Electronic Waste Treatment by High Enthalpy Plasma Jet
Aleksandar Mitrašinović1,2, Larry Pershin1, John Wen2and Javad Mostaghimi1
1) University of Toronto, Centre for Advanced Coating Technologies (CACT)
2) University of Waterloo, Laboratory for Emerging Energy Research (LEER)
Abstract: Plasma torches that utilize molecular gases contain high enthalpy and
promise to have practical promise in electronic waste (e-waste) treatment.
Introduction of a methane and carbon dioxide mixture (CH4/CO2) instead of the
traditional inert gasses created plasmas with high thermal conductivity resulting
in efficient heat transfer to treated materials generating high energy value
products.
Initial work is done on the plasma reactor characterization where the base lines of
main process parameters were established by monitoring reactor temperature
profile and offgas chemical composition. Chemical composition and temperature
of the offgas and solid residues during e-waste treatment were monitored
throughout the process and compared with the previously established base lines.
The high offgastemperature allows its usage for heating purposes instead of
disposal into the atmosphere. Beside significant reduction in the e-waste volume,
solid residues are enriched with valuable metals that previously could not be
harvested. In addition, compact and flexible design makes CH4/CO2 plasma
reactor portable unit that in turn diminishes requirement for transportation of the
treated material.
Keywords: plasma treatment, e-waste, metal recovery
1. Introduction
Plasma technology has recently been used for
treatment of solid waste. Solid residues after plasma
treatment are usually disposed in landfills in the
form of solid glass like material. Proper adjustment
of process parameters and utilization of plasma-fed
alternativesto traditional inert gases can make solid
residues easy to separate after the thermal plasma
treatment. The first step in recovering metals and
other valuable solid residues after the thermal
plasma treatment is proper temperature adjustment
in the reaction zone. For the highest yield of
produced off gases all organic material such as
plastics should be decomposed. However,
temperatures should not exceed the melting or
boiling points of valuable metals.
In this work a novel Thermal Plasma Treatment
(TPT) method for recovery of valuable metals is
explained.
Figure 1. Reactor chamber with highlighted thermocouples
positions.
1
The direct current (DC) plasma torch used in the
experiments was designed and built at the Centre for
Advanced Coating Technology, University of
Toronto. A graphite cathode has been used while the
plasma gases were CO2 and CH4 in a volume ratio 3
to 2. A data acquisition, with attached K type
thermocouples measured characteristic temperatures
inside the lab scale batch type plasma reactor. The
offgas sample was collected from the reactor offgas
fume hood using a Hamilton S0500 Syringe. Later,
the gas sample was injected into the argon flow
(100cc/min) and the concentrations of the CO, CO2
and O2 outlet gases were compared by ABB EL3020
gas analyzers. Torch parameters during experiments
were as current 300A, Voltage 135V, Gas flow
30l/min while thermal efficiency was 65% in all
cases.
the reactor, organic compounds are gasified and
burned resulting in a temperature increase as shown
in Figure 3. The highest temperature depends on the
amount of the charged material, the enthalpy
potential of the charged material and the feed rate.
After material flow is stopped, the reactor
temperature
drops
until
pyrolysis/oxidation
processes are finished and the reactor chamber
reaches steady state again. Once the plasma torch is
shut down (point F), the reactor temperature rapidly
decreases to room temperature.
800
Base
700
B
OffGas
C
E
600
Temperature, (oC)
2. Experimental
D
500
400
300
200
A
100
3. Results and Discussion
Light Optical Microscopy images revealed similar
thicknesses of the Cu layer in samples before and
after thermal plasma treatment. Such outcomes
indicate minimal losses of Cu during the proposed
process (Figure 2).
0
-60
0
60
120
180
240
300
360
420
480
540
Time, (s)
Figure 3: Thermal Analysis curves and characteristic
temperatures during e-waste treatment. A – torch ignition; B –
DAQ start; C – lid opening; D – e-waste charging and E –
plasma shutoff.
Reactor chamber temperatures for tests run with the
empty reactor under 30kW and 36kW torch power
are given in Table 1. Results indicated asymmetric
temperature gradient in the reaction zone. However,
the achieved temperatures were high enough to
initiate decomposition of the organic components in
e-waste material.
Figure 2: Cross section of the circuit board before and after
thermal plasma treatment.
Online Temperature Control during TPT
A data acquisition system (DAQ) with K-type
thermocouples measured temperatures inside reactor
throughout the process. The characteristic thermal
analysis curve for the plasma process is shown in
Figure 3. The initial period (Figure 3, A-C) is
characterized by the reactor chamber and system
parameters’ stabilization. When material is fed into
Table 1: Characteristic temperatures inside empty reactor
chamber for different torch power.Temperatures are in oC.
Lid Position
Open
Closed
Distance, cm
30
50
30
50
TC1
430 385
690 500
TC2
510 535
785 615
TC3
505 425
700 595
TC4
365 440
725 670
TC5
115
65
695 655
TC6
530 450
730
n/a
TC7
430 385
690 500
*TC7 were at the base below circuit board and below TC1
2
Obtained results for empty reactor can be compared
with tests run while circuit boards were fed inside
reactor chamber. Table 2 summarizes characteristic
temperatures for the tests run with circuit boards
under 30kW torch power.
Table 2: Maximum temperatures inside reactor chamber during
e-waste plasma treatment. Temperatures are in oC.
Lid Position
Distance, cm
TC1
TC2
TC3
TC4
TC5
TC6
TC7
Open
30
50
505 840
420 715
455 670
200 655
130 290
420 790
505 745
Closed
30
50
470 770
330 435
400 550
285 620
100
95
400 705
470 800
Addition of the circuit board material causes an
increase in temperature which is shown in Figure 4.
The higher amount of charged material produces
higher temperature peak and longer for reactor to
return in steady state. However, the area below the
peak is proportional to amount of material added.
measurements showed that the oxygen content in the
reactor fume hood in all instances did not exceed
10.2vol% (Table 3).
Table 3: Offgas compositions in plasma reactor.
CO,
ppm/sec
CO2,
ppm/sec
O2,
vol%
Distance,
cm
30
50
30
50
30
50
no ewaste
3.6
1.0
3600
4200
10.2
8.3
with ewaste
102
87
2200
3900
9.5
7.9
Ratio
28.3
87.0
0.61
0.93
0.93
0.95
The amount of CO2 and CO for different power and
distances from the torch nozzle are given in Table 3.
The concentration of CO2 is elevated in tests at a
50cm distance between the torch and the base,
probably due to more time that the gas spends in the
chamber that increase the possibility to react with
oxygen intake from the air. The CO in plasma gas is
beneficial for the overall process since CO may
prevent formation of the oxides by reacting with
entrained oxygen. CO is only found in traces in
offgas generated with an empty reactor. However,
the amount of CO in the offgas during plasma
treatment of the circuit boards was 50-100 times
higher. Such results indicate formation of CO due to
chemical reactions between the plasma gas and
circuit board components.
Weight and Compositional Analyses
Figure 4: Temperature peak for 30cm distance between torch
and reaction zone during addition of circuit boards into the
reactor.
Gas Composition during e-waste TPT
Chemical reactions during plasma processes are
predominantly considered pyrolysis based. However,
plasma processes are always accompanied by
entrainment of air into the reactor chamber unless
the system is vacuum sealed. Since carbon is the
cathode material for low power plasma treatment, it
is important to know the oxygen content of the
plasma gas mixtures. The results of the
Overall weight change of the circuit board samples
before and after TPT is given in Table 4. The weight
loss is mainly due to decomposition and
volatilization of the carbon containing components.
High decrease in weight indicates low oxide
formation during the TPT.
Table 4: Sample weight change during TPT for different basetorch distances.
Before TPT
Weight, g
Change, %
9.56±0.07
A f t er T P T
30cm
50cm
8.18±0.66 8.56±0.30
14.4
11.5
Thickness of the Cu foil after the treatment is
measured by image analysis technique (Table 5).
3
A f t er T P T
Before TPT
30cm
50cm
Centre 51.5±0.79 47.3±2.62 48.1±2.06
Edge
51.5±0.79 30.3±5.91 37.1±4.66
SEM analyses revealed local melting and softening
of fiberglass and binder although recorded
temperatures were well below melting point of
fiberglass. Such results indicate a high exothermic
reaction of the e-waste components that further ease
separation of the metallic parts, and fine grinding of
the formed oxide residues. However, quantitative
analyses of the reactions exothermicity, enthalpy
values and consequently total process energy and
mass balance required further investigation.
Fiberglass
Table 5: Cu thickness during the treatment for different sample
sections for different base-torch distances.
Table 6: Concentrations of selected elements infiberglass and
binder layers after the treatment of the circuit board (at%).
Binder
Results revealed that Cu losses in the middle of the
sample were negligible. Closer to the edge, The Cu
foil was ripped perpendicular to the plasma jet
direction due to oxidation and thermal stresses
during TPT.
Layer C
O
Si Al
st
1
25.7 51.5 13.1 3.9
Ca
4.9
Br
n/a
F
n/a
2nd
24.9 51.5 13.2 4.2
5.2
n/a
n/a
3
rd
25.1 51.9 13.4 4.0
4.7
n/a
n/a
1st
27.1 49.7 8.8
3.6 10.0 n/a
n/a
nd
23.6 50.7 10.3 4.3 10.0 n/a
n/a
rd
31.9 44.0 9.1
1.5
2
3
n/a
9.2
3.6
3. Conclusions
The achieved temperatures in the reaction zone
during thermal plasma treatment allow for complete
decomposition of organic material while valuable
metals remained unaffected after the thermal plasma
treatment.
Figure 6: SEM micrographs of the circuit board sample before
and after plasma treatment. Thicker layers are fiberglass while
thinner and brighter layers are areas where epoxy binder was
sited.Sample was carbon coated.
Table 6 shows concentrations of key elements,
acquired by EDS technique, in different layers of
circuit board perpendicular to the plasma jet. Layer 1
wasat the edge while layer 3 was closer to the
middle of the sample. Analysis made in bright fields
revealed only pure Cu.
4