Thermal reduction of mechanically activated cinnabar (HgS) and stibnite (Sb2S3)

Article abstract of DOI:10.1023/A:1011564232088

The reduction of cinnabar (HgS) and stibnite (Sb₂S₃) by hydrogen was investigated. These investigations were performed in the temperature region 636-765 K for cinnabar and in the region 825-954 K for stibnite. It has appeared that the mechanical activation positively affects the thermal reduction of the sulphides. The thermal decomposition of HgS is accompanied by a change in mechanism taking place at 744 K. As to Sb₂S₃, the change in mechanism in the investigated temperature region was not observed.

Full text of DOI:10.1023/A:1011564232088

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 51 57
THERMAL REDUCTION OF MECHANICALLY
ACTIVATED CINNABAR (HgS) AND STIBNITE (Sb2S3)
P. BalÆ and E. Godo¹ikovÆ
Institute of Geotechnics, Slovak Academy of Sciences, 043 53 Ko ice, Slovakia
(Received August 8, 2000; in revised form March 22, 2001)
Abstract
The reduction of cinnabar (HgS) and stibnite (Sb2S3) by hydrogen was investigated. These investiga-
tions were performed in the temperature region 636 765 K for cinnabar and in the region 825 954 K
for stibnite. It has appeared that the mechanical activation positively affects the thermal reduction of the
sulphides. The thermal decomposition of HgS is accompanied by a change in mechanism taking place
at 744 K. As to Sb2S3, the change in mechanism in the investigated temperature region was not ob-
served.
Keywords: cinnabar, decomposition, hydrogen, mechanical activation, reduction, stibnite
Introduction
The need to introduce waste-free and ecologically irreproachable technologies in extrac-
tion metallurgy stimulates the search for the methods of direct reduction of sulphides of
non-ferrous metals for obtaining pure metals. The use of different reducing agents, e.g.
hydrogen, carbon monoxide, methane and carbon has been investigated [1 5]. The appli-
cation of hydrogen to reduction of simple sulphides gives rise not only to elemental metal
but also to hydrogen sulphide, the decomposition of which yields sulphur and the arisen
hydrogen may be recycled into the primary process.
In this study the results of experiments aimed at intensification of the reduction of
cinnabar and stibnite with hydrogen provided these sulphides were subjected to mechani-
cal activation before reduction are presented. Mechanical activation was proved to be
successful also for intensification of other thermal processes of sulphides such as oxida-
tion, decomposition in inert atmosphere as well as sublimation [6 14].
Experimental
The investigations were carried out with cinnabar and stibnite.
Cinnabar originated from Almaden (Spain) was of the following composition:
51.87% Hg, 14.08% S, 9.31% Fe and 8.93% SiO2. It was found by X-ray diffraction
1418 2874/2001/ $ 5.00
2001 AkadØmiai Kiad , Budapest
AkadØmiai Kiad , Budapest
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Kluwer Academic Publishers, Dordrecht

52
BAL` ¦: CINNABAR (HgS) AND STIBNITE (Sb2S3)
phase analysis that this sample contained hexagonal cinnabar (α-HgS) (JCPDS
6-256), as the major component and pyrite FeS2 (JCPDS 6-710), hematite α-Fe2O3
(JCPDS 4-755) and quartz SiO2 (JCPDS 5-490) as minor parts.
Stibnite originated from Rud any (Slovakia) was of the following composition:
65.19% Sb, 23.45% S, 0.04% Fe, 0.33% Mg, 10.12% SiO2 and 0.86% insoluble rest. The
X-ray examinations showed the presence of stibnite Sb2S3 (JCPDS 6-474) and quartz
SiO2 (JCPDS 5-490).
The sample of both minerals were separated and crushed for granularity minus
200 µm. The prepared samples were used for the preparation of mechanically acti-
vated specimens by high-energy milling. The milling was performed in a Pul-
verisette 4 (Fritsch, Germany) planetary mill under the following conditions: volume
of grinding chamber 350 ml; weighed amount of sample, 20 g; grinding balls of tung-
sten carbide (25 pieces of 10 mm diameter and 5 pieces of 25 mm diameter); relative
acceleration of mill b/g, 10.3; grinding time 5 and 15 min.
The thermal decomposition of both minerals was investigated in a dynamic reactor
with a static layer of the solid phase, under the following conditions; weighed amount,
100 mg; volume flow rate of hydrogen, 0.67 10 3 dm3 s 1; temperature: 636 835 K (for
HgS), 851 929 K (for Sb2S3). The details of the apparatus are given in [13].
The experimental results were best fitted to D nvald Wagner equation
6
α =1 e−kt
(1)
π
where the symbols α, k, t stand for the conversion degree, apparent rate constant (s 1)
and reaction time (s), respectively.
Results and discussion
Cinnabar
At temperature exceeding 613 K the reaction between cinnabar and hydrogen [14]
takes place according to the following equation
HgS(s)+H2(g)→Hg(g)+H2S(g)
(2)
We investigated this decomposition in the temperature range 636 835 K for a
non-activated sample as well as for a sample mechanically activated for 15 min. The
dependence of the degree of conversion α on the time of thermal reduction tT for dif-
ferent experiments is given in Figs 1 and 2. While we can observe that the rate of de-
composition increases in the whole interval of the values of reaction time tT, a gradual
retardation of the decomposition appears at higher temperatures and the decomposi-
tion is limited by the degree of conversion α=0.8 0.9. If we compare the above fig-
ures with each other, we can see that the mechanical activation does not probably
change the mechanism of decomposition and accelerates the decomposition rate only
slightly. The influence of mechanical activation decreases with increasing tempera-
ture.
J. Therm. Anal. Cal., 65, 2001

BAL` ¦: CINNABAR (HgS) AND STIBNITE (Sb2S3)
53
Fig. 1 The influence of the reaction time tT on the conversion degree α of non-activated
HgS. Reaction temperatures: 1 – 636 K; 2 – 679 K; 3 – 743 K; 4 – 722 K;
5 – 835 K
Fig. 2 The influence of the reaction time tT on the conversion degree α of mechanically
activated HgS for 15 min. Reaction temperatures: 1 – 636 K; 2 – 679 K;
3 – 743 K; 4 – 722 K; 5 – 835 K
The results of X-ray phase analysis of a sample mechanically activated for
15 min and thermally treated for 10 min revealed the presence of mercury in the prod-
ucts of decomposition.
The Arrhenius plots in Fig. 3 give evidence of a change in reaction mechanism at
T=744 K (1/T=1.345 10 3 K). This change manifests itself by the break at this temper-
ature in the case of non-activated sample as well as of samples mechanically acti-
vated for 5 and 15 min. The change in mechanism can be related with the process of
dissociative sublimation which begins just at this temperature [11]. At temperatures
above 744 K the process involving simultaneous dissociative sublimation and reduc-
tion of cinnabar proceeds. The elemental sulphur formed in this first process immedi-
J. Therm. Anal. Cal., 65, 2001

54
BAL` ¦: CINNABAR (HgS) AND STIBNITE (Sb2S3)
Fig. 3 The influence of mechanical activation of HgS on Arrhenius plot, T=722 765 K.
Time of mechanical activation: 1 0, 2 5, 3 15 min
Fig. 4 The influence of the reaction time tT on the conversion degree α of
non-activated Sb2S3. Reaction temperatures: 1 – 954 K; 2 – 929 K; 3 – 903 K;
4 – 877 K; 5 – 851 K; 6 – 825 K
ately reacts with the flowing dynamic atmosphere of hydrogen to give hydrogen sul-
phide owing to which the reaction surface is set free and the overall process is accel-
erated. For the activated samples the values of experimental activation energy in the
temperature region 744 765 K are equal to 155 162 kJ mol 1 which points out that
the chemical reaction is the rate determining step of the whole process.
Stibnite
The reaction of Sb2S3 with hydrogen [15] obeys the following equation
J. Therm. Anal. Cal., 65, 2001

BAL` ¦: CINNABAR (HgS) AND STIBNITE (Sb2S3)
55
Sb2S3+3H2→2Sb+3H2S
(3)
According to the temperature [15] stibnite can turn into volatile and partially de-
composed forms:
2Sb2S3(s)→Sb4S6(g)
(4)
(5)
Sb2S3(s)→2SbS(g)+0.5S2(g)
The investigation of the decomposition products of Sb2S3 [15] has shown gas-
eous SbS, Sb2S3, Sb2S2, S2, Sb3S3, Sb3S4, Sb4S4 and Sb4S5 occur among the products of
reactions (4) and (5).
Fig. 5 The influence of the reaction time tT on the conversion degree α of mechanically
activated Sb2S3 for 15 min. Reaction temperatures: 1 – 954 K; 2 – 929 K;
3 – 903 K; 4 – 877 K; 5 – 851 K; 6 – 825 K; 7 – 800 K
Fig. 6 Scanning electron micrographs of Sb2S3 (mechanical activation 15 min). Tem-
perature: A – 800 K, B – 825 K; C – 929 K. Magnification: 6000×
J. Therm. Anal. Cal., 65, 2001

56
BAL` ¦: CINNABAR (HgS) AND STIBNITE (Sb2S3)
Fig. 7 The influence of mechanical activation of Sb2S3 on Arrhenius plot,
T=800–851 K. Time of mechanical activation 1 – 0 min, 2 – 15 min
The results of our study of the decomposition of Sb2S3 in hydrogen atmosphere
are illustrated by the kinetic relationships conversion reaches the value α=1 under
given experimental conditions and the temporal course is dependent on temperature.
The X-ray phase analysis applied to a sample mechanically activated for 15 min
and thermally processed at 851, 877 and 929 K for 5 min indicated the presence of
monoclinic sulphur (JCPDS 13-141) and metallic antimony (JCPDS 5-562). The
photomicrographs of the same sample processed at 799, 825 and 928 K are given in
Figs 6A C. For the temperature of 799 K we can observe a particle of stibnite cov-
ered with metallic antimony which volatilizes at higher temperatures [17].
The microstructure of the mineral comprises a considerable number of micro-
pores (Figs 6B C) while the reaction surface is not blocked.
The character of the Arrhenius plots in Fig. 7 indicates that no change in reaction
mechanism takes place in the investigated temperature region. The mechanical acti-
vation brings about a decrease in experimental activation energy from 130 kJ mol 1
for non-activated sample to the value of 58 kJ mol 1 for a sample activated for 15 min.
Both values point out that the reaction at the surface of Sb2S3 is the rate determining
step. The value of 130 kJ mol 1 is in very good agreement with the value of
121 kJ mol 1 found by Liu [17] for equal reaction of non-activated Sb2S3 in the tem-
perature region 723 793 K.
* * *
The author thanks to Miss KozÆkovÆ for carrying out the thermal reduction measurements. This
work was supported, in part, by the Slovak Grant Agency for Science (grant No. 2/6101/99).
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