Experimental Determination and Modeling of Sodium Salt Solubility in High Solids Kraft Black Liquor

摘要

In this study, Na_2CO_3 and Na_2SO_4 solubility data were determined for aqueous solutions, a kraft mill black liquor, and the same liquor spiked with Na_2CO_3 to increase its sodium-to-carbonate ratio. The experiments were carried out in batch evaporators equilibrated at 120 and 140℃. The data obtained from multiple samplings through a 2 μm filter were highly consistent up to 71% total solids content, suggesting the experimental difficulties of determining solubility of highly viscous black liquor were overcome by the methods developed in this work. Solubility data were compared to predicted solubility behavior using a proprietary chemical equilibrium model. For aqueous solutions, the difference between measured and predicted species concentrations was ±15% for 30 of 35 determinations. For black liquors, there was good agreement with measured and predicted solubility of sulfate; agreement for carbonate solubility was not as good. Some of the deviation may be explained by limitations in the model, which predicts burkeite (with a fixed Na_2SO_4 / Na_2CO_3 molar ratio of 2.0) as the precipitating species at the solubility limit and a transition to pure Na_2CO_3 at higher solids levels, after the dissolved sulfate has been depleted. There is experimental evidence that the composition of burkiete can vary, and that a new carbonate-rich species (~2Na_2CO_3·Na_2SO_4) can form at high carbonate-to-sulfate ratios. In addition to updating the thermodynamics database with these species, further experimental work is required to evaluate a wider range of starting carbonate-to-sulfate ratios, total solids content, and temperature. As part of future work, the solids isolated by filtration should be rapidly purified and analyzed for a more complete understanding of the changing solid phase composition during black liquor concentration.