NL8105055A - METAL COMPOSITION FOR INERT ELECTRODE - Google Patents

Description

---------------- --- - - ...... - - - I «S 3399-107 -.

r, .. -., p & c

Metal composition for inert electrode.

The invention relates to the electrolytic preparation of metals such as aluminum, lead, magnesium, zinc, zircon, titanium, silicon, etc. and more particularly to an inert type electrode for use in the preparation of such metals.

For example, when aluminum is prepared by electrolysis of a solution of aluminum oxide in molten salt using carbon electrodes, carbon dioxide is formed at the anode due to the oxygen released in the decomposition of aluminum oxide. That is, the released oxygen reacts and consumes the carbon anode. Thus, per kg of aluminum produced, 0.33 kg of carbon must be used. Usually, such electrodes use carbon, such as that obtained from petroleum coke. Due to the rising cost of such coke, it has become necessary to find a new material for the electrodes. A desired new material will be a material which is not consumed and which will resist bath deterioration of molten material. In addition, the new material should be able to provide high current efficiency, not adversely affect the purity of the metal, and should also meet reasonable requirements in terms of raw material price and manufacturing.

Numerous attempts have been made to provide an inert electrode of the said type, but apparently without the required degree of success to make it economically feasible. That is, the inert electrodes from the literature appear to be reactive to such an extent that contamination of the metal produced occurs, in addition to consumption in the electrode. For example, U.S. Pat. No. 4,039,401 reports that intensive research has been conducted to find non-consumable electrodes for the electrolysis of alumina in molten salt and that oxides of spinel or oxides of perovskite structure exhibit excellent electron conductivity at a temperature of 900 ° -1000 ° C, exhibit a catalytic action to generate oxygen and have chemical resistance. Furthermore, U.S. Pat. No. 3,960,678 discloses a method of operating an alumina electrolysis cell with one or more anodes, the work surface of which is made of a ceramic oxide-35 material. However, according to this patent, this method requires that a current density above a minimum value must be maintained over the entire anode surface, which contacts the molten electrolyte, in order to minimize the corrosion of the anode.

Thus, it appears that there is a great need for an electrode which is 8105055 - 2 - / / t, - /. . · Practically inert or resistant to attack by molten salts or metal, avoiding contamination and all related problems.

The invention provides an electrode which is highly resistant to attack by materials in an electrolytic cell and which is relatively inexpensive to manufacture. <

According to the invention, there is provided an electrode material suitable for use in the preparation of metals, such as aluminum, lead, magnesium and zinc, etc., using electricity. The metal 10 is formed from. a metal compound, for example, its oxide or salt, which is provided in a molten salt. The electrode material is made from at least two metals or metal compounds, which are combined to provide a combination metal compound containing at least one oxide, fluoride, nitride, sulfide, carbide or boride, the combined metal compound being defined by the formula y. 5- {m] (p m "/ n v.

i = 1 J i-3. J 1 r-1 Ki in which p m n \ /% - l! J * "*, + - 1 and p / xr - 11 bright bright bright r = 1 Z is a number with a value of 1.0 - 2.2; K is a number with a value of 2.2 - 4 , 4; is at least one metal with a valence of 1, 2, 3, 4 or 5 and overall where li is used in the material always represents the same metal or metals; M. is a metal with a valence of 3 2, 3, 4 or 5; -X represents at least one of the elements O, F, N, S, C and B; m, p and n relate to a number of components, that M 1, M 2.

30 and X; F ', F', F 'or F are the mole fractions of Μ., M. and r M. _._ M. M. x ï η

i i 3 r J

X and 0 <\ f '„<1, except if M. represents Sn, Ti or Zr or if m = 1 r / M ï or if oxygen is and K = 3, in which cases 0 <\ f'm <1 .

35 ^ - 1

If the metal compound is a metal oxide formed from at least two metals, the material can be defined with the formula M (M'M) x,

x i — y z K.

where Y is a number with a value of less than -1 and more than O and MD is a metal with a valence of 1, 2, 3, 4 or 5 and M 'is a metal with a

n valence of 2, 3, 4 or 5, while Z is the number 2, 3 or 4, X at least 4U

8105055 t 't - 3 - represents one of the elements O, F, S, C or B and K is a number with a value of 2 - 4.4, which material is highly conductive and inert to molten salt.

Also provided is a metal compound as described herein, wherein at least one metal powder is dispersed through the combination metal compound to increase its conductivity, this metal powder Ni, Co, Fe, Cu, Pt, Rh, In, Ir and / or is an alloy thereof.

In the attached drawing is:

Figure 1 is a graph illustrating or illustrating the change in lattice parameter against the percentage of excess metal oxide relative to the stoichiometric amount;

Figure 2 is a schematic of an electrolytic cell showing the test of the inert electrode of the invention;

Figure 3 shows a micrograph showing an electrode material according to the invention;

Figure 4 is another micrograph showing powdered copper dispersed in the electrode composition of the invention.

An inert electrode suitable for use in the preparation of, for example, aluminum, must meet certain criteria.

For example, the electrode must have a high degree of conductivity. Furthermore, it must be resistant to attack by the bath. Furthermore, it must have a high resistance to oxidation. Other considerations include cost and ease of manufacture. This means that the costs must be such that the electrode must be economically suitable. All these areas are important. For example, if the electrode is not resistant to attack, the metal produced with it, for example aluminum, can be contaminated. When the conductivity is too low, the energy costs become too high. Thus it is seen that all these factors are of great importance to obtain a completely satisfactory electrode.

It has now been found that when the electrode is made from metal oxides, nitrides, borides, sulfides, carbides or halides or combinations thereof, it will only meet these requirements if the oxides or other materials are carefully selected and combined to obtain a combination with a specific composition. In other words, it has been found that without careful selection of the components and their combination in controlled amounts, the electrode has no satisfactory resistance to bath attack.

According to the invention, therefore, an electrode material is manufactured from at least two metal or metal compounds, which are combined such that a combined metal compound is obtained, which contains at least one oxide, fluoride, nitride, sulfide, carbide or boride where the combination metal compound is defined by the formula fr'V'v ^ '' 'λΙ / Σν-Λ LJ 7 lr = 1 K)

where "· p m n \ J

10 <= 1? pV + = 1 and 2 ^ rFxr = 1?,.,; fcl: 3¾ i * = l r = l.

• (*. * Z is a number with a value of 1.0 - 2.2; K is a number.with a value: from 2.0 - 4.4; at least one metal with a valence of 1, 2, 3 , 4 or 5 and wherever M. is used in the material, the same metal is 15a or mixture of metals; M_. Is a metal with a valence of 2, 3, 4 or 5; X is at least one of the elements 0 , F, N, S, C and B represents m, r

p and n refer to the number of components that M ^, M_; and can contain; F „, F *, F“ or F are the mole fractions of Μ, M. and X.

1.3 i r 20 Γ ”

and 0iS. sF * <1, except if M. Sn, Ti or Zr. or if m * = 1 or if X

/ Mi. 1 r is oxygen and K = 3, in which cases O <1.

When M. nickel or cobalt, M. iron and X represents oxygen, a representative compound is {Ni * _Co _) (Fe cNi OCo ·) 00. If M. also contains zircon in addition to the said metals, this is an example of a compound; (Ni .CoA _Zr) (Fe. ..Ni Co_ ") _0 .. If one replaces the zircon with V / 4 U; A Uf 4," U / ö O f éi \ J f £ »6 4 tin, is an example of a compound (Ni .Co. 0Sn) (Fe, .Ni., .Co., _h- 0.4 0.2 0.4 0.6 0.2 0.2 · ^., 0 ^. As Already mentioned, according to the invention, it is also possible to use other elements instead of or in addition to oxygen. For example, when M and M are respectively nickel and iron, it is possible, for example, to use fluorine in addition to oxygen to form a metal oxifluoride, for example Ni (FeQ gNiQ 4 ^ 2 ° 3F * It is noted that it is possible to use other metals and other elements to form metal oxysulfides, oxynitrides, oxycarbides and oxyborides, and the like, all of which are included in the invention. Following are a number of representative combination compounds of the invention, which compounds contain metals, at least two of which must be used in such combination compounds; *, · J l · 8105055 .............. ........

Ni (Fe ^ 6NiO4 / 4) 2O4; Ni (Fe0. *) O3F; NiLiF ^ at -4 V (Iitt0i8V0,2jo4; Nl05G ° Q, 95 ^ 2 ° 47 £ C ^ 9Fe0 /, lJ £ Fe2 * ° 47 • (Sn0., 8V0? 2 * Co2 ° 4; C ° (Co0 / ; 05Fe0.95 * 2 ° 47 £ C ° 0 * 9Pe0 „1 * Fe2 ° 47 £ Ni0 *, 5C ° 0.4Pe0. 1 * Fe2 ° 47 '. £ ,0.6 ^ b0 / 4 * (Fe0.6Nl0 .4? 2 ° 4 7 5 (Nl0;, 8Nb0, 2 * £ Fe0, 6C ° 0., 4 * 2 ° 47 (Nx0, 6Ta 0.4 * £ Fe0, 6G ° 0, 4 * 2 ° 4 7 (Nl0 -, 6C ° 0, 2Zr 0, 2 * (Fe0, 8CoQ, 2 * 2 ° 47 £ Nl θ '. 6Hf 0.4 * £ Fe0, 6Nl0 ,; 4 * 2¾7 £ l ^ X0,', 4C ° 0; 2Hf C. 4 * £ Fe0,6Co0,4 * 2 ° 4; (Ni0,4Co0,2Zr0,4,4} (Fe <V 6Coq,> 4 * 2 ° 4 7 10 (Ni0,6Co0,1Sn0 .3J £ F% JCoQ / v3 * 2 ° 47 (Ni0 16bX0 * 1Zr0 * 3 * (Fe0 ^ 7N; L0 ,, 3 * 2 ° 4 7 NlLi2F47.

(Ni0; 7Co0 / 3) L12F47 ^ 0.6 ^ 0 /, 4 ^ (Fe0, / 611¾.4} 2 ° 47 '- {Ge0., 6C ° 0 -4 ^ (Fe0, 6C ° 0.4 *) 2 ° 4? £ l ^, 9GU0,1 * £ Fe0.6Nl0 4 * 2 ° 47 £ Ni0,? 6Zr0 .2 ^ 0.2 * (Fe0, .7Nl0, 3} 2 ° 47 and '' '15 £ Co0 i6Zr0 £ '£ Pe0, .7Zn0, \ 3 * 2 ° 4 *

It should be noted that some of the compounds may be more inert to molten metal salts than others and are therefore preferred. Furthermore, it is clear that only those 20 metal-compound compounds, which have at least a reasonable degree of inertness to molten salts, are important in their use for inert electrodes. That is, compounds which clearly do not have a sufficient degree of inertness to molten salt are not intended to be included in the invention.

According to another aspect of the invention, it is possible to combine at least two metals or metal compounds, for example metal oxides, to provide a material which contains or is a combination metal oxide of the formula M (M 'M) O. That is to say, that after selecting the kor-

Y ± -y z K.

components, such as metals or metal oxides, which are combined in such amounts that a composition of this formula is obtained.

For the purposes of the invention, y must be a number with a value of less than one and greater than zero. It is an important aspect of the invention that one must strictly adhere to these limits. That is, it is important that y is less than one. It has been found that an obtained metal oxide composition, in which y = 1, resulted in an electrode material which, while having some resistance to attack by a molten bath, as used in the aluminum preparation, generally had an unacceptable level of resistance. Materials, which were composed and in which y = 1, were attacked by the bath, for example cryolite 40 with aluminum oxide dissolved therein, which obviously leads to an unacceptable ... 81 05 055 ____ V | - 6 -% contamination of the metal to be produced, so that it has to be purified again, while it is also necessary to replace the electrode frequently. For example, U.S. Pat. No. 3,960,678 discloses that anodes composed of Fe2 ^ 3 and SnC ^ ZnO conduct large amounts of contamination, for example, Sn 0.80%, Fe 1.27%, Ni 0.45%,

Fe 1.20%, Zn 2.01%, Fe 2.01%, so that such materials were considered unsuitable for anodes because of the problem of the impurities and also because the anodes were consumed. Thus it is seen that such or similar materials should be avoided. In the present formula, if y = O, that no suitable electrode material is obtained. Thus, according to a preferred aspect of the invention, the value of y should be controlled in the range of about 0.1-0.9, wherein a particularly suitable range is about 0.3 - 0.7, especially if the valence of M has a value of 1, 2, 4 or 5 and M 'is a metal with a valence of 3. When M 15 is composed from only two metals, it should also have two metals over the entire formula. It should be noted that M may consist of three or more metals, but in such cases M need not contain all of these metals over the entire composition.

The value of z should be in the range of 1.0-2.2. Furthermore, the value of K should be in the range 2 - 4.4, with a representative value in the range 3 - 4.1. This means that for the purposes of the invention, M and M 'are used in the electrode material in non-stoichiometric amounts according to the principles of the invention.

For the purposes of the invention in M a metal with a valence 1, 2, 3, 4 or 5 and M 'a metal with a valence 2, 3, 4 or 5. Normally according to the invention M and M' are different metals , of which some combinations are mentioned as examples below.

Although in the electrode defined by the formula M (M 'M,) O.

y 1-y z K

Although material has been reported primarily in oxides of such compounds, the oxygen component may be replaced in whole or in part by fluorine, nitrogen, sulfur, carbon or boron. Therefore, one can conveniently define the material of the formula "X", where X can be at least one of the components just mentioned, including oxygen.

Within the scope of the invention it is possible to derive the electrode material from both metals and metal oxides. That is, metals are eligible as a source of material leading to the composition of the invention. For example, M and M1 may be metals 8105055 »ί>> - 7 - which are suitable for forming an alloy and the proportions of which are such that when subjected to oxidation a layer is formed at least on the surface, which is for example a composition with the formula

M (M 'M,) contains or consists of 0. It is clear that further y 1-y z K.

Alloying elements can be provided in the alloy to modify the properties of the resulting oxide. Additional elements can be added to change the electrical conductivity or the resistance of the resulting oxide to attack by the bath, for example, of molten salt.

Figure 1 shows the effect that can be obtained when two metal oxides are combined into an electrode material according to the invention.

This means that in order to obtain the materials suitable for electrodes of the invention, when using two metal oxides, it is necessary to have one of these oxides present in an excess of the stoichiometric amount. When using two metal oxides such as ZnO and Fe + 0, the normal stoichiometric equation is as follows:

Fe2 ° 3 + Zn0 - ^ ZnFe2 ° 4 and the resulting compound is considered as stoichiometric equilibrium.

The compound formed according to such a comparison has a formula called a spinel which, while exhibiting some resistance to a bath of, for example, molten salt, does not exhibit sufficient inertness, as can be seen from US Pat. No. 3,960,678. Therefore, dissolving and corroding an electrode made of such material leads to contamination of the metal produced and the need to replace the electrode frequently, which is economically unsatisfactory, as has already been stated above. Due to the difficulties with stoichiometric spinels containing two metal oxides, it is seen that they are best avoided. According to the invention, it has been found that compositions of the formula M (M 'M') 0 'have a better inertness

y 1-y z K

for molten salts then possess such spinels. As already mentioned, in the case of metal oxides, a material according to the invention can be obtained by providing one of the oxides in excess, as can be seen from Figure 1. In the case of a system of Nio and Fe, it can contain 35 NiO or the be kept in excess. In a preferred embodiment, the components are mixed according to the general formula given to provide a mixture containing one of the components in excess up to the maximum solubility limit for the solid solution represented by points D or E in figure 1.

8105055

IV

- 8 -

Although the invention is not limited by theoretical considerations, it is believed that the effect of keeping one of the metal oxides in excess will cause the excess of the metal atoms to displace the other metal atoms in the lattice structure. If the metal atoms present in excess 5 are smaller than those other metal atoms, the result is a decrease in the distance between the atoms in the structure and thus a decrease in the lattice parameter, as shown by the line AE in Figure 1. It is clear In other systems, it may be the effect that the lattice parameter increases just by using an excess of one of the oxides. This effect is obtained if the size of the excess metal atom is larger than that of the other atom. An increase in grid spacing is represented by line A-D in. Figure 1. It should be noted that point A in Figure 1 indicates stoichiometric equilibrium mixture, eg spinel or perovskite type structures.

In addition to the above, it is believed that only a certain amount of substitution from one atom to another can take place to provide a composition of the invention. This limit is shown in Figure 1 at points D or E, depending on which metal or metal oxide is used in excess relative to the stoichiometric amount. The dotted line from D or E to B or C respectively indicates the change in lattice spacing when the substitution proceeded without interruption. If no further substitution takes place, there is practically no change in lattice spacing, as represented by the lines D-B 'and E-C'.

It can be seen from figure 1 that the lines A-D and A-E represent a composition according to the invention. It is noted that the lines D-B 'and E-C' represent a further material, for example a metal oxide, which may be present in the mixture. Thus, according to another aspect of the invention, a composition is provided with a first part or phase with the

formula M (M 'M,) 0' as defined above and a second part or phase y 1-y z K

of a material consisting essentially of metal oxide, for example as shown in Figure 3. Preferably, in this aspect of the invention, the components of the formula are mixed to obtain a composition in which one of the components is in excess is present relative to the maximum solubility limit for solid solution.

It is clear from the figure that such a limit is represented by the point D or E. In addition, Figure 3 shows a composition according to the formula, in which one of the components is provided in excess relative to 8105055 * f - 9 - of the maximum solubility limit for the solid solution. When using metal oxides to provide the electrode material and the amount of metal oxide used is an excess over that needed for substitution or an excess over the maximum solubility limit for solid solution, the combination can be represented with the formula M (M 'Mn) 0 + MO, where the different symbols in the

y 1-y z K

formula are defined above and MO represents the second phase. If the electrode material is made of two metal oxides, it is preferred that the second phase contain at least the metal oxide 10 in excess.

Figure 3 is a micrograph of an electrode material according to the invention at 400x magnification. An examination of figure 3 shows that different phases are present. A phase designated as a first phase has a composition according to the formula of the invention. That means that the first phase in the micrograph shown in the substantially gray areas has a composition defined by the formula M (M 'M') O '. The second phase, which can be seen as dark gray areas, represents the material that is present in excess of the amount with which substitution in the grid pressure is still possible. That is, the dark areas of the second phase are represented by the line D-B 'or E-C' of Figure 1. The darkest areas in the micrograph represent voids in the material.

The material shown in Figure 3 was composed of NiO and Fe 2 O 4, with 51.7 wt% NiO mixed with 48.3 wt% Fe 2 O 2 to give a material consisting essentially of Ni (Fe "Ni.) O, where O / J Z.4 was present in the NiO in an excess of about 20% by weight relative to the stoichiometric amount.

The said compositions are important embodiments of the invention. That is, said compositions are important, since if a second phase is present it must be carefully chosen so as not to adversely affect the properties of the material.

It is important that the first phase forms the major part of the composition and the second phase a smaller part. It can be seen from Figure 1 that the percentage of excess of the material, for example metal oxide, can determine the amount of the second part.

When the electrode mixture is composed of a first and second phase, as discussed above, it is important that the metal oxide provided to form the smallest part is carefully selected. It has been found that better results can be obtained if the second phase 40 has a grid structure compatible with the first phase.

8105055

v 'W

- 10 - * t

In the material having the above formulas, ML should be at least one of the metals Ni, Sn, Zr, Zn, Co, Mn, Ti, Nb, Ta, Li, Fe, and Hf. M can also be a metal from this list. If Ni contains a tetravalent metal, such as Sn, Ti or Zr, m ^ 3, at least one of the metals is Fe, V, Cr, Mn, Al, Nb, Ta, Zr, Sn, Zn, Co, Ni, Hf or Y and M 'may also be a metal of this list. Preferably, the material is composed of at least two metal oxides of these metals. A preferred material is composed of NiO and Fe 2 O. A representative

u J

tative material obtained by using NiO and Fe o, 10 Ni (Fey_0 ^ yNiy, _o 3) 2 ° 4 or gFej 4 ° 4 · In the system of NiO and Fe ^^, y can have a value of 0.2 - 0.95 and y 'a value of 0.05-0.80.

As other materials which can be formulated according to the invention are to be mentioned Co (Fe Co,.) O for which the starting components are y-ofd y su / 424

Co304 and Fe 2 O 3 are. In the system of Co2 O2 and Fe2 O3, Y may have a value of 0.4-0.95 and y '0.05-0.80. In addition, a three-component system can also be used, which depends to some extent on the properties desired for the final material. For example, according to the invention it is possible to combine Fe 2 O 3, NiO and C 30 J. Also Fe2C> 3,

Sn02 and Co ^^ combine into a valuable material. It is clear from the above that other combinations can also be carried out within the scope of the invention.

With regard to electrodes made of a material according to the invention, it should be noted that varying degrees of inertness may exist. That is, inertness can be defined in one respect by the metal to be produced. For example, even if the physical dimensions of an electrode are not significantly changed, it can still be considered to have no inertness if the metal produced contains an unreasonable amount of impurities. In the case of aluminum, the commercial quality should contain about 99.5% by weight of aluminum and the rest impurities. Thus, with respect to aluminum, an inert electrode is defined as an electrode capable of forming a product with a content of 99.5% by weight and the remainder impurities.

Known ceramic manufacturing methods can be used to manufacture electrodes according to the invention.

The electrode material of the invention is particularly suitable for use as an anode in a cell for the production of aluminum. According to a preferred aspect, the material is particularly suitable as an anode for a Hall cell in the production of aluminum. That is, when the anode 8105055-11 is used, it has been found to have a very high resistance to the bath used in a Hall cell. For example, the electrode material has been shown to resist attack by cryolite-type (Na AlF) electrolyte baths when operated at temperatures of 3-5 ° C around 970 ° C. Typically, such baths have a weight ratio between NaF and AlF 1 of about 1.1: 1 to 1.3; 1. It has also been found that the electrode has excellent resistance to cryolite type baths at a lower temperature, the NaF / AlF ratio being from 0.5: 1 to 1.1: 1. These baths can in representative cases be operated at temperatures of about 800 ° - 850 ° C. Although such a bath may consist exclusively of A 2 O 2 / NaF and AlF 2, it is possible to include in the bath at least one halide compound of the alkali and alkaline earth metal compounds (other than sodium) in an effective amount to lower the operating temperature . Suitable alkali and alkaline earth metal halides are LiF, CaF 2 and MgF 2. In one embodiment, the bath can contain LiF in an amount of 1-15%.

A cell of the type in which anodes having compositions according to the invention were tested is shown in Figure 2. Figure 2 shows an aluminum oxide crucible 10 in a protective crucible 20. In the crucible 10, the bath 30 is arranged and the bath contains a cathode 40. An anode 50 with an inert electrode is also located in the bath. The member 60 is shown for supplying alumina to the bath. The anode-cathode distance is shown at 70. The metal 80 produced in a test is shown at the cathode and at the bottom of the cell.

In some instances, it may be desirable to use a ceramic material of the invention as a coating. For example, when used for bipolar electrodes, the electrode of the invention may be an assembly, the cathode side being made of carbon or titanium diboride or the like and separated from the anode side (which is made of the ceramic material of the invention) by a stronger conductive metal, for example nickel, nickel-iron alloys, nickel-chromium alloys or stainless steels. When such an arrangement is used, it may be desirable to protect the ends of such a composite electrode with an inert non-conductive material, for example, silicon nitride, silicon oxynitride, boron nitride, silicon aluminum oxynitride, etc. It is clear that in the composite electrode interlayers can be used of other metals or materials, for example copper, cobalt, platinum, indium, molybdenum or 40 carbides, nitrides, borides and silicates.

8105055

"I

- 12 -

In electrolytic cells, such as Hall cells, coatings of the material of the invention can also be applied to highly conductive members, which can then be used as an anode. For example, a material of the formulas described above can be applied by spraying, for example, plasma spraying, to the conductive member to form a coating or coating thereon. This embodiment can have the advantage that the length of the resistance path between the highly conductive member and the molten salt serving as the electrolyte is reduced, so that the overall resistance of the cell is significantly reduced. As highly conductive organs which can be used in this embodiment are mentioned metals, such as stainless steels, nickel, iron-nickel alloys, copper, etc., whose resistance to attack by molten salt used as electrolyte is still to be regarded as insufficient. but whose conductive properties are considered to be highly desirable.

As other highly conductive members to which the material of the invention can be applied are generally mentioned sintered materials of heat-resistant hard metals, including carbon and graphite.

The thickness of the coating applied to the conductive member should be sufficient to protect this member from attack and yet be kept sufficiently thin to avoid an undesirably high resistance when conducting electric current. The conductivity of the coating -1 -1 should be at least 0.01 cm.

According to another embodiment of the invention, it has been found that the conductivity of the above-defined electrode material can be significantly increased by at least one of the metals Co, Fe, Ni, Cu, Pt, Eh, In, Ir or alloys thereof therein to provide or disperse. When the metal is provided in the electrode material, the amount thereof should not exceed 30 volume% metal, the remainder being the above material. According to the preferred embodiment, the amount of metal incorporated in the material can be about 0.1-25 volume% and suitable amounts range from 1 to about 20 volume%.

When the electrode material is composed of NiO and Fe 2 O 3, nickel is a particularly suitable metal to disperse in the material.

In the NiO and nickel system, nickel may be present in an amount of about 5-30% by weight, preferably 5-15% by weight. It has been found that the addition of nickel to it can increase the conductivity of the material by up to thirty times.

Metals that can be added to the electrode material should have favorable effects on conductivity and, on the other hand, should not adversely affect the material in terms of its resistance to molten salts or the bath. Metals with these properties are those which are normally not preferentially oxidized at the operating temperatures relative to the electrode material or ceramic material.

It should be noted that in order to optimize the conductivity of the metal contained in the electrode material, it is important to minimize the amount of oxide the formation of which is permitted on the metal during manufacture. That is, it has been found that the metal tends to oxidize during the assembly of the electrode material and the metal. This can interfere with the conductivity and is therefore best avoided. The tendency to oxidation has been observed, for example, in the Nio system and when nickel was added.

In order to combine the electrode material and metal, the electrode material obtained, for example, by combining N 10 and Fe 2 O 3, is comminuted to a particle size of 37-700 µm, while the metal, for example powdered nickel or copper, is pulverized , is provided in a particle size of 37 - 150 µm. It has been found that the powdered metal should be treated with a binder, for example, carbowax, before combining. This treatment should be such that the particles of the powdery nickel are mainly coated with a wax layer. Upon mixing, the ground electrode material adheres to the carbowax, thereby forming a layer on the metal particles, which are believed to prevent the metal particles from being oxidized during the manufacturing steps, such as sintering. In a representative case, the electrode material and the powdered metal or metal compound to be added are mixed together, pressed under a pressure of about 275,790 kPa and sintered at about 1300 ° C.

Although copper has been mentioned above as suitable for greatly increasing the conductivity of electrode materials, copper has been found to be extremely useful as a sintering aid in inert electrode materials such as that of the invention. That is, it has been found that copper greatly increases both the conductivity and the density of the electrode material of the invention. The use of powdered copper with a particle size of at most 1.68 mm and preferably at most 0.15 mm can considerably increase the density of an inert electrode material. For example, the density of the electrode material shown in Figure 3 was increased from 4.6 g / cm to 5.25 g / cm, a density increase of 14%.

In addition to the marked increase in density, it has been found that the use of powdered copper in inert electrode materials has the effect of eliminating virtually all voids therein. This means that the use of powdered copper in inert electrode materials means that such a material is practically void-free. The elimination of voids or the provision of a practically void-free inert electrode 5 is important, since this can provide the advantage that the electrode's ability to resist the highly corrosive environment in an electrolytic cell is greatly increased. This result is obtained by practically eliminating places or cavities to which the bath, for example electrolyte with metal oxide dissolved therein, can migrate. The degree of voiding of voids can be seen by comparing Figure 3 (already mentioned above) with Figure 4, showing copper as a separate white colored phase. The electrode material shown in figure 400 (magnification 400x) was made from the same materials and by practically the same method as in figure 3, except that powdered copper, which was passed through a 149 µm particle size screen, was added. The powdered powder was added in an amount of 5% by weight of the material shown in Figure 4. Powdered copper can even make up 30% by weight of an electrode material; however, the copper content should preferably be in the range of 0.5 - 20% by weight. It should be noted that 6 ,, Ο ^ 611 V2 ° 5 can also be used to increase the density of inert electrode materials in the same way as copper, but these materials are less preferred because none of these compounds significantly reduce conductivity. improve. Likewise, the above-described addition of nickel can be used, but this is also less preferred, since nickel does not appear to significantly promote compaction. Obviously, combinations of nickel, copper, Bi2 ° 3 and can be used to provide densified inert electrode materials with a high degree of conductivity, which materials are practically void free.

The examples below illustrate the invention in more detail.

EXAMPLE I

Fe2O4 having a particle size such that it was passed through a 149 Pa aperture sieve was first heated to remove moisture. Then 58 g of the dried Fe 2 O 4 was mixed with 62 g of NiO, which was also passed through a 149 µm mesh sieve. Mixing was carried out for about half an hour. After mixing, the combination of oxides was pressed in a mold at room temperature under a pressure of 17,237 kPa into a rod-shaped electrode with a density of about 4.0 g / cm 2.

8105055 - 15 - * t

The rod was sintered in air at a temperature of 1125 ° C for 16 hours. The sintered rod was then comminuted to such a particle size that the powder was passed through a 149 Pa orifice screen and pressed again under a pressure of 17,237 kPa and sintered at 1400 ° C. A rod-shaped electrode with a density of about 4.6 g / cm was obtained.

The electrode was examined as an anode in an electrolytic cell as shown in Figure 2. The cell contained a bath containing 90 wt% Na / AlF 2 in a ratio of 1.1: 1 5 wt% and 5 wt% CaF 2 and kept at 960 ° C. The distance between anode and cathode in the cell was 38.1 mm and a platinum wire was used to connect the anode to a voltage source · The voltage in the cell was approximately 5.V and the current density was 0.99 A / cm. The cell was run for 24 hours and aluminum collected on the carbon cathode. Upon analysis, the aluminum was found to contain 0.03 wt% Fe and 0.01 wt% Ni. At 950 ° C, the conductivity of the anode was about 0.4 (-f ^ -. Cm)

EXAMPLE II

In this example, the anode was prepared and tested as in Example 1, except that after the product of NiO / Fe 2 O 2 was first sintered and ground, the mixture (containing 51.7 wt% NiO) was and contained 48.3 wt.% Fe 2 O 3) 10% nickel powder having a particle size such that it was passed through a 149 µm aperture sieve. Before mixing with the NiO / Fe 2 O 3 mixture, however, the nickel powder was first treated with carbowax to obtain a coating thereof on the nickel particles; this wax was applied to ensure that a coating of the NiO / Fe 2 O 2 mixture would adhere to the mickle particles. The combination was pressed and sintered as in Example I, however, the sintering and conductivity measurements were performed in an argon atmosphere. The cell was allowed to run for 17 hours and the aluminum collected on the cathode was analyzed; this was found to contain 0.15 wt% Fe and 0.15 wt% Ni. At 950 ° C, the conductivity of the anode was about 4 (JX.cm) - *, which is an increase to about ten-fold over the electrode of Example I.

EXAMPLE III

In this example, the anode was prepared and treated as in Example I, except that the anode was 29.73 wt% NiO, 31.78 wt% Fe 2 O 3 and 38.49 wt% NiF 3 contained. After mixing, this mixture was calcined at 800 ° C, sieved, pressed at 17,237 kPa, sintered for 20 hours at 1100 ° C, crushed until the powder was passed through a 149 µm mesh sieve, pressed again at 17,237 kPa and sintered at 1300 ° C for 16 hours.

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The density of the sample was 5 µg / cm 2 and the electrical conductivity was 0.03-23 cm at 960 ° C. The electrode was tested as an anode in an electrolytic cell for 26 hours. Analysis of the Ni and 5 Fe present as impurities in the aluminum metal produced during the test showed that these impurities together constituted only 0.2% by weight of the product.

EXAMPLE IV

In this example, a calcined mixture of 51.7 wt% NiO and 48.3 wt% Fe2 ° 3 was applied by plasma spraying to a stainless steel substrate. 446, thereby obtaining an oxide coating with a thickness of 380 µm. The stainless steel substrate was cylindrical and had a hemispherical bottom portion to avoid sharp edges to facilitate coating. An anode connection was made by tapping thread in the stainless steel and screwing a threaded rod of Ni 200 into the substrate. The assembled anode was examined as in Example 1 and the duration of the test was 11 hours. The metal produced contained less than 0.03 wt% Ni and about 0.05 wt% Fe and the substrate was unaffected by the bath.

EXAMPLE V

In this example, the anode was prepared as in Example II, except that 10% by weight of copper powder was added to the mixture of 51.7% by weight NiO and 58.3% by weight Fe 2 ° 3. The combination was pressed and sintered as in Example II. The addition of copper to the mixture increased the conductivity. to about eightfold. The anode was examined and found to contain three phases, as shown in Figure 4.

This means that metallic copper was found to exist as a separate phase. The copper-containing material was examined for 23 hours and an examination showed that no significant corrosion had taken place, while the amount of copper in the aluminum produced was approximately 0.27% by weight. The same anode was left in one. fresh bath working for another 25 hours. The amount of copper in the aluminum produced was 0.18% by weight. Then, the same anode was operated a third time in a new bath for 12 hours and now the aluminum produced contained about 0.18 wt% Fe, 0.012 wt% Cu and 0.027 wt% Ni. This result shows that after some conditioning the corrosion or attack of the anode is very low.

Furthermore, the analysis shows that an anode of this composition has the ability to produce commercial grade aluminum (99.5 wt% Al).

Of course, numerous modifications 40 are possible within the scope of the invention.

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Claims (18)

Metal material suitable for use as an inert electrode in the electrolytic formation of metal from a metal compound dissolved in a molten salt, characterized in that the material is defined by the formula jy «vFv, \" · · J ^ 3 = 1 i-1 JZ {r-1 K j • - * »- 1 and ^ Xr \ = lj ··. : 'fel' pi * fel r = l “· *. ·%. Z is a number with a value of 1.0 - 2.2; K is a number with a value of 2.0 - 4.4; at least one metal having a valence of 1, 2, 3 , 4 'or 5 and wherever the formula is used represents the same metal or metals; M_. Is a metal with the valence' 2, 3, 4 or 5; X ^ .at least one of the elements 0, P, N , S, C or B; m, p and n are the 'number of items that form Μ., M. and X; F', F ', F' 20 1 3 r Mj Mi and P are the mole fractions of M. , M. and X represent and O F 'less than Xr i' 3 r H ± 1 IS. 2. Material according to claim 1, characterized in that M has a valence of 1, 2, 4 or 5 and II, a valence of 2 or 3. 3 Material according to claim 1 or 2, characterized in that it is oxygen. Material according to claim 1, 2 or 3, characterized in that! at least one of the metals is Ni, Sn, Zr, Zn, Co, Mn, Ti, Nb, Ta, Li, Fe, 1, 1, or Hf. J. Material according to claim 1, 3 or 4, characterized in that M_. ten- "! at least one of the metals is Fe, V, Cr, Al, Zn, Co, Ni, Hf or Y. ; Metal material according to one or more of the preceding claims, characterized in that the material is defined by the formula M (M '^ M ^ y) zXk' in which y is a number with a value less than one and. 55 greater than O and M is a metal with a valence of 1, 2, 3, 4 or 5 and M 'is a metal with a valence of 2, 3, 4 or 5, z represents the number 2, 3 or 4, X at least one of the elements 0, F, N, S, C or B and K is a number with a value of 2 - 4.4, which material is highly conductive and inert to molten salt. 7. The material according to claim 6, characterized in that K has a value of 1 y 8 1 Ö 5 0 t 0 0 5 0 0 - - ““ JC JC JC van van van van van - - - - - - van JC JC JC JC JC JC JC JC JC JC JC JC JC. 8. Material according to claim 6 or 7, characterized in that the material is composed of a first and a second phase, the first phase having the composition according to claim 6 or 7. Material according to claim 8, characterized in that the second phase has the composition MO. K 10. A material according to claim 8 or 9, characterized in that the second phase is formed from at least one of the metal oxides used to provide the composition of the first phase. Material according to claims 6-10, characterized in that M is one of the metals Ni, Sn, Zr, Zn, Co, Mn, Ti, Nb, Ta, Fe, Hf or Li. Material according to claims 6-11, characterized in that M 'is one of the metals Fe, V, Cr, Mn, Al, Nb, Ta, Sn, Zn, Co, Ni, Hf or Y. Metal material according to one or more of the preceding claims, characterized in that at least one metal powder is dispersed in the metal material, in order to increase its conductivity, said metal powder being Ni, Co, Fe, Cu, Pt, Rh, In or Ir or an alloy thereof. Metal material according to claim 13, characterized in that the metal powder constitutes an amount of 0.1 - 25 volume%. Metal material according to claim 13 or 14, characterized in that the dispersed metal powder has a particle size of at most 0.15 mm. 16. A method of electrolytically producing metal 25 from a metal compound, in particular a metal oxide or a metal salt dissolved in a molten bath, characterized in that: (a) an electrolytic cell is provided which contains molten salt to dissolve a metal compound therein and (b) provides at least one electrode therein, which is composed of a combination metal material according to any of the preceding claims. 17. Process according to claim 16, characterized in that aluminum metal is used as metal oxide and aluminum salt as metal salts and aluminum is produced therefrom as metal. Process according to claim 17, characterized in that the aluminum produced has a purity of at least 99.0% by weight. 8105055