between 0.05 and 0.80. In addition to the above, a three component system can be used depending, to some degree, on the desired characteristics in the final composition. For example, Fe203, NiO and Co304 can be combined according to the invention. One can also combine Fe203, Sn02 and Co304 to obtain a usable composition. From the above, it is understood that other combinations can be prepared and they fall within the scope of the invention.

As regards the electrodes made from a composition according to the invention, it should be understood that there may be variable degrees of inertia. That is, the inertia in a respect can be defined with respect to the metal that is produced. For example, even if an electrode does not see its physical dimensions appreciably changed, it can still be considered to lack the appropriate inertia if the metal produced contains an unreasonable amount of impurities. In the case of aluminum, industrial grade contains approximately 99.5% aluminum, the remainder being impurities. Consequently, an inert electrode, as defined with respect to aluminum, is an electrode making it possible to produce aluminum at 99.5% by weight, the remainder being impurities.

The ceramic manufacturing methods well known to those skilled in the art can be used to manufacture electrodes according to the present invention.

The electrode composition of the present invention is particularly suitable for use as an anode in an aluminum production cell. In a preferred aspect, the composition is particularly useful as an anode in a Hall cell in the production of aluminum. That is, when the anode is used, it has been found to have very high resistance to the bath used in a Hall cell. For example, the electrode composition has been found to be resistant to attack by electrolyte baths of the cryolite type (Na3AlF6) when operating at temperatures of about 970 ° C. Typically, such baths have a ratio by weight of NaF to AIF3 of between approximately 1.1: 1 to 1.3: 1. It has also been found that the electrode has remarkable resistance to baths of the cryolite type of lower titer where the NaF / AlF3 ratio can be between 0.5 and up to 1.1: 1. These baths can typically operate at temperatures of about 800 to 850 ° C. Although such a bath may contain only A1203, NaF and A1F3, it is possible to provide in the bath at least one halide of alkali metals or alkaline earths other than sodium, in amounts to reduce the operating temperature. Such alkali and alkaline earth metal halides are LiF, CaF2 and MgF2. In one embodiment, the bath can contain LiF at a rate of 1 to 15%.

A cell of the type in which anodes having compositions according to the invention are tested is shown in FIG. 2. In fig. 2, an alumina crucible 10 is shown inside a protective crucible 20. The bath 30 is provided in the alumina crucible and a cathode 40 is provided in the bath. An anode 50 having an inert electrode is also shown in the bath. There is shown a means 60 for supplying the bath with alumina. The distance 70 between the anode and the cathode is shown. The metal 80 produced during a test is represented on the cathode and at the bottom of the cell.

In some cases, it may be appropriate to use a ceramic composition of the present invention in the form of a plating. That is to say, in a bipolar application for example, the electrode of the invention can be a composite with the cathode side made of carbon or titanium diboride, or a similar product, and separated from the anode side (which is made from a ceramic composition of the present invention) by a superior conductive metal such as nickel, nickel-iron alloys, nickel-chromium alloys or stainless steels. When such a device is used, it may then be advisable to protect the ends of this composite electrode with an inert non-conductive material such as silicon nitride, silicon oxynitride, boron nitride, silicon oxynitride and aluminum, etc. We will see that intermediate layers of other metals or materials such as copper, cobalt, platinum, indium, molybdenum, or carbides, nitrides, borides and silicates, can be used in the composite electrode .

In addition, in electrolytic cells, like Hall cells, veneers having the composition of the invention can be used on the highly conductive elements which can then be

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use as anode. For example, a composition as defined by the formulas mentioned above can be sprayed, for example sprayed under plasma, on the conductive element to provide a coating or plating thereon. This solution can have the advantage of reducing or reducing the length of the resistance path between the highly conducting element and the electrolyte constituted by the molten salt and thus significantly reducing the overall resistance of the cell. The highly conductive elements that can be used in this application can include metals such as stainless steels, nickel, iron-nickel alloys, copper, etc., whose resistance to attack by the electrolyte constituted by the molten salts could be considered as insufficient but whose conductive properties can be considered as highly sought after. Other highly conductive elements to which the composition of the invention can be applied generally include sintered compositions of refractory hard metals comprising carbon and graphite.

The thickness of the coating applied to the conductive element must be sufficient to protect it from attack and must however be kept thin enough to avoid undue high resistances when electrical current is passed through it. The conductivity of the coating must be at least 0.01 fì_1cm_ 1.

In another embodiment of the present invention, it has been discovered that the conductivity of the electrode composition as defined above can be significantly increased by providing or dispersing therein at least one of the metals Co, Fe, Ni, Cu, Pt, Rh, In, Ir or their alloys. When the metal is provided in the electrode composition, the quantity should not represent more than 30% by volume of metal, the rest being the composition. In a preferred embodiment, the metal provided in the composition can represent from about 0.1 to 25% by volume, suitable quantities being between 1 and about 20% by volume.

When the electrode composition is prepared from NiO and Fe 2 O 3, a metal quite suitable for dispersion in the composition is nickel. In the NiO and Fe 2 O 3 system, nickel can be present in an amount of approximately 5 to 30% by weight, a preferred amount being between 5 and 15% by weight. It has been found that adding nickel to it can increase the conductivity of the composition up to 30 times.

The metals which can be added to the electrode composition must have advantageous results with regard to conductivity and, however, must not modify the composition in a detrimental manner with respect to the resistance to molten salts in the bath. Metals having such characteristics are those which are not normally preferentially oxidized with respect to the electrode composition or ceramic at operating temperatures.

It should be noted that to optimize the conductivity of the metal provided in the electrode composition, it is important to minimize the amount of oxide which is allowed to form on the metal during manufacture. That is, it has been found that during the preparation of the metal composite and the electrode composition, the metal tends to oxidize. This can interfere with conductivity and is preferably avoided. The tendency to oxidation has been observed, for example in the NiO and Fe 2 O 3 system when nickel is added.

In order to combine the electrode composition and the metal, a suitable method consists in grinding the electrode composition, resulting for example from the combination of NiO and Fe 2 O 3, up to a particle size between approximately 37 and 720 microns and in using metal with a particle size between 37 and 147 microns, that is to say nickel or copper powder for example. It has been discovered that before the combination, the powdered metal must be treated with a binder like Carbowax. This treatment must be such that the powdered nickel particles are substantially coated with a layer of wax. By mixing, the composition for the ground electrode adheres to the wax, forming a layer around the metal particles which, it is believed, prevents the metal particles from oxidizing during the manufacturing steps such as sintering. Typically, the electrode composition and the metal compound or powdered metal to be added are mixed together, compressed to about 2760 10 10 Pa, and sintered to about 1300 ° C.

Although it has been stated previously that copper is particularly useful for significantly increasing the conductivity of electrode compositions, it has been found that copper has great utility in compositions for inert electrodes, such as those of the invention , as a sintering aid. That is, it has been found that copper significantly increases the conductivity and also increases the density of the electrode composition of the present invention. The use of powdered copper having a particle size which is not more than 1.65 mm and which preferably is not more than 147 microns can significantly increase the density of the composition for inert electrodes. For example, the density of the electrode composition shown in FIG. 3 has been increased from 4.6 g / cm3 to 5.25 g / cm3, i.e. an increase of 14%.

In addition to the substantial increase in density, it has been found that the use of powdered copper in inert electrode compositions has the effect of removing substantially all of the voids therefrom. That is, the use of powdered copper in inert electrode compositions results in compositions essentially free of voids. Eliminating voids or obtaining an inert electrode essentially free of voids is important in that it can have the benefit of significantly increasing the ability of the electrode to withstand highly corrosive environments in cells electrolysis. This is achieved by substantially eliminating the sites or voids through which the bath, i.e. the electrolyte in which the metal oxide is dissolved, can migrate. We can see the degree of void elimination by comparing fig. 3 (mentioned above) and fig. 4 where copper is represented in the form of a separate phase of white color. The electrode composition shown (at 400 x) in fig. 4 was prepared or manufactured from the same materials and with substantially the same procedures as those of FIG. 3, but powdered copper having a particle size less than 147 microns was added. The copper powder was added in an amount representing 5% by weight of the composition as shown in FIG. 4. Copper powder can represent up to 30% by weight of an electrode composition; however, preferably the copper content should be between 0.5 and 20% by weight. It will be noted that Bi203 and V205 can also be used to increase the density of inert electrode compositions in the same way as copper but in a less indicated manner since neither of these compounds increases the conductivity in a way significant. Likewise, the addition of nickel as indicated above can be used but less preferably since nickel does not seem to significantly improve densification. It is obviously understood that compositions of nickel, copper, Bi203 and V2Os can be used to obtain inert denser compositions for electrodes having high conductivity degrees and being essentially free of vacuum.

The following examples further illustrate the invention.

Example 1:

First heating Fe203 with a particle size less than 147 microns in order to remove moisture. Then 58 g of the dried Fe 2 O 3 are mixed with 62 g of NiO also having a particle size less than 147 microns. The mixing is carried out for approximately 1 h. After mixing, the combination of oxides is pressed in a mold at room temperature at a pressure of 1725-105 Pa to obtain a bar-shaped electrode having a density of about 4.0 g / cm3. The bar is sintered in air at a temperature of 1125 ° C for 16 h. Then the sintered bar is crushed or ground to a particle size less than 147 microns and it is again compressed to 1725 • 105 Pa and sintered at 1400 ° C.

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to obtain a bar-shaped electrode having a density of approximately 4.6 g / cm3.

The electrode is tested as an anode in an electrolysis cell as shown in fig. 2. The cell contains a bath comprising 90% by weight of NaF / AlF3 in a ratio of 1.1.5% by weight of A1203 and 5% by weight of CaF2, maintained at 960 ° C. The anode-cathode distance in the cell is 38 mm and a platinum wire is used to connect the anode to the source of electric current. The voltage in the cell is approximately 5 V and the current density is approximately 1.0 A / cm2. The cell is operated for 24 h and aluminum is collected on the carbon cathode. On analysis, aluminum contains 0.03% by weight of iron and 0.01% by weight of nickel. At 950 ° C, the conductivity of the anode is approximately 0.4 O-1 cm-1.

Example 2: 15

In this example, the anode is made and tested as in Example 1, except that after having sintered and ground for the first time the Ni0 / Fe203 mixture, the mixture which contains 51.7% by weight is added to the mixture. of NiO and 48.3% by weight of Fe203, 10% of nickel powder having a particle size less than 147 microns. However, before mixing with the Ni0 / Fe203 mixture, the nickel powder is first treated with Carbowax wax to obtain a coating of this wax on the nickel particles, the wax being used to ensure that a coating of the Ni0 / Fe203 mixture adheres to the nickel particles. The combination is compressed and sintered as in Example 1, except that the sintering and the conductivity measurements take place under an argon atmosphere. The cell is operated for 17 h and the aluminum collected on the cathode is analyzed; it is found to contain 0.15% by weight of Fe and 0.15% by weight of Ni. At 950 ° C, the conductivity of the anode is about 4 µ "'cm-1, which represents an increase of about ten times over the electrode of Example 1.

Example 3:

In this example, the anode is made and treated as in Example 1, but the anode contains 29.73% by weight of NiO, 31.78% by weight of Fe203 and 38.49% by weight of NiF2. This composition is mixed, it is calcined at 800 ° C., it is sieved, it is compressed at 1725-105 Pa, it is sintered at 1100 ° C for 20 h, it is ground to less than 147 microns, it is compressed to 1725 • 105 Pa and sintered at 40 1300 ° C for 16 h. The density of the sample is 5.3 g / cm3 and the electrical conductivity is 0.03 f2-Icm-1 at 960 ° C. The electrode is tested for 26 h as an anode in an electrolysis cell. By analyzing the impurities (Ni + Fe) in the metallic aluminum produced in this test, it is found that Ni and Fe combined represent only 0.2% by weight.

Example 4:

In this example, a calcined mixture of 51.7% by weight of NiO and 48.3% by weight of Fe 2 O 3 is sprayed under plasma on a 446 stainless steel substrate to obtain an oxide coating thickness of 380 microns. . The stainless steel substrate has a cylindrical shape and has a hemispherical lower part to avoid sharp edges in order to facilitate coating. An anode connection is made by tapping threads in stainless steel and screwing a threaded rod of Ni 200 into the substrate. The assembled anode is tested as in Example 1 and the duration of the test is 11 h. The metal produced contains less than 0.03% by weight of Ni and about 0.05% by weight of Fe and the substrate is not attacked by the bath.

Example 5:

In this example, the anode is made as in Example 2, but 10% by weight of copper powder is added to the mixture containing 51.7% by weight of NiO and 48.3% of Fe 2 O 3. The combination is pressed and filtered as in Example 2. The addition of copper in this composition increases its conductivity by about eight times. We examine the anode and find that it contains three phases, as shown in fig. 4. That is, metallic copper is found to exist as a separate phase. The copper-containing material is tested for 23 h and examination shows that no significant corrosion has occurred and that the copper in the aluminum produced corresponds to approximately 0.27% by weight. The same anode is tried again with a cool bath for an additional 25 h. The copper in the aluminum produced represents 0.18% by weight. The same anode is tested a third time in a new bath for 12 h and the aluminum produced contains approximately 0.18% by weight of Fe, 0.012% by weight of copper and 0.027% by weight of Ni. This result shows that after a certain conditioning, corrosion or attack by the anode is very low. In addition, the analysis shows that an anode of this composition makes it possible to produce aluminum of industrial quality (aluminum at 99.5% by weight).

Although the invention has been described by its preferred embodiments, it is understood that the appended claims are the only ones to delimit other embodiments which fall within the scope of the invention.

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2 sheets of drawings

Claims (10)

651,857 2 1. A metal composition suitable for use as an inert electrode in the electrolytic production of a metal from a compound of this metal dissolved in a molten salt, characterized in that the composition is defined by the formula: an aluminum salt and the metal produced is aluminum which preferably has a purity of at least 99.0% by weight. Z is a number between 1.0 and 2.2; K is a number between 2.0 and 4.4; M, is at least one metal having a valence of 1, 2, 3, 4 or 5 and is the same metal or the same metals whenever M, is used in the formula; Mj is a metal with a valence of 2, 3,4 or 5; X, is at least one of the elements O, F, N, S, C or B; m, p and n are the number of components which constitute M „Mj and Xr; Fm ,, F'm., F'm. where FXr are the molar fractions of M „Mj and Xr and 0 <Z F'M | <1. 2. Composition according to claim 1, characterized in that M, has a valence of 1, 2.4 or 5 and Mj at a valence of 2 or 3, and preferably X, is oxygen. 3. Composition according to claim 1, characterized in that M, is at least one of the metals Ni, Sn, Zr, Zn, Co, Mn, Ti, Nb, Ta, Li, Fe or Hf, and / or Mj is at least one of the metals Fe, V, Cr, Al, Zn, Co, Ni, Hf or Y. 4. Metal composition according to one of claims 1,2 or 3, characterized in that said composition is defined by the formula M (M ', M, _y) 2XK where y is a number less than 1 and greater than 0 and M is a metal having a valence of 1, 2, 3, 4 or 5 and M is a metal having a valence of 2, 3, 4 or 5, Z is the number 2, 3 or 4, X is at least one of O, F, N, S, C or B , K is a number between 2 and 4.4, preferably 3.9 and 4.4, the composition being very conductive and being inert with respect to the molten salt. 5. Composition according to claim 4, characterized in that said composition comprises a first and a second phase and each of said phases has the formula of claim 4, or in that the second phase has the formula MOK or preferably comprises at least one of the metal oxides according to claim 4. 6. Composition according to either of Claims 4 or 5, characterized in that M is Ni, Sn, Zr, Zn, Co, Mn, Ti, Nb, Ta, Fe, Hf or Li and / or M 'is Fe, V, Cr, Mn, Al, Nb, Ta, Sn, Zn, Co, Ni, Hf or Y. 7. Metal composition according to one of claims 1 to 6, characterized in that at least one metal powder is dispersed in the metal composition in order to increase its conductivity, the metal powder consisting of Ni, Co , Fe, Cu, Pt, Rh, In or Ir or their alloys. 8. A metal composition according to claim 7, characterized in that the metal powder represents from 0.1 to 25% by volume, and preferably the dispersed metal powder has a particle size which is not more than 147 microns. 9. A method of producing metal by electrolytic means from a metal compound which is a metal oxide or a salt of the metal dissolved in a molten bath, the method being characterized in that it comprises the steps which consist: a) using an electrolysis cell containing a molten salt to dissolve a metallic compound therein, and b) using at least one electrode in the cell, the electrode having the metallic composition according to one of claims 1 to 8. 10. Method according to claim 9, characterized in that the metal oxide is aluminum oxide, the metal salt is 30