Mills can expect even more variability in recovered paper quality
By Pulp & Paper Canada
Recyclers from Canada, the United States, Europe and as far away as Malaysia met in Ottawa, ON, September 28 to 30. Here, they discussed fundamental research, process development and optimization of p...
By Pulp & Paper Canada
Recyclers from Canada, the United States, Europe and as far away as Malaysia met in Ottawa, ON, September 28 to 30. Here, they discussed fundamental research, process development and optimization of paper recycling processes at PAPTAC’s 5th Research Forum on Recycling. In the tradition of past Recycling Forums, participants heard of the latest recycling research, exchanged ideas and success stories, debated theories, and also found time to enjoy themselves. The conference dinner was held amid the totems in the Grand Hall of the Museum of Civilization, situated next to the Scott tissue mill in Hull, QC. Participants chose between two post-conference mill tours: Bowater Pulp and Paper Canada’s mill in Gatineau, QC, or Domtar Inc.’s recycled bleached corrugated (RBC) facility in Cornwall, ON.
Ink, stickies and contaminants
Recycling paper is conceptually simple. Reslurry the paper, detach ink from the fibre, separate and remove the ink along with other contaminants, and incorporate the now clean pulp in new paper products.
If only it were that easy…
Inks are designed for good print quality, not recyclability. They are meant to stay on the fibre. And, what is now viewed as a contaminant was once an integral part of the paper product.
Understanding inks and contaminants was front and centre at the Forum. Studies ranged from the fundamental to the practical. They included the behavior of flexographic ink during repulping; quantifying the force of ink attachment to fibre; atomic force microscopy of interaction between talc and toner particles; and, examining the role of agglomerating agents for removal of electrostatic toners. Daniel Haynes, Akzo Nobel Eka Chemicals, documented the incidence of the “summer ink effect” — the decrease in ink removal that accompanies the summer months. Pulp quality suffers unless measures are taken to counteract this effect, such as using additional chemicals or reducing production through the process. Haynes correlated the summer ink effect with ambient temperature — finding the problem more severe in the US south than in Canada.
Gilles Dorris, Paprican, reported on thermal ageing of cold-offset printed ink. He found that a time-dependent induction phase preceded the auto-oxidation of ink leading to ageing. At room temperature, the induction period lasted two to five months, but at 60C it was reduced to hours. As oxidation proceeds, ink is held more strongly to fibre and, when removed, is more fragmented. While this explains the summer ink effect, it does not explain the regional disparity found by Haynes. For example, much of the recovered ONP used in Canada is shipped from the US and the temperature inside a boxcar can peak well above the ambient level.
Stickies are the anathema of the deinked pulp user. “Stickies” — a catchall name for a multitude of components present in recovered paper that cause operating problems on paper machines — include adhesives, hot melts, binders, wood resins and extractives. Process conditions encountered in deinking plants – including deinking chemicals, pH and temperature variations, to the imposition of shear forces in processing equipment — can create or exacerbate a stickies problem. Tracking down the source of a problem or predicting if one will occur can be difficult, as each process situation is unique. However, the incentive to do so is significant. Stickies-caused problems in paper machine runnability and paper quality cost between US$600 and 700 million each year in the US alone.
Angeles Blanco, University of Madrid, described a new test method for quantifying stickies deposition caused by dissolved and colloidal material in white water. The device, called a deposition rotor tester, was used to examine stickies deposition in a model system caused by changes in polymer type and dose.
Chafiq Belouadi, BASF Canada, described a modified polyamine that fixes pitch to fibres. However, one recycled paper producer commented that he didn’t like contaminants fixed to the pulp used to make paper. Indeed, material carried from the deinking system to the papermaking system can play havoc in many ways, and methods to mitigate these effects are needed.
Gunar Laivins, Paprican, studied how PEO/co-factor retention-aid systems were affected by anionic trash carried from the deinking system. He was able to relate co-factor performance to its solubility in the white water. The PEO/co-factor systems failed when the co-factor precipitated.
While we struggle to deal with contaminants in our raw material, what we really need are inks and other materials designed with recyclability in mind. Ian McLennan, EcoSynthetix, reported research on corn-based adhesives designed to work effectively in their primary application and be easily removed during recycling. While not currently used commercially, this is certainly a step in the right direction. In the meantime, our deinking systems must cope with the contaminants we receive.
Deinking unit operations
Repulping is the first and, arguably, most important operation in the deinking process. Its objectives are to reslush the paper and detach ink from the fibre surface. A number of studies examined the repulping process. Benjamin Fabry, CTP, used a shear factor to characterize the overall friction imposed on fibre during repulping, and correlated it with ink fragmentation and subsequent ink removal. Yuxia Ben, Paprican, used model systems and scanning electron micrographs to show that ink particles were irreversibly deposited within the fibre lumen during repulping. She advocated short time repulping to minimize lumen loading.
Chad Bennington, Paprican/UBC, described a kinetic model of ink detachment during repulping. The total ink remaining on fibre can be attributed to three factors: the ink that cannot be removed due to insufficient mechanical force imposed during repulping; the potentially removable ink still attached to fibre (whose rate of removal depends on equipment and processing factors); and, the amount of ink redeposited in the fibre lumen.
Contaminants are separated from fibre during screening operations and improved removal of stickies, in particular, is seen as key to increased recycled paper quality. A number of papers focused on this. Peter Seifert, Thermo Black Clawson, described tests in which model contaminants were screened in a small commercial screen using several design configurations over a range of operating conditions. The study identified several factors that affected stickies removal (for example, screen slot width — smaller being better), but did not examine interactions between variables.
Oliver Heise, Voith Sulzer Paper Technology, focused on the rotor’s effect on the stickies particles themselves. If the shear imposed in the screen was greater than the tensile stress of the particles, disintegration occurred. As shear increased with suspension consistency, so did particle disintegration. A maximum consistency of 2.5% was identified above which disintegration increased substantially, although differences were found among the rotors tested. Indeed, Tomas Wikstrom, Valmet Fibertech, showed that screening performance was greatly affected by rotor geometry, concluding that even small changes could have marked effects on efficiency and power consumption.
Ink must also be separated from the fibre. During flotation, air is bubbled through dilute suspensions. The ink particles impinge on the rising air bubbles and are swept upwards to be separated with the froth. However, fibre can be entrained as well, reducing pulp yield.
Michael Ajersch, McMaster University, studied the entrapment of gas bubbles in model pulp suspensions. Bubble entrapment increases floc buoyancy, carrying them upward in the flotation cell and making them more likely to be removed with the froth. Ajersch found that bubbles having diameters on the order of a few hundred micrometres were trapped under flocs, while smaller bubbles were trapped within them. Bubbles of less than 10 micrometres in diameter passed through the flocs.
Bruno Carre, CTP, showed that the long fibre content of flotat
ion froth was generally quite low. Instead, the froth contains fines and filler elements that typically have a greater amount of ink associated with them. Thus, to improve flotation yield, these materials would have to be retained. This would necessitate that a method, like high-consistency kneading, be included in the process to remove ink bound to fines.
A new method to increase the efficiency of ink capture by gas bubbles was described by Caesar Gomez, McGill University. Using a pilot flotation column, brightness gains were improved by sparging with air bubbles coated with a thin film of silicone oil. The improvement was attributed to increased gas hold-up (due to smaller and/or slower rising bubbles) or improved ink attachment efficiency to the oil-coated bubbles.
Jukka Heimonen, Valmet Mechanical Pulping, and Alain Serres, E&M Lamort, each described how to attain enhanced performance from flotation cells. In both studies, computational fluid dynamics (CFD) was used to guide understanding of flow created within the cells. However, significant simplification in the treatment of the suspension was made. For example, pulp suspensions were treated as homogeneous fluids without incorporation of the gas phase. While CFD studies will be increasingly used to gain insight into flotation, as well as other deinking unit operations, ways to describe the flow more completely are needed.
Prepare for more contaminants
The panel From paper supply to process performance stimulated wide-ranging discussion. Bill Moore, Moore and Associates, opened the session by forecasting growing demand for recovered paper in North America. Increasing demand will increase waste paper recovery and paper cost, and deinking plants will have to prepare for increasing contamination levels in certain paper grades.
Mills, however, already deal with variability in recovered paper quality. The mill panel members: John Allen, Bowater Pulp and Paper Canada, Garnet Bremner, Atlantic Packaging Products, Roger Hare, Malaysian Newsprint Industries, and Dan Orlando, Newstech Recycling, discussed recovered paper quality and how they deal with it. Being prepared is essential. Some surges in contaminant level can be predicted. For example, in the back-to-school and Christmas advertising periods, the amount of four-color printed supercalendered grades substantially increases. Process modifications must be made to handle this. Avoidance is also important. Flotation deinking systems avoid purchasing ONP printed with water-based ink. When flexo does find its way into the paper stream, it is bled into the process slowly, typically at about 5% of the feed.
With more contaminants finding their way into recovered paper streams, either the complexity of the deinking processes must increase or the efficiency of the unit operations must improve. The plant operator must use his equipment in the most advantageous manner possible. However, it is sometimes difficult to know exactly what a process operation is doing for you. Dispersers are a case in point. Of the four mills represented, one bypasses the disperser, one runs the system with the plates wide open because they can’t be bypassed, and two use dispersers to improve pulp quality.
Other questions remain. For example, does old magazine really improve flotation? One panelist found that reducing OMG content from 35% to 25% improved pulp quality although he had expected it to deteriorate. What about dissolved colloidal materials, water reuse issues, etc?
Additional work is needed to answer these and other questions. Further, our deinking processes will require continued improvement to keep pace with the changes and challenges ahead. Gilles Dorris announced that the 6th Research Forum on Recycling will take place in autumn 2001. The tradition of sharing research, debating theories, exchanging ideas and seeking solutions to recycling problems will continue.
Chad Bennington is a research engineer of Paprican at the University of British Columbia Pulp and Paper Centre. He is also an adjunct professor in the Department of Chemical and Bio-Resource Engineering.