TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies [611270]

TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
289OVERVIEW OF LATEST MINERAL CARBONATION
TECHNIQUES FOR CARBON DIOXIDE SEQUESTRATION
Bodor Marius1, Vlad Maria1, Baltăùtefan1
1Universitatea “Dun ărea de Jos” din Gala Ġi,
[anonimizat] ,[anonimizat] ,[anonimizat]
Abstract: This relatively new domain, of carbon dioxide sequestration for mitigation purposes, is one
of the carbon dioxide capture and storage (CCS) alternatives. CO 2sequestration could be used in the
future where geological storage, for example, is not an option due to the lack of suitable locations or
could represent a hazard to the surrounding areas. Also could be used in those locations where the needed
material (raw minerals or industrial wastes) are abundant.
The interest in this technique of CO 2 storage is growing fast and results of researches on this topic are
coming from a larger number of countries every year. What is more interesting is that the research of the
industrial wastes carbonation is developing, giving in the future new purposes for these wastes.
Keywords: mineral carbonation, carbon dioxide, calcium carbonate, storage
1. Introduction
The greenhouse effect is known to be created
by different gases in the atmosphere. One of these
gases is CO 2 and because its concentration is
increasing dangerously for the past one hundred
years is now considered to be the main cause of the
temperature rise on Earth.
In order to avoid the potentially devastating
consequences of global warming and climate
c h a n g e , t h e C O 2 emissions into the atmosphere
caused by human activities should be reduced
considerably [1]. For this goal to be reached some
carbon dioxide capture and storage (CCS)
technologies were proposed but for the moment
none of them were efficient enough in reducing the
CO 2 emissions. Despite the negative results,
researches are still ongoing and in some cases the
results are promising as for example the storage in
underground cavities. The same is hoped to be
happening with the CO 2 storage in minerals and
wastes and hopefully all the CCS technologies will
contribute together to a more efficient CO 2
reduction.
2. Carbon dioxide sequestration techniques
2.1. Geological storage
One of the technologies, that have already been
employed on a significant scale, but not large
enough to have a global CO 2 emissions mitigationimpact, is storage of CO 2 in underground cavities.
This includes the so-called Enhanced Oil Recovery
( E O R ) a n d a l s o E n h a n c e d G a s R e c o v e r y ( E G R ) ,
which are concepts aimed at improving the oil/gas
recovery potential of an oil/gas field by flooding it
with CO 2 [2].
Perhaps the greatest problem related to
underground storage is the permanency of the
solution, as there will always be a risk of leakage.
Therefore, this solution would require continuous
monitoring of storage sites for thousands of years.
2.2. Ocean storage
Another widely studied option for CO 2
sequestration involves injecting CO 2 into the ocean
at great depths, where the gaseous CO 2 reacts to
f o r m c a r b o n i c a c i d ( H 2CO 3). The carbonic acid
then dissociates into a (bi)carbonate ion and
hydrogen ion in accordance with the equation
below: [2]
CO 2(g) + H 2O(l)ർ H2CO 3(aq)ർ HCO-
3 + H+ർ
ർ CO2-
3 + 2H+(1)
Although ocean storage could provide a fast
and relatively easy alternative for CO 2 emissions
reduction it has lost its appeal in recent years,
largely due to the uncertainty when considering the
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
290environmental consequences (e.g. decreasing pH
of ocean water) and the lack of permanency [1].
2.3. Storage below sea bed
An alternative to both geological storage and
ocean storage described above is CO 2 storage
below the ocean floor at depths of at least 3000 m
of ocean and several hundred meters of marine
sediment. In contrast to the previously mentioned
options this option does not suffer from lack of
permanency (ocean storage) or the demand for
monitoring the storage site (geological storage).
T h e i d e a i s b a s e d o n t h e f a c t t h a t C O 2 becomes
denser than water at sufficient depths ( §3000 m),
but that it still needs to be trapped in order to
prevent it from being released by ocean currents or
e.g. earthquakes. Therefore, it should be stored
below the seabed [3].
This alternative is still new and further research
is ongoing in order to verify the theories.
2.4. Mineral carbonation
The reaction between a metal oxide bearing
material and CO 2 is called carbonation and can be
expressed by the following reaction:
MO + CO 2ർ MCO 3 + heat (2)
where in practice M describes a (metallic) element
such as calcium, magnesium or iron. The reaction
in Equation (2) is exothermic and the heat released
is dependent on the metallic element bearingmineral at hand (for the magnesium- or calcium-
based silicate minerals- olivine: 89 kJ/mol CO 2,
serpentine: 64 kJ/mol CO 2 and wollastonite: 90
kJ/mol CO 2 at 298 K). [2]
O n e m a j o r b e n e f i t o f C O 2 sequestration by
mineral carbonation consist of the environmentally
benign and virtually permanent trapping of CO 2 in
the form of carbonated minerals by using abundant
mineral resources such as Mg-silicates [2]. Unlike
other CO 2 sequestration routes it provides a
leakage-free long-term sequestration option,
without a need for post-storage surveillance and
monitoring once the CO 2 has been fixed.
In addition to the benefits of mineral
carbonation, this option is the only CO 2
sequestration option available where large
underground reservoirs do not exist and ocean
storage of CO 2 is not feasible.
Another benefit of mineral carbonation is that,
at least theoretically, the carbonation process could
proceed without energy input, but this has not yet
been accomplished.
Attempts to speed up the carbonation reaction
include using both dry and wet methods, additives,
heating and pressurizing the carbonation reactor,
dividing the process into multiple steps,
pretreatment of the mineral source and more
(fig.1).
There are several different elements that can be
carbonated, but alkaline earth metals, calcium and
magnesium, have proven to be the most suitable
due to their abundance and insolubility in nature.
Figure 1: Main carbonation processes and variants.
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
291In addition to the abundant magnesium and
calcium containing minerals, there are also
industrial solid residues that contain large amounts
of Mg, Ca and even Fe.
Currently the most investigated mineral
resources are olivine, serpentine and wollastonite.
F r o m t h e s o l i d r e s i d u e s , s t e e l s l a g h a s r e c e n t l y
received a lot of attention but other industrial
residues also have been studied: asbestos-mining
tailings, electric arc furnace (EAF) dust, cement-
kiln dust, waste concrete, coal fly ash etc.
Figure 1 displays the various carbonation routes
that are currently being investigated.
3. Directions of mineral carbonation
Trapping carbon dioxide in carbonates can be
achieved through various process routes as
described further, ranging from the most basic
accelerated weathering of limestone to advanced
multi-step processes.
3.1. Direct carbonation
Direct carbonation is the simplest approach to
mineral carbonation and the principal approach is
that a suitable feedstock, e.g. serpentine or a
Ca/Mg rich solid residue is carbonated in a single
process step. For an aqueous process this means
that both the extraction of metals from the
feedstock and the subsequent reaction with the
dissolved carbon dioxide to form carbonates takes
place in the same reactor.
3.1.1. Direct gas-solid carbonation
Gas-solid carbonation is a simple approach
towards mineral carbonation. Here particulate
metal oxides are brought into contact with gaseous
CO 2 at a p a rti c ul ar te m pe rature an d pre ssure . The
dry process has the potential of producing high
temperature steam or electricity while converting
CO 2 into carbonates.
Unfortunately, the reaction rates of such a
process have been too slow and the process suffers
from thermodynamic limitations and further
studies around this alternative have mostly been
abandoned.
One of the important benefits of using industrial
solid residues as feedstock for carbonation
compared to the carbonation of mineral ores is the
possibility of utilizing a waste stream. Thepossibility of simultaneously binding CO 2 and
lowering the hazardous nature of e.g. municipal
solid waste incinerator (MSWI) ash makes this
carbonation route interesting [4]. However, the
potential CO 2 storage capacity for this option is
limited, simply because the amounts of material
that may be carbonated are too small [4].
Nevertheless, where these wastes are present
the carbonation should be taken into consideration
b e c a u s e a s s h o w n b y R . S a n t o s et al. [5] during
the carbonation the strength of the stainless steel
slag is increased. This will give an added value to
the carbonated stainless steel slag wherefore the
material can be used in secondary applications.
The sale price of this product might increase.
3.1.2. Direct aqueous carbonation
The direct aqueous mineral carbonation-route
referring to carbonation preformed in a single step
in an aqueous solution, appears to be the most
promising CO 2 mineralization alternative to date
[6]. High carbonation degrees and acceptable rates
have been achieved but the process is (still) too
expensive to be applied on a larger scale [7].
Ranging from 40–80 €/t CO 2 mineralized (includes
energy use) compared to 0.4–6 €/t CO 2 [ 2] s to re d
for geological storage.
Direct aqueous mineral carbonation can be
further divided into two subcategories, depending
on the type of solution used. Studies focusing on
carbonation in pure aqueous solutions have quickly
made way for additive-enhanced carbonation
experiments and a common solution type used
today, originally presented by O’Connor et al.
(2000) [8], consists of 0.64 M NaHCO 3 + 1.00 M
NaCl. However, it has been reported [9] that there
are still improvements to be made regarding the
above mentioned solution.
When it is necessary to use additives in
carbonation processes it is extremely important to
recycle these, due to the large scale of any
industrial application [7].
Work on finding optimal aqueous carbonation
conditions is ongoing and even though it has been
studied extensively, some questions still remain
unanswered. For example increasing the
liquid/solid ratio (L/S) has been reported to have
both a positive and a negative effect on CO 2
conversion.
The advances made to aqueous solution
chemistry by McKelvy et al. [9] were significant,
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
292but unless the (expensive) additives used cannot be
recycled the process route becomes unattractive.
Nevertheless, the studies conducted on direct
aqueous carbonation have improved the overall
knowledge of aqueous carbonation reactions
considerably.
3.2. Indirect carbonation
When the mineral carbonation process is
divided into several steps it is classified as indirect
carbonation. In other words, indirect carbonation
means that the reactive component (usually Mg or
Ca) is first extracted from the feedstock (as oxide
or hydroxide) in one step and then, in another step,
it is reacted with CO 2 t o f o r m t h e d e s i r e d
carbonates.
3.2.1. Multistage gas-solid carbonation route
Direct gas-solid carbonation of silicate minerals
has been shown to be too slow for any large scale
implementations, but a staged gas-solid
carbonation process could overcome the slow
reaction kinetics. The process involves extraction
of magnesium (oxide or hydroxide) in an
atmospheric pressure step followed by a
carbonation step at elevated temperature (>500 °C)
and pressure (>20 bar) [10].
It has been found that the carbonation of MgO
is significantly slower than the carbonation of
Mg(OH) 2 [11]. Using this observation Zevenhoven
et al. [10] suggested (noting that Mg(OH) 2
production from serpentine in one step cannot be
done because of thermodynamic limitations) that
the direct gas-solid carbonation process should be
divided into three-steps: 1) MgO production
(Equation 3) in an atmospheric reactor followed by
2) MgO hydration (Equation 4) and 3) carbonation
(Equation 5) at elevated pressures:
Mg 3Si2O5(OH) 4(s) ĺ
ĺ 3MgO(s) + 2SiO 2(s) + 2H 2O (3)
MgO(s) + H 2O ļ Mg(OH) 2(s) (4)
Mg(OH) 2(s) + CO 2ļ
MgCO 3(s) + H 2O (5)
In addition to the faster carbonation kinetics in
the three-step gas-solid carbonation route
described above, the process is also preferable
from an energy efficiency point of view comparedto the two step carbonation of MgO [10]. However,
the three-step process is still too slow for large
scale implementation, as preliminary tests
performed in up to 45 bar pressures have shown.
Progress has been made in enhancing the direct
gas-solid reaction rates, primarily by increasing
pressure. Experiments showing that all three-steps
in Equations 3–5 are fast enough for industrial
implementation are still required.
Dividing the gas-solid carbonation route into
several steps could be beneficial, but there is not
enough evidence yet for industrial viability.
3.2.2. Acetic acid route
In order to speed up the aqueous carbonation
process, the use of acetic acid for the extraction of
calcium from a calcium-rich feedstock has been
suggested by Kakizawa et al. [12]. In principal it
consists of two-steps as given in Equations 6 and
7:
CaSiO 3 + 2CH 3COOH ĺ
ĺ Ca2+ + 2CH3COOí + H 2O + SiO 2 (6)
Ca2+ + 2CH 3COOí + CO 2 + H 2O ĺ
ĺ CaCO 3 + 2CH 3COOH (7)
Equation 6 describes the extraction step and
Equation 7 the precipitation step. In principal the
acetic acid used in the extraction step could be
recovered in the following precipitation step.
Inspired by the concept of binding CO 2 in
calcium extracted from a calcium silicate such as
wollastonite using acetic acid [12], Teir et al. [13]
investigated the possibility of producing a high
value PCC material from calcium silicates and
later from other calcium-containing materials [14].
Steelmaking slags then became the centre of
attention as they can contain significant amounts of
both CaO and MgO. Eloneva S. [15] reported that
80–90 % pure calcite was produced from blast
furnace slag using acetic acid. However,
significant amounts of sodium hydroxide were
required for promoting the precipitation of
carbonates from the acidic solution.
Research in Finland has focused on steel slag
carbonation and especially the possibility of
producing valuable precipitated calcium carbonate
(PCC) [14, 15]. The annual CO 2 binding potential
of steelmaking slags is in the order of 70 – 180 Mt
CO 2. Glo bally the CO 2 sequestration potential for
this option is small, but for individual steel plants,
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
293however, the method could provide significant
economical benefits.
3.2.3. Two step aqueous carbonation
Two-step aqueous carbonation has been
investigated because the overall carbonation
reaction can easily be divided into two-steps:
extraction and precipitation that may be
investigated and optimized separately.
By upgrading a waste product into a product of
high commercial value, expensive CO 2
sequestration processes could become
economically feasible. One such approach has
been investigated by Katsuyama et al. [16] who
studied the use of waste cement for the
development of high-purity CaCO 3 b y C O 2
carbonization.
Katsuyama et al. [16] studied the feasibility of
producing CaCO 3 f r o m w a s t e c e m e n t b y f i r s t
extracting calcium from pulverized waste cement
i n a w a t e r s l u r r y a t h i g h C O 2 pressure (several
M P a ) , f o l l o w e d b y t h e p r e c i p i t a t i o n o f C a C O 3
from the extracted solution at lower CO 2 pressures,
producing high purity CaCO 3 (up to 98%) from
waste cement at relatively high reaction rates. The
estimation of the cost of producing high-purity
CaCO 3 c o u l d b e a s l o w a s 1 0 5 € / m3 when
compared to the commercial price of 154–269
€/m3. In addition, if the produced CaCO 3 could be
purified to meet the requirements of ultra-high
purity CaCO 3 ( >99% C aCO 3) the potential profits
could increase substantially. The current cost of
ultrahigh purity CaCO 3 is around 7,700 €/m3, while
Katsuyama et al. [16] estimated a production cost
of only 250 €/m3.
Gorset et al. [17], describing a way of
producing pure MgCO 3 from olivine, claims that
the process consisting of one dissolution step and
two precipitation steps is rapid enough for large
scale implementation. The process does not require
the use of strong mineral or organic acids even
though the dissolution step requires an acidic
environment. The required acidity (pH 3–5) is to
be achieved using pressurized CO 2 (50–150 bar)
and a temperature around 100–170 °C, while the
following step consisting of MgCO 3 precipitation,
takes place in another reactor with preferably a
lower CO 2 pressure (50 – 80 bar) and a higher
temperature (140 – 250 °C) favoring the
precipitation of carbonates. Experimental results
showed a high degree of purity, between 99.28 and
99.44% MgCO 3, of the precipitated carbonate.The pH-swing process developed in Japan is
another two-step aqueous carbonation process
w h e r e a t f i r s t t h e p H o f t h e s o l u t i o n i s l o w e r e d
thereby enhancing the extraction of divalent metal
ions. In the second step the pH is raised to enhance
the precipitation of carbonates.
The principal reactions taking place inside the
extractor (Equation 8) and the precipitator
(Equation 9) are:
4NH 4Cl + 2CaO·SiO 2ĺ
ĺ 2CaCl 2 + 4NH 3 + 2H 2O (8)
4NH 3 + 2CO 2 + 2H 2O + 2CaCl 2ĺ
ĺ 2CaCO 3 + 4NH 4Cl (9)
Equation 9, taking place inside the precipitator,
consists of both CO 2 absorption and CaCO 3
precipitation. In their study, Kodama et al. [18]
investigated a CO 2 sequestration process that
utilizes pH swing using NH 4Cl. The energy input
requirement for the investigated process using steel
making slag as the mineral source was estimated at
around 300 kWh/t CO 2, but the loss of a chemical
additive (NH 3) was considerable.
Different approaches of a two-step aqueous
carbonation process have been presented. All
options are good in theory, but it remains uncertain
whether or not these processes could lead the way
to any significant scale long-term storage of CO 2 in
the future. More experiments for large scale
viability are required.
3.3. Other processes of carbon dioxide
sequestration
Beside the process routes mentioned above,
there are other processes and applications that
resemble mineral carbonation and are described in
the following section.
3.3.1. The precipitated calcium carbonate
production
The production of valuable products ( e.g.PCC)
by utilizing CO 2 was studied extensively. An
example of this concept is Two-step aqueous
carbonation of solid residues. Various methods to
obtain a product of desired properties have been
used and one of the simplest methods is that of
direct aqueous carbonation without the use of
additives.
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
2943.3.2. Brines used in carbonation
T h e s a l i n e – b a s e d s o l u t i o n f o r m e d a s a w a s t e
product during the oil or natural gas extraction is
called brine and it is found in large quantities in
storage tanks. Due to its large amounts available
and a high concentration of elements fit to form
carbonates (mostly calcium and magnesium) this
brine represents an option for carbon dioxide
storage by a carbonation process. Although this
brine is capable of forming carbonates, its use in a
large scale process is for the moment not
appropriate due to the slow kinetics. If the pH of
the brine is raised the carbonation process is faster,
but unknown factors about the parameters
(temperature, pressure, brine composition and pH)
need to be further studied.
3.3.3. Accelerated weathering of limestone
Another option is carbon dioxide capture and
storage by accelerated weathering of limestone
(AWL). This option imitates the natural carbonate
weathering according to the following reaction:
CO 2(g) + H 2O(l) +CaCO 3(s) ĺ
ĺ Ca2+(aq) + 2HCO 3í(aq) (10)
The product of an AWL plant would be a
calcium bicarbonate solution that could readily be
released and diluted into the ocean with a minimal
environmental impact [19]. However, there are still
many issues to deal with, such as the energy
demand of transporting large amounts of calcium
containing (waste or mineral) material to the AWL
plant that preferably should be located near a CO 2
point source as well as a possible disposal site ( e.g.
t h e o c e a n ) . I n a n i d e a l c a s e ( w i t h a c c e s s t o f r e e
limestone, e.g. waste fines, and a “free” water
source, e.g. power plant cooling water) the CO 2
mitigation cost by means of AWL could be as low
as 2.3–3.1 €/ton CO 2. Rau et al. [19] suggests that
some 10-20% of the United States point-source
CO 2-emissions could be mitigated this way.
D e s p i t e t h e p o t e n t i a l p o s i t i v e e f f e c t o f
bicarbonate disposal Rau et al [19] concludes that
further research is needed to fully understand the
impacts of AWL effluent disposal in the ocean.
Some of the most important applications
and techniques for mineral storage of CO 2 are
d e s c r i b e d a b o v e , y e t a l m o s t a l l o f t h e s e
options need further research before starting alarge scale process. Despite the inconvenience
given by results lacking, all of them should
represent an option when CO 2 emission
reduction is taken into account.
4. Conclusions
It can be concluded that the carbonation
techniques using additives for reactivity rising
gave better results than those not using additives
and for that, studies were performed with the
purpose of a better understanding on the reactions
complexity regarding the carbonation. Despite
some good results the performed studies have not,
for the moment, came with significant discoveries
regarding the reactivity in mineral carbonation
process.
For the techniques using additives in
carbonation the biggest problem represents the
recycling of these additives and for the moment no
such breakthrough was reported and if this will
never be accomplished, maybe method’s that don’t
need them will be implemented.
B e s i d e s t h e c o s t o f t h e p r o c e s s t h a t i s f o r t h e
moment larger than of those other carbon dioxide
capture and storage methods, the direct aqueous
carbonation seems to be the most promising option
of mineral carbonation. Other studies, however,
show that the dissolution and the precipitation
steps should be separated, even if the costs will be
higher. This separation would take out the need to
balance these reactions that are opposite. So,
taking that into consideration, the indirect aqueous
carbonation is the most attractive option.
Despite the good results on the high purity
precipitated calcium carbonate process, for the
moment, no large scale production was realized. If
this will be accomplished it is expected for the
price of carbonation process to be acceptable even
if is at a highe r rate , due to the gre at value of the
high purity PCC.
Before one or more mineral carbonation
techniques could be implemented on an industrial
scale, much more research should be done and
important improvements of existing options are
needed. Another way for the large scale
carbonation to become reality would be
represented by completely new techniques.
5. References
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
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Acknowledgements
Bodor Marius would like to acknowledge
the support provided by the European Union,
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TEHNOMUS – New Technologies and Products in Machine Manufacturing Technologies
296Romanian Government and “Dun ărea de Jos”
University of GalaĠ i, through the project
POSDRU – 107/1.5/S/76822.
ùtefan Balt ă would like to acknowledge the
support provided by the European Union,
Romanian Government and “Dun ărea de Jos”
University of GalaĠ i, through the project
POSDRU – 6/1.5/S/15.
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