Paper Bleaching Auxiliaries
These products enable a more efficient bleaching of the chemical, mechanical and deinked pulps and lead to increased whiteness. They are generally based on sur¬face active agents, i. e. wetting agents (see Section 3.6.7.) which are resistant to bleaching chemicals as well as chelating (complexing) agents (see Section 3.6.3.). Some of the transition metals (e. g. Fe, Mn, Cu) in pulp can reduce the bleaching effect due to catalyzed decomposition reactions of dithionite and the re-oxidation of reduced chromophore groups.
Chelating agents also prevent transition metal ions from catalyzing peroxide decomposition. Since the beginning of peroxide bleaching of mechanical and deinked pulps, sodium silicate (Na2O 3–4 SiO2) has been used as an additive. Addition of 0.5 to 3 % (41 °Be’ solution calculated on dry pulp) stabilized peroxide, acts as a buffer and has the ability to form metal com¬plexes. Small amounts (0.05–0.1 %) of magnesium sulfate (usually in the form of Epsom salts, MgSO4 7H2O) are commonly included in the peroxide bleach liquor or added separately. Magnesium acts as a stabilizer of peroxide, probably together with sodium silicate.