A novel process for recovering iron, niobium and scandium from Bayan Obo tailings has been developed. In this paper, the treatment of Bayan Obo tailings by Ca(OH)₂-coal roasting and sulfuric acid leaching process was investigated. In the Ca(OH)₂-coal roasting process, niobium-bearing minerals are converted to CaTiO₃ structure of Ca(Ti_0.8,Fe_0.1,Nb_0.1)O₃ and Ca₂Nb₂O₇ which are soluble in sulfuric acid. The pyroxene and amphibole that are Sc-bearing silicates are mainly decomposed and reduced into metallic iron which can be recycled by weak magnetic separation. Scandium in the silicates is converted to Sc₂O₃. In the sulfuric acid leaching process, Ca(Ti_0.8,Fe_0.1,Nb_0.1)O₃ and Ca₂Nb₂O₇ are converted to Nb(OH)₅ that could easily dissolve in sulfuric acid by dissociating into Nb(OH)⁴⁺ and OH⁻ when Sc₂O₃ is dissolved into heat sulfuric acid. Bayan Obo tailings were roasted with Ca(OH)₂-coal at elevated temperature, followed by magnetic separation and sulfuric acid leaching. The optimized experimental parameters are proposed as follows: Ca(OH)₂-coal-tailings mass ratio of 20: 5: 100; roasting at 1200 °C for 2 h; the magnetic field (magnetic separation) of 270 mT; the liquid-solid ratio of 4:1 (ml/g); leaching at 245 °C for 1 h. Iron concentrate with a grade of 88.39% and a recovery rate of 91.92% is obtained. The leaching results show that the leaching rates of niobium and scandium could achieve 95.52% and 95.75%, respectively.

A novel environmentally friendly one-step decomposition strategy for mixed rare earth concentrate of Bayan Obo in sulfuric acid solution was proposed in this work. In this process, more than 84% of bastnasite and monazite were decomposed in the leaching step at a temperature lower than the boiling point of sulfuric acid solution. So, the dilapidation of sulfuric acid in this current proposed process will be reduced to a large extent. The stability region of rare earth ion in the RE(La, Ce, Nd)-F-P-SO₄-H₂O system at 170 °C has been proven through Eh-pH diagrams. The factors influencing decomposition of rare earth concentrate in this process were also investigated and the optimal leaching conditions were determined to be a leaching temperature of 170 °C with an ore/acid ratio of 1:5 (g/mL), a sulfuric acid concentrate of 75% and a leaching time of 80 min. The mineralogical changes occurring during the H₂SO₄ leaching process were investigated by X-ray diffraction and SEM-EDS. The analysis results showed that bastnasite and most of monazite had been decomposed, leaving only a small amount of monazite in the leaching residue.

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.