MATERIALS: RECYCLINGDESIGN

APPLICATION

Different papers and plastics can be sort-ed efficiently using a combination ofelectrostatic techniques such as coro-

na discharge charging, charging by inductionand rubbing things together – all have theirplace in different stages of the process. Similartechniques can also be applied to a variety ofmaterials, including agricultural seeds andproducts, medical feedstock and diamonds.

With the growing drive to recycle as muchas possible, it is clear that low-cost machinemethods offer the only practicable means ofachieving the goals that EU member stateshave set themselves.To this end, the EU set upa Brite-EuRam III project called ELREC,short for ELectrostatic RECovery of paperand plastic packaging wastes.The project wascompleted in December 2000.

Project leader was ERA Technology(www.era.co.uk), with seven other partici-pants: Hamos in Germany, Centro Tecnologi-co Gaiker in Spain, HLC Henley Burrowes inMalvern, KTH Polymerteknologi and Ragn-Sells in Sweden, Komotini Paper in Greece,and the University of Southampton. Theobjective was to develop and demonstrate apracticable household-waste recycling plant.Removal of metal and glass is relativelystraightforward, but paper and plastics presentproblems, hence the move to electrostatics.

Jeremy Smallwood, a consultant with Elec-trostatic Solutions, originally became involvedwhen he was with ERA Technology. He saysthat the paper-separation side of the projectwas less than 100% successful, but that plasticseparation was “most successful”, and that oneof the industrial participants has brought out anew range of commercial products with awide range of applications as a result.

The problem with the paper separation wasnot so much the technology but the circum-stances of the challenge. The method chosenwas to apply a 0 to 30kV corona dischargefrom an array of needle electrodes to materi-als on a conductive conveyor belt.The mate-rials with the highest conductivity are the firstto discharge and fall off as the belt runs roundthe end roller, while more insulating particlestake longer to discharge and are retained,

falling off later. The problem with applyingthis method to sort paper is that electricalproperties vary by orders or magnitudeaccording to humidity, which means that,unless the feedstock is very carefully dried, orthe climate is fairly dry, the process is veryweather dependent.

Tests at Malvern in the UK during a periodof wet weather were discouraging, but a pilot-scale paper-separating plant is planned for the

Tom Shelley reports on a novel way of sorting materials,crucial for recycling, with many other applications

25

Electrostatics sorts and saves

Eureka August 2001

Electrostatic techniques are proving useful forsorting a range of materials

■ Plastics and many other materials can be effectively and economically separated by electrostatic methods

■ Suitable charging techniques are: rubbing together (triboelectric), corona discharge and induction

■ It may be necessary to use combinations of different electrostatic techniques in complex separation flow sheets, especially those required to handle mixed domestic waste, to acquire a more negative charge

Conveyor belt

Tribochargedparticle feeder

Bin2

Bin1

Highvoltage

electrode

ParticlesMaterial 1Material 2

Conductiveconveyor belt

Particlefeeder

Bin1

Bin2

Bin3

Bin4

Bin5

Bin6

Coronacharge generator

ConductiveParticles

IntermediateInsulating

Conveyor belt

Particlefeeder

Bin1

High voltage electrode

Airextractionhood

ParticlesTetrabrikPlastic

Tribocharged belt separator(CBS) system

Levitating belt separator

The Corona charged belt (CBS) separator

Komotini site in Greece.The plastic/plastic separation technology

involves charging shredded plastics by tum-bling them together.This results in some itemsbecoming positively charged and others neg-atively charged.A suitably charged, high-volt-age electrode will then attract particles of onetype and repel others.Which charge materialsacquire when rubbed together is determinedby the triboelectric series.However,unlike theelectrochemical series, the exact order ofmaterials given by different sources seems tovary, as charging properties are strongly influ-enced by the nature of the material surface.

An ELREC plant, run by Gaiker in Spain,has been used to separate plastics and Tetrabrik(milk-carton material) output streams from itsexisting domestic waste separator. PVC andPET, which have similar densities, appear as amixed heavy fraction from a flotation processand are readily separated, as are PE and PP,which appear as a light fraction.Gaiker has alsobeen able to demonstrate separation of ABSfrom electronic scrap.

A mixed Tetrabrik and polymer output frac-tion can be separated by an entirely newELREC technology.This technique, the Lev-itating Belt Separator, passes particles under apositively charged electrode on an earthedconveyor, charging them by induction.

An earthed particle in an electric field redis-tributes charge to become an equipotentialsurface, and thus acquires a charge opposite tothat of the high-voltage electrode generatingthe electric field.This opposing charge leads toattraction towards the electrode. A dielectricparticle remains polarised because of its inabil-ity to redistribute electrons. Non-conductingparticles (polymers, in this case) have no netcharge and so are neither attracted norrepelled. The Tetrabrik particles, however,retain a net charge and lift off.

Ragn-Sells, one of the other project partic-ipants, has demonstrated a 250kg/hour plantfor separating polyethylene and polypropylene

from pre-sorted Swedish domestic waste. Ini-tial results show output purities in the range:95 to 99.7%, with good recovery rates.

The biggest beneficiary of the whole pro-gramme, however, seems to be Hamos, whichhas brought out a new EKS range of produc-tion triboelectric charged separators based onELREC designs, capable of throughputs of upto 1,500kg/hour. According to Hamos, Chi-tosan, a Chitin derivative with growing use infood and medical industries, may be electro-statically separated from contaminants such asplastic foil packaging, and plastic fibres fromthe fishing nets used to catch them.

During 2000,Hamos says it has sold four EKSmachines for separating rubber sealing fromshredded PVC window profiles. A clean PVCfraction with a purity of more than 99.9% maybe reached from an input fraction containing 5to 7% rubber.The few rubber particles remain-ing can be separated by an opto-electroniccolour sorter, to reach near 100% pure PVC.

A leading recycler in Southern Germany hasinstalled an EKS system to sort plastics fromrecycling bottle caps. In the first step, a PEfraction of more than 99.5% purity is dry sep-arated from a mixture of expanded PVC andPE. In a second step, a clean PVC product isproduced with a purity of more than 99%.

A leading maker of automotive plastics is saidto be using an EKS separator to separate mixedABS and PMMA production wastes resultingfrom making tail lights. An ABS fraction witha purity of 99.8% can be achieved after shred-ding.A second separation step results in a cleanPMMA fraction of 99.7% purity.

Solvay is apparently going to commission aHamos electrostatic separator to remove for-eign matter prior to chemical recycling PVCin a plant to be commissioned in NorthernItaly in autumn this year. Similarly Butler-MacDonald has ordered Hamos EKS equip-ment to electrostatically separate differentplastics in its computer recycling plant in Indi-anapolis, Indiana. More information can befound on the web (www.hamos.com).

An Israeli company, TechnoSeed, hasannounced an effective electrostatic process forseparating plant seeds from themselves andother matter. This idea has proved difficult toestablish commercially, and the originallyenthusiastic announcements have suddenlygone quiet and links on the web have been dis-connected, but this does not mean that the ideahas been given up. TechnoSeed says that itsequipment, details of which it will not divulge,can separate seeds and grains, separate stalk andleaf portions from cut grass and herbs, and cleanand sort tea, coffee kernels and cocoa beans.

ERA Technology Enter 450Electrostatic Solutions Enter 451Hamos Enter 452TechnoSeed Enter 453

26 Eureka August 2001

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ONMATERIALS: RECYCLING

Low-cost machinemethods of recyclingoffer the only practicablemeans of achieving thegoals of EU members