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Designing grinding tools to control and understand fibre release in groundwood pulping
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.ORCID iD: 0000-0002-7432-592X
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mechanical pulping is a very energy demanding process in which only a fraction of the energy is used for the actual separation of wood fibres. The rest of the energy is lost, partly in damaging already separated fibres and partly as heat during viscoelastic deformation of the wood. Groundwood pulping is one of the major mechanical pulping processes. In this process, a piece of wood is pressed against a rotating grinding stone. The stone surface has traditionally been made of grinding particles fused to a vitrified matrix. Though the process is close to 200 years old, the detailed mechanisms of the interactions between the grinding particles and the wood surface are still not fully understood. The random nature of the grinding stones combined with the heterogeneous nature of wood creates a stochastic process that is difficult to study in detail. This work utilizes well-defined tools, that facilitate testing and analysis, to increase the understanding of the tool-wood-interaction. In-situ tomography experiments were performed with such well-defined tools, to study the deformations and strains induced in the wood as the tool asperities engage the wood surface. Numerical simulations were used to study the influence of asperity shape, and to show how the induced strains promote intercellular cracks and fibre separation. Several well-defined tool surfaces were designed and tested in a newly developed lab-scale grinding equipment, to study their performance in terms of energy consumption and the quality of the produced fibres. It was shown that the well-defined grinding surfaces, with asperities the same size as a fibre diameter, can be designed both to achieve drastically lower energy consumption compared with that of traditional stones and to produce long and undamaged fibres. This thesis shows that it is possible to design future tools that can help reducing the energy consumption in industrial pulping.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 59
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1817
Keywords [en]
Groundwood pulping, Grinding mechanisms, Diamond grinding tool, Energy efficiency
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Paper, Pulp and Fiber Technology
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
URN: urn:nbn:se:uu:diva-382719ISBN: 978-91-513-0672-8 (print)OAI: oai:DiVA.org:uu-382719DiVA, id: diva2:1312993
Public defence
2019-08-23, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 37206-2Available from: 2019-05-29 Created: 2019-05-02 Last updated: 2019-08-15
List of papers
1. Initiation of wood defibration in groundwood pulping, single asperity indentation and scratching
Open this publication in new window or tab >>Initiation of wood defibration in groundwood pulping, single asperity indentation and scratching
2016 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 31, no 3, p. 401-406Article in journal (Refereed) Published
Abstract [en]

To understand how the energy requirements of the mechanical pulping process can be reduced, the fundamental mechanisms behind fiber separation in Norway spruce were studied experimentally and analytically. Single tip scratching in heated water was used to reproduce initial defibration mechanisms found industrially. The resulting scratches were then compared with surfaces ground in a real industrial process. Moreover, the mechanical behavior of the wood microstructure was monitored with X-ray computed microtomography as a single hard tip was pressed into it. Subsequent digital image correlation was applied to estimate the strain field in the region around the indenting tip. Regions in the wood with high tensile or shear strains were identified, i.e. where cracking and fiber separation is believed to initiate.

Keywords
Defibration, Scratching, Strain analysis
National Category
Engineering and Technology Wood Science
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-310478 (URN)000387974800001 ()
Funder
Swedish Energy Agency
Available from: 2016-12-16 Created: 2016-12-16 Last updated: 2019-05-02Bibliographically approved
2. Investigating tool engagement in groundwood pulping: finite element modelling and in-situ observations at the microscale
Open this publication in new window or tab >>Investigating tool engagement in groundwood pulping: finite element modelling and in-situ observations at the microscale
(English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434XArticle in journal (Refereed) Accepted
National Category
Paper, Pulp and Fiber Technology Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Identifiers
urn:nbn:se:uu:diva-382716 (URN)
Funder
Swedish Energy Agency, 37206-2
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-11-12
3. Defibration mechanisms and energy consumption in the grinding zone – a lab scale equipment and method to evaluate groundwood pulping tools
Open this publication in new window or tab >>Defibration mechanisms and energy consumption in the grinding zone – a lab scale equipment and method to evaluate groundwood pulping tools
2019 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669Article in journal (Refereed) Epub ahead of print
Abstract [en]

Groundwood pulping is a process that employs large machines, making them difficult to use in research. Lab scale grinders exist, but even though they are smaller, the sizes of the grinding stones or segments make them cumbersome to exchange and tailor. This study presents a method and an apparatus for investigating the detailed mechanisms and the energy requirements behind the fibre separation process. A well-defined grinding tool was used at three different temperatures to demonstrate that the equipment can differentiate levels of energy consumption and defibration rates, confirming the well-known fact that a higher temperature facilitates defibration. It is also shown how the equipment can be used to study the influence of grinding parameters, exemplified by the effect of temperature on the way fibres are separated and the character of the produced fibres. A key feature of the equipment is the use and evaluation of small grinding surfaces, more readily designed, produced, evaluated and studied. This reduces both the cost and time necessary for testing and evaluating. At the same time, a technique to produce well defined grinding surfaces was employed, which is necessary for repeatability and robust testing, not achievable with traditional grinding stones.

Keywords
Computed tomography, Energy efficiency, Groundwood pulping, Lab scale equipment, Test method
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:uu:diva-382698 (URN)10.1515/npprj-2019-0063 (DOI)
Funder
Swedish Energy Agency, 37206-2
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-11-12
4. Influences of load and temperature on groundwood pulping with well-defined tools
Open this publication in new window or tab >>Influences of load and temperature on groundwood pulping with well-defined tools
2019 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 438-439, article id 203051Article in journal (Refereed) Published
Abstract [en]

Groundwood pulping is an industrial process with a high energy demand, although only a minor part of the energy is used for actually separating the fibres and the rest for working them. Traditionally, the separation process employs a grinding stone having particles embedded in a softer matrix. The position and distribution of the particles have been random, causing their interaction with the wood to also be random. This makes studies of the mechanisms during the separation process difficult. Knowledge of the mechanisms in the separation process helps when designing future tools aimed at energy efficiency or tailored fibre properties. Recently, grinding surfaces having diamond particles brazed to a steel backing at fixed positions have been developed and commercialised. In this work, individual particles are not positioned at the tool surfaces. Instead, well-defined structured diamond films, soldered to a backing, are used as grinding tools. The grinding asperities of such films can be tailored to shapes that are not possible to achieve by using particles. Using this kind of tool in a lab scale grinding equipment, confirms the well-known fact that increased load or increased temperature both lead to lower energy consumption for fibre separation and longer, less damaged fibres.

Keywords
Grinding, Pulping, Wood tomography, Energy savings, Tool design
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:uu:diva-382700 (URN)10.1016/j.wear.2019.203051 (DOI)
Funder
Swedish Energy Agency, 37206-2
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-11-12
5. Evaluation of well-defined tool surface designs for groundwood pulping
Open this publication in new window or tab >>Evaluation of well-defined tool surface designs for groundwood pulping
2019 (English)In: BioResources, ISSN 1930-2126, E-ISSN 1930-2126, Vol. 14, no 4, p. 9575-9587Article in journal (Refereed) Published
Abstract [en]

Groundwood pulping is a process in which logs are pressed against a rotating grinding stone. A conventional grinding stone is generally made of grinding particles in a vitrified matrix. As the particles are practically round, their contact with the wood is limited to occasional point contacts. The interaction between the particles and the wood occurs at random positions and at random times, only intermittently contributing to the defibration process. In this work, well-defined grinding tools with asperities giving line contacts rather than point contacts were tested. The tool surface asperities were elongated in shape and positioned with different density over the surface. The tools were tested in a lab-scale equipment at elevated temperatures, and their performance was compared to that of a conventional grinding stone. The grinding mechanisms varied between the different tools, and the specific grinding energy was reduced compared to the conventional tool.

Keywords
Groundwood pulping, Diamond tools, Energy consumption, Tomography, Grinding mechanisms
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:uu:diva-382701 (URN)10.15376/biores.14.4.9575-9587 (DOI)
Funder
Swedish Energy Agency, 37206-2
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-10-21
6. Influence of alignment between extended tool ridges and the wood structure on the defibration mechanisms in groundwood pulping experiments
Open this publication in new window or tab >>Influence of alignment between extended tool ridges and the wood structure on the defibration mechanisms in groundwood pulping experiments
(English)Manuscript (preprint) (Other academic)
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:uu:diva-382709 (URN)
Funder
Swedish Energy Agency, 37206-2
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-09-10

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