Practical Method for Studying NPE in a Kraft Mill, 1998 International Chemical Recovery Conference Proceedings

Closing the water circulation at a kraft pulp mill removes one of the exit routes for NPEs (non-process elements): the bleach plant effluent. A simple method has been developed for investigating the distribution of NPEs in pulp mills to provide background information prior to any process changes. The distribution of NPEs can be determined by combining process data with analytical data from three sampling points. The samples required are unbleached pulp entering the bleach plant, weak black liquor going for evaporation, and white liquor. Also required are flow data for weak black liquor and total white liquors for cooking and oxygen delignification as well as production rate. Using this data, it is possible to calculate the amounts of NPEs that 1) enter the bleach plant, 2) enter the recovery cycle, and 3) return from the recovery cycle. It is then possible to calculate the distribution of NPEs in the fibre line between pulp and black liquor, and to calculate the NPE reduction in the recovery cycle. An estimate can also be made of certain NPE contents of chips. This method has been used to study 16 different elements at all kraft pulp mills in Finland. The main results from these investigations are presented and discussed in this paper.