A process for the partial oxidation of pumpable slurries of solid carbonaceous fuels in which the pumpable slurry of solid carbonaceous fuel in a liquid carrier is passed in liquid phase through one passage of a burner comprising a retracted central coaxial conduit, an outer coaxial conduit with a converging orifice at the downstream tip of the burner and, optionally, an intermediate coaxial conduit. The downstream tips of the central conduit and the intermediate conduit, if any, are retracted upstream from the burner face a distance of respectively two or more say 3 to 10 for the central conduit, and about 0 to 12 say 1 to 5 for the intermediate conduit times the minimum diameter of the converging orifice of the outer conduit at the burner tip. A pre-mix zone is thereby provided comprising one or more, say 2 to 5 coaxial pre-mix chambers in series. The free-oxygen containing stream is passed through a separate passage of the burner into the pre-mix zone, in which mixing takes place with the slurry of solid carbonaceous fuel and liquid carrier. From 0 to 100, say about 2 to 80, volume % of the liquid carrier may be vaporized in the pre-mix zone. The multiphase mixture of reactants is then discharged into the reaction zone of the free-flow partial oxidation gas generator by way of the converging orifice of the outer conduit at the burner tip. Synthesis gas, fuel gas, or reducing gas is thereby produced.