NO340277B1 - A method of reducing a solid metal oxide in an electrolysis cell. - Google Patents

Description

Method for reducing a metal oxide in a solid state in an electrolytic cell

The present invention relates to the reduction of metal oxides in a solid state in an electrolysis cell.

The invention was made during an ongoing research project on the solid-state reduction of titania, titanium dioxide (Ti02), carried out by the present applicants.

During the research project, the applicant carried out experimental work on the reduction of titania using an electrolysis cell that included a graphite crucible that formed an anode in the cell, a pool of molten CaCl2-based electrolyte in the crucible, and an area of cathodes that included solid titania.

One purpose of the experimental work was to reproduce the results reported in W099/64638 in the name of Cambridge University Technical Services Limited and in technical papers published by the inventors in this international application.

The application in the name of Cambridge International describes two potential applications of a "discovery" in the field of metallurgical electrochemistry.

One application is the direct production of a metal from a metal oxide.

Within the context of this application, the "discovery" is the realization that an electrolysis cell can be used to ionize oxygen contained in a metal oxide so that the oxygen dissolves in an electrolyte. The application in the name of Cambridge International describes that when a suitable voltage is applied to an electrolytic cell with a metal oxide as a cathode, a reaction occurs whereby oxygen is ionized and is then able to dissolve in the electrolyte in the cell.

EP 99955507.1 which is derived from the application in the name of Cambridge International, has been approved by the European Patent Office, as patent EP1088113 Bl. The approved claims of the European application define inter alia a method of electrolytically reducing a metal oxide (such as titania) which includes operating an electrolytic cell at a voltage on an electrode formed by the metal oxide which is lower than the deposition potential of cations in the electrolyte on the surface of the electrode .

The European application in the name of Cambridge does not define what is meant by deposition potential and does not include any specific examples giving values of the deposition potential for particular cations.

However, submissions of 2 October 2001 to the EPO by the Cambridge patent attorneys, which preceded the filing of the later allowed claims, indicate that they assumed that the decomposition potential of an electrolyte is the deposition potential of a cation in the electrolyte.

In particular, it is said on page 5 of the post that:

"The second advantage as described above is achieved in part by carrying out the claimed invention below the decomposition voltage of the electrolyte. If higher voltages are used, as indicated in D1 and D2, the cation in the electrolyte will be deposited on the metal or semi-metal compound. In the example of D1 this leads to calcium deposition and therefore to the consumption of this reactive metal. During the implementation of the method, the electrolytic cation is not deposited on the cathode". (Own translation."

Contrary to the findings of Cambridge, experimental work carried out by the present applicants has determined that it is essential that the electrolytic cell be operated at a voltage above the voltage at which Ca<++->cations in the electrolyte can be deposited as Ca metal on the cathode .

Accordingly, the present invention provides a method for reducing a metal oxide in a solid state in an electrolytic cell, wherein the electrolytic cell includes an anode, a cathode formed at least in part of the metal oxide, and a molten electrolyte that includes cations of calcium (Ca< ++>), wherein calcium metal is capable of chemically reducing the metal oxide, wherein the metal oxide is immersed in a solid state in the electrolyte, and which method includes a step of operating the cell at a voltage above the voltage at which the calcium cations (Ca<++>) is deposited as calcium metal on the cathode, whereby the calcium metal chemically reduces the metal oxide.

Present applicants cannot provide a clear definition of the electro-light cell mechanism at the present time. Without wishing to be bound by any particular theory, one will, however, attempt to outline a possible cell mechanism in the following comments.

The experimental work carried out by the applicants gave evidence that Ca metal dissolved in the electrolyte. Applicants hypothesize, at least during the early working stages of the cell, that the Ca metal was the result of electrodeposition of Ca<++->cations, as Ca metal on electrically conductive parts of the cathode.

The experimental work was carried out using a CaCl2-based electrolyte at a cell voltage below the decomposition voltage for CaCl2. Applicants suggest that this initial deposition of Ca metal on the cathode is due to the presence of Ca<++->cations and 0~~-anions which originated from CaO in the electrolyte. The decomposition voltage for CaO is less than the decomposition voltage for CaCl2.

In this cell mechanism, cell operation, at least during its early stages, depends on the decomposition of CaO with Ca<++->cations migrating to the cathode and deposition as Ca metal and 0~~-anions migrating to the anode to form CO and/or CO2 (in a situation where the anode is a graphite anode).

Applicants hypothesize that the Ca metal deposited on electrically conductive parts of the cathode was deposited predominantly in a separate phase in the early stages of cell operation and then dissolved in the electrolyte and migrated to the vicinity of titania in the cathode and participated in the chemical reduction of titania.

The applicants also assume that at later stages of the cell operation, part of the Ca metal that had been deposited on the cathode was employed directly on partially deoxidized titanium and then participated in the chemical reduction of titanium.

The applicant also assumes that the 0~~ anions, once pulled away from the titania present, migrated to the anode and reacted with anode carbon to give CO and/or CO2 (and in some cases CaO) and released electrons which facilitated electrolytic deposition of Ca metal on the cathode.

In a situation where the metal oxide is a titanium oxide, such as titania, it is preferred that the electrolyte is a CaCl2-based electrolyte that includes CaO as one of the constituents of the electrolyte. In this context, it should be pointed out that the invention does not require the addition of significant amounts of CaO to the electrolyte.

In such a situation, it is preferred that the cell voltage is above the voltage at which Ca metal can be deposited on the cathode, that is to say at a voltage that is above the deposition voltage for CaO.

The decomposition voltage for CaO can vary over a considerable range depending on factors such as the composition of the anode, the electrolyte temperature and the electrolyte composition.

In a cell containing CaO-saturated CaCl2 at 1373 K (1100 °C) and a graphite anode, this would require a minimum cell voltage of 1.34 V.

It is also preferred that the cell voltage is below the voltage at which Cl~ anions can be deposited on the anode and give chlorine gas, i.e. the decomposition voltage for CaCl2-

In a cell containing CaO-saturated CaCl2 at 1373 K (1100 °C) and a graphite anode, this would require the cell voltage to be less than 3.5 V.

The decomposition voltage for CaCl2 can vary over a considerable range depending on factors such as the composition of the anode, the electrolyte temperature and the electrolyte composition.

For example, a salt containing 80% CaCl2 and 20% KC1 at a temperature of 900K (657 °C) decomposes to Ca (metal) and CI2 (gas) above 3.4 V and a salt containing 100% CaCl2 at 1373K (1100 ° C), decomposes at 3.0 V.

Generally speaking, in a cell containing CaO-CaCl2 salt (not saturated) at a temperature in the range 600-1100 °C and a graphite anode, it is preferred that the cell voltage is between 1.3 and 3.5 V.

The CaCl 2 -based electrolyte may be a commercially available source of CaCl 2 , such as calcium chloride dihydrate, which partially decomposes on heating to give CaO or otherwise includes CaO.

Alternatively, or in addition, the CaCl 2 -based electrolyte may include CaCl 2 and CaO added separately or premixed to form the electrolyte.

It is preferred that the anode is graphite or an inert anode.

Søker has found in the experimental work that there were relatively significant amounts of carbon that were transferred from the graphite anode to the electrolyte and to a lesser extent to the titanium that was produced at the cathode under a wide range of cell operating conditions.

Carbon in titanium can be an unwanted contaminant. In addition, the carbon transfer was partly responsible for low energy efficiency in the cell. Both problems could prove to be significant barriers to the commercialization of electrolytic reduction technology.

The applicant has also found that the dominant mechanism for carbon transfer is electrochemical rather than erosion and that one way of minimizing carbon transfer and thereby reducing contamination of the titanium produced at the cathode by electrochemical reduction of titania is to position a membrane that is permeable for oxygen anions and is impermeable to carbon in ionic and non-ionic forms between the cathode and anode thereby preventing the migration of carbon to the cathode.

Accordingly, in order to minimize contamination of titanium produced at the cathode as a result of carbon transfer, it is preferred that the electrolytic cell includes a membrane permeable to oxygen anions and impermeable to carbon in ionic and non-ionic forms, positioned between the cathode and the anode thereby preventing the migration of carbon to the cathode.

The membrane can be formed from any suitable material.

Preferably, the membrane is formed from a solid electrolyte.

One solid electrolyte tested by applicant is yttria (yttrium oxide) stabilized zirconia (zirconium oxide).

The invention will be explained in more detail with reference to the following example.

I. Experimental method and electrolysis cell

The electrolysis cell is shown in Figure 1.

Referring to Figure 1, the electrochemical cell includes a graphite crucible equipped with a graphite lid. The crucible was used as the cell anode. A stainless steel rod was used to ensure electrical contact between a direct current energy source and the crucible. The cell cathode consisted of Kanthal or platinum wire connected at one end to the energy supply and Ti02 pellets suspended at the other end of the wire. An alumina tube was used as an insulator around the cathode. The cell electrolyte was a commercially available source of CaCl 2 , namely calcium chloride dihydrate, which partially decomposed on heating at the operating temperature of the cell to give CaO. A thermocouple was immersed in the electrolyte near the pellets.

Two types of pellets were used. One type was drop-cast and the other type pressed. Both types of pellets were made from TiO2 powder of analytical quality. Both types of pellets were sintered in air at 850 °C. One pressed and one drop-cast pellet was used in the experiment.

The cell was positioned in an oven and the experiment carried out at 950 °C. Voltages up to 3 V were applied between the crucible wall and the Kanthal or platinum wire. The voltage of 3 V is below the voltage at which Cl~ anions can be deposited on the anode at this temperature. In addition, the 3 V voltage is above the decomposition voltage for CaO and below the decomposition voltage for CaCl2.

The energy supply was kept at a constant voltage during the experiment. The voltage and resulting cell current were logged using LabVTEW(TM) data acquisition programs.

At the end of the experiment, the cell was removed from the oven and quenched in water. The solid CaCl2 was dissolved in water and the two pellets recovered.

II. Test results

Referring to Figures 2 and 3, the constant voltage (3 V) used in the experiment gave an initial current of around 1.2 A. A continuous drop in the current was observed during the first 2 hours. After this, a gradual increase of the current up to 1 A was observed.

SEM images of the cross-sections of the two recovered pellets are shown in Figures 4 and 5. The SEM images indicate the presence of metallic titanium in both pellets and thereby establish that the method has successfully reduced titania electrochemically.

The presence of virtually pure metallic titanium in both pellets was confirmed by EPMA analysis. The analysis also showed areas of partially reduced titania. The EPMA results are shown in Figures 6 and 7.

Carbon was detected at different locations in these pellets and the content varied up to 18% by weight.

For example, while the above description focuses on the reduction of titania, it is not limited to this compound and also applies to the reduction of other titanium oxides and oxides of other metals and alloys. Examples of other potentially important metals are aluminium, silicon, germanium, zirconium, hafnium, magnesium and molybdenum.

Furthermore, while the description is focused on CaCl2-based electrolyte, the invention is not limited to this compound and also applies to any other suitable electrolyte (and mixtures of electrolytes). In general, suitable electrolytes will be salts and oxides which are soluble in salts. One example of a potentially suitable electrolyte is BaCl2.

Claims (9)

1. Method for reducing a metal oxide in a solid state in an electrolytic cell, wherein the electrolytic cell includes an anode, a cathode formed at least in part of the metal oxide, and a molten electrolyte including cations of calcium (Ca++), wherein calcium metal is in capable of chemically reducing the metal oxide, where the metal oxide is immersed in a solid state in the electrolyte, characterized in that the method includes a step of operating the cell at a voltage that is above the voltage at which the calcium cations (Ca<++>) are deposited as calcium metal on the cathode, whereby the calcium metal chemically reduces the metal oxide. 2. Method according to claim 1, characterized in that the metal oxide is a titanium oxide, and that the electrolyte is a CaCl2-based electrolyte which includes CaO as one of the constituents of the electrolyte. 3. Method according to claim 2, characterized in that the cell voltage is below the decomposition voltage for CaCl2 in order to minimize formation of Cl2 gas at the anode. 4. Method according to claim 2 or 3, characterized in that the cell voltage is less than or equal to 3.5 V in a cell that works with the electrolyte at 600-1100 °C. 5. Method according to any one of claims 2 to 4, characterized in that the cell voltage is at least 1.3 V in a cell that works with the electrolyte at 600-1100 °C. 6. Method according to any one of claims 2 to 5, characterized in that the CaCl2-based electrolyte is a commercially available CaCl2 source which forms CaO upon heating or otherwise includes CaO. 7. A method according to any one of claims 2 to 6, characterized in that the CaCl 2 -based electrolyte includes CaCl 2 and CaO added separately or pre-mixed to form the electrolyte. 8. Method according to any one of the preceding claims, characterized in that the anode is graphite. 9. Method according to any one of the preceding claims, characterized in that the anode is graphite and that the electrolysis cell includes a membrane that is permeable to oxygen anions and impermeable to carbon in ionic and non-ionic form, positioned between the cathode and the anode to thereby prevent the migration of carbon to the cathode.