Irreversible Black Liquor Viscosity Reduction by High Intensity Shearing, 1998 International Chemical Recovery Conference Proceedings

The benefits of firing high solids black liquor in the recovery boiler are well established. Unfortunately, the high viscosity of black liquor at high solids ( greater than 75 -80%) is a limiting factor in conventional recovery boilers. Improved black liquor processability and the possibility of increasing solids content by high shear mixing has been demonstrated in pilot scale tests on both hardwood and softwood black liquors. Shear rates of about 25,000 reciprocal seconds at solids concentrations from about 60 to 70% resulted in permanent viscosity reductions of up to 90%. Plant scale tests have been performed in a high-shear mill at flow rates of about 37 Us (SW gpm) and at shear rates of at least 100,000 reciprocal seconds. At solids concentrations of 40 to 45%, and at process temperatures from 100 to 170 °C, permanent viscosity reductions (about 15 - 20% measured at 90 °C were achieved in short term tests. Preliminary data suggest that the viscosity reduction at constant temperature increases as the solids content increases These positive results have justified long term tests which are planned in for the Spring of 1998.