Abstract: Apparatus (10) for the two stage oxidation of white liquor. The reaction vessel includes a central first stage chamber (24) and a second stage annular outer chamber (26). Heated air is first introduced through the outer second stage chamber (26) by pipes (36) and then directed through elements (38, 40, 44) to the central first stage chamber (24) for completion of the oxidation reaction. Unoxidized white liquor with a small amount of black liquor to catalyze the process is introduced to the first stage of the reaction vessel at a temperature in excess of 185 degrees F. Oxidized white liquor is removed from the second stage chamber and air is venter to atmosphere through the stack at the top of the first stage chamber. The second stage is preferably pressurized for greater efficiency.