Abstract: An agitator or mixer installed in a solid-liquid-gas/slurry reactor in which gas removal from the slurry and foam destruction is promoted. The reaction mixer includes a vessel and an agitator assembly. The vessel is for containing the solid-liquid-gas mixture and defines two mixing zones within a given volume; a first mixing zone and a second mixing zone located above the first mixing zone. The agitator assembly is positionable within the vessel and comprises a rotatable shaft and a first and second impeller coupled to the shaft. The first axial impeller is locatable within the first mixing zone and is configured to pump the liquid in a downward direction along a vertical axis of rotation. The second impeller is locatable within the second fluxing zone and is configured to pump the liquid in an upward direction along the vertical axis of rotation.