Chapter 1

Introduction

1.1 General

Refining refers to the mechanical treatment of chemical pulp in preparation for papermaking. It is an important process for improving pulp properties. In the process of refining, fibers are trapped in the gaps between bars during bar crossings where they are subjected to cyclic compression and shear forces which modify the fiber properties. [Heymer, 2002]. The main target of refining is to modify surface characteristics as well as fiber flexibility in order to develop stronger and smoother paper with good printing properties,. In addition, sometimes the purpose is to develop other pulp properties such as absorbency, porosity, or visual appearance. [Yan Li, 2005].

Low Consistency (LC) refining at 3-5% has many benefits because the pulp suspension acts as an incompressible fluid and therefore may be pumped through the refiner using an external pump. This mixture is more homogeneous than pulp at high consistency (30%) and consequently the refining treatment more uniform. This is evident in the smaller, more uniform gap between the plates, and more stable refiner power consumption. Another benefit of LC refining, related to the above, is that the intensity of treatment and pulp throughput are decoupled, allowing them to be independently controlled and optimized. [Luukkonen, 2011]

The possibility of having an optimum condition for refining intensity arises from the fact that low intensity refining imposes gentle refining effect, thus flexibilizing fibers through internal fibrillation without further disrupting the fiber structure, while on the other hand, high intensity disrupts the fiber structure harshly and creates fibre shortening. However, the literature on an optimum intensity is inconclusive and occasionally contradictory. .Nazhad et al (2001) studied the effect of refining of chemical pulp on paper formation. The authors concluded that fiber shortening caused by refining had strong effect on reducing fiber flocculation, thus improving formation. The author also discussed the fact that the optimum refining intensity should vary depending on raw material properties.

The refining process is usually described by two factors, refining intensity and refining amount. The amount of refining is represented by specific energy. The intensity is represented in different ways. One approach is by a machine intensity (Kerekes 2010). The most used parameters for this is Specific Edge Load (SEL) (Baker 1995). Modifications of the specific edge load have been suggested, for example the Modified Edge Load (MEL) by (Melzer 1995), Specific Surface Load (Lumiainen 1995). Even more complex expressions were developed by (Joris 1995) (Radoslava, Roux et al. 1997)]. Another approach to characterizing intensity is by a fibre intensity . This is based on energy expended on fibre rather than by bar crossings as is the case for machine intensities. An example is the C-factor (Kerekes 1990) which takes into account the properties of the fiber suspension. However, apart from the SEL, none of the other methods have gained wide acceptance.

Of the simpler models, Lumiainen (1994) studied the refining intensity on the hardwood pulp. He concluded that the lower the intensity is better for fiber development and gives lower the energy consumption. Kerekes (2010) compared tensile strength increase for hardwoods and softwoods using both the SEL and the Specific Intensity from the C-Factor. He showed that the that SEL had limitations, but for Specific Intensity the data for both hardwood and softwood fell on one line, and that the optimum SELs for each occurred at the same Specific Intensity. Eileen Joy and Desaeada (2010) concluded the ultra-low intensity refining plate would have benefits for hardwood chemical pulps because gentle refining action increased the specific surface area of the fibers by internal fibrillation, leading to greater strength development. On the other hand, Soupajarviel at, (2009) reported higher refining intensity results fiber fibrillation (external) while fiber length remained unchanged by using high intensity dispergator with LC refining.

Koskenhely et al, 2005 compared the fillings in refining of softwood and hardwood pulp fibers. Their results showed a better dewatering-tensile strength combination when SW was refined with conical fillings. Also, the reduction in fiber length was inversely proportional to gap size because fibers are squeezed and crushed between the bars.

Some work suggested that the refiner gap would be better indication of the effective refining intensity than the conventional Specific Edge Load (SEL) and that the power gap relationship governs the refining result [Luukkonen, 2011]. Moreover, changes in fiber length and fiber curl were also controlled by plate gap and can be related to water retention value (WRV) better than energy and power in refining of chemical pulp. [Mohlin, 2002].

1.2 Objectives of the research

The overall objective of this research is to determine optimum conditions for refining Northern Bleached Softwood Kraft (NBSK) pulp ensure that Canadian pulp customers are optimally refining their pulp. The objective is divided into three parts, which are:

1. To compare gap size and SEL for characterizing the refining intensity.

2. To study the effect of refining intensity on internal fibrillation (tensile, tear, water retention value) and fibre cutting.

3. To determine the optimal refining conditions for chemical pulps in terms of maximum tensile strength at a given specific energy.

1.3 Scope of Study

The scope of this experiment is limited to the pilot scale refiner to achieve the above objectives. These studies focused on chemical pulp (bleached Softwood Kraft pulp).

Chapter 2

Literature Review

2.1 Refining mechanism

The refining mechanism is based on the following picture. It is assumed that the fibers are captured in the form of fiber flocs and that the effective refining action starts when the fiber bundle is pressed between the leading edgesthe edge-to-edge phase in Figure 2.1. This phase is followed by and edge-to-edge surface phase, which continues until the leading edges reach the tailing edges of the opposite bars. The length and the strength of the fiber flocs depend on the physical dimensions and the bonding ability of various fibers in the mixture.

Figure 2.1: Refining mechanism (Paulapuro et al., 2000)

2.2 Refining theories

The utilization of new theories, since 1967, has enabled a better understanding of what happens inside the refiner, and allowed better means for optimization. Fundamental to all the theories is the understanding that the refining result is a function of two major factors, among others:

The amount of applied energy

How is the energy applied

More rigorously, the refining process is a cyclic application of energy to pulp, and therefore it can be described in terms of number and intensity of impacts on fibres. The product of these equals specific energy. Only two of these parameters are independent. It is common to only use specific energy and intensity.

The most commonly used intensity is the Specific Edge Load (SEL).This machine intensity has provided a significant contribution in the search for an index representing the intensity of the beating performance, describing the beating absorption by a pulp, at least with respect to its nature. Although developed empirically, it has been shown by Kerekes & Senger (2006) to have a rigorous scientific meaning: its is the energy expended per bar crossing per bar length. This concept has been developed into other refining theories, all involving the character of the beating action as described by the type (or intensity) of refining, and the extent of action, which is related to the amount of refining imposed on the fiber flocs.

The goal of refining theory is to predict changes in pulp properties from known refining conditions and to allow for the comparison of different refining plates, or fillings, under various operating conditions. There are currently seven main refining theories as shown below:

2.2.1 Specific Edge Load Theory

The specific edge load theory is the most commonly used and simple theory in practice today. The characterization of the refining process is given by the specific energy, E (kW*hr/ton), and the refining intensity parameter, specific edge load (SEL). The SEL is a measure of the energy expended per bar crossing per bar length, having unit of J/m. Its is obtained form equation 2.1.

Eq. 2.1

PNET is the net input power (W). The cutting edge length, CEL (m/s), is the product of the plate factor, Kp (m/rev) and the angular velocity,(rev/s). Kp is the sum of the product of the number of bars on the rotor, Zr, the number of the bars on the stator, Zs, and the length of a bar, L(m), over increments, i, in the refining zone.

2.2.2 Modified Edge Load Theory

The modified edge load theory is an extension of the specific edge load theory that takes into account additional filling parameters. The characterization of the refining process is given by E and the refining intensity parameter, modified edge load (MEL), see Equation 2.2:

Eq. 2.2

is the average bar angle (the angle between the bar and a radial line drawn through the center of the bar section in degrees), B is the bar width (m) and G is the groove width (m). Note that the modified edge load theory is an empirically derived theory which accounts for both operating conditions and filling parameters.

It should be noted that the benefit of the MEL theory is that it allows refining results to be forecast as a function of freeness at a constant refining intensity is not considered