China Nonferrous Metallurgy

During the extraction of nickel and cobalt from limonitic laterite nickel ore using the sulfation roastingwater leaching process， the selective leaching of nickel and cobalt significantly affects subsequent extraction separation. However， the phase transformation mechanism of iron and its conversion efficiency have been insufficiently investigated in existing literature. In this study， a limonitic laterite nickel ore from Myanmar was treated by a concentrated sulfuric acid curing-roasting followed by atmospheric water leaching to selectively extract nickel and cobalt. The roasting conditions were optimized through single-factor experiments. Combined with thermodynamic analysis and characterization of the raw material and process intermediates obtained at different temperatures， the phase transformation mechanism of iron was elucidated. The main conclusions are as follows. The optimal roasting conditions were determined as： sulfuric acid concentration of 90%， sulfuric acid-to-ore mass ratio of 0. 7∶1， roasting temperature of 650℃， and roasting time of 30 min. Under these conditions， the leaching rates of iron， cobalt， and nickel were 20. 06%，84. 33%， and 78. 15%， respectively， achieving preliminary separation of iron from cobalt and nickel. Characterization of the roasted residues revealed that at a roasting temperature of 300℃ ， goethite， serpentine， dolomite， chromite， and talc were transformed into corresponding sulfates. The concentrated sulfuric acid destroyed the original iron mineral structure， releasing lattice-bound cobalt and nickel， which then reacted with the acid to form cobalt and nickel sulfates. At 650℃， Fe₂( SO₄ )₃ was further converted into water-insoluble Fe₂O₃， which remained in the residue during subsequent water leaching， thereby enabling selective leaching of nickel and cobalt. The decomposition and conversion of ferric sulfate directly determine the efficiency of nickel-cobalt selective leaching. Nevertheless， the iron leaching rate remained as high as 20. 06% in this experiment. Future work could employ fluidized roasting or maintain the material in a tumbling state to enhance iron phase conversion and reduce iron leaching.