Electrolytic cell

An electrolytic cell for production of alkali metal chlorates has pairs of spaced flat parallel perforate cathodes and has flat imperforate anodes resident within each pair of cathodes, with each cathode having a plurality of horizontal slots therethrough, within an electrically conductive tank. Electrically insulative chemically resistive bumpers on either side of each anode maintain the anode spaced from the pair of cathodes within which the anode resides. The cell bottom and two sides are formed of a single member; the cell top is electrically insulated from the remainder of the cell.

BACKGROUND OF THE INVENTION 
This invention relates to diaphragmless electrolytic cells for manufacture 
of alkali metal chlorates. 
DESCRIPTION OF THE PRIOR ART 
An electrolytic cell of the type to which this invention relates is 
illustrated in United States Pat. No. 3,824,172, Electrolytic Cell for 
Alkali Metal Chlorates. The 172 patent cell has substantially flat hollow 
electrode modules in an electrically conductive tank, with each module 
including cathodes on either side of anodes such that the pattern of 
cathodes and anodes is 
cathode-anode-cathode-cathode-anode-cathode-cathode-anode-cathode, etc. 
The 172 cathodes are foraminous, having vertically oriented slots. 
SUMMARY OF THE INVENTION 
This invention provides a cell for producing alkali metal chlorates which 
consumes less power than prior cells while operating at high electrode 
current density, thereby producing alkali metal chlorates more efficiently 
than cells known heretofore. The cell includes pairs of spaced perforate 
cathodes with flat imperforate anodes residing within each pair of 
cathodes. Each cathode has horizontal slots therethrough; the cathode and 
anode electrodes are in an electrically conductive tank. Electrically 
insulative chemically resistive bumpers on either side of each anode 
maintain the anodes proximate yet spaced from a pair of cathodes within 
which an anode resides. The tank bottom and two sides are a single member; 
the tank top is electrically insulated from the remainder of the tank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 3 and 5, the cell is designated 10 and includes a can 
designated 12 which forms three of five sides of a tank within which the 
anode and cathode electrodes of the cell reside. Can 12 is a single piece 
of electrically conductive material, preferably carbon steel, and has a 
bottom 14 and two sides each designated 16. To form the tank, two side 
plates designated 18 are secured, preferably by welding, along their 
vertical and bottom margins to can sides 16 and can bottom 14. The tank 
top is closed by bolting headboard 26 (1) to flanges 54 which are secured, 
preferably by welding, to can sides 16 and (2) to horizontal upper 
portions of side plates 18 formed by bending. Headboard 26 is electrically 
conductive, preferably carbon steel. 
Headboard 26 is electrically insulated from the lower portion of the tank 
by a gasket 52 mounted on horizontal upper portions of side plates 18 and 
on horizontal portions of flanges 54. Gasket 52 is maintained between 
headboard 26 and horizontal portions of flanges 54 and side plates 18 by 
nuts 68 in threaded engagement with bolts 66, as best illustrated in FIG. 
6 Nuts 68 and bolts 66 also maintain the headboard on the tank. Each nut 
68-bolt 66 combination is insulated from both the tank and the headboard 
by an insulative collar 70 and an insulative spacer 72. Collar 70 and 
spacer 72 may be any dielectric material which can withstand temperatures 
of up to 100.degree. C occuring during cell operation. 
Within the tank are a plurality of pairs of vertically oriented 
horizontally spaced substantially flat parallel perforate cathodes; each 
cathode has been designated 20. The cathodes are welded at their 
horizontally extreme vertically extending margins to the inside surfaces 
of can vertical sides 16. The cathodes are electrically conductive and 
preferably are carbon steel. Three anodes, each designated 22, reside 
within each pair of individual cathodes 20, with the anodes each 
substantially equally spaced from each cathode of the surrounding pair. 
The anodes are mechanically and electrically connected to headboard 26. 
The anodes are electrically conductive, preferably titanium, and are 
coated with a highly conductive precious metal coating. Although titanium 
is the preferred metal for the anodes, any metal from the titanium group, 
i.e. titanium, zirconium, tantalum and hafnium, may be used to fabricate 
the anodes. One supplier of a suitable precious metal alloy anode coating 
is Englehard Minerals and Chemicals Corporation, located in Union, N.J. 
The precious metal anode coating may also be platinum, a platinum-iridium 
alloy or ruthenium oxide. 
Referring to FIG. 3, anodes 22 are secured to headboard 26 by bolts 24 
which pass through anode bus plate 36. Each anode bus plate 36 on top of 
headboard 26 has a plurality of anodes connected thereto by bolts 24. All 
the anodes enter spaces between cathodes disposed in pairs when headboard 
26 is lowered into place and secured to the tank, as shown by the arrows 
in FIG. 3. Thus when the cell is assembled a 
cathode-anode-cathode-cathode-anode-cathode-cathode-anode-cathode pattern 
results. 
Anode-headboard assembly is best shown in FIG. 7. The top portion of each 
anode 22 resides within a channel in an anode header bar 40 with the anode 
and the header bar preferably welded together. Header bar 40 has a tapped 
hole for receipt of bolt 44. Bolt 44 secures an anode bus plate 36, 
Headboard 26, the preferably titanium headboard liner 42 and an anode 
header bar 40 together. An O-ring 48 seals each anode header bar-titanium 
liner interface. The anode header bars are preferably titanium. 
Each anode has several holes therethrough with anode spacer buttons 46 
secured therewithin. Anode spacer bottons 46 may be any electrically 
insulative material which can withstand high temperatures and the 
corrosive effects of alkali metal chlorides and alkali metal chlorates in 
solution. Both polyvinylidene fluoride and polytetrafluoroethylene are 
suitable materials for the anode spacer buttons. Buttons 46 serve as 
bumpers to maintain the anodes spaced from the surrounding cathode pair 
within which a particular anode resides. Anode spacer buttons 46 are 
preferably shaped so that an externally facing hemispherical configuration 
is provided for contacting a cathode 20. This is best shown in FIG. 8. 
Referring to FIGS. 3, 4 and 5, pairs of cathodes are designated 21, with 
each cathode electrically connected, preferably by welding, to can sides 
16 along vertically extending edges. Each cathode of a pair is 
horizontally spaced from and parallel to its paired cathode mate. 
Stringers 50 welded to cathodes 20 along the cathode lower margins 
reinforce the assembly of cathodes 20 in the tank. Each cathode 20 has a 
plurality of horizontal slots 24 therethrough with the percentage of open 
area in each cathode about 30%. At the upper extremity of each pair of 
cathodes are cathode spacers 62 which may be any electrically insulative 
material which can withstand high temperatures and the corrosive effects 
of alkali metal chlorides and alkali metal chlorates in solution. Both 
polyvinylidene fluoride and polytetrafluoroethylene are suitable materials 
for spacers 62. The spacers 62, along with anode spacer buttons 46, 
maintain the anodes spaced apart from each cathode of the surrounding 
cathode pair. This is best illustrated in FIGS. 7 and 8. 
Secured to the exterior of each of the two can sides 16 are cathode bus 
plates 34. This is best shown in FIG. 3. The cathode bus plates are 
preferably welded to the can sides and improve distribution of electrical 
current over the can side. 
Two or more cells may be electrically connected in series as illustrated in 
FIGS. 1 and 2. (All three of the cells illustrated in FIGS. 1 and 2 embody 
the invention. Some of the structural details, particularly some of bolts 
44, have been omitted from the two end cells illustrated, as unnecessary 
for understanding the invention.) Cell interconnection bus 38 and cell 
interconnection bar 39, which is secured to a side plate 18 and to the two 
cathode bus plates 34, allow series connection of cells. Each cell 
interconnection bus 38 is connected to the anode bus plate of a first cell 
and the cell interconnection bar of a second, adjacent cell by bolts which 
have not been numbered. 
A gas manifold 32 having a gas outlet pipe 33 is formed in headboard 26. 
Gas produced during electrolysis of alkali metal chloride brine collects 
in manifold 32 and is removed therefrom through outlet pipe 33. This 
structure is best shown in FIGS. 3 and 5. 
Referring to FIG. 6, alkali metal chloride brine is introduced into the 
cell through a liquid inlet pipe 28. Feed pipe 58 conveys incoming liquid 
to the bottom area of the cell below the anodes and cathodes; the cell is 
allowed to fill with liquid to a level above the cathodes but slightly 
below titanium liner 42 of headboard 26. The liquid circulates through the 
cell and is removed through a liquid outlet feed pipe 64 connected to 
liquid outlet pipe 30; this structure is best shown in FIG. 5. 
Liquid-tight fittings are provided where the liquid inlet and outlet pipes 
pass through the cell walls. 
During operation, a high current output DC voltage source is connected 
across the cell anodes and cathodes, with the DC voltage source positive 
leads connected to the anode bus bars (and therefore to the anodes) on the 
insulated top of one cell and the DC voltage source negative leads 
connected to either the cell interconnection bar 39 and therefore the 
lower tank portion of that cell, if only one cell is operated, or to cell 
interconnection bar 39 and therefore the lower tank portion of a second 
cell to which the first cell is electrically connected in series when two 
or more cells are operated. The number of cells which can be connected 
together in series is limited only by the available output current of the 
DC voltage source. 
The cell has been constructed with adjacent pairs of cathodes separated, as 
shown by dimension "A" in FIG. 7, a distance of about 21/2 inches, and 
with the cathodes forming a cathode pair separated from each other, as 
shown by dimension "B" in FIG. 7, a distance of about three-fourth of an 
inch. The cathodes have been fabricated of carbon steel and have been 
about 25 .times. 44 .times. 3/8 inches. The anodes have been fabricated of 
titanium, then coated with a precious metal coating and have been about 24 
.times. 12 .times. 3/16 inches. As used herein, the term "about" when 
modifying dimensions means within engineering and fabrication tolerances. 
Slots 24 in the cathodes have been 1/2 inch high with slot ends configured 
as one-fourth inch radius circles. The centers of these circles have been 
separated by 3 inches, making for an overall slot maximum length of 31/2 
inches. Adjacent slots have been separated, in the vertical direction, by 
11/2 inches, measured between slot horizontal centerlines. Horizontally 
adjacent slots have been separated by 11/2 inches, measured between 
centers of most adjacent slot end circles. The tank and headboard have 
been fabricated of one-half inch carbon steel while the headboard liner 
has been 18 gauge titanium. Cathode bus plate 34 has been fabricated of 
13/4 inch carbon steel; cell interconnection bar 39 has been fabricated of 
1 inch carbon steel with the surface facing away from the cell 
explosion-clad with one-eighth inch copper. Anode bus plates 36 have been 
fabricated of copper as has the cell interconnection bus 38. 
Use of titanium as the anode base metal insures that an anode will not 
erode should a portion of the precious metal coating wear away during cell 
operation or be accidentally damaged during cell fabrication, assembly of 
maintenance. If the titanium base metal is exposed to the cell liquor, 
chlorine in the cell liquor attacks the titanium and a titanium oxide film 
forms at the titanium-liquid interface. The titanium oxide prevents 
further attack of the titanium base metal by the chlorine, thereby halting 
deterioration of the anode. 
Surprisingly, cells embodying the invention, when operating at moderate to 
high cathode current densities and at moderate to high cell liquor 
temperatures produce alkali metal chlorates more efficiently than cells 
known heretofore, including the cell disclosed in the U.S. Pat. No. 
3,824,172 patent, the most efficient previously known cell. Even more 
surprisingly, the efficiency advantage of cells embodying the invention 
over previously known cells increases both as cathode current density (and 
hence chlorate production rate) increases and as cell liquor temperature 
increases. 
Interpolating between the curves illustrated in FIG. 9, at cathode current 
densities in excess of 1 ampere per square inch, cells embodying the 
invention produce alkali metal chlorate, specifically sodium chlorate, 
more efficiently than the cell disclosed in the U.S. Pat. No. 3,824,172 
patent, so long as cell liquor temperature is maintained above 136.degree. 
F. The efficiency advantage of cells embodying the invention increases as 
either or both cell liquor temperature and cathode current density 
increase, i.e. as the cell operating point is moved towards the upper 
right-hand corner of FIG. 9. 
Cells embodying the invention also have a substantially longer service life 
than cells known heretofore. The limiting factor on service life of any 
diaphragmless electrolytic cell is loss of precious metal coating from 
cell anodes. To the extent that anode coating loss is minimized, service 
life of any diaphragmless electrolytic cell is maximized. Anode coating 
loss is primarily a function of current density at the anode surface. 
Current density, in turn, is a function of many variables, a primary one 
of which is metallic salt concentration over the anode area. If salt 
concentration is high, electrical conductivity of the solution is high and 
high amperage current flows between the cathodes and anodes. It may be 
desirable to maintain a high salt concentration and therefore high current 
density in the cell to obtain a high rate of chlorate production, at a 
cost of reduced anode coating life. No matter what the current density and 
no matter what the chosen rate of chlorate production, to produce 
chlorates efficiently with maximum anode life it is necessary to minimize 
salt concentration gradients in the cell. If a significant salt 
concentration gradient is allowed to exist within the cell, wherever a 
relative maximum salt concentration occurs, electrical conductivity of the 
liquid solution is high and more current flows from the cathode to the 
anode at that location. Such local high current flow can quickly consume 
the anode coating. Thus, to the extent metallic salt concentration 
gradients over the anode surface are minimized, current density variations 
over the anode surface are minimized and the anode coating wears evenly. 
This leads to longer anode life, no matter what the average salt 
concentration and average anode current density. 
Anode coating loss has been measured in an experimental cell embodying the 
invention. Coating loss has been uniform over the anode surface. 
Extrapolation of measured anode coating loss rates indicates an expected 
anode coating life, and hence an expected cell service life, of from eight 
to ten years. This is substantially greater than service life of cells 
known heretofore. 
Improved efficiency and longer service life of cells embodying the 
invention result from a minimized metallic salt concentration gradient 
within the cell, produced by the combination of (1) closely spaced 
adjacent cathode pairs and (2) horizontal slots through individual 
cathodes. Close spacing of cathode pairs and the horizontal slots through 
the cathodes bring about improved cell efficiency and longer cell service 
life by providing improved hydraulics within the cell and improved gas 
disengagement from the cathodes. The close spacing of the cathode pairs 
and the horizontal slots through the cathodes each independently 
contribute to both the improved cell hydraulics and the improved gas 
disengagement. 
Close spacing of adjacent cathode pairs improves cell hydraulics by forcing 
rapid movement of cell liquor along cathode and anode surfaces as the 
liquor flows through the tank. The cell liquor, moving at a high flow 
rate, efficiently removes gas which forms at and adheres to the cathodes 
during electrolysis of alkali metal chloride brine. Removal of gas as the 
liquid rises through the cell assures maintenance of a uniform metallic 
salt concentration gradient along the vertical length of the anodes. If 
gas were allowed to remain on the cathodes, no current could flow to an 
anode from the cathode area covered by gas, an effect known as "gas 
blinding." With gas effectively removed from between the anodes and 
cathodes, electrical resistance through the brine from the cathodes to the 
anodes is uniform, and the entire area of each cathode transmits current 
through the brine. This effectively reduces cathode-anode voltage drop at 
any current density, thereby reducing cell power consumption for the 
chosen chlorate production rate, and assures that electrolysis of brine is 
reasonably uniform along the vertical length of the anodes and cathodes. 
Horizontal slots through the cathodes improve gas disengagement from the 
cathodes. Since the cell liquor moves generally vertically through the 
cell, from below the cathodes to the cell top, gas formed on the cathodes 
tends to move vertically up the cathode surfaces. When gas bubbles moving 
along a cathode surface encounter a horizontal slot, the gas bubbles 
disengage from the cathode and float upward through the cell liquor. 
Indeed, the rising gas bubbles act as a pump, accelerating liquid flow 
upward through the cell. As gas bubbles disengage from the cathodes, the 
gas bubbles, being less dense than the liquid, reduce effective local 
density of the liquid. The portion of lighter liquid floats upward between 
the cathodes or between a cathode and an anode, effectively forcing more 
liquid to flow along cathode and anode surfaces thereby "pumping" liquid 
through the cell. 
Horizontally-slotted cathodes provide a major improvement over 
vertically-slotted cathodes and over cathodes with no slots. Gas can flow 
upward along the surfaces of both vertically-slotted cathodes and cathodes 
having no slots, with gas thereby remaining in contact with the cathode 
when the gas reaches the cathode vertical wetted extremity. This results 
in gas blinding with consequent reduction in cell efficiency. The 
horizontal slots prevent this in cells embodying the invention. 
As a further advantage, horizontal slots in the cathodes eliminate the need 
for cathode current distribution bars such as bars designated 30 in the 
U.S. Pat. No. 3,842,172 patent. This allows adjacent cathode pairs to be 
very closely laterally spaced and also facilitates improved current 
distribution over the horizontal length of the cathode. In cells embodying 
the invention each portion of solid cathode area between vertically 
adjacent rows of horizontal slots acts as an individual current 
distribution bar, distributing current uniformly along the horizontal 
length of the cathode. 
The cell layout, featuring close spacing both of cathodes about anodes and 
of adjacent cathode pairs, and the horizontal cathode slots 
synergistically contribute to improved cell hydraulics and better gas 
disengagement (with resultant minimized metallic salt concentration 
gradients, minimized current density variations and maximized cell 
efficiency) than known heretofore in diaphragmless electrolytic cells. The 
synergism begins with the horizontal slots promoting gas disengagement 
from the cathode. As the gas disengages, it reduces effective density of 
liquid surrounding the cathode, in the neighborhood where gas 
disengagement occurs. The resulting low density lightweight liquid rises 
through surrounding higher density liquid, effectively increasing rate of 
liquid flow along the cathode. Since the cathodes are closely spaced, 
liquid velocity along the cathodes is high, higher than in prior art 
cells. Higher liquid velocity in turn promotes more gas disengagement 
which in turn further increases liquid velocity. All of this 
synergistically contributes to greater cell efficiency than available in 
cells known heretofore.