Pulp bleaching method - involves multistage bleach sequence using ozone, oxygen@, hydrogen peroxide and chlorine di:oxide or peracetic acid

摘要

The bleaching sequence consists of a first stage in which ozone is used, a second stage in which the pulp is alkaline-extracted in the presence of oxygen and possibly hydrogen peroxide, and at least one final stage in which chlorine dioxide and/or peracetic acid are applied. Ozone is added in the first stage in an amt. of 1 - 20 kg., pref. 2 - 8 kg per ton of dry pulp, chlorine dioxide, when used, in an amt. of at most 20 kg., pref. at most 15 kgs., dry pulp reckoned as active chlorine, and peracetic acid, when used, in an amt. of at most 30 kg., pref. at most 20 kg per ton of dry pulp. Pulp with a consistency of 6 - 15% is treated with acid and then in the first stage with a oxalic acid ina ddition to ozone, whereby oxalic acid and ozone are added to the pulp when it is in a fluidised state. No washing occurs between the addition of these two agents. Oxalic acid is added in an amt. of 2 - 20 kg., pref. 5 - 10 kg. per ton dry pulp, producing a pulp with a lightness of at least 85% ISO, pref. at least 87% ISO.