Thermal analysis of mechanoactivated mixtures of Tunisia phosphorite and ammonium sulfate

Article abstract of DOI:10.1023/A:1025090804049

The thermal behavior of Tunisia phosphorite and ammonium sulfate mixtures are examined by non-isothermal thermogravimetry in air atmosphere. It has been shown that the thermal stability of mixtures have different behavior after mechano-chemical treatment

Full text of DOI:10.1023/A:1025090804049

Journal of Thermal Analysis and Calorimetry, Vol. 72 (2003) 967–980
THERMAL ANALYSIS OF MECHANOACTIVATED
MIXTURES OF TUNISIA PHOSPHORITE AND
AMMONIUM SULFATE
Y. Pelovski 1*, V. Petkova 2 and I. Dombalov 1
1University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Blvd, 1756 Sofia, Bulgaria
2Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences,
Acad. G. Bonchev St., Bl. 107, 1113 Sofia, Bulgaria
Abstract
The thermal behavior of Tunisia phosphorite and ammonium sulfate mixtures are examined by non-
isothermal thermogravimetry in air atmosphere. It has been shown that the thermal stability of mix-
tures have different behavior after mechano-chemical treatment due to phase changes in the mix-
tures. New solid phases are confirmed by the stages and rate of mass changes and also by the heat ef-
fects observed. New phases are also confirmed by using X-ray diffraction method. Based on the data
obtained by the mechanism of chemical transformations is proposed.
The kinetics of mechano-activated mixtures decomposition is significantly influenced by the
time of treatment and proceeds in few stages. Stage I is attributed to the effect of partially ammonia
releasing, when the next stages are associated with the decomposition of apatite structures. Because
of the interfacial interactions during mechano-chemical treatment insoluble phosphorous forms
from apatite structures are transformed to the soluble forms suitable for plants nutrition. The results
have shown that the soluble form can be controlled by the initial components ratio and conditions of
treatment on the way to obtain slow releasing nutrition products.
Keywords: phosphate of ammonium and calcium, solid state reaction, thermal decomposition,
tribo-activation
Introduction
Thermo-tribo-chemical method for treatment of raw materials became a new way for
processing raw materials with low content of useful components and unsuitable for treat-
ment using traditional technologies. Structure defects, created during activation of solids
increases the reactivity of the systems and makes the efficiency higher [1, 2]. Energy ac-
cumulated during activation process often is released when temperature rises and the rate
of chemical reactions takes place [3–5]. Sometimes the energy released is even higher
than needed for certain processes and the optimization of activation conditions is re-
quired. Topochemical character of the activated systems decomposition makes the sys-
tems unique and concrete studies may describe their behavior.
*
Author for correspondence: E-mail: pelovsky@uctm.edu
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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PELOVSKI et al.: MECHANOACTIVATED MIXTURES
Natural phosphate-ammonium sulfate mixtures have been studied as a promis-
ing system for fertilizer production [6–11]. Such fertilizers may supply plants with
few nutrients – nitrogen, phosphorous, sulfur and calcium. By different chemical and
physical methods it is confirmed that different forms of P2O5, suitable for assimila-
tion by the plants could be obtained [11]. Thermomechanical activation of the mix-
tures selected may accelerate the reactions decomposing the insoluble forms of phos-
phorous from natural phosphates to soluble one and it is the aim of the present work.
Thermochemical reactions take place in the mixtures of Tunisia phosphorite and
ammonium sulfate with mass ratio 1:1, activated from 10 min to 50 h in a planetary
mill with Cr–Ni milling balls are studied by use of thermal analysis and X-ray diffrac-
tion methods.
Methods and raw materials
Tunisia phosphorite contain 29.58 mass% P2O5, 48.40 mass% CaO, 6.48 mass% CO2,
3.58 mass% SO3, 2.74 mass% F, 1.88 mass% SiO2, 1.38 mass% Na2O,
1.07 mass% R2O3, 0.07 mass% K2O, 0.056 mass% Cl and traces of Cd, Pb, Cr, Ni. Am-
monium sulfate is p.a. grade from Alerus Ltd. containing 99.5 mass% (NH4)2SO4. As
impurities 0.001% NO–3 , 0.0003% Cl–, 0.0005% PO34– , 0.0002% Fe, 0.00002% As and
0.0002% the supplier specifies as other heavy metals.
Pulverisette-5 of Fritsch Ltd. planetary mill with Cr–Ni milling balls (total vol-
ume 120 mL; total mass 0.562 kg; number 140) is used for tribo-activation of 0.02 kg
samples. For thermal analysis Stanton Redcroft thermal analyzer in temperature
range 288–1373 K is used. Heating rate is 10 K min–1 and mass of samples is
15.20±0.4 mg. DRON X-ray analyzer with CuKα radiation is also used.
Results and discussions
Figures 1–6 and Tables 1–4 represent TG-DTA and X-ray diffractograms of the ini-
tial reagents and mixture and activated mixtures after different time of treatment.
X-ray data for (Fig. 1) identified phase structures in the raw materials and mixture are
Ca5F(PO4)3, β-Ca3(PO4)2, SiO2 and (NH4)2SO4.
Thermal decomposition of ammonium sulfate (AS)
Thermal decomposition of ammonium sulfate is a multistage process (Table 1 and
Fig. 2), where the first stage is in the temperature range 543–673 K and from the sam-
ple NH3 is released as a result of transformation to NH4HSO4. In the temperature
range 723–823 K are the next step reactions where NH4HSO4 is partially transformed
to (NH4)2S2O8. Last stage is in the temperature range 913–1033 K where full decom-
position takes place and only the volatile products are obtained.
J. Therm. Anal. Cal., 72, 2003

PELOVSKI et al.: MECHANOACTIVATED MIXTURES
969
Table 1 Mass losses of ammonium sulfate at different stages of decomposition
Mass losses/%
Stage’s mass losses/%
Temperature/K
Stage
Start
0.00
End
–
Start
Endo
386.7
810.0
1026.4
I
16.50
87.55
91.81
16.50
543.5
659.7
921.2
II
III
16.50
89.45
71.10
2.36
Total mass losses – 93.54%
Fig. 1 X-ray diffractograms of the raw materials and mixtures: a – X-ray diffractogram
of the raw TPh; b – X-ray diffractogram of the raw AS; c – X-ray diffractogram
of the initial mixture mass ratio 1:1
Analysis of the results from TG-DTA curves and X-ray diffraction data show
that decomposition process of ammonium sulfate could be described by the following
reactions:
Stage I – 543–663 K, 16.5 mass loss/%
(NH4)2SO4=NH4HSO4+NH3
(1)
J. Therm. Anal. Cal., 72, 2003

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PELOVSKI et al.: MECHANOACTIVATED MIXTURES
2(NH4)2SO4=(NH4)3H(SO4)2+2NH3
(2)
Stage II – 653–813 K, 71.1 mass loss/%
(NH4)3H(SO4)2=2NH4HSO4+NH3
(3)
(4)
(5)
(NH4)3H(SO4)2=(NH4)2S2O8+NH3+H2O
NH4HSO4=NH3+SO3+H2O
Stage III – 913–1033 K, 2.4 mass loss/%
(NH4)2S2O8=2NH3+2SO3+H2O+½O2
SO3=SO2+½O2
(6)
(7)
Thermal behavior of Tunisia phosphorite (TPh)
During thermal treatment of the Tunisia phosphorite up to 1373 K total mass losses
are about 12% (Fig. 2, Table 2). Up to 423 K free water and volatile organic impuri-
ties are released; in the temperature range 573–923 K oxidation of heavy organic im-
purities takes place and up to 1223 K the release of carbon dioxide, because of car-
bonate’s impurities decomposition is completed.
Fig. 2 TG-DTA curves of raw materials and initial AS+TPh mixture
J. Therm. Anal. Cal., 72, 2003

PELOVSKI et al.: MECHANOACTIVATED MIXTURES
971
Table 2 Mass losses at different stages during thermal treatment of Tunisia phosphorite
Mass losses/%
Start
Stage mass losses/%
Temperature/K
Stage
End
1.68
5.74
11.28
–
Start
End
422.3
927.2
1192.7
I
0.00
2.37
5.74
1.68
3.37
5.54
293.0
578.6
927.2
II
III
Total mass losses – 11.95%
Untreated mixtures of AS and TPh
Registered mass losses and thermal effects (Fig. 2, Table 3) confirmed complicated
mechanism of transformation, related from one side to the individual behavior of the
main components and to the reactions between from the other side. As it was confirmed
in the other papers [6–11] some of the released intermediates, like NH+4 , SO3, take part
in a secondary reactions and new solid phases are confirmed in the final product, like
orto- or polyphosphates of ammonia and calcium. Mass losses registered correspond
well with the step-by-step formation of (NH4)2SO4 2CaSO4, CaHPO4, NH4CaP3O9,
α-Ca2P2O7, Ca3(PO4)2, CaSO4. Formation of CaHPO4 may come on the base of the re-
action between Ca5F(PO4)3 and the intermediate acidic phase NH4HSO4, formed during
first stage of AS decomposition. NH4CaP3O9, α-Ca2P2O7, Ca3(PO4)2 and CaSO4 appear
at higher temperatures as a result of dehydration and polymerization of the phosphate
complex and CaSO4. Welsh [6] stated that NH4CaP3O9 decomposes to (NH4)2P2O6 and
CaP2O6, but the mass losses registered are closer to the transformation of NH4CaP3O9
to α-Ca2P2O7. On the basis of our studies we suppose that in the temperature range
293–1373 K chemical mechanisms of transformations in the AS–TPh mixture can be
described by the following reactions:
523–643 K, 11.75 mass loss/%
(NH4)2SO4=NH4HSO4+NH3
(1)
(2)
2(NH4)2SO4=(NH4)3H(SO4)2+2NH3
633–673 K, 5.43 mass loss/%
2Ca5F(PO4)3+(NH4)2SO4=3Ca3(PO4)2+CaSO4+2HF+2NH3
(8)
(9)
Ca5F(PO4)3+3NH4HSO4=(NH4)2SO4 2CaSO4+3CaHPO4+HF+NH3
673–708 K, 9.92 mass loss/%
(NH4)2SO4 2CaSO4=NH4HSO4+2CaSO4+NH3
3CaHPO4+2NH4HSO4=NH4CaP3O9+2CaSO4+NH3+3H2O
2CaHPO4=Ca2P2O7+H2O
(10)
(11)
(12)
J. Therm. Anal. Cal., 72, 2003

Table 3 Mass losses and temperature ranges for initial components and activated AS+TPh mixture in the planetary mill; time of activation –0.12–50 h
Stage I
Temperature
range/K
Stage II
Temperature
range/K
Stage III
Temperature
range/K
Stage IV
Temperature
range/K
Time of
activation/h
Mass
loss/%
Mass
loss/%
Mass
loss/%
Mass loss/%
531.5–634.7
infl. p.–607.6
634.7–670.7
infl. p.–649.7
0.00
0.12
–
–
–
–
11.75
5.34
–
493.4–562.5
infl. p.–544.5
562.5–644.7
infl. p.–588.8
303.1–363.2
infl. p.*–332.1
423.3–493.4
infl. p.–464.4
3.61
10.09
1.51
0.88
1.05
1.12
1.45
–
–
495.4–564.5
infl. p.–552.5
564.5–623.6
infl. p.–578.6
301.1–363.2
infl. p.–328.1
434.3–495.4
infl. p.–464.4
7.56
6.24
0.50
1.00
5.00
4.01
2.31
0.67
–
495.4–564.5
infl. p.–551.5
564.5–594.7
infl. p.–571.5
303.1–355.2
infl. p.–323.1
440.3–495.4
infl. p.–465.4
7.75
4.32
594.6–633.7
infl. p.–611.6
2.31
1.96
494.4–566.5
infl. p.–552.5
566.5–597.6
infl. p.–575.6
293.0–343.1
infl. p.–308.1
434.3–493.4
infl. p.–467.4
6.97
3.64
597.6–631.7
infl. p.–614.6

Table 3 Continued
Stage I
Temperature
range/K
Stage II
Temperature
range/K
Stage III
Temperature
range/K
Stage IV
Temperature
range/K
Time of
activation/h
Mass
loss/%
Mass
loss/%
Mass
loss/%
Mass
loss/%
494.4–552.5
infl. p.–543.5
552.5–586.6
infl. p.–567.5
301.1–354.2
infl. p.–316.1
421.3–494.4
infl. p.–195.4
5.14
3.51
586.6–617.6
infl. p.–602.6
10.00
0.87
2.40
3.66
1.41
4.78
2.31
2.32
2.04
3.48
2.03
1.67
1.37
3.05
1.81
1.31
502.4–555.5
infl. p.–543.5
555.5–595.6
infl. p.–568.5
308.1–391.2
infl. p.–334.1
426.3–502.4
infl. p.–461.3
4.11
3.94
595.6–627.6
infl. p.–614.6
15.00
30.00
40.00
50.00
491.4–560.5
infl. p.–539.45
560.5–593.6
infl. p.–567.5
313.1–403.2
infl. p.–65.1
446.3–491.4
infl. p.–470.4
4.48
2.41
593.6–649.7
infl. p.–633.7
491.4–541.5
infl. p.–535.5
541.5–588.6
infl. p.–555.5
297.0–372.2
infl. p.–324.1
402.2–491.4
infl. p.–456.3
2.70
3.13
588.6–628.7
infl. p.–615.6
500.4–558.5
infl. p.–554.5
558.5–610.6
infl. p.–571.5
302.1–401.2
infl. p.–345.1
446.3–500.4
infl. p.–467.4
3.13
3.14
610.6–644.7
infl. p.–631.7

Table 3 Continued
Stage V
Temperature
range/K
Stage VI
Temperature
range/K
Stage VII
Temperature
range/K
Time of
activation/h
Total mass loss/%
Mass
loss/%
Mass
loss/%
Mass
loss/%
670.7–706.8
infl. p.–696.8
706.8–788.9
infl. p.–717.8
1037.4–1153.6
infl. p.–1096.5
1153.6–1273.8
infl. p.–1200.7
9.92
8.48
883.1–1037.4
infl. p.–966.3
2.08
2.96
0.00
0.12
0.50
5.39
2.91
49.51
43.24
46.50
644.7–763.9
infl. p.–690.8
931.2–1011.3
infl. p.–974.3
1125.6–1240.8
infl. p.–1192.7
12.72
12.02
3.77
825.0–928.2
infl. p.–904.2
928.2–1001.3
infl. p.–954.2
1092.5–1158.6
infl. p.–1127.6
1158.6–1235.8
infl. p.–1189.7
623.6–755.9
infl. p.–686.8
2.91
2.74
1.83
2.91
1011.4–1148.6
infl. p.–1128.6
1148.6–1234.8
infl. p.–1196.7
633.7–730.8
infl. p.–677.7
876.1–991.3
infl. p.–946.2
2.35
3.43
1.00
5.00
9.76
4.32
44.10
39.66
1069.5–1146.6
infl. p.–1121.5
1146.6–1196.7
infl. p.–1176.6
1196.7–1266.8
infl. p.–1219.7
824.0–905.2
infl. p.–860.1
905.2–988.3
infl. p.–926.2,
943.2
631.7–677.7
infl. p.–668.7
677.7–746.9
infl. p.–693.8
2.03
2.19
3.09
3.84
4.74
2.07
2.98

Table 3 Continued
Stage V
Temperature
range/K
Stage VI
Temperature
range/K
Stage VII
Temperature
range/K
Time of
activation/h
Total mass loss/%
Mass
loss/%
Mass
loss/%
Mass
loss/%
776.9–906.2
infl. p.–843.0
906.2–1032.4
infl. p.–935.2,
947.2
617.6–673.7
infl. p.–666.7
673.7–745.9
infl. p.–685.8
1032.4–1142.6
infl. p.–1119.5
1142.6–1205.7
infl. p.–1162.6
4.53
4.71
3.00
3.25
3.55
2.62
10.00
15.00
39.48
1045.4–1152.6
infl. p.–1129.6
1151.6–1180.7
infl. p.–1170.6
1180.7–1233.8
infl. p.–1190.69
2.98
1.68
1.75
627.6–759.9
infl. p.–692.8
893.1–978.3
infl. p.–937.2
9.11
2.53
2.04
38.62
1081.5–1162.6
infl. p.–1139.6
1162.6–1195.7
infl. p.–1167.6
649.7–751.9
infl. p.–689.8
897.1–997.3
infl. p.–936.1
4.84
1.28
30.00
40.00
50.00
6.49
7.60
6.012
36.83
35.69
34.25
841.0–903.2
infl. p.–887.1
903.2–972.3
infl. p.–929.2
1057.4–1143.6
infl. p.–1124.6
1143.6–1208.7
infl. p.–1156.6
628.7–737.9
infl. p.–678.7
1.32
1.20
4.20
2.98
737.9–886.1
infl. p.–558.0
886.1–999.3
infl. p.–941.2
1065.4–1136.6
infl. p.–1116.5
1136.6–1233.8
infl. p.–1177.6
644.7–737.9
infl. p.–687.8
1.93
1.13
2.04
5.21
infl. p.* – inflexed point

976
PELOVSKI et al.: MECHANOACTIVATED MIXTURES
708–793 K, 8.49 mass loss/%
(NH4)3H(SO4)2=2NH4HSO4+NH3
(3)
(4)
(5)
(NH4)3H(SO4)2=(NH4)2S2O8+NH3+H2O
NH4HSO4=NH3+SO3+H2O
883–1038 K, 5.39 mass loss/%
Ca5F(PO4)3+NH4CaP3O9=3Ca2P2O7+HF+NH3
(NH4)2S2O8=2NH3+2SO3+H2O+½O2
SO3=SO2+½O2
(13)
(6)
(7)
1093–1203 K, 5.10 mass loss/%
Ca2P2O7+CaSO4=Ca3(PO4)2+SO2+½O2
(14)
and/or
Ca2P2O7+CaCO3=Ca3(PO4)2+CO2
CaCO3=CaO+CO2
(15)
(16)
Triboactivated AS–TPh mixture
Data in Fig. 3 and Table 4 demonstrate the differences registered on the base of
changes in the studied system during process of activation. The system is more sensi-
tive during the first 60 min of treatment. The shape of TG-DTA curves and new solid
structures, found by X-ray diffraction method, confirms that circumstance.
Diffractograms confirm that during the first 60 min of activation double salt
(NH4)2SO4 2CaSO4 and Ca3(PO4)2 xH2O are formed. So it is obvious that the local en-
ergy in the mixture makes possible transformations between main components in the sys-
tem – Ca5F(PO4)3 and (NH4)2SO4. The same time release of NH3 and SO3 is confirmed.
When the time of activation increases to 5–15 h (NH4)2H2P2O7, NH4CaP3O9,
CaH2P2O7 and α-Ca2P2O7 are registered in the mixture. Such solid phases are identi-
fied in the untreated mixture at very high temperatures. So it is obvious that activation
energy is used for increasing the reactivity of the components in the studied system.
When the time of activation is between 30 to 50 h intensities of CaH2P2O7, α-Ca2P2O7
and NH4CaP3O9 are also confirmed, but the same time as new phases CaSO4 2H2O
and Ca3(PO4)2 xH2O are formed. (NH4)2H2P2O7 could not be detected by the X-ray
study, but it is likely to be formed in the amorphous phase. At the same time it could
be transformed to CaH2P2O7 and α-Ca2P2O7, according to reactions 17 and 18.
3(NH4)2H2P2O7+2Ca5F(PO4)3=4CaH2P2O7+2HF+6NH3+2Ca3(PO4)2+H2O (17)
(NH4)2H2P2O7+2Ca5F(PO4)3=2Ca2P2O7+2Ca3(PO4)2+2HF+2NH3+H2O (18)
J. Therm. Anal. Cal., 72, 2003

PELOVSKI et al.: MECHANOACTIVATED MIXTURES
977
Fig. 3 X-ray diffraction patterns of the activated mixture
Formation of gypsum structures is another confirmation for the reactions between
apatite structure and ammonium sulfate intermediates. TG curves show that by increas-
ing the time of activation from 10 min to 50 h the mass losses decrease from 49.5 to
34.3% (Figs 4–6). It is another confirmation for the reactions taken place during activa-
tion process. At the same time the thermal effects characterizing reactions proceed are at
lower temperatures by some 35 to 100 K. The differences are higher for high temperature
transformations. For activated samples the new endo-effects at 323–348 and 493–563 K
indicate the dehydration of formed CaSO4 2H2O and the formation of the double salt
(NH4)2SO4 2CaSO4 and Ca3(PO4)2 xH2O. Thermal effects are in a good agreement with
mass changes registered.
J. Therm. Anal. Cal., 72, 2003

978
PELOVSKI et al.: MECHANOACTIVATED MIXTURES
Fig. 4 TG-DTA curves of activated initial AS+TPh mixture; activation time –10 to 60 min
Fig. 5 TG-DTA curves of activated initial AS+TPh mixture; activation time –5 to 15 h
In the temperature range 663–723 K for the initial mixture 2 endothermic effects
are typical, when for activated samples in most of the cases there is overlapping. It
could be explained by the acceleration of the rate of decomposition of NH4HSO4 and
(NH4)3H(SO4)2 as intermediates and formation of (NH4)2H2P2O7, CaH2P2O7,
α-Ca2P2O7 and NH4CaP3O9 at lower temperatures. Reactions 17 and 18 partially rep-
resent this mechanism.
J. Therm. Anal. Cal., 72, 2003

PELOVSKI et al.: MECHANOACTIVATED MIXTURES
979
Table 4 Influence of the time of activation on the solid phases, identified by X-ray
Time of
Identified solid phases
mechanoactivation
0 min
Ca5F(PO4)3, β-Ca3(PO4)2, (NH4)2SO4
Ca5F(PO4)3, (NH4)2SO4, β-Ca3(PO4)2, Ca3(PO4)2 xH2O,
(NH4)2SO4 2CaSO4
10 min
Ca5F(PO4)3, (NH4)2SO4, β-Ca3(PO4)2, Ca3(PO4)2 xH2O,
(NH4)2SO4 2CaSO4
30 min
60 min
5 h
Ca5F(PO4)3, (NH4)2SO4, β-Ca3(PO4)2, Ca3(PO4)2 xH2O,
(NH4)2SO4 2CaSO4, α-Ca2P2O7
Ca5F(PO4)3, (NH4)2SO4, β-Ca3(PO4)2, Ca3(PO4)2 xH2O,
(NH4)2SO4 2CaSO4, (NH4)2H2P2O7, CaH2P2O7, α-Ca2P2O7, NH4CaP3O9
Ca5F(PO4)3, (NH4)2SO4, β-Ca3(PO4)2, Ca3(PO4)2 xH2O, CaH2P2O7,
α-Ca2P2O7, NH4CaP3O9, CaSO4 2H2O
10 h
Ca5F(PO4)3, (NH4)2SO4, β-Ca3(PO4)2, Ca3(PO4)2 xH2O, (NH4)2H2P2O7,
α-Ca2P2O7, NH4CaP3O9
15 h
Ca5F(PO4)3, (NH4)2SO4, Ca3(PO4)2 xH2O, CaH2P2O7, α-Ca2P2O7,
NH4CaP3O9, CaSO4 2H2O
30 h
Ca5F(PO4)3, (NH4)2SO4, Ca3(PO4)2 xH2O, CaH2P2O7, NH4CaP3O9,
CaSO4 2H2O
40 h
Ca5F(PO4)3, (NH4)2SO4, Ca3(PO4)2 xH2O, CaH2P2O7, α-Ca2P2O7,
CaSO4 2H2O
50 h
In the temperature range 843–943 K mass losses are only about 3%, but two
thermal effects, one endothermic and one exothermic, are registered. The endother-
Fig. 6 TG-DTA curves of activated initial AS+TPh mixture; activation time –30 to 50 h
J. Therm. Anal. Cal., 72, 2003

980
PELOVSKI et al.: MECHANOACTIVATED MIXTURES
mic effect we understand as a transformation of CaH2P2O7 to α-Ca2P2O7 (reaction 19)
and the releasing of some volatile impurities from TPh. Reaction 13 could also par-
tially explain such an effect. The exothermic effect is observed only for activated
samples and it could be just the relocation process of the crystal deformations.
CaH2P2O7+Ca3(PO4)2=2Ca2P2O7+H2O
(19)
The last stage changes are in the temperature range 973–1173 K and for untreated
mixture it is due to the decomposition of the free CaO3 from the natural phosphate. For
activated samples it is the result of interaction between CaSO4 and α-Ca2P2O7.
Conclusions
New data for inactivated and activated mixture/ (activation time is from 0.12 to 50 h)
of ammonium sulfate and Tunisia phosphorite is obtained. Thermal stability and be-
havior up to 1373 K confirm that during activation process different reactions are tak-
ing place and formation of new solid phases obtained, like (NH4)2H2P2O7, CaH2P2O7,
α-Ca2P2O7, NH4CaP3O9 and Ca3(PO4)2 is a fact. Such solid phases may appear after
thermal treatment of the initial mixture at higher temperature. During activation pro-
cess formation of the new solid phases demonstrate that the insoluble apatite phos-
phate structure could be transformed to other phosphorous compounds water or citric
acid soluble, so the activation of such mixtures can be used in the practice for fertiliz-
ers production. Needed soluble forms could be obtained only after 10 min of treat-
ment at lower temperatures and the impurities do not affect the process efficiency.
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