Towards kraft mill closure: influence of non-process elements on green liquor dregs settling and filterability

This study developed a laboratory method producing smelt and green liquor containing up to ten times normal levels of inorganic process contaminants. Green liquor dregs composition, settling behaviour and filtration behaviour were determined. Dregs settling rates for laboratory green liquors were compared to that of a base case green liquor (similar to current processes). Elevated levels of magnesium or calcium in green liquor enhanced dregs settling. Liquors containing elevated iron levels produced the greatest negative effect on dregs settling rate. Dregs filtration properties were determined through calculation of bed permeability K, calculated from Darcy's Law. Dregs permeability showed significant reductions from base case behaviour when silicon and iron contents were elevated. Validation of the laboratory simulation was completed by comparison with mill smelt and green liquor samples. Results showed the laboratory simulation produced green liquor similar to that produced in the mill process.