Flotation process and device for separating solid particles from a paper fibre suspension

摘要

The flotation action, to separate solid particles from a suspension of paper fibers, forms a flotation foam (6) which traps the particles. A fluid (F) is fed to the foam/particles so that at least part of the hydrophilic particles (13) are washed out of the foam. The fluid (F) is delivered to the foam (6) at a saturation rate of max. 50 l/m<2>/min. or max. 9 l/m<2>/min. The fluid (F) is fed as a liquid film, and is so finely distributed that it forms a spray mist with a droplet dia. of max. 1.5 mm. The flotation vessel is structured so that the gas bubbles rise over a height of max. 3 m to form the foam (6). The foam (6) generally fills the vol. between the suspension surface and the walls of the flotation vessel. The fluid (F) is applied to the foam (6) from a point above it or within it or at the boundary between the foam (6) and the suspension, using a sieve distributor (14) which is saturated, and transfers the fluid to the foam (6) in a fine distribution, and the mesh distributor (14) is moved to support the foam transport. The added fluid (F) flow is at right angles to the flow direction of the foam (6), to use a cross flow principle, or against the foam flow in a counter flow action. The foam (6) is taken from the suspension surface against the gravity field, or across the gravity. The added fluid (F) washes out paper fibers trapped in the foam (6), and the suspension of recovered paper fibers is returned to the suspension for mixing or is taken off separately. Fluid (F) is fed to the flotation as long as foam (6) is present,. Foam (6) is taken from the flotation vessel before fluid (F) is delivered. The delivered fluid (F) is pref. water which can contain added chemicals to break up foam. The fluid (F) can also be steam or over-saturated air which condenses on or in the foam (6). An Independent claim is included for a flotation assembly where the foam (6) is treated with an application of a fluid (F) for stabilizing and with a low energy to break up the bubbles in the foam. Preferred Features: At least one feed system delivers the fluid (F) to the foam (6) so that at least part of the hydrophilic particles present are washed out of the foam (6), using a sieve mesh (14) which floats on the foam for an additional fine distribution of the fluid (F). The fluid (F) distribution is structured so that the foam (6) is not destabilized. The level of the suspension in the flotation vessel is at least 2 m. Flat inserts within the vessel prevent the clean suspension mixing with the foam. Part-flows of the suspension are taken from the flotation vessel, to be returned to the vessel at different points after degassing. The height difference between the outlet from the flotation vessel and the inlet into the vessel and the under side of the foam layer is max. 2 m. The foam (6) is extracted by suction upwards from the surface of the suspension. The flotation vessel is generally cylindrical, with an inlet at a tangent and a central foam extraction. The fluid feed unit is a tube with fine holes or pores, a tube with slits or a tube fitted with at least one spray jet.