Method for purification of lithium carbonate using combustion exhaust

Abstract

A process for purifying crude lithium carbonate includes carbonation of a slurry of the crude lithium carbonate feed in water with exhaust flue gas to form an aqueous solution of lithium bicarbonate, followed by separation of the solution from insoluble impurities, and depressurization and heating of the separated solution to precipitate solid lithium carbonate and evolve carbon dioxide. The process produces battery-grade purified lithium carbonate and purified carbon dioxide which can be captured for further use, while reducing carbon dioxide emissions from exhausted flue gas and avoiding the need to use pre-purified carbon dioxide as a reactant.

Description

METHOD FOR PURIFICATION OF LITHIUM CARBONATEUSING COMBUSTION EXHAUSTField

[0001] The present application is directed to a method of purifying crude lithium carbonate. More specifically, the present application provides a method for using combustion exhaust containing carbon dioxide to remove impurities from crude lithium carbonate while capturing carbon dioxide from the combustion exhaust.Background

[0002] Lithium carbonate (Li2CO3), an important precursor to other lithium compounds used in the manufacture of lithium-ion batteries, can be obtained from brine reservoirs or by refining lithium-containing ores such as the hard rock, pegmatite, which contains the lithium aluminosilicate mineral, spodumene. After crushing and sorting the pegmatite ore, spodumene concentrate is separated from the gangue and calcined by heating at temperatures up to 1200°C to convert the naturally occurring refractory a-polymorph to the more tractable p-polymorph. The p-spodumene is then roasted with sulfuric acid at 250°C to generate soluble lithium salts such as lithium sulfate (Li2SO4) which can be leached into water. Treating the aqueous leachate with soda ash (sodium carbonate, Na2CO3) causes precipitation of insoluble crude Li2CO3.

[0003] To obtain lithium carbonate which is sufficiently pure (> 99.5 wt% Li2CO3) to be used in the manufacture of lithium-ion batteries, the crude Li2CO3is typically purified by a carbonation process. A slurry of the crude lithium carbonate in water is treated with pressurized CO2at low temperature (for example, 2°C) to convert the insoluble Li2CO3into soluble aqueous lithium bicarbonate (LiHCO3), enabling separation of the aqueous lithium bicarbonate solution from impurities which are less soluble in water by filtration, for example. Subsequent depressurization of the aqueous lithium bicarbonate solution results in decomposition of LiHCO3to regenerate Li2CO3and CO2gas. The insoluble lithium carbonate then precipitates or crystallizes from the aqueous phase, enabling separation of the purified solid Li2CO3from impurities which are more soluble in water by filtration, for example.

[0004] Conventionally, purified CO2is often purchased for use in the carbonation process, adding a significant expense. In addition, the process of calcining thespodumene concentrate usually requires combustion of a fuel to generate the heat required, thereby producing exhaust containing CO2. The produced CO2may either be released, with negative environmental consequences, or may require treatment such as scrubbing at additional cost to prevent or reduce release into the environment.Summary

[0005] One aspect of the present application provides a method of purifying crude lithium carbonate. The method includes:a) exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure to provide an aqueous solution of lithium bicarbonate; b) separating the aqueous solution of lithium bicarbonate from insoluble impurities; c) reducing the pressure applied to the aqueous solution of lithium bicarbonate and / or increasing the temperature of the aqueous solution of lithium bicarbonate, to produce a mixture comprising a residual aqueous solution, solid lithium carbonate and gaseous carbon dioxide;d) capturing the gaseous carbon dioxide; ande) separating the solid lithium carbonate from the residual aqueous solution to provide purified solid lithium carbonate.

[0006] In at least one embodiment, the gas containing carbon dioxide is a flue gas effluent. In at least one embodiment, the gas contains up to about 21% by volume of CO2. In at least one embodiment, the gas contains up to about 15% by volume of CO2. In at least one embodiment, the gas contains from about 1% to about 21% by volume of CO2.ln at least one embodiment, the gas contains from about 5% to about 15% by volume of CO2.

[0007] In at least one embodiment, the crude lithium carbonate contains up to about 99% lithium carbonate by weight. In at least one embodiment, the crude lithium carbonate contains at least about 75% lithium carbonate by weight. In at least one embodiment, the crude lithium carbonate contains about 90% to about 99% lithium carbonate by weight. In at least one embodiment, the aqueous mixture of the crude lithium carbonate is a slurry. In at least one embodiment, the aqueous mixture of the crude lithium carbonate contains from about 5% to about 15% by weight of the crude lithium carbonate.

[0008] In at least one embodiment, the step a) of exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises introducing the gas containing carbon dioxide into a headspace of a vessel containing the aqueous mixture of the crude lithium carbonate to provide a headspace gas at a headspace pressure. In at least one embodiment, the headspace pressure of the gas is from about 650 kPa to about 3000 kPa.

[0009] In at least one such embodiment, the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure further comprises allowing the headspace gas to remain in contact with the aqueous mixture for a predetermined period of time. In at least one embodiment, the predetermined period of time has a duration of up to about 120 minutes. In at least one such embodiment, allowing the headspace gas to remain in contact with the aqueous mixture for a predetermined period of time further comprises agitating the aqueous mixture.

[0010] In at least one such embodiment, the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out one or more exposure cycles, wherein each exposure cycle comprises: introducing the gas containing carbon dioxide into the headspace of the vessel; allowing the headspace gas to remain in contact with the aqueous mixture for the predetermined period of time; andventing at least a portion of the headspace gas from the vessel.

[0011] In at least one such embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure further comprises carrying out a plurality of exposure cycles.

[0012] In at least one embodiment, the step a) of exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises bubbling the gas containing carbon dioxide through the aqueous mixture.

[0013] In at least one embodiment, the aqueous mixture of the crude lithium carbonate is exposed to the gas containing carbon dioxide at a temperature of from about -10°C to about 25°C.

[0014] In at least one embodiment, the aqueous solution of lithium bicarbonate is separated from the insoluble impurities by filtration. In at least one embodiment, the pressure applied to the aqueous solution of lithium bicarbonate is reduced to betweenabout 1 kPa and about 101 kPa. In at least one embodiment, the temperature of the aqueous solution of lithium bicarbonate is increased to about 77°C to about 82°C. In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at the increased temperature for a period of from about 15 minutes to about 90 minutes.

[0015] In at least one embodiment, the method comprises:a) exposing the aqueous mixture of the crude lithium carbonate to gas containing up to 21% by volume of carbon dioxide under a pressure of about 650 kPa to about 3000 kPa at a temperature of about -10°C to about 25°C for at least one exposure cycle comprising:introducing the gas containing carbon dioxide into a headspace of a vessel containing the aqueous mixture to provide a headspace gas; agitating the aqueous mixture;allowing the headspace gas to remain in contact with the aqueous mixture for up to 120 minutes; andventing at least a portion of the headspace gas from the vessel;b) separating the aqueous solution of lithium bicarbonate from the insoluble impurities; andc) reducing the pressure applied to the aqueous solution of lithium bicarbonate to a reduced pressure from about 1 kPa to about 101 kPa and / or increasing the temperature of the aqueous solution of lithium bicarbonate to an increased temperature of about 77°C to about 83°C.

[0016] At least one embodiment of the present method can have at least one of the following advantageous properties:• embodiments of the method can provide lithium carbonate at a level of purity suitable for use in the manufacture of lithium batteries;• embodiments of the method can conveniently and cost-effectively use flue gas effluent in the carbonation process, avoiding the cost of using purified carbon dioxide;• embodiments of the method can prevent release of carbon dioxide in flue gas effluent to the environment, reduce lifecycle greenhouse gas emissions and / or increase eligibility for carbon tax credits where available; andembodiments of the method can produce carbon dioxide gas of increased purity which is suitable for re-sale or re-use.Brief Description of the Drawings

[0017] Further features of the present invention will become apparent from the following written description and the accompanying figures, in which:

[0018] Figure 1 is a diagrammatic illustration of an apparatus used to carry out an embodiment of the present method;

[0019] Figure 2 is a graph showing the variation of the concentration of lithium bicarbonate equivalents ([LiHCO3]e) in g per kilogram of water in an aqueous solution formed by reacting carbon dioxide with a slurry of Li2CO3in water at varying temperatures with the pressure of carbon dioxide used in the reaction; and

[0020] Figure 3 is a graph showing the variation of the composition and pressure of constructed flue gas sampled from the headspace of a vessel of the apparatus of Figure 1 with the number of cycles of reaction with a slurry of crude Li2CO3in water.Detailed Description

[0021] One aspect of the present application provides a method of purifying crude lithium carbonate, including exposing an aqueous mixture of the crude lithium carbonate to a gas containing carbon dioxide under pressure to provide an aqueous solution of lithium bicarbonate, according to the reaction indicated by the chemical equation below:Li2CO3(s) + CO2(g) + H2O(I) - ► 2 LiHCO3(aq)

[0022] As used herein, the term “aqueous mixture” is intended to mean a mixture containing water and at least one other component, which may be soluble, partially soluble or insoluble in the water. Aqueous mixtures include but are not limited to solutions of one or more soluble components in water, slurries of water and one or more partially soluble or insoluble components and mixtures containing water and a plurality of soluble, partially soluble and / or insoluble components.

[0023] In at least one embodiment, the crude lithium carbonate contains at least about 75% lithium carbonate by weight. In at least one embodiment, the crude lithium carbonate contains up to about 99% lithium carbonate by weight. In at least one embodiment, the crude lithium carbonate contains about 90% to about 99% lithium carbonate by weight. In at least one embodiment, the crude lithium carbonate contains from about 93% to about 97% lithium carbonate by weight. In at least one embodiment,the aqueous mixture of the crude lithium carbonate is a slurry. In at least one embodiment, the aqueous mixture of the crude lithium carbonate contains about5% (w / w) to about 15% (w / w) of the crude lithium carbonate. In at least one embodiment, the aqueous mixture of the crude lithium carbonate contains from about 6% (w / w) to about 10% (w / w) of the crude lithium carbonate.

[0024] In at least one embodiment, the gas containing carbon dioxide contains up to 21% (v / v) of carbon dioxide. In at least one embodiment, the gas containing carbon dioxide contains up to 15% (v / v) of carbon dioxide. In at least one embodiment, the gas containing carbon dioxide contains between about 1% (v / v) and about 21% (v / v) of carbon dioxide. In at least one embodiment, the gas containing carbon dioxide contains between about 5% (v / v) and about 15% (v / v) of carbon dioxide. In at least one embodiment, the gas containing carbon dioxide is a flue gas effluent, including but not limited to flue gas effluent produced during the calcination of a-spodumene to form P-spodumene.

[0025] As used herein, the terms “exposing” or “exposed” in the context of an aqueous mixture of the crude lithium carbonate being exposed to a gas containing carbon dioxide under pressure are intended to mean that the aqueous mixture and the gas are brought into contact with each other such that the carbon dioxide present in the gas can react with the lithium carbonate contained in the aqueous mixture. In at least one embodiment, the aqueous mixture can be stirred or agitated in the presence of the gas, or the gas can be bubbled through the aqueous mixture, to increase the surface area of contact between the aqueous mixture and the gas.

[0026] In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises introducing the gas containing carbon dioxide into a headspace of a vessel containing the aqueous mixture of the crude lithium carbonate to provide a headspace gas at a headspace pressure. In at least one embodiment, the headspace pressure of the gas is from about 650 kPa to about 3000 kPa. In at least one embodiment, the headspace pressure of the gas is from about 650 kPa to about 2600 kPa. In at least one embodiment, the headspace pressure of the gas is from about 650 kPa to about 800 kPa. In at least one embodiment, the headspace pressure of the gas is from about 690 kPa to about 800 kPa.

[0027] In at least one such embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure further comprises allowing the headspace gas to remain in contact with the aqueous mixture for a predetermined period of time. The predetermined period of time is selected to allow the carbon dioxide in the headspace gas to react effectively with the lithium carbonate in the aqueous mixture and can be readily determined by the skilled person. In at least one embodiment, the predetermined period of time can be determined by measuring the pressure of the headspace gas and determining the time at which the pressure reaches a substantially minimum value. In at least one alternative embodiment, the predetermined period of time can be determined by taking samples of the headspace gas over time and determining the time at which the concentration of carbon dioxide reaches a substantially minimum value.

[0028] In at least one embodiment, the predetermined period of time has a duration of up to 120 minutes. In at least one embodiment, the predetermined period of time has a duration of up to 90 minutes. In at least one embodiment, the predetermined period of time has a duration of up to 60 minutes. In at least one embodiment, the predetermined period of time has a duration of about 10 minutes to about 120 minutes. In at least one embodiment, the predetermined period of time has a duration of about 15 minutes to about 90 minutes. In at least one embodiment, the predetermined period of time has a duration of about 15 minutes to about 60 minutes. In at least one embodiment, the predetermined period of time has a duration of about 30 minutes. In at least one embodiment, the predetermined period of time has a duration of about 60 minutes. In at least one embodiment, the predetermined period of time has a duration of about 120 minutes.

[0029] In at least one embodiment, allowing the headspace gas to remain in contact with the aqueous mixture for a predetermined period of time further comprises agitating the aqueous mixture. The agitation can facilitate reaction by increasing the surface area of contact between the carbon dioxide and the lithium carbonate and can be carried out by methods well known in the art including but not limited to shaking, swirling, vibrating and stirring, and / or by agitators well known in the art, including but not limited to impellers, propellers, turbines, paddles, blades, magnetic stirrers and other agitators known to the skilled person.

[0030] In at least one such embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out one or more exposure cycles, wherein each exposure cycle comprises:introducing the gas containing carbon dioxide into the headspace of the vessel; allowing the headspace gas to remain in contact with the aqueous mixture for the predetermined period of time; andventing at least a portion of the headspace gas from the vessel.

[0031] In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out at least 1 exposure cycle. In at least one such embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out a plurality of exposure cycles. In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out at least 2 exposure cycles. In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out at least 5 exposure cycles. In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out at least 6 exposure cycles. In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out at least 7 exposure cycles. In at least one embodiment, exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out from 1 to 18 exposure cycles.

[0032] In at least one embodiment, the temperature at which the aqueous mixture of Li2CO3is exposed to the gas is from about -10°C to about 25°C. In at least one embodiment, the temperature at which the aqueous mixture of Li2CO3is exposed to the gas is from about 0°C to about 4°C. In at least one embodiment, the temperature at which the aqueous mixture of Li2CO3is exposed to the gas is about 2°C. In at least one embodiment, the temperature at which the aqueous mixture of Li2CO3is exposed to the gas is about 1°C.

[0033] In at least one embodiment, the aqueous mixture of Li2CO3is exposed to the gas containing up to 21% (v / v) of carbon dioxide at a pressure of about 650 kPa to about3000 kPa and at a temperature of about -10°C to about 25°C for at least one exposure cycle as described herein. In at least one embodiment, the aqueous mixture of U2CO3 is exposed to the gas containing from about 5% to about 15% (v / v) of carbon dioxide at a pressure of about 600 kPa to about 2600 kPa and at a temperature of about 0°C to about 4°C for about 1 to about 18 exposure cycles. In at least one embodiment, the aqueous mixture of Li2CO3is exposed to the gas containing about 15% (v / v) of carbon dioxide at a pressure of about 690 kPa to about 790 kPa and at a temperature of about 0°C to about 4°C for about two to about seven exposure cycles.

[0034] The method further includes separating the aqueous solution of lithium bicarbonate (LiHCO3) from insoluble impurities. In at least one embodiment, separating the aqueous solution of lithium bicarbonate from insoluble impurities comprises decanting the aqueous solution from the insoluble impurities. In at least one embodiment, the mixture containing the aqueous solution of lithium bicarbonate and the insoluble impurities can be centrifuged prior to decanting the aqueous solution from the insoluble impurities. In at least one embodiment, separating the aqueous solution of lithium bicarbonate from insoluble impurities comprises filtration of the aqueous solution. In at least one embodiment, separating the aqueous solution of lithium bicarbonate from insoluble impurities comprises passing the aqueous solution through one or more filters. In at least one embodiment, the solution is filtered through one or more filters having a pore diameter ranging from 40 pm to 0.5 pm. In at least one embodiment, the solution is filtered sequentially through a series of filters having pore diameters of 10 pm, 7 pm and 0.5 pm, respectively. In at least one embodiment, the solution is filtered sequentially through a series of filters having pore diameters of 40 pm, 15 pm and 7 pm, respectively. In at least one embodiment, the solution is filtered sequentially through a series of filters having pore diameters of 40 pm and 7 pm, respectively. In at least one embodiment, the solution is filtered sequentially through a series of filters having pore diameters of 40 pm and 2 pm, respectively. In at least one embodiment, the solution is filtered sequentially through a series of filters having pore diameters of 10 pm and 0.5 pm, respectively.

[0035] The method further includes reducing the pressure applied to the aqueous solution of lithium bicarbonate and / or increasing the temperature of the aqueous solution of lithium bicarbonate, to produce a mixture comprising a residual aqueous solution, solid lithium carbonate and gaseous carbon dioxide. Without being bound by theory, it is contemplated that reducing the pressure applied to the aqueous solution of lithiumbicarbonate and / or increasing the temperature of the aqueous solution of lithium bicarbonate will destabilize the LiHCO3present in the solution, causing the production of CO2and Li2CO3according to the reaction indicated by the chemical equation below:2 LiHCO3(aq) - ► Li2CO3(s) + CO2(g) + H2O(I)Thus, as CO2is formed in this reaction, it will be evolved from the mixture as a gas and can be captured. In addition, as Li2CO3is formed in the reaction, it will crystallize or precipitate from the mixture as a solid and can be recovered. Furthermore, any soluble impurities present in the aqueous solution of lithium bicarbonate will remain dissolved in the residual aqueous solution which can be separated from both the gaseous CO2and the solid Li2CO3.

[0036] In at least one embodiment, the pressure applied to the aqueous solution of lithium bicarbonate is reduced to between about 1 kPa and about 101 kPa. In at least one embodiment, the aqueous solution of lithium bicarbonate is allowed to pass from a pressurized vessel to a depressurized vessel at a pressure lower than the pressure of the pressurized vessel. In at least one embodiment, the pressure in the pressurized vessel is between about 650 kPa and about 3000 kPa. In at least one embodiment, the pressure in the depressurized vessel is between about 1 kPa and about 101 kPa. In at least one embodiment, the aqueous solution of lithium bicarbonate is allowed to pass from the pressurized vessel to the depressurized vessel through one or more filters as described herein.

[0037] In at least one embodiment, the temperature of the aqueous solution of lithium bicarbonate is increased. In at least one embodiment, the temperature of the aqueous solution of lithium bicarbonate is increased to about 350 K (about 77°C) to about 355 K (about 82°C). In at least one embodiment, the temperature of the aqueous solution of lithium bicarbonate is increased to about 353 K (about 80°C). In at least one embodiment, the temperature of the aqueous solution of lithium bicarbonate is increased and the pressure applied to the aqueous solution of lithium bicarbonate is reduced.

[0038] In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at the increased temperature for a period of up to about 90 minutes. In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at the increased temperature for a period of from about 15 minutes to about 90 minutes. In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at theincreased temperature for a period of from about 25 minutes to about 80 minutes. In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at the increased temperature for a period of from about 30 minutes to about 60 minutes. In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at the increased temperature for a period of about 30 minutes. In at least one embodiment, the aqueous solution of lithium bicarbonate is maintained at the increased temperature for a period of about 60 minutes.

[0039] The method further includes capturing the gaseous carbon dioxide. In at least one embodiment, the gaseous carbon dioxide has a purity of at least 94% (v / v).ln at least one embodiment, the gaseous carbon dioxide has a purity of at least 95% (v / v). In at least one embodiment, the gaseous carbon dioxide has a purity of at least 97% (v / v). In at least one embodiment, the gaseous carbon dioxide has a purity of at least 99% (v / v). In at least one embodiment, the gaseous carbon dioxide has a purity of at least 99.5% (v / v). It will be apparent to the person of skill in the art that in embodiments in which the aqueous solution of lithium bicarbonate is transferred from a pressurized vessel to a depressurized vessel, including embodiments in which the aqueous solution of lithium bicarbonate is passed through one or more filters, the aqueous solution of lithium bicarbonate should be transferred so as to prevent passage of the gas containing carbon dioxide from the pressurized vessel to the depressurized vessel. In this way, contamination of the captured gaseous carbon dioxide by the gas containing carbon dioxide can be avoided.

[0040] The method further includes separating the solid lithium carbonate from the residual aqueous solution to provide purified solid lithium carbonate. In at least one embodiment, the purified solid lithium carbonate has a purity of at least 95% (w / w). In at least one embodiment, the purified solid lithium carbonate has a purity of at least 97% (w / w). In at least one embodiment, the purified solid lithium carbonate has a purity of at least 99% (w / w). In at least one embodiment, the purified solid lithium carbonate has a purity of at least 99.5% (w / w). In at least one embodiment, the purified solid lithium carbonate has a purity of at least 99.8% (w / w).

[0041] As used herein, the terms “a” and “an” are intended to mean “at least one”, and include both singular and plural, unless otherwise indicated.

[0042] As used herein, the terms “about” or “approximately” as applied to a numerical value or range of values are intended to mean that the recited values can vary within anacceptable degree of error for the quantity measured given the nature or precision of the measurements, such that the variation is considered in the art as equivalent to the recited values and provides the same function or result. For example, the degree of error can be indicated by the number of significant figures provided for the measurement, as is understood in the art, and includes but is not limited to a variation of ±1 in the most precise significant figure reported for the measurement. Typical exemplary degrees of error are within 20 percent (%), preferably within 10%, and more preferably within 5% of a given value or range of values. Alternatively, and particularly in biological systems, the terms "about" and "approximately" can mean values that are within an order of magnitude, preferably within 5-fold and more preferably within 2-fold of a given value. Numerical quantities given herein are approximate unless stated otherwise, meaning that the term "about" or "approximately" can be inferred when not expressly stated.

[0043] As used herein, the term “substantially” refers to the complete or nearly complete extent or degree of an action, characteristic, property, state, structure, item, or result. For example, an object that is “substantially” in a given position including but not limited to vertical, horizontal, or adjacent to or aligned with another object, would mean that the object is either completely in that position or nearly completely in that position. The exact allowable degree of deviation from absolute completeness may in some cases depend on the specific context. However, generally speaking the nearness of completion will be so as to have the same overall result as if absolute and total completion were obtained.

[0044] The use of “substantially” is equally applicable when used in a negative connotation to refer to the complete or near complete lack of an action, characteristic, property, state, structure, item, or result. For example, a composition that is “substantially” free of an ingredient or element would either completely lack that ingredient or element, or so nearly completely lack that ingredient or element that the effect would be the same as if it completely lacked that ingredient or element. In other words, a composition that is “substantially” free of an ingredient or element may still actually contain such item as long as there is no measurable or significant effect thereof.

[0045] As used herein, terms indicating relative direction or orientation, including but not limited to “upper”, “lower”, “top”, “bottom”, “vertical”, “horizontal”, “outer”, “inner”, “front”, “back”, and the like, are intended to facilitate description of the present invention by indicating relative orientation or direction in usual use, and are not intended to limit the scope of the present invention in any way to such orientations or directions.EXAMPLES

[0046] Other features of the present invention will become apparent from the following non-limiting examples which illustrate, by way of example, the principles of the invention.Example 1: Process parameters and executionPreparation of materials:

[0047] Cylinders of pure CO2and constructed flue gas were provided by Air Liquide. Constructed flue gas consisted of nitrogen (N2, 81.0 vol%), CO2(15.0 vol%), oxygen (O2, 3.0 vol%), carbon monoxide (CO, 1.0 vol%), and nitrogen dioxide (NO2, 500 ppmv). Water vapour, although a common flue gas component, was considered unnecessary and impractical within the mixture owing to the use of water in the process slurry. Certain trace gas components, such as SO2and HCI were also omitted to improve chemical compatibility and operational safety.

[0048] The crude solid lithium carbonate feed (93.4 wt% Li2CO3) was prepared by mixing the following reagent-grade salts, obtained from Fisher Scientific, together with 2.0 L of de-ionized water in a 4-L Pyrex beaker: Li2CO3(950.0 g), NaCI (10.0 g), KCI (10.0 g), MgCI2-6H2O (10.0 g), CaSO4(10.0 g), Na2B4O7-10H2O (5.0 g), and Fe(NO3)39H2O (5.0 g). The resulting slurry was stirred occasionally while heating to 80°C over several days until a dry solid was obtained. The solid was pulverized, homogenized, then passed through a 500-pm sieve, producing a beige powder, which was stored in a well-sealed container to prevent moisture uptake prior to use.Apparatus:

[0049] The apparatus used for the batch process is shown in Figure 1. A cylinder of pressurized source gas (either pure CO2or constructed flue gas) is connected at gas inlet 12 to a CO2-absorption reactor 10, in this case a jacketed, glass pressure vessel 14 (Parr Instruments, model 5112) having an internal volume of 1.5 L. The reactor vessel 14 was charged with crude feed (15.00 g, 93.4 wt% Li2CO3) and de-ionized water (225 mL, 18.2 MQ-cm), leaving ca. 1.3 L of headspace. The mixture in the reactor vessel 14 was maintained at 2°C and stirred by a bladed impeller 16 at 200 RPM under an atmosphere of N2(101 kPa). A dip-tube 18 connected to a 10 pm filter 20 within the reactor leads to an external fluid-transfer line 22, sequentially through a 7 pm filter 24 and a 0.5 pm filter 26, and into a pre-evacuated CO2-desorption vessel 30, in this case a sealed, 250-mL,3-necked, round-bottom flask 32 immersed within in a water bath 34 atop a heating / stirring plate 36. The desorption vessel 32 was pre-evacuated through vent 38 to remove environmental gases that would otherwise contaminate desorbed CO2. The vessel 30 was initially kept at ambient temperature, under static vacuum, and was fitted with a vacant gas-collection bag 40 (8-L capacity) to capture the desorbed gas.Water solubility of Li2CO3 / LiHCO3

[0050] A mass of pure Li2CO3(30.00 g) in a volume of de-ionized water (225 mL) was stirred at 200 RPM in the CO2-absorption reactor 10 maintained under varying pressure (ca. 0, 400, or 800 kPa; Pco2) of pure CO2at varying temperature (275 K or 295 K) for 18 h. The aqueous phase 50 (ca. 215 mL) was transferred from the reactor vessel 14 via the dip-tube 18 through the series of filters 20 and 24, 26 in the fluid-transfer line 22 and into a pre-weighed jar (not shown in Figure 1). The jar was re-weighed to determine the mass of the aqueous phase collected, then left open until its water content had completely evaporated. The jar was again re-weighed to determine both the mass of water that had evaporated and the mass of Li2CO3remaining therein. The latter was used to calculate the equivalent mass of LiHCO3originally dissolved, expressed as the concentration of LiHCO3equivalents ([LiHCO3e]; g-kgwater"1) initially in the solution. Solubility tests were performed in triplicate. The results are presented in Table 1 below and in Figure 2.Table 1 : Variation of LiHCO3solubility in water

[0051] As can be seen from the results presented above and in Figure 2, the concentration of LiHCO3equivalents in the aqueous solution increased with increasing pressure of CO2and with decreasing temperature. Without wishing to be bound by theory, these results are consistent with a shift in the equilibrium reaction indicatedbelow towards the formation of the more water-soluble product LiHCO3as the pressure of the reagent CO2increases.Li2CO3(s) + CO2(g) + H2O(I) < - ► 2 LiHCO3(aq)Furthermore, an increase in temperature is contemplated to shift the equilibrium in the opposite direction towards formation of the reactants, as the thermally unstable LiHCO3decomposes to form Li2CO3and CO2. Thus, decreasing the temperature is contemplated to also favour production of LiHCO3.Optimization of conditions

[0052] Process conditions were identified that best balanced operational feasibility with the highest quantities of Li2CO3that could be dissolved and recovered. Optimal process feed quantities for each batch were found to be 15.00 g of crude salt (about 193 mmol at 93.4 wt% Li2CO3), 225 g of de-ionized water, and about 450 mmol of CO2. During the conventional process variant, pressurization of the 1.3-L headspace of the reactor with pure CO2to 790 kPa at 275 K (450 mmol CO2) was sufficient to completely dissolve the Li2CO3content of the crude salt while stirring with an impeller at 200 RPM for 30 min. However, the flue gas process variant required 7 times this quantity of gas to completely dissolve the Li2CO3content of the crude feed under these conditions owing to its lower CO2content (15.0 vol%). As a result, the flue gas process variant required 7 cycles of reactor pressurization and venting.Process Execution

[0053] The reactor vessel 14 was pressurized with source gas (pure CO2or constructed flue gas) to 790 kPa through gas inlet 12, the CO2content of which was partially absorbed into the water where it reacted with Li2CO3, causing the internal pressure of the reactor vessel 14 to gradually decrease (e.g., to 690 kPa over 30 min, as with flue gas). Once the pressure stopped decreasing, the headspace 52 of the reactor was sampled before venting any excess pressure (e.g., partially scrubbed flue gas) to the atmosphere through vent 28. When flue gas was used, this pressurization cycle was repeated 6 more times (for 7 cycles total) until the feed stopped dissolving and the internal pressure stopped decreasing. When pure CO2was used, no further pressurization cycles were needed (1 cycle total). A valve 21 on the fluid-transfer line 22 was opened to allow nearly the entire volume of aqueous phase 50 in the reactor(ca. 215 mL) to flow through the series of filters 20, 24, 26 and into the vacant CO2-desorption vessel 32, leaving solid residue 54 in the reactor vessel 14. Once fluid transfer was complete, the valve 21 was closed and the vessel containing the aqueous filtrate 56 (F1) was gently heated to 353 K for 30 min, which caused precipitation of a white solid 58 from the aqueous phase as CO2evolved from the solution and accumulated within the gas-collection bag 40.Example 2: Analysis of CO2captured from reaction of crude l_i2CO3with flue gas

[0054] Gas sample composition was determined via gas chromatography with thermal conductivity detection (GC-TCD) using an Agilent 3000 Micro GO instrument. The volume of captured CO2was determined using a Ruska gasometer.

[0055] During the CO2-absorption step of the process described in Example 1 wherein the crude Li2CO3feed was exposed to flue gas, 7 portions (cycles) of flue gas were sequentially supplied to the reactor vessel 14, where each was partially scrubbed before venting. During each such cycle, the reactor was pressurized with flue gas through gas inlet 12, then sealed off from the gas source. After 30 min, the headspace 52 was sampled and vented through vent 28 prior to beginning the next absorption cycle. The pressure and composition of the partially scrubbed and vented gas portions obtained over the series of absorption cycles are shown in Figure 3.

[0056] As shown in Figure 3, the headspace pressure within the reactor decreased as the CO2content of the flue gas dissolved. The CO2content of the scrubbed gas portion vented from the first cycle (#1) was found to be 3.7 vol% CO2, indicating that 75% of the original CO2content of the flue gas was removed by absorption into the slurry during that cycle. As absorption cycling progressed, the CO2content of the partially scrubbed gas portion increased, approaching that of the original flue gas (15.0 vol%). Without being bound by theory, it is contemplated that the effectiveness of CO2scrubbing from the flue gas decreased as absorption cycling progressed and the amount of unreacted Li2CO3in the reactor decreased.

[0057] Following the CO2absorption step, the aqueous phase was separated from the slurry in the reactor by transferring it under the applied pressure differential through a series of filters to the desorption vessel, where its CO2content was stripped under reduced pressure and elevated temperature and collected in a gas-collection bag, asdescribed in Example 1. Once CO2stopped evolving from the mixture in the desorption vessel, the gas-collection bag was sealed, heating was discontinued, and the volume and composition of the captured gas were determined. The gas evolved during this desorption step was found to consist almost entirely of CO2(99.6 vol%) but did contain traces of N2and O2(0.3 and 0.1 vol%, respectively), attributable to the source gas. The volume of captured gas, measured at 295 K and 101 kPa, was found to be 3.48 L (144 mmol CO2), which represented 31% of the CO2contained in the total quantity of flue gas used during this process (472 mmol).Example 3: Compositional analysis of product and by-product fractions from purification of crude l_i2CO3with flue gas

[0058] After pressurizing the reactor with flue gas in each cycle of the CO2-absorption step of the process described in Example 1, stirring was discontinued to allow the suspended solid to settle over 30 min. Visual observation revealed not only that the quantity of undissolved solid decreased as cycling progressed, but also that the colour of the settled solid darkened concomitantly. The crude feed itself was a light-coloured, off-white or beige powder, the slight pigmentation of which is believed to be due to the Fe3+content (647 ppm) of the crude feed. Upon adding de-ionized water to the feed, the colour of the feed did not change perceptibly. Upon repeated exposure to CO2and transfer of the resulting aqueous phase to the desorption vessel, as described in Example 1 , the solid portion of the reaction mixture gradually darkened to a brownorange colour. After the final cycle of exposure to flue gas and transfer of the aqueous filtrate to the desorption vessel, the solid by-product fraction (B1) remaining in the CO2-absorption vessel was recovered, dried, and weighed prior to compositional analysis.

[0059] Following filtration and transfer of the aqueous phase (fraction F1) to the vacant desorption vessel as described in Example 1, the filtrate F1 was heated briefly to allow evolution of CO2and precipitation of Li2CO3. After the mixture had stopped effervescing, the heat source was removed. Once the vessel had cooled to ambient temperature, the mixture was quantitatively transferred, using minimal de-ionized water (ca. 10 mL), onto an 11-pm filter paper (Whatman #1) in a vacuum-filtration apparatus to separate the purified, solid, product fraction (P1) from the second aqueous filtrate (F2) containing dissolved impurities. Product fraction P1 was rinsed with minimal de-ionized water, airdried, and weighed prior to compositional analysis.

[0060] Filtrate F2 was allowed to stand in an open vessel at ambient temperature for several days until 70 wt% of the original water content had evaporated, causing a second product fraction (P2) to crystallize. This extent of evaporation from F2 was chosen to optimize the balance between recovery and purity of P2, as informed by the results of tests whereby fraction F2 was divided into equal-volume aliquots among a series of vessels, each of which was then allowed to stand in an open vessel for varying periods of time to allow for varying amounts of water to evaporate. The supernatant was decanted from fraction P2 and was allowed to stand in an open vessel until evaporation of the water content was complete, yielding a second solid by-product fraction (B2).

[0061] After each product or by-product fraction was obtained as a dry powder, its mass and composition were measured to determine its recovery and purity. Product recovery (RP, in wt% Li2CO3) was determined according to Equation 1, where m denotes the mass, and X denotes the purity (in wt% Li2CO3) of a fraction or feed:Equation 1: RP = 100Sample contents of the elements Ag, Al, As, B, Ba, Be, Ca, Cd, Ce, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Se, Si, Sn, Sr, Ti, Tl, V and Zn were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of a microwave acid-digested sample solution (ca. 250 mg in 50.0 mL of 20% HNO3) using a Thermo Scientific iCAP™ PRO X Duo instrument in accordance with the method of Bartsch, N. (Spectrosc. Asia (2020), 15: 17-18). Sample contents of the elements C, H, and N were determined via combustion analysis using an elemental analyzer (Elementar) as per American Standards for Testing and Materials (ASTM) method D5291. Sample contents of the elements O and Cl were determined using an elemental analyzer (Elementar) using in-house methods. Recovered fraction purity (X, wt% Li2CO3) was determined from total measured contents of impurity elements. Product purity (XP) was calculated as per Equation 2.Equation 2:

[0062] The results of the analysis are shown in Table 2 below.Table 2: Composition of feed and recovered product (P1, P2) and by-product (B1, B2) fractions

[0063] As seen from the results presented in Table 2, the total mass of product obtained in product fractions P1 and P2 (n?pi + mp2 = 11.89 g) had a purity of 99.5 wt% Li2CO3, indicating a good recovery (84.4 wt% Li2CO3) of a battery-grade product. Product fraction P1 was of particularly high purity (99.7 wt% Li2CO3). Compared to the feed, its concentrations of major contaminants (Na, K, Mg, Ca, B, Fe, Si) were reduced by 1-2 orders of magnitude. A comparison of compositional data for the various product and byproduct fractions listed in Table 2 revealed how the different process steps separated different types of ionic impurities from Li2CO3.

[0064] Notably, by-product fraction B1 (the undissolved solid remaining in the reactor after the CO2adsorption step and separation of filtrate F1) had greater concentrations of multivalent cationic impurities (Mg2+, Ca2+and Fe3+) and lower concentrations of monovalent impurities (Na+and K+) than fraction B2, obtained from filtrate F2, which was collected after CO2desorption. This observation confirmed that monovalent cations (including Li+) dissolved preferentially over multivalent cations in the aqueous phase and that even under pressure of CO2, the solubility of multivalent cations remained limited. During desorption of CO2from the aqueous phase, purified Li2CO3precipitated from solution, leaving other monovalent cationic impurities in the aqueous phase. Product recovery and purity did not differ significantly between the processes described in Example 1 wherein the crude Li2CO3feed was exposed to flue gas or to pure CO2. Example 4: General process for purification of crude lithium carbonate

[0065] A pressurizable absorption vessel, having an internal volume of 1.50-4.00 L, was charged with 15.0-60.0 g of crude, solid feed containing 93.4-96.4 wt% lithium carbonate (Li2CO3). Pure deionized water (225-600 g) was added to the absorption vessel, which was then sealed. The resulting slurry mixture contained 6.25-9.09 wt% crude solid feed and had a volume of 0.23-0.63 L, leaving a headspace volume in the vessel of 1.27-3.38 L. The slurry in the absorption vessel was continuously mixed by an impeller rotating at a frequency of 200-500 min-1and the internal temperature of the absorption vessel was maintained at 1-2°C.

[0066] Source gas containing 5-15 vol% carbon dioxide (CO2), in addition to nitrogen (N2) and other combustion exhaust gas components such as oxygen (O2), carbon monoxide (CO), and nitrogen dioxide (NO2), was introduced into the absorption vessel through a valve until the pressure within the headspace reached 791-2,514 kPa. The lithium carbonate slurry was stirred in the presence of the headspace gas for 10-120 min and a sample of the spent headspace gas was transferred into a gas-sampling bag for compositional analysis by gas chromatography with thermal conductivity detection (GC-TCD). The spent headspace gas was then vented to the atmosphere until the headspace pressure in the absorption vessel reached atmospheric pressure, about 101 kPa. The process of introducing fresh source gas into the headspace, stirring the slurry in the presence of the headspace gas and venting the spent headspace gas was repeated for up to 18 absorption cycles or until the concentration of CO2measured in thesample of spent headspace gas approached the concentration in the fresh source gas, indicating that no further CO2was being consumed in the absorption cycle. In the overall process including all absorption cycles carried out, 0.46-1.12 mol of CO2were provided in the headspace gas and 0.21-0.74 mol CO2were vented to the atmosphere.

[0067] Impeller mixing was discontinued and a portion (0.22-0.50 L) of the aqueous phase of the slurry was transferred via a dip tube positioned in the absorption vessel, through a series of successively finer filters into an external desorption vessel with an initial internal pressure of 1-101 kPa and an internal volume of 0.30-0.60 L, leaving slurry residue B1 remaining in the absorption vessel. The coarsest filter has a pore diameter of 10-40 pm, and the finest filter has a pore diameter of 0.5-7.0 pm. Additional crude feed may be added to the slurry residue B1 in the absorption vessel and the mixture may be subjected to additional absorption cycles to improve the recovery of purified Li2CO3. Alternatively, the slurry residue B1 can be sent for disposal if its content of impurities is high enough (for example, greater than 25% impurities by weight) that recovery of further Li2CO3from the slurry residue is not cost effective.

[0068] The desorption vessel was heated to a temperature of 80-82°C for 26-90 min, over which time a white solid formed as a precipitate and a colourless gas evolved from the filtrate. The entire volume (3.48-9.93 L at 20°C and 101 kPa as measured by a gasometer) of evolved gas was captured in a gas-sampling bag. The captured gas contained 94.5-99.6 vol% of CO2as determined by GC-TCD analysis.

[0069] The absorption and desorption vessels were brought to a temperature of 20°C and unsealed. The product-rich slurry remaining in the desorption vessel after evolution of CO2was quantitatively transferred to a glass jar by washing with de-ionized water, then was concentrated to a volume of 150 mL by evaporation. The concentrated slurry was filtered using a vacuum filtration apparatus to separate the solid purified Li2CO3-rich product (P) from the filtrate (B2). The solid product P collected in the filtration apparatus was dried and was found to have a mass of 11.9-32.7 g, to contain 99.5-99.9 wt% Li2CO3, as determined by microwave-assisted acid digestion (MAAD) and inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, and to contain a recovery of 56.4-84.4 wt% of the Li2CO3contained in the original crude feed.

[0070] The slurry residue B1 remaining in the absorption vessel was quantitatively transferred to a glass jar by rinsing with de-ionized water, and the water was removed byevaporation, leaving a mass of solid B1 of 1.4-19.3 g, containing 85.6-97.1 wt% of Li2CO3. Similarly, the filtrate B2 was quantitatively transferred from the vacuum filtration apparatus to another glass jar by rinsing with de-ionized water, and the water was removed by evaporation, leaving a mass of solid B2 of 1.4-6.5 g, containing62.8-81.1 wt% of Li2CO3. Thus, the total recovery of Li2CO3from the original crude feed was 91.0-99.5 wt%.

[0071] The embodiments described herein are intended to be illustrative of the present compositions and methods and are not intended to limit the scope of the present invention. Various modifications and changes consistent with the description as a whole and which are readily apparent to the person of skill in the art are intended to be included. The appended claims should not be limited by the specific embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

CLAIMS1. A method of purifying crude lithium carbonate comprising:a) exposing an aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure to provide an aqueous solution of lithium bicarbonate;b) separating the aqueous solution of lithium bicarbonate from insoluble impurities;c) reducing the pressure applied to the aqueous solution of lithium bicarbonate and / or increasing the temperature of the aqueous solution of lithium bicarbonate, to produce a mixture comprising a residual aqueous solution, solid lithium carbonate and gaseous carbon dioxide;d) capturing the gaseous carbon dioxide; ande) separating the solid lithium carbonate from the residual aqueous solution to provide purified solid lithium carbonate.

2. The method of claim 1 wherein the gas containing carbon dioxide is flue gas effluent.

3. The method of claim 1 or 2 wherein the gas containing carbon dioxide contains from about 1% to about 21% by volume of CO2.

4. The method of any one of claims 1 to 3 wherein the crude lithium carbonate contains up to 99% lithium carbonate by weight.

5. The method of any one of claims 1 to 4 wherein the aqueous mixture of the crude lithium carbonate contains from about 5% to about 15% by weight of the crude lithium carbonate.

6. The method of any one of claims 1 to 5 wherein the aqueous mixture is a slurry.

7. The method of any one of claims 1 to 6 wherein the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises introducing the gas containing carbon dioxide into a headspace of a vessel containing the aqueous mixture to provide a headspace gas at a headspace pressure.

8. The method of claim 7 wherein the headspace pressure is from about 650 kPa to about 3000 kPa.

9. The method of claim 7 or 8 wherein the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure further comprises allowing the headspace gas to remain in contact with the aqueous mixture for a predetermined period of time.

10. The method of claim 9 wherein the predetermined period of time is up to about 120 minutes.

11. The method of claim 9 or 10 further comprising agitating the aqueous mixture.

12. The method of any one of claims 9 to 11 wherein the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out one or more exposure cycles, wherein each exposure cycle comprises:introducing the gas containing carbon dioxide into the headspace of the vessel; allowing the headspace gas to remain in contact with the aqueous mixture for the predetermined period of time; andventing at least a portion of the headspace gas from the vessel.

13. The method of claim 12 wherein the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises carrying out a plurality of exposure cycles.

14. The method of any one of claims 1 to 6 wherein the step a) of exposing the aqueous mixture of the crude lithium carbonate to gas containing carbon dioxide under pressure comprises bubbling the gas containing carbon dioxide through the aqueous mixture.

15. The method of any one of claims 1 to 14 wherein the aqueous mixture of the crude lithium carbonate is exposed to the gas containing carbon dioxide at a temperature of from about -10°C to about 25°C.

16. The method of any one of claims 1 to 15 wherein the aqueous solution of lithium bicarbonate is separated from the insoluble impurities by filtration.

17. The method of any one of claims 1 to 16 wherein the pressure applied to the aqueous solution of lithium bicarbonate is reduced to between about 1 kPa and about 101 kPa.

18. The method of any one of claims 1 to 17 wherein the temperature of the aqueous solution of lithium bicarbonate is increased to an increased temperature of about 77°C to about 82°C.

19. The method of claim 18, wherein the aqueous solution of lithium bicarbonate is maintained at the increased temperature for about 15 minutes to about90 minutes.

20. The method of claim 1 comprising:a) exposing the aqueous mixture of the crude lithium carbonate to gas containing up to 21% by volume of carbon dioxide under a pressure of about 650 kPa to about 3000 kPa at a temperature of about -10°C to about 25°C for at least one exposure cycle comprising:introducing the gas containing carbon dioxide into a headspace of a vessel containing the aqueous mixture to provide a headspace gas;agitating the aqueous mixture;allowing the headspace gas to remain in contact with the aqueous mixture for up to 120 minutes; andventing at least a portion of the headspace gas from the vessel;b) separating the aqueous solution of lithium bicarbonate from the insoluble impurities; andc) reducing the pressure applied to the aqueous solution of lithium bicarbonate to a reduced pressure from about 1 kPa to about 101 kPa and / or increasing the temperature of the aqueous solution of lithium bicarbonate to an increased temperature of about 77°C to about 83°C.

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