Advances in Chemical Engi neering and Science , 2011, 1, 191-197
doi:10.4236/aces.2011.14028 Published Online October 2011 (
Copyright © 2011 SciRes. ACES
Copper and Cyanide Recovery in Cyanidation Effluents
José R. Parga1, Jesús L. Valenzuela2, Héctor Moreno3, Jaime E. Pérez1
1Department of Metall ur gy an d Mat eri al s Science, Institute Technology of Saltillo, Saltillo, México
2Departament of Chemistry and Metallurgy, University of Sonora, Hermosillo, México
3Chemistry Department, Institute Technology of Laguna, Torreón, México
Received August 26, 2011; revised Septembe r 13, 2011; accepted Se pt em b e r 23, 2011
Cyanidation is the main process for gold and silver recovery from its ores. In this study, a process is pro-
posed to recover copper and cyanide from barren solutions from the Merrill-Crowe cementation process with
zinc dust. This technology is based on inducing nucleated precipitation of copper and silver in a serpentine
reactor, using sodium sulfide as the precipitator, and sulfuric acid for pH control. Results show that pH value
has a significant effect on copper cyanide removal efficiency, and it was determined the optimal pH range to
be 2.5 - 3. At this pH value, the copper cyanide removal efficiency achieved was up to 97% and 99%, when
copper concentration in the influent was 636 and 900 ppm. respectively. In this process (sulphidiza-
tion-acidification-thickening-HCN recycling), the cyanide associated with copper cyanide complexes, is re-
leased as HCN gas under weakly acidic conditions, allowing it to be recycled back to the cyanidation process
as free cyanide. Cyanide recovery was 90%. Finally, this procedure was successfully run at Minera William
in México.
Keywords: Precipitation, Cyanide Removal, Copper Recovery, Cyanidation
1. Introduction
Actually, the most common process for gold and silver
recovery from ores is cyanidation, due to the selectivity
of free cyanide for both metals, and the stability of the
cyanide complex (2
Au , k = 2 × 1038) [1 ]. Chemical
recovery of gold from Merrill-Crowe process cyanide
solution, involves two different operations: 1) gold dis-
solution, where it is oxidized and dissolved to form Au (I)
ion and cyanide complex 2, and 2) precipitation
by reduction of metallic gold . In the cyanidation pro cess,
free cyanide ions in solution can only be provided at a
pH of 9.0. The pH of the pulp can be increased adding
alkalis (e.g. Ca(OH)2, NaOH, etc.), known as protective
alkalis. It is accepted that gold dissolution in cyanide
solutions occurs as a sequence of two reactions, as
shown in Equations (1) and (2). These reactions apply to
silver as well.
( CN)
Au(C N)
22 2
22 2
 
Elsner’s Equation (1) shows that oxygen is critical for
the dissolution of gold. Stoichiometry of the process
shows that 4 moles of cyanide are needed for each mole
of oxygen present in solution. At room temperature and
standard atmospheric pressure, approximately 8.2 mg of
oxygen are present in one liter of water. This corre-
sponds to 0.27 × 10–3 mol/L. The corresponding sodium
cyanide concentration for a complete reaction (molecular
weight of NaCN = 49) sh ould be equa l to 4 × 0.27 × 10–3
× 49 = 0.05 g/L or approximately 0.01%. This was con-
firmed in practice at room temperature by a very dilute
solution of NaCN of 0.01% - 0.5% for ores, and 0.5% -
5% for gold and silver concentrates [2]. Gold dissolution
is an electrochemical reaction in which oxygen takes up
electrons at one section of the metallic surface [cathodic
zone], while the metal gives them up in another section
[anodic zone]. Details of this electrochemical reaction
have received considerable attention and under certain
circumstances the reaction is limited by the coupled dif-
fusion of CN and O2 to the gold surface. Dissolution
rate is normally mass-transport controlled in cyanide
solutions and the activation energy is 8 - 20 kJ/mol [3].
The concentration of cyanide used to dissolve gold in
ores is typically higher than the stoichiometric ratio, due
192 J. R. PARGA ET AL.
to the solubility o f other minerals. Free cyanide produces
complexes with several metallic species, especially tran-
sition metals, which show a broad variation in both sta-
bility and solub ility [4]:
 (3)
Many common copper minerals are soluble in the di-
lute cyanide solutio n, typical of leach conditio ns found in
the gold cyanidation process. Minerals such as azurite
and malaquite, are fast leached and soluble in dilute cya-
nide solutions. Enargite and chalcopyrite leach more
slowly but are sufficient soluble to cause excessive cya-
nide loss and contamination of leach solutions with arse-
nic [5]. For example, in the cyanidation of malachite and
azurite minerals, the copper carbonate component lea-
ches as follows:
Malachite and Azurite (Leaching rate = Fast):
2CuCO8NaCN2Na CuCN2Na COCN 
Some others possible reactions are shown below:
Cuprite (Leaching rate = Fast):
Cu O6NaCNH O2NaCuCN2NaOH  (6)
Tenorite (Leaching rate = Fast):
 (7)
Chalcocite (Leaching rate = Fast):
Covellite (Leaching rate = Fast):
The cyanidation of Cu (II) minerals with the conse-
quent formation of cynogen, (CN)2 results in the loss of
cyanide in the proportion of 0.5 mol of cyanide per mole
of Cu (II) leach ed, i.e., 0.39 kg NaCN/kg Cu (II). Cupr ic
cyanogen complexes are first formed and then they are
broken down to the cuprous form liberating cyanogen,
which in turn reacts with alkali to form cyanide and cy-
anate [6].
1.1. Copper Removal after the Merrill-Crowe
The presence of cyanide-soluble copper affects gold and
silver recovery from the cyanide solutions. In the
Merrill-Crowe process, the copper is precipitated along
with gold and silver, resulting in a higher consumption of
zinc dust, fluxes in the smelting of the precipitate and
shorter life for crucibles. For these reasons, copper must
be separated from the precious metals by digesting the
silver/gold/zinc precipitated in sulfuric acid prior to
smelting. This is a common practice in William Mining
Co., which produ ces a copper sulfate acidic solution that
goes to the zinc and arsenopyrite froth flotation circuit or
to an iron cementation process before disposal. Cementa-
tion of copper using scrap iron is practiced when the
quantity of copper makes recovery worthwhile. Also, to
increase the recovery of silver and gold, with less copper,
it is necessary to use conditions which result in the for-
mation of 2
and . High pH values
and high free-cyanide concentrations stabilize copper in
solution resulting in lower levels of copper. Increment of
copper in the barren solution poses serious metallurgical
problems in the cyanidation circuit and it is necessary to
include a process to strip the copper, prior to the gold
and silver leaching step. Failure to do so will result in
lower dissolution of precious metals and production of
high-copper-silver/gol d bullion.
This research fo cuses on the removal of copper before
smelting the gold and silver precipitate and in prevention
and/or minimization of the impact of copper in the barren
solution after the filter press in the Merrill-Crowe proc-
ess. Treatment of high-copper silver/gold leach solutions,
before or after precious metals recovery, is focused on
precipitation of copper as chalcocite (Cu2S) and cyanide
1.2. Cyanide and Copper Recovery Processes
There is a growing interest for the recovery of both cop-
per and cyanide from silver and gold barren solutions
due to high cyanide consumption costs. William Mining
Co. is also interested in reducing costs in this way. The
cost of recovering and recycling cyanide from the barren
leach solution will be lower than the cost of purchasing
new cyanide. It has been almost a century since the
Mills-Crowe process for cyanide regeneration was de-
veloped by the Mining Company Beneficiadora de
Pachuca in México (England Patent No. 241669, 3.9.24)
[7] and until today no significant changes to the process
have been made. The simplest process for cyanide recy-
cling involves acidifying the barren clarified solution
(pH between 2 and 5). During acidification, free cyanide
Copyright © 2011 SciRes. ACES
and relatively weakly complexed cyanide (Ag, Cu, Zn,
Fe) are converted into HCN gas, which is then volatil-
ized by passing a stream of air bu bbles through the solu-
tion. The air/HCN gas stream is scrubbed in a caustic
solution in a second tower reactor to convert the HCN
back into free cyanide ions for recycling [8]. In this
process copper and silver are not recovered for resale.
This has prompted interest to also recover copper by se-
lective metal sulphide precipitation. The copper sulphide
precipitate is then recovered by conventional clarifica-
tion and filtration to produce a filter cake (45% to 60%
Cu) which can be shipped to a copper smelter.
Among the chemical precipitation methods, precipita-
tion of metal hydroxides is the most conventional, but it
suffers from shortcoming, such as high solubilities for
some metals. Sulphide precipitation of metals is a viable
alternative process for copper recovery from the barren
cyanide solutions because of the possible high degree of
metal removal over a broad pH range. However; hydro-
gen sulfide is odorous and highly toxic. It tends to accu-
mulate in poorly ventilated spaces because it is heavier
than air. Exposure to low level concentrations of this gas
can result in eye irritation, sore throat and cough, short-
ness of breath, and fluid in the lungs [9]. Sulphide pre-
cipitation of metals has several advantages over hydrox-
ide precipitation, such as low solubility, high stability of
metal sulphides, fast reaction rates, better settling prop-
erties and potential for re-use of sulphide precipitates by
smelting. The thermodynamic equilibrium involved in
metal sulphide precipitation has been proposed as [10]:
p1 2
 
 
, 1
Kp2 2p2 2
HSSH ,K,pK17.4
 
22 s
 (12)
 
These equations show that concentration of sulphur
species is a strong function of pH. The pK2 value is cur-
rently the most reliable value.
The use of sulphide precipitation process for copper
and cyanide recovering after cyanidation has a key ad-
vantage, the ability to operate in the barren solution to
first recover copper/silver and after that establish acidic
conditions in the solution. This results in rapid release of
free cyanide (HCNgas) that is easily recoverable by vola-
tilization at lowered pH value.
If cyanide ions are present in the barren solution after
precipitation from the Merrill Crowe process as free cya-
nide (pKa = 9.4), it is possible to convert 99% of the
cyanide into HCN gas by lowering the pH value of the
solution to about 6:
 (14)
On the other hand, if cyanide ions are present as me-
tal-cyanide complexes, pH must be lowered to more aci-
dic values to break down the complex and produce HCN
gas. As an example, the weak zinc-cyanide complex (log
B4 = 17.4) breaks down completely at a pH close to 5,
producing zinc sulfate as an aqueous soluble species,
plus HCN gas:
 
24 4
4s g
  (15)
The copper cyanide complex does not break down
completely, even in strong acid solution, unless there is
an oxidant present in the solution. In the absence of an
oxidant, the copper tricyanide (which is the most stable
copper complex under normal cyanidation conditions:
log B3 = 28) decomposes to form a CuCN precipitate,
plus HCN gas (Equation (16)), at pH values lower th an 3.
Hence, 33% of potentially recoverable cyanide is lost to
the precipitate:
 
 g
Barren solution in the William Mining Co. process
also contains ferrocyanide and cuprous cyanide that, a
pH = 4, produces doub le metal cyanide precip itates such
as Cu2Fe(CN)6 and Cu4Fe(CN)6:
 
From the stoichiometry, it can be seen that the ferro-
cyanide molecule releases the third molecule of CN from
the copper tricyanide complex. Therefore, the presence
of ferrocyanide results in increased recovery of cyanide
from the copper-cyanide species.
When thiocyanate is present, as it is often the case
when leaching sulphide-bearing ores, insoluble CuSCN
may also be responsible for copper precipitation and
HCN gas formation in acid conditions, the following
reaction show this behavior:
 
 (18)
Addition of sulphide ions (Na2S) to the acidified cya-
nide solution results in the precipitation of cuprous sul-
phide (chalcocite), which is favored because of its ex-
tremely low solubility (Ksp = 2.3 × 10–48) [11]. The fol-
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lowing reaction takes place:
  
24 22
Stoichiometric rate of sulfide is approximately 0.25
grams S2– per gram of copper, 0.44 grams NaHS per
gram of copper or 0.61 grams Na2S per gram of copper.
However, the actual sulfide dosage required for near-
complete copper precipitation is normally in excess of
200% due to additional ions in the barren solution. In
precipitating copper, sulfide addition also results in the
near-complete precipitation of silver, as shown in the
following reaction:
  
  (20)
Based upon reactions 15 - 20, acid conditions may
cause the dissociation of the complexes, due to the for-
mation of some copper precipitate and subsequent libera-
tion of HCN by volatilization, considering these reac-
tions, up to 99% of copper could be recovered and HCN
gas could be stripped from the barren solutions and ad-
sorbed in an alkali solution of NaOH. The simplified
chemistry of the process is presented in the following
HCNNaOHNaCNH O (21)
The precipitate is a sellable copper product on its own,
or can be blended with the arsenopirite flotation concen-
trate from the flotation sulphide plant.
2. Materials and Methods
Experiments were carried out on barren cyanide solution
after the filter press on the Merrill Crowe process. The
pregnant solution came from the cyanidation leach plant
(500 ton/day), where the ores, from the Minera William
mines, are a mixture of oxides and sulfides, with the
copper ranging from 0.04% to 0.25% as the norm an
average sample contains: 1.7 g/ton Au, 100 g/ton Ag,
0.6% Pb, 0.61% Zn, 0.12% Cu, 2.3 % Fe and 2% of As.
A wet screen analysis of the plant sample indicated that
the granulometry was 80%—74 m. The leaching practice
in the plant was: leach pulps containing 40% solids over
a period of 72 hours leaching at Ph = 11.0, O2 = 5 ppm
and 2 kg/ton NaCN; leached residue: 0.20 g/ton Au and
0.18 g/ton Ag. Then, copper precipitation and cyanide
regeneration experiments were performed to determine
the effect of different process conditions on the solids of
copper/silver sulphide produced by sulphide precipitation.
Precipitation experiments were carried out in a 1 liter
round-bottomed reaction vessel with ports for an over-
head stirred, a gas sparger and a pH electrode. The pH
meter is VWR 8005 Scientific and stirring motor with a
glass impeller driven BDC 1850 CAFRAMO and cone
size settler (1000 ml). The barren solutions used had
copper, silver, zinc and iron ions of varying concentra-
tion. The pH of the barren was adjusted to the required
level with sulfuric acid and then a mixture of Na2S/water
was added. All experimental samples of the liquor and
solid were taken at known times, solutions and solids
from the process were separated by filtration through
cellulose filter paper. The sludge from the precipitation
was dried either in an oven or under vacuum at room
temperature. Analysis of copper, silver, zinc, iron, and
arsenic were performed by digestion of the precipitate
and subsequent ICP/Atomic Emission Spectrometry de-
termination and free cyanide content was determined
directly via titration, whereas the total cyanide was
measured by means of titration after distillation. At the
end of the experiment, HCN volatilization reached effi-
ciencies above 97% and the capture of cyanide gas by
NaOH (1 M) solution was almost 95%.
3. Results and Discussion
The experimental results of the copper, silver, zinc and
iron precipitation as well as CN removal (%) at different
pH values are presented in Table 1.
Results show that pH has a significant effect on copper
cyanide removal efficiency, and it was determined the
optimal pH range to be 2.5 - 3. With these pH values,
when influ en t copp er co n c entr atio n was 636 pp m, co pp er
cyanide removal efficiency was 99%. Some black pre-
cipitates were observed in the solution of experiments 2
to 6; which suggested the presence of copper, silver, ar-
senic, zinc and iron as sulphides. The presence of these
sulphides was confirmed in Figure 1. The measured
sample, which was collected from experiments of pH 2,
3 and 4 (see Table 2), gives rise to peaks corresponding
to covellite, esfalerite and pyrite. The size, EDAX and
morphology of the solids are also shown in Figure 1 by
SEM micrograph and EDAX analysis. The solids in the
precipitate are spherical and approximately 100 nm in
The SEM micrograph confirms the excellent crystal-
linity of synthetic covellite (CuS) formed during the sul-
phide precipitation process. The EDAX chemical analy-
sis pattern of the precipitate at different pH values is
shown in Table 2.
Results of Table 2, indicate that pH = 2 to 3 is the best
condition for the sulphide precipitation of copper, be-
cause the high recoveries > 99% of Cu and excellent
Copyright © 2011 SciRes. ACES
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Table 1. Results of copper, silver, zinc and iron sulphide precipitates and CN removal at different pH values.
Ag Zn Cu Fe Na2S (gr s) pH CN Removal (%)
Feed Barren Solution(ppm) 0.1 184 636 4 0 10.95
Solution 1 (ppm) 0 73 389 2 1.0 6 68
Precipitate 1 (%) 124 17.60 44.90 1.2- -
Solution 2 (ppm) 0 93 420 2 0.5 6.0 63
Precipitate 2 (%) 121 21.40 40.70 1.0- -
Solution 3 (ppm) 0 22 31 2 1.0 5.5 75
Precipitate 3 (%) 114 11.90 51.70 1.0- -
Solution 4 (ppm) 0 8 0 0 1.0 5.0 80
Precipitate 4 (%) 119 13.4 51.27 1.0- -
Solution 5 (ppm) 0 32 0 0 1.0 4.5 95
Precipitate 5 (%) 138 9.92 56.34 0.9- -
Solution 6 (ppm) 0 54 0 0 1..0 4 96
Precipitate 6 (%) 118 1.49 62.68 1.1- -
Solution 7 (ppm) 0 40 0 0 1.0 3.0 99
Precipitate 7 (%) 129 9.53 62.24 1.1- -
Solution 8 (ppm) 0 134 0 0 1.0 2.5 99
Precipitate 8 (%) 106 11.24 60.5 0.9- -
Table 2. EDAX analysis of solids precipitates at different pH values.
pH = 2 pH = 3 pH = 4
Element Weight% Atomic% Weight% Atomic% Weight% Atomic%
O K 7.69 20.37 9.64 24.35 7.73 20.03
Na K 2.51 4.52
S K 27.85 36.80 29.02 36.57 26.44 34.18
Ca K 0.60 0.61 0.45 0.46
Fe K 2.06 1.56 1.96 1.42 1.90 1.41
Cu K 43.46 28.98 41.19 26.19 39.94 26.06
Zn K 18.94 12.28 17.58 10.87 21.04 13.34
Totals 100.00 100.00 100.00
Figure 1. SEM micrograph (×5000) and Chemical analysis of the powder as determined by EDAX, shows the presence of
copper, sulphur, zinc and ir on in a sulphide particle.
3.1. Industrial Application A feed pump located in the precipitation area.
A line carrying barren solution at a rate of 10 li-
ters/second, with 1500 ppm of cyanide, 600 to 900
ppm of copper complexes and 0.1 ppm of silver and
at a pH of 11. At this flow rate precipitation of cal-
cium sulphate (scale) would not occur.
Based on the experimental evidence, obtained with the
sulphide precipitation study for copper and cyanide
removal from the barren solution after the Merrill-
Crowe process, this process was installed on a mine
site at full scale. A Serpentine. Barren solution is currently feed
along with Na2S solution and sulphuric acid, to a 4
inch plastic pipe section in the shape of a SER-
PENTINE, (with inside tripack rings mixers as tur-
bulence promoters).
A simplified process flow diagram, which uses so-
dium sulphide to precipitate copper/silver, and to con-
vert cyanide to HCN gas, under acid conditions (pH 2
to 3) is shown in Figure 2. Three enclosed vacuum vessels of various sizes/ The system consists of:
Copyright © 2011 SciRes. ACES
Figure 2. A schematic diagram, showing the SERPENTINE process for Cu, Ag precipitation and cyanide recovering.
shapes meant to be sulfide precipitate collectors.
Formulation of poly-electrolyte conditioners that
effectively flocculate the fine metal sulfide particles
has eliminated the difficulty in separating the pre-
cipitate from the discharge and has resulted in
sludges that are easily dewatered.
A HCN gas collection system, located over all ves-
sels with a gas adsorption tower, with sodium hy-
droxide as the absorbant.
A pump in the treated barren solution line to feed
the filter press.
In five continuous working days the treated solution
exited the circuit at a pH value of 4, carrying about 0 to
10 ppm of copper and 200 ppm cyanide and was
pumped to two neutralizing (pH 7) tanks.
4. Conclusions
The SERP ENTINE system is a viab le technology for th e
recovery of copper, silver and subsequent recovery of
HCN gas by scrubbing in NaOH.
Advantages of the SERPENTIN include: an odor free
hermetic process and compact treatment facility, high
precipitation rate of copper and silver (99%) and rela-
tively low operation cost, and also the precipitate is a
sellable copper/silver product. However; the main ad-
vantages of using the SERPENTIN system are: low en-
ergy consumption, production of high grade copper sul-
phide precipitate in the range of 40% to 55% of Cu with
130 gr/ton Ag, and recoveries of cyanide of 90%.
5. Acknowledgements
The authors acknowledge the support of this project to
Minera William in México, the National Council of Sci-
ence and Technology (CONACYT) and to the Dirección
General de Educación Superior Tecnológica (DGEST)
from México.
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