Features Research & Innovation
Is There Untapped Potential in Pulp Refining?

Next to the selection of species and the choice of pulping method, pulp refining offers the greatest opportunity for papermakers to enhance the raw material of their product.

Surprisingly, this opportunity is often neglected in modern papermaking. In contrast to earlier years when “paper was made in the beater room”, the process of pulp refining is often given little attention these days and sometimes none at all. This is a lost opportunity for adding value to paper products. In today’s economic climate, no such opportunity can be overlooked.

Like the process itself, research in pulp refining has ebbed and flowed over the years. Currently, the research level is less widespread than in former years, but much good work is underway in various parts of the world.

To draw this research community together, a seminar about progress in pulp refining research was held this past September at the University of British Columbia in Vancouver. The event was organized by the author, in collaboration with Hannu Paulapuro of Helsinki University of Technology, Finland. Eighteen speakers gave presentations to an audience of 65 people. These presentations, in turn, elicited much lively discussion throughout the day and during the evening dinner.

Morning sessions

The morning was largely devoted to the effect of refining on pulp and paper properties. Raj Seth, formerly of Paprican, led with a presentation showing that, in most cases, internal fibrillation and fibre straightening were the main targets of refining. External fibrillation was of doubtful value for many applications, as was fines generation. However, cases where it can be of value were suggested from the audience: filler retention and bulk preservation.

Next, Anders Moberg of Stora Enso Research, Sweden, reported on recent findings from the Wood Ultrastructure Research Centre in that country. He described significant differences in beating result between high yield and low yield commercial never-dried kraft pulps. The high yield pulp consumed more energy, was more sensitive to refining conditions and had more external fibrillation than the low yield pulp. Both pulps responded to refiner fillings less than expected from earlier experiences with commercial, dried, fully-bleached pulps.

The next two papers gave insights on deformation of individual fibres. Ho Fan Jang of Paprican described his pioneering work on measuring fibre collapse using confocal microscopy, and describing this collapse by an equation having a dimensionless parameter consisting of fibre dimensions and modulus.

In other single fibre work, Peter Wild of the University of Victoria described a novel apparatus to impose cyclic compression on individual fibres. He and his colleagues developed parameters to describe compressive strain curves encompassing lumen collapse and cell wall compression.

Beating softwood reinforcement pulps for LWC grades was next described by Eero Hiltunen of KCL, Finland. He showed that refining did not improve the flaw-resisting ability of paper as measured by in-plane fracture energy, but did improve tensile strength and elastic modulus. The findings were attributed to better inter-fibre bonding as well as fibre segment activation resulting from fibre straightening. He also showed that the fracture properties of papers consisting of mixtures of mechanical and chemical wood pulps could not be estimated from the components alone.

Mike McCaw of Weyerhaeuser, presented some insightful thoughts on the possible role of chemical gradients in refining. He suggested that in the case of unbleached pulps, a role of refining may be to abrade off re-deposited lignin from pulping and thereby improve bonding by this mechanism, as well as by the other well known surface structural changes.

Next, Kari Koskenhely of Helsinki University of Technology in Finland described recent work on novel bar designs for refiner plates. The aim is to achieve controlled compression and shear by contouring the bar surfaces to give a converging gap. They showed that pulp properties differing from those created by conventional fillings could be attained at ultra-low powers and refining energies.

The next two papers dealt with refiner operation. Ulla-Britt Mohlin of STFI, Sweden, described some recent results from industrial refiners operated in a single stage mode as opposed to multiple-pass re-circulation common in laboratory refiners. She focused on bonding (WRV), fibre length, and fibre curl changes as key targets of refining, and showed gap size along with power were important operating variables. Smaller gaps gave more efficient refining, but below a critical gap size, fibre shortening increased significantly.

In the last paper of the morning, Bill Sampson of UMIST, United Kingdom, discussed refining strategies for recycled furnishes. He showed that by first fractionating the pulp and separately pre-refining the short fraction (as opposed to the long fraction common in current practice), subsequent co-refining of the mixture of long and short fibres gave superior tensile strength without density loss at the same energy as current practice. He postulated that swelling after pre-refining enabled short fibres to take up more load in the mixed refining.

Afternoon sessions

The afternoon sessions were largely devoted to the “process” of refining. This author led off with an overview of forces and energy in refining. It was pointed out that the current characterizations of refining action are based on energy, but bond-breaking in fibres occurs as a result of strain imposed by forces, and that energy is only a consequence of bar forces applied over distance. By a simple example, it was showed that as little 1% of the bar energy may end up as internal strain energy in fibres, and that external fibrillation was the only refining effect for which bar movement was theoretically necessary. Considering these factors, and the important role of heterogeneity, it was suggested that some re-thinking of the refining process was needed.

Jocco Dekker of the Agrotechnical Research Institute, Netherlands, provided some fresh thinking on the subject of refining. He described a sequence of experiments, which led to the conclusion that internal fibrillation with minimal fine creation should be the target of refining, and that this is best achieved by compression rather than shear action. A new refiner based on this principle is under development, but details could not be revealed at this time.

Peter Antensteiner of Andritz, described a new approach based on combining the major refining variables into four dimensionless numbers to describe two main factors of importance: level of compression and number of loading cycles. This approach was used to predict property changes caused by refining, and has been packaged in a program called MAGNUS.

Jean-Claude Roux of Ecole Francaise Papeterie et des Industries Graphiques, France, reviewed the extensive work carried out over the years at EFPG on characterizing refining action. A significant part of the work has been devoted to the formidable problem of accurately describing bar crossing patterns in refiners and thereby refining intensity. One outcome of this work is the Reference Specific Edge Load. Other work in this laboratory has focused on lubrication and hydro-mechanical theories to described the energy transfer to fibres.

Georges Joris of Matech Europe, France, described an extension of his objective diagram theory in an expert system called “Checkdiag”. He demonstrated the use of this comprehensive expert system for predicting property changes in pulps as well operational constraints in refiners.

The last four papers dealt with improving understanding of refining action. James Olson of UBC described the application of comminution theory in low consistency refiners to determine a ‘selection’ function which represented probability of fibre shortening during a bar crossing. He showed that it was related to refining intensity and fibre length, and correlated with tensile strength of paper, which passed through a maximum at a given value o
f the selection function. This work is part of a larger project at UBC aimed at interpreting changes in fibre length distributions to measure homogeneity of treatment in refiners.

Next, Tom Lundin of bo Academy, Finland described recent work on refiner loadability (power vs. bar gap clearance) in their laboratory refiner. Tests were conducted to determine bar clearance at varying rotational speeds and pulp consistencies. Loadability was found to decrease at lower consistencies, with significant differences occurring between 1% and 2%.

In other work in collaboration with bo Academy, Warren Batchelor of Monash University, Australia, addressed the question of how fractional coverage of bars by fibres influences refining intensity. He proposed a bar coverage factor dependent on consistency to give an “effective” Specific Edge Load to reflect the true load on pulp. As an example, he estimated that a nominal SEL=3 J/m at 3% consistency would produce the same refining effect as a nominal SEL=5 J/m at 5% consistency.

In the last presentation of the seminar, John Senger of Paprican, described recent measurements of forces on bars in refiners using a new sensor embedded in a stator bar of a lab refiner at UBC. Normal and shear force distributions over the bar surface at high consistency (20%) were measured. made. The distributions were shown to have a very sharp peaks at the bar leading edge, decreasing in magnitude over the length of the bar, similar in shape to those measured by Gonchorov in 1971. This new technique will soon be tested in low consistency pulp refining in a Conflow JC-00 refiner in Paprican’s Vancouver Laboratory.

Conclusion

Professor Hannu Paulapuro closed the seminar with a few remarks on the good turnout and high level of interest. He pointed out that the last research-oriented meeting on this topic was in 1980 at the Institute of Paper Chemistry and said that clearly more were needed. Another seminar will be held in the not too far future at the Helsinki University of Technology.

All in all, this seminar showed that interest in pulp refining exists and appears to be growing. Good research is underway in a number of laboratories around the world, but it is clear that many questions remain on both the effect of refining and the process itself.

On the effect of refining, internal fibrillation without length loss and fines creation is often, though not always, the major target in refining, along with fibre straightening. How much of the straightening results from swelling due to internal fibrillation, and how much comes from the mechanical forces in refining, remains in question. In applications where external fibrillation is not desired, would some form of compression refining be better that the current compression/shear attained by bar crossings?

On the process of refining, there is still a need to characterize refining action by forces and strains in fibres rather than energy during bar crossings. There also remains the question of heterogeneity of the process. It is well established that many fibres see no treatment at all in industrial refiners, which raises an even more fundamental question: Is the cyclicality of the process needed for fatigue weakening if individual fibres as is commonly thought, or is it merely to expose many fibres to some loading? Does the entire process need re-thinking?

Clearly, many questions remain, as is always the case in research. Nevertheless, this seminar showed that some good progress has been made in understanding and fresh thinking in refining and that new concepts are under exploration.

Dr. Richard J. Kerekes is the Paprican professor of Pulp & Paper Engineering and the Director of the UBC Pulp & Paper Centre.