Further investigation into lithium recovery from salt lake brines with different feed characteristics by electrodialysis
Introduction
Recognized as a “critical material” [1], lithium's primary applications have been lithium-ion batteries (35%), ceramics and glass (32%), lubricating greases (9%), air treatments (5%), continuous casting mold flux powders (5%), and polymer production (4%) [2]. Among these, the extensive use of lithium-ion batteries, which represent the largest potential growth area for lithium compounds in global end-use markets, has significantly increased the consumption of lithium resources in recent years [2]. Minerals and brines are currently the major types of primary lithium resources [3]; among them, the average brine deposit is significantly larger than the average pegmatite deposit [4] (brines and minerals, respectively, account for 62% and 38% of the lithium-rich resources [5]). As currently the most abundant sources of lithium [6], brine deposits have drawn increasing interest for purposes of lithium extraction. In an aqueous system, it is difficult to separate lithium from magnesium due to their similar ionic properties. Thus, lithium extraction from a brine with a low Mg/Li ratio is easier and more economical [7]. Several methods, such as adsorption [8], [9], extraction [10], [11], [12], nanofiltration (NF) [13], [14], [15], [16], [17], and electrodialysis (ED) [18], have been developed to recover lithium from brines with high Mg/Li ratios.
As a highly selective separation process [19], ED based on ion-exchange membranes is indispensable for separation of ionic species [20]. Ion separation between monovalent and multivalent ions can be achieved by ED with ion-selective exchange membranes [21], [22], [23], [24], [25], [26]. Van der Bruggen et al. [22] indicated that a membrane's selectivity may also be influenced by the applied voltage, but variation in permselectivity versus applied voltage was not considered in their work. The concentration ratio between divalent and monovalent cations was considered to determine separation performance in Kabay's study [25]; however, the range of concentration ratio studied was narrow. Lambert [27] noticed that the transfer behavior of sulfate and chloride was unusual due to the existence of cation–sulfate complexes, but the effect of anion species on cation transfer behavior was not considered. In these current studies, separations were achieved with relatively simple electrolyte-solution systems, which have low ionic strengths and in which each ionic species has similar ionic activity. Thus, the general separation principle suggested in these studies may not be suitable for a salt lake brine system, as the existing forms of ion species in these aqueous systems with a high concentration of electrolytes are complex. Additionally, the brine system has an extremely strong total ionic strength, and thus, the ionic interactions and charge effects are strong. The concentration of co-existing ions, such as sodium, potassium, and sulfate, differ considerably in different types of brine, which may influence the applicability of ED for lithium recovery. In our previous study [18], it was shown that the ion-fractionation of Li+/Mg2+ can be effectively achieved by ED from a binary feed solution with a high Mg/Li ratio. Nevertheless, the experimental study considering the feed characteristic diversity of brines was remained to be further perfected. In particular, the effect of the anion species on separation performance has not yet been discussed. And the mass transfer considering the composition complexity of brine system should be further explained.
In this study, constant-voltage ED using a batch mode of operation was performed to recover lithium from brines with different feed characteristics. As derived from our previous study, the power mode was optimized. In addition, the feed solutions characterized by different Mg/Li ratios, Na/Li ratios, and sulfate concentrations were treated by constant-voltage ED, wherein the partitioning principle was further explained via a thermodynamic analysis of the aqueous species distribution of ions and the ion-exchange membrane's properties. Then, a typical natural brine with a high Mg/Li ratio was treated to validate the applicability of lithium recovery via ED.
Section snippets
Experimental setup
An Asahi Glass Co. DW-1 ED stack was used in a batch recycling setup comprising 40 repeating cells, each consisting of a cation exchange membrane (Asahi Glass Selemion CSO) and an anion exchange membrane (Asahi Glass Selemion ASA). Each membrane had dimensions of 18×55 cm2 with an effective area of 0.0507 m2 and electrical resistance of 2.0–2.3 Ω-cm2. As shown in Fig. 1, the same setup as our previous study [18] was used, which consisted of four separated circuits for the dilute, concentrate,
Ion-exchange isotherm
The ion-exchange isotherm of a CSO membrane at 25 °C is shown in Fig. 2. As shown, divalent counter-ions were preferentially adsorbed by the CSO cation-exchange membrane. It was only when the fraction of monovalent counter-ions in the binary electrolyte solution system became undeniably dominant (close to 1.0) that the adsorption ratio for monovalent ions increased significantly. It has been reported that the ions that are better retained in the ion-exchange membrane are transported more slowly
Conclusions
This study aimed to verify the adaptability of ED with monovalent selective ion-exchange membranes for recovering lithium from salt lake brines with different feed characteristics. The ion-exchange isotherm of a Selemion CSO membrane was initially determined, which verified that divalent counter-ions were preferentially adsorbed by the CSO cation-exchange membrane without the influence of an electric field. Then, constant voltage ED was performed to treat lithium-containing solutions with
Notes
The authors declare no competing financial interests.
Acknowledgment
The research was supported by Natural Science Foundation of China (U1407120) and the National 863 Program (2012AA061601).
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