the pressure oxidation of slurried metal sulfide containing materials the metal sulfides that can be effectively utilized include without limitation gold sulfides iron sulfides copper sulfides zinc sulfides nickel sulfides and arsenic sulfides referring to fig1 and 2 the agitation assembly 10 is depicted the agitation assembly 10 includes a rotatable shaft 14 a gas injecting impeller 18 and a mixing impeller 22 connected to the lower end of the shaft 14 and a motor not shown connected to the upper end of the shaft 14 for rotating the shaft 14 during pressure oxidation the rotatable shaft 14 includes a gas inlet 26 in communication with a conduit 30 extending longitudinally along the shaft 14 the conduit 30 is in communication with a number of conduits 34 a d in the gas injecting impeller 18 for dispersing the gas substantially uniformly throughout the slurry a fresh oxygen containing gas 37 from an oxygen supply plant or the ambient atmosphere can be introduced to the slurry via an inner conduit 25 the conduit 30 and finally radially outward through the conduits 34 a d an oxygen containing gas 38 is recycled from the autoclave atmosphere via inlet 26 which is open to the autoclave interior because rotation.