An apparatus (100) for mixing a liquid (160) containing particulates (106, 108) comprising: a vessel (102) for containing the liquid (160) with a sidewall (120) and bottom (124); an impeller (300) rotates about a substantially vertical axis (X-X), the impeller submerging below the liquid surface (162) by a distance approximately one-tenth to one-half of the liquid (129) height; at least two spaced apart blades (310) extending radially outwardly of the vertical axis, the blades including back-swept blades pitched substantially parallel to the vertical axis, at least 50% of the length of each blade comprising an angled section (312) extending through a chord angle of 20 to 60 degrees to produce: an inner, upward flow region (164) along said vertical axis, a transition flow region (166) around the impeller in which liquid moves radially outwardly toward the vessel sidewall, and an outer, downward flow region (168) along the sidewall.