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Methods for reducing solid waste by reusing discarded materials to make new products. The three integral phases of recycling are the collection of recyclable materials, manufacture or reprocessing of these materials into new products, and purchase of these products. Various techniques have been developed to recycle plastics, glass, metals, paper, and wood.
Plastic discards represent an estimated 10% by weight and up to 26% by volume of the municipal solid waste in the United States discarded after materials recovery. About 2% by weight of discarded plastics is recovered. Approximately half of plastic waste consists of single-use convenience packaging and containers. Many manufacturers prefer plastic for packaging because it is lightweight, resists breakage and environmental deterioration, and can be processed to suit specific needs. Once plastics are discarded, these attractive physical properties become detriments.
The collected plastic waste is usually separated manually from the waste stream, and often it is cleaned to remove adhesives or other contaminants. It is sorted further, based on different resins. Mechanical separation techniques can be used to sort plastics based on unique physical or chemical properties.
A significant problem is the presence of contaminants such as dirt, glass, metals, chemicals from previous usage, toxicants from metallic-based pigments, and other materials that are part of or have adhered to the plastic products. Other constraints involve inconsistencies in the amount of different plastic resins in commingled plastic wastes used for recycling, and engineering aspects of recycled plastic products, such as lessened chemical and impact resistance, strength, and stiffness, and the need for additional chemicals to counteract other types of degradation for reprocessing. There may be limitations to the number of times that a particular plastic product can be effectively recycled as compared to steel, glass, or aluminum, which can be recycled many times with no loss of their properties and virtually no contamination.
Glass containers are a usual ingredient in community recycling programs; they are 100% recyclable and can be recycled indefinitely. In 1993 in the United States, glass containers, which constitute 6% of the solid-waste stream by weight, were recycled at a rate of 35%. Nearly one-third of the glass containers available for consumption in the United States were cycled back into glass containers and other useful items such as glasphalt, or were returned as refillable bottles.
The process of recycling glass is straightforward. Cullet (scrap glass) in the form of used glass bottles and jars is mixed with silica sand, soda ash, and limestone in a melting furnace at temperatures up to 2800°F (1540°C). The molten glass is poured into a forming machine, where it is blown or pressed into shape. The new containers are gradually cooled, inspected, and shipped to the customer. Before glass can be recycled, however, it must be furnace ready, that is, sorted by color and free of contaminants.
Cullet must meet a standard of quality similar to that of the raw material it replaces. Contamination from foreign material will result in the cullet being rejected by the plants, as it poses a serious threat to the integrity and purity of the glass packaging being produced. Contaminants include metal caps, lids, stones, dirt, and ceramics. Paper labels do not need to be removed for recycling, as they burn off at high furnace temperatures.
Metals must be recycled to alleviate the need to mine more ore, to reduce energy consumption, to limit the dissemination of metals into the environment, and to reduce the cost of metals. In the United States a substantial portion of these needs are met by recycling metals.
The extensive recycling is important for three reasons. (1) The energy required to recycle a metal is considerably less in comparison to producing it from ore. (2) Extracting the metal from ore produces a tremendous amount of waste material. (3) Metals that are not recycled become dissipated throughout the environment; since many metals are toxic, this can result in the pollution of water and soil. See Hazardous waste
While it is beneficial to recycle, two important problems hinder recycling: collection and impurity buildup. When a metal becomes scrap and is a candidate for recycling, it must be collected at a cost that makes it attractive to recyclers. It is useful to divide scrap into three categories—home scrap, new or prompt scrap, and old or obsolete scrap—whose methods for collection differ significantly. Home scrap is waste produced during fabrication, and includes casting waste (for example, risers), shearings and trimmings, and rejected material. This scrap is usually recycled within the plant, and therefore it is not recognized as recycled material in recycling statistics. New or prompt scrap is waste generated by the user of semifinished material, that is, scrap from machining operations (such as turnings or borings), trimmings, and rejected material. This material is collected and sold to recyclers and, if properly labeled and segregated, it is easy to recycle and is valuable. Old or obsolete scrap is waste derived from products that have completed their life cycle, such as used beverage cans, old automobiles, and defunct batteries. The collection and the impurity buildup problems are most severe when considering old or obsolete scrap.
Paper and paperboard for recycling come from a variety of sources, including offices, retail businesses, coverters, printers, and households. Paper products that have been distributed, have been purchased, and have served their intended purposes are considered postconsumer waste. Other sources, such as scrap paper generated in the papermaking process (mill broke) or converting operations (such as trimmings from envelopes and boxes), are considered preconsumer waste.
Recycled paper fibers are used in the manufacture of many recycled-content paper products such as paperboard, corrugated containers, tissue products, newspapers, and printing and writing paper. They can also be used in other products such as insulation, packing materials, and molded egg cartons and flowerpots.
Collection is the crucial first step in recycling. It occurs in curbside programs, in drop-off centers, in paper drives, and increasingly in commercial collection systems run side by side with waste collection for landfill or incineration.
Reprocessing begins by sorting waste papers by grade and level of cleanliness. Next, the waste paper (usually in bales) is mixed with water in a slusher or pulper to produce a fiber-and-water slurry. In this pulping stage the paper is agitated until broken down into fibers, and large-size contaminants (greater than about 5 mm or 0.2 in.) are removed when the pulper is emptied through the screen plate. Depending on the intended product, chemicals such as surfactants are added to the pulper to help remove undesirable materials from the fibers for separation in later operations.
The pulp is then pumped through several different-size slotted or perforated screens to separate medium-size contaminants (usually 5–0.2 mm or 0.2–0.05 in.) from the pulp. Screening is generally followed by centrifugal cleaning, where the pulp is subjected to a vortex in a tapered cone. Using specific designs, cleaners separate high-specific-gravity materials, such as dirt and sand, and low-specific-gravity materials, such as styrofoam and some plastics, from the pulp. See Paper
Waste is generated at every stage of the process by which a forest tree is turned into consumer and industrial products. Additional waste is generated in the disposal of those products. Wood waste is also produced by the homeowner and by large and small businesses and is generated from landscaping and agricultural operations such as pruning and tree removal. While these processes do not strictly return wood to the economy in its original form, they have the effect of diverting wood residues from the landfill, and thus they may be included under a broadly interpreted definition of recycling.
Wood recycling begins with wood separation from the waste stream. Recovered materials can be processed into various products, including fuel, raw material for particleboard or other wood-composite panel products, compost, landscaping mulch, animal bedding, landfill cover, amendments for municipal solid waste and sludge compost, artificial firewood, wood-plastic composite lumber and other composite products, charcoal, industrial oil absorbents, insulation, and specialty concrete.
Most of these products require that the wood be ground into small particles. A typical grinder is a hammermill, although a variety of grinders are used. The size of the particles is determined by the end use of the wood; sizes smaller than about 20 mesh are called wood flour (the particles passing the 20-mesh screen are usually less than 0.8 mm in size). Wood may then be passed over an electromagnet which removes items made of ferrous metals such as nails or staples. If additional processing is performed, it is typically to separate wood particles by size. This is accomplished in two ways: the particles can be passed through a series of screens of different mesh size and the various-sized particles can be collected from the screens; or the particles may be separated in a tower with air blown in the bottom and out the top; the particles distribute themselves in the tower, with the small, light particles on top and the large, dense ones at the bottom. See Wood products