Further, the need for near- and on-shoring critical minerals to secure a domestic supply chain has led to proposed lithium extraction projects in the United States. There are only two commercially active operations in the United States for primary lithium production: sourced from continental brines in Nevada and magnesium waste tailings in Utah. The Salton Sea Known Geothermal Resource Area (SS-KGRA) in southern California has the potential to be an additional source. This geothermal brine in this region is currently being utilized for geothermal energy production but is rich in lithium. The geothermal reservoir is expected to have a total dissolved lithium content of 18 million metric tonnes of lithium carbonate equivalent (LCE). It is expected that if lithium was extracted from brines at current geothermal production rates, the resource would provide127,000 metric tonnes of LCE per year.
Existing Li recovery from similar brines relies on the solar evaporation process. In this process, Li-containing brine is stored in large ponds, where evaporation over multiple years increases the concentration of lithium in the brine and sequential precipitation separates Li from competing constituents such as sodium, magnesium, and calcium. However, the solar evaporation method is extremely time intensive, water intensive, and inefficient. For the SS-KGRA, a novel direct lithium extraction (DLE) technology involving the adsorption of Li via lithium/aluminum layered double hydroxides (Li/Al-LDHs) has been identified as the most promising technology by industry actors. Adsorption via Li/Al-LDH can provide rapid, selective, and efficient lithium separation and extraction. Despite the promise of Li/Al-LDH for lithium recovery, there are outstanding questions around how this material will perform under the highly saline and high temperature conditions of the Salton Sea geothermal brine. Additionally, there is little to no information on the water requirements for implementing this technology, which is crucial for decision making in the SS-KGRA, which sources all of its water from the Colorado River. Recent years of drought have led to severe water shortages throughout the Colorado River basin. National agreements between states have mandated a reduction of water consumption for all users of the Colorado River, including in the SS-KGRA. This, in combination with other water requirement needs in the region such as agriculture, means there will be limited water available for the proposed DLE plants. Other potential water sources in the region (groundwater, local surface water, and the Salton Sea itself) are typically sustained by contaminated agricultural runoff and are not considered viable for industrial or potable use. Therefore, careful planning around the allocation of the Colorado River water in the region is crucial. Despite this, very few experimental studies have been conducted on the water use impact of DLE processes. Further, life cycle assessments and environmental impact assessments of these processes have repeatedly indicated that there is a lack of water use data to properly evaluate the impact DLE processes would have at full-scale. As a result, DLE projects in the region face challenges with permitting, which requires assessment of the environmental impacts of the projects.
This work seeks to conduct preliminary evaluation of Li/Al-LDH materials for Li adsorption from Salton Sea geothermal brine and establish relevant and needed data for understanding water consumption during the DLE processes. This data is not only relevant for understanding current DLE technologies, but for informing new process design that can minimize resource use. Li/Al-LDH materials were synthesized via co-precipitation method in which lithium hydroxide and aluminum hydroxide were combined under basic conditions to precipitate a white powder. The resulting material was characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), BET analysis, and particle size analysis to confirm the production of the correct material and evaluate surface characteristics. The material was ground and sieved to achieve a consistent particle size. Initial bench scale testing was conducted. Adsorption isotherms were measured in Li solution via batch adsorption experiments and results were fit to Langmuir and Freundlich isotherms to determine key parameters such as maximum adsorption capacity. Kinetic modelling was conducted similarly via batch adsorption in Li solution to determine kinetic parameters and data was fit to pseudo-first and -second order kinetic models. Separation factors between Li and key components of the Salton Sea geothermal brine (Na+, K+, Ca2+) were measured. Elution experiments were similarly conducted at bench scale by batch method using dilute LiCl in deionized water as the eluent. This is consistent with existing literature that reports decomposition of Li/Al-LDH to the gibbsite phase under low-Li conditions. Recyclability of Li/Al-LDH was tested by repeated adsorption and desorption cycles. Water consumption during each step of these preliminary bench scale experiments was measured and reported. This includes water consumption during adsorbent synthesis, adsorbent washing and conditioning, eluant volume, and reconditioning of adsorbent materials. Consistent with the goal of minimizing water consumption, each of these phases was optimized for reducing water use while maintaining adsorbent characteristics, Li adsorption, and Li recovery. Concentrations of starting materials for adsorbent synthesis were altered, as well as concentration and volume of eluant and adsorbent conditioning solutions.
This work will establish the baseline of using Li/Al-LDH for Li separation and provide initial data on water consumption requirements. It will optimize parameters for adsorbent synthesis, conditioning, and Li elution to enable more efficient and environmentally friendly manufacture of Li/Al-LDH and implementation of the adsorption process in water-constrained regions. Establishing empirical water use needs will inform policy makers, local citizens, and DLE industry partners on how to best implement DLE for minimal impact on the region. Subsequent work will measure solutions of higher complexity designed to more closely model the composition of Salton Sea geothermal brine. Similarly, the effect of technology scaling from bench scale to fixed beds of increasing size on adsorbent performance and water consumption will be measured. This will establish the performance of Li/Al-LDH for Li separation in more industrially relevant conditions, as well as the water consumption requirements in these conditions.