15 Environmental Concerns in PCB Industry
15.6 Recycling of Printed Circuit Boards
In recent years, there has been an increasing concern about the growing volume of end-of-life electronics. The problem is already acute in developed countries and is likely to become so in the developing countries in the very near future. For example, the European Commission (2001) estimates that electronics contribute 4 per cent of the municipal waste stream, and it is growing at the rate of 3 to 5 per cent per annum, three times the rate of growth of other wastes. Over 90 per cent of this waste is land-filled. Four hundred million cell phones are produced a year, and perhaps 500 million personal computers (PCs) will become obsolete by 2007.
In Europe, the precedent for product take-back and recycling has been established with the ELV (vehicle end-of-life) directive, which places the responsibility for disposal firmly on the importer or manufacturer. The directive also sets restrictions on a range of hazardous materials in manufacturing, a list that is being reportedly revised to include electronics assemblies (Rae, 2003). The WEEE (Waste Electrical and Electronic Equipment) directive and RoHS (Reduction of Hazardous Substances), adopted on May 15, 2001, follow this pattern, setting recycling targets and limiting lead, mercury, cadmium, hexavalent chromium and some brominated flame retardants.
Some legislation at the national levels is already in place. Sweden’s recycling law was enacted in 2001. Japan’s electronic appliance recycling law, enacted in April 2001, covers larger domestic appliances and is expected to be extended progressively to computers and other areas. A user fee of
$30 is charged, and recycling plants have been set up by major electronics manufacturers. The awareness about environmental protection is fast developing in India also and the day is not far when a similar legislation would be introduced.
In the light of an eventual take-back legislation for electronic products, which is expected to come into force in the industrialized countries the world over, disposal processes for electronic products are obviously expected to receive special attention (Legarth, et al., 1995). Electronic products are often defined as complex products with a content of printed boards. PCBs today constitute an environmentally problematic fraction in disposal, on the one hand. On the other hand, PCBs contain most of the elements in the periodic table and may thus be seen as a source of some rare and valuable resources.
15.6.1 Present Approach to PCB Scrap Disposal
The PCB scrap is generated at various sources such as PCB manufacturers, OEMs (Original Equipment Manufacturers), individuals, corporate and equipment dismantlers. The scrap from these sources can be directly sent for recycling, recovery operations or for land-fill. On an average, about 85 per cent of all the PCB scrap board waste is subject to land-fill and only 15 per cent is currently subjected to any form of recycling, (Goosey and kellner; 2002).
Scrap PCBs can be categorized into three grades depending upon the inherent precious metal content. These are:
a Low Grade Material: This comprises power supply units and television boards having ferrite transformers, large aluminium heat sink assemblies and laminate offcuts.
a Medium Grade Scrap: This contains precious metal content, generally from pin and edge connectors used in high reliability equipment.
a High Grade Material: This comprises high precious metal content boards, gold containing integrated circuits, discrete components, opto-electronic devices, gold and palladium pin boards, etc.
These grading materials help to determine the economics of applying recovery operations.
However, it is possible to regrade the material from low to medium category through selective manual disassembly of high percentage mass ferrous and aluminium components.
Recycling involves the disassembly of scrap PCBs followed by sorting, grading and shredding operations. Iron and aluminium metals are removed from the final ground product by using magnetic and eddy current separation. The output from the recycler is either sent for land-fill or to a smelter.
However, only those boards which contain sufficient gold or precious metal content are subject to smelting, otherwise all non-precious metal bearing board scrap is consigned to land-fill. About one per cent of the scrap is subject to specialized recycling operations solely for the purpose of precious metal recovery. Figure 15.10 shows a general scheme for PCB scrap disposal /treatment.
PCBs
Recycling Disassembly
Useful components
Sort
Grade
Shred Secondary metals
iron, alumininum ferrite
Smelter
Land fill
Residue for destruction
Recovered metal
Fig. 15.10 PCB scrap disposal/treatment methodology (redrawn after Goosy and Kellnr, 2002)
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15.6.2 Characteristics of PCB Scrap
PCB scrap, particularly populated scrap board, is highly heterogeneous and complex. These boards contain diverse levels of inorganics with relatively low levels of precious metals in conjunction with copper, solders, various alloy compositions, and non-ferrous and ferrous metals (Feldman and Scheller, 1994). The materials and components present in the scrap PCBs have widely differing intrinsic physical and chemical properties. The recycling techniques primarily depend upon various characteristics, which are detailed below.
15.6.2.1 Density Differences
The typical values of the specific gravity of materials contained within scrap PCBs are:
Materials Specific Gravity range (g/cm3) Gold, Platinum group, tungsten 19.3–21.4
Lead, silver, molybdenum 10.2–22.3
Magnesium, aluminium, titanium 1.7–4.5
Copper, nickel, iron, zinc 7.0–9.0
Non-metallic Materials 1.8–2.0
It is evident that various materials can be separated by density-based separation systems, normally employed in the process industry (Barsky, et al., 1991).
15.6.2.2 Magnetic and Electrical Conductivity Differences
The application of low intensity magnetic separators is well developed in the minerals processing industry and can be used to separate ferrous materials from PCB scrap. On the other hand, non- ferrous metals may be separated by means of electrostatic and eddy current separators which are well developed within the recycling industry (Iji and Yokoyama, 1997).
15.6.3 Disassembly of Equipment
In most of the operations for the recycling of printed circuit boards, disassembly is an essential part.
The disassembly of components facilitates a selective and profitable recovery of metals and noble metals. In addition, the concentration of valuable materials could reduce the re-processing cost.
Another advantage of disassembly is the isolation of hazardous components in order to prevent them from contaminating the shredded waste. Electronic components can be re-used after disassembly for economic and ecological reasons. However, the cost of disassembling, testing and selling the re- usable electronic components has to be seen in relation to the cost of a new product (Keimeier, 1994).
The disassembly of scrap is mostly carried out manually by using simple hand tools, which itself sometimes places limits on all such operations due to the costs involved. Disassembly is considered to be an area of great importance in case of recovery of low cost components and is considered to have a great impact on the overall future recycling strategies. In order to ensure safety during
disassembly and take into consideration the cost factors, mechanical dismantling, and automated and robotic dismantling techniques have been practised (Feldmann and Scheller, 1994). The automated component disassembly operation basically involves scanning the board to read all component identification data; reading stored component database to determine their value;
determining how the identified components are soldered or mounted; if mounted, disassembling is done via robot and if soldered, de-soldering is done by using laser or infra-red energy.
Yokoyama and IJi (1995) describe a recycling process of printed circuit boards with electronic components, which is shown in Figure 15.11. A practical process for pulverizing the PCB waste and separating the resulting powder into copper rich powder and glass fibre-resin powder was developed.
With this process, up to 94 per cent of the copper was recovered in the pulverized PCBs of 100-300 microns average particle size. The recovered glass fibre-resin powder improved the mechanical strength and thermal expansion properties for epoxy resin type paints and adhesives. For the next step, the recycling of PCBs with electronic components by disassembling the components from the PCB, and by applying the PCB waste recycling process to the remaining board is being investigated.
PCB with electronic components
Disassembling
Pulverizing and separating
Electronic components Re-material
Glass fiber-resin powder Copper rich powder
Used as a fillerfor polymer products Re-material
Fig. 15.11 Recycling process of a PCB with electronic components (after Yokoyama and Iji, 1995)
15.6.4 Technologies of Recycling of PCBs
Two approaches are emerging as the potential techniques for separation of materials in the recycling process. These are based on mechanical and hydrometallurgical methods.
15.6.4.1 Mechanical Methods
Mechanical systems for the treatment of a wide range of electronic scrap materials including populated and non-populated PCBs are commercially available. A practical process for pulverizing PCB waste and separating the resulting powder into copper rich powder and powder consisting of glass fibre and resin is described by Yokoyama and Iji (1995). The process, which involves pulverizing and separating, is shown in Figure 15.12.
Environmental Concerns in PCB Industry 627
PCB waste
Pulverizing Process
Glass fiber-resin powder Copper rich powder
Used as a fillerfor polymer products Re-material
Crushing step
Fine-pulverizing step
Gravity separation step
Electrostatic separation step Separating process
Airvortex type
Compressive and shearing type Cutting and shearing type
Fig. 15.12 Pulverizing and separating Process for the PCB wastes (adapted from Yokoyama and Iji, 1995)
In this process, the PCB waste is pulverized in a process consisting of a crushing step which uses cutting and shearing forces, and a fine pulverizing step, which uses compressive and shearing forces.
This pulverizing process is highly effective, and is shown to have very high abrasion resistivity. The copper rich powder and the glass fibre-resin powder are recovered with a separating process consisting of a gravity separating step using an air vortex, and an electrostatic separation step. The effective average particle size for the pulverized waste was found to range from 100 to 300 mm. Up to 94 per cent of the copper was recovered from the PCB waste at this size.
While the copper content in the PCB waste was 7 per cent, the content in the copper rich powder obtained from the gravity separation step was more than 20 per cent. Figure 15.13 shows the size distributions of each component.
103 102
1001 50 100
Resin
Glass Fiber
Copper
Particle size [ m]m
Cumulativeweight[wt%]
Fig. 15.13 Size distribution of pulverized PCB components (redrawn after Yokoyama and Iji, 1995)
15.6.4.2 Hydrometallurgical Methods
A number of hydrometallurgical methods have been developed which indicate the potential for the recovery of all materials from the PCB scrap. One such method is the recovery of gold from pins and edge connectors, which have been manually separated from the scrap board via the use of air knives, etc. In USA, a methodology based on solvolysis has been developed to enable both the more efficient recovery of metals and the recovery of plastic materials such as epoxides at high quality, with the additional benefit of the capability to extract both halogens and brominated hydrocarbon derivatives. (www.recyclers-info.de/de/bigat/prasengl.htm).
In addition, various studies have been undertaken to assess the viability of utilizing dilute mineral acids in conjunction with subsequent metal recovery techniques based on concentration and separation, such as solvent extraction, ion exchange, adsorption and cementation (Saito,1994).