The present invention relates to reduction of metal oxides
in a solid state in an electrolytic cell.
The present invention was made during the course of an
on-going research project on solid state reduction of titania (TiO2)
carried out by the applicant.
During the course of the research project the applicant
carried out experimental work on the reduction of titania using an electrolytic
cell that included a graphite crucible that formed an anode of the cell, a pool
of molten CaCl2-based electrolyte in the crucible, and a range of cathodes
that included solid titania.
The CaCl2-based electrolyte was a commercially
available source of CaCl2, namely calcium chloride dihydrate, that partially
decomposed on heating and produced CaO.
The applicant operated the electrolytic cell at a potential
above the decomposition potential of CaO and below the decomposition potential of
The applicant found that the cell could electrochemically
reduce titania to titanium with very low concentrations of oxygen.
The applicant does not have a clear understanding of the
electrolytic cell mechanism at this stage. Nevertheless, whilst not wishing to be
bound by the comments in this and the following paragraphs, the applicant offers
the following comments by way of an outline of a possible cell mechanism.
The experimental work carried out by the applicant produced
evidence of Ca metal dissolved in the electrolyte. The applicant believes that,
at least during the early stages of operation of the cell, the Ca metal was the
result of electrodeposition of Ca++ cations as Ca metal on electrically
conductive sections of the cathode.
The experimental work was carried out using a CaCl2-based
electrolyte at a cell potential below the decomposition potential of CaCl2.
The applicant believes that the initial deposition of Ca metal on the cathode was
due to the presence of Ca++ cations and O-- anions derived
from CaO in the electrolyte. The decomposition potential of CaO is less than the
decomposition potential of CaCl2. In this cell mechanism the-cell operation
is dependent, at least during the early stages of cell operation, on decomposition
of CaO, with Ca++ cations migrating to the cathode and depositing as
Ca metal and O-- anions migrating to the anode and forming CO and/or
CO2 (in a situation in which the anode is a graphite anode).
The applicant believes that the Ca metal that deposited
on electrically conductive sections of the cathode was deposited predominantly as
a separate phase in the early stages of cell operation and thereafter dissolved
in the electrolyte and migrated to the vicinity of the titania in the cathode and
participated in chemical reduction of titania.
The applicant also believes that at later stages of the
cell operation part of the Ca metal that deposited on the cathode was deposited
directly on partially deoxidised titanium and thereafter participated in chemical
reduction of titanium.
The applicant also believes that the O-- anions,
once extracted from the titania, migrated to the anode and reacted with anode carbon
and produced CO and/or CO2 (and in some instances CaO) and released electrons
that facilitated electrolytic deposition of Ca metal on the cathode.
However, notwithstanding that the cell could electrochemically
reduce titania to titanium with very low concentrations of oxygen, the applicant
also found that there were relatively significant amounts of carbon transferred
from the anode to the electrolyte and to the titanium produced at the cathode under
a wide range of cell operating conditions.
Carbon in the titanium is an undesirable contaminant. In
addition, carbon transfer was partially responsible for low energy efficiency of
the cell. Both problems are significant barriers to commercialisation of electrolytic
The applicant carried out experimental work to identify
the mechanism for carbon transfer and to determine how to minimise carbon transfer
and/or to minimise the adverse effects of carbon transfer.
The experimental work indicated that the mechanism of carbon
transfer is electrochemical rather than erosion and that one way of minimising carbon
transfer and therefore contamination of titanium produced at the cathode by electrochemical
reduction of titania at the cathode is to position a membrane that is permeable
to oxygen anions and is impermeable to carbon in ionic and non-ionic forms between
the cathode and the anode and thereby prevent migration of carbon to the cathode.
Accordingly, the present invention provides an electrolytic
cell for reducing a metal oxide in a solid state, which electrolytic cell includes
an anode formed from carbon, a cathode formed at least in part from the metal oxide,
and a membrane that is permeable to oxygen anions and is impermeable to carbon in
ionic and non-ionic forms positioned between the cathode and the anode to thereby
prevent migration of carbon to the cathode.
Preferably, the anode is formed from graphite.
The membrane may be formed from any suitable material.
Preferably, the membrane is formed from a solid electrolyte.
One suitable solid electrolyte tested by the applicant
is yttria stabilised zirconia.
Preferably, the cathode also includes an electrical conductor.
The present invention also provides a method of reducing
a metal oxide in a solid state using the above-described electrolytic cell.
Preferably, the method includes a step of operating the
cell at a potential that is above a decomposition potential of at least one of the
constituents of the electrolyte so that there are cations of a metal other than
that of the metal oxide in the electrolyte.
In a situation in which the metal oxide is a titanium oxide,
such as titania, it is preferred that the electrolyte be a CaCl2-based
electrolyte that includes CaO as one of constituents.
In such a situation it is preferred that the cell potential
be above the decomposition potential for CaO.
It is also preferred that the cell potential be below the
decomposition potential for CaCl2.
It is preferred that the cell potential be less than or
equal to 3.0 V.
It is preferred particularly that the cell potential be
below 2.5 V.
It is preferred more particularly that the cell potential
be below 2.0 V.
It is preferred that the cell potential be above 1.5 V.
The CaCl2-based electrolyte may be a commercially
available source of CaCl2, such as calcium chloride dihydrate, that partially
decomposes on heating and produces CaO or otherwise includes CaO.
Alternatively, or in addition, the CaCl2-based
electrolyte may include CaCl2 and CaO that are added separately or pre-mixed
to form the electrolyte.
The present invention is described further with reference
to the following Example that relates to experimental work on the above-described
As indicated above, the cell included a high density graphite
crucible that formed the anode of the cell, a pool of molten CaCl2 electrolyte
in the crucible, and a cathode that included solid titania. In the initial experimental
set-up the solid titania was in the form of titania pellets connected to a lower
end of a Kanthal or stainless steel electrically conductive wire.
As indicated above, experimental work on the cell identified
carbon transfer as a significant issue in terms of contamination of cathode titanium
and causing low energy efficiency of the cell. In addition, as indicated above,
the experimental work established that carbon transfer was caused by an electrochemical
reaction at the anode.
Thereafter the applicant carried out experimental work
to investigate whether it was possible to prevent migration of carbon from the anode
to the cathode.
One experiment investigated the impact of a solid ionic
barrier on carbon migration.
The ionic barrier was in the form of a yttria stabilised
zirconia membrane positioned between the anode and the cathode, thereby dividing
the cell into an outer anode chamber and an inner cathode chamber.
Figure 1 is a schematic of the cell set-up for the experiment.
With reference to the Figure, the cell included a graphite crucible 3 that formed
the anode, a pool 19 of molten CaCl2 electrolyte in the crucible, titania
pellets 5 and an electrically conductive wire 7 that formed the cathode immersed
in the electrolyte, and a yttria stabilised zirconia membrane 9 immersed in the
electrolyte between the anode and the cathode. The cell was located in a resistance
furnace 11 heated to a temperature to maintain the electrolyte in a molten state.
The experimental set-up also included gas monitoring, cleaning, and analysis equipment.
The cell was operated at an applied potential of 3V for a period of 35 hours, during
which time there was continuous monitoring of the off-gas from the furnace. At the
conclusion of the experiment, the cell was cooled and the solidified electrolyte,
the membrane, the anode and the cathode were analysed.
Figure 2 is a summary of the results of the experiment.
Figure 2 shows measured voltage, current, CO and CO2
composition of the off-gas for the experiment.
Visual and analytical examination of the cathode and the
cathode chamber indicated that there was no carbon on the cathode and in the cathode
In addition, the visual and analytical examination of the
cathode indicated that titania was reduced to titanium. It follows from this finding
that the yttria stabilised zirconia membrane did not restrict migration of O--
anions from the cathode to the anode.
Many modifications may be made to the present invention
as described above without departing from the spirit and scope of the invention.
By way of example, whilst the above description of the
invention focuses on reduction of titania, the invention is not so limited and extends
to electrolytic reduction of other titanium oxides and to oxides of other metals
Examples of other potentially important meals are aluminium,
silicon, germanium, hafnium, magnesium, and molybdenum.
Furthermore, whilst the above description focuses on CaCl2-based
electrolyte, the invention is not so limited and extends to any other suitable electrolytes.
Generally, suitable electrolytes will be salts and oxides
that are soluble in salts. One example of a potentially suitable electrolyte is