Thermoanalytical studies of natural potassium, sodium and ammonium alunites

Article abstract of DOI:10.1007/s10973-009-0581-7

Dynamic and controlled rate thermal analysis (CRTA) has been used to characterise alunites of formula [M(Al)₃(SO₄) ₂(OH)₆] where M⁺ is the cations K⁺, Na⁺ or NH₄⁺. Thermal decomposition occurs in a series of steps (a) dehydration, (b) well-defined dehydroxylation and (c) desulphation. CRTA offers a better resolution and a more detailed interpretation of water formation processes via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of water formation reveal the subtle nature of dehydration and dehydroxylation. Akademiai Kiado, Budapest, Hungary 2010.

Full text of DOI:10.1007/s10973-009-0581-7

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J Therm Anal Calorim (2010) 100:961–966
DOI 10.1007/s10973-009-0581-7
Thermoanalytical studies of natural potassium, sodium
and ammonium alunites
•
Janos Kristof Ray L. Frost Sara J. Palmer
•
•
´
Erzsebet Horvath Emma Jakab
´
•
´
´
Received: 25 May 2009 / Accepted: 21 October 2009 / Published online: 28 November 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract Dynamic and controlled rate thermal analysis
(CRTA) has been used to characterise alunites of formula
[M(Al)3(SO4)2(OH)6] where M? is the cations K?, Na? or
NH4?. Thermal decomposition occurs in a series of steps:
(a) dehydration, (b) well-defined dehydroxylation and (c)
desulphation. CRTA offers a better resolution and a more
detailed interpretation of water formation processes via
approaching equilibrium conditions of decomposition
through the elimination of the slow transfer of heat to the
sample as a controlling parameter on the process of
decomposition. Constant-rate decomposition processes of
water formation reveal the subtle nature of dehydration and
dehydroxylation.
Introduction
Interest in alunites stems from a number of reasons:
(a) firstly there is the possible discovery of alunites on
Mars and possibly other planets [1, 2]. Such findings imply
the presence of water either at present or at some time in
the planetary past, since alunites are only formed from
solution through precipitation and crystallisation processes.
Interest in such minerals and their thermal stability rests
with the possible identification of these minerals and
dehydrated paragenetically related minerals. There have
been many studies on related minerals such as the Fe(II)
and Fe(III) sulphate minerals and jarosites [3–7]. (b) The
importance of alunite formation and its decomposition
depends upon its presence in soils, sediments and evaporite
deposits [8–10]. These types of deposits have formed in
acid soils where the pH is less than 3.0 pH units. Such
acidification results from the oxidation of pyrite which may
be from bacterial action or through air-oxidation.
(c) Thirdly alunites are important from an environmental
point of view. Alunites are minerals which can function as
collectors of heavy metals and low concentrations can be
found in the natural alunites. Such minerals can act as a
significant environmental sink.
Keywords Alunite thermal analysis ꢀ
Controlled rate thermal analysis ꢀ Dehydration ꢀ
Dehydroxylation
´
J. Kristof
Department of Analytical Chemistry, University of Pannonia,
´
PO Box 158, 8201 Veszprem, Hungary
One of the difficulties associated with the analysis of
alunites is that they are often poorly crystalline, making
detection using XRD techniques difficult. Another problem
associated with the study of alunites is their thermody-
namic stability. Often the minerals are formed from acid-
sulphate rich environments such as acid mine drainage and
acid-sulphate soils and as such their solubility is controlled
by climatic conditions, in particular the temperature. Such
minerals lend themselves for thermal analysis. Interest-
ingly, there have been few recent studies on the thermal
analysis of alunites. The reason for this is unclear. Perhaps
R. L. Frost (&) ꢀ S. J. Palmer
Inorganic Materials Research Program, School of Physical and
Chemical Sciences, Queensland University of Technology, 2
George Street, GPO Box 2434, Brisbane, QLD 4001, Australia
e-mail: r.frost@qut.edu.au
´
E. Horvath
Institute of Environmental Engineering, University of Pannonia,
´
PO Box 158, 8201 Veszprem, Hungary
E. Jakab
Central Research Institute for Chemistry, Hungarian Academy of
Science, PO Box 17, 1525 Budapest, Hungary
123

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J. Kristof et al.
962
it is because of the variation in composition which occurs
with natural minerals. Such variation can be overcome with
the use of synthetic minerals. The first recorded thermal
analysis of alunite occurred in 1919. Early studies focussed
on the effect of alunite impurity on the thermal treatment of
other chemicals. Many studies occurred when new equip-
ment became available. Recently thermal analysis has been
used to enhance the understanding of the stability of min-
erals [11–23].
conditions at a uniform rate of 1.0 °C min-1). With the
quasi-isothermal, quasi-isobaric heating program of the
instrument the furnace temperature was regulated precisely
to provide a uniform rate of decomposition in the main
decomposition stage.
Results and discussion
In this work, as part of our studies of secondary mineral
formation and their stability, we report the comparison of
the thermal analysis of selected natural alunites using both
dynamic and controlled rate thermal analysis techniques.
The dynamic TG, DTG and DTA patterns of potassium,
sodium and ammonium alunites are shown in Figs. 1, 2 and
3 respectively. The results of the conventional constant rate
heating experiment are reported in Table 1. The controlled
rate thermal analysis results are reported in Table 2 and
the CRTA patterns of potassium, sodium and ammonium
alunites are shown in Figs. 5, 6 and 7.
Experimental
Minerals
Dynamic thermal analysis of potassium alunite
The alunites were obtained from the Mineralogical
Research Company, Australia. The minerals were phase
analysed by powder X-ray diffraction and were analysed
for chemical composition by EDX methodology.
The behaviour of the thermal decomposition is different for
the potassium, sodium and ammonium alunites. The ther-
mal decomposition of potassium alunites takes place in
four mass loss steps as shown in Fig. 1. The mass loss
values belonging to the individual decomposition steps are
summarised in Table 1. DTG peak maxima are observed at
246, 513, 797 and 908 °C. The DTA patterns shows min-
ima at 246, 517 and 795 °C indicating endothermic steps in
the thermal decomposition process.
Thermal analysis
Conventional thermal analysis experiment
Thermal decomposition of the alunites was carried out in a
Derivatograph PC type thermoanalytical equipment (Hun-
garian Optical Works, Budapest, Hungary) capable of
recording the thermogravimetric (TG), derivative thermo-
gravimetric (DTG) and differential thermal analysis (DTA)
curves simultaneously. The samples were heated in a
ceramic crucible in static air atmosphere at a rate of
5 °C min-1. The thermoanalytical equipment is coupled to
a mass spectrometer. In order to understand the gas evo-
lution pattern, mass spectrometric identification of gaseous
decomposition products has been carried out in argon
atmosphere in a Perkin Elmer TGS-2 type thermobalance
connected to a Hayden HAL 2/301 PIC type mass spec-
trometer. The heating rate was 10 °C min-1. As water
produces m/z 17 fragment ion, the contribution of water
was subtracted from the intensity curve of m/z 17. Thus, the
Step 1: dehydration The loss of crystallization water
takes place at 246 °C according to the following equation:
KAl3ðSO4Þ2ðOHÞ6xH2O ! KAl3ðSO4Þ2ðOHÞ6 þxH2O
Based on the TG curve, the exact amount of crystallization
water was 1.31 moles.
Step 2: dehydroxylation Dehydroxylation occurs at
513 °C resulting in the evolution of 3 moles of water:
0
TG
0.5
–20
DTA
–0.5
–1.5
–2.5
–3.5
–4.5
–5.5
–40
–60
517
513
?
795
797
m/z 17 ion represent only NH3 molecular ion.
DTG
246
908
–80
Controlled rate thermal analysis experiment
–100
–120
Thermal decomposition of the alunite samples under
CRTA conditions was carried out in the Derivatograph
under static air in an open ceramic crucible at a pre-set,
constant decomposition rate of 0.1 mg min-1. (Below this
threshold value the samples were heated under dynamic
0
100 200 300 400 500 600 700 800 900 1000
Temperature/°C
Fig. 1 TG–DTG–DTA curves of potassium alunite under dynamic
heating
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Thermoanalytical studies of natural potassium, sodium and ammonium alunites
963
0
Dynamic thermal analysis of sodium alunite
TG
Sodium alunite
0.5
271
266
–20
–40
–60
–0.5
–1.5
–2.5
–3.5
–4.5
–5.5
DTA
The thermal decomposition behaviour of the sodium alu-
nite appears significantly different to that of the potassium
alunite (Fig. 2). The three mass loss steps that appear in the
DTG curve at 119, 163 and 266 °C belong to the liberation
of water, while desulphation occurs in a single process at
770 °C.
DTG
–80
–100
–120
119
163
770
Step 1: dehydration Dehydration is believed to take place
in two steps resulting in all together the release of
7.88 moles of water. Therefore, the following dehydration
mechanism is proposed:
0
100 200 300 400 500 600 700 800 900 1000
Temperature/°C
Fig. 2 TG–DTG–DTA curves of sodium alunite under dynamic
heating
NaAl3ðSO4Þ2ðOHÞ68 H2O ! NaAl3ðSO4Þ2ðOHÞ66 H2O
þ 2 H2O
0
TG
0.5
DTA
–20
NaAl3ðSO4Þ2ðOHÞ66 H2O ! NaAl3ðSO4Þ2ðOHÞ6
þ 6 H2O
–0.5
–40
–1.5
Step 2: dehydroxylation Dehydroxylation is believed to
take place at 266 °C as follows:
–60
–2.5
–80 DTG
478
–3.5
–4.5
–5.5
NaAl3ðSO4Þ2ðOHÞ6! NaAl3O3ðSO4Þ2 þ 3 H2O
(the theoretical mass loss in this process is 9.54%, while
the actual one is 10.41%).
307
–100
–120
98
759
117
0
100 200 300 400 500 600 700 800 900 1000
Temperature/°C
Step 3: desulphation Sulphur trioxide is released at
770 °C in accordance with the following equation based on
stoichiometric calculations:
Fig. 3 TG–DTG–DTA curves of ammonium alunite under dynamic
heating
NaAl3O3ðSO4Þ6! NaAlO2 þ Al2O3 þ 2 SO3
(the theoretical mass loss in this reaction is 46.54%, while
the actual amount of SO3 released is 53.17%).
KAl3ðSO4Þ2ðOHÞ6! KAl3O3ðSO4Þ6 þ 3 H2O
(the theoretical mass loss is 13.0%, the observed loss is
11.6%).
Dynamic thermal analysis of ammonium alunite
Step 3: desulphation The desulphation reaction occurs in
two steps at 797 and 908 °C leading to the formation of
K2SO4ꢀAl2O3 and alumina as follows:
The thermal decomposition of ammonium alunite shows
peaks in the DTG curve at 98, 117, 307, 478 and 759 °C
(Fig. 3). Endothermic peaks in the DTA curves occur at
similar positions. The TG-MS pattern (Fig. 4) is essential
to reveal the complicated nature of decomposition. In the
2 KAl3O3ðSO4Þ6! K2SO4 ꢀ Al2O3 þ 2 Al2O3 þ 3 SO3
(the theoretical mass loss in this reaction is 33.34%, the
actual loss is 36.37%).
Table 1 Temperature and mass loss data of the decomposition of alunite samples under dynamic conditions
Sample: NH4 alunite Sample: K alunite Sample: Na alunite
Temperature
range/°C
Mass loss (sample mass:
86.18/mg)
Temperature
range/°C
Mass loss (sample mass:
115.04/mg)
Temperature
range/°C
Mass loss (sample mass:
95.12/mg)
mg
%
mg
%
mg
%
32–106
106–259
259–388
388–529
577–791
9.3
22.9
6.0
10.8
26.6
7.0
115–399
399–563
563–851
851–992
6.2
12.6
30.5
4.5
5.4
35–135
135–227
227–403
403–973
6.8
18.2
7.3
7.1
11.0
26.5
3.9
19.1
7.7
3.5
4.1
33.4
35.1
28.6
33.2
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J. Kristof et al.
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Table 2 Temperature and mass loss data of the decomposition of alunite samples under CRTA conditions
Sample: NH4 alunite
Sample: K alunite
Sample: Na alunite
Temperature
range/°C
Mass loss (sample mass:
139.12/mg)
Temperature
range/°C
Mass loss (sample mass:
168.11/mg)
Temperature
range/°C
Mass loss (sample mass:
158.38/mg)
mg
%
mg
%
mg
%
33–66
66–90
17.9
14.7
22.5
7.1
12.9
10.6
16.2
5.1
118–245
245–302
302–379
379–520
520–597
4.1
3.3
2.4
24–91
5.6
3.5
2.0
1.3
91–112
8.3
24.9
10.6
3.3
5.2
15.7
6.7
90–235
235–368
368–506
2.2
112–167
167–234
234–296
296–412
412–569
18.1
1.1
10.8
0.7
6.1
4.4
2.1
2.3
1.5
0.8
0.5
liberated simultaneously. With faster heating the two
processes merge together as it is witnessed in Fig. 4. Thus,
the proposed mechanism of decomposition is as follows:
first step water is released, while the two decomposition
steps between 300 and 550 °C belong to the simultaneous
evolution of water and ammonia. In the last step of
decomposition sulphur trioxide is liberated, only. However,
sulphur trioxide is not stable under mass spectrometric
conditions; therefore, the evolution of sulphur dioxide and
oxygen can be monitored.
6 NH4Al3ðSO4Þ2ðOHÞ6! 6 NH3 þ 21 H2O
þ 4 Al2ðSO4Þ3þ 5 Al2O3
(the theoretical mass loss in this process is 20.37%, while
the actual loss is 17.60%).
Step 1: dehydration Since water elimination in the first
two steps (at 98 and 117 °C) belongs to dehydration, the
total amount of crystallization water is 13.0 moles. Thus,
the following dehydration mechanism is proposed:
Step 3: desulphation The decomposition step of the
mineral observed at 759 °C is interpreted by liberation of
sulphur trioxide as follows:
Al2ðSO4Þ3! Al2O3 þ 3 SO3
NH4Al3ðSO4Þ2ðOHÞ 13 H2O
! NH4Al3ðSO4Þ6ðOHÞ 9 H2O þ 4 H2O
2
6
The controlled rate thermal analysis experiment
NH4Al3ðSO4Þ2ðOHÞ69 H2O ! NH4Al3ðSO4Þ2ðOHÞ6
þ 9 H2O
The thermal decomposition patters of the potassium, sodium
and ammonium alunite samples recorded under CRTA
decompositions are shown in Figs. 5, 6 and 7. The mass loss
data are summarised in Table 2. The dehydration process of
potassium alunite (Fig. 5) under the slow heating rate of
1 °C min-1 can be resolved to four overlapping steps
between 100 and 200 °C, at 222, 257 and 326 °C. During the
Step 2: dehydroxylation and de-ammonification At 307
and 478 °C dehydroxylation water and ammonia are
DTG
0
TG
TG
500
SO2
469
H2O
DTG
400
–10
326
300
200
NH3
O2
–20
–30
257
100
0
222
200
T
100 200 300 400 500 600 700 800 900
Temperature/°C
0
100
300
400
500
600
700
Time/min
Fig. 4 Thermogravimetric and mass spectrometric ion intensity
curves of ammonium alunite (m/z 17: NH3?, m/z 18: H2O?, m/z 32:
Fig. 5 Thermal decomposition pattern of potassium alunite under
CRTA conditions
?
O2?, m/z 64: SO2
)
123