Effects of White Water Fractionation by Two-Step Hydrocyclone Cleaning on Drainage and Strength Properties of OCC Recycled Paper

In recycled old corrugated container (OCC) papermaking, the accumulation of ash and fines in white water adversely affects dewatering efficiency, runnability, and paper strength.Excessive inorganic ash interferes with fiber-to-fiber bonding, whereas organic fines can contribute to sheet consolidation and strength development.An effective white water treatment strategy should therefore selectively remove detrimental ash while retaining fines beneficial to paper properties.In this study, OCC process white water was fractionated using a hydrocyclone system, and two methods were compared: fractionation using two accept-recirculation cleaning loops and fractionation using a Tetradeca-cleaning system followed by an accept-recirculation cleaning loop.The distribution of suspended solids was evaluated through thickening factor and ash content analyses, and the fractionated white water streams were reintroduced into recycled OCC furnish to assess drainage and paper strength.Freeness and wet-web air permeability resistance were measured as dewatering indicators, and burst index, compressive index, and internal bonding strength were evaluated for handsheets prepared with each fraction.Both methods achieved effective separation, concentrating ash in reject streams while accept streams retained relatively lower ash content.Both methods also improved drainage, particularly when secondary-pass cleaner reject fractions were applied, owing to the higher proportion of coarse and highdensity ash particles favorable for water removal.Paper strength was generally improved when secondarypass cleaner accept fractions were added, indicating that fines beneficial to fiber bonding were selectively retained.Among all conditions, the secondary-pass accept fraction exhibited the highest burst, compressive, and internal bonding strengths.Comparing the two methods, the Tetradeca-cleaning system followed by an accept-recirculation cleaning loop provided more stable and greater strength improvements than the two accept-recirculation cleaning loops, although both exhibited comparable fractionation efficiency and drainage enhancement.This was attributed to the Tetradeca-cleaning system's ability to preserve useful fines more effectively while progressively removing strength-reducing ash particles.These results indicate that two-step hydrocyclone treatment of OCC white water can be an effective strategy for improving recycled paper quality.The two accept-recirculation cleaning loops offer advantages in operational simplicity, whereas the Tetradeca-cleaning system followed by an accept-recirculation cleaning loop is more beneficial when im-