Selective Iron Suppression through Sulfation, Oxidative Calcination, and pH-Controlled Atmospheric Leaching of a Ferruginous Oxidized Copper Ore

Abstract : Iron-rich oxidized copper ores are challenging to treat by atmospheric leaching because iron oxides and oxyhydroxides may consume acid and increase impurity loading in the pregnant leach solution. This study evaluated a three-stage route comprising sulfuric-acid sulfation, oxidative calcination, and pH-controlled atmospheric leaching, with the primary objective of minimizing net Fe transfer rather than maximizing Cu extraction. Six distinct operating conditions were investigated using a ferruginous oxidized copper ore from northern Bahia, Brazil, containing 19.6 wt.% Fe, 5.37 wt.% Al, 1.47 wt.% Mg, 0.524 wt.% Mn, and 1.05 wt.% Cu. Commercial sulfuric-acid solution additions during sulfation ranged from 9.75 to 99.69 kg solution/t dry ore. The sulfated products were calcined through staged temperatures from approximately 250 to 700 °C over 6 h and subsequently leached at approximately 90 °C for 180 min, with final solution pH values of 2.33–2.77. Reconciled Fe transfer remained between 0.026% and 0.340%, corresponding to Fe suppression of 99.66–99.97%. By contrast, Mg, Mn, and Al transfers ranged from 6.39% to 29.78%, 38.14% to 59.98%, and 6.95% to 16.28%, respectively, confirming that Fe suppression did not result from a general inhibition of mineral dissolution. The measured Fe transfers corresponded to calculated stoichiometric acid equivalents of only 0.13–1.76 kg H₂SO₄/t ore, compared with an upper-bound requirement of 516.34 kg H₂SO₄/t ore for complete conversion and dissolution of feed Fe as Fe(III) sulfate. The response is consistent with initial formation of ferric sulfate, basic ferric sulfate, or related Fe-bearing sulfate intermediates, followed by thermal destabilization to poorly soluble Fe(III)-oxide-bearing phases and release of sulfur oxides. Released SO₃ may have promoted sulfate redistribution to neighboring reactive metal–oxygen sites, while controlled-pH leaching was consistent with an additional aqueous Fe-rejection barrier through limited Fe-oxide attack and Fe(III) hydrolysis or reprecipitation. Mg behaved as a persistent sulfate sink, Mn showed redox-sensitive transfer, and Al exhibited intermediate behavior. The route therefore selectively decoupled the large Fe inventory from the final aqueous products. Direct-leaching controls and stage-resolved mineralogical, sulfur-speciation, and off-gas analyses are required to quantify actual acid savings and confirm the proposed mechanism.