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Applied Clay Science 165 (2018) 124?134
Contents lists available at ScienceDirect
Applied Clay Science
journal homepage: www.elsevier.com/locate/clay
Research paper
E?ects of carbonaceous matter additives on kinetics, phase and structure
evolution of coal-series kaolin during calcination
T
?
Shuai Yuana, Yanjun Lia,b, , Yuexin Hana, Peng Gaoa
a
b
College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China
Research Center of Coal Resources Safe Mining and Clean Utilization, LNTU, Fuxin 123000, China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Coal-series kaolin
Carbonaceous matter
Kinetic parameters
Phase transformation
Structure evolution
In this study, the in?uence of carbonaceous matter on the kinetics, phase, and structure evolution of coal-series
kaolin during the calcination process was systematically investigated. The pyrolysis characteristics were studied
by thermogravimetric?di?erential-thermal-gravity?di?erential-scanning-calorimetry (TG?DTG?DSC), and the
kinetic parameters were calculated by the Coats?Redfern (CR) method and the Flynn?Wall?Ozawa (FWO)
method. Furthermore, the changes in the mineral composition, chemical structure, and morphology of the
samples during calcination were characterized by means of X-ray di?raction (XRD), Fourier transform infrared
spectroscopy (FTIR), and scanning electron microscope (SEM) analyses. The kinetic analysis demonstrates that
the calcination reaction behaviors of the coal-series kaolin without and with carbonaceous matter additives were
according to the third-order reaction model (F3); the di?erential expression of the F3 mechanism is g(?) = (1 ?
?)?2 ? 1. The activation energy (Ea) and pre-exponential factor (lnA) increased by 7?14% and 13?23%, respectively, with carbon content addition increased from 2% to 6%. The relative decomposition rate of the
kaolinite in the coal-series kaolin increased, the absorption peaks of the kaolinite decreased gradually, and the
scale-shaped lamellar structures of the coal-series kaolin samples became irregular and amorphous with increasing carbon content under similar calcination conditions. Carbonaceous-matter additives can promote the
thermal decomposition of coal-series kaolin during the calcination process.
1. Introduction
of the active components (Liu et al., 2017a). High utilization could not
be obtained with low activity of raw coal-series kaolin. Hence, e?ectively improving the reactivity of CSK is the key to e?cient utilization.
The main components of coal-series kaolin are kaolinite, quartz,
carbonaceous matter, and other impurities such as iron minerals.
Kaolinite (Al2O3�iO2�2O) is the primary mineral with low activity
exhibiting a 1:1 layer stable crystal structure, formed by SiO4 tetrahedral sheets and AlO2(OH)4 octahedral sheets (Claverie et al., 2015;
Tang et al., 2017). Calcination is a common means of achieving high
chemical reactivity of CSK prior to further applications as mentioned
above. The elevated chemical reactivity of CSK, which is generally ascribed to the formation of amorphous reactive SiO2 and Al2O3 from the
decomposition of clay minerals, is closely related to the mineral
transformation and structural evolution of CSK during calcination
(Alujas et al., 2015).
Extensive studies are reported to determine the optimal activation
process and conditions to further achieve the high chemical reactivity
of CSK. However, few reports are regarding carbonaceous matter as
another important component in?uencing CSK's e?ect on the activation
Coal-series kaolin (CSK) is one of the main types of coal gangue; CSK
has a substantial discharge and severely pollutes the environment in
China (Bu et al., 2017). CSK is the solid waste generated during coal
mining and bene?ciation, and it accounts for approximately 10?15% of
coal production. The established reserves of CSK in China are 16.7
million tons, and the prospective reserves are 180.51 million tons. This
large amount of CSK occupies substantial areas of lands and poses
problems of severe air, water, and soil pollution (Huang et al., 2017).
Therefore, the e?cient utilization of CSK is an environmental concern
and has drawn signi?cant attention in China. With its high content of
clay minerals and carbonaceous matter, CSK could also serve as a
source of useful by-products that could be developed and utilized as
important industrial raw materials; it has a wide variety of applications
in numerous ?elds of the industry such as in aluminum extraction,
paper, rubber, plastics, cement, glass, ceramics, and refractory (Liu
et al., 2017b; Liu et al., 2018). The di?erent environments of mineralization result in the diversity of the CSK components and the content
?
Corresponding author at: College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China.
E-mail address: liyanjun@mail.neu.edu.cn (Y. Li).
https://doi.org/10.1016/j.clay.2018.08.003
Received 8 June 2018; Received in revised form 25 July 2018; Accepted 3 August 2018
0169-1317/ � 2018 Published by Elsevier B.V.
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
kinetics, phase, and structure evolution of coal-series kaolin during the
calcination process was investigated. The purpose of this study is a
detailed comparison between containing and not containing carbonaceous matter with regard to the regularity of the kinetics parameters,
mineral composition, crystal structure, and microstructure, by means of
TG?DTG?DSC characterization, kinetic study, XRD analysis, FTIR
spectroscopy, and SEM analysis.
Table 1
Chemical composition of coal-series kaolin (mass %).
Al2O3
SiO2
Fe2O3
TiO2
CaO
MgO
K2O
Na2O
C
S
37.83
47.38
0.24
0.39
0.04
0.12
0.07
0.02
1.68
0.01
2. Experimental process
2.1. Experimental materials and sample preparation
The coal series kaolin powder used in this study was obtained from
Shuozhou (ShanXi Province, China). The chemical compositions of the
coal-series kaolin were analyzed and are presented in Table 1. It is
evident that the proportions of Al2O3 and SiO2 in the raw coal-series
kaolin samples were 37.83% and 47.38%, respectively. X-ray di?raction patterns were adopted to determine the mineral composition of the
raw coal-series kaolin, and the results are shown in Fig. 2. It indicates
that kaolinite (Al2O3�iO2�2O) and quartz (SiO2) are the major
crystallized minerals present in the raw coal-series kaolin. The results
reveal that the raw coal-series kaolin has high purity (95%).
Activated carbon (an analytical reagent) was used as the carbonaceous-matter additive to further investigate its e?ects on the kinetics,
phase, and structure evolution of the coal-series kaolin during the calcination process. The coal series kaolinite and carbon matter are
weighed by certain ratios, and mixed with mechanical agitators until
well combined. The mixed samples were dried in a drying oven at 90 癈,
and coal-series kaolin without carbonaceous matter (CSK0) and those
with 2%, 4%, and 6% of carbonaceous matter (CSK2, CSK4, and CSK6)
Fig. 1. XRD pattern of coal-series kaolin.
of kaolinite during calcination (Cao et al., 2016; Haibin and Zhenling,
2010; Li et al., 2016; Qiao et al., 2008; Si et al., 2012; Zhou et al.,
2012). In this study, the in?uence of the carbonaceous matter on the
Fig. 2. TG?DTG ?DSC curve of each sample at heating rate of 15 癈/min (a: CSK0, b: CSK2, b: CSK4, b: CSK6).
125
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 3. Reaction fraction curve of each sample at di?erent heating rates (a: CSK0, b: CSK2, b: CSK4, b: CSK6).
Table 2
Evaluation of the most probable mechanism based on calculation of R2.
Equation name
Avrami?Erofeev eq., n = 1
Avrami?Erofeev eq., n = 2
Avrami?Erofeev eq., n = 3
Avrami?Erofeev eq., n = 3 / 2
Parabola law
Valensi equation
Jander equation
Ginstling?Brounstein eq.
Reaction order eq., n = 1
Reaction order eq., n = 3
Reaction order eq., n = 3 / 2
Reaction order eq., n = 1 / 4
Power law?cylinder
Power law? sphere
Symbol of g(?)
A1, F1
A2
A3
A3/2
D1
D2
D3
D4
F2
F3
F3/2
F3/4
R2
R3
Expression of g(?) function
?ln(1 ? ?)
[? ln(1 ? ?)]1/2
[? ln(1 ? ?)]1/3
[? ln(1 ? ?)]2/3
?2
? + (1 ? ?)ln(1 ? ?)
[1 ? (1 ? ?)1/3]2
1 + 2? / 3 ? (1 ? ?)2/3
(1 ? ?)?1?1
(1 ? ?)?2?1
(1 ? ?)?1/2?1
1 ? (1 ? ?)1/4
1 ? (1 ? ?)1/2
1 ? (1 ? ?)1/3
were dry milled to 85% passing 74 ?m, for use in the following studies
(Fig. 1).
R2
CSK0
CSK2
CSK4
CSK6
0.995
0.992
0.980
0.994
0.962
0.979
0.991
0.959
0.996
0.996
0.998
0.990
0.981
0.988
0.986
0.980
0.948
0.986
0.926
0.958
0.981
0.909
0.996
0.994
0.995
0.979
0.959
0.974
0.993
0.986
0.959
0.990
0.926
0.961
0.985
0.906
0.997
0.993
0.998
0.983
0.963
0.978
0.993
0.986
0.962
0.990
0.926
0.962
0.986
0.906
0.996
0.992
0.998
0.984
0.965
0.979
Ltd., SelbCity, Germany). Samples placed in corundum crucibles were
calcined at heating rates of 5, 10, 15, 20, 25, and 30 癈/min from the
initial 20?1200 癈. The TG?DTG?DSC analysis was performed under airatmosphere with a ?ow rate of 40 mL/min.
The phases of the calcined products were identi?ed using
PANalytical X'pert PW3040 (PANalytical B.V. Ltd., Almelo,
Netherlands) operated at 40 kV and 30 mA; the experimental condition
2.2. Sample characterization
Comprehensive thermal analysis experiments were conducted using
the STA449F3 thermal analyzer (Netzsch Scienti?c Instruments Trading
126
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 4. Fitting curve of samples based on A1/F1 at di?erent heating rates (a: CSK0, b: CSK2, b: CSK4, b: CSK6).
The most probable mechanism and kinetic parameters of the process
were identi?ed by employing the logarithmic form of the modi?ed
Coats and Redfern equation:
was as follows: the di?raction angle range of 2? was from 5� to 90� with
a scanning rate of 7�/min.
Fourier transform infrared spectroscopy (FT-IR) analysis was carried
out with a Nicolet 380 FT-IR spectrometer (Thermo Fisher Scienti?c
Ltd. Manufacturer, Waltham City, America Country) to further investigate the structure transformation (de la Villa et al., 2017). One
milligram samples were generally accompanied by 100 mg KBr, and the
spectra were recorded in the range of 4000?500 cm?1.
The microstructure and morphologies were observed by adopting a
scanning electron microscope (SEM) with S-3400 N (Hitachi, Ltd.,
Tokyo, Japan). The samples were coated with gold using a sputter
coater to increase their conductivity prior to SEM characterization.
ln
2.3.1. The Coats?Redfern (CR) method
The apparent activation energy (Ea) and the pre-exponential factor
(A) of the investigated process were evaluated by the mechanism-free
method based on the Coats?Redfern (CR) kinetic approach (Ptacek
et al., 2010). In addition to that, generally, the change in the extent of
reaction (reaction fraction-?) is used to study the solid state reaction
kinetics:
m 0 ? mt
m 0 ? m?
(2)
where ? is the extent of reaction, g(?) is an integral form of the kinetic
function, Ea is activation energy, R is the gas constant, A is a pre-exponential factor, and ? is the heating rate. The straight line over the
wide interval of ? was obtained by plotting ln[g(?)/T2] versus reciprocal temperature (1/T) for the proper mathematical model g(?) of
the reaction mechanism. The overall activation energy and pre-exponential factors were calculated from the slope and y-intercept of the
plot, respectively.
2.3. Kinetic study and methods
?=
g (? )
AR ?
Ea
= ln ??
? ?
T2
RT
? ?Ea ?
2.3.2. The Flynn?Wall?Ozawa (FWO) method
Flynn, Wall, and Ozawa proposed the so-called isoconversional
method (FWO) when TG curves were employed to determine the kinetic
parameters of reactions (Opfermann and Kaisersberger, 1992). Furthermore, this method relies on the Doyle approximation for heterogeneous chemical reactions:
AEa ?
Ea
lg(? ) = lg ??
? ? 2.315 ? 0.4567
RT
? R g(a) ?
(1)
(3)
In Eq. (3), ? is the extent of reaction, g(?) is an integral form of the
kinetic function, Ea is activation energy, R is the gas constant, A is a
where m0, mt, and m? are the initial sample mass, sample mass at time
t, and sample mass at the end of the reaction, respectively.
127
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 5. Fitting curve of samples based on F2 at di?erent heating rates (a: CSK0, b: CSK2, b: CSK4, b: CSK6).
pre-exponential factor, and ? is the heating rate.
For di?erent heating rates at a constant degree of conversion ?, a
linear relationship is observed between lg(?) and (1/T), and the activation energy (Ea) is obtained as the slope of the corresponding straight
line. The FWO method is based on the assumption that the reaction rate
at a speci?ed degree of conversion is a function of only the temperature.
Hence, this method can be adopted to validate the apparent activation
energy obtained by the CR method (Kuang et al., 2016; M黶ellim et al.,
2018).
indicated that carbonaceous matter apparently promotes the dehydroxylation of kaolinite and formation of metakaolin. A signi?cant
endothermic peak is observed at 560 癈 in the DSC curve of the sample
CSK0, and the endothermic peak at 560 癈 is weakened for the sample
CSK2 and has vanished in the samples CSK4 and CSK6; moreover, an
intensive exothermic peak appears at 490?510 癈 in each curve of CSK4
and CSK6. The endothermic peak at 560 癈 resulted from the kaolinite
dehydroxylation and the exothermic peak at 490?510 癈 from the
combustion of the carbonaceous matter (Cheng et al., 2010). With the
increase in the carbon content, the endothermic peak is gradually
covered by the exothermic peak. An exothermic peak is observed at
1010?1015 癈 in each DSC curve because of the early formation of
mullite.
3. Results and discussion
3.1. TG, DTG, and DSC characterization
3.2. Reaction fraction of samples
The thermal analysis of the samples is critical for understanding the
e?ect of the carbon and mineral content on the calcination process.
Fig. 2 shows the TG, DTG, and DSC curve of the samples CSK0, CSK2,
CSK4, and CSK6 in the temperature range from room temperature to
1200 癈. According to the TG curves, surface water, bound water, and
void water were the major substances released at internal temperature
of 50?150 癈 resulting in a mass loss. The TG curves exhibit a radical
weight loss e?ect at 400?800 癈, and the loss was 13?16%; this was
attributed to the high-temperature thermal decomposition of kaolinite
and combustion of carbonaceous matter (Xu et al., 2015). The DTG
curve of the samples reveals that the weight-loss of coal gangue attained the maximum at 560 癈 and then reduced to 540 癈; this
Five constant heating rates (5, 10, 15, 20, and 25 癈/min) were
applied to mimic the reaction behavior observed during the calcination
process. The reaction fraction (?) results of samples at di?erent heating
rates are provided in Fig. 3. It is observed that a higher reaction fraction
(?) could be obtained to decrease the heating rates, the same reaction
fraction (?) shifts to a higher temperature, and the mass loss rate increases with the increase in the heating rate. The e?ect of the heating
rates on the reaction fraction (?) is approximately similar. Moreover, it
is noteworthy that the higher reaction fraction (?) is observed at similar
heating rate and similar calcination temperature with the increase in
128
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 6. Fitting curve of samples based on F3 at di?erent heating rates (a: CSK0, b: CSK2, b: CSK4, b: CSK6).
obtained by other models (Table 2).
A comparison between the experimental and simulated data of A1
(F1), F2, F3, and F3 / 2 reaction models by adopting the CR method is
shown in Figs. 4?7. By adopting the CR method, all the four reaction
models exhibit signi?cantly linear correlation between ln[g(?)/T2] and
1 / T. The activation energy (Ea) of each sample in the calcination
process is calculated by the slope of the ?tting line according to Eq. (2).
The distributions of the activation energy obtained from the CR method
are presented in Table 3. It is apparent that the four reaction models
possess di?erent distributions of activation energy, and the average
activation energy of the di?erent samples obtained by A1 (F1), F2, F3,
and F3 / 2 reaction models are 54.1?65.7 kJ/mol, 117.8?122.2 kJ/mol,
184.2?210.7 kJ/mol, and 83.9?89.7 kJ/mol, respectively. It is critical
to verify that the activation energy obtained from the di?erent reaction
models can be employed to simulate the reaction mechanism (Ptacek
et al., 2010). Hence, the activation energy obtained from the FWO
method should be used as a validation of the apparent activation energy
obtained by the CR method.
the carbon content. It can be concluded that the carbonaceous-matter
additives in?uence the reaction fraction at the di?erent heating rates as
well as the calcination processes. Therefore, it is highly important to
determine the kinetic behavior of the thermal reaction.
3.3. Calculation of kinetic parameters
3.3.1. Coats?Redfern method
The most probable mechanism of the dehydroxylation, i.e., the
mathematical expression of the g(?) function, was evaluated by comparing it with mathematical models published in works, including
processes de?ned by the rate of chemical reaction, nucleation, reaction
on the phase boundary, di?usion, and certain other kinetic functions.
The values of the linear regression coe?cient (R2) were calculated for
? from 0.30 to 0.90 with the kinetic equations for the di?erent mechanisms. According to the degree of closeness of the determination
coe?cient (R2) in the ?tting result to one, the controlling mechanism
function of the reaction is determined; this implies that the most
probable mechanism function model is adopted for the calculation
when R2 was closer to one. Among all the applied kinetic functions,
Table 1 displays the results of only the most probable mechanism
(marked by bold) and several common or frequently published kinetic
functions. As illustrated in Table 1, the values of the determination
coe?cient (R2) in the di?erent samples' ?tting results obtained through
calculation using the rate of the A1(F1), F2, F3, and F3/2 reaction
models are all above 0.99, and the ?tting results are better than those
3.3.2. The Flynn?Wall?Ozawa method
The plot of lg (?) versus 1/T should be a straight line; ? denotes the
temperature at the peak obtained in the thermal analysis experiment at
di?erent heating rates. The results are shown in Fig. 8, which indicate
that the determination coe?cient (R2) of each sample calculated by the
FWO method is above 0.99. This implies that the best results of the
linearization procedure for the process are obtained with the FWO
129
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 7. Fitting curve of samples based on F3/2 at di?erent heating rates (a: CSK0, b: CSK2, b: CSK4, b: CSK6).
method (Ptacek et al., 2014). Calculated using the slope of the ?tting
line according to the FWO method, the activation energy ranges from
170.6 to 211.9 kJ/mol, which approach the activation energy (Ea) of
the F3 reaction model obtained by the CR method. It can be inferred
that the third-order chemical reaction F3 was evaluated as the most
probable mechanism of the calcination process. The range of the kinetic
parameters calculated using the di?erent methods is summarized in
Table 4. The distributions of the activation energy obtained using the
CR method and the FWO method have been plotted in Fig. 9. It was
observed that the activation energy increases with the increase in the
carbon content. The calculation results of the activation energy in the
calcination process and the pre-exponential factor of each sample by
the most probable mechanism F3 correspond to the following description: the activation energy (Ea) of the coal-series kaolin is 184.2 kJ/mol,
and the Ea of the coal-series kaolin added with 2%, 4%, and 6% carbonaceous matter corresponds to 197.8, 209.0, and 210.7 kJ/mol,
which correspond to an increase of by 7.38%, 13.5%, and 14.4%, respectively. The pre-exponential factor (lnA) of the coal-series kaolin is
19.87, whereas lnA of the coal-series kaolin added with 2%, 4%, and
6% carbonaceous matter corresponds to 22.50, 24.36, and 24.20, which
correspond to an increase of. 13.2%, 22.6%, and 21.8%, respectively.
shown in Fig. 10. With the increase in the carbon content, the intensity
of the character di?raction peak of the kaolinite in the samples decreased gradually, which is caused by the destruction of crystalline
structure caused by the dehydroxylation (Yan et al., 2017). Whereas the
intensity of the character di?raction peak of the quartz in the samples
increased gradually. The relative decomposition rate of the kaolinite in
the samples was calculated using the following formula:
(Ik / Iq ) uncalcined ?
? = ?1 ?
� 100%
?
(Ik / Iq )calcined ?
?
?
(4)
where ? is the relative decomposition rate, Ik is the intensity of the
characteristic di?raction peak (001) of the kaolinite at 12.53� (Liew
et al., 2012), and Iq is the intensity of the characteristic di?raction peak
(100) of quartz at 20.9�.
The e?ect of the carbonaceous-matter additives on the kaolinite
phase-transformation is presented in Fig. 11. The relative decomposition rate of the coal-series kaolin without carbonaceous matter additives is 78.94%, and the coal-series kaolin added with 2%, 4%, and
6% carbonaceous matter corresponds to 84.22%, 85.69%, and 88.73%,
which correspond to an increase of 5.28%, 6.75%, and 9.79%, respectively. It can be inferred that the kaolinite decomposition reaction could
be improved by increasing the carbon content.
3.4. Phase transformation
3.5. Molecular structure changes
To determine the phase transformation, the samples were calcined
at 600 癈 for 40 min. The XRD patterns of the calcined products are
The infrared spectrum provides important information about the
130
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Table 3
Kinetic parameters of samples obtained by di?erent models.
? /K/min
Reaction model
A1 (F1) reaction
model
CSK0
CSK 2
CSK 4
CSK 6
F2 reaction model
CSK0
CSK 2
CSK 4
CSK 6
F3 reaction model
CSK0
CSK 2
CSK 4
CSK 6
F3/2 reaction
model
CSK0
CSK 2
CSK 4
CSK 6
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Ea
R2
Average
5
10
15
20
25
44.3
0.996
43.2
0.993
41.5
0.996
44.1
0.997
102.1
0.997
112.9
0.998
114.3
0.996
127.6
0.988
178.3
0.997
205.8
0.994
211.6
0.992
242.1
0.982
70.3
0.999
74.2
0.999
73.7
0.999
80.9
0.994
67.3
0.997
50.5
0.992
48.3
0.996
47.5
0.995
124.0
0.993
116.7
0.997
117.7
0.995
115.1
0.996
196.8
0.993
204.3
0.993
209.8
0.989
207.8
0.991
93.3
0.997
80.2
0.998
79.3
0.998
77.7
0.998
69.8
0.994
62.5
0.969
53.4
0.990
55.6
0.990
120.7
0.998
120.9
0.989
116.1
0.998
118.6
0.998
185.3
0.998
196.7
0.989
198.5
0.994
204.2
0.995
93.3
0.998
89.1
0.984
81.7
0.998
84.1
0.998
75.4
0.996
67.3
0.988
74.5
0.990
60.2
0.991
123.6
0.996
125.8
0.999
143.7
0.998
121.4
0.999
183.9
0.996
201.1
0.996
233.5
0.992
204.3
0.994
97.8
0.998
94.1
0.997
106.0
0.998
88.0
0.998
71.9
0.994
61.7
0.990
63.4
0.991
62.9
0.991
118.4
0.999
113.9
0.999
119.5
0.999
119.0
0.999
176.8
0.999
181.0
0.999
191.7
0.999
195.3
0.999
93.5
0.999
85.7
0.999
89.1
0.999
88.6
0.999
65.7
0.995
57.0
0.986
56.2
0.993
54.1
0.993
117.8
0.996
118.1
0.996
122.2
0.997
120.3
0.996
184.2
0.996
197.8
0.994
209.0
0.993
210.7
0.992
89.7
0.998
84.7
0.995
85.9
0.998
83.9
0.998
Fig. 9. Activation energy comparison of di?erent samples obtained using CR
method and FWO method.
molecular structure, particularly about the functionality of the kaolinite
in the samples (Yuan et al., 2017). The FTIR results of the di?erent
samples are presented in Fig. 12. In the uncalcined coal-series kaolin,
the absorption peak at 3693 cm?1 belonged to outer OH stretching
vibration (Gong et al., 2018), that at 3620 cm?1 belonged to inner OH
stretching vibration (Zhang et al., 2018), those at 1030 cm?1 and
1009 cm?1 belonged to SieO stretching vibration of SieOeSi and
SieOeAl, that at 911 cm?1 belonged to AleOH vibration, those at
792 cm?1 and 693 cm?1 belonged to AleOH vibration, that at
535 cm?1 belonged to SieOeAl deformation, and those at 468 cm?1
and 432 cm?1 belonged to SieO bending (Zhang et al., 2015). To sum
up, all these absorption peaks were identical to the characteristic absorption peaks of the kaolinite. It is apparent that the characteristic
absorption peaks of the kaolinite decreased and vanished gradually,
which indicates that the dehydration reaction of the kaolinite was
proceeding. It was particularly noteworthy that the absorption bands at
3620 cm?1 corresponding to OH stretching vibration decreased gradually with the increase in the carbon content. Therefore, the incorporation of increased carbon content accelerated the dehydroxylation of the kaolinite, which was in accordance with the calculation
results of the relative decomposition rate of samples.
3.6. Morphology structure characterization
For a better understanding of the morphology structure changes in
the calcined products with carbonaceous matter, the microstructural
evolution was studied using SEM (Zhou et al., 2012). The morphological characteristics of the calcined products obtained by SEM are
shown in Fig. 13. The SEM images (A-1 and A-2) reveal that the CSK0
samples exhibit smooth surface and intact lamellar structures. When the
carbon content is increased from 2% to 6%, there are numerous differences in the SEM image when compared to that of CSK0 at similar
calcined conditions, which lose more edge sheet structure of the kaolinite. The AlO2(OH)4 octahedral layer of kaolinite could exist before
1000 癈 and completely destroyed after 1000 癈 with the formation of
mullite (Xu et al., 2015). With higher carbon content, the samples release more heat according to the DSC characterization, and more
structures of the AlO2(OH)4 octahedral layer were disrupted (Sperinck
et al., 2011; Peng et al., 2018), which caused more scale-shaped lamellar structures to separate from the group particles and become irregular and amorphous, as shown in the SEM images (B-1 to D-2). The
SEM results reveal that the mineral and structural changes of the kaolinite with the di?erent carbon content as well as the DSC results are
highly consistent with the XRD and FTIR results. It can be veri?ed that
Fig. 8. Fitting curve of each sample obtained using FWO method.
Table 4
Kinetic parameters obtained using FWO method and CR method.
Sample
C-0
C-2
C-4
C-6
FWO method
CR method
Ea (kJ/mol)
Ea(kJ/mol)
lnA (1/min)
?R
170.6
205.3
210.6
211.9
184.2
197.8
209.0
210.7
19.87
22.50
24.36
24.20
0.9982
0.9968
0.9962
0.9958
131
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 10. XRD patterns of samples CSK0, CSK2, CSK4, and CSK6 calcined at 600 癈 for 40 min.
the carbonaceous-matter additive is favorable for the thermal decomposition of the coal-series kaolin.
4. Conclusions
The e?ects of the carbonaceous-matter additives on the kinetics,
phase, and structure evolution of the coal-series kaolin during the calcination process were systematically investigated to promote more effective utilization. The following conclusions can be drawn from this
study:
The pyrolysis characteristics and thermal properties were studied by
TG?DTG?DSC. The endothermic peak caused by the dehydroxylation of
the kaolinite at 560 癈 weakened and vanished with the increase in the
carbon content. Meanwhile, an intensive exothermic peak appeared at
490?510 癈 as a result of the combustion of the carbonaceous matter
with the increase in the carbon content. The release of heat would be
bene?cial to the dehydroxylation of the kaolinite.
The Coats?Redfern integral method was adopted to solve the kinetic
parameters, and the activation energy results were veri?ed by the
Flynn?Wall?Ozawa method. The kinetic analysis reveals that the calcination reaction behaviors of the coal-series kaolin without and with
carbonaceous-matter additives are determined by the rate of the thirdorder reaction model (F3), and the di?erential expression of the F3
mechanism is g(?) = (1 - ?)-2?1. The activation energy (Ea) of the coalseries kaolin without carbonaceous matter during calcination was
184.2 kJ/mol, and the Ea of the coal-series kaolin added with 2%, 4%,
and 6% carbonaceous matter corresponded to 197.8, 209.0, and
210.7 kJ/mol, respectively, which correspond to an increase of 7.38%,
13.5%, and 14.4%, respectively.
XRD, FTIR, and SEM experiments were carried out to investigate the
phase transformation and structure evolution with the carbonaceousmatter additives during the calcination process. The relative decomposition rate of the coal-series kaolin without carbonaceous-matter
additives is 78.94%, whereas the coal-series kaolin added with 2%, 4%,
and 6% carbonaceous matter corresponds to 84.22%, 85.69%, and
88.73%. The characteristic absorption peaks of the kaolinite decreased
and vanished gradually, and the scale-shaped lamellar structures separated from the group particles and became irregular and amorphous
with the increase in the quantity of carbonaceous matter. Hence, it is
concluded that carbonaceous matter additives could promote the
thermal decomposition of kaolinite in coal-series kaolin.
Fig. 11. Decomposition rate of samples CSK0, CSK2, CSK4, and CSK6 calcined
at 600 癈 for 40 min.
Fig. 12. FT-IR spectrum of uncalcined coal-series kaolin and calcined samples
(CSK0, CSK2, CSK4, and CSK6) at 600 癈 for 40 min.
132
Applied Clay Science 165 (2018) 124?134
S. Yuan et al.
Fig. 13. Morphological characterization of samples calcined at 900 癈 for 5 min (A: CSK0, B: CSK2, C: CSK4, D: CSK6).
Acknowledgement
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de la Villa, R.V., Garc韆, R., Mart韓ez-Ram韗ez, S., Fr韆s, M., 2017. E?ects of calcination
temperature and the addition of ZnO on coal waste activation: a mineralogical and
morphological evolution. Appl. Clay Sci. 150, 1?9.
Gong, B., Wu, P., Ruan, B., Zhang, Y., Lai, X., Yu, L., Li, Y., Dang, Z., 2018. Di?erential
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Huang, T., Lei, S.M., Liu, Y.Y., Ji, M.J., Fan, Y.M., 2017. Bene?ciation and in?uencing
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This research was supported by the Open Projects of Research
Center of Coal Resources Safe Mining and Clean Utilization, Liaoning
(LNTU16KF14), Natural Science Foundation of China (51674064;
51674065) and Fundamental Research Funds for the Central
Universities (N170107004).
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