The iron resources in nickel slag were recycled by oxidation and magnetic separation. The effects of holding time, temperature, air flow rate and basicity on the crystallization of magnetite were investigated systematically. Moreover, the influence of particle size and magnetic flux density on the recovery and grade of iron during the magnetic separation was also explored. Results showed that the magnetite particles were significantly influenced by holding time, and the average diameter size reached about 20 μm after holding for 20 min at 1623 K. The holding temperature obviously affected the microstructure of magnetite phases: with the increase in holding temperature, the shapes of the magnetite particles changed from polyhedral form to skeletal particles. As the air flow rate was increased to 170 mL/min, the magnetite developed into tiny spherical particles due to the strong stirring. It was also found that the crystallization of magnetite was slightly effected by basicity. The iron recovery reduced with the decrease of particle size, while the iron grade first increased to a maximal value of 38 μm, and then decreased. As the magnetic flux density increased, the iron recovery initially increased rapidly, reaching a maximal value at 120 mT, while the iron grade remained almost constant. The final iron recovery and grade were 75.99% and 54.08%, respectively, via multi-step magnetic separation instead of single magnetic separation. Iron in concentrate mainly exists in the form of magnetite and magnesium ferrite, and contents of siderophile elements (Ni, Co) in final concentrate were also higher than that of raw slags.

To recover iron from water-quenched nickel slags, CaO was added. Thermodynamic analysis showed that CaO promotes the reaction between fayalite (Fe₂SiO₄) and O₂. Phase diagrams of the FeO-SiO₂-MgO-CaO slag with various CaO contents in an air atmosphere drawn by FactSage 7.1 showed that the phase components can be significantly affected by the CaO contents. With increasing CaO content, the fusion characteristic temperatures decreased rapidly to a minimum and subsequently increased slightly. The oxidization of Fe₂SiO₄ in nickel slags was accelerated significantly by the addition of CaO, which led to an increase of FeO activity and decrease of Fe₂O₃ activity to promote the formation of MgFe₂O₄. Excess addition of CaO led to the formation of more silicates. In addition, the crystallization temperature was also reduced with increasing CaO content, causing less spinel to crystalize. With increasing CaO content, the iron recovery and yield of concentrate first increased and subsequently decreased, while the total iron (TFe) content was almost not influenced and maintained a relatively stable value.

The use of photocatalytic CO₂ reduction as a green technology has attracted the attention of scholars. Nevertheless, the lower visible-light utilisation and photocatalytic efficiency of catalysts remain a challenge. In this work, Bi_xO_yBr_z photocatalysts were synthesised using a hydrothermal method by adjusting the molar ratio of Bi(NO₃)₃·5H₂O and C₁₉H₄₂BrN (Bi:Br ratio) and the pH value of the precursor solution. The obtained samples were characterised, and the CO₂ reduction performance was tested. The results showed that the phase composition for most of the samples was Bi₄O₅Br₂, and BiOBr or Bi₅O₇Br was also confirmed in a small number of samples. Owing to the effects of pH and the Bi:Br ratio on the reaction process, BiOBr→Bi₄O₅Br₂→Bi₅O₇Br transformation occurred. Acidic conditions are conducive to the formation of BiOBr. In alkaline environments, bismuth-rich Bi₄O₅Br₂ or even Bi₅O₇Br easily forms. Bi₄O₅Br₂ has self-assembled microsphere and irregular polyhedron morphologies. The polyhedron Bi₄O₅Br₂ results in CO and CH₄ yields of 10.34 μmol·g⁻¹·h⁻¹ and 1.86 μmol·g⁻¹·h⁻¹ in CO₂ reduction, respectively. Although the microsphere Bi₄O₅Br₂ has a maximum light absorption wavelength of 438 nm, the polyhedron Bi₄O₅Br₂ has the best photocatalytic CO₂ reduction performance and CO selectivity. This work describes the controllable preparation of Bi₄O₅Br₂ at various pH values and Bi:Br ratios and the optimisation of its photocatalytic performance.

In order to decrease the oxidation temperature of fayalite in molten nickel slags, the influence of CaO addition, FeO/SiO₂, and MgO/SiO₂ on melting characteristic temperatures of FeO-SiO₂-MgO-CaO system were investigated. The melting characteristic temperatures, including softening temperature (T_s), hemispherical temperature (T_h), and flow temperature (T_f),were measured with hot stage microscope. Experimental results showed that the melting characteristic temperatures first decreased sharply and then increased slowly with increasing w(CaO) for samples at given FeO/SiO₂ and MgO/SiO₂ ratios. The melting characteristic temperatures of samples without CaO addition were much higher than that of samples with different CaO addition, which was mainly caused by the phase components at different compositions. The variation of melting characteristic temperatures were described in detail. Phase components and liquid fractions were calculated by FactSage 7.1, which was good agreement with the XRD patterns.