Y.-J. WANG et al.: LEACHING CHARACTERISTICS AND MINERALOGICAL CONTROL OF CHROMIUM IN ...
127–133
LEACHING CHARACTERISTICS AND MINERALOGICAL
CONTROL OF CHROMIUM IN ELECTRIC-ARC-FURNACE
STAINLESS-STEEL SLAG
LU@ENJE IN MINERALO[KA KONTROLA KROMA V @LINDRI
NERJAVNEGA JEKLA IZ ELEKTROOBLO^NE PE^I
Ya-Jun Wang
1,2
, Ya-Nan Zeng
1*
, Jun-Guo Li
1*
, Zhi-Yuan Gao
3
1
School of Metallurgy and Energy, North China University of Science and Technology, 063009 Tangshan, P. R. China
2
School of Materials and Metallurgy, Northeastern University, 110819 Shenyang, P. R. China
3
Tangshan Research Academy of Environmental Planning, 063000, Tangshan, P. R. China
Prejem rokopisa – received: 2020-08-10; sprejem za objavo – accepted for publication: 2020-10-26
doi:10.17222/mit.2020.156
This study focuses on evaluating the leachability of chromium in Electric-Arc-Furnace stainless-steel slag (EAFS) from the per-
spective of mineralogical control. Mineral-phase identification results showed that EAFS mainly comprised merwinite, magne-
tite, dicalcium silicate, calcium aluminosilicate, eskolaite, spinel, and perovskite. The chromium in the EAFS was mainly pres-
ent in the Mg-Cr spinel, Fe-Cr alloy, or distributed in the form of oxide in the matrix phase, merwinite. When undergoing
leaching, the leachate of the EAFS was alkaline and reducing, and a portion of the chromium in the slag was released into the
leachate and existed mainly in a trivalent state. In the early stage of leaching, the rapid dissolution of chromium-bearing silicate
minerals led to a rapid increase in the concentration of trivalent chromium. However, in the middle and late stages (> 15 days) of
leaching, the chromium concentration reached stable values. The geochemical model of chromium leaching from the EAFS
demonstrated that the simulated values of the chromium concentration were consistent with the batch-leaching test values. The
primary phase Cr2O3 in the EAFS controlled the chromium concentration in the leachate. After entering the leachate, trivalent
chromium could be presented as Cr(OH)4
–
because Cr(III)-hydroxide was unstable in the alkaline leachate.
Keywords: EAF slag, chromium, mineral phase, geochemical model, leaching, PHREEQC
V {tudiji so se avtorji osredoto~ili na ovrednotenje izlu`enja kroma iz `lindre nerjavnega jekla v elektro oblo~ni pe~i (EAFS;
angl.: electric arc furnace slag) s stali{~a mineralo{ke kontrole. Rezultati identifikacije mineralne faze so pokazali, da EAFS
vsebuje v glavnem mervinite, magnetit, dikalcijev silikat, kalcijev alumosilikat, eskolait, {pinel in perovskit. Krom je v EAFS
prisoten predvsem v Mg-Cr {pinelu, zlitini Fe-Cr ali pa je porazdeljen v obliki oksida v matrici mervinitne faze. Med lu`enjem
je nastali izlu`ek EAFS bazi~en (alkalen) in reduciran. Spro{~eni dele` kroma iz `lindre je v glavnem v trivalentnem stanju. V
za~etni fazi lu`enja pride do hitrega raztapljanja mineralov v katerih se nahaja krom, kar vodi do mo~nega pove~anja
koncentracije trivalentnega kroma. Vendar pa v srednji in zadnji fazi lu`enja (ve~ kot 15 dni) vsebnost kroma dose`e stabilno
vrednost. Geokemijski model izlu`enja kroma iz EAFS je pokazal, da se simulirane vrednosti vsebnosti kroma ujemajo s
prakti~nimi vrednostmi v {ar`i. Primarna Cr2O3 faza v EAFS kontrolira vsebnost kroma v izlu`enem. Tri valentni krom je v
izlu`enem lahko prisoten kot Cr(OH)4
–
, ker je bil Cr(III)-hidroksid nestabilen v alkalnem izlu`ku.
Klju~ne besede: elektrooblo~na pe~, `lindra nerjavnega jekla, krom, mineralna faza, geokemijski model, lu`enje, programsko
orodje PHREEQC
1 INTRODUCTION
EAFS is a by-product generated during the prelimi-
nary smelting of stainless steel using an electric arc fur-
nace, and its chromium content is typically higher than
in ordinary steel slag.
1
Over time, rainwater can leach
chromium, in the form of trivalent chromium (Cr(III)) or
hexavalent chromium (Cr(VI)), from the slag present in
landfills.
2
The ease of leaching and the toxicity of the
Cr(VI) from EAFS has led to extensive ecosystem deteri-
oration after contaminating water and soil.
3–5
Cr(III) is
generally harmless to the environment because of its
lower mobility and toxicity.
6
However, long-term expo-
sure of Cr(III) to humans can lead to allergic skin reac-
tions and even cancer.
7
During the stacking and recycling of EAFS, concerns
about the leaching of chromium mainly focused on the
leaching toxicity and the mineralogical control of chro-
mium. Extensive research has been performed on EAFS
to study chromium leaching behavior, aiming at assess-
ing its toxicity.
8–10
R.Wang et al.
11
tested chromium
leachability when EAFS was used as the cement admix-
ture and stated that the ecological risk of chromium re-
leased from EAFS is minimal because chromium in the
leachates is mainly present as Cr(III) at a concentration
of 0.01 mg/L to 0.05 mg/L. However, because Cr(VI)
has strong carcinogenic properties,
12
if Cr(VI) is ob-
served in the leachate of EAFS, the risk of leaching can-
not be ignored.
Since the smelting stage of an electric arc furnace is a
reducing environment, the low oxygen potential controls
the oxidation of metals and metalloids, and chromium in
EAFS mainly exists in the metallic state and as Cr(III).
Materiali in tehnologije / Materials and technology 55 (2021) 1, 127–133 127
UDK 621.794.4:622.34:625.821.4:691.714.018.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(1)127(2021)
*Corresponding author's e-mail:
zengyanann@126.com (Ya-Nan Zeng)

Metallic chromium can be separated and recovered
through crushing, fine grinding, and magnetic separa-
tion. The remaining Cr(III) is mainly present in
Ca-Mg-Al silicates and spinel minerals. Ca-Mg-Al sili-
cates can be hydrolyzed to release chromium, while
spinel minerals are insoluble.
13
Researchers considered
enriching chromium to insoluble spinel through the
hot-slag-modification method. According to the leaching
index Factor sp (Factor sp = 0.2MgO + 1.0Al
2
O
3
+
nFeOx – 0.5Cr
2
O
3
) constructed by M. Kuhn
14
when Fac-
tor sp is higher than 5 %, the chromium leaching ratio
decreases. By melting at high temperatures, adding
MgO, Al
2
O
3
, FeO
x
, etc. to the slag can reduce the leach-
ing rate of chromium in the stainless-steel slag.
PChaurand et al.
4
stated that the leachability of chro-
mium is controlled by the formation of spinel phases
similar to chromite. Moreover, W. J. Deutsch
15
indicated
that such secondary phases as Cr(III)-hydroxide may de-
termine the release of chromium from the solid phases
into a liquid phase.
To explore the leaching characteristics of solid
wastes, researchers have established some geochemical
models based on detailed mineralogical analyses in con-
junction with leaching tests. V. Ettler et al.
16
applied
PHREEQC to analyze Pb leaching from the secondary
Pb smelter air-pollution-control residues and concluded
that anglesite was the main solubility-controlling phase
for Pb leaching. A. M. Fällman
17
found that the dissolu-
tion of a BaSO
4
and Ba(S,Cr)O
4
solid solution controlled
the barium and chromium concentrations in the leachates
according to the results of the PHREEQC simulation.
These studies provided an effective means to establish a
leaching model of chromium from EAFS.
This work was aimed at assessing the leachability of
chromium in EAFS and exploring its leaching mecha-
nism. X-ray diffraction (XRD) was used to determine the
mineralogical composition of EAFS. Scanning electron
microscopy with energy-dispersive spectroscopy
(SEM/EDS) microscope system was used to observe the
occurrence state of chromium in EAFS. Sequential
leaching tests under three liquid-to-solid ratios were per-
formed to obtain the experimental data. A thermody-
namic model was established using PHREEQC software
to predict the leaching concentration of chromium in the
leachate and to determine the mineralogical phase that
controls the release of chromium in EAFS.
2 EXPERIMENTAL PART
2.1 Materials
The EAFS used for this work was collected from a
Chinese stainless-steel plant. The obtained EAFS was
first crushed by a crusher, and the Fe-Cr alloy metal drop
was extracted by a magnet. Then it was sieved using a
200-mesh sieve. After being dried for6hat105°Ctore -
move the free moisture, the EAFS’s chemical composi-
tion was analyzed by X-ray fluorescence (XRF, DE
2000) and then quantified with the appropriate software
and exported as oxide form. The results are shown in Ta-
ble 1.
As shown in Table 1, the main chemical components
of EAFS were CaO, SiO
2
, MgO, and Al
2
O
3
, with CaO
and SiO
2
(38.64 % and 24.01 %, respectively) being sig-
nificantly higher than the other components. The triple
alkalinity of EAFS calculated by Equation (1) was 2.14,
which indicated that the EAFS should be classified as al-
kaline steel slag. In addition, EAFS contained a certain
amount of Fe
2
O
3
, FeO, TiO
2
, and MnO
2
. The chromium
content of EAFS was 4.21 %, which was higher than
other common types of steel slag.
R =
+ (CaO % MgO %)
SiO %
2
× 100 % (1)
2.2 Characterization methods
The mineral phase compositions of the EAFS were
analyzed by X-ray diffraction (XRD) using an X-ray
diffractometer (D/Max-RB, Rigaku) with Cu-K
 radia-
tion at 40 kV and 100 mA, a scanning rate of 0.03° (2 )
every 6 seconds, and diffraction angles that ranged from
10° to 90°. MDI Jade 9 software coupled with the ICDD
PDF 2009 database was utilized for qualitative and quan-
titative analyses of the mineral phases in the EAFS.
The microstructure of the EAFS was examined using
SEM (Hitachi S-4800) on an instrument equipped with
energy-dispersive spectrometry (EDS, Bruker). The
EAFS powder was inlaid on the bottom surface of a
(1×1) cm phenolic resin cube to prepare the SEM/EDS
sample. After the sample was completely solidified, the
side inlaid with EAFS was polished and sprayed with a
gold film to give it conductivity. The electron beam oper-
ating voltage of the SEM was 20 kV, and the count rate
of the EDS was 4000-5000 counts/s.
2.3 Sequential leaching
The sequential leaching test used in this research was
designed according to the EN12457-2, which was rec-
ommended by the European Committee for Standardiza-
tion as the preferred choice to conduct the compliance
test for the leaching of granular waste materials and
sludges because of its simplicity, reproducibility, and re-
liability.
Three liquid-solid (L/S) ratio leaching systems were
prepared by adding 10 g, 50 g, and 100 g, respectively,
of EAFS into1Lofdeionized water in an Erlenmeyer
Y.-J. WANG et al.: LEACHING CHARACTERISTICS AND MINERALOGICAL CONTROL OF CHROMIUM IN ...
128 Materiali in tehnologije / Materials and technology 55 (2021) 1, 127–133
Table 1: Chemical composition of the EAFS
Oxides CaO SiO2 MgO Al2O3 Fe2O3 FeO TiO
2 MnO
2 Cr others
Contents (w/%) 38.64 24.01 12.63 9.55 2.50 1.82 1.61 0.88 4.21 4.15

flask. The flasks were then tightly stoppered to prevent
air from entering during the leaching process. The
Erlenmeyer flasks were fixed in a horizontal shaker and
subjected to continuous shaking at a rate of 20 min
–1
for
20 days. During the sequential leaching process, 80 mL
of the leachate supernatant was sampled every 24 h, and
the same volume of deionized water was supplemented
to maintain the L/S ratio. The sampled leachates were fil-
tered through 0.22-μm membrane filters. The pH and re-
dox potential (Eh) of the sampled leachate was measured
immediately. After filtering through a 0.22-μm mem-
brane filter, chromium-ion concentration detection was
performed on the sampled leachate. Hexavalent chro-
mium and total chromium concentration in the leachate
were analyzed using UV-visible spectrophotometry by
the diphenylcarbazide method (GB/T 15555.4-1995) in
conjunction with the oxidation of ammonium peroxy-
disulfate.
18
The trivalent chromium concentration was
calculated by the subtraction method.
2.4 Geochemical modeling
To explore the leaching characteristics and identify
the controlling phases of chromium in the EAFS, a geo-
chemical model was established using PHREEQC soft-
ware based on the dissolution of the primary phases and
the precipitation of secondary phases. The supplemen-
tary thermodynamic data used in this model were derived
from the MINTEQ database and extended with minor
modifications as described by J. W. Ball et al.
19
To estab-
lish the model, an input file that specified the structure of
the modeling problem, the input data, and the required
computed results were set up and used in the PHREEQC
software.
To simulate the chromium concentration in the
leachates, data about the element content and mineralog-
ical composition in the EAFS should be proposed. Ac-
cording to the XRD results, the primary phases were
merwinite (Ca
3
Mg(SiO
4
)
2
), dicalcium silicate (Ca
2
SiO
4
),
magnetite (Fe
3
O
4
), gehlenite (Ca
2
Al
2
SiO
7
), eskolaite
(Cr
2
O
3
), magnesium chromate (MgCr
2
O
4
), and perov-
skite (CaTiO
3
). During the leaching process, the primary
phases undergo hydrolysis, dissolution, or precipitation
to form amorphous or crystalline secondary phases. Ac-
cordingly, element concentrations in the leachates were
governed by the dissolution/precipitation equilibrium of
the primary and secondary phases in the aqueous solu-
tion. Generally, the saturation index (SI) is a parameter
calculated by PHREEQC based on mass balance to de-
termine the dissolution or precipitation of a mineral
phase. SI is defined as log(Q/K), where Q is the activity
product, and K is the thermodynamic constant for a
given dissolution reaction.
20
The saturation index is a
useful quantity to determine whether the water is satu-
rated, undersaturated, or supersaturated for the given
mineral. A positive SI (supersaturated state) indicates
that the given mineral may be precipitated as a secondary
phase. A negative SI (undersaturated state) indicates that
the given mineral is not stable in the leachate and may
dissolve. Minerals with –1.5 < SI < 1.5 were considered
to be the potential solubility-controlling minerals.
21,22
In
this way, the mineral phases controlling the chromium
concentration in leachate could be determined.
3 RESULTS AND DISCUSSION
3.1 Mineralogical composition
Figure 1 presents the XRD pattern of the EAFS. The
EAFS mainly consisted of merwinite (Ca
3
Mg(SiO4)
2
),
magnetite (Fe
3
O
4
), dicalcium silicate (Ca
2
SiO
4
), calcium
aluminosilicate (Ca
2
Al
2
Si
2
O
8
), eskolaite (Cr
2
O
3
), spinel
(Mg(Cr, Al)
2
O
4
), and perovskite (CaTiO
3
). This mineral-
ogical characterization is similar to the results reported
by other researchers.
23,24
3.2 Micromorphology
Table 2: Element compositions of representative microdomains in the
EAFS (a/%)
Micro-
domain
Ca Si Mg Al O Cr Fe Ti
1 21.86 10.31 6.04 0.25 61.54 – – –
2 56.98 11.21 0.28 – 30.15 1.10 0.29 –
3 – 12.99 – – – 45.78 34.18 –
4 20.69 13.47 7.33 1.56 47.21 4.94 3.59 1.21
5 21.39 10.64 6.94 45.84 – – –
6 33.46 15.49 2.28 3.71 44.15 – 0.29 0.62
7 – – 42.87 10.32 41.30 5.51 – –
8 27.26 13.84 7.65 0.63 50.14 – – 0.48
9––––– 5.31 94.69 –
Note: – indicates less content
Figure 2 depicts the micromorphology of the EAFS.
It can be observed from Figure 2 that the cross-section
of the EAFS particles mainly existed in an irregular plate
form. Areas with different color contrast were observed
at the slag particle interface, indicating that the chemical
composition of these areas was different. Elemental
compositions of representative points were tested using
Y.-J. WANG et al.: LEACHING CHARACTERISTICS AND MINERALOGICAL CONTROL OF CHROMIUM IN ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 127–133 129
Figure 1: XRD pattern of the EAFS

EDS detection based on color contrast, and the results
are listed in Table 2.
The Ca-Mg silicate phases were the matrix mineral
phases of EAFS (Microdomains 1, 4, 5, and 8), and other
phases were encapsulated in it. Dicalcium silicate existed
as a minor mineral phase in the merwinite matrix, and
there were some trace mineral phases dissolved in it,
such as eskolaite, spinel, or Fe-Cr alloy (Microdomains 2
and 6). Moreover, there were some miniscule dot-shaped
occurrences of Fe-Cr alloy (Microdomains 3 and 9) and
Cr-bearing spinel (Microdomain 7) unevenly distributed
in the merwinite matrix.
Figure 3 shows the distribution of the main elements
in a typical EAFS particle. From these images, calcium,
magnesium, and silicon were evenly distributed through-
out this slag particle. Chromium was mainly concen-
trated in certain areas, and the phases in these areas were
mainly dicalcium silicate, Fe-Cr alloy, and spinel.
3.3 Characteristics of leachates
The pH and redox potential (Eh) are two predominant
parameters controlling the speciation and concentration
of chromium in aqueous solution. Experimental mea-
surements were performed to monitor the change in pH
and Eh over time in the sequential leaching test, and the
results are illustrated in Figure 4. On the first day of
Y.-J. WANG et al.: LEACHING CHARACTERISTICS AND MINERALOGICAL CONTROL OF CHROMIUM IN ...
130 Materiali in tehnologije / Materials and technology 55 (2021) 1, 127–133
Figure 2: Microscopic morphology of the EAFSo
Figure 4: a) Evolution of pH and b) Eh of the leachates with different
L/S ratios
Figure 3: Elemental distribution of Ca, Mg, Si, O, Cr, Fe, Ti in the
EAFS powder

leaching, the pH of the leachates rose sharply to above
11 (Figure 4a). This was mainly due to the dissolution
of alkaline minerals, which resulted in the production of
hydroxide ions.
25
By day 6, the pH value of the leachates
reached equilibrium, and the equilibrium pH value de-
creased as the liquid-solid ratio increased. As the leach-
ing time progressed, the soluble alkaline mineral phases
in the EAFS reached their hydrolytic equilibrium states,
and the hydroxide ion release rates reached their constant
values. As the L/S ratio increased, the dilution effect of
more deionized water on the alkalinity was the main rea-
son for the lower pH value of the larger L/S ratio leach-
ing system.
On the first day of leaching, the Eh decreased rapidly
to a minimum value that was lower than –60 mV and
then increased slightly thereafter (Figure 4b). During the
entire leaching cycle, the leachates’ Eh value change in-
tervals were between –30 mV and –76 mV, indicating a
slight reducing property. During the leaching process,
the Eh of the leachate was mainly controlled by the
Fe
2+
/Fe
3+
redox pair.
25
Since EAFS was a Fe-bearing
slag, the dissolution of Fe
2+
ions caused the leachate to
exhibit a certain degree of reduction. As the L/S ratio in-
creased, the Eh gradually increased from the dilution
from deionized water reducing the concentration of Fe
2+
ions in the leachate.
3.4 Chromium leachability
Leaching toxicity and ecological risk assessment rely
on knowledge of the speciation of heavy metals and their
concentrations in the leachates from the landfilled solid
waste. In the sequential leaching test, the concentrations
of trivalent and total chromium in the leachates were
measured, and the results are illustrated in Figure 5.
Concentrations of hexavalent chromium in the leachates
ranged from 0 to 5.2×10
–3
mg/L, while concentrations of
trivalent chromium in the leachates ranged from 1.7×10
–2
to 8.4×10
–2
mg/L. It can be concluded that due to the
high alkalinity and reduction properties of the leachate,
chromium in the leachate was mainly present in the tri-
valent form. The experiments performed by Chaurand et
al.
26
showed that chromium was trivalent in steel slag,
and its speciation changed very little during water leach-
ing and natural aging.
After leaching for one day, the concentrations of tri-
valent chromium in the leachates with L/S ratios of (0,
20, and 100) g/mL were 8.2×10
–2
mg/L, 3.9×10
–2
mg/L,
and 2.1×10
–2
mg/L, respectively. When the L/S ratio was
10 g/mL, there was a tendency for the trivalent chro-
mium concentration to decrease continuously over a pe-
riod of 15 days and then stabilize at 5.2×10
–2
mg/L.
When the L/S ratio was 20 g/mL, the trivalent chromium
concentration increased slightly during the first five days
and then declined for 15 days until stabilizing at
3.8×10
–2
mg/L. When the L/S ratio was 100 g/mL, the
trivalent chromium concentration remained steady at
2.4×10
–2
mg/L throughout the leaching cycle.
The evolution of trivalent chromium under different
L/S ratios was different. The trivalent chromium in the
leachates originated from the chromium-bearing miner-
als. As analyzed previously, chromium in EAFS was
mainly dispersed in silicates, spinel-type phases, and
Fe-Cr alloy. Spinel-type phases were insoluble both in
neutral and alkaline solutions,
14
whereas the Cr
2
O
3
dis-
solved in the Ca-Mg-silicates, known as pseudo-binary
phases, were freely soluble both in neutral and alkaline
solutions.
27
The rapid dissolution of these pseudo-binary
phases led to changes in the concentration of the trivalent
chromium in the leachate in the early stage of leaching.
In the sequential leaching process, some secondary
phases that were difficult to dissolve might be generated
in the EAFS, and the chromium-bearing primary miner-
als could be subsequently sequestered. The chromium
concentration in the leachates was dependent on the dif-
fusion rate of chromium passing through the secondary
phase layer rather than the dissolution of chro-
mium-bearing primary minerals.
Consequently, steady chromium concentrations were
achieved under different L/S ratios during the latter
leaching period. After a 15-day leaching period, the
chromium concentrations stabilized at 5.2×10
–2
,
3.8×10
–2
, and 2.4×10
–2
mg/L when the L/S ratios were
(10, 20, and 100) g/mL, respectively. This indicated that
the concentration of chromium in the leachates would
not decrease proportionally with the increase of L/S ra-
tio, which suggested that the chromium leaching from
EAFS could be controlled conjunctively by the dissolu-
tion of primary phases and the precipitation of secondary
phases.
3.5 Geochemical model of chromium leaching
Based on the dissolution/precipitation, thermody-
namic, and kinetic control of the primary and secondary
phases, a geochemical model of the leaching concentra-
tion of chromium from EAFS was established using the
PHREEQC software. The PHREEQC speciation-solubil-
Y.-J. WANG et al.: LEACHING CHARACTERISTICS AND MINERALOGICAL CONTROL OF CHROMIUM IN ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 127–133 131
Figure 5: Chromium concentration in the leachate

ity module was used to estimate the chromium species
and the solubility of the mineral phases in the leachates.
Firstly, the equilibrium concentrations of chromium
under various L/S ratios were simulated by PHREEQC.
For comparison, the experimental data were obtained by
averaging the chromium concentrations after stabiliza-
tion for each of the L/S ratios. Figure 6 shows the simu-
lated and the experimental chromium concentrations as a
function of the L/S ratio. The simulated chromium equi-
librium concentration values in the leachates were con-
sistent with the results of the leaching test. This result
suggested that the geochemical model established by
PHREEQC could function as a potential method to as-
sess the toxicity of the heavy metals released from solid
wastes. The small deviation between the simulated and
the measured values could be explained for two main
reasons. First, chromium existed as separated and dis-
solved Cr
2
O
3
in the EAFS, while the real compounds
were taken into account when the geochemical model
was established. Second, the solubility constants of the
amorphous phases present in the EAFS were difficult to
determine. Therefore, only the crystal phases and their
solubility constants could be determined and used in the
geochemical model. To improve the geochemical model,
the solubility constants of amorphous and crystal phases
in the EAFS should be determined.
To explore the mineral phases controlling the chro-
mium concentration in the leachates, the major chro-
mium-bearing original and secondary minerals were se-
lected. Next, the saturation index (SI) of these minerals,
which could be used to indicate the minerals’ dissolution
or precipitation in leachates, was calculated using the
PHREEQC software (Figure 7). Spinel phases in the
EAFS, such as Mg-Cr spinel (MgCr
2
O
4
), Fe-Cr spinel
(FeCr
2
O
4
), are original chromium-bearing minerals in
the EAFS and insoluble during leaching under normal
conditions. Therefore, the SI values of the MgCr
2
O
4
and
FeCr
2
O
4
were negative and far below –1.5. The SI of
Cr
2
O
3
was 0 under the L/S ratios of (10, 20, and
100) g/mL, and the Cr
2
O
3
that was separated or dissolved
in the pseudo-binary phases could release Cr
3+
into the
leachate and control the chromium concentration at vari-
ous L/S ratios. After extracting into the leachates, chro-
mium would exist in the chromium-bearing secondary
phases, mainly as Cr(III)-hydroxides, such as amorphous
Cr(OH)
3(A)
and crystalline Cr(OH)
3(C)
. The SI of
Cr(OH)
3(A)
and Cr(OH)
3(C)
were –4.83 and –7.28, respec-
tively. This indicated that these phases were unstable in
the leachate and could be converted into Cr(OH)
4
–
.
28
Ac-
cording to the solubility model of chromium established
by R. Baciocchi et al.,
21
chromium release could be con-
trolled by either the chromium oxide or hydroxide
phases. The dynamic leaching mechanism of chromium
in EAFS slag can be expressed by Equations (2-4):
Cr
2
O
3(s)
+H
2
O
(l)
= 2CrO
2
–
+2H
+
(2)
CrO
2
–
+H
2
O
(l)
= Cr(OH)
3(s)
(3)
Cr(OH)
3(s)
+H
2
O
(l)
= Cr(OH)
–
+4H
+
(4)
4 CONCLUSIONS
EAFS was found to contain mostly merwinite, mag-
netite, larnite, gehlenite, eskolaite, spinel, and perov-
skite. Chromium was mainly dispersed in the magne-
sia-chromium spinel and the Fe-Cr alloy, while a small
amount was dispersed in the merwinite matrix.
During the sequential leaching process, the leachate
was present as an alkaline and reductive solution. As the
L/S ratio increased, the pH decreased, while the Eh in-
creased. Trivalent chromium was the predominant spe-
cies in the leachates.
After 15 consecutive days of growth, the chromium
concentration in the leachate stabilized. The dissolution
of primary phases and the precipitation of secondary
phases conjunctively controlled the concentration of the
chromium in the leachate.
Y.-J. WANG et al.: LEACHING CHARACTERISTICS AND MINERALOGICAL CONTROL OF CHROMIUM IN ...
132 Materiali in tehnologije / Materials and technology 55 (2021) 1, 127–133
Figure 7: The saturation indices of Cr-bearing phases in the leachate
Figure 6: Simulated and experimental chromium concentration under
different L/S ratio

The geochemical model of chromium leaching devel-
oped using the PHREEQC software was found to be a re-
liable method for assessing the toxicity of heavy metals
released from solid wastes. Lastly, the dissolution of the
primary phase-Cr
2
O
3
in the EAFS controlled the dissolu-
tion of chromium.
Acknowledgments
The authors gratefully acknowledge support from the
National Natural Science Foundation of China (No.
51704119, 51574108) and the Key Research and Devel-
opment Project of Tangshan (No. 19140205F).
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