1666 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
RESULTS AND DISCUSSION
Conversion of ZnFe2O4 to ZnO by CaO-treatment
The zinc, calcium, and iron concentrations of as-supplied
EAFD and CaO-treated EAFD measured by ICP-AES are
shown in Table 1. Zinc and iron were found as the main
components of EAFD at about 16% and 23%, respec-
tively. Excluding the calcium originally contained in the as-
supplied EAFD, the Ca:Fe molar ratio of the CaO-treated
EAFD corresponds well with the CaO addition, at approxi-
mately 1.3.
Although minor quantities of carbon, halides, and
heavy metals are sometimes present, these components
were not measured in the present investigation. The reac-
tion of EAFD with oxygen in air at 1100°C during CaO-
treatment is expected to lead to the combustion of carbon,
while halides and heavy metals are selectively volatilized
into fumes, leaving only ZnO and Ca2Fe2O5 (Chairaksa et
al., 2015, 2016).
Figures 2(a–b) and 3(a–b) show the XRD patterns
and SEM micrographs of the EAFD samples before and
after CaO-treatment, respectively. It was found that while
ZnO peaks remain, those of ZnFe2O4 disappeared and was
replaced by new peaks indicating the formation of dicalcium
ferrite (Ca2Fe2O5). SEM micrographs showed the charac-
teristic spherical morphology of EAFD, which is attrib-
uted to the splashing action during EAF steelmaking from
CO bubble rupture (Han and Holappa, 2003). Elemental
analysis of the as-supplied EAFD samples by EDS before
CaO-treatment confirms the presence of ZnFe2O4 in larger
particles (Point 1) and a mixture of ZnO and ZnFe2O4 in
smaller ones (Points 2 and 3)
After CaO-treatment, it was found that the particle sur-
face became less spherical and rougher in appearance. EDS
analysis indicated a Ca:Fe molar ratio of 1.3, consistent
with the added amount of CaO, suggesting the presence
of both Ca2Fe2O5 and potentially unreacted CaO. Overall,
these findings support the complete reduction of ZnFe2O4
to ZnO and Ca2Fe2O5 using a non-carbothermic process
at 1100°C in air, as previously described in Equation 1.
Compared to existing pyrometallurgical methods, non-
carbothermic CaO-treatment of EAFD presents significant
environmental and economic advantages. Although excess
CaO addition was necessary for complete reduction to
occur, lowering the concentration of iron in the recovered
EAFD by employing a dust injection technology (Tsubone
et al., 2012) may further lower CaO requirements.
Selective Leaching of Zn by Alkaline Media
The influence of temperature and leaching time on zinc,
calcium, and iron removal from CaO-treated EAFD using
NaOH, KOH, and LiOH solutions was experimentally
investigated at 25°C, 50°C, and 70°C and the results are
shown in Figure 4. As expected, zinc extraction increased
with both temperature and leaching time. Meanwhile, it
was found that dissolution of iron and calcium remained
negligible at all temperatures for all three alkaline solu-
tions, suggesting high selectivity to zinc. Among the solu-
tions, NaOH solution achieved the highest zinc recovery,
reaching 99% after 2 hours at 70°C, followed by KOH and
LiOH solutions with 94% and 72% recoveries, respectively.
Notably, untreated EAFD yielded only 45% zinc recovery
when leached with a similar solution of NaOH at 70°C for
2 hours (Chairaksa et al., 2016). The considerable difference
in zinc dissolution values between treated and untreated
EAFD highlights the importance of converting ZnFe2O4 to
ZnO via CaO-treatment for more efficient zinc recycling.
At 25°C and 50°C, it was found that zinc dissolution
steadily increased in all solutions. On the other hand, at
70°C, all solutions showed rapid initial zinc dissolution,
followed by a slowdown. Zinc recovery stabilized around
60 minutes for NaOH, while KOH and LiOH continued
to slowly increase. These results suggest fastest and highest
zinc dissolution with NaOH solution compared to KOH
and LiOH solutions. However, KOH may be a more prac-
tical choice due to its potentially higher zinc extraction
capacity (Gamutan et al., 2024).
The remaining zinc concentrations in the leach residues
after 2 hours at 70°C also matched the trend observed in
terms of the experimental dissolution values in the leachate,
with NaOH showing the highest zinc leaching efficiency,
followed by KOH and LiOH (Table 1). Notably, calcium
and iron concentrations remained relatively unchanged,
confirming selectivity of the alkaline leaching process
towards zinc. As shown in Figure 2(c-e), XRD patterns of
the same leach residues showed no ZnO peaks but revealed
the formation of iron ettringite (Ca3Fe2(OH)12). The cal-
culated Ca:Fe molar ratio in the leached residues (1.4–1.5)
also supports the formation of Ca3Fe2(OH)12. This sug-
gests that excess CaO in the CaO-treated EAFD likely
reacted with Ca2Fe2O5 and water in the solution, as shown
in the following equation:
6H2O Ca2 Fe2O CaO Ca3 Fe2 OHh12
5 ++=^(2)
The formation of Ca3Fe2(OH)12 by the above reaction may
be prevented by minimizing the amount of CaO addition
during the CaO-treatment step such that no unreacted CaO
remains. Meanwhile, negligible dissolution of iron and cal-
cium in the leachate suggests excellent selectivity of the pro-
cess to zinc with minimal contamination and the potential
to recover iron and calcium from the leach residues.
RESULTS AND DISCUSSION
Conversion of ZnFe2O4 to ZnO by CaO-treatment
The zinc, calcium, and iron concentrations of as-supplied
EAFD and CaO-treated EAFD measured by ICP-AES are
shown in Table 1. Zinc and iron were found as the main
components of EAFD at about 16% and 23%, respec-
tively. Excluding the calcium originally contained in the as-
supplied EAFD, the Ca:Fe molar ratio of the CaO-treated
EAFD corresponds well with the CaO addition, at approxi-
mately 1.3.
Although minor quantities of carbon, halides, and
heavy metals are sometimes present, these components
were not measured in the present investigation. The reac-
tion of EAFD with oxygen in air at 1100°C during CaO-
treatment is expected to lead to the combustion of carbon,
while halides and heavy metals are selectively volatilized
into fumes, leaving only ZnO and Ca2Fe2O5 (Chairaksa et
al., 2015, 2016).
Figures 2(a–b) and 3(a–b) show the XRD patterns
and SEM micrographs of the EAFD samples before and
after CaO-treatment, respectively. It was found that while
ZnO peaks remain, those of ZnFe2O4 disappeared and was
replaced by new peaks indicating the formation of dicalcium
ferrite (Ca2Fe2O5). SEM micrographs showed the charac-
teristic spherical morphology of EAFD, which is attrib-
uted to the splashing action during EAF steelmaking from
CO bubble rupture (Han and Holappa, 2003). Elemental
analysis of the as-supplied EAFD samples by EDS before
CaO-treatment confirms the presence of ZnFe2O4 in larger
particles (Point 1) and a mixture of ZnO and ZnFe2O4 in
smaller ones (Points 2 and 3)
After CaO-treatment, it was found that the particle sur-
face became less spherical and rougher in appearance. EDS
analysis indicated a Ca:Fe molar ratio of 1.3, consistent
with the added amount of CaO, suggesting the presence
of both Ca2Fe2O5 and potentially unreacted CaO. Overall,
these findings support the complete reduction of ZnFe2O4
to ZnO and Ca2Fe2O5 using a non-carbothermic process
at 1100°C in air, as previously described in Equation 1.
Compared to existing pyrometallurgical methods, non-
carbothermic CaO-treatment of EAFD presents significant
environmental and economic advantages. Although excess
CaO addition was necessary for complete reduction to
occur, lowering the concentration of iron in the recovered
EAFD by employing a dust injection technology (Tsubone
et al., 2012) may further lower CaO requirements.
Selective Leaching of Zn by Alkaline Media
The influence of temperature and leaching time on zinc,
calcium, and iron removal from CaO-treated EAFD using
NaOH, KOH, and LiOH solutions was experimentally
investigated at 25°C, 50°C, and 70°C and the results are
shown in Figure 4. As expected, zinc extraction increased
with both temperature and leaching time. Meanwhile, it
was found that dissolution of iron and calcium remained
negligible at all temperatures for all three alkaline solu-
tions, suggesting high selectivity to zinc. Among the solu-
tions, NaOH solution achieved the highest zinc recovery,
reaching 99% after 2 hours at 70°C, followed by KOH and
LiOH solutions with 94% and 72% recoveries, respectively.
Notably, untreated EAFD yielded only 45% zinc recovery
when leached with a similar solution of NaOH at 70°C for
2 hours (Chairaksa et al., 2016). The considerable difference
in zinc dissolution values between treated and untreated
EAFD highlights the importance of converting ZnFe2O4 to
ZnO via CaO-treatment for more efficient zinc recycling.
At 25°C and 50°C, it was found that zinc dissolution
steadily increased in all solutions. On the other hand, at
70°C, all solutions showed rapid initial zinc dissolution,
followed by a slowdown. Zinc recovery stabilized around
60 minutes for NaOH, while KOH and LiOH continued
to slowly increase. These results suggest fastest and highest
zinc dissolution with NaOH solution compared to KOH
and LiOH solutions. However, KOH may be a more prac-
tical choice due to its potentially higher zinc extraction
capacity (Gamutan et al., 2024).
The remaining zinc concentrations in the leach residues
after 2 hours at 70°C also matched the trend observed in
terms of the experimental dissolution values in the leachate,
with NaOH showing the highest zinc leaching efficiency,
followed by KOH and LiOH (Table 1). Notably, calcium
and iron concentrations remained relatively unchanged,
confirming selectivity of the alkaline leaching process
towards zinc. As shown in Figure 2(c-e), XRD patterns of
the same leach residues showed no ZnO peaks but revealed
the formation of iron ettringite (Ca3Fe2(OH)12). The cal-
culated Ca:Fe molar ratio in the leached residues (1.4–1.5)
also supports the formation of Ca3Fe2(OH)12. This sug-
gests that excess CaO in the CaO-treated EAFD likely
reacted with Ca2Fe2O5 and water in the solution, as shown
in the following equation:
6H2O Ca2 Fe2O CaO Ca3 Fe2 OHh12
5 ++=^(2)
The formation of Ca3Fe2(OH)12 by the above reaction may
be prevented by minimizing the amount of CaO addition
during the CaO-treatment step such that no unreacted CaO
remains. Meanwhile, negligible dissolution of iron and cal-
cium in the leachate suggests excellent selectivity of the pro-
cess to zinc with minimal contamination and the potential
to recover iron and calcium from the leach residues.