Ammonia Leaching: A New Approach of Copper Industry in Hydrometallurgical Processes

Abstract

Ammonia and ammonium salts have been recognized as effective leaching agents in hydrometallurgical processes due to low toxicity and cost, easy recovery and high selective recovery of metals. New research findings on considerable advantages of leaching by these agents and elimination of problems associated with acid leaching have resulted in a new approach in the world to this method. The investigations in this field indicate more frequent use of this method for extracting copper from ore and concentrate relative to other basic metals. In this paper, an attempt was made to describe the basis and different ammonia leaching methods and present the major research activities in this field for copper. Also latest findings and related novel processes have been presented. Comparisons including assessment of advantages and disadvantages of this method relative to acid leaching method, kinetic study of copper ammonia leaching and evaluation of Eh–pH diagrams in a system containing water and ammonia are other parts of this study. Finally, by describing the studies on copper extraction from the resulting pregnant solutions, the applicable extraction agents have been reviewed.

Full text

REVIEW ARTICLE
Ammonia Leaching: A New Approach of Copper Industry
in Hydrometallurgical Processes
Vahid Radmehr •Seyed Mohammad Javad Koleini •
Mohammad Reza Khalesi •Mohammad Reza Tavakoli Mohammadi
Received: 26 November 2012 / Accepted: 30 October 2013 / Published online: 10 December 2013
ÓThe Institution of Engineers (India) 2013
Abstract Ammonia and ammonium salts have been
recognized as effective leaching agents in hydrometallur-
gical processes due to low toxicity and cost, easy recovery
and high selective recovery of metals. New research find-
ings on considerable advantages of leaching by these
agents and elimination of problems associated with acid
leaching have resulted in a new approach in the world to
this method. The investigations in this field indicate more
frequent use of this method for extracting copper from ore
and concentrate relative to other basic metals. In this paper,
an attempt was made to describe the basis and different
ammonia leaching methods and present the major research
activities in this field for copper. Also latest findings and
related novel processes have been presented. Comparisons
including assessment of advantages and disadvantages of
this method relative to acid leaching method, kinetic study
of copper ammonia leaching and evaluation of Eh–pH
diagrams in a system containing water and ammonia are
other parts of this study. Finally, by describing the studies
on copper extraction from the resulting pregnant solutions,
the applicable extraction agents have been reviewed.
Keywords Leaching Extraction Ammonia 
Copper
Introduction
The use of sulfuric acid in an acid leaching process is a
conventional method for recovery of copper, but in some
cases it is necessary to use other reactants such as ammo-
nia. Ammonia has been known as an effective leaching
agent in hydrometallurgical procedures, although it has
been less studied than the chemical agents used in leaching.
Ammonia leaching was first used for recovery of non-
ferrous metals such as copper from oxide ores or ores
containing pure copper, but gradually the use of this
technology was developed from more traditional elements
of copper, nickel and cobalt to the extractive metallurgy of
zinc, cadmium, silver and gold [1].
The aim of this paper was to review studies in various
fields of ammonia leaching of copper and extraction of
copper from the resulting pregnant solution by solvent
extraction process.
History
The first industrial application of this process related to
hydrometallurgical recovery of copper was put into oper-
ation in 1916, and the two factories of Kennecott in Alaska
(Fig. 1) and Calumet in northern Michigan were separately
established for copper recovery.
Although copper oxide and pure copper were first used
to recover copper due to high solubility in ammonia, later
on ammonia leaching of sulfide ores was also considered.
The common method was the use of oxide roasting at high
V. Radmehr (&)S. M. J. Koleini M. R. Khalesi 
M. R. Tavakoli Mohammadi
Department of Mining Engineering, Faculty of Engineering,
Tabiat Modares University, Jalal Ale Ahmad Highway,
P.O. Box 14115-111, Tehran, Islamic Republic of Iran
e-mail: v.radmehr@ut.ac.ir
S. M. J. Koleini
e-mail: koleini@modares.ac.ir
M. R. Khalesi
e-mail: mrkhalesi@modares.ac.ir
M. R. Tavakoli Mohammadi
e-mail: mr.tavakolimohammadi@modares.ac.ir
123
J. Inst. Eng. India Ser. D (October 2013–March 2014) 94(2):95–104
DOI 10.1007/s40033-013-0029-x

temperatures to convert sulfides to oxides in order to
facilitate dissolution in ammonia solutions [1].
In 1947, a process (Fig. 2) in this regard was invented
by Sherritt Gordon. In this process, ammonia leaching of
sulfide minerals of copper, nickel, cobalt and iron was
conducted at a temperature of about 105 °C and air pres-
sure of 0.8 MPa. Under these conditions, copper, nickel
and cobalt were dissolved but iron was precipitated in
hydroxide form. By separating the unsolved solids, the
pregnant solution was heated to evaporate excess ammonia.
After that, first the solution was allowed to react with air to
oxidize all its thiosulfates, and was then reduced using
hydrogen. Thus, metal nickel was obtained through
reduction of amine nickel ion. It was necessary to add some
metallic nickel powder to the solution as budder to start the
reaction, or add an appropriate catalyzer to the solution to
accelerate the reaction. Reduction operations continued up
to precipitation of nickel without affecting the cobalt ions
in the solution [3]. The good results of this innovative
process led to the construction of a factory in Canada in
1954 [4].
Another innovative process in this field is the Arbiter
process. This process was introduced in 1970 in a pilot
plant in Arizona under the supervision of Arbiter Com-
pany. A few months later, a plant with 91 t of daily
copper production capacity was commercially set up in
Anaconda. In this leaching factory, ten reservoirs were
used equipped with powerful mixers connected to five
thickeners as opposite flow, each with a volume of 14 m
3
,
of which the first overflow was filtered for the pregnant
solution. The basis of this process was ammonia leaching
of chalcopyrite concentrates in presence of oxygen at
70–80 °C temperature to form amine copper sulfate and
iron hydroxide.
Fig. 1 Flow sheet of Kennecott plant
Fig. 2 Flow sheet of Gordon
process [2]
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123

The charged output solution from the filter was extracted
using LIX65N, and stripped by sulfuric acid. The resulting
concentrated solution was subject to the electrolysis to
produce metal copper. Finally, lime was added to the
output raffinate and the pulp was distilled to recover
ammonia and precipitate gypsum for use.
Flow sheet of Arbiter process is shown in Fig. 3.This
process was not welcomed by the industry due to technical
and economic problems in the final stage, the recovery of
ammonia and due to weak elimination of sulfate ions from
the solution at the investigated temperature [4].
Another process to extract nickel, cobalt and copper
from pyrite flotation concentrate was developed by INCO
of Copper Cliff, Ontario, Canada with satisfactory results.
This process involves oxide roasting of sulfide concentrate
followed by rapid quenching of the calcine under reducing
conditions and eventual leaching in an ammonia–ammo-
nium carbonate solution [5].
Although most studies in the past, focused on the disso-
lution of sulfide concentrates in reactors and many of them
led to low process kinetics and high temperatures and pres-
sures being needed, most recent studies have been directed
towards the dissolution of oxide ores including malachite.
Table 1lists the titles of studies conducted in recent
decades, along with chemical agents, parameters under
study and the resulting recoveries.
In new research work [12], suitable parameters of
ammoniacal column leaching for Meskani Mine copper ore
in Iran were determined. Taguchi method was used to
design experiments and evaluate the behavior of the
parameters like ammonia concentration, acid flow rate, size
of ore and leaching time. By investigating the effect of
above parameters on copper oxide recovery rate, the opti-
mum condition for concentration, acid flow rate, size of ore
and leaching time were found to be 40 g/l, 60 cm
3
/h,
1–4.75 mm and 10 days, respectively. Studies showed that
the ammonia concentration and size of ore were more
important parameters. The final experiment was conducted
to confirm the optimal conditions obtained from the
Taguchi method in which the copper recovery was 80.6.
Methods
In general, ammonia leaching methods include:
Neutral method: In this method, the metals are dissolved
without any oxidizing or reducing agents. Leaching of
copper, zinc or molybdenum oxides are of this type.
Oxide method: In this method, leaching requires the use
of an oxidizer to oxidize solids such as metals or sulfide
minerals.
Reduction method: In contrast to the oxide leaching
method, a reducing agent is used in this method. It is
mainly used for dissolving metals from highly oxidized
ores such as ocean floor manganese nodules and lateritic
ores [1].
Fig. 3 Arbiter process in
Anaconda mine [4]
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123

Advantage
Ammonia leaching has many advantages over acidic
leaching. The main advantages are as follows:
a. Marked difference of the two methods. Operations in
the alkaline solution enable the use of ores with high
carbonation, which cannot be used in acidic leaching
due to high consumption of acid.
b. The problems associated with equipment corrosion are
eliminated due to acid replacement.
c. This type of process is more appropriate for mass
leaching of low-grade ores and reservoir leaching of
high grade ores, although this choice also depends on
the grade and the amount deposited.
d. Since metals such as iron and manganese are not
soluble in ammonia or their complexion capacity with
ammonia is low, ammonia leaching has a suitable
selective capacity for the desired metal relative to the
mentioned metals, while the high solubility of these
elements in acidic factors leads to high consumption of
these agents and a non-economic leaching process.
e. In the leaching solution, ammonia prevents calcium
solubility in the presence of carbonate and small
amounts of sulfate. This results in reduced permeabil-
ity in heap due to jarosite or gypsum precipitation, and
high acid consumption in acid leaching is also avoided.
f. Another element causing problem in permeability and
drainage of the heap is silica. Unlike acid types,
ammonia does not react with different compounds of
this element such as alumina silicates and
ferrosilicates.
g. Elimination of the problems associated with formation
of non-filterable precipitates during pH adjustment at
acid leaching factory and non-requirement of viscosity
solutions are other advantages of ammonia leaching.
h. After completion of ammonia leaching, problems
associated with heap washing, neutralization and
long-term monitoring to prevent acid runoff are
minimized. Moreover, the residual ammonia in the
soil can act as fertilizer for growing plants.
In addition to the above advantages, characteristics such
as low toxicity, low cost, easy recyclability and economic
gain are the main factor for increased use of ammonia in
hydrometallurgical processes particularly leaching [1,13].
Disadvantages
a. Lower capacity of leaching of acidic compounds,
although not significantly.
b. The use of ammonia due to its high evaporation
capacity is more difficult than acidic compounds,
Table 1 Activities performed in the past decade
Topic Lixivent Parameters Recovery in
optimum
condition
Reference
Dissolution kinetics of an oxidized copper
ore in Ammonium chloride solution
Ammonium
chloride
Ammonium chloride concentration, particle size, solid/
liquid ratio, stirring speed and reaction temperature
–[6]
Leaching of malachite ore in NH3-
saturated water
Ammonia Ammonia concentration, particle size, temperature,
stirring speed, and solid-to-liquid ratio
–[7]
Dissolution kinetics of malachite in
ammonia–ammonium carbonate
leaching
Ammonia–
ammonium
carbonate
leaching
Leaching time, ammonium hydroxide, and ammonium
carbonate concentration, pH, stirring speed, solid/
liquid ratio, particle size and temperature
98 [8]
A study on the oxidative ammonia–
ammonium sulphate leaching of a
complex (Cu–Ni–Co–Fe) matte
Ammonia–
ammonium
sulphate
– 93.8 [9]
Ammonia pressure leaching for LUBIN
SHALE MIDDLINGS
Ammonia and
ammonium
sulfate
Temperature, oxygen partial pressure, ammonia and
ammonium sulfate concentration and stirring rate
95 [10]
Leaching of copper from tailings using
ammonia–ammonium chloride solution
and its dynamics
Ammonia–
ammonium
chloride
Leaching time, the concentration of ammonia, solid/
liquid ratio and temperature
75 [11]
Dissolution kinetics of low grade complex
copper ore in ammonia–ammonium
chloride solution
Ammonia–
ammonium
chloride
Concentration of ammonia and ammonium chloride,
ore particle size, solid-to-liquid ratio and temperature
––
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123

although through hydrolysis of urea, the risk of its
transport can be minimized [13].
c. Extractors so far used for copper recovery from
ammonia medium do not have high efficiency.
Kinetics
The kinetics of copper dissolution in ammoniacal solution
has been comprehensively studied. Lu and Graydon [14]
showed that the total concentration of ammonia and oxy-
gen play an important role in determining the overall rate.
They defined the overall expression of rate as follows:
Rate ¼2ADO2 O2
½ðKNHþ
4½NHþ
4þKNH3½NH3Þ
8DO2 O2½þdKNHþ
4½NHþ
4þdKNH3½NH3ð1Þ
In this regard, KNH3and KNHþ
4are the rate constants and
dis the diffusion layer thickness that can be determined
based on the continuity equation. According to their
studies, whenever the overall reaction was of the limited
diffusion type, penetration layer thickness was found to be
0.6 910
-3
cm. They also showed that the dissolution
mechanism is such that due to the formation of a kind of
copper oxide during leaching process, the dissolution of
copper is inhibited. However, the cupric ion had a catalytic
effect on the dissolution of metal copper known as the self-
catalytic phenomenon.
Meng et al. [15] evaluated the effect of oxygen on
copper dissolution kinetics in ammoniacal solution. Fig-
ures 4and 5show the results of their studies. It can be
clearly seen that under low oxygen pressure, the dissolution
of copper is a linear function of leaching time. In fact, the
dissolution process is electrochemical in nature. When air
or oxygen was used as an oxidizer, in low pressure, the
reaction was controlled with diffusion phenomenon, and in
high pressure, it was controlled by surface reaction.
Similar results were obtained in studies by other
researchers such as Habashi [17] and Luo et al. [18]. Some
researchers also studied the dissolution of copper minerals
in different ammonia solutions. In a study on malachite
leaching in ammonium chloride solution [6], it was deter-
mined that the dissolution rate is controlled by a compound
kinetics. Their mathematical model for the reaction kinet-
ics is as follows:
Kt¼121XðÞ
1=3þ1XðÞ
2=3ð2Þ
According to their calculations, the activation energy for
malachite dissolution reaction was found to be 71 kJ/mol.
In another study [8] on malachite leaching in ammonium
carbonate solution, it was established that there are two
steps for the dissolution reaction. Firstly, the malachite was
rapidly dissolved, but after completion of 10 % of reaction,
leaching rate was reduced with the formation of needle-
structured middle phase causing surface blockage. This
phase was mainly Cu(OH)
2
. This stage continue to exist
with the concurrent dissolution of malachite and the middle
phase. In the second stage of the reaction and after reaching
Fig. 4 The dissolution of copper in ammonia solution as a function
of time at various concentrations of ammonia (Stirring speed of
600 rpm and oxygen pressure of 304 kPa) [16]
Fig. 5 Effect of oxygen pressure on the dissolution rate of copper in
ammonia solution (Various concentrations of NH
3
and 26 °C
temperature) [16]
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123

90 % of the reaction, essentially all of malachite was dis-
solved and only the remaining middle phase was subject to
dissolution. Equations 3and 4show the reaction kinetics
for the first and second stages:
K1t¼11XðÞ
1=3ð3Þ
K2t¼11XðÞ
1=2ð4Þ
where X represents the solid reactant part, K
1
and K
2
rate
constants at the leaching time. According to their calcula-
tions, the activation energy for the first and second stages
were 64 and 75 kJ/mole, respectively.
Eh–pH Diagram for the Copper–Ammonia–Water
System
In Cu–NH
3
–H
2
O system, several soluble species are pres-
ent such as NH
3
,NH
4?
,H
?
,Cu
2?
,Cu
?
,Cu(OH)
3-
,Cu
(OH)
4
2
,Cu
2
(OH)
2
2?
, Cu (NH
3
)Cu(NH
3
)
2?
,Cu(NH
3
)
3
2?
,
Cu (NH
3
)
2
2?
, Cu (NH
3
)
3
2?
and Cu (NH
3
)
4
2-
. There are
therefore many equilibrium equations in the solution
relating these components in the solution to each other. To
use these equations to understand the different reaction
mechanisms in ammonia solutions, the Eh–pH diagram is
used. Eh–pH diagram shown in Fig. 6has been determined
for 25 °C temperature, Cu activity of 0.5 and total
ammonia (NH
3
and NH
4?
) concentration of 7 kmol/m
3
.
The broken lines indicate the following reactions:
2Hþþ2e¼H2ð5Þ
O2þ4Hþþ4e¼2H2Oð6Þ
According to this diagram, Cu (I) and Cu (II) complexes
with NH
3
are stable ionic species in neutral and alkaline
solutions. In the presence of excess ammonia, Cu(I) and
Cu(II) are also stable as Cu(NH
3
)
2
and Cu(NH3)
4
2?
. The
following equations show oxidation–reduction reactions of
Cu (II)/Cu (I) and Cu (I)/Cu:
Cu NH3
ðÞ
2þ
4þe¼Cu NH3
ðÞ
2þþ2NH3ð7Þ
Cu NH3
ðÞ
2þþe¼Cu þe¼Cu þ2NH3ð8Þ
The oxidation–reduction potential of Cu (NH3)
4
2?
/Cu
(NH3)
2
?
is more positive than that of Cu (NH
3
)
2?
/Cu. This
indicates that Cu (NH
3
)
4
2?
can oxidize metallic copper in an
ammoniacal alkaline solution. Moreover, the oxidation–
reduction potential of Cu (I)/Cu is more positive than that
of hydrogen evolution (Eq. 5), indicating that Cu(I) can be
preferentially reduced to metallic copper [17]. Figure 7
shows the schematic presentation of the process of
electrochemical dissolution of copper in ammonia solution.
There are limited electrochemical studies about ammo-
nia leaching of copper. Arzutug et al. [7] were among the
few people who used the chemical equilibrium in solution
to evaluate the general reactions in this process. Based on
their studies, during the process of ammonia leaching of
copper ore (malachite) to Cu(OH)
2
.CuCO
3
, first CuCO
3
was dissolved and was then converted to Cu(OH)
2
based on
the following reactions:
NH3gðÞþH2O$NH3aqðÞ ð9Þ
CuCO3aqðÞ$Cu2þaqðÞþCO2
3aqðÞ ð10Þ
NH3aqðÞþH2O$NHþ
4aqðÞþOHaqðÞ ð11Þ
Cu2þaqðÞþ2OHaqðÞ$Cu OHðÞ
2sðÞ ð12Þ
The formation of Cu(OH)
2
is succeeded by the
following reactions:
Fig. 6 Eh–PH diagram for Cu–NH
3
–H
2
O system (25 °C tempera-
ture, Cu activity of 0.5 and total ammonia concentration of 7 kmol/m
3
)
[19]
Fig. 7 Schematic presentation of the process of electrochemical
dissolution of copper in ammonia solution [17]
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123

Cu OHðÞ
2sðÞ$Cu2þaqðÞþ2OHaqðÞ ð13Þ
Cu2þaqðÞþ2NH3aqðÞ$Cu NH3
ðÞ
2þ
2aqðÞ ð14Þ
Cu NH3ðÞ
2þ
2aqðÞþ2NH3aqðÞ$Cu NH3
ðÞ
2þ
4aqðÞð15Þ
Firstly, the intermediate compound Cu(NH
3
)
4
2?
and
finally the Cu(NH
3
)
4
2?
complex was formed. These
researchers [7] proposed the following general reaction
for ammonia leaching of copper:
Cu OHðÞ
2:CuCO3sðÞþ8NH3aqðÞ
$2Cu NH
3
ðÞ
4

2þaqðÞþCo2
3aqðÞþ2OHaqðÞ
ð16Þ
Copper Extraction from Ammoniacal Solutions
Case Studies
Copper leaching from ore using ammonia should be fol-
lowed by selective extraction of copper using the extraction
agent in the solvent extraction process. LIX63 was the first
extraction agent used in this field. The low extraction
capacity of this extractor for copper transfer from the acidic
solution led to studying its extraction capability from the
ammonia solution. Although this extractor had a high
efficiency in the extraction of copper from ammoniacal
solutions, due to difficult stripping process, it was not
considered by the industry. Other extractor under study was
LIX64N. This extractor which is cheaper and more prac-
tical than LIX63, was commercially only used for the
recovery of copper from an ammoniacal solution in Arbiter
Process and in melting process. However, the low extrac-
tion ability of copper (12 g/l) and high ammonia loading of
it led to further studies of new extraction agents from the
ammoniacal solutions [20].
These efforts led to synthesis of P204 (D-2-ethyl hexyl
phosphate), which is an organophosphorous acid extractor.
It is reported that under 25 °C temperature, the two phase
contact time of 10 min and phase ratio of 1:1, an ammonia
solution with pH 10 and 20 % P204 concentration (volume
fraction), the copper extraction rate was found to be 93.9 %
[21]. However, P204 is highly soluble in alkaline solution,
and its commercial use in ammoniacal solution (9 \pH) is
impossible.
Subsequently, a beta-D-ketone extractor called LIX54
was used. This extractor had a high extraction potential for
copper and a low loading capacity for ammonia, and
copper strip from it needed low concentrations of the acid
[22]. In 1995, ESCONDIDA in Chile used this extractor for
extracting copper from an ammoniacal solution on a pilot
scale. However, due to a reaction between a keto group of
LIX54 and ammonia and production of ketamine, the
extraction agent disappeared. Therefore, copper strip from
the charged organic phase was difficult, and finally the
pilot plant was closed [23]. So, the choice of LIX54 was
considered inappropriate for copper extraction in ammo-
niacal solution.
In addition to beta-D ketones, ketoxymes and hy-
droxymes were also used for the extraction of copper from
ammoniacal solution. The results of studying some of these
compounds such as LIX84 indicate a very high extraction
capacity of copper (more than 50 g/l) from ammoniacal
solutions [24–28]. However, a hydroxeme like LIX84 had
serious problems of ammonia extraction in the ammoniacal
solution [29,30]. The ammonia present in the charged
organic phase must be eliminated by the electrolyte used
before attempting to strip copper. Otherwise, ammonia in
the charged organic phase is transferred to the pregnant
electrolyte after the stripping process, resulting in the
accumulation and precipitation of (NH
4
)
2
SO
4
in the
charged electrolyte [29]. Recently, in another research, to
decrease ammonia extraction in the charged organic phase,
stearically hindered beta-D ketone was used. In this study,
under the temperature 25 °C, 30 min contact time of two
phases, phase ratio of 1:1, Cu concentration of 3 g/l,
3 mmol/l total ammonia concentration, water pH of 8.43
and beta-D ketone concentration in organic phase of 20 %
(volume fraction), ammonia in the water phase was far less
extracted by the organic phase (only 14.5 mg/l), while the
copper extraction rate was 95.09 %. Table 2show the
general characteristics of the chelate type extractors used in
the extraction of copper from an ammoniacal solution [20].
Extraction Mechanism
In general, extraction equilibrium between Cu
2?
and a
chelation extractor is as follows:
Cu2þaqðÞþ2HR orgðÞ$CuR2orgðÞþ2HþaqðÞð17Þ
Table 2 Characteristics of copper extractors from ammoniacal
solutions [20]
Beta-D-ketone Hydroxeme Property
Moderate Strong Extractive strength
Excellent Good Ease of stripping
Low Moderate Viscosity
Very low Low Ammonia loading
Very good Very good Selectivity of copper
Fast Fast Kinetics
Good Good Stability
Very good Good Phase separation
Very high Very high Copper complex solubility
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123

If the aqueous solution contains copper and aluminum
ions, in sufficiently high pH levels where free ammonia is
released, amine complexes will be formed in aqueous
phase as follows:
Cu2þaqðÞþmNH3aqðÞ$Cu NH3
ðÞm½
2þaqðÞ ð18Þ
When the phases contact, ammonia in aqueous phase
may be extracted in the form of [Cu(NH
3
)m]R
2
(org)
expressed as follows:
Cu NH3
ðÞ
2þ
m
hi
aqðÞþ2HR orgðÞ
$Cu NH3
ðÞ
2þ
m
hi
R2orgðÞþ2HþaqðÞ ð19Þ
in which HR(org) represents extractor in the organic phase,
CuR
2
(org) extractor-copper complex in organic phase,
[Cu(NH
3
)
m
2?
](aq) amine-copper ion complex in aqueous
phase and [Cu(NH
3
)
m
2?
]R
2
(org) extraction compound of
Cu(NH
3
)
m
2?
and extractor in the organic phase [31].
Ammoniacal Leaching of Other Elements
In addition to studies on copper, ammonia leaching has
been used to recover some other basic elements.
Manganese nodules are in fact Ferromanganese oxide
ores containing copper, nickel and cobalt. These metal
oxides have been reported mainly in lateritic iron and
manganese minerals. Crushing these laterites using pyro-
metallurgy reduction or hydrometallurgical reductive dis-
solution is an important step for proper recovery of
precious metal. Therefore, the recovery process of man-
ganese nodules has been studied in two ways:
a. Initial pyrometallurgical preparation and then perform-
ing the metallurgical process.
b. Performing the hydrometallurgical process alone.
Initial pyrometallurgical preparation operations often
involve melting, reduction-roasting, sulfidation and chlo-
rination. Reductive dissolution takes place by Hydrochloric
acid, sulfuric acid, ammonia, etc. in the presence or
absence of reducing reactants such as sulfur dioxide, pyrite,
sodium sulfide, charcoal, alcohol, etc. [32–34].
In research conducted for processing of manganese
nodules by ammonia leaching [35–37], after reductive
roasting of the sample for releasing precious metal oxides
from manganese and iron oxide phase, reduction operations
were done to form copper, nickel and cobalt metals and
facilitate their ammonia leaching. During the ammonia
leaching process, in addition to dissolution of precious
metals such as copper, nickel and cobalt as stable amine
complexes, the dissolution of unwanted metals such as iron
and manganese was also prevented. Overall flowsheet of
this process is shown in Fig. 8.
Ammonia leaching has also been used for recovery of
nickel and cobalt from oxide, sulfide and complex ores. For
example, the so-called Caron process was developed to
recover nickel and cobalt from low-grade ores such as
lateritic ores with an ammonia–ammonium carbonate
solution. This process involves reductive roasting of nickel
ore followed by leaching using ammonia–ammonium
Fig. 8 Flowsheet of precious
metals’ recovery from
manganese nodules of the ocean
floor [35]
102 J. Inst. Eng. India Ser. D (October 2013–March 2014) 94(2):95–104
123

carbonate compound, commercially adopted in Nicaro in
Cuba, Marinduque in the Philippines and Queensland in
Australia [38].
Cadmium, gold and silver are other precious metals of
which ammonia leaching has been studied, although none
of the innovative processes used for their recovery has
found industrial application [39–43].
Summary
In this review, various topics related to ammonia leaching
of copper have been studied. The history of this process,
different methods involved in it, the advantages and dis-
advantages, kinetics, Purbeh diagram, the way to extract
copper from the pregnant solution obtained and extraction
agents used are different issues described in this paper.
Evaluation of the studies shows that, despite a long history
of using this process, easy use of acidic leaching agents and
their higher leaching capacity have prevented ammonia
leaching from being accepted as an acid leaching process.
In recent years, however, due to decrease in proper reserves
for acid leaching and considerable advantages of this
method such as lower toxicity, easier recovery and eco-
nomic gain, ammonia leaching has again been looked upon
as a possible option by researchers and industrialists.
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