Handling Alkali and Sulfur Balances in Closed-Cycle Bleached Kraft Pulp Mills -Options, Technology and Costs, 1996 Minimum Effluent Mills Symposium Proceedings

Björn Warnqvist

AF-IPK AB

Recycling bleach plant effluent to the kraft recovery system typically results in an excess of sodium and/or sulfur in the chemical balance of the mill. This is an important operational problem, requiring at least as much long-term attention and solutions as problems associated with NPE’s (non-process elements) accumulation. Case studies show that controlled removal of sodium (Na) and sulfur (S), primarily discarding a portion of the recovery boiler precipitator catch would be a typical re-quirement, assuming the rest of the mill has low losses. These studies refer primarily to TCF bleaching and recycle. The purge requirement would depend on the charge of bleach chemicals (NaOH, H2 S04 , etc.) and on the relative amount of fully oxidised white liquor used as alkali in the bleach plant. In some cases, purge of sulfur-free alkali (e.g., soda from green liquor), or sulfur (e.g., by heat treatment of black liquor) also may be required. Technologies for these operations are under development, and are being introduced industrially. These purge processes involve investments of the order of 3-15 MSEK (0.5-2.5 million USD, MUSD) in a kraft mill of 1000 ADt/d, and...