Note: Descriptions are shown in the official language in which they were submitted.
125~0~9
METHOD OF CYANIDE DESTRUCTION
This invention relates to an improved method for the
destruction of cyanide in waste waters which contain
unacceptably high levels thereof, such as effluents or
tails from gold and silver mill operations. The invention
particularly relates to those effluent treatments wherein
cyanide is oxidized using hydrogen peroxide in the
presence of copper as catalyst.
It is already known to oxidize cyanide by H2O2 in
aqueous solutions where copper is added as a catalyst.
For example, U. S. Patent No. 3,617,567 discloses a
process for the destruction of cyanide in aqueous
solutions using hydrogen peroxide and a soluble metal
compound catalyst such as Cu, which is used in a
concentration of from about 5 to 1,000 ppm. The solution
in this case must be maintained at a pH between about 8.3
and 11 and at a temperature of about 20C to 75C to
achieve the desired destruction of cyanide anions.
In such a process the cost of the copper catalyst
(which is usually copper sulphate because this is the
cheapest and most readily available copper compound
soluble in water) represents a very significant portion of
the total reagent costs. For example, the annual reagent
costs to treat 200 m3/h of reclaim water containing 10
ppm of cyanide would be about Can. $ 300,000 for
H2O2 plus copper sulphate treatment. If copper sulphate
,.
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additions were decreased by 50% the annual costs would be
reduced to Can. $ 220,000, a saving of Can. $ 80,000.
The principal obj ect of the present invention is,
therefore, to achieve a substantial reduction of copper
sulphate additions within the H202 cyanide oxidizing
systems, while performing the same or improved cyanide
destruction operation.
A further object of the present invention is to provide
a continuous cyanide destruction process which can operate
at the natural temperatures of the waste waters twithout
preheating~, i.e. between about 0C and 20C, at a
satisfactory rate.
Other objects and advantages of the present invention
will become apparent from the following description.
According to the present invention, at least a portion
of the copper sulphate reagent required for the treatment
of the waste waters to remove excess cyanide with the use
of hydrogen peroxide is withdrawn by recycling to the
treatment vessel the sludge from the clarifier into which
the waste waters are discharged after treatment. This
treatment of waste waters can be carried out at the
natural temperatures of such waste waters, namely between
ooc and 20C and a pH of about 6 to 10.5. The weight ratio
of H202 to CN normally required during treatment is
between about 2:1 and 4:1 and that of Cu to CN - between
about 0.1:1 and 2:1 or a Cu content of between about 5
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and 100 ppm. Thus, in accordance with the present
invention, rather than using fresh CuS04 as catalyst for
the treatment, at least a portion of this reagent can be
replaced by the sludge from the clarifier which, it has
been found, contains a copper compound, believed to be
copper hydroxide (Cu(OH)2), produced during the treatment
operation. This sludge can, therefore, be effectively
recycled and reused to replace all or a portion of the
required copper reagent, thereby resulting in considerable
savings as already mentioned above. It is also surprising
that such operation can be carried out at temperatures
between about 0C and 20~C rather than at higher
temperatures as indicated in U. S. Patent No. 3,617,567.
This makes it possible to operate at a natural temperature
of the waste waters, without pre-heating, resulting in
additional operating savings.
Thus, the present invention provides a novel method for
the destruction of cyanide in waste waters containing
unacceptably high levels thereof, which comprises: feeding
said waste waters into a suitable treatment vessel under
agitation; controllably introducing hydrogen peroxide into
said treatment vessel in such amount as to oxidize
sufficient cyanide and thereby reduce its content to an
acceptable level; and adding sufficient copper sulphate
into said treatment vessel to catalyse the reaction; the
waters from the treatment vessel being discharged to a
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clarifier where sludge is formed at the bottom, which
sludge is recycled to the treatment vessel in lieu of at
least part of the required copper sulphate. The operation
is preferably carried out at a pH between 6 and 10.5 and
at a natural temperature of the waste waters. If the pH
drops below the desired level, it can be raised to such
level by addition of lime or a similar alkaline material
as is well known in the art. The hydrogen peroxide can be
added as a 30-70% H202 solution of commercial grade.
The agitation of the treatment vessel may also be
carried out or assisted by air sparging.
The residence time of the waste waters in the treatment
vessel depends on many factors, such as the temperature at
which the treatment is carried out, the size of the
vessels compared to the feed rate of the solution, the
levels of cyanide in the feed and, of course, the final
acceptable level of cyanide which must be achieved by the
treatment and which, in most cases, is below 1 ppm. The
usual retention time is of the order of ~ to 1 hr.
Moreover, instead of one treatment vessel, there may be
used a plurality of such vessels in series.
The present invention will now be described with
reference to the appended drawing which illustrates
schematically the novel method.
As shown in the drawing, tank 10 is provided as the
treatment vessel and it is agitated by stirring means 11.
12~i~019
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Into this tank the waste water is fed together with
appropriate quantities H202 and CUSO4, and the tank is
agitated for a sufficient period of time to reduce the
amount of cyanide in the waste water to an acceptable
level. Once this is achieved, the waters from the tank are
discharged to a clarifier 12 to which a flocculant is also
added as is well known in the art. In the clarifier, the
sludge accumulates at the bottom which contains a source
of Cu which is believed to be Cu(OH)2 produced by
reaction of originally employed CuS04. The sludge is then
recycled to tank 10 to replace all or part of the CUSO4.
It should be noted that CuS04 can always be used to
supplement the Cu requirement if the amount of Cu in the
sludge is insufficient. Such CuS04 can be added directly
into the tank or into the sludge or waste water conduits,
as desired.
As already mentioned, instead of one tank 10, several
such tanks in series can be used and also instead of
clarifier 12 any other suitable equipment which will
; 20 produce the desired sludge can be employed. Any excess
sludge from the clarifier can be returned back to the
clarifier and/or sent to disposal as shown in the drawing.
Also, the clarifier overflow is sent to disposal or to
further treatment which may be required to remove other
impurities present in the waste waters.
The novel system may be operated batchwise or
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continuously, but the preferred mode is a continuous
operation.
EXAMPLE
Hemlo reclaim water, (CN: 2 ppm) which is the water
from the tailings pond at the Hemlo Golden Giant gold
mill, was charged into a 150 m3 tank at the rate of
350 m3/hr. The temperature of the water depends on the
time of the year when treatment takes place, but at the
time of this test it was about 10C. The pH of the reclaim
water was about 9.
A 35% solution of H202 was added at a rate of 6
liters/hr and Cu~04~5H20 was added at a rate of 3 kg/hr
(in the form of an aqueous solution). The tank was
continuously agitated with an average retention time of 25
minutes and its contents were continuously passed to a
second identical tank where agitation continued with a
further retention of 25 minutes and then to a third
identical tank where agitation continued for another
retention time of 25 minutes. From the third tank the
water was fed to a clarifier and the operation was
continued until sufficient sludge accumulated at the
bottom of the clarifier. The overflow from the clarifier
contained about 0.2 ppm CN which more than met the
environmental requirement and was discharged from the
property. When enough sludge was accumulated, the addition
25 of copper sulphate to the first tank was entirely stopped
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and in its stead the sludge was recycled to the first tank
at a rate of 80 liters/min.
This produced the same reduction of CN to about 0.2 ppm,
while entirely eliminating the use of the copper sulphate
reagent and thereby resulting in considerable savings.
Also, if there is insufficient sludge available, it can
be recycled at a lower rate so that only part of CUSO4
originally employed is added. This also would result in
substantial savings for the operation.
Obviously, various waste waters and effluents may be
treated in accordance with this invention to reduce their
CN content to an environmentally acceptable level.
