Journal of Material Cycles and Waste Management (2025) 27:1901–1913
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report 0.96–1.42 μm for thin fiber and 2.15–3.3 μm for thick fiber wheat straw [20]. These differences in coarseness and estimated fiber wall thickness are expected to correlate with the maintenance of average fiber length in the face of mechanical actions during recycling.
Response of Pulps to PFI Refining
It is interesting to understand the effect of refining on the freeness of each pulp. The freeness has a strong correlation with the fine content in a pulp. It is expected that pulps that degrade rapidly with respect to mechanical refining will also be broken during recycling cycles (due to pulping, pump- ing, screening, refining, etc.) forming fines and smaller- length fibers. These fines and small fibers are expected to be washed away from the good pulp stock during recycling and papermaking processes and eventually be removed dur- ing water clarification and end up as sludge. To investigate, the pulps were refined with a PFI mill at different levels and the freeness (Figure S1) and fiber properties (Table S1) determined. Refining had the expected result of densifying the sheets produced from the pulps, increasing the bond- ing area, and thus improving the physical properties of the sheets (Table S1). Refining also considerably increased the water retention value (WRV) of the pulps, which indicates higher water content in the fiber wall which leads to more flexible fibers and more extensive bonding and improved physical paper properties. The UBKP had a starting freeness of 740 mL CSF for the unrefined pulp and decreased to 662 mL CSF at the 9,000-revolution refining condition, corresponding to an 8.7 × 10 –3 mL CSF/revolution decrease. In contrast, SCHW and wheat straw pulps showed orders of magnitude greater freeness drops versus refining level. The SCHW had a reduc- tion of freeness from 707 mL CSF for unrefined condition to 458 mL CSF at the 4,500-revolution refining condition or 5.5 × 10 –2 mL CSF/revolution). The WS dropped from 437 mL CSF to 147 mL CSF for 750 revolutions (3.9 × 10 –1 mL CSF/revolution decrease). This data suggests that the WS and SCHW are more likely to break during recycling rela- tive to UBKP. Additionally, the generation of fines (as defined by the FQA instrument as particles between 70–200 microns) was tracked versus energy input for the refining experiments. The length of weighted fines content did not appreciably change for the UBKP but increased by 67% for the wheat straw, and by 106% for the SCHW due to refining, in agreement with the changes in freeness (Figure S2). More significant decreases in fiber length are also observed for the wheat straw relative to the SCHW and UBKP in Fig. 2. This clearly indicates the fragile nature of WS pulps and would indicate that during recycling the yields of the WS would be expected to be much lower than the UBKP or SCHW.
Fig. 2 Changes in length weighted average fiber length versus energy consumption for UBKP, SCHW, and WS pulps
Recycling Yield of the Pulps with Heavy Refining
Multiple recycling cycles as per Fig. 1 were conducted to reflect the relative yields of the different types of pulps on recycling. This lab-scale recycling incorporates mechanical actions on the fibers such as in the disintegration and the PFI refining (4,500 for the virgin pulp and 1,500 for each recy- cling cycle) that attempts to mimic the forces during indus- trial recycling. Table 2 shows the energy consumption of some industrial recycling operations and relevant laboratory methods. It can be observed that a typical industrial pulper requires 1.75 times higher energy consumption per ton of fiber (392 kWh/ metric ton) than the Technical Associa- tion of the Paper and Pulp Industry (TAPPI) disintegration for dispersing fibers. Industrial operations may not have as efficient use of energy and also have contaminants present, such as plastic, metal pieces, and glue that can complicate the pulping process, but are not present in lab-scale work. It is acknowledged that the geometry, gaps between moving parts, velocities, etc., are different between lab and indus- trial environments. In general, the 4,500-rev PFI used in this study is in the order of magnitude as industrial pulping and the 1,500-rev PFI is similar to the normal old corrugated container (OCC) refining. Kerekes [21] defined the term of “specific intensity” which is refining intensity as the energy expended on a fiber during one impact with a refining bar and showed that PFI mill is a high-energy refining device but it is very low intensity, which is approximately one-tenth of an industrial refiner. Therefore, the results herein are not expected to be a perfect match to the widely spread industrial mechanical actions expected but are considered to be reasonable and within a meaningful range. It was decided to use these lev- els of energy input so that differences can be recognized between the pulps. Later in the research lower energy levels
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