Caro's Acid Brightening of Chemical Pulp, 1998 Pulping Conference Proceedings

Caro’s acid can be used in alkaline pH as in an extraction stage (E) of a DEopDED bleaching to improve pulp brightness. This is analogous to peroxide in the E-stage. A brightness increase as high as 13% ISO was obtained when a DEopD pulp having a brightness of 64.5% ISO was treated with 0.15% Caro’s acid as active oxygen (AO) in the following E-stage. Under similar conditions 0.2% peroxide (as H2O2) gave a moderate brightness increase of about 9 points. However, most of the brightness gained in the extraction stage was lost when a final D-stage was carried out, resulting in only 2% overall increase in brightness in the final pulp. On the other hand, when 0.2 - 0.3% Caro’s acid (as AO) was applied to a DEopD bleached, 81.5% ISO pulp, a brightness of about 89 - 89.5% was achieved. Similarly, when 0.05-0.1% Caro’s acid (as AO) was applied as a final treatment as in high density application to a DEopD bleached, 86.7% ISO pulp, a brightness gain of about 2-2.7% ISO was achieved. About the same magnitude of brightness gain was obtained from an equivalent amount of peroxide application as well. However, the major advantages of using Caro’s acid in the high density chest are maximum brightness in short time with very little or no residual chemical in the pulp and the final pH remaining in the near neutral range. The benefits of Caro’s acid outweighs peroxide and makes Caro’s acid a suitable bleach chemical for final stage bleaching and for High Density bleaching where control for a fixed retention time is not feasible.