Note: Descriptions are shown in the official language in which they were submitted.
S ~ ~
6825-3/HHHH05B
METHOD FOR THE ~EMOVAL OF FREE
AMD COMPLEX CY~NIDES FROM WATER
This invention relates to the chemical treatment
of agueous solutions such as waste water to remove dissolved
cyanides.
It is known that concentrations of a few parts per
million of soluble cyanide~ (e.g~ sodium cyanide) ~re toxic
to the microflora and microfauna which comprise the food-
chain of higher forms of aquatic life such as fish, water-
foul, and eventually man. For this reason the United States
Environmental Protection Agency (E.P.A.) has enacted strict
laws to regulate the amount of soluble cyanides which may be
discharged from any source into natural waters.
For certain industrial opera~ions, such as the
extraction of gold and silver from their ores, soluble
cyanide compounds (e.g. sodium cyanide, potassium cyanide)
are essential reagents used in the extraction process. In
earlier year~, mining and other industrial companies tradi-
tionally discharged their waste waters, containing sometimes
as much as 50-100 parts per million (ppm~ of soluble
cyanide, into streams or rivers. It was assumed that the
r~latively small concentrations of solubl~ cyanides in the
waste water would be greakly diluted, dissipated and inacti-
vated by the natural stream or river waters.
Numerous studies by ecologists, limnologists, and
envir~nmental scientists have demonstrated that concentra-
tions of free, chemically uncomplexed, cyanide ion (~N ) as
low as 1 ppm are toxic to microflora and microfauna com-
prising the food-chain of fish and other animals.
For opexations such as mining, electroplating, and
similar industries which produce large volumes of waste
waters containing soluble cyanides in concentrations in the
range 1-50 ppm or more, the E.P.A. has enacted regulations
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which prohibit the discharge of waste waters that contain
more than 0.02 ppm (i.~., 1 part in 50 m.illion~ of such
cyanides.
Compliance with this extremely low concentration
of cyanide in industrial waste waters which are discharged
to the environment has presented enormous problems to the
industries that must meet such standards.
Aeration of acidified solutions containing free
cyanide ion results in only limited removal of cyanide as
gaseous hydrogen cyanide. This method is not effective for
the xemoval of complex cyanides with metals such as zinc,
nickel, copper, cobalt, iron, etc. In acidified and aerated
solutions, these complexes gradually decompose and free
cyanide ion increases exponentially. What is urgently
needed, therefore, is a process that will remove free or
complex cyanide ions from solution as a stable precipi-tate
and which will leave no detectable soluble cyanide species
in solution. Such a process should also be rapid, economic-
ally feasible and industrially practicable. The present
invention provides a process which attains these criteria.
The use of ferrous sulfate to precipitate soluble
cyanide ion as the very insoluble compound, Prussian blue
(ferric ferrocyanide) has been known for many years. This
process has been shown to be effective in reducing the
concentration of free or complex cyanide ion from relatively
high initial concentrations (.e.g 100, 500, 1000 ppm) to
very low concentrations of total cyanide in the supernatant
solution. Unfortunately, the supernatant solution over the
Prussian blue precipitate is ~ound always to contain approx~
imately 0.5-3.0 ppm cyanide ion, depending upon the condi-
tions of treatment of the solution with ferrous ion. This
concentration of cyanide is well above that which is allowed
by current ~.P.A. regulations in waste waters.
Treatment of the solution resulting from Prussian
blue precipitation by passage through suitable ion exchange
resins has met with only partial or limited success.
Although some ion exchange resins can reduce the total
cyanide concentration from the initial 0.5-3.0 ppm to less
1 1 ~7~
than 0.02 ppm, industrial practice has demonstrated that the
efficiency of removal of total cyanide ion rapidly deterio-
rates as the active adsorption sites on the ion exchange
resin become covered. The result is that while some ion
exchange resins initially have the ability to reduce the
total cyanide concentration in the supernatant solukion fxom
Prussian blue precipitation from approximately 0.5-3.0 ppm
to less than 0.02 ppm, it is found that the ef~iciency of
these resins falls off rapidly, and the legally permitted
upper limit of 0.02 ppm cyanide is soon exceeded.
A further disadvantage of ion exchan~e resins is
their need for regeneration, to desorb the adsarbed complex
ions resulting from the treatment of the original solution
with ferrous ion. While ferrocyanide and ferricyanide ions
can usually be stripped from the resins fairly easily by the
passage of suitable concentrated eluting solutions, o-ther
complex cyanides (e.g. cuprocyanide, cupricyanide, cobalti-
cyanide) present major problems of removal, requiring ex-
tremely long and industrially impractical contact times with
the eluting solutions. In many cases it has been found that
not all of the adsorbed complex cyanide ions can be removed
by eluting solutions. This, of course, progressively re-
duces the adsorption sites and consequently the efficiency
of the regenerated resin.
There exists an urgent requirement, therefore, for
a process in which the supernatant solution resulting from
Prussian blue precipitation is treated chemically (not
physicochemically, as in ion exchange) to reduce the cyanide
concentration dependably to less than 0.02 ppm, or to
"undetectable" levels.
One object oE this invention is to prov.ide a
process by the use of which an a~ueous solution containing
free or complex cyanide ions is treated chemically to reduce
the concentration of such ions to analytically undetectable
levels.
Another object of this invention is to provide a
process by the application of which an aqueous solution
containing cyanide ions which are either uncomplexed with
11 5753 B
metals or complexed with me~al ions as coordination com
pounds are treated to reducb the concentration of such
uncomplexed or complexed cyanide ions to analytically un-
detectable levels in an industrially practicable and
economic manner.
A further objec~ of this invention is to provide a
process by ~he use of which complexed or uncomplexed c~anide
ions are treated to reduce the concentration of such cyanide
ions to analytically undetectable levels and to produce a
solution which is not toxic to aguatic or terrestrial
organisms.
A further object is to provide a process the
application of which is environmentally acceptable.
The above and other ecologically and environmental-
ly useful objects are attained by this invention which
provides a process for the removal of cyanide from an
aqueous solution comprising: treating said solution with
sufficient ~oluble ferrous ion in the presence of soluble
bisulfite ion at an acidic pH compatible with said ferrous
and bisulfite ions to react with the cyanide to produce
Prussian blue and cyanide-free reaction products.
By a compatible acidic pH is meant a p~ value
which will maintain the ferrous and bisulfite ions in a form
and state needed for the desired reaction. In the preferred
embodiment it is desirable to maintain the pH of the aqueous
solution at about 4-5.5. If the pH is more acidic, for
example less than about 4, the bisulfi~e begins to decompose
with the liberation of gaseous sulfur dioxide. If the pH is
above about 5.5, the ferrous ion begins to presipitate as
the hydroxide.
A conveni~nt method for establishing the desired
acidic pH in the aqueous solution is to add the bisulfite
prior to the addition of the ferrous ion. By making the
additions in that order ~he reguisite acidity can be
achieved which will prevent precipitation of the ferrous
on .
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Any suitable materials may be added to the solu-
tion which will provide soluble bisulfite and ferrous ion in
the solution without otherwise adversely affecting the
stability of these ions in solution. For example, where the
ferrous ion is added as a $errous salt, its anion should be
non-oxidizing.
Thus, the present process contemplates that
bisulfite ions may be added directly to the solution or the
bisulfite can be formed ln situ by adding sulfite ions which
are converted to bisulfite under the acidic conditions
employed, or the bisulfite may be formed by dissolving
gaseous sulfur dioxide in the solution. Similarly, the
ferrous ion may be added as a soluble ferrous salt such as
ferrous sulfate, ferrous chloride, ferrous ammonium sulfate,
etc.
In the preferred embodiment, after the ferrous
bisulfite has reacted with the cyanide in solution (both
free cyanide ion and cyanide in solution as a complex ion
with a metal) to form Prussian blue and other environmental-
ly harmless reaction products, it is desirable to removeexc~ss ferrous ion remaining in the solution. To this end
the solution may be treated with base to make the solution
alkaline and thereby pxecipitate residual ferrous ion. Any
suitable base which will form an insoluble ferrous precipi-
tate may be employed. For example, an alkali metalhydroxide such as sodium hydroxide may be added to adjust
the pH to about 8-8.5. Alternatively, ferrous ion may be
precipitated as a carbonate by adding, for example, sodium
or potassium carbonate to raise the pH to about 8.5~9Ø It
is preferred to adjust the pH to the alkaline ranges indi-
cated to facilitate recycling of the precipitated ferrous
ion, for industrially economic reasons. As the pH is raised
substantially higher, such as in the range of pH 9-10, it is
more difficult economically to recover and recycle the iron.
While the process is operable on solutions o any
cyanide concentration by appropriate adjustment of reactant
concentrations, in general the process will be utilized to
opera-te on solutions containing a concentration of free
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cyanide¦ion of ~bout .02 1000 ppm, and on solutions con-
taininglcomplex cyanide of the same concentration. Fre-
quently the process will operate on solutions wherein the
free cyanide and complex cyanide have a combined concentra-
tion of about .02-1000 ppm. A u~eful concentration of
bisulfite ion and ferrous ion for treating such solutions is
about 50-10,000 ppm of each of the bisulfite and ferrous
ions.
In carrying out the process it is advanta~eous to
avoid oxidation of the added ferrous ion prior to the
desired reaction. If such oxidation occurs it will unneces-
sarily consume the bisulfite reducing agent to reverse the
oxidation state of the iron.
The following examples are given in illustration
of the invention.
Example I
To prevent atmosphexic oxidation, a 2 cm. layer of
water-immiscible oil is placed on the surface of 500 milli-
liters (ml.) of water, of pH 8, containing 60 ppm freecyanide ion (as potassium cyanide). The aqueous solution is
then treated with a concentrated solution of sodium
bisulfite in an amount necessary to result in a bisulfite
ion concentration in the mixture of 300 ppm. The solution
is stirred briefly ~o insure complete mixing, then is
allowed to stand unstirred for 5-10 minutes. Concentrated
ferrous sulfate solution is then added in an amount neces-
sary to result in a ferrous ion concentration in the mixtuxe
of 200-300 ppm. The pH value of the mixture falls from 8.0
to about 5.0, and the solution becomes blue due to the
immediate formation of Prussian blue. AftPr standing for 15
minutes at 20~C, unstirred, the blue solution is then
treated with concentrated sodium hydroxide ~olution (e.g. 10
normal~ to raise the pH value to 8.0-8.5 in order to precip-
3~ itate ~errous hydroxide, which encapsulates the colloidallydispersed particles of Prussian blue. After standing un-
stirred for a further 10~15 minutes, to allow the ferrous
hydroxide-Prussian blue precipitate to form and settle~ the
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supernatant solution is then either filtered or decanted
carefully from the precipitate. The clear, colorless solu-
tion is found to contain a concentration of cyanide (free
and complex) which is undetectable when analyzed by the
so-called standard reflux method, as recommended by the
E.P.A., and as described in "Standard Methods for the
Analysis of Waste Water", 14th edition, 1977.
Example II
A 2 cm. layer of water-immiscible oil is placed on
the surface of 500 ml. of water of pH 8.0 containing lC ppm
free cyanide ion (added as sodium cyanide), and 40 ppm
ferrocyanide ion (added as potassium ferrocyanide). The
aqueous solution is then treated with 200 ppm sulfi-te ion
(added as sodium sulfite), stirred briefly to insure thor-
ough mixing, and then allowed to stand unstirred for 5-10
minutes. Ferrous sulfate, or ferrous ammonium sulfate,
solution is then added, with stirring, to give a ferrous ion
concentration of 200-300 ppm. The pH value of the mixture
falls to approximately 4~5, and the solution becomes blue
due to the formation of Prussian blue. After standing,
unstirred, for approximately 10-15 minutes at 20C, the blue
solution is treated with a saturated (or nearly saturated)
solution of sodium or potassium carbonate, to raise the pH
value to 8.5-9.0, and to precipitate ferrous carbonate which
encapsulates the colloidally dispersed particles of Prussian
blue. The mixture is allowed to stand, unstirred, for
approximately 10-15 minutes, to allow the ferrous
carbonate-Prussian blue precipitate to form and settle. The
clear, colorless supernatant solution is then either
filtered or carefully decanted from the precipitate. The
supernatant solution is found to contain a concentration of
cyanide ion ~free or complex) which is undetectable when
analyzed by the so-called reflux methodr as recommended by
the E.P.A., and as described in "Standard Methods for the
Analysis of Waste Water", 14th edition, 1977.
A detailed survey of the chemical literature
reveals very little information on the reaction of sulfite
or bisulfite ions, or of ferrous sulfite, with cyanide ion
1~$~
at very low concentrations (e.g. 1-50 ppm). While the
present reactions are as yet not fully understood, it is
believed that in addition to being precipitated as the very
insoluble compound, Prussian blue (ferric ferrocyanide), the
cyanide ion is concurrently hydrolyzed b~ the bisulfite ion,
at pH 4-6, to ammonium formate:
HCN ~ 2 H20 = HCOONH~
Thereafter the ammonium formate dissociates to
water, carbon dioxide and ammonia.
It is essential to understand that the use of
soluble ferrous ion alone, or of bisulfite ion alone, does
not completely remove free cyanide ion from the solution.
The essence of the present invention is that it is only by
the use of ferrous bisulite that free cyanide ion is sub-
stantially destroyed and removed from solution. This inven-
tion will thus provide a very useful solution to the long-
standing problem confronting industry of reducing cyanide
ion concentrations in waste waters to undetectable levels,
and allowing these waste waters to be safely discharged to
the environment. The relatively minute concentrations of
sulfite ion remaining in the waste waters pose no environ-
mental problems as sulfite anA bisulfite ions are rapidly
oxidized to sulfate and bisulfate ions by atmospheric oxygen
which is always present in the waters. In fact, the oxida-
tion of residual sulfite and bisulfite ions, if present, isaccelerated by the presence of residual ferrous ion, as this
ion is well known to be an "oxygen carrier" ~e.g. in the
hemoglohin of blood). Thus, traces of sulfite or bisulfite
ion remaining in the waste waters will rapidly disappear.
The sulfate and bisulfate ions resulting from the oxidation
of sulfite and bisulfite ions are well known not to be
environmentally harmful when in such minute concentrations
as they are in the present invention.
