Sustainability 2023 , 15 , 2850
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• Mixed wastepaper composed of unsorted wastepaper, formally classified as the entire spectrum of the second type of paper, that is medium-sized varieties. These included newspapers and printed office wastepaper, among others (ranked 3.19 according to theEN643 ‘List of European standard types wastepaper’) (five samples). The wastepaper was crushed manually (pieces of approximately 2–5 cm) and mixed to ensure that the sample was mixed homogeneously. The wastepaper prepared in this way was placed in the described PP foil bags, which were subsequently stored in barrels with tight covers to protect the samples from moisture and contamination. After mechanical shredding, the wastepaper samples were packed in tight containers and stored at a constant temperature of approximately 15 ◦ C. 2.2. Non-Fibrised Substances and Chemical Analysis of Pulp To determine the amount of non-fiberized substances in the tested wastewater, the rewetted pulp samples (22.5 g dry weight samples soaked in water for 24 h) were subjected to disintegration using a laboratory JAC SHPD28D propeller pulp disintegrator (Danex, Katowice, Poland) with 23,000 revolutions following ISO 5263-1 (2004). Non-defibered substances versus fibers and water were removed using a membrane screener (PS-114; Danex, Katowice, Poland) at an amplitude of 25 mm and a frequency of 2 Hz. The screener is equipped with a gap screen (gap width = 0.50 mm). To identify the dissolved substances in the tested wastepaper samples, the association between the amount of oxygen required for the oxidation of organic substances in the tested samples and that of soluble substances was used. The MERCK COD test was applied to quantify these substances. Organic compounds were oxidized following the manufacturer’s instructions. The amount of oxygen consumed to oxidize these substances was determined using a spectrophotometer (UV-1280; Shimadzu, Japan) at 620 nm. Analyses of the chemical composition of cellulosic pulp included the quantification of ash, extractives, holocellulose, and lignin. Ash content was determined using a gravimetric method in compliance with the Tappi T211 standard (ash in wood, pulp, paper, and paperboard; combustion at 525 ◦ C). The amounts of extractives were determined according to the Tappi T204 standard (solvent extractives from wood and pulp). Holocellulose content was determined according to Tappi Useful Method 249 (cellulose in pulp). As indicators of lignin content in the pulp, kappa numbers for dried pulps were examined according to ISO 302 (2015). The average polymerization degree of cellulose (DP) in the pulp was determined using viscometry following ISO 5351 (2010). All chemical analyses were performed in triplicate for each pulp sample. 2.3. Analysis of Fibre and Pulp Properties Pulps were characterized in terms of fiber dimension, fine content, water retention value (WRV), and freeness. The dimensions of fibers were measured according to ISO 16065-2:2014 using the Morfi Compact Black Edition apparatus (Techpap, Grenoble, France). WRV was determined according to ISO 23714:2014. Freeness was measured using the Schopper–Riegler apparatus (Thwing-Albert Instrument Company, West Berlin, NJ, USA), according to PN-EN ISO 5267-1 (2002). All analyses were performed on both unbeaten and refined pulp samples. 2.4. Pulp Refining Before processing, the pulp was soaked in water for 24 h. Then, wastewater was treated in the Danex JAC SHPD28D propeller pulp disintegrator (Danex, Katowice, Poland) according to PN EN ISO 5263-1 (2006) with 23,000 revolutions. The refining process was performed in the Danex JAC PFID12X PFI mill (Danex, Katowice, Poland), with a single batch of dried pulp (22.5 g), according to PN-EN ISO 5264-2 (2011). Pulp was refined to 30 ± 1 ◦ SR.
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