Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to production of magnesium by electrolysis.
Conventional electrolytic production of magnesium from magnesium
chloride dissolved in a molten salt electrolyte in an electrolytic cell
results in
formation of magnesium at the cathode and chlorine gas at the cathode. The
molten salt electrolyte typically comprises one or more alkali metal or
alkaline
earth metal chlorides in which the magnesium chloride is dissolved.
The production of chlorine as a by-product of the production of
magnesium requires auxiliary equipment for recovery and storage of the by-
product chlorine gas which typically is reacted with hydrogen gas to form
hydrochloric acid. Electrolytic methods for producing magnesium are
described in U.S. Patents 4,073,703; 4,192,724; 5,089,094 and 5,665,220. :
This invention seeks to provide a new electrolytic process for the
production of magnesium from magnesium chloride, in which hydrogen
chloride is produced as the by-product.
This invention also seeks to provide a new electrolytic process for the
production of magnesium from magnesium chloride at a lower energy
requirement.
In accordance with one aspect of the invention there is provided in a
process for the electrolytic production of magnesium from magnesium chloride
in an electrolytic cell having an anode and a cathode, and in which magnesium
is generated at the cathode, the improvement wherein hydrogen gas is fed to
the
anode and hydrogen chloride is formed in situ at the anode.
In accordance with another aspect of the invention there is provided a
process for the electrolytic production of magnesium comprising: i)
electrolysing magnesium chloride in a molten salt electrolyte in an
electrolysis
cell having a cathode and an anode, with formation of magnesium metal at said
cathode, ii) feeding hydrogen gas to said anode and reacting chloride ions at
said anode with the hydrogen gas to form hydrogen chloride, iii) recovering
the
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magnesium metal from said cell, and iv) recovering the hydrogen chloride from
said cell.
In accordance with still another aspect of the invention there is provided
an electrolytic cell for production of magnesium metal from magnesium
chloride comprising: a) a cell for housing magnesium chloride in a molten salt
electrolyte, said cell having a cathode and an anode, b) means for feeding
hydrogen gas to said anode, c) means for recovery from said cell of
magnesium metal developed at said cathode, and d) means for recovery from
said cell of hydrogen chloride developed at said anode.
In accordance with yet another aspect of the invention there is provided
use of hydrogen in an electrolytic cell for the production of magnesium from
magnesium chloride with production of by-product hydrogen chloride at the
anode.
In particular the anode is a high surface area anode, for example, a
porous anode in which case the hydrogen gas permeates the pores of the anode,
such as by diffusion, or molten electrolyte containing the magnesium chloride
permeates the pores of the anode, to provide the contact between the hydrogen
gas and the chloride ions. The hydrogen gas may be fed along a non-porous
tube or conduit to the porous anode. If this tube or conduit is in contact
with
the bath it should not be of a material which will function as an anode for
the
electrolysis.
As an alternative to a porous anode, any anode having a structure
permitting delivery of hydrogen to the cell bath at the anode may be employed,
for example, an anode having drilled channels for communication with a source
of hydrogen gas. The requirement is that the anode structure deliver hydrogen
gas to the cell bath at the anode, so that chloride ions at the anode react
with the
hydrogen gas to form hydrogen chloride, rather than discharging as chlorine
gas.
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By way of example, suitable anodes may be of graphite, silicon carbide
or silicon nitride.
It has been found that introducing hydrogen at the anode in the
electrolytic cell for magnesium metal production results in a lower energy
requirement for the cell, and the cell can be operated at a cell voltage lower
than the cell voltage of a corresponding cell having a conventional carbon or
graphite anode, without hydrogen gas.
In addition it is found that hydrogen chloride is formed directly at the
anode by the reaction:
2C1- + H2(g) = 2HChg) + 2e
where (g) indicates the gas phase.
Furthermore, the method has the advantage that this hydrogen chloride
gas is produced with minimal, if any, production of chlorine gas.
In conventional cells in which chlorine gas is produced as the by-
product, the anode is graphite, and at the high temperatures of operation some
chlorinated hydrocarbons are produced by reaction between the chlorine gas
and the carbon anode, and this presents environmental problems. Eliminating
production of chlorine gas in the present invention can be expected to
alleviate
these problems.
Table I below shows how the decomposition voltage of the electrolysis
decreases, with the process of the invention, as compared with the
conventional
process and how the minimum voltage required to maintain energy balance
changes.
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TABLE I
Reaction E Eadiab. Eadiab. - E
MgC12 4 Mg + C12 2.50 3.60 1.1
MgCl2 + H2 4 Mg + HCI 1.46 2.74 1.28
Difference -1.04 -0.86 0.18
In Table I, Eadiab is the minimum voltage required to carry out the
process, assuming 100% current efficiency and that the MgC12 and H2 are fed at
room temperature.
In particular, Table I shows the calculated decomposition voltage
(1000 K) and adiabatic voltage required to cover the energy requirements of
the
process without heat losses.
Table I further shows that the decomposition voltage decreases by 1.04V
and that the overall energy requirement decreases by 0.86V. This means that
with HCI formation, another 0.18V per mole can be dissipated in the cell
without causing overheating. The decrease of 0.86V translates to a reduction
of about 25% less electricity consumption for magnesium production. With
magnesium cells currently requiring an average of 12.5 MW-hr per tonne, and
an average energy cost of 4 cents per KW-hrs, this translates to a savings of
about $125 per tonne of magnesium produced in electrical consumption.
Another major cost saving comes from the fact that the cell is producing
HC1 rather than chlorine, requiring no HC1 synthesis plant. Chlorine treatment
and handling as well as HC1 synthesis can provide for further cost savings.
Environmetal problems associated with chlorine gas production are
expected to be alleviated.
The hydrogen gas may be considered to form a hydrogen anode in the
cell, for discharge of the chloride ions. In such case an anode structure is
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provided which, can be of any suitable material, for example, graphite,
silicon
carbide or silicon nitride.
