US3330756A - Current conducting elements - Google Patents

Description

July l1, 1967 c. E. RANsLEY 3,330,756

CURRENT CONDUCT I NG ELEMENT S Original Filed Dec. 16, 1960 3 Sheets-Sheet l IN V EN TOR.

A TTORNEY Jly 11, 1967 c. E. RANsLEY 3,330,755

CURRENT CONDUCTING ELEMENTS Original Filed Deo. 16. 1960 5 Sheets-Sheet 2 IN V EN TUR.

4 TTEWE Y July 11, 1967 c. E. RANsLEY CURRENT CONDUCTING ELEMENTS .'5 Sheets-Sheet 5 Original Filed Deo.

mk m 0 T m N T E VM A N Ilm.

United States Patent O 3,330,756 CURRENT CONDUCTING ELEMENTS Charles Eric Ransley, Chesham Bois, England, assignor to British Aluminium Company Limited, London, England, a company of Great Britain Continuation of application Ser. No. 76,265, Dec. 16, 1960. This application Mar. 21, 1966, Ser. No. 535,902 Claims priority, application Great Britain, May 4, 1951, 10,548/51, 10,549/51; Ang. 3, 1951, 18,490/51; Apr. 5, 1952, 9,474/52; Jan. 14, 1954, 1,154/54, 1,155/54; Mar. 10, 1955, '7,135/55, 7,136/55, 7,137/55; Nov. 28, 1960, 40,853/60 2 Claims. (Cl. 2044-279) This application is a continuation of my application Ser. No. 76,265 tiled Dec. 16, 1960, and now abandoned.

This invention relates to improvements in solid current conducting elements -for use in the production of aluminum and methods of manufacturing such elements and is particularly concerned with such elements intended to extend into the interior of an electrolytic reduction cell for the production of aluminum or a three-layer refining cell, such an element constituting the cathode of a reduction cell or taking part in the supply of electrolyzing current to a body of molten metal either in a reduction cell or in a refining cell. Such elements, whether used as cathodes or current supply leads, all come into contact with molten aluminum at some part of their surface.

In the specifications of British Patents Nos. 784,695, 784,696, and 802,471 there are described various such current conducting elements which have the common feature that they are largely composed of at least one of the materials titanium carbide and zirconium carbide. In the specification of British Patent No. 802,905 it is stated that such a current conducting element may also be made having at least part of its surface intended to be exposed to the interior of the cell largely composed of at least one of .the materials in the group consisting of the borides of titanium, zirconium, tantalum and niobium and particular reference is made to the excellent properties of titanium diboride TiB2. In particular, it has a much lower electrical resistivity than titanium carbide, is more resistant to oxidation in the temperature range 300 to 800 C., and has a lower solubility in molten aluminum at the temperatures found in aluminum producing electrolytic cells7 i.e., 950 to 1000 C. This last property is extremely important in the commercial exploitation of such current conducting elements since the life of the elements is ultimately determined by the rate at which the material thereof dissolves in the aluminum in the electrolytic cell.

Although the borides of the elements referred to are significantly superior to the carbides thereof for the purposes in View they were, until hitherto, more expensive to produce than the carbides. In the specication of British Patent No. 826,635 l have described and claimed a curent conducting element rfor use in an electrolytic cell for the production of aluminum, which element is intended to come into contact with molten aluminum during its use, characterized in that at least that portion of the element which is to contact the aluminum is largely composed of a mixture of titanium carbide and titanium boride. Such a mixture has various advantages which could not be anticipated from a study of .their properties detedmined separately and special reference is made to the surprising reduction in the solubility of titanium carbide in aluminum at high temperatures (e.g., at 970 C.) which can be achieved by small percentage additions of titanium horide, additions of to 25% by weight of titanium boride being particularly mentioned.

It was found that the oxygen content of the carbides 3,330,755 Patented July 11, 1967 ICC of titanium, zirconium, tantalum, and niobium is an important factor in the solubility of the compounds in molten aluminum. In my co-pending application Ser. No. 764,725 filed Oct. 1, 1958 now Patent No. 3,215,615 I have described and claimed a solid current conducting element for use in an electrolytic cell for the production or purification of aluminum and adapted to have at least a part thereof exposed to a body of molten aluminum or aluminum alloy within the cell wherein at least said part of said element consists essentially of at least one of the materials in the group consisting of the carbides of titanium, zirconium, tantalum and niobium and the oxygen content of said materials is less than 1% by weight.

The expressions largely composed of and consisting essentially of as used hereinafter in the specification and the claims, mean that the current conducting element, or at least the portion thereof which is adapted to be in contact with molten aluminum, made of at least one of the carbides and/or borides referred to above does not contain other substances in amounts sucient to materially affect the desirable characteristics of the current conducting element, although other substances may be present in minor amounts which do not materially affect such desirable characteristics, for example, small proportions of carbon, nitrogen, or iron. In order for the element to consist essentially or be largely composed of at least one of the carbides and/ or borides as described above the oxygen impurity content should be limited as hereafter prescribed. Further, these expressions are used to denote that the current conducting element, or at least the portion thereof adapted to contact molen aluminum during use, desirably, but not essentially contains at least by weight of at least one of the carbides and/or borides referred to.

Although the borides of the chemical elements refered to are markedly superior to .the carbides thereof for the purposes in view, my experiments on the manufacture of solid current conducting elements largely composed of titanium diboride and titanium diboride-titanium carbide mixtures, which are technically of great importance, have shown that unexpected diiliculties can occur in the control of the quality of such elements. Thus in my experimental work, a proportion of the elements were found to be quite unsuitable for use in reduction or rening cells because they failed prematurely under cell operating conditions, this proportion being suihciently high to reliect adversely on the possibility of manufactun ing such elements commercially. This failure was associated with a clearly defined tendency to crack and disintegrate within the cell. Micrographic examination of elements or bars after service showed that in poor materials exhibiting this deficiency, a characteristic intergranular penetration of aluminum had occurred which eventually led to complete disruption of the structure. This eiiect was absent in materials which behaved in a satisfactory manner, even when such materials were sufliciently porous to allow some aluminum to infiltrate into the interior in use, because even under these conditions the aluminum remained localized in the pores, .and did not spread between the grains. These effects could be produced in molten pure aluminum as Well as under electrolytic conditions. This quality factor could in fact be characterized in terms of the so-called dihedral angle (as dened, for example, in the paper by C. S. Smith in the Transactions of the American Institute of Mining and Metallurgy 1948, volume 175, page l5) made by the aluminum phase at its yconjunction with a grain 'boundary in the structure. When this angle was about 60 or more, no penetration between the grains occurred and the material was found suitable for cell applications; when, on the other hand, this angle was less than this (eg. 20-30) disintegration of the structure inevitably occurred and the material was quite unsuitable for cell use.

In order to indicate how good and poor materials may be. distinguished by appropriate chemical analysis and to give an understanding of the complex constitution of these materials, it is desirable Vto indicate the processes by which they may be manufactured.

Continually improving techniques have materially reduced the cost of preparing titanium diboride, TiB2, and various processes are available for its preparation. The most important ones are considered to be the carbothermic process, electrolytic separation and direct reaction between the chemical elements. The carbotherrnic process may involve any one of several reactions, one of which involves reacting TiO2 (anatase or rutile), B203 and carbon according to the following equation:

An alternative reaction is one in which the boron is supplied in the form of boron carbide (nominally B4G):

Elemental boron may also be used in accordance with the equation:

TiO2+2B|2C TiB+2CO (3) Boron, however, is diicult to produce in a pure enough form for the purposes in view, and this is not at present regarded as an economic route.

An alternative reaction which may be employed is ments:

Tia-2B m32 (5) In this case again, however, the main objection is the difficulty of Ipreparing elemental boron of the necessary purity at an economical price.

Various electrolytic processes have also been proposed for the preparation of the borides. These do not normally offer any technical or economic advantage over the carbothermic or direct reaction processes; however, even for this product, the considerations to be detailed below are still applicable.

It is extremely stoichiometric balance in diicult in practice to achieve an exact a carbothermic, or any other reaction for the preparation of TiB2, as is implicit, for example, in Equation 2 above. There are technical diiculties, for example, in providing exactly the correct amounts of boron and carbon; this is because somewhat variable losses can occur during the reaction, accidental contamination may take place, and the atmosphere in which the process is carried out may also have an effect on the composition of the nal product. A further possibility is that the reaction may not have proceeded as far to completion as the make-up of the reaction mass would allow under appropriate conditions.

The product obtained from such reactions will thus contain, to a greater or lesser degree, components other that the simple compound TiBZ. Thus, if the reaction mixture contains an inadequate supply of boron or boron compounds to combine with all the titanium present, and carbon is present over and above that required for the elimination of oxygen, the product will contain an appreciable amount of titanium carbide, TiC. In fact, it is probably not inaccurate to regard TiC as a primary product of the carbothermic preparation; it tends to be unstable in the presence of boron, however, in conformity with the reaction:

Tic+2n ri2+c (6) Similarly, if the mixture is decient in both boron and carbon, the product will be contaminated with oxygen in some form or other. The yother possible variations in composition which can occur do not need detailed description, but it will be evident that the final product may possibly contain, for example, exces lboron carbide and free (or unccmbined) carbon. In addition, a number of contaminating elements may be present which are derived from the raw materials or from the atmospheres used; these may include, for example, relatively small quantities of nitrogen, iron, calcium, silicon, and aluminum.

rhe final product of such carbothermic or other processes is normally in the form of a relatively ne powder, which has been subjected to milling or other operations to render it suitable for further fabrication.

The solid conducting elements referred to are produced by either cold pressing followed by sintering, or by hot-pressing this powder, an appropriate protective atmosphere being used in both these operations. For example, it may be hot-pressed in a graphite die, in a protective atmosphere of hydrogen, and at temperatures of the order of 1800-2100" C. It will be understood that the chemical composition of the final element thus obtained will be determined not only by the exact composition of the powder used, but also by the conditions under which the element is manufactured from it. Precise control of the final composition of the element is essential, however, if the latter is to have a useful economic life in an electrolytic cell.

We have found, unexpectedly, that the elements exhibiting poor behavior do so because of the presence therein of a relatively small percentage of oxygen combined in a certain manner, and it has further been found that the permissible content of oxygen of this type is related to the so-called soluble, or combined, carbon content of the material. The discovery of this inter-relation between the oxygen and carbon contents of such elements has enabled us for the first time to control their composition in such a way that no cracking or disintegration occurs in service.

According to the present invention, a solid conducting element for use in the electrolytic production or purification of aluminum having at least a portion consisting essentially of titanium diboride or a mixture of titanium diboride and titanium carbide and has an oxygen impurity content, defined as that oxygen present in the element in chemical combination with titanium which is less than 0.1% by weight when the combined (or acid-soluble) carbon content is less than 0.4% by weight, and is less than (0.1+0.04 n)% by Weight when the combined carbon content is 11% by weight, and the value of n is greater than 0.4.

Preferably the element contains titanium carbide in the proportion of not more than 40% by weight and desirably not less than 2% by weight.

From the analytical point of view, the proportion of acid-soluble carbon present defines the combined carbon content, which is normally ascribable to the formula TiC. In analysis, this figure is usually derived by earring out a determination of the total carbon content of the material, and a separate determination of the acidinsoluble or ltrable carbon residue remaining after the mass of the sample is dissolved in an appropriate acid or acid mixture; this latter gure thus includes graphitic and elemental carbon, and also carbon in the residual boron carbide. The acid-soluble carbon is then obtained from the difference between these two figures, and this is the value of carbon content which is specified above to which the tolerable oxygen content is related.

The oxygen content of the nal element may conveniently be determined by the well-known vacuum fusion method, in which a sample is reduced in a high temperature bath of molten iron or platinum contained in a graphite crucible, and the consequent evolution of carbon oxide measured. The sample may, for example, comprise a relatively massive chip or cut section of the element. If the element is powdered in order to obtain a representative sample and for convenience in general analysis, spurious oxygen will be introduced in the form of titanium oxide and boric oxide (B203) on the surface of the particles,` and also as absorbed water; appropriate correction must therefore be made in order to obtain the oxygen content which is significant in the present context. Methods are available for this; thus the B203 content can be determined by aqueous extraction of the powder, and the water content can be deduced from the hydrogen evolved in the vacuum-fusion measurement.

It is clearly essential to be able to assess the quality of element which will be obtained by hot-pressing or otherwise consolidating a given batch of powder into the said element. Since it is desired that the oxygen content, as dened above, of the final product shall be as low as possible, a minimum requirement is that the proportion of reducing agents present shall be adequate to eliminate this oxygen from the powder.

Both carbon and boron are capable of functioning as reducing agents. For reasons which are explained in more detail below, it has been found preferable to adjust the composition of the charge in the carbothermic reaction so that the powder for pressing and also the nal element contains an appreciable percentage, e.g., 0.2% or more, of acid-soluble carbon, or about 1% or more of TiC. Oxygen will be evolved from a powder of this type during hot-pressing or sintering by reaction with the free, or other available, carbon with a loss of carbon monoxide from the system. Oxygen present as titanium oxide (e.g., TiO) is able to react as follows:

and the carbon required render these conditions such that the weight ratio O/C is not greater than YZF- 0.66. However, if boron is available in the powder either as unreacted boron or boron carbide, or even as B202, further carbon is made available by reaction of this boron with TiC to form TiB2 and releasing carbon as expressed in Equation 6. Under these conditions the basic carbon requirement is then given by the carbon available from reaction (6) in percent by weight is 0.56 times the percentage of boron present in the powder other than as TiB2. A requirement for the control of quality of the powder is thus:

Total O to be reduced Total C available for reduction Total O by Vacuum fusion Free C +0.56 X B content other than TiB2 If a mixture is used in which an excess of boron is intended, the TiC content of the carbothermic powder tends to vbe low by virtue of the reaction shown in Equation 6, ie., the carbon is displaced by boron. The undesirable oxygen content can then be reduced by reaction with boron; the product obtained, B203, is not harmful to quality and since it is quite volatile at high temperatures, some is lost from the element during the hotpressing or sintering operation. There are some very inconvenient consequences of this type of compositional control, however, which render it undesirable. Thus, a high B202 content tends to cause sticking of the powder in the die during hot-pressing so that a high degree of compaction cannot be obtained without adopting special precautions. In addition, it is found that the electrical resistivity of the final element is higher than in materials with an appreciable TiC content and that the oxidation resistance is also poorer.

Accordingly, the present invention also provides a powder for use in the manufacture of a solid current conducting element for use in an electrolytic cell for the production of aluminum which powder consists essentially of at least one of the materials titanium diboride and titanium carbide and wherein the ratio of total oxygen present in the powder in percent by weight to the total carbon available in the powder for reduction in percent by weight s less than 1.33.

It is preferred that the powder should contain boron in an amount less than that required bythe stoichiometric ratio for the titanium present and carbon in addition to that required for reduction in an amount sucient to compensate for this deficiency of boron.

The invention also provides as a further feature, a method of producing a solid current conducting element for use in an electrolytic cell for the production of aluminum which comprises hot-pressing or cold-pressing and subsequently sintering a powder according to either of the two immediately preceding paragraphs.

It is preferred to hot-press the powder to a density such that the porosity does not exceed 10% by volume although a greater porosity can be tolerated provided it does not exceed 20% by volume. Good results have been achieved with a powder having a particle size which averages 6/,u with a deviation of 2.5/p. but higher particle sizes can be tolerated and smaller particle sizes are preferred.

As mentioned above, commercial production of titanium diboride (e.g., using reaction (2) above) automatically introduces a number of contaminating elements of which the principal ones are iron, carbon, nitrogen and oxygen. Traces of other elements such as calcium, silicon and aluminum are also present, usually totalling not more than 0.5 by weight and these, in such small proportions, do not appear to be significant. Carbon and oxygen usually derive from the basic material and nitrogen is principally derived from air contamination and adsorption in the carbon black employed as one of the starting materials. Iron is derived from two main sources (a) original impurity in the starting material and (b) pick-up in grinding and ball-milling operations. Generally speaking, the iron content should be less than 0.5% by weight although a higher content can be tolerated. The nitrogen content should be less than 0.3% by weight for a percentage by weight of combined carbon of up to 0.4%, not more than 0.6% by weight at 10% combined carbon (corresponding to 50% by weight of titanium carbide) and not more than 1% by weight at 20% cornbined carbon (corresponding to by weight of titanium carbide) ,although a higher content can be tolerated. v

It is preferred that the free carbon content of the final element should not exceed 0.8% by weight.`

Six examples of the manufacture of solid current conducting elements will now be given:

EXAMPLE I (Powder M lS) Weight Equivalent Percent Weight Compounds Percent Acidsoluble boron Titanium Soluble carbon Iron Nitrogen Total oxygen Total In the above analysis, and in those given subsequently, the so-called acid-soluble boron includes a small proportion of water-soluble boron, which may be present as 7 B203 or hydrolysable borides of impurity elements, or both.

The powder was pressed in a 2 in. diameter graphite die in an atmosphere of hydrogen, with an applied pressure which was increased in appropriate steps from 1A -ton/sq. in. to a final value of 1 ton/sq. in., the temperature being raised from cold to 200G-205 C. in about 31/2-4 hours.

The rod obtained, which was approximately 20 in. long, had the following properties which could be determined by non-destructive testing:

Density (overall) (89% theoretical) 4.02 Electrical resistivity microhm cm-- 19.4

The chemical analysis of the material, carried out on a sample of the rod crushed to -90 mesh B.S.S. (British standard Screen scale) was as follows:

(Rod B. U/414) Weight Percent Equivalent Compounds Weight Percent Nitrogen Total oxygen 1 As titanium oxide."

The titanium oxide content was deduced by subtracting the spurious oxygen (associated with water vapor and surface oxidation produced by the powdering process) from the total oxygen. A check igure obtained by measurement of the total oxygen content of a large fragment or chip of bar (which required only a very small correction for spurious oxygen) gave 0.11% oxygen as ttanium oxide.

This rod was provided with an aluminum lead at one end in the manner specified in British Patent No. 825,443, and was used as a top-entering lead in an experimental reduction cell as described in my U.S. application Serial No. 660,994, tiled May 23, 1957. It carried current for a period of 122 days without any cracking or serious deterioration within the molten flux and metal zone of the cell, and with very little solution (a reduction in diameter of only 0.008 n.).

This was thus representative of a iinal fabricated material suitable for cell application.

EXAMPLE II (Powder 211./2) Weight Percent E qulvalent Weight Compounds Percent The powder was hot-pressed on a similar schedule to Example I, and a 2 in. diameter rod approximately 20 in. long obtained with the following properties:

Density (overall) (91% theoretical) 4.11 Electrical `resistivity microhm cm 13.6

The chemical analysis of the crushed rod was determined as follows:

(Rod B 1GO/235) Weight Percent E quivalent Compounds Weight Percent Aeidsoluble boron Titanium Acid-insoluble boron Frce'y carbon Soluble carbon Iron Nitrogen Total oxygen Total 1 As titanium oxide.

The titanium oxide was deduced as in Example I. The rod was provided with an aluminum lead as previously described, and was used as a top-entering lead in a reduction cell in the same manner as in Example I. It failed to carry current after a period of immersion of less than 2 days, and when removed for examination showed multiple cracking, mostly of a radial character, which rendered it useless.

Material of this nature was thus clearly quite unsuitable for cell applications.

EXAMPLE III 2250 g. of titanium boride powder of similar physical characteristics to that specied in Examples I and II were taken; this batch had the following analysis:

This powder was hot-pressed on a similar schedule to that described for Example I, except that because of the smaller weight of charge it was pressed in a different die assembly, and the time taken to reach the maximum temperature of about 2050 C. was only 11/2 hours.

The rod obtained Was approximately 10 in. long and had the following properties:

Density (overall) (89% theoretical) 4.03 Electrical resistivity microhm om-- 18.0 Transverse rupture strength tons/ sq. in 15.4

The chemical analysis of the crushed rod was ldetermined as follows:

(Rod B. 10C/297) Weight percent Equivalent Weight Compounds percent Soluble Carbon.

Iron Nitrogen Total oxygen Total to be quite sound and free from cracks. The dia-meter had decreased by 0.11 in. during this immersion. The rather higher rate of solution than that quoted in Example I arose from the conditions of test. In the test described in Example I, suicient TiB2 rods were inserted into the molten metal to produce saturated solution conditions, whereas in the immersion tests the metal was not fully saturated.

Material of this composition was thus judged suitable for cell applications.

EXAMPLE IV 2250 g. of titanium boride powder of similar physical characteristics to that speciiied in Examples I, II and III This powder was hot-pressed on a similar schedule to that described for Example III.

The rod obtained was approximately in. long and had the following properties:

Density (overall) (90.5% theoretical) 4.10 Electrical resistivity microhm cm 14.2 Transverse rupture strength tons/sq. in 5.7

The chemical analysis of the crushed rod was deter- 10 z200 g. of the powder were hot-pressed on a similar schedule to that described in Examples III and IV.

The rod obtained was approximately 10 in. long and had the following properties:

Density (overall) (95.2% theoretical) 4.36 Electrical resistivity microhm cm 16.6 Transverse rupture strength tons/ sq. in 14.6

The chemical analysis of the crushed rod was determined as follows:

(Rod B. 90/3) Weight Equivalent (Weight Percent Compounds Percent) Acid-soluble boron 27. 36 Titanium 68.8 Acid-insoluble boron- 0. 08 Free carbon 0. 48 Soluble carbon 2. 37 Iron 0.15 Nitrogen. 0. 07

Total oxygen 0. 42

Total judged suitable for cell appli- EXAMPLE VI Titanium diboride powder was ball-milled with 10% by weight of titanium carbide powder and the iinal milled material was analyzed as follows:

uned as follows' (Powder ses) weight Equivalent weight Percent Compounds Percent (Rod B. 10G/241) Weight Equivalent Weight percent Compounds percent 40 Acid s01ub1e boron n en ium Agia-soluble boron 29.01 TiBn 93.2 frflllffff'u jjj: Tlamum- 68- 2 Soluble carbon- {tord-insoluble boron 0-04 Iron Free Nitrogen Total oxygen- Nitrogen Teta] Oxygen 1 0.59 Total oxygen 0.79

Total 99.40 2000 g. of the powder was hot-pressed on a similar schedule to that described in Examples III, IV and V. lAs titanium oxide." The rod obtained was approximately 10 in. long and The titanium oxide content was deduced as in Examples I, ]I and III.

Half of this bar was subjected to an immersion test in a reduction cell in the same manner as described in Example HI. This was recovered after 6 days and was found to be badly cracked and virtually disintegrated.

Material of this composition was thus clearly unsuitable for cell applications.

EXAMPLE V Titanium diboride powder was ball-milled with 10% by weight of titanium carbide powder, and the inal milled material was analyzed as follows:

had the following properties:

Density (overall) (97.3% theoretical) 4.46 Electrical resistivity microh-m cm 14.6 Transverse rupture strength tons/sq. in..- 17.7

The chemical analysis of the crushed rod was deter- .mined as follows:

(Rod B. /1) T/Veight E quivalent Percent Weight Compounds Percent Acid-soluble boron Titanium Acid-insoluble boron Free carbon Soluble carbon on N trogen Total oiwgen Total l As "titanium oxide.

Half of this rod was subjected to an immersion test in a reduction cell in the same manner as previously described. It was recovered after 21 days and was found to be badly cracked.

l l Material of this composition was thus clearly unsuitable tor cell applications.

The oxygen content of a solid current conducting element produced as described above, other than that due i2 will now be briefly described with reference to the accompanying drawings in which:

FIGS. 1 to 7 are sectional views of reduction cells, and FIGS. 8 and 9 are sectional views of three-layer reiining to B203 or oxygen absorbed during powdering of the elecells.

ment to analyze it, is considered to be mainly present in In the example illustrated in FIG. 1 pairs of opposed the form of oxygen associated with the metal, e.g., tisolid cathodes 1 of plate form and consisting essentially tanium oxide and it will be observed from the above exof at least one of the materials titanium diboride and amples that the elements produced in accordance with titanium carbide are supported by walls 2 at a small angle Examples I, III and V, all of which were suitable for the to the vertical one on each side of an appropriately purposes in view, conform to the requirement that this shaped anode 3. The right-hand wall 2 is omitted in this oxygen content in percent by weight, in relation to the figure. The cathodes 1 are immersed in molten electrolyte combined or acid soluble carbon content of the element 4 containing dissolved alumina and extend at their lower in percent by weight is less than 0.-l% when the combined end into the pool of molten aluminum 5 which forms on carbon content is no greater than 0.4% and is less than the floor of the cell. Current leads 6 which also consist (0. l-l-0.04 n)% where nis said combined carbon conessentially of at least one of the materials titanium ditent and is greater than 0.4%. This relationship is due to boride and titanium carbide extend through the wall of the fact that when the combined carbon is greater than the cell in the pool of molten aluminum S and are con- 0.4%, the oxygen, in the form of titanium oxide TiO, nected at their outer ends to the negative pole of the D.C. can exist in solid solution in the carbide, which appears SUPPlY- The P001 0f mOteD aluminum 5, in this eXamPle, as a second phase. forms part of the current supply system and also acts as An analysis of the powders used in and of the final ele. a cathode. The usual crust of solidified llux which forms ments or rods produced by the above examples is set out Over the Cell iS indicated at 7. in the following table. In this table, total carbon available The arrangement illustrated in FIG. 2 is very similar for reduction is calculated on the basis Total carbon=Free t0 that illuStrated in FIG- 1 and like references are USed t0 carbon+0.56 (acid-insol. B-i-water-sol. B). denote like parts. In this case, the current leads 6 are TABLE I Powder Analysis, Percent by Weight Rod Analysis,

Percent by Weight Ratio Example O/C Acid Water Total C Total O as Residual Free C" insol. sol. B available O "titanium Free C Quality B for reduction oxide 0.70 0.23 0.15 0.91 1.11 1.2 0. 07 0.29 Good. 0.20 0. 07 0.47 0.50 1.33 2.05 0. 63 0.02 Bad. 0. 48 0.00 0.07 0.54 0.02 1.15 0.08 0. 00 Good. 0.20 0.04 0.45 0.52 1.27 2.4 0. 59 0.04 Bad. 0.84 0.44 0.12 1.15 1.20 1.05 0.17 0.48 Good. 0.25 0.08 0.10 0.38 0.85 2.3 0. a0 0. 03 Bad.

It will be observed from this table that the powders omitted and replaced by extensions 1a of the cathodes from which a good element was produced conform to the which extend through the crust 7 for connection to the requirement that the ratio of total oxygen to total carbon negative pole of the D.C. supply. These extensions are available for reduction should be less thanv 1.33. provided with protective sleeves 8 where they extend It is apparent from our investigations that in a solid through the crust 7, these sleeves being of aluminum or a current conducting element consisting essentially of at refractory material. least one of the materials titanium diboride and titanium The arrangement illustrated in FIG. 3' is similar to that carbide, the quantity of oxygen present in the form of the illustrated in FIG. 1 but in this case, the solid cathodes 1 oxide of the metal is a determining factor. It would ap- 50 are omitted together with their supporting walls 2 and the pear that when this quantity of oxygen is higher than a pool of molten aluminum 5 constitutes the sole cathode. predetermined value which is a function of the combined The arrangement illustrated in FIG. 4 is similar to that or acid-soluble carbon content of the element, the ele- ShOWn in lFIG. 3 but in this case the current leads 6 do ment is attacked by molten aluminum which penetrates not extend through the wall ofthe cell but extend into the the element and causes it to crack and/ or disintegrate. pool of molten aluminum 5 through the crust 7 at the side It will be apparent from the above disclosure that an of the cell and are provided with a protective sleeve 8. It unsuitable powder can, under appropriate conditions, be is preferred, however, that the current leads 6 should be made suitable by increasing the free carbon content therearranged as indicated in FIG. 5 substantially centrally of of and/ or by increasing the boron content thereof (other the cell between anodes 3 as this arrangement leaves the than in the form of the boride of the metal). As higher sides of the cell free for replenishing the cell and the crust oxygen contents can be tolerated with increasing propor- 7 is thinner at the center than at the sides. Additionally tions of the carbide of the metal, a bad powder can somethis arrangement lends itself to an arrangement of bustimes be changed to a good powder by increasing the bars (not shown) whereby magnetic effects can be reamount of titanium carbide present. This is not a disadduced to a minimum. vantage as I have found that the very pure borides present FIG. 6 shows an arrangement in which the current diiculties in pressing to the iinal form as they do not leads 6 extend through the door of the cell. densify readily and there is a tendency for sticking to FIG. 7 shows how an existing cell may be modified to occur in the graphite die. It is therefore preferred that a include current leads 6 according to the invention, these solid current conducting element consisting of at least one leads being embedded in the carbon floor 9 of the cell of the materials titanium diboride and titanium carbide which is connected to the negative pole ofthe D.C. supply should contain from 2 to 40% by weight of titanium di by iron bars 10 embedded therein. The current leads exboride or titanium carbide and desirably not less than tend up into the pool of molten aluminum 5 through any 10% by weight. sludge layer which may be formed on the floor of the cell Some examples of electrolytic cells embodying solid and so avoid the power loss which such a layer otherwise current conducting elements according to the invention introduces.

FIG. 8 shows a three-layer refining cell having the usual lower layer 11 of molten aluminum alloy, intermediate layer 12 of electrolyte and upper layer 13 of puriiied aluminum. The lower layer 11 is connected to the positive pole of a D.C. supply by current leads 6a extending through the Wall of the cell and the upper layer 13 is connected to the negative pole of the D.C. supply by current leads 6b extending through the wall of the cell, the current leads 6a and 6b consisting essentially of at least one of the materials titanium diboride and titanium carbide.

In a three-layer cell illustrated in FIG. 9 'the current leads 6b are shown extending into the upper layer 13 from the top of the cell and are provided with protective sheaths 8.

It will be obvious that various modications and alterations may be made in this invention without departing from the spirit and scope thereof and it is not to be taken as limited except by the appended claims.

What is claimed is:

1. A solid current-conducting element for use in the electrolytic production of aluminum wherein at least a portion consists essentially of titanium diboride, an oxygen impurity and combined carbon content consisting essentially of titanium carbide, the carbon of said combined carbon content being present in an amount greater than 0.4% by weight of said portion, the relationship of the oxygen impurity and the carbon of said combined carbon content being such that the oxygen impurity content is less than (0.1-1-0.04 n) percent by Weight wherein n is the carbon of said combined carbon content.

2. A solid current conducting element according to claim 1 having a titanium carbide content of from 2%- 40% by weight.

References Cited UNITED STATES PATENTS 2,722,509 11/ 1955 Wainer 204-64 2,915,442 12/ 1959 Lewis 204-67 FOREIGN PATENTS 802,905 10/ 1958 Great Britain.

JOHN H. MACK, Primary Examiner. D. R. JORDAN, Assistant Examiner.

Claims (1)

1. A SOLID CURRENT-CONDUCTING ELEMENT FOR USE IN THE ELECTROLYTIC PRODUCTION OF ALUMINUM WHEREIN AT LEAST A PORTION CONSISTS ESSENTIALLY OF TITANIUM DIBORIDE, AN OXYGEN IMPURITY AND COMBINED CARBON CONTENT CONSISTING ESSENTIALLY OF TITANIUM CARBIDE, THE CARBON OF SAID COMBINED CARBON CONTENT BEING PRESENT IN AN AMOUNT GREATER THAN 0.4% BY WEIGHT OF SAID PORTION, THE RELATIONSHIP OF THE OXYGEN IMPURITY AND THE CARBON OF SAID COMBINED CARBON CONTENT BEING SUCH THAT THE OXYGEN IMPURITY CONTENT IS LESS THAN (0.1+0.04XN) PERCENT BY WEIGHT WHEREIN N IS THE CARBON OF SAID COMBINED CARBON CONTENT.

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