Abstract: The invention relates to the concentration of particulate matter by froth flotation and in particular to improvements in hydraulic-pneumatic flotation apparatus. The apparatus includes an upstanding flotation compartment adapted to contain a relatively quiescent body of aqueous pulp. Aqueous pulp is introduced into and float fraction is collected from the upper portion of the floatation compartment, and a hydraulic chamber is disposed near the bottom of the compartment for supplying aerated water thereto. Retardation plate means spans the flotation compartment intermediate the ends thereof for retarding the descent of pulp therewithin. The plate means is provided with a plurality of apertures to receive rising air bubbles and descending pulp therethrough. Retarding or prolonging the descent of the aqueous pulp within the flotation compartment enhances the probability of floatable particles becoming attached to bubbles and thereby being recovered.