Note: Descriptions are shown in the official language in which they were submitted.
5~3
This invention rela~es ~o a novel electrochemical
process for oxidizing thiocyanate ~SCN ). In particularp
this invention relates to a process fnr recovering
cyanide (CN ) from aqueous solutions containing thio-
cyanate by controlled partial electrooxidation ofthiocyanate.
Aqueous solutions containing thiocyanate arise
from many industrial processes, the principal sources
being hydrometallurgical pxocessing of gold and silver
10 ores and concentrates and certain unit operations related
to base metal processing. Very large volumes of effluent
containing somewhat lower levels of thiocyanate emanate
from coking operations either from the quenching waters
or gas cleaning installations. The refining of petroleum
15 produces dilute thiocyanate solutions and thiocyanate is
a common component of many inorganic waste streams
generated by the chemical industry. Waste effluents
rontaining thiocyanate are environmentally objectionable
because in the natural environment thiocyanate is o~idized
20 by various pathways yielding highly toXic cyanide com-
pounds.
It is helpful to consider an example of a
typical thiocyanate containing waste li~uor that could
be treated by the present process. In gold recovery
25 by cyanidation of sulfidic concentrates obtained by
Eroth flotation of copper ore tailings, the waste liquor
effluent may contain 1000-1200 milligrams of CN per
litre and 1200-1400 milligramsper litre of SCN . The
presence of thiocyanate in the effluent represents a
30 significant loss of reagent cyanide.
The formation of thiocyanate is a result of the
release of sulfide (S2 ~ present in compounds of copper,
ironr nickel and other metals during the cyanidation
S23
leaching of the tailings. Sulfide undergoes chemical
oxidation in the oxygen rich leach liguor to form a
series of oxysulfur species including thiosulfates and
thionates. It is believed that thiocyanate is formed by
reaction of cyanide with thionates. A reaction sugyested
for the formation of thiocyanate by the action of tri-
thionate ion (S302 6) with cyanide is shown in equation
11) .
(1) S3O 6 + CN SCN ~ S2O6
In addition to the irreversible consumption of
reagent cyanide~ there is evidence to suggest that the
presence of thiocyanate in gold cyanidation solution
inhibits the oxidation of gold and therefore retards its
solubilization. This effect could possibly be due to the
formation of unstable gold sulfides on the metallic gold
surface thereby reducing the rate of mass transport of
the reactants, cyanide and dissolved oxygen resulting in
a reduction of the gold leaching rate. A common practice
in gold mills which serves to maintain the thiocyanate
at an appropriate and acceptably low level is to discharge
up to 20% of the thiocyanate fouled leach liqu~r from the
cyanidation circuit pex day. The remaining liquor is
then regenerated by additio'n of reagent cyanide. Let
us assume for the purposes of this example that the volume
of fouled leach solution discharged per day is 250 metric
tons. This represents approximately 350 kilogxams of free
and complexed cyanide per day.
Another source of waste effluent occurs in the
processing of a concentrate fraction obtained from complex
zinc-copper-lead sulfide ores. In this example, it is
necessary to use a cyanide concentration of twenty times
~2~5~3
the conventional level in order to effect dissolution
of contained silver values. Under these conditions,
it is found that a significant fraction of the cyanide
is converted to thiocyanate. The barren discharge
solution can be acidified allowing the expurgation of
cyanide as hydrocyanic acid ~HCN). The cyanide depleted
residual acidic solution may contain up to 1000 milli-
grams/litre of thiocyanate. The silver recovery process
may produce up to 1800 kilograms of thiocyanate pex day.
The two examples given above demonstrate the
large quantity of thiocyanate bearing waste liquor
produced by cyanidation of sulfide ores and concentrates.
The conventional method of processing this type of effluent
(aside from natural oxidation in holding ponds which is
reported to be relatively slow when compared to the
natural oxidation of cyanide) is by chemical oxidation
using aqueous hypochlorite or using chlorine gas and
aqueous caustic - the latter is usually termed alkaline
chlorination. The stoichiometry for the alkaline
chlorination of thiocyanate to cyanate (CNO ) and sulfate
(SO4 ) is often represented by equation ~2).
(2) SCN ~ 4C12 + 10 OH -~ CNO ~ 8Cl ~ So2 + 5H20
The cyanate species (CNO ) may undergo further
oxidation with additional chlorine and base but will
also dissociate via a hydrolysis reaction producing in
receiving waters, ammonia and carbonate. Using the
stoichiometry of equation (2), an estimate of the
chemical requirements can be made for treating by
conventional means the thiocyanate contained in the
effluent of example 1. If a typical 10% reagent excess
is assumed, approximately 0.85 - 1.0 metric tons per day
~2~5~3
of chlorine is required together wi~h 2.3 - 2.7 metric
tons of sodium hydroxide per day (a portion of the
base requirement may already be available in the effluent).
The treated waste would contain approximately 2.4 - 2.8
metric tons per day of sodium chloride which often is
unacceptable in receiving waters. For the purposeSof
comparison, the chemical re~uirements for oxidation of
300 kilograms per day of cyanide would be 0.9 metric
tons per day of chlorine and 1.0 metric ton per day of
sodium hydroxide. The s~oichiometry of the alkaline
chlorination of cyanide is given by equation (3).
(3) CN + C12 ~ 20H -~ CN0 + 2C~ ~ H20
These es~imates of reagent requirements indicate that
the oxidation of thiocyanate by chemical means is an
inherently expensive and hazardous proposition and is
generally regarded a~ being much more expensive than
alkaline chlorination of the cyanide which often
accompanies the thiocyanate oxidation.
It is an object of the present invention to
provide a process whereby thiocyanate can be electro-
chemically oxidized more economically than by conven-
tional means and to recover, for credit and reuse,
cyanide which forms as an intermediate product of the
electrooxidation. The process of the present invention
can be carried out on a batch or continuous basis with
a variety of effluent compositions. With many thio-
cyanate effluents no chemical pretreatment such as p~
adjustments or adjustment of the buffer index or capacity
of the effluent before electrochemical treatment is
required. Also, when thiocyanate or cyanide is treated
in the conventional manner by chemical oxidation, the
5 ~
waste contains a large amount of sodium chloride and may
very well contain undesirable levels of free chlorine or
sodium hydro~ide ~rom chemical overdosage. In additiony
when treated in the conventional manner, the ~olume of
the effluent may be considerably increased by the large
volume of reagents added.
A process for the racovery of cyanide from thio-
cyanate, said process comprising introducing an a~ueous
solution containing thiocyanate into a suitable electro-
chemical reactor, applying a direct current electricalpotential to said reactor, aarrying out a reaction under
controlled conditions around room temperature for an
appropriate time period so that during the period
shortly after the electrochemical reaction begins and
for the remainder of said process, the p~ of the a~ueous
solution is maintained in an acid range to facilitate
conversion of a major proportion of the thiocyanate to
cyanide and recovering the cyanide so formed~
Pre~erably, the process of the present invention
includes the steps of introducing the aqueous solution
into th~ electrochemical reactor at a temperature
around room temperature and carrying out the reaction
without significant heat input.
Still more preferably, the pH of the aqueous
solution shortly after the electrochemical reaction
begins, is maintained in the range of 1 to 4.
There is further provided a process for electxo-
chemically oxidizing thioqyanate. An a~ueous solution
containing thiocyanate ions is introduced into a suit-
able reactor. A direct current electrical potential isapplied to said reactor to convert the thiocyanate ions
to relatively harless reaction products, while main-
taining the pH of the solution in a range ~rom 10 to 12.
~Z~$5;~3
Whether the pro~ess in accordance with th~ pxesent
invention i8 utilized to recover cyanide or to con~ert
the thiocyanate solution into relatively harmless
reaction products, as described abo~e, depends on the
level of thiocyanate prese~t in the efflu~nt. A waste
liquor with a high concentration of thiocyanate would
normally be treated under conditions to allow maximum
recovery of the intermediate cyanide formed during the
electrooxidation process. ~owever, if the waste liquor
contains only low levels of thiocyanate, ~wo options for
processing would be possible. The dilute thiocyanate
containing li~uor may be completely electro-
$5Z3
oxidized producing an environmentally acceptable wasteor the dilute thiocyanate containing liquor may be
concentrated by a convenient physical or chemical method.
The concentrated thiocyanate solution then may be treated
by the method of the presen~ invention which allows for
cyanide recovery.
In discussing the invention in greater detail,
it is helpful to re~er to the possible electrochemical
reactions that occur. In the elec~rochemical treatment
of thiocyanate, electrooxidation of thiocyanate occurs
at anodic surfaces, and at cathodic surfaces electro~
reduction of hydrogen ion occur to produce hydrogen gas.
If the thiocyanate solution contains other electrooxidizable
species such as cyanide, thiosulfate, thionates, etc.,
the reactions at the anodic surfaces will consist of a
number of parallel electrooxidation reactions. Further,
the parallel electrooxidation of water (or hydroxyl ions)
will also occur at anodic surfaces. Similarly, if the
thiocyanate solution contains platable metals such as
copper, zinc, nickel etcr, the reactions at the
cathodic surfaces will consist of number of parallel
electroreduction reactions comprising the simultaneous
production of hydrogen and the cathodic deposition of
metals. For the purpose of explaining the electro-
oxidation of thiocyanate it is useul to consider thatthe solution is essentially a pure thiocyanate solution.
Since the cyanide moiety in thiocyanate can be
anodically converted to a series of products such as
cyanide ion, cyanate ion, ni~rogen gas and carbon dioxide
or carbonate and bicarbonate ion, it is helpful to
considex the electrooxidation reactions in sequence.
Although the stoichiometrics of the various thiocyanate
reactions have not been uneguivocally established,
~L2!t~523
-- 8 --
considerable analysis of anodic products of electro-
oxidation of thiocyanate indicates that under a range
of electrolysi~ conditions the fate of thiocyanate may
be represented by the following equations:
s
Electrooxidation of SCN to CN and SO4
(4) SCN + 4H20 ~ CN + SO4 + 8H ~ 6e
Electrooxidation of SCN through to CNO and So2
2--
(5) SCN + 5H~O-~ CNO ~ SO4 + lOH + 8e
Electrooxidation of SCN throuqh to CO2r.N2 and SO~
(6) SCN + 6H20~ O-5N2 + C2 + 12H + SO4 + lle
The above reactions represent stoichiometries and ~he
form of the species in solution will, of course, depend
on the pH. For example, cyani.de in acidic solution will
be present almost entirely in the neutral HCN form while
in highly basic solution it will he present almost
entirely as CN ion. Similarly, the weak base sulfate
ion will partially protonate in acidic solutions, and
except in low pH solutions, carbon dioxide will be
present as a mixture of bicarbonate and carbonate ions.
The stiochiometry of the anodic production of
oxygen gas by the electrooxidation of water (or hydroxyl
ion) is represented bY equation (7) or equation (8)
~7) 2H2~ 2 + 4H + 4e
(8) 4 -~ + 2H2 + 4e
(g)
.523
In the absenee of electroreducible species sueh as
plata~le metals, the predominant reaciion at the
cathode is the production of hydrogen gas by the
electroxeduetion of hydrogen (hydronium) ion or,
equivalently, from the stoichiometric viewpoint, the
electroreduction of water. The reaction may be written
as follows
(9) 2H20 2e--~H2(g)
From the standpoint of recovering cyanide from
thiocyanate, the relevant electrode reactions are (4)
and ~8). From the standpoint of converting thiocyanate
to relatively nontoxic cyanate and to nontoxic nitrogen
gas and carbon dioxide, the relevant electrode reactions
are respectively (5) and (9) and (6) and (9). The anodie
formation of oxygen gas operates in parallel with all
thiocyanate anodic reactions. At high thiocyanate
concentrations,the current efficieney for oxygen pro-
duction is relatively low. At low thiocyanate concen~trations (and cyanide),oxygen production becomes the
predominant anodic reaction.
The overall electrochemi~al cell reaction leading
to the production of cyanide from thiocyanate is obtained
b~ combining e~uations (4) and (9) to yield reaction
equation (10),
(10~ SCN + 4H20-~ CN + H2S04 + 3H~
When considering the overall reaction (10) and assumin~
a current efficiency of 100% ~that is no other anodic
and cathodic reactions of significance are occurring),
there is a net acid production of 0.33 moles o H+ per
$S~3
-- 10 --
Faraday o charge through the cell. Therefore, as the
electrochemical processing of thiocyanate solution
proceeds the solution tends ~o become more and more
acidic. Reaction (10) stoichiometry has been verified
by analysis for thiocyanate, cyanide and acid during
the course of electrolysis.
~ he production of acid is beneficial from the
standpoint o~ the specific cyanide yield since (except
where the thiocyanate solution has a high buffering
capacity) it has the efect of preserving the cyanide
produced from undergoing further rapid electrooxidation
to cyanate or through to nitrogen gas and carbon dioxide~
I~itially, the conversion of thiocyanate at the anode
can be represented by the reaction ~4). When the thio-
cyanate solution does not have a high huffering capacityin the acidic direction, the large amount of acid
produced (8 moles of H~ per mol of cyanide produced) will
tend to cause a substantial decxease in the pH of the
anolyte ~olution adjacent the anode surace. Similarly,
~0 the hydroxyl ion produced by the cathodic reaction will
increase the p~ în the catholyte adjacent the cathode
surfaces although this effect will be resisted if the
thiocyanate solution has substantial buffering capacity
in the basic direction. This suggests that an acidic
anode boundary layer and a basic cathode boundary layer
may exist.
It is the establishment of an acidic anode
bollndary layer which is believed to he the main reason
why the cyanide product is protected from rapid electro-
oxidation at the anode. It has been established thatthe free anionic CN is much more easily electrooxidi~ed
than the neutral protonated HCN form of cyanide. As
thiocyanate is ~lectrooxidized at the anode to produce
~2~SZ3
cyanide ion, the cyanide ion is immediately protonated
by the anodically produced acid. Conse~uentl~, the
acidic anode boundary layer functions to preserve
cyanide from rapid electrooxidation a~ the anode by
converting the cyanide ion into the much more dificult
to electrooxidi~e neutral protonated ~orm. This
explanation is considered in a quantitative way in the
discussion below on data Tables 1 - 4.
The protonated form of thiocyanate is similarly
made less easily electrooxidized than the free anionic
SCN form of thioc~anate. However, in this case the
acidic anode boundary layer appears to have little efect
on the current efficiency of thiocyanate conversion to
cyanide. An explanation is found in the fact that HSCN
lS is an extremely strong acid CQmpare~ to HCN. The pKa
of HSCN is less than 1.0 (pKa of HCN is 9.32) which means
that even if the pH of the acidic anode boundary layer
dropped as low as pH 1.0~ more than 50% of th~ thio-
cyanate in the acidic boundary layer would still exist
in the SCN , which is much less difficult to electro-
oxidize.
Th~ explanation given above relating to the
boundary layers appears to have some validity as demon-
strated by the data in the following tables, each re-
presenting a separate run. There may be different, butequally plausible, theories to explain why the process
of the present invention occurs. The explanation given
above is not intended to be conclusive.
5~3
-- 12 --
TABLE 1
Run ~1; Bulk pH_- 11.1; 0 . 5~1 in carbonate buf fer
t ~min)SCN ~mg/l ) CN (mg/l)
o 2920
2160 195
1~ 1910 199
1226 73
100 285 9
200 B ~0
TABLE 2
Rlm ~t 2; Bulk pH = 9 . 5; o . 5M carbonate buf f er
_ _ _
t ~min) SCN (mg/l) CN (mg/l)
02860 0
102090 286
5~1180 482
65852 503
100 303 420
2û011 12
- 13 -
TABLE 3
Rurl #3; Bulk pH = 9~6; 0.05M carbonate buffer
t (min ) SCN (mg/l ) CN ~mg/l )
0 ~868 0
2195 311
5Q 1330 621
~0 54~ û14
200 13 410
TABLE 4
Run ~4; Bulk pH = 4.2; no buffer salts added
t (min) SCN (m~/l) CN (mg/l)
n 2930 0
2285 322
1402 64û
100 482 952
120 310 1085
20~ 37 950
250 ~9 8~2
~2~ 3
- 14 -
The above dat~ were obtained by processin~ 300 litre
batches o~ thio~yanabe solution in an industrial size
electrochemical reactor described below and xeferred to
as Reactor 2. The solutions were made up using tap water
5 and technical grade salts. Electrochemical processing
was carried out on a batch recirculation basis. The
temperature was maintained in the range 24 - 29C. The
operating current and the recirculation flow rate were
the same in all runs.
The buffer capacities for the bulk solutions
were respectively 0.25, 0.14, 0.014 and 0.002 mol H
per litre per unit decrease in pH for runs 1 to 4. Com-
paring the rate of thiocyanate electrooxidation, it is
apparent that there is no significant difference in the
15 rates in all four runs.
Considering the extreme runs 1 and 4, the higher
buffer index in run 1 would effectively prevent signifi-
cant acidication of the anode ~oundary layer. However,
in run 4 the absence ~f buffer would result in strong
20 acidification of the anode boundary layer - estimated
drop in pH is about3 pH unit~ to pH 1.2 the fact that
thiocyanate electrooxidation rate is esse~tially the same
in anode boundary layers at pH 11 and 1.2 suggests the
explanation given above for th~ lack of variation of
- 25 thiocyanate electrooxidation rate with pH might be ~alid.
Comparing the accumulation rates of cyanide in the
four runs, it is seen that very little cyanide accumu~ates
in run 1 and close to the theoretical amount calculated
from equation (10) accumulates in run 4 - at least in the
30 first paxt of the run. A possible explanation is as
ol~ows. In run 1, the acidification of the boundary
layer will be resisted by the strong buffering capacity
of the solution and consequently the boundary layer
will not drop much below p~ 11. At pH 11 the fractio~ of
~2~ Z3
- 15 -
cyanide product in the more easily electrooxidizable
form, cyanide ion, will approach 100%. Therefore
conditions are ideal in run 1 for electrooxidation of
cyanide. Thus as cyanide is produced from thiocya~abe,
it is electrooxidized in a parallel anodic reaction,
hence the low rate of cyanide accumulation and the
rapid disappearance of cyanide as the run proceeds.
In run 3, the fraction of cyanide in the cyanide ion
form in the bulk solution at pH 9.6, is equivalent to
about 60%. However, the moderate buffering capacity of
the solution will no~ greatly resist the acidification
o the anode boundary layer. It is estimated for this
case that the pH of the boundary layer can drop about
1 pH units to about pH 8.6. A~ pH 8.6 approximately 13%
of the cyanide product will exist in the more easily
electrooxidizable cyanide ion form. Thus the rate and
level of cyanide accumulation in run 3 should be more than
in run 1, which is apparent from the data. This
explanation is validated by the data of run 2. In run 2
the buffer capaci~y is 10 times higher than in run 3 and
very little anode boundary layer acidiication would be
expected. If this obtains, then the rate and level of
cyanide accumulation/according to our proposed theory,
should be greater in run 3 than in run 2 which the data
confirms. In run 4 as noted above, the anode boundary
layer acidification down to an estimated pH 1.2 could
occur because the solution is acidic initially and the
buffering capacity of the solution is essentially neg-
ligible. In this run it would be expected that essen-
tially all the cyanide found will be in the less easilyelectrooxidiable HCN form in the bul~ solution and in the
acidic boundary layer. Thereore, the rate and level
of cyanide accumulation should be highest in this run,
~9~ ~ 3
- L6 -
which is confirmed by the data.
The process o the present invention has an
additional advantage in that the sulphur present in
thiocyanate appears in the stoichiometry of the half-cell
reaction (4) and ~6) in the form of sulphate (50~ ).
Chemical analyses on process solutions after both partial
and complete oxidation has determined that virtually
all sulphur is present as sulate, which is an environmentally
acceptable form. This is important where it is desired
to use the process of the present invention to treat
industrial effluents that initially co~tain intermediate
oxy-sulfur species as well as thioc~anate.
Various electrochemical reactors will be suit-
able for use with the process according to the presentinvention. For example, the electrochemical reactor or
electric cell described in Canadian Patent No. 1,016,495
is a suitable reactor that can be used to carry out the
pxocess in accordance with the present invention. Various
other sui~able reactors will be readily apparent to those
skilled in the art. However, while it will be possible to
use various electrochemical reactors including a conventional
electrochemical reactor, the efficiency of the process
will vary yreatly with the type of reactor used.
While the reactor described in Canadian Patent
No. 1,016,~95 is suitable to carry out the process according
to the present invention, when the process is to be carried
out on a laxge scale, this reactor IS presently too
expensive and too fragile to be economically feasible.
Since the process of the present invention will often
be utilized in a large scale operation~ the reactor is
preferably one that has durable components and is cap-
able of being fully erected at the site.
~2'~S~
- 17 -
A second suitable reactor that can be used to
carry out the process in accordance with the present
invention is a discrete, fixed layer, particulate, bi-
polax reactor ~hencefoxth referred to as reactor NOr 2)~
Reactor No. 2 has at least two layers of electrically
conductive particles, each layer being discrete in that
it is separated from adjacent layers by an electrically
insulating spacer or screen wedged between adjacent
layers of particles. Electrically insulating spacers
are also located immediately beneath the lower most
layer and immediately above the upper most layer of
particles~ The various layers are maintained in a
fixed relationship by said spacers. Except for that
taken by the spacers themselves, there is no gap, distance
or space between adjacent layers of particles. Of
course, the reactor vessel must contain means for
suppor~ing the various layers within it. Preferably,
the base of the reactor vessel is strong enough to
support the variou~ layers.
In reactor No. 2, the reactor vessel can be
constructed of virtually any suitable material and any
reasonable shape but is preferably circular in cross
section. For example, the reactor vessel can be made of
steel with the inside wall being ruhber-lined so that it
is electrically insulated. Also, the reactor wall
could be mada of concrete. The reactor v~ssel could
also be constructed in modular form so that additional
sections could be added as required. The two primary
electrodes can be fabricated from various materials
for example, graphite plates, stainless steel, lead or
even mild steel.
The conducting or semi-conducting material for
use as layers of particles in reactor No. 2 can be
-
~12~
- 18 -
various material~r f~r ~xample~ graphite, metallurgical
coke or anthracite. The particles can be specifically
arranged in a ~ixed relationship to ~orm a layer, or,
where crushed particles are used, su~ficient particles
can simply be poured on~o an insulating spacer to form
one layer. One type of particle that works well consists
of 2.5 X 2.5 cm graphite cylinders that have been tumbled
wet in a rotating drum. The rotating drum produced
graphite nodules approaching spherical shape as the
edges are rounded by the tumbling action. These nodules
are placed on what remains of the flat portion sf the
cylinder (ie. in an upright position~ in a fixed rela-
tionship forming one layer of particles. ~ach layer i5
topped by a poly-vinyl chloride coated Fibreglas (a trade
mark) mesh and then the next layer of tumbled cylinders
is placed immediately on top of that Fibreglas ~a trade
mark) mesh. Ultimately, a series of ixed conducting
layers is created, each separated by a non-conducting
membrane, all interposed between a primary anode and
cathode. Particle sizes are screened so that no
particles are smaller ~han . 25cmO
With reactor No. 2, in addition to Fibreglas (a
trade mark) mesh, various other materials can be used as
the insulating spacer. For example, crushed ston~,
coarse granular plastic nodules, ceramic burl saddles or
similaxly shaped ceramic or plastic shaped or glass
fabric with poly-vinyl chloride coating.
There are various ways that the cyanide formed
as an intermediate product in accordance with the
process of the present invention, can be recovered ~or
re-use. Also, it is sometimes necessary to pre-treat
the effluent or aqueous solution prior to carrying out
the electrochemical reaction within the suitable reactor.
~l2~ 3
- 19
Some of these procedures are di cussed in the following
examples. Other processes for recovering the cyanide
formedor pre-treating the aqueous solution will be
readily apparent to those skilled in the art; but will
still be within the scope of the claims.
EXAMPLE 1
Cyanide can be recovered by expurgation as hydro-
cyanic acid. As stated abov0, the condition of low pH,
while not influencing the rate of thiocyanate oxidation
promotes the protonation of cyanide ion, which in turn
inhibits its further oxidation. By allowing the pH of
the processing solution to decrease as acid is generated,
the hydrocyanic acid may be continuously recovered by
expurgation.
EXAMæLE 2
_ .
A portion of the thiocyanate containing cyanidation
leach solution is continuously fed to a suitable electro-
chemical reactor where partial electrooxidation takesplace forming cyanide as an intermediate product. This
leach solution with its enriched cyanide concentration
is returned to the cyanidation circuit. With appropriate
process control, a steady state thiocyanate/cyanide
concentration is maintained in the leach circuit.
EXAMPLE 3
Cyanide can be recovered using a strong base
ion axchanger on a batch or semi-continuous basis.
EXAMPLE 4
Cyanide can be recovered by utilizing the
electrochemical reactor in conjunction with an air
5~3
- 20 -
stripper to recover the cyanide as hydrocyanic acid.
The electrochemical reaction products are fed into an
air stripper where air, hydrocyanic acid, water and
hydrogen are separated from the electrochemical
reaction products. The cyanide can then be recovered
from the hydrocyanic acid by neutralization with lime
water or sodium hydroxide in an adsorption tower.
EXAMPLE 5
Cyanide ~an be recovered by utilizing the
electrochemical reactor in conjunction with a steam
stripper to recover the cyanide as hydrocyanic acid.
This is similar to the use of the air stripper except
that steam and air are used with steam stripping. Once
the hydrocyanic acid is recovered, it can be neutralized
with lime or sodium hydroxide to recover the cyanide.
EXAMPLE 6
Cyanide can be recovered by dixectly recycling
it in solution to a cyanide leaching process. Since the
conversion of thiocyanate to cyanide results in virtually
all sulphur species being converted to sulfate, the
acceptability of sulfate must be considered. In the
leaching of zinc sulfide containing residues, the acidic `
zinc-thiocyanate solution is treated electrochemically to
convert most of the thiocyanate to cyanide and sulfate
and simultaneously to recover a large portion of the zinc
cathodically. The electrochemically converted acidic
solution is then treated with lime to neutralize the
sulphuric acid and the hydrocyanic acid. The solid
calcium sulphate is thickened by settling and the clear
supernatant Ca(CN)2 solution is used for make up for
further cyanidation and ~he zinc collected in the reactor
~r'2 ~ 5;~3
- 21 -
is leached out with sulphuric acid.
EXAMPLE 7
Where the effluent contains an acidic solution
of æinc and thiocyanate, a c~tion exchanger could be
used operating on the acid cycle to remove the zinc from
the solution. The essentially zinc free solution is then
treated electrochemically to convert most of the thio-
cyanate to cyanide and sulphate and to cathodically
deposit any residual zinc.
The electrochemically converted acidic solution
is then treated with lime to neutralize the sulphuric
acid and hydrocyanic acid formed. l~he solid calcium
sulphate is thickened by settling and the clear super-
lS natant Ca(CN~4 solution is used for make up for furthercyanidation.
Any cathodic zinc that ha~ been deposited is
removed from the reactor by sulphuric acid. Zinc is
eluted from the cation exchanger with sulphuric acid.
~0
EXAMPLE 8
_
An acidic zinc-thiocyanate solution is treated
electrochemically to convert most of the thiocyanate to
cyanide and sulphate and simultaneously recover a good
portion of the zinc cathodically.
The electxochemically treated solution is then
rendered essentially zinc free by using a cation exchanger
operating on an acid cycle.
The solution is then treated with lime to
neutralize the sulphuric acid and hydrocyanic acid. The
solid calcium sulphate i5 thickened by settling and the
clear supernatant CatCN)2 solution is used for make up
for further cyanidation.
5i2:~
The zinc collected in the reactor is leached
out with sulphuric acid and zinc is eluted from the
cation exchanger with sulphuric acidn
EXAM2LE 9
Pxior to carrying out the electrochemical
reaction, where the cyanidation waste is basic, it is
acidified to a pH ranging from ~ to Ç.5 and any solids
are filtered out.
The waste is then treated on a weaX base anion
exchanger to extract the anionic metal cyanide
species (eg. copper, nickel, iron and/or cobalt) and
the thiocyanate is collected on a second-weak base
anion exchanger. The weak base anion exchanger con~
taining ~ssentially thiocyanate is then eluted with base
such as sodium hydroxide or lime water to produce an
effluent with a low buffer index.
The electrochemical reaction can then be carried
out together with air stripping or steam stripping as
~0 set out in Examples 1 and 2.
This would be necessary only where the alkaline
cyanidation waste has a high buffer index.
EXAMPLE 10
Where the effluent or aqueous solution contains
a high buffer index based on the bi-carbonate/carbonate
concentration~ the buffer capacity can be substantially
reduced by adding calcium chloride to precipitate the
carbonate as calcium carbonate.
The electrochemical reaction to convert the
thiocyanate to cyanide can then be carried out on the
resulting solution.
" ll2~S~3
- 23 -
EXAMPLE _
Where the effluent or aqueous solution has a
high buffering index because of the bi-carbonate/car-
bonate concentration, the buffering index can be
substantially reduced by adding acid to substantially
convert all of the bi-carbonate and carbonate to carbon
dioxide and then expurgating the carbon dioxide to
produce a solution with a low buffer index.
The electrochemical reaction of the present
invention can then be carried out on the resulting
solution to convert the thiocyanate to cyanide. and the
cyanide so for~ed can be recovered.
