Sodium salt scaling in black liquor evaporators and the effects of lignin separation

Lignin separation is an attractive way to create additional value from wood in kraft pulp mills, but its effects on black liquor evaporation must be understood before implementation. In this study, black liquor and filtrates from pilot-scale lignin separation were combined to represent different lignin separation scenarios, and the resulting liquors were evaluated in evaporation experiments using a newly developed scaling test rig. Lignin separation was found to lower the dry solids content at which precipitation starts and to increase Na–CO 3 –SO 4 salt scaling, primarily by increasing the combined carbonate and sulphate content. Scaling increased strongly with these ion concentrations, whereas no clear independent effect of lignin content, wood extractives, or the molar ratio CO 3 /(CO 3 +SO 4 ) was identified from the experimental data. Although most of the Na–CO 3 –SO 4 salts crystallised in the bulk phase, even a small fraction precipitating on the heat transfer surface caused measurable fouling, highlighting the importance of bulk crystallisation in industrial evaporators. The experimental setup produced consistent results and provides a useful platform for future studies of continuous crystallisation under industrially relevant conditions. The results show that lignin separation can increase evaporator scaling unless the additional salt load is considered in evaporator design and operation.