Enrichment of lithium isotopes

Abstract

A method for lithium isotope (6Li, 7Li) enrichment comprising obtaining a lithium salt 101, adding said salt to an algal suspension 102, and allowing the algal species to feed on the salt for a pred

Description

Field of the Invention

[01] The present disclosure relates to a method of enriching lithium isotopes. In particular the disclosure is concerned with lithium isotope enrichment using a biological based cascade process. Moreover, the process may be applied to other elements to likewise enrich those elements. Background

[02] Lithium has many industrial uses, including Li-ion batteries (e.g., for electric vehicles), ceramics, lubricants, and glass. Lithium also has nuclear applications, particularly when in the forms of (enriched) lithium-6 and lithium-7.

[03] Lithium-6 is valuable as the source material for the production of tritium and as an absorber of neutrons in nuclear fusion reactions.

[04] Highly enriched lithium-7 (more than 99% purity) may be used as a coolant in molten salt reactors (MSRs) and pH stabilizer in pressurized water reactors (PWRs).

[05] Accordingly, various methods of lithium isotope enrichment are already in use, including diffusion methods in liquid mercury, electro-potential methods, and chemical methods. Liquid mercury diffusion in particular is still the most industrially viable approach, but has significant drawbacks including toxicity, formation of dangerous mercury-containing waste, and high energy consumption.

[06] More recently, enrichment of Li using algae has been attempted but viability of this approach has so far been limited with low yield and purity, with a focus only on lithium-6 but not lithium-7.

[07] Hence a new approach to lithium enrichment is highly desirable. Summary

[08] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches to lithium enrichment, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[09] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[10] In an aspect of the present invention there is described a method for lithium isotope enrichment. The method comprises (i) obtaining a lithium salt, (ii) adding the lithium salt to an algal suspension comprising algae, (iii) separating the algae and supernatant fluid from the suspension after the algae has fed on the lithium salt for a predetermined feeding time, (iv) extracting lithium isotopes from the separated algae, (v) inputting the extracted lithium isotopes and the separated supernatant fluid to a counter-current enrichment chain, wherein the countercurrent enrichment chain yields a first fluid comprising a first lithium isotope and a second fluid comprising a second lithium isotope, and (vi) extracting the first lithium isotope and second lithium isotope from the respective enrichment chain fluids.

[11] In an example, the first isotope may be lithium-6 and the second isotope may be lithium-7. Preferably, the algae enriches lithium 6 as a result of feeding on the lithium salt.

[12] In an example, the algae may be pre-grown to a plateau stage of its lifecycle.

[13] In an example, the lithium salt may comprise a water soluble lithium compound. For example, the lithium salt may comprise lithium chloride.

[14] In an example, a concentration of the lithium salt in the solution may be from about 0.01g / litre to 10g / litre.

[15] In an example, a species of algae in the algal suspension may be selected from a micro-algal sea water species, a fresh water species, estuarine species, or brackish species.

[16] In an example, the predetermined feeding time may be based on the species of algae, for example between 1 minute and 72 hours, preferably about 60 minutes.

[17] In an example, separating the algae comprises first reducing a feeding rate of the algae.

[18] Reducing the feeding rate may comprise cooling the algal suspension; for example, within about 1°C to about 5°C of the freezing temperature of the suspension.

[19] Reducing the feeding rate may comprise changing a pH level of the suspension. For example, the change in pH level may be at least pH 4.

[20] In an example, separating the algae may comprise vacuum filtration. A mesh size may be based on a species of algae in the suspension, and may be less than about 10 microns.

[21] In an example, extracting the lithium isotopes from the algae comprises lysing the algae. Lysing the algal biomass may comprise at least one of ultrasonic sonication, mechanical chopping or grinding, liquid homogenisation, osmotic stress, and freeze thawing.

[22] In an example, extracting the lithium isotopes comprises adding the algae to a lithium free fluid.

[23] In an example, the counter-current enrichment chain may comprise a counter-current symmetric cascade, where the released lithium isotopes are input to the counter-current enrichment chain in a first direction, and the separated liquid fraction is input to the counter-current enrichment chain in a second direction opposite to the first direction.

[24] In an example, extracting the lithium isotopes from the respective enrichment chain fluids comprises forming and separating lithium hydroxide. The lithium hydroxide may be suitably reacted to obtain at least one of lithium chloride and lithium metal.

[25] The method may further comprise reperforming the method in enrichment cycles, wherein each enrichment cycle comprises steps (i) to (vi), and obtaining the lithium salt at step (i) for a cycle comprises obtaining lithium chloride from step (vi) of a preceding enrichment cycle.

[26] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. The term “about”, or substantially, when used herein means + / - 5% of the stated value. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’. Brief Description of the Drawings

[27] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[28] Fig. 1 shows a flow diagram for a method of lithium enrichment;

[29] Fig. 2 shows an optional step in the method; and

[30] Fig. 3 shows a schematic for a counter-current symmetric cascade for lithium enrichment. Detailed Description

[31] Referring the Figure 1, there is shown a flow diagram for an improved method 100 for lithium enrichment using algae. The technique is convenient to implement, avoids many of the hazards associated with non-biological enrichment processes, and provides improved yields and purity compared to other proposed biological techniques. Many other advantages and improvements will be appreciated from the discussion herein.

[32] At step 101, the method comprises obtaining a lithium salt. The lithium salt is preferably soluble, so that the lithium salt may readily dissolve when combined with fluid solutions used elsewhere in the described method. More particularly, the lithium salt may be water soluble, so that the lithium salt is dissolvable in either salt water or fresh water based solutions. In a preferred example the lithium salt may be lithium chloride.

[33] At step 102, the method comprises adding the lithium salt to an algal suspension. That is, the lithium salt is added to a substantially heterogeneous mixture of algae and fluid.

[34] The fluid may be selected based on the solubility of the lithium salt. For example, where the lithium salt is water soluble, the fluid part of the algal suspension may sea water or fresh water. The choice of fluid may correspond to the choice of algae used in the selection. For example, where the algae is a fresh water species of algae, the fluid may be fresh water. Where the algae is a salt water species of algae, the fluid may be sea water.

[35] In general, the species of algae may be selected from a micro-algal sea water species, a fresh water species, estuarine species, or brackish species (and the fluid selected accordingly). The specific species of algae used in the present technique may be selected based on its ability to absorb (and therefore enrich) particular isotopes of lithium. For example, the species of algae may be selected based on its ability to preferentially absorb at least one of a first isotope or a second isotope. In particular, it will be appreciated that the typical natural form of lithium present in the lithium salt may contain a natural spread of lithium-6 and lithium-7; typically about 5% lithium-6 and about 95% lithium-7, such that the first isotope may be considered lithium-6 and the second isotope lithium-7; although the described techniques are not limited thereto. For ease of explanation, in the following examples it is assumed that the algae preferentially absorbs lithium-6; i.e., lithium-6 may be considered the first isotope hereafter. Examples of algae species which may be selected for lithium enrichment, particularly lithium-6, are known in the art: tisochrysis lutea, chaetoceros sp. and Chlamydomonas reinhardtii.

[36] It can also be desirable to control the growth conditions of the algae to maximise the efficiency of the lithium absorption / enrichment. For many species of algae it has been determined that the optimum time for enrichment is when the algae is in a plateaux stage of its development cycle (as opposed to a growth stage or decline stage). In particular, efficient enrichment may be obtained when the algae is in a latter part of the plateaux stage; i.e., cell count has reached equilibrium. Here, the latter part of the plateaux stage may be taken to be a predetermined time during which the algae has matured since the completion of the growth stage. Thus, in some examples, it may be possible to extend the plateau stage indefinitely with appropriate feeding, lighting, etc. Suitably, growing conditions including wavelength(s) of light illumination of the algae, temperature of the ambient environment, and pH level of the algal suspension, may be adapted to optimise growth and optimise the time during which the algae is in the relevant part of the plateau stage. For example, for growing chlorophyll-b containing algae such as Chlamydomonas reinhardtii, pH can be maintained between 7.5-8.5, temperature from about 1°C to about 35°C, light level between about 10,000 and about 150,000 lux, and wavelengths associated with sunlight (i.,e. 380-800 nm) may be suitable. These growth parameters will likewise be suitable to many other species of algae.

[37] The amount of lithium salt added to the algal suspension may be based on the particular species of algae being used for the enrichment (and therefore also the fluid). In general, lithium salt may be added to the fluid in a concentration from about 0.01g / litre to about 10g / litre, further preferably about 0.0001 g / litre to about 0.01g / litre. About 100 pg / l LiCI is particularly suitable for Chlamydomonas reinhardtii.

[38] The lithium salt may be allowed to reside in the suspension for a predetermined feeding time (between 10 minutes to 72 hours, depending on species) to allow for enrichment by the algae to occur (i.e., for the algae to feed). In some instances the predetermined time is based primarily on manufacturing decisions, such as obtaining a particular concentration of a particular isotope within the algae; this may be particularly the case when the time scale for efficient enrichment is much shorter than the time scale of optimum growth conditions such as the length of the plateaux stage. Alternatively the predetermined time may be based on the growth conditions of the algae species; e.g., how long the optimum part of the plateaux stage of the algae lasts. For example, the optimum feeding time for Chlamydomonas reinhardtii, a fresh-water species, is approximately 60 minutes.

[39] Referring to Figure 2, following the algal feeding at step 102, optionally the method 100 comprises a step 103 of halting, or at least substantially reducing, the algae from feeding on the lithium, which may prevent a reversal of enrichment in the algae. That is, step 103 comprises slowing, and potentially stopping, the biological processes of the algae. A variety of techniques may be employed although one of two are preferred in the present disclosure.

[40] In one example, step 103 may comprise reducing the ambient environment of the algae to about freezing temperature. It will of course be appreciated that freezing temperature may be different depending on the fluid being utilised. E.g., when the fluid is salt water, freezing temperature may be -1.8°C, and when the fluid is fresh water, freezing temperature may be 0°C. Suitably, bring the environment to about freezing temperature may include setting a temperature to within 1°C of the freezing temperature, or between 1°C and 5°C of the freezing temperature. For many algae species, for example, Chlamydomonas reinhardtii, the target temperature may be about 4°C.

[41] In another example, step 103 may comprise changing the pH level of the algal suspension. For example, the pH level may be changed by about 1, or from about 1 to about 2, or from about 2 to about 4. Algae species generally have a preferred pH range for survivability, and therefore in one example the pH change may correspond to at least twice the algae pH growth range (for example, if the growth range is 8.2 to 8.7, such that the absolute value of the range is 0.5, then the pH may be changed by at least 1). Suitably, the pH change may be in either direction (i.e., more alkaline or more acidic), although preferably the change is acidic. For example, target pH for Chlamydomonas reinhardtii would be pH 4, representing a pH change of at least 4.

[42] Returning to Fig. 1, at step 104 the method comprises separating the algae from the algal suspension. That is, the algal biomass is removed from the algal suspension so that only the supernatant fluid, or liquid fraction, remains. A variety of techniques for separating substantially solid matter from a fluid suspension will be appreciated by those in the art.

[43] For example, the algae may be separated via a process of flocculation, thereby involving a step of adding flocculant to the suspension. An example flocculantto be used for Chlamydomonas reinhardtii is slate dust.

[44] In another example, the algae may be separated using a continuous centrifuge.

[45] In yet another example, the separation may be achieved by vacuum filtration. The vacuum filtration may utilise a separating mesh of a size determined by the species of algae. In general, however, a mesh size for the vacuum filtration will be typically less than about 10 microns. Vacuum filtration is preferred due to the speed and efficiency of separation compared to other techniques. By way of example, an appropriate mesh size for Chlamydomonas reinhardtii may be about 5 microns.

[46] The algae biomass will comprise a greater content of one isotope over another based on the species of algae, as mentioned above. For example, the algal biomass may comprise predominantly lithium-6, while the supernatant fluid comprises predominantly lithium-7 (the supernatant fluid is retained for later).

[47] At step 105, the method comprises extracting the first lithium isotope (e.g., lithium-6) from the separated algae. This may involve creating a new suspension by adding the lithium to a fluid which does not comprise any lithium, followed by applying a suitably extraction process. In the present example the extraction process may be lysing. Lysing of the algal biomass may be achieved by any suitably means known in the art, for example, ultrasonic sonication, mechanical chopping or grinding, liquid homogenisation, osmotic stress, and freeze thawing. The preferred technique is ultrasonic sonication, as it is rapid at lysing the algae, releasing the contents of the cells.

[48] At step 106, the method comprises inputting the now lithium rich suspension (i.e., obtained from the lysing) to an enrichment process. Suitable fluid based enrichment processes will be known in the art. In particular, the enrichment process may be a process for enriching lithium-6, especially where that is the isotope enriched by the algae, of the process may be for enriching lithium-7, again preferably where that isotope has been previously enriched by the algae. Optionally the lithium rich suspension may be filtered to remove dead biomass before inputting to the enrichment process.

[49] Preferably, however, step 106 comprises using the lysed (and lithium rich) suspension in a counter-current enrichment chain, in particular a counter-current symmetric cascade. In such a cascade, the lithium rich suspension from lysing the algae is used in the chain in a first direction, while (previously retained) supernatant fluid is used in the chain in a second, opposite, direction. In this way are the first and second isotopes further enriched.

[50] Figure 3 shows an example schematic of such a cascade. Here is shown two cascade steps, cascade 0 and cascade 1, although in general any number of cascade steps may be applied from step -m to step +n. The general principles of counter-current cascade will be familiar to those in the art, however the following provides a brief overview.

[51] At each cascade step, enriched lithium solution which is output by a previous cascade step is combined with algae to form an algal suspension, analogous to a starting algal suspension as at step 102. E.g., lithium rich solution used at the start of cascade step 1 is lithium rich solution output by cascade step 0.

[52] At each step, the algae is allowed to feed on the enriched lithium solution, and is subsequently separated from the fluid solution before being processed to form another lithium enriched solution which is used in subsequent cascade steps (or is processed to extract the lithium).

[53] Similarly, the supernatant fluid comprising lithium enriched with the second isotope (e.g., lithium-7) is obtained from a preceding cascade step from the positive cascade direction. That is, for cascade 0, the lithium solution to be used is the supernatant fluid that results from cascade step 1.

[54] Suitably, the result of the symmetric cascade is a first fluid comprising predominantly the first lithium isotope, e.g., lithium-6, and a second fluid comprising predominantly the second lithium isotope, e.g., lithium-7.

[55] At step 107, the method comprises extracting the lithium isotopes (i.e., the first and second isotopes e.g., lithium-6 and lithium-7) from the respective enrichment chain fluids.

[56] When using water based algae and therefore water based fluid for the lithium solutions, the combination with the lithium salt (e.g., LiCI) forms a lithium hydroxide solution. Suitably, in such examples, the solution may be processed to extract the solid form of lithium hydroxide, for example by evaporation of the fluid.

[57] In some examples, the lithium hydroxide can be further reacted to obtain lithium carbonate, lithium metal or lithium chloride.

[58] For example, adding sodium carbonate to a lithium hydroxide solution causes the solution to crash out insoluble lithium carbonate which can then be extracted in solid form. From lithium carbonate, it is possible to produce lithium chloride via a reaction with hydrochloric acid. The lithium chloride can then be subsequently used to produce lithium metal via molten salt electrolysis.

[59] The lithium metal will of course be a pure form of either lithium-6 or lithium-7.

[60] Production of lithium-chloride is useful because, as a lithium salt, this can be used to repeat the method 100 to further purify the lithium isotopes.

[61] That is, the method may further comprise repeating the method in enrichment cycles, wherein each enrichment cycle essentially corresponds to the method steps 101 to 107 described above. For each enrichment cycle, the step 101 of obtaining the lithium salt for that cycle may comprise the lithium chloride output at step 107 from the previous enrichment cycle.

[62] Put another way, the step 101 of obtaining lithium salt may comprise obtaining a lithium salt by performing a suitable enrichment process (e.g., the steps 102 to 107).

[63] In summary, exemplary embodiments of an improved process for lithium enrichment have been described. The described process may be utilised industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[64] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

[65] In particular, although the above method has been described with reference to lithium isotope enrichment, it should be appreciated that the enrichment process may be applied to enrich isotopes of many other elements without departing from the scope of the disclosure.

[66] That is, in the above description, the element “lithium” may be replaced with one of carbon, nickel, boron, iron, nitrogen, chromium, cobalt, zirconium, uranium, hafnium, gadolinium, holmium, molybdenum and germanium, while still preserving the essence of the disclosure. Although it should be appreciated the isotopes of interest to enrich may be different than ‘element-6’ or ‘element-7’ isotopes discussed for lithium, and the enrichment of specific elemental isotopes will require a suitable selection of algae.

[67] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[68] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[69] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[70] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method for lithium isotope enrichment, comprising:(i) obtaining a lithium salt,(ii) adding the lithium salt to an algal suspension comprising algae,(iii) separating the algae and supernatant fluid from the suspension after the algae has fed on the lithium salt for a predetermined feeding time,(iv) extracting lithium isotopes from the separated algae,(v) inputting the extracted lithium isotopes and the separated supernatant fluid to a countercurrent enrichment chain, wherein the counter- current enrichment chain yields a first fluid comprising a first lithium isotope and a second fluid comprising a second lithium isotope, and(vi) extracting the first lithium isotope and second lithium isotope from the respective enrichment chain fluids.

2. The method of claim 1, wherein the first isotope is lithium-6 and the second isotope is lithium-7.

3. The method of claim 2, wherein the algae enriches lithium 6 as a result of feeding on the lithium salt.

4. The method of any preceding claim, wherein the algae is pre-grown to a plateau stage of its lifecycle.

5. The method of any preceding claim, wherein the lithium salt comprises a water soluble lithium compound.

6. The method of claim 5, wherein the lithium salt comprises lithium chloride.

7. The method of any preceding claim, wherein a concentration of the lithium salt in thesolution is from about 0.01 g / litre to 10g / litre.

8. The method of any preceding claim, wherein a species of algae in the algal suspension is selected from a micro-algal sea water species, a fresh water species, estuarine species, or brackish species.

9. The method of any preceding claim, wherein the predetermined feeding time is based on the species of algae.

10. The method of any preceding claim, wherein the predetermined time is between 1 minute and 72 hours.

11. The method of any preceding claim, wherein separating the algae comprises first reducing a feeding rate of the algae.

12. The method of claim 11, wherein reducing the rate of feeding comprises cooling the algal suspension.

13. The method of claim 12, wherein a temperature is from about 1 °C to about 5°C above the freezing temperature of the suspension.

14. The method of claim 11, wherein reducing the rate of feeding comprises changing a pH level of the suspension.

15. The method of claim 14, wherein the change in pH level is at least pH 4.

16. The method of any preceding claim, wherein separating the algae comprises vacuumfiltration.

17. The method of claim 16, wherein a mesh size is based on a species of algae in the suspension.

18. The method of claim 16 or 17, wherein the mesh size is less than about 10 microns.

19. The method of any preceding claim, wherein extracting the lithium isotopes from the algaecomprises lysing the algae.

20. The method of claim 19, wherein lysing the algal biomass comprises at least one of ultrasonic sonication, mechanical chopping or grinding, liquid homogenisation, osmotic stress, and freeze thawing.

21. The method of any of claim 19 or 20, wherein extracting the lithium isotopes comprises adding the algae to a lithium free fluid.

22. The method of any preceding claim, wherein the counter-current enrichment chain comprises a counter-current symmetric cascade, and wherein the released lithium isotopes areinput to the counter-current enrichment chain in a first direction, and the separated liquid fraction is input to the counter-current enrichment chain in a second direction opposite to the first direction.

23. The method of any preceding claim, wherein extracting the lithium isotopes from the respective enrichment chain fluids comprises forming and separating lithium hydroxide.

24. The method of claim 23, further comprising reacting the lithium hydroxide to obtain at least one of lithium chloride and lithium metal.

25. The method of claim 24, further comprising repeating the method in enrichment cycles, wherein each enrichment cycle comprises steps (i) to (vi), and obtaining the lithium salt at step (i) for a cycle comprises obtaining lithium chloride from step (vi) of a preceding enrichment cycle.

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