Pulping and Bleaching
PSE 476
Lecture #2
Mechanical Pulping
Agenda
• Mechanical pulping
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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
General 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.
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
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
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.
Mechanical Pulping Overview
Stone 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.
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.
Mechanical Pulping Overview
Stone Groundwood: Mechanism
This is a poor reproduction of a drawing showing the action of
the stone on the surface of the wood.
Source: Handbook of Pulping and Papermaking
, Biermann, C, 1996, page 65
Mechanical Pulping Overview
Stone 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.
Mechanical Pulping
Groundwood Flowsheet
Logs
Drum Washer
There is a latency tank before
bleaching which serves to
Grinder
Bull Screen
Refiner
Fine Screen
Press
Refiner
Centrifugal Cleaner
Thickener
“uncurl” the fibers.
Screen
Bleaching
Storage
Paper Machine
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.
Small laboratory refiner
Notes
Mechanical Pulping
Process Conditions
Process
Temp
(C)
Chemicals
pH
RMP
Heating
Time
(min)
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
1-10
110-140
9-12
6
12
60
130-170
3 - 30
minute
reactions
Steamed
SCMP/BCMP
ARTMP
1-10
12-17% Na2SO3,
12-17% NaHSO3
110-140
0.65% DTPA,
0.05% Mg2SO4,
3.1% NaSiO3, 5.1%
NaOH, 4.1% H2O2
Pretreatment Pretreatment
Time
Temp
(min)
(C)
NA
NA
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.
Refiner Plates (2)
Directional
Variable pitch
Chicane
Refiner Pulping
Typical Refiner Flowsheet
Oversize
Chip Screen
Fines
Rechipper
Chip Washer
First Stage
Refiner
Reject
Refiner
Second Stage
Refiner
Screens
Screens
Centrifugal
Cleaners
Rejects
Thickener
Thickener
Bleaching
Storage
Paper Machine
Refiner Pulping
CMP (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.
Refiner Pulping
CMP 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.
Mechanical Pulping
Determination 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.
Mechanical Pulping Overview
Endpoint 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