Molten salt bath circulation patterns in electrolysis

A method for producing metal by electrolysis in a molten salt bath containing superimposed electrodes, at least one of which is a bipolar electrode. The arrangement of the electrodes creates interelectrode spaces between them. Bath is swept through these interelectrode spaces. This method is improved by providing circulation of the bath from one interelectrode space to the next at a location inwards of the outer peripheries of the electrodes. This can be accomplished e.g. by boring holes through the electrodes. It can also be accomplished by breaking the electrodes into individual, mutually separated stacks of electrodes, the circulation of the improvement then occurring e.g. in the space between the stacks.

BACKGROUND OF THE INVENTION 
The present invention relates to methods of producing metal by electrolysis 
in a molten salt bath. More particularly, the present invention relates to 
methods for operating bipolar cells for carrying out such electrolysis. 
U.S. Pat. No. 3,822,195 issued July 2, 1974 in the name of M. B. Dell et 
al. for "Metal Production" illustrates a method for producing metal by 
electrolysis of aluminum chloride in a molten salt containing superimposed 
electrodes. Bipolar electrodes are included. The bath circulates 
peripherally of the electrodes upwards on one side and downwards on 
another side. 
U.S. Pat. No. 3,554,893 issued Jan. 12, 1971 in the name of G. DeVarda for 
"Electrolytic Furnaces Having Multiple Cells Formed of Horizontal Bipolar 
Carbon Electrodes" illustrates likewise a method for producing metal by 
electrolysis in a molten salt bath containing superimposed electrodes. 
This time the substance being electrolyzed is aluminum oxide. The 
electrodes are separated into two stacks. The type of bath circulation 
achieved is not discussed. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a new type of bath 
circulation in a method for producing metal by electrolysis in a molten 
salt bath containing superimposed electrodes, at least one of the 
electrodes being a bipolar electrode, bath being swept through 
interelectrode spaces between the electrodes. 
This as well as other objects which will become apparent in the discussion 
that follows are achieved according to the present invention by 
circulating the bath between interelectrode spaces inwards of the outer 
peripheries of the electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A cell for electrolytically producing aluminum by the electrolysis of 
aluminum chloride dissolved in a molten salt bath utilizing one form of 
the present invention is illustrated in FIG. 1. The cell structure 
includes an outer steel cooling jacket 10, which surrounds the steel sides 
12 of the cell. A cooling fluid (coolant), for example water, flows 
through jacket 10 for withdrawing heat from the cell. The coolant enters 
the cooling jacket at coolant inlet ports 11, and is removed at exit 
nozzles 15. A similar cooling jacket 14, with representative coolant inlet 
port 14a and coolant outlet port 14b, covers the lid 16 of the cell. Lid 
16 is exposed directly to chlorine and salt vapors and is made of a 
suitably chlorine resistant metal such as the alloy nominally containing 
80% Ni, 15% Cr, and 5% Fe and sold under the trademark Inconel. All water 
pipes running to and from the ports of the cooling jackets are provided 
with rubber hose electrical breaks, so that electrical current cannot move 
to or from the cell along the otherwise metallic pipes. A structural 
containment 18, for example of steel, encloses and supports the cell and 
the cooling jacket. In general, it has been found to be good practice to 
isolate the cell from the floor, for instance by setting containment 18 on 
an insulating material such as a thermoset plastic material made from 
fabric or paper impregnated with phenol-formaldehyde resin, for instance 
the material supplied under the trademark Micarta by Westinghouse Electric 
Corp. 
The bath containing cell interior surfaces, i.e. those formed by sides 12 
and steel bottom 20, are lined with a continuous, corrosion-resistant, 
electrically insulating lining (not shown) of plastic or rubber material. 
Good results have been obtained with a lining composed of alternating 
layers of thermosetting epoxy-based paint and glass fiber cloth. Other 
plastic or rubber materials are possible. 
Inwards of the lining is interposed a glass barrier (not shown). For 
further information concerning this glass barrier, see the above-mentioned 
U.S. Pat. Nos. 3,773,643 and 3,779,699. 
The cell is also lined with refractory side wall brick 24, made of 
thermally insulating, electrically nonconductive, e.g. nitride material 
which is resistant to a molten aluminum chloride-containing halide bath 
and the decomposition products thereof (see U.S. Pat. No. 3,785,941 issued 
Jan. 15, 1974, in the name of S. C. Jacobs for "Refractory for Production 
of Aluminum by Electrolysis of Aluminum Chloride"). 
An additional lining 36 of graphite is positioned on the side walls 
alongside and above the anodes 46 to provide further protection against 
the corrosive influence of the bath and the chlorine gas produced by the 
operation of the cell. It may be advantageous not to extend this lining 36 
right up to lid 16. Rather, ending its upwards reach short of lid 16 can 
eliminate a danger of short circuiting. 
The cell cavity includes a sump 26 in its lower portion for collecting the 
aluminum metal produced. The sump is bounded by a tub 28 made of graphite. 
The upper part of tub 28 extends up alongside the cathodes 50. Tub 28 sits 
on refractory floor 32 including the glass barrier. 
The cell cavity also includes a bath reservoir 34 in its upper zone. A 
first port, tapping port 38, extending through the lid 16 into bath 
reservoir 34, provides for insertion of a vacuum tapping tube (see British 
Patent No. 687,758 of H. Grothe, published Feb. 18, 1953.) down into sump 
26, through an internal passage to be described with reference to FIG. 2, 
for removing molten aluminum. A second port, feeding port 42, provides 
inlet means for feeding aluminum chloride into the bath. A third port, 
vent port 44, provides outlet means for venting chlorine. These ports are 
shown open in FIG. 1 just as a matter of convenience. During cell 
operation, port 38 may have vacuum tapping apparatus associated with it 
while port 42 will have a feeder mechanism attached to it and port 44 will 
be connected to a pipeline for carrying-away the chlorine-rich effluent. 
Within the cell cavity are a plurality of plate-like electrodes divided up 
into two stacks. In the direction perpendicular to the plane of FIG. 1, in 
which direction the depth of the electrodes lies, the electrodes extend 
such that they abut against the lining of the cell. Each stack includes an 
upper anode 46, desirably an appreciable number of bipolar electrodes 48 
(11 being shown), and a lower cathode 50, all being made, for example, of 
graphite. These electrodes are arranged in superimposed, spaced 
relationship defining a series of interelectrode spaces within the cell. 
Each electrode is preferably horizontally disposed within a vertical 
stack. 
Each cathode 50 is supported by a plurality of graphite lateral support 
pillars (e.g. pillars 60) and central support pillars (e.g. pillars 61). 
In the direction of the depth of the electrodes, there are other pillars 
behind those shown. These hidden pillars are spaced from those shown and 
from one another, so that bath circulation through sump 26 is possible. 
The remaining electrodes are stacked one above the other in a spaced 
relationship maintained by refractory spacers 53 in the interelectrode 
spaces, and are connected to, and spaced from, the side walls by 
individual insulating pins 54. These spacers 53 are dimensioned to closely 
space the electrodes, as for example to space them with their opposed 
surfaces separated by less than 3/4 inch. 
Above the stacks, hold-down blocks 47 bear on the upper surfaces of the 
anodes 46 to maintain the stacks in place. 
In the illustrated embodiment, 12 interelectrode spaces are formed between 
opposed electrodes in each stack, one interelectrode space between cathode 
50 and the lowest of the bipolar electrodes, 10 between successive pairs 
of intermediate bipolar electrodes, and one between the highest of the 
bipolar electrodes and anode 46. Each interelectrode space is bounded 
above by an electrode lower surface (which functions as an anodic surface) 
and below by an electrode upper surface (which functions as a cathodic 
surface). The spacing therebetween is referred to as the anode-cathode 
distance (the electrode-to-electrode distance is the effective 
anode-cathode distance, due to the sweeping action of the bath, which 
removes the aluminum as it is formed; this sweeping is the subject of the 
above-mentioned U.S. Pat. No. 3,822,195). As brought out in U.S. Pat. No. 
3,822,195, the anodic surfaces may have chlorine removing channels for 
getting the chlorine rapidly out of the electrolysis-effective 
interelectrode spaces. 
The molten salt bath has been omitted from the cell for the purpose of 
better exposing the internal structure of the cell. The bath level in the 
cell will vary in operation but normally will lie above the anode 46 to 
fill all otherwise unoccupied space below within the cell. 
Inwards of the outer peripheries of the electrodes, i.e. in this embodiment 
between the separate stacks of electrodes, is located a gas-lift passage 
55, maintained by spacers 57. The widths of the electrodes in the stacks 
are so chosen that the gas-lift passage 55 has its greatest breadth 
between the anodes 46, the breadth decreasing as one moves down the 
stacks, with the smallest breadth being between the lowest bipolar 
electrodes. The gas-lift passage 55 provides for the upward circulation of 
the bath between the interelectrode spaces inwards of the outer 
peripheries of the electrodes to the reservoir 34 after passage of the 
bath through the interelectrode spaces between the electrodes. The flow is 
induced by the gas-lift effect of the chlorine gas internally produced by 
electrolysis in the interelectrode spaces. 
The above-mentioned chlorine removing channels may be extended right into 
the passage 55, while being blocked-off on their opposite ends. It has 
been found that this aids in getting the chlorine started in the right 
direction, i.e. toward, and into, passage 55. Once the chlorine gets 
started flowing in the desired direction and provided the various flow 
cross sections in the cell have been properly dimensioned, the chlorine 
keeps going in that direction. Thus, the blocking-off of one side of the 
channels is not indispensible. The gas flow can be gotten started in the 
desired direction by other means, for example by using a mechanical 
pumping of the bath or by introducing a pulse of gas at the bottom of 
passage 55. The dimensioning of passage 55, and the remainder of the flow 
cross sections in any particular cell, is advantageously carried out using 
water modeling techniques. 
Upcomer dams 59, located adjacent the exit end of the gas lift passage 
above the anodes, serve to prevent unwanted rechlorination of the 
electrolyzed metal. The upper portions of the dams protrude above the 
upper level of the bath and force the lateral flow of the bath above the 
electrodes to be through passageways 63 in the direction of arrows C and 
D. Passageways 63 open on both sides of each dam 59 below the surface of 
the bath, while the bath surface lies below the top of dam 59. The 
resulting flowpath resists the tendency of pieces of molten metal, which 
are brought upwards in the passage 55, from breaking the bath surface and 
getting rechlorinated by the metal-oxidizing chlorine in reservoir 34 
above the surface of the bath. It would be best if most of the metal 
produced on the cathodic surfaces would fall in passage 55 to sump 26, 
because any metal which is swept upwards can get rechlorinated if it 
breaks through the upper surface of the bath. This would adversely affect 
current efficiency. It is to guard against this eventuality that dams 59 
are provided. Preferably, the bath flow velocity in the directions of 
arrows C and D is great enough to perform the sweeping action of U.S. Pat. 
No. 3,822,195 on the top of anodes 46 in the same manner that the cathodic 
surfaces in the interelectrode spaces are swept. 
Between each electrode stack and the refractory side walls 24, i.e. at the 
outer peripheries of the electrodes, are two bath supply passages 56 
extending past each interelectrode space and past the bipolar electrodes, 
anode 46 and cathode 50. Each passage 56 is maintained by pins 54, by 
which there is on each side of the cell a series of aligned gaps between 
the cell walls and the electrodes, these aligned gaps forming the two 
passages 56. The movement of bath in the passages 56 is first downwardly 
past anodes 46, thus passing first into the outside regions of the 
uppermost interelectrode spaces where portions of the bath split-off to 
supply and sweep the uppermost interelectrode spaces. Focussing on either 
of the two sides, the remainder of the bath then flows downwardly past the 
outside of the next electrode to the outside of the next interelectrode 
space, and so on. A final portion of the bath may flow on through the 
openings on the outside of the cathodes 50 into, through the sump 26, then 
up into passage 55. It will thus be seen that passages 56 make it possible 
for the bath to circulate downwards peripherally of the electrodes, with 
the motivating circulatory force being created by the gas-lift action in 
passage 55 inwards of the outer peripheries of the electrodes. 
As brought out above, design of the dimensions of the various parts of the 
gas-lift and bath supply passages can be carried out advantageously using 
the principles of water modeling to assure that the forming metal is swept 
out of each interelectrode space without substantial accumulation of the 
metal on the cathodic surfaces. For the braoder aspect of the present 
invention, however, it is not necessary that the bath sweep velocity be 
high enough to sweep out metal. It need only be sufficient to prevent 
exhaustion of the dissolved aluminum chloride at the end of the trip of 
the bath through the particular interelectrode space under consideration. 
The anode has a plurality of electrode bars 58 inserted therein which serve 
as positive current leads, and the cathode has a plurality of collector 
bars 62 inserted therein which serve as negative current leads. The bars 
extend through the cell and cooling jacket walls and are suitably 
insulated therefrom. (See e.g. U.S. Pat. No. 3,745,106 issued July 10, 
1973, in the name of S. C. Jacobs for "Fluid Sheathed Electrode Lead for 
Use in a Corrosive Environment".) 
FIG. 2 is a schematic diagram of the case opposite to that illustrated in 
detail in FIG. 1. Here, as shown, by the arrows representing the 
circulatory flow paths, the bath circulation is downwards inwards of the 
outer peripheries of the electrodes and upwards peripherally of the 
electrodes. The blocks arranged in two stacks provide a schematic 
representation of electrodes such as shown in more detail in FIG. 1. Again 
the circulatory force is created by gas-lift pumping but this time the 
pumping is carried out peripherally of the electrodes. 
According to the general concept of the present invention, it is not 
necessary that the circulatory force be created by gas-lifting pumping. 
For example, a mechanical pump may be used as illustrated in U.S. Pat. No. 
2,830,940 issued in the name of R. S. Hood on Apr. 15, 1958 for 
"Production of Metals". 
While the passages in either of the two modes of the invention disclosed 
herein may be advantageously created by breaking the electrodes into two 
separate stacks, it is within the broader concept of the invention to 
provide holes in the electrodes to create the passages. 
An advantage common to the two embodiments disclosed herein is that, by 
providing for some circulation from interelectrode space to interelectrode 
space inwards of the outer peripheries of the electrodes, bath flow 
through interelectrode spaces between the electrodes is a shorter trip 
than would be the case if the bath were circulated between interelectrode 
spaces only at the electrode peripheries as in U.S. Pat. No. 3,822,195. 
This is apparent from a consideration of FIG. 1 for instance. If the 
electrodes in the two stacks were extended inwards to close-up passage 55, 
with e.g. the right passage 56 then being the gas-lift passage, the bath 
must sweep twice the distance, before it emerges from any given 
interelectrode space. A result of the present invention is that the bath 
sweep velocity in the interelectrode spaces need not be as great as would 
otherwise be necessary to prevent exhaustion of AlCl.sub.3 at the end of 
any given trip through an interelectrode space. Another result is that 
evolved chlorine builds up to e.g. only half the volume that it would 
otherwise at the exit ends of trips of bath through interelectrode spaces. 
The two embodiments of circulation disclosed herein for the invention also 
have their own sets of advantages. In the case where the bath is 
circulated upwards inwards of the outer peripheries of the electrodes, the 
bath flow for sweeping the electrode cathodic surfaces free of metal as it 
is created is inwardly directed toward the centrally located passage. In 
this case, the sweeping bath collides with oppositely directed sweeping 
bath in the center of the electrodes, whence the bath rises upwards. This 
has the advantage that the refractory bricks 24 do not have to stand up 
against the erosive impact of the sweeping flow of bath and entrained 
metal. 
In the case where the bath flows downwards inwards of the outer peripheries 
of the electrodes, there is the advantage that the peripherally situated 
gas-lift passages need only each accommodate one-half of the total upwards 
gas volume flow as compared with FIG. 1. The danger of large gas bubbles, 
for instance, flinging the produced molten metal particles upwards into 
the chlorine in the upper part of bath reservoir 34 is less. There is the 
additional advantage here that aluminum chloride fed through an off-center 
port 42 is brought first to the centrally located passageway, so that the 
interelectrode spaces get a uniform feeding of newly charged aluminum 
chloride. In the opposite case, the newly charged aluminum chloride tends 
to move down the right hand passage 56 first, so that the interelectrode 
spaces in the stack on the right get a better replenishment of aluminum 
chloride than do their corresponding spaces in the stack on the left. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes, and adaptions and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.