Monitoring of the evolved gases in apatite-ammonium sulfate thermal reactions

Article abstract of DOI:10.1007/s10973-005-0709-3

Thermal reactions in natural fluorapatite or fluorcarbonate apatite and ammonium sulfate mixtures with mole ratio 1:4 at calcination up to 500°C were studied by simultaneous thermogravimetry and FTIR analysis of the evolved gases. The composition of natur

Full text of DOI:10.1007/s10973-005-0709-3

Journal of Thermal Analysis and Calorimetry, Vol. 80 (2005) 655–658
MONITORING OF THE EVOLVED GASES IN APATITE–AMMONIUM
SULFATE THERMAL REACTIONS
Kaia Tõnsuaadu1*, J. Pelt2 and Maria Borissova1
1Tallinn University of Technology, 19086 Tallinn
2Tartu Observatory 61602 Tõravere, Estonia
Thermal reactions in natural fluorapatite or fluorcarbonate apatite and ammonium sulfate mixtures with mole ratio 1:4 at calcination
up to 500°C were studied by simultaneous thermogravimetry and FTIR analysis of the evolved gases. The composition of natural
apatite has little impact on the release of NH3. Upon the evolution of NH3 nitrous oxides were found in minor amounts. The release
of SO2 at temperatures above 400°C is more intensive and occurs at lower temperatures in the case of fluorapatite than of carbonate
containing apatites. Evolution of CO2 starts at 250°C with maximum at 350–360ºC.
Keywords: apatite, CO2, evolved gas analysis, FTIR, NH3, (NH4)2SO4, N2O, SO2
Introduction
impurity minerals in natural apatite [7, 8]. Carbonate
apatites decompose more readily at thermal treatment
and then CO2 is added to the evolved gases.
Thermal reactions in the mixture of apatite and am-
monium sulfate (AS) have been studied since the
early 1960s, with the main focus on the composition
of solid product. It has been established that at calci-
nation of apatite – AS mixture several simultaneous
reactions take place that lead to the evolvement of
NH3, H2O, SO2, HF, and nitrous oxides [1–5].
In our study the evolvement of gases during apa-
tite – AS thermal interaction at temperatures up to
500ºC was investigated by means of coupled
TG/DTA-FTIR analysis. AS reaction with natural
Kola (Russia) and Sokli (Finland) fluorapatites, Ka-
bala (Estonia) phosphorite (fluorcarbonate-apatite)
and synthetic fluorapatite was studied.
Starting at 220°C, decomposition of (NH4)2SO4
and a reaction between apatite and the products of
(NH4)2SO4 decomposition takes place resulting in NH3,
HF and H2O release and the formation of calcium poly-
phosphate, Ca2(NH4)2(SO4)3 and CaSO4. In the tempera-
ture interval 290–450°C Ca2(NH4)2(SO4)3 could react
with calcium polyphosphate forming CaNH4P3O9, or
decomposition could take place (1)
Experimental
The composition of the natural apatite concentrates
and the synthetic fluorapatite (FAp) used in the experi-
ments is given in Table 1. In Kola apatite the main im-
purity is nepheline, while in Sokli apatite it is iron sili-
cates and carbonates, and in Kabala apatite calcite and
quartz. The chemical composition of apatites was de-
termined by the standard methods of chemical analyses
and the specific surface area (SSA) by BET method.
The (NH4)2SO4 used was a chemical pure grade
(NH3 – 25.76; SO3 – 60.61%). The mole ratio in the
apatite – (NH4)2SO4 mixtures was 1:4. The mixtures
of apatite and (NH4)2SO4 were mixed in an agate ball
mill for 30 min.
Ca2(NH4)2(SO4)3→2CaSO4+
+2NH3↑+SO3↑+ H2O↑
Also the simultaneous reaction of SO3 decompo-
sition (2) [6] takes place
(1)
SO3→SO2+1/2O2
(2)
Calcination of the mixture above 350°C causes
decomposition of CaNH4P3O9 resulting in a decrease
in the content of water-soluble phosphorus and a re-
lease of NH3 and SO2 by the reactions (3) and (2):
The thermal analysis experiments were performed
in the airflow 50 mL min–1 at the heating rate
10 K min–1 in a corundum crucible with Setaram
LabSys 2000 equipment. The on-line gas composition
was monitored using FTIR gas analyser (Inter-
spectrum). The Ranger-AlP Gas cell S/N 23790 (Re-
2CaNH4P3O9+CaSO4→3Ca(PO3)2+SO3↑+
+2NH3↑+H2O↑
The reactivity of apatite is strongly influenced
by its structural properties and the composition of the
(3)
*
Author for correspondence: kaiat@staff.ttu.ee
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

TÕNSUAADU et al.
Table 1 Chemical composition and specific surface area of apatites
Apatite
P2O5/%
CaO/%
MgO/%
CO2/%
F/%
Fe2O3/%
SiO2/%
SSA/m g–1
Fluorapatite
Kola
56.03
39.27
38.00
30.57
46.40
52.32
53.45
45.27
0
0
3.00
2.96
2.70
2.27
0
0
1.00
0.57
1.49
4.94
0.12
0.31
1.56
0.06
1.80
5.54
0.60
2.21
1.58
1.53
0.14
11.35
Sokli
Kabala
flex Analytical Co.) with 8.8 m path length was main-
tained at temperature 150ºC. Spectra were recorded in
the 600–4000 cm–1 region with a resolution of 4 cm–1
as 4 scan averages. The sample mass was about 20 mg.
The gases evolved were identified by using char-
acteristic infrared absorption wavelengths: for NH3 at
930 and 963 cm–1, SO2 at 1345 and 1378 cm–1, H2O
1520, 1700 and 3855 cm–1, CO2 at 2359 and 669 cm–1
and N2O at 2358 and 2242 cm–1 [9, 10]. As the spectra
of the components of the gas mixture partially overlap,
the less overlapped regions were chosen for the analy-
sis of the evolvement of the components (Fig. 1). The
profiles of evolvement of gases were obtained as tem-
perature derivatives of the peak area integrated above
the baseline from 900 to 984 cm–1 for NH3, from 1309
to 1400 cm–1 for SO2, from 2260 to 2406 cm–1 for CO2
and from 2141 to 2286 cm–1 for N2O. The shape of the
SO2 evolvement profile is slightly affected by releasing
water. The high noise level in the water vapor spec-
trum and its overlapping with the NH3 and SO2 spectra
makes the monitoring of the release of water useless.
HF was not identified. Condensation of NH4F in a
cooler part of the furnace (<150°C) of the thermal ana-
lyzer could also take place.
[4, 5, 11]. Remarkable thermal changes do not take
place in studied apatites at temperatures up to 500°C.
TG-DTG and DTA curves (Fig. 2) of the apa-
tite-AS mixtures as well as the curves of gas evolution
reveal the differences of the apatites nature. The mass
loss starts at calcination at 220–240°C and occurs at
the highest speed mainly at 310–320 and 415–440°C
in the mixtures studied. In the mixtures with apatites
with a higher content of impurities, particularly car-
bonates (Kabala and Sokli) (Table 1) an additional
relatively intensive mass loss area is observed at
350–400°C. Therefore, the total mass loss of these
mixtures is up to 5–10% higher.
The evolved NH3 profile shows the release of
NH3 in two steps (Fig. 3). The largest part of NH3
evolves in the temperature interval of 230–320°C that
corresponds to the first mass loss step – to AS decom-
position [4]. The second maximum occurs in the in-
terval of 320–400°C for natural apatites and at about
40°C higher temperature for synthetic FAp. There-
fore, the impurities lower the temperature of NH3 re-
Results and discussion
(NH4)2SO4 thermal decomposition occurs mainly in
two steps. The first step with maximum at 330°C was
assigned to the loss of NH3 and the second one at
420°C to the decomposition of the (NH4)HSO4 or
H2SO4 formed, with release of NH3, SO2 and H2O
Fig. 2 Thermoanalytical curves of apatite – AS (mole ratio 1:4)
mixtures. Heating rate 10 K min–1, air flow 50 mL min–1;
1 – FAp+AS; 2 – Kola+AS; 3 – Sokli+AS; 4 – Kabala+AS
Fig. 1 Standard FTIR spectra of NH3, H2O, SO2, N2O and
CO2 [8]
656
J. Therm. Anal. Cal., 80, 2005

EVOLVED GASES IN APATITE–AMMONIUM SULFATE
gas phase. For the same reason, no evolution of SO2
was observed at temperatures below 400°C when the
amount of NH3 released exceeded the amount of SO2
needed for the formation of AS in the gas phase.
Another explanation for an intensive release of
SO2 above 400°C is that a reaction between calcium
polyphosphate and calcium sulfate could also occur.
The detection limit of the gas cell allowed moni-
tor the release of CO2 only in case of Kabala apatite
where the content of CO2 was 5.54%. In case of Sokli
apatite the noise level was identical to the signal mag-
nitude. The release of CO2 was established in the tem-
perature interval of 250–380°C, with two maximums
(Fig. 5). The first maximum at 300°C corresponds to
the first mass loss step and could be the result of cal-
cite decomposition. The second maximum at 340°C
could be related to carbonate-apatite decomposition.
In addition to the expected components of the gas
phase, nitrogen oxides were also detected in the case of
mixtures with FAp and Kola apatite (Fig. 6). The high-
est content of N2O was observed above 390°C when the
emission of SO2 was the highest. The emergence of ni-
trous oxides could be a result of the secondary reaction
in the gas phase. The oxidation of NH3 could occur
when the SO3 released decomposes into SO2 and O.
Fig. 3 Profile of evolved NH3 from apatite+AS mixtures (mole
ratio 1:4). Heating rate 10 K min–1, air flow 50 mL min–1;
1 – FAp+AS; 2 – Kola+AS; 3 – Sokli+AS; 4 – Kabala+AS
Fig. 4 Profile of evolved SO2 from apatite+AS mixtures (mole
ratio 1:4). Heating rate 10 K min–1, air flow 50 mL min–1;
1 – FAp+AS; 2 – Kola+AS; 3 – Sokli+AS;
4 – Kabala+AS
lease from the products of the reaction between apa-
tite and AS.
More remarkable differences were found in SO2
profiles for different apatites (Fig. 4). The release of
SO2 takes place with maxima at 430–435 and 480°C in
the mixture with pure fluorapatites. For carbonate con-
taining samples (with Sokli and Kabala apatites) the
temperature of SO2 release is at about 40°C higher.
The shift of the temperature of SO2 evolution in the
mixtures of natural apatites is also attributed to the in-
fluence of the impurities in natural apatite concen-
trates. Due to the reactions with impurities the other
phosphates could form [1, 12]. In the mixture with Ka-
bala calcined up to 330°C, in addition to CaNH4P3O9,
the presence of MgNH4P3O9 and FeNH4P2O7 were es-
tablished by XRD analysis.
Fig. 5 Profile of CO2 evolvement from apatite+AS mixtures
(mole ratio 1:4). Heating rate 10 K min–1, air flow
50 mL min–1; 3 – Sokli+AS; 4 – Kabala+AS
The temperature of SO2 release coincides with
the third mass loss step that should correspond to the
reaction (3) but no release of NH3 was detected. On
the contrary, the content of NH3 decreases in the gas
phase, beginning at 400°C when the content of SO2
increases. This could be explained with the condensa-
tion, in a cooler part of furnace before IR analyzer, of
AS formed as a result of the secondary reaction in the
Fig. 6 Profile of N2O evolvement from apatite+AS mixtures
(mole ratio 1:4). Heating rate 10 K min–1, air flow
50 mL min–1; 1 – FAp+AS; 2 – Kola+AS; 3 – Sokli+AS;
4 – Kabala+AS
J. Therm. Anal. Cal., 80, 2005
657

TÕNSUAADU et al.
Conclusions
References
1 A. V. Ekshtelis and A. K. Il’yasova, Izvestiya Akademii
Nauk Kazakhskoi SSR, Seriya Khimicheskaya,
24 (1974) 5 (in Russian).
The composition of natural apatite has little impact on
the release of NH3. Minor amounts of nitrous oxides
were detected simultaneously with the evolvement of
NH3 due to the oxidation reaction of NH3. The release
of SO2 at temperatures above 400°C is more intensive
and occurs at lower temperatures in the case of
fluorapatite than that of carbonate containing apatites.
The evolvement of CO2 starts at 250°C and the maxi-
mum occurs at 350–360ºC.
2 M. Marraha, M. Heughebaert, J.-C. Heughebaert and
G. Bonel, Biomat. Biomechanics, 5 (1983) 445.
3 A. J. E. Welch, Proceedings - International Congress on
Phosphorus Compounds, 2nd (Boston, USA, 1980),
pp. 363–369.
4 K. Tõnsuaadu, M. Borissova, V. Bender and J. Pelt,
Phosphorus, Sufur, Silicon Relat. Elem., 179 (2004) 2395.
5 Y. Pelovski, V. Petkova and I. Dombalov,
J. Therm. Anal. Cal., 72 (2003) 967.
Despite the imperfection of the analytical sys-
tem, where partial condensation of the products of the
reactions, like ammonium sulfate, occurs, monitoring
of the evolved gases by FTIR makes it possible to de-
fine the thermal reactions in natural apatite – ammo-
nium sulfate mixture more precisely than it is possible
in the case of thermal analysis alone.
6 K. H. Stern and E. L. Weise, Nat. Stand. Ref. Data Ser.,
Nat. Bur. Stand. (U.S.) NSRDS-NBS 7 (1966).
7 J. C. Elliot, Structure and Chemistry of the Apatites and
Other Calcium Orthophosphates, Elsevier, Amsterdam 1994,
p. 389.
8 M. Veiderma, Proc. Estonian Acad. Sci. Chem.,
49 (2000) 5.
9 NIST/EPA Gas-Phase Infrared Database.
www.http://www.nist.gov/srd/nist35.htm.
10 Gases and Vapours Database (code GS), BioRad (Sadtler).
11 M. Statheropoulos and S. A. Kyriakou, Anal. Chim. Acta,
409 (2000) 206.
Acknowledgements
The help provided by M. Einard in carrying out chemical anal-
yses and the support of the Estonian Science Foundation
(Grants No. 5648 and 4697) are gratefully acknowledged.
12 M. Arasheva and I. Dombalov, J. Thermal Anal.,
43 (1995) 359.
658
J. Therm. Anal. Cal., 80, 2005