Particle size distribution of magnesium precipitates from solutions with different Mg/Li ratios

ALEMÁN SÁNCHEZ, ARNALDO DAVID; BERTIN, DIEGO ESTEBAN; PIÑA, JULIANA

Buenos Aires

Congreso; WCCE11 - 11th World Congress of Chemical Engineering; 2023

Asociación Argentina de Ingenieros Químicos

Lithium is one of the most in-demand commodities in the world, mainly because it is one of the key components in electric vehicle batteries. Approximately 60% of the world´s lithium reserves are found in the salt flats of northern Argentina and Chile and southern Bolivia. Currently, lithium from brines is obtained by the conventional process that comprises a sequence of steps to separate various elements from the brine prior to the precipitation of lithium carbonate.The brine is originally found underground, pumped into shallow ponds, and exposed to natural atmospheric conditions. Under these conditions, solar radiation produces the evaporation of 60 to 80% of the water to increase the concentration of saline compounds. Finally, the concentrated solution is reacted with sodium carbonate to obtain lithium carbonate. In the carbonation stage, magnesium is one of the compounds competing with lithium; therefore, its presence must be minimized. Usually, this is done in solar ponds by adding calcium hydroxide or quicklime, which promotes the precipitation of magnesium hydroxide. In particular, the precipitation of Mg(OH)2 using NaOH produces a very fine precipitate (An et al., 2012), which makes difficult the solid/liquid separation stage necessary for its recovery.This work investigates the effect of the Mg/Li ratio and the concentration of NaOH (precipitant) on the particle size distribution of the precipitate. To this end, aqueous solutions with different concentrations of MgCl2 and LiCl were prepared and the Mg/Li ratios were determined. Subsequently, different samples were prepared by adding NaOH in different proportions. The NaOH concentration varied from 10% below stoichiometric to 20% in excess. Additional samples were prepared by increasing the Mg/Li mass ratio from 1 to 10. Each sample was allowed to stand long enough to form a stable precipitate. The precipitate was extracted from the sample and its particle size distribution was measured by laser diffraction (Horiba LA-950V2). Furthermore, the concentration of the liquid phase was determined by atomic absorption. The obtained results indicate that the precipitate is magnesium hydroxide. The particle size of the precipitate increases considerably as the Mg/Li ratio increases. The concentration of NaOH in the solution also has an impact on the particle size of the precipitate, giving a linear trend. Indeed, it is found that the mean diameter of the particle size distribution reaches its greatest value when the amount of added NaOH is greater than the stoichiometric proportion given by the precipitation reaction.