Scaling, the formation of hard deposits, in green liquor handling systems is a persistent problem in many kraft pulp mills. Scaling is commonly believed to be a result of pirssonite (Na2CO3-CaCO3-2H2O) formation when the green liquor TTA is high. In this study, we characterized twelve scale samples obtained from ten kraft pulp mills using various analytical methods including TGA/DSC (Thermogravimetric Analysis/Differential Scanning Calorimetry), XRF (X-ray Fluorescence Spectrometry, XRD (X-ray Diffraction Spectrometry), SEM (Scanning Electron Microscopy), and EMPA (Electron Micro Probe Analyzer). The analysis identified only four of these samples as pirssonite; the remaining consisted mostly of calcite (CaCO3). The reason for the predominant presence of calcite in the majority of the scale samples is not known, although one may speculate that it was a result of selective dissolution of Na2CO3 from the pirssonite scale during the time when the green liquor TTA was low, leaving the insoluble CaCO3 behind.
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