Preflotation - Process Info
Preflotation - introduction and background
Prior to the preflotation the accept pulp from the pre screening system is diluted and chemicals as soap and calcium hydroxide may be added. Soap helps to remove ink from the fibres. Soap gives, together with calcium hydroxide, calcium soaps. The calcium soap works as a flocculant, it collects ink patricles and makes it easier for the ink to rise to the surface.
The flotation consists of two stages, a primary and a secondary stage.
The primary flotation stage consists of one mixing cell and 4 – 6 flotation. The pulp is pumped from cell to cell. Pulp is led together with air in the bottom of each cell. Part of the cell wall is opened to allow a backflow through the cells. Ink accumulated at the surface flows over an weir to the sludge chan- nel, from where the sludge is led to a deaeration sludge tank. To clean the overflow wall and to knock down the foam in the sludge channel there are white water showers which are controlled by time (all cells). The accept from the primary stage is led to a deaeration accept chest prior to a centrifugal cleaner system.
From the deaeration sludge tank sluge/pulp is pumped to the secondary flotation stage, consisting of one or two flotation cells. Sludge from the sludge channel is led to a sludge tank from where it is pumped to the sludge treatment. Part of the accept is led back to the deaeration sludge tank for level control. The other part of the accept is led back in cascade to the primary stage.