International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 6, 2012 ISSN 2277 – 4394
Effect of Process Conditions on Crystal
Structure of Precipitated Calcium Carbonate
(CaCO3) From Fly Ash: Na2CO3 Preparation
Conditions
Sabriye1. Pişkin1, Özgül2. Dere Özdemir2
Abstract— The precipitated calcium carbonate was
produced by adding the sodium carbonate to fly ash leachate
with distilled water and the effects of process conditions on
crystal structure were investigated. The products were
characterized with scanning electron microscopy (SEM), and
powder X-ray diffraction analysis (XRD).
The reaction time and temperature, sodium carbonate
concentration, mixing have no significant effect on calcium
carbonate polymorph. However, the mode of mixing the
reactants has a remarkable effect on the type of polymorph
formed and its morphology.
Fly ash leachate was generated during the evaluation of fly
ash. When sodium carbonate is added to fly ash leachate at 65
°C, mostly calcite is formed. However if the sodium carbonate
is added in the solid form, calcite crystals are in rhombohedral
habits. On the other hand calcite crystal structure is in
scalenohedral form when the sodium carbonate solution is
added.
Fig.1. World net electricity generation by fuel type, 2008-2035 [1]
The properties and chemical composition of fly ashes vary
depending on coal source and power-plant operation, which make
these materials even more versatile [2]. Fly ash leachate is generated
during the fly ash evaluation process. It can be used as a source of
calcium for CaCO 3 precipitation.
Precipitated calcium carbonate is formed by dissolving limestone
and precipitating CaCO 3 as very fine particles using CO 2 (Aroganite
Process) or sodium carbonate (Solvay Process) [3].
The main sources of particulate CaCO 3 are ground calcium
carbonate (GCC) mined from calcite deposits and precipitated
calcium carbonate (PCC) produced by means of a chemical reaction.
PCC is more versatile than GCC in terms of its industrial
applications, since a large number of particulate CaCO 3 parameters,
such as the particle size, the morphology, the polymorphic form or
the purity, can be tailored by carefully controlling the reaction
conditions [4].
In addition to the amorphous phase, calcium carbonate occurs in
three main crystal polymorphs: calcite, aragonite, and vaterite. Of
these, calcite, which forms rhombohedral crystals, is the most
thermodynamically stable under ambient conditions.
Aragonite usually forms needle-like orthorhombic crystals and is
favoured at high temperatures and pressures. It is metastable,
converting slowly to calcite.
Vaterite is the thermodynamically least stable polymorph, and its
hexagonal crystals are rarely seen in the naturally occurring mineral.
Since different polymorphs impart different properties, control over
the crystal form is important to end users [5]. This particular
industrial production is increasingly important because its products
Keywords— Crystal, Fly Ash, Morphology, Precipitated
Calcium Carbonate
I. INTRODUCTION
T
HE fast economic development occurring in the world would
heavily rely on the use of energy supplies from fossil fuels. In
this context, coal as a major source for energy production of
global energy consumption in coming years (Fig. 1).
In the electric power sector, a more rapid rate of increase in the
use of other fuels, particularly renewables, leads to a decline in coal's
share of total energy consumption for power generation from 43
percent in 2008 to 37 percent in 2020 [1]. However, coal-fired power
plants are producing larger quantities of fly ash annually, resulting in
the problem of solid waste disposal.
Sabriye1. Pişkin1 is with the Chemical Engineering Department of Yildiz
Technical University, Istanbul, 34210 Turkey (corresponding author to
provide phone: +90 212 383 4729; fax: +90 212 383 4725; e-mail:
[email protected]).
Özgül2. Dere Özdemir2, is with Chemical Engineering Department of
Yildiz Technical University, Istanbul, 34210 Turkey.
(e-mail:
[email protected]).
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International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 6, 2012 ISSN 2277 – 4394
are used for the paper, plastics and pharmaceutical industries, in
personal care and cosmetics, for food and beverages, nutritional
supplements, adhesives and sealants and for some other industries
[6].
In this work, production of precipitated calcium carbonate in two
different morphological character, rhombohedral and scalenohedral
habits, were investigated. Fly ash leachate was used as a calcium
source. Rhombohedral habits were obtained when the Na 2 CO 3 was
added in the solid form besides scalenohedral habits were obtained
Na 2 CO 3 was added in the solution form.
II. MATERIALS AND METHODS
A. Materials
Fly ash samples were collected directly from Turkey coal-fired
power plants: Çan Thermal Power Plant located in Çanakkale.
Na 2 CO 3 (99.95%) obtained from Sigma–Aldrich and pure water
were used in precipitation of CaCO 3 .
b)
Fig.2. XRD patterns of precipitated calcium carbonate precipitated
by a) solid Na 2 CO 3, b) solution Na 2 CO 3
B. Methods
According to literature, calcium carbonate shows three different
crystal structures: vaterite, calcite and aragonite (Fig. 3). Calcite,
easy to find in nature, is more stable than other polymorphs at
ambient temperature and atmospheric pressure whereas vaterite is
most unstable. Aragonite is less stable than calcite and commonly
found in marine organisms [7].
Fly ash samples were leached with pure water at room
temperature, 1/50 solid:liquid ratio and 60 minutes. After filtration
leachate was used as a calcium source.
Calcium carbonate precipitation was performed at 65°C, 60
minutes with different form of Na 2 CO 3 as 1/10 solid:liquid ratio and
10 % Na 2 CO 3 solution. Precipitations were filtered and dried at 105
°C.
Products were characterized with X-ray diffraction (XRD) (Philips
PanAnalitical, X’pert Pro) using Cu Ka radiation at 40 kV and 40
mA. The scan rate was 0.03 °/s in a range of 5-60 ° (2θ). Phase
identification was made by searching the ICSD powder diffraction
file database. To verify the morphology, samples were covered with
a thin layer of gold to make them conductive and examined using a
scanning electron microscope (CamScan, Apollo 300).
III. RESULT AND DISCUSSION
The effect of mixing conditions of reactants was investigated in
this study. The products were characterized by XRD and SEM
instruments. Figs. 2 shows X-ray patterns of precipitated calcium
carbonate. According to the diffractograms, the synthetic product
presented only one phase of calcium carbonate. Both of the patterns
were matched with the same pdf card (01-085-1108).
a)
b)
a)
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International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 6, 2012 ISSN 2277 – 4394
Precipitated calcium carbonate produced from Turkish fly ash
leachate can be clearly seen in scalenohedral (solution Na 2 CO 3 ) and
rhombohedral (solid Na 2 CO 3 ) structure, Fig. 5. The SEM
micrographs confirm the presence of calcite.
c)
Fig. 3. The polyphorms of calcium carbonate crystals: a) Vaterite
[8], b) Calcite [9], c) aragonite [10]
Calcite crystal has three different polymorphs, rhombohedral
(cubic), prismatic (barrel-shaped) and scalenohedral (triangular) (Fig.
4).
a)
a) scalenohedral (triangular)
b)
Fig.5. SEM micrographs of precipitated calcium carbonate
precipitated by a) solid Na 2 CO 3, b) solution Na 2 CO 3
b) prismatic (barrel-shaped)
IV. CONCLUSION
Fly ash is solid waste which is a problem for environment.
Its water leachate can be used as a calcium source for CaCO 3
precipitation.
These results indicated that a selective type of different
habits can be produced simply by changing the mode of mixing
the reactants. Various morphologies of calcite crystals such as
rhombohedral and scalenohedral were produced, depending on
the preparation conditions.
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