Materials Transactions, Vol. 47, No. 9 (2006) pp. 2421 to 2426
#2006 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
A Kinetic Study of the Carbothermic Reduction
of Zinc Oxide with Various Additives
Byung-Su Kim* , Jae-Min Yoo, Jin-Tae Park and Jae-Chun Lee
Minerals & Materials Processing Division, Korea Institute of Geoscience & Mineral Resources, Daejeon, Korea
Most processes for recovering zinc from electric arc furnace (EAF) dust employ carbon as a reducing agent for zinc oxide in the dust. In the
present work, the reduction reaction of zinc oxide with carbon in the presence of various additives was kinetically studied. The effects of
temperature and the additives of Fe2 O3 , mill scale, and CaCO3 on the kinetics of the reduction reaction were measured in the temperature range
of 1173–1373 K under nitrogen atmosphere. The mill scale is one of byproducts generated from the steel rolling process. It was found from the
experimental results that all three additives enhance the reaction rate of zinc oxide with carbon, but the effect of CaCO3 addition is the highest.
The increase in the reaction rate is because Fe2 O3 , mill scale, and CaCO3 in the reduction reaction promote the carbon gasification reaction. The
spherical shrinking core model for a surface chemical reaction control was also found to be useful in describing the kinetics of the reaction,
which had an activation energy of 224 kJ/mol (53 kcal/mol) for ZnO-C reaction system, 175 kJ/mol (42 kcal/mol) for ZnO-Fe2 O3 -C reaction
system, 184 kJ/mol (44 kcal/mol) for ZnO-mill scale-C reaction system, and 161 kJ/mol (39 kcal/mol) for ZnO-CaCO3 -C reaction
system. [doi:10.2320/matertrans.47.2421]
(Received February 24, 2006; Accepted August 1, 2006; Published September 15, 2006)
Keywords: recycling, zinc oxide, electric arc furnace dust
1.
Introduction
2.
In the viewpoints of environmental protection and resources conservation, the recovery of zinc metal from the electric
arc furnace (EAF) dust has been well known to be worth.
Therefore, many processes for recovering zinc metal from
EAF dust have been developed, and some of them were
already commercialized.1–7) So far, the major processes
applied for EAF dust treatment are known to be pyrometallurgical processes. All the processes applied involve the
reduction and volatilization of zinc metal from the dust by the
carbothermic reduction method, leaving an iron rich residue.
Thus, many investigations had been conducted on the
kinetics of the reduction reaction of zinc oxide with
carbon.8–10) The reaction of zinc oxide with carbon can be
expressed as:
ZnO(s) þ C(s) ¼ Zn(g) þ CO(g)
ð1Þ
This reaction is a combination of the following reactions:
ZnO(s) þ CO(g) ¼ Zn(g) þ CO2 (g)
CO2 (g) þ C(s) ¼ 2CO(g)
ð2Þ
ð3Þ
However, very little fundamental information is available
in the literature on the effect of additives such as mill scale
and CaCO3 on the above reaction rate. Usually, the additives
are used to promote the zinc recovery ratio from the dust by
minimizing or avoiding the formation of accretions in the
kiln furnace.11,12) Therefore, in the present work, the
reduction reaction of zinc oxide by carbon with additives
such as Fe2 O3 , mill scale, and CaCO3 under condition in
which the effects of external mass transfer and interstitial
diffusion were eliminated was studied using a weight-loss
technique in nitrogen atmosphere. The mill scale is one of
byproducts generated from the steel rolling process, which
mainly contains over 70% for iron metal. This study aims at
investigating the effect of additives for zinc recovery from
EAF dust by conventional pyrometallurgical processes.
*Corresponding
author, E-mail: [email protected]
Experimental Work
2.1 Materials
Materials used in this study were ZnO, Fe2 O3 , CaCO3 ,
carbon, and mill scale. The ZnO, Fe2 O3 , and CaCO3 were
99.9 mass% pure, and the carbon 99.99 mass% pure. All of
the reagents ranged 74 mm in size, except where the size of
the mill scale was 100 mm. The mill scale that is one of
byproducts generated from a steel rolling process of special
steel company in Korea mainly consists of 59.20 mass% FeO
and 39.03 mass% Fe2 O3 .
2.2 Procedure
Experiments were carried out in a horizontal tube furnace
with cooling system, described in detail elsewhere.13) A
covered alumina crucible that has 1 mm hole on the cover
surface was also used to hold the powder sample, the reason
being to eliminate the effect of concentration gradient of
CO(g) and CO2 (g) in the bed height of powder sample by
minimizing the reaction zone as small as possible. The
sample tray is a cylindrical alumina crucible having 2.7 cm
diameter and 2.7 cm depth, and the size of hole on the cover
surface of the alumina crucible was fixed at 1 mm diameter.
In the experiment, a weighed amount of solid reactants was
first mixed thoroughly and placed in the alumina crucible,
and then the sample crucible was introduced in the less hot
zone of the reactor. After 10 min, the sample crucible was
pushed into the central hot zone of the reactor that was
maintained at the desired temperature. The temperature of
this hot zone was constantly maintained by a temperature
controller connected to a R-type (Rh-Rh 13%/Pt) thermocouple located in the constant temperature zone on the inside
of the reactor tube. The temperature in the hot zone was
maintained within 2 K under a steady flow of nitrogen
(purchased by Air Products Company in Korea). The reactor
system was made oxygen-free with nitrogen gas flow inside
the reactor. Zero time was counted when the crucible
containing the sample was pushed into the reactor that was
maintained at the desired temperature. After a stipulated time
2422
B.-S. Kim, J.-M. Yoo, J.-T. Park and J.-C. Lee
0.8
Bed height (cm)
0.25
0.50
1.00
1.50
0.6
0.4
0.2
Temp. = 1373 K
Reduction ratio, X
Reduction ratio, X
0.8
0.0
Temp. (K)
1173
1273
1323
1373
0.6
0.4
0.2
0.0
0
20
40
60
0
20
Time, t/min
Effect of bed height on the ZnO-C reaction system.
period, the reaction was stopped by sliding the crucible from
the central hot zone to the outer cool zone. The cooled
residual solid was weighed and analyzed by X-ray diffractometer (Rigaku D-max-2500PC, Rigaku/MSC, Inc., TX,
U.S.A) and induction coupled plasma method (JY-38 plus,
Horiba Ltd, Kyoto, Japan). In addition, Fe analysis was
performed by wet chemistry.
The reduction ratio at a particular time was determined by
dividing the mass change of zinc in the solid sample at the
time by the initial zinc mass in the solid sample.
3.
Results and Discussion
60
Time, t/min
Fig. 2
Effect of temperature on the ZnO-C reaction system.
ZnO(g) : Fe2O3(g)
1 : 0.00
1 : 0.05
1 : 0.10
1 : 0.15
1 : 0.20
0.6
Reduction ratio, X
Fig. 1
40
0.4
0.2
Temp. = 1323 K
0.0
It is generally expected that the rate of the reduction
reaction of zinc oxide by carbon is dependent on the bed
height of solid sample because the zinc oxide is reduced
carbothermically either with solid carbon or with carbon
monoxide gas according to the reactions (2) and (3).8,9) Thus,
to evaluate the effect of bed height on the rate of the reduction
reaction, sample beds of different height (0.25–1.50 cm) were
tested at 1373 K in nitrogen of 1.2 L/min. Figure 1 shows the
reduction ratio versus time relationships. However, as shown
in Fig. 1, the reaction rate was independent of the bed height
tested in the experiment. The unexpected results may be
due to eliminating the concentration gradient of CO(g) and
CO2 (g) in the sample bed by using a covered sample holder
as explained in the previous part. Thus, in all of the subsequent runs, a bed height below 1.5 cm was chosen to avoid
the diffusion effects. The bed height was obtained by mixing
1.0 g (0:0002 g) of ZnO and 0.1475 g (0:0002 g) of carbon
for ZnO-C reaction system, 1.0 g (0:0002 g) of ZnO, 0.1475
g (0:0002 g) of carbon and 0.2 g (0:0002 g) of Fe2 O3 for
ZnO-Fe2 O3 -C reaction system, 1.0 g (0:0002 g) of ZnO,
0.1475 g (0:0002 g) of carbon and 0.2 g (0:0002 g) of
mill scale for ZnO-mill scale-C reaction system, and 1.0 g
(0:0002 g) of ZnO, 0.1475 g (0:0002 g) of carbon and
0.2 g (0:0002 g) of CaCO3 for ZnO-CaCO3 -C reaction
system. Here, the mole ratio of carbon and ZnO was fixed at
1, which is the stoichiometry amount for ZnO reduction. On
the other hand, through preliminary experiments for the
effect of nitrogen flow rate in the range of 0.2–1.2 L/min, it
can be seen that there is no effect of the gas flow rate on the
0
20
40
60
Time, t/min
Fig. 3
Effect of Fe2 O3 addition on the ZnO-C reaction system.
reaction rate. Thus, in all of the subsequent runs, a working
gas flow rate of 1.2 L/min was chosen to ensure that the rate
of the reduction reaction of zinc oxide by carbon was not
dependent on the rate of zinc vapor removal from the reaction
zone in the covered sample holder.
The effect of reaction temperature on the reduction
reaction of zinc oxide by carbon without additives is shown
in Fig. 2. From the figure, it was observed that the reduction
ratio of zinc oxide is very low at 1173 K and the reduction
rate increases with increasing temperature. The results will
be used for comparison with them obtained from the reduction reaction of zinc oxide by carbon with Fe2 O3 , mill
scale, or CaCO3 . Shown in Figs. 3 and 4 are the effects of
the addition of Fe2 O3 and mill scale. Here, the input amount
was 1.0 g (0:0002 g) of ZnO, 0.1475 g (0:0002 g) of
carbon and 0–0.20 g (0:0002 g) of Fe2 O3 for the ZnOFe2 O3 -C reaction system and 1.0 g (0:0002 g) of ZnO,
0.1475 g (0:0002 g) of carbon and 0–0.20 g (0:0002 g) of
mill scale for the ZnO-mill scale-C reaction system. As
shown in the figures, the addition of a proper amount of
Fe2 O3 and mill scale enhanced the reduction rate of zinc
oxide. However, the reduction rate decreased when the
weight ratio of ZnO and Fe2 O3 is over 1:0.05 and that of ZnO
A Kinetic Study of the Carbothermic Reduction of Zinc Oxide with Various Additives
0.8
ZnO(g) : Mill-scale(g)
1 : 0.00
1 : 0.05
1 : 0.10
1 : 0.15
1 : 0.20
0.6
0.4
Reduction ratio, X
Reduction ratio, X
0.8
0.2
Temp. = 1323 K
ZnO(g) : CaCO3(g)
1 : 0.00
1 : 0.05
1 : 0.10
1 : 0.15
1 : 0.20
0.6
0.4
0.2
Temp. = 1323 K
0.0
0.0
0
20
40
0
60
20
Fig. 4 Effect of mill scale addition on the ZnO-C reaction system.
2000
40
60
Time, t/min
Time, t/min
Fig. 6 Effect of CaCO3 addition on the ZnO-C reaction system.
2000
1
1 FeO
2 Fe 3O4
3 ZnO
1500
1000
1
2
3
3
2
3
500
3
0
10
20
30
1 - ZnO
2 - CaO
1600
3
40
50
60
70
2 θ
Intensity
Intensity
2423
1200
1
800
1
1
400
Fig. 5 X-ray diffraction pattern of the reacted solid from ZnO-Fe2 O3 -C
reaction system for 60 min at 1373 K. (The weight ratio of ZnO and Fe2 O3
is 1:0.05.)
1
1
2
2
2
2
1
1
0
10
20
30
40
50
60
70
2θ
and mill scale over 1:0.10. The increase in the reaction rate
may be because Fe2 O3 and mill scale easily produce CO(g)
and CO2 (g) by reacting with solid carbon, and thus reaction
(3), Boudouard reaction, is promoted on the carbon surface,
resulting in a fast reduction rate. Otherwise, the decrease in
the reaction rate may be because the contact possibility
between CO(g) produced by Boudouard reaction and the
added Fe2 O3 and mill scale increases with increasing the
addition amount, the CO(g) being consumed to reduce the
ZnO as well as the iron oxides. That is indirectly verified by
the fact that after reacting, unreacted ZnO, FeO, and Fe3 O4 in
the cooled residual solid phase are detected. Figure 5 shows
the X-ray pattern of the cooled residual solid obtained from
the ZnO-Fe2 O3 -C reaction for 60 min at 1373 K. On the other
hand, it should be expected in the ZnO-Fe2 O3 -C and ZnOmill scale-C reaction systems that the iron oxides is reduced
to iron by CO(g), and the reduced iron is, in turn, oxidized
by ZnO according to the solid-solid reaction of which the
reaction rate is relatively slow.2) Thus, the reduction behavior
of iron oxide through the quantitative analysis of iron oxides
was not investigated due to the complication in the present
research.
Figure 6 shows the effect of CaCO3 addition. Here, the
input amount was 1.0 g (0:0002 g) of ZnO, 0.1475 g
(0:0002 g) of carbon and 0–0.20 g (0:0002 g) of CaCO3 .
The results indicate that the addition of CaCO3 accelerates
Fig. 7 X-ray diffraction pattern of the reacted solid from ZnO-CaCO3 -C
reaction system for 60 min at 1373 K. (The weight ratio of ZnO and
CaCO3 is 1:0.1.)
the reduction reaction of ZnO. The increase in the reaction
rate may be the reason why CaCO3 is decomposed to CaO
and CO2 (g), and thus Boudouard reaction is promoted on the
carbon surface. This was verified by X-ray analysis. Otherwise, the increase was no significant difference when the
weight ratio of ZnO and CaCO3 is over 1:0.1. That might be
the reason why carbon as a reducing agent is limited in the
reaction system. Thus, it was considered that over the weight
ratio of ZnO and CaCO3 of 1:0.1, the reduction reaction rate
of ZnO is not affected by the addition amount of CaCO3 .
Figure 7 shows the X-ray pattern of the final solid obtained
from the ZnO-CaCO3 -C reaction for 60 min at 1323 K. It is
seen that the solid contains only unreacted ZnO and CaO.
This result indicates that CaCO3 is easily decomposed to
CaO and CO2 (g), and thus the produced CO2 (g) promotes
Boudouard reaction with solid carbon. It was thus considered
that the increase in the reaction rate would be due to forming
a relatively high reduction atmosphere by Boudouard
reaction in the reaction system.
Figure 8 presents, for comparison, a plot of the reduction
ratio curves with the various additives at 1323 K. The results
2424
B.-S. Kim, J.-M. Yoo, J.-T. Park and J.-C. Lee
0.8
For ZnO-C reaction
73
0.3
0.4
K
13
(1/3)
1-(1-X)
0.6
Reduction, X
0.4
Control
5 % Fe2O3
10 % mill scale
10 % CaCO3
3K
132
0.2
1273 K
0.1
1173 K
0.0
0.2
0
Temp. = 1323 K
0.4
0.0
40
40
Time, t/min
Fig. 8 Effect of various additives on the ZnO-C reaction system.
60
ZnO-Fe 2O3 -C reaction
73
60
(1/3)
20
1-(1-X)
0
20
K
13
0.3
3K
132
0.2
1273
K
0.1
1173 K
0.0
0
20
40
60
0.5
For ZnO-mill scale-C reaction
1-(1-X)
(1/3)
0.4
73
K
13
0.3
K
23
13
0.2
1273
K
0.1
1173 K
0.0
0
0.5
20
40
(1/3)
60
For ZnO-CaCO3 -C reaction
0.4
1-(1-X)
shown there indicate that the effect of the addition of CaCO3
on the reaction rate is the highest. Reduction of zinc oxide
without any additives was around 60% in 60 min. After the
same amount of time, the reduction of zinc oxide with CaCO3
was about 70%. This is the reason that as explained above,
CO(g) produced by Boudouard reaction in the ZnO-CaCO3 C reaction system is consumed to only reduce ZnO. On the
other hand, the effects of temperature on the reduction
reaction of zinc oxide by carbon with Fe2 O3 , mill scale, or
CaCO3 were investigated. Here, the input amount was 1.0 g
(0:0002 g) of ZnO, 0.1475 g (0:0002 g) of carbon and
0.05 g (0:0002 g) of Fe2 O3 for the ZnO-Fe2 O3 -C reaction
system, 1.0 g (0:0002 g) of ZnO, 0.1475 g (0:0002 g) of
carbon and 0.10 g (0:0002 g) of mill scale for the ZnO-mill
scale-C reaction system, and 1.0 g (0:0002 g) of ZnO,
0.1475 g (0:0002 g) of carbon and 0.10 g (0:0002 g) of
CaCO3 for the ZnO-CaCO3 -C reaction system. The results
are shown in Fig. 9.
It is generally accepted that the reduction reaction of zinc
oxide with carbon is one of the gas-solid reactions in which
no solid product is formed.8–10) The reduction reaction
mechanism includes the following sequential steps: 1) carbon
is vaporized with carbon dioxide to form carbon monoxide as
in reaction (3), 2) the carbon monoxide diffuses towards the
surface of the zinc oxide, 3) the zinc oxide is reduced by
carbon monoxide to zinc as in reaction (2), and 4) the zinc
vapor diffuses into the bulk gas and the carbon dioxide takes
part in the reaction as in reaction (3) again. The overall
reaction rate is controlled by the slowest step. Thus, the
particle size of zinc oxide and carbon as reactants is
diminished as the reduction reaction proceeds. In general,
for such a system, spherical shrinking core model (SCM) is
very well known to be useful to analyze the reaction rate data.
For reactions following the SCM, two resistances may
influence the reaction rate: mass-transfer and surface chemical reaction. However, in the study, the mass-transfer could
be neglected by using a covered sample holder as explained
in the previous part and using a sufficiently high gas flow rate.
Thus, the reaction rate data obtained in the study were
measured in the absence of the mass-transfer effects. Based
on these observations, the interpretation of the rate data was
73
K
13
0.3
23
13
0.2
K
1273
0.1
K
1173 K
0.0
0
20
40
60
t/min
Fig. 9 Plot of the reduction ratios obtained from the ZnO-C, ZnO-Fe2 O3 C, ZnO-mill scale-C, and ZnO-CaCO3 -C reaction systems according to
eq. (4). (The dot lines including symbol are for experimental data, the
solid lines being for the best-fit lines according to eq. (4).)
carried out using a number of different rate expressions such
as Jander equation and nucleation and growth equation, from
which the SCM that is useful for a surface chemical reactioncontrolled process was proved to yield the best results. In the
SCM rate equation for a surface chemical reaction-controlled
process, the reduction ratio of zinc oxide is related to the
reaction time by14)
1 ð1 XÞ1=3 ¼ kr t
ð4Þ
Here, X is the reduction ratio of zinc oxide, t is the reaction
A Kinetic Study of the Carbothermic Reduction of Zinc Oxide with Various Additives
6
1
kr ¼ 2:36 10 exp½26;900=T (min )
E = 224 kJ/mol (53 kcal/mol)
-1
ln (k r /min )
-5
-6
-7
-8
For ZnO-C reaction
7.5
-1
8.0
8.5
E = 175 kJ/mol (42 kcal/mol)
-5
ln (k r /min )
time (min), and kr is a rate constant (min1 ) that is a function
of temperature. It is apparent from eq. (4) that a plot of 1 ð1 XÞ1=3 versus t should be linear with kr as the slope.
The applicability of the SCM rate expression for the effects
of temperature on the reduction reaction of zinc oxide by
carbon with Fe2 O3 , mill scale, or CaCO3 were verified by
first plotting the rate data obtained from each reaction
systems according to eq. (4). Figure 9 presents the results.
Examination of these figures reveals that the rate data follow
relatively well eq. (4), as shown in Fig. 9. The values of kr
were thus determined from the slopes of the figures. The
slopes were calculated by regression analysis. Figure 10 is
Arrhenius plots of the rate constants. The slopes of the
straight line placed through the experimental points yield an
activation energy of 224 kJ/mol (53 kcal/mol) for ZnO-C
reaction system, 175 kJ/mol (42 kcal/mol) for ZnO-Fe2 O3 -C
reaction system, 184 kJ/mol (44 kcal/mol) for ZnO-mill
scale-C reaction system, and 161 kJ/mol (39 kcal/mol) for
ZnO-CaCO3 -C reaction system. These lines are represented
by the following equations:
For ZnO-C reaction system
2425
-6
-7
For ZnO-Fe 2O3-C reaction
-8
ð5Þ
7.5
8.0
8.5
For ZnO-Fe2 O3 -C reaction system
kr ¼ 3:38 104 exp½21;100=T (min1 )
ð6Þ
E = 184 kJ/mol (44 kcal/mol)
-5
For ZnO-CaCO3 -C reaction system
kr ¼ 1:15 104 exp½19;400=T (min1 )
ð8Þ
The activation energies obtained are relatively high, which
are less than those obtained for the pure zinc oxide with
CO(g) (253 kJ/mol)10) and for the pure zinc oxide with iron
metal (230 kJ/mol).2) The results indirectly indicate that the
reduction reaction of zinc oxide with solid carbon without
additives or with additives such as Fe2 O3 , mill scale, and
CaCO3 is controlled by a surface chemical reaction.8–10)
However, the effects of reaction (2) and (3) on the reduction
rate of zinc oxide with carbon were not distinguished in the
present research.
4.
Conclusions
The reduction reaction of zinc oxide with carbon in the
presence of various additives such as Fe2 O3 , mill scale, and
CaCO3 was studied in the temperature range of 1173 K–
1373 K under nitrogen atmosphere using a weight-loss
technique. The addition of a proper amount of Fe2 O3 and
mill scale enhanced the reduction rate of zinc oxide, but the
reduction rate decreased when the weight ratio of ZnO and
Fe2 O3 is over 1:0.05 and that of ZnO and mill scale over
1:0.10. The effect of the addition of CaCO3 on the reaction
rate was the most effective, but the increase in the reaction
rate was no significant difference when the weight ratio of
ZnO and CaCO3 is over 1:0.1. The increase in the reaction
rate is because Fe2 O3 , mill scale, and CaCO3 in the reduction
reaction of zinc oxide with carbon promote the carbon
gasification reaction. The spherical shrinking core model
-1
-6
-7
For ZnO-mill scale-C reaction
-8
7.5
8.0
8.5
E = 161 kJ/mol (39 kcal/mol)
-5
-1
ð7Þ
ln (k r /min )
kr ¼ 7:74 104 exp½22;100=T (min1 )
ln (k r /min )
For ZnO-mill scale-C reaction system
-6
-7
For ZnO-CaCO3-C reaction
7.5
8.0
-1
-4
T /10 K
8.5
-1
Fig. 10 Arrhenius plot of the rate constants.
for a surface chemical reaction control was found to be useful
in describing the kinetics of the reaction over the entire
temperature range. The reaction has an activation energy of
224 kJ/mol (53 kcal/mol) for ZnO-C reaction system, 175
kJ/mol (42 kcal/mol) for ZnO-Fe2 O3 -C reaction system, 184
kJ/mol (44 kcal/mol) for ZnO-mill scale-C reaction system,
and 161 kJ/mol (39 kcal/mol) for ZnO-CaCO3 -C reaction
system.
2426
B.-S. Kim, J.-M. Yoo, J.-T. Park and J.-C. Lee
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