Recycling Technologies

Definition of the Subject and Its Importance

Recycling technologies can be defined as the whole of procedures designed to set up physical-chemical actions, at an industrial scale, that perform the recovery of materials and end-use products resulting from the collection of household or industrial wastes. The materials to be recovered and recycled, obviously, influence both processing technologies and plant layouts. In this section an in-depth analysis of the problems arising when suitable recycling technologies must be designed, implemented, and set up is presented with particular reference to paper, glass, metals, plastics, and textiles (not organics or C onstruction and D emolition (C&D) waste ). Recycling technologies must be approached from a processing perspective, that is, by defining a sequence of steps and actions where the waste flow stream feed, and the different products resulting from the different sequential processing steps, are handled in order to produce one or more outputs of materials to reuse. Obviously, processing strategies and equipment must be selected with both low environmental impact and positive economic perspectives in mind. Dealing with waste often means dealing with complex products, that is, products constituted of one or more materials of interest but also of polluting material. The economic value, per unit of weight, of the materials to recover is usually low: recycling technologies thus must assure high production, while minimizing plant investments and management costs. From this perspective, a full characterization of the input waste streams and complete control of the different phases of the recycling process are a key issue when recycling technologies are selected and applied. In this section, for each of the different materials, methodologies, procedures, and logics are presented to preliminarily identify and quantitatively assess recycling technologies according to the characteristics of the materials to be recovered.

Introduction

Recycling can thus be considered as something related to nature, and as consequence, to humans. Since the beginning of time people have needed to find a way to dispose of and/or to recycle waste. Obviously, technology influenced and continues to influence recycling strategies. In early pre-industrial times waste was mainly constituted of combustion residues, wood, bones, bodies, and vegetable waste. A simple recycling approach, mimicking what happens in nature, was to dispose of these wastes in the ground. Wastes thus became compost, helping to improve soil. Nearly 4,000 years ago there was a recovery and reuse system of bronze scrap in Europe. Composting is known to have been a part of life in China (2000 BCE). Ancient rubbish dumps excavated in archeological digs reveal only tiny amounts of ash, broken tools, and pottery. In Knossos (Crete, Greece), traces of landfill sites exist, dating from 3000 BCE, where waste was placed in large pits and covered with earth at various levels. The evolution of humans from nomadic hunter-gatherers to farmers increased waste production. Waste could no longer be left behind, and it soon became a growing problem.

The First World War, the Great Depression, and the Second World War each contributed to recycling. Recycling, in fact, was a necessity for many people to survive or for a nation to support the war effort. Nylon, rubber, and many metals began to be recycled during this period. The practice of recycling continued in many countries after the Second World War came to an end, especially those nations with a high dependence on resources, such as Japan. However, in other countries, with the post-war years' economic boom, consciousness about the importance of applying policies addressed to recycling and recovering materials and products at the end of their life cycle rapidly decreased. In the 1960s and 1970s, the importance of recycling returned with the Environmental Movement (e.g., the first celebration of Earth Day was in 1970), and a constant and continuous growth followed.

The mass production of the Industrial Age is the main cause of the past low level of consciousness about the importance of recycling: when products can be produced, and/or purchased, at low cost, at least if compared with the income, it often seems more economical to just throw away old items and purchase new ones. This "disposable goods" mentality and the corresponding problems of waste disposal have created environmental problems that today all countries face today when adopting recycling technologies.

What are the benefits related to a wider use of recycling technologies?

The first benefit is obviously related to the reduction of the environmental impact of human activities. The possibility of significantly decreasing the industrial use of non-renewable resources, such as primary raw materials and fossil fuels, by utilizing products resulting from recycling represents an important step forward toward environmental protection and energy savings; both aspects strongly contribute to a reduced exploitation of natural resources. The second benefit is the reduction of the environmental impact related to waste dumping . Less waste disposed of means less natural sites to select and manage for waste storage and less risk to the environment in terms of soil contamination and surface water and groundwater pollution. The third benefit is related to better design and manufacturing of products, from the simplest (e.g., paper, glass, metals, or plastic containers) to more complex ones (e.g., household appliances, cars, etc.) from the perspective of recycling (e.g., ease in dismantling) at the end of their life cycle.

What problems are to be faced in increasing the use of recycling technologies both quantitatively and qualitatively?

The lack of the public acceptance toward recycling and the subsequent low growth of the related W aste-D erived-P roduct s (WDPs) market are the main factors negatively affecting recycling in quantitative terms [4]. Such problems have progressively decreased thanks to (1) the definition of a clear traceability route of the WDPs, (2) the technical-economical improvement of recycling products, and (3) new legislation at the national and/or international level stimulating recycling and utilization of recycled products. In qualitative terms, an obstacle to wider "up-to-date" utilization of recycling technologies is related to concerns about the application of innovative technologies inside existing recycling layouts. Today, a great amount of equipment (e.g., comminution, classification, and separation units), and related operative technologies, is easily available on the market. Sometimes, lack of knowledge of specific control tools, necessary for correct handling and control of the equipment, can affect final recycling plant layouts and overall quality of the plant itself in terms of capacity to adapt to new possible future market requirements. From this perspective, the utilization of equipment and inspection tools for waste products quality control are fundamental for a modern, efficient, and profitable recycling technologies implementation. Both equipment and control device systems must be selected and fully integrated according to: (1) waste feed attributes, (2) possible feed variation, and (3) required final products characteristics. These three aspects must always be taken into account in the definition and analysis of the recycling technologies utilized for all the materials described in this section.

For paper, glass, metals, plastic, and textiles, the above-mentioned aspects play a different role and assume a different importance. In paper recycling feed characteristics are relatively easy to control. Final product characteristics, that is, the quality of the recovered fibers, is preeminent. In glass recycling, the main problem is feed quality, especially for the recycling of glass collected from M unicipal S olid W aste (MSW) , where the presence of "ceramic glass" can strongly impact the further processing and final quality of recovered glass fragments. With metal recycling, the aim is to maximize the "correct" recovery of non-ferrous metal alloys. The achievement of this goal strongly influences the development of innovative sorting/detection logic in order to assure the requested final products' characteristics. In plastic recycling, both feed and final products influence the selection of the separation devices and strategies, as well as the sensing technologies required for quality assessment during the different processing stages. Finally, dealing with fiber recycling requires maximizing source material identification and its preliminary separation. Process and technologies are quite different for textiles and carpets; carpets are much more difficult to recycle than textiles. Here, we have briefly outlined the different problems that must be taken into account in the recycling and recovery of the most common waste materials. In the following, such procedures will be analyzed and the related processing/control devices and actions illustrated.

Recycling and Materials to Recycle

Recycling Technologies : Paper

Paper is usually made from raw material wood pulp and fiber. Vegetable fibers are mixed and "cooked" until the fibers are sufficiently softened, chemicals (e.g., lye) are added to enhance and accelerate softening. The pulp is then "screened" over a screening media. Water is dropped off and/or evaporated. The material is then pressed for further water removal in order to obtain the "paper sheet." The quality and arrangement of the fibers affects the overall quality of the final manufactured paper material [5]. With this in mind, recycling technologies applied to waste paper are primarily aimed at maximizing recovery of the fibers.

Paper production dates back to the ancient Egyptians (e.g., papyrus paper). Around 200 BCE Cai Lun, a Chinese court official, made paper from tree bark and old fish netting. Its production was considered as a remarkable secret and only 500 years later were the Japanese able to acquire the secret. Papermaking spread to the West when some Chinese paper makers were captured by Arabs after the defeat of the Tang troops in the Battle of Talas River (751 AD). The first European paper mill was built at Jativa (Valencia, Spain) around 1150. From that time to the fifteenth century, paper mills were located mainly in Italy, France, Germany, and England; by the end of sixteenth century they were located all over Europe. In 1719, Rene de Reaumur, a French scientist, observed wasps chewing slivers of wood and building their nest starting from such a fiber paste. The use of wood fibers for papermaking started from this observation.

Waste Paper Characteristics

Waste paper is usually subjected to sorting according to its origin and characteristics. Such characteristics have been quantified at in Europe with the definition of the European List of Standard Grades of Recovered Paper and Board [6]. Waste graded papers are then pressed and handled as bales. Bales can be thus assumed as the secondary raw materials fed to waste paper recycling plants.

As a result, waste paper characterized by high quality grades (e.g., paper-mill production scrap and office waste) requires simpler processing and can be profitably applied as a primary paper pulp substitute in applications such as paper printing and tissues. Waste paper of intermediate grades (e.g., newspapers) must be subjected to a stronger processing, mainly for de-inking, and can be used again by the newspaper industry. Finally, waste paper of lower grade is utilized for packaging and board.

Waste Paper Recycling Technologies

Re-pulping

Re-pulping can be carried out in batch or continuous conditions. In batch conditions waste paper, H₂O, and chemicals are all charged at the beginning of the process and are removed, all at once, at the end of the process, then the process starts again. In continuous conditions waste paper, H₂O, and chemicals are continuously added to the pulper, as the pulped product is continuously removed.

Screening

Cleaning

Deinking

Pulp deinking removes printing ink and "stickies" (sticky materials like glue residue and adhesives). Deinking is usually performed in two steps: (1) washing and (2) flotation. Small particles of ink are thus preliminarily rinsed from the pulp with water by washing, then large particles and "stickies" are removed with the help of chemicals and air bubbles by flotation.

Froth flotation technology has been developed and used for many decades in the mineral processing industry before the technology was adopted by the pulp and paper industry for the deinking of waste papers at the beginning of the 1960s [10]. A flotation process is based on the surface properties of particulate solids systems when suspended in a fluid. Particles according to their, natural or caused, hydrophobic or hydrophilic characteristic tend to adhere to bubbles and float. During flotation deinking, pulp is thus fed to one, or to a bank of, flotation cells, where air (e.g., bubbles) and chemicals (e.g., surfactants) are also present. The surfactants cause flotation of the ink and sticky materials. Air bubbles carry the ink particles to the top of the cell/s, where the foam is continuously removed, realizing the required pulp deinking.

After flotation, if necessary, pulp is further beaten, or "refined," in order to separate, as much as possible, fibers, avoiding fibers bundles. When white recycled paper is required, pulp is bleached with hydrogen peroxide or chlorine dioxide.

Water and Solid Waste Treatment

Production of both virgin and recycled paper gives rise to pollutants that are discharged to water (e.g., effluents). Studies providing comprehensive comparative evaluation of the environmental impact linked to the effluents generated from recycling plants and those from paper mills demonstrated the environmental impact of the former is lower than that of the latter. In any case, environmental problems related to paper waste recycling are, with reference to the other recycling technologies, further described in this chapter, those presenting a higher impact [7]. The different waste paper processing stages, and related utilized technologies, are, in fact, always carried out in wet conditions and with a large quantities of water and chemicals.

Water. Four key parameters have to be fully monitored in the waste water resulting from waste paper processing : total suspended solids (TSS), biological oxygen demand (BOD), chemical oxygen demand (COD), and chlorinated organic compounds (AOX). De-inking is the main cause of TSS and BOD, and sometimes these parameters are comparable with the same produced processing virgin pulp. On the other hand, COD and AOX are always lower in effluents resulting from waste paper processing. Waste water must be properly processed before it can be re-utilized or before release in the environment. The significant decrease, in recent years, of Cu, Cr, Pb, Ni, and Cd in printing inks dramatically contributed to reduce heavy metal presence in waste water, sludge, and final recycled-paper-based-products.

Solid wastes. The sludge resulting from waste paper processing contains a solid fraction ranging between 30% and 50%. It is mainly constituted by short fibers, fillers, and ink from the de-inking process. Their relative proportion depends on waste paper source characteristics and processing strategies applied to obtain a final product of the required characteristics. Usually the wastes are sent to dumps. In recent years, different attempts have been made to further process and/or re-use them: composting [11], and removal of clay [12] and other fillers [7] for re-use or their utilization for energy production [13].

At the end of all the above-described processing stages recycled pulp fiber finally enters the machine for manufacturing recycled paper sheets. Waste-paper-recovered-fibers can be used alone, or blended with virgin ones to achieve better strength, or smoothness, of the final paper product.

Recycling Technologies: Glass

Glass is made of three relatively simple raw materials, silica sand, limestone, and sodium carbonate, which are melted together at high temperatures (about 1,500°C). Additives can be included to modify some properties, such as color, refractive index, durability, etc. [14].

Examples of glass manufactured goods can be found from several thousand years BCE, when such material was used for ornaments. In the Renaissance period, glass use increased. Vessels, bottles, and other glass containers started to be produced and utilized for both decorative and everyday use. At that time glass manufactured goods were expensive to produce. Large-scale production started with the Industrial Revolution and mass production of glass containers began at the onset of the twentieth century. Together with the increase in production and larger use came the problem of handling glass waste. Glass manufacturers produce a large quantity of products of different characteristics that are addressed to different uses. Glass's physical properties, at high temperature, are close to that of a viscous fluid, and as a consequence it can be worked, by craftsmen or on an industrial scale, to obtain final products of practically nearly infinite number of shapes and characteristics. For this reason glass can be found in, according to its composition and use, several products, ranging from those commonly used at home (e.g., bottles, vases, jars, mirrors, etc.), to those utilized in the automotive sectors (e.g., windscreen) and in industry (e.g., fiberglass for the production of Glass Reinforced Plastics (GRP), Glass Reinforced Cement (GRC), special thermal and/or acoustic insulating panels, X-Ray and cathode tubes, etc.). It is thus easy to understand that waste glass production, and its recycling to produce mainly "new" glass containers, assumes great importance.

Recycled glass mainly comes from the selective collection of solid urban waste (bottles, jars, various containers, etc.), usually done by citizens, and only partially from the refuse of glass goods manufacturing and/or glass-based products at the end of their life-cycle. As a consequence, waste glass collection represents one of the most critical steps of the entire recycling process, and the following recycling technologies and separation strategies are strongly conditioned by the criteria and the methods followed during collection. The quality of the collected materials can be quite different, according to the level of knowledge and, more generally, the "education" of the people involved. As a matter of fact, the quality of the glass collected for recycling can strongly differ from region to region or, in the same city, from district to district.

The following discussion is based on urban waste collection as the source of the glass. It is important to consider the final destination of the recycled glass, which can be identified by the classical market categories where glass in commonly utilized, that is: (1) container production, (2) construction industry, (3) special concrete production (e.g., partial substitution of aggregates by glasses), (4) road pavement (e.g., special asphalt where the coarse fraction is partially substituted by glass), (5) abrasive products, (6) wool glass, etc.

The recycling technologies described for glass recycling will be primarily addressed to producing an economically valuable cullet to use to make new containers. Recycled glass is not equally re-utilized in all the above-mentioned market sectors. Only a small fraction is, in fact, re-utilized in fiberglass, bricks, concrete, and asphalt production. This is mainly for two reasons: (1) cullet quality sometimes does not fit well with the quality standards required in some of the these sectors and (2) the glass container industry is the most interested in waste glass reuse (due to the high cost of primary raw materials versus the relatively lost cost of each single glass container).

Cullet characteristics must satisfy strict conditions to be re-utilized for container production. These characteristics are primarily related to both presence of polluting elements and color of the fragments. Furthermore cullet size class distribution is another important parameter to control. Usually particles around 1 or 2 cm are preferred both for handling and quality control purposes.

Contaminant removal and cullet color sorting are the main goals when recycling glass. Furthermore, such goals must be reached using a process that does not produce too fine particles. As a consequence recycling technologies must be designed to fulfill these goals.

Cullet Contaminants Definition

There are two classes of contaminants: materials not constituted by glass (e.g., ceramics, stones, masonry, organics, and heavy metals) and glass fragments of the wrong color, that is cullets whose color characteristics are different from that of the class they belong to (cross-contamination).

Non-glass materials . Ceramics and stones, which have melting points higher than that of the glass, remain un-melted inside the vitrified matter and as a consequence, even if present in a small amounts, degrade the mechanical characteristics (resistance) of the manufactured products (bottles, jars, etc.). Furthermore, they can seriously damage glass processing equipment, increasing maintenance costs. Lead and heavy metals, according to their high volume weight, settling on the bottom of the fusion crucibles and have a corrosive effect on the refractory material, causing, in some conditions, the perforation of the refractory material itself. Optical sorting devices are commonly used to identify and automatically remove non-glass materials. Among polluting materials special attention has been addressed, in recent years, to ceramic glass. This material rapidly increased its presence in waste glass products, mainly due to the introduction on the market of a large amount of ceramic glass manufactured goods, such as dishware, cookware, etc. [15]. Such material, even if seems quite similar to classic glass, is characterized by a different behavior (i.e., higher fusion point) when melted inside glass furnaces, where cullets are usually fed together with natural raw materials (quartz sands) [16]. As a consequence, the presence of ceramic glass reduces the production rates of the furnace, which needs to be shut down to be cleaned more frequently, and sometimes causes damage that requires the furnace to be rebuilt or replaced. Classical optical sorting devices are practically "blind" to ceramic glass, as its physical-chemical characteristics are similar to those of glasses.

Cullet cross-contamination by color. Glass has, according to its color, a different destination of use and, as a consequence, different market value. The use affects the value of the glass containers; as a consequence, white glasses have higher value than the so-called half-white or colored glass (brown, yellow, green). Cullets, that are collected without distinction of color can be primarily used for the production of green glass and only in part for the production of yellow glass. The production of white glass requires that only cullet of that color be employed. Cross-contamination can thus represent a problem because it always contributes to depreciate the cullet's value. For this reason cullet optical sorting by color is extensively utilized.