An innovative method for the efficient and selective extraction of Co, Ni, Cu, and Mn from oceanic cobalt-rich crusts by ammonium sulfate roasting: Behavior, roasting kinetics and mechanism

The exploitation of deep-sea mineral resources is an unavoidable choice for sustainable human development, given the diminishing or even depletion of land-based mineral resources and the increasing demand for critical metals in global industrialization. This work presents a new method for the selective extraction of Co, Ni, Cu, and Mn from oceanic cobalt-rich crusts through the ammonium sulfate roasting-water leaching process. The metal oxides in oceanic cobalt-rich crusts were transformed to sulfates or ammonium sulfate salts by sulfation roasting and then by selective decomposition roasting to transform Fe2(SO4)3 to Fe2O3, while the other metals remained in sulfate forms. Under the sulfation roasting temperature of 450 degrees C for 120 min and the mass ratio of (NH4)2SO4 to cobalt-rich crusts of 1.2:1, the decomposition roasting temperature of 650 degrees C for 90 min and the air flow rate of 2 L/min, the leaching efficiencies of Mn, Co, Ni, and Cu reached 98.3 %, 93.7 %, 88.7 %, and 66.3 %, while the Fe leaching efficiency was only 0.9 %. The analysis of the mechanism shows that the Mn in oceanic cobalt-rich crusts first formed (NH4)2Mn2(SO4)3 during roasting and then decomposed to MnSO4; Co, Ni, and Cu first formed (NH4)2Co2(SO4)3, (NH4)2Ni2(SO4)3, (NH4)2Cu3(SO4)4 and then decomposed to their respective sulfates; Fe first formed (NH4)3Fe(SO4)3 and finally decomposed to Fe2O3. Ammonia released during the roasting process can be absorbed with dilute sulfuric acid and then regenerated (NH4)2SO4 with an ammonia recovery of 87.3 %. The process proposed in this study is not only efficient, clean, and selective, but also achieves full component re-covery from oceanic cobalt-rich crusts, with obvious environmental and economic advantages over the tradi-tional reduction roasting-acid/ammonia leaching.