Study on kinetics of the pyrolysis process of aluminum sulfate

Article abstract of DOI:10.1080/10426507.2019.1686377

Using high aluminum gangue as a raw material, aluminum sulfate 18 hydrate was made by the sulfuric acid leaching method under certain conditions. The product was characterized using X-ray diffraction (XRD) analysis of the phases, x-ray-fluorescence (XRF) analysis of the aluminum and sulfur content and thermogravimetric analysis of the crystallized water [Al₂(SO₄)₃·18H₂O]. Change characteristics of the crystal form and morphology during pyrolysis of octadecahydrate aluminum sulfate were studied by thermogravimetric Analysis (TGA), differential scanning calorimetry (DSC), differential thermogravimetry (DTG), XRD and scanning electron microscope (SEM). The theoretical basis for the preparation of metallurgical alumina from octadecahydrate aluminum sulfate was provided. According to the characteristics of the crystal structure change, the pyrolysis process of octadecahydrate aluminum sulfate can be separated into three stages. The first stage (dehydration stage 87–250 °C) had a weight loss rate of 40.5% and a loss of 15 water molecules; the weight loss rate of the second stage (dehydration stage 280–414 °C) was 8.1% with three water molecules lost; the weight loss rate of the third stage (decomposition stage 770–900 °C) was 36.1%, where three SO₃ molecules were lost. The pyrolysis products were mainly Al₂O₃. The activation energies of the three reaction stages were calculated using the Coats-Redfern method as 90.02 kJ/mol, 205.74 kJ/mol and 284.40 kJ/mol, respectively.

Full text of DOI:10.1080/10426507.2019.1686377

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: https://www.tandfonline.com/loi/gpss20
Study on kinetics of the pyrolysis process of
aluminum sulfate
Guangya Zheng, Jupei Xia, Zhengjie Chen, Jin Yang & Chenglong Liu
To cite this article: Guangya Zheng, Jupei Xia, Zhengjie Chen, Jin Yang & Chenglong Liu (2019):
Study on kinetics of the pyrolysis process of aluminum sulfate, Phosphorus, Sulfur, and Silicon and
the Related Elements, DOI: 10.1080/10426507.2019.1686377
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2019.1686377
Study on kinetics of the pyrolysis process of aluminum sulfate
a
a
b
a
c
Guangya Zheng , Jupei Xia , Zhengjie Chen
, Jin Yang , and Chenglong Liu
a
b
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, China; Faculty of Metallurgical and Energy
c
Engineering, Kunming University of Science and Technology, Kunming, China; Faculty of Chemical Engineering, Ningxia Normal University,
Guyuan, China
ABSTRACT
ARTICLE HISTORY
Using high aluminum gangue as a raw material, aluminum sulfate 18 hydrate was made by the
sulfuric acid leaching method under certain conditions. The product was characterized using X-ray
diffraction (XRD) analysis of the phases, x-ray-fluorescence (XRF) analysis of the aluminum and sul-
Received 29 July 2019
Accepted 25 October 2019
KEYWORDS
Aluminum sulfate; pyrolysis;
kinetics; alumina
fur content and thermogravimetric analysis of the crystallized water [Al
2
(SO
4
)
3
ꢀ18H
2
O]. Change
characteristics of the crystal form and morphology during pyrolysis of octadecahydrate aluminum
sulfate were studied by thermogravimetric Analysis (TGA), differential scanning calorimetry (DSC),
differential thermogravimetry (DTG), XRD and scanning electron microscope (SEM). The theoretical
basis for the preparation of metallurgical alumina from octadecahydrate aluminum sulfate was
provided. According to the characteristics of the crystal structure change, the pyrolysis process of
octadecahydrate aluminum sulfate can be separated into three stages. The first stage (dehydration
ꢁ
stage 87–250 C) had a weight loss rate of 40.5% and a loss of 15 water molecules; the weight
ꢁ
loss rate of the second stage (dehydration stage 280–414 C) was 8.1% with three water molecules
ꢁ
lost; the weight loss rate of the third stage (decomposition stage 770–900 C) was 36.1%, where
three SO
of the three reaction stages were calculated using the Coats-Redfern method as 90.02 kJ/mol,
05.74 kJ/mol and 284.40 kJ/mol, respectively.
3 2 3
molecules were lost. The pyrolysis products were mainly Al O . The activation energies
2
GRAPHICAL ABSTRACT
Introduction
However, the technology of alkaline aluminum extraction is
limited to small-scale development because it cannot realize
At present, there are two main processes for extracting alu-
mina from aluminum-bearing minerals, namely the alkali the effective reduction of aluminum-bearing minerals.
[
1–3]
[4–5]
method
and acid method.
As the technology and Aluminum extraction using the acid method mainly includes
[6–9]
equipment for extracting aluminum by the alkali method the Hydrochloric Acid Method and Sulfuric Acid Method.
was developed from mature bauxite sintering and the Bayer The prominent advantage of these is that the reduction of
Process, there are already demonstration production lines. aluminum-containing minerals can be achieved.
CONTACT Jupei Xia
xjp6661@163.com; Zhengjie Chen
czjkmust@126.com
Faculty of Chemical Engineering, Kunming University of Science and
Technology, Kunming 650500, Yunnan, China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
ß 2019 Taylor & Francis Group, LLC

2
G. ZHENG ET AL.
Table 1. Composition of aluminum sulfate crystal products.
3þ
4-
Composition
w(Al
)
w (SO )
2
w (H O)
Theoretical value
Actual value
8.11
8.10
43.24
43.19
48.65
48.71
was in a state of thermal equilibrium of kinetic parameters
and ln A . Comparing E with E , the most
E
b!0
b!0
b!0
a!0
likely mechanism of Al (SO ) ꢀ18H O dehydration is 2 and
2
4 3
15]
2
[
1
/3 reactions two process.
However, due to the influence of the instrument parame-
ters and the properties of the crystalline aluminum sulfate
used in the experiment, there is no unified understanding of
[
16]
Figure 1. XRD pattern of aluminum sulfate crystal products.
the dehydration process.
Moreover, less attention has
been paid to the changes of crystal form and macro-morph-
However, in the process of preparing alumina from crys- ology in the dehydration process of aluminum sulfate, so
tallized aluminum chloride or aluminum sulfate, corrosion that it is impossible to provide effective guidance for the
of equipment caused by the simultaneous production of preparation of metallurgical sand alumina with crystallizing
[
10]
[17–18]
acidic gas and water vapor is an important problem.
aluminum sulfate.
In this study, aluminum sulfate octadecahydrate was pre-
Separating the process steps of crystallized water removal
and pyrolysis to prepare alumina becomes the focus of acid pared from high alumina coal gangue. The XRD diffraction
extraction. As sulfuric acid is less corrosive to equipment pattern and theoretical analysis further showed that the
than hydrochloric acid and more stable and inexpensive, product was aluminum sulfate octadecahydrate. The pyroly-
more attention has been paid to the pyrolysis of crystalline sis process of aluminum sulfate was studied by thermogravi-
aluminum sulfate. In addition, the adaptability of raw mate- metric analysis (TGA). Combined with kinetic analysis and
rials is wide, the dissolution rate of acid solution is high, its correlated kinetic parameters, the XRD diffraction pattern
and the content of silicon oxide in acid residue is high, and SEM micrograph of the pyrolysis products in different
which is beneficial to the preparation of related silicon prod- stages were analyzed in order to provide theoretical basis
ucts by acid slag at the later stage, and finally realizes the and technical guidance for the preparation of metallurgical
utilization of all aluminum rich resources. However, due to alumina via crystallization of aluminum sulfate.
the small market capacity of aluminum sulfate, the large-
scale treatment of aluminum sulfate is limited. If aluminum
sulfate is decomposed into alumina by an appropriate pro-
Results and discussion
cess, and the sulfur resources can be recycled in the system,
Composition of Al (SO ) ꢀ18H O prepared by
2
4 3
2
the treatment scale can reach millions of tons. Meanwhile,
acid method
[
11–13]
there is no new solid waste in the process.
The product obtained by the acid method was analyzed by
XRD, and the results are shown in Figure 1. The mass frac-
tion of the constituent elements in Al (SO ) ꢀ18H O was
Research on the dehydration and decomposition of alu-
minum sulfate dates back to the 1970s. The research focuses
on the kinetic mechanism function of the dehydration reac-
tion and the lattice arrangement analysis of the crystal struc-
ture of aluminum sulfate. Chai studied the variation features
of crystalline shape and morphology of hydrated aluminum
sulfate in the dehydration process using molecular simula-
tion, and the theoretical data for alumina preparation from
hydrated aluminum sulfate was developed. The dehydration
was separated into three steps according to the transform-
2
4 3
2
calculated by chemical analysis of the product. The experi-
mental and theoretical data can be found in Table 1. Based
on the data, the amount of crystalline water was calculated
to be 18.
Analysis of the thermal decomposition process of
Al (SO ) ꢀ18H O
st
ꢁ
2
4 3
2
ation of the crystalline structure. In the 1 step (30–80 C),
.7 moles of water were dehydrated, and the structure was The TG and DSC curves of Al
2
2
(SO
triclinic. In the 2 stage (80–322 C), 12.65 moles of water Figure 2. As shown in Figure 2, there were three steps in
were dehydrated, and the structure was monoclinic system. the thermal decomposition process of Al (SO O,
ꢀ18H
The 3 stage occurred at 322–383 C with a 1.15 mole water in which the dehydration process was carried out in two
4
)
3
ꢀ18H
2
O were shown in
nd
ꢁ
)
3
2
4
2
rd
ꢁ
[
14]
ꢁ
ꢁ
loss, and the structure became hexagonal.
Xu studied the stages. In the first step, from 87 C to 250 C, the weight
kinetic mechanism model of aluminum sulfate dehydration loss rate was 40.5% and 15 water molecules were lost, corre-
based on DTA/TGA, where the type of dehydration process sponding to the endothermic peak of the DSC curve at
ꢁ
ꢁ
ꢁ
of aluminum sulfate hydrate and the dehydration tempera- 127 C. The second step was from 280 C to 414 C, where
ture characteristics were discussed. Using the assumptions of the weight loss rate was 8.1% and three water molecules
the mechanistic function of the corresponding kinetic were lost, corresponding to the endothermic peak of the
ꢁ
parameters, 30 different functions were derived. When the DSC curve capping temperature was 327 C. The third step
heating rate was extrapolated to zero, in theory, the system was from 770 C to 900 C, where the weight loss rate was
ꢁ
ꢁ

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
of thermal decomposition of aluminum sulfate in octadeca-
hydrate was defined as X, and the reaction kinetics was
studied by means of the dynamic method.
m0 ꢂ mt
X ¼
(4)
m0 ꢂ m1
where m (mg) is the initial mass of the material; m (mg) is
0
t
the mass of the sample at time t (min); and m (mg) is the
1
final mass of the material.
The dynamic method was used to study the kinetics of the
solid-phase reaction of Al (SO ) ꢀ18H O. The A ! B þ C
2
4 3
2
s
s
g
[
21]
was expressed by the following formula
:
Figure 2. TG-DSC curves of aluminum sulfate eighteen-hydrate dehydration-
decomposition process.
dx
¼
kgðxÞ
(5)
dt
The relationship between k and reaction temperature T
(thermodynamic temperature) can be expressed using the
annotated Arrhenius equation:
ꢀ
ꢁ
Ea
k ¼ A exp ꢂ
(6)
RT
ꢂ1
Where A is the pre-factor, min ; E is the activation
a
ꢂ1
ꢂ1 ꢂ1
energy, kJꢀmol ; R is the molar gas constant, kJꢀmol ꢀK
;
and T is the temperature, K.
The dynamic formula using the integral method can be
expressed as follows:
ð
ð
ꢀ
ꢁ
Figure 3. DTG curve of pyrolysis system of 18-water aluminum sulfate.
1
A
Q
Ea
x
0
T
Ti
GðxÞ ¼
dx ¼
exp ꢂ
dT
(7)
gðxÞ
RT
3
6.1% and three SO molecules were lost, corresponding to
3
The reaction was carried out at a constant heating rate (Q),
Q ¼ dT/dt, binding formula (7). According to the Arrhenius
formula, the integral kinetic formula was treated by the Coats-
the endothermic peak of the DSC curve capping tempera-
ꢁ
ture at 857 C, and the residue was mainly Al O . The ther-
2
3
[
19–20]
mal decomposition process was derived as follows
:
[
22–24]
Redfern method,
obtained.
and the integral kinetic formula was
Step 1:
ꢂ
ꢀ
ꢁꢃ
gðxÞ
AR
2RT
1 ꢂ E
Ea
Al ðSO Þ ꢀ 18H O ! Al ðSO Þ ꢀ 3H O þ 15H OðgÞ
ln
¼ ln
ꢂ
(8)
2
4 3
2
2
4 3
2
2
2
T
QEa
RT
a
(1)
For most reactions, 2RT/E is far less than 1. Therefore, a ¼
a
ln[AR/QE ꢀ (1 –2RT/E )] can be regarded as a constant. Let y ¼
a
a
Step 2:
2
ln[g(x)/T ], a¼ ln[(AR)/(QE )], b ¼ –E /R, x¼ 1/T, and the for-
a
a
Al ðSO Þ ꢀ 3H O ! Al ðSO Þ þ 3H OðgÞ
(2) mula (8) can be simplified to
2
4 3
2
2
4 3
2
y ¼ a þ bx
(9)
Step 3:
The corresponding function image of formula (9) is a
2
line of ln[g(x)/T ], a is the intercept of the line, b is the
Al2ðSO4Þ ! Al2O3 þ 3SO3ðgÞ
(3)
3
slope of the line, and the dependent variable is a line with
2
an independent variable of 1/T, and shown to be ln[g(x)/T ].
Taking a series of points on the TG curve, the corresponding
x and y values can be obtained according to the thermal
parameters of each point. The x and y values obtained by lin-
ear regression analysis can be used as a straight line. The slope
of the line is b, and the intercept is a. Activation energy Ea
and the pre-exponential factor A can be reverse-calculated.
The function g(x) is determined by the reaction mechanism,
Thermal decomposition kinetics of Al (SO ) ꢀ18H O
2
4 3
2
ꢁ
ꢂ1
At a temperature rate of 10 Cꢀmin , the temperature was
ꢁ
raised to 1200 C for 20 min, the quality of the pyrolysis
process was automatically recorded, and the continuous dif-
ferential thermogravimetry (DTG) curves of the samples
were obtained. Figure 3 was the differential weight-loss
DTG curve of the thermal decomposition of aluminum sul- particle size, and particle morphology. This function is a theor-
ꢁ
ꢂ1
fate at different stages in the heating rate of 10 Cꢀmin
.
etical model for studying the kinetics of the solid-state reaction
By means of thermogravimetric analysis (TGA), the kin- and a function in determining the reaction mechanism. The
etics of thermolysis of aluminum sulfate in different stages kinetic parameters and mechanism of the reaction must be
was studied using the dynamic mode. The conversion rate predicted by using the theoretical kinetic model and the TG

4
G. ZHENG ET AL.
data obtained from the thermogravimetric analysis experiment. is evenly distributed inside the solid particles, which causes the
In order to represent the dynamics of mass loss at each stage particle density to uniformly change. In addition to this reac-
of the reaction and its model, the function g(x) should have a tion mechanism, boundary controlled reactions or the shrink-
higher linear correlation coefficient.
ing core model (SCM) (R1 and R2 models) are commonly
[28]
If the reaction kinetics of the pyrolysis process is applied to describe the solid-state reactions. The SCM shows
described accurately, it is difficult to achieve a single kinetic that the reaction occurs only on the surface of spherical react-
model for Al (SO ) ꢀ18H O. Due to the large number of ant particles and that the reaction is subject to the un-reacted
2
4 3
2
solid-state reaction kinetic models, there are 20 correspond- surface area or remaining amount of reactant.
[
25]
ing solid-state reaction mechanism functions.
Different
In the common dynamic model, the correlation coeffi-
model functions (9) were fitted by the computer and cient is close to 1, and the three higher kinetic model func-
selected optimally. The function with correlation coefficient tions are selected, as shown in Table 2. The simulation
close to one and low activation energy was selected as the results were shown in Figure 4. The kinetic parameters of
[
26–27]
mechanistic model of the reaction.
the reaction were calculated in Table 3. Table 3 showed that
It was possible to estimate the reaction mechanisms of the fitting degree was more than 98%, and the calculated
thermal reduction from the thermal gravimetric (TG) results can describe the thermal decomposition process of
curves, by means of these functions. In most dynamic aluminum sulfate. The reaction orders for the three reaction
research that uses thermogravimetric analysis (TGA), the stages were different. With the increase of the reaction coef-
first-order chemical reaction or homogeneous model (HM) ficient, the pyrolysis process reached a later stage, the weight
(A1 model) assumption is most frequently used. The HM loss rate increased, and the reaction time was longer. At the
supposes that the particle size remains constant and the activity same time, it was found that the differences in activation
energy among the three reaction stages was significant and
Table 2. Expression of g(x) for the kinetic model function for solid- showed an increasing trend, which further indicated that the
state reactions.
temperature in the late stage of pyrolysis had a dominant
effect on the weight loss rate of the reaction.
Tab
Symbol
g(x)
Chemistry reaction
Phase boundary
controlled reactions
First-order
Two dimensions
(Contracting Cylinder)
Three dimensions
Contracting Sphere)
A1
R2
-ln(1-x)
1-(1-x)
The calculated activation energy, E , and pre-exponential
1
/2
a
factor, A, of all samples, for the various reaction stages, were
listed in Table 4. It can be seen that the lowest and highest val-
1
/3
R3
1-(1-x)
(
ues of activation energy (E ) for the first reaction stage were
a
2
Figure 4. Plot of ln[g(x)/T ] against 1/T of (a) First stage dehydration, (b) Second stage dehydration, (c) Aluminum sulfate decomposition.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
5
Table 3. Kinetic parameters of all samples obtained at three different systems.
ꢁ
ꢁ
ꢂ1
Stage
T( C)
T
max( C)
tmax(min)
DTGmax(%/min)
1.7915
Model
E
a
(kJ/mol)
A(min
)
R-squre
First stage dehydration
87-250
280-414
770-900
127
327
857
11
A1
R2
R3
A1
R2
R3
A1
R2
R3
90.02
91.41
92.45
215.36
209.58
205.75
284.40
314.00
321.49
4.16E þ 08
3.01E þ 08
2.96E þ 08
3.02E þ 15
4.13E þ 14
1.24E þ 14
1.38E þ 10
1.16E þ 11
2.01E þ 11
0.9894
0.9909
0.9970
0.9817
0.9995
0.9702
0.9879
0.9985
0.9939
Second stage dehydration
31
84
1.6227
1.3243
Aluminum sulfate decomposition
Table 4. Main chemical composition of raw materials.
Constitute
SiO
49.41
2
Al
2
O
3
Fe
2
O
3
TiO
1.75
2
CaO
0.16
MgO
0.05
Mass percentage
43.08
4.37
Figure 6. XRD pattern of pyrolysis products of aluminum sulfate at different
temperatures.
for all systems. However, models A1, R3, A1 may be more
suitable to describe the three reaction stage mechanisms
used here, respectively.
Figure 5. XRD pattern of pyrolysis products of aluminum sulfate at different
temperatures.
obtained by mode A1 and R3, respectively, the second reaction
stage were obtained by mode R3 and A1, respectively, and the
third reaction stage were obtained by mode A1 and R3,
Characteristics of pyrolysis products at different stages
At a scanning voltage of 36 kV, a current of 40 mA and a
ꢁ
scanning step of 2 /min, the slag phase was detected by D/
respectively. The values of E for all samples varied with differ-
a
max-3B X-ray diffractometer. The pyrolysis products of the
60 min system at different temperatures were analyzed by
XRD. The results were shown in Figure 5.
ent pyrolysis products at different stages. For the three reaction
ꢂ1
ꢂ1
stages, the lowest values (90.02 kJꢀmol , 205.75 kJꢀmol , and
ꢂ1
2
84.40 kJꢀmol , respectively) were detected by models A1, R3,
As can be seen from Figures 2 and 5, the characteristic
peak of Al (SO ) ꢀ18H O decreased gradually with the tem-
and A1, respectively. Xu studied the kinetic mechanism
model of aluminum sulfate dehydration based on DTA/
TGA. The activation energy of the two stages of dehydration
2
4 3
2
perature increased, mainly due to the accelerated dehydration
rate caused by the increase of temperature. Aluminum sulfate
lost free water on the surface first, and then lost interlayer
chemisorbed water. It can be seen from the diagram that
ꢂ1
ꢂ1
[15]
was 80.27 kJꢀmol , and 248.82 kJꢀmol , respectively.
However, Wu also studied the decomposition kinetics of
aluminum sulfate crystals. The activation energy of the three
ꢁ
Al (SO ) still contained crystalline water at 400 C. When the
2
4 3
ꢂ1
ꢂ1
stages of dehydration was 94.72 kJꢀmol , 200.58 kJꢀmol
,
ꢁ
temperature increased to 425 C, the last crystalline water mol-
ecule between the molecules of Al (SO ) has been completely
ꢂ1
[29]
and 318.16 kJꢀmol
,
respectively.
Comparing our
2
4 3
research results with these of others, this would be closer to
Wu’s data.
ꢁ
removed. At a temperature of 450 C, although the product
was still anhydrous aluminum sulfate, there were diffraction
In conclusion, when comparing with the calculated kin-
etic parameters of the mixtures, it can be clearly observed
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
peaks at 2h ¼ 15.1 , 21.3 , 25.6 , 34.1 , and 34.8 .
ꢁ
For further analysis, the calcined products at 800 C,
that the value of E obtained at the first reaction stage was
ꢁ
ꢁ
a
850 C and 900 C were analyzed by XRD. The results were
shown in Figure 6. The results showed that when the tem-
perature increased to 800 C, amorphous phase material,
lower than the values obtained at the second reaction stage,
ꢁ
and the value of E obtained at the second reaction stage
a
was lower than the values obtained at the third reaction that was, amorphous Al O , was the main reason for the
2
3
ꢁ
stage, indicating that the thermal decomposition of alumi- disappearance of Al (SO ) . At 850 C, the disordered peak
2
4 3
num sulfate increased with an increase of temperature, and changed to the regular peak, and the result showed that the
the activation energy and decomposition difficulty increased. amorphous phase was the main reason for the disappearance
2
ꢁ
As shown in Figure 4, all models showed high R (> 0.98), of Al (SO ) at 850 C. The characteristic peak of crystalline
2
4 3

6
G. ZHENG ET AL.
ꢁ
phase appeared at 900 C, but it was not obvious. Therefore, observed. As can be seen from Figure 8a, there were two
ꢁ
the calcined products at 950 C were analyzed by XRD, and main types of particle morphology: one was the aggregate
the results were shown in Figure 7. As can be seen from of spherical particles of different sizes, where the diameter
Figure 7, the crystalline form of c-Al O has been generated was nanometer to sub-micron; the other was small hex-
2
3
ꢁ
by decomposition at 950 C.
In Figure 8, the (a), (b), (c) images of the calcined prod- sized and adhere to the surface of the spherical particles.
agonal flake particles, which were sub-micron to micron-
ꢁ
ꢁ
ꢁ
In addition, it was found that the particles showed a densi-
fication trend, mainly because the crystalline water in the
crystalline aluminum sulfate was removed and the volume
of the particles decreased. Figure 8 showed the average
particle size in (a) was no much larger than 45 lm.
Therefore, it was possible to produce a product with par-
ticle size meeting the requirements of metallurgical sand
alumina by preparing crystalline aluminum sulfate with
appropriate size, and then controlling the corresponding
dewatering process conditions.
ucts at 425 C, 800 C, and 950 C, respectively, were
It can be seen from (b) in Figure 8 that when the tem-
ꢁ
perature rose to 800 C, clear small block-like particles
appear, the number was large and no agglomeration
occurred, and the cells were scattered and began to transi-
tion from amorphous alumina to c-Al O . As the calcination
2
3
ꢁ
ꢁ
Figure 7. XRD pattern of calcined product at 950 C.
temperature rose to 950 C, long vertebral, octahedral and
ꢁ
ꢁ
ꢁ
Figure 8. SEM image of pyrolysis products at different temperatures (a) 425 C, (b) 800 C, (c) 950 C.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
7
small granular c-Al O appeared, as shown in (c) of Figure Scanning electron microscopy
2
3
8
, which was studied with Yan and others by XRD and The micromorphology and particle size of the samples were
SEM. The crystal structure and microstructure of alumina observed using a Quanta-200 scanning electron microscope
[
30–31]
were consistent at different temperatures.
made by FEI Company (magnification: 30–100 000).
Conclusions
Experimental and methods
During the calcination and decomposition of Al (SO ) ꢀ18H O,
Materials
2
4 3
2
the dehydration was carried out in two steps and the decom-
position process was completed in one step. A spherical inter-
facial reaction model were developed with activation energies
of 90.02 kJ/mol, 205.74 kJ/mol, and 284.40 kJ/mol, and were
detected by models A1, R3, and A1, respectively. After the
dehydration of aluminum sulfate, the transformation of alu-
The raw material used in the experiment was high alumina
coal gangue, and its main chemical composition was shown
in Table 4. Sulfuric acid concentration of 98% (analytical
pure) was also used.
ꢁ
mina crystal began. At 800 C, amorphous phase material
Methods
appeared, and then changed to a crystalline phase substance at
ꢁ
8
50 C, and then the characteristic peak of crystalline phase
Experimental methods
After fine grinding and removal of iron from high alumina
coal gangue, aluminum sulfate solution was obtained by
ꢁ
material appeared at 900 C, but it was not obvious. When the
temperature increased to 950 C, the crystalline form of
ꢁ
ꢁ
c-Al O was generated. During the pyrolysis of aluminum sul-
2
3
reacting with 98% concentrated sulfuric acid at 240 C for
ꢁ
fate, the morphology of aluminum sulfate changed from lamel-
lae to block and then to vertebra. It was possible to prepare
sand alumina with appropriate particle size using the dewater-
ing process of aluminum sulfate, which provided a new way
and research method for the preparation of metallurgical alu-
mina by crystallizing aluminum sulfate. Meanwhile, the adapt-
ability of raw materials is wide.
1
h, cooling to about 100 C, and adding water for solvation
and filtering. High purity aluminum sulfate crystals were
obtained by evaporation filtrate crystallization at 110 C.
After filtration, the mixture was dried at 65 C. The analyt-
ꢁ
ꢁ
ical phase composition was by XRD, the contents of Al and
S were analyzed by XRF, and the number of crystallized
water molecules was analyzed using a thermogravimetric
method. It was confirmed that it was Al (SO ) ꢀ18H O.
2
4 3
2
Al (SO ) ꢀ18H O (approx 1 g) was placed in corundum Funding
2
4 3
2
magnetic boat, and then it was then placed in the constant
temperature zone of the high temperature tube furnace filled
The authors thank the National Natural Science Foundation of China
(
No. 21566018) and the Key research and development projects in the
with argon, in advance, and having reached a predetermined Ningxia Hui Autonomous Region (No. 2018BEE03021).
temperature. It was taken out and cooled to the room tem-
perature in a dryer after reaching the reaction time. After
weighing the residue, it was ground through 180 mesh
screen and the screen margin was less than 5%, which was
ORCID
Zhengjie Chen
http://orcid.org/0000-0002-1519-275X
used as the raw material for analysis and characterization.
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