Elemental Chlorine-Free Bleaching of Soda, Kraft, and Sulphite Pulps

Abstract

Soda pulp cooked from rapeseed straw was subjected to a four-stage elemental chlorine-free bleaching under laboratory conditions using chloride dioxide, hydrogen peroxide, and peracetic acid as bleaching agents. The alkali extraction enhanced with hydrogen peroxide addition followed the first chlorine dioxide delignification. For comparison, an oxygen-predelignified kraft softwood and sulphite spruce pulps, as well as once-dried kraft softwood pulp were undergone to the same bleaching sequence DEpPPaa. The brightness, zero-span breaking length, and degree of polymerisation were measured for unbleached and bleached pulps. The preliminary results showed that bleachability of soda rapeseed pulp was lower in comparison with kraft softwood pulps and sulphite spruce pulp. The final brightness of 80.0, 84.1, 83.4, and 88.3 % ISO was achieved for soda, once-dried kraft, oxygen-predelignified kraft and sulphite pulps, respectively. However, an increase in brightness was accompanied by a decrease in strength of soda rapeseed pulp fibres. The zero-span breaking length decreased from 4.2 km to 3.4 km for soda pulp, while, for oxygen-predelignified kraft softwood and sulphite pulps, a noticeable decrease in fibre strength was not found.