Note: Descriptions are shown in the official language in which they were submitted.
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~33~13
MEANS FOR RECOVERING SILVER FROM PHOTO CHEMICALS
Back~round of the Invention
This lnvention relates to recovery of precious
metal from a solution conta.ining the same, and is
illustratively described in connec-tion with silver-
recovery apparatus for use with waste photographic-
fixer solu-tion.
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Conventional apparatus of the character
indicated employs carbon anodes which are subject
to gradual disintegration, thus providing a con-
taminant for the solution and giving rise to a
variable plating-current density, as the electrode
is consumed. The solution is not reusable, and the
recovery of silver is only in the order of 40 to 60
percent of that which is available. Moreover, the
silver that is recovered is not of the best quality,
so that further refining steps are needed. In
.
general, the shortcomings of the conventional
technique limit its use essentially to large
commercial photo processing firms and laboratories,
and the matter of anode servicing and replacement
is a major maintenance factor.
Brief Statement of the Invention
It is accordingly an object of the invention
.
to provide improved apparatus of the character
indicated.
~ ~ Another object is to provide such apparatus
~ . -
whlch~will~inherently produce substantially greater
yields of metallic silver, far exceeding 90 percent
of that available, and at the same time yielding
higher-qua1ity~silver than heretofore, namely,
silver of merchantable quality.
A;further obj~ect is to meet the above objects
with apparatus~that LS relative1y small, odor-free,
and simple tQ maintain and Which can therefore
serve institutions, such as hospitals, which only
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; 30 incidentally~must perform their photographic processing
of X-ray negatives.
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A specific object is to provide a device of the character
indicated which will permit recycled use of fixer solution from which
substantially all silver has been recovered.
Another specific object is to provide a device of the character
indicated with means whereby it can function in conjunction with automatic
or semi-automatic photo-processing installationsO
Other objects and various further features of the invention are
illustrative realized in a system for recovery of silver from waste photo-
graphic-fixer solution, the apparatus being so devised and controlled that
high purity is achieved in the reclaimed metal without contamination of the
remaining solution. As a result, the remaining solution may be recycled,
and requirements for replenishment of fixer chemical are held to minimum
quantitiesO
Accordingly, the present invention provides an electrochemical
device for recovering a precious metal from an electrolyte solution
containing ions of said precious metal, comprising a tank with a bottom and
sidewall having a predetermined upper level of liquid capacity, first and
second electrodes having radially spaced cylindrical surfaces and mounted
on a common upstanding axis, one of said electrodes being totally beneath
20~ ~ said predetermined level and above the bottom of said tank, said one electrode
being~also open at its ends for vertical passage of solution therewithin, and
impeller means operative beneath said level to develop a recirculatory flow
of liquid in said tank and toroidally about said one electrode, the direction
of operatlon of said impeller means being such as to induce upward flow in
the~space between said electrodes.
Detailed~Descrlptlon
The invention will be illustratively described in conjunction with
the accompanying drawings, in which:
~ Figure 1 is a simplified diagram s~hematically indicating
; 50 components of a silver-recovery system of the invention;
Figure 2 is a perspective view of a silver-recovery tank unit,
forming part of the system of Figure l;
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Fig. 3 is an exploded view in perspective
showing removal of the cover assembly from the
tank unit of Fig. 2, as for servicing or
inspection;
Fig. 4 is a similar view of an alternative
cover assembly;
Fig. 5 is an electrical and hydraulic circuit
diagram, to show means for implementing use of the
system of Fig. l;
Fig. 6 is a simplified view of a silver-
recovery unit, in the style of Fig. 1, to illustrate
inherent recirculatory flow by reason of electro-
plating action;
Fig. 7 is a fragmentary simplified diagram
to lllustrate a modified silver-recovery unit and
; its adaptability to use in multiple;
~;; Fig. 8 is a diagram similar to Fig. 6 to show
lnherent recirculatory flow by reason of electro-
plating action in each unit of the modification
20~ of~lg. 7;~
~`; r~ Pig. 9 is another simplified diagram to show
a further silver-recovery unit; and
Fig. 10 is an~electrical and hydraulic circuit
diagram~to~show~control means operative in the
25~modl~fication of~Fig. 7.
In Fig. 1, the invention;is shown in application
to an intermittently operative or semi-automatic
; photo-development system, such as a machine 10
customarily~installed in hospitals for development
~ of X-ray negatives, promptly after exposure. ~he
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11~3330
machine is installed in the wall 11 of a dark
room where means 12 provides loading access,
i.e., access for insertion of each exposed
negative to be developed. Within the machine
is a succession of tanks, for developer solution
at 13, fixer solution at 14, and first and
second washing or rinsing steps at 15-16. The
machine 10 will be understood to include means
(not shown) for the automatic transport of the
inserted negative, into the developer at 13,
thence to the successive baths 14-15-16, in timed
sequence appropriate to the desired processing of
the negative. The machine 10 will also be under-
stood to include drying means ~suggested at 17)
preparatory to automatic delivery of the dried
film at a delivery-access tray or door 18 outside
the dark room.
The fixer-related part of machine 10 may
, ~ :
;; include means such as a,pump 19 connected to a
20 ~ supply 20 of fixer solution, for make-up or
replenishment of the contents of bath 14, the same
being shown connected via line 21 to discharge
directly into bath 14. An overflow or exhaust
line 33, which may be operated by pump means 23,
25~ de~livers excess used (silver-laden) fixer solution
to a plating tank 25 of the invention, and tank 25
exhausts in a line 26 to a holding tank 27 forming
i part of return line 28-30, with pump means 29 for
29 recycling the use of fixer solution at bath 14.
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Basically, the plating tank 25 comprises an
upwardly open cupped body 31 having a radial flange
32 for support and fastening of the flanged rim of
a removable cover 33. The tank parts 31-32 present
electrically insulated walls, both inside and outside,
and may conveniently be of glass-fiber reinforced
epoxy or other resin, molded to desired generally
cylindrical internal contour. An open-ended cylindri-
cal anode 34 is centrally suspended from the cover 33,
one suspension point being provided with a terminal-
lead connection 35; the effective cylindrical length
L of anode 34 is fully submerged in fixer solution,
i.e., spaced from the tank bottom and beneath the
liquid level 36 established by placement of the
overflow port 26' to line 26. An open-ended cylin-
drical cathode 37 concentrically surrounds the anode
and is shown adjacent the tank wall, spaced from the
tank bottom and having an upper flange by which it
seats upon the tank-body flange 32; a lead connection
~to the cathode~flange is externally accessible at 38.
Inlet liquid from means 22-23 enters the lower part
of tank 25~at a port 39 beneath the cathode, said
port being~shown as the bent lower~end of a vertical
pipe 40 which extends above the liquid level 36 and
;Z5~ which ls preferably embedded Ln the tank body of the
course of manufacture; suoh construction will be
recognized~as permitting simple detachable connection
of hose or o~ther~supply plumbing, without having to
drain the tank~25,~ and without~weakening the anchorage
"
of port-39.~ Finally, an electric motor 41 secured to
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cover 33 includes a shaft and impeller 42 extending
concentrically within the anode, to promote a gentle
toroidal flow as long as plating potential is applied
at 35-38; preferably, the direction of such impelled
flow is as suggested by arrows, namely, downward
within the anode 34, radially outward beneath the
anode, upward between the electrodes 34-37, and
radially inward above the anode.
The holding tank 27 may be a simple covered
vessel having inlet and outlet ports 43-44 forming
part of the recycling circuit already described.
However, the periodic addition of make-up solution
from supply 2~0 creates a need for discharge of
excess processed solution, preferably at tank 27.
This need may be met by a simple overflow drain
port 45 or, if desired, automatic valve means (not
shown) may be provided to permit accumulation to an
upper level H1 before release via port 45, thus
effecting relatively infrequent discharge of the
20~ head or difference ^H between upper and lower levels
Figs. 2 and 3 show greater detail of a plating
tank which differs only slightly from the plating
tank descrlbed in connectlon with ~ig. l; for this
25~ reason, the ~same reference numbers are adopted for
the same or~corresponding parts. The inlet pipe 40
is seen to be ~nbedded in the tank-body material and
~ ~ :
to present a standard fitting 40' for detacha~le
plumbing connection~ A window 47 in cover 33 permits
,
; 30 viewing of tank contents without disturbing a secured
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closure of the tank, and spaced handles 48 on
the cover provide simplified manipulation of
the cover and all parts subassembled thereto.
In Figs. 2 and 3 there are differences in
detail (compared to Fig. 1) as to the manner of
support of the anode and cathode members 34-37,
but their placement and effective areas remain
the same. Specifically, the cathode 37 fits
closely to the inner wall surface of the tank
body 31, and is positioned just below the level
of flange 32, being held in place by spaced
.
radial bolts 49 which are above the level 36 and
which extend through the body 31; one of these
bolts 49 has an extended shank 49' and serves as
the cathode-lead terminal. The anode 34 is sus-
pended from a spider structure comprising an upper
ring 50 and spaced radially outward arms 51; ring
50 is of substantially the diameter of the anode,
and spaced~straps 52 tie the inner wall of the
~anode~to the~suspension ring 50. A projection or
bracket formatlon 51' on one of the arms 51 provides
electrical lead-connection access to the anode 34.
Mounting holes in arm9 51 locate on upstanding
studs 53, which are preferably anchored in the
~plastic body of flange 32.
;The tank assembly is;secursd by applying a
suitable gssket~(not~shown) to flange 32 before
re~istration of cover holes 54~with studs 53, at
which time stud nuts may be applied. The application
of nuts to studs 53 is found to be necessary only to
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- discourage tampering, because entirely satisfactory
performance is achieved by merely placing the
cover assembly over the tank-body assembly.
As to the impeller 42, good toroidal flow
is achieved for a variety of axial placements.
Generally, it is preferred that the impeller be
located within the anode, at least below the upper
end of the anode, and it may be as low as the
bottom surface of the tank. If the impeller is
located near the lower end of the anode, it is
desired to use the tank bottom as a stabilizing
reference for impeller-shaft rotation. Thus, in
Fig. 3, a baaring 55 such as a nylon or Mylar
bushing is shown embedded at the center of the
lS tank bottom, for guided reception of the pro-
jecting end 42' of the impeller shaft.
The drawings reflect preference for the use
of stainless sheet-metal electrodes 34-37 and
associated suspension structure. In a highly
20 ~satisfactory empl~oyment of the invention, each
electrode is of type 316 stainless steel, approxi-
mately 1/16-Lnch thick, although thickness in the
range of Q.015 to 0.150 inch will also be satis-
factory. A;cathode diameter of 16 inches is well
; 25~accommodated in a tank bore of 16-1/8 inches, and
an associated~anode of 8-inch diameter provides an
effective~rel;ationship. Of course, electrode length
:lS a function of tank~capacity; for five or ten-gallon
capacity, anode length is approximately five or ten
: ~ ~
inches, respectively, and cathode lengths are;scaled
accordingly. Titanium bolts are preferred at 49-53.
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Fig. 4 illustrates a slightly modified cover
assembly wherein the anode 34 and its suspension
structure 50-51-52 are secured by bolt means 56
to the flange of cover 33. Mounting holes 57 in
this flange register with studs 53, to secure the
full tank assembly. And the shaft for impeller 42
is short enough to be fully contained within the
included volume of the anode 34. Thus, upon removal
of the cover assembly, it may be stood to the side
of the tank body 31, resting upon the base of the
anode 34.
~ In the periodic servicing of the described
! plating-tank structures, the cover assembly is
removed and the anode assembly is removed. Access
is then presented for removal of the cathode, the
bore of which may be laden with as much as a one-
inch thickness of high-quality metallic silver; in
Fig. 3, inward brackets 53 at diametrically opposite
`: :
locations on the cathode provide lifting access for
~removal of the cathode, while positioning feet 59
main~tain a desired spacing from the tank bottom
and also provide a convenient footing when the
` cathode is removed from the tank. Metal silver is
then removed from the cathode by fracturing, as by
25 ~sharp applicatlon of one or more mallet blows to
the outer~surface of the cathode; alternatively,
the silver may~be removed~by~melting. Both electrodes
may~then be~immediately restored to service, although
as a practic~al matter, a substitute cathode will
p~robably be installed, to permit the loaded cathode to
; be shipped remotely, for'silver removal and then for reuse.
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The circuit diagram of Fig. 5 shows two
control arrangements, either or both of which
may be used in the automatic operation of the
described system. One of these control techniques,
shown available for the "up" position of a selector
switch 71, reli-es upon a timer 60, which may be
set to deliver a control pulse once for each
given selected interval, for example one pulse
every four hours. This pulse is shown connected
to a bistable flip-flop relay 61, the connection
being such that a starting signal is imparted via
delay means 62 to another flip-flop relay 63; it
being understood that passage of a predetermined
-
delay at 62 is a condition precedent to establishing
a starting condition of relay 63. Once relay 63
~ operates, excitation controls are established for
;~ the impeller motor 41 and for the plating supply
~ ~ 64 to the electrode circuit 35-38. Included in one
~:
arm of this clrcuit is current-responsive means 65,
set by selectively adjustable means 66, to produce
an;~output signal in a control~line~67 should the
detected plating-circuit current be less than a
predetermined level. Such a signal in line 67 is
used, by connection to the relays 61-63, to disable
25~;~the starting mechanism, thereby avoiding a start up
should the current level be of pre-selected insufficient
;amplltude. It will be understood that that delay at
; ;62 should be~at least sufficient to assure termination
of~ the timer pulse at 69, before excitation of the
plating cirouit. By this means, one is assured that,
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if inadequate plating-circuit current flow is
detected, then the timer 60 will be ineffective
to attempt another plating start-up, until passage
of the selected timeinterval for the next pulse,
here assumed to be another four hours.
The other control technique, which may be
concurrently operative with timer 60, but which
through selector-switch operation may be the sole
operating control for the plating circuit, relies
upon operation of the developing machine 10. As
shown, this machine includes a limit switch 68
having a probe arm poised to respond to a film
; insertion at 12. Such a switch 68 or its equivalent
is to be found in most photo-developing machines and
is relied upon, via suitable control means 69, to
operate valves or to drive displacement pumps for
the predetermined incremental supply o replenishment
developer and fixer, from make-up supplies of stock
solution; the present situation is concerned solely
20 ~ with adaptation to the fixer solution and its supply
20, which w~ be understood to be briefly drawn, to
the extent of a predetermined volume Vl by operation
of pump 19 under the control of means 69. Concurrently,
control means 69~is connected to pump means 29 to
25 ~deliver into bath 14 a predetermined volume V2 of
recycling fixer solution, fror.l the holding tank 27;
and c~ntrol means 69 is further connected to provide
~ a~starting~input~to the delay means 62. It will be
`~ understood that adjustments are made to assure a
correct proportionlng of the Lndicated volumes, such
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that the minimum necessary fresh chemicals are
drawn from the supply 20. At the same time,
means 70 responsive to fixer level in bath 14 is
operatively connected to pump means 23 to draw off
used, silver-laden fixer from tank 14, in a
quantity to maintain the desired fixer-bath level.
The used fixer is of course supplied directly
to tank 25, and it may contain enough silver
enrichment to the~contents of tank 25 to enable a
plating operation. This condition is tested by the
current-responsive means 65, already descrlbed. If
there is an inadequate current, the plating circuit
is restored to shut-down condition, but i the
current is at an adequate level, the described
toroidal 10w and plating action proceed, until
::
;~ means 65 functions;to shut down the plating through
having detected the minimum acceptable current level.
It will be seen that the invention meets all
,~
stated objects with a relatively simple and
inherently~clean plating-tank structure. The
arrangement, aontrol and choice of materials are
such that~electrodes are repeatedly reusable; more
than 98 percent of the available silver is reclaimable;
; the processed~fixer is recycled; no gases~are given
25~ off to the surroundlng space (whlch in the case of
a hospital`dark room is very confined); and no solution
: LS ~"burned" by excessive current at high voltage. For
example, ~or the indicated ~anode and cathode dimensions,
and for a relatively high plating potential of 5 volts,
:
ourrent can range up to 15 amperes without bbrning the
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silver, but preferably the current is held to a
level in the range 2 to 7.5 amperes; the maximum
current being determined by assuring sufficiently
frequent attempts at plating, as by shortening
the pulse interval at timer 60 should the fixer
bath be called upon to carry heavy and continuous
use.
While the invention has been described in
detail for the preferred forms shown, it will be
understood that modifications may be made without
departing from the invention. For example, by
reversing the polarity of plating potential applied
to the electrodes 34-37, the outer electrode becomes
the anode and the inner electrode becomes the
cathode, in which case metallic silver is plated
upon the outer surface of the inner electrode. Use
; ~ ~ of the expressions "cathode" and "anode" in applica-
tion to the electrodes 34-37 will thus be understood
, .
to be illustrative, ràther than limiting, in the
20 ~ present context.
Devélopment~of the described toroidal flow
is~inherent Ln the platlng action of the described
ele~ctroplating tank unit 25, the function of impeller
42 being merely to~produce flow in the same direction
25~ and~therefore~to~provlde a means for enhancing such
flow. And it is~helpful to briefly discuss the
inherent flow-inducement platlng function of tank 25
in~conj;unction with Fig. 6, as;an introduction to
description~of the further embodiments of Figs. 7
30~ to 10.~
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In Fig. 6, the mechanical impeller 42 has
been omitted, in order to limit discussion to
the toroidally recirculating flow-inducement
function inherent in application of plating
potential to the leads 35-38 for the concentric
electrodes 34-37 of the tank unit 25 of Fig. 1.
The flow arrows of Fig. 6 delineate the direction
of toroidal flow, namely, downward in an inner
zone within the inner electrode 34, and upward
in the annular zone between electrodes 34-37,
radially outward crossover between these zones
being possible in the bottom clearance beneath
the lower end of inner electrode 34, and radially
inward crossover between these zones being possible
in the upper region where the upper end of electrode
34 is beneath the tank-capacity level of spillway
26'. Before application of plating potential, the
: ~ : waste photo-chemicals which fill the tank provide
a relatively dense electrolyte solution containing
silver ions, and this density is locally reduced in
the annular~reglon between electrodes 34-37 by
reason of plate-out to the cathode surface. ~aving
thus:locally reduced solution density, the heavier
;solution within the inner electrode 34 gravitationally
25~ seeks:equilibrium~with remaining solution, inherently
di~splacing less-dense solution upwardly in and from
:the plating zone, to then spill over into the inner
:zone within electrode 34~ For the relatively low
current densities involved in the described plating
operation, only some of the silver ions are pIated-out
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1~43330
for each pass upward through the plating ~one.
As a result, the toroldal flow indicated by arrows
proceeds continuously until substantial exhaustion
of the silver content of the solution, and if new
waste solution is introduced via inlet 40, the
toroidal-flow process continues.
In the arrangement of Fig. 7, a plurality of
plating-electrode pairs A, B, etc. are supported in
laterally spaced relation, from each other and from
wall structure 70. The inner electrode 34' of each
pair may be as described at 34 in connection with
Fig. 1, but for purposes of initial discussion it
will be assumed to be closed internally but neverthe-
less presenting an operative outer cylindrical surface
for establishment of plating potential with respect to
the concentrically disposed cylindrical open-ended
outer electrode 37'. In contr~ast to Fig.;l, it is the
outer electrode 37' which is of length L and fully
submerged a dlstance ~ below the capaclty level
2~0 ~71 and~a dLstance '~ H~2 above the tank bottom.
Upon~applic~ation of plating potentlal, as via bus
bars 72-73 to corresponding electrodes of each pair
A~, B, etc., it is again the annular plating zone
(between electrodes~of each pair) which develops
25~ ~reduced~denslty through~plate-out action, and s~ince
; solution external~to the outer~electrode 37' is more
dense,~toroidal flow develops around electrode 37'
by reason of~the density differential and by~reason
of the freedom ~or~solution to crossover radially
inward bene~ath electrode 37', with radially outward
~,
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11~3330
spillover above electrode 37', all as indicated
by directional arrows for the plural electrode
pairs A, B, etc.
Fig. 8 illustrates the nature of double-toroidal
S flow, induced by plating action when both electrodes
34'-37' of a given electrode pair (or of each
electrode pair in Fig. 7) are open-ended, fully
immersed ~Hl below the spillway level 71 and spaced
~H2 above the tank bottom. In this case, the first
toroidal path may be as described for Fig. 6, namely
about the inner electrode 34' as the core of the
first toroid, and the second toroidal path may be
as described for Fig. 8, namely about the outer
electrode 37' as the core of the second toroid, it
being noted that the upward flow in the intermediate
annulus or plating zone between electrodes 34'-37'
is a flow that is shared by the two toroidal-flow
: :
paths. In all cases (Figs. 6, 7 and 8), it matters
not~whether the inner electrode is an anode or a
~ cathode, but preference is indicated for the outer
électrode to be~the cathode, in view of the larger
plate-out area it affords.
It will be understood that for enhancement
of the toroidal-flow action in an electrode con-
25~ figura~ion in~which~the inner electrode 34' is open-
ended~and fully immersed~and spaced from the tank
bottom, as for~the case of Fig. 8, the centra1
impeller~42~may~be~provided in the manner~discussed
for inner~electrode 3 4 in the tank unit 2S of Fig. 1.
30: ~ In such event, it iS preferred that the impelier-drive
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11~3330
means be located above the capacity level 71.
However, for the case of a closed-end inner
electrode, there is no reliance upon flow inside
the inner electrode and therefore the upper end
S of such inner electrode may be above the capacity
level 71, to permit non-corrosive electric-cable
access to an impeller motor contained within the
inner electrode. Such an arrange~ent is depicted
in Fig. 9, wherein the inner electrode 34" is of
length L' greater than the length L of the fully
immersed outer electrode 37'. ~he two electrodes
are spaced ~H2 from the tank bottom (as ln Fig. 8),
but the greater length L' of electrode 37" is enough
to place the upper closed end thereof above level
15 71. A motor 75 within electrode 37" will be under-
stood to incorporate adequate gear reduction so that
only the most gentle and slow enhancement of flow is
attributable to the action of its impeller blades 76.
In situations in which multiple pairs of
20~electrodes~A,~ B, etc. are arrayed in laterally
spaced relation, as in the tank 70 of Fig. 7, it is
desirable~that each plating zone be given substantially
equal shares of new electrolyte solution via inlet
40.~ To provide 9uch sharing, the inlet 40 in Fig. 7
25~ is~shown serving plural individual inlet-discharge
poitlts, each at the outlet of a check valve, such as
the valve 77a servLng the electrcde pair A and the
valve 77b serving the electrode pair B. It will be
understood that by appropriate design and/or adjustment,
; 30 equal-volume injections of new silver-laden fixer
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solution will occur at 77a, 77b, etc. for each
operation of the pump 23 which serves inlet 40.
In spite of efforts made, as just described,
to assure equal sharing of plating load at each
electrode pair, it is possible that one or more
electrode pairs may locally exhaust their supply
of silver-laden fixer solution before one or more
other electrode pairs are able to achieve the same
yield. This circumstance can be noted by having
separate current-responsive means 65 associated
with the plating circuit to each electrode pair,
thus enabling each such means 65 to locally terminate
plating action when the predetermined low current
level is locally achieved.
Alternatively, and as illustrated in Fig. lO,
; current-responsive means 80 associated with each
plating circuit may monitor the instantaneous level
of current to develop~a suitable current-indicating
output ;sign~aL,~and further means 81 may be connected
20~ for;response~to a plurallty of such output signals
to sense whether current consumption in any~parti-
cular~circuit exceeds a predetermined amount less
than the current~consumption in one or more other
circuits~. ~For~the case shown, in which two electrode
25~ pairs A-B are served,~the means~81 is a polarity-
sensitive difference~detector, having separate control
autpu~t connectlons tc~ first and~second operatlng
solenoi~ds of~a directional valve 82~in the~fixer
supply line~40~to~hath 7~0; and~depend1ng on which
solenoid is excited, fixer solution will be
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1143330
preferentially delivered via line 83 to the
electrode pair A or via line 34 to the electrode
pair B, it being understood that if neither
solenoid is excited, any operation of pump 23
will deliver substantially equal division of the
new silver-laden electrolyte solution, in both
lines 83 and 84. Thus, if the current differential
is observed by means 81 to be in the polarity sense
indicative of lower current consumption by electrode
pair A (than the current consumption of pair s),
and if the magnitude of this differential exceeds
the predetermined amount, then the correct solenoid
of valve 82 will be actuated to divert the next
discharge by pump 23 into the line 83, or at least
to a greater extent into line 83 than into line 84;
and when the current consumed at A is seen to rise
such that the predetermined differential at 81 no
: longer exists, then the solenoid of valve 82 will
be de-energized to allow valve 82 to return to its
20: mid-position wherein lines 83 and 84 share equally
in the flow from pump 23.
It will be understood that the principle of
:Flg. 10, where1n fresh silver-laden solution is
preferentially~routed to the electrode pair doing
2~5 ~the least work, can be applied to contexts wherein
: more than two electrode pairs are operating con-
currently. In such case, for example, automatic
electrical sensing means exist whereby any electrode
pair which is~detected to be drawing, to a predetermined
~: 30 extent, less current than the pair drawing the greatest
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current, will be automatically served with a
greater share of silver-laden solution next-
delivered by pump 23. The Fig. 10 arrangement
thus merely serves to illustrate the multiple-
pair situation wherein the number of electrodepairs is two.
The invention has been particularly described
for the specific case of recovering silver from
waste photographic fixer solution. But the invention
will be seen to be broadly applicable to the recovery
of elementary metals such as precious metals from
any electrolytic solution which contains ions of
the metal to be recovered. Electrode surfaces may
be of materials other than described, but stainless
steel is preferred for durability, for non-contamination
o~ high-quality plating accumulation, and for ease
of removal of the plating accumulation.
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