Manipulation of Crystallization to Resolve Severe Concentrator Scaling, 2004 International Chemical Recovery Conference

Three of four bodies of a new falling film concentrator have suffered severe capacity loss from soluble scale since start up. Recovery boiler ash is added to the black liquor at the inlet to the 1A effect, and although this effect attains the highest solids content, it is the only concentrator body that does not significantly scale. The practical requirement to have one of the other three concentrator bodies on continuous wash has forced the second effect to operate above 50% solids which has caused it to rapidly foul.

Analysis of black liquor samples revealed that the concentrator feed had a Na2CO3/Na2SO4 weight ratio exceeding 3.7. Recent advances in our understanding of crystallization during black liquor evaporation suggest that the first crystals being formed at this composition are dicarbonate (2Na2CO4·Na2SO4). High levels of soluble calcium in the black liquor, 300-400 ppm, about 90% of total calcium, are expected to delay dicarbonate nucleation and allow very high supersaturation to develop. The combination of these two conditions suggests that massive nucleation of dicarbonate is responsible for rapid scaling. After the boiler ash is mixed with the black liquor, the crystals produced are expected to be burkeite (Na2CO3·2Na2SO4), which has less tendency to form tube surface deposits.

Causes for the high carbonate-to-sulfate ratio have been identified and several recommendations are under evaluation for modifying chemical make-up strategy to manipulate liquor composition within the concentrator and for recirculation of appropriate seed crystals to relieve supersaturation in the problem effects.