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Journal of Mineral, Metal and Material Engineering, 2017, 3, 71-79 71

E-ISSN: 2414-2115/17 © 2017 Synchro Publisher

Kinetics and Mechanism of Thermal Decomposition of Calcite and Aragonite

Petr Ptáček*, František Šoukal and Tomáš Opravil

Institute of Materials Chemistry, Faculty of Chemistry, Brno University of Technology, Purkyňova 118, Brno, CZ-621 00, Czech Republic

Abstract: The influence of polymorph crystal structure on the thermal decomposition process of trigonal (calcite) and orthorhombic (aragonite) calcium carbonate was investigated by means of Thermogravimetric-Differential Thermal Analysis (TG-DTA) under non-isothermal conditions and inert atmosphere of nitrogen. The experiments were conducted to the temperature of 900°C using the heating rate from 1 to 10°C·min-1. The kinetic triplet, i.e. the activation energy, the frequency factor and the kinetic exponent were determined via the Kissinger and Augis-Bennett equations. The activation enthalpy and the entropy of activated state were determined from the Van’t Hoff plot. The results indicate that there is not any significant effect of polymorphism or origin on the value of activation energy of thermal decomposition of calcium carbonate for the specimens of comparable purity.

Keywords: Decomposition kinetics, calcite, aragonite, calcium carbonate precipitate, CaCO3, thermal analysis, heterogeneous kinetics.

1. INTRODUCTION

Carbonate rocks, which may be sedimentary, detrital, or metamorphic, are represented by two principal members, i.e. limestone and dolomite (Figure 1(a)) [1]. Limestone is composed of more than 50 wt% of CaCO3 in the form of mineral calcite [2, 3] (Figure 1(b-d)) and aragonite (Figure 1(e,f)). The main constituent of dolomite rock is a carbonate mineral of the same name (CaMg(CO3)2 [4]). Limestone consists of calcium carbonate (CaCO3) more or less impure, and it occurs in many forms of very diverse origin. The variety of limestone known as calcareous tufa or travertine is a chemical precipitate, but in its larger masses the rock is generally of organic origin. Chalk is probably derived from marine ooze and other limestones are made up of shells and corals [1, 5].

The polymorphs of calcium carbonate include calcite (trigonal), aragonite (orthorhombic) and vaterite (hexagonal). Their occurrence depends on small changes in temperature and pH during the precipitation [1]. Mineral calcite (Figure 1(a-c)) is known since antiquity under a number of names, e.g. the ancient name calx “burnt lime” was given to this mineral by Pliny the Elder (Gaius Plinius Secundus). Calcite was also termed as calcareous spar, but it appears to have first been called calcite by Freiesleben [2]. Aragonite (Arragon Spar), which occurs as an inorganic constituent of many invertebrate skeletons, such as shells of snails, ammonites, clams, corals, sponge

*Address correspondence to this author at the Brno University of Technology, Faculty of Chemistry, Purkyňova 118, 621 00 Brno, Czech Republic; E-mail: [email protected]

spiccules, etc. [6], is metastable at room temperature [7, 8]. As was proved by Boeke [9, 10] aragonite undergoes a slow transformation to calcite with the inversion temperature at about 470°C. Vaterite is also a metastable polymorph that transforms to calcite at ambient conditions [3].

Figure 1: The mineral dolomite (Shangbao, Hunan, China, (a); calcite: (Štramberk, Czech Republic, (b); Iceland spar (Leiping, China, (c); calcite (Lažánky, Czech Republic, (d); aragonite (La Pesquera, Spain, (e); and aragonite (Karlovy Vary, Czech Republic, (f); The photography from author’s mineralogical collection.

The structure of calcite [11] and aragonite [12] is shown in Figure 2. In the structure of calcite (a), the CO3

2- units are planar and occur in layers perpendicular to the c-axis. The CO3

2- groups in one layer have identical orientation, whereas those in adjacent layers rotate by 180° towards the other. The coordination for Ca atom is six. The idealized structure of aragonite (b) has approximate hexagonal close-packing, with C atoms from the CO3

2- groups. Ca atoms are coordinated by nine oxygen atoms of six different CO3

2- units that are slightly a planar [6].

The process of thermal decomposition of calcium carbonate has both, the technological importance, i.e.

72 Journal of Mineral, Metal and Material Engineering, 2017, Vol. 3 Petr, et al.

the manufacturing of lime, Portland cement (raw material [13,14], admixture [15-17]), basic refractory materials [18-20], metallurgy of iron, etc., as well as the scientific importance. For example, the structure of mineral calcite was one of the first which were solved by X-ray diffraction method (Bragg [21]). The rhomb (rhombic crystal form) of transparent variety of calcite, known as Iceland-spar (Figure 1(c)), shows double refraction and perfect cleavages (Bartholinus [22] in 1669). The study of this mineral led to the discovery of the laws of double refraction (Huygens [23], 1690) and the polarization of light (Malus [24], 1808). Iceland spar was the first type of polarizing prism, which was invented in 1828 by Nicol [25].

The mechanism and the thermodynamics of processes, which take place during the heating of carbonates and clay minerals, are the subject of study since the times of Le Châtelier [26]. He was also the first to show that chalk can be fused at 1000°C under partial pressure of carbon dioxide until it exceeds 1000 kilograms per square centimeter [7]. The kinetic studies dealing with the thermal decomposition of limestone show that the particle of CaCO3 becomes covered by continuous layer of CaO in the early stage of the process of thermal decomposition:

CaCO3(s1)! CaO(s2 ) + CO2 (g) , (1)

where s1 denotes some of polymorphs of CaCO3 that does not form solid solution with the phase s2 (lime). The reaction interface then moves into the particle with constant rate which depends on the transfer of heat and diffusion of CO2 through CaO layer. This geometric

model is known in heterogeneous kinetics as “shrinking core” [27], where the decomposition reaction takes place on a definite boundary between the undecomposed carbonate, and the layer of porous lime formed outside it.

The kinetics of thermal decomposition of calcium carbonate has been studied more often than the decomposition of any other solids [28]. The process has been investigated in the terms of heating rate [29], particle size [30,31], atmosphere [31]/ CO2 pressure [32,33], vacuum [28], impurities [34], treatment of CaCO3 particles surface [35] and sample preparation [36] using both isothermal [38,38] as well as non-isothermal methods [39-42]. The process can also be influenced by the reactant self-cooling, sample mass and geometry [43]. Despite this scientific effort, the satisfactory general agreement with the value of activation energy and the mechanism of decomposition process of calcium carbonate was not achieved [33]. The statistical survey of published values for the activation energy was carried out by Maciejewski [44].

The studies presented in this work deal with the mechanism, kinetics and thermodynamics of the process of thermal decomposition of trigonal (calcite) and orthorhombic (aragonite) calcium carbonate investigated via non-isothermal thermogravimetric experiment at constant heating rate. Analyzed specimens include a synthetic sample of precipitated calcium carbonate and two mineral samples of both, calcite and aragonite polymorph from different locations.

Figure 2: The structure of calcite, (a); according to D.L. Graf [11] and aragonite, (b); according to J.P.R. de Villiers [12].

Kinetics and Mechanism of Thermal Decomposition of Calcite and Aragonite Journal of Mineral, Metal and Material Engineering, 2017, Vol. 3 73

2. EXPERIMENTAL

2.1 Samples and Sample Treatment

The specimens of trigonal calcium carbonate analyzed in this work include precipitated calcium carbonate (PCC) of analytical purity grade (Penta), transparent variety of calcite rhomb (Iceland spar, Figure 1(c)) from Leiping, China) and non-transparent rhomb of calcite (d) from Lažánky, Czech Republic). Two analyzed samples of orthorhombic polymorph of calcium carbonate are pseudohexagonal crystal of violet aragonite from La Pesquera, Spain (e) and pisolitic/oolitic variety aragonite from Karlovy Vary, Czech Republic (f).

The material taken from mineral specimens (Figure 1(c-f)) is fine ground in fine glazed porcelain dish. Fine powder of synthetic sample of (precipitated CaCO3) precipitate was used without any further treatment.

2.2. Investigation of Process of Thermal Decomposition

The process of thermal decomposition of the specimens of CaCO3 described above was investigated by non-isothermal TG-DTA (Thermogravimetric-Differential Thermal Analysis) kinetics experiments performed at several constant rates of heating (CRH) ranging from 1 up to 10°C·min-1. 30.0 ±0.5 mg of fine ground sample were introduced into the alumina cup (50 µdm3) and placed on the sample holder of the thermal analyzer (Q600, TA Instruments). The specimen was then heated up to the temperature of 900 °C in inert atmosphere of nitrogen (N2 4.0, 100 cm3·min-1). All experiments were repeated at least four times to obtain average results without outlier values.

2.3. Determination of Kinetics and Thermodynamics

The mechanism, the kinetics and thermodynamics of the process of thermal decomposition of calcium carbonate specimens (Eq.1) were evaluated by numerical differentiation of thermogravimetric data (DTG curve) via the Kissinger equation [45-47]:

ln!

Tm

2

"

#$

%

&' = ln

AR

Ea

n(1()m)n(1"

#$

%

&' (

Ea

RTm

= const.(Ea

RTm

, (2)

where Ea is the apparent activation energy, A is the pre-exponential (frequency) factor, Θ is the heating rate, n is the empirical reaction order (kinetic

exponent), R is the universal gas constant, αm is the conversion degree (fractional conversion) reached for the peak temperature Tm. The value of Ea is determined graphically from the slope (−Ea/R) of the plot of ln ! /T

m

2( ) Vs. reciprocal temperature (T−1.).

The value of n, which is required for the calculation of frequency factor from the ordinate of the intersection point of Kissinger plot, was calculated from the DTG peak parameters according to the Augis-Bennett equation [48]:

n =2.5 R T

m

2

w1/2

Ea

; (3)

where w1/2 is the width at half high of peak (Full width at half maximum, FWHM).

The correlation between kinetic and thermodynamic parameters can be expressed by the Eyring-Polanyi equation [49-53];

k(T ) =kBT

hexp

! S#

R

"

#$

%

&' exp (

!H #

RT

"

#$

%

&' = ) exp (

!G#

RT

"

#$

%

&' = ) K # ;(4)

where kB, h, v=and K# are the Boltzmann, the Plank constant, the universal frequency (frequency factor) and the equilibrium constant, respectively. The thermodynamic parameters of activated complex in Eq.4, i.e. enthalpy (ΔH#), entropy (ΔS#) and Gibbs energy of activation, can be calculated as follows [50, 51]:

!H#= E

a" RT ; (5)

!S# = R lnh A

kBT

"#$

%&'

(

)*

+

,- ; (6)

and;

!G#= !H

#" T !S

#= "RT ln K

# . (7)

Since Eq.7 can be treated as follows:

ln K#= !

"H#

R

1

T+"S

#

R; (8)

the values of ΔH# and ΔS# can be determined graphically from the slope and from the ordinate of intersection point of Van’t Hoff graph, i.e. the plot of ln K# vs. T-1. It should be pointed that the same results can be determined from Eyring plot, where the dependence of ln (k/T) on T-1 provides


straight line with the slope and the intersection point of .

3. RESULTS AND DISCUSSION

Figure 3(a) shows the TG-DTA results for synthetic sample of PCC with trigonal structure of calcite. Since there is not any other processes than the endothermic process of thermal decomposition of calcium carbonate (Eq.1), only the relevant temperature range from 400 to 900°C is displayed. Within the investigated interval of heating rates from 1 to 10°C·min-1, the DTG and DTA peak temperatures are increased from 679.0 ±4.9 and 684.6 ±1.9°C to 786.1 ±1.8 and 781.1 ±1.7°C, respectively. The mass of sample was reduced by 43.97 ±0.16 wt%, i.e. the value which is exactly equal to the theoretical one: (100×44.01)/100.09= 43.97wt%.

The kinetics of the process of thermal decomposition of PCC was evaluated from the

Kissinger plot (Figure 3(b)). The slope of dependence leads to the activation energy of 182.6 ± 2.0 kJ·mol-1. The average value of kinetic exponent calculated from Eq.3 equals to 2.06 ±0.08. That indicates the process with decreasing nucleation rate, which is driven by the diffusion controlled growth of new phase. The value of frequency factor, which was calculated from the intercept of Kissinger plot, is (1.0 ±0.2)·107 s-1.

The thermodynamics of activated state was determined from the Van´t Hoff plot (Figure 3(b)). These results enable to write the relation:

!G#= !H

#"T!S

#= 174.23#10

3+120.92 T . (9)

The overview of determined kinetic and thermodynamic data of thermal decomposition of PCC and other samples of calcite are summarized in Table 1.

Figure 3: TG-DTA curves of the process of thermal decomposition of PCC, (a); Kissinger and Van´t Hoff plot, (b); for the determination of kinetics and thermodynamics there of.

Table 1: Kinetics and Thermodynamics of the Process of Thermal Decomposition for Samples with Trigonal Structure of Calcite

Sample Kinetic triplet Thermodynamics of activated state Note

Ea [kJ·mol-1] 182.6 ±2.0 ΔH# [kJ·mol-1] 174.23 A [s-1] (1.0 ±0.2)·107 ΔS# [J·(K·mol)-1] -120.92 Precipitated CaCO3 (PCC)

n --- 2.06 ±0.08 ΔG# [J·mol-1] Eq.9

Synthetic sample

Ea [kJ·mol-1] 185.7 ±4.2 ΔH# [kJ·mol-1] 177.29 A [s-1] (6.3 ±3.1)·106 ΔS# [J·(K·mol)-1] -124.94 Transparent rhomb of calcite n --- 1.62 ±0.06 ΔG# [J·mol-1] Eq.10

Iceland spar, Figure 1(c)

Ea [kJ·mol-1] 196.6 ±2.7 ΔH# [kJ·mol-1] 190.87 A [s-1] (1.8 ±0.6)·107 ΔS# [J·(K·mol)-1] -110.28 Non-transparent rhomb of

calcite n --- 1.52 ±0.05 ΔG# [J·mol-1] Eq.11

Figure 1(d)


The TG-DTA results from the investigation of kinetics of thermal decomposition of well developed transparent rhomb of mineral calcite (Eq.1) in the variety of Iceland spar (Leiping, China) are shown in Figure 4(a). The DTA peak temperature of this endothermic process increases from 684.3 ±1.4 (1°C·min-1) to 795.7 ±2.0°C (10°C·min-1) and the DTG peak temperature increases from 693.4 ±1.3 to 790.8 ±2.4°C at the same time.

The mass of sample was reduced by 44.12 ±0.15 wt%. Since the above calculated theoretical mass loss (43.97 wt%) ranges within the reliability interval of this result, the specimen can be considered as high purity sample (more than 99.99 wt% of CaCO3). These results are in an agreement with well developed rhombic crystal form (crystal habit) and optical transparency of the sample.

The Kissinger plot for the process of thermal decomposition of Iceland spar variety of calcite

(Figure 1(c)) is shown in Figure 4(b). With regard to an experimental error, the activation energy, which is required for the thermal decomposition of this sample (185.7 ±4.2 kJ·mol-1), is almost identical to the PCC of comparable purity (Table 1). On the other hand, there can be a slight difference in the mechanism of the process. Lower determined value of kinetic exponent (1.62 ±0.06) indicates, that the thermal decomposition of Iceland spar is the process with zero or decreasing nucleation rate, which is driven by the diffusion controlled growth of new phase.

The values of activation enthalpy and entropy (Table 1), which were determined from the Van´t Hoff plot (Figure 4(b)), enable to express the temperature dependence of Gibbs energy of activation as follows:

!G#= !H

#" T!S

#= 177.29 #10

3+124.94 T . (10)

The last investigated specimen of calcite is also well developed rhomb, it is non-transparent mineral from

Figure 4: TG-DTA curves for the process of thermal decomposition of Iceland spar, (a); Kissinger and Van´t Hoff plot, (b); for the determination of kinetics and thermodynamics thereof.

Figure 5: TG-DTA curves for the process of thermal decomposition of non-transparent calcite rhomb, (a); Kissinger and Van´t Hoff plot, (b); for the determination of kinetics and thermodynamics thereof.


Lažánky, Czech Republic (Figure 1(d)). The TG-DTA results for the process of thermal decomposition (Eq.1) are shown in Figure 5(a). Within the investigated interval of heating rates from 1 to 10°C·min-1, the DTA and DTG peak temperatures increase from 700.9 ±2.9 and 703.5 ±1.4°C to 801.5 ±3.0 and 796.4 ±3.7°C, respectively.

The mass of sample was reduced by 43.93 ±0.15 wt% during the process of thermal decomposition. This value is again almost identical to the theoretical one, but in the agreement with sample appearance, the non-transparent calcite rhomb is of slightly lower purity than both previous samples. Using the loss on ignition, the concentration of CaCO3 in this specimen was determined to be about 99.91 ±0.70 wt%.

The Kissinger and Van´t Hoff plot for the process of thermal decomposition of this sample is shown in Figure 5(b). The process of thermal decomposition of this sample requires slightly higher activation energy (196.6 ±2.7 kJ·mol-1) than those determined for both PCC and Iceland spar (Table 1). The frequency factor and the kinetic exponent show the values of (1.8 ±0.6)·107 s-1 and 1.52 ±0.05, respectively. The

comparison of the kinetic triplet data from Table 1 indicates similar mechanism for the process of thermal decomposition as for the transparent rhomb of Ice land spar.

With regard to the values of activation enthalpy and entropy graphically determined from the Van´t Hoff plot (Figure 5(b)), it is possible to write the relation:

!G#= !H

#" T!S

#= 190.87 #10

3+110.28 T . (11)

The results of TG-DTA for the process of thermal decomposition of CaCO3 (Eq.1) in orthorhombic modification of aragonite (Figure 1(e)) are shown in Figure 6(a). Within the investigated interval of heating rates from 1 to 10°C·min-1, the DTA and DTG peak temperatures increase from 698.0 ±1.9 and 707.7 ±0.5°C to 800.4 ±2.9 and 795.1 ±3.0°C, respectively. The mass of sample is reduced by 43.62 ±0.15 wt%. This value is as for all previously described samples close to the theoretical one. Using the loss on ignition, the concentration of CaCO3 in this specimen was determined to be about 99.21 ±0.66 wt%, i.e. similar to previous samples of calcite.

Figure 6: TG-DTA for the process of thermal decomposition of aragonite from Spain, (a); Kissinger and Van´t Hoff plot, (b); for the determination of kinetics and thermodynamics thereof.

Table 2: Kinetics and Thermodynamics of the Process of Thermal Decomposition for Samples with Orthorhombic Structure of Aragonite

Sample Kinetic triplet Thermodynamics of activated state Note

Ea [kJ·mol-1] 199.4 ±8.2 ΔH# [kJ·mol-1] 190.88 A [s-1] (3.7 ±3.7)·107 ΔS# [J·(K·mol)-1] -110.27

Aragonite from Spain

n --- 1.82 ± 0.06 ΔG# [J·mol-1] Eq.12

Figure 1(d), hexagonal outline, violet

Ea [kJ·mol-1] 198.1 ±1.2 ΔH# [kJ·mol-1] 191.16 A [s-1] (8.7 ±3.0)·107 ΔS# [J·(K·mol)-1] -109.99 Aragonite from

Czech Republic n --- 1.88 ± 0.06 ΔG# [J·mol-1] Eq.13

Figure 1(f), pisolitic/ oolitic variety


The Kissinger and Van´t Hoff plot for the process of thermal decomposition of this sample is shown in Figure 6(b). The overview of kinetic and thermodynamic data for both investigated samples of aragonite can be found in Table 2. The activation energy of the process of thermal decomposition was determined to be 199.4 ±8.2 kJ·mol-1. Therefore, the process of thermal decomposition of aragonite from Spain requires the activation energy, which is comparable with previous sample of non-transparent rhomb of calcite.

According to the determined value of kinetic exponent (1.82 ±0.06), the mechanism of the process has zero or decreasing nucleation rate, which means that it is driven by the diffusion controlled growth of new phase. The value of activation enthalpy and entropy (Table 2), which were determined from the Van´t Hoff plot (Figure 6(b)), enable to express the temperature dependence of Gibbs energy of activation as follows:

!G#= !H

#" T!S

#= 190.88 #10

3+110.27 T . (12)

The kinetics and thermodynamics of thermal decomposition of this specimen seems to be similar to calcite.

The TG-DTA results of the process of thermal decomposition (Eq.1) of the last investigated mineral sample of aragonite from Czech Republic (Figure 1(f)) are presented in Figure 7(a). Within the investigated interval of heating rates from 1 to 10°C.min-1, the DTA and DTG peak temperatures increase from 669.6 ±2.1 and 671.6 ±1.4°C to 764.4 ±1.6 and 758.8 ±2.0°C, respectively. The mass of sample is reduced by 39.94 ±0.26 wt%. This value is then significantly lower than

the theoretical one and also the lowest from all samples investigated in this work. Using the loss on ignition, the concentration of CaCO3 in this specimen was determined to be about 90.84 ±1.20 wt%.

The Kissinger and Van´t Hoff plot for the process of thermal decomposition of aragonite from Czech Republic is shown in Figure 7(b) and the overview of kinetic and thermodynamic data is listed in Table 2. The process requires the activation energy of 198.1 ±1.1 kJ·mol-1, i.e. the value very similar to the previous sample of aragonite from Spain. That indicates that the impurities in the sample do not interfere to process of thermal decomposition of aragonite. The determined value of kinetic factor is (8.7 ±3.0)·107 s-1.

As results from the value of kinetic exponent (1.88 ±0.06), both aragonite samples have very similar mechanism of thermal decomposition. In actual fact, there is not any significant difference in the mechanisms of thermal decomposition of all investigate samples, perhaps with exception of PCC. The thermodynamic data of activated state, which were determined from the Van´t Hoff plot (Figure 7(b)), enable to write the relation for the Gibbs energy of activation as follows:

!G#= !H

#" T!S

#= 191.16 #10

3+109.99 T . (13)

CONCLUSION

The activation energy of the process of thermal decomposition of precipitated calcium carbonate (PCC) with trigonal structure of calcite (synthetic sample) was determined to be 182.6 ±2.0 kJ·mol-1. This value is very similar to the activation energies for the decom-

Figure 7: TG-DTA curves for the process of thermal decomposition of aragonite from Czech Republic, (a); Kissinger and Van´t Hoff plot, (b); for the determination of kinetics and thermodynamics thereof.


positions of specimens of mineral calcite in the variety of Iceland spar (185.7 ±4.2 kJ·mol-1) and well developed, but non-transparent calcite rhomb (196.6 ±2.7 kJ·mol-1). The activation energies of thermal decompositions of both investigated samples of orthorhombic polymorph, including well developed pseudohexagonal crystal (199.4 ±8.2 kJ·mol-1) and the pisolitic/oolitic variety of aragonite (198.1 ±1.2 kJ·mol-1), are similar to each other and to trigonal polymorph (calcite) as well. Therefore, there is not any significant effect of the polymorph of calcium carbonate or its origin on the value of activation energy for the specimens of comparable purity. The mechanism of the process of thermal decomposition, which includes zero or decreasing nucleation rate of new phase and the diffusion controlled growth of new phase, is also very similar for all investigated samples.

ACKNOWLEDGMENTS

The paper has been supported by the project Materials Research Centre at FCH BUT- Sustainability and Development, REG LO1211, with financial support from National Programme for Sustainability I (Ministry of Education, Youth and Sports).

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Received on 15-12-2017 Accepted on 21-12-2017 Published on 31-12-2017 © 2017 Petr, et al.; Licensee Synchro Publisher. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

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