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https://doi.org/10.1595/205651317X696676 Johnson Matthey Technol. Rev., 2018, 62, (2) In summary, Li-Mn-O ion sieves exhibited a high ion exchange capacity and high selectivity for lithium ions from various aqueous resources. The acid generated during lithium uptake can be recycled for regenerating the sorbents. This could potentially reduce the cost of the acid consumption itself. However, the dissolution of Mn2+ during the regeneration process with acid degrades the ion exchange capacity and results in a poor cycling stability. This key issue seriously limits Li-Mn-O’s potential for upscaling. Further studies are needed to improve the stability during cycling to realise a stable ion exchange capacity. Simplicity of the regeneration process is also desirable. 2.2 Lithium Titanium Oxides (Li-Ti-O) Titanium-based spinel oxides share most of the advantages with the manganese-based spinel oxides, with an addition of being more environmentally friendly, as the titanium is an earth abundant element, is stable and does not dissolve in acid. In particular, metatitanic acid (H2TiO3) has been considered as an emerging environmentally friendly sorbent for lithium extraction from aqueous resources. The precursor lithium titanate (Li2TiO3) was first synthesised in 1988 and various synthesis methods are now available in the literature, including solid-state reaction (43–47), hydrothermal (48) and sol-gel (49, 50). Debate persists about the crystal structures of Li2TiO3 and H2TiO3, in which Chitrakar et al. (43) indexed both compounds as monoclinic with a space group C2/c, but later Yu et al. (51) pointed out that H2TiO3 should be more reasonably indexed with the 3R1 space group with an LDH structure. Typically, layered H2TiO3, derived from a layered Li2TiO3 precursor upon treatment with HCl solution, will go through ion exchange with lithium ions from the geothermal brines at a pH >7 to form Li2TiO3 (H2TiO3 + 2LiOH → Li2TiO3 + 2H2O). Lithium can be recovered from Li2TiO3 by treating with HCl solution (Li2TiO3 + 2HCl → H2TiO3 + 2LiCl). The theoretical ion exchange capacity of H2TiO3 is up to 142.9 mg g–1 (48), whereas the highest experimental ion exchange capacity so far is 94.5 mg g–1 (46). This is actually the maximum achievable capacity, as only 75% of the H+ occupied ion exchange sites in H2TiO3 are exchangeable with Li+ (44). Table IV summarises the adsorptive behaviours of H2TiO3 synthesised under different conditions from various research groups. It was first demonstrated in 2014 that H2TiO3 exhibits an extremely high selectivity toward lithium ions in the sodium bicarbonate (NaHCO3)-added salt brine and the ion exchange capacity reached 32.6 mg g–1 at a pH of 6.5 (43). However, the ion exchange rate is slow, taking 24 hours to get to equilibrium. This work has since stimulated great efforts investigating the ion exchange behaviour of this emerging ion sieve (44–49, 52, 53). The isotherm of H2TiO3 exhibited a Langmuir type behaviour, following the pseudo-second order rate model (45, 46). The ion exchange capacity of H2TiO3 increases with increasing Li+ concentration and decreasing pH values of the aqueous resources (46, 49). Specifically, the ion exchange capacity of H2TiO3 increased from 11.26 to 31.27 mg g–1 when initial concentration of Li+ was increased from 500 to 2500 ppm (pH = 13.46) (49). To further elucidate the effects of other factors on the ion exchange capacity of H2TiO3, a comprehensive orthogonal test with five factors (pre-calcination temperature, Li:Ti molar ratio, reaction temperature, ion exchange temperature, Li+ concentration) was performed (52). The highest ion exchange capacity of 57.8 mg g–1 is achieved under the optimum conditions: Li+ concentration = 4.0 g l–1 (highest among the tested), ion exchange temperature = 60°C (highest among the tested), molar ratio of Li:Ti = 2.2, reaction temperature = 650°C, pre-calcination temperature = 25°C. To make H2TiO3 more economically efficient, low‐grade titanium slag was used as the starting material and the optimal capacity reached 27.8 mg g–1 (47). Li4Ti5O12 is one of the common anode materials used in LIB (54) and the related H4Ti5O12 is a common ion sieve for lithium extraction from aqueous solutions. H4Ti5O12 derived from Li4Ti5O12 nanotubes (~70 nm in diameter) exhibited an ion exchange capacity of 39.43 mg g–1 from LiCl solution (120 mg l–1 Li+, pH = 9.17). In summary, H2TiO3 is an attractive sorbent for selective lithium extraction with superior advantages including high ion exchange capacity, high selectivity, high stability, environmental friendliness and economic efficiency. However, it is still at the laboratory scale, partly due to the acid requirement during the regeneration process, which produces secondary wastes. 2.3 Lithium Aluminium Layered Double Hydroxide Chloride While the Li-Mn-O and Li-Ti-O sorbents have attracted significant attention from academia, LiCl·2Al(OH)3.xH2O (referred to as Li/Al LDH) is an attractive candidate for application in large scale 166 © 2018 United States GovernmentPDF Image | Lithium Recovery from Aqueous Resources
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