Abstract: The mixer circuit comprises a vertically oriented, open-topped mixer vessel having a cylindrical side wall terminating with a shallow conical bottom. The bottom wall forms a central bottom outlet. Recycled slurry and fresh water streams are fed tangentially to the inner surface of the vessel, thereby forming a vortex. The oil sand enters as a continuous, free-flowing stream moving along a downward trajectory; the stream impinges the vortex, wherein it is dispersed and mixed to create slurry. The slurry exits through the bottom outlet, is screened to remove oversize material, and enters a holding vessel. Part of the slurry in the holding vessel is recycled to the mixer vessel through a pipe loop incorporating a pump. The slurry is energized by the pump and functions to maintain and partly create the rapidly moving vortex that carries out the mixing and lump-disintegration actions. The balance of the slurry in the holding vessel is pumped out as product.