Determination of Vanadium Valency in Roasted Stone Coal by Separate Dissolve-Potentiometric Titration Method
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Determination of Vanadium Valency in Roasted Stone Coal by Separate Dissolve-Potentiometric Titration Method
Bao Shenxu1, Zhang Yimin1, 2, Hang Jing2, Yang Xiao2 and Hu Yangjia1 1
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Wuhan, 430070, PR China. E-mail: [email protected] 2 College of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan, 430081, PR China. ABSTRACT Stone coal is an important vanadium-bearing resource in China. Most vanadium exists in stone coal as V(III), which is stable and not easily to be extracted. The V(III) should be oxidized to V(IV) and/or V(V) by roasting with additives at high temperature and then extracted by acid leaching and/or water leaching. Hence, the vanadium valency in roasted stone coal can reflect the roasting efficiency and leaching rate. In traditional digestion process, the V(V) can oxidize V(III) in solution and this causes great error to the determination of vanadium valency. In this study, the V(IV) and V(V) in roasted stone coal is dissolved firstly in 5% of hydrochloric acid at room temperature for 1h because the V(III) embedded in crystal lattice can not dissolve in dilute acid. The acid solution containing V(IV) and V(V) is titrated by 0.02 M ammonium ferrous sulfate (AFS), and the jump in titration curve indicates the reducing of V(V) to V(IV) by ferrous ion. The volume of V(V) can be calculated according to the consumption of AFS. The total volume of vanadium can be determined by potassium permanganate oxidation-ammonium ferrous sulfate titrimetric method. Hence, the volume of V(IV) can be obtained by deducting the quantity of V(V) from the total vanadium. Secondly, the undissolved residue is digested in Teflon vessel by phosphoric acid and hydrofluoric acid at 90 °C for 2h. The digestion solution is also titrated by AFS under nitrogen atmosphere, and the jump in titration curve denotes the reducing of V(IV) to V(III) by ferrous ion in phosphoric acid medium. So, the volume of V(III) and V(IV) can be obtained in the same way. This method is characterized by high measuring accuracy and excellent reproducibility. INTRODUCTION Vanadium, one of the important rare elements, is mainly used in steel industry at present. About 85% of vanadium consumption comes from the production of high-strength and low-alloy steels (HSLA) together with tool and die steels due to its excellent physical properties [1]. Besides the vanadium-titanium magnetite, stone coal is another important vanadium-bearing resource in China and the vanadium extraction from stone coal is attracting increasing attention due to its enormous reserves [2-3].
Vanadium generally is found as V(III), V(IV) and little V(V) in stone coal, of which V(III) is dominant because stone coal originates from reducing environment. Most V(III) with little V(IV) in stone coal exists as isomorphic form by partly replacing Al(III) from dioctahedral structure of mica group minerals, and other V(IV) can combine with organic compounds
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