An aqueous dispersion of unreacted particulate petroleum coke substantially comprising carbon and containing a small amount of ash substantially comprising vanadium constituents and other materials from a process for the partial oxidation of petroleum coke to produce synthesis gas is mixed with a K.sup.+ amyl xanthate vanadium collection agent and a frothing agent. After pH adjustment, if necessary, the mixture is subjected to aeration with or without mechanical agitation in a froth flotation zone. About 70 to 95 wt. % of the unconverted particulate petroleum coke particles (basis carbon) float in the froth layer that is formed. The remainder of the unconverted petroleum coke comprising coarser particles is suspended in a bottom aqueous dispersion on which the froth layer floats. In addition, selective separation of the vanadium constituents in the unconverted petroleum coke ash takes place. The vanadium content of the ash in the floated portion of unconverted petroleum coke is unexpectedly decreased about 50 to 85 weight percent. Simultaneously, the vanadium content of the ash in the unconverted petroleum coke suspended in the bottom layer of aqueous dispersion is unexpectedly increased about 50 to 85 weight percent. Water may be recovered from this aqueous dispersion and recycled to the gas quench cooling and scrubbing zones. At least portions of the unconverted particulate petroleum coke from the foam layer and the bottom layer are recycled to the partial oxidation gas generator and to a vanadium recovery zone, respectively. By the subject process, the vanadium content in the recycle stream of unconverted petroleum coke to the gas generator is reduced. The life of the refractory lining of the gas generator is thereby extended.