Experimental results here can give some new insights into the mechanism of selective recovery of scandium and sodium from high alkali Bayer red mud（RM） through sulfation-roasting-leaching process. Effects of roasting and leaching conditions including roasting time, roasting temperature, concentrated H_2SO_4 addition, leaching temperature, leaching time and liquid to RM solid ratio on the leaching rates of calcium, iron, aluminum, silicon, sodium, titanium, scandium and gallium were studied and analyzed, suggesting that roasting temperature and roasting time were the two primary constraints on selective recovery of Sc and Na. High leaching temperature also brought a negative effect on the iron leaching rate. Phase transitions and thermal behaviors of sulfated RM indicated that sodium had an inhibitory action on the liberation of SO_2 or SO_3 from metal sulfates, which should follow the decomposition order of TiOSO_4〉Ga_2（SO_4）_3〉Fe_2（SO_4）_3〉NaFe（SO_4）_2〉NaAl（SO_4）_2~Al_2（SO_4）_3〉Na_3Sc（SO_4）_3〉Na_2SO_4〉CaSO_4. After water leaching, solid-liquid separation could be carried out extremely smoothly and 〉95 wt.% Na＋, ~60 wt.% Sc in [Sc（H_2O）_x（SO_4）_n]^（3–2n）（x≤6） with impurities of 0 wt.% Fe^（3＋）, 0 wt.% Ti4＋, 0 wt.% Ga^（3＋）, 7 wt.% Al^（3＋）, ~29 wt.% Ca^（2＋） and ~3 wt.% Si^（4＋） could be leached into leachant under the optimized roasting and leaching conditions. The alkali-free residue obtained could then be employed as iron-making or building materials.
A large amount of coal gangue from coal mining and processing is regarded as waste and usually stockpiled directly. In order to recycle the valuable elements from the coal gangue, an integrated process is proposed. The process consists of three steps: 1concentrating alumina from the coal gangue via activation roasting followed by alkali leaching of Si O2 which produces alumina concentrate for alumina extraction by the Bayer process; 2) synthesizing tobermorite whiskers from the filtrated alkali liquo containing silicate via a hydrothermal method and reusing excess caustic liquor; and 3) enriching titanium component from the Baye process residue by sulfuric acid leaching. Alumina concentrate with 69.5% Al_2O_3 and mass ratio of alumina to silica(A/S) of 5.9pure 1.1 nm tobermorite whisker and TiO_2-rich material containing 33% TiO_2 are produced, respectively, with the optimal parameters Besides, the actual alumina digestion ratio of alumina concentrate reaches 80.4% at 270 oC for 40 min in the Bayer process.
To elucidate the intrinsic reaction mechanism of cementitious materials composed of red mud and coal gangue（RGC）, the hydration kinetics of these cementitious materials at 20°C was investigated on the basis of the Krstulovi？-Dabi？ model. An isothermal calorimeter was used to characterize the hydration heat evolution. The results show that the hydration of RGC is controlled by the processes of nucleation and crystal growth（NG）, interaction at phase boundaries（I）, and diffusion（D） in order, and the pozzolanic reactions of slag and compound-activated red mud-coal gangue are mainly controlled by the I process. Slag accelerates the clinker hydration during NG process, whereas the compound-activated red mud-coal gangue retards the hydration of RGC and the time required for I process increases with increasing dosage of red mud-coal gangue in RGC.
