Flocculants for precipitated calcium carbonate in newsprint pulps

The retention of precipitated colcium carbonate (PCC) in newsprint pulp (15 wt.% semibleached kraft + 85 wt. % peroxide-bleached mechanical) using polymeric flocculants was investigated. Both commercially available and experimental polymeric flocculants were evaluated, and the combination of cationic polyacrylamide and bentonite was found to be the most effective commercial retention aid. However, the experimental poly(AM/MPEGMA), a copolymer of acrylamide and methoxy poly(ethylene glycol) methacrylate, when used in conjunction with phenolic resin (PFR), was the most effective retention aid. The retention of precipitated calcium carbonate by the PFR/poly-(AM/MPEGMA) system was further increased by pretreatment of the pulp furnish with Na2CO3 or by using acid-tolerant PCC instead of conventional PCC. Both Na2CO3-treated pulp and acid-tolerant PCC have lower equilibrium concentrations of Ca⁺⁺ ions.