An effective brown stock washing process is a crucial part of the pulp mill operation due to its impact on downstream processes such as bleaching, black liquor evaporation, and caustic recovery. An optimized brown stock washing process is necessary to remove the maximum amount of black liquor solids from the pulp while minimizing wash water usage (1). Due to the presence of soap and lignin in black liquor, a significant amount of foam is generated in the washing process. In order to combat this foam, a foam control agent is necessary in order to increase production rates, as well as optimize and continue operations (2). The surface active components found in black liquor, such as surfactants, fatty acid soaps and other particles, create foam once energy is added to the system. Air bubbles are separated by a thin liquid-air interface called the lamella and stabilized by the surfactants. Defoamers, in the form of emulsified droplets, approach the foam surface. A three phase film consisting of oil droplets, water and air is formed. Once the defoamer droplet penetrates the lamella, it spreads into a lens at the liquid-air interface, thus destabilizing the bubble wall and disrupting the foam structure. Several options are available to combat foam in brown stock washing applications. Water based silicone defoamers, oil based defoamers, and polymeric antifoams will be explored in this paper, including their impacts on the brown stock washing process and fundamental differences. Water based silicone defoamers are very effective but may cause deposit issues with silicone carryover in certain grades of product. Oil based defoamers are used in mills where a silicone based defoamer would have detrimental effects to the end product. In order to address these issues, Kemira has developed a novel polymeric processing aid that contains no silicone and is very effective at foam control. This study will present lab evaluations and trials to highlight the positive benefits and drawbacks of each defoamer technology. References: (1) Handbook for Pulp and Paper Technologists, 3rd Edition, G. Smook (2) Emulsions, Foams, and Suspensions: Fundamentals and Applications, L. Schramm

Evaluation of defoamer chemistries for brown stock washing

249th National Meeting (2015)

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