A system and method is provided for direct condensing steam heating of oil sands process slurry streams including viscous bitumen froth and tailings products streams. Slurry viscosities greater than that of water increase cavitation and vibration issues. High solids content exacerbate component erosions. Difficult, and competing, steam and slurry interactions are managed by steam nozzle arrangements and management of steam injection at sub-sonic velocities based on a ratio of the slurry back-pressure Pb and steam supply delivery pressure Po.