Reduction of PbS and Sb2S3 with elemental Fe and Mg in dusty plasma environment created during electrical discharge assisted mechanical milling (EDAMM)

Article abstract of DOI:10.1016/j.jallcom.2007.12.032

The newly developed synthesis technique of electrical discharge assisted mechanical milling (EDAMM) is used to reduce Pb and Sb sulfides using Fe and Mg as reduction agents. It is demonstrated that both Fe and Mg can successfully be used to fully reduce P

Full text of DOI:10.1016/j.jallcom.2007.12.032

Journal of Alloys and Compounds 467 (2009) 477–484
Reduction of PbS and Sb2S3 with elemental Fe and Mg in dusty
plasma environment created during electrical discharge
assisted mechanical milling (EDAMM)
A. Calkaa, A. Mosbaha, N. Stanfordb,∗, P. Balazc
a Engineering Materials Institute, Faculty of Engineering, University of Wollongong, Northfields Avenue Wollongong 2522, Australia
b Centre for Material and Fibre Innovation, Deakin University, Pigdons Road, Geelong 3217, Australia
c Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 043 53 Kosice, Slovakia
Received 19 October 2007; accepted 6 December 2007
Available online 23 December 2007
Abstract
The newly developed synthesis technique of electrical discharge assisted mechanical milling (EDAMM) is used to reduce Pb and Sb sulfides
using Fe and Mg as reduction agents. It is demonstrated that both Fe and Mg can successfully be used to fully reduce PbS and Sb2S3 in only 5 min
using EDAMM. This is the fastest sulfide reduction technique so far developed, and has the added advantage of not requiring external heating
of the reactants during processing. The reaction products are a solid mixture of larger crystal sizes than would be achieved through conventional
milling.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Sulfide; Mechanical alloying; Reduction
1. Introduction
a solid or a liquid product and not to a gas as in other reduction
processes, e.g., reduction by carbon or hydrogen. In general,
such processes are called metallothermic [1].
In typical pyrometallurgical processes that are employed to
obtain metals from ores (such as oxides or sulfides), the solid
raw material is transformed to the product by a liquid and/or gas
phase reaction. These processes require high reaction tempera-
tures and pressures. The final products then require an additional
processing step to separate the desired metal product from any
by-products, remaining starting materials and/or from the sol-
vent. Generally, the final product is solid again [1].
Metal sulfides can be reduced by high temperature reaction
with reducing metals such as Al, Ca, Mg, Na and Si (as FeSi).
The most common reduction agent is scrap iron, because of its
low cost. Balaz et al. [2–4] systematically studied the appli-
cation of Fe as a mechanochemical reducing agent for copper
(Cu2S), lead (PbS), and antimony (Sb2S3). The reduction reac-
tions resulted in the synthesis of nanocrystalline Cu, Pb and Sb in
the range of 12–18 nm. During the milling process, these reac-
tion products formed nanocomposites of Cu/FeS, Pb/FeS and
Sb/FeS, and these are of interest as prospective materials for
precursors of catalysts for heterogeneous chemical reactions.
Apart from using Fe, there are also several examples of Mg
being used as a reducing agent. Mechanical milling with Mg
utilizes the high affinity of magnesium metal for oxygen to
enable the reduction of oxidic compounds. Nasu et al. [5] milled
hematite with Mg powder, and observed the formation of ␣-Fe
together with MgO as the product phases. El-Eskandarany et al.
[6] studied the kinetics and mechanism of the mechanochemical
reaction between Fe2O3 and Mg and confirmed the single step
The preparation of metals and alloys by reduction of their
salts can be expressed in general by the equation
M1X + M2 → M1 + M2X
(1)
where M1 is the reduced metal, M2 is reducing metal and X
anions as Cl−1,O−2 or S−2. The reducing metal must have a
strong affinity for the compounds to be reduced. This method is
characterized by the fact that the reducing metal is converted to
∗
Corresponding author.
E-mail address: stanford@deakin.edu.au (N. Stanford).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.12.032

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A. Calka et al. / Journal of Alloys and Compounds 467 (2009) 477–484
Fig. 1. Schematic of electrical discharge assisted mechanical milling (EDAMM).
reactionprocedurewith␣-FeandMgOastheonlyreactionprod-
ucts. In that case the reaction proceeds with one-dimensional
growth and is diffusion controlled [7]. Welham [8] has also used
Mg metal for the reduction of ilmenite FeTiO3 by high-energy
mechanical milling. Matteazzi and Le Caer [9] were the first to
study the solid-state reduction of metal sulfides with pure met-
als by applying high-energy mechanical milling. It was shown
that the reduction of metal sulfides with a suitable reducing
metal is feasible using room temperature vibratory milling. Met-
als, alloys, intermetallic compounds and nanocomposites were
synthesized, and had crystallite sizes in the range 10–30 nm [9].
These examples of mechanical milling being used to reduce
sulfides and oxides to metal using a reducing agent have proved
the ability of this technique to synthesize many metals from
their compounds. Recently, a new high-energy milling tech-
nique has been developed [10]. Electric discharge assisted ball
milling (EDAMM) is a synthesis technique based on mechan-
ical milling, but includes an additional repeated electrical
discharge and/or plasma that is formed around the react-
ing particles during milling. This new technique has been
shown to increase the rate of reaction compared to conven-
tional milling, and is also known to initiate reactions that
are not possible using conventional mechanical milling. This
increased reactivity results primarily from formation of complex
plasma generated by electric discharge. High-energy mechani-
cal milling offers great potential for the rapid synthesis of metals
and compounds [11]. During mechanical milling, the diffu-
sion rates of the reacting powders are substantially increased
by the introduction of a large number of solid defects, and
also by a reduction in the crystallite size of the reactants.
This technique has been applied to the synthesis of a wide
variety of nanocrystalline compounds, for example, see refs.
[12,13].
The following paper describes the use of Fe and Mg in
the reduction of PbS and Sb2S3 using EDAMM. It is shown
that EDAMM is a rapid and highly energy efficient synthesis
Fig. 2. XRD patterns of (a) PbS + Fe starting powder after pre-mixing and (b)
FeS2 + Pb powder produced by EDAMM.
Fig. 3. XRD patterns of (a) PbS + Mg starting powder after pre-mixing and (b)
MgS + Pb powder produced by EDAMM.

A. Calka et al. / Journal of Alloys and Compounds 467 (2009) 477–484
479
Fig. 4. XRD patterns of (a) Sb2S3 + Fe starting powder after pre-mixing and (b)
FeS + Sb + Sb2O3 powder produced by EDAMM.
technique. Moreover, this is the first use of Mg metal for the
reduction of metal s ever reported.
2. Experimental method
PbS and Sb2S3 powders were mixed with stoichiometric portions of both
Fe and Mg to obtain complete reduction of PbS/Sb2S3 and produce MgS or
FeS. These starting powder mixtures were first mechanically pre-mixed in a
conventional ball mill for 1 h under a high purity argon atmosphere. This pre-
mixture was then subjected to 5 min of electric discharge assisted ball milling.
Electric discharge assisted ball milling was performed in a modified vibra-
tional laboratory rod mill. This technique produces a milling mode that combines
Fig. 6. Scanning electron micrographs of (a) PbS + Fe starting powder after pre-
mixing and (b) FeS2 + Pb powder produced by EDAMM. Scale bars = 20 ␮m.
repeated impact of a hardened curved rod end on powder particles, under elec-
trical conditions of pulsed arc discharges (Fig. 1). The electric discharges were
generated during milling in the gaps between the vibrating mill base, the powder
particles and a loosely suspended conducting plunger. The power supply used
in this study generated radio frequency impulses within the kV/mA range. Dur-
ing vibration, small gaps between the stainless steel rod and the chamber wall
resulted in an electric discharge (Fig. 1) and formation of dust cloud between
electrodes. Electric discharge induces high chemical reactivity in between the
powder particles that are suspended between electrodes (the plunger and the
chamber). The mechanism of chemical reactions between powder particles
surfaces induced by electric discharge is complex and poorly understood. How-
ever, the most important factors that contribute to enhanced reactivity are the
formation of plasma environment between powder particles, formation and
acceleration of ions and free radicals under fast moving stream of electrons,
and bombardment (implantation and deep penetration) of ions and free radicals
on powder particle surfaces. This process also causes the formation of chan-
nels on the particles’ surface. This results in an increase in the total surface
area and therefore an increase in the rate of diffusional processes. In addition,
the powder particles are constantly being mixed and fractured by the vibrating
electrode. This process, which is analogous to conventional mechanical milling,
also increases the surface area and reactivity of the powder mixture.
EDAMM was carried in
a high purity Ar flowing at a rate of
0.3 m3/min. In all experiments, a milling time of 10 min was used. X-
ray diffraction (XRD) analysis of the as-milled powders was performed
using a Phillips PW1730 diffractometer with a graphite monochroma-
tor and Cu K␣ radiation. Phase identification was carried out using the
International Centre for Diffraction Data (JCPDS-ICDD 2000) powder diffrac-
tion files (PDF). The morphology of the powders was observed by a
Leica 440 Stereoscan scanning electron microscope (SEM). This SEM is
equipped with an Oxford Instruments ISIS energy dispersive spectroscopy
Fig. 5. XRD patterns of (a) Sb2S3 + Mg starting powder after pre-mixing and
(b) Sb + MgS powder produced by EDAMM.

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Fig. 8. Scanning electron micrographs of (a) Sb2S3 + Fe starting powder after
pre-mixing and (b) FeS + Sb + Sb2O3 powder produced by EDAMM. Scale
bars = (a) 10 ␮m and (b) 20 ␮m.
Fig. 7. Scanning electron micrographs of (a) PbS + Mg starting powder after pre-
mixing and (b) MgS + Pb powder produced by EDAMM. Scale bars = 20 ␮m.
(EDS) system which was used to examine the composition of the pow-
ders.
a small amount produced during EDAMM. The pre-mixing and
EDAMM were carried out in an inert atmosphere, so it is likely
that the small amount of oxygen present in the reaction products
was introduced into the system from the Fe powder.
The reaction between Sb2S3and Mg did not produce any
oxides. The pre-mixture, which remained unreacted after con-
ventional milling (Fig. 5a) formed Sb and MgS after EDAMM
(Fig. 5b).
3. Experimental results
3.1. X-ray diffraction
An X-ray diffraction spectrum of the pre-mixed PbS and Fe
powders is shown in Fig. 2a. It can be seen that after 1 h of con-
ventionalballmillingtherehasbeennoreactionbetweenthePbS
and the Fe. However, after 5 min of EDAMM, strong diffraction
peaks are evident for the FeS2 and Pb phases (Fig. 2b). There is
complete reaction from PbS + Fe to FeS2 + Pb, since no diffrac-
tion peaks from the reactant powders are evident after EDAMM
(Fig. 2b). A very similar result is obtained from the mixture
of PbS and Mg. Pre-mixing for 1 h does not show any reaction
between the PbS and Mg (Fig. 3a), but after 5 min EDAMM,
XRD shows complete reaction to Pb + MgS (Fig. 3b).
The reaction between Sb2S3 and Fe during EDAMM was
slightly more complicated. XRD confirmed that the pre-mixing
of Sb2S3 and Fe did not induce any chemical reaction (Fig. 4a).
However, after EDAMM, XRD revealed that although elemental
Sb was produced, the other reaction products were a mixture of
FeS and Sb2O3 (Fig. 4b). The diffraction peaks from the Sb2O3
were not high in intensity, and this suggests that there was only
3.2. Scanning electron microscopy
After conventional ball milling, the PbS and Fe mix-
ture consisted of small particles ∼10 ␮m in diameter. After
EDAMM, the reacted powder formed large agglomerates that
were 200–300 ␮m in diameter (Fig. 6).
The mixed PbS and Mg powders formed large unreacted
agglomerates during conventional ball milling (Fig. 7a). Fol-
lowing EDAMM, the MgS + Pb mixture formed relatively small
agglomerates of around 25 ␮m in diameter (Fig. 7b).
The conventionally milled Sb2S3 and Fe powders did not
agglomerate, but remained as individual particles in the mixture.
After EDAMM, the products formed larger particles of ∼30 ␮m
in diameter. These particles typically displayed smooth outer
surfacesindicativeoflocalizedmeltingduringEDAMM(Fig. 8).

A. Calka et al. / Journal of Alloys and Compounds 467 (2009) 477–484
481
AsimilarbehaviourwasdisplayedbytheSb2S3 andMgpowders
(Fig. 9
).
The distribution of the reaction products within individual
powder particles was further examined using compositional
mapping in the SEM, and an example is shown in Fig. 10. The
backscattered electron (BSE) image is shown on the top left.
The size and morphology of the constituents in the agglomerate
are visible. The compositional mapping confirms that the lighter
regions are Sb, and that these Sb regions are depleted in S. The
Sb regions are surrounded by MgS. X-ray mapping for the other
samples examined in this study are shown in Figs. 11–13.
4. Discussion
The XRD results shown here in Figs. 2 and 3 show that PbS
and Sb2S3 can successfully be reduced by EDAMM using either
Fe or Mg as the reducing agent. Using conventional pyromet-
allurgical techniques, the reduction of these sulfides by Mg is
not possible. However, it has been shown here that complete
reduction can be achieved in 5 min using EDAMM. In fact, Mg
appears to be a better reducing agent than Fe – the samples of
Sb2S3 milled with Fe formed some oxides as a result of con-
tamination from the Fe powder. Oxide contamination was not
detected when Mg was used as the reducing agent (Fig. 5).
There has been previous research into the reduction of metal
sulfides by mechanical milling. For example, Balaz et al. [3]
have reduced PbS and CuS by conventional milling using Fe as
the reducing agent. In that case, the reduction reactions required
up to 20 min to reach the maximum reaction of 90%. Mechanical
Fig. 9. Scanning electron micrographs of (a) Sb2S3 + Mg starting powder
after pre-mixing and (b) Sb + MgS powder produced by EDAMM. Scale
bars = 20 ␮m.
Fig. 10. Elemental mapping of a particle of Sb + MgS after EDAMM. The backscattered electron (BSE) image is shown, along with the composition maps for Mg,
S and Sb as indicated.

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Fig. 11. Elemental mapping of a particle of Pb + FeS2 after EDAMM. The backscattered electron (BSE) image is shown, along with the composition maps for Fe, S
and Pb as indicated.
Fig. 12. Elemental mapping of a particle of Pb + MgS after EDAMM. The backscattered electron (BSE) image is shown, along with the composition maps for Mg,
S and as indicated.

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483
Fig. 13. Elemental mapping of a particle of Sb + FeS after EDAMM. The backscattered electron (BSE) image is shown, along with the composition maps for Fe, S
and Sb as indicated.
milling reduces the size of the reactant particles to around 15 nm.
This dramatic increase in the surface area of the reacting pow-
ders is the reason that mechanical milling is successful in driving
the reduction reaction. EDAMM combines the increase in sur-
face area afforded by mechanical milling, with rapid chemical
reactions generated by plasma around the reactants.
It might be expected that localized heating in our device gen-
erates high temperature plasma, and that the precursor powders
are vaporized and then rapidly quenched throughout the con-
densation process. However, SEM results suggest a different
reaction mechanism. Only in Fig. 8b can melted particles can
be identified, all other SEM micrographs showed agglomerates
of small particles that did not exhibit melting. Although, some
powder particles can be melted during EDAMM, the purpose of
using the impulse mode is to prevent massive melting, and to
keep the precursor powders in solid form from the beginning up
to the end of the process. This leads to the formation of what we
have termed a “dusty cloud” between the electrodes. The dust
particles become suspended in the plasma in a region between
the plunger and the mill base. The general aim is to create dusty
plasma environment which, we believe, will be more effective
in generation of reduction reactions than thermal plasma.
In the milling device used in this study, the plunger and the
base vibrate independently. The purpose of vibrating the base
is to ensure the powder does not lie stationary on the plunger
base. If the base was not vibrating, the powder would stay at
the bottom of the reaction chamber and would not be suspended
between the base and the plunger. In that case the powder lay-
ing on the base is subjected to both mixing (or milling) by the
plunger and discharges in the gaps between the plunger and the
stationary powder. We found that in this condition only about
10% reaction can be achieved after 5 min of EDAMM. However,
if both the plunger and the base vibrate, full reaction is achieved
within 5 min. This means that the effectiveness of EDAMM is
strongly dependent on powder movement during milling. This
represents a significant improvement in our understanding of the
parameters that promote high reaction rates during EDAMM.
Fig. 14. Schematic representation of “dusty plasma” developed during
EDAMM.

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Although, the reactions occurring during EDAMM are poorly
Acknowledgements
understood, we may say that the first step in plasma processing
is the production of plasma by ionization. Primary ions of a car-
rier gas (Argon) are produced, which then transfer excitation
and ionization to powder particle surfaces, producing ions and
radicals – the reactant precursors. The plasma flow of gas ions,
free radicals and reactant ions bombard the powder surfaces with
high energy causing ion implantation. The interaction between
the particle surface and the arriving radicals leads to the nucle-
ation and growth of new phases by deep penetration of ions and
subsequent diffusion. A possible mechanism is shown in Fig. 14.
Additionally, under the flux of electrons and ions, a thin surface
layer on the outside of some particles may be melted. These can
form agglomerates under the influence of electrostatic forces,
and these agglomerates are seen in the SEM micrographs (e.g.,
Fig. 6). The localized heating also results in the crystallite size
of the reaction products being much larger than those formed
by conventional milling. This is evidenced by the XRD patterns
after EDAMM that show sharp peaks indicative of larger crystal
sizes, as compared to the diffuse peaks observed by Balaz et al.
after conventional milling [3].
This investigation was supported by funding from the
Australian Research Council (Grant Nos. A00103022 and
DP0451907) and Slovak Science and Technology Assistance
Agency (Grant No. LPP-0196-06).
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5. Conclusions
PbS and Sb2S3 have been successfully reduced with both Fe
and Mg using the novel synthesis technique of electrical dis-
charge assisted ball milling. This is the first ever report of the
reduction of PbS and Sb2S3 using Mg as the reducing agent. In
addition, EDAMM has been shown to be the fastest synthesis
route for the reduction of these sulfides with Fe, requiring only
one quarter of the time compared to conventional ball milling.
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