Journal of Minerals & Materials Characterization & Engineering, Vol. 1, No.2, pp111-119, 2002
Printed in the USA. All rights reserved
111
Oxygenated Leaching of Copper Sulfide Mineral under Microwave-
Hydrothermal Conditions
Jiann-Yang Hwang, Shangzhao Shi, Zhiyong Xu and Xiaodi Huang
Institute of Materials Processing, Michigan Technological University
Houghton, MI 49931
Abstract
A study on microwave assisted chalcocite leaching was carried out with a
microwave hydrothermal reactor. The leaching was conducted with thick mineral
slurries (50-100g/l). The leaching media is a mixed solution of CuCl
2
and NaCl.
The investigation included the effect of leaching temperature, quantities of the
minerals per unit slurry volume as well as the initial concentration of cupric ions.
The results were discussed and compared with using conventional leaching
method.

Jiann-Yang Hwang, Shangzhao Shi, Zhiyong Xu and Xiaodi Huang Vol. 1, No. 2
112
1. Introduction
As the demands for environment protection are getting more and more urgent, extracting of
metals from their sulfide minerals by means of hydrometallurgical approaches becomes more
and more attractive. The well-demonstrated advantage of the hydroprocess over the conventional
pyroprocess is its avoidance of producing SO
2
, one of the top industrial pollutants. Another
potential advantage of the hydrometallurgical process is the direct electrolysis of copper sulfide
[1]. The direct electrolysis of copper sulfide minerals is viewed as the most cost effective and
environmentally clean process which industry has developed for over one hundred years [2].
Industrial transformation from the conventional pyrometallugical process to the
hydrometallurgical process depends on the overall advantages of a number of technical and
economic factors. One such factor is productivity. Only the process with high production rate can
be practicable.
Microwave energy has an amazing capability of accelerating chemical reactions. Successful
demonstrations include organic and inorganic synthesis [3,4], mineral digestion [5] and
extraction [6]. While the microwave assistance in hydrothermal synthesis of ceramic powders
has achieved the kinetic acceleration in one to two order of magnitudes [7], microwave assisted
chloride leaching of copper minerals at reflux temperature has achieved about three times
enhancement [8].
In this work the study of microwave-assisted leaching under hydrothermal conditions was
conducted. Oxygen was used as the exogenous pressure supply, since it had been employed as
the primary oxidizing agent for decomposition of the copper sulfide minerals [9, 10].
2. Experimental
The copper sulfide mineral was a chalcocite from Butte, Montana, supplied by Ward’s
Natural Science Establishment, Inc. Its chemical composition was analyzed by ALS Chemex
Labs, Inc., which showed that it contains 47.7% Cu, 21.4% S and 11.5 % Fe. X-ray diffraction
patterns (Fig.3) showed the mineral is actually composed of spionkoptite (Cu
39
S
28
), geerite
(Cu
8
S
5
), an unnamed phase (Cu
5
FeS
4
) and some pyrite (FeS
2
) and quartz as impurities. The
mineral was crushed and ball-milled to –200 mesh before leaching procedure. Possible iron
contamination from the milling was removed by means of a magnet. Aqueous solution of CuCl
2
-
NaCl-HCl was used as the leaching agent.
Leaching of the mineral was carried out in a self-installed microwave-hydrothermal
reactor, as shown in Fig.1. For each batch, 4000ml of leaching solution is charged into the
reactor. While the concentration of NaCl was kept constant (0.4M), the concentration of CuCl
2
varied for different leaching trials. The concentration of HCl was also varied accordingly in
order to keep the total concentration of Cl
-
constant (1.9M).
Leaching slurries were exposed to 2.45GHz microwave radiation. With a power of 4 kW
irradiation, the slurries were heated to their leaching temperatures. The power was then turned
down to an appropriate value to keep the leaching temperature constant. Oxygen was supplied
before the irradiation of microwaves and the pressure was kept constant throughout the
procedure. The slurries were stirred constantly at 300 rpm during leaching, which was detected
by a tachometer (Omega, HHT11).

Vol. 1, No. 2 Oxygenated Leaching of Copper Sulfide Mineral under Microwave-Hydrothermal
113
The leached solids were separated from their solutions and washed first with 1N HCl, then
with deionized water several times. After being dried in vacuum at 75°C, they were dissolved
into concentrated HCl for ICP analysis of copper in the solids. XRF and XRD characterization of
the solids were also performed in order to assist in the assessment of the leaching process.
3. Result and discussion
Fast leaching kinetics
The study shows that the microwave hydrothermal leaching is a fast process. Under
leaching conditions of temperature, 135°C; oxygen pressure, 45 psi; weighed-in CuCl
2
concentration, 0.25M; NaCl concentration, 0.4M; HCl, 1.0M, leaching of 400g of mineral was
completed in 3 hours and leaching of 200g of mineral was completed in 1 hour. XRF results
showed no signs of copper in these solids. Complete decomposition of the mineral was also
indicated by ICP analysis (Fig.2). XRD shows that the sulfur exists in the solids as elemental
sulfur and no other forms of copper sulfide compounds appeared (Fig.3).
It has been reported that conventional leaching has the capability to achieve over 90%
copper extraction within 30 minutes [10]. However, the weighed-in mineral in Ruiz, etc. was
considerably less than that in this experiment, 10g/l vs 50-100g/l. While the initial ratio of the
oxidant copper to the mineral copper in Ruiz’s case was about 1.21, the corresponding ratio in
this experiment was 0.33. The oxidation by Cu
2+
is a fast process, high starting [Cu
2+
]-to-mineral
ratio is able to dramatically increase the initial leaching rate [11]. Also, the ratio of the total
chloride ions to the mineral copper in their case was about 5.09, but the corresponding ratio in
this study was about 2.53. Higher chloride ion concentration also contributes to higher leaching
rate [12]. This study, however, selected to use low [Cu
2+
] and [Cl
-
] starting concentrations, in
order that more minerals can be charged and leached before [Cu
2+
] and [Cl
-
] reach saturation.
Effect of temperature
Fig.4 shows the leaching rate increase with temperature, although conventional leaching
observed a maximum at around 90°C. This different leaching profile in this study may be due to
the microwave-hydrothermal effect. At 90°C and below, much less microwave energy was
required to keep the temperature constant. The leaching results were similar to the conventional
approach. However, microwave energy required for keeping the leaching temperature increases
as the experimental temperature increases and reached to about 0.8 kW at 135°C.
Among a number of mechanisms dealing with the microwave enhancement effect, one
common recognition is that the high frequency electromagnetic waves have the capability to
concentrate their energy on charged ions (selectively super heating [13] these ions). For aqueous
system, although water is a good microwave absorber, its dielectric loss factor drops from above
20 to below 5 as the temperature rises from 3°C to 95°C [14,15]. We may assume a further
decrease of the dielectric loss factor of water at temperatures above its boiling point. High
temperatures tend to cause water molecules to become more symmetric, while the increasing
symmetry means the decrease of polarity and therefore the decrease of dielectric loss factor. The
decrease of the loss factor of water may facilitate the concentrating of microwave energy on the
reaction species and thus accelerate the leaching process.

Jiann-Yang Hwang, Shangzhao Shi, Zhiyong Xu and Xiaodi Huang Vol. 1, No. 2
114
Effect of CuCl
2
concentration
Fig.5 shows the effect of the weighed-in cupric chloride concentration on the mineral
decomposition. While the after-0.5h-leaching decomposition was affected by the concentration
of cupric chloride appreciably, the after-1.5h leaching decomposition remained almost
unaffected. This phenomenon implies that the leaching process was controlled by different
mechanisms in these two stages.
The oxygenated chloride leaching of copper sulfide minerals can be represented by the
following reactions [10]:
Cu
2
S + 2Cu
2+
= 4Cu
+
+ S
0
(1)
Cu
+
+ H
+
+0.25O
2
= Cu
2+
+0.5H
2
O (2)
Cu
2
S +2H
+
+ 0.5O
2
= 2Cu
+
+ H
2
O + S
0
(3)
Previous researchers have observed fast kinetics both for decomposition of copper sulfide
minerals by Cu
2+
[11] and for oxidation of Cu
+
to Cu
2+
by oxygen [16,17]. We therefore assume
that reaction (1) and (2) were responsible for the leaching in this case. Based on the
complexation preference of Cu
+
-Cl
-
to Cu
2+
-Cl
-
[18], we further assume that reaction (1) was
even faster than reaction (2) in the presence of substantial chloride ions in the solution.
At the initial stage, the oxidation ions, Cu
2+
, were directly weighed in and the different
weighed-in concentrations affected the rate of reactions, i.e. decomposition of the mineral. As
the leaching proceeded, the weighed-in Cu
2+
ions were consumed and further proceeding of
reaction (1) relied on the new Cu
2+
ions generated by reaction (2). The lower rate of the reaction
overshadowed the effect of weighed-in Cu
2+
concentration on leaching in the late stage.
The existence of microwave effect can not be identified from this aspect and more
informative experiments should be designed and carried out, which include the change of
microwave radiation intensity.
4. Conclusion
Fast kinetics of oxygenated chloride leaching of chalcocite has been observed under
microwave hydrothermal conditions. Thick mineral slurries (100g/l) can be completely
decomposed within 3 hours with relatively low weighed-in Cu
2+
and Cl
-
concentrations.
The fast leaching kinetics was attributed to the microwave-induced super heating of copper
ions, which becomes more significant as the leaching temperature rises.
The weighed-in Cu
2+
concentration affected the leaching kinetics at the early stage but had
little effect in the later. This is explained in comparison of the rates of the reactions involved in
the leaching. Whether microwave had an effect on this aspect is not clear and needs further
study.

Vol. 1, No. 2 Oxygenated Leaching of Copper Sulfide Mineral under Microwave-Hydrothermal
115
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