Separation of vanadium and molybdenum from aqueous solution using PEG2000 + sodium sulfate + water aqueous two-phase system
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The separation of vanadium (V) and molybdenum (VI) was studied by co-extraction with an aqueous two-phase system formed by PEG2000 + sodium sulfate + water and by selective stripping with another aqueous two-phase system formed by ammonium sulfate solution and PEG2000 phase loaded vanadium and molybdenum. The effect of aqueous pH, concentration of vanadium and molybdenum, temperature on the co-extraction and concentration of ammonium sulfate, temperature and phase ratio on selective stripping separation of vanadium and molybdenum was investigated. The experimental results on co-extraction of vanadium and molybdenum indicated that the co-extraction rate of both metals is sensitive to aqueous pH, and their extraction rate can achieve above 97.54% and 99.49%, respectively, when the pH value of aqueous solution is at 2.0, the molar ratio of vanadium to molybdenum is about 1.5, the concentration of PEG2000 is 20%, and the temperature is 313.15 K, respectively. The co-extraction rate of both metals decreases slightly with an increase in temperature from 313.15 to 343.15 K. The extraction isotherm and McCabe–Thiele method showed that two theoretical stages are needed when concentration of vanadium and molybdenum decreases from initial 12 g/L and 10 g/L to 0.1 g/L under the optimal conditions. The results on selective stripping of both metals from loaded PEG2000 phase to aqueous phase indicated that the stripping rate and precipitation rate of vanadium are evidently impacted by aqueous pH within 8.0 ~ 11.0 and then a little influenced by other parameters. The stripping rate of molybdenum is relatively lower than that of vanadium and is influenced by all kinds of stripping conditions. The stripping rate and precipitation rate of vanadium achieve 99.88% and 98.84%, respectively, and stripping rate of molybdenum achieves 81.99% under the optimal stripping conditions of O/A = 2:1, stripping temperature 343.15 K, (NH4)2SO4 (w/w) 40% and the pH of aqueous solution at 10.0.
KeywordsAqueous two-phase system Separation Polyethylene glycol Sodium sulfate Vanadium Molybdenum
Vanadium and molybdenum are widely used in aerospace, electronic information, industrial production and many other technical fields for their unique properties of corrosion resistance, high temperature resistance and wear resistance, which have greatly stimulated the demanding in recent years. At the same time, it is urgent to develop and reuse its secondary resources as the world’s resources continue to be mined and high-grade ore is increasingly depleted. Among the secondary resources, waste catalysts are undoubtedly more important. The waste hydrodesulfurization catalysts replaced in petroleum refineries contain higher precious metals than precious metal ores. They are considered as an important source of strategic metals such as vanadium, molybdenum, nickel and cobalt [1, 2]. The reasonable recovery of spent catalysts not only has great economic benefits, but also solves the environmental pollution problems associated with their disposal . At present, there are many reports on the recovery of metal vanadium and molybdenum from spent catalysts [4, 5, 6, 7, 8, 9].
In recent decades, many researchers have studied and developed methods for separating and recovering vanadium and molybdenum from leaching solutions. The main methods for the separation of vanadium and molybdenum are ion exchange [10, 11], solvent extraction [12, 13, 14] and chemical precipitation . Among them, the solvent extraction method is the most widely used depending on its simple operation, selective control of separation and complete separation. Some researchers use different extractants to extract and separate vanadium and molybdenum, such as Alamine-336 , EHEHPA , LIX 63  and Cyphos IL.
However, most organic solvents are toxic, flammable and volatile. Using of organic solvents is very harmful to human health and the environment and does not conform to the concept of green chemistry . In addition, although the ion exchange method can effectively separate metals and obtain high-quality products, the ability to load metals is limited, and it cannot meet the scale production of the factory, which leads to the application limited. Therefore, it is necessary to develop a green and environmentally friendly method to separate vanadium and molybdenum. On this basis, we found that the aqueous two-phase system can effectively extract and separate vanadium and molybdenum. Moreover, up to now, it is noteworthy that there is no report on the separation of vanadium and molybdenum by aqueous two-phase system. Compared with the traditional separation methods mentioned above, this method not only balances or even exceeds its efficiency, but also satisfies the outstanding advantages of energy saving, simplicity and time saving. It is worth mentioning that the organic solvents are absent, which actively responds to the safety concept of “green chemistry” for aqueous two-phase extraction systems.
Aqueous two-phase systems has become more and more attractive in recent years due to its mild operating conditions, scale-up easily and no pollution to the environment. It is a promising new separation technology . At present, many researchers have successfully applied this technology to bioengineering [21, 22], fermentation engineering , separation of organic matter  and other fields. There are also many studies on the application of this technology to extraction and separation of metal ions, for example the separation of Hg(II), Zn(II) and Co(II) by Dehghani , the separation of Ca(II), Mg(II) and Ni(II) by Rodrigues  and the extraction of Bi(III) subsalicylate by Sen . The extraction and separation performance and the extraction mechanism of molybdenum and tungsten with the aqueous two-phase system were reported by our early research work [28, 29], and we also used the same system to investigate the extraction and separation of vanadium. It was found that for the extraction of alone vanadium and molybdenum, molybdenum is more easily extracted into the PEG-rich phase, while vanadium is difficult to extract into the PEG-rich phase without the addition of a charge modifier under the acidic conditions . In addition, the conclusions of the partition of Molybdate ion in aqueous two-phase system formed by CuSO4 + PEG 4000 + H2O were obtained by Duran .
In this work, the purpose is to investigate the co-extraction and separation performance of vanadium(V) and molybdenum(VI) with aqueous two-phase system composed of PEG2000 + Na2SO4 + H2O without any extractant on the basis of our previous research work. By a series of basic experimental studies, we found that pH, concentration and T have great influence on the experimental results, while other factors (such as stirring rate and aging time) have little or no influence, so we will not give a single narrative here. The article mainly narrates the effect of aqueous pH, initial vanadium and molybdenum concentration, temperature on co-extraction of vanadium and molybdenum, concentration of ammonium sulfate and ammonium hydroxide and temperature on selective stripping separation of vanadium and molybdenum.
PEG2000 with an average molar mass 1900 ~ 2200 g/mol was obtained from Xilong Chemical Co. Ltd. (GuangDong, China). The stock solutions of molybdenum and vanadium were prepared by dissolving Na2MoO4 and NaVO3 in distilled water, respectively. Sodium sulfate, ammonium sulfate, sodium metavanadate and sodium molybdate were obtained from West Long Science Co. Ltd. (GuangDong, China), while sulfuric acid was supplied by Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin, China). All reagents were analytical grade with the purity of above 99% and directly used. Distilled water was used in all experiments.
2.2 Experimental procedure
The aqueous two-phase system was prepared with PEG2000, sodium sulfate stock solution and vanadium and molybdenum stock solutions, and the system was composed of 20% (w/w) PEG2000, 10% (w/w) sodium sulfate, 0.2% (w/w) vanadium and 0.5% (w/w) molybdenum. The pH of mixture solutions was adjusted to 1.5, 2.0, 3.0, 4.0 and 5.0, respectively, by acidometer (PHS-3C, Shanghai, China) with 2 mol/L sulfuric acid. The mixture was stirred vigorously at 313.15 K for 30 min in a thermostatic magnetic stirrer for fully blending and then placed in a constant temperature water bath at the same temperature for 24 h. All aqueous two-phase extraction systems were placed in 50-mL glass graduated tubes for phase separation. After the phase separation was finished, the volume of the upper phase (extract phase) and lower phase (salt-rich phase) was accurately recorded for calculating the volume ratio. The molybdenum and vanadium in salt-rich phase were appropriately diluted with distilled water and analyzed by inductively couple plasma atomic emission spectrometry (ICP-AES 6300 Radial, Thermo Scientific, USA). Parallel experiments were carried out during the experiments to ensure the accuracy of the data.
3 Results and discussion
3.1 Co-extraction of vanadium and molybdenum
3.1.1 Effect of aqueous pH
3.1.2 Effect of vanadium concentration
3.1.3 Effect of molybdenum concentration
3.1.4 Effect of temperature
3.1.5 Extraction isotherm
It was found from the figures that under the experimental conditions, the possibility to co-extraction of two metals needs only two theoretical extraction stages when molybdenum concentration decreases from initial 12 g/L to 0.1 g/L and vanadium concentration decreases from initial 10 g/L to 0.1 g/L.
3.2 Stripping separation of vanadium and molybdenum
3.2.1 Effect of pH
3.2.2 Effect of (NH4)2SO4 concentration
3.2.3 Effect of temperature
3.2.4 Effect of phase ratio
Stripping efficiency of both metals in a single-stage contact
Precipitation rate (%)
In the absence of any extractant, a method for the efficiently co-extraction and selective stripping separation of vanadium(V) and molybdenum(VI) by aqueous two-phase systems consisting of PEG2000 + sodium sulfate + H2O and ammonium sulfate + loaded PEG2000 phase was proposed in this study. The main conclusions are as follows: The co-extraction rate of vanadium and molybdenum is highest at pH = 2.0, showing that these heteropolyacid anions have relative high hydrophobicity and increase mutual solubility with PEG2000-rich phase having hydrophobic environment, in the meanwhile, the electrostatic attraction generated between heteropolyacid anion and quasi-cation of C–O–C or –OH protonated in PEG2000 molecule promotes the extraction rate of heteropolyacid anion. What’s more, the increase in the heteropolyacid content caused by the increase in the concentration of molybdenum can lead to the increase in the co-extraction rate of vanadium and molybdenum. Increasing extraction temperature is not in favor of the co-extraction of vanadium and molybdenum Under the appropriate conditions of maintaining the pH value of the aqueous solution at 2.0, the molar ratio of vanadium to molybdenum is about 1.5, the concentration of PEG2000 at 20% and 313.15 K, 97.54% of vanadium and 99.49% of molybdenum can be extracted into the PEG2000-rich phase. Two theoretical extraction stages are demanded when molybdenum concentration decreases from initial 12 g/L to 0.1 g/L and vanadium concentration decreases from initial 10 g/L to 0.1 g/L by McCabe–Thiele method. For selective stripping of vanadium and molybdenum, when the pH of the system varies from 8.0 to 10.0, the charge density in the solution increases, the hydrophilicity increases, and the stability of the micelles deteriorates, causing vanadium and molybdenum to be easily dissociated from the micelle phase. Under alkaline conditions and adding ammonium sulfate, the heteropolyacid anion of vanadium and molybdenum was broken and ammonium molybdate and ammonium metavanadate were formed, resulting in the removal of vanadium and molybdenum from PEG2000-rich micelle phase because of increasing hydrophilic nature of both metals. However, the stripping rate and precipitation rate of vanadium are not significantly changed by the above factors. By using a 40% (NH4)2SO4 solution, above 99.88% of vanadium and 81.99% of molybdenum can be stripped and 98.84% of vanadium can be precipitated from surfactant loaded with vanadium and molybdenum in one step under conditions of O/A = 2:1, stripping temperature 343.15 K and the pH of aqueous solution 10.0. An efficient separation of vanadium and molybdenum in aqueous solution is achieved. We hope that this new separation method can be used as an alternative scheme for industrial production in the future.
The authors wish to thank natural science foundation of Hebei Province, China, for their financial support (E2019203538).
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no conflict of interests
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