Content uploaded by Valdek Mikli
Author content
All content in this area was uploaded by Valdek Mikli on Oct 13, 2016
Content may be subject to copyright.
MECHANICAL RECYCLING OF ELECTRONIC WASTES FOR
MATERIALS RECOVERY
Viktor Laurmaa
1
,Jaan Kers
1
, Kaspar Tall
1
, Valdek Mikli
1
, Dmitri Goljandin
1
, Kristiina
Vilsaar
1
, Priidu Peetsalu
1
, Mart Saarna
1
, Riho Tarbe
1
, Lifeng Zhang
2
1
Tallinn University of Technology, Tallinn, Estonia
Ehitajate tee 5, Tallinn 19086, Estonia
Email: jaan.kers@ttu.ee
2
Missouri University of Science and Technology (Missouri S&T)
223 McNutt Hall, Rolla, MO, 65409-0330
Email: zhanglife@mst.edu
Keywords: Printed Circuit Boards, Mechanical Recycling, Material Recovery
Abstract
In this paper, the mechanical milling of the Printed Circuit Boards (PCB) was carried out.
First goal of the work was to examine the management of the WEEE, in particular the re-use
of PCB. Firstly, recycling methods of PCBs were summarized. During the study mechanical
separation methods (magnetic-, density- and air separaton), electrical, and chemical methods
were examined. Secondly the optimal particle size for air-classification was determined.
Several tests were carried out to find the most effective separation method for separation of
different material groups from PCB scrap. The new air classification stand was developed for
testing the separation of lightweight particles like tinfoil stripes and plastics. The test results
showed sufficiently good separation of heavier Al and Cu. For milled materials
characterization the SA, IA, laser diffraction analysis and SEM were used. The chemical
composition of the PCB powders was studied by means of energy dispersive X-ray
microanalysis (EDS).
Introduction
Technological innovation and intense marketing is accelerating the update rate of electric and
electronic equipment (EEE) and shorten the average lifespan. As a result, the amounts of
wastes of electric and electronic equipment (WEEE) containing 3% of printed circuit boards
(PCB) are dramatically increasing. The UN Environment Program estimates that the world
generates 20–50 million tones of WEEE each year and amounts are rising three times faster
than other forms of municipal waste [1]. The typical composition of PCB is non-metals
(plastics, thermosets, glass fibre, ceramics) >70%, copper ~16%, solder ~4%, iron, ferrite
~3%, nickel ~2%, silver 0.05%, gold 0.03%, palladium 0.01%, others (bismuth, antimony,
tantalum, etc.) <0.01% [2] Significant quantities of nonmetallic materials in PCBs (up to 70
wt.%) present an especially difficult challenge for recycling [4]. The nonmetallic materials of
PCBs mainly consist of thermosetting plastics (TS), thermoplastics (TP), glass fibers and
ceramic fractions. Thermosets cannot be remelted or reformed because of their cross-linked
polymeric structure. Incineration is not the best method for treating nonmetallic materials
because of the presence of inorganic fillers such as glass fiber, which significantly reduces the
fuel efficiency [2]. Disposal in landfill is the main method for treating non-metallic materials
of PCBs, but it may cause secondary pollution and resource-wasting [3]. Since the metallic
elements are covered with or encapsulated by various plastic or ceramic materials on printed
circuit boards, a mechanical pre-treatment process allowing their liberation and separation is
3
Recycling of Electronic Waste II, Proceedings of the Second Symposium
Edited by: Lifeng Zhang and Gregory K. Krumdick
TMS (The Minerals, Metals & Materials Society), 2011
first needed in order to facilitate their efficient extraction with acid or alkali by
hydrometallurgical methods [4]. Electronic scrap from printed circuit board can be processed
by mechanical methods like stamp, hammer or cutting mill [4, 5, 6]. Particle size, shape and
liberation degree play crucial roles in mechanical recycling processes [7]. Almost all
mechanical recycling processes have a certain effective size range for material separation [5-
8].
Some recent research has been done for recovery of nonmetallic fraction of powderized PCBs
in molding electronic components [7], phenolic compounds [9] and as a substitute for wood
fillers in thermoplastic matrix composites [10]. In [11] the recycled epoxy resin powder from
electronic industry was used as filler for epoxy resin products, such as paints, adhesives,
decorating agents, and building materials, and this improved the mechanical and thermal
expansion properties of the products when compared to the usual fillers (talc, calcium
carbonate). The amount of PCB nonmetals used in these products is varying from 10 to 40 wt.
%, so these applications will not promote large scale recycling.
It has been indicated that the non-metallic fraction of PCB powder has following advantages:
is lighter than mineral fillers for the composites; has finer granularity which makes the
microstructure more reliable, and contains coarse glass fibers which improve mechanical
strength of the composite material [12]. These superior properties of the non-metallic fraction
of PCB powder can be used for development of new composite materials for engineering
applications. A number of these applications are concentrated on tribological components,
such as gears, cams, bearings, sliders and seals, where the self-lubrication of polymers is of
special advantage [13]. In particular, short fiber reinforcements, such as carbon, glass and
aramid fibers, have been successfully used to improve the strength and therefore the load
carrying capacity of polymer composites subjected to various wear modes [13, 14].
The value of metals contained in PCB scrap is economic incentives for the recyclers.
Recyclers use different methods (pyro- and hydrometallurgy) to reclaim metals with high
purity, which can be sold at a high price. However the remarkable amount of (up to 70 wt. %)
nonmetallic materials are generated inevitably, which are not recovered because of lack of
proper materials separation and product characterization methodology. Mechanical recycling
and air separation technology enables to recover both metallic and non-metallic fractions of
milled PCB powders.
Materials and Methods
Preparation of PCB powders for classifying and materials separation
The fracture of particles in collision with the milling component of one of the rotating rotors
is called disintegration. The theoretical studies on milling by the collision method, which were
conducted at Tallinn University of Technology (TUT), were followed by the development of
the appropriate devices, called disintegrators [15, 16]. Depending on the design of the
disintegrator systems the direct, separative and selective types of milling are available and
useful in powder production. Milling by collision means that the mechanisms of the particle
size reduction of the ductile and brittle materials are different. The milling of brittle materials
by collision results in a direct fracture [17]. During milling of ductile metallic materials, the
metal will be hardened and the fatigue fracture will occur [18].
The Disintegrator Technology Laboratory of TUT has several disintegrator milling equipment
(preliminary crushing, continuous milling and final milling) for production powders in
different size from 1-2 mm into powders 5-10 micron. This laboratory equipment was used
with pre- and after processing of powders – sieving into fractions, analyzing and measuring
the particle size distribution and shape. For size reduction of the PCB scrap the different
disintegrator mills (DSA-158, DSA-2, DSL-115, DSL-160 were used in direct and selective
4
milling conditions to prepare the powders for classification into fractions. For the air
separation the developed inertial-centrifugal air-classification stand was used (see Fig.1). For
the magnetic separation of ferrous fraction the several types of magnets were used.
The fraction sizes of high-energy milled PCB powders were validated to obtain the best air-
separation results of metallic and organic fraction.
Fig.1. Principal scheme of air separation stand with 5 separation pockets
Characterization of the milled PCB powders
Following the milling and separation processes (air, magnetic) of heavy fractions (metallic
materials) and light fractions (organic an inorganic materials) particle size and shape, powder
morphology properties were identified.
- For milled and air-separated materials characterization, sieving analysis (particle size
more than 100 µm), laser diffraction particle-size distribution analyzer (Analyzette 22
Compact) and the stereomicroscope Zeiss Discovery V20 with image analysis
software Omnimet and scanning electron microscope (SEM) JEOLJSM-840A was
used. For evaluation of chemical composition of metallic materials the
materialographic devices for preparation micropoliches (Struers, Bühler sample
preparation system for cutting, encapsulation and poliching), the energy dispersive X-
ray microanalysis (EDS) with the Link Analytical AN10000 system were used. The X-
ray mapping technique will be used to evaluate element (metallic, nonmetallic)
distribution inside powder particles.
Experimental
PCB scrap was recycled by using mechanical methods. The reprocessing technology of the
PCBs by disintegrator milling devices consisted of the following stages:
- the preliminary size reduction of the PCB plates by the experimental DSL-158
disintegrator (up to 2 times);
- the intermediate milling for the size reduction in the semi-industrial disintegrator
DSA-2 (up to 8 times);
- the final milling by the DSL-115 disintegrator system
After intermediate milling the milled product was classified by sieving with vibration into
three different fractions d
50
>10mm and 2 < d
50
<10mm and fraction d
50
< 2mm.
5
Results and Discussion
Size reduction of the PCB powders
The results of the preliminary size reduction, intermediate and final milling are given in Fig 1.
The medium particle size of the plastic component from PCB after a 2-stage milling is about
5-10 mm, after 1-2 times of milling in the disintegrator DSA-2 it is around 1 mm. The
subsequent continuous milling (6 times) in DSA-2 reduced the medium particle size to 0.45
mm. As the medium particle size and mass distribution were similar after 6th and 8th times of
milling in DSA-2, the new equipment DSL-115 for further size reduction was used. Next
remarkable size reduction occurred after the 4 times milling in DSL-115 the medium particle
size was 0.12mm.
The powder particles from PCB after the preliminary size reduction were mainly lamellar
after preliminary milling and they stay lamellar after the multi stages milling (up to 8 times).
The mechanism of the fracture of PCB particles was the same after preliminary and final
milling.
Fig. 2. Dependence of the particles medium size of PCBs on the specific energy of treatment
The particle size and distribution of the fine material (70 wt. %) obtained from 8
th
milling in
DSL-115 (0-0.3 mm) was determinated by Laser Granulometry (see Fig. 2). The arithmetic
mean diameter of the particle is 74 µm (see Fig.2).
Air separation of metallic and organic fractions
The air separation of milled PCB powder fraction d
50
>10mm was not effective due to the
large diversion of fraction size and high content of metallic inserts in organic fraction. During
the first separation the light-fraction of tin-foil stripes were collected in fifth pocket of the air
separation stand. The following separation tasks were performed with material collected from
the first pocket of the air-separation stand. After the two times air separation the metallic and
organic materials were still mixed in the first and second pockets of the stand.
The air separation of milled PCB powder fraction 2mm<d
50
<10mm was also not so effective
due to the large diversion of fraction size and particles shape. Most obstructing were the Cu-
wires with 10-35mm length. During the first separation the light-fraction of tin-foil stripes
were collected in fifth pocket of the air separation stand. The following separation tasks were
0.1
1
10
100
0.1 0.4 5.2 10.0 14.8
Specific energy of treatment E
s
, kWh/T
Medium size d50, mm
6
performed with material collected from the first pocket of the air-separation stand. After the
three times air separation the colour of the metallic fraction collected into first pocket was
turning to similar to light-red colour of Cu, but still containing the organic particles with black
colour. The second and third pockets of the stand contained mostly organic particles, so for
the last step the separation of metals was quite effective.
The best air separation results gave the milled PCB powder < 2mm. After the first separation
he colour of the metallic fraction collected into first pocket was turning to similar to light-red
colour of Cu. The light-fraction of tin-foil stripes were collected mostly in pockets 3-5.
The colour of the material in the first pocket was similar to the Cu and only few plack and
grey organic particles were detected. After the three times air separation the separated
metallic and organic materials were weighed. The obtained powder (See Fig. 3.) had 80 wt.%
of metallic content and 20 wt.% of organic materials.
Fig. 3. Results of air separation of fine fraction d
50
< 2mm
The air separaton results of the milled PCB powder are promising. In order to reduce the
manual labour and energy consumption the air separation tests should be continued. the 5-
pocket air separation stand should be re-designed to obtain two fractions of organic and
metallic fractions with one processing step.
I separation
II separation
III separation
7
EDS study of PCB milled powders
The sample of the PCB milled powder was separated into plastic and metallic fractions and
weighed. The powder contained 29 wt.% of metallic content. The micro polish of the non-
metallic fraction sample was made for further EDS analysis. Then the X-ray microanalysis
was performed. Oxygen was calculated by the difference of 100% with the results given in
weight percentages. Most of the plastic particles contained different metallic crystals or grains
inside the matrix.Samples of the separated plastic particles are in Fig 4 and prepared probes
for EDS analysis are in Fig 5.
Fig 4. Separated organic fractions Fig 5. Probes of material for EDS analysis
In Fig. 6 PCB powder particle with red plastic matrix and 5-10 µm BrO4 crystals inside is
presented. In Fig. 7 PCB powder particle with
black plastic matrix containing Al, Si, Ca and fibers
is presented.
Plastic particles reinforced with fibres contained mainly carbon (60%) and oxygen
(35%) and metals (5%). As it can be seen the plastic particles from PCB powder are
composite materials containing different type of filler and fibrous reinforcement materials in
polymer matrix.
Fig. 7. Black plastic matrix with fibers Fig. 8. Red plastic matrix with 5-10 µm
BrO4 crystals
EDS analysis showed that most of the separated organic particles are composite materials,
with plastic matrix and metallic, ceramic or fibre reinforcements (see Table I).
8
Table I. The composition of the milled PCB powder particles
Object No.
Composition, wt %
Description
11
19
23
25
32
41
61
Ca-38; Mg-0.4; O-61.4
Al-33; O-67
Si-45; O-55
Al-7; Si-24; Ca-15; Ti-0.4; O-53.6
Cu-65; Zn-35
Sn-84; Pb-15,8; Al-0,2
Cu-98
Pure Al
Green plastic matrix , 5-10 µm CaCO3 crystals
inside
Blue plastic matrix,10-100 µm Al particles inside
Black plastic matrix, 10-100 µm SiO
2
grains inside
Black plastic matrix with Al-7; Si-24;Ca-15 fibres
CuZn35 brass, on the edge Sn-90; Pb-10
Sn80-Pb20 solder
20 µm thick Cu stripe with white plastic particle
5-10 µm thick Al stripe
The precious metals like Au, Ag and Pt were not detected in milled PCB powders. The milled
PCB powders consisted in larger amounts of non-metallic elements C, O Cl. S followed by
metals (Cu, Al, Zn, Fe, Sn). In minor amounts alkali earth metals (Mg, Ca, Ba), alkali metals
(K, Na) and non-metallic elements (Br, P, F, S) were detected. EDS analysis is useful for
determining the elementary consistence of milled PCB powders.
Summary
The developed air separation stand is useful for separation of smaller than 2mm fractions of
the metallic and organic particles. These tests are good basis for determining the optimum
milling parameters and designing of new air or classifiers accounting for the densities of
plastic and metallic particles.
The study of the chemical composition of the PCB powder particles showed that organic
particles (polymers) are having metallic grains or crystals in the matrix and because of that
they are difficult to be separated by air-classification system. EDS analysis is useful
determining the elementary consistence of the selected particles, but for analyzing the
powdered material consistence other methods should be considered. One of the methods to be
considered is image analysis (IA) technique. IA enables to analyze the shape and the colour of
the particles to estimate separation effectiveness by calculating the surface area of the metallic
and organic materials.
The developed PCB powder milling, separation technologies and powder characterization
methods are economical by enabling the competitive solutions in practice. The recycling
companies can recover the organic materials into new products, by reducing their waste
management costs. The experimentally obtained data of this study will be the input for
numerical modeling of the particle filled composite properties to determine the influence of
the filler content, resin properties and etc. to the properties of composite material.
The further experimental studies of PCB organic product recovery in thermosetting resin
matrix composite will provide feasible solution in future industrial applications to use the
developed material in demanding conditions. This development is the significant win for
WEEE recycling companies and for environment by reducing amount of hazardous wastes in
sent to landfill.
Acknowledgements
This current research is supported by Estonian Science Foundation grantprojects ETF7705
and ETF8531
9
References
1. M. Burke, The gadget scrap heap, Chemistry World, 4, 6 (2007), 45–48.
2. M. Goosey, R. Kellner, Recycling technologies for the treatment of end of life printed
circuit boards (PCB), Circuit World 29 (3) (2003), 33–37
3. J. Guo, J.Y. Guo, B. Cao, Y. Tang and Z. Xu, Manufacturing process of reproduction
plate by nonmetallic materials reclaimed from pulverized printed circuit boards, Journal
of Hazardous, Materials, 163 (2009), 1019–1025.
4. J.-M. Yoo, J. Jeong, K. Yoo, J. Lee and W. Kim, Enrichment of the metallic
components from waste printed circuit boards by a mechanical separation process
using a stamp mill, Waste Manage. 29 (2009), 1132-1137
5. H.M. Veit, C.C. Pereira, A.M. Bernardes, Using mechanical processing in recycling
printed wiring boards. Journal of the Minerals and Materials Society, 54, 6, (2002), 45–
47.
6. C. Eswaraiah, T. Kavitha, S. Vidyasagar, S. Narayanan, Classification of metals and
plastics from printed circuit boards (PCBs) using air classifier, Chemical Engineering
Processes, 47 (2008), 565–576.
7. J. Cui, E. Forssberg, Mechanical recycling of waste electric and electronic equipment: a
review, Journal of Hazardous Materials, 99 (2003), 243–263
8. L. Zhang, X. Zuo, J. Kers, D. Goljandin, P. Peetsalu, Mechanical and Pyrometallurgical
Recycling of Electronic Wastes, REWAS 2008 (Global Symposium on Recycling,
Waste Treatment and Clean Technology), Cancun, Mehhiko, 12-15.10.2008. (Eds.) B.
Mishra, C. Ludwig, S. Das. Mexico, The Minerals, Metals and Materials Society,
(2008), 699 - 708.
9. J. Guo, Q. Rao, Z. Xu, Application glass-nonmetals of waste printed circuit boards to
produce phenolic moulding compound, Journal of Hazardous Materials, 153, (2008),
728–734.
10. Y. Zheng, Z. Shen, C. Cai, S. Ma, Y. Xing, The reuse of nonmetals recycled from waste
printed circuit boards as reinforcing fillers in the polypropylene composites, Journal of
Hazardouz Materials, 163 (2009), 600–606.
11. M. Iji, Recycling of epoxy resin compounds for moulding electronic components,
Journal of Materials Sci. 33 (1998) 45–53.
12. J. Guo, J. Guo, Z. Xu, Recycling of non-metallic fractions from waste printed circuit
boards: A review, Journal of Hazardous Materials, 168, 2-3, (2009), 567-590
13. I.Crivelli Visconti, A. Langella and M. Durante, The wear behaviour of composite
materials with epoxy matrix filled with hard powder, Applied Composite Materials, 8,
(2001), 179–189
14. L. Chang, Z. Zhang, L. Ye, K. Friedrich, Synergistic effects of nanoparticles and
traditional tribo-fillers on sliding wear of polymeric hybrid composites, Tribology of
Polymeric Nanocomposites, (Eds.) K. Friedrich and A. K. Schlarb, 55, (2008) 35-58
15. B. Tamm, A. Tümanok, “Impact grinding and disintegrators”, Proceedings of the
Estonian Academy of Sciences, Engineering, 2, 2, (1996), 209-223
16. A. Tümanok, P. Kulu, Treatment of Different Materials by Disintegrator Systems,
Proceedings of Estonian Academy of Sciences, Engineering, 5, (1999), 222 – 242.
17. S. Zimakov, D. Goljandin, P. Peetsalu, P. Kulu, Metallic powders produced by the
disintegrator technology. International Journal of Materials and Product Technology,
28, 3/4, (2007) 226 - 251.
18. P. Kulu, R. Tarbe, H. Käerdi, D. Goljandin, Abrasivity and grindability study of
mineral ores. Wear, 267(11), (2009), 1832 – 1837
10
