Effects of calcium on peroxide bleaching and washing of thermomechanical pulp

Since the white water from the paper machine is currently reused in mechanical pulping, high concentrations of calcium ions released from fillers and coating pigments enter the mechanical pulping process. Calcium ions may interact with wood-derived anionic substances, such as pectic acids that are formed and released in peroxide bleaching. In this study, the influence of calcium on retention of pectic acids in pulp, drainage of pulp, and sheet brightness was studied. Presence of calcium ions strongly increased the retention of pectic acids and wood pitch in pulp. Thus, the cationic demand of thermomechanical pulp (TMP) water was much lower when calcium was present. By washing the pulp at pH 8 before final acidification, the anionic dissolved and colloidal (DisCo) substances were effectively removed especially at low calcium concentrations. Calcium ions were washed out to a larger extent at pH 5.5 than at pH 8. Small amounts of calcium slightly elevated the brightness of peroxide-bleached TMP, but higher calcium concentrations did not increase the effect. The drainage of pulp was only moderately affected by addition of calcium.