Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to the preparation of zeolite A
which is a synthetic three-dimensional crystalline zeolite of
the molecular sieve type. More particularly, an improved and
more efficient process is provided.
Description of the Prior Art
:,
The properties, formula and preparation of zeolite A
are well known being described for example in U. S. Patent
3,114,603, and in U. S. Patent 3,119,659. The references
cited in both patents are also of interest.
According to the process of U. S. Patent 3,114,603,
synthetic zeolite A is prepared in a two-step process wherein
the reactants are combined and digested at a temperature of
20-55C for a period of at least 2 hours in a first step which
is followed by a second step of heating at at least 75C for
a prolonged period. In general, the preferred total time
required for the two steps in the examples of U. S. Patent
3,114,603 ranges from 7.5 to 48 hours.
Summary of the Invention
The process of the present invention comprises
producing sodium zeolite A by reacting metakaolin and NaOH in
an aqueous solution at a temperature of from about 70 to about
100 Cfor from about 1 hour to about 15 hours although reaction
periods considerably shorter are preferred. Preferably the
temperature is from about 80 to about 85C, especially about
85C. Preferably the metakaolin, water and NaOH are used in
lO'f'4Z~4
ratios of zeolite A product, the mol ratio of H2O/Na2O in the
system being from about 15 to about 200, preferably f.rom
about 25 to about 50, and typically 40.
Description of the Preferred Embodiments
Zeolite A is represented by the formula:
1.0 + 0.2 Na20:A1203:2.0 + 0.5 SiO2:YH2O
wherein Y may have any value up to about 5.1.
A preferred zeolite is represented in Belgian Patent
814,874 by the formula:
Nal2(Alo2-sio2)l2 xH2o
wherein x is a number in the region of 20 to 30, preferably
about 27. Such are the compositions desired to be produced
by the process of the present invention.
From prior art such as U. S. Patent 3,114,603,
it is known that one way the foregoing zeolites can be prepared
is by reacting metakaolin and NaOH in aqueous solution;
however, such prior art processing utilizes a two-step
process for high purity product (above about 80 percent
zeolite A purity) wherein the reactants are combined and
digested at room temperature (20-25C) for several hours
and then the temperature is raised to a temperature in the
region of 75 to 110C for a prolonged crystallization
operation. Apparently the prior art teaches
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that the two step operation is required to avoid or minimize
the presence in ~e product ~f by-products or contaminants
such as other zeolites, hydroxy sodalite or amorphous substances.
In contrast to the foregoing, it has been found
that a single step process is possible using a reaction time of
the order of two to four hours and that with such shortened time
one step processing, zeolite A product purity of 90 percent or
better are readily obtained. It will be appreciated that the
present discovery makes it possible to simplify the design of
plants used to produce zeolite A, such simplification being
reflected in lower cost product. The present inventors have
found, for example, that a reaction time of 2 hours at 85C is
adequate to achieve substantially complete reactions to a final
product purity of 9S percent or higher.
Product thus obtained is readily recovered by
filtering or centrifuging following which the recovered product
is washed to remove residual mother liquor and impurities. Washed
product is then subjected to drying to remove residual water.
Although mother liquor removed from the product
as well as the washwater may be recycled wholly or in part to the
reaction step for combination with fresh fed reactants, such
generally is less preferred than performing one or more additional
crystallizations to recover the product value contained therein.
With such preferred processing, the present process forms a water
solution of NaOH and reacts that with metakaolin rather than
forming an aqueous NaOH solution containing recycle mother liquor
for the reaction with metakaolin.
Of course, it is evident that the present process
does not require the exclusion of recycle mother liquor and
washings from the reaction system and that under appropriate
cm/
- 1074~
conditions such recycle may be advantageous. It is also evident
that total recycle of the mother liquor is not usually desired
in such systems since the purge of a portion of the recycle
stream is usually desirable to avoid the build-up of impurities.
The present process utilizes the meta form of
amorphous kaolin, called metakaolin, or the equivalent, prefer-
ably produced by the calcination of kaolin at a temperature of
from about 550 to about 900C. Such calcination is well known
in the art and need not be discussed in greater detail here since
it appears in U.S. Patent 3,114,603. As explained in U.S. Patent
3,114,603, it is usually desired to avoid prolonged time delay
between the calcination of the feed kaolin and its use in the
present process.
The following examples indicate preferred embodi-
ments and aspects of the present invention.
EXAMPLE 1
250 grams H20 was added to a l-liter round bottom
flask equipped with a heating mantla and an agitator at room
temperature. 34.5 grams of NaOH beads was then added and the
mixture was stirred until the NaOH was dissolved. 40 grams of
metakaolin was added and the mixture was heated to 85C. The
flask and contents was held at 85C for 2 hours while stirring.
The heating mantle was removed and the contents of
the flask filtered with a Buchner funnel to recover the solid
product. The product was washed with 500 grams of water and oven
dried at 100C for 5 hours or more.
~ sample of the product was analyzed by X-ray
diffraction and found to be 95 percent or higher zeolite A
(ASTM X-ray powder diffraction pattern File Card No. 11-590).
The mol ratios of the foregolng example are:
cm~ ' '
10'74~
Na20/A1203 -= 2 .
SiO2/A1203 = 2
H20/A1203 = 78
H20/Na20 = 39
EXAMPLE I I :
Example I was repeated using 125 grams of water
instead of 250 grams and a temperature of 80C instead of 85C.
Similar results were obtained. The H20/Na20 ratio was 20.
EXAMPLE I I I
153 gallons of deionized water was added to a
steam jacketed, stirred, 250 gallon reactor. 184 pounds of
sodium hydroxide beads was then added to the reactor. The
mixture was heated to 179C and agitated until the beads dissolved.
Then 20~ pounds of metakaolin was added to the reactor and the
temperature was increased to 185F (85C). This temperature
was held 2-1/2 hours while the mixture was agitated. The reaction
mass was cooled to approximately room temperature and discharged
in four batches into a centrifuge for removal of the supernatant
liquor. The centrifuge cake was washed with 262 gallons of
deionized water. 286 lbs of zeolite A was recovered. The
product was essentially 100 percent zeolite A as determined by
X-ray diffraction.
EXAM2LE IV
Example I was repeated in a series of runs under
various conditions. Results are tabulated with the results of
Examples I and II included as Runs I and II, respectively.
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