Operating experience with an ozone-based ECF bleaching sequence - Results have exceeded initial expectations

A medium-consistency ozone stage was incorporated in a (ZD) configuration a s part of a complete hardwood fibre line modernization at the Espanola mill . The major expectations from ozone were improved bleaching economy, a high er brightness ceiling, and reduced environmental impact. Operating results have exceeded expectations. Ozone has a chlorine dioxide replacement ratio of 3.5 (1 kg/t ozone replaces 3.5 kg/t chlorine dioxide) on hard to bleach species and a replacement ratio of 2.5 on easily bleached species, signific antly improving bleaching economy. Effluent AOX, COD and colour have decrea sed significantly. Pulp extractives and TOX content have decreased. Pulp cl eanliness is exceptional. Pulp strength and viscosity have been maintained with the new bleaching sequence.