radial flow 34 to the fluid an impeller imparts a 34 substantially radial flow 34 when at least 50 and preferably 80 of the total flow leaving the tip s of the impeller is in the plane of the impeller 39 s rotation see for example 34 suspension of solids 34 by j e lyons in 34 mixing 34 vol ii edited by v w uhl and j b gray academic press 1967 hereby incorporated by reference for further discussion of radial flow impellers 34 significant portion 34 of the residual oxygen means at least 50 of such oxygen preferably at least 70 of the residual oxygen is removed through use of the present invention 34 residual oxygen 34 is that which remains in the slurry after gross separation of oxygen and slurry following the oxygen bleaching step such gross separation may be accomplished by conventional means in a section of the bleaching vessel in a separate vapor liquid separator or in a section of a vessel employing the present invention preferably at least some of the gross separation occurs in the top section of a vessel having a radial flow impeller in accordance with the present invention the amount of residual oxygen in the untreated slurry may be great as 350 ppm by weight of the slurry usually up to 150 ppm but preferably not greater than 100 ppm the residual oxygen may be dissolved in the slurry or entrained therein as fine gas bubbles or both usually 50 to 60 of the residual oxygen will be in the form of entrained bubbles but it may be more or less depending on.