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Analytical Reviews

What people have to do with polymeric waste

The 20th century is characterized not only by impressive achievements in nuclear physics and molecular biology and a breakthrough of a man in outer space, but also by the intensive commercial production of new synthetic polymer-based materials which had never existed on the Earth before. The excellent properties of these new materials let them penetrate in all spheres of human life; now they are completely irreplaceable and absolutely necessary for people. However, together with intrinsic outstanding positive qualities, synthetic polymers have a significant disadvantage: unlike many natural materials, they, having worked out their resource and functions, are not quickly decomposed under the influence of the environment aggressive factors as light, heat, atmosphere gases, microorganisms and continue to exist in the form of a long-living waste, inflicting an irreparable damage to the nature. Numerous cases of a death of rare sea animals, for instance, giant sea turtles which swallow plastic bags floating on the sea surface mixed them up with medusas, their food, are known.

People have to help the nature: as though namely people manufacture polymeric materials to satisfy their needs, they also have to protect the nature from a negative influence of synthetic materials. However, polymeric waste utilization process is not simpler and cheaper than the polymer production process. People so far prefer to choose the simplest way: they stock these waste together with other rubbish on the Earth’s surface and thus produce another magnificent hand-made creation, a dump. At present, each man annually generates about 200 kg (700 kg in the USA) of waste products, out of which 10% to 15% are polymers. A share of polymeric waste is increasingly growing. If people continue to dispose waste to dumps, they will create a principally new landscape: all big settlements will be surrounded with an uninterrupted ring swell of buried waste. As a width of the swell is permanently growing, the swell will roll on cities, fields, meadows, forests and even break in the seas and oceans. Growth rate of a dump size in developed countries exceeds the population growth rate. While the world population is annually increased by 1.5-2.0%, the size of waste dumps rises by 6%. Waste products accumulated in dumps are gradually decomposing and poison the environment with decomposition products. Yet polymers are more or less inert components of rubbish, they are step by step decomposing and release substances, including super-toxic compounds of dioxin and furan range, which are hazardous to living beings. Therefore, to continue using of polymeric material growing volumes, mankind have urgently develop effective methods of polymeric materials utilization and disposal.

Let’s consider current situation in this field. Since polymeric materials are rather expensive, polymeric waste are considered as valuable products, subject to material recycling, i.e. to treatment with further production of: 1) original polymers, fillers, armoring elements 2) monomers 3) other chemical compounds suitable for application.

The first recycling method seems the most promising but difficult to be implemented. Even though polymeric wastes are carefully separated from other rubbish, it’s practically impossible to process the polymeric wastes and turn them into a polymeric recycle product with satisfactory properties due to an intrinsic feature of polymers, namely, their inability to mix with each other or, in scientific terms, their thermodynamic incompatibility. Upon mixing of polymers with even a similar chemical nature (for instance, polyethylene and polypropylene), two-phase dispersed systems with the properties much more worse than those of the original compounds are formed. So, before processing polymeric wastes, packages, for instance, through melting in granules, suitable for casting of new polymeric products, careful sorting of wastes by their chemical composition is required. For example, in processing polyetheleneterftalate bottles, a number of polyvilylchloride bottles shouldn’t exceed 500 bottles per million. It’s clear that this result can only be obtained through an exhausting manual sorting of bottles which, in addition, should be specially marked. Otherwise, it’s impossible to sort them up only by their exterior. Women from poor Philippine families are often involved in the bottle sorting process; they gather and sell for processing various polymeric package thrown onshore with the sea waves.

Sophisticated industrial polymer sorting schemes, in which process of polymer separation is based on negligible differences in physical and chemical properties of various polymers, are being developed.

In one of these schemes, package waste are crushed to small particles (a few millimeters in size), delivered to a magnetic separation unit to remove ferrous metals and then to an electric precipitator (operation principle is based on Foucault (eddy) current) to remove nonferrous metals. To remove solvable contamination, washing process is then used. After that, the following operations are performed one after another: tossing, flotation (separation by density), and then electrostatic separation. To effect more precise electrostatic separation, improved cartridge drum can be used. The drum is made from a dielectric material and rotates on a horizontally-installed shaft. An arc electrode (the circle arc angle is 150° ) is located inside the drum. Another electrode with a circle arc angle of 90° is located outside the drum, with a big-end-down gap between the electrode and the drum. Passing through the drum, polymer mixture is split into two fraction by chemical composition, and one mixed fraction. Electrostatic separation is used to separate thermoplastic from elastomers. The process goes on inclined surface, on which rubber particles move abruptly, while thermoplastic particles slide with a speed which depends on electric charge value, typical for each sort of thermoplastic. Flotation in the presence of special surface-active substances allows to effectively separate polyethylene and polyvilylchloride.

In automatic sorting system, plastics are separated based on signal received from a sophisticated optic device operable in infrared bands.

Considering the above-mentioned difficulties in collecting and sorting polymers, in Germany a tone of polyetheleneterftalate, a recycled product which is broadly used for package production, costs DM 3,000, while a tone of the original polymer costs only DM 1,600. According to the data of the German company RIGK, reprocessing of polymeric wastes from economic and ecological point of view is feasible only if the wastes are separated by sorts and if it’s possible to manufacture products which are in high market demand. Since profitability of this industry seems doubtful, European authorities took another way. They force polymer package producers and consumers to get busy with the issue of proper package utilization or destruction. The EU has adopted Directive 94/62/CE which stipulates the main requirements on package in respect of negative impact on the environment. All countries, producing package for Europe or in Europe, have to comply with this standard to avoid rigid economic sanctions to be imposed. The EU member countries, in turn, approved the statutes which regulate procedures of polymer wastes collection, processing or destruction in these countries. For instance, Decree 22/97 which allows to sell only the package meeting the European standards has been adopted in Italy. Package producers and consumers on parity basis have set up the National Package Consortium (CONAI) which coordinates collection, sorting and transportation of materials, establishes general conditions to return the wastes to the producers, and develops and adds programs on package and wastes management. Similar bodies were established and successfully operate in other European countries.

Given a drop in budget spending on implementation of the ecology conservation measures in the USA, commercial enterprises are recommended to strengthen information activity to assist municipal authorities in organizing the process of wastes gathering by types. The society should clearly know the final target of waste gathering. American Society for Plastics is realizing a program of municipal polymer wastes collection and utilization. Major attention is paid to selection of wastes collectors and signing contracts with them, submission of convenient containers to them, and organization of wastes transportation to the destination (processing plants).

Germany’s government activity is a good example of nature conservation. In 1993, this country allocated DM 60 bln to realize environmental protection programs; the stated figure is the largest in the world. 956,000 people were occupied in this field in 1994. In 1998, a new law was adopted. The law drastically reduces the possibility of burring the wastes in which organic components content exceeds 5%. The law is effective since 2005. In Germany, the DSD system (Dual System Deutchland) which functions similar to an ordinary municipal wastes gathering system is implemented. The system unites about 600 leading companies involved in manufacture consumer goods and appliances, furnish as well as in production of polymer package (containers, package tape, film, inlays, etc.). The DSD system organizes package collection and processing centers and equips plants. In 1997, a new plant for polymer wastes processing was built. The plant uses industrial and municipal wastes (films, containers, bottles, glasses, etc.) as a raw material. The processing process includes preliminary sorting of package by separate fractions: bottles and containers, films and glasses. These fractions are separately washed and delivered to a mill where the wastes are wetly ground to the size of about 15 mm. Then wastes are additionally treated from adhered particles of paper, glue, wood, and mineral contamination as well. Purification efficiency is very high. After that, a plastic mixture is directed to a centrifuge and sorted by specific weight. The plant is capable to output up to 25 sorts of a highly treated (with efficiency of 99.9%) grained plastic. Upon the consumers request, colorants, pigments, stabilizers, plasticizers and other additives can be added to the grains. Despite rather high costs, volumes of recycling polyetheleneterftalate rapidly grow. While in 1997, only 91,000 tones of this polymer were utilized in Europe, in 2002 the figure is expected to rise up to 300,000 tones, with the major share of the volume to be used for beer bottles manufacture.

20 European companies have entered the utilization process. They plan to build 10 to 15 new plants for polyetheleneterftalate secondary processing. Wide application of polymers for polyvinylchloride window block construction have set a new target - utilization of these window blocks after 20 years of their operation. 15 companies are involved in manufacture of parts for these plastic window blocks, and 3,000 companies annually install about 12 mln window blocks. The results of a research show that polyvinylchloride can be processed into granulate as many as 8 times and addition of recycled products in original plastic doesn’t worsen the properties of the latter. VEKA Umwelttechnick built a large plant to utilize old polyvinylchloride window blocks and arranged the gathering of these blocks across the country. Granulate is delivered to the window block manufacturers. Germstmeier Recyclate outputs over 100 items, made from recycled materials, for Porsche cars; the product range includes guides, supports, lids, etc. Materials are selected depending on the purpose of a part and may include recycled products made from polyamide, polypropylene, mixture of polycarbonate and ABC-plastics, mixture of polypropylene and rubber. Recycled products processing process is similar to the above-described process and has only minor modifications in respect of technological parameters.

Other examples of successful plastics recycling are available.

Multiport Recycling has been given an award for development and implementation of the technology for production of cable channels laid along railroads in Germany and Switzerland. Introduction of fireproofing compounds has allowed to significantly increase fire-resistance of the items: there service life now is 30 years, but might be increased up to 60-100 years. The said channels are of a half-cylinder design, with flanges and a lid, which can withstand heavy loads.

Ford has mastered a manufacture of air filter housings with application of polyamide recycled products which are obtained in processing of a fluff of worn floor sheets. So far, about 3 mln filters have been produced and no deviation in technology or problems in operation have been registered.

In the USA, a new method of recycling polystyrene and polystyrene foam has been developed in the USA. Under this method, raw materials are dissolved, glutinous gel is formed and then granulated.

In Australia, a technological process for production of a new polymeric material from polyethylene film cuts and cellulose residues (60 to 80%) has been developed. This material allows to produce items in extrusion and injection processes.

In 1996 in Austria, material recycling of 38,000 tones of package saved 34,000 tones of original plastic and 38 mln liters of oil and reduced carbon dioxide emission to the atmosphere by 23,800 tones. Thermal recycling of additional 20.1 thousand tones of polymeric wastes saved 15.7 mln liters of oil and cut emissions by 23,000 tones.

Ecological effect is considered the top criteria of recycling efficiency.

Wastes of many polymeric materials can be subjected to thermal recycling with consequent output of useful non-polymeric products. Polyetheleneterftalate can be de-polymerized to the origin materials, namely, ethylene glycol and terephtalic acid, with the application of a “supercritical” water which effects as an acid catalyst. Terephtalic acid is 100 percent separated at the temperature of 350-400° C, ethylene glycol is separated with lower efficiency owing to passing of secondary reactions. Under critical conditions, strong acids or bases are not required. The process goes more or less rapidly and is economically-efficient enough. Polyurethane wastes could be processed in a similar way.

Utilization of rather complex and expensive products as worn tires is a crucial problem for all industrially developed countries. The number of worn tires is huge. The USA, Europe and Japan annually produce 4.3, up to 9 and 0.9 mln tones of worn tires, respectively. In Russia, an automobile pool has reached 30 mln and a number of worn tires is also sufficient.

Of all the applied tire utilization methods, it’s necessary to mention the technology which provides for crushing of tires to particles of some millimeters in size. The crushed particles are used as an additive to road pavements and allow to improve pavement frost resistance and to increase road traffic safety owing to better coherence of car tires with a pavement. The process is rather energy intensive: to crush a tone of tires, 800-1,500 kWh of electricity is required.

The second tire utilization technology is a thermal decomposition (pyrolysis). Liquid fraction being formed in pyrolysis process can be used as an additive to rubbers or plastics, while solid fraction can be processed in activated carbon with a high adsorption capability. This tire utilization method seems very attractive and promising, since it enables to obtain useful product which are in short supply on the market. However, it’s economic effectiveness and ecological safety should be confirmed by pilot tests.

The third tire utilization technology is use or tires as a fuel to generate heat and power.

Processing of worn tires in regeneration products widely spread before is now proved economically inefficient, and regenerate production process is suspended in many countries of the world. Recent information on the possibility to break vulcanized sulfuric cohesion nets in a rubber by means of microbiological method with the use of the Sulfolobus bacteria encourages the advocates of this method. Pilot bioreactor with the capacity of 200 liters daily processed at a room temperature up to 32 kg of tires.

Owing to continuous rise in nonrenewable organic fuel price, big expectations are laid on generation of energy on plants where wastes, including polymers, are used as a fuel. In this case, there’s no need to sort wastes; sometimes it’s required only to grind wastes to large enough pieces to ensure their effective mixing with carbon-containing fuel (mainly, coal), and to inject oxygen in combustion area. To eliminate costs on grinding during combustion of worn tires, it’s proposed to effect preliminary gasification of tires. For this purpose, Termex Technologies, a Canadian company, has developed a tire gasification installation which consists of 2 independent gasifiers each rated on 400-550 tires. To initiate the process, small burners, which are turned into operation in 10 to 15 minutes before auto-combustion, are used. The process takes 6 to 8 hours. 2.8 tones of gaseous and liquid fuel with a calorific value of oil are obtained from 4.5 tones of tires. The installation is capable to process up to 500,000 tires annually; production costs are recovered in 24-30 months.

For the time being, about 2,000 garbage disposal plants of various capacity are in operation worldwide. Many of them don’t meet existing stiff ecological standards. Nevertheless, the most advanced installations are highly mechanized and automated plants which practically don’t contaminate the environment. In Germany, power plants using municipal wastes as a fuel are planned for construction. One of these plants has already been build and brought into operation near the town of Braunschveig. Brief description of the plant is given below. The installation comprises the following major components: a supply system, a hopper for waste, a boiler room, a turbine house, a gas treatment system and a stack, and a system of slag processing. Wastes are delivered to the plant in the form of pressed piles with a volume of up to 3 cubic meters (cum) or in containers. The 20 cum hopper is a storage where grinding and mixing of wastes takes place. The wastes are then delivered from the hopper to the combustion line, where they are first dried at the temperature of 80° C and then are burnt on the boiler grating at the temperature of 1,200° C. Slag is directed to a water bath wherein salts are dissolved. Then, rough impurities and metals are removed. After crushing and a 3-month storage, slag is used in road building. A 30 MW turbine is installed in a turbine house and is fed with 71 tones of live steam entering it with a temperature of 400° C and pressure 400 bars. To clean up flue gases, an atomizing drier, bag filters, and an acidic and alkaline gas washer out of which cleaned gases are rejected and withdrawn via a 120-meter stack to the atmosphere, are provided. To reduce NOx emissions, a method of non-catalytic reduction with injection of ammonia water in the first gas duct of the boiler is used. After wet washing, a pulverized puddling coke is blown in and, together with salts, dusts, heavy metals, dioxins and furans, is precipitated in bag filters.

In should be noted that a risk of the environment contamination with super-toxins such as halogenated dioxins and furans upon combustion of polymeric waste seems significantly overestimated and in a major part relates to old garbage disposal plants (incinerators). At temperatures of 1,200-1,400° C, typical for modern installations, these substances are irreversibly decomposed; the undecomposed fraction is adsorbed in adsorbing filters. Indeed, dioxin emission level at old garbage disposal plants typically reach 300 mkg per a fuel tone; dioxin emissions at modern disposal plants are only 0.6 mkg per a fuel tone, i.e. the emissions has been reduced 500-fold. To compare with, a tone of burning coal releases 1-10 mkg of dioxins, and a tone of burning petrol releases 10-2,000 mkg of dioxins.

According to data obtained from foreign resources, application of worn tires to generate heat and power is one of the most rapidly-developing markets in the waste processing industry. Combustion of tires separately or in mixture with coal (combined combustion) provides the required heat capacity of boilers and cuts total fuel consumption. Combined combustion is preferable, since ecological performance in this case is better, than in the case when only tires are burnt. A rubber share in a fuel shouldn’t exceed 20%. Fuel mixture should evenly be distributed over a grating surface; intensive injection of air is to be provided. Tire pieces size shouldn’t exceed 50 mm.

In Switzerland, France, the Netherlands and North Europe, 50-80% of wastes are burnt to produce heat and power. In Germany, 53 large plants annually burn 14.5 mln tones of wastes and generate 2.1 trln Wh of electricity. The cost of burning a tone of wastes, on average, is DM 316 for old plants and DM 120-230 for new plants. At the same time, storage of a tone of garbage on polygons costs DM 200-400. For the time being, in Germany only 10% of produced garbage are disposed to dumping grounds. In Germany, a volume of wastes per each citizen is decreasing. The example of Germany demonstrates that active position of the government and society helps to effectively resolve the most complicated environment protection issues.

However, new challenges are arising. Meteorologists are deeply concerned of a continuous average temperature growth on the planet, which rises as many as 0.2K a decade. The phenomenon is called “a greenhouse effect”, and it is solely anthropogenic in nature. This phenomenon is a result of increased carbon dioxide concentration in the atmosphere due to grown up emissions and undergoing deforestation process. To reduce global warming rate, an aggregate amount of fuel people burn worldwide has to be diminished. To hit the target, industry output has to be reduced. The last International conference on this issue didn’t give any positive result, since representatives of the USA and Japan didn’t support the proposals to cut carbon dioxide emissions. World society has to endeavor to make a breakthrough in this field.

Source: -
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Author: Ph.D. B. N. Anfimov
Publishing date: November 30, 2000



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