Scaling in black liquor evaporators has been studied for decades, yet the mechanisms governing scaling behaviour remain difficult to explain and predict. Current understanding of Na–CO₃–SO₄ scaling mainly focuses on three factors: crystal species, operating conditions, and wood extractives such as soap, tall oil, and spent acid (brine) from tall oil production. This study provides new experimental data addressing all three aspects.

Laboratory- and pilot-scale evaporation experiments with black liquor were carried out to investigate the effects of various additives on Na–CO₃–SO₄ crystallisation. The additives included four tall oil brine fractions, three fatty and resin acids (oleic, linoleic, and abietic acid), calcium sources (CaCl₂ and CaSO₄), and three scaling inhibitors. Bulk crystals and deposited scales were characterised using SEM, XRD, light microscopy, and X-ray microtomography.

The results showed that scales are heterogeneous, porous, and mechanically weak, leading to stochastic detachment. Tall oil rich brine increased extractives content in the scales by 15–20 times, produced soft, flaky, layered structures, and significantly reduced scaling. A scaling inhibitor designed for the Na–CO₃–SO₄ systems was also effective. Multiple crystal species were identified, confirming greater system complexity than commonly assumed. Calcium incorporation increased with CaSO₄ addition but not with CaCl₂.