How to Refine our Pulps
Refining is the name given to the process in which
fibres are mechanically treated to alter their intrinsic
properties. In basic terms, paper made from fibre
that has not been refined will be poorly bonded, low
in strength and porous. Once fibres have been refined
they will produce paper that is better bonded, has higher
tensile strength and is less porous. In this discussion
we are considering low-consistency refining as is routinely
practiced on papermachines.
Typically refining affects fibres in the following
manner:
- Fibres are cut
- External fibrillation takes place as
the primary
wall is damaged
- Internal de-lamination and swelling occurs
- Micro-compressions occur along the fibre length
- Fines are generated
- Fibre shape is changed - depending on conditions,
it
may curl or it may straighten
- Colloidal material is removed
Most users of our pulps are trying to maximise the
reinforcing contribution of the fibres, and this means
they are trying to develop the best tensile strength
without shortening the fibres or slowing down the paper-machine
drainage. By careful selection of reinforcing fibres
these papermakers have been able to optimise other aspects
of their machine furnishes, such as formation, opacity,
recycled content and of course cost.
Because the natural attributes of our fibres (thin
walls, highly collapsible) favour the rapid development
of tensile strength, the fibres now enjoy a world-wide
reputation for their reinforcing abilities. However,
the type of refining used to develop the reinforcing
strength can make a significant difference too, and
to understand this we need to consider the impact of
refining intensity.
Intensity of Refining
The traditional way of characterising the process of
refining was through net Specific Refining Energy (SRE),
calculated as the useful energy imparted to the stock
after allowing for the no load energy needed to run
the refiner. This is still commonly used today, the
units are kWh/tonne or kWs/kg or hpdays/tonne.
[ 1 kWh/tonne = 3.6GWs/kg =
17.9 hpday/ton ]
Several theories of refining have been developed over
the years in an effort to better understand the process
by which energy is applied to the actual fibre and to
better control the process. Some of these theories focus
on the work being done by the refiner itself, while
others consider what happens at the level of the individual
fibres. An example of the former approach is the Specific
Edge Load (SEL). This idea of refining intensity was
introduced by Wultsch & Flucher (1)and
developed further by Brecht and Siewert(2).
The idea is to apply the SRE term (calculated as above)
to the number of bar crossings within the refiner. The
result is expressed in Ws/m or J/m. A refinement of
the SEL approach, Specific Surface Load (SSL), was suggested
by Lumiainen in 1990(3). This approach considers
not just the length of the bar edges, but also the area
of the bars, its units are Ws/m2 or J/m2.
| Characterising
Refining |
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In this diagram the areas EA
and EB represent the specific energy
imparted in the refiner. They represent the product
of the number of impacts and the intensity per
impact, the two areas are identical.
When the intensity per impact is high and
the number of impacts low, it is common to refer
to the refining operation as being "high
intensity". In the case of the opposite situation,
it is referred to as "low intensity".
For tensile strength development in fine softwood
kraft fibres like those from Canfor, low intensity
refining gives the best results. |
For more information
on refining theories, the interested reader is encouraged
to contact their Temap representatives, or email us
at this site. For a very interesting and relevant discussion
of the effect of different refining intensities on reinforcing
pulps, click on the link below.
References cited:
1 - Wultsch, F., Flucher, W., Das Papier 12(13):334
(1958)
2 - Brecht, W., Siewert,W.H., Das Papier 20(1):4 (1958)
3 - Lumiainen, J., "A new approach to the critical
factors effecting on refining intensity and refining
results in low consistency refining", TAPPI 1990
Papermakers Conference Proceedings, TAPPI Press, Atlanta
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