Direct reduction of mechanically activated galena and sphalerite with hydrogen

Article abstract of DOI:10.1023/A:1011911728724

In this paper the reduction of lead and zinc sulphide by hydrogen is described. It has been found that the rate of formation of elemental lead or zinc is favourably affected by mechanical activation of PbS and ZnS produced by intensive grinding. This effe

Full text of DOI:10.1023/A:1011911728724

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 769 776
DIRECT REDUCTION OF MECHANICALLY
ACTIVATED GALENA AND SPHALERITE WITH
HYDROGEN
P. BalÆ 1 and J. BrianŁin2
1Institute of Geotechnics, Slovak Academy of Sciences, 043 53 Ko ice, Slovakia
2Institute of Material Research, Slovak Academy of Sciences, 043 57 Ko ice, Slovakia
(Received April 6, 2001; in revised form May 4, 2001)
Abstract
In this paper the reduction of lead and zinc sulphide by hydrogen is described. It has been found that
the rate of formation of elemental lead or zinc is favourably affected by mechanical activation of PbS
and ZnS produced by intensive grinding. This effect was observed in the region 678 1048 K for ga-
lena and in the region 851 1023 K for sphalerite. It has appeared that disordering in the structure of
both minerals results in the decrease in experimental activation energy.
Keywords: galena, hydrogen, lead, mechanical activation, sphalerite, thermal reduction, zinc
Introduction
The classical pyrometallurgical reduction of non-ferrous metals is based on applica-
tion of solid state reducing agents such as carbon and lime. As for gaseous reducing
agents, methane, carbon monoxide and lately hydrogen have been used. A survey of
application of different reducing agents is given in paper [1].
In contrast to utilization of SO2 in the form of H2SO4 involving problematic sale
of sulphuric acid, the utilization of reduction products in the form of H2S (which can
be further utilized in the form of elemental sulphur and hydrogen) is more prospec-
tive and ecologically less objectionable.
In this study the results of experiments aimed at intensification of the reduction
of galena (PbS) and sphalerite (ZnS) with hydrogen provided these sulphides were
subjected to mechanical activation before reduction are presented. Mechanical acti-
vation was proved to be successful in our previous works for intensification of other
thermal processes of sulphides such as oxidation, decomposition in inert atmosphere
(pyrolysis) as well as sublimation [2 10].
1418 2874/2001/ $ 5.00
2001 AkadØmiai Kiad , Budapest
AkadØmiai Kiad , Budapest
'
Kluwer Academic Publishers, Dordrecht

770
BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
Experimental
The investigations were carried out with galena and sphalerite.
Galena originated from BanskÆ tiavnica (Slovakia) was of the following com-
position: 78.41% Pb, 13.74% S, 0.20% Cu, 0.71% Fe, 2.31% Zn and 1.56% insoluble
rest. It was found by X-ray diffraction phase analysis that this sample contained ga-
lena (JCPDS 5-592) and minor amount of quartz (JCPDS 5-490).
Sphalerite originated from Zabaykal (Russia) was of the following composition:
44.60% Zn, 15.53% Fe, 33.58% S, 3.81% SiO2 and 2.48% insoluble rest. It was found
by X-ray phase analysis that the sample contained sphalerite (JCPDS 5-0566) and mi-
nor amounts of quartz (JCPDS 5-490) and pyrite (JCPDS 6-0710).
The samples of both minerals ( 200 µm granularity each) were separately me-
chanically activated in a Pulverisette 4 (Fritsch, Germany) planetary mill under the
following conditions: volume of grinding chamber 350 ml; mass of sample, 20 g;
grinding balls of tungsten 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, 10 and
15 min.
The thermal decomposition of both minerals was investigated in a dynamic reac-
tor 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:
937 1049 K for PbS and 851 1023 K for ZnS. The experimental details of the appa-
ratus are given in [9].
The following conversion functions were used for primary kinetic characteriza-
tion
α =1−6πe–k t
(1)
1
for galena reduction and
α =k2t1/2
(2)
for sphalerite reduction, where α is the conversion degree calculated from the mass
difference of sulphide samples before and after reduction, k1, k2 are rate constants and
t is the reaction time.
Results and discussion
Galena (PbS)
In the eighties some papers dealing with the kinetics of reduction of galena with hy-
drogen appeared [11 14]. It was found on the basis of thermogravimetric investiga-
tions that a loss of mass of galena heated in hydrogen flow emerges already at tem-
peratures over 773 K [11, 14]. The isothermal study [12] in the region 948 1098 K
showed that the maximum rate of reduction took place at the commencement and was
constant. At higher reaction times the rate decreased. The stoichiometry of the pro-
cess in this region can be expressed by the following equations
J. Therm. Anal. Cal., 65, 2001

BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
771
PbS(s)+H2(g) → Pb(1)+H2S(g)
Pb(s) → PbS(g)
(3)
(4)
At temperature over 1023 K the reduced lead vaporizes. Reaction starts at
1058 K.
The degree of conversion of galena due to the reaction with hydrogen in our case
is represented for a non-activated sample and a sample mechanically activated for
15 min in Figs 1 and 2. The reduction was investigated in the temperature interval
937 1048 K. Some differences between the above samples appeared only in the tem-
perature range between 937 and 980 K. In agreement with Eq. (3), the surface of re-
acting particles was subjected to fusion at higher temperatures, as it is documented by
the attached photomicrographs. In all cases, however, we can observe the maximum
rate in the initial stage of reaction and the retardation at higher reaction time.
Fig. 1 The influence of the reaction time tT on the conversion degree α of non-activated
PbS. Reaction temperatures: 1 679 K; 2 980 K; 3 1023 K; 4 1048 K
The photomicrographs in Fig. 3 were taken for the samples of mechanically acti-
vated galena which were reduced at the temperatures of 937 K (B), 1048 K (C,E) and
1109 K (D,F). The photomicrograph of the sample before reduction is represented for
comparison in Fig. 3A. If we apply the lowest temperature (B), we can observe that
the porosity has remained preserved, but in comparison with non-ignited sample the
proportion of smaller grains significantly decreased because of preferential reaction
and consumption of these grains. We can observe that the pores are gradually closed
at 1048 K (C) and this process continues at higher temperatures (D). If we apply
smaller magnification (E, F) we can observe that the whole surface of particles is sub-
jected to fusion. Owing to this fact, the reduction is retarded which is consistent with
Eq. (1) and character of the kinetic curves in Figs 1 and 2. On the basis of X-ray phase
analysis we identified lead (JCPDS 4-686) and monoclinic sulphur (JCPDS 13-141)
in the products of decomposition.
J. Therm. Anal. Cal., 65, 2001

772
BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
Fig. 2 The influence of the reaction time tT on the conversion degree α of mechanically
activated PbS for 15 min. Reaction temperatures: 1 679 K; 2 980 K;
3
1023 K; 4 1048 K
Fig. 3 Scanning electron micrographs of PbS (mechanical activation 15 min). Temper-
ature: A laboratory, B 937 K, C and E 1048 K, D and F 1109 K. Magni-
fication: A D 6000 , E 950 , F 450
J. Therm. Anal. Cal., 65, 2001

BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
773
Fig. 4 The influence of mechanical activation of PbS on Arrhenius plot T=980 1048 K.
Time of mechanical activation: 1 0 min, 2 15 min
The Arrhenius plots for the reduction of galena represented in Fig. 4 were used
for calculation of experimental activation energy which amounts to 42 kJ mol 1 for
non-activated sample and 38 kJ mol 1 for the sample activated for 15 min (Table 1).
The character of plots indicates that no change in mechanism due to temperature
and/or structure disordering by mechanical activation takes place in the investigated
temperature interval.
Table 1 Numerical processing of temperature-conversion data for PbS reduction
Mechanical activation/
1
1
1/T•103/K
Calculated parameters
lnk1/s
min
1
1.0203
0.9774
0.9533
5.9932
5.7733
5.6531
E=42 kJ mol
A= 5.086 s
r=0.99997
1
1
1.0203
0.9774
0.9533
5.8872
5.7396
5.5697
E=38 kJ mol
A= 4.587 s
r=0.97899
1
15
E
activation energy, A pre-exponential factor, r regression coefficient
Sphalerite (ZnS)
In comparison with galena the reducibility of sphalerite is worse [15]. JovanoviŁ et
al. has alleged that the degree of reduction of ZnS reaches the value α=0.35 for the
temperature of 1070 K [14]. The process is complicated by the fact that zinc is evapo-
rated in the hydrogen flow at high temperatures. At the same time hydrogen sulphide
originating in the reaction of hydrogen with the sulphur atoms of sphalerite leaves the
surface. At the temperatures between 1173 and 1223 K hydrogen sulphide can react
with zinc vapour to form secondary ZnS [15 16].
Owing to the complicated mechanism, we chose the temperatures between 673
and 1023 K to study the influence of mechanical activation on the rate of reduction of
J. Therm. Anal. Cal., 65, 2001

774
BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
sphalerite. In this temperature interval zinc does not vaporize because and the reac-
tion can be described by the overal equation
ZnS(s)+H2(g) → Zn(s)+H2S(g)
(5)
Fig. 5 The influence of the reaction time tT on the conversion degree α of mechanically
activated ZnS. Time of mechanical activation: 1 0 min, 2 5 min, 3 10 min,
Fig. 6 The influence of mechanical activation of ZnS on Arrhenius plot,
T=851 1023 K. Time of mechanical activation: 1 0 min, 2 5 min, 3 10
In Fig. 5 the dependence of the degree of conversion α on reaction time tT is
given for the sphalerite samples mechanically activated for 5 30 min in a planetary
mill. In the case of all samples we can observe the parabolic course of reduction with
the maximum rate at the commencement. The presence of zinc (JCPDS 4-831) and
elemental sulphur (JCPDS 8-247) was detected by X-ray phase analysis of the sample
mechanically activated for 15 min and subsequently reduced by hydrogen for 20 min
at the temperatures of 937 and 1023 K.
J. Therm. Anal. Cal., 65, 2001

BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
775
The presence of elemental sulphur in reaction products may be a result of hydro-
gen sulphide decomposition. In paper [15] it is alleged that reverse reaction of zinc
with hydrogen sulphide takes place in the cooler part of the reaction tube which could
lead to the explanation of the retardation of the process.
The values of experimental activation energy calculated from the plots in Fig. 6
are 49, 12, 7 and 4 kJ mol 1 for non-activated sample and samples activated for 5, 10
and 30 min, respectively (Table 2).
Table 2 Numerical processing of temperature-conversion data for ZnS reduction
Mechanical activation/
1
1
1/T•103/K
Calculated parameters
lnk1/s
min
1
1.1749
1.1184
1.0671
0.9774
5.5363
4.9630
4.9124
4.7063
E=49 kJ mol
A= 5.861 s
r=0.89313
1
1
1.1749
1.1184
1.0671
0.9774
4.8505
4.7659
4.6303
4.6000
E=12 kJ mol
A= 1.452 s
r=0.86303
1
5
1
1.1749
1.1184
1.0671
0.9774
4.5086
4.4566
4.3742
4.3457
E=7 kJ mol
A= 0.854 s
r=0.95222
1
10
30
1
1.1749
1.1184
1.0671
0.9774
4.3645
4.3228
4.2776
4.4279
E=4 kJ mol
A= 0.438 s
r=0.85504
1
E
activation energy, A pre-exponential factor, r regression coefficient
The disordering of sphalerite by mechanical activation brings about reductions
in the values of activation energy. These values give evidence that the diffusion re-
gime [17] likely involving the secondary ZnS originating from the above-mentioned
recombination of Zn and H2S determines the rate of reaction (5).
* * *
The authors thank to S. KozÆkovÆ for carrying out the thermal decomposition measurements. This
work was supported, in part, by the Slovak Grant Agency for Science (grant No. 2/6101/99).
References
1 Y. K. Rao and S. K. El-Rahaiby, Metall. Trans. B, 16B (1985) 465.
2 P. BalÆ , K. TkÆŁovÆ and E. G. Avvakumov, J. Thermal Anal., 35 (1989) 1325.
3 K. TkÆŁovÆ, P. BalÆ and Z. Bastl, Thermochim. Acta, 170 (1990) 277.
4 P. BalÆ and I. Ebert, Thermochim. Acta, 180 (1991) 117.
5 P. BalÆ , H.-J. Huhn and H. Heegn, Thermochim. Acta, 194 (1992) 189.
J. Therm. Anal. Cal., 65, 2001

776
BAL` ¦, BRIAN¨ IN: REDUCTION OF LEAD AND ZINC SULPHIDE
6 P. BalÆ , J. BrianŁin, V. epelÆk and I. HocmanovÆ, Thermochim. Acta, 196 (1992) 349.
7 P. BalÆ , E. Post and Z. Bastl, Thermochim. Acta, 200 (1992) 371.
8 P. BalÆ and M. BalassaovÆ, J. Thermal Anal., 41 (1994) 1101.
9 P. BalÆ , J. BrianŁin and L. TurŁÆniovÆ, Thermochim. Acta, 249 (1995) 375.
10 P. BalÆ , Extractive Metallurgy of Activated Minerals, Elsevier, Amsterdam 2000.
11 G. I. Zviadadze, I. S. Turgenev, I. Ch. Kabisov and O. J. Vasiljeva, Izv. VUZOV,
Cvet. metall., 2 (1980) 42.
12 G. I. Zviadadze, I. S. Turgenev, I. Ch. Kabisov and O. J. Vasiljeva, Izv. VUZOV,
Cvet. Metall., 1 (1985) 60
13 S. JovanoviŁ, D. SinadinoviŁ and B. ˇ urkoviŁ, Rud. Geolog. Metal., 37 (1986) 594.
14 S. JovanoviŁ, B. ˇ urkoviŁ and D. SinadinoviŁ, Rud. Geolog. Metal., 37 (1986) 1247.
15 I. A. Onajev and V. S. SpitŁenko, Reduction of Sulphides, Nauka, Alma-Ata 1988 (in Rus-
sian).
16 D. M. ¨ i ikov, Metallurgy of Non-ferrous Metals, Nauka, Moskva 1976 (in Russian).
17 F. Habashi, Principles of Extractive Metallurgy, Vol. I General Principles, Gordon and
Breach, New York 1974.
J. Therm. Anal. Cal., 65, 2001