Paper Recycling Technology

Paper Recycling Technology

Dr. Richard A. Venditti

Dept. of Wood and Paper ScienceNorth Carolina State University

Dr. Richard Venditti

Faculty member in the Wood and Paper Science Dept at NCSUPhD in Chemical Engineering, BS in Pulp and Paper Science and Chemical EngineeringResearch areas:

Paper recyclingUtilization of forest/agricultural materials for new applications

Classes Taught:Process ControlUnit Operations for Pulp and PaperPaper Recycling (Distance)

Director of Hands On Workshop for Pulp and Paper Basics, co-sponsored through TAPPI, on-campus and in-millTechnical services projects for over 20 companies

Dr. Richard Venditti: Research Projects in Paper Recycling

The detection of adhesive contaminants, tracking through millsThe changes in fibers upon recyclingAutomatic sorting of recovered papersFlotation deinking surfactantsAgglomeration deinkingScreening phenomena and pressure sensitive adhesivesDeposition of adhesive contaminantsAlternate recycling processes

Ultrasonic deinkingSupercritical carbon dioxide to extract wax from OCC

Course Outline

The Paper Recycling US IndustryContaminantsEffect of Recycling on Fibers/PaperUnit Operations

Pulping, Cleaning, Screening, Washing, Flotation, Dispersion, Bleaching,

Image Analysis, Deinking ChemicalsSystem Design

Course Activities

Viewing of the Videos of LecturesBase lectures by VendittiGuest lectures from industry leaders

Reading assignments from Recycled Fiber and Deinking, Book 7 of the Series: Papermaking Science and TechnologyReading assignments: selected research papersHomeworks: 6 assignmentsFinal Project: Literature Review and Research Proposal

Course Objectives

Broad understanding of paper recycling science and technologyDevelop an expertise in a selected research topic in paper recycling

Critical Issues in Recycling:

Going deeper into the wastepaper stream => poor quality materialMeeting paper specifications, which are getting more stringentIncreased demands for paper from emerging countriesEnvironmentally benign operations at the same time both of the above are being satisfied

Recovered fiber, not “Wastepaper”

Recovered Fiber, not “Wastepaper”

Learning objectivesUnderstand the trends of paper recycling in the industryIdentify the major grades of recovered fiberIdentify the major types of contaminants in recovered fiber

Global Paper and Board DemandOverall Positive Growth

Slide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU Source: RISI

GDP Growth Major World EconomiesChina, Eastern Europe Dynamic Growth Regions

China

EasternEurope

Source: RISISlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU

Paper & Paperboard Consumption vs GDPIncreases with Wealth … Levels Off

Source: 2007 FAO Forest Resource AssessmentSlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU

Asian Supply has Grown to Match DemandLittle Incentive for others to develop export strategy


Use of Recovered PaperGrowing in China, Europe … flat in USA


Use of Recovered fiber Flat in USAMoving increasingly to Asia


Recycled Fiber DefinitionsSecondary Fiber: fibers that have previously been used in a manufacturing process and have been reclaimed as raw material for another process.Pre-consumer waste: any waste, printed or unprinted, generated in the fabrication or conversion of finished paper. Before use by a consumer as a final end product. Post-consumer waste: Paper that has passed through the end usage as a consumer product.Internal broke: off-specification paper that is repulped and used at the same site, not considered secondary fiber.

Recycled Fiber DefinitionsRecovery Rate (RR)

how much paper is diverted from landfill

Utilization Rate (UR)fraction of recycled fibers contained in paper

Consumed Paper of TonsCollected Wastepaper of Tons%100RR =

duced Paper Proof Tonsat Mills Consumed Wastepaper of Tons%100UR =

Supply1

(000 tons)

Recovered (000 tons)

Recovery Rate2

1993 91,538 35,460 38.70%

1994 95,718 39,691 41.50%

1995 95,971 42,189 44.00%

1996 94,529 43,076 45.60%

1997 99,557 43,989 44.20%

1998 101,183 45,076 44.60%

1999 105,316 46,818 44.50%

2000 102,810 47,311 46.00%

2001 97,395 46,996 48.30%

2002 98,949 47,645 48.20%

2003 98,016 49,255 50.30%

2004 101,882 50,287 49.40%

2005 99,618 51,272 51.50%

2006 100,198 53,488 53.40%

Source: AF&PA, 2006 RecoveredPaper Annual Statistics


Utilization of Recovered Paper in the US

Paper Recovered (000 tons)

Paper Landfilled(000 tons)

1993 35,460 38,399

1994 39,691 38,118

1995 42,189 36,118

1996 43,076 34,331

1997 43,989 37,174

1998 45,077 38,329

1999 46,818 41,255

2000 47,311 39,568

2001 46,996 35,894

2002 47,645 37,601

2003 49,255 36,032

2004 50,287 39,107

2005 51,272 36,846

2006 53,488 35,660


Pkging Material Recovered, 2006

Paper 22.9Glass 2.8Metal 2.2Plastic 1.3Other 1.3Total 30.5

(000,000 tons)


Recovered Paper Statistical Highlights, 2005 Edition, AF&PA

Exports and Imports of Recovered Paper

0

2

4

6

8

10

12

14

16

18

1990 1992 1994 1996 1998 2000 2002 2004 2006

Year

Mill

ions

of T

ons

ExportsImports

Major Recovered Paper Grades

Mixed Papers: mixed papers, low quality office waste, magazines, catalogs, telephone directories, recycled boxboard cuttings, tissue paper converting scraps if mainly composed of recycled fiber, mill wrappers, specialty grades, all other grades not specifiedNewspapers: old newspapers, special news, groundwood computer printout, coated groundwood sections, publication blanks, mixed groundwood and flyleaf shavings

Major Recovered Paper Grades

Corrugated: old corrugated containers, container cuttings, kraft paper and bags, old solid fiber containers, kraft bag clippings, carrier stock and its clippings Pulp Substitutes and high grade deinking: bleached chemical pulped office papers and CPO suitable for deinking, or as a pulp substitute if unprinted, bleached sulfite and sulfate cuttings including tissue paper converting scrap if predominantly composed of bleached chemical pulp fiber, coated book stock

Grades of Recovered PaperMixed PaperSuper Mixed PaperBoxboard CuttingsMill WrappersNewsSpecial NewsSpecial News De-ink QualityOver-Issue News]MagazinesCorrugated ContainersDouble Sorted Corrugated New Double-Lined Kraft Corrugated CuttingsUsed Brown KraftMixed Kraft CuttingsCarrier Stock New Colored KraftGrocery Bag ScrapKraft Multi-Wall Bag ScrapNew Brown Kraft Envelope CuttingsMixed Groundwood ShavingsTelephone DirectoriesWhite Blank NewsGroundwood Computer Printout

Publication BlanksFlyleaf ShavingsCoated Soft White ShavingsHard White ShavingsHard White Envelope CuttingsNew Colored Envelope CuttingsSemi Bleached CuttingsManila Tabulating CardsSorted Office PaperSorted Colored LedgerManifold Colored LedgerSorted White LedgerManifold White LedgerComputer PrintoutCoated Book StockCoated Groundwood Sections Printed Bleached Board CuttingsMisprinted Bleached BoardUnprinted Bleached Board#1 Bleached Cup Stock#2 Printed Bleached Cup StockUnprinted Bleached Plate StockPrinted Bleached Stock

Scrap Specifications Circular, Guidelines for ….Paper Stock…By: Institute of Scrap Recycling Industries, Inc.

DefinitionsOutthrow:

“all papers that are so manufactured or treated or are in such a form as to be unsuitable for consumption as the grade specified”

Prohibitive Material: material in excess of specified maximum make the recovered paper unusable as the grade specified Any materials that may be damaging to the equipment

News, De-ink Quality #7. Consists of sorted, fresh newspapers, not sunburned, containing not more than the normal amount of rotogravure and colored sections.

Prohibitive Materials None PermittedTotal Outhrows may not exceed ¼ of 1%

Scrap Specifications Circular, Guidelines for ….Paper Stock…By: Institute of Scrap Recycling Industries, Inc.

Grade-Year At mills

Export/Molded Prod

Total Recov

Total Supply

RR%

OCC-1993 13.5 2.3/0 15.9 26.9 59.1OCC-2006 20.0 5.2/0 25.2 33.0 76.4ONP-1990 3.8 0.9/0.7 5.5 15.9 35.0ONP-2006 5.5 2.5/1.5 9.6 13.2 72.3Mixed-1993 5.3Mixed-2006 12.7 49Pulp Sub-1993 3.3 100Pulp Sub-2006 2.4 100HG DI-1993 3.7 100HG DI-2006 3.6 100

Recovery of Paper and Paperboard (000 tons)


16.4 million t/yr

3.5 million4.2 million

7.0 million

1.6 million


Ctn Bd 15,153NP 0Packg 398Ind CvtPaper Bd 3,946Tissue 167Export 5,218Other 303Total 25,185

(000 tons)


Ctn Bd 1RecPBd 1,172Tissue 698News 3,213P&W 353Other 1,615Export 2,513Total 9,565

(000 tons)


Newsprint 380Tissue 2,680P&W 1,200Other 1,000Export 7,300Total 15,690

(000 tons)


Recovered Paper Prices, $/tonUS Paper Spot Market Prices May 24, 2007Corrugated Container 105Old Newsprint 85Boxboard 71Old Magazines 19Mixed Paper 28Sorted Office Paper 104White Ledger 266White Envelope 370 ScrapIndex.com

Recovered Paper Prices

0

50

100

150

200

250

300

350

400

450

1970 1975 1980 1985 1990 1995 2000 2005

Year

$/to

n

Sorted White LedgerONPOCCMixed Paper

Cost of DIP

SystemMOW to deinked, bleached pulp (DIP)Flotation (2), washing, oxidative bleaching, complementary processesProduce 200 ODTPDYield = 67%

Total Capital Cost Installed= $42 MM$6 MM/yr depreciation for 7 yearsMOW Cost =$220/ton delivered

Cost of DIP: Variable CostsDescription Quantity Unit Unit Cost, $ Contribution,

$/tonFiber 1.5 ton 220 330NaOH 0.024 ton 400 9.6Peroxide 0.024 ton 600 14.4Sodium Silicate 0.022 ton 500 11.0Flotation Aid 0.0027 ton 1000 2.7Sludge Dewater Polymer 0.00115 ton 1200 1.4Clarifier Flocculant 0.00014 ton 1600 0.2Clarifier Bentonite 0.00085 ton 1600 1.4Electricity 310 kWhr 0.045 14.0Steam 790 kPa 0.25 mton 6 1.5Waste Water Treatment 1.236 m3 0.53 0.7Waste Disposal 0.984 mton 36 35.4Process Water 4.8 m3 0.53 2.5Total Variable Prod. Cost 424.8

Cost of DIP: Fixed Costs

Fixed Costs $ Per Year $ / ton

Maintenance 4,000,000 57

Labor 2,000,000 29

Operating Materials 1,700,000 24Depreciation (7 yr straight line) 6,000,000 86

Business Overhead 1,100,000 16

Total Fixed Costs, $/ton 211

Cost of DIP: ROI

Total Costs, $/ton produced 636Total Costs per year, $ 44,532,716Selling Price of ton DIP, $/ton 600Income, $/yr 42,000,000Profit before taxes, $ -2,532,716Taxes (25%) -399,537Profit after taxes, $ -2,133,179

ROI (%) -1

Paper Recycling ReviewOf the about 100 million tons of paper consumed in the US, ___ % is recycled

Name 3 contaminants in recovered paper: _________________ _________________ _______________

Name 4 major categories of recovered paper ____________ ___________ ___________ ____________

____________ is a recycling operation used to remove large contaminants

____________ is a recycling operation used to remove heavy contaminants

Name the two major de-inking operations: _________ and _________

The largest variable production cost in paper recycling is typically _____________True or False. Basically, all recycle systems have the same # and type of unit operations.The yield of useable fiber

A. Is about 100% for all systemsB. Ranges from 20-80% depending on the systemC. Ranges from 60-94% depending on the systemD. Is 50% for all systems

Common Contaminants in Waste Paper

Common Contaminants in Wastepaper

Large Junkmetals: nuts, screws, foil, cansplastics: films, bags, envelopesdirtcloth, yard waste, leather, etc.,

Inks & tonersStickiesCoatingsFillersPapermaking additives

Inks & Toners

Type Component Drying System Ink Resin Film Particle Size(microns)

End Products

Simple LetterPress

Pigment +Mineral Oil

Absorptioninto Web

Weak 1-15 LetterpressEarlyNewsprint

Newsprint andOffset

Pirgment +Soft Resin &Mineral Oil

Penetration ofVehicle intoWeb + ResinOxidation

Soft Film

Hard Film

2-30 Newsprint,Books

We Offset,Letter Press

Rotogravure Pigment+Hard Resin &Solvent

SolventEvaporation

Hard Film 2-250 Magazines,Catalogues

Flexographic Pigment +Resin & WaterEmulsification

AmineAbsorption,Evaporation

WaterResistant Film

N/A Newsprintinserts,Corrugated

UV Cured Pigment+Monomer

UVPhotopolymerization

Non swelling,Non saponif.Hard Film

50-100 High SpeedCoatedPapers

Specialty VariousPgiments andRosins

Heat set orOther

Hard,CoherentFilms

40+ XerographyLaser PrintersElectronicForms

StickiesCurrently the most challenging problem in paper recyclingStickies: contaminants in pulp that have the potential to deposit on solid surfacesTypically organic materials:

Man made stickies: adhesives, coatings…Natural stickies: pitch, resins..

May deposit on papermachine wires, press felts, dryer fabrics, calendar rolls and cause significant down-time on machineAre hard to remove in recycling due to often having a neutral density, and an ability to flow and change shape

Coatings as a Contaminant

Coatings typically contain inorganic fillers and polymeric binders

Coating binders can behave as stickies, sometimes termed white pitchCoating fillers are washed out of the pulp and lower the overall yield of the recycling process

Board is often coated with waxWax can cause recycled paper to be weak and slipperyWax can deposit on machinery

Fillers as a Contaminant

Fillers are washed out of the pulp and lower the overall yield of the recycling process

Common fillers:– clay– CaCO3– TiO2

Fillers are not desirable in some paper products such as tissue paper

Papermaking Additives

Additives such as starch, gums, retention aids, etc., are lost during recycling and lower yieldAdditives such as dyes can cause the fibers to not make color specifications, this is also a problem with fluorescent dyesWet strength additives cause the paper to be unpulpable in many cases, making the entire paper product unusable and thus, a contaminant

Contaminant Removal

Contaminant Size vs. Removal Efficiency

Recovered fiber, not “Wastepaper”: Review-Quiz

True/False: The percentage of paper that is recycled overall is about 50% The four major grades of recovered paper are:

True/False: Basically contaminants are similar in shape, strength, density, etc.,

Effect of Recycling on Fiber Properties

Effect of Recycling on Fiber Properties

Learning objectivesUnderstand how recycling affects chemically pulped fiberUnderstand how recycling affects mechanically pulped (lignin containing) fibers

Effects of Recycling on Chemical Pulps

Mechanical damageLoss of finesHardening and stiffening (hornification)Weakening of bonding

less fiber conformabilityless absorptiveloss of hemicellulose

Decreased cleanliness

Effect of Chemical Pulping

Fiber wall structure as in the tree. Chemical pulping removes lignin from cell wall

Hornification: Irreversible

A Wet kraft fiber before dryingB 30% consistency (Irreversible processes begin to occur)C 30-75% consistencyD >75% consistency

Hornification: Irreversible

Never DriedFiber

Fiber SwollenWith Water

Dried CollapsedFiber - Hornified

Rewetting Does not Re-swell

Effects of Recycling on Chemical Pulps

Effect of Recycling on Chemical Pulps

Chemically pulped fibers that have never been dried: have the ability to swell with water which makes the fibers flexiblecan be mechanically treated (refining) to increase the flexibility

The flexible never dried fibers are able to conform in the papersheet which increases the fiber bonded area making strong paperWhen dried, the fibers become rigid, termed hornification

Effect of Recycling on Chemical Pulps

Previously dried fibers, upon exposure to water, do not swell and do not become flexibleThese rigid fibers break upon refining: causing fines When paper is made, the rigid fibers do not conform in the papersheet and the resulting low fiber bonded area produces a weak sheet

Flexible Fibers Stiff fibers

Effect of Recycling on Mechanical Pulps

1st Use

80 %

20%

2nd Use

70 %

30%

3rd Use

60 %

40%

Effects of Recycling on Mechanical Pulps

Effect of Recycling on Mechanical Pulps

Wood containing pulps from mechanical pulping processes have lignin (a three-dimensional crosslinked polymer) which makes fiber walls stiff and deters water swellingThe stiff, somewhat round cross section fibers make paper with low fiber bonded area and weak fiber bondsTherefore, the strength of never dried mechanical pulps is generally lower than of never dried chemical pulpsRecycling may improve properties of mechanical pulps by flatening and flexiblizing the fibers“Different behavior than chemical pulps on recycling”

Effects of Recycled Fibers on the Papermaking Process

Lower freeness: decrease machine speeds or add drainage aid Lower paper strength: more sheet breaksLow efficiency of chemical additives (fines and anionic trash)Increased depositsDecreased cleanliness

Paper Strength vs. Number of Times Recycled(Howard and Bichard, 1st Res Forum on Recycling, Oct 1991, CPPA)

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0 1 2 3 4 5

Number of Recycles

Bur

st In

dex

(kPa

m^2

/g)

Bleached Kraft (Chemical Pulp)

Groundwood (Mechanical Pulp)

Effect of Recycling on Fiber Properties: Review Quiz

True or False: A pulp produced for newsprint using a CTMP process should have almost equal or better strength properties after recycling.

Paper Recycling Operations

An Example: OCC Recycling

Paper Recycling OperationsAn Example: OCC Recycling

Learning objectivesIdentify major sub-operations in a typical OCC recycle millUnderstand the purpose and how each sub-operation works

Why is contaminant removal so difficult?

Contaminates vary bysizedensityshapesurface propertiessolubilitystrength

No single separation device can remove all of the different types of contaminantsThus, recycling processes consist of many sub-operations that complement each other

Major Recycling StepsPulpingCleaningScreeningDeinking --- used to produce printing or tissue

WashingFlotation

Dispersion and KneadingBleaching --- used to produce printing or tissueWater TreatmentSolid Waste Handling

Example: An OCC Recycling Process

In the next section, we will take a look at the sub-operations in an OCC recycling processThe process has the following operations:

PulpingHigh Density CleaningScreeningForward CleaningThrough Flow CleaningThickeningDispersionWater Clarification

Pulping of Recovered Paper

Pulping of recovered paper

Definition

Pulper : A device whose main objective is to

convert recovered paper into a slurry of well

separated fibers and other waste paper

components.

Pulping of recovered paper

The pulping operation is the first and probably themost critical operation in paper recycling.

Proper pulping is a requirement if unit operationsdownstream (cleaning, screening, flotation…..) areto be effective.

Incorrect pulping conditions can irreversibly damagefibers making them inappropriate for papermakinguses.

PulpingMain Function : Disperse recovered paper into separated fibers.

Several sub-objectives that are also important:1. Detach contaminants from fibers.

2. Mix paper with water and chemicals at the correct ratios.

3. Maintain contaminants as large as possible to aid subsequent removal processes.

4. Avoid damage to the fibers (fiber cutting).

5. Removal of large debris from system.

Basic Pulping Categories : Batch vs. Continuos Pulping

Batch Pulping : The feed recovered paper, water and chemicals are all charged at the beginning of the process and are removed all at once at the end of the process. The batch process is repeated.

Continuos Pulping : The feed recovered paper, water and chemicals are continuously added to the pulper and at the same time, the pulped product is also being continuously removed.

Basic Pulping Categories : Low vs High Consistency

Low Consistency Pulping: Typicallyfrom 3-6 % K. Produces a relatively easily pumpable fluid. The fluid is“pourable”.

High Consistency Pulping: Typicallyfrom 8 - 18 % K. Produces a thick, slurry that will not flow under the influence of gravity alone.

Consistency (solids) 100 * solids wtsolids wt + liquid wt

= % K

General Parts of a Pulper

1. Wastepaper feed method (conveyor).2. Pulper tub.3. Rotor - spinning device for agitation, mechanical energy input

to the system.4. Baffles - protrusions to assist in mixing and prevent swirling.5. Dilution water.6. Pulper exit.

Forces in a Pulper

Mechanical ForcesThese are caused when the fast moving rotor impacts material in the relatively slower body of pulp stock around it.Faster rotor speeds cause more intense mechanical forces in the pulper.

Pulper Forces

Hydraulic Forces : These are caused by the motion of fluid that is caused by the spinning rotor (not by the direct impact of the rotors).When two adjacent portions of a fluid are moving in different directions ( or at different speeds) a shear force is present. An example in the picture would be at point A.

A

Forces in a PulperAttritionMechanical shearing forces that occurs between the moving rotor and a static extraction plate near the rotor.The rotor forces fiber bundles between the rotor and extraction plate. Intense hydraulic forces act to cut the fiber bundles and fibers. This can cause significant damage to fibers.Used only for low % K pulping because the pulp must be screenable.

Low Consistency Pulping

Consistency from 3 -6 %.Low profile rotor that rotates at high speeds.Motion of rotor causes a vortex of pulp stock. The baffles are used to improve mixing.High mechanical force due to impacts of rotor can damage fiber and break contaminants.

Pulper Types: High Consistency

Typically 8-18 %.High profile rotor used. The helical screw type rotor is needed to “pull down” the non-fluid like high % K stock, from the top to the bottom of the pulper.At the high % K, fiber-fiber (solid-solid) rubbing dominates the forces experienced in the pulper.

Comparison of Low vs High Consistency Pulping

Rotor/ tank volume is much higher for high % K pulping. This is needed to maintain proper motion of non - fluid pulp stock at high % K. Specific power is higher for high % K due to the higher viscosity pulp stock that must be pulped. However, the specific power consumption per ton of paper is significantly lower for high % K pulping.This is due to high % K pulping having more tons of fiber for the amount of same pulping volume as a low % K pulper. Also, the relatively less amount of water at high % K pulping causes less energy to be expended on moving water.


Rotor speed is slower for high % K, causing less damage to fibers via mechanical forces of rotor.Attrition forces are not used for high % K pulping. This decreases fiber cutting and contaminant breakage.RESULTS of above: higher tensile, burst and tear strength for high %K pulpingHigh consistency pulping includes more fiber to fiber rubbing. RESULTS of above: This action increases detachment of contaminants from fiber surfaces. The detachment of ink from fibers is especially important for washing and flotation deinking.


Printing and writing grades consist of a high content of fully bleached hardwood and softwood fibers that are susceptible to damage => gentle high consistency pulping is preferredFurther printing and writing grades need ink detachment => high consistency pulping with lots of fiber-fiber rubbing is preferredOCC recycling, a historically older technology, typically has low consistency pulping because unbleached fibers are less susceptible to damage

Screening and Junk Removal in Pulper

One of the sub- objectives of pulping is to remove large debris that enters the system.

Examples of large debris :wood wet-strength paperplastics baling wirenails and bolts

The removal of debris serves two important functions.Protects equipment downstream from damage.Prevents plugging of downstream equipment.

Examples of Debris Removal Methods

Different pulpers have different methods to remove debris, examples of common methods follow.

High Consistency Batch PulpingHC pulper with Dilution ZoneHC pulper with Detrasher

Low consistency PulpingContinuos Low consistency pulper with Ragger and junk tower.Continuos Low consistency pulper with a de-trashing system

High Consistency Batch Pulper with Dilution Zone

The pulper is designed so that during pulping at 15 -18% K the pulper volume is only partially full.At the end of the pulping cycle, dilution water is added to achieve a 5 - 6 % K.After dilution, accepted stock passes through an extraction plate with holes about 3/4 - 1 inch diameter.Finally, large debris is flushed from the pulper through a large rejects opening on the side.

High Consistency Batch Pulper with External Detrasher

The pulper is “full” at high consistency during pulping.At the end of the pulping dilution water is added at the bottom of the pulper diluting the pulp in the bottom to less than 6 %.A large opening on the bottom/side of the pulper is used as the exit for the pulper contents.The pulp and debris are separated by an external detrasher.Note : There is no extraction plate in the pulper.

Note: drawing not to scale.

Continuos Low Consistency Pulper with Ragger and Junk Tower

Low consistency continuos pulper typically have an extraction plate that accepts pulped fibers and rejects debris and unpulped flakes.The extraction plate/ rotor can cause attrition, resulting in fiber cutting.A junker is used to collect unpulpables such as bolts or rocks. This debris is thrown out of the pulper into a junk tower where it is removed.A ragger is also used in many cases to remove bale wire, strings, plastics, etc. The ragger is a continuos “rope” formed by entangled debris. The “rope” is continously pulled out of the pulper and cut into sections and disposed. Common in OCC mills.

Recovered OCC bale storage

Loading OCC bales on pulper conveyor

Wire bale cutter

Pulper conveyor

Bale falling into pulper

Pulper

Ragger removing debrisfrom the pulper surface

Ragger pulling rejectsout of pulper

Junker Claw

Continuos Low Consistency Pulper with Ragger and Junk Tower

Low consistency continuos pulper typically have an extraction plate that accepts pulped fibers and rejects debris and unpulped flakes.The extraction plate/ rotor can cause attrition, resulting in fiber cutting.A junker is used to collect unpulpables such as bolts or rocks. This debris is thrown out of the pulper into a junk tower where it is removed.A ragger is also used in many cases to remove bale wire, strings, plastics, etc. The ragger is a continuos “rope” formed by entangled debris. The “rope” is continously pulled out of the pulper and cut into sections and disposed. Common in OCC mills.

Example Detrashing Process

Accepts

Light rejects

recycle

Heavyrejects

Rotor with screen plate

Drum PulpingA continuos, high

consistency pulping method.Most often used for pulping old newsprint.Consists of an inclined rotating drum 11 -17 rpm through which the paper/ pulp travel down. The drum is very large approximately 10 feet high and 100 feet long.

High Consistency Drum Pulper

High % KZone Screening

ZoneRejects

AcceptPulp

Conveyor

Drum PulpingTwo Zones

High Consistency pulping zoneLow consistency pulping zone.

High Consistency Pulping ZonePaper ,water and chemicals added to ^ 15% K.Baffles on the walls of the drum lift the paper and drop causing defibering in a gentle manner.

Low Consistency Screening zoneWater is added to dilute stock 3-4% K.Pulped fibers pass through 6 mm holes and are accepted from the pulper.Large rejects continue through the pulper and are4 discharged at the end.

Drum Pulping

Two ZonesHigh Consistency pulping zoneLow consistency pulping zone.

High Consistency Pulping ZonePaper ,water and chemicals added to ^ 15% K.Baffles on the walls of the drum lift the paper and drop causingdefibering in a gentle manner.

Low Consistency Screening zoneWater is added to dilute stock 3-4% K.Pulped fibers pass through 6 mm holes and are accepted from the pulper.Large rejects continue through the pulper and are4 discharged at the end.

Overview

Rejects

Drum Pulping

AdvantagesGentle pulping keeps contaminants large and minimizes fiber degradation.Simple operation that includes screening.

DisadvantagesHigh capital cost.Not an aggressive pulping method (example: cannot pulp wet strength papers).

Paper Feed

rejects exit

Pulping SummarySeveral methods to pulpMain objective: defiberizeSecondary Objectives:

Remove Large DebrisDetach contaminantsNot destroy fibersMix

Final Thought: If pulping is not done properly, subsequent processing steps will be ineffective and product quality will be unacceptable

High density cleaners

High Density Cleaner: removes large heavy rejects from pulp

Objective: separate large heavy contaminants from fibers to protect downstream equipment from damage and pluggage

How it works: centrifugal forces separate materials mainly due to density/size

Detrasher

Accepts

Feed

Light weightRejects

HeavyRejects out

the back

Example detrashing unit

Top View

Objective: pulp unpulpedpieces of paper/board and separate contaminants using a screen or centrifugal forces to protect downstream equipment from damage and pluggage

Deflakers

Objective: impart mechanical energy to break up flakes of unpulped material.

Screening

Screening separates contaminants based mainly on size, but also on shape and deformabilityPerformed by presenting a barrier for large contaminants (slots or holes) that allow fibers to pass through

Screening

Modes of Removal1. Stiff particles with all 3 dimensions larger than width of

slot or diameter of hole are rejected

2. Stiff particles with one or two dimensions smaller than width/diameter have a probability of rejection.

Number of screen contacts

Rejection Probability, %

1 2 3 4 5

100%

50%

0%

1-dimension (rod)< slot size

2-dimension (plate) < slot size

Screening Types and ConditionsConsistency RangeScreen

TypeScreen

Openings, mm

Rotor circumference

speed,m/sMC<6%

MC<4.5%

LC<1.5%

Disk Hole2-3

20-30 Yes

Cylindrical Hole0.8-1.5

10-30 Yes

Cylindrical Slot0.1-0.4

10-30 Yes Yes

Screening

Types of perforationscoarse holes .110 in or 2.7 mmfine holes .060 in or 1.52 mmcoarse slots .010 in or .254 mmfine slots .006 in or .152 mm

Also, the fibers offer a resistance to passage, related to the consistency

Example of Disk Screen

Pressure screen

Pressure Screen Principle to Prevent Blinding of Screen

The leading edge of the rotating foil accelerates the stock.

The negative pulse under thesweeping foil momentarily reverses the flow, effectivelypurging the screen openings.

Pressure Screen Flow Configurations

Screen Plates

Holes

Slots

Contoured

Effect of Reject Rate & Plate Opening on Screen Cleanliness

Screening Factors

Screen Performance VariablesStock characteristics

fiber type, debris characteristics, debris levelScreen design

flow configuration, plate cleaning mechanism, perforation type (holes or slots), rotor speed

Operating variablesstock flow rate (pressure drop across screen), feed consistency, reject rate, screen plate perforation size, stock temperature, dilution flow to screen

Screening

Screen Layout:Always have cascaded screens to save fiber.

Primary Screen

Secondary Screen

Tertiary Screen

Accepts

Rejects

Open Gravity Screen

Summary Pressure Screen:

Objective: separate large contaminants from fibers

Can act as barrier screen or probability screen

Typically cascaded to save fiber

Typical conditions to promote increased throughput can have negative impact on cleanliness efficiency.

Cleaning

Centrifugal Cleaning

Remove impurities from the pulp stream Centrifugal cleaners remove

metalsinkssandbarkdirtetc.,

Centrifugal Cleaning

Principles of operationCentrifugal cleaner uses fluid pressure to create rotational fluid motion in a tapered cylinderRotational movement causes denser particles to move to the outside faster than lighter particlesGood fibers carried inward and upward to the accepted stock inletDirt held in the downward current and removed from the bottom

Three Basic Cleaner Types:

High Density Cleaner: separates very large, heavycontaminants such as rocks, staples, glass. Used after pulping (early in the process) to protect downstream equipment. Diameter = 300-700 mm.Forward Cleaners: separates fine, heavycontaminants such as a sand and inks. Also called cyclones, hydrocyclones, or cleaners. (Described above) Diameter = 70-400 mmThrough Flow Cleaner: separates fine, lightcontaminants such as glues, adhesives, plastics, foam. Also called light-weight cleaners or reverse cleaners. Diameter = 100-400 mm

Types of Cleaners: Functional Differences

ConsistencyHC cleaner: 2-4.5% K, MC: 1-2, LC: 0.5-1.5

Centrifugal Acceleration (acceleration due to gravity =9.8 m/s2)

HC cleaner: <60 g, MC: <100g, LC: <1000 g

Reject Rate by mass/stageHC cleaner: 0.1-1% , MC: 0.1-1, LC: 3-30

Centrifugal Cleaner: Features and Flow

“Bank Arrangement” of Cleaners

Several cleaners are piped in parallel fashion. A single cleaner is not capable of providing enough

through put for typical industrial flows.

Accepts Header

Feed Header

Rejects Header

Forward Cleaners

Cleaners

Canister

Cleaners Pump

Cascade Arrangement of Cleaners

REJECTS

Dilution Water

ACCEPTSDilution Water

Dilution Water

FEEDPrimary Cleaners

Tertiary Cleaners

Secondary Cleaners

Typical Cleaner “Curve”

Reject Ratio: OD mass flow reject / OD mass flow inlet

Separation Ratio:

m(in) – m(acc) / m(in)

m= mass flow contaminant

EqualDistribution

EffectiveSeparation

High Density Cleaner

High density cleaners

Through Flow Cleaner

AcceptsRejects

Feed

Thru-flow cleaners

Centrifugal Cleaner Performance Variables

Stock Characteristicsfiber typecontaminant characteristics (size, shape, density), dirt level

Cleaner Designbody diameter, feed inlet configuration, accept diameter, cylindrical section height, cone angle, spiral grooves application, reject rate control method (fixed orifice and back pressure)

Parameters Affecting HydrocycloneCleanliness Efficiency

OperatingVariable increase in:

Cleanliness Efficiency

Sensitivity to Variable

Pressure Difference Incr/Decr HighVolumetric Flow Incr/Decr MediumCyclone Diameter Decrease HighConsistency Decrease HighFlake Content Decrease MediumTemperature Increase LowReject Rate Increase MediumFlushing Flow Decrease Medium

Effect of Particle Properties on Separation

Particles with large density differences wrtwater are removed more effectivelyParticles with density near 1 g/cm3 may separate from fibersLarger particle at same density will be removed more effectively than smaller particleParticles of the same density but with favorable hydrodynamic shape (cwAp) separate more effectively, eg, a sphere is better than a flat plate

radial

axial

Fg

Fd

Fc

Fd

Force balance on a single particle in a hydrocyclone

Radial direction: – Net centrifugal force ↔Drag force

Axial direction: – Net gravitational force ↔Drag force

Tangential direction: – Assume: particles move along with fluid

Fundamentals

One dimension analysis of single particle

Fc (or Fg)Fd

( )

0 ( )

( )

sc g d

c g d

c g d

or

or

or

dUm F F Fdt

F F F

F F F

⋅ = −

= −

=

Assume the time for particle to reach its terminal velocity is very brief

Fundamentals

Particle slip velocity, Us = velocity of water –velocity of particle

2 22

2

12

12

t tp l p l s p d

p p lts

p l d

u uV U A C

r r

VuU

r A C

ρ ρ ρ

ρ ρρ

⎛ ⎞− =⎜ ⎟⎜ ⎟

⎝ ⎠

−⎛ ⎞= ⎜ ⎟

⎝ ⎠

Net centrifugal force Drag force

for radial direction

dragcoefficient , densityof particleand fluid

radialposition tangential velocityof fluidvolumeof particle projectedarea of particle

d p l

t

p p

C

r uV A

ρ ρ= =

= == =

Why is the slip velocity important ?

Us“Reject stream”

Us“Accept stream”

For high Us

For low Us

High ut (tangential velocity)

High ρp (particle density)

Low Cd (drag coefficient)

2 12 p p lts

p l d

VuU

r A Cρ ρρ−⎛ ⎞

= ⎜ ⎟⎝ ⎠

Fiber tends to be rejected

Fundamentals

Cleaner Summary

Several types of cleanersObjective: remove high/low density contaminantsMust reject material to operate effectivelySeveral forces/operational variables/particle characteristics that combine to determine effectiveness in removal

Paper Recycling OperationsAn Example: OCC Recycling Process: Review Quiz

Match the following:

high density cleaner removes light weight contaminants pulper tries to hide contaminantsscreens thickens pulp stockdisperser breaks paper/board into fibersthru flow cleaner uses size differencesvacuum disk filter uses centrifugal forcesclarifier allows us to re-use process water

Paper Recycling Technology

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