Second Critical Solids Black Liquor Scaling, 2000 Engineering Conference Proceedings

Jason B. Smith, Dr. Jeffery S. Hsieh--As the pulp and paper industry continues to move forward in its efforts to maximize available raw materials, many processes are being pushed beyond their design maximums. This is especially true for a kraft pulp mill’s evaporator set, which is being asked to produce black liquor with solids contents as high as 80% on a continuous basis. At this level of operation inorganic salt scaling remains a serious impediment to continued operation of high solids evaporators and concentrators. Burkeite scales common at lower concentrations, are being replaced by more tenacious sodium carbonate scales that are yet to be fully understood.

This research used bench-top falling film evaporators to do a preliminary investigation into a method of studying scaling after second critical solids (the transition between the precipitation of burkeite and the precipitation of sodium carbonate). Batch evaporation was used to slowly concentrate black liquor through both first and second critical solids. This technique indicates that the first critical solids of this kraft black liquor is 50% solids and that the second critical solids is 56% for this black liquor sample. An established literary technique, boiling point rise elevation, has been used to verify first critical solids and an analysis of the deposited scale from the batch experimentation has been used as supporting evidence of the second critical solids point. This batch technique provides a simple approach to quantifying the problematic scaling occurring in high solids evaporation equipment.