Fibre development in an intensified mechanical pulping process

Christer Sandberg

doi:10.1515/hf-2020-0242

Abstract

Abstract Mechanical pulp for printing paper can be produced with a process that involve much less equipment and that require much lower specific energy compared to conventional processes. Even though common evaluation methods, e.g. handsheet testing, have shown that the pulp quality is similar for the simplified and the conventional processes, it is not known how fibre properties, at the microscopic level, is developed with the simplified process. In this mill scale study, the fibre properties attained with an “intensified” mechanical pulping process, consisting of single stage high consistency double disc refining followed by two stage low consistency refining and no reject treatment was investigated. The simplified process was compared to a process with a reject system. The simplified process rendered fibres with higher degree of fibrillation, higher share of axial splits, lower fibre wall thickness but slightly lower length than the conventional process. The fibrillar fines size distribution of the two processes was different. The conventional process generated more of small fibrillar fines which probably explains the higher tensile index at given density for that process. The results show that it is possible to simplify the production process for mechanical pulp and reduce the specific energy with over 700 kWh/adt.

Highlights

In this work, a process concept with extreme refining conditions for production of newsprint grade pulp was compared in mill scale to a more conventional process
Even though common evaluation methods, e.g. handsheet testing, have shown that the pulp quality is similar for the simplified and the conventional processes, it is not known how fibre properties, at the microscopic level, is developed with the simplified process. In this mill scale study, the fibre properties attained with an “intensified” mechanical pulping process, consisting of single stage high consistency double disc refining followed by two stage low consistency refining and no reject treatment was investigated
The results show that it is possible to simplify the production process for mechanical pulp and reduce the specific energy with over 700 kWh/adt

Summary

Introduction

A process concept with extreme refining conditions for production of newsprint grade pulp was compared in mill scale to a more conventional process. The point highlighted by him regarding the pulping (refining) temperature is still relevant and a good starting point of the introduction. The comparably low temperature at fibre separation in the production of stone groundwood and non-pressurised refining (RMP) render pulps with low long fibre content and relatively low strength properties. As refining temperature was increased by the introduction of thermomechanical pulping (TMP), a higher share of unbroken fibres, a decreased shives content and higher strength properties were attained (Asplund and Bystedt 1973; Breck et al 1975; Butcher 1975). Further increased preheating and refining temperature (160–180 °C) leads to almost intact fibres but with the surface covered by middle lamellae which results in poor bonding and optical properties (Atack 1972). There is obviously an optimum temperature for the refining, which depend on the refiner configuration

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Discussion

Conclusion