Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~Z40634
--1--
EXPANDED METAL-SILVER CATHODE FOR
ELECTROLYTIC REDUCTION OF
POLYCHLOROPICOLINATF ANIONS
U.S. Patent No. 4,242,183 discloses a highly
active silver cathode having utility for the stepwise
electrolytic reduction of pentachloropyridine to
2,3,5-trichloropyridine. U.S. Patent No. 4,217,185
discloses the use of the same cathode for the stepwise
reduction of tetrachloropicolinate anions to 3,6-dichloro-
picolinate anions - which are readily converted to
3,6-dichloropicolinic acid (3,6-D), an increasingly
important herbicide.
The foregoing patents teach that the silver
cathode (which must be activated, as by anodization)
may be a monolithic silver conductor or a silver-plated
conductor. The conductor can be of any configuration -
such as a screen, plate, rod, etc. A cylindrical
cathode - such as a cylindrical silver wire screen - is
generally preferred and the cathode utilized in the
pilot plant example described in the ' 7 85 patent was
a planar silver wire screen, bolted against a wall of
a plastic cell body.
30,919-F -1- ~ ~.
1240634
-2-
A high ratio of absolute to projected planar
surface areas - a well known desideratum for any electrode -
is afforded by the wire screening configuration.
However, such screening is a more expensive form of
silver and would require a quite substantial capital
investment if employed in a commercial scale electrolytic
plant. Thus, alternative, possibly less expensive
configurations of comparable absolute surface areas
were considered - with due regard for the well known
fact that other configuration-determined factors, such
as the shape and size of openings, "edge" effects,
etc., can strongly affect the activity, selectivity and
efficiency of the electrode.
The cost of electrolytic plants based on
plate and frame cells designed for continuous, flow-
through operation was estimated by K. B. Keating and
V. D. Sutlic to be substantially lower when expanded
metal sheet electrodes ~anodes and cathodes) were
employed, rather than solid plate electrodes: The Cost
of Electrochemical Cells, AIChE Sym~osium 185, Electro-
o~anic Synthesis TechnoloqY, pp. 76-88, Vol. 75 (1979).
Cells utilizing expanded metal electrodes were indicated
to be more economical to fabricate and assemble. This
is largely attributable to the fact that the expanded
metal electrodes could be exposed on both sides to the
electrolyte - thereby affording full utilization of
their 70-80% higher actual surface areas - whereas, in
the plate and frame design used, solid electrodes would
be exposed on one side only. Some advantage was also
indicated for the turbulence promoting effects of the
expanded metal configuration on gas release from the
electrolyte. Wire screen type electrodes of course
would be advantageous over solid electrodes in the same
30,919-F -2-
3~240634
-3-
respects but would generally be more expensive than
expanded metal sheets.
Both screens and expanded metal sheets are
foraminous and therefore must be thicker than solid
sheets in order to maintain an adeguately low ohmic
resistance through the electrode. However, this is not
a significant cost factor when relatively inexpensive
expanded metals of the type mentioned by Keating and
sutlic may be used.
Among the several known electrolytic cell
types, a plate and tank cell design was selected as
most suitable for manufacture of 3,6-dichloropicolinic
acid. In this type of cell, both sides of whatever
type of generally planar electrodes can usually be
exposed to the electrolyte and foraminous electrodes
ordinarily offer no advantage in this respect. Neverthe-
less, the higher actual surface areas afforded by
screens and expanded metals is still an important
desideratum.
Expanded silver sheet is available and is
about half as expensive as silver wire screening of
comparahle dimensions and ohmic resistance. However,
it is also so "limp" - at least as a monolithic sheet -
as to reguire support by a backboard when of a size
appropriate for use as an electrode in a commercial
scale cell. This not only reguires use of cell space
to accommodate the backboards but also results in less
than full utilization of the surface area of the expanded
metal, due to contact between it and the bac~board.
Furthermore, the actual surface area of an expanded
metal mesh is only about 60% of that for the same size
sheet of comparable mesh wire screening.
30,919-F -3-
lZ40634
Finally, there is the question - in view of
the differences between the shapes of the openings and
the elements defining them in the two forms - as to
whether the expanded form (once activated) would perform
as well as a cathode for the reduction of polychloro-
picolinate anions. For example, the expanded metal
pattern necessarily will result in a somewhat lower
degree of field uniformity between electrode/counter
electrode pairs than when a screen-form electrode is
used; according to an experienced electrochemist
(D. K. Kyriacou; Basics of Electroorganic Synthesis,
page 15; John Wiley and Sons, N.Y., N.Y. 1981),
uniformity in the distribution of the electric field
between the electrodes is not only very desirable but
often is essential for efficient operation and for
avoidance of over (or under) reduction. The possible
importance of even small differences in this regard is
made amply evident by the discussion of field effects
at page 103 of the same book. Thus, it cannot be
presumed that the expanded form of silver is inherently
as active and selective as the wire screen form.
Further, the cost advantage of the expanded form would
appear to be largely or even completely counter-balanced
by the higher effective surface area and greater rigidity
of the screen form. Some further advantage for the
expanded form would then need to be apparent before it
could be considered a viable alternative to the screen
form.
It has now been found that the expanded metal
form silver electrode is considerably more active than
the screen form. That is, on an absolute surface area
30,91~-F -4-
~L24[)~34
--5--
basis, the voltametric current density exhibited by an
activated, expanded silver electrode immersed in a 1%
aqueous sodium tetrachloropicolinate solution is about
5 times that exhibited by a comparable screen-form
electrode in the same solution, under the same
potential. In practical terms, this translates to at
least a 59% higher rate of production of 3,6-
dichloropicolinic acid ("3,6-D", henceforth) per unit
of cell cost. A difference of this magnitude in
production rate is highly significant to the economic
feasibility of commercializing any process.
The present invention provides an improvement
in the process of electrolytically reducing polychloro-
picolinate anions in a basic aqueous medium at anactivated silver cathode characterized in that the
cathode employed is an expanded silver sheet.
The invention also provides an electrode
assembly comprising a platelike supporting member
having two opposed faces, an expanded silver sheet
generally coextensive with the supporting member, a
current collecting/distributing means and a fastening
means~ the sheet being conductively connected to the
collecting means and joined by the fastening means to
the member in a fixed position, uniformly closely
adjacent to the faces.
Preferably, in the foregoing assembly, the
supporting element is a polypropylene or Fiberglass~/-
epoxy composite board, the expanded sheet is disposed
as a wrap around the board and is uniformly in light
contact therewith except along one edge of the board
against which it is compressed by a flat, silver bar
30,919-F -5-
~Z4~)634
-6-
interposed between the edge and a stiff compression bar
bolted to the board through the silver bar, which is
welded to a silver rod extending through the compression
bar away from the rest of the assembly. The two bars, the
bolts and the rod constitute the collecting means. In
this embodiment of the invention, the fastening means com-
prises the latter bolts and preferably also include a
number of soft rivets - preferably plastic rivets.
The invention is further illustrated by the
accompanying drawings in which:
Figure IA is a perspective view depicting an
electrode assembly constituting the above-described
preferred embodiment of the invention. The assemb~y
comprises a composite backing board, an expanded silver
sheet wrapped around it and fastened to it by plastic
rivets, and a bolted-on, sub-assembly which comprises a
compression bar, a silver bar and a silver rod and
functions as a current collecting/distributing means.
Figure IB is a magnified perspective view of
a portion of the expanded silver sheet, seen obliquely
from above at about a 45 angle to the horizontal.
Sheet silver in expanded form ~made by die-
slitting and stretching perpendicularly to the slit
lines) is available from Exmet Corporation, 355 Hanover
Street, Bridgeport, Ct., U.S.A. It may be ordered in
the ranges of dimensions tabulated below. Reference
may be had to Figure lB of the drawings for the meaning
of the following dimensional symbols, as used in the
Table. Ts is the thickness of the strands defining
the generally diamond-shaped openings and is equal to
30,919-F -6-
lZ40634
the thickness of the silver sheet before it was expanded.
Ws is the strand width, SWD is the "short way" distance
across the diamonds and LWD is the "long way" distance
across them - the latter two distances being measured
from center-to-center of the intersections of the
strands.
30,919-F -7-
~z~0634
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30, 919-F -8-
124~)634
g
Sizes 1 through 3 are available in 18"
(45.72 cm~ wide rolls and sizes 4 through 6 in 16"
(40.64 cm) wide rolls, the LWD always being crosswise
of the roll. The method of specifying the expanded
metals is by giving in sequence the sheet thickness
(Ts) in mils, the chemical symbol for the metal (Aq
for silver), the strand width (Ws) in mils and the size
("mesh designation"): 5 Ag 7-5/0, for example.
Referring to Figure l-A, the preferred version
of the electrode assembly will now be described. A
polypropylene backboard (1) of rectangular platelike
shape is conformingly wrapped with a piece (2) of
expanded silver sheet which is as wide as the height of
the backboard and terminates in overlapping end-flaps
(not separately numbered) which are disposed between
the left edge of the backboard and a thin silver bar
(3) to which is attached a silver rod (4) by an annular
weld bead (5), shown in phantom. The bar - which is
somewhat annealed as a result of the welding - is urged
against the end flaps of the silver sheet by an overlying
stainless steel compression bar (6) attached to the
backboard by a series of six stainless steel bolts (7)
passing through aligned holes (not numbered) in bars
(3) and (6) and threaded into tapped bores (not numbered)
in the backboard. A series of plastic rivets (8; only
a few shown) passing through the backboard from face to
face (not numbered) serves as an additional fastening
and positioning means for the silver sheet. Rod (4)
extends from the silver bar (3) through a close-fitting
bore (not numbered) in bar (6) which is countersunk at
its imler end to accommodate the weld bead (5). Although
the expanded sheet is not spaced from the adjacent
backboard surfaces, it is of such a shape that it makes
30,919-F -9-
lZ4(~634
--10--
essentially only minimal (point) contact with the board
and most of its inner surface is accessible to electrolyte
contact.
The use of a scft silver bar in compressive
contact with the expanded (work hardened) silver is
considered a highly preferable way of establishing an
adequately low resistance electrical connection to the
electrode. The expanded silver is so thin and flimsy
that it is difficult to weld a conductive lead to or to
hold in good contact with a wire, however intertwined
with the strands of the mesh.
Welding a silver rod to the bar provides a
corrosion-proof contact with a conductive lead which
can be passed - by means of a conventional seal assembly -
through a cell wall.
The use of a plastic riveting material isdistinctly advantageous in that a number of suitable
plastic materials can be worked at low temperatures and
in a manner such that the expanded silver will not be
damaged by the riveting procedure. It has been found
possible to fasten the metal firmly to the backboard,
even though a plastic rivet will not contract when
cooled after forming. That is, when the rivet heads
are formed, the plastic material flows through the
openings in the expanded metal sheet and becomes inter-
locked with it upon cooling.
The riveting procedure, as employed with a
1/2" (12.7 mm) thick bac~board, is as follows. A 7/'8"
(22.225 mm) length of 1/8" (3.175 mm) diameter poly-
propylene rod is placed in a 1/8" (3.175 mm) hole, 3/4"
(19.05 mm) deep, in a metal bar and the protruding
30,919-F -10-
24(~34
1/8" (3.175 mm~ of the rod is "mushroomed" to the
surface of the bar with a concave, bronze die, kept at
the melting point of the polypropylene by a thermosta-
tically controlled heating element. The resulting
single-headed unit is then inserted through the expanded
metal on one face of the backboard into a 1/8" ~3,175 mm)
bore through the backboard and through the expanded
metal on the opposite face of the board. The already-
-formed rivet head is supported and heat and pressure
applied, with the same die, to the protruding 1~8"
(3.175 mm) to 3/16" (4.762 mm) of the rod, thereby
forming the second head of the rivet.
The platelike supporting member of the elec-
trode assembly can be fashioned from any suitably rigid
material which will not detrimentally react with any of
the reactants or products it will contact in use in an
electrolytic cell. Thus, although polypropylene materials
of the above-described type are preferred, the use of
other materials such as inert ceramics or even metals
(preferably silver plated) is considered feasible.
The process of the present invention is
practiced essentially according to U.S. Patent 4,217,1~5.
However, the expanded metal form of the silver cathode
disclosed herein is employed in place of the foil or
wire screen forms of silver cathode used in the examples
in the patent. Preferably, the cathode is comprised in
an electrode assembly as above defined which is one of
a number of such assemblies disposed in alternating
array with a like number of generally co-extensive,
platelike counter electrodes (anodes) in a plate and
tank type, full-scale cell adapted for circulation of
a basic, aqueous solution of a polychloropicolinic
30,919-F -11-
1240634
-12-
acid salt through it and provided with means for dis-
tributing the solution flow evenly to the spaces
between the electrodes.
~he expanded silver electrode may be acti-
S vated as known in the art and as described in thefollowing examples which further illustrate this
- invention.
Examples
Exam~le 1 - Comparison of Different Silver Cathode
Configurations
The following experiment was made to compare
the cathodic activities of foil, wire screen, expanded
metal and loose-woven mesh configuration silver elec-
trodes.
Two rigid, rectangular, polypropylene blocks
about 1/2" (12.7 mm) thick were bolted together,
drilled through with a 5/8" (15.875 mm) bit and
unbolted. The test electrode specimen was formed as a
disc, about 1" t25.4 mm) in diameter, with a "handle"
20 about 1/4" x 3" (6.35 x 76.2 mm) long extending from it
as an electrical lead. An annular band of a silicone
sealant about 1/8" (3.175 mm) wide was applied to each
face of the disc, which was then clamped ~etween the
blocks so that the uncoated portion of it was exposed
in the bore through the blocks, the unexposed portion
being made electrolyte-inaccessible by the sealant.
The loose-woven mesh specimen was prepared as
follows: an ordinary pot-scru~ber formed by gathering
30,919-F -12-
~z4V634
-13-
a double-walled sleeve, woven from 2 mil x 25 mil (0.05
x 0.635 mm) copper ribbon, into a ball, was "ungathered"
and the resulting sleeve electroplated with sil~er.
The sleeve was then flattened and folded several times
until the openings through the resulting compressed wad
were judged about equal in size to the openings in a
20-mesh (sieve opening 0.84 mm) wire screen. The
specimen was then cut out of the wad in the above-
-described shape. The actual electrolyte-accessible
area of the specimen was determined, after testing, by
carefully cutting out the exposed portion of the still
mounted specimen, weighing it and multiplying by the
surface area to weight ratio determined for a single
length of ribbon unraYelled from the silvered sleeve.
The electrolyte-accessible surface areas of
the screen and expanded metal specimens were calculated
from their dimensions. The screen was a square pattern,
20-mesh (sieve opening 0.84 mm) screen formed from
16 mil (0.406 mm) silver wire and had an actual to
projected surface area ratio of 2.52 to 1. The
expanded metal had a strand thickness of 8 mils (0.2
mm), a strand width of 10 mils (0.254 mm) and 625
openings per in2 (6.45 sq. cm.) (Exmet designation
8 Aq 10-4/0; see Table 1, page 9). The ratio of actual
to projected areas for the expanded metal was 1.38.
A single compartment beaker-cell comprising a
test cathode, a platinum anode and a saturated calomel
reference electrode was used, the anode being positioned
relative to the cathode such that the difference in
front-side and back-side currents was essentially nil.
The cell was connected to a Princeton Applied Research
(PAR) Model 173 potentiostat equipped with a PAR
30,919-F -13-
lZ4~63~
-14-
Model 175 Universal programmer and a Huston Instruments
Model RE0074 X-Y recorder.
The test cathode was activated in 2% aqueous
NaOH by repeated anodizations, i.e., by cycling it five
times between potentials (relative to the reference
electrode) of +1.0 volts and -1.0 voltsj at a rate of 5
millivolts per second. Cycling was discontinued at the
-1 volt limit and the cell contents replaced with a 2%
solution of "tet-acid" (tetrachloropicolinic acid~ in
2% aqueous NaOH. Voltage/current curves were then
recorded by scanning from an initial cathode potential
of 0.0 volts to a potential of -1.8 volts, at a rate of
5 mV per second. Current onset was at -0.9 volts. The
current at -1.4 volts (and the projected and actual
surface areas of the cathode~ was used to calculate the
nominal and actual current densities for each test
cathode.
The test data are given in Table 1, below.
30,919-F -14-
lZ4~634
--15--
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30, 919-F -15-
~z4V634
-16-
The ratio of cell currents (at -1.4 volts) with
the expanded metal- and screen-form cathodes was
50/18=2.78. In other words, the rate of reduction of
tetrachloropicolinate ions (to trichloropicolinate
ions) at the expanded metal was 2.78 times the rate at
the screen.
Example 2 - Laboratory Scale Reductions of
Tetrachloropicolinate Ions to 2,6-
Dichloropicolinate Ions with Screen and
Expanded Metal Form ~ilver Cathodes
A cell was assembled from a 200 ml beaker, a
Teflon~-coated magnetic stirring bar, a cylindrical
silver cathode (20 mesh (sieve opening 0.84 mm), 16 mil
(.406 mm) wire screen or 8 Aq 10-4/0 expanded metal;
projected area 16 in2 (103.2 sq. cm), an anode rod, a
thermometer and a Luggin capillary fitted with a
standard calomel reference electrode. The cell was set
on a magnetic stirrer and charged with 108.24 grams of
7.0 wt. % caustic solution (mercury grade NaOH in
reverse osmosis-purified water).
The cathode was activated by anodization for 12
minutes at a relative potential gradually raised from
0.0 to +0.7 volts and the potential was then decreased
gradually to a final value of -1.3 volts. 11.76 Grams
(0.0451 gram mole~) of tet-acid was added portionwise
over a period of about 2 hours by withdrawing a portion
of cell liquor, masticating about a 3 gram portion of
3 the tet-acid with it and returning the resultant slurry
to the cell. The reduction was continued (at a
temperature of 25-29C and a cathode potential of
-1.3 volts) until the cell current dropped to about
0.6-0.7 amps.
30,919-F -16-
~.z40634
-17-
The cell liquor (pH ~ 13) was filtered by
suction through celite, acidified with aq. HCl to pH ~1
and extracted with CH2C12 repeatedly. The combined
extracts were dried over anhydrous Na2S04, filtered and
S stripped at reduced pressure. The solid residue was
weighed and analyzed by Gas Liquid Partition Chromatography
against known standards. The current efficiency for the
reduction was calculated from the yield of 2,6-D and
the coulomb count (obtained from a cumulative counter
in the power circuit).
A comparison of results obtained with the
screen and expanded metal-form cathodes in otherwise
essentially identical runs by the foregoing procedure
are given in Table 2.
30,gl9-F -17-
~2401~34
--18--
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.,1 ~
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30, 919-F -18-
lZ40~i34
--19--
It is apparent from the Table that the expanded
metal cathode was as good or better in other respects
than the screen and gave an 18% higher average reaction
rate.
Example 3 - Effect on Reaction Rate of Differences in
Dimensions of Expanded Metal-form Silver
Cathodes
A series of laboratory scale reductions of
tet-acid with three different sizes of expanded silver
was carried out. Although other variations were also
involved, the differences in rate observed for the best
run with each size are believed to be largely attributable
to the size effects. The experimental set-up and
procedure were generally the same as in the preceding
example. The results are given in Table 3 below.
30,919-F -l9-
lZ40634
--20--
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30, 919-F -20-
~.z4V634
-21-
It was not possible to make a comparison with
a finer mesh silver screen; silver wire screening
having on the order of 2000 openings per in2 (6.45 sq.
cm) is not available. However, it is believed that even
the 2 Aq 6-6/0 expanded metal would exhibit greater
activity than a comparable mesh screen. 10 Aq 10-4/0
expanded metal is more rigid, longer lived and has a
higher actual to projected surface area than 8 Aq 10-4/0
and is accordingly now preferred.
ExamPle 4 - Pilot Plant Scale Comparison of Screen and
Expanded Metal Form Silver Cathodes
Two series of experimental tet-acid reductions
were carried out in an electrolytic cell comparable in
scale to that described at column 20 of the above-
15 referenced '185 patent. In the first series (33 runs),
a 20-mesh (sieve opening 0.84 mm), 16 mil (0.406 mm)
silver wire cathode was used. In the second series (at
least 40 runs) a cathode of the same nominal size but
consisting of 8 Aq 10-4/0 expanded silver was used. In
both series of runs, the tet-acid (1.4 gram moles) was
charged to the cell all at once, as a 2% solution in 2%
aqueous NaOH. The runs within each series were varied
in some respects but the maximum rate (maximum cell
current) attainable with the screen form cathode was
25 only 46 ampere~, as compared to 109 amperes with the
expanded metal cathode; a ratio of 2.37 in favor of the
latter.
The corresponding maximum production rates
were about 0.10 and 0.24 lbs. of 3,6-D per hour per
square foot of cathode surface (nominal). The best
overall average production rate - which drops
30,919-F -21-
lZ40634
-22-
off as the (batch) reduction proceeds - with the expanded
form cathode was about 0.19 lbs. (.455 kgs) per hour
per ft (.093 sq. meters~, vis-a-vis about 0.10 (.045
kg) with the screen cathode.
In both series of runs, the cathodes were
edge-supported only; no backboards were used.
xam~le 5 - Typical 3,6-D Production Run in Cell
with Expanded Silver Cathode
A prototype, production-scale cell was set up
10 with a total of five parallel-connected, 4'xll" (1.219
x .274 meter) expanded sil~er (8 Aq 10-4/0) cathodes,
supported by composite backboards of the preferred type
described earlier herein. The total nominal cathode
area (counting both sides of the silver sheets) was
15 36.7 ft2 (3.409 sq. meters). The cathodes were washed
with aqueous HCl, rinsed with reverse-osmosis purified
water and activated by anodization in a 2.4% solution
of tet-acid in a 2.3% aqueous solution of NaOH (50%
plant concentrate, diluted) at +0.6 volts (relative to
SCE) for 1/6 hour. The solution was circulated, by
means of a centrifugal pump, from the cell to a mixing
tank and back, and passed from a flow distributor
through the spaces (1/4" (6.35 mm) spacing) between the
cathodes and (six) counter electrodes of the same shape
and area as the cathodes. Additional tet-acid, to make
a total of 200 lbs. (90.72 kgs), was charged to the
reaction by incremental addition to the mixing tank
over a period of 13 hours. The reduction was discon-
tinued after a total time of 26-1/2 hours and the
reaction mixture was worked up.
30,919-F -22-
~.Z40634
-23-
The amount of 3,6-D in the recovered product
was equivalent to about 96 percent of the theoretical
yield (146.7 lbs. (66.54 kgs)). The overall average
3,6-D production rate was 0.145 lbs/hr/ft2 I.0653 kgs/-
hr/.093 sq. meters). The cell current was initially
3000 amperes and dropped during the first five hours of
the run to 2100 amperes.
30,919-F -23-
