Pulp Quality Pulp Strength Delivery

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Pulp Quality Pulp Strength Delivery Martin
MacLeod
Qualities of Kraft Pulp

• Strong Kraft means strength
• Can pulp any wood type
• Sulfite pulp is sensitive to extractives
• Good recovery system
• Hard to bleach

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Pulp Quality Pulp Strength Delivery
Delivering strong pulp is a goal in every mill
but the commercial digester appears to be our
worst enemy
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How is a mill pulp different than pulp made in a
pilot reactor?
There can be several differences
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How is a mill pulp different than pulp made in a
pilot reactor?
Most importantly strength of commercial pulp is
always lower
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How is a mill pulp different than pulp made in a
pilot reactor?
Case A shows considerable strength loss
Spruce/fir pulp of Kappa 31 Case B shows much
less strength loss is characteristic of Kamyr
continuous digesters (but not all Kamyrs)
Western hemlock Most importantly strength of
commercial pulp is always lower
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How is a mill pulp different than pulp made in a
pilot reactor?
All Types of Commercial Digesters Produce Weaker
Pulp
Solid lines refer to pilot-plant pulps, dashed
lines to commercial B Batch, C Continuous
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Do Hardwoods Have This Problem?
NO
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Do Hardwoods Have This Problem?
NO Shorter and more flexible hardwood fibers
appear to be able to escape the digester without
damage
Hardwood
Softwood
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Explicit definition of strength delivery
Tear index of mill made pulp relative to its
pilot-plant reference pulp at a constant
(mid-range) tensile strength
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Where in the fiber line is the strength loss?
The digester appears to be the culprit
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Where in the fiber line is the strength loss?
The digester appears to be the culprit The
digester is where we get significant changes in
chemistry and severe mechanical action on the
fibers during the blow
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How many mills have this problem?

• MacLeod and others have looked at lots of mill
  with regard to strength loss.
• All kraft mills pulping softwood show some
  strength loss
• A bisulfite mill producing bleachable grade
  softwood pulp did not yield any strength loss

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How many mills have this problem?
The mills average about 75 performance but none
come near perfection.
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Where in the digester is the problem?
Do conditions in the digester lead to weaker
pulp? Is it the blow that weakens the pulp? Is
there something else going on here?
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Where in the digester is the problem?
To investigate strength loss in batch digesters,
baskets were hung in the digester to perform
in-situ pilot cooks
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Where in the digester is the problem?
Pulp Sample Strength Delivery ()-Basket in upper position Strength Delivery ()-Basket in lower position
Pilot Plant 100 100
Basket 98 87
Washer 68 69
Blow line 58 59
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Where in the digester is the problem?

• Lessons learned
• The pulp from the basket was somewhat weaker but
  not the major source of strength loss
• Most (about 2/3) of the strength loss appears to
  occur across the blow
• Blow line sampling can be trouble and should
  probably be avoided

We might expect the same thing from a continuous
digester but no studies have been done at least
that I know of.
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A Happy Accident

• A basket broke in one experiment causing the pulp
  to be cooked twice
• Kappa 21 Viscosity of 21mPa.s
• Compared over cooked with blown pulp
• Pilot and mill cooked

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A Happy Accident
Effect of overcooking much less than that of
damage in blow
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What is happening to the fibers?
A - Pilot made fibers have mild cell-wall
damageB Blown mill fibers show severe damage
and frequent collapse of the cell wall
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How do you minimize the damage?

• Displacement batch digesters (RDH, SuperBatch)
  leave pulp cool and low consistency at end of the
  cook
• Ideal for pumping out digester
• Applied in Sunds SuperBatch systems
• Potentially can be applied to conventional
  digesters if digester can be cooled

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How well did pumping work?
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How well did pumping work?