Mittuniversitetet

This study explores the effects of adjusting production rate and plate gaps on fibre distributions and handsheet properties in a double disc (DD) refiner, and examines how these factors influence pulp characteristics and energy use. Experimental results indicate that elevating production rates can reduce specific energy demands at a given tensile index (TI). Beyond individual properties, chi-squared tests were used to compare full fibre property distributions. These analyses demonstrated that pulps with equal TI are statistically indistinguishable across operating conditions, whereas pulps with different TI exhibit clear distributional differences. Overall, findings suggest that improved energy efficiency observed at higher production rates is primarily due to more effective refining mechanisms, rather than differences in the resulting fibre distributions. Further research is needed to clarify the complex interplay between production rate, specific energy, fibre properties, and handsheet properties, particularly the role of fibrillation in TI development.

This study focuses on the development of a compact model with improved interpretability compared to similar approaches, relating thermomechanical pulp (TMP) properties, quantified using a fiber analyzer, to Canadian standard freeness and handsheet properties. The data used in this study are obtained from TMP produced by a conical disc refiner. Utilizing the LASSO-regularized Latent Variable Regression (LASSO-LVR) model, we identified three key latent variables – representing shives content, fibrillation, and slender fines content – that accurately predict eight distinct handsheet properties. In a subsequent analysis, we investigated the linkage between refiner settings and Specific Refining Energy (SRE) to these key analyzer readings and, consequently, to handsheet properties. The inclusion of SRE as an internal state variable in the model significantly enhanced predictive accuracy, providing a foundation for more precise and energy-efficient control strategies in refining processes. 

Increased knowledge on the correlation between pulp processing, fibre-properties and paper properties is required to improve fibre-based products. Part 1 of this investigation deals with the effects of HC and LC refining on fibre properties development. LC refining reduced curl and increased tensile index in a manner similar to hot disintegration whereas HC refining increased curl slightly. In this second part, the correlation between fibre curl and handsheet properties of thermomechanical pulp, subjected to low consistency (LC) refining and hot/cold disintegration is examined. Fibre curl decreased by laboratory disintegration and LC refining and showed a linear correlation with increased tensile index and tensile stiffness. Evaluation of fibre property distributions gave a more detailed description of the development of fibre properties. These revealed that disintegration and LC refining gave different fibre curl versus fibre length distributions, even when their average values were similar. These results confirm that analysing fibre property distributions contributes to a more detailed knowledge of the development of pulp quality. Hot disintegration before laboratory testing exaggerated pulp quality and increase internal fibrillation and can therefore be questioned. When hot disintegration is performed before pulp analyses, the impact of LC refining on paper properties may be misjudged. 

We present a method for pulp particle characterization based on a truncated lognormal mixture (TLM) model, as motivated by size statistics of organisms. We use an optical fiber analyzer to measure the length–width distribution of kraft-cooked roundwood or sawmill sources, of chemi-thermomechanical pulp (CTMP) samples from roundwood or sawmill sources, and the same CTMP samples after kraft post-processing. Our results show that bimodal TLMs capture salient features of the investigated pulp particle distributions, by decomposition into a large-particle and a small-particle fraction. However, we find that fibers from sawmill sources, which have not undergone mechanical treatment, cannot be described by TLM, likely due to non-random sampling. Within the confines of our dataset, the TLM characterization predicts laboratory sheet properties more effectively than conventional averaging methods for pulp particle size distributions. The TLM characterization is intended as a tool for controlling the pulp production process towards higher product quality, uniformity, and energy efficiency, pending further mill trials for validation. 

Particles in mechanical pulp show a wide variety but are commonly described using averages and/or collective properties. The authors suggest using distributions of a common bonding factor, BIND (Bonding INDicator), for each particle. The BIND-distribution is based on factor analysis of particle diameter, wall thickness, and external fibrillation of several mechanical pulps measured in an optical analyser. A characteristic BIND-distribution is set in the primary refiner, depending on both wood and process conditions, and remains almost intact along the process. Double-disc refiners gave flatter distributions and lower amounts of fibres with extreme values than single-disc refiners. More refining increased the differences between fibres with low and high BIND. Hence, it is more difficult to develop fibres with lower BIND. Examples are given of how BIND-distributions may be used to assess energy efficiency, fractionation efficiency, and influence of raw material. Mill scale operations were studied for printing-grade thermomechanical pulp (TMP), and board-grade chemi-thermomechanical pulp (CTMP), both from spruce.

Particles in mechanical pulp are a heterogeneous popu-lation, but commonly described using averages based on wide and skewed distributions. It was found that these aver-ages may lead to erroneous conclusions regarding the char-acter of the material and also how the material has been de-veloped along the process. This study is based on measure-ments of individual particle dimensions (length, curl, and ex-ternal fibrillation) in mill operation of CTMP and TMP as detected in an optical analyser.

Effects of hot-pressing on anisotropic sheets with less good formation was here investigated. The main objective was to study water absorption capacity in relation to the wet strength of hot-pressed paper. A pilot paper machine was used to produce papers from TMP and CTMP furnishes. The results indicate that it is not only the high dry content after wetting that contributes to the high wet strength of the paper hot-pressed at 200C. If it is required to have a paper with both low absorption of water and high wet strength, hot-pressing at 200C seems to be more desirable than using con-ventional drying and adding wet chemical agents to the furnish.

Production of mechanical pulps requires high specific electrical energy compared to many other attrition processes. In Scandinavia, the lowest specific refining energy for production of thermomechanical pulp is around 1800 kWh/t for newsprint quality, which is roughly 60 times higher than for crushing of stone to a similar size distribution. The high specific energy demand for refining has naturally motivated large efforts in the search for improved efficiency. It is always practical to be able to quantify improvements in efficiency for comparison of process designs and of different machine types. However, there is no commonly accepted definition of efficiency for mechanical pulping processes. In published work within mechanical pulping, energy efficiency has been presented in different ways. In this paper, we discuss definitions of energy efficiency and aspects that ought to be considered when energy efficiency is presented. Although focus of this work is on energy efficiency for refiner processes, the principles can be applied to other types of mechanical pulping processes such as stone groundwood. 

A novel process based on low-energy mechanical pulp and deep eutectic solvents (DESs) was evaluated with the goal of producing fibers suitable for papermaking. Ideally, these fibers could be produced at much lower costs, especially when applied to an existing paper mill equipped with a thermomechanical pulp (TMP) production line that was threatened with shutdown due to the decreasing demand for wood-containing paper grades. The efficiency of DES delignification in Teflon-coated autoclaves and in a specially designed non-standard flow extractor was evaluated. All tested DESs had choline chloride ([Ch]Cl) as the hydrogen bond acceptor. Lactic acid, oxalic acid, malic acid, or urea acted as hydrogen bond donors. The temperatures and times of the delignification tests were varied. Chemical analysis of the pulp samples revealed that DESs containing lactic acid, oxalic acid, or urea decreased the lignin content by approximately 50%. The DES delignification based on [Ch]Cl and urea exhibited good hemicellulose retention while DES systems based on organic acids resulted in varying hemicellulose losses. The [Ch]Cl / urea mixture did not appear to be corrosive to stainless steel, which was another advantage of this DES system. 

The aim of this study was to evaluate changes in fibre properties with high (HC)- and low consistency (LC) refining of TMP and determine how these contribute to tensile index. Two process configurations, one with only HC refining and another with HC refining followed by LC refining were evaluated in three TMP mainline processes in two mills using Norway spruce. An increase in tensile index for a given applied specific energy was similar for all LC refiners in the three lines, despite differences in the fibre property profiles of the feed pulps. Compared with only HC refined pulps at a given tensile index, HC+LC refined pulps had greater fibre wall thickness, similar fibre length, strain at break and freeness, but lower light scattering coefficient, fibre curl and external fibrillation. The degree of internal fibrillation, determined by Simons' stain measurements, was similar for both configurations at a given tensile index. The results indicate that the increase in tensile index in LC refining is largely influenced by a decrease in fibre curl and in HC refining by peeling of the fibre walls. Compared at a given tensile index, the shive content (Somerville mass fraction) was similar for both HC+LC and HC refining.