as shown the ratio of the draft tube diameter to the inner diameter of chamber 8 can be varied within a range of approximately 0 4 to 0 9 or more preferably a range of approximately 0 2 to 0 9 to achieve optimal fluid flow characteristics the arrangement of draft tubes within the respective stages of the flotation column as described provides for repetitive loop flow of the material being processed within the flotation column as generally indicated by r in the figures a sparger 30 is preferably made of any suitable material such as sintered porous metal or polymer with a pore size in the range of 5 to 25 microns preferably about 10 microns to produce gas bubbles in a size range of about 20 to 500 microns diameter the sparger 30 is centrally mounted near the bottom of chamber 8 the porous structure of the sparger allows air or other suitable gas to be forced through the pores of the sparger and into the mass of feed material surrounding the sparger whereby from the outset very fine gas bubbles are introduced into the feed material a gas feed 34 supplies gas to sparger 30 and is one of two input streams to the system the other being a liquid slurry feed 32 two output streams are also provided for the system a clean water output 36 and a foam or froth overflow 38 the overflow 38 is disposed in froth launder 9 which has an inclined bottom plate 40 extending circumferentially around a top portion 41 of chamber 8 bottom plate 40 collects foam or froth which overflows the upper end of flotation chamber 8 the froth thus is delivered by gravity to a foam outlet 42 located at the lowest elevation of.