U.S. patent number 3,859,195 [Application Number 05/407,248] was granted by the patent office on 1975-01-07 for apparatus for electrochemical processing.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to John M. Williams.
United States Patent |
3,859,195 |
Williams |
January 7, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
APPARATUS FOR ELECTROCHEMICAL PROCESSING
Abstract
An apparatus for electrochemical processing characterized by
extended surface electrodes maintained at a substantially uniform
electrical potential while presenting a relatively low pressure
drop to liquids in treatment passed through the apparatus.
Inventors: |
Williams; John M. (Newark,
DE) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
26966313 |
Appl.
No.: |
05/407,248 |
Filed: |
October 17, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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290642 |
Sep 20, 1972 |
|
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Current U.S.
Class: |
204/272 |
Current CPC
Class: |
C02F
1/46109 (20130101); C25B 9/19 (20210101); C25B
11/03 (20130101); C02F 2201/4611 (20130101); C02F
1/4678 (20130101); C02F 2001/46157 (20130101); C02F
2101/20 (20130101); C02F 2201/46115 (20130101) |
Current International
Class: |
C25B
9/08 (20060101); C02F 1/461 (20060101); C25B
11/03 (20060101); C25B 11/00 (20060101); C25B
9/06 (20060101); C02F 1/467 (20060101); B01k
003/00 (); B01k 001/00 () |
Field of
Search: |
;204/130,140,153,151,272 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Journal of Applied Electrochemistry 2, (1972), pgs 113-122. .
Porous Cathode Cell for Metals Removal from Aqueous Solutions, by
G. A. Carlson and E. E. Estep, Advance Copy of paper of
Electrochemical Society Meeting, Houston, Texas May 10,
1972..
|
Primary Examiner: Tufariello; T. M.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This Application is a continuation-in-part of U.S. application Ser.
No. 290,642 filed on Sept. 20, 1972 now abandoned.
Claims
What is claimed is:
1. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing provided, at the lower end,
with an inlet port for electrolyte introduction, and at the upper
end with an outlet port for discharge of said electrolyte, at least
one of said electrodes being of uniformly reticulated open
construction so as to oppose low resistance to electrolyte flow
therethrough and having an extended surface area over which a
substantially uniform reaction-producing electrical potential is
maintained with respect to surrounding electrolyte throughout the
portion of said area in confrontation with the electrolyzing area
of the remaining electrode, and said remaining electrode being
disposed within an electrically insulator separator envelope closed
on all sides except at the edge adjacent to said outlet port
fabricated from an electrolyte-inert material which is relatively
liquid-tight in construction so as to bar the ready passage of said
electrolyte therethrough but which is permeable enough to permit
ionic passage between said electrodes.
2. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing according to claim 1 wherein
said separator envelope is fabricated from an electrically
insulative web consisting of one of the group made up of ion
exchange membranes, textile fabrics, paper and spunbonded products
having a pore size barring penetration of dendrites built up by
metal depositing on the cathode electrode of said interfunctioning
electrodes.
3. An electrolytic cell according to claim 2 wherein the
permeability to said ionic passage is equivalent to a pore size of
about 0.2 mil.
4. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing according to claim 2 wherein
said separator envelope is a spunbonded high-density polyethylene
having a density of substantially 1.6 oz./yd..sup.2 (corresponding
to 0.0054 gm/cm.sup.2), and 6 mils (corresponding to 152.mu.m)
thick, made up of individual fibers in the range of 0.2 mil
(corresponding to 5.mu.m) diameter.
5. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing according to claim 1 wherein
said uniformly reticulated electrode having an extended surface
area over which substantially uniform reaction-producing electrical
potential is maintained with respect to said surrounding
electrolyte throughout the portion of said area in confrontation
with the electrolyzing area of said remaining electrode is a
composite made of a plurality of double layers of electrically
conductive metal having good corrosion resistance to said
electrolyte, each of said double layers being opposite sides of a
flattened tubular knit formation fabricated from wire measuring in
the range of about 2-4.5 mils (corresponding to a range of about
0.51- 0.115 mm) diameter.
6. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing provided with a separator
envelope according to claim 1 wherein said electrodes and said
separator envelope are sufficiently flexible to be wound into a
layered spiral composite fitting tightly within said leak-tight
housing and being closed off centrally and peripherally to bar
electrolyte bypass flow around said electrodes and said
separator.
7. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing according to claim 1 wherein
there is interposed between said separator envelope and said
remaining electrode a relatively thin spacer fabricated from
material substantially inert with respect to said electrolyte
having a ribbed structure defining full-length open vertical
passages confronting said remaining electrode and isolating said
separator envelope from contact with said remaining electrode.
8. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing according to claim 7 wherein
said spacer constitutes an extruded netting of high density
polyethylene having a thickness of between about 15-30 mils (0.76
mm) made up of about eight strands/iunch with said strands
intersecting each other in a 90.degree. diamond pattern.
9. An electrolytic cell comprising at least two interfunctioning
electrodes disposed at a common transverse level within a
vertically oriented leak-tight housing according to claim 1 wherein
said electrodes are generally planar in form.
Description
BRIEF SUMMARY OF THE INVENTION
Generally, this invention comprises an apparatus for the
electrochemical treatment of an electrically conductive solution by
effecting separation, reaction or other processing of a given
ingredient of the solution. The apparatus is an electrolytic cell
comprising at least two inter-functioning electrodes disposed
within a vertically oriented leak-tight housing, at least one of
which electrodes has an extended surface area over which a
substantially uniform reaction-producing electrical potential is
maintained throughout that portion of the area of the electrode in
confrontation with the electrolyzing area of the remaining
electrode, the separatory (or reactive) one of the electrode pair
being relatively openn to access of electrically conductive
solutions passed through the cell, whereas the remaining electrode
is isolated frorm the solution by enclosure within an electrically
insulative envelope permitting ionic conduction while barring ready
passage of the soluution therethrough.
DRAWINGS
The invention is described with reference to the following
drawings, in which:
FIG. 1 is a partially schematic side elevation sectional view
through a first (cylindrical) embodiment of cell constructed
according to this invention,
FIG. 2 is a block diagram of an arrangement of apparatus effecting
separation of heavy metal from a dilute electrically conductive
solution as process liquid,
FIG. 3 is a somewhat schematic partially opened view of a preferred
embodiment of spiral wrap electrode structure for use with this
invention,
FIGS. 4A-4C are partially schematic representations of two
different knitted and woven electrode element designs of uniformly
reticulated open construction which can be utilized with this
invention,
FIG. 5A is a partially schematic side elevation view of a third
electrode structure of "cyclone fence" design,
FIG. 5B is an end view of the structure of FIG. 5A taken on lines
5B--5B thereof,
FIG. 5C is a partially schematic fragmentary perspective view of a
fourth electrode structure fabricated from expanded metal,
FIGS. 6A and 6B are, respectively, diagrammatic flow sheets of
continuous mode operation and of batch mode operation with solution
recycle according to this invention,
FIG. 7 is a plot of copper removal according to this invention
wherein Cu content in ppm is plotted as ordinate versus pass number
through the apparatus as abscissa,
FIG. 8 is a plot of copper removal according to this invention from
a typical industrial effluent containing Cu as adulterant wherein
Cu concentration is plotted in ppm as ordinate versus time in
minutes as abscissa,
FIG. 9 is a partially schematic perspective view of a rectangular
electrode embodiment of this invention.
DETAILED DESCRIPTION
This invention will be described primarily as applied to pollution
abatement in the cathodic removal of heavy metals at low
concentrations from dilute electrically conductive solutions, or
waste effluents; however the invention is equally applicable to
high concentration uses, anodic processing and also the
effectuation of electrochemical reactions, such as oxidation and
the like, all as hereinafter detailed.
In ordinary electrochemical processing of aqueous solutions, e.g.,
in electroplating, the ionic components are present in relatively
high concentrations and thus migrate to the electrodes at rates
consistent with practical current densities and at relatively high
coulombic efficiencies for the desired reactions. However, in many
situations, such as most aqueous wastes from industrial plant
operations, the ionic components are present at levels two or three
orders of magnitude lower than those encountered in plating
practice. If electrochemical treatments are to be applied to the
low concentrations of contaminants in such aqueous wastes, it is
found that conventional apparatus known to the art possesses very
low electrical efficiency, coupled with excessive cell residence
time, which renders it impractical to reduce the concentration of
the contaminants to the safe levels prescribed by law, e.g., one
part per million.
Research on conventional electrochemical cell operation reveals
that, when the concentrations of species involved in
electrochemical reactions are very low, other competing
electrochemical reactions can concomitantly ensue, proportionately
reducing the efficiency of the desired reactions. Thus, in aqueous
metal solutions, incidental production of hydrogen at the cathode
instead of plateout of the desired metal component, such as copper,
constitutes electrical current waste. In some instances
objectionable mixed metal deposits are also produced, instead of
the desired pure metal, whereas, in other instances, the deposits
are of inferior physical character, such as loose powders rather
than adherent films.
Attempts have been made to solve the problems described by
utilizing porous carbon electrode equivalents, including fixed and
fluidized beds of carbon particles surrounding the metallic bus
supplying current, thereby relying only on point-to-point contact
of the carbon particles to maintain conduction. Electrodes
fabricated from porous carbon have also been used, through the
pores of which the liquid electrolyte is forced to flow; however,
these suffer from high pressure drop as well as rapid blinding
resulting from accumulation of the plated-out component within the
fine pores.
Blocks of carbon can be arranged to provide a high area electrode
surface per unit volume of the bed; however, there are
disadvantages, such as: (1) carbon has a higher specific
resistivity than most metals, so that an electron current flowing
within a solid carbon matrix displays a substantial voltage drop
between the supply bus and the remote regions of electrode surface.
Non-uniformity of electrode surface potential permits concomitant
multiple electrochemical processes at different electrode regions,
instead of restricting operation to a desired single process. (2)
Conventional porous carbon has a very fine pore size of, typically,
0.001 inch- 0.006 inch (25 to 150 .mu.) and is, therefore, quickly
clogged with an electrochemical deposit. (3) Fluid flow resistance
is very high as a result of both fine pore size and low porosity
(e.g., typically, the void fraction is 0.5 or less), requiring
either high pressure operation or a very low flow rate.
Packed beds of carbon particles can, by careful selection of
particle size and shape, afford a more open deposit area for the
deposition of electrochemical product, which has a longer servce
life, before blinding, together with a lower pressure drop.
However, a new difficulty is encountered, namely, reliance on
point-to-point particles contact for the electrode circuit path,
which evinces a higher electrical resistance between the supply bus
and the bed extremities, so that current efficiency is reduced even
below that of a block carbon electrode structure. Moreover, the
void fraction of packed beds is usually only about 0.50, making
only half of the bed section available for passage of the solution
in treatment.
Fluidized beds of carbon particles display even more severe
disadvantages, in that: (1) there is a substantially higher
particle-to-particle electrical resistance, because the area of
particle contact are smaller and there exist only low interparticle
pressure, since the bed is at least partially floated by the
process stream throughput, (2) part of the time some particles are
completely out of contact with any others, contributing zero
electrode activity and, where corrosive solutions are being
treated, actually reducing overall efficiency by loss of previously
deposited material through corrosive attack, and (3) deposits built
up on individual fluidized particles change the fluidization
properties of the particles, making for objectionable inconstancy
and nonuniformity of electrode behavior.
Use of metallic extended surface electrode structures is disclosed
in U.S. Pat. No. 2,588,450 in the form of a loose stainless steel
wool pad as cathode disposed within a cylindrical basket fabricated
from an insulative material which is perforated on the periphery to
divert the electrolyte flow to an anode disposed coaxially with
respect to the cathode. The cell is utilized for the
electrodeposition recovery of gold from caustic solution, and the
electrolyte is introduced first into the center of the cathode,
after which it flows generally laterally through the basket side
wall and thence to the encircling anode, so that contact of
electrolyte with the electrodes is sequential from cathode to
anode.
U.S. Pat. No. 3,244,604 shows electrolyte flow-through woven mesh
cathodes 1/4 to 4 inches thick, used for removing metal ion
impurities which are present at concentrations below about 500 ppm
in aqueous caustic solutions. The cathodes are pads of woven nickel
wire stiffened by a nickel screen to which electrical connection is
made. Anodes and cathodes alternate in vertical array in the
direction of electrolyte flow and there is, of course, no separator
inhibiting electrolyte flow-through with respect to the anodes,
since the electrolyte has to flow with equal facility through all
of the electrodes in the vertically disposed stack.
Neither of the disclosed constructions employ simultaneous
subjection of electrolyte to both anode and cathode operation at
common transverse levels, and this is an important feature of this
invention.
THIS INVENTION
For the conduct of electrochemical reactions, the essential
electrical potential of a point in an electrolyte, hereinafter
referred to as the "reactionproducing potential," is equal to the
sum of the following:
1. The equilibrium reversible half-cell potential at the conditions
of the electrolysis (i.e., temperature, pressure, concentrations of
species at the electrode-electrolyte interface, etc.),
2. The activation overpotential, this being the extra electrical
potential over and above the equilibrium reversible half-cell
potential required to drive the desired reaction at a given rate.
This overpotential is a function of the real current density at the
electrode-electrolyte interface, and
3. Ohmic voltage drop in the electrode material. This voltage drop
is the integral sum of the ohmic voltage drops along each of the
paths of current flow leading from the electrode-electrolyte
interfaces to the said point in the electrode.
This invention advantageously influences all three of the factors
determinative of electrode point potential in the following
respects:
a. The electrodes, constructed of extremely fine filaments which
provide large surface area and enhance the mass transfer
coefficient, minimize the concentration difference between the bulk
of the electrolyte and the electrode-electrolyte interface at any
given level of operation. Correspondingly, the departure from the
equilibrium reversible half-cell potential predicted from the bulk
concentrations is minimized.
Accordingly, in a situation where competing reactions can occur,
the potential region corresponding to this undesirable condition is
less closely approached, so that the specificity, or coulombic
efficiency, for the desired reaction is enhanced.
b. The activation overpotential, which is a function of the current
density, is minimized at any given level of operation by the high
real surface area of the extended surface electrode. This
contributes even morre importantly than (a) supra to selectivity
and enhanced coulombic efficiency for the desired reaction.
c. The high conductivity of the preferred electrode materials,
together with the use of welded or other low resistance connections
to buses, minimizes ohmic voltage drop for any given level of
operation.
By reduction of each of the enumerated voltage contributions, the
power required for a given level of cell operation is substantially
reduced. A further contribution to higher power efficiency is the
close juxtaposition of anode to cathode in cells of this invention.
This is achieved by enclosing one of the interfunctioning
electrodes of the pair within a separator presenting both sides of
this isolated electrode in electrolyzing disposition with respect
to a co-functioning electrode.
Referring to FIG. 1, a cylindrical embodiment of electrochemical
cell according to my invention comprises the high surface cathodic
design shown, which is here utilized to remove heavy metals (e.g.,
Cr, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, Co, Hg and Pb) from dilute
aqueous solution. In this service, only a relatively small anode is
required and this can, accordingly, constitute a compact rolled-up
metal screen 10 which is centrally located along the longitudinal
axis of the cell enclosure which, in this instance, can typically
be a tube 11 fabricated from glass, polymeric resin or the like. If
desired, a metallic enclosure 11 can be employed, provided that the
anode supply bus 12 is electrically insulated therefrom, in which
case the inside wall surface of the enclosure constitutes
additional cathodic area.
The anode supply bus 12 of FIG. 1 is introduced in fluid-tight
relationship through a passage drilled radially in the wall of
enclosure 11 and is bent downwardly into firm contact with adjacent
plies of the screen over the full length thereof, preferably being
weld-attached thereto.
The cell of FIG. 1 is provided with two concentric cathode element
sleeves 14 and 15 coaxially disposed with respect to the common
axis of enclosure 11 and anode 10, although a greater or lesser
number can be utilized if desired. An essential feature of this
invention is the utilization of cathode elements which have a very
large extended surface, while at the same time insuring
substantially equipotential maintenance over the electrochemically
effective surface as well as presenting an open structure
permitting low pressure drop passage of electrolyte therethrough.
Bulky open mesh designs of the structures shown in FIGS. 4A and 4B
have proved highly effective because of their uniformity in
construction and because of their physical flexibility in
assembling apparatus such as those shown in FIGS. 1 and 3. However,
uniformly reticulated structures generally, such as the species
shown in FIGS. 4C and 5A-5C, can also be used.
Extended surface electrodes employed in my cells can be made from
evenly distributed highly conductive continuous run
corrosion-resistant metal filaments 2 to 4.5 mil dia., fabricated
into knitted form having a void volume in excess of about 85
percent and a surface area in excess of about 5 cm.sup. 2 /cm.sup.3
of electrode volume. While FIG. 1 details extended surface
cathodes, it will be understood that, where anodic electrodes are
to be utilized in electrochemical processing, the same electrode
structures are ideal for anodic service and, in fact, the
electrical supply leads can be simply reversed in polarity to
convert cathodic operation into anodic operation. Cathode bus
connections are effected through branched upright bus connectors
17a, weld attached to radially opposed pairs of cathodic elements
14, 15 and attached at the bottom ends to a cathodic bus 17
extending radically out through the wall of enclosure 11 to a
suitable conventional d-c source, not detailed.
As shown in FIG. 1, anode 10 is mounted within a spacer cage 20,
which can be of generally open cylindrical form constituting
circular strips 20a joined on the inside surfaces to upright strips
20b, pairs of which latter elements define vertical passages
between them for escape of gas released at anode 10. A porous
construction is preferred for spacer cage 20, a preferred material
of construction for an electrically insulating cage being a unitary
bulky polymeric product (e.g., polyethylene) produced by extrusion
through a rotating grooved annular extrusion die. The producct is
an open net-like structure made up of coarse filaments laid over
one another at right angles and bonded at crossing points.
Optionally, an electrically conductive (metal) spacer cage of
generally similar construction as that hereinabove described can be
employed, in which case cage 20 constitutes a radial extension of
anode 10 since it is in conductive contact therewith.
Enclosing the anode-spacer cage subassembly is a porous cup-like
nonconducting electrode separator 23 having its base end 23a
disposed in the direction of process liquid input which, in FIG. 1,
is shown as from the bottom as denoted by the directional arrows.
Separator 23 can be typically fabricated from spunbonded
polyethylene.
The purpose of separator 23 is to forestall electronic conduction
between anode 10 and cathode 14, 15 by isolating against
metal-to-metal electrode contact, while still furnishing an ionic
passage through the solution spanning the electrodes. The
permeability of separator 23 should be moderately high for good
ionic conductivity while still being low enough to prevent gross
mixing between the anolyte and catholyte solutions.
It is also permissible to fabricate separator 23 from electrically
insulative ion-exchange membrane materials, such that ions of at
least one component in the solution can pass through the separator,
thereby completing the ionic current path, while forestalling
direct metal-to-metal electronic conduction between the electrodes
within the cell as well as direct solution flow between electrolyte
chambers.
In summary, the cell structure described supra utilizes very large
extended surface electrodes arranged so as to (1 ) accommodate
liquid electrolyte flow therepast with only moderate pressure drop,
(2) provide short path distance from any point in the electrolyte
in process to the effective extended surface electrode in order to
facilitate rapid ionic discharge and (3) insure exceedingly uniform
potential maintenance between bus connectors 17a and cathodes 14,
15 by keeping the electrical resistivity to a minimum not only
within the electrode structure per se but also from point-to-point
throughout the electrode structure en route to buses 17.
The absolute magnitudes and relative quantitative importance of the
physical parameters of the extended surface electrodes described
depend, importantly, upon the particular electrochemial process to
be effected. For example:
1. In the cathodic deposition of heavy metals such as Cu existing
in low concentrations in dilute H.sub.2 S0.sub.4 solutions, the
current densities required for stoichiometric equivalence to the
metal content removed, with conventional liquid flow velocities
through the cathode, will be relatively low. The voltage drop
across the conductive path between supply bus 17, bus connectors
17a and the extremity of any filamentary path in the cathode mesh
is the product of current (I) and path resistance (R). If fine
filaments are used in the cathode structure in order to secure
large surface per unit volume of the mesh, the increase in R
resulting from the small diameter filaments is largely offset by
the high conductivity. Thus, the existing IR drop is relatively
minimal, so that the voltages at all points on the cathode surface
are approximately the same as the supply voltage of bus 17 and bus
connectors 17a.
2. In the cathode removal of one metal while preserving an
accompanying metal of nearly equal deposition voltage in solution,
it becomes far more important that in (1) supra to assure a close
approach to equipotential cathode surfaces, and filaments of
heavier gage are preferred in this regard.
3. When the concentrations of materials being treated are higher
than in the usual wastes, e.g., in electrowinning, the operation is
less affected by rate of mass transfer of ions to an electrode
because more ions are available and the current densities are
higher in meeting the stoichiometric requirements, so that it
becomes even more important that in (1) and (2) supra to assure
equipotential electrode surfaces. Here it is often feasible to
sacrifice some electrode open fluid space, concomitantly increasing
the conductive filament cross-sections.
4. As an extension of case (3) supra, in situations where no
deposit forms on the electrodes, e.g., anodic oxidation or cathodic
reduction forming soluble products, likelihood of plugging porous
electrodes is greatly reduced, therefore it is practicable to
substitute lower porosities in favor of heavier electrode
cross-sections.
Referring now to FIG. 3, there is shown a preferred design of cell
according to this invention employing one-piece spirally wrapped
cathode, anode electrode structures which are particularly
advantageous in providing a large electrode surface in form adapted
to insertion into a tubular, leak-tight housing, such as a conduit.
Elements in FIG. 3 corresponding to the same elements in FIG. 1 are
denoted by the same reference numerals, except that these are
primed. The external housing 11 is omitted in the showing of FIG. 3
and, to more clearly expose the anode-spacer structure, the outer
terminal portions of the interwrapped elements are pulled apart
radially to bring sizable surface portion into better view.
The cathode, or outer member shown in FIG. 3 is portrayed as having
three schematically represented cathode elements, i.e., 14',15',
and 25, each consisting of a flattened stainless steel wire tubular
knit structure pressed into tight electrically conductive contact
with its neighbor. Also, since anode 10', which can be a screen or
foil of electrochemically inert material (e.g., platinum), is
effective on both of its side surfaces, individual spacers 20' and
20" are employed on opposite sides of the anode, the pair
constituting the equivalent of spacer cage 20, FIG. 1. The
equivalent of cup-like separator 23 is the sharply folded double
ply element 23'bottom, which encases the subassembly 20', 10', 20"
on the bottom edge and both sides, it being understood that the
process liquid again is supplied from the botton, as denoted by the
directional arrow, whereas exit of treated liquid and any gaseous
products is via the top.
Positive supply bus branches 12', 12' are advantageously spaced
along the length of anode 10', to better distribute the supply of
electrolyzing current and voltage in accordance with the demand
imposed by equal incremental lengths of anode surfaces. The same
principle is applied with respect to negative supply bus connectors
17a', 17a'. The structure of FIG. 3 is shown in somewhat loosely
rolled condition, so that there exists a longitudinal passage 26 at
the inner extremity of the spiral. With tighter roll wrapping, this
bore virtually disappears, or can be plugged by a non-conducting
rod, so that no bypass through the cell is presented to process
solution flowing axially thereof.
The structure of FIG. 3 can be neatly fitted into a circular tube
enclosure and, even though it is tightly wrapped into very compact
space, only moderate resistance is interposed to solution flow.
Referring now to FIGS. 4 and 5, there are shown details of extended
surface electrode formation. Thus, FIG. 4A (and end view 4B) show a
knitted bulky wire structure, such as used in the apparatus of FIG.
1, made by interlooping adjacent courses of long length wire
denoted 27, which are weld-joined to bus connectors 17a.
Very satisfactory operation has been obtained using cathode mesh
structures of the configuration shown in FIGS. 4A and 4B employing
from about twenty to about one hundred and twenty individual wire
layers. It is a convenience to utilize flattened tubular knit
sleeves fabricated from Type 316 stainless steel wire having two
filaments in each loop, the filament diameters being any of 2 mil,
3 mil or 4.5 mil (corresponding, respectively, to 0.051 mm, 0.076
mm and 0.115 mm diameters), as marketed commercially by Metex
Corporation, Edison, N.J. Referring to FIG. 3, and utilizing a
single spacer made up of Du Pont Vexar polymeric netting measuring
about 15 mils (0.38 mm) thick and a separator 23' measuring about 6
mils (0.152 mm) thick, it was practicable to obtain a four-wrap
cathode structure, each wrap of which constituted twenty-five
overlaid double layer flattened knit sleeves, or a total of two
hundred radially disposed individual mesh layers, which assembly
fit snugly into a Pyrex pipe enclosure having an inside diameter of
51 mm.
A great variety of materials having a wide range of dimensions can
be utilized in extended surface electrodes as taught in this
invention, however, the following general combination has given
exceptionally good results: (1) spiral arrangement of electrodes as
shown in FIG. 3, (2) use of a pair of spacers 20' (one on each side
of the anode) insuring unhindered egress of any gases generated,
especially at the anode 10', (3) a separator constituting an
envelope open only along the cell electrolyte outlet side made up
of a strong web material such as ion exchange membranes, textile
fabrics, paper and spunbonded products having a permeability to
ionic passage equivalent to a pore size of about 0.2 mil, (4)
knitted mesh of continuous metallic filaments as basis for the
extended surface electrode such as shown in FIGS. 4A and 4B, (5)
electrode mesh size 6-20 courses/inch (2.4-8 courses/cm), (6)
extended surface electrode area 30-50 cm.sup.2 /cm.sup.3, (7) ratio
of electrode area cathode/anode 5-50 in service of removing trace
heavy metals from waste solutions, (8) void volume in extended
surface electrode, 90-95 percent, (9) extended surface electrode
filament size, 2-5 mils (50 .mu.), (10) treated solution flow
velocity, 1-10 cm/sec, and (11) anode current density, 5-200
ma/cm.sup.2, based on the anode' s simple projected area (i.e., the
product of width x height).
Cells constructed and operated as described supra have the
following characteristics: (1) average distance from any point in
liquid in process flowing through extended surface electrode to a
filament surface is 0.7 mm., (2) when operated at a high anode
current level (e.g., 60 ma/cm.sup.2) the maximum voltage drop from
the bus bar to the most remote location in the extended surface
electrode structure is less than about 0.160 volt and (3) the
pressure drop through a typical 51 mm. dia. "jellyroll" spiral
electrode assembly for a 75 mm. active length of the roll of
extended surface electrode (cathode) is as follows for two sizes of
filament in knitted mesh (FIG. 4A design):
TABLE I ______________________________________ PRESSURE DROP VS.
FLOW RATE ______________________________________ Flow Rate Filament
Size gpm cm.sup.3 /min. 2 mils(0.051 mm) 4.5 mils(0.115 mm)
______________________________________ 1.0 3,785 9.8 cm H.sub.2 O
3.3 cm H.sub.2 O 2.0 7,570 20.7 7.6 3.0 11,355 38.5 13.4 4.0 15,140
60.5 20.4 4.5 17,032 70.8 24.0
______________________________________
In FIG. 4C there is shown a "Turkish towel-like" wire cloth mesh
structure having continuous conductive filaments 30 and 31 forming
a looped pile extending outwardly from both sides of the woven
structure. The ends of filaments 30 and 31 are securely bonded to
the bus connector 17a by welding, brazing or the like for low
electrical resistance.
In this structure the loops are woven in as part of the warp and
constitute the wire structure of the electrode. Filler fibers 32
run at right angles to the loops and can, or need not, be
electrically conductive. The purpose of the filler fibers is
primarily to complete the weave and hold the entire structure
together for mechanical integrity. If filler fibers 32 are
conductive, it is preferred that their ends be joined to an
additional bus structure in a manner as hereinbefore described for
filaments 30 and 31 in order to insure lowest possible electrical
resistance within the cathode structure materials. Warp filament
33, and others not shown but reversed in orientation with respect
to opposite individual loops, are companionate threads binding the
filler fibers 32 in place. If the warp filament 33 is conductive,
it is preferably welded at the end to bus connector 17a. However,
it need not be conductive and, in this case, circuit contact with
the bus is dispensed with.
If a cut pile is employed instead of a loop pile, the entire
structure is preferably electrically bonded together by a dip
soldering or brazing.
FIG. 5A, a plan fragmentary view, and FIG. 5B, an end view,
together show a "cyclone-fence" type electrode structure made up of
helically interwound adjacent metal filaments, such as 39a, 39b,
weld-joined at their ends to cathode bus connector 17a. The
relatively large diameters of the helices (refer FIG. 5B)
constitute, in multi-layer assemblages, an exceptionally open
electrode structure not readily plugged by suspended solids in the
electrolyte in process.
Finally, FIG. 5C shows, in perspective, a conventional expanded
metal electrode structure in which the sheet 40 is first provided
with a line of slits and thereafter pulled longitudinally to open
the slits into diamond apertures 41. The outwardly projecting ends
are weld-joined to cathode 17a and successive sheets 40 are creased
or corrugated transversely along lines 42 to furnish the desired
separatory offset between adjacent sheet members.
The foregoing description is particularly directed to the preferred
embodiment "jelly-roll", or spiral assembly, cell construction;
however, extended surface flow-through electrodes having high
conductivity and porosity can equally well be assembled in flat
stack form, resembling a "club sandwich" arrangement. Thus, this
invention is readily applicable to commercial plate-and-frame
electrolysis cell designs and the like.
Referring to FIG. 9, there is shown a cell embodiment according to
this invention utilizing planar electrodes which, in this instance,
are disposed in vertical coparallel rectangular array with the
solution to be subjected to electrochemical processing introduced
from the bottom, as indicated by the flow directional arrow. The
containment housing is omitted in this FIG. for simplicity in the
showing as is also the right-hand cathode sub-assembly, it being
preferred to associate each anode with a pair of cathode
assemblies, one on each side.
The anode 70 is, in this instance, a solid metal plate, typically
fabricated from platinum about 25 mils thick. Anode 70 is disposed
within a separator envelope 71 of good ionic permeability, as
hereinbefore described, which nevertheless effectively isolates the
anode from ready circulation of electrolyte liquid through the
anode region. Spacers 72, also constructed as hereinbefore
described, are interposed between each face of anode 70 and the
inside faces of separator envelope 71.
Electrical conductor tabs 70a, welded, brazed or otherwise secured
in good electrical conductive joinder with anodes 70, afford
circuit connection points with an anode current supply bus, not
detailed.
The cathode sub-assemblies denoted generally at 75 are here shown,
for simplicity, as each made up of only four contiguous layers of
wire mesh construction as hereinbefore described (e.g., two
superposed pieces of knitted stainless steel sleeve construction
each providing a double layer cathode element, electrical
conduction being maintained throughout by tight physical contact
between adjacent layers). Stainless steel plates 76, typically,
40-60 mils thick, each having an upstanding tab 76a for electrical
connection with a negative polarity current supply bus 17, not
shown, are provided on the sides of the cathode sub-assemblies
remote from the associated anodes, thereby completing the overall
cell assembly.
Referring to FIG. 2 there is shown a schematic flow diagram for
treatment of a typical industrial waste stream such as, for
example, a dilute aqueous sulfuric acid solution containing small
quantities of copper sulfate, which, in this case, is
advantageously initially accumulated in hold-up tank 45. A pump 46
propels the waste stream through an extended surface electrolysis
unit of this invention, denoted generally at 47, from whence
treated product is exhausted to outfall, or other desired
destination, via discharge line 48.
For purposes of simplification of the following description, it is
assumed that unit 47 is a single cell. Then, after a given period
of operation, depending on the size of the cell of unit 47 and the
quantity of metal which is removed from the waste solution, the
cathode space of the cell becomes partially filled with the deposit
of the metal which plates out upon the elements of the cathode
structure. This increases the pressure differential across the cell
to a point where it is economical to remove the cell from service
briefly for regeneration. This is conveniently effected by the
balance of apparatus shown in FIG. 2.
Thus, a moderately concentrated acid solution, such as nitric acid,
is stored in leachate tank 51 so that, when electrolysis is halted
in unit 47, the acid is forced, by pump 52, through the
electrolytic cell of unit 47 with return back to tank 51. The
leaching solution speedily dissolves plated-out metal from the
cathode structure to produce a highly concentrated acid leachate
solution thereof. The leachate solution can be accumulated in tank
51 until a convenient time arrives to subject it to electrolytic
recovery in a conventional electrolytic recovery unit, denoted
generally at 54, to which the solution is supplied via pump 53. The
acid, stripped of its metal content, is recycled back to tank 51
via line 56 for repeated regeneration service. The conventional
direct current power supply, denoted generally at 58, is shown as
furnishing electrolyzing current independently to both units 47 and
54.
Others methods exist for removing accumulated metal from the
extended surface electrode of unit 47. Thus, a plugged cell can be
taken out of service and a fresh cathode substituted for the filled
cathode structure, after which operation is restored. Then, in a
separate system, the clogged electrode is made anodic with respect
to another electrode immersed in a small volume of concentrated
electrolyte, and metal is anodically dissolved away to produce a
highly concentrated metal solution resembling the leachate
solution.
It should be mentioned that the electrolytic cells of this
invention are well-suited to multiple use and, in this connection,
can be readily employed in either series, parallel or combination
series-parallel liquid flow convention as dictated by the
circumstances. Thus, in one instance where the waste stream
involved has a very large volumetric flow rate, a multiplicity of
small-sized cells were employed in parallel connection in
preference to designing a single large cell to carry the entire
load. similarly, in cases where a considerable quantity of metal is
to be removed from a waste solution, it can be convenient to treat
the waste stream by series flow through a succession of extended
surface electrode cells, the first of which, in order, remove the
large quantities of the metal whereas later ones in the series
clean up the traces.
The series arrangement described has the advantage that the first
cells gradually become loaded with metal, and can then be removed
for regeneration, while later cells in the series chain take over
the burden. Fresh cells progressively added at the end of the chain
constantly maintain the metal removal capability; however, the
particular point of maximum metal removal shifts progressively
along the cell chain during prolonged operation.
The electrical supply for the hereinabove described cell
arrangements is completely independent of the liquid flow
conventions. Thus, if multiple cells are operated in liquid flow
parallel, all of the potentials required for equal cell plateout
would be the same, so that electrical parallel power supply is
advantageous. Conversely, if the cells are arranged in liquid flow
series, each separate stage usually has a particular optimal
operating voltage, in which case separate power supplies of
preselected voltage output are usually preferred.
In the following examples, two general modes of operation were
utilized: (1) single pass and (2) recirculation.
Referring to FIG. 6A, the single pass mode utilized a solution
supply vessel 60 from which solution to be treated was withdrawn
via pump 61 and routed to the extended surface (ESE) electrolysis
cell denoted generally at 47', with discharge therefrom into a
receiving vessel 62 from which solution samples for analysis could
be withdrawn at will via stopcock 63.
Referring to FIG. 6B, the recirculation mode utilized a solution
supply vessel 60' from which solution to be treated was withdrawn
via pump 61' and routed to the extended surface electrolysis cell
denoted generally at 47", with recirculation therefrom, via line
64, back to supply vessel 60'. Stopcock 63', connected with vessel
60', permitted drawoff of solution samples for analysis as
desired.
EXAMPLE I
A cell was assembled as detailed in FIG. 3 incorporating an
extended surface cathode, a polymeric separator, two polymeric
spacers and a screen anode layered together as a sandwich-like
stack which was then rolled into a spiral for insertion in tubular
methyl methacrylate polymer tube.
For this test the cathode (14',15' and 25) was knitted sleeve
material 0.002 inches (50.8 microns), 2-filament, S.A.E. 316
stainless steel mesh having ten courses/in (3.9/cm). The separator
23' was Du Pont Tyvek 1058, a spun-bonded high-density polyethylene
of 1.6 oz/yd.sup.2 (0.0054 gm/cm.sup.2), about 6 mils (152 .mu. )
thick, with individual fibers in the range of 0.2 mil (5 .mu. )
dia. The spacers 20' and 20" were Du Pont Vexar 30 CDS 89, an
extruded netting of high density polyethethylene, having a
thickness of 30 mils (0.76 mm) with eight strands/in. crossing each
other at a 90.degree. angle, giving a diamond pattern. The anode
10' was 80 mesh (31.5 mesh/cm) woven platinum screen having
individual filaments 0.0042 inch (107 .mu. ) dia.
The cathode was made up of sixty folds of sleeve 2.5 inches (6.35
cm) wide (i.e., four pieces, each folded 15 times constituting 120
individual layers), weighing a total of 60 gms.
The anode dimensions were 2.5 (6.35 cm) wide .times. 6 inches (15.2
cm) long. The two Vexar spacers were the same length as the anode,
but 2.75 inches (7.0 cm) wide. A piece of Tyvek 7 (17.8 cm) long
.times. 6.0 inches (15.2 cm) wide constituted the separator. It was
folded around the spacers and the anodes and then placed on the
layered cathode. The electrodes and interleaved components were
then rolled into a tight spiral and inserted into the 2.0 inches
(5.08 cm) inside diameter methyl methacrylate polymer tube.
The solution treated was a synthetic aqueous "waste" having a
starting concentration of 20 ppm by weight of copper (added as
copper sulfate) and sufficient sulfuric acid to give a pH of 1.0.
The effluent concentration of copper was determined by atomic
absorption spectroscopy after each pass, with the copper
concentration decreasing as tabulated infra.
Operation was conducted pursuant to mode 1 (FIG. 6A) except that,
after each analysis, the treated solution was returned to vessel 60
for another pass through the cell 47'. This cycle was repeated four
times.
Operating conditions for this Example were as follows:
The solution was at room temperature and required no temperature
adjustment as a result of the electrolytic treatment.
The solution velocity (superficial) was 100 cm/min, giving a
residence time of 4.2 secs/pass.
Electrical current was supplied at a rate stoichiometrically
sufficient on the first pass to remove all copper from the
solution, and this current was maintained at the same lever (2000
ma) throughout all subsequent passes.
Table 2 ______________________________________ Effluent
Concentrations Pass No. Copper Concentration (ppm)
______________________________________ 0 20 1 8.2 2 3.4 3 1.3 4 0.6
______________________________________
EXAMPLE II
The same cell and synthetic waste employed in Example I was treated
here with operation in the same mode but at progressively higher
flow rates and using proportionately higher electrical currents.
Thus
Feed Rate, Velocity, Current Run liters/min. cm/sec. ma
______________________________________ A (Example I) 2.0 1.63 2,000
B 6.0 4.9 6,000 C 10.0 8.2 10,000
______________________________________
The corresponding effluent concentrations are plotted in FIG. 7 as
a function of the number of passes through the cell at the
different solution velocities reported.
EXAMPLE III
A cell was constructed as described for Example I, except that the
cathode consisted of twenty layers (18.1 gms total) of 316
stainless steel knitted mesh made from 0.002 inch dia. filaments
with two strands/loop and ten courses/inch.
The separator was Tyvek 1058 and the spacer was Vexar PDS 89, a
polypropylene netting 15 mils (380.mu. ) thick. Due to space
limitations, only one layer of Vexar spacer was used with the anode
in the Tyvek separator envelope. As in EXample I the anode was a
2.5 .times. 6.0 inch piece of 80 mesh woven platinum screen. The
components were layered into a sandwich, rolled into a tight spiral
and inserted into a glass tube 1.0 inch in dia.
The solution treated was a synthetic waste composed of copper
sulfate, sulfuric acid and water (copper content = 103 ppm and pH =
1.0). The cell was operated in mode 1 (FIG. 6A) by passing the
fresh copper-containing solution in approximately one liter
quantities at a flow rate of 500 cc/min, corresponding to a
superficial velocity of 1.67 cm/sec, while maintaining the current
supply at the listed levels with the results tabulated as
follows:
Table 3 ______________________________________ Effect of Current on
Copper Removal (starting with an initial Cu conc'n. = 103 ppm.)
______________________________________ Current Voltage Copper
Conc'n. (ma) (volts) (ppm) ______________________________________
100 2.1 98 200 2.4 93 500 3.4 83 1,000 4.5 67 2,000 6.6 61
______________________________________
EXAMPLE IV
A cell was assembled exactly as described in Example III, and
operated as in Example I. Two different alkaline solutions, each
containing approximately 20 ppm of dissolved copper, were
individually treated. Their compositions were as follows:
A. 17.0 ppm of copper from copper sulfate, 2.75 gms/liter of
(NH.sub.4).sub.2 SO.sub.4, pH adjusted to 8.0 with NaOH; and
B. 17.9 ppm of copper from copper sulfate, 3.0 gms/liter of
(NH.sub.4).sub.2 SO.sub.4, and 1.0 gm/liter of NaCl, pH adjusted to
8.9 with NaOH.
The solution to be treated was pumped from vessel 60 through cell
47' (mode 1) at a flow of 300 cc/min. (1.0 cm/sec superficial
velocity) in repetitive passes and current was supplied at 300 ma.
The effluent copper concentration was determined by atomic
absorption spectroscopy after each pass through the cell and is
tabulated as follows for the two synthetic waste solutions:
Table 4 ______________________________________ Effluent
Concentrations from Alkaline Electrolysis
______________________________________ Pass Solution A Solution B
No. (Cu in ppm) (Cu in ppm) ______________________________________
0 17.0 17.9 1 8.8 12.2 2 4.2 6.8 3 1.8 2.7 4 1.0 1.4 5 0.4 0.9
______________________________________
EXAMPLE V
A cell was constructed as described for Example III, except for the
following: SAE 304 stainless steel was used as cathode, no spacers
were included in the separator envelope and the anode was 80 mesh
platinum screen measuring 2 (5.1 cm) .times. 4 inches (10.2
cm).
The solution treated was a sample of an actual industrial effluent
"A". It had previously been determined that the composition of this
waste stream fluctuated over a wide range, depending upon the
particular plant operations in progress at any given time. However,
the sample tested here had an initial soluble copper content of
15.5 ppm, a pH = 2.8 and a chloride content of approximately 900
ppm. There were traces of other heavy metals, e.g., Fe, Ni and Cr,
and a heterogeneity of dissolved organics, each in low
concentrations. Sulfate was the principal anion present.
After filtering to remove suspended solids the solution was fed at
500 cc/min (1.67 cm/sec superficial velocity) in repetitive passes
to the cell supplied with applied current as listed:
TABLE 5 ______________________________________ Treatment of
Industrial Effluent "A" ______________________________________ Pass
Current Copper Conc'n. No. (ma) (ppm)
______________________________________ 0 -- 15.5 1 500 5.6 2 250
2.8 3 150 1.8 4 100 0.7 5 100 0.6
______________________________________
EXAMPLE VI
The cell of Example V was operated in the same manner as described
for Example V but used to treat a sample of a different actual
industrial plant effluent "B". This aqueous effluent was variable
and heterogeneous in composition, but, as received for test,
contained 48.5 ppm of copper, an approximately equal content of
iron and had a pH = 1.5 due largely to the presence of sulfuric
acid.
After a period of aeration to remove dissolved sulfur dioxide, the
solution was pumped to the cell in repetitive passes at a rate of
500 cc/min (i.e., 1.67 cm/sec superficial velocity). The current
fed during each pass, the effluent copper concentrations and the
attendant coulombic efficiencies are tabulated as follows:
Table 6 ______________________________________ Treatment of
Industrial Effluent "B" ______________________________________ Pass
Current Voltage Cu Conc'n. Efficiency No. (ma) (volts) (ppm) (%)
______________________________________ 0 -- -- 48.5 -- 1 1,000 3.0
15.5 82 2 500 2.5 5.4 51 3 300 2.3 2.1 55 4 200 2.2 0.9 30
______________________________________
In order to determine what disposition was made of the iron,
Example VII was conducted.
EXAMPLE VII
The cell of Examples V and VI was used to treat industrial effluent
"B" in the manner of mode 1 (FIG. 6A). Here the object was to
remove the toxic copper ions from the solution selectively, leaving
the iron in solution for either subsequent treatment or discharge.
The results of two tests, each consisting of one pass through the
extended surface cathode cell, are as follows:
Table 7 ______________________________________ Treatment of
Industrial Effluent "B" ______________________________________ Test
No. B-1 B-2 ______________________________________ Flow Rate,
cc/min 500 300 Current, ma 1500 750 Voltage, volts 4.0 3.4 Initial
iron content, ppm 48.5 48.5 Effluent iron content, ppm 49.5 49.3
Initial copper content, ppm 44.0 44.0 Effluent copper content, ppm
18.2 12.5 ______________________________________
This Example shows that the extended surface cathode was effective
over a current range of at least 2:1 as regards selective removal
of copper without accompanying iron removal. No significance is
attached to the fact that effluent iron content is slightly higher
than influent iron content, as this could be due to analytical
difficulties.
Hydrogen gassing at the cathode is, however, a natural consequence
of the relatively current supply rate of Test B-1, and it is
possible that the generation of hydrogen bubbles screened the
cathode, making it somewhat less effective than in Test B-2 as
regards copper removal. In any case, considering both Tests
together, it is clear that there exists an optimal current for a
particular solution processed in a particular cell. In this
Example, it is believed that this optimal current lies below the
Test B-2 level rather than above it, because the coulombic value
for the copper content of the as-received effluent "B" is 660
ma.
EXAMPLE VIII
The cell of Example III was operated in mode 2 (FIG. 6B), that is,
the solution treated was circulated from solution accumulation
vessel 60' through cell 47", and thence back to vessel 60', which
latter was well-stirred during the test. Samples of solution were
withdrawn via stopcock 63' for analysis at preselected time
intervals denoted.
The solution employed here was 2400 cc of a synthetic
lead-containing waste prepared by dissolving sufficient
Pb(NO.sub.3).sub.2 in water to bring the lead concentration to 23
ppm. The pH of the solution was adjusted to 1.33 with nitric acid.
The solution flow rate through the cell during the test was
maintained at 300 cc/min, while the applied current was maintained
at 500 ma. The progressive lowering of lead content is shown by the
following.
Table 8 ______________________________________ Electrolysis of
Synthetic Lead Wastes ______________________________________
Elapsed Time Lead Concentration (min) (ppm)
______________________________________ 0 23 4 13.8 8 10.3 12 9.5
______________________________________
EXAMPLE IX
The cell of Example III was operated as in Example VIII. The
initial aqueous feed solution in this case was 2,400 cc of acidic
mercuric chloride solution containing 30 ppm of mercury at a pH of
1.3 as adjusted with hydrochloric acid. The solution circulation
rate through the cell was 300 cc/min and the applied current was
500 ma. The reduction of mercury content achieved was as
follows:
Table 9 ______________________________________ Electrolysis of
Synthetic Mercury Waste ______________________________________
Elapsed Time Mercury Concentration (min) (ppm)
______________________________________ 0 30 4 15 8 10 12 5 16 2.5
20 2.0 24 1.5 28 0.7 32 0.5 36 0.4 40 0.3
______________________________________
EXAMPLE X
The cell of Example III was operated as in Example VIII. The
initial aqueous feed solution consisted of 2,400 cc of acidic
silver nitrate solution containing 20 ppm of silver at a pH of 1.35
as adjusted with sulfuric acid. The solution was circulated at a
rate of 300 cc/min and the applied current was 500 ma. The
reduction of silver content achieved was as follows:
Table 10 ______________________________________ Electrolysis of
Synthetic Silver Waste ______________________________________
Elapsed Time Silver Concentration (min) (ppm)
______________________________________ 0 20 4 9.8 8 4.2 12 2.6 16
1.7 20 1.0 24 0.8 28 0.7 32 0.6 36 0.5
______________________________________
EXAMPLE IX
A cell was constructed identical to that described in Example III,
except that 22 gms of knitted mesh were used as the cathode instead
of 18.1 gms. Operation was as in Example VIII, with the initial
feed solution being 1,200 cc of wash water from a Kodak M-6 X-ray
film processing machine. This solution contained 234 ppm of silver
and was at a pH of 4.7. The solution was circulated through the
cell at a rate of 300 cc/min. with an applied current of 1,000 ma.
The silver content of the solution was reduced as follows:
Table II ______________________________________ Electrolysis of
Film Wash Water ______________________________________ Elapsed Time
Silver Concentration (min) (ppm)
______________________________________ 0 234 5 95 10 50 15 36 20 30
25 23 30 20 35 13 40 10 ______________________________________
EXAMPLE XII
The cell of Example III was operated as in Example VIII. The
initial feed solution was a sample of an actual aqueous industrial
effluent "C". Experience showed that the effluent composition could
be expected to fluctuate, as do most industrial wastes, but this
sample contained 425 PPM of copper at a pH of 0.3. There were
negligible suspended solids present, while the dissolved solids
were in the range of 10 to 15 percent. The low pH resulted from a
mixture of hydrochloric and sulfuric acids. This solution was
circulated through the cell at 300 cc/min with an applied current
of 1,000 ma.
The reduction in copper content is shown in the plot of FIG. 8.
EXAMPLE XIII
A cell was constructed without an extended surface cathode to
demonstrate the comparative advantages of such electrodes.
Thus, two platinum foil electrodes, each 5 mils (127 microns) thick
with dimensions of 2 cm .times. 5 cm, were positioned
longitudinally 1.2 cm apart and facing each other in a 1 inch dia.
glass tube. The liquid to be treated was circulated through the
tube and past both electrodes.
The cell was operated in the manner of Example VIII (FIG. 6, mode
2) with a recirculation rate of 300 cc/min and an applied current
of 200 ma. The aqueous solution under test was 200 cc of pH = 1.7
containing initially 105 ppm of copper from copper sulfate and 10
cc/liter of phosphoric acid. Copper was removed from solution, but
at low efficiency, as shown in the following Table:
Table 12 ______________________________________ Electrolysis with
Foil Electrodes ______________________________________ Elapsed Time
Copper Concentration Current Efficiency (min) (ppm) (%)
______________________________________ 0 105 -- 60 67 3.2 120 42
2.1 180 25 1.4 ______________________________________
EXAMPLE XIV
To demonstrate a process application of extended surface
electrodes, the system shown in FIG. 2 was assembled.
Five cells constructed as described in Example I were assembled in
vertical stack or column formation, one on top of the other, to
make up the ESE unit. The liquid in treatment was pumped into the
bottom of the column and exited out the top. A line from the top of
the ESE unit carried the effluent back to the feed (hold up) tank
45 where the copper concentration was adjusted to maintain a
constant feed composition in the cell stack. Each of the five cells
was powered by a separate D-C power supply, and provision was made
for sampling the liquid stream between adjacent cells in the
stack.
The aqueous solution treated was an acidified dilute solution of
copper sulfate containing 20 ppm of copper at a pH = 2.5. Sulfuric
acid was used for pH adjustment. The solution resistivity was
adjusted to 40 ohm-centimeters by the addition of sodium sulfate.
This solution was pumped through the ESE unit at a rate of 3 gpm
(11.7 liters/min), giving a superficial fluid velocity of 9.7
cm/sec. Thus, for the five cells in series, each cell having a
working length in the flow direction of about 7 cm, the contact
time for the solution treated was approximately 3.6 seconds.
Each cell was operated at constant current by its individual power
supply. The applied voltages were low; for example, throughout the
14 hours of the run, the voltage to the first cell never exceeded
5.25 volts, even though the current 17 amps. Because lower currents
were applied to the other cells, their voltages were
correspondingly lower.
The effluent copper concentration was measured at intervals for
both the feed solution and for the solution exiting from each of
the five cells. The results are set out in appended Table 13, along
with the pressure drop (.DELTA.P) across the cells. Thus, the
column headed "Cell I" reports the copper concentration, in ppm, of
the solution exiting the first cell, the column under "II" gives
the same information for fluid exiting the second cell, etc. The
pressure drop across the first cell is .DELTA.P.sub.1, whereas the
total pressure drop across all five cells is .DELTA.P.sub.5. Of
course, the pressure drops increase with passage of time due to the
gradual accumulation of copper in the cathode structures.
Table 13
__________________________________________________________________________
MULTI-CELL ELECTROLYSIS
__________________________________________________________________________
Flow Rate: 3.0 gpm Pressure Drop, psi Feed Cell I Cell II Cell III
Cell IV Cell V * .DELTA.P.sub.1 **
__________________________________________________________________________
.DELTA.P.sub.5 Current Amp. -- 17 11 8 5 3.5 Copper Concentrations
Elapsed Time, hrs. 2 20.9ppm 15.5ppm 11.5ppm 7.7ppm 5.9ppm 4.1ppm
1.25 5.75 4 20.2 16.0 9.1 7.5 4.3 3.0 1.5 6.25 6 19.5 14.2 8.4 6.3
4.0 2.4 2.0 7.0 8 20.6 15.8 11.4 9.3 5.8 4.1 2.5 8.2 10 19.7 14.5
10.3 7.9 5.0 4.0 2.8 8.8 12 20.0 16.5 10.3 8.3 4.1 3.1 3.2 9.4 14
20.4 16.4 11.5 8.2 4.5 3.1 3.5 10.0
__________________________________________________________________________
*Pressure drop across first cell. **Pressure drop across all five
cells.
At this point in the operation of the system, the input of fresh
solution (20 ppm of Cu content) was halted and the power supplied
switched off. The column of cells was drained, flushed with water
for 30 seconds, drained again, then contacted with an acidic
leaching solution to remove the accumulated copper from the
stainless steel cathodes.
For the leach operation, a volume of 9.0 liters of approximately 20
percent nitric acid was used. It had been used once previously for
a similar leaching operation, so that its copper content was 22,800
ppm (2.28 percent) as initially fed to the cell column. The
leachant was circulated through the cell group assembly at a rate
of approximately 6 liters/min for a period of 24 minutes. At that
point, no further traces of copper were seen to remain in the cell
column and leaching was discontinued, the column was drained, then
flushed with water for 30 seconds and drained again.
Flow of the synthetic aqueous waste solution was resumed through
the cell column and electrical power again supplied to the
individual cells. The copper content of the leachate was determined
to be 40,700 ppm. Nitrate analysis of the solution showed the
leaching operation to be essentially stoichiometric.
The cycle hereinabove described was thus 14 hours of extended
surface electrode operation; drain, leach, etc., approximately 30
minutes.
The leachate was then circulated at the relatively low flow rate of
about 1 liter/min from leachate tank 51, FIG. 2, to recovery unit
54 and thence back to the leachate tank, which latter was a 5
gallon (19 liter) polyethylene jug.
Recovery unit 54 was a conventional electrolytic copper recovery
cell housed in a methyl methacrylate polymer tank 15.2 cm wide
.times. 15.2 cm deep .times. 36.8 cm long. The leachate was pumped
into the tank at one end near the bottom and flowed over a weir
12.7 cm above the tank bottom before exiting at the opposite end of
the tank. A multiplicity of alternated flat anode and cathode
plates constituted the electrodes of the recovery cell, these being
hung transverse the tank from polymeric holders, so that the plates
were at 90.degree. to the general direction of solution flow. Two
cathode plates were interposed between three anode plates, all
electrode plates measuring 10.2 cm wide .times. 12.7 cm high,
separated one from another a distance of 1.9 cm and immersed to a
depth of 10.2 cm. The anodes were 80 mesh (31.5 mesh/cm) platinum
screens made up from filaments 107 um dia. The cathodes were 16
gauge type 316 stainless steel plates.
The three anode screens were connected in parallel electrically, as
were also the two cathode plates.
Power was supplied to the electrode of recovery unit 54 from a
separate d-c supply at a rate of 22 amperes. This corresponds to a
cathodic current density of 50 ma/cm.sup.2. After 5 hours of
operation, a total of 99 gms of copper had been deposited on the
two cathodes. The copper concentration of the leachate had been
reduced to 29,800 ppm. Overall, the electrical efficiency for
recovery of copper from the leachate was 76 percent.
In similar test conducted in the same general manner, but with
mechanical agitation of the solution, electrical efficiencies as
high as 98 percent were obtained. Such solutions, after copper
depletion, are ready, after appropriate NHO.sub.3 make-up addition,
for reuse in the next leaching operation.
Example XV
Industrial effluent "G", an aqueous solution of a cationic type red
dye, containing approximately 300 ppm of that dye in addition to
0.1 percent by weight of glycolic acid, 2 percent dimethyl
formamide and 150 ppm of chloride ion, was circulated through an
energized extended surface area cell for the purpose of
decolorizing the solution.
For this test an extended surface cathode cell was assembled as
follows: the anode was 80 mesh (31.5 mesh/cm) woven platinum screen
made up from individual filaments 0.0042 inch (107 .mu.m) dia. The
anode was a single sheet measuring 2.5 (6.4 cm) .times. 5.0 inch
(12.7 cm). The cathode was 15 layers of knitted 0.002 inch (50.8
.mu.m) 2-filament, type 316 stainless steel mesh 2.5 (6.35 cm)
.times. 5.0 inch (12.7 cm), weighing 14.3 gms. A single piece of
Vexar screen, of the type described for Example III, was placed
beside the described anode into a Tyvek envelope, of the type
described in Example III, the whole subassembly being placed in a
60 mm dia. whatman extraction thimble cut off at the closed end to
form an open cylinder. The whatman extraction filter is marketed by
w. & R. Balston, Ltd., Maidstone, Kent, England, and has the
shape of a hollow cylinder closed at one extremity by a
hemispherical end. It is made from heavy paper pulp, having a
thickness of about one mm, and is used in standard laboratory
extractions by the Soxhlet technique. The cell separator layer was
cut to fit smoothly once around the cylindrical cathode with no
overlap at the ends. The Whatman filter served as a peripheral seal
against electrolyte bypassing around the electrolyzing apparatus.
The entire assembly was placed inside a 1.0 inch (2.54 cm) inside
diameter glass tube so as to give the configuation shown in FIG.
3.
To start this run, 400 cc of effluent "G" solution was placed in a
reservoir vessel 60', FIG. 6B, and recirculated (mode 2) through
the cell hereinabove described at a rate of 300 cc/min
(correspoding to a superficial linear velocity of 1.0 cm/sec).
Current to the cell was then supplied at 500 ma, which required
approximately 10 volts across the cell. Every hour, 5-10 cc samples
of the solution in the reservoir 60' were taken for
spectrophotometric analysis by measuring the absorption peak at 525
m.mu.. During the course of this experiment, the destruction of the
red-colored dye was seen to occur visually. The data taken were as
follows:
Table 14 ______________________________________ Decolorization of
Industrial Effluent "G" ______________________________________
Elapsed Time from Start Real Dye of Current Relative Dye Conc'n.
Application Conc'n. of Solution (approximate) (hrs.) in Reservoir
60' ppm ______________________________________ 0 1 300 1 0.684 205
2 -- -- 3 -- -- 4 0.113 34.2 5 0.051 15.1 6 0.025 7.5 7 0.025 7.5
______________________________________
An independent experiment confirmed that destruction of the dye
probably occurred at the cathode; however, because of lack of
knowledge of the precise mechanism of decolorization current
efficiencies were not calculated.
EXAMPLE XVI
A synthetic aqueous cyanide waste was circulated through an
energized extended surface anode cell to demonstrate the oxidative
destruction of cyanide ion through the following probable
mechanism:
2 CN.sup.-+ 4 OH.sup.-= 2 CNO.sup.-+ 2 H.sub.2 O + 4 e.sup.-, which
reaction has a potential of 30 0.970 v. vs. the standard hydrogen
electrode (refer "Standard Aqueous Electrode Potentials and
Temperature Coefficients at 25.degree.C." by A. J. de Bethune and
N.A.S. Loud).
The synthetic waste was made by dissolving sodium hydroxide to a
concentration of 0.01 N NaOH solution (pH = 12) hydroxide to a
concentration of 0.01 N NaOH solution (pH = 12). To this solution
was added NaCN to a concentration of 274 ppm, which corresponds to
145 ppm cyanide ion content.
The extended surface area was fabricated as follows: The extended
surface area anode was 80 mesh (31.5 mesh/cm) woven platinum screen
fabricated from filaments 0.0042 inch (107 .mu.m) dia. Five layers
of this screen, each formed by folding a 12 inch long piece back on
itself to form a 6 inch length, were used for the anode, forming an
assembly 2.5 (6.35 cm) .times. 6 inch (15.2 cm) and weighing 59.5
gms. The cathode was made from a single piece of the same platinum
screen measuring 2.5 (6.35 cm) .times. 6 inch (15.2 cm). A single
piece of Vexar screen of the type described in Exampale III was
placed, as a spacer, along with the described cathode, into a Tyvek
envelope separator of the type described in Example III. The whole
subassembly was placed in a Whatman extraction thimble of the type
described in Example XV and cut to the dimensions of the
subassembly in the manner hereinbefore described in Example XV. The
entire assembly was placed inside a 1.0 inch (2.54 cm) inside
diameter glass tube to give a configuration such as shown in FIG.
3.
At the start of this experiment, 1 liter of solution was placed in
feed vessel 60' and recirculated through the cell 47" in mode 2
(FIG. 6B) at a rate of 100 ml/min. (corresponding to a linear
velocity of 0.33 cm/sec.). Current to the cell was then supplied at
500 ma. Samples of 10-15 ml were taken via stopcock 63' at specific
intervals for cyanide ion concentration analysis by titration with
AgNO.sub.3 solution in the presence of
paradimethylaminobenzalrhodamine as indicator.
Table 15 ______________________________________ Destruction of
Cyanide in Synthetic Cyanide Waste Experiment No. 1
______________________________________ Elapsed Time from CN.sup.-
Concentration Start of Current Appl'n. of Solution in mins. Feed
Vessel 60' (ppm) ______________________________________ 0 145 10
125 20 129 40 117 50 115 60 116.5 80 115 100 112
______________________________________
A second experiment was made under the same conditions on a
solution prepared exactly as described for Experiment No. 1 supra,
but containing only 103 ppm CN.sup.- ion.
The data obtained was as follows:
Table 16 ______________________________________ Destruction of
Cyanide in Synthetic Cyanide Waste Experiment No. 2
______________________________________ Elapsed Time from
CN.sup.-Concentration Start of Current Appl'n. of Solution in mins.
Feed Vessel 60' (ppm) ______________________________________ -- 103
10 101 20 102 30 103 40 94.6 50 95.7 70 92.5 90 95.6 110 85 130
88.0 150 86.2 ______________________________________
EXAMPLE XVII
The cell of Example III was operated as in mode 2, FIG. 6B. The
solution treated was non-aqueous, composed of dimethylformamide in
which 53.4 gms/liter of tetraethyl ammonium perchlorate were
dissolved. The solution had a resistivity of 98 ohm-centimeters.
Copper sulfate was added to produce a copper concentration of
approximately 20 ppm. The pH of this solution was measured
initially, and readings of 8.5 and 8.8 obtained; however, it is
doubtful that these values are true pH levels, for the reason that
only a relatively small number of hydrogen ions probably existed in
the solution.
A total of 300 cc of the solution was circulated through the cell
at a flow rate of 300 cc/min (superficial velocity 1.0 cm/sec). The
applied current was 300 ma. Copper concentrations as determined by
atomic absorption were as follows:
Table 17 ______________________________________ Electrolysis of
Non-Aqueous (Dimethylformamide) Solution
______________________________________ Elapsed Time Copper
Concentration mins. ppm ______________________________________ 0 20
2 1.4 4 0.9 6 0.6 8 0.3 10 0.2
______________________________________
EXAMPLE XVIII
The cell of Example XVI, with the extraction thimble wrapping
replaced by a single wrap of Teflon fluorocarbon sheet measuring
0.031 inch (795 .mu.m) thick and 2.5 inch (6.4 cm) wide, was used
to reduce an aqueous quinone solution of approximately 100 ppm
concentration to hydroquinone by making the extended surface
electrode the cathode.
The solution was prepared by dissolving 400 mgs. of quinone (first
dissolved in methanol) in four liters of distilled water, so that
the final methanol content was 0.30 percent and the pH = 5.5.
At the start of the experiment, one liter of the solution
hereinabove described was placed in feed vessel 60' (FIG. 6B, mode
2) and recirculated through ESE cell 47" at a rate of 100 ml/min
(corresponding to a linear velocity of 0.33 cm/sec). Current was
supplied to the cell as 125 ma. Samples of 10-15 ml volume were
withdrawn via stopcock 63' at preselected intervals for
spectrophotometric analysis. The gradual formation of hydroquinone
was traced by measuring its characteristic absorption peak at 2930
A.
The results obtained were as follows:
Table 18 ______________________________________ (Experiment No. 1)
______________________________________ Elapsed Time, Quinone
Conc'n., Hydroquinone mins. ppm Conc'n., ppm
______________________________________ 0 97.5 15.0 20 68.3 36.0 40
47.5 56.0 60 34.0 67.7 80 18.5 80.7 100 11.0 108.0
______________________________________
The overall coulombic efficiency was 36 percent.
Another experiment was carried out under the same conditions as
hereinabove described, except that the current supplied to the cell
was 250 ma.
Table 19 ______________________________________ (Experiment No. 2)
______________________________________ Elapsed Time, Quinone
Conc'n., Hydroquinone mins. ppm Conc'n., ppm
______________________________________ 0 113.0 4.5 20 68.8 47.0 40
39.0 80.0 60 19.0 94.0 80 9.8 101.0 100 4.4 106.0 120 2.5 108.0
______________________________________
The overall efficiency was 26.5 percent.
EXAMPLE XIX
This Example demonstrates the necessity for spacers 20, 20' and
20", as shown in FIGS. 1 and 3, respectively.
Seven cells of the design of Example I (FIG. 3) were inserted in
vertical stack formation within a tubular glass conduit with
electrical connections made to each as detailed in Example XIV
supra. Through this extended surface electrolysis unit, the
solution treated was pumped from the bottom inlet with exit out of
the top at a rate of approximately 3 gallons/min.
The aqueous solution treated was an actual industrial plant waste
containing a varying amount of copper ions in the range of 1 to 10
ppm. The solution was filtered and adjusted to a pH of usually
about 3 before it was passed to the cells. Each cell was operated
at a current density appropriate to the copper level in the
solution at the point of cell entrance, and the current supply
ranged from about 2 to about 8 amperes/cell, giving current
densities from about 10-40 ma/cm.sup.2.
The cell stack reduced the copper concentration of the solution
treated by at least 90 percent. The stack was operated continuously
for 600 hours without loss of performance, except for one cell in
which a Vexar spacer slipped, apparently during assembly of the
cell, which permitted the Tyvek separator 23 (FIG. 1) to contact
the platinum screen anode. It was found that the Tyvek separator
degraded seriously at those points where it contacted the anode,
allowing the cell to short out before the 600 hours operation was
achieved.
The cause of the degradation is not known for certain; however, it
could have been oxidation occurring at the anode surface.
Another problem to be safeguarded against is that of electrical
shorting due to metallic dendrite growth occurring on the cathode
and extending toward the anode. Dendrite formation is somewhat
unpredictable, depending as it does on the metal ion concentration
of the treated solution, the existence of localized high current
density paths between the electrodes and, probably, other
variables.
In any case, dendrite penetration through the separator elements
23, 23', FIGS. 1 and 3 respectively, must be prevented, as it has
been found that electrical short-out dendrites can form in as brief
a period as 45 minutes when electrolyzing copper solutions
containing from 500 to 1,000 ppm Cu.
As hereinbefore described, Tyvek 1058 has worked well as a
separator material, this being a spunbonded sheet formed by the
random distribution of very fine translucent continuous fibers
(about 0.0002 inch dia.) which are self-bonded by heat and
pressure. The composite Tyvek 1058 is about 0.006 inch thick and
constitutes approximately 30 fiber layers. Under visual
examination, each of these layers presents average triangular
openings defined by any three randomly oriented fibers presenting
maximum openings measuring 0.0008 to 0.0016 inch. However, since
there is a multiplicity of superposed layers, the effective
openings for ionic transport are equivalent to devious pores of
approximately 0.00016 inch size. Separator structures, as
described, have the appearance of translucent sized book paper and
are strong enough to prevent dendrite penetration therethrough
while still permitting good electrode-to-electrode ionic
transport.
In comparison, a Tyvek 1621 sheet, which has been treated to
possess openings of about 0.012 inch maximum dimension visible to
the unaided eye, i.e., about 100 times larger than the effective
size of Tyvek 1058 sheet, did permit dendrite penetration and
consequent electrical cell short out.
* * * * *