WO2008101290A1

WO2008101290A1 - Electrochemical reduction of metal oxides

Info

Publication number: WO2008101290A1
Application number: PCT/AU2008/000228
Authority: WO
Inventors: René Ignacio OLIVARES; Gregory David Rigby; Ivan Ratchev
Original assignee: Metalysis Limited
Priority date: 2007-02-20
Filing date: 2008-02-20
Publication date: 2008-08-28

Abstract

A cathode element for use in an 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 cathode element includes a honeycomb-shaped structure that includes a series of interconnected walls (103) that define a plurality of passages (97) for an electrolyte in the cell to penetrate the structure to contact internal exposed surfaces of the structure.

Description

- I -

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 (Tiθ₂> .

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.

The applicant has considered the issue of rraling 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 .

One of the issues considered by the applicant is the issue of selecting cathode elements that include the metal oxides to be reduced in the cell and a method of preparing the elements . The applicant has found that this issue involves considering a range of engineering issues covering areas of manufacturing the elements , handling the elements, assembling cathodes that include the elements, and post-reduction handling and processing of the elements in a reduced form. In addition, the applicant has found that in the case of a number of metal oxides , such as titania, this issue also involves considering how to accommodate volume changes that occur during phase changes as the metal oxide reduces .

The present invention is based on a realisation that a honeycomb structure is a suitable structure for cathode elements for 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.

According to the present invention there is provided a cathode element for use in an electrochemical cell for carrying out an electrochemical process for reducing a metal oxide such as a titanium oxide in a solid state, the cathode element including a honeycomb-shaped structure that includes a series of interconnected walls that define a plurality of passages for an electrolyte in the cell to penetrate the structure to contact internal exposed surfaces of the structure.

Preferably the cathode element includes passages that extend through the structure .

Preferably the passages are parallel passages .

The cathode element may be any suitable shape and size .

Preferably the cathode element is in the shape of a block .

Typically, the block has a width, a height, and a depth, with the passages extending through the depth of the block, and the depth being selected so that a maximum distance between the cathode element and an anode in the electrochemical cell can be maintained in use of the cathode element in the electrochemical cell . The two other global dimensions (i.e. block width and block height) may be selected to be sufficiently large to allow efficient handling of the cathode element. One suitable block shape is a cube shape.

The passages may have any suitable cross- section .

Preferably the passages are any transverse cross- section that gives a regular shape.

Typically, the passages are any polygonal shape such as a pentagon or a hexagon.

It is preferred particularly that the passages have a square transverse cross-section.

Preferably the walls are relatively tbin compared to the transverse cross-sectional width of the passages .

Preferably the walls are less than 4 mm thick.

More preferably the wall thickness is less than 2.5 mm.

It is preferred particularly that the wall thickness be 2.0 mm or less .

Preferably the maximum thickness of the walls , measured at intersections of the walls, is less than 2.5 mm, more preferably less than 2.0 mm.

Preferably the transverse cross-sectional width of the passages is less than 70 mm.

More preferably the transverse cross-sectional width of the passages is less than 55 mm.

According to the present invention there is also provided a method of preparing the above-described cathode element that includes the steps of: (a) preparing a slurry of a metal oxide powder;

(b) extruding the slurry through a die and forming a continuous extrudate having the transverse cross-section of the above-described cathode element;

(c) cutting the continuous length of the exrudate and forming a green element having the shape of the above-described cathode element; and

(d) sintering the green element and forming the above-described cathode element.

The slurry may be formed using any suitable liquid carrier.

Preferably the carrier is water or an organic liquid.

The slurry may include additives to provide the correct properties for the extrusion step (b) .

Preferably one consideration for the selection of the additives is that the additives should leave minimal, preferably no, residue after sintering step (d) .

Preferably the method includes preparing the cathode element with a wall porosity of 35-60% by volume.

More preferably the method includes preparing the cathode element with a wall porosity of 40-50% by volume.

Preferably step (d) includes sintering the green element at a temperature in a range of 1150-1250⁰C.

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 CaCl₂-based electrolyte containing CaO, a plurality of anodes extending into the cell chamber and at least partially immersed in the electrolyte, and a plurality of cathodes including a plurality of the above-described cathode elements extending into the cell chamber and at least partially immersed in the electrolyte with the metal oxide contacting the electrolyte.

Preferably the metal oxide is a titanium oxide, such as titania.

According to the present invention there is provided a process for eleσtrochemically reducing a metal oxide such as 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 CaO in the molten electrolyte in the cell and electrochemically reducing the metal oxide in contact with the electrolyte and producing reduced material .

Preferably the metal oxide is a titanium oxide and the process includes producing 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 perspective view of an embodiment, although not the only embodiment, of an electrochemical cell in accordance with the present invention with only one anode/cathode module positioned in the cell for clarity;

Figure 2 is an enlarged view of -the anodes and the cathode of the anode/cathode module shown in Figure 1;

Figure 3 is a perspective view identical to that of Figure 2 but with one anode removed for clarity;

Figure 4 is a front elevation of a part of the cathode of the anode/cathode module shown in the Figures, with one cathode element shown properly and the other cathode elements shown in outline for clarity;

Figure 5 is a side elevation of the part of the cathode shown in Figure 4;

Figure 6 is an enlarged front view of the cathode element shown in the Figures ; and

Figure 7 summarises the results of an experiment to evaluate the performance of an embodiment of a cathode element in accordance with the present invention in a laboratory electrolytic cell .

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 .

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.

With reference to Figure 1, 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 1 for clarity) extending through openings in the lid 61 into the cell chamber 41 and at least partially imroesed in a molten bath of electrolyte 71 located in the cell chamber 41.

Whilst only one anode/cathode module 31 is shown in Figure 1, 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-by-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 as blocks 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 rod? 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 aligned openings in the support plates 51a, 51b 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 is a block that includes a series of interconnected walls 103 that define a plurality of passages 97 for the electrolyte in the cell chamber 41 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 erf the cathode and during and after reduction in the cell 3. Typically, the postreduction handling includes (a) washing the cathode elements 95 after the elements have been removed from a cell 3 to remove retained electrolyte from the elements and (b) 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, typically at a temperature of 1200°C to increase the strength of the elements .

The cathode element 95 shown in the Figures is a cube, with a length/width/height dimension of 25 mm, a wall thickness of 2 mm and passages that have a transverse width of 5 mm. The applicant has evaluated the cathode element 95 shown in the Figures in a laboratory electrolytic cell .

The cell comprised a reaction vessel, a furnace, a crucible assembly, an electrode assembly, and a power supply .

The reaction vessel was manufactured from a high temperature stainless steel and had an internal diameter of 110mm and height of 430mm, and a water-cooled flange.

The vessel was contained within a resistance- heated furnace capable of reaching 1400⁰K. A positioning pedestal within the vessel allowed for the crucible containing the molten salt to be properly fixed within the vessel .

A water-cooled lid was a critical feature of the reaction vessel and had provision for a viewing port which facilitated accurate positioning of the electrodes and the thermocouple within the molten salt bath. The viewing port also allowed the surface of the molten salt to be monitored during the reaction .

Carbon monoxide and carbon dioxide are the main by-products of the electrochemical reduction process . In order to monitor the progress of reduction and to ensure that the only source of oxygen was the titanium dioxide, special care was taken in controlling the gas atmosphere . A measured flow of high purity argon gas was passed through a furnace containing copper turnings at 873°K, then through a furnace containing magnesium flakes at 673°K before entering the vessel. After passing through the vessel, the gas stream was stripped of any chlorine- containing species and moisture and continuously analysed for CO and CO₂. A solid electrolyte based oxygen analyser monitored the partial pressure of O₂ in the off-gas immediately after the furnace.

Data logging and control was performed using a

LabView software interface.

Furnace pressure, applied voltage, reference voltage, cell current, electrolyte temperature, oxygen, carbon monoxide and carbon dioxide content of the off-gas were monitored and logged for subsequent analysis as a function of time .

As is indicated above, the chemistry of the molten electrolyte was found to have a significant impact on the process operation. Consequently, the applicant ensured that the composition of the electrolyte at the start of a run was well controlled.

The electrolyte was prepared from analytical grade dihydrate, CaCl₂.H₂θ obtained from APS Chemicals. Typically, 68Og of molten salt were used in the experiments. Prior to melting, a dehydration step was carried out as follows .

The CaCl_2-H₂O was slowly heated under vacuum to 525K and kept at this temperature for at least 12 hours during which time the weight loss due to water removal was monitored. Once there was no more weight loss, the anhydrous salt was transferred from the vacuum oven into a platinum crucible and placed in a melting furnace . The salt was melted at 1275°K and kept at this temperature for 30 minutes to allow further removal of any residual water. The molten salt was then cast into a preheated steel mould, removed once solid, and transferred while hot to a drying oven held at 400K. By using this standardised procedure for preparing the CaCl₂ electrolyte, it was possible to obtain reproducible quality material.

In the majority of experiments the titania used was 99.5% minimum purity rutile (Alfa Aesar) although occasionally other titania sources were used.

A plurality of the cathode elements 95 shown in the Figures were formed from pigment grade titania powders and reduced in the above laboratory electrolytic cell operated under the following conditions in a specific experiment to evaluate the performance of the cathode element .

• Voltage : 3V.

• Electrolyte temperature : 1000⁰C .

• Run time : 8 hr s .

• Cathode elements: 17 g of the cathode elements 95, with the walls of the elements having a porosity of 48 vol.%, i.e. highly porous .

• Salt: feed ratio (wt.%) > 50:1.

The cathode elements 95 were manufactured by a method that includes the following steps :

(a) preparing a slurry of pigment grade titania powder, a plasticiser, and water;

(b) extruding the slurry through a die and forming a continuous extrudate having the transverse cross-section of the cathode element 95; (c) cutting the continuous length of the extrudate and forming a green element having the shape of the cathode element 95; and

(d) sintering the green element and forming the cathode element 95.

At the end of the experiment the cathode elements 95 were removed from the cell , washed to remove as much retained electrolyte as possible, and then grit blasted to remove any remaining accretions , which were found to be carbides .

The cathode elements were analysed and found to comprise titanium containing of the order of 2000 ppm oxygen .

The results of the experiment are summarised in Figure 7.

Figure 7 plots current, voltage, electrolyte temperature, and concentrations of selected off-gases (CO and CO₂) during the course of the experiment.

Figure 7 also includes a series of photographic images that show one of the cathode elements 95 at different stages in the experiment.

The image on the left side of the Figure shows the element 95 suspended at one location of the element by a support member prior to the assembly of the element and the support member being placed in the cell .

The next image shows the element 95 after it has been removed from the cell. It is evident from this image that a considerable amount of the electrolyte is retained on the element 95 as it is removed from the cell . The next image shows the element after it has been washed in water. Again, it is evident from this image that a considerable amount of the electrolyte is retained on the element 95 after the washing step.

The final image in the sequence shows that grit blasting the element 95 removed a significant proportion of the retained carbide accretions on the element.

It is evident from the right hand image in particular that th^ shape of the cathode element changed during the experiment. The shape change was due to phase changes , and consequential column changes , as the titania reduced to titanium.

Significantly, it is evident from the right hand image that, notwithstanding the exposure to a high temperature environment and the volume changes that occurred during reduction, the reduced cathode element 95 is largely structurally intact. This indicates that the honeycomb structure is sufficiently robust and otherwise suitable for use in an electrochemical process . In addition, the applicant has found that the above-described method of preparing the cathode elements 95, which involves extruding, cutting, and sintering, is an effective and efficient method of preparing large numbers of cathode elements for reduction in an electrochemical cell.

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

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A cathode element for use in an electrochemical cell for carrying out an electrochemical process for reducing a metal oxide such as a titanium oxide in a solid state, the cathode element including a honeycomb-shaped structure that includes a series of interconnected walls that define a plurality of passages for an electrolyte in the cell to penetrate the structure to contact internal exposed surfaces of the structure.

2. The CP⁺"'lode element defined in claim 1 includes passages that extend through the structure.

3. The cathode element defined in claim 2 wherein the passages are parallel passages.

4. The cathode element defined in claim 2 or claim

3 being in the shape of a block.

5. The cathode element defined in any one of the preceding claims wherein the passages are a polygonal shape such as a pentagon or a hexagon in transverse cross- section .

6. The cathode element defined in any one of the preceding claims wherein the walls are relatively thin compared to the transverse cross-sectional width of the passages .

7. The cathode element defined in any one of the preceding claims wherein the walls are less than 4 mm thick .

8. The cathode element defined in claim 7 wherein the wall thickness is less than 2.5 mm.

9. The cathode element defined in any one of the preceding claims wherein the maximum thickness of the walls, measured at intersections of the walls, is less than 2.5 mm, more preferably less than 2.0 mm.

10. The cathode element defined in any one of the preceding claims wherein the transverse cross-sectional width of the passages is less than 70 mm.

11. The cathode element defined in claim 10 wherein the transverse cross-sectional width of the passages is less than 55 mm.

12. A method of preparing the cathode element defined in any one of the preceding claims that includes the steps of:

(a) preparing a slurry of a metal oxide powder;

(b) extruding the slurry through a die and forming a continuous extrudate having the transverse cross-section of the cathode element;

(c) cutting the continuous length of the exrudate and forming a green element having the shape of the cathode element; and

(d) sintering the green element and forming the cathode element.

13. The method defined in claim 12 includes preparing the cathode element with a wall porosity of 35-60% by volume .

14. The method defined in claim 13 includes preparing the cathode element with a wall porosity of 40-50% by volume .

15. The method defined in any one of claims 12 to 14 wherein step (d) includes sintering the green element at a temperature in a range of 1150-1250⁰C.

16. 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 CaCl₂-based electrolyte containing CaO, a plurality of anodes extending into the cell chamber and at least partially immersed i^ the electrolyte, and a plurality of cathodes including a plurality of the cathode elements defined in any one of claims 1 to 11 extending into the cell chamber and at least partially immersed in the electrolyte with the metal oxide contacting the electrolyte .

17. The cell defined in claim 17 wherein the metal oxide is a titanium oxide, such as titania.

18. A process for electrochemically reducing a metal oxide such as titanium oxide in a solid state in the electrolytic cell defined in claim 16 or claim 17, which electrochemical 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 in the cell and electrochemically reducing the metal oxide in contact with the electrolyte and producing reduced material .

19. The process defined in claim 18 wherein the metal oxide is a titanium oxide and the process includes producing titanium having no more than 0.2% by weight oxygen .