NO801022L - ANODE COMPOSITION. - Google Patents

Description

General description of the invention

The present invention relates to a system for the electrolytic production of aluminum from a number of raw materials R containing aluminum in a low-temperature electrolytic bath B. The basis of the invention is a unique anode A as the only source of the aluminum applied to the cathode H. The anode A comprises a combination of an aluminum-containing source that usually contains aluminum oxide, Al^O^, and a reducing agent such as carbon. Conductors D can be supplied to the anode to increase the conductivity of the anode and a membrane M can be used to contain the raw materials R.

An electrolytic cell C containing the anode A and cathode H can have a variety of structural forms with electrodes with inclined walls or electrodes with vertical walls. In the preferred embodiment, inclined wall electrodes are more economical and practical when used at low temperatures with the single anode.

The electrolytic bath B may be composed of chlorides or fluorides or mixtures thereof and does not require an initial addition of aluminum salt to the bath. In one form of the invention, the aluminum chloride cycle, aluminum chloride is present in the bath, and is maintained at a constant concentration due to reaction of the composite anode A in the bath to produce aluminum ions for reduction at the cathode. In another embodiment, where a bath containing only fluorides is used, aluminum is also ionized at the anode A for application to the cathode H.

Detailed description of the invention

The electrolytic cells

The electrolytic system according to the invention uses an electrolytic cell C, which is produced in one of the figures for the unique, continuous production of aluminium. " «•-<->

In Figures 1-6, one form is an electrolytic cell structure generally shown at 10 composed of an outer steel shell with a refractory coating 1k which can serve only as thermal insulators or as both insulator and electrode. The refractory lining can be of any material that is ex° resistant to the influence of the molten electrolytic bath 16. The refractory lining at conventional, vertical sides 15 and a bottom 17 are constructed so that the desired thermal balance is maintained in the cell during operation and are therefore very thin in cross-section to produce a small thermal gradient, which leads both to a thin layer solidified salt on the surface of the refractory lining and a hot outer wall on the surface of the steel shell 12. The refractory lining can also be quite thick to get a solidified layer of salt on the inside of the refractory lining, leading to a cold surface on the steel shell although this is not necessary in the cell with vertical walls in Figures 1-6. In contrast, the cathode 19 is a conductive liner placed on both sides of the anode in the inclined wall electrode cell, shown in Figure 7. A thermal and electrical insulating liner may be placed between the cathode 19 and the shell 12 if desired. The solidification line should be within the boundaries of the conductive liner or cathode 19 to prevent a solid salt layer from collecting on the bath side of the electrode. Such a salt layer will act as an electrical insulator and prevent effective current flow.

The lid 18 on top of the cell forms an air-tight closure and is only necessary in baths containing chlorine. This lid thus prevents air and moisture from entering the cell or any vapors of the salt compound 16 leaking out and affecting the surrounding environment. Lid 18 can be lined with refractory material 20 which can be the same as the refractory lining lk or any other refractory material which is compatible with maintaining the temperature balance in the cell and is chemically inert to the salt preparation 16. Gaskets 22 are attached to the lid 18 and towards the electrodes 2k and

25 and 26 to prevent atmospheric air and moisture from entering the cell and vapors from the cell escaping to the surrounding environment. The sealing of lid 18 and around the electrodes may be by any means that prevents vapor from leaking out and may be standard or conventional packing material capable of withstanding the temperatures during operation while being resistant to the electrolyte vapors. Acceptable materials for such gaskets include asbestos, fibrous, ceramic materials, teflon, vitron, silicones, liquid metal sealants such as mercury, liquid solder materials, tin, lead etc.

The electrodes 2k, 25 and 26 can be anodes, cathodes or bipolar electrodes. They may include solid or coated conductors to carry electrical current for cell operation. " These conductors may be of any material capable of withstanding the temperature of the cell which is in the range of 150° to 1060°C, stable to the halide composition 16 and good electrical conductors. Materials useful for this purpose are carbon, graphite and titanium carbides, nitrides or borides and aluminum metal suitably treated for heat transfer balance The materials of choice for these conductors have been found to be graphite and titanium dichloride when operating in bipolar mode.

The aluminum chloride cell also comprises a pipe or an outlet pipe 28 with a valve 30 which will control the flow of any gaseous elements from the pipe and establish the pressure build-up in the cell for continuous operation. Gases that come out of the cell are those that arise from the oxidized, reducing agent and it is remarkable that no chlorine gas can be detected at all from a salt containing aluminum chloride. If any chloride is produced, this will be reacted with the anode 26 and recycled as aluminum chloride. The molten aluminum 32 is tapped off by means of a conventional tapping tube }" h or extracted by vacuum through standard methods which are known in and of themselves.

Figure h shows modification of the cell construction in Figure 1 again with vertical electrodes. The cell structure, including the shell 12 and the refractory material lk, is the same as described above, the electrode kk which serves as anode has either one of the anodes shown in figures 2, 2A or 2B, but preferably in figure 3 "The anode kk is immersed in the electrolyte which contains fluoride or fluoride salts or formation thereof and is heated to a temperature which is usually 5. the range 67O<0>to 8lO°C. At the bottom of the cell, and resting on the cathode beam 45 placed over refractory insulation 14, is a block 46 which is preferably somewhat further than the anode 44 and serves as the cathode through suitable electrical connection with the cathode beam 45.

The block 46 may be made from any of the materials for electrodes described above. The block 46 should extend close to the bottom 50 of the anode 44 which is its only surface for erosion into the anode. Closer anode-cathode spacing for such an electrode configuration is possible when the block 46 also goes above the level of the molten aluminum 32. As aluminum is applied to the cathode block 46, the surface is wetted and the aluminum flows off the block and into the pool 32 at the bottom of the cell and drained as desired at 3^»

Figure 4 also shows the power supply clamp 47 shown schematically, in contact with the anode 44 either above but preferably below bath level and near the bottom of the anode to reduce power loss due to resistance in the anode. The anode 44 can be constructed, for example, as shown in figures 3, 12 and 13. The clamp does not act as an anode because the composite anode preferably dissolves in the bath. The clamp 47 can partially or completely enclose the anode 44 as it is continuously supplied to the bath. The clamp consists of any suitable inert material which is electrically conductive. Among these materials are graphite, carbon, TiB^ or mixtures of these. The electrical contact between the clamp and the anode may be through a projecting contact point 48. The power supply to the clamp 47 is through suitably split cylindrical conductors 49 > which extend over the top of the cell.

Instead of changing the anodes periodically to supply new aluminum-containing material, the present invention can be adapted to a supply mechanism for continuous operation as shown in Figure 5 or continuous supply of an electrode as shown in Figure 4 of the Søderberg type.

Projecting up through cell C in figure 5»

an anode electrode 52 which penetrates deep into the melt 16 but remains above the pool of molten aluminum 32, or cathode block 46. The anode raw materials shown generally at R surround the anode 52, constituting the aluminum containing material and

the reducing agent. This anode mixture can be prepared as small particles ranging in size from 0.025 to approx. 25 mm or more and can be produced by extrusion, melting or the like and supplied to the cell through the funnel 5k. The particles of raw material of aluminum containing material and reducing agent are identified specifically at 58 and are in close contact with the anode 52 to provide the required source of aluminum and reducing agent.

These anode materials are kept in close contact with each other and with the anode 52 by being enclosed in a porous membrane container 60 which encloses the anode 52. As the anode materials 58 are used up and the level falls substantially below the level in the molten bath 16, the feed is brought 5^ in driTt to supply additional, anodic materials 58 to the container of the porous membrane 60.

In the embodiment in Figure 6, a bipolar cell is shown. Again, similar structures have been given the same numerical designations.

The same basic principle is used in the bipolar cell except that there are a pair of electrodes at each end of the cell which are in contact with suitable electrical sources. One of the electrodes 64 is a cathode and at the opposite end an anode 66. Between the electrodes 64 and 66 is a group of separate electrodes 68 which are not connected to each other or to another electrical source. Attached to each of the electrodes 68 and the anode 66 is a porous membrane container 60 of the same type as described at 60 in Figure 5. However, the porous membrane 60 has in the bipolar cell as one side one of the electrodes 66 or 68 which forms the container for anodic raw materials 58.

In the bipolar cell, the side of the electrode 68 closest to the anode 66 is negatively charged and the side of the electrode 68 facing the cathode 64 is positively charged. This side 72 of the electrode 68 will act as the anode and it is in the side that is in contact with the anodic raw materials 58. The electrolysis then produces aluminum on the negative side of the electrode 68 and C02 on the positive or anodic side of the same electrode. Aluminum falls into the basin 32 at the bottom and is collected in the usual manner.

In Figures 7 to 11, the electrolytic cell with slanted sides is shown in combination with the anode preparation according to the invention, which leads to significant savings in electrodeposition of aluminium.

In aluminum reduction cells using the Hall process, one has an anode-cathode distance that must take into account the magnetic field effect and "back-reaction" due to wave movements in the aluminum pool. Such considerations preclude any closer distance than from 3.7 to 5.0 cm between the bottom surface of the anode where all erosion takes place, and the top of the aluminum basin or cathode electrode. Another and equally important reason for the requirement for a greater distance between cathode and anode, whether in a Hall cell construction that uses vertical sides or in an attempt to use an electrode with sloping sides, is the great difficulty in maintaining dimensional stability due to of the high temperatures required and the corrosive salts which were necessarily added to maintain the high temperature for dissolution of alumina. In combination with beveled electrode cells and anodes using alumina and in reducing agent to produce the sole source of aluminum, the use of temperatures so low as to be just above the melting temperatures of aluminum will reduce any dimensional problems lack of stability and therefore makes it possible for cells according to the present invention to be constructed with a smaller anode-cathode distance than has previously been possible. It is thus the special combination of the anode and the inclined walls for the production of the cell that produces a lower IR voltage number in the salt due to the smaller distance that is possible between the inclined walls and reduction in the current density at the anode.

In their main features, the cells in Figures 7 to 11 are similar to those described above, except for the inclined surfaces which form the electrodes. With this cell structure, the anode 47 is provided with inclined sides 46 which are shown here as outer and which are directed downwards inwards, although the direction of the angle is not critical. The slope of the sides can be in any direction or at any angle from the level of the bath B. The angle can even vary from 10° to 80°C or more from the bath level. When using the anode base of the electrode with inclined sides where the part of the inclined anode side is immersed in the bath 16, the anode will erode over a larger surface and add aluminum for final application to the cathode.

The cathode 78 has the surface 79 of complementary shape to the beveled sides 76 of the anode, to provide an electrode distance on the sides shown by the distance Y. This distance can be between 0.8 and 6.25 cm. Greater distance results in greater energy consumption. The distance between the bottom 80 of the anode 74 in Figure 7 and the surface of the aluminum layer 82 which forms part of the aluminum pool 8k is shown at X and can be between 0.8 and 6.25 cm. Preferably the distances X and Y should be between 0, 8 and 2.5 cm. The distance between the anode and the cathode above the solidified bath layer 86 is not essential to the use of the invention. However, the distances X and Y between the anode and the cathode should be equal or different depending on the desired current density and anode erosion, but when it is placed close as indicated above, this will lead to significant energy savings eu

The liner 78 which forms the cathode in the cell can be

of typical materials used for electrolytic cells such as carbon, titanium diboride or the like, and is shaped as mentioned above to conform to the outer shape of the anode "] h. In addition, the bottom of the liner has an inclined floor 90 for the aluminum basin leading into the aluminum collection well 92. As can be seen from the inclined floor 90, this is such that it retains only a limited depth of the aluminum layer which can be regulated by removal devices (not shown) for aluminum from the collection well. this thin aluminum layer under the base 80 in the anode, is to eliminate wave effects in the molten aluminum layer due to the magnetic effect in the cell.

In other respects, the cell in Figure 7 is similar to that in Figure 1 in that the lid 18 is provided with an outlet opening 80 which is part of the shell 12. Refractory insulation in suitable form as shown at 14 may also be present.

The combination of using the anode according to the invention with inclined sides which are in accordance with inclined anodes, enables the configuration of the cells and the anode to vary significantly as shown in figures 8 to 11.

In Figures 8 and 9, the shape of the anode 74 is varied and has centrally placed inclined sides $k which diverge and which form a tip 96 in the anode. Carbon or other lining material such as TiB£, etc, which acts as a cathode, protrudes to conform to the internal shape of the anode as shown in Figure 8,~The operation of such a cell as shown in Figure 8 and 9© is substantially similar to that described in figure 7, especially with regard to the increased erosion of the surfaces 94.

In Figures 10 and 11, double anodes 100 and 102 with oppositely shaped, inclined sides 104 and 106 are placed in a cell with cathode 98, shaped largely as described in Figure 8.

The use of inclined cathodes in electrolytic cells shown in Figures 7 to 11 has been found to require that one cannot have any solidified salt layer on the surfaces of the inclined cathode wall immersed in the bath and facing part of the anode surface. Otherwise, the desired distance between cathode and anode cannot be maintained. In addition, the solidified saJ.t which would be attached to the wall of the cathode, would be a good electrical insulator, and could therefore inhibit the current from the anode to the inclined cathode sidewall. In previous use of such electrodes with inclined walls, problems with salts solidifying on the sides and likewise the dimensional lack of stability in the lining prevent widespread use of such cells. However, with the anode preparation according to the present invention and lower bath temperatures, a number of low-melting salt preparations which will not freeze out on the side walls can easily be used. Ideally, the melting point of the salt and the thermal balance of the cell are adjusted so that the freeze-out line for the salt is in the liner or in the outer steel shell, rather than on the liner or at the cathode-bath interface. It is not important where the freeze-out line is located, as long as the freeze-out line is inside the liner and the salt is kept in a liquid state on the surface of the cathode liner immersed in the bath. In such cases, the desired cathode-anode distance is maintained without difficulty.

The process

a. Baths containing chloride.

The electrolytic processes of the invention for the continuous production of aluminum ions at the anode employ the closed-top electrolytic cell shown in Figure 1, or any of the other cells described herein, if the top is closed or if appropriate precautions to prevent: (a) moisture from contacting the chloride electrolyte;

or

(b) oxidation of aluminum chloride, while the evaporated bath salts are retained. The advantage of the present invention when using baths containing chloride is not only from the continuous in situ production of aluminum ions at the anode, but also from the use of significantly lower energy to produce aluminum of high quality in the total absence of chlorine gas from the cell. Continuous production of aluminum ions at the anode is achieved by the anode being produced from an aluminum-containing material that contains aluminum oxide and a reducing agent. This anode is immersed in a molten bath containing alkali metal and/or alkaline earth metal halide salts of any composition provided aluminum chloride is present in the bath. In electrolysis, ionized aluminum is applied in the bath as aluminum metal to the cathode, while the reaction at the anode also forms CO^ in addition to aluminum ions. Aluminum is collected as molten aluminum and drawn away, but it is the reaction at the anode to convert the aluminum ions that constitutes the most important part of the present invention.

It is possible that halogen chlorine, either as chloride ion, chlorine atoms or chlorine gas, can participate in the chloride reaction" with aluminum oxide from the aluminum-containing material and the reducing agent in the anode to produce aluminum ions plus oxide of the reducing agent. Aluminum from the anode is ionized in the molten bath to continue the cycle, and the anions, which may be chloride, oxide or others, maintain the charge balance with the aluminum ions.

Aluminum produced on the cathode is usually as pure as the aluminum-containing material that forms the anode. It is possible to produce ultrapure aluminum in accordance with the invention by using a very pure alumina source or to produce a slightly impure aluminum by direct use of aluminium-containing materials such as bauxite or clays containing aluminium, such as kaolin or mixtures of such materials. Usually it is possible to achieve a purity of aluminum of at least 99>5%.

In the Hall-Heroult cell reaction, it is known that the carbon at the anode contributes to the general reaction when extracting aluminum by reducing the decomposition voltage for Al^O^. For example, decomposition of AI2O-J in cryolite with a platinum anode is approx. 2.2 volts, but with a carbon electrode where it is assumed that 50 volume percent CO is produced and 50$> CO2, the decomposition voltage is approx. 1.2. You get about the same decomposition voltage from Al^O-^ if methane is blown under the platinum electrode to produce mainly CO2.

In the present invention, the use of the composite anode results in a lower decomposition voltage than would be obtained if AlCl^ were composed with the production of Cl^ gas on the anode. In any electrochemical reaction, if the current-voltage curve is extrapolated to 0 current, one obtains a number that gives the approximate decomposition voltage. When a graphite anode is used in an electrolysis process with aluminum chloride, a decomposition of 1.8 to 2.0 volts can be obtained, which is in accordance with the values given in the literature and the theoretical value calculated from thermodynamite.

It was demonstrated that the decomposition voltage according to the invention varies slightly with electrolyte composition. With pure NaAlCl^, the decomposition voltage is lowest, but gradually as the AlCl^ component. as the electrolyte was reduced, the decomposition voltage tended to increase slightly. The lowest decomposition voltage obtained was 0.5 volts and the highest 1.5 volts. The average value was 1.2 volts. If one uses the most common average value of 1.2 decomposition voltage, one can see that in the present invention the decomposition voltages are 0.6 volts less than for AlCl^, when chlorine is produced, and the value obtained here approaches the value of Al ^O^ and carbon, suggesting that the same general reaction mechanism takes place both in a Hall-Heroult cell and in the present invention.

This lower decomposition voltage leads to a significant energy saving for the electrolytic production of aluminum, not only compared to classic aluminum-chloride systems where chlorine is produced at the anode, but also when considering the additional energy required to produce AlCl^ from Al^O^ , carbon and chlorine.

The process conditions for the electrolytic production of aluminum have not been shown to be critical with regard to the voltage used or the current density. The temperature in the bath can vary considerably and is simply the temperature necessary to maintain, bath melting, which depends on the composition of the halide salts present, can be achieved in a temperature range of from 150° to 1000°C, but which will usually be in the range between the melting point of aluminum and the boiling point of the cell components, preferably 10° to 400°C and more preferably 10° to 150°C up to less than 250°C above the melting point of aluminum. The pressure conditions in the closed cell are not particularly critical since no chlorine gas will escape as in previously known aluminum chloride salt processes. While CO or COg or both may be produced in the present process, these gases are not as corrosive as chlorine. The pressure conditions which are not important may vary from atmospheric pressure to 0.7 or more atmospheres.

b. Baths that only contain fluoride.

The Hall cell is based chemically on the fact that aluminum oxide will dissolve in the cryolite-fluoride salt bath at temperatures of 950° to 1000°C. Bayer alumina is soluble in baths containing cryolite at a minimum temperature of at least 900°C or more. Any bath containing fluoride at a temperature below approx. 900°C will not readily dissolve commonly prepared Bayer alumina, and therefore, alumina as a source of aluminum cannot enter the reduction reaction, nor is it possible for aluminum to be applied to the cathode. Without this general solubility of aluminum oxide in the fluoride salt bath, it is not possible to extract aluminum electrolytically.

As an aside to the present invention, it has been demonstrated that in all baths containing fluoride, the temperature can be in the range of from between the melting point of aluminum to the boiling point of the cell components and preferably 10°-400°C and preferably 10° to 150° C up to less than 250°C above the melting point of aluminium. In order to electro-reduce aluminum from its corresponding oxide or other compound containing oxygen, the range of bath temperatures will generally be from 670° to 800°C and preferably from 700° to 750°C.

The important aspect of this discovery that distinguishes it from conventional methods in the Hall-Heroult cell is that the composite anode containing a mixture of alumina and reducing agents produces a transformation of alumina and produces ionic aluminum in the fluoride bath at low temperature. However, the general reaction is assumed to be largely the same as the Hall cell reaction as mentioned above. Aluminum is produced in liquid form on the liquid metal pool that forms the cathode. It is assumed that a reaction takes place on the anode surface in a special way which leads to the reaction of aluminum oxide to produce aluminum ions in the same way as the mechanism that takes place in the Hall cell, even if the temperature is only slightly above the melting point of aluminum.

The importance of using the composite anode according to the invention should be quite clear since under the same conditions as under the present invention but using carbon or another non-consumable anode, the addition of alumina to the bath will neither lead to dissolution of alumina or electrodeposition of aluminum. An important aspect of the present invention is that by using a composite anode at a low temperature from 670<0> to 800°C in a bath containing only fluoride, the Hall cell can be operated in such a way as in figure k without the closed top which is necessary when operating the chloride bath as shown in Figure 1. Bath composition, current density and other process parameters are not critical to the operation of a cell containing a chloride bath or a fluoride bath.

The anode

The most important reason for achieving the favorable results according to the invention lies in the use of the unique, composite anode which consists of an aluminum compound containing oxygen, usually aluminum oxide and a reducing agent.

The anode provides the sole source of aluminum ions for electrolytic reduction to aluminum at the cathode and likewise, since carbon is a reducing agent, the means of conducting electrical current through the electrical alumina to the reaction site of alumina in contact with immersing them in the electrolyte. The anode also preferably provides, at least in part, a necessary source of reducing agent which enables aluminum oxide to react in the anodic environment and produces aluminum upon application to the cathode as aluminum metal.

The reducing agent is preferably, at least partially, mixed with aluminum oxide to produce a close contact between the reducing agent and the aluminum oxide. The reducing agent which should be conductive if it is selected correctly, when mixed with aluminum oxide, can also act as a conductor of electric current to the reaction area of the aluminum oxide. After the reaction of each aluminum oxide particle at a specific location in contact with the electrolyte at a specific current, another particle at the same location will be exposed and can react. This pattern takes place on the surface of the anode and continues until there is no more alumina that can be converted. If the reducing agent is not conductive and not mixed with alumina, the electrical conducting function must be achieved by conductive rods to keep alumina anodic at the reaction zone.

In an aluminum chloride salt bath, the anode must provide a reducing agent that supports the theoretical reaction of the aluminum source with chloride or oxygen or both to maintain a constant concentration of aluminum chloride. Maintaining a constant concentration of aluminum chloride is an important part of the chloride cycle in the present invention, since this eliminates the need for an external supply of aluminum chloride to be electrolyzed or the production of chlorine gas at the anode."

In the process where a pure fluoride bath is used, the anode according to the invention, in the same way as in the chloride cycle, gives aluminum oxide which reacts in the chloride bath to form aluminum ions at a completely unprecedented low temperature in the range of 670° to 800°C. The cell can also be open as shown in figures k, 5 or 7«

The source of aluminum is alumina, A^O^, but can also be any material containing alumina, such as bauxite or clay, such as kaolin, or other materials that can react at the anode to produce aluminum ions that can be reduced to molten metal at the cathode as in the fluoride or chloride processes.

When mixtures form the anode, the proportions are from at least 1>5 and up, with acceptable upper limits of 7»5»20.0 and even 50.0 or more parts by weight of alumina in the aluminum-containing material per weight fraction reducing agent.

In the present invention, the amount of aluminum oxide in the mixture of aluminum-containing material is preferably 2.0 to 6.5 and preferably 2.5 to 6.0 parts by weight of aluminum oxide per part reducing agent.

The reducing agent which may be used in accordance with the invention is not limited to a particular material, but may be any of such materials which are effective in reacting with alumina. The reaction in the fluoride and chloride baths is not clearly defined, but it may be that the reducing agent reacts with A^O-} and produces aluminum ions which are finally applied to the cathode and C0£ at the anode. The reaction mechanism may be the same in all chloride, fluoride or mixed chloride/fluoride salt electrolytes.

Among the reducing agents particularly useful for alumina and other oxides is carbon or a reducing carbon compound used in the mixture. Sulphur, phosphorus or arsenic can also be used independently or together with carbon. Carbon is particularly preferred since it characteristically has the dual ability to conduct current to the reaction areas for alumina and likewise maintain a reducing function and produce a gaseous product at the anode. The carbon sources in the mixture can be any inorganic material and especially those that are fossil such as tar, coal and coal products, reducing gases, for example carbon monoxide, and can also contain natural or synthetic resinous materials, such as wax, rubber, phenols, epoxies, vinyls, etc., etc. which can, if desired, also be pre-coked in the presence of the aluminium-containing material. Coking of the carbon source which is mixed with the aluminum oxide compound can be carried out in a manner known per se, such as with the methods used when baking the anodes used in a Hall-Heroult cell. This is achieved by casting, forming, extruding, etc., a composite anode such as A A^O^-tar in the desired ratio, for example 6.5 parts aluminum oxide for every part aluminum in the coked state, after which the formed anode is slowly heated in a non-oxidizing atmosphere to a coking temperature of 700° to 1200°C. After coking, the composite anode is ready for use.

It is also considered, for example, within the framework of the present invention to produce carbon as a reducing agent in mixture with aluminum oxide by coking the carbon source in the molten electrolyte bath both before and during the electrolysis. Bath temperatures in the range of 670° to 850°C are sufficient to coke the carbon source to produce the required carbon. The time required to achieve such coking is not critical, but it may require several minutes and up to several hours depending on the temperature of the molten bath and the mass of the mixture of aluminum-containing material

and reducing material.

Continuous coking is possible when using the supply clamp in figure k by introducing an anode on top of the last one and as consumption takes place continuously lowering the anode until "one is completely used up and the next takes the place of the first etc. The anode can be supplied continuously to the cell in the raw state which is common with the traditional Søderberg electrode. In this case steel rods are traditionally used to make contact, but the contacts can also be graphite, carbon, Til^, aluminum or mixtures of these. The unbaked, composite anode material is gradually coked from the heat in the cell so that the end of the anode in the salt is always completely coked to the temperature of the cell." Søderberg-style coking in the cell at 670° to 800°C produces an anode with lower conductivity compared to composite anodes prebaked at much higher temperatures.

The entire source of reducing agent, as mentioned above, should possibly not be mixed with the alumina source and form the anode. It has been shown that, for example, the only requirements for the reducing agent are that it be in contact with the anodic alumina and present in sufficient quantities to produce aluminum of metal on the anode.

However, it is clear that the electric current must be transferred to the reaction area for the reaction to take place.

If the reducing agent such as carbon is not mixed to carry the current, it is possible that another conductor, compatible with the cell and its contents, can be used.

For example, combinations of aluminum and noble metals and high-melting, conductive oxides such as silver-tin oxide or TiB2, alone or together with carbon or graphite, can be mixed with aluminum oxide in sufficient quantities to conduct the electric current to the reaction area. Such amounts are not critical provided that alumina is made anodic at the reaction site. Quantities as small as approx. 0.001 and up to at least 0.75 parts conductive materials per part aluminum oxide can be used. Larger amounts increase conductivity at the expense of reactive material availability, but are possible without actual upper limits. Of course, a reducing agent must still be present to produce the necessary reaction.

When alumina is the aluminum-containing material, the use of hydrated or calcined alumina can be used. Anodes made from hydrated alumina may show improved conductivity compared to calcined alumina, but hydrated alumina, Al^O^x 3H£0 or Al(OH)^tends to crack during prebaking and coking, and when placed in the hot bath, due to water carried off during the coking When having a salt containing aluminum chloride and using coking in the bath of the hydrated alumina, water carried off may undesirably hydrolyze the AlCl^.

Any cracking or breaking up of the anode due to driven moisture causes no difficulty provided that the membrane as shown in Figure 5 surrounds the anode. Any particles of the anode that fall off will be held up by the membrane for continuous reaction. The anode can also be any proportion of hydrated and calcined oxide to reduce cracking. The maximum amount of hydrated oxide that can be used produces an energy saving during the calcination.

The size and surface of the particles that make up the anode containing aluminum oxide have not shown any sensitivity with respect to the reaction rate of the anode. These characteristic features of the present invention stand in contrast to previous experience in the reduction of A^O-j and carbon with chlorine as a gas-solid phase reaction in a furnace. It has previously been shown that the reaction temperature and speed are very sensitive to particle size and the specific surface of the particles.

It is usually desired in the prior art to use an alumina with a specific surface area of 10 to 125 m/g for the AlCl 2 reaction. In the present invention, however, no sensitivity has been demonstrated with respect to the reaction rate at the anode based on particle size or specific surface area. That is, A^O^with a surface area of 0.5 m/g or less apparently reacted as readily as A120-^with a surface area of 100 m<2>/g. These results are based on experiments with anodes containing aluminum oxide with particles of different surface and size. Anode current densities from 0.32 to 6.k amp/cm were run in the cells with an outlet line connected to a rigid, else-iodine indicator for chlorine detection. No chlorine gas was detected regardless of current density or surface of aluminum oxide. This suggests that if any chlorine is produced at the anode, this will completely react and form aluminum chloride or that only aluminum ions are formed at the anode from AlgO^ while oxygen from Al^O^ combines with carbon and forms CO2. It is assumed that in order to produce chlorine at the anode, it will be necessary to set the potential so high that one overcomes the decomposition potential of AlCl^, but even then the chlorine produced will presumably react with Al£0^ and carbon and produce more AlCl ^rather than evolving chlorine at the anode.

Anodes for use in electrolytic cells can be produced in a number of forms and by a number of methods. A mixture of alumina material and reducing agent may form the anode in any convenient manner. For example, a mixture may bond to a typical electrode and form a coating that surrounds all or one side of the electrode as shown in Figure 2 of the drawings. It is also considered that the anode material can form the anode by being formed into any suitable shape which is attached to one end of the electrode rod or pin in a manner shown in Figure 3 of the drawing. It is also possible to meet the requirements according to the invention to produce the anode in a different way than having a physical bond directly to the electrode. However, it is desirable that the aluminum-containing material is in intimate physical contact with the carbon-containing material or another reducing agent. The last ratio can be used if mixtures of aluminum-containing materials and reducing agents are in the form of a homogeneous mixture of powders, small balls of mixed powders or larger, composite bouquets of such formed materials which can be produced by casting or extrusion in varying sizes from 0.025 to 25 mm or more. Uniformity in the distribution of carbon and aluminum oxide has been shown to be desirable in order to obtain maximum anode efficiency during the dissolution or reaction during the electrolysis. In order to keep the aluminum-containing material and the reducing agent that make up the anodic materials in the area of the electrode and thus in a combination that makes up the anode, a container in the form of a porous membrane can be used. ;For commercial use, the anode should be as conductive as possible. Since the anode according to the invention is not solid or pure carbon which is traditionally used in the Hall cell, it will be less conductive due to the presence of the aluminum compound. If the anode were to become as little conductive as the salt electrolyte, the water balance would be affected due to overheating which could take place as a result of the same current being passed through the anode with more resistance. When, for example, a fixed, composite anode is used as shown in Figure 3 in the cell in Figure 1, it is necessary for the electric current to pass through the anode from top to bottom, where the current losses are transferred to heating the bath. It is therefore desirable to produce an anode that has as high a conductivity as possible. The more conductive the anode material is, the lower the energy consumption to extract the metal will be, but in any case the conductivity of the anode should be greater than the conductivity of the salt for optimal operation. In particular, it is desirable to reach a target for maximum production of aluminum with minimal energy consumption, the resistance in the anode becomes significant. It has been shown that the conductivity of the anode varies considerably depending on the manufacturing process. The parameters that have been shown to affect the conductivity are the ratio between carbonaceous binder materials such as tar, carbon or coke particles which are supplied to the composite anode as a source of reducing agent and the type of alumina. The greater the carbon content in the anode, within the aforementioned ratios between aluminum oxide and reducing agent, the greater the conductivity. It is possible, for example, when using a ratio in the range k/l to G/l of aluminum oxide and carbon, to produce a solid, composite anode that has at least one-tenth the conductivity of a standard Hall-Heroult anode. To reduce the energy loss through the composite anode, several alternatives are also shown in figures 2, 2S, 2B and kA. several conductive cores 36 and 37 placed in the anode as shown in figures 2, 2A and 2B or a series of vertically placed conductor rods 39 as shown in figure hA.

The composite anode 26A shown in Figure 2 has a conductive central core 36 which may be carbon or graphite cast into the composite anode or the composite anode material 38 cast or coated on a conductive core prepared in advance. The central core 36 can also be a metal such as the same metal that is applied, for example aluminum. The exterior of the conductor 36 coated on one side for bipolar use or surrounded on both sides for multipolar use by the matrix 38 of composite anode material consisting of a mixture of aluminum oxides and reducing agent as described above. When coating on a single side, a bipolar operation is assumed. The term "oxides" shall also include silicates, which are often a combination of metal oxide and silicon oxide and any other oxygen-containing compounds of aluminum to be applied.

For large size anodes, another embodiment is shown in Figures 2A and 2B. In order to improve the conductivity, it is preferred to use rods 36 and 37 of pure aluminum as electrical wire bushings in a matrix of the composite anode material 38 which may be of the Søderberg type. Since pure aluminum is used to make the lead bars, it will melt as the anode is consumed and be carried along with the cathode metal in a continuous cycle. The rods are placed so that the voltage loss is reduced in relation to the conductivity of the composite anode. In Figure 2B, the wire rods 36 are connected to a plate hO, attached to a central conductor hl.

The number and size of the conductors 36 and 37 are chosen on the basis of anode size, current density at the anode, cell size, operating temperature and heat transfer so that the aluminum conductors 36 and 37 melt at the same rate as the matrix 38 of the anode is consumed. The unique advantage of the anode designs shown in Figures 2A and 2B is that large voltage drops are avoided in anodes with relatively high resistance, so that the process can be operated with significantly reduced energy consumption. The size of the aluminum rods lies in a diameter range ranging from 0.16 to 7.5 cm and preferably from 0.3 to 5.0 cm and most preferably from 0.6 to 2.5 cm.

To achieve the desired conductivity in the anode, the distance between the outer surface of the composite anode 38 and the surface of any aluminum rod in Figure 2, 2A or 2B and the mutual distance between the outer surfaces of these aluminum rods in Figures 2A and 2B, not critical and can vary from 0.3 to 60 cm and preferably from 2.5 to 15 cm, and preferably from 3.8 to 10 cm. As an example, if the conductivity of the composite anode is approx. 0.1 of a standard, pre-baked Hall-cell anode, a distance between the aluminum pins of approx. 7"5cm result in an acceptable voltage drop.

Since the operating temperature of the cell is usually 700° to 750°C, the aluminum rods can be made so that they will melt at about the same rate as the anode is consumed and will thus transfer energy to the base of the anode. If the diameter of the aluminum rod is too large, it will not melt and salt will freeze over the surface with the result that the anode that is consumed leaves behind an aluminum stub that will short out to the cathode as the anode is advanced. If the rod diameter is too small, it will melt back so far into the anode that you get a large voltage loss due to an I-finger wire path. It is desirable that the aluminum bars melt back into the anode to some extent, rather than being flush with the bottom surface of the anode. This is because anodic oxidation of the aluminum rods will then be reduced. Desirable remelting distance is based on what gives a minimal voltage drop, coupled with minimal anodic oxidation of the aluminum bars. If the rods remain level with the bottom surface of the anode, there will be a tendency for aluminum ions to be introduced into the bath from the rods (as in a refining operation) and likewise from the composite anode material, which will reduce the Faradaic efficiency of the cell. The heat can be balanced so that the line from the bath up through the anode and energy generated through the conductors are balanced so that you get the desired degree of melting in the conducting

aluminum pliers.

Figure 3 describes another alternative in the composition of an anode electrode as shown at 26B. In this embodiment, the electrode 26B consists of a composite material 38, which may be the same as the coating 38 in Figure 2, but which is formed into a suitable shape for use as an electrode. This

the shape of the electrode may be molded around a stub or rod electrode 42 extending from the upper end of the body of the electrode 26B for connection to the common electrical circuit. Optionally, the electrode 26B is molded and the stub 42 inserted using known methods which are used for pre-baked Hall cell anodes.

Another alternative embodiment is that shown in Figure 4A where an anode 26B is illustrated which has the same composition as described above but which is provided with a series of pairs of vertically placed conductor bars 39 and to the lowest pair of bars a bushing 39A is placed directly as in a Søderberg-type connection. The position of the bars 39 is not critical and they can be vertical to the anode axis or stand at an angle as shown. As the composite anode material is consumed, the wire connection 3 9 A is carried upwards to the next, higher pair of bars. The rods consist of aluminum which is also consumed and, as mentioned above, supplied to the pool of manufactured aluminium. Optionally, the rods can be made of iron, which is usually used in the Søderberg connections, and the rods are removed as the anode is consumed.

The designs in Figures 12 and 13 show a variation when it comes to conducting electrical energy to the working surface of the anode. As shown, the block 9^- of the composite anode A is laminated with sheets of aluminum metal 108. These sheets act exactly like the aluminum rods in Figures 2A and 2B. The shape and number of laminates are not critical. The block can lie in a continuous form and is introduced into the cell through the clamp 47 as in Figure 4, where together with the aluminum plates 108, the electrical connection is made.

The number, spacing and thickness of the aluminum plates 108 are determined by the same factors described with respect to the conductors 36 and 37. Typically, the thickness of the aluminum plates will vary from 0.025 to 12.5 mm and preferably from 0.25 to 9>k mm and preferably from 0.25 to 6.25 mm. The aluminum plates must be of sufficient thickness to carry the necessary current to avoid significant voltage drops and must also melt into the cathode pool as the anode is consumed. The distance between the aluminum plates 108 is such that excessive stress is avoided through the composite block as described with respect to the conductors 36 and 37. Usually the distance will vary from 0.3 to 60 cm and preferably from 2.5 to 15 cm and preferably from 3>7 to 10 cm.

Membranes

The membrane as shown in figure 5 must have a threefold function or ability.

First, the membrane acts as a separator or a barrier between the molten, cathodic metal phase and the source of the anode material to be electrolysed. When using the membrane according to the invention, the distance can be significantly reduced to produce a significant increase in conductivity and efficiency without any turbulent effects that could otherwise produce a reduction in the efficiency or quality of the aluminum product.

Secondly, the membrane according to the invention physically holds in place materials in the composite anode which, for example, may include aluminium-containing raw materials and the reducing agent. This keeps these materials close to the electrode and forms an anode for producing aluminum ions in the most efficient way. The membrane also prevents mixing of the raw materials with molten aluminum in the cell bottom. If a hydrated metal oxide such as hydrated aluminum oxide were to be used as one of the anode materials, the membrane would hold in place any pieces of the anode material that could break off due to the development of moisture from the aluminum oxide during coking. These pieces continue to be a source of aluminum through the reduction reaction as long as they are within the anode circuit of the membrane.

Thirdly, the membrane allows the free passage of ionic compounds and dissolved solids in the electrolyte, but will not pass through and will largely reject molten aluminum and undissolved solid materials that constitute such impurities as are present in the aluminum source and prevent contamination of the cathode coating.

The external shape of the membrane is not important and it can be in the form of a cylinder, prism etc. or parts of these. For example, the membrane can have a three- or four-sided shape with a bottom and thus form an enclosed container. This container is designed to hold the anodic raw materials in place for reaction in the salt bath.

Due to the corrosive nature of the molten salt bath, the choice of materials that will make up the membrane is important for the life of the cell and for the success of the process. If the electrolyte to be used is a pure chloride bath, the choices for membrane are somewhat greater due to the reduced corrosive nature of such a bath compared to a bath containing fluorides. However, baths containing some fluorides are preferred because of their lower volatility.

Pure fluoride baths have other advantages mentioned above. Materials suitable for use in fluoride baths will of course be useful in the less corrosive, chloride baths.

Among the materials that have been found useful are carbon glass foam, carbon or graphite as a porous solid or porous solids of refractory hard metals, such as nitrides of boron, aluminium, silicon (including oxy-nitrite), titanium , hafnium, zirconium and tantalum; silicides of molybdenum, tantalum and tungsten; carbides of hafnium, tantalum, niobium, zirconium, titanium, silicon, boron and tungsten; and borides of hafnium, tantalum, zirconium, niobium, titanium and silicon. Other refractory hard metals known may prove useful in forming the membrane provided they are resistant to the molten salt bath.

The refractory, hard metals that form the membrane according to the invention can be produced in the form of a cloth, mat, felt, foam, porous sintered solid base or as a coating on such a base, and all are known in and of themselves. The membrane must also meet special standards with regard to through porosity and connected pore size.

These two properties can be defined as follows: through porosity - the percentage of the total volume in the membrane that is made up of passages passing through from one side of the membrane to anotherj

connected pore size - the smallest diameter in a passage through the membrane.

Through-hole porosity varies with the type of membrane material, the temperature in the molten bath and the salt composition, but a common property for useful membranes is that the porosity must be sufficient to pass all metal ions such as aluminium, and all electrolyte salts without undissolving -te impurities are passed through. It has been demonstrated that the greater the porosity, the greater the current flow, and thus the greater the electrical efficiency in the cell. The porosity can vary from 1 to 971?0 or more, but usually it is in the range of 30 to 70$>. The preferred porosity to obtain the greatest efficiency is from the 90 to 97$ range. A carbon glass foam, for example, is able to provide such a high porosity and at the same time retain sufficient mechanical strength.

The connected pore size must be small enough to reject undissolved solid impurities, but large enough to pass through ionic and dissolved particles. Generally, the acceptable pore size is between one micron and one cm.

The thickness of the membrane material is a function of its porosity, pore size and ability to retain undissolved, impure solids and molten metal. The thicker the membrane, the greater the electrical resistance is obviously. It is therefore desirable to use such a thin membrane which is practically compatible with the porosity and pore size and which is in accordance with the mechanical strength of the membrane in position in the cell. Preferred sizes are from 3 to 12.5 mm, but it can be as thick as 50 mm or more.

Typical membrane materials that have been found to be useful include, but are not limited to, carbon glass & foam, carbon or graphite in the form of a porous solid, felt or cloth, aluminum nitride, silicon nitride, silicon carbide, silicon oxynitride, boron nitride and titanium nitride as a porous solid, as a cloth or as a coating on the surface of carbon glass foam or porous graphite. Aluminum nitride appears to be the most desirable material. It has been demonstrated that aluminum nitride can conveniently be produced in a porous structure by first creating a porous aluminum oxide structure which is then impregnated with carbon followed by heating to 1750°C in a nitrogen atmosphere to convert aluminum oxide to aluminum nitride. Such a process leads to a strong, porous structure that is chemically compatible with the corrosive salt environment and molten aluminum?

Composition of the molten bath.

The electrolytic bath according to the invention can vary considerably compared to a typical Hall cell salt composition. In the present invention, the bath composition may comprise any halide salt, and in particular chloride and fluoride are preferred. Any alkali or alkaline earth metal such as especially sodium, potassium, lithium, calcium, magnesium, barium and the like can be used to prepare the halide salts.

There is no critical composition or range of properties that are desirable or necessary. It has been observed that no aluminum salt needs to be present at the start of the electrolysis to produce aluminum during an electrolysis using the composite anode. For example, a salt effect electrolyte containing only alkali and/or alkaline earth halides will produce aluminum metal at the anode when using the composite anode and without any "anode effect".

It is usually preferred that the salt bath initially contains an aluminum halide, although this is not necessary in accordance with the invention. In the case of baths containing AlCl^ which may contain only chloride anions or both chloride and fluoride anions, the aluminum chloride concentration may be 2 to 60$>, but may also be in the range of 1 to 95 weight percent AlCl^. The fluoride-only bath may contain the same fluoride salts mentioned above and may likewise contain aluminum fluoride in any desired proportion.

Among the advantages and disadvantages of the different electrolyte types, one finds that all chloride baths have a very low tolerance to oxide contamination, but have very high conductivity and are the least corrosive to refractory materials and cell components.

Electrolytes containing fluoride will develop aluminum as droplets that easily flow together, but the corrosive effect of the electrolyte towards the refractory materials and cell components increases "a lot".

A lithium component in any electrolyte will increase conductivity, but this is expensive and increases the cost of the electrolyte. In operation, a balance must be found with regard to the electrolyte cost, the conductivity of the electrolyte and the resulting energy consumption to produce aluminium.

The preferred electrolyte is a balance of economy with respect to salt components, conductivity, corrosiveness to refractories and cell components, tolerance to oxide contamination, and pooling of the produced aluminum in a pool for easy utilization.

THE EXAMPLES

Example 1

In Figure 1, the anodes are graphite plates where the cathode electrode is a titanium diboride plate. The anode was prepared with a coating 38 as shown in Figure 2 which consisted of Al^O^ purified by the Bayer process, calcined at 1000°C and mixed with a weight ratio of five parts Al^O^ to one part carbon in a coking step . The carbon was provided by mixing A^O^ with a phenolic resin and gradually heating to 1000°C

in an inert atmosphere to coke the phenolic resin to carbon. The electrode coating was prepared by mixing Al₂O₂ and phenolic resin, applying the mixture to the electrode and heating to coking temperature.

The electrolyte consisted of an equimolar mixture of sodium chloride and aluminum chloride which formed the double salt NaAlCl^ at approx. 150°C. The temperature in the cell was increased to 700°C and electrolysis of A2O-j was carried out for several hours, which produced a layer of molten aluminum on the bottom of the cell. Examination of the anode showed that the coating had dissolved and aluminum applied to the cathode. This coating of aluminum corresponded to the aluminum content of Al^O^ which was dissolved in the anode. The general, controlling reaction is assumed to be ionization of the Al20rji anode with carbon which is converted to primarily form COg. During the electrolysis, no sign of any chlorine gas released on the anodes and in the outlet pipe was seen. The outlet gas was analyzed and found to be mainly CC^.

Example 2

The electrolyte consisted of 63% NaCl, 17% LiCl, 10% LiF, 10% AlCl^ and the electrode coating according to Figure 2 was prepared from standard bauxite A^O-^ and a petroleum tar which was fcr-coked to produce a ratio of A^O^ and carbon (coked) of 5.7 to 1. Electrolysis was carried out in a cell according to Figure 1 at a temperature of 750°C. The distance between the anode and the cathode was 1.25 cm, which gave a current density on the electrode of -2.4 amp/cm<2>at an applied voltage of 2.5 volts. No chlorine gas could be detected from the anode. Aluminum was produced. The collected aluminum was produced at a Faradaic efficiency of 92% with an energy consumption of 8.07 Kwh/kg.

Example 3

The electrolyte composition was 10% NaCl, 50% CaCl2, 20% CaF2, and 20% AICI3. The electrode coating in figure 2 was produced as in example 2, but only on one side of the electrode. The electrical connections were made so that the anode next to the outlet pipe was connected to the positive pole and the negative pole to the electrode that had been removed from the outlet pipe. The coated sides of the electrodes 25 and 26 each faced away from the discharge tube and towards the cathode. Electrode 2^-, ^the cathode, was not coated. This led to the electrode 25 not being physically connected to the direct energy supply. The electrode then became bipolar. The side coated with AlgO^-C among none has a positive charge in this way. The side of the bipolar electrode 25 nearest the discharge tube becomes negatively charged, and aluminum is developed here and sinks into the molten pool. Aluminum is also developed on the negatively charged electrode 24 and sinks into the molten pool. The temperature in the cell was 800°C and the applied voltage was 3 volts with respect to each electrode or a total of 6 volts across the poles. The applied voltage with an electrode spacing of 1.9 cm resulted in an electrode current density of 1.9 amps/cm 2 .

Example 4

The anode electrodes were composed of titanium diboride rods and the cathode electrode was also titanium diboride. The anodes were coated with bauxite as in figure 2 which was calcined at 600°C, mixed with phenolic resin and coked at 800°C. The ratio between alumina in bauxite and carbon after coking was 6 to 1. The composition of the electrolyte was 20% NaCl, 30% CaCl2, 10% CaF2, 4% NaF, 36% AlCl^ and the cell was operated at 750°C and an electrode- density of 2.4 amp/cm^. This led to 4 volts at an electrode distance of approx. 1.9 cm. No chlorine gas was observed. The composition of aluminum fed to the molten pool was 97% aluminium, 0.5% Si, 1.5% Fe and 0.9% Ti with minor amounts of other constituents.

Example 5

The cell in figure 5 used a porous membrane of aluminum nitride material which was 4.7 mm thick with 50% porosity and a pore size in the range of 12 to 24 microns. Aluminum nitride was produced by impregnating a porous body of aluminum oxide with carbon and then heating it to 1750°C in a nitrogen atmosphere. The anode conductor was a graphite rod and the aluminum-containing material in the anode was a Bayer A120^and carbon mixed powder in a ratio of 6 to 1. The electrolyte composition was 20% NaCl, 25% LiCl, 30% LiF, 25% AlCl^and the electrolysis was carried out at 720° C. The distance between the membrane and the aluminum pool was approx. 1.25 cm and the electrolysis was carried out at a current density of 1.6 amp/cm. This resulted in a voltage of 2.8 volts. Aluminum was produced with an efficiency of 92% and a purity of 99.5%.

Example 6

A salt preparation consisting of 12% NaF, 25% LiF, 28% NaCl, 15% LiCl, 10% AlF^ and 10% AlCl^ was melted in a cell with straight side walls. One;. 5 cm thick aluminum sole was melted on the bottom of the cell and the operating temperature was adjusted to 700°C. Using an anode as shown in Figure 2A, a distance between the bottom of the anode and the aluminum sole was set to 4.4 cm. At one current density of 0.96 amp/cm<2>, the cell potential was 3>5 volts.

After 8 hours of electrolysis, the anode was removed from the cell and placed in a cell with 45° sidewalls as shown in

figure 7'After a few hours of electrolysis, the anode was eroded so that the sides were parallel to the cathode side walls. The anode immersion in the salt was 7"5cm. Applying the same total current as in the straight-walled cell, the potential was 2.45 volts. This reduced potential due to side anode erosion at 90% current efficiency, and constant production rate, reduces the energy consumption from 10.5 kwh/kg to 8.1 kwh/kg which is a reduction of 3>45 kwh/kg.

Example 7

A salt preparation consisting of 20% NaCl, 25% LiCl, 25% LiF, 10% NaF, 10% AlF^ and 10% AlCl^ was melted in a cell with straight side walls. A 5 cm thick aluminum sole was melted on the bottom of the cell and the operating temperature adjusted to 700°C. Using a composite anode 30 cm long with a copper feed line attached to one end in the traditional manner, the anode-cathode distance was adjusted to 4.4 cm. At a current density on the anode of 0.96/cm, the cell potential was 5.75 volts after equilibrium was reached.

An identical anode but with a 45° slope at the end opposite the feed rod was inserted into a cell with k5° sidewalls as shown in Figure 7. The immersion depth of the anode in the salt was 7.5 cm. After a few hours of electrolysis to ensure that the angle of the anode was the same as that of the side walls of the cell, the potential required to obtain the same total current used in the straight wall cell was 4.4 volts. This reduced potential due to side anode erosion at 90% current density and at a constant production rate between the two cell types reduces the energy consumption from 19 kwh/kg to 14.52 kwh/kg which is a reduction of 4.5 kwh/kg .

The reduction in energy consumption at constant current from that used in a traditional cell where only the base of the anode is eroded, and in a cell with an inclined anode, is obvious from this example.

Example 8

An anode was prepared using Alcoa A-I A^O^ mixed with coal tar and phenol and cast in a closed mold with heat applied to cure the phenolic component. The ratio between the components was such that after coking the composite anode contained 17% carbon and 83% A^O^. The electrolyte consisted of 20% NaCl, 25% LiCl, 30% LiF and 25% NaF and the cell was operated at 700°C. A cell as shown in Figure 7 was used, but no aluminum sole was added. After several hours of electrolysis at approx. 700°C, aluminum was collected in the well, which showed that the composite anode will produce aluminum during electrolysis, without the original use of an aluminum salt in the electrolyte.

Example 9

A cell with straight side walls as shown in figure 5 was used, but without the membrane 68 and the anode rod 52. Instead, a carbon cylinder was mounted just above the salt-electrolyte layer in which a short anode part was immersed as shown in figure 2A. On top of the pre-baked short anode and around the wire rods 37, a mixture of Al 2 O 3 , petroleum coke powder and a mixture of tars was added. The mixture of A^O^, coke powder and tar was such that 18% carbon and 82% AlgO^ were obtained when the anode was coked in the salt electrolyte at temperatures of 740°C. A salt electrolyte consisting of 10% NaF, 25% LiF, 20% NaCl, 15% LiCl, 20% AlF 3 and 10% AlCl 3 was used. Electrical connections were made to lead bars 37 by means of clamps in the cool areas above the level of a Søderberg type anode assembly in the carbon cylinder. The electrolysis was carried out at 1.6 amp/cm current density at the anode and with a distance of 4.4 cm between the aluminum pool and the anode. The electrolysis was continued with the addition of AlgO^-carbon tar in the carbon cylinder, a continuous supply of anode which hardened and coked as it entered the salt electrolyte. The aluminum rods melted as the anode was consumed and floated together with the aluminum cathode pool 32.

A similar test was made using a carbon rectangle, pre-baked rectangular blocks and aluminum sheets as shown in Figure 13. The aluminum sheets were 1.5 mm thick and the pre-baked blocks were 5 cm thick. Electrical connections were made using rollers on each aluminum plate. As the anode was advanced, further, pre-baked blocks were inserted between the aluminum plates.

THE DRAWINGS

Figure 1 is a schematic sketch showing in cross-section a part of an electrolytic cell according to the invention which contains a klox-id bath and which illustrates the closed top of the cell together with the relative position of the electrodes. Figure 2 is a sketch showing a cross section electrode and which is used as an anode and which is coated with a mixture of an aluminum-containing material and a reducing agent. Figure 2A is a sketch showing in perspective an alternative embodiment of the electrode in Figure 2 which shows a series of conductive cores in the matrix of aluminum-containing material and reducing agent. Figure 2B is a schematic perspective sketch of a variation of the electrode shown in Figure 2A. Figure 3 is a sketch showing a partially cut-away alternative electrode. Figure k is a schematic cross-sectional illustration of an open top electrolytic cell containing a pure fluoride bath and an anode clamp providing a source of electrical current to a continuously supplied anode. Figure kA is a schematic sketch of another alternative electrode which corresponds to the electrode in Figure 3. Figure 5 is a schematic sketch of an embodiment of the invention illustrating the use of a porous membrane to contain the various anodic materials and including a material which contains aluminum and a reducing agent. Figure 6 is a cross-sectional schematic diagram of another embodiment of an electrolytic cell illustrating the use of bipolar electrodes. Figure 7 is a schematic cross-section of a combination of electrolytic cells with electrodes with inclined sides and a composite anode of similar shape. Figure 8 is a schematic cross-section of a modification of a combination electrolytic cell and composite anode. Figure 9 is a perspective sketch of the anode in Figure 8. Figure 10 is a schematic cross-section of another embodiment of the combination in Figure 8. Figure 11 is a perspective sketch of the anode in Figure 10. Figure 12 is a schematic perspective sketch of another embodiment of a composite anode according to the invention illustrating a laminar construction. Figure 13 is also a schematic perspective sketch of the anode from Figure 12 and the anode clamp in Figure k.

Claims (110)

1. Electrodes for use in an electrolytic cell for the electrodeposition of aluminum from a molten electrolyte, an electric current source and a cathode for depositing aluminum, characterized in that said anode contains an aluminum compound containing oxygen in a sufficient amount to provide aluminum during electrolysis and a reducing agent in contact with said aluminum compound containing oxygen and which is present in a sufficient amount to react at the anode and then form aluminum metal at the cathode where the weight ratio between said oxygen compound expressed as Al2 0^ and said reducing agent is at least approx. 1.5 parts by weight. 2. Electrolytic cell for electrodeposition of aluminum characterized in that it comprises the electrode according to claim 1 and said electrolyte comprising ions selected from the group consisting of chlorides, fluorides or a mixture thereof, and said compound which is capable of reacting at the anode for to produce aluminum ions for reduction to metal at the cathode. 3. Cell according to claim 2, characterized in that the temperature in the electrolyte lies between the melting point of aluminum and the boiling point of the liquids in the cell. k. Cell according to claim 2, characterized in that the temperature in the electrolyte is between approx. 10° to 150°C above the melting point of the metal. 5. Electrode and cell according to claim 1, 2, 3 or h, characterized in that said reducing agent is selected from the group consisting of a compound containing carbon or carbon in the cell, sulphur, phosphorus and arsenic. 6. Electrode and cell according to claim 1, 2, 3 or k, characterized in that said reducing agent before coking is selected from the group consisting of an organic material which produces carbon during coking. 7. Electrode and cell according to claim 1, 2, 3 or k, characterized in that said aluminum compound containing oxygen is mixed with said reducing agent in an amount of at least 1.5 parts by weight of A^O <y> per part reducing agent. 8. Electrode and cell according to claim 1, 3 or 4, characterized in that said electrolyte is produced largely from fluoride salts and said compound which is capable of reacting at the anode to produce aluminum ions for reduction to metal at the cathode. 9. Electrode and cell according to claim 1, 3 or 4, characterized in that said electrolyte largely consists of fluoride salts, said compound containing aluminum and which is capable of reacting at the anode to produce aluminum ions for reduction to aluminum metal at the cathode and in that the temperature in the electrolyte is between approx. 700° and 800°C. 10. Electrode and electrolytic cell according to claim 9" characterized in that said reducing agent is selected from the group consisting of carbon and a compound containing carbon and in that said metal compound containing oxygen is selected from the group consisting of aluminum oxide, bauxite, clay and oxides containing metal. 11. Electrolytic cell for electrodeposition of aluminium, characterized in that it contains the electrode according to claim 1, and a membrane placed in contact with said compound largely to prevent the passage of undissolved, solid substances through the membrane. 12. Electrolytic cell according to claim 11, characterized in that said membrane containing said compound forms the anode. 13. Cell according to claim 11 or 12, characterized in that said membrane is made from a material selected from the group consisting of carbon-glass foam, graphite or solid carbon, nitrides of boron, aluminium, silicon (including oxynitride), titanium, hafnium, zirconium and tantalum; silicides of molybdenum, tantalum and tungsten; carbides of hafnium, tantalum, niobium, zirconium, titanium, silicon, oor and tungsten and borides of hafnium, tantalum, zirconium, niobium, titanium and silicon, and said materials have a continuous porosity which is sufficient for all liquid materials to pass, except for molten aluminum in said electrolytic bath, including ionic compounds and dissolved solids and largely discards undissolved solids that constitute impurities, so that an electric current is created, and in that it has a connected pore size with a diameter sufficiently small to admit undissolved particles. 14. Cell according to claim 13"characterized in that said overall porosity is between 1 to 97% and the connected pore size is between 1 micron and 1 cm. 15. Cell according to claim 11, characterized in that it contains a membrane which is porous and is placed in said electrolyte in such a way that said compound and said reducing agent are retained for reaction as anode. 16. Cell according to claim 15, characterized in that it contains supply devices placed on said cell, to continuously renew said compound and said reducing agent in said membrane. 17. Electrode and cell according to claim 7, characterized in that said anode is formed from a central, conductive core with a coating of said aluminum compound. 18. Electrode and cell according to claim 1 or 3" characterized in that said anode has a number of pairs in vertically placed conductor bars for connection to electrical bushings. 19. Cell according to claim 13, 14, 15 or 16, characterized in that said electrolyte is mostly made of fluoride salts and said compound which is capable of reacting at the anode to produce aluminum ions for reduction to aluminum at the cathode. 20. Cell according to claim 2, 3 or 4, characterized in that said cell has a lid to close the top of the cell to prevent loss of gases and hydrolysis of the electrolyte. 21. Cell according to claim 2, 3 or 4, characterized in that the distance between the anode and the cathode is less than approx. 2.5 cm. 22. Cell according to claim 21, characterized in that the distance between the anode and the cathode is less than approx. 1.25 cm. 23. Electrode and cell according to claim 1, 2, 3 or 4', characterized in that said anode consists of at least one internally located conductive material, surrounded by anodic material containing the aluminum compound. 24. Cell according to claim 2, 3 or 4, characterized in that said anode consists of at least one internally placed conductive material, surrounded by anodic materials containing the alunrinium compound and a membrane placed in contact with said aluminum compound, largely to prevent the undissolved solids are passed through said membrane and in that said electrolyte mostly consists only of fluoride salts, said aluminum compound, and in that it is capable of reacting at the anode to produce aluminum ions for reduction to aluminum metal at the cathode and in that the temperature in the electrolyte is between 700° and '800°C. 25. Cell according to claim 24, characterized in that said membrane contains a mixture of said aluminum compound and said reducing agent to produce an anode. 26. Cell according to claim 25, characterized in that said membrane is made of a metal selected from the group consisting of carbon-glass foam, graphite or solid carbon, nitrides of boron, aluminium, silicon (including oxynitride), titanium, hafnium, zirconium and tantalum; silicides of molybdenum, tantalum and tungsten; carbides of hafnium, tantalum, niobium, zirconium, titanium, silicon, boron and tungsten; and borides of hafnium, tantalum, zirconium, niobium, titanium and silicon, wherein said materials have a through porosity sufficient for all liquid material except molten aluminum in said electrolyte bath to be passed through, including ionic compounds and dissolved solids .substances and in that materials largely discard undissolved solids that constitute impurities, creating electrical current and* have a connected pore size with a diameter sufficiently small to keep out dissolved particles. 27. Cell according to claim 26, characterized in that said throughout porosity is between 1% and 97% and connected pore size between 1 micron and 1 cm. 28. Cell according to claim 2, 3 or k, characterized in that a porous membrane is placed in the electrolyte in such a way that it can contain a mixture of said aluminum compound and said reducing agent for reaction as anode. 29. Electrode and cell according to claim 9, characterized in that said anode consists of at least one internally placed, conductive material surrounded by anodic material containing the aluminum compound. 30. Electrode and cell according to claim 2k, characterized in that said membrane forms a space containing a mixture of said aluminum compound and said reducing agent and forms a bipolar electrode. 31. Electrode and cell according to claim 30, characterized in that said cell comprises a space for metal collection placed under said membrane and said cathode. 32. Electrode and cell according to claim 30 or 31, characterized in that said membrane space contains the retained impurities and forms the bipolar electrode. 33. Electrode and cell according to claim 30 or 31, characterized in that said electrolyte contains ions selected from the group consisting of chlorides, fluorides and a mixture thereof, and said compound which is capable of reacting at the anode to produce metal- ions for reduction to metal at the cathode. 34. Electrode and cell according to claim 30 or 31, characterized in that the temperature in the electrolyte lies between the melting point of the aluminum metal and the boiling point of the liquids in the cell. 35. Electrode and cell according to claim 30 or 31, characterized in that the temperature in the electrolyte is between 10° and 150°C above the melting point of the metal and in that said electrolyte mostly only consists of fluoride salts and in that said metal is directly reduced to molten metal at the anode. 36. Method for the electro-deposition of aluminum using an electrolytic cell having an electric current source, a molten electrolyte for conducting electric current and a cathode for the deposition of aluminum, characterized in that a combination of an aluminum compound is used as the anode contains oxygen in sufficient quantity to give aluminum during the electrolysis and a reducing agent in contact with said compound which is present in sufficient quantity to react at the anode and produce aluminum metal at the cathode, the weight ratio of said aluminum compound being expressed as AlgO^ and said reducing agent is at least about 1.5 to 1, reacts the compound at the anode, produces aluminum metal at the cathode and collects aluminum at the cathode. 37• Method according to claim 36, characterized in that said electrolyte comprises ions selected from the group consisting of chlorides, fluorides and mixtures thereof and said compound which is capable of reacting at the anode to produce aluminum ions for reduction to aluminum by the cathode. 38. Method according to claim 36, characterized in that said compound is selected from the group consisting of aluminum oxide, bauxite, clay and oxides containing aluminum. 39. Method according to claim 36, characterized in that the temperature in the electrolyte lies between the melting point of aluminum and the boiling point of the liquids in the cell. hO. Method according to claim 36, 37, 38 or 39, characterized in that the temperature in the electrolyte is between approx. 10° and 250°C above the melting point of the metal and the electrolyte containing only fluorides. hl. Method according to claim 36, 37" 38 or 39" characterized in that said reducing agent before coking is selected from the group consisting of an organic material which produces carbon during coking. 42. Method according to claim 36, 37 or 39" characterized in that said aluminum compound containing oxygen is mixed with said reducing agent in an amount of 1.5 to 50 parts by weight of Al^O^ per part reducing agent. 43• Method according to claim 36, characterized in that said compound and said reducing agent are well mixed and largely in mutual contact. 44. Method according to claim 36 or 39" characterized in that said reducing agent is selected from the group consisting of - a carbon compound or carbon cell, sulphur, phosphorus and arsenic and that said compound is selected from the group consisting of aluminum oxide, bauxite, and 1st ire. 45. Method according to claim 36, characterized in that said reducing agent is immersed in said electrolyte both at a temperature and for a sufficiently long time to coke said reducing agent. 46. Method according to claim 45"characterized in that said reducing agent before coking is selected from the group consisting of an organic material that gives carbon during coking. 47» Method according to claim 45, characterized in that said organic material is selected from the group consisting of tar, coal, coal products and natural or synthetic resin materials. 48. Method according to claim 36, characterized in that said electrolyte is mostly only produced from fluoride salts and said compound which is capable of reacting at the anode to produce aluminum ions for reduction to aluminum at the cathode and in that the temperature in the electrolyte is between 10° and 150°C above the melting point of aluminium. 49. Method according to claim 42, characterized in that the temperature in the electrolyte is between 10° and 150°C above the melting point of the metal and that said electrolyte is made largely only of fluoride salts <*> and said compound which is capable of reacting at the anode to produce aluminum ions for reduction to aluminum at the cathode. 50. Method according to claim 36, characterized in that it comprises a porous membrane placed in said electrolyte in such a way that said compound and said reducing agent are held back for reaction at the anode. 51. Method according to claim 36, characterized in that it comprises an electrolyte containing only fluoride and said reducing agent capable of reacting at the anode to produce metal ions for reduction to metal at the cathode. 52. Method - according to claim 48, characterized in that said temperature is in the range 700° to 800°C and the metal compound is aluminum oxide. 53«" Process for the electrodeposition of aluminum from aluminum chloride using an electrolytic cell having a current source, an electrolyte containing aluminum chloride for conducting electric current and a cathode for depositing aluminum, characterized in that a combination of a aluminum compound containing oxygen in a sufficient amount to yield aluminum during electrolysis and a reducing agent in contact with said compound wherein the weight ratio between said aluminum compound expressed as A^O^ and said reducing agent is at least about 1.5 to 1, electrolyzes aluminum ions from the electrolyte to deposit aluminum at the cathode, and by continuously renewing the aluminum ions that are electrolysed by reacting the aluminum compound with the reducing agent at the cathode. 54. Method according to claim 53" characterized in that said aluminum compound is an aluminum compound containing oxygen which is capable of reacting with the reducing agent at the anode to produce aluminum ions. 55. Method according to claim 53"characterized in that said aluminum compound is selected from the group consisting of aluminum oxide, bauxite and clay <*>" 56. Method according to claim 53 in 5^ or 55" characterized in that said ratio is flour lom 1.5 to 50 parts by weight Al^ O^ per part reducing agent. 57 • Method according to claim 53» 5^-» 55 or. 56" characterized in that said reducing agent before coking is selected from the group consisting of an organic material which gives carbon during coking. 58. Method according to claim 57" characterized in that said organic material is selected from the group consisting of tar, coal, coal products and natural or synthetic resin materials. 59"F method according to claim 53" 5^" 55 or 5°, characterized in that said aluminum compounds and said reducing agent are well mixed and largely in mutual contact. 60.. Method according to claim 53" characterized in that said reducing agent is selected from the group consisting of a compound containing carbon or carbon-celium, sulphur, phosphorus and arsenic, said aluminum compound is selected from the group consisting of aluminum oxide, bauxite and clay, and in that said aluminum compound and said reducing agent are well mixed and largely in mutual contact. 61. Method according to claim 53" characterized in that said reducing agent is immersed in said electrolyte both at a temperature over a sufficiently long time to coke said reducing agent. 62. Method according to claim 6l, characterized in that said reducing agent is selected before the coking from the group consisting of an organic material which gives carbon during coking. 63. Method according to claim 62, characterized in that said organic material is selected from the group consisting of tar, coal, coal products and natural or synthetic resin materials. 6k. Method according to claim 25, 26 or 27, characterized in that said aluminum compound and said reducing agent are well mixed and mostly in mutual contact. - 65. Method according to claim 25, 26, 27 or 28, characterized in that a membrane is placed in contact with said aluminum compound, largely to prevent the passage of undissolved ions through said membrane. 66. Method according to claim 25, 26, 27 or 29, characterized in that said membrane contains said aluminum compound to form an anode thereof. 67. Method according to claim 16, characterized in that the cell voltage is lowered by having aluminum chloride in the electrolyte bath. 68. Method according to claim 3.1, characterized in that said electrolyte bath contains a mixture of large amounts of NaAlCl^. 69. Electrode for use as an aluminum source in electropolishing of aluminum, characterized in that said electrode contains a mixture of an aluminum-containing material containing aluminum oxide and a reducing agent in contact with said aluminum oxide of said aluminum-containing materials, and at least one metal conductor in contact with said electrode to conduct electric current through said mixture, said conductor being placed inside said electrode and extending at least to the end of the electrode to be immersed in said electrolyte. 70. Electrode according to claim 69" characterized in that said metal conductor is placed in the longitudinal direction as a central core in said electrode. 71. Electrode according to claim 69> characterized in that said metal conductor is a series of pairs of rods which are placed transversely along the electrode. 72. Electrode according to claim 69> characterized in that said mixture is in the form of at least two adjacent laminates, said metal conductor is in the form of a plate placed between said laminates, so that a unit is formed. 73. Electrode according to claim 69, 70, 71 or 72, characterized in that the ratio between aluminum oxide in said aluminum-containing material and reducing agent is between 1.5 and 50. <**> " 74. Electrode according to claim 73" characterized in that the ratio between aluminum oxide in said aluminium-containing material and said reducing agent is between 2.5 and 6.0. 75. Cell according to claim 2, 3 or k, characterized in that it contains an electrode placed between said anode and cathode, and said electrode is in electrical contact with said aluminum compound and said reducing agent for bipolar electrolysis. 76. Cell according to claim 75> characterized in that said aluminum oxide compound is mixed with said reducing agent in an amount of 1.5 to 50.0 parts by weight of AlgO^ per part reducing agent. 77" Cell according to claim 75" characterized in that said reducing agent is selected from the group consisting of carbon and a compound containing carbon or carbon-celium, sulphur, phosphorus and arsenic. 78. Cell according to claim 75, 76 or 77, characterized in that said electrolyte largely consists of fluoride salt, said aluminum compound and in that the temperature in the electrolyte is between 700° and 800°C. 79" Cell according to claim 751 characterized in that said reducing agent is selected from the group consisting of carbon and a carbon compound or carbon cell, sulphur, phosphorus and arsenic, said aluminum oxide compound is well mixed with said reducing agent in an amount of from 1.5 to 20 parts by weight of A^O^ per part reducing agent, and in that said electrolyte consists largely only of fluoride salt, said aluminum compound, and is capable of reacting at the anode to produce aluminum ions for reduction to aluminum metal at the cathode and in that the temperature of the electrolyte is between 700° and 800°C. 80. Cell according to claim 75, characterized in that said electrode is coated only on one side with said compound and said reducing agent. 81. Cell according to claim 75" characterized in that said electrode is placed inside a membrane containing said compound and said reducing agent which is in contact with said compound. 82. Cell according to claim 81, characterized in that said membrane largely prevents the passage of insoluble, solid substances. 83. Method according to claim 36, characterized in that an electrode is placed between said anode and cathode, said electrode is brought into contact with said aluminum compound and said reducing agent and in that said electrode is allowed to be bipolar during the electrodeposition. 84. Method according to claim 83, characterized in that said aluminum oxide compound is mixed with said reducing agent in an amount of from 1.5 to 50 parts by weight Al^O^ per part reducing agent. 85. Method, according to claim 83" characterized in that said reducing agent is selected from the group consisting of a compound containing carbon or carbon cell, sulphur, phosphorus and arsenic. 86. Method according to claim 83, 84 or 85"characterized in that said electrolyte mainly consists only of fluoride salt, said aluminum compound and is"able to react at the anode to produce aluminum ions for reduction to aluminum metal at the cathode and in that the temperature in the electrolyte is between 700O and 800°C. 87. Method according to claim 84, characterized in that said reducing agent is selected from the group consisting of a carbon compound or carbon cell, sulphur, phosphorus and arsenic, said aluminum oxide compound is mixed with said reducing agent in an amount of from 1.5 to 50 parts by weight Al^ O^ per part reducing agent, said electrolyte being mostly only fluoride salt, said aluminum compound and capable of reacting at the anode to produce aluminum ions for reduction to aluminum metal at the cathode and in that the temperature of the electrolyte is between 700° and 800° C. 88. Method according to claim 84, characterized in that only one side of the electrode is coated with said compound and said reducing agent. 89. Method according to claim 83" characterized in that said electrode is placed inside a membrane that contains said compound and said reducer end agent which is in contact with said compound. 90. Method according to claims 36, 39 and 48, characterized in that there is an inclined surface on the surface of said anode and a complementary shaped surface on said cathode and in that a distance is selected in advance between said anodic surface and said cathodic surface. 91. Method according to claim 90, characterized in that said distance between said complementary shaped surface is up to approx. less than 6.25 cm. 92. Method - according to claim 90, characterized in that said distance is between 0.6 to 2.5 cm. 93» Method according to claim 90, characterized in that said inclined surface has an angle to the surface of said bath of approximately 10° to 80°C. 94. Method according to claim 90, characterized in that the temperature of said cathode surface which is immersed in said electrolyte is kept sufficiently high to prevent said electrolyte from solidifying on said cathode surface and thereby maintains said distance. 95" Electrolytic cell for electrodeposition of aluminium, characterized in that it comprises an electrode according to claim 1, immersed in an electrolytic bath and in that said cell comprises said electrode as an anode to produce aluminum for the deposition, where said anode has at least an inclined surface immersed in said electrolyte to increase the anodic surface which is exposed to erosion, and in that said cathode has at least part of a cathodic surface complementary shaped to said inclined anodic surface and wherein said complementary shaped cathodic surface and said anodic surface are placed in relation to each other at a pre-determined close distance in order to significantly reduce the energy consumption during said deposition. 96. Cell according to claim 95> characterized in that said distance between said complementary shaped surface is up to less than approx. 6.25 cm. 97" Cell according to claim 96, characterized in that said distance is between 0.6 and 2.5 cm. 98. Cell according to claim 95" characterized in that said inclined surface has an angle to the surface of said bath of from 10° to 80°C. 99" Cell according to claim 95" characterized in that said inclined surface has an angle which slopes downwards and inwards. 100. Cell according to claim 95"characterized in that a bottom surface of said anode is inclined at an angle in relation to the surface of said bath and in that the bottom surface of said cathode facing said anode is complementary shaped, at least in part in relation to to the said bottom over let on the anode.' 101. Cell according to claim 95, 96, 97, 98, 99 or 100, characterized in that the temperature in the electrolyte is between the melting point of aluminum and the boiling point of the liquids in the cell. 102. Cell according to claim 95, 96, 97, 98, 99 or 100, characterized in that said anode consists of at least one internally placed conductive material surrounded by anodic material containing the aluminum compound. 103. Cell according to claim 95, 96, 97, 98 or 99, characterized in that it contains a collection well for aluminum placed under said anode, where said collection well has a sloping floor under said anode to reduce the depth of the aluminum pool. 104. Cell according to claim 95, 96, 97, 98, 99 or 100, characterized in that a lining in said cell forms part of said cathode surface in contact with said electrolytic bath, where said cathode surface has a temperature that does not is less than the freezing temperature of said bath, thereby keeping a guard in advance, close distance. 105. Membrane for use in electrodeposition of aluminum and capable of withstanding the corrosive action of an electrolytic bath containing halide salts, characterized in that it comprises a material selected from the group consisting of carbon glass foam*" graphite or solid carbon, nitrides of boron, aluminium, silicon (including oxynitride), titanium, hafnium, zirconium and tantalum; silicides of molybdenum, tantalum and tungsten; carbides of hafnium, tantalum, niobium, zirconium, titanium, silicon, boron and tungsten; and borides of hafnium, tantalum, zirconium, niobium, titanium and silicon, where said material has a through porosity that is sufficient for all liquid materials in said electrolyte to pass through again, including ionic compounds and dissolved solids, while largely all undissolved solids that constitute impurities are pushed away to create electric current , and where the membrane has a continuous pore size with a sufficiently small diameter to keep out undissolved particles. 106. The membrane according to claim 105, characterized in that the said through porosity is between 1% to 97% and the connected pore size is between 1 micron and 1 cm. 107. The membrane according to claim 105, characterized in that the membrane is from 3>1 to 50 mm thick. 108. The membrane according to claim 105, characterized in that the membrane is between 3.1 and 12.5 mm thick. 109» The membrane according to claim 105, characterized in that said through porosity is between 30% and 70%. 110. The membrane according to claim 105, 107 or 109, characterized in that the connected pore size is between 1 micron and 1 cm.