Effect of coexisting ions on recovering lithium from high Mg2+/Li+ ratio brines by selective-electrodialysis
Introduction
Lithium and its compounds, as important materials, have excellent physical and chemical performance, which makes their application more and more widespread [1], [2] in metallurgy, glass, medicine and energy field [3]. In particular, the demand for lithium used in lithium ion batteries is huge. Worldwide lithium production increased by an estimated 13% to 43,000 tons in 2017 in response to increasing lithium demand for battery applications [4], which can increase to 66% of the global lithium production by 2025 [5]. Thereby the demand for lithium and its compounds has been accelerated in recent years.
The majority of world lithium production is derived from minerals, like three spodumene operations in Australia, and salt lake brines such as two brine operations each in Argentina and Chile [4]. Extracting lithium from lithium ores is not only costly but also polluted seriously. What’s more, after exploiting about a hundred years, lithium ores have been on the verge of dried up. In recent years, lithium recovery from salt lake brine is an important means for bridging the gap between the supply and demand of lithium material, because about 70–80 wt% of the total lithium resources (about 2.69 × 1010 kg) exists in brines [5], [6]. In recent years, most countries in the world turned the direction of lithium extraction into the salt lake brine. Additionally, about 2.5 × 1014 kg lithium is reserved in the seawater, which is about ten thousand time than that on land. Nevertheless, both salt lake brine and seawater are mixed systems which usually contain various coexisting ions like Na+, K+, Ca2+, Mg2+, and so on. As we all know, Li+ (radius ≈ 0.076 nm) has similar ionic property to Mg2+ (radius ≈ 0.072 nm) since their ionic size is nearly identical [7], making it difficult to separate Li+ and Mg2+. Besides, the methods for extracting lithium from salt lake brine are various on account of different ion content in diverse salt lakes. For example, precipitation has been developed to separate and recover lithium from salt lake, in which a certain amount of precipitant needs to be added to remove the coexisting calcium and magnesium, so that its economic efficiency is relatively poor for lithium recovery from brine with high Mg2+/Li+ ratio [8], [9]. Somrani et al. [10] reported that in the process of nanofiltration, the magnesium was partially entrapped and the separation was not exhaustive. At present, other technologies such as solvent extraction [11], [12], ion exchange and adsorption [13], [14], [15] have been developed to extract lithium from brine, and the key of these methods is to find the right extractant and adsorbent. Considering the strong electrolyte of aqueous lithium salt solution and the advantages such as simple operation, environment-friendship, and energy saving of electrodialysis (ED) process [16], it should be economic and feasible for selective-electrodialysis (S-ED, a kind of ED) technology to condense lithium from brine and even (concentrated) seawater [17].
As a new-type electro-membrane process, selective-electrodialysis (S-ED) uses monovalent selective ion-exchange membrane as separation medium [18], [19], [20], [21], [22], [23]. In S-ED process, monovalent cations in desalting compartment pass though the selective cation exchange membrane and arrive in concentrating compartment, while a mass of multivalent cations such as Mg2+ and Ca2+ are intercepted in desalting compartment, thus achieving the purpose of separating Mg2+/Li+ and concentrating lithium. Dall and Boateng [24] concentrated lithium from the high Mg2+/Li+ brines with the Mg2+/Li+ = 60. First, the Mg2+ in brine was precipitated with CaCO3, which reduced the ratio of Mg2+/Li+ to 5, and then the recovery ratio of Li+ was up to 78.9% after multistage electrodialysis. Furthermore, Nie et al. [25] used electrodialysis (ED) to treat a bittern with a high Mg2+/Li+ ratio and the Li+ recovery ratio could be over 90%. Previously, we have explored some factors like the voltage, liner velocity of desalting and concentrating cell, pH and Mg2+/Li+ ratio in the separation process of lithium and magnesium in binary cations solution system. Under the optimized experiment conditions, lithium resource could be recovered from brine with the high Mg2+/Li+ of 60, and the Mg2+/Li+ ratio could reduce to 7 [26], classified as low Mg2+/Li+ ratio category [27]. Simultaneously, we found that the effect of coexisting ions on lithium extraction, especially for the ions whose concentration is far above that of Li+, cannot be ignored. In salt lake brine the concentration of coexisting monovalent cation like Na+ or K+ is much higher than that of Li+, and divalent anion SO42− has the highest concentration in the divalent anion. Nie et al. proposed that free ion proportions of both lithium and magnesium varied as sulfate was added [28]. These previous studies directly focus on the extraction of lithium from liquid lithium resources mainly with binary cation system: some influences of parameters on the lithium migration, lithium recovery ratio, and specific energy consumption. There is little concern of the migratory differences between lithium and magnesium, which are the key difficulty for lithium extraction from high Mg2+/Li+ ratio brine, and the synchronous effects of other coexisting ions on Li+ and Mg2+. Additionally, HCO3− is a nonnegligible chemical component in some salt lake brine. Sometimes the concentration of HCO3− is more than that of CO32− [27], [29], so the influence of HCO3− should not always be ignored for the recovery of lithium. However, there is little study on the effects of HCO3− on the Mg2+/Li+ separation.
To sum up, the coexisting ions like Na+, K+, SO42− and HCO3− should have some direct effect on the migrations of Li+ and Mg2+, and then affect the separation of Mg2+/Li+ in the brine. In this study, some investigation was carried out, facing lithium recovery from different ternary cation systems (Li+, Mg2+, M+//Cl−—H2O, M+: Na+, K+) and binary anion systems (Li+, Mg2+//Cl−, SO42−—H2O; Li+, Mg2+, Na+//Cl−, HCO3−—H2O). And the separation coefficient of magnesium and lithium, recovery ratio of Li+, current efficiency and specific energy consumption were used as evaluation index. The results will hopefully provide some theoretical basis and data support for further application of S-ED technology to produce lithium salt from salt lake brine.
Section snippets
Experimental apparatus and procedure
Electrodialysis apparatus mainly consists of membrane stack, desalt solution tank, concentrate solution tank, electrolyte solution tank, tube pump and direct-current (DC) power, as shown in Fig. 1. The core component of S-ED is membrane stack, which contains 10 pairs of monovalent selective ion-exchange membranes with an effective surface area of 0.01407 m2 in this study as before in reference [26]. The desalt solution tank and concentrate solution tank were filled with 2.5 L artificial brine
Voltage optimization
The voltage drop is one of most significantly parameter affecting the performance of ED process. Occasionally, the voltage drop across one cell pair (V/cp) of ED stack is in the range of 0.5–2 V/cp [32]. In order to explore the influence of coexisting ions on the ion fractionation of lithium and magnesium and ensure the current density below the limiting current density, different applied voltages in the range of 3–8 V considering the pairs of ion exchange membranes in the S-ED apparatus were
Conclusions
An S-ED process was used to investigate the effect of coexisting ions (including Na+, K+, SO42− and HCO3−) on the ion fractionation of lithium and magnesium in Mg2+/Li+ ratio brines. Through investigating the influence of applied voltage on the separation of Mg2+ and Li+ in the ternary cation systems of Li+ (Li+, Mg2+, Na+//Cl−—H2O), 6 V was considered as the optimum one with overall consideration. With investigating the effect of different cations concentration (expressed by molar ratio M+/Li+
Acknowledgment
The research was supported by Program for the Top Young Innovative Talents of Hebei Province, Tianjin Research Program of Application Foundation and Advanced Technology (12JCQNJC03300), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT14R14). And the work is also supported by National Natural Science Foundation of China (20806019), Natural Science Foundation of Hebei Province (B2009000024 and B2017202246), Education Department of Hebei Province (2008112),
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