Controlled wettability graphite electrodes for selective use in electrolysis cells

Metal such as aluminum is produced electrolytically from metal chlorides or other halides dissolved in a molten solvent bath of higher decomposition potential in a cell including one or more graphite cathode surfaces spaced from opposed anodes, particularly a bipolar cell, with bath flow through the spaces between the anodes and cathodes. The wetting characteristics of the carbonaceous cathode with respect to the metal deposited there by electrolysis are selectively balanced with the bath flow over the cathode and with the anode-to-cathode distance. Cathode surface wear rate is substantially reduced if the surface is wettable by the metal in regions of low bath flow velocity or regions of greater anode-cathode distance. The wear rate is also reduced by using non-wettable cathode surfaces in regions of higher bath flow velocity or regions of closer anode-cathode distance. Conditions of graphite manufacture, including raw material selection and graphitization temperature, are specified to achieve controlled wettability of graphite electrodes to enable the selective production of either condition for the particular cell operation involved.

BACKGROUND OF THE INVENTION 
This invention relates to the production of metal such as aluminum from 
metal chloride dissolved in molten halide solvent bath by electrolyzing 
the bath in a monopolar or bipolar cell. More particularly, the invention 
relates to graphite electrodes used in such cells and to selective use 
thereof with respect to their wetting or non-wetting characteristics so as 
to prolong useful electrode life in such cells and to controlled methods 
of graphite electrode manufacture to achieve the desired wetting or 
non-wetting characteristics for such selective use. 
One type of electrolytic cell used in the production of metal, such as 
aluminum, from metal chloride dissolved in a solvent salt bath includes a 
terminal anode, at least one intermediate bipolar electrode and a terminal 
cathode. These electrodes are typically situated in relatively closely 
spaced, generally parallel relationship wherein opposed anode-cathode 
faces provide interelectrode spaces through which the molten bath can move 
and be electrolyzed by passage of current from anode to cathode. 
Electrolysis of the metal chloride occurring within the interelectrode 
space results in molten metal depositing at the cathode and chlorine gas 
collecting at the anode. Cells of this type are described in U.S. Pat. 
Nos. 3,755,099 and 3,822,195, incorporated herein by reference. One of the 
important features of these cells is that the anode-to-cathode space or 
distance should be carefully maintained at a preselected level in order to 
achieve the high current efficiency and lower power consumption 
capabilities of the bipolar chloride electrolysis process. Obviously, any 
amount of wear occurring on either the anode or the cathode surface, as by 
erosion or other removal of electrode material, tends to increase the 
distance and, accordingly, increase the electrical resistance across the 
distance between anode and cathode. For the most part, the anode presents 
little problem since under most conditions chlorine is relatively 
non-corrosive to the carbonaceous materials employed for electrodes. 
However, experience has shown that some amount of electrode wear does 
occur on the cathode surface, and considerable effort has been expended to 
reducing or relieving this wear condition. Excessive cathode surface wear 
is a problem, not only in increasing power consumption as just explained, 
but can increase the resistance so much that the cell is considered 
uneconomical to operate, thus necessitating a costly shut-down, repair or 
replacement of the electrodes, and restarting the cell. In addition to the 
electrical resistance problems resulting from cathode wear, the 
carbonaceous material removed from the cathode surface can contaminate the 
bath. This alone can reach such an extreme as to necessitate shutting down 
the cell. 
SUMMARY OF THE INVENTION 
In accordance with the invention, it has been discovered that graphite 
electrode surfaces can exhibit either wetting or non-wetting behavior with 
respect to the metal deposited at the cathode, and that such behavior can 
be utilized in association with bath flow velocity and anode-cathode 
distance to minimize cathode surface wear. It has further been discovered 
that the wettability or the non-wettability of graphite electrodes can be 
established by carefully controlling the graphite manufacturing process. 
Accordingly, it is an object of the present invention to provide for 
decreased cathode electrode wear in halide electrolytic cells used in 
producing metal such as aluminum from metal chlorides. 
Another object is to provide a means for selectively positioning graphite 
cathode material based on its wetting characteristics so as to balance 
such with other cell operating conditions to minimize cathode wear. 
Another object is to provide for selectively controlling the wetting 
characteristics of graphite electrode material by controlling the steps in 
manufacturing the graphite. 
These and other objects will be apparent from the drawing, specification 
and claims appended hereto. 
In accordance with the invention, it has been found that graphite cathode 
surface wear is reduced if the cathode surface is selected and controlled 
with respect to its wettability and with respect to bath flow rate over 
the cathode surface. Cathode graphite surfaces wetted by the metal 
deposited from the bath are used when the bath is moving over the cathode 
at a relatively low velocity. However, graphite cathode surfaces which are 
not wetted are used in regions of high velocity bath flow. It is to be 
appreciated that in electrolytic cells of the type here concerned, bath 
flow velocity can vary from cell to cell and within a single cell. Thus, 
in some electrolytic cells the bath flow velocity through the 
anode-cathode interelectrode space is relatively slow and in others it is 
more rapid. Moreover, there are cells which include regions wherein each 
effect occurs. In general, in the electrolytic cells of the type depicted 
herein and in the patents above referred to featuring one or more 
horizontal bipolar electrodes between an upper terminal anode and a lower 
terminal cathode providing more or less horizontal bath flow therebetween, 
it is difficult to avoid the occurrence of both fast and slow regions. In 
these cells a more rapid interelectrode bath flow velocity can occur in 
the upper interelectrode spaces and a lower flow velocity can occur in 
lower interelectrode spaces. Hence, one practice of the invention includes 
in single electrolytic cell the use of non-wettable cathode surfaces in 
regions of the cell where the higher flow rates occur, typically regions 
higher or further away from the terminal cathode and the use of wettable 
cathode surfaces in regions where low flow rates occur, typically regions 
lower or closer to the terminal cathode.

DETAILED DESCRIPTION OF THE INVENTION 
Electrolytic Cell 
A suitable cell structure for producing metal in accordance with the 
invention is illustrated in FIG. 1. The cell illustrated includes an outer 
steel shell 1, which is lined with refractory sidewall and end wall brick 
3, made of thermally insulating, electrically non-conductive material 
which is resistant to molten alkali metal and metal chloride-containing 
halide bath and the decomposition products thereof. The cell cavity 
includes a sump 4 in the lower portion for collecting the metal produced. 
The sump bottom 5 and walls 6 are preferaly made of graphite. The cell 
cavity also accommodates a bath reservoir 7 in its upper zone. The cell is 
enclosed by a refractory roof 8, and a lid 9. A first port 10, extending 
through the lid 9 and roof 8, provides for insertion of a vacuum tapping 
tube down into sump 4, through an internal passage to be described later, 
for removing molten metal from the sump. A second port 11 provides inlet 
means for feeding the metal chloride into the bath. A third port 12 
provides outlet means for venting chlorine. 
Within the cell cavity are a plurality of plate-like electrodes which 
include an upper terminal anode 14, desirably an appreciable number of 
bipolar electrodes 15 (four being shown), and a lower terminal cathode 16, 
all of graphite. These electrodes are shown arranged in superimposed 
relation, with each electrode preferably being horizontally disposed 
within a vertical stack. Sloping or vertically disposed electrodes can 
also be employed, however, in either monopolar or bipolar electrode cell 
arrangements. In FIG. 1, the cathode 16 is supported at each end on sump 
walls 6. The remaining electrodes are stacked one above the other in a 
spaced relationship established by interposed refractory pillars 18. Such 
pillars 18 are sized to closely space the electrodes, as for example to 
space them with their opposed surfaces separated by 3/4 inch or less. In 
the illustrated embodiment, five interelectrode spaces 19 are provided 
between opposed electrodes, one between terminal cathode 16 and the lowest 
of the bipolar electrodes 15, three between successive pairs of 
intermediate bipolar electrodes 15, and one between the highest of the 
bipolar electrodes 15 and terminal anode 14. Each interelectrode space 19 
is bounded by an upper surface 20 provided by the bottom of one electrode 
(which surface 20 functions as an anode surface) opposite a lower surface 
21 provided by the top of another electrode (which surface 21 functions as 
a cathode surface). The spacing between anode and cathode surfaces is the 
anode-cathode distance in the absence of a metal layer of substantial 
thickness. When a layer of metal is present on the cathode surface, the 
effective anode-cathode distance is shorter than the distance between the 
graphite electrode surfaces 20 and 21. The bath level in the cell will 
vary in operation but normally will lie well above the anode 14, thus 
filling all otherwise unoccupied space therebelow within the cell. 
Anode 14 has a plurality of electrode bars 24 inserted therein which serve 
as positive current leads, and cathode 16 has a plurality of collector 
bars 26 inserted therein which serve as negative current leads. The bars 
24 and 26 extend through the cell wall and are suitably insulated from the 
steel shell 1. A suitable voltage is imposed across the terminal anode 14 
and the terminal cathode 16, and this imparts the bipolar character to 
bipolar electrodes 15. 
As indicated earlier, the sump 4 is adapted to contain bath and molten 
metal, and the latter may accumulate beneath the bath in the sump, during 
operation. Should it be desired to separately heat the bath and any metal 
in sump 4, an auxiliary heating circuit may be established therein. 
A bath supply passage indicated by arrow 30 generally extends from the 
upper reservoir 7 down along the right-hand side (as viewed in FIG. 1) of 
the electrodes and into each interelectrode space 19. Thus, each of the 
interelectrode spaces 19 is supplied with a continual supply of the molten 
bath which travels across each interelectrode space 19 (moving right to 
left in FIG. 1) and exits the interelectrode space 19 turning upwardly as 
generally indicated by arrows 34 and 35. 
THE MOLTEN BATH 
The electrolyte employed for producing aluminum in accordance with the 
present invention typically comprises a molten salt bath composed 
essentially of aluminum chloride dissolved in one or more halides, 
particularly chlorides, of higher decomposition potential than aluminum 
chloride. By electrolysis of such a bath, chlorine is produced on the 
anode surfaces and aluminum on the cathode surfaces of the cell 
electrodes. The metal is conveniently separated by settling from the 
lighter bath, and the chlorine rises to be vented from the cell. In such 
practice of the present invention, the molten bath may be positively 
circulated through the cell by the buoyant gas lift effect of the 
internally produced chlorine gas, and aluminum chloride is periodically or 
continuously introduced into the bath to maintain the desired 
concentration thereof. 
The bath composition, in addition to the dissolved aluminum chloride, will 
usually be made up of alkali metal chloride, although, other alkali metal 
halide and alkaline earth halide, may also be employed. A presently 
preferred aluminum chloride containing composition comprises an alkali 
metal chloride base composition made up of about 50-75 percent by weight 
sodium chloride and 25-50 percent lithium chloride. Aluminum chloride is 
dissolved in such halide composition to provide a bath from which aluminum 
may be produced by electrolysis, and an aluminum chloride content of about 
1.kappa. to 10 percent by weight of the bath is generally desirable. As an 
example, a bath analysis as follows (in percent by weight) is 
satisfactory: 53 percent NaCl, 40 percent LiCl, 0.5 percent MgCl.sub.2, 
0.5 percent KCl, 1 percent CaCl.sub.2, and 5 percent AlCl.sub.3. In such 
bath, the chlorides other than NaCl, LiCl and AlCl.sub.3 may be regarded 
as incidental components or impurities. The bath is employed in molten 
condition, usually at a temperature above that of molten aluminum and in 
the range between 660.degree. and 730.degree. C., typically at about 
700.degree. C. 
OPERATION 
As described hereinabove, bath supplied from reservoir 7 through bath 
supply passage 30 is electrolyzed in each interelectrode space 19 to 
produce chlorine on each anode surface 20 and aluminum on each cathode 
surface 21. Electric current applied between the upper anode 14 and the 
bottom cathode 16 causes the interdisposed bipolar electrodes 15 to 
exhibit their bipolar behavior and effect electrolysis within each 
interelectrode space 19. The electrode current density can conveniently 
range from about 5 to 15 amperes per square inch, but preferred current 
density can vary from one particular cell to another and is readily 
determined by observation. 
The chlorine produced at the anode is buoyant in the bath and its movement 
through the bath may be employed to effect bath circulation. That is, the 
chlorine rising up along the left side, when viewed in FIG. 1, of the cell 
creates a bath circulating effect including a sweeping of the bath through 
the interelectrode spaces 19. This sweeping action sweeps the aluminum 
produced on each cathode surface through an out of each interelectrode 
space 19 in the same direction as the bath, toward the left as viewed in 
FIG. 1, and permits the aluminum to then settle down into the sump 4. 
As indicated hereinabove, the spacing between electrodes and the bath 
velocity through those spaces can vary from cell to cell and within a 
given cell. For the type of cell shown in U.S. Pat. No. 3,755,099, it will 
usually be found that the lower zones closer to the terminal cathodes 16 
exhibit a lower bath velocity through the interelectrode spaces, whereas 
the higher zones closer to terminal anode 14 tend to exhibit higher bath 
flow rates through the interelectrode spaces 19. 
DETERMINING WETTABILITY 
In accordance with the invention, the wettability of a given graphite 
electrode material is readily determined by a test now described. 
Referring to FIGS. 2 and 3, there are schematically shown convenient 
arrangements for determining the wettability characteristics of electrode 
materials. In this type of arrangement, a small laboratory type 
electrolytic cell 200 has positioned therein an anode 314 together with 
two cathodes 316. The cathodes 316 may be identical or they may be 
different where it is desired to test two different electrode samples. 
Since the area of concern is the cathode surface, it is important that the 
surface 321 of the cathode 316 correspond to the cathode surface to be 
used in a production cell. That is, the cathode 316 should be taken from a 
larger electrode, or at least be representative of such material removed 
from a larger electrode, and be such that its surface 321 is 
representative of the cathode surface for the production electrode. It is 
also significant that the bath 213 contained within the cell 200 is 
preferably of substantially the same composition and temperature as 
anticipated in the production cell so as to minimize departures from 
production cell conditions. 
A suitable size for the cathode blocks 316 is about 11/2 inches long by 5/8 
inch thick by about 3/4 inch wide, and the cathodes are spaced from the 
anode 314 by a distance "d" which can suitably be 9/16 inch. The surface 
321 should be aligned with the opposite surface 315 on the anode to be 
parallel and oppositely facing. The cell is operated at about 710.degree. 
C. at a current density of about 8 amperes per square inch. As is the case 
with production cells, a suitable bath contains 70% sodium chloride, 30% 
lithium chloride, to which is added about 7% aluminum chloride. The 
aluminum chloride content is maintained by periodic or continuous addition 
of aluminum chloride. The operating conditions are continuously maintained 
for a period of about 5 days during which aluminum is made continuously. 
After about 5 days, the entire bath is drained from cell 200 and the 
cathodes are removed. The cathode surfaces 321, i.e. those closest to and 
oppositely facing the anode surfaces, are examined. The largest drop or 
droplet of aluminum found on the cathodic surface 321 is measured as an 
index of wettability. If this droplet is greater than one millimeter in 
its largest dimension in this test, the cathodic surface is considered to 
be wetted by the aluminum in the electrolyte bath. If, on the other hand, 
the largest droplet is one millimeter or less in its major dimension, the 
cathodic surface 321 is considered to be non-wetting. 
ELECTRODE SELECTION 
As indicated hereinabove, the invention involves selection of cathode 
electrodes based on the wettability or non-wettability of the cathode 
surface in association with the electrolyte bath flow velocity over the 
cathode surface. The bath flow velocity is readily determined using a 
simulated water model of the cell, either full size or scaled down. 
In accordance with the invention, cathode surfaces which exhibit wetting 
behavior are positioned to contact the bath where bath flow velocity over 
the cathode surface is relatively low, 1.5 feet per second or less, for 
instance, 0.3 or 0.5 to 1.4 or 1.5 feet per second. These will typically 
be found in the lower regions in cells of the type depicted in U.S. Pat. 
No. 3,755,099. One practice of the invention involves the use of 
relatively widely spaced electrodes in the cell regions which exhibit 
relatively low bath flow, especially where significant amounts of aluminum 
can accumulate on the cathode surfaces. In these regions the electrode 
gap, that is the distance between the anode surface and the opposed 
cathode surface, can be greater than 1/2 inch, for instance 5/8 to 3/4 
inch, although distances of up to one inch can be useful, particularly 
where a significant collection of molten aluminum occurs on the cathode 
surface, such as sometimes can happen in the lower bath portions in a cell 
of the type depicted in FIG. 1 and in U.S. Pat. No. 3,755,099, that is 
lower regions of the cell closer to terminal cathode 16. 
In those regions of electrolytic cells where the bath flow velocity at the 
cathode surface is relatively high, over 1.5 feet per second, for 
instance, 1.5 to 3 feet per second, the cathode surface should be 
non-wetted by the aluminum depositing there from the bath. Regions of high 
flow typically occur in the relatively higher regions of electrolytic 
cells of the type depicted in FIG. 1 and in U.S. Pat. No. 3,755,099, that 
is, regions closer to terminal anode 14. In regions of higher bath flow 
velocity, a preferred practice is to use relatively closely spaced 
electrodes, 1/2 inch or less, for instance 3/8 inch. 
The practice of the invention includes the use in a single electrolytic 
cell of both high flow and low flow regions and the selective use of 
graphite electrodes in those respective regions based on the 
non-wettability or wettability of their cathode surfaces. Hence, one 
embodiment of the invention features the use of both high and low flow 
velocity regions in an electrolytic cell such that the bath flow between 
the anode and cathode in one or more interelectrode spaces 19 is 
relatively high, for instance greater than 1.5 feet per second. That same 
cell also includes a lower flow rate of about 1.5 feet per second or less 
in one or more other interelectrode spaces.The relatively high flow 
velocity can be 11/2 or 2 or more times the relatively low flow velocity. 
The practice of the invention places cathodes with non-wettable surfaces 
in the high flow regions and one or more cathodes with wettable surfaces 
in the lower flow regions, all in the same cell. The use of greater 
anode-cathode distances for the low flow regions and lesser anode-cathode 
distances for the high flow regions as just described can also be employed 
within a single cell. 
ELECTRODES 
The electrodes, including the bipolar electrodes 15, are comprised of 
graphite grade carbon, which can be produced from coke derived from coal 
or petroleum. In the case of petroleum coke, such is typically calcined at 
a temperature of about 800.degree. to 1600.degree. C. in order to drive 
off volatile impurities. In making an electrode, the calcined coke is 
blended with a pitch binder to provide a mixture having a pitch content of 
about 10 to 30%. This mixture is shaped such as by extrusion to provide a 
suitable size and configuration for use as an electrode or for cutting 
into electrodes. A shaped member can be cut to provide two or more 
electrode block pieces, after which the electrode is baked at about 
700.degree. to 1600.degree. C. to drive off volatiles from the pitch 
binder. The next step usually involves immersing the baked block to 
impregnate it with liquid pitch to increase the density, after which it is 
again baked at about 700.degree. to 1600.degree. C. The baking and pitch 
treatment can be repeated one or more times to further increase the 
density. Finally, the carbonaceous material is graphitized at a typical 
temperature of about 2000.degree. to 3100.degree. C. 
In the manufacture of graphitic carbonaceous electrode materials, 
non-wetting surface characteristics are generally favored by the use of 
higher graphitization temperatures, higher crystallinity of the graphite 
structure, higher graphite density and by the use of acicular or 
non-acicular coke as the starting material as distinct from isotropic 
coke. Onthe other hand, wetting characteristics are generally favored by 
lower graphitization temperatures and lower crystallinity and, to some 
extent, by lower density and by the use of isotropic coke as a starting 
material. 
As just indicated above, the internal structure of the coke starting stock, 
the density and crystallinity of the graphite produced therefrom and 
expecially the graphitization temperature have a marked influence on the 
wettability or non-wettability of the graphite in contact with aluminum in 
a chloride reduction cell, and these aspects are now discussed in greater 
detail. In general, coke exhibits one of three internal structures, 
isotropic, acicular and non-acicular. The isotropic structure, as the name 
implies, is generally characterized by equiaxed grains or cells. Acicular, 
on the other hand as its name implies, is characterized by elongate, 
needle-like grains or cells. Non-acicular can be viewed as between the 
extremes represented by the isotropic and acicular structures. In the 
non-acicular structure, the grains or cells are non-equiaxed so as to be 
discernible from the isotropic, but are also clearly discernible from the 
needle-like character of the acicular structure. These characteristics are 
generally recognized in the art and the terms, as used herein, correspond 
to the general understanding in the art. 
A significant consideration as to whether a particular specimen of graphite 
exhibits wettable or non-wettable behavior has been found to be the degree 
of crystallinity in the graphite structure. It is generally recognized 
that several useful measures of graphite crystallinity can be obtained 
from wide angle X-ray diffraction of the crystallite size and the 
interlayer spacing of graphite samples. The diameter, L.sub.a, and the 
height, L.sub.c, of the crystallite can be obtained from measurement of 
the broadening of the appropriate X-ray diffraction peaks. The interlayer 
spacing, d.sub.002, and d.sub.10, and the crystallite diameter, L.sub.a, 
remain more or less the same despite substantial changes in crystallinity. 
However, the degree of crystallinity correlates well with the crystallite 
height, L.sub.c, thus providing a simplified approach for the X-ray 
determination of the comparative crystallinity of graphite. This 
correlation is considered valid despite a simplified analysis to determine 
L.sub.c which is based principally on "size broadening" without allowing 
for strain effects or for distribution of layer spacings. That is to say 
that determination of L.sub.c can be made without accurate determination 
of the broadening parameters by a rigorous analysis of X-ray data which is 
complicated by a number of corrections as it generally recognized in the 
art of X-ray diffraction. It is suitable for purposes of the invention to 
evaluate the broadening parameters directly from experimental 
diffractometer traces and a smooth curve drawn through the profile of the 
trace. To determine L.sub.c, a base value of intensity is determined and a 
line parallel to the base line drawn at one-half of the peak height above 
the base line. Scherrer's equation can then be used to determine the value 
of L.sub.c. 
EQU Lc=(0.089.lambda./B cos .theta.) 
In this equation, .lambda., B and .theta. are, respectively, X-ray 
wavelength, half width in radians, and peak angle in degrees. This method 
is described in a publication entitled "Measurement of Interlayer Spacings 
and Crystal Sizes inTurbostratic Carbons" by M. A. Short and P. L. Walker, 
Jr., Carbon, Vol. 1 (1963), pp. 3-9. 
In general, a lower degree of crystallinity as reflected by a lower L.sub.c 
value correlates with a wetting characteristic, whereas a higher degree of 
crystallinity as reflected in a higher L.sub.c value correlates with a 
non-wetting characteristic. For instance, an L.sub.c of 350 angstrom units 
(A) or more correlates with non-wettable performance, whereas an L.sub.c 
value less than 350 angstrom units tends to characterize wettable 
performance. 
Where isotropic coke serves as the starting material, the resulting 
graphite will, for all practical purposes, always exhibit a wettable 
characteistic with respect to aluminum in chloride reduction cells. The 
carbonaceous material can be graphitized at almost any temperature between 
1800.degree. and 3000.degree. C. and still exhibit a wetting behavior 
which is more or less insensitive to density changes. Further, the L.sub.c 
value will practically always be less than 350 angstroms (A) and generally 
range from less than 100 to a maximum of about 300 angstroms. 
Where acicular coke serves as the starting material, non-wetting behavior 
is favored where the graphitization temperature is equal to or greater 
than 2300.degree. C. This tends to produce an L.sub.c which exceeds 350 
angstroms. Acicular coke can be produced to exhibit wetting behavior by 
graphitizing at a temperature of less than 2300.degree. C. which tends to 
result in a crystallinity characterized by an L.sub.c value of less than 
350 angstroms. In the case of acicular coke as the starting material in 
producing the graphite, the density of the final graphite product can 
exert some influence on its wetting or non-wetting behavior. In general, a 
higher density tends to favor non-wetting behavior, whereas a lower 
density tends to favor wetting behavior. In general, the density can be 
controlled by the pitch impregnation employed in manufacturing the 
graphite. Repeating the pitch impregnation one or more times tends to 
increase the density. 
In the case of non-acicular coke as the starting material, non-wettable 
behavior is favored by a graphitization temperature of 2500.degree. C. or 
higher which tends to result in a crystallinity characterized by an 
L.sub.c value of 350 angstroms or more. Graphite produced from 
non-acicular coke can be produced to exhibit wettable behavior by 
graphitizing at a temperature of less than 2500.degree. C. which tends to 
result in a crystallinity characterized by an L.sub.c value of less than 
350 angstroms. Density is not as important as with acicular coke. 
From the foregoing explanation, it is readily apparent that the 
graphitization temperature is of marked significance with respect to 
acicular and non-acicular coke in the production of graphite. In the case 
of acicular coke, density control becomes a factor but to a much lesser 
extent than graphitization temperature. Isotropic coke practically always 
results in wetting performance irrespective of graphitization temperature. 
The highest temperature to which the graphite has been heated is readily 
determined by subsequent X-ray diffraction analysis. As is known in the 
art, a standard curve relating X-ray parameter to highest temperature 
encountered can be developed for a given coke type and manufacturing 
sequence. Hence, this analysis is considered to reliably indicate the 
highest temperature employed in manufacturing graphite, that is, the 
graphitization temperature. Of significance in the use of wettable 
graphite is the fact that it can be less expensive to produce than 
non-wettable graphite, thus reducing costs, provided it is properly 
employed in accordance with the invention. 
To this point, the invention has been described with an eye to starting 
with a single type of coke for a given graphite electrode production since 
this is the normal practice in commercial production. However, it is 
possible to use more than one grade of coke in producing a graphite 
electrode. In such case, the guidelines discussed above apply based on the 
dominant type of coke employed based principally on proportion and 
secondarily on comparative influence. With respect to comparative 
influence, isotropic coke is more influential than either acicular or 
non-acicular, and non-acicular is more influential than acicular. If a 
mixture of coke types includes 60 or 70% or more of any particular type, 
that type dominates. However, where different types are present in more or 
less equal amounts, then the above-stated order of influence applies. 
Obviously, as the degree of dominance diminishes, the certainty of the 
result can be likewise diminished. Hence, it is preferable in practicing 
the invention to use but a single type of coke as the starting material or 
at least, where a mixture is used, it is preferred to use a mixture 
characterized by a clear dominance such as a dominance of at least 80% 
proportion. 
The invention and the improvements achieved thereby are illustrated in the 
following examples listed in table form. The data in Tables I and II show 
cathode wear rate as it varies with cathode graphite wettability and bath 
flow velocity in baths containing around 70% NaCl and 30% LiCl to which is 
added about 7% AlCl.sub.3. Wettability is determined in accordance with 
the herein-described test (FIG. 2 and particularly FIG. 3). The baths 
operating at about 710.degree. C. are electrolyzed to produce aluminum and 
the wear rate is determined for a measured time and converted to mm. of 
wear per year to provide a comparative wear estimate. 
TABLE I 
__________________________________________________________________________ 
Droplet Bath 
Graphite size velocity 
Wear rate 
Example 
Coke L.sub.c (A) 
(mm) Wetting (ft/sec) 
(mm/year) 
__________________________________________________________________________ 
1 acicular 
330 2.2 wettable &lt;0.1 4.6 
2 acicular 
330 2.2 wettable 2.5 19.1 
3 acicular 
430 0.8 non-wettable 
&lt;0.1 6.1 
4 acicular 
430 0.8 non-wettable 
2.5 7.4 
__________________________________________________________________________ 
Table I illustrates the sensitivity of wettable graphite to a relatively 
high bath flow velocity of 2.5 feet/sec. (Example 2) but indicates a much 
lower wear rate for a low bath flow velocity of less than 0.1 feet per 
second (Example 1). A similar test at 1.4 feet per second bath veolcity 
resulted in a comparative wear rate estimate of only 3 mm. per year on a 
wettable graphite cathode surface. Non-wettable graphite (Examples 3 and 
4) in this test had acceptable wear rates for either flow rate but not as 
good as the wettable graphite under low bath flow rate conditions. 
TABLE II 
______________________________________ 
Graphite Production Wetting Test 
Exam- Graphit. Droplet 
ple Coke Temp. L.sub.c (A) 
Size (mm) 
Wetting 
______________________________________ 
5 acicular 2000.degree. C. 
200 2.0 wettable 
6 acicular 2600.degree. C. 
360 0.1 non- 
wettable 
7 non-acicular 
1800.degree. C. 
92 9.0 wettable 
8 non-acicular 
2800.degree. C. 
370 0.5 non- 
wettable 
9 isotropic 1800.degree. C. 
82 9.0 wettable 
10 isotropic 2800.degree. C. 
300 5.0 wettable 
______________________________________ 
Table II shows Examples 5 to 10 wherein starting with acicular coke 
(Examples 5 and 6) or with non-acicular coke (Examples 7 and 8), the 
graphite produced can be either wetting or non-wetting by molten aluminum 
in accordance with the invention. For instance, in Examples 5 and 6, 
increasing graphitization temperature from 2000.degree. C. to 2600.degree. 
C. changes the graphite from wettable to non-wettable. However, with 
isotropic coke (Examples 9 and 10) graphitization at either 1800.degree. 
or 2800.degree. still results in a wettable surface. 
While the invention has been described with particular reference to 
electrolytic cells of the type shown in FIG. 1 featuring horizontal 
electrodes and horizontal interelectrode spaces therebetween for 
essentially horizontal bath flow through the interelectrode spaces, it is 
believed that the invention may also be useful in cells featuring 
non-horizontal electrodes such as vertical electrodes. In such case, the 
non-wettable cathode surfaces are to be used with higher velocity bath 
movement whereas wettable cathode surfaces are to be used in conjunction 
with lower bath velocity over the cathode surface.