US 3321392 A
Description (OCR text may contain errors)
C. J. M MINN ET AL ALUMINA REDUCTION CELL AND METHOD FOR MAKING REFRACTORY LINING THEREFOR 2 Sheets-Sheet 1 Filed Sept. '7, 1962 LJLJLJLJLJ INVENTORS CURTIS J. M NIINNEL JOHN L. DEWEY 4 /azg zww ATTORNEY5 May 23, 1967 c J c gN ET AL 3,321,392
ALUMINA REDUCTION CELL- AND METHOD FOR MAKING REFRACTORY LINING THEREFOR 1962 2 Sheets-Sheet 2 Filed Sept. 7
' 1NVENTOR5 CURTIS J. MCMINN & JOHN L. DEWEY fim/ flfl ATTORNEYS United States Patent C) ALUMINA REDUCTION CELL AND METHOD FOR MAKING REFRACTORY LINING THEREFOR Curtis J. McMinn and John L. Dewey, Florence, Ala., as-
signors to Reynolds Metals Company, Richmond, Va.,
a corporation of Delaware Filed Sept. 7, 1962, Ser. No. 222,079 8 Claims. (Cl. 204243) This invention relates to a novel apparatus and method for the control of stresses and shrinkage in refractory linings of electrolytic cells. More particularly, the invention concerns a system of divider elements for controlling the shrinkage of a refractory lining in an electrolytic cell for the reduction of alumina.
Aluminum metal is conventionally 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 as one electrode, namely the cathode. Large car-hon blocks presented at the top of the bath function as the anode. Molten aluminum metal at a temperature of about 1800 F. collects at the bottom of the cell and is siphoned therefrom periodically.
The carbon cell linings involve high initial cost of construction and require periodic shutdowns for repairs, maintenance, or replacement. In cells lined with carbon, swelling and heaving of the lining are attributable in part to the absorption of molten salts from the electrolytic bath, either in the carbon itself or in the underlying layer of insulative material due to permeability of the carbon, and this necessitates special bracing of the steel cell walls and supporting structure. There is also present a detrimental tendency toward formation of undesirable aluminum carbide, causing wastage of carbon and loss of electrode conductivity.
The recent advent of refractory hard metal current collectors has made it possible to employ nonconductive lining materials having more desirable properties. Thus, in copending application Ser. No. 847,594, filed Oct. 20, 1959 (now US. Patent No. 3,093,570), there was disclosed the concept of a special type of lining for alumina reduction cells which consists chemically of the same elements as the cell bath itself. There was specifically disclosed :as a refractory lining material a high melting mixture of a refractory oxide and a double fluoride of aluminum and an alkali metal, for example, a mixture of "between about 20% and 75% alumina, and the balance sodium cryolite.
A refractory lining of the character described maybe installed by prefusing the refractory oxide and the double fluoride, allowing the molten mixture to cool, crushing the cooled melt, moistening the crushed mixture with water and tamping it into place to form the bottom of the reduction pot. Alternatively, the two ingredients are mixed in powder form, the dry powder mixture is placed in the bottom of the reduction cell, and the cell is charged with a sufficient amount of aluminum to furnish a pool of molten aluminum covering the current collector bars. The aluminum is melted with a gas flame, then the electrolyte is added and the current applied. During con tinued operation of the cell, the top few inches of the refractory fuses or sinters to a hard, dense, light grey solid.
Further experience with reduction cells in which a number of current collector bars are disposed within a refractory lining has shown that thermal effects in the course of normal operations produce shrinkage in the lining and stresses upon the bats. With the aluminacryolite refractory, such shrinkage effects are most pronounced in that portion of the lining where the refractory material is in a fused condition.
It is, therefore, an object of this invention to provide a ice method and means for the control of shrinkage in refractory cell linings and to improve the usefulness and stability of such linings. A further object of the invention is the provision of means to protect refractory hard metal current collectors from stresses attributable to movement of the cell lining.
In accordance with the invention there is provided a novel method for controlling the shrinkage of a body of a refractory lining material which occurs upon cooling of the refractory from an elevated temperature, which comprises providing in the body of the refractory at the time of its formation a transverse parting plane to subdivide the refractory body into segments and isolate the shrinkage movement of each segment from adjacent segments.
The parting plane is defined by a solid divider made of material having a fusion point higher than that of the cell lining, and to which the latter will not adhere. The divider is advantageously a refractory material having good mechanical properties and adequate resistance to corrosion by molten aluminum and cry-olite, such as the nitrides and carbides of silicon, boron and titanium, or combinations of these materials. Examples of suitable materials include silicon carbide, silicon nitride, boron nitride, and titanium boride; also, carbon and graphite. The presently preferred material is silicon carbide bonded with silicon nitride, one type of which is available commercially under the designation Crystolon N.
The manner in which the apparatus and the method of the present invention are constructed and operated will appear in the following description which, considered in connection with the accompanying drawings, sets forth the presently preferred embodiments of the invention.
Referring to the drawings:
FIG. 1 is a plan view of an aluminum reduction cell embodying bottom entry current collectors and sets of divider strips in a refractory lining of the cell;
FIG. 2 is a longitudinal elevation taken on the line 22 of FIG. 1;
FIG. 3 is :a transverse elevation of a modified cell, showing the various zones formed in an alumina-cryolite refractory lining;
FIG. 4 is a detailed view in cross-section showing the construction and relationship of the current collector and divider strips.
Referring to FIG. 1, the plan view shows a general arrangement of current collectors and divider strips in a reduction cell 1%. A chamber to hold the molten contents of the cell is defined by a refractory bottom lining 13 (see FIG. 2) and four side walls 11 lined with insulating material 12. Cathode current collector bars 14 are arranged as shown in FIG. 1, and in more detail in FIGS. 2, 3 and 4, said collector bars 14 being electrically connected to corresponding bus bars 15, which lead to the general cathode bus bar system of the reduction cell bank.
The body of the refractory bottom lining 13 is subdivided into segments 16 and 16' by divider strips designated generally as 17 and 17, the divider 17 extending longitudinally of the cell bottom lining, and the dividers 17 extending laterally.
The construction and mounting of the collector bars 14 and the dividers 17 and 17' is shown in more detail in FIGS. 2, 3 and 4. Each collector bar 14 includes a titanium diboride current collector 18 which protrudes into the cell interior for contact with the aluminum pad. An iron stem portion 20 of each bar 14 is welded to member 18 and connected to the collector bus 15. The current collector 18 is protected by sleeve 21 to guard against corrosive action along its surfaces within the bottom lining. Details of such a protective sleeve are disclosed in US. Patent 3,287,247 of Nov. 22, 1966, issued on applicants copending application Ser. No. 215,234, filed July Q '24, 1962, as a continuation-in-part of his application Ser. No. 7,681, filed Feb. 9, 1960, now abandoned.
In the presently preferred embodiment of the invention, cell bottom lining 13 is the alumina-cryolite mixture previously referred to in connection with copending application Ser. No. 847,594. A layer of insulating material 19, such as alumina, underlies the bottom lining 13. The alumina-cryolite lining 13 is advantageously installed by filling the bottom of the cell with a powdered mixture of alumina and cryolite. In operation, the cell action causes this mixture to form roughly three zones, as depicted in FIG. 3. The topmost zone 22 is a fused layer extending across the upper surface of the bottom lining. Extending immediately beneath layer 22 is a sintered refractory layer 23, while underlying both zones 22 and 23 is a zone in which the refractory mixture remains as a loose powder 24. Zone 22 will usually be up to about four inches thick.
Another method of forming the refractory lining 13 in situ is by filling the bottom of the cell with a bed of alumina, then putting the cell in operation to infiltrate the alumina with electroylte and cause the formation of a fused alumina-cryolite layer.
The manner of construction and mounting of the divider :strips 17 and 17' is shown in more detail in FIGS. 3 and 4. The dividers are arranged in the form of a lattice as shown in FIG. 1, and each set 17 and 17 includes a strip or slab 25 of silicon carbide bonded with silicon nitride. The refractory divider strip should be of suflicient depth (at least about 6 inches) to extend below the bottom of the fused layer 22, the shrinkage of which it acts to con- .trol. The divider strip 25 is conveniently supported by a steel sheet 26 of suitable thickness, such as approximately 12 gage.
The cell is started up by heating the cell interior with :a gas flame, gradually raising its temperature and that of the collector bars to approximately 500 C. The exposed tops of the collector bars may be provided with aluminum caps to protect them during this step. Molten aluminum is poured into the cell in an amount sufficient to cover the upper ends of the collector bars, and the reduction electrolyte of cryolite and alumina is then added. The anode is inserted and reduction current is applied to the cell.
The dividers form parting planes to permit controlled shrinkage of the lining and to prevent the collector bars from being subjected to excessive lateral stresses caused by contraction of the fused layer of the lining. When the cell is shut down in the course of operation, the refractory hard metal current collectors remain upright and intact.
In some instances, furthermore, it is desirable to extend the dividers above the lining-bath interface to help protect the collector bars from stresses similarly induced by contraction of the aluminum pad.
The invention is broadly applicable to the protection of refractory bodies in electrolytic cells for the production of aluminum, including refining cells as well as reduction cells, and in similar environments where the refractory is subjected to thermal stresses. It is to be noted, furthermore, that the beneficial results of the invention extend to refractory cell linings, whether or not the refractory itself contacts the molten contents of the cell. In addition, the practice of the invention with respect to refractory lining materials of an alumina reduction cell does not depend on the use of bottom-entry collector bars, since other electrode arrangements are recognized as being within the skill of the art.
Thus, while present preferred embodiments of the .invention have been illustrated and described, it will be appreciated that the invention may be otherwise variously embodied and practiced within the scope of the following claims.
What is claimed is: 1. An alumina reduction cell having a chamber to hold the molten contents of the cell, a refractory lining having a surface underlying said chamber, and a refractory divider element subdividing said lining into segments along said surface and providing parting planes between adjacent segments thereof, said divider element having a fusion point above that of said lining.
2. An alumina reduction cell having a chamber to hold the molten contents of the cell, including a pad of molten aluminum, a refractory lining having a surface underlying said chamber, an electrode disposed within said lining and having a refractory hard metal current collector in electrical contact with the aluminum pad, and a refractory divider element subdividing said lining into segments along said surface and providing parting planes between adjacent segments thereof, said divider element having a fusion point above that of said lining.
3. An alumina reduction cell having a chamber to hold the molten contents of the cell, including a pad of molten aluminum, a refractory lining having an upper surface disposed in contact with substantially the entire lower surface of said molten contents, a plurality of electrodes entering the chamber through said lining and disposed in parallel rows, each electrode having a refractory hard metal current collector protruding above said upper surface of the lining for electrical contact with the aluminum pad, and a system of longitudinal and transverse divider elements between adjacent electrodes, subdividing said lining into segments along said upper surface and providing parting planes between adjacent segments thereof, said divider elements having a fusion point above that of said lining.
4. An electrolytic cell for the production of aluminum, including a cell lining having a fused layer of refractory material consisting essentially of alumina and cryolite, and a refractory divider element subdividing said fused layer of the lining into segments and providing parting planes between adjacent segments thereof, said divider element being capable of withstanding the operating temperature of the cell.
5. An electrolytic cell for the production of aluminum, having a refractory lining consisting essentially of alumina and cryolite, said lining including a fused layer contacting the molten contents of the cell, and a refractory divider element subdividing said fused layer of the lining into se ments and providing parting planes between adjacent segments thereof, said divider element having a fusion point above that of said lining.
6. Method of constructing an alumina reduction cell, having an interior lining which defines a chamber to hold the molten contents of the cell and includes a layer of fused refractory material underlying said chamber and disposed in contact with said molten contents, with provision for controlled shrinkage of said lining upon cooling from an elevated temperature, which method comprises installing a layer of unfused refractory material and a divider element separating such material into laterally adjacent segments, said divider element having a fusion point above that of said refractory material, thereby providing a transverse parting plane in the body of said refractory material to isolate the movement of that portion of the refractory bounded by said parting plane, and to prevent the transmission of strain from said portion to the remainder of the refractory of layer, and then heating said refractory material to form said fused layer.
'7. Method of forming a refractory lining in an alumina reduction cell, which comprises the steps of subdividing the cell interior along the bottom into a plurality of segments by locating divider elements therein capable of withstanding the operating temperature of the cell; filling the spaces bounded by said elements with alumina in pulverulent form; and putting the cell in operation to infiltrate the alumina with electrolyte, thereby forming a discontinuous layer of lining material consisting essentially of alumina and cryolite and causing said layer to fuse said di- 5 6 vider elements providing parting planes subdividing said References Cited by the Examiner lg fid f f f t 1 UNITED STATES PATENTS e o o orming a re rac ory ining 1n an a umina 1,769,298 7/1930 Lauber 204243 reduct1on cell, which comprises the steps of placing Withm 2,124,865 7/1938 Winklar et a1 26 30 the cell a bed of lining material comprising a mixture of 5 2 378 142 6/1945 Hurter alumina and cryolite in pulverrulent form; disposing Within 2666975 1/1954 Farnsworth 264 3O said mlxture a system of divider elements of rnatenal hav- 2 846 388 9/1958 Morel 2O4 243 ing a fusion point above that of said alumina-cryolite mix- 3:093:57O 6/1963 Dewey ture; and putting the cell in operation to cause said miX- ture to fuse while maintaining said divider elements in 10 JOHN MACK Pnmwy Examiner solid condition. A. B. CURTIS, G. KAPLAN, Assistant Examiners.