WO2006084318A1

WO2006084318A1 - Supplying solid electrolyte to an electrolytic cell

Info

Publication number: WO2006084318A1
Application number: PCT/AU2006/000161
Authority: WO
Inventors: Sergey Alexander Bliznyukov; Ivan Petkov Ratchev; Gregory David Rigby; David Thomas Musgrave; Rene Ignacio Olivares; Andrew Arthur Shook
Original assignee: Bhp Billiton Innovation Pty Ltd
Priority date: 2005-02-08
Filing date: 2006-02-08
Publication date: 2006-08-17

Abstract

A process for supplying solid chloride-containing electrolyte to an electrochemical cell (3) is described. The process includes supplying solid electrolyte into a section of the cell through at least one delivery means (13) in the cell that at least substantially isolates the solid electrolyte from the remainder of the cell and melting the solid electrolyte in the delivery means. The delivery means is formed and positioned in the cell so that HCl gas that is produced as the solid electrolyte melts is confined at least initially within the delivery means.

Description

SUPPLYING SOLID ELECTROLYTE TO AN ELECTROLYTIC CELL

The present invention relates to electrochemical reduction of metal oxides .

The present invention relates particularly, although by no means exclusively, to electrochemical reduction of metal oxides in the form of powders and/or pellets in an electrochemical cell containing a molten electrolyte to produce reduced material .

The present invention is concerned with supplying solid electrolyte to an electrochemical cell . This is a particularly important issue when starting up an electrochemical reduction process in a cell . It is also an important issue after start up in situations when it is necessary to add further solid electrolyte to an operating cell , as may be the case for example in a continuous process .

The present invention was made during the course of an on-going research project on electrochemical reduction of metal oxides being carried out by the applicant. The research project has focused on the reduction of titania (TiO₂) .

During the course of the research project the applicant has carried out a series of experiments , initially on a laboratory scale and more recently on a pilot plant scale, investigating the reduction of metal oxides in the form of titania in electrochemical cells comprising a pool of molten CaCl₂-based electrolyte, an anode formed from graphite, and a range of cathodes .

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 electrochemical 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 electrochemical cells under a wide range of different operating parameters and conditions .

The applicant operated the laboratory electrochemical 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 electrochemical cells on a batch basis with other metal oxides .

Recent pilot plant work carried out by the applicant has been on a pilot plant cell arrangement set up to operate initially on a continuous basis and subsequently on a batch basis .

During the course of the pilot plant work the applicant started up each run of the process by supplying solid electrolyte to the pilot plant cell and melting the electrolyte in the cell and forming a molten electrolyte bath .

The applicant found that the addition of solid electrolyte, namely commercially available CaCl₂, to the cell and subsequent melting of the electrolyte to form the molten bath resulted in significant evolution of HCl gas at high temperature .

The applicant has recognised that the corrosiveness of the hot HCl gas is a concern and that, in particular, it is important to avoid contact of HCl gas with structural components of the cell , such as the walls of the vessel that contain the electrolyte .

The applicant has recognised that the hot HCl gas is also a concern from an occupational health and safety viewpoint and that, in particular , it is important to avoid contact of hot HCl gas at the concentrations produced in the cell with individuals .

The applicant has also recognised that the problem of HCl gas evolution is not confined to process start up and also extends to any situation in which it is necessary to add solid electrolyte to an operating cell .

The applicant has found that the problem of HCl gas evolution can be at least substantially avoided by supplying solid chloride-containing electrolyte into a section of the cell through a delivery means , such as a tube positioned in the cell , that at least substantially isolates the solid electrolyte from the remainder of the cell and melting the solid electrolyte in the delivery means , with the delivery means being formed so that HCl gas that is produced as solid electrolyte melts is confined at least initially within the delivery means and, for example , is extracted from the delivery means by an extraction means , and does not contact the cell walls and other components of the cell that could be adversely affected by contact with HCl gas .

In general terms , the present invention relates to a process for electrochemically reducing a metal oxide in a solid state in an electrochemical cell and to the cell per se . In general terms , the present invention relates to a cell that includes an anode, a cathode , and a molten chloride-containing electrolyte containing cations of a metal that is capable of chemically reducing the metal oxide . In general terms , the present invention relates to an electrochemical process that includes a step of applying an electrical potential across the anode and the cathode of the cell and electrochemically reducing metal oxide feed material in contact with molten chloride- containing electrolyte and producing reduced material .

According to the present invention there is provided a process for supplying solid chloride-containing electrolyte to the above-described electrochemical cell during start up of the above-described electrochemical process and/or during the course of operating the electrochemical process in the cell , which process includes supplying solid electrolyte into a section of the cell through a delivery means in the cell that at least substantially isolates the solid electrolyte from the remainder of the cell and melting the solid electrolyte in the delivery means , with the delivery means being formed and positioned in the cell so that HCl gas that is produced during the melting process is confined at least initially within the delivery means and thereby at least minimizes contact with walls and other components of the cell that could be adversely affected by contact with HCl gas .

Preferably the process includes extracting HCl gas from the delivery means .

By way of example , the process may include extracting HCl gas from the delivery means by using a suitable gas extraction means . For example, the extraction means may be a venturi assembly .

With such an arrangement, the process includes extracting HCl gas from the delivery means by supplying gas that is in the delivery means to a throat section of a venturi of the venturi assembly and entraining the HCl gas in a stream of air moving through the venturi .

The venturi assembly is an effective means of extracting the HCl gas from the delivery means .

In addition, the venturi assembly is an effective means of extracting other gas from the delivery means . By way of particular example , this an advantage in terms of extracting water vapour from the delivery means .

In addition, the venturi assembly is an effective means of mixing the HCl gas in a larger gas stream and thereby reducing the temperature and the concentration of the HCl gas .

The process may also include extracting HCl gas from the delivery means by injecting a purge gas into the cell and forming a flow of purge gas into the delivery means and thereafter from the cell . This purge gas option has the same advantages of reducing the temperature and the concentration of the HCl gas as the venturi assembly.

In a situation in which the process is concerned with supplying solid electrolyte to the electrolytic cell during start up of the above-described electrochemical process , preferably the process includes a first step of pre-heating the electrolytic cell to a temperature that is higher than the melting temperature of the solid electrolyte prior to supplying solid electrolyte into the cell via the delivery means .

The delivery means may be of any suitable configuration and be formed from any suitable material .

The delivery means may be any suitable configuration that defines a passageway or a chamber in the cell for supplying solid electrolyte into the cell and for removing HCl gas from the cell .

In the case of a process for electrochemically reducing titania with a CaCl₂-based electrolyte, the material of the delivery means should be capable of withstanding high temperatures , typically of the order of 900⁰C , be capable of withstanding substantial temperature variations , and be at least substantially inert with respect to the electrolyte .

Graphite is one example of a suitable material for the delivery means in the situation described in the preceding paragraph .

One example of the delivery means is an open- ended tube having an open upper end and an open lower end.

Preferably the process includes positioning the tube so that the upper end of the tube is above the cell and the tube extends into the cell and there is a gap between the lower end of the tube and a base wall of the cell that is sufficiently small to at least substantially confine solid electrolyte and HCl gas that evolves as solid electrolyte melts within the tube and is sufficiently large to allow molten electrolyte to flow from the lower end of the tube into the cell .

Another , although not the only other , example of the delivery means is a chamber in the cell that includes an inlet for supplying solid electrolyte into the chamber, an outlet for allowing HCl gas to flow from the cell , and an outlet that connects the chamber to the remainder of the cell so that molten electrolyte can flow from the chamber into the remainder of the cell .

Preferably the electrolyte is a CaCl₂-based electrolyte containing CaO .

According to the present invention there is also provided a process for electrochemically reducing a metal oxide in a solid state which includes the above-described process for supplying solid chloride-containing electrolyte to an electrolytic cell during start up of the electrochemical reduction process and/or during the course of operating the electrochemical process in the cell .

Preferably the metal oxide is in a powder and/or a pellet form.

Preferably the metal oxide is a titanium oxide .

More preferably the titanium oxide is titania.

Preferably the electrolyte is a CaCl₂-based electrolyte containing CaO .

Preferably the electrochemical process includes applying an electrical potential across an anode and a cathode of the cell that is above a potential at which cations of the metal that is capable of chemically reducing the metal oxide can deposit as the metal on the cathode , whereby the metal chemically reduces the metal oxide .

In the case of a CaCl₂~based electrolyte containing CaO, preferably the electrochemical process inclυdes applying a potential across the anode and the cathode of the cell that is above the decomposition potential of CaO and below the decomposition of CaCl₂.

The electrochemical process may be carried out on a batch basis , a semi-continuous basis , and a continuous basis .

The electrochemical process may be carried out as a single stage or a multi-stage process .

According to the present invention there is also provided an electrochemical cell for electrochemically reducing a metal oxide in a solid state as described above which includes at least one delivery means in the cell for supplying solid chloride-containing electrolyte to the cell during start up of an electrochemical process in the cell and/or during the course of operating the electrochemical process in the cell .

The delivery means may include a wall that is formed to absorb at least some of the HCl gas .

The delivery means may be any suitable configuration that defines a passageway or a chamber in the cell for supplying electrolyte into the cell and for removing HCl from the cell .

Graphite is one example of a suitable material for the delivery means .

One example of the delivery means is an open- ended tube in the cell . Preferably the tube has an open upper end that is above the cell and an open lower end in the cell .

Preferably there is a gap between the lower end of the tube and a base wall of the cell that is sufficiently small to at least substantially confine solid electrolyte and HCl gas that evolves as solid electrolyte melts within the tube and is sufficiently large to allow molten electrolyte to flow from the lower end into the cell .

Another, although not the only other, example of the delivery means is a chamber in the cell that includes an inlet for supplying solid electrolyte into the chamber, an outlet for allowing HCl gas to flow from the cell, and an outlet that connects the chamber to the remainder of the cell so that molten electrolyte can flow from the chamber into the remainder of the cell .

Preferably the cell includes a means for extracting HCl gas from the delivery means .

By way of example , the extraction means may be a venturi assembly .

The present invention is described further by way of example with reference to the accompanying drawings , of which :

Figure 1 is a vertical cross-section of an electrochemical reduction cell of the above-mentioned pilot plant of the applicant at an early stage of starting up an electrochemical reduction process in the cell , with a number of components of the cell that are not important from the viewpoint of the present invention removed from the drawing to simplify the drawing; and Figures 2 and 3 are vertical cross-sections of embodiments of electrochemical reduction cells in accordance with the present invention of the type shown in Figure 1 which include different means for extracting HCl gas from the cells and show the cells at early stages of starting up an electrochemical reduction process in the cells , with a number of components of the cells that are not important from the viewpoint of the present invention removed from the drawings to simplify the drawings .

The following description of the cells shown in the Figures is in the context of electrochemical reduction of titania to titanium in the cells using a CaCl₂-based electrolyte containing CaO. The present invention is not confined to this metal oxide and to this electrolyte .

With reference to Figure 1 , the cell , generally identified by the numeral 3 , is a quadrilateral shape with a flat base wall 5 and four side walls 7 (only two of which are shown in the drawing) extending upwardly from the base wall 5.

The cell 3 also includes a lid 9 that has a plurality of openings 11 (only two of which are shown in the drawing) that allow cell components to be inserted into the cell 3.

The cell 3 includes two such cell components in the form of tubes 13 for delivering pellets 15 of solid electrolyte into the cell 3 during start up of an electrochemical process in the cell and thereafter during the course of operating process in the cell 3 when additional electrolyte is required to replace electrolyte lost from the cell 3.

The tubes 13 can be inserted into and removed from the cell 3 via the openings 11 in the lid 9. The tubes 13 have open lower ends 17 and open upper ends 19.

The tubes 13 are formed from graphite with a coating of a heat resistant paint. The tubes 13 may be made form any other material that is capable of withstanding the cell operating temperatures of the order of 900 ° C and is resistant to hot HCl gas that is evolved as the pellets 15 of solid electrolyte melts in the tubes 13.

The tubes 13 are positioned at start up, as shown in the drawing, so that there are gaps 6 between the lower ends 17 of the tubes 13 and the base wall 5 of the cell 3. It is evident from the drawing that the gaps G are relatively small gaps .

The size of the gaps 6 is selected to :

(a) confine the solid pellets 15 within the tubes 13 ;

(b) allow molten electrolyte that forms in the tubes 13 to flow from the tubes 13 via the lower ends 17 and fill the cell 3 to a required level ; and

(c) minimize the risk of hot HCl gas that evolves in the tubes 13 as the solid electrolyte pellets 15 melt flowing into the cell 3 via the lower ends 17 of the tubes 13.

In use, during start up of an electrochameical process in the cell 3 , the cell 3 is sealed with an air tight seal and, while there is no electrolyte in the cell 3 , the cell 3 is heated to a predetermined temperature that is higher than the melting point of the solid electrolyte , typically a temperature of the order of 900⁰C . Thereafter , pellets 15 of solid electroyte are supplied into the tubes 13 via the upper ends 19 of the tubes 13 and melt in the tubes 13. The molten electrolyte flows into the cell 3 via the lower ends 17 of the tubes 13 and hot HCl gas that is evolved as the solid electrolyte melts flows upwardly in the tubes 13 and is either absorbed into the tube walls or is extracted and thereafter processed via the upper ends 19 of the tubes 13. Several options for extracting the hot HCl gas are described hereinafter in relation to Figures 2 and 3.

The above described process for supplying solid electrolyte has been operating successfully by the applicant during start up of the above-mentioned pilot plant .

Specifically , the start up process in the pilot plant work included the following steps :

(a) while the cell 3 was sealed with an air tight seal and there was no electrolyte in the cell 3 , heating the cell 3 to a predetermined temperature higher than the melting point of the electrolyte ; and

(b) when the predetermined temperature was reached, supplying pellets (in the form of discs having a 25 mm diameter) of solid electrolyte into the tubes 13 via the open upper ends 19 of the tubes 13 and forming beds of the pellets in the tubes 13 , with the solid electrolyte progressively melting and flowing from the lower ends 17 of the tubes 13 into the cell 3.

The applicant found that the above-described tubes 13 prevented hot HCl gas that evolved in the tubes 13 from escaping from the tubes 13 and contacting the side walls 7 and other components of the cell 3 that would be adversely affected by the HCl gas .

The cells 3 shown in Figures 2 and 3 are of the same basic construction as the cell 3 shown in Figure 1 and the same reference numerals are used to describe the same features of the cells 3.

The cell 3 shown in Figure 2 includes a venturi assembly for extracting hot HCl gas from the delivery tube 13 shown in the drawing.

The venturi assembly includes a venturi 31 that has an inlet 33 and an outlet 35 for airflow into and from the venturi 31 and an inlet 37 for flow of gas (including hot HCl gas) from the tube 13 into a throat 39 of the venturi 31.

In use , HCl gas that evolves in the tube 13 as solid electrolyte melts in the tube 13 is drawn into the throat 39 of the venturi 31 via the inlet 37 and is entrained in air (at ambient temperature) flowing through the venturi 31 from the inlet 33 to the outlet 35. As a consequence , the hot HCl gas is cooled and diluted as it mixes with the air . This is an effective option for reducing the occupational health and safety issues associated with hot HCl gas .

The cell 3 shown in Figure 3 uses a different approach to extracting (and cooling and diluting) hot HCl gas from the delivery tube 13.

Specifically, rather than using a mechanical extraction means , such as the above-described venturi assembly, the lid 9 of the cell 3 includes an inlet 41 for injecting a purge gas , such as an inert gas , into the cell 3.

In use, the injected purge gas ultimately forms a flow of gas from the cell 3 into the lower end 17 of the delivery tube 13 , as indicated by the arrowed flow lines in the drawing, and thereafter up the tube 13 to and thereafter from the upper end 19 of the tube 13. The flow of the purge gas mixes with the hot HCl gas in the tube 13 and cools and dilutes the gas as moves the gas upwardly from th tube 13.

Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention .

By way of example , whilst the above-described embodiments include a delivery means in the form of one or more than one tube 13 in the cells 3 shown in the drawings , the present invention is not limited to this specific form of delivery means .

Another, although not the only other , example of the delivery means is a chamber in the cell that includes an inlet for supplying solid electrolyte into the chamber, an outlet for allowing HCl gas to flow from the cell , and an outlet that connects the chamber to the remainder of the cell so that molten electrolyte can flow from the chamber into the remainder of the cell . The chamber forms a dedicated part of the cell , for example located at one end of a cell with quadrilateral walls , and is defined at last in part by one or more than one of the walls of the cell . The outlet that connects the chamber to the remainder of the cell may include , by way of example , an underflow weir and an overflow weir that allows molten electrolyte to flow from the chamber into the remainder of the cell .

Claims

CLAIMS :

1. A process for supplying solid chloride-containing electrolyte to an electrochemical cell described herein during start up of an electrochemical process described herein and/or during the course of operating the electrochemical process in the cell , which process includes supplying solid electrolyte into a section of the cell through at least one delivery means in the cell that at least substantially isolates the solid electrolyte from the remainder of the cell and melting the solid electrolyte in the delivery means , with the delivery means being formed and positioned in the cell so that HCl gas that is produced as the solid electrolyte melts is confined at least initially within the delivery means and thereby at least minimises contact with walls and other components of the cell that could be adversely affected by contact with HCl gas .

2. The process defined in claim 1 includes extracting HCl gas from the delivery means .

3. The process defined in claim 2 or claim 3 wherein , in a situation in which the process is concerned with supplying solid electrolyte to the electrolytic cell during start up of the electrochemical process , the process includes a first step of pre-heating the electrolytic cell to a temperature that is higher than the melting temperature of the solid electrolyte prior to supplying solid electrolyte into the cell via the delivery means .

4. The process defined in any one of the preceding claims wherein the delivery means defines a passageway or a chamber in the cell for supplying solid electrolyte into the cell and for removing HCl gas from the cell .

5. The process defined in any one of -the preceding claims wherein the delivery means is in the form of an open-ended -tube having an open upper end and an open lower end.

6. The process defined in claim 5 includes positioning the tube so that there is a gap between the lower end of the tube and a base wall of the cell that is sufficiently small to at least substantially confine solid electrolyte and HCl gas that evolves as solid electrolyte melts within the tube and is sufficiently large to allow molten electrolyte to flow from the lower end of the tube into the cell .

7. A process for electrochemically reducing a metal oxide in a solid state includes the process for supplying solid chloride-containing electrolyte to an electrolytic cell during start up of the electrochemical reduction process and/or during the course of operating the electrochemical process in the cell as defined in any one of the preceding claims .

8. The electrochemical reduction process defined in claim 7 wherein the metal oxide is in a powder and/or a pellet form.

9. The electrochemical reduction process defined in claim 7 or claim δ wherein the metal oxide is a titanium oxide .

10. The electrochemical reduction process defined in any one of claims 7 to 9 wherein the electrolyte is a CaCl2~based electrolyte containing CaO .

11. The electrochemical reduction process defined in any one of claims 7 to 10 includes applying an electrical potential to an anode and a cathode of the cell that is above a potential at which cations of the metal that is capable of chemically reducing the metal oxide can deposit as the metal on the cathode, whereby the metal chemically reduces the metal oxide .

12. The electrochemical reduction process defined in claim 11 wherein, in the case of a CaCl₂-based electrolyte containing CaO, the process includes applying a potential across the anode and the cathode that is above the decomposition potential of CaO and below the decomposition of CaCl₂.

13. An electrochemical cell for electrochemically reducing a metal oxide in a solid state includes at least one delivery means in the cell for supplying solid chloride-containing electrolyte to the cell during start up of an electrochemical process in the cell and/or during the course of operating the electrochemical process in the cell .

14. The cell defined in claim 13 wherein the delivery means includes a wall that is formed to absorb at least some of the HCl gas .

15. The cell defined in claim 13 or claim 14 wherein the delivery means defines a passageway or a chamber in the cell for supplying electrolyte into the cell and for removing HCl from the cell .

16. The cell defined in any one of claims 13 to 15 wherein the delivery means includes an open-ended tube that extends into the cell .

17. The cell defined in claim 16 wherein the tube has an open upper end that is above the cell and an open lower end that is in the cell .

18. The cell defined in claim 17 wherein there is a gap between the lower end of the tube and a base wall of the cell that is sufficiently small to at least substantially confine solid electrolyte and HCl gas that evolves as solid electrolyte melts within the tube and is sufficiently large to allow molten electrolyte to flow from the lower end into the cell .

19. The cell defined in any one of claims 13 to 19 includes a means for extracting HCl gas from the delivery means .