Aluminum Reduction Cell And System For Energy Conservation Therein

Abstract

A compact electrolytic potcell for the reduction of aluminum in fluoride fusions including a multiplicity of spaced and sized electrical conductors extending from the reduced aluminum pad underlying the fluoride fusion, through the refractory potlining, and to an aluminum slab heat-sink on the potshell, the conductors being molten where in contact with the pad and solid where in contact with the aluminum slab and the temperature and voltage difference of the slab relative to the pad being adjusted to as low as feasible with cell operation to conserve energy for reduction.

Parent Case Text

RELATED APPLICATION

This application is a continuation-in-part of my patent application Ser. No. 528,503 filed Feb. 18, 1966, now U.S. Pat. No. 3,434,957.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrolytic cells and particularly to an aluminum reduction cell having partially molten aluminum conductors and a potlining of noncarbonaceous composition.

2. Description of the Prior Art

The electrolytic cell of the Hall-Heroult type in universal use employs carbonaceous linings to provide a cathode vessel or pot which is substantially, but not completely, nonreactive with the components undergoing electrolysis or resulting therefrom. Such linings are generally formed of baked carbon paste or blocks made from calcined anthracite coal or foundry coke particles mixed with coal tar pitch. It is common practice to embed iron or steel bars in the carbonaceous lining to collect the cathode current and lead it to copper or aluminum bus bars exterior to the cell. Although such carbonaceous linings have been in use for about 80 years, results have not been entirely satisfactory. Some of the drawbacks known throughout the industry include wasteful losses of heat and electric power (which increase with the age of the potlinings) uneven current density resulting in local overheating, uneven and excessive expansion and cracking, absorption of sodium causing graphitization and injury to the refractory lining and penetration of aluminum into cracks reaching the embedded iron or steel bars or casing of the cell and damaging these, also impairing the purity of the electrolyzed aluminum. The above cells have potlining lives limited to a few years when the economics make relining necessary.

The characteristic expansion of a carbonaceous cell lining requires reinforcement of the steel potshell by steel buttresses or the like having a weight and cost roughly equal to the potshell itself besides occupying more valuable potroom floor space and requiring longer pot to pot bus connections.

Many attempts have been made to obviate the need of carbonaceous linings and thus overcome the above and other disadvantages. Collector bars have been made at least in part of titanium diboride and similar compounds resistant to molten aluminum and fluorides and electrically conductive to a degree 100 or 1,000 times that of carbonaceous lining so these could project through the potlining and into the molten aluminum overlaying the potlining. With such bars the potlining theoretically need not be electrically nonconductive so that carbon may be eliminated and materials refractory to attack but not conductive of electricity may be used. However, as a practical matter refractory conductive compounds such as titanium diboride have thus far been so expensive, subject to such short life from breakage and erratic in amounts of current carried from day to day that they have not been adopted on a significant commercial scale.

Cells for reduction or refining such as described in U.S. Pat. Nos. 2,584,565, 2,685,566, and 2,866,743 suggest making connection with the molten layer utilizing metal wells or troughs connected with the cathode layer by an aperture and connected with the cathode bus by metal connectors. So far as is known none of the above have led to substantial commercial application due perhaps due to the added cost of the wells and troughs and heat losses therefrom which might exceed any electrical savings. In present commercial cells of 50,000 to 250,000 amperage capacity the electromagnetic circulation and metal pileup caused by withdrawing current in a few large cross section branching or curving cathode conductors would not be conductive to a uniform anode-cathode spacing and hence uniform current density over every square inch of the 10,000 to 50,000 square inches of fluoride fusion undergoing electrolysis. As a practical matter the very essence of reduction cell efficiency is a compact cell having uniform current density and temperature over all anode and cathode areas and low heat losses and low electrical losses which this invention provides.

SUMMARY OF THE INVENTION

This invention avoids the disadvantages of prior art by providing a multiplicity of spaced and sized conductors housed in refractory potlining with the part of the conductor adjoining the molten aluminum cathode (hereinafter for brevity called the pad) being molten and mixing therewith and the balance of the conductor being in the solid state due to the outward conductance of heat therefrom to a heat-sink such as a thick aluminum potshell or slab preferably forming the ends and sides as well as the bottom of the cell and cooled just enough by just enough circulating air on water to maintain the lowest practical and feasible voltage and temperature difference between the pad and slab to conserve the energy needed for the aluminum reduction. The aluminum slab on the relatively hotter sides and ends carries the heat (conventionally wasted to the atmosphere there) down to the cell bottom, increasing the temperature enough to oppose the flow of heat through the bottom and the flow of heat through the aluminum conductors to the slab heat-sink. Heat removal from the slab heat-sink may be regulated by controlling the circulation of a heat absorbing fluid, such as air into contact therewith. Slab temperature may be increased to as much as 600.degree. C. but usually to about 150.degree. to 400.degree. C. Over a period of years the side and end potlining exposed to the fluoride fusion usually wears thin enough so the heat release from the fusion is just enough to keep a film of fluoride fusion frozen against the sidelining sufficient to prevent further attack. By this invention the unavoidable heat loss from sides and ends is carried by the aluminum potshell or continuous aluminum slab attached to a steel potshell to the potbottom where refractory corrosion downward from the aluminum pad does not occur and increased slab temperatures enormously decreases heat losses from the cell bottom.

In addition, as a further aspect of the present invention, the potlining is a noncarbonaceous material and preferably consists of a layer of hard, impervious refractory compounds such as fused alumina and calcium fluoride or oxide beneath the cathode layer of molten aluminum. The hard, impervious potlining composition adjacent the molten pad may be underlain by a heat insulating layer having disconnected pores consisting, for example, of small, fused, hollow spheres of alumina bound together by a refractory of lower melting point such as alumina and calcium fluoride or oxide or a castable refractory such as calcium aluminate cement.

The cell of the invention includes a multiplicity of aluminum conductors depending from the aluminum pad to the aluminum slab at the bottom, each conductor being encased, at least over part of its length, in a hard refractory material. The conductors are carefully spaced and sized for withdrawing current as uniformly as possible over the entire aluminum pad cathode area, and consist preferably of upright shafts of aluminum varying from 0.1 to 3 square inches or more in cross-sectional area, the conductor or conductors providing about one square inch of conductor cross section for each square foot of pad directly beneath the anode. The lower size limit of conductor is governed by the type of hard refractory channel or tube surrounding the conductor as regards its surface tension, and ability to wet but not react with molten aluminum containing sodium. The upper limit of size is governed by the permissible intensity of electromagnetically induced swirling action where the conductor enters the pad. Too intense swirling prevents the desired temperature gradient along the conductor length. In any size of molten conductor having uniform cross section throughout its length, the current density per unit area of aluminum cross section is limited by that at which, on the high side, the electrical resistance (voltage drop) and swirling is excessive, and on the low current density side, by that at which the outward heat loss through the aluminum conductors is excessive and freezes fluoride fusion on the cell bottom. Electrical loss which is saved by making the current density lower in the aluminum conductors can be converted to the needed internal heat in the electrolyte itself by merely increasing potline amperage, and hence production, or alternately increasing the anode-cathode distance of an individual cell (raising the anodes further from the pad) and this added anode-cathode distance beneficially increases ampere efficiency (aluminum production per cell). The range of current density per square inch of aluminum conductor economically suitable to meet the above conditions usually will be found to lie between 100 and 1000 amperes and most often between 400 and 1000 depending on the length of conductor as Examples I, II, and III given herein illustrate. The aluminum slab heat-sink which preferably constitutes a continuous aluminum potshell on the order of 1 to 4 inches thick forming the bottom, ends, sides and top deckplate distributes heat so that any possibility of holes being eaten through the shell is eliminated so higher average shell temperatures are safe. In fact, the outside of the potshells may be insulated to restrict heat loss to just that desired by the potroom operators to get best results at the potline amperage carried. No sidelining has been found entirely resistant to fluoride fusions. This invention uses resistant, high-alumina refractories and in addition insures sidewall temperatures uniform enough to prevent potshell damage but high enough to allow much thinner sidelining than conventionally used and without the loss of greater amounts of heat but actually less heat. The thinner sidelinings and endlinings allow more anode area (larger anodes) thus permitting greater amperage to be carried in a cell of any given inside dimensions of width and breadth. Furthermore, the compact cells of this invention may have less depth for the reason bottom potshell temperatures may safely be higher and the heat loss is slowed by such higher temperatures. Ideally, all porous refractories (which deteriorate by absorption of fluorides) are eliminated from the interior of the compact aluminum potcell and only dense refractories used therein and porous refractories of low heat conductivity used only external to the potshell where insulating value is permanently retained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross section through a cell utilizing the invention;

FIG. 2 is a plan view of the cell of FIG. 1 but with the fused fluoride fusion and molten aluminum pad and anodes omitted for better illustration;

FIG. 3 is a longitudinal vertical cross section at 3--3 of FIG. 2;

FIG. 4 is a vertical cross section of another type of cell utilizing the invention;

FIG. 4a is a cross-sectional view taken along lines 4a--4a of FIG. 4;

FIG. 5 is an electrically downstream side view of the cell of FIG. 4;

FIG. 6 is a partial vertical cross section of a variation of the cell of FIG. 1;

FIG. 7 is an enlarged vertical cross section of one of the current conductors of the cell of FIG. 1;

FIG. 8 is an isometric view of a refractory block of the cell of FIG. 1;

FIG. 9 is a cross-sectional view of a modified embodiment of the electrical conductor of the invention;

FIG. 10 is a cross-sectional view taken along the lines 10--10 of FIG. 9; and

FIG. 11 is a cross-sectional view of another type of cell utilizing the invention with a very compact aluminum potshell externally heat insulated.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The compact cell embodiment illustrated in FIGS. 1, 2 and 3 comprises a steel potshell bottom 1 tightly attached to an aluminum slab 2 on the under bottom of the cell which may have fins 3 to extend its surface for cooling by atmospheric convection or air blowers. An aluminum slab 2a is attached to the sides 4 and 5. The electrically upstream side 4 and downstream side 5 and ends 6 and 7 may be made of steel or aluminum or preferably steel attached to aluminum slabs inside or outside the steel shell. If the potlinings do not contain carbonaceous material, the entire potshells may be made of aluminum several inches thick (as shown in the compact cell of FIG. 11) without the need of conventional steel buttresses to keep a cell from deforming. Of course, if very high shell temperatures of 300.degree. to 500.degree. C. or more are carried a steel shell outside the aluminum slab is advisable for strength. The aluminum slab equalizes temperatures and electrical potentials over the entire shell and permits heat to be dissipated as desired from the most convenient location. It also permits the heat to be converted to steam power. The steam pipes of this invention are electrically insulated from the potcells by a thin, heat conductive refractory, so that the steam can be joined from many potcells at various electrical potentials without the pipes constituting a "short circuit" hazard. Since it usually takes less than 1 gallon of water per minute evaporated to steam to recover all the waste heat possible to recover from a 100,000 ampere cell, it is very convenient to use this form of cooling on cells; although not previously feasible without proper electrical insulation. On very large cells this invention provides that the steam pipes be in direct metallic contact with the potshells to absorb highest temperature heat from them and each cell have its own high pressure steam turboelectric generator which feeds electric power back into the potline. It is theoretically possible to reduce electric power requirements in a reduction plant by about 10 percent in this manner assuming that about 25 percent of the heat now being wasted can be converted to electric power. In any case, whatever saving can be made in heat now wasted from the potshells constitutes an addition to a direct saving in voltage of about 10 percent made by this invention since cells often operate at about 4.5 volts and have potlining drops conventionally of about 0.5 volts which this invention reduces to about 0.05 volts. Of course, elimination of the waste heat from the potroom atmosphere greatly improves working conditions therein.

The potshell may be insulated against loss of heat from the molten contents by a relatively lightweight heat insulation 9 having disconnected pores which may advantageously be made of hollow spheres of fused alumina suspended in an impervious refractory of 70 percent to 80 percent calcium fluoride and the balance alumina which melts at a temperature about 700.degree. C. lower than the fused alumina spheres but about 300.degree. C. above the fluoride fusion used in reduction cells. Less positive disconnected porosity may be obtained in the fused refractories of this invention by gas bubbles released and purposely retained by rapid freezing during manufacture. The impervious fused refractories prevent absorption into the pores or spheres and thus slow loss of heat insulating qualities desired during the 10 or 20 years of potlining life sought by this invention. In FIG. 11 no porous refractory is used inside the potshell but only on the exterior where it should last indefinitely.

The bottom layer of hard, impervious refractory 10 on which the molten aluminum cathode pad 12 rests and is contained may be made of fused alumina or such fused mixture containing 70 percent to 80 percent calcium fluoride or oxide and sodium oxide (as soda ash) and the balance alumina as will melt below 1300.degree. C. compared to 2000.degree. C. for alumina and is resistant to attack by molten alumina containing sodium.

The pot sidelining of hard, impervious refractory 10 which surrounds and contains the molten aluminum fluoride fusion may be made of fused alumina or a fused mixture containing 70 percent to 80 percent of calcium fluoride or oxide and aluminum fluoride and the balance alumina which is resistant to attack by the fluoride fusions undergoing electrolysis which often contain cryolite plus an excess of aluminum fluoride necessary to the cryolite formula Na.sub.3 A1F.sub.6.

The cooled cathode bus 14 is attached to the aluminum outer potshell 2. The cathode bus is advantageously formed of flexible sheets of copper or aluminum and is connected to an anode bus 15, of the next electrically downstream cell in the potline (not shown). The anode bus 15 is supported above the potshell on adjustable jacks 16 and 17 resting on the steel deckplate which covers the sidelining. Steel stubs 18 carry the current from bus 15 to the anodes 20 and thence through the fluoride fusion 21 on which the crust 22 forms, then down into the molten aluminum cathode layer or pad 12. As shown in FIG. 6 the hard potlining refractory 10 may have bowl-shaped depressions 23 adjoining the conductor assemblies 24. As shown in FIG. 7, these assemblies each consist of a hard, fused refractory tube or coating 25 and an aluminum core conductor 26.

FIG. 4, 4a, and 5 illustrate how aluminum conductor assemblies 40 may be used in the electrically downstream side of a compact cell to carry all the current a relatively short aluminum conductor path through the pot sidelining either horizontally or inclined somewhat upward or downward from the horizontal. As shown in FIGS. 4 and 4a the molten aluminum pad should be thicker on the electrically downstream side of the cell where a metal well or trough 43 may serve to feed the electric current accumulated in the pad to the horizontal aluminum conductors 40 which lead to the cathode bus exterior to the potcell.

If desired, and particularly where the aluminum conductor is molten, the aluminum conduit may be partly filled with fused refractory rods or refractory fibers 41 of FIGS. 9 and 10 which may be formed by casting or drawing fused refractory material such as that mentioned above. The rods or fibers are laid parallel to the conduit to restrain metal circulation and to distribute the current carrying paths over a greater area of the side of the cell without greatly increasing the amount of heat lost from the cell as shown in FIGS. 4 and 4A. Such a mixture of refractory rods or fibers 41 and aluminum 42 may have as little as one-sixth the heat and electrical conductivity as a solid mass of aluminum, and more importantly, they screen out unwanted suspended solids from entering the conduit and likewise restrain metal circulation which would otherwise wear the conduit and prevent the desired high temperature-gradient therein. Where the conductor is molten, any refractory granules or pebbles mixed with the metal have the undesired feature of causing eddy current circulation of metal and local overheating. Likewise particles may bring on the "pinch" phenomenon as originally explained by Carl Hering about 1907. If the aluminum cathode connection is not attached to the cell sidewall, which acts as a heat sink since it is cooled by the atmosphere, it must be attached to an aluminum bus 27 connecting all the cathode connections of the cell together to eliminate slight voltage differences between sad individual cathode connections. This bus 27 may be an aluminum tube or attached to an aluminum tube 44 and cooled by forced air or water circulation. The heat from the anode bus 15 may be carried back by the flexible leaves 14 to the cathode bus 27 having a hollow interior duct 44 cooled by air but preferably by water which boils to produce byproduct steam with only a fraction of a gallon per minute of water being required per potcell to recover the heat as discussed previously. To cause less heat to be lost from the molten fusion the potshell temperatures are raised by restricting atmospheric airflow over the bottom, sides and ends by, shields, baffles, or the heat insulation 46 illustrated in FIG. 11. The higher the potshell temperatures used the larger are the aluminum conductors used to keep down the voltage losses within the conductors as illustrated in Example II. In FIG. 11 water is evaporated to make steam in the steel pipes 44 to which are welded steel plates 45 to pick up heat from the potshell through a thin electrical insulator but heat conductive refractory 8 which electrically insulates the potcell from the steam pipes. The thin refractory 8 can be made of a thickness necessary to carry just the steam pressure desired in the system. At temperatures up to about 200.degree. C. gasket materials are available to electrically insulate steam lines gathering steam from potcells operating at different electrical potentials.

The aluminum conductors may be housed in grooves in the fused refractory blocks 10 instead of making special conduits. Such grooves may be horizontal as grooves in the bottom lining of the potcell of FIG. 4 or be vertical in the bottom lining of the potcells of FIGS. 1, 3, 6 or 11. Rather than make grooves in the potlining blocks, the edges of adjoining blocks may be beveled or rounded to provide T-shaped or star-shaped apertures bounded by inward curves which slow circulation of the molten portion of aluminum conductor housed therein. FIG. 8 shows a block 10 with vertical groove 24a.

Where conduits are used to house the aluminum conductor, FIG. 7 illustrates in vertical cross section an aluminum conduit assembly 24 of the invention comprising a fused refractory tube or coating 25 on the aluminum conductor core 26. The fused refractory tube may be fused alumina or lower melting point fusible mixtures of the oxides, nitrides or fluorides of calcium, sodium, aluminum or magnesium which best resist attack of molten aluminum and dissolved sodium therein present in the reduction cells. Small additions of zirconium, titanium or boron may be used in the refractory mixtures to the extent refractory or wetting properties are improved. For example, titanium boride added to or coated by rubbing or application of slurries on a tube or fiber of alumina makes aluminum wet the tube or fiber better so smaller tubes or channels may be used. Other coatings which may be used are the carbides of titanium, zircon, hafnium, vanadium, columbium or tantalum. The refractory tube may be formed first and the aluminum forced therein by pressure or aluminum wire or rod may be dipped into fused refractory one or more times to acquire a coating. The assembly 24 may then be inserted into a groove 24a in the vertical side of a fused refractory block as illustrated in FIG. 8 or such grooves may simply be filled with aluminum without another refractory 25. In any case the aluminum conductor 26 extends through a hole in the steel bottom and is welded to the aluminum outer potshell 2 which is shown in FIGS. 1, 3 and 6 to make mechanical and electrical connection therewith. The lower end of the refractory tube may be protected against bending or breakage and its contained aluminum further protected against possible oxidation as well as bending and breakage by a steel pipe casing (not shown) which can be welded to the steel potshell. Any such steel pipe should be protected inside and outside by a coating such as aluminum silicate commercially sold as fiberfrax cement coating.

The aluminum slab 2 may be poured as a molten mass on the bottom (or sides if desired) of the steel potshell by turning it upside down (or on the proper side or end) for this casting operation, or the slab may be poured inside the shell as disclosed in my copending Pat. Application Ser. No. 607,330, filed Jan. 4, 1967, now U.S. Pat. No. 3,434,958. The projecting aluminum conductor 26 of each assembly 24 is thereby fused or welded into an aluminum outer or inner potshell which constitutes a heat sink. The upper end of conductor 26 extends into the molten aluminum pad and melts and mixes with it as soon as the cell is started.

Instead of the aluminum conductors being made of commercial primary grade aluminum of 99.50 percent to 99.90 percent grade and the balance mostly iron and silicon impurities, the conductors may be made of heavier aluminum alloys and preferably those which have higher melting points and higher viscosities than commercial primary aluminum. Moreover, conductors as installed in the potlining may be made of steel similar to those described and illustrated in my copending Pat. application Ser. No. 528,503 filed Feb. 18, 1966, now U.S. Pat. No. 3,434,957, but with the conductors extending upward through the potlining into the molten aluminum pad with the result that the aluminum first reduced in starting the potcell operation alloys with each steel conductor in at least the upper inch or so of its length until a stable liquid-solid interface is formed. This results in at least some part of the liquid or semiliquid portion of the conductor being an alloy of iron and considerably different in composition from the solid conductor which is principally steel. In the preferred construction, however, the conductors are the same approximate chemical composition as the primary aluminum or aluminum alloy produced in the potcell where used since thereby the aluminum produced is not contaminated, high electrical conductivity is achieved and the needed simplicity in construction and operation is secured. "Aluminum" as used herein is intended to include all aluminum base alloys.

FIGS. 9 and 10 show a variation in construction. The conductor 40 of FIGS. 9 and 10 consists of a plurality of refractory rods or fibers 41 surrounded by molten aluminum 42 and encased in a refractory tube conductor 44. This type of conductor 40 gives the added advantages that circulation of the molten metal is constrained, and granular material such as aluminum oxide and aluminum carbide may be kept from settling into the tube conductors. Such granular foreign matter tends to cause eddy currents in molten metal and thus prevents the desired temperature gradient developing in the conductor as the distance from the molten aluminum cathode increases and the distance to the heat sink of the potshell or exterior to the potshell decreases. On the other hand, refractory fibers or rods running parallel to the conductors keep the electrical current running parallel to the conductor and restrain the circulation of molten metal. The rods or fibers may be made of graphite or graphite impregnated with aluminum or made from the oxides, nitrides, borides and fluorides of aluminum, sodium, titanium and zirconium. The rods may be made principally of fused aluminum oxide which is not wet by aluminum but this can be overcome as previously explained by coatings on the aluminum oxide which will make aluminum wet it. Consequently, the wettable fibers result in the aluminum being drawn between the fibers by the capillary action of a wick. The finer fibers wet by the aluminum then practically filter out foreign particles which would otherwise tend to work down into the conductors. Aluminum nitride or boron nitride resist attack by molten aluminum and are examples of a useful refractory body for the above rods or fibers or coating thereon. The rods or fibers may be 0.01 to 0.3 inches in diameter and need be no longer than the length of the conductor which remains molten.

The refractory 10 may be made of hard, fused alumina or lower melting mixtures previously mentioned and pressure cast in graphite molds to form blocks and then grooves for conductors or made more true to size with a diamond saw. As stated above the use of grooves may eliminate the need for the refractory casing 25; and if many small grooves are used the need for fibers or rods therein is eliminated. The face of a CaF.sub.2 -A1.sub.2 0.sub.3 block cooled by casting against graphite will be higher in alumina and should be used adjacent to molten aluminum or fluoride fusion as the cooled face is more resistant to corrosion. The heat insulating refractory 9 may be alumina powder or other type of heat insulating powder or brick suitable for the temperatures and possible chemical attack involved by breakthroughs in 10 or 11 permitting molten aluminum and cryolite fusion to percolate downward. The preferred refractory 9 of this invention provides maximum permanent insulation by providing as an aggregate, fused alumina spheres and as a binder for the aggregate a castable refractory such as calcium aluminate cement of a fused refractory composed chiefly of calcium fluoride or oxide and alumina. Such fused refractory is relatively cheap to make since calcium fluoride is mined as fluorspar or calcium oxide as limestone and alumina may be provided as low-silica bauxite with iron impurities being removed in the fusion process or provided in dross from skimmings from aluminum reduction plant crucibles, holding furnaces or remelt operations after contained aluminum particles are screened out.

In the operation of cells of the invention it will generally be found desirable to carry about 3.5 to 7 amperes per square inch of current density in the anode area corresponding to about 500 to 1,000 amperes per square foot of cathode area pad laying under the anodes. An average of about one or two or more conductors per square foot will ordinarily be required to carry 100 to 1,000 amperes of current per conductor. To prevent metal circulation and pile up in the pad, conductors are confined to areas directly under the anodes and are spaced uniformly there. The conductor of aluminum carrying current from the molten aluminum at 950.degree. C. to a heat sink at 150.degree. C. is appreciably more efficient than conventional carbon as regards power losses of heat and electricity providing the conductors are properly proportioned in size and uniformly spaced as exemplified in FIG. 1. Table I shown below shows the approximate advantage of conducting electricity out of the cathode pad with aluminum instead of carbon at the different temperature zones through which the conductor must pass. ##SPC1##

In making the comparison of Table I it must not be overlooked that the conventional carbon potlining serves the double purpose of conducting cathode current from the pad as well as containing the fused contents at 950.degree. C. and that substituting aluminum for carbon as the electrical conductor outward from the molten cathode pad makes desirable substituting a suitable refractory for the carbon potlining that will have a much lower specific heat conductivity. In this respect fused alumina refractories can have a heat conductivity about equal or less than conventional fluoride saturated carbon potlining. Fused calcium fluoride-alumina mixtures containing discrete and unconnected pore spaces afforded by fused alumina bubbles can have as little as one-sixth of the heat conductivity of conventional potlining. Protected by a layer of fused alumina the porous refractories 9 should preserve heat insulating qualities for perhaps 10 or 15 years whereas porous heat insulation used beneath the carbon potlining suffers great deterioration in insulating power in 5 years due to the ready absorption of sodium and fluorides by the carbon and any conventional porous insulation backing it. The cell of FIG. 11 not having porous refractories inside the aluminum potshell eliminates absorption.

Although carbonaceous potlining is made unnecessary by this invention and is not to be desired, it will be understood that carbonaceous potlining can be used with the aluminum conductors of this invention with the resulting improvement over conventional potlinings that the electrical loss is greatly reduced between the molten cathode pad and the cathode bus. To appreciate the advantage of the aluminum conductors of this invention it must be realized that conventional potlinings suffer not only from the poorer electrical conductivity of carbon compared to aluminum shown in Table I but also in the contact drops inherent between the molten aluminum pad and carbon and between carbon and the steel collector bars as well as a further contact drop between the steel collector bar and the aluminum cathode bus. These contact drops may constitute one-third or one-half of the 0.4 to 0.6 volt drops between the molten aluminum pad and cathode bus of conventional cells. With the present invention the above total voltage drop may be only 5 percent to 10 percent of conventional values. Also this invention eliminates the highly undesirable characteristic of carbon potlinings to "hog" or concentrate current in any portion of the potlining that becomes overheated and persists since the electrical resistance of carbon decreases greatly with increasing temperature which is opposite to the behavior of most metals including aluminum whose electrical resistance increases about eight times in going from solid aluminum at room temperature to cathode pad temperature near 1000.degree. C. The value of the aluminum conductors is further illustrated in the following discussion on conductor design.

The heat conductivity of some molten aluminum between 960.degree. C. and its freezing point just below 660.degree. C. can average about 2.4 watts/inch-1/.degree. C. in comparison to 4.5 identical units in solid aluminum from about 660.degree. C. down to 160.degree. C. potshell (heat sink) temperature. As regards electrical conductivity or inversely electrical resistance (RI.sup.2 loss) this aluminum would average about 11 watts per inch of length when carrying 1,000 amperes per square inch of conductor cross section while the solid portion average about 3 watts per inch length RI.sup.2 loss. With such information it is easy for those versed in the art to calculate the length of conductor illustrated in FIG. 7 that will be molten in the end joining the pad at 960.degree. C. and the remaining length that will be frozen solid towards the outer aluminum potshell at 160.degree. C. If it is assumed that the liquid-solid interface is stable, then the heat carried down from the molten metal pad taking into account that generated by RI.sup.2 loss will be equal to that carried away from the stable interface taking into account the RI.sup.2 generated in the solid portion. If high enough current density is carried in the aluminum conductor, the RI.sup.2 heat generated in the molten portion may be made enough so temperature gradient near the upper end of the conductor is small and there is practically no heat conducted out of the pad by the conductors. However, this is rarely desirable since it does not result in the lower range of electrical losses which are more desired since heat losses can be saved by better heat insulation in the potlining or outside the potshell as described above.

The following examples are given to show the advantages and superior operating results obtained with the invention.

EXAMPLE I

A 100,000 ampere cell of this invention has 100 vertical conductors each about 1 square inch in cross section and each about 24 inches long reaching from the cathodic pad to the 2 inch thick, finned, aluminum potshell bottom maintained at about 160.degree. C. by controlled atmospheric convection. By the methods disclosed above it is computed that about 8 inches of the upper end of each conductor remains molten and the remaining lower 16 inches solidify so that practically no heat leaves the metal pad and only about 9 kilowatts of RI.sup.2 heat are carried into the finned potshell bottom. There is a total electrical RI.sup.2 loss in the molten and solid portions of the aluminum conductor corresponding to a voltage loss of 0.14 volts which is about one-third or one-fourth that common in conventional cells.

EXAMPLE II

A 100,000 ampere capacity cell of this invention is built with 200 vertical conductors each 1 inch square in cross section and 24 inches long terminating in aluminum bottom slab maintained as a heat sink initially at 160.degree. C. By calculation, about 6 inches of each conductor remains molten and the remaining 18 inches solid. The conductors as a whole carry about 22 kilowatts of heat out of the molten pad and about 23.5 into the slab heat sink. There is an RI.sup.2 heat loss in the conductors of only about 3 kilowatts corresponding to a voltage loss of about 0.03. With such a low voltage difference between molten metal and potshell there is practically no danger remaining from a crack in the potlining allowing molten metal from the pad to eat a hole through the potshell. Consequently the steel shell, which is covered on sides and ends as well as bottom with a continuous aluminum slab 3 inches thick, is brought up to a temperature of 360.degree. C. by enough baffles to restrict atmospheric convection cooling as well as by partial covering with heat insulation. By this change in heat sink temperature the length of conductor molten changes to 9 inches and only about 13.5 kilowatts of heat are carried out of the molten metal pad and about 15 delivered to the heat sink. The RI.sup.2 heat loss is increased to about 4 kilowatts corresponding to a voltage loss of 0.04 volts compared to the 0.03 initially when shell temperature was 160.degree. C. instead of 360.degree. C. Likewise there is a heat saving of about 25 percent of about 60 kilowatts initially lost by conduction through the potlining to the bottom, sides and ends. Consequently by raising the shell (heat-sink) temperature about 200.degree. C. the heat loss from the pad via the aluminum conductors is fully compensated by lower conduction losses through the potlining while the electrical saving amounts to about 10 percent of normal cell voltage of about 4.5 volts so that 10 percent more potcells are then added to a potline of this invention to add 10 percent more production from the same amount of potline power as employed by a conventional cell potline.

EXAMPLE III

A 100,000 ampere capacity cell of this invention and the type illustrated in FIG. 4 is equipped with horizontal side conductors having a length of about 10 inches and a total cross-sectional area of about 100 square inches of aluminum conductor. In operation about 3.5 inches length of a conductor is molten and 6.5 inches solid. Under these circumstances the heat lost from the pad amounts to about 15 kilowatts. There is an electrical loss of about 6 kilowatts RI.sup.2 loss corresponding to a voltage loss of about 0.06 volts from the molten pad to the heat sink. The steel shell is heat insulated sufficiently so that the 15 kilowatts of heat lost through the conductors is more than compensated, compared to conventional cells, by lower heat loss conducted through the potlining.

EXAMPLE IV

A conventional potline of 100,000 amperes having 100 cells operating in series at 4.4 volts each and thus utilizing a total of 44,000 kilowatts is converted to the process of this invention by the use of thinner walls of potlining on the sidelinings and endlinings, aluminum slabs 3 inches thick on top of the deckplates as well as on all outsides of ends, sides and the bottom of the shell, 200 aluminum conductors each having 1 square inch cross section and heat insulation outside the potshell to raise shell temperature from a previous average of 200.degree. C. to a new average of 400.degree. C. Enough greater fusion area is created so anodes of 10 percent greater cross section may be used than previously in conventional practice and the cells only average 4.0 volts at 110,000 amperes and aluminum production is increased about 10 percent with the same total amount of power as previously. No shell burnouts are experienced with the thinner sidelinings and higher amperage and shell temperatures because the thick aluminum slabs freeze all fusion and molten aluminum penetration through cracks, and because pad-slab voltage difference is only 0.04 volts.

Claims

I claim:

1. In a cell for the electrolytic reduction of aluminum from fluoride fusions having an exterior metal potshell, an interior refractory potlining for containing a reduced aluminum pad and the overlying fusion, at least one anode for supplying current to the fluoride fusion, a cathode heat-sink comprising an aluminum slab and a multiplicity of spaced and sized conductors in the refractory extending from the pad to the cathode heat-sink which is electrically connected to an anode for another cell, said conductors being molten where in contact with the pad, said refractory being nonreactive with the fusion and aluminum, and said slab being sufficiently thick to remove heat from the conductors to maintain their attached portions solid.

2. A cell as defined in claim 1 which comprises aluminum conductors encased in an electrically nonconductive refractory.

3. A cell as defined in claim 1 wherein said refractory potlining consists essentially of hard, impervious, electrically nonconductive, fused refractory.

4. A cell as defined in claim 3 wherein said refractory potlining is formed of at least one of the materials alumina, calcium fluoride, calcium oxide, aluminum nitride, aluminum fluoride, sodium oxide and mixtures thereof.

5. A cell as defined in claim 4 in which the refractory is formed essentially of a fusion of at least one of the materials dross, bauxite, fluorspar, limestone and mixtures thereof.

6. A cell as defined in claim 1 wherein the cathode heat-sink constituted at least part of the bottom of the potshell to which the aluminum conductors are attached.

7. A cell as defined in claim 6 which comprises aluminum side and end walls which are contiguous with the bottom slab and form a part of the heat-sink the aluminum of the heat-sink being of such mass as to freeze any of the fusion which may penetrate the potlining refractory adjacent the potshell covered by the aluminum.

8. A cell as defined in claim 6 in which the heat-sink is insulated to suppress the heat flow into it and has baffles restricting atmospheric convection and heat insulation exterior to the potshell.

9. A cell as defined in claim 7 in which the aluminum slab averages at least 1 inch in thickness and covers at least most parts of sides and ends and bottom so as to carry higher temperature heat in the refractory enclosing the sides of the fluoride fusion to lower temperature slab areas when they exist.

10. A cell as defined in claim 7 in which the entire potshell is essentially aluminum and only such steel as needed for reinforcement.

11. A cell as defined in claim 1 in which the current density carried in the aluminum conductors averages between 200 and 1,000 amperes per square inch of cross section.

12. A cell as defined in claim 1 in which the cross section of an aluminum conductor averages between 0.25 and 2 square inches.

13. A cell as defined in claim 1 in which the conductors are essentially located in areas of the pad beneath the anode areas and depend downward therefrom.

14. A cell as defined in claim 1 in which the number of conductors averages at least 1 for every 2 square feet of anode area.

15. The cell of claim 1 in which the heat-sink is protected against the attack of molten material in the cell by a thin, adherent refractory coating.

16. In a cell for the electrolytic reduction of aluminum from fluoride fusions having an exterior metal potshell and a bottom, a lining within the potshell for containing a reduced aluminum pad and the fluoride fusion, the improvement which comprises a first refractory lining in contact with the pad and fluoride fusion which is hard, impervious and electrically nonconductive, a second layer of heat insulating refractory underlying said first refractory lining and overlying the cell bottom, a slab of aluminum in contact with the cell bottom, and a multiplicity of spaced and sized aluminum conductors in contact with the pad which pass through both refractories and engage the aluminum slab.

17. In a cell as defined in claim 16 in which the slab is in contact with aluminum side and end portions adjacent the areas of the shell sides and ends, and means for restricting the escape of heat from the aluminum slab.

18. In the reduction of aluminum by electrolysis of a fluoride fusion overlying a reduced aluminum pad wherein cathode current is withdrawn in a multiplicity of conductors in contact with the pad and terminating in an aluminum slab heat-sink the improved process which comprises restricting the escape of heat from the heat-sink sufficiently that there is less than 20 kilowatts of heat removed from the pad by the conductors.

19. The process of operating the cell of claim 1 in which the voltage difference between the aluminum slab and the pad is less than 0.2 volt.

20. The process of operating the cell of claim 1 in which the temperature of the heat-sink in at least some parts exceeds 200.degree. C.

21. The process of operating the cell claim 1 in which at least part of the heat from the heat-sink is utilized to evaporate water into steam.

22. The combination of cells for the type defined in claim 1 which comprises at least two cells arranged side-by-side the longitudinal sides of which are close together, each cell having a plurality of anodes transversally spanning the cell and being supported on means providing upward and downward movement whereby each anode is adjustable to unify current distribution, and a separate flexible bus connecting the cathode heat-sink of one cell to the nearest anodes transversally spanning the next cell.