Pressure Leach of Β-Spodumene with Carbonic Acid: Weak Acid Process for Extraction of Lithium

This contribution develops a process for recovering lithium from ss-spodumene by sequential pressure leach with solutions of carbonic acid, as a replacement for digestion of ss-spodumene with concentrated sulfuric acid. Six sequential steps, each operated at 200 degrees C and 100 bar for 1 h, retrieve 75 % of lithium from the initial feed material. This compares with a 12.6 % Li recovery from a single-stage (batch) operation. We develop both the thermodynamic and mass-transfer models of the leach process and argue that the lithium extraction is masstransfer controlled. The underlying mechanism involves: (i) near-instantaneous ion exchange between H+ for Li+ at the onset of the digestion; (ii) direct formation of amorphous Li-depleted rinds; (iii) precipitation of secondary minerals, boehmite (AlO(OH)) and skin-forming amorphous silica (SiO2), the latter only in the batch operation; (iv) counter-current diffusion of H+ and Li+ through the surface layer (surface layer = depleted sublayer + precipitation sublayer); (v) dissolution of the surface layer at the solid-solution interface, through the reactions with water. The evidence comes from the near-proportionality between the lithium recovery and mass of dissolved ss-spodumene, no detection of keatite-HAlSi2O6, particle morphology, precipitation of amorphous skins of SiO2 decorated with boehmite specks observed by electron microscopy and predicted from thermodynamic modelling as well as from a mass-transfer analysis of the Li-recovery rates. Direct observations confirm the existence of a three-phase reacting system, as predicted from thermodynamic calculations, with the absence of a supercritical fluid phase. Lithium recoveries, of at least 28 % in a single step, are attainable, but only for long contact time of 48 h. Digestion lasting hundreds of hours is needed for the batch process to reach Li extraction corresponding to that of the sequential leach. The challenges for industrial implementation of the process comprise recycling of large amounts of water, avoiding the precipitation of silica that forms rinds, accelerating the hydrolysis of Li-depleted sublayer, concentrating the dilute solutions of Li+ and overcoming hesitance of industry to consider high-pressure treatment.