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.
Leaching of roasted stone coal with sulfuric acid results in a mixed vanadium solution containing Fe(III), Fe(II), Al(III) and Mg(II). Separation of V from synthetic sulfate solution containing these impurities has been investigated by ion exchange and solvent extraction . In ion exchange experiments, anion exchange resin ZGA414 was tested as its optimum adsorption capacity compared with D202, D453, D301FC and ZGA351 resins, and D2EHPA and TBP diluted with kerosene were employed in solvent extraction. Ion exchange tests indicated that only V(V) was loaded from the synthetic solution at pH > 1.5, while it was difficult to separate V(V) from Fe(III), which also made resin toxic. Solvent extraction experiments revealed that V(IV) had a better extraction ratio than that of V(V), while Fe(III) had a serious effect on the extraction of V(IV). The co-extraction ratio of Al(III) and Mg(II) can be decreased by controlling their concentrations lower than 10 g/L. Counter-current experiments with D2EHPA presented that 99% of V(IV) was extracted from the real leach solution after reduction process, leaving most of Fe(II), Al(III) and Mg(II) in the raffinate.
To improve separation of V(IV) and Al(III) from aluminum-rich sulfuric acid leaching solution of stone coal, the two-stage separation by crystallization and solvent extraction methods have been developed. A co-extraction coefficient ( k ) was put forward to evaluate comprehensively co-extraction extent in different solutions. In the crystallization stage, 68.2% of aluminum can be removed from the solution. In the solvent extraction stage, vanadium was selectively extracted using di-2-ethylhexyl phosphoric acid/tri-n-butyl phosphate from the crystalline mother solution, followed by H 2 SO 4 stripped efficiently. A V 2 O 5 product with purity of 98.39% and only 0.10% Al was obtained after oxidation, precipitation, and calcination. Compared with vanadium extraction from solution without crystallization, the counter-current extraction stage of vanadium can be decreased from 6 to 3 and co-extraction coefficient ( k ) decreased from 2.51 to 0.58 with two-stage separation. It is suggested that the aluminum removal by crystallization can evidently weaken the influence of aluminum co-extraction on vanadium extraction and improve the selectivity of solvent extraction for vanadium.
Qihua Shi,Yimin Zhang,Jing Huang,Tao Liu,Hong Liu,Luyao Wang
New extractant and an extractant system are attractive for liquid-liquid extraction. To improve the extraction efficiency and selectivity, the binary extractant system consisting of bis-2-ethylhexyl phosphoric acid (D2EHPA) and 2-ethylhexylphosphonic mono-2-ethylhexyl (PC88A) was explored to separate vanadium (V) via synergism from multi-element sulphuric acid leaching solution in this study. The optimal proportion and concentration of extractant was investigated and results demonstrated that the maximum synergistic co-efficient was 1.52 by adjusting the volume ratio to 14:6 (D2EHPA/PC88A) when the extractant concentration was 20% (v/v). Next, the selectivity of the mixture was investigated at different pH values and contact time, which demonstrated better separation of vanadium from primary impurities compared with extraction by D2EHPA or PC88A alone. Furthermore, the Fourier Transform Infrared (FT-IR) spectra were employed to analyse the mechanism of extraction. The results suggested that the cation exchange (P OH bands) and coordination reaction (P O bands) contribute to vanadium extraction, and the structure of extracted complex was proposed. The formation of a dimer between D2EHPA and PC88A was demonstrated. Finally, after multi-stage counter-current extraction and stripping, V 2 O 5 product with a high purity of 99.07% was obtained by oxidation, precipitation and calcination.
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