Black Liquor Properties, Crystallization, and Sodium Salt Scaling in Concentrators, 2001 Pulping Conference Proceedings

T. N. Adams--Sodium salt scale formation on heat transfer surfaces is a common problem in black liquor concentrators. The dry solids content of black liquor fired in recovery boilers is between 60% and 85%. This is well above the dry solids level where the sodium salts in the liquor begin to precipitate. Precipitation either can occur on sodium salt crystals existing in the liquor or on the heat transfer surfaces, depending on the level of supersaturation of the salts. To avoid equipment fouling due to precipitation on the heat transfer surfaces, an amply concentration of sodium salt crystals in the liquor is required.

Liquor chemistry and concentrator design affect the formation and growth of sodium salt crystals. Very little data is available on black liquor sodium salt crystallization at this time. The purpose of this paper is to examine information that is available on black liquor properties and concentrator design in order to identify operating strategies to reduce sodium salt scaling, and methods for troubleshooting scaling problems when they occur.

The available information on black liquor inorganic chemistry is briefly reviewed, along with the basic concepts of crystallization. Both nucleation and crystal growth are identified as potential limitations to developing adequate sodium salt crystal concentration in black liquor. The black liquor organic chemistry and physical properties are then examined with respect to their potential impact on sodium salt nucleation and crystal growth. This information is then put first into a check list for good operating strategies for reducing fouling and second into a check list for troubleshooting potential causes when scaling rate increases.