How to improve bleached pulp yield measurement

Kraft pulping yield can be improved by using yield-preserving chemicals suc h as anthraquinone or polysulphide. It is not clear from the literature, ho wever, how much of the yield increase from improved pulping is maintained t hrough bleaching because it is difficult to do direct gravimetric measureme nt accurately and reproducibly. In this paper we evaluate a laboratory grav imetric procedure and its precision, and make recommendations on how to mea sure a meaningful yield from a bleaching sequence. We found that variabilit y in handling procedures caused most of the variation in bleaching yield me asurements and that careful pulp homogenization before measuring the pulp c onsistency was important. We also found that fines loss, occurring when was hing is done without filtrate recycling, caused a loss in yield of 0.38%. T he 95% confidence limit of the yield obtained after full bleaching using tr iplicate determinations was +/-0.70% for the OD(0)EoDED sequence and +/-0. 65 % for the OD(0)EoDED sequence. These 95% confidence limits can be decrea sed to +/-0.25% by increasing the number of determinations to 8.