CHARACTERIZATION OF HARD SCALE FORMED IN THE KRAFT MILL GREEN LIQUOR PROCESSING EQUIPMENT

摘要

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 (Na_2CO_3·CaCO_3·2H_2O) 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 (CaCO_3). 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 Na_2CO_3 from the pirssonite scale during the time when the green liquor TTA was low, leaving the insoluble CaCO_3 behind.