5. Liquid–Solid Separation: De-Sanding—Flocculation, Sedimentation and Liquor Filtration

Post-digestion, the solids and aluminum bearing liquor need to be separated, and the solids washed to remove and recover the bulk of the liquor. Sedimentation and filtration equipment are commonly used for this. High temperatures, high scaling rates, and very fine particle size solids make these applications challenging. De-sanding equipment technologies and combinations thereof are described which frequently have been installed to protect flat bottom thickeners from high loads of coarse solid material. Today’s thickener technology providing relatively small diameters, steep bottom cones and a high mud bed mostly are operating without sand separation units. Flocculation of the fine solids into larger agglomerates is a key aspect of thickening, along with diluting the feed to an optimal concentration for the flocculation reaction. Good flocculation allows both the production of both clean overflows and high underflow densities that result in better recoveries and washing efficiencies in the CCD circuit. Scale growth is a major operating issue and eventually requires equipment shut down and descaling, so all thickeners are designed to be bypassed during the descaling operation. Historically, descaling was done largely by hand, but newer equipment is frequently designed for use with mechanical descaling arms that improve operator safety. Filtration technologies are described that have been developed by various companies for pregnant/green liquor filtration along with their specific features. A novel filter design is described that is operated completely without the addition of tri-calcium aluminate (TCA) as a filter aid. That specific characteristic is achieved due to a unique filter media cleaning technology comprising an automated hydro jet system.