A thickening tank has a peripheral overflow launder and a centrally disposed feed well. Feedstock slurry is fed to a mixing box which also receives supernatant liquid and is supplemented by flocculated material flowing into the interior of the feed well onto a distributor baffle plate. The supernatant liquid is delivered at a controlled rate from a receiving vessel formed by an upright cylinder provided at its lower end with an impeller of a lift pump which is operated by a controller positioned at the upper end of a vertical drive shaft extending axially through the receiving vessel. The feed well is surrounded by a second launder which is closed at its base and formed with a cup. The lower end of the receiving vessel is spaced from the floor of the cup to draw supernatant liquid exclusively from the second launder rather than from the thickener tank.