Pilot-scale mechanical production of cellulose nanofibrils: Energy-structure trade-offs during disc refining.

Sustainable, chemical-free production of cellulose nanofibrils (CNFs) remains constrained by high mechanical energy demand during fibrillation. In this study, pilot-scale disc refining of bleached pine kraft (BPK) pulp was performed under water-only conditions (2 wt% consistency) using a Regmed MD-3000 refiner to evaluate the relationship between plate gaps, energy consumption, fibrillation behaviour, and suspension properties. The disc gap was progressively reduced from 10 mm to 0.2 mm in closed-loop recirculation mode. Specific energy consumption increased with decreasing gap, reaching an effective cumulative SEC of ~15,647 kWh/t over the 1.0-0.2 mm fibrillation window, with more than 60% of the energy input occurring below 0.4 mm. Brecht-Holl screening showed that the fines fraction increased from ~11% in unrefined pulp to ~88% after refining at 0.2 mm. SEM analysis confirmed a transition from sub-micrometre fragments to nanofibrils. Rheological gel-point analysis revealed a maximum effective aspect ratio (~618) at 0.4 mm plate gap, indicating peak network connectivity at intermediate refining intensity. Further refinement reduced effective aspect ratio due to fibre shortening. Nanofibre zeta potential remained approximately constant (~-15 mV), demonstrating that mechanical treatment altered morphology without modifying cellulose surface chemistry. These results quantify energy-structure trade-offs and identify a system-specific operational fibrillation window for scalable CNF processing.