Note: Descriptions are shown in the official language in which they were submitted.
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The present învention relates to a process for
the preparation of lithium silicate according to which
lithium carbonate and silica are mixed intimately and the
mixture obtained is then submitted to a thermal treatment.
The lithium silicate can, e.g., be used as a
covering material in fusion reactors with the object of
producing tritium.
A usual process for the preparation of lithium
silicate consists in mixing lithium carbonate and silica
in suitable proportions and in calcining said mixture at a
temperature near the melting point of lithium carbonate
(718C + 2C). The thus obtained silicate has poor
sintering properties and is not suited for the manufacture
of dense pellets (more than 85% of the theoretical
density) by cold pelletizing and sintering.
An object of the invention is to remedy this
drawback and to provide a process for preparing lithium
silicate allowing a grade of lithium silicate to be
obtained that can be sintered and hence can be used for
the manufacture of high-density pellets by cold
pelletizing and sintering.
Accordingly, the invention provides a process
for the preparation of lithium silicate comprising the
steps of: mixing lithium carbonate and silica, and
submitting the thus obtained mixture to a thermal
precalcination treatment at a temperature between 410C
and 440C for a period of at least 10 hours, so as to form
in said mixture at least 25% by weight of a hexagonal
phase composed of lithium, silicon and oxygen, whose
30 crystal parameters are a = 3.06 A and c = 4.9~ A, and
submitting directly the product obtained by the thermal
precalcination to a high-density sintering treatment or to
a calGination treatment.
By precalcination a thermal treatment is to be
understood at a temperature below 440C.
The above phase not yet described in technical
literature and called phase X hereinafter could be
isomorphous to lithium aluminate (LiAl02), described by
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- la -
T.I. Barry and collaborators in "The crystallization of
glasses based on the Eutectic composition in the System
Li20 - A1203 - SiO2, Part II, Lithium Metasilicate-beta
Eucryptite"
, :
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-- 2
(Journal of Materials Science, 5,1970, 117-126). However,
the authors are not sure at all about the composition of the
above-mentioned compound and the corresponding descriptive
card has been removed from the "Powder Diffraction File"
in 1983.
According to a particular embodiment of the
invention the mixture is maintained at a temperature
between 410C and 440C for at least 10 hours.
According to an advantageous embodiment of the
invention the thermal precalcination treatment is carried
out at a temperature comprised between 430C and 435C.
According to an efficient embodiment of the
invention the thermal precalcination treatment is carried
out in moist air.
Preferably the thermal treatment is carried out
in moist air containing at least 10 % of relative humidity.
After the thermal precalcination treatment the
product rich in phase X can be pelletized directly and
sintered at relatively high density, or it can also be
20 calcined at low temperature (from 450C to 600C) before
being pelletized and sintered at high density. Precalcination
and calcination are preferably carried out in one single
treatment, e.g., by a 91ow rise between 410C and 470C
in 60 hours.
Other particular and advantages of the invention
will appear from the description of a process for preparing
lithium silicate according to the invention; this
description is only given by way of example without limiting
the scope of the invention.
For preparing lithium silicate that can be
sintered, suitable amounts of lithium carbonate and silica
are fixst mixed intimately.
This mixture may be in the form of powder,
granules or pellets.
,,
_ . . . .
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-- 3
Then this mixture is submitted to a thermal
treatment in moist air for several hours in order to form
a hexagonal intermediate phase X composed of lithium,
silicon and oxygen, whose crystal parameters are
a - 3.06 R and c = 4.97 R.
This phase X i8 ideally formed by a thermal
precalcination treatment ox the mixture carried out
between 430C and 435C, but it i9 also formed in a
sufficient amount between 410C and 430C or between 435C
and 440C on condition that the mixture is kept between
these temperatures for at least 10 hours.
It is indeed not necessary to work under the
optimal conditions of formation of phase X.
For achieving the expected results it suffices
to form at least 25 % by weight of it.
Besides, this phase X is gradually transformed
into metasilicate.
Consequently said mixture of lithium carbonate
and silica is submitted for several hours to a thermal
precalcination treatment at a temperature between 410C
and 440C and preferably between 430C and 435C in moist
air.
The treatment is preferably effected in moist
air containing at least 10 % of relative humidity. The
best results, however, are obtained with air saturated
with humidity at room temperature. The precalcination
treatment is preferably effected for at least 10 hours,
especially if it takes place at temperatures comprised
between 410C and 430C or between 435C and 440C.
Evidently, the thermal treatment may take place
either at constant temperature, or in successive levels,
or in a continuous slow rise.
The thermal treatment modifies the sintering
properties of the mixture.
After said thermal precalcination treatment
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the product rich in phase X is pelletized and sintered at
relatively high density, or the product it first calcined
at relatively low temperature (from 450C to 600C) for
transforming phase X into metasillcate whereafter it is
pelletized and sintered at high density
Precalcination and calcination are preferably
effected in one single treatment, e.g., by a 910w rise
between 410C and 460C to 480C within 50 to 70 hours
and preferably from 410C to 460C within 60 hours.
The formation of 25 % of phase X i8 essential
for obtaining a product having a high specific surface
area and thus being very æuited for sintering, even after
calcination. The table hereinafter illustrates these
results : it lists the values of specific surface area
of mixtures of lithium carbonate and silica ground under
water, spray-dried and then treated according to the
invention or not.
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_ _
THERMAL TREATMENT spec.surf.
(BET) in
m2 /g
According to the invention
granules precalcined in moist air .
successively at 410C for 6 h, at 420C for 56
1 h and at 430C for 8 h
powders precalcined in air of 60 % of 60
relative humidity at 430C for 15 h
pellet precalcined at 421C for 24 h, then 34
calcined at 529C for 24 h
granules precalcined at 430C for R h,then 38
calcined at 550C for 12 h
powder precalcined at 430C for 30 h,then 15
calcined at 605C for 16.5 h
Calcination of a product containinq
no phase X
powder precalcined at 380C for 60 h,then 3
calcined at 550C for 24 h
powder calcined at 600C for 24 h 3.5
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It is evident that the process according to the
invention i5 suited for obtaining lithium silicates that can
be sintered and that are destined for manufacturing high-
density sintered parts as well as for making sintered parts
however may be the method of shaping and the final form of
said parts. Said parts, e.g., are shaped in the form of
pellets or granules.
The process according to the invention warrants
a high specific surface area (see table above) and
favourable sintering properties as well : 90, it i9 possible
to make dense parts by sintering at not too high temperatures
and during short periods of time. This allows the
realization of a fine-grain structure.
The process also allnws to improve the sintering
propertie9 of solely precalcined products or of precalcined
and calcined products by improving the fineness of the raw
materials. This i9 not possible without forming phase X.
According to the above described process one
can obtain after the thermal precalcination process a
mixture formed of phase X and metasilicate still containing
some residual carbonate but being free from disilicate
and orthosilicate, even if the general composition of the
mixture deviates in one of both senses from the
stoichiometric composition corresponding to the meta-
silicate. After calcination at low temperature thecomposition of the obtained product is relatively near the
nominal composition of the mixture. This is not at all
the case by direct calcination at low temperature without
the said thermal precalcination process : a mixture is then
obtained rich in disilicate that has to be transformed
into metasilicate during sintering, which occasions
deformation of the pellets and a fall of density.
Some examples illustrate the invention.
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Example 1
190 g of lithium carbonate powder and 160 g of
silica powder (AEROSIL of Degussa - 300 m /g) are mixed
with distilled water so as to form a smooth paste, This
paste is ground for 60 min under water in an attritor
whose receptacle, balls and arms are made of alumina. The
suspension thus obtained is continuously stirred in order
to avoid any demixing, then spray-dried.
The resulting fine powder has a specific
surface area of 110 sq.m/g.
The powder i9 precalcined in moist air at 430C
for 30 h, then calcined at 605C for 16 h and finally
pelletized under 2.5 t/cm2 in a matrix of 6 mm diameter.
After sintering at 950C in the air for 9 h
the pellets have a density of 91.4 % T.D. .
The same mixture ground for -2 h instead of 1 h
and treated under the same conditions yields sintered
pellets of 95 % T.D. (starting from green pellets of
48 % T.D.) and of 98 % T.D. (starting from green pellets
of 50 % T.D.) .
Example 2
272 g of lithium carbonate powder is mixed with
228 g of silica (AEROSIL of Degussa) and water, ground for
60 min in an attritor and spray-dried.
The dried powder is precompacted under 1.2 t/cm2
in pellets of 12 mm diameter. The pellets are granulated
on a grid of 1 mm; the obtained granules are successively
treated at 410C for 6 h, at 420C for 16 h and at 430C
for 8 h in moist air, then pelletized under 2.5 t/cm2.
The pellets attain a density of either 87.6 % T.D. at
950C after 3 h or of 81.1 % T.D. at 950C after 1 h.
Example 3
The precalcined granules of Example 2 are
calcined at 525C for 12 h before being pelletized under
2.5 t/cm . The pellets attain a density of 93 % T.D. at
* trademark
-- 8 --
925C after 3 h and of 9o % T.D. at 950C after 1 h.
It should be understood that the invention
i9 not at all limited to the above described embodiment3
and that many modification can be made without departing
from the scope of the present patent application.
