I. Soda ash method ― Consider sodium carbonate addition to cold brine; yet just enough to precipitate both Ca2+ and Mg2+ as CaCO3 and MgCO3, but not sufficient to also precipitate Li2CO3. Conversion to Ca and Mg carbonates occurs: e.g. CaCl2​ + Na2​CO3 ​→ CaCO3↓ ​+ 2NaCl. Further carbonation to Ca and Mg bicarbonates should be avoided, namely by pH control. If, nonetheless, any Li2CO3 is still able to co-precipitate, it can be redissolved by washing the precipitate with icy water. The mother solution would then contain Li2CO3, which could be later precipitated after further Na2CO3 addition, and/or converted to LiCl by HCl (aq).

May it reduce the lithium concentration by precipitation as a lithium carbonate?!

Or such precipitation can be prevented by controlling the temperature?!

II. Lime (or causticization) method ―

One could consider to add slaked lime slurry and heat the brine (but not boiling); a precipitate containing Mg(OH)2 and Ca(OH)2 is obtained: MgCl2 + Ca(OH)2 → Mg(OH)2↓ + CaCl2. Slaked lime is able to convert alkaline-earth carbonates to bicarbonates: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O; Mg(HCO3)2 + Ca(OH)2 → CaCO3↓ + MgCO3 + 2H2O. CaO can substitute for slaked lime. LiCl also reacts, to form LiOH (aq): 2LiCl (aq) + Ca(OH)2 (aq) → 2LiOH (aq) + CaCl2 (aq). Mg(OH)2 precipitates with lime. CaSO4·2H2O may precipitate for brines containing sulfate. By washing the precipitate with icy water, Ca(OH)2 can be selectivity dissolved, thus being separated from Mg(OH)2 if that is found convenient. Mother liquor is obtained, containing unpreciptated Ca(OH)2, LiOH, and CaCl2.

LiOH·H2O can be then precipitated from the mother liquor by means of: (i) cooling the mother liquor (e.g. by adding ice); (ii) adding NaOH to the mother liquor (common ion effect); (iii) adding some suitable water-miscible non-solvent or poor solvent for LiOH ― such as isopropyl alcohol ― to the mother liquor; (iv) adding what may be recycled LiCl (common ion effect); (v) adding Li2CO3 (common ion effect while contributing to alkalinity, yet with possible CaCO3 coprecipitation); (vi) adding NaCl (generates NaOH at the alkaline mother liquor while increasing ionic strength).

After neutralization of the precipitated LiOH·H2O with HCl (aq); LiCl can be obtained.

III. Lime-Soda method ― Combines both soda ash (cf. §I; above) and slaked lime (cf. §II; above) addition to the brine, so that Ca2+ precipitates as calcium carbonate while Mg2+ precipitates as magnesium hydroxide. This approach is analogous to the lime-soda water softening process. Cf. US Patents 6,921,522 B2 (Jul. 26, 2005) and US 8,691,169 B2 (Apr. 8, 2014).

IV. Electrolysis using diaphragm or membrane cell ― Partially recovers LiCl from the brine as lithium hydroxide mono hydrate (LiOH.HO). Cf. US Patents 3,597,340 (Aug. 3, 1971) and 4,036,713 (July 19, 1977).

V. Ion Exchange ― Aminophosphonic cationic chelating resin can be used in the sodium form to selectively adsorb divalent Ca2+ an Mg2+ cations, over monovalent Li+ or Na+ cations, from alkalized brine ― after Ca(OH)2/Na2CO3/NaOH addition. This kind of resin has been used for brine softening in the chlor-alkali industry. Regeneration can be achieved with mineral acid followed by conversion to sodium form using NaOH (aq). Cf. US Patents 9,677,181 B2 (Jun. 13, 2017) and US 10, 066,305 B2 (Sep . 4 , 2018); European Patents EP 2 855 735 B1 (07.03.2018), EP 3 363 930 A1 (22.08.2018), and EP 3 060 522 B1 (12.06.2019).

VI. Bicarbonation by pressurized CO2 ― By means of sodium carbonate addition to cold brine (cf. soda ash method), conversion to carbonates and their precipitation can occur (as CaCO3; possibly as MgCO3). Non-precipitated carbonates can be converted to bicarbonates in autoclave, under pressurized dissolved CO2: Li2CO3 + CO2 + H2O → 2 LiHCO3; MgCO3 + CO2 + H2O → Mg(HCO3)2. Metastable LiHCO3 selectively dissolves in hot pressurized aqueous solution. Mg(HCO3)2 precipitates readily upon depressuring the former pressurized liquour, mainly as carbonate (hydrate). Mg(HCO3)2 can decompose to MgCO3 while dried.

@sewar