Materials Transactions, Vol. 44, No. 12 (2003) pp. 2489 to 2493
Special Issue on New Systems and Processes in Recycling and High Performance Waste Treatments
#2003 The Japan Institute of Metals
Leaching of Zinc Oxide in Acidic Solution
Takashi Yoshida
MS Zinc Co., Ltd., Tokyo 105-0004, Japan
Leaching mechanism of zinc oxide is important not only to understand zinc smelting process but also to develop of new zinc recycling
process. Recycling of zinc is mainly done by treatment of electric arc furnace dust (EAF dust). In Japan pyro metallurgical process is enhanced
for EAF dust treatment, however only zinc oxide can be recovered by pyro metallurgical process. Hydro metallurgical process makes it possible
to recover high purity zinc directly from EAF dust. One of the main components of zinc included in EAF dusts is zinc oxide. Thus the
mechanism of zinc oxide leaching is important to develop the new hydro metallurgical zinc recycling process. Leaching test of zinc oxide in
hydrochloric acid and sulfuric acid solution has been carried out kinetically. Zinc oxide disk was used for leaching test which the reaction area is
clear. The experimental parameters of pH, temperature, leaching time and rotating speed of the disk specimen were varied. The results obtained
are as follows;
(1) The activation energy of leaching reaction by sulfuric acid was 17.5 kJmol1 and 11.6 kJmol1 by hydrochloric acid solutions.
(2) The leaching rate increases in proportion to the square root of rotating speed of test specimen.
These results indicate that the rate controlling factor of zinc oxide leaching reaction in acidic solution is mass transportation.
(Received July 14, 2003; Accepted September 5, 2003)
Keywords: leaching, zinc oxide, mass transfer, disk specimen, recycling and acid solution
1.
Introduction
The amount of zinc production in the world have increased
and reached more than eight million tons. The major zinc
production process is the hydro-metallurgical process which
consists of roasting, leaching and electro-winning. The
kinetics of leaching is very important to fully understand
the zinc smelting process. Thus several studies1–4) have been
done in the leaching kinetics of zinc oxide. On the other hand
the recycling has become the major issue for almost every
industry. The one of the major recycling of zinc is done by
treatment of electric arc furnace dusts (EAF dust). Pyrometallurgical processes such as Wealz kiln process, MF
process and electric thermal process are enhanced for
recycling of zinc from EAF dust in Japan.5) These processes
recover zinc as zinc oxide. Thus further treatment is
necessary to produce high purity zinc metal. Hydro-metallurgical process makes it possible to produce high purity zinc
directly from EAF dust. Several hydro-metallurgical processes have been proposed6) however they are not sufficient
from the industrial point of view. Thus development of new
hydro-metallurgical process for zinc recycling is very much
concerned. Zinc included in EAF dust is mainly zinc oxide.
Thus the basic study on leaching kinetics of zinc oxide
becomes more important for development of new recycling
process.
Relatively many studies have been done for leaching
kinetics of metal sulfide and oxide which controlled by
chemical reaction.7–9) Few studies have been made on
leaching kinetics which controlled by mass transportation.
Leaching of zinc oxide by acidic solution is typical example
that the reaction is controlled by mass transportation.
Previous studies on leaching of zinc oxide in acidic solutions
are carried out mainly by using powder samples. In these
cases the surface area is not well understood. In this study
disk type zinc oxide specimen is used to make sure the
surface area and fully understand of leaching kinetics of zinc
oxide. Furthermore the changing of surface morphologies
was investigated. The observation of the surface morphologies is very few in previous studies.
2.
Experimental Procedure
2.1 Samples
Reagent grade zinc oxide powders are used for making
zinc oxide disk. Zinc oxide powders were dried at 473 K for
7.2 ks then pressed under the pressure of 490 GPa. The
pressed disk was sintered at 1523 K for 28.8 ks. The diameter
of sintered disk is 10 103 m. The specific density of the
disk was approximately 97 pct of the theoretical value. The
disk specimen was coated in an epoxy resin. The one surface
of the disk was exposed and polished by emery paper where
leaching reaction occurred. Chemical reagent grade acids and
deionized water was used for every leaching test.
2.2 Experimental apparatus for leaching test
A glass flask containing 0.5 dm3 of leaching solution was
used for reaction vessels of all leaching experiments. The
disk specimen of zinc oxide attached to the rod and
submerged to the leaching solution. The rod and the
specimen were rotated by the variable rotating speed motor.
The temperature was measured by thermocouple and controlled in the water bath. The temperature difference was
controlled within 0:5 K during leaching experiments.
Sulfuric acid and hydrochloric acid were used for leaching
test. The pH value was maintained in same value during the
leaching test because the volume of acid solution has large
enough and the amount of dissolved zinc oxide was rather
small. It was cleared by preliminary experiments to maintain
pH value constant during the leaching tests. The samples of
leaching solution were withdrawn periodically for quantitative analysis to determine the amounts of dissolved zinc by
using ICP (Inductively coupled plasma) mass analyzer. The
surface of zinc oxide disk specimen was observed before and
after leaching tests by scanning electron microscope (SEM).
The experimental conditions are summarized in Table 1.
2490
T. Yoshida
Acid
H2 SO4 , HCl
Temperature (K)
303, 323, 333, 353
pH
1, 2, 3
Rotating speed of specimen (min1 )
0, 100, 300, 1200
Experimental Results and Discussions
3.1 The morphology of the leaching tests
Figure 1 shows the typical morphologies of test specimens
observed by SEM. The surface morphology of the zinc oxide
specimen before leaching test is almost flat and it is
understandable because the specific density of the zinc oxide
specimen was 97 pct of the theoretical value. After leaching
tests, the surface morphologies changed that surface shown
the bumpy shape in both hydrochloric acid and sulfuric acid
leaching test.
3.2 Leaching curves at pH 1, 2 and 3.
The leaching curves of zinc oxide disks, leached by
hydrochloric and sulfuric acid at pH 1, 2 and 3 are illustrated
in Figs. 2 and 3 respectively. In these figures C represents the
amounts of dissolved zinc to the liquid phase. The leaching
temperature was controlled at 333 K and the rotating speed of
the disk specimen was 10 s1 . It is clear that the amounts of
dissolved zinc increase according to the leaching time at each
pH value and the leaching kinetics are linear for 7.2 ks. When
the pH value was lower the amounts of dissolved zinc
increased in both acids. Figure 4 illustrates the relationship
between R (Leaching rate) and pH value in hydrochloric and
sulfuric acid solutions. The kinetics of leaching rate shows
linear relations with pH value. Hydrolytic reactions of Zn2þ
hardly occur in the pH 1 to 3, thus pH value can be the
parameter of aHþ (activity of hydrogen). So it can be said that
the leaching kinetics of zinc oxide show linear relationship to
activity of Hþ in the sulfuric acid and hydrochloric acid
solutions when the pH value maintained 1 to 3.
3.3 Temperature dependence on leaching rate
Temperature dependence on the leaching rate of zinc oxide
was examined. The leaching curves when the leaching
temperature was 303, 323, 333 and 353 K are shown in Figs. 5
and 6. Figure 5 shows the leaching curves in hydrochloric
(a)
10
9
C / 10-3 mol . dm-2
3.
Experimental conditions
pH1
8
pH2
7
pH3
6
5
4
3
2
1
0
0
1200
2400
3600
4800
6000
7200
Leaching Time, t / s
Fig. 2 Zinc oxide leaching curves in hydrochloric acid solutions at 333 K.
The rotating speed was 10 s1 .
16
pH1
14
C / × 10-3 mol . dm-2
Table 1
pH2
12
pH3
10
8
6
4
2
0
0
1200
2400
3600
4880
6000
7200
Leaching Time, t / s
Fig. 3 Zinc oxide leaching curves in sulfuric acid solutions at 333 K and
the rotating speed of specimen was 10 s1 .
acid solution at pH 2 and Fig. 6 shows the leaching curves in
sulfuric acid at pH 2. The rotating speed of the specimen was
controlled at 10 s1 in each experiment. The leaching kinetics
(b)
50 µ m
50 µ m
Fig. 1 Surface morphology of Zn oxide disk leached in hydrochloric acid solutions at 333 K. The rotating speed was 10 s1 : (a) before
leaching (b)after leaching for 7.2 ks.
Leaching of Zinc Oxide in Acidic Solution
-5
H2SO4
353K
0.15
HCl
333K
-6
323K
C / 10-3 mol . dm-2
Log ( R / mol- dm-2 - s-1 )
2491
-7
-8
303K
0.1
0.05
1
2
3
pH
Fig. 4 Effect of pH value on the reaching rate of zinc oxide in hydrochloric
acid and sulfuric acid at 363 K. Rotating speed of specimen was 10 s1 .
0
0
1200 2400 3600 4800 6000 7200
Leaching Time, t / s
1.0
Fig. 6 Leaching curves of zinc oxide in sulfuric acid solutions at pH 2 and
rotating speed was 10 s1 .
353K
333K
323K
0.6
H2SO4
303K
Ln ( R / mol . dm -2 . min-1 )
C / 10-3 mol . dm-2
0.8
0.4
0.2
HCl
–12.5
17.5kJ . mol-1
–12
11.6kJ . mol-1
–11.5
0
0
1200 2400 3600 4800 6000 7200
2.9
Leaching Time, t / s
Fig. 5 Leaching curves of zinc oxide in hydrochloric acid solutions at pH 2
and rotating speed was 10 s1 .
of both leaching tests by hydrochloric acid and sulfuric acids
in temperature range from 303 to 353 K are linear. The higher
temperature enhanced the leaching volume in both acidic
solutions. The data are summarized in the Arrhenius plot in
Fig. 7. The leaching rates were determined by the gradient of
the leaching curves of each leaching test of different
temperature. The logarithmic value of the leaching rate and
1=T where T is the leaching temperature shows good linear
relationship. The activation energy (Ea) can be expressed as
follows;
k ¼ k0 expðEa=RTÞ
ð1Þ
ln k ¼ ln k0 Ea=T
ð2Þ
In above equations k is the overall reaction coefficient and k0
is constant.
Based on these equations the activation energy can be
3.0
3.1
3.2
3.3
Temperature T -1/ 10-3 K-1
Fig. 7 Arrhenius plot of zinc oxide leaching in hydrochloric acid and
sulfuric acid solutions at pH 2 and rotating speed was 10 s1 .
calculated by the gradient of each line in Fig. 7. The
activation energy for leaching of zinc oxide disk by hydrochloric acid at pH2 is 11.6 kJmol1 and 17.5 kJmol1 by
sulfuric acid solutions at the temperature range from 303 to
353 K. These activation energies show good agreement
obtained by previous studies.1,4) Generally the activation
energy of diffusion in liquid phase is less than 20 kJmol1
and the activation energy of chemical reaction is approximately 50 to 100 kJmol1 . These small values of the
activation energy suggest the leaching rates of zinc oxide
disks in hydrochloric acid and sulfuric acid solutions are
controlled by mass transportation such as diffusion of
chemicals through the liquid boundary layer.
2492
T. Yoshida
3.4 Effects of the rotating speed
The leaching reaction of zinc oxide by hydrochloric acid
and sulfuric acid solutions can be expressed as follows;
ZnO þ H2 SO4 ¼ ZnSO4 þ H2 O
ð4Þ
The mechanism of leaching reaction can be describe that the
diffusion of the chemical reagent to the surface of the zinc
oxide disk through the liquid boundary layer and the leaching
reaction occurs on the surface of the specimen. Then the
products of the chemical reactions move from the surface to
the leaching solution through the liquid boundary layer. Thus
the kinetic of the leaching reaction is basically mixtures of
chemical reactions and mass transportations. When the
chemical reactions of eqs. (3) and (4) occur fast enough the
leaching reaction is controlled by mass transportations. If
leaching reactions of zinc oxide in hydrochloric acid and
sulfuric acid solutions are controlled by the mass transportations the leaching rates should depend upon the rotating
speed of the zinc oxide specimen. Basically the thickness of
the liquid boundary layer on the disk specimen decreases
when the rotating speed of the specimen increases. Therefore,
to determine the rate controlled factor the effects of the
rotating speed of the zinc oxide specimen on leaching
reactions were carried out.
The leaching curves of zinc oxide in different rotating
speeds by hydrochloric acid solutions and sulfuric acid
solutions are illustrated in Figs. 8 and 9 respectively. The
rotating speed changed from 0 to 20 s1 and pH value was
maintained at 2. The leaching temperature was 333 K in each
test. The leaching kinetics are linear in each rotating speed of
zinc oxide specimen. As illustrated in Figs. 8 and 9, the
amounts of dissolved zinc oxide increased by leaching time
and rotating speed as well.
The thickness of liquid boundary layers decreases in
proportion to the square root of the rotating speed of the
600 min-1
300 min-1
1.2
100 min-1
C / 10-3 mol . dm-2
ð3Þ
0 min-1
1.0
0.8
0.6
0.4
0.2
0
0
1200 2400 3600 4800 6000 7200
Leaching Time, t / s.
Fig. 9 Leaching curves of zinc oxide in sulfuric acid solutions at pH 2 and
temperature was 333 K.
R / 10-6 mol - dm-2 - s-1
ZnO þ 2 HCl ¼ ZnCl2 þ H2 O
1200 min-1
1.4
0.2
H2SO4
HCl
0.15
0.1
0.05
0
01
1.0
1200 min-1
2
Rotating
3
Speed1/2,
ω/s
4
1/2
600 min-1
300 min-1
C / 10-3 mol . dm-2
0.8
100 min-1
Fig. 10 Effect of rotating speed of specimen on zinc oxide leaching in
hydrochloric acid and sulfuric acid solutions at the 333 K and the pH value
was 2.
0 min-1
0.6
0.4
0.2
0
0
1200 2400 3600 4800 6000 7200
Leaching Time, t / s.
Fig. 8 Leaching curves of zinc oxide in hydrochloric acid solutions at pH 2
and temperature was 333 K.
specimen. The leaching rates of zinc oxide and square root of
the rotating speed is illustrated in Fig. 10. The leaching rates
show the linear relationship with square root of rotating speed
in both hydrochloric acid and sulfuric acid where the rotating
speed was between 0 to 20 s1 . It indicates that the leaching
rates of zinc oxide disk increase with increasing rotating
speed and these phenomena suggests the rate of zinc oxide
leaching is controlled by diffusion through the liquid
boundary layer.
In Fig. 9 when the rotating speed is zero, the reaching rate
is approximately 0:01 106 mol1 dm2 s1 . If the liquid is
idealized solution the leaching rate should be zero when the
zinc oxide specimen is stationary. The leaching rate is not
zero because even rotating speed of the specimen is zero the
natural convection occurs by the difference of concentrate of
Leaching of Zinc Oxide in Acidic Solution
chemicals between on the surface of the specimen and bulk
solutions.
These experimental data that the activation energy is rather
low and the leaching rate depends on the rotating speed of
zinc oxide disk indicate that the rate controlling step of zinc
oxide leaching in hydrochloric and sulfuric acid is mass
transfer through liquid boundary layer. Further investigation
is necessary to determine the chemicals whose diffusion rate
controls the leaching reaction rate.
4.
Conclusions
The leaching of zinc oxide disk in hydrochloric acid and
sulfuric acid solutions was studied kinetically. The effect of
pH value, temperature and rotating speed of zinc oxide disk
on leaching rate were investigated. The followings are the
results obtained from experimental data;
(1) Leaching rate increases at lower pH value.
(2) Leaching rate increases at higher leaching temperature.
By the Arrhenius plot, the activation energy of the zinc oxide
leaching by hydrochloric acid is 11.6 kJmol1 and
17.5 kJmol1 by sulfuric acid leaching.
(3) Leaching rate increases in proportion to square root of
2493
rotating speed of zinc oxide disk.
(4) The relatively lower activation energy and relationship
between leaching rate and rotating speed of test specimen
indicate that the reaction rate of zinc oxide leaching is
controlled by mass transfer through the liquid boundary
layer.
REFERENCES
1) K. Matsumoto, S. Taniguchi and A. Kikuchi: J. Japan Inst. Metals 55
(1991) 853-859.
2) F. M. Doyles, N. Ranjan and E. Peters: Trans. Instn. Min. Metall. (Sect.
C: Mineral Process. Extr. Metall.) 96 (1987), C69.
3) H. Gerischer and N. Sorg: Electrochemical Acta. 5 (1992) 827-835.
4) A. J. Monhenius and G. Katungu: Inst. Chem. Eng. Symp. Ser. 87 (1984)
635-42.
5) T. Yoshida: Shigen-to-Sozai 113 (1997) 967-971.
6) Y. Umetsu: Shigen-to-Sozai N5 (1995) 14-17.
7) H. Majima, Y. Awakura and T. Mishima: Metall. Trans. B 16B (1985)
23-30.
8) T. Hirato, H. Majima and Y. Awakura: Metall. Trans. B 18B (1987)
31-39.
9) T. Hirato, H. Hiai, Y. Awakura and H. Majima: Metall. Trans. B 20B
(1989) 485-491.