Modeling and Minimization of Barium Sulfate Scale, 2006 Engineering, Pulping, & Environmental Conference

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The majority of the barium present in the pulping process exits the digester as barium carbonate. Barium carbonate dissolves in the bleach plant when the pH drops below 7 and, if barium and sulfate concentrations are too high, begins to precipitate as barium sulfate. Barium is difficult to control because a mill cannot avoid this carbonate to sulfate transition using common bleaching technology. An advantage in controlling barium scales is that the sulfate, once formed, will not redissolve to any appreciable extent, and this reduces the rate at which barite scale can form in other locations in the bleach plant. A mill has relatively few options for controlling barium sulfate scale. The first is to ensure very good debarking to minimize barium entering the mill with the wood. The second option is to minimize sulfate content of the first chlorine dioxide stage in the bleach plant. Sulfate is minimized through good brownstock washing and by using sulfuric acid for pH control instead of chlorine dioxide generator spent acid. Sulfate can also be reduced by operating the initial chlorine dioxide stage at higher pH, but this will work only in mills where calcium scales are not a concern. A third option is to use crystal modifiers (scale inhibitors). Scale inhibitors can be an expensive solution but are effective at delaying precipitation, and if the location can be moved from pumps and process piping into a tower, the impact of the scale is reduced. Because barium sulfate will not redissolve in the bleach plant, a barite deposit is less likely to reoccur somewhere else in the bleach plant.