US3457158A - Cell lining system - Google Patents

Description

v. l.. BULLOUGH 3,457,158

CELL LINING SYSTEM l Filed Oct.

July 22, 1969 ATTORNEYJ' United States Patent O 3,457,158 CELL LINING SYSTEM Vaughn L. Bullough, Florence, Ala., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware v Filed Oct. 2, 1964, Ser. No. 401,067 Int. Cl. C22d 3/12, 3/02 U.S. Cl. 204-243 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electrolytic cells for the production of aluminum and other metals and to a novel method for controlling the thermal expansion of cell insulating materials. lMore particularly, the invention concerns a novel method for controlling the thermal expansion of powdered alumina.

Aluminum metal is conveniently produced in electrolytic cells by passing a current through a bath of molten cryolite containing dissolved alumina in a large tank lined with carbon which serves las one electrode, namely the cathode. Large carbon blocks presented at the top of the bath function as the anode. Molten aluminum metal at a temperature of about 1800" F. collects to the bottom of the cell `and is siphoned therefrom periodically. The cells may be lined with rammed rcarbon and may have a row of horizontal graphite bars extending from one side of the cell which serve to contact the pad of molten aluminum which collects at the bottom of the cell when in operation, thus acting as cathode current collectors. In newer types of construction of electrolytic reduction cells, refractory linings made of alumina and cryolite are employed, `as illustrated, for example, in U.S. Patent 3,093,570, and in copending application Ser. No. 222,079, now U.S. 3,321,392, led Sept. 7, 1962. In cells of this type, current collectors extending into the aluminum metal layer of the cell are made of substances such as graphite or the borides, nitrides and carbides of elements of Groups 4, and 6 of the Periodic System, particularly zirconium and titanium, for example, titanium diboride.

In the construction of aluminum reduction cells it is the practice to provide a powdered insulating -layer between the cathode and the shell of the cell. The insulating powder underlies the molten metal pad and surrounds the cathode contacts, whether they be of graphite or titanium boride. When the cell is in operation a glazed layer forms on the upper sur-faceof the insulating material, adjacent the molten metal pad, and provides a boundary between the molten metal and the bulk of the insulating powder.

Whether the cathode collectors are made of graphite or of titanium boride, it is of importance to match the thermal expansion of the insulating layer of the cell as closely as possible to the thermal expansion of the cathode collector in order to minimize thermal stresses which may tend to produce failure of the cathode collectors.

It has been found that the thermal expansion characteristics of the cell insulation material can be tailored to correspond closely with those of the cathode collector material. This is accomplished, in accordance Iwith theV invention, by incorporating in the powdered alumina used SAS'LISS Patented `luly 22, 1969 for the cell insulating material a minor proportion of a lluoride of aluminum.

The iluoride of aluminum may ibe aluminum fluoride, AlF3, or it may be a double iluoride of aluminum and an alkali metal, such as cryolite NaFAlF. These iiuorides are intimately admixed with powdered alumina in a proportion ranging Ifrom about 0.1% to about by weight. In proportion to the amount of fluoride added, it has been fund, surprisingly and unexpectedly, that the thermal expansion of the powdered alumina may be controlled, with the result that it may be either diminished or increased. Thus, for example, the addition of from about 0.1% to `about 5% of cryolite by weight to alumina powder brings about a decreased thermal expansion, whereas the addition of from 10% to 20% produces an increase in thermal expansion.

The effect of additions of cryolite to alumina in diminishing or increasing its linear thermal expansion may rbe seen from the curves of the accompanying drawing. The relationship of linear expansion to temperature for pure alumina is shown in the broken line curve. The addition of 5% of cryolite is reflected in the group of curves lying below the pure alumina curve, `while the addition of larger amounts of cryolite elevates the thermal expansion curve.

The curves of the drawing are lbased upon experimental data obtained by measuring thermal expansion of compacted alumina powder at temperatures to 1000 C. in a silica dilatometer. The dilatometer was constructed by modifying a porcelain lter crucible (Selas No. 6010) by installation of a thermocouple well through its bottom and extending into the Crucible approximately one inch,

v the Well being cemented into place. The powdered material to be tested for thermal expansion was placed in the test crucible and packed by tapping the bottom. The crucible containing the sample was placed within the cavity of a wire-wound silica dilatometer furnace and heated to 1000 C. at the rate of about 2.8 C. per minute. The linear expansion of the sample was measured to the nearest 0.0001 inch by means of a dial indicator, the expansion being transferred to the dial lby means of a fused silica rod placed on top of the sample within the dilatometer furnace.

In accordance with the novel method of the invention, the admixing of the fluoride with the alumina powder is performed in any suitable type of mixing apparatus, the mixing being carried on until a uniform mixture is obtained.

The practice of the invention is illustrated by the yfo-llowing example, which is not, however to be regarded as limiting:

Example Where -an aluminum reduction cell is constructed so as to include bottom entry graphite cathode collectors, in the manner illustrated, for example, in FIG. `6 of Patent 3,093,570, the thermal expansion of the alumina insulating layer may be controlled to match that of the graphite, which is substantially less than for powdered alumina. The mean temperatures of the graphite and the alumina lining will depend upon the cell design. However, the thermal conductivity of the graphite is about 1000 times that of the powdered alumina. In practice, the graphite temperature is about 900 C. A reasonable mean temperature for the alumina may be assumed to lbe `about 550 C. The junction of the glazed layer previously men- -tioned and the graphite collector is rbelow the top of the graphite collector, the relative position depending upon the design and operation of the cell. The longitudinal thermal expansion of graphite at 900 C. is about 0.3% (Carbon Products Handbook p. 19, 1964 Ed., Union Carbide Corp., New York). Assuming that about of the graphite member is embedded in the alumina layer, the

problem is one of employing a refractory mixture that has the same average thermal expansion at 550 C. over its length as the graphite member has over its length, i.e. about 0.225%.

In 4accordance with the present invention, it has been found that a mixture of 99% alumina and 1% cryolite has an expansion coecient of 0.21%. Thus, for the design in question, a mixture of 99 parts by weight alumina to l part by Weight of cryolite `will prevent tension stresses in the graphite, providing an expansion of the alumina powder which is compatible with the graphite collector.

Employing similar considerations, the thermal expansion of the alumina insulating layer may be controlled to match that of a titanium diboride cathode collector.

There may also tbe employed a suitable mixture of powdered alumina and aluminum fluoride as a bed material for a reduction cell.

It will be appreciated, furthermore, that the insulating or bed layer may be subdivided into two or more layers, each adjusted by addition of cryolite or aluminum fluoride so as to provide suitable expansion characteristics for a particular area.

What is claimed is:

1. An yelectrolytic reduction cell having an interior lining consisting essentially of a powdered mixture of alumina and from about 0.1% to about 20% by weight of `a fluoride of aluminum, with a conductive cathode extending at least partially therethrough for Contact with the molten contents of the cell, said lining having a thermal expansion substantially similar to that of said cathode.

2. An electrolytic reduction cell having an interior -lining consisting essentially of a powdered mixture of alumina and from about 0.1% to about 5% by weight of cryolite, with a graphite cathode extending at least partially therethrough, -said lining having a thermal expansion substantially similar to that of said graphite cathode.

3. An electrolytic reduction cell having an interior lining consisting essentially of a powdered mixture of alumina and from about 10% to about 20% by weight of cryolite, with `a titanium diboride cathode extending at least partially therethrough, said lining having a thermal expansion substantially similar to that of said titanium diboride cathode.

4. The cell of claim 1 in which the iiuoride of aluminum is cryolite.

5. The cell of claim -1 in which the fluoride of aluminum is aluminum triiluoride.

6. Method of lining an electrolytic reduction cell, which consists essentially of intimately admixing powdered alumina with from vabout 0.1% to about 20% by weight of a uoride of aluminum and then tamping said powdered mixture into the bottom of the cell so that a cathode of conductive material extends at least partially herethrough, said mixture having a thermal expansion substantially similar to that of said `conductive material.

7. The method of claim 6 in `which the conductive cathode material is graphite.

8. The method of claim 6 in which the conductive cathode material is titanium diboride.

9. The method of claim 6 in which the fluoride of aluminum is cryolite.

References Cited UNITED STATES PATENTS 3,028,324 4/ 1962 Ransley 204-243 3,093,570 6/1963 Dewey 204-243 3,261,699 7/1966 Henry 204-243 3,267,183 8/1966 Feinleib 204-243 JOHN -H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner

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