GB2483627A

GB2483627A - A bipolar electrolysis cell and method of operation

Info

Publication number: GB2483627A
Application number: GB1005736.2A
Authority: GB
Inventors: Lee Shaw
Original assignee: Metalysis Ltd
Current assignee: Metalysis Ltd
Priority date: 2010-04-06
Filing date: 2010-04-06
Publication date: 2012-03-21
Also published as: GB201005736D0

Abstract

A bipolar electrolysis cell 100 has a housing 110 defining a chamber 120 for containing molten salt electrolyte 160, first and second terminal electrodes and a plurality of bipolar elements 10 disposed between the first and second terminal electrodes 140,130. The bipolar elements 10 are removably loadable into the cell 100 and each bipolar element is capable of movement within the cell relative to the terminal electrodes 140,130. The electrolysis cell can be used for the reduction of a solid feedstock and the solid electrolyte 160 may comprise molten calcium chloride. In use, a potential is applied between the first and second terminal electrodes 140, 130 is such that a surface of each of the bipolar elements 10 becomes cathodic. One or more bipolar elements 10 are then removed from the cell adjacent the first terminal electrode and the remaining bipolar elements are moved within the cell towards the first terminal electrode 140.

Description

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Electrolysis cell and method of operation The invention relates to an electrolysis cell and a method of operating the cell.

The cell and method may have particular applicability to the reduction of a solid feedstock, for example for the production of metal by electrolytic reduction of a solid feedstock.

Backqjound The prior art comprises a number of electrolysis cells that may be used, for example, to reduce metal compounds or semi-metal compounds to metals, semi-metals or partially-reduced compounds, or to reduce mixtures of metal compounds to alloys. In order to avoid repetition, the term metal will be used in this document to encompass all such products, such as metals, semi-metals, alloys, intermetallics and partially reduced products.

In recent years there has been great interest in the direct production of metal by reduction of a solid feedstock, for example, a solid metal oxide feedstock. One such reduction process is the Cambridge FFC electro-decomposition process (as described in WO 99/64638). In the FFC method a solid compound, for example a solid metal oxide, is arranged in contact with a cathode in an electrolytic cell comprising a fused or molten salt. A potential is applied between the cathode and an anode of the cell such that the solid compound is reduced. In the FFC process the potential that reduces the solid compound is lower than a deposition potential for a cation from the fused salt. For example, if the fused salt is calcium chloride then the cathode potential at which the solid compound is (educed 5 lower than a deposition potential for depositing calcium from the salt.

Other reduction processes for reducing feedstock in the form of cathodically-connected solid metal compounds have been proposed, such as the Polar process described in WO 031076690.

Conventional implementations of the FFC and other electrolytic reduction processes are batch processes. A feedstock in the form of powder, particles or pellets is generally brought into contact with a cathode element and reduced within an electrolytic cell. Once the feedstock has been reduced it is removed from the cell and the process repeated.

While a batch process implementation of the FFC process may be very successful in terms of reducing a metal oxide to a metal, there are inherent inefficiencies in such processes. One issue is that a product is only produced intermittently, for example in a large volume at the end of a batch run. In many commercial situations it may be preferable for product to be produced on a near continuous basis. I0

It is an aim of the invention to provide an electrolysis cell capable of operating more efficiently than conventional batch process cells.

Summary of the invention

is The invention provides a bipolar electrolysis cell and a method of operating a bipolar electrolysis cell as defined in the appended independent claims to which reference should now be made. Preferred or advantageous features of the invention are defined in various dependent sub-claims.

Thus, in a first aspect a bipolar electrolysis cell comprises a housing defining a chamber for containing electrolyte. First and second terminal electrodes are disposed within the chamber and a plurality of bipolar elements is disposable between the first and second terminal electrodes. The cell comprises means for allowing a potential to be applied between the two electrodes such that one surface of each bipolar element becomes cathodic. Each bipolar element is capable of being loaded into and unloaded out of the electrolysis cell.

Furthermore, when loaded in the electrolysis cell each bipolar element is capable of linear displacement between the first and second terminal electrodes.

It may be advantageous that each bipolar element is a discrete bipolar element and can, thus, be loaded into the cell and unloaded out of the cell separately from other elements.

Alternatively, it may be desirable that the bipolar elements can be unloaded and loaded in groups consisting of more than I bipolar element. In this case the groups may be formed by discrete elements that are loaded or unloaded in groups of the desired number of elements, or may be bundles of elements that are permanently associated with each other. Where the elements are loaded s and unloaded in groups, the number of elements in a group will be lower than the total number of elements that can be loaded into the cell.

A group of bipolar elements may be loaded into the cell in series, i.e. where each of the elements is spaced apart such that one of the group is closer to the anode and another of the group is closer to the cathode. Alternatively, a group of bipolar elements may be loaded into the cell in parallel, i.e. where each of the elements of the group are adjacent each other and at approximately the same distance from both the anode and cathode terminals. This latter arrangement may be used to increase the capacity of a cell and may require the use of terminal electrodes that have greater surface area than each individual element, or the use of multiple terminal anodes and multiple terminal cathodes within the same cell.

It is described that a potential is applied between the terminal electrodes such that one surface of each bipolar element becomes cathodic. Likewise, this may be described as a potential being applied such that one surface of each element become anodic.

Advantageously, a bipolar cell as defined herein may be of particular advantage as cell for the reduction of a solid feedstock. In such a cell,a feedstock, for example a powdered feedstock or a granular feedstock or a feedstock consisting of a plurality of preformed particles or pellets, is preferably capable of being retained in contact with the cathodic surface of one or more of the bipolar elements. Preferably feedstock can be retained in contact with the cathodic surface of each bipolar element.

For the reduction of a solid feedstock it is preferable that the electrolyte is a molten or fused salt, for example a molten halide salt. A number of suitable molten salts are disclosed in WO 99/64638, the contents of which are incorporated by reference. Other molten salt compositions may also be used. It is particularly preferable that the molten salt is a calcium chloride salt or comprises calcium chloride.

The first terminal electrode of the bipolar electrolysis cell maybe an anode and the second terminal electrode may be a cathode. Alternatively, the first terminal electrode may be a cathode and the second terminal electrode may be an anode. Preferably these terminal electrodes are connected to or couplable to a power supply external to the chamber such that a potential can be applied between the terminal electrodes.

For some applications it may be beneficial if one of the terminal electrodes is capable of making a physical electrical connection with one of the bipolar elements when in use. Thus, it may be advantageous, for use in a cell for the reduction of solid feedstock, that the cathode is capable of a physical electrical connection with a bipolar element. Thus, the or each bipolar element, when connected physically to one of the terminal electrodes, for example the cathode, acts as a terminal electrode. For example, if the cathode terminal is physically electrically connected to a bipolar element, that bipolar element would in effect act as a terminal cathode for the period of time that it is coupled to or connected to the cathode terminal.

Each bipolar element may be capable of being separately loaded into, and separately unloaded out of, the cell. Alternatively, groups of elements may be loaded and unloaded simultaneously. In some cell configurations it may be preferable for the housing to define a first opening for loading bipolar elements into the chamber and second opening for unloading bipolar elements out of the chamber. For example, the cell may comprise means for unloading a bipolar element adjacent to the first terminal electrode and a means for loading a bipolar element into the cell adjacent to the second terminal electrode. Such means may include separate chambers adjacent to the bipolar electrolysis cell for holding a bipolar element prior to introduction to the electrolysis cell and for holding a bipolar element subsequent to withdrawal from the electrolysis cell.

It may be desirable to preheat bipolar elements prior to loading them into the s cell. Such preheating may involve ramping the temperature of the bipolar

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element up to the working temperature of the salt. In this way, sudden temperature changes in the bipolar element are prevented and the element is less likely to freeze the molten salt on introduction into the cell. It may be advantageous to preheat the bipolar elements in a holding chamber adjacent to s the cell.

Likewise1 it may be desirable to cool bipolar elements removed from the cell in a controlled manner. Such cooling may be achieved in a holding chamber.

The cell may advantageously include means for moving bipolar elements towards the first terminal electrode or, alternatively, towards the second terminal electrode. For example, the bipolar elements may be mounted within the cell by means such as a rail along which the bipolar elements can slide.

Such a rail is preferably mounted in a line between the terminal electrodes such is that the bipolar elements are capable of linear movement between the terminal electrodes. Alternative means can be envisaged including moving belts, moving chain drives and walking beam mechanisms. Thus, the cell preferably includes a means or mechanism for moving the bipolar elements along this line while loaded in the cell.

Preferably the bipolar elements have length and width dimensions substantially greater than their thickness dimension. For example the bipolar elements may be described as substantially plate-like in shape.

The thickness of the elements may preferably be in the range of between 2 cm and 10cm, for example 3cm, 4 cm, or 5cm.

The bipolar elements preferably have length and breadth (or diameter) of the order of between 50 cm and 500 cm, for example about 75 cm or 100 cm or 3o 150 cm.

Each bipolar element need not be a solid element but may be formed from a mesh, or comprise a mesh or a plurality of rods defining the overall shape of the bipolar element. The bipolar elements may be formed comprising a plate or a sheet defining a plurality of through-holes.

Each bipolar element may comprise a composite structure. For example, the bipolar element may have a first material comprising the cathodic surface or side of the bipolar element and a second material comprising the anodic s surface or side of the bipolar element.

Advantageously, the first terminal electrode and the second terminal electrode may be substantially horizontally spaced from each other within the electrolysis cell chamber. In this arrangement the bipolar elements may be disposed within io the chamber between the terminals such that they can move in a horizontal direction.

Alternatively, the first terminal electrode and the second terminal electrode may be substantially vertically spaced from each other within the electrolysis cell chamber. In this arrangement the bipolar elements may be disposed between the terminals such that the elements can move in a vertical direction.

It may be advantageous that the cell comprises spacer elements in order to maintain a predetermined gap between bipolar elements within the cell.

Preferably any spacer elements are associated with, or attached to, each bipolar element such that when the bipolar elements are loaded within the chamber of the cell they can be maintained at a predetermined distance apart.

It is preferred that the spacing between bipolar elements is maintained at a distance of between 4 cm and 20 cm, for example between 5 cm and 15 cm, or between 6cm and 10cm.

Spacer elements need not be required if a predetermined gap can be maintained between plates by other means. The plates may be suspended within the cell at discrete intervals such that a required spacing between the plates is maintained. As an example, if the plates are suspended by a walking beam structure then a discrete spacing will be maintained between separate plates as each plate. As a further example, separation may be achieved by a frame structure, rather than by a physical spacer associated with each plate.

Where the electrolysis cell is to be used for the reduction of a solid feedstock it may be advantageous that each bipolar element comprises a means for retaining the feedstock in contact with its cathodic surface (i.e. the surface that becomes cathodic when a potential is applied between the terminal electrodes).

Examples of such means or mechanisms include a tray, basket, or a clamping means. Pins, hooks, or pegs may also be used to retain suitably shaped feedstock, for example feedstock that has been extruded in the form of a honeycomb. Furthermore, the bipolar elements may comprise shelf-like structures that allow feedstock to be poured into the cathodic surface of the element and retained. Where the bipolar elements are furnished in a substantially horizontal orientation, gravitational forces may be sufficient to retain the feedstock in contact with the element.

It may be advantageous that the bipolar electrolysis cell is used for a is continuous process, with bipolar elements being loaded into one portion of the cell and sequentially removed from another portion of the cell. In order to facilitate a continuous process it may be advantageous that the electrolyte is capable of being constantly refreshed and the composition of the electrolyte is capable of being monitored, and then maintained within certain predetermined limits. Thus, it may be advantageous that the housing of the electrolysis cell defines an electrolyte inlet, for example a molten salt inlet, and an electrolyte outlet, for example a molten salt outlet, situated such that electrolyte can enter the chamber through the inlet and exit the chamber through the outlet. The inlet and outlet may be coupled to a reservoir for the electrolyte and a constant flow may be provided through the cell by an electrolyte pumping means.

A second aspect provides a method of operating a bipolar electrolysis cell, the cell comprising first and second terminal electrodes disposed in contact with an electrolyte within a chamber of the cell and plurality of bipolar elements each occupying a position between the first and second terminal electrodes. The method comprises the steps of: a) -Applying a potential between first and second terminal electrodes such that a first surface of each of the bipolar electrodes becomes cathodic. The potential may be applied only for a predetermined period of time.

b) -Removing a bipolar element occupying a final position, closest to the first terminal electrode, from the cell.

s c) -Moving the remaining bipolar elements towards the first terminal electrode such that a new bipolar element occupies the final position and a first position closest to the second terminal becomes unoccupied.

d) -Loading a new bipolar element into the first position, closest to the second io terminal electrode.

Advantageously the method may comprise the step of repeating steps a to d.

Steps a to d may be repeated as often as necessary.

Where reference is made to removing a bipolar element, a single bipolar element or a group of bipolar elements that are a subset of the plurality of bipolar elements could be removed. Likewise, reference to loading a new bipolar element may refer to loading a single element or a group of elements.

Preferably the new bipolar element (i.e. the bipolar element introduced into the cell in the first position or an equivalent group of elements) migrates from the first position to the final position via a plurality of intermediate positions. The number of intermediate position preferably depends on the number of bipolar elements disposed between the terminals. For example, if the electrolysis cell has a capacity of ten discrete bipolar elements between the first and second terminal eiectrodes then each bipolar element will occupy ten different positions as it migrates through the cell, starting with the first position adjacent to the second terminal electrode and ending in the tenth and final position adjacent to the first terminal electrode.

The method may be advantageously used for the reduction of a solid feedstock. In such a method the electrolyte is preferably a molten salt electrolyte and the solid feedstock is arranged in contact with the first surface of at least one of the bipolar elements prior to that element being loaded into the s cell. The potential applied between the terminal electrodes in such a method is sufficient to reduce the solid feedstock.

It is particularly preferable to use this method for the reduction of a solid oxide feedstock to metal, for example by an FFC type electro-deoxidation process as described in WO 99/64638. In this case the potential applied between the terminal electrodes is such that the cathodic potential at the cathodic surface of each bipolar element is sufficient to reduce the metal oxide feedstock that is in contact with the cathodic surface of the bipolar element.

io Thus, the sold feedstock may be reduced in a plurality of stages, and preferably reduced to metal. As an example, if the electrolysis cell has a capacity of ten bipolar elements and a potential is applied for a period of one hour between removing an element from one part of the electrolysis cell and loading a bipolar element into another part of the electrolysis cell, then each bipolar element will is undergo a total of ten hours of reduction while it processes or migrates through the electrolysis cell.

It may be particularly advantageous that the electrolyte is cycled between the electrolysis cell and an electrolyte reservoir, for example a molten salt reservoir.

This process allows the composition of the electrolyte to be monitored and for the composition to be controlled within predetermined limits.

While the method may be particularly advantageous for use in the reduction of a solid feedstock comprising a metal oxide, the method may also advantageously be used to remove dissolved oxygen from a metal containing excess dissolved oxygen. For the removal of dissolved oxygen from a metal the overall processing time may be considerably shorter than would be required for removing oxygen from a metal oxide.

Preferred Embodiments of the Invention A specific embodiment of the invention will now be described with reference to drawings in which; Figure 1 is a schematic illustration showing a side view of a bipolar element for the reduction of a solid oxide feedstock; Figure 2 is a schematic illustration showing a front view of the bipolar element of figure 1; Figure 3 is a schematic illustration of a cell having four bipolar elements (each as illustrated in figure 1) suspended in a horizontal line between a terminal anode and a terminal cathode; Figure 4 illustrates the removal of a bipolar element from the cell of figure 3; I0 Figure 5 illustrates the movement of bipolar elements within the cell of figure 3 towards the terminal anode; Figure 6 illustrates the loading of a new bipolar element into the cell of figure 3; Figure 7 illustrates the movement of the terminal cathode to contact a newly loaded bipolar element in the electrolysis cell of figure 3.

An embodiment of the invention will be described both in general terms and as an example suitable for the reduction of tantalum oxide.

Figures 1 and 2 illustrate a bipolar element 10 for use in an electrolysis cell according to an embodiment of the invention. The bipolar element 10 is viewed from the side in figure 1 and from the front in figure 2. The element comprises a bipolar plate 20, a spacing element 30 and a hook 40 for suspending the bipolar element from a rail. The bipolar plate 20 has a surface which, in use, becomes anodic 22 and a surface which, in use, becomes cathodic 23.

A metal oxide feedstock 50, in the form of oxide performs, is arranged in contact with the cathodic surface 23 of the bipolar plate 20 by means of a basket structure 60.

In use in an electrolysis cell the bipolar element 10 is disposed in contact with a molten salt contained within the cell, and between terminal electrodes in the cell. Which surface of the plate 20 becomes anodic and which becomes cathodic depends on the orientation of the element between the terminal electrodes. The surface of the plate 20 nearest the cathode terminal will become anodic when a potential is applied between the two terminals, and the surface nearest the anode terminal will become cathodic. In this specific example, the element is oriented so that the surface of the plate comprising the basket structure 60 for retaining the feedstock 50 is arranged such that it faces the terminal anode.

For the avoidance of doubt, the elements of the bipolar element 10 are only illustrated schematically in figures 1 and 2.

The bipolar plate may be formed from a single material, for example a dimensionally stable anode material or an oxygen evolving anode material.

The bipolar plate may also be formed entirely from a carbon material. As an alternative, the bipolar plate 20 may be formed such that its anodic surface 22 is formed from one type of material, for example carbon, and its cathodic surface is formed from another type of material, for example a metallic material.

Figure 3 is a schematic illustration of an electrolysis cell 100 having a housing defining a chamber 120. The chamber 120 contains a terminal cathode 130 and a terminal anode 140, these terminal electrodes being situated on opposite sides of the chamber. The terminal anode 140 is electrically coupled to a terminal anode plate 145. Four bipolar elements 10 are disposed between the terminal anode 140 and terminal cathode 130. Each of these bipolar elements 10 has the same construction as the bipolar element illustrated in figure 1 and described above.

Each of the bipolar elements 10 is suspended via hooks 40 from a rail 150 that extends across the cell. The bipolar elements 10 are suspended such that they hang between the terminal anode 140 and the terminal cathode 130.

The chamber 120 preferably contains a molten calcium chloride based salt 160 and the terminal anode, terminal cathode, and all four of the bipolar elements are arranged to be substantially below the surface of the molten salt 165 when the cell is in use.

The terminal anode 140 and the terminal cathode 130 are coupled to a power supply to enable a potential to be applied across the cell such that a cathodic surface and an anodic surface are formed on the bipolar elements 10.

Each of the bipolar elements is maintained at a predetermined spacing between adjacent anodic and cathodic surfaces by means of the spacer elements 30. Spacer elements 30 are electrically insulating and preferably made from a material that does not react with its environment under cell operating conditions. Examples of suitable spacer materials may include io yttrium oxide, aluminium oxide and silicon nitride. The range of suitable spacer materials may be increased if the spacer is arranged so that it does not contact the molten slat during cell operation.

The cathodic terminal 130 impinges upon the bipolar element in a first position (this element has been denoted as A in the figures). This bipolar element (A) is thus in a physical electrical communication with the cathodic terminal.

The bipolar element (A) in the first position impinges on the bipolar element (B) in a second position via its spacing element 30. This maintains the predetermined gap between the cathodic surface 23 of bipolar element (A) and the anodic surface 22 of the bipolar element (B). Likewise the bipolar element in the second position (B) impinges on the bipolar element (C) in a third position via its spacing element 30. Likewise the bipolar element (C) in the third position impinges on a bipolar element (D) in a fourth position via its spacing element 30. The bipolar element in the fourth position (D) impinges on the anode plate of the anode terminal 140 such that the cathodic surface of the fourth element is maintained at the predetermined distance from the anode plate.

In use in this arrangement, a potential can be applied between the terminal anode 140 and the terminal cathode 130 such that the cathodic potential at the cathodic surface 23 of each bipolar element is sufficient to reduce the tantalum oxide performs, which are arranged in contact with the cathodic surface of each bipolar element and in contact with the molten salt electrolyte.

It is clear that many different numbers of bipolar elements may be used for an electrolysis cell according to various aspects of the invention. For example it is likely that a cell for industrial use would comprise at least between four and twenty bipolar elements, and may have many more elements.

As each bipolar element can be separately removed from the cell, and can be linearly moved within the cell between the terminal anode 140 and the terminal cathode 130, it is envisaged that the cell may be advantageously used for a continuous or semi-continuous electrolytic process. In such a process bipolar to elements holding a reduced feedstock will be repeatedly removed from one portion of the cell and bipolar elements holding fresh feedstock loaded into another portion of the cell, each bipolar element migrating through the cell over a period of time. During the overall period of time in which the element migrates through the cell the oxide preforms attached to that element are is reduced, preferably to metal. This method is illustrated by figures 4 to 7.

After a predetermined period of electrolysis, the potential applied between the terminal anode 140 and the terminal cathode 130 may be removed. The bipolar element adjacent to the anode 140 can then be removed from the molten salt by lifting the element (the element denoted as D in the figures) out of the cell. This is illustrated in figure 4. The remaining bipolar elements (A, B & C) are then moved through the cell towards the anode. This movement may be advantageously accomplished by pushing the bipolar elements, for example with a movable cathode terminal 130, such that the elements slide along the supporting rail 150 towards the terminal anode 140. There may, of course, be other mechanisms for moving the bipolar elements within the cell.

When the separating element 30 of the bipolar element (C) contacts the anode plate of the anode terminal 140 the movement of the bipolar elements is halted.

This is illustrated in figure 5. This cathode terminal 130 can then be withdrawn and a new bipolar element (denoted as element Z) can be inserted into the electrolysis cell by lowering into the position between the bipolar element (A) and the cathode terminal 130 until the bipolar element (Z) is suspended on the rail 150. This is illustrated in figure 6. The cathode terminal 130 may then be s moved towards the bipolar elements until it contacts the bipolar element (Z) that currently occupies the first position in the cell. This is shown in figure 7.

The result of this method is that the bipolar element closest to the anode has been removed from the cell. The remaining bipolar elements have migrated s within the cell, by one position, towards the anode. A new bipolar element has been inserted into the cell adjacent to the cathode. Once the cathode terminal has contacted the new bipolar element a potential can be applied once more between the terminal anode and the terminal cathode and a further reduction for a further predetermined period of time can be carried out.

Each cycle of the method of this specific embodiment comprises the steps of; applying a potential for a predetermined period of time, removing the potential, removing the bipolar element closest to the anode, moving remaining bipolar elements towards the anode, and inserting a new bipolar element adjacent to is the cathode. If the electrolysis cell holds N bipolar elements (where N is any whole number) then a bipolar element adjacent to the cathode must go through N cycles before it is removed from the position adjacent to the anode.

Assuming the period of time allowed for reduction in each cycle (T) is the same, then the bipolar element will undergo a total period of reduction given by the product of N multiplied by T. The apparatus and method described above may be used for the reduction of a large number of metal oxides and mixtures of metal oxides. Preferable metals that can be produced by such reduction include tantalum, titanium, aluminium, silicon and alloy materials such as Ti6AI4V or ferrotitanium. For the reduction of metal oxide preforms the total reduction time for preforms attached to any bipolar element is likely to be somewhere between 8 and 56 hours.

If the composition of the salt is maintained within predetermined limits then it may be possible to keep the cell operating for extended periods of time during which many tens or hundreds of bipolar elements pass through the electrolysis cell. This may advantageously reduce or eliminate dead time associated with heating the cell to working temperature and cooling the cell after electrolysis.

The nature of the process also provides a continuous stream of reduced feedstock for further processing.

The apparatus and method described above rely on the bipolar elements being moved along a slide rail by a force provided by a movable cathode. In an alternative embodiment, the bipolar elements within the cell may be moved by a walking beam mechanism. For example, the elements may be supported within the cell by a pair of static rails. Each static rail may be associated with a walking beam, or indexing rail, that is able to move with a cyclical motion relative to the static rails. To move the elements towards the anode, the walking beam moves upwards and lifts the elements from the static rail. The io beam then moves forward, thereby moving the elements forward by a distance, and down, depositing the elements back onto the static rail. The walking beam then returns to its starting point. By repeating this motion the elements may be moved forward in a series of discrete steps. An advantage of such movement it that the need for a sliding surface along which the elements can move is is eliminated.

A further advantage or a walking beam mechanism is that, as each bipolar element is located at an indexed position on the beam mechanism, spacers may not be required to maintain elements at a predetermined spacing.

Movement of the cathode contact may still be required in a cell with a walking beam arrangement, but the movement need only be sufficient to create contact with the closest bipolar element, where this is required.

The specific embodiments described above describe the operation of a cell in which a single bipolar element is replaced at each step. It is clear that more than one element may be replaced at any step, and indeed this may be desirable for high volume throughput in a large cell. Thus, two or three or four or more elements may be removed from the cell at any one time, and likewise, two or three or four or more may be loaded into the cell at any one time to replace the elements that have previously been removed.

The following specific example provides specific dimensions of the bipolar elements and cell operating conditions that could be used for the reduction of a tantalum oxide feedstock to metallic tantalum. The description refers to the same figures described in more general terms above. The same arrangement would also be suitable for the reduction of many other metal oxides to metal, for example for the reduction of titanium oxide to titanium, and for the removal of excess oxygen from metallic feedstock.

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The exemplary cell for the reduction of tantalum oxide 100 contains ten bipolar elements or plates 10. Each bipolar element 10 is formed as a composite structure having a graphite portion with dimensions of 100cm by 100cm by 5 cm thick. The graphite portion forms the anodic surface 22 of the element 10.

The cathodic surface 23 of each element is formed from a stainless steel plate having dimensions of 100 cm by 100 cm by 2 mm. This stainless steel plate is fastened to the graphite plate to form the cathodic surface 23 of the element and feedstock 50 is retained at the surface of the plate by means of a stainless steel basket structure 60.

The spacer elements 30 are made from silicon nitride and hold the bipolar elements 10 such that the feedstock 60 on the cathodic surface 23 is held at a distance of 10 cm from the anodic surface of an adjacent bipolar plate.

The molten salt within the cell for the reduction process is molten calcium chloride containing between 0.3 % and 0.6 % calcium oxide. A working temperature of 800 °C is suitable for the reduction of tantalum oxide.

To effect reduction, a potential of between about 25 to 50 volts is applied between the terminal cathode and terminal anode to form a cathodic potential of about 2.5 volts on the cathodic surface 23 of each bipolar element 10. The exact potential required to form 2.5 volts at the surface of each bipolar element will depend on the efficiency of the cell. Reduction of the oxide to metal proceeds according to the FFC process. The mechanism for FFC reduction may be as follows.

Current is passed between the terminal cathode and terminal anode primarily by means of ionic transfer through the melt. For example, 02. ions are removed from the feedstock supported on the element that is coupled to the terminal cathode 130 by electro-deoxidation and are transported to the anodic portion 22, of the next bipolar element. The reaction of the oxygen ions with the carbon anode surface results in the evolution of a mixture of gaseous carbon monoxide, carbon dioxide and oxygen.

s Electrons transported through the melt by the 02 ion are transferred to the carbon portion 22 of the element 10 and into the cathodic stainless steel portion 23 of the element where they are available for the electro-decomposition reaction of the tantalum pentoxide retained here. The electro-decom position reaction causes the removal of oxygen from the tantalum oxide in the form of io an Q2. ion, and this ion is then transported to the next bipolar element away from the terminal cathode 130. The process is repeated until 2 ions are transported to the terminal anode 140.

Reduction of the feedstock may be carried out using processes other than the is FFC process. For example, electro-decomposition could be carried out using the higher voltage process as described in WO 03076690.

The tantalum oxide feedstock on each bipolar element is reduced for a total time period of 36 hours in order to reduce the feedstock to tantalum metal.

During the reduction, each bipolar element migrates through the cell as described above. Thus, each element migrates through a total of ten positions within the cell and is reduced for a total time of 3.6 hours in each position. it is intended that a cell may remain operational for many weeks, or months, during which hundreds of individual elements may pass through the cell.

Claims

CLAIMS1. A bipolar electrolysis cell comprising,Sa housing defining a chamber for containing molten salt electrolyte, first and second terminal electrodes disposed within the chamber, io a plurality of bipolar elements disposable between the first and second electrodes within the chamber, and means for applying a potential between the electrodes such that, in use, is one surface of each bipolar element becomes cathodic, in which the bipolar elements are removably loadable into the cell, and each bipolar element is capable of movement within the cell relative to the terminal electrodes.
2. A cell according to claim I in which each bipolar element is capable of linear movement between the terminal electrodes.
3. A cell according to claim 1 or 2 for the reduction of a solid feedstock, in which feedstock can be retained in contact with the cathodic surface of each bipolar element.
4. A cell according to claim 1, 2, or 3 in which the electrolyte is a molten salt.
5. A cell according to any preceding claim in which the first terminal electrode is a cathode and the second terminal electrode is an anode.
6. A cell according to claim 5 in which the cathode is capable of physical electrical connection with a bipolar element.
7. A cell according to any preceding claim in which bipolar elements are capable of being loaded into the cell individually, or in groups consisting of fewer elements than the plurality of bipolar elements.
8. A cell according to any preceding claim in which the housing defines a first opening for loading a bipolar element or bipolar elements into the chamber and a second opening for unloading a bipolar element or bipolar elements from the chamber.
9. A cell according to any of claims 5 to 8 comprising means for unloading a bipolar element or elements adjacent the anode, means for moving remaining bipolar elements towards the anode, and means for loading a bipolar element or etements adjacent the cathode.
12. A cell according to any of claims 5 to 8 comprising means for unloading a bipolar element or elements adjacent the cathode, means for moving remaining bipolar elements towards the cathode, and means for loading a bipolar element or elements adjacent the anode.
11. A cell according to any of claims 5 to 8 in which, in use, one of the bipolar elements is in physical electrical connection with the cathode.
12. A cell according to any preceding claim in which the bipolar elements are substantially plate-like, having length and width dimensions greater than a thickness dimension.
13. A cell according to any preceding claim in which the first terminal electrode and the second terminal electrode are substantially horizontally spaced from each other within the chamber.
14. A cell according to any preceding claim comprising spacer elements associated with each bipolar element to maintain a predetermined space between bipolar elements loaded in the chamber.
15. A cell according to any preceding claim in which each bipolar element comprises a tray, basket or clamping means for retaining solid feedstock in contact with its cathodic surface.
16. A cell according to any preceding claim comprising a molten salt inlet and a molten salt outlet situated such that molten salt can enter the chamber through the molten salt inlet and exit the chamber through the molten salt outlet.
17. A method of operating a bipolar electrolysis cell, the cell comprising first and second terminal electrodes disposed in contact with a molten salt electrolyte within a chamber of the cell, and a plurality of bipolar elements each occupying a position between the first and second terminal electrodes, the method comprising the steps of, a) applying a potential, for a predetermined period of time, between the first and second terminal electrodes such that a first surface of each of the bipolar elements becomes cathodic, b)removing a bipolar element or a group of bipolar elements, occupying a final position closest to the first terminal electrode, from the cell, c) moving the bipolar elements remaining within the cell towards the first terminal electrode such that a new bipolar element or group of bipolar elements occupies the final position and a first position closest to the second terminal becomes unoccupied, and d) loading a new bipolar element or group of bipolar elements into the first position closest to the second terminal electrode.
18. A method according to claim 17 comprising the further step of repeating steps a to d.
19. A method according to claim 17 or 18 in which the new bipolar element or group of bipolar elements migrates from the first position to the final position via a plurality of intermediate positions.
20. A method according to claim 17, 18, or 19 in which a solid feedstock is arranged in contact with the first surface of at least one of the bipolar elements prior to that element being loaded into the cell, the potential applied between the terminal electrodes being sufficient to reduce the solid feedstock.to
21. A method according to claim 20 in which solid feedstock is reduced by progressively greater degrees as the bipolar element it is in contact with migrates through the cell.
22. A method according to any preceding method claim in which electrolyte is cycled between the cell and an electrolyte reservoir.
23. A method according to any of claims 20 to 22 in which the solid feedstock comprises a metal containing dissolved oxygen or a metal oxide and the reduction reaction removes oxygen from the feedstock.
24. A bipolar electrolysis cell substantially as described herein and with reference to the figures.
25. A method of operating a bipolar electrolysis cell substantially as described herein and with reference to the figures.