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Impact of brownmillerite hydration on Cr(VI) sequestration in chromite ore processing residue


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Experimental and modeling studies were conducted to delineate the reaction progress of chromite ore processing residue (COPR) upon hydration and the roles of brownmillerite and calcium aluminum chromium oxide hydrates (CAC) in the scavenging of hexavalent chromium. A kinetic study was conducted by preparing slurry samples with both synthetic brownmillerite and actual COPR samples at ambient temperatures. The hydration reaction of brownmillerite using synthetic brownmillerite was very fast (within 1 hour) and was completed within 2 days. However, the hydration of brownmillerite embedded in COPR to its hydration byproducts was not clearly observed after 7 days of aging. Newly formed Ca4Al2O6(CrO4)·14H2O (CAC-14) was observed after 1 hour of aging in both samples. However, the rate of formation of CAC-14 with synthetic brownmillerite was much faster than the COPR embedded brownmillerite. The reaction progress of synthetic brownmillerite and COPR upon chromate influx was simulated by a reaction path modeling program. The phase transformation of both samples can be predicted by the constructed model. Moreover, the formation of CACs upon chromate addition was predicted by the model, suggesting an effective sink for Cr(VI). Key wordschromite ore processing residue–brownmillerite–calcium aluminum chromium oxide hydrates–hexavalent chromium
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Geosciences Journal
Vol. 15, No. 3, p. 287
296, September 2011
DOI 10.1007/s12303-011-0023-y
The Association of Korean Geoscience Societies and Springer 2011
Impact of brownmillerite hydration on Cr(VI) sequestration in chromite
ore processing residue
ABSTRACT: Experimental and modeling studies were conducted
to delineate the reaction progress of chromite ore processing res-
idue (COPR) upon hydration and the roles of brownmillerite and
calcium aluminum chromium oxide hydrates (CAC) in the scav-
enging of hexavalent chromium. A kinetic study was conducted by
preparing slurry samples with both synthetic brownmillerite and
actual COPR samples at ambient temperatures. The hydration
reaction of brownmillerite using synthetic brownmillerite was very
fast (within 1 hour) and was completed within 2 days. However,
the hydration of brownmillerite embedded in COPR to its hydra-
tion byproducts was not clearly observed after 7 days of aging. Newly
formed Ca
O (CAC-14) was observed after 1 hour
of aging in both samples. However, the rate of formation of CAC-
14 with synthetic brownmillerite was much faster than the COPR
embedded brownmillerite. The reaction progress of synthetic
brownmillerite and COPR upon chromate influx was simulated
by a reaction path modeling program. The phase transformation
of both samples can be predicted by the constructed model. More-
over, the formation of CACs upon chromate addition was pre-
dicted by the model, suggesting an effective sink for Cr(VI).
Key words: chromite ore processing residue, brownmillerite, calcium
aluminum chromium oxide hydrates, hexavalent chromium
Millions of tons of chromite ore processing residue
(COPR) which is an alkaline material (pH > 12) were depos-
ited over a span of 50 years at numerous urban areas in the
USA, the UK, and elsewhere in the world (Burke et al.,
1991; Weng et al., 1994; James et al., 1996; Farmer et al.,
1999; Graham et al., 2006). There are more than 200 active
COPR sites in Hudson County, New Jersey, USA (Wazne et
al., 2007). COPR was beneficially used as structural fill
because of its favorable structural quality as a granulated
material. The COPR was generated by the high-temperature
roasting process of chromite ore with lime in order to extract
chromium as soluble sodium chromate. Specifically, the
roasting process was conducted at approximately 1200 °C
and the lime was added to act as a mechanical separator
allowing oxygen to react with the chromite and sodium car-
bonate. Lime also served as a sequestering agent, combin-
ing with various ore impurities to form insoluble compounds
(Allied Signal, 1982). The sodium chromate formed during
the roasting process was extracted with hot water as a weak
yellow liquor solution. The sodium chromate was then con-
verted into sodium dichromate by reaction with sulfuric
acid. After draining, the residue was discarded. The disposed
COPR contains unreacted chromite ore and un-extracted
chromate. Even though the high lime process has ceased in
the USA and the UK, it is still being used in China, Russia,
Kazakhstan, India and Pakistan (Darrie, 2001).
Chromium exists mainly in two oxidation states as
hexavalent [Cr(VI)] and trivalent chromium [Cr(III)] in common
environments. Cr(VI) is highly mobile, severely toxic at mod-
erate doses, and classified as a respiratory carcinogen in
humans. In contrast, Cr(III) is used as a dietary element at
low doses, and in most environmental systems is immobile
(Higgins et al., 1998). The COPR, which contains both Cr(III)
and Cr(VI), was discovered not as benign as initially
thought; yellow chromate solutions are often observed to
leach from locations where COPR is deposited and elevated
Cr(VI) concentrations are measured in ground water and
water bodies in the proximity of these sites (Geelhoed et al.,
2002). Additionally, structures built on sites where COPR
was used as fill experienced catastrophic failures due to
heave and uncontrolled expansion of the COPR material.
Consequently, COPR has become a major geoenvironmen-
tal and geotechnical hazard in many urban areas.
The main solid phases that comprise COPR at the New
Jersey site are brownmillerite (Ca
), periclase (MgO)
and quicklime (CaO) with minor impurities (Chrysochoou et
al., 2005b). Moreover, approximately 31% of brownmillerite
was observed among those phases (Jagupilla et al., 2009).
Brownmillerite is known to be unstable under environmental
aqueous conditions and should hydrate to hydrogarnet
], portlandite [Ca(OH)
] and hematite [Fe
according to the following reaction:
+7 H
+ Ca
. (1)
Portlandite Hematite Hydrogarnets
Deok Hyun Moon*
Mahmoud Wazne
Department of Environmental Engineering, Chosun University, Gwangju 501-759, Republic of Korea
W.M. Keck Geoenvironmental Laboratory, Center for Environmental Systems, Stevens Institute of Tech-
nology, Hoboken, NJ 07030, USA
*Corresponding author:
288 Deok Hyun Moon and Mahmoud Wazne
However, brownmillerite is still present at numerous sites
some 80 years after its deposition. It has been reported that
the major COPR phases (brownmillerite, periclase, quicklime)
are unstable in aqueous environments and eventually must
react entirely to form hydration products (Moon et al., 2005b).
Among these three phases, only lime hydrates quickly to
form portlandite whereas brownmillerite and periclase react
much slower to form their respective hydration products.
Especially, brownmillerite hydration in actual COPR material
is extremely slow at ambient temperature (Moon et al.,
2008). This could be due to the iron coating on the actual
COPR which delays the dissolution process (Taylor, 1997).
Calcium aluminum chromium oxide hydrates [Ca
O, CACs] were also identified in COPR mate-
rials as the main reaction products of brownmillerite. The
CACs could be found at different hydration states (9, 12, or
14 H
O) (Chrysochoou et al., 2005a). The transformation of
brownmillerite to CAC-14 [Ca
O] upon
chromate influx can be written as:
. (2)
In COPR materials, Cr(VI) is found mainly in CACs and
is partially present in hydrotalcites [Mg
O] and hydrogarnets through anionic substitution. These
minerals could be embedded in brownmillerite (Kremser et al.,
2005), and therefore, the rate of Cr(VI) release to the environ-
ment would be dependent on the rate of brownmillerite hydration.
Therefore, the hydration pathway of brownmillerite in
COPR during long-term weathering is not well understood. It
is worth studying the hydration pathway of synthetic brown-
millerite which does not have an iron coating or other impu-
rities to determine the time needed for hydration to occur. This
information can be used as reference for long-term hydration
of brownmillerite in COPR at ambient temperature. Also,
hydration of brownmillerite is needed in order to understand
Cr(VI) sequestration in COPR and the reaction pathway of
brownmillerite to form CAC compounds upon hydration.
The purpose of this study was to investigate brownmil-
lerite hydration reaction pathways using synthetic brown-
millerite and field COPR samples to assess the hydration
process under controlled conditions. The hydration of
brownmillerite was also investigated in the presence of
chromate in order to examine Cr(VI) sequestration in COPR
materials. Reaction path kinetics geochemical modeling
using the geochemical modeling code, EQ3/6 (Wolery and
Jarek, 2003), was used to simulate the synthetic brownmil-
lerite and the COPR-water interactions.
2.1. Materials and Reagents
Synthetic brownmillerite samples and COPR samples
containing high brownmillerite concentrations were used to
investigate the hydration of brownmillerite and the formation
of CAC. The synthetic brownmillerite was obtained from
CTL (Skokie, IL) with a mean particle size of 20 µm. The
COPR samples were obtained from Study Area 7 (SA 7),
located in Hudson Country, New Jersey, USA. All the COPR
samples were pulverized and sieved by hand through a 100-
mesh sieve (0.15 mm diameter opening). Deionized (DI)
water was all used in this study. Potassium chromate
) salts obtained from Fisher Scientific (Suwanee,
GA) dissolved in DI water were used as a chromate source
for the investigation of CAC formation.
2.2. Slurry Sample Preparation
In order to investigate katoite [Ca
(x =
1.5 to 3.0), a hydrogarnet] and CAC formation upon
brownmillerite hydration, slurry samples were prepared
using 10 grams of either synthetic brownmillerite or actual
COPR material with DI water and potassium chromate
solution using a 2:1 liquid to solid ratio. The liquid to solid
ratio of 2 to 1 was used to provide sufficient water and to
promote full hydration (Moon et al., 2005a). The concen-
tration of chromate was 1 wt% of the dry sample, to rep-
resent the average Cr(VI) concentration in COPR. The
method of slurry sample preparation used for CAC formation
was similar to the method used for ettringite formation
where sodium sulfate decahydrate (Na
O) was used
as a sulfate source (Moon et al., 2005a). The time intervals
for aging at ambient temperature were 1 hour, 8 hours, 1
day, 2 days, and 7 days. All of the slurry samples were con-
tinuously mixed for 1 hour using a Toxicity Characteristic
Leaching Procedure (TCLP) tumbler at 30 rpm. After 1 hour
of continuous mixing, samples were periodically shaken
during aging.
2.3. X-ray Powder Diffraction (XRPD) Analyses of the
Slurry Samples
About one gram of homogenized slurry sub-samples
were drawn after the designated curing time using a 5 mL
pipette. The samples were then filtered using a 0.4 µm pore
size membrane filter. The residue was analyzed using XRPD.
Step-scanned X-ray patterns were collected with a Rigaku
DXR-3000 computer-automated diffractometer. XRPD anal-
yses were conducted at 40 kV and 40 mA using a diffracted
beam graphite-monochromator with Cu radiation. XRPD
data was collected in a range of 5° to 65° 2θ with a step size
of 0.02° and a count time of 3 seconds per step. Qualitative
analysis of XRPD patterns were conducted using the JADE
software version 7.1 (MDI, 2005) using the International
Center for Diffraction Data database, (ICDD, 2002) refer-
ence patterns.
Impact of brownmillerite hydration on Cr(VI) sequestration in chromite ore processing residue 289
2.4. Mineralogical Characterization of the COPR Sample
The initial COPR sample was homogenized and air dried
for 24 hours. Two grams of the homogenized air dried sam-
ple was pulverized by a McCrone micromill for 10 min
together with 7 mL cyclohexane. The resulting slurry was
air dried then mixed with 20% by weight of corundum (α-
) as an internal standard. The sample was then sub-
jected to XRPD analysis. The qualitative and quantitative
analyses of the XRPD patterns were performed using the
Jade software (MDI, 2005), version 7.1 and the Whole Pat-
tern Fitting function of Jade, which is based on the Rietveld
method (Rietveld, 1969). The reference databases for pow-
der diffraction and crystal structure data were the Interna-
tional Center for Diffraction Data database, (ICDD, 2002)
and the Inorganic Crystal Structure Database (ICSD, 2005),
2.5. Scanning Electron Microscopy (SEM) Analyses
Prior to SEM analysis, all samples were dried for 3 days
in a nitrogen-purged chamber and prepared using double-
side carbon tape. SEM analyses were conducted using a
LEO-810 Zeiss microscope equipped with an ISIS-LINK
2.6. Geochemical Modeling
Geochemical modeling was used to simulate the reaction
progress of brownmillerite and COPR. The modeling results
can be used to assess the experimental results and to predict
the hydration progress of brownmillerite and COPR. The
reaction progress of synthetic brownmillerite and COPR in
the presence of a chromate solution was simulated by using
the EQ3/6 software package, version 8.0 (Wolery and Jarek,
2003). In this modeling package the reactants are added
incrementally and thermodynamic equilibrium is enforced
at every incremental step. This modeling procedure allows
the reactants to have different reaction rates and facilitates
the assessment of the reaction products at each incremental
step and not necessarily only at equilibrium.
The transformation of synthetic brownmillerite to katoite
was clearly observed after 1 hour of continuous mixing, and
was completed after 2 days of curing (Fig. 1). The SEM
images of synthetic brownmillerite (initial condition) and
Fig. 1. XRPD patterns of hydrating synthetic brownmillerite for curing time up to 7 days.
290 Deok Hyun Moon and Mahmoud Wazne
katoite (2 days of curing) are presented in Figures 2 and 3,
respectively. In addition to katoite, paraalumhydrocalcite
O) and iron carbonate hydroxide
)] were also identified.
Compared to that of synthetic brownmillerite, the major
phases identified in the COPR samples before and after
hydration were brownmillerite, periclase, brucite, katoite,
hydroandradite, quartz, Ca
O (CAC-12),
CAC-14 and stichtite [Mg
O] (Fig. 4).
Upon hydration, the transformation of brownmillerite was
not clearly observed within 7 days (Fig. 4).
Fig. 2. SEM image of synthetic brownmillerite as received (irreg-
ular shape).
Fig. 3. SEM image of katoite (hexagonal in shape) derived from
synthetic brownmillerite hydration (2 days of curing).
Fig. 4. XRPD pattern of the actual COPR materials upon hydration.
Impact of brownmillerite hydration on Cr(VI) sequestration in chromite ore processing residue 291
Upon addition of chromate to synthetic brownmillerite,
the major phases identified after 1 hour of continuous mix-
ing were brownmillerite, CAC-14, bernalite [Fe(OH)
and calcium aluminum iron carbonate hydroxide hydrate
O, CAFC] (Fig. 5). The SEM
image of the transformed CAC-14 upon chromate influx is
presented in Figure 6. Katoite formation was observed with
minor peaks after 1 day of aging.
Upon the addition of chromate to the COPR samples,
no obvious changes in peak intensities for most of the
phases were identified, except for portlandite and CAC
compounds. Portlandite dissolved and newly formed CAC-14
was observed, after 1 hour of aging (Fig. 7).
Paraalumhydrocalcite was identified upon the hydration
of synthetic brownmillerite. It was stable up to 8 hours of
aging, but then it dissolved after 1 day of aging. One pos-
sible explanation is that paraalumhydrocalcite is a meta-sta-
ble phase, and that the Ca
and Al
from this phase would
be consumed to form katoite. However, it should be noticed
that this carbonate phase was more thermodynamically
favored than calcite because no calcite was observed after
7 days. Even though it has been reported that iron does not
Fig. 5. XRPD pattern of synthetic brownmillerite upon chromate addition.
Fig. 6. SEM image of CAC-14 (hexagonal plates) formed by
exposing synthetic brownmillerite to K
solution (7 days).
292 Deok Hyun Moon and Mahmoud Wazne
form phases separately during brownmillerite hydration
(Chatterji and Jeffery, 1962), iron carbonate hydroxide was
stable throughout the experiment (7 days), suggesting that
iron liberated by the synthetic brownmillerite hydration pre-
cipitated to form iron carbonate hydroxide though released
iron may have also resulted in the formation of amorphous
iron compounds.
It appears that CAC-12 was transformed into CAC-14
upon the hydration of the COPR samples. It has been reported
that CAC hydration states can be fluctuated depending on
available moisture (Chrysochoou et al., 2005). No apparent
changes in the peak intensity for brownmillerite and katoite
were observed upon the hydration of the COPR material.
This indicated that the dissolution of brownmillerite in the
COPR samples is very slow. This could be due to the iron
coating on the COPR samples which may hinder or slow
down the dissolution process (Taylor, 1997). Research also
showed that Mg
substitution for Ca
in brownmillerite
results in a slower hydration process (Jupe et al., 2001).
Alternatively, this could be due to the relatively larger par-
ticle size of the COPR samples used in this study compared
to the synthetic brownmillerite samples (150 µm versus 20
µm). However, if the relatively larger particle size was the
cause, then some brownmillerite hydration should have
been observed after aging the samples for 7 days.
Upon chromate influx to synthetic brownmillerite, CAC-
14 was identified after 1 hour of mixing. The formation of
CAC-14 is mainly due to the reaction mentioned in Equation
2. The SEM image of transformed CAC-14 upon chromate
influx (Fig. 6) indicated that the Fe
liberated by the syn-
thetic brownmillerite accumulated as bernalite and CAFC,
while Ca
was converted to CAC-14 and CAFC. Moreover,
the quantities of CAC-14 and CAFC increased upon aging.
However, it was not clear whether the peak intensities for
bernalite increased or whether this was due to small peak
intensities. Katoite formation observed with minor peaks
after 1 day of aging suggested that CAC, bernalite and CAFC
formation was thermodynamically favored over katoite upon
chromate addition. Moreover, it was noticed that brown-
millerite was not completely transformed into other phases
at 7 days of aging, suggesting that the reaction is slow but
Upon chromte influx to the actual COPR material, Port-
landite dissolved and CAC-14 was newly formed (Fig. 7)
after 1 day of aging. This phase formation is mainly due to
hydration. Moreover, CAC-14 intensities increased with
Fig. 7. XRPD pattern of the actual COPR materials upon chromate addition.
Impact of brownmillerite hydration on Cr(VI) sequestration in chromite ore processing residue 293
curing, indicating that chromate addition mainly contributed
to the formation of this phase.
Overall, following chromate addition to the synthetic
brownmillerite and the COPR samples, a newly formed CAC-
14 compound was observed in both samples. However, the
rate of CAC-14 formation was much faster in synthetic
brownmillerite than in the COPR samples. No apparent peak
reductions of brownmillerite were observed in the COPR
samples, indicating that synthetic brownmillerite was more
thermodynamically or kinetically unstable than brownmil-
lerite in the COPR samples.
The mineralogical characterization of the COPR sample
indicated a high brownmillerite content at approximately
39% of the crystalline phases (Table 1). Even though brown-
millerite is not stable thermodynamically under aqueous
conditions it is still present at the site some 80 years after
its deposition. During the model simulation, the rate of
brownmillerite hydration was assumed two orders of mag-
nitude slower than the reaction rate of the other minerals.
This was incorporated into the model using the relative
kinetic rates feature of EQ3/6. Similarly, periclase is also
not stable at the site conditions but its slow hydration
explains its presence at the site. Its reaction rate was treated
similar to that of brownmillerite. The amorphous content
was determined at 30% using a corundum internal standard.
This amorphous fraction was assumed to consist of the iron
oxide hydroxide phase and goethite during the simulation
There is no separate thermodynamic data for CAC-12
and CAC-14 in the thermodynamic database of EQ3/6 or in
the surveyed literature. Therefore, Cr(VI)-hydrocalumite
thermodynamic data was used to represent both of these
minerals during the model simulation. Similarly, there is no
thermodynamic data for stichtite but the Cr(III)-hydrotalcite
phase and Al-hydrotalcite thermodynamic data were used
instead. It is also worth mentioning that cationic and anionic
substitution may exist and some solid phases may be solid
solutions rather than pure solids; however, the extent of
these substitutions was not investigated in this study.
The model predicted the transformation of brownmillerite
to katoite, portlandite, hematite and Cr(VI)-Hydrocalumite
(a CAC). As shown in Figure 8, these phases formed as
soon as brownmillerite started to dissolve. The model
differed from the experimental data by not predicting the
formation of bernalite and calcium aluminum iron carbon-
ate hydroxide hydrate. It is no surprise that these two phases
were not predicted by the model since their thermodynamic
data is not present in the thermodynamic data base of EQ3/
Table 1. The percentages of the crystalline phases for the initial
COPR sample as quantified by Rietveld analysis
Species Molecular formula Percent
(Al, Fe)O
3CaO Al
Fig. 8. EQ3/6 simulation of the reaction progress of 500 g of synthetic brownmillerite with chromate solution with chromate content
at 1 percent of dry weight brownmillerite and liquid to solid ratio of 2.
294 Deok Hyun Moon and Mahmoud Wazne
6, which is the most reliable and extensive mineral database
available. Furthermore, no thermodynamic data was avail-
able in the surveyed literature for these two phases. One
model simulation was run without chromate (simulation not
presented) and the model predicted katoite, portlandite and
hematite. During this run, paraalumhydrocalcite and iron
carbonate hydroxide were not predicted by the model due to
the unavailability of their thermodynamic database in the
EQ3/6 thermodynamic data base in a similar fashion to the
first simulation.
The presence of CAFC and paraalumhydrocalcite in the
experimental data indicated a carbonate source; however,
carbonate was not added to the system. This may be caused
by atmospheric carbon dioxide contamination enhanced by
the high alkaline pH of the slurry. Conversely, when 0.1%
of carbonate was added in the model simulation, calcite was
predicted as the carbonate phase (simulation not shown)
rather than CAFC and paraalumhydrocalcite. As for the
iron phases, the experimental data showed the sequestration
of iron in bernalite and iron carbonate hydroxide whereas
the model predicted the formation of hematite. Bernalite and
iron carbonate hydroxide thermodynamic data were also not
available in the EQ3/6 thermodynamic database.
As soon as chromate was introduced into the system, all
of the chromate (0.043 moles, 1% w/w) was sequestered
immediately by the newly formed Cr(VI)-hydrocalumite.
Cr(VI)-hydrocalumite remained stable throughout the reac-
tion progress of the hydration of brownmillerite. This is sig-
nificant as the model predicted that Cr(VI)-hydrocalumite,
which is a hydration byproduct of the dissolution of brown-
millerite, is expected to scavenge all of the chromate in the
system constrained only by stoichiometric limitations.
For the reaction progress of COPR in the presence of a
chromate solution, the model predicted the complete disso-
lution of quartz, periclase, brownmillerite, brucite and goet-
hite (the surrogate amorphous phase). The new phases
predicted by the model are andradite (Ca
) and
hematite. The model predicted an increase in the content of
portlandite, katoite and hydrotalcite. However, the portlan-
dite content decreased initially at reaction progress less than
10% before it started to increase again. Portlandite may have
served as an immediate calcium source for the formation of
andradite and Cr(VI)-hydrocalumite before the start of the
dissolution of brownmillerite. As soon as a significant per-
centage of brownmillerite dissolved the percentages of port-
landite and katoite started to increase.
The model also predicted the increase in the concentra-
tion of Cr(VI)-hydrocalumite upon the introduction of the
chromate solution which is similar to the experimental results
in Figure 6. The total number of moles of chromate (0.059)
was sequestered in Cr(VI)-hydrocalumite and remained sta-
ble throughout the simulation, similar to the hydration of
synthetic brownmillerite in the presence of a chromate solu-
tion in the previous section.
The model simulation of COPR hydration without the
addition of a chromate solution (simulation not shown) was
similar to the run with the chromate solution except for the
increase in the content of Cr(VI)-hydrocalumite and a smaller
drop in the portlandite concentration upon chromate solu-
tion addition, which indicates that the increase in Cr(VI)-
hydrocalumite content in the previous simulation upon the
addition of chromate partially derived its calcium from port-
Fig. 9. EQ3/6 simulation of the reaction progress of 500 g of COPR with chromate solution with chromate content at 1 percent of dry
weight COPR and liquid to solid ratio of 2.
Impact of brownmillerite hydration on Cr(VI) sequestration in chromite ore processing residue 295
landite. Also, during the same run, katoite content increased
slightly at a reaction progress of 10% instead of the small
drop shown in Figure 9, which indicates that katoite may
have served as the aluminum source for the formation of
The transformation of brownmillerite to katoite and CAC
compounds was specifically investigated using synthetic
brownmillerite and actual COPR materials. DI water and a
chromate solution were introduced to synthetic brownmil-
lerite and actual COPR materials to investigate the reaction
pathways of katoite and CAC compounds, respectively. The
geochemical model was used to predict the product phases
of COPR upon complete hydration. Upon hydration of syn-
thetic brownmillerite, katoite formation was thermodynam-
ically favored after 1 hour of continuous mixing. The trans-
formation of brownmillerite to katoite was completed after
2 days of aging. However, no major changes in katoite peak
intensities in the actual COPR materials were observed
within 7 days of aging. This indicated that the dissolution of
brownmillerite in the actual COPR materials was very slow.
Upon chromate influx to the synthetic brownmillerite and
the actual COPR materials, CAC-14 was identified in both
samples after 1 hour of continuous mixing. The peak inten-
sities of CAC-14 increased upon aging in both samples.
However, the rate of CAC-14 formation was much faster in
synthetic brownmillerite than in the actual COPR material,
suggesting that the dissolution of brownmillerite in the
actual COPR material was slower than in synthetic brown-
millerite. The geochemical model predicted the formation
of Cr(VI)-hydrocalumite upon the introduction of the chro-
mate solution, and more importantly, the model predicted
that the byproducts of the hydration of brownmillerite and
COPR would scavenge chromate and thus control its sol-
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Manuscript received February 1, 2010
Manuscript accepted June 24, 2011
... Ici, deux formes sont identifiées, la forme « classique » détectée pour l'ensemble des essais à 60°C et une forme contenant du Cr de formule 3 CaO·Cr 2 O 3 ·CaCO 3 ·11 H 2 O détectée pour les essais à 40°C de 24h, 7 jours et 1 mois. La katoite et l'hydrocalumite sont des phases fréquemment observées dans les résidus miniers de chromite (Chromite Ore Processing Residues ou COPR) et sont identifiés comme produits d'hydratation de la brownmillérite [35,145,157,207]. Toujours dans les COPR, l'hydrotalcite se forme à partir du Mg libéré par le périclase et la brucite, à condition que suffisamment d'Al soit libéré par la brownmillérite (Wazne et al., 2008). ...
... Toujours dans les COPR, l'hydrotalcite se forme à partir du Mg libéré par le périclase et la brucite, à condition que suffisamment d'Al soit libéré par la brownmillérite (Wazne et al., 2008). Moon et Wazne [157] proposent les équations suivantes pour la formation de l'hydrocalumite (IV. Les analyses DRX montrent elles aussi des résultats très comparables à ceux obtenus pour les essais sur poudre en réacteur statique. ...
... La formation de ces phases a pu être favorisée lors des essais de lixiviation sur poudres d'une part grâce au pH légèrement plus élevé qui a été mesuré, et d'autre part grâce à une hydratation accélérée de la brownmillérite. En effet, l'hydratation de la brownmillérite est connue pour être particulièrement lente [157,207], mais le broyage a pu considérablement accélérer le processus grâce à une augmentation de la surface de réaction. L'origine du K et du Na n'est pas connue. ...
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Plus de 400 millions de tonnes de laitiers sidérurgiques sont produits chaque année dans le monde. Si une majorité est aujourd’hui recyclée, principalement sous forme de granulats, des quantités importantes sont encore mises en décharges tous les ans (de 1 à 4 millions de tonnes en Europe). La présence d’éléments traces métalliques potentiellement toxiques (Cr, V, Mo) est l’un des principaux obstacles à la valorisation de ces laitiers. Il est nécessaire de connaitre le comportement sur le long terme de ces matériaux, afin de prédire le lessivage des métaux qu’ils contiennent et leur dispersion dans l’environnement. Dans cette thèse on s’intéresse à un site sidérurgique appartenant au groupe Industeel - ArcelorMittal et situé à Châteauneuf dans la Loire. Plusieurs milliers de tonnes de laitiers de four à arc électrique y ont été accumulés pendant plusieurs décennies, exposés à l’altération météorique. L’objectif est de caractériser le plus finement possible, à l’échelle de l’échantillon mais aussi du crassier, les variations minéralogiques et de comprendre davantage les processus de transformations qui se produisent au cours de l’altération de ces matériaux. Différentes techniques analytiques sont utilisées à la fois en laboratoire et sur le terrain. Le couplage d’analyses chimiques, minéralogiques et magnétiques a permis une caractérisation détaillée de ces laitiers déjà partiellement altérés. Puis l’installation de lysimètres et des tests de lixiviation ont aidé à évaluer la mobilité des métaux présents et à comprendre les transformations minéralogiques qui s’opèrent au cours de l’altération.
... Chromate ore process residues (COPR) were generated by the process of roasting chromite ore at approximately 1200°C to oxidize the trivalent Cr in the chromite ore to the hexavalent state with the addition of lime. The mineralogical characteristics of COPR have been studied extensively (Hillier et al. 2003(Hillier et al. , 2007Chrysochoou et al. 2009aChrysochoou et al. , 2010Moon and Wazne 2011). The mineral phases are divided into three main categories: unreacted feedstock ore (chromite), high temperature phases produced during Cr extraction (brownmillerite, periclase and larnite), and minerals formed under ambient weathering conditions at the disposal sites (brucite, calcite, aragonite, ettringite, hydrocalumite, hydrogarnet). ...
... b/searc h/Cr). All the reference materials were selected based on its possible occurrence in the COPR reported by other studies (Hillier et al. 2003;Chrysochoou et al. 2010;Moon and Wazne 2011). Spectra of the samples and reference compounds were processed using Athena program (version 0.8.056). ...
... Brownmillerite (Fig. 2b), a typical COPR parent mineral, was found containing a high abundance of Cr. The hydrated minerals including stichtite (Fig. 2a), paraalumhydrocalcite (Fig. 2c) and brucite (Fig. 2f) were also identified to bearing Cr in the sample, which have been reported in other studies as well Hillier et al. 2007;Moon et al. 2007;Moon and Wazne 2011). However Cr bearing in magnesiochromite (Fig. 2d) and hashemite (Fig. 2e) has been rarely reported in the previous studies. ...
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The chromate ore process residues (COPR) polluted soil was physically separated into coarse sand (2.000–0.425 mm), fine sand (0.425–0.053 mm) and silt to clay (< 0.053 mm) fractions. The Cr speciation was characterized by synchrotron based micro X-ray fluorescence (µ-XRF) and micro X-ray absorption near-edge spectra (µ-XANES). The results indicated that Cr was bearing both in COPR parent minerals and hydrated products and was dominated by Cr(III) in three size-fractions. The synchrotron results indicated that Cr(III) was dominated by chromite, organic matter bound Cr(III) and particle adsorbed Cr3+ in the selected hotspots from the coarse sand, fine sand and silt to clay sized fraction, respectively. While Cr(VI) occurred in the form of CrO42− in the selected hotspots from three size fractions. The difference of Cr(III) species in the size-fractions suggested that higher edaphic effects occurred in the fine size-fractions than in the coarse size-fraction for the weathered COPR.
... Hence, chromium salt is produced by roasting in absence of lime which can reduce the COPR generation. The COPR has high content of Cr(VI) which is highly mobile and can contaminate nearby the soil, surface waters and ground waters as a result of precipitation and runoff [3][4][5]. Moreover, its toxicity is severe than Cr(III). ...
Chromite ore processing residue (COPR) is a hazardous waste due to the presence of highly mobile Cr(VI). In this paper, the reduction/immobilization of COPR using composite materials (fly ash, blast furnace slag and metakaolin) based geopolymer coupled with zero-valent iron (ZVI) was investigated. The effects of ZVI and acid dosages on the reduction of Cr(VI) in COPR had been analysed. The immobilization of original COPR samples and reduced COPR samples were prepared. The immobilization effect was evaluated by compressive strength and leaching tests. The results showed that geopolymer with reduced COPR had better immobilization effect as compared to original COPR. According to mechanical and leaching properties of geopolymer with reduced COPR samples as well as X-ray diffraction, scanning electron microscope with energy dispersive spectrometer and Fourier transform infrared spectrometry analyses, COPR was effectively immobilized.
Excessive reductants are used in engineering to ensure a reliable remediation effect of chromite ore processing residue (COPR), however, re-yellowing phenomenon of remediated COPR occurs after some time though the Cr(VI) content meets regulatory requirements after curing period. This problem is due to a negative bias on Cr(VI) determination using USEPA method 3060A. To address this issue, this study tried to reveal the interference mechanisms and proposed two methods to amend the bias. Results of ion concentrations, UV-Vis spectrum, XRD, and XPS together showed that Cr(VI) was reduced by ions (Fe2+, S52-) in the digestion stage of USEPA method 3060A, and as a result, method 7196A would not reflect the true Cr(VI) concentration. The interference on Cr(VI) determination generated by excess reductants mainly occurs during the curing period of remediated COPR, but it decreases over time as reductants being oxidized gradually by the air. Compared with the thermal oxidation, the chemical oxidation with K2S2O8 prior to alkaline digestion performs better to eliminate the masking effect brought by excess reductants. This study provides an approach on how to accurately determine the Cr(VI) concentration in the remediated COPR. It might be helpful to reduce the occurrence possibility of re-yellowing phenomenon.
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Linking chromium (Cr) speciation with its stability in soils is vital because insoluble Cr(VI) and chemical adsorbed Cr(VI) could hinder the remediation efficiency and release Cr(VI) for a prolonged period of time. In this study, we investigated key Cr species to probe the mechanisms controlling the release of insoluble Cr(VI) at Cr-contaminated sites using synchrotron-based X-ray absorption near-edge structure (XANES) for the first time. Chromite, stichtite and Cr-silicate were predominant forms of Cr(III). Insoluble Cr(VI) was hosted by layered double hydroxides (LDHs) such as brownmilerite and hydrotalcite. Anion competition tests documented a substitution of absorbed Cr(VI) by SO42- and NO3-. Acid extraction released 6.7-25.7% more Cr(VI) than anion extraction, possibly attributing to the erosion of LDH and CaCrO4 in calcite rather than Cr-bearing minerals. Brown and red soils released maximally 62% and 44% of total Cr(VI) by 10 mol/(kg soil) and 2 mol/(kg soil) of H+, respectively. SO42-, H2O and H+ contributed to more release of total Cr(VI) in brown soils (22%, 33% and 7%) than red soils (25%, 17% and 2%). More crystalline Cr structures were found after chemical stabilization, indicating a higher Cr stability in chemically stabilized soils. Cr and Mn exhibited an overlapped distribution pattern in both contaminated and chemically stabilized soils, hinting at the re-oxidation of Cr(III). Insoluble Cr(VI) could be released by acidic rainfalls and soil organic matters, posing potential threats to Cr long-term stability in field-scale remediation.
Layered double hydroxide (LDH), the only anionic clay in the environment, plays a key role in natural ion transportation. The ion retention effect of LDHs was traditionally attributed to ion exchange with low affinity. Here, we demonstrated an ultrastrong interaction between anions and LDHs induced by their inherent nanoconfinement using chromium ore processing residue (COPR) that contained several Cr(VI)-bonded LDHs as a probe. Hydrocalumite (Ca/Al-Cl LDH) was verified as the primary phase for Cr(VI) retention through two types of interactions such as ion exchange and Cr-Ca coordination. More significantly, the confined spacing between two layers of hydrocalumite provided spatial restriction and shielding effects to the intercalated Cr(VI), which enhanced Cr-Ca coordination by shortening the bonding distance and modulating the binding angle to achieve the lowest bonding energy. Such enhancement boosted Cr(VI) affinity up to 3.2 × 105 mL/g, which was 1-3 orders of magnitudes higher than ion exchange. The universality of this mechanism was verified using another Mg/Al-Cl LDH and various anions. This study broke the traditional awareness of low ion affinities of LDHs limited by single ion exchange and disclosed an essential mechanism for unexpected ion retention effects of anionic clays in nature.
This paper presents the microstructural properties of KOH-activated slag pastes produced with natural seawater under different salinity levels. The seawater-mixed samples exhibit higher 91-day strength compared to the control sample produced with deionized water. Their reduced total porosity and decrease in average pore size are closely related to the strength improvement. The dense microstructure is composed of reaction products resulting from the inclusion of seawater; these include Cl-hydrocalumite, Cl-hydrotalcite, AlClO, K2SO4, CaCO3, calcium aluminum iron oxide carbonate hydroxide hydrate, and aluminum chloride hydrate. In addition, CO3²⁻ in the CO3-hydrotalcite and CO3-hydrocalumite is replaced by chloride ions, and then a Cl-bearing phase is formed, leading to a higher bound chloride content. Lastly, in the single ocean current investigated in this study, the strength of seawater-mixed samples is weakly vulnerable to salinity variation.
Chromite ore processing residue (COPR) poses a serious Cr(VI) pollution to the environment, and ascertaining the Cr speciation in COPR is significant for guiding remediation. In this study, a systematic investigation on the Cr speciation in fresh COPR was carried out by multiple quantification methods as follows: i) via X-ray absorption spectroscopy (XAS), it was determined that 35% of the total Cr is Cr(VI); ii) the host phases of Cr(VI) and Cr(III) were identified and their Cr content were analyzed by X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS) and leaching tests; iii) the weight percent of each Cr host phase was determined by EDS-assisted quantitative phase analysis; iv) the Cr occupancy percentage of each Cr host phase was determined by integrative calculation based on the above analysis. Results indicate that brownmillerite, hydrogarnet and amorphous phase are the key host phases of Cr(VI), which hold 24.2%, 19.6% and nearly 50% of the total Cr(VI), respectively; spinel and amorphous phase are the key host phases of Cr(III), which hold 25.4% and 71.9% of the total Cr(III), respectively. This study has improved the understanding of Cr speciation in COPR, which is significant for developing effective and practical remediation technology. The quantification methods employed in this study can be extended to research on the speciation of Cr or other metals in other solid wastes.
The physical and chemical characteristics of MSWI bottom ashes from four different areas were investigated. The effects of sprinkling water and aeration on the elution characteristics of bottom ash were also researched. Treatments using the same liquid-solid ratio for two differing periods (1 day or about 50 days) were set up to study the effects of hydraulic intensity and ventilation volume. The quantities of Na and TOC eluted by the water-sprinkling treatment (L/S=0.5-0.6) were found to be about the same as for mechanical washing (L/S=10) if the intensity of the water sprinkling was optimized for each ash type. The results suggested that aeration for 50 days accelerated elution of Cl and inhibited elution of TOC. The elution of Cl was particularly affected by the initial chemical and mineral composition. This research also showed that aeration was more effective at inhibiting the elution of Pb and Ca than washing out by sprinkling water.
It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from chromium ore processing residue (COPR). We modified various parameters of EPA 3060A toward understanding the transformation of COPR minerals in the alkaline extraction and improving extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si were the major elements dissolved from NIST 2701, and their concentrations in solution were correlated with Cr(VI). The extraction fluid leached additional Al and Si from the method-prescribed borosilicate glass vessels which appeared to suppress the release of Cr(VI). Use of polytetrafluoroethylene vessels and intensive grinding of NIST 2701 increased the amount of Cr(VI) extracted. These modifications, combined with an increased extraction fluid to sample ratio of ≥900 mL g–1 and 48-h extraction time resulted in a maximum release of 1274 ± 7 mg kg–1 Cr(VI). This is greater than the NIST 2701 certified value of 551 ± 35 mg kg–1 but less than 3050 mg kg–1 Cr(VI) previously estimated by X-ray absorption near edge structure spectroscopy. Some of the increased Cr(VI) may have resulted from oxidation of Cr(III) released from brownmillerite which rapidly transformed during the extractions. Layered-double hydroxides remained stable during extractions and represent a potential residence for unextracted Cr(VI).
Conference Paper
Full-text available
Chromite ore processing residue (COPR) is a solid waste that was generated by the high temperature process of chromium extraction from chromite ore using soda ash and lime. The purpose of this study was to evaluate whether brownmillerite and periclase could be transformed into other minerals to understand and predict phase transformation during weathering in COPR. In this study, brownmillerite hydration to hydrogarnets and periclase to brucite in COPR materials were respectively assessed at elevated temperatures (100°C and 200°C) for 30 days because these reactions are very slow at ambient temperatures. The results showed that no apparent phase transformation was observed at 100°C. However, brownmillerite and periclase started dissolving the first day at 200°C, forming hydrogarnets and brucite, and completely dissolving after 15 days. Moreover, hydrogarnets were the main phase after 30 days of reaction. This indicated that the phase transformation could be possible when the initial COPR materials with high content of brownmillerite are exposed to long-term weathering.
A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.
The paste hydration of C4AF with and without the addition of lime and/or gypsum was studied by means of electron-optical and X-ray diffraction techniques. It was found that in the paste hydration of C4AF iron did not separate out but remained in the structure of the hydrated compounds, probably substituting for aluminum. In a neat paste of C4AF and H2O, at first di- and tetracalcium aluminate hydrate crystals formed, but later cubic C3AH6 crystals appeared. In the presence of Ca(OH)2, C2AHx crystals did not form; otherwise the course of the reaction was the same as without lime. Any CO2 present formed monocarboaluminate. When gypsum alone was present, ettringite crystals first appeared which later changed to monosulfate hydrate. In the presence of both lime and gypsum, the reactivity of C4AF was much reduced. Ettringite crystals were somewhat stabilized by lime. The formation of monocarboaluminate was hindered in the presence of gypsum.
Chromite ore processing residue (COPR) contains very high levels of chromium as Cr(III) and Cr(VI) and has a pH of ∼11.5 to 12. Millions of tonnes of COPR have in the past been deposited in urban areas. We have studied the factors that control leaching of Cr(VI), Ca, Al, Si, and Mg from COPR by means of batch experiments, mineralogical characterization of COPR via X-ray powder diffraction and scanning electron microscopy, and chemical equilibrium modeling. Batch experiments at a range of pH values and two liquid:solid ratios showed that mineral solubility control exists for aqueous concentrations of Cr(VI) above pH 10. Calculations indicate that the solid phases that control the solubility of Cr(VI) at pH values above 11 are Cr(VI)-substituted hydrogarnet (Ca3Al2(H4O4,CrO4)3) and Cr(VI)-hydrocalumite (Ca4Al2(OH)12CrO4·6 H2O), a layered double-hydroxide clay with chromate anions held in the interlayers. In the pH range 9.5 to 11, the description of the Cr(VI) concentration in solution was strongly improved by the incorporation in the model of Cr(VI)-ettringite (Ca6Al2(OH)12(CrO4)3·26 H2O), which precipitates as a secondary phase when hydrocalumite dissolves. The proposed model for leaching of COPR at high pH includes Cr(VI)-bearing hydrogarnet, Cr(VI)-hydrocalumite, Cr(VI)-ettringite, brucite, calcite, Ca2Al2(OH)10·3 H2O, CaH2SiO4, and gehlenite hydrate (Ca2Al2(OH)6SiO8H8·H2O). The model accurately predicts the concentrations of Cr(VI), Ca, Al, Si, and Mg in solution in the pH range 10 to 12 as well as the pH-buffering behavior. Below pH 8, a decrease in the Cr(VI) concentration in solution is observed, which may be attributed to sorption of chromate onto freshly precipitated Al and Fe hydroxide surfaces. Sulfate and carbonate show the same type of behavior as chromate. The chemistry of COPR shows similarities with cement and high-pH municipal waste incinerator bottom ash.
To obtain information for the remediation design of chromium waste sites, the leaching behavior of chromium in chromium-contaminated soil (Cr-soil) derived from chromium ore processing residue (COPR) was investigated. Batch leaching experiments were conducted using simulated rainwater as the leaching solution with pH adjusted to cover a range from 2.0 to 12.0 No Cr(VI) was detected in the leachate at low pH (<2.5). This may be attributed to adsorption of Cr(VI) onto the soil surface and/or reduction of Cr(VI) to Cr(III) by organic matter and/or by ferrous iron (two other major components of the soil); these processes are favored at low pH. Significant amounts of Cr(VI) were leached between pH 4.5 and 12. Results from leaching experiments indicate that approximately 1% of total Cr (26 mg/g) is readily leachable. The major chromium form in Cr-soil was identified as chromite using X-ray diffraction analysis (XRD). The forms of leachable chromium cannot be identified. It can only be hypothesized that Cr(VI) is leached by a dissolution of the chromate salts and attenuated by adsorption/desorption, precipitation, and redox processes that may occur in the soil-water system. Removal of the organic matter from the Cr-soil increases the amount of Cr(VI) leached over the entire pH range, suggesting that the organic matter can reduce Cr(VI) present in the solution. Cr(III) leaching behavior was also investigated as a function of pH. Cr(III) was found in solution at pH < 5. The amount of Cr(III) leached was controlled by the solubility of Cr(III) precipitates, the extent of Cr(VI) reduction, and the magnitude of Cr(III) adsorption onto the soil surface. No Cr(III) was detected between pH 4.5 and 12 which can be attributed to the presence of insoluble precipitates such as Cr(OH)3(s) and CrxFe1−x(OH)3(s) and the adsorption of Cr(III) species onto the soil particle surface.