Influence of alkaline additives on main minerals in Baotou rare earth tailings in the process of coal-based magnetizing roasting

The influence of NaOH-Ca(OH)2 on magnetizing roasting of iron minerals and on the decomposition reactions of rare earth minerals during coal-based magnetizing roasting is explored in the present work. The effect of alkaline additives was determined by means of SEM-EDS and X-ray diffraction methods in NaOH-Ca(OH)2-coal-tailings mixtures under optimal magnetizing roasting conditions (mass ratio of NaOH, Ca(OH)2, coal and tailings equal to 2.5: 4.5: 2.5: 100, roasting at 650 °C for 2.5 h). Although NaOH results in an adverse impact on magnetizing roasting of hematite, iron in silicate minerals could be released by NaOH-Ca(OH)2 additive as a form of hematite, which is further reduced to ferromagnetic magnetite. Moreover, the reduction ability of gas is improved in presence of coal and water from decomposition reaction of Ca(OH)2. Regarding the decomposition of bastanasite, no evident effect of NaOH-Ca(OH)2 is observed. The roasted products of pristine bastnasite particles are mainly composed of rare earth oxides (REO) with little fluorine, independent on the use of alkaline additives. No high purity REO were determined in the monazite particle by SEM-EDS, which is mainly decomposed into REO and REFeO3.