Cellulose
Experimental
Materials
An industrial, never dried, unbleached softwood kraft pulp containing 85% spruce, 10% pine and 5% larch with Kappa number 27 was used in all trials. The primary fines content of the pulp was 5% determined according to SCAN-CM 66:05. A part of the primary fines fraction of this pulp was separated using a laboratory scale pressure screen implemented at the Institute for Bioproducts and Paper Technology at Graz University of Technology.
Fig. 2 Micro perforated 100 l m hole screen installed in the laboratory pressure screen
Samples containing 30g dry matter (pulp ? primary fines) were diluted to a consistency of 1,5% w/w for disintegration according to ISO 5263-1. For subse- quent PFI refining the samples were thickened to a consistency of 10% w/w by filtration. To avoid loss of fines during filtration, the filtrate from thickening was added to the filter cake several times. After thickening the reference pulp and the pulp/primary fines blends were refined in a PFI mill for 1000, 4500 and 6000 revolutions (ISO 5264-2).
Methods
In order to separate the primary fines fraction, a laboratory scale pressure screen (Fig. 1) equipped with a 100 l m hole screen (Fig. 2) was used. The reference pulp was fed to the pressure screen at a volume flow _ V F of 10 l/min at a consistency of 0.5% (w/w). The accept passing the screen with a volume flow _ V A of 4 l/min and a consistency of approximately 0.015% (w/w), was collected in vessels for sedimen- tation. With this process 12.5 g dry of primary fines per 1 kg dry pulp was separated. The supernatant was decanted after adequate time for sedimentation to reach a dry content of 0.5% for subsequent applica- tion. The coarse fraction was discarded.
P ulp p ro p er t ies
Fibre morphology was determined using a L&W fibre Tester ? . This optical flow microscopy analyzer has an optical resolution of 3 : 3 l m = p ixe l . Samples contain- ing 0.1 g dry matter were analyzed and measurements were repeated three times. For each repetition a minimum of 100.000 particles was detected. Results are discussed based on length weighted fibre length distributions according to ISO 16065-2. The total fines content was determined using the Britt Dynamic Drainage Jar (BDDJ) according to SCAN-CM 66:05 using a 200mesh screen. The degree of beating of the samples was determined according to ISO 5267-1 (Schopper Riegler method). The separated primary fines were characterized using a light microscopy, based imaging method developed by Mayr et al. (2017a). In this method 5g of fines suspension at 0,01% consistency is prepared with deionized water. This sample is mixed with 0,06g of an emulsion working as a fixing agent for subsequent staining with methylene blue. This emulsion is prepared by mixing 5g of deionized water with 0.01g crude tall oil. The mixture is emulsified at 80 C in an ultrasonic bath for
P ulp / p rimar y fines b l ends
Samples with a total fines content of 9 and 12% w/w were obtained by adding the necessary amount of previously separated primary fines to the reference pulp having a primary fines content of 5% w/w.
Fig. 1 Laboratory scale pressure screen
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