SOFTWOOD BLEACHING PRACTICES INCANADA: ANALYSIS OF THE 2003PAPTAC BLEACHING COMMITTEE “BESTPRACTICES” SURVEY

摘要

ECF bleaching is now firmly established in Canada. Bleachedrnkraft pulp mills are no longer concerned with the transition to chlorine dioxide-based bleaching, but with optimization of chemical consumption and cost reduction.rnSince the last survey in 1999, softwood bleach plants haverndecreased total chlorine dioxide use by 5%. However, thernkappa number of the pulp entering the bleach plant has alsorndecreased: by 8% for oxygen-delignified softwood, and byrn7% for “conventional” softwood (I.e. without oxygenrndelignification). Furthermore, hydrogen peroxide use hasrnincreased by 13%. This means that although pulp mills arernsending less lignin into the bleach plant, they are actually using 5-6% more oxidizing chemical per unit of lignin, and to reach a lower final brightness than before.rnHowever, more detailed analysis of the data showed that most of these changes are primarily due to mills with “high” relative chemical consumption, especially in the case of the conventionally-delignified softwood bleach plants. When the mills were sorted and grouped by their overall Sequence Kappa Factor (the relative amount of ClO2 and H2O2 used in bleaching), there are clear differences between the “low” and “high” relative chemical consumption mills. For conventional softwood mills, the low relative consumptionrngroup had an average Sequence Kappa Factor of 0.34,rnessentially unchanged since the last survey, while the highrnrelative consumption mills have increased their SequencernKappa Factor by 8% to 0.44. For mills producing oxygendelignifiedrnsoftwood, both groups have increased their relativernchemical use by 9%: the low relative consumption averagernSequence Kappa Factor has increased to 0.42, while that of thernhigh relative consumption group has increased to 0.57.rnThese differences between the two groups equate to anrnaverage bleaching cost advantage of more than $5/BADMtrn(bleached, air-dry metric tonne of pulp) for the low relativernconsumption conventional softwood mills, and a costrnadvantage of more than $7/BADMt for the low relativernconsumption mills with O2 delignification. This is not due torna lower bleach feed kappa number; in fact, the low relativernconsumption mills with oxygen delignification bleach a pulprnwith 1-point higher kappa number on average, and the lowrnrelative consumption conventional softwood mills bleach arnpulp with an average 3-point higher kappa number, eachrncompared to the high relative consumption groups.rnFor each pulp type, one of the key differences between the tworngroups is a low Kappa Factor in the first bleaching stage forrnthe low relative consumption mills. This correlation isrnespecially strong for the conventional softwood mills. Inrnaddition, analysis of the relative amount of chlorine dioxidernused for pulp brightening, compared to the post-Eop-stagernkappa number, and the pH control strategies in the D1 stage,rnsuggest that the high relative consumption mills mayrnexperience more carryover from the Eop stage washer into thernD1 stage.rnOverall, the following additional trends have also beenrnobserved since 1999:rn? More bleach plants with on-line Kappa Analyzers; 75% ofrnmills with oxygen delignification and 50% ofrnconventional lines now have Kappa Analyzers. However,rnthey are not yet being fully-utilized; less than 50% ofrnmills are using some form of Kappa Factor Control in thernD0 stage, and many mills are still controlling the D1 stagernmanually.rn? An increase in the D0 stage Kappa Factor from 0.22 torn0.26 for bleach plants processing oxygen-delignifiedrnsoftwood.rn? An increase in Eop pressure for bleach plants withoutrnoxygen delignification.rn? More high-shear mixers have been installed in Eop stages,rnand in bleach plants which previously used MC pumps forrnD1 stage ClO2 mixing.rn? Final brightness has decreased by about one point.rnOn average, bleached softwood kraft pulp mills haverndecreased the kappa number of the pulp entering the bleachrnplant, and have also decreased the final brightness of the pulprnleaving the bleach plant, but have not proportionally decreasedrnchemical use, despite the installation of more equipment andrntechnology to decrease chemical consumption. In 1999, thernsurvey team reported a slight increase in the Sequence KapparnFactor since the 1996 survey, and this trend has surprisingly continued.