Techniques for separation of plastic wastes

Techniques for separation of plastic wastes

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Silvia Serranti, Giuseppe Bonifazi Department of Chemical Engineering, Materials & Environment, Sapienza University of Rome, Rome, Italy

2.1

Introduction

Mechanical recycling, which is the processing of waste by physical means, represents the main approach to follow in order to recover plastics. This process typically includes different actions, such as collection, screening, manual and/or automatic sorting, size reduction, washing, extrusion, and granulation that may occur in different sequences and more than one at a time, according to the characteristics of the feed plastic waste, in terms of origin, size, shape, and composition (Hopewell et al., 2009; Ragaert et al., 2017). Foundation of each mechanical process, finalized to separate a specific material inside a flow stream containing other materials also, is to know the different properties of the target material, with respect to the actions to be applied (i.e., comminution, classification, separation). Important material properties useful to select the best separation strategies for segregation of plastic waste include: particle size, class distribution, density, magnetic and electric properties, color, shape, etc. Density usually represents one of the most utilized properties to perform material separation. Unfortunately, some polymers are characterized by very close values of density (Al-Salem et al., 2009); in these cases this property cannot be successfully utilized, especially to obtain high-quality single polymer streams. The need of powerful technologies to perform plastic waste separation, being at the same time cost-effective and able to guarantee high quality of products in terms of purity is more and more stringent in order to produce secondary plastics that are competitive in the market in comparison with the virgin polymers. In fact, the actual economic and environmental constraints dramatically increase the interest of many players (i.e., industries, recyclers, technology developers, engineers, etc.) both in waste-sorting technologies, for the production of high-quality secondary polymers, and in developing automatic sensors for quality assessment of waste-derived secondary polymers. On December 2015 plastic was in fact identified by the European Commission as a key priority in the “EU Action Plan for a circular economy” (COM, 2015) and in January 2018 a “European strategy for plastics in a circular economy” (COM, 2018) was adopted in order to use such a resource in a more sustainable way, including measures for the improvement in plastic sorting and recycling capacity and in quality of recycled plastics.

Use of Recycled Plastics in Eco-efficient Concrete. https://doi.org/10.1016/B978-0-08-102676-2.00002-5 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Use of Recycled Plastics in Eco-efficient Concrete

A mechanical process aimed to perform plastic waste recycling is based on the utilization of fast, accurate, and reliable tools and equipment specifically addressed to separate and recover single polymer streams, eliminating polluting elements (i.e., other polymers or other materials) present in the feed. As already stated, recycling plant layout has to be developed and managed taking into account the different polymers in the feed as well as the presence of other materials, both aspects in relation to the plastic waste sources (Ignatyev et al., 2014), that is: virgin and used ones. The polymer-based products that belong to the first source class (i.e., virgin waste) never reached the consumer (i.e., runners from injection molding, waste from production, changeovers, fall-out products, cuttings, and trimmings). These start-of-life plastic wastes are usually uncontaminated both from other polymers and/or nonpolymers. Obviously, they represent the higher-quality grades of polymer waste. End-of-life plastic wastes belong to the second source class (i.e., postconsumer waste). These latter can strongly vary both in quantity and in quality according to the collecting source and/or the adopted collecting strategies. Mechanical recycling can be applied to plastic waste sorting following two different approaches, that is, at macro- or microscale. Plastic macrosorting is usually performed when the waste flow stream contains the polymers to be recovered as macroobjects easy to be identified and separated. In this case, any specific mechanical action (i.e., size reduction/screening) has to be preliminary applied and waste plastics, usually bottles and containers, are separated. Specific polymer attributes are first detected by specialized sensing devices and according to their characteristics further separated, usually following air-blowebased strategies. Manual separation strategies are also applied and human knowledge is at the base of the separation, It is a labor-intensive, costly, and inefficient option, even if today plastic containers are labeled according to the constituting polymer and/or blend of polymers. Plastic microsorting is usually applied when waste plastics are recovered as flakes, that is, individuals resulting from milling actions, inside a flow stream of mixed waste characterized by different physical chemical attributes. In this case, handling costs decrease and the quantity of waste strongly increases, but more complex, and often also sophisticated technologies have to be designed, implemented, set up, and applied. These technologies (e.g., size reduction, screening, separation, etc.) are usually sequentially applied. In the latter case, sorting units and related logics, both addressed to separation and/or recovered polymer flow stream quality assessment play a preeminent role.

2.2 2.2.1

Plastic waste sources and typologies Production of plastic waste

Over the last 50 years the role and importance of plastics in our economy have grown steadily. World plastic production has increased twentyfold compared to the 1960s reaching 335 million tonnes in 2016 (Plastics the Facts, 2017), and should double in the next 20 years. In the EU, plastic production reached 60 million tonnes in

Techniques for separation of plastic wastes

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2016. The largest plastic producers are China (29%), followed by Europe (19%) and NAFTA (18%). Despite the global increase in plastic production, the potential for recycling plastic waste is still largely unexploited. The reuse and recycling of plastic at the end of life are very low, especially compared to other materials such as paper, glass, and metal. The European plastics converter demand by segment in 2016 is reported in Fig. 2.1, showing that the packaging sector accounts for 39.9%, followed by building and construction (19.7%); automotive (10%); electrical and electronic equipment (6.2%); household, leisure, and sports (4.2%), agriculture (3.3%). Other sectors, including appliances, mechanical engineering, furniture, medical, etc., account for the remaining 16.7% (Plastics The Facts, 2017). In Fig. 2.2 the European distribution of plastic waste generation by segment in 2015 is reported. It is evident that the main source of plastic waste is packaging, accounting for 59% of the total plastic waste. It can be noticed that from production to waste, different plastic products are characterized by different life cycles, depending on their use, for example, plastic packaging has a service life of less than 1 year, plastic for industrial equipment can have a service life of 40 years or more. That is the reason why the volume of collected plastic waste in 1 year usually does not match the volume of plastic production. About 27.1 million tonnes of plastic waste were collected in Europe in 2016 (Plastics The Facts, 2017), of which 31.1% was collected for recycling, 41.6% for energy recovery and 27.3% still went to landfill. Even if the percentage of recycled plastics is quite low, a positive aspect is that in the past 10 years (from 2006 to 2016) plastic waste recycling has increased by 79% and landfill has decreased by 43%. Unfortunately, even if the EU situation is improving, in many countries landfill is still the first or second option for plastic waste. Concerning plastic packaging waste treatment, in 2016 recycling was the first option accounting for 40.9%, followed by energy recovery (38.8%) and landfill (20.3%). Plastic demand by different market sectors (%) Packaging 39.90% Building & construction 19.70% Automotive 10.00%

Electrical & electronic 6.20% Household, leisure & sports 4.20% Agriculture 3.30% Others 16.70%

Figure 2.1 Plastic demand by different market sectors in 2016. Plastics The Facts, 2017.

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Use of Recycled Plastics in Eco-efficient Concrete EU plastic waste generation (%) Others 14.00% Agriculture 5.00% Non packaging household 4.00% Electrical & electronic 8.00%

Packaging 59.00% Automotive 5.00%

Building & construction 5.00%

Figure 2.2 EU plastic waste generation in 2015. COM, 2018. A European Strategy for Plastics in a Circular Economy. p. 28.

It was estimated that plastic production and the incineration of plastic waste generate a total of about 400 million tonnes of CO2 per year (Ellen MacArthur Foundation, 2016). Increased use of recycled plastics can reduce dependence on fossil fuel extraction for plastic production and contain CO2 emissions. According to estimates (Rahimi and García, 2017), recycling of plastic waste from around the world could result in annual energy savings of 3.5 billion barrels of oil. Alternative types of raw materials are also being developed (for example, bio-based plastics or plastics produced from carbon dioxide or methane), which offer the same functionalities of traditional plastics with a potentially lower environmental impact, but currently represent a very small slice of the market. Very large quantities of plastic waste, generated both on land and at sea, are dispersed in the environment, causing considerable economic and environmental damage. Worldwide, between 5 and 13 million tonnes of plastics end up in the oceans each year, representing between 1.5% and 4% of the world production of this material (Jambeck et al., 2015). Plastic is estimated to account for over 80% of marine litter. The plastic residues are transported by sea currents, sometimes even for very long distances, and can be deposited on land, break up into microplastics, or form dense areas trapped in oceanic gyres. The phenomenon is accentuated by the increasing amount of plastic waste generated every year, also due to the growing diffusion of “single-use” plastic products, for example, packaging or other consumer products thrown away after only one short use, rarely recycled, and subject to being dispersed in the environment. These products include small packaging, bags, disposable cups, lids, straws, and cutlery, in which the plastic is widely used for its lightness, low costs, and practical features. New sources of plastic dispersion are also increasing, generating further potential risks to the environment and human health. Microplastics, defined as tiny plastic fragments smaller than 5 mm, accumulate in the sea, where, due to their small size, they can be easily ingested by marine fauna, and can also enter the food chain. Recent studies have found the presence of microplastics in the air, in drinking water, and in foods, and their impact on human health is still unknown.

Techniques for separation of plastic wastes

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Furthermore, the increase in the market share of plastics with biodegradable properties creates new opportunities but also generates risks. In the absence of a clear labeling for consumers and without proper collection and processing of waste, it could lead to an increase in the dispersion of plastics and create problems for mechanical recycling. On the other hand, biodegradable plastics can certainly be useful for some applications and innovation in this sector is welcomed.

2.2.2

Typologies of polymers, characteristics, and uses

The term “plastic” is derived from the Greek word “plastikos,” meaning fit for moulding. This refers to the material’s malleability or plasticity during manufacture, which allows it to be cast, pressed, or extruded into a variety of shapesdsuch as films, fibers, plates, tubes, bottles, boxes, and much more. There are two categories of plastics: thermoplastics and thermosets. Thermoplastics can be melted when heated and hardened when cooled, the process is reversible. Due to their characteristics, they can be reheated, reshaped, and frozen many times Thermoplastics include polyethylene terephthalate (PET), low-density polyethylene (LDPE), polyvinyl chloride (PVC), high-density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS) among others. On the contrary, thermosets undergo a chemical change when heated, so they cannot be remelted and reshaped. Thermosets are widely used in electronics and automotive products. Thermoset plastics include epoxy, polyester, melamine, phenol formaldehyde, vulcanized rubber, silicone, polyurethane (PUR), etc. Each plastic is identified by a resin code that was introduced to facilitate recycling operations (ASTM, 2014). In Table 2.1 a list of the main plastic types, with their typical applications, is reported. The most diffused polymers, according to plastic converter demand, are (Fig. 2.3): PP, LDPE, HDPE, PVC, PUR, PET, and PS. Such polymers are also the most abundant in plastic waste with some variations according to different lifespan of products. Polyethylene (LDPE and HDPE) is the most abundant polymer in plastic waste, due to their dominance in packaging applications, followed by PP, forming together the polyolefin family, accounting for 56.1% of plastic production demand. Other polymers, accounting for 19.3% of the total, are mainly represented by acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polymethyl methacrylate (PMMA), polybutylene terephthalate (PBT), polytetrafluoroethylene (PTFE), utilized in many different fields, such as medical, electronics, aerospace, etc.

2.3

The plastic recycling chain

The plastic recycling chain can be divided in the following operations: Collection

Manual sorting

Material/ Screening polymer sorting

Size reduction

Washing

Extrusion & granulation

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Use of Recycled Plastics in Eco-efficient Concrete

Table 2.1 Main plastic types and their typical applications Resin code

Polymer name

Applications

Polyethylene Terephthalate

Drink bottles, detergent bottles, clear film for packaging, food trays, carpet fibers

High-Density Polyethylene

Detergent bottles, mobile components, agricultural pipes, pallets, toys

Polyvinyl Chloride

Packaging for food, medical materials, pipes, window frames, cable insulation

Low-Density Polyethylene

Foil and films for dry cleaning, bread, frozen food, fresh produce and household garbage, toys, squeezable bottles

Polypropylene

Containers for food, medicine bottles, bottle caps, bins, automobile applications

Polystyrene

Disposable cutlery, cups and plates, meat trays, protective packaging for furniture, electronic items and toys

Other. Use of this code indicates that a package is made with a resin other than the six listed above, or is made of more than one resin and used in a multilayer combination.

Other packaging

European plastics demand by polymer types 19.3%

19.3%

17.5%

12.3% 10.0% 6.7%

PS

7.4%

7.5%

PET

PUR

PVC

HDPE

LDPE

PP

Figure 2.3 European plastics converter demand by polymer types in 2016. Plastics The Facts, 2017.

Others

Techniques for separation of plastic wastes

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Each step of the chain affects the others. For example, the selection of the sorting technology will depend on the characteristics of collected plastic waste (types, composition, etc.) and the final destination of the recovered product will depend on its quality. Collection is carried out adopting different systems, depending also on the different sources, such as plastics from household waste and from industrial waste. Collection can be, for example, monomaterial, if plastic is collected as source-separated fraction, or multimaterial, if plastic is collected with other packaging materials (aluminum, glass, etc.). Manual sorting is usually necessary at the beginning of the recycling process for the preliminary removal of films, cardboard, and bulky items and is usually carried out by operators checking the waste stream on the conveyor belt. Screening is applied to remove small objects such as glass and stones. Typical screening equipment are drum or vibrating screens. Usually waste is divided into three fractions: undersize (<50 mm), middle size (from 50 to 300 mm), and oversize (>300 mm). Usually plastic is concentrated in the middle size fraction. Material/Polymer Sorting has the aim to obtain high-quality recycled plastic products, preferably single polymer stream. Sorting technologies are based on different physical-chemical properties of waste materials, such as shape, density, size, color, or chemical composition of objects. Material sorting consists in the removal of the unwanted contaminants such as pieces of metals, glass, paper, etc., from the plastic waste stream. Polymer sorting is applied to separate polymers by type; this step is of paramount importance in order to obtain high-quality single polymer stream. The different plastic waste separation technologies are described in Section 2.4. Size reduction is usually carried out by shredding or cutting techniques; such operations can be present before or after the sorting step, depending on the plant layout and on the typology of plastic waste stream. Plastics are usually shredded in flakes having a size of 5e10 mm. Extrusion and granulation: this step is necessary to produce a granulate which is easier to use for converters than flakes. The polymer flakes are fed into the extruder, are heated, and then forced through a die to form a continuous polymer product (strand) which can then be cooled in a water bath before being pelletized. The granulation process is used to reduce the strands to pellets which can then be used for the manufacture of new products.

2.4

Plastic waste separation technologies

Plastic waste separation has the main aim to remove unwanted contaminants (such as metals, glass, etc.) and to obtain high purity polymers. In the following sections the different separation technologies usually adopted in plastic recycling plants are described. The choice of the technology (or combination of more than one technology) will depend on the feed characteristics and on the quality requirements for the output products, for example, single polymer stream or mixed polymer stream.

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2.4.1 2.4.1.1

Use of Recycled Plastics in Eco-efficient Concrete

Gravity separation Dry

Air classifier Air classifiers use air as the medium to separate lighter materials from heavier ones. The waste stream enters the column with a raising current of air and lighter objects are blown upward whereas heavier ones are dropped down (Fig. 2.4). Air classifiers are usually utilized to remove light contaminants such as dust, small foam particles, paper, glass powders, and polymer foils from the main plastic waste stream. To reach this goal, aspirators, wind sifters of air-cyclonesebased techniques are utilized. The separation occurs based on the different behavior of particles when subjected to a stream of air. Even if the separation principle is quite simple, air-based classification has to take into account different parameters (i.e., particle density, morphological and morphometrical characteristics) that interact with single polymer terminal velocity, thus affecting separation efficiency and corresponding operative setup (Shapiro and Galperin, 2005). Air classifiers can be utilized after a gravity separation step or at the beginning of the process before or after a preliminary comminution stage, to properly handle/separate complex plastic-rich parts from end-of-life durables (e.g., automotive-derived parts, electrical and electronic devices, appliances, etc.).

Ballistic separator Ballistic separation is based on a simple principle, that is, the different movement characteristics of particles of different size, shape, and weight, spatially defined as 2D or 3D structures (Christensen and Fruergaard, 2011). Ballistic separation can be successfully utilized both for mixed waste containing plastics and for plastic waste streams. In the first case, film, paper, cardboard, textiles, and fibrous materials can be assigned to a 2D flat and light class of products; on the contrary, plastic containers, bottles, stone, Light

Feed

Air stream

Heavy

Figure 2.4 Schematic representation of an air classifier.

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wood, cans, and ferrous materials can be assigned to a 3D class of rolling and heavy products. In the second case, films and flakes belong to 2D individual domains; on the contrary, containers and/or crumpled containers belong to 3D individual domains. In both cases separation occurs thanks to the utilization of a so-called ballistic separator, or ballistic screen. Such a device is usually constituted by a series of screening paddles, or perforated plates, whose number, size, and shape profile can vary according to the feed rate and physical characteristics of the waste materials, affected by an orbital motion and characterized by an inclined position, usually ranging from 10 degrees to 20 degrees. The materials fed to this separator, according to their 2D or 3D structure and physical characteristics (i.e., weight, morphological and morphometrical characteristics) follow different trajectories with respect to the orbital blades movement. The 2D and light materials are conveyed to the upper part of the ballistic separator, whereas 3D, heavier, and “rolling” individuals move toward the lower part of the separator. The continuous shaking of the waste produces also a screening effect: particles characterized by a size smaller than the distance between the different screening paddles pass through, generating a third flow stream.

2.4.1.2

Wet

Sink-float separation Sink-float separation processes are based on the utilization of the different density properties of materials. Separation is based on the fact that when materials are introduced in a tank containing a fluid of a specific density, lighter materials will float and heavier ones will sink (Fig. 2.5). A sink-float separation unit is efficient when materials are characterized by quite different density values (Callister and Rethwisch, 2010). Therefore this method can be used to separate plastics from heavier materials, or polymers characterized by different densities (i.e., PET from PP/PE or ABS from

Floating plastic flakes

Sinking plastic flakes

PP, HDPE, LDPE Float plastics with specific gravity < 1 g/cm3 Sink plastics with specific gravity < 1 g/cm3

Figure 2.5 Sink-float separation.

PVC, PET, ABS Water specific gravity = 1 g/cm3

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Use of Recycled Plastics in Eco-efficient Concrete

PS), whereas it cannot be utilized for separation of polymers characterized by quite close density values, as for example polyolefins (PP, LDPE, HDPE). Presence of contaminants, air bubbles on polymer surface, polymer alteration, fillers, and additives can affect separation efficiency.

Jigging Jigging is one of the oldest methods of gravity concentration (Hori et al., 2009). Jigging can be defined as an “enhanced gravity based separation” method: a water stream is pulsed, or moved by pistons upward and downward, through the material bed. Individuals are separated according to their densities, but also thanks to the systematic and repetitive applied pulsation, whose frequency and amplitude is strictly related to physical, morphological, and morphometrical attributes of materials. With reference to plastic waste, this procedure is quite efficient in many cases, allowing to enhance polymer separation with respect to their relatively low density differences. Thanks to the repetitions of these actions, particles stratify, across the bed height, according to their specific density: the heaviest form the lowest layer and the lightest constitute the highest.

Hydrocycloning Hydrocycloning is a density sorting technology based on the centrifugal/centripetal forces and fluid resistance of different particles having different characteristics (Bradley, 1965). A slurry is usually fed to the cyclone. A selected solid/liquid ratio and operative pressure is adopted. As a result, the fluid pressure transfer produces, inside the device, a rotational fluid motion, thus permitting separation among the different materials (i.e., polymer-contaminant or polymer-polymer characterized by different densities). Lighter fractions will be transported to the upper part of the cyclone, the heavier ones to the bottom (Fig. 2.6). Light plastics stream Plastic waste feed

Heavy plastics stream

Figure 2.6 Plastic hydrocyclone separator.

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Materials have to be properly milled before hydrocyclone-based sorting. The separation mechanism can be synthetically summarized as follows. The slurry is tangentially fed to the inlet, causing the material to rotate within the vessel and ultimately to form a vortex. Heavier materials are forced outward by centrifugal force and down from the barrel section into the cone section. Materials heavier than fluid (i.e., usually water) flow down the inner wall and exit through the apex; the lighter materials sweep into the center vortex by inward fluid motion and are carried out to the outlet. When hydrocyclones are selected, several factors have to be considered in order to reach an optimal and efficient utilization: the need of relatively complex fluiddynamic circuits (i.e., presence of pumps, storage bins, pipes, valves, etc.) and the need to perform a strict feed characteristics control (i.e., constancy of water/solids ratio, polymers particle morphological and morphometrical characteristics, polymers surface status).

2.4.2

Electrostatic separation

Electrostatic separation is usually applied when dielectric particles are handled. Dielectric particles, when electrostatically charged, can be separated according to their polarity charge (Reinsch et al., 2014). The electrostatic separation architecture is commonly constituted by two electrodes: one positive and the other negative. Particle charging conditions and modality are of paramount importance in this kind of separation. Plastic particles charging is usually carried out utilizing the triboelectric effect. This effect is based on rubbing together plastic waste particles of different characteristics; as a result they transfer their electrical charge and surfaces are thus affected by different electrical charges allowing to perform separation inside an electric field where also charged electrodes are present. For example, charged plastics falling down freely in the area between two electrodes change their trajectory due to the mobilized attractive/repulsive electrostatic forces and as a consequence can be “easily” collected and separated (Fig. 2.7). This separation can be successfully applied with reference to several polymers, according to the triboelectric charging sequence (Dodbiba et al., 2001): ðþ Þ ABS  PP  PC  PET  PS  PE  PVC  PTFE ð Þ When two plastics in this sequence are rubbed against each other, the plastic closer to the positive end is charged positively and the one closer to the negative end is charged negatively. For example, if PVC is rubbed against PET, PVC is charged negatively and PET positively. On the contrary, when PET is rubbed against PP, PET is charged positively and PP negatively. Main disadvantages of this separation are linked to: (1) the operative conditions (i.e., plastics and more in general the waste stream have to be dry), (2) particle size and shape (i.e., particle surface characteristics and particle size affect the “chargeability”), (3) presence of additives/fillers (Albrecht et al., 2011) and, finally, (4) presence of dirtiness on particle surface that can change or inhibit particle surface charging.

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Use of Recycled Plastics in Eco-efficient Concrete

Plastics waste feed

Tribocharger

Negatively charged particles Positively charged particles

Positive electrode (+)

Negative electrode (–)

Output products

Figure 2.7 Schematic representation of a triboelectric separation process.

2.4.3

Magnetic density separation

Magnetic density separation (MDS) is a density-based sorting process realized utilizing a “magnetic fluid” constituted by a liquid (i.e., water) and magnetic particles (i.e., iron oxide particles of about 10e20 nm) suspended in the liquid (Bakker et al., 2009). Through a special magnetic field (Rem et al., 2013) an artificial gravity is produced, as a magnetic force. Such a force varies exponentially in the vertical direction, and the effective density of the liquid also varies accordingly in the same direction. The result is that waste particles (i.e., plastic particles) will float in the liquid at a level where the effective density is equal to their own density. In other words, particles characterized by different densities are suspended at different heights (Hu et al., 2013). Adopting this strategy, it is thus possible to separate plastic particles characterized by very close density values, such as PP and PE (Serranti et al., 2015) (Fig. 2.8) and PVC and rubber from construction and demolition waste (Luciani et al., 2015). Particular care has to be addressed to the correct fulfilling of the following fundamental steps: (1) wetting, to make the polyolefin surface hydrophilic (Hu et al., 2010), (2) feeding, separating, and collecting to avoid turbulence in the flow stream, before, during, and after the separation, negatively affecting particle flow inside the magnetic fluid.

Techniques for separation of plastic wastes

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Magnet PP

Plastic waste feed

Splitter PE Ferro fluid

PP PE

Mixing zone

Separation zone

Collection zone

Figure 2.8 Schematic representation of a magnetic density separation system for PP and PE.

Such technology is very useful to produce high-quality secondary raw materials, that is, single polymer stream with a very low presence of impurities.

2.4.4

Flotation

Flotation processes are based on the different surface wettability properties of materials (Wang et al., 2015). In principle, flotation works very similarly to a sink and float process, where the density characteristics of the materials, with respect to that of the medium where they are placed are at the base of the separation. Sometimes a centrifugal field is applied to enhance separation. Flotation works in a different way in the sense that in a liquid medium, usually water, a “carrier” is introduced, air bubbles, responsible to float hydrophobic particles that adhere to the bubbles with respect to the hydrophilic ones that sink. According to surface plastic characteristics, this technique can be profitably applied, in principle, to separate waste polymers (Fraunholcz, 2004). To enhance or reduce plastic surface characteristics (i.e., hydrophobic or hydrophilic) appropriate collectors, conditioners (Singh, 1998; Shen et al., 2002), and flotation cell operative conditions (i.e., air flow rate, agitation) can be utilized. Usually plastic flotation is carried out in alkaline conditions (Takoungsakdakun and Pongstabodee, 2007). Once floated, hydrophobic polymers are recovered as well as the sunk ones (i.e., hydrophilic) at the bottom of the cell. This technique, even if it is well-known (Buchan and Yarar, 1995) and in principle quite powerful is not widely used mainly for three reasons: (1) it is a wet technique, this means that water has to be recovered and processed before reutilization, due to the presence of the reagents and contaminants, (2) polymer surface status (i.e., presence of dirtiness/pollutants and/or of physical/chemical alteration) can strongly affect floatability, and (3) large variation of waste plastics feed in terms of composition. Flotation allows to separate PS, PVC, PET, PC, and mixed polyolefins (MPO).

2.4.5

Sensor-based sorting

Plastic sorting, with respect to other materials, and/or different polymers, is usually carried out utilizing specific materials’/polymers’ physical properties allowing separation. Separation often occurs defining handling architectures (i.e., separation

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Use of Recycled Plastics in Eco-efficient Concrete

equipment) designed and set up in order to enhance how waste particles behave in respect of the selected property to perform separation. This behavior is usually accomplished, as previously stated, through particle trajectory changes at the device output/s and/or through concentration in different section of the separation equipment. The mechanical removal of these different streams generates concentrates, wastes and, in some cases, one or more intermediate compositional product classes, called “middlings.” Material physical property can be thus considered as the “direct” responsible of separation. The adoption of sensors to perform sorting means to follow a different approach, requiring the utilization and the implementation of online analytical logics and robotic units to perform the separation. Materials in fact, have to be first detected, then identified and topologically assessed in the stream; after these steps automated devices realize the sorting. Following this approach, it substantially means to take into account two aspects. The first one is linked to the sensing principle selected to perform materials identification and the second one is related to the required actuators logics/architectures utilized to collect the materials of interest from the investigated waste flow stream. Sensorbased sorting techniques are thus substantially classified according to these principles (i.e., sensing and collection). In all cases there are three main components of the sorting architecture: a conveyor belt for materials feeding, a sensor connected to a computer analyzing data collected from the waste stream on the conveyor belt, and a pneumatic system to mechanically separate materials (Fig. 2.9). Sensors do not require contact with the materials and are nondestructive.

2.4.5.1

Visible spectroscopy

Sorting in the visible range is mainly focused on the utilization of spectroscopic analytical techniques, performed in the wavelength range 400e700 nm, or on the adoption of digital imaging. Both the approaches are not particularly efficient in the case of plastic recycling, being that the detection principle is mainly based on what is detectable according to investigated individuals’ pictorial attributes, that is, visible spectra, when spectroscopy analysis is performed, or digital color components (RGB, HSY, etc.), size, shape, and surface textural attributes when imaging is applied (J€ahne, 1993). Feed waste

Sensor

Air gun

Conveyor belt Material 1 Material 2

Figure 2.9 Schematic representation of a sensor-based sorting system.

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Both the approaches are not very efficient to perform polymer sorting; for this reason, they are not widely utilized except sometimes at the beginning of the process and when polymers are constituted by large products (i.e., several centimeters) whose color and/ or shape (Zhu and Basir, 2006) can be associated to a known specific polymer-based manufactured product (i.e., container, pipe, frame, etc.). They can be used also at the end of a plastic recycling process to sort by color a monomaterial stream of plastics, as for example, green, blue, and transparent PET.

2.4.5.2

Near infrared spectroscopy

Near infrared spectroscopy (NIR) is probably the most utilized technology in plastic recycling. It is based on the collection of reflected spectra of polymers properly energized by a light source (Beigbeder et al., 2013). The investigated wavelength range is usually 1000e700 nm; in some cases it is extended to the SWIR region (1000e2500 nm). The NIR sensor sorting system includes: a conveyor belt, illumination system and optical sensor, a separation unit with compressed air nozzles. Many different polymers can be separated by near infrared sensors as they are characterized by different spectral signature in such wavelength range (i.e., PP, PE, PVC, PET, PS, etc.). Plastics can be also separated from other materials, as paper, wood, glass, stones, etc. The reasons of the wide use of this technique are mainly related to: (1) NIR does not require any direct contact with the investigated object, (2) it can be applied defining very flexible architectures, thanks to the possibility to largely utilize fiber opticse based architectures both to energize and to collect the spectral response of plastics (i.e., analysis and identification, in respect of previously set up reference spectral libraries of the different polymers’ infrared absorption bands), (3) high detection/identification speed, (4) multiple detection (i.e., multiple check of the same sample), and (5) no color interference. The size of flakes to be sorted as well as flakes disposal on the conveyor belt plays an important role, affecting the sensor detection. Sensing probes, in fact, are characterized by a physical dimension that influences the investigated image field and, as a consequence, the analytical spatial resolution of the single sensing unit (i.e., usually installed as an array). Black or very dark polymers are almost impossible to be identified due to their low surface reflectance.

2.4.5.3

Hyperspectral imaging

Hyperspectral imaging (HSI) is an innovative fast, and nondestructive technique able to collect both spectral and spatial information from an object. The collected information generates a data structure defined “hypercube,” that is a dataset containing both spatial data (i.e., pixel coordinates: x and y axis) and spectral data (i.e., z axis, representing the spectrum associated to each pixel). The investigated spectral range depends on the sensor mounted on the device and can vary from VIS to NIR, SWIR, or MWIR regions, depending on applications. The application of this technique dramatically grew in these last years in many sectors (i.e., chemical, pharmaceutical, agricultural, food industry, etc.) and also in recycling: glass recycling (Bonifazi and Serranti, 2006), compost product quality control (Dall’Ara et al., 2012), recycled aggregates

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Use of Recycled Plastics in Eco-efficient Concrete

from concrete (Serranti and Bonifazi, 2014; Bonifazi et al., 2015, 2018b) and characterization of different plastic waste (Serranti et al., 2012a,b; Hu et al., 2013; Ulrici et al., 2013). This large use is intimately linked to some intrinsic characteristics of the HSI sensing device (Bonifazi and Serranti, 2014) as: (1) the possibility to perform a continuous monitoring of waste large flow streams as disposed on a conveyor belt thanks to the scan line camera architecture, (2) easy topological definition of the individual to sort, (3) utilization of different time scaleerelated sampling strategies, in case of specific product oriented control/quality actions to develop, (4) implementation of fast and reliable recognition logics, strongly linked to HSI detectors characteristics (e.g., possibility handle spectra, images, or both spectra and images), (5) total absence of environmental impacts and/or safety constraints related to the HSI utilized device, and finally (6) relatively low cost of the device. With reference to polymer recycling, HSI is particularly powerful (Jansen et al., 2012) allowing to implement online sorting and/or quality control strategies, thanks to the possibility to identify the spectral regions, in the NIR/SWIR range (1000e1700 or 1000e2500 nm), where polymer molecules absorb light by overtone or combination vibrations (Workman and Weyer, 2007). This behavior produces spectral signature characteristics of the polymer thus allowing its identification (Bonifazi et al., 2018a). In the last years, high-speed spectral cameras working in the MWIR wavelength range were introduced in the market in order to sort black polymers that are not classified by sensors working in the commonly investigated spectral ranges (400e2500 nm) due to the higher light absorption and the consequent low reflectance (e.g.,: Rozenstein et al., 2017). HSI-based sorting architectures are usually constituted by a conveyor unit (i.e., belt) carrying materials to sort (Serranti et al., 2006). A sensing unit inspects and continuously acquires spectra at a fixed rate. Spectra are then processed by a classification engine previously set up, according to a material spectral reference library, and individual/s recognition is performed. An array of compressed air nozzles mounted at the end of the conveyor belt provides to separate through a shot of air the recognized individuals (Tatzer et al., 2005; Pic on et al., 2010).

2.4.5.4

X-ray fluorescence

Sorting based on X-ray fluorescence (XRF) is based on the detection of the emitted wavelengths, as well as of the released energy, by a sample previously energized by X-ray whose atoms release energy generating an X-ray fluorescence radiation. The elements contained in the sample influence the emission both in terms of wavelengths and energy. Such a technique is quite powerful and it is largely used in the waste sector, mainly in wood recycling (Blassino et al., 2002) to evaluate the presence of potential harmful elements (i.e., arsenic, chromium, copper, etc.). In the waste plastic sector, this technique is primarily utilized to sort PVC from PET (Brunner et al., 2015). The use of this technique is expected to increase in the future, as it can be applied to the separation of brominated plastic from an input stream of shredded plastics. Bromine is in fact largely used as flame-retardant, especially in electronic devices. Other XRF-based approaches have been proposed as the Energy

Techniques for separation of plastic wastes

25

Dispersive X-ray Fluorescence (EDXRF) (Bezati et al., 2011) adding tracers to the polymer matrix. Sorting architectures are similar in both cases (i.e., XRF and EDXRF). Referring to EDXRF, the operative unit is constituted by an X-ray beam energizing the waste flow stream (i.e., particles transported on a conveyor belt) to analyze and sort. X-ray beam is focused and passed through the material until it reaches the detector. The signal collected by the detector is processed, the presence of tracers identified, their amount evaluated, and according to predetermined rules, the corresponding plastic individuals blow out by air. XRF does not require any sample preparation/collection; it can identify black and/ or very dark polymers, as well the presence of contaminants on polymers surface and as individuals. The disadvantages of this technique in plastic sorting is that it is not able to distinguish between polymers. Furthermore, there are some safety constraints related to the utilization of X-ray sources.

2.4.5.5

Laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an analytical technique based on the utilization of high power laser pulse that performs an ablation of the sample to analyze, thus producing plasma plumes (Cremers and Radziemski, 2006). The radiation produced by the ablated portion of the investigated material is then analyzed by a CCD-based spectrometric device (Gondal et al., 2007). Following this approach (i.e., identification of the atomic emission lines), it is thus possible to analyze the properties of waste in terms of constituent materials (Sattler and Yoshida, 1993) and/or, as in the case of polymers, elements present as carbon and hydrogen and their resulting line intensity ratio (C/H) (Anzano et al., 2008). Recently a compact and reliable architecture was proposed to perform the recognition of polymer particles following a statistical analysis starting from the information collected by the so-called laser-induced plasma spectroscopy (LIPS) and processed performing linear and rank correlations, in the wavelength interval: 200e800 nm (Anzano et al., 2006) or performing, as previously outlined, instant ratio analysis of molecular bands to identify the different energetic materials (Anzano et al., 2011). Following this latter approach PVC, PS, PET, PP, LPDE, and HDPE can be identified. Sorting architecture is constituted by mechanical units that, according to the LIPS based detection, sort polymers into their respective bins.

2.4.6

Auxiliary separation technologies

Auxiliary separation technologies, referred to as plastic waste recycling, are those allowing to clean the plastic waste stream from the presence of other materials with different characteristics and/or nature. They are usually applied at the beginning (i.e., scalping) or at the end (i.e., refining) of the process. Materials usually removed through such techniques are: (1) ferrous metals, as low-grade stainless steel, nickel alloys, etc., (2)nonferrous metals, as aluminum. Magnets (Wills, 2016) and Eddy current (Rem, 1999) based separators, respectively, are commonly utilized.

26

2.4.6.1

Use of Recycled Plastics in Eco-efficient Concrete

Magnetic separation

The magnetic separation is commonly applied utilizing belt magnets, magnetic head pulleys, and drum magnets (Svoboda, 2004). Schematic representations of the different typologies of magnetic separators are reported in Fig. 2.10. In belt magnets, the magnet is usually installed above the plastic waste flow stream (Fig. 2.10a). Belt magnet

Feed waste

(a)

Conveyor belt Magnetic particles Non magnetic particles Feed waste

(b)

Magnetic head pulley

Conveyor belt Magnetic particles Non magnetic particles Feed waste

(c)

Conveyor belt Magnetic particles Non magnetic particles Magnetic drum

Figure 2.10 Different typologies of magnetic separators. (a) Overbelt magnetic separator; (b) Magnetic head pulley separator; (c) Magnetic drum separator.

Techniques for separation of plastic wastes

27

The overhead magnetic field has a belt moving across its surface at approximately a 90 degree angle to the material flow. Ferrous metal particles are thus attracted, removed from plastics, and discharged, as the moving belt of the separator turns away from the magnetic field. Magnetic head pulleys are usually installed at the end of a conveyor belt, beneath the belt (Fig. 2.10b). Ferrous metal particles are thus held to the belt, while plastics can be downloaded. Drum magnets are commonly installed inside feeder chutes, between chutes and conveyors (Fig. 2.10c). Ferrous metals are held by the drum, until a divider provides to its discharge; on the contrary, plastic wastes continue their flow. All the previous mentioned devices are normally positioned at the beginning of the plastics recycling plant having the aim to remove large magnetic polluting individuals. To perform a strong refining/control of the final products, high-intensity permanent magnets are usually utilized (Svoboda and Fujita, 2003.).

2.4.6.2

Eddy current separation

Eddy current separation is based on the use of a high speed magnetic rotor system and is used to remove nonferrous metals (i.e., aluminum and copper) from waste plastic streams. Due to the high speed of the rotor, an electric current, called Eddy current, is induced into conducting metals. The induced electric current produces a magnetic field, opposed by the field created by the rotor, repelling the conducting metals. The remaining materials such as plastics, glass, and other dry recyclables will simply freefall over the rotor, separating them from the repelled metals (Fig. 2.11). Eddy current separators are usually located in the preliminary stages of the process. Such a choice is mainly due to the fact that Eddy current separation process is highly dependent on the size of the feed particles according to its separation principle. The magnitude of the repulsive forces depends on the specific conductivity, mass, morphological and morphometrical characteristics of the particles, and on the intensity and distribution of the magnetic field (Van der Walk et al., 1986).

Feed waste Magnetic high-speed rotor

Conveyor belt

Non metal particles Non ferrous metal particles

Figure 2.11 Schematic representation of an eddy current separator.

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2.5

Use of Recycled Plastics in Eco-efficient Concrete

Recycled plastics quality control

The quality certification of recycled plastic products is fundamental to increase their economic value and to foster their penetration in the market. Product quality assessment is an important aspect in all industrial manufacturing sectors, but this is particularly true for the recycled materials whose market is still hindered by many barriers. Recycled polymeric materials are expected to have the same high quality and performance characteristics of those of the corresponding virgin polymers; however, the achievement of high quality levels requested by the end users through the mechanical recycling process is not an easy task. Furthermore, many plastic brand owners and manufacturers distrust recycled plastics and fear that they cannot assure their need of high volumes of reliable material with clearly defined constant quality specifications. As a consequence, there is a low demand for recycled plastic, especially in high value products, and its use is often limited to low-value or niche applications (COM, 2018). It is evident that there is a strong need not only to ensure and to improve quality of plastic recycled products through technological innovation in the recycling process, but also to develop and define specific quality standards for recycled plastic products. The quality requirements must be related to the different applications of plastic products such as food contact or durable goods like electronics and automobiles. Recycled plastics can vary from virgin resins in a number of ways, including contamination originated from multiple sources (e.g., impurities, the use-phase, misuse, degradation, improper separation of materials, legacy substances, or crosscontamination during waste collection). Such incidental contaminants can affect the quality and safety of recyclates and therefore fast, cost-effective quality control strategies should be developed and implemented at plastic recyclers’ premises, in order to produce a certified output that meet customer specifications (Vilaplana and Karlsson, 2008).

2.5.1

Recycled plastics quality measurements

The quality assessment of recycled plastic products should be based not only on the same testing equipment commonly utilized for virgin resins, but also on specific characterization measurements related to the possible contamination and degradation of a plastic recycled material. Traditionally the basic measurements to evaluate plastic product quality and performance in different applications are rheological and mechanical properties (e.g., melt flow rate, tensile and impact strength, etc.). For recycled plastics it is important to assess also other important properties that can affect quality, such as the degree of mixing, in terms of presence of polymeric impurities in a single polymer stream, the level of degradation (chemical and structural/textural alteration), and the presence of low-molecular weight compounds such as contaminants, additives, etc. (Karlsson, 2004; Vilaplana et al., 2007). One of the main challenges related to the production of plastic recycled materials lies in the fact that different polymers are incompatible and immiscible at molecular level; even low levels of contaminations will affect the quality of the target stream,

Techniques for separation of plastic wastes

29

for example, the presence of small amounts of PVC in a PET stream will make it brittle and yellowish when recycled (Hahladakis and Iacovidou, 2018). It follows that plastic must be recycled as much as possible in single polymer streams. For a plastic producer, stability in composition of the plastic raw material fed to the plant is very important, since even small variations in melting point or other properties can affect the production, in terms of functionality, strength, or durability of products that is not acceptable for some high-tech applications such as medical devices or automobile components. This means that a constant and stable composition of a recycled plastic stream must be assured. The methods commonly utilized to check the quality of a single polymer recycled stream in terms of presence of other polymers are applied at laboratory scale, which means time-consuming operations, involving the presence of a trained operator, a sample collection, and preparation step. Examples of commonly adopted techniques at laboratory scale are DSC (Differential Scanning Calorimetry) and FT-IR (FourierTransform Infrared Spectroscopy). An alternative solution to check the quality of the recycled plastic products is the use of hyperspectral imaging that can be applied online directly on the conveyor belt without any sample preparation (Serranti et al., 2011; Luciani et al., 2015) (Fig. 2.12). Polymer mixing evaluation can be achieved through the definition of classification models, allowing the identification of different plastics at the same time. HSI in the NIR/SWIR wavelengths ranges (1000e1700/2500 nm) coupled with chemometrics were successfully applied to set up fast and reliable quality control strategies at recycling plant scale (i.e., better and more strict control of sorting and separation process stages) with reference to many different polymers, including PP, HDPE, LDPE, PET, PVC, etc. (Bonifazi et al., 2018a).

Spectral-imaging instrumentation Illuminant

Monitor

Moving belt

Console

PC

Figure 2.12 HSI platform working in the NIR range (1000e1700 nm) developed for quality control of different recycled polymers.

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Use of Recycled Plastics in Eco-efficient Concrete

The possible presence of harmful substances can also limit the use of recycled plastic as secondary raw materials, especially in applications related to food packaging, due to possible dangerous contamination. X-ray fluorescence can be used to check the presence of hazardous materials and elements, such as brominated flame retardants and chlorine-containing materials, both of which can only be detected at the elemental level through X-ray analysis. Other analytical techniques can be used to determine the presence of additives in plastics, such as inductively coupled plasma optical emission spectrometry (ICP-OES) or LIBS (Vilaplana and Karlsson, 2008).

2.6

Technical challenges in plastic recycling

In the following sections, some of the main current hot research topics on plastic recycling are introduced.

2.6.1

PP-PE separation

Polypropylene and polyethylene, belonging to the family of polyolefins, are the most produced plastics at global level. A mixture of PP-PE is commonly an output of a recycling plant treating plastics, especially when dealing with household waste due to their wide use in packaging. However, in order to produce high-quality secondary polyolefins that means single polymer streams, a purity of at least 97% must be reached (Bakker et al., 2009). MDS, as already mentioned in Section 2.4.3, was proposed as a powerful technology to separate PP and PE (Fig. 2.8). Its innovation is based on the use of a medium characterized by a gradient of density, allowing to separate not only materials with very low differences in density, as PP and PE, but also more than two materials in one single step (Serranti et al., 2015).

2.6.2

LDPE-HDPE separation

LDPE and HDPE are both semicrystalline polymers but the degree of crystallinity is higher for HDPE and lower for LDPE, due to the different number of polymer branches (da Silva and Wiebeck, 2017). Their mechanical separation is not an easy task, due to their very similar physical characteristics, and especially their density (i.e., LDPE: 0.926e0.939 g/cm3 and HDPE: 0.940e0.965 g/cm3). The possibility to correctly identify these two polymers by fast and reliable methods working online is still a challenge and represents an important goal to reach. In a recent study, an innovative strategy based on SWIR-HSI was explored allowing LDPE, HDPE, and other polymers to be recognized in one shot in a plastic waste flow stream (Serranti et al., 2018) (Fig. 2.13). The study was carried out at laboratory-scale, but it is very promising and it could be applied in the near future also at plant-scale, thanks to the fast growing of HSI technologies and corresponding computing power.

Techniques for separation of plastic wastes

31

(a) 0.9 PVC PP PS LDPE HDPE

0.8

Reflectance

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1000

1500 2000 Wavelength (nm)

2500

(b) PP LDPE HDPE PS

PVC

Figure 2.13 a) Average reflectance spectra in the SWIR range (1000e2500 nm) of different polymers; (b) source image and corresponding prediction map of the different polymers obtained by the application of classification algorithms on hyperspectral images.

2.6.3

Black or dark color polymers

Recycling of end-of-life black or dark color plastics is hampered by the availability of suitable technology to sort them by polymer type (Turner, 2018). NIR sensorbased sorting, commonly adopted in plastic recycling plants, is unable to identify black or dark color plastics, usually colored with carbon black, due to their very low reflectance in this investigated spectral region. Recent studies explored the potential of MWIR detection (3e12 mm) in black plastic waste sorting (Rozenstein et al., 2017). Black or dark color polymers are in fact characterized by specific absorption features in this spectral range. The first integrated efficient industrial equipment to sort black polymers using MWIR technology have started to be commercialized in the last years.

32

2.6.4

Use of Recycled Plastics in Eco-efficient Concrete

Biopolymers

Biodegradable plastics can be mistaken for and mixed with conventional plastics, contaminating recyclate streams, as they cannot be recycled using conventional mechanical recycling techniques. In recent years, the biopolymer PLA (polylactic acid) has been introduced in the market as an environmentally friendly packaging solution alternative to the very popular PET. PLA is biodegradable and compostable, being entirely made from corn or sugarcane, and it is characterized by a look and feel very similar to that of PET. As a consequence of its diffusion, the recycling industry started to be concerned by the use of this biopolymer, since the potential contamination of PLA in the PET recycling stream can have a negative impact on the physical properties, for example, on molecular weight, of extruded rPET, making the material unfit for use (La Mantia et al., 2012). The use of HSI in the NIR range (1000e1700 nm) was successfully applied to recognize and classify PET and PLA polymer flakes, in order to develop an innovative strategy for quality control and/or sorting action in plastic recycling streams (Ulrici et al., 2013) (Fig. 2.14).

2.6.5

Marine plastics

The quantity of marine plastics dramatically grew accordingly to the increased production of polymer-based goods (C ozar et al., 2014, 2015, 2017) and their improper disposal at their end-of-life (Barnes et al., 2009; Hidalgo-Ruz et al., 2012; Andrady, 2017). Marine plastic litter originates from several sources, both land- and sea-based. It can be found on beaches, seafloor, and water (Pham et al., 2014; Suaria and Aliani, 2014; Munari et al., 2016). Once plastic litter enters the ocean, it undergoes degradation processes due to UV radiation, oxidation, and wave action (Claessens et al., 2013), inducing fragmentation and the generation of abundant small plastic particles (Cozar et al., 2014), the so called microplastics, that is waste polymers individual below 5 mm. These small particles can easily enter the marine food chain, transferring

(a)

(b)

(c)

White tile Transparent PLA flakes Transparent PET flakes Expanded PLA flakes Expanded PET flakes

1 cm

Figure 2.14 (a) Acquisition scheme of different PLA and PET flakes, both transparent and expanded; (b) corresponding RGB image; (c) prediction map obtained by the application of classification algorithms on hyperspectral images.

Techniques for separation of plastic wastes

33

hazardous substances to the biota and causing important environmental impacts (Anderson et al., 2016). Their recovery is thus of primary importance even if their characteristics, due to the previous mentioned alteration effects, can influence both their behavior in the marine environment (i.e., flowing and floating properties) and the further potential collecting/recycling strategies. Polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC) are the main sources of microplastics (Rocha-Santos and Duarte, 2015; Suaria et al., 2016). Their fast, reliable, and robust identification thus represent the first step to better understand generating sources and transport mechanisms in the seas, as well as the setup of correct separation/classification strategies to maximize the recovery in different classes of products. The previously mentioned HSI approach can dramatically contribute to fulfill both the goals, as presented in a recent study (Serranti et al., 2018).

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