EP2162393A1

EP2162393A1 - A process for the production of eutectic li2co3-likco3

Info

Publication number: EP2162393A1
Application number: EP07736870A
Authority: EP
Inventors: Annalisa Sabattini; Elena Bergaglio; Paolo Capobianco
Original assignee: Ansaldo Fuel Cells SpA
Current assignee: Ansaldo Fuel Cells SpA
Priority date: 2007-05-28
Filing date: 2007-05-28
Publication date: 2010-03-17
Also published as: WO2008146312A1; WO2008146312A8

Abstract

The present invention relates to a process for the production of a 1:3 Li₂CO₃/LiKCO₃ eutectic mixture with a potassium carbonate content less than the detection limits of X-ray analysis, comprising the step of mixing potassium carbonate and lithium carbonate in a ball mill, characterised in that the potassium carbonate is hydrated and mixing is conducted for a period of time greater than or equal to 80 hours.

Description

DESCRIPTION "A process for the production of eutectic Li₂CO₃-LiKCO₃"

[0001] The present invention relates to an improved process for obtaining highly pure eutectic Li₂CO₃/LiKCO₃, preferably for use in molten carbonate fuel cells.

[0002] A number of processes are known for producing the eutectic mixture consisting of Li₂CO₃/LiKCO₃ phases in a molar ratio of 1:3 starting from a eutectic mixture of Li₂CO₃/K₂CO₃ in a molar ratio of 62:38. [0003] Patent US 7,060,219 describes equipment and a process for the continuous production of a eutectic mixture consisting of Li₂CO₃/LiKCO₃ in a molar ratio of 1:3. The production method comprises, among others, the steps of mixing Li₂CO₃ and K₂CO₃, melting the mixture obtained, maintaining the mixture at a temperature greater than that of the melting point, cooling the mixture in a crucible, breaking up the block of carbonates obtained, pulverising and sieving the powder in order to obtain the desired granulometry. [0004] Hence, the above described process includes a step of melting the mixture of carbonates, and processing, following cooling of the molten carbonates, thus making the method costly and laborious . [0005] Patent WO 2005/096429, from the same applicant, describes a process for obtaining a eutectic Li₂CO₃/LiKCO₃ mixture in a molar ratio of 1:3 consisting of mixing Li₂CO₃ and K₂CO₃ in powder form and grinding them in a ball mill for periods of time comprised of between 20 and 28 hours . [0006] With respect to the method described in patent US 7,060,219, this process has the advantage of not having the steps of melting the mixture of carbonates and processing, following cooling, of the molten carbonates and, thus being a faster and less costly method. [0007] In any case, the inventors of the present patent application have surprisingly found that the process in WO 2005/096429, despite being an optimal method for obtaining highly pure eutectic Li₂CO₃/LiKCO₃, could be improved in terms of the purity of the eutectic obtainable. Indeed, it has been verified that said process does not always give the desired result, i.e. a eutectic mixture that is free, or essentially free, of potassium carbonate. [0008] The problem that has been dealt with by the inventors, and that lies at the heart of the present invention, is thus that of providing an improved method for preparing a highly pure eutectic Li₂CO₃/LiKCO₃ mixture in a molar ratio of 1:3, preferably a eutectic mixture where the content of K₂CO₃ and hydrated K₂CO₃ is below the X-ray analysis detection threshold. [0009] Said problem is resolved by an improved process for preparing a highly pure eutectic Li₂CO₃/LiKCO₃ mixture in a molar ratio of 1:3, as described in the appended claims . [0010] It has surprisingly been found that, using hydrated potassium carbonate as a starting product, and mixing the potassium carbonate and lithium carbonate in a ball mill for a period of time greater than or equal to 80 hours, a 1:3 eutectic Li₂CO₃/LiKCO₃ mixture is obtained with a potassium carbonate content less than the limits detectable by X-ray analysis.

[0011] Preferably, the starting potassium carbonate is K₂CO₃-3/2H₂O. Furthermore, the potassium carbonate and lithium carbonate are mixed in a ball mill, preferably for a period of time greater than or equal to 90 hours. Preferably, both the grinding spheres and the mill are made of alumina .

[0012] This result is much more surprising considering that by using a hydrated potassium carbonate a eutectic mixture is obtained where the potassium carbonate is more or less non-existent.

[0013] The Li₂CO₃ and K₂CO₃ starting salts are mixed in a ball mill in such a stoichiometric ratio as to obtain, by means of mechanical processing, the 62/38 Li₂CO₃/K₂CO₃ eutectic mixture in which Li₂CO₃ and the corresponding LiKCO₃ fusion compound are present in a ratio of 1:3. Preferably, Li₂CO₃ and K₂CO₃ will be mixed in stoichiometric ratios comprised of between 3:1 and 1:1, preferably in a ratio of 1.7:1. [0014] Hence, in the eutectic mixture of the present invention, water soluble K₂CO₃ is absent, whilst LiKCO₃, which has greatly reduced water solubility, is present. The high purity, of the eutectic mixture of the present invention allows its use in the production of a homogeneous electrolytic mixture to be used in molten carbonate fuel cells, wherein the carrier solution is a mixture of water with one or more organic solvents, according to the description in WO 2005/096429. [0015] EXPERIMENTAL TESTS [0016] In order to obtain approx. 3 Kg of powdered mixture of 62/38 Li₂CO₃/K₂CO₃ eutectic mixture, a 10 L alumina jar is filled to approx. 1/3 of its volume with alumina grinding spheres (diameter - 25 mm) , 1266 g of Li₂CO₃ and 1734 g of K₂CO₃-3/2H₂O equal to a Li₂CO₃/K₂CO₃ weight ratio of 42.2/57.8 (in order to obtain a molar ratio of 62/38) . [0017] The jar is placed on a "trundler" and the powder ground at a speed greater then or equal to 70 rpm for 96 hours . [0018] On completion of the process, the powder is unloaded from the mixture and X-rays used to verify that the eutectic mixture is formed exclusively of Li₂CO₃ and LiKCO₃.

[0019] As may be deduced from Table 1, potassium carbonate and hydrated potassium carbonate are absent (example 1) . [0020] Table 1 also reports the comparative tests where the eutectic mixture has been prepared by following the above described process, but with a grinding time of 72 hours

(example 2) . As may be observed from the comparison, with equal purity of starting potassium carbonate, times of over 80 hours are required, preferably greater than 90 hours in order to obtain a 1:3 Li₂CO₃/LiKCO₃ eutectic mixture free of potassium carbonate and hydrated potassium carbonate impurities. [0021] Table 1 also shows a comparison example (example 3) wherein the eutectic mixture is produced by following the above described process, but using anhydrous potassium carbonate. Again in this case, it may be observed that the eutectic mixture obtained, despite mixing being carried out for 96 hours, contains potassium carbonate impurities .

[0022] Table 1 also shows a comparison example (example 4) wherein the eutectic mixture is produced by following the above described process, but using a different purity of lithium carbonate (the RE and RPE-ACS codes indicated in table 1 refer to salts with different purity, as indicated in the Carlo Erba catalogue) . In this case, it may be observed that potassium carbonate and anhydrous potassium carbonate are absent from the eutectic mixture obtained, and hence, mixing is complete. This shows that the desired eutectic mixture is attained when any kind of lithium carbonate is used with hydrated potassium carbonate .

[0023] In conclusion, grinding times greater than 80 hours, preferably greater than 90 hours, and the use of hydrated potassium carbonate, preferably K₂CO₃-3/2H₂O, are essential for obtaining the eutectic mixture of the invention.

Table 1

[0024] Table 2 below shows a series of experiments preparing the eutectic mixture, performed by using the above described process, wherein various different -potassium carbonates (all hydrated) have been used. In particular, K₂CO₃ (MERCK, hydrated in the laboratory) , K₂CO₃ (Carlo Erba RPE-ACS, hydrated in the laboratory) and K₂CO₃-3/2H₂O (SIGMA-ALDRICH, purchased in sesquihydrate form) have been used.

Table 2

0 [0025] The results obtained show that whatever kind of potassium carbonate used, both laboratory hydrated and purchased already as potassium carbonate sesquihydrate, leads to the formation of the mixed salt. [0026] The process described in the present invention has 5 the advantages of being faster and cheaper than the method described in US 7,060,219. Furthermore, it is more reproducible and reliable than the process described in

Claims

Claims l.A process for the production of a 1:3 Li₂CO₃/LiKCO₃ eutectic mixture with a potassium carbonate content less than the detection limits of X-ray analysis, comprising the step of mixing potassium carbonate and lithium carbonate in a ball mill, characterised in that the potassium carbonate is hydrated and that the mixing is conducted for a period of time greater than or equal to 80 hours.

2. The process according to claim 1, wherein said potassium carbonate has a water content equal to K₂CO₃-3/2H₂O.

3. The process according to claim 1, wherein said mixing time is greater than or equal to 90 hours.

4. The process according to any claim 1 to 3 , wherein said grinding spheres and said mill are made of alumina.

5. The process according to any claim 1 to 4, wherein Li₂CO₃ and K₂CO₃ are mixed in a stoichiometric ratio comprised of between 3:1 and 1:1, preferably in a ratio of 1.7:1.

6. Use of the 1:3 Li₂CO₃/LiKCO₃ eutectic mixture, with a potassium carbonate content less than X-ray analysis detection limits, to prepare a homogeneous electrolytic mixture to be used in molten carbonate fuel cells .

7. The use according to claim 6 , wherein the carrier solution of said electrolytic mixture is a mixture of water with one or more organic solvents .