Impact of the ethylene content on poly (ethylene-co-vinyl alcohol) membrane morphology and performance via immersion precipitation for lithium extraction
Graphical abstract
Introduction
Global lithium consumption has grown over 75% from 2008 to 2016 according to statistics [1]. Increasing demand of lithium chemicals has been mostly contributed from the worldwide expanding lithium battery market. Therefore, continuous supply of lithium is a key issue for the sustained development. There are three main resources for lithium in nature: seawater, Li-containing ores, and salt-lake brines [2]. Brine resources recently attracted increasing research interests because of the relatively high lithium abundance and that the ores are becoming depleted. In addition, the extraction technology may be used as a viable tool for lithium recycle. However, most current extraction technologies are facing severe challenges.
The ionic compositions of brine are generally complicated with coexistence of a wide spectrum of ions. Some of the cations, such as magnesium, calcium, and sodium, have similar ion hydration radii to that of lithium. This complicates the separation and purification process. In particular, for the Qinghai salt lake brine located in the northwest of China, the high concentration of magnesium hampers the exploitation of lithium [2]. To extract lithium from salt-lake brine at an economical scale, many methods have been explored and summarized in a recent review paper [2]. Among these methods including adsorption [3,4], precipitation [5], ion exchange and calcination leaching [6], extraction [[7], [8], [9]], nanofiltration (NF) [[10], [11], [12], [13], [14]], and electrodialysis (ED) [15]. Membrane extraction [[16], [17], [18], [19]] is promising because it is highly selective, scalable, of a small footprint, and potentially scalable.
In a membrane extraction process the organic extractant is circulated between the extraction and stripping stages and metal ions transfer from the feed to the stripping solutions; the membrane is located between the aqueous phase and the organic extractant phase [16,17], which prevents loss of organic phase into aqueous phase incurring negative environmental impact. Solvent stability of the membrane is a key issue for a stable operation. A good membrane should be hydrophilic, microporous (or nanoporous), and chemically resistant to the extractant. Although various strategies have been used to develop solvent resistance membrane, such as blending [16], coating and surface modification, it is desirable to obtain the membrane in a straightforward approach [20].
Recently, poly (ethylene-vinyl alcohol) (EVAL) was reported as such a candidate via immersion phase separation. EVAL is a block copolymer consisting of hydrophobic ethylene segment and hydrophilic vinyl alcohol segment which shows a wide spectrum of applications in various fields. Our preliminary results showed that EVAL membrane is organic extractant resistant in lithium extraction from brine [17]. It was believed that the hydrophilic segment in EVAL polymer is responsible for the ion permeability and the hydrophobic segment contributes to the structure stability. However, the effect of the ratio of the hydrophobic and hydrophilic segments on membrane morphology and extraction performance has not yet been documented.
In this work, we aimed to study the relationship between the content of the ethylene segments in EVAL polymer and the membrane morphology as well as lithium permeance in membrane extraction. By using commercially available EVAL polymers, the variation of ethylene segments with membrane wettability (due to the change in the vinyl alcohol segments), morphology, mechanical properties and degree of crystallinities were explored. The impact of the ethylene blocks on the phase separation kinetics was analyzed. The membrane ion transport performance was evaluated in extracting lithium using real salt-lake brine. The contribution of hydrophilic/hydrophobic segments was analyzed from both membrane formation and performance points. The results provide guidelines for further development of the hollow fiber membranes and modules for lithium and other rare element extraction. The methodology and concepts developed in this work are expected to be applicable to control the morphology and performance of the membranes prepared from similar semi-crystalline polymers.
Section snippets
Chemicals and materials
The EVAL materials with different ethylene contents were purchased from Sigma-Aldrich as listed in Table S1 (Supporting Information). The molecular weight and the polymer dispersity index (PDI, Mw/Mn) of all EVAL polymers were in a similar range. EVAL was dried 24 h at 65 °C before usage. Dimethylacetamide (DMAc, Analytical grade) and lithium chloride (Analytic grade) were purchased from Titan chemical Reagent Co (Shanghai, China). Analytical tributyl phosphate (TBP and kerosene were supplied
Morphology and crystallinity of EVAL membranes
As a semi-crystalline block-co-polymer consisting of hydrophobic ethylene and hydrophilic vinyl alcohol segments, EVAL polymer tends to form nanoporous microstructure upon immersion precipitation [17]. Fig. 1 indeed shows similar morphology with a wide variation in the polymer ethylene content from 27% to 44%. Fig. 1 A1-D1 show compact top skin layer consisting of much smaller nanopores identified at higher magnifications; Fig. 1 A3-D3 series show larger interconnected cellulose porous
Conclusions
In this report, the immersion phase separation was investigated for EVAL membranes in order to obtain a stabilizing barrier in membrane extraction of lithium ions from salt-lake. EVAL materials with varied ethylene chain contents from 27 to 44 mol.% were compared in terms of crystallinity, water uptake, membrane structure, mechanical properties and performance. It was found that the gelation temperature of the EVAL solutions changes with the ethylene content where a higher ethylene content
Acknowledgment
The authors would like to thank the partial financial support from the National Natural Science Foundation of China (No. U1507117, 21676290, 51861145313, 21808236), and Newton Advanced Fellowship (Grant No. NA170113).
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