Estudo das causas de reversão de alvura de polpas kraft de eucalipto

Abstract

Some high brightness eucalyptus Kraft pulps have shown poor brightness stability. In most cases, the causes have not been identified and permanent solutions have not been found. This work focused on evaluating the brightness stability profile of pulps bleached by in sequences such as O(DC)(PO)DD, O(DC)(PO)DP, OD(PO)DD, OD(PO)DP, ODHT(PO)DD, ODHT(PO)DP, OA/ D(PO)DD, OA/D(PO)DP, OAD(PO)DD and O(Ze)D(PO). Brightness stability tests induced by according to Tappi UM200 procedure on samples bleached to 90±0.5% ISO. Brightness stability was measured after each bleaching stage of the various sequences and expressed as brightness loss in % ISO. The results indicate that pulps bleached with sequences ending with a peroxide stage have higher brightness stability compared to those ending with a chlorine dioxide stage. Pulps bleached with a standard sequence, initiating with a (DC) stage, show brightness stability similar to that of pulp bleached by an ECF (Elementary chlorine free) sequence initiating with a regular D0 stage. ECF sequences, initiated with hot stages produce pulps with higher brightness stability than sequences initiating with a regular D0 stage. The profile across the bleaching sequences shows a tendency of increased brightness stability in alkaline stages containing peroxide and decreased stability in those stages containing chlorine and/or chlorine dioxide, paralleling pulp carbonyl group content.