Abstract: A process is described for the recovery, in metallic form, of mercury present in roasting gases, containing SO.sub.2, so that the purified gas has less than 0.1 mg/m3 of this element. Essentially, the system consists of a conventional device for the cooling and scrubbing of the gas; two irrigated towers with an acid solution of Hg(SCN).sub.4.sup.2- free SCN.sup.- and active carbon; two wet electrostatic precipitators, one in front of the first tower and another between them and a reactor for scrubbing the emulsion (soot) obtained from the condensation, with ammonium. All mercury-bearing tailings are recycled to the roasting oven at appropriate intervals, until under certain conditions the hot gases attain a mercury content of more than 40 mg/m3.