Pirssonite, a double salt of sodium and calcium carbonate (Na₂CO₃.CaCO₃.2H₂O) often deposits in smelt dissolving tanks, green liquor lines, clarifiers and storage tanks; the scale has a stratified appearance. The scaling tendency depends on the concentration of total titratable alkali (TTA) in the green liquor, the liquor temperature, the calcium concentration, and the efficiency of mixing in the dissolving tank. Pirssonite forms at temperatures above 35 C in solutions containing high concentrations of Na₂CO₃, other sodium salts and calcium (i.e., CaCO₃). The solubility of pirssonite decreases as the NaOH concentration increases. Other sodium salts have the same effect as NaOH. The solubility also decreases as green liquor cools. The change in Na₂CO₃ solubility is about 0.5 lb/ft3 (8 g/L) as Na₂O per 10 C change in temperature*.

Pirssonite solubility curves (Na₂CO₃ concentration versus other Na salts concentration) are available at different temperatures. These curves can be used to determine the maximum TTA level at which the pirssonite will precipitate in green liquor (TTA sat). This level will vary from mill to mill because of differences in the composition of the green liquors. To find out the TTA sat, the mill operating line has to be defined. The intersection of this line with the solubility curves determines the upper limit for green liquor concentration. It is recommended that the operating TTA max should be 5-10% below TTA sat.

* Frederick, Jr., W.J. et al, “Pirssonite Deposits in Green Liquor Processing”, Tappi J., 73(2): 135-140, 1990.