Decreased Water Usage in a Softwood ECF Bleaching Sequence – Full Mill Simulations, 2017PEERS

In this study, an Elemental Chlorine Free bleach plant with a D(EOP)D(EP)D 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 brown stock 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 t/ADt in the base case to about 2 t/ADt, without increasing the carryover of COD to the pulp machine.

Non-process 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 brown stock washer and by the grade of the sodium hydroxide used in the mill.  

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