Decreased water usage in a softwood ECF bleaching sequence— full mill simulations, TAPPI JOURNAL June 2018

ABSTRACT: In this study, an elemental chlorine free (ECF) bleach plant with a D0(EOP)D1(EP)D2 sequence was studied with the aim of identifying options for significantly decreasing (fresh) water usage in the bleach plant and decreasing the effluent volume. A base-case simulation model for a softwood kraft market pulp mill was made based on a reference model representing the best available techniques as well as data produced in an extensive  laboratory pulp bleaching study. This model was used to evaluate increased closure within the bleach plant and the recirculation of bleach plant effluent to the brownstock system and their effects on both the bleach plant and the recovery cycle. The results indicate that it is possible to reduce the fresh water consumption from 15 metric tons/a.d. metric ton in the base case to about 2 metric tons/a.d. metric ton, without increasing the carryover of chemical  oxygen demand (COD) to the pulp machine. Nonprocess elements in wood contribute to the levels of metals found in the bleach plant and thus to the risk of  precipitates such as calcium oxalate, barium sulfate, and calcium carbonate. The risk of precipitates forming is a key factor determining the possible degree of closure. In addition, chloride concentration in the black liquor is another important factor that is affected by recirculating bleach plant filtrate to the brownstock washer and by the grade of the sodium hydroxide used in the mill.

Application: The intention of this work is to give input for possible steps than can be taken in order to reduce the water use in a pulp mill applying ECF bleaching technology.

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Author: Pia Jour, Karin Lindgren, Katarina Gutke, and Johan Wallinder
Decreased water usage in a softwood ECF bleaching sequence—
Decreased water usage in a softwood ECF bleaching sequence— full mill simulations, TAPPI JOURNAL June 2018

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