Characterization and Treatment of Dolomite as Raw Material for Producing Magnesium Metal

Abstract

Dolomite is one of many mineral resources that are abundant in Indonesia. It can be processed into many chemical products and metal commodities through calcination. Calcination process can be understood by analyzing thermal behavior of dolomite in calcination temperature. In this experiment, the thermal behavior of dolomite obtained from West Java Province in Indonesia was evaluated using FactSage® simulation and Thermogravimetric Analysis (TGA). The thermal behavior is analyzed at the temperature range of 25-1000 o C in nitrogen atmosphere. The component of the dolomite was also analyzed using X-ray Diffractometer (XRD). The experimental data is then used to determine the kinetic model and activation energy of dolomite decomposition. XRD results showed that dolomite has two main components, CaCO 3 and MgCO 3. The thermal behavior result showed that dolomite decomposed through single step decomposition in nitrogen atmosphere. The decomposition began at the temperature above 700 o C and finished at the temperature of above 850 o C. The power law nucleation model closely described the dolomite decomposition mechanism, proven by experimental data fitting with kinetic model. Using this model as the base for energy activation calculation, the activation energy of Indonesian dolomite decomposition obtained in this experiment is 144,426 – 149,568 kJ/mol.

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Prosiding Seminar Nasional Teknik Kimia Indonesia 2015 ISSN
Sustainable Energy and Mineral Processing for National Competitiveness
Yogyakarta, 12-13 Oktober 2015
! !
Characterization and Treatment of Dolomite as Raw Material for
Producing Magnesium Metal
Winny Wulandari*, Subagjo, Adnanta Rio, Pratama Istiadi
Chemical Engineering Program, Faculty of Industrial Technology, Institut Teknologi Bandung
Jalan Ganesa No. 10, Bandung 40132
*Corresponding Author : winny@che.itb.ac.id
Abstract
Dolomite is one of many mineral resources that are abundant in Indonesia. It can be processed into
many chemical products and metal commodities through calcination. Calcination process can be
understood by analyzing thermal behavior of dolomite in calcination temperature. In this experiment,
the thermal behavior of dolomite obtained from West Java Province in Indonesia was evaluated using
FactSage® simulation and Thermogravimetric Analysis (TGA). The thermal behavior is analyzed at
the temperature range of 25-1000oC in nitrogen atmosphere. The component of the dolomite was also
analyzed using X-ray Diffractometer (XRD). The experimental data is then used to determine the
kinetic model and activation energy of dolomite decomposition. XRD results showed that dolomite has
two main components, CaCO3 and MgCO3. The thermal behavior result showed that dolomite
decomposed through single step decomposition in nitrogen atmosphere. The decomposition began at
the temperature above 700oC and finished at the temperature of above 850oC. The power law
nucleation model closely described the dolomite decomposition mechanism, proven by experimental
data fitting with kinetic model. Using this model as the base for energy activation calculation, the
activation energy of Indonesian dolomite decomposition obtained in this experiment is 144,426 –
149,568 kJ/mol.
Keywords : activation energy, FactSage®, kinetic model, thermogravimetric analysis,calcination
1. Introduction
Indonesia is blessed with a various amount of mineral resources. One of those abundant mineral
resources is dolomite. Dolomite is a carbonate based mineral that rich in magnesium and calcium
content. According to the Indonesian Ministry of Energy and Mineral Resources, there are
approximately 2,397,707 thousand tonnes of dolomite deposit in Indonesia. Dolomite can be
processed into various products, such as refractory brick, soil fertilizer, and magnesium metal. The
key important step in dolomite processing is calcination. Calcination of dolomite in industries is
carried out at 1000oC to 1200oC. Since it is energy intensive and the key step in dolomite processing, a
comprehensive study regarding dolomite calcination behavior is important.
The aim of this study is to examine the kinetic behavior of Indonesian dolomite during calcination in
ideal condition. This ideal condition is represented with the small amount of dolomite sample used in
experiment and an inert eluting gas used to swift the product gas away from the reaction atmosphere,
thus minimizing the effect of mass transfer and heat transfer within the calcination process. While
there are several studies in dolomite calcination, the study of Indonesian dolomites is scarce. Through
this experiment, it is expected that the behavior of Indonesian dolomite calcination can be understood
well, so that a larger scale experiment can be constructed further.
Dolomite will decompose into its oxide during calcination. According to Samtani, et al. (2000), in
nitrogen (N2) atmosphere, dolomite decomposes through one stage of decomposition. This
decomposition is shown at Equation (1):
!𝐶𝑎𝑀𝑔 𝐶𝑂!!!(!)↔𝐶𝑎𝑂.𝑀𝑔𝑂(!)+𝐶𝑂!!(!) (1)
As in carbon dioxide (CO2) atmosphere, dolomite decomposes through two steps of decomposition, as
shown in Equation (2) and (3):

Prosiding Seminar Nasional Teknik Kimia Indonesia 2015 ISSN
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Yogyakarta, 12-13 Oktober 2015
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!𝐶𝑎𝑀𝑔 𝐶𝑂!!!(!)→𝐶𝑎𝐶𝑂!!(!)+!𝑀𝑔𝑂!(!)+𝐶𝑂!!(!) (2)
𝐶𝑎𝐶𝑂!!(!)→𝐶𝑎𝑂(!)+𝐶𝑂!!(!) (3)
Dolomite decomposition is also affected by other several factors, such as dolomite composition,
dolomite density, dolomite particle size, and calcination temperature.
The dolomite decomposition reaction follows the general model of solid reaction kinetic, which is
shown in Equation (4):
!"
!"
=𝐴𝑒!!"
!"!𝑓(𝛼) (4)
whereas A is the Arrhenius constant, Ea is the activation energy, R is the ideal gas constant, T is the
temperature of reaction, and f(α) is the reaction model.
Although Equation (4) is generally used in isothermal reaction, it is also can be used to model the non-
isothermal reaction. This can be carried out by transforming the solid reaction kinetic into a model
with temperature function, as shown in Equation (5)
!"
!"
=!!"
!"
!"
!"
! (5)
Equation (5) is then substituted to Equation (4), which results in Equation(6).
!"
!"
=!!
!
!𝑒!!!
!" 𝑓(𝛼) (6)
Whereas dα/dT is the non-isothermal reaction rate, dα/dt is the isothermal reaction rate, and β is the
factor of heating rate (dT/dt).
Table 1. Reaction Models of Solid Reaction Kinetics
No
Reaction Model
f(α)
g(α)
Nucleation Model
1
Power law
4α3/4
α 1/4
2
Power law
3α2/3
α 1/3
3
Power law
2α1/2
α 1/2
4
Power law
2/3α-1/2
α 3/2
5
Avrami-Erofeev 1
4(1- α)[-ln(1-α)]3/4
[-ln(1-α)]1/4
6
Avrami-Erofeev 2
3(1-α)[(-ln(1-α)]2/3
[-ln(1-α)]1/3
7
Avrami-Erofeev 3
2(1-α)[-ln(1-α)]1/2
[-ln(1-α)]1/2
Contracting Geometry Model
8
Contracting sphere
3(1-α)2/3
1-(1-α)1/3
9
Contracting cylinder
2(1-α)1/2
1-(1-α)1/2
Diffusion Model
10
1-dimensional diffusion
1/2α-1
α 2
11
Valensi-Barrer equation (2-D diffusion)
(-ln(1-α))-1
α + (1-α)ln(1-α)
12
Jander equation (3-D diffusion)
(3/2)(1-α)2/3[1-(1-α)1/3]-1
[1-(1-α)1/3]2
13
Three dimensional diffusion
2(1-α2/3)(1-(1-α)1/3)-1
[1-(1-α)1/3]2
14
Gintsling-Brounshtein equation
(3/2)[(1-α)-1/3-1]-1
[1-(2/3)α]-(1-α)2/3
Reaction Order Model
15
Mampel 1st order
1-α
-ln(1-α)
16
Second order
(1-α)2
(1-α)-1-1
17
One-third order equation
(1-α)1/3
(3/2)[1-(1-α)2/3]
18
One-half order equation
(1-α)1/2
2[1-(1-α)1/2]
19
Two-thirds order equation
(1-α)2/3
3[1-(1-α)1/3]

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Yogyakarta, 12-13 Oktober 2015
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The reaction model f(α) that is used in the kinetic model depend on the reaction mechanism. Several
models has been developed by Vyazoykin and Wight (1999) and Ersoy-Mericboyu and Kucukbayrak
(1994) to explain the solid reaction kinetic. These models are shown in Table 1.
2. Methodology
In this experiment, dolomite sample was obtained from PT Polowijo Gosari in Gresik City, West Java.
The experiment was carried out by comparing two methods, which are thermochemical software
simulation using FactSage® and Thermogravimetric Analysis (TGA). Before the experiment,
dolomite sample content was analyzed using X-ray Diffractometer (XRD).
1. XRD analysis
Dolomite sample obtained from Gresik was analyzed its content using XRD. The XRD
results would show what kind of crystal phase that is contained inside the dolomite.
2. FactSage ® simulation
The simulation was used to predict the thermal behavior of dolomite decomposition using
thermochemical principles. The dolomite sample amount was determined and the
composition of dolomite was obtained from the dolomite provider, as shown in Table 2,
Tabel 2. Dolomite Sample Composition
Compound
Composition (%-mass)
CaCO3 (calcite)
54,37
MgCO3 (magnesite)
40,95
H2O (water vapor)
4,39
SiO2 (silica)
0,17
Fe2O3 (hematite)
0,09
Al2O3 (alumina)
0,03
The temperature range of the simulation was inputted at 25oC to 1000oC. The nitrogen
atmosphere was also inputted to the simulation to match the TGA experiment condition.
3. Thermogravimetric Analysis (TGA)
TGA was used to determine the thermal behavior of dolomite decomposition experimentally.
15 mg of 100 mesh dolomite sample was used in this analysis. The nitrogen as an eluting gas
was introduced to the system at the rate of 50 mL/min. The analysis was also done at the
temperature range of 25oC to 1000oC with variation of heating rate at 5, 10, 15 mL/min. The
conversion order (α) is calculated using Equation 7.
𝛼=!!!!!!!
!!!!!!
(7)
Whereas mo is the initial sample mass, mt is the sample mass at the time t, m∞ is the final
sample mass.
3. Results and Discussion
3.1. Result Analysis of XRD
The XRD analysis of Indonesian dolomite is shown in Figure 1. Figure 1 shows that the main
component of Indonesian dolomite is calcite (CaCO3) with 2θ of 29-31o and magnesite (MgCO3) with
2θ of 31-32oC. The dolomite sample used in this experiment is quite pure, since the impurities content
detected in the XRD is relatively low. It means that the dolomite in Gresik can be considered as high
quality.

Prosiding Seminar Nasional Teknik Kimia Indonesia 2015 ISSN
Sustainable Energy and Mineral Processing for National Competitiveness
Yogyakarta, 12-13 Oktober 2015
! !
Figure 1. XRD Result of Indonesian Dolomite
3.1. Result Analysis of FactSage ® Simulation.
The FactSage® simulation resulted in curve that is shown in Figure 2.
Figure 2. Simulation Result of Dolomite Decomposition
Figure 1 shows several key results, which are:
• Magnesium oxide (MgO – periclase) begins to form at the temperature of 325oC to 425oC
through decomposition reaction of MgCO3 which also results in CO2 gas.
• Calcium oxide (CaO – lime) begins to form at the temperature of 862.5oC to 875oC through
decomposition reaction of CaCO3 which also results in CO2 gas
• Several byproducts such as Ca2SiO4 dan Ca2FeO5, are also formed during dolomite
decomposition. However these byproducts are formed in small amount (< 2 x 10-7 mol).

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Yogyakarta, 12-13 Oktober 2015
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• Dolomite decomposes through two steps of decomposition, which is the magnesium oxide
formation at temperature of 325-425oC, followed by formation of calcium oxide at the
temperature of 862,5-875oC.
3.2. Result Analysis of TGA
The TGA experiment resulted in thermal behavior curve of dolomite decomposition in various heating
rate. The resulting curve is shown in Figure 3.
Figure 3. The Thermal Behavior of Dolomite Decomposition
Figure 3 shows that as the temperature risen, the dolomite mass was also decreased due to dolomite
decomposition process, which forms magnesium and calcium oxide alongside with carbon dioxide
gas. Dolomite began decomposing after 700oC, noted by its significant loss of mass, and stopped
decreasing after 850oC, indicating that the decomposition reaction had finished. The decomposition
process is carried out in single step decomposition, just like Samtani, et al. (2001) reported.
The thermal behavior curve was then used to predict the solid reaction kinetic model of dolomite
decomposition. Data fitting of models in Table 1 yielded various correlation coefficient value, as
shown in Table 3:
Tabel 3. Correlation Coefficient of Models Fitting to Experimental Data
Model
f(α)
R2
5oC/min
10oC/min
15oC/min
1
4α3/4
0,916
0,9427
0,852
2
3α2/3
0,9319
9,9542
0,8775
3
2α1/2
0,9517
0,9679
0,913
4
2/3α-1/2
0,9669
0,9737
0,9649
5
4(1- α)[-ln(1-α)]3/4
0,8856
0,9542
0,9198
6
3(1-α)[(-ln(1-α)]2/3
0,8829
0,9561
0,9327
7
2(1-α)[-ln(1-α)]1/2
0,878
0,9566
0,9486
8
3(1-α)2/3
0,9191
0,961
0,9618
9
2(1-α)1/2
0,9431
0,9662
0,9615
10
1/2α-1
0,9619
0,9684
0,9643
11
(-ln(1-α))-1
0,9451
0,9591
0,9612
40!
50!
60!
70!
80!
90!
100!
110!
0! 100! 200! 300! 400! 500! 600! 700! 800! 900! 1000!
%"Mass&
Temperature&(oC)&
5!C!per!menit!! 10!C!per!menit! 15!C!per!menit!
5oC/minute
10oC/minute
15oC/minute

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Model
f(α)
R2
5oC/min
10oC/min
15oC/min
12
(3/2)(1-α)2/3[1-(1-α)1/3]-1
0,9011
0,9467
0,9565
13
2(1-α2/3)(1-(1-α)1/3)-1
0,9011
0,9467
0,9565
14
(3/2)[(1-α)-1/3-1]-1
0,9315
0,955
0,9597
15
1-α
0,8665
0,9494
0,9608
16
(1-α)2
0,7313
0,9112
0,95
17
(1-α)1/3
0,9622
0,9708
0,9605
18
(1-α)1/2
0,9431
0,9662
0,9615
19
(1-α)2/3
0,9191
0,961
0,9618
Table 3 shows that the model number 4, or the power law, is the model that best describes the
dolomite decomposition reaction, since it has the highest R2 value in all heating rate variation.
However, as there is no significant difference between the model and other high R2 models, it cannot
be said that the model number 4 is the single best model that can definitively describe the dolomite
calcination.
The power law model showed that solid decomposition follows the nucleation model (Khawam,
2006). The model assumes that the nucleation process occurs constantly during reaction, without any
effects that may affect the nuclei growth, such as ingestion (the elimination of potential area of
nucleation due to existing nuclei growth) and coalescence (the compounding of two or reaction
interface due to the joining of two nuclei).
After obtaining the model that closely represent the dolomite decomposition, the reaction rate constant
and the activation energy of dolomite decomposition is readily calculated from the TGA data using
Equation (8)
ln !"
!"
=−!"
!"
ln !
!+ln 𝑓(𝛼) (8)
The activation energy values resulted from this experiment are shown in Table 4.
Table 4. Activation Energy and Reaction Rate Constant
Heating Rate (per minute)
Activation Energy (kJ/mol)
Reaction Rate Constant
ln (A/β)
5oC
148,263
12,996
10oC
149,568
12,844
15oC
144,426
12,010
Table 4 shows that with different heating rates, the resulting activation energy values of dolomite
decomposition is almost the same with no significant difference. The activation energy obtained in
this experiment is 144,426-149,568 kJ/mol. This value is significantly lower than other values
reported from earlier studies, such as Samtani, et al. (2001) with 175,05 kJ/mol and Halikia, et al.
(2007) with 242,05 kJ/mol. These difference is due to the dolomite specific characteristics that differ
in every region. This lower value of activation energy in Indonesian dolomite decomposition indicates
that the dolomite is easier to process compared to another dolomites from other regions.

Prosiding Seminar Nasional Teknik Kimia Indonesia 2015 ISSN
Sustainable Energy and Mineral Processing for National Competitiveness
Yogyakarta, 12-13 Oktober 2015
! !
4. Conclusion
Through this study, it is concluded that:
1. Gresik dolomite has two main components, which are calcite (CaCO3) and magnesite
(MgCO3)
2. The FactSage® simulation predicted that dolomite decomposes through two step process,
while TGA results shows that dolomite decomposes via single step decomposition. This
difference in result is due to atmosphere condition cannot be taken into account in FactSage®
simulation.
3. The power law model number 4 closely describes the dolomite decomposition mechanism.
4. With the nucleation model, the activation energy of Gresik dolomite decomposition is
144,426 – 149,568 kJ/mol, which is lower than several earlier studies.
Symbols
m mass [g]
T temperature [K]
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