minerals
Article
Kinetics of Iron Leaching from Kaolinitic Clay,
Using Phosphoric Acid
Román A. Hernández-Hernández 1 , Felipe Legorreta-García 1, *, Leticia Esperanza Hernández-Cruz 1 ,
Edgar A. Chavez-Urbiola 2 and Eleazar Salinas-Rodríguez 1
1
2
*
Laboratorio de Tecnología de Cerámicos, Universidad Autónoma del Estado de Hidalgo,
Área Académica de Ciencias de la Tierra y Materiales, Carretera Pachuca—Tulancingo, Km 4.5 s/n,
Mineral de la Reforma, Hidalgo C.P. 42184, Mexico; [email protected] (R.A.H.-H.);
[email protected] (L.E.H.-C.); [email protected] (E.S.-R.)
Consejo Nacional de Ciencia y Tecnología (CONACyT)—Universidad Autónoma del Estado de Hidalgo,
Área Académica de Ciencias de la Tierra y Materiales, Carretera Pachuca—Tulancingo, Km 4.5 s/n,
Mineral de la Reforma, Hidalgo C.P. 42184, Mexico; [email protected]
Correspondence: [email protected]; Tel.: +52-771-717-2000 (ext. 2297)
Academic Editor: William Skinner
Received: 3 May 2016; Accepted: 15 June 2016; Published: 24 June 2016
Abstract: Kaolin is important because it has many applications in the paper, paint-coating, functional
filler, extender and pigment industries. This material needs to be bleached by iron leaching to
be useful for these applications because this element can be the principal cause of its undesirable
coloration. In this work, the kinetics of leaching of iron contained in kaolin clays from the community
of Agua Blanca Iturbide, Hidalgo, Mexico, was studied. The leaching experiments were carried
out using phosphoric acid as the reagent of leaching for the iron dissolution process. Temperature,
acid concentration, and particle size were investigated to determine the most important kinetics
parameters during iron leaching. It was determined that the rate of iron dissolution increases with
increasing of phosphoric acid concentration, the temperature, and decreased particle size of clay.
Data obtained showed that iron dissolution from kaolin clay is due to the diffusion of iron through
the product layer. The results revealed that the activation energy of the process was 10.18 kJ¨ mol´1 .
Keywords: iron; leaching; kaolinitic clay; kinetics; phosphoric acid
1. Introduction
The term kaolin has a different meaning to geologists, mineralogists, and ceramists. To geologists,
is a rock comprised primarily of one of the kaolin groups of clay minerals. To mineralogists, the
term refers to the group for the kaolin minerals; namely kaolinite, halloysite, dickite, and nacrite.
To ceramists, the term is synonymous with “china clay” meaning that it is a white ceramic raw material.
Industrially, the term refers to a clay consisting of substantially pure kaolinite, or related clay minerals.
They are of natural origin, or can be beneficiated to reach white or nearly white color. Its beneficiation
by known methods is desirable to make it suitable for use in a white product such as paper, rubbers,
paints, etc. [1,2].
Clays possess organic matter, generally lignite, as well as other colloidal minerals that confer
them its characteristic colors. The red clays have ferric oxide, in minerals such as hematite, magnetite,
and amorphous hydrated oxides, among others. Iron is the main contaminant in minerals of clay
and kaolin. The removal of the iron from kaolin is of particular importance in the paper industry,
among others, where purity requirements are high. In these minerals, iron may or may not form
part the kaolinite crystalline lattice. When iron is inside the lattice, low concentrations do not affect
coloration [3]. The iron oxyhydroxides are often deposited along with the kaolinite, contaminating
Minerals 2016, 6, 60; doi:10.3390/min6030060
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Minerals 2016, 6, 60
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and making much of the kaolin unusable for commercial applications due to the leaking of whiteness.
On the other hand, other secondary minerals accompanying the clay and kaolin such as hematite,
maghemite, and pyrite, iron can be removed by magnetic separation [4,5]. According to Mexican
Geological Service studies, the kaolin reserves in the state of Hidalgo exceed 25 million tons; those
of the neighboring states of Puebla and Veracruz are unquantified. Conduct leaching studies for
bleaching kaolin in this region opens the way to applied research with the aim that in the medium
term can be an impetus for the economic development of these localities currently considered to be in
high poverty.
Different inorganic and organic acids or complexing reagents have been used in the dissolution of
iron from these kinds of kaolin minerals. Most of these are focused on the mechanism of dissolution on
hematite and magnetite, testing different acids and experimental conditions [6,7]. Different researchers
have used oxalic acid for the dissolution of iron in the kaolin [6–10]. Early works, phosphoric acid was
reported as leaching iron in kaolin clay [11]. Similarly, the influence of phosphoric acid in iron removal
from quartz sand bleaching was also studied [12]. Also, statistical analysis was conducted to determine
the most influential factors in the dissolution with organics acids used as a leaching reagent to remove
iron from a kaolin mineral [13,14]. Recently, the general concepts and fundamentals of the biological
dissolution of iron from kaolin clays had been studied by Hosseini et al. [15]. Among the organic and
inorganic acids [6–13,16], the biological beneficiation, [15,17,18] a combination of both [19–21] and the
anionic surfactants, the first are the most common reagents to remove iron oxides and oxyhydroxides
contained in kaolin clay [22]. However, goethite species have been removed with anionic surfactants,
increasing the whiteness index [23]. The advantages of using phosphoric acid as the leaching agent
if compared with other acids are: the whiteness index that is achieved is higher, they have rapid
dilution in water, and simple operation as well as ease of adding directly to the pulp. Bioleaching is
an environmentally friendly method and it is also an alternative; however, bacteria are usually quite
sensitive to changes in the environment in which they develop. It is crucial to consider some factors
such as atmosphere, temperature, and pH which leads to a more complex process.
Considerable attention has been to the studies of mechanisms of dissolution, which employ
synthetic minerals such as hematite, goethite, and magnetite and ignoring the complex interactions
that can take place in iron dissolution from industrial minerals such as kaolin. The contribution of
kinetics data for industrial-use minerals like kaolin clays it is of high importance as they are clays with
complex interactions, variable iron content, and in other words very different each other due to the
different geological origin they have. The kinetic data for the iron extraction from kaolin clay using
phosphoric acid are very important for industrial applications such as the supply of raw material for
the industry of paper and the ceramic. The aim of this work was to examine the iron leaching kinetics
from kaolin clay using phosphoric acid solutions.
2. Experimental
For performing the experiments, Kaolin from the city of Agua Blanca Iturbide, Hidalgo State,
Mexico, was used. This mineral was firstly grinded and classified by particle size. For the experimental
procedure, particles of average size of 35 µm were used. To characterize the mineral samples
(Mineralogy and composition) X-ray diffraction (XRD) and Atomic Absorption Spectroscopy (AAS)
were used.
The leaching experiments were carried out in a round bottom reactor (500 mL) with mechanical
stirring and constant temperature. The temperature was regulated with a thermo-regulator, which is
part of the equipment. In the leaching tests at 94 ˝ C, the reactor, equipped with a thermometer and
a reflux condenser, was heated with a thermostatically controlled heating mantle. All leaching tests
were made at atmospheric pressure for 120 min.
Two different experiments were carried out. Firstly, the effect of the concentration (0.1, 0.5, 1.0,
and 3.0 M) of the phosphoric acid at a fixed temperature of 367.15 K (94 ˝ C) was evaluated, adding the
kaolin to the solution. Secondly, the acid concentration was fixed at 3.0 M varying the temperature in
3. Results and Discussion
The results found through mineralogical and chemical analysis of the kaolin are shown in
Table 1, Figures 1 and 2. The chemical composition is shown in Table 1. The values of SiO2, Al2O3 and
low contents of Fe3O4, indicates that this material has an ideal composition for the leaching kinetic
Minerals 2016, 6, 60
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study. Figure 1 shows the X-ray diffraction spectrum; for (a) natural kaolin mineral and (b) leached
kaolin; upon a comparison of both diffractograms, it is possible to appreciate the reduction of the
between 273
353oxyhydroxide
K. For both experiments,
thepeak
pH was
kept constant
at 1.0 adding
small
amounts
compound
ofto
iron
(FH). A major
corresponding
to kaolinite
can be
appreciated,
of saturated
NH4 OH
or H2in
SOthe
solutions.
Early
works
confirmed
that
optimum
dissolution
rate is
with
minor signals
of silica
forms
of
quartz
and
tridymite.
4
at a pH value of 1, approximately [1]. The progress of extraction reaction was carried out by taking
Table 1.the
Chemical
Analysisand
of the
kaolinfor
sample.
samples from solution throughout
experiments,
testing
Fe content by atomic absorption
spectrometry. Variations in the mass balance due to the sampling take and the addition of reagent
Components
wt %
were taken into account for the calculations.
SiO2
65.00
Al2O3
27.00
3. Results and Discussion
Fe3O4
0.70
The results found through mineralogical
analysis of the kaolin are shown in Table 1,
CaO and chemical
0.04
Figures 1 and 2. The chemical composition
is
shown
in
Table
MgO
0.57 1. The values of SiO2 , Al2 O3 and low
contents of Fe3 O4 , indicates that this material
has
an
ideal
composition for the leaching kinetic study.
Na2O
2.65
Figure 1 shows the X-ray diffraction spectrum;
K2O for (a) natural
2.42 kaolin mineral and (b) leached kaolin;
upon a comparison of both diffractograms,
it
is
possible
to
appreciate the reduction of the compound
TiO2
0.67
of iron oxyhydroxide (FH). A major peakLIO*
corresponding0.95
to kaolinite can be appreciated, with minor
signals of silica in the forms of quartz and
tridymite.
LIO*
= Loss of ignition.
FH= [99-200-3045] HFeO2 Iron oxide hydroxide
K= [99-101-0871] Al2Si2O9H4 Kaolinite
K
K
K
K
a)
K
K
K
K
Q
K K
FH
FH
Q
K
K
K K
FH
b)
11
12
13
14
15
16
17
18
19
20
26
27
28
29
30
31
32
33
34
35
36
37
2 Theta
−1
Figure
kaolin mineral
mineral and
and (b)
(b) leached
leached kaolin
kaolin at
at373
373K,
K,3.0
3.0mol¨
mol·L
Figure 1.
1. X-ray
X-ray diffractogram
diffractogram of
of (a)
(a) kaolin
L´1 of
of
phosphoric
acid,
pH
1.0,
and
average
particle
size
of
35
μm.
phosphoric acid, pH 1.0, and average particle size of 35 µm.
Minerals 2016, 6, 60
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3.0 M
1.0 M
0.5 M
0.1 M
100
Fe Diffusion (%)
80
60
40
20
0
0
20
40
60
80
100
120
Time (Min)
Figure
2. Effects
of phosphoric
acid
molar
concentration
in Fe
diffusion
through
time
(0.10,
0.50,
1.01.0
Figure
2. Effects
of phosphoric
acid
molar
concentration
in Fe
diffusion
through
time
(0.10,
0.50,
andand
3.03.0
M)M)
(pH
1.0,
373
K,
35
μm).
(pH 1.0, 373 K, 35 µm).
For this work, the phosphoric acid has been used as leaching reagent due to their theoretical
advantage for this use. Regarding the iron leaching, hydrogen ions play an important role in the
dissolution of iron oxides with inorganic acids. The H3PO4 in solution provides H+, that reacts with
iron oxide, and the possible reaction according Zhang et al. [12], might be:
6H + + Fe 2 O 3 → 2Fe 3+ + 3H 2 O
(1)
These hydrogen ions are adsorbed on the activated surface of the iron oxyhydroxide species.
Minerals 2016, 6, 60
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Table 1. Chemical Analysis of the kaolin sample.
Components
wt %
SiO2
Al2 O3
Fe3 O4
CaO
MgO
Na2 O
K2 O
TiO2
LIO*
65.00
27.00
0.70
0.04
0.57
2.65
2.42
0.67
0.95
LIO* = Loss of ignition.
For this work, the phosphoric acid has been used as leaching reagent due to their theoretical
advantage for this use. Regarding the iron leaching, hydrogen ions play an important role in the
dissolution of iron oxides with inorganic acids. The H3 PO4 in solution provides H+ , that reacts with
iron oxide, and the possible reaction according Zhang et al. [12], might be:
6H` ` Fe2 O3 Ñ 2Fe3` ` 3H2 O
(1)
These hydrogen ions are adsorbed on the activated surface of the iron oxyhydroxide species. The
hydrogen ions are adsorbed on sites of the solid surface, thus creating a surface of active centers on
which the main reaction of dissolution takes place. As the hydrogen ion concentration in the solution
increases, the amount of adsorbed hydrogen ions also increases, according to the adsorption theory. An
increase in the number of active centers results in a corresponding increase in the dissolution rate [3].
The H3 PO4 not only provides more H+ ions (pKa1 = 2.12), which will react with iron oxide; but
also provides PO4 3´ ions, which are produced while the H+ is being ionized, and also have a larger
complexing ability to iron ions. All these features lead to high leaching percentages of the H3 PO4 .
The possible reaction according to Zhang et al. [12], might be:
8H` ` 4PO4 3´ ` Fe2 O3 Ñ rFe pHPO4 q2 s´ ` rFe pPO4 q2 s3´ ` 3H2 O
(2)
These multiple synergisms of the H3 PO4 , lead to the best leaching results [2].
4. Kinetic Study
The results obtained during kinetics study of iron extraction, are related with the effect of
phosphoric acid concentration and temperature in the range 298 to 367 K and were treated with
the shrinking core model, for heterogeneous reactions both for diffusive and chemical control.
Extraction of iron from kaolin clay can be explained with the shrinking core model [4], in which:
aAFluid ` bBParticle Ñ Product
(3)
If diffusion controls the reaction rate through an inert product layer, the integrated rate equation
is as follows:
2
2
1 ´ X ´ p1 ´ Xq 3 “ Kd t
(4)
3
The above equation represents the shrinking core model for diffusive control. Additionally, if the
reaction is controlled by a chemical reaction, Equation (4) is transformed to Equation (5) [24].
1
1 ´ p1 ´ Xq 3 “ Kc t
(5)
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where X is the fraction that has reacted, t is the reaction time and Kd and Kc are the rate constants,
which are calculated from Equations (4) and (5), respectively.
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5. Effect of the Acid Concentration
whereInXorder
is thetofraction
reacted,
t is the reaction
time and Kin
Kc areofthe
rate
constants,
d and
observethat
the has
effect
of phosphoric
acid concentration
the range
0.10,
0.50,
1.0 and
which
are
calculated
from
Equations
(4)
and
(5),
respectively.
3.0 M, experiments were performed in the temperatures ranging of 298 to 367 K. The results are
shown in Figure 2. It shows that increasing phosphoric acid concentration increases the iron
5. Effect of the Acid Concentration
dissolution rate in the kaolin clay, especially in lower concentrations.
In
to observe
thethat
effect
phosphoric
concentration
the range
1.0 and
It order
was also
observed
theofleaching
rateacid
of iron
at 1.0 M isinhigher
thanofat0.10,
0.100.50,
M; the
iron
3.0
M,
experiments
were
performed
in
the
temperatures
ranging
of
298
to
367
K.
The
results
are
shown
extractions were of 98.65% and 19.81%, respectively.
in Figure
It shows
thatinincreasing
concentration
increasesstudy
the iron
rate
The 2.
results
found
this studyphosphoric
are in goodacid
accordance
with another
[2], dissolution
where the high
in
the
kaolin
clay,
especially
in
lower
concentrations.
efficiency of iron removal was reported; but unlike our study, this work was carried out in quartz
It was
observedacid.
that the leaching rate of iron at 1.0 M is higher than at 0.10 M; the iron
sands,
usingalso
phosphoric
extractions
were of rate
98.65%
and 19.81%,
respectively.
The apparent
constants
Kd, and
Kc obtained from Equations (4) and (5), and the regression
The
results
found
in
this
study
are
in
good accordance
with anotherThe
study
[2], where
the high4
coefficients, were calculated from the graph
at each concentration.
graph
of Equation
efficiency
iron straight
removallines
waswith
reported;
butregression
unlike our
study, this
carried
out in quartz
(Figure 3a)ofyield
a global
coefficient
R2 work
equal was
to 0.99
for phosphoric
acid
sands,
using
phosphoric
acid.
concentrations of 3.0 M. The Figure 3b displays the graph of the Equation (5), it exhibits straight lines
apparent rate
constants
Kd , andthan
Kc obtained
from Equations
(4) and (5),
andlinearity.
the regression
withThe
a regression
coefficient
R2 higher
0.98 showing
a slight deviation
from
These
coefficients,
were calculated
at each concentration.
Theis graph
of Equation
(4)
values of regression
coefficientfrom
showthe
thatgraph
iron dissolution
from kaolin clay,
controlled
by diffusion
2 equal to 0.99 for phosphoric acid
(Figure
3a)
yield
straight
lines
with
a
global
regression
coefficient
R
through an inert product layer.
concentrations
of 3.0
M. The
3b displays
the graphaccording
of the Equation
(5), it exhibits
straight
The obtained
slopes
forFigure
the graph
were calculated
to the model
1 − (2X/3)
− (1 − lines
X)2/3,
2
with
regression
coefficient
higher
than
0.98
a as
slight
from linearity.
values
fromawhich
a graph
of log(KRd) vs.
log[PO
43−]
wasshowing
obtained
candeviation
be seen further
in FigureThese
4. A straight
of
regression
coefficient
show
that
iron
dissolution
from
kaolin
clay,
is
controlled
by
diffusion
through
line from this graph gave a slope from where was calculated a reaction order of n = 0.168, according
an
inertEquation
product (4)
layer.
to the
and that is shown in Figure 3.
0.8
3.0 M
1.0 M
0.5 M
0.1 M
0.6
3.0 M
1.0 M
0.5 M
0.1 M
0.30
0.25
Model 1-(1-X)1/3
Model 1-(2X/3)-(1-X)
2/3
0.7
0.5
0.4
0.3
0.2
0.1
0.20
0.15
0.10
0.05
0.00
0.0
10
20
30
40
Time (min)
(a)
50
60
10
20
30
40
50
60
Time (min)
(b)
Figure 3. Graph of 1 − (2X/3) − (1 − X)2/3 (a) and 1 − (1 − X)2/3 (b) vs. time for the iron dissolution from
Figure 3. Graph of 1 ´ (2X/3) ´ (1 ´ X)2/3 (a) and 1 ´ (1 ´ X)2/3 (b) vs. time for the iron dissolution
kaolin
clayclay
at different
acid(0.10,
(0.10,
0.50,
3.0(pH
M)1.0,
(pH
from
kaolin
at differentconcentrations
concentrationsof
of phosphoric
phosphoric acid
0.50,
1.0 1.0
and and
3.0 M)
3731.0,
K,
373
K,
35
μm).
35 µm).
6. Effect of Temperature
The obtained slopes for the graph were calculated according to the model 1 ´ (2X/3) ´ (1 ´ X)2/3 ,
´ ] temperature
aim ofof
determining
effect4 3of
kinetics
of further
the ironinextraction,
from With
whichthe
a graph
log(Kd ) vs. the
log[PO
was obtainedinasthe
can
be seen
Figure 4.
several
experiments
in an
acid gave
solution
0.3 M
were
performed,
varying athe
temperature
in nthe
range
A
straight
line from this
graph
a slope
from
where
was calculated
reaction
order of
= 0.168,
from
298
to
373
K.
Typical
rate
curves
with
a
strong
temperature
dependence
were
found.
It
denoted
according to the Equation (4) and that is shown in Figure 3.
a very slow rate in the temperature range of 273 to 353 K exhibiting a very strong growth rate above
353 K. This suggests that it is reasonable to use temperatures above 353 K. That means that iron
dissolution using phosphoric acid could be thermally activated to improve its efficiency. The results
in Table 2 show that iron dissolution from kaolin clay in the temperature range of 298 to 373 K, fit
well with the diffusion through an inert product layer model shown in Equation (4).
corresponding regression coefficients were calculated from the graph at each temperature tested.
These values were treated with Equation 4, and the resulting data is shown in Figure 4. The analysis
of these graph revealed straight lines for 298, 313, 333, 353 and 373 K with regression coefficients (R2)
of around 0.99 in average. It is important to mention that the results obtained from the graph of the
temperature effect were also analyzed by chemical control model (Equation (5)) and the data did not
Minerals
60 this model. Therefore, it can be dismissed that the controlling stage being the chemical 6 of 8
fit2016,
well6,with
reaction [25].
-2.20
-2.25
Log Kd
-2.30
-2.35
-2.40
-2.45
-2.50
1.8
2.1
2.4
2.7
3.0
3.3
3.6
Log[PO-3
]/mol L-1
4
Figure 4. Graph of log(Kd) vs. log[PO43−]/mol·L−1 showing the order of reaction of n = 0.168.
Figure 4. Graph of log(Kd ) vs. log[PO4 3´ ]/mol¨ L´1 showing the order of reaction of n = 0.168.
The apparent rate constants, Kd, for iron dissolution were calculated from the slopes in the graph
6. Effect
of Temperature
of Figure
5. On the other hand, a graph was made, showing the variation of lnKd vs. 1000/T where T
is thethe
temperature
in the rangethe
of 298
to 373
K and from which
theof
straight
lineextraction,
shown in the
With
aim of determining
effect
of temperature
in theyields
kinetics
the iron
several
2 was 0.99. The apparent activation energy calculated from the
Figure
6.
The
regression
coefficient
R
experiments in an acid solution 0.3 M were performed, varying the temperature in the range from
Arrhenius plot was, 10.18 kJ·mol−1. This result confirmed that iron dissolution from the kaolin clay is
298 to 373 K. Typical rate curves with a strong temperature dependence were found. It denoted a very
controlled by diffusion through an inert product layer. In fact, this low energy is considered within
slow rate in the temperature range of 273 to 353 K exhibiting a very strong growth rate above 353 K.
the stated range for the controlling step of diffusion through a layer of products (below
This suggests
20 kJ·mol−1that
) [6].it is reasonable to use temperatures above 353 K. That means that iron dissolution
using phosphoric acid could be thermally activated to improve its efficiency. The results in Table 2
show that iron dissolution from kaolin clay in the temperature range of 298 to 373 K, fit well with the
diffusion through an inert product layer model shown in Equation (4).
Table 2. Kd and Kc values and regression coefficients found for each temperature for a 3.0 M
concentration of phosphoric acid is displayed with an average size of 35 µm.
Temperature (K)
Kd ˆ 10´3 (s´1 )
R2
Kc ˆ 10´3 (s´1 )
R2
298
313
33
353
373
2.70
3.30
4.30
5.40
6.00
0.995
0.999
0.990
0.993
0.997
6.70
7.60
9.20
11.80
13.40
0.980
0.992
0.981
0.993
0.997
The apparent rate constants, Kd and Kc obtained from Equations (4) and (5) and their
corresponding regression coefficients were calculated from the graph at each temperature tested.
These values were treated with Equation 4, and the resulting data is shown in Figure 4. The analysis of
these graph revealed straight lines for 298, 313, 333, 353 and 373 K with regression coefficients (R2 )
of around 0.99 in average. It is important to mention that the results obtained from the graph of the
temperature effect were also analyzed by chemical control model (Equation (5)) and the data did not
fit well with this model. Therefore, it can be dismissed that the controlling stage being the chemical
reaction [25].
The apparent rate constants, Kd , for iron dissolution were calculated from the slopes in the graph
of Figure 5. On the other hand, a graph was made, showing the variation of lnKd vs. 1000/T where T
is the temperature in the range of 298 to 373 K and from which yields the straight line shown in the
Figure 6. The regression coefficient R2 was 0.99. The apparent activation energy calculated from the
Arrhenius plot was, 10.18 kJ¨ mol´1 . This result confirmed that iron dissolution from the kaolin clay is
controlled by diffusion through an inert product layer. In fact, this low energy is considered within the
stated range for the controlling step of diffusion through a layer of products (below 20 kJ¨ mol´1 ) [6].
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0,35
373K
353K
333K
373K
313K
353K
293K
333K
313K
293K
0,35
0,30
2/3
1-(2X/3)-(1-X)
ModelModel
1-(2X/3)-(1-X)
2/3
0,30
0,25
0,25
0,20
0,20
0,15
0,15
0,10
0,10
0,05
0,05
0,00
10
0,00
10
20
30
20
30
40
Time (Minutes)
40
50
60
50
60
70
70
Timefor
(Minutes)
Figure 5. Graph of 1 − (2X/3) − (1 − X)2/3 vs. time
iron dissolution from kaolin clay at different
2/3 vs. time for iron dissolution from kaolin clay at different
Figure 5.
Graph of 1(298–373
´ (2X/3)
´ (1 ´ X)
temperatures
(3.0 mol·L
of2/3
phosphoric
acid,
1.0, 35 μm).
Figure 5. Graph
of 1 −K)(2X/3)
− (1 −−1 X)
vs. time for
ironpH
dissolution
from kaolin clay at different
´1
temperatures
(298–373 K) (3.0 mol¨ L of phosphoric acid, pH 1.0, 35 µm).
temperatures (298–373 K) (3.0 mol·L−1 of phosphoric acid, pH 1.0, 35 μm).
-1
-1
(min
d
LnKLnK
(min
) )
d
-5.5
-5.5
-5.6
Ea=10.18KJ mol-1
-5.6
-5.7
Ea=10.18KJ
R2=0.9912 mol-1
-5.7
-5.8
R2=0.9912
-5.8
-5.9
-5.9
-6.0
-6.0
-6.1
-6.1
-6.2
-6.2
-6.3
-6.3
-6.4
-6.4
-6.5
-6.52.4
2.4
2.5
2.5
2.6
2.7
2.8
2.9
3.1
3.2
3.3
3.4
3.5
2.6
2.7
2.8
2.9 3.0 3.1
1000/T
(K-1)
1000/T (K-1)
3.0
3.2
3.3
3.4
3.5
3.6
3.6
Figure 6. Graph of the iron dissolution from kaolin clay (298–373 K), 3.0 M of phosphoric acid, and 35 μm.
Figure 6. Graph of the iron dissolution from kaolin clay (298–373 K), 3.0 M of phosphoric acid, and 35 μm.
Figure 6. Graph of the iron dissolution from kaolin clay (298–373 K), 3.0 M of phosphoric acid, and 35 µm.
7. Conclusions
7. Conclusions
In this work, the kinetics of iron dissolution from kaolin clay in phosphoric acid solution was
7. Conclusions
In this work, the kinetics of iron dissolution from kaolin clay in phosphoric acid solution was
studied. The results showed that the temperature, concentration of phosphoric acid, and particle size,
studied. The results showed that the temperature, concentration of phosphoric acid, and particle size,
of importance
for the
determination
of the
reaction
effect
of
In were
this work,
the kinetics
of iron
dissolution
from
kaolinkinetics.
clay in Importantly,
phosphoricthe
acid
solution
was
were of importance for the determination of the reaction kinetics. Importantly, the effect of
onshowed
iron dissolution
is temperature,
more significantconcentration
at 94 °C. Also, the
iron
dissolution
increased
with
studied.temperature
The
results
that
the
of
phosphoric
acid,
and
particle
size,
temperature on iron dissolution is more significant at 94 °C. Also, the iron dissolution increased with
acid
and temperature.
Leaching
data showed
that the iron
dissolution
from
were of phosphoric
importance
forconcentration
the determination
of the reaction
kinetics.
Importantly,
the
effect offrom
temperature
phosphoric acid
concentration
and temperature.
Leaching
data showed
that the iron
dissolution
kaolin clay occurs by diffusion through the
product layer, showing an order of reaction n = 0.168.
˝
kaolin clay occurs
by diffusion
through
showing
an orderincreased
of reaction with
n = 0.168.
on iron dissolution
is more
significant
at 94theC.product
Also, layer,
the iron
dissolution
phosphoric
Finally, the activation energy of the process was found to be 10.18 kJ·mol−1−1.
Finally, the activation
energy of the
process was
found
to be 10.18
. dissolution from kaolin clay
acid concentration
and temperature.
Leaching
data
showed
that kJ·mol
the iron
Acknowledgments:
The authors
to thank
the CONACyT
FOMIX
HGO ofof
thereaction
México Government
for Finally,
their
occurs by
diffusion through
the want
product
layer,
showing
an order
n = 0.168.
the
Acknowledgments: The authors want to thank the CONACyT FOMIX HGO of the México Government for their
financial support to project 192265.
´
1
activation
energy
of the
process
was found to be 10.18 kJ¨ mol .
financial
support
to project
192265.
Author contributions: Roman A. Hernández-Hernández performed the experiments as part of the doctoral thesis
Author contributions: Roman A. Hernández-Hernández performed the experiments as part of the doctoral thesis
work, Felipe Legorreta-García
performed
the chemical
and crystallographic
analysis,
Leticia
E. Hernández-Cruz
Acknowledgments:
The authors want
to thank
the CONACyT
FOMIX HGO
of the
México
Government for their
work, Felipe Legorreta-García
performed
the chemical
and crystallographic
analysis,
Leticia
E. Hernández-Cruz
Edgar to
A. project
Chavez-Urbiola
wrote the paper and Eleazar Salinas-Rodríguez advised for the kinetic studies.
financialand
support
192265.
and Edgar A. Chavez-Urbiola wrote the paper and Eleazar Salinas-Rodríguez advised for the kinetic studies.
Conflicts
The
declare
Author Contributions:
Roman
A.
Hernández-Hernández
performed the experiments as part of the doctoral thesis
ConflictsofofInterest:
Interest:
Theauthors
authors
declare no
no conflict
conflict of
of interest.
interest.
work, Felipe Legorreta-García performed the chemical and crystallographic analysis, Leticia E. Hernández-Cruz
and Edgar
A. Chavez-Urbiola wrote the paper and Eleazar Salinas-Rodríguez advised for the kinetic studies.
References
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