An on-line air content probe was installed in a number of recycling mills to obtain insights into the role of aeration on the flotation kinetics of ink particles. A survey of air contents in various commercial flotation cells revealed considerable variations from one type of cell to another. Though injector designs affected air content, furnish type, flotation chemistry and pulp consistency were also found to be very important. Based on mill trials and mill distributed control system data, we have established that changes in bubble size in a given flotation machine were affected principally by the pulp consistency and by the recovered paper furnish. Certain papers, such as old magazines, release surface active agents and filler particles during repulping, which minimizes bubble coalescence in a flotation cell. Smaller bubbles were shown to be often, but not always, beneficial for ink removal. Indeed, a switch from one blend of fatty acid soaps to another increased flotation rate despite forming larger bubbles. This observation indicated that not only bubble size, but also collection efficiency, was a very significant parameter of ink flotation rate. The on-line monitoring of air content in commercial cells allows for a better understanding of the role of aeration in flotation, and can be used to optimize flotation operation.
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