GB1571264A - Production of finely divided zeolite - Google Patents

Description

(54) PRODUCTION OF FINELY DIVIDED ZEOLITE A (71) We, BAYER AKTIEN GESELLSCHAFT, a body corporate organised under the laws of the Federal Republic of Germany, of 509 Leverkusen, Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for the production of finely divided zeolite A using kaolin as starting material.

Zeolites of type A with their special properties have long been known (cf. for example German Patent No. 1,038,017; German Auslegeschrift No. 1,095,795; "Zeolite Molecular Sieves" by D. W.

Breck, 1974, John Wiley & Sons, New York). These molecular sieves have an average composition of approximately (1.0+0.2)Na20 . A1203.

(l.8510.2)SiO2 . (6)H2O, the sodium ions being replaceable by ion exchange, for example with potassium, calcium, strontium, lithium, caesium, thallium, silver and ammonium ions; a crystal structure characterized by the following most important X-ray interferences: 12.3 8.7 7.1 4.1 3.7 3.4 3.3 3.0 : and a pore width of approximately 4 a in the sodium form, approximately 5 in the calcium form and approximately 3 A in the potassium form.

Molecular sieves have found significant commercial applications both as adsorbents (for example as intensive drying agents for gases, liquids, lacquers and plastics; as agents for the removal of triatomic molecules from gases, for example for the production of nitrogen; as agents for the separation of n- and iso-alkanes) and as ion exchangers for use, (for example, for water softening, or for desalination of sea water).

They are generally produced by combining sodium silicate and sodium aluminate solutions and heating the resulting mixture for several hours at a temperature of from SOOC to 1000C.

With systems such as these, the zeolite is generally formed in crystal sizes of from 1 to 10 microns (cf. "Zeolite Molecular Sieves", D. W. Breck, John Wiley & Sons, 1974, pages 384 et seq). However the individual crystallites often coalesce into agglomerates and/or aggregates, resulting in the formation of relatively large particles which generally have a particle size in the range of from 10 to 40 microns.

Unfortunately, the presence of coarse particles gives rise to problems in a number of applications, for example in the lacquer sector.

In the softening of water, coarse particles also show a lower exchange capacity and an undesirably slow exchange of the alkaline earth metal ions. Accordingly, coarse particles are particularly troublesome in detergents. In this case, excessively coarse particles also show a tendency towards sedimentation, where as small particles are readily dispersible and remain in suspension both in the washing solution and, hence, in waste pipes as well.

In order to obviate these disadvantages, it is necessary to reduce the number of coarse particles in the production of zeolites from waterglass-aluminate solutions by additional measures which are often extremely expensive.

The use of kaolin as starting material for the production of zeolites is also known per se. Kaolin has the composition Al203. 2SiO2. 2H2O and is a constituent of numerous clays.

In 1926, German Patent No. 426,083 disclosed a process by which a baseexchanging material for softening water was produced from clay. After heating to 500"C--700"C, the clay shaped into granules or needles is boiled for 1 to 2 hours with a 10 to 20% lye.

Example 1 of US Patent No. 2,544,695 (1951), discloses the calcination of China Clay (=kaolin) for 3 hours at 8000 C, with addition of 10% NaOH-solution to the calcine and heating of the resulting suspension with stirring for 16 hours at 104"C. According to US Patent No.

2,992,068, kaolin is calcined for 16 hours at about 760"C.

According to Journ. Amer. Ceram. Soc.

35, 205 (1955), the kaolin calcines obtainable at up to 9500C are called metakaolin.

According to Example II of US Patent No. 3,119,660, kaolin granulates are obtained by calcination for 3 hours at 7000C and hydrothermal treatment with NaOHsolution gives 87% of zeolite A in the form of granules.

In Example VII of the same patent, kaolin is calcined for 21 hours at 7000 C, the calcine is mixed with one and a half times the quantity of uncalcined kaolin and water to form a kneadable mass, the mass thus obtained is pre-granulated, the granules are dried and then calcined for 15.2 hours at 700"C. Hydrothermal treatment gives 92% of zeolite A according to X-ray analysis and 100 of zeolite A according to adsorption analysis.

It would appear from these disclosures that the yield of zeolite A is better, the longer the kaolin is calcined.

The present invention relates to a process for the production of zeolite A having a crystal size of from 0.1 to 6 microns, and preferably from 0.5 to 3 microns, by hydrothermally reacting calcined kaolin with aqueous alkali hydroxides, wherein kaolin is scattered into a gaseous stream heated to a temperature of from 600"C to 1000"C and, after a residence time in this gaseous stream of from 0.01 to 5 seconds, is suspended at a temperature above 300"C in a aqueous solution containing from 4 to 20 , by weight of sodium hydroxide and the resulting suspension is maintained for 0.5 to 6 hours at a temperature in the range of from 70"C to 1000C.

It is surprising that pure zeolite A having the particularly advantageous properties referred to above can be obtained from a kaolin which has only been heated for 0.01 to 5 seconds in a gaseous stream with a temperature of from 600 to 1000 C.

The process according to the present invention gives crystallites in a very uniform size of from 0.1 m to 6 ym, preferably from 0.5 to 3 pm, which show hardly any intergrowth. Instead, they are largely in the form of cube-shaped individual crystals.

Any agglomerates formed during drying may readily be broken up again by grinding, for example in a conventional pin disc mill, hammer mill or other impact mill, without any damage to the ion exchange or adsorption capacity.

The size and shape of the crystallites can be discerned very clearly from electron microscope photographs. Sedimentation analyses by Andreasen's method confirm that aggregation, agglomeration and also flocculation are all minimal. Measurements such as these show that more than 90 X" of the particles and, in many cases, more than 95 X" have a particle size of less than 5 pm.

The kaolin may be introduced into the hot gaseous stream for example by means of a vibrating chute, a rotary feeder, a feed screw, a fan blower or a compressed-air spray nozzle. Since kaolin is generally present in readily divisible form, it is not absolutely essential to grind it beforehand.

However, this can be advantageous in order, for example, to obtain better distribution in the gaseous stream and hence better utilization of the energy introduced with the gaseous stream. The most advantageous size of the starting material has proved to be a particle size of from 0.1 micron to 10 microns.

Pin disc mills, hammer mills or other impact mills have proved to be suitable grinding units. Any type of gas may be used.

In most cases, the most economical gas is the waste gas of a natural gas or oil burner.

The required temperatures may readily be controlled by additionally introducing air in the appropriate quantity. The temperature of the gas before it reaches the point at which the kaolin is introduced should not exceed 1000"C. The introduction of the kaolin should be controlled so that the product of calcination has a temperature of more than 600"C. Outside these temperature limits, secondary products can be formed to the detriment of the output of end product.

The heat treatment may be carried out in a simple tube. The cross-section and length of the tube from the point at which the kaolin is introduced to the point at which the calcine is discharged should be adapted so as to guarantee a residence time of the solid in the gaseous stream of from 0.01 to 5 seconds. For example, for a throughput of 3600 m3/h of heating gas through a tube having a cross-section of 0.25 m2, the length of the tube between the point at which the solid is introduced and the point at which it is discharged should amount to 2 meters in order to obtain a residence time of the solid in the gaseous stream of 0.5 second.

Tests were also conducted with a heating chamber in the form of an inverted cone, in which the heating gas is introduced tangentially above the tip of the cone while the kaolin powder is introduced by means of a spray nozzle arranged in the middle of the flat cone surface. The calcined product is discharged tangentially at the point of largest diameter i.e. at the upper end of the cone. The calcined kaolin is best separated by means of one or more cyclones. After cooling of the gaseous stream, for example by means of a steam or warm-air generator, the residual constituents of calcined kaolin are removed by means of an electrostatic separator, a filter unit or a washing unit.

One suitable apparatus for carrying out the process according to the invention is described for example in British Patent No.

869,966 and in US Patent No. 3,021,195.

In the process according to the present invention, the calcined kaolin is separated by means of a cyclone and is delivered directly, i.e. while still hot, into the stirrerequipped vessel containing the NaOHsolution required for the hydrothermal synthesis of the zeolite. The hot solid brings with it most of the heat which is required for heating the mixture.

It is surprising that this so-called hot input contributes not only to a high yield of zeolite A but also to the advantageous properties of the product referred to above.

The required final crystallisation temperature may be adjusted by means of a heating coil or even directly by the introduction of steam. Relatively high temperatures should be selected, particularly for low concentrations of NaOH, for example temperatures in the range of from 80"C to 1000C for 4% to 10% by weight NaOH-solutions, because otherwise relatively long crystallisation times are necessary. Above a concentration of 15 ,; by weight of NaOH, the temperatures should not exceed 80"C because otherwise the formation of sodalite as an undesirable secondary constituent is promoted. With a concentration of this order, however, the crystallisation rate is high enough, even at relatively low temperatures. A crystallisation time of from 0.5 to 3 hours is normally sufficient.

The quantity of NaOH-solution should be at least sufficient so that the suspension is stirrable. The Na2O/SiO2-ratio in the suspension preferably amounts to between 0.5 and 10 and most preferably to between 0.7 and 1.5.

There is no upper limit to the quantity of solution. However, it is uneconomical to use excessively large quantities. In general, it is best to use 3m3 to 6m3 of NaOH-solution per ton of zeolite.

On completion of the synthesis, the mother liquid is separated off in the conventional manner. The mother liquor is returned to the starting vessel where additional concentrated sodium hydroxide solution is added to it in order to adjust the required final concentration and, after mixing with fresh hot calcined kaolin, is used for another synthesis.

In one preferred embodiment of the present invention, the ratio of kaolin to NaOH is kept constant during the introduction of hot kaolin into the receiving vessel. This result may be achieved for example by adapting the inputs of hot kaolin and 4 /"20% sodium hydroxide solution, preferably 10-t6% sodium hydroxide solution, into the receiving vessel to one another and by simultaneously introducing the two components into the receiving vessel.

In one particularly preferred embodiment of the process according to the present invention, the hot kaolin is deposited into the receiving vessel filled with l%-10% sodium hydroxide solution. The concentration of NaOH is increased to 10--20 by the addition of concentrated NaOH. It is preferred to use the mother liquor and/or NaOH-containing washing water in the form of dilute sodium hydroxide solution. In a variant of this embodiment, the deposition of the hot kaolin into the receiving vessel and the input of dilute sodium hydroxide solution take place simultaneously so that the ratio of kaolin to NaOH remains constant during mixing.

As already mentioned, the final ratio of Na2O to SiO2 in the suspension preferably amounts to between 0.5 and 10 and most preferably to between 0.7 and 1.5 in all embodiments of the process.

The solid may be directly used in the form of the filter cake obtained, for example, for the production of detergents. It may also be washed by standard methods to remove the excess alkali adhering to it. The washing water may also be used, for example, for the described waste-gas wash and may then be reused for the synthesis process. The washed zeolite may be dried and, in the powder form which accumulates, may be used, for example, for the production of detergents or, following the addition of clays, silica sol or other binders, even for the production of granules. Zeolite granules are known to be widely used for drying, purifying and adsorbing gases or liquids.

The zeolite powder obtained after drying is present in the advantageous crystallite size mentioned above of from 0.1 to 6 um preferably from 0.5 to 3 pm. The crystallites show hardly any intergrowth and also hardly any agglomeration.

If the filter cake is heated to temperatures higher than those normally used for drying, for example to between about 300 and 4000 C, the zeolite A is obtained in the form of an adsorption-active poweder because, at temperatures of this order, not only the external water, but also the water molecules are removed from the voids inside the individual zeolite crystals. The active zeolite powder may advantageously be used for example for drying liquids, particularly solvents or lacquers, for example solventfree polyurethane coatings.

The process according to the invention is distinguished from known processes for the production of zeolite A from kaolin by its particular economy. The volume-time yield is considerably increased by the process according to the invention. The utilization of the energy of the heated kaolin for heating the reaction mixture is also particularly worth mentioning. Another particular advantage of the process according to the present invention is that no polluted effluents are formed.

The process according to the invention is illustrated by the following Examples of which Examples 2 and 3 are included for the purposes of comparision.

EXAMPLE 1 Kaolin was introduced by means of a spray nozzle into a natural gas or petroleum fired shock-calcination furnace of the type described in US Patent No. 3,021,195. The temperature in the cone-shaped heating chamber was 850"C. After an average residence time of about 2 seconds, the calcined kaolin was introduced through a battery of cyclones at a temperature of 700"C into 16% sodium hydroxide solution accommodated in a 60 cubic meter receiving vessel.

The calcining furnace had a separation capacity of 5 t/h. The final ratio of Na2O to SiO2 amounted to 1. After the kaolin had been added, the temperature of the sodium hydroxide in the receiving vessel rose from the initial 40"C to 800 C. The suspension was then heated with steam to 90--1000C and stirred at that temperature for 2 hours. On completion of crystallization, the zeolite A obtained is separated from the liquid phase by a conventional method. The end product may be put to its further use, for example as a water softener in detergents, in the form of a wet cake. However, it may also be washed, dried and, optionally, even ground, activated granulated or made into a paste by standard methods.

The mother liquor separated off which had a residual NaOH-concentration of around 8% was returned to the receiving vessel together with washing waters and concentrated to a 16 /n sodium hydroxide solution by the addition of concentrated NaOH.

The zeolite A separated off had the following particle size distribution: 92 Z" by weight smaller than 4 microns 96 /n by weight smaller than 6 microns.

EXAMPLE 2 (Comparative) The shock-calcination furnace of Example 1 was operated at 5000 C. The formation of metakaolin was incomplete so that, after reaction with NaOH, the end product contained kaolin and sodalite.

EXAMPLE 3 (Comparative) The shock-calcination furnace of Example 1 was operated at a temperature of 1100"C as a result of which mullite was partly formed from the kaolin and likewise contaminated the end product.

EXAMPLE 4 The shock-calcination of kaolin into metakaolin was carried out in the same way as described in Example 1, except that deposition into the receiving vessel was accompanied by introduction of the 16% NaOH. The input of NaOH was controlled by measuring instruments so that the ratio of Na2O to SiO2 remained constant at 0.9 during the simultaneous introduction. On completion of deposition, the further procedure was as described in Example 1.

EXAMPLE 5 The procedure was as described in Example 4, except that deposition of the hot metakaolin is accompanied by the introduction of sodium hydroxide solution in the form of dilute sodium hydroxide (mother liquor). The ratio of Na2O to SiO2 during deposition was again constant, but only amounts to 0.5. After the simultaneous introduction of kaolin and mother liquor, the ratio of Na2O to SiO2 was increased to 1.1 by the introduction of concentrated NaOH. The final suspension was then further processed in the same way as in Example 1.

In all the Examples according to the invention, the final zeolite had a crystallite size of from 0.1 to 6 ,xlm. Comparative Example 2 and 3 demonstrate incomplete formation of the desired metakaolin with the formation of sodalite or mullite contaminating the end product.

WHAT WE CLAIM IS: 1. A process for the production of zeolite A having a crystallite size of substantially from 0.1 to 6 microns by the hydrothermal reaction of calcined kaolin with aqueous alkali hydroxides, wherein kaolin is scattered into a gaseous stream heated to a temperature of from 600"C to 10000C and, after a residence time in this gaseous stream of from 0.01 to 5 seconds, the hot kaolin is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. for example to between about 300 and 4000 C, the zeolite A is obtained in the form of an adsorption-active poweder because, at temperatures of this order, not only the external water, but also the water molecules are removed from the voids inside the individual zeolite crystals. The active zeolite powder may advantageously be used for example for drying liquids, particularly solvents or lacquers, for example solventfree polyurethane coatings. The process according to the invention is distinguished from known processes for the production of zeolite A from kaolin by its particular economy. The volume-time yield is considerably increased by the process according to the invention. The utilization of the energy of the heated kaolin for heating the reaction mixture is also particularly worth mentioning. Another particular advantage of the process according to the present invention is that no polluted effluents are formed. The process according to the invention is illustrated by the following Examples of which Examples 2 and 3 are included for the purposes of comparision. EXAMPLE 1 Kaolin was introduced by means of a spray nozzle into a natural gas or petroleum fired shock-calcination furnace of the type described in US Patent No. 3,021,195. The temperature in the cone-shaped heating chamber was 850"C. After an average residence time of about 2 seconds, the calcined kaolin was introduced through a battery of cyclones at a temperature of 700"C into 16% sodium hydroxide solution accommodated in a 60 cubic meter receiving vessel. The calcining furnace had a separation capacity of 5 t/h. The final ratio of Na2O to SiO2 amounted to 1. After the kaolin had been added, the temperature of the sodium hydroxide in the receiving vessel rose from the initial 40"C to 800 C. The suspension was then heated with steam to 90--1000C and stirred at that temperature for 2 hours. On completion of crystallization, the zeolite A obtained is separated from the liquid phase by a conventional method. The end product may be put to its further use, for example as a water softener in detergents, in the form of a wet cake. However, it may also be washed, dried and, optionally, even ground, activated granulated or made into a paste by standard methods. The mother liquor separated off which had a residual NaOH-concentration of around 8% was returned to the receiving vessel together with washing waters and concentrated to a 16 /n sodium hydroxide solution by the addition of concentrated NaOH. The zeolite A separated off had the following particle size distribution: 92 Z" by weight smaller than 4 microns 96 /n by weight smaller than 6 microns. EXAMPLE 2 (Comparative) The shock-calcination furnace of Example 1 was operated at 5000 C. The formation of metakaolin was incomplete so that, after reaction with NaOH, the end product contained kaolin and sodalite. EXAMPLE 3 (Comparative) The shock-calcination furnace of Example 1 was operated at a temperature of 1100"C as a result of which mullite was partly formed from the kaolin and likewise contaminated the end product. EXAMPLE 4 The shock-calcination of kaolin into metakaolin was carried out in the same way as described in Example 1, except that deposition into the receiving vessel was accompanied by introduction of the 16% NaOH. The input of NaOH was controlled by measuring instruments so that the ratio of Na2O to SiO2 remained constant at 0.9 during the simultaneous introduction. On completion of deposition, the further procedure was as described in Example 1. EXAMPLE 5 The procedure was as described in Example 4, except that deposition of the hot metakaolin is accompanied by the introduction of sodium hydroxide solution in the form of dilute sodium hydroxide (mother liquor). The ratio of Na2O to SiO2 during deposition was again constant, but only amounts to 0.5. After the simultaneous introduction of kaolin and mother liquor, the ratio of Na2O to SiO2 was increased to 1.1 by the introduction of concentrated NaOH. The final suspension was then further processed in the same way as in Example 1. In all the Examples according to the invention, the final zeolite had a crystallite size of from 0.1 to 6 ,xlm. Comparative Example 2 and 3 demonstrate incomplete formation of the desired metakaolin with the formation of sodalite or mullite contaminating the end product. WHAT WE CLAIM IS:

1. A process for the production of zeolite A having a crystallite size of substantially from 0.1 to 6 microns by the hydrothermal reaction of calcined kaolin with aqueous alkali hydroxides, wherein kaolin is scattered into a gaseous stream heated to a temperature of from 600"C to 10000C and, after a residence time in this gaseous stream of from 0.01 to 5 seconds, the hot kaolin is

suspended with a temperature of above 3000C in an aqueous solution containing from 4 to 20% by weight of sodium hydroxide and the resulting suspension is maintained for 0.5 to 6 hours at a temperature of from 70"C to 100"C.

2. A process as claimed in claim I, wherein the hot kaolin is suspended in 1 to 100 NaOH and the NaOH is subsequently adjusted to a total NaOH concentration of from 100 to 20% by the addition of concentrated NaOH.

3. A process as claimed in claim 1, wherein the aqueous solution contains from 40 to 10% by weight of sodium hydroxide and wherein the suspension is maintained at a temperature of from 80"C to 1000C.

4. A process as claimed in claim 1, wherein the aqueous solution contains from 15 to 20% by weight of sodium hydroxide and wherein the suspension is maintained at a temperature of from 70"C to 80or.

5. A process as claimed in any of claims 1 to 4, wherein the final ratio of Na2 to SiO2 in the suspension is between 0.5 to 10.

6. A process as claimed in claim 5, wherein the ratio is between 0.7 to 1.5.

7. A process for the production of zeolite substantially as herein described with reference to any of the specific Examples 1, 4 and 5.

8. Zeolite A having a crystallite size of substantially from 0.1 to 6 microns when produced by a process as claimed in any of claims 1 to 7.

9. Zeolite A as claimed in claim 8 having a crystallite size of from 0.5 to 3 microns.