Kinetic study and mechanism of chalcocite and covellite oxidation process

Article abstract of DOI:10.1007/s10973-005-0601-1

Results of the experimental investigation of the copper natural sulfide minerals (chalcocite and covellite) oxidation process in the air atmosphere are presented in this paper. Based on data obtained using thermal analysis methods and by construction of P

Full text of DOI:10.1007/s10973-005-0601-1

Journal of Thermal Analysis and Calorimetry, Vol. 79 (2005) 715–720
KINETIC STUDY AND MECHANISM OF CHALCOCITE AND
COVELLITE OXIDATION PROCESS
ꢀ. ꢀivkoviº1*, Nada Štrbac1, Dragana ꢀivkoviº1, V. Velinovski2 and I. Mihajloviº1
1University of Belgrade, Technical Faculty Bor, Serbia and Montenegro
2TIR–RTB, Bor, Serbia and Montenegro
Results of the experimental investigation of the copper natural sulfide minerals (chalcocite and covellite) oxidation process in the
air atmosphere are presented in this paper. Based on data obtained using thermal analysis methods and by construction of PSD dia-
grams for the Cu–S–O system at different temperatures, mechanisms of the investigated processes were determined. DTA-TG-DTG
and RDA methods were used for the investigations given in this paper. Kinetic parameters as well as activation energies of investi-
gated processes were determined using Sharp’s method of reduced halftime of reaction.
Keywords: activation energy, copper minerals, oxidation process, thermal analysis
Introduction
Experimental
Because of great practical importance in copper pyro-
metallurgy, oxidation processes of different copper-
sulfides were subject of considerable interest [1–8].
Physicochemical characteristics of different
minerals and concentrates in Cu–S system were in-
vestigated during oxidation roasting [1–3]. Also, tem-
peratures of combustions for different sulfides were
determined and presented in [4]. Special interest was
exerted toward investigations of kinetics and mecha-
nism of different pure sulfide minerals [5, 6] and con-
centrates oxidation processes [7, 8]. The oxidation of
chalcocite processed by flash smelting methods, was
investigated by Dunn et al. [9].
For experimental investigations presented in this pa-
per samples of natural minerals–chalcocite and covel-
lite from Bor mine (Eastern Serbia) were used.
Experimental procedure for obtaining kinetic pa-
rameters under isothermal conditions was performed
in a following manner: heating of investigated sam-
ples, during which roasting process occurred, was
performed in Mars’s furnace. The distinct amount of
air was introduced into the reaction area, while gas-
eous products of reaction (SO2 and SO3) were gener-
ated from the tube to the absorption tanks, filled with
aqueous solution of hydrogen peroxide, producing
sulfuric acid. Produced sulfuric acid, in the presence
The results of kinetic study and mechanism of
chalcocite and covellite oxidation process are pre-
sented in this paper. Contributing to the knowledge
and understanding of copper natural sulphide minerals,
complex comparative kinetic investigations were done,
as well as further characterization of the oxidation
products. The mechanism determined is complex, and
the relationship between the kinetic conclusions and
Arrhenius parameters based on TG and DTA are dis-
cussed, that is the method vide used in [10, 11].
Phase diagram for the Cu–S system [1] is pre-
sented in Fig. 1. The positions of chalcocite, Cu2S and
covellite CuS, are shown while their crystallo-
chemical properties [1] are given in Table 1.
Fig. 1 Cu–S phase diagram [1]
*
Author for correspondence: jmm@eunet.yu
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

¦IVKOVI‡ et al.
Table 1 Crystallochemical properties of investigated copper sulfides [1]
Lattice spacings/m×10–10
Sulfid
Crystal structure
c/a
Density/g cm–3
a
b
c
5.783
5.785
Cu2S
rhombic
11.90
27.28
13.41
1.717
3.89
3.95
–
–
6.68
6.75
–
–
–
–
Cu2S
CuS
hexagonal
hexagonal
3.75
–
16.23
4.320
4.680
Results and discussion
of indicator, reacted with measured standard solution
of sodium hydroxide for the purpose of the calcula-
tion of sulfur insulated content and further degree of
desulphurization during oxidation roasting.
The results of the chemical analysis for investigated
copper sulfides are given in Table 2, while results of
the X-ray analysis for the starting samples are shown
in Fig. 2.
Obtained X-ray results confirm the presence of
Cu2S in the starting sample of chalcocite, while the
presence of covellite and small quantity of chalcocite
was noticed in the starting sample of covellite.
The results of thermal analysis application,
which include DTA-TG-DTG recordings of investi-
gated sulfides, are shown in Fig. 3.
Thermal analysis of the investigated samples was
done using Derivatograph 1500 (MOM, Hungary) un-
der following conditions: air atmosphere, heating rate
of 10 K min–1 and Tmax=1000°C, while alumina was
used as reference material. Mineralogical characteris-
tics of the starting sample and products of oxidation
were determined by X-ray apparatus (Siemens), with
Cu-anticathode and Ni-filters at 40 kV voltage and
18 mA current. Chemical analysis of the starting sam-
ples was done using a standard procedure.
Table 2 Results of the chemical analysis
Chalcocite
Covellite
Component
content/%
Cu
Fe
72.04
2.40
5.24
19.50
0.81
0.007
0.014
0.0032
<0.001
0.003
–
65.10
1.53
SiO2
S
0.49
32.31
1.88
Cu-ox
Ag
Mg
Pb
0.0045
0.026
Sn
0.0010
Bi
–
–
Ni
Ti
0.035
0.074
<0.003
–
Co
V
0.01
0.066
–
Ca
Sb
<0.01
<0.01
<0.01
0.1
Zn
Mn
As
Ge
Al
–
0.003
<0.1
0.06
0.008
<0.003
<0.003
–
–
Mo
Cr
–
Fig. 2 X-ray results for the starting samples: a – chalcocite,
b – covellite; (Hk – chalcocite Cu2S; Kv – covellite CuS)
–
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J. Therm. Anal. Cal., 79, 2005

CHALCOCITE AND COVELLITE OXIDATION PROCESS
In order to determine mechanisms of chalcocite,
Based on characterization of starting sulfides,
DTA-TG-DTG analysis results and phase stability in-
vestigations of Cu–S–O system, following mecha-
covellite and white metal oxidation processes, depend-
encies DGT0 vs. temperature for characteristic reactions
were determined using [12–14] and shown in Fig. 4,
while constructed 3D phase stability diagram for the
system Cu–S–O, based on known thermodynamic
data [8, 14, 17] for characteristic reactions occurring at
the investigating temperatures is given in Fig. 5. As
can be seen, sulfate formation is typical for copper sul-
fides oxidation at lower temperatures, while at higher
temperatures oxides occurs as oxidation products.
Fig. 4 DGT0 vs. temperature for the investigated reactions:
a – chalcocite,
1– 4/9Cu2SO2=4/9CuSO4+2/9Cu2O,
2 – 0.4Cu2S+O2=0.4CuO+0.4CuSO4,
3 – 2Cu2O+O2=4CuO,
4 – Cu2O+O2+SO2=CuO×CuSO4,
5 – 4CuO+2SO2+O2=2CuO×CuSO4,
6 – 2CuSO4=2CuO+2SO2+O2,
7 – 4/3CuSO4=2/3Cu2O+4/3SO2+O2,
8 – 2CuO×CuSO4=4CuO+2SO2+O2,
9 – CuO×CuSO4=Cu2O+SO2+O2;
b – covellite,
1 – 2Cu2O+O2=4CuO,
2 – Cu2O+O2+SO2=CuO×CuSO4,
3 – 4CuO+2SO2+O2=2CuO×CuSO4,
4 – 2CuSO4=2CuO+2SO2+O2,
5 – 4/3CuSO4=2/3Cu2O+4/3SO2+O2,
6 – 2CuO×CuSO4=4CuO+2SO2+O2,
7 – CuO×CuSO4=Cu2O+SO2+O2,
8 – 0.8CuS+O2=0.4Cu2O+0.8SO2,
9 – Cu2O+SO2+O2=2CuO+SO3,
10 – 2/3Cu2O+4/3SO2+O2=4/3CuSO4
Fig. 3 DTA-TG-DTG analysis of the starting samples:
a – chalcocite, b – covellite
J. Therm. Anal. Cal., 79, 2005
717

¦IVKOVI‡ et al.
2CuSO4=Cu2O+2SO2+1.5O2
CuO×CuSO4=2CuO+SO2+0.5O2
CuO×CuSO4=Cu2O+SO2+O2
(5)
(6)
(7)
- for covellite:
• the two exothermal peaks occurring in the tempera-
ture interval 280–475°C, with the decrease of mass
at TG curve, respond to the oxidation of CuS to
Cu2O and CuO:
2CuS+2.5O2=Cu2O+2SO2
Cu2O+SO2+O2=2CuO+SO3
(8)
(9)
• occurring of the exothermal peaks, with increase of
mass at TG curve, in the temperature interval
475–680°C respond to reaction (3) and (3a) as well
as reaction:
Cu2O+2SO2+1.5O2=2CuSO4
(10)
• the endothermal peak occurring over 680°C, with de-
crease of mass at TG curve, respond to the dissocia-
tion of the formed sulfates, defined by Eqs (4)–(7).
In Fig. 6, results of RDA analyses of products of
oxidation of chalcocite are shown. Results of X-ray
analyses of products of oxidation of covellite are
shown in Fig. 7.
Fig. 5 3D Phase stability diagram for Cu–S–O system
nisms of chalcocite and covellite oxidation processes
could be defined:
- for chalcocite:
• The exothermal peak occurring in the temperature
interval 300–460°C, with the increase of mass at
TG curve, responds to the oxidation of Cu2S to
CuSO4 and Cu2O [3]:
As can be seen, from Figs 5–7 obtained results
are in a good agreement with suggested mechanisms
2Cu2S+4.5O2=2CuSO4+Cu2O
(1)
• further temperature increase (460–500°C) responds
to the decrease of mass at TG curve, with exothermal
peak, according to reaction of oxidation of Cu2S to
CuO and CuSO4:
Cu2S+2.5O2=CuO+CuSO4
(2)
which is the sum of reaction (1) and reaction
Cu2O+0.5O2=2CuO
• the occurrence of mass increase at TG curve at
T>500°C (temperature interval 500–700°C), fol-
lowed with exo-peak respond to:
Cu2O+O2+SO2=CuO×CuSO4
(3)
as well as:
2CuO+SO2+0.5O2=CuO×CuSO4
(3a)
Parallel with reactions (1) and (2) until all of
Cu2S is reacted.
• the endothermal peak at temperatures over 700°C,
with the mass decrease at TG curve, respond to the
dissociation of sulfates and oxisulfates to Cu2O and
CuO (Eqs (4)–(7)):
Fig. 6 X-ray analysis of products of oxidation of chalcocite at
different temperatures;
CuSO4=CuO+SO2+0.5O2
(4)
(Cp – Cu2O, Hk – Cu2S; z – CuSO4; Tn – CuO)
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J. Therm. Anal. Cal., 79, 2005

CHALCOCITE AND COVELLITE OXIDATION PROCESS
Fig. 8 Degree of desulphurisation (D) vs. time for the investi-
gated reactions at different temperatures; a – chalcocite,
b – covellite
netic function F1, for both investigated compounds,
which means that kinetic function for linearisation of
experimental results should be F1 [–ln(1–a)=kt].
The values for rate constant of reactions were de-
termined using the gradient of lines representing
linearised experimental data, while the values of kinetic
parameters were calculated using Arrhenius equation.
Arrhenius diagrams for investigated process of
oxidation of chalcocite, and covellite are presented in
Fig. 9.
Fig. 7 X-ray analysis of products of oxidation of covellite at
different temperatures;
(Cp – Cu2O, Tn – CuO, x – CuO×CuSO4, z – CuSO4)
for oxidation of all three sulfides as well as with the
results presented in [9].
Experimental results for coefficient of desul-
phurisation (D) of investigated sulfides as function of
time are presented at Fig. 8.
Selection of kinetic model for linearisation of ex-
perimental isotherms was done using Sharp’s method
of reduced halftime of reaction [15], where experimen-
tal results are presented in the form of degree of desul-
furisation dependence on reduced halftime of reaction
and afterwards compared with the theoretically derived
curves and the usability of the selected kinetic models
for description of the kinetics in such systems was esti-
mated [16].
Results obtained by kinetic investigations of
chalcocite and covellite oxidation processes are sum-
marized in Table 3.
As can be seen from Table 3, in both cases change
of a reaction mechanism occurs at following degrees of
desulphurization:
• for chalcocite at a=30–40%
• for covellite at a=30–40%
In first period, up to the change of a reaction mech-
anism, values of activation energies decrease as follows:
Values of reduced halftime of reaction for exper-
imental data were approximate to values represent ki-
J. Therm. Anal. Cal., 79, 2005
719

¦IVKOVI‡ et al.
Conclusions
Results of thermal analysis of oxidation process for
natural copper sulfides chalcocite and covellite are
presented in this paper. Based on the results of
DTA-TG-DTG, RDA and chemical analysis, mecha-
nisms of oxidation processes were defined and con-
firmed by thermodynamic analysis. Kinetic parame-
ters and activation energies of investigated processes
were determined using Sharp’s method of reduced
halftime of reaction.
Values of activation energies indicate that for
both investigated samples oxidation process begin in
kinetic, which means that temperature has overall in-
fluence on oxidation rate, and moves to diffusion area
with change of a reaction mechanism.
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¾¾¾¾¾¾® ¾¾¾¾¾¾®
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while in second period, this sequence is given as:
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¾¾¾¾¾¾® ¾¾¾¾¾¾®
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ning of oxidation processes are going on in a kinetic
area. It should be mentioned that higher values for Ea
obtained for chalcocite show dominant influence of
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cess is near to the transition area.
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Table 3 Summarized results of kinetic investigations of Cu2S
and CuS oxidation processes
Activation energy/kJ mol–1
Mineral
Received: January 6, 2004
until fraction
after the fraction
In revised form: October 25, 2004
chalcocite
covellite
77
33
19
29
720
J. Therm. Anal. Cal., 79, 2005