Macroscopic Flow Structures in a Bubbling Paper Pulp-Water Slurry

The hydrodynamic characteristics of three-phase slurry columns containing water, paper fibers, and air have been experimentally investigated. Such systems are relevant to flotation deinking, a critical but poorly understood separation process in the production of recycled paper. Two transparent bubble columns were used, one for quiescent liquid and one for cocurrent air and liquid flow. Water and dilute aqueous pulp slurries were studied using γ-densitometry to obtain local gas holdup (void fraction). Results in pulp slurries were significantly different than in pure water. In the quiescent flow system, network formation alters bubble size distributions and promotes transition to regimes with lower interfacial area. In contrast, with cocurrent flow, gas holdup and interfacial area can be greater in pulp slurries than in pure water. Results are explained in terms of the effects of fibers and flocs on flow behavior. Potential applications of this work for improved deinking performance are discussed.