JP2007509232A - Electrochemical reduction of metal oxides - Google Patents

Abstract

  A method for electrochemical reduction of a solid state metal oxide feed in an electrolytic cell is disclosed. The electrolysis cell includes an electrolyte molten bath, an anode, a cathode, and a means for applying a potential between the anode and the cathode. The method requires supplying an amount of electrolyte into the bath that is greater than the amount of electrolyte required to replace the electrolyte removed from the bath with the reduced material, and the height of the bath. Characterized in that the molten electrolyte is removed from the bath so that it is maintained at or within the required height range.

Description

  The present invention relates to electrochemical reduction of metal oxides.

  The invention particularly relates to continuous or semi-continuous electrochemical reduction of metal oxides in powder form to produce metals having a low oxygen concentration, typically less than or equal to 0.2% by weight.

The present invention has been made during ongoing research projects for the electrochemical reduction of metal oxides being carried out by the applicant. The research project focuses on the reduction of titania (TiO ₂ ).

During the course of the research project, Applicants have investigated a series of titania reductions in an electrolysis cell that includes a pool of molten CaCl ₂ based electrolyte, an anode formed from graphite, and a series of cathodes. Have carried out experiments.

The CaCl ₂ electrolyte used in the experiment was commercially available CaCl ₂ , so-called calcium chloride dihydrate, which decomposed upon heating to produce very small amounts of CaO.

Applicants above the electrolytic cell than the decomposition potential of CaO, and was operated at a potential below the decomposition potential of CaCl _2.

  Applicants have determined that electrolysis cells can electrochemically reduce titania at these potentials to titanium with a low concentration, i.e., less than 0.2 wt% oxygen.

Applicants operated the electrolytic cell in a batch basis with titania in the form of pellets and larger solid blocks early in the study and later in the study with titania powder.
Applicants have also operated the electrolysis cell in batch mode with other metal oxides.

  Although research has established that it is possible to electrochemically reduce titania (and other metal oxides) in such electrolysis cells to metals with low oxygen concentrations, the applicant has identified It was understood that there is significant practical difficulty in industrially operating the electrolytic cell with the formula.

  In the course of considering the results of the study and the possibility of industrialization of the technology, the Applicant, while the metal oxide powders and pellets are transported through the cell in a controlled manner and discharged from the cell in reduced form, It has been found that commercial production can be achieved by operating the electrolysis cell continuously or semi-continuously.

  In the international application PCT / AU2003 / 001657 in the name of the Applicant, the invention is broadly described as a metal oxidation such as solid state titania in an electrolytic cell comprising a bath of molten electrolyte and a cathode and anode. A method for electrochemically reducing an object, wherein (a) a cell potential capable of electrochemically reducing a metal oxide supplied to a bath is applied between an anode and a cathode, and (b) a metal Supplying the oxide continuously or semi-continuously in the form of powder and / or pellets into the bath; (c) conveying the powder and / or pellets along a path in the bath and along the path Described as the above process comprising the steps of reducing the metal oxide as the metal oxide powder and / or pellets move and (d) continuously or semi-continuously removing the metal from the bath That.

  This international application defines the term “powder and / or pellets” to mean particles having a particle size of 3.5 mm or less.

  As used herein, the terms “powder” and “pellet” are understood to mean particles having a major dimension of less than 5 mm.

  The terms “powder” and “pellet” as used herein are not intended to limit the scope of patent protection to specific particle manufacturing procedures.

  The term “semi-continuous” is used in the international application and in the present specification, where the method includes (a) the period during which the metal oxide powder and / or pellets are fed to the cell and such metal oxide powder and / or Or includes a period of no supply of pellets to the cell, and (b) a period of removal of the reduced material from the cell and a period of no removal of such reduced material from the cell. .

  Within the Australian provisional application and throughout this specification, the general intent of the use of the terms “continuous” and “semi-continuous” is to describe the operation of cells other than batch.

  In contrast, the term “batch” is used in the international application and in this specification where the metal oxide is continuously fed to the cell and the reduced metal is fed to the end of the cell cycle. It is understood that the situation accumulates within, for example, as disclosed in the international application WO 01/62996 in the name of the Defense Minister.

  After making the first invention described above, the Applicant has conducted further research into the possibility of commercial production based on operating the electrolysis cell in a continuous or semi-continuous manner. Applicants have stated that the cell cathode to be contained in a cell for commercial production has a top surface for supporting the metal oxide in the form of pellets as described herein, and is horizontal. Placed or slightly tilted and immersed in an electrolyte bath and supported for movement, preferably for longitudinal movement, with the metal oxide pellets facing the front end of the cathode. I noticed that it is in the form of a plate-like member that can be moved.

  International application PCT / AU2004 / 000809 in the name of the applicant describes this so-called “shaker table” cathode invention in a broad way.

  The Applicant has conducted further research and development on the “shaker table” invention and has now designed a specific electrolysis cell according to the invention. The invention of the specific electrolysis cell design is the subject of an international application filed September 27, 2004 in the name of the present applicant claiming priority from the Australian provisional application 20000305261. Specific electrolysis cell designs feature multiple anodes and feature a support structure that separates and supports the “shaker table” cathode and the anode from above the cell, the support structure on the top surface of the “shaker table” cathode. The spacing of the upper anode can be adjusted.

  In the course of research and development on commercial production possibilities based on operating electrolytic cells in a continuous or semi-continuous manner, the applicant is required to supplement the electrolyte discharged from the bath with the reducing metal. We have found that it is important for efficient operation of the cell to purge more than one electrolyte from the bath continuously or periodically and to carefully add electrolyte to make up for this purge.

  In the course of its research and development, the Applicant can also clean the purged electrolyte to remove contaminants (eg, carbides and carbonates) and return the cleaned electrolyte to the bath. I realized that I could do it.

  In accordance with the present invention, the solid form metal oxide feed material is electrochemically contained in an electrolytic cell of the type comprising a molten bath of electrolyte, an anode, a cathode, and means for applying a potential between the anode and cathode. (A) a potential capable of electrochemically reducing the metal oxide supplied to the molten electrolyte bath is applied between the anode and the cathode, and (b) continuous or semi-solid. Continuously supplying the metal oxide feed into the bath; (c) conveying the metal oxide feed along a path in the bath; and as the feed moves along the path An electrolyte that is reduced and (d) continuously or semi-continuously removes at least partially reduced material from the bath and (e) reduced material lost from the bath and removed with the reduced material from the bath To compensate Supply an amount of electrolyte larger than the required amount of electrolyte into the bath and (f) maintain the height of the bath at the required height or within the required height range. There is provided a method as described above comprising the steps of removing molten electrolyte from the bath.

The method described above exceeds the amount of electrolyte required to compensate for the reduced material removed from the bath and the electrolyte retained by the reduced material during cell operation (typically CaCl _2). ) Is characterized by the careful addition (s) of step (e).

  The electrolyte addition in step (e) may be continuous or periodic.

  The electrolyte added to the bath in step (e) may be in the melt or solid phase.

  Preferably, step (e) includes supplying the electrolyte in an amount that is 70% to 100% of the amount of metal oxide feed supplied to the bath in step (b), based on a time average.

  The method also features step (f) of carefully removing the electrolyte from the bath so as to maintain the height of the bath at the required height or within the height range.

  In one, but not limited, embodiment, electrolyte removal is by an overflow weir in the cell. The removed electrolyte is treated to remove contaminants such as carbides and carbonates. The treated electrolyte is returned to the cell.

  Preferably, the metal oxide feed is in the form of powder and / or pellets as described herein.

  Preferably, the method includes treating the electrolyte removed from the bath in step (f) to remove contaminants and feeding the treated electrolyte to the bath.

  Preferably, the cathode has a top surface for supporting the metal oxide in powder and / or pellet form, is horizontally disposed or slightly tilted, and is immersed in an electrolyte bath. And is in the form of a plate-like member that is supported to face forward and backward movement so that the metal oxide powder and / or pellet can be moved toward the front end of the cathode.

  In this arrangement, preferably step (b) supplies a metal oxide and / or pellet form metal oxide feed to the bath such that the powder and / or pellets are deposited on the top surface of the cathode at the rear end of the cathode. Including that.

  Preferably, the method moves the metal oxide powder and / or pellets over the top surface of the cathode in contact with the molten electrolyte toward the front end of the cathode, thereby moving the powder and / or pellets toward the front end. A process in which electrochemical reduction of the metal oxide occurs.

  Preferably, step (b) comprises feeding the metal oxide powder and / or pellets into a molten electrolyte bath such that the powders and / or pellets form a monolayer on the top surface of the cathode.

  Preferably, step (c) carries the metal oxide powder and / or pellets as a layer of powder and / or pellet-filled monolayer by moving the metal oxide pellets from the top surface of the cathode toward the front end of the cathode. Including that.

  Preferably, step (c) includes selectively moving the cathode to move the metal oxide powder and / or pellets over the top surface of the cathode toward the front end of the cathode.

  Preferably, step (c) causes the powder and / or pellets to move at the same speed across the width of the cathode so that the metal oxide powder and / or pellets have substantially the same residence time in the bath. And carrying the metal oxide powder and / or pellets by moving the cathode.

  Preferably, the method electrochemically reduces the metal oxide to a metal having an oxygen concentration of 0.3% by weight or less.

  More preferably, the oxygen concentration is 0.2% by weight or less.

  The method may be a one-step method or a multi-step method involving one or more electrolysis cells.

  In situations where the metal oxide feed is in powder and / or pellet form, preferably the method in step (d) cleans the powder and / or pellets removed from the bath with the powder and / or pellets. It involves separating the electrolytes carried from the cell together.

  The method includes recovering the washed electrolyte from the powder and / or pellet and recycling the electrolyte to the cell.

  Preferably, the method includes maintaining the temperature of the cell below the temperature of electrolyte vaporization and / or decomposition.

  Preferably, the method includes applying a cell potential above the decomposition potential of at least one component of the electrolyte so that there are cations of a metal other than the cathode metal oxide cation in the electrolyte.

In the situation where the metal oxide is titania, the electrolyte is preferably a CaCl ₂ electrolyte containing CaO as one of the components.

  In such a situation, the method preferably includes maintaining the cell potential above the decomposition potential of CaO.

  In accordance with the present invention, an electrolytic cell for electrochemical reduction of a metal oxide feedstock comprising (a) a bath of molten electrolyte, (b) a cathode, (c) an anode, (d) an anode and a cathode (E) means for supplying a metal oxide feed material to the electrolyte bath; (f) removing at least partially electrochemically reduced metal oxide from the electrolyte bath. Means (g) supplying an amount of electrolyte in the bath that is greater than the amount of electrolyte required to compensate for the reduced material lost and the electrolyte retained by the reduced material removed from the bath. Said cell comprising means and (f) means for removing the molten electrolyte from the bath so as to maintain the height of the bath at or within the required height range. .

  Preferably, the electrolysis cell further includes means for treating the electrolyte removed from the bath in step (f) to remove contaminants from the electrolyte and feeding the treated electrolyte to the bath. As described above, target contaminants include carbides and carbonates.

  Preferably, the means for applying a potential between the anode and the cathode includes (a) a power source and (b) an electric circuit for electrically connecting the power source, the anode and the cathode.

  Preferably, the electrolysis cell includes means for treating the gas released from the cell.

  The gas processing means may include means for removing any one or more of carbon monoxide, carbon dioxide, chlorine-containing gas and phosgene from the gas.

  The gas treatment means may also include means for burning carbon monoxide gas in the gas.

In the situation where the metal oxide is titania, the electrolyte is preferably a CaCl ₂ electrolyte containing CaO as one of the components.

  Preferably, the particle size of the powder and / or pellet is in the range of 1-4 mm.

  Typically, the particle size of the powder and / or pellet is in the range of 1-3 mm.

  The invention is further described by way of example with reference to the drawings.

  In the following description, titania is electrochemically reduced to titanium metal having an oxygen concentration of less than 0.3% by weight. However, it should be noted that the present invention is not limited to this metal oxide and extends to other metal oxide powders and / or pellets and / or other forms.

The method involves an electrolyte (typically CaCl2) that exceeds the amount of electrolyte required to compensate for the reduced material and the electrolyte retained by the reduced material removed from the bath during cell operation. ₂ ) is characterized by a careful addition step.

  The method also features the step of carefully removing electrolyte from the bath to maintain the height of the bath in the cell.

  In this embodiment, the addition of electrolyte is in the form of an appropriately sized casting block, and electrolyte removal is by an overflow weir in the cell. The removed electrolyte is treated to remove CaO (by washing with HCl) and possibly to remove contaminants such as carbon. The treated electrolyte is returned to the cell. In practice, removal of the electrolyte and treatment of the electrolyte remove oxygen ions from the electrolyte. This is advantageous in that it improves the diffusion rate of oxygen ions from the cathode to the anode.

  The usual suggestion is to add titania to the cell at 0.8 kg / hour and electrolyte at 1.0 kg / hour. This is a relatively small amount of electrolyte addition in situations where the bath holds about 700 kg of salt, but it is a relatively significant addition in relation to the amount of titania added to the cell.

  The main component of the apparatus shown in FIG.

  The cell shown in the drawing is an enclosed chamber that is rectangular in plan view, and has a base wall 3, opposing end walls 5, opposing side walls 7, And a top cover 9.

The cell contains a molten electrolyte bath 21. A preferred electrolyte is CaCl ₂ containing at least some CaO.

  The cell includes an inlet 59 (see FIG. 2) for the solid electrolyte block in the top cover 9, at the right end when viewed in FIG. 2 and at the left end when viewed in other views.

  The apparatus includes a furnace 45 for melting the commercially supplied electrolyte and a casting station 47 for casting the molten electrolyte from the furnace into appropriately sized blocks.

  The cell has a series of inlets for titania pellets in the top cover 9, near the left end of the cell when viewed in FIGS. 1, 3, 4 and 5, and near the right end of the cell when viewed in FIG. , Including. This end of the cell is hereinafter referred to as the “rear end” of the cell. These inlets are identified by reference numeral 11 in FIG.

  The apparatus includes a pan pelletizer 51 that forms titania pellets in a “green” state, and a sintering furnace 53 that sinters the green pellets to provide sufficient strength to withstand subsequent processing steps. Contains. The sintered pellet is stored in a storage bin 55 as it is, and is supplied to the cell inlet 11 via a vibration feeder 57. Typically, the pellet has a size range of 1-4 mm.

  The cell further includes an outlet in the form of an overflow weir 49 (see FIGS. 3, 4 and 5) for removing excess electrolyte from cell 1. The overflow weir 49 is located at the end of the cell 1 opposite to the electrolyte inlet 59. The overflow weir 49 is an effective option to ensure that the electrolyte bath does not exceed a predetermined maximum height.

  The apparatus also includes a tank 57 for electrolyte removed from the cell 1 via the overflow weir 49.

  The apparatus also includes a processing station (not shown) for processing the electrolyte from tank 57 prior to recycling to cell 1. For example, the electrolyte may be treated to remove carbides and carbonates from the electrolyte. The treatment may also involve treating the electrolyte to remove other contaminants.

  The cell further has an outlet 13 for titanium metal pellets in the bottom wall 3 near the right end of the cell when viewed in FIGS. 1, 3, 4 and 5 and on the right side of the cell when viewed in FIG. Contains near the edge. This end of the cell is hereinafter referred to as the “front end” of the cell. The outlet 13 is in the form of a sump defined by a side 15 converging downward and an auger 35 inclined upwards, or the titanium pellets and the retained electrolyte are drawn from the bottom of the sump. Other suitable means arranged to accept and to remove the pellet from the cell.

  The cell further includes the cathode 25 in the form of a plate or other suitable member immersed in the bath 21 and disposed at a short distance above the bottom wall 21. The cathode plate 25 is supported in the cell by the support structure so that the upper surface of the cathode plate 25 is horizontal or slightly inclined downward from the rear end to the front end of the cell. The length and width dimensions of the cathode plate 25 are selected to be as large as possible to fit conveniently within the cell. The cathode plate 25 is supported so as to move in the front-rear direction by an oscillating motion as will be described later. The cathode plate support structure is described in detail in an international application filed September 27, 2004 in the name of the applicant, and the disclosure in the international application is hereby incorporated by reference. Built in.

  The cell further includes six anodes, generally identified by the numeral 19, extending into the bath 21. The anode 19 includes a graphite block 23 mounted at the end of a rod or other suitable support member 27. The anode block 19 includes a longitudinally extending slot 91 (see FIG. 2) that allows gas generated in the electrolyte bath 21 to escape from the bath. The anodes 19 are arranged in pairs, and the size of the anode block 23 is selected so that the anodes are positioned directly above substantially the entire upper surface of the cathode plate 25. The anode 19 is supported by a support structure so that the anode block 23 can be gradually lowered into the bath as the lower section of the anode graphite is consumed by the cell reaction at the anode. The cell top cover 9 includes an opening 95 for the support member 27 (see FIGS. 2 and 3). The anode support structure is described in detail in the above international application filed September 27, 2004 in the name of the applicant.

  Applicants believe that the movement of the cathode plate 25 in a repetitive sequence including a short back and forth motion, ie, a vibration motion and a short pause, is a series of short steps on the top surface of the pellet on the top surface of the cathode plate 25. Invented that it is possible to move the cell from the rear end to the front end.

  In addition, Applicants have stated that the type of motion described above can be moved at a constant speed across the width of the cathode plate 25 with the pellets so that the pellets have substantially the same residence time in the bath 21. Was revealed.

  The cell further includes a power source 31 for applying a potential between the anode block 23 and the cathode plate 25, and an electrical circuit (including the cathode support member 79 described above) connects the power source 31, the anode 23 and the cathode. Connect electrically. The size and / or position of the cathode support member 79 is selected to provide a preselected current distribution to the cathode plate 25 to optimize the electrochemical reduction of the titania pellets on the cathode plate 25. Yes. Depending on the environment, there will be a range of current distribution required for cell operation.

  The operation of the cell generates carbon monoxide and carbon dioxide and possibly chlorine containing gases in the anode block 23, so it is important to remove these gases from the cell. In addition, the cell includes an exhaust gas outlet 41 in the cell top cover 9 and a gas processing unit 43 for processing the exhaust gas, which releases the treated gas to the atmosphere. Gas treatment includes removing carbon dioxide and some chlorine gas, and may involve burning carbon monoxide to generate heat for the process.

  In use of the cell, titania pellets are fed to the upper surface of the cathode plate 25 at the rear end of the cell to form a monolayer on the cathode plate 25, and the cathode is moved as described above, and the pellets are plated. Step forward on the surface of the cell to the front edge of the cell, and finally drop from the front edge of the cathode. As the pellet is moved over the surface of the cathode plate 25, the pellet is progressively electrochemically reduced in the cell. The operating parameters of the cathode plate 25 are selected so that the pellet has sufficient residence time to achieve the required level of reduction of titania pellets in the cell. Typically, 2-4 mm titania pellets require a residence time of 4 hours to be reduced to titanium with an oxygen concentration of 0.3 wt% at a cell operating voltage of 3V.

  Applicants have shown that the above arrangement results in substantial reduction of the titania pellets within a short distance from the front edge of the cell.

  The titanium pellets are continuously or semi-continuously removed from the cell at the outlet 13 along with the electrolyte retained in the pores of the titanium pellets. The discharged material is conveyed by an auger 35 to a water spray chamber 37 and quenched to a temperature that is below the solidification temperature of the electrolyte, thereby preventing the electrolyte from direct metal exposure, This limits the oxidation of the metal. The discharged material is then washed to separate the retained electrolyte from the metal powder. The metal powder is then processed as required to produce the final product.

  More electrolyte than is needed to compensate for the net loss of electrolyte lost with the discharged titanium pellets is added to the cell at the inlet 59 in the form of a cast electrolyte block. As mentioned above, the purpose of the additional electrolyte is to purge the cell to maintain optimal operating conditions. When electrolyte is added to the cell from the inlet 59 at the back end of the cell, the electrolyte overflows from the weir 49 at the front end of the cell and flows into the storage tank 57 and then washed with HCl to remove CaO. And the treated electrolyte is returned to the cell as part of the electrolyte addition to the cell.

The anode block 23 is gradually consumed by the reaction between the carbon in the anode block 23 and the O ⁻⁻ anion generated in the cathode plate 25 during cell operation, and the reaction occurs mainly at the lower end of the anode block 23. The distance between the upper surface of the cathode plate 25 and the lower end of the anode block 23 is maintained as required to maintain optimum operating conditions within the cell. Preferably, the distance between the upper surface of the cathode plate 25 and the lower end of the anode block 23 is selected such that sufficient resistance heating occurs to maintain the bath 21 at the required operating temperature.

Preferably, the cell is operated at a potential higher than the decomposition potential of CaO in the electrolyte. Depending on the environment, the potential may be as high as 4-5V. According to the above mechanism, the operation above the decomposition potential of CaO is due to the presence of Ca ⁺⁺ cations, which promotes the deposition of Ca metal on the cathode plate 25 and as a result of the applied electric field. O to the anode block 23 ^- to facilitate the movement of the anion, and O ^- to promote the reaction between the carbon of the anion and the anode block 23 to generate carbon monoxide and carbon dioxide and to emit electrons. In addition, according to the above mechanism, the deposition of Ca metal resulting chemical reduction of titania via the mechanism described above, and O ^- anions to generate, the O ^- anions further and the electric field was applied electronic As a result of the discharge, it moves to the anode block 23. Operating the cell below the decomposition potential of CaCl ₂ minimizes the generation of chlorine gas, and this is an advantage of this scheme.

  The above cell and method is efficient and effective for electrochemically reducing metal oxides in powder and / or pellet form continuously or semi-continuously to produce reduced materials with low oxygen concentration Means.

  Numerous modifications may be made to the above-described aspects of the invention without departing from the spirit and scope of the invention.

  In particular, the electrolysis cell shown in the figure is only one example of the many possible cell configurations that are within the scope of the present invention.

FIG. 1 is a schematic diagram illustrating one embodiment of an electrochemical method and apparatus according to the present invention. FIG. 2 is a perspective view of the electrolysis cell of the apparatus shown in FIG. 1, omitting the cathode support bar for clarity of illustration. FIG. 3 is a longitudinal sectional view of the electrolysis cell shown in FIGS. 1 and 2. FIG. 4 is a longitudinal cross-sectional view shown in FIG. 3, but the top cover, anode, and anode support structure have been omitted to more clearly illustrate the cathode and cathode support structure. FIG. 5 is a longitudinal cross-sectional view shown in FIG. 3 with the top cover, cathode and cathode support structure omitted to more clearly illustrate the anode and anode support structure.

Claims (12)

  A method for electrochemical reduction of a solid form metal oxide feedstock in an electrolytic cell of the type comprising an electrolyte molten bath, an anode, a cathode, and means for applying a potential between the anode and cathode. (A) a potential capable of electrochemically reducing the metal oxide supplied to the molten electrolyte bath is applied between the anode and the cathode, and (b) the metal oxide continuously or semi-continuously. Feeding the feed into the bath; (c) conveying the metal oxide feed along a path in the bath and reducing the metal oxide as the feed moves along the path; (d) Required to continuously or semi-continuously remove the at least partially reduced material from the bath and (e) compensate for the reduced material lost from the bath and the electrolyte removed with the reduced material from the bath Amount taken A larger amount of electrolyte than the electrolyte is fed into the bath and (f) the molten electrolyte from the bath so as to maintain the height of the bath at or within the required height range. The above method comprising the steps of removing   The method of claim 1, wherein the electrolyte addition in step (e) is continuous or periodic.   The method according to claim 1 or 2, wherein the electrolyte added to the bath in step (e) is in a molten phase or a solid phase.   Step (e) comprises feeding electrolyte in an amount that is 70% to 100% of the amount of metal oxide feed fed to the bath in step (b) based on a time average. 4. The method according to any one of 3.   5. A method according to any one of claims 1 to 4, wherein the metal oxide feed is in the form of powder and / or pellets.   6. The method according to any one of claims 1 to 5, comprising treating the electrolyte removed from the bath in step (f) to remove contaminants and feeding the treated electrolyte to the bath.   7. Applying a cell potential above the decomposition potential of at least one component of the electrolyte so that there are metal cations other than those of the cathode metal oxide in the electrolyte. The method according to one item. Metal oxide is titania and the electrolyte is a CaCl ₂ based electrolyte containing CaO as one of the components, the method according to any one of claims 1 to 7.   9. A method according to any one of claims 1 to 8, comprising maintaining the cell potential above the decomposition potential of CaO.   An electrolysis cell for electrochemical reduction of a metal oxide feed, wherein (a) a bath of molten electrolyte, (b) a cathode, (c) an anode, (d) an electric potential is applied between the anode and the cathode (E) means for supplying a metal oxide feed material to the electrolyte bath; (f) means for removing the at least partially electrochemically reduced metal oxide from the electrolyte bath; Means for supplying an amount of electrolyte into the bath that is greater than the amount of electrolyte required to compensate for the reduced material and the electrolyte retained by the reduced material removed from the bath; and (f) An electrolysis cell as described above, comprising means for removing molten electrolyte from the bath to maintain the height of the bath at or within the required height range.   The cell of claim 10, further comprising means for treating the electrolyte removed from the bath in step (f) to remove contaminants from the electrolyte and supplying the treated electrolyte to the bath.   12. A cell according to claim 10 or claim 11, wherein the means for applying a potential between the anode and the cathode includes (a) a power source and (b) an electrical circuit electrically connecting the power source and the anode and cathode.

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