A flotation machine for de-inking waste paper stock comprises a horizontally extending flotation cell (5) defining a reservoir for a stock liquid which forms a free liquid surface in the cell. A froth receptacle (4) extends along the entire length of the cell and receives froth floating up to the liquid surface. At opposite ends of the cell are a stock inlet (1) and a stock outlet (2). Situated within the lower portion of the cell is a rotary air bubble generator (3) which extends between the two ends of the cell with its axis of rotation extending generally horizontally whereby stock liquid introduced through the inlet (1) flows along the length of the cell towards the outlet (2) in at least one spiral flow pattern (11) with a generally horizontal axis.