Treatment and disposal of wastewaters containing carbon black and ash

A process for treating and disposing of wastewater which contain carbon black and ash and originate from high temperature processes and in which the carbon black is present in a very finely divided state and in which the ash contains vanadium and/or nickel, wherein

• (a) calcium carbonate or calcium hydroxide is added, in an amount of from 2 to 10 kg/m.sup.3, to the wastewaters,
• (b) an aqueous solution of an organic flocculant is thoroughly dispersed in the wastewaters in the course of from 1 to 2 second to flocculate said carbon black.
• (c) the wastewaters are flocculated in a flow-through stirred container for from 30 to 180 seconds while stirring at speeds of from 5 to 20 min.sup.-1,
• (d) immediately thereafter the wastewaters are subjected to gravity filtration to obtain a carbon black sludge,
• (e) the carbon black sludge is then immediately dewatered further by pressure filtration under from 0.5 to 15 bar, filter cake being obtained, and
• (f) the resulting filter cake is aftertreated to convert the vanadium and nickel present into a water-insoluble or non-elutable form by dripping onto it, or spraying it with, an aqueous trivalent iron salt solution, in an amount corresponding to 1-20 kg of iron salt (calculated as 100%) per 1000 kg of filter cake, while retaining the solid, lumpy state.