Dissolving behavior and calcium release from fibrous wollastonite in acetic acid solution

Article abstract of DOI:10.1016/j.tca.2009.10.002

The degradability of fibrous wollastonite (CaSiO₃) in an aqueous solution of acetic acid and leaching of Ca²⁺ ions were investigated in the temperature range from 22 to 50 °C. The Inductively Coupled Plasma Atomic Emission Spectrosco

Full text of DOI:10.1016/j.tca.2009.10.002

Thermochimica Acta 498 (2010) 54–60
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Thermochimica Acta
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Dissolving behavior and calcium release from fibrous wollastonite in acetic
acid solution
∗
ˇ
Petr Ptácˇek , Magdaléna Nosková, Jirˇí Brandsˇtetr, Frantisˇek Soukal, Tomásˇ Opravil
Institute of Materials Science, Faculty of Chemistry, Brno University of Technology, Purkynˇova 464/118, Brno CZ-621 00, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 May 2009
Received in revised form 2 October 2009
Accepted 5 October 2009
Available online 13 October 2009
The degradability of fibrous wollastonite (CaSiO3) in an aqueous solution of acetic acid and leaching of
Ca2+ ions were investigated in the temperature range from 22 to 50 ◦C. The Inductively Coupled Plasma
Atomic Emission Spectroscopy (ICP-OES) was used for the assessment of calcium and other selected
cations in the leaching medium. The amount of calcium in the solvent can be significantly enhanced
through leaching at higher temperature. Fibrous silica particles are the main by-product of the leaching
process. The properties of by-product were examined by thermal analysis (simultaneous TG–DTA–EGA),
infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The formation of silica layer on
the surface of fibrous wollastonite particles is an important factor in the leaching process. Particles were
covered by the silica layer and wollastonite core size was continually decreasing during leaching. The
shape of resulting silica particles shows no significant changes during this process. Specific surface of the
formed fibrous silica particles strongly depends on the leaching temperature.
Keywords:
Wollastonite
Calcium silicate
Silica
Acetic acid
Dissolution of silicates
Carbon dioxide sequestration
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
many other minerals, traps CO2 as environmentally stable solid car-
bonates that could provide considerable storage capacity within a
The 20th century has been experiencing the rapid increase of
population and huge growth in energy consumption. As more coun-
tries become industrialized, it is expected that more energy will be
consumed in the 21st century. Fossil fuels have been the world’s
primary source of energy for well over a century. They currently
account for nearly 85% of world’s energy supply. This trend is
expected to continue throughout the 21st century [1,2].
The increasing carbon dioxide content in the atmosphere and
its long-term effect on the climate has led to increasing interest
and research on the possibilities of capture, utilization and long-
term storage of CO2. One of the major options for reduction of
anthropogenic CO2 emissions in the near future is the carbon diox-
ide capture and storage (CCS) technologies. This concept includes
capture (separation and compression) of CO2 from large central-
ized CO2 emitters (such as power plants, metallurgy and cement
producers), and transportation to an adequate storage site (such as
depleted oil and gas fields or saline aquifers). Carbonation of natu-
ral silicate minerals and fixing of carbon dioxide as carbonates is an
interesting alternative option for long-term storage of CO2 because
suitable calcium and magnesium rich rocks are distributed all over
the world. Carbonation of calcium and magnesium silicates, such
as wollastonite (CaSiO3), enstatite (MgSiO3), olivine (Mg2SiO4) and
geological time scale. The silicate rock could be turned into envi-
ronmentally stable carbonates by following mechanism [1–8]:
(Ca, Mg)xSiyOx+2y + xCO2 → x(Ca, Mg)CO3 + ySiO2.
(1)
The proposal to remove greenhouse gases by pumping lique-
fied CO2 several kilometers under the ground (geosequestration)
implies that many carbonate minerals will be formed. Among these
minerals brugnatellite and coalingite with the hydrotalcite struc-
ture are probable. The study of the magnesium carbonates is of
importance in the development of technology for the removal of
greenhouse gases [9–11]. For wollastonite, overall carbonation pro-
cess can be written as [8,12,13]:
CaSiO3 + CO2 → CaCO3 + SiO2.
(2)
Industrial residues, such as steel slag and municipal solid waste
incinerator bottom ash, are promising materials for mineral car-
bonation too [3,8,14–17].
The most efficient processes suggested for carbonation involve
leaching or dissolution of calcium or magnesium silicates in liquid
media and precipitation of carbonates or hydroxides during sub-
sequent carbonation. The process consists of two main steps. The
first one, calcium or magnesium ions are extracted from natural
silicates by leaching with acetic acid [3,7]:
CaSiO3 + 2CH3CO2H → Ca2+ + 2CH3CO2− + SiO2+H2O;
MgSiO3 + 2CH3CO2H → Mg2+ + 2CH3CO2− + SiO2 + H2O.
(3)
(4)
∗
Corresponding author. Tel.: +420 541 149 389; fax: +420 541 149 361.
E-mail address: ptacek@fch.vutbr.cz (P. Ptácˇek).
0040-6031/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2009.10.002

P. Ptácˇek et al. / Thermochimica Acta 498 (2010) 54–60
55
Table 1
Properties and results of the chemical analysis of wollastonite.
Specific humidity
Ignition loss
Calcitea
Sample composition (%)b
c
Ca
Al
Fe
Ni
Zn
SiO2
49.18
0.13%
2.36%
5.57%
32.30
0.24
0.17
0.07
0.08
a
Admixture quantity was calculated from TGA results (please see Fig. 5).
The sample was fused with Na2CO3 in Pt crucible and melt was leached out by diluted HCl. The precipitated silica was filtered out and composition of the filtrate was
b
analyzed by ICP-OES. Collected data were corrected to results of reference sample. Alkaline fusion was used due to assumed content of aluminosilicates in the sample.
c
Amount of precipitated SiO2 was determined by gravimetric analysis.
The behavior of some other fibrous silicates, such as amosite,
crocidolite, chrysotile, sepiolite, palygorskite, in acid media has
been reported [18–20]. After filtrating silica out of the solution,
carbon dioxide is pumped into the solution, forming calcium or
magnesium carbonates that precipitate from the solution:
by the Inductively Coupled Plasma Atomic Emission Spectroscopy
(ICP-OES; ICP IRIS Iterdip II XSP duo). Filter cake was dried at 110 ◦C,
its properties and composition were subsequently investigated by
simultaneous TG–DTA–EGA, FT-IR, BET and SEM.
Thermal analyses (simultaneous TG–DTA) were made with
TG–DTA analyzer TA Instruments Q600 connected to heated gas
cell of FT-IR spectrometer Thermo Nicolet iS10 via heated capillary
due to investigation of gases releasing from the sample (EGA). All
methods were carried out at the same time on one sample heated
up at rate of 20 ◦C min−1 under flow of argon (100 cm3 min−1).
Infrared spectra were collected with FT-IR spectrometer Thermo
Nicolet iS10 via KBr pellets technique. Specimens were ground with
dried spectroscopic grade KBr powder and the mixture was com-
pressed to pellets for FT-IR measurements. The sample to KBr mass
ratio was 1:100. All spectra are collected in the 4000–400 cm−1
range at 8 cm−1 resolution.
Ca2+ + 2CH3CO2− + CO2 + H2O → CaCO3 + 2CH3CO2H;
Mg2+ + 2CH3CO2− + CO2 + H2O → MgCO3 + 2CH3CO2H.
(5)
(6)
Acetic acid is recovered in this step and recycled for use in the
next extraction step [3,7]. Most of these processes have been con-
sidered to be too expensive for CO2 storage, since they require
energy for crushing, grinding or preheating and possible chem-
ical additives which cannot be recovered. However, published
experimental data on these processes are scarce and occasionally
contradictory [7].
The present paper is aimed at the study of leaching process of
calcium from wollastonite in the aqueous solution of acetic acid.
The effect of temperature on the amount of extracted calcium and
properties of residual fibrous silica particles was evaluated by ther-
mal analysis, infrared spectroscopy and BET. Specific surface area
of silica by-products is very important property for its potential
application. Research on exploitation of residual silica is required
due to its large contents in minerals which are supposed for mineral
carbonation process. A paper dealing with the kinetics of leaching
process is being prepared.
3. Results and discussion
3.1. Leaching test
The typical time dependence of pH is shown in Fig. 1. The pH
of leaching medium increased during dissolution of wollastonite.
Nevertheless, when the solution of acetic acid was introduced into
the reactor, the fast dissolution of calcite began and next the pH
value stayed almost constant during a short induction period (see
details in Fig. 1). The reaction began probably immediately after
wetting process, but the amount of dissolved material remained
very small for a certain time. The induction period is not rare phe-
nomenon for heterogeneous processes. Some authors explain this
delay at the beginning of the process by the nucleation incubation
period, i.e. such as the transient time in which the steady rate of for-
mation of nuclei is reached [21–23]. Other published explanations
of induction period depend on the chemistry of dissolution pro-
cess or crystal lattice stability [24–27]. Our experiments show that
induction period takes a longer time in case of lower temperatures
and lower acid concentrations.
2. Experimental procedure
The wollastonite obtained from Ankerpoort NV: The Mineral
Company (Netherlands), with specific surface area of 0.66 m2 g−1
(Chembet 3000) was used in this study. The 90 ␮m undersize parti-
cles of sample were employed without other treatment. According
to X-ray crystallography results (Siemens D500) the calcite, quartz
and small amount of mica or chlorite group minerals are acces-
sory minerals of wollastonite. Some properties and composition of
applied wollastonite are listed in Table 1.
Dissolution experiments were carried out on stirred suspension
of wollastonite in aqueous solution of acetic acid (Lachema, p.a.).
Temperature of leaching bath ranged from 22 to 50 ◦C. The temper-
ature of double wall glass reactor was adjusted using external water
flow of temperature controlled water batch (thermostat). Sample
was poured on by solution of acetic acid that was preheated to the
applied leaching temperature in water batch of thermostat. The sta-
bility of suspension is very poor due to low value of electrokinetic
(zeta) potential that is only −19.7 mV (Zeta meter Brookhaven 90
Plus). Hence, the stirring of system by magnetic stirrer was used.
Suspension contained 12.5 g of wollastonite per liter of leaching
solution with concentration of acetic acid of about 3 mol dm−3. The
pH value of dispersing medium for 24 h leaching experiment was
continuously measured by pH meter connected to PC.
The buffer system of weak acid (CH3CO2H) and its salt
(Ca(CH3CO2)2) with a strong base, i.e. Ca(OH)2, was formed dur-
Solid part of suspension was separated by filtration through
dense filter paper (red strip) after leaching. Filter cake was washed
three times by slightly acidified (acetic acid) distilled water. The
quantities of ions in original sample and leachate were determined
Fig. 1. Dissolution of wollastonite in acetic acid at 22 ◦C including the induction
period.

56
P. Ptácˇek et al. / Thermochimica Acta 498 (2010) 54–60
Table 2
The temperature dependence of calcium and some other elements leaching efficiency per gram of wollastonite raw material after 24 h. Significant values of Pearson correlation
coefficient (R) are typed by bold.
Element
Temperature (◦C)
22
25
30
35
40
45
50
R
Ca
Al
Fe
Ni
Zn
67.6
0.4
9.8
2.4
18.3
72.1
0.4
11.3
2.7
79.0
0.3
13.5
3.4
84.7
0.2
16.0
4.7
89.4
0.2
17.4
4.5
93.6
0.3
19.7
5.3
94.4
0.2
21.6
5.9
0.986
P (%)
−0.772
0.998
0.982
−0.363
29.1
3.9
13.5
10.3
16.3
12.4
ing dissolution of wollastonite (Eq. (3)). With respect to reaction
stoichiometry, the amount of formed acetate ions was double the
concentration of Ca2+ ions released from wollastonite. Therefore
following subform of Henderson buffer equation may be used for
estimation of the course of leaching process:
structure, admixtures of other minerals in the sample as well as in
consequences of adsorption events.
Calcium is not the only element among elements noted in
Table 2 which shows linear temperature dependence of concentra-
tion. The Fe and Ni contents in leaching medium were increasing
with increasing leaching temperature. Acceleration of diffusion
phenomenon with increasing temperature gives the reason for
this fact. Furthermore, according to correlation analysis results, the
amount of calcium correlates with quantities of Fe and Ni. That
implies that lattice of utilized wollastonite may contain cations of
these elements.
2[Ca2+
[CH3CO2H]
]
pH(T) = pKa(T) + log
(7)
Nevertheless, the measured data must be corrected to Ca2+ ions
released during dissolution of calcite before Eq. (7) can be applied
to dissolution of wollastonite.
Silica or more exactly mainly silica containing gel phase was
formed during dissolution of wollastonite in acetic acid (Eq. (3)).
The mass of original sample was reduced to 58 2% during this
process. Hence besides the application of leaching process in CCS
technology the problem of by-products utilization must be solved
as well. In other words proper exploitation of by-product is an
integral part of leaching technology.
It is evident that the course of wollastonite dissolution and
leaching of calcium through the formed silica layer can be mon-
itored via pH of system, because the content of calcium is much
higher than the amount of other ions, which can be released from
wollastonite (see Table 1). Naturally there must be sufficient excess
of acetic acid in the reaction mixture so that its concentration is
practically constant during the leaching process.
The specific surface area is one of the most important param-
eter for potential application of by-products, for example such as
adsorbent, carrier of catalysts or reactive admixture in concrete
technology. The influence of temperature on specific surface area
of nascent SiO2 xerogel is presented in Table 3. Acquired data
show that specific surface area is strongly influenced by temper-
ature. Over studied temperature range from 22 to 50 ◦C the specific
surface area increased from 73 to 146 m2 g−1, i.e. its value grew
approximately twice. Furthermore, the significant statistical corre-
lation has been found between temperature of leaching bath and
specific surface area.
Filtrates gained from 24 h lasting dissolution of wollastonite in
the aqueous solution of acetic acid within temperature range from
22 to 50 ◦C were analyzed by ICP-OES to determine the concen-
trations of calcium and some other relevant elements, such as Fe,
Ni and Zn. The leaching efficiency (P) can be evaluated as a mass
ratio of element extracted from unit weight of wollastonite to the
amount of this component in the original sample. Table 2 shows
the results.
Acquired data confirm that the amount of extracted calcium is
markedly increasing with growing temperature of leaching batch.
The amount of extracted calcium has increased up to about 27%
within the investigated temperature interval. Moreover, a signif-
icant linear relationship between temperature and quantity of
calcium was found using correlation analysis. The effect of temper-
ature on leaching efficiency of calcium is shown in Fig. 2. It must be
pointed out that only simple extrapolation to 100% efficiency pro-
vides incorrect value due to other ions present in the wollastonite
The fact must be pointed out that behavior of silica based disper-
sion systems can be influenced by many factors (temperature, pH,
presence of salt, concentration, etc.). This area of colloidal chem-
istry was described in details by Iler [28].
3.2. Infrared spectroscopy
The infrared spectrum of wollastonite is given in Fig. 3. The
peaks at 1081, 1060, 1016, 965, 925, 900, 682, 643, 568, 472 and
451 cm−1 belong to ␤-CaSiO3 [29]. The high-wavenumber band
at 1250–800 cm−1, mid-wavenumber band at 800–600 cm−1 and
low-wavenumber band 600–400 cm−1 were assigned to antisym-
metric (ꢀ3), symmetric (ꢀ1) and bending mode (ꢀ4) of the silicate
tetrahedron vibrations, respectively [30].
The water adsorbed onto the surface of sample shows a broad
stretching band (ꢀ1) and weak deformation band (ꢀ2) at 3435
and 1626 cm−1, respectively [9,10,31]. The bands located at 2512,
1798, 1431 and 920 cm−1 indicate that CaCO3 is the only sig-
Table 3
Effect of temperature on the specific surface of silica xerogel remaining after dissolu-
tion of wollastonite and value of the correlation coefficient (R) between temperature
and specific surface of the sample.
T (◦C)
22
73
25
77
30
35
96
40
96
45
50
R
Fig. 2. The effect of temperature on the leaching efficiency of calcium from wollas-
tonite raw material.
Sspec (m2 g−1
)
102
133
146
0.927

P. Ptácˇek et al. / Thermochimica Acta 498 (2010) 54–60
57
Fig. 4 shows typical infrared spectra of silica based by-products
formed during dissolution of wollastonite at 22 and 50 ◦C. The
higher specific surface area of silica gives the reason for growing
intensities of bands at 3451 and 1632 cm−1 belonging to water
adsorbed onto the surface. The OH vibration region was fitted by
Gauss function (see details in Fig. 4). Stretching of free and bonded
surface hydroxyl groups at 3647 and 3521 cm−1 participate on the
structure of band [33]. Bending of silanol groups (SiO–H) is located
at 940 cm−1 [31].
Two most intensive bands at 1095 and 463 cm−1 are attributed
to antisymmetric stretching (ꢀ3) and bending (ꢀ4) vibration of Si–O
bond in the SiO4 tetrahedron, respectively. The Si–O symmetric
stretching vibration (ꢀ1) is located at 805 cm−1 [31,34]. It explains
the reason that silica is the main part of sample after leaching pro-
cess. Other bands belong to residual wollastonite and remaining
acetic acid adsorbed on the silica surface.
The spectrum of pure SiO2 is also plotted in Fig. 4. The com-
parison of all spectra leads to conclusion that certain amount of
wollastonite is still present after leaching at 22 ◦C, whereas the
sample after leaching at 50 ◦C consists of silica only.
Fig. 3. The baseline corrected FT-IR spectrum of wollastonite. Admixtures bands are
designated as following: Calcite, C and adsorbed water, W.
3.3. Simultaneous TG–DTA–EGA
Thermal analysis of wollastonite is given in Fig. 5. The total
mass loss of the sample over whole analyzed temperature interval
(25–1250 ◦C) is 2.80%. Wollastonite undergoes the phase transition
of low-temperature wollastonite (␤-CaSiO3) to high-temperature
phase (␣-CaSiO3) at 1125 ◦C. Therefore, any effects on the thermal
curves and infrared spectra of evolved gases belong to impurities.
The weak endothermic DTA peak with maximum at 98 ◦C is
due to the loss of water molecules. The experimentally determined
mass loss for this peak is 0.07%. The two following inexpressive
endotherms at 207 and 374 ◦C appeared probably due to presence
of chlorite or mica group admixtures in the sample. Dehydration
and dehydroxylation are connected with mass loss of about 0.10
and 0.04% on TG curve. Thermal decomposition of CaCO3 at 710 ◦C
is the most expressive process in Fig. 5. The observed mass loss
of sample is 2.45%. Calcination of calcite (identified by infrared
spectroscopy) is distinctly visible in the EGA pattern.
Silica xerogel was formed during drying step with a total mass
loss of 2–4% over the temperature range from 25 to 180 ◦C (Fig. 6).
Endothermic peak at 112 ◦C corresponds to the elimination of
water from silica gel, which is formed during leaching of calcium
from wollastonite. Chemisorbed water, i.e. various kinds of silanol
groups ( Si–OH) on the surface of silica, is disappearing over wide
temperature interval. The EGA results show that bands of formed
water can be recognized almost to 1000 ◦C.
Fig. 4. The baseline corrected FT-IR spectrum of silica based xerogel.
nificant impurity of wollastonite. These bands are assigned as
2ꢀ2 + ꢀ4 combination mode, ꢀ1 + ꢀ4 combination mode, antisym-
metric stretching (ꢀ3) and out-of-plane bending (ꢀ2) of CO3
anion, respectively. The absence of symmetric stretching (ꢀ1) band
2−
in infrared spectrum identifies the CaCO3 as calcite [9,10,32].
Fig. 5. TG–DTA and EGA of wollastonite.

58
P. Ptácˇek et al. / Thermochimica Acta 498 (2010) 54–60
Fig. 6. The typical TG–DTA and EGA plot for by-product of leaching process.
The EGA peaks 1337 and 1185 cm−1 show that acetone is
being formed within the temperature interval ranging from 535
to 620 ◦C. Studies concerned to thermal decomposition of acetates
describe several possible routes of this process where CH3COCH3
was formed [36–39]. The mass of sample was reduced for about
5.9% during all these processes.
The porous structure of xerogel contains a certain amount of
residual calcium acetate monohydrate. The thermal decomposition
of Ca(CH3COO)2·H2O is divided into four steps: two-stage dehydra-
tion (75 and 120 ◦C), formation of CaCO3 (392 ◦C) and its calcination
to CaO (595 ◦C) [35]. Hence it proves that dehydration participates
on the mass loss of sample during drying step. The pyrolysis of
residual amount of calcium acetate begins at 225 ◦C. The CO2 and
weak CO bands are visible from EGA. Carbon monoxide implies
reducing condition and formation of calcite according to Eq. (8).
Produced calcite is subsequently calcinated (Eq. (9)) and further
CO2 appears in EGA:
3.4. Electron scanning microscopy
Microphotographs of wollastonite grains before leaching of cal-
cium are in Fig. 7a. The sample consists of fibrous fine grain forms.
The surface of particles shows splinters and uneven fractures. Any
other shapes of aggregates belong to admixtures. Some of them
show clearly layered structure. That corresponds to thermal analy-
Ca(CH3COO)2 → CaCO3 + 1.5H2O
CaCO3 → CaO + CO2
(8)
(9)
Fig. 7. SEM pictures of fibrous wollastonite particles before (a) and after (b) leaching of calcium. Some of admixtures show layered structure.

P. Ptácˇek et al. / Thermochimica Acta 498 (2010) 54–60
59
sis and FT-IR results, because these methods indicate the presence
of phyllosilicates as well.
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Typical SEM picture of leaching products is shown in Fig. 7b.
Leaching product consists of slightly bent and cracked fibrous parti-
cles of silica or silica coated wollastonite core. The cracks of particles
probably took place during drying of filter cake due to drying forces
leading to shrinkage of the sample. The surface of particles is very
different from the wollastonite one. There is not any fine detritus
placed on the surface that is much smoother.
These observations show that the shape of particles is preserved
during leaching process. The SiO2 gel layer increased while wollas-
tonite core was gradually disappearing, i.e. there is a topotactical
relationship to the parent wollastonite particle. The sizable defor-
mation was observed for sample prepared at higher temperatures.
The reason can be found within the absence of the wollastonite
core. Thus the behavior during dissolution can be utilized for the
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All tests show that acetic acid is able to extract a significant
amount of calcium from wollastonite and that efficiency of the
process can be improved through higher temperature of leaching.
Dissolution of wollastonite in the aqueous solution of acetic acid is
a steady process. The continuous silica layer formed on the surface
of wollastonite particles during leaching. Thickness of this layer
increased during leaching, while wollastonite core was gradually
disappearing. That led to the silica particles with similar shape like
original particles of the wollastonite.
The temperature has a significant effect on the dissolution of
wollastonite in acetic acid. Furthermore, temperature of the sys-
tem is an important factor determining the specific surface area of
resulting silica xerogel. The system shows huge specific surface area
that increases with the higher temperature of leaching. The specific
surface area of SiO2 is one of the most important properties for its
potential utilizations, e.g. as an adsorbent, carrier of catalysts, filler
or reactive admixture to Portland cement or geopolymers.
There is also another important possibility resulting from the
course of dissolution. It is concerned to preparation of fibrous par-
ticular composite consisting of wollastonite core and silica shell
with a huge, reactive surface. These particles are formed during
leaching process at the lower temperatures. The properties of sur-
face silica layer may be controlled by temperature, time and other
factors, such as concentration of acid and intensity of stirring during
dissolution process.
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