Why does Canfor use a pilot refiner to evaluate pulps?
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Since
we started exclusively testing pulp using a pilot refiner,
this is a question we have often been asked.
The answer is simple, we believe that the pilot
refiner gives us a better indication of how a given
pulp or
blend of pulps will actually perform in a paper mill,
than we could get from using devices like PFI mills,
Jokkro mills or Valley beaters. We encourage other
pulp suppliers to do likewise.
Most readers of this
website know how a pulp refiner operates, energy is
applied to the pulp fibres by passing
them between two surfaces separated by a narrow gap.
One or both of the surfaces is rotating and both are
covered in multiple fine patterns of bars. The energy
applied to the refiner, rotational speeds, dimensions
of the bars and distances between the plates are all
known. The intensity with which the energy is actually
applied to the fibres can be calculated.
When we consider
how the standard laboratory instruments such as PFI
or Jokkro mills operate, it is clear that
we have no way to easily know how much energy they
actually apply to the pulp, nor the intensity at which
it is actually applied. The only parameter we measure
is the number of revolutions. When dealing with properties
such as tensile strength, the values obtained on PFI
or Jokkro mills are usually very high, the devices
are acting as “ideal” refiners. Needless
to say, such values never appear in the papermill!
Several
studies have been conducted to learn more about the
mode of action of these devices, and to compare
them with industrial and pilot refiners. In the case
of PFI mills it was found that 6000 revolutions of
the rotor is roughly equivalent to 1000 kWhr/t of
pulp, almost 10 times what most refiners in a papermill
might
be expected to apply. What was more, the actual energy
applied varied with the type of pulp.
Some results
from one of these studies are shown graphically below.
The graph in Figure 1 is a comparison
of breaking
length versus freeness for pulp refined on a PFI
and on a conical Escher Wyss, the EW refiner was operated
at specific edge-loads of 1 and 3 J/m. It is immediately
obvious that the response of freeness and breaking
length to the refining process is quite different.
In Figure 2 the actual energy
applied to the two types
of device is plotted and again it is clear just
how different the response of the pulp is to the applied
energy. Why should that be?
 |
 |
Figure 1 |
Figure 2 |
There are several
reasons why, but one major one is that PFI and Jokkro
types of devices tend to compress the fibres resulting
in lots of internal fibrillation and very little fines
generation or fibre cutting. Actual refiners are much
more prone to cut the fibres with consequent creation
of fibre debris and fines. This is illustrated in Figure
3 where the different responses of a Douglas fir fibre
to PFI and conical refining are shown.
 |
Unrefined fibre
|
PFI mill 6000 revs
|
E.Wyss 3J/m, 240 kWh/t |
Figure 3 |
|
|
As a result of the
different responses of fibre to the different refining
approaches it is perhaps not surprising that the results
from PFI and Jokkro are often misleading. A good example
of this effect was illustrated in a paper from Dr.
Mohlin of STFI. In this paper she shows how two pulps
which gave promising results on a PFI mill delivered
very different results when actually run in a paper
mill.
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Figure 4
|
Conclusions
So, PFI devices refine differently, have
extremely low intensity, do minimum fibre shortening,
and may
not identify sub-standard pulp. In short they are
very poor predictors of ultimate pulp performance.
Pilot scale refiners avoid these drawbacks. So
in 2001, Canfor explained to our customers that we
were
contemplating switching all testing from PFI to
a pilot disk refiner click here
for details. Today
all of our specifications have been revised to
reflect
the switch, and all our pulp testing and competitive
analyses are done with the lab refiner.
References
1) R.J. Kerekes, Characterizing refining
action in PFI Mills, TAPPI Journal, vol 4, No 3,
pp 9-13 (March
2005) |
