Abstract Gas removal from the papermaking process is currently a standard practice, whereas purification of the internal water circulation has become common only recently. Both unit processes have progressed greatly during recent decades and new concepts are constantly being developed. The aim of this thesis was to analyse the efficiency and applicability of a channel flow design introduced by Metso for passive white water deaeration and to study the dynamics of passive bubbly gas removal. In addition, separation of the detrimental process water components by selective flotation during deaeration was studied to add further functionality to the channel flow design.

Turbulent mixing at the flow discharge and the consequent air entrainment were seen to limit the gas separation efficiency. Also, the properties of different white waters notably affect their deaeration through viscous forces, the concentration of surface active components and bubble-particle interactions. Thus similar levels of gas separation cannot be achieved with all process waters. The analysis showed that the drag of small microbubbles is mostly caused by hydrophobic contamination and the dispersed particles that readily attach to the bubbles. Correlations were derived based on experimental data to provide new information on the drag force experienced by small bubbles in white waters.

Chemically unaided flotation of white water in the channel flow was shown to be efficient in separating hydrophobic contaminants that have adverse effects on paper machine production and product quality. Both good reductions in contaminant content and high selectivity in their removal were achieved. Channel flow with an overflow can be considered well suited for the first stage of froth separation, while further treatment of the channel flow reject may consist of a secondary flotation or other process that enables the recirculation of fines and fillers. Although a certain level of losses of fines and fillers must be expected, substantial fraction of these solid components can be returned to the process stream.

The proposed multifunctional process, channel flow deaeration and frothing of white water, was seen to be straightforward, economical and feasible while also providing benefits in terms of total process efficiency that are not delivered by any current process scheme. The experimental parameters presented here regarding bubble dynamics and flotation efficiency can be used to achieve better models of these processes.