WO2008101283A1

WO2008101283A1 - Electrochemical reduction of metal oxides

Info

Publication number: WO2008101283A1
Application number: PCT/AU2008/000220
Authority: WO
Inventors: Gregory David Rigby; Ivan Ratchev; Andrew Arthur Shook
Original assignee: Metalysis Ltd
Current assignee: Metalysis Ltd
Priority date: 2007-02-20
Filing date: 2008-02-20
Publication date: 2008-08-28
Anticipated expiration: 2009-08-20

Abstract

A commercial scale electrochemical cell for carrying out an electrochemical process for reducing a metal oxide such as a titanium oxide in a solid state is disclosed. The cell includes a plurality of cathodes that are formed at least in part from the metal oxide and can be removed from the cell and replaced by other such cathodes as the process continues to operate in the cell and thereby continues to reduce the metal oxide in the cell.

Description

ELECTROCHEMICAL REDUCTION OF METAL OXIDES

The present invention relates to electrochemical reduction of metal oxides in a solid state.

The present invention relates particularly, although by no means exclusively, to electrochemical reduction of metal oxides , such as titanium oxides , in any suitable solid state form in an electrolytic cell containing a molten electrolyte to produce a reduced material .

In the case of titanium oxides, typically the reduced material is titanium having a low oxygen concentration, typically no more than 0.2% by weight.

The present invention was made during the course of a research project on electrochemical reduction of metal oxides carried out by the applicant.

The research project focussed on the reduction of titanium oxides in the from of titania (TiO₂) .

The following description refers particularly to the reduction of titanium oxides. Nevertheless, it is understood that the present invention is not so confined and extends to the reduction of other metal oxides .

During the course of the research project the applicant carried out a series of experiments, initially on a laboratory scale and more recently on a pilot plant scale, investigating the reduction of titanium oxides in the form of titania in electrolytic cells comprising a bath of molten CaCl₂~based electrolyte, an anode formed from graphite, and a range of cathodes formed at least in part from titania. The CaCl₂-based electrolyte used in the experiments was a commercially available source of CaCl₂, which decomposed on heating and produced a very small amount of CaO.

The applicant operated the electrolytic cells at a potential above the decomposition potential of CaO and below the decomposition potential of CaCl₂.

The applicant found in the laboratory work that the cells electrochemically reduced titania to titanium with low concentrations of oxygen, i.e. concentrations less than 0.2 wt.%, at these potentials.

The applicant operated the laboratory electrolytic cells under a wide range of different operating parameters and conditions .

The applicant operated the laboratory electrolytic cells on a batch basis with titania in the form of pellets and larger solid blocks in the early part of the laboratory work and titania powder in the later part of the work.

The applicant also operated the laboratory electrolytic cells on a batch basis with cathodes formed at least in part from other metal oxides .

Recent pilot plant work carried out by the applicant was carried out on a pilot plant electrolytic cell that was set up to operate initially on a continuous basis and subsequently on a batch basis .

One result of the research project is a process for electrochemically reducing a metal oxide, such as a titanium oxide, in a solid state in an electrolytic cell that includes a molten bath of a CaCl₂-based electrolyte containing CaO, an anode at least partially immersed in the electrolyte, and a cathode formed at least in part from the metal oxide and at least partially immersed in the electrolyte with the metal oxide contacting the electrolyte, which electrochemical process includes applying an electrical potential across the anode and the cathode that is above the decomposition potential of CaO and electrochemically reducing the metal oxide in contact with the molten electrolyte and producing reduced material. In the case of reducing a titanium oxide, preferably the reduced material is titanium having no more than 0.2% by weight oxygen.

The applicant has considered the issue of scaling up the laboratory and pilot plant cells to a commercial scale electrochemical cell for carrying out the above- described electrochemical process .

The applicant has found that this is a complex, multi-faceted problem that involves consideration of a series of engineering and commercial issues .

The present invention is based on a realisation that a viable commercial scale electrochemical cell for carrying out an electrochemical process for reducing a metal oxide such as a titanium oxide in a solid state includes a plurality of cathodes that are formed at least in part from the metal oxide and can be removed from the cell and replaced by other such cathodes as the process continues to operate in the cell and thereby continues to reduce the metal oxide in the cell .

According to the present invention there is provided an electrochemical cell for carrying out an electrochemical process for reducing a metal oxide such as a titanium oxide in a solid state, which cell includes a cell chamber that contains a molten bath of an electrolyte, a plurality of anodes extending into the cell chamber and at least partially immersed in the electrolyte, and a plurality of cathodes formed at least in part from the metal oxide and extending into the cell chamber and at least partially immersed in the electrolyte with the metal oxide contacting the electrolyte, and with the cathodes being replaceable cathodes in that the cathodes can be removed from the cell chamber as required and replaced by other such cathodes during operation of the electrochemical process in the cell .

In use of the cell of the present invention, with the process operating in the cell and reducing the metal oxide, each cathode is retained in the cell in an operative position for a time period required to reduce the metal oxide of the cathode to a required extent, the cathode is then removed from the cell chamber and thereafter processed as required to separate metal from the cathode, and a replacement cathode formed at least in part from the metal oxide is inserted into the cell chamber in place of the removed cathode while the process continues to operate in the cell . This sequence of steps is repeated for each replacement cathode while the process continues to operate in the cell .

The above-described cell does not "transport" metal oxide through an electrolyte, with progressive reduction of the metal oxide, as is the case with the "continuous" production cells described and claimed in International applications PCT/AU2003/001657,

PCT/AU2004/000809, and PCT/AU2004/001331 in the name of the applicant.

Preferably the anodes are also replaceable anodes in that the anodes can be removed from the cell chamber and replaced by other such anodes while the process is operating in the cell. Preferably the cell is a sealed cell to minimise air infiltration into the electrolyte in the cell chamber.

Preferably the cell includes a lid for the cell chamber which can close the cell chamber with an air-tight seal.

Preferably the lid includes a plurality of openings for allowing removal of existing cathodes from the cell chamber and insertion of replacement cathodes into the cell chamber.

Preferably the lid includes a plurality of openings for allowing removal of existing anodes from the cell chamber and insertion of replacement anodes into the cell chamber .

Preferably the openings and the anodes and the cathodes are formed so that there are air-tight seals when the anodes and the cathodes are positioned to extend through the openings in the lid in operative positions in the cell chamber.

Preferably the cell includes an apparatus for purging air from the cell chamber with a gas that is inert with respect to the reduced material produced in the cell at the operating temperature of the cell .

In the case of a cell that is operated to produce titanium metal , preferably the inert gas is argon or helium.

Preferably the purge apparatus is arranged to purge the cell chamber via the openings in the lid.

The pilot plant work carried out by the applicant found that consumption of electrolyte in the above- described cell is substantially confined to electrolyte that is retained in the pores and on exposed surfaces of cathodes that are removed from the cell . Depending on the physical characteristics of the cathodes, i.e. porosity, etc, the amount of electrolyte removed with the cathodes may be relatively significant. Nevertheless, and somewhat surprisingly, it is thought that the cell may be able to be operated on a commercial basis with minimal changeover of electrolyte beyond that required to replace the electrolyte removed with the cathodes .

The cell chamber may be internally heated, self- heated or externally heated to maintain the electrolyte in a molten state. Typically, it is preferred that the electrolyte be heated to a temperature in a range of 940- 1100⁰C.

The term "internally heated" is understood herein to mean that the cell chamber is heated with one or more heating means extending into the electrolyte in the cell chamber .

The term "self-heated" is understood herein to mean that the cell chamber is heated by resistance heating within the cell chamber.

The pilot plant work has found that it may be possible to maintain the electrolyte temperature in the cell chamber by internal (rather than external) heating or by self-heating. This is a potentially significant finding because external heating is not as practical an option as internal heating or self-heating.

External heating leads to issues of containment and heat transfer. By way of example, the applicant has found that it is very difficult to use refractory-lined cell chambers with external heating because of heat transfer issues .

Accordingly, if external heating is used a cell chamber must be made from materials that allow heat transfer and this constraint places limitations on operating temperature limits for the cell .

Internal heating and self-heating open up a possibility for the use of cell chambers with water-cooled staves and/or refractory-lined cells, leading to operation at higher temperatures .

Preferably the cell chamber is internally heated or self-heated.

It is preferred particularly that the cell chamber be self-heated.

The cell chamber may be any suitable shape and size.

By way of example, the cell chamber may be a rectilinear shape with a base and parallel sides and parallel ends extending upwardly from the base.

Typically, the rectilinear-shaped cell chamber is 12-18 m, preferably 15 m, long and 1-3 m, preferably 2 m, wide.

The anodes and the cathodes may be any suitable shape and size.

The cell may include any suitable arrangement of the anodes and the cathodes in the cell chamber.

In practice, the optimum spacing of the anodes and the cathodes in a cell chamber of any given shape and size is governed by a number of factors including, by way of example, optimising current efficiency and controlling self-heating caused by resistance heating in the cell chamber .

Preferably the cell includes a plurality of pairs of the anodes and the cathodes arranged in the cell chamber whereby the metal oxide of the cathode of each anode/cathode pair is exposed to substantially the same reduction conditions .

In one embodiment the anode/cathode pair includes (a) an assembly of a line of spaced-apart downwardly extending anode rods and (b) an assembly of a line of a plurality of cathode elements, such as disks or pellets, adjacent the anode assembly.

In the case of a rectilinear cell chamber, preferably each anode/cathode pair extends across the cell chamber from one side to the other side of the cell chamber and is supported on the sides of the cell chamber.

Preferably the metal oxide forms exposed surfaces of the cathodes.

In one embodiment, the anode/cathode pair includes an anode and a cathode that are in the form of plates .

In the case of a rectilinear cell chamber, preferably the plate anodes and the plate cathodes extend across the cell chamber from one side to the other side of the cell chamber and are supported on the sides of the cell chamber.

Preferably the anode/cathode pairs are arranged in side-by side parallel relationship in the cell chamber, with the plate anodes and the plate cathodes alternating in the cell so that the plate anodes are adjacent the plate cathodes and vice versa.

Preferably the surface-to-surface spacing of the plate anode and the plate cathode in each anode/cathode pair is in a range of 10-100 mm.

More preferably the anode/cathode surface-to- surface spacing is 30-80 mm.

Typically, the anode/cathode surface-to-surface spacing is 50 mm.

Preferably the metal oxide forms exposed surfaces of the cathodes .

In the case of the plate cathodes , preferably the metal oxide of the cathodes forms opposite sides of the plates .

With this arrangement, preferably each plate cathode includes an assembly of an electrically conductive plate and the metal oxide on both surfaces of the plate.

The electrically conductive plate may be a solid plate .

By way of further example, the electrically conductive plate may be a mesh.

Preferably the cell includes a plurality of anode/cathode modules that can be removed from the cell chamber and replaced with a replacement cell module, with each anode/cathode module including a plurality of the anodes and a plurality of the cathodes . Preferably each anode/cathode module includes a support member that supports the anodes and the cathodes and forms a sealed section of the lid of the cell chamber .

In one embodiment each anode/cathode module includes a support member and a plurality of the plate anodes extending from the support member as a series of parallel , spaced apart anodes .

Preferably each anode/cathode module further includes a plurality of cathode plates positioned in gaps between adjacent anode plates and arranged to be separately removable from the module and replaced with replacement cathode plates.

Preferably the support member of each anode/cathode module includes an upper section that has a plurality of openings that allow insertion and removal of the cathode plates into gaps between adjacent anode plates of the module .

Preferably the openings in the support member of each anode/cathode module and the cathode plates are shaped and sized so that the cathode plates close the openings when the cathodes are positioned in an operative position in the cell, and thereby form a sealed section of a lid of the cell chamber.

Preferably the cell includes a busbar assembly that electrically connects the anodes and the cathodes to a source of electrical power.

Preferably the anodes and the cathodes are connected together in a parallel electrical connection.

Preferably the anodes are formed from graphite. Preferably the metal oxide is a titanium oxide, such as titania.

The metal oxide may be in any suitable form.

The metal oxide may be in the form of powders , pellets formed by agglomerating powders into pellet shapes, or other products formed by other processes. By way of example, the pellets may be disc-shaped. By way of further example, the other products may be foamed metal oxide blocks . By way of further example, the other products may be honeycomb-shaped products that include a series of interconnected walls that define a plurality of passages for the electrolyte to penetrate the structure to contact internal exposed surfaces of the products .

Preferably the electrolyte is a salt.

More preferably the electrolyte is CaCl₂-based electrolyte containing CaO.

According to the present invention there is also provided a plant for producing a metal such as titanium having no more than 0.2% by weight oxygen, from a metal oxide such as a titanium oxide that includes a plurality of the above-described electrochemical cell.

According to the present invention there is provided a process for electrochemically reducing a metal oxide such as a titanium oxide in a solid state in the above-described electrolytic cell, which electrochemical process includes applying an electrical potential across the anodes and the cathodes of the cell that is above the decomposition potential of a component in the molten electrolyte in the cell and electrochemically reducing the metal oxide in contact with the electrolyte and producing reduced material, and periodically removing the cathodes from the cell chamber and inserting replacement cathodes into the cell chamber while the process continues to operate in the cell .

Preferably the process includes removing no more than 30%, more preferably no more than 20%, of the cathodes from the cell chamber at any point in time.

Preferably the process includes timing the removal of the cathodes and the insertion of the replacement cathodes into the cell chamber to even out power consumption in the cell over a cell operating period.

Preferably the process includes removing the anodes from the cell chamber and inserting the replacement anodes into the cell chamber while the process continues to operate in the cell .

Preferably the process includes timing the removal of the anodes and the insertion of the replacement anodes into the cell chamber to even out power consumption in the cell over a cell operating period.

Preferably the process includes processing the removed cathodes to recover reduced material from the cathode .

Preferably the electrolyte is CaCl₂-based electrolyte containing CaO and the process includes applying an electrical potential across the anodes and the cathodes of the cell that is above the decomposition potential of CaO in the molten electrolyte.

Preferably the reduced material is titanium having no more than 0.2% by weight oxygen. The present invention is described further by way of example with reference to the accompanying drawings , of which :

Figure 1 is a partially sectional, end view of one embodiment of an electrochemical cell in accordance with the present invention;

Figure 2 is a top plan view of the anode/cathode cell module of the cell shown in Figure 1 ;

Figure 3 is a cross-section of the cell module shown in Figure 2 along the line 3-3 in Figure 2 ;

Figure 4 is a side elevation of the anode/cathode module shown in Figures 1-3 with the cathode plates of the module removed for clarity;

Figure 5 is a cross-section along the line 5-5 of

Figure 4 ;

Figure 6 is a side elevation of a cathode plate of the anode/cathode module shown in Figures 1-5;

Figure 7 is an end view of the cathode plate shown in Figure 6;

Figure 8 is a partially cut-away, diagrammatic view of the cell chamber shown in Figure 1 with a plurality of the anode/cathode module shown in Figures 2-7 positioned in the cell chamber, with one cathode plate shown in a raised position to illustrate removal/insertion from the cell chamber ;

Figure 9 is an enlarged view of the right-side end of the Figure 8 arrangement; Figure 10 is a perspective view of the anode/cathode module shown in Figures 2-7;

Figure 11 is a perspective view of another, although not the only other, embodiment of an electrochemical cell in accordance with the present invention, with only one anode/cathode module positioned in the cell for clarity;

Figure 12 is an enlarged view of the anodes and the cathode of the anode/cathode module shown in Figure 11;

Figure 13 is a perspective view identical to that of Figure 12 but with one anode removed for clarity;

Figure 14 is a front elevation of a part of the cathode of the anode/cathode module shown in Figures 11 to 13, with one cathode element of the cathode shown properly and the other cathode elements shown in outline for clarity;

Figure 15 is a side elevation of the part of the cathode shown in Figure 14 along the line 15-15; and

Figure 16 is an enlarged front view of the cathode element shown in Figures 11-15.

Figure 1 shows a transverse cross-section of one embodiment of an electrochemical cell (generally identified by the numeral 3) in accordance with the present invention located in an electrochemical plant (only partly shown) .

Typically, the electrochemical cell 3 is one of a substantial number of such cells 3 in a commercial electrochemical plant that produces a metal (such as titanium) by electrochemical reduction of a metal oxide (such as a titanium oxide, for example titania) in the cells .

The cells 3 may be arranged, by way of example, along the lines of the cells in an aluminium plant or a copper electro-winning plant.

The electrochemical cell 3 includes (a) a cell chamber (generally identified by the numeral 41) , (b) a molten bath of a salt electrolyte, such as CaCl₂-based electrolyte containing CaO, in the cell chamber 41 at a temperature of the order of 940-1100⁰C, (c) a plurality of pairs of anodes and cathodes extending into the cell chamber 41 and at least partially immersed in the electrolyte, with the cathodes being formed at least in part form the metal oxide, and (d) an apparatus (not shown) for purging air from the cell chamber 41.

As is shown in Figure 1 , the electrochemical cell 3 also includes a busbar assembly for supplying electricity from a remote power source (not shown) to the anodes and the cathodes to apply a potential between the anodes and the cathodes that is above the decomposition potential of CaO and thereby facilitate electrochemical reduction of the metal oxide. The busbar assembly includes (a) a pair of busbars 11a, lib extending parallel to and outboard of the side walls 7 of the cell and (b) a plurality of "fingers" 13 extending inwardly from the busbars 11a, lib through the side walls 7 into the cell chamber 41 and electrically connecting the power supply to the anodes and the cathodes in the cell. One of the busbars 11a, lib supplies electricity to the anodes and the other of the busbars 11a, lib supplies electricity to the cathodes . Furthermore, as is partly shown in Figure 1, the electrochemical plant includes an anode/cathode handling apparatus for (a) removing the anodes and the cathodes from the cell while the cell is operating and reducing metal oxides in the cell chamber, (b) transporting the removed anodes and cathodes away from the cell, (c) transporting replacement anodes and cathodes to the cell, and (d) inserting the anodes and the cathodes into the cell while the cell is operating and reducing metal oxides in the cell chamber,

With reference to Figure 1, the anode/cathode handling apparatus includes (a) suitable vehicles 21 (one of which only is shown in Figure 1) adapted to move along the lines of the cells 3 and to transport anodes and cathodes to and from the cells, and (b) an overhead gantry (generally identified by the numeral 23) for facilitating removal of anodes and cathodes from the cell chamber 41 and insertion of anodes and cathodes into the cell chamber 41.

Furthermore, whilst not shown, the anode/cathode handling apparatus includes a removal/insertion assembly that can be positioned above anodes and cathodes to be removed from the cell chamber 41 or above the positions for inserting replacement anodes and cathodes into the cell chamber 41 and can be operated to remove or insert anodes or cathodes, as required.

Furthermore, whilst not shown, the electrochemical plant also includes an apparatus for removing metal from the cathodes that have been removed from the cell .

With reference to Figures 2-9, and particularly in the perspective views of Figures 8 and 9, the electrochemical cell 3 includes a rectilinear cell chamber (typically 15 m long, 2 m wide, and 2 m high) defined by a base 5 and parallel side walls 7 and parallel end walls 9 extending outwardly from the base 5 formed from a suitable refractory material .

With reference to Figures 2-9, the anodes and the cathodes in the electrochemical cell 3 are in the form of anode plates 33 and cathode plates 35 that are assembled together into anode/cathode modules (generally identified by the numeral 31) . The anode/cathode modules 31 are positioned in side by side relationship in the cell chamber 41.

Specifically, each anode/cathode module 31 includes a plurality of the anode plates 33 and a plurality of the cathode plates 35 in side by side relationship separated by a selected spacing, typically 40-60 mm. Preferably the spacing is 50 mm.

Each anode plate 33 includes a main anode member

49 formed from graphite.

Each cathode plate 35 includes a main cathode member 59. The main cathode member 59 includes an assembly of an electrically conductive plate (which may be a solid plate or a mesh sheet) and metal oxide on both surfaces of the plate .

Each anode plate 33 and each cathode plate 35 includes a busbar hanger 39 that extends along the length of each plate. The outer ends of the busbar hangers 39 are shaped to contact the inner ends of the fingers 13 of the busbar assembly. Specifically, as is shown in Figure 1, the ends of the fingers 13 and the ends of the busbar hangers 39 are angled in a complimentary way, whereby there is effective electrical contact between these elements when the anode plates 33 and the cathode plates 35 are in operative positions in the cell chamber 41. Specifically, the inner ends of the fingers 13 are inclined to have upwardly facing contact surfaces, as viewed in Figure 1, and the ends of the busbar hangers 39 are inclined to have complementary downwardly facing contact surfaces , as viewed in Figure 1.

Each cathode plate 35 includes an upper member 69.

Each cathode plate 35 includes tie rods 73 that connect the upper member 69 and the busbar hanger 39 together .

Each anode/cathode module 31 includes a support plate 51 that supports the anode plates 33 so that the anode plates 33 extend downwardly in a parallel , spaced- apart arrangement. The support plate 51 defines a part of an airtight lid of the cell chamber 41.

The support plate 51 includes a plurality of parallel spaced-apart slots 53 that define openings through which the cathode plates 35 can be removed from and inserted into cell chamber 41.

The openings 53 and the cathode plates 35 are shaped and sized so that the cathode plates 35 form airtight seals when the cathode plates 35 are in an operative position in the anode/cathode module 31. The operative position is shown in Figures 2, 3, and 8-10, with the cathode plates 35 positioned in the gaps between adjacent, parallel anode plates 33.

In use of the cell 3, the cell contains a bath of the electrolyte at a selected temperature, typically at

1000⁰C, a selected electrical potential of 3 V (or selected current) is applied to the anodes and the cathodes of the cell, and the titania of the cathode plates 35 progressively reduce in the cell. The electrolyte is maintained at the required temperature by self-heating caused by resistance heating in the cell.

From time to time, while the cell is operating, selected cathode plates 35 that have been reduced to a required extent are removed from the cell and are replaced with titania replacement cathode plates 35 using the above-described anode/cathode handling apparatus .

. A key consideration in the change-over process is to achieve removal of a current cathode plate 35 and insertion of a replacement cathode plate 35 without allowing air to enter the cell chamber 41. Minimising entry of air into the cell chamber 41 during normal operation of the cell , i.e. at times other than during a cathode (and anode) changeover, is also a key consideration. Water (which may be present as moisture in air) contacting the electrolyte is undesirable. In addition, in the case of titanium metal, air (both oxygen and nitrogen components) contacting the titanium in the cell is undesirable. Accordingly, the purge apparatus (not shown) is provided to purge air from the cell chamber during the changeover process and during normal operation of the cell.

The cathode plates 35 may be individually removed at different locations along the entire length of the cell 3. Alternatively, the four cathode plates 35 in a given anode/cathode module 31 may be removed at a given changeover time.

Any suitable sequence of removal and replacement may be followed. One consideration in this regard is to limit the numbers of cathode plates 35 to be removed (and replaced) at any given time to ensure that there are no significant variations in operating conditions in the cell. Typically, no more than 20% of the cathode plates 35 are removed at any given time.

In addition to the removal and replacement of cathode plates 35 , the anodes are removed and replaced from time to time. The anodes are consumed in the process and, consequently, must be replaced. Typically, the anodes require replacement once every 10 times that a cathode plate 35 is replaced.

The embodiment of the electrochemical cell 3 shown in Figures 11-16 is similar to the electrochemical cell 3 shown in Figures 1-10 in terms of the basic cell construction.

With reference to Figure 11, the cell 3 includes (a) a cell chamber 41 defined by a base (not shown) , side walls 7 and parallel end walls 9, (b) a lid (generally identified by the numeral 61) positioned on the cell chamber 41 and forming an air-tight seal with the cell chamber 41, and (c) a plurality of anode/cathode modules 31 (only one of which is shown in Figure 11 for clarity) extending through openings in the lid 61 into the cell chamber 41 and at least partially immersed in a molten bath of electrolyte 71 located in the cell chamber 41.

Whilst only one anode/cathode module 31 is shown in Figure 11, it can readily be appreciated that under normal operating conditions the cell 3 includes a plurality of such anode/cathode modules 31 positioned in side-side relationship along the length of the cell chamber 41.

Each anode/cathode module 31 includes (a) an upper horizontal support plate 51a and a lower horizontal parallel support plate 51b that are connected together by vertical posts 91 and (b) two anode assemblies and a cathode assembly supported by the support plates 51a, 51b.

Each anode assembly includes a line of nine vertical, graphite anode rods 73.

Each anode assembly also includes an anode support assembly. Specifically, groups of three of the anode rods 73 in each line are supported by (a) a horizontal cross member 75 connected to upper ends of the anode rods 73 and (b) a support rod 77 that extends vertically upwardly from the cross member 75 and the anode rods 73.

The cathode assembly includes (a) a support frame

(generally identified by the number 87) and (b) two arrays of honeycomb-shaped cathode elements 95 formed from the metal oxide supported by the support frame 87.

The support frame 87 of the cathode assembly includes a main support body that includes (a) a horizontal cross member 81, (b) a plurality of parallel, vertical support rods 83 extending from the cross member, (c) a plurality of pins 85 extending outwardly from opposite sides of the support rods 83 and defining mounting posts for the cathode elements 95, and (d) three support rods 89 that extend vertically upwardly from the cross member 81.

The support rods 77 of the anode assemblies and the support rods 89 of the cathode assembly extend vertically through openings in the support plates 5a, 5b and are supported by the support plates and are electrically isolated from the support plates .

The support rods 77 are electrically connected to a power source (not shown) to supply electricity to the anode assemblies and the cathode assembly to apply a potential between the assemblies that is above the decomposition potential of CaO and thereby facilitate electrochemical reduction of the metal oxide of the honeycomb-shaped cathode elements 95.

The honeycomb-shaped cathode elements 95 are formed from the metal oxide to be reduced in the cell 3.

In general terms, each cathode element 95 includes a series of interconnected walls 103 that define ■'- plurality of passages 97 for the electrolyte to penetrate the structure to contact internal exposed surfaces of the structure. More specifically, each cathode element 95 shown in the Figures has a relatively thin walled structure that defines sixteen parallel passages 97. The arrangement is such that the electrolyte can readily penetrate the structure and contact exposed interior surfaces via the passages 97. In overall terms , the structure is such that there is a substantial surface area of the metal oxide that is exposed to the electrolyte .

The cathode elements 95 are formed to have sufficient mechanical strength to withstand handling after manufacture and during assembly of the cathode and during and after reduction in the cell 3.

Typically, the post-reduction handling includes washing the cathode elements 95 after the elements have been removed from a cell 3 to remove retained electrolyte from the elements and grit blasting the washed elements to remove any solidified accretions not removed in the washing step.

Moreover, in the case of some metal oxides, such as titania, the cathode elements 95 have to be sufficiently tough to withstand physical size changes that occur as a consequence of phase changes during reduction.

In the case of metal oxides in the form of titania, preferably the cathode elements 95 are formed by extruding a slurry of powders of the metal oxide and water into a continuous length of the honeycomb shape shown in the Figures, cutting the continuous length into the required length, and then sintering the elements to increase the strength of the elements .

Many modifications may be made to the embodiment? of the electrochemical cell shown in the Figures without departing from the spirit and scope of the invention.

Claims

1. An electrochemical cell for carrying out an electrochemical process for reducing a metal oxide such as a titanium oxide in a solid state, which cell includes a cell chamber that contains a molten bath of an electrolyte, a plurality of anodes extending into the cell chamber and at least partially immersed in the electrolyte, and a plurality of cathodes formed at least in part from the metal oxide and extending into the cell chamber and at least partially immersed in the electrolyte with the metal oxide contacting the electrolyte, and wit^τ - the cathodes being replaceable cathodes in that the cathodes can be removed from the cell chamber as required and replaced by other such cathodes during operation of the electrochemical process in the cell .

2. The cell defined in claim 1 wherein the anodes are also replaceable anodes in that the anodes can be removed from the cell chamber and replaced by other such anodes while the process is operating in the cell .

3. The cell defined in claim 1 or claim 2 is a sealed cell to minimise air infiltration into the electrolyte in the cell chamber.

4. The cell defined in any one of the preceding claims includes a lid for the cell chamber which can close the cell chamber with an air-tight seal.

5. The cell defined in claim 4 wherein the lid includes a plurality of openings for allowing removal of existing cathodes from the cell chamber and insertion of replacement cathodes into the cell chamber .

6. The cell defined in claim 5 when dependent on claim 2 wherein the lid includes a plurality of openings for allowing removal of existing anodes from the cell chamber and insertion of replacement anodes into the cell chamber .

7. The cell defined in claim 6 wherein the openings and the anodes and the cathodes are formed so that there are air-tight seals when the anodes and the cathodes are positioned to extend through the openings in the lid in operative positions in the cell chamber .

8. The cell defined in any one of claims 4 to 7 wherein the lid includes the cell includes an apparatus for purging air from the cell chamber with a gas that is inert with respect to the reduced material produced in the cell at the operating temperature of the cell.

9. The cell defined in claim 8 wherein the purge apparatus is arranged to purge the cell chamber via the openings in the lid.

10. The cell defined in any one of the preceding claims wherein the cell chamber is internally heated, self-heated or externally heated to maintain the electrolyte in a molten state.

11. The cell defined in any one of the preceding claims wherein the cell includes a plurality of pairs of the anodes and the cathodes arranged in the cell chamber whereby the metal oxide of the cathode of each anode/cathode pair is exposed to substantially the same reduction conditions .

12. The cell defined in claim 11 wherein the anode/cathode pair includes (a) an assembly of a line of spaced-apart downwardly extending anode rods and (b) an assembly of a line of a plurality of cathode elements, such as disks or pellets, adjacent the anode assembly.

13. The cell defined in claim 12 wherein, in the case of a rectilinear cell chamber, each anode/cathode pair extends across the cell chamber from one side to the other side of the cell chamber and is supported on the sides of the cell chamber.

14. The cell defined in any one of the claims 11 to 13 wherein the anode/cathode pair includes an anode and a cathode that are in the form of plates.

15. The cell defined in claim 14 wherein, in fre case of a rectilinear cell chamber, the plate anodes and the plate cathodes extend across the cell chamber from one side to the other side of the cell chamber and are supported on the sides of the cell chamber.

16. The cell defined in claim 15 wherein the anode/cathode pairs are arranged in side-by side parallel relationship in the cell chamber, with the plate anodes and the plate cathodes alternating in the cell so that the plate anodes are adjacent the plate cathodes and vice versa .

17. The cell defined in claim 16 wherein the surface- to-surface spacing of the plate anode and the plate cathode in each anode/cathode pair is in a range of 10-100 mm.

18. The cell defined in any one of the preceding claims includes a plurality of anode/cathode modules that can be removed from the cell chamber and replaced with a replacement cell module, with each anode/cathode module including a plurality of the anodes and a plurality of the cathodes.

19. The cell defined in claim 18 wherein each anode/cathode module includes a support member that supports the anodes and the cathodes and forms a sealed section of the lid of the cell chamber.

20. The cell defined in claim 18 or claim 19 wherein each anode/cathode module includes a support member and a plurality of the plate anodes extending from the support member as a series of parallel , spaced apart anodes .

21. The cell defined in claim 20 wherein each anode/cathode module further includes a plurality of cathode plates positioned in gaps between adjacent anode plates and arranged to be separately removable from the module and replaced with replacement cathode plates .

22. The cell defined in claim 21 wherein the support member of each anode/cathode module includes an upper section that has a plurality of openings that allow insertion and removal of the cathode plates into gaps between adjacent anode plates of the module.

23. The cell defined in claim 22 wherein the openings in the support member of each anode/cathode module and the cathode plates are shaped and sized so that the cathode plates close the openings when the cathodes are positioned in an operative position in the cell, and thereby form a sealed section of a lid of the cell chamber.

24. The cell defined in any one of the preceding claims includes a busbar assembly that electrically connects the anodes and the cathodes to a source of electrical power.

25. A plant for producing a metal such as titanium having no more than 0.2% by weight oxygen, from a metal oxide such as a titanium oxide that includes a plurality of the electrochemical cell defined in any one of the preceding claims .

26. A process for electrochemically reducing a metal oxide such as a titanium oxide in a solid state in the electrolytic cell defined in any one of claims 1 to 24, which electrochemical process includes applying an electrical potential across the anodes and the cathodes of the cell that is above the decomposition potential of a component in the molten electrolyte in the cell and electrochemically reducing the metal oxide in contact with the electrolyte and producing reduced material, and periodically removing the cathodes from the cell chamber and inserting replacement cathodes into the cell chamber while the process continues to operate in the cell .

27. The process defined in claim 26 includes removing no more than 30%, more preferably no more than 20%, of the cathodes from the cell chamber at any point in time.

28. The process defined in claim 26 or claim 27 includes timing the removal of the cathodes and the insertion of the replacement cathodes into the cell chamber to even out power consumption in the cell over a cell operating period.

29. The process defined in any one of claims 26 to 28 includes removing the anodes from the cell chamber and inserting the replacement anodes into the cell chamber while the process continues to operate in the cell .

30 The process defined in any one of claims 26 to 28 includes timing the removal of the anodes and the insertion of the replacement anodes into the cell chamber to even out power consumption in the cell over a cell operating period.