Hydrodynamics of deinking flotation

Abstract

The technological parameters relating to the aeration, mixing and separation steps of the flotation cells currently used in the field of waste paper recycling are reviewed in this paper. A laboratory flotation cell was designed to study the effects of the hydrodynamic flotation parameters, and tested on different deinking furnish. The physico-chemical aspects of flotation deinking were not investigated and the flotation efficiencies were measured for the ink particles detached from the fiber surface. Experimental evidence concerning the first-order kinetics of ink removal, with respect to flotation time and consumed air, was obtained for different particle and bubble sizes. Small bubbles were shown to be more effective, but led to higher fiber losses than large bubbles for a given ratio of consumed air. The design of the experimental cell was then improved in such a way as to produce calibrated air bubbles of different sizes between 0.5 and 2 mm diameter, and to investigate separately the effect of turbulence generated in the flotation cell. The results showed that the relation between the flotation rate constant and bubble size depends on particle size and shape. The optimum ink particle size showed to be in the range of 10–100 μm. The flotation efficiencies decreased strongly with particle size in the case of flat shaped ink particles, while no significant efficiency drop was observed for the large particles in the case of calibrated laser inks. A turbulence increase improved significantly the removal efficiency except for the largest laser ink particles. The experimental results were compared with the theory developed in the field of mineral flotation and showed quite good agreement with the theoretical relations as far as the effects of air ratio, bubble size and turbulence with respect to relatively small particle sizes are concerned.