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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. 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
Pulp Quality Pulp Strength Delivery Delivering strong pulp is a goal in every mill but the commercial digester appears to be our worst enemy
How is a mill pulp different than pulp made in a pilot reactor? There can be several differences
How is a mill pulp different than pulp made in a pilot reactor? Most importantly – strength of commercial pulp is always lower
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
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
Do Hardwoods Have This Problem? NO – Shorter and more flexible hardwood fibers appear to be able to escape the digester without damage Hardwood Softwood
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
Where in the fiber line is the strength loss? The digester appears to be the culprit
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
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
How many mills have this problem? The mills average about 75% performance but none come near perfection.
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?
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
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.
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
A Happy Accident Effect of overcooking much less than that of damage in blow
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
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