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THE REFINING OF NON-WOOD FIBRES

Author

Colin Baker

Company

IPT Ltd

Keywords

refining, non-wood, hemp, straw, bamboo, specific edge load

 

 

 

ABSTRACT

The main direction of refining research is directed toward the optimisation of long and short fibred wood species but there is an increasing demand to use non wood long fibred pulps such as flax, hemp bamboo and straw and little is known of the refining characteristics of these pulps. The most widely used of the non wood fibres i.e. cotton can give problems in modern refiners e.g. many fine paper mills can only use the shortest cut lintres because of problems with knotting. Additionally, as raw material costs have increased over the last decade the trend towards use of cheaper raw materials has escalated with a corresponding increase in the energy used to manufacture the product.

Changes in agricultural policy and the pressure of the green lobby have encouraged the papermaker to take a fresh look at his source of raw material. There is an ever growing pressure to use non woods, e.g. cereal crops (straw), stem fibres (Bagasse, flax) and materials such as elephant grass as well as the traditional cotton and rag particularly in countries without forest resource e.g. India and China in particular. To maximise the potential development of long fibred pulps, in particular those made from non wood raw materials, a special type of refining will be required. In the case of non wood materials there is much less knowledge of how to treat them for optimum performance than there is for wood fibres.

This talk describes some experiments to optimise the refining of the more common non-wood fibres and discusses potential for use in fine paper products. Optimised conditions for refining some types of non wood fibres have been found.

The conclusions are that it is possible to define an optimum set of refining conditions for non -wood fibres. For hemp and bamboo the need is for more cutting than for softwoods and hardwoods. However standard refiner fillings can cope.

When used as a replacement for both softwoods and hardwoods, calculations show that there should be few problems or loss of performance. The results demonstrate that the performance of a given fibre type at the optimum conditions of specific edge load and net energy are remarkably similar.

1. Background

In the paper making process the refining of fibrous raw materials is the first stage and the most important, as it greatly influences the final properties of the pulp. It provides the papermaker with the greatest scope for modifying the quality of his paper. In many cases the paper is made "in the refiners".

The increased demands on the performance of paper and board in terms of speed increases in its manufacture and conversion has led to a greater emphasis on machine runnability.  The greater the production rates of a paper or printing machine, the greater the effects of machine downtime through breaks.

The main direction of refining research is directed toward the optimisation of long and short fibred wood species but there is an increasing demand to use non wood long fibred pulps such as flax, hemp bamboo and straw and little is known of the refining characteristics of these pulps. The most widely used of the non wood fibres i.e. cotton can give problems in modern refiners e.g. many fine paper mills can only use the shortest cut lintres because of problems with knotting (1). Additionally, as raw material costs have increased over the last decade the trend towards use of cheaper raw materials such as chalk or clay has escalated with a corresponding increase in the energy used to manufacture the product. (2)

For example, over the last ten years most fine paper manufacturers have reduced softwood content in favour of hardwood (which can often be weaker) by 20-40% and increased filler contents by 5-10%. (2)

As a specific example, the raw materials used to manufacture copier papers have changed in the following way :

1980s

60:40 softwood:hardwood 90%

Filler content   10%

1990s

20:80 softwood:hardwood 80%

Filler content 20%


Similarly the production of fine papers e.g printings and writings have also changed as many grades have a higher level of recycled fibre and ash content.

1980s

60:40 softwood:hardwood 90%

Filler content   10%

1990s

12:48:20 softwood:hardwood:waste 80%

Filler content 20%


Over the same period the level of specific energy used to treat the pulp has increased from 100 kWh/tonne to 140 kWh/tonne to maintain the properties perceived to influence runnability. The ability to increase the filler level, thereby saving on raw material costs, is now believed to be restricted again because of perceived effects on runnability. As a result of the changes in raw material use and increased treatment of energy other processes of paper making are affected, usually for the worse.  This is illustrated in Figure 1.

Figure 1. The Runnability Cycle (4)

Figure 1

A change in agricultural policy and the pressure of the green lobby has encouraged the papermaker to take a fresh look at his source of raw material. There is an ever growing pressure to use non woods, e.g. cereal crops (straw), stem fibres (Bagasse, flax) and materials such as elephant grass as well as the traditional cotton and rag. To maximise the potential development of long fibred pulps, in particular those made from non wood raw materials, a special type of refining will be required. In the case of non wood materials there is much less knowledge of how to treat them for optimum performance than there is for wood fibres. (3)

 The refining of any fibrous raw material is a complex process.  The most important variables being shown diagrammatically in Figure 2:

Figure 2. Refining Parameters (5)

Figure 2

This talk describes some experiments to optimise the refining of the more common non-wood fibres and discusses potential for use in fine paper products.

2. Refining equipment

The system at Pira International used for the trials is described in figure 3. A Pilao Single Disc Refiner (12" disc) is included in the 10 kg rig which was used for all pulp evaluations.

 Figure 3. Research pilot refiner (12" Pilao single disc)

Figure 3

The stock container has 'plug-flow' which prevents inter-pass mixing.  The refiner can be operated at Specific Edge Loads at between 0.2 and 5.0 Ws/m. There is the possibility to measure flow, power, temperature and refiner inlet and outlet pressure.

3. Terminology

Until recently refining was more an art than a science with the process qualitatively measured by reference to time or wetness development.  Power was measured very occasionally but there was little reference to energy used. This changed with the advent of formulae which described the refining process in a quantitative way and allowed the measurement of fibre treatment in terms of power and energy.

There are several theories which describe the severity and number of impacts received by a fibre slurry.  The simplest to use is specific edge load (SEL). (6) Severity is the most easily measured by the specific edge load, while the number of impacts can be related to the specific net energy used in refining. The two equations best describing the refining process are thus: 

  Bs = Pn/(Ls.1000)  (1)

  We = Pn/m (2)

  Where:

  Bs = specific edge load (Ws/m)
  Pn = net power (kW) ie operating less backed off (no load) power
  Ls = cutting edge length (km/s) obtainable from the refining manufacturer
  We = specific net energy (kWhr/tonne)
  m =  dry fibre flow (tonne/hr)

These formulae are not exact mathematical expressions but are used to visualise and quantify the refining process and are particularly useful for comparitive operational alternatives.

A high SEL, Bs, denotes a tendency to cut whereas a low SEL gives fibrillation.  Using this theory a clearer picture emerges as to how to treat different fibres.  A tough softwood fibre will require a high SEL whereas a relatively weaker hardwood fibre will require a much lower SEL.

From equations (1) and (2) it can be seen that the definition of SEL and specific energy depends particularly on the following refiner and stock parameters :

  • refiner power (kW)
  • no load power (kW)
  • type of refiner tackle (for Ls) ie pattern, angle, bar length and width of the fillings
  • rotation speed (for Ls)
  • consistency of stock (for m)
  • volumetric flow (for m).

The number and length of bars in a set of refiner tackle will determine the ability of that tackle to cut or fibrillate in an efficient manner. Bar width, apart from affecting the number of bars which a refiner filling can have, is also thought by some to define a further refining attribute, the specific surface load (7) which divides Bs by a bar width factor.

4. Experimentation

All pulps used were dried and commercially available because there was no sufficiently large pilot plant pulping operations at Pira International (or in any other research institute. For this reason bagasse could not be included.

The pulp was slushed in a hydrapulper for 20 minutes at a stock consistency of 3.5% then pumped into the stratifying tower. The pulp was then pumped at the required flow through the refiner back to the top of the tower via the Saunders valve installed for pressure regulation.  A sample was taken corresponding to unrefined stock.  The plates were wound in to the required gross power and samples were taken at relevant times for 50, 100, 150 and 200 kWht-1.

Results presented here are for softwood (Scandinavian), hardwood (Brazilian eucalyptus), straw, hemp and bamboo. The basic data is attached as Tables 1-5, following each table is a set of figures showing, with the exception of hardwood, the percentage change in property with energy for each SEL. The reason for this method of presentation is to give a fairer comparison given that zero energy points in a pilot plant tend to be more variable.

4.1. Softwood (Table 1, Figure 4)

Refining intensities (SEL) were set to be 1.0, 1.5, 2.0 and 3.0 Wsm-1 and energy inputs 0. 50, 100, 150 and 200 net kW.h/t.

For this softwood kraft change in wetness (SR) and freeness (CSF is slower at the lower refining intensity). There is a maximum strength development for each energy level at a specific edge load of 1.0 Wsm-1. Also at a specific edge load of 1.0 Wsm-1 the percentage reduction in tear overall is slightly less than at the edge loads of 1.5, 2.0 and 3.0 Wsm-1.

For non-destructive properties, bulk decreases during refining at each specific edge load as expected.  At edge loads of 1.0 and 1.5 Wsm-1 the decrease in bulk is more rapid. The results for opacity show that, as expected, opacity decreases as the refining intensity increases. The porosity figures show that results for each SEL are very similar.

The overall optimised level of refining for this pulp occurs at a specific edge load of 1.0 Wsm-1. This results in an increase in strength development and the slowest development in drainage. The use of lower intensity refining will also lead to real savings in energy.

Table 1 (for the full-size table, please click here)

Table 1

 

Figure 4 Softwood kraft

Refining energy versus breaking length

Figure 4a

Refining energy versus tear index

Figure 4b

Refining energy versus bulk

Figure 4c

Refining energy versus opacity

Figure 4d

Refining energy versus Bendtsen porosity

4.2. Hardwood (Table 2, Figure 5)

Refining intensities (SEL) were set to be 0.2, 0.6, 1.0, and 3.0 Wsm-1 and energy inputs 0. 50, 100, 150 and 200 net kW.h/t.

For a Eucalyptus kraft change in wetness (SR) and freeness CSF is higher at the lower refining intensity. There is a maximum strength development for each energy level at a specific edge load of 0.2 Wsm-1. For Eucalyptus the tear strength increases with refining, the change in tear overall is similar for all specific edge loads, the highest tear figures are achieved at 0.2 and 0.6 Ws m-1.

For non-destructive properties, bulk decreases with refining at each specific edge load as expected but there is little difference to be seen between results. The results for opacity show that opacity decreases as the refining intensity decreases. The porosity figures show that the decrease in porosity is highest at the lowest specific edge load.

The overall optimised level of refining for this pulp occurs at a specific edge load of 0.2 Wsm-1. This results in an increase in strength development to almost the level of the softwood but a higher development of wetness (decrease in freeness).  The use of lower intensity refining will also lead to real savings in energy.

Table 2 (for the full-size table, please click here)

Figure 5 Hardwood

Figure 5a

Figure 5b

Figure 5c

Figure 5e

Figure 5f

4.3. Hemp (Table 3, Figure 6)

Refining intensities (SEL) were set to be 1.0, 2.0, 2.5 and 3.0 Wsm-1 and energy inputs 0. 50, 100, 150 and 200 net kW.h/t.

The pulp tested was a Manilla Hemp pulp. Results for drainage (SR and CSF) show that the development of drainage is slower at the lower refining intensity (1.0 Wsm-1). There is a maximum strength development at a specific edge load of 3.0 Wsm-1 at each net energy level, for tensile index/breaking length. However for burst index the greatest development is at a specific edge load of 2.5 Wsm-1.

As expected the tear index decrease with refining. At an edge load of 1.0 Wsm-1 the decrease is slightly less compared to the other edge loads.

Bulk decreased during refining at each specific edge load. The decrease in bulk is less rapid at an edge load of 3.0 Wsm-1. Results for opacity are more varied at each edge load and net energy.  At an edge load of 2.5 Wsm-1 opacity has the lowest decrease. Porosity decreases as refining intensity increases.

The optimised level of refining for Manilla Hemp pulp occurs at a specific edge load of 2.5 Wsm-1 as the increase in strength development is greater for burst index. However the development of drainage is more rapid at 2.5 Wsm-1 compared to 1.0 Wsm-1 which has the slowest development in drainage.

Table 3 (for the full-size table, please click here)

Table 3

Figure 6

Figure 6a

Figure 6b

Figure 6c

Figure 6d

Figure 6e

4.5. Bamboo (Table 4, Figure 7)

Refining intensities (SEL) were set to be 1.0, 1.5, 2.0 and 3.0 Wsm-1 and energy inputs 0. 50, 100, 150 and 200 net kW.h/t.

The pulp tested was a non wood Bamboo pulp.  Properties have been plotted as a percentage increase or decrease against net energy.

Results for 'wetness' (SR) show that at the lower refining intensity (2.0 Wsm-1) the development of drainage is slower. However, the development of 'freeness' (CSF) is slower at a refining intensity of 3.0 Wsm-1.

There is a minimum strength development at an edge load of 3.0 Wsm-1.  The greatest overall maximum strength development is at 2.5 Wsm-1 and at a net energy level of 200 kWht-1. As expected tear index decreases with refining.  The smallest decrease is at a specific edge load of 3.0 Wsm-1.  The greatest overall decrease in tear is at a specific edge load of 2.5 Wsm-1 and at a net energy level of 200 kWh/t.

Bulk decreases during refining at each specific edge load. The decrease in bulk is least rapid at a specific edge load of 2.0 Wsm-1. Results for opacity are more varied at each specific edge load and each net energy. At a specific edge load of 1.0 Wsm-1 opacity shows an increase.  However, at a specific edge load of 2.5 Wsm-1 opacity decreases at each net refining energy level. Porosity results show that as refining intensity increases the porosity decreases with a specific edge load of 3.0 Wsm-1 showing the lowest overall decrease.

The optimised level of refining for Bamboo pulp occurs at a specific edge load of 2.5 Wsm-1 as at this specific edge load there is an increase in strength development and development of drainage is not as rapid.

Table 4 (for the full-size table, please click here)

Table 4

Figure 7

Figure 7a

Figure 7b

Figure 7c

Figure 7d

Figure 7e

4.5. Straw (Table 5, Figure 8)

Refining intensities (SEL) were set to be 0.2, 0.5, 1.5 and 3.0 Wsm-1 and energy inputs 0. 50, 100, 150 and 200 net kW.h/t.

Results for wetness (SR) show that at the lower refining intensity (0.5 Wsm-1) the development of drainage is slower overall. However for freeness a specific edge load of 3.0 Wsm-1 has the lowest overall decrease.  As expected the drainage was much higher for the unrefined pulp.

In common with the eucalyptus there is a maximum strength development at a specific edge load of 0.5 Wsm-1 at each net energy level, for tensile index and breaking length. However for burst index the greatest development is at a specific edge load of 0.2 Wsm-1. Like the softwood the tear index decreases with refining. At an edge load of 3.0 Wsm-1 the decrease is less compared to the other specific edge loads.

Bulk decreases during refining at each specific edge load as expected. The decrease in bulk is less rapid at an edge load of 0.2 Wsm-1. Opacity also decreases with refining.  At a specific edge load of 3.0 Wsm-1 the decrease is less rapid. Porosity could only be measured for 50 kWht-1 at an edge load of 0.2, 0.5 and 1.5 Wsm-1 as they started at so low a level.  These low porosity results are due to the fact that the initial drainage is much higher.

The optimised level of refining the straw pulp is at a specific edge load of 0.5 Wsm-1 as the increase in strength development is greater for tensile index and breaking length. Also tear index does not decrease so rapidly.  However because of the extremely high drainage, approximately 52 SR and 250 CSF, it is probably advisable to use this pulp as part of a mixed furnish.

Table 5 (for the full-size table, please click here)

Table 5

Figure 8

Breaking length versus refining energy

Figure 8a

Burst index versus refining energy

Figure 8b

Tear index versus refining energy

Figure 8c

Bulk versus refining energy

Figure 8d

Opacity versus refining energy

Figure 8e

Bendtsen porosity versus refining energy

Figure 8f

5. End use of non wood fibres

From the figures for the individual fibre types we can now see what happens if we replace a proportion of a standard furnish with some of these non wood materials.

The most commonly used methods of refining a mixture of softwood and hardwood are mixed and separate systems. In mixed refining all components are treated equally in the same refiners. In separate refining all components are refined individually to their best advantage using optimum treatment.

Given the very different refining characteristics needed by different types of fibre it is evident that as more short fibre is used a change in refining methods will be necessary. It is also evident that refining softwood and hardwood fibres in mixture will not give the best development for either fibre.

In order to predict the effects of the refining of different mixtures linear blending theory can be used to calculate the properties of the mixtures. For a mixture of pulps A, B and C the equation will be :-

Property (ABC) = %Property A x %Property B x %Property C

However there are some areas for caution :-

  • When a furnish is refined in mixture there is a tendency for the larger softwood fibres to 'protect' the thinner hardwood fibres and receive most of the treatment.  This has been found to be true in practice where unrefined hardwood fibres have been found in a furnish at 30oSR.
  • For properties such as Bulk and porosity the properties of the mixture are closer to the lowest result of any pulp type used.
  • When attempting to calculate the properties of a furnish from its components the theoretical results differ greatly from the actual results when refining in mixture. However, if results are calculated as though the softwood component has received more energy than the hardwood component the results are much closer.

For the purposes of this paper it will be assumed that the components are treated separately at their optimum refining conditions. A comparison is given in figure 9.

Figure 9. Comparison with wood fibres

Figure 9a

Figure 9b

Figure 9c

Figure 9d

Figure 9f

A good example of a bulk grade of communications paper is Copier paper. This grade of paper relies on bonding a toner to the surface of the paper, usually by melting the toner, the molten toner then grips into the surface fibres. The surface of the paper must not be hard sized otherwise the molten toner will not wet the surface and a poor bond will result. Paper qualities required are stiffness, dimensional stability (curl) and the correct surface properties . The paper is usually so highly loaded that opacity is not a problem.

The furnish is taken for the purpose of calculation to be 70:30 softwood hardwood with each component refined at the optimum intensity to 100 net kW.h/tonne. Because of the wetness straw is not included.

 Optimum intensity for softwood is taken as  1.5 Ws.m-1

 Optimum intensity for hardwood is taken as  0.2 Ws.m-1

 Optimum intensity for hemp is taken as  2.5 Ws.m-1

 Optimum intensity for bamboo is taken as  2.5 Ws.m-1

Raw data for the above is shown in table 6. Results for the replacement of both hardwood and softwood by hemp are shown in table 7 and by bamboo in table 8.

Table 6 (for the full-size table, please click here)

Table 6

5.1. Replacement by hemp (Table 7)

When used as a hardwood replacement the end result is one of increasing tensile, burst and especially tear. Wetness and freeness show little change so drainage will not be a problem. The sheet is very slightly bulkier but there is no difference in porosity or opacity.

When used as a softwood replacement the tensile and burst are similar for all conditions. The tear is higher by a significant amount. Wetness and freeness again show little change so drainage will not be a problem. The sheet is bulkier and more porous and slightly more opaque.

In general there are no significant detrimental changes to the sheet and some benefits.

Table 7 (for the full-size table, please click here)

Table 7

5.1. Replacement by bamboo (Table 8)

When used as a hardwood replacement the end result is one of slightly lower tensile, burst and tear but not dramatically so. Wetness and freeness show little change so drainage will not be a problem. The sheet is very slightly bulkier and just higher in porosity and opacity.

When used as a softwood replacement the tensile and burst are just lower for all conditions. The tear is also lower. Wetness and freeness again show little change so drainage will not be a problem. The sheet is bulkier, more porous and more opaque porosity or opacity.

In general there are no really significant detrimental changes to the sheet and some benefits.

Table 8 (for the full-size table, please click here)

Table 8

6. Conclusions

It is possible to define an optimum set of refining conditions for non-wood fibres. For hemp and bamboo the need is for more cutting than for softwoods and hardwoods. However standard refiner fillings can cope.

When used as a replacement for both softwoods and hardwoods, calculations show that there should be few problems or loss of performance. The results demonstrate that the performance of a given fibre type at the optimum conditions of specific edge load and net energy are remarkably similar.

7. References

  • Pulp and paper manufacture vol. 3 "Secondary fibres and non-wood pulping" Chapter IX, page 112.
  • "Practical ways forward to achieving higher filler content in paper", C.F.Baker and B. Nazir, Use of Minerals in papermaking, Pira Conference, Manchester February 1997
  • "Non-wood fibres past, present and future?",Trevor Dean, Non-Wood Fibres and Crop Residues, Pira Conference, Amsterdam October 2001.
  • Refining Technology Chapter 3, page 65, Edited by C.F.Baker, Published by Pira International 2000, ISBN 1 85802 182 0
  • Paperi Ja Puu Paper and Timber, vol 72, no. 2, 1990
  • Wulsh F. and W. Flucher. Das Papier, 1958, 12(13/14), p334
  • Lumiainen J, Pulp Pap. Int. vol. 32.no 8, Aug. 1990, pp 46-47, 54

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