Gaseous and Solid By-Products Generated from Thermal Plasma CellPhone Waste Treatment
O.L. Li1,2, B. Ruj2,3, J.S. Chang2,3,N. Saito1, T. Takai1, T. Inaba4 and G.Ｊ. Pietsch2,5
1
EcoTopia Science Institute & Graduate School of Engineering, Nagoya University, Japan
2
3
Department of Engineering Physics, McMaster University, Hamilton, Canada
Thermal Engineering Group, Central Mechanical Engineering Research Institute (CMERI) [CSIR], Durgapur713209, India
4
Department of Electrical and Electronics Engineering, Chuo University, Tokyo, Japan
5
RWTH Aachen University, Aachen, Germany
Abstract: Recently disposal and recycle of cell phones become a significant
problem. Although the potential life span of a mobile phone (excluding batteries)
is over 10 years, most of the users upgrade their phones around four times during
this period mainly due to technological and fashionable obsolescence. The
disposal of cell phones thus needs to be considered in an effective, economical
and environmental friendly way. In this work, the treatment of cell phone wastes
by thermal plasma under reducing atmosphere has been investigated. Four types
of sample were treated as follows; a crashed cell-phone without display unit with
and without magnetic separation and cell-phone with display unit with and
without magnetic separation. During the process, the combustible reformed gas
was recovered. The results show that the main gaseous by-products are CO, H2
and CxHy. SO2 was also present to a relative high degree, while the concentration
of CO2 was below the detection limit. Other toxic gases such as NOx and H2S
were not observed. The effect of magnetic separation not only affects trace metal
concentration, but also gaseous by-products. The results also show the presence
of high Cx liquid by-products in the exhaust gas. The results indicate that two of
the major by-products, C8H8 and C6H6O were observed in the on-line by-product
gas. In addition, the molecules become heavier after cooling down the by-product
gas.
Keywords: cell phone waste, thermal plasma treatment, waste treatment
1. Introduction
The number of cell phones worldwide in use
increases exponentially, not only in countries with
well-developed telephone lines, but also in countries
with limited wireless excess. Typical average life of
cell phone is around two years and hence
inexpensive recycle technique should be developed
for recovering valuable resources used in cell phones
such as expensive metals and plastics. In addition,
the disposal of cell phone generates a significant
amount of complex electronic wastes. Cell phone
waste, which is also one type of E-waste, is much
more hazardous than other municipal wastes. It
contains numerous components made of toxic
chemicals and metals like lead, cadmium, chromium,
mercury, polyvinyl chlorides (PVC), brominates
flame-retardants, beryllium, antimony and phthalates
[1-2]. On the other hand, cell phone consists of
many expensive metals such as gold, platinum and
silver and valuable metals such as lithium, titanium,
chromium, manganese, cobalt, nickel and etc.
Primitive recycling or disposal of cell phone waste
to landfills and incinerators cause irreversible
environmental damage by polluting air, water and
soil. Cell phones are made up of plastics, metals,
ceramics, and other trace substances. In general, a
wireless phone handset consists of 40 % metals,
40 % plastics, and 20 % ceramics and trace materials.
The disposal of cell phones thus needs to be
managed in an environmental friendly way to
minimize releases into the environment and threat to
human health. When recycled responsibly, the
metals can be recycled, decreasing the need for
metal mining.
In this work, the treatment of cell phone wastes by a
thermal plasma system in reducing atmosphere for
generation of syngas and metal recovery from the
slag/residue has been investigated. The effect of
magnetic separation and display unit will be
discussed in detail.
thermocouple was introduced in the environmental
chamber to measure the near reactor wall
temperature. An on-line combustion gas analyzer
(Eurotron Greenline 8000) and a hydrogen gas
analyzer (Beacon 200) were installed at the exit of
the reaction chamber behind a heat exchanger. All
samples were treated 30 minutes.
Table 1: Weight percentages of four different cell phone waste
samples
Magnetic
separation by
weight (%)
Non-magnetic
separation by
weight (%)
Display
15
85
Non-display
(main body)
30
70
2. Sample Separation and Experimental
Set-up
Cell phone waste was first separated into display and
non-display part. After crashed, the samples were
cut into small pieces of around 5 mm size by sheet
metal cutting scissor. With a magnet ferromagnetic
were separated from non-ferromagnetic components.
Cell phone waste was divided into four categories:
Display magnetic (dm), display non-magnetic (dnm),
non-display magnetic (ndm), and non-display nonmagnetic (ndnm). Their percentages by weight are
shown in Table 1. Magnetic separated parts can be
directly recycled since only ferromagnetic materials
are present.
A schematic of the plasma torch type cell phone
waste treatment system is shown in Figure 1. Cell
phone waste samples were placed into a 99.8 % pure
alumina reactor with maximum operating
temperature of 1950 ºC in reducing atmosphere [3-4].
The ceramic reactor with a diameter of 7.5 cm and
depth of 2.6 cm was placed 5 cm below the torch.
The sample weights were mostly in between 7 g and
10 g. The thermal plasma was generated by a DC 10
kW plasma torch and was ejected vertically through
the top of the environmental chamber. The power
was limited to 1.5 kW. Pure argon gas with a fixed
flow rate of 35 L/min was used in order to produce a
reducing atmosphere. A constant-voltage power
supply, in series with a resistor bank, was applied to
control the power of the plasma torch. K-type
Figure 1. A schematic of the plasma torch system
3. Results and Discussion
3.1 Solid Analyses
Physical analyses including the examination of the
appearance of the cell phone waste, weight reduction
after plasma treatment and x-ray diffraction
elemental analyses were conducted. The Scanning
Electron Microscope (SEM) image of treated cell
phone ashes is shown in Figures 2. Slight melting
around the edge of some ash particles was observed.
conducted in a reducing atmosphere, the oxygen
percentage remained at a high level after treatment.
70
Weight %
60
50
40
ndnm
30
dnm
20
10
0
Table 2: Weights of cell phone waste samples before and after
treatment
Samples
Sample
weight (g)
Weight after
treatment (g)
Weight loss
(%)
dm
7
6.61
5.57
dnm
10
9.30
7.0
ndm
10
9.94
0.6
ndnm
10
9.65
3.5
C
O
Al
Si
Ti
Fe
Figure 3: Weight percentage of elements of an original
(untreated) non-display non-magnetic and display non-magnetic
cell phone waste
40
35
30
Weight %
Figure 2: Cell phone ashes after thermal plasma treatment
(argon flow rate: 35 L/min; treatment time: 30 min; power: 1.5
kW)
25
20
15
10
5
0
The weight reduction of the waste samples by
plasma treatment is shown in Table 2. The
maximum weight removal of 7 % was achieved for
the dnm sample. The removal percentages of the
display wastes are higher in general. From the
results of the x-ray diffraction (XRD) solid analyses
of non-treated cell phone wastes in Figure 3 follows
that the display unit contains a higher percentage of
carbon and the non-display unit a higher percentage
of metals. Thermal plasma treatment will convert
carbon and other volatile materials into gaseous byproducts.
Figure 4 shows the results of the solid analysis of the
products remained in the cell phone ashes by XRD.
The results of various samples varied greatly. Hence,
the weight percentages were averaged from four
samples. It is observed that the weight percentage of
carbon decreases drastically, while other trace
metals and elements are being concentrated in the
solid by-products. Since the treatment was
C O Na Mg Al Si S Cl K Ca Ti Fe
Figure 4: Mean weight percentage of elements of four treated
cell phone waste samples
3.2 Gaseous By-Products Analyses
The present investigation was carried out in a
reducing atmosphere. The exhaust gas was analyzed
by an on-line combustion gas analyzer, a hydrogen
gas analyzer and Fourier transform infrared
spectroscopy (FTIR). Since the plastic part of the
cell phone waste was converted to gases, it is
necessary to investigate the gas content of the outlet
to prevent toxic gases from being emitted into the
environment. Ten gases (CO, CO2, NO, NO2, NOx,
SO2, H2S, CxHy, O2 and H2) were analyzed. The
system was purged by argon gas prior to the
experiment. Since no oxygen was applied to the
system, gasification started. The concentrations of
CO, CxHy and H2 during thermal plasma treatment
are presented in the Figures 5, 6 and 7, respectively.
150
Figure 7 shows that the emission of H2 reaches the
detection limit of the hydrogen gas analyzer of 1200
ppm after 8 min treatment. Since CO, H2 and CxHy
are combustible gases and have their own heat
values; they are also referred as syngas and can be
used for the generation of electricity. However, the
gas should be desulfurized prior to the use of
electricity generation. FTIR spectroscopy indicates
the presence of C2H2, C2H4, C8H8 and C6H6O as
hydrocarbons in the exhaust gas. The mass numbers
of the group (94>66>65) and (104>78>51>103>77)
were observed during on-line measurements, while
the group (149>167 >57>71>43) was detected after
cooling the by-product gas. The results let one
assume that two of the major by-products might be
liquefied C8H8 and C6H6O in the on-line by-product
gas. Heavier molecules were observed after cooling
the exhaust gas.
100
4. Concluding Remarks
70
dm
dnm
60
ndm
ndnm
[CO] in ppm
50
40
30
20
10
0
0
10
20
Time (mins)
30
40
Figure 5: Concentration of CO as a function of treatment time
during thermal plasma treatment (argon flow rate: 35 L/min,
treatment time: 30 min, power; 1.5 kW)
300
dm
dnm
250
[CxHy] in ppm
ndm
ndnm
200
50
0
0
5
10
15
20
25
30
Time (mins)
Figure 6: Concentration of CxHy as a function of treatment time
during thermal plasma treatment (argon flow rate: 35 L/min,
treatment time: 30 min, power; 1.5 kW)
1400
1200
[H2] in ppm
1000
800
dm
Plasma treatment of cell phone waste in reducing
atmosphere generated gaseous components such as
H2, CO and CxHy, which are combustible gases.
Formation of complex molecules, such as dioxins,
was not observed. The metals in the cell phone waste
were concentrated in solid by-products. Complete
decomposition of plastic parts in cell phone waste
expected after optimization of the system. The
system provides energy recovery with volume
reduction of cell phone waste, reduction of toxic
gases and potential recovery of metals.
dnm
600
ndm
Acknowledgement
ndnm
400
200
0
0
5
10
15
20
25
30
35
Time (mins)
Figure 7: Concentration of H2 as a function of treatment time
during thermal plasma treatment (argon flow rate: 35 L/min,
treatment time: 30 min, power; 1.5 kW)
Carbon and hydrogen in plastic were converted
mainly into CO, H2 and CxHy. Concentration of CO2
was below detection limit in reducing atmosphere.
Minor concentrations of NO, NO2, NOx and SO2
were emitted. As the display parts contain a higher
percentage of carbon, higher concentrations of CO
and CxHy was generated during the treatment as well.
Authors are thankful to CSIR, New Delhi, India and
McMaster University, Canada for supporting this
research project under the Raman Research
Fellowship Program.
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