Note: Descriptions are shown in the official language in which they were submitted.
This invention rela~tes to a novel electrochemical
process for oxidizing -thiocyanate (SCN ). In particular,
this invention relates to a process for reco~ering
cyanide (CN ) from aqueous solutions containing thio-
cyanate by controlled partial eiectrooxidation ofthiocyanate.
~ queous solutions containing thiocyanate arise
from many industrial processes, the principal so~lrces
being hydrometallurgical processing 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
containing 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 liquor that could
be treated by the present process. In gold recovery
25 by cyanidation of sulfidic concentrates obtained by
froth flotation of copper ore tailings, the waste liquor
effluent may contain 1000-1200 milligrams of CN per
litre and 1200-1400 milligra~sper 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 (S ) presen-t in compounds of copper,
iron, nickel and other metals during the cyanidation
~`
leaching of the tailings. Sulfide undergoes chemical
oxidation in the oxygen rich leach li~uor to form a
series of oxysulfur species incll-ding thiGsulfates and
thionates. It is believed that thiocyanate is formed by
reaction of cyanide with thionates. A reaction suggested
for the formation of thiocyanate by the action of tri-
thionate ion (S302 6) with cyanide is shown in equation
(1)
(1) 3 6 SCN ~ S2O6
In addition to the irreversible consumption of
reagent cyanide, there is evidence to suggest that the
presence ofthiocyanate 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 liquor from the
cyanidation circuit per 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 ~50 metric
tons. This represents approximately 350 kilograms 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
5~
the conventional level in order to effect dissolution
of contained silver values. Under these cGnditions,
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 per day.
The two examples given above demonstrate the
large quantity of thiocyanate bearing waste liquor
produced by cyanidation of sulfide ores and concentrates.
The ~onventional 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 ~ SO~ ~ 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 (~), an estimate of the
chemical reguirements 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
513
- 4 -
of chlorine is required together with 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 receivin~ 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 l.0 metric ton per day of
sodium hydroxide. The stoichiometry of the alkaline
chlorination of cyanide is given by equation ~3).
(3) CN + C12 + 20H -~ CN0 + 2Cl + H~0
These estimates of reagent requirements indicate that
the oxidation of thiocyanate by chemical means is an
inherently expensive and hazardous proposition and is
generally regarded as being much more expensive than
alkaline chlorination of the cyanide which often
~0 accompanies the thiocyanate oxidation.
Through the use of the process of the present
invention it is possible to electrochemically oxidize
thiocyanate more economically than by conventional means
and,in addition, it is possible to recover for credit
and reuse, cyanide which forms as an intermediate product
of the electrooxidation. In addition, the present process
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 pH
3~ adjustments or adjustment of the buffer index or capacity
o~ the e~fluent before electrochemical treatment is
re~uired. Also, when thiocyanate or cyanide is treated
in the conventional manner by chemical oxidation, the
? 5 (3~
waste contains a lar~e amount of sodium chloride and may
very well contain undesirable levels of free chlorine or
sodium hydroxide from chemical overdosage. In addition,
when treated in the conventional manner, the volume of
the effluent may be considerably increased by the large
volume of reagents added.
A process for the reco~ery of cyanide from thio-
cyanate, said process comprising introducing an aqueous
solution containing thiocyanate into a suitable electro-
chemical reactor, applying a direct current electricalpotential to said reactor, carrying 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 pH of the aqueous
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~
Preferably, the process of the present invention
includes the steps of introducing the aqueous solution
into the 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
2S solution shortly after the electrochemical reaction
begins, is maintained in the range of 1 to 4.
There is further provided a process for electro-
chemically oxidizing thiocyanate. An aqueous solution
containing thiocyanate ions is introduced into a suit-
able reactor for an appropriate time period. The pHof the aqueous solution is maintained in a range from
lO to 12 during the reaction and the aqueous solution
is removed from the reactor once the thiocyanate has
been converted to cyanate and sulfate.
- 5~ -
Whether the process in accordance with the pxesent
invention is utili~,ed to recover c~anide or to convert
the thiocyanate solution into relatively harmless
reaction products, as described above, depends on the
level of thiocyanate present in the effluent. ~ waste
liquor with a high concentration of thiocyanate would
normally be treated under conditions to allow maxi.mum
recovery of the intermediate cyanide formed during the
electrooxidation process. However, if the waste liquor
contains only low levels of thiocyanate, two options for
processing would be possible. The dilute thiocyanate
containing liquor may be completely electro-
38i~5(~
oxidized producing an environmen-tally acceptable waste
or the dilute thiocyanate containing liquor may be
concentrated by a convenient physical or chemical method.
The consentrated thiocyanake solution then may be treated
by the method of the present invention which allows for
cyanide recovery.
In discussing the invention in greater ~etail,
it is helpful to refer to the possible electrochemical
reactions that occur. In the electrochemical treatment
of thiocyanate, electrooxidation of thiocyanate occurs
at anodic surfaces, and a~ cathodic surfaces electro-
reduction of hydrogen ion occur to produce hydrogen gas.
I the thiocyanate solution contains other electrooxidizable
species such as cyanide, thiosulfate, thionates, etc.
the reactions at the anodic surfaces will consist of a
nu~ber of parallel eleetrooxidation 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 etc., the reactions at the
cathodic surfaces will consist of number of parallel
electroreduetion reactions comprising the simultaneous
; production of hydrogen and the cathodic deposition of
metals. For the purpose of explaining the electro-
~5 oxidation of thiocyanate it is useful to consider that
the solution is essentially a pure thiocyanate solution.
Since the cyanide moiety in thiocyana~e can be
anodically converted to a series of products such as
cyaniae ion, cyanate ion, nitrogen gas and carbon dioxide
or earbonate and bicarbonate ion, it is helpful to
considex the electrooxidation reactions in sequence~
~lthough the stoichiometrics of the various thiocyanate
reactions have not been unequivocally established,
5(3
considerable analy~is of anodic products of electro-
oxidation of thiocyanate indicates that under a range
bf electrolysis conditions the fate of thiocyanate may
be represented by the following equations:
Electrooxidation of SCN to CN and S04
(4) SCN + 4H20 ~ CN + S04 + 8H + 6e
Electrooxidation of SCN through to CNO and S04
(5) SCN + 5H20-~ CNO + SO~ + lOH -~ 8e
Electrooxidation of SCN through to C02,N2 and S04
; 16) SCN '~ 6H2~ 0-5N2 + C02 + 12H -~ SO~ + lle
The above reactions represent stoichiometries and the
form of the ,species in solution will, of course, depend
on the pH. For example, cyanide in acidic solution will
be present almost entirely in the neutral HCN form while
in highly basic solution it will be 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) 2H20~2 + 4H + ~e
: (g)
(8) 4 ~ + 2H20 + 4e
(g)
;'5~
- 8 -
In the absence of electroreducible species such as
platable ~etals the predominant reaction at the
cathode is the pxoduction of hydrogen gas by the
electroreduction of hydrogen (hydronium) ion or
equivalently, from the stoichiometric viewpoint, the
electroreduction of water. The reaction may be written
as
~9) 2H20 ~ 2e -~H2 -~ 20H
; 10 (g)
From the standpoint of recovering cyanide from
thiocyanate, the relevant electrode reactions are (4)
and (8)o From the standpoint of ~onverting thiocyanate
to relatively nontoxic cyanate and to nontoxic nitrogen
gas and carbon dioxide, the relevant electrode reactions
are recpectively (5) and (9) and (6) and (9). The anodic
formation of oxygen gas operates in parallel with all
thiocyanate anodic reactions. At high thiocyanate
concentrations the current efficiency for oxygen pro
duction is relatively lowO At low thiocyanate concen-
trations (and cyanide) oxygen production becomes the
predominant anodic reac~ion.
The overall electrochemical cell reaction leading
to the production of cyanide from thiocyanate is obtained
by combing ~quations ~4) and (9) to yield reaction equation
(1~3
(10) SCN + 4H2O-~ CN + H2SO~ + 3H2
When considering the overall reaction (10) and assuminy
a current efficiency of 100% (that is no o~her anodic
and cathodic reactions of significance are occurring),
there is a net acid production of 0.33 moles of H~ per
5~3
Faraday of charge through the cell. Therefore, as the
electrochemical processing of thiocyanate solution
proceeds the solution tends to become more and more
acidic. Reaction (10) stoichiometry has been verified
by analysis for thiocyanater cyanide and acid during
the course of e]ectro]ysisO
The production of acid is beneicial from the
standpoint of the specific cyanide yield since (except
where thethiocyanate solution has a high buffering
capacity) it has the effect of preserving the cyanide
produced from undergoing further rapid electrooxidation
to cyanate or through to nltrogen gas and carbon dioxide.
Initially, the conversion of thiocyanate at the anode
can be represented by the reaction (4). When the thio-
cyanate solution does not have a high buffering capacityin the acidic direction, the large amount of acid
produced (~ moles of H+ per mol of cyanide produced) will
tend to cause a substantial decrease in the pH of the
anolyte solution adjacent the anode surface. Similarly,
the hydroxyl ion produced by the cathodic xeaction will
increase the pH in the catholyte adjacent the cathode
suxfaces although this effect will be resisted if the
thiocyana~e solution has substantial buf-fering capacity
in the basic direction. This suygests that an acidic
~5 anode boundary layer and a basic cathode boundary layer
may exist.
It is the establishment of an acidic anode
boundary layer which is believed to be the main reason
why the cyanide product is protected from xapid electro-
o~idation 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 electrooxidi2ed at the anode to produce
o
-- 10 --
cyanide ion, the cyanide lon is immediately protonated
by the anodically produced acid. Consequently, the
acidic anode boundary layer functions to preserve
cyanide from rapid electrooxidation at the anode by
converting the cyanide ion into the much moxe diEEicult to
electrooxidize neutxal protonated form. This explanation
is considered in a quantitative way in the discussion
below on data Tables 1 - ~.
The protonated form of thiocyanate is similarly
made less easily electrooxidized than the free anionic
SCN form of thiocyanate. However, in this case the
acidic anode boundary layer appears to have little effect
on the current efficiency of thiocyanate conversion to
cyanide. An explanation is found in the fact that HSCN
is an extremely strong acid compared 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 the thio-
cyanate in the acidic boundary la~er would still exist
in the much less difficult to electrooxidize SCN form.
The 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, but
equally plausible, theories to explain why the process
of the present invention occurs. The explanation given
above is not intended to be conclusive~
:..,
~ 5 ~
TABLE 1
Run #l; Bulk pH = 11.1; 0.5M in carbonate buffer
t (min)SCN (~g/l) CN (mg/l)
0 2920 0
2160 ~195
12 1910 199
1226 73
100 285 9
200 8 ~0
TABLE 2
:
Run #2; Bulk pH ~ 9.5; 0.5M carbonate buffer
t ~min)SCN tmg/l) CN (mg/l)
o 2860 0
2090 286
. 50 1180 ~82
852 503
100 303 . ~20
200 11 12
8~5~(~
-- 12 -
TABLE 3
Run #3; Bulk pH = 9. 6? - 05M carbonate buffer
(rr.in ) SCN (mg/l ) CN (mg/l )
o 2868 0
2195 311
1330 ~21
548 814
200 13 410
TABLE 4
Run #4; Bulk pH = 4.2; no buffer salts added
t ~min)SCN (mg/l ) CN (mg/.l )
0 2930 0
2285 322
1a~02 640
- 100 ~82 952
120 310 1085
200 37 950
250 - ~ 9 892
5~3
- 13 -
The ~boye data were obtained b~ processin~ 300 litre
batches of thiocyanate solution in an industrial size
electrochemical reactox described below and xe~erred to
as Reactor 2. The solutions were made up using tap watPr
5 and technical grade salts. Electrochemical processing
was caxried out on a batch recirculation basis. The
temperature was maintained in the range 24 - 29C. The
operatinq current and the recirculation flow rate ~ere
the same in all runs.
The buffer capacities for the bulk solutions
are 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 differencP in the
15 rates in all four runs.
Considering the extreme r~ms 1 and 4, the higher
buffer index in run 1 would effectively prevent signifi-
cant acidication of the anode houndary layer~ However,
in run 4 the absence of buffPr would result in strong
20 acidification of the anode boundary layer - estimated
drop in pH i5 about3 pEI units to pH 1.2 the fact that
thiocyanate electrooxidation rate is essentially the same
in anode bo~mdary layers at pH 11 and 1.2 suggests the
explanation given above for the lack of variation of
25 thiocyanate electro~xidation rate with pH might be valid.
~ omparing the accumulation rates of cyanide in
four runs, it is seen that very little cyanide accumulates
in run 1 and close to the theoretical amount calculated
from equation (10) accumulates in run 4 ~ at least in the
30 first part of *he run. A possible explanation is as
follows. In run 1, the acidification of the boundary
layer will be resisted by the strong bufferin~ capacicy
of the solution and consequently the boundary layer
will not drop much below pH 11. At p~ 11 the fraction of
5~
- 14 -
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 thiocyanate,
it is electrooxidized in a parallel anodic reaction,
hence the low rate of cyanide accumulation and the
rapid disappearance o~ 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 oE
the solution will not greatly resist the acidification
of 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. At 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 capacity is 10 times higher than in run 3 and
very little anode boundary layer acidification would be
expected. If this obtains, then the rate and level of
cyanide accumulation according to our proposed theory
should be greater in run 2 than in rur. 3 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 solu-tion is essen~ially 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 bulk solution and in the
acidic boundary layer. Therefore, the rate and level
of cyanide accumulation should be highest in this run
3;Z5~.~
which is confirmed by the data.
The process of 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 (S0~ ).
Chemical analyses on process solutions after both partial
and complete oxidation has determined that virtually
all sulphur is present as sulfate, which is an environmentally
acceptable form. This is important where it is desired
to use the process of the present invention to treat
indus-trial effluents that initially co~tain intermediate
oxy-sulfur species as well as thiocyanate.
~arious electrochemical reactors will be suit~
able for use with the process according to the present
invention. 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
process in accordance with the present invention. Various
other suitable reactors will be r~adily apparent to those
skilled in the art. However, while it will be possible to
use various electroch~mical reactors including a conventional
electrochemical reactor, the efficiency of the process
- will vary greatly with the type of reactor used.
~5 While the reactor described in Canadian Patent
No . 1~ 016 r 495 is suitable to carry out the process according
to the present invention, when the process is to be carried
out on a large scale, this reactor ~s presently too
expensive and too fragile to be economically feasible.
Since the process oE 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~
- 16 -
A second suitable reactor that can be used to
carry out the process in accordance with the present
invention is a discrete, fixed layer, particulater bi-
polar reactor (henceforth referred to as reactor No. 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 particlesO Of
course, the reactor vessel must contain means for
supporting the various layers within it. Preferably,
the base o the reactor v~ssel is strong enough to
support the various layers.
In reactor No. 2, the reactor vessel can be
constructed of virtually any suitable material and any
reasona~le shapP but is preferably circular in cross
section. For example, the reactor vessel can be made of
steel with the inside wall being rubber-lined so that i~
is electrically insulated. Also, the reactor wall
could be made of concrete. The reactor vessel could
- also be constructed in modular form so that additional
sections could be added as required. The two primary
electroaes 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
various materials~ for exam~le~ graphite, metallur~.ical
coke or anthracite. The particles can be specifically
arranged in a fi~ed relationship to form a layer, or,
where crushed particles are used, sufficient particles
can simply be poured onto an insulating spacer to form
one layer. One type o~ 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 of the
cylinder ~ie. in an upright position) in a fixed rela-
~tionship forming one layer of particles. Each layer is
topped by a poly-vinyl chloride coated Fibreglas (a trade
mark) mesh and therl the next layer of tumbled cylinders
is placed immediately on top of that Fibreglas (a txade
mark) mesh. Ultimately, a series of fixed 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 than .25cm.
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
similarly shaped cexamic 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 o~ the present invention, can be recovered ~or
, re-use. Also, it is sometimes necessary to pre-treat
the ef~luent or aqueous solution prior to carrying out
the electrochemical reaction within the suitable reactor.
' :'
Some of these procedures are discussed 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 above, 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.
EXAMPLE 2
A portion of the thiocyanate containing cyanidation
leach solution is continuously fed to a suitable electro~
chemical raactor where partial electrooxidation takes
place 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.
EX~MPLE 3
Cyanide can be recovered using a strong base
ion exchanger on a batch or semi-continuous basis.
EXAMæLE 4
Cyanide can be recovered by utilizing the
electrochemical reactor in conjunction with an air
5(3
-- 19 -- . '
stripper to recover the cyanide as hydrocyanic acid.
The elec-trochemical reaction products are fed into an
air stripper where air, hydrocyanic acid, water and
hydrogen are separated from the electrochemical
reaction produc-ts. The cyanide can then be recovered
from the hydrocyanic acid by neutrali~ation with lime
water or sodium hydroxide in an adsorption tower.
~XP~LE 5
Cyanide can be recovered by utilizing the
electrochemical reactor in conjunction with a steam
stripper to recover the cyanide as hydrccyanic acid.
This is similar to the use of the air stripper except
that steam and air are used with steam stripping. Once
lS the hydrocyanic açid is recovered, it can be neutralized
: with lime or sodium hydroxide to recover the cyanide.
,
EXAMPLE 6
Cyanide can be recovered by directly 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)4 solutlon is used for make up for
further cyanidation and the zinc collected in the reactor
5~3
-- ~o - ,
is leached out with sulphuric acid.
EXAMPLE 7
__
Where the effluent contains an acidic solution
of zinc and thiocyanate, a cation 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 ~he sulphuric
acid and hydrocyanic acid formed. The solid calcium
sulphate is thickened by settling and the clear super-
natant Ca(CN~4 solution is used for make up for further
- cyanidation.
Any cathodic zinc that has been deposited is
removed from the reactor by sulphuric acid. Zinc is
eluted from the cation exchanger with sulphuric acid.
E ~PLE 8
-
An acidic zinc-thiocyanate solution i6 treated
electrochemically to convert most of the thiocyanate to
cyanide and sulphate and simultaneously recover a good
~5 portion of the zinc cathodically.
The electrochemically 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
neutral~ze the sulphuric acid and hydrocyanic acid. The
solid calcium sulphate is thickened by settling and the
clear supernatant Ca~CN)4 solution is used for make up
for further cyanidation.
5~
The zinc collected in the reactor is leached
ou~ with sulphuric acid and zinc is eluted from the
cation exchanger with sulphuric acid.
EXAMPI.E 9
Prior to carrying out the electrochemical
reaction, where the cyanidation waste is basic, it is
acidified to a pH ranging from 5 to 6.5 and any solids
are filtered out.
The waste is then treated on a weak 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 essentially 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
set out in Examples 1 and 2.
This would be necessary only where the alkaline
cyanidation waste has a high buffer index.
EXP~LE 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.
~: '
l:L~50
- 22 -
EXAMæLE 11
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 dioxid~ to
produce a solution with a low buffer index.
The electrochemicàl reaction of the present
invention can then be carried out on t~e xesulting
solution to convert the.thiocyanate to cyanide and the
cyanide so formed can be recovered.
