Note: Descriptions are shown in the official language in which they were submitted.
065%7Z
The present invention relates to the treatment of dilute
cyanide solutions.
In many industrial operations such as, for example, electro-
plating, articles are dipped into concentrated cyanide solutions.
After removing from the cyanide solutions, the articles require
rinsing to free them of cyanide contamination. This rinsing is
- commonly performed in flowing water and produces an aqueous efflu-
ent containing cyanide in solution. Although the concentration of
cyanide in such effluents is relatively low (typically less than
1~ 1000 milligrammes per litre), these effluents still present a
; very considerable disposal problem due to the high toxicity of
even dilute cyanide solutions.
Further, the cyanide present may be that of a heavy metal,
such as copper, zinc, cadmium etc., as in many plating solutions.
Since heavy metals are toxic, they also must be removed from the
effluent before it can be discharged to a sawer.
Hitherto, it has been the normal practice to treat effluents,
such as wash solutions from electroplating plant, in a separate
effluent treatment plant. The cyanide is u~ually destroyed by
alkaline chlorination. In thi~ proces~, alkali i8 added to adjust
the pH and then either gaseous chlorine or sodium hypochlorite is
added. The treated effluent is then allowed to stand for about half
an hour and checked for excess chlorine, the presence of which
indicates completeness of cyanide removal. The pH is then adjusted
to precipitate the heavy metals which are allowed to settle as a
- sludge of metal hydroxides. This sludge i8 subsequently removed
and disposed of by tipping, possibly after being dewatered.
This effluent treatment process has a number of disadvantages.
A separate effluent treatment plant has to be built independently
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of the electroplating or other industrial process plant. The
effluent treatment process involves delaying discharge of the
effluent for a sufficient time to ensure completeness of the
various treatment reactions. The necessary chemicals for the
treatment process have to be purchased. Further, the treatment
process produces a sludge, disposal of which is becoming increasing-
ly difficult due to environmental considerations. Also, the value
of the heavy metals in the wash solution is lost since they are
removed as worthless hydroxide sludge.
Another method of destroying cyanides in solution employs
an electrochemical treatment process. If a cyanide solution is
used as the electrolyte in an electrolytic cell, the cyanides can
be oxidized to cyanates at the anode of the cell. Cyanates are
considerably le~s toxic than cyanides. However, the cyanide solu-
tions in the wash waters from, for example, an electroplating
plant are of an extremely low concentration and tend to be inef-
ficiently destroyed in normal electrolytic cells.
In Canadian Patent No. 1,050~478 issued on March 13, 1979 to
The Electricity CouncilJ there is disclosed an electrolytic cell
having a flat or curved rigid plane electrode in a liquid electro-
lyte, the electrode having a non-smooth surface, for example being
formed of apertured material or material having surface irregu-
larities and there being provided a fluidized bed of non-conducting
particles adjacent the surface of the electrode. The effect of
the fluidized bed in conjunction with the particular form of
electrode is to break up the surface layer of electrolyte ad-
jacent the electrode and to cause mixing of the electrolyte. This
mixing of the electrolyte prevents the formation of a surface layer
depleted of ions until a much higher current density is reached
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compared with what occurs in the cell without any such mechanical
mixing. This enables the cell to be used successfully for
electrolysing substances in dilute solutions.
According to the present invention there is provided apparatus
for treating dilute metal cyanide solutions comprising an electro-
lytic cell having a cathode and an anode, the cathode and anode
having irregularly shaped effective surfaces, and being disposed
relative to one another so that the ratio between the effective
surface areas of the cathode and the anode is between 1.3:1 and
2:1, a bed of non-conducting particles in the cell adjacent the
effective surfaces of the cathode and anode, means for circulating
a dilute metal cyanide solution through the cell as the electrolyte
to flow upwardly through the bed to fluidise the bed and means
for passing a current between the cathode and anode in the cell
to deposit metal at the cathode and oxidise cyanide at the anode.
With this apparatus, the metal in the solution will be de-
pO8 ited at the cathode of the cell at the same time as the cyanide
is oxidised at the anode. Thus, the apparatus of the invention
has the significant advantage of recovering the heavy metals from
the wash solution as well as de~troying the cyanides. Because,
the effective area of the cathode is greater than that of the
anode, there i8 a higher current density at the anode than at the
cathode. These conditions tend to optimize the oxidisation of
cyanide at the anode and deposition of metal at the cathode. The
anode and the cathode may be constructed of mesh materials. Then
the difference in effective area may be achieved by the anode
having a greater open area than the cathode. The anode should be
made of an inert material such as platinised titanium or lead
dioxide coated titanium. However~ the cathode may be constructed
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of other materials as well. For example, if copper cyanide is
being electrolysed, the cathode may be of copper if a homogeneous
material is required after metal deposition thereon.
According to another aspect of the invention, there is pro-
vided a method of treating a dilute metal cyanide solution com-
prising the steps of providing an electrolytic cell having a
cathode and an anode, the cathode and anode having irregularly
shaped effective surfaces and being disposed relative to one
another so that the ratio between the effective surface areas of
the cathode and the anode is between 1.3:1 and 2:1, providing a
bed of non-conductinq particles in the cell adjacent the effective
surface~ of the anode and the cathode, circulating the dilute
metal cyanide solution to be treated through the cell as the
electrolyte to flow upwardly through the bed to fluidise the bed,
and pa~sing a current between the cathode and the anode to deposit
metal on the cathode and oxidise cyanide at the anode.
The invention may be used, for example, in an electroplating
shop where a cyanide solution is used as the plating electrolyte.
Then, the invention may be used to ~eat the effluent from rinsing
baths to destroy cyanide~ in solution therein before discharge
to a sewer. Preferably, however, the invention is used to maintain
a low level of cyanide concentration in a wash solution in a primary
treatment or rinsing bath for articles being plated, by recirculat-
ing the wash solution through the cell.
Thus, the apparatus of the invention may include a treat-
ment bath for rinaing articles contaminated with metal cyanides,
said means for circulating being adapted to recirculate dilute
metal cyanide solution from the rinse bath through the cell.
This apparatus may readily be incorporated into the
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processing line of the industrial procesæ in which the articles
become contaminated. In the electroplating example, the plated
articles are removed from the plating solution and given a primary
rinse in the rinse bath. The level of cyanide and metal ioas in
the rinse bath is maintained low by recirculating the wash solution
through the electrolytic cell and thereby oxidising the cyanide
to cyanate and plating off the metal. Further rinsing of the
articles after removal from the primary rinse bath can be carried
out in the normal way in flowing water and due to the low level of
contaminants on the articles after primary rinsing, the secondary
rinse water may have sufficiently low toxicity to be discharged
directly to the sewer.
Examples of the present invention will now be described
with reference to the accompanying drawings, in which:
Figure 1 is a schematic representation of a treatment
apparatus according to the invention, and
Figures 2 and 3 illustrate the anode and cathode respective-
ly of the cell in the apparatus of Figure 1.
In the drawings, an electrolytic cell 10 is operative to
destroy cyanide9 in a washing ~olution contained in a treatment
tank 11. The solutlon in the tank 11 is recirculatod through the
cell 10 by means of a pump 12. The cell 10 has an anode 13 and
two cathodes 14 which become immersed in the recirculated solution
in the cell. The solution pumped from the tank 11 by the pump 12
enters the bottom of the cell 10 and passes through a distributor
plate 15. The distributor plate 15 supports a bed 16 of non-
conducting particles and thi-C bed is fluidized by the upward
passage therethrough of the colution from the tank 11 under the
influence of the pump 12. The pumped solution passes from the top
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of the cell 10 through a duct 17 feeding it back to the treatment
tank 11. The bed of particles when fluidized fills the spaces
between the anode and the cathodes up to a level just below the out-
let point of the cell leading into the duct 17.
The effective cathode area provided by cathodes 14 is larger
than the effective area provided by anode 13 so that the current
density at the anode is larger than that at the cathode. This is
conueniently done by forming the anode 13 and the cathodes 14
as meshes 25 and 26 respectively (shown enlarged in Figures 2
and 3) and making the open area of mesh 25 greater than that of
mesh 26. Then, as the wash solution contains metal cyanides, both
the oxidisation of cyanide at the anode and the deposition of
metal at the cathode can be optimised. To energise the cell, a
power supply 18 is connected to pass an electrolysing current be-
tween the anode 13 and cathodes 14.
Anode 13 and cathodes 14 are conveniently formed of platinis-
ed titanium or lead dioxide coated titanium. The particles of
the fluidized bed 16 may be spherical particles formed of glass,
~-sand or plastics material or any other material which i8 inert
with respect to the electrolyte. The apertures in the meshes 25
and 26 of the electrodes are made larger than the diameter of the
particles in bed 16 so that the particles can pass through. The
distributor plate 15 may be formed with apertures or slots of
sufficiently small sizes to prevent passage therethrough of the
bed particles. Alternatively plate 15 may be a porous plate.
-In operation, articles contaminated with a concentrated
cyanide solution, for example from a plating bath, are dipped in
the tank 11 to rinse off the contaminating substance. Excess
build up of cyanide in the treatment tank 11 would reduce the
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efficiency of the rinsing operation. However, this excess build
up is prevented by recirculating the washing solution in tank 11
through the cell 10 in which the cyanides are oxidised around anode
13 to become relatively non-toxic cyanates. Further, metal such
as copper in a copper plating plant is deposited on cathodes 14.
After rinsing in tank 11 articles can be further rinsed in flowing
water without the flowing water being sufficiently contaminated
with cyanides or metals to require treatment before disposal to
the sewers.
In a first test example, the effect of repeated rinsing of
contaminated articles in the wash solution was imitated by con-
tinuously pumping into the tank 11 a made-up concentrated plating
solution contained in a vessel 20. This solution had the approxi-
mate composition: copper cyanide 26 g/l, sodium cyanide 35 g/l,
~odium carbonate 30 g/l and Rochelle Salt 45 g/l. In the test
example, the treatment tank 11 had the dimensions 16 inches by 18
inches by 30 inches. The total volume of wash solution was 100
litres. The anode was formed of a titanium mesh coated with lead
dioxide and had an overall size of 90 s~uare inches and a~ open
area of approximately 53%. Thus as both ~aces o~ the anode 13
were effective the effective anode area was 84.6 square inches.
Two copper mesh electrodes were used as the cathodes 14, one on
each side of the anode. The size of each cathode was 105 square
inches and the open area was approximately 43%. Only one face
of each cathode was effective so the effective cathode area was
120 square inches. Thus, the ratio of cathode to anode area was
approximately 1.4 to 1 producing a correspondingly higher anode
current density to cathode current density.
Before starting a test run, plating solution from vessel 20
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was added to the wash water in the treatment tank to give a copper
concentration of approximately 200 mg/1 and a cyanide concentration
of approxLmately 370 mg/l. In addition 10 g/l of sodium sulphate
was added to the solution in the treatment tank, thereby to raise
the conductivity of the wash solution and lower the power costs
of the process.
The power supply 18 was arranged and connected to set up a
potential difference of 3 volts between the anode and the cathodes.
; This voltage produced a current of 7 amps, giving an anode current
density of about 12 amps per square foot and a cathode current
density of approximately 8.4 amps per square foot. Plating solu-
tion wa~ pumped from vessel 20 into the treatment tank 11 so as to
add copper thereto at approximately 2~ mg per minute and cyanide
at approximately 46 mg per minute. The test was continued for
seven hours. The level of copper in the treatment tank at the
start of the test was 207 mg/l and after seven hours was reduced ~;
to 163 mg/l. The cyanide ConCentratiQn was reduced from 370 mg/l
to 298 mg/l. Thus it can be seen that, in this example, the
electrolysis had not only kept pace with the addition of copper
cyanide and sodium cyanide but had in fact reduced their concen-
trations in the treatment tank. During the seven hour test period,
12.8 g of copper were removed and 26.7 of cyanide destroyed by
49 ampere hours of electricity. The copper recovered was of high
purity.
In a second test example, a silver cyanide solution was
provided in the vessel 20 instead of copper cyanide. The solution
contained 90.9 grams per litre of -cilver cyanide salts, 85.86
grams per litre of potassium cyanide plus small quantities of two
commercial brighteners. The two cathodes were formed of copper
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mesh as for the first example, but the anode was formed of a
platinised titanium mesh.
The apparatus was run for seven hours with solution being
added ~o the treatment tank 11 at an average rate of 82 mls/hr.
The power supply 18 was set and the open area of the mesh electrodes
was such that the anode current density was 10 amps per square
foot and the cathode current density was 6 amps per square foot.
- During the run, the cyanide concentration in the treatment tank
was an average value of 469 ppm and the silver concentration 146
ppm. The pH was 9.6. In total 575 mls of solution from vessel
20 were added, 4.~54 grams of high purity silver recovered and
7.973 grams of cyanide destroyed.
A third test example employed a zinc cyanide plating
solution from an electroplating shop. The solution contained
52.98 grams/litre of zinc and 42 grams/litre of cyanide. A total
of 320 mls of this solution was added to the treatment bath over
a 5 hour run. The zinc concentration was 268 ppm at the start
of the run and 136 ppm at the end; the respective figures for
cyanide being 343 ppm and 306 ppm. The current efficiency for
~0 zinc recovery was 49.8~ and the rate of cyanide destruction was
0.34 grams/ampere hour.
In practice the rate of addition of cyanides to the solution
in the treatment tank i8 unlikely to remain constant and in order
to simulate this a further experiment was carried out in which
the rate of addition of the strong cyan~ e solution was varied.
In addition two large volumes of strong cyanide solution were
added to simulate a 'shock' loading.
The results are presented in the table.
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RATE OF STRONG TOTAL VOLUME
TDME (hrs) SOLUTION ADDEDOF STRO~G Zn CONCn CN CONCn
Lml/h~ SOLUTION ADDED (ppm)- (pp~)
97 0 136 306
7 91 680 99 Not measured
24 - 2230 90 289
At this point 500 mls of the strong solution was added and then the
rate of addition via the peristaltic pump was also increased
24 248 2730 354 499
29 17 3970 264 780
49 - 4290 3 81 -~
At this point 200 mls of the strong solution was added
49 87 4490 109 165 :
77 ~ 6930 166 320 ~
The overall current efficiency for zinc deposition over : ~ -
the 77 hours was 39% and the rate of cyanide destruction O. 376 g ~ :
CN /A~. -
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