- 上一篇：多足步行混合机器人英文文献和中文翻译
- 下一篇：数控系统在平面磨床上应用英文文献和中文翻译
HDPE milkbottles, which can be positively identified and sortedout of a co-mingled waste stream. Conversely, thereis limited recycling of multi-layer/multi-componentarticles because these result in contamination betweenpolymer types. Post-consumer recycling thereforecomprises of several key steps: collection, sorting,cleaning, size reduction and separation, and/orcompatibilization to reduce contamination byincompatible polymers.(a) CollectionCollection of plastic wastes can be done by ‘bring-schemes’ or through kerbside collection. Bring-schemestend to result in low collection rates in the absence ofeither highly committed public behaviour or deposit-refund schemes that impose a direct economicincentive to participate. Hence, the general trend isfor collection of recyclable materials through kerbsidecollection alongside MSW. To maximize the cost effi-ciency of these programmes, most kerbside collectionsare of co-mingled recyclables (paper/board, glass,aluminium, steel and plastic containers).While kerbsidecollection schemes have been very successful at recover-ing plastic bottle packaging from homes, in terms of theoverall consumption typically only 30–40% of post-consumer plastic bottles are recovered, as a lot of thissort of packaging comes from food and beverage con-sumed away from home. For this reason, it is importantto develop effective ‘on-the-go’ and ‘office recycling’collection schemes if overall collection rates for plasticpackaging are to increase.(b) SortingSorting of co-mingled rigid recyclables occurs by bothautomatic and manual methods. Automated pre-sorting is usually sufficient to result in a plasticsstream separate from glass, metals and paper (otherthan when attached, e.g. as labels and closures).Generally, clear PET and unpigmented HDPE milkbottles are positively identified and separated out ofthe stream. Automatic sorting of containers is nowwidely used by material recovery facility operatorsand also by many plastic recycling facilities. These systems generally use Fourier-transform near-infrared(FT-NIR) spectroscopy for polymer type analysis andalso use optical colour recognition camera systems tosort the streams into clear and coloured fractions.Optical sorters can be used to differentiate betweenclear, light blue, dark blue, green and other colouredPET containers. Sorting performance can be maxi-mized using multiple detectors, and sorting in series.Other sorting technologies include X-ray detection,which is used for separation of PVC containers, whichare 59 per cent chlorine by weight and so can beeasily distinguished (Arvanitoyannis & Bosnea 2001;Fisher 2003).Most local authorities or material recovery facilitiesdo not actively collect post-consumer flexible packagingas there are current deficiencies in the equipmentthat can easily separate flexibles. Many plasticrecycling facilities use trommels and density-based air-classification systems to remove small amounts offlexibles such as some films and labels. There are, how-ever, developments in this area and new technologiessuch as ballistic separators, sophisticated hydrocyclonesand air-classifiers that will increase the ability to recoverpost-consumer flexible packaging (Fisher 2003).(c) Size reduction and cleaningRigid plastics are typically ground into flakes andcleaned to remove food residues, pulp fibres andadhesives. The latest generation of wash plants useonly 2–3 m3of water per tonne of material, aboutone-half of that of previous equipment. Innovativetechnologies for the removal of organics and surfacecontaminants from flakes include ‘dry-cleaning’,which cleans surfaces through friction without usingwater.(d) Further separationAfter size reduction, a range of separation techniques canbe applied. Sink/float separation in water can effectivelyseparate polyolefins (PP, HDPE, L/LLDPE) fromPVC, PETand PS.Use of differentmedia can allow sep-aration of PS from PET, but PVC cannot be removedfromPET in thismanner as their density ranges overlap.Other separation techniques such as air elutriation canalso be used for removing low-density films fromdenser ground plastics (Chandra&Roy2007), e.g. inremoving labels from PET flakes.Technologies for reducing PVC contaminants inPET flake include froth flotation (Drelich et al.1998; Marques & Tenorio 2000)[ JH1], FT-NIR orRaman emission spectroscopic detectors to enableflake ejection and using differing electrostatic proper-ties (Park et al. 2007). For PET flake, thermal kilnscan be used to selectively degrade minor amounts ofPVC impurities, as PVC turns black on heating,enabling colour-sorting.Various methods exist for flake-sorting, but tra-ditional PET-sorting systems are predominantlyrestricted to separating; (i) coloured flakes from clearPET flakes and (ii) materials with different physicalproperties such as density from PET. New approachessuch as laser-sorting systems can be used to remove‘Laser-sorting’ uses emission spectroscopy to dif-ferentiate polymer types. These systems are likelyto significantly improve the ability to separatecomplex mixtures as they can perform up to860 000 spectra s21and can scan each inpidualflake. They have the advantage that they can be usedto sort different plastics that are black—a problemwith traditional automatic systems. The applicationof laser-sorting systems is likely to increase separationof WEEE and automotive plastics. These systems alsohave the capability to separate polymer by type orgrade and can also separate polyolefinic materialssuch as PP from HDPE. However, this is still a verynovel approach and currently is only used in a smallnumber of European recycling facilities.(e) Current advances in plastic recyclingInnovations in recycling technologies over the lastdecade include increasingly reliable detectors andsophisticated decision and recognition software thatcollectively increase the accuracy and productivity ofautomatic sorting—for example current FT-NIRdetectors can operate for up to 8000 h betweenfaults in the detectors.Another area of innovation has been in findinghigher value applications for recycled polymers inclosed-loop processes, which can directly replacevirgin polymer (see table 3). As an example, in theUK, since 2005 most PET sheet for thermoformingcontains 50–70% recycled PET (rPET) through useof A/B/A layer sheet where the outer layers (A) arefood-contact-approved virgin resin, and the innerlayer (B) is rPET. Food-grade rPET is also nowwidely available in the market for direct food contactbecause of the development of ‘super-clean’ grades.These only have slight deterioration in clarity fromvirgin PET, and are being used at 30–50% replace-ment of virgin PET in many applications and at 100per cent of the material in some bottles.A number of European countries includingGermany, Austria, Norway, Italy and Spain are alreadycollecting, in addition to their bottle streams, rigidpackaging such as trays, tubs and pots as well as limitedamounts of post-consumer flexible packaging such asfilms and wrappers. Recycling of this non-bottle pack-aging has become possible because of improvementsin sorting and washing technologies and emerging mar-kets for the recyclates. In the UK, the Waste ResourceAction Programme (WRAP) has run an initial studyof mixed plastics recycling and is now taking this tofull-scale validation (WRAP 2008b). The potentialbenefits of mixed plastics recycling in terms of resourceefficiency, persion from landfill and emission savings,are very high when one considers the fact that in theUK it is estimated that there is over one million tonneper annum of non-bottle plastic packaging (WRAP2008a) in comparison with 525 000 tonnes of plasticbottle waste (WRAP 2007).4. ECOLOGICAL CASE FOR RECYCLINGLife-cycle