EXTRACTION OF LITHIUM IONS AND OTHER RARE ALKALINE METAL IONS FROM GEOTHERMAL WATER WITHIN A BINARY GEOTHERMAL POWER PLANT

Description

[0001] The present invention relates to an apparatus and a method for extracting lithium ions and other rare alkali metal ions from geothermal water within a binary geothermal power plant.

[0002] In highly saline deep waters of the Upper Rhine Graben, but also in other geothermal areas (e.g. Salton Sea, California / USA; Clayton Valley, Nevada / USA; Lazio, Italy), lithium occurs in relatively high concentrations (Williams et al., Geochimica et Cosmochimica Acta 1989, 53, 8, 1905-1920; Sanjuan et al., Chemical Geology 2016, 428, 27-47).

[0003] A very high enrichment of rare metals (lithium, cesium, rubidium) is also found in geothermal waters along the south-north direction of the Shiquanhe-Yarlung Zangbo geothermal area, near the Yarlung Zangbo Suture Zone, southern Tibet, China (W. Wang et al., Scientific Reports 2019, 9, 7918).

[0004] A major challenge is the quantitative, economical, and environmentally friendly extraction of rare alkali metal ions, such as lithium, cesium, and rubidium, from geothermal waters. In particular, lithium extraction from geothermal brines used within geothermal power plants is of great economic interest.

[0005] Some processes are described in the literature on how lithium, cesium and rubidium can be extracted from brines: Meshram et al. describe absorption methods (ion exchangers), liquid-liquid extraction methods, and precipitation processes for the extraction of lithium from ores, minerals, and brines. (P. Meshram et al., Hydrometallurgy 2014, 150, 192-208).

[0006] WO2014172032A2 describes a general process for the selective or combined removal of rubidium and / or cesium from brines using tetrafluoroborates. Also disclosed are processes for the production of rubidium and / or cesium chlorides using ionic liquids and exchange media.

[0007] However, chemical precipitation processes for the separation of divalent cations from the brine can lead to the precipitation of compounds containing heavy metals or radionuclides in addition to the precipitation of magnesium and calcium salts.

[0008] WO2016116687A1 describes a technical process based on liquid-liquid extraction of ions from brines using an ion-absorbing liquid and has so far only been used under atmospheric pressure.

[0009] Huang et al.describe a liquid-liquid extraction process for the extraction of rubidium and cesium from brines using 1,4,7,10,13,16-hexaoxacyclooctadecane (18C6) and 1-butyl-3-methylimidazolium hexafluorophosphate (D. Huang et al., Polish Journal of Chemical Technology 2018, 20, 2, 40-46).

[0010] AA Rahman et al. disclose a simulated combined heat and power system comprising an integrated geothermal power plant based on double-flash binary technology with a subsystem for lithium extraction by precipitation of Li 2 CO 3 . The schematic sketch of the simulation system in AA Rahman et al. involves pumping water from a geothermal reservoir, passing the water through a heat exchanger, transporting the water to the lithium extraction unit, and reinjecting the water through an outlet well. The simulation conditions in AA Rahman et al.are adapted to a geothermal power plant in the Salton Sea Geothermal Field, California, USA (AA Rahman et al., Int J Energy Res. 2020, 44, 9586-9597).

[0011] US10604414B2 describes a system and sequential process for the recovery of lithium, zinc, and manganese from a geothermal brine, in particular from a brine from the Salton Sea Known Geothermal Resource Area, within a flash steam power plant. The system and process comprise: 1) the removal of impurities such as silica and metals by oxidation and coprecipitation; 2) the selective recovery of lithium chloride from the brine stream and its concentration using continuous countercurrent ion exchange at atmospheric pressure; 2a) the optional sequential use of further lithium extraction processes such as liquid extraction, reverse osmosis / nanofiltration, and electrolysis; and 3) the conversion of lithium chloride to lithium carbonate (with cooling to near room temperature) or lithium hydroxide.

[0012] The French company Eramet has developed a process that uses a linear arrangement of ion exchangers, reverse osmosis, nanofiltration and liquid-liquid extraction to extract lithium from salt brines, for example from the salt deserts of Argentina (https: / / www.eramet.com / en / activities / - development-projects / lithium-project-new-growth-domain, accessed on January 29, 2020).

[0013] Ion exchangers and liquid-liquid extraction methods must achieve very high selectivity for the alkali metal ions to avoid subsequent, potentially very complex chemical separation processes. For the use of ion exchangers and liquid-liquid extraction methods in geothermal power plants, a portion of the volume flow would have to be extracted from the thermal water cycle, as ion exchangers and liquid-liquid extraction cannot be easily integrated into the material cycle of a geothermal power plant, and thus the large volume flows (up to 100 l / s) in geothermal plants cannot be utilized. Interaction of geothermal water with ion exchangers or ionic liquids is not easy to achieve if the geothermal water is to be kept under pressure and at temperatures of ≥ 50 °C to avoid precipitation.

[0014] Ion exchange processes and liquid-liquid extraction methods still require large amounts of non-recyclable acid, usually hydrochloric acid, for the remobilization or leaching of, for example, lithium ions (X. Zhao et al., Fluid Phase Equilibria 2018, 459, 129-137; Meshram et al., Hydrometallurgy 2014, 150, 192-208).

[0015] In Li et al.The applicability of nanofiltration membranes for the recovery of lithium from salt lake brine with a high magnesium-to-lithium mass ratio is evaluated under varying operating pressure, temperature, the concentration of additional cations (Na +< , K +< , Ca 2+< ), and the magnesium-to-lithium concentration ratios. A pressure range of 15–35 bar, a temperature range of 20–40 °C, and a low pH value (pH = 4 ± 0.2) were chosen for the experiments. The results show that the yield of lithium and its separation from magnesium increases with pressure, while the separation efficiency decreases with increasing temperature. The results also show that the addition of an excess of Na +< ions and K +< ions to the test solution increases the retention rate of Li +< ions, whereas the addition of Ca 2+< ions decreases the Li +< ion retention rate (Li et al., Desalination 2019, 468, 114081).

[0016] Pramanik et al.conducted experiments on the enrichment of lithium from a simulated salt lake brine using an integrated nanofiltration and membrane distillation process under an operating pressure of ≤ 10 bar. Two types of nanofiltration membranes, namely NF90 and NF270, were used to compare their performance for lithium and magnesium separation under different operating conditions. The NF90 and NF270 nanofiltration membranes achieved lithium separation efficiencies of 23% and 44%, respectively. After nanofiltration treatment, the separated lithium could be further concentrated to 80% using a direct contact membrane distillation system (BK Pramanik et al., Journal of Environmental Chemical Engineering 2019, 7, 5, 103395).

[0017] The extraction of rare alkali metal ions, such as lithium, caesium or rubidium ions, from geothermal water within a geothermal power plant using the geothermal water cycle, the volume flow and the high temperatures of ≥ 50 °C, in which the prevailing water pressure is maintained, the increased osmotic pressure present in geothermal water due to the very high salinities (total dissolved solids > 90 g / l) is overcome and in which chemical precipitation of undesirable ingredients, such as compounds containing heavy metals or radionuclides, is avoided, has not yet been described.

[0018] The present invention is therefore based on the object of providing a method which enables the extraction of lithium, cesium or rubidium ions from the geothermal water circuit of a binary geothermal power plant using the volume flow and the existing temperature and pressure conditions as efficiently as possible, without restricting the ongoing operation of the geothermal power plant and the production of electricity and / or heat.

[0019] The above technical problem is solved by the embodiments characterized in the claims.

[0020] The method according to the invention comprises the following steps: A) Pumping up geothermal water with a temperature of 120 °C - 180 °C from a geothermal reservoir (11) via a production well (1) into a heat exchanger (2)under a pressure of 15 - 35 bar; B) Passing of the geothermal water from step A) through the heat exchanger (2); C) Feeding 100% of the geothermal water from step B) into at least one nanofiltration module (3); D) Nanofiltration of the at least one nanofiltration module (3) supplied geothermal water for the separation of alkali metal ions and other monovalent ions from multivalent ions under a pressure of 15 - 35 bar and a temperature of 50 °C - 80 °C and direct reinjection of the residual solution enriched in multivalent ions into the geothermal reservoir (11) via at least one reinjection well (4); E) Transfer of the permeate of the nanofiltration from step D) into a precipitation unit (5a or 5b) for the precipitation of the remaining divalent alkaline earth ions; F) precipitation of the remaining divalent alkaline earth ions in the precipitation unit (5a or5b) from the permeate of the nanofiltration of step D); G) Transport of the permeate depleted of divalent ions from step F) by a circulation pump (6a or 6b) to a second heat exchanger (7a or 7b); H) Heating the permeate from step G) in the second heat exchanger (7a or 7b) to 65 - 85 °C; I) Membrane distillation (8) of the permeate heated in step H) under normal pressure and a temperature of 65 °C - 85 °C; J) separation of lithium ions concentrated in step I) in a separation unit (9) by precipitation, ion exchange, absorption processes, liquid-liquid extraction or electrodialysis.

[0021] In a particular embodiment, the residual solution from step J) enriched in caesium ions and rubidium ions is further processed in a separation unit (10) to extract caesium ions and rubidium ions.

[0022] Features of the invention lie in the development of a new process for the extraction of lithium ions and optionally of cesium and rubidium ions from geothermal water within a binary geothermal power plant.

[0023] A binary geothermal power plant is a geothermal dual-fuel cycle power plant in which the thermal energy of geothermal water pumped up from a geothermal reservoir is transferred through a heat exchanger to a secondary (binary) fluid (working fluid). The working fluid evaporates and is fed into the turbine / generator unit. The vapor pressure of this working fluid drives the turbines. (I) and the generators (II) to generate electricity. The working fluid cools due to expansion in the turbine and the steam pressure decreases. The suction of the cooling systems or condensers (III) leads to a negative pressure of less than 1 bar. In the condensers (III)The working medium is further cooled and liquefied again before it is circulated by a circulation pump (IV) re-enters the heat exchanger. In a binary geothermal power plant, the geothermal water (primary circuit) does not come into contact with the working fluid and the turbine / generator unit (secondary circuit). (https: / / orkustofnun.is / gogn / unu-gtp-report / UNU-GTP-2013-20.pdf, accessed on March 27, 2020).

[0024] The process according to the invention allows the technical integration of nanofiltration and membrane distillation without significantly limiting the electricity and heat production in the binary geothermal power plant and relies exclusively on plant-internal technical and energy resources - i.e., the process does not require any additional external energy and takes into account a small space requirement.

[0025] The decisive advantage over the state of the art lies in the recyclability of the process according to the invention, in the utilization of the entire volume flow while largely avoiding unwanted chemical precipitation, and in the utilization of the energetic and technical constraints of geothermal plants, i.e., the utilization of pressure maintenance and the heat contained in the system, without restricting operations and, if applicable, electricity production. This enables a minimally invasive, economical, and environmentally friendly extraction of lithium ions and other rare alkali metal ions.

[0026] In the process according to the invention, in a first step, the geothermal water is extracted from a production well at a temperature of 120 °C - 180 °C, preferably at a temperature of 135 - 180 °C, particularly preferably at a temperature of 150 - 170 °C (1) into a heat exchanger (2)within a binary geothermal power plant. The geothermal water and the working fluid, which preferably has a boiling point of < 40 °C, then pass through the heat exchanger (2).

[0027] In the process according to the invention, 100% of the geothermal water after passing through the heat exchanger (2) transferred into at least one nanofiltration module (3).

[0028] The process according to the invention comprises, in a further step, the extensive separation of lithium, cesium and rubidium ions and other monovalent ions from multivalent ions in the at least one nanofiltration module (3) after the geothermal water passes through the first heat exchanger (2), where the multivalent ions critical for precipitation (e.g. Ba 2+< , Ca 2+< , Pb 2+< , Cd 2+< , Fe 2+< , Mn 2+< , Sb 3+< , Ra 2+< , SO 4 2-< etc.) are separated as retentate (concentrate) and returned to the geothermal reservoir (11)be reinjected.

[0029] Nanofiltration is a pressure-driven membrane process in which membranes are used that, by definition, have pore sizes of 0.1 - 10 nm, which distinguishes them from coarser membranes used in ultrafiltration and microfiltration, and from finer membranes used in reverse osmosis (https: / / www.fiw.rwth-aachen.de / neo / fileadmin / pdf / membranbuch / D_Membranbuch_300106.pdf, accessed on February 18, 2020).

[0030] In the process according to the invention, the pipeline pressure within the binary geothermal power plant is used for nanofiltration. Depending on the water chemistry, the water pressure prevailing in the pipelines of binary geothermal power plants in the primary circuit (geothermal water circuit) is typically used in addition to scaling inhibitors (inhibitors of mineral deposits) to prevent precipitation and degassing during and after the geothermal water passes through the heat exchanger. Sanjuan et al. describe the water chemistry of geothermal waters based on the analysis of water samples from the Upper Rhine Graben (B. Sanjuan et al., Chemical Geology 2016, 428, 27-47).

[0031] The most significant precipitations that can occur during and after the passage of geothermal water through the heat exchanger, depending on the water chemistry, are precipitations of CaCO 3 (calcite, aragonite), CaCO 3 • MgCO 3 (dolomite), BaSO 4 (barite) and CaSO 4 (anhydrite).

[0032] CaCO 3 and CaCO 3 • MgCO 3 precipitations are largely prevented in the process according to the invention by choosing correspondingly high operating pressures and temperatures (S. Speil, Master's thesis in Earth Sciences at Graz University of Technology 2018, https: / / diglib.tugraz.at / download.php?id=5aaa6e3ba95bf&locati on=browse, accessed on November 5, 2020).

[0033] In a specific embodiment of the process according to the invention, BaSO 4 and CaSO 4 precipitations are prevented by adding (2)Scaling inhibitors can be added. Suitable scaling inhibitors are organophosphates, such as diethylenetriaminepenta(methylenephosphonic acid), 1-hydroxyethane-(1,1-diphosphonic acid), aminotrimethylenephosphonic acid, or ethylenediaminetetra(methylenephosphonic acid), which shield the Ba 2+< and Ca 2+< ions from the sulfate ions by complexing them, preventing them from reacting with each other. In addition, they prevent the crystal growth of BaSO4 or CaSO4 crystals (J. Schreiber et al., PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-202, https: / / pangea.stanford.edu / ERE / pdf / IGAstandard / SGW / - 2014 / Scheiber.pdf, accessed on November 5, 2020; Master's thesis C. Otten, Department of Engineering and Natural Sciences, Merseburg University of Applied Sciences 2019, https: / / opendata.uni-halle.de / bitstream / 1981185920 / 14189 / 1 / OttenChristoph_Charakte risierung_eines_organischen_Scalinginhibitors.pdf, accessed on November 5, 2020).

[0034] Polycarboxylic acids, such as polymaleic acid, polyacrylic acid or polymethacrylic acid, are also suitable as scaling inhibitors.

[0035] Organophosphates and polycarboxylic acids belong to the so-called "threshold inhibitors" because they can be used in substoichiometric amounts, typically in concentrations of 1 - 50 ppm (M. Mpelwa et al., Petroleum Science 2019, 16, 830-849; J. Zotzmann et al., Geotherm Energy 2018, 6, 18).

[0036] Organophosphates and polycarboxylic acids also act as corrosion inhibitors and thus prevent corrosion in the pipelines of the geothermal water cycle (https: / / www.irohedp.com / best-organic-phosphonic-antiscaleand-corrosion-inhibitor / , accessed on November 5, 2020; DE Arthur et al., Int. J. Ind. Chem. 2013, 4, 2).

[0037] Other suitable corrosion inhibitors, as described in US9688903B2, are ether compounds such as ethylene glycol monobutyl ether or ethylene glycol monopropyl ether, quaternary ammonium compounds such as benzyldimethyldodecylammonium chloride or benzyldimethyltetradecylammonium chloride, and condensates of fatty acid amines. In US9688903B2, these corrosion inhibitors are added at a concentration of approximately 1 ppm to approximately 100 ppm.

[0038] Degassing (CO2 leakage) must be prevented during the passage of the geothermal water through the heat exchanger, since otherwise the heat transfer in the heat exchanger is inefficient or prevented and CaCO3 precipitation can occur in all parts of the thermal water circuit.

[0039] In the process according to the invention, the water pressure in the pipelines is built up by a feed pump and is between 15 and 35 bar. Under the pressure and temperature conditions prevailing in the process according to the invention, neither precipitation of CaCO3 nor release of CO2 from the geothermal water is to be expected.

[0040] In a preferred embodiment, the water pressure in the pipelines is 20–25 bar. By adding additional pumps or increasing the pumping capacity of the feed pump, the pressure can be increased to ≤ 35 bar as needed. Maintaining the pressure is necessary, on the one hand, to convey the hot geothermal water (> 120 °C) as a liquid phase while simultaneously leaving other gases – especially CO2 – dissolved in the water. The pressure therefore depends primarily on the thermodynamic boundary conditions (https: / / www.internetchemie.info / chemielexikon / daten / w / was-serdampfdruck.php, accessed on March 18, 2020). For example, for geothermal water with a temperature of 160–170 °C, 6–8 bar is sufficient to convey it as a liquid phase. However, to keep the CO2 dissolved in the geothermal water at these temperatures and the salinities present in the geothermal water, pressures of ≥ 20 bar are required.

[0041] Nanofiltration membranes can retain ions and other dissolved substances, such as low-molecular-weight compounds, dissolved in water. Of particular interest in nanofiltration membranes is their ability to largely separate monovalent from multivalent ions. This is due to interactions between the ions and the membrane material and the different hydration radii of the ions. The radius of a hydrated ion determines the permeability of ions and the associated ion rejection of nanofiltration membranes (B. Tansel et al., Separation and Purification Technology 2006, 51, 1, 40-47). The hydration shell of ions depends on the charge of the ions, the presence of other ions in the solution, and the crystal radius (F. David et al., Journal of Molecular Liquids 2001, 90, 1, 45-62). Despite smaller ionic radii (crystal radius), divalent cations (e.g.Magnesium ions) have a larger hydration radius than monovalent cations (e.g. lithium ions) and are therefore better separated by the nanofiltration membrane.

[0042] The negative charges in the pores and on the surface of the nanofiltration membrane are immobile. However, they are shielded by free positive monovalent ions, such as Na +< and K +< , in the feed solution, resulting in the formation of an electrical double layer on the pore wall. If the concentration of salts in the feed solution is increased, the negative solid ions in the membrane are shielded even more strongly, and monovalent anions can also pass through the membrane. Compared to monovalent ions, polyvalent cations hardly penetrate the membrane pores and are largely retained.

[0043] Another property of nanofiltration membranes is the Donnan effect, which is also due to the concentration dependence of ion rejection (AE Childress et al., Environmental Science & Technology 2000, 34, 17, 3710-3716; T. Melin and R. Rautenbach, Membrane Processes: Fundamentals of Module and System Design (2007), Springer-Verlag Berlin, Heidelberg). Furthermore, other mechanisms play a role in the transport of ions through the nanofiltration membrane, such as sieving effects or diffusion within the membrane and in the solution. Sieving effects arise from the convective transport of ions through the membrane as a result of the pressure difference between both sides of the membrane and depend on the pore size distribution of the membrane. Diffusion of ions through the membrane occurs due to concentration potential gradients (https: / / www.fiw.rwth-aachen.de / neo / fileadmin / pdf / membranbuch / D_Membranbuch _300106.pdf, accessed on 18.02.2020).

[0044] Due to the high ion concentrations (~ 1.5 mol / l) in geothermal waters, very high osmotic pressures (>> 40 bar) prevail, which limit the use of alternative, denser membranes (e.g. reverse osmosis membranes).

[0045] The high salinities in geothermal waters are primarily caused by sodium and chloride ions. However, sodium and chloride ions permeate very well through the nanofiltration membrane. This significantly lowers the effective osmotic pressure that must be overcome during nanofiltration.

[0046] In the process according to the invention, monovalent ions, such as lithium, cesium and rubidium ions, flow through the nanofiltration membrane used to an extent of more than 80%, whereas the multivalent ions, such as Mg 2+< and Ba 2+< ions, are retained to an extent of more than 80%.

[0047] Membrane permeability increases with increasing temperature due to the exponential temperature dependence of water viscosity according to the Arrhenius-Andrade equation (EN da C. Andrade, London Edinb. Dub. Philos. Mag. J. Sci. 1934, 17, 112, 497-511). Higher temperatures thus favor the extraction of alkali metal ions, such as lithium, cesium, and rubidium ions, in that a smaller total surface area of ​​the nanofiltration membrane modules is required at higher temperatures. After passing through the first heat exchanger (2), the temperature of the geothermal water is between 50 °C and 75 °C, depending on the prevailing outside temperature, and in exceptional cases of extreme outside temperatures (> 35 °C), even up to 80 °C.

[0048] The nanofiltration membrane used in the process according to the invention is a polymer membrane or a ceramic membrane with a pore size of 0.1 nm - 10 nm, a pressure resistance of up to 55 bar and a temperature resistance of up to 80 °C.

[0049] The use of an asymmetric polymer nanofiltration membrane consisting of an active layer of polyamide and a porous support layer of polysulfone is preferred.

[0050] Particularly preferred is the use of a nanofiltration membrane NF245 (DuPont) with a pressure resistance of up to 54.8 bar at 45 °C and up to 27.5 bar at 70 °C, and a temperature resistance of up to 70 °C. This membrane has a 0.3-3 µm thick active layer made of polyamide and a porous support layer made of polysulfone. The NF245 membrane retains organic substances with a molecular weight > 300 g / mol while simultaneously allowing increased permeability of monovalent ions.

[0051] In one specific embodiment of the process according to the invention, the nanofiltration membranes are technically arranged to form a connectable unit, a flat channel module. The flow conditions in the flat channel modules are similar to those in commercially available wound modules. The wound modules themselves consist of flat membranes stacked in several layers, separated by a mesh grid (spacer), and wound around a perforated central tube.

[0052] In another embodiment, tubular membranes in tubular or hollow fiber modules are also used for nanofiltration in the process according to the invention (https: / / www.fiw.rwth-aachen.de / neo / fileadmin / pdf / membranbuch / D_Membranbuch_300106.pdf, accessed on 18.02.2020).

[0053] In the method according to the invention, the nanofiltration unit consists in one embodiment of one and in another embodiment of several nanofiltration modules (3).

[0054] The pressure losses on the retentate side of the nanofiltration membrane are kept low, so that the retentate can be reinjected via at least one reinjection hole (4) into the geothermal reservoir (11) can be reinjected.

[0055] The permeate of the nanofiltration from step D) is fed into a precipitation unit (5a or 5b) to precipitate divalent alkaline earth ions.

[0056] In a further step of the process according to the invention, the remaining low concentrations of divalent alkaline earth ions, such as Mg 2+< -, Ca 2+< -, Sr 2+< - and Ba 2+< - ions, are removed from the permeate of the nanofiltration in the precipitation unit (5a or 5b)precipitated and filtered. Due to the low solubility products of alkaline earth carbonates and alkaline earth hydroxides compared to the solubility products of alkali carbonates and alkali hydroxides, chemical precipitation of alkaline earth ions as carbonates or hydroxides is preferred. The permeate is depleted of multivalent ions after nanofiltration.

[0057] On the permeate side, atmospheric pressure conditions prevail in the process according to the invention, so that the permeate is circulated (6a or 6b) to a further process stage. The permeate passes through a second heat exchanger (7a or 7b), and by absorbing heat energy, the temperature of the permeate is increased to up to 85 °C.

[0058] In a further step of the process according to the invention, the permeate is distilled by membrane (8) concentrated.

[0059] Membrane distillation is a thermally driven membrane process that enables the removal of water and other volatile substances from a solution at temperatures of up to 85 °C and a temperature difference of ≥ 15 K. In membrane distillation, a hydrophobic, microporous membrane serves as a barrier to prevent contact between the hot and cold sides and to allow the passage of water vapor (Smolders et al., Desalination 1989, 72, 249-262).

[0060] Due to the hydrophobic properties of the membrane, liquid water cannot penetrate the membrane pores, while water vapor molecules can pass easily. With a sufficient temperature gradient between the raw water and product streams, a water vapor partial pressure difference develops across the membrane, which acts as a driving force for the permeation of water vapor molecules through the membrane, enabling the transfer of water from the raw water stream to the product stream. The raw water stream is concentrated by the water removal, and the water vapor is condensed in the cooler product stream (Alkhudhiri et al., Desalination 2012, 287, 2-18).

[0061] In the process according to the invention, a direct contact membrane distillation, an air-gap membrane distillation or a vacuum membrane distillation is preferably used (D. Winter et al., Journal of Membrane Science 2011, 375, 1-2, 104-112).

[0062] In the process according to the invention, the membrane distillation is carried out under normal pressure and a temperature of 65 °C - 85 °C.

[0063] Direct contact membrane distillation is particularly preferred. In the direct contact membrane distillation process, both sides of the membrane are exposed to liquid. The hot feedwater is located on the first side, the evaporator side, while the cooled permeate is located on the second side. The condensation of the vapor permeating through the membrane takes place directly in the liquid phase at the membrane boundary layer. Since in this case the membrane is the only obstacle to mass transfer, relatively high area-specific permeate fluxes are achieved (BB Ashoor et al., Desalination 2016, Volume 398, 222-246).

[0064] In the process according to the invention, in a specific embodiment, individual membrane distillation modules are interconnected so that they can be adapted to the respective boundary conditions - in particular the flow rates - in a binary geothermal power plant.

[0065] In the process according to the invention, the membrane distillation is followed by further processes for lithium separation in a separation unit (9) Examples of separation processes include precipitation, ion exchange, absorption processes, liquid-liquid extraction, and electrodialysis.

[0066] The separation of lithium ions by precipitation is preferred. Lithium ions are particularly preferably chemically precipitated in the salt solution enriched by membrane distillation by adding Na 2 CO 3 as lithium carbonate (Li 2 CO 3 ) or by adding sodium phosphate (Na 3 PO 4 ) as lithium phosphate (Li 3 PO 4 ).

[0067] Lithium carbonate exhibits an inverse solubility behavior, so that lithium carbonate can be precipitated more efficiently at higher temperatures than at lower temperatures (U. Wietelmann and RJ Bauer, Ullmann's Encyclopedia of Industrial Chemistry, 6th ed., 2002, 339-366, Weinheim: Wiley-VCH).

[0068] The chemically precipitated lithium carbonate or lithium phosphate is separated in the process according to the invention, for example by filtration or centrifugation.

[0069] In a particular embodiment, the residual solution after the lithium ion separation in step J) is used to recover rubidium ions and cesium ions in a separation unit (10) further processed, for example by further concentrating the aqueous solution containing caesium ions and rubidium ions.

[0070] The residual solution remaining after the separation of lithium ions and, if applicable, of caesium and rubidium ions is injected via at least one reinjection well (4) into the geothermal reservoir (11) reinjected.

[0071] Deionized water is produced as a by-product of membrane distillation, which is either reinjected into the reservoir or used for internal plant technical purposes or commercially as a by-product.

[0072] The invention is explained in more detail with reference to the following figures, exemplary embodiments, and descriptions. All illustrated features and their combinations are not limited to these figures and exemplary embodiments and their configurations. Rather, they should be considered representative of other possible configurations that are not explicitly illustrated as exemplary embodiments.

[0073] Figure 1describes a flow diagram of the integrated, recyclable alkali metal ion extraction process in a binary geothermal power plant. From a production well (1) Geothermal water (120 °C - 180 °C) is extracted from a geothermal reservoir (11) under a pressure of more than 20 bar into a first heat exchanger (2) pumped. In the heat exchanger (2) The working fluid of the geothermal power plant is heated and the geothermal water is cooled to 50 °C - 75 °C. The working fluid is then used to drive the turbine (I) and the generator (II) used.

[0074] The working fluid cools down due to expansion in the turbine and the steam pressure decreases. The working fluid then passes through the condensers ( III ), is further cooled and liquefied again. In a specific embodiment, the working fluid passes through the heat exchanger (7a), before the capacitors (III)goes through.

[0075] Via a circulation pump ( I V) the working medium enters the heat exchanger again (2) a.

[0076] While maintaining the pressure, 100% of the geothermal water is fed to at least one nanofiltration module (3) supplied.

[0077] The retentate from the nanofiltration module is fed through at least one reinjection well (4) into the geothermal reservoir (11) returned.

[0078] From the permeate of nanofiltration (a or b) the remaining divalent alkaline earth ions are separated in a precipitation unit (5a or 5b) failed.

[0079] The permeate of the nanofiltration, which is depleted of divalent ions, is circulated (6a or 6b) into a second heat exchanger (7a or 7b)which - depending on the heat demand - is located either between the turbine / generator unit (I, II ) and the capacitors (III) or between production well (1) and first heat exchanger (2) is located.

[0080] From the heat exchanger (7a or 7b) the permeate enters the membrane distillation module (8). From the membrane distillation module (8) The salt solution is subjected to a lithium separation process in a separation unit (9) and the residual solution is further used in a particular embodiment for the recovery of rubidium ions and cesium ions in a separation unit (10) The residual solution remaining after the separation of alkali metal ions is injected via at least one reinjection well (4) into the geothermal reservoir (11) returned.

[0081] The deionized water produced during membrane distillation can either be fed via a line to at least one reinjection well (4) or used for other purposes.

[0082] Figure 2 describes a flat channel cell system and is state of the art from https: / / publikationen.bibliothek.kit.edu / 1000029879, DOI: 10.5445 / IR / 1000029879, accessed on March 17, 2020.

[0083] From a storage container (12) With a capacity of approximately 250 liters, the water is continuously brought to operating pressure (PI: pressure gauge) by a centrifugal pump (M) and finally flows into the six membrane modules connected in parallel. A liquid level control (LC) and a temperature indicator (TI) are installed in the storage tank.

[0084] The concentrate from all modules is recombined and flows back into the tank. This pipeline contains two valves (FI) used to adjust the flow rate, as well as one through which a sample of the concentrate can be taken.

[0085] The permeate from each of the modules is used to determine the flow in a container whose weight is continuously measured by a balance (WI) (13) The permeate is collected in a cyclically changing tube to measure the flow of all modules. The remaining permeate is drained from the system via a hose. Six valves allow permeate samples to be taken from each individual module.

[0086] Furthermore, a bypass allows the pump to be started up gently and a valve in front of the pump allows sampling from the tank.

[0087] Figure 3illustrates the retention (in %) of different ions (Li +< , Mg 2+< , Ba 2+< and Cl -< ) and the electrical conductivity through two different membranes NF270 (DuPont) and NF245 (DuPont) at a pressure of 15 bar, an inlet concentration of lithium ions of 200 mg / l Li +< and a temperature of 20 °C (see Example 2, solution c).

[0088] Figure 4 illustrates the rejection (in %) of different ions (Li +< , Mg 2+< , Ba 2+< and Cl -< ) and the electrical conductivity through two different membranes NF270 (DuPont) and NF245 (DuPont) at a pressure of 15 bar, an inlet concentration of lithium ions of 100 mg / l, an inlet concentration of sodium ions of 2000 mg / l and a temperature of 40 °C (see Example 2, solution d).

[0089] Figure 5 illustrates the influence of Na +< concentration on Li +< rejection (membrane NF245, Example 3).

[0090] Figure 6illustrates the influence of Na +< concentration on Mg 2+< rejection (membrane NF245, example 3).

[0091] Figure 7 describes the functioning of direct contact membrane distillation and is state of the art from Bauer et al., Journal of Membrane Science 2019, 577, 145-152).

[0092] The membrane distillation experimental setup consists of two circuits separated by a hydrophobic, microporous membrane. In the condensate circuit, the condensate tank is heated to a temperature of 45 °C using a thermostat.

[0093] The water is pumped to the direct contact membrane distillation cell using a gear pump. In the feed circuit, the feed tank is heated to a temperature of 65 °C using a thermostat. The water is then transported to the direct contact membrane distillation cell using a gear pump with a suction shoe pump head.

[0094] The extraction of lithium, cesium and rubidium as by-products within binary geothermal power plants makes geothermal energy production significantly more economically profitable.

[0095] Lithium compounds are used, for example, in energy storage devices, in the glass and ceramics industry, in specialty lubricants, in air conditioning systems, in the metal industry and in the pharmaceutical industry.

[0096] Cesium compounds are used, for example, as scintillation materials in scintillation counters, as electrolytes in galvanic cells, for the production of density gradients in ultracentrifuge separation, as windows and prisms for IR and FIR spectroscopy, as dopants in vanadium catalysts and as bases in organic chemistry.

[0097] Rubidium compounds are used as catalysts, electrolytes for accumulators, for the production of special glasses, as painkillers and sedatives, as antidepressants and as strong bases in organic chemistry.

[0098] Cesium and rubidium compounds continue to be used in laser and vacuum technology.

[0099] Deionized water as a by-product of membrane distillation can be used for technical purposes in the geothermal plant itself or sold commercially. Examples Materials and methods Membrane experiments:

[0100] Experiments on nanofiltration of lithium ions were carried out using a flat channel cell system ( Figure 2). Water was continuously brought to operating pressure from a storage tank with a capacity of approximately 150 liters using a centrifugal pump and then fed into six membrane modules connected in parallel. The flow conditions in the membrane modules were similar to those in a commercially available spiral-wound module.

[0101] The results of the filtration with the flat channel cell system were used to select a membrane for the subsequent experiments with increased salt concentrations.

[0102] For experiments with elevated salt concentrations, stirred cells of the HP4750 model from STERLITECH™ were used. The stirred cells were equipped with a stirring unit, with one stirrer driven by an external magnetic drive and the other stirrer driven by an electric motor located below the cell. The cell capacity was 300 ml. The effective membrane area in the cell was 14.6 cm².

[0103] In the experiments, two different membranes from DuPont made of polyamide with a support layer made of polysulfone were used: NF270 with a salt rejection of > 97%, a maximum pressure load of 41 bar and a maximum temperature load of 45 °C and NF245 with a salt rejection of > 98.5%, a maximum pressure load of 54.8 bar at 45 °C and 27.5 bar at 70 °C and a temperature resistance up to a maximum of 70 °C.

[0104] The salt rejection was determined using a solution of 2000 ppm MgSO4 at 25 °C and 15% yield at 4.8 bar (https: / / www.dupont.com / content / dam / dupont / amer / us / en / watersolutions / public / documents / en / 45-D00719-en.pdf, accessed on February 21, 2020). Membrane distillation:

[0105] The experimental setup of the membrane distillation consists of two circuits separated by a hydrophobic, microporous membrane ( Figure 7In the condensate circuit, the condensate tank is heated to a temperature of 45 °C using a thermostat (Julabo, F25-HE). The water is pumped to the direct contact membrane distillation cell using a gear pump (Ismatec, MCP-Z Standard) with a cavity-style pump head (Ismatec, Z-130).

[0106] In the feed circuit, the feed tank is heated to a temperature of 65 °C using a thermostat (Julabo, F34-He). The water is pumped to the direct contact membrane distillation cell using a gear pump (Ismatec, MCP-Z Standard) with a suction shoe pump head (Ismatec, Z-201). The pump head is specially designed for higher temperatures up to 177 °C (Bauer et al., Journal of Membrane Science 2019, 577, 145-152). ICP-OES for cation analysis:

[0107] The chemical analyses to determine the permeate behavior of the nanofiltration membrane in the various experiments were performed using a Varian 715-ES ICP-OES with a radial inductively coupled plasma as the excitation source and a CCD simultaneous detector with an echelle grating, coupled with an autosampler for analyzing larger sample volumes. The measurement accuracy varied from 5 µg / l to 500 µg / l, depending on the element being analyzed.

[0108] Both the feed solutions and the permeate solutions were measured for all experiments. The feed solutions were measured as a control, since slight deviations from the planned concentration (initial weight, water volume, hygroscopy) can occur during solution preparation. All samples with Na +< < 2 g / l were diluted 1:10 and all ions were measured simultaneously. For samples with 2 g / l < Na +< < 35 g / l, two measurements were carried out. The first measurement of the ions Li +< , Ba 2+< , Mg 2+< was carried out on a sample diluted 1:10. The measurement of the Na +< concentration was carried out on a sample diluted 1:100. For Na +< = 35 g / l, the Na +< concentration was measured in a solution diluted 1:1000. The other ions were again measured in a solution diluted 1:10. All measuring solutions were mixed with 1% HNO3.

[0109] Ion chromatography to determine anion concentrations: The Cl ion concentrations were determined using a Metrohm 790 Personal IC ion chromatography system to verify charge neutrality and to perform mass balance calculations. For the measurements, the samples were diluted 1:50 and the Cl concentration was measured in Metrosep A Supp 5 columns at flow rates of 0.8 ml / min. pH value and electrical conductivity determinations:

[0110] Before, during and after the experiments, pH values ​​and electrical conductivities were measured. Example 1: Experiments to characterize Li retention

[0111] Membrane system: Flat channel cell system ( Figure 2), cross-flow method. Model solutions (quantities refer to 1 liter of water): a. 100 mg Li +< - dissolved as chloride. b. 150 mg Li +< - dissolved as chloride. c. 200 mg Li +< - dissolved as chloride. Temperature: 20 °C and 40 °C. Pressure: 7.5 bar and 15 bar.

[0112] All experiments were conducted at a flow velocity of 0.22 m / s. For each individual experiment, at least two modules of the flat channel cell system were equipped with the same membrane and tested (duplicate measurements).

[0113] The results of nanofiltration with the flat channel cell system were used to select a membrane for the experiments in Examples 2 and 3 with increased salt concentrations. Example 2: Experiments to characterize Li retention in the presence of other ions:

[0114] Membrane system: Flat channel cell system ( Figure 2), cross flow method Model solutions (the quantities refer to 1 liter of water): a. 100 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< - dissolved as chloride. b. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< - dissolved as chloride. c. 200 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< - dissolved as chloride. d. 100 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 2000 mg Na +< - dissolved as chloride. e. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 2000 mg Na +< - dissolved as chloride. f. 200 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 2000 mg Na +< - dissolved as chloride. Temperature: 20 °C and 40 °C. Pressure: 7.5 bar and 15 bar.

[0115] All experiments were conducted at a flow velocity of 0.22 m / s. For each individual experiment, at least two modules of the flat channel cell system were equipped with the same membrane and tested (duplicate measurements).

[0116] Figure 3illustrates the rejection (in %) of different ions (Li +< , Mg 2+< , Ba 2+< and Cl -< ) by the membranes NF270 (DuPont) and NF245 (DuPont) at a pressure of 15 bar, an inlet concentration of lithium ions of 200 mg / l and a temperature of 20 °C (see Example 2, solution c).

[0117] In Figure 3 The NF270 membrane shows a rejection of approximately 8% for all measured ions, largely independent of the ion's charge and size. In contrast, the NF245 membrane shows significant differences between the rejections of different ions: magnesium and barium, as divalent cations, are more strongly rejected (approx. 80%) than monovalent lithium (approx. 10%).

[0118] Furthermore, it was found that temperature had little influence on ion retention. An increase in temperature primarily affects the viscosity of the water and thus the permeability of the membrane.

[0119] Ion retention is also influenced by the membrane charge. The organic amides present in the surface impart a single negative charge to the membrane, which ensures that anions are repelled while simultaneously retaining them more effectively. Therefore, the NF245 membrane retains approximately 45% of chloride ions.

[0120] The influence of other ions on the retention of lithium is Figure 4 shown. For this purpose, NaCl was added to the solution.

[0121] Figure 4 illustrates the rejection (in %) of different ions (Li +< , Mg 2+< , Ba 2+< and Cl -< ) by two different membranes NF270 (DuPont) and NF245 (DuPont) at a pressure of 15 bar, an inlet concentration of lithium ions of 100 mg / l, an inlet concentration of sodium ions of 2000 mg / l, and a temperature of 40 °C (see example 2, solution d).

[0122] When NaCl is added (Na +< -influent concentration 2000 mg / l), in Experiment Example 2, solution d ( Figure 4 ), compared to Experiment Example 2, Solution c ( Figure 3), regardless of the membrane, the rejection of all ions with the exception of chloride, the rejection of which remains approximately the same when using the NF245 membrane within the measurement error. The NF270 membrane still shows a lower rejection of all ions in the solution, which does not allow the separation of monovalent and multivalent ions. As an anion, chloride is most strongly retained by the NF270 membrane with up to approx. 20% due to the repulsion between the membrane surface charge and the anions. The NF245 membrane, on the other hand, continues to show good rejection of the divalent ions of > 80%, but the rejection of the lithium ions also increases because sodium ions have a smaller hydration radius and therefore pass through the membrane more easily. The retention of chloride is also influenced by the maintenance of charge neutrality in the permeate and retentate. Example 3:

[0123] Based on the results from Examples 1 and 2, experiments with increased Na +< influent concentrations (up to 35000 mg / l) were carried out using the NF245 membrane. Membrane system: Dead-end process (stirred cells) Model solutions (the solutions refer to 1 liter of water): a. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 2000 mg Na +< - dissolved as chloride. b. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 5000 mg Na +< - dissolved as chloride. c. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 10000 mg Na +< - dissolved as chloride. d. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 20000 mg Na +< - dissolved as chloride. e. 150 mg Li +< , 10 mg Ba 2+< , 400 mg Mg 2+< , 35000 mg Na +< - dissolved as chloride Temperature: 25 °C Pressure: Pressures between 7 bar and 25 bar were applied with increasingly higher Na +< - and Cl -< concentrations. The applied pressures were calculated from the respective osmotic pressure, which in turn could be calculated from the respective ion concentrations of the retentions resulting from the data from Example 2.

[0124] The experiments realistically represent the concentrations of Mg 2+< and Ba 2+< – as proxies for divalent ions – as well as Na +< and Li +< – as proxies for monovalent ions – in geothermal waters. To this end, the Na concentrations were gradually increased until the model solution in Example 3, Solution e (approximately 1.5 molar NaCl solution), reached real concentrations of Na and Cl.

[0125] For the composition of the model solution, the data from Sanjuan et al. (Sanjuan et al., Chemical Geology 2016, 428, 27-47). To exclude the influence of anions on the selectivity of the membrane for divalent cations, only chloride ions were used.

[0126] All experiments were performed at least twice (duplicate measurement).

[0127] In the Figures 5 and 6The influence of increasing NaCl concentration on the retention of lithium and magnesium is shown in approximation to realistic concentrations in high-saline geothermal waters.

[0128] With increasing NaCl concentration, the retention of both lithium ( Figure 5 ) and magnesium ( Figure 6 ) generally decreases. At low ionic strengths and neutral pH values, the membrane is negatively charged. The increase in the ionic strength of the solution increasingly compensates for the negative membrane charge, so that the retention of anions due to repulsion effects is minimized. As a result, chloride is barely retained due to its small hydration radius, and the sieving effect becomes more important, allowing both lithium and sodium to pass through the membrane more easily.

[0129] Magnesium is retained due to the sieve effect and is therefore less affected by the increase in the ionic strength of the solution ( Figure 6 ).

[0130] In Example 3, it was shown that the initially already low Mg / Li molar ratio of 0.75 in Example 3, Solution e, could be reduced by a factor of 4.4 to a Mg / Li molar ratio of 0.17 by nanofiltration. Example 4:

[0131] The membrane distillation experiments were conducted using a membrane distillation system equipped with a direct contact membrane distillation module at a starting temperature of 65 °C. The starting solution was a solution whose concentrations were identical to the concentration of the permeate from Experiment Example 3, Solution e, since the results from this experiment most closely correspond to the expected real-world conditions.

[0132] The experiments achieved an enrichment of the lithium concentration by a factor of 4 after 7 days (Table 1). Table 1: Presentation of the analytical data of the membrane distillation experiments over time. Analyses Li +< (mg / l) Mg 2+< (mg / l) Ba 2+< (mg / l) Na +< (g / l) Starting solution 128 66 2 31,6 Day 4 197 96 3 47,1 Day 5 331 159 5 76,2 Day 7 522 254 8 126,5

[0133] A greater lithium enrichment is possible. However, care must be taken not to exceed the solubility of the salts contained in the solution. Exceeding the respective solubility products leads to precipitation, which hinders the membrane distillation process by fouling and reducing hydrophobicity (D. Winter, Membrane Distillation. A Thermodynamic, Technological and Economic Analysis. (2014), Dissertation D386, Technical University of Kaiserslautern, Shaker Verlag, Aachen, pages 325 ff.).

[0134] The electrical conductivity of the condensate was very low, ranging between 1 and 2 µS / cm during the enrichment process. This also makes further technical use of the condensate (deionized water) in various applications (including boiler water) conceivable.

Claims

1. A process for extracting lithium ions from geothermal water within a binary geothermal power plant, comprising the steps A) Pumping geothermal water with a temperature of 120 °C - 180 °C from a geothermal reservoir (11) via a production well (1) into a heat exchanger (2) under a pressure of 15 - 35 bar; B) Passing the geothermal water from step A) through the heat exchanger (2); C) Feeding 100% of the geothermal water from step B) into at least one nanofiltration module (3); D) Nanofiltration of the geothermal water supplied to the at least one nanofiltration module (3) in step C) for the separation of alkali metal ions and other monovalent ions from multivalent ions under a pressure of 15 - 35 bar and a temperature of 50 °C - 80 °C and direct reinjection of the residual solution enriched in multivalent ions into the geothermal reservoir (11) via at least one reinjection well (4); E) Transferring the permeate of the nanofiltration from step D) to a precipitation unit (5a or 5b) for the precipitation of the remaining divalent alkaline earth ions; F) Precipitation of the remaining divalent alkaline earth ions in the precipitation unit (5a or 5b) from the permeate of the nanofiltration of step D); G) Transporting the permeate depleted of divalent ions from step F) by a circulation pump (6a or 6b) to a second heat exchanger (7a or 7b); H) Heating the permeate from step G) in the second heat exchanger (7a or 7b) to 65 - 85 °C; I) Membrane distillation (8) of the permeate heated in step H) under normal pressure and a temperature of 65 °C - 85 °C; J) Separation of lithium ions concentrated in step I) in a separation unit (9) by precipitation, ion exchange, absorption processes, liquid-liquid extraction or electrodialysis.

2. Process according to claim 1, wherein scaling inhibitors and corrosion inhibitors are added to the geothermal water in step A) before entering the heat exchanger (2).

3. The process according to claim 2, wherein the added scaling inhibitors and corrosion inhibitors are selected from organophosphates or polycarboxylic acids.

4. Process according to any one of the preceding claims, wherein the nanofiltration is carried out with a polymer membrane or a ceramic membrane.

5. The process according to claim 4, wherein the nanofiltration is carried out in one or more wound modules, hollow fiber or tubular modules.

6. Process according to claims 4 or 5, wherein during the nanofiltration in step D) at least 80% of the alkali metal ions and the other monovalent ions are separated from multivalent ions.

7. Process according to any one of the preceding claims, wherein the membrane distillation is carried out as direct contact membrane distillation, as air-gap membrane distillation or as vacuum membrane distillation.

8. Process according to any one of the preceding claims, wherein lithium ions are separated in step J) by chemical precipitation.

9. The process according to claim 8, wherein lithium ions are precipitated as lithium carbonate or lithium phosphate and separated from the residual solution by filtration or centrifugation.

10. Process according to any one of the preceding claims, wherein the residual solution after the separation of lithium ions in step J) is used for the recovery of rubidium and caesium ions in a separation unit (10).

11. Process according to any one of the preceding claims, wherein the residual solution after the separation of lithium ions in step J) and optionally after the recovery of caesium and rubidium ions in the separation unit (10) is reinjected into the geothermal reservoir (11) via the at least one reinjection well (4).

12. Device for lithium extraction from a binary geothermal power plant, comprising a) At least one production well (1) from which geothermal water from a geothermal reservoir (11) at a temperature of 120 °C - 180 °C under a pressure of 15 - 35 bar can be conducted into a heat exchanger (2); b) One or more nanofiltration modules (3) connected downstream of the heat exchanger (2), in which, under a pressure of 15 - 35 bar and a temperature of 50 °C - 80 °C, the monovalent ions largely permeate and the divalent ions of the geothermal water can be retained; c) At least one reinjection well (4) through which the retentate (concentrate) enriched in multivalent ions can be reinjected into the geothermal reservoir (11); d) A precipitation unit (5a or 5b) in which the remaining divalent alkaline earth ions of the nanofiltration permeate can be precipitated; e) A circulation pump (6a or 6b) which pumps the permeate of the nanofiltration depleted of divalent ions to a second heat exchanger (7a or 7b); f) A second heat exchanger (7a or 7b), the second heat exchanger being located either between the turbine / generator unit (I, II) and the condensers (III) or between the production well (1) and the heat exchanger (2); g) A membrane distillation module (8) in which the nanofiltration permeate depleted in multivalent ions is concentrated under normal pressure and a temperature of 65 °C - 85 °C; h) A separation unit (9) in which lithium ions can be separated by precipitation, ion exchange, absorption processes, liquid-liquid extraction or electrodialysis.

13. Device according to claim 12, wherein the separation unit (9) for the separation of lithium ions is followed by a separation unit (10) for the extraction of rubidium and caesium ions.