Strategies for Controlling Sodium Salt Scaling in Black Liquor Evaporator: The Influence of the Bulk Crystal Population

Journal article, 2018

Black liquor evaporation is an essential unit operation in the chemical recovery cycle at a pulp mill. Because black liquor contains high
concentrations of sodium carbonate and sodium sulphate, these salts will start to crystallize when dry solids content typically exceeds 50%.
The concentrator effect will therefore work as a crystallizer, and its operation is challenging because crystallization is unavoidable, which in
turn often causes scaling. In this paper, the relationship between scaling and bulk crystals is investigated experimentally on a pilot scale by
using hydrocyclone separation to change the bulk crystal population. Recommendations for scale control are given based on the observed
results. It was found that large crystals contributed to a lower degree of scaling and are therefore more important in the system than small
crystals. Provided that a minimum quantity of bulk crystals is available during normal operation (continuous crystallization), it was also found
that further increasing the quantity of bulk crystals will not decrease the scaling rate.