Full text of DOI:10.1080/10473289.2000.10464191

This article was downloaded by: [142.58.101.27]
On: 11 October 2013, At: 10:18
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer
House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of the Air & Waste Management Association
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/uawm20
Preparation of Calcium Silicate Absorbent from Iron
Blast Furnace Slag
Lia F. Brodnax a & Gary T. Rochelle b
a Bay Area Air Quality Management District , San Francisco , California , USA
b Department of Chemical Engineering , University of Texas at Austin , Texas , USA
Published online: 27 Dec 2011.
To cite this article: Lia F. Brodnax & Gary T. Rochelle (2000) Preparation of Calcium Silicate Absorbent
from Iron Blast Furnace Slag, Journal of the Air & Waste Management Association, 50:9, 1655-1662, DOI:
10.1080/10473289.2000.10464191
To link to this article: http://dx.doi.org/10.1080/10473289.2000.10464191
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose
of the Content. Any opinions and views expressed in this publication are the opinions and views of the
authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should
not be relied upon and should be independently verified with primary sources of information. Taylor
and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses,
damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection
with, in relation to or arising out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

TECHNICAL PAPER
Brodnax and Rochelle
ISSN 1047-3289 J. Air & Waste Manage. Assoc. 50:1655-1662
Copyright 2000 Air & Waste Management Association
Preparation of Calcium Silicate Absorbent from Iron Blast
Furnace Slag
Lia F. Brodnax
Bay Area Air Quality Management District, San Francisco, California
Gary T. Rochelle
Department of Chemical Engineering, University of Texas at Austin, Texas
ABSTRACT
INTRODUCTION
Calcium silicate hydrate (CSH) solids were prepared from
hydrated lime and iron blast furnace slag in an aqueous
agitated slurry at 92 °C. While it was hoped a minimal
lime/slag ratio could be used to create near-amorphous
CSH, the surface area of the product improved by increas-
ing the lime/slag weight ratio to 2. The addition of gyp-
sum to the lime/slag system dramatically improved the
formation of surface area, creating solids with 139 m2/g
after 30 hr of reaction when only a minimal amount of
lime was present. The SO2 reactivity of solids prepared
with gypsum greatly exceeded that of hydrated lime,
achieving greater than 70–80% conversion of the alkalin-
ity after 1 hr of reaction with SO2. The use of CaCl2 as an
additive to the lime/slag system, in lieu of gypsum, also
produced high-surface-area solids, 115 m2/g after 21 hr of
reaction. However, the SO2 reactivity of these sorbents was
relatively low given the high surface area. This empha-
sized that the correlation between surface area and SO2
reactivity was highly dependent on the solid phase, which
was subsequently dependent on slurry composition.
The use of an alkaline solid to remove SO2 from a gas
stream is one of several options for tail-end SO2 control
from a variety of smaller sources. Typically, hydrated lime
has been used as the industrial sorbent for dry or semidry
SO2 scrubbing. Unfortunately, the precipitated calcium
sulfite product tends to coat the lime surface, leading to
significantly slower rates of reaction. The result is low
conversion of the lime and, therefore, an increased cost
of buying and disposing of largely unreacted reagent.
Calcium silicate hydrates (CSHs) have been shown to
be highly effective reagents in dry scrubbing systems. The
silicates have a higher surface area than lime and are able
to attain higher conversions of the alkalinity present in
the solids.1,2 Previous calcium silicate sorbent research has
focused on the preparation of solids from lime and fly ash,
though it has been shown that glass, silica fume, and other
amorphous silicas can be used in lieu of fly ash.3–7
The formation of silicates from lime and an amorphous
SiO2 source in an aqueous environment below 100 °C is de-
scribed in reactions 1–3. The dissolution of the lime releases
Ca2+ and OH– into solution. The increased solution pH fa-
cilitates the dissolution of the amorphous (SiO2)x shown in
reaction 2. Dissolved Si(OH)5– and Ca2+ then precipitate to
form high-surface-area, nearly amorphous CSH. Above 100
°C, the precipitated CSH is believed to be comprised of less
amorphous, more crystalline tobermorite phases.8
IMPLICATIONS
Calcium silicate solids prepared from iron blast furnace
slag were substantially more reactive with SO2 than hy-
drated lime, the typical industrial sorbent used in dry de-
sulfurization processes. Sorbent with 122 m2/g and an
alkalinity equivalent to 0.63 g Ca(OH)2 was prepared un-
der agitated conditions at 92 °C from a 0.5/1/0.1 lime/
slag/gypsum (wt) mixture. While the total alkalinity per
mass was lower, SO2 at 58% relative humidity (RH) was
able to react 79% of the alkalinity of these solids com-
pared with an average conversion of 30% for hydrated
lime. It is believed that solids prepared from 0.1/1/0.1 lime/
slag/gypsum, with a surface area of 139 m2/g, would ex-
hibit similar SO2 reactivity. This presents the optimum case,
preparing highly reactive solids using a minimal quantity
of additional lime and gypsum.
Ca(OH)2 ↔ Ca2+ + 2OH–
(1)
(SiO2)X + 2H2O + OH– ↔ (SiO2)X–1 + Si(OH)5– (2)
Ca2+ + ySi(OH)5– + (2 – y)OH– + (z – 2y –1)H2O →
(CaO)(SiO2)y(H2O)z (CSH)
(3)
This research investigated the formation of calcium sili-
cates using iron blast furnace slag as the amorphous SiO2
Volume 50 September 2000
Journal of the Air & Waste Management Association 1655

Brodnax and Rochelle
source. Slag is the secondary product of an iron blast fur-
nace, consisting of lime flux and the SiO2, sulfur, and other
impurities separated by the flux from the iron ore. Slag
was chosen for study due to the large intrinsic Ca con-
centration, shown in Table 1, roughly equal parts Ca to Si.
This presents an interesting possibility: the activation of
slag as an alkaline sorbent for desulfurization applications
with a minimal amount of additional lime, thereby reduc-
ing the need to acquire (and ship) raw material. Bench-
scale experiments conducted with slag focused on the
effects of the lime/slag/additive recipe on the aqueous slurry
chemistry and, therefore, the final product solids.
at 90 °C. The liquid filtrate was filtered again through a
0.45-µm filter and stored in polypropylene bottles for later
analysis.
Nonagitated Solids Preparation. Solids were prepared with-
out agitation in single glass sample vials (~8 mL). The
unreacted solids and water were mixed and weighed in the
vial, which was then capped. Sets of vials were placed in a
Blue M convection oven that controlled the temperature
at 92 °C, and individual samples were removed at the ap-
propriate reaction time. They were weighed for water loss
(<5%) and vacuum-dried at 90 °C for 6–24 hr. Since the
samples were not filtered prior to drying, any dissolved
species in the slurry solution must have precipitated on
the solid surfaces. As expected, the glass vials also reacted
with the slurry as an amorphous SiO2 source. However, this
effect was minimized by the large relative surface area of
the ground slag sample compared with the vial walls.
EXPERIMENTAL
This section describes the experimental apparatus and
procedures used to prepare calcium silicates from lime and
blast furnace slag. The apparatus utilized to measure the
gas/solid reaction between the sorbents and SO2 is also
described. A more detailed description of the experimen-
tal procedures is published elsewhere.6,7
Sorbent Preparation Sample Analysis. All the solid samples
were analyzed for BET surface area by nitrogen adsorp-
tion. Nitrogen adsorption was also used to determine the
porosity of a few samples. Selected samples were also char-
acterized using scanning electron microscopy (SEM) and
X-ray diffraction. The “divalent alkalinity” (molar equiva-
lent to Ca2+) was measured by dissolving the solids in 0.1
M HCl and back-titrating with 0.2 M NaOH to the phe-
nolphthalein end point. Liquid filtrate samples were ana-
lyzed for pH at ambient temperature and for Ca2+, K+, and
Na+ by atomic absorption (AA) spectrophotometry. Aque-
ous sulfide was measured by titrating with AgNO3 to the
potentiometric end point as measured with a silver sul-
fide electrode. The composition of the solid product was
measured by “selective dissolution,” in which the solids
were dissolved in 0.1 M HCl and the resulting liquid ana-
lyzed for Ca2+, Si, and Na+ by AA.
Sorbent Preparation
The ground granulated iron blast furnace slag used
in this work was a sample donated by Koch Industries,
Inc., with a measured Brunauer-Emmett-Teller (BET) sur-
face area of 3.62 m2/g. The lime was a rotary mill hy-
drate (Code MR200) donated by the Mississippi Lime Co.
with a measured BET surface area of 21.6 m2/g. The gyp-
sum (CaSO4•2H2O) and CaCl2 additives were reagent
grade material.
Agitated Solids Preparation. Sorbents were prepared at 92 °C
from agitated batch slurries in a 500-mL jacketed Pyrex re-
actor connected to an ethylene glycol temperature-control
bath. As expected, the Pyrex reactor also reacted with the
slurry as an amorphous SiO2 source. However, this effect
was minimized by the large relative surface area of the
ground slag sample compared with the reactor walls. The
slurry was stirred at 350 rpm with a mechanical agitator
placed through a plexiglass lid on the reactor. Though
the reactor was covered to minimize water loss, it was not
airtight. Several samples were withdrawn via syringe at
various reaction times. The samples were filtered through
a medium filter paper, and the solids were vacuum-dried
SO2 Reactivity
The reaction between solid sorbents and SO2 was mea-
sured using a packed fixed-bed glass reactor. The sorbent
(0.1 g) was mixed with an inert SiO2 sand (30 g) for good
dispersion across the bed, and the mixture was suspended
on a glass frit within the reactor. The reactor was immersed
in a water bath for temperature control at 50 °C.
Table 1. Comparison of a typical fly ash with iron blast furnace slag.9,10
Composition (wt %)
MgO Na2O
SiO2
Al2O3
CaO
K2O
Fe2O3
TiO2
S
Fly Ash
Slag
51.6
24.7
5.2
1.8
0.5
3.3
7.8
1.4
–
30–45
5–14
30–45
8–15
0–1
0–1
0–0.75
0.1.5
0–3.5
1656 Journal of the Air & Waste Management Association
Volume 50 September 2000

Brodnax and Rochelle
A synthesized flue gas was created by diluting com-
pressed SO2/N2 with N2 utilizing mass flow controllers.
The gas stream was humidified to 58% relative humidity
(RH) using a syringe pump and furnace. The synthesized
flue gas flowed downward through the packed bed reac-
tor where the sorbent reacted with and removed the SO2.
The outlet was diluted (~20×) with house air to prevent
downstream condensation, and the SO2 concentration was
measured and recorded using a Thermo Electron Pulsed
Fluorescent Analyzer (Series 43).
RESULTS
Added 1 mL H2O2
Added 0.5 mL H2O2
This work investigated the formation of calcium silicate
solids from the reaction between iron blast furnace slag
and lime. Since SO2 reactivity for calcium silicate sorbents
from fly ash was correlated with the amount of moisture
the solids can hold within the pore structure,11 and this
moisture was shown to correlate with surface area,12 sur-
face area was used as the convenient initial measurement
of the reactivity of the product solids. This simplification
should hold true only as long as the solid composition
and phase remains constant. For the most promising sol-
ids, that is, those with the highest surface area, SO2 reac-
tivity was measured directly.
Reaction Time (hr)
Figure 1. Effect of additives on sulfide concentration. 92 °C; agitated;
400-mL water; 75-g lime; 25-g slag. H2O2 addition: intermittent addition
as shown on graph. Fe2+: 0.13M FeSO4•7H2O added at start. SO32–
S2O32–: 0.1M Na2SO3/0.1M Na2S2O3 added at start.
/
the slag. The H2O2 was added intermittently, as shown in
Figure 1. The addition of H2O2 eliminated sulfide below
detection by either the electrode method or by lead ac-
etate paper, which has a detection limit of 25 ppm. The
addition of Fe2+ or H2O2 did not significantly affect the
rate of surface area formation.
Sulfide Control
It was decided to utilize the procedure of H2O2 addi-
tion for the bulk of experiments with slag. The reaction
slurry was tested periodically (every 1–2 hr) by lead ac-
etate paper. If a positive reading was obtained, H2O2 was
added in 3- to 5-mL increments until the lead acetate tested
negative. While H2O2 might be considered a relatively
expensive additive in industrial settings, the amounts used
could easily be minimized, and the benefits of prevent-
ing sulfide release would have to be weighed.
One of the more unique components in slag, as compared
with fly ash or container glass, is sulfide. The nominal
concentration of sulfide was expected to be 0–3.5 wt %
(Table 1) of the slag. Gaseous H2S is a nuisance at 0.13
ppm, an irritant at 20 ppm, and a dangerous gas at 150
ppm.13 Therefore, it was decided that finding a suitable
method of controlling the sulfide release during sorbent
preparation would be useful for both bench-scale and in-
dustrial applications.
Solution sulfide was measured by potentiometric ti-
tration. Figure 1 shows the effects of several different ad-
ditives on the measured sulfide concentration. Na2S2O3
and Na2SO3 were added, at a level of 0.1 M each at the
start of the experiment, with the expectation that the
sulfide might convert to elemental sulfur or thiosulfate.
It is clear from Figure 1, however, that the SO32–/S2O32–
mixture, in fact, increased the measured sulfide. Further-
more, the presence of these additives decreased the sur-
face area of the product solids.
Effect of Lime/Slag Ratio
As mentioned previously, an interesting concept regard-
ing SiO2 sources with extremely high intrinsic alkalinity
is the possibility of creating a reactive alkaline sorbent
with a minimum of lime. An initial OH– level would be
required to initiate the breakdown of the SiO2 matrix as
shown in eq 2. This initial OH– concentration may come
from added lime, NaOH, or perhaps from the dissolution
of alkali from the SiO2 itself, thereby creating a self-cata-
lyzing effect. Once the SiO2 matrix starts to dissolve, the
intrinsic Ca2+, which is simultaneously dissolving, should
fuel the formation of calcium silicates according to eq 3.
Peterson14 and Peterson and Rochelle15 showed that
high-calcium fly ash could be activated for reaction with-
out the addition of lime. The addition of NaOH, how-
ever, increased the initial dissolution of fly ash, and
thereby increased the SO2 reactivity. A tradeoff was ob-
served between the increased dissolution rates of the SiO2
Figure 1 also shows the results of the addition of FeSO4
at an initial level of 0.13 M. It was hypothesized that the
Fe2+ would precipitate the sulfide as FeS. While it was clear
that Fe2+ reduced the amount of sulfide in solution, it
appeared that there was a measurable equilibrium limita-
tion to the sulfide concentration.
H2O2 was added to the system in an attempt to oxi-
dize the sulfide directly into SO42– as it was released from
Volume 50 September 2000
Journal of the Air & Waste Management Association 1657

Brodnax and Rochelle
and the suppression of the Ca2+ concentration by reac-
tion 1 caused by high OH– concentration. Stroud12 also
showed that a small amount of hydrated lime (0.1 g
Ca(OH)2/g fly ash) would activate high-calcium fly ash.
The effect of the lime/slag ratio on surface area for-
mation was investigated in this research to determine if
sorbent could be prepared with a minimum amount of
additional lime. Figure 2 shows that slag alone (0/1 lime/
slag) increased in surface area from 3.6 to 25 m2/g within
the first 4 hr, but remained constant thereafter. The ad-
dition of lime to the system increased the rate of sur-
face-area formation over reaction times up to 50 hr. It
appeared, within the experimental scatter and the ratios
tested, that the absolute surface area increased with lime
addition up to a lime/slag wt ratio of 2/1. At higher ra-
tios, the surface area of the product decreased, indicat-
ing that the excess hydrated lime reduced the average
surface area of the solids.
exhibited considerable experimental scatter. The scatter
may have resulted from filtrate containing small particles
of lime or other Ca-containing solids. However, identical
procedures in the preparation of glass/lime sorbents7 did
not produce the same level of scatter. In spite of the scat-
ter, the average Ca concentration increased from 5.3 mM
(5 observations/σ = 2.06) to 11.1 mM (8 observations/σ =
3.35) when the lime/slag ratio increased from 0.5 to 3,
supporting the hypothesis that the system was lime dis-
solution limited at low lime/slag ratios.
Arthur and Rochelle6 showed that agitation had a posi-
tive effect on the lime/glass/gypsum only after a fairly high
surface area was formed. Figure 3 shows that within the ex-
perimental scatter, agitation would not affect the lime/slag
system for reaction times less than 50 hr. The apparent in-
crease in surface area with an increasing lime/slag ratio from
2 to 3 is indicative of the normalized surface area axis, which
increases with relative decreases in slag loading. The nor-
malized axis was used in Figure 3 to spread the data and
facilitate the observation of the experimental scatter.
While it was hoped that a minimal amount of lime
would be sufficient, it is clear that a larger lime concen-
tration enhanced the rate of surface area formation. Near-
amorphous CSH is defined by a Ca/SiO2 molar ratio of
0.8–2.0.8 For the low lime limit of a Ca/Si ratio of 0.8, and
45 wt % of Ca in the slag, it is conceivable to form CSH
without an additional Ca source. One hypothesis to ex-
plain the larger lime requirement is that at low lime load-
ing, the system becomes lime dissolution limited. The pH
and Ca concentration in the bulk solution drop, and the
product precipitates on the surface of the lime rather than
on the surface of the slag. Therefore, the remaining un-
dissolved lime is essentially coated, causing a further re-
duction in lime dissolution, and the reaction shown in
eq 2 slows to a very low rate.
Effect of Additives on Sorbent Formation
This section presents results from experiments in which
gypsum or CaCl2 was added to the lime/slag system. All
experiments discussed in this section were conducted at
92 °C with agitation, so that slurry solution analysis would
be possible. The sulfide was controlled by the addition of
H2O2 as mentioned previously.
Effect of Gypsum. While hydrated lime and SiO2 are the
primary reactants of interest, it has been shown that add-
ing other compounds to the reaction slurry, such as gyp-
sum or CaCl2, produced sorbents from fly ash or glass with
increased surface area and reactivity.3,6,7,16,17 The additives
were added to counter the “calcium effect” seen in both
fly ash and glass systems.
While the surface area data shown in Figure 2 presented
a fairly clear trend, the Ca concentration measurements
Reaction time (hr)
Reaction time (hr)
Figure 2. Effect of lime/slag ratio on surface area formation. 92 °C;
agitated and nonagitated; 13–50-wt % solids. Labels indicate lime/
slag wt ratio; no additives.
Figure 3. Effect of agitation on surface area formation. 92 °C; 20-wt
% solids. Labels indicate lime/slag wt ratio.
1658 Journal of the Air & Waste Management Association
Volume 50 September 2000

Brodnax and Rochelle
The calcium effect was reported by Peterson14 and
Kind3 as a result of either the addition of NaOH to the
slurry or KOH dissolution from the fly ash. NaOH or KOH
in solution share OH– as a common ion with lime. As eq 1
shows, if the OH– concentration becomes too large, the
concentration of Ca2+ in solution will be depressed. Kind3
showed gypsum and CaCl2 to be effective additives to
counter this common ion effect.
It was shown that the presence of a gypsum solid phase
produced an increased Ca2+ concentration over the dura-
tion of the experiment by providing a common SO42– ion
for Na+ and K+ released from the glass or fly ash to associate
with, as seen in eqs 4 and 5. It is believed that this allowed
for a higher reaction rate, producing solids with much
higher surface area than systems without gypsum.
Reaction time (hr)
Figure 5. Effect of gypsum or CaCl2 on Ca2+ concentration. 92 °C;
agitated; 16–21% solids. Labels indicate lime/slag/additive wt ratio.
CaSO4 + 2KOH ↔ K2SO4 + Ca(OH)2
CaSO4 + 2NaOH ↔ Na2SO4 + Ca(OH)2
(4)
(5)
addition of gypsum did maintain a higher concentration
of Ca2+ overall.
The data imply that with gypsum present, only a
minimal amount of lime was initially required. It is clear
from Figure 4 that the experiment with 0.1/1/0.1 lime/
glass/gypsum had a very high rate of surface formation;
however, at longer reaction times, the rate decreased. It is
thought that the Ca2+ from the 0.1/1/0.1 lime/slag/gyp-
sum recipe was depleted and beginning to limit the reac-
tion after 22 hr. Unfortunately, the liquid filtrate, and,
therefore, a measurement of solution Ca2+, was not avail-
able to support this.
X-ray data support the hypothesis that the 0.1/1/0.1
lime/slag/gypsum reaction was limited by Ca2+ after 134
m2/g (22 hr of reaction). Figure 6 shows the diffraction pat-
terns for a range of lime/slag/gypsum recipes. It is clear
that excess hydrated lime was still present after 30 hr, and
that 122 m2/g have been formed with the 0.5/1/0.1 recipe.
While gypsum was present in the pattern at 3 hr for the
0.1/1/0.1 recipe, no excess lime was visible as Ca(OH)2 at
this early time, nor throughout the reaction. Results from
the lime/glass/gypsum system showed that surface area
would continue to form after X-ray diffraction no longer
detected Ca(OH)2.7 This was true in this system as well.
However, it appears that the higher concentration of lime
maintained the reaction for a longer time.
Figure 4 shows that the presence of gypsum increased
surface area formation dramatically for lime/slag ratios of
0.5 and 1. In fact, the addition of a small amount of gyp-
sum to the system at 0.5/1 lime/slag increased the surface
area by almost 3-fold.
Figure 5 shows the Ca2+ concentration in solution
from experiments with and without gypsum. For the fly
ash and glass systems, it was shown that without gypsum
present, the Ca2+ concentration would fall to ~1 mM.3,7
For the slag system shown in Figure 5, the experiments
without gypsum sustained a Ca2+ concentration of ~5 mM.
In light of experience with fly ash and glass, this concen-
tration was expected to be sufficient to maintain a rea-
sonable rate of surface area formation. However, it is
evident in Figure 4 that the reaction was relatively slow.
While there was substantial scatter in the Ca2+ data, the
Taylor8 stated that a Ca2+ concentration between 1
and 20 mM is necessary to produce the C-S-H solid phase
with a Ca/Si ratio of 0.8–1.5, believed to be one of the
desired, high-surface-area phases. After 30 hr of reaction,
the product (~139 m2/g) prepared from 0.1/1/0.1 lime/
slag/gypsum was analyzed by “selective dissolution” and
found to have a Ca/Si ratio of 1.1. Increasing the lime
concentration to 0.5/1/0.1 lime/slag/gypsum produced a
30-hr product with 122 m2/g and a Ca/Si ratio of 1.9. While
this ratio was outside of the 0.8–1.5 range described by
Reaction time (hr)
Figure 4. Effect of gypsum or CaCl2 on sorbent surface area. 92 °C;
agitated; 16–21% solids. Labels indicate lime/slag/additive wt ratio.
Volume 50 September 2000
Journal of the Air & Waste Management Association 1659

Brodnax and Rochelle
a considerably larger concentration of Ca2+; however, the
surface area in Figure 4 did not increase likewise. There-
fore, while a sufficient Ca2+ concentration must be main-
tained, either by the addition of gypsum or CaCl2, an
excess beyond the 20 mM suggested by Taylor8 is not nec-
essarily beneficial.
Release of Alkali from Slag
If the rate of surface area formation is proportional to the
dissolution of the slag, it might follow that it is also pro-
portional to the dissolution of individual components of
the slag. This was shown to be true for the dissolution of
Na+ from the lime/glass system.7
Figure 7 shows the release of Na+ and K+ from slag for
several different reactant recipes. The experiments repre-
sented in Figure 7 include various lime/slag ratios, with-
out additives, with gypsum, and with CaCl2. Except for a
couple of outlying points that are not shown (from the
90 mM CaCl2 experiment), the release of alkali is quite
linear with surface area. Figure 7 also shows that the K+
and Na+ diffuse out from the SiO2 matrix at proportional
rates.
While it is clear that the dissolution of alkali from
the SiO2 matrix is proportional to the formation of sur-
face area, this does not imply that the dissolution of the
SiO2 matrix is rate limiting. Rather, the rate of both pro-
cesses is probably controlled by diffusion through the
calcium silicate product layer.
Figure 6. X-ray pattern: sorbents prepared with additives. 92 °C;
agitated; 20-wt % solids. 0.1/1/0.1 lime/slag/gypsum – 3 hr – 55 m2/g.
0.1/1/0.1 lime/slag/gypsum – 30 hr – 139 m2/g. 0.5/1/0.1 lime/slag/
gypsum – 30 hr – 80 m2/g. 1/1/0.1 lime/slag/gypsum – 30 hr – 122
m2/g. 0.5/1 lime/slag – 50 mM CaCl2 – 21 hr – 115 m2/g. L: Ca(OH)2;
G: gypsum (CaSO4•2H2O).
Taylor, the X-ray diffraction pattern in Figure 6 showed
this sample to have excess lime present. Therefore, the
actual Ca/Si ratio of the high-surface-area solids in this
sample was undetermined, and it was likely that the ratio
was in fact within the 0.8–1.5 range.
Figure 6 also shows the diffraction pattern hump cre-
ated by the CSH amorphous phase at ~28–33 2-theta.
The patterns after 30 hr of reaction also show the smaller
CSH peak at 2-theta of 50. The hump at 2-theta from 28
to 36 measured from the sample which had only 3 hr of
reaction time is indicative of the glassy phase of slag. It
is clear from the figure that the product phase is near-
amorphous CSH. This conclusion is supported to a greater
extent by Arthur.7 The peak at 2-theta of 11 was an
anomaly that appeared in a few of the long-time slag-
based sorbents. It was not identified, as it was not con-
sistently present.
SORBENT CHARACTERIZATION–SEM
SEM was utilized to observe overall structural characteris-
tics of the slag sorbents. Figure 8a shows the sorbent cre-
ated from lime, slag, and gypsum. In appearance, it was
very similar to the solid produced from glass, lime, and
gypsum.7
Figure 8b shows solids prepared from lime and slag
with 50 mM CaCl2 in solution. It is quite clear from the
Effect of CaCl2. Presuming that the results observed with
the addition of gypsum were strictly a function of Ca2+
concentration, it seemed obvious to observe the effects of
boosting the Ca2+ concentration with CaCl2. Figure 4
shows the formation of surface area from slag with 50
and 90 mM of CaCl2 in the slurry. The addition of 50 mM
CaCl2 created a product with the same surface area be-
havior as that created with 0.1 g gypsum/g slag. The addi-
tion of 90 mM CaCl2 caused an unexpected upward
curvature in the formation of surface area curve. This is
probably due to either experimental scatter or the exces-
sively high ionic strength of the solution slurry.
Reaction time (hr)
Figure 7. Release of Na+ and K+ from slag during preparation. 92 °C;
agitated. Experiments without additives, with gypsum, and with CaCl2.
Figure 5 presents the Ca concentration data for the
experiments with CaCl2 added. The Cl– experiments had
1660 Journal of the Air & Waste Management Association
Volume 50 September 2000

Brodnax and Rochelle
figure that the solids appear to have a different surface
structure than those prepared with gypsum, even though
the surface areas were essentially the same. X-ray patterns
shown in Figure 6 failed to show any pronounced struc-
tural differences between the two solids, such as the pres-
ence of CaCl2 precipitate. While both have excess lime
present, the overall structures appeared to be that of CSH.
Porosity measurements of the two solids also supported a
difference in solid phase, however. While both solids had
very similar surface areas (115 vs 122 m2/g), the solid
sample prepared with Cl– had a porosity of 0.49 compared
with 0.66 cm3/g for the sample prepared with gypsum.
much higher maximum solids conversion after 1 hr of
reaction. The two sorbents prepared with gypsum indi-
cate an increased SO2 reactivity with increasing surface
area. Similar results have been published for sorbents pre-
pared from recycled container glass.7
As shown earlier, sorbents prepared with CaCl2, in
lieu of gypsum, formed solids of very high surface area.
The SEM micrographs presented in Figure 8, however,
suggest that the silicates prepared with CaCl2 were differ-
ent than those prepared with gypsum present. Figure 9
shows that the SO2 reactivity of sorbents prepared with
CaCl2 was substantially lower than that expected from
the high surface area of the solids. It appeared that even
though CaCl2 was successful in producing a high-surface-
area product, that product was not as reactive with SO2.
One hypothesis is that the CaCl2 in the liquid solu-
tion remained in the pores of the solid after filtering and
precipitated during the drying process. This precipitate
has a high surface area but is not reactive toward SO2. The
X-ray pattern of this sorbent shown in Figure 6, however,
did not show any crystalline CaCl2, which would be the
expected precipitate. Determining the exact solid phase
of the slag sorbent prepared with CaCl2 was not within
the scope of this project.
SO2 Reactivity of Slag Sorbents
The goal of silicate sorbent preparation is to optimize re-
activity with acid gases such as SO2. The SO2 reactivity
experiments with slag sorbents were conducted with a
simplified synthesized flue gas consisting of only SO2, N2,
and H2O for screening purposes. A typical desulfurization
application might treat a flue gas stream including O2,
CO2, NOx, and HCl, in addition to SO2.
Figure 9 shows the results from reacting several slag
sorbents with SO2 in a humid gas stream at 50 °C. “Solids
conversion” is defined as the cumulative moles of SO2
removed, normalized by the total amount of initial alka-
linity in the packed bed reactor. The fractional SO2 re-
moval was determined in 30-sec intervals by normalizing
the moles of SO2 removed during that interval, as mea-
sured in the gas phase, by the measured moles of SO2 in
the inlet. It is clear that the silicate sorbents are substan-
tially more reactive than hydrated lime, the typical in-
dustrial sorbent. This was indicated by the sustained rate
of SO2 removal as the solids were utilized, resulting in a
CONCLUSIONS
Calcium silicate solids were prepared from hydrated lime
and iron blast furnace slag in an aqueous agitated slurry
at 92 °C. Without additives present, the surface area of
the product improved by increasing the lime/slag weight
ratio to 2. At higher lime/slag ratios, the dilution effect of
the excess lime caused a decrease of the product surface
area. The addition of gypsum and CaCl2 dramatically
(a)
(b)
Figure 8. SEM: calcium silicate sorbents from slag. 92 °C; agitated. (a) 0.5/1/0.1 g lime/slag/gypsum – 20% solids; 30-hr reaction time; SA = 121.5
m2/g. (b) 0.5/1 g lime/slag – 20% solids; 50 mM CaCl2; 21-hr reaction time; SA = 114.5 m2/g.
Volume 50 September 2000
Journal of the Air & Waste Management Association 1661

Brodnax and Rochelle
REFERENCES
1. Rochelle, G.T. Discovery 1993, 13(1), 33-37.
2. Jozewicz, W.; Rochelle, G.T. Environ. Prog. 1986, 5(4), 219-224.
3. Kind, K.K. Ph.D. Thesis, University of Texas at Austin, Austin, TX,
1994.
4. Jozewicz, W.; Jorgensen, C.; Chang, J.S.; Sedman, C.B.; Brna, T.G. J.
Air Pollut. Control Assoc. 1988, 38, 796-805.
5. Jozewicz, W.; Gullett, B.K.; Tseng, S.C. Presented at the Second Inter-
national Conference on Municipal Waste Combustion, Tampa, FL,
April 16–19, 1991.
6. Arthur, L.F.; Rochelle, G.T. Environ. Prog. 1998, 17(2), 86-91.
7. Arthur, L.F. Ph.D. Thesis, University of Texas at Austin, Austin, TX,
1998.
8. Taylor, H. The Chemistry of Cements; Academic: New York, 1964; Vol.
1.
9. Jozewicz, W.; Chang, J. Evaluation of FGD Dry Injection Sorbents and
Additives, Vol. 1: Development of High Reactivity Sorbents; EPA-600/7-
89-0006a; Prepared for the U.S. Environmental Protection Agency by
ACUREX Corp.: 1989.
Solids conversion (mol SO2 rem./mol. alk. inlet)
10. Koch Industries. Material Safety Data Sheet: Ground Granulated Blast-
Furnace Slag; National Fire Protection Association: Washington, DC,
1980.
11. Jozewicz, W.; Rochelle, G.T.; Stroud, D.E. Presented at the 1991 SO
Control Symposium, Washington DC, December 1991; Paper 7-122;
EPRI TR-101054.
Figure 9. SO2 reactivity for silicate sorbents prepared from slag.
50 °C; 58% RH; ~1000-ppm SO2. 1.521 slpm total gas rate; 0.1-g
sorbent. Labels indicate lime/slag/additive wt ratio and surface area.
Solids preparation: 92 °C; agitated; 20% solids.
12. Stroud, D.E. M.S. Thesis, University of Texas at Austin, Austin, TX,
1991.
13. Sigma-Aldrich; H2S Material Safety Data Sheet, 1994.
14. Peterson, J.R. M.S. Thesis, University of Texas at Austin, Austin, TX,
1987.
15. Peterson, J.R.; Rochelle, G.T. Environ. Sci. Technol. 1988, 22(11), 1299-
1304.
16. Peterson, J. Ph.D. Thesis, University of Texas at Austin, Austin, TX,
1990.
17. Kind, K.K.; Wasserman, P.D.; Rochelle, G.T. Environ. Sci. Technol. 1994,
28, 277-283.
improved the formation of high-surface-area solids. Us-
ing a 0.1/1/0.1 lime/slag/gypsum ratio produced solids
with the highest absolute surface area of 139 m2/g (167
m2/g slag) after 30 hr of reaction. There was evidence,
however, that the reaction rate was slowing due to the
consumption of the hydrated lime. Increasing the lime
concentration to 0.5/1/0.1 lime/slag/gypsum produced
solids with 194 m2/g slag (122 m2/g), and the reaction ap-
peared to continue past 30 hr. The silicate sorbents were
consistently more reactive with SO2 than hydrated lime,
the typical commercial sorbent. The SO2 reactivity of sol-
ids prepared with CaCl2, in contrast to gypsum, was unex-
pectedly low given the high surface area. This emphasized
the understanding that the surface area/SO2 reactivity cor-
relation is highly dependent on the solid phase. H2O2 was
shown to be an effective additive to oxidize sulfide released
from the slag during sorbent preparation.
About the Authors
Lia F. Brodnax, Ph.D., is an air quality engineer with the Bay
Area Air Quality Management District (BAAQMD), 939 Ellis
St., San Francisco, CA 94109. She was formerly with Ra-
dian International. Ms. Brodnax’s maiden name was L.F.
Arthur. Gary T. Rochelle, Ph.D., is the Carol and Henry
Groppe Professor in Chemical Engineering and graduate
advisor in Chemical Engineering, Department of Chemical
Engineering, The University of Texas at Austin, Austin, TX
78712.
1662 Journal of the Air & Waste Management Association
Volume 50 September 2000