Modeling of Bleach Plant Washer Mineral Scale, 2004 Engineering, Pulping, and PCE&I Conference

One of the more common areas of mineral scale formation in bleach plants is on washer face wires and the extraction rings of diffusion washers. Whereas most scale problems can be understood as a mixing of two streams that increase the concentrations of anions and cations and exceed a solubility product, washer problems are often more difficult to understand. The behavior of the pulp mat as a cation exchange resin makes these devices operate as an ion exchange column. This feature has the effect of delaying the release of cations into the filtrate and can cause a high concentration of a scaling cation to be released into a filtrate stream containing a higher concentration of the scaling anion. This ion exchange column process is proposed as a critical element in understanding scaling problems on washers. To test the hypothesis, a finite element model of a bleach plant washer has been prepared. This model divided the mat into thickness increments and modeled the ion exchange and precipitation equilibrium for each increment. The soluble fraction was passed on to the next increment using a washing efficiency factor to apportion the displacement and mixing of the input and output streams for each increment. The model has been tested against laboratory washing data and has been validated against two extraction stage washers in the MeadWestvaco Covington, VA mill.