Note: Descriptions are shown in the official language in which they were submitted.
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The present invention concerns a novel process for the production of
a silico-aluminate with a high capacity for absorption of water and with an
SiO2/A1203 molar ratio higher than 1.85.
It is known to produce zeolites of type A, which are enriched in
respect of SiO2. Thus French Patent No 1 536 ~47 to WEY, GUTH and COLLIN
describes a process for producing zeolites of this kind, from mixtures containing
the oxides forming the zeolites in molar ratios within the following limits:
SiO2/A1203 5 to 15
Na20/A1203 7.5 to 17.8
~i2/H2 0.0152 to 0.0734
Na20/H20 0.038 to 0.070
The production process described comprises raising a mixture of sili-
cate and aluminate to a temperature which is close to boiling temperature, and
maintaining a high temperature under conditions of agitation for a period of
time to produce crystallites of type A.
However, such a process suffers from two disadvantages. Firstly,
beczuse it involves operating in a relatively dilute medium and secondly because
it does not afford the means for continuous production.
French Patent No 1 257 034 describes a process for directly producing
a silico-aluminate of type A, corresponding to the formula Na20. A1203. 2 SiO2.
4.6 H20. However, in this case, the process uses a highly dilute solution and
a long periof of time is required for achieving the result.
French Patent No l 310 548 also recommends using a reactive system with
a SiO2/A1203 ratio which is equal to or higher than 2, starting from kaolin in
a more highly concentrated medium, but the resulting product contains only from
85 to 87% by weight of zeolite A, and the period o-f time required for pToduction
thereof is still long.
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It is ound that, even though the literature includes a large number
of ~ocuments and patents dealing with the production o~ crystalline silico-
aluminates of type A in a discontinuous mode, few documents deal with a method
of producing them in a continuous mode. According to United States Patent No.
3,071,~34, this difficulty is due to the complex structure of the crystals which
ordinarily require a relatively long period of time to form from the highly
disordered reactants which are initially distributed in a random configuration
through the liquid and solid phases of the reaction medium,
A first approach in an effort to overcome this type of problem
consisted in providing for recirculation or recycling of the liquors. Thus,
United States No. 3,071,434 claims recycling of a part of the suspension contain-
ing the crystallite. It should be noted however that the process is limited to
a fairly dilute medium, corresponding to a system of the following molar ratios:
Na2O/SiO2 0.8 to 3.0
SiO2/A12O3 0.5 to 2.5
H2O/Na2O 35 to Z00
Attempts have been made to modify the process by directing it towards
a process of the type involving successive regions, as in United States Patent
No. 3,~25,800 or as in Canadicm Patent No. L,132,526 and ~rench Patent 2,38~,716
to the present applicants. In those cases, however, the process encounters either
limits in regard to the concentration of the medium, as before, or the presence
of an alumina-rich mother liquor.
Indeed~ it is known that, in the prior art, operation is generally
effected with an excess of aluminate, which means that the SiO2/A12O~ ratio is
in actuality lower than 2. This gives a disadvantage from the process point of
~iew, namely alumina-rich mother liquors.
It has also been proposed that operation may be under substantially
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stoichiometric conditions as in Vnited Kingdom Patent No. 2,005,653, but this
involves relatively dilute solution conditions.
A similar sltuation a~ises in regard to United States Patent No.
3,674,426, which describes a process Eor producing zeolite A also in a dilute
medium and with SiO2/A12O3 molar ratios which are higher than 2. Ilowever, it
should be noted that, under these conditions, the water absorption capacity
remains low and it is necessary to resort to salts of heavy metals, such as
bichromate, vanadate or permanganate, in order to raise the water absorption
capacity.
It is also known that, as regards the product, the other requirements
in regard to uses thereof become increasingly severe: fine and confined
granulometries, well-isolated particles and good thermal stability.
~lowever, we have now found a process which makes it possible directly
to produce a zeolite of type A with an SiO2/A1203 molar ratio higher than 1.85
and advantageously at least equal to 2, with a high capacity for the absorption
of water.
Accordingly, the invention provides a process for producing a zeolite
of type A with a high water absorption capacity and an SiO2/A1203 molar ratio
higher than 1.85, which comprises: a) producing a mixture of solutions of silica,
sodium silicate and sodium aluminate from a cooled aluminate solution cmd a
silicate solution at a temperature such that the temperature of the medium ob-
tained is substantially lower than the ambient temperature, while maintaining
sufficient agitation to permit a homogenous solution to be produced before a gel
appears and while maintaining the molar ratio in the reaction medium at a value
at least equal to 1.85; b) raising the temperature of the reaction medlum to a
value in the range of from 60 to 100C; c) maintaining the medium at that temp-
erature for a sufficient period of time of from 0.2 to about 5 hours so as to
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effect crystallisation and redispersion of the sillco-alumina-te in the form of a
suspension of crystals in the liquid phase; and d) separating and drying the
crystals thus obtained.
Thus7 the process of the invention comprises firstly producing a
homogenous medium of the solutions of silicate and aluminate before-formation of
a gel, then gelling the mixture, then causing it to evolve until crystallites
are formed, and separating said crystallites. In accordance with the present
invention, the SiO2/A12O3 molar ratio in the initial reaction medium is at
least 1.85 and is preferably at least 2.
The homogeneity of the initial medium before gelling is achieved
using any known means, for example by means of a severe shearing action. However,
it is known that this means may ~e unsatisfactary, in particular in concentrated
media. It is for this reason that advantageously at least one of the solutions
of reactants is introduced at low temperature so that the initial medium is at
a temperature whi.ch is less than the ambient temperature, as described in
Canadian Patent No. 1,132,526.
It will be appreciated that this temperat~re must be determined by
the man skilled in the art so as to represent a compromise between the necessit~
of not attaining excessive viscositles which are detrlmental to homogenisation
by mechanical means and the obligation to delay the gelling action until a
homogenous condition is achie~ed,
Indeed, it can be considered that mixing should generally take place
in at most about fifteen minutes.
As stated hereinbefore, t~e process is applied to concentrated media,
that is to say media containing more than 200 g/l of zeolites.
In accordance w-ith a preferred embodiment of the invention:
a) an initial medium is produced, which corresponds to the following
7~
system expressed in respect o:E molar ratios:
Na20/SiO~ 1.05 to 1.50
SiO2/A12O3 2 to 2 5 and preferably
H2O/Na2O 20 to 40
by producing a mixture from an alumlnate solution which is advantageously cooled
at a temperature of from -10 to ~10C, and an addition of a silicate solution
at a temperature such that the resulting temperature of the medium is substantial-
ly lower than am~ient temperature, and maintaining suficient agitation to
permit a homogenous solution to be produced before a gel appears,
b) the temperature of the reaction medium is raised to a temperature
in the range of from 60 to 100C,
c) the medium is maintained at that temperature for a sufficient
period of time of from 0.2 to about 5 hours so as to effect crystallisation and
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redispersion of the silico-aluminate in the eorm of a suspension of crystals in
the liquid phase, and
d) the crystals produced in this way are separated and dried.
In the process of the invention, it is important to pay attention to
the amount of the initial mixture so as to permit homogenisation oE the medium
in a period of time which is less than the gelling time at the equilibrium
temperature of the mixture. Advantageously, said period of time is less than
15 minutes.
It is observed that, unexpectedly, a silico-aluminate is produced which
has molar ratios equal to those of the starting composition, without the necessity
to operate with an excess of alumina. Moreover, it is noted that the resulting
product has a high level of thermal stability and a very substantial capacity
for the absorption of water.
Moreover, in this manner of operation, the levels of concentration in
respect of reactants at the end of evolution in the liquid phase may be kept very
low in respect of A12O3 and SiO2, less than 10 g/l and advantageously less than
S g/l, while maintaining a very high level of concentration in respect Oe
zeolite, of the order of at least 200 g/l, the concentration in respect oE
Na20 being at least 70 g/l when the alkali metal is sodium.
The process according to the invention may be perEormed continuously
or discontinuously.
In the continuous mode, in practice, operation is advantageously as
follows:
- First of all, there is produced the mixture of a solution of sodium
aluminate, which is cooled to the above-specified range, and a solution of sodium
silicate, which is at a temperature close to ambient temperature.
- Before gelling, said mixture is sprayed in a first region, at a
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density which is less than the density of the aqueous mixture, which is heated
at a temperature such that, after contact, said aqueous mixture is raised to the
selected reaction temperature, said first region comprising a water-immiscible
heat-exchange medium such as petroleum or oil bath.
- The temperature in the bath is maintained in a second downstream
region until conversion to the crystalline phase is complete, while providing
for a piston-type advance movement in said downstream region.
- The reaction medium containing the crystallites of silico-aluminates
is collected in the suspension state, and the crystallites are separated from
the suspension by any known means such as crystallisation or centrifuging, washed
and collected.
Advantageously, the first region is a transfer region in which the
medium is subjected to agitation for a very short residence time of from 1 to 2
seconds, while the second region is of piston-type advance movement kind and
corresponds to a much longer residence time.
The present invention will be more easily appreciated by means of the
following Examples which are given purely by way of illustration and without any
limitation whatsoever. The accompanying drawing shows diagrammatically an
apparatus for carrying out the process of the invention.
Example_
A solution of sodium aluminate containing 219.5 g of aluminate is
dissolved in 757.3 g of a solution containing 10 g/l of NaOH and is cooled to a
temperature of -5C in a tubular exchanger 1, at a flow rate of 10 l/hour. The
cooled flow is continuously mixed with a flow 3 of 4 l/h of a solution of sodium
silicate which is at a temperature of 20C and which titrates 26.9% of SiO2 and
39.46% of Na20 by weight, in an agitated reaction vessel 2.
The homogenous mixture, the temperature of which is established in the
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region of 15C, is used to supply, by means of a peristaltic pump 4, an injector5 having capillaries with ~ of 0.5 mm, for continuously forming drops which fallinto the upper part of a reaction vessel 6 which is filled with petroleum
maintained at a temperature of 85C by a circulation 7 of heated brine.
The density of the bath is so adjusted that the mean time taken by the
drops formed by the capillaries to fall is 3 seconds. At the end of that period,the spherical particles have gelled and accumulate at the bottom of the reactionvessel and are gradually converted into a fluid suspension of silico-aluminate
which accumulates in the conical part 9 of the reaction vessel. The suspension
is continuously drawn off by means of a suction pipe 8 at a rate of 14 l/hour
after 1 hour of continuous supply of the reactants, in order to define a mean
residence time for the reactants in the reaction vessel, of 2 hours ]5 minutes.
The suspension which is drawn off by suction is then mechanically dried
and washed using any known means, and dried in a drying oven at 100C.
The conditions of the Example correspond to an initial system as
follows:
SiO2/A12O3 = 2.00
Na2O/SiO2 = 1.19
~l2O/Na2O = 26.00
Finally, it is noted that the mother liquors contain 70 g/l of Na2O,
2.6 g/l of SiO2 and 3.0 g/l of A12O3.
The theoretical yield defined as:
theoretical weigh-t of anhydrous zeolite is 19%.
~ starting materials
The chemical formula of the resulting product is 1.06 Na2O. A12O3.
2.04 SiO2.
It will be seen that the ratio SiO2 is retained. The X-ray spectrum
A12O3
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is characteristic of type 4A. The granulometry as measured on a Coulter counter
gives a mean diameter in respect of the crystallites of 3.6~.
The grain distribution is as follows:
Diameter > % by weight
15~ 2.5
10~ 4
5~ 22
~ 68
2~ 93
1~ 98
The cation exchange capacity is 105 mg Ca/g of anhydrous product.
The water absorption capacity is then determined by measuring the
capacity for the absorption of water, defined as being the rate of increase in
the weight of a sample which is maintained in a desiccator at a given relative
humidity level, after having dried the sample for a period af two hours at a
given temperature.
The Tables, in summarized formJ will denote this value by
E 0.1 tlO% RH)
E 0.6 (60% ~I),
and the drying te~peratures will also be specified.
The following Table summarizes the results:
heat
treatment 250C 650C
E 0.1 20% 22.4%
E 0.6 22.3% 24.9%
E 0.9 25% 29%
Attention should be paid on the one hand to the high values in respect
of absorption of water and on the other hand to the remarkable behaviour of the
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silico-aluminates produced in accordance with the process o this invention~
after drying at temperatures of 650C.
Example 2
This Example is similar to the previous Example except that the
Na20/SiO2 ratio is somewhat higher, the system being as follows:
SiO2/A1203 = 2.00
Na20/SiO2 = 1.30
H20/Na20 = 23.8
It will be noted that setting is rapid, of the order of 5 min. The
mother liquors are of the following composition:
Na2 : 0.85 g/l, SiO2 : 1.5 g/l, A1203 : 2.3 g/l.
The theoretical yield is 18.8. The mean diameter is 2.8~ and the
granulometry is as follows
>15~ 1% >3~ 45%
>10~ 2% >2~ 85%
> ~ 7% >1~ 98%
This product once again is a zeolite ~A having the following formula:
l.OS Na20. A1203. 2.05 SiO2.
The SiO2/A1203 ratio is 2.05.
The exchange mechanism as a function of time was observed in regard to
the resulting product. The results obtained are summari~ed in the following
table:
Time in minutes 2 5 15 30
mmg of Ca exchanged
per g of anhydrous 83 106 115 117
product
After drying at a temperature of 250C, the following values in respect
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of water absorption are found:
E 0.1 = 20.5%S E 0.6 = 22.4%, E 0.9 = 24%.
This Example is similar to the previous Example, except that the degree
of dilution is increased slightly:
SiO2/A1203 = 2.0
Na20/SiO2 = 1.30
H20/Na20 = 25
The mother liquors are of the following composition:
Na20 = 85 g/l, SiO2 = 2 g/l, A12O3 = 2.2 g/l.
The mean diameter of ~he zeolite is 2.6~ and its granulometry corre-
sponds to the following distribution:
>15~ 1.5%
>10~ 2.5%
> 5~ 7 %
> 3~ 38 %
> 2~ 78 %
> 1~ 98 %
AEter drying at a temperature of 250C, the water absorption values
are as follows:
E 0.1 = 20.3%, E 0.6 = 22.6%, E 0.9 = 23.5%.
This once again is a zeolite 4A having the formula:
1.09 Na20 Al23 2-02 Si2
ExaDIpIe 4
This Example is identical to Example 3 as regards the reactants, but
instead of using the device of Example 3, the medium is caused to develop in a
double-jacket 2 litre balloon flask provided wi~h an agitator, by introducing the
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cooled aluminate-silicate mixture in the form of a sol into the flask at a
temperature of 85C. The crystallisation time is 2 hours 15 minutes. The
result is a silico-aluminate o~ type 4A without hydroxysodalite. The crystalline
silico-aluminate corresponds to the following formula:
1.09 Na2 Al23 2-02 Si2
The composition of the mother liquors is as f~llows:
Na20 85 g/l
SiO2 2 g/l
A1203 2.2 g/l
Exchange mechanism:
minutes 2 5 15 30
mg Ca/g of
produc~ 74 103 112 116
Granulometry in ~ Mean ~ 2.6
>15 3%
>10 6%
> 5 17%
> 3 38%
> 2 73%
> 1 95%
The theoretical yield is the same as in the previous Example.
Example 5
The starting material in this Example is a system having the following
initial ratios:
SiO2/A1203 = 2.20
Na20/SiO2 = 1.30
H20/Na20 = 25.00
~s~
The process started from a solution of sodium silicate containing
554.5 g of silicate with a molar ratio o~ 3.~, 82.4 g of caustic soda, 315.4 g
of water (that is to say, 2.49 ~SiO2) 1.74 (Na20) 38.68 H20) and a solution of
sodium alu~inate containing 219.5 g of dry sodium aluminate in 737.7 ml of a
10 g/l soda solution ~namely 1.13 A1203 . 1.49 Na20 . 42.1 H20).
The mode of operation is as follows: The two solutions are adjusted
to a temperature of about -5C. The silicate solution is slowly poured into the
aluminate solution, while maintaining rapid agitation in the medium ~Ultra turax~
so as to form a homogenous solution which rapidly gels ~in a time of the order
of one minute).
The other operating conditions are as set forth in the other Examples.
The final product which is 100% of type 4A has an SiO2/A1203 molar
ratio which is maintained at a value of 2.02 and, after drying at a temperature
of 250C, the following water absorption values:
E 0.1 = 20%, E 0.6 = 22.1%, E 0.9 = 24.8%.
Example 6
The starting materials are a silicate solution comprising 9.26 Na20.
29.1 SiO2 which is diluted in a proportion of 352 g of water per 82.4 g of
silicate and an aluminate solution, so as to produce the following reaction
medium, which is an initial composition corresponding to that set forth in United
States Patent No 3 679 426, Example 17:
Na20/SiO2 = 0.65
SiO2/A12O3 = 2.0
H2O/Na2~ = 150
This therefore is a highly dilute solution, but after the two solutions
have been coo]ed to a temperature of about 0C, the silicate solution is fed into
the alumina solution, maintaining strong mechanical agitation so as to achieve
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homogeneity of the mixture before a gell is formed. The g01 is then maintained
at a temperature of 50C for 8 hours, while maintaining mechanical agitation,
and is left to crystallize for 20 hours at a temperature of 100C.
The resulting product corresponds to the following formula:
1 Na2O. 1 A1203. 2.10 SiO2
and has a mean particle diameter of 2.5~ with a variation:
60% _ l~
86% ~ 2~
and a 5% refusal at a lO sieve.
The following water absorption levels at a temperature of 250C are
found:
E 0.1 = 22%
E 0.6 = 25%
E 0.9 = 30%
These Examples clearly illustrate the merit of the present invention
and reveal the unexpected character o~ the process which results in zeolites
with a high moisture absorption capacity.
