ECF bleaching practices in Canada-part 2: Analysis of paptac bleaching committee survey for softwood kraft pulps

Chemical pulp bleaching in Canada continues to evolve. In 1988. bleach plants began adopting substantial substitution of chlorine dioxide for chlorine. In addition. mills began practicing complete replacement of chlorine with chlorine dioxide, so-called Elemental Chlorine-Free (ECF). The transition to chlorine dioxide-based bleaching (ECF) is essentially complete. In 1998, on a production-weighted basis, chlorine dioxide substitution averaged 96% for all bleached chemical hardwood and softwood pulp. Over 92% of bleach plants in Canada produced ECF in 1998, totaling an estimated 9.6 million tonnes of bleached pulp. ECF now accounts for 86% of Canadian bleached chemical pulp production. The balance is produced using high substitution of chlorine dioxide for chlorine in the first stags of bleaching, i.e.. greater than or equal to 50%. Oxygen delignification of softwood pulp is growing. In 1996, 28% of softwood kraft pulp was oxygen-delignified, In 1998, this figure grew to 39%, representing 3.3 million tonnes of production. Analysis of bleaching conditions shows a number of developing trends since 1996: An increase in post oxygen kappa no. (17.3 to 18.0) A doubling of the number of on-line kappa analyzers A modest increase in the number of medium consistency Do stages (typically associated with introduction of oxygen delignification) An increase in the temperature of the Eop stage (76 degreesC to 81 degreesC) Fewer mills using NaOH for DI pH control A doubling of the mills using hydrogen peroxide in the second extraction stage There has not been a significant change in the overall bleaching chemical consumption for conventionally-delignified softwood pulps. The average sequence kappa factor increased from 0.44 in 1996 to 0.46 in 1998. Bleaching costs are unchanged at similar to $51/ADMt (CAD). Similarly there has not been a significant change in the overall bleaching chemical consumption for oxygen-delignified softwood pulps. The average sequence kappa factor increased from 0.57 in 1996 to 0.59 in 1998. In addition, bleaching costs are unchanged at similar to $40/ADMt (CAD). Oxygen-delignified softwood pulps require more oxidizing equivalents per unit of unbleached pulp kappa number. In spits of this, on the average. oxygen-delignified pulps have a bleaching chemical cost advantage of similar to $11/ADMt (CAD) when compared to conventionally-delignified bleached pulps. An assessment of bleaching practices for both conventionally- and oxygen-delignified pulps, showed the difference between mills with relatively "high" chemical consumption and mills with relatively "low" chemical consumption was similar to $5/ADMt (CAD). In some cases, conventionally-delignified mills had the same bleaching cost as some oxygen-delignified mills. The data showed that the group of oxygen-delignified mills with significantly better bleaching efficiency had an average kappa no, of 20. The group of oxygen-delignified mills with the lower bleaching efficiency had an average kappa no, of 16. The bleaching cost For the 20 kappo no, pulps was $4.63/ADMt (CAD) lower than for the 16 kappa no. pulps due to the much better bleaching efficiency. This is a surprising result. Some contributing factors were identified such as carryover to the Do stage and D1 stages and mixing technology. The result also raises questions regarding pulp bleachability.