Note: Descriptions are shown in the official language in which they were submitted.
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
1
CONTROL OF BY-PASS CURRENT IN
MULTI-POLAR LIGHT METAL REDUCTION CELLS
TECHNICAL FIELD
This invention relates to the control of by-pass current in multi-polar metal
reduction cells, particularly those used for the production of light metal
such as
magnesium, aluminum, sodium, lithium, etc.
BACKGROUND ART
Magnesium and other light metals are often produced by electrolysis from
metal salts in mono-polar electrolysis cells. However, multi-polar
electrolysis cells
may be used, where such cells have at least one, and usually several, multi-
polar
electrodes positioned in the space between an anode and a cathode forming a
series connection of sub-cells. The multi-polar electrodes increase the number
of
electrolysis steps at which electrolysis may take place, and thus increase the
cell
energy and production efficiency compared to mono-polar cells of the same
amperage. A gas is normally generated in the electrolyte during the process of
electrolysis (for example, chlorine is generated when magnesium is derived
from
magnesium chloride). In a vertical electrode cell, the resulting gas bubbles
rise
between the electrodes, lifting the surrounding electrolyte with them. This
causes
fresh electrolyte to flow between the electrodes from below and ensures that
fresh
metal salt is made available for further reduction. The rising volume of gas
and
electrolyte emerges from between the electrodes at the top of the cell and the
gas
separates from the liquid to fill the head space above the electrodes. The
electrolyte circulates back to a reservoir of electrolyte in the cell for
replenishment
of salt feed and eventual recirculation to the space between electrodes. Cells
of this
kind are said to operate on the "gas-lift" principal.
Multi-polar electrolysis cells that employ the gas-lift principal may be of
two
main kinds. In a first kind of cell, the anode, cathode and multi-polar
electrodes are
planar and are arranged face-to-face in a row with suitable gaps therebetween.
In a
second kind of cell, the anode is in the form of a solid upright rod and the
cathode
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
2
and multi-polar electrodes are in the form of hollow cylinders of different
diameters
encircling the rod at increasing distances, the cathode being the most distant
from
the anode rod. In both cases, the electrodes are encased in a container lined
with
refractory material that provides thermal insulation for the molten
electrolyte and
the molten metal. An arrangement for the collection and removal of generated
gas
is also provided at the upper end of the cell.
A persistent problem with cells of this kind is that bypass current travels
around the ends of the multi-polar electrodes following a more direct path
through
the electrolyte between the cathode and the anode rather than passing between
the multi-polar electrodes through the series-connected sub-cells formed
between
adjacent electrodes. This leads to reduced cell productivity, increased power
consumption and decreased current efficiency. The by-pass current may
typically
reduce current efficiency by 3 to 5% or more.
A by-pass current forms most readily at the upper end of the electrodes if
the electrolyte overflows the electrodes driven by the rising volume of gas.
In some
cell designs, the by-pass current has been kept in check by designing the
electrode
cassettes (i.e. electrode assemblies) in such a way that the thickness of the
electrolyte layer passing over the tops of the electrodes is minimized. This
requires
careful electrolyte level control to ensure that adequate, but not excessive,
flow is
maintained. Despite such measures, by-pass currents still flow and there is a
reduction of current efficiency. Examples of prior patents in which level
control is
suggested to reduce the by-pass current are U.S. patent 4,514,269 which issued
on
April 30, 1985 to Sivilotti, and U.S. patent 5,935,394 which issued on August
10,
1999 to Sivilotti et al. (and which are both assigned to the same assignee as
the
present application). Other solutions have included the provision of electrode
extensions on the upper end of the multi-polar electrodes. These extensions
project well above the surface of the electrolyte and thus prevent overflow of
the
electrolyte. However, this solution requires that the electrolyte rising in
the inter-
electrode gaps be diverted, generally to the ends of the electrode array where
channels are provided to return the electrolyte to the reservoir. This reduces
the
efficiency of the electrolyte recirculation and it is a solution that cannot
be
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
3
employed with cylindrical electrodes, as there are then no exits for
electrolyte flow
except over the tops of the electrodes. Examples of prior patents in which
electrode extensions are suggested are U.S. Patent 4,401,543 which issued on
August 30, 1983 to Ishizuka, and Japanese Patent Application 02-258993A which
published on October 19, 1990.
There is therefore a need to minimize current by-pass while maintaining
good electrolyte recirculation.
SUMMARY OF THE INVENTION
Certain exemplary embodiments can provide a multi-polar electrolytic cell
for producing a light metal by electrolysis of a corresponding metal salt. The
cell
comprises a molten electrolyte containing a metal salt that produces a light
metal
and a gas when electrolyzed, and an arrangement of generally vertical
electrodes
surrounded by the molten electrolyte, including an anode, a cathode and at
least
one current-conducting multi-polar electrode interposed between the anode and
the cathode. At least one of the multi-polar electrode(s) has an upper end
with an
electrical insulator positioned over the upper end (at least partially
covering the
upper end surface of the electrode). In use of the cell, the insulator(s) is
(are)
immersed beneath the electrolyte as it overflows the electrodes. The cell may
minimize or eliminate by-pass current normally encountered in multi-polar
electrolysis cells.
Cells of this kind preferably lack outlets or channels for the rising
electrolyte
that would permit the electrolyte to be diverted around and prevented from
rising
over the upper ends of the electrodes. Thus, the electrode structure (whether
planar or cylindrical) is preferably such that all or substantially all of the
rising
electrolyte passes over the insulated upper ends of the multi-polar
electrode(s), as
well as the cathode, during normal operation of the cell. Thus, the cells are
preferably of a kind that, but for the insulation of the upper ends of the
electrodes
as disclosed herein, would develop a significant by-pass current during use.
Other exemplary embodiments can provide a method of minimizing or
eliminating by-pass current between an anode and a cathode in a multi-polar
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
4
electrolysis cell suitable for production of a light metal, the method
comprising
electrically insulating an upper end of at least one multi-polar electrode of
the cell,
and conducting electrolysis with the insulated upper end maintained below an
upper surface of molten electrolyte containing a metal salt to be electrolyzed
held
within the cell.
By the term "by-pass current" we mean current that flows around (above,
below and beside) a multi-polar electrode resulting in a skipping of at least
one
multi-polar electrolysis step between electrodes (anode, cathode or multi-
polar
electrode) without contributing to the electrolysis reaction. It is sometimes
referred
to as current leakage and it represents a loss of current efficiency of the
cell.
By the term "immersed beneath" to describe the condition of the insulators
at the upper ends of the electrodes during normal use of the cell, we mean
that the
insulators are covered by electrolyte either by virtue of the overflow of the
rising
electrolyte and/or by virtue of the insulators being positioned below the
upper level
of the electrolyte that it adopts when current is not flowing through the
cell. In the
former case, the upper ends of the electrodes, or at least the insulators, may
stand
proud of the upper level of the electrolyte when no current is flowing. In the
latter
case, they may be immersed.
In certain embodiments, it is advantageous to electrically insulate part of
the
anode as well. Such insulation may be placed on the face of the anode (or
surrounding a cylindrical anode). The insulation can be effective even if it
is entirely
below the electrolyte surface, but it is particularly preferred that it extend
above
the electrolyte surface. This insulation minimizes further the bypass currents
that
would flow from the anode across the adjacent multi-polar electrode even when
the multi-polar electrode has an insulator present.
The anode and multi-polar electrodes used in the present invention may be
made of graphite, metals, cermets, composites, and laminates of these
materials.
The cathode is generally made of steel. The electrodes are generally non-
consumable in that the electrode is not consumed by the main electrolysis
reaction.
However, side reactions may contribute to some deterioration of the electrode
material. The electrical insulators should preferably be stable in the cell
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
environment and resistant to attack or degradation by electrolyte and the
products
of electrolysis at the cell operating temperature for the duration of the cell
life.
When magnesium is the metal being produced, alumina is the preferred material
for
the insulators. Other materials may alternatively be employed, e.g. magnesia,
Mg-
5 aluminate spinel, aiuminum nitride, silicon nitride, SIALON (a fine grain
non-porous
technical grade engineering material comprising a silicon nitride ceramic with
a
small percentage of aluminum oxide added), etc. The insulator may be a solid
sintered or fused-cast ceramic block keyed into the edge of the graphite
electrode
or held in place by ceramic spacers. Alternatively, the insulators may
comprise a
coating layer applied to the appropriately shaped electrode edge by a ceramic
coating process, such as plasma spraying, sputter deposition and chemical
vapour
deposition. Still another electrode edge insulator may comprise thin ceramic
tiles
adhesively bonded to the electrode by cement.
One way of providing the upper ends of the multi-polar electrodes with
electrically insulating surfaces is to add pieces of insulating refractory
material to
the upper ends of the electrically conductive parts of the electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is plan view of a multi-polar electrolysis cell with the top removed
suitable for use with the present invention, the cell containing a cassette of
electrodes of upright planar shape;
Fig. 2 is a plan view of a multi-polar electrolysis cell with the top removed
suitable for use with the present invention, the cell containing a plurality
of
cassettes or electrodes of upright cylindrical shape;
Fig. 3 is a vertical cross-section of an electrode cassette along the line A-A
in
either Fig. 1 or Fig. 2 in which the multi-polar electrodes (either planar or
cylindrical)
are insulated in accordance with one preferred form of the present invention;
Fig. 4 is a vertical cross-section of an electrode cassette along the line A-A
on
either Fig. 1 or Fig. 2 in which the multi-polar electrodes (either planar or
cylindrical)
are insulated in accordance with a preferred form of the present invention,
but with
the upper ends at different heights;
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
6
Fig. 5 is a vertical cross-section of an electrolysis cell provided with one
or
more cylindrical cassettes of the kind shown in Fig. 4;
Figs. 6A to 6G illustrate various ways in which insulating material may be
fixed to the electrically-conductive material of electrodes;
Fig. 7A is a partial perspective view of an alternative design of insulating
block suitable for insulating upper ends of planar or electrodes;
Fig. 7B is a partial perspective view of an alternative design of insulating
block suitable for insulating upper ends of cylindrical electrodes;
Fig. 8 is a graph showing the variation of by-pass currents in one example of
a cell with insulators of various heights on the tops of the multi-polar
electrodes;
and
Fig. 9 is a graph of the measured current efficiency of a cell using
insulators
on the tops of some multi-polar electrodes relative to a cell without
insulators over
an extended period of cell operation.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
The present invention is capable of being used with multi-polar cells of all
kinds, but mainly multi-polar cells that have vertical (or sloping) electrodes
that are
planar or cylindrical. However, the exemplary embodiments relate in particular
to
such cells in which electrolyte circulation is achieved by use of a "gas-lift"
principal
in which a gas generated during electrolysis over the entire active surfaces
of the
electrodes causes upward flow of electrolyte in the inter-electrode gaps. When
the
electrolyte reaches the tops of the gaps, it overflows the adjacent tops of
the
electrodes (the multi-polar electrodes and cathode) and returns to the body of
the
cell. In the cells utilized for the exemplary embodiments, normally no
channels are
provided to divert the electrolyte and to avoid such overflow. The indicated
flow of
electrolyte allows for efficiency of electrolyte circulation, but permits
communication between electrodes and hence the possibility of the development
of bypass current. A good circulation in the cell is needed to keep the inter-
electrode gaps supplied with fresh salt to be electrolyzed, permit efficient
removal
of the anode gas which is generated and collection of the metal cathode
product
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
7
and thus allow productivity to be maintained. It is therefore desired to
reduce
bypass current (current efficiency loss) without reducing the efficiency of
circulation.
Figures 1 and 2 are simplified plan views of electrolysis cells 10 each with a
top wall removed. The cell of Fig. 1 has an electrode cassette made up of
planar
electrodes, whereas the cell of Fig. 2 has several electrode cassettes made up
of
cylindrical electrodes. In each case, the cell 10 includes an outer wall 11
provided
with a refractory lining 12 that provides insulation against heat loss and
flow of
electricity. The cell is divided into an electrode compartment 13 and a
reservoir
compartment 14 separated from each other by a vertical refractory wall 15. The
compartments 13 and 14 communicate with each other via channels (not shown in
these figures, but shown later in relation to Fig. 5) provided near the top
and
bottom of the wall 15. This allows molten electrolyte 16 to circulate freely
between
the two compartments. Fresh metal salt may be added to the electrolyte in
reservoir compartment 14 from time to time to replenish the salt consumed
during
the process of electrolysis.
The electrode compartment 13 of Fig. 1 contains an arrangement of
electrodes in the form of an electrode cassette 18 made up of a vertical
planar
anode 19 flanked on each side at a distance by a pair of vertical planar
cathodes 20.
Interposed between the anode and the cathodes are eight vertical planar multi-
polar electrodes 21, four on one side of the anode and four on the other side
of the
anode.
Vertical ends 22 and 23 of the cassette 18 are closed by ceramic insulators
made up of a ceramic anode side insulator 25, ceramic cathode side insulators
26,
ceramic side edge insulators 27 and ceramic spacers 28. These ceramic
insulators
and spacers prevent the development of bypass current at the vertical side
edges 22
and 23 of the electrode cassette 18. They also prevent movement of electrolyte
16
into or out of the cassette 18 at the side edges 22 and 23 so the flow of
electrolyte
is kept vertical. In some embodiments, it is convenient if these insulators
are made
part of the adjacent outer wall 11, 12 and the vertical refractory wall 15.
The
electrode cassette 18 is positioned between short vertical refractory walls
24.
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
8
The electrode compartment of Figure 2 contains a plurality of cylindrical
electrode assemblies or cassettes 18 made up of an outer cylindrical cathode
20, an
inner rod-like cylindrical anode 19, and four multi-polar electrodes 21 in
between
the cathode and anode. The electrodes are nested together in the manner shown.
Figure 3 is a vertical cross-section through an electrode cassette 18 along
line A-A in either Figure 1 or in Figure 2 as these cross-sections of both
figures look
essentially the same. In Figure 3, the cassette 18 of electrodes is made up of
the
anode 19 (which may be planar as in Figure 1 or a circular rod as in Figure 2)
and the
cathode 20 (again either planar or cylindrical). The gap between the anode and
the
cathode on each side is filled by four different spaced multi-polar electrodes
21 (the
embodiment of Fig. 1) or different parts of the four cylindrical multi-polar
electrodes (the embodiment of Fig. 2) separated by gaps 36. In this view, it
can be
seen that the upper ends 29 of the multi-polar electrodes 21 are capped by
insulators 33 which are blocks made of an electrically-insulating refractory
material.
The method of attachment of the blocks to the electrodes is discussed in
detail later.
Referring again to Figure 3, the anode 19 rests on a ceramic spacer block 40
that in turn rests on the floor of the cell or more preferably on a support
within the
cell (not shown in this view). Similarly, the cathode 20 rests on a refractory
ceramic
spacer 41 and the multi-polar electrodes 21 rest on aligned ceramic refractory
spacer blocks 42 which rest on a support within the cell (not shown). The
blocks 40,
41 and 42 are provided with gaps 43 that align with the gaps 36 between the
electrodes so that electrolyte may enter the cassette 18 of electrodes from
below.
In the embodiment of Figure 3, the anode 19 is also provided with a ceramic
refractory circumferential insert 45 on its exterior surface in the region
confronting
the upper ends of the multi-polar electrodes and the insulators 33. This
insert
provides an insulating refractory shield that further minimizes the risk of
bypass
current.
In operation, the cassettes 18 as shown in Figure 3 are immersed in the
electrolyte 16 of the cell such that the tops of the insulators 33 are, at
least in use,
positioned below the surface of the electrolyte 16. The gas generated in the
gaps
36 between the anode, cathode and multi-polar electrodes causes electrolyte
(now
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
9
containing droplets of metal resulting from the electrolysis) to flow upwards,
and to
form a layer 50 of electrolyte on the top of the electrodes which flows
outwardly in
a transverse manner to the openings or channels around the cassette. The
electrolyte then flows into the electrode compartment 13 (Fig. 1 and Fig. 2)
and via
openings previously mentioned in the upper portion of the separating vertical
refractory wall 15 to the reservoir compartment 14 where metal separation
occurs.
The electrolyte then returns via openings in the lower portion of the vertical
refractory wall 15 to the underside of the electrode cassette where it re-
enters the
inter-electrode gaps 36 via the gaps 43 provided between the supports 40, 41
and
42.
Figure 4 shows an alternative embodiment having cylindrical electrode
cassettes 18 in which the electrodes are configured as in Figure 3, except
that the
upper ends of the insulators 33 on the top of the multi-polar electrodes 21
are
arranged in a stepwise manner of increasing height towards the anode 19. Such
a
cassette can be operated with the electrodes immersed well below the surface
of
the electrolyte, but can also be advantageously operated with the electrolyte
level
controlled so that the electrolyte containing the metal dropiets cascades over
the
tops to achieve a relatively thin layer of electrolyte as shown at 50. This
type of
stepped arrangement is shown for cassettes without insulators on the top of
the
electrodes in U.S. Patent 5,935,394 (the disclosure of which is incorporated
herein
by reference). Such a stepped arrangement has been used to reduce the bypass
currents with some effectiveness, but the addition of insulators adds a still
further
significant reduction of bypass currents in arrangements of this kind.
Figure 5 shows a vertical cross-section of a cell 10 similar to that of Figure
2
but provided with at least one electrode cassette 18 of the type shown in
Figure 4.
The cell 10 comprises a vessel provided with outer walls 11 having a
refractory and
insulating lining 12. The vessel has a lined cover 17 that is sealed against
gas leaks
from the cell but has a gas vent 24 which may be connected to a conduit (not
shown) for gas delivery to other equipment. The electrode cassette 18 is
connected
to a current source (not shown) by an anode busbar 51 at the top of the anode
19
and by a cathode busbar 52 welded to the cathode 20 of the electrode cassette
18,
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
and passing through the wall 11 of the cell, where it is sealed against
electrolyte
leakage.
The cell is divided by vertical refractory wall 15 into an electrode
compartment 13 occupied by the electrode cassette or cassettes 18, and
reservoir
5 compartment 14 where metal collection takes place. The vertical refractory
wall 15
is provided with an upper opening 31 and lowery opening 32 (or more than one
of
such openings). The bottom of each electrode cassette 18 is supported above
the
floor of the electrode compartment 13 by refractory supports 34. These
supports
are sufficiently open that they do not impede the flow of electrolyte to and
within
10 the cassette.
The cell is filled with molten electrolyte 16 to a level 35 in the reservoir
compartment 14. During operation, the electrolyte flows upwards between the
electrodes in the electrode cassette 18, flows over the tops of the insulators
33 to
the electrode compartment 13, and into the reservoir compartment via the
opening
31. The molten metal is entrained as droplets in the electrolyte stream and
the
droplets float to the surface in the reservoir compartment, where they
coalesce to
form a floating layer 30. The electrolyte eventually flows back to the
electrode
compartment 13 via the lower opening 32. Metal is periodically removed by
vacuum tapping via an opening (not shown) in the top cover 17. The cover 17
may
also have a closable opening (not shown) for the introduction of metal salt
into the
cell from time to time.
Means (not shown) are also preferably provided to maintain the upper level
of the electrolyte 16 at a predetermined level 35 in the cell. Such means are
known
in the prior art, for example in U.S. Patent 4,518,475 (the disclosure of
which is
incorporated herein by reference). In the embodiments where the electrolyte
level
is set so that the tops of the insulators are fully immersed in the
electrolyte at all
times, the control of the electrolyte level is less critical.
In all embodiments, the insulators 33 are preferably positively secured to the
electrodes 21 to prevent their dislodgment during operation of the cell.
Depending
on the material used for the insulators, they may experience buoyancy when
submerged in the electrolyte, so there is therefore usually a need to attach
the
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
11
insulators by means other than gravity alone. The electrical insulation helps
to
prevent the development of bypass current between the anode and the cathode.
The insulators 33 effectively provide the multi-polar electrodes with
electrical
insulation extending completely along the top edges 29 of the electrodes.
Figures 6A through 6G illustrate some ways in which the insulators may be
attached to the multi-polar electrodes, or to an anode or cathode. These
methods
employ pins or inserts 61 (Figures 6A, 6B, 6C, 6D), tongue and grooves 63
(Figures
6A, 6B and 6C), dovetails 62 (Figures 6E, 6F, 6G), adhesive or the like, or a
combination of two or more of these mounting features. In this way, the
ceramic
materials can be fixed securely to the conductive electrode material to
prevent
dislodgement of the ceramic during operation of the cell. The fixtures can be
asymmetric or "off-centre" as shown, or symmetric. If off-centre, the thicker
electrode material is preferably positioned on the anode face of the electrode
since
that surface is more likely to experience wear and loss of material in use,
and the
off-centre positioning thus increases the effective operating life of the
cassette.
Figure 7A is a partial view of the top of an electrode cassette which
illustrates another preferred embodiment of the invention in which the
insulators
33 are interconnected to form a unitary block 33A. As in the embodiment of
Figure
3, the multi-polar electrodes 21 are each capped at their upper ends 29 with
an
elongated insulator 33 that covers the entire upper end surface of the
electrode.
However, the insulators 33 are joined together by spacers 34 and end plates
35.
The gaps 36' between the spacers and end plates align with the inter-electrode
gaps
36 between the electrodes 21. While the spacers 34 and end plates 35 extend
across the inter-electrode gaps, they are narrow enough not to impede the flow
of
electrolyte unduly. However, they support the insulators 33 and make the
cassette
more rigid and secure.
Fig. 7B shows a similar arrangement for cylindrical electrode cassettes.
Again, as in the previous embodiment, the insulators 33 may be joined to form
a
block 33B of ceramic material having the shape as shown in Figure 7B. For
simplicity,
this figure shows only two multi-polar electrodes 21 and only one half of the
electrode cassette. It can be seen that the block 33B consists of insulators
33 that
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
12
are circular (in plan view) with rounded upper ends 55. A circular gap 36'
aligns
with the inter-electrode gap 36 between the electrodes 21. Spacers 34
interconnect
the insulators in diametrically opposed positions, thus unifying the block
33B. Again,
the spacers 34 do not unduly restrict the flow of electrolyte.
For all exemplary embodiments, the following generally comments can be
made about the dimensions of the insulators 33. Clearly, the bigger an
insulator is,
the lower the consequent bypass loss will be. However, it is desirable not to
change
the fluid flow of a cell dramatically and to ensure that the electrolyte still
flows over
the tops of the multi-polar electrodes (and insulators). There is a decrease
in gas lift
as more of the multi-polar electrode is replaced with an insulator as a
fraction of the
face of the multi-polar electrode will no longer be electrochemically active.
There is
therefore a trade-off between the size of the insulator blocks and the
efficiency of
cell operation. Along the top and bottom edges of multi-polar electrodes,
electrical
resistance to the bypass current is determined by the ratio of the length of
the gap
between the adjacent insulators and the cross-sectional area of the gap which
is
proportional to its width. Optimization is preferred to achieve the best
balance
between the hydrodynamic resistance to electrolyte flow and the electrical
resistance to bypass currents.
While any amount of insulation at the top ends of the multi-polar electrodes
offers an advantage, the preferred dimensions for the insulators can be stated
as
follows, in which the terms "width", "length" and "height" have the following
meanings for cells having both planar and cylindrical electrodes:
Width: The through-thickness of the multi-polar electrode, parallel to the
direction that the current is traveling though the multi-polar
electrode.
Length: The horizontal direction, parallel to the electrode face, generally
normal to the current flow.
Height: The vertical direction, parallel to the face of the electrode face.
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
13
To define the height dimension of the insulators, reference is made to the
gaps between the multi-polar electrodes (referred to as ACD). The electrodes
are
often equally spaced within an electrode cassette. Although a continuous
improvement is obtained with increasing insulator height, a compromise in gas
lift
(electrolyte flow) and available room in the cell must be made. A first
preferred
range of dimensions of the insulators is as follows:
Width: Between 0.1 and 1.5 times the width of the underlying multi-polar
electrode, and more preferably more than 0.5 times the width.
Length: Equal to the length of the multi-polar electrode (does not extend
beyond the ends of the multi-polar electrodes).
Height: 1 to 20 times the electrode gap (ACD).
A more preferred range of dimensions is as follows:
Width: Between 0.5 and 1.0 times the width of the underlying multi-polar
electrode.
Length: Equal to the length of the multi-polar electrode.
Height: 5 to 10 times the electrode gap (ACD).
In cells having more than one multi-polar electrode, it is desirable to
provide
insulators on the upper ends of all the multi-polar electrodes. However, the
provision of insulators on only one or some of the multi-polar electrodes is
better
than having no insulators at all. Bypass currents over the tops of the multi-
polar
electrodes are affected by electrolyte overflow depth, number of multi-polar
electrodes, thickness of multi-polar electrodes and gap between multi-polar
electrodes, decomposition potential and electrolyte conductivity. The effect
of
placing insulators at the electrode upper ends is to add an additional degree
of
resistance to the leakage current pathway. This extra electrical resistance,
which is
developed by increasing the bypass current pathway through the electrolyte,
has
the effect of decreasing the overall bypass current.
CA 02697396 2010-02-22
WO 2009/033260 PCT/CA2008/001544
14
developed by increasing the bypass current pathway through the electrolyte,
has
the effect of decreasing the overall bypass current.
An example of the variation of bypass current for a range of insulator heights
(expressed as multiples of the ACD) has been calculated and is shown in Figure
8.
Insulator heights as low as about 1 times the ACD are effective, and beyond
heights
that are 20 times the ACD, the improvement with increased height is less. For
practical reasons a height of more than 10 times the ACD is not preferred, and
a
height of at least 5 times the ACD ensures an economically useful effect on
the
bypass currents without a significant negative impact on the electrolyte flow
or cell
productivity.
EXAMPLE
A test was carried out to demonstrate the suitability of the design,
ruggedness and materials employed in one form of the present invention. In the
test, an electrolytic reduction cell having 24 planar multi-polar electrodes
was
provided with elongated rectangular insulators attached to the tops of four of
the
multi-polar electrodes. The refractory material used for the insulators was
made
from a 93% alumina / 5% silica castable mixture. The width of each insulator
was
the same as that of the multi-polar electrode on which it rested, the length
was the
same as that of the multi-polar electrode length and the height was 6 times
the
average ACD.
The cell was operated for 685 days under normal conditions. The test
showed that the cell operated properly, and, although the multi-polar
electrodes
did erode over the operational lifetime of the cell, the insulator dimensions
did not
change and the insulators remained fixed to the top of the multi-polar
electrodes.
Based on the design and dimensions of the insulators, the current efficiency
was calculated, compared to a cell lacking such insulators and the results are
plotted against cell life in Figure 9. This shows a consistently higher
current
efficiency with even a few insulators over the case with no such insulators.
