US3088906A - Cathode bar for aluminum reduction cell - Google Patents

Description

United The invention relates to the flame spray coating of materials and involves coating the materials with magnesla.

One object of the invention is successfully to flame spray and to coat materials with magnesia. Another object is to protect materials from molten metal such as molten zinc, molten aluminum and molten lead. Another object is to line melting pots for protection from molten metal. Another object is to provide coatings for cathode bars which are used in aluminum reduction cells. As an example, in relatively new types of cells cathode bars made out of zirconium boride and out of titanium boride have been found to be promising. These bars extend through furnace walls and to avoid loss of electric energy it is desirable to insulate the bars from the walls. Magnesia is not only resistant to attack by molten aluminum, but it is also an electrical insulator. Currently these cathode bars made out of zirconium boride and alternatively made out of titanium boride are being coated with magnesia by means of my process throughout that area of the bar which desirably should be insulated from the wall.

Another object of the invention is to produce oxide coatings on metals which have a thermal coelficient of expansion not too far different from that of the metal involved. Magnesia has a thermal coeflicient 01f expansion close to that of many metals.

Other objects will be in part obvious or in part pointed out herein.

I provide a flame spraying gun that fuses and sprays powder-s. I am not aware of any patents so cannot cite any. Satisfactory guns of this type can be procured commercially, however. These guns have hoppers and feeding channels to feed the powder to a blast of flame produced by burning acetylene and oxygen. They have a slight resemblance to flame spraying guns for spraying metal fed into the guns in the form of wire such as disclosed in US. Patent 1,100,602 to Erika Morf, dated June 16, 19:14.

I provide powder of magnesia, MgO, containing from 1% to 10% of silica, SiO with not more than 5% impurities. I mean by the above that the particles of powder are made of magnesia with the amount of silica specified, etc. Another specification is that the powder should be flowable, meaning that it shall readily flow from the hopper through the channel and orifices of the flame spray gun. In addition to this it should have a particle size finer than 90 mesh.

To produce the coating of my invention, I simply charge the hopper of the gun with the magnesia powder defined, light .the flame and spray the molten particles of magnesia since the flame melts them. The object to be coated is placed in the zone of molten particles and not too close to the nozzle of the gun to be injured by the flame. 'Flame spraying of powders is now well known technology and need not be further described. The apparatus provided by .the manufacturer of the flame spray gun will naturally be used as he directs.

in the hopper of a flame spraying powder gun and, having lit the flame, produced a coating on a zirconium boride cathode .bar for an aluminum reduction cell. The gun was supplied with oxygen and acetylene under moderate pressure and was operated according to the directions of the manufacturer. A coating of 015 inch was produced. This coating was very .ahderent and hard. Cross sectioned and examined under the microscope, it proved to be laminate in structure. It had a porosity of about 20.4%.

This magnesia that was used undoubtedly had other material besides magnesia and silica but only to the extent of very small percentages properly classified as traces.

I repeated this operation using the same composition of magnesia but of mesh particle size and achieved equally good results. I again repeated the operation using the same composition of magnesia .and silica but of 275 mesh particle size. Again the coating was good and so far as I could see of the same quality as the other two.

Example II Using mesh particle size magnesia powder containing :1.2% of silica, I produced with a flame spray powder gun a coating on steel. I 'found this coating to be usable but somewhat soft. This is where I get the low limit of 1% of silica.

Example III I procured magnesia powder of 180 mesh particle size containing as closely as I can estimate 3 /270 of silica, impurities merely traces. Using this powder in a flame spray powder .gun, -I produced good, adherent and hard coatings on plates of steel. This is the best example (best mode). However I should state that the material coated depends upon the use and so there is no best mode for that.

Relative to the upper limit of 10% silica, since the principal use Olf my process .and my coatings is now believed to be for the coating of cathode bars, the upper limit of 10% is arrived at because a coating with more than that amount of silica would react with molten aluminum and other melts. In my opinion, commercially pure silica in powder form can be flame sprayed in a powder gun but my invention is limited to the flame spraying of magnesia with the specified range of silica. With more than 10% of silica the coatings would be interior for the principal purpose mentioned and inferior to alumina or zirconia coatings for most other purposes.

The coating according to my invention is .a coating of magnesia with from 1% to 10% of silica, and having not more than 5% of material selected from the group consisting of metal oxide and metal and mixtures and not more than .5% of other matter and preferably not more than 11% of other matter. Metal oxides and metals are compatible with my coatings and therefore that is the reason for the specification of a possible amount thereof up to 5% Other materials are not so compatible and that is the reason for limiting them to smaller amounts. For the best mode of the invention, so far as I now know, the coating should be all magnesia and silica but it is impossible to avoid traces of impurities.

Before the invention was made I attempted to flame spray magnesia powder of the various mesh sizes, 40, 90, 100 and 250, each one containing finer particles. This powder was chemically pure magnesia and probably contained about 20% silica. I could produce no coatings with these powders. 11 also tried .to produce magnesia coatings using 99% pure sintered magnesia rods in a flame spraying process according to my colleague Wheildons U.S. Patent No. 2,707,691, but the rod spat badly and did not form a spray. Magnesia does not appear to be a stable metal oxide as described by Wheildon in his patent. Apparently it dissociates or volatilizes rather than form-ing a liquid spray. Frequently it appears to be reduced to some extent. Therefore it is surprising that success was achieved using powders. It is even more surprising because in general the flame spraying of powders, as \for example metal oxides, produces inferior coatings to those made by flame spraying rods.

Rods made out of magnesia bonded with clay bond were also tried by me in a flame spray gun of the type disclosed in the Mor-f patent. In one case the rods were made out of 96% magnesia and the rest clay and in another case out of 92% magnesia .the rest clay. The rods were fired to cone 16 in the usual manner. No adherent coatings were produced with the rods containing 96% of magnesia. Only inferior coatings were produced with the rods containing 92% magnesia. The use of more clay bond would produce coatings of inferior quality for the most important uses mentioned herein. In these experiments using magnesia with clay bond, the spray rate was extremely slow and the rods spat causing non-uniform coatings.

Almost all compounds of titanium are chemically very close to corresponding compounds of zirconium, and that includes Zirconium with its usual hafnium content. For example, zirconium oxide with the normal content .of hafnium oxide is referred to as zirconium oxide and zirconium boride with its normal content of 2% or 3% hafnium boride is referred to as zirconium boride. But recently it has become possible to make rather pure zirconium boride and that is included within the scope of my claims.

Titanium, zirconium and hafnium are all of them in group IV of the periodic table and they are the only transition metals in this group. Therefore it will be seen that they are extremely closely related to each other.

It is not yet apparent whether, for aluminum reduction cells, the winner will ultimately be titanium boride bars or zirconium boride bars. It is perfectly possible that bars made out of mixtures of titanium boride and zirconium boride will be the winner, and these bars will have the normal content of hafnium boride because it will be too expensive and to no purpose to eliminate the hafnium component. Accordingly when I use the term cathode bar in the claims, the term is intended to cover all of these Varieties of bars coated with magnesia throughout the .area juxtaposed the wall of the reduction cell as described.

Any solid material can be coated with magnesia in accordance with my invention. Even flexible materials can be coated although the desirability of doing this is not apparent. Metals which melt at a low temperature can be protected by cooling, and materials like wood which burn can be protected by an atmosphere of non-oxidizing gas. The invention is useful for coating borides, other ceramics, and metals so far as now known and other uses will be developed in due course.

Coatings according to my invention should be at least one mil thick but it doesnt matter how much thicker they are provided specifications of size, weight and cost are met and so the upper limit is indefinite. In this specification all percentages are by weight. In the best mode of the invention the amount of silica is from 2% to 4% It will thus be seen that there has been provided by this invention a method of flame spray coating with magnesia and the coatings so produced in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A cathode bar for insertion through the wall of an aluminum reduction cell made of boride selected from the group consisting of titanium boride and zirconium boride and mixtures thereof, said bar having a laminate adherent coating of magnesia over an area intermediate the ends of the bar and being at least one mil thick, said magnesia having from 1% to 10% silicia, not more than 5% of other material selected from the group consisting of metal oxide and metal and mixtures thereof and not more than .5% of other matter, all by weight.

2. A cathode bar for aluminum reduction cells according to claim 1 in which the silica is from 2% to 4% by weight.

References Cited in the file of this patent UNITED STATES PATENTS 1,570,929 Udale et al J an. 26, 1926 2,285,952 Vogel et al. June 9, 1942 2,695,242 Woodward Nov. 23, 1954 2,707,691 Wheildon May 3, 1955 2,915,442 Lewis Dec. 1, 1959 FOREIGN PATENTS 217,737 Australia May 23, 1957

Claims (1)

1. A CATHODE BAR FOR INSERTION THROUGH THE WALL OF AN ALUMINUM REDUCTION CELL MADE OF BORIDE SELECTED FROM THE GROUP CONSISTING OF TITANIUM BORIDE AND ZIRCONIUM BORIDE AND MIXTURES THEREOF, SAID BAR HAVING A LAMINATE ADHERENT COATING OF MAGNESIA OVER AN AREA INTERMEDIATE THE ENDS OF THE BAR AND BEING AT LEAST ONE MIL THICK, SAID MAGNESIA HAVING FROM 1% TO 10% SILICIA, NOT MORE THAN 5% OF OTHER MATERIAL SELECTED FROM THE GROUP CONSISTING OF METAL OXIDE AND METAL AND MIXTURES THEREOF AND NOT MORE THAN .5% OF OTHER MATTER, ALL BY WEIGHT.