Ce-doped nano-α-Fe₂O₃ was successfully synthesized via the hydrothermal method. The properties of the prepared particles were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. It was found that the Ce element can be doped into the α-Fe₂O₃ lattice resulting in lattice distortion, which can refine the grain and improve the crystal surface’s integrity significantly. In addition, doping of Ce element can shorten the Fe–O bond length in the α-Fe₂O₃ crystal, cause a blue shift of the stretching vibration band, enhance binding energy of Fe–O and the chemical stability of the α-Fe₂O₃ crystal.

Because of the acute toxicity of Cr(VI)-bearing substances, the pollution problem caused by chromite process residue has become a worldwide concern. In the view of relevant studies, the technologies based on the alkali treatment cannot fundamentally resolve the pollution problem, because the oxidation of Cr(III) to Cr(VI) is unavoidable during chromite decomposition. In contrast, the oxidation of Cr(III) to Cr(VI) can be controlled by the sulfuric acid treatment of chromite, and the Cr(VI) pollution can be eliminated from the original source of production. Many research studies focusing on the resolutions of the key obstacles hindering the development of the sulfuric acid treatment process have been carried out, and significant progress has been achieved. In this study, a clean hydrometallurgical process without the generation of hexavalent chromium is demonstrated. First, the chromite was decomposed and leached by sulfuric acid solution in the presence of an oxidant. Then, iron was hydrothermally removed from the acid solution as the precipitate of jarosite. Finally, chromium salts were obtained by adjusting the basicity of the solution, separation and drying. With the aim of realizing industrialization, future research emphasis on the development of the sulfuric acid treatment process is proposed in this study.

Oxide layer formed on stainless steel surface can result in a decrease in surface quality. Thus, it must be removed. In this paper, we investigated the effect of different pickling processes on removal of the oxide layer. Ferritic stainless steel was used as raw material. After it received annealing treatment, the components and structure of the oxide layer formed on its surface were studied. Then, the annealed stainless steel was pickled in hydrochloric acid solution system and mixed acid solution system, respectively. Results are as follows. The oxide layer on stainless steel surface has a thickness of 15–35 μm. With increase in hydrochloric acid concentration in hydrochloric acid pickling solution, the weight loss rate of stainless steel increases and is always greater than that in mixed acid pickling system. In addition, the surface of the stainless steel pickled in hydrochloric acid is smoother than that pickled in mixed acid. Furthermore, compared with channels in the oxide layer created during mixed acid pickling, those created during hydrochloric acid pickling are more, leading to shorter removal time. These results are important for increasing the pickling efficiency and improving the surface quality of stainless steel.

Sulfuric acid-based leaching is a promising cleaner method to produce chromium salts, but its feasibility for treating low Fe(II)-chromite still remains to be proven. A Box–Behnken design (BBD)-based set of experiments for sulfuric acid leaching of low Fe(II)-chromite was utilized in this work for generating an experimental dataset for revealing the functional relationships between the processing parameters and the extraction yields of Cr and Fe. The dependent variables were found to exhibit strong intercorrelations and the models developed on the basis of statistical criteria showed excellent prediction accuracy. The optimum process conditions of leaching treatment were found to be a temperature of 176 °C, a dichromic acid/chromite mass ratio of 0.12, and a sulfuric acid concentration of 81%. Furthermore, the dissolution behavior of chromite in the leaching process and the effect of dichromic acid were experimentally investigated. It was found that the decomposition efficiency was highly dependent on the Fe(II) content of chromite, and that the dichromic acid acted both as an oxidant and a catalyst in the leaching process. On the basis of the results of this study, a novel process for treating low-Fe(II) chromite was proposed.

The sodium salt roasting process was widely used for extracting vanadium due to its high yield rate of vanadium. However, the serious pollution was a problem. The calcium roasting process was environmentally friendly, but the yield rate of vanadium was relatively lower. Focusing on the calcium metavanadate produced in the calcium roasting process of vanadium minerals, the mechanism of the carbonate leaching for calcium metavanadate and its leaching kinetics of calcium metavanadate were studied. With the increase of the leaching agent content, the decrease of the particle size, the increase of the temperature and the increase of the reaction time, the leaching rate of vanadium increased, and the constant of reaction rate increased. In the carbonate leaching process, the calcium carbonate was globular and attached to the surface of calcium metavanadate. In the solution containing bicarbonate radical, lots of cracks formed in the dissolution process. However, the cracks were relatively fewer in the solution containing carbonate. In the present study, the carbonate leaching for calcium metavanadate was controlled by diffusion, the activation energy reached maximum and minimum in the sodium bicarbonate and the sodium carbonate solution, respectively. The activation energy value in the ammonium bicarbonate solution was between those two solutions. The kinetic equations of the carbonate leaching for calcium metavanadate were as follows: 1 − 2/3η − (1 − η)^2/3 = 4.39[Na₂CO₃]^0.75/r₀ × exp(−2527.06/T)t; 1 − 2/3η − (1 − η)^2/3 = 7.89[NaHCO₃]^0.53/r₀ × exp(−2530.67/T)t; 1 − 2/3η − (1 − η)^2/3 = 6.78[NH₄HCO₃]^0.69/r₀ × exp(−2459.71/T)t.