Experimental assessment of sustainable lithium extraction from geothermal reservoirs

The sustainable extraction of critical metals, including lithium, from geothermal fluids offers an environmentally responsible alternative to traditional mining practices, addressing the rising global demand for these materials. This study examines the interaction of reinjected geothermal fluids with rhyolite and pumice rocks from the Taupō Volcanic Zone under simulated geothermal reservoir conditions. Using a continuous-flow system capable of maintaining pressures up to 500 bar and temperatures up to 400 °C, we investigate the mechanisms and timescales of metal mobilization and the roles of fluid chemistry, temperature, and rock composition on timescales of 1 to 2 months. The reactant, reinjection fluid is silica-depleted, which enhances reaction rates with the silica-rich rocks. Product fluids from our experiments were analysed for a large number of major and trace elements and run product solids were characterised by SEM and XRD. The results demonstrate that lithium release is significantly influenced by temperature, with elevated temperatures (>200 °C) enhancing lithium mobilization through the devitrification of volcanic glass. At the end of each experiment, the total recovery (as a percentage of that originally in the rock) of lithium is 90 % for the pumice and 80 % for the rhyolite. The only other elements with significant (>5 %) recovery rates are tungsten (29 % for rhyolite), cesium (100 % rhyolite), fluorine (59 % pumice; 11 % rhyolite), silver (30 % pumice) and germanium (40 % pumice; 100 % rhyolite). Some elements, notably antimony for both rhyolite and pumice, are quantitatively stripped from the reactant fluid, presumably into secondary minerals. Differences in behaviour between rhyolite and pumice for tungsten and cesium may reflect sequestering of these phases in secondary minerals, such as zeolites, in the pumice experiments. Secondary minerals did not sequester lithium in either of the experiments; its very high recovery rates probably reflect rapid diffusion through the silicate glass that operates at a faster rate than silicate glass breakdown. The study provides insights into fluid-rock interactions within volcanic geothermal reservoir rocks and highlights the ability of silica-poor reinjection fluids to enhance lithium extraction from the reservoir, a potential strategy for sustainable lithium extraction from geothermal systems.