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Energy Procedia 00 (2010) 000–000
Energy
Procedia
www.elsevier.com/locate/XXX
GHGT-10
The CO
2
-binding by Ca-Mg-silicates in direct aqueous carbonation
of oil shale ash and steel slag
Mai Uibu
a
1*, Rein Kuusik
a
, Lale Andreas
b
, Kalle Kirsimäe
c
a
Laboratory of Inorganic Materials, Tallinn University of Technology, 5 Ehitajate S.,Tallinn 19086, Estonia
b
Division of Waste Science &Technology, Luleå University of Technology, SE 971 87 Luleå, Sweden
c
Department of Geology, University of Tartu, 14A Ravila St., 51014 Tartu, Estonia
Elsevier use only: Received date here; revised date here; accepted date here
Abstract
This study was focused on carbonation of waste materials having low water-solubility in which Ca and Mg are generally bound
as silicates. Here, pulverized firing oil shale ash (PFA from Narva Power Plants, Estonia), electric arc furnace slag (EAFS, types
1 and 2 from Uddeholm Tooling, Sweden) and ladle slag (LS from Uddeholm Tooling, Sweden) were studied as sorbents for
binding CO
2
from flue gases in direct aqueous mineral carbonation process. The experiments were carried out at room
temperature and atmospheric pressure.
Results showed that Ca-Mg-silicate phases bound up to 9 g of CO
2
per 100 g of initial ash, which formed 30% of the total CO
2
bound in direct aqueous carbonation of PFA. The CO
2
uptakes for steel slags (EAFS1, EAFS2 and LS) were 8.7g CO
2
/100 g
EAFS1, 1.9 g CO
2
/100 gEAFS2 and 4.6 g/100g LS. Quantitative XRD analysis indicated that Ca
2
SiO
4
and Ca
3
Mg(SiO
4
)
2
were
the main CO
2
binding low solubility components of oil shale ash as well as steel slags. The main carbonation product was calcite
(CaCO
3
), indicating that Mg-compounds were not reactive towards CO
2
at these mild conditions.
Based on multifaceted studies on carbonation of oil shale ash, a new method for eliminating CO
2
from flue gases by Ca-
containing waste material was proposed. The process includes contacting the aqueous suspensions of Ca-containing waste
material with CO
2
containing flue gas in two steps: in the first step the suspension is bubbled with flue gas keeping the pH levels
in the range of 10–12 and in the second step keeping the pH levels in the range of 7–8. The water-soluble components such as
free lime are carbonated in the first step and the components of low solubility, in which Ca is generally contained in the form of
silicates, are carbonated in the second step.
© 2010 Elsevier Ltd. All rights reserved
Keywords: PF ash; EAF slag; ladle slag; aqueous carbonation; quantitative XRD analysis
1. Introduction
Atmospheric emissions of CO
2
originating from the fossil fuels based heat and power production is a serious
problem worldwide. Fixation of CO
2
in the thermodynamically stable form of inorganic carbonates, also known as
mineral carbonation is a prospective option for CO
2
storage [1]. Although the CO
2
storage capacity of the natural
* Corresponding author. Tel.: +372-620-2812; fax: +372-620-2801.
E-mail address: maiuibu@staff.ttu.ee.
2 Author name / Energy Procedia 00 (2010) 000–000
Ca-Mg-silicate minerals is sufficient to fix the CO
2
emitted from the combustion of the fossil fuels, the
technological carbonation of these minerals (for instance serpentinite, olivine) is slow and energy demanding. One
way to evade some of the negative aspects of the technological carbonation of natural minerals is to utilize some
alkaline waste residues (ashes from coal- and oil shale-fired power plants [2-5], steel slags [6-8], MSWI ashes [9,
10], APC residues [11], etc) as CO
2
sorbents. These materials are often associated with CO
2
point source emissions
and tend to be chemically more active than geologically derived minerals. Consequently, they require not as much
of pre-treatment and less energy-intensive operating conditions to achieve sufficient carbonation rates [6].
Combustion of low-grade carbonaceous fossil fuel oil shale is characterized by high specific carbon emissions as
well as huge amounts of waste ash. Carbonation of oil shale ash has previously been investigated in the context of
its relatively high content of free lime (up to 30% depending on combustion technology). In addition to free lime,
PFA also contains up to 30% of Ca-Mg-silicates (CaSiO
3
, Ca
2
SiO
4
, Ca
3
Mg(SiO
4
)
2
) as potential CO
2
binders [12].
Previous experiments with synthetic model compounds (CaSiO
3
, Ca
2
SiO
4
, Ca
3
Mg(SiO
4
)
2
and
(Ca,Na)
2
(Mg,Al)(Si,Al)
3
O
7
) have shown that Ca-silicates displayed a good CO
2
-binding efficiency under mild
operating conditions (atmospheric pressure and room temperature): CaSiO
3
reached up to 88.7% and Ca
2
SiO
4
up to
76.4% of their theoretical CO
2
-binding potential. The CO
2
-binding ability of Ca
3
Mg(SiO
4
)
2
was considerably lower
and (Ca,Na)
2
(Mg,Al)(Si,Al)
3
O
7
was not active toward CO
2
[13, 14].
Iron and steel slags are byproducts from iron and steel manufacturing, and consist mainly of calcium,
magnesium, and aluminum silicates in various combinations [15]. Accelerated carbonation of steel slags is in most
cases carried out in water slurry phase (S/L<1 w/w) at elevated pressure and temperature [16]. CO
2
uptake is
influenced by residue composition (iron and steel slags are highly variable with respect to their composition [17])
and operational parameters (pressure, temperature, particle size distribution) [16].
This study was focused on comparative carbonation of industrial wastes in which Ca and Mg are generally bound
as silicates. Waste materials such as pulverized firing oil shale ash, electric arc furnace slag (types 1 and 2) and ladle
slag were studied as sorbents for binding CO
2
from flue gases in direct aqueous mineral carbonation process at mild
operating conditions (room temperature and atmospheric pressure).
2. Materials and Methods
2.1. Characterization of the samples
The samples were characterized by chemical analysis and quantitative X-ray diffraction (XRD) methods as well
as by BET (absorption theory by Brunauer, Emmet and Teller) and scanning electron microscopy (SEM) methods.
For XRD analysis in a Dron-3M diffractometer using Ni-filtered Cu-Kα radiation, powdered non-oriented
preparations were made. Diffractograms were digitally registered within 2-50
o
2θ range and analyzed by Sirquant
[18] code using full-profile Rietveld analysis [19]. Specific surface area (SSA) was estimated with BET method at
Sorptometer KELVIN 1042 (Costech Microanalytical Ltd.). Scanning electron microscope Jeol JSM-8404 was used
for surface observations.
As a pre-treatment, the EAFS types 1 and 2 were ground in a ball mill (d <100 µm). LS and PFA were used as
received basis (d
mean
=24 µm and d
mean
=42 µm, respectively).
2.2. Experimental setup
The aqueous suspensions of initial samples (Table 1) were treated at S/L = 0.1 with CO
2
containing model gas
(50 L/h; 15% CO
2
in air) in an absorber for 65 minutes (Figure 1a). The carbonation process was carried out at room
temperature and atmospheric pressure. Carbonation products (cPFA, cEAFS1, cEAFS2 and cLS) were characterized
by chemical analysis and quantitative XRD methods as well as by observations with scanning electron microscope.
Theoretical extent of carbonation (g/100g) was calculated according to Huntzinger et al. [20]:
ThCO
2
=0,785(%CaO-0,56·CaCO
3
-0,7·%SO
3
)+1,091·%MgO+0,70·%Na
2
O+0,468(%K
2
O-0,632·%KCl) (1)
To estimate the CO
2
-binding potential of oil shale ash components of low water-solubility (in which Ca and Mg
are bound as silicates) exclusively the aqueous carbonation process was also carried out with the lime depleted
material (LDM). PFA (20 g) was repeatedly (7 times) treated with distilled water (1000 mL) to prepare LDM. The
Author name / Energy Procedia 00 (2010) 000–000 3
aqueous suspension of PFA was stirred for 15 minutes and filtered to separate the solid phase. The separated solid
phase was then contacted with distilled water again to repeat the procedure (7 times). The final product (LDM) was
analyzed for its chemical and phase composition. As a next step the aqueous suspension of LDM (S/L=0.1) was
treated with CO
2
containing model gas (50 L/h; 15% CO
2
in air) in an absorber for 37 minutes (Figure 1a) at room
temperature and atmospheric pressure. Carbonated lime depleted material (cLDM) was separated by filtering and
analyzed for its chemical and phase composition as well as by observations with scanning electron microscope.
Figure 1. Laboratory batch setup (a) and process diagram for continuous mode aqueous carbonation of Ca-containing waste material (b)
3. Results and discussion
3.1. Characterization of the initial samples
The initial samples were characterized by chemical analysis (Table 1) and quantitative XRD methods (Table 2)
as well as by BET (Table 1) and SEM methods (Figures 2a,e,g).
PFA contained 51% of total CaO, from which 44% (22.4 abs-%) was in the free form (CaO, Ca(OH)
2
) and 37%
bound into Ca-Mg-silicates (CaSiO
3
, Ca
2
SiO
4
, Ca
3
Mg(SiO
4
)
2
) (Tables 2, 3). Minor amounts of CaO were bound into
sulphates and carbonates. Previous studies about the composition of oil shale ash have shown relatively good
correlation between the chemical and quantitative XRD analysis and the latter can be used for preliminary express
analysis [12]. However, some inconsistencies may occur (content of CaO, Table 2). The particles of PFA were
characterized by regular spherical shape with smooth surface (Figure 2a) and relatively low SSA (Table 1).
The mineralogy of the EAFS and LS samples was very complex and consisted of a number of silicate phases.
EAFS1 contained up to 60% of various Ca-Mg-silicates such as merwinite (Ca
3
Mg(SiO
4
)
2
), montecilillite
(CaMgSiO
4
) and cuspidine (Ca
4
Si
2
O
7
(F,OH)
2
) (Table 2). EAFS2 contained predominantly Mg-compounds:
pyroxene ((Mg,Fe)
2
Si
2
O
6
), spinel (MgAl
2
O
4
) and Mg-olivine ((Mg,Fe)
2
SiO
4
) (Table 2). Total CaO content was the
highest in the case of LS (42%), which contained mainly calcium silicate (Ca
2
SiO
4
), mayenite (Ca
12
Al
14
O
33
) and
akermanite (Ca
2
MgSi
2
O
7
) (Tables 1, 2).
The particles EAFS1 and LS were characterized by sharp edges and smooth non-porous surface (Figure 2e,g).
The specific surface areas of LS and ground EAFS was on the same level with PFA (SSA=1.23-1.71 m
2
/g).
Table 1 Chemical composition* and physical characteristics of the initial materials
SiO
2
t
, % Al
2
O
3
t
, % CaO
t
, % CaO
f
, % MgO
t
, % Fe
2
O
3
t
, % CO
2
, % SSA, m
2
/g
PFA 21.90 5.25 51.19 22.40 4.93 3.98 5.41 1.84
EAFS 1 32.34 5.28 36.12 0.15 18.95 2.68 1.47 1.71
EAFS 2 39.76 18.98 26.91 0.13 18.95 2.87 1.06 1.63
LS 15.02 22.34 42.22 0.37 14.99 0.79 1.69 1.28
*: Chemical analysis performed at the accredited Laboratory of the Geological Survey of Estonia
Characterization of solid samples:
q-XRD; chemical analysis; SEM; BET
(b)
Reactor 1
Reactor 2
Separator
Ash
Aqueous
phase
Neutralized
ash
Flue gas
Treated
gas
Aqueous
phase
PFA
LDM
EAFS1
EAFS2
LS
cPFA
cLDM
cEAFS1
cEAFS2
cLS
(a)
Carbonation treatment
Magnetic
stirrer
CO
2
AIR
pH, TDS
4 Author name / Energy Procedia 00 (2010) 000–000
a) b)
c) d)
e) f)
g) h)
Figure 2. SEM images of the initial (a - PFA, e - EAFS1, g - LS) and treated materials (b - cPFA, c - LDM, d - cLDM, f - cEAFS1, h - cLS).
Author name / Energy Procedia 00 (2010) 000–000 5
Table 2 Phase composition of the initial materials
Component, % PFA EAFS 1 EAFS 2 LS
Calcite CaCO
3
9.55 2.5 2.3 1.9
Dolomite CaMg(CO
3
)
2
3.34
Portlandite Ca(OH)
2
1.42 tr
Lime CaO 29.52
Periclase MgO 4.27 3.8 tr 11.3
Brucite Mg(OH)
2
0.9 2.2 tr
alpha-Ca
2
SiO
4
Ca
2
SiO
4
alpha 1.99 18.3
beta-Ca
2
SiO
4
Ca
2
SiO
4
beta 16.92 14.8
Melilite (Ca,Na)
2
(Mg,Al)(Si,Al)
3
O
7
4.99
Merwinite Ca
3
Mg(SiO
4
)
2
6.81 19.8 1.4 1.5
Anhydrite CaSO
4
4.48
Gypsum CaSO
4
*2H
2
O 0.76
Wollastonite 2M CaSiO
3
2M 3.88 1.4 2.3 1.5
Hematite Fe
2
O
3
1.19 tr tr tr
Quartz SiO
2
7.38 0.7 1.4 tr
Orthoclase KAlSi
3
O
8
3.51 1.5 2.2 1.0
Montecillite CaMgSiO
4
32.9
Cuspidine Ca
4
Si
2
O
7
(F,OH)
2
15.8 14.8
Akermanite Ca
2
MgSi
2
O
7
3.8 tr 11.3
Mayenite Ca
12
Al
14
O
33
20.2
Pyroxene (Mg,Fe)
2
Si
2
O
6
5.4 42.0 1.8
Spinel MgAl
2
O
4
9.7 13.6 6.4
Mg-olivine Mg
2
SiO
4
10.6
Brownmillerite Ca
2
(Al,Fe)
2
O
5
1.4 2.3 0.7
Bredigite Ca
7
Mg(SiO
4
)
4
3.5
Garnet Ca
3
Al
2
(SiO
4
)
3
6.1
alfa-Fe 0.8
Σ 100.00 99.7 99.0 98.9
3.2. The CO
2
-binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash
The quantitative chemical and phase composition as well as distribution of Ca-compounds for initial (PFA) and
treated materials (cPFA, LDM, cLDM) was determined (Table 3, Figure 3a,b). The Ca-compounds consistent in the
initial and treated ash were divided into four groups: 1) free CaO (CaO, Ca(OH)
2
), 2) CaO bound into silicates (α-
Ca
2
SiO
4
, β-Ca
2
SiO
4
, (Ca,Na)
2
(Mg,Al)(Si,Al)
3
O
7
, Ca
3
Mg(SiO
4
)
2
, CaSiO
3
), 3) CaO bound into sulfates (CaSO
4
,
CaSO
4
•2H
2
O), 4) CaO bound into carbonates (CaCO
3
, CaMg(CO
3
)
2
). To compare the CO
2
-binding efficiency of
different groups the quantitative changes in every step of the treatment were recalculated on the basis of the initial
material (PFA).
Results of quantitative XRD indicated that the main CO
2
binding component of oil shale ash was as expected
CaO (16.2 g CO
2
/100 g PFA) (Table 3, Figure 3a). An additional amount of CO
2
was bound by Ca-silicates (9.6 g
CO
2
/100 g PFA) which formed 33% of the total CO
2
bound in direct aqueous carbonation of PFA (29 g CO
2
/100 g
6 Author name / Energy Procedia 00 (2010) 000–000
PFA, counting also CO
2
bound into CaMg(CO
3
)
2
and K
2
Mg(CO
3
)
2
by Mg and K compounds). The theoretical extent
of carbonation (Eq.1.) was 35 g CO
2
/100g PFA.
CaO
MgO
CaSiO
3
Ca
3
Mg(SiO
4
)
2
Ca
2
SiO
4
Ca(OH)
2
K
2
Mg(CO
3
)
2
CaMg(CO
3
)
2
CaCO
3
0
10
20
30
40
50
60
%
PFA
cPFA
(Ca, Na)
2
(Mg,Al)(Si,Al)
3
O
7
a)
CaO
MgO
CaSiO
3
Ca
3
Mg(SiO
4
)
2
Ca
2
SiO
4
Ca(OH)
2
CaMg(CO
3
)
2
CaCO
3
0
10
20
30
40
50
60
%
LDM
cLDM
(Ca,Na)
2
(Mg,Al)(Si,Al)
3
O
7
b)
Figure 3. Distribution of Ca-Mg compounds in the initial (PFA) and treated materials cPFA, LDM, cLDM) according to quantitative XRD
measurements.
According to the results of quantitative XRD the LDM contained residual lime only in fractional quantity. After
carbonation treatment (cLDM) most of the CO
2
was bound by Ca-silicates (5.4 g CO
2
/100g PFA), predominantly on
account of Ca
2
SiO
4
(Table 3, Figure 3b). The portion of Mg-compounds participating in CO
2
binding process was
insignificant: about 0.15 g of CO
2
per 100 g of PFA was bound by MgO and Ca
3
Mg(SiO
4
)
2
. The amount CO
2
bound
by Ca-silicates in the cycle of PFA→LDM→cLDM was to some extent lower as compared to direct carbonation of
PFA→cPFA (Table 3). Intensive chemical reactions like slaking and carbonation of lime trigger reaction heat and
the internal expansive forces cause ash particles to fracture and disintegrate [21], creating thereby more favorable
conditions for deeper carbonation of ash (including carbonation of Ca-Mg-silicates).
Table 3 Distribution of CaO bound into different groups of Ca-compounds in initial (PFA) and treated materials (cPFA, LDM, cLDM) and
bound CO
2
(according to chemical analysis and quantitative XRD measurements).
PFA cPFA Bound CO
2
LDM cLDM Bound CO
2
Content of CaO bound into
different groups of Ca-compounds
g/100g PFA
Free CaO 22.40 2.92 16.15 1.15 0.58 0.4
Sulphates 2.09 0.64 0.00 0.00 0.00 0.0
Silicates 19.24 7.00 9.61 17.53 10.71 5.4
Carbonates 6.36 40.12 Sum:25.8 13.00 20.43 Sum: 5.8
Leaching and carbonation treatment caused significant changes in the structure and surface characteristics of
PFA: the particles of cPFA and LDM were fractured and disintegrated and covered with porous and permeable
product layer (Figure 2b,c), as SSA increased from 1.8 to 13-16 m
2
/g. The particles of cLDM were partly covered
with tighter layer of CaCO
3
crystals (Figure 2d).
Based on recent studies on the carbonation of oil shale ash, a new method for eliminating CO
2
from flue gases by
Ca-containing waste material was proposed [22]. The process includes contacting the aqueous suspensions of Ca-
containing waste material with CO
2
containing flue gas in two steps: in the first step the suspension is bubbled with
flue gas keeping the pH levels in the range of 10–12 and in the second step keeping the pH levels in the range of 7–8
(Figure 1b). The water-soluble components such as free lime are carbonated in the first step and the components of
low solubility, in which Ca is generally contained in the form of silicates, are carbonated in the second step. This
enables optimal conditions for treating different phases of multicomponent waste materials. As another process
route, the free lime could without difficulty be separated from ash by leaching it into the aqueous solutions in order
to produce precipitated calcium carbonate as a commercial product.
Author name / Energy Procedia 00 (2010) 000–000 7
3.3. The CO
2
-binding by Ca-Mg-silicates in direct aqueous carbonation of steel slag
Quantitative XRD analysis of the carbonation products indicated that Ca
3
Mg(SiO
4
)
2
was the main CO
2
binding
component in EAFS1 (Figure 4a). The main carbonation product was calcite (CaCO
3
), indicating that Mg-
compounds were not reactive towards CO
2
at these mild conditions (Figure 4a). Consequently, the CO
2
binding
ability of EAFS2 was also marginal. The total amount of CO
2
bound by EAFS1 was 8.7g CO
2
/100 g EAFS1, which
formed 18% of the theoretical extent of carbonation calculated according to Eq. 1. EAFS2 bound only 1.9 g
CO
2
/100 gEAFS2 (5% of the theoretical extent of carbonation).
Although LS contained substantial amount of CaSiO
4
(Figure 4b) which according to the model experiments [14]
showed quite good CO
2
-binding characteristics under atmospheric pressure and room temperature, the actual
carbonation extent remained low. The CO
2
uptake of LS was 4.6 g/100g LS (10% of the theoretical extent of
carbonation). As LS contained a number of Ca-Mg-Al-silicate phases, it was not clear which ones were the main
participants in the CO
2
binding reactions. Grinding of the EAFS1 prior to carbonation treatment probably worked as
a mechanical activation method and enhanced the carbonation process [23]. This would explain the higher carbonate
contents of cEAFS1 as compared to cLS (LS was used as received basis). Carbonation treatment changed
considerably the structure and surface characteristics of EAFS1: the particles were covered with spindle-shaped
product layer (Figure 2f) and SSA increased from 1.7 to 13.9 m
2
/g. Changes in the shape and surface of cLS
particles were not as noticeable (Figure 2h).
CaMgSiO
4
Ca
4
Si
2
O
7
(F,OH)
2
(Mg,Fe)
2
Si
2
O
6
(Mg,Fe)Al
2
O
4
Ca
2
(Al,Fe)
2
O
5
Ca
2
MgSi
2
O
7
Ca
3
Mg(SiO
4
)
2
CaCO
3
Mg(OH)
2
MgO
CaSiO
3
0
5
10
15
20
25
30
35
40
45
50
%
EAFS1 cEAFS1
a)
MgO
(Mg,Fe)
2
Si
2
O
6
MgA
l2
O
4
; FeA
l2
O
4
Ca
12
Al
14
O
33
Ca
3
Al
2
(SiO
4
)
3
Ca
7
Mg(SiO
4
)
4
Ca
2
MgSi
2
O
7
Ca
3
Mg(SiO
4
)
2
Ca
2
SiO
4
CaSiO
3
CaCO
3
0
5
10
15
20
25
30
35
40
45
50
%
LS cLS
b)
Figure 4. Distribution of Ca-Mg compounds in initial (EAFS1 and LS) and treated materials (cEAFS1, cLS) according to quantitative XRD
measurements.
4. Conclusions
Direct aqueous carbonation of Ca and Mg silicates which were derived from the actual industrial wastes like PF
oil shale ash and steel slags (EAFS types 1 and 2 and ladle slag) was demonstrated. The experiments were carried
out at mild operating conditions: room temperature and atmospheric pressure. Comprehensive mineral composition
of the initial samples as well as the carbonation products was presented.
Quantitative XRD analysis indicated that Ca
2
SiO
4
and Ca
3
Mg(SiO
4
)
2
were the main CO
2
binding low solubility
components of PF oil shale ash as well as steel slags. The main carbonation product was calcite (CaCO
3
), indicating
that Mg-compounds were in most cases not reactive towards CO
2
at these mild conditions.
Results showed that Ca-Mg-silicate phases bound up to 9 g of CO
2
per 100 g of initial ash, which amounted to
30% of total CO
2
bound in direct aqueous carbonation of PFA. The total amount of CO
2
bound by PFA was 29 CO
2
g/100 g PF, which formed 83% of the theoretical extent of carbonation (46% bound by lime and 27% bound by Ca-
silicates). The CO
2
uptakes for steel slags (EAFS1, EAFS2 and LS) were 8.7g CO
2
/100 g EAFS1, 1.9 g CO
2
/100
gEAFS2 and 4.6 g/100g LS, which formed 18%, 5% and 10% of the theoretical carbonation extents, respectively.
Comparative carbonation of different Ca-Mg-silicates containing waste materials confirmed that the CO
2
-binding
ability depends significantly on the origin of the material as well as on the pretreatment conditions.
8 Author name / Energy Procedia 00 (2010) 000–000
Based on multifaceted studies about carbonation of oil shale ash, a new method for eliminating CO
2
from flue
gases by Ca-containing waste material was proposed. The process includes contacting the aqueous suspensions of
Ca-containing waste material with CO
2
containing flue gas in two steps: in the first step the suspension is bubbled
with flue gas keeping the pH levels in the range of 10–12 and in the second step keeping the pH levels in the range
of 7–8. The water-soluble components such as free lime are carbonated in the first step and the components of low
solubility in which Ca is generally bound as silicate are carbonated in the second step.
Acknowledgements
Authors express their gratitude to Dr. V. Mikli for SEM analysis. The research was supported by the Estonian
Ministry of Education and Research (SF0140082s08) and the Estonian Science Foundation (Grant No 7379).
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