Materials Transactions, Vol. 50, No. 5 (2009) pp. 1225 to 1230
#2009 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Dissolution of Magnesium from Serpentine Mineral in Sulfuric Acid Solution
Kyoungkeun Yoo, Byung-Su Kim, Min-Seuk Kim, Jae-chun Lee* and Jinki Jeong
Minerals & Materials Processing Division, Korea Institute of Geoscience & Mineral Resources (KIGAM), Daejeon 305-350, Korea
As the volume of CO2 , one of greenhouse gases linked to global warming, in the atmosphere increases, there has been an increasing interest
in CO2 sequestration. Aqueous carbonation, which involves the extraction of Mg from serpentine minerals and the subsequent carbonation
reaction with CO2 to form the geologically stable mineral MgCO3 , has been proposed as a promising CO2 sequestration technology. This study
investigates the dissolution of Mg from serpentine mineral in H2 SO4 solution. The study is part of a major research project aimed at developing
an effective CO2 sequestration technology using the serpentine mineral which is readily available in Korea. Complete dissolution of Mg from
natural serpentine was achieved in 30 min at a temperature of 90 C under 0.5 M H2 SO4 . The rate of dissolution of Mg was independent of the
agitation speed at speeds above 300 rpm. The fraction of Mg dissolved from milled serpentine was found to be a little higher than that from
natural serpentine up to 70 C in 0.5 M H2 SO4 . The Jander equation was used to explain the dissolution rate data. The rate of Mg dissolution
seemed to be limited by diffusion through the thin channels formed between the silica layers in the serpentine particles.
[doi:10.2320/matertrans.M2009019]
(Received January 16, 2009; Accepted March 11, 2009; Published April 22, 2009)
Keywords: carbon dioxide sequestration, mineral carbonation, serpentine, magnesium, Jander equation
1.
Introduction
Serpentine mineral [Mg3 Si2 O5 (OH)4 ] is a hydrous magnesium-rich silicate mineral which generally occurs in three
types: antigorite, chrysotile, and lizardite. It commonly
contains 32–38% MgO and 35–40% SiO2 with minor
amounts of Fe, Al, Ca, Cr, and Ni.1–4) Currently, serpentine
has mainly been used as: the flux in blast furnaces in the iron
and steel industry, a road construction material, foundry sand,
fertilizer, and for soil amendment etc.4,5)
Several studies have been carried out to extend the
utilization of serpentine mineral. Pundsack has developed a
method for recovering silica, iron oxide and magnesium
carbonate from serpentine using ammonium bisulfate.6)
Noranda has proposed the Magnola process to produce
magnesium metal from chrysotile serpentine and asbestos
tailing.7) The Magnola magnesium plant with a capacity of
58,000 t/yr was started up in 2000, but was closed in 2003
after operating for about two years. Kosuge et al. have
extensively studied the preparation of magnesium sulfate,
amorphous silica and siliceous porous materials by acid
treatment of the serpentine mineral.8) Kim et al. have
conducted research on the synthesis of zeolite using highly
porous amorphous silica obtained from serpentine by hydrochloric acid treatment.9)
As interest increases in the capture and storage of CO2 , one
of greenhouse gases linked to global warming, the availability of serpentine mineral for sequestering CO2 emissions
generated from fossil fuel combustion has been investigated.10–14) Sequestration of CO2 using serpentine mineral
provides safe, long-term storage for CO2 as CO2 is
immobilized into the mineral by the carbonation reaction to
form thermodynamically stable MgCO3 . Among the carbonation routes for serpentine that have been studied, the most
promising one seems to be the aqueous mineral carbonation
process, which includes the extracting of Mg from serpentine
minerals and the subsequent carbonation reaction with CO2
to form the geologically stable mineral MgCO3 .
*Corresponding
author, E-mail: [email protected]
The rate-determining step in the aqueous carbonation of
serpentine minerals is the dissolution of Mg from serpentine.11) Thus, the fundamental characteristics of the dissolution of Mg from serpentine, which is very important to the
optimization of the mineral CO2 sequestration using serpentine, should be well understood. There have been several
studies on the dissolution mechanisms and kinetics of Mg
from serpentine in acid solutions, but most of them involved
the pre-treatment of the mineral, which affected the dissolution of the Mg.15–17) Teir et al. have recently reported that
the dissolution kinetics of Mg from natural serpentine in
acids follows a shrinking unreacted-core model with product
layer diffusion as the rate-determining step.18) In that study,
the serpentine particles were assumed to be spherical in
shape,18) but serpentine is known to have a layered crystal
structures formed by the alternate stacking of the tetrahedral
silica and octahedral brucite layers.1) Therefore, taking the
crystal structure of serpentine into account,19,20) further
studies are required to completely elucidate the dissolution
of Mg from serpentine in acids.
The present study aimed to examine the behavior of Mg
during H2 SO4 dissolution of the serpentine mineral distributed in Korea. The effects of H2 SO4 concentration, dissolution
temperature, and agitation speed on the dissolution of Mg
were investigated. The dissolution kinetics of Mg reported in
this paper was determined by minimizing the effects of the
film boundary diffusion and particle size. In addition, the
mechanochemical effect on the dissolution of Mg was
investigated. The study is expected to provide useful data
for the optimization of H2 SO4 dissolution of Mg to
effectively utilize Korean serpentine mineral in CO2 sequestration through the aqueous carbonation process.
2.
Experimental
2.1 Materials
The serpentine ore distributed in the Andong area, Korea,
was selected for this study. It was ground by a ball mill, and
then sieved to smaller than 45 mm (average particle size
20.6 mm). The X-ray pattern (see Fig. 1) of the serpentine
1226
K. Yoo, B.-S. Kim, M.-S. Kim, J. Lee and J. Jeong
carried out using a 0.5 M H2 SO4 solution by the procedure
described above.
The sample solutions were analyzed by an atomic
absorption spectrometry (AA, SpectrAA400, Varian Inc.)
and an inductively coupled plasma-atomic emission spectrometry (ICP-AES, JY-38 plus, Jobin Yvon Ltd.). The
serpentine sample was also characterized by X-Ray Diffraction (XRD) (D-max-2500PC, Rigaku Co.).
A
Intensity (cps)
A : Antigorite C : Chlorite
D : Diopside F : Forsterite
M : Magnetite T : Tremolite
A
3.
F
C
0
C F
C
T
10
C
F
20
T F
D
A
F
F
30
F
F
M
40
A
D
50
60
70
2 Theta
Fig. 1
Results and Discussion
F
Serpentine is a sheet silicate mineral with a 1 : 1 structure
by the alternate stacking of the tetrahedral silica and
octahedral brucite [Mg(OH2 )] layers linked into sheets.
Under acidic conditions, brucite layers of Mg(OH2 ) dissolves
from serpentine, leaving the silica as a product layer.8,19) The
dissolution reaction of Mg from serpentine in H2 SO4 solution
can be expressed as follows:
XRD pattern of the serpentine ore used in this study.
Mg3 Si2 O5 (OH)4 þ 3H2 SO4
! 3Mg2þ þ 3SO4 2 þ 2SiO2 þ 5H2 O
ð1Þ
Table 1 Chemical composition of serpentine ore used in this study
(mass%).
Content
Mg
Si
Fe
Al
Ca
Ni
Cr
Na
K
19.96 17.52 7.99 2.81 2.62 0.10 0.01 0.07 0.01
Ig. loss
8.90
ore shows that it is composed mainly of antigorite with
minor lizardite, magnetite (Fe3 O4 ), tremolite [Ca2 Mg5 Si8 O22 (OH)2 ], and diopside (MgCaSi2 O6 ). The chemical
composition of the serpentine sample which was used
is shown in Table 1. It contained 19.96 mass% Mg,
17.52 mass% Si, 7.99 mass% Fe, and 2.81 mass% Al. The
weight loss of serpentine on ignition for 1 h at 900 C under a
nitrogen atmosphere was 8.90 mass%.
2.2 Procedures
Experiments on the dissolution of serpentine in H2 SO4
solution were carried out in a 1 L three necked Pyrex reactor
immersed in a thermostatic water bath. The reactor was fitted
with an agitator and reflux condenser. The reflux condenser
was inserted in one port to avoid solution losses at high
temperatures. In a typical run, 500 mL of acid solution (0.10–
3.00 M H2 SO4 ) was poured into the reactor and allowed to
reach thermal equilibrium (25–90 C). 2.00 g of serpentine
was then added to the reactor, and the agitator was set to
400 rpm. During the experiment 4 mL of the solution sample
was taken periodically at a desired time interval (5–120 min)
with a syringe. The sample was filtered and then 3 mL of the
filtrate was diluted with 5% HCl solution for the chemical
analysis.
The milling of serpentine was conducted using a planetary
ball mill (Pulverisette 5, Fritsch GmbH) to investigate the
mechanochemical effect on the dissolution of Mg in H2 SO4
solution. 25 g of the serpentine sample was placed to a
jar (250 mL inner volume) with 13 zirconia balls (20 mm
diameter). The milling was carried out batch-wise for
different periods of time (30–120 min). The rotation speed
of the mill was kept at approximately 150 rpm. Dissolution
experiments with the mechanically activated serpentine were
3.1 Dissolution of natural serpentine ore
The dissolution of Mg at agitation speeds in the range 200–
600 rpm was tested to investigate the effect of liquid film
boundary diffusion surrounding solid particles on the rate of
dissolution of Mg from natural serpentine particles in 0.5 M
H2 SO4 at 50 C. As can be seen from Fig. 2, the results
indicate that the dissolution rate is independent of the
agitation speed at speeds higher than 300 rpm. Therefore, in
all subsequent dissolution experiments, a working agitation
speed of 400 rpm was selected to ensure effective particle
suspension in the solution while minimizing the effect of
liquid film boundary diffusion surrounding the solid particles.
The dissolution of Mg from natural serpentine ore was
investigated at H2 SO4 concentrations of 0.1–3.0 M at 50 C.
Figure 3 shows the effect of H2 SO4 concentration on the
dissolution of Mg. The fraction of Mg dissolved increased as
the concentration of H2 SO4 increased. As shown in Fig. 3,
the rate of dissolution of Mg was initially (in the first 20–
0.8
Fraction of Mg dissolved, XMg/-
Elements
0.6
0.4
200 rpm
300 rpm
400 rpm
600 rpm
0.2
0.0
0
20
40
60
80
100
120
Time, t /min
Fig. 2 Effect of agitation speed on the dissolution of Mg from the natural
serpentine ore in 0.5 M H2 SO4 at 50 C.
0.8
Fraction of Mg dissolved, XMg /-
Fraction of Mg dissolved, XMg /-
Dissolution of Magnesium from Serpentine Mineral in Sulfuric Acid Solution
0.6
0.4
H2SO4 conc. (M)
0.2
0.1
0.5
2.0
0.25
1.0
3.0
1.0
0.8
0.6
0.4
Milling time (min)
0
30
60
120
0.2
0.0
0.0
0
20
40
60
80
100
0
120
20
40
Time, t /min
80
100
120
Temperature (°C)
50
25
90
70
0.6
0.4
0.2
0.0
Fig. 5 Effect of milling time on the dissolution of Mg from the natural
serpentine ore in 0.5 M H2 SO4 at 50 C.
Fraction of Mg dissolved, XMg /-
1.0
0.8
60
Time, t /min
Fig. 3 Effect of H2 SO4 concentration on the dissolution of Mg from the
natural serpentine ore at 50 C.
Fraction of Mg dissolved, XMg /-
1227
1.0
0.8
Temperature (°C)
25
50
70
90
0.6
0.4
0.2
0.0
0
20
40
60
80
100
120
Time, t /min
0
20
40
60
80
100
120
Time, t /min
Fig. 4 Effect of dissolution temperature on the dissolution of Mg from the
natural serpentine ore in 0.5 M H2 SO4 .
Fig. 6 Effect of dissolution temperature on the dissolution of Mg from the
milled serpentine ore in 0.5 M H2 SO4 (Milling time = 30 min).
30 mins) found to be a little fast, but then slowed down
slightly as the dissolution time increased. The fraction of Mg
dissolved increased from about 45% to about 85% as the
H2 SO4 concentration increased from 0.1 M to 3.0 M for a
dissolution time of 120 min.
Figure 4 shows the effect of temperature on the dissolution
of Mg from the serpentine in 0.5 M H2 SO4 solution. The
dissolution temperature varied in the range 25–90 C, while
all other parameters were kept constant. It can be seen from
Fig. 4 that higher temperatures yielded higher dissolution
rates of Mg from the serpentine. At 50 C the fraction of
Mg dissolved was about 60% in 120 min, while complete
dissolution of Mg was achieved in 30 min when the
dissolution temperature was increased to 90 C.
time. The milling time for the mechanical activation of
natural serpentine ore was varied up to 120 min. As shown in
Fig. 5, appreciable enhancement of Mg dissolution was
achieved through the mechanical activation of natural
serpentine ore by the milling. The speed of the dissolution
of Mg in 0.5 M H2 SO4 solution increased initially (for the
first 20 min) as the milling time for the natural serpentine ore
increased. Based on the interpretation by Zhang et al.,21) the
somewhat faster dissolution rate of the milled serpentine ore
may explain why the structure of natural serpentine ore was
transformed from a crystalline state into an amorphous one
through the mechanical activation, which was not clearly
verified in this work. Zhang et al. have reported that the
considerable enhancement of the dissolution of Mg from
milled serpentine arises from the structural transformation of
natural serpentine by milling, and that structural transformation is caused by the disordering around the Mg local
structure caused by the release of the OH base from the Mg
octahedron.21)
Figure 6 shows the effect of the dissolution temperature on
the dissolution of Mg from the milled serpentine ore in 0.5 M
3.2 Dissolution of milled serpentine ore
In order to enhance the rate of dissolution of Mg from
natural serpentine ore in H2 SO4 solution, it was mechanically
activated by a planetary ball mill. Figure 5 shows the
dissolution behavior of Mg from the milled serpentine ore in
0.5 M H2 SO4 solution at 50 C as a function of the milling
1228
K. Yoo, B.-S. Kim, M.-S. Kim, J. Lee and J. Jeong
0.24
H2SO4 conc. (M)
Experimental data
0.1
0.25
1.0
0.5
2.0
3.0
[ 1 - (1 - XMg)1/3 ]2
0.20
0.16
Best fit line
0.1
0.5
2.0
0.12
0.25
1.0
3.0
0.08
0.04
0.00
0
20
40
60
80
100
120
Time, t /min
Fig. 8
MgO6 octahedral layer
Si2O5 tetrahedral layer
Fig. 7 Schematic model of the structural texture of serpentine ore
(Antigorite type).
H2 SO4 . The serpentine ore was mechanically activated by the
milling at a rotational speed of 150 rpm for 30 min. Figure 6
shows that the dissolution rate of Mg from the milled
serpentine increased remarkably as the dissolution temperature increased. Based on the results shown in Figs. 4 and 6,
it was found that the fraction of Mg dissolved from the milled
serpentine ore was a little higher than that from the natural
serpentine ore in 0.5 M H2 SO4 at a dissolution temperature of
25–70 C. However, it was observed that there was little
mechanochemical effect on the dissolution of Mg in H2 SO4
solution when the dissolution temperature was increased to
90 C. This suggests that more information is required about
the feasibility of using mechanical activation to enhance the
rate of dissolution of Mg from serpentine.
3.3 Kinetic analysis
The interpretation of the dissolution kinetics of Mg has
been based on the experimental data for the dissolution of Mg
from serpentine and its textural structure. The textural
structure of serpentine mineral with a 1 : 1 structure due to
the alternate stacking of the tetrahedral silica and octahedral
brucite is shown in Fig. 7.8) In the H2 SO4 dissolution of
serpentine ore, Mg in the brucite layer between the silica
layers dissolves in the acid solution, while the silica which
has not dissolved in the acid solution forms thin channels by
maintaining its own morphology. Thus, it was assumed that
the dissolution of Mg from serpentine ore in the acid solution
proceeds by the diffusion of the acid solution throughout the
thin channels formed between the silica layers which may act
as a diffusion barrier of H3 Oþ ions. Based on that
observation, the Jander rate equation is expected to best
represent the dissolution reaction of Mg from serpentine ore
Plot of the results in Fig. 3 according to eq. (2).
in H2 SO4 solution. This was verified after trying a number of
different rate expressions such as: the spherical shrinking
unreacted-core expression, and the nucleation and growth
equation. The Jander rate equation was derived from the
reactions controlled by a fluid diffusion mechanism.
This rate expression can be expected to be applicable
because the volume change of serpentine particles before
and after the dissolution of Mg in H2 SO4 solution is
negligible, as can be seen indirectly from the texture of the
serpentine ore in Fig. 7. In this rate expression for a spherical
particle of serpentine ore, the fraction of Mg dissolved
from the serpentine is related to the dissolution time by the
expression:
½1 ð1 XMg Þ1=3 2 ¼ kapp t
ð2Þ
In this equation, XMg is the fraction of Mg dissolved, t is
the dissolution time (min), and kapp is the apparent rate
constant (min1 ) which is given by
kapp ¼ bk f ðCH2 SO4 Þ ¼ bkCH2 SO4 n ðmin1 Þ
ð3Þ
Where b is the stoichiometry factor (b ¼ 1=3 in the
serpentine–H2 SO4 system, according to the formulation by
Sohn22)) for eq. (1), k is the intrinsic rate constant
(min1 moln ), CH2 SO4 is the concentration of H2 SO4 , f is
the dependence of the concentration of the rate, and n is the
reaction order for the H2 SO4 concentration. It is apparent
from eq. (2) that a plot of ½1 ð1 XMg Þ1=3 2 vs. t should be
linear with kapp as the slope. The validity of the Jander rate
expression for the dissolution of Mg from the serpentine in
H2 SO4 solution was verified by plotting curves of the
dissolved Mg versus time of Figs. 3, 4 and 6 according to
eq. (2), as shown in Figs. 8–10. In the present study, the
fraction of Mg dissolved up to 0.90 was investigated for the
kinetic analysis of the dissolution reaction. Figures 8–10
indicate that the rate of dissolution of Mg is limited by the
diffusion of Mg2þ , H3 Oþ , and SO4 2 ions throughout the thin
channels formed between silica layers that are not dissolved
by H2 SO4 solution, as explained previously.
In order to evaluate the dependence of the H2 SO4
concentration of kapp , the values of kapp obtained in Fig. 8
was plotted in Fig. 11 as ln kapp against ln CH2 SO4 . As shown
Dissolution of Magnesium from Serpentine Mineral in Sulfuric Acid Solution
-6.0
ln (apparent rate constant), k app /min
-1
0.28
0.24
[ 1 - (1 - X Mg)
]
1/3 2
Temperature (°C)
0.20
Experimental data
25
50
70
90
0.16
Best fit line
25
70
0.12
50
90
0.08
0.04
-7.0
-7.5
-8.0
Reaction order = 0.57
-2.0
-1.0
0.0
1.0
2.0
-3
0
20
40
60
80
100
In (H2SO4 concentration), C H SO /kmol m
2
120
Time, t /min
4
Fig. 11 Dependence on H2 SO4 concentration of the dissolution of Mg
apparent rate constants calculated from the results of Fig. 8.
Plot of the results in Fig. 4 according to eq. (2).
0.25
Experimental data
25
50
70
90
0.15
E For the milled serpentine = 76.7 kJ/mol
50
90
-4.0
-1
Best fit line
25
70
0.20
-2.0
)
Temperature (°C)
-0.57
0.30
ln (k ) (min mol
Fig. 9
1/3 2
-6.5
-8.5
-3.0
0.00
[1-(1-XMg) ]
1229
0.10
0.05
-6.0
E For the natural serpentine = 82.0 kJ/mol
-8.0
0.00
0
20
40
60
80
100
2.8
120
3.4
Fig. 12 Arrhenius plot of the rate constants obtained using eq. (4) from the
slopes of each straight line in Figs. 9 and 10.
Fig. 10 Plot of the results in Fig. 6 according to eq. (2).
in Fig. 11, the reaction order n in eq. (3) is obtained to be
0.57 from the slope. Thus, eq. (3) can be rewritten as:
1
kCH2 SO4 0:57 ðmin1 Þ
3
3.2
103/T (K)
Time, t /min
kapp ¼
3.0
ð4Þ
The values of k at different temperatures were calculated
using eq. (4) from the slopes of each straight line in Figs. 9
and 10. Figure 12 shows the Arrhenius plots of the rate
constants. The slopes of the straight line placed through
the experimental points yield activation energies of 82.0 kJ/
mol for the natural serpentine–H2 SO4 system and 76.7 kJ/
mol for the milled serpentine–H2 SO4 system, respectively.
As shown in Fig. 12, the activation energy obtained from
the Mg dissolution experiments of the milled serpentine
ore is a little low compared with that obtained from the
experiments with the natural serpentine ore. Thus, it was
considered that the fraction of Mg dissolved from the
milled serpentine is a little higher than that from the natural
serpentine, as explained previously. The rate constants k
for the natural serpentine–H2 SO4 system and the milled
serpentine–H2 SO4 system at different temperatures calculated on the basis of eq. (4) can be expressed by the
following equation:
For the natural serpentine–H2 SO4 system
9;862:4
10
k ¼ 7:2 10 exp
ðmin1 mol0:57 Þ ð5Þ
T
For the milled serpentine–H2 SO4 system
9;225:0
10
ðmin1 mol0:57 Þ ð6Þ
k ¼ 1:5 10 exp
T
Where all the activation energy obtained is a little higher
than measured in many cases of diffusion-controlled leaching. In this research, the reason for that was not clearly
determined.19,23) It may be due to some forms of surface
diffusion of Mg2þ , H3 Oþ , and SO4 2 ions in the thin
channels formed between the silica layers during the
dissolution of Mg, which has a high activation energy. On
the other hand, a number of researchers have also reported a
high activation energy for the diffusion-controlled leaching.
Teir et al. have reported that the activation energies for the
diffusion-controlled leaching of Mg from natural serpentinite
in several acid solutions are 68–74 kJ/mol.18) Hernández
et al. have also reported that the leaching of Mg from
sepiolite with H2 SO4 has an activation energy of 63.5
kJ/mol,19) which is controlled by diffusion of the acid
1230
K. Yoo, B.-S. Kim, M.-S. Kim, J. Lee and J. Jeong
solution. It has also been reported that the activation energies
for the diffusion-controlled leaching of iron oxides with HCl
are 62–79 kJ/mol.23)
Using eqs. (2) and (4)–(6), the fraction of Mg dissolved
from serpentine ore in H2 SO4 solution is, respectively,
represented by the following equation:
For the natural serpentine–H2 SO4 system
½1 ð1 XMg Þ1=3 2 ¼ kapp t
Where
k ¼ 2:4 1010 exp
ð7Þ
9;862:4
CH2 SO4 0:57 ðmin1 Þ
T
For the milled serpentine–H2 SO4 system
½1 ð1 XMg Þ1=3 2 ¼ kapp t
Where
k ¼ 5:0 109 exp
4.
ð8Þ
9;225:0
CH2 SO4 0:57 ðmin1 Þ
T
Conclusions
The behavior of Mg during H2 SO4 dissolution of the
serpentine mineral which occurs in Korea was investigated
under differing conditions in a stirred batch reactor. In 0.5 M
H2 SO4 , complete dissolution of Mg from the natural
serpentine was achieved after 30 min at a dissolution
temperature of 90 C. The fraction of Mg dissolved from
the milled serpentine was a little higher than that from the
natural serpentine in 0.5 M H2 SO4 at dissolution temperatures up to 70 C, whereas the increase in the fraction of Mg
dissolved was negligible at 90 C. The dissolution kinetics of
Mg were found to follow the Jander equation with the
diffusion of Mg2þ , H3 Oþ , and SO4 2 ions throughout the thin
channels formed between the silica layers in the serpentine
particles acting as the rate-controlling step. The activation
energies of the dissolution were calculated to be 82.0 kJ/mol
for the natural serpentine–H2 SO4 system and 76.7 kJ/mol for
the milled serpentine–H2 SO4 system, respectively. The
reaction order with respect to H2 SO4 concentration in the
solution was found to be 0.57.
Acknowledgements
This research was supported by the Basic Research Project
of the Korea Institute of Geosciences and Mineral Resources
(KIGAM) funded by the Ministry of Knowledge and
Economy of Korea. The authors would like to thank Dr.
Young-nam Jang and Dr. Sujeong Lee for providing valuable
comments.
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