On Electrochemical oxidation of thiocyanates in solutions for cyanidation of gold-containing ores and concentrates

Article abstract of DOI:10.1134/S1070427210090156

Effect of hydrogen peroxide on the electrochemical oxidation of thiocyanate-containing solutions was studied.

Full text of DOI:10.1134/S1070427210090156

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 9, pp. 1589–1592. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © T.A. Kenova, V.L. Kormienko, S.V. Drozdov, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 9, pp. 1489–1492.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
On Electrochemical Oxidation of Thiocyanates in Solutions
for Cyanidation of Gold-containing Ores and Concentrates
a
a
b
T. A. Kenova , V. L. Kormienko , and S. V. Drozdov
a
Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, Russia
b
Polyus Private Company, Krasnoyarsk, Russia
Received July 9, 2009
Abstract—Effect of hydrogen peroxide on the electrochemical oxidation of thiocyanate-containing solutions
was studied.
DOI: 10.1134/S1070427210090156
The operation of gold-recovery plants results in the
formation and accumulation of solutions containing
noxious impurities, such as free and complexed
cyanides and thiocyanates and compounds of
nonferrous metals. One of ways to diminish the
detrimental effect of these aqueous solutions on the
environment is their recycling into various processes.
However, this leads to an increased amount of im-
purities in sewage and higher consumption of pure
natural water. As a result, the amount of water in the
balance of plant becomes larger and the capital
expenditure and operating costs grow.
It is known that electro-oxidation of the thiocyanate
ion at pH > 4 yields hydrogen cyanide and(or) the
cyanide and sulfate ions [3]. In further oxidation of the
cyanide ion, the cyanate ion being formed is either
gradually hydrolyzed in accordance with the equation
–
2–
3
+
4
CNO + 2Н
2
О → СО
+ NН
(1)
or can be further oxidized to elementary nitrogen and
carbon dioxide [4]. At low pH values, the electro-
–
oxidation of SCN proceeds to give hydrogen cyanide
2–
4
+
SCN¯ + 4Н
2
О → SO
+ HCN + 7H + 6е,
(2)
which is considerably less subject to further oxidation
to the cyanate ion.
Leaching of sulfide gold-containing ores and
concentrates yields aqueous solutions with a large
amount of thiocyanate compounds. Depending on the
Depending on the solution pH and H O concentra-
2
2
tion, the interaction of the thiocyanate ion with hydro-
gen peroxide may lead to its oxidation to the cyanate
or cyanide ion [5, 6]. It was found that the intermediate
product is hypothiocyanate formed by the reaction
–
content of sulfur and number of recycles, the SCN
concentration in technological solutions may range
from several milligrams to several grams per liter. The
formation of thiocyanate compounds leads to an
increased expenditure of NaCN in cyanidation and
adversely affects the sorption leaching of gold [1]. At
SCN¯ + Н
2
О
2
→ ОSCN¯ + Н
2
О,
(3)
which is further rapidly oxidized at pH 4–12 to sulfate
and hydrocarbonate ions and an ammonium ion:
–
–1
an insignificant concentration of SCN (<0.15 g l ),
solutions are recycled into the technological process.
Solutions with a larger content of thiocyanate ions
should be purified before being discharged into the
tailing dump. Because approximately 50% of the
cyanide is irretrievably lost via formation of
thiocyanates, the problem its recovery from thio-
cyanate solution is not only ecologically, but also
economically important [2].
OSCN¯ + Н
2
О
2
→ ОS(О)CN¯ + Н
2
О,
(4)
(5)
(6)
(7)
ОS(О)CN¯ + Н
2
О
2
→ Н
2
SО
3
+ CNO¯,
–
3
–
4
SО
+ Н
2
О
2
→ SО
+ Н
2
О,
+
4
–
3
НCNО + 2Н
2
О → NН
+ НСО
.
At low pH values, hypothiocyanate is rapidly
oxidized to the cyanide ion, which can interact with
1
589

1
590
KENOVA et al.
–1
cSCN, g l
ηCN, %
–1
–1
cCN, mg l
cSCN, g l
2
1
1
0
6
2
8
4
100
80
60
40
20
1400
7
1
2
'
'
3
4
'
'
1000
5
1
'
6
00
2' 3
1
4
3'
4
3
'
2
1
200
1
3
2
4
0
.5
1.5
2.5
τ, h
1
2
3
τ, h
–
Fig. 2. Residual concentrations of (1–4) SCN , cSCN, and
(
–
Fig. 1. (1–4) Residual SCN concentration cSCN and
–
1'–4') CN , cCN, vs. the electrolysis duration τ. Current
–
(
1'–4') yield ηCN of CN vs. time τ at various H
2
O
2
–2 – –1
density 1000 A m , Initial SCN concentration 5.76 g l .
–
–1
concentrations. Initial SCN concentration 18.6 g l . c(H
2
2
O )
–1
(1, 1') Without addition of H
2 2 2 2
O ; c(H O ) (g l ):
2, 2') 5.53, (3, 3') 5.53 (in portions), and (4, 4') 10.89.
–
1
(
g l ): (1, 1') 0, (2, 2') 1.86, (3, 3') 7.3, and (4, 4') 17.8.
(
–
OSCN to give sulfur dicyanide S(CN) . In addition, it
hydrogen peroxide was added in amounts of 0.54,
0.22, and 0.06 of that required by the stoichiometry of
reaction (2). The electrochemical oxidation of thio-
cyanates was carried out in the galvanostatic mode in a
2
has been noted [5] that, in the presence of copper(I)
ions, CN is rapidly catalytically oxidized to CNO :
–
–
–
–
CN + Н
2
О
2
→ CNO + 2H
2
О.
(8)
cell with a Pb/PbO anode at current densities of 500
2
–
2
Previously, it has been shown on model and
technological solutions with SCN concentrations
exceed-ing 20 g l that the electrochemical method
can be employed for effective recovery of cyanides
from acid thiocyanate solutions in a sufficiently high
yield (70–80%) [7].
and 1000 A m , with and without blow-off of hyd-
rogen cyanide being formed. The initial and residual
–
–
1
–
concentrations of SCN in solutions were determined
by permanganatometry. The content of cyanide ions in
the electrolyte and absorbing vessel was found by
argentometry with (4-dimethylaminobenzylidene)rhod-
amine as indicator [8].
The goal of the present study was to examine the
effect of hydrogen peroxide on the regeneration of the
cyanide in electrochemical oxidation of acid solutions
containing thiocyanate ions. The study was not aimed
to determine the oxidation mechanism of thiocyanate
ions under these conditions. The reason was that all
experiments were carried out using technological
solutions containing, in addition to SCN ,
considerable amount of other impurities, including
complex compounds of nonferrous metals.
The rate of the electrochemical oxidation of
thiocyanates is determined by the current density at the
anode and the concentration of substrates in the elec-
trolyte. At a lower content of thiocyanate ions, other
oxidation processes, including oxidation of water, may
occur at the anode. Introduction of hydrogen peroxide
into the solution enables oxidation of both thiocyanates
and products of their electrochemical decomposition at
the anode. In addition, in the presence of mixed-
valence metal ions and hydrogen peroxide, hydroxy
radicals are formed and involved in oxidation
processes [9].
–
a
EXPERIMENTAL
The study was performed with solutions containing
.76 and 18.6 g l
concentration of free cyanide ions was 0.1 g l . The
solutions were preliminarily acidified to pH 2–3 and
–1
5
of thiocyanate ions. The
The effect of the hydrogen peroxide concentration
on the oxidation of thiocyanates is illustrated by Fig. 1.
It can be seen that, as this concentration increases, the
–
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 9 2010

ON ELECTROCHEMICAL OXIDATION OF THIOCYANATES
1591
–1
cSCN, g l
ηCN, %
–1
cCN, mg l
6
100
1
2
3
'
'
'
7
00
1
2
4
2
4
'
6
2
0
0
500
300
100
1
2
3
4
1
2
3
τ, h
0
.4
0.8
Q, A h
–
Fig. 3. (1–4) Residual SCN concentration cSCN and
–
(
1'–4') yield ηCN of CN vs. the electrolysis duration τ at
various current densities. Initial SCN concentration 5.76 g l .
Current density (A m ): (1, 1', 2, 2') 500 and (3, 3', 4, 4') 1000.
1, 1', 3, 3') Without addition of H
– –1
–2
–
Fig. 4. CN concentration cCN in the absorbing vessel vs.
the quantity Q of passed electricity at different current
densities. Current density (A m ): (1) 500 and (2) 1000.
(
5
2 2 2 2
O ; (2, 2', 4, 4') c(H O ) =
–
1
–2
.53 g l .
–
–
oxidation rate of SCN grows and the yield of CN
becomes lower. For example, at a H O concentration
cyanide lower and this decrease is the more pro-
nounced, the higher the H O concentration. A deter-
mination of the concentration of sodium cyanide in the
absorbing vessel demonstrated that presence of
hydrogen peroxide in solution affects the amount of
blown-off hydrogen cyanide only slightly.
2
2
2
2
–1
of 17.8 g l in solution, the residual concentration of
–
–1
SCN decreased in 1 h of electrolysis to 2.7 g l and
the yield of the substance was 56.2%, whereas in the
absence of hydrogen peroxide, these values were
–1
1
1.8 g l and 78.5%, respectively. In 3 h of electrolysis
With the data on the current efficiency and oxida-
in the presence of H O , the yield of the cyanide was
–
2
2
tion rate of thiocyanate ions and on the yield of CN
6
8.6% at a total oxidation of thiocyanate ions.
taken into account, it can be suggested that the yield of
hydrogen cyanide in the acid medium decreases either
In the experiments, the current efficiency by
–
thiocyanate ions was determined, with its values
exceeding 100% in all cases upon 2 h of electrolysis.
For example, with the electrolysis performed for 0.5 h,
the current efficiency was higher than 200%,
depending on the concentration of hydrogen peroxide,
and 150% in its absence. The same tendency was
due to its further interaction with OSCN to give
S(CN) or, in the presence of mixed valence metal ions
2
and hydrogen peroxide in solution, because of its
deeper oxidation to the cyanate ion. Moreover, it is not
improbable that there occurs destructive oxidation of
·
the thiocyanate ion by OH radicals.
–
preserved for electrolysis of solutions with a SCN
–1
A possible reason for the increased current effi-
concentration of 5.76 g l . It should be noted that,
upon addition of hydrogen peroxide, this phenomenon
is characteristic of processes performed both with and
without blow-off of the cyanide. The departure from
the Faraday law in the presence of hydrogen peroxide
is observed because thiocyanate ions are oxidized not
only at the anode, but also in the bulk of the
electrolyte. In the absence of hydrogen peroxide and
without blow-off of HCN being formed, the current
efficiency did not exceed 100% during the whole
electrolysis.
ciencies in the absence of H O is that, with hydrogen
2
2
cyanide blown-off, processes occurring in solution in
the presence of atmospheric oxygen lead to additional
–
oxidation of SCN , except that at the anode. This
assumption is in rather good agreement with pre-
viously obtained data [10] and with the results of
experiments carried out without blow-off of HCN
being formed.
The effect of hydrogen peroxide on the electro-
oxidation of thiocyanate ions without blow-off of
hydrogen cyanide were performed in solutions
It follows from Fig. 1 that addition of hydrogen
peroxide to the solution makes the yield of hydrogen
–
1
–
containing 5.76 g l of SCN . Figure 2 shows how the
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 9 2010

1
592
KENOVA et al.
residual concentrations of the thiocyanate ion depend
on the electrolysis duration at a current density of
in the absorbing vessel is the higher, the lower the
current density. At Q = 0.25 A h, the amounts of blown-
off hydrogen cyanide are approximately the same; this
value corresponds to approximately 50% oxidized
thiocyanate ions. Further increase in the quantity of elec-
tricity results in that, at high current densities, HCN
has not enough time to be removed into the absorbing
level and is further oxidized in the electrolyzer.
–
2
1
000 A m . It can be seen that a nearly twofold
increase in the H O concentration does not lead to any
2
2
–
noticeable rise in the oxidation rate of SCN . At the
same time, the CN concentration becomes three times
–
lower in 1 h of electrolysis, compared with the electro-
oxidation in the absence of hydrogen peroxide. It
should be noted that portionwise addition of hydrogen
peroxide during electrolysis hardly affects the
CONCLUSIONS
–
oxidation rate of thiocyanate ions, but the CN
(
1) The efficiency of the electrochemical oxidation
concentration decreases somewhat faster (Fig. 2,
of thiocyanate ions markedly grows upon addition of
hydrogen peroxide into the working solution. The
oxidation rate is the faster, the higher the initial
curves 3 and 3') The decrease in the concentration of
–
cyanide ions in electro-oxidation of SCN without
hydrogen peroxide is, in all probability, due to its
oxidation in the near-electrode layer by the highly
–
–
concentration SCN at the same SCN : H O ratio.
2
2
·
(
2) If hydrogen cyanide formed in oxidation of
reactive OH radicals. Hydroxy radicals can be formed
–
SCN in the presence of hydrogen peroxide is blown-
off, the yield of the substance remains sufficiently high
for its recovery. Making the current density lower as
the concentration of thiocyanate ions decreases also
favors an increase in the extraction of HCN.
in oxidation of water at Pt,Pb/PbO anodes by the
reaction [11]:
2
·
+
2
Н
2
О → 2НО + 2Н + 2e.
(9)
Thus, it can be assumed that, as the concentration
of thiocyanate ions in solution decreases, the share of
reaction (9) markedly grows, which results in an
increase in the oxidation rate of hydrogen cyanide.
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