In this report, the vanadium recovery mechanisms by novel BaCO₃/CaO composite additive roasting and acid leaching technology, including the phase transformations and the vanadium leaching kinetics, were studied. The purpose of this manuscript is to realize and improve the vanadium recovery from stone coal using BaCO₃/CaO as the composite additive. The results indicated that during the composite additive BaCO₃/CaO roasting process, the monoclinic crystalline structure of muscovite (K(Al,V)₂[Si₃AlO₁₀](OH)₂) was converted into the hexagonal crystalline structure of BaSi₄O₉ and the tetragonal crystalline structure of Gehlenite (Ca₂Al₂SiO₇), which could, therefore, facilitate the release and extraction of vanadium. Vanadium in leaching residue was probably in the form of vanadate or pyrovanadate of barium and calcium, which were hardly extracted during the sulfuric acid leaching process. The vanadium leaching kinetic analysis indicated that the leaching process was controlled by the diffusion through a product layer. The apparent activation energy could be achieved as 46.51 kJ/mol. The reaction order with respect to the sulfuric acid concentration was 1.1059. The kinetic model of vanadium recovery from stone coal using novel composite additive BaCO₃/CaO could be finally established.

In this study, a novel NaOH molten roasting and water leaching technology, an eco-friendly process with good selectivity and good availability, was investigated for vanadium recovery from refractory stone coal. In addition, the phase transformations during the vanadium recovery process were also studied. During the NaOH molten roasting process, the monoclinic crystalline structure of muscovite (K(Al,V)₂[Si₃AlO₁₀](OH)₂) was converted into the orthorhombic crystalline structure of Na₂SiO₃ and the tetragonal crystalline structure of gehlenite (Ca₂Al₂SiO₇); this could promote the liberation and recovery of vanadium. During the water leaching process, the tetragonal crystalline structure of gehlenite (Ca₂Al₂SiO₇) was converted into the monoclinic crystalline structure of Ca₃Si₂O₇·H₂O. Ca₃Si₂O₇·H₂O, a kind of C–S–H gel, produced during the water leaching process probably covered the surface of the roscoelite and thus prevented effective vanadium recovery from roscoelite.

A novel staged leaching process has been reported in this paper to selectively extract vanadium from roasted stone coal and the mechanisms have been clarified. Results showed that the leaching efficiency of V, Al, P and Fe was 80.46%, 12.24%, 0.67% and 3.12%, respectively, under the optimum dilute sulfuric acid dephosphorization (DSAD)-two-stage pressure acid leaching (PAL) conditions. The efficient separation of V from Fe, Al and P was realized. As apatite could be leached more easily than mica, the apatite could completely react with sulfuric acid, while the mica had almost no change in the DSAD process, which was the key aspect in realizing the effective separation of V from P. Similarly, the hydrolyzation of Fe and Al could be initiated more easily than that of V by decreasing the residual acid of leachate. The alunite and iron-sulphate compound generated in the first-stage PAL process resulted in the effective separation of V from Fe and Al.

The particle size of the precursor is reduced, while it is easier for N ₂ to enter the material. A new porous structure appeared and the specific surface area increased, which increases the reaction interface.

In view of the current situation wherein acid resources and valuable components in titanium dioxide waste acid cannot be effectively extracted and are prone to secondary pollution, our research team proposed a new technique consisting of step extraction and the comprehensive utilization of titanium dioxide waste acid. In this paper, the thermodynamics of selective precipitation and the preparation of doped iron phosphate from waste acid were studied. The thermodynamics results show that the content of Al³⁺, Mn²⁺, Mg²⁺, and Ca²⁺ in the reaction system can be tuned by adjusting the pH during the pre-precipitation process. In the first step, these impurity ions should be settled as much as possible; then, Fe²⁺ should be oxidized to Fe³⁺ so as to obtain iron phosphate with higher purity in the next step of the precipitation process. The effects of the reaction temperature, seed crystals, pH value, and P/M on the precipitation process were investigated in detail. The experimental results show that in the reduced state, the optimal precipitation conditions are a temperature of 75 °C, an initial pH value of 4.5, and an optimal P/M molar ratio of 1.1. In the oxidized state, the optimal precipitation conditions are a temperature of 60 °C, a solution pH = 2.5, and a reaction time of 25 min. After calcination, the precipitate mainly consists of iron phosphate, which basically meets the requirements of an iron phosphate precursor.

The problem of spent fluid catalytic cracking (SFCC) catalyst resource utilization, draws more and more attention to system analysis. SFCC was leached in an oxalic solution for comprehensive utilization. The results showed that for a D50 ≤ 17.34 μm, the catalyst leached for 240 min at 95 °C in the presence of a 2 mol/L oxalic acid solution, and the extent of leaching of V, Ni, Fe, and Al was 73.4%, 32.4%, 48.2%, and 36.8%, respectively. Studies on the occurrence state of the main ions (V, Ni, Fe, and Al) in the leaching solution were presented. Additionally, the separation of the main ions from such a solution by the “solvent extraction-stripping-hydrothermal precipitation-comprehensive recovery of valuable metal” process was studied. The immobilization rates of vanadium and nickel in geopolymers can be obtained using the toxicity characteristic leaching procedure (TCLP) test, and the geopolymers prepared by SFCC leaching residues can be considered a non-hazardous material. A process diagram of the comprehensive utilization of SFCC catalysts is presented.