J. Cent. South Univ. Technol. (2011) 18: 1865−1870
DOI: 10.1007/s11771−011−0915−z
Preparation of basic magnesium carbonate and
its thermal decomposition kinetics in air
LIU Xin-wei(刘欣伟)1, 2, FENG Ya-li(冯雅丽)1, LI Hao-ran(李浩然)2
1. Civil and Environmental Engineering School, University of Science and Technology Beijing, Beijing 100083, China;
2. National Key State Laboratory of Biochemical Engineering, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing 100190, China
© Central South University Press and Springer-Verlag Berlin Heidelberg 2011
Abstract: The thermal decomposition process of basic magnesium carbonate was investigated. Firstly, Basic magnesium carbonate
was prepared from magnesite, and the characteristics of the product were detected by X-ray diffraction (XRD) and scanning electron
microscopy (SEM). Subsequently, the thermal decomposition process of basic magnesium carbonate in air was studied by
thermogravimetry-differential thermogravimetry (TG-DTG). The results of XRD confirm that the chemical composition of basic
magnesium carbonate is 4MgCO3·Mg(OH)2·4H2O. And the SEM images show that the sample is in sheet structure, with a diameter
of 0.1−1 μm. The TG-DTG results demonstrate that there are two steps in the thermal decomposition process of basic magnesium
carbonate. The apparent activation energies (E) were calculated by Flynn-Wall-Ozawa method. It is obtained from Coats-Redfern’s
equation and Malek method that the mechanism functions of the two decomposition stages are D3 and A1.5, respectively. And then,
the kinetic equations of the two steps were deduced as well.
Key words: basic magnesium carbonate; TG-DTG; thermal decomposition; kinetics; mechanism function
1 Introduction
Basic magnesium carbonate (xMgCO3·Mg(OH)2·
yH2O) is considered as one of the most important
compounds of the magnesium industry. It has been
widely used in various industries, such as toothpaste,
painting, cosmetic manufacturing, plastic, rubber, and as
precursors for other magnesium-based chemicals [1−2].
Basic magnesium carbonate can also be used as the
chemical coolant when the heat is absorbed during the
thermal decomposition [3]. In recent years, a series of
xMgCO3·Mg(OH)2·yH2O with different morphologies
have been obtained via different synthesis method or in
different preparation conditions. However, to the best of
our knowledge, the report on the thermal decomposition
of basic magnesium carbonate is rare. The decomposition
reported in previous studies showed that the thermal
decomposition of basic magnesium carbonate occurs in
at least two stages, dehydration and magnesium
carbonate decomposition. KHAN et al [4] studied the
influence of heating rate, sample size and atmospheric
conditions on the origin of exothermic peak.
The aim of the present work is to investigate the
thermal decomposition of basic magnesium carbonate,
which was prepared from magnesite, by thermogravimetry-differential thermogravimetry (TG-DTG)
technology [5−6]. Based on the analysis of the dynamic
parameters in thermal decomposition, the basic
magnesium carbonate decomposition mechanism was
investigated and the key dynamic parameters were also
tested.
2 Experimental
2.1 Raw material
The magnesite ore used in the experiment was
collected from Haicheng in Liaoning Province, China.
Chemical analysis result of the ore is given in Table 1. As
can be seen from Table 1, the main component of the ore
is MgCO3.
Table 1 Chemical analysis of magnesite (mass fraction, %)
MgO
CaO
SiO2
Fe2O3
Al2O3
Ignition loss
46.7
1.35
0.52
0.41
0.05
50.97
2.2 Experimental procedures
2.2.1 Preparation of basic magnesium carbonate
The light-burned magnesia was prepared from
magnesite, which was calcinated at 1 023 K for 1.5 h.
Then, basic magnesium carbonate was obtained from the
light-burned magnesia. Process flow chart is shown in
Fig.1.
Foundation item: Project(20876160) supported by the National Natural Science Foundation of China
Received date: 2010−12−02; Accepted date: 2011−03−11
Corresponding author: FENG Ya-li, Professor, PhD; Tel: +86−10−82627064; E-mail: [email protected]
1866
J. Cent. South Univ. Technol. (2011) 18: 1865−1870
Fig.1 Preparation flow chart of basic magnesium carbonate
2.2.2 Analysis of basic magnesium carbonate
The produced basic magnesium carbonate was
analysed by X-ray diffraction (Rigaku D/max-RB type
X-ray) with Cu Kα radiation (λ=1.541 8 Å at 40 kV and
70 mA). Scanning electron microscope (SEM) images
were taken with SSX-550 field-emission scanning
electron microscope.
2.2.3 Thermal decomposition of basic magnesium
carbonate
A NETZSCH STA 449C (manufactured by Netzsch
in Germany) thermogravimetry differential scanning
calorimetry (TG-DSC) system was used to determine the
TG and differential TG (DTG) experimental points for
basic magnesium carbonate at different heating rates. A
given amount of sample was put into an alumina crucible
and its mass loss was recorded at heating rate of 15, 20
and 25 K/min in air atmosphere, with a carrier gas flow
rate of 50 mL/min. In the TG-DSC system, the masses of
samples are 3.271−4.077 mg, and the results obtained are
repeatable.
(JCPDS25-0513).
Figure 3 shows the SEM images of the basic
magnesium carbonate. The products exist as independent
flaky units, with a diameter of 0.1−1 μm, which are
generally consistent with Ref.[7]. Some flaky units
aggregate like roseleaf-shaped basic magnesium
carbonate. It is superposed layer by layer, from inside to
outside, and the flakes exhibit uniform thickness.
3 Results and discussion
3.1 Detection and analysis of basic magnesium
carbonate
The XRD pattern of the product is shown in Fig.2.
All the peaks in Fig.1 can be indexed to the crystalline
phase of 4MgCO3·Mg(OH)2·4H2O, with unit cell
parameters of a=10.11 Å, b=8.94 Å, c=8.38 Å and β=
114.58°, which are in agreement with literature values
Fig.2 XRD pattern of product synthesized in accordance with
Fig.1
Fig.3 SEM micrographs of 4MgCO3·Mg(OH)2·4H2O
3.2 Thermal decomposition of basic magnesium
carbonate
Figure 4 shows TG-DTG curves of the thermal
decomposition of 4MgCO3·Mg(OH)2·4H2O.
The TG curves indicate that there are two stages in
the thermal decomposition process of basic magnesium
carbonate. The first decomposition stage is in the range
of 423−573 K attributed to the water loss of
crystallization, and this decomposition is accompanied
by an endothermic peak at 508.4 K. The mass loss is
measured to be about 14.98 % in this stage. The second
decomposition stage is in the range of 573−823 K, and
this decomposition is accompanied by an endothermic
peak at 729 K. The mass loss is measured to be about
J. Cent. South Univ. Technol. (2011) 18: 1865−1870
1867
Fig.4 TG-DTG curves of 4MgCO3·Mg(OH)2·4H2O thermal
decomposition at different heating rates
The plot of ln β vs 1/T was established based on the
same conversion at different heating rates and activation
energy was calculated from the slope of the regression
line. In this work, the experiments were conducted at
three heating rates of 15, 20 and 25 K/min. And nine
conversion values were chosen from 10% to 90% with an
increment of 10% at each heating rate. The regression
lines of ln β vs 1/T are plotted in Fig.5. From Fig.5, it can
be observed that when basic magnesium carbonate
conversion rates are 10%, 20% and 30%, the three
regression lines are parallel to each other. However,
when the conversion rates are between 40% and 90%,
this part of regression lines are parallel to each other.
This also shows that there are two steps in the thermal
decomposition process of basic magnesium carbonate.
56.88%, which is slightly smaller than the theoretical
value (57.08%). Basic magnesium carbonate has been
completely decomposed into magnesium oxide, carbon
dioxide and water in the second stage. The thermal
decomposition of basic magnesium carbonate can be
expressed as follows.
In first stage:
4MgCO3·Mg(OH)2·4H2O→4MgCO3·Mg(OH)2+4H2O
In second stage:
4MgCO3·Mg(OH)2→5MgO+H2O+4CO2
3.3 Calculation of dynamic parameters
According to non-isothermal kinetic theory, the
kinetic equation of solid-state thermal transformation
under linear temperature increasing condition could be
generally described as [8−9]
d A
 E
  exp  
dT 
 RT

  f ( )

(1)
where α is the extent of conversion, dα/dT is the reaction
rate and f(α) is the reaction model; A is the
pre-exponential term; β is the heating rate; E is the
energy of activation; R is the universal gas constants and
T is the absolute temperature.
Methods for solving kinetic parameters can be
attributed to the approximate treatment of Eq.(1). In this
work, Flynn−Wall−Ozawa (FWO) method [10] was used.
FWO equation can be written as:
 AE 
0.456 7 E
 2.315 
lg   lg 

RT
 RG ( ) 
Fig.5 ln β−1/T curves at different decomposition rates
The plot of basic magnesium carbonate
decomposition activation energy versus conversion rate
is constructed, as shown in Fig.6. From Fig.6, it can be
seen that the decomposition apparent activation energy is
divided into two parts. The first part of the activation
energy is about 51.84 kJ/mol, and the second part is
about 191.97 kJ/mol, which further explains that the
(2)
From Eq.(2), it can be seen that, when the reaction
mechanism function is unchanged, by plotting ln β vs 1/T,
straight lines can be produced, and the slopes can be used
to calculate the activation energy E, which was obtained
without the mechanism functions [11].
Fig.6 Thermal decomposition activation energy variation curve
of 4MgCO3·Mg(OH)2·4H2O
J. Cent. South Univ. Technol. (2011) 18: 1865−1870
1868
thermal decomposition of basic magnesium carbonate is
divided to two-step reactions.
3.4 Mechanism functions determination
In order to study the non-isothermal reaction
kinetics of basic magnesium carbonate thermal
decomposition from single TG curve. Coats−Redfern
equation [12−13] was used. The equation can be written
as
ln[ F ( ) / T 2 ]  ln( AR /  E )  E / RT
(3)
in the first stage of basic magnesium carbonate
decomposition process, D3, D4 and 3D mechanism
functions have higher correlation coefficient and are
close to each other. In the second stage, 3D and A1.5
mechanism functions have higher correlation coefficient
and are close to each other. In the case of correlation
coefficients close to one another, Malek method is an
effective method to further determine the most probable
mechanism function [19]. The standard curve equation of
mechanism function can be written as
y ( ) 

where F ( )   1/ f ( )d .
0
Linear-regression was used to analyze the kinetic
characteristics and reaction mechanism of the entire
basic magnesium carbonate decomposition. According to
Eq.(3), a plot of ln[F(α)/T2] vs 1/T was constructed.
Eight different values of F(α) from documents [14−18]
were tested. The form of F(α) that gave the best straight
line with high correlation coefficient (R-squared value)
of linear regression analysis was selected and the
mechanism corresponding to this value of F(α) was
assigned as the mechanism for the reaction. The slope of
this particular plot yielded the energy of activation while
the y-intercept provided the pre-exponential factor
(lgA-value) for the reaction. The values obtained are
listed in Table 2.
From the regression results, it can be deduced that
f ( )  F ( )
f (0.5)  F (0.5)
(4)
Experimental curve equation can be written as
2
 T  d  d 
y ( )  



 T0.5  dT  dT 0.5
(5)
where F(α) and f(α) are the reaction models, and dα/dt is
obtained from the TG curve. Plotting y(α) vs α curves, if
the experimental curve and the standard curve overlap or
all experimental data points fall on a standard curve, the
corresponding standard curve, F(α) or f(α), is the most
probable mechanism function.
Taking 20 K/min as an example, the two stages y(α)
vs α curves are given in Fig.7 and Fig.8. Experimental
curve is expressed by the dashed line and labeled by “p”.
Based on the above analysis, it can be determined
Table 2 Kinetics parameters and regression calculation results
Heating rate/(K·min−1)
Stage
First
stage
D3
D4
3D
A2
A1.5
A1
C2
R2
E/(kJ·mol−1)
52.54
45.41
40.70
5.95
29.45
106.53
34.16
18.8
lg(A/s−1)
7.82
7.95
8.48
9.15
7.96
9.03
7.65
8.98
0.998 2
0.996 9
0.996 7
0.987 7
0.996 0
0.992 8
0.996 2
0.995 5
208.2
185.62
156.71
124.8
192.39
136.9
197.78
142.48
16.50
15.29
18.04
18.52
15.84
17.78
16.80
17.85
0.979 0
0.966 5
0.990 8
0.945 8
0.995 0
0.955 4
0.966 5
0.979 2
50.42
59.35
44.64
9.44
38.66
15.29
44.49
25.75
7.76
6.88
7.41
9.15
7.35
8.86
6.92
8.58
0.997 5
0.996 8
0.996 8
0.991 7
0.966 3
0.994 3
0.996 5
0.995 7
E/(kJ·mol )
204.65
183.43
158.14
123.94
192.21
135.84
195.33
142.22
lg(A/s−1)
16.44
15.49
18.12
18.70
15.73
18.01
16.66
18.03
0.970 0
0.952 5
0.989 9
0.924 5
0.998 4
0.938 2
0.953 8
0.969 9
51.78
42.98
40.49
5.29
27.36
9.71
31.77
17.62
7.33
8.42
8.83
9.32
8.36
9.25
8.09
9.26
0.998 5
0.992 5
0.992 0
0.971 4
0.990 0
0.983 0
0.990 4
0.989 7
200.8
180.52
150.99
122.71
185.20
134.28
192.09
138.75
15.92
16.02
18.62
18.79
15.46
18.12
16.36
18.27
0.985 4
0.975 2
0.987 0
0.959 2
0.998 9
0.967 2
0.975 9
0.984 6
R
15
2
−1
Second
stage
E/(kJ·mol )
−1
lg(A/s )
R
2
−1
First
stage
E/(kJ·mol )
−1
lg(A/s )
R
20
2
−1
Second
stage
R
2
−1
First
stage
E/(kJ·mol )
−1
lg(A/s )
R
25
2
−1
Second
stage
Mechanism
Kinetic
parameter
E/(kJ·mol )
−1
lg(A/s )
R
2
J. Cent. South Univ. Technol. (2011) 18: 1865−1870
1869
In the second stage:
d

dT
1015.68
 23 089.6 
1/ 3
  exp 
  {1.5(1   )[ ln(1   )] }
T


(7)
4 Conclusions
1) Basic magnesium carbonate is prepared from
magnisite, and the product is in sheet structure, with
diameters of 0.1−1 μm, and the chemical formula is
4MgCO3·Mg(OH)2·4H2O.
2) The thermal decomposition process of basic
magnesium carbonate in air is studied by TG-DTG. The
results demonstrate that there are two steps in the thermal
decomposition process of basic magnesium carbonate,
and decomposition products are mainly magnesium
oxide, carbon dioxide and water. Dehydration reaction is
in the first reaction, and complete decomposition
reaction is in the second stage.
3) It is obtained from Coats−Redfern equation and
Malek method that the mechanism functions of the two
decomposition stages are D3 and A1.5, respectively. The
apparent activation energy (E) are 51.84 kJ/mol and
191.97 kJ/mol, respectively, and pre-exponential factor
(lgA) are 7.64 s−1 and 15.68 s−1, respectively.
Fig.7 y(α)−α curves of first phase
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