This paper focuses on the importance of having good washing in fiberlines for chemical pulp. A new generation of
wash presses is presented, the Compact Press. The main features of the new wash press are described and its washing principles are compared with other types of washers. Based on mill results it is concluded that the
Total washing efficiency of the new wash press is above 95 %.
It is well-known that the global pulping industry is facing greater pressure to continuously improve its operation.
The reasons may vary but market forces, financial objectives and environmental standards are some often referred to. A positive outcome of this situation is that new and better technologies are fostered. One example
is the recent development of a new wash press, the Compact Press. In this paper the principles used in the new wash press will be discussed. The paper will also deal with the basics of chemical pulp washing and why
efficient washing is an important objective in the pulp manufacturing process.
TRENDS IN THE PULPING INDUSTRY
Cost efficiency is the alpha and omega of all production. Cost efficiency to a large extent relates to efficient
washing. Generally, the need for efficient washing is highest closest to the cooking plant, where the COD content is highest. A low COD content for the pulp entering oxygen delignification is important for this
process. However, also directly after oxygen delignification special attention should be paid to efficient washing, which here acts as a tool for reducing the environmental impact of the bleaching as long as the
bleach plants remain at least semi-open. Needless to say, in the bleaching itself the more dissolved organic substance that is present at a given position, the greater the need for efficient washing. In the
brownstock washing as well as in the post-oxygen washing there is generally a need for two subsequent washing machines.
Looking into the future it is not hard to foresee that a steadily increasing system
closure will be the path. The driving forces behind this development is of course sometimes given by imposed regulation on a limited water through-flow1, but there are also plenty of incentives why to
decrease the water through-flow also from purely economical points of view. In fact, steadily decreased water through-flow in the bleaching is a necessary pre-condition for the growing size of new mills. For purely
practical reasons, e.g. to avoid scaling inside the washer, the water through-flow in a bleach plant is governed by the washing machines used.
1So far the amount of water flowing through the pulp mill and being used for various purposes in the mill before being let out
to the recipient has often been referred to as "water consumption". However, this is a confusing term since it implies that the water is actually consumed in the mill. Better is to use the term water need or water
WASHING TECHNOLOGY AVAILABLE
Several different types of washing machines exist today, including drum washers, atmospheric diffusers,
drum-displacement washers, wire presses, dewatering presses, pressure diffusers and wash presses, each of these operating according to a unique set of principles but also demanding a unique set of preconditions in
terms of water through-flow, power consumption, feeding consistency etc.
Perhaps the most obvious difference between a wash press and other washing machines is
the high consistency of the outgoing pulp, reaching up to 32–35 % for a press, in contrast to some 10–14 % for other washers. This means that at a given dilution factor, e.g. 2.0 m³/ADt, the wash water needed in the press is about 3.9 m³/ADt, while it is 8.6 m³/ADt for a drum washer with 12 % outgoing consistency. Interestingly, in the case of an open stage, these figures also refer to the effluent volume from each stage, assuming internal filtrate for dilution before the press indicating one important benefit of a wash press bleach plant. Steffes has concluded that the effluent volume from a bleach plant operating with wash presses with 30 % outgoing consistency is less than half that of a drum washer-based wash plant , whereas Stål has reported a range of between 50 and 75 % . This argumentation is of course only valid for washing between stages, where a change in process conditions inside the machine should be avoided, e.g. between D0 and E1 and between E1 and D1,
whereas the argumentation is irrelevant for counter-current washing e.g. post-oxygen washing or for the washing between D1 and D2.
Harper and Grengg have identified a number of important issues to be met by a new
generation of washing machines , emphasising on:
- improved washing efficiency
- readiness to handle temperatures above 90 °C
- insensitivity towards scaling
- high specific through-put, i.e. a compact machine design
These factors were all in focus when the new wash press was developed.
Figure 1. Wash Press installed at the Billerud Gruvön mill, Sweden
WASH PRESS DEWATERING PRESS
A very important issue in discussions on presses is the distinction between a press with and without washing. Both kinds are today available on the market, but both the installation cost
and the operational costs for e.g. bleaching chemicals, resulting from their unlike washing results, differ. Expressed in a more theoretical way, the difference between the two
categories is between a pure dilution/extraction washing and a displacement washing , where the latter apart from pressing the pulp also adds wash water inside the press. In
Figure 2 a schematic drawing showing the difference between the two categories of presses is given.
Figure 2. Principal difference between a wash press and a dewatering press.
Adoption of a washing philosophy with minimal water through-flow will also reduce the costs for equipment for secondary treatment of BOD, COD and AOX in effluents from the bleaching.
It is today practically feasible to build a kraft mill with a water through-flow of 5–15 m³/ADt. Among others, Stål has concluded the obvious, that the lower the carry-over into the bleaching and in particular to a D0-stage the lower the AOX discharge to the effluent from
the bleaching since the D0-stage is the one mainly responsible for the AOX formation. Even with extensive use of chlorine dioxide a modern ECF bleach line can have an AOX discharge
of 0.2–0.5 kg/ADt, which is much below the values discussed in the days of chlorine bleaching. The recently developed DualD-stage (D*), performed at high temperature and
long time has proven to be one very efficient mean in decreasing the AOX discharge from an ECF bleaching line, by as much as 50 %.
The importance of efficient washing seems hard to overestimate. The question is rather how
to compare the washing efficiency of different washers in a proper way. It has been concluded that the displacement ratio, DR-value, is a misleading concept to use when
comparing washers with different discharge consistency . In this respect the Total washing efficiency or the E10-value are much better measures. However, when comparing two
washers with the same discharge consistency, the DR-value is still a powerful and reliable tool, e.g. in the comparison of different wash presses or of wash press and dewatering press
. For example, the maximum theoretical DR-value for two different wash presses, in which the washing is carried out at 10 and 15 % pulp consistency respectively, could be compared.
Assuming a dilution factor of 2.0 m³/ADt, Figure 3 shows the different flows entering and leaving the wash press, here divided into its three subsequent unit operations, dewatering, displacement and pressing.
Figure 3. Theoretical representation of a wash press and its internal flows.
L is the liquor flow in m³/ADt, where L2 for pulp washed at 15 % consistency e.g. can be calculated to
The maximum theoretical DR-value is now given by
i.e. DRmax for a wash press adding the wash water at 10 % pulp consistency is 0.48, whereas it is as high as 0.76 for a wash press adding the wash water at 15 % pulp consistency. As long as an actual displacement is taking place in the press, the DR-value will
be relatively close to the calculated maximum. As a consequence, very low or even negative DR-values reported for certain wash presses in fact tell that actual displacement does not
take place. In practice, the Total washing efficiency as well as the DR-value also depend on the amount of COD in the ingoing pulp, so that a higher amount will give a higher Total washing efficiency.
In our new wash press the incoming pulp is fed into the two distribution screws, one for
each drum. The distribution screws distribute the pulp evenly along the whole working width of the machine. The good distribution of the pulp suspension by the distribution screws at
the inlet lays the ground for excellent washing result.
Figure 4. The patented distribution screw is a key component.
In the decreasing space between the drum and the flap, into which the pulp is forced by drum rotation, the pulp is compressed and dewatered to approximately 15 % consistency.
This pulp at relatively high consistency is washed in the zone between the vat and the drum. Wash liquor is added to the pulp through nozzles in two rows per drum and filtrate is
pressed out through the perforated drum. Separate liquors can be used for each row. The nozzles are placed to ensure a good distribution of wash liquor. The result is a very sharp
and even displacement of the wash liquid through the pulp bed. Decreasing distance between drum and vat causes further thickening all the way to the press nip, where the two
pulp webs meet. Finally, the press nip will squeeze out the remaining old liquor from inside the lumen and a final consistency of above 30 % is reached.
Doctor blades remove the pulp from the drums to a shredding and conveying screw, which
transports it axially to the dilution screw where the pulp is diluted with liquor to the desired consistency. In the free drum area between the shredder screw and the distribution screws,
spray pipes clean the drum holes by utilising internal filtrate pumped at high pressure from the filtrate tank. All filtrate, which is displaced and dewatered from the pulp web, flows
inside drum channels and is collected in chambers at the end of the drums. From these chambers the filtrate flows down to the filtrate tank.
Figure 5. Working principle.
Both distribution screws as well as drums have variable speed in order to give optimal washing at varying production levels.
Thanks to the three washing principles, dewatering, displacement and pressing used in the
new press, the washing efficiency is very high.
As the name implies, the new wash press has a compact design and requires very little
space. This is possible due to the excellent performance of the press, relating to the washing being carried out at a high pulp consistency (15 %), which has been made possible
through the patented distribution screw  and the use of a larger part of the circle segment of the drums. Being a completely new machine, the wash press is also designed for
contemporary demands brought forward from occupational health and safety people upon maintenance issues.
Altogether there are now eight Compact Presses in operation worldwide. The first installation
was at the Billerud Gruvön mill in Sweden and started up in May 2000 (Figure 5). The fiberline makes SW kraft pulp.
Figure 6. Flowsheet of the installation at Billerud Gruvön mill in Sweden.
The wash press in Gruvön (Figure 6) has operated with an ingoing consistency of 2.5–5.5 % at a production range between 400–1200 ADt (without reaching the upper capacity limit).
With a DF of up to 2.5 m³/ADt a typical value for the outgoing COD has been 3–4 kg/ADt, corresponding to a Total washing efficiency of 95 %. The outgoing consistency has been above 32 %.
In Figure 7 the flowsheet for the installation at Smurfit Munksjö Aspa mill in Sweden is shown
. The mill makes SW kraft pulp.
Figure 7. Flowsheet of the installation at Smurfit Munksjö Aspa mill in Sweden.
The wash press in Aspa has operated with an ingoing consistency of 2.5–5.0 % at a production range between 300–750 ADt. With a DF of up to 3.0 m³/ADt (half the wash liquor
volume is here made up of alkaline bleach plant filtrate), a typical value for the outgoing COD has also been 3–4 kg/ADt. The outgoing consistency has been above 32 %. In Figure 8
results from the operation in Aspa are shown.
Figure 8. The DR-value plotted against the dilution factor for mill operation of a Compact Press in the Smurfit Munksjö Aspa mill in Sweden.
As can be seen, the Total washing efficiency is in the range 0.96–0.98, corresponding to E10-values from 5.6–7.2.
In Figure 9 the flowsheet for the installation of two wash presses at Daishowa Suzukawa mill
in Japan is shown. The mill makes HW (mixed Eucalyptus) kraft pulp.
Figure 9. Installation at Daishowa Suzukawa mill in Japan.
At the Daishowa Suzukawa mill in Japan a DR-value of 0.69 has been measured at a DF of 2.2 m³/ADt.
To summarise, the results from the installations herein presented show a Total washing
efficiency ranging from 95 to 98 % for the Compact Press, corresponding to a DR-value of 0.65 at a DF of 2.0 m³/ADt.
As described in Figure 10 the wash presses can be installed in many applications of a fiberline.
Figure 10. Wash press applications.
Particularly interesting is the use of the new wash press in bleach plants. The high outlet consistency means that the wash press serves as a liquid barrier between stages. This
function is important in the bleaching where normally pH varies from acidic to alkaline and different temperatures apply to different stages. The wash press thus enables significant
savings in alkali and acid for pH adjustment and savings of steam. A bleaching line with wash presses also facilitates reduction of effluent volumes since the risk of scaling inside the washer is reduced.
Yet one interesting example of the wash press application is in short bleaching sequence
concepts. Today, full brightness can easily be reached in only 2 stages for HW kraft pulp as shown in Figure 11.
Figure 11. A 2-stage and 2-reactor concept for manufacture of fully bleached HW kraft pulp.
The performance of the washing equipment utilised in a chemical pulp mill has a direct impact
on pulp manufacturers' operation costs, quality of produced product and environmental impact. Due to these reasons pulp manufacturers worldwide will continue to focus on
achieving good performance of their washing systems. The press features open new possibilities achieving these objectives. Mill operations of the Compact Press have shown a typical DR-value of 0.65 at DF 2.0 m3/ADMT and outgoing consistency above 32 %, resulting
in Total washing efficiencies of above 95 % on both softwood and hardwood operation. The high DR-value for the new wash press compared to other wash presses is a result of the fact
that washing is carried out at a higher pulp consistency (15 %), which has been made possible through the patented distribution screw, optimised vat configuration and the use of
a larger part of the circle segment of the drums. Consequently, the new wash press requires very little space.
Bror Ek and Axel Lämås are acknowledged for valuable criticism of the manuscript.
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Bleach Effluent Reduction", Pap. Celul., 54(3): 65–69 (1999).
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3. Harper, S. and Grengg, M., "Resin Extraction and Effects on Pulp Quality", proceedings
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4. Ragnar, M., "COMPACT BLEACHING – A Concept for Fully Bleached HW Kraft Pulp in only 2
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Kraft Pulp a Reality", proceedings from the 2002 International Pulp Bleaching Conference (IPBC), Portland, USA, Vol. Papers: 237–244.
6. Ragnar, M. and Törngren, A., "Ways to Reduce the Amount of Organically Bound Chlorine
in Bleached Pulp and the AOX Discharges from ECF Bleaching", Submitted to Nord. Pulp Pap. Res. J.,
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Treatment of a Fibrous Suspension", USA patent nr. US6306259 (2001 (publ.))
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