Reject Refining - Miotti Consulting

Reject Refining 

Appita Mechanical Pulping Course 

3 April 2001 

Metso RGP 82 

CD refiner at 

Stora Enso 

Skoghall

Outline 

� Review of key facts and concepts 

� Reasons for reject refining 

� Techniques of separation of reject material 

� Example systems 

� Modern rejects system design

Morphology of fibre fractions 

Spruce TMP at at 90 mL CSF 

Law, K-N, “Rethinking chip refining”, Appita Conference, 101-106 (2000)



� R14, R28 and R48 contain mostly whole 

fibres with smooth surfaces 

� Some splitting, fibrillation and peeling of S 1 layer, 

especially in R48 

� High freeness and low strength properties 

� R100 includes ribbon-like cell wall lamellae 

and short fragments 

� Medium freeness and strength properties



� R200 contains mostly ribbons, very short 

fragments and fibrillar elements 

� Low freeness and high strength properties 

� P200 contains ray cells, flakes (S 1 layer) 

and fibrillar fines (S 2 layer) (magnification 

5x that of other fractions) 

� Fibrillar fines contribute to sheet consolidation

Why reject refine? 

� About 2 / 3 of the fibres (R14, R28, R48 and 

R100) in a typical TMP pulp at low freeness 

has poor bonding potential and needs 

further development 

� This can be done by separating those fibres 

(rejects or low quality material) and treating 

them in a separate refiner

Why reject refine? 

� Protection against operational upsets 

� May allow lower overall refining energy 

� Better overall pulp properties 

� Better paper machine and pressroom 

runnability 

� Better paper printability and smoothness

Let’s put things into perspective

Let’s put things into perspective

What are these “rejects”? 

Old focus 

� Removal of shives with screens 

� Removal of dirt (hence the name “cleaners”) 

and minishives with hydrocyclones 

� Long fibre was considered all “good” and to 

be accepted

What are these “rejects”? 

Modern focus 

� Separation of “good” from “bad” material 

(i.e. fractionation of low quality material) 

� Removal of thick-walled, collapse-resistant 

fibres, unfibrillated fibres, shives, 

minishives, chop and ray cells 

� Further treatment of this material by using 

as little energy and equipment as possible

Collapsible vs flexible fibres 

Property Collapsible fibres Flexible fibres 

Wall thickness Small Large 

Outer diameter Large Small 

Fibre type Earlywood Latewood 

Deformability Easily collapsible Do not collapse 

Density/ Strength/ Smoothness High Low 

Fibre rising (“spring back”) in printing & coating Low High 

Moment of inertia High Low 

Flexibility/ Stiffness Stiff Flexible 

Specific surface area High Low 

Split during refining Yes No 

Form crack-inducing shives No Yes 

Desirability in accepts Desirable Undesirable

Collapsible vs flexible fibres 

� Fibres of high 

resistance to 

collapse tend to be 

those of low 

moment of inertia. 

Fibres shown have 

the same wall area 

(i.e. coarseness) 

Wakelin, R.F., Jackman, J.K. and Bawden, A.D., “Changes in mechanical pulp fibre cross-sectional dimension distributions caused by 

screens, hydrocyclones and reject refining”, Appita Conference (1999).

Splitting gives > conformability and 

bonding 

Collapsible fibres 

Collapsed fibres have > intrafibre and 

interfibre bonding

Morphological properties of major 

high yield pulping species 

Radiata pine: L = 3.3 mm - Width = 32 microns - Wall thickness = 

4.3 microns => less flexible and collapsible than spruce

� Schematic picture of 

fibre fractionation 

with screens and 

hydrocyclones. 

Dashed material is 

well fibrillated and 

has high specific 

surface and low 

density. Base = 

accept, apex = reject 

Separation mechanisms 

Sandberg, C., Nilsson, L. and Nikko, A., “Fibre fractionation - a way to improve paper quality”, 1997 International Mechanical Pulping 

Conference.

Separation mechanisms for 

screens 

� Length (main) 

� Longitudinal flexibility (minor) 

� Surface development/ fibrillation (minor)

� Screen 

system 

separation. 

Accepts are 

mixed 

primary and 

secondary 

accepts 


screens 


Conference.


hydrocyclones 

� Surface development/ fibrillation 

� Fibre length and flexibility are irrelevant


hydrocyclones 

� Hydrocyclone 

system 

separation 


Conference.

Conclusion on separation 

mechanisms 

� Screens and cleaners are, therefore, 

complementary in the separation of low 

quality material 

� The optimal fractionation system for fibre 

development and fibrillation may require: 

� Wedgewire slotted screens 

� Smooth hole screens 

� Cleaners

The optimal fractionation system 

� Relatively limited amount of mainline 

refining before fractionation 

� P1 or P1/P2 (in series) with wedgewire 

slotted baskets (accepts forward) 

� S (optional) with smooth perforated baskets 

(accepts forward)


� R1 or R1/R2 (cascaded) with smooth 

perforated baskets (accepts forward) 

� Either mainline or reject cleaners, or both 

� One or more intermediate cleaner stage 

accepts stream(s) to reject refining


Pulp mill cleaners 

� Generally considered necessary for market 

CTMP mills and integrated mills for MFC, 

LWC and SC 

� Stora Port Hawkesbury and Norske Skog 

Saugbrugs Halden have recently installed 

hydrocyclones for SCA paper


Pulp mill cleaners 

� Generally considered unnecessary for SNP 

and INP grades 

� Ponderay Newsprint Usk has recently 

installed wedgewire slotted baskets and 

by-passed the cleaners


Pulp mill cleaners - Exceptions 

� SNP mills with cleaners: 

� Norske Skog Albury, HSPP Port Mellon and Alliance 

Dolbeau with mainline cleaners 

� Bowater Dalhousie with 5-stage reject cleaners 

� Specialty mills without cleaners: 

� Inpacel Arapoti (market CTMP) 

� Irving Paper St John, Stora Enso Langerbrugge (SC) 

� Pacifica Port Alberni (MFC)


Are pulp mill cleaners really necessary? 

� An important question: 

� Are pulp mill cleaners really necessary if the 

paper machine has a full-fledged cleaning 

system? 

� Open for debate as fractionation and long 

fibre development are to be achieved with 

least amount of energy and equipment


LC or MC screening? 

� Another important question open for debate: 

� Screen at low (1.5-2% OD) or medium (4-4.5% 

OD) consistency? 

� OZ and NZ mills generally LC (except NS 

Boyer in CCS plant) 

� Canadian mills generally LC (except Abitibi- 

Consolidated Alma, Irving Paper St John, 

Pacifica Powell River)


LC or MC screening? 

� Finnish mills generally MC 

� Norwegian mills generally MC with recent 

trend towards LC (NS Saugbrugs Halden) 

� Swedish mills generally LC (except 

Rottneros Rockhammars) 

� USA mills generally LC (except Alabama 

River News Perdue Hill and Augusta News)

Reject refiner systems 

� The rejects are thickened to 30-35% OD 

consistency in either screw or twin roll 

presses and are then fed to the reject refiners

Andritz screw press

Metso twin roll press

Andritz SB 150 

refiner at Norske 

Skog Golbey, 

France 

Andritz reject refiner

Metso RGP 

262 refiner at 

Stora Enso 

Langerbrugge, 

Belgium 

Metso reject refiner

Refining conditions for pine TMP 

Chip size mm 15-16 

Pre-steaming pressure kPag 0-70 

Refining pressure kPag 380 

Primary refiner consistency % OD 45-55 

Secondary refiner consistency % OD 55-60 

Overall reject rate % 50-60 

Reject refiner consistency % OD 35-40 

Reject refiner SEC MWh/ODt 1-1.6 

Mainline refiner SEC MWh/ODt 2.5

Effects of reject refiners 

� Break down shives in individual fibres 

� “Peel” the fibre wall, reduce its thickness 

and form fibrillar fines 

� Generate fibres that are at the same time 

more flexible and collapsible 

� Increase light scattering coefficient


� There is conflicting evidence on whether 

refining preferentially fibrillates the 

latewood (Norway) or the earlywood fibres 

(NZ) 

� The refined long fibre develops better 

strength properties than the shorter fibre in 

the screen accepts


� Long fibre development and high quality 

fibrillar fines generation will give a stronger 

combined pulp and result in a dense, wellbonded 

sheet with low porosity, and high 

strength, elongation and smoothness

Pulp properties before and after 

reject refiner - Spruce TMP 

Property Unit Before 

Reject 

Refiner 

After 

Reject 

Refiner 

Percent 

Change 

Freeness mL CSF 647 262 -60% 

Density kg/m 3 

265 310 +17% 

Burst index kPa . m 2 /g 0.97 2.23 +130% 

Tensile index Nm/g 21.6 41.9 +94% 

Tear index mN . m 2 /g 7.9 9.5 +20% 

Wet web 

tensile 

N/m 51 94 +84% 

Wet web 

stretch 

% 2.8 4.2 +50%

Reject refiners cannot do 

everything 

� The reject refiner is “transparent” to ray cell 

material => will not increase its bonding 

potential and reduce its linting propensity 

� Norske Skog Saugbrugs has installed 

cleaners to purge ray cells 

� Could this be a justification for cleaners, 

despite their high capital and running costs?

Example systems 

� Metso Thermopulp TMP plant for SNP at 

Papier Masson, PQ 

� 740 ODt/d - black spruce/ white spruce/ balsam fir 

� Metso Thermopulp TMP plant for SNP at 

Inforsa Nacimiento, Chile 

� 660 ODt/d - radiata pine 

TMP technology, Sunds Defibrator

Modern reject system design 

� 1 reject refiner even for large mainline 

tonnages (Papier Masson, Inforsa) 

� Constant pulp feed rate 

� Attention to plate pattern and alloy 

� Regular plate changes 

� Uniform refining consistency 

� High for wider plate gap, > strength and < cutting


� Adequate steam exhaust to avoid motor load 

fluctuations 

� High throughput 

� Adequate SEC 

� Adequate latency removal


Series reject refining 

� Series reject refining is carried out for 

LWC/SC grades 

� SCA Ortviken (LWC) 

� Stora Enso Kvarnsveden, Langerbrugge and Port 

Hawkesbury (SC) 

� The refining energy is distributed between two refiners 

with < refining intensity in each unit, < fiber cutting and 

< linting propensity


Series reject refining 

Properties Single Series Reject 

Stage Refining 

Freeness mL CSF 42 80 42 

Density, kg/m 3 

403 377 410 

Burst Index kPa.m 2 /g 2.46 2.41 3.0 

Tear Index mN.m 2 /g 9.2 11.3 10 

Tensile Index N.m/g 44.7 46.6 53.3 

Opacity % 91.6 89.9 90.7


Pressurised RR 

� More long fibres and < debris level 

� Loss in printability and opacity due to < 

scattering coefficient 

� High consistency operation is a must 

� Heat recovery is common even if less than 

that from mainline refiners


Open discharge vs pressurised RR 

7.0 

6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

0.0 

120 mL 

CSF 

200 mL 

CSF 

Tensile, km - Pressure 

Tensile, km - O.D. 

Burst index, kPa.m2/g - Pressure 

Burst index, kPa.m2/g - O.D.


Burst index, kPa.m 2 /g 

4.5 

4 

3.5 

3 

2.5 

2 

1.5 

1 

0.5 

0 

Burst index vs pressure 

100 mL 

CSF 

150 mL 

CSF 

200 mL 

CSF 

450 mL 

CSF 

310 kPag 

240 kPag 

380 kPag

� Co refining 


Co-refining vs separate refining of rejects 

� Rejects fed back to the 

secondary refiner 

� No preferential energy uptake 

by long fibre (PAPRO) 

� All fibre fractions will develop 

with detrimental effect on 

scattering and opacity 

� Relatively low cost if power is 

available in the secondary 

� Separate refining 

� Allows excellent control and 

fibre development 

� Rejects can be used as a 

separate furnish component 

to the paper machine 

� More capital intensive 

� Preferred option, despite > 

cost

Co-refining of rejects

Conclusions 

� RR can provide the highest quality pulp 

within the mechanical pulp mill 

� RR improves paper machine and pressroom 

runnability and sheet printability 

� Attention to design and operation of the 

reject system is fundamental to the 

production of high quality high yield pulp

Reject Refining - Miotti Consulting

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