Separation of molybdenum from spent HDS catalysts using emulsion liquid membrane system
Graphical abstract
Introduction
The production of dangerous waste and effluents from various industries will become a severe problem worldwide in the future. These sources are composed of inorganic solids or metal effluents produced by many industries [1]. In the oil refining operation, the spent HDS catalysts are produced as solid wastes, which can have a significant impact on the environment. The environmental hazards associated with these wastes are mainly due to the presence of hazardous metals such as molybdenum, cobalt, and nickel in the spent catalysts [2]. Disposal of these wastes without the removal of hazardous metals has been actively prohibited by Environmental Protection Agencies (EPAs), and therefore proper operations are required to remove metals from the catalysts before discharge into the environment [3]. In addition, economically viable catalysts are crucial because not only there are large quantities, most often, the value of recovered metals is ten times the cost of recovery [4].
Molybdenum in the spent HDS catalysts is a valuable element for all living organisms due to its functional role in common factors related to bacteria, plant and animal enzymes and has specific geochemical behavior [5]. Mo is also of great industrial importance due to its high melting point, high strength at higher temperatures, high heat conductivity and good corrosion resistance [6]. It is widely used in a variety of industrial processes including the production of stainless-steel tools, the manufacture of catalysts, the production of pigments for corrosion inhibition and the production of lubricants and etc [[7], [8], [9], [10]]. Therefore, the research is necessary to recover Mo from the spent HDS catalysts more efficiently.
As one of the very promising techniques, liquid membranes have attracted researchers for the past two decades [11]. Generally liquid membranes can be divided into three major groups: bulk, supported and emulsion liquid membrane (ELM) [11]. ELMs usually achieve the highest mass transfer area among these three types of membranes and only the ELM has been industrialized, thus it is preferred in many applications [10]. The liquid membrane system usually has an immiscible or semi-permeable liquid layer between the feed and the receiving phases [12]. These membranes have two properties that distinguish them from the conventional solvent extraction system. The first feature is the high mass transfer area [13]. This not only improves the mass transfer rate compared to solvent extraction but also helps to recover elements from dilute solutions with low organic-aqueous volume ratio and save energy and materials [14]. The second feature in the separation process is that it can recycle the extractant material. This feature dramatically increases the capacity of extracting the metal ions [15]. The emulsion liquid membrane, also known as a double emulsion, was first used by Norman Li to separate hydrocarbons from effluents in 1968 [16,17]. There are usually two types of ELMs: water-in-oil-in-water (W/O/W) emulsion and oil-in-water-in-oil (O/W/O) emulsion, in which the immiscible membrane phase as water or oil separates the internal and external phases [18]. It is perhaps depends on the quiddity of external and internal phases [19]. To recover metal ions from spent catalysts, a water-in-oil-in-water emulsion is usually used. Since extraction and stripping co-occur in the one step in the ELM process, the selected extractant and stripping agents determine the reactions at the liquid membrane interfaces. In the ELM, one of the factors that strongly influences the emulsion stability and transfer rate is the type of surfactant and its concentration [20]. Emulsion breakage and swelling are two essential characteristics that influence metal extraction and recovery. Swelling is controlled by choice of the organic phase dilution and the surfactant. Swelling occurs when the external feed phase penetrates the internal phase due to osmotic pressure difference or physical breakage. This phenomenon reduces the stripping agent concentration in the internal phase and leads to a reduction of metal ion recovery efficiency. In the breakage phenomena, the internal phase leaks into the external phase because of high turbulent eddy kinetic energy [21].
In the past few decades, many examples of ELM techniques have been used to remove and recover metal ions such as copper, nickel, zinc, cobalt, chromium, uranium, cadmium, mercury, rare-earth elements and more [[22], [23], [24], [25], [26], [27], [28]]. A comprehensive study has been done by Kulkarni et al. for the recovery of Mo from diluted synthetic solution by ELM process [29]. They used Aliquat 336 as an extractant and found that the pH of the feed phase plays a crucial role in the process of ELM. They also concluded that by raising the concentration of the stripping agent and surfactant, the swelling would increase. Further research work was performed by Hirato et al. to recovery of Mo from a sulfuric acid aqueous solution by ELM [30]. In this work, the possibility of using tri-normal octyl amine (TNOA) as an extractant was investigated. Hirato et al. found that the extraction efficiency in the ELM process is higher than that of conventional solvent extraction. Sodium carbonate (Na2CO3) was preferred among the various materials tested as a stripping agent. Finally, under operating conditions, about 90 % of Mo was recovered. Another study was investigated by Singh et al. for the separation of Mo (VI) ions through the membrane phase consisting of Tris (2-Ethylhexyl) phosphate (TEHP) as extractant, span 80 as a surfactant and toluene as diluent with different concentrations of hydrochloric acid in the feed phase, NaOH and sodium carbonate in the recovery phase. The highest Mo transfer observed by using 50–70% TEHP in toluene and 7–8 wt.% span 80 [31]. Singh et al. observed that the use of NaOH alone in the internal phase leads to emulsion instability. However, the emulsion was stable with sodium carbonate but could not alone lead to a maximum recovery of Mo. Therefore, the mixture of NaOH and sodium carbonate was selected as the stripping agent, and it was observed that a stable emulsion occurs with a rapid and transparent transfer of Mo.
In the present study, the selective extraction of Mo from other metal ions present in the spent HDS catalysts leach solution using the Cyanex 272 as an extractant in the ELM system was investigated for the first time. All valid parameters including extractant and surfactant concentration, stripping agent concentration, speed of agitation, contact time, treat ratio, and internal to membrane phase volume ratio were optimized to achieve the highest level of Mo separation with minimum breakage and swelling of ELM.
Section snippets
Materials
The extractant used, Cyanex 272 (85 %), was obtained from Sigma Aldrich. Span 80 (≥ 99.5 %) from ACECR; branch of Tehran university was manufactured locally. The catalysts used in this study belong to the Tehran Refinery. To improve the membrane phase stability, a dough polymer called polyisobutylene (PIB) ((≥98.0 %) with an average molecular mass of 500,000 g/mol was provided by Sigma Aldrich. Alpha Acer Laboratory kerosene (< = 100 %) was used as the organic phase solvent. Other chemicals
Mechanism of Mo transport through the emulsion liquid membrane
To recover the metal ions from spent HDS catalysts using ELM system, the external aqueous phase is the leach solution of spent catalysts, and the internal aqueous phase is a potent complexing agent, while the intermediate organic phase contains organic diluent with the extractant and the surfactant. First, the metal ions in the external phase penetrate to the interface of the feed-membrane phase, where it forms complex rapidly with extractant molecules. This metal-extractant complex move freely
Conclusion
This study presented the emulsion liquid membrane technique for selective extraction of Mo from the leach solution of spent HDS catalysts. In this process, Cyanex 272 as an extractant and ammonium fluoride as a stripping agent were well adapted to extract Mo from the spent catalysts. Other parameters such as surfactant concentration, speed of agitation, contact time, treat ratio, and internal to membrane phase ratio were effective on the emulsion liquid membrane system efficiency and stability.
CRediT authorship contribution statement
Seyed Hamid Reza Rouhani: Investigation, Resources, Writing - original draft, Visualization. Reza Davarkhah: Investigation, Resources, Writing - review & editing. Parisa Zaheri: Conceptualization, Methodology, Validation, Writing - review & editing, Supervision, Project administration. Seyed Mohammad Ali Mousavian: Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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