Textbooks sometimes don't provide current technical information, or don't assemble existing knowledge in a way that can be applied readily to industrial kraft pulping operations. Research articles, on the other hand, can be narrowly focused and hard to digest. Here are some reference articles which provide a generalist's view of some core topics in kraft pulping science and technology.
Tappi Journal 53(1):35 (1970) pdf file
Abstract: Knowledge concerning the kraft pulping process is evaluated from the point of view of industrial application. Reported influences of pulping variables on delignification rate, pulp yield, and pulp properties are examined. Special emphasis is placed on methods for increasing pulp yield and the feasibility of adopting these methods by industry. Water and air pollution problems related to kraft pulping and possible solutions are discussed. Developments in process technology and control are described.
[This is the classic article on how the key independent variables of kraft pulping affect pulping rate and yield.]
The Top Ten Factors in Kraft Pulp Yield
Paperi ja Puu 89(7-8):417 (2007) pdf file
Abstract: Kraft pulp yield depends on a plethora of factors: the nature of the wood and the quality of the chips, the cooking recipe (especially the key independent variables -- alkali charge, sulfidity, temperature, and kappa target), the pulping equipment, and so on. Here, the factors have been assembled into a "top ten" list, and are assessed in terms of relative importance, potential to influence yield values, and contribution to practical knowledge of how pulp yields can be improved. The top ten factors can be re-ordered at will, to rank the magnitude of the yield changes they can produce, for example, or to see which factors have the highest potential for yield improvements at modest cost.
[Almost everything affects pulp yield. This article explains the key factors.]
Chemical and Physical Performance of Kraft Cooking: The Impact of Process Alternatives
Panu Tikka, Martin MacLeod, & Kari Kovasin
Tappi Journal 70(11):47 (1991) pdf file
Abstract: Research carried out during the 1980s revealed that the average mill-made, unbleached kraft softwood pulp is only 75% as strong as pilot-plant reference pulp made from the same chips. There were also reports that pulp strength could be improved considerably by changing the means of digester discharge and by using liquor displacement technology in batch digesters. This paper reviews the results from studies of pulp strength delivery and cooking uniformity in North American and Scandinavian kraft mills. The information covers a wide range of kraft operations. Improvements, and the mechanisms behind them, are discussed.
[Digester systems and their modes of operation have a big influence on the outcome of kraft pulping, especially pulp strength.]
Pulp Strength Delivery Along Complete Kraft Mill Fiber Lines
Martin MacLeod, Jim McPhee, Kevin Kingsland, Robert Tristram, Tim O'Hagan, Roma Kowalska, and Brent Thomas
Tappi Journal 78(8):153 (1995) pdf file
Abstract: Kraft pulps are prized for their superior physical properties. Some strength is lost in digester operations, but what happens along the rest of the fiber line? Not surprisingly, that depends on the design of the mill and on how it is operated. When pulp performance is measured from samples taken along a complete fiber line, papermakers can see how the pulp changes physically as it progresses through oxygen delignification and bleaching. Such a characterization of a kraft mill's fiber line can identify where improvements are needed.
[The basic story of where kraft pulp strength originates, and is lost, along fiberlines in mills.]
Making and Losing Pulp Strength in Bleached Kraft Mills
2006 PAPTAC Annual Meeting Preprints, Montreal, p. C45 pdf file
Abstract: A research-based methodology is useful in addressing the challenge of where kraft pulp strength comes from and how it might be made better. In most kraft mills today, digester systems fail to achieve the pulp strength potential inherent in their original wood furnishes. Of the strength which goes “missing”, the biggest part happens in digester operations, usually for a combination of chemical and physical reasons. In strength delivery terms, the deficits are usually 25% ± 10%. The same analysis, applied to downstream operations, shows that oxygen delignification and bleaching typically make pulp weaker, while pulp drying dramatically changes its refining characteristics. Because kraft pulp production is complex and expensive, continual attention is required on how to do it well, and how – when the rare opportunity arises – to make changes for the better in choosing process equipment and operating practices. Combining all the best factors can result in impressive gains in the physical performance of kraft pulps.
[Where kraft pulp strength is made and lost along fiberlines in mills, 10 years and 20 mills after the article immediately above.]
The physical properties of kraft pulp along a brownstock line
Tappi Journal 70(7):135 (1987) pdf file
Abstract: Mill-made sftwood kraft pulps always have significant strength deficits relative to their potential, represented by pilot-plant reference pulps made from mill chips. Might some of the missing strength be the result of damaging the pulp fibres in the unit operations typically found on the unbleached side of a mill? We systematically sampled pulp after each operation during stable mill operating conditions, then thoroughly washed and screened the samples. Physical testing revealed that the key strength parameters were essentially unaffected by brownstock washing, screening, centrifugal cleaning, and thickening.
[Brownstock processing operations in a bleachable-grade kraft mill -- de-knotting, washing, screening -- do not affect the overall strength of the unbleached pulp exiting a brownstock decker.]
Basket cases: kraft pulps inside digesters
Martin MacLeod and Lorraine Pelletier
Tappi Journal 70(11):47 (1987) pdf file
Abstract: The hanging basket(s) technique is the sole way to retrieve cooked chips from kraft batch digesters in mills without exposure to the violent hydrodynamics of discharge. There is no comparable technique available for continuous digesters. In experiments in half a dozen directly and indirectrly-steamed batch digester plants, basket pulps retained ~90-95% of the overall strength of their corresponding pilot-plant reference pulps from mill chips, whereas hot-blown mill brownstock emerged at strength delivery values of ~70-75%.
[This knowledge led to the development of low-consistency pumped discharge from liquor-displaced batch digesters after terminal displacement for heat reduction].
New, improved kraft pulp quality
Martin MacLeod pdf file
Paperi ja Puu 72(8):780 (1990)
Abstract: Relative to the physical properties of pilot-plant unbleached softwood kraft pulps, mill-made pulps were found to have considerable and consistent strength deficits. For example, they were usually about 25% weaker in tear-tensile strength. This finding led to a borad characterization of "pulp strength delivery" in kraft mills. Hanging basket experiments then established that the strength losses were mainly due to fibre degradation during hot-blown discharge. An alternative -- pumped discharge -- was engineered for a batch digester operating with liquor displacement technology, and three-quarters of the missing pulp strength was regained. Pulp strength delivery studies are now being extended to bleaching, to the lingering puzzle of continuous digester operations, and to a much closer look at the interaction between variability in cooking and its consequences in pulp performance.
[Research between 1980 and 1995 characterized why, and to what degree, a considerable amount of physical strength was missing from mill-made unbleached softwood kraft pulps. Eventually, an industrial-strength solution was found to the problem in conventional batch digesters -- converting to pumped discharge from liquor-displaced digesters. The pulp strength problem remains in continuous digester oeprations].
Measuring Pulp Strength Delivery
Martin MacLeod and Bertil Strömberg pdf file
Abstract: The determination of pulp strength delivery at a kraft mill includes the production and testing of reference pulps made from the mill's chips. Although the experimental practices used in chemical pulping research laboratories might be construed as "standard", this contention had not been tested. We did so by providing a common source of wood chips and a set of pulping guidelines to seven laboratories. At each, duplicate pulps were produced, and all were tested for physical properties at two central laboratories. In six of the seven cases, the tear-tensile results and many other physical properties were within 10% of one another, and the variability between samples at any one lab was small. Strength delivery measured at a single laboratory can provide a valid comparison of two pulps that have a tear-tensile difference of 10% or greater.
[How good is the pilot-plant or laboratory reference pulp making on which pulp strength delivery in mills is based? The answer: very reproducible indeed in this co-operative study at seven different labs in North America and Europe. For example, six of the labs made twelve reference pulps whose tear indexes at 8 km breaking length averaged 21.2 ± 0.3 mNm2/g at the 95% confidence level.]
Pulp Strength Delivery Along Research Fibrelines
Martin MacLeod pdf file
Abstract: If unbleached kraft pilot-plant reference pulps (made from a mill's chips, using the mill's basic pulping conditions) provide the standard for comparing the overall strength levels of unbleached mill pulps, are they still suitable when the focus moves downstream along a fibreline? Why not make oxygen-delignified laboratory reference pulps and fully-bleached reference pulps too? That way, a "research fibreline" would be produced, allowing a systematic comparison of reference pulps with mill-made pulps along complete fibrelines (i.e., after pulping, O2 delignification, and ECF bleaching). Surprise -- it's unnecessary! The strength of softwood kraft pulp does not change when careful and appropriate experimental procedures are used in lab O2 delignification and bleaching, so the tear-tensile strength of the unbleached reference pulp serves equally well at all fibreline positions. Beware, though, that it is possible to degrade pulps when using inappropriate laboratory procedures, leading to artificially inflated pulp strength delivery results.
[Doing a complete research-scale fibreline's worth of reference pulp making consumes significant time and money but provides no scientific gain over simply making standard unbleached pilot-plant reference pulps from mill chips. These unbleached pulps provide the proper anchor points for pulp strength comparisons anywhere along a mill's fibreline.]
Crushing: Is This Any Way to Treat Overthick Softwood Chips fof Kraft Pulping?
Martin MacLeod, Arnold Dort, Joel Young, Desmond Smith, Kendall Kreft, Marc-Andre Tremblay, and Pierre-Alexandre Bessette
Pulp & Paper Canada 106(2):44 (2005) pdf file
Crushing: Is This Any Way to Treat Overthick Hardwood Chips fof Kraft Pulping?
Martin MacLeod and Arnold Dort
Pulp & Paper Canada 109(10):17 (2008) pdf file
Abstract: Chip crushing machines are used in some mills to destructure the overthick material from thickness screening into an acceptible form for kraft pulping. These articles describe a technique to measure the pulpability of crushed overthick chips versus that of accept chips of ideal thickness, applying it to two softwood cases and three hardwood ones. The crushing machines were able to reduce the rejects which would otherwise be generated in kraft pulping by 90% or more. They can also cause losses in fiber length and pulp strength. The machines should be tested regularly and maintained well to deliver consistently high efficiency.
[Crushed overthick chips cook pretty much like accept chips of proper thickness. Even so, this stream of material should be minimized in chip thickness screening systems. Read why...]
Kraft Softwood Pulp Strength vs. Kappa Number and Pulp Viscosity
Based on Paperi ja Puu 73(8):773 (1991) pdf file
Abstract: The overall strength of bleachable-grade kraft softwood pulp is remarkably insensitive to both kappa number and pulp viscosity -- it is essentially constant across the kappa range 20-35. This behavior is the same for mill-made and research-made pulps. Above and below this kappa range, pulp strength falls off, but for different reasons.
[Do you believe that the strength of bleachable-grade softwood kraft pulp changes significantly if kappa number is raised or lowered by only a few units? In fact, in the target kappa range of bleachable-grade pulping, overall strength is constant (assuming that the wood chips and pulping conditions don't change)]
The Half-Life of Biological Knots in Kraft Pulping
Martin MacLeod and Arnold Dort
2008 PAPTAC Annual Meeting Preprints, Montreal, p. A299 pdf file
Abstract: How many times do you have to cook biological knots and the knotter rejects derived from them in a bleachable-grade kraft mill before they are completely reduced to fibrous material and dissolved organics? And what is the yield of pulp from such material? To find the answers, we experimented with stockpiled knotter rejects and with fresh biological wood knots. With either material, re-cooking demonstrated that knotter rejects continued to beget further knotter rejects. Relative to normal wood chips, biological knots cooked far slower and to lower pulp yields; after re-cooking, the pulps became progressively worse in average fibre length and overall strength. Re-cooked knotter rejects from biological knots had a half life of two complete cooks in bleachable-grade kraft pulping. Because mixing biological knots or their knotter rejects descendants with normal chips makes kraft pulping much more heterogeneous and impairs pulp yield and product quality, they should be purged as efficiently as possible from chip furnishes going to digesters, and knotter rejects containing them should go to hog fuel boilers or landfills rather than being recycled to digesters.
[Knotter rejects are often sent back to digesters. Those derived from biological wood knots remain very resistant to delignification. They should not be recycled.]
Bark in Kraft Pulping
Marianne Harder, Dean Einspahr, and Russell Parham
Tappi J. 61(11):121 (1978) pdf file
Bark accompanying wood chips into a kraft pulping operation will often contaminate the pulp with extractives and dirt particles (hard to bleach or to screen out), and may increase the consumption of pulping and bleaching chemicals. Sadly, bark provides essentially no yield of suitable papermaking fibres at all, either from softwoods or hardwoods.
[In bleachable-grade kraft pulping, bark content in chips needs to be minimized, as it does nothing good for pulp yield or quality.]
21st Century Kraft Pulp Mills
Tony Johnson, Beca Consultants
2008 International Pulp Bleaching Conference, Quebec City, Canada
PAPTAC (and other sponsoring technical associations)
At the 2008 International Pulp Bleaching Conference, Tony Johnson was the keynote speaker. This interview appeared in Pulp & Paper a month before the conference. In it, Tony describes the main characteristics of the world's newest kraft pulp mills. They're huge (3000+ tpd). They're pulping plantation eucalypts. They use ECF sequences for bleaching.
Read more about the leading edge of kraft pulping technology... pdf file