Solubility of sodium oxalate in kraft black liquors below 100°C

Precipitation of sodium salts from kraft black liquor leads to scale formation in the evaporation plants of kraft pulp mills. Sodium oxalate scaling is most prevalent in mills pulping tropical hardwoods such as eucalyptus and acacia, but it has also been found in the scales in high solids lines after the concentrators in mills pulping softwoods. There is very limited solubility data for sodium oxalate in black liquor, especially below 100°C. This study measures the solubility limit in four different black liquors from 14% to 47% dry solids between 50 and 90°C. The newly generated solubility data in black liquors enhance our understanding of sodium oxalate scale formation in low solids effects of the evaporation plant. An example of an application to a kraft pulp mill with sodium oxalate scaling in the seventh effect is given.