Minimizing BASO4 Scale Formation In ECF Bleach Plants, 2003 Fall Technical Conference

Increased barium sulfate scale deposits have been a problem for some mills as they switch from chlorine based bleaching to elemental chlorine free (ECF) bleaching. These deposits typically show up in the tower dilution zone and in the washer vat of the D0 stage. When a mill changes from elemental chlorine to ECF bleaching, the pH in the first stage increases from under 2.0 to usually above 3.0. Most ECF mills add sulfuric acid or sodium sesquisulfate from the chlorine dioxide generator to control the D0 stage pH. At a pH below 4.0 most of the Ba is desorbed from the pulp, and at pH’s above 2.0 most of the sulfate is free to react with the Ba to form an insoluble BaSO4 deposit.

BaSO4 solubility has been modeled using a general chemical equilibrium solver with corrections for the effects of temperature and ionic strength. The effects of cation complexation with pulp and inorganic ligands has been included in the modeling. From these simulations and laboratory data we have been able to formulate an operating strategy to minimize BaSO4 scale formation within the bleach plant.