Sodium Salt Scaling in Black Liquor Evaporators and Concentrators, 2000 Engineering Conference Proceedings

Terry N. Adams, Ph.D.--Scale formation on the heat transfer surfaces of evaporators and concentrators due to precipitation of sodium salts is a chronic problem during black liquor evaporation. Data on the composition and mineral structure of these scale deposits shows that there can be as many as five different sodium minerals. The data also show that there is a rapid change in the scale deposit composition for Na2SO4 contents in the black liquor solids below 3wt%.

When the liquor composition changes due to recovery, recaust, or make-up chemical changes, the preferred crystal types for the precipitating sodium salts also change. Crystallization technology shows that this is effectively like restarting the unit without seed crystals. The crystal nuclei required to relieve supersaturation conditions are not initially available, so supersaturation rises to the point that heterogeneous nucleation on the equipment surfaces occurs. This leads to deposit and growth of scale.

The primary method for avoiding evaporator and concentrator scaling is to avoid heterogeneous nucleation by ensuring a continuous supply of the full suite of sodium salt crystals that can form from black liquor. Controlling the Na2SO4 content of the liquor is one way of doing this. Two other methods are suggested by practical approaches developed for crystallization processes elsewhere.