When a fluff pulp lot performs poorly, attention usually goes to the pulp machine or the converting line. In most cases the cause was set earlier, in screening and cleaning, where contaminants and oversized fibre are removed — or not removed.
What screening removes
Screening separates fibre by size. Modern mills run multi-stage systems:
- Coarse screening removes shives — bundles of fibres that were under-cooked and never separated.
- Fine screening removes smaller debris and controls the upper end of the fibre length distribution.
- Rejects treatment recovers usable fibre from the reject stream, which is where most of the yield difference between mills is decided.
What cleaning removes
Cleaning uses centrifugal force to remove high-density contaminants — sand, metal, knots — and, in some configurations, low-density contaminants like plastic and pitch.
Contamination in fluff pulp shows up in the converting line as holes in the formed sheet and as visible specks in the finished product. Both are rejection triggers, and both are almost impossible to fix downstream.
Why shives are the most expensive defect
A shive does not defibrate in the hammer mill. It passes through as a hard knot, damages the forming screen, and ends up as a visible defect in the finished product. A single shive per square metre is enough to fail a quality inspection.
Shive content is therefore one of the most useful things to ask about in a supplier qualification, and one of the least commonly published.
What to ask a supplier
- How many screening stages, and what is the reject rate?
- Is there a rejects treatment stage, and what is the recovery rate?
- What is the measured shive content, and by which method?
- What contamination specification is guaranteed, and how is it verified?
The commercial consequence
Two mills can run the same wood, the same digester and the same pulp machine, and produce different material because one has a better screening line. That difference does not appear in brightness or CSF — it appears in dust, in screen life and in rejection rate at your plant.
Which is why the technical audit of a new pulp supplier should start at the screening stage, not at the finished bale.