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Mech 450 – Pulping and Papermaking Topic 3 – Mechanical Pulping

James A. Olson

Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia

Mechanical Pulping

Comparison of Mechanical and Chemical Pulps

Debarking

Stone Groundwood

Refiner Mechanical Pulp

Thermo mechanical pulping (TMP)

Chemi thermo mechanical pulping (CTMP)

Brightening

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Mechanical Pulping

Fibres mechanically removed from wood matrix

Chemical Pulping

Lignin holding fibres together is dissolved

Lignin

Fibres

In addition to fibre removal, fibres are broken and fines (fibres <

0.5mm) are created

About 1/3 of pulp mass is in form of fines

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In contrast, chemical pulping produces intact fibres

Chemical Mechanical

Yield Fibre/Wood - Low 40-70% - High 90-98%

Cellulose Purity - High - lignin - Low - lignindissolved remains

End Uses - Dissolving pulp - Low quality- High quality paper - High volume

paper(e.g. book) (e.g. newsprint)

- Reinforcement pkg. - Molded products

Raw Material Sensitivity - Low - High

General Parameters

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Chemical Mechanical

Strength - High - fibres intact - Low - fibres damaged

Bulk - Low - more and - High - few and lessflexible fibres flexible fibres

Optical - Dark but bleachable - Bright but hard- Poor light scattering to bleach high

- Good light scattering

Drainability - Good - long fibres, - Poor - short fibres,few fines many fines

Permanence - Good - Poor (optical)

Quality Parameters

Chemical Mechanical

Raw Material - High - low yield - Low - high yield

Capital - High - Low

Operating - High - Low - becoming(chemicals, energy etc) lower - high for

electrical energy

Auxiliary - High - Low - for slush pulp(pollution recovery etc)

Mechanical pulps are generally used forshort-life, inexpensiveproducts, e.g. newsprint

Cost Parameters

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History

Pre-mid 1800’s paper made of rags.

1841, Friedrick Keller “inventor” 1848 Johan Voith in Heidenheim

made first commercial grinder. 1859 Voith developed “Raffineur”

to break up any course material not properly ground. First success.

1867 Full plant powered by steam. Paper made with 70% wood (Worlds fair Paris)

1868 Tampella (finish company) started making grinders.

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Debarking Drum

Ring Debarking

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Debarking resistance

Factors:

Species

Moisture content

Felling season

Storage duration

Temperature

Jan Dec

4

12

May Sept0

Deb

arki

ng r

esis

tanc

e N

/cm

^2

Stone Groundwood (SGW)

Pulp produced by pressing

logs against rotating

grindstone

Unchanged for 150 years.

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Action of grinder

Circumferential speed 30 m/s Grinding pressure 250kPa Grits deform fibre-lignin matrix Repeated visco-elastic deformation

creates heat Increased heat in wood

Heat softens lignin that’s found in between fibres and helps to release the fibres

Action of grinder

Fibres are peeled back in layers

Grits pass over partially removed

fibres

Develops surface and flexibility of

fibres … paper strength.

Fibres are released

Next layer peeled off

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Operating Parameters

Species and property of wood

Amount of spray water

Temperature of spray water

Rate of wood feed

Pressure applied

Speed of grinder

Structure of stone

Pulp Constituents

Shives: fibre bundles (3%)

Long, intact fibres (20%)

Short, broken fibres (35%)

Fines (45%)

Flour 30x30

Fibrils 30x1

Dust 1x1

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Pulp Properties

Higher strength as

more energy applied

CSF drops 150-50 ml

as energy applied

Brightest of

unbleached pulps up

to 65 ISO

Stone Sharpening

Stones wear due to constant high-

speed abrasion

Ceramic stones

Sharpening every 6-14 days

Sharpness affects energy and

production

2.5mm

grits

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Continuous Grinding

Pressure Ground Wood (PGW)

Higher pressure leads to

higher temperatures

Softer lignin, easier to detach

whole fibres

Stronger pulp

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Example

The quality of the pulp produced during grinding is dependent on the

temperature in the grinding zone. Fibre can be liberated largely intact if

the lignin has been softened by temperature, however, if the temperature

is too low the fibres will be largely broken or if the temperature is too high

the wood will start to darken.

Since virtually all of the grinding power is dissipated as heat in the

grinding zone, it follows that temperature in that zone is controlled by the

addition of shower water.

For a given grinding operation, wood, F, and dilution, D, (kg/s) enter the

grinder at Tin degrees C. The suspension leaving the grinder at Tout and

at a consistency, C. Assume that the steam is not formed. Determine the

electrical energy applied, E (J/kg), to maintain these outlet conditions.

C

F

D

E

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Mech 450 – Pulping and Papermaking Topic 3b – Refiner Mechanical Pulping

James A. Olson

Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia

Refiner Mechanical Pulp (RMP)

Wood chips are comminuted into fibres by bars on rotating

and stationary discs

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History

1957 Stora (Sweden) installed a Defibrator “raffinator”. Bauer shortly after

1963 Both companies modified to operate under pressure to make Thermo-mechanical pulp

1970’s First 100% TMP newsprint 1980’s 2-stage refining and heat recovery 1985 Large refiners 15MW. Chemicals added to further soften lignin

(CTMP). Mechanical pulps are replacing chemical pulps

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Chip Handling

Wood is typically chipped in a disc chipper

Goal is to have a high proportion of acceptable chips

3-16 knives on a disc 4 m diameter 450 m^3 / hr of solid wood Low cutting speed (20 m/s) as pin

chips increase with speed

Effect of chip size

Over size chips

Uneven feed in refiner

Reduces quality

Over thick fraction

Contains most of the knots

Decreases fibre length and long fibre portion

Decreases strength and brightness

Fines Fraction

Lowers energy consumption

Decreases strength, sheet density, brightness and light

scattering

Creates linting problems and increases shive content

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Chip washing

Immersed in a tank fed by a paddle wheel (Sunds). Removes: Rocks, metal, sawdust, bark Adds moisture Raises temperature

Chip Screening

Chips are passed through a series of screens Oversize: left on screen with 45 mm holes Overthick: left on screen with 7 mm slots Accept: left on screen with 7 mm holes Pin chips: left on screen with 3 mm holes Fines: pass through last screen

Overthick chips don’t react well to pre-treatments, lower yield

Fines and pin chips produce too many shives (not refined)

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Chip Steaming/Preheating

Atmospheric type Steam to 80 - 95 C

Most are pressurized (50kPa to 110kPa over pressure) Objective is to warm chip and equalize the moisture

content Can optimize a bit:

Higher temperature gives longer fibres, higher tensile Lower temperatures give better optical properties

Chip impregnation systems Used in CTMP Processes Compresses chips

• Water is removed and is high in extractives… fed to effluent• 4:1 compression ratio or higher

Passes chips into a pool liquor containing chemicals Increase moisture content by 6-7%

Disc Refiner

Refining Equipment

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Self Pressurization

Refining imposes cyclic

compression of visco-elastic

material

Generates tremendous amount

of heat and steam

Dilution required to maintain

approx 30% consistency

Steam pressure reaches max

and flows both ways

Can cause blow-back

Types of Refiners

Single disc, Moving rotor staionary stator 1.7m Dia. 15 MW

Double Disc Two counter-rotating discs More power delivered Less energy required per ton

• Higher shives, less long fibres, (similar to SGW)

Twin refiner One rotor, two stators… more

refining surface• Low intensity refining possible

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Refiner size over time

Conical Disc Refiners

Flat disc section and

conical section

Increases grinding surface

without increasing

diameter

Power: CD70, 76, 82 uses

15, 24 32 MW

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Refining Action

Chips are preheated to soften lignin Chips hit breaker bars and undergo a

series of normal and shear forces Rapid Breakdown in screw feeder,

entrance zone and breaker bars section. Fractures along grains, mostly along

fracture planes initiated in chipping Match stick size fragments accumulate

in refining zone with major axis along tangential direction

Match sticks defibred by longitudinal grinding and brooming

Fibres form flocs and flow out by steam drag and inertial forces

Flocs caught on bar edges and repeatedly compresssed by passing bars.

Breakerbars

Refining action

Fibre development

step

Fibres undergo cyclic

compressions

between bars

Internally and

externally delaminates

the fibres

Increases flexibility

and surface area

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Refiner Segment Design Parameters

Width of Grooves and Bars Traditionally the main parameter Wide grooves - narrow bars

• reduce specific energy consumption in refiner• Open volume allows gap to be narrower and can result in lower pulp

quality Wide bars / narrower grooves

• Increase specific energy consumption and improve quality• When Volume in groove is reduced steam flow is impeded and axial

load is higher and infeed of fibres is more difficult. This can lead to unstable feed

Height of the bars Higher the more open the groove volume, the better steam

removal Low bar height forces fibres to the plate gap an pulp quality

improves.

Dam number, height, and placement Forces pulp from the grooves to the plate gap Residence time increases. Hinders steam removal

Bar taper and angle When bars form a pumping angle fibre are forced through, lower

residence time which reduces energy consumption

Thermo-mechanical Pulp (TMP)

Pulping carried out in two refiners in

tandem

First refiner - pressurized with steam

(along with pre-steamer)

Second refiner is atmospheric

Produces longer fibre (stronger paper) and

fewer shives (small bundles of fibres)

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Theory

Specific Energy

Intensity:

Number of impacts

Intensity of each impact:

Specific energy per

impact

No LoadP PE

QC

I

“High Intensity”

BE

AE

“Low Intensity”

N

Ee

How do we calculate residence time?

Force balance on element of pulp

1 2r rF C F F bS

224 ( ) ( ) ( )( ) ( )

2r m

f p

rP r c rdv r b c rU r C A r

dr v m v

2

1

r

r

dr

v

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Operating parameters

Refiner speed (increase)

Increase intensity at same power

Lower energy at same freeness, lower length, and tear

Inlet Consistency (increase)

Increase moisture content and fibre length

Production rate (increase)

Reduce energy and lower length and strength

Preheating and steaming temperature

Not too critical

Plate Gap

Increases intensity

Lead to pad collapse

Effect of refining on coarseness

Coarseness:

Decreasing coarseness support

delamination theory

Lower coarseness of small fraction

indicate they are created from

fragments of cell wall

Not always evident if we measure

coarseness of whole pulp

Difficult to measure coarseness of pulp

with fines

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Effect of refining on long fibres

Effect of refining on fibre width

Refining reduces fibre

width by removing outer

wall material.

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Effect of refining on wall thickness

High intensity refining

reduces wall thickness more

at same energy

Outer part of fibre wall is

being peeled away

Effect of refining on fibre collapse

X-section measured by CLSM

Collapse index is an indication

of fibres ability to form ribbons

High intensity process creates

more collapsed fibres at same

energy

Wall stiffness about the same

Therefore wall thickness is

less for high intensity

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Effect of refining on fibre flexibility

Effect of increasing

energy plateaus at

moderate energies

Fibre development is

mostly through removal

of outer wall

Not through internal

delamination

Comparison of Pulp Properties

SGW RMP TMP

Energy required (GJ/ton) 5.0 6.4 7.0

Freeness 100 130 100-150

Burst index 1.2 1.6 1.8-2.4

Tear index 3.5 6.8 7.5-9.0

Breaking length (km) 3.2 3.5 3.9-4.3

Shive content (%) 3 2 0.5

Long fibre content (R48) 28 50 55

Fines content (P100) 50 38 35

Brightness (unbleached) 61.5 59 58.5

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Miscellaneous Other Data

Typical Production Rate 300 Bdt/d

(of one refiner) 800 Bdt/d - modern

Typical gap between plates 0.5-1 mm

Typical Specific Energy 7 GJ/t

Typical Power to Refiners 20-30 MW

(27,000 – 42,000 horsepower

10-15 train diesel locomotive)

Latency Removal

After refining fibres are kinked and

curled and not suitable for

papermaking

Lignin cools and holds kinked shape

Latency removal straightens fibres

Low consistency

30 minutes

90 degrees C

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Latency removal

Latency removal result in:

Chemi Thermo Mechanical Pulping (CTMP)

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Chemi-Mechanical Pulps

• To decrease energy cost or to improve pulp quality, chemical treatments are often added to mechanical pulping

• Pretreatment of chips• to lower energy

• Interstage treatment• lower energy, fibre flexibilization

• Post-treatment• fibre flexibilization

Sulphonation reactions

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1. increase in tear index2. increase in freeness

Increasedlong fibrecontent

Decreasedshive content

Improved fibreseparation

Softening ofmiddle lamella

lignin

Low sulphonatecontent (0-1%)

Usual means is sulphonation using sodium sulphite or sodium bisulphite

Decrease in freenessIncrease in breaking length

Decrease in specific scattering

Increase infibre flexibility

and conformability

Softening offibre wall

lignin

High sulphonatecontent (1-2%)

Pulp Properties

RMP fibres broken

TMP separated at primary wall,

some fibre broken

CTMP Middle lamella very soft,

almost all fibres separated at

M.L.

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Pulp Properties

Light scattering reflects

fines content

Tensile reflects surface

area and flexibility of

long fibres.

Pulp Properties Changes during Refining

Strength increase

Corresponds to energy

increase without cutting

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“Alphabet” Pulps

Many combinations of treatment and pulping processes are

possible

PUREMECHANICAL

SGWPGWRMP

TRMPPRMP

TMP

CHEMICALLYMODIFIED

HEAVYFRACTIONAL

LIGHT

HEAVY

LFCMPCTLF

TCMPCRMPCTMP

OPCOSCMPBCMP

UHYBSUHYS

MO

NO

PU

LP

S

PR

INT

ING

PU

LP

S

RE

INF

OR

CE

ME

NT

PU

LP

S

Effect of sulphonation on Lignin softening temperature

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Effect of yield on fibre stiffness

Effect of sulphonation on fibre length

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Effect of sulphonation on tensile

Effect of Sulphonation Energy required

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Effect of sulphonation on light scattering

Scattering vs Energy

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Mechanical Pulp Brightening

Often desirable to make pulp brighter (whiter)

Do not want to remove lignin to keep yield high

Use “brightening” chemicals, e.g. hydrogen peroxide

Problem: If lignin not removed, brightness not permanent

(reversion, yellowing)

Example: BCTMP (Bleached Chemi-Thermo-Mechanical

Pulp)

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Screening and Cleaning

Pulping process imperfect

Small bundles of fibres (shives) remain

These must be removed and further refined

Mechanical pulping is therefore follows by an elaborate

screening system

Subject of next lecture (after LC-refining)

TMP System

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Process may also include “cleaners” (hydrocyclones)

Energy Recovery

Enormous volume of steam produced from heat created

in mechanical pulping

This steam can be recovered and used for mill process

steam, e.g. for paper drying

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Energy balance

About 65% of electrical energy can be

recovered in this manner

RTS results

Retention: Short retention in pre-

heater (10-20s).

The short time at elevated temperature

reduces the brightness losses

Temperature: increase pressure

to 5.5-6.0 bar

Speed: Increase speed to 2000-

2500 RPM. Decreases specific

energy to get same ‘quality of

pulp’. 15% energy reduction.

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Conclusions

Refining characterized by specific energy and intensity

Refining removes outer wall material

Thin wall, collapsible fibres

Smoother, stronger paper

Heat softens lignin

More long fibres and less fines

CTMP softens lignin in fibre wall

Even more long fibres, less fines

Makes fibres more collapsible at same wall thickness

Less fines

The end