Characterization of Hard Scale Formed in the Kraft Mill Green Liquor Processing Equipment, 2011 PEERS Conference

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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.