A froth flotation method and apparatus for extracting a valuable substance from a slurry comprising a mixture of solid phase, liquid phase and said substance. A gas dispersing mechanism (4) feeds gas into the slurry (1) in a flotation vessel (2) to infuse gas bubbles into the slurry. The slurry is agitated in the flotation vessel (2) for dispersing the gas bubbles into the slurry. The gas bubbles capture the valuable substance from the slurry and form a froth phase F above the slurry phase S. The froth phase F exits the vessel by overflow. Continuously moving negative pressure zones (7) are mechanically generated by rake blades (9) in the froth phase (F). The negative pressure zones (7) extend from the froth phase (F) to the slurry phase (S). The negative pressure zones (7) capture unwanted slurry and liquid pockets (8) entrained from the slurry phase to the froth phase (F) and form paths for guiding said unwanted slurry and liquid pockets (8) via said paths from the froth phase (F) downwardly back to the slurry phase (S).