Pilot evaporation experiments were conducted with black liquors of different composition, to evaluate the relationship between the crystallization behavior of Na_2SO_4 and Na_2CO_3 and fouling of the heat-transfer surface by crystalline species composed of these compounds. The results showed that the onset of fouling corresponded closely to the onset of crystallization of a double salt of composition approximately 2Na_2CO_3·Na_2SO_4, at total solids contents of 65-70%. No significant fouling occurred during crystallization of burkeite (~2Na_2SO_4·Na_2CO_3), which began near 50% total solids content. Options for controlling deposition of dicarbonate scale during evaporation of black liquor at high solids content are considered.
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