The beneficial effect of leaching of carbonate gangue of flotation middlings (tailings from 1st cleaning at Lubin Concentrator) with sulfuric acid prior to their final flotation, leaching or bioleaching has been presented. The leaching of flotation feed with sulfuric acid decomposes selectively the carbonate gangue and leads to liberation of sulfide minerals and enhanced flotation or leaching results. It was shown, that after decomposing of 50-70% of carbonates in flotation feed, both flotation recovery and concentrate grade increased considerably versus the results observed for unleached feed. Products of acidic leaching comprise gypsum, soluble magnesium sulfate and gaseous carbon dioxide. Carbon dioxide evolving during the reaction creates the non-oxidizing atmosphere in the pulp during leaching and therefore effectively prevents the digestion of metals from sulfide minerals. The beneficial effect of acidic leaching on flotation was evidently confirmed in pilot tests performed on several feed materials. The application of sulfuric acid leaching of flotation feed produced an enhancement of flotation results (metal recovery and concentrate grade) and provided a rational use of H2SO4, which becomes a troublesome and difficult-to-sale product of copper metallurgy. Non-oxidative leaching has to be unquestionably applied prior to the atmospheric, pressure or bio-leaching in acidic conditions.