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Membranes 2022, 12, 343 5 of 27 as grinding, filtration of slurries, and water recovery processes, Li may be selectively produced and extracted from mineral ores by leaching processes either in acidic or alkaline aqueous solutions [5,11,17,18]. The first stage of the chemical processing on hard rock Li mineral-bearing ores these days typically involves the sulfuric acid pug roasting of the mineral ore at a temperature between 250 and 400 ◦C to support the decomposition of the silica mesostructure and convert the Li contained in the minerals into a water-soluble form [32]. Alkaline processes, whereby minerals are reacted with a mixture of calcium sulfate and calcium oxide/hydroxide at ~250 ◦C to convert the silicate into water soluble Li aluminate can also be performed to yield LiOH or Li2CO3 salts [33]. Ion exchange processes are sometimes required to support the extraction of undesired components and increase the purity of the Li product liquor [6,34,35]. During acid/sulfonation processes, alkali metal sulfates, sulfuric acid, or SO3 gas mixed with water and oxygen are employed as reagents to produce highly water-soluble Li sulfates that are less prone to precipitation compared to other Li compounds. However, drawbacks include the large volumes of reagent chemicals required and challenges in producing high-purity Li carbonates from such brines resulting from the capacity of sulfate reagents to bind to Al, Na, Mg, Fe, and K [2,36]. The sulfate roasting of lepidolite followed by water leaching has been studied widely using Na2SO4/K2SO4/CaO, Na2SO4, and FeSO4 and has yielded Li extraction extents up to 99.5% at 1000 ◦C [37–39]. The use of Na2SO4 and H2SO4 with the zinnwaldite, petalite, and montmorillonite ores has yielded Li extraction extents up to 90, 97.3, and 90%, respectively [5,37,40,41]. However, this approach normally requires sodium carbonate dosing to precipitate Li carbonates [5]. The alkaline Li extraction process is a more economical and more environmentally benign process that involves Li extraction from minerals with lime as an active leaching reagent. The roasting of Li ores in the temperature ranges 100–205 ◦C and 825–1050 ◦C will convert Li ores to LiO2, a precursor to LiOH. The lithium hydroxide produced can be further converted to LiCl or LiCO3 by a reaction with hydrochloric acid or carbon dioxide [5]. The lithium precipitates may be further upgraded while the mother liquor, such as the liquor obtained after lithium crystallization, is looped to the first stage of the process. The chlorination of lithium concentrates takes place between 800 and 1100 ◦C in the presence of hydrochloric acid, sodium chloride, calcium chloride, or chlorine gas, depending on the original ore chemistry. The process is used to convert the lithium compounds into lithium chloride (LiCl), which can be solubilized in water and thus purified. As an example, an acid baking process involving roasting of β-spodumene with Cl2 gas at 1100 ◦C for 2.5 h resulted in almost complete extraction of Li as LiCl2 [42]. The systems utilized for lithium recovery from minerals are summarized in Table 4. Table 4. Comparison of leaching processes for lithium extraction from minerals [5,36,43]. Process Active reagents Time pH Temperature Disadvantages Advantages Acid/Sulfonation Alkali metal sulphates, sulfuric acid, SO3 at water or oxygen 1–3 h 2–3 200–1000 ◦C Non-selective method; A lot of leached solution is needed; Impurities such as Al, Na, Mg, Fe, and K High rate of Li extraction Alkali Lime or limestone 1–2 h 8–10 100–200 ◦ C 800–1000 ◦C Need to decompose lime and limestones to CaO High rate of Li extraction without corrosion agents Chlorination Hydrochloric acid, sodium chloride, calcium chloride, or chlorine gas Upto2.5h ~5 800–1100 ◦ C Toxic chloric reagents; aggressive environment of leaching Selective for lithium chloride productionPDF Image | Electro-Driven Materials and Processes for Lithium
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