Geothermal power plants produce a large volume of brine for power generation. Since these brines are the product of long-term water-rock interactions at elevated temperatures at depth, they contain dissolved chemical components including critical and strategic mineral commodities at various concentrations. Despite the low concentrations for many of these minerals, significant quantities of select minerals could be recovered due to the large volumes of brine utilized by geothermal power plants. Over the years, the U.S. Department of Energy (DOE) Geothermal Technologies Office (GTO) has been funding research activities focusing on characterization of brine resources and development of extraction technologies for the rare earth elements, lithium, and other critical minerals from geothermal brines. The primary goal of the GTO’s efforts is to secure domestic sources for these critical materials as well as provide an additional revenue stream to the geothermal developers and make geothermal energy more competitive against other types of renewable energy.

Our assessment of the US geothermal brines for their mineral contents indicates that several mineral commodities (e.g., Li, Mn, SiO₂, etc.) are present in high enough concentrations and sufficient flow rates to be economically recovered from geothermal brines. In this presentation, we will provide a summary for mineral contents in geothermal brines in the US. Also we will provide a summary of past as well as current mineral extraction status, opportunities, and challenges for the commercial-scale deployment of mineral extraction technologies.

Geothermal brines may contain a plenty of chemical elements in a wider range of concentrations ranging from trace to bulk concentration level. This makes such brines interesting for the winning of metals as well as of bulk chemicals. Main constraints are the high temperatures and pressures that should be maintained during processing, the accompanying gases like Methane and/or Hydrogen Sulfide and the enrichment of naturally occuring radionuclides during processing.

This can make such processes troublesome.

Coming from research in radioactive scales abatement a process and a galvanic high pressure cell were devoloped for electrochemical separation of valuable trace metals like Sb, Tl, Te from geothermal brine and testes over a longer period at our test facility at Neustadt-Glewe, Northern Germany. We also examined the extraction of numerous metals from brine sample from the Großschönebeck geothermal site by means of electrochemical and selective adsorption methods. In Addition based on brines from the Stassfurt geological series the winning of base chemical like hydrochloric acis and sodium hydroxide solutions was also shown to be possible by means of electromembrane processes. The economic potential of such methods will bei evaluated based on experimental data.

In order to increase the economic potential of geothermal power plants in view of environmental protection and sustainability, there are considerations to extract critical raw materials (CRM), such as lithium, from the geothermal fluids. The Upper Rhine Graben (URG) in southern Germany is of particular interest as it represents a hydrothermal fluid reservoir with large CRM potential.

Apart from the sedimentary strata with different reservoir rocks and associated fluids, their Variscan basement is exhumed in the Vosges and the Schwarzwald. Unconformity related hydrothermal vein-type and Mississippi-Valley-Type deposits evolved from paleofluids in the region. Recent fluids precipitate mineralogically and chemically similar scalings in the power plants indicating their potential for CRM extraction from these fluids. These access to all parts of a geological system in the URG area accounts for its consideration as a natural laboratory. Therefore, this study deals generally with metal provenance, reservoir processes, transport and precipitation mechanisms.

One of the hypotheses regarding those processes that lithium is released into the geothermal fluid by hydrothermal alteration of basement feldspars and mica. To corroborate the hypothesis, different reservoir rocks and fluids in various depths are mineralogically and geochemically investigated. The fluids analyzed to this point show different trace element distributions, which are associated with changes in Cl/Br and Rb/Cs ratios and might thus indicate an interaction of the fluids with the reservoir rocks. A comparison of modern fluid chemistry with the geochemistry of altered minerals, will help to link CRM provenance and content in modern fluids with economic potential.

Lithium is one of the critical elements for the realization of electric mobility and energy transition. With a contribution to global Li-production and recycling of less than 1% (2017), Europe depends almost entirely on Li-import. To reduce the dependency, new and unconventional Li resources are explored in the EU. One resource are highly saline brines from geothermal reservoirs of the Upper Rhine Graben (URG), characterized by Li concentrations of up to 200 mg/L. Due to space restrictions, conventional production methods are not suitable and technologies for the direct lithium extraction (DLE), such as Li-Mn oxide sorbents, is needed. Lithium-Mn oxide sorbents have a comparably high Li adsorption capacity and due to ion-sieve properties a high selectivity for Li. Batch experiments with synthetic Li⁺-solutions and natural geothermal brines were conducted to investigate the sorption capacity and kinetics, and the role of competing ions on Li sorption. The batch experiments reveal fast sorption kinetics with an adsorption of >70% of the maximum Li sorption capacity within several minutes. In relation to Li⁺-solutions, Li sorption decreases for geothermal brines due to sorption of competing ions. While alkaline metals show a relatively little influence, Mn and Ba are the major competing ions. We prove successfully that Li-extraction from geothermal brines in the URG by Li-Mn oxide sorbents is technologically feasible at the laboratory scale. Upscaling into a pilot plant integrated into a power plant is in progress.

The energy transition and the associated need for non-energy, mineral raw materials have prompted the German government to expand research and development activities along the entire value chain. It is well known that the highly mineralized thermal waters that circulate during the extraction of geothermal energy have, in some cases, significant enrichments of economically strategic elements such as lithium, rubidium, antimony or magnesium. The extraction of mineral resources from thermal waters is still challenging in terms of process technology, but new sustainable methods are paving the way for economic extraction as an alternative to conventional hard rock mining. Due to the overall high salt concentrations, selective separation of scale-forming minerals in a pretreatment stage is necessary to avoid scalings or membrane fouling in the later process steps.

The focus of this study is on controlling silicate precipitation, which is expected to occur due to temperature changes and proceeding concentration cycles, which are required for raw material and freshwater extraction. The treatment and an associated precipitation process must be cost-effective, integrable into the power plant process, and selective for silica but must not affect the concentration of valuable elements. In a multi-step and interdisciplinary process, a treatment strategy was developed and implemented in a large-scale prototype. In this study, the development of the silica processing strategy from laboratory to prototype design is described. Finally, the construction and implementation of a large-scale prototype with promising field results are presented.