Prediction of Crystal Species Transition in Aqueous Solutions of Na2CO3 and Na2SO4 and Kraft Black Liquor, 2004 International Chemical Recovery Conference

Many of the fouling problems that occur in black liquor evaporators are the result of crystallization of Na2CO3 and Na2SO4. Shi (2002) measured the compositions of Na2CO3 and Na2SO4 in water and in black liquor at the onset of crystallization during evaporation. He also measured the composition of the first crystals produced for a range of relative concentrations of Na2CO3 and Na2SO4. The crystals that produce scale are the sulphate-rich, double salt burkeite, and a carbonate-rich double salt referred to as dicarbonate.

We used Shi’s data to obtain the apparent changes for crystallization of burkeite and dicarbonate as a function of the relative concentrations of Na2CO3 and Na2SO4 in solution. The apparent equilibrium relationships obtained were solved with the constraints of Shi’s experimentally determined relationship between solution and crystal composition and the conservation of mass and charge. The resulting model predicts the changing compositions of the sodium salts in solution and the changing composition of the crystals formed as evaporation proceeds. This model can also be used for predicting the solubility limit of these sodium salts in black liquor.

The compositions of Na2SO4 and Na2CO3 predicted by the model are slightly different than measured equilibrium values due to some constraints. However with regard to understanding and controlling fouling during evaporation of black liquor, the compositions at the onset of crystallization are the ones of most interest.