A process and device for the material utilization of soot from the waste water of a gasification appliance (heavy oil POX) in which a hydrogen- and carbon monoxide-containing (crude synthesis gas) is generated from relatively high-boiling hydrocarbons by partial oxidation, is disclosed. The soot-loaded waste water from the heavy oil POX is mixed with naphtha and is subsequently introduced into a separator (decanter) from which a substantially soot-free water fraction and a substantially water-free naphtha/soot mixture are taken off separately, where the naphtha/soot mixture is fed as feed to a further gasification appliance (naphtha POX), in which appliance predominantly naphtha is converted into a crude synthesis gas by partial oxidation.