Gamma lithium aluminate product and process of making

Details Gamma lithium aluminate product and process of making Gamma lithium aluminate product and process of making

1998-12-01

2001-09-18

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

Reexamination Certificate

active

06290928

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lithium aluminate which is particularly useful for an electrolyte-retainer plate in a Molten Carbonate Fuel Cell (MCFC) and to an industrial preparation process thereof.
2. Description of the Related Art
An electrolyte-retainer plate of MCFC is required to have such characteristics as high molten carbonate retaining property, alkali resistance and heat resistance since it is used for retaining a mixed molten carbonate such as Li
2
CO
3
and K
3
CO
3
in an elevated temperature region around 650° C. As a material which satisfies the required characteristics, presently lithium aluminate has been used as a structural material of the electrolyte-retainer plate, particularly finely divided &ggr;-type lithium aluminate having a relatively large specific surface area and excellent electrolyte retaining property has been used.
As for the technologies of producing lithium aluminate having such a large specific surface area, many suggestions have been made including those disclosed in Japanese Patent Laid-Open Nos.60-65719, 60-151975, 61-295227, 61-295228, 63-270311, 1-252522, and 2-80319. The main points of these known methods lie in increasing the specific surface area by controlling the denseness of the structure through burning a mixture of alumina and lithium hydroxide or lithium carbonate at a temperature in a range of from 600 to 1000° C., or by carrying out secondary treatment such as hydration treatment or a treatment to impart porosity.
In the specification of Japanese Patent Publication No.6-37292, a process for preparing porous lithium aluminate coarse particles has been disclosed in which silica particles and alumina particles are bonded to prepare coarse particles then they are heated to 500-1000° C. in a carbonate containing lithium ion so that the silica particles in the coarse particles are eluted into the carbonate to form pores and the alumina having pores is combined with lithium to form lithium aluminate.
However, when the &ggr;-type lithium aluminate produced by the above-mentioned conventional technique is exposed to a high temperature in a molten electrolyte for a long time, such phenomenon appears that the &ggr;-type structure is partly transformed into &agr;-type, or the specific surface area is decreased by grain growth. Accordingly, when it is formed into an electrolyte-retainer plate for MCFC, it has a defect that the electrolyte retention capability may drop suddenly during its use to deteriorate the battery life.
The process of preparing porous lithium aluminate disclosed in the above-mentioned Japanese Patent Publication No.6-37292 has been proposed as a technique to alleviate such defect, however, it has problems such as being a long and complicated process since it requires special process to carry out ion exchange of lithium after obtaining coarse particles of silica-alumina, and difficulty in carrying out ion exchange completely.
As can be seen from the above description, the requirements for improving the long-life of MCFC which are becoming more and more severe, including high level of retention capability for the molten carbonate, high alkali resistance and high heat resistance, cannot be sufficiently satisfied with the conventional &ggr;-type lithium aluminate preparation technique, besides, the conventional techniques still have some problems to be cleared before they can be used as industrial production means.
Accordingly, an object of the present invention is to solve the above-mentioned conventional problems, and in i particular, to provide &ggr;-type lithium aluminate having excellent heat stability and chemical stability in a molten carbonate when used for an electrolyte-retainer plate of MCFC, and an industrial preparation process thereof.
SUMMARY OF THE INVENTION
As a result of an intensive study carried out in order to solve the above-mentioned problems, the present inventors found out that lithium aluminate, particularly having a specific BET specific surface area, and having a diffraction intensity ratio by lithium aluminate X-ray diffraction spectral analysis in a specific range, shows excellent properties as a material for an electrolyte-retainer plate of MCFC. The present inventors also confirmed the fact that the lithium aluminate obtained from such an alumina source as cluster particles of &agr;-alumina which are produced by burning a finely divided aluminum compound, or porous &agr;-alumina particles which are obtained by burning relatively coarse particles of an aluminum compound at a high temperature and thereby having a stabilized particle structure, does not change its particle structure even after being exposed to a high temperature in a molten carbonate for a long time, and shows excellent alkali resistance, heat resistance and high level of retention capability.
Therefore, according to one aspect of the present invention, there is provided &ggr;-type lithium aluminate, a particle of which has a BET specific surface area in a range of from 1 to 15 m
2
/g and a synthesized degree (P) calculated according to the following equation (1) of not less than 80%.
Synthesized degree (
P
)=(
I
2
/I
1
)×100  (1)
In the equation (1), I
1
and I
2
represent diffraction intensity in the X-ray diffraction (X-RD) spectral analysis of lithium aluminate and I
1
represents the height of a (101) peak, I
2
represents the height of a (200) peak.
According to another aspect of the present invention, there is provided the above-mentioned &ggr;-type lithium aluminate which has a BET specific surface area change rate (R) obtained according to the following equation (2) in a range of not more than 25%.
R
=(
S
2
−S
1
)/
S
1
=100  (2)
In the equation (2), S
1
represents the BET specific surface area of lithium aluminate (m
2
/g) prior to heating, and S
2
represents the BET specific surface area of lithium aluminate (m
2
/g) after the heating, and the heating conditions for lithium aluminate are such that a sample and an electrolyte (constituent composition is LI
2
CO
3
:K
2
CO
3
=62:38 mol %) are mixed at a ratio by weight of 1:3, and subjected to treatment at 700° C. for 200 hours in an electric oven wherein the atmosphere is maintained to be air/CO
2
=70/30.
According to a further aspect of the present invention, there is provided a method of producing &ggr;-type lithium aluminate characterized in that cluster particles of &agr;-alumina obtained by burning a finely divided aluminum compound are mixed with a lithium compound, where an Al/Li molar ratio is 1/0.95-1.05, by dry mixing, and the mixture is subjected to burning treatment.
According to a still further aspect of the present invention, there is provided a method of producing &ggr;-type lithium aluminate characterized in that porous &agr;-alumina particles and a lithium compound are mixed, where an Al/Li molar ratio is 1/0.95-1.05, by dry mixing and the mixture is subjected to burning treatment.


REFERENCES:
patent: 3878296 (1975-04-01), Vine et al.
patent: 3998939 (1976-12-01), Mason et al.
patent: 4704266 (1987-11-01), Kadokura et al.
patent: 5217702 (1993-06-01), Frianeza-kullberg
patent: 5252315 (1993-10-01), Higashiyama et al.
patent: 5434031 (1995-07-01), Nakao et al.
patent: 5580673 (1996-12-01), Farooque et al.
patent: 60-65719 (1985-04-01), None
patent: 60-151975 (1985-08-01), None
patent: 61-295227 (1986-12-01), None
patent: 61-295228 (1986-12-01), None
patent: 63-260812 (1988-10-01), None
patent: 63-270311 (1988-11-01), None
patent: 1-252522 (1989-10-01), None
patent: 2-80319 (1990-03-01), None
patent: 2-233519 (1990-09-01), None
patent: 5-294614 (1993-11-01), None
Kinoshita et al., “Synthesis of fine particle size lithium aluminate . . . ” Mat. Res. Bull. vol. 14, pp. 1357-1368, 12/79.

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