Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TITLE
Improved Cathode, Method for Making Same,
and Method ~or Lowering Hydrogen
Overvoltage in a Chlor-alkali Cell
BACKGROUND OF TEE INVENTION
This invention concerns improvements in and
relating to a cathode for a chlor-alkali cell, a
method for making said cathode, and a method for
lowering the hydrogen overvoltage of a chlor-alkali
cell.
Production of caustic and chlorine by elec~ro-
lysis of brine is well known in the art. The
electrolysis is carried out in an electrolytic cell
which consists in general of an anode, a cathode, an
anode compartment and a cathode compartment. In one
of the more recent types of such an electrolytic cell,
the two compartments are separated from one another
by a fluorine-containing cation exchange membrane.
Such an electrolytic cell can be operated more
efficiently and economically as the current efficiency
is i~creased, and as the operating voltage is lowered.
Inasmuch as very large quantities of caustic and
chlorine are produced by electrolysis of brine daily,
even very small improvements in the current efficiency
and operating voltage of chlor-alkali cells will lead
to saving of large amounts of money and conservation
of large amounts of energy.
The opera~ing voltage of a chlor-alkali cell is
made up of a number of component parts, of which one
is the voltage drop at the cathode, known as the hydro-
gen overvoltage. A lowering of the hydrogenovervoltage will result in lowering of the overall cell
voltage and consequently make the process more economi~
cal.
It is thereore an object of this invention to
provide an improved cathode for a chlor-~lkali cell and
a method for making tha~ cathode.
It is another object of this invention to
AD-5022
provide a method for lowering the hydrogen overvoltage
of a chlor-alkali cell,
SUMMARY OF THE INVENTION
The above objects are accomplished by the present
invention which, briefly, comprises in one embodiment a
ca~x~e having crystals of alpha-iron adherer.t to its surface.
More speci~ically, according to the present
invention there is provided a cathode for use in elec-
trolysis of an alkali metal halide comprislng an
electrically conductive cathode substrate and crystals
of alpha-iron adherent to the surface of said cathode
substrate.
There is also provided according to the present
invention an electrolytic cell which includes that
cathode.
There is also provided according to the present
invention a process for making a cathode in an elec-
trical apparatus which comprises a housing and an anode,
said process comprising
(a) placing in said housing an electrically conductive
cathode substrate, an aqueous electrolyte having a pH
no less than about 7, and particles comprising grains
of alpha-iron in an amount of about lg or more per dm2
of the included area of said cathode substrate, and
(b) passing an electrical current between said anode
and said cathode substrate until at least some of
said grains of alpha-iron have adhered to said
cathode substrate.
There is further provided a method for lowering
the operating voltage of a chloralkali electrolysis
cell whlch comprises an anode, a cathode, an anode
compartment, a cathode compartment, and a fluorine-
containing cation exchange membrane which separates the
compartments, said me~hod comprising adding particles
comprising grains of alpha-iron in an amount of a~out lg e~
more per dm~ o~ membrane area to said cathode compart-
ment.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the inVention~ ~ cathode
having crystals of alpha-iron on its surface provides
a lowering of the hydrogen overvoltage at the cathode
of a chloralkali cell~
Such a cathode is suitably made by adding to an
electrical cell, which may be a chloralkali cell, a
small amount of particles which comprise alpha-iron.
Such particles may be of alpha-iron or may contain
grains of alpha~iron along with other substances.
One suitable type of particle is filings of grey
cast iron. Grey cast iron is predominantly
alpha-iron, as determined by X-ray diffraction, and
also contains about 5~ Fe2O3 and about 1 to 2% each of
carbon and silicon. When filings of grey cast iron
are used to make the novel cathode, the particles of
alpha-iron seen on the surface of the cathode arP
found to be smaller than the filings used as the
source of alpha-iron. Although the explanation of
this observation is not known, it is believed that the
initial particles break down in the aqueous medium or
caustic solution to provide grains of alpha-iron which
deposit on the cathode. It is also possible that iron
from the added particles is being reformed in some
manner not yet understood to provide crystals of
alpha-iron on the surface of the cathode. In any case,
the invention as claimed below is not bound by any
theory as ~o how or why the cathode~cell or processes
operate as they do.
The particles of iron employed can be of
various shapes, one suitable form being filings, as
mentioned above. The particles can be of various sizes,
for ~le, fram a size which passes ~rough a screen with 1.7-~m
openings (10 mesh) or larger, to a size which passes
through a screen with 0.15 .~m openings (100 mesh) or
smaller, preferably from a size which passes through
a screen with 0.6-mm openings (30 mesh) to a size which
passes through a screen with 0.24-mm openings ~60 mesh).
A suitable convenient size passes through a screen with
0.38-mm openings (40 mesh).
Crystals of alpha-iron from as small as about
0.1 micrometer in each dimension up to as large as
about 10 micrometers in the largest dimension have
been observed on the cathode of the invention. It is
believed, howe~er, that a cathode having crystals of
considerab]y smaller or larger dimension adhered to
its surface also exhibits a reduced hydrogen overvoltage.
The particles of alpha-iron deposited on the
cathode are not permanently bonded to the cathode,
and the deposit can easily be scraped from the cathode,
yet the deposit is sufficiently adherent that the
cathode can be removed from the cell and placed in a
second cell without unduly disturbing the deposit,
and the performance of the second cell is also improved.
The amount of iron particles employed does not
appear to be critical. ~mounts of about lc3 or more
per dm of membrane area separating the compartments
of the cell are suitable. In the case of an electrical
cell which does not contain an ion-exchange membrane,
amounts of about 1 g or more per dm2 of the included
area of the cathode are suitable. By "included area"
of a cathode is meant the overall area included by the
outline of the cathode (of generally flat configuration
or deformed into a generally flat configuration).
If the iron particles employed are covered with
a surface of iron oxide, little effec~, if any, is
observed in lowering the hydrogen overvoltage. Thus,
the iron particles should have a surface which is at
least in part iron ~etal. The greatest effect is
observed if the surface of the particles is all iron
metal, i.e., if there is no oxide on the ~urface. If
; 35 there is oxide on the surface of the iron particles,
the oxide should either be reduced to iron, for example
with hydrogen, or removed from the particles. It is
easier to remove the oxide, which can be accomplished
by treatiny the particles of iron with an acid which
will dissolve the oxide, such as phosphoric acid.
Use of the cathode of the invention in a chloraLkali
cell,or a,~ ticn of iron p~rticles to a chloralkali cell as
described above, results in a lowered overall cell
voltaye, generally in the ranye of between 0.05 and
0.4 volts below that of the unmodified cathode, or
below that beore the addition was made, respectively.
The improvement diminishes only slightly with time,
if at all. Once a modified cathode has been prepared
in this manner, the cathode can be removed and used
in a different cell. A lower than normal voltage will
also be observed in the second cell. Again the
improvement diminishes only slightly, if at all, with
time.
~hen particles of iron as described herein are
added to an operatiny chloralkali cell, or when placed
in a cell at startup, a reduction in voltaye associated
with the presence of alpha-iron on the cathode surface
is in most cases observed almost immediately, but, for
reasons not yet understood, in some cases the reduction
in voltage has not been realized until after operation
of the cell for a period up to about one day.
The cathode of the invention, made either
in a cell having no cation-exchange membrane or in a
membrane cell, both as exemplified in examples below,
can be removed rom the cell in which made and placed
for use in a chloralkali cell of either the membrane
type or the diaphragm type, both of which are well
known in the art.
The invention is applicable with a wide variety
of different typ~s and shapes of cathoc'es used as the cathode
substrate. Examples include mild steel and mild
steel haviny a nickel surface, such as nickel platins
S
or Raney nickel, in the form of sheets, ro~s or expanded
metal. Electrodes having a Raney nickel surface are
described, for expamplet in U.S. Patents 4,116,804,
4,169,025, and 3,637,437.
The invention is useful with chloralkali cells
containing any of the known types of cation exchange
membranes suitable for use in that type of cell. Such
membranes of fluorine-containing polyrners include those
disclosed in U.S.P.'s 3,282,875, 4,085,071, and 4,176,215,
and South African patent publications 78/002224 of W.G. Grot
et al, published 1979 April 25, 78/002225 of D.C. England et
al, published 1979 April 25, and 78/002221 of W.G. Grot et
al, published 1979 May 30, or fahricated from polymers
describecl therein, but are not limited thereto.
Although the invention is applicable over a wide
range of cell operating conclitions, it ordinarily finds
greatest use in cells operating at a current density of 7.5-
50 Ar~s per dm2 (0.5-3 amps ~er s~uare inch), at 75-90C.,
while producing caustic at a concentration of 10-40% by
weight, with an exit brine concentration of 15-25% by weight.
In most typical chloralkali cells of the membrane
type of commercial size, the spacing between the anode and
cathode is of the order of 3 to ~ mm. When the cell is
assemb1ed for use, the membrane can be mounted equidista~t
from the two electrodes, or closer to the anode or cathode,
but is preferably closer to the anode. Because of possible
swelli~g of the membrane in the aqueous medium and/or
deformation of the membrane (~ue to varying pressure in
either the catholyte or anolyte, the membrane may act~ally
3Q contact either the anode or cathocle. It is preferred to
operate in s~lch manner that the cathode is spaced away from
the membrane by a nominal Aistance, generally ~bout 1 to
6 mm, which spacing can be maintained, e.g., by maintaininy
the catho]yte at a suitably hiyher pressure than the
anolyte. In the examples to follow, the cathodes used are
of expanded metal mesh ha~inc3 a few spacer bars ca. 3 mm
L46
thick mounted on the surface which faces the membrane,
and the cells are assembled witn the membrane ca. 1.5 ~m
from the anode surface, and ca. 1.5 mm from the spacer
bars of the cathode, i.e., ca. 4.5 mm from the expanded
met~l portion of the cathode~
To further illustrate the innovative aspects
of the present invention, the following examples are
provided. The iron filings used in the examples are
of the grey cast iron described above.
Example 1
An electrolytic mem~rane cell for electrolysis
of brine wlth 0.45 dm2 active membrane area, using a
nickel-plated, mild steel cathode, was started up at 31
A/dm and 80C., producing 31-32 wt ~ NaOH. The ~embrane
mounted in the cell during asseDbly comprised a 0.127 r.lm
(5-mil) film of a copolymer of perfluoro(3,6-dioxa-4-
methyl-7-octenesulfonyl fluoride) and tetrafluoroethylene
having an equivalent weight of 1100, and a 0.051-mm
(2-mil) film of a copolymer of methyl perfluoro(4,7-
dioxa-5~methyl-8-nonenoate) and tetrafluoroethylene
having an equivalent weight of 1025, laminated to
opposite sides of a fabric of fluorocarbon yarns (T28C)
such that the two films contact one another in the
openings in the fabric, and hydrolyzed so that the func~
tional groups were sulfonic and carboxylic acid potas-
sium salts. The surface of the membrane having carboxy-
lic functionality faced toward the cathode. Over the
first 14 days of operation, which was stable at 90 92%
current efficiency, the cell voltage increased from
3.73 volts to 3.93 volts, at which point a small amount
of iron filings (approx. 1 g, 40 mesh) was added to the
cathode compartment. The ~-oltage started to decrease
immediately, and on the 20th day was 3.63 volts, with no
change in current efficiency.
E ~
The cathode was removed from the cell of
Example 1 at the end of ~he 20th day, and placed in
- : 7
another like cell started up in the same manner. No
additional iron filings were added. After an initial
four days at 3.68-3.76 volts, this cell operated at
3.55-3.65 volts for 83 days at 91-96~ current
efficiency.
Example 3
An electrolytic membrane cell for electrolysis
of brine with 0.45 dm2 active membrane area, using a
Ni-plated, mild steel cathode, producing 32 wt ~ caustic,
waS started up at 31 A/dm and 80C. Themembrane mounted
in the cell during assembly was a 0.051-mm (2-mil)
film of a copol~mer of methyl perfluoro(4,7-dioxa-5-
methyl-8-nonenoate) and tetrafluoroethylene having an
equivalent weight of 1012, and hydrolyzed so that the
functional groups were carboxylic acid potassium salt.
After three weeks, stable operation resulted in 96~97%
current efficiency and 3.8 to 3.9 volts. At the 25th
day of operation, a small amount of iron filings (approx.
0.5 g, 40 me~h) treated wi~h phosphoric acid and washed
with water was added to the cathode compartment. An
immediate decrease in operating voltage from 3.83 to
3.74 volts occurred. By the 28th day, the voltage
was 3.40 volts, with no change in current efficiency.
The cell continued this level of performance through
60 days on line at which time it was shut down.
Example 4
An electrolytic membrane cell for the
electrolysis of brine, with 0.45 dm2 active membrane
araa, using a Ni-plated, mild steel cathode, producing
30 28 wt ~ caustic, was started up at 31 A/dm2 and 80C.
The membrane mounted in the cell during assembly was a
0.178-mm (7-mil) film of a copolymer of perfluoro(3,6-
; dioxa-4-methyl-7-octenesulfonyl fluoride) and tetra-
fluoroethylene having an equivalent ~7eight of 1150,
treated on one side with ethylene diamine (EDA) to a
depth of 0.038 mm (1.5 mils), havlng a fabric of
fluorocarbon yarns (T 900G) embedded in the remaining
sulfonyl fluoride layer, and having the remaining
sulfonyl fluoride groups throughout hydrolyzed to
sulfonic acid potassium salt. The EDA-treated side of
the membrane faced toward the cathode. While the cell
was operating at 3.90 volts, approx. 0.5 g of iron
filings treated with phosphoric acid and washed
thoroughly with water was added to the cathode compart-
ment; two days later the cell voltage was 3.74 volts.
In a duplicate parallel experiment, the
initial cell voltage was 4.00 volts, and two days
after addition of iron filings was 3.84 volts.
Example 5
An electrolytic membrane cell for the electro-
lysis of brine, with 0.45 dm2 active membrane area,
using a Ni-plated mild steel cathode, producing 20~
caustic,~s s~.ed up at 31 AJdm2 and 80C. The m~mbrane mounted
in thecell during asser.~ly was a 0.127-mm (5-mil) film of a copoly-
~,er of per1uoro(3,6-dioxa-4-methyl-7-octenesulfonyl
fluoride~ and tetrafluoroethylene having an equivalent
weight of 1200, having a fabric of fluorocarbon yarns
~ (T-12) embedded therein, and having the sulfonyl
fluoride groups hydrolyzed to sulfonic acid potassium
salt. While the cell was operating at 4.16 volts,
0.5 g o~ iron filings (40 mesh) treated with
phosphoric acid and washed with wa~er was added to
the cathode compartment. By the next morning the
voltage had decreased to 4.00 volts, with no change
in current efficiency.
Example 6
An electrolytic membrane cell for the elec-
trolysis of brine, with 0.45 dm2 active membrane area,using a Ni-plated mild steel cathode, producing 10~
caustic, ~Jas s~ted up at 31 A/d~2 and 80C. The n~brane mounted
in the oe ~ duringassembl~ was a laminate of a 0.025-mm(1-mil)f~m of
a copolymer or perfluoro(3,6~dioxa-4-methyl-7-octene-
sulfonyl fluoride) and tetrafluoroethylene having anequivalent weight of 1500 and a 0.127~mm (5-mil) film
of a copolymer of the same two monomers having an
.. ~ 9
iL4~
equlvalent weight of 1100, having a fabric of fluoro-
carbon yarns (T 24C) embedded in the layer or
equivalent weight 1100, and having the sulfonyl groups
throughout hydrolyzed to sulfonic acid potassium salt.
S The surface of the membrane of 1500 equlvalent weight
polymer faced toward the cathode. While the cell was
operating at 4.60 volts, 0.5 g of iron filinss
(40 mesh) treated with phosphoric acid and washed
with water was added to the cathode compartment. The
voltage decreased immediately to 4.45 volts, and by
the next morning had decreased to 4.33 volts, with no
change in current efficiency.
Example 7
An electrolytic membrane cell for electrolysis
of brine, with 0.45 dm2 active membrane area, using a
cathode of mild steel having a Raney nickel surface
of the kind described in U.S. Patent 4,116,804, producing 31-32g6
caustic, was started up at 31 A4~m2 and 80C. me membrane
mounted in the cell during assembly was a fabric of fluoro-
carbon polymer filaments ~0.127 mm, or 5 mil, diameter)in a ~eno weave having 68% open area, having laminated
to one side a 0.102~mm (4-mil) film of a copolymer of perfluoro(3,6-
dio~a-4-methyl-7~octenesl1fonyl fluoride) and tetrafluoroethylene
ha~ing an equivalent weight of 1100 and to the other side a
0.051-mm (2~mil) film of the same copolymer
such that the two films contact one another in the openings in the
fabric, Irther having a 0.051-mm (2-mil) film of a copolymer of
methyl p~luoro(4,7-dioxa-5-methyl-8-no~ te) and tetrafluoro-
ethylene having an equivalent weight of 1012 laminated to the
indicated 4~mil film, and hydrolyzed so that the
functional groups were sulfonic and carboxylic acid
potassium salts. The side of the membrane having
carboxylic functionality faced towaxd the cathode.
The cell started up at 3.76 volts, but by the next
morning the voltaye was 3.53 volts and for three days
operation was stable at 3.46-3.56 volts and 96-97~
current efficiency. On the 4th day of opera~ion, a
~84~
small amount (a~proximately 0.5 g, 40-mesh) of iron
filings treated with phosphoric acid and washed with
water was added to the cathode compartment. By the
next day the voltage had dropped to 3.32 volts, at
the same current efficiency of 96-97~. For the next
se~en days the voltage remained in the range of 3.23-
3.40 volts at current efficiencies of 94-97~.
Example 8
An electrolytic membrane cell for electrolysis
lQ of brine, with 0.45 dm2 active membrane area, using a
cathode of nickel-plated mild steel, producing 31-32
caustic, was started up at 31 A/dm2 and 80C. The
membrane and its orientation were like those specified
in Example 7. After twenty days of stable operation
at 4.03-4.08 volts and 94-95~ current efficiency, the
cell was shut down. While the cell was shut down,
approximately 1 gof iron filings t40 mesh) which had
been treated with phosphoric acid and washed with water
was added to the cathode compartment. The cell was
restarted; during the first day after restarting the
voltage fell from 4.02 volts to 3.86 volts, and for
the next three days the voltage was 3.78-3.84 volts
at 94-96% current efficiency.
Example 9
An electrolytic membrane cell for electrolysis
of brine, with 0.45 dm2 active membrane area, using a
cathode of nickel-plated mild steel, producing 31-32%
caustic; was started up at 31 A/dm2 and 80C. The
membrane and its orientation were like those specified
30 in Example 7. After a few days of unsettled operation,
stable operation was attained for 31 days at 3.97-4.02
volts and 95-96~ current efficiency. Then, with the
cell still in operation, approximately lg of iron
filings (which passed through a 100 mesh screen) which
35 had been treated with phosphoric acid and washed with
water was added to the cathode compartment. Although
there was considerable loss of iron particles from the
11
cell in the caustic stream ~lowing from the cathode
_ompaL~ment, some or she iro~ particles remalned in the
cell, and the cell operated at 3.~4-3.37 volts and
93-97% current efficiency for six davs, after which the
cell was briefly shutdown for removal of the cathode
so that the cell could be used for another experiment.
Example 10
An electrical cell was assembled using two
nickel-plated, expanded mesh, mild steel electrodes,
each being 7.6 cm in diameter, one to serve as an
anode and the other to serve as a cathode substrate.
In this case, no membrane was used to separate the two
cell compartments. Into the cell were placed a 30%
by wt. solution of sodium hydroxide in water, and
approximately 2g of iron filings (40 mesh) which had
been treated with phosphoric acid and washed with water.
A current of 8.4 amps was passed between the electrodes
for 2 hours, and after shutting down overnight, for
another 2 hours the next morning. The cell was dis-
mantled and the cathode (cathode A) was found to havea particulate deposit derived from the iron filings
adherent to its surface.
The procedure o~ the previous paragraph was
substantially repeated, except that the current of 8.4
amps was passed between the electrodes for 5 hours on
the first day, and after interruption overnight, for
6 hours the next day, to provide a similar cathode
(~athode B).
When the chloralkali cell of Example 9 was shut
down at the end of that example, the cathode of that
cell was removed and replaced with ea~hode A from above,
the cell was again started up immediately, and
elect~olysisof brine was continued. The cell operated
at 3.78-3.84 volts at 94-95% current efficiency for
8 days. This voltage is approximately 0.2 volts ~elow
the voltage of 3.97-4.02 volts at which this cell
had operated with a plain nickel-plated mildsteelca~hode.
12
~18~
The chlora'kali cell was shut down for one hc~r,
during which time cathode A was replaced by cathode B.
The cell operated for three days at 3.83-3.90 volts
mostly at 3.83-3.86 volts, and 95-96% current
efficiency.
The cell was again shut down for one hour,
during which time cathode B was replaced by a standard
nickel-plated mild steel cathode. The voltage rose,
and the cell operated at 3.89-4.01 volts, mostly at
3.95 4.00 volts, and 95-96% current efficiency, for
the next 40 days.
Example 11
An electrolytic membrane cell for electrolysis
of brine, with 0.~5 dm2 active membrane area, ùsing a
cathode of mild steel, producing 31-32% caustic, was
started up at 31 A/dm2 and 80C. The membrane and its
orientation were like those specified in Exarnple 7.
On the 13th day of operation, after stable operation at
3.98-3.99 volts and 97-98% current efficiency had been
attained, approximately 1~ of iron filings (40 mesh)
which had been treated with phosphoric acid and
washed with water was added to the cathode compartment.
For the next 94 days the cell operated at 3.70-3.88
volts and 92-98% current efficiency (with the exception
of a 3-day period at 3.64-3.68 volts and 81-92%
current efficiency when another chloralkali cell
connected in series with this cell shorted out). At
the end of this period, the cell was briefly shut down,
the cathode (ca~hode C) was replaced by a standard
nickel plated cathode, and the cell was restarted, after
which the voltage rose to 3.90-3.95 volts at 95~97%
curren~ efficiency.
Photomicrographs of cathode C as removed from
the cell revealed on i~s surface crystals of alpha-iron.
The crystals ranged in size from small ones about 0.1
rnicrometer in each of its three dimensions to laryer
ones about 10 micrometers long in their longest dimen-
tion. 13
' ----
Industrial Applicability
The invention is useful broadly in thechloralkali industry for providing a more efficient and
economical operation of chloralkali cells. For example,
for a plant producing 1000 metric tons of caustic per
day, operating at 90% current efficiency with power
costs of $0.02/kilowatt hour, there is an annual
savings of $544,000 for each reduction in operating
voltage of 0.1 volt. Thus, if an average voltage reduc-
tion of 0.2 volt is achieved, the saving for such aplant would be in excess of one million dollars per year.
Beyond the actual monetary savings, there is also a
corresponding saving in the world's energy reserves.
; 15
