Abstract: The object of this invention is a process for separating metal compounds, even in small percentages, from dusts and sludges, by reduction.The process may be successfully applied to the selective separation of vanadium pentoxide contained in small fractions in the ashes of heavy fuels, as well as to the separation and/or passivation of chromium compounds, of which the polluting characteristics are well-known.Separation is obtained by means of a reduction tower containing preferably iron scraps or shavings, adjusting on one side the pH of the input solutions according to their compositions, and on the other side the speed with which the solution passes through the reduction tower, so as to obtain a specific pH value of the output, again depending on the type of metal to be treated (FIG. 2).