Process and method for purifying lithium carbonate starting from an impure lithium chloride solution

Abstract

A method for purifying lithium from raw brine includes charging the raw brine to a feeder tank maintained at a temperature T1 and containing a carbonate source sufficient to precipitate any carbonate-forming solids in the raw brine to form a precipitation mixture and a crystal-free supernatant, and pumping the crystal-free supernatant from the feeder tank to a first crystallization reactor maintained at a temperature T2 to crystallize lithium carbonate salt from the crystal-free supernatant, wherein temperature T1 is less than temperature T2 and the flow rate is controlled to maintain a steady-state concentration of lithium carbonate salt in the solution phase of the crystallization reactor.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This non-provisional application claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Application No. 62 / 946,767, filed December 11, 2019, which is expressly incorporated herein by reference.

[0002] 1.Technical Field The present invention relates generally to a method for purifying high purity lithium carbonate from raw brine, and more particularly to a method for purifying high purity lithium carbonate from impure lithium chloride brine utilizing a lithium carbonate intermediate. [Background technology]

[0003] 2. Description of the Background Art The global lithium mining industry is struggling to keep up with the rapidly growing demand for battery-grade lithium carbonate raw material used to make cathodes and electrolytes in lithium-ion battery manufacturing. This demand is driven by the rapid adoption of battery, electric, and hybrid electric vehicles to reduce vehicle emissions, and the adoption of large lithium-ion batteries to balance the power supply and demand for renewable power generation systems such as solar, wind, and tidal.

[0004] "Battery grade" lithium carbonate is a generic term, but is typically used to indicate lithium carbonate purity greater than 99.5%. Lithium's properties make it one of the most attractive battery materials of all elements. Around the world, rechargeable lithium-ion batteries power most mobile phones and laptop computers, and most heavy-duty power tools, and automakers are adopting and deploying improved lithium-ion batteries for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Rechargeable lithium-ion batteries are also used in power grid storage applications.

[0005] Although lithium is a ubiquitous element, the majority of the world's lithium chemical supplies come from two main regions: the lithium-rich continental brine deposits of Chile and Argentina, and the hard rock spodumene pegmatite deposits of Western Australia. Most typically, the brine deposits are processed domestically to produce lithium carbonate chemicals, while the hard rock spodumene deposits are mined and concentrated domestically, and the concentrate is then shipped to China where it is further processed and converted to lithium hydroxide monohydrate.

[0006] More specifically, with regard to the brine deposits in Chile and Argentina, they are generally pumped to the surface and circulated through a series of evaporation ponds over a period of 12 to 24 months. Three main processes occur in these ponds: evaporation of water to the atmosphere and subsequent concentration, precipitation of various salt species when saturation limits are reached, and forced precipitation of various salt species when reagents are added to the ponds. Typically, the resulting lithium chloride-rich solution is then further purified by various industrial processes and then converted to the final lithium carbonate product. The classical processes commonly used to produce lithium carbonate can be difficult to control and frequently produce a relatively impure form of lithium carbonate called "industrial grade". This industrial grade lithium carbonate is suitable for some industrial applications of lithium, but is typically not suitable for use in modern lithium-ion batteries.

[0007] Typically within the industry, technical grade lithium carbonate is upgraded using CO2 or bicarbonation processes, which results in additional technical and operational complexities, as well as additional operational costs. At every stage of producing lithium carbonate using the industry standard batch crystallization process, it is difficult to form pure lithium carbonate crystals (without the inclusion of other salts).

[0008] Although conventional processing of brine to produce lithium-enriched brines or solutions allows for the removal of most of the interfering ions, there remains a need for simple removal of the interfering ions from raw solid isolated lithium salts to produce battery grade lithium carbonate. Current processes suffer from inefficiencies in terms of the energy, time, and materials required to process raw (industrial grade) lithium carbonate to a level of purity sufficient for modern battery applications.

[0009] To produce lithium carbonate of acceptable quality from the brine, techniques must be employed to remove certain cations and anions associated with the lithium in solution and then concentrate and extract the lithium.

[0010] Although not substantiated, it is accepted that lithium chloride crystals containing 0.07 wt. % Mg or more may be too high in magnesium for use in the production of lithium metal and subsequent applications in the manufacture of lithium organometallic compounds. Thus, the industry is looking for low magnesium organolithium catalysts in polymerization reactions. Lithium chloride with high magnesium content may also adversely affect the operation of lithium electrolytic cells when producing lithium metal.

[0011] Similarly, in producing lithium metal, sodium impurities in the lithium chloride crystals will appear directly in the metal. Thus, low sodium lithium salts are desirable. More than 0.6% sodium in the lithium chloride crystals by weight will produce a metal containing 1% or more sodium by weight. Sodium concentrations of about 1% by weight or greater in the lithium metal will make the lithium metal more reactive to natural components of air, making the metal more difficult and dangerous to handle. Table 1 provides data regarding sodium limits and tolerances in various lithium sources.

[0012] TIFF0007602275000001.tif35170

[0013] Commercial methods used to produce low-calorie lithium carbonate and lithium chloride on a commercial scale include the extraction of lithium compounds from mineral deposits such as spodumene-containing ores and natural brines. Many processes have been described and several commercialized to produce lithium carbonate from these sources.

[0014] One such commercial process involves the extraction of lithium from lithium-containing ores or brines to produce a pure lithium sulfate solution, as described, for example, in U.S. Patent No. 2,516,109, or a lithium chloride solution, as described, for example, in U.S. Patent No. 5,219,550. After purifying these solutions, sodium carbonate is added, either as a solid or in solution, to precipitate lithium carbonate crystals. The lithium carbonate is then filtered from the spent liquid (mother liquor), and the lithium carbonate is washed, dried, sized and packaged.

[0015] Lithium carbonate is often used as a feedstock to produce other lithium compounds such as lithium chloride, lithium hydroxide monohydrate, lithium bromide, lithium nitrate, lithium sulfate, lithium niobate, various lithium-containing cathode materials, and electrolyte salts. Lithium carbonate itself is used as an additive in the electrolytic production of aluminum to improve battery efficiency, and as a source of lithium oxide in the manufacture of glass, enamel, and ceramics. High purity lithium carbonate is used in medical and modern battery applications.

[0016] For example, a commercial procedure currently used to produce chemical grade lithium chloride is to react a lithium base, such as lithium carbonate or lithium hydroxide monohydrate, with concentrated hydrochloric acid to produce pure lithium chloride brine. The resulting lithium chloride brine is evaporated in a vacuum crystallizer at above 101 °C to produce anhydrous lithium chloride crystalline product. This procedure results in a product that meets most commercial specifications for chemical grade lithium chloride, but not most commercial specifications for low sodium grade lithium chloride.

[0017] Chemical grade lithium chloride is suitable for air drying applications, as a flux, an intermediate in the manufacture of mixed ion exchanged zeolites, and as a feed to electrolytic cells to produce chemical grade lithium metal.

[0018] Chemical grade lithium metal is used, among other things, to produce lithium organometallic compounds, which are used as catalysts in the polymerization and pharmaceutical industries.

[0019] Chemical grade anhydrous lithium chloride should contain less than 0.16% sodium to produce a metal containing less than 1% sodium. The importance of minimizing the sodium content in the metal and its associated costs is the primary reason for using lithium hydroxide monohydrate or lithium carbonate as a feedstock for producing lithium chloride and subsequently lithium metal. With this in mind, low sodium lithium chloride typically contains less than 0.0008% sodium by weight and is commercially produced to produce low sodium lithium metal suitable for battery applications and alloy production.

[0020] Commercially, low-sodium lithium chloride is produced indirectly from chemical grade lithium carbonate, which is converted to lithium hydroxide monohydrate by reaction with hydrated lime. The resulting slurry contains precipitated calcium carbonate and a 2-4 wt.% lithium hydroxide solution, which are separated by filtration.

[0021] The lithium hydroxide solution is concentrated in a vacuum evaporative crystallizer, causing lithium hydroxide monohydrate to crystallize, leaving soluble sodium in the mother liquor solution.

[0022] Crystalline lithium hydroxide monohydrate is separated from the mother liquor and dried. This salt typically contains 0.02-0.04% sodium. To further reduce the sodium level, the lithium hydroxide monohydrate must be dissolved in pure water, recrystallized, and subsequently reacted with pure hydrochloric acid to form concentrated lithium chloride brine containing less than 10 ppm sodium. The resulting lithium chloride solution is then evaporated to dryness to obtain anhydrous lithium chloride suitable for producing battery grade lithium metal containing less than 100 ppm sodium. The above process requires seven main processing steps, which are described as follows: 1) Extracting and purifying a low-boron aqueous solution containing 6% or less by weight of Li from lithium-containing ore or natural brine. 2) Purifying and filtering the brine for magnesium and calcium 3) Precipitating lithium carbonate from the purified brine by addition of Na2CO3, then filtering and drying the lithium carbonate. 4) Reacting hydrated lime with lithium carbonate to produce LiOH solution and filtering it. 5) Crystallization of LiOH·H2O in a vacuum crystallizer 6) dissolving LiOH·HO crystals and recrystallizing LiOH·HO from the solution; and 7) Reacting high purity HCl with recrystallized LiOH·HO to produce high purity lithium chloride brine from which low sodium lithium chloride is crystallized and dried.

[0023] Low sodium lithium carbonate can be prepared from recrystallized LiOH·H2O using the first part of the process described above. The recrystallized LiOH·H2O is then mixed with water and reacted with CO2 to precipitate lithium carbonate. The process steps are described below: 1) Extracting and purifying a low-boron aqueous solution containing 6% or less by weight of Li from lithium-containing ore or natural brine. 2) Purifying and filtering the brine for magnesium and calcium 3) Precipitating Li2CO3 from the purified brine by adding Na2CO3, filtering and drying. 4) Reacting hydrated lime with Li2CO3 to produce LiOH solution and filtering it 5) The process of crystallizing LiOH·H2O in a vacuum crystallizer 6) Dissolving LiOH·H2O and recrystallizing from the solution 7) The CO2 gas is reacted with a slurry containing recrystallized LiOH·H2O to produce high purity lithium carbonate crystals with low sodium content, which are then filtered and dried.

[0024] Production of lithium carbonate

[0025] A common method of producing lithium carbonate precipitate from lithium chloride solution is by mixing sodium carbonate (Na2CO3) or a slurry of sodium carbonate into the lithium chloride solution in a reaction vessel and heating the mixture to about 85-100°C to batch precipitate lithium carbonate. A "heel" or residual slurry from a similar reaction is included as part of the sodium carbonate addition to improve the separation of lithium carbonate. The residual slurry acts as a set of nucleation seeds to aid in batch crystallization to produce a lithium carbonate precipitate with good settling, filtration, and washing properties. The addition of sodium carbonate is controlled when the lithium chloride solution is reacted with sodium carbonate to form lithium carbonate and dissolved sodium chloride (NaCl). In other words, its concentration is controlled so that the process is carried out within the solubility range of NaCl to prevent sodium chloride from co-precipitating with the lithium carbonate.

[0026] Reaction with sodium carbonate precipitates lithium carbonate according to the following equation: 2LiCl+Na2CO3=>Li2CO3+2NaCl

[0027] Adding sodium carbonate (dry) directly to the lithium salt solution in this manner results in lithium carbonate of very low purity, where significant amounts of other ions, especially sodium, are present. A significant portion of the lithium carbonate is recovered from the brine, which also contains high levels of sodium, making the production of low sodium lithium salts difficult and costly.

[0028] The evaporated concentrated lithium chloride solutions used to precipitate lithium carbonate typically have lithium ion concentrations in solution ranging from 0.6 to 7.3% by weight and sodium carbonate concentrations in solution ranging from 25 to 56%, the latter higher range being a slurry of hydrated sodium carbonate (Na2CO3·H2O).

[0029] The reactants are mixed at a temperature ranging from room temperature to about 100° C., and the resulting mixture is heated. The lithium carbonate precipitate formed is then separated by centrifugation at a temperature near 100° C., the temperature at which lithium carbonate precipitate has the lowest solubility in water.

[0030] The product solution after centrifugation contains a significant amount of soluble Li2CO3. The presence of large amounts of sodium chloride makes further recovery of unprecipitated Li2CO3 difficult, and subsequent cooling of the remaining solution after removal of the lithium carbonate solids to 0 °C removes only a small amount of sodium chloride by precipitation.

[0031] Recycling the mother liquor to the early stages of the brine process, whether the mother liquor is discarded or recycled, is often done to improve the efficiency of lithium recovery and to achieve the highest economic recovery possible from the initial precipitation.

[0032] Currently, the most common and lowest cost process for producing low sodium content battery grade lithium carbonate directly from lithium-containing natural brines involves some pre-treatment to obtain industrial grade lithium carbonate, which is purified via a chemical conversion cycle whereby the lithium carbonate reacts with carbon dioxide and water to produce a soluble lithium bicarbonate solution.

[0033] Li2CO3+CO2+H2O=>2LiHCO3

[0034] The fully solubilized lithium bicarbonate is separated from the insoluble impurities. The lithium bicarbonate is then reconverted to lithium carbonate by adjusting the pH and heating, releasing carbon dioxide. Unfortunately, this process requires hardware to handle a gas-liquid reaction whose reaction rate is proportional to the pressure and concentration of dissolved CO2. That is, to run most efficiently, pressurized reaction hardware is required, which significantly increases the equipment and process complexity, energy requirements, hazards, and maintenance costs.

[0035] Direct recrystallization (without chemical interconversion) of impure lithium carbonate offers a potential purification process. However, this process has not been adopted for two main reasons.

[0036] First, lithium carbonate, unlike sodium chloride and most other salts, has a very low "inverse solubility" property, where its solubility in water decreases as the solution temperature increases. Increasing the temperature of the solution increases the precipitation and recovery of lithium carbonate while minimizing co-precipitation and contamination of sodium chloride. While this property is beneficial from the standpoint of selective solubility (least soluble in temperature regions where other salts are more soluble), a saturated solution can only contain 18 mg of lithium carbonate per ml of simulated brine solution (20°C). Thus, large amounts of solvent are required to process the material (to fully solubilize the raw product).

[0037] Second, recrystallization of industrial grade lithium carbonate cannot be effectively accomplished from the mother liquor (containing high levels of dissolved sodium salts) because no net reduction of sodium impurity is realized. This shortcoming is due to the formation of crystalline aggregates (sodium entrapment) that contain amorphous or crystalline impurities. This particular type of crystalline impurity cannot be effectively addressed by washing because they are inclusion bodies and are trapped inside the lithium carbonate crystal matrix. Thus, purification requires separation of the crude lithium carbonate from the mother liquor, washing, and then resuspension and recrystallization from clean water.

[0038] Examples of lithium carbonate aggregates imaged by optical microscopy are shown in Figures 1A and 1B, and examples of lithium carbonate single crystals without aggregates are shown in Figures 2A and 2B. Summary of the Invention

[0039] For the above reasons, there remains a need for improved methods for purifying lithium from lithium carbonate intermediates to overcome the shortcomings of conventional processes.

[0040] In a first embodiment, the present invention provides a method for purifying lithium from raw brine, comprising the steps of: dosing the raw brine into a feeder tank maintained at a temperature T1 and containing sufficient carbonate source to precipitate all carbonate-forming solids in the raw brine to form a precipitation mixture and a crystal-free supernatant; pumping the crystal-free supernatant from the feeder tank to a first crystallization reactor maintained at a temperature T2 to crystallize lithium carbonate salt from the crystal-free supernatant; Including, Temperature T1 is lower than temperature T2, and controlling the flow rate to maintain a steady state concentration of lithium carbonate in the solution phase of the crystallization reactor. method.

[0041] The method may also include pumping the solution phase in the first crystallization reactor back to the feeder tank. The method may also include pumping the solution phase in the first crystallization reactor into a second crystallization reactor maintained at a temperature T3, where temperature T3 is greater than temperature T2. Similarly, the method may include pumping the solution phase in the first crystallization reactor into a second crystallization reactor maintained at a temperature T3, where temperature T3 is greater than temperature T2, and continuing the method of pumping the solution phase from the "n" crystallization reactor to the "n+1" crystallization reactor, where the "n+1" crystallization reactor is maintained at a temperature greater than the temperature of the "n" crystallization reactor, where "n" is an integer greater than or equal to 2.

[0042] The first crystallization reactor may be seeded with high purity lithium carbonate crystal seeds. The carbonate source may be sodium carbonate, potassium carbonate, or carbon dioxide. The flow rate may be sufficient to maintain a steady state concentration of dissolved lithium carbonate in the solution phase, so that the overall mass flow rate of the crystallization is faster than the maximum flow rate used. The flow rate may be adjusted to accommodate the rate of addition of solution to achieve 50-250 g Li2CO3 per hour, if the process requires and the hardware allows.

[0043] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. [Brief description of the drawings]

[0044] The invention will be more fully understood from the detailed description given below and the accompanying drawings, which are given by way of example only and therefore not as a limitation of the invention, in which like reference numbers are used to denote like features in the various figures.

[0045] [Figure 1A] 1 shows examples of lithium carbonate aggregates imaged by optical microscopy. [Figure 1B] 1 shows examples of lithium carbonate aggregates imaged by optical microscopy. [Figure 2A] 1 shows an example of a single crystal of lithium carbonate without agglomerates imaged by optical microscopy. [Figure 2B] 1 shows an example of a single crystal of lithium carbonate without agglomerates imaged by optical microscopy. [Figure 3A] 1 is a graph showing the variation in solubility of Li2CO3. [Figure 3B] 1 is a graph showing the variation in solubility of Li2CO3. [Figure 4] 1 is a graph showing the solubility values ​​of Li2CO3 at 95° C. as a function of Na2CO3 concentration for three levels of NaCl concentration. [Diagram 5] FIG. 1 is a diagram illustrating an example of a system according to the present invention. [Figure 6] 1 is a graph showing the solubility swing of lithium carbonate at different temperatures. [Figure 7]1 is a graph showing the theoretical purity over time of samples of the solid phase from both the feeder tank (dissolver) and the crystallization reactor (crystallizer) as analyzed by ICPOES. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] The present invention is a new process that uses selective crystallization to produce battery grade or other high purity lithium carbonate, greater than 99.5% lithium carbonate (Li2CO3). While other exemplary processes use lithium carbonate crystallization as a purification method, the process of the present invention provides a significantly simplified and direct route to high purity lithium carbonate from impure groundwater brines containing multiple dissolved salts.

[0047] The present invention provides an integrated novel process that reduces the number of major process steps to produce high purity lithium carbonate with 0.01-0.002 wt.% sodium directly from natural lithium-containing brines enriched to 0.2-10.0 wt.% Li. The process of the present invention avoids: 1) One and two recrystallization steps of lithium hydroxide monohydrate 2) Lithium carbonate becomes lithium bicarbonate and then recrystallizes back into the carbonate twice. 3) Careful titration of carbonate addition to effect initial precipitation of undesired cations (e.g., Mg and Ca) (selective precipitation based on solubility)

[0048] Not relying on a carbon dioxide / bicarbonate cycle (used in conventional processes) results in dramatic simplifications in terms of required hardware (e.g., no need for pressurized carbon dioxide and associated industrial infrastructure for chemical interconversion), reduced materials required (water and sample loss), and reduced energy requirements to complete the process. In terms of the physical plant, the reduction in required process steps, simplified process flow, and continuous operation design dramatically reduce the physical footprint of the pilot plant. Thus, the present invention provides significant operability and usability advances, allowing for small scale operation to be considered and brought online with less capital and infrastructure investment.

[0049] Experimental precipitation of Li2CO3 from LiCl has shown that it is generally difficult to obtain Li2CO3 with good settling, filtration, and washing properties. The following observations and conclusions are relevant to the precipitation of Li2CO3 from LiCl solutions.

[0050] (1)Effect of NaCl concentration on the solubility of Li2CO3 Ions in solution have "cross effects" that affect the solubility and precipitation of ions. A study of the effect on the solubility of Li2CO3 is shown in Figures 3A and 3B. At 95°C, as the concentration of NaCl is increased to about 9%, the solubility of Li2CO3 increases from 7.5 grams / liter in pure water to about 9.3 grams / liter. As the concentration of NaCl is further increased, the solubility of Li2CO3 decreases, and at 25 weight percent NaCl, the solubility of Li2CO3 is only about 6.5 grams / liter, less than its solubility in water. KCl has a similar effect on the solubility of Li2CO3 to about the same extent.

[0051] (2a) Concentration of reactants Considering the above data on the effect of the concentration of NaCl on the solubility of Li2CO3, it is desirable to work with reactant concentrations that result in a high concentration of NaCl in the mother liquor. Precipitation of NaCl due to exceeding the maximum solubility of NaCl must be avoided. When working with aqueous solutions of LiCl and Na2CO3, precipitation of NaCl is not possible. However, when Na2CO3 in the form of a slurry of solid Na2CO3·H2O is used in its saturated solution, care must be taken to ensure that the reactant concentrations do not result in precipitation of NaCl. Even in such cases, due allowance must be made for the effect of excess Na2CO3, which moderately reduces the solubility of NaCl in the mother liquor, and for the presence of NaCl in the LiCl solution (see 2b below).

[0052] (2b) Lithium chloride solution In the recovery of lithium from natural chloride brines, the recovered LiCl solution produced by sun drying, after appropriate purification, usually contains moderate amounts of NaCl and KCl. Both NaCl and KCl (especially NaCl) can be reduced to fairly low levels by simple sun drying concentration. Table 2 shows the solubility of NaCl in solutions of various LiCl contents at 25°C and 100°C.

[0053] TIFF0007602275000002.tif59170

[0054] Under properly controlled process conditions, lithium carbonate precipitates with good physical properties can be obtained over the range of LiCl concentrations listed above. However, recovery rates vary widely and increase with increasing LiCl concentration. Increased solar drying and increased residence times in evaporation ponds are required to achieve higher LiCl concentrations.

[0055] (2c) Sodium carbonate solution In addition to achieving Li2CO3 precipitation with concentrated Na2CO3 solutions (26 to 33 weight percent Na2CO3), it is also possible to use slurries of soda ash and water having a total Na2CO3 content of more than 33 weight percent. Such slurries contain a mixture of hydrated sodium carbonate (Na2CO3·H2O) solids in a saturated sodium carbonate solution. Slurries containing as much as 56 weight percent total Na2CO3, which can be pumped without difficulty, have been used successfully. The use of these slurries, in conjunction with a LiCl solution of suitable concentration, allows the achievement of the most practical concentration of NaCl in the mother liquor. The resulting recovery of Li2CO3 is high. However, one drawback of using slurries is the inability to remove certain insoluble impurities in the soda ash by filtration.

[0056] (3) Mixing order of reactants In LiCl solutions, 0.5 to 5% by weight Li + By utilizing a dilute lithium carbonate solution containing 20% ​​by weight or less LiCl, good settling and filtration precipitation can be obtained by adding the solution cold (25°C) to a cold sodium carbonate solution, followed by heating to about 95-100°C before centrifugation. This method was only successful when using LiCl solutions containing 20 weight percent LiCl or less.

[0057] The preferred procedure for mixing the reactants is to add the lithium chloride and sodium carbonate solutions (or slurry) simultaneously to a moderately sized heel of unfiltered slurry (or mother liquor) from a previous precipitation. Typically, each solution is added at a flow rate approximately proportional to its volume, such that the total time of addition is about 1 hour. Good agitation is maintained throughout. This method results in a precipitate with excellent physical properties and is applicable to LiCl solutions concentrated as low as 45% LiCl by weight.

[0058] (4) Precipitation temperature It has been previously noted that the solubility of lithium carbonate in either water or sodium chloride solution is retrograde (i.e., solubility decreases as temperature increases), which is opposite to the normal effect of temperature on the solubility of salts. Therefore, to improve the recovery of lithium, centrifugation or filtration at high temperatures is necessary. When using the simultaneous method described above, the reactants may be mixed at room temperature or some intermediate temperature, e.g., 50°C, rather than at 95°C. The temperature is then raised to about 95°C, followed by centrifugation to obtain a precipitate with superior properties. This method is advantageous over using an addition temperature near 95°C, because it eliminates the undesirable evaporation and heat loss that occurs by maintaining a temperature near 95°C during the relatively long addition step.

[0059] (5) Use of excess NaCO3 It is desirable to use about 10% excess of Na2CO3 over the equivalent amount of LiCl, thereby decreasing the solubility of Li2CO3 in the mother liquor due to the increased carbonate ion concentration. The decrease in solubility of Li2CO3 with increasing concentration of Na2CO3 is not linear (i.e., the rate of decrease decreases with increasing Na2CO3). Therefore, the cost of the excess Na2CO3 used must be balanced against the increased recovery of Li2CO3 to ensure that the excess is justified. Usually, 5-10% Na2CO3 over stoichiometric is advantageous.

[0060] The three curves in Figure 4 give the experimentally determined values ​​of Li2CO3 solubility at 95°C as a function of Na2CO3 concentration for three NaCl concentration levels. These curves graphically illustrate the effect of both Na2CO3 and NaCl concentrations on Li2CO3 solubility. These curves can be used to calculate the expected loss of Li2CO3 in the mother liquor for any given set of reactant compositions and concentrations.

[0061] Currently, there are several methods for extracting lithium from lithium chloride-rich salts or other lithium-containing liquids. However, these methods are not currently used to produce liquids containing low concentrations of magnesium and lithium carbonate. Conventional methods for extracting lithium carbonate from mineral sources using spodumene or lithium aluminum silicate ore (LiAlSi2O6) also produce material of insufficient purity. Thus, there is a need to extract lithium from lithium-containing brines in sufficient purity to produce high-purity lithium metal in the form of lithium salts such as lithium chloride or lithium carbonate.

[0062] In this regard, a key advantage of the method of the present invention is the ability to control the crystal growth rate of lithium carbonate in the presence of natural (high sodium salt) brine mother liquor. This advantage can be achieved by understanding the relationship between the crystal growth rate and the concentration of dissolved lithium carbonate as a function of solution temperature. By controlling the crystallization process, the size, shape and quality of the lithium carbonate crystals can be controlled and the formation of undesirable agglomerates can be prevented.

[0063] One embodiment of the overall process of the present invention can be broadly described using the schematic diagram of an exemplary system shown in FIG.

[0064] First, a raw brine 1 is obtained. Preferably, the raw brine 1 is mainly lithium chloride. The composition of the raw chloride brine 1 is not limited, but typically contains 0.1-6 wt% Li, 0.1-3 wt% Na, 0.001-0.4 wt% K, 0.01-2.6 wt% Ca, 0.01-0.5 wt% Mg, 0.1-0.22 wt% B, and 0.1-0.3 wt% Si. A typical composition of the raw chloride brine 1 is 1 wt% Li, 2.5 wt% Na, 0.01 wt% K, 0.01 wt% Mg, 0.01 wt% Ca, 0.12 wt% B, and 0.26 wt% Si.

[0065] Raw brine 1 is charged to feeder tank 4 via line 2 and treated with sufficient sodium carbonate to precipitate all carbonate forming solids 5 (Mg, Ca, Li). This precipitation mixture is lower grade than industrial grade lithium carbonate formed in other processes because it contains all carbonate forms of the corresponding cations. Feeder tank 4 contains mixer 6. Mixer 6 agitates the contents of feeder tank 4 to keep solids dissolved and accelerate mass transfer. Mixer 6 does not have to be a physical mixer, mixer 6 can simply be agitation by liquid flow.

[0066] The purpose of this first step is to drive off both Li2CO3 and CaCO3 / MgCO3 etc. as raw solids. In plant trials, only Ca / Mg (and possibly boron and Si) precipitates initially, giving a metastable, supersaturated Li2CO3 solution. If left unattended, the solution will eventually precipitate Li2CO3, which is uncontrollable. Ion chromatography measurements can tell the exact [Li+] in the input brine. For example, a feed of about 3000 mg of Li+ as LiCl would give about 16 g Li2CO3 / L, but the saturation limit at 10°C is only about 12-14 g / L. Thus, after softening, supersaturation occurs. This kinetically stable condition leads to spontaneous crystallization of Li2CO3 in the low temperature reactor over time. The process of the present invention can use this metastable condition to make the recovery more efficient, both in terms of energy consumption and process time. A typical process would simply heat the batch to force crystallization, but the already high supersaturation, combined with the lack of control due to thermal fluctuations, prevents control of the crystallization process. In the present invention, the supersaturated solution is added in a controlled manner, applying the principle of continuous crystallization in a well-mixed and seeded crystallization reactor (at about 70°C). This protocol results in stable and reproducible crystal growth that can be maintained to avoid agglomeration, resulting in a purer material that is easier to process. Thus, the method of the present invention is not adversely affected by the high supersaturation that complicates previous protocols that utilize batch recrystallization of supersaturated lithium carbonate using thermal or chemical supersaturation. These previous processes that use batch crystallization cannot control the process, leading to equipment failures and impure products. The method of the present invention can also utilize chemical supersaturation to improve mass throughput. This feature is likely the reason for the very wide tolerance to chemical inputs.

[0067] Secondly, the feeder tank 4 is kept at a low temperature T1. The lower temperature T1 can be between -10°C and 40°C, preferably between 5°C and 25°C, more preferably between 10°C and 20°C. At this temperature, lithium carbonate has the highest solubility. Specifically, the solubility can be between 10mg / ml and 30mg / ml, more preferably between 15mg / ml and 21mg / ml. The feeder tank 4 further includes a waste solid discharge line 8 that discharges any solid material (possibly MgX and CaX) from the spent material. MgX can be magnesium hydroxide, magnesium silicate, magnesium borate or magnesium carbonate. CaX can be calcium hydroxide, calcium silicate, calcium borate or calcium carbonate.

[0068] The crystal-free supernatant in the feeder tank 4 is then pumped via transfer line 10 to the first crystallization reactor 14. This crystal-free supernatant can be obtained using any standard filtration or settling technique suitable for a pilot plant, including, but not limited to, any standard means for solid / liquid separation, such as filtration using membranes or sintered media, centrifugal settling, and gravity settling. The first crystallization reactor 14 is held at an elevated temperature T2. The elevated temperature T2 can be 40°C to 100°C, preferably 55°C to 85°C, and more preferably 65°C to 75°C. This temperature results in a lower solubility environment for lithium carbonate salts. Specifically, the solubility can be 2mg / ml to 20mg / ml, preferably 5mg / ml to 10mg / ml. The first crystallization reactor 14 is further seeded with high purity lithium carbonate crystal seeds to provide a growth medium for lithium carbonate. This seeding can involve the spontaneous generation (primary nucleation) of Li2CO3 upon heating the solution phase to produce a supersaturated solution, or it can be accomplished by the addition of commercially available high purity (>99.95%) Li2CO3. An example of this solubility swing is shown in Figure 6, a solubility diagram for lithium carbonate in water.

[0069] In principle, the temperature difference between the two tanks can be as little as 1° C., but a minimum of 30° C. is most reasonable to provide a time- and energy-efficient process. In this regard, a higher temperature difference between T1 and T2 provides less control over the crystallization of the lithium carbonate salt, but produces more lithium carbonate salt. On the other hand, a smaller temperature difference between T1 and T2 provides more control over the crystallization, but produces less lithium carbonate salt. Nevertheless, the process of the claimed invention works with any temperature difference between T1 and T2, and the crystallization rate is controlled by controlling a combination of (i) the flow rates into and out of each tank, and (ii) the temperature difference between the reactors. By controlling the crystallization rate, the invention can control the size, morphology, and agglomeration of the crystals. The invention also has the advantage that (i) the flow rates into and out of each tank, and (ii) the temperature difference between the reactors at which the process occurs can be controlled, so that the crystallization rate can be altered as needed. The inclusion of high purity lithium carbonate seeds provides a further means of mitigating supersaturation and controlling the crystallization rate and isolated crystal morphology.

[0070] Samples were processed using the general procedure listed above for Figure 6. Solubility measurements allow prediction of mass transfer efficiency per volume. Samples were measured using gravimetric analysis in triplicate to determine the total mass of lithium carbonate in solution.

[0071] The first crystallization reactor 14 may include a mechanical mixer 16 that is utilized to effectively agitate and mix the reactor and maintain a uniform concentration distribution throughout the reactor. This agitation allows for uniform crystal growth rates and prevents agglomeration and clumping of the purified Li2CO3.

[0072] As an example of the process of the present invention, the feeder tank 4 may be held at 20° C., while the first crystallization reactor 14 is held at 90° C. This temperature difference of 70° C. provides a theoretical mass throughput of 7 mg of lithium carbonate per ml of brine circulating between the two environments. When the brine enters the first crystallization reactor 14 at the higher temperature T2, it becomes supersaturated relative to the new temperature environment. This temperature difference promotes crystallization, which drives the lithium carbonate out of solution, resulting in the depletion of lithium carbonate from the solution.

[0073] The purified lithium carbonate 15 collects at the bottom of the first crystallization reactor 14 and is filtered through a filter 18 (e.g., a bag filter or filter press, or another filtration or settling technique suitable for a pilot plant, including any standard means for solid / liquid separation, such as, but not limited to, filtration using membranes or sintered media, centrifugal settling, and gravity settling) to recover purified solid lithium carbonate.

[0074] In the simplest process of the invention, the crystal-free solution phase in the first crystallization reactor 14 is pumped back into the feeder tank 4 via return line 21 and cooled in-line as it is returned. Thus, the liquid phase is now undersaturated to its cooled temperature, so lithium carbonate (impurity) from the solid phase can dissolve and return to the solution phase at equilibrium concentration. The impurities (such as Ca or MgCO3) remain in the solid phase as they have the lowest solubility in the feeder tank 4 (lower temperature T1) and do not further contaminate the solution phase.

[0075] The solution phase cycle process then continues from the feeder tank 4 (where lithium carbonate has a high solubility) to the first crystallization reactor 14 (where lithium carbonate has a low solubility). This process results in a net mass transfer of lithium carbonate moving from the feeder tank 4 to the first crystallization reactor 14. In this way, the solution phase acts as a conveyor belt.

[0076] If the purity of the lithium carbonate in the first crystallization reactor 14 is insufficient, a second crystallization reactor 24 can be added in the flow path. The crystal-free supernatant containing lithium carbonate in the first crystallization reactor 14 is pumped to the second crystallization reactor 24 via a transfer line 20. In this embodiment, three temperatures are used. The temperature T1 of the feeder tank 4 is the lowest temperature, the temperature T3 of the second crystallization reactor 24 is the highest temperature, and the temperature T2 of the first crystallization reactor 14 is between the temperatures T1 and T3. As mentioned above, the process will work if there is only a 1°C temperature difference between T3, T2, and T1, but a minimum temperature difference of 10 to 20°C (T3-T2 and T2-T1) is preferred to allow operation at reasonable efficiency. The second crystallization reactor is substantially the same as the first crystallization reactor with the same hardware (filtration, mixer, and temperature control) as the first crystallization reactor.

[0077] The second crystallization reactor 24 includes a mixer 26. The mixer 26 agitates the contents of the second crystallization reactor 24 to maintain a constant concentration (i.e., to prevent the formation of gradients). The mixer 26 does not have to be a physical mixer; the mixer 26 can simply be agitated by a liquid flow. However, in a preferred embodiment, the mixer 26 is a physical mixer.

[0078] The purified lithium carbonate 25 collects at the bottom of the second crystallization reactor 24 and is filtered through a filter 28 (e.g., a bag filter or a filter press) to recover the purified solid lithium carbonate. The final isolated lithium carbonate is further subjected to washing 50 using clean water (reverse osmosis or distilled) in a filtration stage before drying and final packaging.

[0079] The crystal-free solution phase in the second crystallization reactor 24 is pumped back into the feeder tank 4 via transfer line 32 and cooled in-line as it is returned. Thus, the liquid phase is now undersaturated to its cooled temperature, so lithium carbonate (impurity) from the solid phase can dissolve and return to the solution phase at equilibrium concentration. The impurities (such as Ca or MgCO3) remain in the solid phase as they have the lowest solubility in the feeder tank 4 (lower temperature T1) and do not further contaminate the solution phase.

[0080] The input selection valve 31 allows the operator to select whether (i) material is to be recirculated from the first crystallization reactor 14 and / or the second crystallization reactor 24, or (ii) the unused brine 1 to be treated is to be directed to the feeder tank 4. Thus, the input selection valve 31 allows the operator to select whether more Li is to be input to the process. + One can choose to add fresh virgin material containing Li2CO3 or add used brine to dissolve remaining impure Li2CO3.

[0081] The addition of the second crystallization reactor 24 adds a fractional crystallization sequence in which highly impure solids from the feeder tank 4 are dissolved in the first crystallization reactor 14 and once crystallized create purer solids, which is a continuous dynamic flow (dissolution and deposition) with a final crystallization event in the second crystallization reactor 24 to give a high purity material. The purified crystals are gradually (continuous operation) or in stages (batch operation) isolated by filtration or some other solid-liquid separation means. They are withdrawn from the reactor and isolated as the final purified product.

[0082] In another embodiment, multiple stages can be added as needed to accommodate almost any brine chemistry, making the entire process modular, flexible, and highly adaptable (dramatically increasing the number of different brines that can be processed with this technology) without requiring major redesign to the overall design. In other words, more crystallization reactors can be added (N+1) as needed.

[0083] Each of the N+1 crystallization reactors includes a mixer 36. The mixer 36 agitates the contents of the N+1 crystallization reactors to maintain a constant concentration (i.e., to prevent the formation of gradients). The mixer 36 does not have to be a physical mixer; the mixer 36 can simply be agitated by liquid flow.

[0084] The purified lithium carbonate 35 collects at the bottom of the N+1 crystallization reactors and is filtered through a filter 38 (e.g., a bag filter or a filter press) to recover the purified solid lithium carbonate. The final isolated lithium carbonate is further subjected to washing 50 using clean water (reverse osmosis or distilled) in a filtration step before drying and final packaging.

[0085] The crystal-free solution phase in the N+1 crystallization reactors is pumped back into the feeder tank 4 via transfer line 40 and cooled in-line as it is returned. Thus, the liquid phase is now undersaturated to its cooled temperature, so lithium carbonate (impurity) from the solid phase can dissolve and return to the solution phase at equilibrium concentration. The impurities (such as Ca or MgCO3) remain in the solid phase as they have the lowest solubility in the feeder tank 4 (lower temperature T1) and do not further contaminate the solution phase.

[0086] The waste stream 55 from washing the pure crystalline solids can be recycled to recover Li using conventional means such as, but not limited to, solvent extraction.

[0087] Following isolation of the crystalline product in the filter, two separate waste streams are produced. The first waste stream is the supernatant discharged from the high temperature reactor. The second waste stream is the wash, which is clean reverse osmosis or deionized water at 70°C used to wash the filter cake. The wash water strips off the attached supernatant and dissolves the outer layer of the crystals. These waste streams are very pure, with the only other components being [Na+] or [K+]. By combining the waste streams with a Li-selective solvent extract, the Li in the wastewater can be adequately recovered, dramatically increasing efficiency. The solvent extract can then be used as feedstock for the recrystallization process.

[0088] After washing, the isolated lithium carbonate is transferred to a dryer via line 60 and then removed from the dryer via line 65.

[0089] As an additional process, impure Li2CO3 (crystalline material less than 99.5% pure) can be fed via line 19 to the first crystallization reactor 14. The impure crystalline material can be dissolved and then recrystallized (reprocessed) to produce pure crystalline material 15. This pure crystalline material is then isolated as the final purified product by filtration or some other means of solid-liquid separation. Again, multiple stages can be added to accommodate any desired purity of the Li2CO3 fed via line 19.

[0090] In recrystallization, a slurry is generated (dry charge and clean water, or thick slurry from the hot crystallizer) and held in a cold tank. The cold tank allows net dissolution. The supernatant obtained (filtered) from the cold tank is sent to the hot crystallizer at a controlled flow rate as described above. Clean water with lower [Na] can be used, making it possible to isolate a purer product. The supernatant (after filtering) from the hot tank is returned to the cold tank to redissolve the less pure crystals. This recrystallization can be done as cascading stages of MSMPR (with successively increasing temperatures) or by batch isolation, purging of the solvent, and recharging of the dried crystals into pure water. This latter case is the most likely way that solvent-extracted material is reintroduced into the system. With sufficient adjustments, the entire process can be run in a fully continuous mode, adding continuous reintroduction of the sample.

[0091] The present invention has several advantages over conventional processes.

[0092] First, less working solvent is required. Because the solution phase is primarily used as a conveyor of mass from the feeder to the crystallizer, the process may utilize a smaller working volume of brine, dramatically reducing the size of the processing reactor and the physical space required for the plant without compromising the potential of the mass recovered. Thus, the process of the present invention is more flexible and can be located in a greater variety of locations. If some form of in-line evaporation is added to the return circuit, three other advantages can be realized: (i) the feedstock can be added as raw brine, so the process is fully continuous; (ii) refined / waste solids can be collected from the crystallizer or feeder; and (iii) the spent brine can be concentrated and recycled, allowing maximum recovery of lithium from a lower initial Li input. Thus, the present invention can be a continuous process rather than a batch process.

[0093] Second, the crystallization rate can be tightly controlled by the temperature difference between the reactors and the flow rates into and out of the tank. In all other processes, lithium carbonate crystallization is driven by a high degree of supersaturation to maximize recovery of the desired salt. This aids in rapid purification, but results in less control over the crystallization rate and forces the formation of agglomerates. In the present invention, the overall degree of supersaturation is kept at a relatively low level because the overconcentrated solution metered into the larger reactor constitutes a relatively small proportion (1%-25% by volume) of the total holding environment. Thus, although the net mass recovered per unit time is less, a degree of crystal control (size, morphology, and agglomeration) is easily achieved.

[0094] Third, the process of the present invention is highly adaptable. The ability of the present invention to vary the flow allows the crystallization rate to be controlled even if the chemistry of the input feedstock is suddenly changed. In one embodiment, this advantage is achieved by incorporating in-line analytical techniques. In another embodiment, a process crystallization microscope probe can be used to continuously monitor the size, shape, and quality (single crystals vs. aggregates) of the flow stream. Another embodiment can incorporate an in-line ion chromatography system, allowing the cationic composition and concentration to be monitored as a function of time. These tools allow the process conditions (e.g., flow rates and temperatures) to be continuously adapted and modified to ensure the plant is performing optimally, and can respond to changes in input brine composition or other upstream material variations.

[0095] With regard to in-line analytical techniques, the concentration of other ions dramatically affects the solubility of Li2CO3 of interest. In-line analytical techniques work to isolate pure material despite salt-rich environments. In-line analytical techniques (e.g., ion chromatography) can be used to accommodate variable input streams (e.g., adding too much or too little Na2CO3 causes variations in solubility or crystallization behavior).

[0096] For example, turbidity tracking with a Blaze tool (Blaze 900 from Blaze Metrics) prevents the system from fouling, allows the crystallization rate to be estimated, and the values ​​are used to lock flow ranges and temperatures. Overall, turbidity tracking is an important in-line process metric to ensure that the system is operating within normal limits and that crystallization is as expected. The Blaze tool allows tracking of turbidity versus time, which gives the change in mass of crystals per unit time to ensure adjustments are made to the crystallization (temperature and addition flow rates). Generally, if crystallization occurs too quickly, the crystals will clump together. The Blaze tool also visualizes the crystals as they pass through the filter press to correlate the crystal shape with the isolated purity. The Blaze microscope reports back code langth, which gives a time-dependent measurement of crystal size and shape throughout the crystallization process. It is therefore possible to (1) trigger changes in temperature, flow rate, or filtration time (plant operation) based on the Blaze data, and (2) modify retention times and possibly make purity decisions based on the crystal image data.

[0097] As another example, the brine composition can be monitored by ion chromatography. Specifically, the concentration and composition of the brine can be monitored and quantified at several key control points. For example, by monitoring the input, the exact weight of sodium carbonate required to adequately soften the input brine can be identified and the initial condition confirmed. In this regard, too much carbonate also means that the [Na+] is too high, increasing the solubility of the Li2CO3 product. By monitoring the softened filtered supernatant, the [Li+] as it leaves the cold tank is obtained, confirming that only CaCO3 and other trace impurities are removed as solids in the cold tank (i.e., chemical supersaturation is maintained without losing Li2CO3 in the cold tank). By monitoring the hot filtered supernatant, the [Li+] after crystallization is obtained, proving that the hot tank is high enough to drive crystallization.

[0098] Fourth, considering the effect of NaCl concentration on the solubility of Li2CO3, very high salt concentrations can be used to increase the recovery rate of the crystallization process. The maximum total mass of Li2CO3 recovered is determined by the solubility difference between the two different temperatures of the reactor. For pure Li2CO3 at 95°C, the solubility is about 10mg / ml. However, as the NaCl content increases to more than 25 wt%, the solubility of Li2CO3 drops to less than about 6mg / ml. Therefore, it is advantageous to use high concentrations of NaCl to drive more Li2CO3 out of solution and increase the recovery yield. In contrast, in conventional crystallization processes, operating at high NaCl levels results in high contamination and low purity of the final Li2CO3. The continuous crystallization of the present invention uses this complex solubility behavior while still achieving high purity Li2CO3 in the process.

[0099] Because of these advantages, the present invention reduces the final cost of production through reduced energy requirements for processing the materials, and also provides advantages in terms of greenhouse gas equivalents emitted by processing.

[0100] Working Example Solid phase samples from both the feeder tank (dissolver) and the crystallization reactor (crystallizer) were isolated and analyzed by ICPOES. The results are shown in Figure 7. The solid phase sample from the dissolver was found to gradually decrease in purity as the Li2CO3 was extracted, leaving primarily CaCO3. The sample from the crystallizer remained at high purity. The current 3x error rate of the ICPOES data confirms that the samples are >99% pure (i.e. >99.5% pure).

[0101] References herein to terms such as "vertical", "horizontal", etc. are made by way of example and not by way of limitation to establish a frame of reference. It is understood that various other frames of reference may be used to describe the present invention without departing from the spirit and scope of the present invention. It is also understood that features of the present invention have not necessarily been shown to scale in the drawings. Furthermore, to the extent that the terms "composed of", "includes", "having", "has", "with" or variations thereof are used in either the detailed description or the claims, such terms are intended to be inclusive and open-ended in the same manner as the term "comprising".

[0102] References herein to terms modified by approximation terms such as "about," "approximately," and "substantially" are not intended to be limited to the exact value stated. Approximation terms may correspond to the precision of the instrument used to measure the value and may indicate + / - 10% of the stated value unless specific reliance is placed on the precision of the instrument.

[0103] A feature that is "connected" or "coupled" to another feature may be directly connected or coupled to the other feature, or alternatively, there may be one or more intervening features present. A feature may be "directly connected" or "directly coupled" to another feature if there are no intervening features. A feature may be "indirectly connected" or "indirectly coupled" to another feature if there are at least one intervening feature present. A feature that is "on" or "contacting" another feature may be directly on or in direct contact with the other feature, or alternatively, there may be one or more intervening features present. A feature may be "directly on" or "in direct contact" with another feature if there are no intervening features present. A feature may be "indirectly on" or "indirectly in contact" with another feature if there are at least one intervening feature present.

[0104] The terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting of the present invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising", as used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0105] Although the present invention has been illustrated by the description of various embodiments and these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict the scope of the appended claims to such details or in any way limit them. Further advantages and modifications will be readily apparent to those skilled in the art. Thus, the broader aspects of the present invention are not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. The applicant has provided information regarding both advantages and disadvantages of various detailed embodiments in order to fully enable those skilled in the art to make and use the claimed invention. Those skilled in the art will appreciate that in some applications, the disadvantages of the specific embodiments as detailed above may be entirely avoided or may be outweighed by the overall advantages provided by the claimed invention. Thus, departures may be made from the above detailed teachings without departing from the spirit or scope of the applicant's general inventive concept.

Claims

1. 1. A method for purifying lithium from raw brine, comprising the steps of: Temperature T １ and charging the raw brine (1) to a feeder tank (4) containing a carbonate source sufficient to precipitate all carbonate-forming solids in the raw brine (1) to form a precipitation mixture and a crystal-free supernatant; The crystal-free supernatant is removed from the feeder tank (4) at temperature T ２ to a first crystallization reactor (14) held in said first crystallization reactor (15) to crystallize lithium carbonate salt from said crystal-free supernatant; Including, The temperature T 1 is −10° C. to 40° C., the temperature T 2 is 40° C. to 100° C., and the temperature T １ is the temperature T ２ and there is a temperature difference of at least 1° C. between the temperature T 2 and the temperature T 1 ; and controlling the flow rate of the crystal-free supernatant from the feeder tank (4) to the first crystallization reactor (14) to maintain a steady-state concentration of lithium carbonate in the solution phase of the first crystallization reactor (14). method.

2. 2. The method of claim 1, comprising pumping the solution phase in the first crystallization reactor (14) back to the feeder tank (4).

3. 3. The method of claim 1 or 2, wherein the first crystallization reactor (14) is seeded with high purity lithium carbonate crystal seeds.

4. The solution phase in the first crystallization reactor (14) is heated to a temperature T ３ and pumping the mixture into a second crystallization reactor (24) held in The temperature T ３ is the temperature T ２ Higher than 4. The method according to any one of claims 1 to 3.

5. 5. The method of claim 1, wherein the carbonate source is sodium carbonate, potassium carbonate or carbon dioxide.

6. The solution phase in the first crystallization reactor (14) is heated to a temperature T ３ and pumping the mixture into a second crystallization reactor (24) maintained at temperature T ３ is the temperature T ２ and continuing the method of pumping the solution phase from the "n" crystallization reactor to the "n+1" crystallization reactor, said "n+1" crystallization reactor being maintained at a temperature higher than the temperature of said "n" crystallization reactor, where "n" is an integer equal to or greater than 2; The method of claim 1 , further comprising:

7. 7. The method of any one of claims 1 to 6, wherein the flow rate is sufficient to maintain a steady state concentration of dissolved lithium carbonate in the solution phase such that the overall mass flow rate of crystallization is faster than the maximum flow rate used.

8. The flow rate is 50 to 250 g of Li per hour. ２ CO ３ 8. The method according to claim 1 , wherein the addition rate of the solution is adjusted accordingly to achieve

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