US3034972A - Electrolytic production of aluminum - Google Patents

Description

May 15, 1962 R. A. LEws ELECTROLYTIC PRODUCTION OF ALUMINUM Filed March 28, 1958 I v I INVENTOR.

ROBERT A. LE W/S BY &2:

ATTORNEV 3,@34372 Patented May 15, 1962 This invention relates to` the production of aluminum by electrolytic reduction of an aluminum-containing compound, e.g. alumina. More particularly, the invention relates to an improved method for the electrolytic production of aluminum.

The production of aluminum by electrolysis of an aluminum-containing compound, e.g. alumina, dissolved in a molten salt electrolyte, e.g. cryolite, and deposition at the cathode is a very o ld process. The alumina is broken down into its components; the oxygen is liberated at the anode and the aluminum -is deposted at the cathode, which forms the bottom of the electrolytic cell. In practice, the pool of molten aluminum which is ormed in the bottom portion of the cell in eftect constitutes the cathode of the cell. Conventionally, use has been made of two types of electrolytic cells, namely, that commonly referred to as a pre-bake cell and that eornmonly referred to as a Soderberg cell. With either cell, the reduction process involves precisely the same chemical reactions. The principal difference between the two cells is one of structure. In the pre-bake cell the carbon anodes are prebaked before being installed in the cell, while in the Soderberg cell or continuous anode cell the anode is baked in situ, that is, it is baked during operation of the electrolytc cell, thereby utilizing part of the heat generated by the 'reduction process.

Conventionally, the total voltage drop across the cell is about 4.5 to about volts with a voltage drop between the anode and cathode of about 3.5 to about 4 volts. The Voltage drop between the anode and cathode is composed of (1) the voltage required to overcome the ohmic resistance of the electrolyte and (2) the sum of the reversible decompositon voltage required for the cell reaction and the polarization voltages. The voltage required, eg., in (l) above may be about 2 volts While the voltage of (2) may be about 1.7 volts. The remainder of the cell voltage is required to overcome the resistance of the lining, the external conductors and connections, the anode, anode effects, and various contact resistances. Power Consumption per pound of aluminum produced generally falls within the range of `from about 7.5 to 8.5 kilowatt-hours (lswh).

Although the theory of the electrolysis of alumina in molten cryolite has not been fully developed, a considerable amount of knowledge has been gained over the past seventy years in rega'd to the electrochemical factors, such as voltage relationships and current efliciency, and the thermal aspects, such as heat generation and heat dissipation, of the process. The knowledge of these factors has been essential in the development of the electrolysis of alumina into a commercially feasible process. The thermal aspects, in particular, are of utmost importance for they determine the size, the design and the operational conditions of the reduction cell.

The current efliciency (or Faraday effieiency) of the aluminum reduction process is only about 80 to 88% of the theoretical. This relatively low efiiciency is due to a number of causes, particularly the formation of a "metal tog" at the aluminum cathode and recombination of the anodic and cathodic products, which are brought together by stirring and difl usion. The formation and reoxidation of cathodic metal tog are more active the higher the 'temperature of the metal and the electrolyte, and the current efficiency thus becomes lower. Oonsequently, the

highest efiicency is favored by the lowest possible temperature of the molten electrolyte consistent with other requirements, such as the solublity of alumn-a in the electrolyte and general workability of the cell. In order to maintain low electrolyte temperatures and prevent the excessive generation of hea t, it is necessary to operate reduction cells employing conventional electrolytes at low current densities.

The minimum power required to produce alurninum metal may be calculated by the following equation:

where P=the power invkw.

% C.E.=the percent current efi'ciency of the operation of the cell 2.*O4=the voltage required to sustain the cell reaction I=cell current in amperes The voltage required to sustan the reaction is calculated, utilizing the beat of the overall cell reaction, by the following equation:

Where njF E=the Voltage required to sustan the cell reaction AH-:the heat of reaction in calories n=the valence change or number of Chemical equivalents of substance reacting j=constant for changing joules to calories=0.239

F=96,50O coulombs of electricity passing through the cell for each Chemical equivalent of substance.

Equation 2 may be expressed by AH (3) 23,070n

The overall cell reaction in the reduction of alumina is as follows:

( 2A12Oa+3C asc O2+4A1 (room (room (cell (cell p temp.) temp.) temp.)

The heat of reaction evolved in Equation 4 is expressed by:

Assuming a cell temperature of 975 C. and room temperature of 2S C.

AH=4AH I+3AH -2AH 1 0 -3AH AH =AH +sensible heat (room temp. to cell The power efliciency, that is, the percentage of the E: 2.04 volts %C.E. 2.041( 100 1s0 where V=total voltage drop I V=total power into the cell In Equation 6, assuming a current etficiency of about 80 to 85% and a voltage drop across the cell of about 4.5 to 5 volts, which figures are representative for operation in the aluminum reduction industry, it is seen that the power eficiency is on the order of 30 to 35%. Consequently, the major part of the total power supplied to the cell is not utilized in producing aluminum.

In order to improve the power efiiciency, a reduction of the heat losses caused by the various ohmic resistances in the cells is necessary. Consequently, many studies of the heat balance, that is, the heat generated and heat dssipated, of alumnum reduction cells have been-made.

Much experimenting has been done in an effort to decrease the resistances (and herce improve thepower eiiiciency) of the various elements of the call, particularly the electrolyte which accounts for about tWo-thirds of the total heat generated. Only a portion of the heat generated in the electrolyte is essential in heating and maintaining the electrolyte at the Operating temperature. The rest of the generated heat must be dissipated somehow in order to prevent the cell from running at too high a temperature which would reduce the current efficiency.

In an efiort to reduce the heat generated in the electrolyte, prior work has been drected to lower Operating temperatures of the electrolyte and increased electrolyte conductivity; the desired result being decreased cell voltages giving lower power consumption per pound of aluminum metal produced, as distinguished from increased productive Capacity of the cell. A lower limit to Operating temperature is fixed -by the liquidus temperature of the cryolite electrolyte. The liquidus of cryolite is lowered by additions of sodium fluoride, aluminun fluoride, and alurnina. Sodium fluoride additions are undesirable in that they promote the formation of cathodic "metal fog," and it is therefore usual practice to operate with electrolytes containing an analytical excess of aluminum fiuoride. However, aluminum fluoride, due to its volatilty, can be used only in restricted amounts, e.g. up to in excess of the stoichiometric amount of aluminurn fluoride in cryolite. Therefore, calcium fiuoride has been used in amounts of up to l2.5% to further lower the liquidus which has been partly lowered by the addition of aluminum fluoride. The practical limit of solublity of alumina is about 8% and, also, the alumina content diminishes as the electrolysis proceeds. However, the addition of calciurn fiuoride as well as the additions of aluminum fluoride and alumina, undesirably reduce the conductivity of the electrolyte. Furthermore, ealeium fluorde undesirably reduces the density differential between the electrolyte and the molten aluminum.

It was recognized in the prior art that some increase in current elficiency and decrease in carbon Consumption should result if the Operating temperatures could be lowered. consequently, the prior art is replete with suggestions or proposals for modification of the basic electrolyte composition in order to accomplish this objective. In general, these proposals have been to add other alkali fluorides, cryolites, or chlorides as an equivalent or substitute for the sodium fluoride or sodium cryolite. It has been proposed to use sodium chloride as an additive or as a substitute for part of the sodium fluoride of the conventional electrolyte. Also, it has been proposed to use potassium fluoride or cryolite and lithium fluoride or cryolite as a substitute for the sodium fluoride or sodium cryolite constituent of the electrolyte or as an additive thereto. It should also be noted that in an attempt to reduce heat generation of the cell, in order to increase power efiiciency, there has been a steady reduction in the industry in current density used in operation of electrolytic cells.

The usual electrolyte composition consists essentially of a mixture of the fluorides of sodium and aluminum in which alumina is dissolved. Generally speaking, it may be said that most electrolyte compositions consist essentially of cryolite and alumina and fall within the ranges, in percent by weight of electrolyte, of sodium cryolite (Na AlF to alum'num fiuoride (AlF )-O to 10%, calcium fiuorde (CaF )-5 to and alumina (Al O -2 to 6%. In operation, the electrolyte composition is generally restricted to a sodium fiuoride to aluminum fiuoride weight ratio in the range of between 1.20 to 1.50 and the calcium fluoride content restricted to an amount within the range of 7 to 9%.

The freezing point of electrolyte compositions in the ranges set forth above is on the order of about 950 C. to 960 C. In operation, the temperature of the electrolyte generally is maintained above about 965 C. to avoid Operating difficulties due to excessive freezing and thickening of the electrolyte crust which forms over the rnolten electrolyte and the occurrence of ledges covering the interor sides and part of the bottom of the lning. Operating temperatures on the order of 965 C. to 975 C. are generally used in the industry today. Higher temperatures present difliculties due to reduced current eficiency resulting from two factors. One factor is the effect of temperature itself which reduces the current efliciency as temperature is increased. The other factor is the interaction of electrolyte and aluminum metal with the carbon side walls of the cell lining and stray currents to the side walls which result when the frozen electrolyte ledge covering the side walls melts off as the temperature increases. consequently, practical considerations of maximum current eflciency dictate that reduction cells be operated with the electrolyte being at a temperature of not more than about 10 to 15 C. above the freezing point. This results in a rather thick crust forming over the top surface of the electrolyte and ledges forming over the sides and partially over the bottom of the cell lining, all of which tend to limit the heat dissipation from the cell itself. This in turn, limits the power input to the cell in order to prevent the operating temperatures from rising above the desired level. This means that there is a definite limit to the current density which can be used in practice for a given cell design which in turn limits the productive capacty of the cell in question.

The voltage drop in the electrolyte, caused by the ohmic resistance of the electrolyte and by which the corresponding resistance heat generation can b measured, is expressed by the equations:

where R =the resistance of the electrolyte and (s V= I=-ld a where r=the specific resistivity of the molten electrolyte l=the anode-cathode distance zz=average cross-sectional area of electrolytic path over distance l d average current density in the electrolyte In Equation 8 all of the factors are theoretically varable, but from a practical standpoint only d, current density, may be varied a considerable amount. The temperature cannot be increased for the purpose of reducing the specific resistivity because, as stated previously, an increase in temperature will result in lower current efficiency. The reduction of anode-cathode distance, l, would result in increased reoxidation of the cathodic metal. consequently,

the current density becomes the basic characteristic of the cell, and a reduction of current density has been the means used for reducing the heat generated in the cell;

In View of the numerous and complex interrelated factors affecting the operation of an altuninun reduction cell, it is only natural that the philosophy of those engaged in the reducton field has been to limit the current densityin order to reduce the heat generated in the cell, thereby improving the current efiiciency and the power elficiency. This course of action, although it directly limits the productive capacity of the cell, has been deemed expedient and necessary in order to have economic harmony among the many interrelated factors of the cell. In order to increase the production of aluminum, cells of larger and larger dimensions are being built. With cells of these larger dirnensions, the problem of power efficiency becomes greater. The rate of heat generation of a cell increases faster with increase in size than its ability to dissipate heat for the same electrolyte temperature. Consequently, under the prior mode of operation, further reductions in current density are`.mandatory in order not to generate excess heat in the electrolyte, even though this measure is at the expense of cell productivity.

Accordingly, it is the prmary purpose and object of this invention to provide an improved method for Operating an electrolytic cell for the production of aluminum which results in a substantial increase in the productive capacity or output of the cell.

A further object of this invention is to provide an improved method for the electrolytic production of aluminum metal Wherein the electrolytic cell productive capacity is increased with little or no increase in power Consumption per pound of aluminum metal produced and With a decrease in capital investment, maintenance, administration, and labor costs per pound of aluminum metal produced.

A further object of this invention is to provide an improved method or the electrolytic production of aluminum which results in better heat dissipation, greater current efficency and lower carbon Consumption per pound of aluminum produced.

Other objects and advantages of the present invention will be apparent from the following detailed description.

According to the present invention, it has been found that an electrolytic cell Operating with a given Voltage drop across the cell can be operated with significantly greater productive `Capacity or output and with better heat dissipation, greater current efiiciency, and lower carbon consumption by carrying out the reducton of the aluminum-containing compound, eg. alumina, by a method Wherein a molten salt electrolyte consisting essentially of cryolite and alumina is modified by the presence of or containing a predetermined amo-unt of a lithium-containing compound and wheren use is made of a substantally increased current or, in other Words, Where there is established a current flow so that the resultant voltage drop across the cell is maintained at least substantially as great as that existing in the cell devoid of the lithium addition to the electrolyte. The power 'Consumption per pound of aluminum produced 'generally is substantially the same as that for conventional cells. However, the present invention also contemplates in some instances using a cell voltage and power Consumption greater than that of conventional cells but wherein the extent of pro-fit from increased production far eXceeds the added cost of power.

An even greater increase in produotive capacity of electrolytic reduction cells can be achieved by, in addition to the above features, passing electric current through such electrolyte from the anode to at least one cathodic currentconducting element exposed to the pool of molten aluminum at the base of the cell, which pool in effect constitutes the cathode, and wherein at least that portion of such element in 'contact with the molten aluminum consists essentially of a material possessing a low electrical resistivity, a low solubilty in molten aluminum and molten electrolyte under cell Operating conditions, is wettable by molten aluminum under cell Operating conditions, and has good stability under the conditions existing at the cathode of the cell. It has been found that the carbides and borides of titanium, zirconium, tautalum and niobium and mixtures thereof, eXhibit all or substantially all of the properties hereinabove set forth. These compounds are referred to as refractory hard metals. If desired, the who-le of the current-conducting element may consist essentially of one or more of such materials.

The expression consists essentially, as used hereinafter in the specification and claims, means that the portion of the element made of one or more of the carbides and :borides referred to above does not contain other substances in amounts sufficient materially to aflect the desirable characteristics of the current-conducting element, although other substances may .be present in minor amounts which do not materially affect such desirable char acteristics, for example, small proportions of oxygen, iron, or nitrogen in titanium boride. It is preferred that the refractory materials in the current-conducting elements be essentially free from elements or compounds which Would lead to undesirable contamination of the aluminum produced. Nevertheless, the current-conducting elements may contain initially, among others, certain compounds which dissolve out when the element is first put into use and which do not materially afiect the element.

esrably, that portion of the element consisting essentially of one or more of the above mentioned refractory materials should be composed of at least by Weight of such materials. The elements have been found to possess a relatively high electrical conductivity (substantially better than that of canbon), a good resistance to attack by molten electrolyte, a very low solubility in molten aluminum at cell Operating temperatures, are Wet by molten aluminum under cell Operating conditions, and have a resistance to oxidation considerably better than that of carbon. Such elements can he produced in suitable forms With good mechanical properties.

The modification of the electrolyte or bath by the presence of lithiurn can be accomplished by an addition or incorpor-ation of a suitable lithiu-m-containing material to the conventional molten electrolyte in the cell or as a substitnte for calcium fiuon'de or soclium uoride or both. Alternatively, the lithium-containing material may be incorporated by making up the electrolyte completely outside of the cell. By practice of the instant invention the oalcium fluoride can be maintained at a minium and in the preferred operation of the invention the electrolyte should not include intentionally added calciurn fluon'de.

According to the invention, lithium should he present in the electrolyte in an amount equal to that resulting from the addition of lithiurn fluoride in an amount from about 2% to 20% by Weight of the molten electrolyte, preferably from about 3 to 8%. The lithium material may take the form of various compounds which are compatible With the other electrolyte con-stituents and do not introduce excessive amounts of impurities into the cell. For example, use can be made of lithium fluoride, lithium aluminum fluoride (lithiurn cryolite), lithium canbonate, lithium hydroxide, or lithium aluminate. In providing the necessary lithium content in the electrolyte, such compounds can be used singly or in combirration. Regardless of the lithium compound used from the above list, the end result in the molten electrolyte, as far as composition is concerned, will be the same. Although various lithium-containing compourds :are suitable. for purposes of the present invention, from the standpont of handling, moisture pick-up, gas evolution, etc., the presently preferred source materials are lithium fluoride, lithium cryolite, and lithium aluminate.

As discussed hereinbefore, the present invention invoives the presence of lithium in the electrolyte in a predetermiued amount coupled with maintenance of a voltage drop across the cell at least substantially equal to that existing in the cell devoid of the lithium content in the electrolyte. Or, stated another way, the present invention utilizes a substantially increased current so that the resultant voltage drop is at least substantially as great as the first named voltage drop of the cell devoid of the lithium content. The current is maintained at greater than a increase over normal operation, that is, operation of the cell devoid of the lithium content in the electrolyte. The anode current density used in the operation of the conventional pre-bake cells is in the range of from about 6.0 to 8.0 arnperes per square inch and in the case of the conventional Soderberg cells, in the range of fromabout 4.5 to 6.5 amperes per square inch. The anode current density determinations cited herein are calculated by dividing the current supplied to the cell by the nominal bottom\ area of the anodes.

The nanges given above for current densities used in conventional cells `are due to the size and heat balances of various cells. Smaller cells in general operate at higher current densities than do larger cells because of better heat dissipation. With the use of the lithium addition of the instant invention, the current density (and the current) for any given cell can be maintained at greater than a 5% increase in order to increase the pro ductive capacity. It appears possible, through practice of the instant invention, that the increase in current density (and current) may be 25% or higher.

Practice of the invention has produced important and unexpected results in operation of aluminum electrolytic reduction cells, the most important being that of producing significant increases in production capacity of a conventional reduction cell, eig. 12% increase in production per cell. Still greater increases in productive capacity have been found, according to the invention, when use is also made of cathodic current-conducting elements made of refractory hard metals, as discussed hereinbefore, e.g. 25% increase or greater, in productive capacity.

Moreover, the presence of lithium ion in the electrolyte results in little effect on the deposition efficiency for aluminum. Any adverse efiect is more than oil-set by the corresponding decrease in Operating temperature made possible by the lower freezing point of the electrolyte resulting from the addition of lithium. Moreover, lithium shows no signs of increasing attack on the lining.

The significant increase in productive Capacity of the reduction cell is believed primarily due to the lower electrolyte resistance and to an increase in heat dissipation of the cell. Moreover, it is believed that the increased heat dissipitation of the cell is due to less bottom and side ledging :and a thinner top crust.

It is to be noted that the initial freezing points of the lithium-'containing electrolytes are substantially lower than those of the conventional electrolytes. The freezing point of an electrolyte containing 3% lithium fiuoride is about 940 C., while an electrolyte containing 7% lithium fiuoride has a freezing point of about 890 C. Therefore, with the lithium addition it is practical to use a lower Operating temperature. It has been found that with an electrolyte consisting of 34% lithium cryolite (corresponding to about 165% lithium fiuoride), 65% sodium cryolite and 1% alumina, operation of the cell could be car'ied out at 845 C. With an electrolyte containing 15% lithium cryolite (corresponding to about 7 .5% of lithium fiuoride), it was found that the freezing point was such that operation of the cell could be carried out at 910 C. At the lower temperatures possible with the higher lithium additions, the alumina solubility and solution rate is decreased; however, with the improvement of feeding techniques these higher additions will be of considerable value to the reduction operation. Utilizing a lithium addition corresponding to 3 to 8% lithium fiuoride, the cell Operating temperature nay be from about 910 C. to 955 C.

The amount of super-heat (that is, the heat in excess of that required for melting) in the electrolyte of the invention, While still maintaining a lower Operating temperature, may be substantially greater than that existing in a conventional cell This may account at least in part for less bottom ledging, lower cathode resistance and thinner top crust permitting better disspation of heat. In addition, the lower Operating temperature of reduction cells according to the present -invention results in a lower net carbon consumption from anodes per pound of aluminum produced. It will be understood that the above discussion is in no way to be a limitation on the invention but is given merely as a possible explanation for the significant results achieved -by the present invention.

It has been found also that because of the marked etfect of lithium on the freezing point and electrical conductivity of the electrolyte, it is possible, by making relatively small changes in `lithium content, to fully compensate for difierences in -heat dssip'ation with age of a given reduction cell in a cell line. This can be of substantial significance in reduction cell line operation in compensating for irregularities and poor current elficiency oftentimes encountered in individual cells in a given line of cells, particularly with regard to new cells placed in operation.

Also, the lithium additions to the electrolyte elfect a greater density differential between the electrolyte and the aluminum metal. A greater density differential between the electrolyle and the aluminum metal reduces the tendency for metal to rise into the electrolyte under forces caused 'by electromagnetic fields, discharge of anode gases, and thermal convection. This not only allows for better se'paration of the metal from the electrolyte but also has a clamping effect on the tunbulence induced in the metal and the electrolye by the magnetic fields resulting from the heavy electrical Currents. Turbulence from electromagnetic forces, which becomes particularly acute in large size cells, results in reoxidation of the metal which will seriously impair the current efliciency. With the clamping of the turbulence by the lithium-containing electrolyte, the working distance, that is, the anode-Cathode distance, can be decreased.

For purposes of further understanding the invention, Table I below sets fcrth comparative, average Operating data between two groups of nine conventional horizontalstud Soderberg cell-s in a commercial operation, one group Operating with and one group without the presence of lithium in the electrolyte or bath.

Table I Horizontal Horizontal Stud Cclls Stud Cclls (N 0 Li) (XVith Li) No. of Calls 9 9 Perigd:

ays 77 77 Pot Days 638 632 Total Production, Lbs 660, 568 708, 639 Production, Lbs. per cell daym 1, 035 1 2 Increase in Produstion, Perccnt S. 2 Lnmg Material... Carbon carbon Current, Amperes 114 72 236 Current Increase, Percent.. Y G Voltage Drop Across Cell 5. 06 5. 03 Voltage Drop Between Anodc and Cathodc 3. 70 3. 64 Anode Current Density, Amps/sq, in 5. 94 0. 3 Bath Temperatura, C 970 9.51 Bath Ratio (Nai /AIF@--- 1.33 1.33 Bath Frcezing Point, C.. 957 930 Bath, Pcrccnt Cam." 6. 5 1 3. 9 Bath, Percent AlaO 5. 5 5. 5 Bath, Perccnt LiF (Source oi L 4 35 Li CO Anode-Cathode Distanca, Inchcs 1. 2 1. Lbs., C/Lb. A l 0. 521 0. 508 Current Eeiency, Perccnt.. 85. 6 87. 5 K.w.h. per Lb. Al 7. 7. 78 Cathode Voltage Drop, Volts 0. 574 0.610 Bath Depth, Inches 5 5 Metal Depth, Inches 10 10 CaFz not intcntionally added-introduccd into electrolyte as impurities from raw materials.

It will -be apparentfrom the above table that the present invention, as applied to conventional electrolytic reduction cells for the production of aluminum, results in a Very significant increase in the productive Capacity' of the cell and, also, a significant decrease in the consumption of canbon per pound of aluminum produced as well as other advantages.

Calculating the power efficiencies by use of Equation 6 above, the percent power efiiciency of the Sodenberg cells in Table I Operating without the lithium electrolyte 34.6% while those Operating with the lithium bath is 362% a substantial improvement.

Table I'I records data which shows a comparison between pre-bake cells, and without the presence of lithium in the electrolyte.

Table Il Pre-Bake Pre-Bake Cell Cell (No Li) (With Li) No. of Cells 1 1 Period, Days 252. 6 251.4 Total Production, Lbs 37, 450. 8 41, 742. 8 Production, Lbs. per Cell Day.. 148. 2 166. Increase in Production, Pereent 12 Lining Material Carbon Carbon Current, Amperes 9, 716 10, 783 Current Increase, Percen 11 Voltage Drop Across Cell-- 5. 18 5. 11 Voltage Drop Between Anode and Cathode 4.01 4. 13 Anoda Current Density, Amps/sq. in 7. 64 8. 48 Bath Temperature, 974 961 Bath Ratio, NaF/A1F 1. 39 1. 42 Bath Freezing Point, C 965 942 Bath, Pereent CaF2. 7. 7 I 2. 8 Bath, Pereent Alaoa (Average) 3, 0 2.1 Bath, Pereent LiF (Source of LiF) 4. 93 (LiF, LOH and LiCO Anoda-cathode Distanee, Inches 2. 11 2.15 Lbs., C/Lb. Al 0. 473 0. 450 Current Eifieiency, Percet 86. 0 86.8 K w.h. Per Lb. 8. 15 7. 96 Cathode Voltage Drop, Volts- 0. 68 0. 457 Bath Depth, Inches 6. 7 7.0 Metal Depth, Inehes 5 5. 1

1 CaF not intentioally added-introduced into electrolyte as impnriies rrom raw materials.

From Table II it is seen that with the practice of the invention a substantial increase in productive Capacity is realized. Also, a sgnificant decrease in the Consumption of carbon is realized. The power efficiencies (based on Equation 6 above) are 338% for the cell Operating without lithium and 34.6% for the cell Operating with lithium.

It is not the principal aim of the present invention, however, to improve the power efficiency of the reduction operation. There are instances in the practice of the invention wherein the power efcency may be less than that of the conventional cell operation, but the sacrifice of power eiciency in these cases with the use of the present invention will be more than off-set by the increase in productive Capacity.

As set forth hereinabove, it has also been found, according to the invention, that the use of the lithium-containing electrolyte combined With use of a cathodic current-conductng element or elements of the refractory hard metals, described above, results in still further significant increases in productive capacity of the aluminum reduction cell.

One embodiment of a reduction cell suitable for carryingout the method of this invention is shown schematically in the drawing. In this embodiment, 1 is a metal shell, generally steel, within which is disposed in the usual manner an insulating layer 2 which can be any desired material, e.g. alumina, bauxite, clay, aluminum silicate brick, etc. Within the insulating layer 2 is disposed cell lining 3 which can be of any desired material, eg., carbon, alum'na, fused alumina, silicon carbide, silicon nitride bonded silicon carbide or other desired materials. Most commonly the lining is made up of a plurality of carbon blocks or is a rammed carbon mixture or a combination of a ramrned carbon rniXture for the bottom of the -lining with side and end walls constructed of blocks of carbon. Alternatively, the side and end walls can be constructed of blocks of silicon carbide or other suitable refractory. The -lining 3 defines a chamber which contains a pool of molten aluminum 4 and a body of molten electrolyte or bath 5, as described.

When carrying out the method of this invention and at the time when aluminum is being produced, electrolyte 5 and aluminum pool 4 are both in the molten state. Suspended from above the electrolyte, and partially immersed therein, is anode 6 of the conventional carbon type and which can be either of the "pre-bake" or Soderberg (self-baking) type known to the art. Molten electrolyte 5 is covered by a crust 7 which consists essentially of frozen electrolyte constituents and additional alumina. As alumina is consumed in electrolyte 5, the frozen crust is broken and more alumina fed into the electrolyte.

The anode is connected by suitable means (not shown) to the positive pole of a source of supply of electrolyzing current. For purposes of completing the electric circuit use is made of cathodic current-conducting elements 8. The elements 8 extend through suitable openings provided in the metal shell insulation layer and lining with the inner end thereof projecting into the pool of molten aluminum. The outer ends of such elements are connected by suitable means to negative bus-bars 9.

It will be understood that the drawing is but one of many apparatus embodiments using such cathodic currentconducting elements that can be used for carrying out the method of this invention. For example, other modifications involve the current-conducting elements projecting upwardly from the base of the cell lining with the upper end of such elements projecting into the molten aluminum pool or wherein the elements extend downwardly from above the cell through the electrolyte and with the lower end of the elements immersed in the pool of molten aluminum. In addition, it is not necessary that the elements project completely through the lining, insulating layer and metal shell of the cell structure. Instead, such elements may terminate short of the metal shell or insulation lining and suitable electrical connection means made between the outer end of the element and the negative bus bar. Moreover, as mentionecl hererbefore, such elements may be made up entirely of the refractory hard metal, as described, or only in part, it being essential that that portion of the surface of the end of the element which is in contact with the pool of molten aluminum and electrolyte consists essentially of such materials. Furthermore, the negative currentconducting elements can take the form of a sheet, plate, or other suitable shape and, also, Where in efiect it would function as the cathode of the electrolytic cell in place of the pool of molten aluminum.

Examples of practice of the present invention, and involving use of a reduction cell structure similar to that schernatically shown in the drawings, are set forth in Table III below. In Table III data are given for two pre-bake cells (A and B) employing refractory hard metals as cathodic current-conducting elements and having lithium present in the electrolyte. For comparison, Operating data are given for a prebake cell not employing refractory hard metal cathodic current-conducting elements and not involving a lithium-containing electrolyte.

Table lll Prebake Prebake Cell Prcbake Cell Cell Reiractory Refractory (No Ll) Hard Metal Hard Metal Cathodic Cathodic Elements 1 Elements 1 (With Li) (With Li) No. oi Cells. 1 1 1 Period, Days 140. G 144. 143 Total Production, Lbs 19, 587 27, 725. 4 27, 105. 2 Production, LbsJCell Day 139. 3 191. 9 189. 5 Increase in Production, Percent.. 37.8 36 Llning Material Carbon Bottom-Car- Bottom-Sic bon Wall-C Walls-SIC Current, Amperes 9, 366 ll, 895 11,806 Current Increase, Pereent 27 26 Voltage Drop Across Cell, Volts 5. 46 5. 02 Voltage Drop Between Anode and Cathode,

Volts 34 93 4. 73 4. 27 Anode Current Density, Amps /s 74 37 9. 35 9. 30 Batl Tenipemture, C 974 941 946 Bath Ratio, NaF/AIFL 1.49 1.32 1.32 Both Freezing Point, 0. 963 937 937 Both, Percent CaF 7. 48 2 2. 42 2 2. 5 Bach, Percent AlzOa (Average). 3. 76 2. 48 2.49 Both, Perccnt LiF (Source of LlF) 4. 97 4. 97

(LlF) (LiF) Anode-Cathode Distanca, Inches 2. 12 2.05 2. 08 Lbs., C/Lb. Al 0. 482 0.407 0. 423 Current Efliciency, Percent.. 84, 7 90. 9 90. 5 K.w.h. per Lb. Al 8. 19 8.13 7. 51 Cathode Voltage Drop, Volts. 0. 679 0. 269 0. 208 Batl Depth, lnches 6. 8 6, 5 6. 5 Metal Depth, Inches 5. 4 5. 3 6.2

Carbon-0.20%.

2 OaFz not intentionally added-introduced into electrolyte as impuritics from raw materlals.

lt is thus seen from a comparison of the examples in Table IlI that the productive Capacity of the cell is vastly increased, the carbon Consumption per pound of aluminum produced is reduced, the current eificiency increased, and wherein the power per pound of aluminum produced is substantially the same as that resulting from operation of a conventional electrolytic reduction cell.

The power efliciences of the examples in Table III are 34%, 34% and 368% for the prebake cell without refractory hard metal cathodc current-conducting elements, Cell A, and Cell B, respectively.

Although the invention is not so limited, it will be noted by way of the examples heretofore discussed that the cell voltage drops are within the range of about 4.5 to 5.5 volts.

lt Will be apparent from the above description that by practice of the present invention sgnificant increases in productive Capacity or output can be accomplished with existing conventional electrolytic reduction cells, eg., prebakd cells and Soderberg cells of either the horizontal stud or Vertical stud type, wherein power consumption per pound of aluminum produced is substantially the same coupled with lower net carbon Consumption per pound of aluminum and higher cell current and power efllciencies. Moreover, by modification of the cell structure to include the provision of cathodic current-conducting elements, as described, still further increases in productive capacity can be accomplished. In addition to conventional electrolytic cells, the invention s also applicable to other electrolytic cells, for example a multiple-cell aluminum reduction furnace having inclined bipolar electrodes as disclosed in French Patent l,1l9,832 and a cell employing sloping cathode walls as disclosed in French Patent l,ll9,82i. Manifestly, from these significant increases in productive Capacity or output stern important economic advantages. Capital investment in electrolytic furnaces per pound of aluminum produced will be decreased. Also, labor, maintenance, and administration costs per pound of aluminum produced will be substantially reduced.

lt will be understood that various changes, modifications and altcrations may be made in the instant inventi on without departing from the spirit and scope thereof and, `as such, the invention is not to be limited except by the appended claims, wherein What is claimed is:

1. The method of increasing the output of an electrolytic aluminum reduction cell, which cell comprises a carbon anode, a molten aluminum cathode and a molten salt electrolyte conssting essentially of cryolite and alumina, and in the operation of which current is passed from said anode through said electrolyte to said cathode with a given voltage drop across the cell, which method comprises incorporating lithium-containing material in said electrolyte in an amount to provide in said electrolyte a lithium content equal to that resulting from an addition of lithium fluoride in an amount from about 2 to 20 percent by weight of the electrolyte, and establishing a current flow so that the resultant voltage drop across the cell is at least substantially as great as said first named voltage drop, the temperature of the lithium-modified electrolyte being above about 845 C.

2. Method according to claim 1 wherein the lithium content of the lithium-containing material incorporated in the electrolyte is equal to that resulting from the addition of lithium fluorde in an amount from about 3 to 8% by weight of the electrolyte.

3. Method according to claim 1 wherein said lithiumcontaining material is at least one substance selected from the group consisting of lithium fluoride, lithium cryolite, lithium aluminate, lithium carbonate and lithium hydroxide.

4. The method of increasing the output of an electrolytc aluminum reduction cell which cell comprises a carbon anode, a solid cathodic current-conducting element wherein at least the operative surface thereof consists essentially of at least one of the materials selected from the group consisting of the borides and carbides of titanium, zirconium, tantalum and niobium, a pool of molten aluminum and a molten salt electrolyte consisting essentially of crylolite and alumina, and in the operation of which current is passed from said anode through said electrolyte to said element with a given voltage drop across the cell, which method comprises incorporating &034,972

lithium-containing material in said elect'olyte in an amount to provide in said electrolyte a ljthium content equal to that resulting from an addition of lithium fluoride in an amount from about 2 to 20% by Weight of the electrolyte and establishing a current flow so that the voltage drop is at least substantially as great as said first named voltage drop, the temperature of the lithummodified electrolyte being above about 845 C.

5. Method according to claim 4 wherein the lithium content of the lithium-containing material incorporated in the electrolyte is equal to` that resulting from the addition of lithium fiuoide in an amount from about 3 to 8% by weight of the electrolyte.

6. A method of increasing the output of an electrolytic alumirum reduction cell comprising the steps of passing a current from a carbon anode through a m'olten salt electrolyte consistng essentially of cryolite and alumina, and to a molten aluminurn cathode, `said electrolytevcontaining a lithiurn content equal to` that resulting from the addition of lithum fluoride in an amount from about 2 to 20% by weight of the electrolyte; and increasing the current u References Cited in the file of this patent UNITED STATES PATENTS 400,664 Hall Apr. 2, 1889 886,757 Blackmore May 5, 1908 1,566,694 Railsback Dec. 22, 1925 1,709,759 Weber et al Apr. 16, 1929 2,062,340 Weaver Dec. 1, 1936 2,919,234 Slatin Dec. 29, 1959 FOREIGN PATENTS 687,758 Great Britain Feb. 18, 1953 1,149,468 France Dec. 26, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. &034.972 May 15, 1962 Robert A. Lewis ppears in the above numbered pat- It is het-aby certified that error a id Letters Patent should read as ent requiring correction and that the sa oorrected below.

Column 3, line 26, for "call" read cell column 8 line 29, for "electrolyle" read electrolyte line 35 for "electrolye" read electrolyte same column 8, Table I "Produstion" read Production first, column, Seventh item, for line 19 ther-cof, for

column 9. Table II, column 3, "LiCO read Li CO Signed and sealed this lst day of January 1963.

(SEAL) Attes:

DAVID L. LADD ERNEST W. SWIDER Attestiug Officer Commissioner of Patents

Claims (1)

1. THE METHOD OF INCREASING THE OUTPUT OF AN ELECTROLYTIC ALUMINUM REDUCTION CELL, WHICH CELL COMPRISES A CARBON ANODE, A MOLTEN ALUMINUM CATHODE AND A MOLTEN SALT ELECROLYTE CONSISTING ESSENTIALLY OF CRYOLITE AND ALUMINA, AND IN THE OPERATION OF W HICH CURRENT IS PASSED FROM SAID ANODE THROUGH SAID ELECTROLYTES TO SAID CATHODE WITH A GIVEN VOLTAGE DROP ACROSS THE CELL, WHICH METHOD COMPRISES INCORPORATING LITHIUM-CONTAINING MATERIAL IN SAID ELECTROLYTE IN AN AMOUNT TO PROVIDE IN SAID ELECTRO LYTE A LITHIUM CONTENT EQUAL TO THAT RESULTING FROM AN ADDITION OF LITHIUM FLUORIDE IN AN AMOUNT FROM ABOUT 2 TO 20 PERCENT BY WEIGHT OF THE ELECTROLYTE, AND ESTABLISHING A CURRENT FLOW SO THAT THE RESULTANT VOLVAGE DROP ACROSS THE CELL IS AT LEAST SUBSTANTIALLY AS GREAT AS SAID FIRST NAMED VOLTAGE DROP, THE TEMPERATURE OF THE LITHIUM-MODIFIED ELECTROLYTE BEING ABOVE ABOUT 845*C.

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