Pulping and BleachingPSE 476

Lecture #2Mechanical Pulping

Lecture #2Mechanical Pulping

Agenda

• Mechanical pulping» Grinding-logs» Refining-chips» Mechanical pulp properties» Process conditions (chemicals, steam, etc.)» Canadian Standard Freeness Test

• Mechanical pulping» Grinding-logs» Refining-chips» Mechanical pulp properties» Process conditions (chemicals, steam, etc.)» Canadian Standard Freeness Test

Mechanical Pulping

• Mechanical pulping is the process by which fibers are produced through mechanical methods:

• Grinding-Stone Groundwood (SGW)» Logs (very occasionally chips) are pressed into a

turning stone thus releasing fibers.

• Refining-Refiner Pulp » Chips are fed between 2 disks. One disk is always

turning while the other can be fixed or turning.» Often heat or a chemical pretreatment is utilized.

• Mechanical pulping is the process by which fibers are produced through mechanical methods:

• Grinding-Stone Groundwood (SGW)» Logs (very occasionally chips) are pressed into a

turning stone thus releasing fibers.

• Refining-Refiner Pulp » Chips are fed between 2 disks. One disk is always

turning while the other can be fixed or turning.» Often heat or a chemical pretreatment is utilized.

Mechanical PulpingGeneral Info

• Raw Material Quality» Pulps cannot be brightened very much;

therefore good quality material must be used- Chips used should be less than 2 weeks old

(oxidation and biological decay darken chips after this point).

- Low bark and dirt tolerance (color and machinery wear issues).

• Species Dependence» Different wood species work better in

different processes.

• Raw Material Quality» Pulps cannot be brightened very much;

therefore good quality material must be used- Chips used should be less than 2 weeks old

(oxidation and biological decay darken chips after this point).

- Low bark and dirt tolerance (color and machinery wear issues).

• Species Dependence» Different wood species work better in

different processes.

Properties of Mechanical Pulp

• Relatively cheap» High yield of product (85-95%)» Low capital costs relative to Kraft mill

• High opacity product» Large amount of fines in product scatter light» Allows printing on both sides of thin sheet

• Good printing surface» Broad fiber size distribution gives smooth

surface» Good bulk

• Relatively cheap» High yield of product (85-95%)» Low capital costs relative to Kraft mill

• High opacity product» Large amount of fines in product scatter light» Allows printing on both sides of thin sheet

• Good printing surface» Broad fiber size distribution gives smooth

surface» Good bulk

Properties of Mechanical Pulp

• Relatively weak product» Not strong enough to get through printing press» Need to add chemical fibers to product

- Can use recycle fibers

• Limited brightness» Bleached with lignin retaining bleaching agents

• Photoyellowing» Formation of chromophores from lignin

compounds through the reaction with light and oxygen

• Relatively weak product» Not strong enough to get through printing press» Need to add chemical fibers to product

- Can use recycle fibers

• Limited brightness» Bleached with lignin retaining bleaching agents

• Photoyellowing» Formation of chromophores from lignin

compounds through the reaction with light and oxygen

Stone Groundwood: Equipment

• Not used at all in the Pacific Northwest—all our wood leftovers from saw mills as chips.

• Different designs.• Logs are pressed into a rotating stone. • The fibers are washed off the stone with water

(cools the stone).- Running this system under a slight pressure

improves the process.

• Not used at all in the Pacific Northwest—all our wood leftovers from saw mills as chips.

• Different designs.• Logs are pressed into a rotating stone. • The fibers are washed off the stone with water

(cools the stone).- Running this system under a slight pressure

improves the process.

Mechanical Pulping OverviewStone Groundwood: Equipment

• The surface of the stones are cut with patterns (burrs) using a metal burr.

• Patterns are 1.6 mm deep.• The stones must be sharpened every 50-150

hours.

• The surface of the stones are cut with patterns (burrs) using a metal burr.

• Patterns are 1.6 mm deep.• The stones must be sharpened every 50-150

hours.

Pulpstone “sharpening”

• Sharpening= fracturing the softer bond posts.

• This causes the worn grains to be removed, uncovering new sharp abrasive grains.

• Why sharpen?» To expose fresh abrasive

grit. » To control the compression /

decompression frequency on the wood fibres.

» To control fibre length. » To clean stone pores. » To carry water into the

grinding zone and pulp out of the grinding zone.

• Sharpening= fracturing the softer bond posts.

• This causes the worn grains to be removed, uncovering new sharp abrasive grains.

• Why sharpen?» To expose fresh abrasive

grit. » To control the compression /

decompression frequency on the wood fibres.

» To control fibre length. » To clean stone pores. » To carry water into the

grinding zone and pulp out of the grinding zone.

Mechanical Pulping OverviewStone Groundwood: Mechanism

Source: Handbook of Pulping and Papermaking, Biermann, C, 1996, page 65

This is a poor reproduction of a drawing showing the action of the stone on the surface of the wood.

Mechanical Pulping OverviewStone Groundwood: Mechanism

• The burrs on the stone alternately compress and decompress the fibers. This loosens the fibers. Additionally, the heat developed softens the lignin in the middle lamella which helps the process.

• The application of heat or chemical will also soften the lignin and improve this process.

• The burrs on the stone alternately compress and decompress the fibers. This loosens the fibers. Additionally, the heat developed softens the lignin in the middle lamella which helps the process.

• The application of heat or chemical will also soften the lignin and improve this process.

Mechanical PulpingGroundwood Flowsheet

Drum Washer

Logs

Grinder

Bull Screen Refiner Refiner

Fine Screen Press

Centrifugal Cleaner Screen

Thickener Bleaching Storage Paper Machine

There is a latency tank before

bleaching which serves to

“uncurl” the fibers.

Refiner Pulping

• Refiner Mechanical Pulping (RMP)

• Thermo-mechanical Pulping (TMP)

• Chemi-mechanical Pulping (CMP)

• Chemi-thermomechanical Pulping (CTMP)

• The differences between these systems are discussed in the notes section and on the next slide.

• Refiner Mechanical Pulping (RMP)

• Thermo-mechanical Pulping (TMP)

• Chemi-mechanical Pulping (CMP)

• Chemi-thermomechanical Pulping (CTMP)

• The differences between these systems are discussed in the notes section and on the next slide.

Small laboratory refiner

Notes

Mechanical PulpingProcess Conditions

Process Heating Time (min)

Temp (C)

Chemicals pH Pretreatment Time (min)

PretreatmentTemp (C)

RMP NA NA NA NA NA NA

TMP 1-10 110-140 NA NA NA NA

CTMP 1-10 110-140 2-5% Na2SO3 9-12 2-5 120-130

CMP - Cold NaOH NA NA NaOH Bath 12+ 120 30

- Sulfite SCMP/BCMP

1-10 110-140 12-17% Na2SO3,

12-17% NaHSO3 9-12

6 60 130-170

ARTMP 1-10 110-140 0.65% DTPA, 0.05% Mg2SO4,

3.1% NaSiO3, 5.1% NaOH, 4.1% H2O2

12 3 - 30 minute

reactions

Steamed

Refiner Plates

• The refiners contain opposing plates that shred the chips. These plate are separated into different sections» 1st section (inlet) rips

chips into wood slivers.» Each subsequent step

reduces the size of the bundles.

» Water carries the fibers through the disk.

• The refiners contain opposing plates that shred the chips. These plate are separated into different sections» 1st section (inlet) rips

chips into wood slivers.» Each subsequent step

reduces the size of the bundles.

» Water carries the fibers through the disk.

Refiner Plates (2)

Variable pitch Chicane  Directional

Refiner PulpingTypical Refiner Flowsheet

Chip Screen Rechipper

Chip Washer

Oversize

Fines

First StageRefiner

Second Stage Refiner

Screens Thickener

Screens

RejectRefiner

CentrifugalCleaners

Thickener Bleaching Storage Paper Machine

Rejects

Refiner PulpingCMP (CTMP) Process Chemicals

• Hot Sulfite (NaOH + Na2SO3)» NaOH swells wood for easier refining.» Sulfite prevents the wood from darkening

upon this treatment.

• Cold Soda (NaOH)» NaOH swells the wood.» Some hemicelluloses and all resins lost.» Softwoods resist this treatment so used only

for hardwoods; a very very minor process.

• Hot Sulfite (NaOH + Na2SO3)» NaOH swells wood for easier refining.» Sulfite prevents the wood from darkening

upon this treatment.

• Cold Soda (NaOH)» NaOH swells the wood.» Some hemicelluloses and all resins lost.» Softwoods resist this treatment so used only

for hardwoods; a very very minor process.

Refiner PulpingCMP Process Chemicals

• Alkaline Peroxide (NaOH + H2O2)» Pulping and bleaching together.» Difficult with Softwoods.» Need to stabilize peroxides against:

- Thermal degradation and degradation by metals

» Silicates used to stabilize peroxides can deposit on refining equipment.

• CTMP (2-5% Na2SO3)» Most common of the chemical processes. The

lignin is sulfonated to only a very slight extent. This softens the lignin making fiber removal easier.

• Alkaline Peroxide (NaOH + H2O2)» Pulping and bleaching together.» Difficult with Softwoods.» Need to stabilize peroxides against:

- Thermal degradation and degradation by metals

» Silicates used to stabilize peroxides can deposit on refining equipment.

• CTMP (2-5% Na2SO3)» Most common of the chemical processes. The

lignin is sulfonated to only a very slight extent. This softens the lignin making fiber removal easier.

Mechanical PulpingDetermination of Endpoint

• In chemical pulping, the pulping reaction is allowed to go until a certain level of lignin is reached as determined through the determination of Kappa number.

• In mechanical pulping, the material is refined until a specific freeness is reached.» Freeness is the ease with which water leaves the

fiber mat formed on a wire mesh from a dilute slurry.

• In chemical pulping, the pulping reaction is allowed to go until a certain level of lignin is reached as determined through the determination of Kappa number.

• In mechanical pulping, the material is refined until a specific freeness is reached.» Freeness is the ease with which water leaves the

fiber mat formed on a wire mesh from a dilute slurry.

Mechanical Pulping OverviewEndpoint Determination

• Canadian Standard Freeness» Measurement of ease at

which water leaves a fiber mat

» Simple, quick, reproducible but not always meaningful

• High Freeness» 750 ml water: Unrefined -

chemical pulp

• Low Freeness» 20 ml water: Heavily refined

- mechanical pulp

• Canadian Standard Freeness» Measurement of ease at

which water leaves a fiber mat

» Simple, quick, reproducible but not always meaningful

• High Freeness» 750 ml water: Unrefined -

chemical pulp

• Low Freeness» 20 ml water: Heavily refined

- mechanical pulp