Materials Transactions, Vol. 51, No. 6 (2010) pp. 1136 to 1140
#2010 The Japan Institute of Metals
Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge
by Chlorination Treatment with Ammonium Chloride
Osamu Terakado*1 , Takashi Saeki*2 , Ryoji Irizato*3 and Masahiro Hirasawa
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
In the present paper we address the novel chlorination process for recovery of indium selectively from dental metal recycling sludge which
contains considerably high amount of indium. The process is based on the utilization of ammonium chloride, NH4 Cl, as chlorination reagent. It
was found that indium could be successfully recovered from the sludge in the form of volatile indium chloride by heating the mixture of sludge
and NH4 Cl at the temperature of 400 C under inert atmosphere. The influence of process parameters, such as composition of NH4 Cl, was
investigated in order to achieve high process efficiency. [doi:10.2320/matertrans.M2010020]
(Received January 21, 2010; Accepted March 3, 2010; Published April 15, 2010)
Keywords: indium, recovery, chlorination, dental metal recycling sludge, ammonium chloride
1.
Introduction
Indium is a trace element in some zinc, lead, copper, and
tin ores, and produced mainly as by-product of zinc refining
process.1,2) The global demand of indium metal is steadily
increasing with the increase in the production of flat panel
display, in which indium is used in transparent electrical
conductor film as indium tin oxide, ITO. Recently, indium is
also utilized as an alloying element of lead-free electronic
solders. The establishment of the recovery process from
indium-containing waste is, therefore, of great importance.
Various techniques have been proposed so far, mainly based
on the hydrometallurgical processes.3)
Among the wastes containing indium, dental metal recycle
sludge is one of the attractive indium resources. The sludge is
a by-product of precious metal recycling from dental alloy
scraps and contains 0:08 mass% of indium in some cases.
In the present paper we report the novel pyrometallurgical
method of the indium recovery from the indium-rich sludge
based on a selective chlorination reaction.
Chlorination process plays an important role in the
extractive metallurgy. Typical chlorination reagents are Cl2
or HCl gas and metal salts such as NaCl and CaCl2 . For
example, Ohwa et al.4) reported the development of a
commercial plant for the production of high-purity metals
such as gallium, indium and bismuth by chlorination process
using Cl2 gas as chlorination reagent. Besides the good
effectiveness and usefulness of the processes, chlorine and
hydrogen chloride gases are very corrosive in nature, so that
special attention must be paid for the leakage of the reactor.
As for the metal salts, the handling of reagent is quite easy,
but one needs higher treatment temperature, usually over
1000 C, than that in the case of chlorine gas in order to
achieve sufficient reaction rate. Thus, exploring alternative
chlorination reagents that can be used at lower temperature
with convenient handling is intriguing to establish simpler
and effective process. As an example, Jena et al.5) examined
*1Corresponding
2.
Experimental
2.1 Materials
Dental metal recycling sludge rich in indium was supplied
from Yamamoto Precious Metal Co., Ltd. A SEM image of
the sludge is shown in Fig. 1. The morphology is quite
complex: fibrous and spherical materials with different sizes
are observed. For the analysis of the contents of metallic
elements in the sludge, we have leached 600 mg of the sludge
with 40 mL of aqua regia, and the obtained solution has been
analyzed with ICP spectrometer (SPS 1500VR, Seiko Instruments). The result of the quantitative analysis of some
metallic elements is summarized in Table 1. Obviously in the
table, the sludge consists of various compounds of metal
elements. Typical phases of the sludge were characterized
by scanning electron microscopy with energy dispersive
spectrometer, SEM-EDS, (JSM-6330F&JED-2140, JOEL) as
shown in Fig. 2. The presence of SiO2 , alkaline and alkaline
earth oxides as well as the complex oxides of indium and tin
was observed.
author, E-mail: [email protected]
*2Graduate Student, Nagoya University. Present Address: Nagoya Railroad
Co., Ltd., Nagoya 450-8501, Japan
Student, Nagoya University. Present Address: BOSCH Co.,
Ltd., Tokyo 150-8360, Japan
*3Graduate
the chlorination of vanadium pentoxide at the temperature
between 553 and 788 K by carbon tetrachloride gas, that
shows, however, also corrosive property.
In the present study we have successfully applied
ammonium chloride as chlorination reagent for the chlorination of indium in the sludge. The process is based on the
thermal decomposition of NH4 Cl into NH3 and HCl gases,
and the consequent chlorination of indium specie in the
sludge. Indium chloride has high vapour pressure at 400 C,
and it deposits at colder part of 280 C in the reactor.
The treatment is especially profitable with respect to the
relatively low treatment temperature (400 C) as well as
simpler handling of the reagent in comparison with the
cases of conventional chlorination reagents. Various process
parameters including the composition of the sludge–NH4 Cl
mixture, reaction atmosphere, and deposition temperature are
discussed.
2.2 Chlorination treatment
The chlorination reaction was carried out in a quartz tube
equipped in a horizontal electric furnace. The size of the
Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge by Chlorination Treatment with Ammonium Chloride
Table 1
1137
Metal content of supplied sludge (in mg metal/g sludge, dry matter).
In
Pd
Pt
Sn
Cu
Ni
Co
Ag
Au
Ti
78 4
1:9 0:2
0:5 0:2
38 2
22 1
10 1
2:0 0:2
1:5 0:3
1:4 0:1
0:70 0:03
Si
10 µ m
O
300 µ m
10 µ m
Fig. 1 Representative SEM image of the sludge employed for chlorination
reaction in the present study.
quartz tube was inner diameter of 26 mm, outer diameter
of 30 mm, and the length of 800 mm. Samples for the
chlorination reaction were prepared by mechanical mixing of
the sludge and ammonium chloride powder (99.5%, Wako
pure chemicals). The total weight of the mixture was 600 mg.
The powder mixture was pelletised with the load of ca.
2:5 108 Pa. Reactions were conducted under helium
(99.9999%) or air gas flow with the flow rate of 100
mL/min. After the temperature of high temperature zone
of the furnace was stabilised, sample mixture contained in a
mullite reaction boat was introduced quickly from the cold
part to the reaction zone inside the reaction tube. Typical
reaction temperature was 400 C and the reaction time was
30 min, if not specified in the present report. A water trap
was set in the off-gas line connected to the reaction tube in
order to collect the evolved gas.
After a reaction run the condensation of yellowish material
was observed at the cold part of the quartz reaction tube.
This material is hereafter denoted as reaction products.
The product was carefully leached by pure water or dilute
hydrochloric acid aqueous solution (ca. 0.6 M). The solution
was then filtered with mixed cellulose filter paper with the
average pore size of 0.1 mm. The amount of metal ions in the
filtrate was analyzed by ICP spectrometer. The amount of
ammonium and chloride ions were also measured for the
water-leached solution of the reaction products, the water
trap as well as the water-leached solution of the residue
remaining in the reaction boat with an ion meter (IM-40,
TOA-DKK).
3.
Results and Discussion
3.1 Chlorination treatment with ammonium chloride
Table 2 shows the typical results of the ICP analysis for
the reaction products. They are presented in terms of metal
Sn
In
O
10 µ m
Na
Ca
Fig. 2
O
Elemental mapping of representative phases of the sludge.
concentration of the water-leached solution of the reaction
products, whereby the sample solution is prepared to the total
volume of 100 mL precisely. The deposition of reaction
products occurs obviously at reaction temperature above
280 C in the presence of ammonium chloride. The analytical
1138
O. Terakado, T. Saeki, R. Irizato and M. Hirasawa
Table 2
Representative results of the recovery of metals at different reaction conditions (in mg/L ).
Composition (mass%)
(Sludge : NH4 Cl)
Reaction temperature
( C)
In
Pd
Pt
Sn
Cu
Ni
Co
Au
Ti
2:3
400
152.7
n.d.
0.2
n.d.
1.3
n.d.
0.2
n.d.
0.1
2:3
280
0.8
n.d.
0.2
n.d.
n.d.
n.d.
0.2
n.d.
0.0
1:0
400
n.d.
n.d.
0.2
n.d.
n.d.
n.d.
0.2
n.d.
0.0
Metal concentration in aqueous solution where reaction products from sludge sample of 600 mg were collected with pure water and the ICP sample was
prepared to the volume of 100 mL.
Table 3 Results of Cl , NH4 þ and pH of the leached solutions of residue
and reaction product as well as water trap .
NH4 þ
pH
Residue leached with pure water
39 1
0:3 0:0
4.4
Reaction product leached with pure water
61 2
31 2
2.8
Water trap
12 1
55 3
10
Total
112 2
87 1
Results for ions are presented as (amount of ion)/(total amount the
corresponding ion in the initial sample) 100.
80
Recovery rate (%)
Cl
100
60
40
20
0
results of the amount of Cl and NH4 in the water-leached
solutions of reaction residue and reaction product as well
as in the water trap, together with the value of pH, are
summarized in Table 3. A reasonable mass balance has been
observed for the ions in the present reaction. The results show
that the main product in the water trap is obviously ammonia,
while the reaction residue contains mainly HCl. The reaction
product is considered as indium chloride together with
hydrogen chloride.
It is well known that ammonium chloride decomposes into
HCl and NH3 above 332 C, and the latter defuses faster than
the former decomposition product. The reaction mechanism
is thus summarized briefly as follows: (1) decomposition of
NH4 Cl, (2) evaporation of ammonia, (3) chlorination of
indium, and the consequent evaporation (with the simultaneous HCl evaporation) and the deposition at the cold part of
the reaction tube.
3.2
0
20
þ
Influence of the concentration of ammonium chloride
One of the most important process parameters for
chlorination treatment is the partial pressure of chlorine as
well as that of oxygen. This can also affect the concentration
of impurities in the reaction product, most probably tin
chloride. We have, therefore, studied the dependence of
composition of the ammonium chloride in order to examine
the influence of the partial pressures. Figure 3 shows the
result of the recovery ratio, i.e. 100 (Recovered amount
of metal)/(Initial amount of metal in the sludge sample), of
indium and tin as a function of concentration of ammonium
chloride in the initial mixture. The experiments were
performed at 400 C for 30 min under helium atmosphere.
Clearly, the recovery ratio of indium increases with
increasing amount of the chlorination reagent. As for the
influence of the reaction atmosphere, the recovery rate
of indium decreases at higher oxygen partial pressure,
as shown in Fig. 4 for the reaction under helium and air
atmosphere.
40
60
80
100
NH4Cl (mass%)
Fig. 3 Recovery ratio of indium and tin as a function of NH4 Cl
concentration in the mixture. The products are leached with either pure
water or dilute hydrochloric acid aqueous solution. The reaction is
conducted at 400 C for 30 min.
100
In, Helium
In, Air
80
Recovery rate (%)
In, Leaching with HCl soln.
In, Leaching with H2O
Sn, Leaching with HCl soln.
Sn, Leaching with H2O
60
40
20
0
0
20
40
60
80
100
NH4Cl (mass%)
Fig. 4 Recovery ratio as a function of ammonium chloride composition
under helium and air reaction atmosphere (Reaction temperature at 400 C,
Reaction time = 30 min). The products are collected by pure water.
An interesting observation is the different leaching
behaviour of tin and indium by pure water and HCl solution.
While the leaching behaviour of indium does not change by
different leaching solutions, tin could not be detected by ICP
measurement after leaching with pure water. This is due to
the formation of white colloidal precipitates during the
leaching treatment; the precipitates are removed from the
leached solution by filtration prior to ICP measurement.
The compounds are water-insoluble tin oxide resulting from
the hydrolysis of tin chloride in water solution of pH 3
(Table 3). On the other hand, the precipitation of tin
compounds does not occur in the acid-leached solution.
The result indicates the possibility of separation of indium
Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge by Chlorination Treatment with Ammonium Chloride
0
1139
100
280
In2O3 (s)
-30
260
60
In
Sn
40
240
20
Temperature, T/°C
InCl (l)
2
log(p Cl )
-20
Recovery rate (%)
InCl3 (s)
-10
80
220
In (l)
0
14
16
18
20
22
Distance from the center of the furnace, d /cm
-40
-50
-50
-40
-30
-20
-10
0
Fig. 6 Recovery ratio and the corresponding average temperature at deposition point (Reaction temperature = 400 C, Reaction time = 30 min.)
log(pO )
2
0
6
SnCl4 (g)
SnCl2 (l)
log(P/Pa)
2
log(p Cl )
SnCl2
2
-20
SnO2 (s)
-30
0
InCl3
-2
-4
Sn (l)
-6
-40
-8
100
-50
-50
SnCl4
4
-10
-40
-30
-20
-10
2
Fig. 5 Potential stability diagram of the In-Cl-O and Sn-Cl-O systems at
400 C. A circle inside the figure is the partial pressure of oxygen and
chlorine in the present experimental condition calculated with FactSage
software with assumption where indium and tin exists as pure solid oxides,
i.e. In2 O3 and SnO2 .
and tin by leaching of chlorination product with water and
the consequent filtration.
Thermodynamic aspects of the chlorination reaction of
the sludge have been investigated with the software package
FactSage Ver.6.0 (Thermfact and GTT-Technologies).
Figure 5 shows potential stability diagrams of the In-Cl-O
and Sn-Cl-O systems at 400 C. We have also evaluated the
oxygen and chlorine partial pressure for sample mixture with
composition of 50 mass% of NH4 Cl under inert atmosphere,
assuming the coexistence of indium and tin solid oxides,
namely In2 O3 and SnO2 , in the sludge. The calculation result
of the partial pressure is displayed as circle in the figure.
As clearly seen in the figure, both indium and tin can be
chlorinated under the equilibrium condition. The increase
in oxygen partial pressure is, on the other hand, unfavourable for the formation of indium and tin chloride. The
thermodynamic consideration can explain the experimental
observation of the influence of the reaction atmosphere
reasonably. The potential phase stability diagram indicated
that selective chlorination of indium was difficult, so that
other possibility of the selective recovery of indium should
be considered.
300
400
Temperature, T /°C
0
log(pO )
200
Fig. 7 Vapour pressure of some indium and tin chlorides as a function of
temperature that is calculated with thermochemical data.6)
3.3
Possibility of the selective recovery of indium by
control of the condensation temperature
The influence of condensation temperature of the reaction
product was examined. For this purpose we have collected
the reaction products with HCl solution at different positions
along the reaction tube; the average temperature of each
position was measured prior to the reaction. Figure 6 shows
the recovery ratio of indium and tin and the corresponding
temperature along the reaction tube. The chlorination
reaction was carried out at a temperature of 400 C for the
composition of ½sludge : ½NH4 Cl ¼ 3 : 2 by weight. The
separation of indium and tin inside the reaction tube is
obvious in the figure. Indium chloride is mainly collected at
270 C, while the tin chloride is collected at lower temperature (250 C). Figure 7 shows the vapour pressure of
some indium and tin chlorides as a function of temperature,
that is calculated from thermochemical data.6) At temperatures between 200 and 300 C, the vapour pressures of tin
chlorides are higher than those of indium chloride. Thus,
selective collection of indium is possible with the control of
the deposition temperature.
With respect to the reaction mechanism, our preliminary
experiment of the chemical analysis of tin products showed
that they deposited in oxidation state 4 even for the product
formed in helium atmosphere. At the tin-rich deposition
temperature, i.e. 250 C, tetravalent tin chloride should
1140
O. Terakado, T. Saeki, R. Irizato and M. Hirasawa
evaporate, as indicated by the vapour pressure data shown in
Fig. 7. Further study is needed in order to clarify the reaction
mechanism, since many factors are involved including the
formation of ammonia, a possible reductant, that can play a
complex role in the reaction. Moreover, indium forms variety
of chloride compounds with complex valency and different
thermal properties which can affect the process performance
of the separation. The existence of tin compounds can also
affect the chlorination behaviour of indium.
3.4 Outlook of the process
We have experimentally shown the selective chlorination
of indium from the sludge by the reaction with ammonium
chloride at relatively low temperature (400 C). The process
is quite simple: satisfactory recovery of indium is possible
by heating of the simple mechanical mixture of sludge and
NH4 Cl powder. The handling of the chemicals is easy, and
the process does not require severe gas-tight reaction line in
contrary with the processes using Cl2 or HCl gas. Furthermore, in comparison with other salt type chlorination
reagents such as NaCl and KCl, the reaction temperature is
much lower in the present system. A possible contamination
of tin can be eliminated by the leaching of chlorination
product with pure water as well as by a careful control of
condensation temperature.
Moreover, there is another advantage of the present
process which results from the fact that both the decomposition products of ammonium chloride, i.e. acidic HCl and
alkaline NH3 , are employed in metal recovery. The formation
of In(OH)3 is feasible by a treatment of reaction products
with ammonia aqueous solution, namely water trap in the
present experimental setup. The hydroxide can be utilised by
conventional metal recovery processes.
A disadvantage of the current process is that high amount
of the chlorination reagent is required. More than 50 mass%
of ammonium chloride is needed to achieve the indium
recovery ratio of 70%. The increase in the contact area
between the sludge and the chlorination reagent can be a key
point for the improvement of the process efficiency.
It is considered that the present simple method can be
applied not only for the sludge but also for the recovery of
indium from other kinds of indium-containing wastes.
Further studies including ITO glass slide wastes are currently
under investigation.
4.
Summary
In the present study we have shown that ammonium
chloride can be successfully applied as chlorination reagent
for the recovery of indium from dental metal recycling
sludge. Heat treatment of the mixture of sludge and
chlorination reagent at 400 C results in a sufficient indium
recovery. This simple process has potential application for
the recovery of indium from other waste. Further fundamental researches such as reaction kinetics are required to
optimize the process parameters.
Acknowledgements
Part of the present work was supported by a grant-in-aid
for scientific research (K2130) from the Ministry of Environment, Japan.
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