BLEACHING OF EUCALYPT KRAFT PULPS FROM AN ENVIRONMENTAL POINT-OF-VIEW

Modern bleaching sequences were used to bleach kraft pulps from mature and plantation eucalypts and their effect on the pulp and efflient qualities were evaluated. Kraft pulps were prepared from three export woodchip samples and two Eucalyptus globulus plantation wood samples. All the kraft pulps were easily delignified under oxygen bleaching conditions and their responses were similar. The bleaching sequence O(D70C30)(EO)DD was used as the reference sequence. The pulps from the plantation woods were bleached to a higher brightness (91% ISO) than those from the mature woods (88-90%) using the same bleaching sequence and the same chemicals. The use of 100% chlorine dioxide also gave pulps with high brightness levels and there are no technical reasons why this sequence cannot be used to bleach eucalypt kraft pulps. The major part of the total AOX was formed in the first bleaching stage (DC) or D, whereas most of the chlorate is formed in the DD stages. The AOX level in the untreated filtrate was in the range 0.7-0.9 kg/o.d.t of pulp with 70% chlorine dioxide substitution and 0.4-0.7 when 100% chlorine dioxide was used. The Germgard equation over-estimated the amount of AOX formed during bleaching of the pulps. The effect of carryover of oxygen-bleaching filtrate (equivalent to 10 kg COD/t pulp) with the (DC)(EO)DD sequence was to increase the AOX in the filtrate by 0.13 kg/t and slightly lower the pulp brightness.