The decomposition kinetics of mechanically activated alunite ore in air atmosphere by thermogravimetry

Article abstract of DOI:10.1016/j.tca.2006.07.003

The thermal decompositions of both non-activated and mechanically activated alunite ore have been studied by thermogravimetry (TG). The ore was activated mechanically in an attritor for 15 min and amorphisation in the structure was studied by X-ray diffra

Full text of DOI:10.1016/j.tca.2006.07.003

Thermochimica Acta 448 (2006) 107–110
The decomposition kinetics of mechanically activated alunite
ore in air atmosphere by thermogravimetry
∗
Furkan K u¨ c¸ u¨ k, Kenan Yildiz
Sakarya University, Metallurgy and Materials Engineering, 54187 Sakarya, Turkey
Received 12 June 2006; received in revised form 10 July 2006; accepted 11 July 2006
Available online 16 July 2006
Abstract
The thermal decompositions of both non-activated and mechanically activated alunite ore have been studied by thermogravimetry (TG). The
ore was activated mechanically in an attritor for 15 min and amorphisation in the structure was studied by X-ray diffraction analysis. It can be
verified that alunite decomposes in two steps, which are dehydration and desulphation. It was also established that the mechanical activation
affected especially on the temperature range of dehydration reaction. The activation energies of dehydration and desulphation reactions have
−
1
been calculated from the thermogravimetric data at heating rates of 5, 10, 15 and 20 K min involving isoconversional methods of Ozawa and
Kissenger–Akahira–Sunose (KAS).
©
2006 Elsevier B.V. All rights reserved.
Keywords: Alunite; Decomposition; Kinetics; Mechanical activation; Thermogravimetry
1
. Introduction
possible to reduce their decomposition temperature or causes
such a degree of disordering. The mineral activation leads to
a positive influence on the leaching reaction kinetics and to an
increase in the measured surface area [6]. Mechanical activa-
tion of minerals by intensive grinding is a non-traditional way
of influencing the processes in extractive metallurgy. Creation
of fine particles, increase in specific surface area and forma-
tion of defective structures are among the main factors, which
accelerate leaching in hydrometallurgy [7].Decomposition of
solids is the subject of many kinetic studies. In many meth-
ods of kinetic estimation, isoconversional method is recommend
as trustworthy way of obtaining reliable and consistent kinetic
information [8]. It is a ‘model-free’ method, which involves
measuring the temperatures corresponding to fixed values of
α from experiments at different heating rates, β. Recently, we
have initiated a study of dehydration and desulphation kinetics
of non-activated and activated alunite by thermogravimetry. The
activation energies are calculated according to the isoconver-
sional methods of Ozawa [9] and Kissenger–Akahira–Sunose
Alunite is one of the minerals of the jarosite group. It is a
potassiumalumoccurring generallyin rhombohedralandhexag-
onal crystals. Its chemical formula is KAl3(SO4)2(OH) and as
6
a source of alum, potassium and aluminium, it is a commer-
cial raw material for the chemical industry. In Turkey, alunite
reserves are located in S¸ ebinkarahisar-Giresun (15–20 million
tonnes), S¸ aphane-K u¨ tahya (7 million tonnes) and Fo c¸ a-Izmir (5
million tonnes) regions [1–3].
The structure of alunite contains aluminium sulphate and
potassium sulphate, both of them dissolve in water. However,
it is not soluble in water, acids and bases unless calcined. Alu-
nite, calcined at temperatures over 500 C, dissolves due to the
decomposition of the structure. It is widely used in the manu-
facturing of aluminium sulphate, potassium alum, alumina and
potassium sulphate. The ore calcined at 600–650 C is con-
verted to the products, mentioned above, by acidic [4] and basic
extractions [5].The mechanical activation of minerals makes it
◦
◦
(KAS) [10]. In literature there is no report on the thermal
decomposition kinetics of mechanically activated alunite. So
the aim of the research is to investigate the effect of mechan-
ical activation on the dehydration and desulphation kinetics of
alunite.
∗
Corresponding author. Tel.: +90 264 2955770; fax: +90 264 2955601.
E-mail addresses: furkankucuk80@hotmail.com (F. K u¨ c¸ u¨ k),
kenyil@sakarya.edu.tr (K. Yildiz).
0
040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.07.003

1
08
F. K u¨ c¸ u¨ k, K. Yildiz / Thermochimica Acta 448 (2006) 107–110
2
. Experimental
Alunite ore used in this study was provided from Dostel
Aluminum Sulphate Ltd. in S¸ aphane-K u¨ tahya, Turkey. It was
crushed, ground and sieved to −100 ␮m size. The chemicals
used in the analysis were obtained from Merck Chemical. The
components SiO2, H2O, SO3 and Al2O3 in the alunite ore were
analysed by gravimetric method. Fe2O3, CaO and MgO were
analysed by complexometry with EDTA and K2O was analysed
by flame photometry. The chemical analysis of alunite ore is
2
2.98% Al2O3, 4.61% K2O, 45.56% SiO2, 18.08% SO3, 0.61%
Fe2O3, 0.16% CaO + MgO and 8.00% H2O.
Mechanical activation of alunite ore was performed in an
attritor (stirring ball mill), which is designed and built in our lab,
under following conditions: weight of the sample: 10 g, weight
of balls: 200 g of stainless steel balls with 5.5 mm in diameter,
grinding time: 0 and 15 min, stirring rate: 850 rpm.
X-ray powder diffraction patterns were obtained with a
Rigaku Ultima X-ray diffractometer and Cu K␣ radiation was
used. Thermogravimetric analysis curves were obtained using a
Netzsch RT-1500C TGA Instrument in air atmosphere at dif-
−
1
ferent heating rates (5, 10, 15 and 20 K min ) from room
temperature to 1273 K, with 130 ± 2 mg samples. The TG data
were used to obtain the apparent activation energies using both
Ozawa and Kissenger–Akahira–Sunose (KAS) method.
◦
Fig. 1. X-ray diffraction patterns (10–40 ) of non-activated and activated alu-
nite.
3
. Results and discussion
3
.1. X-ray diffraction analysis
The X-ray diffraction analysis of non-activated and activated
alunite is given in Fig. 1. Considering the peaks of non-activated
and activated alunite, it can be clearly seen that all diffraction
peaks of alunite get shorter after mechanical activation. This is
due to partial amorphisation and structural disordering in alunite
structure. In literature [6,7], it was reported that the mechani-
cal activation resulted in the amorphisation of mineral particles.
Tromans and Meech [11] indicated that mechanical activation
resulted in development of large numbers of dislocations and
their associated strain fields, which might lead to an overall
decrease in long-range lattice periodicity. This may be inter-
preted as the formation of a metastable “amorphous phase”,
because line broadening and disappearance of diffraction peaks
takes place on X-ray patterns after extended milling.
3
.2. Thermal decomposition of non-activated and activated
alunite
TG and DTG curves of non-activated and mechanically acti-
vated alunite ore are given in Fig. 2(a and b). The DTG curves
for both alunite ores show two peaks. The first peak is at
◦
◦
about 560 C for non-activated and 545 C for activated alunite
ores. This is due to the dehydration of the structural water and
the transformation of alunite into KAl(SO4)2 and amorphous
Al2O3, as given in Eq. (1) [12]:
KAl3(SO4)2(OH) → KAl(SO4)2 + Al2O3 + 3H2O
(1)
Fig. 2. TG and DTG curves for (a) non-activated and (b) activated alunite.
6

F. K u¨ c¸ u¨ k, K. Yildiz / Thermochimica Acta 448 (2006) 107–110
109
Fig. 3. TG analysis of (a) non-activated alunite and (b) activated alunite at different heating rates.
◦
various heating rates (β = 5, 10, 15 and 20 K min 1) are shown
in Fig. 3 for (a) non-activated alunite and (b) activated alunite.
According to isoconversional method, the basic data of α and T
collected from Fig. 3(a and b) are illustrated in Tables 1 and 2,
respectively.
−
The second peak is at about 820 C for both non-activated and
activated alunite ore. This is due to the desulphation, as given in
Eqs. (2)–(4) [12]:
2
2
3
2
KAl(SO4) → K3Al(SO4) + Al2(SO4)
(2)
(3)
(4)
2
3
3
3
2
3
1
3
According to the above-mentioned equations, the plots of
log β versus 1000/T and ln(β/T ) versus 1000/T corresponding
K3Al(SO4) → K2SO4 + Al2(SO4)
3
3
2
2
3
2
3
Al2(SO4) → Al2O3 + 2SO2 + O2
The TG curve for non-activated alunite ore shows two steps
of weight losses. The weight losses are about 8% for dehydra-
tion and about 22% as the total weight loss after desulphation. In
non-activatedalunitedehydrationtakesplaceatacertaintemper-
ature range. This is an expected property for crystalline alunite.
However, activated alunite exhibited dehydration reaction start-
to different conversions α can be obtained by a linear regress
of least-square method, respectively. The activation energies E␣
can be calculated from the slopes of every line with better linear
correlation coefficient r. The slopes change depending on the
degree of conversion, α, for the dehydration and desulphation
3
Table 1
α–T data at different heating rates, β (K min 1), for non-activated alunite
−
◦
ing from 100 C. This is result of amorphisation and structural
disordering, providing with mechanical activation.
α
T (K)
Dehydration
Desulphation
β = 10 β = 15 β = 20
3
.3. Activation energy of dehydration and desulphation
β = 5 β = 10 β = 15 β = 20 β = 5
steps
0
0
0
0
0
.1 764.6 785.2 796.1 801.6 1017.7 1047.6 1061.5 1069.1
.2 792.8 806.8 814.2 819.3 1038.5 1061.4 1073.3 1080.1
.3 804.5 817.1 824.1 828.7 1043.8 1070.3 1081.9 1088.6
.4 811.9 824.5 831.7 836.4 1055.5 1077.4 1089.1 1095.8
.5 818.1 831.1 838.6 843.5 1062.9 1083.7 1095.3 1102.1
Decomposition of solids is the subject of many kinetic stud-
ies. It helps to understand the following decomposition mecha-
nism: A(s) → B(s) + C(g). This is a model-free method, which
involves measuring the temperatures corresponding to fixed val-
ues of α from experiments at different heating rates (β). The
activation energies (E␣) can be calculated according to the iso-
convertional methods. In kinetic study of both non-activated and
activated alunite, Ozawa and KAS equations were used to deter-
mine the activation energy of the dehydration and desulphation
reactions.
0.6 823.9 837.4 845.5 851.1 1068.5 1089.2 1100.9 1107.9
0.7 830.1 844.5 853.8 860.2 1073.5 1094.2 1106.1 1113.4
0
.8 842.6 853.9 864.9 872.6 1078.1 1099.1 1111.3 1119.1
Table 2
α–T data at different heating rates, β (K min 1), for activated alunite
−
The equations used for E␣ calculation are:
α
T (K)
ꢀ
ꢁ
AE␣
0.4567E␣
Dehydration
Desulphation
β = 10 β = 15 β = 20
Ozawa equation : log β= log
− 2.315 −
Rg(α)
RT
β = 5 β = 10 β = 15 β = 20 β = 5
(5)
0
0
.1 393.6 413.6 424.2 435.3
.2 442.0 456.1 466.0 477.9
0.3 488.6 511.1 518.7 531.3
885.1
951.1
927.5
986.1
932.7
997.8 1014.5
958.6
ꢂ
ꢃ
ꢀ
ꢁ
ꢂ
ꢃ
β
AE␣
E␣
986.7 1019.2 1031.5 1045.2
KAS equation : ln
= ln
−
(6)
2
0.4 556.9 587.8 594.3 610.7 1011.7 1042.0 1053.3 1065.5
T
Rg(α)
RT
0
0
0
0
.5 652.1 691.1 694.3 713.4 1030.0 1057.1 1068.3 1078.6
.6 731.1 754.4 760.1 771.1 1042.6 1067.5 1078.1 1087.2
.7 766.3 783.7 788.2 794.9 1050.9 1074.7 1084.9 1093.8
.8 784.7 800.1 803.9 809.9 1056.9 1080.7 1091.1 1100.4
ꢄ
α
−1
where g(α) =
f(α) dα is the integral form of the f(α).
At the constant condition of other parameters, the TG curves
for dehydration and desulphation of alunite in air atmosphere at
0

1
10
F. K u¨ c¸ u¨ k, K. Yildiz / Thermochimica Acta 448 (2006) 107–110
Table 3
E␣ vs. r calculated by Ozawa and KAS methods for non-activated alunite
of KAS kinetic equation. The values calculated by Ozawa
method are close to those obtained by KAS method. The
mechanical activation promoted some amorphisation, but not
completely, and structural disorders in alunite. This pretreat-
ment decreased the activation energy values to the range of
α
Ozawa method
KAS method
E␣ (kJ mol 1)
−
r
E␣ (kJ mol
−1
)
r
Dehydration
41–76 kJ/mol, up to α (degree of conversion) = 0.5 and then
0
0
0
0
0
0
0
0
.1
.2
.3
.4
.5
.6
.7
.8
177.3
267.4
300.1
302.0
296.1
296.1
281.9
259.4
0.993
0.998
0.999
0.999
0.999
0.999
0.999
0.999
173.5
267.8
302.1
303.9
297.6
282.6
258.8
264.3
0.992
0.998
0.999
0.999
0.999
0.999
0.999
0.983
activation energy was increased to actual values. This con-
firmed the present of crystalline structure of alunite in activated
alunite.
The activation energy values of desulphation were between
2
2
20 and 325 kJ/mol for non-activated alunite and 120 and
94 kJ/mol for activated alunite. The values calculated by Ozawa
and KAS method are also close to each other for desulpha-
tion. It can be said that the effect of mechanical activation on
desulphation reaction is less than its effect on dehydration of
alunite.
Desulphation
0
0
0
0
0
0
0
0
.1
.2
.3
.4
.5
.6
.7
.8
226.3
290.7
271.3
309.8
323.4
325.6
325.3
320.3
0.989
0.995
0.987
0.996
0.997
0.998
0.998
0.999
220.7
288.1
267.6
307.9
322.1
324.4
323.9
318.6
0.987
0.994
0.986
0.995
0.997
0.998
0.999
0.999
4. Conclusions
Alunite decomposes in two steps: dehydration and desulpha-
tion. The dehydration of alunite is important for its further treat-
ments. In non-activated alunite, the dehydration reaction starts
◦
after about 500 C. On the contrary, the dehydration of mechan-
Table 4
E␣ vs. r calculated by Ozawa and KAS methods for activated alunite
◦
ically activated alunite ore was started after 100 C. Mechanical
activation results in amorphisation and structural disordering in
alunite. Its effect on desulphation reaction temperature of alunite
was not excessive.
α
Ozawa method
KAS method
E␣ (kJ mol 1)
−
r
E␣ (kJ mol
−1
)
r
Dehydration
For the dehydration and desulphation reactions of non-
activated and activated alunite, the activation energies calcu-
lated by Ozawa and KAS methods are close to each other. In
dehydration of alunite, the activation energies decreased to the
range of 41–83 kJ/mol from the range of 173–303 kJ/mol, due
to amorphisation and structural disordering, up to α (degree of
conversion) = 0.5 and then activation energy was increased to
actual values. This confirmed the present of crystalline structure
of alunite in activated alunite.
0
0
0
0
0
0
0
0
.1
.2
.3
.4
.5
.6
.7
.8
45.7
65.4
68.1
70.4
83.1
0.998
0.983
0.988
0.973
0.946
0.979
0.979
0.978
41.2
61.1
63.2
64.4
76.1
152.2
233.1
278.3
0.997
0.978
0.985
0.965
0.930
0.976
0.977
0.976
156.6
233.9
277.2
Desulphation
0
0
0
0
0
0
0
0
.1
.2
.3
.4
.5
.6
.7
.8
128.9
169.3
194.7
220.8
250.9
278.3
293.8
293.9
0.956
0.989
0.992
0.992
0.994
0.994
0.995
0.996
120.3
161.8
187.9
214.9
246.4
275.0
291.2
291.1
0.945
0.987
0.991
0.990
0.993
0.994
0.994
0.995
References
[
[
1] H. G u¨ lensoy, MTA Enstit u¨ s u¨ Dergisi (in Turkish) 71 (1968) 93–128.
2] H. G u¨ lensoy, I.A. S¸ engil, Kimya ve Sanayii (in Turkish) 31 (157/158)
(
1989) 116–131.
[
[
[
[
3] I.A. S¸ engil, Turk. J. Eng. Environ. Sci. 15 (1991) 436–446.
¨
4] M. Ozdemir, H. C¸ eti s¸ li, Hydrometallurgy 76 (2005) 217–224.
¨
5] M. Ozacar, I.A. S¸ engil, Can. Metal. Quart. 38 (4) (1999) 249–255.
6] P. Balaz, Extractive Metallurgy of Activated Minerals, Elsevier Science
B.V., Amsterdam, The Netherlands, 2000.
reactions of alunite. The α-dependence of the apparent value
of E␣, calculated from the slopes of Eqs. (5) and (6) at various
α, is shown in Table 3 for non-activated alunite and in Table 4
for activated alunite. As shown in Tables 3 and 4, the activation
energies calculated by Ozawa method and KAS method are close
to each other.
[7] P. Balaz, E. Boldizarova, M. Achimovicova, R. Kammel, Hydrometallurgy
57 (1) (2000) 85–96.
[
[
8] S. Vyazovkin, C.A. Wight, Thermochim. Acta 340/341 (1999) 53–68.
9] T. Ozawa, J. Thermal Anal. 2 (1970) 301–304.
[
[
10] H.E. Kissenger, J. Anal. Chem. 29 (1957) 1702–1706.
11] D. Tromans, J.A. Meech, Miner. Eng. 14 (11) (2001) 1359–1377.
The activation energy values of dehydration of non-activated
alunite were calculated between 173 and 304 kJ/mol by using
[12] A. K u¨ c¸ u¨ k, M. S¸ . G u¨ labo g˘ lu, Ind. Eng. Chem. Res. 41 (2002) 6028–
6032.