Addition of Sodium Silicate and Chelant to the FAS Stage to Bleach Recycled Fibers, 1999 Pulping Conference Proceedings

G. J. Kang & Y. Ni

Limerick Pulp and Paper Centre

University of New Brunswick

A.R.P. van Heiningen

Department of Chemical Engineering

University of Maine

A reducing agent is usually needed in bleaching recycled fibers to remove dyes. In such an application FAS (Formamidine Sulfinic Acid) is more effective than sodium hydrosulfite . An optimum FAS treatment is conducted at an initial pH of about 10, which is usually reached with the addition of sodium hydroxide. In this paper we report that when sodium silicate instead of sodium hydroxide is used for pH control during the FAS treatment, a better bleaching result can be obtained. This is explained by the reduced FAS decomposition since sodium silicate can stabilize the transition metal ions present in the system. Furthermore, when both chelant and sodium silicate are added to the FAS stage, followed with a peroxide stage (a FP sequence), the final brightness of the resulting pulp is higher than the control FP sequence. The improved bleaching in the peroxide stage is a result of enhanced removal of transition metal ions due to the combined actions of reduction and chelation in the FAS stage, which precedes the peroxide stage.