Membrane electrolysis for the removal of Mg2+ and Ca2+ from lithium rich brines

Abstract

Lithium is today an essential raw material for renewable energy technologies and electric mobility. Continental brines as present in the Lithium Triangle are the most abundant and the easiest to exploit lithium sources. Lithium is present in diluted concentrations together with different ions, and it is imperative to fully remove both magnesium and calcium before lithium carbonate can be precipitated. Here we use membrane electrolysis as a novel method to generate hydroxyl groups in situ in a two-chamber electrochemical cell with a side crystallizer, omitting the need for chemical addition and not leading to substantial loss of lithium rich brine. Batch electrolysis experiments fully removed more than 99.99% of both Mg²⁺ and Ca²⁺ for three different native South-American brines treated at current densities ranging from 27 to 350 A m^-2 (final concentrations were below ICP detection limit: < 0.05 mg L^-1). For a brine containing 3090 mg L^-1 of Mg²⁺ and 685 mg L^-1 of Ca²⁺, 62 kWh m^-3 are needed for the full removal of both cations when a current density of 223 A m^-2 is employed. Most importantly, the Li⁺ concentration in the brine is not affected. The removed cations are precipitated as Mg(OH)₂ and Ca(OH)₂. Our process has the potential to simultaneously recover lithium, magnesium, and calcium compounds, minimizing waste production.

Keywords: Anion exchange membrane; Hydrometallurgy; Magnesium hydroxide; Raw materials; Sustainable mining; Zero-waste.