Experience in Evaporation of TCF Bleach Plant Filtrates, 1995 Pulping Conference Proceedings

Olli Välttila, Taina Pöllänen

The Finnish Pulp and Paper Research Institute

Erkki Kiiskilä. Marjo Kuusio

A. Ahlström Corporation

Water balances at kraft mills will require substantial reductions in the volume of bleach plant filtrates before the filtrates can be recycled to the chemical recovery system. In aiming at a closed-cycle kraft mill, one way to achieve the necessary water balance would be to treat the filtrates to produce process water clean enough for USC in final washing of bleached pulp or on the pulp drying machine. In this study bleach plant filtrates at a TCF kraft mill were evaporated in a pilot-plant evaporator. Evaporation of chelating stage filtrates as well as alkaline filtrates and their mixes was tested during both softwood and hardwood pulp production. The results obtained during trials showed that filtrates from TCF bleaching are easy to evaporate. A dry solids content of about 30% can be achieved by multiple-effect low-temperature evaporation using only waste heat from a kraft mill. Viscosity or boiling point rise do not limit their further concentration to higher dry solids content but precipitation during evaporation should be taken into consideration. The condensates from evaporation are clean enough to be used in the drying of bleached pulp or in the last washing stages in bleaching. However. because of methanol in the filtrates the condensates have to be steam stripped.