1. Thermal Activation of Antigorite for Mineralization of CO2
Reydick D. Balucan and Bogdan Z. Dlugogorski*
Priority Research Centre for Energy, The University of Newcastle, Callaghan, NSW 2308, Australia
*S Supporting Information
ABSTRACT: This contribution demonstrates the sensitivity of antigorite
dehydroxylation to treatment conditions and discusses the implications of
the observations for scientific (i.e., dehydroxylation kinetics) and
technological (i.e., energy efficient conditions and design of practical
activation reactors) applications. At present, the energy cost of
dehydroxylation of serpentinite ores represent the most important
impediment for a large scale implementation of sequestering CO2 by
mineralization. We have analyzed changes in antigorite’s derivative
thermogravimetric curves (DTG) and deduced factors affecting the mass
loss profiles. The imposed heating rate, type of purge gas, type of
comminution and sample mass all influence the dehydroxylation curve.
However, the results show no influence of material of construction of the
heating vessel and flow rate of the purge gas. We report an important effect
of oxidation of Fe2+
under air purge gas that occurs prior to dehydroxylation and leads to formation of hematite skins on
serpentinite particles, slowing down subsequent mass transfer and increasing the treatment temperature. From the process
perspective, 75 μm particles afford optimal conditions of temperature and rate of dehydroxylation. Overall, the practical
considerations, in thermally activating serpentinite ores for storing CO2 by carbonation, comprise rapid heating, proper size
reduction, prior demagnetisation, and fluidization of the powder bed.
1. INTRODUCTION
Accurate measurements of the dehydroxylation of serpentine
minerals during thermal treatment allow deriving the
thermokinetic parameters and calculating the necessary heat
requirements. Such measurements may also serve to develop
new technologies for activating serpentines, and to design
equipment items, for implementing CO2 sequestration by
mineralization at a realistic scale. However, despite the
numerous thermal studies on serpentine minerals1−15
only a
small number of investigations examined the effect of treatment
conditions.9,14,16,17
To the best of our knowledge, no study
evaluated the influence of these conditions for preparing
activated serpentine minerals for their carbonation. Inves-
tigations are needed to identify and quantify the effect of the
treatment parameters on the thermal activation of serpentines.
Such investigations must supply information of kinetics of
serpentine dehydroxylation, that, in combination with heat
transfer parameters, could serve to design unit operations
(equipment items) for testing the viability of mineral
carbonation at a pilot plant scale.
Outstanding questions include the determination of the
suitable feedstock for heat activation (crushed versus ground),
the appropriate operational sequence and cost efficiency
(kWhe) for each option. Operational viability and cost
efficiency of thermal activation of serpentines for mineral
carbonation must dictate the material’s particle size,14
and
hence comminution technology. Small scale experiments could
assist in identifying the practical particle size among those
currently used (−38 to −75 μm)18−21
in serpentine
carbonation at a laboratory scale.
The influence of the material of construction of a unit
operation and type of a purge gas on efficiency of the
processing operation remain poorly understood. Both variables
require attention due to variable mineralogical and chemical
composition of serpentinite rocks.22−24
Not only do these rocks
host the rock-forming serpentine minerals (lizardite, antigorite,
and chrysotile), they may retain their relict peridotitic minerals
(forsterite and enstatite) as well as contain various amounts of
metal oxides associated with the serpentinisation process (i.e.,
the exothermic hydration of the peridotitic minerals). Cations
such as Fe2+
, Fe3+
, and Al3+
are incorporated into the octahedral
and tetrahedral sheets of the serpentine minerals.23,25−28
Normally, Fe2+
replaces Mg2+
in octahedral sheet, whereas
Fe3+
and Al3+
may appear both in octahedral and tetrahedral
sheets, replacing Si4+
in the latter.29
The oxidation of Fe2+
was
reported to influence the dehydroxylation process.30,31
It is
therefore of practical significance, to the design of a
dehydroxylation reactor, to understand the suitability of a
refractory vessel (e.g., alumina) and purge gases (e.g., CO2,
water vapor or air).
Special Issue: Carbon Sequestration
Received: September 11, 2012
Revised: October 28, 2012
Accepted: November 28, 2012
Published: December 6, 2012
Article
pubs.acs.org/est
© 2012 American Chemical Society 182 dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190

2. The present study examines antigorite, the most thermally
stable serpentine mineral1
which contains the highest
proportion of oxidizable Fe2+
among the three common
polymorphs, and is expected to present higher energy
requirements for thermal processing than those of lizardite
and chrysotile. Because of this consideration, antigorite
provides a conservative benchmark of energy requirement for
thermal processing of serpentines. Previous evaluations
assumed electrical heating rather than the direct use of thermal
heat for activation, leading to unrealistic estimates of energy
intensity of CO2 storage by mineral carbonation.18,32
The overall scientific objective of this work was to gain
insights into the behavior of antigorite undergoing dehydrox-
ylation, and, in particular, to determine how the rate of
dehydroxylation is affected by the experimental conditions and
mineral preparation. Specific objectives were (a) to quantify the
effect of hematite formation from oxidizable Fe2+
present in the
mineral on the rate of dehydroxylation and to investigate the
influence of the atmosphere (i.e., oxidative vs reductive) on the
rate, as we hypothesized that the appearance of a hematite layer
may add mass transfer resistance to the removal of water vapor
from the mineral; (b) to investigate the influence of
comminution type (wet vs dry grinding) on the dehydrox-
ylation of antigorite and examine the feasibility of dehydrox-
ylation of crashed but unground mineral, as there is mounting
evidence that surface properties of mineral grains may modify
their thermal behavior; (c) to study the relationship between
the rate of dehydroxylation and the rate of heating, and the
dependence of the rate of dehydroxylation on particle size; as
well as (d) to investigate the effect of liberated water vapor, as
water vapor may engender backward reactions and its presence
may force the dehydroxylation to take place closer to
thermodynamic equilibrium delaying the onset of the process.
2. EXPERIMENTAL SECTION
Antigorite, obtained near Bingara in the Great Serpentinite Belt
in NSW, Australia (location: 30.122217 S and 150.635966 E)
was prepared into various sample fractions by crushing (sample
E) and subsequent grinding (A, B, C, D). The wet ground
samples (A, B, C) were demagnetised prior to heat activation,
while the dry ground (D) and crushed (E) samples were used
without further demagnetisation. Ground materials (A, B, C,
D) are all in powder form while a single shard represents the
dry crushed sample (E). Table 1 details the chemical and
mineralogical composition as well as the particle sizes of the
sample fractions.
X-ray powder diffraction (XRPD) in a Philips X’Pert Pro
multi purpose diffractometer, using Cu Kα radiation in the
range of 6−90° 2θ, with a step size of 0.02° and collection time
of 1 s step−1
, afforded the identification of crystalline
composition (Figure S4, Supporting Information). The
diffraction patterns were automatched against the International
Center for Diffraction Data using X’pert Highscore, and
confirmed visually for validity.34
Chemical composition of the
natural antigorite was characterized via X-ray fluorescence by
Amdel Laboratories, whereas a Spectro X’lab 2000 at the
University of Newcastle EM-X-ray Unit was used to analyze the
demagnetised samples. Particle size distribution was obtained
via low angle laser light scattering (LALLS) using a Malvern
Mastersizer 2000 laser sizer in aqueous media. Micrographs of
the gold coated samples of the starting and quenched materials
Table 1. Chemical, Mineralogical and Physical Properties of the Sample Fractions; all Particle Sizes are in μm
A B C D E
demagnetised
fractions
magnetic
fractions
chemical composition of starting material, % weightc
SiO2 43.2 41.8
MgO 38.2 38.2
FeOa
3.08 3.08
Fe2O3
b
1.74 3.79
Al2O3 1.04 0.89
CaO 0.13 0.05
Na2O 0.11 0.05
LOI 11.9 12.0
mineral composition (International Center for Diffraction Data reference code) starting material antigorite-8.0 M (00−007−
0417), dimagnesium oxide
dihydroxide (01−070−9187)
antigorite-8.0 M
(00−007−
0417) periclase
(01−077−
2364), triiron
tetroxide
(01−089−
0691)
quenched materiald
forsterite (00−034−0189), enstatite (00−019−
0768)
particle size of starting material, μm (±3.8 μm) d3,2 5.31 4.85 4.37 2.87
d4,3 44.2 31.0 17.9 6.35
d90 125 86.4 47.2 14.2
d50 15.7 11.8 9.03 4.23 ≥2000
d10 1.82 1.73 1.69 1.39
d80
e
75.0 52.0 31.0 10.0
a
Obtained from titration, represents the Fe2+
content of the primary rock. b
Represents magnetite (Fe3O4) produced from serpentinisation. c
Trace
components <0.05% are: SO3, TiO2, K2O, organic C and inorganic C. d
After heating to 1000 °C. e
Derived from the power trendline of the d10, d50,
and d90 value.
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3. were collected using a Philips XL30 scanning electron
microscope (SEM) operated at 15 kV at 20000× magnification.
A Setaram Setsys Evolution 1200 thermogravimetric
analyzer-differential scanning calorimeter (TGA-DSC) using
SeftSoft 2000 software recorded the thermogravimetric (TG)
and derivative thermogravimetric (DTG) curves. The replicate
runs were acquired at random dates within a period of three
months to check for instrumental drift and ensure experimental
reproducibility for the entire duration of the study. The
experimental conditions for the control experiment are as
follows: 5.5 ± 0.1 mg wet ground, d80 = 75 μm demagnetised
antigorite (Sample A), open cylindrical alumina crucible (5 mm
Ø, 8 mm height, 0.10 cm3
capacity), heated from 30 to 1000 °C
at a heating rate (β) of 10 °C min−1
, under argon purge gas
flowing at 20 mL min−1
. The samples, once loaded, were
slightly tapped twice in order to distribute the powder evenly
while avoiding the compaction of the sample bed. Table 2
summarizes the conditions used to evaluate the effects of the
experimental variables on antigorite dehydroxylation.
3. RESULTS AND DISCUSSION
Antigorite dehydroxylation, as seen in the DTG curve in Figure
1, covers a wide temperature region varying from ∼500 to 800
°C. This temperature region defines the removal of structurally
bound water, constituting 11.43 ± 0.03% w/w antigorite
(Δm105−850). The mass loss of <0.5% w/w from 105 to 500 °C
indicates negligible amounts of either adsorbed moisture or
deformed hydroxyl groups. It must be noted that, full
dehydroxylation is attained at temperatures in excess of 850
°C, as such all the quenched products (heated up to 1000 °C,
then cooled) are fully dehydroxylated. Weight normalized DTG
curves show the peak temperatures, Tp, at 715 ± 2 °C (Tp1)
and 736 ± 2 °C (Tp2). The DTG curve of this sample is typical
for antigorite, whereby peak temperatures (Tp) are in excess of
720 °C. Overall, the dehydroxylation profile of this antigorite
specimen (sample A) is in agreement with the previously
reported thermal profiles of other antigorites.1
Although the DTG curves generated by the small samples
size (5.5 mg) are uncontaminated by heat and mass transfer
limitations, the curves’ attributes are not as smooth as those
obtained at higher masses >30 mg.1
The pertinent thermal
signatures include the serpentine doublet which comprises a
low temperature shoulder, Tsh and the first peak temperature,
Tp1, as well as antigorite’s diagnostic high temperature peak, Tp2
(Table S4, Supporting Information). The low temperature
shoulder, Tsh, encompasses 635 to 679 °C and is common
among serpentine minerals, whereas the Tp’s are shifted to
higher temperature with respect to lizardite and chrysotile.
From the present results and those of Viti1
for antigorite, our
specimen’s Tp1, falls around 710 to 720 °C, whereas the
diagnostic peak, Tp2, lies between 730 and 760 °C. At each
respective Tp, the mass loss rate maxima, -(dm/dt)/mo_max_Tp1
and -(dm/dt)/mo_ max_Tp2, are roughly identical at 1.7 × 10−4
±
1.4 × 10−6
s−1
(for both Tp1 and Tp2).
The quenched material, shown in Figure 2, indicates that full
dehydroxylation (∼1000 °C) of antigorite results in a
recrystallized solid. The fully dehydroxylated mineral shows
Table 2. Conditions Employed to Evaluate the Effect of Experimental Variables
heating
material
gas flow rate,
mL min‑1
particle size d80, μm
(±3.8)
sample mass, mg
(±0.1)
purge
gas
heating rate,
°C min‑1
type of comminuted
sample
control alumina 20 75 5.50 argon 10 wet ground
effect of heating material alumina 20 75 5.50 argon 10 wet ground
platinum
effect of purge gas flow
rate
alumina 20 75 5.50 argon 10 wet ground
200
effect of particle size alumina 20 31 5.50 argon 10 wet ground
52
75
effect of sample mass alumina 20 75 2.75 argon 10 wet ground
5.50
11.0
37.7
effect of purge gas alumina 20 75 5.50 argon 10 wet ground
CO2
air
effect of heating rate alumina 20 75 5.50 argon 10 wet ground
20
30
effect of comminution alumina 20 75 37.8 argon 10 wet ground
10 35.6 dry ground
≥2000 49.9 dry crushed
Figure 1. TGA-DTG profile of the antigorite showing a typical
replicate runs for the control experiment. See text for detailed
discussion.
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4. an evident morphological change suggesting that it has
undergone significant structural reorganization and subsequent
particle contraction. The mineral phases identified in the
dehydroxylated material (Table 1), shows forsterite and
enstatite regardless of material preparation, be it either
demagnetised (A, B, C) or magnetic fractions (D, E). This is
as expected, as our previous study on this particular antigorite
specimen33
showed forsterite and enstatite already in existence
by 725 and 825 °C, respectively.
With forsterite present by 725 °C, prior to full dehydrox-
ylation, it is expected that antigorite dehydroxylation
decelerates due to structural reorganization and subsequent
Figure 2. SEM micrographs (a) untreated and (b) heat treated antigorite under argon purge gas showing the general change in particle morphology.
Figure 3. Antigorite DTG curves in various (a) types of heating material, (b) rates of purge gas, (c) particle sizes, (d) sample mass, (e) types of
purge gas, (f) heating rate (g) type of comminution, and (h) heating rates in both inert and oxidizing gas.
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5. particle contraction. External factors such as those evaluated in
this study could further influence the mass loss behavior, risking
further deceleration due to the combined effect of these
experimental variables and solids recrystallization. For example,
the entrapment of water vapor due to passivation by hematite
may further delay antigorite Tp’s to higher temperatures.
It is therefore essential that fundamental kinetic studies and
cross-comparison of reported kinetic parameters recognize both
internal (solids recrystallization) and external (experimental
variables, especially partial pressure of H2O) effects. This means
that, ideally, comparison of kinetic parameters is possible for
studies employing the same experimental conditions (i.e., same
particle sizes and sample mass). This explains the large
variations in the reported kinetic parameters for serpentine
minerals, where, activation energy, Ea ranges from 160 to >543
kJ mol−1
and pre-exponential factor, A, varies from 1 × 10−8
to
1 × 108
s−1
.2−5,9−11,15,33
Figure 3 shows the DTG curves of antigorite under different
experimental conditions. Variations in sample mass, purge gas,
type of comminution, and heating rates result in a marked
departure from the mass loss profile of the base case, while
changes in the type of heating vessel, purge gas flow rate and
particle size do not influence the DTG curves. The crucible
material (alumina vs platinum) does not affect the mass-loss
profiles (Figure 3a). This indicates that a wide range of
materials could be appropriate for constructing the dehydrox-
ylation reactor. In a practical sense, serpentine heat treatment
(i.e., 25 to >800 °C) could involve a vessel made either of a
refractory material (i.e., alumina) or a relatively inert material
(i.e., stainless steel), with the material selection based on
surface erosion rates during treatment.
The identical thermal responses for both purge gas flow rates
(Figure 3b) imply that, at 20 mL min−1
, the dehydroxylation
reaction is neither hindered by entrapment of liberated water
vapor within the sample matrix nor limited by any build up of
product gas above the sample bed. Furthermore, this suggests
that, for the present set of experimental conditions (i.e., 5.5 mg
of 75 μm antigorite heated in alumina crucible from 25 to 1000
°C at 10 °C min−1
under argon purge flowing at 20 mL min−1
),
the dehydroxylation reaction proceeds far from equilibrium;
that is, irreversibly, with a negligible rate of reverse reaction. In
practical situations, with water vapor accumulating in the purge
gas, this deceleration in the reaction may need to be included in
the design calculations.8,9
Figure 3c shows similar thermal curves for the particle sizes
typically employed in mineral carbonation. Unless required by
the carbonation reaction, the result indicates that, stage 2
grinding, to produce feedstocks −38 μm in size at cost of about
Figure 4. Deviations in (a) practical activation temperature, T‑H2O and (b) mass loss rate, r‑H2O of the antigorite sample; β denotes the heating rate.
The deviations were assessed based on the established practical activation temperature, T‑H2O = 725 ± 2 °C and mass loss rate, r‑H2O = 1.7 × 10−4
±
1.4 × 10−6
s−1
.
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6. 63.5 kWhe (tonne mineral)−1
,18
is unnecessary due to similar
thermal curves generated by particles of 31 to 75 μm in size. It
is indeed more practical and cost efficient to subject larger
particles (75 μm) to thermal treatment. Under this option, the
total energy requirement, for prior size reduction to produce
feedstocks 75 μm in size, amounts to 11.8 kWhe (tonne
mineral)−1
. This estimate accounts for electrical power required
for both crushing (1.8 kWhe (tonne mineral)−1
) and stage 1
grinding (10 kWhe (tonne mineral)−1
, to −75 μm).18
This
translates to energy savings of about 53.5 kWhe by avoiding
stage II grinding.
The treatment conditions reported in Figures 3d−h influence
antigorite’s mass loss profile. Employing larger sample mass and
different oxidizing purge gases results in minor variations, while
the use of a crushed sample with no demagnetisation and
employing high heating rates significantly alter the mass-loss
rate.
The process parameters that resulted in significant deviations
in thermal profiles of antigorite were analyzed in detail to assess
the extent of their influence. To simplify the evaluation, we
represented the thermal profile (Table S5, Supporting
Information) by an average value of the two peak temperatures,
Tp_mean, and the average value of the maximum mass-loss rate
for the temperature doublet, −(dm/dt)/mo_max_mean. These
values, redesignated as T‑H2O and r‑H2O, signify the practical
activation temperature (i.e., temperature at which dehydrox-
ylation rate is fastest) and its corresponding mass-loss rate,
respectively.
Figure 4 summarizes the extent of deviations from the
characteristic T‑H2O and r‑H2O of the sample and are expressed
in terms of ΔT (°C) and % change, in that order, with respect
to the base case, for which T‑H2O = 725 ± 2 °C and r‑H2O = 1.7
× 10−4
± 1.4 × 10−6
s−1
. While all deviations favor increase in
T‑H2O (11−39 °C), r−H2O may either decrease by as much as
10% or increase by as high as 192%. This implies that changes
in the processing parameters result in delays in the removal of
evolved gases, and could either enhance or deteriorate the rate
at which escaping product gas leaves the sample matrix. The
probable factors explaining this type of behavior and the
possible implications of the present observations to design of
dehydroxylation reactor are discussed in detail in the
subsequent paragraphs.
3.1. Effect of Sample Mass. Mass increase by ∼600%
elevated the T‑H2O by 11 °C (Figure 4a) as well as increased the
r‑H2O by 10% (Figure 4b). The new Tp mean of 736 °C for 38
mg sample of antigorite falls between the previously reported
Tp for ∼30 and 44 mg of antigorite at 731 and 749 °C,
respectively.1,13
A slight increase in r‑H2O by 4% with 100% mass
increase could be explained in terms of competing effects of the
increase in number of hydroxyl sites and reduction in gas
permeability within the sample matrix that elevates H2O
concentration in void spaces. This means that the evolved gas
becomes entrapped in the stationary matrix, delaying its escape
into the bulk carrier gas. Evolved water vapor entrained within
the powder bed induces the reverse reaction, slowing down the
dehydroxylation process. This phenomenon, known as the
depth effect, operates similarly in the thermal decomposition of
carbonates (50−300 mg),35
owing to increased levels of CO2
that force the reverse reactions.
We estimated the sample loading and bed height, based on a
tap density of 1.53 g mL−1
sample and crucible capacity of 0.1
mL. While sample mass below 11 mg takes no more than 15%
of the crucible’s loading capacity, the ∼38 mg sample occupies
as much 40%. The latter corresponds to a bed height of 3.2
mm, which is almost half of the entire crucible height of 8 mm.
This means that the evolved gases need to negotiate more than
twice the distance through the sample matrix. The entrapment
of water vapor within the sample bed increases the local partial
pressure of water vapor, PH2O. Since serpentine dehydroxylation
kinetics is highly dependent on PH2O,9
one would expect the
delays in Tp, exactly as observed in this study. These results
suggest deployment of fluidized bed reactors for activating
serpentine minerals, in preference to moving bed reactors.
3.2. Effect of Purge Gas. The use of an oxidizing gas
increases heating requirements and slows down the mass loss
rate. Despite the perceived simplicity of air activation, the delay
in T‑H2O by 17 °C (Figure 4a) and the decrease in r‑H2O by 10%
(Figure 4b) makes this purge gas less desirable for operating a
dehydroxylation reactor. Based on the obvious decoloration of
the sample to reddish hue, we suggest that the formation of
hematite layer on the surface of serpentine grains limits the
removal of the liberated water. Hematite formation in
serpentine minerals had been described by other workers as
“fully ferric chrysotile”,31
hematite formation in carlosturanite,36
and oxidized form of magnetite during lizardite heat treat-
ment.37
Under oxidizing atmosphere, the transformation31,36,38
of Fe2+
to Fe3+
most often concludes prior to the onset of
dehydroxylation.
Besides the oxidation of magnetite, Fe2+
, present in the
octahedral sites in serpentinite, oxidizes to Fe3+
. Also, Fe3+
present in tetrahedral sites migrates out to octahedral sites.31,39
By conservation of charge, a third of the converted Fe3+
from
Fe2+
must migrate toward the surface of serpentinite grains to
form Fe2O3 layers, as observed by the reddish hue on surfaces
of serpentine activated under air. Based on the chrysotile
studies by MacKenzie and MacGavin,31
iron present in the
hematite layers corresponds to about 10% of the initial iron
content of the mineral. With antigorite having relatively higher
proportions of Fe2+
than lizardite or chrysotile,40
the effect of
an oxidizing gas is probably more pronounced for antigorite
than for the other two polymorphs of serpentine.
The removal of Fe3O4 via demagnetisation prior to heat
activation reduces the severity of hematite formation. This is
exemplified in the slightly faster mass loss rate of the partially
demagnetised sample (Sample A, despite its relatively larger
size) as compared to the nondemagnetised Sample D. As can
be seen in Figures 3g and 4b, the mass loss rate for the
magnetic sample (Sample D) slightly decreases by about 3%.
However, the ARC (Albany Research Center, now U.S.
Department of Energy’s National Energy Technology Labo-
ratory) investigations reported that oxidizing gas used during
heat treatment of a partially demagnetised antigorite appeared
not to influence the serpentinite conversion during carbo-
nation.37
This means that a sufficiently high amount of Fe2+
,
was removed by demagnetisation prior to thermal treatment,
resulting in formation of minor layers of Fe2O3 during
activation that had no influence on the conversion of activated
serpentine during its carbonation. But the findings of Connor et
al. might be specific to their serpentine mineral (Section 4,
Supporting Information).
In a practical situation, the use of CO2 as purge gas can
provide similar efficiency as that of an inert gas. A possible
scenario may involve bleeding some CO2 delivered from a
capture plant to satisfy the purge requirements of the activation
unit. After exiting the activation unit (calciner), mixture of CO2
and steam could be routed through a heat exchanger for
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7. extraction of heat from the gas. Steam could be then
condensed, separated and used in the subsequent processes
(i.e., water input for carbonation), prior to recycling of CO2 to
the activation unit or its use in the carbonation reactor. While
air is readily available for actual operations, it is not the
preferred purge gas for activating serpentines.
Overall, we conclude that a diffusive barrier of Fe2O3 coating
the antigorite surface forces the temperature shift in T‑H2O to
higher values and the decrease in the r‑H2O. This effect
highlights a need for demagnetisation, especially if thermal
treatment involves an oxidizing gas such as air. In other words,
demagnetisation serves the dual purpose of removing valuable
minerals of iron and chromium for cost offsets, and to decrease
the energy load required for activation.
3.3. Effect of Heating Rate. Figures 4a,b shows that the
increase in heating rate by 100% (20 °C min−1
) and 200% (20
°C min−1
) results in impediment in T‑H2O by 11 and 22 °C,
respectively. Despite these delays, the increase in r‑H2O (Figure
4b) is extremely high, at 89% and 192%, respectively (Section
5, Supporting Information). As can be seen in Figure 5a, the
response in both T‑H2O and r‑H2O with increased heating rates
follows an increasingly linear trend. The shifts in T‑H2O and
r‑H2O toward higher values, as function of the increasing heating
rate, arise as a result of the dependence of the mass-loss rate on
the Arrhenius expression.
Further analysis indicates that while higher heating rates
elevate T‑H2O, a significant increase in r‑H2O drastically reduces
the processing time, to reach target degree of dehydroxylation.
Figure 5b illustrates the decrease in processing time to produce
thermally treated antigorite with 20% residual OH content, %
OHres. The required treatment time appears to scale with the
heating rate as t = 630/β0.942
, based on the measurements
collected between 10 and 30 °C min−1
. Figure 5b extrapolates
the treatment time to higher heating rates (40−100 °C min−1
).
We conclude this section by noting that while slow heating
rates are essential for investigating intrinsic kinetics, operation
of a practical reactor necessitates more rapid thermal
processing.
3.5. Effect of Comminution Type. Figures 4a,b show that
while the r‑H2O of the crushed sample increases by 35%, the
T‑H2O is 36 °C higher than that of the ground sample. This is as
expected for a crystalline solid, evolving water en-bulk at
relatively higher temperature due to the imparted structural
rigidity of the crystal lattice structure. Further comminution
(i.e., grinding) could disrupt this structure,41
rendering the
material susceptible to thermal and/or chemical decomposition
at relatively lower temperatures. The apparent shift in the Tsh
location to ∼725 °C also suggests that, this thermal feature is
neither contaminant chrysotile nor partially amorphised
material but an intermediate phase associated with thermal
dehydroxylation. This is because the reported peak temperature
of high purity chrysotile at ∼650 °C,1
and even our
experimentally determined Tp’s of chrysotile fiber (Figure
S11, Supporting Information, also shown as the table of content
graphic) at ∼690 °C are significantly lower.
While the direct use of crushed samples in thermal activation
may be possible, the apparent savings in electrical power is
negated by the likelihood of further comminution after thermal
activation. This is because dehydroxylation at especially high
temperatures (approaching 820 °C) may induce the formation
of enstatite. Further comminution is also necessary to increase
solid’s surface area. In general, the smaller particles from
ground samples represent the preferred feed for thermal
dehydroxylation due to the relative ease of dehydroxylation.
While there is no significant differences in the thermal profile
among the two types of grinding methods (wet grinding ∼21
min; dry grinding ∼1 min), the significantly faster grind time
required to reduce the particle size makes dry grinding
attractive. It must also be noted that although wet ground
sample initially contains magnesium hydroxide species (Table
1), these hydroxides dehydroxylate prior to antigorite. Hence,
the presence of Mg(OH)2 does not have any discernible effect
on the subsequent serpentine dehydroxylation process.
3.6. Environmental Technology Implications to
Serpentine Activation for CO2 Storage by Mineralisa-
tion. The evaluation of the effects of sample mass, purge gas,
heating rate, and communition type on the behavior of
antigorite has provided new (i) knowledge of the dehydrox-
ylation kinetics of this mineral at elevated temperatures, and (ii)
scientific underpinnings for designing larger unit operations
needed to for scaling up the process of sequestering of CO2 by
mineralization. Particles of less than 75 μm in size afford
optimal conditions of temperature and rate of dehydroxylation,
and allow savings in electrical power for the size reduction
stage. The detrimental effect of oxidation of Fe2+
to Fe3+
, owing
to formation of hematite layers on activated particles, suggests
CO2 as preferred purge gas.
We highlight the need to fluidize the powder bed to avoid
the entrapment of liberated water that engenders the reverse
reaction and results in higher processing temperature.
Otherwise, the effect of bed height on inducing the reverse
Figure 5. (a) Practical activation temperatures (T‑H2O) and estimated
mass loss rates (r‑H2O) with respect to the increase in heating rate, β.
(b) Required time at various heating rates (β) for the production of
thermally treated antigorite containing 20% OHres.
Environmental Science & Technology Article
dx.doi.org/10.1021/es303566z | Environ. Sci. Technol. 2013, 47, 182−190188

8. reactions must be included in the design of a moving bed
reactor, as the results suggest that the build-up of localized PH2O
is likely to occur in moving bed reactors with increase in bed
height. We also recommend the rapid thermal treatment as a
practical way to increase the throughput, and minimize the
reactor’s size.
■ ASSOCIATED CONTENT
*S Supporting Information
Further information on the material standard pretreatment
process, mineralogy, thermal properties and details on the
deviations in T‑H2O and r‑H2O are found in Sections 1−3 of the
Supporting Information. Sections 4−7 provide additional notes
on the effects of purge gas, heating rate and comminution type.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*Phone: +61 2 4985 4433; fax: +61 2 4921 6893; e-mail:
bogdan.dlugogorski@newcastle.edu.au.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This study was funded by an internal grant (ref. No.
G0189103) from the University of Newcastle. We gratefully
acknowledge valuable discussions with Prof. Eric Kennedy
during the course of this research. The first author thanks the
University of Newcastle for a Postgraduate Research Scholar-
ship. Material and analytical assistance from Prof. Erich Kisi, Dr.
Judy Bailey, Ms. Monica Davis, and Ms. Jennifer Zobec (EM-X-
ray Unit) are greatly appreciated.
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10. S1
Supporting Information
Environmental Science & Technology
28-Oct-2012
Thermal Activation of Antigorite
for Mineralisation of CO2
Reydick D. Balucan1
and Bogdan Z. Dlugogorski1*
1
Priority Research Centre for Energy
The University of Newcastle, Callaghan, NSW 2308, Australia
*corresponding author:
bogdan.dlugogorski@newcastle.edu.au
Number of pages: 21
Figures: 11
Tables: 5
Table of Contents:
Section 1. Standard Pretreatment
Section 2. Material and Thermal Characteristics
Section 3. Deviations in T-H2O and r-H2O
Section 4. Effect of Purge Gas
Section 5. Effect of Heating Rate
Section 6. Combined Effect of Heating Rate and Purge Gas
Section 7. Effect of Comminution
Section 1: Standard Pretreatment

11. S2
Antigorite (Sample BI4 obtained near Bingara in the Great Serpentinite Belt in NSW, Australia, at the
location of 30.122217 S and 150.635966 E), was dry crushed with a jaw crusher to ~5 mm, then in a roll
crusher to reduce its size to ~2 mm. This 2 mm dry crushed sample (Sample E) was further reduced in
size by wet grinding in a rod mill (13 rods, 60 % solids, 21 min grind time) specifically to target ground
material with 80 % passing the -75 m sieve. From this wet ground sample, we extracted three (3) size
fractions during the sieve analysis at ~ 10 g each and designated these as Samples A (-75 + 53 m), B (-
53 + 38 m) and C (-38 m). Each of these ~ 10 g portions was then wet demagnetised by repeated
removal of the magnetic particles appended on a PTFE magnetic stirring bar (107 mm × 27 mm) using a
PTFE retriever. The demagnetised slurries were dewatered and oven dried for 1 day at 110 °C.
On the other hand, a ring mill (tungsten carbide rings, ~ 60 % solids, 1 min grind time) served to dry
ground the crushed sample to produce a size fraction of d80 = 10 m (Sample D). The wet ground
samples (A, B, C) were demagnetised prior to heat activation, while the dry ground (D) and crushed (E)
samples were used without further demagnetisation. Ground materials (A, B, C, D) are all in powder
form whilst a single shard represents the dry crushed sample (E).

12. S3
Figure S1. Standard pretreatment process employed to the antigorite sample BI4. This pretreatment
stage comprises crushing, homogenisation, grinding, and wet demagnetisation.

13. S4
Table S1. Trial wet grinds to establish the grind time for 80 % passing 75 m.
Seive,
m
Trial Grind 1
Seive,
m
Trial Grind 2
Seive,
m
Trial Grind 3
Duration 6 min Duration 12 min Duration 24 min
%
mass
%
passing
%
mass
%
passing
%
mass
%
passing
710 3.04 97.0 710 710
600 4.30 92.7 600 600
500 12.3 80.4 500 500
400 11.6 68.7 400 400
300 10.8 57.9 300 1.09 98.9 300 100
250 6.65 51.3 250 3.85 95.1 250 100
180 7.99 43.3 180 14.1 81.0 180 0.05 100
106 9.77 33.5 106 24.9 56.1 106 1.43 98.5
90 2.29 31.2 90 4.83 51.2 90 2.00 96.5
75 2.27 29.0 75 4.69 46.6 75 4.14 92.4
63 63 63 5.37 87.0
53 53 53 6.27 80.7
38 3.94 25.0 38 7.96 38.6 38 5.04 75.7
-38 25.0 -38 38.6 -38 75.7

14. S5
Figure S2. Wet grind time curve of the P80 values for the three trial grinds. The estimated time to
achieve P80 75 m is about 21 min.
496
53
176
y = 9155.7x-1.6124
r 2
= 0.9982
0
100
200
300
400
500
0 5 10 15 20 25
t , min
P80,µm
X

15. S6
Table S2. Sieve analysis of the final grind designed to achieve P80 of 75 m.
Seive, m
Final Grind
21 min
% mass % passing
710
600
500
400
300 100
250 0.75 99.3
180 3.00 96.3
106 7.00 89.3
90 3.69 85.6
75 4.97 80.6
63 4.03 76.6
53 4.70 71.9
38 7.15 64.7
-38 65.2

16. S7
Table S3. Resultant particle size and power requirements of the standard pre-treatment stages.
Size Reduction
Stages
d80, m
Power requirements,
kWhe tonne-1
Crushing 2000 1.80
Stage 1 Grinding 75 10.0
Stage 2 Grinding 37 63.5
Stage 3 Grinding < 10 136

17. S8
Section 2: Material and Thermal Characteristics
Figure S3. Particle size distribution plots showing the (a) trendlines used to calculate and designate the
samples in terms of d80 from the measured d10, d50 and d90 values, (b) cumulative volume distribution,
and (c-f) the differential volume distribution of the various size fractions of the antigorite sample.
A, wet ground (d80,75 µm)
0
2
4
6
8
10
0 50 100 150 200 250 300
Particle size, µ m
Differential
volume
distribution
,%
c
B, wet ground (d80,52 µm)
0
2
4
6
8
10
0 50 100 150 200 250 300
Particle size, µm
Differential
volume
distribution
,%
d
C, wet ground (d80,31 µ m)
0
2
4
6
8
10
0 50 100 150 200 250 300
Particle size, µm
Differential
volume
distribution,
%
e
0
20
40
60
80
100
0 50 100 150 200 250
Particle size, µ m
Cumulativevolumedistribution,%
A (wetground, d80 75 um)
B(wetground, d80 75 um)
C(wetground, d80 75 um)
D (dry ground, d80 75 um)
b
d80, 10 µm)
d80, 31 µm)
d80, 52 µm)
d80, 75 µm)
y =1.0848e0.0529x
r2
=0.9999
y =1.2060e0.047x
r2
=0.9989
y =1.0961e0.0416x
r2
=1.0000
y =0.9984e0.0291x
r2
=0.9994
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80 90
dx
Particlesize,µm
A (wet ground, d80 75 um)
B (wet ground, d80 52 um)
C (wet ground, d80 31 um)
D (dry ground, d80 10 um)
a
d80, 10 µm)
d80, 31 µm)
d80, 52 µm)
d80, 75 µm)
D, dry ground (d80,10 µm)
0
2
4
6
8
10
0 50 100 150 200 250 300
Particle size, µm
Differential
volume
distribution
,%
f

18. S9
Figure S4. X-ray powder diffraction of antigorite showing both (a) natural, dry ground sample D and
(b) wet ground, demagnetized sample A.
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
5 10 15 20 25 30 35 40 45 50 55 60 65 70
o
2θ
intensity,counts
*
*
*** *
*
*
* Antigorite
** **
b
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
5 10 15 20 25 30 35 40 45 50 55 60 65 70
o
2θ
intensity,counts
+ +
*
*
*
*
*
* Antigorite
+ Magnetite
**
+
*
*
*
* *
*
a

19. S10
Figure S5. DTG curves of the control experiments. The DTG peak temperatures and shoulder are
designated as TP1, TP2 and Tsh, respectively.
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
100 200 300 400 500 600 700 800 900
T, o
C
-(dm/dt)/mo,s-1
Run 1
T p2: 734 o
C, 170 s-1
T p1: 716 o
C, 172 s-1
T sh start: 629 o
C, 60 s-1
T sh end: 683 o
C, 110 s-1
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
100 200 300 400 500 600 700 800 900
T, o
C
-(dm/dt)/mo,s-1
Run 2
T p2: 737 o
C, 168 s-1
T p1: 713 o
C, 174 s-1
T sh start: 641 o
C, 53 s-1
T sh end: 675 o
C, 96 s-1
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
100 200 300 400 500 600 700 800 900
T, o
C
-(dm/dt)/mo,s-1
Run 3
T p2: 738 o
C,162 s-1
T p1: 713 o
C, 168 s-1
T sh start: 631 o
C, 47 s-1
T sh end: 674 o
C, 92 s-1

20. S11
Table S4. Summary of the DTG profile for the antigorite specimen.
TGA
Runs
mass
loss, %
Tp,
o
C –(dm/dt)/mo_max, s-1†
∆m 105-
850
Tsh
start
Tsh
end
Tsh
range
Tp1 Tp2
Tsh
start
Tsh
end
Tsh
range
Tp1 Tp2
Run 1 11.46 629 683 54 716 734 60 110 50 172 170
Run 2 11.41 641 675 34 713 737 53 96 43 174 160
Run 3 11.40 631 674 43 713 738 47 92 45 168 162
mean 11.43 635 679 44 715 736 57 103 47 173 169
σ 0.03 8 6 14 2 2 5 10 5 1 1
†
listed values are to be multiplied by 1.0 × 10-6

21. S12
Table S5. Summary of antigorite’s thermal profile.
Thermal profile
T-H2O, °C
(Tp_mean) †
r-H2O, s-1
-(dm/dt)/mo_max_mean
††
Tp1
725 1.7 × 10-4
Tp2
σ 2 1.4 × 10-6
†
Average value of Tp1 and Tp2.
††
Average value of the -(dm/dt)/mo_max_mean from each Tp1 and Tp2.

22. S13
Section 3. Deviations in T-H2O and r-H2O
Figure S6. Analysis of the deviations in the practical activation temperature, T-H2O and its respective
estimated mass lost rate, r-H2O resulting from variations in sample mass.
-1
0
4
15
-6
-4
-2
0
2
4
6
8
10
12
14
16
18
decreased by
50% (2.75 mg)
control:
(5.50 mg)
increased by
100%
(11.0 mg)
increased by
590%
(37.7 mg)
ΔT-H2O,o
C
a
mass decreased by
50 % (2.75 mg)
mass increased by
100 % (11.0 mg)
mass increased by
~600 % (37.7 mg)
delayed by 11 o
C
3
0
6
12
-3
-1
1
3
5
7
9
11
13
decreased by
50% (2.75 mg)
control:
(5.50 mg)
increased by
100%
(11.0 mg)
increased by
590%
(37.7 mg)
%change,r-H2O
b
mass decreased by
50 % (2.75 mg)
mass increased by
~600 % (37.7 mg)
mass increased by
~100 % (11.0 mg)
faster by 1 %
faster by 4 %
faster by 10 %

23. S14
Figure S7. Analysis of the deviations in the practical activation temperature, T-H2O and its respective
estimated mass lost rate, r-H2O resulting from variations in purge gas.
0 1
21
-6
-4
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
control: argon carbon dioxide air
ΔT-H2O,o
C
a
CO2 Aircontrol: Argon
delayed by 17 o
C
0
3
-12
-14.0
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
control: argon carbon dioxide air
%change,r-H2O
b
CO2control: Argon Air
faster by 1 %
slower by 10 %

24. S15
Figure S8. Analysis of the deviations in the practical activation temperature, T-H2O and its respective
estimated mass lost rate, r-H2O resulting from variations in heating rate.
0
91
194
-20
30
80
130
180
230
control:
10 oC/min
in argon
heating rate
increased by 100% in
argon
heating rate
increased by 200% in
argon
%change,r-H2O
b
control:
10 o
C min-1
, Argon
increased by 100 %,
(20 o
C min-1
), Argon
increased by 200 %,
(30 o
C min-1
), Argon
faster by 89 %
faster by 192 %
0
15
26
-6
-2
2
6
10
14
18
22
26
30
control:
10 oC/min
in argon
heating rate
increased by 100% in
argon
heating rate
increased by 200% in
argon
ΔT-H2O,o
C
a
10 o
C min-1
, Argon
increased by 100 %,
(20 o
C min-1
), Argon
increased by 200 %,
(30 o
C min-1
), Argon
delayed by 11 o
C
delayed by 22 o
C

25. S16
Figure S9. Analysis of the deviations in the practical activation temperature, T-H2O and its respective
estimated mass lost rate, r-H2O resulting from variations in the type of comminution.
0
-5
37
-20
-10
0
10
20
30
40
50
control : wet ground dry ground dry crushed
%change,r-H2O
b
faster by 35 %
slower by 3 %
0
-3
40
-6
-2
2
6
10
14
18
22
26
30
34
38
42
46
control : wet ground dry ground dry crushed
ΔT-H2O,o
C
a
delayed by 36 o
C

26. S17
Figure S10. Analysis of the deviations in the practical activation temperature, T-H2O and its respective
estimated mass lost rate, r-H2O resulting from variations in heating rate in both argon and air atmosphere.
0
21
26
43
-6
4
14
24
34
44
54
control:
10 oC/min
in argon
10 oC/min
in air
heating rate
increased by
200% in argon
heating rate
increased by
200% in air
ΔT-H2O,o
C
a
heating rate
increased by 200 %,
(30 o
C min-1
), Argon
heating rate
increased by 200 %,
(30 o
C min-1
), air
(10 o
C min-1
),
Argon
(10 o
C min-1
),
air
delayed by 17 o
C
delayed by 22 o
C
delayed by 39 o
C
0 -12
194 191
-30
20
70
120
170
220
control:
10 oC/min
in argon
10 oC/min
in air
heating rate
increased by
200% in argon
heating rate
increased by
200% in air
%change,r-H2O
b
heating rate
increased by 200 %,
(30 o
C min-1
), Argon
heating rate
increased by 200 %,
(30 o
C min-1
), air
control:
(10 o
C min-1
),
Argon
(10 o
C min-1
),
air
slower by 10 %
faster by 192 %
faster by 189 %

27. S18
Section 4. Effect of Purge Gas
In the work of O’Connor et al,(1) removal of 54 % of the magnetite content, resulted in the reduction of
the total oxidisable species to about 57 % of its original content.(37) The removal of similar amount of
magnetite in our sample (54 % the total magnetite content, Table S1, Fe2O3
*
), however, only decreased
the total oxidisable Fe to 70 % of its original value (Fe2O3_total 6.87 % down to 4.82 % w/w antigorite).
Although similar amounts of magnetite were removed in both studies, our demagnetised sample
contained more oxidisable Fe due to a relatively higher amount of Fe2+
in octahedral sites than in
magnetite grains. As such, partial demagnetisation (~ 54 % of the original Fe3O4 content remains) may
be sufficient for the mineral used by O’Connor et al., but it seems insufficient for the antigorite of the
present investigation.
1. O'Connor, W.; Dahlin, D. C.; Nilsen, R. P.; Rush, G.E.; Walters, R.P.; Turner, P. C. In CO2
storage in solid form: A study of direct mineral carbonation. 5th International Conference on
Greenhouse Gas Technologies, DOE/ARC-2000-01. Cairns, Australia, Cairns, Australia, August 14-18,
2000.

28. S19
Section 5. Effect of Heating Rate
Based on our energy studies, this energy demand corresponds to ~15 kJ (kg antigorite)-1
for every 10 °C
delay in Tp. Overall, the negative cost implications of this delay are minor in comparison to the offset in
process intensification (throughput), as a consequence of increased mass-loss rate. The practicality of
implementing rapid thermal processing warrants further examination to gain insights into heat transfer
during serpentine activation, and to avoid heat transfer becoming a step limiting phenomenon.

29. S20
Section 6. Combined Effect of Heating Rate and Purge Gas
Figures 3a-b emphasise the cumulative effects of high heating rates and the use of an oxidising gas.
Both effects elevated T-H2O, but exerted contradictory influence on the rate of dehydroxylation. The
highest deviation in the T-H2O by 39 °C represents the additive effect of the increase in heating rate by
200 % (increase in T-H2O by 22 °C) and application of air as purge gas (increase by 17 °C). Although
the higher heating rates significantly improve r-H2O (by almost 200 % for 30 °C min-1
), the application of
air activation of non-demagnetised samples results in overall slowing down of the activation rate (by 3
% in comparison to the base case).

30. S21
Section 7. Effect of Comminution
Figure S11. Thermal profile of chrysotile asbestos.
Chrysotile Fibre
-1.8
-1.55
-1.3
-1.05
-0.8
-0.55
-0.3
-0.05
0.2
0.45
0.7
0 100 200 300 400 500 600 700 800 900
T , o
C
Heatflow,mWmg-1
0.E+00
1.E-03
2.E-03
3.E-03
4.E-03
5.E-03
6.E-03
7.E-03
8.E-03
9.E-03
1.E-02
DTG,-(dm/dt),mgs-1
Heatflow
DTG
Exo
697
810
697
Sample Mass: 22 mg
Heating Rate : 10 o
C min-1
Purge Gas: Argon
Gas Flow rate: 20 mL min-1