Materials Sciences and Applicatio ns, 2011, 2, 521-525
doi:10.4236/msa.2011.26070 Published Online June 2011 (http://www.SciRP.org/journal/msa)
521
Kinetics of Indium Extraction from Mechanically
Activated ITO Scrap
Xuanhai Li1, Yanjuan Zhang1, Jinhuan Yao2, Xiumin Li1, Liuping Pan1
1School of Chemistry and Chemical Engineering, Guangxi University, Nanning, China; 2Department of Material and Chemical En-
gineering, Guilin University of Technology, Guilin, China.
Received February 23rd, 2011; revised March 15th, 2011; accepted April 6th, 2011.
ABSTRACT
The indium tin oxide scrap (ITOS) was mechanically activated by a stirring ball mill and subsequently studied for the
leaching behavior and kinetics of extracting indium from ITOS in hydrochloric acid solution. The X-ray diffraction
(XRD) analysis showed that MA caused the decrease in crysta lline phase and increase in lattice distortion. The effects
of reaction temperature and hydrochloric acid concentration on the leaching rate of indium were also investigated,
which showed that the ind ium extraction from ITOS had significan t depend ency on tempera ture and HCl concen tration.
The equal-recovery method was used for kinetics analysis. When ITOS was mechanically activated for 15 and 30 min,
the apparent activation energy decreased from 90.6 kJ/mol to 70.3 and 53.0 kJ/mol, respectively, which indicated that
MA could enhance the reactivity of ITOS and accelerated the reaction. The reaction orders of extracting indium from
the nonactivated, milled for 15 and 30 min ITOS with respect to HCl concentration were 2.30, 1.44, and 1.31, respec-
tively.
Keywords: ITO, Mechanical Activation, Indium, Leachin g Kinet i cs
1. Introduction
As a rare metal, indium is widely used in many fields due
to its quite unique physical and chemical properties [1,2].
In recent years, Most of the indium (about 70%) is used
to produce indium tin oxide (ITO), which plays an im-
portant role in optoelectronic devices, transparent elec-
trodes for liquid crystal display and solar cells [3-6].
With the rapid increase in demand of liquid crystal dis-
play, the production of ITO becomes blooming. In the
manufacture of transparent conduction oxide (TCO),
only about 15% of ITO is consumed in making TCO, and
the rest (85%) becomes scrap [7]. In addition, some left-
over material, smear metal and wasters are also produced
during the production of ITO targets. Due to the increas-
ing demand and over-exploitation, the resource of indium
is gradually exhausted. So, it is very important to recover
indium from indium-contained wastes. Current indium
sources estimate that 65% of the indium from used ITO
targets is recovered via reclaim processing. Therefore, it
is of great importance to study the techniques of recy-
cling indium from these wastes. In the production of ITO
targets, different proportion of In2O3 and SnO2 forms
different crystal structure phases under the high tem-
perature and high pressure conditions. The difference in
crystal structure leads to the difference in recycling in-
dium from ITO scraps (ITOS). For some crystal phases,
it is very difficult to be dissolved in acid solution.
Therefore, the investigation of enhancing indium extrac-
tion from ITOS has great significance both in theory and
practice.
Mechanical activation (MA) may increase the internal
energy and reaction activity due to the formation of lat-
tice distortion, the defect in crystal lattice, and amorphi-
zation in solid material via the mechanical force by
high-energy milling [8-10]. In recent years, many re-
searchers have investigated the effect of MA on hydro-
metallurgical extraction of valuable metals from minerals
by using different methods and activation equipment,
such as vibrating mill, planetary mill, and drum ball mill,
showing that MA plays a positive role in the dissolution
of minerals [11-13]. In this study, MA was used for the
pretreatment of ITOS. The purpose of this work was to
investigate the effects of MA on the change in crystal
structure of ITOS and the kinetics of extracting indium
from ITOS in hydrochloric acid.

Kinetics of Indium Extraction from Mechanically Activated ITO Scrap 522
2. Experimental
2.1. Materials
The ITOS (containing 42.5% of indium), which was the
acid leaching residue of waste ITO target, was selected
as the raw material. The grain size distribution of the
sample used for this study was narrow with the average
diameter of 160.6 μm. The sample was characterized by
X-ray diffraction (XRD, Rigaku model D/max-2500) and
the result is illustrated in Figure 1, which shows that
indium tin oxide (In4Sn3O12, PDF 88-0773) and alumin-
ium phosphide (AlP, PDF 80-0013) were the main com-
ponents in the ITOS.
2.2. Mechanical Activation
MA of ITOS samples was performed using a customized
stirring mill driven by a commercial available drill press
equipped with a speed-tuned motor. In each milling ex-
periment, the stainless steel chamber (1200 mL) was
filled with 10,000 g of 4 mm-diameter stainless steel
balls and 200 g of as-received ITOS, then the samples
were subjected to dry milling under air atmosphere and
no other additives were used during the milling. Me-
chanically activated ITOS samples were obtained after
milled for 15 and 30 min, and the activated samples were
sealed for XRD analysis and leaching experiments.
2.3. Leaching Experiment
The leaching reactor was a 1000 mL three-neck flask,
equipped with a mechanical stirrer, thermometer, reflux
condenser and sampler and immersed in a thermostati-
cally controlled water bath. In a typical experiment, the
liquid-to-solid ratio and stirring speed were fixed as 66:1
(mL/g) and 700 rpm. 660 mL of hydrochloric acid solu-
tion was first added in the reactor for preheating. When
the temperature reached the pre-set value, 10.0 g of sam-
ple was put into the reactor. At appropriate time intervals,
solution samples of 2 mL were withdrawn and the in-
dium concentration was determined by a polarographic
analyzer (Model JP-303).
3. Results and Discussion
3.1. Change of Crystal Structure
Figure 2 shows the XRD patterns of nonactivated ITOS
and samples milled for different time. It can be observed
that the intensity of the main planes of ITO (211, 122,
214, 413) decreased with increasing the activation time,
and the diffraction peaks broadened after MA. No new
phase was found according to the XRD patterns of acti-
vated ITOS. The decrease in intensity of diffraction lines
is the result of decrease in crystalline phase, and the
broadening of X-ray diffraction line profile obtained in a
Figure 1. XRD pattern of the nonactivated ITOS.
Figure 2. XRD patterns of the (a) nonactivated; (b) 15 min
and (c) 30 min milled ITOS.
diffractometer attributes to instrumental and physical
(crystallite size and lattice strain) factors [14]. So MA
caused obvious changes in crystalline phase, crystallite
size and lattice strain, which may subsequently enhance
the chemical reactivity of ITOS in hydrochloric acid.
3.2. Kinetics Modeling
If the leaching rate is very fast at the initial stages of re-
action and decreases with reaction time, the equal-re-
covery method is usually used for kinetics analysis [15].
The apparent activation energy Ea can be determined by
the following equation:


ln1 atTE

R (1)
where t is the leaching time, R is the ideal gas constant, T

Kinetics of Indium Extraction from Mechanically Activated ITO Scrap523
is reaction temperature. As a result, the plot of ln t to 1/T
can be drawn, and the slope of the line is –Ea/R or Ea/R.
As to calculate the reaction order n under the same ki-
netic conditions, the similar equation also can be ob-
tained as follows:
0
ln lntnC (2)
where C0 is the initial concentration of HCl (mol/L).
Therefore, by constructing the plots of ln t versus ln C,
the reaction order can be determined.
3.3. Calculation of Activation Energy
The experiments of extracting indium from the nonacti-
vated and milled ITOS samples were carried out with a
temperature range of 50˚C to 90˚C in acid solution con-
taining 5 mol/L HCl. The results are shown in Figure 3,
which indicates that indium extraction increases with the
increase of reaction temperature for all these samples. So
the dissolution of ITOS was quite sensitive to the reac-
tion temperature.
According to Equation (1), the leaching time (t) of ex-
tracting 10%, 15% and 20% of indium from nonactivated
and activated ITOS at different temperature was obtained
from Figure 3. Taking the nonactivated sample as an
example, Figure 4 plotted the straight lines of ln t versus
1/T. From the slopes (Ea/R) of the straight lines, the ap-
parent activation energy of extracting indium from
nonactivated ITOS was calculated to be 90.6 kJ/mol.
Similarly, the activation energies of extracting indium
from the samples activated for 15 and 30 min were de-
termined to be 70.3 and 53.0 kJ/mol, respectively. The
values of activation energy show that the process of ex-
tracting indium from ITOS was significantly influenced
by reaction temperature and chemical reaction was the
controlling step [16]. The activation energy decreased
obviously after milling, which demonstrates that MA
reduced the dependency of ITOS dissolution on tem-
perature.
3.4. Calculation of Reaction Order
The influence of hydrochloric acid concentration on the
indium leaching rate of unmilled and activated ITOS was
examined in the range of 1.0 to 4.0 mol/L with the tem-
perature of 80˚C. The results, shown in Figure 5, indi-
cate that the HCl concentration has significant effect on
the indium extraction. The leaching rate of indium in-
creased with increasing the HCl concentration. It is nota-
ble that the indium extraction of milled ITOS samples
increased significantly with the increase in HCl concen-
tration when lower than 3.0 mol/L, but the increasing
trends became slow with the HCl concentration higher
than 3.0 mol/L, especially the 30 min milled one.
(a)
(b)
(c)
Figure 3. Effect of temperature on indium extraction from
nonactivated and milled ITOS (5.0 M HCl; liquid-to-solid
ratio: 66 mL/g; stirring speed: 700 rpm). (a) Nonactivated;
(b) 15 min; (c) 30 min.

Kinetics of Indium Extraction from Mechanically Activated ITO Scrap 524
Figure 4. Arrhenius plot for extracting indium from nonac-
tivated ITOS.
The use of the kinetic model also allows for the deter-
mination of the dependency of indium extraction on the
concentration of H+ ions. According to Equation (2), the
leaching time of extracting 10%, 15% and 20% of in-
dium from different ITOS samples at different HCl con-
centrate was obtained from Figure 5. Plots of lnt versus
lnCHCl in Figure 6 were drawn for the nonactivated sam-
ple and exhibiting an excellent linear relationship. The
reaction order of nonactivated ITOS dissolution with
respect to HCl was proportional to 2.30 power of HCl
concentration ([HCl]2.30). Similarly, the reaction orders
for the indium extraction from 15 and 30 min milled
samples were found proportional to 1.44 and 1.31 power
of HCl concentration, respectively. The decrease in reac-
tion order after milling indicates that MA could decrease
the dependency of indium leaching from ITOS on HCl
concentration.
4. Conclusions
The effects of MA on crystal structure, leaching behavior
and kinetics of extracting indium from ITOS were inves-
tigated in this study. The results showed that MA caused
the decrease in crystalline phase and increase in lattice
distortion. The leaching experiments of unmilled and
mechanically activated samples showed that MA en-
hanced the reactivity of ITOS and increased the leaching
rate and kinetics. After milled for 15 and 30 min, the
apparent activation energy of extracting indium from
ITOS decreased from 90.6 kJ/mol to 70.3 and 53.0
kJ/mol, respectively, while the reaction order decreased
from 2.30 to 1.44 and 1.31, respectively. The decrease in
activation energy and reaction order indicated that MA
decreased the dependency of ITOS dissolution on tem-
perature and initial concentration of hydrochloric acid.
(a)
(b)
(c)
Figure 5. Effect of HCl concentration on indium extraction
from nonactivated and milled ITOS (conditions as Figure 3).
(a) Unactivated; (b) 15 min; (c) 30 min.

Kinetics of Indium Extraction from Mechanically Activated ITO Scrap 525
Figure 6. Determination of reaction order for extracting
indium from nonactivated ITOS.
5. Acknowledgements
This work was financially supported by National Natu-
ral Science Foundation of China (No. 51064002) and
Natural Science Foundation of Guangxi, China (No.
0728238).
REFERENCES
[1] A. M. Alfantazi and R. R. Moskalyk, “Processing of In-
dium: A Review,” Minerals Engineering, Vol. 16, No. 8,
2003, pp. 687-694.
doi:10.1016/S0892-6875(03)00168-7
[2] X. Tong, S. X. Song, J. He and A. Lopez-Valdivieso,
“Flotation of Indium-Beard Marmatite from Multi-Me-
tallic Ore,” Rare Metals, Vol. 27, No. 2, 2008, pp.
107-111. doi:10.1016/S1001-0521(08)60096-0
[3] S. Calnan, H. M. Upadhyaya, M. J. Thwaites and A. N.
Tiwari, “Properties of Indium Tin Oxide Films Deposited
Using High Target Utilisation Sputtering,” Thin Solid
Films, Vol. 515, No. 15, 2007, pp. 6045-6050.
doi:10.1016/j.tsf.2006.12.063
[4] A. B. Chebotareva, G. G. Untila, T. N. Kost, S. Jorgensen
and A. G. Ulyashinc, “ITO Deposited by Pyrosol for
Photovoltaic Applications,” Thin Solid Films, Vol. 515,
No. 24, 2007, pp. 8505-8510.
doi:10.1016/j.tsf.2007.03.097
[5] J. Liu, D. Wu, N. Zhang and Y. Wang, “Effect of Surfac-
tants on the Structure and Photoelectric Properties of ITO
Films by Sol-Gel Method,” Rare Metals, Vol. 29, No. 2,
2010, pp. 143-148. doi:10.1007/s12598-010-0025-3
[6] W. S. Jung, S. G. Yoon, S. M. Kang, S. Kim and D. H.
Yoon, “Electrical and Optical Properties of ITO: Ca
Composite Thin Films for TEOLED Cathode,” Thin Solid
Films, Vol. 516, No. 16, 2008, pp. 5445-5448.
doi:10.1016/j.tsf.2007.07.070
[7] S. Hsieh, C. Chen and W. C. Say, “Process for Recovery
of Indium from ITO Scraps and Metallurgic Microstruc-
tures,” Materials Science and Engineering B, Vol. 158,
No. 1-3, 2009, pp. 82-87.
doi:10.1016/j.mseb.2009.01.019
[8] C. Sasikumar, D. S. Rao, S. Srikanth, B. Ravikumar, N. K.
Mukhopadhyay and S. P. Mehrotra, “Effect of Mechani-
cal Activation on the Kinetics of Sulfuric Acid Leaching
of Beach Sand Ilmenite from Orissa, India,” Hydrometal-
lurgy, Vol. 75, No. 1-4, 2004, pp. 189-204.
doi:10.1016/j.hydromet.2004.08.001
[9] M. Achimovičová and P. Baláž, “Influence of Mechanical
Activation on Selectivity of Acid Leaching of Arsenopy-
rite,” Hydrometallurgy, Vol. 77, No. 1-2, 2005, pp. 3-7.
doi:10.1016/j.hydromet.2004.09.008
[10] E. Godočíková, P. Baláž and E. Boldižárová, “Structural
and Temperature Sensitivity of the Chloride Leaching of
Copper, Lead and Zinc from a Mechanically Activated
Complex Sulphide,” Hydrometallurgy, Vol. 65, No. 1,
2002, pp. 83-93. doi:10.1016/S0304-386X(02)00094-4
[11] A. M. Amer, “Investigation of the Direct Hydrometallur-
gical Processing of Mechanically Activated Low-Grade
Wolframite Concentrate,” Hydrometallurgy, Vol. 58, No.
3, 2000, pp. 251-259.
doi:10.1016/S0304-386X(00)00134-1
[12] P. Baláž, “Mechanical Activation in Hydrometallurgy,”
International Journal of Mineral Processing, Vol. 72, No.
1-4, 2003, pp. 341-354.
doi:10.1016/S0301-7516(03)00109-1
[13] Y. J. Zhang, X. H. Li, L. P. Pan, Y. S. Wei and X. Y.
Liang, “Effect of Mechanical Activation on the Kinetics
of Extracting Indium from Indium-Bearing Zinc Ferrite,”
Hydrometallurgy, Vol. 102, No. 1-4, 2010, pp. 95-100.
doi:10.1016/j.hydromet.2010.02.003
[14] R. Ebrahimi-Kahrizsangi, M. H. Abbasi and A. Saidi,
“Mechanochemical Effects on the Molybdenite Roasting
Kinetics,” Chemical Engineering Journal, Vol. 121, No.
2-3, 2006, pp. 65-71. doi:10.1016/j.cej.2006.04.011
[15] Z. W. Zhao, G. Zhang, G. S. Huo and H. G. Li, “Kinetics
of Atmospheric Leaching Molybdenum from Metallifer-
ous Black Shales by Air Oxidation in Alkali Solution,”
Hydrometallurgy, Vol. 97, No. 3-4, 2009, pp. 233-236.
doi:10.1016/j.hydromet.2009.02.004
[16] M. Ashraf, Z. I. Zafar and T. M. Ansari, “Selective
Leaching Kinetics and Upgrading of Low-Grade Cal-
careous Phosphate Rock in Succinic Acid,” Hydrometal-
lurgy, Vol. 80, No. 4, 2005, pp. 286-292.
doi:10.1016/j.hydromet.2005.09.001