Kinkajou : Sorting : Our Next Recycling Topic
Once the waste has been collected it must be sorted into streams of like materials suitable for industrial recycling processes.
Erasmus :The most common process used today for recycling is commingled recyclates being collected and delivered to a central recycling facility. Then typically in a series of stages, automatic processes extract materials into “like streams” of recyclables.
- Firstly, large items such as timber or large chunks of metal are removed to prevent machinery from jamming.
- Secondly, automated machinery such as disk screens and air blowers separate the recyclables by weight, essentially separating lighter paper, cardboard and plastics from heavy items such as glass and metals.
- Cardboard is usually differentiated from the light items stream.
- Strong magnets may be used to separate ferrous materials (magnetic).
- Nonferrous materials can be ejected by magnetic eddy currents. In this process a rotating magnetic field induces an electric current in items such as copper or aluminium, in turn creating a magnetic eddy current which allows the item to be ejected from the rest of this recyclate stream.
- Finally, glass is ejected and sorted automatically, generally by colour. Small glass fragments less than 10 mm in size often need to be extracted manually and are collected as “glass fines”.
Kinkajou : Tell us about some of the problems that arise in sorting process?
Erasmus : The collection process itself can cause substantial problems in recycling. For example, the compaction phase in rubbish collection often results in many glass materials being crushed.
This creates problems in extracting glass particulates from processed waste streams such as paper pulp streams and in isolating these particulates and redirecting them to the glass recycling stream. It would obviously be easier for glass to be recycled if it were not crushed by rubbish trucks forced to compact their load to improve the economics of garbage transport.
Soiling of the various garbage streams, especially paper or plastic, is significant for these types of materials. High-quality of sorted waste in terms of purity of the type of waste and freedom from contamination or soiling is important if these materials are to be suitable for use as feedstock for recycling processes.
Rubbish Bin Day
If materials such as paper cardboard become wet, the usefulness of recycling can be substantially reduced due to the growth of contaminating microorganisms such as moulds.
Wet materials cannot be stored wet as this encourages decay and growth of spoilage organisms. When paper and cardboard are wet at the start of the recycling process, they are processed immediately to minimise decay and spoilage.
Kinkajou : PLASTIC SORTING: This sounds difficult.
Erasmus : A number of technologies are used to differentiate plastics. A flotation process may sort plastics by density, the heavier plastics sinking and the lighter plastics floating. These plastic streams are then further differentiated. These can be by mechanical procedures.
Another technique involves x-ray fluorescence to detect chlorine atoms in a sample, allowing other plastics with similar numbers of chlorine atoms to be identified and separated from the mixed stream of plastics recyclables. Infrared sorting is another technique used.
Exposure to infrared light causes a plastic to omit light of specific wavelength unique to its chemical composition, again allowing separation of plastics into streams of plastics with similar chemical composition.
These spectroscopic techniques generally allow the identification of PET (a.k.a. polyethylene terephthalate = type I or resin code 1) plastics and HDPE (Type II or resin code 2 plastics).
Plastic Polymer Life Cycle
Kinkajou : But don’t plastics carry a code to tell you about the recyclability?
Erasmus : The resin identification code (: letters or numbers) is stamped or printed on the bottom of containers and surrounded by a triangle of arrows. It is used to facilitate identification of plastics and thus to facilitate recycling.
Many plastics have characteristics which cannot be discerned by human senses. Codes can therefore assist recycling. However, if this code is to be used it must be used on intact recyclable products (e.g. plastic soft drink bottles).
Erasmus : The situation recycling plastics is more complex than just the consideration of the type of plastic.
Erasmus : Difficulties with these methods arise from the addition of colourings, adhesives, glues, and additives at the time of plastics manufacture.
This can result in manual methods needing to be used whereby a stream of recyclable debris moves along a conveyor belt past a human operator for identification. Obviously this method is very labour intensive and subject to human error.
Kinkajou : What other waste collection processes are used?
Erasmus : Another collection process is mixed waste collection. In this process recyclers are collected mixed in with the waste. Contamination of recyclable materials is unavoidable.
It is cheaper for city councils because there is no requirement to pay for separate collections of recyclates/ general waste and no public education is needed. Typically, this process gives a higher yield to landfill dump streams and carries higher costs than other methods of recycling. Sorting occurs in a central location, typically the dump.
Such recycling of mixed waste or commingled recyclables represents a “single stream” system. Recyclates are identified by automated systems and separated from general waste. Soiling and contamination of the recyclable waste stream is a problem.
Kinkajou : Tell us about source separation sorting.
Erasmus : Currently source separation is the preferred method for initiating recycling today. Residential or industrial uses identify and sort recyclables from the general waste stream, storing recyclable items in separate containers.
Separate Containers for Recycling
Kinkajou : Collection and transportation
Erasmus : Collection and transportation costs can represent 70% to 80% of the total cost of recycling. Different sorts of recycling events are planned to reduce these costs. In Brisbane, weekly collection of general waste occurs and second weekly collection of recyclable waste occurs.
Some householders may opt to have a green waste bin for an extra fee payment, which is collected second weekly. An annual curb-side collection event is planned for each household to allow for the disposal of larger items, which typically would not fit inside waste disposal bin without substantial processing (e.g. cutting up).
In some regions a point-of-purchase collection model has been set up. Typical examples include retailers being required to receive and organise electronic products for collection and recycling. This can be implemented in formats ranging from annual to daily incremental. (I.e. People drop stuff off daily at retailers or do a once a year drop off at manufacturers/ retailers).
Collection depots such as petrol/gasoline stations have also been used to assist hydrocarbon/oil recycling and lead acid battery recycling.
Recycling is an activity which appears to have strong support from the community due to a perceived favourable environmental effects and because it appears to conserve valuable resources. Most community level recycling is done voluntarily by householders with minimal government involvement or legislation.
Legislation has however been useful in changing the behaviour of some manufacturers and retailers. For example in Australia, legislation has been used to restrict the use of polystyrene in food distribution. Waxed paper wraps which are much more easily recyclable are now commonly used, a different situation to that existing approximately 20 years ago.
Erasmus :Though we have talked a lot about residential recycling, is important to realise that approximately 60% of waste generated arises in industry such as manufacturing or retailing.
Due to high volumes at more specific sites, legislation has a high impact and recycling activities can be more formal and structured, and hence more efficient.
Although many government programs are concentrated on recycling at home, 64% of waste in the United Kingdom is generated by industry.
Quality of recyclate
The three areas of focus are:
- Collection systems and input contamination
- Sorting facilities – material sampling and transparency
- Material quality benchmarking and standards
Paper Recycling during war
Kinkajou : Let’s talk about Glass.
Erasmus :Recycling based on returning a deposit to people for recycling specific bottles to a collection point, is becoming less common.
Most glass tends to be recycled as a system issue by waste disposal organisations such as city councils and their employees or sub-contractors.
Key issues are removal of contaminants such as: metal caps, plastic lids, dirt, organic waste, associated ceramic materials, or paints and adhesives. (Paints often contain metal particles as part of the pigment). Paper does not generally need to be removed as it readily burns off readily at the high temperatures used in melting process.
Glass must be sorted according to colour to facilitate recycling. Glass fragments are generally accumulated to a separate “fines” collection process.
Scrap glass is mixed with silica sand, soda ash and limestone in a furnace and melted at temperatures up to 1600°C. Molten glass is then fed into specific machines which form, blow or press new glass shapes.
Glass can be recycled indefinitely. But there are complexities in such a simple process as well. To facilitate the re-manufacturing of clear glass products there are tight restrictions for colored glass going into the re-melt process.
Recycled product labeling aims to utilise consumer financial support and encourage recycling by purchasing recycled products, thereby altering the economics of the recycling process.
Scrap Metal Recycling
Kinkajou : Let’s talk about Metals
Erasmus : Metals are also obviously desirable materials to recycle. By using scrap metal as a feedstock for metal production, substantially less waste material is generated then if virgin ore were used from a mining process.
Energy usage for recycling is substantially lower than in mining. Most metals corrode and can contaminate environments, so recycling also makes sense for this reason.
The big problem with recycling metals focuses on the cost economics of collection and sorting. Most metal recycling programs focus on source sorting. Specific recyclers collect specific materials. For example some recyclers collect cars and car parts. Parts are often stripped and vehicles crushed to enable easier transport for recycling. Some recyclers extract copper.
Aluminium cans are differentiated, and crushed. Nonferrous metals such as tin bronze or brass are accounted for and stored separately prior to reprocessing. Once sufficient stores have accumulated, they are sold from collectors to processors.
Some materials such as batteries are collected separately again, to allow differentiation of their lead content from other metals to facilitate easier conversion back to raw materials.
Metal Smelting recycling
Kinkajou : Let’s talk about Paper
Erasmus : Collection and differentiation are crucial in recycling paper. Cardboards are especially important due to the long cellulose fibre lengths they contain which make recycling paper into other products much easier. Recycled papers often require the addition of long fibre cellulose to enhance the quality of the paper.
Papers generally need to be sorted based on cleanliness or contamination. Once sorted into bales, paper is wet and pulped, then screened to remove particular contaminants, notably particulate soil, glass fragments and material such as plastics or styrofoam.
Once cleaned paper is pulped or mixed it is then heated, then combined with additives and chemicals to facilitate papermaking processes. Air is pumped through the mixture to create foam removing approximately 50% of ink.
Further chemicals are then added to facilitate de- inking. Cellulose fibre length needs to be adjusted to ensure a useful paper pulp mix. Paper pulp slurry is then wire screened to drain off water and to form a sheet.
The sheet is then passed under heavy rollers to squeeze out the water and to form a paper sheet. Generally heated rollers are used to dry the paper and iron rollers are used to straighten the paper.
Kinkajou : Tell us about the Advantages of recycling paper:
Erasmus : Making paper from recycled materials uses 99% less water and 50% less energy than producing paper from raw materials.
Recycling 1 kg of paper or cardboard reduces the production of greenhouse gases by 1 kg
The production of 26 sheets of paper produces the same amount of greenhouse gas as driving a car 1 km .
It takes approximately 2.5 tons of raw radiata pine from forestry to make just one ton of newsprint.
One tree can make about six reams (approximately 3000 sheets) of A4 paper.
Paper breaks down slowly in landfills due to low oxygenation. This results in the production of methane, a greenhouse gas. Paper in landfills produces more significant greenhouse gas than simple incineration.
Compressed Paper for recycling
Kinkajou : Let’s talk about Wood
Erasmus : Recycling wood is important to divert this material from landfills. Wood is however, difficult to recycle as wood from residential uses may be contaminated (for example: CCA Pine used in building contains heavy metals i.e. copper, chrome and arsenic), or may not be economic or suitable to recycle due to volumes generated from collection.
Recycling is possible for many of the wood waste products generated by industrial processes used on timber raw materials in manufacture.
Most recycling processes concentrate on reducing wood into small particles or woodchips. These can then be further processed. Paper production is a prime example of this type of process.
Where building materials can be recovered in significant lengths, they can be resold into domestic or residential building markets. Some timbers have specific characteristics such as colour, grain or appearance.
As wood timber production has shifted to the use of plantation timbers, some specific timbers to match those in existing dwellings, can only be acquired from recycled sources. So there is a ready market for recycled building timber.
Erasmus : Timber can be recycled directly after being cleaned and de-nailed. Typically specialist recyclers: “size” and provide recycled timbers for sale to the community.
Other unusual uses of waste timber are for example, inclusion of small particles with cement aggregate or other clay materials prior to firing. This produces a very lightweight block aggregate.
Timber can be burned providing energy, which is a form of recycling.
Timber can be processed into cellulose materials suitable for encouraging grass growth, due to its water retaining characteristics.
Woodchip mulch can also be produced. Some wood fibres can also be recycled into processed timber boards with the application of binders under temperature and pressure.
These combo boards have replaced many raw timbers in sheeting walls. Typically they have a cleaner tongue and groove structure with less need for extra filling then do standard timber slats.
Skip Hire Brisbane
Kinkajou : Let’s talk about Construction Materials
Erasmus : Construction generates significant environmental waste. The hierarchy of available options for recycling include: reduce, reuse, recycle, compost, burn or incinerate, and finally landfills.
Typical waste types generated in construction include: brick or concrete or masonry, asphalt, ferrous metals, nonferrous metals, glass, paper and cardboard, plastic and timber. Generally ferrous materials form about 30% to 40% of waste and timber materials also represent 30% to 40% of waste generated.
Erasmus : To be reused, bricks need to be “cleaned” and the more useful “facing” bricks separated. This is a labour-intensive process. Not all bricks are suitable for cleaning processes.
I remember in my youth, one my friends operated a quite lucrative brick cleaning business. He would earn upward of a thousand dollars per weekend in cleaning bricks, substantially more than many people would earn in an average job for a week.
Erasmus : Concrete can be recycled through two main processes. Firstly, it can be broken up and remixed as a component of the aggregate into new cement or concrete (this is called “bound”).
Alternatively it can be crushed or broken up (and used unbound) such as in road base. Masonry is often crushed and may be used in situations such as a replacement for clay or sand in brick making, contamination of masonry with lime can limit its ability to be recycled.
Adherent cement needs to be removed by either mechanical or thermal processes.
Erasmus :The key barrier to the processing of ferrous or nonferrous materials is their sorting and collection.
Erasmus : Asphalt is easily recombined after heating with new asphalt.
Erasmus :On a building site glass is probably one of the higher value and potentially recyclable materials. Windows can be resold or reused directly. Glass can then be collected for disposal or reprocessing.
Some reprocessed uses include manufacture of glass fibre for use in thermal or acoustic insulation, filling material in concrete or cement mixers, paving blocks produced from recycled glass aggregate, mixing glass fibre or particulates with asphalt in road, mixing with aggregate in road making, mixing with aggregate in concrete production, and man-made soil e.g. in Japan.
Erasmus :Plastic High level reuse of Polyethylene (PE), Polypropylene (PP), Polystyrene (PS) and Polyvinylchloride (PVC) is possible if these materials are collected separately and cleaned.
Plastic can be difficult to recycle as old plastics deteriorate. This means that old plastics generally need to be added to at least some virgin new plastic material when used for recycling. Recycling is difficult if plastic wastes are mixed with other contaminants.
We have discussed timber recycling above.
Kinkajou : Let’s talk about E-waste recycling
Microprocessors retrieved from waste stream ILLUSTRATION
Erasmus : E-waste is derived from a range material such as computer screens, computer boards, TVs, VCRs, camcorders, DVDs, mobile phones, many consumer appliances including for example heaters containing chipboards for electronic control, audio devices, fax machines, and telephones.
Erasmus : E-waste is the newest recycling frontier. CRTs (cathode ray tubes) are little used these days but were a substantial problem in old days. They have been one of the largest sources of lead in common waste.
Some states in America banned their disposal and legislated for compulsory recycling of all CRTs. A typical format is legislation for EPR (extended producer responsibility). This means that manufacturers must take responsibility for the products at the end of their useful life.
Most electronics contain significant quantities of hazardous materials such as mercury, lead or cadmium, and complex plastic such as PCBs, (polychlorinated biphenyls).
Some equipment such as mobile phones can be tested and repaired. However, the growth of technology essentially mandates disposal of older equipment, as it is commercially unusable.
Technology changes would even preclude recycling of parts such as electronic integrated circuits or chipsets as their use changes with time, resulting in most parts having no resale value as parts.
Unfortunately many consumers retain their obsolete products as they believe they may have some value. Unfortunately with outdated electronic devices, the residual value evaporates very quickly with time, precluding resale.
Most technology more than two generations old is essentially only suitable for dumping or recycling. Furthermore, with increasing environmental focus over the years it is often the older devices that contain larger quantities of hazardous materials such as plastics or lead.
Erasmus : Generally e-waste is shredded into pieces approximately 10 cm x 10 cm. The fragments are manually checked to remove toxic batteries and capacitors which may contain toxic or heavy metals. The remaining waste is then further shredded to approximately 10 mm x 10 mm size.
At this point ferrous materials are magnetically separated nonferrous materials are separated by any currents generated within a spinning drum or centrifuge or vibrating plates. Precious metals can be dissolved in acid and then recovered.
Magnetic Can Sorters
Kinkajou : Recycling of e-waste is one of those activities that is best done either properly or not at all.
Erasmus :Yes you are right. Recycling of waste electrical and electronic parts in Third World countries such as India and China can generate substantial hazardous waste.
Incineration of complex plastics such as polybrominated diphenylethers (PBDEs), polychlorinated dioxins and furans, as well as polybrominated dioxins and furans (PCDD/Fs and PBDD/Fs) requires high temperatures to minimise environmental contamination via air or ash by products of combustion.
Lead is a significant problem, and persists in the environment. It is difficult to extract from broader contaminated soil sites. Many workers are exposed to substantial hazards (for poor pay) due to inappropriate safety standards at recycling sites.
Plastics Recycling Symbols
Kinkajou : Let’s talk about Plastics
Erasmus : A plastic is an organic polymer. Is made up of individual molecules called monomers which are linked via a chemical reaction into long chains known as polymers. This chemical reaction can be difficult to reverse, causing difficulties in recycling some plastics. Many plastics and many products are also not designed to be recycled which further complicates the process of recycling. Contaminants such as dirt, organic contaminants (e.g. food residuals), paints and adhesives are major problems for plastics recycling.
Plastics are usually disposed of in one of three ways: discarded, combusted, or recycled. Of the three options, recycling is least implemented.
Plastics are difficult to dispose of as well. When discarded they form up to 40 % of the volume of household rubbish but only 10% of the weight of household rubbish. Much household plastic waste consists of single use convenience packaging or containers.
The ability to dictate the properties of plastics and manufacturing makes them easy to tailor to specific household purposes. However the variety of plastics generated and their different chemical structures, then makes them difficult to recycle. The same properties that makes them attractive in manufacturing, becomes a disadvantage in recycling.
One form of recycling
Kinkajou : So tell us about how plastics are recycled.
Erasmus : The primary process in recycling plastics begins with the manual separation of plastics from the waste stream. Generally plastics and paper are separated by mechanical processes such as blowing. Further processes are used to differentiate plastics' types based on their monomer resin component.
Plastics can only be recycled a limited number of times to the degradation of the plastic polymer component. This is in contrast to the situation for metals or glass which can be recycled essentially indefinitely.
Plastics are generally more suited to be recycled to different purposes than that of their original usage. This is often known as “down cycling”.
Changes in polymer size (monomer repeat length) and changes in chemical composition due to inclusion of contaminants that the physical properties of recycled plastics can be difficult to predict.
As their physical properties dictate their potential usage, it can be difficult to use recycled plastics in manufacturing as it can be difficult guarantee that the properties of recycled plastic material (product) will match specifications required.
Erasmus : A further alternative to the recycling of plastic polymers is their usage in combustion to generate energy. Since plastic is derived from petroleum products, their energy yield is almost as high as that of oil.
Both yield approximately 46,000 kJ per kilogram as fuel, oil being approximately 10% more energy dense.
Erasmus : One significant problem is that many plastics are fairly resistant to degradation. Many remain in the environment for long periods of time. There are parts of the ocean where current eddies coalesce plastic debris in significant quantities forming a substantial bio- hazard to marine life. Plastic bags and open plastic rings are especially hazardous.
Some sources have calculated that up to 10 million tonnes of waste plastic enters the Earth’s oceans every year. A solution to this problem has been the proposal that plastics be designed to be biodegradable.
Kinkajou : Yes I have certainly stored some plastic bags in the past, (away from UV). When I attempted to reuse those years later, I found the plastic shopping bags essentially falling apart into plastic confetti.
It makes an awful mess but it gets rid of the problem of plastics persisting in the environment/biosphere. Biodegradability could certainly be a useful feature for materials such as plastic bags.
Kinkajou : I remember a school student who won a science prize at high school. His project was directed towards biological recycling of plastic materials. He had realised that plastics do biodegrade, although slowly, within landfill dump sites.
This was obviously occurring due to the action of microbes on the plastic. His project simply obtained soil from around plastics in landfills and used this to inoculate a bioreactor he had built for his project, attempting to accelerate the biological degradation of plastic materials.
Erasmus : Clever! So obvious when you think about it. Still if a school student can dig some germs up out of the ground that are capable of eating plastics, human scientists should do better.
Culturing bacteria and selecting them for their ability to biodegrade complex plastics is an obvious path which I think we have little appreciated and little used. I believe that for every organic material existing on the planet, there are some bacteria somewhere with the appropriate molecular processing chemistry to use the organic material as a food or fuel source.
Kinkajou : Identify your Bacteria and use their traits to achieve your purpose. Next obvious development of this process, is to identify the optimum environmental characteristics under which these bacteria can perform their task of by degrading waste. Then this environment needs to be provided to encourage the destruction of waste.
Kinkajou : Many plastics over the years have been discarded to landfills.
Erasmus : They continue to cause problems here due to their propensity to occupy a disproportionate amount of space and due to their biological hazard characteristics. For example, phthalates from plastics degradation have infiltrated groundwater, thereby becoming a hazard to human as well as wildlife health.
Plastics that are discarded into landfills may release toxic chemicals that pose risks for human and wildlife health.
Kinkajou : Plastics are convenient to use but they can be a very costly item in terms of their environmental effects.
Erasmus : Due to their chemical structure, many plastics have unique chemical and physical properties. This means that some resins are not compatible with other resins for purposes of recycling.
Plastics that are easier to separate into their respective resins such as PET or HDPE generally have higher recycling rates. PVC resin (resin code 3) contains chlorine atoms. HDPE and LDPE resin is contained only carbon and hydrogen.
These different molecular structures can result in recycled polymer mixtures layering or repelling each other (when melted or dissolved) rather than mixing to form a feedstock for manufacturing processes.
Other chemicals that make plastic recycling difficult are additives to make plastics flame retardant, or flexible, or resistant to UV damage.
Such chemicals may also impair mixing reactions essential to underpin recycling processes. Additives are less commonly used in plastic bags and in beverage containers such as soft drink bottles, enabling these to be recycled more easily and more often.
Because it is economically unviable to reclaim dyes from recycled polymer mixes, generally plastics are also sorted for colour after being sorted for resin type.
Having a recycling code or the chasing arrows logo on a material is not an automatic indicator that a material is recyclable but rather an explanation of what the material is. Types 1 and 2 are the most commonly recycled plastics.
Resin code 7 includes plastics which combine mixed plastics and other less commonly used polymers, accounting for approximately 20% of total plastic waste but are much more difficult to recycle, due to the difficulty of separating mixed plastics. PET plastics (resin code 1) are widely used in drinking bottles hence making them easily identifiable and easy to sort.
Identification of plastic resin type can be complex. Many small plastic items such as takeaway restaurant cutlery often lack the universal triangle and number to facilitate plastic recycling.
Manual processes will always be part of plastics recycling activities. For example bottle caps must be removed from recycled bottles, and this must be undertaken manually. However over the last few decades there has been a substantial introduction of automation into the recycling process as method of reducing manual handling and processing, and as a means to reduce costs.
recycling Plastics by Dissolution
Rensselaer technology using selective dissolution, eliminating the sorting step in the current recycling process
Erasmus : Processes used in the recycling plastics include:
- Physical recycling.
- Thermal depolymerisation (pyrolysis)
- Heat compression
- Selective dissolution:
Erasmus: Physical recycling. PET plastics can be converted into polyester and reused as clothing.
Chemical recycling: PET can be treated with an alcohol and a catalyst to form a dialkyl- terephthalate. The terephthalate diester can be used with ethylene glycol to form a new polyester polymer, thus making it possible to use the pure polymer again.
Using recycled feedstock makes it much more difficult to predict the degree of monomer polymerization and therefore much more difficult to predict the final properties of the plastic produced. We have mentioned laser spectroscopy and infrared plastics resin type identification and sorting systems already.
Erasmus : Thermal depolymerisation (pyrolysis) to hydrocarbon and products. This type of process can use plastics, rubber tyres, and chemical polymers such as in animal feathers or agricultural waste. Heating by itself is often not sufficient to dissolve and mix different plastics.
Plastics must generally be of nearly identical competition to mix efficiently. When different plastics are melted together they tend to phase separate, in a manner akin to mixed oil and water. The resultant phase boundaries cause weaknesses in resulting plastics limiting the use of plastic polymer blends.
Given below is the list of suitable plastic raw materials for pyrolysis:
- Mixed plastic (HDPE, LDPE, PE, PP, Nylon, Teflon, PS, ABS, FRP, etc.)
- Mixed waste plastic from waste paper mill
- Multi-layered plastic
Erasmus : Heat compression is a process that takes unsorted cleaned plastics, and mixes them in large rotating drums while heating to produce an undifferentiated plastic output. This type of plastic is only suitable for general applications, where specific properties such as transparency and strength are not critical.
Erasmus :Selective dissolution: this is a newer process in which plastics are shredded, washed, and dried. They then placed in a “dissolver” chamber with at a xylene solvent. Polystyrene (resin code 5) dissolves about 15°C and can be drained from the dissolver tank.
The temperature is increased and other plastics resins are progressively removed until only PET and PVC are left in the mix. These commingled plastics are then pumped to a smaller dissolver chamber with enhanced mixing capacity and in combination with the different solvent (xylene cyclohezanone) at even higher temperatures.
The process is repeated with a xylene-cyclohezanone solvent and a higher temperature . Liquid Resins produced are held in tanks matching resin type, and then solvents are removed.
This Rensselaer process has a higher output due to its capacity to use a greater range of feedstocks and has been shown to have a favourable cost profile to producing plastics from virgin hydrocarbon fuel stocks. It is capable of processing resin code 7 mixtures as well. In commerce, yield is everything and costs rule.
Physical process such as gasification, briquetting, pelletisation, shredding, compaction, flotation separation and swarf drying may be used to further process waste plastics or processed feedstocks. (Swarf is the name for plastic chips or shavings that result from manufacturing or recycling processes.)
Kinkajou : I’ve heard that some plastics can be used to replace bitumen in road making.
Erasmus :Plastics have been incorporated into road surfaces (notably the bitumen asphalt layer). It has been claimed the road surface has improved durability and water resistance due to the incorporated plastics.
Typically approximately 60 kg of plastic are used for a half a kilometre of two-lane roadway. Tests have suggested that roads made with this material have a better resistance to heat, cold, cracking and formation of ruts by factor of up to times three.