Membrane electrolysis for the removal of Na+ from natural brines

CÉSAR HORACIO DÍAZ NIETO; FLEXER, VICTORIA

Mikulov

Congreso; 29th Topical Meeting of ISE; 2021

International Society of electrochemistry (ISE)

Lithium is a fundamental raw material in the production of rechargeable batteries, ceramics, glass, lubricating greases, casting and alloy industries, air conditioning, and aluminium, amongst others 1.Lithium can be extracted from either hard rock minerals and brines: geothermal, oilfield and continental brines. The latter are highly saline solutions, with total dissolved solids (TDS) in the range 170-330 g L-1, containing typically large quantities of Na+, and lower concentrations of Li+, K+, Mg2+, and Ca2+. Anions are principally Cl- with minor amounts of SO42-, HCO3-, and different borates 2.Na+ is the most concentrated salt that could be recovered. And the need to considerably increase the Li+/Na+ ratio is a must in order for the successful recovery of lithium salts at a later stage. We propose the use of membrane electrolysis coupled to crystallization as a new methodology to separate a large share of Na+ present in brines. The brine (without Mg2+ and Ca2+, these cations were removed in a previous stage 3) is fed into a three-compartment water electrolyzer with an attached side-crystallizer, and Na+ is recovered as NaHCO3 (Figure 1). NaHCO3 is later transformed into Na2CO3, can be precipitated with high purity, and thus no valuable Li+ ions are lost in the process.In a 3-compartment reactor, cations selectively migrate from the middle compartment through a cation exchange membrane to the cathodic compartment, where water electrolysis produces alkalinisation, and through CO2 spurging sodium bicarbonate precipitation is triggered. By following the real time concentrations in middle and cathodic compartments, we were able to prove the proposed methodology, and recover a solid with a purity of 99.5 %. A coulombic efficiency of (95 ± 5) % was calculated, and the separation coefficients showed a preferential migration in the order K+ > Na+ > Li+. The methodology produces fresh water as a by-product, with total salinity 2 orders of magnitude lower than the feed. 3 different applied current density values were tested. 331.3 kWh at 10 A m-2 is needed to treat 1 m3 of brine.