Paper ID 33
Application of Reductive Melting Process of CRT Glass for
Recovering Valuable Metals from PCB Waste
Naoki Hiroyoshi1*, Hanthanon Prin2, Yoshihiro Takaya3, Mayumi Ito4
1
Faculty of Engineering, Hokkaido University, Japan
Graduate school of engineering, Hokkaido University, Japan
3
Graduate school of engineering, Hokkaido University, Japan
4
Faculty of Engineering, Hokkaido University, Japan
*Authors to correspondence should be addressed via e-mail: [email protected]
2
ABSTRACT
The amount of electronics waste (E-waste) has been dramatically increasing, and print circuit board
(PCB) in the E-waste contains high concentrations of valuable metals. Cathode Ray tube (CRT) glass used as
TV component contains a large amount of lead. Reductive melting processes have been reported to be
effective to remove lead from CRT glass.
In this study, the reductive melting process of CRT glass was applied for recovering valuable metals
from PCB. Reductive melting experiments were carried out with 20g of crushed CRT glass containing 25
wt% lead (particle size less than 1 mm), 10g of Na2CO3, 3 g of activated carbon, and 0.2g of metal
compounds (Au, Ag, Cu, Ni, and In2O3) in alumina crucible at maximum temperature of 1200oC. It was
found that Au, Ag, Cu and Ni were recovered in metallic Pb obtained by the reduction, while In was
remained in glass phase.
KEY WORDS: Cathode Ray Tube (CRT) / Print Circuit Board (PCB)/ Reductive Melting / Lead Recovery/Recycling
1. INTRODUCTION
Funnel glass from cathode ray tubes (CRT)
has been recycled to make a new CRT. In Japan, the
waste CRT generation in 2007 was approximately 60
thousand tons which was transported mainly to the
CRT production facilities in Malaysia [1]. However,
this closed-loop recycling (waste CRT to new CRT)
is becoming difficult because flat-panel displays are
rapidly replacing CRT. The funnel glass of CRT
(Fig. 1) contains a considerable amount of lead (2030 wt% as PbO) [2]. Because lead is a toxic element,
direct disposal of CRT funnel glass containing lead
in landfill would cause the contamination of soils.
It was reported that reductive melting
method is effective to remove lead from CRT funnel
glass [3]. In this method, crushed funnel glass is
melted down under an elevated temperature (1200 1400 oC), and PbO contained in the funnel glass is
reduced to elemental lead by adding reducing agent
such as carbon. Heavier molten lead settles down
through the lighter molten glass phase, and they are
separated.
To melt the CRT funnel glass, a significant
amount of thermal energy is required, and this may
cause a high treatment cost. Molten lead has potential
to contain metals like gold (Au) and copper (Cu).
This implies that the reductive melting of CRT
funnel glass can be applied for recovering valuable
metals from metal containing waste such as printing
circuit board (PCB) in E-waste. In this case, lead
containing valuable metals could be recovered from
the process, and this may contribute to develop more
economical process.
In this study, distribution of several metals in
molten glass and lead in the reductive melting
process of CRT funnel glass was investigated by
using reagent metals instead of real PCB.
2. EXPERIMENTAL
2.1 Materials
Fig. 1 Schematic view of the CRT components
CRT funnel glass used in this study was
collected from crushing and grinding process in a
CRT monitor treatment facility in Japan. The particle
size was under 100 μm. Chemical composition of
glass analyzed by X-Ray Fluorescent (XRF) is
shown in Tab. 1 CRT glass contains about 25 wt% of
Resources Recycling 8
PbO. In the reductive melting experiments, Na2CO3
was added to reduce viscosity of molten glass, and
activated carbon were used for reducing agent.
Reagent Cu, Ni, Au, Ag and In2O3 were used as
metal sources.
Tab. 1 CRT glass chemical composition analyzed by
XRF
Component
Mass%
SiO2
45.9
PbO
25.2
K2O
8.9
Na2O
5.8
CaO
4.0
Al2O3
3.3
BaO
2.0
SrO
1.9
2.2 Reductive melting experiment
A 20 g of CRT funnel glass was mixed in
alumina crucible with 10 g of Na2CO3, and 3 g of
activated carbon. In the most of the expreriments,
0.2 g of the metal source (Cu, Ni, Au, Ag, or In2O3)
was also added. In a experiment, as a metal source,
0.2 g of Cu, Ni, Au, Ag, and In2O3 were mixed and
added. Alumina crucible with the mixture described
above were placed in an electric furnace.
Temperature profile applied in the reductive melting
experiments is shown Fig. 2,
Fig. 2 Temperature profile used in this study.
After the temperature of the sample became
room temperature, product was removed from
alumina crucible. Glass phase was separated from
elemental lead, crushed and ground in a mortar and
chemical composition of the glass phase was
analyzed by XRF. Elemental lead content in the glass
phase was determined by differential scanning
calorimetory (DSC).
3. RESULTS AND DISCUSSION
3.1 Lead removal from glass
In the reductive melting process, carbon (C)
is react with oxygen (O2) and carbon dioxide (CO2)
to produce mono carbon oxide (CO), according to
2C + O2 → 2CO
C + CO2 → 2CO
PbO contained in the glass is reduced to elemental
lead by CO as follows
PbO + CO → Pb + CO2
Density of metallic lead is higher than that of
glass, so reduced elemental lead settle to the bottom
of the alumina crucible as shown in Fig. 3 during the
melting process.
Elemental
lead
Fig. 3 Bottom view of a sample obtained after
reductive melting process without metal addition.
In the following part, the effect of metal
addition on the removal of Pb from CRT funnel glass
will be disscussed. The metals selected here are Cu,
Ni, Au, and Ag, which are valuable metals and
contained in PCB. In2O3 is also selected because it is
used as transparent electrodes in flat panel
display(FPD), and there is a possiblity that FPD may
mixed with CRT during the waste processing.
Tab. 2 shows lead removal determined from the
XRF data for the glass phase. When one of the
metals was added to CRT funnel glass, 94 - 97 wt%
of lead was removed from the glass phase, and these
values are almost the same as lead removal obtained
in the experiment witout metal addition. When 0.2 g
of Au, Ag, Cu, Ni, and In2O3 were added(total
amount of addition, 1.2g) only 74.3 wt% of lead was
removed. This result indicates that (i) larger amount
of metal addition or (ii) addition of various metals
suppress the lead removal.
Tab. 2 Effect of metal addition on the lead removal
in reductive melting method
Sample
Pb removal (wt%)
Without metals
94.3
With Au
94.7
With Ag
94.7
With Cu
95.9
With Ni
96.7
With In2O3
95.9
With Au, Ag, Cu, Ni,
74.3
and In2O3
For removing lead from CRT funnel glass, (1)
reduction of PbO to Pb, and (2) settling of Pb from
glass phase is needed. DSC analysis of glass residue
after reductive melting treatment indicates that Pb
remained in the glass residue was 0.17 wt% without
metal addition. However, Pb remained in the glass
Resources Recycling 9
residue was 2.03 wt% when Au, Ag, Cu, Ni, and
In2O3 were added. This suggest that even with the
addition of the metals the PbO reduction to Pb
occurs, but that the reduced Pb did not settle from the
glass phase.
Tab. 3 shows distribution of metals in lead. The
XRF data for glass phase shows that only small
amount of metal left in the glass phase, indicating
that significant amount of metals was distributed to
elemental lead. When 0.2 g of Au, Ag, Cu, or Ni
were added, more than 85 wt% of added metals were
distributed to elemental lead. In comparison with
these metals, distribution of Indium to elemantal lead
was lower (61.0 wt%). This may be due to
imcomplete reduction of Indium.
Fig. 3 shows the Ellingham diagram of selected
oxides (the relation between standard free energy of
formation (∆G°) and temperature). At 1200℃, Gibbs
free energy of In2O3 formation is almost same as that
of CO2 formation from CO, indicating that complete
reductiom of In2O3 by CO is difficult. Without the
reduction indium cannot be distributed to molten
elemental lead. Therefore the imcomplete reduction
of In2O3 may be the reason of low indium
distribution in elemental lead.
When Au, Ag, Cu, Ni, and In2O3 were added,
distributions of Ag and Ni to elemental lead were
85.5 wt% and 85.0 wt%, respectively, and these
valuses are lower than that obtained with single
addition of theses metals (93.5 wt% for Ag, 99.0
wt% for Ni). On the other hand, distribution of
Indium to elemental lead increased from 61 wt% to
75 wt%, when Au, Ag, Cu, Ni, and In2O3 were
added. In the presence of various metals and metal
oxide, complex redox reaction between them may
occur. For example, the oxidation of Ni to NiO
combined with the reduction of PbO to Pb may occur
because Gibbs free energy of NiO formation is more
negative than that of PbO (Fig.4). This kind of
complex redox reaction may cause the change in the
metal distributions and Pb removal, because the
formed metal oxide (ex.NiO) will affect the structure
of SiO2 network in the glass phase and its viscosity.
Further study is need to clarify the details of the
phenomenon.
Tab. 3 Metal recovered in lead in reductive melting method
Metal Recovery(%)
Sample
Au
Ag
Cu
Ni
In
With Au
100
With Ag
93.5
With Cu
87.0
With Ni
99.0
With In2O3
61.0
With Au, Ag, Cu, 100
85.5
88.0
85.0
75.0
Ni, and In2O3
∆G° (KJ/mol)
3.2 Distribution of Metals
Temperature (oC)
Fig. 4 Ellingham diagram of selected metals used in
this study.
5. CONCLUSIONS
The effects of the addition of various metal species
(Au, Ag, Cu, Ni, and In2O3) on the reductive melting
of CRT funnel glass were investigated. PbO
contained in CRT was reduced to elemental lead
even in the presence of the metals. Metals were
recovered together with elemental lead. More than
85% of Au, Ag, Cu, and Ni were distributed in the
elemental lead, while a significant amount of Indium
was remained in glass phase.
REFERENCES
[1] Japan Ministry of Economy, Trade and Industry,
Investigation of recycling systems of cathode
ray tube television sets for establishment of
recycling networks of cathode ray tube,(2009)
(in Japanese).
[2] F. Mear, P. Yot, M. Cambon and M. Ribes, The
characterization of waste cathode-ray tube glass,
Waste Manage, pp.1468-1476, Vol. 26 (2006).
[3] Inano, H. (2009), Pb Recovery from the Waste
CRT Glass by Reduction Melting Method,
Proceeding of the 6th International Symposium on
Environmentally Conscious Design and Inverse
Manufacturing (Eco Design 2009), Sapporo, Dec,
2009, on CD-ROM.
[4] A. Paul, Chemistry of Glasses, 2nd ed. (Chapman
and Hall, London 1982), Chap.7, pp.219-243.
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