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Hydrothermal leaching vanadium using oxalic acid is a novel method reported recently to overcome the serious environmental problems caused by traditional extracting processes. In view of its promising application potential, the hydrothermal leaching kinetics of vanadium from a concentrate mainly composed of Fe3−xVxO4 mineral via oxalic acid were investigated in this study. Firstly, the effects of the temperature and concentration of oxalic acid on the leaching behavior of vanadium were studied by measuring the leaching efficiency of vanadium at various times. Then, by fitting the measured leaching efficiency data to the proposed kinetic model, the leaching mechanism was analyzed and the rate-controlling step of the leaching process, the apparent activation energy, and the order of the chemical reactions were determined. Finally, a kinetic model was proposed to describe the present investigated leaching process. Detailed results are as follows: (1) an interfacial chemical reaction was the rate-controlling step of the present hydrothermal leaching process within temperatures ranging from 363 to 403 K, and the leaching efficiency was less than 85%; (2) the apparent activation energy of the interfacial chemical reaction was 45.6 kJ/mol; (3) the order of the interfacial chemical reaction to the concentration of oxalic acid was around 1.66. [ABSTRACT FROM AUTHOR]

In the present study, a process of separating high-quality TiO2 from an oxalic-acid leachate of vanadium slag was proposed. It consists of two steps; oxalic acid was firstly recovered from the leachate by the cooling-crystallization method, and subsequently TiO2 was separated from the oxalic-acid recovered leachate by the hydrothermal precipitation method. The experimental results indicate that oxalic acid can be recovered from the leachate by cooling crystallization at 5 °C, and after the recovery of oxalic acid, the purity of final TiO2 product can also be improved. For example, when the leachate was cooled directly at 5 °C for 5 h, about 7% of oxalic acid was recovered, and the purity of final TiO2 product improved from 95.7% to 96.6%. Furthermore, it was found that when some HCl solution was added to the leachate, both the recovery percentage of oxalic acid and the purity of TiO2 product increased. For instance, when 15 vol% of HCl solution relative to pregnant leachate was added, about 35% oxalic acid was recovered by cooling crystallization at 5 °C for 3 h, and the anatase TiO2 product with a purity of 99.2% was obtained by hydrothermal precipitation at 140 °C for 2.5 h. [ABSTRACT FROM AUTHOR]

Although vanadium slag contains various valuable metals, including vanadium, titanium, chromium, iron, etc., it is only used to extract the vanadium due to technical limitations. In order to realize the comprehensive utilization of vanadium slag, a new approach was proposed in this work. First, the hydrogen reduction of vanadium slag was investigated. The phase and micromorphology evolution of the vanadium slag under different reduction conditions were discussed in detail. The results showed that the pyroxene and olivine surrounding the spinel in vanadium slag were reduced selectively into metallic iron and silica under appropriate reduction conditions, and the structures were destroyed. Then, the method of recovering metallic iron from the reduced vanadium slag with ferric chloride solution was investigated. The results showed that more than 98 pct of metallic iron in the reduced vanadium slag can be leached selectively by ferric chloride solution, and the vanadium, titanium, and chromium were left in the deironized intermediate. Finally, the extraction of vanadium, titanium, and chromium from the deironized intermediate by the oxalic acid hydrothermal leaching method was studied, and the leaching recoveries were 96.8, 94.7, and 95.4 pct, respectively. This approach provides insights into the comprehensive utilization of vanadium slag, which is especially favorable for low-grade vanadium slag. [ABSTRACT FROM AUTHOR]

To co-extract vanadium, titanium, and chromium from vanadium slag, a cleaner process featuring oxalic acid hydrothermal leaching with synergy of Fe powder was proposed in this paper. With the leaching temperature of 125 °C, oxalic acid concentration of 25 wt pct, liquid-to-solid mass ratio of 8:1, leaching time of 90 minutes, and iron powder addition of 3.2 wt pct, the leaching extents of vanadium, titanium, and chromium can reach up to 97.9, 98.4, and 93.3 pct, respectively. The non-toxic residue is mainly composed of FeC2O4·2H2O, MnC2O4·2H2O, and amorphous SiO2. Compared with the traditional sodium roasting-water leaching and calcification roasting-acid leaching processes, this novel process is possessed of two prominent advantages. Firstly, without the roasting step, the process is simplified significantly and the energy consumption is reduced greatly. Secondly, the valuable components titanium and chromium which cannot be extracted by the traditional processes are leached synchronously and efficiently. [ABSTRACT FROM AUTHOR]

The first- and second-order interaction coefficients of V in Fe-C-V melts were accurately measured using the chemical equilibrium technique, by equilibrating CaO-MgO-Al2O3-VOx slags with Fe-C melts under controlled oxygen potential for 24 hours at 1873 K (1600 °C). The argon was employed as the protective gas and a purification device was used to control the oxygen potential in the atmosphere. The values of the interaction coefficients were determined as follows: e V C = - 0.468 , r V C = 0.286 , r V V,C = - 0.213 [ABSTRACT FROM AUTHOR]

To ascertain the thermodynamic conditions for the enrichment of vanadium in V-bearing steelmaking slag and V-bearing black shale, the effects of alumina content and basicity on the formation of phases and distribution of vanadium in CaO-SiO2-FeOt-MgO-Al2O3-V2O3-P2O5 slag system at 1573 K (1300 °C) were investigated by adopting the high-temperature quenching method. The phases formed in the samples were characterized by X-ray diffractometry and scanning electron microscopy combined with energy dispersive spectroscopy. For slags with a basicity of 1.7, it was found that nearly all of the vanadium were concentrated in the (Fe, Mg)O·(Al, Fe, V)2O3 spinel. For slags with basicities of 1.5 and 1.3, vanadium was almost not found in the supercooled glass phase when the alumina content was less than 6 mass pct. However, some vanadium was present in the supercooled glass phase when the alumina content was more than 6 mass pct. For slags with a basicity of 1.0, vanadium migrated between the V-rich spinel and the supercooled glass phase. With an increase in the alumina content from 2 to 10 mass pct, the vanadium content in the spinel phase decreased from 28 to 33 mass pct to approximately 10 mass pct. From these results, it can be concluded that the increase in alumina content and decrease in basicity of the slag are unfavorable for the enrichment of vanadium present in the spinel. [ABSTRACT FROM AUTHOR]

In this study, two aspects concerning with the thermodynamics of CaO–SiO2–VOx system were investigated. One aspect is about the effects of oxygen partial pressure on the phase relationships in CaO–SiO2–VOx system at 1873 K, and the other is the measurement of the standard Gibbs energy of formation of the vanadium compounds. In the first aspect, the phase relationships in CaO–SiO2–VOx system at 1873 K under oxygen partial pressure of 1.7 × 10−9 atm were determined, and the isothermal section diagram was constructed. Furthermore, by comparing this diagram with that under 6.9 × 10−11 atm, the effects caused by oxygen partial pressure on this system were elucidated. With the increase in oxygen partial pressure, the compound CaV2O4 that exists stably under 6.9 × 10−11 atm is oxidized to V2O3 and CaVO3. The solubility limit of V2O3 in liquid phase also increases remarkably, which results in the enlargement of the single liquid area. In the second aspect, the standard Gibbs energy of formation of CaV2O4, CaVO3, V2O3, and Ca2Si1−δVδO4 (0 < δ < 0.1) at 1873 K were measured. The determined values for CaV2O4 and CaVO3 are −1 259 468.9 ± 5090.5 J/mol and −963 479.8 ± 3298.6 J/mol, respectively. Then, the limit of oxygen partial pressure for CaV2O4 existing stably at 1873 K is determined to be 4.4 × 10−10 atm. [ABSTRACT FROM AUTHOR]

The dissolution of vanadium and potassium from waste vanadium catalyst using FeSO4-H2SO4 solution was studied. The effects of stirring time, temperature, stirring speed, liquid-to-solid mass ratio, Fe2+ concentration and waste vanadium catalyst particle size on leaching of vanadium and potassium were examined. The results show that the optimum conditions are as follows: stirring time of 5 min, water bath temperature of 20~30 °C,stirring speed of 200 r/min, liquid-to-solid mass ratio of 6 : 1, FeSO4 * 7H2O mass concentration of 0.028 g/mL, the proportion of waste vanadium catalyst particle with porticlal size -200 mesh of 91%. [ABSTRACT FROM AUTHOR]

Stone coal, a kind of important vanadium-bearing resources in China, accounts for about 87% of the domestic reserves of vanadium. But the currently developed techniques to extract vanadium from stone coal are all confronted with environmental problems. For example, large quantities of caustic or greenhouse gases discharged in the oxidizing roast process, enormous consumptions of acid or alkali in the leaching step and massive tailings containing toxic V 5+ ions. With a view to solve those problems, an innovative process for pre-concentration of vanadium from stone coal was designed and confirmed on a laboratory scale. The distinct features of this design are as follows. Firstly, the environment-friendly Fe 2 O 3 is selected as a capturer of vanadium. Secondly, the natural reducing property of stone coal is utilized to promote the formation of magnetic V-rich phase, Fe 2 VO 4 . Thirdly, a high-grade vanadium concentrate is obtained by magnetic separation method. The practicability of this design was confirmed using industrial stone coal on a laboratory scale. When 10 mass% Fe 2 O 3 relative to stone coal was added and roasted at 1200 °C for 3 h, a high-grade concentrate containing vanadium higher than 14 mass% was obtained, and more than 91% of the total vanadium in stone coal was recovered. [ABSTRACT FROM AUTHOR]

The separation of titanium, vanadium and chromium in vanadium slag (VS) is a difficult problem restricting the comprehensive utilization of VS. This paper presents the first study on the separation of titanium, vanadium and chromium from oxalic acid leachate of VS. Firstly, the separation of titanium from the leachate by hydrothermal method was studied. The results show that more than 99% of titanium in the leachate was precipitated in the form of spherical anatase TiO2 with the purity of 95.7%. Then, the extraction separation of vanadium and chromium from the titanium-free filtrate by three-stage extraction of acidified N235 extractant and four-stage stripping of HCl solution was investigated. The extraction mechanism was identified as the anion exchange reaction between acidified N235 extractant and vanadium and chromium complex anions, which were further stripped by HCl solution in the stripping process. After obtaining the concentrated and purified stripping solution containing vanadium and chromium, the separation of vanadium and chromium from the stripping solution by hydrothermal method was studied, and the product was mainly composed of VO2 and Cr2O3. This process provides an idea for the comprehensive utilization of titanium, vanadium and chromium in oxalic acid system. [ABSTRACT FROM AUTHOR]

To alleviate the energy consumption and the effluent of traditional oxidizing roasting and leaching process of vanadium extraction, a novel method to extract the vanadium from iron vanadate spinel mineral (Fe 3 -x V x O 4) by one leaching step was developed. In this method, the Fe powder was found as a synergist of oxalic acid to leach the vanadium as [V(C 2 O 4) 3 ]3− complex under hydrothermal condition. With the synergy of Fe powder, the leaching efficiency of V3+ ions from iron vanadate spinel concentrate was enhanced significantly, and simultaneously Fe3+ ions present as impurity in the pregnant leachate were eliminated effectively by forming FeC 2 O 4 ·2H 2 O precipitate. Using the most suitable conditions, the leaching efficiency of vanadium can reach 95%, reducing Fe3+ to less than 100 mg/L. Furthermore, FeC 2 H 2 O 4 ·2H 2 O with 98% purity could be obtained as a by-product by washing the filtered cake. • V3+ ions in the Fe 3- x V x O 4 mineral was extracted directly by one leaching step. • Fe powder was found as a synergist of oxalic acid to enhance the V3+ leaching. • Simultaneously, Fe3+ ions in solution were transformed to FeC 2 H 2 O 4 ·2H 2 O by-product. • The V leaching rate of 95%, and the FeC 2 O 4 ·2H 2 O precipitation with 98% purity were obtained. [ABSTRACT FROM AUTHOR]

Aiming at alleviating the difficulties of extracting vanadium from high-carbon stone coal and solving the relevant pollution problem, a new synergetic and cleaner method combining concentration of vanadium with clean utilization of carbon was proposed and corroborated. Firstly, Fe 2 O 3 , acting as the source for the catalytic of carbon gasification and the capturer of vanadium, was added to the raw ore. Then, the carbon within the raw ore was gasified by water vapor at 800–900 °C under the catalysis of reducing product of Fe 2 O 3 (Fe 3 O 4) to generate synthesis gas (CO + H 2), and subsequently the vanadium dispersive occurred in the ore was captured by Fe 3 O 4 at 1100–1200 °C under Ar atmosphere to form Fe x V 3-x O 4. Finally, the Fe x V 3-x O 4 phase was separated magnetically, and a high-grade vanadium concentrate was obtained. The experimental results show that the Fe 3 O 4 effectively promoted the gasification rate and ratio of carbon. When 10 mass% Fe 2 O 3 relative to raw ore was added and roasted at 850 °C for 2 h, the gasification ratio of carbon increased by 26%. The gasification ratio of carbon could reach up to 90.2%. Besides, the Fe 3 O 4 could react with the V3+ components to form magnetic V-rich phase, Fe x V 3-x O 4 , and the vanadium concentrate was obtained by magnetic separation. Under the optimal conditions, the vanadium concentrate with a metallic vanadium content of 6.35 mass% was obtained by magnetic separation, which was 23.5 times higher than the raw ore. The recovery ratio of vanadium reached 85.6%. • A synergetic method for cleaner utilization of high-carbon stone coal is proposed. • Fe 2 O 3 acts as the source for the catalytic of C gasification and the capturer of V. • Water vapor is used to gasify carbon, and CO and H 2 can be formed at 800–900 °C. • Magnetic V-rich phase can be formed at 1200 °C and extracted by magnetic separation. • C/V in the raw ore can be cleanly and synergistically utilized by one-step roasting. [ABSTRACT FROM AUTHOR]

• V can be precipitated by hydrothermal decomposition of [V(C 2 O 4) 3 ]3− complex ions. • The VO 2 hydrothermal synthesis method was introduced into V precipitation process. • Based on the vanadium precipitation, the VO 2 product with 97% purity was obtained. • The hydrothermal vanadium precipitation method can achieve a zero emission process. To achieve the clean extraction of vanadium from iron vanadate spinel minerals, vanadium has been successfully leached as [V(C 2 O 4) 3 ]3− complex by using oxalic acid as a leaching agent in our previous study. However, the precipitation of vanadium from the V-bearing oxalic acid leaching solution has never been reported. Therefore, an environment-friendly hydrothermal precipitation method was applied in vanadium precipitation process in this study. Firstly, vanadium was effectively precipitated from the V-bearing oxalic acid leaching solution by hydrothermal decomposition of [V(C 2 O 4) 3 ]3− complex ions at the temperature above 200 °C to obtain a mixture of vanadium oxides. Based on the vanadium precipitation, to further obtain high purity vanadium oxide product, the hydrothermal precipitation of VO 2 from the V-bearing oxalic acid leaching solution was investigated. Before the hydrothermal reaction, an excess hydrogen peroxide solution was employed to oxidize the [V(C 2 O 4) 3 ]3− complex ions to form VOC 2 O 4 in advance. Then, the effect of temperature, the quantity of added V 2 O 5 , and reaction time on the VO 2 product preparation and vanadium precipitation extent were investigated. Finally, the uniform and well-crystallized VO 2 (B) with 97% purity was obtained, and higher than 99% of vanadium precipitation extent was achieved under the optimum precipitation conditions. [ABSTRACT FROM AUTHOR]

In order to recover vanadium from vanadium-bearing black shale efficiently, a hydrometallurgical process is developed in this paper to treat the concentrate beneficiated from stone coal ore, in which the vanadium mainly occurs in the form of Fe 3- x V x O 4 spinel. At the first step, the leaching behaviors of Fe and V from the concentrate by using sulfuric acid solution are investigated in detail. The extraction rates of vanadium and iron could reach 92.5% and 98.5% respectively under the optimum leaching conditions of leaching temperature of 90 °C, sulfuric acid concentration of 25 wt%, leaching time of 90 min, L/S mass ratio of 30 and stirring speed of 600 rpm. Secondly, the iron impurity in the pregnant solution is removed efficiently by the precipitation method with Fe 2 O 3 as nucleating agent. The effect of pH value, the amount of Fe 2 O 3 nucleating agent and the precipitating time on the removal of iron and losing of vanadium are systematically investigated, and thereby >99.7% iron impurity can be removed with the loss rate of vanadium not higher than 2.5%. Finally, vanadium is precipitated from the purified leach solution as ammonium poly-vanadate by adding ammonium sulfate, and then the product of V 2 O 5 is obtained by calcining the ammonium poly-vanadate. The purity of V 2 O 5 powder can reach 92.0% approximately. • The leaching behaviors of V and Fe from a concentrate beneficiated from stone coal were investigated systematically. • By using powder Fe 2 O 3 as nucleating agent, the iron impurities in the pregnant solution were removed effectively. • V 2 O 5 powder with purity about 92.0 % was prepared, and the total recover rate of vanadium reached 85.0 %. [ABSTRACT FROM AUTHOR]