Phase transformation of iron in limonite ore by microwave roasting with addition of alkali lignin and its effects on magnetic separation

Phase transformation and magnetic properties of limonite ore with 40.10% Fe via microwave roasting with addition of alkali lignin were investigated by chemical phase analysis and vibrating sample magnetometer (VSM) analysis, respectively. The results of chemical phase analysis indicated that phase compositions and contents of iron in microwave roasted limonite ore were varied with the dosage of alkali lignin, the roasting temperature, the roasting time and the microwave power, and among them the dosage of alkali lignin exerted more significant influence. Iron oxides in limonite ore could be reduced to magnetic iron oxides including gamma-Fe2O3 and Fe3O4 in the following sequence during microwave roasting process by evenly distribution of alkali lignin below 5%: FeOOH/alpha-Fe2O3 ->gamma-Fe2O3/Fe3O4, accompanied with small amount of FeO, and as the dosage was over 5%, gamma-Fe2O3 and Fe3O4 could be in turn successively transformed into alpha-Fe2O3. Magnetic property studies demonstrated that an iron concentrate containing 88.72% magnetic iron oxides with a maximum saturation magnetization of 41.393 emu/g could be produced from roasted ore which was obtained by microwave roasting at 200 degrees C and 600 W with 5% alkali lignin for 30 min. In addition, the roasted ore was further used for magnetic separation, and the results showed that combining microwave roasting with addition of 5% alkali lignin could improve the iron recovery from the roasted ore distinctively. It was concluded that the microwave roasting process in the presence of alkali lignin could be a promising approach to effective utilization of limonite ore resources. (C) 2017 Elsevier B.V. All rights reserved.