Extracting Vanadium from Stone Coal by a Cyclic Alkaline Leaching Method
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VANADIUM, as a strategic metal, has been attractive for a variety of applications, including iron industry, catalysis, flow batteries.[1-3] It was reported that the reserve of vanadium (V2O5 as the unit) in stone coal is more than 117.97 million tons in China.[4] With such large reserves, extracting vanadium from stone coal has obtained great consideration in recent decades. Generally, there are two methods to extract vanadium from stone coal: acid leaching and alkaline leaching. At present, the studies about vanadium extraction from stone coal are mostly concentrated on the acid leaching method.[5] The acid treatment attracts attentions largely due to the low cost of acid.[6] Nevertheless, the acid method usually suffers from high consumption of acid[7,8] as well as the long reaction times.[9] Meanwhile, several impurities (Fe, Al, Mg, Mn, P, Ca, etc.) are also dissolved into the leachate, requiring a relatively complex impurity removal process at the subsequent stage.[10] Besides, some additives, such as CaF2, HF, NaClO3, are added in the acid leaching process to enhance the leaching efficiency,[11-13] which will bring about not only an extra cost but also pollution problems. In order to overcome the above shortcomings, researchers began investigating the leaching of vanadium from stone coal under alkaline conditions. Compared with the acid leaching, alkaline leaching has advantages of faster kinetics as well as a simpler impurity removal process.[14-17] Nevertheless, the higher price of caustic soda makes the alkaline leaching KAILONG HU, XUHENG LIU, and QINGGANG LI are with the School of Metallurgy and Environment, Central South University, Changsha, 410083, Hunan, China. Contact e-mail: [email protected] Manuscript submitted September 22, 2016. Article published online January 3, 2017. 1342—VOLUME 48B, APRIL 2017
method less competitive. Therefore, the question of how to reduce the cost of production becomes a key point for the application of the alkaline leaching method. It is worthwhile to note that the cyclic utilization of leachates, used in the Bayer process,[18] was an economical and efficient way to decrease the caustic soda dosage. However, the cyclic alkaline leaching method for extracting vanadium from stone coal has not been reported so far. In stone coal, vanadium, mainly exists as V(III), taking the place of Al(III) in illite (a sort of minerals in the stone coal).[19] Illite is one of the common gangue in the manufacturing process of alumina, decomposing easily [20] Inspired by into SiO2 3 and AlO2 in the Bayer process. the Bayer process, we hypothesized that the vanadium-bearing illite in stone coal might also be decomposed by the alkaline cyclic leaching for the extraction of vanadium. However, V(III) is insoluble in alkaline solutions. In order to leach vanadium, V(III) must be oxidized to form soluble V(IV) or V(V). Roasting can be used for this purpose.[21,22] During roasting at high temperature, the structure of vanadium-bearing illite is gradually destroyed as V(III) in vanadium-bearin
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