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Pulp QualityPulp 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 QualityPulp 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 commercialB = 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

Softwood Hardwood

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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?