Free carbon soot entrained in the product gas of a free-flow synthesis gas generator is recovered in a sedimentation vessel as part of a thickened dispersion of solids in water from which clear supernatant water is easily separated by supplying particulate petroleum coke to the sedimentation vessel or to the reaction zone of said gas generator as a portion of the fuel, or by mixing the carbon soot-water dispersion in the sedimentation vessel with a thickened water slurry of unconverted petroleum coke such as produced by quenching in water the product gas of a second free-flow synthesis gas generator in which the fuel comprises raw particulate petroleum coke and the thickened underflow from said sedimentation vessel.