Field of the Invention

The present invention concerns a new and improved electrode assembly system or unit for electrolytic cells used for electrolysis in molten salts, especially for electrolysis of alumina dissolved in molten cryolite.

Background of the Invention

The technology for the production of aluminum by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950 ^* C is more than one hundred years old.

This process, conceived almost simultaneously by Hall and Heroult, has not evolved as many other electrochemical processes. It is difficult to understand why, despite the tremendous growth in the total production of aluminum that in fifty years has increased almost one hundred fold, the process and the cell design have not undergone any great change or improvement.

The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by anthracite-based carbon blocks at the wall and at the cell floor bottom which acts as cathode and to which the negative pole of a direct current source is connected by means of steel conductor bars embedded in the carbon blocks.

The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still approximately 950 ^* C in order to have a sufficiently high alumina solubility and rate of dissolution which decreases rapidly at lower temperatures.

The carbonaceous materials used in Hall-Heroult cells as anode and as cell lining are certainly not ideal for resistance under the existing adverse operating conditions-

The anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form CO ₂ and small amounts of CO. The actual consumption of the anode is approximately 450 KG/Ton of aluminum produced which is more than 1/3 higher than the theoretical amount of

355 Kg/Ton corresponding to that of the stoichiometric reaction.

The carbon lining of the cathode bottom has a useful life of a few years after which the operation of the entire cell must be stopped and the cell relined at great cost. In spite of an aluminum pool having a thickness of more than 20 mm maintained over the cathode, the deterioration of the cathode carbon blocks cannot be avoided because of penetration of cryolite and liquid aluminum, as well as intercalation of sodium ions which causes swelling and deformation of the cathode carbon blocks and displacement of such blocks.

In addition, when cells are rebuilt, there are problems of disposal of the carbon which contains toxic compounds including cyanides.

The carbon blocks of the cell wall lining do not resist attach by cryolite, and a layer of solidified cryolite has to be maintained on the cell wall to extend its life. The major drawback, however, is due to the fact that irregular electromagnetic forces create waves in the molten aluminum pool and the anode-cathode distance (ACD), also called intereiectrode gap (IEG), must be kept at a safe minimum value of approximately 50 mm to avoid short circuiting between the cathodic aluminum and the anode. The high electrical resistivity of the electrolyte, which is about 0.4 Ohm.cm. , causes a voltage drop which alone represents more than 40% of the total voltage drop with a resulting energy efficiency which reaches only 25% in the most modern cells. The high incidence of the cost of energy, which has become even a bigger item in the total manufacturing cost of aluminum since the oil crisis, has decreased the rate of growth of this important metal.

In the second largest electrochemical industry following aluminum, namely the chlorine and caustic industry, the invention of dimensionally stable anodes (DSA ^β )

which were developed around 1970 permitted a revolutionary progress in chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine caustic purity.

The substitution of graphite anodes with DSA ^® increased drastically the life of the anodes and reduced substantially the cost of operating the cells. The rapid increase in chlorine caustic growth was stopped only by ecological concerns.

In the case of aluminum production, pollution is not due to die aluminum produced, but to the materials used in the process and to d e primitive cell design and operation which have remained the same over the years. Progress has been made in die operation of modern plants which utilize cells where die gases emanating from the cells are in large part collected and adequately scrubbed and where die emission of highly polluting gases during d e manufacture of the carbon anodes is carefully controlled.

However, die frequent substitution of the anodes in die cells is still a clumsy and unpleasant operation. This cannot be avoided or greatly improved due to die size and weight of me anode and die fact that the cathode is formed by die cell floor and is not removable during cell operation. Recently, progress has been made in the anode and die cathode composition, primarily with the development of non-carbon, substantially non-consumable anodes (NCA) and cadiodes (NCC). The life of these NCA and NCC is nevertheless limited and even d ese electrodes need occasional replacement or reconditioning.

Background Art

U.S. Patent 4,560,448-Sane et al discloses a recent development in molten salt electrolysis cells concerning making materials wettable by molten aluminum. However, the carbon or graphite anodes are of conventional design wid no suggestion leading to die present invention.

U.S. Patent 4681671-Duruz illustrates another improvement in molten salt electrolysis wherein operation at lower than usual temperatures is carried out utilizing permanent anodes, e.g. metal, alloy, ceramic or a metal-ceramic composite as disclosed in European Patent Application NO. 0030834 and U.S. Patent 4397729.

While improved operation is achieved at lower temperatures, there is no suggestion of die subject matter of the present invention.

PCT Application WO 89/06289 - La Camera et al deals widi molten salt electrolysis wherein attention is directed to an electrode having increased surface area. However, again, diere is no disclosure leading to the present invention.

The following references disclose several od er proposals to improve cell operation:

European Patent Application No. 0308015 de Nora discloses a novel current collector; European Patent Application No. 0308013 de Nora deals widi a novel composite cell bottom; and

European Patent Application No. 0132031 Dewing provides a novel cell lining-

While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none deal widi or suggest die present invention.

Summary of the Invention

This invention aims to overcome problems inherent in die conventional operation of electrolysis cells used in the production of aluminum via electrolysis of alumina dissolved in molten cryolite.

The invention permits more efficient cell operation particularly by modifying me electrode configuration, die materials of construction, and by utilizing a multi- double-polar cell employing a new me iod of operating die cell by means of me removal and reimmersion of an anode-ca iode double-polar electrode assembly system which, according to die invention, forms a single assembly. This assembly can be removed from the cell as a unit whenever the anode and/or me cadiode or any part of die electrode assembly unit needs reconditioning for good cell operation. The invention proposes a single anode-cadiode double polar electrode assembly system or unit including at least two assembly units of anodes and cathodes connected to a single source of electrical direct current, die assembly system being removable

or immersible or reimmersible as such into the molten electrolyte during operation of die electrolysis cell.

In particular the invention concerns an anode-cadiode double-polar electrode assembly forming an anode-cadiode electrode assembly system or unit of a new configuration to be utilized in multi-double-polar cells or continuous double-polar configurations for the production of aluminum, by the electrolysis of alumina dissolved in cryolite based molten salts.

In this assembly, the anode and cadiode materials are electrically conductive and their surface or coating is resistant to the electrolyte and to die respective products of electrolysis. The anode-cathode gap is maintained substantially constant and die anode and die cadiode are held togedier by means of connection elements made of material of high electrical, chemical and mechanical resistance, thus permitting the removal from and reimmersion in the molten electrolyte of a double- polar electrode assembly unit during operation of die multi-double-polar cell for the production of aluminum whenever the anode and/or die cadiode or any part of the electrode assembly unit may need reconditioning for efficient cell operation.

In the anode-camode double-polar electrode assembly units d e anode and d e cadiode surfaces may be substantially parallel in configuration whereby the current density across the gap is completely balanced. On d e other hand, d e anode-cadiode gap may slighdy be changed along a line at a 90° angle widi respect to the current path in order to balance die voltage drop in difference current paths and so as to maintain a more uniform current density over the entire active surface area of the electrodes. The lines of current path may of course be changed to be at any angle to die horizontal or vertical directions, i.e. substantially vertical, substantially horizontal or at an angle with die vertical.

The invention contemplates using a package, i.e. , a plurality of spaced apart anodes and cathodes connected by suitable electrically insulating means such as a bar or insulating layer The number of anode-cadiode combinations in a package can be varied as desired; generally from 4 to 100 are considered practical. The electrical contacts in such double-polar electrode assembly units or packages may taken on different configurations. For example die electrical contacts to die anode and cadiode of die double-polar electrode assembly unit may be both

made from the top of die multi-double-polar electrode assembly unit may be made from the top and diat to die cadiode may be made from the bottom.

In me double-polar electrode assembly unit die anodes may be made of porous material for greater active surface area and better evolution of die gas produced. Similarly die double-polar electrode assembly unit may contain cadiodes made of porous materials for better drainage of die aluminum produced. In fact porous materials may be used for die anodes, me cathodes, and/or for the non-conductive connections for better chemical and mechanical resistance.

Advantageously, die gas evolution and its guided displacement is utilized for better electrolyte circulation in the space between die anode and cadiode active surfaces.

Additionally the anodes of die anode-cadiode double-polar electrode assembly unit may be made from non-carbon, substantially non-consumable refractory materials resistant to the electrolyte, to the oxygen produced, and to odier gases, vapors, and fumes present in me cell. Such refractory materials normally may be selected from me group consisting of metals, metal alloys, intermetallic compounds and metal- oxyborides, oxides, oxyfluorides, ceramics, cermets, and mixtures thereof. The anode materials may also be made from metals, metal alloys, intermetallic compounds and/or metal-oxycompounds which contain primarily at least one of nickel, cobalt, aluminum, copper, iron, manganese, zinc, tin, chromium and lithium and mixtures thereof. Oxides and oxyfluorides, borides, ceramics and cermets which contain primarily at least one of zinc, tin, titanium, zirconium, tantalum, vanadium, lidiium, cerium, iron, chromium, nickel, cobalt, copper, yttrium, lanthamdes, and Misch metals and mixtures thereof may be also used. Adherent refractory coatings may be coated on anodes comprising an electrically conductive structure.

The cadiodes may be made of or coated widi an aluminu -wettable refractory hard metal (RHM) with litde or no possibility of molten cryolite attack. The refractory hard material may be a borides of titanium, zirconium, tantalum, chromium, nickel, cobalt, iron, niobium, and/or vanadium. Thus, the cathode may comprise a carbonaceous material, refractory ceramic, cermet, metal, metal alloy, intermetallic compound or metai-oxycompound having an adherent refractory coating

made of an aiuminum-wettable refractory hard metal (RHM). The carbonaceous material could be a andiracite based material or carbon or graphite.

Doping agents may be added to d e anode and cadiode materials to improve meir density, electrical conductivity, chemical and electrochemical resistance and odier characteristics.

All die materials mentioned above may be made by micropyretic reactions described in an earlier US patent application SN (ref. MOL0508, filed

April 1st 1992) whose contents are incorporated herein by way of reference.

The connections utilized to bind die anode to die cathode to form a single or multiple double-polar anode-cadiode electrode assembly may be made of any suitable electrically non-conductive material resistant to the electrolyte and die products of electrolysis. These include silicon nitride, aluminum nitride and odier nitrides as well as alumina and other oxides, and oxynitrides.

Micropyretic reactions starting from slurries may become the methods of making the anode-cadiode double-polar electrode assembly systems The slurries may contain reactant and non-reactant fillers. The non-reactant fillers may contain paniculate powders made of materials obtainable by the micropyretic reaction.

Micropyretic memods may be utilized to form die double-polar or multi- double-polar assemblies in a single operation. Multi-double-polar cells and packages are also contemplated containing two or more anode-cadiode double-polar single electrode assembly units. The multi- double-polar cells could have plates, cylinders or rods to optimize die voltage efficiency and work within the current density limitations of die materials being used. For instance, the anodes can be substantially cylindrical hollow bodies and die cadiodes can be rods placed inside such bodies. As stated before, porous materials may be employed. Memods of operating such cells are also envisaged widi various configurations of anodes and cadiodes in rod, V or cylindrical formation For instance, the anodes can have the shape of an inverted V and die cathodes have die shape of a prism placed inside d e anodes. All die assemblies are contemplated to be environmentally superior to current designs as die amount of CO ₂ and CO emissions are minimized to avoid pollution problems which dismrb the atmosphere and which delay die growdi or production of

aluminum- Computer monitoring of electrode gaps is also envisaged. All die assemblies described herein are expected to be immersible and/or reimmersible in the electrolyte. A continuous replacement strategy for the electrodes is also envisaged.

Brief Description of the Drawings

Reference is made to die drawings wherein:

Figure 1 is a schematic drawing of a molten salt electrolysis cell illustrating both a conventional anode and packages of anodes and cadiodes employing this invention.

Figure 2 is a schematic drawing of an anode-cathode double-polar cell utilizing a porous cadiode.

Figure 3 is a schematic drawing of another form of double-polar cell utilizing a porous cadiode. Figure 4 is a schematic drawing of another anode-cadiode configuration.

Figure 5 is a schematic drawing of another configuration where die anode active surface area is continuously replaceable.

Detailed Description of d e Drawings Referring to the drawings, in Figure 1 there is shown an electrolytic cell 10 containing molten cryolite 11 and aluminum 13 and containing both a conventional pre-baked carbon anode 12 as well as tiiree removable anode-cathode packages 14 of tiiis invention comprising alternate anodes 16 and cadiodes 18 held in spaced-apart relationship by a transverse electrically insulating bar 15. The anodes and cadiodes can be closely spaced to improve cell voltage and energy efficiency and overall good cell operating conditions. The anode-cadiode removable units or packages 14 offer substantially greater electrochemical active surfaces compared to currentiy employed anodes such as 12. Moreover, the electrically insulating bar 15 can be designed to be continuously adjustable to insure optimum distance and best performance. In Figure 2 there is shown an anode-cadiode double-polar cell 20 containing molten cryolite 22, aluminum 23 and an anode-cadiode assembly system 24 consisting of an anode 26 and a porous cathode 28 separated by mechanically strong electrically

insulating material 27 resistant to attack by molten cryolite. The pieces of materials 27 serve both as means for suspending die porous cathode 28 and as spacers leaving between the facing anode and cadiode surfaces a space containing die electrolyte, or the insulating material 27 could form a porous diaphragm with pores of sufficient size. Electrolysis circulation can be induced in die anode-cadiode gap. In operation, catiiodically-produced aluminum drips through the pores in cathode 28, and drips into die pool aluminum 23.

A preferred anode-cadiode double-polar electrode assembly is as set forth in Figure 3. In Figure 3 tiiere is shown an anode-cadiode double-polar cell 30 containing molten cryolite 32 and molten aluminum 34. The anode-cadiode double- polar single electrode assembly 36 includes an anode 38 and a porous cadiode 40. One or more horizontal insulating bars 42 separates the anode 38 and cadiode 40. d e cadiode 40 having a U-section as shown and being suspended from die insulating bar(s) 42. Note that the insulating bar 42 holding die anode 38 and cadiode 40 togedier is above the cryolite. The cathode 40 also may be formed of materials containing a plurality of holes.

Figure 4 illustrates an anode-cadiode configuration which can be fitted in a conventional aluminum production cell or in a cell of completely new design. In tiiis design, carbon prisms of inverted V shape or wedges 50 are fitted on a carbon cell bottom 52, preferably fixed tiiereon by bonding when die cells is being built or reconstructed. These carbon wedges 50 have inclined side faces, for instance at an angle of about 45° to 10° to the vertical, meeting along a top ridge 54. The wedges 50 are placed side by side, spaced apart at their bottoms to allow for a shallow pool 56 of aluminum on the cell bottom 52. The ridges 54, which can be rounded, are all parallel to each other across or along the cell and spaced several centimeters below the top level of die electrolyte 58.

The inclined side faces of die wedges 50 can be coated widi a permanent dimensionally stable aluminum-wettable coating, preferably one produced by a micropyretic reaction. The application of micropyretic reactions to produce electrodes for electrochemical processes, in particular for luminum production is d e subject of co-pending US patent applications SN 07/648,165 and SN 07/715/547, the contents of which are incorporated herein by reference.

Over die catiiode-forming wedges 50 are fitted anodes 60, each formed by a pair of plates which together fit like a roof over die wedges 50, parallel to the inclined surfaces of the wedges 50, providing an anode-cadiode spacing of about 10 to 60 mm, preferably 15 to 30 mm. At their tops, the pairs of anode plates 60 are joined togedier and connected to a positive current supply. Holes are provided towards die top of die anode for better escape of the gas evolved and useful electrolyte circulation. The anode plates 60 are made of or coated widi any suitable non-consumable or substantially non-consumable, electronically-conductive material resistant to die electrolyte and to die anode product of electrolysis, which is normally oxygen. For example, the plates may have a metal, alloy or cermet substrate which is protected in use by a cerium-oxyfluoride-based protective coating produced and/or maintained by maintaining a concentration of cerium in the electrolyte, as described in U.S. patent 4614569.

Odier refractory surfaces on carbonaceous or refractory substances can be produced by die methods described in co-pending U.S. patent application SN (ref MOL0508, filed April 1st 1992), die disclosures of which is incorporated herein by reference.

Adjacent pairs of anode plates 60 and their cathode wedges 50 are assembled togedier as units by an adequate number of horizontal bars 65 of insulating material, suspended from one or more central insulating posts 67. By this means, die entire unit can be removed from and replaced in die cell when required.

In all cases, the current flow is, of course, from anode to cadiode through the molten cryolite. In utilizing an anode-cathode double-polar electrode assembly of this invention, me voltage and energy efficiency can be singularly improved since the anode-cathode spacing can be minimized and significant numbers of assemblies put togedier to provide high efficiency while permitting easy removal of the anode- cadiode double-polar electrode assembly during cell operation from die molten electrolyte and reimmersion therein.

Since no conventional massive carbon anode is required, the electrode assembly of this invention can be significandy lighter in weight tiian conventional anodes, further, the materials of fabrication and technique of construction are readily available and can be produced and utilized in large quantities using relatively

inexpensive procedures. Since the anode-cadiodes double-polar electrode assembly can be formed of various configurations, it is available to retrofit existing aluminum production cells widi all the advantages set forth herein.

Figure 5 illustrates another embodiment of die invention disclosing a cell trough containing cryolite 72, aluminum 73, an upwardly-curved cadiode section 74 and a corresponding downwardly curved anode 76. The cathode has a central opening into which the produced aluminum can drain. The anode 76 can consist of flexible wire or a bundle of flexible wires or can be in the form of a flexible sheet. The anode and cadiode are made of materials as previously described herein. As shown, die anode 76 can be replaced continuously, e.g. by rotation, or at predetermined intervals as desired. The or each insulating bar 75 in this case has holes for the movement of the anode. This configuration is called die continuous double-polar construction.

The insulating bar 75 may be above or below the cryolite line. The insulating bar 75 serves to guide and space die anode(s) 76 from the cadiode 74. There can be several insulating bars 75 across the cell, and bars 75 at different levels. By means of the central upwardly projecting post or extension 77, die insulating bars 75 can be lifted out of the cell with its associated anodes 76 and cadiode 74, for servicing when required. Many of these continuous electrode assemblies or units can be set side by side in an electrolytic cell.

It will be understood tiiat die anode-cadiode electrode assembly can have other configurations such as cylindrical bodies (or of other shaped open cross section) wherein, e.g. the anodes are formed to surround cadiodes which are solid (or hollow) cylinders or of other cross sectional shape.

Further, whatever configuration is used, die anodes and/or cadiodes can be provided wid cooling means, e.g., internal fluid conduits to contain and permit the flowdirough of coolants.

In the practice of operating a multi-double-polar cell for the electrowinning of aluminum, it is one of die advantages of tiiis invention that one anode-cathode unit or a package of anode-catiiodes can be removed from the molten electrolyte while the cell is in operation and replaced by anodier anode-cadiode unit or package. This

- provides a singular improvement over conventional molten cell anode replacement operations. Further, this invention permits monitoring of anode-cathode performance under computer control to permit automatic removal of a faulty anode-cadiode package and automatic reimmersion of a new or renovated anode-cathode package. It is further feature of this invention tiiat the anode-cadiode gap can be maintained constant or made variable, e.g., where any lowering of the electrolyte bath electrical conductivity which occurs due to change in electrolyte bath composition or drop of the operating temperature can wholly or partially be compensated by decreasing die anode-cadiode gap witiiin limits permitted by an acceptable current efficiency.

The materials used to form the anode-cadiode can be and preferably are, porous, or contain a plurality of holes.

The anodes preferably are substantially non-consumable refractory materials resistant to die oxygen produced and die other gases, vapors and fumes present in die cell, and resistant to chemical attack by the electrolyte.

Useful refractory materials include metals, metal alloys, intermetallic compounds, metal oxyborides, oxides, oxyfluorides, ceramics, cermets and mixtures thereof. In the case of the metals, metal alloys, intermetallics and/or metal- oxycompounds, it is preferred that die component metals be selected from at least one of nickel, cobalt, aluminum, copper, iron, manganese, zinc, tin, chromium, lithium, and mixtures in a primary amount, i.e., at least 50% by weight.

In the case of oxides, oxyfluorides, borides, ceramics and cermets, it is preferred that they contain a primary amount, i.e., at least 50% by weight, of at least one of zinc, tin, titanium, zirconium, tantalum, vanadium, lithium, cerium, iron, chromium, nickel, cobalt, copper, yttrium, lanthanides, Misch metals and mixtures thereof.

The cathodes can be formed of or coated with an aluminum- wettable refractory hard metal (RHM) having litde or no solubility in aluminum and having good resistance to attach by molten cryolite. Useful RHM include borides of titanium, zirconium, tantalum, chromium, nickel, cobalt, iron, niobium and/or vanadium.

Useful cadiode materials also include carbonaceous materials such as anthracite, carbon or graphite.

It is preferred diat such a material be coated widi a RHM. Further information on RHM coatings is set forth co-pending in U.S. Patent Application SN (ref. MOLO508, filed on April 1st 1992), which is incorporated herein by reference. The anode and cadiode materials or at least their surfaces may also contain a small but effective amount of a dopant such as iron oxide, lithium oxide, or cerium oxide to improve their density, electrical conductivity, chemical and electrochemical resistance and odier characteristics.

Reference is now made to two examples of specific embodiments of the invention.

Example 1 A cell in die new configuration shown in Figure 1 was run in a small bath at 960°C containing molten cryolite. The anode plate material was made of a nickel alloy and die cadiode plate was made from antiiracite coated widi a TiB ₂ coating.

The anode and cadiode distance in the double-polar configuration was kept at 10 mm. Ceil voltage was 3. IV at a current of 1 Amp which translates to a current density of 0.7 Amp/cm ² . The anode-cadiode double-polar assembly is removed after 4 hours, cleaned to regenerate a fresh anode surface, the gap adjusted to 10 mm and die assembly reimmersed. The cell voltage returns to die original value of 3.1V at the same current. The test of removing and further reimmersion was carried out 24 times to establish the concept of die double-polar cell. The insulating bar in this test was made out of alumina.

Example 2

An electrode assembly in the configuration of Figure 3 was made and tried as a anode-cadiode double-polar electrode assembly. The anode was a solid block of nickel aluminide and die porous cathode was made of TiB ₂ . Stable and constant conditions were noted at a current density of 0.7 Amp/cm ² with an average anode- cadiode gap of 15 mm. This system was removed and reimmersed once every hour for 24 hours and a stable and constant cell voltage of 3.4 V was measured each time. The insulating bar in diis test was made out of alumina.

In conclusion, it has been shown that new anode-ca hode double-polar assemblies are possible and advantageous.