Method and system for organic solvent recovery from a lithium-containing solution

Abstract

Provided herein are a method and a system for the recovery of organic solvents from a lithium-containing solution. In particular, the method includes a membrane-based process to treat electrolyte wastewater generated in lithium-ion battery recycling.

Description

[0001]DI83910-WO-PCT PATENT TITLE OF THE INVENTION METHOD AND SYSTEM FOR ORGANIC SOLVENT RECOVERY FROM A LITHIUM- CONTAINING SOLUTION CROSS-REFERENCE TO RELATED APPLICATION The present application claims priority under 35 U.S.C. § 365(c) to U.S. Provisional Appln. No.63 / 700,035, filed on September 27, 2024, which is incorporated herein by reference in its entirety. FIELD OF THE INVENTION The present invention relates to a method and system for the recovery of organic solvents from a lithium-containing solution. In particular, the method includes a membrane- based process to treat electrolyte wastewater generated in lithium-ion battery recycling. BACKGROUND OF THE INVENTION Several patents, patent applications and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents, patent applications, and publications is incorporated by reference herein. Lithium-ion batteries (LiB) are widely used in electric vehicles, energy storage systems, computers, and other electronic devices. At present, lithium (Li) is primarily extracted from natural sources such as from salt lakes, salars, geothermal reservoirs, clay mining, or from hard rock deposits. An increasing global demand necessitates that Li and other valuable chemicals be recycled from spent or used batteries. A battery comprises four key components: a cathode, an anode, an electrolyte, and a separator. In lithium-ion batteries, electrolytes generally comprise Li salts of Lewis acid ions (such as BF4–, PF6–, ClO4–, AsF6–) dissolved in an organic solvent or in a mixture of two or more organic solvents. Additives that are often added to the electrolyte solution include film-forming additives, conductive additives, flame retardant additives, overcharge protection additives, additives to improve low-temperature performance, and other functional or multifunctional additives. The purpose of electrolytes is to conduct ions between the electrodes, and a high ionic conductivity is desired. Therefore, organic solvent molecules commonly used as electrolyte solvents are a mixture of a low-viscosity acyclic carbonate ester and a high dielectric constant cyclic carbonate ester. Handling of these organic solvent molecules during battery recycling is challenging as they are volatile and toxic. In addition, fluorine-containing electrolyte salts, e.g. lithium hexafluorophosphate (LiPF6), hydrolyze in the presence of water and generate hazardous HF gas. This hydrolysis imposes additional restrictions on handling and disposal of electrolyte wastewater generated during the recycling process. Various recovery methods have been described to recover lithium-containing salts (electrolyte salts) and organic solvent molecules (electrolyte solvents) from LiB electrolyte wastewater. U.S. Pat. No.5,882,811 describes a method that includes opening a LiB using a water jet under an inert gas. The resultant aqueous solution is filtered to separate it into a retained (undissolved solids) part and a filtrate (solution with dissolved solids) part. An organic solvent (e.g. methyl alcohol, CH3OH) is added to the filtrate part to recover Li in the form of lithium hydroxide (LiOH) precipitations. The remaining filtrate is distilled under reduced pressure to separate other dissolved solids from liquids and, finally, organic solvent molecules are recovered through fractional distillation. JP 6,311,877 (’877) describes a recovery method for use when fluorine-containing electrolyte salts (e.g. LiPF6, LiBF4) are present. A step described in ’877 uses an organic solvent, preferably one of the LiB electrolyte solvents, as an extractant. Water is added to the extracted solution. The liquid mixture is heated under reduced pressure and organic solvent molecules are collected as a condensate. Finally, the remaining fluorine is removed from the condensate liquid by adding an alkali which reacts to form precipitates of alkali-metal fluorides such as CaF2or NaF. Recovery methods that include membrane-based separation steps have been used to recover lithium-containing salts (electrolyte salts) and organic solvent molecules (electrolyte solvents) from LiB electrolyte wastewater. CN 117,642,909 describes the use of a pervaporation membrane to dehydrate the electrolyte solvents obtained from a reduced pressure distillation operation. CN 107,768,760 describes a step to send the electrolyte wastewater to a nanofiltration unit, where a nanofiltration membrane intercepts multivalent cations (Al, Fe) dissolved in wastewater and permeates a part of wastewater with dissolved Li ions and organic solvent molecules. Further, the permeate solution is passed through an ion exchange resin column to capture lithium ions. The remaining permeate solution is distilled under reduced pressure to recover the organic solvent molecules as a condensate. Lithium ions are recovered in chloride form by washing the ion exchange resin column with hydrochloric acid. CN 113,611,939 describes the use of the ultrasonication of battery components in organic solvents for extraction. Metallic powders, preferably calcium, are added to remove fluorine in the form of CaF2precipitations. After the precipitated solids are removed, the unreacted calcium dissolved in the extracted solution is separated from the solution by using a nanofiltration membrane. Nevertheless, conventional treatment methods for LiB electrolyte wastewater have challenges. An energy-intensive distillation process can be used for separation and recovery of the organic solvent molecules. In addition, the presence of even a small amount of fluoride anions in the distillation feed solution generates hazardous HF gas during distillation, creating EH&S issues as well as corrosion problems. Therefore, less energy-intensive methods for recovery of organic solvent molecules are needed. An improved method would lower the need for fresh organic solvent molecules (electrolyte solvents), which are expensive, for new LiB manufacturing. It would also reduce the volume of hazardous wastewater generation and other disposal challenges. SUMMARY OF THE INVENTION Accordingly, provided herein is a method of recovering organic solvent comprising providing a process stream comprising at least 0.05 wt.% of organic solvent molecules selected from a group consisting of carbonate esters and carboxylate esters, and at least 0.03 wt.% of dissolved lithium ions. The method of recovery also provides a first filtration assembly comprising at least one nanofiltration stage, and a second filtration assembly comprising at least one reverse osmosis stage. The method of recovery further comprises treating the process stream with the first filtration assembly to generate a nanofiltration permeate stream and a nanofiltration reject stream, wherein the molar concentration of solvent molecules in the nanofiltration permeate stream is more than 50 mol% of the molar concentration of solvent molecules in the process stream, and treating at least a portion of the nanofiltration permeate stream with the second filtration assembly to generate a reverse osmosis permeate stream and a reverse osmosis reject stream, wherein the molar concentration of the solvent molecules in the reverse osmosis permeate stream is less than 50 mol% of the molar concentration of solvent molecules in the process stream. The advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. For a better understanding of the invention, its advantages, and the objects obtained by its use, however, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described one or more preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a schematic diagram that illustrates a filtration system comprising first and second connected filtration assemblies, wherein the permeate stream from the first filtration assembly is further partitioned by the second filtration assembly. Fig.2 is a schematic drawing that illustrates one configuration of a filtration system that is useful for recovering organic solvent molecules from a process stream comprising organic solvent molecules and dissolved lithium ions. Fig.3 is a schematic diagram that illustrates a configuration for the second filtration assembly that is suitable for batch and semi-batch operations, including intermittent production of a reverse osmosis reject stream. Fig.4a is a schematic diagram that illustrates a preferred configuration for the first filtration assembly. Figs.4b through 4f are schematic diagrams that illustrate preferred configurations for the first filtration assembly that are suitable to permeate the majority of organic solvent molecules through a nanofiltration membrane at least twice, or through at least two nanofiltration membranes. Fig.5 is a schematic drawing of a configuration of the first filtration assembly that includes an additional membrane-based enrichment operation suitable to increase the concentration of lithium ions from the nanofiltration reject stream. Fig.6 is a schematic diagram of a configuration of the second filtration assembly that includes a heat-exchanger after the second filtration assembly to treat the reverse osmosis reject stream. DETAILED DESCRIPTION OF THE INVENTION Provided herein is a method of recovering organic solvent from an aqueous solution or “process stream”. The method of recovery comprises providing a process stream comprising organic solvent molecules and dissolved lithium ions. The process stream may be obtained from Lithium-Ion battery (LiB) recycling, and the process stream is preferentially obtained from an aqueous extraction of LiB electrolytes from the battery. A possible source of organic solvent molecules in the process stream may be electrolyte solvents used in the LiB. An example of the LiB electrolyte solvent is dimethyl carbonate, DMC. A possible source of lithium ions in the process stream may be a Li-containing electrolyte salt. An example of the Li-containing electrolyte salt is lithium hexafluorophosphate, LiPF6. An aqueous extraction of LiB electrolytes may be used directly to form the process stream, or the process stream may first undergo other unit operations. To produce the process stream, non- limiting examples of such operations include chemical additions, dilutions, filtering, and distillation. Other possible sources of lithium ions in the process stream may be the LiB cathode and anode components, especially when all the key components of a battery are extracted together. An example of a Li-containing cathode material is lithium nickel manganese cobalt oxide, for example of the family LiNixMnyCozO2, wherein x, y, and z are positive real numbers. When ^, ^, and ^ are the oxidation states of the nickel, manganese and cobalt, respectively, ^x+^y+^z=3. An example of a Li-containing anode material is lithium titanate, for example Li4Ti5O12. The process stream may be generated directly by extracting all the key components of a LiB together or by treating the extracted solution using different unit operations to produce the process stream. The method of recovery provided herein comprises providing a process stream comprising organic solvent molecules selected from a group consisting of carbonate esters and carboxylate esters. The term ‘carbonate esters’ as used herein refers to organic esters of carbonic acid (H2CO3). A carbonate ester typically consists of a polar carbonyl (C=O) group flanked by two alkoxy (-O-) groups. Carbonate esters are particularly preferred as the LiB electrolyte solvents since they are (a) non-reactive to the reducing cathode and the oxidizing anode components, (b) their polar C=O group helps to dissolve electrolyte salts, (c) their low viscosity offers facile ion transport, and, (d) they remain in liquid form for a wide temperature range (i.e. a low melting temperature and a high boiling temperature). In the method of recovery described herein, carbonate esters are preferably selected from a group consisting of: (a) an acyclic carbonate ester represented by the following formula (1): wherein R1and R2are each consisting of an alkyl group and an haloalkyl group, and (b) a cyclic carbonate ester represented by the following formula (2): wherein R3is selected from a halogen atom, an alkyl group, and a haloalkyl group. As used herein, the term "alkyl group" refers to a linear, branched, or cyclic saturated hydrocarbon-based chain preferably comprising from 1 to 10 carbon atoms, more preferably from 1 to 4 carbon atoms; the term "haloalkyl group" refers to an alkyl group comprising at least one halogen atom; and the term "halogen" refers to an atom selected from F, Cl, Br, and I atoms. Non-limiting examples of acyclic carbonate esters are dimethyl carbonate or DMC (where both R1and R2are -CH3), diethyl carbonate or DEC (where both R1and R2are -CH2CH3), and ethylmethyl carbonate or EMC (where R1is -CH3 and R2is -CH2CH3or vice versa). Acyclic carbonate esters are suitable for use as electrolyte solvents due to their low viscosity and a wide gap between their melting and boiling temperatures. Non-limiting examples of cyclic carbonate esters are ethylene carbonate or EC (where R3is H) and propylene carbonate or PC (where R3is -CH3). Cyclic carbonate esters are widely used as electrolyte solvents due to their high dielectric constant. Substitution of a H atom in R3by a halogen atom, specifically by F, lowers the melting temperature of cyclic carbonate esters and also improves their protective film formation ability on carbonaceous anodes. Similar benefits are obtained when one or more of the alkyl groups in R3are substituted by the haloalkyl group, specifically a fluorine-containing haloalkyl group. Non- limiting examples of suitable haloalkyl groups appear in Table 12 of Xu, Kang “Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries,” Chem. Rev.2004, 104, 4303-4417, for example. The term ‘carboxylate esters’ as used herein refers to organic esters in which at least one hydroxyl group (-OH) is replaced by an alkoxy group or a haloalkoxy group (-O-R, wherein R is as described below with respect to R4and R5). The carboxylate esters are preferably selected from a group consisting of: (a) an acyclic carboxylate ester represented by the following formula (3): wherein R4and R5are each group consisting of an alkyl group and a haloalkyl group, and (b) a cyclic carboxylate ester selected from a group consisting of a γ-butyrolactone and a γ-valerolactone. Non-limiting examples of acyclic carboxylate esters are ethyl acetate (where both R4and R5are -CH3), methyl butyrate (where R4is -CH3and R5is -CH2CH2CH3), ethyl butyrate (where R4is -CH2CH3and R5is -CH2CH2CH3), and methyl propionate (where R4is -CH3and R5is -CH2CH3). The low-temperature performance of LiB electrolytes improves when acyclic carboxylate esters are used as organic solvents. Further improvement in low- temperature performance and protective film formation ability on carbonaceous anodes are gained when haloalkyl group-containing acyclic carboxylate esters are used as organic solvents. Among the haloalkyl groups, selection of R5as (CF2)nY, where, Y is a H or F atom and n is a positive integer, are specifically beneficial. In special cases, cyclic carboxylate esters, such as γ-butyrolactone or γ-valerolactone, are used as electrolyte solvents. For instance, a combination of γ-butyrolactone as electrolyte solvent and LiBF4as electrolyte salt offers synergistic performance improvements. The process stream may contain only one type of organic solvent molecule. For example, the process stream may contain only propylene carbonate, PC, as the organic solvent molecule. Alternatively, the solvent molecules in the process stream may be a mixture having two or more types of organic solvent molecules present. For example, the process stream may contain both DMC and EC as organic solvent molecules. In another case, the process stream may contain DMC, EMC, and EC as organic solvent molecules. Accordingly, as used herein, the term ‘organic solvent molecules’ refers to either a pure (single type) or a mixture (two or more types) of organic solvent molecules. The methods described herein are particularly advantageous when the content of organic solvent molecules in the process stream is at least 0.05 wt.%. For a given process stream, the content of organic solvent molecules expressed in wt.% is the mass of the organic solvent molecules (in ‘g’ unit) present in 100 g of the process stream. The method of recovery described herein is particularly advantageous when 50 mg or more of organic solvent molecules are present per 100 g of process stream. Preferably, the content of organic solvent molecules in the process stream of interest is above 1 wt.%, and more preferably, is above 2 wt.%. Preferably, the content of organic solvent molecules is lower than their solubility limit in the process stream, such that only a single phase is present. For a mixture having two or more organic solvents present, the organic solvent molecules content (in wt.%, as defined above) in the process stream may be calculated by adding the individual mass of organic solvent molecules present in 100 g of the process stream. For example, if a 300 g of process stream contains 150 mg of DMC and 60 mg of EC, then the organic solvent molecules content of said process stream is 0.07 wt.%. Stated alternatively, these weight percentages are based on the total weight of the process stream, and the sum of the weight percentages of the organic solvent and the other components of the process stream, such as for example water and lithium salts, is 100 wt.%. The process stream, and any stream derived therefrom, can also be characterized in terms of their molar concentration of organic solvent molecules. For a given process stream, the molar concentration (in mol / L unit) of organic solvent molecules may be calculated as the number of moles of organic solvent molecules present per L of said process stream. The number of moles of organic solvent molecules may be calculated as the mass of the organic solvent divided by the its molecular weight. For example, the molar concentration of solvent molecules that corresponds to 150 mL (or 0.15L) of a process stream carrying 1.5g of dimethyl carbonate (DMC, molar mass 90.08 g / mol), is 0.11 mol / L. For a mixture of two or more organic solvents present, the total molar concentration (in mol / L) of organic solvent molecules in the process stream may be calculated by adding the individual organic solvent molecules’ molar concentrations (in mol / L). The process stream, and any stream derived therefrom, can also be characterized in terms of both the wt.% of dissolved lithium ions (Li+) and a corresponding molar concentration of lithium ions. The process stream preferably comprises at least 0.03 wt.% of dissolved lithium ions (Li+). Therefore, a 100 g of process stream contains at least 30 mg of lithium ions. The lithium-ion content in the process stream is preferably between 0.03 and 3.0 wt.%, more preferably between 0.1 and 1.0 wt.%. Again, these weight percentages are based on the total weight of the process stream, and the sum of the weight percentages of the components of the process stream is 100 wt%. The molar concentration (in mol / L unit) of lithium ions may be calculated as the measured mass of lithium ion present in per L of a stream divided by the lithium-ion molar mass (6.941 amu). The process stream preferably comprises at least 70 wt%, more preferably at least 80 wt%, even more preferably at least 90 wt% water, based on the total weight of the process stream. The process stream may also contain non-aqueous protic solvents. Such protic solvents may be added to battery components with water during extraction or during any unit operation preceding to the method of recovery disclosed here. Non-limiting examples of non- aqueous protic solvents are methanol, ethanol, and n-butanol. In addition to Li+, the process stream may include monovalent metal ions of Na, K, or both Na and K. The process stream may include one or more multivalent metal ions of Ca, Ni, Cu, Mn, Co, Fe, Al, Ti, and Sn. The process stream may include one or more dissolved non-metallic elements such as, for example, B, C, and Si, or salts or soluble compounds of B, C, or Si. The process stream may further include one or more anions. Some anions that may be found in the process stream are F-, CO32-, HCO3-, OH-, SO42-, HSO4-, HPO42-and PO43-. The types of anions present in the process stream depend on the extracted LiB compositions, specifically, the electrolyte salt compositions; preceding unit operations, if any; and pH-adjusting species, if any. For example, it is particularly advantageous to remove most of the F- ions from the process stream before treating it according to the methods described herein. The F- ions may be removed by adding an alkaline substance, e.g. caustic (NaOH), slaked lime (Ca(OH)2), CaCO3, or quick lime (CaO), to generate alkali-metal fluoride (e.g. CaF2, NaF) precipitations. It is preferred that the added alkaline substance contains multivalent cations. It is desirable to remove the precipitated solids (alkali-metal fluorides and others) before the process stream undergoes the method of recovery. The precipitated and / or suspended solids may be removed by any suitable method, such as for example by a settling operation, by centrifugation, or by using filtration equipment (e.g. media filter, sand filter, ultrafiltration membrane, or microfiltration membrane), or by a combination of two or more of these methods. In particular, an advantaged means for removal of solids is ultrafiltration. In the method provided herein for recovering an organic solvent, battery extraction to produce a battery extract and optional subsequent operations carried out upon the battery extract, such as those described above, may provide a process stream to be treated. The recovery method further comprises providing a first filtration assembly comprising at least one nanofiltration (NF) stage. The nanofiltration stage comprises a nanofiltration membrane. As used herein, the term “nanofiltration membrane” refers to a semi-permeable membrane that passes 50% or more of DMC molecules when operated at a flux of 25 LMH (liter / m2 / hr) and at a temperature of 25oC. A preferred nanofiltration membrane also passes 50 mol% or more of monovalent anion-based salts (e.g. LiCl) and rejects 50 mol% or more of divalent anion-based salts (e.g. Li2SO4), based on the number of moles of the divalent anion(s) in the nanofiltration feed, which is preferably the process stream. The recovery method still further comprises providing a second filtration assembly comprising at least one reverse osmosis stage. The reverse osmosis stage comprises a reverse osmosis membrane. As used herein, the term “reverse osmosis membrane” refers to a semi- permeable membrane that passes 50 mol% or less of DMC molecules, based on the total number of moles of DMC molecules in the reverse osmosis feed, when operated at a flux of 25 LMH and at a temperature of 25oC. A preferred reverse osmosis membrane also rejects 50 mol% or more of monovalent and divalent anion-based salts, based on the number of moles of the divalent anion(s) in the reverse osmosis feed, which is preferably the nanofiltration permeate stream. It is preferred that both the nanofiltration membrane and the reverse osmosis membrane are thin-film composite membranes. The composite membrane typically comprises a nonwoven backing layer that provides mechanical strength, a porous middle layer of 25 µm to 125 µm in thickness that supports the discriminating layer and provides paths for permeation, and a discriminating layer of 0.01 µm to 1 µm in thickness that enables separations. The backing layer is commonly a scrim or web made of polyolefins or PET. The porous support layer has small pore sizes (from about 0.001 to 0.5 µm). Non-limiting examples of porous supports include those made of polysulfone, polyether sulfone, polyimide, polyamide, polyetherimide, polyacrylonitrile, poly(methyl methacrylate), polyethylene, polypropylene, and various halogenated polymers such as polyvinylidene fluoride. The discriminating layer preferably comprises a polymeric layer that is responsible for the desired separation. The discriminating layer polymer may be selected from a group consisting of a fully aromatic polyamide, a semi-aromatic polyamide, a sulfonated polysulfone, a sulfonated polyethersulfone, and a polysulfonamide. In the nanofiltration membrane, the discriminating layer polymer may preferably be a semi-aromatic polyamide. The discriminating layer polymer may more preferably be a semi- aromatic polyamide made by interfacial polymerization of monomers that include piperazine and trimesic acid chloride. Trimesic acid is also known as benzene-1,3,5-tricarboxylic acid. In the reverse osmosis membrane, the discriminating layer polymer may preferably be a fully aromatic polyamide. The discriminating layer polymer may more preferably be a fully aromatic polyamide made by interfacial polymerization of monomers that include m-phenylenediamine and trimesic acid chloride. The nanofiltration membrane may be of different configurations. For example, it may be housed within a nanofiltration membrane module. Similarly, the reverse osmosis membrane may be of different configurations. For example, it may be housed within a reverse osmosis membrane module. The membrane may be in a flat-sheet, hollow-fiber, or tubular configuration. A membrane module may comprise one or more test cells containing a membrane in a flat sheet configuration with a few cm2of total active area. Preferably, the membrane module may be a spiral-wound module containing the membrane(s) with more than 30 m2of total active area. Suitable nanofiltration modules and reverse-osmosis modules are commercially available from DuPont de Nemours, Inc., of Wilmington, DE, under the FilmTec™ trademark. A plurality of membrane modules is preferably axially aligned in serial arrangement within the chamber of a cylindrical pressure vessel (forming a nanofiltration stage or a reverse osmosis stage depending on the type of membrane used) to increase the available active membrane area (preferably to >100 m2). The first and second filtration assemblies may further comprise multiple pressure vessels arranged in parallel or in series. The first filtration assembly is suitable to treat the process stream and to generate a nanofiltration permeate stream and a nanofiltration reject stream. In the methods described herein, the process stream is treated with the first filtration assembly to generate the nanofiltration permeate stream and the nanofiltration reject stream. Fluid from the process stream that is sent to the nanofiltration membrane is pressurized. The process stream or fluid from the process stream that is sent to the nanofiltration membrane may be pressurized using a pump located either before or within the first filtration assembly. Under pressure, a part of the process stream permeates through the nanofiltration membrane to form the nanofiltration permeate stream. The remaining part of the process stream, which is intercepted by the nanofiltration membrane, forms the nanofiltration reject stream. In the methods described herein, the molar concentration of organic solvent molecules in the nanofiltration permeate stream is more than at least 50% of the molar concentration of the organic solvent molecules in the process stream. For instance, if the molar concentration of organic solvent molecules in the process stream is 0.11 mol / L, then the molar concentration of organic solvent molecules in the nanofiltration permeate stream must be more than 0.055 mol / L. The molar concentration of organic solvent molecules in the nanofiltration permeate stream is preferably more than 70%, more preferably at least 85%, and even more preferably at least 90% of the molar concentration of organic solvent molecules in the process stream. In preferred embodiments, the molar concentration of lithium ions in the nanofiltration permeate stream is less than 75% of the molar concentration of lithium ions in the process stream. The molar concentration of lithium ions in the nanofiltration permeate stream is preferably less than 70%, more preferably less than 60%, even more preferably less than 50% of the molar concentration of lithium ions in the process stream. The first filtration assembly comprises at least one nanofiltration stage. The first filtration assembly is used to treat the process stream. The first filtration assembly partitions the process stream into the nanofiltration permeate stream and the nanofiltration reject stream. In preferred embodiments, the nanofiltration permeate stream is an aqueous solution that contains the majority of organic solvent molecules that were present in the process stream. In more preferred embodiments, the nanofiltration permeate stream is an aqueous solution that contains the majority of organic solvent molecules that were present in the process stream and the nanofiltration reject stream is an aqueous solution that contains the majority of lithium ions that were present in the process stream. The second filtration assembly is suitable to treat a portion of the nanofiltration permeate stream and to generate a reverse osmosis permeate stream and a reverse osmosis reject stream. In particular, a portion of the nanofiltration permeate stream, obtained from the first filtration assembly, is passed to a second filtration assembly to generate the reverse osmosis permeate stream and the reverse osmosis reject stream. Within the second filtration assembly, the nanofiltration permeate stream is pressurized, preferably using a pump, before it is sent to the at least one reverse osmosis stage. Further, the portion of the nanofiltration permeate stream, that comes into contact with the reverse osmosis membrane, is partitioned into the reverse osmosis permeate stream and the reverse osmosis reject stream, both of which exit the second filtration assembly. A part of the nanofiltration permeate stream that permeates through the reverse osmosis membrane forms the reverse osmosis permeate stream. The remaining part of the nanofiltration permeate stream that is intercepted by the reverse osmosis membrane forms the reverse osmosis reject stream. A reverse osmosis membrane typically has high rejection of all but water and small neutral molecules. In the methods described herein, the molar concentration of organic solvent molecules in the reverse osmosis permeate stream is less than 50% of the molar concentration of the organic solvent molecules in the process stream. For instance, for a process stream entering the first filtration assembly with a molar concentration of organic solvent molecules of 0.11 mol / L, the molar concentration of organic solvent molecules in the reverse osmosis permeate stream exiting the second filtration assembly is less than 0.055 mol / L. The molar concentration of organic solvent molecules in the reverse osmosis permeate stream is also lower than the molar concentration of organic solvent molecules in the nanofiltration permeate stream. In preferred embodiments, the majority of the organic solvent molecules are rejected by the reverse osmosis membrane. The molar concentration of organic solvent molecules in the reverse osmosis permeate stream is preferably less than 30%, more preferably less than 15%, and even more preferably less than 10% of the molar concentration of organic solvent molecules in the process stream. The second filtration assembly comprises at least one reverse osmosis stage. The second filtration assembly is used to treat the nanofiltration permeate stream. The second filtration assembly partitions the nanofiltration permeate stream into the reverse osmosis permeate stream and the reverse osmosis reject stream. In preferred embodiments, the majority of organic solvent molecules in the process stream are recovered in the reverse osmosis reject stream. In more preferred embodiments, the majority of the organic solvent molecules in the process stream are recovered in the reverse osmosis reject stream and majority of lithium ions in the process stream are recovered in the nanofiltration reject stream. In still more preferred embodiments, the majority of the organic solvent molecules in the process stream are recovered in the reverse osmosis reject stream, the majority of lithium ions in the process stream are recovered in the nanofiltration reject stream, and the reverse osmosis permeate stream contains only a trace amount of lithium ions and organic solvent molecules. Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to Fig.1, an organic solvent molecules recovery system 10 is illustrated. This recovery system 10 is suitable to treat a process stream 14 by partitioning it using both a first filtration assembly 20 and a second filtration assembly 50. While not shown in this figure, the first filtration assembly 20 contains at least one nanofiltration stage and the second filtration assembly 50 contains at least one reverse osmosis stage. The first filtration assembly 20 partitions the process stream 14 into a nanofiltration permeate stream 30 and a nanofiltration reject stream 40. The second filtration assembly 50 partitions the nanofiltration permeate stream 30 into a reverse osmosis permeate stream 60 and a reverse osmosis reject stream 70. The recovery system 10 can be used to increase the concentration of organic solvent molecules in the reverse osmosis reject stream 70 compared to the process stream 14. It can also be used to increase the concentration of lithium ions in the nanofiltration reject stream 40 compared to the process stream 14. Fig.2 is schematic drawing that illustrates one simple embodiment that can be useful for recovering organic solvent molecules from a process stream comprising organic solvent molecules and dissolved lithium ions. The first filtration assembly 20 contains a nanofiltration stage 24, symbolically shown by a combination of a rectangle and a diagonal line. The diagonal line can be understood to symbolically represent a nanofiltration membrane 26, so that a nanofiltration transported stream 28 is recognizable as having permeated through the nanofiltration membrane 26 and may be easily differentiated from a nanofiltration intercepted stream 38 that has not permeated through or intercepted by the nanofiltration membrane 26. However, the symbol (rectangle and diagonal line combination) more broadly corresponds to a generalized filtration stage that may include multiple vessels and membrane modules that are arranged in series, in parallel, or both in series and in parallel. Similarly, the second filtration assembly 50 contains at least one reverse osmosis stage 54, symbolically also shown by that combination of a rectangle with a diagonal line. Again, the symbol is useful in distinguishing a reverse osmosis transported stream 58 (that has permeated through a reverse osmosis membrane 56) from a reverse osmosis intercepted stream 68 (that has not permeated through or been intercepted by the membrane 56). However, the symbol for the reverse osmosis filtration stage 54 may stand-in for multiple vessels and membrane modules that are arranged in series, in parallel, or both in series and in parallel. The process stream 14 shown in Fig.2 becomes a nanofiltration feed stream 22, and this is then partitioned into the nanofiltration permeate stream 30 and the nanofiltration reject stream 40. Similarly, the nanofiltration permeate stream 30 becomes a reverse osmosis feed stream 52, and this is then partitioned into the reverse osmosis permeate stream 60 and the reverse osmosis reject stream 70. The nanofiltration feed stream 22 and reverse osmosis feed stream 52 must be in a pressurized state to enable partitioning by the respective membranes. In Fig.2, a first pump 16 is located within the first filtration assembly to provide the pressurized nanofiltration feed stream 22 to the nanofiltration membrane 26. Similarly, a second pump 46 is shown located within the second filtration assembly to provide the pressurized reverse osmosis feed stream 52 to the reverse osmosis membrane 56. Without changing their function, first and second pumps (16, 46) may also have been located external to the dotted lines designating the respective first and second filtration assemblies (20, 50). Alternatively, the process stream 14 may already be sufficiently pressurized, so that the first pump 16 prior to nanofiltration is not required. As depicted in Fig.1, the recovery system 10 may be operated in a continuous mode, where a continuous supply of the process stream 14 is provided directly or from preceding operations. As an alternative, the recovery system 10 in Fig.2 includes a tank 12 which would enable the recovery system 10 to be operated in a batch-wise mode, where a discrete volume of the process stream is treated before re-batch. The discrete volume may be the process stream contained inside the tank 12. Within the first filtration assembly 20 of Fig.2, the first pump 16 is employed to provide sufficient pressure to cause a portion of the nanofiltration feed stream 22 to permeate through the nanofiltration membrane 26 to produce the nanofiltration transported stream 28. In absence of any permeate recycle loop, the nanofiltration transported stream 28 becomes the nanofiltration permeate stream 30. The nanofiltration intercepted stream 38 is also produced that contains the remaining section (that is not permeated through or intercepted by the nanofiltration membrane 26) of the nanofiltration feed stream 22. In absence of any brine recycle loop, the nanofiltration intercepted stream 38 becomes the nanofiltration reject stream 40. Figs.1 and 2 show the nanofiltration permeate stream 30 being forwarded to the second filtration assembly 50. In practice, the nanofiltration permeate stream 30 may be forwarded to the second filtration assembly 50 in any part or portion up to its entirety, so that it may be applied as the reverse osmosis feed stream 52. Within the second filtration assembly 50, the second pump 46 provides sufficient pressure to cause a portion of the nanofiltration permeate stream 30 (or the reverse osmosis feed stream 52) to permeate through the reverse osmosis membrane 56 and to produce the reverse osmosis transported stream 58. With an absence of any permeate recycle loop, Fig.2 shows that the reverse osmosis transported stream 58 becomes the reverse osmosis permeate stream 60. The reverse osmosis intercepted stream 68 is also produced that contains the remaining section of the reverse osmosis feed stream 52 that is intercepted by the reverse osmosis membrane 56. Without any brine recycle loop, Fig.2 shows that the reverse osmosis intercepted stream 68 becomes the reverse osmosis reject stream 70. As is also depicted in Fig.1, the second filtration assembly 50 of Fig.2 partitions the nanofiltration permeate stream 30 into the reverse osmosis permeate stream 60 and the reverse osmosis reject stream 70. In the embodiments represented by Fig.1 and Fig.2, the reject streams (40, 70) and the permeate streams (30, 60) are each preferably produced continuously. However, in some embodiments, a batch-wise operation may be conducted where one or more of the reject streams are produced discontinuously. Fig.3 illustrates a second filtration assembly configuration that is suitable for producing (and discharging) the reverse osmosis reject stream 70 intermittently. In this configuration, a reverse osmosis brine recycle loop 66, a mixing tank 32, and one or more control valves (34’, 34”) are present within the second filtration assembly 50. During the first part of the batch cycle, the reverse osmosis transported stream 58 may be continuously removed from the second filtration assembly 50 as the reverse osmosis permeate stream 60. At the same time, control valves (34’, 34”) are oriented to send the reverse osmosis intercepted stream 68 to the mixing tank 32 through the reverse osmosis brine recycle loop 66. In the mixing tank 32, the reverse osmosis intercepted stream 68 is continuously mixed with all or with a remaining part of the nanofiltration permeate stream 30. In a subsequent part of the batch cycle, one or more control valves (34’, 34”) are re-oriented to discharge the contents of the mixing tank 32 as the reverse osmosis reject stream 70. Such a configuration may be advantageous when limited membrane active area is available to treat a high volume of fluid targeting to produce a concentrated reject stream. Another alternative and efficient configuration may be Desalitech’s Close Circuit Reverse Osmosis (CCRO) design comprising a recirculation pump located within the brine recycle loop. This CCRO configuration, described in U.S. Patent Nos.7,695,614 and 8,025,804, both issued to Efraty, is energy efficient as the existing pressure of the reverse osmosis brine recycle loop is not lost when it is discharged downstream of the high-pressure pump. In the Desalitech CCRO design, a concentrated reject stream discharge or collection is also controlled by a pair of valves. The nanofiltration permeate and reject streams, 30 and 40, respectively, from the first filtration assembly 20 and the reverse osmosis permeate and reject streams, 60 and 70, respectively, from the second filtration assembly 50 may change in composition with time. For example, this time-dependent variation is inherent for the recovery method configurations with discontinuous production of the reverse osmosis reject streams 70 shown in Fig.3. Similar time-dependent variation is inherent for the Efraty CCRO design as well. However, even for the more stable batch and continuous processes, the composition of the permeate stream (or the reject stream) will vary to some extent (e.g. due to changing pump pressure or other conditions such as for example temperature, which may be less tightly controlled). Accordingly, in the present description, the composition of a reject or permeate stream is understood to be equal to an average composition obtained through combining all the fluids collected as that stream continuously or discontinuously. For example, for a given time period, the molar concentration of organic solvent molecules in the reverse osmosis permeate stream 60 is the molar composition of the organic solvent molecules in a mixed fluid of the stream 60 collected within the specified time period. Preferably, the molar concentration of organic solvent molecules in the nanofiltration permeate stream 30 is more than 50% of the molar concentration of organic solvent molecules in the process stream 14; also preferably, the molar concentration of organic solvent molecules in the reverse osmosis permeate stream 60 is less than 50% of the molar concentration of organic solvent molecules in the process stream 14. In addition, the molar concentration of lithium ions in the nanofiltration permeate stream 30 is preferably less than 75% of the molar concentration of lithium ions in the process stream 14. Fig.4a illustrates a simple configuration of the first filtration assembly 20 where organic solvent molecules are permeated through the nanofiltration membrane 26 only once. Ideally, the nanofiltration membrane 26 should intercept all lithium ions and allow all organic solvent molecules to permeate through it. Under such circumstance, the molar concentration of lithium ions in the nanofiltration transported stream 28 and the subsequent nanofiltration permeate stream 30 would be zero. However, in practice, a small fraction of lithium ions may permeate through the nanofiltration membrane 26. As a result, a fraction of the mass of lithium dissolved in the process stream 14 may not be recovered in the nanofiltration reject stream 40. In addition, lithium ions in the nanofiltration permeate stream 30, when forwarded to the second filtration assembly 50, may be intercepted by the reverse osmosis membrane 56 and may consequently contaminate one or both of the reverse osmosis intercepted stream 68 and the reverse osmosis reject stream 70, which are rich in organic solvent molecules. Therefore, it is advantageous to remove lithium ions from the nanofiltration transported stream 28 before it becomes the nanofiltration permeate stream 30. In some embodiments, the first filtration assembly 20 is suitable to create the nanofiltration permeate stream 30 by permeating the majority of organic solvent molecules through a nanofiltration membrane 26 at least twice. Figs.4b through 4f illustrate non-limiting examples of first filtration assembly configurations 20 suitable to permeate the majority of organic solvent molecules through a nanofiltration membrane 26 at least twice. The majority of organic solvent molecules may be permeated through the same nanofiltration membrane repeatedly using a permeate recycle loop. For instance, in Fig.4b, a fractional nanofiltration transported stream 29, drawn from the nanofiltration transported stream 28, is sent back to the upstream of the first pump 16 using a nanofiltration permeate recycle loop 27. A variation (not shown) of this configuration includes placing a recirculation pump in the nanofiltration permeate recycle loop 27 to return the fractional nanofiltration transported stream 29 to the downstream of the first pump 16. In that configuration, the recirculation pump pressurizes and recycles the fractional nanofiltration transported stream 29 enabling mixing downstream of the first pump 16 to produce the nanofiltration feed stream 22. In both cases, a higher interception of lithium ions by the nanofiltration membrane 26 may be achieved as the majority of organic solvent molecules (and the process stream) are exposed to the nanofiltration membrane 26 at least twice (i.e. repeated nanofiltration operations). In other configurations, the majority of organic solvent molecules may be permeated through two or more separate nanofiltration membranes within the first filtration assembly 20. This may be achieved by forwarding the nanofiltration transported stream produced by a preceding nanofiltration membrane to a succeeding nanofiltration membrane. For example, in Figs.4c through 4f, several non-limiting configurations of the first filtration assembly 20 are presented. Each includes a preceding nanofiltration membrane 26’ housed in a preceding nanofiltration stage 24’ and a succeeding nanofiltration membrane 26” housed in a succeeding nanofiltration stage 24”. The nanofiltration membranes 26’ and 26” within corresponding stages 24’ and 24” may be selected independently with same or different chemistries and performance characteristics. In some cases, more than one membrane type may also be present within a stage. In Figs.4c through f, a preceding high-pressure pump 16’ is located within the first filtration assembly 20 to provide a pressurized preceding nanofiltration feed stream 22’ to the preceding nanofiltration membrane 26’. Without changing function, the preceding high- pressure pump 16’ may also be located external to the first filtration assembly 20 in the process stream 14. Alternatively, the process stream 14 may already be sufficiently pressurized, so that the preceding high-pressure pump 16’ is not required before nanofiltration. The preceding high-pressure pump 16’ is employed to provide sufficient pressure to cause a portion of the preceding nanofiltration feed stream 22’ to permeate through the preceding nanofiltration membrane 26’ to produce a preceding nanofiltration transported stream 28’. The remaining part of the preceding nanofiltration feed stream 22’, that is intercepted by the preceding nanofiltration membrane 26’, forms a preceding nanofiltration intercepted stream 38’. In Figure 4c, a succeeding high-pressure pump 16” advances the preceding nanofiltration transported stream 28’ from the preceding nanofiltration membrane 26’ to the succeeding nanofiltration membrane 26” where lithium ions are further intercepted. In this case, a succeeding nanofiltration transported stream 28”, constituted by a portion of the preceding nanofiltration transported stream 28’ that permeates through the succeeding nanofiltration membrane 26”, becomes the nanofiltration permeate stream 30 and exits the first filtration assembly 20. The remaining part of the preceding nanofiltration transported stream 28’, that is intercepted by the succeeding nanofiltration membrane 26”, forms a succeeding nanofiltration intercepted stream 38”. The preceding and succeeding nanofiltration intercepted streams (38’, 38”), intercepted by nanofiltration membranes 26’ and 26”, respectively, are combined to form the nanofiltration reject stream 40. All of the organic solvent molecules within the nanofiltration permeate stream 30 have permeated through two nanofiltration membranes. Fig.4d is differentiated from Fig.4c by inclusion of a bypass loop 31 that carries a bypass stream 23’ and enables a portion of the preceding nanofiltration transported stream 28’ to bypass the succeeding high-pressure pump 16” and the succeeding nanofiltration stage 24”. The system may be operated with a degree of bypass such that majority of organic solvent molecules within the nanofiltration permeate stream 30 have permeated through two nanofiltration membranes. Fig.4e is differentiated from Fig.4d by a nanofiltration brine recycle loop 39 that returns the succeeding nanofiltration intercepted stream 38” generated from the succeeding nanofiltration membrane 26” to the downstream side of the preceding high-pressure pump 16’. In the configuration depicted in Fig.4e, a recirculation pump 17 is located within the nanofiltration brine recycle loop 39, but the brine recycle loop 39 could alternatively have supplied the succeeding nanofiltration intercepted stream 38” to the upstream side of the preceding high-pressure pump 16’. In this configuration, the succeeding nanofiltration transported stream 28”, constituted by a portion of the preceding nanofiltration transported stream 28’ that permeates through the succeeding nanofiltration membrane 26”, becomes the nanofiltration permeate stream 30. The preceding nanofiltration intercepted stream 38’ from the preceding nanofiltration membranes 26’, becomes the nanofiltration reject stream 40. In this configuration, some organic solvent molecules will have permeated the nanofiltration membranes 26’ and 26” four (or more) times in total to create the nanofiltration permeate stream 30. Fig.4f illustrates a configuration similar to the configuration presented in Fig.4e, with the difference that the nanofiltration brine recycle loop 39 returns a fraction of the intercepted fluids from the succeeding nanofiltration membrane 26” to a point downstream of the preceding nanofiltration transported stream 28’. By inclusion of the recirculation pump 17 in the nanofiltration brine recycle loop 39, this point may be downstream of the succeeding high-pressure pump 16”. In this case, the succeeding nanofiltration transported stream 28”, constituted by a portion of the preceding nanofiltration transported stream 28’ that permeates through the succeeding nanofiltration membrane 26”, becomes the nanofiltration permeate stream 30. The preceding nanofiltration intercepted stream 38’ intercepted by the preceding nanofiltration membranes 26’ forms the nanofiltration reject stream 40. In this configuration some organic solvent molecules permeate through the nanofiltration membranes 26’ and 26” a total of three times to create the nanofiltration permeate stream 30. All the configurations presented in Figs.4b through 4f are engineering options to improve interception of lithium ions by the nanofiltration membrane(s) within the first filtration assembly 20. Alternatively, processing condition options may be employed alone or in conjunction with the engineering options to improve interception of lithium ions by the nanofiltration membrane(s). For instance, despite the fact that lithium ions may have reduced permeability through a positively charged membrane, it has been found that a negatively charged discriminating layer surface can be more effective. A negatively charged discriminating layer intercepts anions over cations, multivalent anions over monovalent anions, and bulky anions over smaller anions. This may be effective in intercepting lithium ions as well, because electroneutrality prevents smaller lithium ions crossing the nanofiltration membrane. Membrane surface charge (or, more precisely the discriminating layer surface charge) depends on the pH of the fluid in contact, and, in the processes described herein, it is preferable that the process stream pH be greater than 7. The process stream pH is preferably between 7 and 14, more preferably between 8 to 12, even more preferably between 9 to 11. For a process stream with CO32- / HCO3- as the major anions, an additional advantage is gained by operating with a process stream at a pH of 9 or above, where bivalent CO32-ions are dominant anions. A continuous operation with an alkaline process stream, specifically pH > 11, may be disadvantageous as it may compromise the nanofiltration membrane durability, especially at a temperature of 35oC or above. The process stream pH may be adjusted by using a common acid (e.g. HCl, H2SO4, H3PO4) to lower the pH or by using a common base (e.g. NaOH, KOH, Mg(OH)2) to raise the pH. In some preferred embodiments, it is advantageous to use an acid with multivalent anions or a base with multivalent cations to reduce the permeation of lithium ions through the nanofiltration membrane. Referring now to Fig.5, in some embodiments the method described herein includes an additional operation to produce an enriched stream 80 from the nanofiltration reject stream 40, such that the enriched stream 80 has increased lithium concentration compared to the nanofiltration reject stream 40. This enrichment operation may be accomplished by various means such as: evaporating a portion of water molecules from the nanofiltration reject stream, or using a lithium specific sorbent to adsorb lithium ions from a given volume of the nanofiltration reject stream and subsequently recovering it with a lesser volume of regenerating fluid, or by using a membrane-based process such as nanofiltration or reverse osmosis, or by a combination of two or more of these means. The membrane-based process is particularly beneficial as it can be run in a continuous mode and has a smaller size or energy footprint compared to the other options. For example, an enrichment operation 72 employing membranes is depicted in Fig.5, comprising at least one reverse osmosis stage 74 housing at least one reverse osmosis membrane 76. The nanofiltration reject stream 40 from the first filtration assembly 20 may be processed through the at least one reverse osmosis stage 74, where the at least one reverse osmosis membrane 76 preferentially intercepts almost all lithium ions and allows the water molecules to permeate through the membrane 76. Therefore, the enriched stream 80 is produced which has greater concentration of lithium ions than the nanofiltration reject stream 40. In some preferred embodiments, the enrichment operation 72 comprises a high-pressure pump (not shown) which may additionally pressurize the nanofiltration reject stream 40 before advancing it to the at least one reverse osmosis stage 74. This configuration is particularly advantageous as it provides the option to run the first filtration assembly 20 and the enrichment operation 72 at different pressures. The enriched stream 80, high in lithium concentration, may further be evaporated, or chemically treated such as by adding Na2CO3to generate Li2CO3precipitations to recover lithium in solid form. The at least one reverse osmosis membrane 76 also produces a dilute transported stream 78 which is primarily clean water and may be reused in the recovery system 10, or in the enrichment operation 72, or in any preceding or succeeding unit operations. For instance, if the enrichment operation 72 is operated in diafiltration mode, then the dilute transported stream 78 may be used to dilute the stream entering the at least one reverse osmosis stage 74. The recovery method described herein is particularly advantageous when organic solvent molecules have a limited aqueous solubility at the processing condition. In particular, the process stream may comprise two or more types of organic solvent molecules, where one type of organic solvent molecules is water soluble and the other type is insoluble or has limited solubility. For example, in an aqueous solution of ethylene carbonate and diethyl carbonate, the former molecule is soluble in water while the latter molecule is insoluble. Among other factors, a solvent’s aqueous solubility depends on the solvent chemistry, concentration in the solution, presence of other dissolved compounds, and temperature. In general, cyclic carbonate esters are more soluble in water than are acyclic carbonate esters. Some organic solvent molecules show a limited solubility. For example, dimethyl carbonate (DMC) solubility in a 100 mL aqueous solution is approximately 13 to 14g at ambient conditions (i.e. an aqueous molar solubility value of 1.44-1.55 mol / L). Therefore, an aqueous solution with a molar concentration of DMC > 1.44-1.55 mol / L will form a two-phase system: an aqueous phase and an organic phase. From this two-phase system, organic solvent molecules can be easily recovered, for example, using a separating funnel. In the second filtration assembly 50, the reverse osmosis membrane intercepts organic solvent molecules and lets the water molecules permeate through the membrane. This dewatering may increase the molar concentration of organic solvent molecules in the reverse osmosis reject stream 70 compared to the process stream 14. In preferred embodiments, organic solvent molecules in the reverse osmosis reject stream 70 are present at a molar concentration that exceeds their aqueous molar solubility at 25oC. This may be accomplished by a volume concentration process, i.e. preferentially intercepting the organic solvent molecules, allowing the water molecules to permeate through the reverse osmosis membrane 56, and subsequently enriching the organic solvent molecules’ concentration in the reverse osmosis intercepted stream 68 before it becomes the reverse osmosis reject stream 70. The volume concentration may be achieved by: (a) exposing the reverse osmosis intercepted stream to the same reverse osmosis membrane repeatedly using a reverse osmosis brine recycle loop (recycle loop 66 as shown in Fig.3), or (b) exposing a preceding reverse osmosis intercepted stream to a succeeding reverse osmosis membrane (multi-stage configuration), or (c) a combination of both (a) and (b). If a sufficient volume concentration of the reverse osmosis intercepted stream is achieved through dewatering to exceed the molar aqueous solubility limit of organic solvent molecules at the operating temperature, a two-phase system will be formed. For instance, from a liter (L) of a nanofiltration permeate stream with a DMC molar concentration of 0.31 mol / L, advanced to the second filtration assembly, one needs to remove approximately 0.8L of water (~5X volume concentration) to surpass the molar solubility limit at ambient temperature and possibly create a two-phase reverse osmosis reject stream. In general, an organic solvent molecule’s aqueous solubility decreases with decreasing temperature. Therefore, in some embodiments, it is advantageous to operate the recovery method or the second filtration assembly at a sub-ambient temperature. A lower temperature will require a lower volume concentration (or less dewatering) before the molar concentration of organic solvent molecules exceeds the molar solubility limit at that temperature. Also, solution osmotic pressure, a thermodynamic minimum pressure one must overcome in membrane filtration processes, decreases at lower temperature which provides additional benefit. One additional challenge, however, may arise from the need for pumping a fluid with higher viscosity. Other methods of enhancing or accelerating phase separation may also be used in the methods described herein. For example, an aqueous solution comprising one or more organic solvents may form a phase-separated product upon resting for a period of time. Alternatively, the addition of a soluble polar material, such as a salt or a more polar solvent, to an aqueous solution comprising one or more organic solvents produces more favorable conditions for phase separation. Combinations of two or more of these methods are also suitable for producing a phase-separated product stream. In other more preferred embodiments, the reverse osmosis reject stream 70 is cooled, for example within a heat exchanger, to produce a phase-separated product stream. In this case, it may be advantageous to operate the second filtration assembly 50 at ambient or elevated temperature. In Fig.6, the reverse osmosis reject stream 70, preferably at ambient temperature, is sent to a heat exchanger 85. It is preferable that the configuration of the heat exchanger 85 is a non-contacting configuration that prevents co-mingling of fluids, such as for example a shell and tube heat exchanger, a plate heat exchanger, or a plate and shell heat exchanger. In Fig.6, a cold, heat-absorbing fluid 82 enters the heat exchanger 85 and absorbs heat from the reverse osmosis reject stream 70 through indirect contact. As a result, the temperature of the heat absorbing fluid 82 rises while the temperature of the reverse osmosis reject stream 70 decreases. Preferably, the stream 70 is cooled sufficiently so that the molar concentration of organic solvent molecules, originally present in the reverse osmosis reject stream 70, exceeds the aqueous molar solubility of the organic solvent at the exit temperature to produce a temperature-adjusted product stream 90 that preferably is (or will in time) become a phase-separated product stream 90 comprising an aqueous phase and an organic solvent phase. More preferably, the aqueous phase and the organic solvent phase are liquid. Further, organic solvent molecules can easily be recovered from the phase-separated product stream 90 through common processes such as processing with a separating funnel. The temperature of the product stream 90 is preferably less than the temperature of at least the reverse osmosis reject stream 70, more preferably 20oC or below, and even more preferably 10oC or below. Preferably, the temperature of the product stream 90 is higher than the organic solvent molecules’ melting temperature, to avoid their precipitation inside the heat exchanger 85 or any pipes and / or tubes connected to it. As used herein, the term “melting temperature” is synonymous with the term “melting point,” and refers to the temperature at which a substance changes state from solid to liquid. Preferably, water is used as the heat absorbing fluid. More preferably, the reverse osmosis permeate stream 60 is used as the heat absorbing fluid 82 (not shown). In some embodiments, where a membrane-based enrichment operation to further enrich the lithium concentration from the nanofiltration reject stream (e.g. the enrichment operation 72 presented in Fig.5) is available, the dilute transported stream 78 which is primarily water can be reused as the heat absorbing fluid 82. If both the stream 60 and stream 78 are available, they can be mixed at any ratio and reused as the absorbing fluid 82. In some preferred embodiments, a heater or second heat-exchanger (not shown) is also used to increase the temperature of fluid within the second filtration assembly 50, enabling higher concentrations to be achieved without phase separation. In some preferred embodiements, at least a portion of the phase-separated product stream 90, selected from either the aqueous phase or an organic solvent phase, may be recirculated upstream. This may be added to either the first recirculation assembly 20 or the second recirculation assembly 50. For instance, the aqueous phase may still include a smaller concentration of organic solvent that can be further recovered after additional concentration and subsequent phase separation. The following examples are provided to describe the invention in further detail. These examples, which set forth specific embodiments and a preferred mode presently contemplated for carrying out the invention, are intended to illustrate and not to limit the invention. EXAMPLES Description of filtration test protocols for Examples 1 through 4 Dimethyl carbonate (DMC) is a volatile compound (vapor pressure 7.4 kPa at 25oC). Therefore, there is a possibility of DMC concentration change during the filtration process. To verify this, a stability study was conducted wherein the DMC concentration in a representative process stream was measured at different time points using a liquid injection gas chromatography with flame ionization detection (LI GC-FID) instrument (Agilent 7890A, available from Agilent Technologies, Inc., of Wilmington, DE). No substantial concentration change was observed for the first 6 hours of measurements. Therefore, the representative process stream was considered stable at least for 6 hours from the time of preparation. In all subsequent examples, all the tests were completed within 6 hours of preparation unless mentioned otherwise. Examples 1 through 4 correspond to nanofiltration (NF) operations suitable for the first filtration assembly comprising at least one nanofiltration stage. Filtration tests in Examples 1, 2, and 3 were performed to measure the molar concentration of organic solvent molecules and lithium ions in the nanofiltration permeate stream compared to the process stream for different nanofiltration membranes. The lithium-ion concentration values were analyzed by an inductively coupled plasma - optical emission spectroscopy (ICP-OES) method using an iCap 7600 ICP_OES analyzer available from Thermo Scientific of Waltham, MA. The process stream and the nanofiltration permeate stream are referred to hereinafter as the NF-feed stream and NF-permeate stream, respectively. A process stream (NF-feed stream) was prepared by dissolving a measured weight of lithium carbonate (Li2CO3) and dimethyl carbonate (DMC) in de-ionized water. The solution contained a ~0.07 mol / L molar concentration of lithium ions (or Li+~ 480 mg / L) and a ~ 0.078-0.094 mol / L molar concentration (or 0.7-0.85 wt.%) of DMC molecules. Tests were performed under a crossflow configuration by pressurizing the NF-feed stream using a pump and passing the feed across membrane coupons housed in standard- membrane filtration cells. Each cell housed a rectangular membrane coupon with an active area of 42 cm2cut from flat membrane sheets. The membrane unit contained four membrane filtration cells, and therefore, the total active area that came in contact to the feed was 168 cm2(0.18 ft2). In these examples, each membrane filtration cell with a membrane is considered as the membrane module and all four cells / modules combined is considered as the membrane stage. These four membrane filtration cells were positioned in a single bank and were connected in series. The membrane discriminating layer was exposed to the feed solution. A feed volumetric flow rate of 1.5 L / minute was maintained during the entire test. The portion of the NF-feed that permeated through the membrane, the NF-permeate stream, was collected from the individual membrane filtration cell permeate line. The remaining portion of the NF-feed forming the NF-reject stream, was recycled back to the feed tank upstream of the pump for the entire test period. Unless otherwise mentioned in any specific example, the NF-permeate streams were also recycled back to the feed tank upstream of the pump except during the NF-permeate stream collection period. All filtration tests were conducted at a feed temperature range of 23-25oC. Individual permeate streams collected from each filtration cell were weighed using an analytical balance and the membrane flux (volume per unit area per unit time) was calculated by adjusting the measured permeate weight to account for the permeate collection time and the membrane active area. Flux values were averaged over four permeate streams collected (one from each membrane filtration cell) and are reported in units of liter / m2 / h (LMH). An approximated permeate stream density of 1 g / ml was used for flux calculations. For example, if 10.5 g of liquid is collected from a NF coupon (active area 42 cm2) in 10 min., the calculated flux value is (10.5g / 1 g / L)*(60 min. / 10 min.)*(100*100 cm2 / 42 cm2) = 15 LMH. Permeate solutions collected from all four test cells were combined to form the NF-permeate stream. The molar concentrations (mol / L) of lithium ions and DMC molecules were analyzed for both the NF-permeate stream and the NF-feed stream. The NF-feed stream was collected from the feed tank at the same time the permeate streams were collected. The results are reported in Table 1. EXAMPLE 1: The membrane used in this example was a composite membrane with a piperazine-based polyamide barrier layer that is used in DuPont™ FilmTec™ LiNE-XD elements, which are commercially available from DuPont de Nemours, Inc., of Wilmington, DE (hereinafter “DuPont”). EXAMPLE 2: In Example 2, the membrane coupon was a composite membrane with a piperazine-based polyamide barrier layer that is used in FilmTec™ NF270 elements, which are commercially available from DuPont. EXAMPLE 3: In Example 3, the membrane coupon was a composite membrane with a piperazine-based polyamide barrier layer that is used in FilmTec™ NF-400 elements, which are commercially available from DuPont. EXAMPLE 4: In Example 4, the membrane coupon was also a composite membrane with a piperazine-based polyamide barrier layer that is used in FilmTec™ NF-400 elements. In this case the NF-feed contained a 0.072 mol / L molar concentration of lithium ions (or Li+~ 500 mg / L) and a ~ 0.22 mmol / L molar concentration (or 2.0 wt.%) of DMC molecules. Table 1 Examples Applied Permeate Molar DMC Molar Molar Lithium ions pressure flux concentration of concentration concentration of Molar on As is apparent from the data in Table 1, the majority of DMC molecules permeated the nanofiltration membrane while the lithium ions were intercepted by the membrane. Description of filtration test protocols for Example 5 Example 5 corresponds to reverse osmosis (RO) operations suitable for the second filtration assembly comprising at least one reverse osmosis stage. In this example a RO-feed stream was prepared by dissolving a measured weight of dimethyl carbonate (DMC) in a measured volume of de-ionized water. The same experimental setup used in Examples 1 through 4 was used, with the following exception. The portion of the feed that was intercepted by the membrane, the reject stream, was recycled back to the feed tank upstream of the pump for the entire test period. However, the RO-permeate streams, from all four test cells, were continuously collected from the system during the test period. In this test, the molar concentration of DMC molecules in the RO-feed changed continuously and an average of the start and end values was taken as the molar concentration of DMC in RO-feed. The molar concentration of DMC in RO-permeate was measured by using the mixed permeate solution collected from four test cells over the experiment duration. In Example 5, the membrane coupons were a composite membrane with an aromatic amine-based polyamide barrier layer that is used in FilmTec™ SWXHR elements, which are commercially available from DuPont. In Example 5A, the starting RO-feed contained ~ 0.22 mmol / L molar concentration (or 2.0 wt.%) of DMC molecules. The starting-RO feed did not contain any added lithium ion. The RO-feed pH and conductivity values were 6.67 and 2.37 μS / cm, respectively. The experiment duration was 5.5 hrs. In Example 5B, the starting RO-feed contained a molar concentration of DMC molecules close to the same measured at the end of Example 5A. The experiment duration was 6.0 hrs. In both Examples 5A and 5B, the applied pressure was adjusted with time to maintain a steady flux value. Results are presented in Table 2. In this experiment, the feed concentration in the feed tank changed continuously. Therefore, an arithmetic average of the Start-RO feed concentration and the End-RO feed was used to calculate the molar concentration ratio which was expressed in %. The molar concentration ratio of DMC in the RO permeate stream was calculated as: 100*Molar concentration of DMC in RO- permeate / (Molar concentration of DMC in Start-RO feed + Molar concentration of DMC in End-RO feed) / 2. The Molar Concentration Ratios data presented in Table 2 demonstrate that the second stage reverse osmosis treatment reduced the concentration of DMC to less than 50% of its original value. Table 2 Examples Average Molar concentration of DMC , mol / L Molar n In summary, the method of recovery described herein finds particular utility in lithium-ion battery electrolyte wastewater handling. It offers advantages by separating lithium ions from organic solvent molecules. These separated lithium ions can be reprocessed and reused. The DMC aqueous solution can be concentrated to reduce waste volume, or it can be further processed to obtain pure DMC solution for reuse. While certain of the preferred embodiments of the present invention have been described and specifically exemplified above, it is not intended that the invention be limited to such embodiments. Rather, it is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

What is claimed is:

1. A method of recovering an organic solvent comprising: (a) providing a process stream comprising: i. at least 0.05 wt.% of organic solvent molecules selected from the group consisting of carbonate esters and carboxylate esters, and ii. at least 0.03 wt.% of dissolved lithium ions, wherein the weight percentages are based on the total weight of the process stream; (b) providing a first filtration assembly comprising at least one nanofiltration stage; (c) providing a second filtration assembly comprising at least one reverse osmosis stage; (d) treating the process stream with the first filtration assembly to generate a nanofiltration permeate stream and a nanofiltration reject stream, wherein the molar concentration of organic solvent molecules in the nanofiltration permeate stream is more than 50% of the molar concentration of organic solvent molecules in the process stream; and (e) treating at least a portion of the nanofiltration permeate stream with the second filtration assembly to generate a reverse osmosis permeate stream and a reverse osmosis reject stream, wherein the molar concentration of the organic solvent molecules in the reverse osmosis permeate stream is less than 50% of the molar concentration of organic solvent molecules in the process stream; wherein the molar concentrations are measured in moles / liter.

2. The method of recovering an organic solvent of claim 1, wherein the carbonate esters are selected from the group consisting of: an acyclic carbonate ester represented by the formula:wherein R1and from a group consisting of an alkyl group and an haloalkyl group; anda cyclic carbonate ester represented by the formula: wherein R3is selectedatom, a halogen atom, an alkyl group, and a haloalkyl group.

3. The method of recovering an organic solvent of claim 1 or claim 2, wherein the carboxylate esters are selected from the group consisting of: an acyclic carboxylic ester represented by the following formula: wherein R4and R5area group consisting of an alkyl group and a haloalkyl group, and a cyclic carboxylic ester selected from a group consisting of a γ-butyrolactone and a γ-valerolactone.

4. The method of recovering an organic solvent of any of claims 1, 2, or 3, wherein the molar concentration of lithium ions in the nanofiltration permeate stream is less than 50% of the molar concentration of lithium ions in the process stream.

5. The method of recovering an organic solvent of any preceding claim, wherein the first filtration assembly creates the nanofiltration permeate stream by permeating the majority of organic solvent molecules through a nanofiltration membrane at least twice.

6. The method of recovering an organic solvent of any preceding claim, further comprising the step of conducting an enrichment operation to increase the concentration of lithium ions in the nanofiltration reject stream.

7. The method of recovering an organic solvent of any preceding claim, wherein the organic solvent molecules in the reverse osmosis reject stream are present at a molar concentration that exceeds their aqueous molar solubility at 25oC.

8. The method of recovering an organic solvent of any preceding claim, wherein a heater or a heat-exchanger is used to increase or decrease the temperature of fluid within the second filtration assembly.

9. The method of recovering an organic solvent of any preceding claim, further comprising the step of cooling the reverse osmosis reject stream within a heat exchanger to produce a temperature-adjusted product stream, and preferably wherein the temperature of the temperature-adjusted process stream is 20oC or below, more preferably 10oC or below.

10. The method of recovering an organic solvent of claim 8 or claim 9, wherein the temperature-adjusted product stream comprises one or more organic solvents; wherein each of the organic solvents has a melting temperature; and wherein the temperature of the temperature-adjusted product stream is higher than the melting temperature of each of the organic solvents.

11. The method of recovering an organic solvent of any of claims 8, 9, or 10, wherein the temperature-adjusted process stream is a phase separated product stream comprising an aqueous phase and an organic solvent phase; and wherein the aqueous phase and the organic solvent phase are liquid.

12. The method of recovering an organic solvent of any preceding claim, wherein the phase separated product stream is produced by one or more methods selected from the group consisting of cooling the reverse osmosis reject stream; operating the recovery method or the second filtration assembly at a sub-ambient temperature; resting the reverse osmosis reject stream or the temperature-adjusted stream for a period of time; and adding a soluble polar material to the reverse osmosis reject stream or to the temperature-adjusted stream.

13. The method of recovering an organic solvent of any preceding claim, wherein the process stream has a pH that is greater than 7.

14. An organic solvent molecules recovery system 10 comprising: a first filtration assembly 20 comprising at least one nanofiltration stage 24; a second filtration assembly 50 comprising at least one reverse osmosis stage 54; a heat exchanger 85; and a heat absorbing fluid 82; wherein the second filtration assembly 50 is suitable to produce a reverse osmosis reject stream 70; and wherein the heat exchanger 85 is suitable to process the reverse osmosis reject stream 70 to produce a temperature-adjusted product stream 90.

15. The organic solvent molecules recovery system 10 of claim 14, wherein the heat exchanger 85 has a non-contacting configuration that prevents co-mingling of the heat-absorbing fluid 82 with the reverse osmosis reject stream 70 or with the temperature-adjusted product stream 90; and preferably wherein the heat exchanger 85 is selected from the group consisting of a shell and tube heat exchanger, a plate heat exchanger, and a plate and shell heat exchanger.

16. The organic solvent molecules recovery system 10 of claim 14 or claim 15, further comprising an enrichment operation 72 that is operated by a procedure selected from the group consisting of: evaporating a portion of water molecules from the nanofiltration reject stream; using a lithium specific sorbent to adsorb lithium ions from a given volume of the reverse osmosis reject stream 70 followed by regeneration using an eluant; using a membrane-based process; and a combination of two or more of these procedures.

17. The organic solvent molecules recovery system 10 of any of claims 14, 15, or 16, wherein the first filtration assembly 20 is suitable to produce a nanofiltration reject stream 40; and wherein the enrichment operation 72 is suitable to process the nanofiltration reject stream 40 to produce: an enriched stream 80 having a concentration of lithium ions that is greater than the concentration of lithium ions in the nanofiltration reject stream 40; and a dilute transported stream 78.

18. The organic solvent molecules recovery system 10 of any of claims 14 through 17, further comprising a high-pressure pump that pressurizes the nanofiltration reject stream 40 within or before the enrichment operation 72.

19. The organic solvent molecules recovery system 10 of any of claims 14 through 18, wherein the heat absorbing fluid 82 is water; or wherein the heat absorbing fluid 82 comprises the reverse osmosis permeate stream 60, or the dilute transported stream 78, or a mixture of the reverse osmosis permeate stream 60 and the dilute transported stream 78 in any ratio.

20. The organic solvent molecules recovery system 10 of any of claims 14 through 19, wherein the temperature of the temperature-adjusted process stream 90 is 20oC or below, more preferably 10oC or below.

21. The organic solvent molecules recovery system 10 of any of claims 14 through 20, wherein the temperature-adjusted process stream 90 is a phase separated product stream 90 comprising an aqueous phase and an organic solvent phase; and wherein the aqueous phase and the organic solvent phase are liquid.

22. The organic solvent molecules recovery system 10 of any of claims 14 through 21, wherein at least a portion of the temperature-adjusted process stream 90 is removed from either the aqueous phase or the organic solvent phase and is recirculated to the first recirculation assembly 20 or the second recirculation assembly 50.

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