Process

Abstract

A continuous process for at least partially converting a metal carbonate (M (CO3) x) to a metal bicarbonate (M (HCO3) y) comprising: supplying a composition comprising a metal carbonate and a composition comprising water into a reactor to form a mixture of water and a metal carbonate; optionally adding a solution of water / metal salt; wherein a gas comprising carbon dioxide (CO2) is supplied to the reactor at high pressure.

Description

Technical Field

[0001] The present invention relates to a method for the continuous bicarbonation of metal salts. The method is particularly suitable for the production of lithium bicarbonate. Background Art

[0002] Lithium salts (such as LiPF6) are used in batteries, such as commercial secondary batteries, which is an application that utilizes their high solubility in non-aqueous, polar solvents.

[0003] Lithium salts can be used as precursors for lithium compounds (such as LiPF6) used in lithium-ion batteries. Lithium bicarbonate (LiHCO3) is commonly used as the precursor compound because it has high solubility in water (compared to other salts, such as lithium carbonate (Li2CO3)), and can be safely transported in an aqueous solution. Other salts with poor solubility require larger transportation containers and thus higher transportation costs.

[0004] Currently, the production of lithium bicarbonate is usually carried out by carbonation with CO2 in an aqueous environment in a reactor.

[0005] The following reaction

[0006]

[0007] produces a lithium bicarbonate solution.

[0008] The bicarbonation reaction is slightly exothermic.

[0009] This method can be batchwise. These batch methods usually operate at low CO2 pressures and are thus inefficient.

[0010] There are also continuous production methods. In currently used continuous production methods, a mixture of water and lithium carbonate is usually charged into a reactor together with a carbon dioxide source (usually bubbled through the lithium carbonate solution).

[0011] This can be challenging because the initial solubility of lithium carbonate in water is lower than that of other lithium salts. At lower temperatures, the solubility of lithium carbonate in water is higher.

[0012] Since the solubility of lithium carbonate is higher at lower temperatures, the reactor is usually cooled. Carbon dioxide can be recycled.

[0013] In currently used continuous production methods, rapid dissolution of CO2 cannot be achieved. This has a negative impact because the slow dissolution of CO2 limits the conversion of lithium carbonate to lithium bicarbonate. The carbon dioxide utilization efficiency can be as low as about 30%-40%. To address the problem of poor conversion, large batch reactors are required for industrial-scale production. However, even so, these large reactors (and related equipment) are usually complex and the operating systems are expensive.

[0014] Therefore, an improved method for producing lithium bicarbonate is needed. SUMMARY OF THE INVENTION

[0015] According to the present invention, there is provided a continuous method for converting at least a portion of a metal carbonate (M(CO3) x ) into a metal bicarbonate (M(HCO3) y ), which comprises:

[0016] Supplying a composition comprising a metal carbonate and a composition comprising water to a reactor to form a mixture of water and the metal carbonate;

[0017] Optionally adding a solution of water / metal salt;

[0018] Wherein a gas comprising carbon dioxide (CO2) is supplied to the reactor under high pressure.

[0019] It has been found that the method of the present invention achieves a high conversion rate of the metal carbonate. It has also been found that the conversion occurs rapidly. It is hypothesized that this is (at least in part) because in the method of the present invention, the dissolution of CO2 is enhanced. These factors contribute to the advantage that only a smaller reactor (compared to the prior art) is required to operate the method of the present invention.

[0020] Generally, the metal carbonate is lithium carbonate or contains lithium carbonate.

[0021] Preferably, the reactor comprises a plug flow reactor (PFR) reactor.

[0022] Preferably, the method of the present invention uses co-current flow through the reactor. Since it has been found that in the method of the present invention, the reaction proceeds rapidly and has a high conversion rate, it has been found that the bicarbonation reaction can be carried out in a pipeline system. This eliminates the need for a rated pressure reaction vessel required for existing distribution / continuous methods.

[0023] The reactor is preferably lined with an elastic material (such as PTFE). The lining is used to avoid (or at least reduce) any metal contamination (such as caused by the reactor) of the metal (lithium) bicarbonate and downstream products. In this regard, avoiding contamination is important not only for product purity but also for ensuring that any downstream steps (such as further processing, such as fluorination of the metal bicarbonate) operate as expected. It has been found that further processing of the metal bicarbonate, such as fluorination, is adversely affected by metal contamination.

[0024] A single reactor can be used, or multiple reactors can be used. In the case of using multiple reactors, these reactors can be in series, where the output from one reactor is supplied to the subsequent reactor in the series. In the case of using multiple reactors, it is possible to introduce process steps between two (or more) reactors. For example, there can be a heating / cooling mechanism between the reactors, such as a heat exchanger (to provide cooling, see below). (Or and / or additionally, one or more of the multiple reactors can have their own cooling mechanism).

[0025] The total residence time through the reactor(s) / series of reactors is from about 5 seconds to about 5 minutes, such as from about 1 minute to about 4 minutes, such as about 3 minutes. Herein, it should be understood that the residence time will depend on many factors, including: the concentration of the reactants, bubbling, the scale of the reaction, and the shear rate. For example, the formation of large bubbles may require a longer residence time.

[0026] Typically, the metal bicarbonate product is in the form of a salt solution.

[0027] Preferably, the metal bicarbonate solution is extracted from the final reactor and transferred to a storage tank.

[0028] Optionally, there can be a purification step. As an example, unwanted metal ions can be removed (e.g., Mg 2+ and / or Ca 2+ ) and their salts when the desired metal is lithium. Preferred forms of ion removal include ion exchange. It has been found that the bicarbonate solution remains stable: it has been found that any unreacted metal carbonate (M(CO3) x / Li2CO3) remains in solution and does not precipitate within several days (although the pressure is usually reduced compared to the bicarbonation step). Additionally, it has been found that little or no decarbonation / reverse reaction to produce metal carbonate occurs.

[0029] Optionally, a dilution step of adding a metal salt solution as a mixture of dilution water and metal carbonate is carried out. The dilution salt preferably does not participate in the metal bicarbonate formation reaction; in other words, the dilution salt is preferably inert with respect to the metal bicarbonate formation reaction. The use of a dilution salt is preferred because a high initial metal carbonate (M(CO3) x ) concentration is used to minimize the size of the slurrying tank / continuous slurry mixer. Such a high initial metal carbonate (M(CO3) x ) concentration may produce a metal bicarbonate (M(HCO3) y ) product that exceeds the solubility limit of the metal (lithium) bicarbonate (M(HCO3) y ). The use of a dilution salt dilutes the slurry to the desired (maximum) bicarbonate concentration.

[0030] As an alternative, pure water can be used to achieve the desired concentration of metal (lithium) bicarbonate. Thus, in the alternative, instead of optionally adding a solution of a metal salt, water can be optionally added.

[0031] The diluting salt preferably comprises a metal salt similar / same to the metal carbonate; for example, in the case where the metal carbonate comprises lithium carbonate, the diluting salt comprises a lithium salt. Preferably, the metal diluting salt comprises a metal halide, such as a metal fluoride salt. In fact, it is preferred to obtain the diluting salt solution from a downstream step (such as further processing, such as fluorination of the metal bicarbonate). Thus, the diluting salt preferably comprises lithium fluoride (LiF). The diluting salt solution can also have the benefit of precipitating unwanted ions, such as Ca 2+ and / or Mg 2+ ions. These ions can then be removed by filtration prior to ion exchange. In this regard, in a preferred embodiment where the diluting salt comprises lithium fluoride, it has been found that unwanted ions, such as Ca 2+ and / or Mg 2+ ions, precipitate in the corresponding fluoride form.

[0032] The initial concentration of the metal carbonate (before dilution) is preferably in the range of 0.1 - 500 g / L, more preferably 120 - 240 g / L (1.6 to 3.3 moles). The concentration of the metal bicarbonate (after dilution) is up to 75 g / L (up to 1.2 moles). The metal diluting salt concentration is preferably about 0.1 to 2 g / L, more preferably about 1.5 g / L (about 0.06 moles).

[0033] Preferably, a gas comprising carbon dioxide (CO2) (preferably (substantially) consisting of CO2) is applied at a pressure of 0.1 to 100 barg, more preferably 5 to 10 barg.

[0034] The amount of the gas comprising carbon dioxide (CO2) supplied to the reactor is preferably such that there is at least a stoichiometric / equimolar amount and more preferably an excess of carbon dioxide (CO2) to effect carbonation of the metal carbonate (M(CO3) x ). Preferably, at least a portion of the gas comprising carbon dioxide (CO2) is recovered from and / or recycled to the reactor.

[0035] Typically, a gas containing carbon dioxide is applied to the reactor by injection with a gas injector. The gas injector is preferably disposed below the expected liquid level in the reactor such that the gas containing carbon dioxide is introduced into the reaction liquid in the form of bubbles / microbubbles. Each reactor may have a single gas injector or multiple gas injectors. In the case of multiple reactors, each reactor may have a different number of injectors and / or injectors of different nature. By using a suitable mixer, such as an in-line mixer, excessive gas dispersion can be reduced / eliminated.

[0036] In the case of multiple gas injectors, the injector / each injector may be different from each other. For example, each injector may have its own mass flow control for optimizing the gas / liquid ratio.

[0037] The gas injector is preferably in the form of a gas sparger. Examples of preferred gas spargers include sintered metal gas spargers. These spargers can be optimized to produce the desired discharge rate of fine bubbles.

[0038] Since the bicarbonation reaction is slightly exothermic, it is preferred to cool the reactor. The reactor is preferably cooled to below 30 °C, and more preferably below 20 °C. Higher temperatures may cause problems with the precipitation of metal carbonate (M(CO3) x ). Cooling is preferably carried out by standard cooling methods, such as using a heat exchanger. Pressurized CO2 injection can also be employed, which benefits from the Joule - Thompson effect of cooling.

[0039] Typically, the conversion of the metal carbonate is carried out at least in part. Preferably, the conversion of the metal carbonate is such that all of the metal carbonate supplied to the reactor is at least converted into a soluble form (i.e., not in the form of a suspension / slurry). In the case where complete dissolution is not achieved (identified by means such as turbidity analysis), the product solution can be redirected back to the reactor for further carbonation steps.

[0040] Example 1 - Preparation of Lithium Bicarbonate (LiHCO3) - Laboratory-Scale Example

[0041] Lithium carbonate powder is added to 25 L of demineralized water (or mother liquor from the LiF process, saturated in LiF at a concentration of about 1.3 g / L) in a plastic container to prepare a slurry containing 40 - 60 g / L of Li2CO3.

[0042] The slurry is mixed using a standard overhead stirrer with a coated impeller to dissolve the Li2CO3 until its solubility limit (about 13 g / L).

[0043] Next, a diaphragm pump was used to pump the Li2CO3 slurry at a discharge pressure of 8 barg and a forward flow rate of 1 L / min to a plug flow reactor (PFR), and the forward flow rate was measured by an in-line rotameter.

[0044] Three PFRs in series were used. Each PFR contained a 1 / 4" ID PFA tube with a length of 30 m.

[0045] The starting point of each PFR was a CO2 injection point, which included a T-tube and a sintered nozzle with a pore size of 2 microns.

[0046] CO2 was supplied by a compressed gas cylinder regulated to 8.5 - 9 barg. It was controlled by in-line mass flow control, with a minimum total CO2 addition of 16 g / min at 40 g / L or 28 g / min at 60 g / L. The CO2 addition rate at each injection point could be controlled independently.

[0047] The total residence time was about 3 minutes, and then all the Li2CO3 was observed to dissolve. This was visually obvious. The concentration was measured by conductivity and IC.

[0048] After passing through the PFR, the solution was passed through a backpressure controller, which maintained a system backpressure of 6 barg. Throughout the process, the pressure drop was about 2 bar.

[0049] Then the bicarbonate solution was collected in 25 L bottles for storage.

[0050] No precipitated impurities were observed because the Li2CO3 powder contained very few impurities. Separate experiments were conducted with quantitatively added impurities to evaluate the IX performance.

[0051] The bicarbonate solution (up to 60 g / L) was found to be stable. Any remaining Li2CO3 was found to remain in solution and did not precipitate over several days despite the pressure drop. This allowed the bicarbonate solution to be stored in an atmospheric storage tank ready for downstream processing steps, eliminating the need for more costly pressure vessels.

Claims

1. A continuous process for converting at least part of a metal carbonate (M(CO3) x ) into a metal bicarbonate (M(HCO3) y ), which comprises: A composition comprising a metal carbonate and a composition comprising water are supplied to a reactor to form a mixture of water and the metal carbonate; Optionally, a solution of water / metal salt is added; Wherein a gas comprising carbon dioxide (CO2) is supplied to the reactor under high pressure.

2. The method according to claim 1, wherein the metal carbonate is lithium carbonate or comprises lithium carbonate.

3. The method according to claim 1 or 2, wherein the reactor comprises a plug flow reactor (PFR) reactor.

4. The method according to claim 1, 2 or 3, wherein a single reactor is used, or multiple reactors are used.

5. The method according to claim 4, wherein in the case of using multiple reactors, these reactors are in series.

6. The method according to any one of claims 1 to 5, wherein the total residence time through the reactor / reactors in series is about 3 minutes.

7. The method according to any one of claims 1 to 6, wherein a metal bicarbonate solution is extracted from the final reactor and transferred to a storage tank.

8. The method according to any one of claims 1 to 7, wherein the initial concentration of the metal carbonate (before dilution) is in the range of 120 - 240 g / L (1.6 to 3.3 moles).

9. The method according to any one of claims 1 to 8, wherein the gas comprising carbon dioxide (CO2) is applied at a pressure of 5 to 10 barg.

10. The method according to any one of claims 1 to 9, wherein the gas comprising carbon dioxide is applied to the reactor by injection with a gas injector.

11. The method according to any one of claims 1 to 10, wherein the reactor is cooled to below 20 °C.

12. A metal bicarbonate produced by the method according to any one of claims 1 to 11.