The objective in this work was to obtain a fine fraction of kraft pulp, with as high concentration as possible, in a pilot-scale fractionation with micro-perforated screen baskets. The influence of screen basket surface, hole size, feed concentration, pulp type and refining segment design was investigated. The results showed that a smooth screen basket surface improved the fractionation efficiency of the unrefined pulp compared to a profiled screen basket, despite a larger hole size. A significantly higher fine fraction concentration was obtained when using refined hardwood pulp compared to when using softwood pulp, which was explained with its lower average fibre length and narrower and thus more flexible fibre fragments. The pilot trials also showed that the screening process could be operated at feed concentrations similar to those directly after a refiner, 30–40 g/l. This was demonstrated in a process layout with partial recirculation where a refiner and a micro-perforated screen basket were operated in series in pilot scale.
Funding statement: The authors are grateful for the financial support from The Knowledge Foundation and the companies participating in the projects CREPs (Production Concepts for Raw Material and Energy Efficient Products) between 2010 and 2014, Fibre, Stock and Product Optimisation between 2012 and 2014, and Fibre and Stock Design between 2015 and 2017.
We thank the staff at the pilot plant.
Conflict of interest: The authors declare no conflicts of interest.
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