In kraft pulping, an alkaline solution of sodium hydroxide and sulfide is used to delignify wood. The dissolved wood components and pulping chemicals are washed from the solid pulp after cooking. This resulting solution is 85+% water and is called black liquor. The first step in recovering the spent pulping chemicals is evaporation of water from black liquor so that it contains less than 35% water. Scaling and fouling of heat transfer surfaces are the major issues by far for black liquor evaporators. Limiting evaporation capacity, plugging flow passages, and frequent cleaning are examples of the consequences of scale build-up. Two historic surveys showed the scales of sodium carbonate and sodium sulfate double salts are the primary contributor to evaporators fouling. Three types of sodium double salts have been identified in black liquor; which can be burkeite (nominally 2Na2SO4.Na2CO3), sodium sulfate carbonate (nominally Na2SO4.1.5Na2CO3), and sodium sulfate dicarbonate (nominally Na2SO4.2Na2CO3). Which double salt precipitates out depends on the ratio of carbonate to sulfate ions in the liquor. The solubility limit for the carbonate rich salts is higher than for burkeite in water-salt solutions, but there is limited data for black liquor for the carbonate rich salts. Due to the improvements in the chemical recovery cycle or kraft pulp mills, the black liquor composition has changed over time to carbonate-rich as supposed to sulfate-rich. This study determines the solubility data for sodium-carbonate-sulfate system in black liquor with an emphasis on the regions where data is lacking. The effect of Na2CO3 to Na2SO4 ratio, the effect of sodium concentration, and the effect of temperature on solubility will be investigated. At 115oC and 0.77 initial molar ratio of (CO3/CO3+SO4), the onset of carbonate-sulfate precipitation is found to take place at 53%. As the dry solids is increased, the concentration of soluble sulfate and carbonate decreases and the change in their concentration is used to estimate the composition of the solids precipitated. An empirical model will be derived from the newly generated data to predict the solubility of Na2CO3 and Na2SO4.

Solubility of Na-CO3-SO4 salts in Kraft Black liquor

ACS Spring 2022

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