Process to prepare a gas mixture of hydrogen and carbon monoxide from an ash containing carbonaceous feedstock by performing the following steps, (i) partial oxidation of the ash containing carbonaceous feedstock with an oxygen containing gas thereby obtaining liquid ash and a gas mixture comprising hydrogen, carbon monoxide and solids, (ii) separating more than 90 wt % of the liquid ash from the gas mixture, (iii) reducing the temperature of the gas mixture, in the absence of the separated ash (iv) scrubbing the cooled gas of step (iii) by contacting with liquid water obtaining a scrubbed gas and a water effluent containing ash, (v) separating the ash from the water effluent by means of a decanter centrifuge thereby obtaining a wet ash and a stream of water poor in ash.