Separation and recirculation of bulk crystals to potentially mitigate sodium salt scaling in black liquor evaporators

Abstract

Black liquor contains high concentrations of sodium carbonate and sodium sulfate. In black liquor concentrators, these salts begin to crystallize and have the potential to rapidly form scales on the surfaces of heat exchangers. The highest potential for scaling is when a supersaturated liquor begins to nucleate, which occurs when no crystals are available in the bulk. A mitigation strategy used in some mills is to recirculate thick liquor, which increases the bulk crystal concentration. However, a more efficient solution would be to separate bulk crystals and add them selectively to mitigate scaling directly. This study investigated the separation of bulk crystal from black liquor using a hydrocyclone. Separation was shown to be possible and was proven for a dry solids content of up to 65%, which corresponded to a viscosity of 20 mPa·s at 120°C. The two most important parameters relating to separation were viscosity and flow velocity. The cyclone Reynolds number encompasses both of these parameters and can be used to estimate the degree of separation. An implementation strategy for bulk crystal recirculation has also been developed and discussed. The recirculation of bulk crystals is most beneficial when operating close to critical solids and when reseeding crystals after cleaning.