Characterization of hard scale formed in the kraft mill green liquor processing equipment

Abstract

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 total titrateable alkali (TTA) is high. In this study, we characterized 12 scale samples obtained from 10 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 microprobe 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. It may be a result of selective dissolution of sodium carbonate (Na2CO3) from the pirssonite scale during the time when the green liquor total TTA was low, leaving the insoluble CaCO3 behind.