Proceeding Articles

The sulphite operation in the Quebec North Shore Paper Company at Baie Comeau was converted to a high-yield sodium bisulphite pulping system in 1970. It consisted of a regular two-stage refining sequence with intermediate washing and a separate reject-refining arrangement.

Batches of 100 g unbeaten, bleached softwood sulphite fibres dispersed in 20 1 distilled water were prepared and mixed with increasing amounts of a commercial filler grade of titanium dioxide. The addition level ranged from 0.5 to 3.0 g TiO₂/g fibres. Each mixture was stirred slowly for 1 h with a blade stirrer (40 rpm) to prevent sedimentation. After this time the adsorption equilibrium was established. Four samples, 100 ml each, of the supernatant liquid were drawn with a plastic tube covered with a fine wire mesh at the bottom. From the TiO₂ content of the samples the amounts of TiO₂ adsorbed on the fibres were calculated.

The rheological properties of fibre-water suspensions are of critical importance in many of the papermaking processes from stock preparation (beating, screening, fractionation, cleaning) through transport and distribution (pumping, pipe flow, headbox flow), to the actual forming process (dispersion in headbox and forming zone, dewatering).

The limitations of existing techniques for measuring velocity and turbulence as well as concentration and flocculation in pulp suspensions are discussed.

The major recent fundamental investigations of fibre suspension flow in pipes are critically reviewed within the framework of the three basic flow mechanisms (plug, mixed and turbulent flow) and the three basic study levels (empirical, network and fibre). It is concluded that the varied and complex flow phenomena exhibited by the suspensions, from plug flow through to turbulent flow, are controlled by the same variables at the fibre level : the volumetric concentration, aspect ratio and modulus of elasticity of the fibres.

Recent investigations of flocculation and turbulence are critically reviewed, and it is found that no reliable measurements of turbulence in fibre suspensions have been reported.

A new investigation of elongational pipe flow is presented.

Finally a new method for the simultaneous noncontact measurement of turbulence and flocculation is described. It is a combination of laser doppler anemometry and light reflection measurement, and offers many new possibilities in the fundamental study of pulp suspension flow.

The most widely used synthetic additives to improve drainage and fines retention during the formation of the fibrous mat are cationic polymers. These polymers are also frequently used to improve paper wet strength and internal bonding.(¹) Their wide applicability is based on their preferential adsorption onto the anionic fibres and fines.

Advances in understanding the factors involved in beating chemical pulps, which have occurred since the symposium on ‘Fundamentals of Papermaking Fibres’ held in 1957, are surveyed and selectively reviewed. The status of current knowledge of refining and grinding is summarised.

One of the problems arising in the utilisation of high-yield pulps in papermaking is their tendency to produce various kinds of dissolved matter during mechanical treatment. One result of this is that the amount of paper producible from a given amount of wood is less than that which is calculated from the pulping yield. Moreover, the hazard element of the effluent from the environment aspect is increased.

The literature contains little discussion of this problem, although Sjbström et al. have observed the dissolution of xylan from birch pulps of both normal and high-yield. ⁽¹,²⁾

Within the framework of a project concerning the optimisation of the defibration and beating of high-yield pulps a study has been made of the material losses that occur during these process stages. The results of this study are presented in the following.

It has long been standard practice to evaluate and assess the usefulness of a papermaking pulp by beating it in a laboratory beater for increasing periods of time and examining the pulp and sheets of paper made from it as the beating process continues. The examination of the pulp can include the inspection under a microscope, fibre fractionation, the Schopper-Riegler evaluation or equivalent, and the water retention value using a standardised centrifuging technique. The examination of the sheets can include any of the physical tests that are of interest for the particular use to which the pulp is to be put. In particular, the rate at which the wetness (in °SR) increases with the beating time is called beatability of the pulp. For example, sulphate pulp is generally more difficult to beat than sulphite pulp, and cotton linters are more difficult still.

This method of pulp evaluation is not only very time consuming, but also does not tell the papermaker much about how best to treat his pulp in the stock preparation. There is a need for a simple method to examine an unbeaten pulp and to assess its behaviour in a beating process without actually carrying out the beating. Such a method can be based on the solubility of the pulp in a suitable solvent. The condition for the success of such a method is that it is sensitive to those structural features of the fibre which affect the beating and to the changes which they suffer during this treatment.

The structural features of the fibre most sensitive to the beating action whilst contributing to the paper properties, are the elements which influence the swelling phenomena in water. Moreover, they are responsible for the solubilisation of low polymers (hemicelluloses) in NaOH-solutions.

Today, several important parameters in sheet forming on the paper machine are changing considerably.

The twin wire concept is about to become the standard for new installations. Here, the drainage zone is drastically reduced compared to the fourdrinier, with corresponding reduction in drainage time. Besides, there is a general increase in paper machine speed. The drainage elements are changed, reducing the degree of pulsed drainage.

Also the papermaking furnish is changing.

New fibre raw materials are introduced by a change from mature to juvenile trees on the one hand and the increasing use of tropical hardwoods on the other. This implies a significant change in the fibre compositions of the furnishes.

This tendency is enhanced by another technical trend, partly caused by scarce wood supplies, towards pulps of higher yields. The trend is significant both within the conventional chemical pulping processes, and for the development of new processes like the thermomechanical pulping methods.

The change in paper forming speed, and in the fibre composition may cause problems for the production process. It therefore seems useful to try to produce new information on the influence of fibre form on the pulp drainage characteristics.

The importance of wet-web strength in the manufacture of newsprint is considered. A number of case histories are presented, which illustrate how the assessment of wet-web strength can be effectively used to clarify practical observations on paper machines. These observations were concerned with the influence on paper-machine operation of normal fluctuations in the properties of the mechanical and chemical pulp components of a newsprint furnish. Wet-web strength measurements were carried out at moisture contents of the web which reflect its ability to release water under standard drainage and pressing procedures. A rapid wet-web burst testing technique was developed that is suitable for evaluating wet-web strength in a mill environment.

The availability of accurate draw measuring equipment makes it possible to measure these two properties of the wet web running on the paper machine. An example has occurred recently, which illustrates the principle of such measurements.