Investigation of thermal decomposition of MgCl2 hexammoniate and MgCl2 biglycollate biammoniate by DTA-TG, XRD and chemical analysis

Article abstract of DOI:10.1016/j.tca.2003.09.018

The decomposition of MgCl₂·6NH₃ and MgCl ₂·2C₂H₆O₂·2NH ₃ in static air was investigated by DTA-TG, XRD and chemical analysis from room temperature to 700°C. The results indicated that MgCl₂·6NH₃ took three steps to decompose to anhydrous MgCl₂ below 450°C and at higher temperatures resulted in the oxidation of anhydrous MgCl₂. This theoretically confirms the validity of the operation in which Ar or N₂ is used in manufacturing anhydrous MgCl₂ at 650°C. MgCl₂·2C ₂H₆O₂·2NH₃ experienced a complicated process of decomposition, Mg(OH)Cl and carbon were obtained at 300°C and MgO at 530°C, respectively, which may decrease the quality of anhydrous MgCl₂.

Full text of DOI:10.1016/j.tca.2003.09.018

Thermochimica Acta 412 (2004) 149–153
Investigation of thermal decomposition of MgCl hexammoniate
2
and MgCl biglycollate biammoniate by DTA–TG, XRD
2
and chemical analysis
a,b,∗
a
a
a
a
Long Guangming
, Ma Peihua , Wu Zhiming , Li Mingzhen , Chu Minxong
a
Institute of Salt Lakes, Chinese Academy of Science, Xining, Qinghai 810008, PR China
b
Graduate school of Chinese Academy of Science, Beijing 100039, PR China
Received 8 April 2003; received in revised form 23 September 2003; accepted 23 September 2003
Abstract
The decomposition of MgCl
from room temperature to 700 C. The results indicated that MgCl
and at higher temperatures resulted in the oxidation of anhydrous MgCl
2
◦
·6NH
3
and MgCl
2
·2C
2
H
6
O
2
·2NH
3
in static air was investigated by DTA–TG, XRD and chemical analysis
◦
2
·6NH
3
took three steps to decompose to anhydrous MgCl
2
below 450 C
2
. This theoretically confirms the validity of the operation in which
·2NH experienced a complicated process of decomposition,
◦
Ar or N
2
is used in manufacturing anhydrous MgCl
2
at 650 C. MgCl
2
·2C
2
H
6
O
2
3
◦
◦
Mg(OH)Cl and carbon were obtained at 300 C and MgO at 530 C, respectively, which may decrease the quality of anhydrous MgCl
2
.
©
2003 Elsevier B.V. All rights reserved.
Keywords: MgCl2·6NH3; MgCl2·2C2H6O2·2NH3; Thermal decomposition; Anhydrous MgCl2
1
. Introduction
magnesium chloride hexammoniate must be calcined at
◦
450–650 C [3,4]. However, up to now, there is no detailed
MgCl2 hexammoniate is useful in producing anhy-
information about decomposition of the complexes men-
tioned above, and confusion of calcination conditions (flow-
ing Ar gas was needed [2] or was not needed [3], at 450,
drous MgCl2, which is the commercially efficient origin
in preparation of metal magnesium. It was also used to
control the temperature of the electrolyte as well as the
content of MgCl2 in the process for electrolytically pro-
ducing magnesium metal [1]. The ammoniate was formed
by treating the anhydrous solution of MgCl2 hydrate (e.g.
MgCl2·6H2O) in ethylene glycol (with or without ammo-
nium chloride) with an excess amount of ammonia [2,3]. It
could also be produced by precipitating MgCl2 in a solution
of MgCl2–NH3–NH4Cl–CH3OH–H2O with ammonium in
the solution saturated by ammonium [4]. In the production
of magnesium chloride hexammoniate in ethylene glycol, it
was pointed out that there were three kinds of complexes,
MgCl2 hexammoniate, MgCl2 biglycollate biammoniate
and magnesium chloride triglycollate, formed in different
reaction conditions, and the XRD pattern of the three com-
pounds were provided [3]. To obtain anhydrous MgCl2,
◦
550 or 650 C) appeared in different patents. In addition, it
is not clear how the other two complexes in MgCl2·6NH3
affect the quality of anhydrous MgCl2 produced thereby.
So it is commercially and academically significant to study
the process of the decomposition of the ammoniates.
In this work, by DTA and TG analysis coupled with XRD
and chemical analysis, we investigated the thermal decom-
position of MgCl2 hexammoniate and MgCl2 biglycollate
biammoniate prepared according to the USA patent [3], and
the possible pathways of the decomposition of the ammoni-
ates were discussed.
2. Experimental
Magnesium chloride hexahydrate (MgCl2·6H2O), ethy-
∗
lene glycol (C2H4(OH)2), methanol (CH3OH), ammonia
(
Corresponding author. Tel.: +86-971-6304066;
NH3) were obtained from commercial sources and all the
solid and liquids are AR regents.
fax: +86-971-6306002.
E-mail address: lgming888@sina.com (L. Guangming).
0
040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2003.09.018

1
50
L. Guangming et al. / Thermochimica Acta 412 (2004) 149–153
2
.1. Preparation of MgCl2 hexammoniate and MgCl2
Table 1
XRD spectrum of MgCl2 hexammoniate
biglycollate biammoniate
This work
Reference
d (Å)
MgCl2 hexammoniate is referred as Example 1 in Best
Modes for Carrying out Invention in previous patent [3].
The preparation of MgCl2 biglycollate biammoniate was ob-
tained by a similar procedure and the difference was that
the dehydrated ethylene glycol magnesium chloride solution
was pumped into the reaction vessel much faster than the
suggested for preparation of magnesium chloride hexammo-
niate, and the temperature of the vessel was maintained at
d (Å)
Intensity
79
Intensity
5.89
5.82
100
3
60
6
85
40
3
20
6
12
4
2
5
2
5
.05
3
3
.605
.071
49
4
100
40
3
11
7
12
5
3.58
3.06
2.93
2.54
2.27
2.07
1.96
1.798
1.719
2.943
2.547
2
2
1
1
.27
.079
.961
.802
◦
about 60 C.
2
.2. Chemical analysis
1.722
1
.694
1.609
.469
1.360
1
1
.611
.362
4
3
Magnesium and chlorine contents were determined by
1
ethylenediaminetetraacetic acid (EDTA) titration, the am-
monia content was determined by the formaldehyde method
and the methanol and glycol content by withdrawing mag-
nesium chloride and ammonia from the total mass.
3
terial are shown in Table 2 with data provided in the report
3] for comparison. It could be confirmed to be magnesium
chloride biglycollate biammoniate with a little of MgCl2
hexammoniate.
[
2
.3. Measurement procedure
DTA and TG were recorded on SP ZRY-1P thermal ana-
◦
−1
lyzer at 10 C min heating rate and sample mass of each
compound was about 5 mg in Al2O3 crucible. The temper-
3
.2. Thermal decomposition of magnesium chloride
◦
hexammoniate
ature range was from room temperature to 700 C and the
thermal analyzer was calibrated with CuSO4·5H2O.
X-ray diffraction (XRD) was carried out in D/MAX-IIIB
diffractometer. The XRD powder diffractograms were
The DTA and TG curves of magnesium chloride hexam-
monia are shown in Fig. 1, and the thermal decomposition
◦
recorded using the Fe K␣ with 2θ angle ranging from 5
◦
to 80 . X-ray measurements were interpreted by compar-
Table 2
ison with the database of JCPDS-International Centre for
Diffraction Data.
XRD spectrum of MgCl2 biglycollate biammoniate
This work
Reference
d (Å)
d (Å)
Intensity
Intensity
100
3
. Results and discussion
6
5
.867
.87
100
9
6.94
a
3
.1. Characterization of the complexes
MgCl2 hexammoniate formed in the procedure men-
5.44
4
8
5
8
10
5
65
30
10
4.87
4
3
3
3
.191
.943
.865
.567
3
4
8
4.23
3.97
3.88
3.57
3.45
3.40
3.26
tioned above was analyzed and found to contain 47.9 wt.%
of magnesium chloride, 51.7 wt.% of ammonia, <1 wt.% of
methanol and glycol. The mole ratio of ammonia to MgCl2
is 6.1:1.0 which is in good agreement with the ratio of
7
3.438
77
20
8
6
3
12
8
4
3
4
3
3
2
2
2
.39
.253
.935a
.841
.378
6
.0:1.0 expected for pure magnesium hexammoniate. XRD
data of this chemical are shown in Table 1 along with the
data provided in the patent [3]. XRD spectroscopy indicated
an absence of MgCl2 glycollate compounds.
The synthetic magnesium chloride biglycollate biammo-
niate was determined to contain 23.9 wt.% of magnesium
chloride, 9.4 wt.% of ammonia and 66.7 wt.% of ethylene
glycol, which includes both free ethylene glycol and glycol
ligands of magnesium chloride glycollate compound. The
ammonia and magnesium chloride contents indicated a mole
ratio of ammonia to MgCl2 of 2.2:1.0. XRD data of this ma-
2.84
2.73
2.46
2.30
2.15
5
20
12
10
8
2.466
2
2
2
2
1
.304
.15
.071a
.009
.88
6
3
5
3
2.01
1.88
1.73
1.58
10
8
10
8
1.73
1
.58
a
XRD signals of MgCl ·6NH .
2
3

L. Guangming et al. / Thermochimica Acta 412 (2004) 149–153
151
Fig. 1. DTA and TG curves of MgCl2·6NH3.
Table 3
Thermal decomposition data for magnesium chloride hexammoniate
◦
Step number
Temperature range ( C)
Thermal effects
Reactions
Residue (%)
TG
Calculated
1
2
3
4
50–180
Endo
Endo
Endo
Endo
MgCl · 6NH3 → MgCl · 4NH3 + 2NH3
82
65
49
26
82.8
65.5
48.3
20
2
2
180–290
290–360
434–565
MgCl · 4NH3 → MgCl · 2NH3 + 2NH3
2
2
MgCl · 2NH3 → MgCl + 2NH3
2
2
1
4
1
2
1
2
MgCl + O2 + H2O → MgO + HCl + Cl2
2
data are given in Table 3. The chemical analyses of products
of each step of the decomposition are shown in Table 4.
Four steps of the decomposition of the complex are ob-
served with corresponding mass losses in the same ranges of
temperature. No endothermal peak in the first step demon-
strates that the two NH3 have a weak bonding with Mg but
to MgCl2·NH3 and the later to anhydrous magnesium chlo-
ride. The fourth step should be ascribed to the oxidation
of anhydrous of MgCl2 to MgO. The oxidation of MgCl2
in O2 investigated by Epstein et al. [6] and Khomenko [7]
indicated that Cl2, HCl and MgO were obtained in the ox-
idation with the participation of H2O. The reaction can be
expressed as
◦
MgCl2·4NH3 is thermally stable below 180 C. Lastra [5] in
his study of equilibrium of magnesium chloride compounds
with ammonia atmosphere provided the evidence of the ex-
istence of the tetrammonia of magnesium chloride, which
was thought not to exist previously. TG analysis shows that
there is no clear boundary between the MgCl2 tetrammoni-
ate decomposing to biammoniate and the later to anhydrous
MgCl2. Because of the existence of monoammoniate of mag-
nesium chloride confirmed by previous investigation [5], the
third step should contain two processes, e.g. MgCl2·2NH3
4MgCl + O2 + 2H2O → 4MgO + 4HCl + 2Cl2
2
We calculated the thermodynamic parameters of the
◦
−1
◦
reaction as ꢀH
= 275.06 kJ mol
and ꢀS
=
298
298
380.3 J K 1 mol with the thermodynamic data of relevant
substances coming from handbook [8]. It is demonstrated
that the reaction is endothermal and it can only take place
−
−1
◦
◦
when the temperature is higher than 450 C because ꢀG
2
98
turns to negative since then. The calculation supports the
identification of the step.
Table 4
XRD measurements show that the products of the first
three steps are all amorphous, and MgO was checked after
the fourth step with a poor crystal.
Chemical analysis of residues of MgCl2·6NH3 and MgCl2·2NH3·
2
C2H6O2 in different steps
MgCl2·6NH3
Step 2Cl/Mg Residues
MgCl2·2NH3·2C2H6O2
In conclusion, thermal decomposition of MgCl2·6NH3
Step
Cl/Mg Residues
contains four steps. Anhydrous MgCl2 can be obtained at
◦
1
+ 2 1.026
MgCl2·2NH3
1
1.042
MgCl2·2NH3·
about 450 C, and this is consistent with the study reported
2
C2H6O2
[3]. For industrial manufacture, higher temperature may be
3
4
0.993
0.086
MgCl2
MgO + MgCl2
2 + 3 0.543
4 0.011
Mg(OH)Cl
MgO
needed, so inert gas like Ar or N2 must be used to prevent
oxidation of MgCl2.

1
52
L. Guangming et al. / Thermochimica Acta 412 (2004) 149–153
Fig. 2. DTA and TG curves of MgCl2·2C2H6O2·2NH3.
Table 5
Thermal decomposition data for magnesium chloride biglycollate biammoniate
◦
Step number
Temperature range ( C)
Thermal effects
Process
Residue (%)
TG
Calculated
1
2
3
4
140–206
206–260
260–330
410–540
Endo
Endo
Endo
Exo
Releasing free glycol
Releasing ammonia
Obtaining Mg(OH)Cl and C
Obtaining MgO
83
70
36
10
81.6
72.2
35.1
8.3
3
.3. Thermal decomposition of magnesium chloride
may be a process of decomposition of Mg(OH)Cl but at a
lower temperature than in the previous one [9].
About the formation of carbon, we hypothesize the fol-
lowing mechanism:
biammonia biglycol
DTA and TG curves of the decomposition of magnesium
chloride biglycollate biammoniate are shown in Fig. 2, the
chemical analysis results of the composition of the products
of the chemical in each step are shown in Table 4 and the
data of its thermal decomposition in Table 5.
From the thermal analysis, it is clear that the decom-
position of magnesium chloride biglycollate biammoniate
is much more complicated than the magnesium chloride
hexammoniate. There are four endothermal peaks and one
exothermal peak in the process of the thermal decomposition
of the complex, accompanied by four mass loss steps. The
MgCl · 2C2H O2 · 2NH3
2
6
◦
2
73 C
−−→ MgCl · 2C2H6O2 + 2NH3 ↑
2
◦
2
70 C
MgCl · 2C2H O2 −−→ MgCl + 2OH–CH2–CH2–OH
2
6
2
◦
300 C
MgCl + H2O −−→ Mg(OH)Cl + HCl
2
◦
3
00 C
OH–CH2–CH2–OH + O2 −−→ C + 4H2O
◦
first step (140–206 C) is attributed to releasing of free ethy-
In conclusion, the formation of Mg(OH)Cl and carbon in
lene glycol by the observation of no difference appearing be-
tween the residue and the original material in XRD diagram
and the chemical analysis by which the composition of prod-
uct of the step is identified to be glycol:ammonia:MgCl2 as
2
the decomposition of MgCl2·2C2H O2·2NH3 indicates that
6
this complex is useless for the production of anhydrous
magnesium chloride, and its formation when precipitating
MgCl2·6NH3 in anhydrous magnesium chloride–ethylene
glycol solution with a poor condition control will define the
quality and appearance of anhydrous MgCl2 product.
◦
.1:1.9:1.0. The second step (206–260 C) may be removal
of two molecules of ammonia from the complex, the next
step which corresponds to the third (260–330 C) endother-
◦
mal peaks obtains Mg(OH)Cl and C identified by XRD and
◦
the black color of the product. The last step (410–540 C),
4
. Conclusion
corresponding to the exothermal peak, is easily assigned to
the reaction of C burning in air. XRD measurement of the
residue indicates that MgO was formed in the step, which
From this work, we concluded that anhydrous MgCl2
can be produced by the decomposition of MgCl hexam-
2

L. Guangming et al. / Thermochimica Acta 412 (2004) 149–153
153
◦
moniate at 450 C or higher temperature in inert gas atmo-
sphere. When manufacturing MgCl2 hexammoniate in the
C2H4(OH)2, the condition must be carefully controlled to
References
[1] O.G. Sivilotti, US Patent 5,439,563 (1995).
[
[
2] K. Amundsen, H.R. Eklund, R. Schmidt, US Patent 6,042,794 (2000).
3] G.J. Sheehan, F.-S. Wong, M.M. Hourn, M. Kodama, D.H. Jenkins,
US Patent 6,143,270 (2000).
decrease the formation of MgCl2·2C2H O2 as little as pos-
6
sible.
[
[
4] O.G. Sivilotti, J.V. Sang, R.J.R. Lemay, US Patent 5,514,359 (1996).
5] R. Lastra, Trans. Inst. Miner. Metall., Sect. C 100 (1991) C110–C114.
Acknowledgements
[6] J.A. Epstein, M. El Feist, A. Kleinmann, J. Zmora, DE Patent
,532,022 (1976).
2
[
[
7] A.Z. Khomenko, Khim. Tekhnol. (Kiev) 1 (1974) 16–18.
8] Y.J. Liang, Y.C. Che, Handbook of Thermodynamic Data of Inorganic
Compounds, Neupress, 1993.
This research was supported by the Chinese Academy of
Science under XIBU ZHI GUANG program.
[9] S. Xia, G. Wang, Qinghai Huagong 2 (1984) 20–28.