Abstract
Pressure screening, nowadays the most widely used method for cleaning pulp, has been traditionally investigated as a debris
removal process. The aim of this thesis, however, was to study it with a view to the fractionation of pulps, examining
systematically and extensively the effects of screening parameters on fractionation under actual working conditions in order
to provide an insight into its possibilities and limitations as a fractionation method.
The experimental work was performed with a full-scale two-stage pressure screen connected to an industrial TMP process.
Fractionation of the pulp was analysed in terms of consistency, freeness, optical fibre length distribution, coarseness index
and Bauer-McNett fractions. Two sampling systems were used during the screening experiments, manual and semiautomatic. The
latter was assessed to be more reliable, as reflected in lower stochastic variation and the absence of a systematic bias in
the mass balance errors over the screen. The poorer reliability of the manual sampling system was offset by the large number
of screening tests, however.
The results of the screening experiments showed that with a given design of the screen plate, the separation of each
fraction
was dependent almost exclusively on the mass and volumetric reject rates. The mass flow of fines, defined as the Bauer-McNett
P200 fraction, was dependent mostly on the volumetric reject rate, while the mass flow of fibrous fractions (R200, R50, R30,
R16) depended mostly on the mass reject rate. The mass reject rate obtained in pressure screening was a result of the choice
of operating parameters, but fractionation efficiency could not be affected by using different combinations of these
parameters (feed consistency, rotor tip speed and slot velocity) if the mass and volumetric reject rates were kept constant.
The slot width together with the contouring of the screen plate affected the fractionation efficiency as compared with the
situation at constant mass and volumetric reject rate. Increased fractionation was obtained by reducing the slot width and
contouring. The pulp passage ratio, which combines the mass and volumetric reject rates into one parameter, was found to be a
expedient way of expressing the fractionation of pulp, as it was possible to present fractionation uniformly as a function of
this ratio. The change in freeness was found to correlate quite well with that in Bauer-McNett fractions, and it was a good
indicator of fractionation efficiency in screening. Apart from fractionation according to length (or Bauer-McNett fractions),
the slotted pressure screen was also found to classify the fibres according to their coarseness. The coarseness difference
was partially dependent on the fibre length, but additionally the coarseness in the accept pulp for any given fibre length
class was always lower than that in the reject pulp. The difference obtained seemed to depend on the passage ratio of the
pulp.
This thesis provides new information for the modelling of pulp quality and the design of fractionation experiments,
fractionation processes and screen room control strategies.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn951-42-5919-X |
| Date | 01 March 2001 |
| Creators | Ämmälä, A. (Ari) |
| Publisher | University of Oulu |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2001 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/0355-3213, info:eu-repo/semantics/altIdentifier/eissn/1796-2226 |
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