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Recovery of precious metals from spent automobile catalytic converters using supercritical carbon dioxide.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2008; 3: 364–367
Published online 10 July 2008 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.156
Research Article
Recovery of precious metals from spent automobile
catalytic converters using supercritical carbon dioxide
Muhammad Faisal,1 Yoichi Atsuta,2 Hiroyuki Daimon3 * and Koichi Fujie4
1
Department of Chemical Engineering, Syiah Kuala University, Banda Aceh, Indonesia
Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi Japan
3
International Cooperation Center for Engineering Education Development (ICCEED), Toyohashi University of Technology, Tempaku-cho, Toyohashi,
Aichi Japan
4
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaka-ku, Yokohama Japan
2
Received 28 October 2007; Revised 12 February 2008; Accepted 14 April 2008
ABSTRACT: Recovery of precious metals (platinum, palladium, and rhodium) from a solid matrix sample by
supercritical carbon dioxide containing a chelating ligand, a tributyl phosphate (TBP), was studied. The effects of
temperature, pressure, and static extraction time on the extraction efficiency were investigated. All experiments were
performed using a supercritical fluid extraction system at a temperature range of 40–80 ◦ C and a pressure of up to
30 MPa. Results showed that addition of the chelating ligand was necessary to extract the metals. Experiments with
pure supercritical carbon dioxide result in no extraction of the metals. It has been observed that the extraction efficiency
of metals was strongly dependent on temperature, pressure, and static extraction time.  2008 Curtin University of
Technology and John Wiley & Sons, Ltd.
KEYWORDS: recovery; precious metals; supercritical carbon dioxide; spent catalytic converter; ligand
INTRODUCTION
Precious metals including platinum, palladium, and
rhodium, which are used as catalyst, jewelry, in
medicine, extrusion device, electrical, and electronic
industry have been in demand recently, in spite of a low
natural abundance in the earth crust. Due to scarcity and
high value of these metals, there is an increased interest
towards their recovery from wastes such as spent catalyst. Spent automobile catalytic converter has emerged
as a major source of these precious metals. Concentrations of the metals in the catalysts vary widely depending on the manufacturer.[1] Concentration of platinum,
which is generally present in larger amounts than palladium and rhodium, ranges from 300 to 1000 µg g−1 ;
in the case of palladium, concentrations vary from 200
to 800 µg g−1 ; and that for rhodium vary from 50 to
100 µg g−1 .[2] These amounts are quite small, but still
the concentrations are often richer than those found in
mined ores.[3] Depending upon the manufacturing quality, the activity of the catalyst is gradually lost due to
its disturbance and thermal shock, limiting its lifetime.
Since the catalyst is important in controlling automotive
*Correspondence to: Hiroyuki Daimon, International Cooperation
Center for Engineering Education Development (ICCEED), Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi
Japan. E-mail: daimon@icceed.tut.ac.jp
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
emissions, replacement of the used catalytic converter
is crucial. Although regeneration and reuse of the spent
catalysts are always preferred, however, if economics
of recovery of precious metals from spent catalysts is
not justifiable, then spent catalysts must be disposed
of.[4] Spent catalyst containing hazardous contaminants
was discarded in landfills in the past, but environmental pollution brought about by the disposal of waste
catalyst became a serious problem. However, at the
same time, accumulated quantities of precious metals
in the spent automobile catalytic converter could potentially be exploited. Moreover, the low production rate
of these metals due to their low concentration in related
ores, and the high costs of production from natural
resources have made recovery of precious metals from
the spent catalysts a viable and cost-effective alternative
to preparation.[5] In this regard, it has been reported that
less than 10% of precious metals from scrapped converters is currently recycled,[6] and the recycling rate of
these metals in USA in 1998 was estimated at 16%.[7]
A number of studies focussed on recovering and separating precious metals from synthetic and industrial
effluents. These include ion exchange methods using
anion exchange resin,[8] combining two hollow-fibre
liquid membrane systems,[2] supported liquid membranes (SLMs) containing a selective carrier,[9] sorption
by chitosan,[10] selective dissolution in aqua regia,[7]
Asia-Pacific Journal of Chemical Engineering
RECOVERY OF PRECIOUS METALS USING SUPERCRITICAL CARBON DIOXIDE
carbochlorination,[11] and high temperature leaching.[5]
Although they are powerful techniques, they may
involve environmental drawbacks such as release of
large quantities of organic solvent. Thus, development
of an alternative recovery process of precious metals
such as with liquid and supercritical carbon dioxide (sc
CO2 ), which is economical and environment-friendly is
still in progress.
The use of liquid and sc CO2 has been gaining interest among researchers because it is considered as an
environmentally benign alternative to organic solvent
for a wide range of applications. CO2 is environmentally acceptable, has low toxicity and convenient critical properties (Tc = 31.1 ◦ C, Pc = 7.38 MPa), is nonflammable, and easy for recycling. CO2 is not currently
regulated as a volatile organic chemical by the US Environmental Protection Agency (EPA).[12] The physical
and chemical properties of liquid and sc CO2 , such
as solvent power, density, polarity, and dielectric constant, can be easily adjusted by varying temperature and
pressure.[13] Those favourable properties of CO2 offer
opportunities for selective extraction and fractionation.
Furthermore, interest in sc CO2 as a solvent for use in
extraction processes has been driven by increased environmental legislation restricting the use of conventional
solvents.[14] Besides the environmental soundness, the
sc CO2 process has at least two advantages over the
conventional solvent process, i.e. easy solvent recovery
and reduction in equipment size.[15] Recently, numerous
papers described the extraction of metal ions from various solid and liquid matrices by sc CO2 modified by
the addition of complexing agents.[13,16 – 25] An excellent
review on sc CO2 extraction of metals from aqueous
solutions has also been provided by Erkey.[26] However, only a few papers focussing on the extraction of
precious metals with sc CO2 have been reported. Powell and Beckman[27] have pioneered the extraction of
platinum in an aqueous liquid matrix of hydrochloric
acid (HCl).
This research deals with a preliminary investigation
of the possibility of extraction of precious metals from
the spent catalyst using sc CO2 modified with a complexing agent. Various parameters such as extraction
time, temperature, and pressure are investigated to optimise the recovery. The extraction behaviour is also
discussed. The results obtained will be of great importance for the development of recycling technology using
a green solvent of sc CO2 , as well as a demand for
precious metals in various applications.
Fig. 1. Liquid carbon dioxide (more than 99.9 v/v%
pure; Suzuki Shokan Co. Tokyo) was delivered from
a cylinder to a high-pressure pump (Intelligent prep.
pump, PU-2086 pluss, Jasco Corp.). The pump head
was cooled to −10 ◦ C with a cool circulator (CH-201,
Scientific Instruments, Scinics Co. Ltd) to liquefy the
gaseous CO2 . CO2 was pumped to an extraction vessel
(cylindrical stainless steel cell, 10-mm i.d × 130 mm
length, and a volume of 10 ml) from the top. The
vessel used was equipped with a stainless steel filter
cap screwed at each end. The vessel was placed in
an oven which controlled the temperature to be within
±1.0 ◦ C of the desired temperature. The inside pressure
was controlled by a back-pressure regulator (SCFBpg, Jasco Corp.) to an accuracy of ±0.1 MPa. In
a typical extraction, 2 mg of pure metals sample and
8 ml of a prepared chelating ligand (tributyl phosphate
and nitric acid) were added to the extraction vessel.
The system was then sealed. The TBP–HNO3 was
prepared by contacting 20 ml of an anhydrous TBP
(Wako Pure Chemicals Co.) with an equal volume of
concentrated HNO3 (70%; Wako Pure Chemicals Co.)
in a centrifuge tube. The mixture was shaken vigorously
on a wrist action mechanical shaker for 5 min followed
by centrifuging for 2 h. After centrifugation, the TBP
phase was used for the experiments.
Subsequently, the extraction vessel was pressurised.
After the temperature and pressure reached the desired
value, the CO2 pump was shut off. This moment
was defined as the beginning of static extraction. The
experiments were then carried out dynamically with
0.8 ml/min flow rate of CO2 for 13 min. When the
extraction was completed, the system was depressurised
to atmospheric conditions and the extractant–metal
complex was collected in a bottle sample. The concentrations of metals were determined by inductively
coupled plasma (ICP) measurement (SPS7700R Plasma
Spectrometer, Seiko Instrument Inc.) at a wave length
Cooling
Unit
Extraction
Vessel
Back Pressur
Regulator
BP
CO2
Oven
EXPERIMENTAL
All experiments were performed using a supercritical
fluid extraction (SFE) system. The SFE system was
constructed in-house and is shown schematically in
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Sample
Collection
CO2 Pump
Figure 1. Schematic diagram of SFE apparatus.
Asia-Pac. J. Chem. Eng. 2008; 3: 364–367
DOI: 10.1002/apj
365
M. FAISAL ET AL.
Asia-Pacific Journal of Chemical Engineering
of 214.423, 340.458, and 233.477 nm for Pt, Pd, and
Rh, respectively.
100
Extraction efficiency (%)
Pt
RESULTS AND DISCUSSION
Pd
Rh
80
60
Various extraction parameters such as pressure, temperature, static extraction time, and the nature of ligand
have been found to affect the extraction efficiency.
Figure 2 shows the effect of the nature of ligands on
extraction efficiency. As can be seen from the figure,
experiments on direct extraction with pure CO2 result
in no extraction of precious metals. A significant obstacle to the application of CO2 to conventional chemical
processes is its low solvent power. Although its solvent
power was once suggested to be comparable to that of
liquid alkanes, recent research has shown that this generalisation is in error.[12] Thus, the presence of a ligand
is necessary to extract precious metals with sc CO2 . The
metal ions are bound to organic ligands and form neutral
species that could increase the solubility of the metals
in sc CO2 . This result agrees with previous findings on
metal extraction with sc CO2 .[28,29] Direct extraction of
metal ions is not feasible due to the charge neutralisation
requirements and the week solute–solvent interactions.
By adding complexing agent to the supercritical fluid
phase, the charge on the metal ions can be neutralised
and lipophilic groups can be introduced into the metal
complex system. Solubilisation of the metal complex
into the supercritical fluid is then possible.[22,28,29]
the solubility of TBP–HNO3 ligand and its complexes
formed with the metals. By increasing the pressure at
a constant temperature, the density of the CO2 phase
increased, and the solvent power of sc CO2 increased
correspondingly. This would account for the increase of
the solubility of metal complexes in sc CO2 , which is
beneficial to the extraction of precious metals. However,
when the density reaches its maximum value, additional
pressure was not able to increase density, thus limiting
the solubility of metal complexes in sc CO2 .[23] It was
found that more than 98% of Pd could be extracted at
30 MPa.
Effect of extraction pressure
Effect of extraction temperature
Pressure, which determined the solvent power of the
supercritical fluid, is an important parameter for the sc
CO2 extraction of metals. Figure 3 shows the results
of the effect of pressure on extraction efficiency. The
amount of extracted metals increased as the pressure of
the system increased. This may be due to an increase in
Another factor that significantly affects the extraction
efficiency of the metals is temperature. As can be
seen from Fig. 4, the extraction efficiency increased
with increasing temperature up to 60 ◦ C. The extraction
efficiency for the precious metals was 12, 62, and 84%
20
10
20
Pressure (MPa)
30
Figure 3. Effect of pressure on extraction efficiency of precious metals. (Ligand: TBP–HNO3 ,
T: 60 ◦ C, static extraction time: 20 min).
100
Pt
Pd
80
60
40
20
Rh
80
60
40
20
0
Pure CO2
Pd
Rh
Extraction efficiency (%)
Pt
0
40
0
100
Extraction efficiency (%)
366
TBP
Ligands
TBP-HNO3
Figure 2.
Effect of the nature of ligands
on extraction efficiency. (60 ◦ C, 20 MPa, static
extraction time: 2 h).
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
40
50
60
Temperature (°C)
70
80
Figure 4. Effect of temperature on extraction efficiency
of precious metals. (Ligand: TBP–HNO3 , P: 20 MPa, static
extraction time: 20 min).
Asia-Pac. J. Chem. Eng. 2008; 3: 364–367
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
RECOVERY OF PRECIOUS METALS USING SUPERCRITICAL CARBON DIOXIDE
◦
at the extraction temperature of 40, 50, and 60 C,
respectively. Then, the amount of extracted metals
decreased with increase of temperature. It should be
noted that by increasing the temperature of the system
at a constant pressure, the vapour pressure of solutes
increases, resulting in its easy dissolution in sc CO2 .
On the other hand, the density of supercritical fluid
decreases gradually with increasing temperature, which
makes the solubilising ability of sc CO2 decreased.
Thus, there is an optimum value for the solubility
and, in turn, the extraction efficiency of the metals.
Experimental results showed that the highest extraction
efficiency was obtained at a temperature of 60 ◦ C.
In order to increase the solvent penetration into the
sample matrix, static extraction was applied. Figure 5
shows the effect of static extraction time on extraction efficiency. The results show that the extraction
efficiency of precious metals increased with increasing
static extraction time up to 60 min. The efficiency then
remains constant even when the static extraction time
was extended to 120 min. More than 96% of Pd could
be extracted at 60 ◦ C, 20 MPa, and 60 min of static
extraction.
The formation of metal complex plays an important
role in the extraction of precious metals with sc CO2 .
Thus, the selection of complexing agent is necessary
in the recovery of metal using sc CO2 technique.
Results of this research have shown that platinum and
rhodium were difficult to extract with sc CO2 because
the TBP–HNO3 was not bound properly with Rh.
These results suggested that TBP–HNO3 has a higher
selectivity to the extraction of Pd.
To improve recovery, effect of other parameters such
as water, pH, modifier, and acid concentration will be
investigated in the near future.
100
Extraction efficiency (%)
Pd
Rh
80
60
40
20
0
0
20
60
Static extraction time (min)
Supercritical CO2 with TBP–HNO3 ligand was shown
to be effective in the extraction of palladium, and
was not favourable for the extraction of Pt and Rh.
An increase in pressure resulted in an increase of
the extraction rate due to the increase in density and
subsequently the solvent power. The highest extraction
efficiency was obtained at 60 ◦ C, while the optimum
static extraction time was 60 min. More than 96% of
Pd could be extracted at 60 ◦ C, 20 MPa, and 60 min of
static extraction time.
REFERENCES
Effect of static extraction time
Pt
CONCLUSION
120
Figure 5. Effect of static extraction time on
extraction efficiency of precious metals. (Ligand:
TBP–HNO3 , T: 60 ◦ C, P: 20 MPa).
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
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Asia-Pac. J. Chem. Eng. 2008; 3: 364–367
DOI: 10.1002/apj
367
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using, dioxide, recovery, supercritical, metali, spent, automobile, catalytic, converter, precious, carbon
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