Detailed characterization of poor settling green liquor dregs

At a northern bleached softwood kraft (NBSK) mill in western Canada, poor settling green liquor dregs caused high non-process element levels in lime mud and white liquor pressure filter plugging. Dregs samples were collected during poor settling and normal settling conditions. Samples were examined by qualitative analysis, elemental analysis, quantitative X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscope/energy dispersion X-ray (SEM/EDX) spectroscopy. Poor settling dregs were caused by an inorganic gelatinous material. The inorganic gel was determined to be an amorphous magnesium silicate compound of approximate composition Mg2(Si1-xAlx)O4, with a molar ratio of silicon to aluminum of approximately 5:1. The density of the inorganic gel was only slightly higher than the green liquor, causing it to settle very slowly. When calcite particles were trapped by the gel, the average density increased, which increased the settling rate. The inorganic gel was present during normal settling, but contained more aluminum (silicon to aluminum ratio of approximately 2:1). During normal settling, the gel was more dense and contained more trapped particles of calcite.