A catalyst rejuvenation vessel assembly for treating particulate used oil-coated catalysts to provide a rejuvenated catalyst material is disclosed. A pressurizable vertically-oriented vessel assembly has inlet and outlet openings for the catalyst and washing fluids, and has a removable lower sub-assembly head portion which contains a conical shaped grid unit located therein. The vessel assembly is arranged to permit solvent washing, vacuum drying and acid treatment and gas drying of the used catalyst in a bed supported above the conical grid, by upward flow and recycle of the washing liquids and fluidization of the catalyst. Following rejuvenation of the catalyst, it is withdrawn form the vessel downwardly through the conical shaped grid and out through a central withdrawal conduit containing a slide valve unit for further processing or use as desired.