6.1 Introduction

Electronic waste has been a growing concern around the world. The amount of e-waste generated has been growing at a rate of 3–4% and is expected to reach 120 million tonnes by 2050. With the development of new technologies, especially in laptops, cellphones, and tablets, older technologies are getting obsolete, reducing the lifespan of electrical and electronics products and thus contributing to a higher rate of waste generation. These staggering facts should be considered as an essential incentive for e-waste recycling. If this waste is recycled correctly, it could offer an opportunity for urban mining for the recovery of copper, gold, silver, palladium, and other metals with an estimated value of 55 billion euros. The concentration of metals in e-waste is also significantly higher than the natural ores.

Various routes are currently being implemented to recover metals from the e-waste stream. The recycling methods for e-waste could be broadly categorized into two sections, preprocessing and end processing (Namias 2013). The preprocessing stage, also known as mechanical recycling, has gained popularity due to its relatively low operating cost. It also recovers the metallic fraction without destroying the nonmetal fraction, which, in turn, helps to promote the recycling of nonmetal fraction and the circular economy (Ning et al. 2017). The physical methods recover the metallic fraction and thus reduce the overall mass to be transported or processed subsequently and are less energy-intensive usually. On the other hand, it requires proper dust control units if processed dry or proper water recycling/treatment systems if processed wet.

The preprocessing techniques deal with physical separation methods focused on separating metals from the nonmetals and thus reduce the amount to be processed in subsequent energy-intensive and costly stages. The end processing uses the metal concentrate obtained from the preprocessing stage and methods such as pyrometallurgy, hydrometallurgy, and biometallurgy to produce high-purity metals.

Mechanical recycling of waste printed circuit boards (PCBs) is usually based on density separation, magnetic separation, or electrostatic separation. The liberation of metal and nonmetals is achieved by using a combination of shredders (coarse crushing), hammer mills, and knife/cutting mills (fine crushing) (Bizzo et al. 2014; Ghosh et al. 2015; Mou et al. 2004; Ogunniyi et al. 2009; Otsuki et al. 2019; Veit et al. 2006; Wen et al. 2005a; Zhang and Forssberg 1999). After the size reduction, the liberated waste is processed using physical separation methods.

This chapter provides the details of the preprocessing processes used in e-waste recycling. The preprocessing step majorly deals with the disassembly, size reduction, and physical separation of various components such as metals, glass, and plastics from the e-waste.

6.2 Dismantling

The dismantling process is mostly a manual disassembly process to remove the hazardous materials out of the e-waste before recycling. The substances removed during dismantling include (Kaya 2018),

• Inks and toners from printers
• Cathode-ray front panels from cathode tube ray (CRT) screens
• Batteries from laptops
• Degassing of chlorofluorocarbons (CFC), hydrofluorocarbons (HFC), and hydrochlorofluorocarbons (HCFC)
• Coolants from refrigerators
• Mercury-containing switchers and PCB capacitors
• Removal of compressor and deoiling

This stage is also used to reduce cross-contamination and to segregate different streams such as plastics, steel, aluminum, copper, and printed circuit boards (Abdelbasir et al. 2018). Automatic and semiautomatic dismantling systems use has increased in recent years; however, manual dismantling is still the most widely used (Hsu et al. 2019). Robotic systems such as Liam and Daisy have been developed by Apple Inc. for dismantling iPhones that can process several cellphones at once. These automatic systems reduce the risk of worker safety and increase the capacity and processing speed for the processing plant. The dismantled streams are sent to the individual processor for the subsequent stage involving size reduction and metal recovery.

Dismantling helps to improve the quality of streams/products for subsequent steps, has low investment, and reduces/eliminates the dust issue. However, it is harder to dismantle new sophisticated technologies, is time consuming, and increases the risk for worker safety. It also is a significant cost concern, especially in high-labor-cost economies.

6.3 Comminution/Size Reduction

Similar to the mining industry, the comminution step involves reducing the particle size to increase liberation and provide proper particle size for subsequent processing/enrichment. The size reduction in e-waste recycling is achieved by using shredders and hammer mills. The particle breakage is achieved by impact and shear forces (Schubert and Hoberg 1997). Recently, high-voltage pulse fragmentation has also gained attention for size reduction as it reduces the dust issue and provides liberation at a coarser size (Duan et al. 2015).

The comminution unit increases the overall plant throughput and reduces the volume for materials to be transported for subsequent process. The significant drawbacks of the comminution process are the generation of dust and loss of valuable metals with the dust and decreased grade/cross-contamination of streams if performed without prior dismantling.

6.3.1 Shredders

Shredders are the first stage of the size reduction process in e-waste recycling. A shredder utilizes sharp cutting blades mounted on a rotating shaft powered by high-power electric motor for cutting materials into smaller pieces. It relies on shear cutting and tearing forces provided in the form of low-speed and high-torque operation. The two main parts of shredders are the rotating shaft and the cutting knives. A shredder can be mounted with single, double, or four heavy-duty rotating shafts depending on the requirement of the operation. The additional shafts provide second-stage size reduction and can further reduce the particle size by increasing the cutting action at a given time (Ekman 2018). The shafts rotate counterclockwise, and the knives mounted on the shaft provide the required shear forces on the material trapped between the knives (Fitzgerald 2009). The knife design depends on the application. Heavy-duty shredders used for cutting cars and metals require thicker and smaller knives. The number of knives dictates the operation of the shredder. More knives not only increase the probability of shredding actions, but also increase the chances of materials to keep bouncing on the surface of the shredder and sometimes clogging the equipment (Ekman 2018).

The rotational speed of industrial shredders ranges from 10 to 50 RPM and can process up to 150 t of materials per hour; however, the throughput capacity changes widely depending on the application (Fitzgerald 2009). The energy consumption ranges from 3 to 11 kWh/t depending on the feed rate and feed properties. A low-power shredder from UNTHA shredding technology has a single shaft (LR520) powered by an 11 kW motor rotating at a speed of 85 RPM, whereas a two-shaft shredder (S120) from the same manufacturer (high torque shredder) has 88 KW drive power and a rotational speed of 10–25 RPM (UNTHA Shredding Technology 2018).

6.3.3 High-Voltage Fragmentation

High-voltage pulse fragmentation is a relatively new technology that has been developed to utilize electrical energy to break the rocks/ore at the grain boundary. The primary benefit of this technology is its higher selectivity. The electrical fragmentation uses fast pulse rise time (<1 μs) and higher voltages (>90 kV) to discharge into a solid material, causing an “explosion” within. The higher selectivity of this technology comes from different electrical properties of different materials. Field distortions caused by adjacent minerals with different dielectric constants provide the fastest route to ground for the discharge (Wielen et al. Van Der Wielen et al. 2014).

A comparison of ores crushed using SELFRAG (a high-voltage pulse fragmentation technology, shown in Figure 6.2) and ground using conventional methods at same specific energy showed that the pulse technology generates less fines and a significantly higher proportion of >95% liberated minerals in coarser size fraction (Wielen et al. Van Der Wielen et al. 2014). Shi et al. (2014) showed that an energy reduction of 5 kWh/t was achieved while simulating a grinding circuit with this technology.

SELFRAG (2012) showed that metals from e-waste could be separated without fine grinding as proper liberation is achieved during the fragmentation process. Since it utilizes a water as a process medium, the loss-to-fines is depressed, and a coarser liberation size also improves the recovery of the subsequent processes. Duan et al. (2015) showed that ∼98% of the copper gets liberated and concentrated in the −2 mm size when a printed circuit board was processed through the SELFRAG lab at 400 pulses.

6.4 Particle Size Analysis

The particle size plays an essential role in the processing of e-waste or ore. All equipment has an optimum particle size for its efficient functionality. Guo et al. (2011) and Ventura et al. (2018) have shown that metals usually get enriched in sizes below 1.25 mm and 1.7 mm, respectively, in crushed waste circuit boards with the highest concentration in the finest fraction, whereas Duan et al. (2015) have suggested that all metals could achieve nearly complete liberation in −1 mm size fraction. Many researchers have suggested a low separation efficiency of the mechanical process for −75 μm particles (Zhao et al. 2012).

Particle size analysis could be conducted using mechanical screens for coarse particles (up to 38 μm) or laser-based sizing for fine particles (below 500 μm). Depending on the particle size, screening can be performed either dry or wet. The efficiency of dry screening decreases with particle size, and wet screening is preferred for fine particles. The particle size of a sample is most widely defined by its 80% or 50% cumulative passing sizes (D₈₀ or D₅₀), the screen size at which 80% or 50% of the total mass would pass through the screen. The D₈₀ or D₅₀ for a sample is obtained using sieve analysis where a given sample mass is placed on a stack of consecutive screens and shaken for 10–20 minutes to allow the particle to segregate based on sizes. The sample retained on each screen is then weighed to determine cumulative passing at each screen size and can be plotted, as shown in Figure 6.4. The D₈₀ or D₅₀ is obtained by interpolating the screen size and cumulative passing data.

The particle size distributions can also be represented using cumulative distribution functions such as Gaudin–Schuhmann equation for fine particles or Rosin–Rammler distribution for coarse particles. Rosin–Rammler distribution has shown a good fit for waste printed circuit boards, shredded electronics products, and solid waste (Quan et al. 2012; Sun et al. 2015; Vesilind 1980).

Where F(x) is cumulative percent passing at size X, X is the particle size, X’ is size modulus defining the theoretical maximum particle size, and n is distribution modulus defining the spread of the distribution (small n represents wide distribution).

6.5 Size Separation/Classification

Size separation is a crucial stage in processing and is used for two main reasons,

• Provide the optimum size range particles to specific downstream equipment
• Maintain a specific top size from the comminution circuit

A typical flowsheet for a comminution circuit is shown in Figure 6.5. It shows that the hammer mill is in the closed circuit with the first screen to provide a fixed top size to the subsequent process. Screen 2 is used to provide an optimum particle size range to individual separators. The two main sizing processes, namely screening and classification (hydrocyclones or air cyclones), could be used for size separation (Menad 2016).

6.5.1 Screening

Screening is the most commonly used process to separate particles by size to provide an optimum size range for the subsequent process to increase their process efficiency. In the case of close circuit comminution, it is used to prevent oversized materials from passing to the next stage. This process is mostly used for coarse applications (up to 1 mm), and the screening efficiency reduces with the reduction in particle size.

The most commonly used screens are the vibratory screens with perforated decks. These screens can be used as single, double, or triple deck depending on the process requirement. The decks have opening holes, and the diameter of these holes depends on the particle cut size. The particle larger than the opening size reports to the oversize, which could be recirculated back to the hammer mill. The particle smaller than the opening size would pass through the screen and report to the undersize, which is sent to the subsequent process. The perforated screen deck could be made of woven wire, polyurethane, rubber, or stainless steel depending on the properties of the materials being processed.

The efficiency of screens is estimated using the recovery of a particle at a given size, and amount of misplaced particles (fine particles reporting to the oversize) and can be calculated using the following equation,

Where R_u is the efficiency of undersize recovery and f_x and O_x are fractions of material smaller than the cut size in feed and oversize.

The efficiency of screening depends on several machine and operating parameters as well as feed material properties. Some of these factors are:

• Feed rate: inversely proportional
• Screen angle: inversely proportional
• Particle shape: inversely proportional to extreme particle shapes
• Opening area: inversely proportional
• Vibration: excessive vibration would reduce screening efficiency
• Moisture: inversely proportional

The size (length and width) of the screen required could be estimated using Vibrating Screen Manufacturers Association (VSMA) equation for screening surface area calculation or Metso equation and is shown in the equation below (Olsen and Carnes 2016):

Where U is undersize tonnes per hour, S is safety factor, A is basic capacity factor, B is oversize percentage factor, C is half-size factor, D is deck location factor, E is wet screening factor, F is bulk density factor, G is screen surface open area factor, H is opening shape factor, and J is screen efficiency factor.

Once the required area is estimated, the screen would be selected based on the screen information available from different vendors.

6.5.2 Classification

Classification is a size separation process based on the particle behavior in a fluid such as air or water. The process is suitable for fine particles (<1 mm) and mostly used in the final stage of comminution as a size control device before beneficiation.

The classification process depends on the forces acting on a particle in a fluid. The net resultant force on the particle dictates the movement of the particle. To achieve proper size classification, the direction of the net resulting force should be different for coarse and fine particles. The forces acting on the particle in the fluid are gravitational force, drag force, and centrifugal force. Depending on the solid density (amount of solids in the slurry by weight), the particle-settling behavior in the fluid can be defined as free settling or hindered settling. If particles of the same density are placed in a water medium, the coarser particle moves faster than finer particles, thus achieving separation. The classification units can be classified as gravitational or centrifugal depending on the forces acting on the particles.

6.5.2.1 Centrifugal Classifier

The most widely used centrifugal classifier in the processing industry is hydrocyclone. These are robust and relatively efficient size separators. The schematic of a hydrocyclone is shown in Figure 6.6. A hydrocyclone consists of a feed inlet, an apex/spigot (underflow discharge), a vortex finder (overflow discharge), and a cylindroconical body. The slurry is tangentially fed through the feed inlet under pressure and high velocity. The tangential feed inlet provides the rotational motion to the slurry that helps to create opposing forces, centrifugal force, and drag force. The high centrifugal force on the coarse particle moves it toward the outside wall, where it spirals downwards and reports to the underflow through apex/spigot. The finer particles remain in the center and move upward with the secondary vortex and then exit as overflow through the vortex finder. The cyclone diameter could vary from 13 mm to 900 mm depending on the requirement and could be used for classification down to 20 μm or sometimes lower.

The cut point of the hydrocyclone is the size that has an equal probability of reporting in underflow or overflow stream. The cut size for a hydrocyclone depends on the cyclone diameter and flow rate. The cut size increases with the cyclone diameter but decreases with flow rate. The efficiency of the cyclone is quantified using partition curves, which defines the percentage of a size fraction in the feed reporting to underflow. Ideally, the particles larger than cut size should report to underflow, whereas particles smaller than the cut size should report to overflow. However, in real operations, some particles are always misplaced. Some coarse particles that did not have sufficient opportunity to move to the underflow would report to overflow as short circuit, whereas some fine particles could become trapped among coarse particles and would report to the underflow as entrainment. The efficiency of separation is estimated using the following equation,

Where d₇₅, d₅₀, and d₂₅ are particle sizes at the partition value of 0.75, 0.50, and 0.25. For the ideal scenario, imperfection would be 0.

Another criterion for cyclone performance is the amount of water in the underflow. In the ideal scenario, no water should report to underflow; however, the water content of <30% in underflow is considered to be good.

6.5.2.2 Gravitational Classifiers

Gravitational classifiers are simple and low-energy consuming classifiers that work on particle settling by gravity. The rake and spiral classifiers are the most common examples of gravitational classifiers. It comprises a tank with an inclined surface and a system to move settled material in an upwards motion. The coarse particles are settled due to their weight and get continuously extracted with the help of a mechanical device (Dunne 2019; Zhou et al. 2012).

In the case of spiral classifiers, one or two screws move the settled materials upward, whereas, in rake classifiers, an array of tines is used to separate the coarser material from the finer ones, as shown in Figure 6.7. The fine particles stay suspended in water and get discharged by overfilling the tank.

Some other gravitational classifiers are hydraulic classifiers such as Linatex, Reflux, Crossflow, Allux, and Floatex classifiers. These devices use gravity for separating coarse particles from fine particles instead of a mechanical system. The coarse particles form a bed at the bottom of the classifiers, and the fine particles are moved upward with water injection through a perforated bottom plate that moves fine particles to the overflow launder. The coarse particles are discharged through the lower part of the tank as thick pulp (Luttrell et al. 2006; Mankosa et al. 2019).

6.6 Magnetic Separation

Magnetic separation has been utilized to remove the ferromagnetic elements, iron and nickel, and other iron scraps. It uses the difference in particle behavior in a magnetic field for separation. All particles can be classified as diamagnetic, paramagnetic, and ferromagnetic, depending on their magnetic properties.

Diamagnetic particles such as copper, gold silver, plastic, fibers are repelled from the magnetic field lines and would report to the nonmagnetic tailings bin. The paramagnetic particles such as platinum, manganese, tungsten are attracted to the magnetic field lines and would report to the magnetic concentrate bin during separation. The ferromagnetic particles such as iron, cobalt, and nickel are a special case of paramagnetic behavior where the particle retains magnetic property even after the removal of magnetic field lines. These particles would also report to the magnetic concentrate bin during separation.

A magnetic separator consists of a permanent magnet or electromagnet to generate the magnetic field and a conveyor to move particle over the field lines. The types of magnetic separators can broadly be classified into low- and high-intensity magnetic separators depending on their magnetic field intensity.

6.6.1 Low-Intensity Magnetic Separators

Low-intensity magnetic separators are used to separate ferromagnetic materials such as iron and nickel from the crushed e-waste. The magnetic field intensity used is <0.3 T, which is sufficient to take advantage of the high magnetic susceptibility of iron particles. The primary utilization of the low-intensity separator is in the form of cross-belt separators located over the conveyor. It is widely used to remove the coarse magnetic iron particles before the materials are fed to the hammer mills.

Another major type to the low-intensity separator is the rare-earth roll separator used in dry applications. The feed is moved using a conveyor, and a rare-earth magnetic roll is installed at one discharge end of the conveyor. The magnetic particles are attracted to the roll, move with the belt, and get discharged into the magnetic bins, whereas the nonmagnetic particles fly away from the belt due to the centrifugal forces from the rotating rolls and fall in the nonmagnetic bin as shown in Figure 6.8 (Wills and Finch 2016c). The roll speed, feed rate, field strength, and particle size are the major factors affecting the separation. In the case of wet applications, drum separators are the most widely used. It consists of a nonmagnetic drum fitted with magnets that rotate in a pool of slurry. The magnetic particles get pinned with the drum and lifted to the concentrate bin, whereas the nonmagnetic particles move with the slurry and get discharged into the tailings (Haldar 2018).

6.6.2 High-Intensity Magnetic Separators

These types of magnetic separators are useful for the separation of weakly magnetic particles (paramagnetic materials). The magnetic field could be generated using rare-earth alloy magnets or electromagnetic coils and is more useful for fine particles and low iron-containing materials.

The induced roll separator is one of the most common dry high-intensity magnetic separators where the material is fed directly on the rotating roll. The magnetic materials get pinned with the roll, whereas the nonmagnetic materials fly off the roll due to the centrifugal forces induced due to the roll rotation (Figure 6.9). Magnetic field strength up to 2 T could be achieved with this separator. Particle size and feed rate are the major factors for particle separation.

Wet high-intensity magnetic separators have successfully been used for fine hematite ores to produce iron ore concentrates and can be used for ultrafine particles too. Other magnetic separators such as high-gradient separators and superconducting separators are used for particles of very low magnetic susceptibility.

Yoo et al. (2009) used magnetic separation to recover 83% of iron and nickel as magnetic concentrate from shredder printed circuit boards. Veit et al. (2005) showed that a high iron and nickel concentration in the magnetic concentrate for printed circuit boards and electronic component scrap could be achieved at a low magnetic field strength of 0.65 T.

The iron concentration in the magnetic materials was increased to 41% compared to 27% in the feed with 89% iron recovery for fine (−0.6 mm) e-waste powder in a study conducted by Lee and Mishra (2018). In another study, up to 89% nickel and 98% iron were recovered for relatively coarse (+0.5 mm) circuit board particles at 0.1 and 0.8 T field strength (Fujita et al. 2014). Recently, Zhu et al. (2020) used a wet low-intensity magnetic separator for fine waste printed circuit board particles (−0.5 mm) to recover iron particles and suggested that magnetic separation is an effective method to recover high-grade magnetic particles.

6.7 Electrical Separation

Electrical separation is one of the most used techniques to recycle metals such as copper from nonmetals such as plastic and fibers. It uses the differences in electrical conductivity among various materials present in e-waste. One of the significant challenges with the process is its limitation to dry feed. For most efficient operation, the feed should be in monolayer, which also restricts its throughput. The separator needs to charge the particles to use the conductivity differences selectively. There are three main mechanisms of particle charging, ion bombardment (corona electrostatic), frictional charging (triboelectric), and eddy current. Corona electrostatic separation has been proven useful to separate metals from nonmetal streams in dry environment, whereas eddy current separation has been successfully utilized to extract aluminum from the nonferrous metal stream (Abdelbasir et al. 2018; Bizzo et al. 2014; Burat and Özer 2018; Das et al. 2009; Duan et al. 2009; Hsu et al. 2019; Kasper et al. 2015; Kaya 2016, 2018; Menad 2016; Veit et al. 2014).

6.7.2 Triboelectric Separation

The triboelectric separator uses particle contact/friction as the method for particle charging. When two particles with different electrical properties come in contact and rub against each other, there is an exchange of electrons resulting in positively and negatively charged particles. If these charged particles are subjected to an electric field, the particles move toward oppositely polarized electrodes and hence provide the needed separation. The rubbing/friction between the particles is achieved by using a fluidized bed, and then the charged particles are allowed to free fall between two oppositely charged electrode plates. This process is more useful for plastics and other nonmetallic materials.

Saeki (2008) used triboelectric separation to recover 95% polyvinyl chloride (PVC) with 99% purity from a mixture of PVC, polyethylene terephthalate (PET), and polyethylene (PE), whereas 90% recovery of acrylonitrile butadiene styrene (ABS) with 99% purity was achieved from a mixture of ABS, polypropylene (PP), and polystyrene (PS). Dodbiba et al. (2002b) listed that the triboelectric charging sequence for various plastics is,

PC is polycarbonate and PTFE is polytetrafluoroethylene.

The sequence suggests that if two plastics (i.e. PE and PS) are rubbed against each other, the one on the left of the sequence (PS) gets a positive charge and one on the right of the sequence (PE) gets a negative charge. In a two-component mix of PET and PE, triboelectric separation could achieve over 95% grade of individual plastic. Other researchers have also concluded high grades and high recoveries for different plastics using triboelectric separation (Lee and Shin 2002; Li et al. 2015; Xiao et al. 2003; Zelmat et al. 2013).

Figure 6.10 shows a simplified schematic for a triboelectric separation system where particle charging is achieved via tribo-cyclone. The charged particles are gravity fed to the separation system and products are collected in the collecting bins.

In the field of nonmetal fraction from printed circuit boards recycling, Zhang et al. (2017) and Yang et al. (2019) used triboelectric separation to separate inorganics from the nonmetal fraction to improve the quality of the product. Overall, a concentrate yield of 47% was achieved with an organic content of 69%, recovery of 59%, and an upgradation ratio of 1.27 by Zhang et al. (2017). In the case of Yang et al. (2019), an upgradation ratio of 1.17 was achieved.

6.7.3 Eddy Current Separation

Eddy current separation is widely used in the recycling industry to recover nonferrous metals such as aluminum and copper from electronic waste (Jujun et al. 2014; Ruan et al. 2017). The process is dictated from the generation of eddy current in particles when passing over a rotating magnetic field. When a nonferrous metal is placed in a high-speed rotating magnetic field, eddy currents generated in the nonferrous materials create a secondary magnetic field around itself. These particles are then repelled due to the same polarity of the two magnetic fields and hence separating the nonferrous metals from the mixture, as shown in Figure 6.11. It can be used to separate particles in size range of 3 mm–350 mm (Smith et al. 2019).

Eddy current separators were used to recovery over 90% aluminum from electronic scrap by Zhang et al. (1998). Bi et al. (2019) used eddy current separation to recover copper and aluminum from the crushed spent lithium iron phosphate batteries in size range of 2–20 mm. Li et al. (2017) used eddy current separator to recover printed circuit boards from crushed cell phones with over 95% separation efficiency. Ruan and Xu (2011) showed that this process is also an efficient method to separate aluminum from plastic in crushed waste toner cartridges.

6.8 Gravity Separation

Gravity separation is one of the most common low-cost physical processing methods and has already being used for the separation of metals from nonmetals (Eswaraiah et al. 2008; Forssberg et al. 1998; Hanafi et al. 2012; He et al. 2015; Meng et al. 2018; Nekouei et al. 2018; Sarvar et al. 2015; Veit et al. 2014; Ventura et al. 2018; Wen et al. 2005b; Zhou et al. 2016). It has been successfully used in the separation of different types of plastic wastes too. Processes such as air classifiers, jigs, hydrocyclones, and float–sink have been widely studied in the separation of different types of plastics (Bauer et al. 2018; Ruj et al. 2015; Serranti and Bonifazi 2019).

A gravity separator utilizes the differences in the density of various metals and nonmetals as the basis of separation. If particles of same size are placed in a fluid, the high-density particles sink faster than the light-density particles. Since the density of most of the metals is higher compared to the nonmetals present in the e-waste, gravity separation is the most successful technique for the physical recycling of e-waste. The ease of gravity separation can be expressed by the concentration criterion and can be estimated using the equation below,

Where ρ_h, ρ_l, and ρ_f are the density of heavy material, light material, and the fluid medium.

Generally, if the concentration criterion is greater than 2.5, the gravity separation is relatively easy, and the efficiency of separation decreases with decreasing value of concentration criterion. Various gravity separation units are available at industrial scale for metal recycling from e-waste. Some of the most common separators are discussed in this section.

6.8.2 Spirals

Spirals are simple, robust, and low-cost gravity separators where the slurry is fed at a solid density of ∼20–25% at the top of the spiral. As the slurry flows down the curved channel, lighter particles due to action of drag forces combined with the water flow report to the outer area of the spiral as tailings, while the heavier particles are pulled inward to the inner area of the spiral as a concentrate, thus effecting the separation. Wash water is continuously added at the inner area to flush any entrained light particles. Modern spirals are constructed from fiberglass and plastic and can treat particles down to 75 μm. The operating parameters are feed rate, pulp density, feed grade, splitter openings, the position of distributors, and the diameter of spirals.

No researchers have yet used spirals in the field of e-waste recycling; however, spiral concentrators have shown success in iron, coal, and rare-earth industry.

6.8.5 Centrifugal Concentrators

Centrifugal concentrates such as Knelson concentrator and Falcon concentrator are highly efficient processes used to enhance the gravitational forces experienced by particle by adding high rotational speed, especially for fine particles. The feed slurry is pumped to the top of the feed inlet of the concentrator. In the centrifugal concentrator, the particles are accelerated up to 60–300 times the regular g-forces from the rotating bowl trapping the heavier particles in the grooves located in the bowl, whereas the lighter particles are flushed with the water and report to the tailing launder as shown in Figure 6.13. The centrifugal concentrator in most of the gold processing plants is batch operated with an optimum concentrate cycle. After the completion of the cycle, the heavy concentrate is flushed from the bowl into the concentrate launder (Chen et al. 2020; Oruç et al. 2010).

Zhang et al. (2018) showed that the high density of lithium cathode oxide could help to achieve a separation of lithium cathode oxide from spent lithium-ion batteries with over 80% grade and recovery using Falcon concentrator. The optimum particle size was determined to be −90 + 45 μm. Duane et al. (Duan et al. 2009), Wen et al. (2005b), and Yue-min (2006) showed that the Falcon concentrator could be used for metal recovery from crushed printed circuit boards with high recovery and high grades. In a study conducted by Kwon et al. (2016), Kelson concentrator was used to recover tantalum from the electronic components of printed circuit boards obtained from a laptop with ∼80% grade and recovery.

6.8.6 Dense Medium Separation (DM Bath/Cyclone)

Dense media separation is the most common approach used in the coal industry to remove heavy inorganic materials from the light cleaner coal particles. The process uses a fluid medium at a given separation density to achieve separation. When a particle is placed in a fluid, it would either float or sink depending on fluid density. If the particle density is less than the fluid density, it floats to the surface, or else it would sink to the bottom. The fluid used for this process is mostly organic liquid of different specific gravities at lab scale; however, at the industrial scale, the required fluid density is achieved by mixing fine heavy media such as magnetite or ferrosilicon into water. This media could then be recycled using simple drum magnetic separators and reused.

The coarse material is mostly processed in a static bath (dense media bath) where the heavy particles are settled at the bottom and then carried out to the heavy launder by a removal mechanism such as a conveyor belt, whereas the light particles overflow or are scraped from the surface. For finer particle size, a dense medium cyclone is used. The working principle is similar to that of a hydrocyclone. The heavy particle moves toward the cyclone wall due to centrifugal forces and move to the bottom of the cyclone and get discharged through the spigot. The light particles are moved to the center and get discharged through the vortex finder. The separation density and theoretical grade and recovery for a gravity separator are established from a float–sink test.

A float–sink test is a simple lab-scale technique used to determine the gravity separation behavior for a given sample where the sample is subjected to increasing density liquids, and the floated and sunk fractions are analyzed from their properties. Previous researchers have utilized heavy liquids of higher densities (∼2.89 g/cm³) to separate the metallic fraction from the nonmetallic fraction (Bizzo et al. 2014). A lab-scale float–sink test was able to recover 75% of metallic fraction using organic liquids (Nekouei et al. 2018). Cui and Forssberg (2007) showed that over 88% of copper in the shredded television scrap could be recovered at a separation density of 1.4 g/cm³ with 42% copper grade.

Float–sink test based on heavy liquids has also been used to separate waste plastics into <1.0, 1.0–1.1, and 1.1–1.2 specific gravity ranges by Lee (2007). Pongstabodee et al. (2008) used a three-stage float-and-sink method to classify six different plastics into low (specific gravity: 0.90–1.00), medium (specific gravity: 1.00–1.20), and high (specific gravity: 1.20–1.40) density ranges. Choi (2004) used gravity separation to recover PVC (specific gravity 1.32–1.42) from a mix of polyolefin plastic waste (specific gravity: 0.90–1.06). Dodbiba et al. (2002a) used a drum separator at a speed of 1 RPM at a specific gravity of 1.11 to separate PET from polypropylene (PP). The schematic of the drum separator used by Dodbiba et al. (2002a) is shown in Figure 6.14. The light product overflows to the discharge launder, whereas the heavy sink product is carried up by the sink lifters and then discharged to the sink launder.

6.9 Froth Flotation

Froth flotation is a physicochemical separation process that utilizes the differences in the surface properties of various materials for separation in the water medium. This process depends on the selective attachment of air bubbles to one specific material/mineral in the sample and brings it to the surface where it is skimmed off to the concentrate launder.

The tendency of the air bubble attachment to a particle can be expressed using the contact angle. The contact angle depends on the work of adhesion, cohesion, and surface tension at the bubble, water, and solid interface. A contact angle of 0^° represents complete wetting rendering the surface hydrophilic (water attracting), whereas a contact angle of 180^° represents complete dewetting, rendering the surface hydrophobic (water repelling). The bubble selectively attaches to the hydrophobic particle leaving the hydrophilic particles in the slurry as tailings.

However, the contact angle measurement is possible for smooth and coarse particles. The surface energy or the bubble particle interaction could also be measured using inverse gas chromatography (IGC). The surface tension at three interfaces is highly affected by the surface properties of the solids and surface energy is one of the most important surface properties (Ali et al. 2013). One of the benefits of using an IGC for estimating the hydrophilicity/hydrophobicity of particles that it can be used on fine particles, powders, and rough/uneven surfaces. Mohammadi-Jam and Waters (2016) and Mohammadi-Jam et al. (2014) showed that decreasing the surface energy of particles decreased the work of adhesion with water and increased the floatability of particles and thus increasing hydrophobicity for different minerals.

Some particles such as plastics and nonmetals in e-waste are naturally hydrophobic, whereas the metals are naturally hydrophilic, making flotation a useful process for metal recycling. Inverse/reverse flotation is most studied for flotation for e-waste recycling where the unwanted particles are selectively floated, and the valuable materials are left behind in the slurry. Various reagents are added to the flotation process to aid with the flotation. The reagents can be classified as,

1. Collectors: These are the chemicals that selectively adsorb on the surface of a particle, making it hydrophobic, and facilitates the bubble particle attachment. Examples: Xanthates, amines, oils, sulfonates, etc.
2. Frother: This chemical helps with the formation of stable bubbles by interacting with the air–water interface and decreasing water/air surface tension. Examples: pine oil, methyl isobutyl carbinol, dow froth, etc.
3. Activators: These are the chemicals that help with altering the chemical structure of the particle surface, thus helping collectors to adsorb on the given particle. Examples: metal ions, amines, etc.
4. pH regulators: This group of chemicals is added to the slurry to alter the pH of the slurry and thus changing the surface charge and help with the adsorption process. Examples: lime, sodium hydroxide, etc.
5. Dispersants: It is used to prevent fine particles to agglomerate by controlling the surface charge. Examples: polyphosphates, sodium silicate, starch, guar gum, etc.
6. Depressants: It is used to prevent the adsorption of collectors on the particles rendering them hydrophilic. Examples: humic acids, sodium bisulfide, cyanide, etc.

A combination of these chemicals is used to help with the froth flotation process. However, researchers have shown high recovery without needing any additional chemical due to the nature of particles in the e-waste system. The nonmetal fractions such as fiberglass and plastics are naturally hydrophobic, whereas the metallic fraction is commonly hydrophilic. It should be noted that the flotation is highly dependent on particle size, reagent type, and dosage; hence, very carefully designed experiments at the lab scale are needed to optimize the process.

The flotation is mostly carried out in mechanical flotation cells in the industry as shown in Figure 6.15. It consists of a tank with a rotating impeller. The impeller design changes widely with the vendors; however, it delivers the same purpose. It helps with the agitation of the slurry and create air bubbles by breaking a stream of air. The air bubble rises in the slurry carrying the hydrophobic particles with it, which is skimmed from the surface. The largest flotation cell from Outotec has a capacity of 630 m³. Usually, flotation cells are installed in banks consisting of several (5–8) individual cells.

Column flotation cells are another type of flotation device commonly used at cleaner and recleaner stage to improve the metal grade. The bubble in the columns is generated directly through air spargers. Normally, the feed to the column enters one-third way down from the top and moves downwards where it encounters upward moving air bubbles. The hydrophobic particles are carried to the surface with the bubbles. Wash water is added at the surface using an array of perforated pipes to remove any entrained hydrophilic particles, thus increasing the grades.

Flotation without any reagent has shown a good metal recovery from waste printed circuit boards (Mäkinen et al. 2015; Ogunniyi and Vermaak 2009). Flotation using kerosene and terpenic oil showed a metallic recovery of over 84% at a particle size below 75 μm (He and Duan 2017), whereas soapy collector was used in the flotation experiments conducted by Zhu et al. (2019). He and Duan (2017) reported a copper recovery of 79%, whereas the recovery for Zhu et al. (2019) was only 47%, most likely due to larger particle size (−1 + 0.5 mm). Vidyadhar and Das (2013, 2012) showed that over 90% of the total metal in printed circuit boards could be recovered with a single-stage flotation process at 1 mm grind size. Frother flotation with fuel oil as collector and pine oil as frother showed over 95% metal recovery (Sarvar et al. 2015). Gallegos-Acevedo et al. (2014) showed that a metal-rich and a nonmetal rich stream could be obtained with the addition of 5 ppm frother with over 90% metal recovery.

Separation of different plastics using froth flotation is another avenue for extensive research. Researchers have shown that the surface properties of plastics can be modified to increase or decrease the hydrophobicity of plastic particles, thus aiding with the flotation process. Table 6.1 lists some of the research conducted in the field of plastics flotation.

6.10 Sensor-Based Sorting

Sensor-based sorting is an automated material detection and removal system that uses high-precision sorter to identify the materials based on their physical and chemical signature and then remove it using an ejection system. It is an online, contactless real-time system that analyzes the particle properties along with its location. The sensor-based system has four major components (Wills and Finch 2016d),

1. Particle presentation: Particle is fed to the sensor using a conveyor belt in a monolayer distribution.
2. Sensing (particle examination/detection): Various sensor systems (listed in Table 6.2) have been developed to analyze the particle and capture the particle properties.
3. Electronic processing (data analysis): The particle properties are processed using a lab-developed algorithm based on sensor type and ore properties.
4. Separation: The particle rejection is achieved by a stream of air jets. Mechanical ejectors have also been employed for some sensing technologies.

6.11 Example Flowsheets

The European Recycling Platform have published the recycling process for some of the common e-waste items as shown in Figure 6.16. It shows that the recycling process starts with manual dismantling and removal of hazardous chemicals followed by the shredding and size reduction process. The separation of various components is achieved by a combination of physical separation processes and the recovered items are then sent for subsequent processes.

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