Mines of the Future: Electronic Waste

Electronic waste consists of devices containing electrical and electronic equipment such as televisions, telephones, printers and computer components, refrigerators, and dryers. Today, thanks to population growth and advancing technology, the production and consumption rates of electronic equipment are steadily increasing. E-waste quantities, which show an average annual increase of 5-8%, constitute approximately 5-6% of annually generated solid waste. The amount of e-waste is directly proportional to the development index and purchasing power, and in developing countries this growth rate is expected to accelerate further due to advanced technology use. Personal computers and communication devices will be leading devices in e-waste matters. Fashion, demands for new functions, and advances in technology and information technology are driving individuals to use new equipment in these areas. The management of these wastes (reuse, recycling, and recovery)—both from an environmental and economic perspective—is emerging as a new research topic worldwide. From an environmental standpoint, these wastes cause serious harm to nature and living organisms due to the heavy metals and toxic compounds they contain [1]. The harmful metals and components found in electronic waste and their effects are presented in Table 1.

Table 1. Harmful Elements and Components in Electronic Waste and Their Effects on Living Organisms [2]

Due to their significant metal content, computer motherboards can be evaluated as secondary metal sources. Components mounted on printed circuit boards generally consist of chips, connectors, and capacitors. Different components have different compositions. For example, Ta is used in capacitors; Ga, In, Ti, Ge, As, Sb, Se, and Te can be found in chips and other units. Solders contain Sn, Pb, and Cd, while bridges and sockets in boards are made of ceramic and contain BeO and silica. Semiconductors contain Ga, Se, Ge, and other elements in smaller quantities. In addition to the positive economic impacts that recovering valuable portions of these wastes would bring to the country's economy, the presence of hazardous compounds that need to be removed is one of the factors that increases the importance of waste management. Table 2 shows the proportions of metals found in various electronic waste. Among these metals, the proportions of valuable metals such as gold, silver, and copper are particularly noteworthy. While gold deposits become economically valuable when they contain around 3-5 g/ton of Au, the Au found in electronic waste, particularly in computer and mobile phone printed circuit boards, is present 20-100 times more. For this reason, metal recovery from electronic waste is of great importance. According to 2015 figures, the value of electronic printed circuit boards worldwide was approximately USD 58.6 billion. As the importance of this subject has increased in our country, work has begun on the recovery and utilization of electronic waste. Companies that have obtained compliance certificates from the Ministry of Environment and Forestry conduct disposal and recovery of electronic waste through various methods. Within this scope, iron, copper, and plastic products are obtained and brought into the economy. Additionally, obtained electronic cards, floppies, and printed circuits are generally not processed and are exported.

Table 2. Metal Contents in Electronic Waste

By using these wastes as secondary sources, it is possible to reduce the depletion rate of primary sources, reduce solid waste quantities, recover non-metal materials (plastics, etc.), and recover ferrous, non-ferrous (steel, Al, Cu), and precious metals (Au, Ag, Pd, etc.) [8]. Metal recovery from printed circuit boards (PCBs) within the waste is generally carried out using pyrometallurgical and hydrometallurgical methods, and applying physical preconcentration processes before both methods can reduce metal losses by 10-35% [9]. Through mechanical preprocessing, components of electronic waste can be dismantled and separated. In this way, metals (iron, copper, aluminum, etc.), plastics, ceramics, batteries, and PCBs can be obtained separately. Electronic waste contains heavy metals such as mercury, lead, cadmium, and chromium, halogenated compounds such as chlorofluorocarbons (CFC), polychlorinated biphenyls (PCB), polyvinyl chloride (PVC) and brominated flame retardants, and dangerous and toxic contaminants such as asbestos and arsenic, creating environmental problems. Efficient separation of plastics from metals before hydrometallurgic and pyrometallurgic processes will increase metal recovery efficiencies, reduce operating costs to minimum levels, and reduce energy costs. In particular, acid consumption in leaching processes will decrease, and the emission of toxic gases resulting from combustion in smelting processes will be minimized. For this purpose, there is a necessity to separate and remove light materials such as metals, plastics, and ceramics. While disposal of these light materials is possible through certain simple separation techniques, valuable metals (Cu, Au, Ag, etc.) can be concentrated together.

Figure 1. Motherboard Components

Metal Recovery from Printed Circuit Boards

Printed circuit boards, which form the foundation of electronics, constitute 3% of all electronic waste [10]. Printed circuit boards consist of capacitors, heatsinks, chips, and connector inputs located on them. Figure 1 shows an example printed circuit board. The content of valuable metals such as Au, Ag, Cu, and Pd found in printed circuit boards (PCBs) is much higher compared to their content in primary sources, and many studies have been conducted in recent years for the recovery of these metals. Printed circuit boards consist of 30% metal, 30% ceramic, and 40% plastic-derived materials [11]. Components on printed circuit boards (capacitors, heatsinks, chips, bridges, sockets) are manually removed and evaluated separately. Figure 2 shows parts dismantled from a printed circuit board [12].

Size Reduction—Classification

The preliminary processes that are of great importance in the recovery of electrical and electronic equipment scrap have three basic objectives. First, smaller particle-sized pieces adhere more easily than larger particle-sized pieces. Second, during electrostatic separation, particles of the same size and shape can be separated more effectively. The final objective is the separation of different materials from one another [13-14]. For crushing operations, cutting-shredding equipment is generally preferred at the first stage, and hammer or impact crushers at the second stage. Due to the different size release properties of metals in printed circuit boards, crushing and grinding operations are performed in stages. Metal, plastic, and ceramic materials that are sufficiently liberated are separated from the plastic portion using physical enrichment methods. Since metals are ductile, they cannot pass below the liberation size and only change shape. For this reason, determining the liberation size is very important so there is no unnecessary capacity and energy waste for crushers or grinders.

Figure 2. Motherboard Components Separated as a Result of Manual Assembly Work

Enrichment Methods

The recovery of these materials, which have a heterogeneous composition, is carried out using methods (physical, physicochemical, and chemical) selected by taking into account the physical and chemical properties of the materials in the composition [15].

1. Physical Enrichment Methods

The fact that electronic waste consists of numerous materials with different physical properties (specific gravity, magnetic susceptibility, conductivity, etc.) makes it possible to utilize these differences to separate metals and plastics [16-18]. The obtained pre-metal concentrate can be evaluated through hydrometallurgical processes to achieve higher metal recovery efficiencies.

• Separation by specific gravity difference

Separation by specific gravity difference is a relatively simple and economical enrichment process applied in liquid or air environments to relatively coarse materials with specific gravity differences. The movement of particles in the medium depends not only on specific gravity but also on particle shape and size. Products separated into similar size groups can be separated from each other more effectively. While jigs and shaking tables can be used for coarse sizes, when particle size is very small, separators using centrifugal force (such as Knelson and Falcon Concentrators) may be preferred [19].

• Magnetic separation method

The magnetic separation method is based on the principle of separating metals with high magnetic susceptibility; such as Fe, Ni, Co, from portions with low magnetic susceptibility. Equipment selection is made according to particle size and magnetic susceptibility degree [13]. In printed circuit board enrichment, ferrous material that becomes free after the first stage crushing operation is generally separated using the magnetic separation method. The remaining portion is subjected to size reduction operations in stages and is separated at the next enrichment stage.

• Electrostatic separation

Electrical conductivity-based separators are used to separate materials with different electrical conductivity (resistivity). With eddy current separators, ferrous-free metal/non-metal separation is performed on particles larger than 5 mm; with corona electrostatic separators, metal/non-metal separation is performed on materials in the 0.1-0.5 mm range; and with triboelectric separators, plastic separation is performed on materials smaller than 5 mm [20-21].

2. Physicochemical Enrichment Method (Flotation)

Flotation is based on causing water-loving minerals (hydrophilic) to sink in water within a flotation cell, while water-repelling minerals (hydrophobic) attach to generated gas bubbles and are carried upward [22]. Although many separation technologies such as automatic classification, separation by density difference, magnetic and electrostatic separation are applied to plastic waste, some difficulties are encountered, and flotation is an alternative method to these [23]. The critical point for success in separation by flotation is determining the surface conditions that will enable selective attachment of materials to be separated to the froth [24]. Flotation is a method applied for fine-sized materials, and for this method to be applicable to printed circuit boards, particle size must be reduced below 100 microns. For fine-sized liberated metals, it is a method that can be applied as an alternative to physical methods.

3. Metallurgical Methods

Metallurgical methods consist of pyrometallurgical and hydrometallurgical processes. To obtain metal from electronic waste, concentration is increased by resorting to metallurgical processes after physical methods or directly, and final products can be reached through certain techniques. The portion with particularly high copper content in electronic waste is sent to pyrometallurgical facilities for processing. In the hydrometallurgy section, after metals are taken into solution, the desired metal is recovered through methods such as solvent extraction, ion exchange, precipitation, and cementation. To completely rid it of remaining impurities, refining is performed through electrometallurgical processes. Within the scope of the Scientific Research Project titled "Recovery of Precious Metals from Printed Circuit Boards by Flotation and Centrifugal Separators" being conducted in the Ore Dressing Engineering Department of Istanbul Technical University, in research on obtaining metal from printed circuit boards, enrichment by specific gravity difference was applied using a shaking table after two-stage crushing operations, and high metal content products were obtained. In the enrichment process using the shaking table shown in Figure 3, a heavy product (metallic brown product) was obtained from the left corner of the table, and a lighter product (gray product with plastic and ceramic density) with lower specific gravity was obtained from the far right. The Au content of 130 ppm in the fed product was increased to 780 ppm in the heavy product, and the Cu content of 12.1% was increased to 73.2%.

Figure 3. Shaking Table and Obtained Products

The fact that computer motherboards are exported abroad as scrap without being recovered in our country is a negative situation for the country's economy. The development of a simple, economical, and environmentally effective recovery process will create a highly profitable sector through the utilization of waste-classified material. This emerging sector will provide direct and indirect benefits to the country's economy through investments it makes. Dr. Fırat Burat Istanbul Technical University Faculty of Mines Ore Dressing Engineering Department Recovery, Separation and Enrichment Research Group   Dr. Mustafa Özer Istanbul Technical University Faculty of Mines Ore Dressing Engineering Department Recovery, Separation and Enrichment Research Group   Research Assistant Esra Tanısalı Istanbul Technical University Faculty of Mines Ore Dressing Engineering Department Recovery, Separation and Enrichment Research Group    

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