Effluent quality at kraft mills that use complete substitution bleaching

The orientation of kaolin pigments is expected to influence the flow behavior during coating and the final properties of the coating layer. This work provides a new computational tool to predict the trajectories and orientation of oblate spheroids in any coating geometry. The motion of oblate spheroids in a coating geometry is simulated using a Stokesian dynamics technique. Computational fluid dynamics is used to calculate velocity fields that are used to estimate the long-range hydrodynamic forces and torques on the particles. The model calculates the trajectories, rotation, forces, and torques on the particles, including particle-particle hydrodynamic and colloidal forces. Process variables, such as coat weight and particle aspect ratio, are studied. In general, the particles are predicted to be aligned with the web, but the blade coater produces more orientation than the metering size press for particle aspect ratios less than 10. Particles with a low aspect ratio are found to have a low orientation. Particles are forced together in the metering zone, which may lead to processing issues. Application : a new method is presented to predict the motion and orientation of disk-shaped particles in any coating geometry. Blade geometries generate more highly aligned particles than roll coaters.