Mechanical pulpers meet Deep in the Heart of Texas
August 1, 1999 By Pulp & Paper Canada
Having been subjected to countless westerns as well as reruns of Dallas, one has the impression of Texas as a sprawling range dotted with cattle ranches and oil fields. So why was the International Me…
Having been subjected to countless westerns as well as reruns of Dallas, one has the impression of Texas as a sprawling range dotted with cattle ranches and oil fields. So why was the International Mechanical Pulping Conference (co-sponsored by PAPTAC and TAPPI) held in Houston from May 24 to 26? Stereotypes die hard, but Texas also has large forested areas and is home to Donohue’s newsprint mills in Sheldon and Lufkin. Houston is also a bustling metropolis and major airport. Many attendees pleasantly found out that there were direct flights to Houston from most major centres in North America and Europe. Social events were held at the Museum of Natural History and Space Centre Houston with customary western hospitality.
Mechanical pulp is characterized by high opacity, which makes it suited to lightweight grades, but its maximum brightness is lower than that for chemical pulps. However, even if the brightness ceiling could be circumvented, mechanical pulps yellow when exposed to light. Until now that is. John Schmidt, Paprican, explained A Breakthrough in yellowing inhibition of mechanical pulp, a joint development with Ciba to produce a chemical system that inhibits yellowing. Unprecedented yellowing inhibition is achieved with a UV absorber combined with a radical scavenger consisting of a nitroxide radical. Initial experiments in the laboratory were followed by pilot coater trials. The response to ultra-violet light was equivalent to fully bleached kraft papers. Trials on commercial machines are imminent.
Tina Hallamaa, Finnish Pulp and Paper Research Institute (KCL), studied the effect of fibre properties on the structure of lightweight-coated (LWC) sheets. The experiments involved preparing base stock on a pilot paper machine from mixtures of kraft and mechanical pulp. The mechanical pulp component consisted of either groundwood or TMP rejects refined to different levels. The average fibre length of each mechanical pulp was kept constant by discarding either fines or shives. The paper was later coated and printed in web form. More refining of the long fibre, which reduced the wall thickness and increased fibre flexibility, lead to denser and smoother paper sheets. However, more refining of the long fibre fraction did not reduce fibre rising induced by wetting the sheet.
Split ends may not be good for your hair but apparently they are good for wood fibres. A number of excellent papers characterized the properties of fibres prepared under different conditions of grinding or refining. Fibre splitting appears to enhance surface properties and strength. There were questions on the details of the measurements, degree of splitting (did the whole fibre split or just part of it?), how to achieve fibre splitting and measurement statistics.
Development of fibres in pressure groundwood was described by Kathrin Morseburg, bo Akademi University. Grinding increases the width and thickness of the fibre by delamination and splitting of the fibre wall. Juvenile wood has the advantage of lower fressness with 20% less energy, lower shives and higher bulk at equal tear. In paper, this pulp gives a smoother surface and higher light scattering coefficient.
Is it possible to tailor raw material selection with defibrating action for particular paper grades? Annikki Heikkurinen, KCL, addressed this question by studying fibre properties as a function of defibration conditions. In a series of experiments, wood with different diameters and growth rates was processed as TMP and PGW (pressure groundwood). The different wood samples required the same energy to reach a given freeness. Log diameter had no effect on wood density but fibre length and coarseness increased with increasing log diameter. The fibre length of the raw material was reflected in the fibre length of both the TMP and PSW. The longest and coarsest fibres from the slow growth were damaged the most by refining. With low-intensity refining, the fibre wall thickness of different material reached similar levels. Bonding and fibre length were generally higher with lower refining intensities.
All fines are not created equal said Kari Luukko, Helsinki Institute of Technology. Building on the classic work of Brecht and Klemm, he examined the effect of two different classes of fines, designated flakes and fibrillar fines, on papermaking potential. Fines were examined by image analysis as they passed through a laboratory flow loop. The ratio of fibrillar fines to flakes is an important parameter because fibrillar fines contribute primarily to strength properties whereas the flakes increase light scattering. Perhaps this technique could be developed into an on-line tool. The discussion centred on whether it would be possible to control this ratio. The flake-like fines are generated by disintegration of the primary wall, which has a finite volume whereas the fibrillar fines come from the secondary wall. Obviously there is room for further research.
Modern equipment helps
Ray cells are pesky, brick-like structures that comprise the major component of lint that collects on the blankets of offset printing presses. Accumulation of lint impairs printing performance and leads to costly stoppages of the printing presses to wash the blankets. Processing by conventional equipment has been ineffective in isolating, concentrating or breaking down ray cells. John Wood described experiments at Paprican in which ray cells were concentrated using a hydrocyclone with a small diameter tip. The concentrated ray cells were then subjected to mechanical and chemical treatment. Even in concentrated form, the ray cells could not be modified by refining, grinding or ball milling. However, alkaline peroxide treatment of ray cells reduced the IGT lint index by 35% and increased the burst by 10%. The finding is probably the reason for the lower linting tendency of peroxide bleached papers. It also suggests that a mild chemical treatment of the fines in the whitewater might be effective in reducing linting. Questions included why isolate the ray cells only to return them to the rest of the furnish after treatment. The ray cells represent 5 to 7% of the pulp by volume and purging this amount of material would be expensive. Although ray cells are transparent to conventional refiners, applying more energy to develop long fibres will hold ray cells more firmly in the sheet and reduce the linting tendency. Behind the closed doors of the conference room, the mechanical pulpers could also admit that a good solution to linting problems was a modern paper machine.
Many years ago in the movie, The Incredible Voyage, a submarine including the crew was miniaturized in order to explore the human body via the blood vessels. There seems to be a similar need to see inside a refiner to understand its internal workings. Although present technology doesn’t follow the movie script, it does draw on sophisticated techniques involving radioactive tracers and sapphire windows with digital imaging systems. Esko Harkonen, UPM-Kymmene, described residence time measurements on commercial refiners running under normal production conditions. Holes were drilled into the stationary plates of the primary, secondary and reject refiners at several radial positions. A radioactive tracer was introduced either by treating chips or injected directly into one of the holes. Samples were withdrawn from the other holes in the plate to monitor pulp flow through the refiner. Total residence time increases with increasing power. The measured velocity was independent of power level and production rate. There is an intensive mixing zone at the entrance to the plate and the back flow follows the steam. The residence time is from 2.5 to 7 seconds through the breaker bars and plate centre. At the periphery of the plate, were most of the fibre is developed, the residence time is only about 0.5 seconds.
Inspired by the pioneering work of Stationwala and Atack of Paprican, Taito Alahautala, Tampere University of Technology, looked inside a refiner through a sapphire window. A stroboscope and a CCD camera were used to measure p
ulp velocity, pulp orientation, area covered with pulp and the presence of flocs. Multi-pulse stroboscopy with xenon lamps was used to follow the motion of flocs and calculate their velocity. In the crush zone the backflow was surprisingly small at approximately 1 cm/s. In the intermediate zone, the backflow was 1 m/s and turbulent. A speed of 30 m/s was recorded in the refining zone. Based on these measurements, should we be satisfied with current refiner plate design? The consensus seems to be that plate designs are needed to give more uniform velocities in the refining zone.
Bill Strand described a model predictive control strategy that has been applied to the mainline refiners at the Ponderay and the Bowater mills in Usk, WA, and Thunder Bay, ON. This approach uses process models to predict future plant behavior to control the process to freeness and fibre length targets. They are working on similar control strategies for the rejects refiner, screen room, and low consistency, post refiner. The challenge in implementing model predictive control is to acquire sufficient process knowledge. Strand said that bump testing would take so long that you would be kicked out of the operators’ control room long before it could be completed. Information to tune the model is obtained from historical data, mass and energy balances combined with some bump testing. In a refiner plant, many important quality factors and process variables are either difficult to measure or cannot be measured quickly enough for a control action. This problem is circumvented by using software sensors. Although this sounds like the creation of a virtual TMP plant, the future predictions of the model are later calibrated to the slower measurements, which assures that it is firmly grounded in reality. The Ponderay mill was able to reduce kraft consumption in its lightweight grade from 10% to 4% and in its newsprint grade from 8% to essentially zero over the past three years while increasing machine speed from 4000 to 4300 ft/min. This was a consequence of reduced variability and more long fibre in the mechanical pulp. Approximately half of these improvements were attributed to the control system. In response to a question, Strand conceded that pulp quality variability could also be reduced by improving wood supply uniformity.
In a conventional pocket grinder the logs are loaded into a pocket where they are compressed by a hydraulic cylinder against the rotating grindstone. This discontinuous cycle limits the production rate because of idle time to reload the pockets. As the hydraulic cylinder reaches the end of its stroke, the production rate increases because the logs are hotter and more conformable, allowing them to deform and fill the intervening spaces. Pulp quality is also more uniform near the end of the stroke. Pekka Haikkala, Valmet, described a New grinder concept for continuous pressure grinding that was conceived to maintain the log packing density at a constant level to maximize production rate and minimize quality variations. The new grinder has two interleaving loading shoes that exert constant load on the logs in the pocket but permit additional logs to be added before the pocket is empty. Pilot trials were conducted with 0.5-m long logs to test the loading mechanism and a full pilot grinder was commissioned in early 1999.
Bolt-through pulp stones as the name implies are bolted directly to the underlying support structure. This leaves a hole in the stone surface for the head of the bolt, which at first glance looks like a bad idea. However, bolt-through pulp stones can increase the production in a conventional groundwood mill. Gord McDowell gave St. Marys Paper experience in a paper entitled Conversion to a BT (bolt-through) pulp stone — Mill trial results. By installing bolt-through pulpstones, St Marys increased its production rate by 20%. Brightness, fines and opacity were higher whereas shives, tear and porosity were lower. Tensile did not change. More importantly, it achieved a more consistent temperature in the grinders and more uniform pulp quality. Time between stone sharpening increased by 30%. It was speculated that the holes in the surface of the stone increased production rate by acting as transportation devices.
The Ergs (Eminent Refiner and Groundwood Scientists) had a dilemma at Houston. Who should receive the Kirkpatrick award for the best paper presented by a young scientist given the number of high quality presentations? In the end, they made the popular decision to present two awards. The winners were Kathrin Morseburg, bo Akademi University, and Tiina Hallamaa, KCL.
David McDonald is director, mechanical pulping division, Paprican, Pointe-Claire, QC.
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