A mixing apparatus is provided for mixing slurries, particularly high density, high viscosity fracturing fluid slurries containing a large proportion of proppant material. A mixing tub has a generally round horizontal cross-sectional shape. A relatively large, low-speed rotating agitator is utilized to mix the slurry. The design of the agitator is such that a radially inwardly rolling toroidal shaped slurry flow zone is created adjacent the upper surface of the slurry within the tub. A stream of clean fracturing fluid is introduced into the tub near the center of the toroidal shaped flow zone. Dry proppant material is introduced into the tube and carried by the radially inwardly rolling flow into contact with the clean fracturing fluid. Foraminous baffles, preferably constructed from expanded metal sheets, are radially oriented within the tub to reduce rotational motion of the slurry within the tub without causing dropout of proppant from the slurry. A double suction vertical sump pump is utilized to pump the slurry from the tub.